arXiv subjects

Julien Chaste

Publications and source records attributed to Julien Chaste.

At least 19 recordsLinked to original sources

arXiv2026

Epitaxial Growth and Electronic Properties of QuasiFreeStanding Rhombohedral WSe2 Bilayers on Cubic W110

Rhombohedral-stacked transition metal dichalcogenides (TMDs) break inversion symmetry between adjacent layers, giving rise to an intrinsic out-of-plane ferroelectric polarization.Controlling the formation of this stacking polytype is therefore essential for harnessing ferroelectric effects in two-dimensional materials. In this work, we demonstrate the epitaxial growth of rhombohedral bilayer tungsten diselenide (3R-WSe2) on a cubic W(110) single crystal by molecular beam epitaxy. We show that selenium passivation of the substrate is key to enable a quasi van der Waals epitaxy effectively suppressing strong interfacial bonding and promoting the growth of quasi free standing bilayer films. The 3R stacking order is confirmed through a combination of Raman spectroscopy and high-resolution angle-resolved photoemission spectroscopy (ARPES), supported by density functional theory (DFT) calculations. ARPES and DFT reveal an indirect-gap electronic structure with the valence-band maximum at the Gamma point, as well as a pronounced spin orbit driven splitting of 520 +- 20 meV at the K point. Analysis of the measured dispersions yields hole effective masses of 0.46 +- 0.04 me and 0.75 +- 0.06 me for the upper and lower valence bands at K point, respectively. These results establish a robust route for synthesizing quasi free standing 3R-WSe2 and provide a platform for exploring the electronic, optical, and ferroelectric functionalities that emerge from inversion symmetry breaking in layered TMDs. Our findings further highlight the potential of cubic substrates for deterministic fabrication of rhombohedral TMD heterostructures and ferroelectric devices at the nanoscale.

cond-mat.mtrl-sci↗

arXiv2025

Tunable electronic band structure in WSSe van der Waals Alloys

The electronic structure of semiconducting 2D materials such as transition metal dichalcogenides (TMDs) is known to be tunable by its environment, from simple external fields applied with electrical contacts up to complex van der Waals heterostructure assemblies. However, conventional alloying from reference binary TMD compounds to composition-controlled ternary alloys also offers unexplored opportunities. In this work, we use nano-angle resolved photoemission spectroscopy (nano-ARPES) and density functional theory (DFT) calculations to study the structural and electronic properties of different alloy compositions of bulk WS2(1-x)Se2x. Our results demonstrate the continuous variation of the band structure and the progressive evolution of the valence band splitting at the K points from 420 to 520 meV in bulk WS2(1-x)Se2x. We also carried out scanning tunneling microscopy (STM) measurements and DFT to understand the possible S or Se substitutions variants in WS2(1-x)Se2x alloys, with different local atomic configurations. Our work opens up perspectives for the fine control of the band dispersion in van der Waals materials and demonstrate how the band structure can be tuned in bulk TMDs. The collected information can serve as a reference for future applications.

cond-mat.mtrl-sci↗

arXiv2025

Hybridization in van der Waals epitaxy of PtSe2/h-BN and PtSe2/graphene heterostructures

Van der Waals (vdW) heterostructures, which combine bi-dimensional materials of different properties, enable a range of quantum phenomena. Here, we present a comparative study between the electronic properties of mono- and bi-layer of platinum diselenide (PtSe2) grown on hexagonal boron nitride (h-BN) and graphene substrates using molecular beam epitaxy (MBE). Using angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT), the electronic structure of PtSe2/graphene and PtSe2/h-BN vdW heterostructures are investigated in systematic manner. In contrast to PtSe2/h-BN, the electronic structure of PtSe2/graphene reveals the presence of interlayer hybridization between PtSe2 and the graphene, which is evidenced by minigap openings in the {\pi}-band of graphene. Furthermore, our measurements show that the valence band maximum (VBM) of monolayer PtSe2 is located at the {\Gamma} point with different binding energies of about -0.9 eV and -0.55 eV relative to the Fermi level on h-BN and graphene and substrates, respectively. Our results represent a significant advance in the understanding of electronic hybridization between TMDs and different substrates, and they reaffirm the crucial role of the substrate in any nanoelectronic applications based on van der Waals heterostructures.

