We investigate the electron band structure of grey arsenic, whose (111) face hosts the topological Shockley state. Interestingly, the bulk band close to the touching point with the surface state exhibits the characteristics of inelastic scattering. Moreover, the band structure analysis reveals linearity in the imaginary part of electron self-energy. These features are analogous to those observed in high-temperature superconductors and marginal Fermi liquid systems, respectively, where strong electronic correlations exist. Our results suggest that correlated many-body states can be connected by non-interacting topological states, providing a viable playground to explore the coupling between topological and correlated states via grey arsenic surface.

In this work, the band structure of quasi-one-dimensional HfSe3 was investigated with nanospot angle-resolved photoemission spectroscopy (nano-ARPES) in both the p-and s-polarization geometries and with density functional theory calculations. HfSe3 has a rectangular surface Brillouin zone where the effective hole mass along the chain direction ( P to Y) measured with p-polarization geometry is-0.27 +/- 0.01 me, which is smaller than the effective hole mass along the direction perpendicular to the chains measured with s-polarization geometry ( P to B), -1.17 +/- 0.01 me, in agreement with the calculated hole masses of-0.25 me ( P to Y) and-1.11 me ( in the experimental band structure along P to Y. This band separation may be partly enhanced due to intrinsic spin-orbit coupling effects, as the band structure calculated with density functional theory shows a significant separation of 0.29 eV, for the two bands closest to the top of the valence band along P to Y, only when spin-orbit interactions are included. P to B ), respectively. A band separation of 0.22 +/- 0.01 eV is observed at the top of the valence band

The photodiode stack is the most effective design geometry for integrating colloidal nanocrystals (NCs) into optoelectronic devices dedicated to light emission and detection. Traditional designs rely on determining the absolute energy band alignment, followed by selecting suitable materials to transport charges (with energy levels resonant to the active material's bands). Because of this method's inherent limitations, we propose to explore an alternative approach where alkali metals are used to tune the absolute energy levels of the optically active layer. We illustrate this concept using lithium and cesium deposition onto narrow band gap NC films (i.e., HgTe and InAs), that are relevant materials for infrared optoelectronics. Our results show that work function shifts up to 0.9 eV can be achieved and that smaller alkalis are more effective at generating this shift. However, different behaviors are observed for HgTe and InAs. In the case of II-VI materials, the alkali acts as a pure dipole (i.e., no shift in the core level) and the film behaves as a bulk effective medium (i.e., no evidence of alkali intercalation). For III-V NCs, the alkali plays a dual role as both a dipole and a redox agent, making the alkali's effect dependent on the NCs' surface-to-volume ratio and the size of the alkali.

Rhombohedral graphene exhibits an exceptionally diverse array of correlated phases that depend sensitively on the displacement field. Compiling reported phases into a unified phase diagram reveals a pronounced field-dependent electron-hole asymmetry: correlated states on the hole-doped side emerge at small displacement fields, whereas the fractional quantum anomalous Hall effect (FQAHE) is observed exclusively on the electron-doped side under large displacement fields. This stark asymmetry highlights the need to understand how flat bands evolve with displacement fields. Here, we directly visualize the field-induced electron-hole asymmetric band flattening in rhombohedral pentalayer graphene (R5G) using nanospot angle-resolved photoemission spectroscopy with electrostatic gating. Beyond gap opening and spectral weight redistribution indicative of layer polarization, the gating field drives a strongly asymmetric modification of the flat bands: the flat valence band (FVB) evolves into an M-shaped dispersion at high field, whereas the flat conduction band (FCB) progressively flattens with increasing field. Comparison with calculations identifies critical parameters governing the band curvature of R5G, from which the resulting finite Berry curvature and near-ideal quantum geometry support the emergence of topological phases under electron doping at large fields. These results establish a direct link between the asymmetric phase diagram, band structure evolution, and quantum geometry, providing a microscopic framework for understanding correlated and topological phases in rhombohedral graphene.

Twisted MoTe_2 hosts intriguing correlated quantum phenomena including the fractional quantum anomalous Hall effect in twisted bilayer (t-BL) MoTe_2 near 3.7^∘, which is sensitive to the twist angle and moiré superlattices. Here, we directly visualize the twist-angle-modulated electronic structure of t-BL and twisted double-bilayer (t-DBL) near this critical angle. We find that the moiré superlattice not only modifies the relative energy between Γ and K valleys in t-BL MoTe_2, but also strongly reconstructs the Γ valley for both t-BL and t-DBL. Specifically, the deep p_z-derived band at Γ exhibits a distinct splitting that systematically varies with increasing twist angle. Theoretical analysis suggests that this modulation arises from the twist-angle-dependent lattice relaxation, especially interfacial corrugations. Our work directly visualizes the moiré-modulated electronic structure and provides key spectroscopic information of lattice relaxation and interlayer interactions underlying the physics of twisted MoTe_2.

