Ultrafast Band Structure Control of a Two-Dimensional Heterostructure

Exploring the effect of the covalent functionalization in graphene-antimonene heterostructures

The growing field of two-dimensional (2D) materials has recently witnessed the emergence of heterostructures, however those combining monoelemental layered materials remain relatively unexplored. In this study, we present the chemical fabrication and characterization of a heterostructure formed by graphene and hexagonal antimonene. The interaction between these 2D materials is thoroughly examined through Raman spectroscopy and first-principles calculations, revealing that this can be considered as a van der Waals heterostructure. Furthermore, we have explored the influence of the antimonene 2D material on the reactivity of graphene by studying the laser-induced covalent functionalization of the graphene surface. Our findings indicate distinct degrees of functionalization based on the underlying material, SiO2 being more reactive than antimonene, opening the door for the development of controlled patterning in devices based on these heterostructures. This covalent functionalization implies a high control over the chemical information that can be stored but also removed on graphene surfaces, and its use as a patterned heterostructure based on antimonene and graphene. This research provides valuable insights into the antimonene-graphene interactions and their impact on the chemical reactivity during graphene covalent functionalization.

Amplified Spontaneous Emission from Electron-Hole Quantum Droplets in Colloidal CdSe Nanoplatelets

Two-dimensional cadmium selenide nanoplatelets (NPLs) exhibit large absorption cross sections and homogeneously broadened band-edge transitions that offer utility in wide-ranging optoelectronic applications. Here, we examine the temperature-dependence of amplified spontaneous emission (ASE) in 4- and 5-monolayer thick NPLs and show that the threshold for close-packed (neat) films decreases with decreasing temperature by a factor of 2-10 relative to ambient temperature owing to extrinsic (trapping) and intrinsic (phonon-derived line width) factors. Interestingly, for pump intensities that exceed the ASE threshold, we find development of intense emission to lower energy in particular provided that the film temperature is ≤200 K. For NPLs diluted in an inert polymer, both biexcitonic ASE and low-energy emission are suppressed, suggesting that described neat-film observables rely upon high chromophore density and rapid, collective processes. Transient emission spectra reveal ultrafast red-shifting with the time of the lower energy emission. Taken together, these findings indicate a previously unreported process of amplified stimulated emission from polyexciton states that is consistent with quantum droplets and constitutes a form of exciton condensate. For studied samples, quantum droplets form provided that roughly 17 meV or less of thermal energy is available, which we hypothesize relates to polyexciton binding energy. Polyexciton ASE can produce pump-fluence-tunable red-shifted ASE even 120 meV lower in energy than biexciton ASE. Our findings convey the importance of biexciton and polyexciton populations in nanoplatelets and show that quantum droplets can exhibit light amplification at significantly lower photon energies than biexcitonic ASE.

Time- and angle-resolved photoemission spectroscopy (TR-ARPES) of TMDC monolayers and bilayers

Many unique properties in two-dimensional (2D) materials and their heterostructures rely on charge excitation, scattering, transfer, and relaxation dynamics across different points in the momentum space. Understanding these dynamics is crucial in both the fundamental study of 2D physics and their incorporation in optoelectronic and quantum devices. A direct method to probe charge carrier dynamics with momentum resolution is time- and angle-resolved photoemission spectroscopy (TR-ARPES). Such measurements have been challenging, since photoexcited carriers in many 2D monolayers reside at high crystal momenta, requiring probe photon energies in the extreme UV (EUV) regime. These challenges have been recently addressed by development of table-top pulsed EUV sources based on high harmonic generation, and the successful integration into a TR-ARPES and/or time-resolved momentum microscope. Such experiments will allow direct imaging of photoelectrons with superior time, energy, and crystal momentum resolution, with unique advantage over traditional optical measurements. Recently, TR-ARPES experiments of 2D transition metal dichalcogenide (TMDC) monolayers and bilayers have created unprecedented opportunities to reveal many intrinsic dynamics of 2D materials, such as bandgap renormalization, charge carrier scattering, relaxation, and wavefunction localization in moiré patterns. This perspective aims to give a short review of recent discoveries and discuss the challenges and opportunities of such techniques in the future.

