Staff directory Francesc Alzina Sureda

Francesc Alzina Sureda

Senior Researcher
francesc.alzina(ELIMINAR)@icn2.cat
Ultrafast Dynamics in Nanoscale Systems

ORCID: 0000-0002-7082-0624

Publications

2022

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
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).

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2021

Electron beam lithography for direct patterning of MoS2on PDMS substrates
Jumbert G., Placidi M., Alzina F., Sotomayor Torres C.M., Sledzinska M. RSC Advances; 11 (32): 19908 - 19913. 2021. 10.1039/d1ra00885d. IF: 3.361

Precise patterning of 2D materials into micro- and nanostructures presents a considerable challenge and many efforts are dedicated to the development of processes alternative to the standard lithography. In this work we show a fabrication technique based on direct electron beam lithography (EBL) on MoS2on polydimethylsiloxane (PDMS) substrates. This easy and fast method takes advantage of the interaction of the electron beam with the PDMS, which at high enough doses leads to cross-linking and shrinking of the polymer. At the same time, the adhesion of MoS2to PDMS is enhanced in the exposed regions. The EBL acceleration voltages and doses are optimized in order to fabricate well-defined microstructures, which can be subsequently transferred to either a flexible or a rigid substrate, to obtain the negative of the exposed image. The reported procedure greatly simplifies the fabrication process and reduces the number of steps compared to standard lithography and etching. As no additional polymer, such as polymethyl methacrylate (PMMA) or photoresists, are used during the whole process the resulting samples are free of residues. © The Royal Society of Chemistry 2021.

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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.

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Quantifying thermal transport in buried semiconductor nanostructures: Via cross-sectional scanning thermal microscopy
Spièce J., Evangeli C., Robson A.J., El Sachat A., Haenel L., Alonso M.I., Garriga M., Robinson B.J., Oehme M., Schulze J., Alzina F., Sotomayor Torres C., Kolosov O.V. Nanoscale; 13 (24): 10829 - 10836. 2021. 10.1039/d0nr08768h. IF: 7.790

Managing thermal transport in nanostructures became a major challenge in the development of active microelectronic, optoelectronic and thermoelectric devices, stalling the famous Moore's law of clock speed increase of microprocessors for more than a decade. To find the solution to this and linked problems, one needs to quantify the ability of these nanostructures to conduct heat with adequate precision, nanoscale resolution, and, essentially, for the internal layers buried in the 3D structure of modern semiconductor devices. Existing thermoreflectance measurements and "hot wire"3ω methods cannot be effectively used at lateral dimensions of a layer below a micrometre; moreover, they are sensitive mainly to the surface layers of a relatively high thickness of above 100 nm. Scanning thermal microscopy (SThM), while providing the required lateral resolution, provides mainly qualitative data of the layer conductance due to undefined tip-surface and interlayer contact resistances. In this study, we used cross-sectional SThM (xSThM), a new method combining scanning probe microscopy compatible Ar-ion beam exit nano-cross-sectioning (BEXP) and SThM, to quantify thermal conductance in complex multilayer nanostructures and to measure local thermal conductivity of oxide and semiconductor materials, such as SiO2, SiGex and GeSny. By using the new method that provides 10 nm thickness and few tens of nm lateral resolution, we pinpoint crystalline defects in SiGe/GeSn optoelectronic materials by measuring nanoscale thermal transport and quantifying thermal conductivity and interfacial thermal resistance in thin spin-on materials used in extreme ultraviolet lithography (eUV) fabrication processing. The new capability of xSThM demonstrated here for the first time is poised to provide vital insights into thermal transport in advanced nanoscale materials and devices. © The Royal Society of Chemistry.

