Molybdenum disulfide (MoS$_2$) attracts attention as a high efficient and low cost photocatalyst for hydrogen production, but suffers from low conductance and high recombination rate of photo-generated charge carriers. In this work, we investigate the MoS$_2$ heterostructures with graphene variants (GVs), including graphene, graphene oxide, and their boron- and nitrogen-doped variants, by using first-principles calculations. Systematic comparison between graphene and graphene oxide composites is performed, and contrary effect of B and N doping on interface function and hydrogen evolution is clarified. We find that upon the formation of the interfaces some amount of electronic charge transfers from the GV side to the MoS$_2$ layer, inducing the creation of interface dipole and the reduction of work function, which is more pronounced in the graphene oxide composites. Moreover, our results reveal that N doping enhances the interface functions by forming donor-type interface states, whereas B doping reduces those functions by forming acceptor-type interface states. However, the B-doped systems exhibit lower Gibbs free energy difference for hydrogen adsorption on GV side than the N-doped systems, which deserves much consideration in the design of new functional photocatalysts.
In this work we study the electronic structure and magnetism of a TiO2 film grown on another non-magnetic oxide such as a LaAlO3 (001) substrate, concentrating on the role played by structural relaxation and oxygen vacancies. Using Density Functional Theory ab-initio methods, we study the free-standing anatase film as well as the interfaces with either the LaO or AlO2 planes of LaAlO3, focusing on the possibility of magnetic solutions. Our results show that the interface LaO/TiO2 is favored against the AlO2/TiO2 one if no oxygen vacancies are present in the interface whereas the contrary happe…
Based on first-principles calculations, the materials properties (structural, electronic, vibrational, and optical properties) of out-of-plane heterostructures formed from the transiti on metal dichalcogenides, specifically MoS$_2$-WSe$_2$ and WS$_2$-MoSe$_2$ were investigated. The heterostructures of MoS$_2$-WSe$_2$ and WS$_2$-MoSe$_2$ are found to be direct and ind irect band gap semiconductors, respectively. However, a direct band gap in the WS$_2$-MoSe$_2$ heterostructure can be achieved by applying compressive strain. Furthermore, the excitoni c peaks in both monolayer and bilayer heteros…
Van der Waals heterostructures are constructed by stacking different atomic layers and can inherit many novel electronic and optical properties from the constituting atomic layers. Mechanical stability is of key importance for the high performance of nano devices based on the van der Waals heterostructure. In particular, buckling instability is a critical mechanical issue for the heterostructure due to its two-dimensional nature. Using graphene/MoS$_2$ heterostructure as an example, the present work demonstrates the relationship between the buckling instability and the inevitable misfit strain…
We propose a tubal van der Waals heterostructure by rolling up the graphene and MoS2 atomic layers into a tubal form. We illustrate that the interlayer space for the tubal van der Waals heterostructure can be varied in a specific range, which is determined by the competition between the interlayer van der Waals force and the mechanical properties of the atomic layers. The variability of the interlayer space can be utilized to efficiently tune mechanical properties of the tubal van der Waals heterostructure. More specifically, we demonstrate that the Poisson's ratio of the tubal van der Waals h…
We combined the bond order length strength and bond charge models and the topological concept to obtain the nonbonding, bonding, and antibonding states of the T type WTe$_2$/MoS$_2$ heterostructure.The energy band projection method and electronic information entropy are remarkable approaches for analyzing the electronic properties of various structures based on DFT calculations. This study provides a new way to describe the electronic properties of T type heterostructures and calculate the electron and bonding state probabilities.
We perform classic molecular dynamics simulations to comparatively investigate the mechanical properties of single-layer MoS2 and a graphene/MoS2/graphene heterostructure under uniaxial tension. We show that the lattice mismatch between MoS2 and graphene will lead to an spontaneous strain energy in the interface. The Young's modulus of MoS2 can be enhanced by a factor of five by sandwiching it between two graphene layers. While the stiffness is enhanced, the yield strain of the MoS2 is reduced due to lateral buckling of the outer graphene layers due to the applied mechanical tension.
