Raman thermometry is a powerful technique for sub-microscale thermal measurements on semiconductor-based devices, provided that the active region remains accessible and is not obscured by metallization. Since pure metals do not exhibit Raman scattering, traditional Raman thermometry becomes ineffective in such cases. To overcome this limitation, we propose the use of atomically thin Two-Dimensional materials as local temperature sensors. These materials generate Raman spectra at the nanoscale, enabling highly precise absolute surface temperature measurements. In this study, we investigate the feasibility and effectiveness of this approach by applying it to power devices, including a calibrated gold resistor and an SiC Junction Barrier Schottky (JBS) diode. We assess the processing challenges and measurement reliability of 2D materials for thermal characterization. To validate our findings, we complement Raman thermometry with thermoreflectance measurements, which are well suited for metallized surfaces. For example, on the serpentine resistor, Raman thermometry applied to the 2D material yielded a thermal resistance of 22.099 °C/W, while thermoreflectance on the metallic surface measured 21.898 °C/W. This close agreement suggests good thermal conductance at the metal/2D material interface. The results demonstrate the potential of integrating 2D materials as effective nanoscale temperature probes, offering new insights into thermal management strategies for advanced electronic components. Additionally, thermal simulations are conducted to further analyze the thermal response of these devices under operational conditions. Furthermore, we investigate two 2D material integration methods, transfer and direct growth, and evaluate them through measured thermal resistances for the SiC JBS diode, highlighting the influence of the deposition technique on thermal performance.

The crystallographic orientation of anisotropic 2D materials plays a crucial role in their physical properties and device performance. However, standard orientation techniques such as transmission electron microscopy (TEM) or X‐ray diffraction can be complex and less accessible for routine characterization. Herein, the orientation of black phosphorus (BP) from bulk crystals to thin layers is investigated using angle‐resolved polarized Raman spectroscopy with a single‐wavelength (514 nm) Raman setup. By incorporating thickness‐dependent interference effects and anisotropic optical indices, this approach provides a reliable framework for orientation determination across different BP thicknesses. The method is validated through direct orientation measurements using TEM and electron backscattering diffraction, confirming its applicability to both thick and ultrathin samples. Given its simplicity and compatibility with widely available Raman setups, this approach offers a practical solution for characterizing BP orientation without requiring advanced structural characterization techniques.

One of the main interests of 2D materials is their ability to be assembled with many degrees of freedom for tuning and manipulating excitonic properties. There is a need to understand how the structure of the interfaces between atomic layers influences exciton properties. Here we use cathodoluminescence and time-resolved cathodoluminescence experiments to study how excitons interact with the interface between two twisted hexagonal boron nitride (hBN) crystals with various angles. An efficient capture of free excitons by the interface is demonstrated, which leads to a population of long-lived and interface-localized (2D) excitons. Temperature dependent experiments indicate that for high twist angles, these excitons localized at the interface further undergo a selftrapping. It consists in a distortion of the lattice around the exciton on which the exciton traps itself. Our results suggest that this exciton-interface interaction causes the broad 4-eV optical emission of highly twisted hBN-hBN structures. Exciton self-trapping is finally discussed as a common feature of sp2 hybridized boron nitride polytypes and nanostructures due to the ionic nature of the B-N bond and the small size of their excitons.

Over the past decade, MRAMs developments have focused on improving magnetic tunnel junctions while using magnetic electrodes with fixed properties as spin sources. Interestingly, 2D semiconductors offer interface tailoring opportunities for spin valve devices, with many atomically thin materials now available. However, integrating them with oxidation-prone spintronics materials remains a challenge. Here, spin devices are fabricated and evaluated with large-scale MoS2 directly grown on a monocrystalline ferromagnetic spin source. While most spin transport experiments with 2D semiconductors focus on their isolated dielectric properties, the presented approach unlocks an additional spin manipulation opportunity from MoS2 hybridization with ferromagnetic electrodes. The experimental results show a substantial tunnel magnetoresistance (TMR) value of over 65%, an order of magnitude higher than previously observed for exfoliated 2D semiconductor-based devices. A non-monotonic dependence of the spin signal on the applied bias, including a sign reversal, is also uncovered, which is attributed to the modulation of the MoS2 band structure by the ferromagnetic electrode. Ab initio calculations support these findings by illustrating how the MoS2 band structure evolves upon hybridization, introducing a pronounced exchange-induced spin splitting and resulting in an unusual bimodal spin response. This study demonstrates the unique spin manipulation opportunities offered by 2D semiconductors unlocked by direct integration.

