Staff directory Emigdio Chávez Angel



  • Application of Synchrotron Radiation-Based Fourier-Transform Infrared Microspectroscopy for Thermal Imaging of Polymer Thin Films

    Chavez-Angel, E; Ng, RC; Sandell, S; He, JY; Castro-Alvarez, A; Torres, CMS; Kreuzer, M Polymers; 15 (3): 536. 2023. 10.3390/polym15030536.

  • Contactless characterization of the elastic properties of glass microspheres

    Maire, Jeremie; Necio, Tomasz; Chávez-Ángel, Emigdio; Colombano, Martín F.; Jaramillo-Fernández, Juliana; Sotomayor-Torres, Clivia M.; Capuj, Nestor E.; Navarro-Urrios, Daniel Apl Materials; 11 (4) 2023. 10.1063/5.0146969.

  • Enhanced Thermoelectric Properties of Misfit Bi2Sr2-xCaxCo2Oy: Isovalent Substitutions and Selective Phonon Scattering

    Chatterjee, A; Banik, A; El Sachat, A; Roque, JMC; Padilla-Pantoja, J; Torres, CS; Biswas, K; Santiso, J; Chavez-Angel, E Materials (Basel); 16 (4): 1413. 2023. 10.3390/ma16041413.


  • Comparison of Brillouin Light Scattering and Density of States in a Supported Layer: Analytical and Experimental Study

    El Abouti, O; Cuffe, J; El Boudouti, E; Torres, CMS; Chavez-Angel, E; Djafari-Rouhani, B; Alzina, F Crystals; 12 (9) 2022. 10.3390/cryst12091212. IF: 2.670

  • Controlling the electrochemical hydrogen generation and storage in graphene oxide by in-situ Raman spectroscopy

    Pinilla-Sánchez A., Chávez-Angel E., Murcia-López S., Carretero N.M., Palardonio S.M., Xiao P., Rueda-García D., Sotomayor Torres C.M., Gómez-Romero P., Martorell J., Ros C. Carbon; 200: 227 - 235. 2022. 10.1016/j.carbon.2022.08.055.

    Hydrogen, generated from water splitting, is postulated as one of the most promising alternatives to fossil fuels. In this context, direct hydrogen generation by electrolysis and fixation to graphene oxide in an aqueous suspension could overcome storage and distribution problems of gaseous hydrogen. This study presents time-resolved determination of the electrochemical hydrogenation of GO by in-situ Raman spectroscopy, simultaneous to original functional groups elimination. Hydrogenation is found favoured by dynamic modulation of the electrochemical environment compared to fixed applied potentials, with a 160% increase of C–H bond formation. Epoxide groups suppression and generated hydroxide groups point at these epoxide groups being one of the key sites where hydrogenation was possible. FTIR revealed characteristic symmetric and asymmetric stretching vibrations of C–H bonds in CH2 and CH3 groups. This shows that hydrogenation is significantly also occurring in defective sites and edges of the graphene basal plane, rather than H-Csp3 groups as graphane. We also determined a −0.05 VRHE reduction starting potential in alkaline electrolytes and a 150 mV cathodic delay in acid electrolytes. The identified key parameters role, together with observed diverse C-Hx groups formation, points at future research directions for large-scale hydrogen storage in graphene. © 2022

  • Effect of crystallinity and thickness on thermal transport in layered PtSe2

    El Sachat A., Xiao P., Donadio D., Bonell F., Sledzinska M., Marty A., Vergnaud C., Boukari H., Jamet M., Arregui G., Chen Z., Alzina F., Sotomayor Torres C.M., Chavez-Angel E. npj 2D Materials and Applications; 6 (1, 32) 2022. 10.1038/s41699-022-00311-x.

    We present a comparative investigation of the influence of crystallinity and film thickness on the acoustic and thermal properties of layered PtSe2 films of varying thickness (1–40 layers) using frequency-domain thermo-reflectance, low-frequency Raman, and pump-probe coherent phonon spectroscopy. We find ballistic cross-plane heat transport up to ~30 layers PtSe2 and a 35% reduction in the cross-plane thermal conductivity of polycrystalline films with thickness larger than 20 layers compared to the crystalline films of the same thickness. First-principles calculations further reveal a high degree of thermal conductivity anisotropy and a remarkable large contribution of the optical phonons to the thermal conductivity in bulk (~20%) and thin PtSe2 films (~30%). Moreover, we show strong interlayer interactions in PtSe2, short acoustic phonon lifetimes in the range of picoseconds, an out-of-plane elastic constant of 31.8 GPa, and a layer-dependent group velocity ranging from 1340 ms−1 in bilayer to 1873 ms−1 in eight layers of PtSe2. The potential of tuning the lattice thermal conductivity of layered materials with the level of crystallinity and the real-time observation of coherent phonon dynamics open a new playground for research in 2D thermoelectric devices and provides guidelines for thermal management in 2D electronics. © 2022, The Author(s).

  • Erratum: “Thermal transport in silver-coated polymer sphere composites by the bidirectional 3ω method” [J. Appl. Phys. 131, 125107 (2022)]

    Sandell, Susanne; Wang, Thorstein; Chávez-Ángel, Emigdio; Kristiansen, Helge; Zhang, Zhiliang; He, Jianying Journal Of Applied Physics; 132 (12): 129903. 2022. 10.1063/5.0123921. IF: 2.877

  • Excitation and detection of acoustic phonons in nanoscale systems

    Ng R.C., El Sachat A., Cespedes F., Poblet M., Madiot G., Jaramillo-Fernandez J., Florez O., Xiao P., Sledzinska M., Sotomayor-Torres C.M., Chavez-Angel E. Nanoscale; 2022. 10.1039/d2nr04100f.

    Phonons play a key role in the physical properties of materials, and have long been a topic of study in physics. While the effects of phonons had historically been considered to be a hindrance, modern research has shown that phonons can be exploited due to their ability to couple to other excitations and consequently affect the thermal, dielectric, and electronic properties of solid state systems, greatly motivating the engineering of phononic structures. Advances in nanofabrication have allowed for structuring and phonon confinement even down to the nanoscale, drastically changing material properties. Despite developments in fabricating such nanoscale devices, the proper manipulation and characterization of phonons continues to be challenging. However, a fundamental understanding of these processes could enable the realization of key applications in diverse fields such as topological phononics, information technologies, sensing, and quantum electrodynamics, especially when integrated with existing electronic and photonic devices. Here, we highlight seven of the available methods for the excitation and detection of acoustic phonons and vibrations in solid materials, as well as advantages, disadvantages, and additional considerations related to their application. We then provide perspectives towards open challenges in nanophononics and how the additional understanding granted by these techniques could serve to enable the next generation of phononic technological applications. © 2022 The Royal Society of Chemistry.

  • Exciton tuning and strain imaging in WS2 supported on PDMS micropillars

    Sledzinska, M; Xiao, P; Vilardell, EP; Angel, EC; Esplandiu, MJ; Torres, CMS Applied Physics Letters; 121 (25) 2022. 10.1063/5.0130927. IF: 3.971

  • On the thermoelectric properties of Nb-doped SrTiO3epitaxial thin films

    Chatterjee A., Lan Z., Christensen D.V., Bauitti F., Morata A., Chavez-Angel E., Sanna S., Castelli I.E., Chen Y., Tarancon A., Pryds N. Physical Chemistry Chemical Physics; 24 (6): 3741 - 3748. 2022. 10.1039/d1cp03679c. IF: 0.000

    The exploration for thermoelectric thin films of complex oxides such as SrTiO3-based oxides is driven by the need for miniaturized harvesting devices for powering the Internet of Things (IoT). However, there is still not a clear consensus in the literature for the underlying influence of film thickness on thermoelectric properties. Here, we report the fabrication of epitaxial thin films of 6% Nb-doped SrTiO3 on (001) (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) single crystal using pulsed laser deposition (PLD) where the film thickness was varied from 2 nm to 68 nm. The thickness dependence shows a subtle increase of tetragonality of the thin film lattice and a gradual drop of the electrical conductivity, the density of charge carriers, and the thermoelectric Seebeck coefficient as the film thickness decreases. DFT-based calculations show that ∼2.8% increase in tetragonality results in an increased splitting between t2g and eg orbitals to ∼42.3 meV. However, experimentally observed tetragonality for films between 68 to 13 nm is only 0.06%. Hence, the effect of thickness on tetragonality is neglected. We have discussed the decrease of conductivity and the Seebeck coefficient based on the decrease of carriers and change in the scattering mechanism, respectively. © the Owner Societies.

