Monolayer Transition Metal Dichalcogenides Research Articles

Direct Hot-Electron Transfer at the Au Nanoparticle/Monolayer Transition-Metal Dichalcogenide Interface Observed with Ultrahigh Spatiotemporal Resolution.

Plasmon-induced hot-electron transfer at the metallic nanoparticle/semiconductor interface is the basis of plasmon-enhanced photocatalysis and energy harvesting. However, limited by the nanoscale size of hot spots and femtosecond time scale of hot-electron transfer, direct observation is still challenging. Herein, by using spatiotemporal-resolved photoemission electron microscopy with a two-color pump-probe beamline, we directly observed such a process with a concise system, the Au nanoparticle/monolayer transition-metal dichalcogenide (TMD) interface. The ultrafast hot-electron transfer from Au nanoparticles to monolayer TMDs and the plasmon-enhanced transfer process were directly measured and verified through an in situ comparison with the Au film/TMD interface and free TMDs. The lifetime at the Au nanoparticle/MoSe2 interface decreased from 410 to 42 fs, while the photoemission intensities exhibited a 27-fold increase compared to free MoSe2. We also measured the evolution of hot electrons in the energy distributions, indicating the hot-electron injection and decay happened in an ultrafast time scale of ∼50 fs without observable electron cooling.

Biaxial strain tuned upconversion photoluminescence of monolayer WS2

Monolayer tungsten disulfide (1L-WS2) is a direct bandgap atomic-layered semiconductor material with strain tunable optical and optoelectronic properties among the monolayer transition metal dichalcogenides (1L-TMDs). Here, we demonstrate biaxial strain tuned upconversion photoluminescence (UPL) from exfoliated 1L-WS2 flakes transferred on a flexible polycarbonate cruciform substrate. When the biaxial strain applied to 1L-WS2 increases from 0 to 0.51%, it is observed that the UPL peak position is redshifted by up to 60 nm/% strain, while the UPL intensity exhibits exponential growth with the upconversion energy difference varying from − 303 to − 120 meV. The measured power dependence of UPL from 1L-WS2 under biaxial strain reveals the one photon involved multiphonon-mediated upconversion mechanism. The demonstrated results provide new opportunities in advancing TMD-based optical upconversion devices for future flexible photonics and optoelectronics.

Electric field induced out-of-plane second-order optical nonlinearity in monolayer transition metal dichalcogenides

Electric field induced out-of-plane second-order optical nonlinearity in monolayer transition metal dichalcogenides

Resonant Band Hybridization in Alloyed Transition Metal Dichalcogenide Heterobilayers.

Bandstructure engineering using alloying is widely utilized for achieving optimized performance in modern semiconductor devices. While alloying has been studied in monolayer transition metal dichalcogenides, its application in van der Waals heterostructures built from atomically thin layers is largely unexplored. Here, heterobilayers made from monolayers of WSe2 (or MoSe2) and MoxW1 - xSe2 alloy are fabricated and nontrivial tuning of the resultant bandstructure is observed as a function of concentration x. This evolution is monitored by measuring the energy of photoluminescence (PL) of the interlayer exciton (IX) composed of an electron and hole residing in different monolayers. In MoxW1 - xSe2/WSe2, a strong IX energy shift of ≈100 meV is observed for x varied from 1 to 0.6. However, for x < 0.6 this shift saturates and the IX PL energy asymptotically approaches that of the indirect bandgap in bilayer WSe2. This observation is theoretically interpreted as the strong variation of the conduction band K valley for x > 0.6, with IX PL arising from the K - K transition, while for x < 0.6, the bandstructure hybridization becomes prevalent leading to the dominating momentum-indirect K - Q transition. This bandstructure hybridization is accompanied with strong modification of IX PL dynamics and nonlinear exciton properties. This work provides foundation for bandstructure engineering in van der Waals heterostructures highlighting the importance of hybridization effects and opening a way to devices with accurately tailored electronicproperties.

Doping Ability Modulated by Interlayer Coupling in AA' and AB Stacked Bilayer V-WS2 Grown with Chemical Vapor Deposition.

