Monolayer Growth Research Articles

Computational insights into electronic characteristics of 2D PtSe2 nanomaterials: Effects of vacancy defects and strain engineering

The epitaxial growth of PtSe2 monolayer has brings new opportunities for the application and development of materials science. Using first-principles calculations, the effect of vacancy defects, and strain engineering on the electronic properties of PtSe2 monolayer are systematically investigated. The results show that the Pt single vacancy induces a large magnetic moment of 4.0 μB on PtSe2 monolayer and transforms it from semiconductor to metal. However, the Se single vacancy systems are nonmagnetic and realize the PtSe2 from an indirect semiconductor to a direct one. Moreover, the band gap of PtSe2 monolayer can be prominently modulated within a appreciable uniaxial strain range, and the band gap are monotonically increase/decrease as decrease/increase the magnitude of the compressive/tensile strain. In particular, when a specific strain applied, a wide and high absorption peak across near-infrared, visible light and ultraviolet region. These findings not only enrich the fundamental understanding of PtSe2 monolayer but also provide useful guidance to design PtSe2-based spintronic, optoelectronic and gas sensing applications.

Immobilized Precursor Particle Driven Growth of Centimeter-Sized MoTe2 Monolayer.

Molybdenum ditelluride (MoTe2) has attracted ever-growing attention in recent years due to its novel characteristics in spintronics and phase-engineering, and an efficient and convenient method to achieve large-area high-quality film is an essential step toward electronic applications. However, the growth of large-area monolayer MoTe2 is challenging. Here, for the first time, we achieve the growth of a centimeter-sized monoclinic MoTe2 monolayer and manifest the mechanism of immobilized precursor particle driven growth. Microscopic characterizations reveal an obvious trend of immobilized precursor particles being consumed by the monolayer and continuing to provide a source for the growth of the monolayer. Time-of-flight secondary ion mass spectrometry verifies the attachment of hydroxide ions on the surface of the MoTe2 monolayer, thereby realizing the inhibition of crystal growth along the [001] zone axis and the continuous growth of the MoTe2 monolayer. The first-principles DFT calculations prove the mechanism of immobilized precursor particles and the absorption of hydroxide ions on the MoTe2 monolayer. The as-grown MoTe2 monolayer exhibits a surface roughness of 0.19 nm and average conductivity of 1.5 × 10-5 S/m, which prove the smoothness and uniformity of the MoTe2 monolayer. Temperature-dependent electrical measurements together with the transfer characteristic curves further demonstrate the typical semimetallic properties of monoclinic MoTe2. Our research elaborates the microscopic process of immobilized precursor particles to grow large-area MoTe2 monolayer and provides a new thinking about the growth of many other two-dimensional materials.

Modeling the initial monolayer formation in thermally localized surface deposition

In atomic layer deposition (ALD), thin layers of materials are deposited on a substrate with atomic layer precision in the vertical direction. The ability to control layer growth in the lateral direction as well is expected to greatly increase the potential of ALD as a path to the bottom-up additive fabrication of electronic devices like solar panels and organic light-emitting diode displays. We explore the possibility of controlling the lateral growth by modifying the temperature profile on the substrate using, for instance, pulsed lasers. This maskless technique keeps the majority of the substrate at a low temperature suppressing one of the chemical half-reactions, while in a small, localized area, the substrate is heated, which allows the reaction to proceed at a higher rate. We test this idea with course-grained computational models that model the control of the temperature by various illumination protocols and simulate the nucleation and growth of the initial monolayer within this inhomogeneous temperature distribution. Our results suggest that the location and the extent of deposition can, in principle, be localized and controlled and address operational regimes in which a thin conducting line may be obtained.

Pharmacological Preconditioning Improves the Viability and Proangiogenic Paracrine Function of Hydrogel-Encapsulated Mesenchymal Stromal Cells.