cond-mat.mtrl-sci↗

arXiv2025

Mexican hat-like valence band dispersion and quantum confinement in rhombohedral ferroelectric alpha-In2Se3

Two-dimensional (2D) ferroelectric (FE) materials offer a large variety of electronic properties depending on chemical composition, number of layers and stacking-order. Among them, alpha-In2Se3 has attracted much attention due to the promise of outstanding electronic properties, attractive quantum physics, in- and out-of-plane ferroelectricity and high photo-response. Precise experimental determination of the electronic structure of rhombohedral (3R) alpha-In2Se3 is needed for a better understanding of potential properties and device applications. Here, combining angle resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations, we demonstrate that 3R alpha-In2Se3 phase exhibits a robust inversion of the valence band parabolicity at the Gamma point forming a bow-shaped dispersion with a depth of 140 +- 10 meV between the valence band maximum (VBM) along the GammaK direction of the Brillouin zone (BZ). Moreover, we unveil an indirect band gap of about 1.25 eV, as well as a highly electron doping of approximatively 5.1012 electrons per cmsquare at the surface. This leads to surface band bending and the formation of a prominent electron accumulation layer. These findings allow a deeper understanding of the rhombohedral alpha-In2Se3 electronic properties underlying the potential of III/VI semiconductors for electronic and photonic technologies.

cond-mat.mtrl-sci↗

arXiv2025

Decoupling thermoelectric coefficients of multilayer graphene by nanomeshing

Nanostructuring materials at small scales enables control over their physical properties, revealing behaviors not observed at larger dimensions. This strategy is particularly effective in two-dimensional (2D) materials, where surface effects dominate, and has been applied in the thermoelectric field. Here, we use multilayer graphene (4-6 nm thick) as a test platform to study the effect of nanomeshing on its thermoelectric properties. The nanomesh consists of a hexagonal array of holes, with a measured diameter and neck-width of ~360 nm and ~160 nm, respectively. The multilayer graphene is integrated into field-effect transistor-like devices supported by hexagonal boron nitride (hBN), allowing simultaneous electric and thermoelectric measurements, with nanomeshing applied to only part of the material. We use modulated thermoreflectance to investigate thermal transport in equivalent nanomeshed and pristine graphene flakes, extracting key parameters that affect thermoelectric performance. The nanomesh geometry suppresses thermal transport without significantly impacting charge transport, highlighting the different scattering lengths of phonons and electrons while enhancing the thermopower response. We observe a twofold improvement in the device power factor, PF = S^2 sigma (with S the Seebeck coefficient and sigma the electrical conductivity), at room temperature, along with a nearly threefold reduction in thermal conductivity k. The results show that nanomeshing can significantly improve the thermoelectric performance of multilayer graphene, paving the way for novel energy conversion strategies using 2D materials.

cond-mat.mes-hall↗

arXiv2025

Growth of Highly Conductive PtSe2 Films Controlled by Raman Metrics for High-Frequency Photodetectors and Optoelectronic Mixers at 1.55 {\mu}m

Two-dimensional PtSe2 exhibits outstanding intrinsic properties such as high carrier mobility, tunable bandgap, broadband absorption and air stability, making it ideal for (opto)electronic applications. In particular, thick PtSe2 is semimetallic and well suited for ultrafast optoelectronics in the infrared domain. However, achieving PtSe2 films of high crystalline quality with controlled properties on low-cost and insulating substrates remains challenging. Here, highly crystalline semimetallic PtSe2 films are grown by molecular beam epitaxy on sapphire substrates. It is shown how an optimized post-growth annealing remarkably improves the out-of-plane crystallinity and leads to record sheet conductances, up to 1.6 mS. In-depth structural analyses reveal the strong influence of the domain arrangement within the films on their electrical properties. Films that are mainly composed of vertically single crystalline domains exhibit high sheet conductance (1.1 - 1.6 mS), whereas films that contain superimposed twisted domains present low sheet conductance (0.5 - 0.6 mS). Moreover, it is demonstrated that the A1g Raman peak width, in addition to the commonly used Eg peak width, are both effective metrics for evaluating the quality of PtSe2: films with narrower Eg and A1g peaks exhibit higher in-plane and out-of-plane crystalline quality, respectively, as well as higher sheet conductance. Finally, coplanar waveguides integrating a semimetallic PtSe2 channel are fabricated on a 2-inch sapphire substrate to demonstrate optoelectronic devices operating at the 1.55 {\mu}m telecom wavelength. This includes photodetectors with a record 60 GHz bandwidth and the first PtSe2-based optoelectronic mixer with a bandwidth above 30 GHz.