  • Spectroscopic and Thermal Characterization of Extra Virgin Olive Oil Adulterated with Edible Oils

    Chavez-Angel E., Puertas B., Kreuzer M., Fortuny R., Ng R., Castro-Alvarez A., Sotomayortorres C. Foods; 11 (9, 1304) 2022. 10.3390/foods11091304.

    The substitution of extra virgin olive oil with other edible oils is the primary method for fraud in the olive-oil industry. Developing inexpensive analytical methods for confirming the quality and authenticity of olive oils is a major strategy towards combatting food fraud. Current methods used to detect such adulterations require complicated time- and resource-intensive preparation steps. In this work, a comparative study incorporating Raman and infrared spectroscopies, photoluminescence, and thermal-conductivity measurements of different sets of adulterated olive oils is presented. The potential of each characterization technique to detect traces of adulteration in extra virgin olive oils is evaluated. Concentrations of adulterant on the order of 5% can be detected in the Raman, infrared, and photoluminescence spectra. Small changes in thermal conductivity were also found for varying amounts of adulterants. While each of these techniques may individually be unable to identify impurity adulterants, the combination of these techniques together provides a holistic approach to validate the purity and authenticity of olive oils. © 2022 by the authors. Licensee MDPI, Basel, Switzerland.

  • Thermal Properties of Nanocrystalline Silicon Nanobeams

    Maire J., Chávez-Ángel E., Arregui G., Colombano M.F., Capuj N.E., Griol A., Martínez A., Navarro-Urrios D., Ahopelto J., Sotomayor-Torres C.M. Advanced Functional Materials; 32 (4, 2105767) 2022. 10.1002/adfm.202105767. IF: 18.808

    Controlling thermal energy transfer at the nanoscale and thermal properties has become critically important in many applications since it often limits device performance. In this study, the effects on thermal conductivity arising from the nanoscale structure of free-standing nanocrystalline silicon films and the increasing surface-to-volume ratio when fabricated into suspended optomechanical nanobeams are studied. Thermal transport and elucidate the relative impact of different grain size distributions and geometrical dimensions on thermal conductivity are characterized. A micro time-domain thermoreflectance method to study free-standing nanocrystalline silicon films and find a drastic reduction in the thermal conductivity, down to values below 10 W m–1 K–1 is used, with a stronger decrease for smaller grains. In optomechanical nanostructures, this effect is smaller than in membranes due to the competition of surface scattering in decreasing thermal conductivity. Finally, a novel versatile contactless characterization technique that can be adapted to any structure supporting a thermally shifted optical resonance is introduced. The thermal conductivity data agrees quantitatively with the thermoreflectance measurements. This study opens the way to a more generalized thermal characterization of optomechanical cavities and to create hot-spots with engineered shapes at the desired position in the structures as a means to study thermal transport in coupled photon-phonon structures. © 2021 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH

  • Thermal Rectification and Thermal Logic Gates in Graded Alloy Semiconductors

    Ng R.C., Castro-Alvarez A., Sotomayor-Torres C.M., Chávez-Angel E. Energies; 15 (13, 4685) 2022. 10.3390/en15134685.

    Classical thermal rectification arises from the contact between two dissimilar bulk materials, each with a thermal conductivity (k) with a different temperature dependence. Here, we study thermal rectification in a Si(1−x)Gex alloy with a spatial dependence on the atomic composition. Rectification factors (R = kmax/kmin) of up to 3.41 were found. We also demonstrate the suitability of such an alloy for logic gates using a thermal AND gate as an example by controlling the thermal conductivity profile via the alloy composition. This system is readily extendable to other alloys, since it only depends on the effective thermal conductivity. These thermal devices are inherently advantageous alternatives to their electric counterparts, as they may be able to take advantage of otherwise undesired waste heat in the surroundings. Furthermore, the demonstration of logic operations is a step towards thermal computation. © 2022 by the authors. Licensee MDPI, Basel, Switzerland.

  • Thermal transport in silver-coated polymer sphere composites by the bidirectional 3 ω method

    Sandell S., Wang T., Chávez-Ángel E., Kristiansen H., Zhang Z., He J. Journal of Applied Physics; 131 (12, 125107) 2022. 10.1063/5.0080682. IF: 2.546

    The bidirectional 3 ω method is an electrothermal technique that is commonly used to obtain the thermal conductivity of materials such as liquids, biological samples, and pastes. In this work, an epoxy-based adhesive was filled with monodisperse 10 μm polymethyl methacrylate spheres coated with silver thin films (AgPS), such that a metallic network that dominated the thermal transport was formed through the composite. The bidirectional 3 ω method was used to obtain the thermal conductivity of the conductive adhesive at different volume fractions of AgPS. For 50 vol.% AgPS, corresponding to 3.4 vol.% silver, the thermal conductivity was 2.03 ± 0.21 W m-1 K-1. The results show that the thermal conductivity is strongly correlated with the AgPS volume fraction, while maintaining a volume fraction of silver far below the commercial silver paste, which has typical filler fractions of 40 vol.% silver. The results of this work were compared to thermal measurements of the same material by other techniques, and advantages and disadvantages of the methods were finally discussed. © 2022 Author(s).

  • Unraveling Heat Transport and Dissipation in Suspended MoSe2 from Bulk to Monolayer

    Saleta Reig D., Varghese S., Farris R., Block A., Mehew J.D., Hellman O., Woźniak P., Sledzinska M., El Sachat A., Chávez-Ángel E., Valenzuela S.O., van Hulst N.F., Ordejón P., Zanolli Z., Sotomayor Torres C.M., Verstraete M.J., Tielrooij K.-J. Advanced Materials; 34 (10, 2108352) 2022. 10.1002/adma.202108352. IF: 30.849

    Understanding heat flow in layered transition metal dichalcogenide (TMD) crystals is crucial for applications exploiting these materials. Despite significant efforts, several basic thermal transport properties of TMDs are currently not well understood, in particular how transport is affected by material thickness and the material's environment. This combined experimental–theoretical study establishes a unifying physical picture of the intrinsic lattice thermal conductivity of the representative TMD MoSe2. Thermal conductivity measurements using Raman thermometry on a large set of clean, crystalline, suspended crystals with systematically varied thickness are combined with ab initio simulations with phonons at finite temperature. The results show that phonon dispersions and lifetimes change strongly with thickness, yet the thinnest TMD films exhibit an in-plane thermal conductivity that is only marginally smaller than that of bulk crystals. This is the result of compensating phonon contributions, in particular heat-carrying modes around ≈0.1 THz in (sub)nanometer thin films, with a surprisingly long mean free path of several micrometers. This behavior arises directly from the layered nature of the material. Furthermore, out-of-plane heat dissipation to air molecules is remarkably efficient, in particular for the thinnest crystals, increasing the apparent thermal conductivity of monolayer MoSe2 by an order of magnitude. These results are crucial for the design of (flexible) TMD-based (opto-)electronic applications. © 2022 The Authors. Advanced Materials published by Wiley-VCH GmbH


  • Anisotropic Thermal Conductivity of Crystalline Layered SnSe2

    Xiao P., Chavez-Angel E., Chaitoglou S., Sledzinska M., Dimoulas A., Sotomayor Torres C.M., El Sachat A. Nano Letters; 21 (21): 9172 - 9179. 2021. 10.1021/acs.nanolett.1c03018. IF: 11.189

    The degree of thermal anisotropy affects critically key device-relevant properties of layered two-dimensional materials. Here, we systematically study the in-plane and cross-plane thermal conductivity of crystalline SnSe2 films of varying thickness (16-190 nm) and uncover a thickness-independent thermal conductivity anisotropy ratio of about ∼8.4. Experimental data obtained using Raman thermometry and frequency domain thermoreflectance showed that the in-plane and cross-plane thermal conductivities monotonically decrease by a factor of 2.5 with decreasing film thickness compared to the bulk values. Moreover, we find that the temperature-dependence of the in-plane component gradually decreases as the film becomes thinner, and in the range from 300 to 473 K it drops by more than a factor of 2. Using the mean free path reconstruction method, we found that phonons with MFP ranging from ∼1 to 53 and from 1 to 30 nm contribute to 50% of the total in-plane and cross-plane thermal conductivity, respectively. © 2021 The Authors. Published by American Chemical Society.

