Light-emitting silicon nanocrystals (Si nc) have attracted much interest due to their possible application as optoelectronic devices. The interest for Si nanopowders is enhanced by their photoluminescence (PL) emission intensity that can be very strong at room temperature. Due to the intrinsic biocompatibility of Si nanoparticles, this strong optical emission intensity as well as the long decay time (mean life time around hundred microseconds) make these powders potential candidates as tracers for in-vivo applications.

Si nanopowders were obtained in gram quantities by CO2 laser pyrolysis of silane. The particles in the produced powders are in the size range 10-15 nm. These nanoparticles exhibit strong red photoluminescence after heat treatment. The appearance of intense PL emission is clearly related to the surface oxidation of the powders which must be carefully controlled. Several steps have been identified in the oxidation process. This paper presents a detailed study of the evolution of both the photoluminescence intensity and spectral dependence and of the crystalline structure as a function of the heat treatment. We also show that the nanopowders can be dispersed in liquids and incorporated in gel samples while keeping their intense photoluminescence. This result opens a route towards the fabrication of novel devices

A systematic study is reported about the amount-dependent morphology change in a series of Au-Pt bimetallic nanoparticles synthesized using chemical reduction. The amount of Au precursor is kept constant throughout whole series of compounds to obtain fixed Au core size (∼7.5 nm). The Au/Pt ratio is varied from 1/1 to 1/4 in order to synthesize Pt shell layer of different thickness. We observed a remarkable shift of surface plasmon band around 410 nm. With the aid of high resolution transmission electron microscope (HRTEM) and energy-dispersive spectrometer (EDS), the composition of shell layer is found to be Pt enriched Au-Pt alloy. As the amount of Pt increases, the Pt clusters formed a string-like shape on the surface of nanoparticles. The average diameter of these Pt clusters is about 2 nm. This special structure may exhibit unique catalytic property.

We fabricated quantum dot arrays using anodic alumina mask. We grew an alumina template on Si wafers by two-step anodization. The porous alumina template thus grown is used as a mask for metal deposition. After thermal evaporation and removal of the mask, we fabricated the quantum dot arrarys on a Si substrate. Using this template-assisted method we obtained a high density array of quantum dots in a large scale. These quantum dots have a narrow size distribution which can be easily controlled by a pore widening of templates from 20 to 60 nm.

Ferromagnetic Resonance experiments are used to investigate the magnetic properties of monodisperse Co/CoO core-shell nanoparticles with diameters of about 10nm. From frequency- dependent measurements at various frequencies of 9-80 GHz the g-value is determined to be 2.13 which suggests an fcc bulk-like environment of the Co atoms within the core of the particles. This result yields a direct measure of the ratio of orbital to spin magnetic moment νL/νS=0.065. Moreover, from temperature-dependent measurements of the resonance field the anisotropy energy is extracted and found much lower than the hcp bulk value.

Memory effect in a metal-insulator-silicon (MIS) structure with the insulator layer consisting of a sputtered capping SiO2 / Ge nanocrystals embedded in SiO2/ rapid thermal oxide structure has been observed. For the devices with a co-sputtered SiO2+Ge middle layer, larger nanocrystals were formed devices with a higher Ge concentration. It was noted that for such devices, the RTO and the capping oxide layers were able to confine the nanocrystals within the middle layer to some extent. However, in devices with pure sputtered Ge middle layer, the RTO and capping oxide layers were not as effective in confining the Ge nanocrystals. In addition, we have consistently observed memory effect from devices annealed in Ar and the absence of memory effect from devices annealed in forming gas. However, Ge nanocrystals were found in devices annealed in both ambient. This implies that having nanocrystals does not necessarily imply the presence of charge storage or memory effect.

We report the successful application of bipolar electrochemistry for depositing metals and conducting polymer nano-structures on isolated template-grown carbon nanopipes (CVD-CNP). Metals such as tin, cobalt and nickel were reduced electrochemically at the tips of isolated CVD- CNP from their chloride or nitrate salts. Polypyrrole dendrites could also be grown on both ends of selected CVD-CNP. The potential applications of thus encapsulated CVD-CNP in nano- fluidics and other areas will be discussed.

Ultrahigh pore density nanoporous films with a pore diameter of less than 10nm and a pore density exceeding 1016 pores/m2 were developed. Nano phase separation of a eutectic Al-Si system was used for the fabrication of these nanoporous films. Co-sputtered AlSi films form Al nano-cylinders, perpendicular to the substrate and parallel to each other, embedded in an amorphous Si matrix during film growth due to phase separation. Removal of the Al nano-cylinders from the co-sputtered AlSi films by chemical etching gives us ultrahigh pore density nanoporous films. The nanoporous films consist of mainly oxidized silicon. Depending on the film compositions and the film preparation conditions, such as RF power and the deposition temperature, the average pore diameter can be varied systematically from less than 5nm to 13nm with the pore density from 1015 to exceeding 1016 pores/m2. Furthermore we have demonstrated a template-assisted growth of ultrahigh-density Ni nanowire arrays with an aspect ratio of ∼100 in the nanoporous films by electrodeposition. The fabrication method for nanowire arrays using the nanoporous films is quite simple and promising for the fabrication of nanostructured devices.

