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Water flow in carbon nanotubes (CNTs) starkly contradicts classical fluid mechanics, with permeabilities that can exceed no-slip Haagen-Poiseuille predictions by 2-5 orders of magnitude. Semiclassical molecular dynamics accounts for enhanced flow rates that are attributed to curvature-dependent lattice mismatch. However, the steeper permeability enhancement observed experimentally at about nanometer-size radii remains poorly understood, and suggests emergence of puzzling non-classical mechanisms. Here, we address water-CNT friction from a quantum mechanical perspective, in terms of water-energy loss upon phonon excitation. We find that combined weak water- phonon coupling and selection rules hinder water-CNT scattering, providing effective protection to water super flow, whereas comparison with a semiclassical theory evidences a friction increase that can exceed the quantum mechanical prediction by more than 2 orders of magnitude. Quasi-frictionless flow up to subnanometer CNTs opens new pathways toward minimally invasive trans-membrane cellular injections, single-water fluidics, and efficient water filtration.

Among the three basic states of matter (solid, liquid, and gas), the liquid state has always eluded general theoretical approaches for describing liquid energy and heat capacity. In this Viewpoint, we derive the phonon theory of liquids and biological fluids stemming from Frenkel's microscopic picture of the liquid state. Specifically, the theory predicts the existence of phonon gaps in vibrational spectra of liquids and a thermodynamic boundary in the supercritical state. Direct experimental evidence reaffirming these theoretical predictions was achieved through a combination of techniques using static compression X-ray diffraction and inelastic X-ray scattering on deeply supercritical argon in a diamond anvil cell. Furthermore, these findings inspired and then led to the discovery of phonon gaps in liquid crystals (mesogens), block copolymers, and biological membranes. Importantly, phonon gaps define viscoelastic crossovers in cellular membranes responsible for lipid self-diffusion, lateral molecular-level stress propagation, and passive transmembrane transport of small molecules and solutes. Finally, molecular interactions mediated by external stimuli result in synaptic activity controlling biological membranes' plasticity resulting in learning and memory. Therefore, we also discuss learning and memory effects─equally important for neuroscience as well as for the development of neuromorphic devices─facilitated in biological membranes by external stimuli.

Crystals respond to perturbations like displacement, electric field and stress. The responses are described by properties like dielectric constant, polarization, phonon frequency and so on. Phonons defined as periodic excitation in solids are seen as acoustic waves. Most important phonon applications use frequencies that are higher than those human can hear. High frequency phonons are used for imaging of many things. The study of phonon frequencies of different solids is therefore essential. Although phonon frequency of many materials have been investigated, phonon frequencies of divalent metal mixed fluorides seem to be missing in the literatures. This paper deals with the calculation of phonon frequencies of mixed divalent metal fluorides by using Density Functional Pertubation Theory (DFPT) within the Generalized Gradient Approximations. Phonon frequencies of some constituent binary fluorides are calculated and compared with experimental values, reasonable agreement is found. Infrared and Raman active phonons of nine ternary alloys (CaxCd1-xF2, CaxMg1-xF2 and CaxSr1-xF2) with composition range 012Hz, indicating their usefulness in building phonon lasers that emit sound in much the same way that optical lasers emit light. The limitation of this research is that there seems to be no experimental data to compare the phonon frequencies of the mixed ternary fluoride alloys with. However, this research can be used as a reference data. Keywords: Phonons, Ternary Fluorides, Density Functional Pertubation Theory, Phonon frequency. Title: Zone Center Phonon Frequencies of Ternary Mixed Fluorides Author: A. B. Olanipekun International Journal of Recent Research in Physics and Chemical Sciences (IJRRPCS) ISSN 2350-1030 Vol. 10, Issue 1, April 2023 - September 2023 Page No: 10-19 Paper Publications Website: www.paperpublications.org Published Date: 03-June-2023 DOI: https://doi.org/10.5281/zenodo.8000895 Paper Download Link (Source) https://www.paperpublications.org/upload/book/Zone%20Center%20Phonon%20Frequencies-03062023-1.pdf, International Journal of Recent Research in Physics and Chemical Sciences (IJRRPCS), ISSN 2350-1030, Paper Publications, Website: www.paperpublications.org

Thin films of colloidal CZTS nanocrystals (NCs) synthesized using a “green” approach in water with a variation of the copper-to-tin ratio are investigated by Raman scattering, mid-infrared (molecular vibrations) and near-infrared (free carrier) absorption, X-ray photoemission spectroscopy (XPS), electrical conductivity, and conductive atomic force microscopy (cAFM). We determined the effect of the actual Cu content on the phonon spectra, electrical conductivity, and spectral parameters of the plasmon band. An increase in the electrical conductivity of the NC films upon annealing at 220 °C is explained by three factors: formation of a CuxS nanophase at the CZTS NC surface, partial removal of ligands, and improved structural perfection. The presence of the CuxS phase is concluded to be the determinant factor for the CZTS NC film conductivity. CuxS can be reliably detected based on the analysis of the modified Auger parameter of copper, derived from XPS data and corroborated by Raman spectroscopy data. Partial removal of the ligand is concluded from the agreement of the core-level XPS and vibrational IR spectra. The degree of lattice perfection can be conveniently assessed from the Raman data as well. Further important information derived from a combination of photoelectron and optical data is the work function, ionization potential, and electron affinity of the NC films.

Despite governing heat management in any realistic device, the microscopic mechanisms of heat transport in all-solid-state electrolytes are poorly known: existing calculations, all based on simplistic semi-empirical models, are unreliable for superionic conductors and largely overestimate their thermal conductivity. In this work, we deploy a combination of state-of-the-art methods to calculate the thermal conductivity of a prototypical Li-ion conductor, the Li3ClO antiperovskite. By leveraging ab initio, machine learning, and force-field descriptions of interatomic forces, we are able to reveal the massive role of anharmonic interactions and diffusive defects on the thermal conductivity and its temperature dependence, and to eventually embed their effects into a simple rationale which is likely applicable to a wide class of ionic conductors.