The efficacy of cell therapy is limited by low retention and survival of transplanted cells in the target tissues. In this work, we hypothesize that pharmacological preconditioning with celastrol, a natural potent antioxidant, could improve the viability and functions of mesenchymal stromal cells (MSC) encapsulated within an injectable scaffold. Bone marrow MSCs from rat (rMSC) and human (hMSC) origin were preconditioned for 1 hour with celastrol 1 μM or vehicle (DMSO 0.1% v/v), then encapsulated within a chitosan-based thermosensitive hydrogel. Cell viability was compared by alamarBlue and live/dead assay. Paracrine function was studied first by quantifying the proangiogenic growth factors released, followed by assessing scratched HUVEC culture wound closure velocity and proliferation of HUVEC when cocultured with encapsulated hMSC. In vivo, the proangiogenic activity was studied by evaluating the neovessel density around the subcutaneously injected hydrogel after one week in rats. Preconditioning strongly enhanced the viability of rMSC and hMSC compared to vehicle-treated cells, with 90% and 75% survival versus 36% and 58% survival, respectively, after 7 days in complete media and 80% versus 64% survival for hMSC after 4 days in low serum media (p < 0.05). Celastrol-treated cells increased quantities of proangiogenic cytokines compared to vehicle-pretreated cells, with a significant 3.0-fold and 1.8-fold increase of VEGFa and SDF-1α, respectively (p < 0.05). The enhanced paracrine function of preconditioned MSC was demonstrated by accelerated growth and wound closure velocity of injured HUVEC monolayer (p < 0.05) in vitro. Moreover, celastrol-treated cells, but not vehicle-treated cells, led to a significant increase of neovessel density in the peri-implant region after one week in vivo compared to the control (blank hydrogel). These results suggest that combining cell pretreatment with celastrol and encapsulation in hydrogel could potentiate MSC therapy for many diseases, benefiting particularly ischemic diseases.

Synthesis of scalable adlayer-free monolayer graphene film on Cu80Ni20 foil

Homogeneity is crucial to material applications. Although monolayer graphene was successfully prepared more than a decade ago, to date, the growth of adlayer-free monolayer with a large area remains challenging. Here, we report the synthesis of metre-size adlayer-free monolayer graphene film on Cu80Ni20 foil via atmospheric pressure chemical vapour deposition. Experimental results show that the growth parameter window of adlayer-free monolayer graphene on Cu80Ni20 is very wide, which makes the growth of adlayer-free monolayer graphene easy and controllable. A growth model of adlayer-free monolayer graphene on Cu80Ni20 substrate is also proposed.

Development of novel MoS2 hydrovoltaic nanogenerators for electricity generation from moving NaCl droplet

Competence of two-dimensional (2D) material e.g. MoS2 for developing efficient seawater energy harvester has been investigated as sea water stores huge amount of clean energy in its dynamic form. Hydrovoltaic nanogenerators has been fabricated using MoS2 monolayer grown by pulsed laser deposition (PLD) technique. Fabricated MoS2 nanogenerators are found to generate voltage>7 V by the dynamic flow of NaCl electrolyte over MoS2 monolayer. Good quality large area growth of MoS2 monolayer using PLD technique was confirmed by Raman spectroscopy, Photoluminescence (PL), in-situ RHEED and XPS. Obtained high voltage from MoS2 hydrovoltaic nanogenerators has been explained using equivalent circuit model considering dynamic charging and discharging process in formed electric double layer with successive movement of NaCl droplet. Substrate dependent nanogenerator performance was investigated and it was concluded that unintentional doping of MoS2 provided by underlying substrates is crucial for enhancing the power generation. Present work opens the possibility of using MoS2 monolayer as practical nanogenerator for energy harvesting from sea water (0.6 M NaCl) as demonstrated.