We have used interference lithography to create surfaces with di-periodic topography with periods less than 400 nm and formed gold particle arrays on these substrates through the dewetting of thin solid films. Under appropriate conditions, we have found that dewetting of gold films on di-periodic arrays of {111} bound pits in silicon leads to periodic square arrays of particles with very narrow size distributions. In addition, we find that topography can significantly reduce the diameter of particles formed by the dewetting process. We present a mechanism for this self-organization process based on modulation of the local curvature of a conformal film on topography.

Controlling the characteristics of nanowires in order to later construct nanoarchitecture and nanocomponents for nanodevice and nanosensor applications is essential. Metal oxide nanowires are aligned using the Langmuir-Blodgett (LB) technique to uniaxially compress the nanowires. A surfactant monolayer of metal oxide nanowires is fabricated, and then compressed on an aqueous subphase. The compression yields an array of aligned nanowires, which is transferred to a planar substrate. Cutting areas of the array are defined by electron beam nanolithography. After an etching process, it is shown that the metal oxide nanowires have been successfully cut. With further refinement of this technique, the nanowires can be used to construct basic building blocks of nanodevices and nanosensors.

The defect formation in surface modified TiO2 has been studied. Anatase TiO2 structures in the 3-20 nm range formed by a wet chemical technique were surface modified and nitridation of the highly reactive TiO2nanocolloid surface was achieved by a quick and simple treatment in alkyl ammonium compounds. The nitriding process was also accompanied by a metal surface coating process using electroless plating techniques, resulting in a thin metal surface layer on the modified TiO2 nanostructures. The crystal structure of the resultant TiO2-xNx nano-colloids remained anatase and the freshly prepared samples exhibited a strong light emission near 560nm (2.21 eV), which red shifted to 660 nm (1.88 eV) and dropped in intensity with aging in the atmosphere. This behavior was also evident in some of the combined nitridized and metal coated TiO2 nano-colloids. Electron Spin Resonance performed on these samples identified a resonance at g = 2.0035, which increased significantly with nitridation. This resonance is attributed to an oxygen hole center created near the surface of the nanocolloid, which correlates well with the observed optical activity.

This paper reports on a new bottom-up technique of forming silicon nanostructures based on natural aggregation of nanocrystalline (nc) -Si dots in the solution. We first study how the nc-Si dots deposited on the Si substrate get mobile in the solution by simply dipping the substrate with the nc-Si dots on into various solutions. We then demonstrate a solution droplet evaporation method that utilizes aggregation of the dots when we evaporate a solution droplet applied onto the nc-Si dots randomly deposited on the Si substrate. It is shown that the nc-Si dots are assembled well in a droplet of the hydrofluoric acid solution, resulting in various regular patterns on the substrate.

Spherical aberration corrected Atomic Number Contrast Scanning Electron Microscopy (Z-STEM) has recently demonstrated an amazing ability to not only obtain sub-angstrom levels of detail but also yield chemical information at that level as well. With an optimal probe size of 0.8 Å, extremely detailed images of CdSe nanocrystals were obtained showing the lattice structure and surface morphology. As an example of the usefulness of this technique, a sample of CdSe nanocrystals prepared using trioctylphosphine oxide (TOPO) as the surfactant was compared to a sample of CdSe prepared using a mixture of TOPO and hexadecylamine (HDA) as the surfactant. The TOPO/HDA nanocrystals exhibit a narrower size distribution and several orders of magnitude greater fluorescence compared to that of the TOPO only nanocrystals. Interestingly, the Z-STEM images show a striking difference in nanocrystal morphology as the result of the addition of HDA to the reaction mixture. This result suggests surface morphology can be tuned through judicious choice of surfactant. A second example of Z- STEM imaging involves the characterization of CdSe/ZnS core/shell nanocrystals. The mass contrast afforded by Z-STEM can easily distinguish between core and shell.

74Ge nanocrystals were formed by ion beam synthesis in SiO2. Transmission Electron Microscopy was used to characterize the structure and properties of these Ge nanocrystals before and after liberation from the matrix. The liberation from the SiO2 matrix was achieved through selective etching in a HF bath. High-resolution micrographs and selective area diffraction confirm that the crystallinity is retained in this process. Transfer of released nanocrystals is achieved through ultrasonic dispersion in methanol and deposition onto lacey carbon films via evaporation of methanol. In an effort to determine the melting point of Ge nanocrystals and observe the growth and evolution of nanocrystals embedded in the amorphous SiO2 during heat treatment, as-grown nanocrystals were heated in-situ up to 1192°C±60°C in a JEOL 200CX analytical electron microscope. Electron diffraction patterns are recorded using a Charge-Coupled Device. A large melting hysteresis was observed around the melting temperature of bulk Ge.