Exciton Manifolds in Highly Ambipolar Doped WS2

The disentanglement of single and many particle properties in 2D semiconductors and their dependencies on high carrier concentration is challenging to experimentally study by pure optical means. We establish an electrolyte gated WS2 monolayer field-effect structure capable of shifting the Fermi level from the valence into the conduction band that is suitable to optically trace exciton binding as well as the single-particle band gap energies in the weakly doped regime. Combined spectroscopic imaging ellipsometry and photoluminescence spectroscopies spanning large n- and p-type doping with charge carrier densities up to 1014 cm-2 enable to study screening phenomena and doping dependent evolution of the rich exciton manifold whose origin is controversially discussed in literature. We show that the two most prominent emission bands in photoluminescence experiments are due to the recombination of spin-forbidden and momentum-forbidden charge neutral excitons activated by phonons. The observed interband transitions are redshifted and drastically weakened under electron or hole doping. This field-effect platform is not only suitable for studying exciton manifold but is also suitable for combined optical and transport measurements on degenerately doped atomically thin quantum materials at cryogenic temperatures.

Ultrafast Momentum-Resolved Hot Electron Dynamics in the Two-Dimensional Topological Insulator Bismuthene

Two-dimensional quantum spin Hall (QSH) insulators are a promising material class for spintronic applications based on topologically protected spin currents in their edges. Yet, they have not lived up to their technological potential, as experimental realizations are scarce and limited to cryogenic temperatures. These constraints have also severely restricted characterization of their dynamical properties. Here, we report on the electron dynamics of the novel room-temperature QSH candidate bismuthene after photoexcitation using time- and angle-resolved photoemission spectroscopy. We map the transiently occupied conduction band and track the full relaxation pathway of hot photocarriers. Intriguingly, we observe photocarrier lifetimes much shorter than those in conventional semiconductors. This is ascribed to the presence of topological in-gap states already established by local probes. Indeed, we find spectral signatures consistent with these earlier findings. Demonstration of the large band gap and the view into photoelectron dynamics mark a critical step toward optical control of QSH functionalities.

Through the Lens of a Momentum Microscope: Viewing Light-Induced Quantum Phenomena in 2D Materials

Van der Waals (vdW) materials at their 2D limit are diverse, flexible, and unique laboratories to study fundamental quantum phenomena and their future applications. Their novel properties rely on their pronounced Coulomb interactions, variety of crystal symmetries and spin-physics, and the ease of incorporation of different vdW materials to form sophisticated heterostructures. In particular, the excited state properties of many 2D semiconductors and semi-metals are relevant for their technological applications, particularly those that can be induced by light. In this paper, the recent advances made in studying out-of-equilibrium, light-induced, phenomena in these materials are reviewed using powerful, surface-sensitive, time-resolved photoemission-based techniques, with a particular emphasis on the emerging multi-dimensional photoemission spectroscopy technique of time-resolved momentum microscopy. The advances this technique has enabled in studying the nature and dynamics of occupied excited states in these materials are discussed. Then, the future research directions opened by these scientific and instrumental advancements are projected for studying the physics of 2D materials and the opportunities to engineer their band-structure and band-topology by laser fields.

Directly Visualizing Photoinduced Renormalized Momentum-Forbidden Electronic Quantum States in an Atomically Thin Semiconductor

Resolving the momentum degree of freedom of photoexcited charge carriers and exploring the excited-state physics in the hexagonal Brillouin zone of atomically thin semiconductors have recently attracted great interest for optoelectronic technologies. We demonstrate a combination of light-modulated scanning tunneling microscopy and the quasiparticle interference (QPI) technique to offer a directly accessible approach to reveal and quantify the unexplored momentum-forbidden electronic quantum states in transition metal dichalcogenide (TMD) monolayers. Our QPI results affirm the large spin-splitting energy at the spin-valley-coupled Q valleys in the conduction band (CB) of a tungsten disulfide monolayer. Furthermore, we also quantify the photoexcited carrier density-dependent band renormalization at the Q valleys. Our findings directly highlight the importance of the excited-state distribution at the Q valley in the band renormalization in TMDs and support the critical role of the CB Q valley in engineering the quantum electronic valley degree of freedom in TMD devices.