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2020

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

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Fracturing of Polycrystalline MoS2 Nanofilms
Marianna Sledzinska, Gil Jumbert, Marcel Placidi, Alois Arrighi, Peng Xiao, Francesc Alzina, Clivia M. Sotomayor Torres Acs Applied Electronic Materials. 2 (4): 1169-1175; 2 (4): 1169 - 1175. 2020. 10.1021/acsaelm.0c00189. IF: 0.000
High-temperature silicon thermal diode and switch
Kasprzak M., Sledzinska M., Zaleski K., Iatsunskyi I., Alzina F., Volz S., Sotomayor Torres C.M., Graczykowski B. Nano Energy; 78 (105261) 2020. 10.1016/j.nanoen.2020.105261. IF: 16.602

A thermal rectifier/diode is a nonreciprocal element or system that enables preferential heat transport in one direction. In this work we demonstrate a single-material thermal diode operating at high temperatures. The diode is made of nanostructured silicon membranes exhibiting spatially and temperature-dependent thermal conductivity and, therefore, falling into the category of spatially asymmetric, nonlinear nonreciprocal systems. We used an all-optical state-of-the-art experimental technique to prove rectification along rigorous criteria of the phenomenon. Using sub-milliwatt power we achieve rectification of about 14%. In addition, we demonstrate air-triggered thermal switching and passive cooling. Our findings provide a CMOS-compatible platform for heat rectification and applications in energy harvesting, thermal insulation and cooling, as well as sensing and potentially thermal logic. © 2020 The Authors

View abstract
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

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.

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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.

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Thermal conductivity in disordered porous nanomembranes
Sledzinska M., Graczykowski B., Alzina F., Melia U., Termentzidis K., Lacroix D., Sotomayor Torres C.M. Nanotechnology; 30 (26, 265401) 2019. 10.1088/1361-6528/ab0ecd. IF: 3.399

In this work we study the effects of disorder on the thermal conductivity of porous 100 nm thick silicon membranes, in which the size, shape and position of the pores were varied randomly. Measurements using two-laser Raman thermometry on both non-patterned and porous membranes revealed more than a 10-fold reduction of the thermal conductivity compared to that of bulk silicon and a six-fold reduction compared to non-patterned membranes for the sample with random pore shapes. Using Monte Carlo methods we solved the Boltzmann transport equation for phonons and compared different possibilities of pore organization and its influence on the thermal conductivity of the samples. The simulations confirmed that the strongest reduction of thermal conductivity is achieved for a distribution of pores with arbitrary shapes that partially overlap. Up to a 15% reduction of the thermal conductivity with respect to the purely circular pores was predicted for a porous membrane with 37% filling fraction. The effect of the pore shape and distribution was further studied. Maps of temperature and heat flux distributions clearly showed that for particular pore placement heat transport can be efficiently blocked and hot spots can be found in narrow channels between pores. These findings have an impact on the fabrication of membrane-based thermoelectric devices, where low thermal conductivity is required. This work shows that for porous membranes with a given filling fraction the thermal conductivity can be further modified by introducing disorder in the shape and placement of the pores. © 2019 IOP Publishing Ltd.

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2017

Angle-Dependent Photoluminescence Spectroscopy of Solution-Processed Organic Semiconducting Nanobelts
Wang M., Gong Y., Alzina F., Sotomayor Torres C.M., Li H., Zhang Z., He J. Journal of Physical Chemistry C; 121 (22): 12441 - 12446. 2017. 10.1021/acs.jpcc.7b02958. IF: 4.536

We report an anomalous anisotropy in photoluminescence (PL) from crystalline nanobelt of an organic small-molecule semiconductor, 6,13-dichloropentacene (DCP). Large-area well-aligned DCP nanobelt arrays are readily formed by self-assembly through solution method utilizing the strong anisotropic interactions between molecules. The absorption spectrum of the arrays suggests the formation of both intramolecular exciton and intermolecular exciton. However, the results of angle-dependent PL spectroscopy indicate that the PL arises only from the relaxation of intramolecular exciton, which has an optical transition dipole moment with an angle of 115° with the long-axis of the nanobelts. The angular dependence of PL signals follows a quartic rule (IPL(θ) ∞ cos4(θ - 115)) and agrees well with the optical selection rule of individual DCP molecules. The measured polarization ratio ρ from the individual nanobelts is on average 0.91 ± 0.02, superior to that of prior-art organic semiconductors. These results provide new insights into exciton behavior in 1D π-π stacking organic semiconductors and demonstrate DCP's great potential in the photodetectors and optical switches for large-scale organic optoelectronics. © 2017 American Chemical Society.

