Advanced Electronic Materials and Devices

Group Leader: Jose Antonio Garrido

Publications

2024

Clinical translation of graphene-based medical technology
Kostas Kostarelos; Carolina Aguilar; José A. Garrido Nature Reviews Electrical Engineering; 1 (2): 75 - 76. 2024. 10.1038/s44287-024-00020-1.
Concurrent functional ultrasound imaging with graphene-based DC-coupled electrophysiology as a platform to study slow brain signals and cerebral blood flow under control and pathophysiological brain states
Zhang, Julie Meng; Masvidal-Codina, Eduard; Nguyen, Diep; Illa, Xavi; Degardin, Julie; Goulet, Ruben; Prats-Alfonso, Elisabet; Matsoukis, Stratis; Guger, Christoph; Garrido, Jose Antonio; Picaud, Serge; Guimera-Brunet, Anton; Wykes, Rob C Nanoscale Horizons; 9 (4) 2024. 10.1039/d3nh00521f. IF: 8.000
Engineered Graphene Material Improves the Performance of Intraneural Peripheral Nerve Electrodes
Rodríguez-Meana B; Del Valle J; Viana D; Walston ST; Ria N; Masvidal-Codina E; Garrido JA; Navarro X Advanced Science; : e2308689. 2024. 10.1002/advs.202308689. IF: 14.300
Ethanol Solvation of Polymer Residues in Graphene Solution-Gated Field Effect Transistors
Merino, Juan Pedro; Brosel-Oliu, Sergi; Rius, Gemma; Illa, Xavi; Sulleiro, Manuel Vazquez; Del Corro, Elena; Masvidal-Codina, Eduard; Bonaccini Calia, Andrea; Garrido, Jose Antonio; Villa, Rosa; Guimera-Brunet, Anton; Prato, Maurizio; Criado, Alejandro; Prats-Alfonso, Elisabet Acs Sustainable Chemistry & Engineering; 2024. 10.1021/acssuschemeng.4c01538. IF: 7.100
Graphene-based microelectrodes with bidirectional functionality for next-generation retinal electronic interfaces
Duvan, Fikret Taygun; Cunquero, Marina; Masvidal-Codina, Eduard; Walston, Steven T; Marsal, Maria; de la Cruz, Jose Manuel; Viana, Damia; Nguyen, Diep; Degardin, Julie; Illa, Xavi; Zhang, Julie M; Bernicola, Maria del Pilar; Macias-Montero, Jose Gabriel; Puigdengoles, Carles; Castro-Olvera, Gustavo; del Corro, Elena; Dokos, Socrates; Chmeissani, Mokhtar; Loza-Alvarez, Pablo; Picaud, Serge; Garrido, Jose A Nanoscale Horizons; 2024. 10.1039/d4nh00282b. IF: 8.000
Nanoporous graphene-based thin-film microelectrodes for in vivo high-resolution neural recording and stimulation
Viana, Damia; Walston, Steven T; Masvidal-Codina, Eduard; Illa, Xavi; Rodriguez-Meana, Bruno; del Valle, Jaume; Hayward, Andrew; Dodd, Abbie; Loret, Thomas; Prats-Alfonso, Elisabet; de la Oliva, Natalia; Palma, Marie; del Corro, Elena; Bernicola, Maria del Pilar; Rodriguez-Lucas, Elisa; Gener, Thomas; de la Cruz, Jose Manuel; Torres-Miranda, Miguel; Duvan, Fikret Taygun; Ria, Nicola; Sperling, Justin; Marti-Sanchez, Sara; Spadaro, Maria Chiara; Hebert, Clement; Savage, Sinead; Arbiol, Jordi; Guimera-Brunet, Anton; Puig, M Victoria; Yvert, Blaise; Navarro, Xavier; Kostarelos, Kostas; Garrido, Jose A Nature Nanotechnology; 2024. 10.1038/s41565-023-01570-5. IF: 38.100
On the Electrochemical Activation of Nanoporous Reduced Graphene Oxide Electrodes Studied by In Situ/Operando Electrochemical Techniques
Bernicola, Maria del Pilar; Lounasvuori, Mailis; Padilla-Pantoja, Jessica; Santiso, Jose; Debiemme-Chouvy, Catherine; Perrot, Hubert; Petit, Tristan; Garrido, Jose A; del Corro, Elena Advanced Functional Materials; 2024. 10.1002/adfm.202408441. IF: 18.500
Single-Step Functionalization Strategy of Graphene Microtransistor Array with Chemically Modified Aptamers for Biosensing Applications
Brosel-Oliu, Sergi; Rius, Gemma; Avino, Anna; Nakatsuka, Nako; Illa, Xavi; del Corro, Elena; Delga-Fernandez, Marta; Masvidal-Codina, Eduard; Rodriguez, Natalia; Merino, Juan Pedro; Criado, Alejandro; Prato, Maurizio; Tkatchenko, Raphaela; Eritja, Ramon; Godignon, Philippe; Garrido, Jose Antonio; Villa, Rosa; Guimera, Anton; Prats-Alfonso, Elisabet Small; : e2308857. 2024. 10.1002/smll.202308857. IF: 13.000
Surface two-dimensional hole gas in Si doped β-Ga2O3 thin film
Chikoidze, Ekaterine; Leach, Jacob; Chi, Zeyu; von Bardeleben, Jurgen; Ballesteros, Belen; Goncalves, Anne-Marie; Tchelidze, Tamar; Dumont, Yves; Perez-Tomas, Amador Journal Of Alloys And Compounds; 971: 172713. 2024. 10.1016/j.jallcom.2023.172713. IF: 5.800

2023

Assessment of large critical electric field in ultra-wide bandgap p-type spinel ZnGa2O4
Chi, ZY; Tchelidze, T; Sartel, C; Gamsakhurdashvili, T; Madaci, I; Yamano, H; Sallet, V; Dumont, Y; Perez-Tomas, A; Medjdoub, F; Chikoidze, E Journal Of Physics D-Applied Physics; 56 (10): 105102. 2023. 10.1088/1361-6463/acbb14. IF: 3.400
Calcium Imaging In Electrically Stimulated Flat-Mounted Retinas
Cunquero, M; Marsal, M; Castro-Olvera, G; Walston, ST; Duvan, FT; Macias-Montero, JG; Puigdengoles, C; Chmeissani, M; Garrido, JA; Loza-Alvarez, P Jove-Journal Of Visualized Experiments; (198): e65705. 2023. 10.3791/65705. IF: 1.200
Native defects association enabled room-temperature p-type conductivity in β-Ga2O3
Chi, ZY; Sartel, C; Zheng, YL; Modak, S; Chernyak, L; Schaefer, CM; Padilla, J; Santiso, J; Ruzin, A; Gonçalves, AM; von Bardeleben, J; Guillot, G; Dumont, Y; Pérez-Tomás, A; Chikoidze, E Journal Of Alloys And Compounds; 969: 172454. 2023. 10.1016/j.jallcom.2023.172454. IF: 6.200
Neural substrates of cognitive impairment in a NMDAR hypofunction mouse model of schizophrenia and partial rescue by risperidone
Delgado-Sallent, C; Gener, T; Nebot, P; Lopez-Cabezon, C; Puig, MV Frontiers In Cellular Neuroscience; 17: 1152248. 2023. 10.3389/fncel.2023.1152248. IF: 5.300
Physics-based bias-dependent compact modeling of 1/f noise in single- to few-layer 2D-FETs
Mavredakis, N; Pacheco-Sanchez, A; Alam, MH; Guimerà-Brunet, A; Martinez, J; Garrido, JA; Akinwande, D; Jiménez, D Nanoscale; 15 (14): 6853 - 6863. 2023. 10.1039/d3nr00922j. IF: 6.700

2022

Electrical properties of p -type Zn:Ga2O3thin films
Chikoidze E., Sartel C., Yamano H., Chi Z., Bouchez G., Jomard F., Sallet V., Guillot G., Boukheddaden K., Pérez-Tomás A., Tchelidze T., Dumont Y. Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films; 40 (4, 043401) 2022. 10.1116/6.0001766.

Ultra-wide bandgap gallium oxide (∼5 eV) has emerged as a novel semiconductor platform for extending the current limits of power electronics and deep ultraviolet optoelectronics at a predicted fraction of cost. Finding effective acceptor dopant for gallium oxide is a hot issue. One element that quite often is considered as a potential candidate is zinc. A number of experimental works have reported the signature of Zn-acceptor, but the direct evidence of hole conductivity was missing. In this work, p-type Zn-doped Ga2O3 thin films were grown by the metal-organic chemical vapour deposition technique on sapphire substrates. By high-temperature Hall effect measurements, Zn related acceptor level ionization energy as 0.77 eV above the valence band maximum was determined. Additionally, we have carried out the simulation study regarding the application of the Zn:Ga2O3 semi-insulating material, to be used as a guard ring for improving the high voltage performance of the Schottky diode structure. © 2022 Author(s).

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Full-bandwidth electrophysiology of seizures and epileptiform activity enabled by flexible graphene microtransistor depth neural probes
Bonaccini Calia A., Masvidal-Codina E., Smith T.M., Schäfer N., Rathore D., Rodríguez-Lucas E., Illa X., De la Cruz J.M., Del Corro E., Prats-Alfonso E., Viana D., Bousquet J., Hébert C., Martínez-Aguilar J., Sperling J.R., Drummond M., Halder A., Dodd A., Barr K., Savage S., Fornell J., Sort J., Guger C., Villa R., Kostarelos K., Wykes R.C., Guimerà-Brunet A., Garrido J.A. Nature Nanotechnology; 17 (3): 301 - 309. 2022. 10.1038/s41565-021-01041-9. IF: 39.213

Mapping the entire frequency bandwidth of brain electrophysiological signals is of paramount importance for understanding physiological and pathological states. The ability to record simultaneously DC-shifts, infraslow oscillations (<0.1 Hz), typical local field potentials (0.1–80 Hz) and higher frequencies (80–600 Hz) using the same recording site would particularly benefit preclinical epilepsy research and could provide clinical biomarkers for improved seizure onset zone delineation. However, commonly used metal microelectrode technology suffers from instabilities that hamper the high fidelity of DC-coupled recordings, which are needed to access signals of very low frequency. In this study we used flexible graphene depth neural probes (gDNPs), consisting of a linear array of graphene microtransistors, to concurrently record DC-shifts and high-frequency neuronal activity in awake rodents. We show here that gDNPs can reliably record and map with high spatial resolution seizures, pre-ictal DC-shifts and seizure-associated spreading depolarizations together with higher frequencies through the cortical laminae to the hippocampus in a mouse model of chemically induced seizures. Moreover, we demonstrate the functionality of chronically implanted devices over 10 weeks by recording with high fidelity spontaneous spike-wave discharges and associated infraslow oscillations in a rat model of absence epilepsy. Altogether, our work highlights the suitability of this technology for in vivo electrophysiology research, and in particular epilepsy research, by allowing stable and chronic DC-coupled recordings. © 2021, The Author(s), under exclusive licence to Springer Nature Limited.

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Ga2O3 and Related Ultra‐Wide Bandgap Power Semiconductor Oxides: New Energy Electronics Solutions for CO2 Emission Mitigation
Chi Z., Asher J.J., Jennings M.R., Chikoidze E., Pérez‐tomás A. Materials; 15 (3, 1164) 2022. 10.3390/ma15031164. IF: 3.623

Currently, a significant portion (~50%) of global warming emissions, such as CO2, are related to energy production and transportation. As most energy usage will be electrical (as well as transportation), the efficient management of electrical power is thus central to achieve the XXI century climatic goals. Ultra‐wide bandgap (UWBG) semiconductors are at the very frontier of electronics for energy management or energy electronics. A new generation of UWBG semiconductors will open new territories for higher power rated power electronics and solar‐blind deeper ultraviolet optoelectronics. Gallium oxide—Ga2O3 (4.5–4.9 eV), has recently emerged pushing the limits set by more conventional WBG (~3 eV) materials, such as SiC and GaN, as well as for transparent conducting oxides (TCO), such asIn2O3, ZnO and SnO2, to name a few. Indeed, Ga2O3 as the first oxide used as a semiconductor for power electronics, has sparked an interest in oxide semiconductors to be investigated (oxides represent the largest family of UWBG). Among these new power electronic materials, AlxGa1‐xO3 may provide high‐power heterostructure electronic and photonic devices at bandgaps far beyond all materials available today (~8 eV) or ZnGa2O4 (~5 eV), enabling spinel bipolar energy electronics for the first time ever. Here, we review the state‐of‐the‐art and prospects of some ultra‐wide bandgap oxide semiconductor arising technologies as promising innovative material solutions towards a sustainable zero emission society. © 2022 by the authors. Licensee MDPI, Basel, Switzerland.

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Heat dissipation in few-layer MoS2and MoS2/hBN heterostructure
Arrighi A., Del Corro E., Urrios D.N., Costache M.V., Sierra J.F., Watanabe K., Taniguchi T., Garrido J.A., Valenzuela S.O., Sotomayor Torres C.M., Sledzinska M. 2D Materials; 9 (1, 015005) 2022. 10.1088/2053-1583/ac2e51. IF: 7.103

State-of-the-art fabrication and characterisation techniques have been employed to measure the thermal conductivity of suspended, single-crystalline MoS2 and MoS2/hBN heterostructures. Two-laser Raman scattering thermometry was used combined with real time measurements of the absorbed laser power. Measurements on MoS2 layers with thicknesses of 5 and 14 nm exhibit thermal conductivity in the range between 12 Wm-1 K-1 and 24 Wm-1 K-1. Additionally, after determining the thermal conductivity of the latter MoS2 sample, an hBN flake was transferred onto it and the effective thermal conductivity of the heterostructure was subsequently measured. Remarkably, despite that the thickness of the hBN layer was less than a hal of the thickness of the MoS2 layer, the heterostructure showed an almost eight-fold increase in the thermal conductivity, being able to dissipate more than ten times the laser power without any visible sign of damage. These results are consistent with a high thermal interface conductance G between MoS2 and hBN and an efficient in-plane heat spreading driven by hBN. Indeed, we estimate G ∼ 70 MW m-2 K-1 for hBN layer thermal conductivity of 450 Wm-1 K-1 which is significantly higher than previously reported values. Our work therefore demonstrates that the insertion of hBN layers in potential MoS2-based devices holds the promise for efficient thermal management. © 2021 IOP Publishing Ltd.