Microscopic Understanding of Ultrafast Charge Transfer in van der Waals Heterostructures

Van der Waals heterostructures show many intriguing phenomena including ultrafast charge separation following strong excitonic absorption in the visible spectral range. However, despite the enormous potential for future applications in the field of optoelectronics, the underlying microscopic mechanism remains controversial. Here we use time- and angle-resolved photoemission spectroscopy combined with microscopic many-particle theory to reveal the relevant microscopic charge transfer channels in epitaxial WS_{2}/graphene heterostructures. We find that the timescale for efficient ultrafast charge separation in the material is determined by direct tunneling at those points in the Brillouin zone where WS_{2} and graphene bands cross, while the lifetime of the charge separated transient state is set by defect-assisted tunneling through localized sulphur vacancies. The subtle interplay of intrinsic and defect-related charge transfer channels revealed in the present work can be exploited for the design of highly efficient light harvesting and detecting devices.

Twist-Angle-Dependent Ultrafast Charge Transfer in MoS2-Graphene van der Waals Heterostructures

Vertically stacked transition metal dichalcogenide-graphene heterostructures provide a platform for novel optoelectronic applications with high photoresponse speeds. Photoinduced nonequilibrium carrier and lattice dynamics in such heterostructures underlie these applications but have not been understood. In particular, the dependence of these photoresponses on the twist angle, a key tuning parameter, remains elusive. Here, using ultrafast electron diffraction, we report the simultaneous visualization of charge transfer and electron-phonon coupling in MoS₂-graphene heterostructures with different stacking configurations. We find that the charge transfer timescale from MoS₂ to graphene varies strongly with twist angle, becoming faster for smaller twist angles, and show that the relaxation timescale is significantly shorter in a heterostructure as compared to a monolayer. These findings illustrate that twist angle constitutes an additional tuning knob for interlayer charge transfer in heterobilayers and deepen our understanding of fundamental photophysical processes in heterostructures, of importance for future applications in optoelectronics and light harvesting.

Time-resolved ARPES Determination of a Quasi-Particle Band Gap and Hot Electron Dynamics in Monolayer MoS2

The electronic structure and dynamics of 2D transition metal dichalcogenide (TMD) monolayers provide important underpinnings both for understanding the many-body physics of electronic quasi-particles and for applications in advanced optoelectronic devices. However, extensive experimental investigations of semiconducting monolayer TMDs have yielded inconsistent results for a key parameter, the quasi-particle band gap (QBG), even for measurements carried out on the same layer and substrate combination. Here, we employ sensitive time- and angle-resolved photoelectron spectroscopy (trARPES) for a high-quality large-area MoS₂ monolayer to capture its momentum-resolved equilibrium and excited-state electronic structure in the weak-excitation limit. For monolayer MoS₂ on graphite, we obtain QBG values of ≈2.10 eV at 80 K and of ≈2.03 eV at 300 K, results well-corroborated by the scanning tunneling spectroscopy (STS) measurements on the same material.

Photodriven Transient Picosecond Top-Layer Semiconductor to Metal Phase-Transition in p-Doped Molybdenum Disulfide

Visible light is shown to create a transient metallic S-Mo-S surface layer on bulk semiconducting p-doped indirect-bandgap 2H-MoS₂ . Optically created electron-hole pairs separate in the surface band bending region of the p-doped semiconducting crystal causing a transient accumulation of electrons in the surface region. This triggers a reversible 2H-semiconductor to 1T-metal phase-transition of the surface layer. Electron-phonon coupling of the indirect-bandgap p-doped 2H-MoS₂ enables this efficient pathway even at a low density of excited electrons with a distinct optical excitation threshold and saturation behavior. This mechanism needs to be taken into consideration when describing the surface properties of illuminated p-doped 2H-MoS₂ . In particular, light-induced increased charge mobility and surface activation can cause and enhance the photocatalytic and photoassisted electrochemical hydrogen evolution reaction of water on 2H-MoS₂ . Generally, it opens up for a way to control not only the surface of p-doped 2H-MoS₂ but also related dichalcogenides and layered systems. The findings are based on the sensitivity of time-resolved electron spectroscopy for chemical analysis with photon-energy-tuneable synchrotron radiation.