Doping in transition metal dichalcogenide (TMD) has received extensive attention for its prospect in the application of photoelectric devices. Currently researchers focus on the doping ability and doping distribution in monolayer TMD and have obtained a series of achievements. Bilayer TMD has more excellent properties compared with monolayer TMD. Moreover, bilayer TMD with different stacking structures presents varying performance due to the difference in interlayer coupling. Herein, this work focuses on doping ability of dopants in different bilayer stacking structures that has not been studied yet. Results of this work show that the doping ability of V atoms in bilayer AA' and AB stacked WS2 is different, and the doping concentration of V atoms in AB stacked WS2 is higher than in AA' stacked WS2. Moreover, dopants from top and bottom layer can be distinguished by scanning transmission electron microscopy (STEM)image. Density functional theory (DFT) calculation further confirms the doping rule. This study reveals the mechanism of the different doping ability caused by stacking structures in bilayer TMD and lays a foundation for further preparation of controllable-doping bilayer TMD materials.

Langmuir-Blodgett organic films deposition for the formation of conformal 2D inorganic-organic heterostructures

2D inorganic materials offer the opportunity to design and build heterostructures with unprecedented atomic control down to a monolayer. Integrating inorganic 2D layered materials, with the unlimited variety of organic molecules, provides a good platform for basic research and the formation of functional 2D organic-inorganic heterostructures, with tailored optoelectronic properties. In this work, we propose to employ a Langmuir-Blodgett (LB) monolayer deposition technique for the large-scale assembly of 2D inorganic-organic heterostructures. First, the transition-metal dichalcogenidemonolayers (TMDCs), MoS2 and WSe2, are grown by a chemical vapor deposition (CVD)-based method. This is followed by the LB deposition of a monolayer(s) of Zn(II)-5-(4-aminophenyl)-10,15,20-(triphenyl)porphyrin (Zn-TPP(1f)). The morphology, optical and electrical properties were systematically studied for the two TMDC systems. Despite the similarities between both TMDCs, their heterostructures with Zn-TPP(1f) monolayers show a very different optical response; a strong quenching of the photoluminescence intensity is measured for the Zn-TPP(1f)/MoS2 system, an opposite trend, PL enhancement, is obtained for the WSe2 based heterostructure. These results are explained by the differences within their electronic band diagrams, as elucidated using a wide variety of spectroscopic methodologies. The orthogonal impact on the PL emission may serve as a general platform for the formation of 2D organic-inorganic heterostructures, with controlled optical properties.

Controlled Sixfold Symmetric Exfoliation of Oriented MoS2 Monolayers by Coulomb Force

AbstractAtoms, molecules, and nanoparticles can be spatially manipulated by an atomic force microscopy (AFM) tip, through van der Waals (vdW) and/or Coulomb forces. These point‐to‐point manipulations are highly accurate at nanoscale, facilitating the construction and modification of nanostructures. Nevertheless, it is difficult to manipulate 2D layers in vdW crystals by an AFM tip, because the tip‐induced attractive force is usually insufficient to outcompete the interlaminar vdW forces. Herein, manipulation of the surface layers on a MoS2 single crystal by a conductive AFM tip is successfully reported. By applying a bias between the tip and MoS2, the Coulomb attractive force allows the topmost MoS2 layers to be picked up. These exfoliated layers are deformed into micron‐sized bubbles with sixfold symmetry, which are composed of high‐quality monolayers and visually reflecting the hexagonal lattice orientation. The underlying mechanisms of the sixfold symmetric exfoliation and the formation of monolayers are discussed by in situ monitoring of the tunneling volt‐ampere characteristics and simulation of the force distribution. The findings open a new route to obtain high‐quality transition metal dichalcogenide (TMD) monolayers and their derived nanostructures on the surface of TMD single crystals for optoelectronic and photonic device applications.

The deep-acceptor nature of the chalcogen vacancies in 2D transition-metal dichalcogenides

Chalcogen vacancies in the semiconducting monolayer transition-metal dichalcogenides (TMDs) have frequently been invoked to explain a wide range of phenomena, including both unintentional p-type and n-type conductivity, as well as sub-band gap defect levels measured via tunneling or optical spectroscopy. These conflicting interpretations of the deep versus shallow nature of the chalcogen vacancies are due in part to shortcomings in prior first-principles calculations of defects in the semiconducting two-dimensional TMDs that have been used to explain experimental observations. Here we report results of hybrid density functional calculations for the chalcogen vacancy in a series of monolayer TMDs, correctly referencing the thermodynamic charge transition levels to the fundamental band gap (as opposed to the optical band gap). We find that the chalcogen vacancies are deep acceptors and cannot lead to n-type or p-type conductivity. Both the (0/−1) and (−1/−2) transition levels occur in the gap, leading to paramagnetic charge states S=1/2 and S = 1, respectively, in a collinear-spin representation. We discuss trends in terms of the band alignments between the TMDs, which can serve as a guide to future experimental studies of vacancy behavior.

