本論文研究過渡金屬二硫屬化合物薄膜的光譜特性。樣品以化學氣相沉積法成長於藍寶石基板，分別為單層MoS2、MoSe2及WSe2薄膜。
首先，我們量測樣品的拉曼散射光譜，樣品皆顯示2個拉曼活性振動膜， MoS2薄膜的 和A1g振動模頻率位置分別為385.8和405.8 cm-1，MoSe2薄膜的 和A1g振動模頻率位置分別242.2和285.6 cm-1，WSe2薄膜的 和2LA(M)振動模頻率位置為250.4和261.5 cm-1。
其次，我們量測單層MoS2與MoSe2薄膜的變溫橢圓偏振光譜，探討複數折射率及吸收係數能譜。吸收係數能譜在低能量區間顯現兩個A和B激子的吸收峰，高能量區間呈現高強度的數個吸收峰。我們使用加寬羅侖茲模型，分析樣品的激子能階，求得單層MoS2與MoSe2薄膜室溫直接能隙分別為1.92 ± 0.01與1.62 ± 0.01 eV，及室溫激子束縛能分別為0.27 ± 0.01與0.25 ± 0.01 eV。單層MoS2與MoSe2薄膜的變溫吸收能譜，直接能隙值隨著溫度升高產生紅移現象。
最後，我們量測單層WSe2薄膜的高磁場穿透光譜，吸收光譜呈現四個吸收峰，分別為A和B激子與A和B吸收邊緣。我們主要探討A激子與A吸收邊緣。以左旋偏振光(σ-)入射時，A激子與A吸收邊緣峰值隨外加磁場呈現藍移，反之，以右旋偏振光(σ+)入射時，A激子與A吸收邊緣峰值隨外加磁場呈現紅移。外加磁場分裂布里淵區能量簡併的K與-K能谷，形成能谷的賽曼效應(Valley Zeeman effect)。

We investigated the optical properties of monolayer transition metal dichalcogenides thin films (MoS2, MoSe2 and WSe2). These samples were deposited on the sapphire substrates by using chemical vapor deposition (CVD).

We first measured the Raman scattering spectra of all thin films. The spectra are composed of two strong phonon modes. For MoS2 thin film, the frequencies of the E2g1 and A1g phonon modes are approximately 358.8 and 405.8 cm-1, while for MoSe2 thin film, they are about 242.2 and 285.6 cm-1. For WSe2 thin films, the frequencies of E2g1 / A1g and 2LA(M) phonon modes are approximately 250.4 and 261.5 cm-1.

Secondly, we measured the temperature dependence of ellipsometric spectra of monolayer MoS2 and MoSe2 thin films. The absorption spectra show two exciton peaks (A and B) at low photon energies and several strong optical absorptions at higher photon energies. We used broadened Lorentzian model to analyze the exciton energy states. At room temperature, the monolayer MoS2 and MoSe2 thin films exhibit direct band gap approximately 1.92 ± 0.02 and 1.62 ± 0.02 eV and exciton binding energy of 0.27 ± 0.02 and 0.25 ± 0.02 eV. With increasing temperature, the band gap is red shifted.

Finally, we measured the optical transmittance spectra of monolayer WSe2 thin film under high magnetic fields. The zero-field absorption spectrum shows four absorption peaks, including A, B exciton and A, B band edge. For the left circularly polarized light, the peak positions of A exciton and A band edge are blue shifted with increasing magnetic field. By contrast, they are red shifted by using the right circularly polarized light. These splittings demonstrate that an applied magnetic field breaks the valley degeneracy (the ±K valley) at the corners of hexagonal Brillouin zone, resulting in the valley Zeeman effect.

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