CdTe nanocrystal quantum dots sequestered in TiO2 thin film matrix have been synthesized by r.f. sputtering from a composite CdTe/TiO2 target. CdTe nanocrystal formation is nucleation controlled as their size (11-25 nm), dispersion and volume fraction (0.065-0.2) increases with film thickness, substrate temperature (100°C) and thermal treatment. The optical band gap derived from the onset of absorption coefficient showed blue shifts concurrent with the CdTe nanocrystal size reduction due to quantum size effects. These shifts, not consistent with theoretical models based on strong or weak confinement regimes, are explained on the basis of anisotropic growth and formation of CdTe nanocrystal clusters. TiO2, in addition to being an ideal passivator and providing a barrier for carrier confinement to observe quantum effects, shows O2 vacancy dependent conductivity modulation. Electrical conductivity variation with CdTe nanocrystal size and density is attributed to electrical coupling and tunneling behavior of carriers between CdTe nanocrystallites.

The results of photoluminescence (PL) and electroreflectance (ER) measurements on InGaAs/GaAs self-organized quantum dots (QDs) in field-effect structure are presented. It has been found that the QDs PL can be completely quenched in reversely biased structure both at room temperature and at T=4.2K. A non-monotonic dependence of QDs PL peak energy with applied bias is observed at low temperature, which is attributed to the band-gap re-normalization due to QDs charging and size distribution effects. The electric field dependence of the QDs ER feature at room temperature has been analysed. A red shift of that feature with increasing electric field has been observed.

Thin (<1000 Å) hydrogenated nanocrystalline silicon films are widely used in solar cells, light emitting diodes, and spatial light modulators. In this work the conductivity of doped and undoped amorphous-nanocrystalline silicon thin films is studied as a function of film thickness: a giant anisotropy of conductivity is established. The longitudinal conductivity decreases dramatically (by a factor of 109 − 1010) as the layer thickness is reduced from 1500 Å to 200 Å, while the transverse conductivity remains close to that of a doped a- Si:H. The data obtained are interpreted in terms of the percolation theory.

Thin film transistors (TFTs) offer the possibility to study the electronic transport properties of an intrinsic semiconductor as a function of the Fermi level position without the introduction of dopants and/or doping related defects. Recently, we reported on the first TFTs incorporating nanostructured silicon deposited with the Hot-Wire Chemical Vapor Deposition technique. These structures offer significant advantages over conventional plasma-deposited amorphous silicon TFTs. First of all, the HW deposited nanocrystalline silicon (nc-Si:H) TFTs do not show any threshold voltage shift upon prolonged gate voltage stress. Therefore, it is now possible to study the transport characteristics at a relatively large gate voltage in a controlled fashion, unhampered by any drift of the characteristics due to the creation of metastable electronic defect states and/or charge trapping. Second, the result of the field effect is that the Fermi energy moves into the conduction band of the virtually defect-free nanocrystalline domains in the channel region of the TFT. As the effective mobility gap of the surrounding amorphous phase is higher than that of the silicon crystallites, the Fermi energy is driven deep into the band-tail distribution of the amorphous phase, a situation that could never be achieved in purely amorphous silicon TFTs nor by heavily doping an amorphous semiconductor. Thus, the nanostructured nature of the silicon thin film near the gate insulator allows to shift the Fermi level far into the tail states region of the amorphous phase. This situation reveals for the first time the inverted Meyer-Neldel relationship in an intrinsic semiconductor.

We report on in situ studies of the vibrational properties of ultra-thin Si layers grown by dc magnetron sputtering in ultrahigh vacuum on amorphous MgO and Ag buffer layers. The average thickness of the Si layers ranged from monolayer coverage up to 200 Å. The interference enhanced Raman scattering technique has been used to study changes in the phonon spectra of Si nanoparticles during the crystallization process. Marked size-dependencies in the phonon density of states of the Si quantum dots and the relaxation of the k-vector conservation condition with decrease in size of the Si nanoparticles have been detected. Electron energy loss spectra have been collected for amorphous and crystallized Si nanoparticles on SiO2 buffer layers and the difference in the onset of the electronic transitions have been found.

