Nanomaterial thermal transport properties and nanothermodynamics

Resume : Magneto-optical materials have been proposed as promising candidates for an active control of the directionality of nanoscale heat radiation. Unexpected and very interesting effects like the thermal radiative Hall effect [1], persistent currents [2], giant magneto-resistance [3,4], and circular heat and momentum fluxes [5] have been highlighted. We review some of the recent developments in this new direction of magneto-optical thermotronics [6]. In particular, we discuss the possibility to rectify nanoscale radiative heat fluxes by means of non-reciprocal surface waves an therewith propose a nanoscale heat flux rectifier or diode which can be controlled actively by means of externally applied fields [7]. [1] P. Ben-Abdallah, ``Photon Thermal Hall Effect,'' Phys. Rev. Lett. 116, 084301, (2016). [2] L. Zhu and S. Fan, ``Persistent Directional Current at Equilibrium in Nonreciprocal Many-Body Near Field Electromagnetic Heat Transfer,'' Phys. Rev. Lett. 117, 134303 (2016). [3] I. Latella and P. Ben-Abdallah, ``Giant Thermal Magnetoresistance in Plasmonic Structures,'' Phys. Rev. Lett. 118, 173902, (2017). [4] R. M. Abraham Ekeroth, P. Ben-Abdallah, J.C. Cuevas, and A. Garcia Martin, ``Anisotropic Thermal Magnetoresistance for an Active Control of Radiative Heat Transfer,'' ACS Photonics 5, 705 (2017). [5] A. Ott, P. Ben-Abdallah, and S.-A. Biehs, ``Circular heat and momentum flux radiated by magneto-optical nanoparticles,'' Phys. Rev. B 97, 205414 (2018). [6] A. Ott, R. Messina, P. Ben-Abdallah, S.-A. Biehs, ``Magnetothermoplasmonics: from theory to applications,'' J. Photon. Energy 9, 032711 (2019). [7] A. Ott, R. Messina, P. Ben-Abdallah, S.-A. Biehs, ``Radiative thermal diode driven by nonreciprocal surface waves,'' Appl. Phys. Lett. 114, 163105 (2019).

Resume : A thermophotovoltaic (TPV) device converts infrared radiation from hot sources into electrical power via a specifically designed photovoltaic cell. Indium antimonide (InSb) is well-fitted to the blackbody spectrum of infrared emitters due to its very-low energy band gap (0.23 eV, i.e. 5.4 µm, at 77 K). In order to increase the generated electrical power, such cells can be coupled with an infrared emitter in the near field (< 4 µm at 700 K) where the radiative heat transfer between the emitter and the cell can be enhanced by several orders of magnitude due to the contribution of evanescent waves. This enhancement is well described theoretically and demonstrated experimentally (e.g [2-4]) but the coupling with a TPV cell was performed recently [5] with very low electrical power and conversion efficiency. In this work, we first characterize experimentally fully-functional InSb TPV cells [1] that operate below room temperature. We then demonstrate a strong electrical power photogeneration by placing a graphite spherical emitter in the near field of the TPV cells. To maximize the electrical power, large temperature differences are investigated: the emitter can be heated up to temperatures higher than 1000 °C. The influence of the emitter material on near-field radiative heat transfer and near-field TPV conversion is studied as a function of distance down to a few nanometers and comparisons to simulations using the Proximity Flux Approximation are made. [1] Cakiroglu et al., submitted (2019) [2] Polder et al., Physical Review B 4, (1971) [3] Rousseau et al., Nat. Photonics 3, (2009) [4] Kim et al., Nature 528, (2015) [5] Fiorino et al., Nano Letters 18, (2018)

Resume : We present a thermodynamic formalism able to describe in general states and processes of thermal radiation at the nanoscale. The approach is based on the definition of a free energy, that considers the nature of the system through its density of states, from which we derive the different thermodynamic potentials. We focus on two cases of current interest: near-field radiation and Casimir forces. In the former, we show the peculiarities of the photon gas in the near field whereas in the later we indicate a way to obtain Casimir forces based on thermodynamic principles. Our formalism provides a sound starting point for further studies in the field of nanoscale energy exchange with a high potential for theoretical studies and technological applications.

Resume : In this contribution, we shall discuss two examples in which the presence of nanoparticles has a dramatic effect on thermal transport in soft materials. The first example concerns ultrafast vaporization in water driven by optically irradiated colloidal nanoparticles. Understanding the physical processes at play in phase change under very large temperature gradients generated by plasmonic nanoparticles has applications in biomedicine, solar energy conversion and for photoacoustic imaging. For all these applications, it is crucial to control the repeatability of the vaporization process, so that nanobubbles may be triggered “on demand” by a laser switch. In this contribution, we investigate nanoparticle mediated pulsed vaporization on the basis of hydrodynamic phase field modeling. We shall illustrate how the combination of a continuous illumination and a well-chosen train of pulses allows to generate highly reproducible bubbles, at a high repetition rate. The second example to be discussed concerns thermal transport properties of polymer nanocomposites. Polymeric materials display low values of the thermal conductivity, and lifting off their intrinsically low conductivity is highly desirable to envision the widespread of polymers in microelectronics and packaging. Here, we model the thermal transport properties of silica/polystyrene nanocomposites using molecular dynamics simulations. We show how the presence of polymer chains permanently grafted on the surface of the silica nanoparticles may enhance its Kapitza conductance, which in turn helps in lifting off the nanocomposite conductivity. The origin of this effect will be discussed based on a microscopic analysis of the vibrational modes involved in interfacial heat transport.

Resume : The study of nanoscale heat transport is of growing interest following the industry’s trend of downsizing chip components. It is of great importance for both the industrial and scientific community to understand heat transfer at low dimensions in order to improve devices performances (thermoelectric materials, effective heat dissipation) as well as understanding physical phenomena emerging from this downsizing (ballistic effects, coherence, reduced mean free path, etc.) A road towards an efficient thermoelectric material is the reduction of its lattice thermal conductivity. This can be achieved by various means, such as size reduction (thin films, nanowires), introduction of strain/defects, doping, etc. In many cases, lattice thermal conductivity reduction is concomitant with a lowered electrical conductivity, reducing the overall thermoelectric efficiency. In this study, we seek to investigate mesoscopic thermal properties of an epitaxial nanostructured semiconductor made of a perfectly crystalline Ge matrix containing Ge3Mn5 nano-inclusions (30-60nm in size). The study is carried out by playing with several lengths, such as the nano-inclusions size and distribution, and probing the consequent thermal transport. In order to do so, we have developed a sensitive 3ω setup [1], enabling us to probe the film thermal conductivity as a function of temperature. We will first present experimental results on the thermal conductivity of amorphous very thin films (30-120nm) to validate our setup. Thermal boundary resistance measurements between the film and substrate will be thoroughly discussed. Then, we will discuss about the thermal conductivity measurements of nanostructured GeMn thin films as a function of temperature. a)jessy.paterson@neel.cnrs.fr b)olivier.bourgeois@neel.cnrs.fr [1] D. Singhal, J. Paterson, D. Tainoff, J. Richard, M. Ben-Khedim, P. Gentile, L. Cagnon, D. Bourgault,D. Buttard, and O. Bourgeois. Measurement of anisotropic thermal conductivity of a dense forest of nanowires using the 3ω method. Review of Scientific Instruments, 89(8) 084902 (2018).

