Niu Li, Zhu Jia-qi, Gao Wei, Du Shan-yi
Center for Composite Materials and Structure, Harbin Institute of Technology, Harbin 150080, China.
Guang Pu Xue Yu Guang Pu Fen Xi. 2009 Sep;29(9):2438-41.
The vibrational density of states and nonresonant reduced Raman spectra of amorphous carbon at densities of 2.6, 2.9 and 3.2 g x 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 x 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.
采用第一性原理平面波赝势方法计算了密度为2.6、2.9和3.2 g·cm⁻³的非晶碳的振动态密度和非共振拉曼光谱。利用Car-Parinello分子动力学通过液体淬火法生成了三种结构模型，采用线性响应方法确定了它们的振动频率和本征模，并使用有限电场方法计算了拉曼耦合张量。计算结果表明，随着密度从2.6 g·cm⁻³增加到3.2 g·cm⁻³，sp³ 分数从50%增加到84.4%，sp² 构型从主要为环变为短链，G峰位置向高频移动，D峰和G峰的强度比降低，T峰位置向低频移动，T峰和G峰的强度比增加。作者计算的拉曼光谱与实验光谱总体上吻合良好。基于原子振动的分析证实，G峰和D峰均来自sp² 碳的贡献，G峰归因于任意一对sp² 原子的拉伸振动，T峰归因于C-C sp³ 振动。作者的分析还证实，G峰和T峰的频散是由于键长变化。链（烯烃）的键长比环的键长短，因此它们的振动频率更高，并且随着sp³ 分数的增加，G峰位置向高频移动。随着sp³ 分数的增加，sp³-sp² 型键的数量减少。这些键比纯sp³-sp³ 键短，因此T峰位置向低频移动。研究结果为分析非晶碳的实验拉曼光谱提供了理论依据。
