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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 gold nanowire is prepared that is thick at its ends but thin in the middle (8). When an electrical current passes through this wire, the thin part of the wire breaks to create the gap. In an alternative strategy, a thin metallic bridge is created across a pit in a silicon wafer (9). The wafer is then strained in order to break the bridge at the thinnest point along its length. Such methods typically require experimental finesse and sophisticated modern microfabrication facilities, and often yield only a small number of functional devices (8).
On-wire lithography. A nanopore membrane (A) is used as the template for synthesizing segmented nanowires (B). Next, the template is dissolved, the nanowires are deposited on a surface, and silicon is deposited selectively onto segmented nanowires (C). Removal of sacrificial metal (green) creates a nanometer-scale gap between the two conducting (gold) segments (D).
What would be the ideal device for fundamental studies in molecular electronics? Ideally, the device would be micrometers in length, such that it could be easily manipulated and positioned. Somewhere along its length, this micrometer-scale structure would have a reproducible, nanometer-scale gap. Finally, it should be possible to prepare the device by a simple and versatile method that allows mass production and provides convenient and reproducible control over the size of the gap. The ability to have different metals on either side of the gap would also be useful (10).
On-wire lithography yields devices that have the potential to satisfy all these criteria (7). The requisite micrometer-scale structure is a microwire (360 nm in diameter and 5 m in length) prepared by template synthesis (11). In this method, cylindrical pores in a membrane are used as templates to prepare wires and tubes that typically have micrometer-scale lengths and nanometer-scale diameters. Methods such as electroplating or chemical polymerization are used to deposit the wires or tubes in the pores. Because the membrane pore densities are high (see the figure), it is easy to make large numbers of these structures. In addition, the pores have uniform diameters, resulting in correspondingly uniform tubes and wires, and the pore diameter can be varied at will. After preparation, the wires or tubes can be liberated by dissolving the template membrane, and manipulated by simple solution-based processing methods.
In on-wire lithography, a segmented microwire--in the simplest case, a long gold segment connected to a very short silver segment connected to another long gold segment--is electroplated into the pores of a commercially available alumina template. Template-based electroplating of such segmented wires is well established (12, 13), and electronic measurements on segmented nanowires have been reported (14). The gap is prepared by dissolving the short silver segment. Electroplating provides a reproducible and quantifiable way to control the quantity of material deposited. Hence, the length of the gap-forming segment can be controlled with great precision. Furthermore, it is easy to prepare wires with more than one gap and with gaps of different lengths.
It would be pointless to have a dissolvable gap between two nondissolvable segments if, upon dissolution, the remaining segments fell apart. Qin et al. solve this problem by dispersing the segmented wires on a surface and then coating one side with a sheath of a nondissolvable material. This sheath holds the wire together after removal of the dissolvable segment. The authors show that these sheathed microwires can be placed between two much larger electrodes and that the gap can be bridged with a test molecular-electronic material--a conductive polymer.
Challenges remain. The smallest gap reported by Qin et al. is 5 nm. There is no fundamental reason why smaller gaps cannot be prepared, but this remains to be demonstrated. Furthermore, the test material consisted of a collection of polymeric chains. The method has yet be demonstrated on a single small-molecule conductor. Nevertheless, the ease with which the devices can be prepared, characterized, and manipulated bodes well for on-wire lithography to become an important tool in the molecular-electronics toolbox.
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10.1126/science.1114663
The authors are in the Department of Chemistry and the Center for Research at the Bio/Nano Interface, University of Florida, Gainesville, FL 32611, USA. E-mail: crmartin@chem.ufl.edu
The editors suggest the following Related Resources on Science sites:
m in length) prepared by template synthesis (
