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MATERIALS SCIENCE
The goal of molecular electronics is the construction of electronic circuit elements (such as transistors and diodes) from individual molecules (1, 2). The molecules of interest have dimensions on the order of a few nanometers, whereas with conventional photolithography, the smallest structures that can be prepared are on the order of 100 nm (3). Therefore, molecular electronics potentially allows a greater number of circuit elements to be packed on a chip than is possible with conventional methods (4). Mainstream electronics companies such as Hewlett-Packard, Motorola, and IBM are pursuing research and development projects in molecular electronics (5).
However, the development of this technology requires electrical contact to the molecule to be made (1, 6). To study the electronic properties of a macroscopic circuit element, such as a resistor, one simply connects electrical leads to each end of the device. How can one connect electrical leads to each end of a molecule? On page 113 of this issue, Qin et al. describe a potentially versatile method, on-wire lithography, for accomplishing this task (7).
Currently, two general approaches are used to make electrical contacts to individual molecules (1). In the first approach, one end of the molecule is connected to a conductive surface, and the ultrafine tip of a scanning-probe microscope is used to make contact to another part of the molecule. However, it can be difficult to find a single molecule on a surface and to know how much force to apply to make good electrical contact (6).
The second approach entails fabrication of a nanometer-scale gap between two electrodes, followed by insertion of the molecule into the gap (1). In one such method, a...