Resume : The miniaturization of electronic devices substantially increases their power density, and as a result, requires effective means for cooling of these devices. Carbon nanotube (CNT) forests is one of the most promising nano-materials for use as a high-end heat dissipation element due to their high thermal conductivity, mechanical compliance, and nano-fabrication process compatibility. However, the lack of a clear understanding of the heat transfer mechanisms of CNT forests has so far impeded their large-scale use as cooling elements. In this work, our thermal analysis revealed that the heat dissipation of CNT forests is determined mainly by their height. The heat dissipation behavior of tall samples was dominated by convection from the CNTs sidewalls. The mechanism of heat transfer in short CNT forests, in contrast, was dominated by their morphology. Short CNT forests with highly aligned CNTs or with high concentration of defects demonstrated dominant CNT sidewalls convective heat dissipation (similar to that of tall CNT forests). Other short CNT forsts exhibited heat transfer dominated by conduction along the CNTs. Therefore, this work provides important guidelines regarding the parameters that can be changed to optimize the performances of CNT forests in thermal applications, such as cooling elements in electronic devices, where convection is desired, or thermal interface materials, where efficient heat conduction is necessary.

Resume : The recent research has pointed to nanostructuring as a highly efficient approach to reducing thermal conductivity. One example of nanostructured materials are porous/holey phononic crystals (PnCs), which thermal conductivity can be engineered by means of the surface-to-volume ratio and surface roughness. Tuneable thermal properties make these structures good candidates for integrated heat management devices, for instance for waste heat recovery or heat rectification. In particular, the thermal rectification means that the magnitude of heat flux changes when the temperature gradient is reversed in direction. As a step towards heat rectification using silicon porous membranes in this work, we studied both thermal conductivity and its temperature dependence on a geometric parameter, i.e. the surface-to-volume ratio. We fabricated and characterized the set of nearly 50 samples with hexagonal lattice and periods from 2µm to 200nm. Furthermore, we report fabrication and measurement of thermal rectifier based on porous membrane PnCs of the linear gradient surface-to-volume ratio.

Resume : Due to the increase in density and performance of electronic and photonic devices, heat dissipation in multilayer assembly has to be managed carefully especially close to the heat sources. Thermally conductive thin film such as aluminum nitride are very appropriate for heat transfer enhancement. Indeed, aluminum nitride is a material with both a high thermal conductivity (theoretically up to 285 W.m-1.K-1) and a very low electrical one which makes it one of the best candidates to be integrated in photonic and electronic devices. However, its elaboration is critical since the obtained thermal conductivities are very sensitive to synthesis condition and growth mechanism of thin film which involve various crystalline qualities and microstructures. In addition as the thermal resistance of aluminum nitride thin film is very small typically in the range 5×10-8 to 10-9 m2.K.W-1, thermal conductivity is not easy to measure. Pulsed photothermal method is one of the few techniques that can provide thin film high thermal conductivity measurement. The heating is provided by a typical 12 ns short pulse YAG laser beam and the temperature measurement by using a metallic thin film deposited on the top of the aluminum nitride thin films. Using a transient thermal model based on the quadruple technique, we have shown the requirement of taking into account the real incident heat flux evolution if we want to avoid a 20% bias compared to its basic description with a Dirac delta function. Finally the estimated results for aluminum nitride layer deposited by magnetron sputtering and with final thicknesses between 785 nm and 3000 nm have provided values from 111 to 175 W.m-1.K-1 respectively. These values are much higher than the ones for silicon dioxide (1.4 W.m-1.K-1) or silicon nitride (16 to 33 W.m-1.K-1), which are traditionally used in microelectronics for semiconductor passivation.

Resume : We present a thermodynamics derivation of the thermoelectric process, based on a close loop approach. Such systems are usually quite generally characterized by two parameters: the output power and the conversion efficiency. We establish that a detailed understanding of the effects of the dissipative coupling on the energy conversion process, only necessitates the knowledge of these two quantities: the system's feedback factor and its open-loop gain, the product of which characterizes the interplay between the efficiency, the output power and the operating point of the system. In particular, we show that the feedback loop approach provides a very efficient tool for the optimization of the thermoelectric devices, including their effective thermal conductivity term.

Resume : The effect of disorder on thermal transport in silicon nanomembranes has been one of the major scientific questions in the recent years. While the up to now the majority of these studies have reported significant results in low temperatures, in this work present our latest experimental findings concerning the role of disorder in pore position and shape at room temperature supported by Monte-Carlo simulation scheme to resolve the Boltzmann equation for phonons. Measurements using two-laser Raman thermometry on both non-patterned, 100 nm-thick and two porous membranes revealed more than a 10-fold reduction of the thermal conductivity compared to that of bulk silicon and a six-fold reduction compared to non-patterned membranes for the sample with random pore shapes. Using Monte Carlo methods, we compared different possibilities of pore organization and its influence on the thermal conductivity of the samples, starting with purely circular pores and gradually introducing complexity in the pore shape by possibility of overlapping. First of all, the simulations confirmed that the strongest reduction of thermal conductivity is achieved for a distribution of pores with arbitrary shapes, for instance up to a 15% reduction of the thermal conductivity with respect to the purely circular pores was predicted. Secondly, temperature and heat flux maps were realized for the disordered samples in the temperature range between 200 and 400 K. These maps clearly showed that for particular pore placement heat transport can be efficiently blocked and hot spots can be found in narrow channels between pores, which persist at temperatures up to 400K. Our findings have an impact on the fabrication of membrane-based thermoelectric devices, where low thermal conductivity is required.

Resume : Phononic crystals are excellent candidates for the control of heat transport by lattice vibrations, in particular due to their tailored periodic structures. This control may come in the form of phononic band gaps or band flattening, and can lead to significant changes in transport coefficients. This behaviour makes silicon phononic crystals potentially excellent materials for thermoelectrics with a high figure of merit. Phononic crystals have additional application potential in the fields of noise control, ultrasound imaging and telecommunication, among others. In this work, bottom-up designed phononic crystals made from silicon nanowires and nanoparticles are simulated using Reverse Non-Equilibrium Molecular Dynamics (RNEMD). Their thermal conductances of phononic crystals with various lengths are calculated using the results of this method, and the length scaling of the conductance will be compared with those of nanowires with similar lengths and number of atoms. The suppression of thermal transport of the phononic crystals is shown to be significantly greater than that of even porous bulk silicon, indicating nanoscale effects in the phononic crystal.