Discovering an efficient spintronic semiconductor workhorse with dual host capabilities as a channel and spin valve barrier remains one of the most elusive endeavors toward the development of spin-logic circuits. Graphene paved the way for two-dimensional (2D) materials, yet engineering a controlled band gap in it remains a challenge. Black phosphorus (BP) was recently unveiled as a potential candidate in the realm of 2D semiconductors, with carrier mobilities among the largest reported for a 2D material and a low spin-orbit coupling reminiscent of graphene. Although promising spin transport properties have already been reported, their potential for tunneling and spin injection remains uncharted. Here, we unveil an unknown spin transport mechanism spin-split in k-space and report on corresponding high magnetoresistance spin signals up to 500% in BP based spin valves. Those findings are analyzed and discussed in light of a first-principles theoretical investigation showing BP's potential for spin filtering beyond its expected role of spin transport channel. This strongly supports BP's vision as an outstanding platform for spintronics, as it could become a versatile workhorse yet unavailable with any other semiconductor.

We present a novel experimental protocol using Cathodoluminescence measurements as a function of the electron incident energy to study both exciton diffusion in a directional way and surface exciton recombination. Our approach overcomes the challenges of anisotropic diffusion and the limited applicability of existing methods to the bulk counterparts of 2D materials. The protocol is then applied at room and at cryogenic temperatures to four bulk hexagonal boron nitride crystals grown by different synthesis routes. The exciton diffusivity depends on the sample quality but not on the temperature, indicating it is limited by defect scattering even in the best quality crystals. The lower limit for the diffusivity by phonon scattering is 0.2 cm$^{2}$.s$^{-1}$. Diffusion lengths were as much as 570 nm. Finally, the surface recombination velocity exceeds 10$^{5}$ cm$^{2}$.s$^{-1}$, at a level similar to silicon or diamond. This result reveals that surface recombination could strongly limit light-emitting devices based on 2D materials.

Van der Waals heterostructures are set as strong contenders for post-CMOS quantum materials engineering. A major step for their systematic exploration and exploitation of technological component demonstrators resides in their eased large-scale design. In this direction, the growth of artificial van der Waals 2D superlattices is presented here such as (MoS2/WS2)(n), (WS2/WSe2)(n), and (MoS2/WSe2)(n) with unit cells repetitions reaching n > 10. The fabrication of these materials is enabled by a fully automated in-situ pulsed laser deposition (PLD) tool. This approach provides cm(2) scale homogeneous superlattices with on-demand material parameters tailoring (layer number, order, and composition). The process is rapid and simple compared to manual pickup exfoliation methods or to sequential transfers of single layers grown by techniques such as chemical vapor deposition, allowing a large repetition of the unit cells in a "mille-feuille" cake configuration. The computational exploration of this family of superlattice materials sheds light on the potential for optoelectronic property design by shaping the band-structure landscape while taking into account the influential effects induced by proximity. Overall, this large-area approach is proposed as an entry point for the systematic design of complex van der Waals heterostructures.

Black phosphorus (BP) stands out from other two-dimensional (2D) materials by the wide amplitude of the band-gap energy (Eg) that sweeps an optical window from visible to infrared wavelengths, depending on the layer thickness. This singularity made optical and excitonic properties of BP difficult to map. Here we report a comprehensive study of the intrinsic (i.e., measured at 4 K) optical properties of 79 passivated BP flakes obtained by mechanical exfoliation of thickness ranging from 4 to 700 nm. By following single- or multistamp exfoliation protocols and by combining micro-Raman and photoluminescence experiments, we demonstrate that the exfoliation step induces line like defects which open radiative recombination paths alternative to those of the crystalline bulk and that actually dominate the emission process. We also show that the evolution of the photoluminescence energy versus thickness follows an inverse square law. We relate this to a quantum well model whose validity is discussed and justified at intermediate thickness. Finally, we report that the emission energy of BP slabs placed in different 2D heterostructures is not significantly modulated by the dielectric environment.

