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Nearly localized moiré 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 WSe2/MoSe2 heterobilayer grown by molecular beam epitaxy. This flat band is localized near the Fermi level and has a width of several hundred meVs. By combining ARPES measurements with density functional theory 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 Γ point. The energy difference between the VBM at Γ and the K point is of −60 meV, which is a stark difference compared to individual single monolayer WSe2 and monolayer WSe2, showing both a VBM at K.

In two dimensional materials, substitutional doping during growth can be used to alter the electronic properties.

Atomically thin two-dimensional (2D) layered semiconductors such as transition metal dichalcogenides 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

In atomically thin transition metal dichalcogenide semiconductors, there is a crossover from indirect to direct band gap 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-layer 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.

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., Comment: Nanotechnology (2022)

The strain in hybrid van der Waals heterostructures, made of two distinct two-dimensional van der Waals materials, offers an interesting handle on their corresponding electronic band structure. Such strain can be engineered by changing the relative crystallographic orientation between the constitutive monolayers, notably, the angular misorientation, also known as the “twist angle”. By combining angle-resolved photoemission spectroscopy with density functional theory calculations, we investigate here the band structure of the WS2/graphene heterobilayer for various twist angles. Despite the relatively weak coupling between WS2 and graphene, we demonstrate that the resulting strain quantitatively affects many electronic features of the WS2 monolayers, including the spin-orbit coupling strength. In particular, we show that the WS2 spin-orbit splitting of the valence band maximum at K can be tuned from 430 to 460 meV. Our findings open perspectives in controlling the band dispersion of van der Waals materials.

Significance The on-demand control of topological properties with readily modifiable parameters is a fundamental step toward the design of novel electronic and spintronic devices. Here, we show that this goal can be achieved in the correlated system BaCo 1 − x Ni x S 2 , where we succeeded in significantly changing the reciprocal space position and shape of Dirac nodes by chemically substituting Ni with Co. We prove that the tunability of the Dirac states is realized by varying the electron-correlation strength and the charge-transfer gap, both sensitive to the substitution level, x . Based on our finding, a class of late-transition metal compounds can be established as prototypical for engineering highly tunable Dirac materials.

International audience; In atomically thin transition metal dichalcogenide semiconductors, there is a crossover from indirect to direct band gap 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 WS 2(1-x) Se 2x 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 WS 2(1-x) Se 2x (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 WS 2(1-x) Se 2x , we performed a nano-angle resolved photoemission spectroscopy study coupled with first-principles calculations. We find that the high micro-PL value for bilayer WS 2(1-x) Se 2x (x = 0.8) is due to the overlay of direct and indirect optical transitions. This peculiar high PL intensity in WS 2(1-x) Se 2x opens the way for spectrally tunable light-emitting devices.

International audience; van der Waals materials offer a large variety of electronic properties depending on chemical composition, number of layers, and stacking order. Among them, As2Te3 has attracted attention due to the promise of outstanding electronic properties, and high photo-response. Precise experimental determinations of the electronic properties of As2Te3 are yet sorely needed for better understanding of potential properties and device applications. Here, we study the structural and electronic properties of α-As2Te3. Scanning transmission electron microscopy coupled to energy X-ray dispersion (STEM-EDX), and micro-Raman spectroscopy all confirm that our specimens correspond to α-As2Te3. Scanning tunneling spectroscopy (STS) at 4.2K demonstrates that α-As2Te3 exhibits an electronic band gap of about 0.4 eV. The material can be exfoliated, revealing the (100) anisotropic surface. Transport measurements on a thick exfoliated sample (bulk-like) confirm the STS results. These findings allows for a deeper understanding of the As2Te3 electronic properties, underlying the potential of V-VI semiconductors for electronic and photonic technologies.