Graphene/MoS$_2$ van der Waals (vdW) heterostructures have promising technological applications due to their unique properties and functionalities. Many experimental and theoretical research groups across the globe have made outstanding contributions to benchmark the properties of graphene/MoS$_2$ heterostructures. Even though some research groups have already made an attempt to model the graphene/MoS$_2$ heterostructures using {\it first-principles} calculations, there exists several discrepancies in the results from different theoretical research groups and the experimental findings. In the …
The interlayer coupling in van der Waals heterostructures governs a variety of optical and electronic properties. The intrinsic dipole moment of Janus transition metal dichalcogenides (TMDs) offers a simple and versatile approach to tune the interlayer interactions. In this work, we demonstrate how the van der Waals interlayer coupling and charge transfer of Janus MoSSe/MoS2 heterobilayers can be tuned by the twist angle and interface composition. Specifically, the Janus heterostructures with a sulfur/sulfur (S/S) interface display stronger interlayer coupling than the heterostructures with a …
Van der Waals heterostructures such as graphene/MoS$_2$ are promising candidates for plenty of optical or electronic applications, owing to advanced properties inherited from the constitutional atomic layers. Thermal expansion is an important phenomenon to be considered for the thermal stability of the van der Waals heterstructure as temperature commonly rises during the operation of nano devices. In the present work, the thermal expansion coefficient for the graphene/MoS$_2$ heterostructure is investigated by molecular dynamics simulations, and the effect from the unavoidable misfit strain on…
Vertical integration of two-dimensional van der Waals materials is predicted to lead to novel electronic and optical properties not found in the constituent layers. Here, we present the direct synthesis of two unique, atomically thin, multi-junction heterostructures by combining graphene with the monolayer transition-metal dichalocogenides: MoS2, MoSe2, and WSe2.The realization of MoS2-WSe2-Graphene and WSe2-MoSe2-Graphene heterostructures leads toresonant tunneling in an atomically thin stack with spectrally narrow room temperature negative differential resistance characteristics.
We designed solution-processed, flexible hybrid graphene flake/2H-MoS2 quantum dot (QD) heterostructures, showing enhanced electrocatalytic activity for the hydrogen evolution reaction (HER) with respect to their native individual components. The 2H-MoS2 QDs are produced through a scalable, environmentally friendly, one-step solvothermal approach from two-dimensional (2D) 2H-MoS2 flakes obtained by liquid phase exfoliation (LPE) of their bulk counterpart in 2-Propanol. This QDs synthesis avoids the use of high boiling point and/or toxic solvents. Graphene flakes are produced by LPE of graphite…
Van der Waals heterostructures are the ideal material platform for tunnel field effect transistors (TFETs) because a band-to-band tunneling (BTBT) dominant current is feasible at room temperature (RT) due to ideal, dangling bond free heterointerfaces. However, achieving subthreshold swing (SS) values lower than 60 mVdec-1 of the Boltzmann limit is still challenging. In this work, we systematically studied the band alignment and heterointerface quality in n-MoS2 channel heterostructure TFETs. By selecting a p+-MoS2 source with a sufficiently high doping level, stable gate modulation to a type I…
Two-dimensional transition metal-dichalcogenides are emerging as efficient and cost-effective electrocatalysts for hydrogen evolution reaction (HER). However, only the edge sites of their trigonal prismatic phase show HER-electrocatalytic properties, while the basal plane, which is absent of defective/unsaturated sites, is inactive. Here, we tackle the key challenge that is increasing the number of electrocatalytic sites by designing and engineering heterostructures composed of single-/few-layer MoSe2 flakes and carbon nanomaterials (graphene or single-wall carbon nanotubes (SWNTs)) produced b…
Interfaces between normal and topological insulators are bound to host metallic states that are protected by time-reversal symmetry and are therefore robust against disorder and interface reconstruction. Two-dimensional topological insulators (quantum spin Hall insulators) offer a unique opportunity to change the local topology by adsorption of atoms or molecules and thus comprise an ideal platform for designing topological heterostructures. Here, we apply first principles calculations to show that the quantum spin Hall insulator 1T'-MoS2 exhibits a phase transition to a trivial insulator upon…
Near-field optical microscopy can be used as a viable route to understand the nanoscale material properties below the diffraction limit. On the other hand, atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs) are the materials of recent interest to study the spatial confinement of charge carriers, photon, and phonons. Heterostructures based on Mo or W based monolayer TMDs form type-II band alignment, and hence the optically excited carriers can be easily separated for applications pertaining to photonics and electronics. Mapping these spatially confined carriers or phot…