X-ray reflectometry is used to study the first stages of formation of thin n-type porous silicon layers. Results on classical n−-type porous silicon prepared under illumination are first reported. Then, the effect of the illumination during the formation is observed by comparing n+/− - type samples prepared in darkness or under illumination. X-ray specular reflectivity measurements allow to observe an increase of the surface porosity even for the short formation times and a macroporous layer under the nanoporous layer is also identified for illuminated samples. The presence of a crater at the top of the layer is observed by profilometer measurements, especially in the case of illuminated samples. Specular and diffuse x-ray scattering results show important effects of light during the porous silicon formation.

Microcrystalline silicon with various crystalline volume fractions was prepared by plasma enhanced chemical vapour deposition. The material was studied by steady state and transient electron spin resonance in the dark and under light illumination. The observed resonances at g-values of 2.01, 2.0052, 2.0043, 1.998 can be attributed to the amorphous and microcrystalline constituents, and their respective intensities change as the ratio of amorphous to crystalline volume is varied. The origin of a fifth resonance at g = 1.995 remains unclear. Smaller crystalline volume fractions lead to lower spin densities and affect the recombination behaviour of photogenerated charge carriers. The recombination behaviour in highly crystalline material is also influenced by moderate Fermi level shifts, where differences show up between n-type (or undoped) and p-type samples. The differences are attributed to trapping of photo-generated holes in deep states within the disordered regions.

We have grown Si nanowhiskers on a Si{1111} surface via the vapor-liquid-solid (VLS) mechanism. The minimum diameter of the crystalline is 3nm and is close to the critical value for the effect of quantum confinement. We have found that many whiskers grow epitaxially or non-epitaxially on the substrate along the 〈112〉 direction as well as the 〈111〉 direction.

In our growth procedure, we first deposited gold on a H-terminated Si{111} surface and prepared the molten catalysts of Au and Si at 500°C. Under the flow of high pressure silane gas, we have succeeded in producing the nanowhiskers without any extended defects. We present the details of the growth condition and discuss the growth mechanism of the nanowhiskers extending along the 〈112〉 direction.

Molecular-dynamics (MD) simulations and the Stillinger-Weber three-body potential are used to study the growth and stability of silicon clusters of diameters from 2 to 5 nm embedded in the melt. Our simulations show that the melting temperature of such nano-clusters is lower than the bulk melting temperature by an amount proportional to the inverse of the cluster size. We also show that the nature of the kinetics of such small Si clusters is essentially the same as that of the homoepitaxial growth. In particular, we show that the mobility of the highly-curved crystalliquid interface is controlled by diffusion in the adjacent melt, and is characterized by the same activation energy.

Ion implantation is a versatile technique by which compound semiconductor nanocrystals may be synthesized in a wide variety of host materials. The component elements that form the compound of interest are implanted sequentially into the host, and nanocrystalline precipitates then form during thermal annealing. Using this technique, we have synthesized compound semiconductor nanocrystal precipitates of ZnS, CdS, PbS, and CdSe in a fused silica matrix. The resulting microstructures and size distributions were investigated by cross-sectional transmission electron microscopy. Several unusual microstructures were observed, including a band of relatively large nanocrystals at the end of the implant profile for ZnS and CdSe, polycrystalline agglomerates of a new phase such as γ-Zn 2SiO4, and the formation of central voids inside CdS nanocrystals. While each of these microstructures is of fundamental interest, such structures are generally not desirable for potential device applications for which a uniform, monodispersed array of nanocrystals is required. Methods were investigated by which these unusual microstructures could be eliminated.

