PatolskyMRS07 - Nanowire-Based Nanowire Field-Effect...

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142 MRS BULLETIN • VOLUME 32 • FEBRUARY 2007 • www.mrs.org/bulletin Introduction Semiconductor nanowires are emerging as remarkably powerful building blocks in nanoscience, with the potential to have a significant impact on numerous areas of science and technology ranging from electronics and photonics to the life sciences and healthcare. 1–9 Critical to the advances now being made worldwide with nanowires has been the well- developed understanding of the growth mechanism, 1–6,10 which has enabled the re- producible synthesis of nanowires of ho- mogeneous composition and diameter with controllable electronic and optical properties. Moreover, predictable elabo - ration of the basic nanowire structural motif has been utilized to produce axial heterostructures and superlattices, radial core–shell and core–multishell heterostruc- tures, and branched nanowire structures with unique functions built in at the stage of synthesis. 2–6 Significantly, these charac- teristics make semiconductor nanowires one of the best defined and most versatile nanomaterial systems available today, thus enabling scientists to move beyond, for ex ample, single - device proof - of - concepts studies to the exploration of new areas of science and technology. One particularly rich area centers on the interface between nanowires and the life sciences. In general, the similarity in size of nanowires and natural nanostructures in biological systems makes nanowires an ob vious choice for creating highly sensi- tive tools that can probe biological sys- tems. Nanowire electronic devices, moreover, enable a detection and sensing modality—direct and label-free electrical readout (i.e., without the use of bound dyes or fluorescent probes)—that is excep- tionally attractive for many applications in medicine and the life sciences. 7–9,11–15 Here, we provide an introduction to the under- lying nanowire nanotechnology and then illustrate the diverse applications of this technology at the interface with the bio- logical sciences. Nanowire Field -Effect Sensors Underlying detection using semicon- ductor nanowires is their configuration as field-effect transistors (FETs), which ex- hibit a conductivity change in response to variations in the electric field or potential at the surface of the device. 11,16–19 In a stan- dard FET, the conductance of the semicon- ductor between the source and drain is modulated between on and off states by a third gate electrode capacitively coupled through a thin dielectric layer to the semiconductor. 20 In the case of a p -type semi- conductor, applying a negative gate voltage leads to an accumulation of majority charge carriers (positive holes) and a correspon- ding increase in conductance. The depend- ence of the conductance on gate voltage and corresponding charge at the gate electrode/dielectric interface makes FETs natural candidates for electrically based sensing, since the binding of a charged or polar biological or chemical species to the gate dielectric is analogous to applying a voltage using a gate electrode. For ex -
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