Intensive research is currently underway to exploit the intriguing optical and electronic behaviour of nano-sized particles. The basis of the unique properties of these particles is the smallness of their size; dimensions on the nanometre scale can result in interesting quantum mechanical phenomena, such as Coulomb blockade. There are currently a number of ways by which the nanoparticles can be deposited onto solid substrates. Here, we report on the use of the layer-by-layer electrostatic method, which has shown much promise in the context of deposition of thin films of certain organic materials. In this technique, layers of oppositely charged materials are generated by dipping an appropriate substrate into solutions of polyelectrolytes. For example, the polybases poly(ethyleneimine) (PEI), when adsorbed on a substrate, produce a positively charged surface. We have deposited carboxylic acid (-COOH) derivatised gold nanoparticles onto a PEI-coated silicon substrate and an amine funtionalised silicon substrate. The distribution of the gold nanoparticles was compared using atomic force microscopy.

We report silicon nitride whisker formation from hydrogenated amorphous silicon (a-Si:H) nanoparticles grown by PECVD for the first time. We compared the results with the kinetics of whisker formation from ball milled crystalline silicon (c-Si) microparticles. Whisker formation is analyzed at different temperatures (900–1440 °C) and oxygen partial pressures. At temperatures equal or above 1350 C and at low oxygen partial pressure we observe monocrystalline α-Si3N4 whiskers having 30–100 nm diameter and several microns length. By increasing the oxygen partial pressure, the structure of whiskers is completely changed, as shown by electron microscopy. In this case we observe α-Si3N4 whiskers covered by an amorphous silica layer at 1350 C. Finally, when the precursor material is silicon microparticles, thicker (170–330 nm) and longer whiskers are formed.

Gold and silver nanorods/wires of different dimensions were obtained by wet-chemical seeded and non-seeded growth methods.

We report on three-dimensional fluorescence imaging of micron-size faceted crystals precipitated from solutions of CdSe nanocrystals. Such crystals have previously been suggested to be superlattices of CdSe quantum dots [1,2]. Possible applications for these materials include their use in optical and optoelectronic devices. The micron-size crystals were grown by slow evaporation from toluene solutions of CdSe nanocrystals in the range of 3–6 nm, produced by traditional wet-chemistry techniques. By using a confocal microscope with laser illumination, three-dimensional raster-scanning and synchronized hyper-spectral detection, we have generated spatial profiles of the fluorescence emission intensity and spectrum. The fluorescence data of the micro-crystals were compared with spectra of individual nanocrystals obtained from the same solution. The results do not support the assertion that these microcrystals consist of CdSe superlattices.

Photocurrent mechanism in a hybrid system of 1-thioglycerol and HgTe quantum dots(QDs) was studied for the first time in the intra-red (IR) range. 1-thioglycerol-capped HgTe QDs were prepared using colloidal method in aqueous solution; the synthesis and size of the HgTe QDs were examined by x-ray diffraction, Raman scattering, and high-resolution transmission electron microscopy. Absorption and photoluminescence spectra of the capped HgTe QDs revealed the strong excitonic peaks in the range from 900 to 1100nm, because of their widened band gap due to the shrinkage of their sizes to about 3 nm. The wavelength dependence of the photocurrent for the hybred system of the 1-thioglycerol and HgTe QDs was very close to that of the absorption spectrum, indicating that charge carriers photoexcited in the HgTe QDs give direct contribution to the photocurrent in the medium of 1-thioglycerol. In this hybrid system, the photo-excited electrons in the HgTe QDs are strongly confined, but the photo-excited holes act as free carriers. Hence, in the photocurrent mechanism of the this hybrid system, only holes among electron-hole pairs created by incident photons in the HgTe QDs are transferred to 1-thioglycerol surrounding HgTe QDs and contribute photocurrent flowing in the medium of 1-thioglycerol.

Time-resolved photoluminescence of single charge tuneable quantum dots allows us to probe the differences in recombination dynamics between neutral and negatively charged excitons. We find that the luminescence decay from a neutral exciton contains a second lifetime component of several nanoseconds that is not present in the luminescence from singly or doubly charged excitons. We attribute the slowly decaying component to excitation cycles in which the initial exciton formed in the dot is dark, with angular momentum M = 2, and which subsequently scatters into the bright state with M = 1.

The nature of the scattering mechanism is revealed by the dependence of the lifetime on the electrical bias applied across the charge-tuneable device. That the lifetime changes by an order of magnitude within a short bias range implies that the dark-to-bright transmutation does not occur through a simple spin flip. Rather it appears to come about by the dot briefly entering a higher energy charging state which allows exchange of the existing electron with another from the n-type contact region. We model the lifetimes and relative intensities of the two decay components using a simple rate equation analysis.

Considerable progress has been made towards creating quantum dot (QD) based nanosensors. The most promising developments have utilized QDs as energy donor in fluorescence resonance energy transfer (FRET) processes. Hybrid QD-protein-dye complexes have been assembled to study FRET, to prototype analyte sensing and even to control or modulate QD photoluminescence. In order to transition the benefits of this technology into the field, QD-based nanosensors will have to be integrated into microtiter wells, flow cells, portable arrays and other portable devices. This proceeding describes two examples of QD-protein-dye assemblies. The first investigates the concepts of FRET applied to QD energy donors and the second describes a prototype biosensor employing QDs. We also introduce the first steps towards implementing surface-tethered QD-bioconjugates, which could potentially serve in the design of solid-state QD-based sensing assemblies.

