摘要/Abstract

表面增强拉曼光谱（SERS）的广泛应用源于优良的基底，目前主要局限在粗糙贵金属及胶体纳米颗粒材料.而半导体的光谱高稳定性和再现性，使其成为制备SERS基底的新型材料，但对于其SERS增强机理的研究仍存在极大挑战.本工作以典型的形貌新颖、尺寸均一的扫帚状n型纳米半导体ZnO为SERS基底材料，通过调节激发光的波长和选用具有不同对位取代基的对硝基苯硫酚（PNTP）、苯硫酚（TP）、对氨基苯硫酚（PATP）为探针分子，系统地研究了纳米ZnO的SERS增强行为，估算了其表面增强因子（EF），分离了化学增强作用中非共振增强效应和电荷转移效应对SERS的贡献.研究表明三种分子在不同激发光作用下的增强因子为10至35，其中PNTP分子约10倍的增强主要来自于因吸附而造成极化率变化的非共振增强效应，TP和PATP分子20~35倍的增强则是由非共振增强效应与光子驱动电荷转移效应共同作用所致，光子能量越高，SERS增强效应越强.且因分子与ZnO间电荷转移的速率较慢导致ZnO表面电荷转移增强效应较贵金属低1~2个数量级.本研究结果为新型半导体SERS基底的制备及调控提供了新思路.
关键词: 表面增强拉曼光谱, ZnO, 表面增强因子, 非共振增强, 电荷转移增强
The promising application of surface-enhanced Raman spectroscopy (SERS) was definitely based on the high quality substrates which were restricted to the rough noble metals and colloidal nanoparticle materials. However, semiconductor has become a potential substrate for the SERS investigation due to its high stability and reproducibility. It remains significant challenges in interpreting the enhancement mechanisms. Herein, broom-like ZnO nanoparticles with novel morphology and uniform size was prepared by pyrolysis of (CH₃COO)₂Zn. By using p-nitrophenylthiophenol (PNTP), phenylthiophenol (TP) and p-aminophenylthiophenol (PATP) as probe molecules, the SERS effect on ZnO surfaces was systematically studied under the irradiation of excitation lines with the wavelength of 532 nm and 638 nm. The different substituents in p-position of TP allowed to change the energy levels by the electron withdrawing or donating group, it was beneficial to match the energy level gap between the probe molecules and semiconductor for triggering the photon driven charge transfer. The surface enhancement factor (EF) of broom-like ZnO nanoparticles were estimated accordingly, and the contribution of non-resonance and charge transfer to SERS effect was distinguished. The results demonstrated that the surface enhancement factor was about 10 to 35 times depending on the probe molecules and excitation wavelengths. Therefore, the different enhancement origination contributed to the different molecules on the ZnO substrate. For the TP and PATP, the charge transfer from the HOMO level of molecule to CB of ZnO was achieved by the assistance of the laser photon with the appropriate energy. Moreover, the higher energy of the photon is, the stronger the SERS enhancement effect. As for the PNTP, the photon driven charge transfer was absent due to the significant change of the HOMO and LUMO level caused by the electron withdrawing group of NO₂. It revealed that the enhancement effect of PNTP molecule about 10 times was contributed by the non-resonance enhancement mechanism which was mainly due to the changes in the polarizability caused by the chemical adsorption. Comparing to the noble metal surface, the enhancement of charge transfer on ZnO was decreased with 1~2 orders of magnitude. The relatively lower rate of charge transfer in semiconductor resulted in the decrease of the charge transfer enhancement. The preliminary studies provided a novel approach for the preparation and regulation of new semiconductor SERS substrates.
Key words: surface-enhanced Raman Spectroscopy (SERS), ZnO, surface enhancement factor (EF), non-resonance enhancement, photon-driven charge transfer enhancement


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