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In situ uniform growth of ZnO nanowires was realized and monitored at real time by heating zinc film in an environmental scanning electron microscope. Better controllability and repeatability were obtained by using zinc film as source material compared to traditionally used zinc powder. Morphology of the as-grown ZnO nanowires was found to depend on both the growth temperature and holding time. Low temperature (500 °C) and short growth time (∼20 min) favor one-dimensional nanowire growth, whereas longer holding time (>40 min) or higher temperature (700 °C) lead to nanosheet growth. The results suggest that the zinc vapor partial pressure is vital in determining the final morphology. These results help to give more insights into the mechanism of ZnO nanowire synthesis. [ABSTRACT FROM AUTHOR]

Highly Cu doped ZnO nanowires have been fabricated by vapor-liquid-solid (VLS) process. The average concentration of Cu in the ZnO nanowires is estimated to be 6 at. %. The ultrafine synthesized nanowires have diameters nearly 80 nm, while their average length lies in the range of 40 to 90 μm. Raman spectroscopy shows that the Cu doped ZnO nanowires have a typical wurtzite structure. High resolution transmission electron microscopy (HRTEM) investigations of individual nanowires demonstrate that the nanowires have single crystalline structure in which the growth direction is oriented along the c axis. Room temperature photoluminescence spectrum of as prepared nanowires shows two emissions in UV and visible regions that can be ascribed to the near band edge (NBE) transition and defects respectively, while the spectrum of the annealed nanowires exhibits a red shift in UV and a suppression in visible bands. Furthermore, the low temperature (10 K) PL spectrum illustrates a novel dominant blue emission relating to the different valence states of Cu atoms in ZnO, which is explained on the basis of Dingle model. [ABSTRACT FROM AUTHOR]

Carbon nanotubes (CNTs) enable large electric field enhancement for an extremely broad bandwidth spanning from the optical domain down to static fields. This is due to their high aspect ratio, small tip radius, and high structural stability. CNTs therefore represent an ideal model-system for the investigation of nonlinear and strong-field phenomena. In this paper, we extend the range of optical-field-emission materials from metal nanostructures to CNTs. Quiver-quenched optical-field-emission (i.e., the transition to a sub-cycle regime) is observed for CNTs tips in a short-wavelength laser field of 820 nm that requires a mid-infrared excitation field of conventional metal tips emitters. This special property relies on the ultrasmall tips radius (~1 nm) and the high optical-field enhancement (~21.6) properties of CNTs. This study suggests that CNTs are excellent candidates for optically driven ultrafast electron sources with both high spatial and high temporal coherence. They also provide more freedom for the manipulation and control of electron dynamics at the attosecond timescale, which extends the bandwidth of light-wave electronic devices. [ABSTRACT FROM AUTHOR]

Nanopore-based sensing technology is considered high-throughput and low-cost for single molecule detection, but solid-state nanopores have suffered from pore clogging issues. A simple Tween 20 coating method is applied to ensure long-term (several hours) non-sticky translocation of various types of bio-molecules through SiN nanopores in a wide pH range (4.0-13.0). We also emphasize the importance of choosing appropriate concentration of Tween 20 coating buffer for desired effect. By coating nanopores with a Tween 20 layer, we are able to differentiate between singlestranded DNA and double-stranded DNA, to identify drift-dominated domain for single-stranded DNA, to estimate BSA volume and to observe the shape of individual nucleosome translocation event without non-specific adsorption. The wide pH endurance from 4.0 to 13.0 and the broad types of detection analytes including nucleic acids, proteins, and biological complexes highlight the great application potential of Tween 20-coated solid-state nanopores. [ABSTRACT FROM AUTHOR]

Impurity doping plays a pivotal role in semiconductor electronics. We study the doping effect in a two-dimensional semiconductor, gapped bilayer graphene. By employing in situ deposition of calcium on the bilayer graphene, dopants are controllably introduced. Low temperature transport results show a variable range hopping conduction near the charge neutrality point persisting up to 50 K, providing evidence for the impurity levels inside the gap. Our experiment confirms a predicted peculiar effect in the gapped bilayer graphene, i.e., formation of in-gap states even if the bare impurity level lies in the conduction band. The result provides perspective on the effect of doping and impurity levels in semiconducting bilayer graphene. [ABSTRACT FROM AUTHOR]

We demonstrate hydrogen assisted growth of high quality epitaxial graphene on the C-face of 4H-SiC. Compared with the conventional thermal decomposition technique, the size of the growth domain by this method is substantially increased and the thickness variation is reduced. Based on the morphology of epitaxial graphene, the role of hydrogen is revealed. It is found that hydrogen acts as a carbon etchant. It suppresses the defect formation and nucleation of graphene. It also improves the kinetics of carbon atoms via hydrocarbon species. These effects lead to increase of the domain size and the structure quality. The consequent capping effect results in smooth surface morphology and suppression of multilayer growth. Our method provides a viable route to fine tune the growth kinetics of epitaxial graphene on SiC. [ABSTRACT FROM AUTHOR]

We propose and demonstrate gold nanoparticle decorated graphene as an ideal system for studying carrier inhomogeneity induced magnetoresistance. Large linear magnetoresistance has been realized in the system and the magnitude can be tuned by a gate. By detailed study, we provide an empirical expression, which reveals the dependence of the magnetoresistance on density fluctuations and mobility. The induced magnetoresistance is temperature independent and can be strongly enhanced by the high mobility of graphene, hence offers potential for magnetic sensor applications. [ABSTRACT FROM AUTHOR]

