Electrostatic charging and manipulation of semiconductor nanowires
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It was observed that silicon and germanium nanowires can exhibit significant electrostatic charging and respond strongly to externally applied electric fields. This includes nanowires in air and dispersed in low-conductivity, low-dielectric-constant solvents such as hexane, toluene, and benzene. The electrostatic charging of semiconductor nanowires was investigated as a tool for nanowire manipulation. By charging a substrate, nanowires could be deposited on surfaces with very high coverage and onto selected locations of the surface. The density of deposited nanowires could be adjusted systematically by varying the strength of the electric field. Alternating electric fields, applied between two electrodes, resulted in nanowires oriented with respect to the field orientation. I. INTRODUCTION
We have developed methods for producing large quantities of silicon (Si) and germanium (Ge) nanowires,1–4 and in the process of manipulating these materials for various applications, we have observed that both Si and Ge nanowires can accumulate a significant amount of electrostatic surface charge. This static charge can be a nuisance. On a day with low humidity, nanowires in a vial can dance around erratically when any surface with static charge— a spatula, tweezers, a gloved hand, or even a bench top— comes near them, making handling difficult. Even nanowires in a solvent can respond dramatically to nearby surfaces that have accumulated static charge. Because the nanowires have an extremely high amount of surface area, they can accumulate large quantities of surface charge. Static charging is known to make handling of particulate materials difficult, and sometimes dangerous. For example, charge accumulation can be a problem in food and chemical industries that store, process, and transport particulate powders, creating the potential for an electrostatic discharge that can cause the powder to explode.5 These hazards are not limited to particulate powders, as low-conductivity liquids can also accumulate large quantities of static charge while flowing or being mechanically agitated.6,7 Static charge can also damage electronic devices if not controlled. On the other hand, static charge can be harnessed for useful processes, such as electrostatic printing. The xerographic process, for example, combines photography with the ability to manipulate the static charge of pigment particles and paper. Herein, we show how dramatic the effects of electrostatic charging can be on semiconductor nanowires and then demonstrate ways of manipulating nanowires using electric fields, enabling a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.152
II. EXPERIMENTAL
Si and Ge nanowires were synthesized by gold nanocrystal–seeded supercritical fluid–liquid–solid growth in supercritical toluene using either monophenylsilane or diphenylgermane as the reactant, as described elsewhere in detail.1–4,8 All solvents were purchased from SigmaAldrich (St. Louis, MO). Nanowire fabric was prepared using a modified version of th
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