Understanding collective phenomena in quantum materials from first principles is a promising route toward engineering materials properties on demand and designing new functionalities. This work examines the quantum paraelectric state, an elusive state of matter characterized by the smooth saturation of the ferroelectric instability at low temperature due to quantum fluctuations associated with anharmonic phonon effects. The temperature-dependent evolution of the soft ferroelectric phonon mode in the quantum paraelectric KTaO$_3$ in the range 0-300 K is modelled by combining density functional theory (DFT) calculations with the stochastic self-consistent harmonic approximation assisted by an on-the-fly machine-learned force field. The calculated data show that including anharmonic terms is essential to stabilize the spurious imaginary ferroelectric phonon predicted by DFT, in agreement with experiments. Augmenting the DFT workflow with machine-learned force fields allows for efficient stochastic sampling of the configurational space using large supercells in a broad and dense temperature range, inaccessible by conventional ab initio protocols. This work proposes a robust computational workflow capable of accounting for collective behaviors involving different degrees of freedom and occurring at large time/length scales, paving the way for precise modeling and control of quantum effects in materials., Comment: 10 pages, 6 figures. To be published in Advanced Quantum Technologies. Editor: Luigi Ranalli

Previous experiments on cubic UO2 have suggested that the temperature dependences of the nearest-neighbour U–O and U–U distances are different. We have acquired total-scattering neutron diffraction patterns out to Q = 23.5 Å−1 for 50 < T < 1023 K and produced via Fourier transform a pair-distribution function P D F ( r ) . The P D F ( r ) shows quite clearly that r ( U–O ) , defined by the maximum of the U–O peak in the P D F ( r ) , does in fact decrease with increasing temperature, whereas r ( U–U ) follows the lattice expansion as expected. We also observe that the r ( U–O ) contraction accelerates continuously above T ≈ 400 K, consistent with earlier experiments by others. Furthermore, by analysing the eigenvectors of the phonon modes, we show that the Δ 5 ( T O 1 ) phonon tends to separate the eight equivalent U–O distances into six shorter and two longer distances, where the longer pair contribute to a high-r tail observed in the U–O distance distribution becoming increasingly anisotropic at higher T. These results have significance for a wide range of materials in which heavy and light atoms are combined in a simple atomic structure.

Aquesta tesi proporciona un nou formalisme per resoldre l'equació de transport de Boltzmann per fonons per a nombres de Knudsen finits que proporciona una equació hidrodinàmica de transport de calor, l'equació de Guyer i Krumhansl, similar a l'equació de Navier-Stokes, per a semiconductors generals. Aquesta generalització de la llei de Fourier s'obté en casos generals, des de sistemes dominats per col·lisions normals que conserven el moment, com és ben sabut, fins a materials cinètics dominats per col·lisions resistives, on captura efectes no locals. La característica clau del nostre marc és assumir que la funció de distribució de fonons fora de l'equilibri es descriu en termes del flux de calor i les seves derivades primeres. Obtenim expressions explícites per a la distribució de fonons fora de l'equilibri i per als paràmetres macroscòpics, independents de la geometria, en funció de les propietats dels fonons que es poden calcular a partir de primers principis. Aquest formalisme es valida des de dues perspectives diferents: la teòrica i l'experimental. Des de la perspectiva teòrica, recuperem dos resultats coneguts en el transport tèrmic. Primer, obtenim la llei de Fourier amb un operador de col·lisions general. En segon lloc, recuperem exactament els resultats originals de l'equació de Guyer i Krumhansl, on s'utilitza que dominen les col·lisions normals. Des d'un punt de vista experimental, es troba que les prediccions del model ab initio coincideixen amb una àmplia gamma d'experiments en silici i germani, considerant diferents geometries, temperatures, mides o situacions dependents i independents del temps. A més, a diferència dels enfocaments basats directament en l'equació de transport de Boltzmann, l'equació hidrodinàmica es pot resoldre en geometries arbitràries, proporcionant així una potent eina per a la modelització de calor a la nanoescala a un baix cost computacional. Finalment, aquest formalisme obre la porta a millorar la seva aplicabilitat a nombres de Knudsen més grans mitjançant la inclusió de derivades d'ordre superior o l'ús de paràmetres efectius a la descripció. Esta tesis proporciona un nuevo formalismo para resolver la ecuación de transporte de Boltzmann de fonones para números de Knudsen finitos que proporciona una ecuación hidrodinámica de transporte de calor, la ecuación de Guyer y Krumhansl, similar a la ecuación de Navier-Stokes para semiconductores generales. Esta generalización de la ley de Fourier se obtiene en casos generales, desde sistemas dominados por colisiones normales que conservan el momento, como es bien sabido, hasta materiales cinéticos dominados por colisiones resistivas, donde captura efectos no locales. La característica clave de nuestro marco es assumir que la función de distribución de fonones fuera del equilibrio se describe en términos del flujo de calor y sus primeras derivadas. Obtenemos expresiones explícitas para la distribución de fonones fuera del equilibrio y para los parámetros macroscópicos, independientes de la geometría, en función de las propiedades de los fonones que se pueden calcular a partir de primeros principios. Este formalismo se valida desde dos perspectivas diferentes: la teórica y la experimental. Desde la perspectiva teórica, recuperamos dos resultados bien conocidos en el transporte térmico. Primero, obtenemos la ley de Fourier con un operador general de colisiones. En segundo lugar, recuperamos exactamente los resultados originales de la ecuación de Guyer y Krumhansl, donde se utiliza que dominan las colisiones normales. Desde un punto de vista experimental, se encuentra que las predicciones ab initio del modelo concuerdan con una amplia gama de experimentos en silicio y germanio, considerando diferentes geometrías, temperaturas, tamaños o situaciones dependientes e independientes del tiempo. Además, a diferencia de los enfoques basados directamente en la ecuación de transporte de Boltzmann, la ecuación hidrodinámica se puede resolver en geometrías arbitrarias, lo que proporciona una poderosa herramienta para el modelado de calor a la nanoescala a un bajo costo computacional. Finalmente, este formalismo abre la puerta a mejorar su aplicabilidad a números de Knudsen más grandes mediante la inclusión de derivadas de orden superior o el uso de parámetros efectivos en la descripción. This thesis provides a new formalism to solve the phonon Boltzmann transport equation for finite Knudsen numbers that supplies a hydrodynamic heat transport equation, the Guyer-Krumhansl equation, similar to the Navier-Stokes equation for general semiconductors. This generalization of Fourier's law is obtained in general cases, from systems dominated by momentum-preserving normal collisions, as is well known, to kinetic materials dominated by resistive collisions, where it captures nonlocal effects. The key feature of our framework is to assume that the nonequilibrium phonon distribution function is described in terms of the heat flux and its first derivatives. We obtain explicit expressions for the nonequilibrium phonon distribution and for the geometry-independent macroscopic parameters as a function of phonon properties that can be calculated from first principles. This formalism is validated from two different perspectives: theoretical and experimental. From the theoretical perspective, we recover two well-known results in thermal transport. First, we obtain Fourier's law with a general collisions operator. Second, we exactly recover the original results for the Guyer and Krumhansl equation, where it is used that normal collisions dominate. From an experimental point of view, the ab initio model predictions agree with a wide range of experiments in silicon and germanium, considering different geometries, temperatures, sizes, or time-dependent and independent situations. Furthermore, in contrast to approaches directly based on the Boltzmann transport equation, the hydrodynamic equation can be solved in arbitrary geometries, thus providing a powerful tool for nanoscale heat modeling at a low computational cost. Finally, this formalism opens the door to improving its applicability to larger Knudsen numbers by including higher-order derivatives or using effective parameters in the description.

