Thinning Segregated Graphene Layers on High Carbon Solubility Substrates of Rhodium Foils by Tuning the Quenching Process

We report the synthesis of large-scale uniform graphene films on high carbon solubility substrates of Rh foils for the first time using an ambient-pressure chemical vapor deposition method. We find that, by increasing the cooling rate in the growth process, the thickness of graphene can be tuned from multilayer to monolayer, resulting from the different segregation amount of carbon atoms from bulk to surface. The growth feature was characterized with scanning electron microscopy, Raman spectra, transmission electron microscopy, and scanning tunneling microscopy. We also find that bilayer or few-layer graphene prefers to stack deviating from the Bernal stacking geometry, with the formation of versatile moiré patterns. On the basis of these results, we put forward a segregation growth mechanism for graphene growth on Rh foils. Of particular importance, we propose that this randomly stacked few-layer graphene can be a model system for exploring some fantastic physical properties such as van Hove singularities

Unravelling Orientation Distribution and Merging Behavior of Monolayer MoS₂ Domains on Sapphire

Monolayer MoS₂ prepared by chemical vapor deposition (CVD) has a highly polycrystalline nature largely because of the coalescence of misoriented domains, which severely hinders its future applications. Identifying and even controlling the orientations of individual domains and understanding their merging behavior therefore hold fundamental significance. In this work, by using single-crystalline sapphire (0001) substrates, we designed the CVD growth of monolayer MoS₂ triangles and their polycrystalline aggregates for such purposes. The obtained triangular MoS₂ domains on sapphire were found to distributively align in two directions, which, as supported by density functional theory calculations, should be attributed to the relatively small fluctuations of the interface binding energy around the two primary orientations. Using dark-field transmission electron microscopy, we further imaged the grain boundaries of the aggregating domains and determined their prevalent armchair crystallographic orientations with respect to the adjacent MoS₂ lattice. The coalescence of individual triangular flakes governed by unique kinetic processes is proposed for the polycrystal formation. These findings are expected to shed light on the controlled MoS₂ growth toward predefined domain orientation and large domain size, thus enabling its versatile applications in next-generation nanoelectronics and optoelectronics

Periodic Modulation of the Doping Level in Striped MoS₂ Superstructures

Although the recently discovered monolayer transition metal dichalcogenides exhibit novel electronic and optical properties, fundamental physical issues such as the quasiparticle bandgap tunability and the substrate effects remain undefined. Herein, we present the report of a quasi-one-dimensional periodically striped superstructure for monolayer MoS₂ on Au(100). The formation of the unique striped superstructure is found to be mainly modulated by the symmetry difference between MoS₂ and Au(100) and their lattice mismatch. More intriguingly, we find that the monolayer MoS₂ is heavily n-doped on the Au(100) facet with a bandgap of 1.3 eV, and the Fermi level is upshifted by ∼0.10 eV on the ridge (∼0.2 eV below the conduction band) in contrast to the valley regions (∼0.3 eV below the conduction band) of the striped patterns after high-temperature sample annealing process. This tunable doping effect is considered to be caused by the different defect densities over the ridge/valley regions of the superstructure. Additionally, an obvious bandgap reduction is observed in the vicinity of the domain boundary for monolayer MoS₂ on Au(100). This work should therefore inspire intensive explorations of adlayer–substrate interactions, the defects, and their effects on band-structure engineering of monolayer MoS₂

Epitaxial Monolayer MoS₂ on Mica with Novel Photoluminescence

Molybdenum disulfide (MoS₂) is back in the spotlight because of the indirect-to-direct bandgap tunability and valley related physics emerging in the monolayer regime. However, rigorous control of the monolayer thickness is still a huge challenge for commonly utilized physical exfoliation and chemical synthesis methods. Herein, we have successfully grown predominantly monolayer MoS₂ on an inert and nearly lattice-matching mica substrate by using a low-pressure chemical vapor deposition method. The growth is proposed to be mediated by an epitaxial mechanism, and the epitaxial monolayer MoS₂ is intrinsically strained on mica due to a small adlayer-substrate lattice mismatch (∼2.7%). Photoluminescence (PL) measurements indicate strong single-exciton emission in as-grown MoS₂ and room-temperature PL helicity (circular polarization ∼0.35) on transferred samples, providing straightforward proof of the high quality of the prepared monolayer crystals. The homogeneously strained high-quality monolayer MoS₂ prepared in this study could competitively be exploited for a variety of future applications