Raman thermometry is well-suited for thermal measurements on semiconductor-based devices when the active area is accessible. To improve the technique, we develop the use of 2D materials as local sensors at the nanoscale for a very precise absolute surface temperature measurement. In this paper, we show the performances and limitations for the application on power devices.

Black phosphorus (BP) stands out from other 2D materials by the wide amplitude of the band-gap energy (Delta(Eg)) that sweeps an optical window from Visible (VIS) to Infrared (IR) wavelengths, depending on the layer thickness. This singularity made the optical and excitonic properties of BP difficult to map. Specifically, the literature lacks in presenting experimental and theoretical data on the optical properties of BP on an extended thickness range. Here we report the study of an ensemble of photoluminescence spectra from 79 passivated BP flakes recorded at 4 K with thicknesses ranging from 4 nm to 700 nm, obtained by mechanical exfoliation. We observe that the exfoliation steps induce additional defects states that compete the radiative recombination from bound excitons observed in the crystal. We also show that the evolution of the photoluminescence energy versus thickness follows a quantum well confinement model appreciable from a thickness predicted and probed at 25 nm. The BP slabs placed in different 2D heterostructures show that the emission energy is not significantly modulated by the dielectric environment. Introduction Confinement effects

Atomic layers of black phosphorus (BP) present unique opto-electronic properties dominated by a direct tunable bandgap in a wide spectral range from visible to mid-infrared (IR). In this work, we investigate the IR photoluminescence (PL) of BP single crystals at very low temperature. Near-band-edge recombinations are observed at 2 K, including dominant excitonic transitions at 0.276 eV and a weaker one at 0.278 eV. The free-exciton binding energy is calculated with an anisotropic Wannier–Mott model and found equal to 9.1 meV. On the contrary, the PL intensity quenching of the 0.276 eV peak at high temperature is found with a much smaller activation energy, attributed to the localization of free excitons on a shallow impurity. This analysis leads us to attribute respectively the 0.276 eV and 0.278 eV PL lines to bound excitons and free excitons in BP. As a result, the value of bulk BP bandgap is refined to 0.287 eV at 2 K.

Black Phosphorus stands out in the 2D-materials panorama by its unique semiconducting properties: direct bandgap which can be tuned by the layer number in a wide range of wavelengths from visible (monolayer) to midinfrared (bulk) [1]. This tunability combined the anisotropy of the structure therefore offers promising perspectives in various fields such as electronics and photonics. However, the fast photooxidation in ambient condition, coupled to a high sensitivity to quantum confinement and dielectric environment in ultrathin BP, make very difficult the investigations on its intrinsic optical properties [2]. Further, as screening effects may strongly affect electronic and spectroscopic properties of 2D materials, it is highly desirable to investigate intrinsic properties of free-standing layers as well the ones of the bulk material which remain poorly known. To start with, we have investigated the infrared photoluminescence of BP single crystals at very low temperature [3]. Near-band-edge recombinations are observed at 2 K, including dominant excitonic transitions at 0.276 eV and a weaker one at 0.278 eV. The free-exciton binding energy is calculated with an anisotropic Wannier-Mott model and found equal to 9.1 meV. On the contrary, the PL intensity quenching of the 0.276 eV peak at high temperature is found with a much smaller activation energy, attributed to the localization of free excitons on a shallow impurity. This analysis leads us to attribute respectively the 0.276 eV and 0.278 eV PL lines to bound excitons (I°X) and free excitons (X) in BP. As a result, the value of bulk BP electronic bandgap is refined to 0.287 eV at 2K, to serve as reference for future work on thin BP layers [3]. As far as the thinnest layers are concerned, which cannot manipulated in air, we have shown that Angular resolved Electron energy loss spectroscopy implemented in Transmission Electron Microscopy (Ar-EELS-TEM) offers a unique way to investigate dielectric response of free-standing layers related to valence band and plasmon excitations with the advantage to get access to their q dispersion and their symmetry properties [4]. By combining this technique with suitable ab initio calculations, we have studied the dielectric response of free-standing BP layers as a function of the number of layers. We found optical bandgap values of 1.9 eV, 1.4 eV and 1.1 eV for the mono- bi- and trilayer respectively. Moreover, by combining our results with a simple variational model, we correlate the exciton energy with the dielectric screening. We hence demonstrate that the variations of the electronic gap are sizeably larger than the variations of the binding energy. Finally, we probe and analyze the volume and surface plasmons dispersion as a function of momentum for the 1-3 BP layers and bulk and highlight a deviation and linearization of the parabolic dispersion with strong anisotropic fingerprints [5]. [1] G. Zhang et al., Nat. Com., 8, 14071, (2017) [2] A. Favron, E. Gaufres et al Nature Mat. 14 (2015) 826. [3] E. Carré et al, 2D Materials (2020) :doi.org/10.1088/2053-1583/abca81 [4] F. Fossard et al, Phys. Rev. B 96, 115304 (2017) [5] E. Gaufres et al, Nanoletters 19, 8303 (2019); DOI: 10.1021/acs.nanolett.9b03928

