A nonlithographic second-order self-assembly process for synthesizing uniform and ordered arrays of nanorods and nanodots is presented and applied to the fabrication of ZnO nanorod arrays. Nucleation sites were defined by patterning Au nanodot catalysts with a self-organized array of nanopores formed in anodized aluminum oxide (AAO). The self-assembled vertically aligned ZnO nanorods grown on GaN exhibit hexagonal facets, and have a uniform diameter of 60 nm and a mean length of 400 nm. The growth technique is simple, robust, and offers a direct control over array and single nanorod configurations. The growth temperature is significantly lower than normal, and yet, the resultant defect level is much lower than normal.

In this article, we introduce and provide details on a large-scale, cost-effective pathway to fabricating ultrahigh dense CuO nanowire arrays by thermal oxidation of Cu substrates in oxygen ambient. The CuO nanowires that are produced at ∼500 °C for ∼150 min feature an average length and diameter of ∼15 µm and ∼200 nm, respectively. The room temperature device-related characteristics such as transport, analyte detection and opto-electronic response of individual CuO nanowires have been probed by fabricating single CuO nanowire devices with the use of lift-off photolithographical techniques. The experiments confirm that as-grown nanowires are of p-type, have a band gap of ∼1.4 eV, and show strong sensitivity to both NO 2 and NH 3 gases. The devices also showed strong response to white light with device responsivity approaching ∼8 A/W for optical power densities of only ∼1 mW/cm 2 . Additionally, a complex interaction of photoproduced electron-hole pairs with the surface-originating chemisorbed agents including O 2 and NO 2 is found to drastically affect the gas sensitivity of CuO nanowire-based devices, where photoinduced adsorption of the analyte enhances the device response.

1D nanostructures of high-performance semiconductors and oxides are emerging as an important class of potential optoelectronic materials, and have been the focus of much recent interest. [1,2] Advances in nanofabrication methodologies have enabled the growth of ZnO nanowires as dislocation-free, highly faceted single crystals. [3] These nanowires show remarkable electrical and optical properties, and thus hold great promise for applications in various types of nanodevices including but not limited to solid-state lighting, photodetectors, and optical switches. Owing to their large bandgap (ca. 3.2 eV for wurtzite ZnO), the intrinsic sensitivity of metal oxides to visible and IR radiation is typically quite limited. Furthermore, the sensitivity of ZnO nanowires to short-wavelength photons also tends to be low because of their relatively small photon-capture efficiency (diameter ( a À1, where a represents the absorption coefficient) and relatively large number of the surface defects (arising from the large surface-to-volume ratio).[4] Nevertheless, crystalline ZnO nanowires with diameters spanning the range from tens to hundreds of nanometers constitute a novel class of nanomaterials with intermediate to weak quantum confinement of excitons and carriers. The latter, however, makes quantum-mechanical-based nanoengineering approaches not practical or economical to improve and broaden the spectral sensitivity of nanostructured ZnO. In this study, orders of magnitude enhancement of the spectral sensitivity has been obtained over both UV and visible spectral ranges by doping ZnO nanowires with transitionmetal impurities such as Cu. where Dn is the photogenerated excess carrier density and n 0 is the equilibrium carrier density.Accordingly, by increasing Dn and decreasing n 0 , it should be possible to produce ultra-high-sensitivity optoelectronic materials. Thus, achieving p-type doping in wide-bandgap semiconductors such as ZnO is critical not only from the standpoint of realizing p-n junction nanodevices based on ZnO nanowires (e.g., electrically driven UV lasers), but also for improving the photosensitivity characteristics by making use of the doping compensation effect.In this work, Cu impurities, which are known to form acceptor states within the bandgap of II-VI compounds [5,6] such as ZnS, CdS, and ZnO, and are also able to act as green luminescence activators, have been intentionally introduced during the growth of ZnO nanowires by the vapor-liquid-solid (VLS) method. The Cu dopants have been found to be both electrically and optically active within the nanowires, and more significantly are also able to act as visible-light photoconduction activators, as discussed in detail below. Combined with the observed effect of strong multiplication of photocarriers, this strategy yields ZnO nanowires with dramatically enhanced sensitivity to optical radiation over multiple spectral ranges the UV and visible regions of the electromagnetic spectrum.To synthesize Cu:ZnO nanowires, high-purity Cu powders (99.99%), Zn (99.999%)...

We report the observation of giant photoresistivity in electrochemically self-assembled CdS and ZnSe nanowires electrodeposited in a porous alumina film. The resistance of these nanowires increases by one to two orders of magnitude when exposed to infrared radiation, possibly because of real-space transfer of electrons from the nanowires into the surrounding alumina by photon absorption. This phenomenon has potential applications in “normally on” infrared photodetectors and optically controlled switches.

We report a Raman spectroscopy investigation of electrochemically self-assembled quasiperiodic arrays of CdS quantum dots with characteristic feature size of 10 nm. The dots were synthesized using electrochemical deposition of CdS into a porous anodized alumina film. Polarization-dependent Raman scattering study over an extended frequency range reveals the quantization of electronic states in the conduction band and intersubband transitions. Raman peaks observed at 2919 and 3050 cm Ϫ1 are attributed to transitions between the lowest two subbands. The results suggest that quantum dot arrays, produced by inexpensive robust electrochemical means, may be suitable for infrared detector applications.

Integrating nanotechnology with experimental biology is paramount to advancing fundamental biological science and technology, and, therefore, of high current interest and importance. In this article, we report on a new possibility of utilizing carbon nanotube probes assembled by a modified dielectrophoretic based technique for single-cell experimentation and delivery. The modified approach permits highly reproducible construction of water-stable, highly-aligned, and electrically-conductive probes several hundred microns in length, which hold a great promise for enhancing previously developed molecular-scale intracellular experimental techniques. The results of this work, in particular, indicate that the minimally invasive nanotube probes could be advantageous for studies involving permeabilization and subsequent desorption of molecules into a cell's interior, thereby obviating permeabilization and diffusion across membranes.

We report ultrafast transient-grating measurements of crystals of the three-dimensional Dirac semimetal cadmium arsenide, Cd 3 As 2 , at both room temperature and 80 K. After photoexcitation with 1.5-eV photons, charge-carriers relax by two processes, one of duration 500 fs and the other of duration 3.1 ps. By measuring the complex phase of the change in reflectance, we determine that the faster signal corresponds to a decrease in absorption, and the slower signal to a decrease in the light's phase velocity, at the probe energy. We attribute these signals to electrons' filling of phase space, first near the photon energy and later at lower energy. We attribute their decay to cooling by rapid emission of optical phonons, then slower emission of acoustic phonons. We also present evidence that both the electrons and the lattice are strongly heated.

We report the growth by molecular beam epitaxy and the optical characterization of GaN films nucleated on a Si(111) surface that has been patterned by dry etching an ordered array of nanometer-scale pores prior to the growth. The etching is performed using an anodized aluminum oxide membrane as a mask. The nanopore array surface with the pore diameter of 60 nm and periodicity of 110 nm exhibits significant effects on emissivity and the optical properties of the resulting film. Room-temperature photoluminescence intensity increases by a factor of 5 for GaN grown on nanoporous Si. Peak shifts in photoluminescence and Raman spectroscopy suggest that the material grown on nanopores may be more relaxed than films grown on flat substrates. The effects of nanopore topography on the nucleation of GaN films offer a potential path to significant improvement of III-nitride heteroepitaxy for device applications.