As the integration of transition metal dichalcogenides (TMDC) becomes more advanced for optoelectronics, it is increasingly relevant to develop tools that can correlate the structural properties of the materials with their electrical output. To do so, the determination of the electronic structure must go beyond the hypothesis that the properties of the pristine material remain unaffected after the device integration, which generates changes in the dielectric environment, including electric fields that are likely to renormalize the electronic spectrum. Here, we demonstrate that nanobeam photoemission spectroscopy is a well-suited tool to unveil the device energy landscape under operando conditions. Both the gate vertical field and the drain in-plane vectorial electric field can be determined with a sub-μm resolution. We provide a correlative description of a field-effect transistor to connect its bias-modified energy landscape with the transistor electrical output. The method appears highly suited to unveil how the actual geometry of the flake (thickness, edge effect, presence of structural defects, etc.) is driving the current flow within the device. Lastly, the method appears fully compatible with traditional device fabrication, therefore making it relevant for systematic rational optimization of TMDC-based electronic devices.

Colloidal nanocrystals are now widely explored for their integration into more advanced electronic and optoelectronic devices. Among the key components enabling this progress is the field-effect transistor (FET). While widely used as a phototransistor, combining both light absorption and gate-induced current modulation, its primary role remains as a tool for extracting material parameters. The electrical output from FETs serves as the main measurement to probe carrier density and mobility in nanocrystal films. However, such an approach suffers from two main flaws: it relies on modeling to link the electrical output to material properties; and second, it can be affected by the presence of defects. Here, we use scanning photoemission microscopy to assess the energy profile in such nanocrystal-based FETs. This method is used to quantify the impact of a local gate defect, which appears to be quite significant, as its impact is stronger and has longer-range effects than the conventional gate operation. We also demonstrate that the method is effective in determining the process at the origin of electrical breakdown. Overall, the method appears well suited to bridge the gap between the material scale and the obtained electrical output and to quantify the impact of potential deviations from ideal behavior.

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.

Controlling the crystal phase of two-dimensional transition metal dichalcogenides (TMDs) is essential for tailoring their electronic and optical properties. Among the polymorphs of WS2, the metastable 1T' phase exhibits semimetallic or narrow-bandgap character and hosts quantum functionalities distinct from the semiconducting 1H phase. Here, we investigate the temperature-induced 1T'/1H phase transition in colloidally synthesized monolayer WS2 nanosheets functionalized with organic ligands. The reducing conditions of the synthesis stabilize the 1T' phase via electron doping. Through in situ analyses of both the structural and electronic properties, we monitor the phase evolution during annealing and find that the 1T' phase remains stable up to 300 °C, accompanied by a relative lattice contraction. Between 300 °C and 350 °C, a mixed 1T'/1H regime appears, where the 1H content can be finely tuned by controlling the annealing time. Above 350 °C, a rapid and complete transformation to the 1H phase occurs. We demonstrate that the decomposition of the reducing ligand serves as the primary trigger of the structural transition, revealing a strong interplay among doping, surface chemistry, and lattice structure. Notably, nanosheets with smaller lateral dimensions exhibit slower phase transition kinetics, suggesting that finite size could influence the structural rearrangement underlying the phase transformation.

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 demonstrates how the band structure can be tuned in bulk TMDs. The collected information can serve as a reference for future applications.

Colloidal nanocrystals (NCs) are a promising platform for infrared optoelectronics. Current efforts focus on designing NCs that absorb in the short- and mid-wave infrared and integrating them into diode stacks. A major challenge is to coupling these sensors to read-out integrated circuits (ROICs) for infrared imaging, which requires infrared-transparent top electrodes. Conventional materials like tin-doped indium oxide lose transparency at longer wavelengths, limiting their effectiveness. Metallic grids have emerged as an alternative but struggle to maintain a uniform potential, as shown by nanobeam X-ray photoemission microscopy. To address this, graphene is explored as a transparent electrode. A novel diode stack is proposed to maintain a backside mirror, accommodate HgTe NCs' chemical constraints, and incorporate electrodes that efficiently extract both electrons and holes. Unlike conventional designs limited to near-zero bias, this stack operates optimally under CMOS read-out-integrated-circuit (ROIC) conditions. Additionally, its transparent electrode allows photoelectron emission from within the diode, enabling in situ electric field analysis. This capability enables to rationalize the optimization process of photodiode design.

