We report on PbSe QDs grown on a few μm thick PbTe(111) layers on Si(111). The QDs form as pyramids with three (100) side faces and (111) base. Their sizes are almost equal, the standard deviation of their heights can be as low as 2–3%. This seems to be the narrowest distribution ever observed for self assembled QDs. Similar QDs of PbSe have first been described by Pinczolits et al. (Appl. Phys. Lett. 73, 250, 1998). These authors used cleaved BaF2 as a substrate, they observed two types of dots with somewhat wider size distributions. The results are compared and explained on the basis of nucleation and growth theories.

The quality of lattice-mismatched semiconductor heterojunctions is often limited by the presence of misfit dislocations. Nanowire geometries offer the promise of creating highly mismatched, yet dislocation free heterojunctions. A simple model, based upon the critical thickness model of Matthews and Blakeslee for misfit dislocation formation in planar heterostructures, illustrates that there exists a critical nanowire radius for which a coherent heterostructured nanowire system is unstable with respect to the formation of misfit dislocations. The model indicates that within the nanowire geometry, it should be possible to create perfect heterojunctions with large lattice-mismatch.

ABSRTACT:An effective nonlinear dielectric permittivity of a heterogeneous composite structure consisting of coated ellipsoidal nonlinear nanoparticles embedded in a linear host medium has been theoretically studied. An expression for the effective dielectric permittivity tensor of such a system has been derived following the Clausius-Mossotti approximation. The conditions for the existence of electromagnetic bistability phenomenon in a single coated ellipsoidal particle with a nonlinear core and a metallic shell have been obtained. The numerical results show more prominent bistability phenomenon in more dense suspensions of nonlinear heterogeneous ellipsoids.

Polymorphous Si nanowires (SiNWS) have been successfully synthesized on Si wafer by plasma enhanced chemical vapor deposition (PECVD) at 440°C, using silane as the Si source and Au as the catalyst. To grow the polymorphous SiNWS preannealing the Si substrate with Au film at 1100 °C is needed. The diameters of Si nanowires range from 15 to 100 nm. The structure, morphology and chemical composition of the SiNWS have been characterized by high resolution x-ray diffraction, scanning electron microscopy, transmission electron microscopy, as well as energy dispersive x-ray spectroscopy. A few interesting nanowires with Au nanoclusters uniformly distributed in the body of the wire were also produced by this technique.

The electroluminescence (EL) properties of Er-doped Si nanoclusters (nc) embedded in metal-oxide-semiconductor devices are investigated. It is shown that, due to the presence of Si nc dispersed in the SiO2 matrix, an efficient carrier injection occurs and Er is excited producing an intense 1.54 μm room temperature luminescence. The EL properties as a function of the current density, temperature and time have been studied in details, elucidating the radiative and non-radiative de-excitation properties of the system. We have also estimated the excitation cross section for Er under electrical pumping finding a value of −1×10 cm. This value is two orders of magnitude higher than the effective excitation cross section of Er ions through Si nc under optical pumping, and quantum efficiencies of ∼1% are obtained at room temperature in these devices. These data will be presented and the impact on future applications discussed.

This paper describes the preparation of alkyl terminated silicon nanoparticles that are air and moisture stable by the oxidation of magnesium silicide with bromine and then the subsequent termination with an alkyl Grignard reagent. The nanoparticles have been characterized by transmission electron microscopy (TEM), high-resolution TEM (HRTEM), energy-dispersive X-ray spectroscopy (EDS), Fourier transform infrared (FTIR) spectroscopy, and fluorescence spectroscopy.

Low temperature measurements of differential conductivity in nanocrystalline Si – amorphous SiO2 superlattices surprisingly reveal a clear double-peak structure associated with tunneling via levels of light and heavy holes. Numerical simulations show not only detailed agreement with the experiment but also predict that the studied system has no stable solutions for carrier concentration higher than 1017 cm−3. According to this prediction, partial screening of the external electric field generates current instabilities and oscillations, and that is experimentally observed. The developed model also suggests that a more uniform electric field and stabilization of carrier transport at a higher level of carrier density can be achieved under transient carrier injection.

In this work, FeSi2 precipitates were formed in Si by iron implantation using a metal vapor vacuum arc ion source. Transmission electron microscopy (TEM) was used to determine the crystal and defect structures of the implanted samples. It was found that the implantation temperature played an important role on the shape and phase formation of the FeSi2 precipitates, as well as the formation and distribution of the dislocation loops. When implantation was performed at an elevated temperature of about 380°C, there were a lot of dislocation loops formed and only β-FeSi2 precipitates were observed. When implanted at a low temperature of about -100°C followed by a dual step annealing process, the samples were found to be free of dislocation loops, and γ-FeSi2 and β-FeSi2 precipitates coexisting and coherent with the silicon substrate were observed. Photoluminescence (PL) spectra of different line shapes from these implanted samples were observed. By combining the PL and TEM results, the origins of the PL peaks in different samples could be distinguished and identified to be from β-FeSi2 precipitates or from crystal defects in the samples.