Large scale, uniform single-layer graphene was transferred onto well-aligned ZnO nanowire arrays to produce high density nanoscale protrusions within graphene for efficient field emission. Polymethyl methacrylate (PMMA) was used as a supporting layer to provide a quasi-flat surface for graphene transfer. Highly efficient (maximum current density of 500 μA/cm²) and stable field emission with low turn-on fields (5.4 V/μm) was observed due to highly localized electric field, which is much better than those without using PMMA. F-N plot showed an unique up-bending feature of single-layer graphene. Our approach provides an efficient way to produce high quality single-layer graphene field emitters. [ABSTRACT FROM AUTHOR]

Well-aligned graphene field emission arrays were fabricated on copper foils via simple photolithography and wet etching techniques. The method offered high-ordered micro sized graphene pattern in the array elements. Field emission measurement revealed a turn-on field of 7.2 V/μm at 100 nA/cm2 and a good emission current stability. Exposed graphene edges were produced on each array element edge and found to be the significant contributors to the emission current. Owing to the obvious advantages of low cost and easy for scale-up production, this method demonstrates the feasibility of utilizing such graphene field emission array configurations in display applications. [ABSTRACT FROM AUTHOR]

A reproducible quasi-three-dimensional structure, composed of top and bottom concentric nanorings with same periodicity but different widths and no overlapping at the perpendicular direction, is built up by a separation-layer method, which results in huge enhancement of surface-enhanced Raman spectroscopy (SERS) due to the coupling of plasmons. Simulations show plasmonic focusing with “hot arcs” of electromagnetic enhancement meeting the need of quantitative SERS with extremely high sensitivities. In addition, the separation-layer method opens a simple and effective way to adjust the coupling of plasmons among nanostructures which is essential for the fabrication of SERS-based sensors. [ABSTRACT FROM AUTHOR]

Uniform TiSi2 nanocables were synthesized on large substrates using a simple physical vapor deposition method. X-ray diffraction, scanning electron microscopy, and transmission electronic microscopy were employed to characterize the samples. It reveals that the as-grown TiSi2 nanocables are of high quality single crystal inside with thin amorphous SiO2 sheathing layer. Electrical properties of these core-shell structure nanowires are also presented. Insulativity of the outer layer and stable metallic characters of the inside single crystal TiSi2 were observed. Moreover, carrying capacity of current density is up to 1011 A/m2. The nanoscale structure and excellent electrical performance make the TiSi2 nanocables good candidate for electrical interconnection in potential nanodevices. [ABSTRACT FROM AUTHOR]

This letter reports the electrical and field-emission properties of Au-decorated boron nitride nanotubes (Au-BNNTs). The insulating BNNTs become metallic after Au coating as the Au coverage exceeds a critical value. The Au decoration modifies the work function of the BNNTs and, as a consequence, the field-emission current densities of Au-BNNTs are significantly enhanced. Correspondingly, the turn-on field of the Au-BNNTs is reduced to one third and the emission current density is increased by four orders in contrast to pure BNNTs. The experimental results demonstrate that such Au-BNNTs are promising electron field emitters. [ABSTRACT FROM AUTHOR]

ZnO nanostructures with peculiar morphology were synthesized in large quantities via catalyst-free thermal evaporation. Both ZnO nanopushpins with very thin and flat cap on top of the nanorods, and regular hexagonal nanotubes, can be obtained in the same run of growth in different regions. The ZnO nanopushpins and nanotubes were characterized by using scanning electron microscopy and transmission electron microscopy. Cathodoluminescence measurements revealed that the emission from oxygen vacancy overwhelmed that of the near band gap in the as-grown ZnO nanotube arrays. [ABSTRACT FROM AUTHOR]

A multistep template replication route was employed to fabricate highly ordered silicon nanotube (SiNT) arrays, in which annular nanochannel membranes were produced first, and then silicon was deposited into the annular nanochannels by pyrolytic decomposition of silane. Electron microscopy revealed that these SiNTs are highly crystalline and the wall thicknesses can be controlled by the spaces of the annular nanochannel. Field emission characterization showed that the turn-on field and threshold field for the SiNT arrays are about 5.1 V/μm and 7.3 V/μm, respectively. These results represent one of the lowest fields ever reported for Si field emission materials at technologically useful current densities. [ABSTRACT FROM AUTHOR]

Highly ordered AlN nanowire arrays were synthesized via a simple physical vapor deposition method on sapphire substrate. The nanowires have an extremely sharp tip <10 nm, with the average length around 3 μm. Raman spectroscopy analysis on the AlN nanowire arrays revealed that the lifetime of the phonons is shorter than that in bulk AlN. The transmission spectra of the AlN nanowires showed a blueshift ∼0.27 eV at the absorption edge with that of the bulk AlN, which is closely related to the small size of the nanowires. [ABSTRACT FROM AUTHOR]

High-density ZnO nanowires (ZnONWs) were aligned onto Au-catalyzed Si substrate through a simple low-temperature physical vapor deposition method. Scanning electron microscope (SEM) observations, x-ray diffraction (XRD) analysis, and photoluminescence spectra showed that the ZnONWs were single-crystalline, with a hexagonal wurzite structure. All of the results inferred from the SEM observations, the XRD rocking curves, and the Raman spectra for the investigated samples confirm that the ZnONWs are well aligned and c-axis oriented. The Raman spectra also indicated that the ZnONWs on Si substrates are under the biaxial compressive stress. Since it takes the advantage of low-cost, easily controlled deposition spot (due to the selective deposition trait of the Au layer), potential for scale-up production, and ability to integrate with Si substrate, this technique has a potential in future for fabricating the ZnONW array-based optoelectronic devices. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]