Abstract-The Role of Photo-Induced Exciton in the Transient Terahertz Conductivity of Few-Layer WS2 Laminate

Xiao Xing, Litao Zhao, Zeyu Zhang, Xiankuan Liu, Kailin Zhang, Yang Yu, Xian Lin, Huaying Chen, Jinquan Chen, Zuanming Jin, Jianhua Xu, and Guo-Hong Ma

http://pubs.acs.org/doi/abs/10.1021/acs.jpcc.7b05345?journalCode=jpccck

The exciton effect in two-dimensional (2D) transition metal dichalcogenides (TMDs) plays a dominated role in describing the optical and optoelectronic properties. However, the interplay between the excitons and free carriers has yet to be understood upon the photo excitation in 2D TMDs. Here we first present a study of the dynamical interplay of excitons and unbound electron-hole pairs using time-resolved terahertz (THz) spectroscopy (TRTS) in a few-layer WS2 laminate. Our experimental results demonstrate that the Auger recombination is observed only in the relaxation process of the mobile charge carriers rather than that of excitons upon photoexcitation. The transient complex THz photoconductivity spectroscopy of WS2 is well described by Lorentz oscillator model of free carriers modulated by the exciton polarization field. Our results provide a comprehensive understanding of non-equilibrium carrier kinetics (both excitons and free carriers) in WS2 laminate, and should be applicable to other 2D system.

Abstract-Photo-Induced Terahertz Conductivity and Carrier Relaxation in Thermal-Reduced Multilayer Graphene Oxide Films

Xiao Xing, Litao Zhao, Zeyu Zhang, Liang Fang, Zhengfu Fan, Xiumei Liu, Xian Lin, Jianhua Xu, Jinquan Chen, Xinluo Zhao, Zuanming Jin, and Guo-Hong Ma

J. Phys. Chem. C, Just Accepted Manuscript

DOI: 10.1021/acs.jpcc.6b10580

Publication Date (Web): January 11, 2017

Copyright © 2017 American Chemical Society

http://pubs.acs.org/doi/abs/10.1021/acs.jpcc.6b10580?journalCode=jpccck

Graphene oxide (GO) is an attractive option for large scale production of graphene. On the other hand, the graphene obtained by the reduction of GO has inevitable structural defects, and the vacant lattice sites will significantly restrict its conductivity. It has been demonstrated that thermal annealing in hydrogen is an efficient method to reduce defects and heal the lattice in GO samples. However, it is still not clear that how the defects and/or disordering influence on the photoelectric conversion efficiency and the carrier relaxation pathway in GO. Herein, the time-domain terahertz (THz) spectroscopy is employed to characterize the properties of the multilayer GO films which were annealed in hydrogen at various temperatures. Upon photo excitation, a transient increase of the conductivity was observed for the reduced graphene oxide (RGO) samples. The ultrafast carrier relaxation process can be well assigned to the carrier-carrier scattering and carrier-phonon coupling. Our results demonstrated that the RGO films with fewer defects and better lattice structure is successfully manufactured. In addition, by fitting to the Drude model, several electron transport parameters, such as the carrier scattering time, carrier plasma frequency and photoinduced conductivity, are obtained in our multilayer RGO films.