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International audience; In this paper, we demonstrate that it is possible to combine two computational approaches, usually used in very different contexts, namely, the Green-Kubo formalism and statistical approaches, to solve the Boltzmann transport equation to calculate the thermal conductivity of semiconductors. In this framework, first the phonon transport is solved by the Monte Carlo method according to the Debye-Callaway or Holland formalisms, then the flux autocorrelation is calculated. This new approach has been implemented to calculate the properties of Si, Ge, GaN, and C (diamond) in an extended temperature range, from 50 to 600 K. The simulation results are compared with those given by experimentation, with very good agreement.

International audience; In the paper, combined bi-modal microscopic visualization of biological objects based on reflection-mode optical and photoacoustic measurements was presented. Gas-microphone configuration was chosen for the registration of photothermal response. Precise positioning of the scanning laser beam with modulated intensity was performed employing acousto-optic deflectors. Photoacoustic images are shown to give complementary details to the optical images obtained in the reflected light. Specifically, the photoacoustic imaging mode allows much better visualization of the features with enhanced heat localization due to the reduced heat outflow. For example, the application of the photoacoustic imaging mode was especially successful to visualize Drosophila fly's micro-hairs. Furthermore, the photoacoustic image quality was shown to be adjusted through modulation frequency.

The characterization of material thermal properties at nanoscales remains a challenge even if progress was achieved in developing outstanding characterization techniques like scanning thermal microscopy (SThM). In the present work, we propose a detailed procedure based on the combined use of a SThM probe characterization and its Finite Element Method (FEM) modeling to recover in operando 3ω measurements achieved under high vacuum. This approach is based on a two-step methodology: (i) a fine description of the probe's electrical and frequency behaviors in “out of contact” mode to determine the intrinsic parameters of the SThM tip and (ii) a minimization of the free parameter of our model, i.e., the contact thermal resistance, by comparing 3ω measurements with the simulations of the probe operating “in contact mode.” Such an approach allows us to measure thermal interface resistances between the tip and the surface. We applied our methodology to three different materials with known thermal properties: Si, SiO2 bulk materials, and a gold thin film. In addition, the FEM modeling provides insights into SThM thermal probes sensitivity, as a function of probe/sample interface resistance and the contact area to measure material thermal conductivity paving the way to quantitative SThM measurements.

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The paper presents the results of thermal transport properties study of CsPbBr3 perovskite at room temperature. The samples were fabricated with the use of Bridgman method. For experimental investigations, the photoacoustic method in gas-microphone configuration was used. The evaluated thermal conductivity was found to be equal 1±0.3 W/(m K). Additionally, thermal conductivity was simulated with the use of equilibrium molecular dynamics approach. The value calculated with the simulation approach was found to be equal to 0.75±0.8 W/(m K). Thus, one can state good agreements of the results of simulations and experimental ones.

Molecular dynamics simulations describing the equilibrium shape of a nanodroplet located on the solid substrate are presented for the cases of a “cylindrical water droplet” on silicon substrates. Several examples of the structuration of the solid substrate surface are simulated, i.e.: atomistic flat substrate and substrates with ordered nanopillars and nanopores. The adhesives forces between molecules of the substrate and the fluid are modified to change the wettability. Three wetting configurations are considered in this work for the smooth surface: (i) hydrophilic (0 = 30∘), (ii) hydrophobic (0 = 136∘), and (iii) an intermediate regime (0 = 80∘). Further, the dependence of the wetting angle as a function of the surface state is studied in details for the above-mentioned configurations., Робота присвячена дослiдженню рiвноважної форми нанокраплi, розташованої на твердiй наноструктурованiй пiдкладинцi, зокрема, було розглянуто випадок “цилiндричної краплi води” на кремнiєвiй пiдкладинцi. Було змодельовано декiлька варiантiв структурування поверхнi твердої пiдкладинки: атомiстично плоска пiдкладинка та пiдкладинка з упорядкованими наностовпчиками та нанопорами. Сили адгезiї мiж молекулами субстрату та рiдини були модифiкованi вiдповiдним чином для того, щоб варiювати змочуванiсть. У данiй роботi було розглянуто три конфiгурацiї змочування поверхнi: 1) гiдрофiльний (0 = 30∘), 2) гiдрофобний (0 = 136∘), та 3) промiжний режим (0 = 80∘). В подальшому залежнiсть кута змочування вiд функцiї стану поверхнi детально вивчалась для вищезгаданих конфiгурацiй.