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Postnatal environmental enrichment enhances memory through distinct neural mechanisms in healthy and trisomic female mice
Alemany-González M., Vilademunt M., Gener T., Puig M.V. Neurobiology of Disease; 173 (105841) 2022. 10.1016/j.nbd.2022.105841.

Stimulating lifestyles have powerful effects on cognitive abilities, especially when they are experienced early in life. Cognitive therapies are widely used to improve cognitive impairment due to intellectual disability, aging, and neurodegeneration, however the underlying neural mechanisms are poorly understood. We investigated the neural correlates of memory amelioration produced by postnatal environmental enrichment (EE) in diploid mice and the Ts65Dn mouse model of Down syndrome (trisomy 21). We recorded neural activities in brain structures key for memory processing, the hippocampus and the prefrontal cortex, during rest, sleep and memory performance in mice reared in non-enriched or enriched environments. Enriched wild-type animals exhibited enhanced neural synchrony in the hippocampus across different brain states (increased gamma oscillations, theta-gamma coupling, sleep ripples). Trisomic females showed increased theta and gamma rhythms in the hippocampus and prefrontal cortex across different brain states along with enlarged ripples and disrupted circuit gamma signals that were associated with memory deficits. These pathological activities were attenuated in their trisomic EE-reared peers. Our results suggest distinct neural mechanisms for the generation and rescue of healthy and pathological brain synchrony, respectively, by EE and put forward hippocampal-prefrontal hypersynchrony and miscommunication as major targets underlying the beneficial effects of EE in intellectual disability. © 2022 The Authors

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The advantages of mapping slow brain potentials using DC-coupled graphene micro-transistors: Clinical and translational applications
Wykes, RC; Masvidal-Codina, E; Guimera-Brunet, A; Garrido, JA Clinical And Translational Medicine; 12 (7) 2022. 10.1002/ctm2.968. IF: 8.554

2021

A 1024-Channel 10-Bit 36-μW/ch CMOS ROIC for Multiplexed GFET-Only Sensor Arrays in Brain Mapping
Cisneros-Fernandez J., Garcia-Cortadella R., Illa X., Martinez-Aguilar J., Paetzold J., Mohrlok R., Kurnoth M., Jeschke C., Teres L., Garrido J.A., Guimera-Brunet A., Serra-Graells F. IEEE Transactions on Biomedical Circuits and Systems; 15 (5): 860 - 876. 2021. 10.1109/TBCAS.2021.3113556. IF: 3.833

This paper presents a 1024-channel neural read-out integrated circuit (ROIC) for solution-gated GFET sensing probes in massive muECoG brain mapping. The proposed time-domain multiplexing of GFET-only arrays enables low-cost and scalable hybrid headstages. Low-power CMOS circuits are presented for the GFET analog frontend, including a CDS mechanism to improve preamplifier noise figures and 10-bit 10-kS/s A/D conversion. The 1024-channel ROIC has been fabricated in a standard 1.8-V 0.18-mum CMOS technology with 0.012 mm 2 and 36 mu W per channel. An automated methodology for the in-situ calibration of each GFET sensor is also proposed. Experimental ROIC tests are reported using a custom FPGA-based muECoG headstage with 16times 32 and 32times 32 GFET probes in saline solution and agar substrate. Compared to state-of-art neural ROICs, this work achieves the largest scalability in hybrid platforms and it allows the recording of infra-slow neural signals. © 2007-2012 IEEE.

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A study on free-standing 3C-SiC bipolar power diodes
Li F., Renz A.B., Pérez-Tomás A., Shah V., Gammon P., Via F.L., Jennings M., Mawby P. Applied Physics Letters; 118 (24, 242101): 1ENG. 2021. 10.1063/5.0054433. IF: 3.791

A low p-n built-in potential (1.75 V) makes 3C-SiC an attractive choice for medium voltage bipolar or charge balanced devices. Until recently, most 3C-SiC had been grown on Si, and power device fabrication had, therefore, been hindered by issues, such as high defect density and limited processing temperature, while devices were necessarily limited to lateral structures. In this work, we present the fabrication and characterization of a vertical PiN diode using bulk 3C-SiC material. A p-type ohmic contact was obtained on Al implanted regions with a specific contact resistance ∼10−3 Ω cm2. The fabricated PiN diode has a low forward voltage drop of 2.7 V at 1000 A/cm2, and the on-off ratio at ±3 V is as high as 109. An ideality factor of 1.83-1.99 was achieved, and a blocking voltage of ∼110 V was observed using a single-zone junction termination design. © 2021 Author(s).

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A walk on the frontier of energy electronics with power ultra-wide bandgap oxides and ultra-thin neuromorphic 2D materials
Prez-Toms A., Chikoizde E., Rogers D. Proceedings of SPIE - The International Society for Optical Engineering; 11687 (2590747) 2021. 10.1117/12.2590747. IF: 0.450
Author Correction: Graphene active sensor arrays for long-term and wireless mapping of wide frequency band epicortical brain activity
Garcia-Cortadella, R.; Schwesig, G.; Jeschke, C.; Illa, X.; Gray, Anna L.; Savage, S.; Stamatidou, E.; Schiessl, I.; Masvidal-Codina, E.; Kostarelos, K.; Guimerà-Brunet, A.; Sirota, A.; Garrido, JA. Nature Communications; 12 (1) 2021. 10.1038/s41467-021-23078-z. IF: 14.919
Bipolar self-doping in ultra-wide bandgap spinel ZnGa(2)O4
Chi Z, Tarntair FG, Fregnaux M, Wu WY, Sartel C, Madaci I, Chapon P, Sallet V, Dumont Y, Perez-Tomas A, Horng RH, Chikoidze E Materials Today Physics; 20 (100466) 2021. 10.1016/j.mtphys.2021.100466. IF: 9.298

The spinel group is a growing family of materials with general formulation AB2X4 (the X anion typically being a chalcogen like O and S) with many advanced applications for energy. At the time being, the spinel zinc gallate (ZnGa2O4) arguably is the ternary ultra-wide bandgap bipolar oxide semiconductor with the largest bandgap (∼5eV), making this material very promising for implementations in deep UV optoelectronics and ultra-high power electronics. In this work, we further demonstrate that, exploiting the rich cation coordination possibilities of the spinel chemistry, the ZnGa2O4 intrinsic conductivity (and its polarity) can be controlled well over 10 orders of magnitude. p-type and n-type ZnGa2O4 epilayers can be grown by tuning the pressure, oxygen flow and cation precursors ratio during metal-organic chemical vapor deposition. A relatively deep acceptor level can be achieved by promoting antisites (ZnGa) defects, while up to a (n > 1019 cm−3) donor concentration is obtained due to the hybridization of the Zn–O orbitals in the samples grown in Zn-rich conditions. Electrical transport, atomic and optical spectroscopy reveal a free hole conduction (at high temperature) for p-ZnGa2O4 while for n-ZnGa2O4 a (Mott) variable range hopping (VRH) and negative magnetoresistance phenomena take place, originated from “self-impurity” band located at Ev+ ∼3.4 eV. Among arising ultra-wide bandgap semiconductors, spinel ZnGa2O4 exhibit unique self-doping capability thus extending its application at the very frontier of current energy optoelectronics.

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Carbon Incorporation in MOCVD of MoS2Thin Films Grown from an Organosulfide Precursor
Schaefer C.M., Caicedo Roque J.M., Sauthier G., Bousquet J., Hébert C., Sperling J.R., Pérez-Tomás A., Santiso J., Del Corro E., Garrido J.A. Chemistry of Materials; 33 (12): 4474 - 4487. 2021. 10.1021/acs.chemmater.1c00646. IF: 9.811

With the rise of two-dimensional (2D) transition-metal dichalcogenide (TMD) semiconductors and their prospective use in commercial (opto)electronic applications, it has become key to develop scalable and reliable TMD synthesis methods with well-monitored and controlled levels of impurities. While metal-organic chemical vapor deposition (MOCVD) has emerged as the method of choice for large-scale TMD fabrication, carbon (C) incorporation arising during MOCVD growth of TMDs has been a persistent concern-especially in instances where organic chalcogen precursors are desired as a less hazardous alternative to more toxic chalcogen hydrides. However, the underlying mechanisms of such unintentional C incorporation and the effects on film growth and properties are still elusive. Here, we report on the role of C-containing side products of organosulfur precursor pyrolysis in MoS2 thin films grown from molybdenum hexacarbonyl Mo(CO)6 and diethyl sulfide (CH3CH2)2S (DES). By combining in situ gas-phase monitoring with ex situ microscopy and spectroscopy analyses, we systematically investigate the effect of temperature and Mo(CO)6/DES/H2 gas mixture ratios on film morphology, chemical composition, and stoichiometry. Aiming at high-quality TMD growth that typically requires elevated growth temperatures and high DES/Mo(CO)6 precursor ratios, we observed that temperatures above DES pyrolysis onset (â 600 °C) and excessive DES flow result in the formation of nanographitic carbon, competing with MoS2 growth. We found that by introducing H2 gas to the process, DES pyrolysis is significantly hindered, which reduces carbon incorporation. The C content in the MoS2 films is shown to quench the MoS2 photoluminescence and influence the trion-To-exciton ratio via charge transfer. This finding is fundamental for understanding process-induced C impurity doping in MOCVD-grown 2D semiconductors and might have important implications for the functionality and performance of (opto)electronic devices. ©

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Characterization of optogenetically-induced cortical spreading depression in awake mice using graphene micro-transistor arrays
Masvidal-Codina E., Smith T.M., Rathore D., Gao Y., Illa X., Prats-Alfonso E., Corro E.D., Calia A.B., Rius G., Martin-Fernandez I., Guger C., Reitner P., Villa R., Garrido J.A., Guimera-Brunet A., Wykes R.C. Journal of Neural Engineering; 18 (5, 055002) 2021. 10.1088/1741-2552/abecf3. IF: 5.379

Objective. The development of experimental methodology utilizing graphene micro-transistor arrays to facilitate and advance translational research into cortical spreading depression (CSD) in the awake brain. Approach. CSDs were reliably induced in awake nontransgenic mice using optogenetic methods. High-fidelity DC-coupled electrophysiological mapping of propagating CSDs was obtained using flexible arrays of graphene soultion-gated field-effect transistors (gSGFETs). Main results. Viral vectors targetted channelrhopsin expression in neurons of the motor cortex resulting in a transduction volume 1 mm3. 5-10 s of continous blue light stimulation induced CSD that propagated across the cortex at a velocity of 3.0 0.1 mm min-1. Graphene micro-transistor arrays enabled high-density mapping of infraslow activity correlated with neuronal activity suppression across multiple frequency bands during both CSD initiation and propagation. Localized differences in the CSD waveform could be detected and categorized into distinct clusters demonstrating the spatial resolution advantages of DC-coupled recordings. We exploited the reliable and repeatable induction of CSDs using this preparation to perform proof-of-principle pharmacological interrogation studies using NMDA antagonists. MK801 (3 mg kg-1) suppressed CSD induction and propagation, an effect mirrored, albeit transiently, by ketamine (15 mg kg-1), thus demonstrating this models' applicability as a preclinical drug screening platform. Finally, we report that CSDs could be detected through the skull using graphene micro-transistors, highlighting additional advantages and future applications of this technology. Significance. CSD is thought to contribute to the pathophysiology of several neurological diseases. CSD research will benefit from technological advances that permit high density electrophysiological mapping of the CSD waveform and propagation across the cortex. We report an in vivo assay that permits minimally invasive optogenetic induction, combined with multichannel DC-coupled recordings enabled by gSGFETs in the awake brain. Adoption of this technological approach could facilitate and transform preclinical investigations of CSD in disease relevant models. © 2021 The Author(s). Published by IOP Publishing Ltd.

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Direct Visualization of Anti-Ferroelectric Switching Dynamics via Electrocaloric Imaging
Vales-Castro P., Vellvehi M., Perpiñà X., Caicedo J.M., Jordà X., Faye R., Roleder K., Kajewski D., Perez-Tomas A., Defay E., Catalan G. Advanced Electronic Materials; 7 (12, 2100380) 2021. 10.1002/aelm.202100380. IF: 7.295

The large electrocaloric coupling in PbZrO3 allows using high-speed infrared imaging for visualizing anti-ferroelectric switching dynamics via the associated temperature change. It is found that in ceramic samples of homogeneous temperature and thickness, switching is fast due to the generation of multiple nucleation sites, with devices responding in the millisecond range. By introducing gradients of thickness, however, it is possible to change the dynamics to propagation limited, whereby a single-phase boundary sweeps across the sample like a cold front, at a speed of ≈20 cm s−1. Additionally, introducing thermostatic temperature differences between two sides of the sample enables the simultaneous generation of a negative electrocaloric effect on one side and a positive one on the other, yielding a Janus-like electrocaloric response. © 2021 Wiley-VCH GmbH

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Graphene active sensor arrays for long-term and wireless mapping of wide frequency band epicortical brain activity
Garcia-Cortadella R., Schwesig G., Jeschke C., Illa X., Gray A.L., Savage S., Stamatidou E., Schiessl I., Masvidal-Codina E., Kostarelos K., Guimerà-Brunet A., Sirota A., Garrido J.A. Nature Communications; 12 (1, 211) 2021. 10.1038/s41467-020-20546-w. IF: 14.919

Graphene active sensors have demonstrated promising capabilities for the detection of electrophysiological signals in the brain. Their functional properties, together with their flexibility as well as their expected stability and biocompatibility have raised them as a promising building block for large-scale sensing neural interfaces. However, in order to provide reliable tools for neuroscience and biomedical engineering applications, the maturity of this technology must be thoroughly studied. Here, we evaluate the performance of 64-channel graphene sensor arrays in terms of homogeneity, sensitivity and stability using a wireless, quasi-commercial headstage and demonstrate the biocompatibility of epicortical graphene chronic implants. Furthermore, to illustrate the potential of the technology to detect cortical signals from infra-slow to high-gamma frequency bands, we perform proof-of-concept long-term wireless recording in a freely behaving rodent. Our work demonstrates the maturity of the graphene-based technology, which represents a promising candidate for chronic, wide frequency band neural sensing interfaces. © 2021, The Author(s).