  • Fabrication and characterization of large-area suspended MoSe2 crystals down to the monolayer

    Varghese S., Reig D.S., Mehew J.D., Block A., El Sachat A., Chávez-Ángel E., Sledzinska M., Ballesteros B., Sotomayor Torres C.M., Tielrooij K.-J. JPhys Materials; 4 (4, 046001) 2021. 10.1088/2515-7639/ac2060. IF: 0.000

    Many layered materials, such as graphene and transition metal dichalcogenides, can be exfoliated down to atomic or molecular monolayers. These materials exhibit exciting material properties that can be exploited for several promising device concepts. Thinner materials lead to an increased surface-to-volume ratio, with mono- and bi-layers being basically pure surfaces. Thin crystals containing more than two layers also often behave as an all-surface material, depending on the physical property of interest. As a result, flakes of layered materials are typically highly sensitive to their environment, which is undesirable for a broad range of studies and potential devices. Material systems based on suspended flakes overcome this issue, yet often require complex fabrication procedures. Here, we demonstrate the relatively straightforward fabrication of exfoliated MoSe2 flakes down to the monolayer, suspended over unprecedentedly large holes with a diameter of 15 µm. We describe our fabrication methods in detail, present characterization measurements of the fabricated structures, and, finally, exploit these suspended flakes for accurate optical absorption measurements. © 2021 The Author(s).

  • Heat transport control and thermal characterization of low-dimensional materials: A review

    El Sachat A., Alzina F., Sotomayor Torres C.M., Chavez-Angel E. Nanomaterials; 11 (1, 175): 1 - 32. 2021. 10.3390/nano11010175. IF: 5.076

    Heat dissipation and thermal management are central challenges in various areas of science and technology and are critical issues for the majority of nanoelectronic devices. In this review, we focus on experimental advances in thermal characterization and phonon engineering that have drastically increased the understanding of heat transport and demonstrated efficient ways to control heat propagation in nanomaterials. We summarize the latest device-relevant methodologies of phonon engineering in semiconductor nanostructures and 2D materials, including graphene and transition metal dichalcogenides. Then, we review recent advances in thermal characterization techniques, and discuss their main challenges and limitations. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.

  • Reversing the Humidity Response of MoS2- And WS2-Based Sensors Using Transition-Metal Salts

    Xiao P., Mencarelli D., Chavez-Angel E., Joseph C.H., Cataldo A., Pierantoni L., Sotomayor Torres C.M., Sledzinska M. ACS Applied Materials and Interfaces; 13 (19): 23201 - 23209. 2021. 10.1021/acsami.1c03691. IF: 9.229

    Two-dimensional materials, such as transition-metal dichalcogenides (TMDs), are attractive candidates for sensing applications due to their high surface-to-volume ratio, chemically active edges, and good electrical properties. However, their electrical response to humidity is still under debate and experimental reports remain inconclusive. For instance, in different studies, the impedance of MoS2-based sensors has been found to either decrease or increase with increasing humidity, compromising the use of MoS2 for humidity sensing. In this work, we focus on understanding the interaction between water and TMDs. We fabricated and studied humidity sensors based on MoS2 and WS2 coated with copper chloride and silver nitrate. The devices exhibited high chemical stability and excellent humidity sensing performance in relative humidity between 4 and 80%, with response and recovery times of 2 and 40 s, respectively. We have systematically investigated the humidity response of the materials as a function of the type and amount of induced metal salt and observed the reverse action of sensing mechanisms. This phenomenon is explained based on a detailed structural analysis of the samples considering the Grotthuss mechanism in the presence of charge trapping, which was represented by an appropriate lumped-element model. Our findings open up a possibility to tune the electrical response in a facile manner and without compromising the high performance of the sensor. They offer an insight into the time-dependent performance and aging of the TMD-based sensing devices. © 2021 American Chemical Society.

  • Review on Sol-Gel Synthesis of Perovskite and Oxide Nanomaterials

    Daniel Navas, Sandra Fuentes, Alejandro Castro-Alvarez, Emigdio Chavez-Angel Gels; 7 (4) 2021. 10.3390/gels7040275. IF: 4.702

    Sol-Gel is a low cost, well-established and flexible synthetic route to produce a wide range of micro- and nanostructures. Small variations in pH, temperature, precursors, time, pressure, atmosphere, among others, can lead to a wide family of compounds that share the same molecular structures. In this work, we present a general review of the synthesis of LaMnO3, SrTiO3, BaTiO3 perovskites and zinc vanadium oxides nanostructures based on Sol-Gel method. We discuss how small changes in the parameters of the synthesis can modify the morphology, shape, size, homogeneity, aggregation, among others, of the products. We also discuss the different precursors, solvents, working temperature, reaction times used throughout the synthesis. In the last section, we present novel uses of Sol-Gel with organic materials with emphasis on carbon-based compounds. All with a perspective to improve the method for future applications in different technological fields.

  • Understanding the molecular basis of 5-ht4 receptor partial agonists through 3d-qsar studies

    Castro-Alvarez A., Chávez-ángel E., Nelson R. International Journal of Molecular Sciences; 22 (7, 3602) 2021. 10.3390/ijms22073602. IF: 5.924

    Alzheimer’s disease (AD) is a neurodegenerative disorder whose prevalence has an incidence in senior citizens. Unfortunately, current pharmacotherapy only offers symptom relief for patients with side effects such as bradycardia, nausea, and vomiting. Therefore, there is a present need to provide other therapeutic alternatives for treatments for these disorders. The 5-HT4 receptor is an attractive therapeutic target since it has a potential role in central and peripheral nervous system disorders such as AD, irritable bowel syndrome, and gastroparesis. Quantitative structure-activity relationship analysis of a series of 62 active compounds in the 5-HT4 receptor was carried out in the present work. The structure-activity relationship was estimated using three-dimensional quantitative structure-activity relationship (3D-QSAR) techniques based on these structures’ field molecular (force and Gaussian field). The best force-field QSAR models achieve a value for the coefficient of determination of the training set of R2 training = 0.821, and for the test set R2 test = 0.667, while for Gaussian-field QSAR the training and the test were R2 training = 0.898 and R2 test = 0.695, respectively. The obtained results were validated using a coefficient of correlation of the leave-one-out cross-validation of Q2LOO = 0.804 and Q2LOO = 0.886 for force-and Gaussian-field QSAR, respectively. Based on these results, novel 5-HT4 partial agonists with potential biological activity (pEC50 8.209– 9.417 for force-field QSAR and 9.111–9.856 for Gaussian-field QSAR) were designed. In addition, for the new analogues, their absorption, distribution, metabolism, excretion, and toxicity properties were also analyzed. The results show that these new derivatives also have reasonable pharmacokinetics and drug-like properties. Our findings suggest novel routes for the design and development of new 5-HT4 partial agonists. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.


  • 2D Phononic Crystals: Progress and Prospects in Hypersound and Thermal Transport Engineering

    Sledzinska M., Graczykowski B., Maire J., Chavez-Angel E., Sotomayor-Torres C.M., Alzina F. Advanced Functional Materials; 30 (8, 1904434) 2020. 10.1002/adfm.201904434. IF: 16.836

    The central concept in phononics is the tuning of the phonon dispersion relation, or phonon engineering, which provides a means of controlling related properties such as group velocity or phonon interactions and, therefore, phonon propagation, in a wide range of frequencies depending on the geometries and sizes of the materials. Phononics exploits the present state of the art in nanofabrication to tailor dispersion relations in the range of GHz for the control of elastic waves/phonons propagation with applications toward new information technology concepts with phonons as state variable. Moreover, phonons provide an adaptable approach for supporting a coherent coupling between different state variables, and the development of nanoscale optomechanical systems during the last decade attests this prospect. The most extended approach to manipulate the phonon dispersion relation is introducing an artificial periodic modulation of the elastic properties, which is referred to as phononic crystal (PnC). Herein, the focus is on the recent experimental achievements in the fabrication and application of 2D PnCs enabling the modification of the dispersion relation of surface and membrane modes, and presenting phononic bandgaps, waveguiding, and confinement in the hypersonic regime. Furthermore, these artificial materials offer the potential of modifying and controlling the heat flow to enable new schemes in thermal management. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

  • A frequency-domain thermoreflectance method for measuring the thermal boundary conductance of a metal-polymer system

    Sandell S., Maire J., Chavez-Angel E., Sotomayor Torres C.M., He J. Proceedings - 2020 IEEE 8th Electronics System-Integration Technology Conference, ESTC 2020; (9229862) 2020. 10.1109/ESTC48849.2020.9229862. IF: 0.000

    The use of nanostructures in electronic devices has opened up a new field of challenges in thermal management, where standard thermal measurement techniques reach their limits. An especially challenging scenario is related to the thermal transport at interfaces between dissimilar materials. In these interfaces, a considerable thermal resistance arises. As the number of interfaces per area increases, the interface thermal resistance becomes a limiting factor in heat dissipation of the device, impeding the heat flow out of the device. In this work, we describe a contactless, optothermal method for measuring the thermal boundary conductance (TBC) of heterointerfaces, the frequency-domain thermoreflectance (FDTR) method. The method is demonstrated by measuring TBC between gold and polymethyl methacrylate (PMMA). By inserting a nickel nanofilm between gold and PMMA, the TBC increases from 59 MW/m2K to 139 MW/m2K. © 2020 IEEE.