The thermal transport properties of porous silicon and nano-hybrid "porous silicon/water" systems are presented here. The thermal conductivity was evaluated with equilibrium molecular dynamics technique for porous systems made of spherical voids or water-filled cavities. We revealed large thermal conductivity enhancement in the nano-hybrid systems as compared to their dry porous counterparts, which cannot be captured by effective media theory. This rise of thermal conductivity is related to the increases of the specific surface of the liquid/solid interface. We demonstrated that significant difference for more than two folds of thermal conductivity of pristine porous silicon and "porous silicon liquid/composite" is due to the liquid density fluctuation close to "solid/liquid interface" (layering effect). This effect is getting more important for the high specific surface of the interfacial area. Specifically, the enhancement of the effective thermal conductivity is 50 % for specific surface area of 0.3 (1/nm), and it increases further upon the increase of the surface to volume ratio. Our study provides valuable insights into the thermal properties of hybrid liquid/solid nanocomposites and about the importance of confined liquids within nanoporous materials.

International audience; The photoacoustic method with piezoelectric detection for the simultaneous evaluation of the thermophysical properties is proposed. The approach is based on the settling of an additional heat sink for redistribution of heat fluxes deposited on the sample surface. Firstly, the approach was tested on the porous silicon with welldefined morphology and well-studied properties. Then, heat capacity and thermal conductivity of silicon nanowires arrays have been investigated by recovering the experimental data through numerical simulations. The decrease of heat capacity and effective thermal conductivity of the samples upon increasing thickness and porosity of the sample is observed. Such behavior could be caused by the increase of the structure heterogeneity. In particular, this can be related to larger disorder (increased density of broken nanowires and larger porosity) that appears during the etching process of the thick layers.

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International audience; We report the thermal transport properties of wurtzite GaN in the presence of dislocations using molecular dynamics simulations. A variety of isolated dislocations in a nanowire configuration are analyzed and found to considerably reduce the thermal conductivity while impacting its temperature dependence in a different manner. Isolated screw dislocations reduce the thermal conductivity by a factor of two, while the influence of edge dislocations is less pronounced. The relative reduction of thermal conductivity is correlated with the strain energy of each of the five studied types of dislocations and the nature of the bonds around the dislocation core. The temperature dependence of the thermal conductivity follows a physical law described by a T À1 variation in combination with an exponent factor that depends on the material's nature, type and the structural characteristics of the dislocation core. Furthermore, the impact of the dislocation density on the thermal conductivity of bulk GaN is examined. The variation and absolute values of the total thermal conductivity as a function of the dislocation density are similar for defected systems with both screw and edge dislocations. Nevertheless, we reveal that the thermal conductivity tensors along the parallel and perpendicular directions to the dislocation lines are different. The discrepancy of the anisotropy of the thermal conductivity grows with increasing density of dislocations and it is more pronounced for the systems with edge dislocations. Besides the fundamental insights of the presented results, these could also be used for the identification of the type of dislocations when one experimentally obtains the evolution of thermal conductivity with temperature since each type of dislocation has a different signature, or one could extract the density of dislocations with a simple measurement of thermal anisotropy.

The paper is devoted to the study of the features of CdTe surface treatment under laser irradiation with both different wavelengths (λ = 300–800 nm) and pulse durations (τp = 7 ns–1 ms). The thermal conductivity of the semi-insulating p-like CdTe semiconductor crystals was evaluated using the photoacoustic gas-microphone method. Simulations of the melting threshold were performed based on the three stage model of the laser induced excitation and relaxation. In particular, the following processes were considered in frames of the model: (i) rapid interband thermalization, (ii) nonradiative interband and (iii) nonradiative surface recombination. It was revealed that in the range of pulse durations from 7 ns to 1 μs, the melting threshold of the CdTe mainly depended on the absorption coefficient α(λ). For pulse durations longer than 1 μs the threshold started to depend also on the spectra of the reflectivity coefficient R(λ). The obtained results have been used for optimization of the laser-assisted techniques of surface processing and stimulated doping of CdTe crystals.