Ultralow Auger-Assisted Interlayer Exciton Annihilation in WS2/WSe2 Moiré Heterobilayers.

Transition metal dichalcogenide (TMD) heterobilayers have emerged as a promising platform for exploring solid-state quantum simulators and many-body quantum phenomena. Their type II band alignment, combined with the moiré superlattice, inevitably leads to nontrivial exciton interactions and dynamics. Here, we unveil the distinct Auger annihilation processes for delocalized interlayer excitons in WS2/WSe2 moiré heterobilayers. By fitting the characteristic efficiency droop and bimolecular recombination rate, we quantitatively determine an ultralow Auger coefficient of 1.3 × 10-5 cm2 s-1, which is >100-fold smaller than that of excitons in TMD monolayers. In addition, we reveal selective exciton upconversion into the WSe2 layer, which highlights the significance of intralayer electron Coulomb interactions in dictating the microscopic scattering pathways. The distinct Auger processes arising from spatial electron-hole separation have important implications for TMD heterobilayers while endowing interlayer excitons and their strongly correlated states with unique layer degrees of freedom.

Gate tunable linear dichroism in monolayer 1T'-MoS2.

The linear dichroism demonstrates promising applications in the fields of polarization-resolved photodetectors and polarization optical imaging. Herein, we study the optical properties of monolayer 1T'-MoS2 based on a four-band effective k · p Hamiltonian within the framework of linear response theory. Owing to the anisotropic band structure, the k-resolved optical transition matrix elements associated with armchair(x) and zigzag(y) direction polarized light exhibit a staggered pattern. The anisotropy of the optical absorption spectrum is shown to sensitively depend on the photon energy, the light polarization and the gate voltage. A gate voltage can continuously modulate the anisotropy of the optical absorption spectra, rendering it isotropic or even reversing the initial anisotropy. This modulation leads to linear dichroism conversions across multiple wavelengths. Our findings are useful to design polarized photodetectors and sensors based on monolayer 1T'-MoS2. Our results are also applicable to other monolayer transition metal dichalcogenides with 1T' structure.

Temperature dependence of photoluminescence in twisted heterobilayers of transition-metal dichalcogenides

Two-dimensional (2D) van der Waals transition metal dichalcogenides (TMDCs) are highly attractive due to their novel phenomena and potential device applications. Especially, twisted heterojunction of different 2D TMDC monolayers for various twist angles (θ) is one of the hot issues in the area of 2D TMDCs because they exhibit exotic quantum behaviors. Here, we report the effect of temperature (T) on the θ-dependent variation of photoluminescence (PL) spectra of MoS2/WS2 heterobilayers. The PL peak-vs-T variations are divided into three θ regions. Near θ = 0 and 60°, the PL peak energies show almost monotonically-decreasing behaviors with increasing T, caused by stronger electron-phonon interactions at higher T. However, in the intermediate θ range, the PL peaks show similar red shifts over almost full T range except anomalous blue shifts in a short T range near ∼150 K. We discuss this anomalous PL behaviors based on hybridization of interlayer and intralayer excitons, resulting in the formation of hybridized excitons, correlated with structural transition.

Tuning the atomic and electronic structures of mirror twin boundaries in molecular beam epitaxy grown MoSe2 monolayers via rhenium doping

Interplay between defects like mirror twin boundaries (MTBs) and dopants may provide additional opportunities for furthering the research on two-dimensional monolayer (ML) transition metal dichalcogenides. In this work, we successfully dope rhenium (Re) into molecular beam epitaxy grown ML MoSe2 and confirm the formation of a new type of MTBs, named 4|4E-M (M represents metal, Mo/Re) according to the configuration. Data from statistic atomic resolution scanning transmission electron microscopy also reveals a preferable MTB enrichment of Re dopants, rather than intra-domain. In conjunction with density functional theory calculation results, we propose the possible routes for Re doping induced formation of 4|4E-M MTBs. Electronic structures of Re doped MTBs in ML MoSe2 are also predicted theoretically and then preliminarily tested by scanning tunneling microscopy and spectroscopy.