This paper is focused on the sol-gel synthesis and characterization of CuO-based nanosystems both in the form of supported films and as guest nanoclusters embedded in a silica matrix. In both cases copper acetate (Cu(CH3COO)2 · H2O) was used as Cu source and, for the CuO :SiO2 nanocomposite systems, tetraethoxysilane (Si(OC2H5)4, TEOS) was adopted as silica precursor. Films were obtained by a dip-coating procedure and subsequently treated in air between 100 and 900°C. The system evolution on thermal annealing was studied by X-ray photoelectron spectroscopy (XPS), Glancing-Incidence X-ray diffraction (GIXRD) and optical absorption. Irrespective of the processing conditions, the formation of tenorite (CuO) crystallites with nanometric dimension was observed. In the nanocomposite samples, copper was homogeneously distributed in the host matrix and stable CuO nanoclusters (φ ≈ 15 nm) were obtained.

Efficient electroluminescence (EL) is obtained at low operating voltages (<3 V) from n+-type silicon- electrochemically oxidized thin nanocrystalline porous silicon (PS)-amorphous carbon-Indium tin oxide (ITO) junctions. The effects of a few nanometer thick amorphous carbon film between PS and ITO on the EL characteristics have been investigated. The carbon film enhances the stability. The EL efficiency is improved due to a reduction of current density and an increase in EL intensity. In addition, the reproducibility from device to device is very much improved by the carbon film. The enhancement in stability should originate from the capping of PS by the carbon film and the high chemical stability of carbon and Si-C bonds, which should prevent PS oxidation. The carbon film acts as an efficient buffer layer between PS and ITO, resulting in enhanced mechanical, electrical and chemical stability of the top contact and providing high reproducibility. The thin carbon film has only positive effects on all the EL characteristics. This is a very important step towards application.

Si nanoparticle linear arrays have been produced by pulsed KrF laser irradiation in inert atmospheres. The Si nanoparticles have a Gaussian size distribution with a mean diameter of 2.9 nm and a FWHM of 2 nm. The self – organized lines are spaced at a distance approximately equal to the laser beam wavelength, λ. The evolution of this self – organization process and that of laser – induced periodic surface structures (LIPSS) are closely related. The nanoparticles scatter light that interferes with the incident light. Similar to LIPSS, the inhomogeneous deposition of laser light due to this interference pattern created at the surface drives the particle formation and clustering. A detailed study of the clustering formation as a function of the laser pulses is presented.

A stochastic distribution of nanocrystalline sizes model is applied to fit photoluminescence (PL) spectra of luminescent Si nanocrystals in a Si/SiO2 matrix synthesized by RF co-sputtering on the top of quartz substrates. With this method, the PL spectra from a diverse set of samples can be resolved mainly as the sum of two components: a contribution from a gaussian-like distribution of sizes of quantum dots (QD) and a similar component from a distribution of quantum wires (QW). These distributions of sizes and their associated PL energies agree well with the so-called Smart Quantum Confinement model (SQC).

We report the systematic characterization of metal nanocrystal formation on ultra-thin tunnel gate oxide (2∼3nm) for memory applications. To get a high density and small average size of nanocrystals, the process parameters including annealing temperature, initial film thickness, and substrate doping are investigated for Au, Ag, and Pt nanocrystal formation with Si nanocrystal structure as control samples. The observation of nanocrystal formation by scanning electron microscope (SEM) shows that annealing below melting temperature of deposited film contributes to the reshaping of nanocrystals, while the initial film thickness to actual nanocrystal growth. In addition, the Schottky charge effect from substrate doping is not negligible if the tunnel oxide is thin. Controlling the process parameters, Au, Ag, and Pt nanocrystals of 4.0×1011cm-2, 2.8×1011cm-2, and 2.4×1011cm-2 can be formed with mean size of 6.2nm, 6.6nm, and 8.0nm, respectively. The observation of nanocrystals by scanning transmission electron microscope (STEM) shows that nanocrystals are spherical and crystalline. Metal contamination to the Si/SiO2 interface is also closely monitored with many process recipes of metal nanocrystal formation on 2∼3nm oxide showing atomically clean interface. Electrical evaluation of nanocrystal formation is carried out by C-V measurements of metal-oxide-semiconductor (MOS) capacitors with embedded metal nanocrystals.

Amorphous silicon has been used for a wide variety of electronic applications including thin film transistors and energy conversion devices. However, these devices suffer greatly from defect scattering and recombination. A method for depositing crystalline silicon would be highly desirable, especially if it can be remotely created and deposited on any kind of substrate. Our work aims at synthesis and deposition of mono-disperse, single crystal silicon nanoparticles, several tens of nm in diameter on varied substrates. Synthesis of nanocrystals of 2–10 nm diameter has been previously reported but larger particles were amorphous or polycrystalline. This work reports the use of an inductively coupled low-pressure plasma to produce nanocrystals with diameters between 20–80 nm. Electron microscopy studies confirm that the nanocrystals are highly oriented diamond-cubic silicon.