The impact of decorated dislocations on the effective thermal conductivity of GaN is investigated by means of equilibrium molecular dynamics simulations via the Green-Kubo approach. The formation of "nanowires" by a few atoms of In in the core of dislocations in wurtzite GaN is found to affect the thermal properties of the material, as it leads to a significant decrease of the thermal conductivity, along with an enhancement of its anisotropic character. The thermal conductivity of In-decorated dislocations is compared to the ones of pristine GaN, InN, and random and ordered InxGa1-xN alloy, to examine the impact of doping. Results are explained by the stress maps, the bonding properties and the phonon density of states of the aforementioned systems. The decorated dislocations engineering is a novel way to tune, among other transport properties, the effective thermal conductivity of materials at the nanoscale, which can lead to the manufacturing of interesting candidates for thermoelectric or anisotropic thermal dissipation devices.

Mesoparticles with characteristic sizes larger than all known molecules and smaller than usual nanoparticles can easily penetrate inside living cells. Thus, they appear to be especially promising for cancer theranostic application. However, presence of mesoparticles can strongly perturb physico-chemical properties of the biological media in which they are incorporated, it is crucial to know a magnitude of such perturbations. For example, thermal properties of liquid solutions with dispersed mesoparticles are crucial for development of theranostic approaches based on photoacoustic effect. In this paper, thermal transport in nanofluids formed by carbon flurooxide (CFO) mesoparticles is reported. A significant non-linear enhancement of the thermal conductivity of the nanofluids (up to 85%, in comparison with initial basic liquids) depending on volume fraction of the dispersed CFO mesoparticles was found. It was revealed that the thermal conductivity rise can be explained by existence of interfacial layers between the mesoparticles and the hosting liquids.

International audience; Features of heat transport in silicon-based porous structures with different morphology before and after thermal annealing are investigated. Specifically, fractal-like porous silicon and silicon nanowire arrays samples were considered. All the structures were annealed in dry air at various temperatures in diapason from 600 °C up to 1000 °C for 20 minutes. The photoacoustic technique with gas-microphone registration was applied for the thermal diffusivity evaluation of the samples. Particularly, the amplitude frequency dependencies of photoacoustic response from the considered samples in the classical gas-microphone configuration were measured in the range from 10 Hz to 3 kHz. The simulation of the experimental dependencies with the use of appropriate model allowed us to evaluate thermal conductivity. It was found that the short time thermal annealing together with increasing annealing temperature leads to significant reduction of thermal transport in mesoporous silicon. On the opposite, the thermal properties of silicon nanowires remain unaffected.

The present work addresses the problem of thermal conductivity simulation in ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ and SnSe thermoelectric nanostructures. It first details phonon lifetime calculation in both thermoelectric compounds assuming polynomial dispersion properties and the Debye-Callaway model for the relaxation-time approximation. For both materials, distinct crystallographic directions are considered, i.e., $\mathrm{\ensuremath{\Gamma}}\ensuremath{-}Z$ (trigonal axis) and $\mathrm{\ensuremath{\Gamma}}\ensuremath{-}X$ (basal plane) for ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ and $\mathrm{\ensuremath{\Gamma}}\ensuremath{-}X$ ($a$ axis), $\mathrm{\ensuremath{\Gamma}}\ensuremath{-}Y$ ($b$ axis), and $\mathrm{\ensuremath{\Gamma}}\ensuremath{-}Z$ ($c$ axis) for SnSe. On this basis, the lifetime model is parametrized and bulk thermal conductivity is computed through the resolution of the phonon Boltzmann transport equation with a Monte Carlo method. Gr\"uneisen parameter and mass-disorder lifetime are adjusted to fit experimental temperature dependence. The second part of the study addresses the calculation of thermal conductivity for these two thermoelectric materials in the case of thin films (cross-plane case) and nanowires. The main goals of this work are to provide a fully parametric description of heat transport in ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ and SnSe nanofilms and nanowires showing reliable behavior on an extended size range, from 20 nm to $2\phantom{\rule{0.0pt}{0ex}}\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{m}$, and large temperature range (100--500 K for ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$; 200--800 K for SnSe). Comparisons to bulk thermal conductivity calculations and measurements as well as to recent investigations on nanowires, demonstrate the effectiveness of the proposed methodology to deal with nanostructured ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ and SnSe.

