Orientational Analysis of Monolayers at Low Surface Concentrations Due to an Increased Signal-to-Noise Ratio (S/N) Using Broadband Sum Frequency Generation Vibrational Spectroscopy
Orientational Analysis of Monolayers at Low Surface Concentrations Due to an Increased Signal-to-Noise Ratio (S/N) Using Broadband Sum Frequency Generation Vibrational Spectroscopy
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DOI：
10.1177/0003702819857139
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发表时间：
2019-07
影响因子：
3.5
通讯作者：
Narendra M. Adhikari;U. I. Premadasa;Zachary Rudy;K. Cimatu
中科院分区：
文献类型：
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作者：
Narendra M. Adhikari;U. I. Premadasa;Zachary Rudy;K. Cimatu
和频生成（SFG）* 贡献者相等。光谱被用来推断在空气-固体和空气-液体界面处的自组装单分子膜（SAM）的末端甲基（CH 3）基团在99%氘代分子的存在下在表面浓度低至1%质子化分子的情况下的取向。利用气固界面上十八烷基硫醇（ODT）和氘代十八烷基硫醇（d37 ODT）自组装膜的SFG光谱进行了分析。然而，在空气-液体界面处分析了在水上的二十烷酸（EA）和氘代EA（d39 EA）SAM。对于1%ODT：99%d37ODT的SAM，末端CH 3基团的倾斜角估计为1.39 °，而1%EA：99%d39EA单层的末端CH 3基团的倾斜角估计为1.32 °。通过测试SFG光谱的灵敏度，验证了在低浓度下取向分析的可靠性。对于1%ODT：99%d37ODT和1%EA：99%d39EA的CH 3对称拉伸（SS），分别获得了1.60和1.45的信噪比（S/N）。S/N比值的估计增加，作为SFG光谱的灵敏度的量度，验证了在环境条件下在低浓度的界面分子下获得SFG光谱的能力。总体而言，取向分析的CH 3 SS振动模式是可行的，在低浓度的质子化分子，由于增加的S/N比。作为支持，还通过实验证明了可见光入射功率密度变化时信噪比的改善。
Sum frequency generation (SFG) * Equal contributors. spectroscopy was used to deduce the orientation of the terminal methyl (CH3) group of self-assembled monolayers (SAMs) at the air–solid and air–liquid interfaces at surface concentrations as low as 1% protonated molecules in the presence of 99% deuterated molecules. The SFG spectra of octadecanethiol (ODT) and deuterated octadecanethiol (d37 ODT) SAMs on gold were used for analysis at the air–solid interface. However, the eicosanoic acid (EA) and deuterated EA (d39 EA) SAMs on the water were analyzed at the air–liquid interface. The tilt angle of the terminal CH3 group was estimated to be ∼39 ° for a SAM of 1% ODT : 99% d37 ODT, whereas the tilt angle of the terminal CH3 group of the 1% EA : 99% d39 EA monolayer was estimated to be ∼32 °. The reliability of the orientational analysis at low concentrations was validated by testing the sensitivity of the SFG spectroscopy. A signal-to-noise (S/N) ratio of ∼60 and ∼45 was obtained for the CH3 symmetric stretch (SS) of 1% ODT : 99% d37 ODT and 1% EA : 99% d39 EA, respectively. The estimated increase in S/N ratio values, as a measure of the sensitivity of the SFG spectroscopy, verified the capacity to acquire the SFG spectra at low concentrations of interfacial molecules under ambient conditions. Overall, the orientational analysis of CH3 SS vibrational mode was feasible at low concentrations of protonated molecules due to increased S/N ratio. In support, the improved S/N ratio on varying incident power density of the visible beam was also experimentally demonstrated.