The ability to synthesise and tune photoresponsive and photoredox II-VI semi-conductor nanoparticles by quantum confinement is a continuously and rapidly developing area. Chemical methods are useful in producing dispersions of nanoparticles of uniform size in many organic solvents. Such synthetic methods employ the use of reverse micelles [1,2], phospholipid vesicles[3,4] or capping agents[5,6]to restrict particle size and growth to the nanometre regime.

The production of super lattice assemblies, through, for example the use of controlled crystallisation [7], Langmuir monolayers[8], Langmuir Blogget films[9], and self assembled short chain dithiol monolayers[10] on gold substrates has also received attention in the past. This latter approach potentially opens the way for self assembled electro-luminescent devices[11].

The ability to couple organic self assembly and inorganic nano-synthesis could also provide a route toward the chemical synthesis of an organised array of quantum-confined semiconductors. An aspect of this synthesis with construction approach is the possible coupling of biological templating systems to produce biomimetic materials [12,13]. In fact, an example would be the stabilised CdS nanoparticles in yeast binding to phytochelatin peptides which are secreted in response to metal toxicification [14].

We present the first demonstration of spectrally selective charge storage in self-assembled quantum dots that utilizes optical charge generation and readout of the stored charge distribution. Resonant optical excitation charges only a sub-ensemble of the inhomogeneous broadened dots. The optical readout process directly recovers the spectral distribution of stored charges. Wavelength selectivity is demonstrated in a two-color experiment exhibiting charge storage in two independent QD sub-ensembles. Resonant charge storage is observed at least up to 85K depending on excitation energy. Lack of time evolution of the spectral emission indicates charge storage times Δt much longer than 25μs.

Quantitative understanding of charging and discharging of Si nanocrystals in SiO2 films on Si substrate is essential to their application in floating gate nonvolatile memory devices. Charge imaging by atomic force microscopy (AFM) or electrostatic force microscopy (EFM) can provide qualitative information on such system, while a further step is needed. We have developed a generalized method of images, which can solve Poisson equation for multiple dielectric layers, to simulate the charge imaging of Si nanocrystals by non-contact mode AFM under different sample geometries. Simulated images can be compared with experimental images thoroughly to estimate the total amount and distributions of trapped charges, which is also useful in the study of time evolution of charges or dissipation problems.

Carbon nanotubes on metallic wires may be act as electrode for the field emission (FE) luminescent devices. Growing nanotubes on metallic wires with controlled density, length and alignment are challenging issues for this kind of devices. We, in the present investigation grow carbon nanotubes directly on the metal wires by a powerful but simple technique. A novel approach has been proposed to align nanotubes during growth. Methane, acetylene and dimethylamine have been used as source gases. With the same growth conditions (viz. pressure, growth temperature and plasma) methane does not produce any nanotube but nanotubes grown with dimethylamine show shorter length and radius than acetylene. The effect of temperature to control the radius, time to control the density, plasma conditions to align the nanotubes has been focused. Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Rutherford Back Scattering (RBS) are used to characterize the nanotubes.

In present report, we investigated the degradation processes in porous silicon light-emitting devices (LED) in different atmospheres (O2, N2, air and vacuum) by photoluminescence (PL), electroluminescence (EL), lifetime (LT) and I-V characteristic measurements as well as by Energy Dispersive X-ray Spectroscopy (EDS). The contacts were made by evaporation of Au and Au/Cu alloy. The LEDs emit in visible range at forward and reverse bias. As a rule, full width at half maximum of EL spectrum is wider than that of PL spectrum. The bias direction of applied voltage during degradation change EL, PL, I-V characteristics, and LT of the LEDs. At forward bias, LT degradation is less than that in reverse bias.

The degradation of LEDs during forward bias did not produce any change in the spectral shape of EL and PL. At reverse bias, degradation led to red shift in the peak of EL and PL. The results show that the lifetime of LEDs with Au contact is longer than Au-Cu. Operating in different atmospheres, the LT in vacuum is longest and is more than 100 hours in reverse bias at room temperature.

Possible mechanisms of degradation of LEDs are discussed. It is proposed that degradation is connected mainly with two processes: oxidation and metal diffusion. It is shown that the oxygen and metal in ionic state can diffuse quickly. Hence, in forward bias, the diffusion of metal would dominate, and in reverse bias, diffusion of oxygen dominates.

Covalently-linked adducts of single-walled carbon nanotubes (SWNTs) with biomolecules have been fabricated. Results are presented here for DNA-SWNT and for biotin-SWNT adducts. DNA-SWNT adducts are shown to be biochemically accessible and to exhibit high selectivity, favoring hybridization with complementary vs. non-complementary DNA sequences. Biotin-SWNT adducts were also prepared and used to direct the assembly of nanotubes onto a biotinylated glass surface.