Resume : Nanowires are ideal candidates for exploring the effect of low dimensionality on thermal transport due to the intrinsic 1D nature of transport in these structures. Furthermore, the reduced thermal conductivity in nanowires with diameters on the order of their phonon mean free paths or smaller is promising for applications in thermoelectric energy conversion. However, measuring the thermal conductivity of single nanowires is a non-trivial task. A wide range of measurement schemes is currently being used, all with their respective advantages and disadvantages. We present a new method based on Raman thermography and a resistive heater. The single nanowire is cantilevered on an electric heater and suspended in air. The heat flowing through the wire is controlled by the convective cooling of the wire, which is modelled as a heat fin. By measuring the temperature profile over the wire using Raman thermography the thermal conductivity of the wire can be extracted. With this approach, interesting information about conductive and convective heat transfer at the nanoscale can be derived. This novel method has been tested on GaAs and InAs nanowires by comparing the results with those we obtained by other Raman, laser-heating based approaches and with those available in literature, obtaining in both cases a good agreement.

Resume : Thermoelectricity was a cornerstone in near-equilibrium irreversible thermodynamics. Conversely, classical irreversible thermodynamics (CIT) with its local equilibrium hypothesis (LEH) remains the dominating framework in thermoelectricity. In this talk two instances where CIT approach fails will be presented. Defect engineering is used to control materials thermal conductivity κ, and multiple morphological defects were reported to effectively serve the scope, suppressing phonons with various mean free paths. We studied how grain boundaries and dispersed nanovoids reduce Si κ. The co-presence of defects with different scattering lengths were confirmed to reduce κ. However, application of Matthiessen's rule even to non-gray phonon models led to inconsistencies, showing that non-local descriptors are needed to account for κ modulation. Non-instantaneous response takes a major share in time-dependent thermoelectric phenomena. While the topic is receiving growing attention, still it is mostly framed within CIT, neglecting the disputable validity of the LEH for stimuli with characteristic times comparable to system relaxation times. An approach based on extended irreversible thermodynamics (EIT) will be sustained. However, complete EIT constitutive relations are not yet available for thermoelectricity. This state of affairs will be reviewed and the hurdles hindering the writing of evolutionary equations for thermoelectricity will be commented upon along with recent results.

Resume : The Single Electron Transistor (SET) attracts currently special interest in the research community of nanoelectronics because it is promising for the future generation quantum devices. Here, we discuss the thermal conductance of the SET calculated using our theoretical model based on the thermodynamic transport equations for a dot. We discuss the dependences of Coulomb oscillations on the energy spectrum, temperature and charging energy from the quantum to the classical transport regimes. We show the effect on the thermoelectric efficiency of the SET. We compare with experimental data and show agreement with them. We explore the operation of the SET as heat switch and show that it can efficiently operate at temperatures below the charging energy.

Resume : In thermionic junctions two reservoirs are connected by an energy barrier, the height of which controls electron thermionic emission. Phonon thermal isolation plays a crucial role in the overall electro-thermal performance and it has been considered to be difficult to achieve sufficient isolation through solid-state thermionic junction due to apparent strong phonon transmission over short distances. Typical thermal isolation schemes of thermionic devices include superlattices, vacuum (gas) barriers, and small electron-phonon coupling occurring at low temperatures. In this work, we focus on our recent discovery [https://arxiv.org/abs/1809.02994] demonstrating that after all it is possible to achieve significant phonon isolation in a solid-state thermionic junction. We demonstrate that a single solid interface can operate both as an efficient thermionic element and heat transfer blockade for phonons. In the experiments we use semiconductor-superconductor (Sm-S) tunnel junctions, where the electronic thermionic emission is controlled by the superconducting energy gap and voltage bias and the phonon thermal boundary resistance at the junction provides the phonon blockade. The Sm-S junctions support, thermally isolate and electrically refrigerate a silicon chip: we observe a significant cooling of ~40 % from the bath temperature at low temperatures. We will also show how the overall electro-thermal performance can be enhanced further by utilizing phonon engineering methodologies.

Resume : We report a method using scanning tunneling microscope (STM) single molecular break junctions (SMBJ) to simultaneously measure the single molecular thermopower and electrical conductance of three test molecules, 1) oligo-phenylene ethynylene (OPE3), 2) octanedithiol (ODT), and 3) 4,4'-bipyridine (44BPY). The three measured Seebeck coefficients agree with literature values, with comparable or improved error. Good materials for thermoelectric applications must have high Seebeck coefficient and high electrical conductivity, but must also have low thermal conductivity, summarized in the thermoelectric figure of merit, ZT. Recently it has been suggested that organic molecules may be tuned to yield high ZT and surpass even the best state-of-the-art thermoelectrics. Our reported method statistically correlates the Seebeck voltage offset, electrical conductance, and stretching displacement of the single molecular junction, introducing a powerful multivariate method for studying the interdependence of two important variables contributing to ZT. This method enables future exploratory work in single molecular thermoelectrics. Our method uses the variance measured during the measurement to correct for offsets in the electronics before calculating the voltage offset due to the single molecular thermopower, reducing the variance in the voltage offset to below 1 microvolt, yielding improved precision in the measured voltage offsets, and reducing error in our calculated Seebeck coefficients.

Resume : In this study, molecular dynamics (MD) was used to simulate a single layer of germanene nanosheets. The mechanical properties of germanene nanosheets under tension were investigated at different temperatures. The results show that Young's modulus of germanene nanosheets in armchair and zigzag directions at a temperature of 300 K were about 169 and 154 GPa, respectively. The simulation revealed that the thermal conductivity of armchair germanene (23.39-60.72 W/mK ) was larger than that of zigzag germanene(12.47-44.23 W/mK). As the size was increased, the thermal conductivity was significantly increased. The effects of size and defect on the thermal conductivity of germanene nanosheets were discussed by using a non-equilibrium molecular dynamics The results are important for understanding scattering in two-dimensional systems and for practical applications of germanene nanosheets.

Resume : Carbon nanotube (CNT) forests have been successfully integrated into a wide range of thermal applications due to their high thermal conductance, mechanical compliance, and compatibility with nano-fabrication processes. However, due to the many parameters that are involved in the growth process, their morphological parameters, and specifically CNT alignment, span over a wide range. As a result, their thermal properties also show a wide range of possible values. We show in this work that CNT alignment is modified when CNT forests are subjected to high temperatures. Consequently, important thermal properties, such as thermal conductance and coefficient of thermal expansion, strongly depends on the environmental temperature. Analysis of CNT forests morphology reveals that high temperatures straighten the CNTs and therefore improve the thermal conductance of CNT forests. Moreover, we show that CNTs that are initially bent experience more significant change in their alignment compared to CNTs that were initially aligned. Therefore, this work sheds light on the interplay between the morphology of CNT forests and their thermal properties.

