Single Molecule STM Nanospectroscopy using Difference Frequency Generation at the Tunnel Gap

本文提出了一种新的单分子分辨纳米光谱（STM- dfg）方案，该方案利用扫描隧道显微镜（STM）在针尖和样品的隧道间隙处局部产生的强红外光。成果如下。(1) STM探针尖端的光场增强测量：由于非线性I-V关系，在隧道间隙处的光场整流产生隧道电流，我们测量到石墨样品的光场增强系数分别为10^3和5×10^2，即使W尖端和Pt-Ir尖端也是如此。(2)证明了隧道隙中差频的产生：将C_<60>薄膜样品与W尖结合，分别用波长固定的ti -蓝宝石激光器和波长扫描的二极管激光器照射，我们成功获得了与C_<60>晶体红外吸收一致的光谱。(3)碳纳米管（CNT）中的场增强效应：我们发现n -二甲基甲酰胺（DMF）分子的拉曼散射只有在DMF中含有碳纳米管并且激发光与碳纳米管共振（633nm）时才会显著增强，这表明碳纳米管具有预期的场增强效应。(4)光场增强效应的STM观测：以金纳米棒为模型样品，我们开发了一种STM技术，通过检测局部整流电流，直接观察单个粒子上发生的光场增强效应。(5) STM- dfg的最佳条件：我们已经得出结论，更强烈地聚焦光束，使用脉冲激光，增加设定的隧道电流，以及使用特殊材料（Ag, Au， CNT）作为STM尖端是最有效的。

We have developed a new scheme of single-molecule-resolved nanospectroscopy (STM-DFG) that uses an intense infrared light locally generated at the tunnel gap of tip and sample in a scanning tunneling microscope (STM). The achievements are as follows. (1)Measurements of optical field enhancement at STM probe tips : From a tunneling current arising from a rectification of optical field at the tunnel gap due to the nonlinear I-V relation, we have measured for graphite samples the optical field enhancement factors as large as 10^3 and 5×10^2 even for W tips and Pt-Ir tips, respectively. (2)Proving difference frequency generation at a tunneling gap : For the combination of C_<60> film sample and W tip that was illuminated with two laser lights from a Ti-sapphire laser (wavelength fixed) and a diode laser (wavelength swept), we have successfully acquired spectra in agreement with infrared absorption by C_<60> crystals. (3)Field enhancement effect in carbon nanotubes (CNT) : We have found a remarkable enhancement of Raman scattering from N-Dimethyl formamide (DMF) molecules only when the DMF contained CNT and the excitation light is resonant (633nm) with CNT, a signature of the expected field enhancement effect of CNT. (4)STM observations of optical field enhancement effect : Using Au nano-rods as a model sample, we have developed an STM technique to observe directly the optical enhancement taking place at individual particles by detecting the local rectified current. (5)Optimum conditions for STM-DFG : We have concluded that it is most efficient to more intensely focus the light beam, to use pulsed laser lights, to increase the set tunneling current, and to use special materials (Ag, Au, CNT) for STM tips.