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., Comment: ACS nano (2020)

Semiconducting monolayers of a 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 MoS

International audience; Hexagonal boron nitride (hBN) is attracting tremendous interest as an essential component in van der Waals heterostructures due to its ability to provide weakly interacting interfaces and because of its large bandgap. Although most of theoretical calculations yield the standard AA′ stacking for few-layer hBN, the exact determination of its structural and electronic properties remains unrevealed to date. Here, we provide the direct observation of structural and electronic transitions in few layers of isotopically pure exfoliated h11BN flakes. Our nanoscopic angle-resolved photoemission spectroscopy measurements combined with density-functional theory calculations indicate that the stacking and the band structure can be strongly affected by the thickness of h11BN. Hence, we show that hBN presents an AA′ stacking in its bulk form and another more exotic stacking for three and four layers. Our findings open perspectives in understanding and controlling the stackings in hBN, which could be of great interest for optoelectronic applications.

International audience; Hybrid heterostructures, made of organic molecules adsorbed on two-dimensional metal monochalcogenide, generally unveil interfacial effects that improve the electronic properties of the single constitutive layers. Here, we investigate the interfacial electronic characteristics of the F4-TCNQ/single-layer GaSe heterostructure. A sharp F4-TCNQ/GaSe interface has been obtained and characterized by x-ray photoemission spectroscopy. We demonstrate that a high electron transfer from 1TL GaSe into the adsorbed F4-TCNQ molecules takes place, thereby yielding a reduction in the excess negative charge density of GaSe. Additionally, the direct band-structure determination of the heterostructure has been carried out using angle-resolved photoemission spectroscopy, shedding light on essential features such as doping and band offset at the interface. Our results indicate that the buried 1TL GaSe below the F4-TCNQ layer exhibits a robust inversion of the valence dispersion at the Γ point, forming a Mexican-hat-shaped dispersion with 120 ± 10 meV of depth. Our experiments also reveal that F4-TCNQ can significantly tune the electronic properties of 1TL GaSe by shifting the band offset of about 0.16 eV toward lower binding energies with respect to the Fermi level, which is a key feature for envisioning its applications in nanoelectronics.

[EN] The combination of monolayers of different two-dimensional (2D) materials into van der Waals hetero-bilayer structures creates unprecedented physical phenomena, acting as a powerful tool for future devices. Understanding and exploiting these phenomena hinge on knowing the electronic structure and the hybridization of hetero-bilayer structures. Here, we show strong hybridization effects arising between the constitutive single layers of a SnS/WSe hetero-bilayer structure grown by chemical vapor deposition. Surprisingly, the valence band maximum position of WSe is moved from the K point for the single layer WSe to the Γ point for the aligned SnS/WSe hetero-bilayer. Additionally, a significant photoluminescence quenching is observed for the SnS/WSe hetero-bilayer structure with respect to the WSe monolayer. Using photoluminescence spectroscopy and nano-angle-resolved photoemission spectroscopy techniques, we demonstrate that the SnS/WSe heterostructure present a type-II band alignment. These findings directly answer many outstanding questions about the electronic band structure and the band offset of SnS/WSe hetero-bilayers for envisaging their applications in nanoelectronics., We acknowledge the financial support by RhomboG and MagicValley grants. This work is supported by a public grant overseen by the French National Research Agency (ANR) as part of the “Investissements d’Avenir” program (Labex NanoSaclay, reference: ANR-10-LABX-0035).

