Layered transition metal dichalcogenides (TMDs), especially molybdenum disulfide (MoS2), have been of great interest for a long time. MoS2 naturally occurs as the mineral molybdenite, which has been involved in diverse research fields, such as electronics, optoelectronics, spintronics, energy storage, lubrication, and catalysis.
In MoS2 crystals, a sheet of molybdenum atoms is sandwiched between sheets of sulfur atoms. The covalent Mo-S bonding is strong, but the interaction between the sandwich-like tri-layers is weak van der Waals force, resulting in easy exfoliation of a single layer or a few layers. These MoS2 ultrathin layers (less than 10 atoms thick) belong to the family of two-dimensional (2D) materials. As dimensionality reduced, these MoS2 ultrathin layers have unique charge transport properties, which solve the limitation of conventional Si and other bulk materials as scaling down to microelectronic and nanoelectronic devices. However, the synthesis of orientated single- or few- layer TMDs with large area remains challenging. The first part of this dissertation focuses on the design and controlled synthesis of 2D TMDs and study of their electronic device applications. A facile vapor-solid method was employed to get single crystalline few-layer MoS2 films on (0001)-oriented sapphires with excellent structural and electrical properties over centimeter length scale. A carrier density of ~2.0E11 cm -2 and a room temperature mobility as high as 192 cm2/Vs were extracted from space-charge limited transport regime in the films. In addition, transition metal doped 2D MoS2 films were successfully synthesized by one-step process. These doped films enable the tuning of the properties 2D MoS2 films. By substituting to other substrates or film transferring, 2D/3D heterojunction diodes have also been made with excellent rectification.
MoS2 has also been explored as catalysts such as for hydrogen evolution reaction (HER). The edges of MoS2 has been identified as the active sites for HER, but the basal planes are catalytically inert. The second part of this dissertation focuses on the design and characterization of Mo-S clusters and study of their HER catalytic activities. Different Mo-S clusters that contain the edge structures of MoS2 have been made to understand and improve the HER efficiency. X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and X-ray absorption spectroscopy (XAS) has been used to reveal the properties of Mo and S atoms at different sites.
The synchrotron X-ray based XAS is one of the most useful technique to determine the local geometric and electronic structure of materials. The XAS technique can characterize both bulk sample in transmission mode, and ~ 100 nm surface in total electron yield mode regardless of the crystallinity of the materials. The XAS includes X-ray absorption near edge structure (XANES) and Extended X-ray absorption fine structure (EXAFS). The XANES is sensitive to the charge transfer, orbital occupancy and symmetry. The bond length and coordination details can be extracted from Fourier transform of EXAFS. The third part of this dissertation focuses on the application and improvement of XAS technique.