VOLUME 83, NUMBER 14 PHYSICAL REVIEW LETTERS 4OCTOBER 1999 Spatially Resolved Tunneling along a Molecular Wire V. J. Langlais, 1 R. R. Schlittler, 1 H. Tang, 2 A. Gourdon, 2 C. Joachim, 2 and J. K. Gimzewski 1, * 1 IBM Research, Zurich Research Laboratory, 8803 Rüschlikon, Switzerland 2 CEMES–CNRS, 29 rue J. Marvig, B.P. 4347, 31055 Toulouse Cedex, France (Received 26 March 1999) We have spatially resolved the electronic penetration of metallic electronic states through a molecular wire connected to an atomically clean contact. The molecular wire, which is 0.3 nm wide and 1.7 nm long, was electronically connected on one side, and a scanning tunneling microscope tip was used as a second movable electronic counterelectrode. The results reveal a clear exponential decay in the transparency (conductance) of the wire with distance from the contacted end. Analysis of the data shows that electrons are transported along the molecular wire by virtual resonance tunneling with an inverse decay length of 4 nm 21 , in excellent agreement with theoretical calculations. PACS numbers: 73.40.Gk, 61.16.Ch, 73.61.Ph The design and synthesis of identical, structurally perfect molecular conductors for tunneling electrons are very appealing for the development of future inte- grated nanoelectronic devices. Using scanning tunneling microscopy (STM), vertical tunnel junctions and point contacts, comprising a tip-apex molecule-surface geome- try, have been used to study the electronic transport through a single molecule [1] and single/double atom wires [2]. Conductance studies using break junctions of gold electrodes coated with benzene-1,4-dithiol have been performed [3]. Recently, electromechanical deformation, induced on STM point contact, of individual C 60 molecules was used to fabricate an electromechanical amplifier [4]. Single wall and multiwall carbon nanotubes (SWNT [5], MWNT [6]) as well as doped MWNTs [7] have been studied in two-, three-, and four-probe configu- rations on planar electrodes, and exhibit a large range of metallic to semiconducting electronic characteristics depending on the tube diameter and their wrapping angle in terms of a rolled-up graphene sheet [8]. To date, no experimental data exist on the length dependence of the conductance of a molecular wire, although theoretical calculations have been reported [9]. Such experiments are crucial for the verification of current theories and the establishing of design rules to enable optimization of novel molecular wires. In this Letter, we report the quantitative determination of the electron transport mechanism along the length of a molecular wire electrically connected to a metallic step at one end. The electronic conductance of the molecule- metal junction was spatially resolved as a function of distance from the step edge and spectroscopic current- voltage (I -V ) and current-distance (I -s) measurements were recorded at different points along the wire. Fig- ure 1 shows a schematic view of the experimental prin- ciple. By adsorbing a molecular wire onto a metallic step under ultrahigh-vacuum (UHV) conditions, an atomically clean contact between the molecule and the metal is cre- ated. This renders spatially resolved measurements along the length of the molecule experimentally accessible. The molecular wire itself was maintained above the substrate by four spacer units, designed to exhibit low leakage cur- rents to the substrate, as shown in Fig. 1(c). Analogous to a microwave stub, the STM tip can be used as a second, movable counterelectrode to probe the junction conduc- tance in a noninvasive manner. FIG. 1. Schematics of the experimental principle together with theoretically calculated STM profiles. (a) Double atomic steps offer a suitable height to adsorb molecular wires. Calculated STM cross-sectional profile on the double atomic step. (b) Molecular wire electronically coupled to the step. The evanescent wave function from the top terrace leaks into the wire. The profile shows an alternation of nodes and antinodes of the molecular orbitals. (c) The molecular wire is electronically decoupled from the lower terrace by using four spacer units, which modify the fine structure of the profile. 0031-9007 99  83(14)  2809(4)$15.00 © 1999 The American Physical Society 2809