The effects of uniaxial strain on the local band structure modification in a single wrinkle in a trilayer (3L) molybdenum disulfide (MoS2) flake have been enlightened by combining complementary atomic force microscopy, Raman and photoluminescence microspectroscopies. Controlled wrinkles were introduced in 3L MoS2 flakes by using buckling instability, inducing local tensile strains of up to 0.07%. The ability to induce and fabricate stable wrinkles in 3L MoS2, which is a nanoscale system whose thickness is smaller than 2 nm, arises from the material's reduced thickness. Submicron-scale spatial mapping of the isolated wrinkle revealed a reduction of the direct bandgap by 10 meV, accompanied by a quenching of the radiative recombination of excitons in the wrinkle compatible with an antifunneling effect, where excitons drift away from lower-bandgap areas before recombination. Finally, angle-resolved photoemission microspectroscopy further demonstrated a shift of the valence band toward higher binding energies in the isolated MoS2 wrinkle. Combining these results with the ones from optical spectroscopies results in a type-II band alignment between the flat and the strained regions, with downward shifts in both the conduction and valence bands. These new insights into the local electronic structure in locally strained 3L MoS2 nanosheets may help the design and performance of nanoscale optoelectronic and photonic devices by enabling precise control over the excitonic properties and the energetic spatial landscape.

The fractional quantum anomalous Hall effect (FQAHE) is a fascinating emergent quantum state characterized by fractionally charged excitations in the absence of a magnetic field. Recently, the FQAHE has been observed in aligned rhombohedral pentalayer graphene on BN (aligned R5G/BN)1 with moiré potential. Intriguingly, the FQAHE preferably emerges when carriers are displaced away from the moiré interface1-3, raising debates about the role of moiré potential4-17. Here, by performing nanospot angle-resolved photoemission spectroscopy, we directly visualize the topological flat band in both aligned and non-aligned R5G/BN. The moiré potential in the aligned sample generates moiré bands and enhances the topological flat band as compared to non-aligned sample. Combined with theoretical calculations, we propose that the moiré bands on the top surface arise through the interlayer Coulomb interaction with the moiré-modulated bottom layer. Our results provide direct experimental evidence for the role of moiré potential in aligned rhombohedral graphene, and establish a foundation for understanding its emergent quantum phenomena.

Van der Waals heterostructures offer a playground for optoelectronics with the promise of coupling a broad range of materials while lifting the constraints of epitaxy. Optimization of such devices also requires careful control of the spatial localization of the in-plane and out-of-plane electric field. Therefore, operando tools that give direct access to the local energy and electric field landscape are necessary. Here, we demonstrate that nanobeam X-ray photoemission imaging is an effective method to map the electric field in a 2D flake-based multielectrode transistor and a WS2/MoSe2 heterostructure, while the electrical biases are applied. We discuss how the shape and geometry of the flake, the electrical contacts, and the overlapping of the flakes impact the field distribution. In the region where flakes overlap, the in-plane electric field distribution aligns in the two materials, whereas the out-of-plane field distribution resulting from the gating charge injection deviates from a homogeneous distribution.

van der Waals (vdW) heterostructures, which combine bidimensional materials of different properties, enable a range of quantum phenomena. Herein, we present a comparative study between the electronic properties of mono- and bilayer 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 is investigated in a systematic manner. In contrast to PtSe2/h-BN, the electronic structure of PtSe2/graphene reveals the presence of interlayer hybridization between PtSe2 and graphene, which is evidenced by minigap openings in the π-band of graphene. Furthermore, our measurements show that the valence band maximum (VBM) of monolayer PtSe2 is located at the Γ point with different binding energies of about -0.9 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.

Coupling Weyl quasiparticles and charge density waves (CDWs) can lead to fascinating band renormalization and many-body effects beyond band folding and Peierls gaps. For the quasi-one-dimensional chiral compound (TaSe4)2I with an incommensurate CDW transition at TC = 263 K, photoemission mappings thus far are intriguing due to suppressed emission near the Fermi level. Models for this unconventional behavior include axion insulator phases, correlation pseudogaps, polaron subbands, bipolaron bound states, etc. Our photoemission measurements show sharp quasiparticle bands crossing the Fermi level at T > TC, but for T < TC, these bands retain their dispersions with no Peierls or axion gaps at the Weyl points. Instead, occupied band edges recede from the Fermi level, opening a spectral gap. Our results confirm localization of quasiparticles (holes created by photoemission) is the key physics, which suppresses spectral weights over an energy window governed by incommensurate modulation and inherent phase defects of CDW.