Dendritic, Transferable, Strictly Monolayer MoS₂ Flakes Synthesized on SrTiO₃ Single Crystals for Efficient Electrocatalytic Applications

Controllable synthesis of macroscopically uniform, high-quality monolayer MoS₂ is crucial for harnessing its great potential in optoelectronics, electrocatalysis, and energy storage. To date, triangular MoS₂ single crystals or their polycrystalline aggregates have been synthesized on insulating substrates of SiO₂/Si, mica, sapphire, <i>etc.</i>, <i>via</i> portable chemical vapor deposition methods. Herein, we report a controllable synthesis of dendritic, strictly monolayer MoS₂ flakes possessing tunable degrees of fractal shape on a specific insulator, SrTiO₃. Interestingly, the dendritic monolayer MoS₂, characterized by abundant edges, can be transferred intact onto Au foil electrodes and serve as ideal electrocatalysts for hydrogen evolution reaction, reflected by a rather low Tafel slope of ∼73 mV/decade among CVD-grown two-dimensional MoS₂ flakes. In addition, we reveal that centimeter-scale uniform, strictly monolayer MoS₂ films consisting of relatively compact domains can also be obtained, offering insights into promising applications such as flexible energy conversion/harvesting and optoelectronics

Modulating the Electronic Properties of Monolayer Graphene Using a Periodic Quasi-One-Dimensional Potential Generated by Hex-Reconstructed Au(001)

The structural and electronic properties of monolayer graphene synthesized on a periodically reconstructed substrate can be widely modulated by the generation of superstructure patterns, thereby producing interesting physical properties, such as magnetism and superconductivity. Herein, using a facile chemical vapor deposition method, we successfully synthesized high-quality monolayer graphene with a uniform thickness on Au foils. The hex-reconstruction of Au(001), which is characterized by striped patterns with a periodicity of 1.44 nm, promoted the formation of a quasi-one-dimensional (1D) graphene superlattice, which served as a periodic quasi-1D modulator for the graphene overlayer, as evidenced by scanning tunneling microscopy/spectroscopy. Intriguingly, two new Dirac points were generated for the quasi-1D graphene superlattice located at −1.73 ± 0.02 and 1.12 ± 0.12 eV. Briefly, this work demonstrates that the periodic modulation effect of reconstructed metal substrates can dramatically alter the electronic properties of graphene and provides insight into the modulation of these properties using 1D potentials

Controllable Growth and Transfer of Monolayer MoS₂ on Au Foils and Its Potential Application in Hydrogen Evolution Reaction

Controllable synthesis of monolayer MoS₂ is essential for fulfilling the application potentials of MoS₂ in optoelectronics and valleytronics, <i>etc.</i> Herein, we report the scalable growth of high quality, domain size tunable (edge length from ∼200 nm to 50 μm), strictly monolayer MoS₂ flakes or even complete films on commercially available Au foils, <i>via</i> low pressure chemical vapor deposition method. The as-grown MoS₂ samples can be transferred onto arbitrary substrates like SiO₂/Si and quartz with a perfect preservation of the crystal quality, thus probably facilitating its versatile applications. Of particular interest, the nanosized triangular MoS₂ flakes on Au foils are proven to be excellent electrocatalysts for hydrogen evolution reaction, featured by a rather low Tafel slope (61 mV/decade) and a relative high exchange current density (38.1 μA/cm²). The excellent electron coupling between MoS₂ and Au foils is considered to account for the extraordinary hydrogen evolution reaction activity. Our work reports the synthesis of monolayer MoS₂ when introducing metal foils as substrates, and presents sound proof that monolayer MoS₂ assembled on a well selected electrode can manifest a hydrogen evolution reaction property comparable with that of nanoparticles or few-layer MoS₂ electrocatalysts