Silicon nanowire (SiNW) arrays were investigated by gas-microphone photoacoustic (PA) technique to evaluate the thermal conductivity and optical absorption . The arrays with SiNW's length from 3 to 25 μm were formed by metal-assisted chemical etching of (100)-oriented monocrystalline silicon (c-Si) substrates with the low and high concentration of doping impurities . PA responses of SiNWs were excited by sources with wavelengths of 460 and 630 nm in the frequency range from 15 to 1150 Hz. An analysis of the experimental data reveals that the thermal conductivities of SiNWs formed on the lightly and heavily doped substrates are about 4–7 and 0.17–0.22 W/(m K), respectively. The latter value is explained by the porous structure of the SiNWs, promoting a strong restriction of the phonon free path. The thermal conductivity and optical absorption of SiNW arrays depend on the doping level and length of SiNWs due to a combination of the phonon and light scattering processes.

In this study, we investigated the thermal transport in porous silicon nanostructures and silicon nanowires fabricated by electrochemical and metal-assisted chemical etching processes, respectively. In particular, the thermal conductivity values for the silicon-based nanomaterials were estimated based on photoacoustic measurements obtained from piezoelectric recordings. The amplitude–frequency and phase–frequency dependencies of the photoacoustic responses of the nanostructured silicon samples were obtained, and simulated successfully with a theoretical model. Correlations were established between the thermal conductivities and etching parameters.

The aim of the chapter is to give physical insights regarding solid/liquid interfaces and solid/liquid nanocomposites via atomistic simulations and improved macroscopic analytical approaches. Particularly, the use of molecular dynamics for the study of a nanodroplet located on a solid substrate will be discussed. The impact of line tension, Tolman length and adsorption layer on the wetting characteristic will be considered in details.

In this work, we investigated the tensile and compression forces effect on the thermal conductivity of silicon. We used the equilibrium molecular dynamics approach for the evaluation of thermal conductivity considering different interatomic potentials. More specifically, we tested Stillinger-Weber, Tersoff, Environment-Dependent Interatomic Potential, and Modified Embedded Atom Method potentials for the description of silicon atom motion under different strain and temperature conditions. It was shown that the Tersoff potential gives a correct trend of the thermal conductivity with the hydrostatic strain, while other potentials fail, especially when the compression strain is applied. Additionally, we extracted phonon density of states and dispersion curves from molecular dynamics simulations. These data were used for direct calculations of the thermal conductivity considering the kinetic theory approach. Comparison of molecular dynamics and kinetic theory simulations results as a function of strain and temperature allowed us to investigate the different factors affecting the thermal conductivity of the strained silicon.

The paper is devoted to the study of the evaporation of water sessile drops resting on a smooth surface consisting of a vanadium dioxide thin film (∼150 nm) covering a sapphire substrate. Vanadium dioxide is considered because of the promising applications it is expected to have in new technologies for heat waste recuperation. Water drops are deposited on the substrate heated at different temperatures from 20 to 80°C, and the time dependence of the droplet shape is registered with the use of shadow imaging. This reveals that the three-phase contact line remains pinned during most of the drop lifetime, while the wetting contact angle as well as the drop volume are continuously changing. With the help of image processing, the vaporization rate of the drop is calculated. It is found that it is almost constant with time, and increases exponentially with the temperature of the substrate, well in agreement with theoretical models developed to describe the evaporation of drops with a pinned contact line.

Tin-induced crystallization of amorphous silicon in thin-film Si–Sn–Si structures under the influence of laser irradiation of various types has been studied, by using Raman scattering. The size and concentration dependences of Si nanocrystals on the power of 10-ns and 150-мs laser pulses with a wavelength of 535 or 1070 nm are experimentally measured and analyzed. A possibility of effective tin-induced transformation of silicon in a-Si layers 200 nm in thickness from the amorphous to crystalline phase within 10 ns time interval under the action of laser light pulses is demonstrated. The theoretical calculation of the spatial temperature distribution and its time evolution in the area of the laser beam action is used to interpret the experimental results., Методом комбiнацiйного розсiювання свiтла дослiджено процеси iндукованої оловом кристалiзацiї аморфного кремнiю в тонкоплiвкових структурах Si–Sn–Si пiд дiєю рiзних видiв iмпульсного лазерного опромiнення. Експериментально визначено та проаналiзовано залежностi розмiрiв та концентрацiї нанокристалiв Si вiд потужностi лазерних iмпульсiв тривалiстю 10 нс та 150 мкс з довжиною хвилi 535 нм та 1070 нм. Показано можливiсть ефективної iндукованої оловом трансформацiї кремнiю iз аморфної фази в кристалiчну за час порядку 10 нс в шарах a–Si товщиною 200 нм пiд дiєю iмпульсу лазерного свiтла. Теоретичний розрахунок просторового i часового розподiлу температур в зонi дiї лазерного променя використано для iнтерпретацiї експериментальних результатiв.