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Hybrid Metal-Phenol Nanoparticles with Polydopamine-like Coating for PET/SPECT/CT Imaging
SuÃirez-GarcÃ-A S., Esposito T.V.F., Neufeld-Peters J., Bergamo M., Yang H., Saatchi K., Schaffer P., HÃfeli U.O., Ruiz-Molina D., RodrÃ-Guez-RodrÃ-Guez C., Novio F. ACS Applied Materials and Interfaces; 13 (9): 10705 - 10718. 2021. 10.1021/acsami.0c20612. IF: 9.229

The validation of metal-phenolic nanoparticles (MPNs) in preclinical imaging studies represents a growing field of interest due to their versatility in forming predesigned structures with unique properties. Before MPNs can be used in medicine, their pharmacokinetics must be optimized so that accumulation in nontargeted organs is prevented and toxicity is minimized. Here, we report the fabrication of MPNs made of a coordination polymer core that combines In(III), Cu(II), and a mixture of the imidazole 1,4-bis(imidazole-1-ylmethyl)-benzene and the catechol 3,4-dihydroxycinnamic acid ligands. Furthermore, a phenolic-based coating was used as an anchoring platform to attach poly(ethylene glycol) (PEG). The resulting MPNs, with effective hydrodynamic diameters of around 120 nm, could be further derivatized with surface-embedded molecules, such as folic acid, to facilitate in vivo targeting and multifunctionality. The prepared MPNs were evaluated for in vitro plasma stability, cytotoxicity, and cell internalization and found to be biocompatible under physiological conditions. First, biomedical evaluations were then performed by intrinsically incorporating trace amounts of the radioactive metals 111In or 64Cu during the MPN synthesis directly into their polymeric matrix. The resulting particles, which had identical physicochemical properties to their nonradioactive counterparts, were used to perform in vivo single-photon emission computed tomography (SPECT) and positron emission tomography (PET) in tumor-bearing mice. The ability to incorporate multiple metals and radiometals into MPNs illustrates the diverse range of functional nanoparticles that can be prepared with this approach and broadens the scope of these nanoconstructs as multimodal preclinical imaging agents. ©

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Novel Graphene Electrode for Retinal Implants: An in vivo Biocompatibility Study
Nguyen D., Valet M., Dégardin J., Boucherit L., Illa X., de la Cruz J., del Corro E., Bousquet J., Garrido J.A., Hébert C., Picaud S. Frontiers in Neuroscience; 15 (615256) 2021. 10.3389/fnins.2021.615256. IF: 4.677

Evaluating biocompatibility is a core essential step to introducing a new material as a candidate for brain-machine interfaces. Foreign body reactions often result in glial scars that can impede the performance of the interface. Having a high conductivity and large electrochemical window, graphene is a candidate material for electrical stimulation with retinal prosthesis. In this study, non-functional devices consisting of chemical vapor deposition (CVD) graphene embedded onto polyimide/SU-8 substrates were fabricated for a biocompatibility study. The devices were implanted beneath the retina of blind P23H rats. Implants were monitored by optical coherence tomography (OCT) and eye fundus which indicated a high stability in vivo up to 3 months before histology studies were done. Microglial reconstruction through confocal imaging illustrates that the presence of graphene on polyimide reduced the number of microglial cells in the retina compared to polyimide alone, thereby indicating a high biocompatibility. This study highlights an interesting approach to assess material biocompatibility in a tissue model of central nervous system, the retina, which is easily accessed optically and surgically. © Copyright © 2021 Nguyen, Valet, Dégardin, Boucherit, Illa, de la Cruz, del Corro, Bousquet, Garrido, Hébert and Picaud.

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Novel transducers for high-channel-count neuroelectronic recording interfaces
Guimerà-Brunet A., Masvidal-Codina E., Cisneros-Fernández J., Serra-Graells F., Garrido J.A. Current Opinion in Biotechnology; 72: 39 - 47. 2021. 10.1016/j.copbio.2021.10.002. IF: 9.740

Neuroelectronic interfaces with the nervous system are an essential technology in state-of-the-art neuroscience research aiming to uncover the fundamental working mechanisms of the brain. Progress towards increased spatio-temporal resolution has been tightly linked to the advance of microelectronics technology and novel materials. Translation of these technologies to neuroscience has resulted in multichannel neural probes and acquisition systems enabling the recording of brain signals using thousands of channels. This review provides an overview of state-of-the-art neuroelectronic technologies, with emphasis on recording site architectures which enable the implementation of addressable arrays for high-channel-count neural interfaces. In this field, active transduction mechanisms are gaining importance fueled by novel materials, as they facilitate the implementation of high density addressable arrays. © 2021 The Authors

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Origin of large negative electrocaloric effect in antiferroelectric PbZr O3
Vales-Castro P., Faye R., Vellvehi M., Nouchokgwe Y., Perpiñà X., Caicedo J.M., Jordà X., Roleder K., Kajewski D., Perez-Tomas A., Defay E., Catalan G. Physical Review B; 103 (5, 054112) 2021. 10.1103/PhysRevB.103.054112. IF: 4.036

We have studied the electrocaloric response of the archetypal antiferroelectric PbZrO3 as a function of voltage and temperature in the vicinity of its antiferroelectric-paraelectric phase transition. Large electrocaloric effects of opposite signs, ranging from an electrocooling of -3.5 K to an electroheating of +5.5K, were directly measured with an infrared camera. We show by calorimetric and electromechanical measurements that the large negative electrocaloric effect comes from an endothermic antiferroelectric-ferroelectric switching, in contrast to dipole destabilization of the antiparallel lattice, previously proposed as an explanation for the negative electrocaloric effect of antiferroelectrics. © 2021 American Physical Society.

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Single and Multisite Graphene-Based Electroretinography Recording Electrodes: A Benchmarking Study
de la Cruz J; Nguyen D; Illa X; Bousquet J; Pérez-Marín AP; del Corro E; Picaud S; Garrido JA; Hebert C Advanced Materials Technologies; 7 (6) 2021. 10.1002/admt.202101181. IF: 7.848
Status and prospects of cubic silicon carbide power electronics device technology
Li F., Roccaforte F., Greco G., Fiorenza P., La Via F., Pérez-Tomas A., Evans J.E., Fisher C.A., Monaghan F.A., Mawby P.A., Jennings M. Materials; 14 (19, 5831) 2021. 10.3390/ma14195831. IF: 3.623

Wide bandgap (WBG) semiconductors are becoming more widely accepted for use in power electronics due to their superior electrical energy efficiencies and improved power densities. Although WBG cubic silicon carbide (3C-SiC) displays a modest bandgap compared to its commercial counterparts (4H-silicon carbide and gallium nitride), this material has excellent attributes as the WBG semiconductor of choice for low-resistance, reliable diode and MOS devices. At present the material remains firmly in the research domain due to numerous technological impediments that hamper its widespread adoption. The most obvious obstacle is defect-free 3C-SiC; presently, 3C-SiC bulk and heteroepitaxial (on-silicon) display high defect densities such as stacking faults and antiphase boundaries. Moreover, heteroepitaxy 3C-SiC-on-silicon means low temperature processing budgets are imposed upon the system (max. temperature limited to ~1400 °C) limiting selective doping realisation. This paper will give a brief overview of some of the scientific aspects associated with 3C-SiC processing technology in addition to focussing on the latest state of the art results. A particular focus will be placed upon key process steps such as Schottky and ohmic contacts, ion implantation and MOS processing including reliability. Finally, the paper will discuss some device prototypes (diodes and MOSFET) and draw conclusions around the prospects for 3CSiC devices based upon the processing technology presented. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.

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Study of Spin-Orbit Interactions and Interlayer Ferromagnetic Coupling in Co/Pt/Co Trilayers in a Wide Range of Heavy-Metal Thickness
Ogrodnik P., Grochot K., Karwacki Ł., Kanak J., Prokop M., Chȩciński J., Skowroński W., Ziȩtek S., Stobiecki T. ACS Applied Materials and Interfaces; 13 (39): 47019 - 47032. 2021. 10.1021/acsami.1c11675. IF: 9.229

The spin-orbit torque, a torque induced by a charge current flowing through the heavy-metal-conducting layer with strong spin-orbit interactions, provides an efficient way to control the magnetization direction in heavy-metal/ferromagnet nanostructures, required for applications in the emergent magnetic technologies like random access memories, high-frequency nano-oscillators, or bioinspired neuromorphic computations. We study the interface properties, magnetization dynamics, magnetostatic features, and spin-orbit interactions within the multilayer system Ti(2)/Co(1)/Pt(0-4)/Co(1)/MgO(2)/Ti(2) (thicknesses in nanometers) patterned by optical lithography on micrometer-sized bars. In the investigated devices, Pt is used as a source of the spin current and as a nonmagnetic spacer with variable thickness, which enables the magnitude of the interlayer ferromagnetic exchange coupling to be effectively tuned. We also find the Pt thickness-dependent changes in magnetic anisotropies, magnetoresistances, effective Hall angles, and, eventually, spin-orbit torque fields at interfaces. The experimental findings are supported by the relevant interface structure-related simulations, micromagnetic, macrospin, as well as the spin drift-diffusion models. Finally, the contribution of the spin-orbital Edelstein-Rashba interfacial fields is also briefly discussed in the analysis. © 2021 The Authors. Published by American Chemical Society

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2020

A First Evaluation of Thick Oxide 3C-SiC MOS Capacitors Reliability
Li F., Mawby P., Song Q., Perez-Tomas A., Shah V., Sharma Y., Hamilton D., Fisher C., Gammon P., Jennings M. IEEE Transactions on Electron Devices; 67 (1, 8935512): 237 - 242. 2020. 10.1109/TED.2019.2954911. IF: 2.913

Despite the recent advances in 3C-SiC technology, there is a lack of statistical analysis on the reliability of SiO2 layers on 3C-SiC, which is crucial in power MOS device developments. This article presents a comprehensive study of the medium-and long-term time-dependent dielectric breakdowns (TDDBs) of 65-nm-thick SiO2 layers thermally grown on a state-of-the-art 3C-SiC/Si wafer. Fowler-Nordheim (F-N) tunneling is observed above 7 MV/cm and an effective barrier height of 3.7 eV is obtained, which is the highest known for native SiO2 layers grown on the semiconductor substrate. The observed dependence of the oxide reliability on the gate active area suggests that the oxide quality has not reached the intrinsic level. Three failure mechanisms were identified and confirmed by both medium-and long-term results. Although two of them are likely due to extrinsic defects from material quality and fabrication steps, the one dominating the high field (>8.5 MV/cm) should be attributed to the electron impact ionization within SiO2. At room temperature, the field acceleration factor is found to be ≈0.906 dec/(MV/cm) for high fields, and the projected lifetime exceeds 10 years at 4.5 MV/cm. © 1963-2012 IEEE.

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Bias dependent variability of low-frequency noise in single-layer graphene FETs
Mavredakis N., Cortadella R.G., Illa X., Schaefer N., Calia A.B., Anton-Guimerà-Brunet, Garrido J.A., Jiménez D. Nanoscale Advances; 2 (11): 5450 - 5460. 2020. 10.1039/d0na00632g. IF: 0.000

Low-frequency noise (LFN) variability in graphene transistors (GFETs) is for the first time researched in this work under both experimental and theoretical aspects. LFN from an adequate statistical sample of long-channel solution-gated single-layer GFETs is measured in a wide range of operating conditions while a physics-based analytical model is derived that accounts for the bias dependence of LFN variance with remarkable performance. LFN deviations in GFETs stem from the variations of the parameters of the physical mechanisms that generate LFN, which are the number of traps (Ntr) for the carrier number fluctuation effect (ΔN) due to trapping/detrapping process and the Hooge parameter (αH) for the mobility fluctuations effect (Δμ). ΔN accounts for an M-shape of normalized LFN variance versus gate bias with a minimum at the charge neutrality point (CNP) as it was the case for normalized LFN mean value while Δμ contributes only near the CNP for both variance and mean value. Trap statistical nature of the devices under test is experimentally shown to differ from classical Poisson distribution noticed at silicon-oxide devices, and this might be caused both by the electrolyte interface in GFETs under study and by the premature stage of the GFET technology development which could permit external factors to influence the performance. This not fully advanced GFET process growth might also cause pivotal inconsistencies affecting the scaling laws in GFETs of the same process. This journal is © The Royal Society of Chemistry.

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Distortion-Free Sensing of Neural Activity Using Graphene Transistors
Garcia-Cortadella R., Masvidal-Codina E., De la Cruz J.M., Schäfer N., Schwesig G., Jeschke C., Martinez-Aguilar J., Sanchez-Vives M.V., Villa R., Illa X., Sirota A., Guimerà A., Garrido J.A. Small; 16 (16, 1906640) 2020. 10.1002/smll.201906640. IF: 11.459

Graphene solution-gated field-effect transistors (g-SGFETs) are promising sensing devices to transduce electrochemical potential signals in an electrolyte bath. However, distortion mechanisms in g-SGFET, which can affect signals of large amplitude or high frequency, have not been evaluated. Here, a detailed characterization and modeling of the harmonic distortion and non-ideal frequency response in g-SGFETs is presented. This accurate description of the input–output relation of the g-SGFETs allows to define the voltage- and frequency-dependent transfer functions, which can be used to correct distortions in the transduced signals. The effect of signal distortion and its subsequent calibration are shown for different types of electrophysiological signals, spanning from large amplitude and low frequency cortical spreading depression events to low amplitude and high frequency action potentials. The thorough description of the distortion mechanisms presented in this article demonstrates that g-SGFETs can be used as distortion-free signal transducers not only for neural sensing, but also for a broader range of applications in which g-SGFET sensors are used. © 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

View abstract
Effect of channel thickness on noise in organic electrochemical transistors
Polyravas A.G., Schaefer N., Curto V.F., Calia A.B., Guimera-Brunet A., Garrido J.A., Malliaras G.G. Applied Physics Letters; 117 (7, 073302) 2020. 10.1063/5.0019693. IF: 3.597

Organic electrochemical transistors (OECTs) have been widely used as transducers in electrophysiology and other biosensing applications. Their identifying characteristic is a transconductance that increases with channel thickness, and this provides a facile mechanism to achieve high signal amplification. However, little is known about their noise behavior. Here, we investigate noise and extract metrics for the signal-to-noise ratio and limit of detection in OECTs with different channel thicknesses. These metrics are shown to improve as the channel thickness increases, demonstrating that OECTs can be easily optimized to show not only high amplification, but also low noise. © 2020 Author(s).