A plasma enhanced chemical vapor deposition technique has been developed to grow single crystal boron carbide nanowires and nanonecklace arrays using the single precursor closo-l,2-dicarbadodecaborane (C2B10H12). Nanowire and nanonecklace growth is expedited by Fe seeding of the substrate. Using the compound Fe-(C5H5)2 as an Fe source, it has been demonstrated that the density of nanowires, as well as the types of nanostructures that grow, can be tailored by controlling the concentration of Fe deposited onto the substrate surface prior to boron carbide deposition.

Quantum confined nanostructures of semiconductors such as Ge and Si are being actively studied due to their interesting optical and electronic transport properties. We fabricated Ge nanostructures buried in the matrix of polycrystalline-AIN grown on Si(111) by pulsed laser deposition at lower substrate temperatures than that used in previous studies. The characterization of these structures was performed using high resolution transmission electron microscopy (HRTEM), photoluminescence and Raman spectroscopy. HRTEM observations show that the Ge islands are single crystal with a pyramidal shape. The average size of Ge islands was determined to be 15 nm, considerably smaller than that produced by other techniques. The Raman spectrum reveals a peak downward shift, upto 295 cm−1, of the Ge-Ge mode caused by quantum confinement in the Ge-dots. Photoluminescence (PL) was observed both with a single layer of Ge nanodots embedded in the AlN matrix and from ten layers of dots interspersed with AIN. The PL of the dots was blue shifted by ˜0.266 eV from the bulk Ge value of 0.73 eV at 77 K, resulting in a distinct peak at ˜1.0 eV. The full width at half maximum (FWHM) of the peak was 13 meV, for the single layer and 8 meV for the ten layered sample, indicating that the Ge nanodots are fairly uniform in size, which was found to be consistent with our HRTEM results. The importance of pulsed laser deposition (PLD) in fabricating novel nanostructures is discussed.

Electrochemical lithium intercalation in nanostructured anatase TiO2 is investigated with in-situ X-ray diffraction. A complete and reversible phase transformation from tetragonal anatase TiO2 to orthorhombic anatase Li0.5TiO2 is observed. The difference of the XRD spectra before and after insertion can be fitted with the lattice parameters of the two phases as fit parameters. The maximum amount of lithium that can be dissolved in anatase TiO2 before the phase transformation occurs is found to be very small.

We have recently demonstrated that screen-printed sensors using a 20 nm- instead of microsized BaTiO3-CuO-additives powder exhibit up to one order of magnitude higher sensitivity to CO2. In this paper, we focus on both the surface chemistry of the nano-BaTiO3- CuO-additives powder (mix-BaTiO3) and electrical changes during O2 and CO2 adsorptions. We show by Fourier transform infrared (FTIR) spectrometry, thus without using electrodes, that the mix-BaTiO3 system behaves like a p-type semiconductor at the operating temperature. The variations of the electrical conductivity versus CO2 concentrations are followed in situ by FTIR spectrometry and prove to be dependent on the surrounding oxygen. These IR results are then correlated to the electrical measurements performed on the sensor. Preliminary electrical response modelling shows a good agreement with the surface barrier layer theory.

Colloidal chemistry techniques were used to synthesize ZnO particles in the nanometer size regime. The particle aging kinetics were determined by monitoring the optical band edge absorption and using the effective mass model to approximate the particle size as a function of time. We show that the growth kinetics of the ZnO particles follow the Lifshitz, Slyozov, Wagner theory for Ostwald ripening. In this model, the higher curvature and hence chemical potential of smaller particles provides a driving force for dissolution. The larger particles continue to grow by diffusion limited transport of species dissolved in solution. Thin films were fabricated by constant current electrophoretic deposition (EPD) of the ZnO quantum particles from these colloidal suspensions. All the films exhibited a blue shift relative to the characteristic green emission associated with bulk ZnO. The optical characteristics of the particles in the colloidal suspensions were found to translate to the films.