We have investigated the thermal transport in anisotropic nanostructured materials. Particularly, porous silicon and silicon nanowires arrays have been considered in this work. For thermal conductivity evaluation Raman technique has been applied. The thermal transport anisotropy is more pronounce in silicon nanowires array than in porous silicon. Yet, the in-plane and cross-plane thermal conductivities of porous silicon differ approximately in twice. Both values are in a good agreement with the literature.

Laser ultrasound enhancement with carbon-based nanomaterials incorporated into phantom tissues is reported. More specifically, impact of nanodiamonds and graphene oxide on the efficiency of photoinduced pressure excitation was investigated. Agarose-based phantom tissues with and without the nanoparticles were compared. The second harmonic of pulsed Nd:Yag laser (532 nm) was chosen for ultrasound generation. Specific acoustic probe was fabricated for laser-induced ultrasound registration. Dependence of the ultrasound amplitude on concentration of the nanoparticles incorporated into the phantom tissues was analyzed in details. The most significant enhancement of the amplitude ensured by graphene oxide was stated. Considering biocompatibility of the both types of carbon-based nanomaterials, they can be potentially used in future as contrast agents for bio-imaging and theranostic applications.

Features of thermal transport in multilayered porous silicon nanostructures are considered. Such nanostructures were fabricated by electrochemical etching of monocrystalline Si substrates by applying periodically changed current density. Hereby, the multilayered structures with specific phononic properties were formed. Photoacoustic (PA) technique in gas-microphone configuration was applied for thermal conductivity evaluation. Experimental amplitude-frequency dependencies were adjusted by temperature distribution simulation with thermal conductivity of the multilayered porous structure as a fitting parameter. The experimentally determined values of thermal conductivity were found to be significantly lower than theoretically calculated ones. Such difference was associated with the presence of thermal resistance at the interfaces between porous layers with different porosities arising because of elastic parameters mismatch (acoustical mismatch). Accordingly, the magnitude of this interfacial thermal resistance was experimentally evaluated for the first time. Furthermore, crucial impact of the resistance on thermal transport perturbation in a multilayered porous silicon structure was revealed.

We present an efficient technique for the evaluation of the Gibbs adsorption of a liquid on a solid substrate. The behavior of a water nanodroplet on a silicon surface is simulated with molecular dynamics. An external field with varying strength is applied on the system to tune the solid-liquid interfacial contact area. A linear dependence of droplet's volume as a function of the contact area is observed. We introduce a modified Young-Laplace equation to explain the influence of the Gibbs adsorption on the nanodroplet volume contraction. Fitting of the molecular dynamics results with the analytical approach allows us to evaluate the number of atoms per unit area adsorbed on the substrate, which quantifies the Gibbs adsorption. Thus, a threshold of a droplet size is obtained, for which the impact of the adsorption is crucial. For instance, a water droplet with 5 nm radius has 3% of its molecules adsorbed on silicon substrate, while for droplets less than 1 nm this amount is more than 10%. The presented results could be beneficial for the evaluation of the adsorption impact on the physical-chemical properties of nanohybrid systems with large surface-to-volume ration.

International audience; Spherical and cylindrical water droplets on silicon surface are studied to tune the silicon–oxygen interaction. We use molecular dynamics simulations to estimate the contact angle of two different shaped droplets. We found that the cylindrical droplets are independent of the line tension as their three phases curvature is equal zero. Additionally, we compare an analytical model, taking into account or not the Tolman length and we show that for spherical small size droplets, this length is important to be included, in contrast to cylindrical droplets in which the influence of the Tolman length is negligible. We demonstrate that the usual convenient way to exclude linear tension in the general case can give wrong results. Here, we consider cylindrical droplets, since their contact angle does not depend on the droplet size in the range of few to 10ths of nanometres. The droplets are stabilised due to the periodic boundary conditions. This allows us to propose a new parameterisation for nanoscale droplets, which is independent the size of the droplets or its shape, minimising at the same time the calculation procedure. With the proposed methodology, we can extract the epsilon parameter of the interaction potential between a liquid and a solid from the nanoscaled molecular simulation with only as input the macrosized experimental wetting angle for a given temperature.