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Elastic Properties of Few Nanometers Thick Polycrystalline MoS2 Membranes: A Nondestructive Study
Graczykowski B., Sledzinska M., Placidi M., Saleta Reig D., Kasprzak M., Alzina F., Sotomayor Torres C.M. Nano Letters; 17 (12): 7647 - 7651. 2017. 10.1021/acs.nanolett.7b03669. IF: 12.712

The performance gain-oriented nanostructurization has opened a new pathway for tuning mechanical features of solid matter vital for application and maintained performance. Simultaneously, the mechanical evaluation has been pushed down to dimensions way below 1 μm. To date, the most standard technique to study the mechanical properties of suspended 2D materials is based on nanoindentation experiments. In this work, by means of micro-Brillouin light scattering we determine the mechanical properties, that is, Young modulus and residual stress, of polycrystalline few nanometers thick MoS2 membranes in a simple, contact-less, nondestructive manner. The results show huge elastic softening compared to bulk MoS2, which is correlated with the sample morphology and the residual stress. © 2017 American Chemical Society.

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Modification of thermal conductivity and phonon dispersion relation by means of phononic crystals
Sledzinska M., Sachat A.E., Reparaz J.S., Wagner M.R., Alzina F., Torres C.M.S. THERMINIC 2017 - 23rd International Workshop on Thermal Investigations of ICs and Systems; 2017-January: 1 - 4. 2017. 10.1109/THERMINIC.2017.8233817.

Heat conduction in silicon can be effectively reduced by means of periodic patterning of free-standing membranes. In this work we show a straightforward method for fabrication of free-standing phononic crystals based on thin silicon membranes. We use the contactless two-laser Raman thermometry method to measure thermal conductivity of the hexagonal phononic crystals. The aim of the study is to understand and control the behaviour of phonons in phononic crystals, with the target of minimizing the thermal conductivity. In particular, we are interested in the influence of the surface-to-volume ratio on the thermal conductivity. © 2017 IEEE.

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Nonlinear dynamics and chaos in an optomechanical beam
Navarro-Urrios D., Capuj N.E., Colombano M.F., Garciá P.D., Sledzinska M., Alzina F., Griol A., Martínez A., Sotomayor-Torres C.M. Nature Communications; 8 ( 14965) 2017. 10.1038/ncomms14965. IF: 12.124

Optical nonlinearities, such as thermo-optic mechanisms and free-carrier dispersion, are often considered unwelcome effects in silicon-based resonators and, more specifically, optomechanical cavities, since they affect, for instance, the relative detuning between an optical resonance and the excitation laser. Here, we exploit these nonlinearities and their intercoupling with the mechanical degrees of freedom of a silicon optomechanical nanobeam to unveil a rich set of fundamentally different complex dynamics. By smoothly changing the parameters of the excitation laser we demonstrate accurate control to activate two- A nd four-dimensional limit cycles, a period-doubling route and a six-dimensional chaos. In addition, by scanning the laser parameters in opposite senses we demonstrate bistability and hysteresis between two- A nd four-dimensional limit cycles, between different coherent mechanical states and between four-dimensional limit cycles and chaos. Our findings open new routes towards exploiting silicon-based optomechanical photonic crystals as a versatile building block to be used in neurocomputational networks and for chaos-based applications. © 2017 The Author(s).

View abstract
Optomechanical coupling in the Anderson-localization regime
García P.D., Bericat-Vadell R., Arregui G., Navarro-Urrios D., Colombano M., Alzina F., Sotomayor-Torres C.M. Physical Review B; 95 (11, 115129) 2017. 10.1103/PhysRevB.95.115129. IF: 3.836

Optomechanical crystals, purposely designed and fabricated semiconductor nanostructures, are used to enhance the coupling between the electromagnetic field and the mechanical vibrations of matter at the nanoscale. However, in real optomechanical crystals, imperfections open extra channels where the transfer of energy is lost, reducing the optomechanical coupling efficiency. Here, we quantify the role of disorder in a paradigmatic one-dimensional optomechanical crystal with full phononic and photonic band gaps. We show how disorder can be exploited as a resource to enhance the optomechanical coupling beyond engineered structures, thus providing a new tool set for optomechanics. © 2017 American Physical Society.