The diverse and intriguing phenomena observed in twisted bilayer systems, such as graphene and transitionmetal dichalcogenides, prompted new questions about the emergent effects that they may host. However, the practical challenge of realizing these structures on a scale large enough for spectroscopic investigation, remains a significant hurdle, resulting in a scarcity of direct measurements of the electronic band structure of twisted transition metal dichalcogenide bilayers. Here, we present a systematic nanoscale angle-resolved photoemission spectroscopy investigation of bulk, single-layer, and twisted bilayer WS2 with a small twist angle of 4.4 degrees. The experimental results are compared with theoretical calculations based on density functional theory along the high-symmetry directions P- K and t'-M. Surprisingly, the electronic band structure measurements suggest a structural relaxation occurring at 4.4 degrees twist angle and the formation of large, untwisted bilayer regions replacing most of the twisted area.

In recent years, the interest for narrow band gap colloidal quantum dots (CQDs) has shifted from materials optimized for solar cells (with E-G approximate to 1.2 eV) to materials with properties at longer wavelengths. This shift necessitates a systematic investigation of their electronic properties to optimize photodiode designs effectively. In this study, we utilized X-ray photoemission to systematically determine how the band gap (with absorption maximum wavelengths of 1000, 1200, 1400, and 1550 nm) and surface chemistry (short halides and thiols) influence the band alignment and the core levels in PbS nanocrystals. Our research provides evidence for a shift from a quasi-intrinsic behavior for the narrowest band gap to an exclusively n-type nature for the particles with the largest size, potentially indicating the emergence of degenerate doping. Core-level analysis reveals that the effect of ligands extends beyond dipole behavior, also inducing charge transfer leading to the formation of metallic islands, which might be detrimental to photodetection. Furthermore, by employing scanning photoemission microscopy, we gained direct access to the photodiode's built-in potential, a parameter typically obtained through indirect modeling. We then discussed how the particle size influences this property. This study lays the foundation for a more rational design of PbS CQD-based photodiodes operating in the short-wave infrared spectrum.

Twisted bilayer graphene (tBLG) provides a fascinating platform for engineering flat bands and inducing correlated phenomena. By designing the stacking architecture of graphene layers, twisted multilayer graphene can exhibit different symmetries with rich tunability. For example, in twisted monolayer-bilayer graphene (tMBG) which breaks the C2z symmetry, transport measurements reveal an asymmetric phase diagram under an out-of-plane electric field, exhibiting correlated insulating state and ferromagnetic state respectively when reversing the field direction. Revealing how the electronic structure evolves with electric field is critical for providing a better understanding of such asymmetric field-tunable properties. Here we report the experimental observation of field-tunable dichotomic electronic structure of tMBG by nanospot angle-resolved photoemission spectroscopy (NanoARPES) with operando gating. Interestingly, selective enhancement of the relative spectral weight contributions from monolayer and bilayer graphene is observed when switching the polarity of the bias voltage. Combining experimental results with theoretical calculations, the origin of such field-tunable electronic structure, resembling either tBLG or twisted double-bilayer graphene (tDBG), is attributed to the selectively enhanced contribution from different stacking graphene layers with a strong electron-hole asymmetry. Our work provides electronic structure insights for understanding the rich field-tunable physics of tMBG.

Magic-angle twisted bilayer graphene exhibits correlated phenomena such as superconductivity and Mott insulating states related to the weakly dispersing flat band near the Fermi energy. Such a flat band is expected to be sensitive to both the moiré period and lattice relaxations. Thus, clarifying the evolution of the electronic structure with the twist angle is critical for understanding the physics of magic-angle twisted bilayer graphene. Here we combine nano-spot angle-resolved photoemission spectroscopy and atomic force microscopy to resolve the fine electronic structure of the flat band and remote bands, as well as their evolution with twist angle from 1.07° to 2.60°. Near the magic angle, the dispersion is characterized by a flat band near the Fermi energy with a strongly reduced band width. Moreover, we observe a spectral weight transfer between remote bands at higher binding energy, which allows to extract the modulated interlayer spacing near the magic angle. Our work provides direct spectroscopic information on flat band physics and highlights the important role of lattice relaxations.