Resume : Knowledge of phonon mean free path (MFP) is essential for advanced thermal engineering by nanostructuring. Over the past decade, researchers have applied various theoretical and experimental techniques to probe the MFP spectra of phonons in different bulk materials. However, many phononic and microelectronic structures are based on thin suspended membranes, which are expected to have a MFP different than that of bulk materials, due to size effect. The reconstruction of the MFP distribution of suspended membranes is thus required and can be done through theoretical methods proposed in the literature from membranes. In this work, we use micro time-domain thermoreflectance technique to experimentally determine the phonon MFP measuring thermal conductivity of 145-nm-thick silicon membranes with nanoslits of the size ranging from 135 to 500 nm. As phonons can only pass through these slits, the phonon MFP can be extracted from the thermal conductivity of these membranes. We propose an improved theoretical model to extract the MFP spectra from the measured thermal conductivity values. Our theoretical approach allows to extract the spectra analytically, without inversion of large matrices, and thus is not computationally demanding. The proposed experimental and theoretical methods callow to directly probing the phonon MFP in various nanostructures and materials, which helps exploring new frontiers in phononics and thermodynamics.

Resume : Nanoimprinting with rigid molds offers almost unlimited pattern resolution, but it suffers from high sensitivity to defects, and is limited to pattering flat surfaces. These limitations can be addressed by nanoimprinting with soft molds. However, soft molds have been used so far with UV resists, and could not achieve a resolution and minimal feature size comparable to those of rigid molds. Here, we explore the miniaturization edge of soft nanoimprint molds, and demonstrate their compatibility with thermal imprint resists. To that end, we produced a pattern with 10 nm critical dimensions, using electron beam lithography, and used it to replicate nanoimprint molds by direct casting of an elastomer onto the patterned resist. We showed that the produced pattern can be faithfully transferred from the mold by thermal nanoimprinting. In addition, we showed that similar nanoimprint molds can also be produced by double replication, which includes nanoimprinting of a thermal resist with an ultrahigh resolution rigid mold, and replication of a soft mold from the imprint pattern. We also demonstrated our novel nanoimprinting approach in two unconventional applications: nanopatterning of a thermal resist on a lens surface, and direct nanoimprinting of chalcogenide glass. Our novel nanoimprint approach pushes the envelope of standard nanofabrication, and demonstrates its potential for numerous applications impossible up to now.

Resume : We have measured the in-plane and the cross plane thermal conductance of a thick Silicon wafer. The experiments are performed between 0.3 K and 2 K using the direct method and the heat pulse technique devised by Maldonado [1]. Since the thermal conductance is dominated by boundary scattering in this temperature range, the specularity factors and the geometrical dependency of the thermal conductance are compared to predictions of the Casimir model [2]. This work is complemented with a simulation study to reveal the importance of the thermal contact resistances between the Si wafer and the Cu sample holder. [1]. O. Maldonado, Cryogenics 32, 908 (1992) [2]. H. B. G. Casimir, Physica V, 6, (1938)

Resume : Heat dissipation is investigated for various confined systems on monocrystalline silicon substrate heated by a local source, the thermal contact between the sample and a scanning thermal microscopy (SThM) probe that is operated in active mode. Three types of samples were studied: (1) bulk materials of well-known roughness and physical properties, (2) samples consisting of several sets of silicon surfaces with out-of-plane Sq roughness parameters of ~0, 0.5, 4, 7 and 12 nm, and (3) deposited amorphous carbon monoatomic layers. Two different surrounding environments and various types of resistive probes with different sizes and materials of probe apex were used. Simultaneous heating and measurement of the probe temperature provide information on the effective thermal conductance at the probe/sample contact and of the probed material. Experimental values of these thermal conductances are compared with those estimated using standard heat diffusion modeling and modeling accounting ballistic heat conduction. Results show that the regime of heat dissipation depends on the ratio of the phonon mean free path in the sample material to the heating source size. They also demonstrate that roughness induces a thermal conductance decrease at the contact up to 30% compared to a flat silicon sample depending on the probe and environment. First results on amorphous carbon monoatomic layers on silicon substrates are discussed

Resume : Nano-composites composed of c-GaN nano-sphere in a a-SiO2 matrix can provide a good thermal insulator without impairing the electrical conductivity. We focus on the thermal conductivity at the nanoscale of such nano-composites, indeed a previous study found that at these sizes the Effective Medium Approach (EMA) fails to predict the thermal conductivity. Molecular Dynamics simulations of such system have shown percolation effects for a given relative orientation of inclusions*. To further study this phenomena, we used a wave packet method, in which we sample the propagation and diffusion of energy over the excitation pulse frequency. Theses characteristics can then be used in conjunction with the Vibrational Density Of States (VDOS) to compute the thermal conductivity. With decreasing nano-spheres radius the gap between acoustic and optic phonons in the VDOS seems to be more populated, relative to the total number of modes. This effect seems to be increased if the inclusion are trapped in a SiO2 matrix. More over in contrast with the metallic nano-sphere the size reduction seems to induce a red shift and not an over all spectral spreading. This VDOS variation with inclusion size can help to understand the failure of EMA to estimate the thermal conductivity, which assumes size independent material properties. *K. Termentzidis, V. M. Giordano, M. Katsikini, E. Paloura, G. Pernot, D. Lacroix, T. Karakostas and J. Kioseoglou,Nanoscale, 2018, 10, 21732–21741.

Resume : With the rapid evolution of the elaboration methods of new materials and novel nano-architectured devices during the last decade, it is nowadays possible to fabricate series of nanoporous materials with functionalized internal surfaces with tunable properties for fluids. Specifically, porous silicon has a very large surface to volume ratio and a thermal conductivity considerably hindered compared to the bulk silicon. The impact of different morphological parameters as: porosity, spatial and size distribution of the pores, their shapes and the amorphisation/oxidation of their free surfaces on the thermal properties have been explored both experimentally and theoretically. However, in all these studies, structures are supposed to be dry, which is not the case in the “real life”. In the latter context heat transfer in such systems depends on the matrix but also on the presence or not of confined water and solid/liquid interfaces. Here, we studied the thermal conductivity of porous silicon and nano-hybrid “porous silicon/water” systems with the use of equilibrium molecular dynamics technique. We revealed large thermal conductivity enhancement in the nano-hybrid systems as compared to dry porous sample which cannot be captured by effective media theory. We found that the rise of thermal conductivity is related to the increases of pore’s specific surface and the presence of adsorbed liquid layer.

