Directed-assembly of nanowire-based devices 1 will enable the development of integrated circuits with new functions that extend well beyond mainstream digital logic. For example, nanoelectromechanical resonators are very attractive for chip-based sensor arrays 2 because of their potential for ultrasensitive mass detection 3-6 . In this letter, we introduce a new bottom-up assembly method to fabricate large-area nanoelectromechanical arrays each having over 2,000 single-nanowire resonators. The nanowires are synthesized and chemically functionalized before they are integrated onto a silicon chip at predetermined locations. Peptide nucleic acid probe molecules attached to the nanowires before assembly maintain their binding selectivity and recognize complementary oligonucleotide targets once the resonator array is assembled. The two types of cantilevered resonators we integrated here using silicon and rhodium nanowires had Q-factors of ~4,500 and 1,150, respectively, in vacuum. Taken together, these results show that bottom-up nanowire assembly can offer a practical alternative to top-down fabrication for sensitive chip-based detection.Our hybrid nanoelectromechanical systems (NEMS) array integration strategy combines deterministic bottom-up nanowire (NW) assembly with conventional top-down microfabrication as illustrated in Fig. 1. Here we show the flexibility of this approach by fabricating resonators using two types of NWs-semiconducting silicon (Si) or metallic rhodium (Rh)-synthesized off-chip with different growth methods. The SiNWs grown by the Au-catalysed vapour-liquid-solid (VLS) technique are predominately single-crystal and oriented in the 〈111〉 or 〈112〉 growth directions 7 . In contrast, RhNWs electrodeposited within the pores of anodic aluminium oxide membranes are polycrystalline, with an average grain size of 5 nm (ref. 8). This assembly method is quite general and can be extended to many other NW materials 1,9 (for example, multisegment metal, magnetic, oxide or piezoelectric materials) and geometries 10 (for example, hollow tube or flat belt).Correspondence and requests for materials should be addressed to R.B.B. and T.S.M. † These authors contributed equally to this work. Bottom-up assembly is used to position single NWs at lithographically defined locations on a Si chip for fabricating multiplexed arrays. Unlike earlier reports of NW assembly 11-15 , three separate mechanisms are combined to achieve high-yield NW integration over centimetre-scale chip areas: electric-field forces, capillary forces and NW lift-off. As illustrated in Fig. 1, we patterned arrays of wells in a sacrificial insulating photoresist layer covering metal guiding electrodes defined on the chip surface. An alternating voltage in the kilohertz range was applied between the guiding electrodes to produce spatially confined electric fields that polarize NWs in suspension 11 . Long-range dielectrophoretic forces attract the NWs to the surface and align them along the electric-field gradient. Single NWs are further centr...

A strong diameter dependence is observed in the interfacial abruptness and growth rates in Si/Si 1- x Ge x axial heterostructure nanowires grown via Au-mediated low pressure CVD using silane and germane precursors. The growth of these nanowires has similarities to that of heterostructure thin films with similar compositional interfacial broadening, which increases with and is on the order with diameter. This broadening may reveal a fundamental challenge to fabrication of abrupt heterostructures via VLS growth.

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Phosphine (PH3) was investigated as an n-type dopant source for Au-catalyzed vapor-liquid-solid (VLS) growth of phosphorus-doped silicon nanowires (SiNWs). Transmission electron microscopy characterization revealed that the as-grown SiNWs were predominately single crystal even at high phosphorus concentrations. Four-point resistance and gate-dependent conductance measurements confirmed that electrically active phosphorus was incorporated into the SiNWs during VLS growth. A transition was observed from p-type conduction for nominally undoped SiNWs to n-type conduction upon the introduction of PH3 to the inlet gas. The resistivity of the n-type SiNWs decreased by approximately 3 orders of magnitude as the inlet PH3 to silane (SiH4) gas ratio was increased from 2 x 10(-5) to 2 x 10(-3). These results demonstrate that PH3 can be used to produce n-type SiNWs with properties that are suitable for electronic and optoelectronic device applications.

There have been extensive studies of germanium (Ge) grown on planar silicon (Si) substrates by the Stranski-Krastanow (S-K) mechanism. In this study, we present S-K growth of Ge on Si nanowires. The Si nanowires were grown at 500 degrees C by a vapor-liquid-solid (VLS) method, using silane (SiH4) as the gaseous precursor. By switching the gas source from SiH4 to germane (GeH4) during the growth and maintaining the growth conditions, epitaxial Ge islands deposited on the outer surface of the initially formed Si nanowires. Transmission electron microscopy (TEM), scanning TEM, and energy-dispersive X-ray spectroscopy techniques were utilized to identify the thin wetting layer and the three-dimensional Ge islands formed around the Si core nanowires. Cross-sectional TEM verified the surface faceting of the Si core nanowires as well as the Ge islands.

Aluminum doped core-shell ZnO/ZnS nanowires: Doping and shell layer induced modification on structural and photoluminescence properties

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Diameter-dependent compositions of Si(1-x)Ge(x) nanowires grown by a vapor-liquid-solid mechanism using SiH(4) and GeH(4) precursors are studied by transmission electron microscopy and X-ray energy dispersive spectroscopy. For the growth conditions studied, the Ge concentration in Si(1-x)Ge(x) nanowires shows a strong dependence on nanowire diameter, with the Ge concentration decreasing with decreasing nanowire diameter below approximately 50 nm. The size-dependent nature of Ge concentration in Si(1-x)Ge(x) NWs is strongly suggestive of Gibbs-Thomson effects and highlights another important phenomenon in nanowire growth.