cond-mat.mtrl-sci↗

arXiv2024

Distinguishing different stackings in WSe2 bilayers grown Using Chemical Vapor Deposition

The stacking order of two-dimensional transition metal dichalcogenides (TMDs) is attracting tremendous interest as an essential component of van der Waals heterostructures. A common and fast approach to distinguish between the AAprime (2H) and AB (3R) configurations uses the relative edge orientation of each triangular layer (theta) from optical images. Here, we highlight that this method alone is not sufficient to fully identify the stacking order. Instead we propose a model and methodology to accurately determine the bilayer configuration of WSe2 using second harmonic generation (SHG) and Raman spectroscopy. We demonstrate that the SHG response of the AB phase (theta = 0) deg layers is more intense than the signal from the single layer structure. However, the SHG totally vanishes in the AAprime and ABprime phases (theta = 60 deg) and 0 deg respectively) of homo-bilayer WSe2. Also, several optical features of homo-bilayer WSe2 are found to depend on the details of the stacking order, with the difference being the clearest in the low frequency (LF) Raman frequencies, as confirmed by DFT simulation. This allows unambiguous, high-throughput, nondestructive identification of stacking order in TMDs, which is not robustly addressed in this emerging research area.

cond-mat.mtrl-sci↗

arXiv2024

High Strain Engineering of a Suspended WSSe Monolayer Membrane by Indentation and Measured by Tip-enhanced Photoluminescence

Straintronics involves the manipulation and regulation of the electronic characteristics of 2D materials through the use of macro- and nano-scale strain engineering. In this study, we utilized an atomic force microscope (AFM) coupled with an optical system to perform indentation measurements and tip-enhanced photoluminescence (TEPL), allowing us to extract the local optical response of a suspended monolayer membrane of ternary WSSe at various levels of deformation, up to strains of 10%. The photoluminescence signal is modelled considering the deformation, stress distribution and strain dependence of the WSSe band structure. We observe an additional TEPL signal that exhibits significant variation under strain, with 64 meV per percent of elongation. This peak is linked to the highly strained 2D material lying right underneath the tip. We discuss the amplification of the signal and its relation to the excitonic funnelling effect in a more comprehensive model. We will also compare the diffusion caused by Auger recombination against the radiative excitonic decay. We use TEPL to examine and comprehend the local physics of 2D semi-conducting materials subjected to extreme mechanical strain. Chemical vapour deposition-fabricated 2D ternaries possess high strain resistance, comparable to the benchmark MoS2, and a high Young's modulus of 273 GPa.

cond-mat.mes-hall↗

arXiv2023

Quasi van der Waals Epitaxy of Rhombohedral-stacked Bilayer WSe2 on GaP(111) Heterostructure

The growth of bilayers of two-dimensional (2D) materials on conventional 3D semiconductors results in 2D/3D hybrid heterostructures, which can provide additional advantages over more established 3D semiconductors while retaining some specificities of 2D materials. Understanding and exploiting these phenomena hinge on knowing the electronic properties and the hybridization of these structures. Here, we demonstrate that rhombohedral-stacked bilayer (AB stacking) can be obtained by molecular beam epitaxy growth of tungsten diselenide (WSe2) on gallium phosphide (GaP) substrate. We confirm the presence of 3R-stacking of the WSe2 bilayer structure using scanning transmission electron microscopy (STEM) and micro-Raman spectroscopy. Also, we report high-resolution angle-resolved photoemission spectroscopy (ARPES) on our rhombohedral-stacked WSe2 bilayer grown on GaP(111)B substrate. Our ARPES measurements confirm the expected valence band structure of WSe2 with the band maximum located at the gamma point of the Brillouin zone. The epitaxial growth of WSe2 on GaP(111)B heterostructures paves the way for further studies of the fundamental properties of these complex materials, as well as prospects for their implementation in devices to exploit their promising electronic and optical properties.