Phonon scattering at grain boundaries (GBs) is significant in controlling the nanoscale device thermal conductivity. However, GBs could also act as waveguides for selected modes. To measure localized GB phonon modes, milli-electron volt (meV) energy resolution is needed with subnanometer spatial resolution. Using monochromated electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM) we have mapped the 60 meV optic mode across GBs in silicon at atomic resolution and compared it to calculated phonon densities of states (DOS). The intensity is strongly reduced at GBs characterized by the presence of 5- and 7-fold rings where bond angles differ from the bulk. The excellent agreement between theory and experiment strongly supports the existence of localized phonon modes and thus of GBs acting as waveguides.

We present a high-pressure investigation of the semiconductor-to-metal transition in MoS2 and WS2 carried out by synchrotron-based far-infrared spectroscopy, to reconcile the controversial estimates of the metallization pressure found in the literature and gain new insight into the mechanisms ruling this electronic transition. Two spectral descriptors are found indicative of the onset of metallicity and of the origin of the free carriers in the metallic state: the absorbance spectral weight, whose abrupt increase defines the metallization pressure threshold, and the asymmetric lineshape of the E1u peak, whose pressure evolution, interpreted within the Fano model, suggests the electrons in the metallic state originate from n-type doping levels. Combining our results with those reported in the literature, we hypothesize a two-step mechanism is at work in the metallization process, in which the pressure-induced hybridization between doping and conduction band states drives an early metallic behaviour, while the band-gap closes at higher pressures., Comment: 8 pages, 3 figures

We report studies on the nonlinear electronic transport properties of copper point contacts. Utilizing the mechanically controllable break junction technique, various contact sizes can be realized to study ensemble-averaged differential conductance spectra at low temperatures. We investigate signatures of phonon excitations for contact sizes down to the atomic scale, where conductance fluctuations arise superimposing the phonon signatures. Applying high bias voltages to atomic-size copper contacts reveal additional features caused by atomic rearrangements. published

The electron self-interaction is a long-standing problem in density functional theory and is particularly critical in the description of polarons. Polarons are quasiparticles involving charge localization coupled with self-induced lattice distortions. Since their prediction by Landau almost a century ago, polarons have drawn a great deal of attention in physics, chemistry, and materials science. The polaron stability results from the competition between the energy gain associated with the charge localization and the energy cost of the involved lattice distortions. Therefore, the polaron localization and its formation energy are sensitively affected by the description of the electron self-interaction. Various competitive correction schemes based on either one-body or many-body descriptions of the self-interaction have been proposed to solve this longstanding problem. At present, it remains unclear which of these two descriptions of the self-interaction needs to be addressed in polaron physics. In this thesis, we address the self-interaction problem in relation to polarons in density functional theory. First, we develop a scheme for correcting finite-size electrostatic effects involving the polaron charge density, which is crucial for achieving energetics of isolated polarons. Then, we study polarons with state-of-the-art hybrid functionals, highlighting the notion of formation energy for determining the polaron stability. Next, we develop a unified theoretical framework encompassing one-body and many-body forms of self-interaction, which confers superiority to the notion of many-body self-interaction over the notion of one-body self-interaction. Given the preeminence of the many-body self-interaction, we introduce an efficient semilocal scheme for localizing polarons based on the inclusion of a weak local potential in the semilocal Hamiltonian to suppress the many-body self-interaction. Taking advantage of these findings, we develop a selection criterion for the Hubbard interaction in Hubbard-corrected functionals. Finally, we apply our methodologies to the case of an anisotropic system, and use semilocal functionals free from many-body self-interaction to calculate polaron hopping rates. In this context, we demonstrate that polaron properties free from many-body self-interaction, including formation energies, hopping energy barriers, and hopping rates are robust upon variation of the functional. This supports the use of our semilocal scheme and of the Hubbard-corrected functional over computationally more expensive hybrid functionals. This thesis advances the conceptual understanding of the self-interaction problem in density functional theory, and paves the way to efficient calculations of polarons in large systems, in systematic studies involving large sets of materials, in molecular dynamics evolving over long time periods, and in charge transfer mechanisms.

Synchrotron radiation based X-ray scattering techniques provide powerful bulk-sensitive probes for actinide materials. Here, selected experiments showing the potential of these techniques in advancing the knowledge of actinides and their compounds are reviewed., By reviewing a selection of X-ray diffraction (XRD), resonant X-ray scattering (RXS), X-ray magnetic circular dichroism (XMCD), resonant and non-resonant inelastic scattering (RIXS, NIXS), and dispersive inelastic scattering (IXS) experiments, the potential of synchrotron radiation techniques in studying lattice and electronic structure, hybridization effects, multipolar order and lattice dynamics in actinide materials is demonstrated.

Gate-controlled supercurrent (GCS) in superconducting nanobridges has recently attracted attention as a means to create superconducting switches. Despite the clear advantages for applications, the microscopic mechanism of this effect is still under debate. In this work, we realize GCS for the first time in a highly crystalline superconductor epitaxially grown on an InAs nanowire. We show that the supercurrent in the epitaxial Al layer can be switched to the normal state by applying similar to +/- 23 V on a bottom gate insulated from the nanowire by a crystalline hBN layer. Our extensive study of the temperature and magnetic field dependencies suggests that the electric field is unlikely to be the origin of GCS in our device. Though hot electron injection alone cannot explain our experimental findings, a very recent non-equilibrium phonons based picture is compatible with most of our results.

