Tungsten oxide nanorod arrays deposited on the cylindrical substrate have been used for fabrication of a two-dimensional (2D) sensor. The sensibility and capacity towards methane and acetone at different temperatures have been examined. Experimental data have indicated that the newly designed sensor is highly sensitive to methane with low cross sensitivity towards possible mixed organic gas acetone, relying on the operating temperature. At room temperature (20°C), a quick response time and recovery time of the sensor, less than 10 ms, have been obtained.

We report on fabrication, structural and infrared optical characterization of nanostructure aluminum sculptured thin films prepared by glancing angle deposition (GLAD) and controlled substrate motion on p-type silicon. We discuss two structures, one with plate-like and one with screw-like (chiral) morphology. While the plate-like sample possesses a metal Drude behavior in the infrared spectral range, the chiral nanowire sample behaves non-metallic and reveals a series of intriguing resonances, which are equally spaced in frequency by ∼7.5 THz. We suggest that formation of 3D nano resonator circuits consisting of inductances and capacitances has occurred within the screw-like conductive aluminum wire sample, which might be responsible for the observed resonances. We suggest conductive GLAD nanostructures in combination with Schottky diodes to facilitate active or passive THz detector and transmitter devices.

Molecular beam epitaxy growth of GaN and InGaN nanowires is accomplished on Si (111) substrates using Ga-droplet nucleation. Typical diameters range from 25-80 nm and lengths can be varied by increasing the growth time; the growth rate is ∼0.25 microns/hour. The nanowires have been characterized structurally and optically. Photoluminescence spectra show band-edge emission of GaN nanowires centered at 362 nm at 290 K. Transmission electron microscopy images unveil that the nanowires are highly crystalline, and grow along the 0001 polar direction. Indium has also been successfully incorporated into GaN nanowires by modifying the growth conditions; the InGaN nanowires emit at ∼520 nm, which provides a possible route to solving strain related problems of high In-composition InGaN based efficient green emitters.

The n-type silicon nanowire MOSFETs with a nanowire shape being triangular or trapezoidal, have been fabricated on SOI substrates and characterized. The height and bottom-width of the triangular nanowire has been 10 nm and 19 nm, respectively. The devices have shown good gate control, such as a nearly ideal subthreshold slope of 63 mV/decade, high Ion/Ioff ratio of 107, and small drain-induced barrier lowering of 5 mV/V at room temperature. The low field mobility of triangular nanowire has been estimated to be 130 cm2/V·s and shown no difference with the change of the nanowire shape and direction within the investigated range.

The 1-nm-wide Bi nanoline has been proposed as a possible template for the growth of very-high-density arrays of atomic-scale nanowires, grown epitaxially on the technologically important Si(001) surface. Indium reacts with the Bi dimers, forming a unique zigzag atomic chain structure. Simulations of the appearance in STM of the lowest-energy isomer of this structure match experimental filled-states images. Calculation of the LDOS for the single-layer islands, finds that the nanowires are semiconducting, with a band gap smaller than that of the substrate, in good agreement with STS. A delocalised LUMO state is created, which may provide a conduction pathway along the nanowire. We have performed dual-probe STM conduction measurements along the In-Bi nanowires to test this prediction.

Semiconductor nanowires are attractive nano- building blocks for microelectronics. However, the requirements for their manufacturing and application in the microelectronics industry are very demanding. Beyond compatibility with Si technology, full control on the characteristics of the grown wires (diameter, location, crystallinity, etc..), homogeneity on wafer –scale and reproducibility are essential. In this study we review critically important challenges for a controlled process of In –mediated growth of Si nanowires. First, we stress the importance of surface type for both particle catalysts and growth substrates. Both selection and preparation of such surfaces have large impact on growth, as they influence the initiation and the driving forces for the VLS growth mechanism. Moreover, wire characteristics such as morphology, crystalline quality and growth orientation appear more difficult to control when growing from particles with sizes below 40-50nm. This limitation arises as a result of both fundamental mechanisms and more specific constrains linked to the In-Si system.

A few perspectives are given for the achievement of a controlled Si nanowire growth in a Si –technology compatible fashion.

Raman spectroscopy was performed on magnesium (Mg) doped InP nanowires bridged between single crystal vertical silicon electrodes using a green laser (λ∼ 532 nm). First order TO-phonon and LO phonon-plasmon peaks observed at 305 cm−1 and 345 cm−1, respectively, are consistent with those for bulk single crystal InP. Misorientation of the nanowires is found to influence the TO and LO peak intensities. Bottom broadening up to ∼20 cm−1 of the TO peak due to the energy dispersion in the shifted Stokes spectrum is observed in long (∼6 μm) nanowires. Raman measurements indicated a trace of uncatalyzed InP on the insulating silicon oxide substrate and was verified through the electrical measurements of leakage currents before and after the nanowire growth. Initial investigation reveals that Raman spectroscopy can be a very useful in the study of nanowire heterostructures.

Comprehensive characterization at nanoscale is needed to create novel nanostructures for high efficiency solar cells. To produce consistent results, wide-ranging characterization procedures for integrated nanostructures have been developed. Characterization of a novel nanowire, nanotube and nanocable system includes: chemical, electrochemical, structural, optical and electrical characterization of nanostructures in relation with growth conditions. We present here results on Au and CdTe/Au system that help understanding how surface composition and properties are modified in this system. New challenges in structural characterization were also identified and improved sample preparation techniques were developed.

