不同厚度的碳六十薄膜之光穿透特性

Optical transmission on C60 films of various thicknesses

蘇志豐(Chih-Feng Su)

指導教授 ： 邱寬城

中原大學/理學院/應用物理研究所/碩士(2008年)

https://doi.org/10.6840/cycu200900189

摘要

本實驗利用物理汽相沉積法，藉由控制成長時間、基板與粉末端溫度，製備不同結晶性和厚度的 C60 多晶薄膜。同時利用掃描式電子顯微鏡觀測薄膜表面形貌與厚度，使用 X-ray 繞射分析薄膜之結晶性，並於變溫條件 (30 ~ 300 K) 下量測光穿透頻譜 (1.64 ~ 2.07eV)來探討電子吸收光後的躍遷行為。由實驗結果可得知 C60 薄膜顆粒大小 (R) 隨薄膜厚度 (L) 之變化關係大致可區分為三部份。當開始成長時，R 會隨 L 變厚而增大，且遵循 R  L0.72 之關係式。直到 L 約介於1 ~ 2 μm 時， R 會趨近飽和於1 μm，此時 C60 顆粒已經覆蓋滿 ITO 表面。若持續成長，則遵循 R  L0.91 之關係式，直到 L 約為 10 ~ 20 μm 時，R 會再度趨近飽和。若再持續成長，則遵循 R  L1.18 之關係式。另外，由膜厚 5 ~ 20 μm 的 C60 薄膜光穿透頻譜可觀測得三對吸收峰，分別為：Ea = 1.848 eV 及 Ea' = 1.880 eV、Eb = 1.940 eV 及 Eb' = 1.968 eV、與 Ec = 2.016 eV 及 Ec' = 2.045 eV。再由膜厚 20 ~ 50 μm 的 C60 薄膜光穿透頻譜可觀測得四根吸收峰，分別為：E1 = 1.688 eV、E2 = 1.719 eV、E3 = 1.741 eV、及 E4 = 1.777 eV。這些有關 C60 有機分子固體的光學吸收峰值變
化，可由Herzberg-Teller 耦合 (即電子與分子的振動模態之作用) 來解釋。基於上述的觀測結果，本文因此提出一個有關固態 C60 分子材料之能階躍遷模型。本躍遷模型假設 C60 多晶薄膜之價電子帶與傳導帶是分別由 S0 與S1 能階經 Davydov 分裂而組成，故 S0 至 S1 的起始躍遷能量即對應為能隙 (Eg)。所以，由變溫之光穿透頻譜可得一系列能隙隨溫度之變化 Eg(T)。最後，本文利用數值模擬來擬合及分析 C60 之 Eg(T)，並探討 C60 多晶薄膜與 C60 單晶之
Eg(T) 擬合參數之差異。

關鍵字

光穿透 ； 能階躍遷 ； Herzberg-Teller 耦合

並列摘要

In this study, by controlling source temperature, substrate temperature, and growth time, C60 polycrystalline films are prepared by physical vapor deposition. Meanwhile, the surface morphology, films thickness, degree of crystallinity, and optical absorption of the as-grown C60 polycrystalline films are characterized by scanning electron microscopy, X-ray diffraction, and temperature-dependent optical　transmission (with incident photon energies from 1.46 ~ 2.07 eV), respectively. Based on the experimental results from C60 polycrystalline films, the variation of averaged grain size (R) versus film thickness (L) can be categorized by three regimes. At the very beginning, R varies with L following the equation: R  L0.72. As L approaches 1 ~ 2 μm, R saturates around 1 μm and at this moment the ITO surface is completely covered by C60 polycrystalline grains. In the second regime, as the growth proceeds, R varies with L0.91 until L reaches 10 ~ 20 μm, and then R saturates around 10 μm again. In the third regime, R varies with L1.18. In addition, form the optical transmission spectra of C60 polycrystalline films with L = 5 ~ 20 μm, three pairs of absorption peaks can be resolved as Ea = 1.848 eV, Ea’ = 1.880 eV, Eb = 1.940 eV, Eb’ = 1.968 eV, Ec = 2.016 eV, and Ec’ = 2.045 eV. Furthermore, four absorption peaks of E1 = 1.688 eV, E2 = 1.719 eV, E3 = 1.747 eV, and E4 = 1.777 eV can be recognized in the spectra of thick C60 polycrystalline films with L = 52 ~ 60 μm. These rich variations of absorption peaks can be explained by the Herzberg-Teller coupling (electronic-vibronic interaction) in this organic molecular solid. Then, an energy terms diagram is proposed for the band structure of solid C60 fullerite. This model assumes that the valence band and conduction band in this polycrystalline sample are composed by the Davydov splitting from molecular levels of S0 and S1, respectively. So, the energy bandgap (Eg) is defined and then Eg(T) is performed for this solid C60 fullerite. Finally, from a numerical fit of Eg(T) to O’Donnell-Chen equation, the fitting parameters are obtained for these C60 polycrystalline thick-films, and a comparison of these fitting
parameters to those obtained from C60 single crystals is made.

並列關鍵字

energy terms diagram ； Optical transmission spectra ； Herzberg-Teller coupling

參考文獻

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