Angle, Spin, and Depth Resolved Photoelectron Spectroscopy on Quantum Materials

The role of X-ray based electron spectroscopies in determining chemical, electronic, and magnetic properties of solids has been well-known for several decades. A powerful approach is angle-resolved photoelectron spectroscopy, whereby the kinetic energy and angle of photoelectrons emitted from a sample surface are measured. This provides a direct measurement of the electronic band structure of crystalline solids. Moreover, it yields powerful insights into the electronic interactions at play within a material and into the control of spin, charge, and orbital degrees of freedom, central pillars of future solid state science. With strong recent focus on research of lower-dimensional materials and modified electronic behavior at surfaces and interfaces, angle-resolved photoelectron spectroscopy has become a core technique in the study of quantum materials. In this review, we provide an introduction to the technique. Through examples from several topical materials systems, including topological insulators, transition metal dichalcogenides, and transition metal oxides, we highlight the types of information which can be obtained. We show how the combination of angle, spin, time, and depth-resolved experiments are able to reveal "hidden" spectral features, connected to semiconducting, metallic and magnetic properties of solids, as well as underlining the importance of dimensional effects in quantum materials.

A quantitative comparison of time-of-flight momentum microscopes and hemispherical analyzers for time- and angle-resolved photoemission spectroscopy experiments

Time-of-flight-based momentum microscopy has a growing presence in photoemission studies, as it enables parallel energy- and momentum-resolved acquisition of the full photoelectron distribution. Here, we report table-top extreme ultraviolet time- and angle-resolved photoemission spectroscopy (trARPES) featuring both a hemispherical analyzer and a momentum microscope within the same setup. We present a systematic comparison of the two detection schemes and quantify experimentally relevant parameters, including pump- and probe-induced space-charge effects, detection efficiency, photoelectron count rates, and depth of focus. We highlight the advantages and limitations of both instruments based on exemplary trARPES measurements of bulk WSe₂. Our analysis demonstrates the complementary nature of the two spectrometers for time-resolved ARPES experiments. Their combination in a single experimental apparatus allows us to address a broad range of scientific questions with trARPES.

Microscopic Coulomb interaction in transition-metal dichalcogenides

The quasi-two dimensional Coulomb interaction potential in transition metal dichalcogenides is determined using the Kohn-Sham wave functions obtained fromab initiocalculations. An effective form factor is derived that accounts for the finite extension of the wave functions in the direction perpendicular to the material layer. The resulting Coulomb matrix elements are used in microscopic calculations based on the Dirac Bloch equations yielding an efficient method to calculate the band gap and the opto-electronic material properties in different environments and under various excitation conditions.

Exciton Polarization and Renormalization Effect for Optical Modulation in Monolayer Semiconductors

The ideal quantum confinement structure of monolayer semiconductors offers prominent optical modulation capabilities that are mediated by enhanced many-body interactions. Herein, we establish an electrolyte-gating method for tuning the luminescence properties that are in transition metal dichalcogenide (TMDC) monolayers. We fabricate electric double-layer capacitors on TMDC/graphite heterostructures to investigate electric-field- and carrier-density-dependent photoluminescence. The exciton peak energy initially shows a slight quadratic red shift of ∼1 meV without carrier accumulations, which is caused by the quantum-confined Stark effect. In contrast, the exciton resonance exhibits a larger red shift up to 10 meV with the accumulated carrier density above 10¹³ cm^-2. These results indicate that the optical transitions can be largely modulated by the carrier density control in S- and Se-based TMDCs, as triggered by the doping-induced band gap renormalization effect. To further inspire this modulation capability, we also apply our method to electrolyte-based TMDC light-emitting devices. Biasing solely in electrolyte-induced p-i-n junctions yields pronounced red shifts up to 40 meV for exciton and trion electroluminescence. Consequently, our approach reveals that the doping effects in the high-carrier-density regimes are potentially significant for efficient optical modulation in monolayer semiconductors.

