Sum-Frequency Spectrum Phase Measurement of the Silica-Octadecyltrichlorosilane Interface and Measurement Accuracy Analysis
LIU Xiao-jie1,2, XU Shuai1,2, LI Yu-qiong1,2, JIN Gang1,2, FENG Ran-ran1,2,3*
1. National Microgravity Laboratory, Institute of Mechanics, Chinese Academy of Sciences, Beijing 100190, China
2. School of Engineering Science, University of Chinese Academy of Sciences, Beijing 100049, China
3. School of Chemistry and Molecular Engineering, Advanced Spectroscopy Center, Peking University, Beijing 100871, China
Abstract：Phase measurement sum-frequency vibration spectroscopy (SFG) can obtain molecular orientation information of the material surface, but there are still some key issues that remain unresolved, including experimental repeatability, experimental design, and interface analysis. Phase error can cause spectral changes and mislead interface structure analysis. Therefore, analyzing and accurately controlling errors is the key technique for phase measurement SFG. We used z-cut quartz as the phase standard, measured the sum frequency vibration spectrum of octadecyltrichlorosilane (OTS) modified on the fused silica substrate in the C—H vibration band, analyzed the phase spectrum of OTS. The results show that in the OTS imaginary spectrum, the two positive peaks at 2 878 and 2 936 cm-1 are the symmetrical vibration (CH3ss) and Fermi resonance (CH3FR) of the terminal CH3, and the negative peak at 2 960 cm-1 is anti-symmetric stretching vibration of CH3 (CH3as), and the spectral characteristics and designation of these three peaks are consistent with the literature. The negative peak near 2 910 cm-1 is CH2 anti-symmetrical stretching (CH2as). Compared with the literature, there is an offset of about 20 cm-1, and a negative peak is also observed near 2 850 cm-1, which belongs to CH2 symmetrical stretching (CH2ss). We think that the difference from the literature may be due to the effect of sample preparation time on the molecular arrangement of OTS. By establishing the relationship between the imaginary part spectrum of OTS and the orientation angle of CH3, it is found that the angle between the c-axis and the surface normal of the three vibration modes of CH3 is less than 90°, and its H is more oriented upward and arranged in order, indicating that the phase measurement can obtain richer surface information, compared with the intensity measurement. At the same time, the influence of the inconsistency of the position of the test sample and the reference sample on the phase measurement accuracy is discussed. By measuring the imaginary part spectrum of OTS at three different positions (12.1, 12.3, and 12.73 mm), and compared with simulated phase error on the imaginary part spectrum, indicate that the 2.5 μm displacement between the measurement position of the test sample and the reference sample corresponds to 1 ° phase error the phase shift of 20° will cause the zero position to move about 6 cm-1, which causes changes in the position and sign of the vibration peaks, leading to incorrect interpretation of the spectrum. Therefore, in order to obtain stable and reliable phase information of the interfacial molecules, it is necessary to strictly control that the measurement positions of the two samples are consistent.The results of this experimental study provide guidance for improving the accuracy of phase measurements and provide an effective means for the detection and analysis of surface states of molecular molecules, including the detection of small signals.
Key words：Second-order nonlinear optical technology; Sum frequency vibration spectrum; Interfacial molecular detection; Phase measurement
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