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

我们开发了一种新的单分子分辨纳米光谱（STM-DFG）方案，该方案使用在扫描隧道显微镜（STM）中的针尖和样品的隧道间隙处局部产生的强红外光。取得的成就如下。（1）STM探针针尖的光场增强测量：利用非线性I-V关系对光场在隧道间隙整流产生的隧穿电流，我们测量了石墨样品的光场增强因子，即使对于W针尖和Pt-Ir针尖，也分别高达10 ^3和5 × 10 ^2。（2）证明了隧道隙处的差频产生：对于C_2薄膜样品和W针尖的组合，用钛蓝宝石激光器（波长固定）和二极管激光器（波长扫描）的两种激光照射，成功地获得了与C_2晶体红外吸收一致的光谱。<60><60>（3）碳纳米管（CNT）的场增强效应：我们发现只有当N-二甲基甲酰胺（DMF）中含有CNT，且激发光与CNT共振（633nm）时，DMF分子的拉曼散射才显著增强，这是CNT场增强效应的特征。（4）光场增强效应的STM观测：以Au纳米棒为模型样品，发展了一种通过探测局域整流电流直接观测单个粒子光场增强的STM技术。（5）STM-DFG的最佳条件：我们得出的结论是，更强的光束聚焦，使用脉冲激光，增加隧道电流，以及使用特殊材料（Ag，Au，CNT）作为STM针尖是最有效的。

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.