Heterostructures play significant roles in modern semiconductor devices and micro/nanosystems in a plethora of applications in electronics, optoelectronics, and transducers. While state-of-the-art heterostructures often involve stacks of crystalline epi-layers each down to a few nanometers thick, the intriguing limit would be heterto-atomic-layer structures. Here we report the first experimental demonstration of freestanding van der Waals heterostructures and their functional nanomechanical devices. By stacking single-layer (1L) MoS2 on top of suspended single-, bi-, tri- and four-layer (1L to…
Two dimensional (2D) van der Waals heterostructures from transition metal dichalcogenide (TMDC) semiconductors show a new class of spatially separate excitons with extraordinary properties. The interlayer excitons (XI) have been studied extensively, yet the mechanisms that modulate XI are still not well understood. Here, we introduce several organic-layer-embedded hybrid heterostructures, MoS2/organic/WSe2, to study the binding energy of XI. We discover that the dielectric screening of the quasi-particle is reduced with organic molecules due to decreased dielectric constant and greater separat…
Two-dimensional (2D) materials viz. transition metal dichalcogenides (TMD) and transition metal oxides (TMO) offer a platform that allows creation of heterostructures with a variety of properties. The optoelectronic industry has observed an upheaval in the research arena of MoS$_2$ based van der Waals (vdW) heterostructures (HTSs) and Janus structures. Therefore, interest towards these structures are backed by the selectivity in terms of electronic and optical properties. The present study investigates the photocatalytic ability of bilayer, MoS$_2$ and Janus (MoSSe) based vdW HTSs viz. MoS$_2$…
We review magneto-optical and magneto-transport effects in GaAs/GaAlAs heterostructures doped in GaAlAs barriers with donors, providing two-dimensional electron gas in GaAs quantum wells, and additionally doped with smaller amounts of acceptors (mostly Be atoms) in the vicinity of 2DEG. One may also deal with residual acceptors (mostly C atoms). The behavior of such systems in the presence of a magnetic field differs appreciably from those doped in the vicinity of 2DEG with donors.
Van der Waals heterostructures (vdWH) provide an ideal playground for exploring light-matter interactions at the atomic scale. In particular, structures with a type-II band alignment can yield detailed insight into free carrier-to-photon conversion processes, which are central to e.g. solar cells and light emitting diodes. An important first step in describing such processes is to obtain the energies of the interlayer exciton states existing at the interface. Here we present a general first-principles method to compute the electronic quasi-particle (QP) band structure and excitonic binding ene…
Graphene/MoS2 heterostructures are formed by combining the nanosheets of graphene and monolayer MoS2. The electronic features of both constituent monolayers are rather well-preserved in the resultant heterostructure due to the weak van der Waals interaction between the layers. However, the proximity of MoS2 induces strong spin orbit coupling effect of strength ~1 meV in graphene, which is nearly three orders of magnitude larger than the intrinsic spin orbit coupling of pristine graphene. This opens a bandgap in graphene and further causes anticrossings of the spin-nondegenerate bands near the …
Recent discovery of a novel hexagonal phase of GeSe (Gamma-GeSe) has triggered great interests in nanoelectronics applications owing to its electrical conductivity of bulk phase even higher than graphite while its monolayer is a semiconductor. For potential applications, construction of functional two-dimensional (2D) contacts is indispensable. Herein, via first-principles calculations, we propose the design of van der Waals heterostructures (vdWHs) of Gamma-GeSe contacting respectively with graphene, 2D h-BN and MoS2, as representatives of metallic, insulator, and semiconductor partners. Our …
Here we report the successful growth of MoSe2 on single layer hexagonal boron nitride (hBN) on Ru(0001) substrate by using molecular beam epitaxy. We investigated the electronic structures of MoSe2 using scanning tunneling microscopy and spectroscopy. Surprisingly, we found that the quasi-particle gap of the MoSe2 on hBN/Ru is about 0.25 eV smaller than those on graphene or graphite substrates. We attribute this result to the strong interaction between hBN/Ru which causes residual metallic screening from the substrate. The surface of MoSe2 exhibits Moiré pattern that replicates the Moiré patte…
The properties of Van der Waals heterostructures are determined by the twist angle and the interface between adjacent layers as well as their polytype and stacking. Here we describe the use of spectroscopic Low Energy Electron Microscopy (LEEM) and micro Low Energy Electron Diffraction (μLEED) methods to measure these properties locally. We present results on a MoS$_{2}$/hBN heterostructure, but the methods are applicable to other materials. Diffraction spot analysis is used to assess the benefits of using hBN as a substrate. In addition, by making use of the broken rotational symmetry of the …