Room temperature photoelectrical characterization with 325-nm ultraviolet and 633-nm visible laser excitations is performed on lateral p-type InP nanowires bridged between vertically oriented heavily p-doped single crystal silicon electrodes. Experimental results under 5 V bias demonstrate persistent photoconductivity through a slow decay of excess photocurrent with relaxation times ∼110 s and ∼50 s for the UV and visible laser illuminations, respectively. Persistent photocurrent originates from the long recombination time due to carrier trapping in vacancies, defect centers, and surface states in the InP nanowires. The study opens a new understanding of trap physics of nanowire heterostructures, a critical investigation for applications of these materials in photonic devices.

We report first results on a new solar cell consisting of a p-i-n hetero-junction formed between n-type transparent nanowires, undoped semiconductor quantum dots and a wide bandgap p-type polymer layer. The overall structure is SnO2/ZnO/CdSe/MEH-PPV with MEH-PPV standing for poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene]. Microscopic studies on the structure of the quantum dot layer before and after anneal indicates a morphology change from a quantum dot assembly to a continuous polycrystalline thin film. The charge transfer between the absorber layer and the adjacent layers is improved as the layer is converted from the quantum dot assembly towards a polycrystalline thin film structure. In optimized devices the spectral photocurrent response shows contributions from the ZnO, the CdSe and the MEHPPV, covering the spectral range from 300 to 700nm with an external quantum efficiency between 30 and 40%. The overall energy conversion efficiency approaches 1%.

Low-density layers of SnO2 nanowires were produced using the vapor solid method and dry-pressed onto surface-oxidized Si-substrates equipped with a set of 39 parallel Pt-electrodes. Current-Voltage (I-V) characteristics of the segments between the electrodes were measured in ambient air at a substrate temperature of 300°C. Statistical analysis of the 38 I-V characteristics allows drawing conclusions, that only Schottky contacts between large nanowires and electrodes are significant for conductometry, and that they have very similar barrier characteristics. The statistical approach and its advantages are demonstrated. The clarity obtained concerning the roles of different resistivity mechanisms involved enables predictions of the nanowire net device behavior in applications, which is demonstrated on an instance of long-term stability examination of gas sensor arrays.

The low frequency noise of individual ZnO nanowire (NW) field effect transistors (FETs) exposed to air is systematically characterized. The measured noise power spectrum shows a classical 1/f type. The noise amplitude is independent of source-drain current and inversely proportional to gate voltage. The extracted Hooge's constant of ZnO NW is found to be 6.52×10−3. In addition, the low frequency noise of ZnO NW according to NW resistance and contact property are investigated. The noise amplitude is proportional to the square of ZnO NW resistance. If a sample shows a nonlinear current-voltage (I-V) characteristic due to a poor electrical contact, the noise power spectrum is proportional to the third power of current instead of the square of current.

Among various ways to produce nanowires; anodic alumina membrane (AAM) based synthesis has constantly received much attention because AAM possess a uniform and parallel porous structure which makes them an ideal template material for creating highly ordered nanostructures. In this paper we report fabrication of InSb nanowire arrays with diameter of 200 nm and 30 nm by direct current electrodeposition inside the nanochannels of anodic alumina membranes without subsequent annealing. The nanowires have four major growth direction, [220] being the most dominant with structure defects such as twins. The transmission electron microscopy (TEM) and scanning electron microscopy (SEM) results demonstrate that these InSb nanowires are uniform with diameters about 200 nm and 30 nm, corresponding to the pore diameter of the AAMs. The nanowires also conduct almost no current in the dark, but when hit with light, they conduct 10,000 times more current. This photoconduction property could lead to a variety of tiny optoelectronic devices potentially useful in future generations of nanoelectronics and chemical sensors. The light-induced conductivity increase and the temperature dependent behavior of the nanowires are also reported.

Field effect transistors(FETs) made of ZnO nanowires are very sensitive to the gas environment, so that the passivation can be a good way to get reliable nanowire FETs with longer lifetime and the better mobility. The studies on the passivation effects with the positive electron-beam resist was investigated by selectively covering the part of nanowire devices between the electrodes. Reproducible electrical characteristics were recorded, reflecting the stable electrical properties by the passivation which deters the degradation of a device. Considering the defect states of oxide nanowires dominate the charge states, the pre-state just before the passivation process will be crucial to understand the reproducible and controllable device characteristics of nanowire devices.

In this study, we report on the synthesis of gallium nitride (GaN) nanopowders on boron nitride (BN) substrates both with and without the use of metal catalyst by chemical vapor deposition (CVD). The synthesis process is based on the reaction between gallium (Ga) atoms from the decomposition of gallium acetylacetonate and ammonia (NH3) gas molecules. Using this process, gallium nitride (GaN) nanopowders have been synthesized at temperatures as low as 400°C, lower than previously reported. The grown nanopowders were characterized by SEM, EDX and TEM. Analysis reveals that higher yields were obtained by treating the BN substrates with Ni catalyst. Experiments to study the effect of growth conditions on the morphology of the nanopowders and analyze the growth mechanism are ongoing.