This work describes tin-induced crystallization of amorphous silicon studied with Raman spectroscopy in thin-film structures Si-Sn-Si irradiated with pulsed laser light. We have found and analyzed dependencies of the nanocrystals’ size and concentration on the laser pulse intensity for 10 ns and 150 μm duration laser pulses at the wavelengths of 535 nm and 1070 nm. Efficient transformation of the amorphous silicon into a crystalline phase during the 10 ns time interval of the acting laser pulse in the 200 nm thickness films of the amorphous silicon was demonstrated. The results were analyzed theoretically by modeling the spatial and temporal distribution of temperature in the amorphous silicon sample within the laser spot location. Simulations confirmed importance of light absorption depth (irradiation wavelength) in formation and evolution of the temperature profile that affects the crystallization processes in irradiated structures.

International audience; An approach for the size evaluation of fine morphological features of fractal-like porous materials is described. The method is based on the measurements of thermal conductivity of porous layers with empty and filled pore networks. Being associated to a heat transfer model, the experimentally obtained values of thermal conductivity allow the size evaluation of very small structural constrictions ensuring interconnections between the main particles constituting the porous material. The method is applied to determine the size values of the structural constrictions interconnecting silicon nanocrystals forming meso-porous silicon layers.

This paper is devoted to the study of photothermal transformations in multilayered structures. As a modeled sample, porous silicon with a periodic distribution of porosity was chosen. The spatial distribution of the optical properties inside the structure was evaluated under Brugmann's approximation. The heat sources arising as a result of electromagnetic radiation absorption in the structure were estimated by solving Maxwell's equations. This allowed us to calculate the temperature profiles of a photoexcited sample. For experimental measurements, a photoacoustic setup with a gas-microphone transduction system was chosen to investigate the thermal properties of the structure. The results of the photoacoustic response simulation based on the gas-piston model demonstrated excellent agreement with experiments. This allows a reliable evaluation of the thermal conductivity by fitting the experimental amplitude-frequency photoacoustic signal with the simulated one.

An efficient technique for low temperature metal-induced nanocrystalline silicon fabrication is presented. The technique is based on laser annealing of thin films of “amorphous silicon-tin” composites combined with in situ control and monitoring with Raman technique. Laser annealing was shown to provide the possibility of fine-tuning the nanocrystals size and concentration, which is important in photovoltaic and thermoelectric devices fabrication.

International audience; Anisotropic nanomaterials possess interesting thermal transport properties because they allow orientation of heat fluxes along preferential directions due to a high ratio (up to three orders of magnitude) between their in-plane and cross-plane thermal conductivities. Among different techniques allowing thermal conductivity evaluation, micro-Raman scattering is known to be one of the most efficient contactless measurement approaches. In this letter, an experimental approach based on Raman scattering measurements with variable laser spot sizes is reported. Correlation between experimental and calculated thermal resistances of one-dimensional nanocrystalline solids allows a simultaneous estimation of their in-plane and cross-plane thermal conductivities. In particular, our measurement approach is illustrated to be applied for anisotropic thermal conductivity evaluation of silicon nanowire arrays.

In this report the study of the elasticity stresses arising in the composite system “porous silicon –etching solution” will be discussed. For this purpose, the modified etching cell with piezoelectric transducer was adopted It was shown that the thermally induced pressures of the liquid confined inside of pores influenced dramatically on the time-domain shape of voltage signal detected on electrodes of the piezoelectric transducer. Therefore, the significant role of these pressures on the formation of general elastic stresses in the system was stated. The dependence of their magnitude and time delay on the thickness of porous silicon layer was found. Thus, the obtained experimental results could be a background for the new technique for the porous silicon in-situ study.