cond-mat.mes-hall↗

arXiv2023

Intrinsic defects and mid-gap states in quasi-one-dimensional Indium Telluride

Recently, intriguing physical properties have been unraveled in anisotropic semiconductors, in which the in-plane electronic band structure anisotropy often originates from the low crystallographic symmetry. The atomic chain is the ultimate limit in material downscaling for electronics, a frontier for establishing an entirely new field of one-dimensional quantum materials. Electronic and structural properties of chain-like InTe are essential for better understanding of device applications such as thermoelectrics. Here, we use scanning tunneling microscopy/spectroscopy (STM/STS) measurements and density functional theory (DFT) calculations to directly image the in-plane structural anisotropy in tetragonal Indium Telluride (InTe). As results, we report the direct observation of one-dimensional In1+ chains in InTe. We demonstrate that InTe exhibits a band gap of about 0.40 +-0.02 eV located at the M point of the Brillouin zone. Additionally, line defects are observed in our sample, were attributed to In1+ chain vacancy along the c-axis, a general feature in many other TlSe-like compounds. Our STS and DFT results prove that the presence of In1+ induces localized gap state, located near the valence band maximum (VBM). This acceptor state is responsible for the high intrinsic p-type doping of InTe that we also confirm using angle-resolved photoemission spectroscopy.

cond-mat.mtrl-sci↗

arXiv2023

Quantum Confinement and Electronic Structure at the Surface of van der Waals Ferroelectric {\alpha}-In$_{2}$Se$_{3}$

Two-dimensional (2D) ferroelectric (FE) materials are promising compounds for next-generation nonvolatile memories, due to their low energy consumption and high endurance. Among them, {\alpha}-In$_{2}$Se$_{3}$ has drawn particular attention due to its in- and out-of-plane ferroelectricity, whose robustness has been demonstrated down to the monolayer limit. This is a relatively uncommon behavior since most bulk FE materials lose their ferroelectric character at the 2D limit due to depolarization field. Using angle resolved photoemission spectroscopy (ARPES), we unveil another unusual 2D phenomena appearing in 2H \alpha-In$_{2}$Se$_{3}$ single crystals, the occurrence of a highly metallic two-dimensional electron gas (2DEG) at the surface of vacuum-cleaved crystals. This 2DEG exhibits two confined states which correspond to an electron density of approximatively 10$^{13}$ electrons/cm$^{3}$, also confirmed by thermoelectric measurements. Combination of ARPES and density functional theory (DFT) calculations reveals a direct band gap of energy equal to 1.3 +/- 0.1 eV, with the bottom of the conduction band localized at the center of the Brillouin zone, just below the Fermi level. Such strong n-type doping further supports the quantum confinement of electrons and the formation of the 2DEG.

cond-mat.mtrl-sci↗

arXiv2022

Unidirectional Rashba Spin Splitting in Single Layer WS$_{2(1-x)}$Se$_{2x}$ alloy

