Jeremie Maire

Investigating the temperature dependence of the Seebeck coefficient in low-dimensional materials provides an effective means to uncover how crystallinity, size, and thickness influence thermoelectric performance. Such insights are crucial for advancing large-scale thermoelectric devices and Seebeck-based sensors. To enhance measurement precision and efficiency, we developed a microchip platform specifically tailored for accurate Seebeck coefficient measurements in low-dimensional systems. The chip accommodates a broad range of samples, including both polycrystalline thin films (chemically or physically deposited) and mechanically exfoliated two-dimensional (2D) crystals. It integrates micro-heaters, temperature sensors, and voltage electrodes, enabling the generation of controlled in-plane temperature gradients and the simultaneous measurement of the induced Seebeck voltage (ΔV). Finite-element simulations support the formation of stable and tunable temperature differences across the chip. To validate the platform, we first measured Bi2Te3-based thin films, obtaining Seebeck coefficients consistent with reported literature values, thereby confirming the accuracy and robustness of the setup. Subsequently, mechanically exfoliated crystalline SnSe2 flakes were investigated. Their Seebeck coefficient exhibited clear dependencies on both thickness and temperature, the absolute value increasing from −349 ± 20 μV/K at 100 K to −521 ± 10 μV/K at 300 K in a 198 nm-thick sample. These findings highlight the strong influence of dimensionality on thermoelectric transport in SnSe2. Overall, this work establishes a versatile platform for evaluating emerging 2D materials and their heterostructures, facilitating both fundamental research and the engineering of efficient 2D material-based thermoelectric devices.

Measuring temperature fields in semitransparent media requires the knowledge of material thermo-optic properties. While the current techniques are well established for isothermal thin films, they remain unexplored for thick media where temperature gradients need to be considered. This study proposes to generalize the current methods of infrared heterodyne photothermal imaging to thick media by combining transmittance measurements and thermo-optic modeling to measure simultaneously the thermoabsorbance and thermoreflectance coefficients in semitransparent material. In this work, we demonstrate the validity of our methodology followed by the measurements of the thermo-optic properties of several materials: soft polymers (polydimethylsiloxane) and borofloat glasses.

La mesure des champs de température dans les milieux semi-transparents nécessite une connaissance des propriétés optothermiques. Si les techniques actuelles sont bien établies pour les couches minces isothermes, elles restent inexplorées pour les couches épaisses présentant des gradients de température. Cette étude propose de généraliser les méthodes actuelles vers les milieux épais en combinant la mesure de transmittance par imagerie photothermique hétérodyne en transmission (TPHI) et sa modélisation optothermique. Dans ce travail, nous montrons ainsi la validité de notre modèle, puis la mesure des coefficients de thermotransmittance et de thermoreflectance pour 3 matériaux différents.

The “Heart of Voh”, immortalized by Yann Arthus Bertrand in his book The Earth from the Air, depicts a sparse, heart-shaped clearing in the mangroves of New Caledonia. This highly poetic image is also the physical representation of thermal diffusion phenomena disrupted by fluid flow. This type of figure is a basic figure for analyzing source fields in a microfluidic channel surrounded by solid walls. Here, several analytical solutions will be presented and used for the estimation of crucial parameters related to the thermal diffusivity of the walls around the channel and the fluid flow inside the channel.

Scanning Thermal Microscopy (SThM) has become an important measurement technique for characterizing the thermal properties of materials at the nanometer scale. This technique requires a SThM probe that combines an Atomic Force Microscopy (AFM) probe and a very sensitive resistive thermometer; the thermometer being located at the apex of the probe tip allows for the mapping of temperature or thermal properties of nanostructured materials with very high spatial resolution. The high interest of the SThM technique in the field of thermal nanoscience currently suffers from a low temperature sensitivity despite its high spatial resolution. To address this challenge, we developed a high vacuum-based AFM system hosting a highly sensitive niobium nitride (NbN) SThM probe to demonstrate its unique performance. As a proof of concept, we utilized this custom-built system to carry out thermal measurements using the 3ω method. By measuring the V3ω voltage on the NbN resistive thermometer under vacuum conditions, we were able to determine the SThM probe’s thermal conductance and thermal time constant. The performance of the probe is demonstrated by performing thermal measurements in-contact with a sapphire sample.

Thermal considerations affect the performance of most microsystems. Although surface techniques can give information on the thermal properties within the material or about buried heat sources and defects, mapping temperature and thermal properties in three dimension (3D) is critical and has not been addressed yet. Infrared thermography, commonly used for opaque materials, is not adapted to semi-transparent samples such as microfluidic chips or semiconductor materials in the infrared range. This work aims at answering these needs by using the variations of transmittance with temperature to obtain information on the temperature within the thickness of the sample. We use a tunable mid-infrared light source combined with an infrared camera to measure these variations of transmittance in a glass wafer. We couple this technique with a thermal model to extract the thermotransmittance coefficient—the coefficient of temperature variation of the transmittance. We then introduce a semiempirical model based on Lorentz oscillators to estimate the temperature-dependent optical properties of our sample in the mid-IR spectral range. Combined with the measurement, this paper reports the spectroscopic behavior of the thermotransmittance coefficient in the mid-IR range and a way to predict it.

Glass microspheres are of great interest for numerous industrial, biomedical, or standalone applications, but it remains challenging to evaluate their elastic and optical properties in a non-destructive way. In this work, we address this issue by using two complementary contactless techniques to obtain elastic and optical constants of glass microspheres with diameters ranging from 10 to 60 µm. The first technique we employ is Brillouin Light Scattering, which yields scattering with longitudinal acoustic phonons, the frequency of which is found to be 5% lower than that measured in the bulk material. The second technique involves exciting the optical whispering gallery modes of the microspheres, which allows us to transduce some of their vibrational modes. The combined data allow for extracting the refractive index and the elastic constants of the material. Our findings indicate that the values of those properties are reduced with respect to their bulk material counterpart due to an effective decrease of the density, resulting from the fabrication process. We propose the use of this combined method to extract elastic and optical parameters of glass materials in microsphere geometries and compare them with the values of the pristine material from which they are formed.

Contactless temperature field measurements in or at the surfaces of semitransparent media are a scientific challenge as classical thermography techniques based on proper material emission cannot be used. In this work, an alternative method using infrared thermotransmittance for contactless temperature imaging is proposed. To overcome the weakness of the measured signal, a lock-in acquisition chain is developed and an imaging demodulation technique is used to retrieve the phase and amplitude of the thermotransmitted signal. These measurements, combined with an analytical model, enable the estimation of the thermal diffusivity and conductivity of an infrared semitransparent insulator (wafer of Borofloat 33 glass) and the monochromatic thermotransmittance coefficient at 3.3 µm. The obtained temperature fields are in good agreement with the model, and a detection limit of ±2 °C is estimated with this method. The results of this work open new opportunities in the development of advanced thermal metrology for semitransparent media.