Exciton-phonon coupling (EXPC) plays a key role in the optoelectronic properties of semiconductor nanocrystals (NCs), but a microscopic picture of EXPC is still lacking, particularly regarding the magnitude and scaling with NC size, the dependence on phonon frequency, and the role of the NC surface. The computational complexity associated with accurately describing excitons and phonons has limited previous theoretical studies of EXPC to small NCs, noninteracting electron-hole models, and/or a small number of phonon modes. Here, we develop an atomistic approach for describing EXPC in NCs of experimentally relevant sizes. We validate our approach by calculating the reorganization energies, a measure of EXPC, for CdSe and CdSe-CdS core-shell NCs, finding good agreement with experimental measurements. We demonstrate that exciton formation distorts the NC lattice primarily along the coordinates of low-frequency acoustic modes. Modes at the NC surface play a significant role in smaller NCs while interior modes dominate for larger systems.

A dual-mode immunoassay strategy based on CdS nanoparticles as signal probes with both of photoluminescent (PL) and multi-phonon resonance Raman scattering (MRRS) properties was developed. Simplified structural design and preparation were achieved due to the intrinsic integration of PL and MRRS dual signals in the single-unit CdS nanoprobes. Human immunoglobulin G (HIgG) was sensitively and specifically detected using the proposed PL-MRRS dual-mode strategy. The linear relationship between the HIgG concentration and the intensity of 707 nm PL peaks/300 cm

We address the degeneracy of the ground state multiplet on the 5d 1 Re6+ ion in double perovskite Ba2MgReO6 using a combination of specific heat measurements and density functional calculations. For Ba2MgReO6, two different ground state multiplets have previously been proposed—a quartet (with degeneracy N = 4) (Hirai and Hiroi 2019 J. Phys. Soc. Japan 88 064712) and a doublet (N = 2) (Marjerrison et al 2016 Inorg. Chem. 55 10701). Here we employ two independent methods for the estimation of phonon contribution in heat capacity data to obtain the magnetic entropy Smag, which reflects the degeneracy of the ground state multiplet N through Smag = R lnN. In both cases, we obtain that in the temperature range covering 2 to 120 K the released entropy is better described by Smag = R ln2. The detailed nature of the ground state multiplet in Ba2MgReO6 remains an open question., Journal of Physics: Condensed Matter, 35 (24), ISSN:0953-8984, ISSN:1361-648X

The layered transition-metal dichalcogenide WTe2 is characterized by distinctive transport and topological properties. These properties are largely determined by electronic states close to the Fermi level, specifically to electron and hole pockets in the Fermi sea. In principle, these states can be manipulated by changes to the crystal structure. The precise impact of particular structural changes on the electronic properties is a strong function of the specific nature of the atomic displacements. Here, we report on time-resolved X-ray diffraction and infrared reflectivity measurements of the coherent structural dynamics in WTe2 induced by femtosecond laser pulses excitation (central wavelength 800 nm), with emphasis on a quantitative description of both in-plane and out-of-plane vibrational modes. We estimate the magnitude of these motions, and calculate via density functional theory their effect on the electronic structure. Based on these results, we predict that phonons periodically modulate the effective mass of carriers in the electron and hole pockets up to 20%. This work opens up new opportunities for modulating the peculiar transport properties of WTe2 on short time scales., npj 2D Materials and Applications, 6, ISSN:2397-7132

The field of nanoscale energy transport, conversion, and storage is at an exciting time with next-generation devices manipulating discrete energy carriers, e.g. phonons, photons, and electrons, in confined dimensions arriving closer to commercialization, such as solid-state flexible electronics and optoelectronics utilizing one dimension (1D) and two dimensional (2D) nanomaterials. The transport dynamics of quasiparticles and their coupling are modified, notably in low dimensional nanomaterials, with the inclusion of disorder and artificial long-range order. Through this lens, it is possible to probe interesting physics and draw out intrinsic properties of the nanomaterials. This is especially important for electronic systems and energy conversion & storage devices where heat generation and dissipation within nano- and microscale locations of nonequilibrium impedes continued advancement. This thesis examines outstanding questions concerning nanoscale thermal and thermoelectric transport in low-dimensional materials to further understanding of crystal disorder and artificial long-range order. Specifically, the material systems investigated are alloy disorder and surface roughness in semiconducting silicon germanium (SiGe) nanowires, microscale rippling in layered molybdenum disulfide (MoS2) flakes, intra- and interlayer interactions in bulk and monolayer MoS2, and artificially created, long-range domain walls in twisted bilayer graphene (TBG). The fundamental questions are addressed through electrothermal, optothermal, and scanning probe metrology techniques. First, eight-probe thermal conductivity measurements of SiGe nanowires show that alloying suppresses thermal transport, and the mean-free-paths of low-frequency phonons are suppressed by diffuse surface roughness scattering in nanowires. The diffuse surface scattering results in length-independent thermal conductivity for lengths over two micrometers. Similarly, four-probe thermal conductivity measurements reveal that microscale ripples have negligible effects on phonon transport in 2D layers as the ripple wavelengths and curvatures are much larger than the phonon mean free paths and wavelengths. The peak thermal conductivity is found to increase with decreasing Raman scattering intensity in the frequency range with vanishing phonon density of states in MoS2 indicating an important role of point defect scattering. In addition, this dissertation presents an experimental effort to employ micro-Raman spectroscopy to investigate local nonequilibrium among different phonon polarizations in MoS2 inside the focused laser spot. It also describes an exploration of ultra-high vacuum scanning probe microscopy for probing the local thermoelectric property of twisted bilayer graphene moiré superstructures.

The hyperbolic phonon polaritons supported in hexagonal boron nitride (hBN) with long scattering lifetimes are advantageous for applications such as super-resolution imaging via hyperlensing. Yet, hyperlens imaging is challenging for distinguishing individual and closely spaced objects and for correlating the complicated hyperlens fields with the structure of an unknown object underneath. Here, we make significant strides to overcome each of these challenges. First, we demonstrate that monoisotopic h11BN provides significant improvements in spatial resolution, experimentally resolving structures as small as 44 nm and those with sub 25 nm spacings at 6.76 μm free-space wavelength. We also present an image reconstruction algorithm that provides a structurally accurate, visual representation of the embedded objects from the complex hyperlens field. Further, we offer additional insights into optimizing hyperlens performance on the basis of material properties, with an eye toward realizing far-field imaging modalities. Thus, our results significantly advance label-free, high-resolution, spectrally selective hyperlens imaging and image reconstruction methodologies.

