Vibrational Sum Frequency Generation Spectroscopy Measurement of the Rotational Barrier of Methyl Groups on Methyl-Terminated Silicon(111) Surfaces
Vibrational Sum Frequency Generation Spectroscopy Measurement of the Rotational Barrier of Methyl Groups on Methyl-Terminated Silicon(111) Surfaces
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DOI：
10.1021/acs.jpclett.9b01487
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发表时间：
2019-09-19
影响因子：
5.7
通讯作者：
Benderskii, Alexander, V
中科院分区：
文献类型：
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作者：
Bhattacharyya, Dhritiman;Montenegro, Angelo;Benderskii, Alexander, V
端甲基Si（111）表面具有3倍面内对称，甲基取向垂直于底物。甲基的螺旋桨状旋转在室温下受到阻碍，并在与晶体硅衬底的三个等能最小值之间通过120度的跳跃进行。我们使用偏振选择振动和频率产生光谱的线形分析来确定表面甲基的旋转弛豫速率，并测量了弛豫速率在20至120℃之间的温度依赖性。我们提取了甲基旋转过程的活化能（旋转势垒）为830 +/- 360 cm(-1)，尝试频率为（2.9 +/- 4.2）× 10(13)s（-1）。与3倍余弦势中甲基的谐波频率比较表明，受阻旋转是通过单个甲基的不相关跳跃而不是协调的齿轮状旋转发生的。
The methyl-terminated Si(111) surface possesses a 3-fold in-plane symmetry, with the methyl groups oriented perpendicular to the substrate. The propeller-like rotation of the methyl groups is hindered at room temperature and proceeds via 120 degrees jumps between three isoenergetic minima in registry with the crystalline Si substrate. We have used line-shape analysis of polarization-selected vibrational sum frequency generation spectroscopy to determine the rotational relaxation rate of the surface methyl groups and have measured the temperature dependence of the relaxation rate between 20 and 120 degrees C. By fitting the measured rate to an Arrhenius dependence, we extracted an activation energy (the rotational barrier) of 830 +/- 360 cm(-1) and an attempt frequency of (2.9 +/- 4.2) x 10(13)s(-1) for the methyl rotation process. Comparison with the harmonic frequency of a methyl group in a 3-fold cosine potential suggests that the hindered rotation occurs via uncorrelated jumps of single methyl groups rather than concerted gear-like rotation.