Atomically thin two-dimensional (2D) layered semiconductors such as transition metal dichalcogenides (TMDs) have attracted considerable attention due to their tunable band gap, intriguing spin-valley physics, piezoelectric effects and potential device applications. Here we study the electronic properties of a single layer WS$_{1.4}$Se$_{0.6}$ alloys. The electronic structure of this alloy, explored using angle resolved photoemission spectroscopy, shows a clear valence band structure anisotropy characterized by two paraboloids shifted in one direction of the k-space by a constant in-plane vector. This band splitting is a signature of a unidirectional Rashba spin splitting with a related giant Rashba parameter of 2.8 0.7 eV . The combination of angle resolved photoemission spectroscopy with piezo force microscopy highlights the link between this giant unidirectional Rashba spin splitting and an in-plane polarization present in the alloy. These peculiar anisotropic properties of the WS$_{1.4}$Se$_{0.6}$ alloy can be related to local atomic orders induced during the growth process due the different size and electronegativity between S and Se atoms. This distorted crystal structure combined to the observed macroscopic tensile strain, as evidenced by photoluminescence, displays electric dipoles with a strong in-plane component, as shown by piezoelectric microscopy. The interplay between semiconducting properties, in-plane spontaneous polarization and giant out-of-plane Rashba spin-splitting in this two-dimensional material has potential for a wide range of applications in next-generation electronics, piezotronics and spintronics devices.

cond-mat.mtrl-sci↗

arXiv2022

Visualizing giant ferroelectric gating effects in large-scale WSe$_2$/BiFeO$_3$ heterostructures

Multilayers based on quantum materials (complex oxides, topological insulators, transition-metal dichalcogenides, etc) have enabled the design of devices that could revolutionize microelectronics and optoelectronics. However, heterostructures incorporating quantum materials from different families remain scarce, while they would immensely broaden the range of possible applications. Here we demonstrate the large-scale integration of compounds from two highly-multifunctional families: perovskite oxides and transition-metal dichalcogenides (TMDs). We couple BiFeO$_3$, a room-temperature multiferroic oxide, and WSe$_2$, a semiconducting two-dimensional material with potential for photovoltaics and photonics. WSe$_2$ is grown by molecular beam epitaxy and transferred on a centimeter-scale onto BiFeO$_3$ films. Using angle-resolved photoemission spectroscopy, we visualize the electronic structure of 1 to 3 monolayers of WSe$_2$ and evidence a giant energy shift as large as 0.75 eV induced by the ferroelectric polarization direction in the underlying BiFeO$_3$. Such a strong shift opens new perspectives in the efficient manipulation of TMDs properties by proximity effects.

cond-mat.mtrl-sci↗

arXiv2022

Electroluminescence of monolayer WS$_2$ in a scanning tunneling microscope: the effect of bias polarity on the spectral and angular distribution of the emitted light

Inelastic electron tunneling in a scanning tunneling microscope (STM) is used to generate excitons in monolayer tungsten disulfide (WS$_2$). Excitonic electroluminescence is measured both at positive and negative sample bias. Using optical spectroscopy and Fourier-space optical microscopy, we show that the bias polarity of the tunnel junction determines the spectral and angular distribution of the emitted light. At positive sample bias, only emission from excitonic species featuring an in-plane transition dipole moment is detected. Based on the spectral distribution of the emitted light, we infer that the dominant contribution is from charged excitons, i.e., trions. At negative sample bias, additional contributions from lower-energy excitonic species are evidenced in the emission spectra and the angular distribution of the emitted light reveals a mixed character of in-plane and out-of-plane transition dipole moments.

cond-mat.mes-hall↗

arXiv2022

Hybridization and localized flat band in the WSe2/MoSe2 heterobilayer grown by molecular beam epitaxy

Nearly localized moire flat bands in momentum space, arising at particular twist angles, are the key to achieve correlated effects in transition-metal dichalcogenides. Here, we use angle-resolved photoemission spectroscopy (ARPES) to visualize the presence of a flat band near the Fermi level of van der Waals (vdW) WSe2/MoSe2 heterobilayer grown by molecular beam epitaxy. This flat band is localized near the K point of the Brillouin zone and has a width of several hundred meVs. By combining ARPES measurements with density functional theory (DFT) calculations, we confirm the coexistence of different domains, namely the reference 2H stacking without layer misorientation and regions with arbitrary twist angles. For the 2H-stacked heterobilayer, our ARPES results show strong interlayer hybridization effects, further confirmed by complementary micro- Raman spectroscopy measurements. The spin-splitting of the valence band at K is determined to be 470 meV. The valence band maximum (VBM) position of the heterobilayer is located at the Gamma point. The energy difference between the VBM at Gamma and the K point is of -60 meV, which is a stark difference compared to individual 1L WSe2 and 1L WSe2, showing both a VBM at K.