Resume : Heat assisted magnetic recording (HAMR) is a future hard disk drive (HDD) technology for extremely increasing in areal density. The tunneling magnetoresistive (TMR) device structure, located inside the magnetic read-write head of a hard drive is the most complex and sensitive component which contains the sensitive components such as a touch down sensor and a near field transducer. The laser using in writing process might cause degradation of magnetic head components. The effects of laser irradiation on TMR device has been investigated using quasi-static test (QST) measurement to observe the characteristic changes of magnetic-response sensitivity function. The results showed only the amplitude degradation, as the resistance and asymmetry parameters were unchanged. Nanostructure of the degradation TMR devices was investigated by using scanning tunneling electron microscopy (STEM) and X-ray energy-dispersive spectrometry (XEDS). It was found that the manganese depletion in the antiferromagnetic layer causes non-active zone due to poor magnetic coupling pinning function and reduce the functioning of the HDD. The ratio of active and non-active zone elements has a strong correlation to the amplitude degradation of individual TMR devices. The TMR structure with thin strip height can have a higher risk on device degradation.

Resume : > > > In the last years several experimental setups using pump-probe or thermoreflectance imaging, have allowed to obtain the behavior of heat transport at reduced sized and in fast heating rates. Most of these new experiments are done on metal-semicoinductor interfaces. Because of this, the thermal boundary resistance (TBR) appearing at the interface between two materials has been an object of extensive study during last years. Nevertheless, present models of this magnitude like diffuse mismatch model (DMM) or acoustic mismatch model (AMM) do not predict the experimental value. In some of these works, phonon hydrodynamics has appeared as an alternative to Fourier law to describe the thermal transport at the nanoscale. The kinetic-collective model (KCM), including hydrodynamic effects in its expressions, allows discern the contribution of the TBR from other phenomena like nopnlocal effects in the heat flux near the contacts. Combining KCM with an expression for the TBR depending on the specularity of the interface and intrinsic properties of the materials we are able to describe the thermal behaviour of Metal-Semiconductos interfaces without using fitting parameters. In the limit of completely diffusive interface, we recover the DMM expression. All these parameters are obtained from ab initio techniques and are independent of the geometry of the experiment. These results give the possibility to go further in the study of the TBR without attributing other phenomena to it.

Resume : The 100 nm, 280 nm, and 380 nm CuPc films were deposited on BK7 substrates by PVD method in a high vacuum. Different thicknesses resulted in their different morphology. The CuPcs were studied by the AFM, then analyzed using the RMS, mean grain size and total perimeter length (TPL). The surface morphology reveals the increase of RMS and the grain size with the increase of CuPc film thickness. The TPL shows opposite relation. The SThM was used to investigate local thermal properties of CuPc films. Two different kind of probes with different resolution were applied. The PSIA XE-70 with nanothermal probe with radius of the tip equal to 100 nm was used to determine its dynamic and static resistance in contact and out of contact with the sample surface. The ratio of static and dynamic resistances differences measured in contact and out of contact with the sample surface depends on the thermal conductivity of this sample. While, the microscope 2990 MicroTA equipped with Wollaston wire with resolution of 1000 nm was used for measurement of a heat flux from the probe to the surroundings, expressed by the power (P) dissipated on this probe. For each CuPc film the difference ΔP between probe signals measured in contact (Pc) with the sample surface and in the air (Pa) was determined. The relative difference ΔP/Pc related with thermal conductivity of the sample was calculated. Both types of SThM reveal the dependence of CuPc thermal properties on the film thickness. Regardless of the type of SThM probe, the thermal properties deteriorate with the increase of film thickness.

Resume : We rederive the Green-Kubo relation establishing a connection between the near- and far-field heat transfer between two objects out of equilibrium to the equilibrium fluctuations of these objects in an arbitrary environment. Employing the scattering approach in combination with the fluctuation-dissipation theorem, we generalize the previously derived Green-Kubo expression to the case of non-reciprocal objects and non-reciprocal environments.

Resume : Ballistic heat conduction in semiconductors is a remarkable but controversial nanoscale phenomenon, which implies that nanostructures can conduct thermal energy without dissipation. Signs of such unusual heat conduction appear in experiments on nanowires, carbon nanotubes, graphene ribbons and other nanostructures. However, the experiments usually involve non-negligible thermal contact resistance, thus the ranges of length and temperature at which ballistic heat conduction takes place remain unclear. Here, we experimentally probed ballistic thermal transport at distances of 400 – 800 nm and temperatures of 4 – 250 K using contactless experimental technique. Measuring thermal properties of straight and serpentine Si nanowires, we found that at 4 K heat conduction is quasi-ballistic with stronger ballisticity at shorter length scales. At higher temperatures, we observed how quasi-ballistic heat conduction gradually turned into diffusive at temperatures above 150 K. Our Monte Carlo simulations show how this transition is driven by different scattering processes and linked to the surface roughness and the temperature. These results demonstrate the length and temperature limits of quasi-ballistic heat conduction in Si nanostructures, knowledge of which is essential for thermal management in microelectronics.

Resume : In the diffusive regime thermal transport is governed by Fourier’s law, which states that a temperature difference across a system results in a linear temperature profile. Contrarily, in the ballistic regime, the wave nature of phonons allows for the transmission of energy without dissipation, leading to a constant temperature profile. However, ballistic phonon transport remains elusive due to the short mean free path of heat carrying phonons at room temperature. Here we report on the experimental verification of ballistic heat transport at room temperature in pure GaP nanowires, grown using the vapor-liquid-solid technique. Thermal conductivity measurements using suspended Pt heater/thermometers show a thermal conductance that decreases with length for thick wires (d≥50 nm), in accordance with Fourier’s law. However, the thermal conductance was found to be independent of the length for thin wires (d≤25 nm), in accordance with ballistic transport. This behavior persists up to the longest nanowires (15 micron). To further corroborate these results, the local temperature profile has been measured using spatially dependent Stokes Raman thermometry. A linear profile along the nanowire was found in the diffusive case, while a constant temperature was found in the ballistic case. This is the first measurement of the local temperature profile in ballistic systems. This research could open up the way for more efficient cooling at the nanoscale, as well as phonon-based logical devices.