View abstract
Raman antenna effect from exciton-phonon coupling in organic semiconducting nanobelts
Wang M., Gong Y., Alzina F., Svoboda O., Ballesteros B., Sotomayor Torres C.M., Xiao S., Zhang Z., He J. Nanoscale; 9 (48): 19328 - 19336. 2017. 10.1039/c7nr07212k. IF: 7.367

The highly anisotropic interactions in organic semiconductors together with the soft character of organic materials lead to strong coupling between nuclear vibrations and exciton dynamics, which potentially results in anomalous electrical, optical and optoelectrical properties. Here, we report on the Raman antenna effect from organic semiconducting nanobelts 6,13-dichloropentacene (DCP), resulting from the coupling of molecular excitons and intramolecular phonons. The highly ordered crystalline structure in DCP nanobelts enables the precise polarization-resolved spectroscopic measurement. The angle-dependent Raman spectroscopy under resonant excitation shows that all Raman modes from the skeletal vibrations of DCP molecule act like a nearly perfect dipole antenna IRaman ∝ cos4(θ - 90), with almost zero (maximum) Raman scattering parallel (perpendicular) to the nanobelt's long-axis. The Raman antenna effect in DCP nanobelt is originated from the coupling between molecular skeletal vibrations and intramolecular exciton and the confinement of intermolecular excitons. It dramatically enhances the Raman polarization ratio (ρ =I∥/I⊥ > 25) and amplifies the anisotropy of the angle-dependent Raman scattering (κRaman = Imax/Imin > 12) of DCP nanobelts. These findings have crucial implications for fundamental understanding on the exciton-phonon coupling and its effects on the optical properties of organic semiconductors. © 2017 The Royal Society of Chemistry.

View abstract
Record Low Thermal Conductivity of Polycrystalline MoS2 Films: Tuning the Thermal Conductivity by Grain Orientation
Sledzinska M., Quey R., Mortazavi B., Graczykowski B., Placidi M., Saleta Reig D., Navarro-Urrios D., Alzina F., Colombo L., Roche S., Sotomayor Torres C.M. ACS Applied Materials and Interfaces; 9 (43): 37905 - 37911. 2017. 10.1021/acsami.7b08811. IF: 7.504

We report a record low thermal conductivity in polycrystalline MoS2 obtained for ultrathin films with varying grain sizes and orientations. By optimizing the sulfurization parameters of nanometer-thick Mo layers, five MoS2 films containing a combination of horizontally and vertically oriented grains, with respect to the bulk (001) monocrystal, were grown. From transmission electron microscopy, the average grain size, typically below 10 nm, and proportion of differently oriented grains were extracted. The thermal conductivity of the suspended samples was extracted from a Raman laser-power-dependent study, and the lowest value of thermal conductivity of 0.27 W m-1 K-1, which reaches a similar value as that of Teflon, is obtained in a polycrystalline sample formed by a combination of horizontally and vertically oriented grains in similar proportion. Analysis by means of molecular dynamics and finite element method simulations confirm that such a grain arrangement leads to lower grain boundary conductance. We discuss the possible use of these thermal insulating films in the context of electronics and thermoelectricity. © 2017 American Chemical Society.

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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).

View abstract
Thermal transport in epitaxial Si1-xGe x alloy nanowires with varying composition and morphology
Sachat A.E., Reparaz J.S., Spiece J., Alonso M.I., Goñi A.R., Garriga M., Vaccaro P.O., Wagner M.R., Kolosov O.V., Sotomayor Torres C.M., Alzina F. Nanotechnology; 28 (50, 505704) 2017. 10.1088/1361-6528/aa9497. IF: 3.440

