Local non-linear spectroscopy using electromagnetic enhancement effect of a metal tip apex

利用金属尖端电磁增强效应的局部非线性光谱

基本信息

批准号：
15204026
负责人：
USHIODA Sukekatsu
金额：
$ 30.37万
依托单位：
Japan Advanced Institute of Science and Technology (2004)Tohoku University (2003)
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (A)
财政年份：
2003
资助国家：
日本
起止时间：
2003 至 2004
项目状态：
已结题

项目摘要

When a laser beam is shined at the tip-sample gap of a scanning funneling microscope (STM), the electromagnetic field is strongly enhanced immediately below the sharp tip apex. Using this phenomenon one can realize optical spectroscopy with very high special resolution beyond the optical diffraction limit. In this study we have constructed a tip-enhanced nonlinear spectroscopy system that consists of an ultra-high-vacuum low-temperature STM, a highly sensitive photon detection system, and a tunable laser system. As a first experiment, we have measured the tip-enhanced Raman spectra of single wall carbon nanotubes (SWNT) deposited on the Au(111) surface with excitation light of 785 nm. In this experiment we used Au tips, which are expected to show large electromagnetic enhancement. The Au tips were fabricated by electrochemical etching in a saturated aqueous solution of NaCl. By adjusting the temperature and pH of the electrolyte, we have succeeded in producing Au tips with the radii of curvature less than 50 nm. Using an Au tip we observed STM images of the sample until an individual SWNT bundle was found. Then the STM tip was fixed over the target SWNT bundle, and subsequently the Raman spectra were measured as a function of the tip-sample distance. The Raman intensity of the G-band (〜1590 cm^<-1>) of SWNT decreases rapidly as the tip-sample distance increases up to 〜10 nm. This result clearly shows the tip-enhanced scattering effect. Furthermore, we have observed optical second harmonic generation from Au nanowire arrays on faceted NaCl(110) substrates with wire widths from 40 to 60nm, in order to investigate the general local field near Au nanostructures. We have found that the amplitude of the incident field polarized perpendicular to the nanowire axes is considerably reduced by the de polarization field induced in the nanowires.

当激光束照射到扫描漏斗显微镜 (STM) 的尖端与样品间隙时，尖端尖端下方的电磁场会强烈增强，利用这种现象可以实现超越光学的极高特殊分辨率的光谱学。在这项研究中，我们构建了一个尖端增强非线性光谱系统，该系统由超高真空低温STM、高灵敏度光子探测系统和可调谐激光器组成。作为第一个实验，我们使用 785 nm 的激发光测量了沉积在 Au(111) 表面上的单壁碳纳米管 (SWNT) 的尖端增强拉曼光谱。为了表现出较大的电磁增强作用，我们通过在饱和氯化钠水溶液中进行电化学蚀刻来制造金尖端，通过调节电解质的温度和 pH 值，我们成功地制备了金尖端。使用金尖端，我们观察样品的 STM 图像，直到找到单个 SWNT 束，然后将 STM 尖端固定在目标 SWNT 束上，随后测量拉曼光谱。当尖端-样品距离增加至约 10 nm 时，SWNT 的 G 波段 (~1590 cm^-1>) 的拉曼强度迅速减小。此外，我们观察了线宽为 40 至 60 nm 的多面 NaCl(110) 基底上的金纳米线阵列的光学二次谐波产生，以研究金纳米结构附近的一般局部场。垂直于纳米线轴偏振的入射场的幅度因纳米线中感应的去偏振场而显着减小。