Nanoscopic analyses: Single molecule characterization in molecular electronics and surface science.

摘要

Time-resolved scanning tunneling microscope (STM) measurements of single molecules are used to determine the behavior and interactions of adsorbed species. This thesis describes the custom design of the scanning tunneling microscope hardware and software used to acquire molecular-resolution images. Using this instrument, two systems are characterized: the conductance switching of oligo (phenylene-ethynylene) (OPE) molecules inserted in n-alkanethiolate self-assembled monolayers (SAMs), and the motion of benzene adsorbed on Au{111} at 4 K. For both systems, the use of digital image processing was applied to the analysis of time-resolved sequential images to determine changes in topographic features or overlayer structure. From these analyses we can determine the mechanism for conductance switching and quantify the weak adsorbate-adsorbate substrate-mediated interactions present in benzene overlayers. These types of analyses must be executed at the single molecule level, as no ensemble technique can observe the discrete, heterogenious behaviors observed by STM in these systems.;One of the ultimate miniaturizations in nanotechnology is molecular electronics, where electronic devices will potentially consist of individual molecules. Some of the molecules being investigated for application in molecular electronics are a family of OPE molecules. Ensemble measurements of these molecules have yielded hysteretic switching and negative differential resistance, both useful properties that enable memory and logic operations. We investigate these systems by inserting the molecule of interest in an inert SAM to isolate single molecules, probing each molecule with the STM tip on a single-molecule basis. We acquire time-resolved sequences of STM images over periods of up to several days or acquire real-time measurements at a rate of 10 kHz for 15 seconds, and observe that these molecules exhibit conductance switching such that their apparent height changes. Due to properties of the materials used to acquire STM images, the field of view may drift. A digital image tracking algorithm based on Fourier transform cross-correlation has been developed to correct for instrumental drift in STM images. Analyzing the apparent height of different molecules in variable SAM matrices as a function of time has enabled us to propose that conductance switching is the result of hybridization and/or conformational changes at the metal-molecule interface.;Using similar analyses, we have applied scanning tunneling microscopy to probe and to quantify the weak substrate-mediated interactions in benzene overlayers on Au{111} at 4 K. We observe that benzene molecules exhibit three types of motion, including 2D desorption, 2D adsorption, and simultaneous dislocations of many molecules (molecular cascades). Correlating the probability of 2D desorption with the number of nearest neighbors of the desorbing molecules enables the calculation of the magnitude of the adsorbate-adsorbate substrate-mediated interactions. We also observe chains of up to 12 molecules simultaneously moving in the same direction at the same time in an event we refer to as molecular cascades. These cascades are the result of translation of the overlayer structure and are highly correlated with 2D desorption and 2D adsorption.