Sharp resonances can strongly modify the electromagnetic response of matter. A classic example is the Reststrahlen effect - high reflectivity in the mid-infrared in many polar crystals near their optical phonon resonances. Although this effect in bulk materials has been studied extensively, a systematic treatment for finite thickness remains challenging. Here we describe, experimentally and theoretically, the Reststrahlen response in hexagonal boron nitride across more than 5 orders of magnitude in thickness, down to a monolayer. We find that the high reflectivity plateau of the Reststrahlen band evolves into a single peak as the material enters the optically thin limit, within which two distinct regimes emerge: a strong-response regime dominated by coherent radiative decay and a weak-response regime dominated by damping. We show that this evolution can be explained by a simple two-dimensional sheet model that can be applied to a wide range of thin media.

Anti-Stokes photoluminescence (ASPL), which is an up-conversion phonon-assisted process of the radiative recombination of photoexcited charge carriers, was investigated in methylammonium lead bromide (MALB) perovskite nanocrystals (NCs) with mean sizes that varied from about 6 to 120 nm. The structure properties of the MALB NCs were investigated by means of the scanning and transmission electron microscopy, X-ray diffraction and Raman spectroscopy. ASPL spectra of MALB NCs were measured under near-resonant laser excitation with a photon energy of 2.33 eV and they were compared with the results of the photoluminescence (PL) measurements under non-resonant excitation at 3.06 eV to reveal a contribution of phonon-assisted processes in ASPL. MALB NCs with a mean size of about 6 nm were found to demonstrate the most efficient ASPL, which is explained by an enhanced contribution of the phonon absorption process during the photoexcitation of small NCs. The obtained results can be useful for the application of nanocrystalline organometal perovskites in optoelectronic and all-optical solid-state cooling devices.

We present the results of a theoretical investigation of the stability and collective vibrations of a two-dimensional hydrodynamic lattice comprised of millimetric droplets bouncing on the surface of a vibrating liquid bath. We derive the linearized equations of motion describing the dynamics of a generic Bravais lattice, as encompasses all possible tilings of parallelograms in an infinite plane-filling array. Focusing on square and triangular lattice geometries, we demonstrate that for relatively low driving accelerations of the bath, only a subset of inter-drop spacings exist for which stable lattices may be achieved. The range of stable spacings is prescribed by the structure of the underlying wavefield. As the driving acceleration is increased progressively, the initially stationary lattices destabilize into coherent oscillatory motion. Our analysis yields both the instability threshold and the wavevector and polarization of the most unstable vibrational mode. The non-Markovian nature of the droplet dynamics renders the stability analysis of the hydrodynamic lattice more rich and subtle than that of its solid state counterpart.

GeTe, as a p-type semiconductor, has been intensively studied in recent years as a promising lead-free mid-temperature-range thermoelectric (TE) material. Herein, we report an improved energy conve...

Cu2ZnSnS4 (CZTS) is one of the promising materials for absorber layers of new-generation thin film solar cells. Various synthetic routes of materials preparation and structural characterization have been explored so far. Further tuning of the CZTS properties is realized via partial substitution of the cations. Here we have used an affordable and scalable method of synthesizing colloidal CZTS nanocrystals (NC) in an aqueous solution. Variation of the synthesis parameters, in particular pH of the solution, was employed to improve the crystallinity of the NCs. Furthermore, CZTS NCs with partial substitution of Cu for Ag were also successfully synthesized. Raman spectroscopy was employed as a prime tool of structural characterization of the NCs obtained, along with optical absorption spectroscopy and ab initio DFT lattice dynamics calculations. An experimentally observed slight upward shift of the main phonon Raman peak upon increase of the Ag content in (AgxCu1-x)2ZnSnS4 NCs is in agreement with the trend predicted by DFT calculation. No pure Ag2ZnSnS4 NCs could be formed, indicating a critical role of Cu in forming the kesterite structure NCs under given synthesis conditions in an aqueous medium.

International audience; Thermoelectric performance largely depends on the reduction of lattice thermal conductivity (κL). The study of the thermal conductivity (κ) of a Sb2Te3/graphite nanocomposite system demonstrates ∼40% reduction in κL with graphite incorporation. A plausible explanation of intrinsic low κL observed in Sb2Te3 based system is presented by modeling experimental specific heat (Cp) data. Raman spectroscopy measurement combined to X-Ray diffraction data confirms the presence of graphite as separate phase in the composite sample. It is found that phonon scattering dominates heat transport mechanism in the nanostructured Sb2Te3/graphite composite. Large reduction in κL is accomplished by intensifying scattering rate of phonons via various sources. Graphite introduces effective scattering sources, i.e., defects of different dimensionalities in synthesized nanocomposite sample. Furthermore, graphite mediates phonon-phonon coupling and enhances lattice anharmonicity, which causes an intrinsic scattering of phonons with all frequencies in the Sb2Te3/graphite nanocomposite sample. Dislocation density and phonon anharmonicity of the synthesized samples are estimated from in depth analysis of temperature dependent synchrotron powder diffraction and Raman spectroscopic data. κL value as low as 0.8 W m−1K−1 at 300 K, achieved with graphite dispersion in Sb2Te3 based composite system makes the present comprehensive study an interesting concept to be developed in thermoelectric materials.

Thermodynamic stability is regularly quantified using the enthalpy of formation () and the Gibbs free energy of formation (ΔfH). Although knowledge of these properties is crucial for many applications, literature data are often missing for many solids. We evaluate the harmonic approximation (HA) for the prediction of ΔfH and ΔfGat constant volume and pressure, based on phonon calculations. Using density functional theory to carry out phonon calculations, we show that the HA excellently describes the temperature-dependence of ΔfH and ΔgH for 14 compounds; mean absolute error (MAE) of 2.1 kJmol−1 and 1.1 kJmol−1, respectively, in the temperature interval 0–2500 K. Moreover, the performance of the HA was evaluated using computational data from the Materials Project database for 69 compounds yielding an MAE of 1.1 kJmol−1 and 1.1 kJmol−1, respectively, in the temperature interval 100–800 K. Very good performance is also observed for a number of additional compounds, including several hydrated salts, at 298.15 K. The model is subsequently applied to a number of phase-transition phenomena that demonstrate the strengths and weaknesses of the model. In addition, it is demonstrated that the HA model can provide quantitative performance that rivals that of the conventional quasi-harmonic approximation for the prediction of formation properties, at a significantly reduced computational effort (∼5–10 times faster).

