Optical studies of two dimensional materials: graphne and transition metal dichalcogenides

• Main Authors: Chih-Chiang Shen, 沈稚強
• Other Authors: Hsiang-Lin Liu
• Format: Others
• Language: en_US
• Published: 2014
• Online Access: http://ndltd.ncl.edu.tw/handle/00904044001582054728

博士 === 國立臺灣師範大學 === 物理學系 === 102 === We present the results of THz absorption and spectroscopic ellipsometric measurements of triazine-doped graphene and monolayer transition metal dichalcogenides (MoS2 and MoSxSey). The triazine-doped graphene thin films were deposited on oxidized silicon substrate (SiO2/Si), using either chemical vapor deposition (CVD) or electrochemical exfoliation (ECE). Monolayer MoS2 and MoSxSey thin films were deposited onto sapphire substrates by CVD. Our aim is to investigate the charge dynamics and electronics structures of these novel materials. THz conductivity of all samples displays a coherent response of itinerant charge carriers at zero frequency. Notably, the CVD-grown graphene thin films with doping show an additional finite frequency peak at about 155 cm-1. A finite-frequency peak, which coexists with a Drude contribution, is likely associated with the significant disorder induced by triazine doping. Furthermore, as the temperature is lowered, the Drude plasma frequency (~ 21 and 7 THz for CVD-grown graphene with doping and MoS2 thin films) decreases, whereas the carrier relaxation time (~ 13 and 26 fs) does not show much temperature variation. These results suggest the semiconducting behavior of the CVD-grown graphene with doping and monolayer MoS2 thin films. Additionally, the Drude plasma frequency of the ECE-grown graphene thin films is three times larger than that of CVD-grown ones. In contrast, the carrier relaxation time of the ECE-grown graphene thin films (~ 10 fs) is shorter than that of the CVD-grown samples (~ 84 fs). Interestingly, the Drude plasma frequency of monolayer MoSxSey thin films is in the range from 6.5 to 8 THz. Carrier relaxation time is in the range from 19 to 26 fs. The optical properties of all samples were also determined by spectroscopic ellipsometry. The absorption spectrum of the CVD-grown graphene thin films exhibits an asymmetric Fano resonance in the ultraviolet frequency region. This excitonic-dominated charge transfer band in the triazine-doped graphene thin films shows a blueshift in comparison with that of undoped analog. The line shape of the ECE-grown graphene thin films displays less asymmetric. Such behavior could be attributed to the changes of the charge distributions in the graphene thin films prepared by different growth methods. Additionally, monolayer MoSxSey films show a direct gap (~ 1.95 eV for MoS2 and ~ 1.62 eV for MoSe2). The ground-state exciton binding energy is found to be about 0.28 eV for MoS2 and 0.24 eV for MoSe2. These findings bring additional understanding of two-dimensional materials with respect to their charge dynamics and electronic structures and provide the foundation for future technological applications of these materials.