International audience; In this paper the results of an experimental study of heat transport peculiarities in a composite system “porous matrix – liquid” are presented where porous silicon with different values of porosity was used as the matrix. For evaluation of the thermal conductivity, a photoacoustic technique in a classical configuration with a gas-microphone registration was used. As a result of the experimental research, the increase of the thermal conductivity (up to two times) of the composite system “porous silicon – viscous liquid” in comparison with porous silicon has been determined. It was shown that such an increase is caused mainly by improved thermal contact between nanocrystallites of the PSi matrix as the result of the filling of second-order pores.

International audience; A new efficient model describing photoacoustic (PA) signal formation with piezoelectric detection is reported. Multilayer sandwich-like systems: heterogeneous studied structure—buffer layer—piezoelectric transducers are considered. In these systems, the buffer layer is used for spatial redistribution of thermoelastic force moments generated in the investigated structure. Thus, mechanical properties of this layer play a crucial role to ensure perfect control of the detected voltage formed on a piezoelectric transducer by contribution of different regions of the studied structure. In particular, formation of the voltage signal strongly depends on the point at which the thermoelastic source is applied. Therefore, use of relatively simple linear Green’s functions introduced in frames of the Kirchhoff–Love theory is chosen as an efficient approach for the PA signal description. Moreover, excellent agreement between the theoretical model and measured results obtained on a heterogeneous “porous silicon-bulk Si substrate” structure is stated. Furthermore, resolving of the inverse problem with fitting of the experimental curves by the developed model allows reliable evaluation of the thermal conductivity of the nanostructured porous silicon layer.

International audience; In this article, we demonstrate that the thermal conductivity of nanostructured porous silicon is reduced by amorphization and also that this amorphous phase in porous silicon can be created by swift (high-energy) heavy ion irradiation. Porous silicon samples with 41%-75% porosity are irradiated with 110 MeV uranium ions at six different fluences. Structural characterisation by micro-Raman spectroscopy and SEM imaging show that swift heavy ion irradiation causes the creation of an amorphous phase in porous Si but without suppressing its porous structure. We demonstrate that the amorphization of porous silicon is caused by electronic-regime interactions, which is the first time such an effect is obtained in crystalline silicon with single-ion species. Furthermore, the impact on the thermal conductivity of porous silicon is studied by micro-Raman spectroscopy and scanning thermal microscopy. The creation of an amorphous phase in porous silicon leads to a reduction of its thermal conductivity, up to a factor of 3 compared to the non-irradiated sample. Therefore, this technique could be used to enhance the thermal insulation properties of porous Si. Finally, we show that this treatment can be combined with pre-oxidation at 300 °C, which is known to lower the thermal conductivity of porous Si, in order to obtain an even greater reduction.

We report on the size dependence of the surface tension of a free surface of an isotropic fluid. The size dependence of the surface tension is evaluated based on the Gibbs-Tolman-Koenig-Buff equation for positive and negative values of curvatures and the Tolman lengths. For all combinations of positive and negative signs of curvature and the Tolman length, we succeed to have a continuous function, avoiding the existing discontinuity at zero curvature (flat interfaces). As an example, a water droplet in the thermodynamical equilibrium with the vapor is analyzed in detail. The size dependence of the surface tension and the Tolman length are evaluated with the use of experimental data of the International Association for the Properties of Water and Steam. The evaluated Tolman length of our approach is in good agreement with molecular dynamics and experimental data.

International audience; In the paper the experimental study of heat transport across the interface “porous silicon/liquid” by photoacoustic technique is reported. Two cases with and without liquid covering of porous silicon surface were considered. Thermal perturbations were excited at the surface of porous silicon as a result of absorption of the light with modulated intensity. The resulting thermal-elastic stresses arising in the system were registered with piezoelectric transducer. The amplitude-frequency dependencies of the voltage on the piezoelectric electrodes were measured. The presence of the liquid film leads to decreasing of the amplitude of photoacoustic signal as a result of the thermal energy evacuation from the porous silicon into the liquid. The experimental dependencies were fitted with the results of simulation that takes into account heat fluxes separation at the porous silicon/liquid interface. With the presented method one can precisely measure heat fluxes transferred from the solid into contacting fluid. Moreover, the presented approach can be easily adopted for the thermal conductivity study of the different nanofluids as well as thermal resistance at the interface nanostructured solid/fluid.