Resume : The development of new experimental techniques designed to measure the thermal response of semiconductors at reduced length and time scales is revealing the requirment of generalized heat transport models beyond the classical Fourier law [1,2]. We present the Kinetic Collective Model (KCM), consisting in the hydrodynamic heat transport equation with ab initio calculated coefficients and the corresponding boundary conditions [3,4]. The model equations can be solved using Finite Elements to predict the thermal response of electronic devices with complex geometries including interfaces at reduced length and time scales. We compare the model predictions with experimental data on Frequency Domain Thermoreflectance Experiments (FDTR) in Silicon and with the effective thermal conductivity of Silicon thin films and holey films [3]. New phenomenology as phonon viscosity and vorticity arise from the transport equation by analogy with the Navier-Stokes equation for fluids, wich allow new physical insight of the non-Fourier effects taking place at the nanoscale due to boundaries and interfaces with arbitrary geometry. References: [1] A. Ziabari et.al., Nat. Comm. 9, 255 (2018). [2] K. M. Hoogeboom-Pot et. al., PNAS 112 16 4851 (2015) [3] A. Beardo et al. Phys Rev. Applied 11, 034003 (2019) [4] P. Torres et al. Phys Rev. Materials 2, 076001 (2018) [4] Y. Guo et. al., Phys. Rev. B 93, 035421 (2018) [5] R.A. Guyer et. al. Phys. Rev. 2, 148 (1966)

Resume : Heat management is essential in many microelectronic devices, the components of which have been scaled down to the nanometer. When the characteristic size becomes comparable to the mean free path of energy carriers (phonons in dielectrics), interactions with boundaries dominate thermal transport, and the quasi-ballistic heat conduction regime arises: Fourier’s diffusive law is no more valid to describe thermal transport. In this contribution, we investigate the deviation from the diffusive regime in different configurations. To this end, we solve the Boltzmann Transport Equation (BTE) for phonons using the Discrete Ordinate Method (DOM) [1] in (i) cylindrical and (ii) 2D Cartesian geometries, in the isotropic-crystal and single relaxation time approximations. This method accounts for the directional and non-local aspects of the energy flux [2]. In the cylindrical geometry, we simulate (a) thermal transport in a nanowire and (b) heat dissipation from a thin disc into a substrate. The second case is reminiscent of surface heating by laser or by the hot tip of a scanning thermal microscope in contact with the sample. There are two effects resulting in deviations to the Fourier temperature profile: ballistic transport and thermal boundary resistances as well. Thus, the dissipated heat fluxes are reduced in comparison to Fourier prediction, and we can estimate the effective thermal conductivity and quantify its reduction comparing to the bulk material. We aim at quantifying the impacts of the two effects. We also consider heat dissipation from a surface finite source in a 2D Cartesian geometry, a situation that mimics thermal losses from metallic lines deposited on top of surfaces [3]. Temperatures jumps at boundaries appear and lead to overprediction of the flux dissipated in the sample. We present a way to consider large spatial domains which cannot be computed only by the resource-costly BTE method. Coupling between small-scale BTE and a large-scale FEM simulations is introduced. The impact of hot spots close to boundaries on the device performances is finally discussed. References: [1] Ref D. Lemonnier Volz [2] Thermal transport phenomena beyond the diffusive regime, P.-O. Chapuis et al., Proceedings of MIXDES 2016. [3] W. Jaber, PhD thesis (2016), INSA Lyon. Acknowledgements: The support of project EFINED (H2020-FETOPEN-1-2016-2017 766853) and ANR TIPTOP is acknowledged. We address a special thank to D. Lemmonier (Pprime).

Resume : Femtosecond time-resolved optical spectroscopy is used to investigate the ultrafast thermal cooling dynamics of nanometric objects after an initial impulsive heating. At this size scale, the thermal resistance (also called Kapitza resistance) at the interface between the nano-objects and their environment (substrate, liquid or glass matrix) plays a major role, limiting the thermal energy flow out of the nanoparticles. Connecting the ultrafast optical response to the temperatures variations of the nano-objects and of their environment is thus essential for quantitative extraction of thermal parameters – such as thermal interfacial resistance and local environment thermal conductivity – from optical experiments. We present here pump-probe optical measurements performed on metal nano-objects (e.g. gold and silver nanospheres) in dielectric environments, together with a full theoretical model which includes both the nano-objects thermal dynamics (modelling of temperature decrease computed by solution of Fourier law in the presence of a thermal interface resistance) and their optical response (transient optical absorption modifications induced by particle and environment temperature variations). This experimental and theoretical approach provides reliable extraction of thermal parameters, thus allowing investigation of the dependence of thermal interfacial resistance on material properties and also opening the way for studying non-Fourier heat conduction at the nanoscale.

Resume : The use of CO2 as a natural refrigerant in data center cooling and CO2 capture and storage is gaining traction in recent years which involves heat transfer between CO2 and the base fluid. Also, the high viscosity CO2 is of interest to the oil and gas industry in enhanced oil recovery and well-fracturing applications. A need arises to improve the thermal conductivity and viscosity (thermo-physical properties) of CO2 to increase the efficiency of the few mentioned applications. Aggregation of nanoparticles, one of the crucial mechanisms to improve the thermal conductivity and viscosity of nanofluid. The nanofluid in this aggregation study consists of alumina (Al2O3) nanoparticles and CO2 as a base fluid. However, the aggregation morphology of nanoparticles remains unclear so far. We have evaluated the stable configurations of the aggregation of nanoparticles by determining potential energy of the different configurations system and by fractal dimension. In this paper, Green-Kubo formalism is used to calculate the thermo-physical properties of the different aggregated nanofluid. The fractal dimensions of the aggregations of different configurations are obtained by Schmidt-Ott equation. Comparisons of the fractal dimension and potential energy of the system with thermo-physical properties of the nanofluid show us that, lower fractal dimension and potential energy can conceive in achieving greater thermo-physical properties. The results also mark that various morphologies of the aggregated nanoparticles have different enhancements of thermo-physical properties of the nanofluid. This study is conducive for the researchers to perceive the importance and influence of aggregation morphology of nanoparticles and their stability on the thermal conductivity and viscosity of nanofluid.

Resume : Optomechanical (OM) structures are potential building blocks for phononic circuits and quantum computing, and a huge focus is put on non-linear applications, with features such as phonon lasing, chaos and high sensitivity readout. Although the coherent phonon modes of OM structures are usually limited to a few GHz, the impact of the thermal properties remains critical in their operation. In this work, I will present where the thermal properties of our OM cavities come into play when it comes to amplifying confined mechanical modes, by comparing the optomechanical properties of crystalline and poly-crystalline silicon, which display largely different thermal conductivities, but also a switching function based on the photothermal effect. We designed one dimensional silicon nanostructures that can emit coherent phonons up to 5 GHz. Here, we use the photothermal effect induced by an external laser to modulate the coherent phonon emission properties of these OM cavities, effectively switching the phonon emission on and off. As this effect depends on the temperature of the cavitiy, it benefits from the reduced thermal conductivity of the nanobeam as compared to bulk silicon. We then assessed the structural and thermal properties of different nanocrystalline-silicon-on-insulator nanobeams, is a more versatile platform, with a TEM analysis and thermoreflectance measurements. More efficient absorption and lower thermal conductivities than crystalline silicon lead to a higher temperature of the OM cavity, and therefore to more stable phonon emission states and higher mechanical quality factors.

Resume : The thermal conductivity of Ge-rich GeSbTe phase change alloy has been measured in the configuration of thin-films using the modulated photothermal radiometry technique. This kind of material allows to postpone the phase-change temperature at higher value that the classical 225 stoichiometric configuration, making the application to phase-change memory reliable within the configurations of high temperature applications. The investigated temperature range, from room temperature up the the melting one, allows observing the phase-change from amorphous to crystalline state. It has been clearly observed that the phase-change temperature varies according to the film thickness. A model, based on the Boltzmann Transport Equation within the the phonon relaxation time assumption shows the contribution of Umklapp and defects scattering processes at high temperature.