We report on structural, compositional, and thermal characterization of self-assembled in-plane epitaxial Si1-xGe x alloy nanowires grown by molecular beam epitaxy on Si (001) substrates. The thermal properties were studied by means of scanning thermal microscopy (SThM), while the microstructural characteristics, the spatial distribution of the elemental composition of the alloy nanowires and the sample surface were investigated by transmission electron microscopy and energy dispersive x-ray microanalysis. We provide new insights regarding the morphology of the in-plane nanostructures, their size-dependent gradient chemical composition, and the formation of a 5 nm thick wetting layer on the Si substrate surface. In addition, we directly probe heat transfer between a heated scanning probe sensor and Si1-xGe x alloy nanowires of different morphological characteristics and we quantify their thermal resistance variations. We correlate the variations of the thermal signal to the dependence of the heat spreading with the cross-sectional geometry of the nanowires using finite element method simulations. With this method we determine the thermal conductivity of the nanowires with values in the range of 2-3 W m-1 K-1. These results provide valuable information in growth processes and show the great capability of the SThM technique in ambient environment for nanoscale thermal studies, otherwise not possible using conventional techniques. © 2017 IOP Publishing Ltd.

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2016

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.

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Fabrication of phononic crystals on free-standing silicon membranes
Sledzinska M., Graczykowski B., Alzina F., Santiso Lopez J., Sotomayor Torres C.M. Microelectronic Engineering; 149: 41 - 45. 2016. 10.1016/j.mee.2015.09.004. IF: 1.277

Free-standing Si films have been and remain an excellent example to study experimentally the effect of the reduction of the characteristic size on the phonon dispersion relation. A step further in geometrical complexity and, therefore, in increasing the control and manipulation of phonons is achieved by introducing periodicity in the medium to form phononic crystals. Here we report on the development of the fabrication process of large-area, solid-air and solid-solid two-dimensional phononic crystals, directly on free-standing, single crystalline silicon membranes. The patterning of the membranes involved electron-beam lithography and reactive ion etching for holes or metal evaporation and lift-off for pillars. The fabrication was possible due to the external strain induced on the membrane in order to reduce the buckling, which is typically found in large area free-standing structures. As a result, we obtained 250 nm thick structured membranes with patterned areas up to 100 × 100 μm, feature size between 100 and 300 nm and periodicity between 300 and 500 nm. The changes in dispersion relations of hypersonic acoustic phonons due to nanopatterning in free-standing silicon membranes were measured by Brillouin light scattering and the results were compared with numerical calculations by finite elements method. Information on phonon dispersion relation combined with a reliable fabrication process for large-scale structures opens a way for phonon engineering in more complex devices. © 2015 Elsevier B.V. All rights reserved.

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Finite element analysis of true and pseudo surface acoustic waves in one-dimensional phononic crystals
Graczykowski B., Alzina F., Gomis-Bresco J., Sotomayor Torres C.M. Journal of Applied Physics; 119 (2, 025308) 2016. 10.1063/1.4939825. IF: 2.101

In this paper, we report a theoretical investigation of surface acoustic waves propagating in one-dimensional phononic crystal. Using finite element method eigenfrequency and frequency response studies, we develop two model geometries suitable to distinguish true and pseudo (or leaky) surface acoustic waves and determine their propagation through finite size phononic crystals, respectively. The novelty of the first model comes from the application of a surface-like criterion and, additionally, functional damping domain. Exemplary calculated band diagrams show sorted branches of true and pseudo surface acoustic waves and their quantified surface confinement. The second model gives a complementary study of transmission, reflection, and surface-to-bulk losses of Rayleigh surface waves in the case of a phononic crystal with a finite number of periods. Here, we demonstrate that a non-zero transmission within non-radiative band gaps can be carried via leaky modes originating from the coupling of local resonances with propagating waves in the substrate. Finally, we show that the transmission, reflection, and surface-to-bulk losses can be effectively optimised by tuning the geometrical properties of a stripe. © 2016 AIP Publishing LLC.

View abstract
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.

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Nanoscale pillar hypersonic surface phononic crystals
Yudistira D., Boes A., Graczykowski B., Alzina F., Yeo L.Y., Sotomayor Torres C.M., Mitchell A. Physical Review B; 94 (9, 094304) 2016. 10.1103/PhysRevB.94.094304.