Direct Determination of Band-Gap Renormalization in the Photoexcited Monolayer MoS_{2}

A key feature of monolayer semiconductors, such as transition-metal dichalcogenides, is the poorly screened Coulomb potential, which leads to a large exciton binding energy (E_{b}) and strong renormalization of the quasiparticle band gap (E_{g}) by carriers. The latter has been difficult to determine due to a cancellation in changes of E_{b} and E_{g}, resulting in little change in optical transition energy at different carrier densities. Here, we quantify band-gap renormalization in macroscopic single crystal MoS_{2} monolayers on SiO_{2} using time and angle-resolved photoemission spectroscopy. At an excitation density above the Mott threshold, E_{g} decreases by as much as 360 meV. We compare the carrier density-dependent E_{g} with previous theoretical calculations and show the necessity of knowing both doping and excitation densities in quantifying the band gap.

Physics of excitons and their transport in two dimensional transition metal dichalcogenide semiconductors

Two-dimensional (2D) group-VI transition metal dichalcogenide (TMD) semiconductors, such as MoS₂, MoSe₂, WS₂ and others manifest strong light matter coupling and exhibit direct band gaps which lie in the visible and infrared spectral regimes. These properties make them potentially interesting candidates for applications in optics and optoelectronics. The excitons found in these materials are tightly bound and dominate the optical response, even at room temperatures. Large binding energies and unique exciton fine structure make these materials an ideal platform to study exciton behaviors in two-dimensional systems. This review article mainly focuses on studies of mechanisms that control dynamics of excitons in 2D systems – an area where there remains a lack of consensus in spite of extensive research. Firstly, we focus on the kinetics of dark and bright excitons based on a rate equation model and discuss on the role of previous ‘unsuspected’ dark excitons in controlling valley polarization. Intrinsically, dark and bright exciton energy splitting plays a key role in modulating the dynamics. In the second part, we review the excitation energy-dependent possible characteristic relaxation pathways of photoexcited carriers in monolayer and bilayer systems. In the third part, we review the extrinsic factors, in particular the defects that are so prevalent in single layer TMDs, affecting exciton dynamics, transport and non-radiative recombination such as exciton–exciton annihilation. Lastly, the optical response due to pump-induced changes in TMD monolayers have been reviewed using femtosecond pump–probe spectroscopy which facilitates the analysis of underlying physical process just after the excitation.

Two-dimensional (2D) group-VI transition metal dichalcogenide (TMD) semiconductors, such as MoS₂, MoSe₂, WS₂ and others manifest strong light matter coupling and exhibit direct band gaps which lie in the visible and infrared spectral regimes.

Spin Structure of K Valleys in Single-Layer WS_{2} on Au(111)

The spin structure of the valence and conduction bands at the K[over ¯] and K[over ¯]^{'} valleys of single-layer WS_{2} on Au(111) is determined by spin- and angle-resolved photoemission and inverse photoemission. The bands confining the direct band gap of 1.98 eV are out-of-plane spin polarized with spin-dependent energy splittings of 417 meV in the valence band and 16 meV in the conduction band. The sequence of the spin-split bands is the same in the valence and in the conduction bands and opposite at the K[over ¯] and the K[over ¯]^{'} high-symmetry points. The first observation explains "dark" excitons discussed in optical experiments; the latter points to coupled spin and valley physics in electron transport. The experimentally observed band dispersions are discussed along with band structure calculations for a freestanding single layer and for a single layer on Au(111).

A Perspective on the Application of Spatially Resolved ARPES for 2D Materials

In this paper, a perspective on the application of Spatially- and Angle-Resolved PhotoEmission Spectroscopy (ARPES) for the study of two-dimensional (2D) materials is presented. ARPES allows the direct measurement of the electronic band structure of materials generating extremely useful insights into their electronic properties. The possibility to apply this technique to 2D materials is of paramount importance because these ultrathin layers are considered fundamental for future electronic, photonic and spintronic devices. In this review an overview of the technical aspects of spatially localized ARPES is given along with a description of the most advanced setups for laboratory and synchrotron-based equipment. This technique is sensitive to the lateral dimensions of the sample. Therefore, a discussion on the preparation methods of 2D material is presented. Some of the most interesting results obtained by ARPES are reported in three sections including: graphene, transition metal dichalcogenides (TMDCs) and 2D heterostructures. Graphene has played a key role in ARPES studies because it inspired the use of this technique with other 2D materials. TMDCs are presented for their peculiar transport, optical and spin properties. Finally, the section featuring heterostructures highlights a future direction for research into 2D material structures.