Atomically thin MoS2/graphene heterostructures are promising candidates for nanoelectronic and optoelectronic technologies. Among different graphene substrates, epitaxial graphene (EG) on SiC provides several potential advantages for such heterostructures including high electronic quality, tunable substrate coupling, wafer-scale processability, and crystalline ordering that can template commensurate growth. Exploiting these attributes, we demonstrate here the thickness-controlled van der Waals epitaxial growth of MoS2 on EG via chemical vapor deposition, giving rise to transfer-free synthesis …
Transition metal dichalcogenides (TMD) possess novel properties which makes them potential candidates for various spintronic applications. Heterostructures of TMD with magnetic thin film have been extensively considered for spin-orbital torque, enhancement of perpendicular magnetic anisotropy etc. However, the effect of TMD on magnetic anisotropy in heterostructures of in-plane magnetization has not been studied so far. Further the effect of the TMD on the domain structure and magnetization reversal of the ferromagnetic system is another important aspect to be understood. In this context we st…
Due to its high carrier mobility, broadband absorption, and fast response time, graphene is attractive for optoelectronics and photodetection applications. However, the extraction of photoelectrons in conventional metal-graphene junction devices is limited by their small junction area, where the typical photoresponsivity is lower than 0.01 AW-1. On the other hand, the atomically thin layer of molybdenum disulfide (MoS2) is a two-dimensional (2d) nanomaterial with a direct and finite band gap, offering the possibility of acting as a 2d light absorber. The optoelectronic properties of the hetero…
We performed density functional theory (DFT) calculations for a bi-layered heterostructure combining a graphene layer with a MoS2 layer with and without intercalated Li atoms. Our calculations demonstrate the importance of the van der Waals (vdW) interaction, which is crucial for forming stable bonding between the layers. Our DFT calculation correctly reproduces the linear dispersion, or Dirac cone, feature at the Fermi energy for the isolated graphene monolayer and the band gap for the MoS2 monolayer. For the combined graphene/MoS2 bi-layer, we observe interesting electronic structure and den…
Semiconductor heterostructures based on layered two-dimensional transition metal dichalcogenides (TMD) interfaced to gallium nitride (GaN) are excellent material systems to realize broadband light emitters and absorbers. The surface properties of the polar semiconductor, such as GaN are dominated by interface phonons, thus the optical properties of the vertical heterostructure depend strongly on the interface exciton-phonon coupling. The origin and activation of different Raman modes in the heterostructure due to coupling between interfacial phonons and optically generated carriers in a monola…
In this paper we propose two transistor concepts based on lateral heterostructures of monolayer MoS$_2$, composed of adjacent regions of 1T (metallic) and 2H (semiconducting) phases, inspired by recent research showing the possibility to obtain such heterostructures by electron beam irradiation. The first concept, the lateral heterostructure field-effect transistor, exhibits potential of better performance with respect to the foreseen evolution of CMOS technology, both for high performance and low power applications. Performance potential has been evaluated by means of detailed multi-scale mat…
Phase transitions of two-dimensional materials and their heterostructures enable many applications including electrochemical energy storage, catalysis, and memory; however, the nucleation pathways by which these transitions proceed remain underexplored, prohibiting engineering control for these applications. Here, we demonstrate that the lithium intercalation-induced 2H-1T' phase transition in MoS2 proceeds via nucleation of the 1T' phase at a heterointerface by monitoring the phase transition of MoS2/graphene and MoS2/hexagonal boron nitride (hBN) heterostructures with Raman spectroscopy in s…
Excitons dominate the optics of atomically-thin transition metal dichalcogenides and 2D van der Waals heterostructures. Interlayer 2D excitons, with an electron and a hole residing in different layers, form rapidly in heterostructures either via direct charge transfer or via Coulomb interactions that exchange energy between layers. Here, we report prominent intertube excitonic effects in quasi-1D van der Waals heterostructures consisting of C/BN/MoS$_2$ core/shell/skin nanotubes. Remarkably, under pulsed infrared excitation of excitons in the semiconducting CNTs we observed a rapid (sub-picose…