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Improved metal-graphene contacts for low-noise, high-density microtransistor arrays for neural sensing
Schaefer N., Garcia-Cortadella R., Calia A.B., Mavredakis N., Illa X., Masvidal-Codina E., Cruz J.D.L., Corro E.D., Rodríguez L., Prats-Alfonso E., Bousquet J., Martínez-Aguilar J., Pérez-Marín A.P., Hébert C., Villa R., Jiménez D., Guimerà-Brunet A., Garrido J.A. Carbon; 161: 647 - 655. 2020. 10.1016/j.carbon.2020.01.066. IF: 8.821

Poor metal contact interfaces are one of the main limitations preventing unhampered access to the full potential of two-dimensional materials in electronics. Here we present graphene solution-gated field-effect-transistors (gSGFETs) with strongly improved linearity, homogeneity and sensitivity for small sensor sizes, resulting from ultraviolet ozone (UVO) contact treatment. The contribution of channel and contact region to the total device conductivity and flicker noise is explored experimentally and explained with a theoretical model. Finally, in-vitro recordings of flexible microelectrocorticography (μ-ECoG) probes were performed to validate the superior sensitivity of the UVO-treated gSGFET to brain-like activity. These results connote an important step towards the fabrication of high-density gSGFET μ-ECoG arrays with state-of-the-art sensitivity and homogeneity, thus demonstrating the potential of this technology as a versatile platform for the new generation of neural interfaces. © 2020 Elsevier Ltd

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Low-frequency noise parameter extraction method for single-layer graphene FETs
Mavredakis N., Wei W., Pallecchi E., Vignaud D., Happy H., Cortadella R.G., Schaefer N., Calia A.B., Garrido J.A., Jimenez D. IEEE Transactions on Electron Devices; 67 (5, 9042866): 2093 - 2099. 2020. 10.1109/TED.2020.2978215. IF: 2.913

In this article, a detailed parameter extraction methodology is proposed for low-frequency noise (LFN) in single-layer (SL) graphene transistors (GFETs) based on a recently established compact LFN model. The drain current and LFN of two short channel back-gated GFETs (L = 300 and 100 nm) were measured at lower and higher drain voltages, for a wide range of gate voltages covering the region away from charge neutrality point (CNP) up to CNP at p-type operation region. Current-voltage (IV) and LFN data were also available from a long-channel SL top solution-gated (SG) GFET (L = 5 μm), for both p- and n-type regions near and away CNP. At each of these regimes, the appropriate IV and LFN parameters can be accurately extracted. Regarding LFN, mobility fluctuation effect is dominant at CNP, and from there, the Hooge parameter αH can be extracted, whereas the carrier number fluctuation contribution which is responsible for the well-known M-shape bias dependence of output noise divided by squared drain current, also observed in our data, makes possible the extraction of the NT parameter related to the number of traps. In the less possible case of a Λ-shape trend, NT and αH can be extracted simultaneously from the region near CNP. Away from CNP, contact resistance can have a significant contribution to LFN, and from there, the relevant parameter SΔ R2 is defined. The LFN parameters described above can be estimated from the low drain voltage region of operation where the effect of velocity saturation (VS) mechanism is negligible. VS effect results in the reduction of LFN at higher drain voltages, and from there, the IV parameter hΩ which represents the phonon energy and is related to VS effect can be derived both from drain current and LFN data. © 1963-2012 IEEE.

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Multiplexed neural sensor array of graphene solution-gated field-effect transistors
Schaefer N., Garcia-Cortadella R., Martínez-Aguilar J., Schwesig G., Illa X., Moya Lara A., Santiago S., Hébert C., Guirado G., Villa R., Sirota A., Guimerà-Brunet A., Garrido J.A. 2D Materials; 7 (2, 025046) 2020. 10.1088/2053-1583/ab7976. IF: 7.140

Electrocorticography (ECoG) is a well-established technique to monitor electrophysiological activity from the surface of the brain and has proved crucial for the current generation of neural prostheses and brain-computer interfaces. However, existing ECoG technologies still fail to provide the resolution necessary to accurately map highly localized activity across large brain areas, due to the rapidly increasing size of connector footprint with sensor count. This work demonstrates the use of a flexible array of graphene solution-gated field-effect transistors (gSGFET), exploring the concept of multiplexed readout using an external switching matrix. This approach does not only allow for an increased sensor count, but due to the use of active sensing devices (i.e. transistors) over microelectrodes it makes additional buffer transistors redundant, which drastically eases the complexity of device fabrication on flexible substrates. The presented results pave the way for upscaling the gSGFET technology towards large-scale, high-density μECoG-arrays, eventually capable of resolving neural activity down to a single neuron level, while simultaneously mapping large brain regions. © 2020 IOP Publishing Ltd.

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P-Type Ultrawide-Band-Gap Spinel ZnGa2O4: New Perspectives for Energy Electronics
Chikoidze E., Sartel C., Madaci I., Mohamed H., Vilar C., Ballesteros B., Belarre F., Del Corro E., Vales-Castro P., Sauthier G., Li L., Jennings M., Sallet V., Dumont Y., Pérez-Tomás A. Crystal Growth and Design; 20 (4): 2535 - 2546. 2020. 10.1021/acs.cgd.9b01669. IF: 4.089

The family of spinel compounds is a large and important class of multifunctional materials of general formulation AB2X4 with many advanced applications in energy and optoelectronic areas such as fuel cells, batteries, catalysis, photonics, spintronics, and thermoelectricity. In this work, it is demonstrated that the ternary ultrawide-band-gap (∼5 eV) spinel zinc gallate (ZnGa2O4) arguably is the native p-type ternary oxide semiconductor with the largest Eg value (in comparison with the recently discovered binary p-type monoclinic β-Ga2O3 oxide). For nominally undoped ZnGa2O4 the high-temperature Hall effect hole concentration was determined to be as large as p = 2 × 1015 cm-3, while hole mobilities were found to be μh = 7-10 cm2/(V s) (in the 680-850 K temperature range). An acceptor-like small Fermi level was further corroborated by X-ray spectroscopy and by density functional theory calculations. Our findings, as an important step toward p-type doping, opens up further perspectives for ultrawide-band-gap bipolar spinel electronics and further promotes ultrawide-band-gap ternary oxides such as ZnGa2O4 to the forefront of the quest of the next generation of semiconductor materials for more efficient energy optoelectronics and power electronics. Copyright © 2020 American Chemical Society.

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Photocurrent spectroscopy of in-plane surface conductive diamond homostructures
Simon P., Beck P., Rathi A., Stutzmann M., Garrido J.A. Physical Review B; 101 (20, 205306) 2020. 10.1103/PhysRevB.101.205306. IF: 3.575

Electrical breakdown at hydrogen/oxygen interfaces is limiting the use of nanoscaled diamond devices for the control of optically active defect centers. In this work electron transport across an oxygen-terminated potential barrier in a hydrogen-terminated surface conductive diamond is investigated. We analyze temperature-dependent current-voltage characteristics for different barrier widths and report on a reduced effective barrier height compared to theoretical expectations. This is ascribed to an inhomogeneous potential landscape, as observed by Kelvin probe and conductive force microscopy. Furthermore, we use photocurrent spectroscopy to discuss possible transport processes and identify a field dependent absorption feature. A defect state involved in transport across the barrier is proposed at the hydrogen-oxygen barrier approximately 1eV above the valence band maximum. The new understanding enabled by our work may help to overcome the current limitations of diamond surface electronics. © 2020 American Physical Society.

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Production and processing of graphene and related materials
Backes C., Abdelkader A.M., Alonso C., Andrieux-Ledier A., Arenal R., Azpeitia J., Balakrishnan N., Banszerus L., Barjon J., Bartali R., Bellani S., Berger C., Berger R., Ortega M.M.B., Bernard C., Beton P.H., Beyer A., Bianco A., Bøggild P., Bonaccorso F., Barin G.B., Botas C., Bueno R.A., Carriazo D., Castellanos-Gomez A., Christian M., Ciesielski A., Ciuk T., Cole M.T., Coleman J., Coletti C., Crema L., Cun H., Dasler D., De Fazio D., Díez N., Drieschner S., Duesberg G.S., Fasel R., Feng X., Fina A., Forti S., Galiotis C., Garberoglio G., García J.M., Garrido J.A., Gibertini M., Gölzhäuser A., Gómez J., Greber T., Hauke F., Hemmi A., Hernandez-Rodriguez I., Hirsch A., Hodge S.A., Huttel Y., Jepsen P.U., Jimenez I., Kaiser U., Kaplas T., Kim H., Kis A., Papagelis K., Kostarelos K., Krajewska A., Lee K., Li C., Lipsanen H., Liscio A., Lohe M.R., Loiseau A., Lombardi L., López M.F., Martin O., Martín C., Martínez L., Martin-Gago J.A., Martínez J.I., Marzari N., Mayoral A., McManus J., Melucci M., Méndez J., Merino C., Merino P., Meyer A.P., Miniussi E., Miseikis V., Mishra N., Morandi V., Munuera C., Muñoz R., Nolan H., Ortolani L., Ott A.K., Palacio I., Palermo V., Parthenios J., Paste 2D Materials; 7 (2, 022001) 2020. 10.1088/2053-1583/ab1e0a. IF: 7.140

We present an overview of the main techniques for production and processing of graphene and related materials (GRMs), as well as the key characterization procedures. We adopt a 'hands-on' approach, providing practical details and procedures as derived from literature as well as from the authors' experience, in order to enable the reader to reproduce the results. Section I is devoted to 'bottom up' approaches, whereby individual constituents are pieced together into more complex structures. We consider graphene nanoribbons (GNRs) produced either by solution processing or by on-surface synthesis in ultra high vacuum (UHV), as well carbon nanomembranes (CNM). Production of a variety of GNRs with tailored band gaps and edge shapes is now possible. CNMs can be tuned in terms of porosity, crystallinity and electronic behaviour. Section II covers 'top down' techniques. These rely on breaking down of a layered precursor, in the graphene case usually natural crystals like graphite or artificially synthesized materials, such as highly oriented pyrolythic graphite, monolayers or few layers (FL) flakes. The main focus of this section is on various exfoliation techniques in a liquid media, either intercalation or liquid phase exfoliation (LPE). The choice of precursor, exfoliation method, medium as well as the control of parameters such as time or temperature are crucial. A definite choice of parameters and conditions yields a particular material with specific properties that makes it more suitable for a targeted application. We cover protocols for the graphitic precursors to graphene oxide (GO). This is an important material for a range of applications in biomedicine, energy storage, nanocomposites, etc. Hummers' and modified Hummers' methods are used to make GO that subsequently can be reduced to obtain reduced graphene oxide (RGO) with a variety of strategies. GO flakes are also employed to prepare three-dimensional (3d) low density structures, such as sponges, foams, hydro- or aerogels. The assembly of flakes into 3d structures can provide improved mechanical properties. Aerogels with a highly open structure, with interconnected hierarchical pores, can enhance the accessibility to the whole surface area, as relevant for a number of applications, such as energy storage. The main recipes to yield graphite intercalation compounds (GICs) are also discussed. GICs are suitable precursors for covalent functionalization of graphene, but can also be used for the synthesis of uncharged graphene in solution. Degradation of the molecules intercalated in GICs can be triggered by high temperature treatment or microwave irradiation, creating a gas pressure surge in graphite and exfoliation. Electrochemical exfoliation by applying a voltage in an electrolyte to a graphite electrode can be tuned by varying precursors, electrolytes and potential. Graphite electrodes can be either negatively or positively intercalated to obtain GICs that are subsequently exfoliated. We also discuss the materials that can be amenable to exfoliation, by employing a theoretical data-mining approach. The exfoliation of LMs usually results in a heterogeneous dispersion of flakes with different lateral size and thickness. This is a critical bottleneck for applications, and hinders the full exploitation of GRMs produced by solution processing. The establishment of procedures to control the morphological properties of exfoliated GRMs, which also need to be industrially scalable, is one of the key needs. Section III deals with the processing of flakes. (Ultra)centrifugation techniques have thus far been the most investigated to sort GRMs following ultrasonication, shear mixing, ball milling, microfluidization, and wet-jet milling. It allows sorting by size and thickness. Inks formulated from GRM dispersions can be printed using a number of processes, from inkjet to screen printing. Each technique has specific rheological requirements, as well as geometrical constraints. The solvent choice is critical, not only for the GRM stability, but also in terms of optimizing printing on different substrates, such as glass, Si, plastic, paper, etc, all with different surface energies. Chemical modifications of such substrates is also a key step. Sections IV-VII are devoted to the growth of GRMs on various substrates and their processing after growth to place them on the surface of choice for specific applications. The substrate for graphene growth is a key determinant of the nature and quality of the resultant film. The lattice mismatch between graphene and substrate influences the resulting crystallinity. Growth on insulators, such as SiO2, typically results in films with small crystallites, whereas growth on the close-packed surfaces of metals yields highly crystalline films. Section IV outlines the growth of graphene on SiC substrates. This satisfies the requirements for electronic applications, with well-defined graphene-substrate interface, low trapped impurities and no need for transfer. It also allows graphene structures and devices to be measured directly on the growth substrate. The flatness of the substrate results in graphene with minimal strain and ripples on large areas, allowing spectroscopies and surface science to be performed. We also discuss the surface engineering by intercalation of the resulting graphene, its integration with Si-wafers and the production of nanostructures with the desired shape, with no need for patterning. Section V deals with chemical vapour deposition (CVD) onto various transition metals and on insulators. Growth on Ni results in graphitized polycrystalline films. While the thickness of these films can be optimized by controlling the deposition parameters, such as the type of hydrocarbon precursor and temperature, it is difficult to attain single layer graphene (SLG) across large areas, owing to the simultaneous nucleation/growth and solution/precipitation mechanisms. The differing characteristics of polycrystalline Ni films facilitate the growth of graphitic layers at different rates, resulting in regions with differing numbers of graphitic layers. High-quality films can be grown on Cu. Cu is available in a variety of shapes and forms, such as foils, bulks, foams, thin films on other materials and powders, making it attractive for industrial production of large area graphene films. The push to use CVD graphene in applications has also triggered a research line for the direct growth on insulators. The quality of the resulting films is lower than possible to date on metals, but enough, in terms of transmittance and resistivity, for many applications as described in section V. Transfer technologies are the focus of section VI. CVD synthesis of graphene on metals and bottom up molecular approaches require SLG to be transferred to the final target substrates. To have technological impact, the advances in production of high-quality large-area CVD graphene must be commensurate with those on transfer and placement on the final substrates. This is a prerequisite for most applications, such as touch panels, anticorrosion coatings, transparent electrodes and gas sensors etc. New strategies have improved the transferred graphene quality, making CVD graphene a feasible option for CMOS foundries. Methods based on complete etching of the metal substrate in suitable etchants, typically iron chloride, ammonium persulfate, or hydrogen chloride although reliable, are time- and resourceconsuming, with damage to graphene and production of metal and etchant residues. Electrochemical delamination in a low-concentration aqueous solution is an alternative. In this case metallic substrates can be reused. Dry transfer is less detrimental for the SLG quality, enabling a deterministic transfer. There is a large range of layered materials (LMs) beyond graphite. Only few of them have been already exfoliated and fully characterized. Section VII deals with the growth of some of these materials. Amongst them, h-BN, transition metal tri- and di-chalcogenides are of paramount importance. The growth of h-BN is at present considered essential for the development of graphene in (opto) electronic applications, as h-BN is ideal as capping layer or substrate. The interesting optical and electronic properties of TMDs also require the development of scalable methods for their production. Large scale growth using chemical/physical vapour deposition or thermal assisted conversion has been thus far limited to a small set, such as h-BN or some TMDs. Heterostructures could also be directly grown. © 2020 The Author(s).

