Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742, United States, USA.
ACS Nano. 2010 Oct 26;4(10):5763-72. doi: 10.1021/nn101484a.
The SERS phenomenon was studied using a large set of silver nanocube dimers programmed to self-assemble in preset locations of a patterned substrate. This SERS substrate made it possible to demonstrate the dependence of the SERS enhancement on the geometry of the silver nanocube dimers and to quantify the dispersion in the SERS enhancement obtained in an ensemble of dimers. In addition to the effects of the gap distance of the dimer and the orientation of the dimer axis relative to the laser polarization on SERS enhancement, the data reveal an interesting dependence of the site-to-site variations of the enhancement on the relative orientation of the nanocubes in the dimer. We observed the highest heterogeneity in the SERS signal intensity with face-to-face dimers and a more robust SERS enhancement with face-to-edge dimers. Numerical calculations indicate that the plasmon resonance frequencies of face-to-face dimers shift considerably with small changes in gap distance. The resonance frequency shifts make it less likely for many of the dimers to satisfy the matching condition between the photon frequencies and the plasmon resonance frequency, offering an explanation for the large site-to-site variations in SERS signal intensity. These results indicate that plasmonic nanostructure designs for SERS substrates for real-world applications should be selected not only to maximize the signal enhancement potential but also to minimize the heterogeneity of the substrate with respect to signal enhancement. The latter criterion poses new challenges to experimentalists and theorists alike.
采用大量经编程自组装在图案化基底预定位置的银纳米立方体二聚体研究了 SERS 现象。该 SERS 基底使我们能够证明 SERS 增强取决于银纳米立方体二聚体的几何形状，并定量获得二聚体集合中的 SERS 增强的分散度。除了二聚体的间隙距离和二聚体轴相对于激光偏振的方向对 SERS 增强的影响外，数据还揭示了增强的局域变化与二聚体中纳米立方体的相对取向之间存在有趣的依赖性。我们观察到面对面二聚体的 SERS 信号强度具有最高的异质性，而面对面二聚体具有更稳健的 SERS 增强。数值计算表明，面对面二聚体的等离子体共振频率随间隙距离的微小变化而显著移动。共振频率的移动使得许多二聚体不太可能满足光子频率与等离子体共振频率之间的匹配条件，这为 SERS 信号强度的大局域变化提供了一种解释。这些结果表明，用于实际应用的 SERS 基底的等离子体纳米结构设计不仅应最大化信号增强潜力，而且还应最小化基底相对于信号增强的异质性。后者的标准对实验家和理论家都提出了新的挑战。
