二維材料石墨烯與過渡金屬雙硫屬化合物之光譜性質研究

Abstract

我們量測摻雜三聚氰胺分子的石墨烯薄膜及單層過渡金屬硫屬化合物(MoS2,MoSxSey)薄膜樣品的兆赫波吸收能譜與橢圓偏光光譜，探究這些樣品的電荷傳輸行為與電子結構。我們使用化學氣相沉積法(CVD)與電化學剝離法(ECE)製作摻雜三聚氰胺分子的石墨烯薄膜樣品，並以化學氣相沉積法製作單層過渡金屬硫屬化合物薄膜樣品。

我們發現摻雜後的石墨烯薄膜樣品，在頻率位置155 cm-1有一個吸收峰，此應與摻雜了三聚氰胺分子後所造成的晶格結構無序性有關。此外，居德電漿頻率(摻雜後的石墨烯薄膜與單層二硫化鉬薄膜分別為21和7 THz) 隨著溫度降低而下降，載子的鬆弛時間(13和26 fs)並不隨著溫度改變有顯著的變化。這些結果顯示摻雜後的石墨烯薄膜與單層二硫化鉬薄膜樣品具有半導體的特性。

此外，以電化學剝離法製作的石墨烯薄膜樣品，其居德電漿頻率大於使用化學氣相沉積法製作的樣品。相反的，以電化學剝離法製作的石墨烯薄膜樣品，其載子的鬆弛時間(10 fs)短於使用化學氣相沉積法製作的樣品(84 fs)。有趣的是，單層MoSxSey薄膜樣品的居德頻率由6.5到8 THz，載子鬆弛時間從19到26 fs。

我們發現以化學氣相沉積法製作的石墨烯薄膜樣品，其吸收能譜在紫外光頻率波段具有一個不對稱的Fano共振吸收。這個吸收峰主要是激子在能帶間的躍遷。相較於未摻雜的樣品，摻雜後的石墨烯薄膜樣品，吸收峰的頻率位置呈現藍移的現象。以電化學剝離法製作的石墨烯薄膜樣品，其吸收能譜的波形較為對稱。我們推測此與使用不同的成長方式，改變了石墨烯薄膜樣品的電荷分佈有關。此外，單層MoSxSey薄膜樣品具有直接能隙(二硫化鉬和二硒化鉬分別為1.95 和 1.62 eV)。二硫化鉬和二硒化鉬的激子束縛能分別為0.28和0.24 eV。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.