We report anomalies observed in photoelectron spectroscopy measurements performed on α-(BEDT-TTF)2I3 crystals. In particular, above its metal-insulator transition temperature (T = 135 K), we observe the lack of a sharp Fermi edge in contradiction with the metallic transport properties exhibited by this quasi-bidimensional organic material. We interpret these unusual results as a signature of a one-dimensional electronic behavior confirmed by DFT band structure calculations. Using photoelectron spectroscopy we probe a Luttinger liquid with a large correlation parameter (α > 1) that we interpret to be caused by the chain-like electronic structure of α-(BEDT-TTF)2I3 surface doped by iodine defects. These new surface effects are inaccessible by bulk sensitive measurements of electronic transport techniques., We acknowledge A. Santander-Syro, M. Gabay and P. Auban- Senzier for fruitful discussions. Work in Spain was supported by the Spanish Ministerio de Econom a y Competitividad (Grants FIS2015-64886-C5-4-P and CTQ2015-64579-C3-3-P) and Generalitat de Catalunya (2017SGR1506, 2017SGR1289 and XRQTC). E.C. acknowledges support of the Spanish MINECO through the Severo Ochoa Centers of Excellence Program under Grant SEV-2015-0496. P.A. acknowledges support from the Maria de Maeztu Units of Excellence Program under Grant MDM-2017-0767.

International audience; One of the most important challenges in the study of topological insulators is the realization of materials that are really insulating in the bulk, in order to emphasize quantum transport in the protected surface states. Irradiation with electron beams is a very promising approach toward this goal. By studying a series of samples of the prototype 3D topological insulator Bi 2 Te 3 , we show that while the topological properties of Dirac surface states are preserved after electron irradiation, their relaxation dynamics are very sensitive to the related modifications of the bulk properties. Using time-and angle-resolved photoelectron spectroscopy, we can reveal two distinct relaxation regimes after optical excitation for non-irradiated and irradiated samples. While the faster regime, corresponding to the first few picoseconds, presents a similar temporal evolution of the photoexcited population for all studied samples, the slower regime is strongly influenced by the controlled generation of defects in the bulk lattice. By adjusting the irradiation parameters in this class of materials, one can thus not only change the bulk transport properties but also tune the ultrafast response of the topological surface states.

Using femtosecond time- and angle-resolved photoemission spectroscopy, we explore the out-of-equilibrium dynamics of surface fermions in the topological system ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{2}\mathrm{Se}$. We show that the presence of localized states from defects at the surface is one of the key material parameters undergirding the long relaxation time of photoexcited Dirac electrons lying in the projected band gap of the bulk-insulating pristine compound. Doping this ternary compound with Sn substituting on Bi sites, while desirably increasing the resistivity at low temperatures decreases decay times of the excited homologous Dirac electrons. On the basis of these observations, we argue that long relaxation times can be ultimately controlled by a charge transfer to the surface.

We present a time- and angle-resolved photoemission study of the transition-metal dichalcogenide ${\mathrm{WTe}}_{2}$, a candidate type-II Weyl semimetal exhibiting extremely large magnetoresistence. Using femtosecond light pulses, we characterize the unoccupied states of the electron pockets above the Fermi level. Following the ultrafast carrier relaxation in distinct parts of the Brillouin zone, we report remarkably similar decay dynamics for electrons and holes. Our results confirm that charge compensation between electron and hole pockets---a key effect to explain the nonsaturating magnetoresistance of this material---is a distinctive feature of ${\mathrm{WTe}}_{2}$ even in an out-of-equilibrium regime.

We present an experimental study describing the effects of surface termination on the electronic structure of the natural topological superlattice phase ${\mathrm{Sb}}_{2}\mathrm{Te}$. Using scanning angle-resolved photoemission microscopy, we consistently find various nonequivalent regions on the same surface after cleaving various ${\mathrm{Sb}}_{2}\mathrm{Te}$ single crystals. We were able to identify three distinct terminations characterized by different Sb/Te surface stoichiometric ratios and with clear differences in their band structure. For the dominating Te-rich termination, we also provide a direct observation of the excited electronic states and of their relaxation dynamics by means of time-resolved angle-resolved photoemission spectroscopy. Our results clearly indicate that the surface electronic structure is strongly affected by the bulk properties of the superlattice.