A thermodynamic assessment of the KF-ThF4 binary system using the CALPHAD method is presented, where the liquid solution is described by the modified quasichemical formalism in the quadruplet approximation. The optimization of the phase diagram is based on experimental data reported in the literature and newly measured X-ray diffraction and differential scanning calorimetry data, which have allowed to solve discrepancies between past assessments. The low temperature heat capacity of alpha-K2ThF6 has also been measured using thermal relaxation calorimetry; from these data the heat capacity and standard entropy values have been derived at 298.15 K: C-p,m(o)(K2ThF6, cr, 298.15K) = (193.2 +/- 3.9) J.K-1.mol(-1) and S-m(o)(K2THF6, cr, 298.15K) = (256.15 +/- 4.8) J.K-1.mol(-1). Taking existing assessments of the relevant binaries, the new optimization is extrapolated to the ternary systems LiF-KF-ThF4 and NaF-KF-ThF4 using an asymmetric Kohler/Toop formalism. The standard enthalpy of formation and standard entropy of KNaThF6 are re-calculated from published e.m.f data, and included in the assessment of the ternary system. A calculated projection of the NaF-KF-ThF4 system at 300 K and the optimized liquidus projections of both systems are compared to published phase equilibrium data at room temperature and along the LiF-LiThF5 and NaF-KThF5 pseudobinaries, with good agreement. (C) 2020 The Authors. Published by Elsevier Ltd.

Atomic layers of Black Phosphorus (BP) have been recently isolated, ten years after graphene. BP stands out in the 2D-materials panorama by its unique semiconducting properties: direct bandgap which can be tuned by the layer number in a wide range of wavelengths from visible (monolayer) to midinfrared (bulk) [1]. This tunability combined the anisotropy of the structure therefore offers promising perspectives in various fields such as electronics and photonics. However, the fast photooxidation in ambient condition, coupled to a high sensitivity to quantum confinement and dielectric environment in ultrathin BP, make very difficult the investigations on its intrinsic optical properties [2]. Further, as screening effects may strongly affect electronic and spectroscopic properties of 2D materials, it is highly desirable to investigate intrinsic properties of free-standing layers as well the ones of the bulk material which remain poorly known. To start with, we have investigated photoluminescence (PL) and absorption properties of high quality BP single crystals at 2K [FTIR]. The PL intensity appears comparable to high quality InAs crystals confirming the suitability of BP for infrared applications. Two peaks are evidenced at 0.275 eV and 0.26 eV. The highest energy peak is particularly narrow (FWHM = 3.8meV) and is tentatively attributed to exciton recombination, thanks to the temperature dependence of PL spectra from 2K to 300K. Further, we have measured the gap related energy shift of the PL spectra as a function of the BP thickness in a series of mechanically exfoliated samples with thicknesses down to 10 nm, complementary to the previous measures done on thinner samples [3]. As far as the thinnest layers are concerned, which cannot manipulated in air, we have shown that Angular resolved Electron energy loss spectroscopy implemented in Transmission Electron Microscopy (Ar-EELS-TEM) offers a unique way to investigate dielectric response of free-standing layers related to valence band and plasmon excitations with the advantage to get access to their q dispersion and their symmetry properties [4]. By combining this technique with suitable ab initio calculations, we have studied the dielectric response of free-standing BP layers as a function of the number of layers. We found optical bandgap values of 1.9 eV, 1.4 eV and 1.1 eV for the mono- bi- and trilayer respectively. Moreover, by combining our results with a simple variational model, we correlate the exciton energy with the dielectric screening. We hence demonstrate that the variations of the electronic gap are sizeably larger than the variations of the binding energy. Finally, we probe and analyze the volume and surface plasmons dispersion as a function of momentum for the 1-3 BP layers and bulk and highlight a deviation and linearization of the parabolic dispersion with strong anisotropic fingerprints [5]. [1] G. Zhang et al., Nat. Com., 8, 14071, (2017) [2] A. Favron, E. Gaufres et al Nature Mat. 14 (2015) 826. [3] C. Chen et al., NanoLett., (2019) [4] F. Fossard et al, Phys. Rev. B 96, 115304 (2017) [5] E. Gaufres et al, Nanoletters (2019); DOI: 10.1021/acs.nanolett.9b03928