The Dielectric Impact of Layer Distances on Exciton and Trion Binding Energies in van der Waals Heterostructures

The electronic and optical properties of monolayer transition-metal dichalcogenides (TMDs) and van der Waals heterostructures are strongly subject to their dielectric environment. In each layer, the field lines of the Coulomb interaction are screened by the adjacent material, which reduces the single-particle band gap as well as exciton and trion binding energies. By combining an electrostatic model for a dielectric heteromultilayered environment with semiconductor many-particle methods, we demonstrate that the electronic and optical properties are sensitive to the interlayer distances on the atomic scale. An analytic treatment is used to provide further insight into how the interlayer gap influences different excitonic transitions. Spectroscopical measurements in combination with a direct solution of a three-particle Schrödinger equation reveal trion binding energies that correctly predict recently measured interlayer distances and shed light on the effect of temperature annealing.

Synthesis, structure and applications of graphene-based 2D heterostructures

With the profuse amount of two-dimensional (2D) materials discovered and the improvements in their synthesis and handling, the field of 2D heterostructures has gained increased interest in recent years. Such heterostructures not only overcome the inherent limitations of each of the materials, but also allow the realization of novel properties by their proper combination. The physical and mechanical properties of graphene mean it has a prominent place in the area of 2D heterostructures. In this review, we will discuss the evolution and current state in the synthesis and applications of graphene-based 2D heterostructures. In addition to stacked and in-plane heterostructures with other 2D materials and their potential applications, we will also cover heterostructures realized with lower dimensionality materials, along with intercalation in few-layer graphene as a special case of a heterostructure. Finally, graphene heterostructures produced using liquid phase exfoliation techniques and their applications to energy storage will be reviewed.

Structural Changes as a Function of Thickness in [(SnSe)1+δ]mTiSe2 Heterostructures

Single- and few-layer metal chalcogenide compounds are of significant interest due to structural changes and emergent electronic properties on reducing dimensionality from three to two dimensions. To explore dimensionality effects in SnSe, a series of [(SnSe)_1+δ]_mTiSe₂ intergrowth structures with increasing SnSe layer thickness (m = 1-4) were prepared from designed thin-film precursors. In-plane diffraction patterns indicated that significant structural changes occurred in the basal plane of the SnSe constituent as m is increased. Scanning transmission electron microscopy cross-sectional images of the m = 1 compound indicate long-range coherence between layers, whereas the m ≥ 2 compounds show extensive rotational disorder between the constituent layers. For m ≥ 2, the images of the SnSe constituent contain a variety of stacking sequences of SnSe bilayers. Density functional theory calculations suggest that the formation energy is similar for several different SnSe stacking sequences. The compounds show unexpected transport properties as m is increased, including the first p-type behavior observed in (MSe)_m(TiSe₂)_n compounds. The resistivity of the m ≥ 2 compounds is larger than for m = 1, with m = 2 being the largest. At room temperature, the Hall coefficient is positive for m = 1 and negative for m = 2-4. The Hall coefficient of the m = 2 compound changes sign as temperature is decreased. The room-temperature Seebeck coefficient, however, switches from negative to positive at m = 3. These properties are incompatible with single band transport indicating that the compounds are not simple composites.