Nanoparticles of ZnS and Mn-doped ZnS capped with TOPO (tri-n-octylphosphine oxide) and close to mono-dispersed have been prepared by a single source route using bis(diethyldithiocarbamato) zinc(II) as a precursor. The nanoparticles obtained show quantum size effects in their optical spectra and ZnS nanoparticles exhibit near band-edge luminescence. Clear difference in photoluminescence results between ZnS and ZnS:Mn samples. Changes in Mn-doping levels are shown by the changes in photoluminescence intensity. The most intense photoluminescence was observed for 1% and the least intense for 5% doping level. ESR spectra and ICP results confirm the presence of Mn in ZnS quantum dots and also correspond to the amount of Mn in each ZnS:Mn sample.

The Selected Area Electron Diffraction (SAED), X-ray diffraction (XRD) pattern and Transmission Electron Microscopy (TEM) show the material to be of the hexagonal phase. The crystallinity of the material was also evident from High Resolution Transmission Electron Microscopy (HRTEM) which gave well-defined images of nanosize particles with clear lattice fringes.

The dynamics of nanoparticle formation by laser ablation into background gases are revealed by gated-ICCD photography of photoluminescence (PL) and Rayleigh-scattering (RS) from gas-suspended nanoparticles. These techniques, along with gated-spectroscopy of PL from isolated, gassuspended nanoparticles, permit fundamental investigations of nanomaterial growth, doping, and luminescence properties prior to deposition for thin films. Using the time-resolved diagnostics, particles unambiguously formed in the gas phase were collected on TEM grids. Silicon nanoparticles, 1-10 nm in diameter, were deposited following laser ablation into 1-10 TorrAror He. Three in situ PL bands (1.8, 2.6, 3.2 eV) similar to oxidized porous silicon were measured, but with a pronounced vibronic structure. Structureless photoluminescence bands were reproduced in the films (2.1, 2.7, 3.2 eV) only after standared annealing. The ablation of metal zinc into Ar/02 is also reported for the preparation of < 10 nm diameter hexagonal zincite nanocrystals, The particles were analyzed by bright field and Z-contrast TEM and high resolution EELS.

CdSe and CdTe nanoclusters were formed in aqueous solutions at moderate temperatures by a wet chemical route in the presence of thiols as effective stabilizing agents. The nature of the stabilizing agent (thioalcohols or thioacids) had an important influence on the particle size and largely determined the photoluminescence properties. The nanoclusters were characterized by means of UV-vis absorption and photoluminescence spectroscopy, powder X-ray diffraction, high resolution transmission electron microscopy, and extended X-ray absorption fine structure measurements. CdSe and CdTe nanoclusters were crystalline, in the cubic zincblende phase, with mean sizes in the range of 2 to 5 nm depending on the preparative conditions and the postpreparative size-selective fractionation, and showed pronounced electronic transitions in the absorption spectra. Thioglycerol-stabilized CdTe nanoclusters possessed sharp band-egde photoluminescence being tunable with particle size.

Good quality, close to mono-dispersed, nanoparticles of CuSe and CuInSe2 have been prepared from thermolysis reactions in tri-n-octylphosphine oxide (TOPO). Bis(diethyldiselenocarbamato)-copper(II) ([Cu(Se2CNEt2] )2») was used to prepare CuSe and TOPSe, CuCI and InCl3 were reacted to form CuInSe2. HRTEM images showed that the particles have reasonable monodispersity and are crystalline. Clear lattice fringes were observed in single dots of CuInSe2 (ca. 4 nm) and CuSe (ca. 16 nm). The particles are capped with TOPO and the SAED pattern showed cubic CuInSe2 and hexagonal CuSe.

Nanocrystalline indium tin oxide (ITO) powder is prepared by gas phase condensation (GPC) and used to produce conductive optical coatings. Materials produced by GPC are commercially available at reasonable cost and bridge the gap between basic science and product development. Nanocrystalline ITO films are transparent (>95%), conductive, and offer distinct device processing advantages. The oxidation state and surface chemistry of ITO may be tailored for particular applications and offers a flexible, affordable alternative to competitive films. The preparation and characterization of nanocrystalline ITO films and device applications will be discussed.