非晶碳薄膜振动拉曼光谱的第一性原理研究

采用基于第一性原理的赝势平面波方法，对3个不同密度(2.6，2.9和3.2 g·cm-3)非晶碳结构的振动态密度和振动拉曼光谱进行了研究。结构模型由快速“液体-淬火”方法模拟得到，振动频率和本征模由线性响应理论决定，拉曼耦合张量由有限电场方法计算。计算结果表明：当密度从2.6增加到3.2 g·cm-3时，sp3碳含量从50%增加到84.4%，G峰向高频区偏移，D峰和G峰的强度之比I_D/I_G减小，T峰向低频区偏移且T峰和G峰的强度之比I_T/I_G增大。该结果与实验结果显示出很好的一致性。依据原子振动的分析结果证实：拉曼光谱的G峰和D峰均来自于sp2碳原子的振动贡献，且G峰是由任何成对的sp2碳原子的伸缩振动产生的，T峰来自于sp3杂化碳原子的振动贡献，G峰和T峰峰位随结构的色散是由键长变化导致的。

First-Principles Study of Vibrational Raman Spectra of Amorphous Carbon

The vibrational density of states and nonresonant reduced Raman spectra of amorphous carbon at densities of 2.6, 2.9 and 3.2 g·cm-3 were calculated by the use of a first-principles plane-wave pesudopotential method. Three structural models were generated by liquid-quench method using Car-Parinello molecular dynamics, their vibrational frequencies and eigenmodes were determined using the linear response approach, and Raman coupling tensors were calculated using the finite electric field method. The calculated results show that the sp3 fraction increases from 50% to 84.4%, the sp2 configuration changes from mainly rings to short chains, the position of the G peak moves to higher frequencies, the intensity ratio of D and G peaks decreases, the position of the T peak moves to lower frequencies and the intensity ratio of T and G peaks increases as density increases from 2.6 to 3.2 g·cm-3. The authors’ calculated Raman spectra show an overall good agreement with experimental spectra. The analysis in terms of atomic vibrations confirms that the G and D peaks both come from sp2 C contribution, G peak is due to the stretching vibration of any pair of sp2 atoms and the T peak is due to the C—C sp3 vibration. The authors’ analysis also confirms that the dispersion of G and T peaks is due to bond-length changes. The bond length of chains (olefins) is shorter than that of rings, so their vibrational frequency is higher and the G-peak position moves to higher frequencies with increasing the sp3 fraction. The number of sp3-sp2 type bonds decreases as the sp3 fraction increases. These bonds are shorter than pure sp3-sp3 bonds, hence the T-peak position moves to lower frequencies. The research results provide a theoretic basis for analyzing experimental Raman spectra of amorphous carbon.