From single to multiple Ag-layer modification of Au nanocavity substrates: a tunable probe of the chemical surface-enhanced Raman scattering mechanism.
From single to multiple Ag-layer modification of Au nanocavity substrates: a tunable probe of the chemical surface-enhanced Raman scattering mechanism.
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DOI：
10.1021/nn200567m
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发表时间：
2011-06
期刊：
影响因子：
17.1
通讯作者：
N. Tognalli;E. Cortés;A. Hernández-Nieves;P. Carro;G. Usaj;C. Balseiro;M. E. Vela;R. Salvarezza
中科院分区：
文献类型：
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作者：
N. Tognalli;E. Cortés;A. Hernández-Nieves;P. Carro;G. Usaj;C. Balseiro;M. E. Vela;R. Salvarezza
我们提出的实验和计算结果揭示了化学对表面增强拉曼散射（SERS）贡献的机制。以电化学方式覆盖单层银或 10-100 个银层的金空隙金属阵列，用 4-巯基吡啶自组装单层进行修饰，作为分子拉曼探针，显示出丰富且意想不到的拉曼响应。观察到光谱红色部分拉曼强度的共振增加，这与腔阵列的等离激元激发无关。值得注意的是，我们发现在 Au 基底上沉积单个 Ag 层时，SERS 放大倍数增加了 10-20 倍，然而，在沉积 10 个原子层时，SERS 放大几乎被淬灭。进一步沉积 100 个原子 Ag 层导致 SERS 信号新增加，这与 Ag 块状结构的等离子体效率提高一致。根据从头计算和基于分子 HOMO 态与金属表面费米能级之间的共振电荷转移过程的 SERS 化学机制的微观模型，分析了 SERS 响应作为 Ag 层厚度的函数。我们发现，与 Au/Ag/分子复合物中 Ag 单层的存在相关的电子电荷密度的重新排列导致 HOMO 电荷中心与金属图像平面之间的距离增加，这导致了所研究的基底之间拉曼增强的变化。我们的结果提供了一个通用平台，用于研究 SERS 的化学贡献，以及通过银膜的电化学修饰来提高定制 Au-SERS 模板的拉曼效率。
We present experimental and computational results that enlighten the mechanisms underlying the chemical contribution to surface-enhanced Raman scattering (SERS). Gold void metallic arrays electrochemically covered either by a Ag monolayer or 10-100 Ag layers were modified with a self-assembled monolayer of 4-mercaptopyridine as a molecular Raman probe displaying a rich and unexpected Raman response. A resonant increase of the Raman intensity in the red part of the spectrum is observed that cannot be related to plasmon excitations of the cavity-array. Notably, we find an additional 10-20 time increase of the SERS amplification upon deposition of a single Ag layer on the Au substrate, which is, however, almost quenched upon deposition of 10 atomic layers. Further deposition of 100 atomic Ag layers results in a new increase of the SERS signal, consistent with the improved plasmonic efficiency of Ag bulk-like structures. The SERS response as a function of the Ag layer thickness is analyzed in terms of ab initio calculations and a microscopic model for the SERS chemical mechanism based on a resonant charge transfer process between the molecular HOMO state and the Fermi level in the metal surface. We find that a rearrangement of the electronic charge density related to the presence of the Ag monolayer in the Au/Ag/molecule complex causes an increase in the distance between the HOMO center of charge and the metallic image plane that is responsible for the variation of Raman enhancement between the studied substrates. Our results provide a general platform for studying the chemical contribution to SERS, and for enhancing the Raman efficiency of tailored Au-SERS templates through electrochemical modification with Ag films.