Nickel titanium, also know as nitinol, is a prototypical shape memory alloy, a property intimately linked to a phase transition in the microstructure, which allows the meso/macroscopic sample shape to be recovered after thermal cycling. Not much is known about the other alloys in this binary system, which prompted our computational investigation of other compositions. In this work, structures are found by probing the potential energy surfaces of NiTi binary systems using a minima hopping method, in combination with ab initio electronic structure calculations. We find stable structures in 34 different stoichiometries and calculate derived physical properties of the low energy phases. From the results of this analysis a new convex hull is formed that is lower in energy than those in the Materials Project and Open Quantum Materials Databases. Two previously unreported phases are discovered for the NiTi2 and Ni5Ti compositions, and two metastable states in NiTi and NiTi2 shows signs of negative linear compression and negative Poisson ratio, respectively., We acknowledge the computational resources awarded by XSEDE, a project supported by National Science Foundation Grant No. ACI-1053575. We acknowledge the support from the Texas Advances Computer Center (with the Stampede2 and Bridges supercomputers). This work was supported by the DMREF-NSF 1434897, NSF OAC-1740111, and DOE DE-SC0021375 projects. M.J.V. acknowledges funding by the Belgian FNRS (PDR G.A. T.1077.15-1/7, T.0103.19, and a sabbatical “OUT” grant at ICN2), ULiege, and the Communauté Française de Belgique (ARC AIMED G.A. 15/19-09) and computational resources from the Consortium des Equipements de Calcul Intensif (FRS-FNRS G.A. 2.5020.11) and Zenobe/CENAERO funded by the Walloon Region under G.A. 1117545.

The response of crystal structure to external stimuli provides potential applications in the areas of detection, diagnosis, and repair. In this work, we fabricate two allotropic organic cocrystals, with different space groups and lattice parameters, with identical donor and acceptor molecules inside them. Under external stimuli, lattice vibration and electron-phonon coupling present pronounced differences in these two types of crystals, where different strengths of spin polarizations are observed. Furthermore, due to pronounced differences in coupling between lattice and spin inside the allotropic charge transfer crystals, the magnetic field presents a discrepant tunability on both transmission and fluorescence lifetimes. Through decreasing temperature or applying external electric field, the electron-phonon coupling coefficient presents a decreasing tendency, which will affect the dipole and dielectric constant in the allotropic crystals differently.

Nanostructured surfaces with designed optical functionalities, such as metasurfaces, allow efficient harvesting of light at the nanoscale, enhancing light-matter interactions for a wide variety of material combinations. Exploiting light-driven matter excitations in these artificial materials opens up a new dimension in the conversion and management of energy at the nanoscale. In this review, we outline the impact, opportunities, applications, and challenges of optical metasurfaces in converting the energy of incoming photons into frequency-shifted photons, phonons, and energetic charge carriers. A myriad of opportunities await for the utilization of the converted energy. Here we cover the most pertinent aspects from a fundamental nanoscopic viewpoint all the way to applications.

Janus Sb and Bi monolayers as a new class of 2D topological insulator materials, which could be fulfilled by asymmetrical functionalizations with methyl or hydroxyl, are demonstrated by first-principles spin–orbit coupling (SOC) electronic structure calculations to conflate nontrivial topology, Rashba splitting and valley-contrast circular dichroism. Cohesive energies and phonon frequency dispersion spectra indicate that all Janus Sb and Bi monolayers possess a structural stability in energetic statics but represent virtual acoustic phonon vibrations of the hydrogen atoms passivating on monolayer surfaces. Band structures of Janus Sb and Bi monolayers and their nanoribbons demonstrate they are nontrivial topological insulators. Rashba spin splitting at G point in Brillouin zone of Janus Bi monolayers arises from the strong SOC px and py orbitals of Bi bonding atoms together with the internal out-of-plane electric field caused by asymmetrical functionalization. Janus Sb and Bi monolayers render direct and indirect giant bandgaps, respectively, which are derived from the strong SOC px and py orbitals at band-valley Brillouin points K and K′ where valley-selective circular dichroism of spin valley Hall insulators is also exhibited.

We demonstrate that the heat capacity Boson peak (BP)-like anomaly appearing in fully ordered anharmonic molecular crystals emerges as a result of the strong interactions between propagating (acoustic) and low-energy quasi-localized (optical) phonons. In particular, we experimentally determine the low-temperature (

We employ a combined experimental-theoretical study of the first- and second-order Raman modes of monoclinic β−Ga2 O3. Gallium oxide has attracted considerable interest due to its deep-UV band gap paired with a high critical field strength, offering promising applications in power electronics. A crucial prerequisite for the future development of Ga2 O3-based devices is a detailed understanding of the lattice vibrations, i.e., phonons, as they govern important material properties such as elasticity, thermal conductivity, temperature-dependence of the band gap, or free-carrier transport. Polarized micro-Raman spectroscopy measurements on the (010) and (¯201) planes of β−Ga2 O3 single crystals enable the determination of the phonon frequencies of all 15 first-order and more than 40 second-order Raman modes. The experimental results are correlated with calculations of the mode frequencies, the phonon dispersion relation, and the phonon density of states using density functional perturbation theory. By applying a group-theoretical analysis, we are able to distinguish between overtones and combinational modes and identify the high-symmetry points in the Brillouin zone that contribute to the observed second-order modes. Based on this information, we demonstrate the simultaneous determination of Raman, IR, and acoustic phonons in β−Ga2 O3 by second-order Raman spectroscopy.

Excitation of localized surface plasmon resonance in metal nanoparticles (NPs) embedded in a glassy matrix generates hot electrons, which can be extracted for different optoelectronic applications. The insights of plasmon relaxation dynamics with varying surrounding dielectric environments and temperature dependence electron-phonon scattering process in gold (Au) NPs are still not very clear. Here, we have employed ultrafast transient absorption (TA) spectroscopy to explore the hot plasmon mediated electron transfer (PMET) and electron-phonon dynamics of photo-excited Au NPs in glassy film matrix with variable SiO

International audience; Understanding how thermal-phonon paths can be shaped is key for controlling heat dissipation at the nanoscale. Thermophononic crystals are periodic porous nanostructures with thermal conductivity deviating from effective medium theory, which is possible if the characteristic sizes are of the order of phonon mean free paths and/or if phonons are forced to flow in privileged directions. We investigate suspended silicon nanomembranes with a periodic array of partially perforated holes of original paraboloid shape, with all characteristic lengths below 100 nm. Results from scanning thermal microscopy, a thermal sensing technique derived from atomic force microscopy, indicate that partial perforation of the membranes impacts heat conduction moderately, with the holey crystals showing a thermal conductivity reduction by a factor 6 in comparison to the bulk and a factor 2.5 in comparison to the non-perforated membrane. The impact of the phononic shapes is analyzed in light of a complementary Monte Carlo ray-tracing estimate of the effective phonon mean free paths that include multiple phonon reflection and highlights phonon backscattering.