et al., Topological insulators are a promising class of materials for applications in the field of spintronics. New perspectives in this field can arise from interfacing metal-organic molecules with the topological insulator spin-momentum locked surface states, which can be perturbed enhancing or suppressing spintronics-relevant properties such as spin coherence. Here we show results from an angle-resolved photemission spectroscopy (ARPES) and scanning tunnelling microscopy (STM) study of the prototypical cobalt phthalocyanine (CoPc)/BiSe interface. We demonstrate that that the hybrid interface can act on the topological protection of the surface and bury the Dirac cone below the first quintuple layer., M.C., L.K., E.P., M.K., and M.M. acknowledge financial support by “Investissement d’avvenir Labex Palm” (Grant No. ANR-10-LABX-0039-PALM) and by the ANR “Iridoti”(Grant No. ANR13-IS04-0001-01). M.P. and A.M. acknowledge support from the Ministerio de Ciencia e Innovación (Grant No. MAT2013-46593-C6-5-P) and the Severo Ochoa Program (MINECO, grant SEV-2013-0295). Work of L.K.-E. is supported by NSF Grants DMR-1420634 and DMR-1312483, and by DODW911NF-13-1-0159. G.D.S. acknowledges Italian MIUR project FIRB “Supracarbon” RBFR10DAK6”. A.G. thanks JU grant Agreement 270722, “Energy for a green society: from sustainable harvesting to smart distribution (ERG)”. A. P. and S. L. thank Elettra-Sincrotrone Trieste S.C.p.A. for economic support. E.V.C. acknowledges funding from the University of Basque Country UPV/EHU (Grant No. GIC07-IT- 756-13), the Departamento de Educacion del Gobierno Vasco (Grant No. FIS2010-275), the Spanish Ministerio de Ciencia e Innovacion (Grant No. 19609-C02-01), the Tomsk State University Academic D.I. Mendeleev Fund Program (Grant No. 8.1.05.2015), the Spanish Ministry of Economy and Competitiveness MINECO (Grant No. FIS2013-48286-C2-1-P), and Saint Petersburg State University (Project No. 15.61.202.2015). We acknowledge the support by NFFA Trieste.

This paper reports on the synthesis and the characterization of CoFe2O4 and CoFe2O4/Co3O4 nanofibers prepared by Electrospinning. The samples with various weight ratios of Co/Fe were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and Raman spectroscopy. The results show that by increasing the weight ratio of Co/Fe, a mix of phases CoFe2O4/Co3O4 has been detected. The one dimensional morphology is conserved till a critical weight ratio Co/Fe of 1.08 above which the nanostructures completely lose their morphology. These results provide a promising lead for the future synthesis of a variety of CoFe2O4 based nanofiber morphologies with controllable diameter and crystalline structure.

Introduction In 2008, the Food and Drugs Administration required manufacturers of TNFα antagonists to strengthen their warnings about the risk of serious fungal infections in patients with rheumatoid arthritis (RA). Sinus aspergilloma occurs occasionally in RA patients and can progress to invasive Aspergillus disease. The purpose of this study was to describe symptomatic sinus aspergilloma in RA patients treated with TNFα antagonists. Methods Retrospective descriptive study of symptomatic cases of sinus aspergilloma in patients with RA followed in three French university hospitals. A systematic literature review was performed. Results Among 550 RA patients treated with TNFα antagonists, six (1.1%) had symptomatic maxillary aspergilloma diagnosed by computed tomography before or during TNFα antagonist therapy. None had chronic neutropenia. Aspergilloma treatment was with surgery only in all six patients. In the literature, we found 20 reports of Aspergillus infection in patients with chronic inflammatory joint diseases (including 10 with RA). Only 5/20 patients were treated with TNFα antagonists (invasive lung aspergillosis, n = 3; intracranial aspergillosis, n = 1; and sphenoidal sinusitis, n = 1). Conclusions Otorhinolaryngological symptoms must be evaluated before starting or switching TNFα antagonists. Routine computed tomography of the sinuses before starting or switching TNFα antagonists may deserve consideration.