Ab initio computation of low-temperature miscibility gap of V(Se,Te)2

Monolayers of quasi-binary transition metal dichalcogenides are a focus of attention as they are expected to exhibit many exciting physical properties, but not much is known about their thermodynamic stability. In this study, we use a combination of global energy landscape exploration, local minimization using density functional theory, and thermodynamic analysis, to compute the composition-temperature phase diagram of the quasi-binary V(Se,Te)2 system, both for a 2H monolayer and for the analogous bulk material. We find that the phase diagram exhibits a miscibility gap, with a critical temperature T c = 500 K and T c = 650 K for monolayer and bulk, respectively, indicating that the system prefers to form solid solution phases. In particular, at room temperature, the thermodynamically stable phase of the monolayer would correspond to a decomposition into two solid solution monolayers, with ca. 90% Se and Te content, respectively.

Nanoscrolls of Janus Monolayer Transition Metal Dichalcogenides.

Tubular structures of transition metal dichalcogenides (TMDCs) have attracted attention in recent years due to their emergent physical properties, such as the giant bulk photovoltaic effect and chirality-dependent superconductivity. To understand and control these properties, it is highly desirable to develop a sophisticated method to fabricate TMDC tubular structures with smaller diameters and a more uniform crystalline orientation. For this purpose, the rolling up of TMDC monolayers into nanoscrolls is an attractive approach to fabricating such a tubular structure. However, the symmetric atomic arrangement of a monolayer TMDC generally makes its tubular structure energetically unstable due to considerable lattice strain in curved monolayers. Here, we report the fabrication of narrow nanoscrolls by using Janus TMDC monolayers, which have an out-of-plane asymmetric structure. Janus WSSe and MoSSe monolayers were prepared by the plasma-assisted surface atom substitution of WSe2 and MoSe2 monolayers, respectively, and then were rolled by solution treatment. The multilayer tubular structures of Janus nanoscrolls were revealed by scanning transmission electron microscopy observations. Atomic resolution elemental analysis confirmed that the Janus monolayers were rolled up with the Se-side surface on the outside. We found that the present nanoscrolls have the smallest diameter of about 5 nm, which is almost the same as the value predicted by the DFT calculation. The difference in work functions between the S- and Se-side surfaces was measured by Kelvin probe force microscopy, which is in good agreement with the theoretical prediction. Strong interlayer interactions and anisotropic optical responses of the Janus nanoscrolls were also revealed by Raman and photoluminescence spectroscopy.

Control of the valley polarization of monolayer WSe2 by Dexter-like coupling

Intervalley scattering mechanisms strongly affect the dynamics of excitonic complexes in transition metal dichalcogenide monolayers. Here, we investigate the excitation energy dependence of the valley polarization of excitons in a WSe2 monolayer. We observe that the valley polarization drastically decreases when the excitation is resonant with the B 1s resonance. This behavior can be explained by a Dexter-like coupling in the momentum space between exciton states residing in opposite valleys but with the same spin configuration. This induces a net transfer of the exciton population from the optically driven valley towards the opposite, undriven valley. We observe the long-term fingerprints of this population transfer as a vanishing valley polarization for the neutral exciton, and a negative valley polarization for biexcitonic complexes, in qualitative agreement with theoretical predictions based on a fully microscopic many-particle approach. This, together with a decrease of the PL energy when the excitation is resonant with the B 1s state, points to the prominent role of the Dexter-like coupling in the exciton dynamics of atomically thin semiconductors.

Upconversion photoluminescence of monolayer WSe2 with biaxial strain tuning.

Mechanical strain can be used to tune the optical properties of monolayer transition metal dichalcogenides (1L-TMDs). Here, upconversion photoluminescence (UPL) from 1L-WSe2 flakes is tuned with biaxial strain induced by cruciform bending and indentation method. It is found that the peak position of UPL is redshifted by around 24 nm as the applied biaxial strain increases from 0% to 0.51%. At the same time, the UPL intensity increases exponentially for the upconversion energy difference that lies within a broad range between -157 meV to -37 meV. The observed linear and sublinear power dependence of UPL emission in 1L-WSe2 with and without biaxial strain at three different excitation wavelengths of 784 nm, 800 nm, and 820 nm indicates the multiphonon-assisted one-photon upconversion emission process. The results of strain-dependent UPL emission from 1L-TMDs pave a unique path to the advances in photon upconversion applications and optoelectronic devices.

