Free-standing aligned carbon nanotubes have previously been grown above 700°C on mesoporous silica embedded with iron nanoparticles. Here, carbon nanotubes aligned over areas up to several square centimeters were grown on nickel-coated glass below 666°C by plasma-enhanced hot filament chemical vapor deposition. Acetylene gas was used as the carbon source and ammonia gas was used as a catalyst and dilution gas. Nanotubes with controllable diameters from 20 to 400 nanometers and lengths from 0.1 to 50 micrometers were obtained. Using this method, large panels of aligned carbon nanotubes can be made under conditions that are suitable for device fabrication.

Highly crystalline, size-selected silicon (Si) nanocrystals in the size range 2-10 nm were grown in inverse micelles and their optical absorption and photoluminescence (PL) properties were studied. High resolution TEM and electron diffraction results show that these nanocrystals retain their cubic diamond stuctures down to sizes -4 nm in diameter, and optical absorption data suggest that this structure and bulk-like properties are retained down to the smallest sizes produced (-1.8 nm diameter containing about 150 Si atoms). High pressure liquid chromatography techniques with on-line optical and electrical diagnostics were developed to purify and separate the clusters into pure, monodisperse populations. The optical absorption revealed features associated with both the indirect and direct bandgap transitions, and these transitions exhibited different quantum confinement effects. The indirect bandgap shifts from 1.1 eV in the bulk to -2.1 eV for nanocrystals -2 nm in diameter and the direct transition at T(Tz -TIS) blue shifts by 0.4 eV from its 3.4 eV bulk value over the same size range.Tailorable, visible, room temperature PL in the range 700-350 nm (1.8 -3.5 eV) was observed from these nanocrystals. The most intense PL was in the violet region of the spectrum (-400 nm) and is attributed to direct electron-hole recombination. Other less intense PL peaks are attributed to surface state and to indirect bandgap recombination. The results are compared to earlier work on Si clusters grown by other techniques and to the predictions of various model calculations.Currently, the wide variations in the theoretical predictions of the various models along with considerable uncertainties in experimental size determination for clusters less than 3-4 nm, make it difficult to select among competing models.-1-

Highly-oriented, multi-walled carbon nanotubes were grown on polished polycrystalline and single crystal nickel substrates by plasma enhanced hot filament chemical vapor deposition at temperatures below 666°C. The carbon nanotubes range fiom 10 to 500 nm in diameter and 0.1 to 50 pm in length depending on growth conditions. Acetylene is used as the carbon source for the growth of the carbon nanotubes and ammonia is used for dilution gas and catalysis. The plasma intensity, acetylene to ammonia gas ratio and their flow rates, etc. affect the diameters and uniformity of the carbon nanotubes. [2]. Nanotube alignment is particularly important to enable both fundamental studies and applications, such as flat panel displays, vacuum microelectronics, chargeable batteries, etc. However, only one report exists on the growth of aligned carbon nanotubes by thermal decomposition of acetylene in nitrogen gas at temperature above 700°C on mesoporous silica containing iron nanoparticles [6] before our report on growth of large arrays of well-aligned carbon nanotubes on glass [16]. Here we report the growth of highly-oriented, multi-walled carbon nanotubes on nickel substrates at low temperatures by the same method (plasma enhanced hot tungsten-filament chemical vapor deposition) described in our previous paper [16]. The motivation to grow carbon nanotubes on Ni substrates is for the applications of using carbon nanotubes as battery electrodes and energy storage. We use acetylene (C2H2) to provide carbon for the growth of the carbon nanotubes and ammonia (NH3) gas for both dilution gas and catalysis. The catalytic role of ammonia is discussed in our previous paper [ 161.The base pressure of the deposition chamber is < 6 x Torr. We grew carbon nanotube films in a pressure of 1 -20 Torr maintained by flowing acetylene and ammonia gases with a total flow rate of 120 -200 sccm. We varied the acetylene-to-ammonia volume ratio fiom 1 : 2 to 1 : 10 for different experimental runs. Both polished polycrystalline and single-crystal Ni substrates were used. After stabilizing the working pressure, the tungsten filament coil powered by a DC source and the plasma-generator were turned on to generate heat and plasma. Under the present experimental set-up, the temperature of samples is estimated to be below 666 "c (which is the strain point of the display glass provided by Corning Inc.) since the display glass sit side by side with the Ni did not show any noticeable deformation after the experiments [ 161 and also Ni is not red-hot by visual observation. Growth durations were fiom 10 min to 5 h depending on the desired carbon nanotube lengths. Samples were examined by scanning electron microscopy (SEM, Hitachi S-4000) to measure tube lengths, diameters, site distributions, alignment, density and uniformity. High-resolution transmission electron microscopy (TEN was used to determine 2 the microstructure of individual tubes. Samples were also examined by x-ray diffraction, Raman spectroscopy, and x-ray photoemission spectroscopy to stud...