View abstract
Self-Pixelation Through Fracture in VO2 Thin Films
Laura Rodríguez, Elena del Corro, Michele Conroy, Kalani Moore, Felip Sandiumenge, Neus Domingo, José Santiso, Gustau Catalan Acs Applied Electronic Materials; 2 (5): 1433 - 1439. 2020. 10.1021/acsaelm.0c00199. IF: 0.000
Switchless multiplexing of graphene active sensor arrays for brain mapping
Garcia-Cortadella R., Schäfer N., Cisneros-Fernandez J., Ré L., Illa X., Schwesig G., Moya A., Santiago S., Guirado G., Villa R., Sirota A., Serra-Graells F., Garrido J.A., Guimerà-Brunet A. Nano Letters; 20 (5): 3528 - 3537. 2020. 10.1021/acs.nanolett.0c00467. IF: 11.238

Sensor arrays used to detect electrophysiological signals from the brain are paramount in neuroscience. However, the number of sensors that can be interfaced with macroscopic data acquisition systems currently limits their bandwidth. This bottleneck originates in the fact that, typically, sensors are addressed individually, requiring a connection for each of them. Herein, we present the concept of frequency-division multiplexing (FDM) of neural signals by graphene sensors. We demonstrate the high performance of graphene transistors as mixers to perform amplitude modulation (AM) of neural signals in situ, which is used to transmit multiple signals through a shared metal line. This technology eliminates the need for switches, remarkably simplifying the technical complexity of state-of-the-art multiplexed neural probes. Besides, the scalability of FDM graphene neural probes has been thoroughly evaluated and their sensitivity demonstrated in vivo. Using this technology, we envision a new generation of high-count conformal neural probes for high bandwidth brain machine interfaces. © 2020 American Chemical Society.

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Ultra-high critical electric field of 13.2 MV/cm for Zn-doped p-type β-Ga2O3
Chikoidze E., Tchelidze T., Sartel C., Chi Z., Kabouche R., Madaci I., Rubio C., Mohamed H., Sallet V., Medjdoub F., Perez-Tomas A., Dumont Y. Materials Today Physics; 15 (100263) 2020. 10.1016/j.mtphys.2020.100263. IF: 10.443

Which the actual critical electrical field of the ultra-wide bandgap semiconductor β-Ga2O3 is? Even that it is usual to find in the literature a given value for the critical field of wide and ultra-wide semiconductors such as SiC (3 MV/cm), GaN (3.3 MV/cm), β-Ga2O3 (~8 MV/cm) and diamond (10 MV/cm), this value actually depends on intrinsic and extrinsic factors such as the bandgap energy, material residual impurities or introduced dopants. Indeed, it is well known from 1950's that reducing the residual doping (NB) of the semiconductor layer increases the breakdown voltage capability of a semiconductor media (e.g. as NB−3/4 by using the Fulop's approximation for an abrupt junction). A key limitation is, therefore, the residual donor/acceptor concentration generally found in these materials. Here, we report that doping with amphoteric Zinc a p-type β-Ga2O3 thin films shortens free carrier mean free path (0.37 nm), resulting in the ultra-high critical electrical field of 13.2 MV/cm. Therefore, the critical breakdown field can be, at least, four times larger for the emerging Ga2O3 power semiconductor as compared to SiC and GaN. We further explain these wide-reaching experimental facts by using theoretical approaches based on the impact ionization microscopic theory and thermodynamic calculations. © 2020

View abstract

2019

Crossover from ballistic to diffusive thermal transport in suspended graphene membranes
El Sachat A., Köenemann F., Menges F., Del Corro E., Garrido J.A., Sotomayor Torres C.M., Alzina F., Gotsmann B. 2D Materials; 6 (2, 025034) 2019. 10.1088/2053-1583/ab097d. IF: 7.343

We report heat transport measurements on suspended single-layer graphene disks with radius of 150-1600 nm using a high-vacuum scanning thermal microscope. The results of this study revealed a radius-dependent thermal contact resistance between tip and graphene, with values between 1.15 and 1.52 × 108 KW-1. The observed scaling of thermal resistance with radius is interpreted in terms of ballistic phonon transport in suspended graphene discs with radius smaller than 775 nm. In larger suspended graphene discs (radius >775 nm), the thermal resistance increases with radius, which is attributed to in-plane heat transport being limited by phonon-phonon resistive scattering processes, which resulted in a transition from ballistic to diffusive thermal transport. In addition, by simultaneously mapping topography and steady-state heat flux signals between a self-heated scanning probe sensor and graphene with 17 nm thermal spatial resolution, we demonstrated that the surface quality of the suspended graphene and its connectivity with the Si/SiO2 substrate play a determining role in thermal transport. Our approach allows the investigation of heat transport in suspended graphene at sub-micrometre length scales and overcomes major limitations of conventional experimental methods usually caused by extrinsic thermal contact resistances, assumptions on the value of the graphene's optical absorbance and limited thermal spatial resolution. © 2019 IOP Publishing Ltd.

View abstract
Electrical characterisation of thick 3C-SiC layers grown on off-axis 4H-SiC substrates
Li F., Jokubavicius V., Jennings M., Yakimova R., Pérez-Tomás A., Russell S., Sharma Y., Roccaforte F., Mawby P., Lavia F. Materials Science Forum; 963 MSF: 353 - 356. 2019. 10.4028/www.scientific.net/MSF.963.353.

300 μm thick 3C-SiC epilayer was grown on off-axis 4H-SiC(0001) substrate with a high growth rate of 1 mm/hour. Dry oxidation, wet oxidation and N2O anneal were applied to fabricate lateral MOS capacitors on these 3C-SiC layers. MOS interface obtained by N2O anneal has the lowest interface trap density of 3~4x1011 eV-1cm-2. Although all MOS capacitors still have positive net charges at the MOS interface, the wet oxidised sample has the lowest effective charge density of ~9.17x1011 cm-2. © 2019 Trans Tech Publications Ltd, Switzerland.

View abstract
High-resolution mapping of infraslow cortical brain activity enabled by graphene microtransistors
Masvidal-Codina E., Illa X., Dasilva M., Calia A.B., Dragojević T., Vidal-Rosas E.E., Prats-Alfonso E., Martínez-Aguilar J., De la Cruz J.M., Garcia-Cortadella R., Godignon P., Rius G., Camassa A., Del Corro E., Bousquet J., Hébert C., Durduran T., Villa R., Sanchez-Vives M.V., Garrido J.A., Guimerà-Brunet A. Nature Materials; 18 (3): 280 - 288. 2019. 10.1038/s41563-018-0249-4. IF: 38.887

Recording infraslow brain signals (<0.1 Hz) with microelectrodes is severely hampered by current microelectrode materials, primarily due to limitations resulting from voltage drift and high electrode impedance. Hence, most recording systems include high-pass filters that solve saturation issues but come hand in hand with loss of physiological and pathological information. In this work, we use flexible epicortical and intracortical arrays of graphene solution-gated field-effect transistors (gSGFETs) to map cortical spreading depression in rats and demonstrate that gSGFETs are able to record, with high fidelity, infraslow signals together with signals in the typical local field potential bandwidth. The wide recording bandwidth results from the direct field-effect coupling of the active transistor, in contrast to standard passive electrodes, as well as from the electrochemical inertness of graphene. Taking advantage of such functionality, we envision broad applications of gSGFET technology for monitoring infraslow brain activity both in research and in the clinic. © 2018, The Author(s), under exclusive licence to Springer Nature Limited.

View abstract
Impact of contact overlap on transconductance and noise in organic electrochemical transistors
Polyravas A.G., Curto V.F., Schaefer N., Calia A.B., Guimera-Brunet A., Garrido J.A., Malliaras G.G. Flexible and Printed Electronics; 4 (4, 044003) 2019. 10.1088/2058-8585/ab4dc4.

Organic electrochemical transistors (OECTs) from poly(3,4-ethylenedioxythiophene) doped with polystyrene sulfonate (PEDOT:PSS) are used as amplifying transducers for bioelectronics. Although the impact on performance of device geometry parameters such as channel area and thickness has been widely explored, the overlap between the semiconductor film and the source and drain contacts has not been considered. Here we vary this overlap and explore its impact on transconductance and noise. We show that increasing contact overlap does not alter the magnitude of the steady-state transconductance but it does decreases the cut-off frequency. Noise was found to be independent of contact overlap and to vary according to the charge noise model. The results show that high-quality contacts can be established in PEDOT:PSS OECTs with minimal overlap. © 2019 IOP Publishing Ltd.

View abstract
Long-Term Functionality of Transversal Intraneural Electrodes is Improved by Dexamethasone Treatment
De La Oliva N., Del Valle J., Delgado-Martinez I., Mueller M., Stieglitz T., Navarro X. IEEE Transactions on Neural Systems and Rehabilitation Engineering; 27 (3, 8633873): 457 - 464. 2019. 10.1109/TNSRE.2019.2897256. IF: 3.478

Neuroprostheses aimed to restore lost functions after a limb amputation are based on the interaction with the nervous system by means of neural interfaces. Among the different designs, intraneural electrodes implanted in peripheral nerves represent a good strategy to stimulate nerve fibers to send sensory feedback and to record nerve signals to control the prosthetic limb. However, intraneural electrodes, as any device implanted in the body, induce a foreign body reaction (FBR) that results in the tissue encapsulation of the device. The FBR causes a progressive decline of the electrode functionality over time due to the physical separation between the electrode active sites and the axons to the interface. Modulation of the inflammatory response has arisen as a good strategy to reduce the FBR and maintain electrode functionality. In this paper, transversal intraneural multi-channel electrodes (TIMEs) were implanted in the rat sciatic nerve and tested for three months to evaluate stimulation and recording capabilities under chronic administration of dexamethasone. Dexamethasone treatment significantly reduced the threshold for evoking muscle responses during the follow-up compared to saline-treated animals, without affecting the selectivity of stimulation. However, dexamethasone treatment did not improve the signal-to-noise ratio of the recorded neural signals. Dexamethasone treatment allowed to maintain more working active sites along time than saline treatment. Thus, systemic administration of dexamethasone appears as a useful treatment in chronically implanted animals with neural electrodes as it increases the number of functioning contacts of the implanted TIME and reduces the intensity needed to stimulate the nerve. © 2001-2011 IEEE.

View abstract
Neural interfaces based on flexible graphene transistors: A new tool for electrophysiology
Guimera-Brunet A., Masvidal-Codina E., Illa X., Dasilva M., Bonaccini-Calia A., Prats-Alfonso E., Martinez-Aguilar J., De La Cruz J.M., Garcia-Cortadella R., Barbero A., Godignon P., Rius G., Del Corro E., Bousquet J., Hebert C., Wykes R., Sanchez-Vives M.V., Villa R., Schaefer N., Garrido J.A. Technical Digest - International Electron Devices Meeting, IEDM; 2019-December (8993433) 2019. 10.1109/IEDM19573.2019.8993433.

The use of graphene transistors for transducing neural activity has demonstrated the potential to extend the spatiotemporal resolution of electrophysiological methods to lower frequencies, providing a new tool to understand the role of the infra-slow activity. © 2019 IEEE.

View abstract
On the Suitability of Raman Spectroscopy to Monitor the Degree of Graphene Functionalization by Diazonium Salts
Sampathkumar K., Diez-Cabanes V., Kovaricek P., Del Corro E., Bouša M., Hošek J., Kalbac M., Frank O. Journal of Physical Chemistry C; 123 (36): 22397 - 22402. 2019. 10.1021/acs.jpcc.9b06516. IF: 4.309

Raman spectroscopy is undoubtedly the most frequently used technique for universal characterization of graphene and related materials. Quantification of parameters like disorder or strain is possible through analysis of particular Raman bands. However, under certain conditions, such evaluation can be jeopardized by - sometimes hidden - convolution of more overlapping effects. In this work, graphene functionalization by the common nitrobenzenediazonium salt under simultaneous biaxial tensile deformation induced by substrate swelling was investigated by Raman spectroscopy. As expected, the disorder-related D band appeared in the spectra documenting the covalent attack on the graphene lattice. However, the strain-induced shift of the graphene bands exposed additional peaks, masked at exactly the same positions as the unstrained graphene bands. The new bands were assigned to vibrations of the diazonium molecule and its decomposition products adsorbed on top of the functionalized graphene. The external strain thus provided means for more correct quantification of the lattice disorder. © 2019 American Chemical Society.