Substrate Facet Effect on the Growth of Monolayer MoS₂ on Au Foils

MoS₂ on polycrystalline metal substrates emerges as an intriguing growth system compared to that on insulating substrates due to its direct application as an electrocatalyst in hydrogen evolution. However, the growth is still indistinct with regard to the effects of the inevitably evolved facets. Herein, we demonstrate for the first time that the crystallography of Au foil substrates can mediate a strong effect on the growth of monolayer MoS₂, where large-domain single-crystal MoS₂ triangles are more preferentially evolved on Au(100) and Au(110) facets than on Au(111) at relative high growth temperatures (>680 °C). Intriguingly, this substrate effect can be weakened at a low growth temperature (∼530 °C), reflected with uniform distributions of domain size and nucleation density among the different facets. The preferential nucleation and growth on some specific Au facets are explained from the facet-dependent binding energy of MoS₂ according to density functional theory calculations. In brief, this work should shed light on the effect of substrate crystallography on the synthesis of monolayer MoS₂, thus paving the way for achieving batch-produced, large-domain or domain size-tunable growth through an appropriate selection of the growth substrate

Controlled Growth of High-Quality Monolayer WS₂ Layers on Sapphire and Imaging Its Grain Boundary

Atomically thin tungsten disulfide (WS₂), a structural analogue to MoS₂, has attracted great interest due to its indirect-to-direct band-gap tunability, giant spin splitting, and valley-related physics. However, the batch production of layered WS₂ is underdeveloped (as compared with that of MoS₂) for exploring these fundamental issues and developing its applications. Here, using a low-pressure chemical vapor deposition method, we demonstrate that high-crystalline mono- and few-layer WS₂ flakes and even complete layers can be synthesized on sapphire with the domain size exceeding 50 × 50 μm². Intriguingly, we show that, with adding minor H₂ carrier gas, the shape of monolayer WS₂ flakes can be tailored from jagged to straight edge triangles and still single crystalline. Meanwhile, some intersecting triangle shape flakes are concomitantly evolved from more than one nucleus to show a polycrystalline nature. It is interesting to see that, only through a mild sample oxidation process, the grain boundaries are easily recognizable by scanning electron microscopy due to its altered contrasts. Hereby, controlling the initial nucleation state is crucial for synthesizing large-scale single-crystalline flakes. We believe that this work would benefit the controlled growth of high-quality transition metal dichalcogenide, as well as in their future applications in nanoelectronics, optoelectronics, and solar energy conversions

Direct Chemical Vapor Deposition Growth and Band-Gap Characterization of MoS₂/<i>h</i>‑BN van der Waals Heterostructures on Au Foils

Stacked transition-metal dichalcogenides on hexagonal boron nitride (<i>h</i>-BN) are platforms for high-performance electronic devices. However, such vertical stacks are usually constructed by the layer-by-layer polymer-assisted transfer of mechanically exfoliated layers. This inevitably causes interfacial contamination and device performance degradation. Herein, we develop a two-step, low-pressure chemical vapor deposition synthetic strategy incorporating the direct growth of monolayer <i>h</i>-BN on Au foil with the subsequent growth of MoS₂. In such vertical stacks, the interactions between MoS₂ and Au are diminished by the intervening <i>h</i>-BN layer, as evidenced by the appearance of photoluminescence in MoS₂. The weakened interfacial interactions facilitate the transfer of the MoS₂/<i>h</i>-BN stacks from Au to arbitrary substrates by an electrochemical bubbling method. Scanning tunneling microscope/spectroscopy characterization shows that the central <i>h</i>-BN layer partially blocks the metal-induced gap states in MoS₂/<i>h</i>-BN/Au foils. The work offers insight into the synthesis, transfer, and device performance optimization of such vertically stacked heterostructures