  • Enhancement of thermal boundary conductance of metal–polymer system

    Sandell S., Maire J., Chávez-ángel E., Torres C.M.S., Kristiansen H., Zhang Z., He J. Nanomaterials; 10 (4, 670) 2020. 10.3390/nano10040670. IF: 4.324

    In organic electronics, thermal management is a challenge, as most organic materials conduct heat poorly. As these devices become smaller, thermal transport is increasingly limited by organic–inorganic interfaces, for example that between a metal and a polymer. However, the mechanisms of heat transport at these interfaces are not well understood. In this work, we compare three types of metal–polymer interfaces. Polymethyl methacrylate (PMMA) films of different thicknesses (1–15 nm) were spin-coated on silicon substrates and covered with an 80 nm gold film either directly, or over an interface layer of 2 nm of an adhesion promoting metal—either titanium or nickel. We use the frequency-domain thermoreflectance (FDTR) technique to measure the effective thermal conductivity of the polymer film and then extract the metal–polymer thermal boundary conductance (TBC) with a thermal resistance circuit model. We found that the titanium layer increased the TBC by a factor of 2, from 59 × 106 W·m−2·K−1 to 115 × 106 W·m−2·K−1, while the nickel layer increased TBC to 139 × 106 W·m−2·K−1. These results shed light on possible strategies to improve heat transport in organic electronic systems. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.

  • Ferromagnetic Resonance Assisted Optomechanical Magnetometer

    Colombano M.F., Arregui G., Bonell F., Capuj N.E., Chavez-Angel E., Pitanti A., Valenzuela S.O., Sotomayor-Torres C.M., Navarro-Urrios D., Costache M.V. Physical Review Letters; 125 (14, 147201) 2020. 10.1103/PhysRevLett.125.147201. IF: 8.385

    The resonant enhancement of mechanical and optical interaction in optomechanical cavities enables their use as extremely sensitive displacement and force detectors. In this Letter, we demonstrate a hybrid magnetometer that exploits the coupling between the resonant excitation of spin waves in a ferromagnetic insulator and the resonant excitation of the breathing mechanical modes of a glass microsphere deposited on top. The interaction is mediated by magnetostriction in the ferromagnetic material and the consequent mechanical driving of the microsphere. The magnetometer response thus relies on the spectral overlap between the ferromagnetic resonance and the mechanical modes of the sphere, leading to a peak sensitivity of 850 pT Hz-1/2 at 206 MHz when the overlap is maximized. By externally tuning the ferromagnetic resonance frequency with a static magnetic field, we demonstrate sensitivity values at resonance around a few nT Hz-1/2 up to the gigahertz range. Our results show that our hybrid system can be used to build a high-speed sensor of oscillating magnetic fields. © 2020 American Physical Society.

  • Large thermoelectric power variations in epitaxial thin films of layered perovskite GdBaCo2O5.5±δwith a different preferred orientation and strain

    Chatterjee A., Chavez-Angel E., Ballesteros B., Caicedo J.M., Padilla-Pantoja J., Leborán V., Sotomayor Torres C.M., Rivadulla F., Santiso J. Journal of Materials Chemistry A; 8 (38): 19975 - 19983. 2020. 10.1039/d0ta04781c. IF: 11.301

    This work describes the growth of thin epitaxial films of the layered perovskite material GdBaCo2O5.5±δ(GBCO) on different single crystal substrates SrTiO3(STO), (LaAlO3)0.3(Sr2TaAlO6)0.7(LSAT) and LaAlO3(LAO) as an approach to study changes in the thermoelectric properties by means of the induced epitaxial strain. In addition to strain changes, the films grow with considerably different preferred orientations and domain microstructures: GBCO films on STO are purelyc-axis oriented (c⊥) with an average 0.18% in-plane tensile strain; GBCO on LSAT is composed of domains with a mixed orientation (c‖andc⊥) with an average 0.71% in-plane compressive strain; while on LAO it isb-axis oriented (c‖) with an average 0.89% in-plane compressive strain. These differences result in important cell volume changes, as well as in the orthorhombicity of thea-bplane of the GBCO structure, which in turn induce a change in the sign and temperature dependence of the thermopower, while the electrical conductivity remains almost unchanged. In general, compressively strained films show negativeSthermopower (n-type) while tensile strained films show a positiveS(p-type) at low temperatures, probing the adaptive nature of the GdBaCo2O5.5±δcompound. These results point to the spontaneous generation of oxygen vacancies to partially accommodate the epitaxial stress as the main cause for this effect. © The Royal Society of Chemistry 2020.

  • Nanoscale Mapping of Thermal and Mechanical Properties of Bare and Metal-Covered Self-Assembled Block Copolymer Thin Films

    Alexandros El Sachat, Jean Spièce, Charalambos Evangeli, Alexander James Robson, Martin Kreuzer, Maria R. Rodríguez-Laguna, Emigdio Chavez, Marianna Sledzinska, Clivia M. Sotomayor Torres, Oleg V. Kolosov, Francesc Alzina Acs Applied Polymer Materials; 2 (2): 487 - 496. 2020. 10.1021/acsapm.9b00924. IF: 0.000

  • Phonon bridge effect in superlattices of thermoelectric tinisn/hfnisn with controlled interface intermixing

    Heinz S., Angel E.C., Trapp M., Kleebe H.-J., Jakob G. Nanomaterials; 10 (6, 1239): 1 - 12. 2020. 10.3390/nano10061239. IF: 4.324

    The implementation of thermal barriers in thermoelectric materials improves their power conversion rates effectively. For this purpose, material boundaries are utilized and manipulated to affect phonon transmissivity. Specifically, interface intermixing and topography represents a useful but complex parameter for thermal transport modification. This study investigates epitaxial thin film multilayers, so called superlattices (SL), of TiNiSn/HfNiSn, both with pristine and purposefully deteriorated interfaces. High-resolution transmission electron microscopy and X-ray diffractometry are used to characterize their structural properties in detail. A differential 3ω-method probes their thermal resistivity. The thermal resistivity reaches a maximum for an intermediate interface quality and decreases again for higher boundary layer intermixing. For boundaries with the lowest interface quality, the interface thermal resistance is reduced by 23% compared to a pristine SL. While an uptake of diffuse scattering likely explains the initial deterioration of thermal transport, we propose a phonon bridge interpretation for the lowered thermal resistivity of the interfaces beyond a critical intermixing. In this picture, the locally reduced acoustic contrast of the less defined boundary acts as a mediator that promotes phonon transition. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.

  • Properties of nanocrystalline silicon probed by optomechanics

    Navarro-Urrios D., Colombano M.F., Maire J., Chávez-Ángel E., Arregui G., Capuj N.E., Devos A., Griol A., Bellieres L., Martínez A., Grigoras K., Häkkinen T., Saarilahti J., Makkonen T., Sotomayor-Torres C.M., Ahopelto J. Nanophotonics; 9 (16): 4819 - 4829. 2020. 10.1515/nanoph-2020-0489. IF: 7.491

    Nanocrystalline materials exhibit properties that can differ substantially from those of their single crystal counterparts. As such, they provide ways to enhance and optimize their functionality for devices and applications. Here, we report on the optical, mechanical and thermal properties of nanocrystalline silicon probed by means of optomechanical nanobeams to extract information of the dynamics of optical absorption, mechanical losses, heat generation and dissipation. The optomechanical nanobeams are fabricated using nanocrystalline films prepared by annealing amorphous silicon layers at different temperatures. The resulting crystallite sizes and the stress in the films can be controlled by the annealing temperature and time and, consequently, the properties of the films can be tuned relatively freely, as demonstrated here by means of electron microscopy and Raman scattering. We show that the nanocrystallite size and the volume fraction of the grain boundaries play a key role in the dissipation rates through nonlinear optical and thermal processes. Promising optical (13,000) and mechanical (1700) quality factors were found in the optomechanical cavity realized in the nanocrystalline Si resulting from annealing at 950°C. The enhanced absorption and recombination rates via the intragap states and the reduced thermal conductivity boost the potential to exploit these nonlinear effects in applications including Nanoelectromechanical systems (NEMS), phonon lasing and chaos-based devices. © 2020 Daniel Navarro-Urrios et al., published by De Gruyter, Berlin/Boston 2020.

