化学贡献在SERS中是否仅起次要作用？
Has the chemical contribution a secondary role in SERS?
AI总结：
本研究通过共振SERS实验发现，化学共振贡献与电磁机制协同作用决定SERS效率，修正了化学贡献仅起次要作用的观点，强调需深入理解两种机制以优化基底分析应用。
摘要：
普遍认为，电磁贡献（即等离子体介导的激光和散射局域电磁场的增强）是表面增强拉曼散射（SERS）的主要机制，而所谓的化学（与分子相关的）贡献充其量仅起辅助作用。我们对多种基于纳米球光刻的金属基底进行了全面的共振研究，使用共价结合的4-巯基苯甲酸单分子层作为探针（该分子在溶液中无共振），结论是这一公认的观点需要修正。我们通过扫描激光波长和通过纳米球直径调节等离子体响应，对金属膜覆盖纳米球（MFON）基底进行了详细的共振SERS研究。通过光学反射率和SERS效率的测量分别表征了远场和局域场特性，并辅以数值模拟。我们证明，SERS效率确实取决于由系统等离子体响应决定的电磁机制，但我们也观察到，它还受到与共价结合探针分子的金属-配体电子跃迁相关的化学共振贡献的强烈影响。当等离子体模式与配体-金属化学共振相交时，两种机制协同作用，实现最佳放大。对于其他基于纳米球光刻的基底，包括球段空腔和六方有序三角纳米颗粒，使用Ag或Au作为等离子体金属，以及商业基底，均观察到相同的普遍趋势。我们得出结论，要充分利用这些基底的分析应用，必须深入理解电磁和化学两种机制。
英文摘要：
It is an established understanding that the electromagnetic contribution (the plasmon-mediated enhancement of the laser and scattered local electromagnetic fields) is the main actor in Surface Enhanced Raman Scattering (SERS), with the so-called chemical (molecule-related) contribution assuming only, if any, a supporting role. The conclusion of our comprehensive resonant study of a broad range of nanosphere lithography based metallic substrates, with covalently attached 4-mercaptobenzoic acid monolayers used as probe (molecules non-resonant in solution), is that this accepted understanding needs to be revised. We present a detailed resonant SERS study of Metal-film over nanosphere (MFON) substrates done both by scanning the laser wavelength, and by tuning the plasmon response through the nanosphere diameter. Far and local field properties are characterized through measures of optical reflectivity and SERS efficiency, respectively, and are supported by numerical simulations. We demonstrate that the SERS efficiency depends indeed on the electromagnetic mechanism, determined by the plasmonic response of the system, but we observe that it is also strongly defined by a chemical resonant contribution related to a metal-to-ligand electronic transition of the covalently bound probe molecule. Optimum amplification occurs when the plasmon modes intersect with the ligand-to-metal chemical resonance, contributing synergically both mechanisms together. The same general trend is observed for other nanosphere lithography based substrates, including sphere-segment void cavities and hexagonally ordered triangular nanoparticles, using both Ag or Au as the plasmonic metal, and also with a commercial substrate (Klarite). We conclude that a deep understanding of both the electromagnetic and chemical mechanisms is necessary to fully exploit these substrates for analytical applications.
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