We report on nanoscale pillar-based hypersonic phononic crystals in single crystal Z-cut lithium niobate. The phononic crystal is formed by a two-dimensional periodic array of nearly cylindrical nanopillars 240 nm in diameter and 225 nm in height, arranged in a triangular lattice with a 300-nm lattice constant. The nanopillars are fabricated by the recently introduced nanodomain engineering via laser irradiation of patterned chrome followed by wet etching. Numerical simulations and direct measurements using Brillouin light scattering confirm the simultaneous existence of nonradiative complete surface phononic band gaps. The band gaps are found below the sound line at hypersonic frequencies in the range 2-7 GHz, formed from local resonances and Bragg scattering. These hypersonic structures are realized directly in the piezoelectric material lithium niobate enabling phonon manipulation at significantly higher frequencies than previously possible with this platform, opening new opportunities for many applications in plasmonic, optomechanic, microfluidic, and thermal engineering. © 2016 American Physical Society.

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Self-pulsing and phonon lasing in optomechanical crystals
Navarro-Urrios D., Capuj N.E., Gomis-Bresco J., Colombano M.F., García P.D., Sledzinska M., Alzina F., Griol A., Martinez A., Sotomayor-Torres C.M. International Conference on Transparent Optical Networks; 2016-August (7550436) 2016. 10.1109/ICTON.2016.7550436.

We report on a novel and efficient strategy that can drive a mechanical mode into the lasing regime by exploiting the radiation pressure force in optomechanical (OM) cavities. The pumping mechanism is based on a self-pulsing limit-cycle, which is a spontaneous process that modulates the intracavity radiation pressure force in resonance with a mechanical eigenmode of the OM cavity. © 2016 IEEE.

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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.

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Thermal conductivity of MoS2 polycrystalline nanomembranes
Sledzinska M., Graczykowski B., Placidi M., Reig D.S., El Sachat A., Reparaz J.S., Alzina F., Mortazavi B., Quey R., Colombo L., Roche S., Torres C.M.S. 2D Materials; 3 (3, 035016) 2016. 10.1088/2053-1583/3/3/035016. IF: 9.611

Heat conduction in 2D materials can be effectively engineered by means of controlling nanoscale grain structure. Afavorable thermal performance makes these structures excellent candidates for integrated heat management units. Here we show combined experimental and theoretical studies for MoS2 nanosheets in a nanoscale grain-size limit.Wereport thermal conductivity measurements on 5 nm thick polycrystalline MoS2 by means of 2-laser Raman thermometry. The free-standing, drum-like MoS2 nanomembranes were fabricated using a novel polymer- and residue-free, wet transfer, in which we took advantage of the difference in the surface energies between MoS2 and the growth substrate to transfer the CVD-grown nanosheets. The measurements revealed a strong reduction in the in-plane thermal conductivity down to about 0.73 ± 0.25 W m-1 K-1. The results are discussed theoretically using finite elements method simulations for a polycrystalline film, and a scaling trend of the thermally conductivity with grain size is proposed. © 2016 IOP Publishing Ltd.

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Two-Dimensional Phononic Crystals: Disorder Matters
Wagner M.R., Graczykowski B., Reparaz J.S., El Sachat A., Sledzinska M., Alzina F., Sotomayor Torres C.M. Nano Letters; 16 (9): 5661 - 5668. 2016. 10.1021/acs.nanolett.6b02305. IF: 13.779

The design and fabrication of phononic crystals (PnCs) hold the key to control the propagation of heat and sound at the nanoscale. However, there is a lack of experimental studies addressing the impact of order/disorder on the phononic properties of PnCs. Here, we present a comparative investigation of the influence of disorder on the hypersonic and thermal properties of two-dimensional PnCs. PnCs of ordered and disordered lattices are fabricated of circular holes with equal filling fractions in free-standing Si membranes. Ultrafast pump and probe spectroscopy (asynchronous optical sampling) and Raman thermometry based on a novel two-laser approach are used to study the phononic properties in the gigahertz (GHz) and terahertz (THz) regime, respectively. Finite element method simulations of the phonon dispersion relation and three-dimensional displacement fields furthermore enable the unique identification of the different hypersonic vibrations. The increase of surface roughness and the introduction of short-range disorder are shown to modify the phonon dispersion and phonon coherence in the hypersonic (GHz) range without affecting the room-temperature thermal conductivity. On the basis of these findings, we suggest a criteria for predicting phonon coherence as a function of roughness and disorder. © 2016 American Chemical Society.