Resume : With 2D materials such as graphene (GR) and hexagonal boron nitride possessing highest known thermal conductivities, one-atom thick nature of these materials makes thermal transport in them drastically dependent on the local environment. Moreover, the equally extraordinary electronic properties of GR such as relativistic carrier dynamics combined with GR highly anisotropic thermal conductance may point to unusual thermoelectric properties. In order to study thermal and thermoelectric phenomena in these nanoscale materials, we applied scanning thermal microscopy (SThM) that uses a sharp tip in contact with the probed surface that can create a controlled local sample temperature rise in the few nm across spot, while measuring the resulting sample temperature and a heat flow. We used high vacuum environment that eliminates spurious heat dissipation channels to boost accuracy and sensitivity and to allow cryogenic measurements. We show that the thermal resistance of GR on SiO2 is increased by one order of magnitude by the addition of a top layer of MoS2, over the temperature range 150-300 K with DFT calculations attributing this increase to the phonon transport filtering in the weak vdW coupling and vibrational mismatch between dissimilar 2D materials. By measuring the heat generated in the nanoscale constrictions in monolayer GR devices, we have discovered unconventional thermoelectric Peltier effect due to geometrical shape of 2D material and not requiring a junction of dissimilar materials, with phenomenon confirmed by measuring the Seebeck thermovoltage map due to local heating by the SThM tip. The novel nonlinear thermoelectric phenomena due to “electron wind”, and effects of GR doping and layer number are also reported.

Resume : The problem of cooling of the microelectronic devices became more important in the last decades due to a miniaturization which requires to remove excess heat on the microscale. This causes an increased interest in finding the microstructures with special thermal properties. Recent theoretical studies [1,2] of heat conduction on the microscale showed that 1D and 2D low defect harmonic structures can have desired properties. In these works, the analysis of lattice dynamics is performed. This approach allows to describe analytically the transient and steady heat conduction in 1D and different 2D crystal lattices. It is shown that on the microscale the heat conduction has wave-like properties and qualitatively differs from what one can observe on the macroscale. This feature can be used to solve technological issues discussed above. This approach can describe wide variety of different materials such as carbon nanotubes, nanowhiskers, graphene, two-dimensional hexagonal boron nitride, two-dimensional molybdenum disulfide. Especially graphene attracts a lot of interest since it shows outstanding heat and electrical conduction properties. Such properties make graphene a good candidate for the industrial applications. Different techniques are used to measure heat conduction properties of graphene. Scanning Thermal Microscopy (SThM) is a good choice to investigate the steady temperature distribution since SThM sensors have high spatial resolution. As obtained in the recent numerical and analytical calculations transient and steady heat profile in a graphene heated by a point source differs from what is predicted by classical theory, in particular the anisotropy is observed. In the current work we focus on the steady heat conduction. From the experimental point of view, we will use SThM to measure the steady temperature profile in the suspended circular graphene membrane which is heated in the center by a laser. The goal of the planned experiment is 1. qualitatively detect anisotropy of temperature profile and 2. distinguish it quantitatively from the Fourier law. The results of the experiments will be compared with ballistic heat equations presented in [1,2]. References: [1] Krivtsov, A. M. (2015). Heat transfer in infinite harmonic one-dimensional crystals. In Doklady Physics (Vol. 60, No. 9, pp. 407-411). Pleiades Publishing. [2] Kuzkin, V. A. (2019). Thermal equilibration in infinite harmonic crystals. Continuum Mechanics and Thermodynamics, 1-23.

Resume : The two-dimensional materials with interlayer van der Waals (vdWs) bonding such as graphene that be exfoliated from graphite have arisen tremendous interests in researching their properties. Recently, layered materials consist of transition metal trichalcogenides with strong in-plane anisotropy have been successfully synthesized so that we can unearth their unusual properties and potential applications through experiment and theory. For instance, titanium trisulfide nanoribbons with intralayer covalent bonding and interlayer vdWs bonding burst extensive research due to theoretically calculated ZT value (3.1) along y-direction at moderate carrier concentration. These results prompt us to disclose the intrinsic physics of lattice thermal conductivity and electron transport through the first-principles calculations and experiment. We obtain thermal properties of titanium trisulfide by means of performing first-principles calculations to get second- and third-order interatomic force constants and implementing ShengBTE code for solving the phonon Boltzmann transport equation to estimate the lattice thermal conductivity. The results reveal that thermal conductivity tinily decreases with the number of titanium trisulfide layers increasing from 1 to 3 and displays a strong anisotropy in the temperature lower than 200K. In particular, the out-of-plane thermal conductivity is around 1.05 W/m-K at the room temperature which is 5 times lower than graphite(5.5 W/m-K) due to its weaker intralayer covalent bonding. Besides the theoretical calculation, we also synthesized the titanium trisulfide nanoribbons to measure the thermal conductivity using a suspended micro-thermometry from 20 to 300 K. The results show that thermal conductivity of titanium trisulfide nanoribbons is decreasing as the thickness increasing and when the thickness reach to 52nm, the thermal conductivity is close to the value of bulk counterpart. Moreover, the thermal conductivity sustainably decline even though thickness up to 52nm rather than maintaining a constant like graphene. We speculated this phenomenon is mainly arise from high frequency phonon induced tardily by the cross-layer coupling. In order to elucidating physical mechanism, we measured more nanoribbons to find the relationship between the thickness and thermal conductivity. In general, Thermal conductivity decays with the thickness up to 272nm.The intriguing thickness dependence is that thermal conductivity rapidly declines as thickness below 60nm and slowly decays beyond 150nm. Due to the computational cost, 4 layers or thicker titanium trisulfide cannot be calculated. The giant large layer thickness dependent in-plane thermal conductivity is surprising and confusing. We speculated that this phenomenon may stem from weak interlayer coupling strength and unique atomic structure, which needs further investigation.

