Study of Ultrasensitive Nanoscale Spectroscopy using Electric Field Enhancement

(1)原位透射电子显微镜实验表明，在聚焦的电子显微镜电子束照射下，六方薄晶体中形成的韧窝在随后的适度散焦电子束照射下恢复，使表面变平。电子受激恢复既不是碰撞损伤，也不是束流加热效应，而是GaN晶体中电子增强的自扩散效应。(2)当STM的样品尖端间隙被斩波光照射时，光产生的调制隧穿电流与伏安-电流曲线的二阶微分和光电场的平方成正比。在针尖样品间隙施加频率为fv的调制偏置电压，产生频率为2fv的调制隧穿电流，该调制电流与伏安-电流曲线的二阶微分成正比。通过比较这两种调制信号，我们可以定量地估计包括增强效应在内的STM针尖下的光电场。我们成功地在原位测量了HOPG表面的光增强强度。(3B)即使使用W尖端和铂尖端，也观察到了超过两个数量级的强烈光增强，尽管从表面等离子激元机制估计这些材料的光增强幅度不到50。结果表明，通常情况下，STM针尖具有较大的增强效应。

(1)In-situ transmission electron microscopy (TEM) experiments revealed that dimples formed in hexagonal thin crystals by irradiation of a focused TEM electron beam recovers to flatten the surface under a subsequent electron irradiation with a moderately defocused beam. The electron-stimulated recovery can be attributed neither to knock-on damage nor beam heating effects but to electronically enhanced self-diffusion in the GaN crystals.(2A)when the sample tip gap of STM was irradiated by chopped light, the light generates modulated tunneling current which is proportional to the second order differential of Voltage-Current curve and the square of light electric field. A modulated bias voltage of frequency fv applied to tip sample gap generates modulated tunneling current of frequency 2fv which is proportional to the second order differential of Vaoltage-Current curve. By comparing these two modulated signals, we can estimate quantitatively the light electric field beneath the STM tip including enhancement effects. We have successfully measured the magnitude of light enhancement on HOPG surface in situ.(3B)Drastic light enhancements over two orders were observed even with W tips and Pt tips, although expected magnitudes of enhancement for these materials estimated from surface plasmon mechanism were less than fifty. It was revealed that ordinally STM tips have a large amount of enhancement effects.