Layer number dependent optical and electrical properties of CVD grown two-dimensional anisotropic WS2

Engineering 2D transition metal dichalcogenides with precise control over layer number enable tuning of exciting optical and electrical properties at the nanoscale level. We report controlled one-step chemical vapour deposition growth of WS2 monolayer, bilayer, and trilayer for large scale manufacturing and demonstrate layer dependent changes in their work function, photoluminescence, and electrical conductivity. Raman, photoluminescence, and fluorescence imaging revealed that the base WS2 monolayer contains alternating triangular domains with different emission properties. It is observed that bilayer and trilayer grow selectively on less luminescent facet leading to fan-like morphology for second and third layers. We have systematically demonstrated that desired growth and areal coverage of bilayer and trilayer can be achieved by controlling WO3 precursor content. Kelvin probe force microscopic studies suggest a higher work function of thicker layers as compared to the monolayer. It was found that work function increases by 0.04 eV when thickness increases from monolayer to bilayer. FET device measurement on mono and bilayer shows n-type characteristics and two-fold higher photo-current in monolayer in comparison to the bilayer. The studied thickness dependence of the work function of WS2 is vital to the fabrication of metal contacts for WS2 based electronic and optoelectronic devices.

Dynamics of Monolayer Growth in Vapor-Liquid-Solid GaAs Nanowires Based on Surface Energy Minimization.

The vapor–liquid–solid growth of III-V nanowires proceeds via the mononuclear regime, where only one island nucleates in each nanowire monolayer. The expansion of the monolayer is governed by the surface energetics depending on the monolayer size. Here, we study theoretically the role of surface energy in determining the monolayer morphology at a given coverage. The optimal monolayer configuration is obtained by minimizing the surface energy at different coverages for a set of energetic constants relevant for GaAs nanowires. In contrast to what has been assumed so far in the growth modeling of III-V nanowires, we find that the monolayer expansion may not be a continuous process. Rather, some portions of the already formed monolayer may dissolve on one of its sides, with simultaneous growth proceeding on the other side. These results are important for fundamental understanding of vapor–liquid–solid growth at the atomic level and have potential impacts on the statistics within the nanowire ensembles, crystal phase, and doping properties of III-V nanowires.

All-Trans Retinoic Acid Fosters the Multifarious U87MG Cell Line as a Model of Glioblastoma.

Glioblastoma multiforme (GBM) is a primary brain cancer of poor prognosis, with existing treatments remaining essentially palliative. Current GBM therapy fails due to rapid reappearance of the heterogeneous neoplasm, with models suggesting that the recurrent growth is from treatment-resistant glioblastoma stem-like cells (GSCs). Whether GSCs depend on survival/proliferative cues from their surrounding microenvironmental niche, particularly surrounding the leading edge after treatment remains unknown. Simulating human GBM in the laboratory relies on representative cell lines and xenograft models for translational medicine. Due to U87MG source discrepancy and differential proliferation responses to retinoic acid treatment, this study highlights the challenges faced by laboratory scientists working with this representative GBM cell line. Investigating the response to all trans-retinoic acid (ATRA) revealed its sequestering of the prominin-1 stem cell marker. ICAM-1 universally present throughout U87MG was enhanced by ATRA, of interest for chemotherapy targeting studies. ATRA triggered diverse expression patterns of long non-coding RNAs PARTICLE and GAS5 in the leading edge and established monolayer growth zone microenvironment. Karyotyping confirmed the female origin of U87MG sourced from Europe. Passaging U87MG revealed the presence of chromosomal anomalies reflective of structural genomic alterations in this glioblastoma cell line. All evidence considered, this study exposes further phenotypic nuances of U87MG which may belie researchers seeking data contributing towards the elusive cure for GBM.

Gas-Phase "Prehistory" and Molecular Precursors in Monolayer Metal Dichalcogenides Synthesis: The Case of MoS2.

