Density control of ZnO nanowires grown using Au-PMMA nanoparticles and their growth behavior.

Au nanoparticles stabilized by poly(methyl methacrylate) (PMMA) were used as a catalyst to grow vertically aligned ZnO nanowires (NWs). The density of ZnO NWs with very uniform diameter was controlled by changing the concentration of Au-PMMA nanoparticles (NPs). The density was in direct proportion to the concentration of Au-PMMA NPs. Furthermore, the growth process of ZnO NWs using Au-PMMA NPs was systematically investigated through comparison with that using Au thin film as a catalyst. Au-PMMA NPs induced polyhedral-shaped bases of ZnO NWs separated from each other, while Au thin film formed a continuous network of bases of ZnO NWs. This approach provides a facile and cost-effective catalyst density control method, allowing us to grow high-quality vertically aligned ZnO NWs suitable for many viable applications.

Formation of metastable liquid catalyst during subeutectic growth of germanium nanowires.

Lattice-resolved, video-rate environmental transmission electron microscopy shows the formation of a liquid Au-Ge layer on sub-30-nm Au catalyst crystals and the transition of this two-phase Au-Ge/Au coexistence to a completely liquid Au-Ge droplet during isothermal digermane exposure at temperatures far below the bulk Au-Ge eutectic temperature. Upon Ge crystal nucleation and subsequent Ge nanowire growth, the catalyst either recrystallizes or remains liquid, apparently stabilized by the Ge supersaturation. We argue that there is a large energy barrier to nucleate diamond-cubic Ge, but not to nucleate the Au-Ge liquid. As a result, the system follows the more kinetically accessible path, forming a liquid even at 240 degrees C, although there is no liquid along the most thermodynamically favorable path below 360 degrees C.

Formation of composite organic thin film transistors with nanotubes and nanowires

Plastic electronics is a rapidly expanding topic, much of which has been focused on organic semiconductors. However, it is also of interest to find viable ways to integrate nanomaterials, such as silicon nanowires (SiNWs) and carbon nanotubes (CNTs), into this technology. Here, we present methods of fabrication of composite devices incorporating such nanostructured materials into an organic matrix. We investigate the formation of polymer/CNT composites, for which we use the semiconducting polymer poly(3,3‴-dialkyl-quaterthiophene) (PQT). We also report a method of fabricating polymer/SiNW TFTs, whereby sparse arrays of parallel oriented SiNWs are initially prepared on silicon dioxide substrates from forests of as-grown gold-catalysed SiNWs. Subsequent ink-jet printing of PQT on these arrays produces a polymer/SiNW composite film. We also present the electrical characterization of all composite devices. © 2007 Elsevier B.V. All rights reserved.

Atomistic Simulations of the Mechanical Behavior of Fivefold Twinned Nanowires

Atomistic simulations are used to investigate the mechanical behavior of metal nanowire with fivefold twinned structure. The twinned nanowires were reported in recent experiments [B. Wu et al., Nano Lett. 6, 468 (2006)]. In the present paper, we find that the yield strength of the fivefold twinned Cu nanowire is 1.3 GPa higher than that of the face-centered-cubic (fcc) < 110 > single crystalline Cu nanowire without fivefold twinned structure, and the microstructure-hardened mechanism is primarily due to the twinned boundaries which act as the barriers for the dislocation emission and propagation. However, we also find that the fivefold twinned Cu nanowire has lower ductility than that of fcc < 110 > single crystalline Cu nanowire without the twinned structure, and this is mainly attributed to the scarcity and low mobility of dislocations. In addition, in our simulations the effect of preexisting stacking faults and dislocations on strength of the fivefold twinned nanowires is investigated.

High-resolution TEM characterization of ZnO core-shell nanowires for dye-sensitized solar cells

Recently ZnO nanowire films have been used in very promising and inexpensive dye-sensitized solar cells (DSSC). It was found that the performance of the devices can be enhanced by functionalising the nanowires with a thin metal oxide coating. This nm-scale shell is believed to tailor the electronic structure of the nanowire, and help the absorption of the dye. Core-shell ZnO nanowire structures are synthesised at low temperature (below 120°C) by consecutive hydrothermal growth steps. Different materials are investigated for the coating, including Mg, Al, Cs and Zr oxides. High resolution TEM is used to characterise the quality of both the nanowire core and the shell, and to monitor the thickness and the degree of crystallisation of the oxide coating. The interface between the nanowire core and the outer shell is investigated in order to understand the adhesion of the coating, and give valuable feedback for the synthesis process. Nanowire films are packaged into dye-sensitised solar cell prototypes; samples coated with ZrO2 and MgO show the largest enhancement in the photocurrent and open-circuit voltage and look very promising for further improvement. © 2010 IOP Publishing Ltd.

Spatial determination of gold catalyst residue used in the production of ZnO nanowires by SIMS depth profiling analysis

In this paper we demonstrate how secondary ion mass spectrometry (SIMS) can be applied to ZnO nanowire structures for gold catalyst residue determination. Gold plays a significant role in determining the structural properties of such nanowires, with the location of the gold after growth being a strong indicator of the growth mechanism. For the material investigated here, we find that the gold remains at the substrate-nanowire interface. This was not anticipated as the usual growth mechanism associated with catalyst growth is of a vapour-liquid-solid (VLS) type. The results presented here favour a vapour-solid (VS) growth mechanism instead. Copyright © 2007 John Wiley & Sons, Ltd.

Strength of nanotubes, filaments, and nanowires from sonication-onduced scission

A model to describe the cavitation-induced breakage of nanofilaments during their sonication in solution is proposed. The model predicts a limiting length below which scission no longer occurs, and accurately describes experimental results for materials ranging from carbon nanotubes to protein fibrils. Sonication-induced breakage can now be used as a probe for the strength of nanostructures. © 2009 WILEY-VCH Verlag GmbH & Co. KGaA.

Raman Spectrum of silicon nanowires

We measure the effects of phonon confinement on the Raman spectra of silicon nanowires (SiNWs). We show how previous reports of phonon confinement in SiNWs and nanostructures are actually inconsistent with phonon confinement, but are due to the intense local heating caused by the laser power used for Raman measurements. This is peculiar to nanostructures, and would require orders of magnitude higher power in bulk Si. By varying the temperature, power and excitation energy, we identify the contributions of pure confinement, heating and carrier photo-excitation. After eliminating laser-related effects, the Raman spectra show confinement signatures typical of quantum wires. © 2003 Elsevier B.V. All rights reserved.

Raman spectroscopy of silicon nanowires

We measure the effects of phonon confinement on the Raman spectra of silicon nanowires. We show how previous spectra were inconsistent with phonon confinement, but were due to intense local heating caused by the laser. This is peculiar to nanostructures, and would require orders of magnitude more power in bulk Si. By working at very low laser powers, we identify the contribution of pure confinement typical of quantum wires.