Therefore, control of GB structure by controlling grain orientation and understanding the boundary-oriented electronic structure provide a basis for the realization of future TMD-based devices. The existence of GB-induced midgap states significantly affects electronic transport across the boundary, leading to reduction of carrier mobility via additional carrier scattering at the GB. For example, a GB with 7-5 and 8-4-4 membered rings forms in a CVD-grown MoS 2, where midgap boundary states appear. Because the orientation of nuclei is normally random, a wide variety of GB structures can be formed. During the CVD growth of TMDs, nuclei form at the beginning of the CVD process, growing to form a large-area continuous sheet of TMD with GBs. In CVD-grown large-area TMDs, grain boundaries (GBs) are inevitably formed, which can significantly alter the electronic and optical properties of TMDs. Recently, the growth of TMDs by metal-organic CVD (MOCVD) with volatile liquid sources has been successfully demonstrated, and MOCVD is a promising method to realize wafer-scale TMDs that are compatible with device applications. In typical CVD growth of TMDs, solid sources such as metal oxides and elemental sulfur are used, and monolayer TMDs film with a lateral size of millimeters have been reported. Crystal growth by CVD is a bottom-up approach to obtain thin films, having been successfully applied to grow various atomic layers, such as graphene, hexagonal boron nitrides (hBN), and TMDs.
Top-down approaches, such as mechanical exfoliation, are not compatible with wafer-scale monolayer TMDs, and a bottom-up approach is required for that purpose. įor future applications of TMDs for electronic and optoelectronic devices, wafer-scale monolayer TMDs grown by chemical vapor deposition (CVD) are indispensable. In addition, monolayer TMDs in 2H form can have valley-degree-of-freedom, which may lead to future novel electronic devices based on valleytronics. In conjunction with the flexibility arising from the ultrathin structure, flexible electronic and optoelectronic devices can also be made. In fact, various TMD-based devices, including high-performance FET devices, light-emitting transistors, and photodetectors, have actually been demonstrated. One of the most distinct in TMDs from graphene is that TMDs can have sizable bandgap (~2 eV), leading to electronic and optoelectronic applications of TMD atomic layers. TMDs have a long research history, but research on properties of monolayer TMDs, three-atom-thick atomic layers, has only recently been started. We also found that even with the perfectly stitched structure, valence band maximum (VBM) and conduction band minimum (CBM) shows significant blue shift, which probably arise from lattice strain at the boundary.Ī post-graphene material, transition metal dichalcogenide (TMD), has recently attracted a great deal of attention. Through scanning tunneling microscopy (STM) and spectroscopy measurements, we have found that the perfectly stitched structure between two different grains of MoS 2 was realized in the case of the 0 degree grain boundary.
The grain boundaries with specific relative angle have been formed with chemical vapor deposition (CVD) growth on graphite and hexagonal boron nitride flakes van der Waals interlayer interaction between MoS 2 and the flakes restricts the relative angle between two different grains of MoS 2. We have investigated atomic and electronic structure of grain boundaries in monolayer MoS 2, where relative angles between two different grains are 0 and 60 degree.