Indium Nanowires Synthesized at an Ultrafast Rate

A challenge in the development of nanometer-scale devices, which are envisioned to impact human life in the near future, is the development of economical techniques for the fabrication of their building blocks. Owing to their unique and exquisite characteristics, nanowires are ideal building blocks for functional nanometer-scale electronics, photonic structures, and nanosurgery devices. Here, we report a novel phenomenon that provides a robust technique for fabrication of single-crystal indium nanowires at an ultrafast rate. Indium nanowires have peculiar temperature-dependent electrical properties, which make them attractive for various applications. For example, the electrical resistance of indium nanowires decreases rapidly when reaching the superconducting transition temperature, which makes them more attractive for making magnetic field generators or superconducting quantum interference devices. On the other hand, post-processing of indium nanowires (e.g., by oxygen or nitrogen plasma treatment) can be used to create composite nanowires made from indium compound materials on the outer surface layer of the wire. Such composite nanowires have versatile applications in electronics and optoelectronic devices, for example in building biosensors, solar cells, electrodes, and even memory devices. The first step in our experiments is to grow indium-rich InGaN layers of 300 nm thickness epitaxially on an GaN/ sapphire substrate having a thickness of 330 mm. Examination of the composition of the fabricated InGaN layers using X-ray diffraction (XRD) (see Experimental) indicated that the layers are 80% indium. The second step subjects the InGaN layers to direct irradiation by a Gaþ focused ion beam (FIB). It is observed that FIB irradiation results in rapid growth of straight nanowires on the surface area of the substrate, as shown schematically in Figure 1A. Movie S1 (see Supporting Information) provides an example of the appearance of nanowires as the substrate is subjected to FIB with an ion current density of 400 nA cm 2 and an accelerating voltage of 10 kV. From the onset of irradiation, nanowires start to appear after 80 s and grow at an average rate of 50 nm s 1 until 260 s, at which point the indium source in the InGaN layer is exhausted as will be explained later. Figure 1B shows four snapshots of the growth sequence for nanowires taken at 50 s time intervals at an ion current density of 200 nA cm 2 and an accelerating voltage of 10 kV, where the synthesized nanowires have lengths as long as 30 mm and diameters in the range 50–200 nm. To examine the material characteristics of synthesized nanowires, we performed transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS). We used these techniques to examine nanowires synthesized at various ion beam accelerating voltages and current densities. Figure 2A and B shows a set of data from these analyses, identifying the synthesized nanowires with diameter smaller than 200 nm as pure indium single crystals. Nanowires with larger diameters are polycrystalline. Moreover, electron energy loss spectroscopy (EELS) analysis was used to investigate the chemical compositions of the nanowires and the surface layer before and after FIB irradiation (Fig. 2C). Prior to FIB irradiation, the InGaN layers are saturated with 80% indium content of (denoted by red color in Fig. 2C). After irradiation, essentially all the indium is diffused through the substrate pores, producing pure indium nanowires. This can be explained as follows: Upon exposure to Gaþ FIB, phase decomposition occurs in the InGaN with the weaker bond between In and N broken, readily producing [*] Prof. K. H. Oh, S. S. Oh, D. H. Kim, Prof. M. Kim, Prof. E. Yoon Department of Materials Science and Engineering Seoul National University Seoul, 151-744 (Korea) E-mail: [email protected]