  • Thermoreflectance techniques and Raman thermometry for thermal property characterization of nanostructures

    Sandell S., Chávez-Ángel E., El Sachat A., He J., Sotomayor Torres C.M., Maire J. Journal of Applied Physics; 128 (13, 131101) 2020. 10.1063/5.0020239. IF: 2.286

    The widespread use of nanostructures and nanomaterials has opened up a whole new realm of challenges in thermal management, but also leads to possibilities for energy conversion, storage, and generation, in addition to numerous other technological applications. At the microscale and below, standard thermal measurement techniques reach their limits, and several novel methods have been developed to overcome these limitations. Among the most recent, contactless photothermal methods have been widely used and have proved their advantages in terms of versatility, temporal and spatial resolution, and even sensitivity in some situations. Among them, thermoreflectance and Raman thermometry have been used to measure the thermal properties from bulk materials to thin films, multilayers, suspended structures, and nanomaterials. This Tutorial presents the principles of these two techniques and some of their most common implementations. It expands to more advanced systems for spatial mapping and for probing of non-Fourier thermal transport. Finally, this paper concludes with discussing the limitations and perspectives of these techniques and future directions in nanoscale thermometry. © 2020 Author(s).


  • Development of low-melting point molten salts and detection of solid-to-liquid transitions by alternative techniques to DSC

    Rodríguez-Laguna M.D.R., Gómez-Romero P., Sotomayor Torres C.M., Lu M.-C., Chávez-Ángel E. Solar Energy Materials and Solar Cells; 202 (110107) 2019. 10.1016/j.solmat.2019.110107. IF: 6.019

    The ‘Solar salt’ (60% NaNO3-40% KNO3, wt. %) is the most used heat transfer and storage material in high temperature CSP systems. The main drawback is its high melting temperature of 228 °C, which requires extra-energy to keep it in the liquid state and avoid damage to pipes at low temperatures. Multi-component salts are combinations of different cations and anions. The difference in size of the ions hinders the crystallization of the material and provides lower melting temperatures. Multi-component salts are considered in this study to replace simpler combinations, such as binary and ternary eutectic mixtures. Herein, we report on two novel six-component nitrates with a melting temperature of 60–75 °C and a thermal stability up to ~500 °C under a linear heating program in N2 atmosphere. Properties such as the thermal conductivity in solid and molten state, heat capacity and vibrational spectra were evaluated. The study of the thermal behaviour of these materials using differential scanning calorimetry was insufficient, hence alternative and complementary techniques were used, such as: the three-omega technique, optical transmission and Raman spectroscopy. Multi-component salts were found to solidify as amorphous solids even at slow cooling rates and water was found to behave as a catalyst of crystallization. © 2019 Elsevier B.V.

  • Enhanced thermoelectric properties of lightly Nb doped SrTiO3 thin films

    Bhansali S., Khunsin W., Chatterjee A., Santiso J., Abad B., Martin-Gonzalez M., Jakob G., Sotomayor Torres C.M., Chávez-Angel E. Nanoscale Advances; 1 (9): 3647 - 3653. 2019. 10.1039/c9na00361d. IF: 0.000

    Novel thermoelectric materials developed for operation at room temperature must have similar or better performance along with being as ecofriendly as those commercially used, e.g., Bi2Te3, in terms of their toxicity and cost. In this work, we present an in-depth study of the thermoelectric properties of epitaxial Nb-doped strontium titanate (SrTi1-xNbxO3) thin films as a function of (i) doping concentration, (ii) film thickness and (iii) substrate type. The excellent crystal quality was confirmed by high resolution transmission electron microscopy and X-ray diffraction analysis. The thermoelectric properties were measured by the three-omega method (thermal conductivity) and van der Pauw method (electrical resistivity), complemented by Seebeck coefficient measurements. A maximum power factor of 8.9 × 10-3 W m-1 K-2 and a thermoelectric figure of merit of 0.49 were measured at room temperature in 50 nm-thick films grown on lanthanum strontium aluminate. The mechanisms behind this high figure of merit are discussed in terms of a possible two-dimensional electron gas, increase of the effective mass of the electrons, electron filtering and change in strain due to different substrates. The overall enhancement of the thermoelectric properties suggests that SrTi1-xNbxO3 is a very promising n-type candidate for room- to high-temperature applications. © 2019 The Royal Society of Chemistry.

  • From thermal to electroactive graphene nanofluids

    Rueda-García D., Del Rocío Rodríguez-Laguna M., Chávez-Angel E., Dubal D.P., Cabán-Huertas Z., Benages-Vilau R., Gómez-Romero P. Energies; 12 (23, 4545) 2019. 10.3390/en12234545. IF: 2.707

    Here, we describe selected work on the development and study of nanofluids based on graphene and reduced graphene oxide both in aqueous and organic electrolytes. A thorough study of thermal properties of graphene in amide organic solvents (N,N-dimethylformamide, N,N-dimethylacetamide, and N-methyl-2-pyrrolidone) showed a substantial increase of thermal conductivity and specific heat upon graphene integration in those solvents. In addition to these thermal studies, our group has also pioneered a distinct line of work on electroactive nanofluids for energy storage. In this case, reduced graphene oxide (rGO) nanofluids in aqueous electrolytes were studied and characterized by cyclic voltammetry and charge-discharge cycles (i.e., in new flow cells). In addition, hybrid configurations (both hybrid nanofluid materials and hybrid cells combining faradaic and capacitive activities) were studied and are summarized here. © 2019 MDPI AG. All rights reserved.

  • Impact of the regularization parameter in the mean free path reconstruction method: Nanoscale heat transport and beyond

    Sanchez-Martinez M.-Á., Alzina F., Oyarzo J., Torres C.M.S., Chavez-Angel E. Nanomaterials; 9 (3, 414) 2019. 10.3390/nano9030414. IF: 4.034

    The understanding of the mean free path (MFP) distribution of the energy carriers in materials (e.g., electrons, phonons, magnons, etc.) provides a key physical insight into their transport properties. In this context, MFP spectroscopy has become an important tool to describe the contribution of carriers with different MFP to the total transport phenomenon. In this work, we revise the MFP reconstruction technique and present a study on the impact of the regularization parameter on the MFP distribution of the energy carriers. By using the L-curve criterion, we calculate the optimal mathematical value of the regularization parameter. The effect of the change from the optimal value in the MFP distribution is analyzed in three case studies of heat transport by phonons. These results demonstrate that the choice of the regularization parameter has a large impact on the physical information obtained from the reconstructed accumulation function, and thus cannot be chosen arbitrarily. The approach can be applied to various transport phenomena at the nanoscale involving carriers of different physical nature and behavior. © 2019 by the authors. Licensee MDPI, Basel, Switzerland.

  • Modification of the raman spectra in graphene-based nanofluids and its correlation with thermal properties

    Rodríguez-Laguna M.D.R., Romero P.G., Torres C.M.S., Chavez-Angel E. Nanomaterials; 9 (5, 804) 2019. 10.3390/nano9050804. IF: 4.034

    It is well known that by dispersing nanoparticles in a fluid, the thermal conductivity of the resulting nanofluid tends to increase with the concentration of nanoparticles. However, it is not clear what the mechanism behind this phenomenon is. Raman spectroscopy is a characterization technique connecting the molecular and macroscopic world, and therefore, it can unravel the puzzling effect exerted by the nanomaterial on the fluid. In this work, we report on a comparative study on the thermal conductivity, vibrational spectra and viscosity of graphene nanofluids based on three different amides: N, N-dimethylacetamide (DMAc); N, N-dimethylformamide (DMF); and N-methyl-2-pyrrolidinone (NMP). A set of concentrations of highly stable surfactant-free graphene nanofluids developed in-house was prepared and characterized. A correlation between the modification of the vibrational spectra of the fluids and an increase in their thermal conductivity in the presence of graphene was confirmed. Furthermore, an explanation of the non-modification of the thermal conductivity in graphene-NMP nanofluids is given based on its structure and a peculiar arrangement of the fluid. © 2019, MDPI AG. All rights reserved.