Two-dimensional MoS2 is one of the most promising materials for nanoelectronics due to its semiconducting nature and plethora of extraordinary properties. The main method for mass production of large-scale, high-quality MoS2 monolayers is chemical vapor deposition (CVD). Yet, the details of the chemistry occurring during the synthesis remain largely unknown, hindering process optimization. Combining ab initio molecular dynamics (AIMD) simulations and first-principles calculations allows us to explore the complete processes of MoS2 monolayer growth at the atomic level. We find that solid MoO3 precursor sublimates forming ringlike molecules, such as Mo3O9, which can later be regarded as gas-phase Mo-carrier reactants, undergoing sulfurization in three main stages: ring opening, chain breaking as the rate-limiting step, and further sulfurization. The fully sulfurized MoS6 molecule emerges as an immediate gas precursor to the crystal growth, as it reacts to join the MoS2-layer edge, with the release of a S4 molecule. Our comprehensive study provides detailed insights into the microscopic reaction mechanisms of MoS2 CVD growth and guidance for optimizing the synthesis parameters for transition metal dichalcogenides.

(Invited) Unidirectional Epitaxy of TMD Monolayers on Sapphire

Wafer-scale synthesis of semiconducting transition metal dichalcogenide (TMDs) monolayers is of significant interest for device applications to circumvent size limitations associated with the use of exfoliated flakes. Promising results have been demonstrated for epitaxial films deposited by vapor phase techniques such as CVD and MOCVD. However, the three-fold symmetry of TMDs such as MoS2 and WSe2, results in two energetically equivalent domain alignments, often referred to as 0o and 60o domains, when grown on substrates such as c-plane sapphire and graphene. The oppositely oriented domains give rise to inversion domain boundaries (IDBs) upon coalescence which exhibit a metallic character and are generally undesirable. In this study, we demonstrate the epitaxial growth of unidirectional TMD monolayers on 2” diameter c-plane sapphire substrates with a significantly reduced density of inversion domains. Steps on the sapphire surface are shown to break the surface symmetry giving rise to a preferred domain orientation.Metalorganic chemical vapor deposition (MOCVD) was used for the epitaxial growth of WSe2 and WS2 monolayers on c-plane sapphire in a cold-wall horizontal quartz-tube reactor. The as-received sapphire substrates, which are miscut ~0.2o toward <112 ̅0>, consist of steps with sub-1 nm step height separated by 50-70 nm wide terraces. A three-step nucleation-ripening-lateral growth process, carried out at temperatures ranging from 850oC to 1000oC, was used to achieve epitaxial films using W(CO)6, H2Se and H2S as precursors in a H2 carrier gas. Nucleation was observed to occur at the terrace edge and the growing domains align epitaxially with the underlying (0001) sapphire lattice. As a result of the nucleation process, the domains grow with a zig-zag edge facing the terrace edge which imparts a preferential direction to the domains. The percentage of domains with a preferred direction ranges from 75%-86% depending on MOCVD growth conditions. Continued lateral growth for times ranging from 10-30 minutes results in fully coalesced TMD monolayers that are epitaxially oriented on the sapphire, as assessed by in-plane x-ray diffraction, with a reduced density of inversion domain boundaries. The results demonstrate the important role of surface structure in nucleation and epitaxial growth of TMD monolayers.

The Versatile Electronic, Magnetic and Photo-Electro Catalytic Activity of a New 2D MA2 Z4 Family*.

The recent successful growth of MoSi2 N4 and WSi2 N4 monolayers led to the discovery of a new class of the two-dimensional (2D) MA2 Z4 materials with no known 3D layered allotropes, which renders great possibilities to integrate diverse properties by proper design of sandwiched "MZ2 " building blocks and "A-Z" passivation layers. In this work, the dynamic stability, electronic properties, and surface reactivity of the new MA2 Z4 family, in which M is Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, A refers to Si or Ge, and Z is N, P or As, is theoretically probed. Among the proposed 54 possible combinations, about 42 compositions are dynamically stable, which vary from non-magnetic, anti-ferromagnetic, to ferromagnetic semiconductors, metals and half-metals. In particular, the VB (V, Nb, Ta) MA2 Z4 possesses robust intrinsic ferromagnetism that is essential for spintronics applications. In regard to surface activity, most MA2 Z4 , particularly N- or P-terminated IVB and VB MA2 Z4 , have high catalytic potential for hydrogen evolution, and the ▵GH of non-magnetic MA2 Z4 is highly correlated to the highest occupied p electronic states of the surface Z atoms. The photocatalytic activity is also evaluated. MoSi2 N4 and WSi2 N4 within 4 % tensile strain are capable of photocatalytic overall water splitting. The findings indicate the new 2D MA2 Z4 family has fascinating properties and possesses strong potential for applications but not limited to electronics, spintronics and catalysts, which will stimulate the interests of experimental synthesis.

