Probing the Molecule-Electrode Interface of Single-Molecule Junctions by Controllable Mechanical Modulations Jianfeng Zhou, Guojun Chen, and Bingqian Xu* Molecular Nanoelectronics, Faculty of Engineering & Nanoscale Science and Engineering Center, UniVersity of Georgia, Athens, Georgia 30602 ReceiVed: February 9, 2010; ReVised Manuscript ReceiVed: March 16, 2010 Presented here is an in-depth study to exploit the molecule-electrode interface effects on electronic transport properties in stabilized molecular junctions under controlled mechanical modulations. By monitoring and analyzing the conductance and force changes corresponding to the modulations, we isolated the effects of both the molecule terminating groups and the different contact configurations on electronic transport properties. The experimental results can be understood by our calculations developed based on simple and straightforward models. The results would be helpful not only to resolve the discrepancies of the recent scanning tunneling microscopy molecular break junction experiment results but also to add new insights into the understanding of the electronic transport mechanisms of molecular junctions. Introduction The ultimate goal of molecular electronics is to use functional electronic devices made from single molecules. 1-4 As a prerequisite to achieving this goal, it is crucial to be able to measure, control, and understand the electronic transport proper- ties of such molecular devices. 5,6 Studies have shown that, in a molecular junction, one of the most intensively studied molec- ular devices, the electronic transport characteristics are deter- mined by not only the molecular core but also the features of metal-molecule interfaces. 6-9 The molecular core depends on what and how the atoms make up the molecule, while the issue of interface includes the chemical nature and the contact geometric configurations 10-14 of the interface atom, which is much more complex, and thus, little has been known about many behaviors of the interface. 15 The main reason for the current difficulties first lies in the fact that many factors in the molecule-electrode interface can contribute to the transport properties; these factors include contact stabilities of molecular junctions, 16,17 contact bond length, 18 contact bond angles, 19 terminating anchoring group of molecule to electrode, 20-22 the arrangement of electrode atom, 23,24 and so on. Second, these interface factors couple with each other to make it difficult to identify their distinct influences on electron transport through molecules. Moreover, the difficulties are aggravated by a lack of effective experimental methods to monitor and control the microstructures of contact geometric configurations. In the previous experimental and theoretical results, 21,25-28 even for the simplest single molecule, it shows several orders variation of single molecular conductance values because of the less control over the contact configurations. 12,29 Therefore, the experimental methods that can be used to isolate the effects of the interfaces on electronic transport properties are expected urgently to result in a deeper understanding of the electronic transport properties in molecular junctions. In this article, we report an in-depth study of electronic transport properties of Au-alkanedithiol/alkanediamine-Au junctions, focusing on metal-molecule interface effects: both the terminating anchoring group of the molecule to the electrode and the contact configurations. Our earlier study revealed multiple conductance values of single-molecule junctions cor- responding to specific stable contact configurations. 30 Here, we monitor and analyze the conductance and force changes corresponding to the modulating of the contact configurations of a specific stable contact configuration by introducing me- chanical perturbations. 8,19,31 Regular ac triangular piezoelectric transducer (PZT) modulations are applied as controllable perturbations 8 in the free-holding processes of the modified scanning probe microscopy break junction technique (SPMBJ). 30 On the basis of the responsive conductance and force signals monitored simultaneously, we aim to build the connection among the conductance fluctuations, force changes, and the PZT modulations. The results would be helpful not only to resolve the discrepancies of the recent STM break junction experiment results but also to add new insights into the understanding of the electronic transport mechanisms of molecular junctions. Experimental Section Chemicals and Materials. 1,8-octanedithiol (HS(CH 2 ) 8 SH) and 1,8-octanediamine (H 2 N(CH 2 ) 8 NH 2 ) were purchased from Aldrich. Other solvents were used directly as received. Au(111) Substrate Preparation. Gold substrates were prepared by evaporating ∼100 nm of gold onto freshly cleaved mica sheets using an evaporator under a vacuum of 10 -7 Torr. The gold beads for Au substrate deposition come from Kurt J. Lesker Company (99.999%), and mica sheets were from Ted Pella, Inc. The surfaces were annealed in a hydrogen flame immediately before immersion in sample solutions. This an- nealing step cleans the surface and allows epitaxial reconstruc- tion of the Au to form large terraces of Au(111). Preparation of Self-Assembly Monolayers (SAMs). C8DT SAMs on gold substrates were prepared by soaking the 1 mM solution of C8DT toluene solution for 3 h. The C8DT molecules formed compact packed monolayers. C8DA SAMs on gold were prepared by soaking the 1 mM solution of C8DT water solution for 3 h. The coated gold substrates were then rinsed in the water solution three times and put in pure toluene after drying with argon. * To whom correspondence should be addressed. E-mail: bxu@engr.uga.edu. J. Phys. Chem. C 2010, 114, 8587–8592 8587 10.1021/jp101257y  2010 American Chemical Society Published on Web 04/02/2010