  • Nanowire forest of pnictogen-chalcogenide alloys for thermoelectricity

    Singhal D., Paterson J., Ben-Khedim M., Tainoff D., Cagnon L., Richard J., Chavez-Angel E., Fernandez J.J., Sotomayor-Torres C.M., Lacroix D., Bourgault D., Buttard D., Bourgeois O. Nanoscale; 11 (28): 13423 - 13430. 2019. 10.1039/c9nr01566c. IF: 6.970

    Pnictogen and chalcogenide compounds have been seen as high-potential materials for efficient thermoelectric conversion over the past few decades. It is also known that with nanostructuration, the physical properties of these pnictogen-chalcogenide compounds can be further enhanced towards a more efficient heat conversion. Here, we report the reduced thermal conductivity of a large ensemble of Bi2Te3 alloy nanowires (70 nm in diameter) with selenium for n-type and antimony for p-type (Bi2Te3-ySey and Bi2-xSbxTe3 respectively). The nanowire growth was carried out through electrodeposition in nanoporous aluminium oxide templates with high aspect ratios leading to a forest (109 per centimetre square) of nearly identical nanowires. The temperature dependence of thermal conductivity for the nanowire ensembles was acquired through a highly sensitive 3ω measurement technique. The change in the thermal conductivity of nanowires is largely affected by the roughness in addition to the size effect due to enhanced boundary scattering. The major factor that influences the thermal conductivity was found to be the ratio of the rms roughness to the correlation length of the nanowire. With a high Seebeck coefficient and electrical conductivity at room temperature, the overall thermoelectric figure of merit ZT allows the consideration of such forests of nanowires as efficient potential building blocks of future TE devices. © 2019 The Royal Society of Chemistry.

  • On the Enhancement of the Thermal Conductivity of Graphene-Based Nanofluids

    Rodriguez-Laguna M.R., Torres C.M.S., Gomez-Romero P., Chavez-Angel E. Proceedings of the IEEE Conference on Nanotechnology; 2018-July (8626244) 2019. 10.1109/NANO.2018.8626244. IF: 0.000

    Heat transfer fluids have been extensively used in both low-temperature and high temperature applications (e.g. microelectronics cooling and concentrated solar power). However, their low thermal conductivity is still a limit on performance. One way to enhance thermal properties is to disperse nanomaterials, such as graphene flakes in the base fluid. In this work, we have developed highly stable DMAc-graphene nanofluids with enhanced thermal properties. Furthermore, the displacement of several Raman bands as a function of graphene concentration in DMAc suggests that the solvent molecules are able to interact with graphene surfaces strongly. © 2018 IEEE.

  • Subamorphous Thermal Conductivity of Crystalline Half-Heusler Superlattices

    Chavez-Angel E., Reuter N., Komar P., Heinz S., Kolb U., Kleebe H.-J., Jakob G. Nanoscale and Microscale Thermophysical Engineering; 23 (1): 1 - 9. 2019. 10.1080/15567265.2018.1505987. IF: 3.323

    The quest to improve the thermoelectric figure of merit has mainly followed the roadmap of lowering the thermal conductivity while keeping unaltered the power factor of the material. Ideally an electron-crystal phonon-glass system is desired. In this work, we report an extraordinary reduction of the cross-plane thermal conductivity in crystalline (TiNiSn):(HfNiSn) half-Heusler superlattices (SLs). We create SLs with thermal conductivities below the effective amorphous limit, which is kept in a large temperature range (120–300 K). We measured thermal conductivity at room temperature values as low as 0.75 W m −1  K −1 , the lowest thermal conductivity value reported so far for half-Heusler compounds. By changing the deposition conditions, we also demonstrate that the thermal conductivity is highly impacted by the way the single segments of the SL grow. These findings show a huge potential for thermoelectric generators where an extraordinary reduction of the thermal conductivity is required but without losing the crystal quality of the system. © 2018, © 2018 Taylor & Francis.


  • Mechanisms behind the enhancement of thermal properties of graphene nanofluids

    Rodríguez-Laguna M.R., Castro-Alvarez A., Sledzinska M., Maire J., Costanzo F., Ensing B., Pruneda M., Ordejón P., Sotomayor Torres C.M., Gómez-Romero P., Chávez-Ángel E. Nanoscale; 10 (32): 15402 - 15409. 2018. 10.1039/c8nr02762e. IF: 7.233

    While the dispersion of nanomaterials is known to be effective in enhancing the thermal conductivity and specific heat capacity of fluids, the mechanisms behind this enhancement remain to be elucidated. Herein, we report on highly stable, surfactant-free graphene nanofluids, based on N,N-dimethylacetamide (DMAc) and N,N-dimethylformamide (DMF), with enhanced thermal properties. An increase of up to 48% in thermal conductivity and 18% in specific heat capacity was measured. The blue shift of several Raman bands with increasing graphene concentration in DMF indicates that there is a modification in the vibrational energy of the bonds associated with these modes, affecting all the molecules in the liquid. This result indicates that graphene has the ability to affect solvent molecules at long-range, in terms of vibrational energy. Density functional theory and molecular dynamics simulations were used to gather data on the interaction between graphene and solvent, and to investigate a possible order induced by graphene on the solvent. The simulations showed a parallel orientation of DMF towards graphene, favoring π-π stacking. Furthermore, a local order of DMF molecules around graphene was observed suggesting that both this special kind of interaction and the induced local order may contribute to the enhancement of the fluid's thermal properties. © The Royal Society of Chemistry.

  • Nanocrystalline silicon optomechanical cavities

    Navarro-Urrios D., Capuj N.E., Maire J., Colombano M., Jaramillo-Fernandez J., Chavez-Angel E., Martin L.L., Mercadé L., Griol A., Martínez A., Sotomayor-Torres C.M., Ahopelto J. Optics Express; 26 (8): 9829 - 9839. 2018. 10.1364/OE.26.009829. IF: 3.356

    Silicon on insulator photonics has offered a versatile platform for the recent development of integrated optomechanical circuits. However, there are some constraints such as the high cost of the wafers and limitation to a single physical device level. In the present work we investigate nanocrystalline silicon as an alternative material for optomechanical devices. In particular, we demonstrate that optomechanical crystal cavities fabricated of nanocrystalline silicon have optical and mechanical properties enabling non-linear dynamical behaviour and effects such as thermo-optic/free-carrier-dispersion self-pulsing, phonon lasing and chaos, all at low input laser power and with typical frequencies as high as 0.3 GHz. © 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement.

  • Raman thermometry analysis: Modelling assumptions revisited

    Jaramillo-Fernandez J., Chavez-Angel E., Sotomayor-Torres C.M. Applied Thermal Engineering; 130: 1175 - 1181. 2018. 10.1016/j.applthermaleng.2017.11.033. IF: 3.771

    In Raman thermometry, several assumptions are made to model the heat conduction and to extract the thermal conductivity of the samples from the measured data. In this work, the heat conduction in bulk and mesa-like samples was investigated by numerical simulation and measured by the temperature-induced Raman shift method, to study the range of applicability of these assumptions. The effects of light penetration depth and finite sample size on the accuracy of the thermal conductivity determination were investigated by comparing the results of the finite element method with the usual analytical approximation for bulk samples. We found that the assumptions used in the analytical model can be applied to extract the thermal conductivity in solids if the following conditions are fulfilled: the ratio of light penetration depth to laser spot radius is smaller than 0.5, the ratio of spot radius to sample thickness is smaller than 0.1, and the ratio of spot radius to sample half width is smaller than 0.01. © 2017


  • Synthesis and optical characterization of Er-doped bismuth titanate nanoparticles grown by sol–gel hydrothermal method

    Fuentes S., Muñoz P., Llanos J., Vega M., Martin I.R., Chavez-Angel E. Ceramics International; 43 (4): 3623 - 3630. 2017. 10.1016/j.ceramint.2016.11.200. IF: 2.986

    The Er3+-doped bismuth titanate (Bi4Ti3O12, BIT) nanoparticles were synthesized by a combined sol–gel and hydrothermal method under a partial oxygen pressure of 30 bar. The composition and morphology were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Raman scattering. They showed pure and homogeneous spherical BIT nanoparticles with a size below the 30 nm. The incorporation of Er ions showed a strong decrease in the lattice parameters, as well as averaged particle size. The photoluminescence up-conversion (excitation wavelength =1480 nm) showed an enhancement of the infrared emission (980 nm) as Er concentration increased, achieving a maximum for 6% mol, while photoluminescence spectra (excitation wavelength =473 nm) showed a strong green emission (529 and 553 nm) with a maximum at 4% mol. © 2016 Elsevier Ltd and Techna Group S.r.l.