Growth of monolayer and multilayer graphene on glassy carbon electrode for simultaneous determination of guanine, adenine, thymine, and cytosine

• Graphene (Gr) was grown vertically on a glassy carbon (GC) electrode with Au catalyst. • Monolayer and multilayer Gr films were deposited on GC to form a novel carbon electrode. • The carbon electrode responds directly to all four DNA bases and various other biomarkers. • The carbon electrode has excellent stability and reliability in DNA base analysis. A novel carbon film composed of glassy carbon (GC), monolayer graphene (Gr), and multilayer Gr was prepared by continuously growing Gr sheets on a GC substrate, using Au nanoparticles as catalysts. The formation of the monolayer Gr sheets and the vertical growth mechanism of the multilayer Gr sheets were investigated by analyzing the morphology, microstructure, and composition variations. The carbon multilayer film can recognize biomolecules and inorganic ions, and can respond to up to 10 species. This function is based on the inherent electrocatalytic activity of the carbon film without modification using additional sensitive materials. The use of the multilayer carbon film as an electrochemical biosensor was investigated with the four DNA bases: guanine (G), adenine (A), thymine (T), and cytosine (C). The carbon film clearly identified G, A, T, and C in a neutral electrolyte environment, with detection limits of 0.3, 0.4, 6.5, and 4.5 μM, respectively; the peak potential differences for A–G, T–A, and C–T were 164, 216, and 184 mV, respectively. This carbon film can replace commercial GC, Au, and carbon paste electrodes and function as a basic electrode to explore new electrochemical sensors with high functionality and sensitivity.

High anisotropic inserted dendritic growth during first stage of Zn monolayer deposition on Ag(111) substrate

Sub-monolayer physical layer deposition of Zn on Ag(111) substrate at room temperature is investigated using Auger electron spectroscopy, low energy electron diffraction and scanning tunneling microscopy. We identified an original Zn highly-anisotropic structure in the shape of finger-like inserted dendrite in front of Zn monolayer growth. On the basis of STM observations, a mechanism of inserted dendritic growth and a mechanism of its transition to layered growth are proposed.

Proteasome Inhibition Suppresses KIT-Independent Gastrointestinal Stromal Tumors Via Targeting Hippo/YAP/Cyclin D1 Signaling

Purpose: Gastrointestinal stromal tumors (GISTs) are the most common malignant tumor of mesenchymal origin of the digestive tract. A yet more challenging resistance mechanism involves transition from oncogenic KIT to a new imatinib-insensitive oncogenic driver, heralded by loss of KIT expression. Our recent studies have shown that inhibition of cyclin D1 and Hippo signaling, which are overexpressed in KIT-independent GIST, is accompanied by anti-proliferative and apoptosis-promoting effects. PRKCQ, JUN, and the Hippo/YAP pathway coordinately regulate GIST cyclin D1 expression. Thus, targeting of these pathways could be effective therapeutically for these now untreatable tumors. Methods: Targeting cyclin D1 expression of small molecular drugs was screened by a cell monolayer growth and western blotting. The biologic mechanisms of bortezomib to KIT-independent GISTs were assessed by immunoblotting, qRT-PCR, cell viability, colony growth, cell cycle analysis, apoptosis, migration and invasiveness. Results: In the initial small molecular inhibitor screening in KIT-independent GIST62, we found that bortezomib-mediated inhibition of the ubiquitin-proteasome machinery showed anti-proliferative effects of KIT-independent GIST cells via downregulation of cyclin D1 and induction of p53 and p21. Treatment with proteasome inhibitor, bortezomib, led to downregulation of cyclin D1 and YAP/TAZ and an increase in the cleaved PARP expression in three KIT-independent GIST cell lines (GIST48B, GIST54, and GIST226). Additionally, it induced p53 and p21 expression in GIST48B and GIST54, increased apoptosis, and led to cell cycle G1/G2-phase arrest, decreased cell viability, colony formation, as well as migration and invasiveness in all GIST cell lines. Conclusion: Although our findings are early proof-of-principle, there are signs of a potential effective treatment for KIT-independent GISTs, the data highlight that targeting of cyclin D1 and Hippo/YAP by bortezomib warrants evaluation as a novel therapeutic strategy in KIT-independent GISTs.