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2015

A self-stabilized coherent phonon source driven by optical forces
Navarro-Urrios D., Capuj N.E., Gomis-Bresco J., Alzina F., Pitanti A., Griol A., Martínez A., Sotomayor Torres C.M. Scientific Reports; 5 ( 15733) 2015. 10.1038/srep15733. IF: 5.578

We report a novel injection scheme that allows for phonon lasing in a one-dimensional opto-mechanical photonic crystal, in a sideband unresolved regime and with cooperativity values as low as 10'2. It extracts energy from a cw infrared laser source and is based on the triggering of a thermo-optical/free-carrier-dispersion self-pulsing limit-cycle, which anharmonically modulates the radiation pressure force. The large amplitude of the coherent mechanical motion acts as a feedback that stabilizes and entrains the self-pulsing oscillations to simple fractions of the mechanical frequency. A manifold of frequency-entrained regions with two different mechanical modes (at 54 and 122MHz) are observed as a result of the wide tuneability of the natural frequency of the self-pulsing. The system operates at ambient conditions of pressure and temperature in a silicon platform, which enables its exploitation in sensing, intra-chip metrology or time-keeping applications.

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Elucidation of the wettability of graphene through a multi-length-scale investigation approach
Amadei C.A., Lai C.-Y., Esplandiu M.J., Alzina F., Vecitis C.D., Verdaguer A., Chiesa M. RSC Advances; 5 (49): 39532 - 39538. 2015. 10.1039/c5ra04397b. IF: 3.840

Univocal conclusions around the wettability of graphene exposed to environmental conditions remain elusive despite the recent efforts of several research groups. The main discrepancy rests on the question of whether a graphene monolayer (GML) is transparent or not to water and more generally what the role is that the substrate plays in determining the degree of wetting of the GML. In this work, we investigate the water transparency of GML by means of a multi-length-scale approach. We complement traditional static contact angle measurements and environmental scanning electron microscopy experiments with atomic force microscopy based force spectroscopy to assess the role that intermolecular interactions play in determining the wetting of GML. To gain deeper insight into the wetting transparency issue, we perform experiments on inert metals, such as gold and platinum, covered or not covered by GML. The comparison of the results obtained for different systems (i.e. GML covered and uncovered inert metals), provides unambiguous evidence that supports the non-wetting transparency theory of GML. This work aims to assist the development of technologies based on graphene-water interaction, such as graphitic membranes for water separation processes. © The Royal Society of Chemistry 2015.

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Phonon dispersion in hypersonic two-dimensional phononic crystal membranes
Graczykowski B., Sledzinska M., Alzina F., Gomis-Bresco J., Reparaz J.S., Wagner M.R., Sotomayor Torres C.M. Physical Review B - Condensed Matter and Materials Physics; 91 (7, 075414) 2015. 10.1103/PhysRevB.91.075414. IF: 3.736

We investigate experimentally and theoretically the acoustic phonon propagation in two-dimensional phononic crystal membranes. Solid-air and solid-solid phononic crystals were made of square lattices of holes and Au pillars in and on 250 nm thick single crystalline Si membrane, respectively. The hypersonic phonon dispersion was investigated using Brillouin light scattering. Volume reduction (holes) or mass loading (pillars) accompanied with second-order periodicity and local resonances are shown to significantly modify the propagation of thermally activated GHz phonons. We use numerical modeling based on the finite element method to analyze the experimental results and determine polarization, symmetry, or three-dimensional localization of observed modes. © 2015 American Physical Society.

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2014

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.

