Wen Wan

Interlayer coupling of a direct van der Waals epitaxial MoS2/graphene heterostructure

Many efforts have been undertaken towards the synthesis of vertically stacked two-dimensional (2D) crystals due to their unique electronic and optical properties. Here, we present direct molecular beam epitaxy (MBE) growth of a MoS2/graphene heterostructure by a strict epitaxial mechanism. By combining Raman, photoluminescence, transmission electron microscopy characterizations and first-principles calculations, we find that there exists a strain effect and strong interlayer coupling between MoS2 and graphene resulting from the intrinsic crystal lattice mismatch, which could generate potential metallic behavior of the heterostructure. The direct epitaxial technique applied here enables us to investigate the growth mechanisms and interlaminar interaction of 2D heterostructures without sample handling and transfer, and offers a new approach to synthesize multilayer electronic and photonic devices.

Temperature-Related Morphological Evolution of MoS2 Domains on Graphene and Electron Transfer within Heterostructures.

Other than the well-known sulfurization of molybdate compound to synthesize molybdenum disulfide (MoS2 ) layers, the dynamic process in the whole crystalline growth from nuclei to triangular domains has been rarely experimentally explored. Here, a competing sulfur-capture principle jointly with strict epitaxial mechanism is first proposed for the initial topography evolution and the final intrinsic highly oriented growth of triangular MoS2 domains with Mo or S terminations on the graphene (Gr) template. Additionally, potential distributions on MoS2 domains and bare Gr are presented to be different due to the charge transfer within heterostructures. The findings offer the mechanism of templated growth of 2D transition metal dichalcogenides, and provide general principles in syntheses of vertical 2D heterostructures that can be applied to electronics.

A visualization method for probing grain boundaries of single layer graphene via molecular beam epitaxy

Graphene, a member of layered two-dimensional (2D) materials, possesses high carrier mobility, mechanical flexibility, and optical transparency, as well as enjoying a wide range of promising applications in electronics. Adopting the chemical vaporization deposition method, the majority of investigators have ubiquitously grown single layer graphene (SLG), which inevitably involves polycrystalline properties. Here we demonstrate a simple method for the direct visualization of arbitrarily large-size SLG domains by synthesizing one-hundred-nm-scale MoS2 single crystals via a high-vacuum molecular beam epitaxy process. The present study based on epitaxial growth provides a guide for probing the grain boundaries of various 2D materials and implements higher potentials for the next-generation electronic devices.

Critical Annealing Temperature for Stacking Orientation of Bilayer Graphene

It is rarely reported that stacking orientations of bilayer graphene (BLG) can be manipulated by the annealing process. Most investigators have painstakingly fabricated this BLG by chemical vapor deposition growth or mechanical means. Here, it is discovered that, at ≈600 °C, called the critical annealing temperature (CAT), most stacking orientations collapse into strongly coupled or AB-stacked states. This phenomenon is governed (i) macroscopically by the stress generation and release in top graphene domains, evolving from mild ripples to sharp billows in certain local areas, and (ii) microscopically by the principle of minimal potential obeyed by carbon atoms that have acquired sufficient thermal energy at CAT. Conspicuously, evolutions of stacking orientations in Raman mappings under various annealing temperatures are observed. Furthermore, MoS2 synthesized on BLG is used to directly observe crystal orientations of top and bottom graphene layers. The finding of CAT provides a guide for the fabrication of strongly coupled or AB-stacked BLG, and can be applied to aligning other 2D heterostructures.

MoS2 materials synthesized on SiO2/Si substrates via MBE

Two-dimensional (2D) MoS2 materials possess indirect-to-direct bandgap tunability, and have enjoyed wide applications in electronics and optoelectronics. Most of investigators have ubiquitously synthesized these materials by using the chemical vaporization deposition (CVD) method. Here we have adopted MoO3 source materials to synthesize MoS2 on 280-nm SiO2/Si substrates via molecular beam epitaxy (MBE). We have obtained triangular nucleation, tens-of-micron domain, and monolayer MoS2. This MBE technique can be applied to synthesizing other members of semiconducting layered transition metal dichalcogenides (TMDCs), such as WS2, MoSe2, and WSe2 materials.

Synthesis of sub-millimeter Bi-/multi-layer graphene by designing a sandwiched structure using copper foils

Bernal-stacked (AB-stacked) bilayer graphene has been receiving significant attention because it has a tunable band-gap under an applied vertical electric field. Herein, we designed a sandwiched structure simply by embedding one piece of Cu sheet into a Cu pocket to establish an environment that suppresses Cu evaporation and ensures that both surfaces of Cu sheet are smooth to grow large-size bilayer graphene (BLG) and multilayer graphene (MLG). Single-diffusion and double-diffusion mechanisms help explain graphene growth on both the Cu pocket and the Cu sheet, respectively. On the basis of the double-diffusion mechanism, we prepared AB-stacked sub-millimeter BLG and MLG with diameters up to 603 μm and 793 μm, respectively. Our work regarding the improvement of the quality and single-crystal size of graphene domains helps broaden the potential applications in materials chemistry and microelectronic devices.