A pair of facing electrodes with a desired gap width ranging from several angstroms to several nanometers is a key structure in the domain of nanoscience and nanotechnology. [1][2][3][4][5][6][7][8][9][10] For example, such a structure supported on a microchip provides a means to interface individual molecules or nanocrystals in integrated circuits. [2][3][4][5] The fabricated gap should have a width that is controllable and can be set to meet the specific needs of the target molecules (0.5-2 nm scale) or nanocrystals (1-10 nm scale) whose electron-transport properties can be characterized in detail. [6][7][8][9][10] In the past few years, three methods have been developed for fabricating electrode pairs with nano-sized or angstromsized gaps for device applications. [10][11][12][13][14][15][16][17][18] The first one, referred to as "break junction", is based on the mechanical breaking of a metal wire into two electrodes with a certain gap. [10,11] The involvement of mechanical forces in the fabrication procedure seems unlikely to be promising for manufacturing microchips, and, moreover, does not have the precision and flexibility to give the desired gap width. The second approach is based on electromigration of metal atoms, which is realized by passing a large electrical current through, and eventually breaking, a gold nanowire. [12,13] This process can yield a stable electrode separation of 1 nm with high efficiency. The above two methods, however, have certain limitations in applicability. The metal nanowire to be broken on a chip should be very thin, around 20 nm, and nanowires of this diameter can only be fabricated by electron-beam lithography, which is not commonly available.
The third method, which is based on the principle of electrodeposition or electrodissolution, can overcome the above-mentioned disadvantages. [14][15][16][17][18][19] By electrodepositing metal atoms onto a specific face of electrodes, one can sequentially narrow the gap from the original micrometer scale down to the domain of a few angstroms, or even connect