View abstract
Velocity Saturation Effect on Low Frequency Noise in Short Channel Single Layer Graphene Field Effect Transistors
Nikolaos Mavredakis, Wei Wei, Emiliano Pallecchi, Dominique Vignaud, Henri Happy, Ramon Garcia Cortadella, Andrea Bonaccini Calia, Jose A. Garrido, and David Jiménez ACS Applied Electronic Materials; 1 (12): 2626 - 2636. 2019. 10.1021/acsaelm.9b00604.

Graphene devices for analog and radio frequency (RF) applications are prone to low frequency noise (LFN) due to its up conversion to undesired phase noise at higher frequencies. Such applications demand the use of short channel graphene transistors (GFETs) that operate at high electric fields in order to ensure a high speed. Electric field is inversely proportional to device length and proportional to channel potential, so it gets maximized as the drain voltage increases and the transistor’s length shrinks. Under these conditions though, short channel effects like velocity saturation (VS) should be considered. The reduction of LFN data due to the VS effect at short channel GFETs operating at high drain potential is for the first time shown in the present work. Carrier number and mobility fluctuations have been proven to be the main sources that generate LFN in GFETs. While their contribution to the bias dependence of LFN in long channels has been thoroughly investigated, the way in which VS phenomenon affects LFN in short channel devices under high drain voltage conditions has not been well understood. In this paper we have proposed a physics-based analytical LFN model that works under both low and high electric field conditions. The implemented model is validated with experimental data from CVD grown back-gated single layer GFETs operating at gigahertz frequencies. The model accurately captures the reduction of LFN especially near the charge neutrality point because of the effect of the VS mechanism. Moreover, an analytical expression for the effect of contact resistance on LFN is derived. This contact resistance contribution is experimentally shown to be dominant at high gate voltages and is accurately described by the proposed model. The noise parameter related to LFN at contacts is found to have an exponential dependence with contact resistance, and to our knowledge, this is shown for the first time.

View abstract
Versatile Graphene-Based Platform for Robust Nanobiohybrid Interfaces
Bueno R., Marciello M., Moreno M., Sánchez-Sánchez C., Martinez J.I., Martinez L., Prats-Alfonso E., Guimerà-Brunet A., Garrido J.A., Villa R., Mompean F., García-Hernandez M., Huttel Y., Morales M.D.P., Briones C., López M.F., Ellis G.J., Vázquez L., Martín-Gago J.A. ACS Omega; 4 (2): 3287 - 3297. 2019. 10.1021/acsomega.8b03152. IF: 2.584

Technologically useful and robust graphene-based interfaces for devices require the introduction of highly selective, stable, and covalently bonded functionalities on the graphene surface, whilst essentially retaining the electronic properties of the pristine layer. This work demonstrates that highly controlled, ultrahigh vacuum covalent chemical functionalization of graphene sheets with a thiol-terminated molecule provides a robust and tunable platform for the development of hybrid nanostructures in different environments. We employ this facile strategy to covalently couple two representative systems of broad interest: metal nanoparticles, via S-metal bonds, and thiol-modified DNA aptamers, via disulfide bridges. Both systems, which have been characterized by a multitechnique approach, remain firmly anchored to the graphene surface even after several washing cycles. Atomic force microscopy images demonstrate that the conjugated aptamer retains the functionality required to recognize a target protein. This methodology opens a new route to the integration of high-quality graphene layers into diverse technological platforms, including plasmonics, optoelectronics, or biosensing. With respect to the latter, the viability of a thiol-functionalized chemical vapor deposition graphene-based solution-gated field-effect transistor array was assessed. © Copyright 2019 American Chemical Society.

View abstract

2018

Dexamethasone Reduces the Foreign Body Reaction to Intraneural Electrode Implants in the Peripheral Nerve of the Rat
De la Oliva N., Navarro X., del Valle J. Anatomical Record; 301 (10): 1722 - 1733. 2018. 10.1002/ar.23920. IF: 1.373

Intraneural electrodes must be in intimate contact with nerve fibers to have a proper function, but this interface is compromised due to the foreign body reaction (FBR). The FBR is characterized by a first inflammatory phase followed by a second anti-inflammatory and fibrotic phase, which results in the formation of a tissue capsule around the implant, causing physical separation between the active sites of the electrode and the nerve fibers. We have tested systemically several anti-inflammatory drugs such as dexamethasone (subcutaneous), ibuprofen and maraviroc (oral) to reduce macrophage activation, as well as clodronate liposomes (intraperitoneal) to reduce monocyte/macrophage infiltration, and sildenafil (oral) as an antifibrotic drug to reduce collagen deposition in an FBR model with longitudinal Parylene C intraneural implants in the rat sciatic nerve. Treatment with dexamethasone, ibuprofen, or clodronate significantly reduced the inflammatory reaction in the nerve in comparison to the saline group after 2 weeks of the implant, whereas sildenafil and maraviroc had no effect on infiltration of macrophages in the nerve. However, only dexamethasone was able to significantly reduce the matrix deposition around the implant. Similar positive results were obtained with dexamethasone in the case of polyimide-based intraneural implants, another polymer substrate for the electrode. These results indicate that inflammation triggers the FBR in peripheral nerves, and that anti-inflammatory treatment with dexamethasone may have beneficial effects on lengthening intraneural interface functionality. Anat Rec, 301:1722–1733, 2018. © 2018 Wiley Periodicals, Inc. © 2018 Wiley Periodicals, Inc.

View abstract
Flexible Graphene Solution-Gated Field-Effect Transistors: Efficient Transducers for Micro-Electrocorticography
Hébert C., Masvidal-Codina E., Suarez-Perez A., Calia A.B., Piret G., Garcia-Cortadella R., Illa X., Del Corro Garcia E., De la Cruz Sanchez J.M., Casals D.V., Prats-Alfonso E., Bousquet J., Godignon P., Yvert B., Villa R., Sanchez-Vives M.V., Guimerà-Brunet A., Garrido J.A. Advanced Functional Materials; 28 (12, 1703976) 2018. 10.1002/adfm.201703976. IF: 13.325

Brain–computer interfaces and neural prostheses based on the detection of electrocorticography (ECoG) signals are rapidly growing fields of research. Several technologies are currently competing to be the first to reach the market; however, none of them fulfill yet all the requirements of the ideal interface with neurons. Thanks to its biocompatibility, low dimensionality, mechanical flexibility, and electronic properties, graphene is one of the most promising material candidates for neural interfacing. After discussing the operation of graphene solution-gated field-effect transistors (SGFET) and characterizing their performance in saline solution, it is reported here that this technology is suitable for μ-ECoG recordings through studies of spontaneous slow-wave activity, sensory-evoked responses on the visual and auditory cortices, and synchronous activity in a rat model of epilepsy. An in-depth comparison of the signal-to-noise ratio of graphene SGFETs with that of platinum black electrodes confirms that graphene SGFET technology is approaching the performance of state-of-the art neural technologies. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

View abstract
Lipid Monolayer Formation and Lipid Exchange Monitored by a Graphene Field-Effect Transistor
Blaschke B.M., Böhm P., Drieschner S., Nickel B., Garrido J.A. Langmuir; 34 (14): 4224 - 4233. 2018. 10.1021/acs.langmuir.8b00162. IF: 3.789

Anionic and cationic lipids are key molecules involved in many cellular processes; their distribution in biomembranes is highly asymmetric, and their concentration is well-controlled. Graphene solution-gated field-effect transistors (SGFETs) exhibit high sensitivity toward the presence of surface charges. Here, we establish conditions that allow the observation of the formation of charged lipid layers on solution-gated field-effect transistors in real time. We quantify the electrostatic screening of electrolyte ions and derive a model that explains the influence of charged lipids on the ion sensitivity of graphene SGFETs. The electrostatic model is validated using structural information from X-ray reflectometry measurements, which show that the lipid monolayer forms on graphene. We demonstrate that SGFETs can be used to detect cationic lipids by self-exchange of lipids. Furthermore, SGFETs allow measuring the kinetics of layer formation induced by vesicle fusion or spreading from a reservoir. Because of the high transconductance and low noise of the electrical readout, we can observe characteristic conductance spikes that we attribute to bouncing-off events of lipid aggregates from the SGFET surface, suggesting a great potential of graphene SGFETs to measure the on-off kinetics of small aggregates interacting with supported layers. © 2018 American Chemical Society.

View abstract
On the use of Parylene C polymer as substrate for peripheral nerve electrodes
De La Oliva N., Mueller M., Stieglitz T., Navarro X., Del Valle J. Scientific Reports; 8 (1, 5965) 2018. 10.1038/s41598-018-24502-z. IF: 4.122

Parylene C is a highly flexible polymer used in several biomedical implants. Since previous studies have reported valuable biocompatible and manufacturing characteristics for brain and intraneural implants, we tested its suitability as a substrate for peripheral nerve electrodes. We evaluated 1-year-aged in vitro samples, where no chemical differences were observed and only a slight deviation on Young's modulus was found. The foreign body reaction (FBR) to longitudinal Parylene C devices implanted in the rat sciatic nerve for 8 months was characterized. After 2 weeks, a capsule was formed around the device, which continued increasing up to 16 and 32 weeks. Histological analyses revealed two cell types implicated in the FBR: macrophages, in contact with the device, and fibroblasts, localized in the outermost zone after 8 weeks. Molecular analysis of implanted nerves comparing Parylene C and polyimide devices revealed a peak of inflammatory cytokines after 1 day of implant, returning to low levels thereafter. Only an increase of CCL2 and CCL3 was found at chronic time-points for both materials. Although no molecular differences in the FBR to both polymers were found, the thick tissue capsule formed around Parylene C puts some concern on its use as a scaffold for intraneural electrodes. © 2018 The Author(s).

View abstract
Photocurrent generation of biohybrid systems based on bacterial reaction centers and graphene electrodes
Csiki R., Drieschner S., Lyuleeva A., Cattani-Scholz A., Stutzmann M., Garrido J.A. Diamond and Related Materials; 89: 286 - 292. 2018. 10.1016/j.diamond.2018.09.005. IF: 2.232

The direct conversion of sunlight into chemical energy via photosynthesis is a unique capability of plants and some bacterial species. Aimed at mimicking this energy conversion process, the combination of inorganic substrates and organic photoactive proteins into an artificial biohybrid system is of a great interest for artificial bio-photovoltaic applications. It also allows to better understand charge transfer processes involved in the photosynthetic chain. In this work, single layer graphene (SLG) and multilayer graphene (MLG) electrodes are used as a platform for the immobilization of reaction centers (RCs) from purple bacteria Rhodobacter sphaeroides, a protein complex responsible for the generation of photo-excited charges. Electrochemical experiments with graphene electrodes and redox molecules reveal fundamental differences in the charge transfer processes for SLG and MLG films. We demonstrate that both graphene-based materials enable the immobilization of RCs without loss of functionality, attested by a photocurrent generation under illumination with IR-light at a wavelength of 870 nm. Furthermore, we report on the dependence of the generated photocurrent on the applied bias voltage, as well as on the presence of charge mediators in the surrounding electrolyte. This work demonstrates that SLG and MLG are a suitable platform for RC immobilization and subsequent photocurrent generation, suggesting a promising potential for graphene-based materials in bio-photovoltaics. © 2018 Elsevier B.V.

View abstract
Segregation of motor and sensory axons regenerating through bicompartmental tubes by combining extracellular matrix components with neurotrophic factors
del Valle J., Santos D., Delgado-Martínez I., de la Oliva N., Giudetti G., Micera S., Navarro X. Journal of Tissue Engineering and Regenerative Medicine; 12 (4): e1991 - e2000. 2018. 10.1002/term.2629. IF: 4.089

Segregation of regenerating motor and sensory axons may be a good strategy to improve selective functionality of regenerative interfaces to provide closed-loop commands. Provided that extracellular matrix components and neurotrophic factors exert guidance effects on different neuronal populations, we assessed in vivo the potential of separating sensory and motor axons regenerating in a bicompartmental Y-type tube, with each branch prefilled with an adequate combination of extracellular matrix and neurotrophic factors. The severed rat sciatic nerve was repaired using a bicompartmental tube filled with a collagen matrix enriched with fibronectin (FN) and brain-derived neurotrophic factor (BDNF) encapsulated in poly-lactic co-glycolic acid microspheres (FN + MP.BDNF) in one compartment to preferentially attract motor axons and collagen enriched with laminin (LM) and nerve growth factor (NGF) and neurotrophin-3 (NT-3) in microspheres (LM + MP.NGF/NT-3) in the other compartment for promoting sensory axons regeneration. Control animals were implanted with the same Y-tube with a collagen matrix with microspheres (MP) containing PBS (Col + MP.PBS). By using retrotracer labelling, we found that LM + MP.NGF/NT-3 did not attract higher number of regenerated sensory axons compared with controls, and no differences were observed in sensory functional recovery. However, FN + MP.BDNF guided a higher number of regenerating motor axons compared with controls, improving also motor recovery. A small proportion of sensory axons with large soma size, likely proprioceptive neurons, was also attracted to the FN + MP.BDNF compartment. These results demonstrate that muscular axonal guidance can be modulated in vivo by the addition of fibronectin and BDNF. Copyright © 2017 John Wiley & Sons, Ltd.

