Semiconductor Nanowires
Abstract
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S. Christiansen, M. Becker, V. Sivakov, and G. Andrae
Silicon nanowires grown by the vapor-liquid-solid (VLS) growth mechanism with gold as the catalyst show gold caps ~50 nm – 400 nm in diameter with an almost ideal hemispherical shape atop a silicon column. These gold caps are extremely well suited to exploit the tip- or surface enhanced Raman effects (SERS/TERS) since they assume the right size on the nanometer scale and a reproducable, almost ideal hemispherical shape. Attaching a nanowire with gold cap to an atomic force microscopy (AFM)-tip the signal enhancement by the gold nanoparticle could be used to spatially resolve a Raman-signal. Applications of this novel nanowire based technical TERS solution are widespread and lie in the fields of bio-medical and life-sciences as well as security (e.g. detection of bacteria and explosives) and in the field of solid state research e.g. in silicon technology where the detection of materials composition, doping, crystal orientation and lattice strain can be probed by Raman spectroscopy. A prerequisite to obtain this spatial resolution in nano-Raman spectroscopy is the attachment of a nanowire with gold cap to an AFM tip. The attachment by welding a nanowire in a scanning electron microscope to an AFM tip is demonstrated.
Fig. 5: Chopping off nanowires from an assembly of nanowires with a moving AFM tip. The nanowires break by mechanical force induced by the moving AFM tip.
Fig. 6: Nanomanipulation sequence for the attachement of an up-side-down nanowire onto an AFM tip; a) Mechanically, some nanowires are broken off the substrate by the movement of an AFM tip; b) A nanowire that lies on the substrate surface is brought into contact with the AFM tip; c) The nanowire is moved into the desired position; d) Welding of the nanowire to the AFM tip by electron beam induced contamination; e) Retraction of the AFM tip; f) Cantilever with tip and welded nanowire assembled to be used as a TERS probe.
Fig. 7: The inset shows the light optical micrograph of a single nanowire gold cap with a diameter of ~ 300 nm. The Raman spectra recorded at the positions indicated at the single gold cap (red, a) and close to the gold cap on the bare substrate surface (blue, b) are shown. The circle represents the size of the focused laser spot. The spectrum shows that even a single nanowire gold cap with even a somewhat larger diameter is capable of producing a significant signal enhancement. Consequently, the gold caps atop silicon nanowires can be assumed to be well suited to be used as TERS probes.
Fig. 8: Setup for our TERS measurements with the Si nanowire AFM tip (left). The AFM tip attached with the Si nanowire with the gold droplet atop is brought close (a few nanometers distance) to a second gold-coated AFM tip (referred to as the sample tip), which is contaminated with malachite green. The sample tip itself produces a tip-enhanced Raman signal due to the gold coating. The gold droplet of the NW tip and the gold-coated sample tip then are in close proximity, and the additional enhancement of the Raman scattered signal due to the gold droplet of the NW tip can be observed (down right). The light optical micrograph (up right) shows an inclined side view of the experimental setup. Both tips and the position of the focused laser spot are visible.
Further reading:
[1] M. Becker, V. Sivakov, G. Andrä, S. Hoffmann, J. Michler, U. Gösele, H.J. Reich, S.H. Christiansen, Nanowires enabling signal enhanced nano-Raman-Spectroscopy, Small, accepted (2006)
[2] M. Becker, V. Sivakov, G. Andrä, R. Geiger, J. Schreiber, S. Hoffmann, J. Michler, A. Milenin, P. Werner, S.H. Christiansen, The SERS and TERS-effect obtained by gold droplets on top of Si-nanowires, Nano Lett., 7(1), 75 (2007)
[3] S.H. Christiansen, M. Becker, V. Sivakov, G. Andrä, S. Fahlbusch, J. Michler, R. Geiger, Signal enhancement in nano-Raman-Spectroscopy by gold caps on Silicon nanowires obtained by vapor-liquid-solid growth, Nanotechnology 18, 035503 (2007)