Spintronic terahertz emitters are broadband and efficient sources of terahertz radiation, which emerged at the intersection of ultrafast spintronics and terahertz photonics. They are based on efficient spin-current generation, spin-to-charge-current conversion, and current-to-field conversion at terahertz rates. In this Editorial, we review the recent developments and applications, the current understanding of the physical processes, and the future challenges and perspectives of broadband spintronic terahertz emitters.

Ensembles of ultra-cold atoms have been proven to be versatile tools for high precision sensing applications. Here, we present a method for the manipulation of the state of trapped clouds of ultra-cold bosonic atoms. In particular, we discuss the creation of coherent and squeezed states of quasiparticles and the coupling of quasiparticle modes through an external cavity field. This enables operations like state swapping and beam splitting which can be applied to realize a Mach–Zehnder interferometer (MZI) in frequency space. We present two explicit example applications in sensing: the measurement of the healing length of the condensate with the MZI scheme, and the measurement of an oscillating force gradient. Furthermore, we calculate fundamental limitations based on parameters of state-of-the-art technology. H2020 Marie Skłodowska-Curie Actionshttps://doi.org/10.13039/100010665

Thermoelectric materials enable the mutual energy conversion of waste heat and electricity, critical to relieve global energy crisis. Hightemperature thermoelectric materials are special species due to their high-temperature stability and noticeable energy conversion efficiency. Here, we report a systematic investigation on high-temperature thermoelectric gadolinium selenides, cubic Gd3-xSe4 (x = 0.16, 0.21 and 0.25) and orthorhombic Gd2Se3-y (y = 0.02, 0.06 and 0.08). High energy synchrotron x-ray diffraction and total scattering are used to investigate the crystallographic and local structures. The atomic-scale cluster of Gd vacancy in cubic Gd2.84Se4 is observed by employing the reverse Monte Carlo simulation. For cubic Gd3-xSe4, its carrier concentration is tuned and multiple conduction bands are incorporated by adjusting Gd vacancy. Experimentally, the gradual increase in effective masses is evidently observed in cubic Gd3-xSe4, which is consistent with the density functional theory (DFT) calculation. A reasonable peak zT value of 0.27 is achieved at 850 K for Gd2.84Se4. On the other hand, adjusting Se vacancy enables the optimization of electron concentration for orthorhombic Gd2Se3-y phase. Its low deformation potential (Ξ = 12eV) with single conduction band gives rise to enhanced electron mobility and higher peak zT value of 0.54 at 850 K for Gd2Se2.98. In addition, a higher zT of 1.2 at1200 K is reasonably predicted for Gd2Se2.98 by using quality factor analysis. This work not just presents a systematic crystallographic investigation of gadolinium selenides, but also provides a deep insight into the charge transport and phonon scattering mechanisms. This study facilitates the exploration of more high-temperature thermoelectric materials. Ministry of Education (MOE) Accepted version The authors would like to acknowledge EFRC Solid-State-Thermal Energy Conversion Center (S3TEC), Grant DE-SC000XXXX. This work is also supported by Japan Society for the Promotion of Science (JSPS) KAKENHI, Grant JP 19F19057. L. H. acknowledge the International Research Fellow of JSPS. Q. F. Y. acknowledge the Chinese Scholarship Council (CSC) for the scholarship in Tokyo Institute of Technology. A. Q. Yan acknowledges Singapore MOE Tier 2 under Grant MOE2018-T2-1- 010. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Science, under Contract No. DE-AC02-06CH11357.

Up-conversion photoluminescence (UCPL) refers to the elementary process where low-energy photons are converted into high-energy ones via consecutive interactions inside a medium. When additional energy is provided by internal thermal energy in the form of lattice vibrations (phonons), the process is called phonon-assisted UCPL. Here, we report the exceptionally large phonon-assisted energy gain of up to ~ 8kBT (kB is Boltzmann constant, T is temperature) on all-inorganic lead halide perovskite semiconductor colloidal nanocrystals that goes beyond the maximum capability of only harvesting optical phonon modes. By systematic optical study in combination with a statistical probability model, we explained the nontrivial phonon-assisted UCPL process in perovskites nanocrystals, where in addition to the strong electron-phonon (light-matter) coupling, other nonlinear processes such as phonon-phonon (matter-matter) interaction also effectively boost the up-conversion efficiency. Ministry of Education (MOE) National Research Foundation (NRF) Accepted version Q. X. gratefully acknowledges financial support from the Singapore National Research Foundation through the NRF Investigatorship Award (No. NRF-NRFI2015-03) and the Singapore Ministry of Education via AcRF Tier 3 Programme (No. MOE2018-T3-1-002), Tier 2 grant (No. MOE2018-T2-2-068)and Tier 1 grants (Nos. RG103/15 and RG113/16). A. G. D. A. gratefully acknowledges the financial support of the Presidential Postdoctoral Fellowship program of the Nanyang Technological University. We acknowledge Dr. Lulu Zhang for his help on the TEM characterization.

The ability to tune the optical response of a material via electrostatic gating is crucial for optoelectronic applications, such as electro-optic modulators, saturable absorbers, optical limiters, photodetectors and transparent electrodes. The band structure of single layer graphene (SLG), with zero-gap, linearly dispersive conduction and valence bands, enables an easy control of the Fermi energy E$_F$ and of the threshold for interband optical absorption. Here, we report the tunability of the SLG non-equilibrium optical response in the near-infrared (1000-1700nm/0.729-1.240eV), exploring a range of E$_F$ from -650 to 250 meV by ionic liquid gating. As E$_F$ increases from the Dirac point to the threshold for Pauli blocking of interband absorption, we observe a slow-down of the photobleaching relaxation dynamics, which we attribute to the quenching of optical phonon emission from photoexcited charge carriers. For E$_F$ exceeding the Pauli blocking threshold, photobleaching eventually turns into photoinduced absorption, due to hot electrons' excitation increasing SLG absorption. The ability to control both recovery time and sign of nonequilibrium optical response by electrostatic gating makes SLG ideal for tunable saturable absorbers with controlled dynamics., Comment: 6 figures

Resonance Raman spectroscopy is one of the spectroscopic methods often chosen for studying linear polyene molecules because the Raman intensities of their υ