View abstract
Single-layer graphene modulates neuronal communication and augments membrane ion currents
Pampaloni N.P., Lottner M., Giugliano M., Matruglio A., D’Amico F., Prato M., Garrido J.A., Ballerini L., Scaini D. Nature Nanotechnology; 13 (8): 755 - 764. 2018. 10.1038/s41565-018-0163-6. IF: 37.490

The use of graphene-based materials to engineer sophisticated biosensing interfaces that can adapt to the central nervous system requires a detailed understanding of how such materials behave in a biological context. Graphene’s peculiar properties can cause various cellular changes, but the underlying mechanisms remain unclear. Here, we show that single-layer graphene increases neuronal firing by altering membrane-associated functions in cultured cells. Graphene tunes the distribution of extracellular ions at the interface with neurons, a key regulator of neuronal excitability. The resulting biophysical changes in the membrane include stronger potassium ion currents, with a shift in the fraction of neuronal firing phenotypes from adapting to tonically firing. By using experimental and theoretical approaches, we hypothesize that the graphene–ion interactions that are maximized when single-layer graphene is deposited on electrically insulating substrates are crucial to these effects. © 2018, The Author(s).

View abstract
Time course study of long-term biocompatibility and foreign body reaction to intraneural polyimide-based implants
de la Oliva N., Navarro X., del Valle J. Journal of Biomedical Materials Research - Part A; 106 (3): 746 - 757. 2018. 10.1002/jbm.a.36274. IF: 3.231

The foreign body reaction (FBR) against an implanted device is characterized by the formation of a fibrotic tissue around the implant. In the case of interfaces for peripheral nerves, used to stimulate specific group of axons and to record different nerve signals, the FBR induces a matrix deposition around the implant creating a physical separation between nerve fibers and the interface that may reduce its functionality over time. In order to understand how the FBR to intraneural interfaces evolves, polyimide non-functional devices were implanted in rat peripheral nerve. Functional tests (electrophysiological, pain and locomotion) and histological evaluation demonstrated that implanted devices did not cause any alteration in nerve function, in myelinated axons or in nerve architecture. The inflammatory response due to the surgical implantation decreased after 2 weeks. In contrast, inflammation was higher and more prolonged in the device implanted nerves with a peak after 2 weeks. With regard to tissue deposition, a tissue capsule appeared soon around the devices, acquiring maximal thickness at 2 weeks and being remodeled subsequently. Immunohistochemical analysis revealed two different cell types implicated in the FBR in the nerve: macrophages as the first cells in contact with the interface and fibroblasts that appear later at the edge of the capsule. Our results describe how the FBR against a polyimide implant in the peripheral nerve occurs and which are the main cellular players. Increasing knowledge of these responses will help to improve strategies to decrease the FBR against intraneural implants and to extend their usability. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 746–757, 2018. © 2017 Wiley Periodicals, Inc.

View abstract
Understanding the bias dependence of low frequency noise in single layer graphene FETs
Mavredakis N., Garcia Cortadella R., Bonaccini Calia A., Garrido J.A., Jiménez D. Nanoscale; 10 (31): 14947 - 14956. 2018. 10.1039/c8nr04939d. IF: 7.233

This letter investigates the bias-dependent low frequency noise of single layer graphene field-effect transistors. Noise measurements have been conducted with electrolyte-gated graphene transistors covering a wide range of gate and drain bias conditions for different channel lengths. A new analytical model that accounts for the propagation of the local noise sources in the channel to the terminal currents and voltages is proposed in this paper to investigate the noise bias dependence. Carrier number and mobility fluctuations are considered as the main causes of low frequency noise and the way these mechanisms contribute to the bias dependence of the noise is analyzed in this work. Typically, normalized low frequency noise in graphene devices has been usually shown to follow an M-shape dependence versus gate voltage with the minimum near the charge neutrality point (CNP). Our work reveals for the first time the strong correlation between this gate dependence and the residual charge which is relevant in the vicinity of this specific bias point. We discuss how charge inhomogeneity in the graphene channel at higher drain voltages can contribute to low frequency noise; thus, channel regions nearby the source and drain terminals are found to dominate the total noise for gate biases close to the CNP. The excellent agreement between the experimental data and the predictions of the analytical model at all bias conditions confirms that the two fundamental 1/f noise mechanisms, carrier number and mobility fluctuations, must be considered simultaneously to properly understand the low frequency noise in graphene FETs. The proposed analytical compact model can be easily implemented and integrated in circuit simulators, which can be of high importance for graphene based circuits' design. © The Royal Society of Chemistry.

View abstract
Uniformly coated highly porous graphene/MnO2 foams for flexible asymmetric supercapacitors
Drieschner S., Seckendorff M.V., Corro E.D., Wohlketzetter J., Blaschke B.M., Stutzmann M., Garrido J.A. Nanotechnology; 29 (22, 225402) 2018. 10.1088/1361-6528/aab4c2. IF: 3.404

Supercapacitors are called to play a prominent role in the newly emerging markets of electric vehicles, flexible displays and sensors, and wearable electronics. In order to compete with current battery technology, supercapacitors have to be designed with highly conductive current collectors exhibiting high surface area per unit volume and uniformly coated with pseudocapacitive materials, which is crucial to boost the energy density while maintaining a high power density. Here, we present a versatile technique to prepare thickness-controlled thin-film micro graphene foams (μGFs) with pores in the lower micrometer range grown by chemical vapor deposition which can be used as highly conductive current collectors in flexible supercapacitors. To fabricate the μGF, we use porous metallic catalytic substrates consisting of nickel/copper alloy synthesized on nickel foil by electrodeposition in an electrolytic solution. Changing the duration of the electrodeposition allows the control of the thickness of the metal foam, and thus of the μGF, ranging from a few micrometers to the millimeter scale. The resulting μGF with a thickness and pores in the micrometer regime exhibits high structural quality which leads to a very low intrinsic resistance of the devices. Transferred onto flexible substrates, we demonstrate a uniform coating of the μGFs with manganese oxide, a pseudocapacitively active material. Considering the porous structure and the thickness of the μGFs, square wave potential pulses are used to ensure uniform coverage by the oxide material boosting the volumetric and areal capacitance to 14 F cm-3 and 0.16 F cm-2. The μGF with a thickness and pores in the micrometer regime in combination with a coating technique tuned to the porosity of the μGF is of great relevance for the development of supercapacitors based on state-of-the-art graphene foams. © 2018 IOP Publishing Ltd.

View abstract

2017

Cryogenic characterisation and modelling of commercial SiC MOSFETs
Woodend L; Gammon P; Shah V; Pérez-Tomás A; Li F; Hamilton D; Myronov M; Mawby P Materials Science Forum; 897 MSF: 557 - 560. 2017. 10.4028/www.scientific.net/MSF.897.557.
Electrochemical characterization of GaN surface states
Winnerl A., Garrido J.A., Stutzmann M. Journal of Applied Physics; 122 (4, 045302) 2017. 10.1063/1.4995429. IF: 2.068

In this work, we present a systematic study of the electrochemical properties of metal-organic chemical vapor deposition and hybrid vapor phase epitaxy grown n-type GaN in aqueous electrolytes. For this purpose, we perform cyclic voltammetry and impedance spectroscopy measurements over a wide range of potentials and frequencies, using a pure aqueous electrolyte and adding two different types of redox couples, as well as applying different surface treatments to the GaN electrodes. For Ga-polar GaN electrodes, the charge transfer to an electrolyte is dominated by surface states, which are not related to dislocations and are independent of the specific growth technique. These surface states can be modified by the surface treatment; they are generated by etching in HCl and are passivated by oxidation. Different surface defect states are present on N-polar GaN electrodes which do not significantly contribute to the charge transfer across the GaN/electrolyte interface. © 2017 Author(s).

View abstract
Frequency response of electrolyte-gated graphene electrodes and transistors
Drieschner S., Guimerà A., Cortadella R.G., Viana D., Makrygiannis E., Blaschke B.M., Vieten J., Garrido J.A. Journal of Physics D: Applied Physics; 50 (9, 095304) 2017. 10.1088/1361-6463/aa5443. IF: 2.588

The interface between graphene and aqueous electrolytes is of high importance for applications of graphene in the field of biosensors and bioelectronics. The graphene/electrolyte interface is governed by the low density of states of graphene that limits the capacitance near the Dirac point in graphene and the sheet resistance. While several reports have focused on studying the capacitance of graphene as a function of the gate voltage, the frequency response of graphene electrodes and electrolyte-gated transistors has not been discussed so far. Here, we report on the impedance characterization of single layer graphene electrodes and transistors, showing that due to the relatively high sheet resistance of graphene, the frequency response is governed by the distribution of resistive and capacitive circuit elements along the graphene/electrolyte interface. Based on an analytical solution for the impedance of the distributed circuit elements, we model the graphene/electrolyte interface both for the electrode and the transistor configurations. Using this model, we can extract the relevant material and device parameters such as the voltage-dependent intrinsic sheet and series resistances as well as the interfacial capacitance. The model also provides information about the frequency threshold of electrolyte-gated graphene transistors, above which the device exhibits a non-resistive response, offering an important insight into the suitable frequency range of operation of electrolyte-gated graphene devices. © 2017 IOP Publishing Ltd.

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GaN surface states investigated by electrochemical studies
Winnerl A., Garrido J.A., Stutzmann M. Applied Physics Letters; 110 (10, 101602) 2017. 10.1063/1.4977947. IF: 3.411

We present a systematic study of electrochemically active surface states on MOCVD-grown n-type GaN in aqueous electrolytes using cyclic voltammetry and impedance spectroscopy over a wide range of potentials and frequencies. In order to alter the surface states, the GaN samples are either etched or oxidized, and the influence of the surface treatment on the defect-mediated charge transfer to the electrolyte is investigated. Etching in HCl removes substoichiometric GaOx, and leads to a pronounced density of electrochemically active surface states. Oxidation effectively removes these surface states. © 2017 Author(s).

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Graphene Field-Effect Transistors for In Vitro and Ex Vivo Recordings
Kireev D., Zadorozhnyi I., Qiu T., Sarik D., Brings F., Wu T., Seyock S., Maybeck V., Lottner M., Blaschke B.M., Garrido J., Xie X., Vitusevich S., Wolfrum B., Offenhäusser A. IEEE Transactions on Nanotechnology; 16 (1, 7782310): 140 - 147. 2017. 10.1109/TNANO.2016.2639028. IF: 2.485

Recording extracellular potentials from electrogenic cells (especially neurons) is the hallmark destination of modern bioelectronics. While fabrication of flexible and biocompatible in vivo devices via silicon technology is complicated and time-consuming, graphene field-effect transistors (GFETs), instead, can easily be fabricated on flexible and biocompatible substrates. In this work, we compare GFETs fabricated on rigid (SiO2/Si and sapphire) and flexible (polyimide) substrates. The GFETs, fabricated on the polyimide, exhibit extremely large transconductance values, up to 11 mS·V-1, and mobility over 1750 cm2·V-1·s-1. In vitro recordings from cardiomyocyte-like cell culture are performed by GFETs on a rigid transparent substrate (sapphire). Via multichannel measurement, we are able to record and analyze both: difference in action potentials as well as their spatial propagation over the chip. Furthermore, the controllably flexible polyimide-on-steel (PIonS) substrates are able to ex vivo record electrical signals from primary embryonic rat heart tissue. Considering the flexibility of PIonS chips, together with the excellent sensitivity, we open up a new road into graphene-based in vivo biosensing. © 2016 IEEE.

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Graphene in the Design and Engineering of Next-Generation Neural Interfaces
Kostarelos K., Vincent M., Hebert C., Garrido J.A. Advanced Materials; 29 (42, 1700909) 2017. 10.1002/adma.201700909. IF: 19.791

Neural interfaces are becoming a powerful toolkit for clinical interventions requiring stimulation and/or recording of the electrical activity of the nervous system. Active implantable devices offer a promising approach for the treatment of various diseases affecting the central or peripheral nervous systems by electrically stimulating different neuronal structures. All currently used neural interface devices are designed to perform a single function: either record activity or electrically stimulate tissue. Because of their electrical and electrochemical performance and their suitability for integration into flexible devices, graphene-based materials constitute a versatile platform that could help address many of the current challenges in neural interface design. Here, how graphene and other 2D materials possess an array of properties that can enable enhanced functional capabilities for neural interfaces is illustrated. It is emphasized that the technological challenges are similar for all alternative types of materials used in the engineering of neural interface devices, each offering a unique set of advantages and limitations. Graphene and 2D materials can indeed play a commanding role in the efforts toward wider clinical adoption of bioelectronics and electroceuticals. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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Interfacing neurons on carbon nanotubes covered with diamond
Seyock S., Maybeck V., Scorsone E., Rousseau L., Hébert C., Lissorgues G., Bergonzo P., Offenhäusser A. RSC Advances; 7 (1): 153 - 160. 2017. 10.1039/c6ra20207a. IF: 3.108

A recently discovered material, carbon nanotubes covered with diamond (DCNTs) was tested for its suitability in bioelectronics applications. Diamond shows advantages for bioelectronics applications (wide electro chemical window and bioinertness). This study investigates the effect of electrode surface shape (flat or three dimensional) on cell growth and behavior. For comparison, flat nanocrystalline diamond substrates were used. Primary embryonic neurons were grown on top of the structures and neither incorporated the structures nor did they grow in between the single structures. The interface was closely examined using focused ion beam (FIB) and scanning electron microscopy. Of special interest was the interface between cell and substrate. 5% to 25% of the cell membrane adhered to the substrate, which fits the theoretical estimated value. While investigating the conformity of the neurons, it could be observed that the cell membrane attaches to different heights of the tips of the 3D structure. However, the aspect ratio of the structures had no effect on the cell viability. These results let us assume that not more than 25% of cell attachment is needed for the survival of a functional neuronal cell. © The Royal Society of Chemistry.

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Mapping brain activity with flexible graphene micro-transistors
Blaschke B.M., Tort-Colet N., Guimerà-Brunet A., Weinert J., Rousseau L., Heimann A., Drieschner S., Kempski O., Villa R., Sanchez-Vives M.V., Garrido J.A. 2D Materials; 4 (2, 025040) 2017. 10.1088/2053-1583/aa5eff. IF: 6.937

Establishing a reliable communication interface between the brain and electronic devices is of paramount importance for exploiting the full potential of neural prostheses. Current microelectrode technologies for recording electrical activity, however, evidence important shortcomings, e.g. challenging high density integration. Solution-gated field-effect transistors (SGFETs), on the other hand, could overcome these shortcomings if a suitable transistor material were available. Graphene is particularly attractive due to its biocompatibility, chemical stability, flexibility, low intrinsic electronic noise and high charge carrier mobilities. Here, we report on the use of an array of flexible graphene SGFETs for recording spontaneous slow waves, as well as visually evoked and also pre-epileptic activity in vivo in rats. The flexible array of graphene SGFETs allows mapping brain electrical activity with excellent signal-to-noise ratio (SNR), suggesting that this technology could lay the foundation for a future generation of in vivo recording implants. © 2017 IOP Publishing Ltd.