The Hippo Pathway Effector YAP/TEAD Regulates EPHA3 Expression and Functions

Cell-cell interaction is critical for tissue development and repair, immunological responses, and cancer cell metastasis. The tyrosine kinase EPHA3 (erythropoietin‑producing hepatocellular carcinoma cell surface type-A receptor 3) regulates cell-cell interaction, cell differentiation, and cancer cell survival. Previously, our published study indicated that the STK4-encoded MST1 signaling, a core kinase component of the Hippo pathway, suppressed EPHA3 expression in the prostate cancer cell models. However, the mechanism is unknown. Here, we have demonstrated that the YAP1 and TEAD1 proteins, critical nuclear effectors of the Hippo pathway, mediate EPHA3 expression. First, we showed that AR-positive cell lines express the highest levels of EPHA3 and its ligand ephrin-A5 transcripts compared with other EPH family members. Second, we demonstrated that the induction of MST1/STK4 attenuated the EPHA3 protein and transcripts, consistent with our initial observation. Next, we demonstrated that knockdown of YAP1 by siRNA suppressed EPHA3 protein and mRNA expression. Similarly, the silencing of the TEAD1-4 proteins, critical mediators of YAP1-dependent gene transcription, revealed that the TEAD1 is a crucial inducer of EPHA3 expression. Moreover, bioinformatics tools allowed the identification of three putative TEAD binding sites (p<0.001) in the promoter region of the EPHA3 gene. Furthermore, CRISPER/Cas9-aided EPHA3 knockout significantly (p<0.01), decreased cell growth in monolayer and sphere formation in 3D cultures and caused androgen-independent cells to become sensitive to enzalutamide, a potent direct inhibitor of AR activity. These observations suggest that the YAP/TEAD1 transcriptionally regulates EPHA3 and its cellular biology.

Controlled Growth of Large-Sized and Phase-Selectivity 2D GaTe Crystals.

GaTe has recently attracted significant interest due to its direct bandgap and unique phase structure, which makes it a good candidate for optoelectronics. However, the controllable growth of large-sized monolayer and few-layer GaTe with tunable phase structures remains a great challenge. Here the controlled growth of large-sized GaTe with high quality, chemical uniformity, and good reproducibility is achieved through liquid-metal-assisted chemical vapor deposition method. By using liquid Ga, the rapid growth of 2D GaTe flakes with high phase-selectivity can be obtained due to its reduced reaction temperature. In addition, the method is used to synthesize many Ga-based 2D materials and their alloys, showing good universality. Raman spectra suggest that the as-grown GaTe own a relatively weak van der Waals interaction, where monoclinic GaTe displays highly-anisotropic optical properties. Furthermore, a p-n junction photodetector is fabricated using GaTe as a p-type semiconductor and 2D MoSe2 as a typical n-type semiconductor. The GaTe/MoSe2 heterostructure photodetector exhibits large photoresponsivity of 671.52 A W-1 and high photo-detectivity of 1.48 × 1010 Jones under illumination, owing to the enhanced light absorption and good quality of as-grown GaTe. These results indicate that 2D GaTe is a promising candidate for electronic and photoelectronic devices.