  • Thermal conductivity and air-mediated losses in periodic porous silicon membranes at high temperatures

    Graczykowski B., El Sachat A., Reparaz J.S., Sledzinska M., Wagner M.R., Chavez-Angel E., Wu Y., Volz S., Wu Y., Alzina F., Sotomayor Torres C.M. Nature Communications; 8 (1, 415) 2017. 10.1038/s41467-017-00115-4. IF: 12.124

    Heat conduction in silicon can be effectively engineered by means of sub-micrometre porous thin free-standing membranes. Tunable thermal properties make these structures good candidates for integrated heat management units such as waste heat recovery, rectification or efficient heat dissipation. However, possible applications require detailed thermal characterisation at high temperatures which, up to now, has been an experimental challenge. In this work we use the contactless two-laser Raman thermometry to study heat dissipation in periodic porous membranes at high temperatures via lattice conduction and air-mediated losses. We find the reduction of the thermal conductivity and its temperature dependence closely correlated with the structure feature size. On the basis of two-phonon Raman spectra, we attribute this behaviour to diffuse (incoherent) phonon-boundary scattering. Furthermore, we investigate and quantify the heat dissipation via natural air-mediated cooling, which can be tuned by engineering the porosity. © 2017 The Author(s).

  • Thermal conductivity of epitaxially grown InP: experiment and simulation

    Jaramillo-Fernandez J., Chavez-Angel E., Sanatinia R., Kataria H., Anand S., Lourdudoss S., Sotomayor-Torres C.M. CrystEngComm; 19 (14): 1879 - 1887. 2017. 10.1039/c6ce02642g. IF: 3.474

    The integration of III-V optoelectronic devices on silicon is confronted with the challenge of heat dissipation for reliable and stable operation. A thorough understanding and characterization of thermal transport is paramount for improved designs of, for example, viable III-V light sources on silicon. In this work, the thermal conductivity of heteroepitaxial laterally overgrown InP layers on silicon is experimentally investigated using microRaman thermometry. By examining InP mesa-like structures grown from trenches defined by a SiO2 mask, we found that the thermal conductivity decreases by about one third, compared to the bulk thermal conductivity of InP, with decreasing width from 400 to 250 nm. The high thermal conductivity of InP grown from 400 nm trenches was attributed to the lower defect density as the InP microcrystal becomes thicker. In this case, the thermal transport is dominated by phonon-phonon interactions as in a low defect-density monocrystalline bulk material, whereas for thinner InP layers grown from narrower trenches, the heat transfer is dominated by phonon scattering at the extended defects and InP/SiO2 interface. In addition to the nominally undoped sample, sulfur-doped (1 × 1018 cm−3) InP grown on Si was also studied. For the narrower doped InP microcrystals, the thermal conductivity decreased by a factor of two compared to the bulk value. Sources of errors in the thermal conductivity measurements are discussed. The experimental temperature rise was successfully simulated by the heat diffusion equation using the FEM. © The Royal Society of Chemistry.


  • Acoustic Phonons in Ultrathin Free-Standing Silicon Membranes

    Torres C.M.S., Alzina F., Shchepetov A., Chavez-Angel E., Cuffe J., Graczykowski B., Prunnila M., Reparaz J.S., Ahopelto J. Silicon Nanomembranes: Fundamental Science and Applications; : 305 - 326. 2016. 10.1002/9783527691005.ch12.

    This chapter reviews phonon research in ultrathin free-standing silicon membranes made of silicon-on-insulator (SOI) wafers. The membranes are stress-free and some are intentionally stressed. Their fabrication is described in both cases. Calculations and experimental work on confined acoustic phonons, dispersion relations, phonon lifetimes, and transport mechanisms in different phonon propagation regimes are gathered to provide a comprehensive view of the underlying physics. © 2016 Wiley-VCH Verlag GmbH & Co. KGaA. All rights reserved.

  • Nanophononics: State of the art and perspectives

    Volz S., Ordonez-Miranda J., Shchepetov A., Prunnila M., Ahopelto J., Pezeril T., Vaudel G., Gusev V., Ruello P., Weig E.M., Schubert M., Hettich M., Grossman M., Dekorsy T., Alzina F., Graczykowski B., Chavez-Angel E., Sebastian Reparaz J., Wagner M.R., Sotomayor-Torres C.M., Xiong S., Neogi S., Donadio D. European Physical Journal B; 89 (1, 15) 2016. 10.1140/epjb/e2015-60727-7. IF: 1.223

    Understanding and controlling vibrations in condensed matter is emerging as an essential necessity both at fundamental level and for the development of a broad variety of technological applications. Intelligent design of the band structure and transport properties of phonons at the nanoscale and of their interactions with electrons and photons impact the efficiency of nanoelectronic systems and thermoelectric materials, permit the exploration of quantum phenomena with micro- and nanoscale resonators, and provide new tools for spectroscopy and imaging. In this colloquium we assess the state of the art of nanophononics, describing the recent achievements and the open challenges in nanoscale heat transport, coherent phonon generation and exploitation, and in nano- and optomechanics. We also underline the links among the diverse communities involved in the study of nanoscale phonons, pointing out the common goals and opportunities. © The Author(s) 2016.

  • Self-sustained coherent phonon generation in optomechanical cavities

    Navarro-Urrios D., Gomis-Bresco J., Alzina F., Capuj N.E., García P.D., Colombano M.F., Chavez-Angel E., Sotomayor-Torres C.M. Journal of Optics (United Kingdom); 18 (9, 094006) 2016. 10.1088/2040-8978/18/9/094006. IF: 1.847

    Optical forces can set tiny objects in states of mechanical self-sustained oscillation, spontaneously generating periodic signals by extracting power from steady sources. Miniaturized self-sustained coherent phonon sources are interesting for applications such as mass-force sensing, intra-chip metrology and intra-chip time-keeping among others. In this paper, we review several mechanisms and techniques that can drive a mechanical mode into the lasing regime by exploiting the radiation pressure force in optomechanical cavities, namely stimulated emission, dynamical back-action, forward stimulated Brillouin scattering and self-pulsing. © 2016 IOP Publishing Ltd.


  • Structural characterisation of slightly Fe-doped SrTiO3 grown via a sol–gel hydrothermal synthesis

    Fuentes S., Muñoz P., Barraza N., Chávez-Ángel E., Sotomayor Torres C.M. Journal of Sol-Gel Science and Technology; 75 (3): 593 - 601. 2015. 10.1007/s10971-015-3730-4. IF: 1.532

    Abstract: A detailed structural study of the incorporation of Fe into SrTiO3 nanoparticles is reported. Slightly iron-doped strontium titanate nanoparticles with 0, 1, 3 and 5 mol% concentration of iron were grown using a sol–gel hydrothermal process and characterised using Raman scattering, X-ray photoelectron and X-ray diffraction spectroscopy. The amorphisation of the nanostructures was observed as the iron content increased, which was confirmed by the TEM images. The XPS results indicated that the oxidation states of the Sr atoms were maintained in 2+. However, a mixture of Fe3+ and Fe4+ atoms was observed as the Fe content increased, resulting in a significant number of oxygen vacancies in the perovskite structure. The analysis of Raman spectra indicated that the intensity, linewidth and frequency shift of the TO4 phonon can be used as an indicator of the Fe content as well as a local temperature probe for future thermal analysis. Graphical abstract: Temperature evolution of the Raman spectra of STO:Fe 1 mol%. The peaks with star correspond to the second-order processes. (b) Temperature dependence of the TO4 phonon mode. Blue dots denote measured Raman spectra, and the red solid lines are the Lorentzian fits to respective spectra.[Figure not available: see fulltext.] © 2015, Springer Science+Business Media New York.

