Application of Electrodeposition and Etching Forms Stable Metallic Nanogaps
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RESEARCH/RESEARCHERS
Application of Electrodeposition and Etching Forms Stable Metallic Nanogaps As research heads toward the age of nanoelectronics, some applications require electrodes with gap sizes on the same order of magnitude as the size of an atom or molecule. Researchers from the Department of Physics at Florida International University have fabricated stable molecular-scale gaps with sub-angstrom precision. As reported in the December 18 issue of Applied Physics Letters, gaps a few nm or less can be fabricated with a 0.5 Å resolution. The gap sizes may be readily increased or decreased by the process of electrodepositioning or etching. The researchers indirectly measured the gap widths by the amplitudes of the electron tunneling current. The initial calibration setup was comprised of a Pt0.8Ir0.2 scanning tunneling microscope (STM) tip coated with Apiezon wax and a 100-nm thick gold film on a mica substrate in an electrical feedback circuit. The researchers C.Z. Li, H.X. He, and N.J. Tao narrowed the initial gap, 50 nm wide, by bathing the STM tip in a CuSO4 and H 2SO 4 electrolytic solution. This caused copper to deposit onto the STM tip. They measured the tunneling current continuously while the gap narrowed. In order to produce long-term stable nanogaps, the researchers fabricated a pair of gold electrodes on oxidized silicon. An expected increase in tunneling current as the gap narrowed was observed. The unexpected event, however, was the observation of step-wise changes during this climb. The researchers said that the discrete nature of atoms may have caused this quantization in tunneling current through a process known as structural relaxation. Relaxed structures supply stable energy wells, which provide a predictable means of gap construction. These discrete changes in currents correspond to 0.5 Å changes to the physical widths of the gap. The researchers further controlled the gap width with a feedback loop through reversibility by electrochemical etching. This is similar to the deposition process except for the reversed electrochemical potential. This method of nanogap fabrication contributes toward the future of nanotechnology due to its precision, stability, and reversibility. JUNE LAU
Atomic Force Microscope Aids Fabrication Control of Gold Nanowires A group of scientists from the National Research Institute for Metals in Japan has developed a technique for controlled 8
growth of gold nanowires. The researchers explained their method as well as their results in measurements of electronic transport in the December 25 issue of Applied Physics Letters. Using an atomic force microscope (AFM) in contact mode with a piezoresistive cantilever coated with gold, they grew the nanowires on top of a Si(111) substrate in ultrahigh vacuum. Prior to Au deposition the silicon is flashed to 1200°C several times until a clear 7 × 7 surface reconstruction is observed. The surface is then checked for cleanliness by Auger spectroscopy and scanning tunneling microscopy. The nanowires are then drawn onto the sur
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