We report experiments using high-resolution size exclusion chromatography (HRSEC), dynamic light scattering, and transmission electron microscopy to investigate the effects of aging of Au nanoclusters in the presence of surfactant ligands. We first describe our observations of the role of thiols as etchants to produce clusters in a micelle-free synthesis by reduction of a metal-organic precursor. Clusters with large abundances are identified using HRSEC, and their sizes and optical properties are reported. The smallest, D c ∼ 1 nm molecular sized Au clusters, with approximately 1 closed atomic shell, N ∼ 13 atoms, have nonclassical features in their room temperature absorbance spectra. The other dominant sub-populations also correspond closely to closed-shell structural stabilities. We show that, contrary to the expectation that aging in solution will always broaden the size dispersion and increase the average size (Ostwald ripening), a narrowing of the size dispersion and change in average size can occur with time under ambient conditions. In the presence of various chain length alkanethiols, an etching and size decrease usually occurs; in the case of weakly bound alkylated poly-(ethylene oxide) surfactants, an increase in size with time is observed.

One of the unusual properties of metal nanoclusters is the size dependence of the melting temperature. Studies show that the melting temperature is a decreasing function of the cluster diameter, although the actual dependence is not known, due to the difficulties associated with the experimental measurements. We have synthesized narrow dispersity alkanethiol-functionalized Au and Pt nanoclusters in inverse micellar solutions and have measured the sintering temperatures of these films using a capacitance technique to determine the onset of film conductivity. Cluster sintering is accompanied by pronounced optical changes and a thermal signature that can be observed in differential thermal analysis. The large optical absorbance of nanocluster films makes laser sintering a practical method for the formation of metallization layers.

'~~1~f~q$ e t.&" P8TJ"A new air-stable electronic surface passivation for GaAs and other III-V comp u semiconductors that employs sulfur and a suitable metal ion, e.g., Zn, and that is ro ust towards plasma dielectric deposition has been developed. Initial improvements in photoluminescence are twice that of S-only treatments and have been preserved for >11 months with SiOXNYdielectric encapsulation. Photoluminescence and X-ray photoelectron spectroscopes indicate that the passivation consists of two major components with one being stable for >2 years in air. This process improves heterojunction bipolar transistor current gain for both large and small area devices.. High surface recombination velocities andiorFermi-level pinning due to a high density of mid-gap surface states (> 10'2/cm2) have diminished the performance of heterojunction bipolar transistors (HBTs) and delayed the realization of metal-insulatorsemiconductor (MIS) devices in III-V compound semiconductors.Reaction of the GaAs surface with sulfur or its compounds (1-11) produces a dramatic decrease in the interface states responsible for surface recombination and Fermi-level pinning. However, most sulfur-treated surfaces rapidly reoxidlze, returning to their original high density of mid-gap states. Previously reported air-stable S-based pa.ssivation techniques include a glow discharge in sulfur vapor with GaAs heated to 400°C (10) and immersion in S,CIJCC1, (1 1). Despite their air-stability, these processes are limited for actual device applications by either the high temperatures employed (10) or the over-etching of GaAs by S,Cl,, which necessitates in-situ measurement of current gain to terminate the process at maximum device performance (1 1). We have developed a new method whereby a high-quality semiconductor surface is preserved against air oxidation by sulfidation followed by reaction with a suitable metal ion in aqueous solution at room temperature. This process has been applied to prefabricated HBTs to drastically reduce the current gain dependence on surface area. It is also robust against silicon oxynitride encapsulation in a high-density plasma deposition system, making it suitable for fabrication of practical devices.Ĩ n this new process, the semiconductor surfaces were first sulfided using the S vapor and UV light as previously described (5,6). Samples were then immersed in an aqueous solution of a suitable metal salt for several seconds, rinsed with DI water, and blown dry with N2. Performing these steps in a N2 atmosphere prevents degradation of photosulfided surfaces by 0, prior to immersion and drying. After treatment with the metal salt, samples were exposed to atmosphere. The compositions of GaAs (100) surfaces before and after sulfur passivation were determined using X-ray photoelectron spectroscopy (XPS) (5). Photoluminescence (PL) intensity was employed as a relative measure of the depth of the surface depletion region, assuming negligible PL emission from the depleted depth , d = NJNa,, where N, is the surface state density an...

High-resolution transmission electron microscopy (HRTEM) shows that amorphous-tetrahedral diamondlike carbon (a-tC) films grown by pulsed-laser deposition on Si(100) consist of three-to-four layers, depending on the growth energetics. We estimate the density of each layer using both HRTEM image contrast and Rutherford backscattering spectrometry. The first carbon layer and final surface layer have relatively low density. The bulk of the film between these two layers has higher density. For films grown under the most energetic conditions, there exists a superdense a-tC layer between the interface and bulk layers. The density of all four layers, and the thickness of the surface and interfacial layers, correlate well with the energetics of the depositing carbon species.

The carbon ion energy used during faltered cathodic vacuum arc deposition determines the bonding topologies of amorphous-carbon (a-C) films. Regions of relatively low density occur near the substrate/film and filmkwface interfaces and their thicknesses increase with increasing deposition energy. The ion subplantation growth results in mass density gradients in the bulk portion of a-C in the growth direction; density decreases with distance from the substrate for fdms grown using ion energies< 60 eV and increases for ftis grown using ion energies >160 eV. Films grown between these energies are the most diamondlike with relatively uniform bulk density and the highest optical transparencies. Bonding topologies evolve with increasing growth energy consistent with 'the propagation of subplanted transformation of 4-fold to 3-fold coordinated carbon atoms.carbon ions inducing a partial