  • Tuning of heat transport across thin films of polycrystalline AlN via multiscale structural defects

    Jaramillo-Fernandez J., Ordonez-Miranda J., Ollier E., Sanatinia R., Kataria H., Chavez-Angel E., Volz S., Sotomayor-Torres M. ECS Transactions; 69 (9): 53 - 64. 2015. 10.1149/06909.0053ecst. IF: 0.000

    The effective thermal conductivity of nanocrystalline films of AlN with inhomogeneous microstructure is investigated experimentally and theoretically. This is done by measuring the thermal conductivity of the samples with the 3-omega method and characterizing their microstructure by means of electron microscopy. The relative effect of the microstructure and the interface thermal resistance on the thermal conductivity is quantified through an analytical model. Thermal measurements showed that when the thickness of an AlN film is reduced from 1460 to 270 nm, its effective thermal conductivity decreases from 8.21 to 3.12 WYm-1?K-1, which is two orders of magnitude smaller than its bulk counterpart value. It is shown that both the size effects of the phonon mean free paths and the intrinsic thermal resistance resulting from the inhomogeneous microstructure predominate for thicker films, while the contribution of the interface thermal resistance strengthens as the film thickness is scaled down. The obtained results demonstrate that the structural inhomogeneity in polycrystalline AlN films can be efficiently used to tune their cross- plane thermal conductivity. In addition, thermal conductivity measurements of epitaxially grown InP layers on silicon using Raman spectroscopy are reported. ©The Electrochemical Society.


  • A novel contactless technique for thermal conductivity determination: Two-laser Raman thermometry

    Reparaz J.S., Chavez-Angel E., Wagner M.R., Graczykowski B., Gomis-Bresco J., Alzina F., Sotomayor Torres C.M. THERMINIC 2014 - 20th International Workshop on Thermal Investigations of ICs and Systems, Proceeding; (6972535) 2014. 10.1109/THERMINIC.2014.6972535.

    We present an extension of the method for thermal characterisation named 'Raman Thermometry' that relaxes the assumption of boundary conditions by spatially resolving the thermal field. The technique is contact-less and suitable to study nanoscale systems unattainable to other by other more invasive thermal characterisation techniques. © 2014 IEEE.

  • A novel contactless technique for thermal field mapping and thermal conductivity determination: Two-Laser Raman Thermometry

    Reparaz, J.S.; Chavez-Angel, E.; Wagner, M.R.; Graczykowski, B.; Gomis-Bresco, J.; Alzina, F.; Sotomayor Torres, C.M. Review of Scientific Instruments; 2014. 10.1063/1.4867166. IF: 1.584

  • A one-dimensional optomechanical crystal with a complete phononic band gap

    Gomis-Bresco, J.; Navarro-Urrios, D.; Oudich, M.; El-Jallal, S.; Griol, A.; Puerto, D.; Chavez, E.; Pennec, Y.; Djafari-Rouhani, B.; Alzina, F.; Martinez, A.; Sotomayor Torres, C.M. Nature Communications; 2014. 10.1038/ncomms5452. IF: 10.742

  • High quality single crystal Ge nano-membranes for opto-electronic integrated circuitry

    Shah, V.A.; Rhead, S.D.; Halpin, J.E.; Trushkevych, O.; Chavez-Angel, E.; Shchepetov, A.; Kachkanov, V.; Wilson, N.R.; Myronov, M.; Reparaz, J.S.; Edwards, R.S.; Wagner, M.R.; Alzina, F.; Dolbnya, I.P.; Patchett, D.H.; Allred, P.S.; Prest, M.J.; Gammon, P.M.; Prunnila, M.; Whall, T.E.; Parker, E.H.C.; Sotomayor Torres, C.M.; Leadley, D.R. Journal of Applied Physics; 2014. 10.1063/1.4870807. IF: 2.185

  • Modification of Akhieser mechanism in Si nanomembranes and thermal conductivity dependence of the Q-factor of high frequency nanoresonators

    Chávez-Ángel, E.; Zarate, R.A.; Gomis-Bresco, J.; Alzina, F.; Sotomayor Torres, C.M. Semiconductor Science and Technology; 2014. 10.1088/0268-1242/29/12/124010. IF: 2.206

  • Reduction of the thermal conductivity in free-standing silicon nano-membranes investigated by non-invasive Raman thermometry

    Chávez-Ángel, E.; Reparaz, J.S.; Gomis-Bresco, J.; Wagner, M.R.; Cuffe, J.; Graczykowski, B.; Shchepetov, A.; Jiang, H.; Prunnila, M.; Ahopelto, J.; Alzina, F.; Sotomayor Torres, C.M. APL Materials; 2014. 10.1063/1.4861796. IF: 0.000

  • Thermal Energy Harvesting

    Mouis M., Chávez-Ángel E., Sotomayor-Torres C., Alzina F., Costache M.V., Nassiopoulou A.G., Valalaki K., Hourdakis E., Valenzuela S.O., Viala B., Zakharov D., Shchepetov A., Ahopelto J. Beyond CMOS Nanodevices 1; 9781848216549: 135 - 219. 2014. 10.1002/9781118984772.ch7.

    This chapter presents some recent advances in the field of thermal energy harvesting, starting with thermoelectric energy harvesting, with a focus on the prospects of materials nanostructuration. Research toward alternative solutions will also be presented. Thermoelectric (TE) conversion is the most straightforward method to convert thermal energy into electrical energy, able to power such systems as autonomous sensor networks. Raman thermometry offers particular advantages for a fast and contactless determination of the thermal conductivity. The highly porous Si material is nanostructured and has the properties of confined systems, including a very low thermal conductivity. The chapter explores an alternative route for thermal energy harvesting (TEH) with composites using the mechanical coupling between a thermal shape memory alloy (SMA) and a piezoelectric material. © ISTE Ltd 2014. All rights reserved.

  • Thermal Isolation Through Nanostructuring

    Leadley D., Shah V., Ahopelto J., Alzina F., Chávez-Ángel E., Muhonen J., Myronov M., Nassiopoulou A.G., Nguyen H., Parker E., Pekola J., Prest M., Prunnila M., Reparaz J.S., Shchepetov A., Sotomayor-Torres C., Valalaki K., Whall T. Beyond CMOS Nanodevices 1; 9781848216549: 331 - 363. 2014. 10.1002/9781118984772.ch12.

    This chapter discusses the cooling of a platform, which requires the electronic coolers to extract heat by coupling to phonons within the platform material. Major results obtained within the nanofunction NoE on the development of nanomodulated magnetic materials and the investigation of their main properties are also presented. The cooling power of the devices becomes paramount, as opposed to the base temperature that could be reached, and must exceed heat leaks into the platform from the surroundings. This indirect cooling is desirable for systems where electrical isolation from the refrigeration elements is required, such as in quantum information applications or superconducting transition edge sensors (TESs). Thick porous Si layers on the Si wafer constitute alternative structures that could replace the rather fragile silicon nitride membranes for use as thermal isolation platforms. The structure and morphology of porous Si determines its electrical and thermal conductivity. © ISTE Ltd 2014. All rights reserved.


  • Influence of reactant type on the Sr incorporation grade and structural characteristics of Ba1-xSrxTiO3 (x=0-1) grown by sol-gel-hydrothermal synthesis

    Fuentes, S.; Chávez, E.; Padilla-Campos, L.; Diaz-Droguett, D.E. Ceramics International; 39 (8): 8823 - 8831. 2013. 10.1016/j.ceramint.2013.04.070. IF: 1.789

  • Lifetimes of confined acoustic phonons in ultrathin silicon membranes

    Cuffe, J.; Ristow, O.; Chávez, E.; Shchepetov, A.; Chapuis, P.-O.; Alzina, F.; Hettich, M.; Prunnila, M.; Ahopelto, J.; Dekorsy, T.; Sotomayor Torres, C.M. Physical Review Letters; 110 (9) 2013. 10.1103/PhysRevLett.110.095503. IF: 7.943

  • Synthesis and structural characterization of nanocrystalline batio3 at various calcination temperatures

    Fuentes, S.; Céspedes, F.; Muñoz, P.; Chávez, E.; Padilla-Campos, L. Journal of the Chilean Chemical Society; 58 (4): 2077 - 2081. 2013. . IF: 0.000


  • Calculation of the specific heat in ultra-thin free-standing silicon membranes

    Chávez, E. ; Cuffe, J.; Alzina, F. ; Sotomayor Torres, C. M. Journal of Physics: Conference Series; 395: 12105. 2012. .

  • Phonons in Slow Motion: Dispersion Relations in Ultra-Thin Si Membranes

    Cuffe, J. ; Chavez, E.; Shchepetov, A. ; Chapuis, P.O.; El Boudouti, E. H.; Alsina, F.; Dudek, D. ; Gomis-Bresco, J. ; Pennec, Y.; Djafari-Rouhani, B.; Prunnila, M.; Ahopelto, J.; Sotomayor Torres, C. M. Nano Letters; 12: 3569 - 3573. 2012. DOI: 10.1021/nl301204u.