One of the major challenges confronting the utilization of nanoparticles in industrial and social applications is that of producing the nanoscale materials. Of the methods of manufacturing nanoscale materials, processes involving plasmas have been shown to be cost-effective and versatile in the production of chemically diverse material. Using transmission electron microscopy, individual nanoparticles produced by a thermal plasma-based production method have been examined. The observations of these studies imply that the thermal history of the nanoparticles during formation is of great importance in the determination of the resulting nanoparticle morphology. Such results have the potential to enable the manufacturing of nanoparticles of a specific size and shape from plasmas.

Self-assembled nanopore templates were fabricated by anodizing aluminum in several electrolytes under specially selected conditions. The size of the nanopores was controlled, and many sizes were fabricated. Y-junction nanopore templates were also fabricated using a two-step anodization. Magnetic nanowires and Y-junction nanowires were then fabricated by electrochemical deposition of magnetic material into the nanopore templates and Y-junction nanopore templates. The magnetic properties of nanowires were investigated by VSM and compared.

The uniformity and reproducibility of the CdTe QD arrays on the GaAs substrates can be improved by using a nanoporous mask. The CdTe QDs on the GaAs substrate were grown by a molecular beam epitaxy (MBE) method. The nanoporous alumina masks used for the fabrication of QD arrays have the thickness from 0.3 νm to 5 νm with the nanochannels of ∼ 80 nm diameter and the pore density of ∼ 1010cm−2. When the thickness of the alumina mask used for the CdTe QD growth was about 300 nm, the CdTe QD arrays formed as a replica of the nanochannels of the mask. Smaller self-assembled CdTe QDs located randomly were produced by using the thicker nanochannel mask than 0.5 νm. The thickness of the nanochannel mask controls the size of the CdTe/GaAs QDs.

ZnO is of great interest for photonic applications due to its wide band gap (3.37 eV) and large exciton binding energy (60 meV). Variety of preparation methods and obtained morphologies (such as nanorods, tetrapod nanorods, nanowires, nanoribbons, hierarchical structures, nanobridges, and nanonails) were reported for this material. In this work, the morphology and optical properties of ZnO nanostructures prepared by three different methods were studied. ZnO nanostructures were prepared by oxidation of Zn (no catalyst) at 950°C, heating of a mixture of ZnO:graphite (1:1) at 1100°C, and chemical method (from solution of zinc nitrate hydrate and hexamethylenetetramine at 90°C). The properties of obtained products were examined using scanning electron microscopy, transmission electron microscopy, selected area electron diffraction, X-ray diffraction, room temperature photoluminescence and electron paramagnetic resonance spectroscopy. Chemical synthesis method produced different morphology compared to heating of Zn and ZnO:graphite. In the former case, straight rods are obtained, while in the latter case ZnO tetrapod structures are formed. The ZnO tetrapods, both from Zn and ZnO:graphite, exhibit similar photoluminescence spectra with UV peak and characteristic broad green emission but they have different EPR spectra. The EPR signal g≈1.96 is clearly visible in ZnO tetrapods synthesized from ZnO:graphite, while it is at noise level in ZnO tetrapods synthesized from Zn. Therefore, it can be concluded that the type of intrinsic defects in ZnO nanostructures is strongly dependent on the fabrication conditions, and that the green photoluminescence is not necessarily related to ≈1.96 EPR peak which is commonly assigned to shallow donors.

Self-assembled hexagonal networks with discrete Au particles on Al2O3 (00.1) and Si (111) have been synthesized. SiOx nanowires were grown on individual Au particles using a vapor transport deposition process. The growth of SiOx nanowires was found to cover completely the surface of Au particles on Al2O3 (00.1). On the other hand, the SiOx nanowires were grown selectively on Au particles on Si (111). Interaction of Au and Si substrate is invoked to explain the difference.

Superparamagnetic magnesium ferrite, MgFe2O4, nanoparticles were synthesized under mild microwave hydrothermal (MH) conditions. Transmission electron microscopic studies showed that the average particle size of the ferrite obtained is ∼3 nm, with a narrow size distribution. Temperature dependent AC magnetic susceptibility measurements at 2 Oe showed characteristic feature of superparamagnetism with blocking temperature, TB, at 47 K. TB decreases with increasing DC magnetic field as evidenced by zero-field-cooled susceptibility studies at 50 and 500 Oe (TB = 38 and 27 K respectively). As a typical superparamagnetic behavior, the zero-field- cooled and the field-cooled magnetizations diverge below TB. Magnetic hysteresis behavior is observed below TB, with a high coercivity of 185 Oe at 12 K, and magnetic hysteresis behavior disappears when measured above TB. The results indicate that MH method is highly suitable for the synthesis of superparamagnetic ferrite nanoparticles of uniform size distribution.

The use of bright-field transmission electron microscopy 3-D tomographic reconstruction allowed us to localize the synthesized nanostructures within the pores of mesoporous nanotemplates. Using this technique, we found that noble metal impregnation resulted in the inclusion of metal nanostructures within the SBA-15 mesoporous silica hexagonal pores (from nanoclusters to nanowires). The method allowed corroborating if the nanostructures were synthesized inside the pores or if they were alternatively on the nanotemplate aggregates exterior surface.