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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
Acoustic phonon propagation in ultra-thin Si membranes under biaxial stress field
Graczykowski, B. ; Gomis-Bresco, J.; Alzina, F.; Reparaz, J.S.; Shchepetov, A.; Prunnila, M.; Ahopelto, J.; Sotomayor Torres, C.M. New Journal of Physics; 2014. 10.1088/1367-2630/16/7/073024. IF: 3.671
Dynamical back-action at 5.5 GHz in a corrugated optomechanical beam
Navarro-Urrios, D.; Gomis-Bresco, J.; El-Jallal, S.; Oudich, M.; Pitanti, A.; Capuj, N.; Tredicucci, A.; Alzina, F.; Griol, A.; Pennec, Y.; Djafari-Rouhani, B.; Martínez, A.; Sotomayor Torres, C.M. AIP Advances; 2014. 10.1063/1.4902171. IF: 1.590
Electrocatalytic tuning of biosensing response through electrostatic or hydrophobic enzyme-graphene oxide interactions
Baptista-Pires, L.; Pérez-López, B.; Mayorga-Martinez, C.C.; Morales-Narváez, E.; Domingo, N.; Esplandiu, M.J.; Alzina, F.; Torres, C.M.S.; Merkoçi, A. Biosensors and Bioelectronics; 61: 655 - 662. 2014. 10.1016/j.bios.2014.05.028. IF: 6.451
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
Hypersonic phonon propagation in one-dimensional surface phononic crystal
Graczykowski, B.; Sledzinska, M.; Kehagias, N.; Alzina, F.; Reparaz, J.S.; Sotomayor Torres, C.M. Applied Physics Letters; 2014. 10.1063/1.4870045. IF: 3.515
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
Nanostructured p-type Cr/V2O5 thin films with boosted thermoelectric properties
Loureiro, J.; Santos, J.R.; Nogueira, A.; Wyczisk, F.; Divay, L.; Reparaz, S.; Alzina, F.; Sotomayor Torres, C.M.; Cuffe, J.; Montemor, F.; Martins, R.; Ferreira, I. Journal of Materials Chemistry A; 2 (18): 6456 - 6462. 2014. 10.1039/c3ta15168a. IF: 0.000
Optical and mechanical mode tuning in an optomechanical crystal with light-induced thermal effects
Navarro-Urrios, D. ; Gomis-Bresco, J. ; Capuj, N. E.; Alzina, F. ; Griol, A. ; Puerto, D. ; Martínez, A. ; Sotomayor-Torres, C. M. Journal of Applied Physics; 2014. 10.1063/1.4894623. IF: 2.185
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.

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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.

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2013

Electrical detection of spin precession in freely suspended graphene spin valves on cross-linked poly(methyl methacrylate)
Neumann, I.; Van De Vondel, J.; Bridoux, G.; Costache, M.V.; Alzina, F.; Torres, C.M.S.; Valenzuela, S.O. Small; 9 (1): 156 - 160. 2013. 10.1002/smll.201201194. IF: 7.823
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
Non Local Corrections to the Electronic Structure of Non Ideal Electron Gases: The Case of Graphene and Tyrosine
García, Y.; Cuffe, J.; Alzina, F.; Sotomayor Torres, C. M. Journal of Modern Physics; 4 (4): 522 - 527. 2013. 10.4236/jmp.2013.44074. IF: 0.000
Ultra-thin free-standing single crystalline silicon membranes with strain control
Shchepetov, A.; Prunnila, M.; Alzina, F.; Schneider, L.; Cuffe, J.; Jiang, H.; Kauppinen, E.I.; Sotomayor Torres, C.M.; Ahopelto, J. Applied Physics Letters; 2013. 10.1063/1.4807130. IF: 3.794

2012

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. .

2011

The morphology of graphene sheets treated in an ozone generator
Tao, H.; Moser, J.; Alzina, F.; Wang, Q.; Sotomayor-Torres, C.M. Journal of Physical Chemistry C; 115: 18257 - 18260. 2011. 10.1021/jp2050756.

2010

Magnetotransport in disordered graphene exposed to ozone: From weak to strong localization
Moser, J.; Tao, H.; Roche, S.; Alzina, F.; Sotomayor Torres, C.M.; Bachtold, A. Physical Review B - Condensed Matter and Materials Physics; 81 2010. 10.1103/PhysRevB.81.205445.
Relaxation and derelaxation of pure and hydrogenated amorphous silicon during thermal annealing experiments
Kail, F. ; Farjas, J.; Roura, P.; Secouard, C.; Nos, O.; Bertomeu, J.; Alzina, F.; Roca i Cabarrocas, P. Applied Physics Letters; 2010. .