Synthesis and photoluminescence property of hexangular star MoSe2 bilayer

Abstract

Transition metal dichalcogenides (TMDs) have received widespread attention because of their excellent performances in the field of optoelectronic, nanoelectronic device and photocatalytic exploration. The structures of TMDs can be expressed by the MX2, M=Mo, W; X=S, Se, Te, etc. As a typical TMD, MoSe2 has a graphene-like two-dimensional periodic structure with perfect physical, photoelcrtonic and catalytic properties. Currently, there are various methods to prepare the nanolevel MoSe2, such as the mechanical exfoliation, physical vapor deposition (PVD), hydrothermal method, chemical vapor deposition (CVD), etc, and most studies focused on regular triangular morphologies of the surfaces of different substrates. The new morphology, such as the hexangular star bilayer, has not been systematically investigated. In this study, the hexangular star MoSe2 nanosheets are successfully synthesized by using a simple CVD method in an atmosphere of mixed H2/Ar with a flow rate ratio of 1:4. Molybdenum trioxide(MoO3) and selenium (Se) powders are chosen to be the Mo and Se source, respectively. Moreover, the structure of the obtained MoSe2 nanosheet is characterized by Raman, SEM, EDS, XRD and TEM. The results of Raman spectrum and SEM indicate that the hexangular star MoSe2 possesses a bilayer structure. The TEM characterization reveals that the MoSe2 is a single crystal with a hexagonal lattice structure and good quality. The heating time at high temperature has a remarkable influence on the MoSe2 bilayer growth process. The growth process of the hexangular star MoSe2 bilayer is inferred to experience a three-step process. First, Mo and Se sources are gasified into gaseous molecules and then the Mo molecules are selenized into the MoSe2 crystal nucleus under high temperature. Next, these crystal nucleus are in a triangular epitaxial growth under the action of carrier gas. As heating time increases, the space steric effect leads to different interlayer separations between the two MoSe2 layers in various stacking configurations, eventually forming a hexangular star bilayer. The PL result shows that the spectra split into two main emission peaks, i.e., the direct and indirect bandgaps of the hexangular star structure appearing at 1.53 eV (810.2 nm) and 1.78 eV (696.9 nm), respectively. It might be due to the spin-orbit coupling interaction between the double MoSe2 molecules. The wide spectral range of the MoSe2 bilayer indicates that it has a potencial application in the photoelectric detectors.