Photo-oxidative Crack Propagation in Transition Metal Dichalcogenides.

Monolayered transition-metal dichalcogenides (TMDs) are easily exposed to air, and their crystal quality can often be degraded via oxidation, leading to poor electronic and optical device performance. The degradation becomes more severe in the presence of defects, grain boundaries, and residues. Here, we report crack propagation in pristine TMD monolayers grown by chemical vapor deposition under ambient conditions and light illumination. Under a high relative humidity (RH) of ∼60% and white light illumination, the cracks appear randomly. Photo-oxidative cracks gradually propagated along the grain boundaries of the TMD monolayers. In contrast, under low RH conditions of ∼2%, cracks were scarcely observed. Crack propagation is predominantly attributed to the accumulation of water underneath the TMD monolayers, which is preferentially absorbed by hygroscopic alkali metal-based precursor residues. Crack propagation is further accelerated by the cyclic process of photo-oxidation in a basic medium, leading to localized tensile strain. We also found that such crack propagation is prevented after the removal of alkali metals via the transfer of the sample to other substrates.

Spectroscopy of monolayer and multilayer tungsten disulfide under high pressure

Recently exfoliated monolayer and multilayered transition metal dichalcogenides have gathered significant interest based on their tunable bandgap and extremely high carrier mobility. We have investigated the Raman and photoluminescence spectra of monolayer and multilayer WS2 as a function of pressure. The Raman-inactive mode B1u, which is activated by structural disorder, was revealed at 6.7 GPa in monolayers, at 8.0 GPa in bilayers, and at 13.7 GPa in multilayers, respectively. With the enhancement of pressure-induced interlayer interaction, the crystal phase transition due to layer sliding like 2Hc to 2Ha occurs at 14.8 and 18.7 GPa in bilayers and multilayers, as evidenced by the split of E12g and B1u. The electronic phase transition of the monolayer is supposed to be a direct K-K bandgap changing to an indirect Λ-K bandgap at 2.6 GPa. These observations contribute to a better understanding of the impact of interlayer interactions on the modulation of WS2 energy bands and structure, as well as fundamental studies of two-dimensional layered materials, which can inform the development of device applications.

Uniaxial Strain Tuning of Upconversion Photoluminescence in Monolayer WSe2

Strain engineering is one of the leading mechanical ways to tune the optical properties of monolayer transition metal dichalcogenides among different techniques. Here, uniaxial strain is applied on exfoliated 1L‐WSe2 flakes transferred on flexible polycarbonate substrates to study the strain tuning of upconversion photoluminescence. It is demonstrated that the peak position of upconversion photoluminescence is redshifted by around 20 nm as the applied uniaxial strain increases from 0% to 1.17%, while the intensity of upconversion photoluminescence increases exponentially for the upconversion energy difference ranging from −155 to −32 meV. The linear and sublinear power dependence of upconversion photoluminescence is observed for different excitation wavelengths with and without uniaxial strain, suggesting the multiphonon‐assisted mechanism in one‐photon regime for the upconversion process. These results offer the potential to advance 2D material‐based optical upconversion applications in night vision, strain‐tunable infrared detection, and flexible optoelectronics.

Electrically Confined Electroluminescence of Neutral Excitons in WSe2 Light-Emitting Transistors.

Monolayer transition metal dichalcogenides (TMDs) have drawn significant attention for their potential in optoelectronic applications due to their direct band gap and exceptional quantum yield. However, TMD-based light-emitting devices have shown low external quantum efficiencies as imbalanced free carrier injection often leads to the formation of non-radiative charged excitons, limiting practical applications. Here, electrically confined electroluminescence (EL) of neutral excitons in tungsten diselenide (WSe2) light-emitting transistors (LETs) based on the van der Waals heterostructure is demonstrated. The WSe2 channel is locally doped to simultaneously inject electrons and holes to the 1D region by a local graphene gate. At balanced concentrations of injected electrons and holes, the WSe2 LETs exhibit strong EL with a high external quantum efficiency (EQE) of ≈8.2 % at room temperature. These experimental and theoretical results consistently show that the enhanced EQE could be attributed to dominant exciton emission confined at the 1D region while expelling charged excitons from the active area by precise control of external electric fields. This work shows a promising approach to enhancing the EQE of 2D light-emitting transistors and modulating the recombination of exciton complexes for excitonic devices.