Electronic properties of single-layer tungsten disulfide on epitaxial graphene on silicon carbide

This work reports an electronic and micro-structural study of an appealing system for optoelectronics: tungsten disulfide (WS₂) on epitaxial graphene (EG) on SiC(0001). The WS₂ is grown via chemical vapor deposition (CVD) onto the EG. Low-energy electron diffraction (LEED) measurements assign the zero-degree orientation as the preferential azimuthal alignment for WS₂/EG. The valence-band (VB) structure emerging from this alignment is investigated by means of photoelectron spectroscopy measurements, with both high space and energy resolution. We find that the spin-orbit splitting of monolayer WS₂ on graphene is of 462 meV, larger than what is reported to date for other substrates. We determine the value of the work function for the WS₂/EG to be 4.5 ± 0.1 eV. A large shift of the WS₂ VB maximum is observed as well, due to the lowering of the WS₂ work function caused by the donor-like interfacial states of EG. Density functional theory (DFT) calculations carried out on a coincidence supercell confirm the experimental band structure to an excellent degree. X-ray photoemission electron microscopy (XPEEM) measurements performed on single WS₂ crystals confirm the van der Waals nature of the interface coupling between the two layers. In virtue of its band alignment and large spin-orbit splitting, this system gains strong appeal for optical spin-injection experiments and opto-spintronic applications in general.

In situ construction of a heterojunction over the surface of a sandwich structure semiconductor for highly efficient photocatalytic H2 evolution under visible light irradiation

Developing a heterostructure on the surface of a "sandwich" structure semiconductor is essential for full utilization of its heterojunction function and hence for designing efficient solar energy conversion systems. Here, we show that 2D-2D MoS₂/MnSb₂S₄ heterostructure composites are designed for the first time and successfully synthesized by a simple in situ calcination pathway. Under visible light irradiation, the ca. 3.3 wt% MoS₂/MnSb₂S₄ samples exhibited the highest activity for H₂ evolution, which was 7.7 times higher than that of the pristine MnSb₂S₄ monolayer. The outstanding photocatalytic performance was attributed to the MoS₂ nanosheets intimately growing on the surface [SbS]⁺ layers of monolayer MnSb₂S₄ nanosheets with the [SbS]⁺-[MnS₂]^2--[SbS]⁺ sandwich substructure to form the 2D-2D MoS₂/MnSb₂S₄ heterojunction structure. More importantly, we prove that this specific heterojunction structure can lead to more weakening of the constraint of the valence electrons in the composited photocatalysts, which can promote the transfer of photogenerated electrons from MnSb₂S₄ to MoS₂. The present study provides a new design strategy for the construction of a heterostructure to improve the photocatalytic H₂ production activity highly efficiently.

Long-Lived Direct and Indirect Interlayer Excitons in van der Waals Heterostructures

We report the observation of a doublet structure in the low-temperature photoluminescence of interlayer excitons in heterostructures consisting of monolayer MoSe₂ and WSe₂. Both peaks exhibit long photoluminescence lifetimes of several tens of nanoseconds up to 100 ns verifying the interlayer nature of the excitons. The energy and line width of both peaks show unusual temperature and power dependences. While the low-energy peak dominates the spectra at low power and low temperatures, the high-energy peak dominates for high power and temperature. We explain the findings by two kinds of interlayer excitons being either indirect or quasi-direct in reciprocal space. Our results provide fundamental insights into long-lived interlayer states in van der Waals heterostructures with possible bosonic many-body interactions.

Angle-resolved photoemission spectroscopy for the study of two-dimensional materials

Quantum systems in confined geometries allow novel physical properties that cannot easily be attained in their bulk form. These properties are governed by the changes in the band structure and the lattice symmetry, and most pronounced in their single layer limit. Angle-resolved photoemission spectroscopy (ARPES) is a direct tool to investigate the underlying changes of band structure to provide essential information for understanding and controlling such properties. In this review, recent progresses in ARPES as a tool to study two-dimensional atomic crystals have been presented. ARPES results from few-layer and bulk crystals of material class often referred as “beyond graphene” are discussed along with the relevant developments in the instrumentation.

The Role of Electronic and Phononic Excitation in the Optical Response of Monolayer WS2 after Ultrafast Excitation

Transient changes of the optical response of WS₂ monolayers are studied by femtosecond broadband pump-probe spectroscopy. Time-dependent absorption spectra are analyzed by tracking the line width broadening, bleaching, and energy shift of the main exciton resonance as a function of time delay after the excitation. Two main sources for the pump-induced changes of the optical response are identified. Specifically, we find an interplay between modifications induced by many-body interactions from photoexcited carriers and by the subsequent transfer of the excitation to the phonon system followed by cooling of the material through the heat transfer to the substrate.