cond-mat.mtrl-sci↗

arXiv2020

Indirect to direct band gap crossover in two-dimensional WS2(1-x)Se2x alloys

In atomically thin transition metal dichalcogenide semiconductors, there is a crossover from indirect to direct bandgap as the thickness drops to one monolayer, which comes with a fast increase of the photoluminescence signal. Here, we show that for different alloy compositions of WS2(1-x)Se2x this trend may be significantly affected by the alloy content and we demonstrate that the sample with the highest Se ratio presents a strongly reduced effect. The highest micro-PL intensity is found for bilayer WS2(1-x)Se2x (x = 0.8) with a decrease of its maximum value by only a factor of 2 when passing from mono- to bi-layer. To better understand this factor and explore the layer-dependent band structure evolution of WS2(1-x)Se2x, we performed a nano-angle resolved photoemission spectroscopy study coupled with first-principles calculations. We find that the high micro-PL value for bilayer WS2(1-x)Se2x (x = 0.8) is due to the overlay of direct and indirect optical transitions. This peculiar high PL intensity in WS2(1-x)Se2x opens the way for spectrally tunable light-emitting devices.

cond-mat.mes-hall↗

arXiv2020

Phase Transition in a Memristive Suspended MoS2 Monolayer Probed by Opto- and Electro-Mechanics

Semiconducting monolayer of 2D material are able to concatenate multiple interesting properties into a single component. Here, by combining opto-mechanical and electronic measurements, we demonstrate the presence of a partial 2H-1T phase transition in a suspended 2D monolayer membrane of MoS2. Electronic transport shows unexpected memristive properties in the MoS2 membrane, in the absence of any external dopants. A strong mechanical softening of the membrane is measured concurrently and may only be related to the phase 2H-1T phase transition which imposes a 3percent directional elongation of the topological 1T phase with respect to the semiconducting 2H. We note that only a few percent 2H- 1T phase switching is sufficient to observe measurable memristive effects. Our experimental results combined with First-principles total energy calculations indicate that sulfur vacancy diffusion plays a key role in the initial nucleation of the phase transition. Our study clearly shows that nanomechanics represents an ultrasensitive technique to probe the crystal phase transition in 2D materials or thin membranes. Finally, a better control of the microscopic mechanisms responsible for the observed memristive effect in MoS2 is important for the implementation of future devices.

cond-mat.mes-hall↗

arXiv2019

Nanomechanical strain concentration on a 2D nanobridge within a large suspended bilayer graphene for molecular mass detection

The recent emergence of strain gradient engineering directly affects the nanomechanics, optoelectronics and thermal transport fields in 2D materials. More specifically, large suspended graphene under very high stress represents the quintessence for nanomechanical mass detection through unique molecular reactions. Different techniques have been used to induce strain in 2D materials, for instance by applying tip indentation, pressure or substrate bending on a graphene membrane. Nevertheless, an efficient way to control the strain of a structure is to engineer the system geometry as shown in everyday life in architecture and acoustics. Similarly, we studied the concentration of strain in artificial nanoconstrictions (~100 nm) in a suspended epitaxial bilayer graphene membrane with different geometries and lengths ranging from 10 to 40 micrometer. We carefully isolated the strain signature from micro-Raman measurements and extracted information on a scale below the laser spot size by analyzing the broadened shape of our Raman peaks, up to 100 cm-1. We potentially measured a strong strain concentration in a nanoconstriction up to 5percent, which is 20 times larger than the native epitaxial graphene strain. Moreover, with a bilayer graphene, our configuration naturally enhanced the native asymmetric strain between the upper and lower graphene layers. In contrast to previous results, we can achieve any kind of complex strain tensor in graphene thanks to our structural approach. This method completes the previous strain-induced techniques and opens up new perspectives for bilayer graphene and 2D heterostructures based devices.

cond-mat.mes-hall↗