International audience; Coating gold nanostructures with a silica shell has been long considered for biomedical applications, including photoacoustic imaging. Recent experimental and modeling investigations reported contradicting results concerning the effect of coating on the photoacoustic response of gold nanostructures. Enhanced photoacoustic response is generally attributed to facilitated heat transfer at the gold/silica/water system. Here, we examine the photoacoustic response of gold core–silica shell nanoparticles immersed in water using a combination of the two temperature model and hydrodynamic phase field simulations. Here, of particular interest is the role of the interfacial coupling between the gold electrons and silica shell phonons. We demonstrate that as compared to uncoated nanoparticles, photoacoustic response is enhanced for very thin silica shells (5 nm) and short laser pulses, but for thicker coatings, the photoacoustic performance are generally deteriorated. We extend the study to the regime of nanocavitation and show that the generation of nanobubbles may also play a role in the enhanced acoustic response of core–shell nanoparticles. Our modeling effort may serve as guides for the optimization of the photoacoustic response of heterogeneous metal–dielectric nanoparticles.

The present article is a short overview of the theoretical modeling of spin transitions in polymetallic compounds. As distinguished from many insightful reviews on this topic, the present work is focused on the nature of cooperative interaction of the metal clusters in molecular crystals with emphasis at the physical role of molecular vibrations and phonons. The underlying model assumes that the cooperativity is triggered by phonons while the metal centers are coupled to molecular vibrations. It is demonstrated that the suggested model gives a satisfactory description of the observed spin transitions in mono-, bi- and tetranuclear compounds. In the framework of the described approach, we discuss the experimental data on spin crossover in the mononuclear [Fe(ptz)6](BF4)2, binuclear [{Fe(bt)(NCS)2}2bpym] and tetranuclear [Fe(tpa){N(CN)2}]4·(BF4)4·(H2O)2 compounds containing iron ions. The approach is also applied to the description of the charge-transfer-induced spin transition in the [{(Tp)Fe(CN)3}{Co-(PY5Me2)}](CF3SO3) complex.

Hexagonal boron nitride (hBN) is widely used as a protective layer for few-atom-thick crystals and heterostructures (HSs), and it hosts quantum emitters working up to room temperature. In both instances, strain is expected to play an important role, either as an unavoidable presence in the HS fabrication or as a tool to tune the quantum emitter electronic properties. Addressing the role of strain and exploiting its tuning potentiality require the development of efficient methods to control it and of reliable tools to quantify it. Here we present a technique based on hydrogen irradiation to induce the formation of wrinkles and bubbles in hBN, resulting in remarkably high strains of ∼2%. By combining infrared (IR) near-field scanning optical microscopy and micro-Raman measurements with numerical calculations, we characterize the response to strain for both IR-active and Raman-active modes, revealing the potential of the vibrational properties of hBN as highly sensitive strain probes.

International audience; Triggering new stable macroscopic orders in materials by ultrafast optical or terahertz pump pulses is a difficult challenge, complicated by the interplay between multiscale microscopic mechanisms, and macroscopic excitation profiles in samples. In particular, the differences between the two types of excitations are still unclear. In this article, we compare the optical response on acoustic timescale of a V2O3 Paramagnetic Metallic (PM) thin film excited by a terahertz (THz) pump or an optical pump, at room temperature. We show that the penetration depth of the deposited energy has a strong influence on the shape of the optical transmission signal, consistent with the modulation of permittivity by the superposition of depth-dependent static strain, and dynamical strain waves travelling back and forth in the sample layer. In particular, the temporal modulation of the optical transmission directly reflects the excitation profile as a function of depth, as well as the sign of the acoustic reflection coefficient between the film and the substrate. The acoustic mismatch between the V2O3 layer and the substrate was also measured. The raw data were interpreted with a one-dimensional analytical model, using three fitting parameters only. These results are discussed in the context of triggering phase transitions by ultrafast pump pulses. To the best of our knowledge, this is the first report of the modulation of the optical transmission of V2O3 with a THz pump within the acoustic timescale.

Funder: Engineering and Physical Sciences Research Council; doi: https://doi.org/10.13039/501100000266, Recently published density functional theory results using the PBE functional (Whaley-Baldwin and Needs 2020 New J. Phys. 22 023020) suggest that elemental sulfur does not adopt the simple-cubic (SC) P m 3 ̄ m phase at high pressures, in disagreement with previous works (Rudin and Liu 1999 Phys. Rev. Lett. 83 3049--52; Gavryushkin et al 2017 Phys. Status Solidi B 254 1600857). We carry out an extensive set of calculations using a variety of different exchange–correlation functionals (both local and non-local), and show that even though under LDA and PW91 a high-pressure SC phase does indeed become favourable at the static lattice level, when zero-point energies (ZPEs) are included, the transition to the SC phase is suppressed in every case, owing to the larger ZPE of the SC phase; thus confirming the transition sequence as R 3 ̄ m → BCC, with no intervening SC phase. We reproduce these findings with pseudopotentials that explicitly include core electronic states, and show that even at these high pressures, only the n = 3 valence shell contributes to bonding in sulfur. We then compare our findings against the all-electron code ELK, which is in excellent agreement with our pseudopotential results, and examine the roles of the exchange and correlation contributions to the total energy. We further calculate anharmonic vibrational corrections to the ZPEs of the two phases, and find that such corrections are several orders of magnitude smaller than the ZPEs and are thus negligible. The effect of finite temperatures is also considered, and we show that the P m 3 ̄ m phase becomes even more unfavourable with an increase in temperature. Finally, the experimental consequences of our results on the equation of state of sulfur and its superconducting critical temperature are explicitly calculated.

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., This work was supported by the European Commission FET Open project PHENOMEN (G.A. Nr. 713450). ICN2 was supported by the S. Ochoa program from the Spanish Research Agency (AEI, grant no. SEV-2017-0706) and by the CERCA Programme / Generalitat de Catalunya. ICN2 authors acknowledge the support from the Spanish MICINN project SIP (PGC2018-101743-B-I00). D.N.U. and M.F.C. acknowledge the support of a Ramón y Cajal postdoctoral fellowship (RYC-2014-15392) and a Severo Ochoa studentship, respectively. E.C.A. acknowledges financial support from the EU FET Open Project NANOPOLY. (GA 829061). A.M. acknowledges support from Ministerio de Ciencia, Innovación y Universidades (grant PGC2018-094490-B, PRX18/00126) and Generalitat Valenciana (grants PROMETEO/2019/123, and IDIFEDER/2018/033).