In this study,we combined time-resolved terahertz spectroscopy along with transient absorption spectroscopy to revisit the interlayer non-equilibrium carrier dynamics in largely lateral size Gr/MoS2 heterostructure fabricated with chemical vapor deposition method. Our experimental results reveal that, with photon-energy below the A-exciton of MoS2 monolayer, hot electrons transfer from graphene to MoS2 takes place in time scale of less than 0.5 ps, resulting in ultrafast formation of interfacial exciton in the heterostructure, subsequently, recombination relaxation of the interfacial exciton o…
Heterostructures made from van der Waals materials provide a template to investigate proximity effects at atomically sharp heterointerfaces. In particular, near-field charge and energy transfer in heterostructures made from semiconducting transition metal dichalcogenides (TMD) have attracted interest to design model 2D "donor-acceptor" systems and new optoelectronic components. Here, using of Raman scattering and photoluminescence spectroscopies, we report a comprehensive characterization of a molybedenum diselenide (MoSe$_2$) monolayer deposited onto hexagonal boron nitride (hBN) and capped b…
The thermoelectric figures of merit of pristine two-dimensional materials are predicted to be significantly less than unity, making them uncompetitive as thermoelectric materials. Here we elucidate a new strategy that overcomes this limitation by creating multi-layer nanoribbons of two different materials and allowing thermal and electrical currents to flow perpendicular to their planes. To demonstrate this enhancement of thermoelectric efficiency ZT, we analyse the thermoelectric performance of monolayer molybdenum disulphide (MoS2) sandwiched between two graphene monolayers and demonstrate t…
Two-dimensional monolayer transition metal dichalcogenides (TMDs) have unique optical and electronic properties for applications pertaining to field effect transistors, light emitting diodes, photodetectors, and solar cells. Vertical interfacing of WS2 and MoS2 layered materials in combination with other families of 2D materials were previously reported. On the other hand, lateral heterostructures are particularly promising for the spatial confinement of charged carriers, excitons and phonons within an atomically-thin layer. In the lateral geometry, the quality of the interface in terms of the…
We demonstrate a graphene-MoS2 architecture integrating multiple field-effect transistors and we independently probe and correlate the conducting properties of van der Waals coupled graphene-MoS2 contacts with the ones of the MoS2 channels. Devices are fabricated starting from high-quality single-crystal monolayers grown by chemical vapor deposition and characterized by scanning Raman and photoluminescence spectroscopies. Transconductance curves of MoS2 are compared with the current-voltage characteristics of graphene contact stripes, revealing a significant suppression of transport on the n-s…
Two dimensional (2D) van der Waals heterostructures (vdWHs) have their unique potential in facilitating the stacking of layers of different 2D materials for optoelectronic devices with superior characteristics at a reduced cost. However, the fabrication of large area all-2D heterostructures is still challenging towards realizing practical devices. In the present work, we have demonstrated a rapid yet simple, impurity free and highly efficient sonication-assisted chemical exfoliation approach to synthesize hybrid vdWHs based on 2D molybdenum disulphide (MoS$_2$) and tungsten disulphide (WS$_2$)…
Photoexcited intralayer excitons in van der Waals heterostructures (vdWHs) with type-II band alignment have been observed to tunnel into interlayer excitons on ultrafast timescales. Such interlayer excitons have sufficiently long lifetimes that inducing dissociation with external in-plane electric fields becomes an attractive option of improving efficiency of photocurrent devices. In the present paper, we calculate interlayer exciton binding energies, Stark shifts, and dissociation rates for six different transition metal dichalcogenide (TMD) vdWHs using a numerical procedure based on exterior…
We present an electron spin resonance (ESR) approach to characterize shallow electron trapping in band-tail states at Si/SiO2 interfaces in metal-oxide-semiconductor (MOS) devices and demonstrate it on two MOS devices fabricated at different laboratories. Despite displaying similar low temperature (4.2 K) peak mobilities, our ESR data reveal a significant difference in the Si/SiO2 interface quality of these two devices, specifically an order of magnitude difference in the number of shallow trapped charges at the Si/SiO2 interfaces. Thus, our ESR method allows a quantitative evaluation of the S…
Combined diverse two-dimensional (2D) materials for semiconductor interfaces are attractive for electrically controllable carrier confinement to enable excellent electrostatic control. We investigated the transport characteristic in heterointerface of multilayer molybdenum disulfide and hexagonal boron nitride (MoS$_2$/h-BN) to reveal that the charge transfer switching (CTS) is highly dependent on both the local gate constriction and bias in the channel. Notably, the CTS is shown to be controlled at a molecular level through electrotunable gated constriction. The resulting significant change i…
