Electronic and mechanical properties of single molecular junctions
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Molecular junctions possess simple structures but promising properties for potential device applications. The central issue in current single molecular junction studies is to achieve comprehensive understanding on the fundamental electron transport mechanisms. Nowadays, the main challenge facing this field is how to fabricate and characterize single molecular junctions under precise controls, which is crucial to study the influences of different factors on the electron transport processes. The approach which can isolate these factors to enable the accurate studies of their influences is greatly needed. In our studies, we developed an integrated scanning probe microscope (SPM) technique. It could fabricate and in situ characterize multiple types of molecular junctions in an identical measurement environment and under precise controls. By applying this technique, we evaluated the factors contributing to the transport properties of molecule junctions independently in identical environments. The results lead to in-depth understanding on electron transport mechanisms of single molecular junctions. Based on the understanding of conduction mechanisms, we tried to mechanically modulate the electron transport through single molecular junctions. We firstly investigated the stability of molecular junctions under regular mechanical perturbations. With the simultaneously measured force and current corresponding to the modulation signals, we constructed a simple quantitative model based on assumptions of multiple retangular potential barriers. This model can accurately evaluate effects of mechanical modulations on electron transport properties of single molecular junctions. Finally, we tested two typical functional molecular junctions: both Ru-terpyridine molecular junctions and the heterojunction with asymmetric contacts. Nonlinear electron transport behaviors of single Ru-terpyridine molecular junctions were attributed to the specific properties of Ru-terpyridine. By analyzing correspondent force evidences, we proposed that the bias induced contact-molecule coupling should be responsible to the observed Negative differential resistance (NDR) features. To understand single molecular properties of heterojunctions, a series of molecular junctions with symmetric or asymmetric contacts were also studied using our integrated technique. Our experimental results demonstrated that the symmetry or asymmetry of transport behavior depends on the nature of the electronic coupling in the contacts.