Nanowire deformation DOI: 10.1002/smll.200700052 Shear Stress Measurements on InAs Nanowires by AFM Manipulation** Michael Bordag,* Aline Ribayrol, Gabriela Conache, LinusE. Frçberg, Struan Gray, Lars Samuelson, Lars Montelius,andHakan Pettersson Nanowires (NWs) have in recent years attracted consider- ACHTUNGTRENNUNGable attention due to their interesting fundamental proper- ties and the exciting prospects for using these materials in future electronic and photonic applications. [1] For example, nanoscale field-effect transistors, [2–4] inverters [5–6] and more complex logic gates [4] have been demonstrated using well- defined NW building blocks. Furthermore, it has recently been shown that it is possible to form heterostructures in NWs [5] facilitating one-dimensional (1D) electronics, for ex- ample, resonant tunneling diodes [7] and single-electron tran- sistors. [8] For photonic applications, LEDs, [3] lasers, [9] and in- frared photodetectors [10–11] have also been assembled using NWs. Devices based on NWs can be of two types: vertical or lateral. Although vertical device designs may be the obvious choice for NWs, which grow vertically from the surface, in- teresting lateral devices such as crosslinked electronics can be made by depositing the NWs horizontally on a substrate using a dry technique or from a dispersion. Placing NWs at predetermined positions in a controlled manner requires an understanding of the adhesion and friction forces between the NWs and the surface. In addition to this, the microscop- ic origin of friction is interesting from a fundamental point of view. It is for instance known that at the sub-micrometer scale the friction force, F lateral , is proportional to the contact area A rather than to the load force, that is, F lateral = sA. [12–14] In this paper, we present a novel method for investigat- ing the shear stress s between InAs NWs and a SiO 2 sur- face. The proposed “method of the most-bent state” rests on the observation that for an ideal elastically deformed wire pinned by adhesion forces to a flat surface and in equi- librium between static friction forces and restoring elastic forces, the most tightly bent regions contain information about the maximal static friction force, that is, about the shear stress. Experimentally, the NWs are bent in a ACHTUNGTRENNUNGcontrolled manner using the tip of an AFM. After the ma- nipulation, the most-bent state can be determined by visual inspection of AFM micrographs. Assuming bulk values for the Young)s modulus, the shear stress can be obtained from a straightforward analysis according to the standard theory of elasticity. We apply the method in this paper only to InAs nanowires, however the method is quite general and it can be expected to work for other wires as well. To fabricate the NWs, size-selected gold particles were produced with an aerosol method, [15] and deposited on an InAsACHTUNGTRENNUNG(111)B substrate. The substrate was subsequently trans- ferred into a chemical beam epitaxy growth chamber. Prior to growth, the substrate oxide was removed by heating the substrate under As pressure. The growth sources used were trimethylindium (TMIn) and precracked tertiarybutylarsine (TBAs), which enter the growth chamber as molecular beams. [5] The resulting nanowires are about 50 nm in diame- ter and 3 mm in length. After growth, the wires were re- moved from the substrate and transferred to a silicon sub- strate with a 330-nm-thick SiO 2 layer. The transfer was car- ried out by mechanically wiping off the wires from the sub- strate using a cleanroom wipe, followed by brushing off the NWs from the wipe onto the SiO 2 layer. The manipulation and imaging of the NWs were performed using a Dimen- sion3100 Nanoscope IIIA microscope from Veeco, ACHTUNGTRENNUNGequipped with rectangular-shaped cantilevers with a nomi- nal spring constant of about 40 Nm 1 . To bend the NWs in a gentle controlled manner, the fol- lowing AFM manipulation scheme was employed. After imaging of the surface with randomly distributed NWs, a single NW was selected and a manipulation force was ap- plied at a predetermined position along the wire. In the trace scan, tapping-mode imaging was carried out at a pre- determined distance above the surface (positive set-point). In the retrace scan, the z-setpoint was decreased (corre- sponding to a smaller tip–surface distance) and the oscilla- tor driving voltage set to zero. This procedure was repeated along the same scan line, continuously decreasing the re- trace z-setpoint in small steps, until a movement of the NW was observed in the AFM line scan data. The distance swept by the tip was sufficient to enable the NW to be bent beyond the point of the “most-bent state”, after which it re- laxes to the equilibrium configuration where the friction forces balance the elastic restoring forces. The shear stress s (i.e., the lateral static friction force, F lateral , per unit area) is considered to be a more fundamen- tal quantity than the friction coefficient at a macroscopic level. While friction at the macroscopic level depends on a variety of factors, for example, the surface roughness, more fundamental physical principles are addressed on a micro- scopic level when we are concerned with friction between atomically flat surfaces. Microscopic friction may depend on a number of factors including residual surface contamina- [*] Dr. M. Bordag Institute for Theoretical Physics, Leipzig University Postbox 100 920, 04009 Leipzig (Germany) Fax: (+ 49)341-973-2548 E-mail: Michael.Bordag@itp.uni-leipzig.de Dr. M. Bordag, G. Conache, Prof. H. Pettersson Center for Applied Mathematics and Physics Halmstad University, Box 823, SE-301 18 Halmstad (Sweden) Dr. A. Ribayrol, G. Conache, L. E. Frçberg, Dr. S. Gray, Prof. L. Samuelson, Prof. L. Montelius, Prof. H. Pettersson Solid State Physics/The Nanometer Structure Consortium Lund University, Box 118, 22100 Lund (Sweden) [**] The authors acknowledge financial support from EC (6th FP NMP4-CT-2005-017071), the Swedish Research Council under Grant No. 621-2004-4439, the Swedish National Board for Indus- trial and Technological Development, the Office of Naval Research, and the Swedish Foundation for Strategic Research. 1398 # 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim small 2007 , 3, No. 8, 1398 – 1401 communications