Nanotribological properties and scratch resistance of MoS2 bilayer on a SiO2/Si substrate

The tribological properties and scratch resistance of MoS2 bilayer deposited on SiO2/Si substrates prepared via chemical vapor deposition are investigated. Friction force microscopy (FFM) is employed to investigate the friction and wear properties of the MoS2 bilayer at the nanoscale by applying a normal load ranging from 200 to 1,000 nN. Scratch resistance is measured using the scratch mode in FFM based on a linearly increasing load from 100 to 1,000 nN. Kelvin probe force microscopy (KPFM) is performed to locally measure the surface potential in the tested surface to qualitatively measure the wear/removal of MoS2 layers and identify critical loads associated with the individual failures of the top and bottom layers. The analysis of the contact potential difference values as well as that of KPFM, friction, and height images show that the wear/removal of the top and bottom layers in the MoS2 bilayer system occurred consecutively. The FFM and KPFM results show that the top MoS2 layer begins to degrade at the end of the low friction stage, followed by the bottom layer, thereby resulting in a transitional friction stage owing to the direct contact between the diamond tip and SiO2 substrate. In the stable third stage, the transfer of lubricious MoS2 debris to the tip apex results in contact between the MoS2-transferred tip and SiO2. Nanoscratch test results show two ranges of critical loads, which correspond to the sequential removal of the top and bottom layers.This study was supported by the Research Program funded by the SeoulTech (Seoul National University of Science and Technology, Republic of Korea)

Experimentally Derived Catalytic Etching Kinetics for Defect-Utilized Dual-Porous Silicon Formation

A kinetic model of defect-utilized dual-porous structure (DPS) formation of silicon, composed of amorphous porous structure (APS) and defect-followed mesoporous structure (DMPS), is proposed. It is found that a defect site is preferentially removed while creating the oblique DMPS of high aspect ratio, and the DMPS is wholly covered with the APS which is gradually grown through applied etching time but finally saturated. It is suggested that the APS growth is progressed by the chemically enhanced-oxygen diffusion, which is driven by catalytic chemical reactions, and the APS dissolution depends on the oxygen concentration of the APS itself. On the basis of these results, we describe the DPS formation using fundamental reaction kinetics and Fick’s law of diffusion. Understanding the APS growth mechanism is profound and potentially useful for prediction and controlling of the porous Si growth in the conventional HF/HNO₃/H₂O etching system. The DMPS development at the defect sites is ca. 22 times faster than the defect-free sites due to varied physicochemical properties. This analytical approach is a new attempt to describe the porous silicon formation mechanism as well as the conventional HF/HNO₃/H₂O etching procedure and further opens a new domain for the viability of defect engineering

Experimentally Derived Catalytic Etching Kinetics for Defect-Utilized Dual-Porous Silicon Formation

A kinetic model of defect-utilized dual-porous structure (DPS) formation of silicon, composed of amorphous porous structure (APS) and defect-followed mesoporous structure (DMPS), is proposed. It is found that a defect site is preferentially removed while creating the oblique DMPS of high aspect ratio, and the DMPS is wholly covered with the APS which is gradually grown through applied etching time but finally saturated. It is suggested that the APS growth is progressed by the chemically enhanced-oxygen diffusion, which is driven by catalytic chemical reactions, and the APS dissolution depends on the oxygen concentration of the APS itself. On the basis of these results, we describe the DPS formation using fundamental reaction kinetics and Fick’s law of diffusion. Understanding the APS growth mechanism is profound and potentially useful for prediction and controlling of the porous Si growth in the conventional HF/HNO₃/H₂O etching system. The DMPS development at the defect sites is ca. 22 times faster than the defect-free sites due to varied physicochemical properties. This analytical approach is a new attempt to describe the porous silicon formation mechanism as well as the conventional HF/HNO₃/H₂O etching procedure and further opens a new domain for the viability of defect engineering