Le phosphore noir est un semi-conducteur à petit gap (environ 0.3 eV) ayant récemment rejoint la famille des matériaux bidimensionnels. Sa bande interdite modulable du moyen infrarouge au visible selon l'épaisseur, sa forte anisotropie dans le plan atomique ainsi que la grande mobilité des porteurs de charges lui promettent un haut potentiel applicatif dans le domaine de l'optoélectronique. L'objectif de cette thèse a été d'étudier les propriétés optiques du cristal de phosphore noir ainsi que de ses feuillets atomiques.Après une description des différents développements instrumentaux réalisés au cours de cette thèse, les méthodes de fabrication des échantillons sont abordées. Deux points sont à maîtriser : l'élaboration de couches fines et leur protection des conditions ambiantes pour éviter leur oxydation. Dans une première partie, plusieurs méthodes dites « Top-Down » (exfoliation mécanique et assistée à l'or, gravure ionique) sont comparées sur la base de la qualité, la taille, l'épaisseur des échantillons obtenus ainsi que de la facilité d'exécution du mode opératoire. La seconde partie présente deux méthodes de protection des couches fines: la passivation à l'alumine (par ALD ou évaporation d'aluminium) et l'encapsulation dans des feuillets de hBN (hétérostructure hBN/BP/hBN).La forte anisotropie du phosphore noir fait que la détermination de l'orientation des axes cristallographiques est un point clé dans l'étude du matériau. Dans ce but, un mode opératoire a été proposé qui utilise la spectroscopie Raman polarisée. Celui-ci a été confronté puis validé par différents moyens expérimentaux (observations TEM, EBSD) et théoriques (modélisation de l'intensité Raman dans des couches fines). Les propriétés vibrationnelles ont également été étudiées en fonction du nombre de couches atomiques. Plusieurs effets ont été remarqués à haute (> 100 cm-1) et basse (< 100 cm-1) fréquences et sont attribués à la réduction de dimensionnalité et à des phénomènes de résonnance. Grâce aux conditions expérimentales d'excitation utilisées, un grand nombre de modes relatifs aux vibrations inter-plans sont mis en évidence pour la première fois et se sont révélés être des indicateurs précis de l'épaisseur des cristallites.La photoluminescence du cristal massif a été étudiée pour la première fois à température ambiante et cryogénique. Plusieurs composantes d'émission en bord de bande de nature excitonique ont été identifiées dont une raie fine due à l'exciton libre. L'analyse de leur comportement en fonction de la température ainsi qu'un calcul de l'énergie de liaison de l'exciton libre prenant en compte l'anisotropie du milieu ont permis d'établir une nouvelle valeur de référence du gap du phosphore noir à 0.287 eV à 2 K. L'étude en photoluminescence des cristaux exfoliés a révélé la disparition de la raie fine de luminescence au profit d'une bande large. Ce changement est attribué à la densité de défauts introduits par l'exfoliation mécanique ainsi qu'en atteste l'élargissement des bandes en spectroscopie Raman. La bande de photoluminescence a été suivie en fonction de l'épaisseur des couches exfoliées jusqu'à 8 couches atomiques. En dessous d'une épaisseur seuil évaluée à 25 nm, un décalage de la bande vers les hautes énergies est mis en évidence, dont le comportement est très bien décrit par un modèle de confinement quantique. Aucune différence significative n'est observée entre les échantillons passivés alumine et encapsulés dans du hBN ce qui indique que les effets de diélectriques ne sont pas prépondérants dans la gamme d'épaisseur étudiée.