Significance of local density of states in the scanning tunneling microscopy imaging of alkanethiol self-assembled monolayers.
Significance of local density of states in the scanning tunneling microscopy imaging of alkanethiol self-assembled monolayers.
复制标题
烷硫醇自组装单层扫描隧道显微镜成像中局部态密度的意义。
作者：
Alexandru Riposan;Gang
用系统的扫描隧道显微镜(STM)研究了烷硫醇自组装单分子膜(SAM)的偏置电压、隧道电流和尖端-末端间距。在超高真空中退火后，得到了稳定的、无刻蚀坑的近堆积十一硫醇/Au(111)自组装膜。STM显示了两个不同的c(4x2)结构，每个单位细胞有四个不等价的分子。对于这两种结构，当系统地调节偏置电压、电流和尖端-终端距离时，会发生可逆的对比度变化。这些对比度跃迁源于探测每个分子对应的局域态密度(LDOS)，而不是烷硫醇链的重新定向。在0.5-2.5A的范围内，扫描隧道显微镜的对比度对针尖-末端的间隔特别敏感，反映了LDOS的距离依赖性。在固定的针尖高度，STM对比度对0.1-1.2V范围内的偏置变化不那么敏感。我们首次证明，对于烷硫醇SAM系统，LDOS可能会覆盖STM地形对比度的物理高度变化。
A systematic scanning tunneling microscopy (STM) study of alkanethiol self-assembled monolayers (SAMs) is presented as a function of the bias voltage, tunneling current, and tip-termini separation. Stable and etch-pit free SAMs of close-packed undecanethiol/Au(111) were obtained after annealing in ultrahigh vacuum. STM revealed two distinct c(4x2) structures with four nonequivalent molecules per unit cell. For both structures, reversible contrast variations occur upon systematically tuning the bias voltage, the current, and the tip-termini distance. These contrast transitions originate from probing the corresponding local density of states (LDOS) of each molecule and not from the reorientation of the alkanethiol chains. The STM contrast is particularly sensitive to the tip-termini separation in the range of 0.5-2.5 A, reflecting the distance-dependence of LDOS. At a fixed tip elevation, the STM contrast is less sensitive to changes in bias within 0.1-1.2 V. For the first time, we demonstrate that LDOS may override the physical height variations in the STM topographic contrast for alkanethiol SAM systems.