Resume : Thermal management in perovskite devices is expected to be of outstanding importance. As their thermal conductivities, thermal diffusivities, and heat capacities are low, the experimental assessment is challenging, and only a few studies exist [1]. Thermal transport investigations on caesium lead bromide perovskite thin films, which are re-crystallized by the application of heat and pressure will be presented. For comparison their single crystalline analogues, which are grown by anti-solvent vapour crystallisation, have been studied. We simultaneously mapped the topography, thermal conductivity(CsPbBr3/CsPb2Br5: 0.43W/mK/0.33W/mK), thermal diffusivity (both 0.3mm2/s), and the volumetric thermal capacity(1.3J/(cm3∙K)/1.1J/(cm3∙K)) with a high spatial resolution (typ. 100nm) to understand the thermal properties in dependence of the dimensionality, using an advanced 3w-technique in the frequency domain applied to a Scanning Near-field Thermal Microscope [2]. In addition to heat transport analyses in dependence of the crystal orientations, the influence of temperature-depended phase-transitions (3D: orthorhombic - tetragonal - cubic) on the thermal properties are measured and the consequences will be discussed. Our results demonstrate, that thermal management of all-inorganic halide perovskite devices is a challenge and requires particular attention. [1] Heiderhoff, R. et al., J. Phys. Chem. C 2017, 121, 28306. [2] Haeger T. et al., J. Phys. Chem. Lett. 2019, Just accepted

Resume : Scanning thermal microscopy (SThM) is a key thermal characterization tool, where a micro to nanometer-scale probe tip is used to measure the temperature close to a surface. The spatial and temperature resolutions of this technique are limited in particular by the size of the tip, the sample properties, and the various tip-sample heat transfer mechanisms [1] depending on the operation conditions. In this communication, we present a study of heat transfer between a sample surface and a SThM probe based on resistive thermometry: the electrical resistance of the probe depends on its temperature. The probe operates in thermal-property measurement (active) mode where significant Joule heating is needed to allow thermal power flowing into the sample, depending on its thermal conductivity. We study especially the hot tip-cold sample (active mode) heat exchange as a function of tip-sample distance [2]. When the tip is far from the surface (>100 micrometers), it is cooled by heat convection and there is no exchange with the sample. At lower distances, the diffusive heat exchange starts. The exact geometry is required to compute accurately the transfer, which can be done by means of Finite Element Modeling (FEM). In the sub-micrometer distance regime, air conduction cannot be modeled by usual FEM because the air mean free path (~60 nm) is on the order of the average distance between the hot object and the sample. In our simulations, we account for this deviation to the diffusive regime by adding thermal resistances on surfaces, which correspond to the ballistic transfer limit in air. This allows us to reproduce numerically the ballistic leveling off seen experimentally. Depending on the probe considered, a 1D approximation [3] or improved considerations based on the Boltzmann transport equation for air molecules are to be taken into account. We discuss in particular the impact of various tips with curvature radii ranging from a micron down to 10 nm. We believe that this work, where both the understanding of the physical mechanisms responsible for thermal transport between the tip and the samples and precise knowledge of the energy balance of the tip-sample systems are targeted, provides a decisive step for determining quantitative data from the experiments.

Resume : The effective thermal conductivity of complex insulating materials like glass wool drastically depends on their micro-structure. There is a need for microscopic measurement of thermal transfer in and between thermal insulating elements, both in air and in vacuum. Numerical studies showed that thermal transfer in fibrous insulating materials is driven by a few number of fibers and their contacts. Besides it is proven that the radiative heat transfer grows drastically near to contact at distances shorter than the thermal wavelength. Understanding the contact and near to contact regimes between micrometer-sized insulating objects is strongly needed, but measuring it is very challenging for low conductivity materials. We propose two complementary characterization techniques to achieve this goal. First, we developed a contactless pump/probe optical method, which uses a scanning heat source produced by short ultra-violet (UV) laser pulses on the sample while measuring the resulting time-dependent infrared (IR) thermal radiation. This allows to beat the diffraction limit for IR thermography. Second, we measure the thermal resistance between a heated micrometer-sized glass bead and a glass planar surface using the probe of a Scanning Thermal Microscope (SThM) with a bead attached to it. The probe serves both as a heat source and as a temperature sensor. In this way, we can vary in a highly controlled manner the bead-plane separation, from hundreds of micrometers to the contact.

Resume : Scanning Thermal Microscopy (SThM) is considered as a key tool for measurements of local thermal properties of nanostructured materials such as: nanowires, nanotubes, thin films (crystalline, amorphous, nanoporous), etc. SThM technique, it is based on the mapping of a sample surface by a sharp temperature sensor (tip). However, one of the major concern with this thermal microscopy is to propose accurate model to relate experimental output (3ω voltage) to a thermal value (temperature, thermal conductivity). In this framework, our purpose is to investigate heat dissipation within nanostructures, image their surfaces and provide a reliable model to measure the thermal conductivity. Although significant improvements have been accomplished in scanning thermal microscopy, achieving quantitative measurements using this technique remained ambiguous. These limitations of SThM often appear when working in ambient environment because of the heat conduction through air and liquid meniscus arising between the tip and the sample. In order to overcome these limitations, we investigate in this work the heat transfer between a resistive nanoprobe (50nm) and different samples under vacuum conditions; in this frame heat transport is dominated by solid-solid contact. First, we investigate thermal transport between the tip and different nanofilms (Si and SiO2 nanofilms, nanoporous Si, etc.) using 3ω-SThM in a large frequency range and for several heating currents. These studies are carried out at ambient pressure and in vacuum putting forward the differences existing between both transport regimes but also theirs perks and limits. In a second part, these results are compared with detailed modelling based on FEM simulations of the tip-sample interaction during 3ω SThM operation. Studying the contact mode in vacuum environment reveals to be challenging but it was shown to provide a larger temperature drop and achieve higher thermal resolution. Detailed modelling is also a main asset of this work, relating experimental data to physical properties.

Resume : The development of micro- and nanofabrication has shown great potential for future heat management technologies. One of the prime example is integrated devices, where removal of heat accumulation is one of the big challenge in order to improve the lifetime of devices. Therefore, realisation of thermal transport in nanoscale devices has become important for the development of novel electronic, optoelectronic and thermal devices. Over many years, various thermal measurement techniques have been developed, but precise thermal measurement at very small length scale is still a challenge. So, there is currently a growing need for new experimental tools with high sensitivity to study temperature and thermal conductivity at low dimensions. In this respect, new instruments have to be developed to fill those requirements. The Scanning Thermal Microscopy (SThM) is one of them. This instrument consists in using an Atomic Force Microscopy (AFM) environment and in having a probe equipped with a highly sensitive thermometer. Here we propose to use a resistive thermometry based on niobium nitride (NbN) as developed at Institut Néel, to functionalize a thermometer at the apex of the AFM tip. This NbN based thermometer has shown temperature coefficient of resistance (TCR) up to 〖10〗^(-2) K^(-1) at room temperature, a value ten times higher than currently existing resistive thermometer on SThM probes. This will allow strong improvements of the thermal sensitivity of SThM. Also, we use a proper thermal design of the cantilever in order to improve the technique. Indeed, currently available cantilevers have not been fully optimized, which results in significant thermal losses in SThM probes. We will show how we optimize the deposition of a NbN thermometer on AFM probes (SThM probes) for nanoscale quantitative thermal measurements. The first calibration measurements will be presented as a function of temperature along with characterization of the thermometry itself. The perspectives rely on demonstration of the new capabilities of this new resistive SThM probe on micro and nanostructured materials and systems.