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Protecting a Diamond Quantum Memory by Charge State Control
Pfender M., Aslam N., Simon P., Antonov D., Thiering G., Burk S., Fávaro De Oliveira F., Denisenko A., Fedder H., Meijer J., Garrido J.A., Gali A., Teraji T., Isoya J., Doherty M.W., Alkauskas A., Gallo A., Grüneis A., Neumann P., Wrachtrup J. Nano Letters; 17 (10): 5931 - 5937. 2017. 10.1021/acs.nanolett.7b01796. IF: 12.712

In recent years, solid-state spin systems have emerged as promising candidates for quantum information processing. Prominent examples are the nitrogen-vacancy (NV) center in diamond, phosphorus dopants in silicon (Si:P), rare-earth ions in solids, and VSi-centers in silicon-carbide. The Si:P system has demonstrated that its nuclear spins can yield exceedingly long spin coherence times by eliminating the electron spin of the dopant. For NV centers, however, a proper charge state for storage of nuclear spin qubit coherence has not been identified yet. Here, we identify and characterize the positively charged NV center as an electron-spin-less and optically inactive state by utilizing the nuclear spin qubit as a probe. We control the electronic charge and spin utilizing nanometer scale gate electrodes. We achieve a lengthening of the nuclear spin coherence times by a factor of 4. Surprisingly, the new charge state allows switching of the optical response of single nodes facilitating full individual addressability. © 2017 American Chemical Society.

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Tuning the electronic properties of monolayer and bilayer transition metal dichalcogenide compounds under direct out-of-plane compression
García Á.M., Corro E.D., Kalbac M., Frank O. Physical Chemistry Chemical Physics; 19 (20): 13333 - 13340. 2017. 10.1039/c7cp00012j. IF: 4.123

The band-gap modulation of atomically thin semiconductor transition metal dichalcogenides (MX2; M = Mo or W, X = S or Se) under direct out-of-plane compression is systematically studied by means of the density functional theory (DFT) formalism including spin-orbit coupling (SOC) and dispersion correction (D3). The out-of-plane compared with other regimes stress regime significantly reduces the pressure threshold at which the semimetal state is achieved (2.7-3.1 and 1.9-3.2 GPa for mono- and bilayer systems, respectively). Structural, electronic and bonding properties are investigated for a better understanding of the electronic transitions achieved with compression. A notable relationship with the formal ionic radius (M4+ and X2-) is obtained. On one hand, the monolayer systems with the smallest transition metal radius (Mo4+ < W4+) reach the semimetal state at lower stress, on the other hand, for bilayer specimens the transition to semimetal is observed earlier for compounds with the smallest chalcogenide radius (S2- < Se2-). Moreover, the appearance of non-covalent interaction (NCI) domains in the semimetal state confirms that the out-of-plane compression promotes the interaction between sulfur atoms in the single layered systems and reduces the interlayer space in bilayer configurations. Our predictions, supported by experimental evidences in the case of monolayered MoS2, demonstrate new alternative methods for tuning the electronic properties of transition metal dichalcogenides under direct out-of-plane compression. © 2017 the Owner Societies.

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2016

Flexible graphene transistors for recording cell action potentials
Blaschke B.M., Lottner M., Drieschner S., Calia A.B., Stoiber K., Rousseau L., Lissourges G., Garrido J.A. 2D Materials; 3 (2, 025007) 2016. 10.1088/2053-1583/3/2/025007. IF: 9.611

Graphene solution-gated field-effect transistors (SGFETs) are a promising platform for the recording of cell action potentials due to the intrinsic high signal amplification of graphene transistors. In addition, graphene technology fulfills important key requirements for in-vivo applications, such as biocompability, mechanical flexibility, as well as ease of high density integration. In this paper we demonstrate the fabrication of flexible arrays of graphene SGFETs on polyimide, a biocompatible polymeric substrate. We investigate the transistor's transconductance and intrinsic electronic noise which are key parameters for the device sensitivity, confirming that the obtained values are comparable to those of rigid graphene SGFETs. Furthermore, we show that the devices do not degrade during repeated bending and the transconductance, governed by the electronic properties of graphene, is unaffected by bending. After cell culture, we demonstrate the recording of cell action potentials from cardiomyocyte-like cells with a high signal-to-noise ratio that is higher or comparable to competing state of the art technologies. Our results highlight the great capabilities of flexible graphene SGFETs in bioelectronics, providing a solid foundation for in-vivo experiments and, eventually, for graphene-based neuroprosthetics. © 2016 IOP Publishing Ltd.

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High surface area graphene foams by chemical vapor deposition
Drieschner S., Weber M., Wohlketzetter J., Vieten J., Makrygiannis E., Blaschke B.M., Morandi V., Colombo L., Bonaccorso F., Garrido J.A. 2D Materials; 3 (4, 045013) 2016. 10.1088/2053-1583/3/4/045013. IF: 9.611

Three-dimensional (3D) graphene-based structures combine the unique physical properties of graphene with the opportunity to get high electrochemically available surface area per unit of geometric surface area. Several preparation techniques have been reported to fabricate 3D graphene-based macroscopic structures for energy storage applications such as supercapacitors. Although reaserch has been focused so far on achieving either high specific capacitance or high volumetric capacitance, much less attention has been dedicated to obtain high specific and high volumetric capacitance simultaneously. Here, we present a facile technique to fabricate graphene foams (GF) of high crystal quality with tunable pore size grown by chemical vapor deposition. We exploited porous sacrificial templates prepared by sintering nickel and copper metal powders. Tuning the particle size of the metal powders and the growth temperature allow fine control of the resulting pore size of the 3D graphene-based structures smaller than 1 μm. The as-produced 3D graphene structures provide a high volumetric electric double layer capacitance (165 mF cm-3). High specific capacitance (100 Fg-1) is obtained by lowering the number of layers down to single layer graphene. Furthermore, the small pore size increases the stability of these GFs in contrast to the ones that have been grown so far on commercial metal foams. Electrodes based on the as-prepared GFs can be a boost for the development of supercapacitors, where both low volume and mass are required. © 2016 IOP Publishing Ltd.

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Suppression of Photoanodic Surface Oxidation of n-Type 6H-SiC Electrodes in Aqueous Electrolytes
Sachsenhauser M., Walczak K., Hampel P.A., Stutzmann M., Sharp I.D., Garrido J.A. Langmuir; 32 (6): 1637 - 1644. 2016. 10.1021/acs.langmuir.5b04376. IF: 3.993

The photoelectrochemical characterization of silicon carbide (SiC) electrodes is important for enabling a wide range of potential applications for this semiconductor. However, photocorrosion of the SiC surface remains a key challenge, because this process considerably hinders the deployment of this material into functional devices. In this report, we use cyclic voltammetry to investigate the stability of n-type 6H-SiC photoelectrodes in buffered aqueous electrolytes. For measurements in pure Tris buffer, photogenerated holes accumulate at the interface under anodic polarization, resulting in the formation of a porous surface oxide layer. Two possibilities are presented to significantly enhance the stability of the SiC photoelectrodes. In the first approach, redox molecules are added to the buffer solution to kinetically facilitate hole transfer to these molecules, and in the second approach, water oxidation in the electrolyte is induced by depositing a cobalt phosphate catalyst onto the semiconductor surface. Both methods are found to effectively suppress photocorrosion of the SiC electrodes, as confirmed by atomic force microscopy and X-ray photoelectron spectroscopy measurements. The presented study provides straightforward routes to stabilize n-type SiC photoelectrodes in aqueous electrolytes, which is essential for a possible utilization of this material in the fields of photocatalysis and multimodal biosensing. © 2016 American Chemical Society.

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Surface State Mediated Electron Transfer Across the N-Type SiC/Electrolyte Interface
Sachsenhauser M., Sharp I.D., Stutzmann M., Garrido J.A. Journal of Physical Chemistry C; 120 (12): 6524 - 6533. 2016. 10.1021/acs.jpcc.5b11569. IF: 4.509

Understanding the mechanisms of charge transfer across the semiconductor/electrolyte interface is a basic prerequisite for a variety of practical applications. In particular, electrically active surface states located in the semiconductor band gap are expected to play an important role, but direct experimental evidence of surface states has proven to be challenging, and further experimental studies are required to verify their influence on the exchange of charge carriers between semiconductor and electrolyte. Due to its wide band gap, chemical stability, and controllable surface termination, silicon carbide (SiC) provides an excellent model system for this purpose. In this report, we provide a fundamental electrochemical study of n-type 6H-SiC and 4H-SiC electrodes in aqueous electrolytes containing the ferricyanide/ferrocyanide redox couple. Cyclic voltammetry and impedance spectroscopy measurements are performed over a wide range of potentials to determine the energetic positions of the SiC band edges and to investigate the electron-transfer kinetics between SiC and the ferricyanide molecules. For both polytypes, a broad distribution of surface states with energy levels close to the conduction band is found to mediate electron transfer, resulting in deviations of the observed charge transport characteristics from the predictions of well-established models. Moreover, a detailed evaluation of the impedance data allows for explicit correlation of the charge-transfer resistance associated with the ferricyanide reduction reaction with the potential-dependent distribution of surface states. In addition to the relevance of our studies for advancing the implementation of SiC in biosensing, electrocatalytic, and photocatalytic applications, the presented methodology can also be adopted for fundamental electrochemical investigations of other semiconductor electrodes. (Figure Presented). © 2016 American Chemical Society.

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α,ω -dihexyl-sexithiophene thin films for solution-gated organic field-effect transistors
Schamoni H., Noever S., Nickel B., Stutzmann M., Garrido J.A. Applied Physics Letters; 108 (7, 073301) 2016. 10.1063/1.4942407. IF: 3.142

While organic semiconductors are being widely investigated for chemical and biochemical sensing applications, major drawbacks such as the poor device stability and low charge carrier mobility in aqueous electrolytes have not yet been solved to complete satisfaction. In this work, solution-gated organic field-effect transistors (SGOFETs) based on the molecule α,ω-dihexyl-sexithiophene (DH6T) are presented as promising platforms for in-electrolyte sensing. Thin films of DH6T were investigated with regard to the influence of the substrate temperature during deposition on the grain size and structural order. The performance of SGOFETs can be improved by choosing suitable growth parameters that lead to a two-dimensional film morphology and a high degree of structural order. Furthermore, the capability of the SGOFETs to detect changes in the pH or ionic strength of the gate electrolyte is demonstrated and simulated. Finally, excellent transistor stability is confirmed by continuously operating the device over a period of several days, which is a consequence of the low threshold voltage of DH6T-based SGOFETs. Altogether, our results demonstrate the feasibility of high performance and highly stable organic semiconductor devices for chemical or biochemical applications. © 2016 AIP Publishing LLC.

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2015

Role of grain boundaries in tailoring electronic properties of polycrystalline graphene by chemical functionalization
Seifert M., Vargas J.E.B., Bobinger M., Sachsenhauser M., Cummings A.W., Roche S., Garrido J.A. 2D Materials; 2 (2, 024008) 2015. 10.1088/2053-1583/2/2/024008. IF: 0.000

Grain boundaries, inevitably present in chemical vapor deposited graphene, are expected to have considerable impact on the development of graphene-based hybrid materials with tailored material properties.Wedemonstrate here the critical role of polycrystallinity on the chemical functionalization of graphene comparing ozone-induced oxidation with remote plasma hydrogenation.Weshow that graphene oxidation and hydrogenation occur in two consecutive stages upon increasing defect density: an initial step in which surface-bound functional groups are generated, followed by the creation of vacancies. Remarkably, we find that hydrogenation yields homogeneously distributed defects while ozone-induced defects are preferentially accumulated at the grain boundaries eventually provoking local cracking of the structure. Supported by quantum simulations, our experimental findings reveal distinct electronic transport regimes depending on the density and distribution of induced defects on the polycrystalline graphene films. Our findings highlight the key role played by grain boundaries during graphene functionalization, and at the same time provide a novel perspective to tailor the properties of polycrystalline graphene. © 2015 IOP Publishing Ltd.

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Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems
Ferrari A.C., Bonaccorso F., Fal'ko V., Novoselov K.S., Roche S., Bøggild P., Borini S., Koppens F.H.L., Palermo V., Pugno N., Garrido J.A., Sordan R., Bianco A., Ballerini L., Prato M., Lidorikis E., Kivioja J., Marinelli C., Ryhänen T., Morpurgo A., Coleman J.N., Nicolosi V., Colombo L., Fert A., Garcia-Hernandez M., Bachtold A., Schneider G.F., Guinea F., Dekker C., Barbone M., Sun Z., Galiotis C., Grigorenko A.N., Konstantatos G., Kis A., Katsnelson M., Vandersypen L., Loiseau A., Morandi V., Neumaier D., Treossi E., Pellegrini V., Polini M., Tredicucci A., Williams G.M., Hee Hong B., Ahn J.-H., Min Kim J., Zirath H., Van Wees B.J., Van Der Zant H., Occhipinti L., Di Matteo A., Kinloch I.A., Seyller T., Quesnel E., Feng X., Teo K., Rupesinghe N., Hakonen P., Neil S.R.T., Tannock Q., Löfwander T., Kinaret J. Nanoscale; 7 (11): 4598 - 4810. 2015. 10.1039/c4nr01600a. IF: 7.394

We present the science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, targeting an evolution in technology, that might lead to impacts and benefits reaching into most areas of society. This roadmap was developed within the framework of the European Graphene Flagship and outlines the main targets and research areas as best understood at the start of this ambitious project. We provide an overview of the key aspects of graphene and related materials (GRMs), ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlighting the steps necessary to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries. We also define an extensive list of acronyms in an effort to standardize the nomenclature in this emerging field. © The Royal Society of Chemistry 2015.

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