Analyzing growth kinematics and fractal dimensions of molybdenum disulfide films

Though the positive role of alkali halides in realizing large area growth of transition metal dichalcogenide layers has been validated, the film-growth kinematics has not yet been fully established. This work presents a systematic analysis of the MoS2 morphology for films grown under various pre-treatment conditions of the substrate with sodium chloride (NaCl). At an optimum NaCl concentration, the domain size of the monolayer increased by almost two orders of magnitude compared to alkali-free growth of MoS2. The results show an inverse relationship between fractal dimension and areal coverage of the substrate with monolayers and multi-layers, respectively. Using the Fact-Sage software, the role of NaCl in determining the partial pressures of Mo- and S-based compounds in gaseous phase at the growth temperature is elucidated. The presence of alkali salts is shown to affect the domain size and film morphology by affecting the Mo and S partial pressures. Compared to alkali-free synthesis under the same growth conditions, MoS2 film growth assisted by NaCl results in ≈81% of the substrate covered by monolayers. Under ideal growth conditions, at an optimum NaCl concentration, nucleation was suppressed, and domains enlarged, resulting in large area growth of MoS2 monolayers. No evidence of alkali or halogen atoms were found in the composition analysis of the films. On the basis of Raman spectroscopy and photoluminescence measurements, the MoS2 films were found to be of good crystalline quality.

Epitaxy of boron nitride monolayers for graphene-based lateral heterostructures

Monolayers of hexagonal boron nitride (hBN) are grown on graphite substrates using high-temperature molecular beam epitaxy (HT-MBE). The hBN monolayers are observed to grow predominantly from step edges on the graphite surface and exhibit a strong dependence of the morphology, including the dominant crystallographic edge, of the hBN monolayers, on the growth temperature, as well as systematic variations in growth rate and coverage, and significant differences in the growth at monolayer and multilayer graphite steps. At graphite monolayer steps hBN grows laterally across the surface on the lower terrace, but hBN growth on the upper side of the graphite step is more limited and is nucleated by three-dimensional clusters. Multilayer graphite steps exhibit a much higher density of non-planar hBN aggregates and growth on both the upper and lower terraces occurs. The results show that the hBN monolayer growth edge type, hBN island shape and the presence of hBN aggregates can be controlled in HT-MBE, with the highest quality layers grown at a substrate temperature of about 1390 °C. Sequential HT-MBE growth of hBN, graphene (G) and a second cycle of hBN growth results in the formation of monolayer thick lateral hBN–G–hBN heterostructures, in which a strip of G is embedded between monolayers of hBN.

Controllable Epitaxial Growth of Large-Area MoS2 /WS2 Vertical Heterostructures by Confined-Space Chemical Vapor Deposition.

The controllable large-area growth of single-crystal vertical heterostructures based on 2D transition metal dichalcogenides (TMDs) remains a challenge. Here, large-area vertical MoS2 /WS2 heterostructures are synthesized using single-step confined-space chemical vapor epitaxy. The heterostructures can evolve into two different kinds by switching the H2 flow on and off: MoS2 /WS2 heterostructures with multiple WS2 domains can be achieved without introducing the H2 flow due to the numerous nucleation centers on the bottom MoS2 monolayer during the transition stage between the MoS2 and WS2 monolayer growth. In contrast, isolated MoS2 /WS2 heterostructures with single WS2 domain can be obtained with introducing the H2 flow due to the reduced nucleation centers on the bottom MoS2 monolayer arising from the hydrogen etching effect. Both the two kinds of the vertical MoS2 /WS2 heterostructures feature high quality. The photodetectors based on the isolated MoS2 /WS2 heterostructures exhibit a high responsivity of 68 mA W-1 and a short response time of 35ms. This single-step chemical vapor epitaxy can be used to synthesize vertical MoS2 /WS2 heterostructures with high production efficiency. The new epitaxial growth approach may open new pathways to fabricate large-area heterostructures made of different 2D TMDs monolayers of interest to electronics, optoelectronics, and other applications.