Although it has long been known that the classical notions of capacitance are altered at the nanoscale, few first principles calculations of these properties exist for real material systems. With a recently developed ab initio formalism, which combines nonequilibrium Greens function techniques with real-space density functional calculations, we have investigated charging effects for carbon nanotube systems, which are described by the capacitance coefficients. Specifically, the capacitance matrix of two nested nanotube armchair nanotubes, the insertion of one nanotube into another, and the properties of a nanotube acting as a probe over a flat aluminum surface were considered.

We report on the photosensitizing properties of optically excited Silicon (Si) nanocrystal assemblies that are employed for an efficient generation of singlet oxygen. Spin triplet state excitons confined in Si nanocrystals transfer their energy to molecular oxygen (MO) adsorbed on the nanocrystal surface. This process results in a strong suppression of the photoluminescence (PL) from the Si nanocrystal assembly and in the excitation of MO from the triplet ground state to singlet excited states. The high efficiency of the energy transfer if favored by a broad energy spectrum of photoexcited excitons, a long triplet exciton lifetime and a highly developed surface area of the nanocrystal assembly. Due to the specifics of the coupled system Si nanocrystal – oxygen molecule all relevant physical parameters describing the photosensitization process are accessible experimentally. This includes the role of resonant and phonon-assisted energy transfer, the dynamics of energy transfer, and its mechanism.

Multi-fold stacks of In0.6Ga0.4As quantum dots embedded into a GaAs matrix were investigated by means of x-ray diffuse scattering. The measurements were done with synchrotron radiation using different diffraction geometries. Data evaluation was based on comparison with simulated distributions of x-ray diffuse scattering. For the samples under consideration ((001) surface) there is no difference in dot extension along [110] and [-110] and no directional ordering. The measurements easily allow the determination of the average indium amount in the wetting layers. Data evaluation by simulation of x-ray diffuse scattering gives an increase of Incontent from the dot bottom to the dot top.

Cu:CdSe nanoparticle alloys were synthesized using inorganic cluster precursor via a lyothermal method. The dots synthesized were approximately 4 nm in size and exhibited doping levels as high as 20% for Cu(I). Cu(I) cations are randomly doped on the Cd-Td site based on a Vegard's law analysis, suggesting that doping arises from a random ion substitution mechanism. The resulting dots exhibited essentially no band edge photoluminescence due to a high level of trap site inclusions in these materials resulting from Se vacancy formation in order to compensate for the Cu+ replacing a Cd2+ ion. Se vacancy formation is evident in the XPS analysis of the Cd:Cu:Se ratios for these materials.

Boron nitride (BN) nanolayers encapsulating iron (Fe) fine particles have been synthesized by annealing mixtures of hematite (α-Fe2O3) and boron powders at 1373 K for 2 hours in nitrogen atmosphere. The Fe particles had an average diameter of ∼300 nm with BN nanolayers coating of ∼10 nm. The α-Fe2O3 was transformed into Fe and then Fe-B on a process of annealing. The Fe-B was decomposed into Fe and BN, and consequently Fe particles coated with BN nanolayers were synthesized. They showed soft magnetic properties with coercivity of 1.5 kA/m. The BN nanolayers encapsulation was effective on improving oxidation resistance. BN nanotubes with diameter of ∼100 nm were also synthesized as a resultant product by this method.

Three kinds of single metal oxide (MnO2, ZnO, and AlO(OH)) and one complex metal oxide (K2O·6TiO2) having nano structure of wire, rod and ribbon were rapidly synthesized by hydrothermal synthesis in supercritical water. Aqueous Mn(NO3)2, Zn(NO)2, Al(NO3)3 solutions and mixtures of TiO2 sols and KOH solutions were used as starting materials, respectively. Syntheses of these nanostructured materials were performed by a flow type apparatus. We investigated the relationship between reaction parameters (temperature, pH and reaction time) and morphologies of the products. Reaction temperatures were 350, 400, and 420 °C. Reaction time is in the range of 1.8 – 116 s. The product was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results show that particle morphologies strongly depend on pH for MnO2 and ZnO. MnO2 nanowires with 10 nm in diameter and ZnO nanorods with 50 nm in diameter were obtained from acidic metal salt solutions. In the case of AlO(OH), temperature and time were key parameters for crystal growth. In the case of K2O6TiO2, larger fibrous particles with 50 nm in diameter were obtained at higher reaction temperature.

We present spectroscopic characterization of InP nanocrystals grown through a fast reaction in a non-coordinating solvent. The photoluminescence (PL) spectra collected from these nanocrystals exhibit a sharp feature associated with the band-edge emission and a broad infrared feature associated with deep level surface trap emission. The emission efficiencies of the as-grown nanocrystals vary between 0.3% and 1% from sample to sample. After undergoing an HF etching process, the emission efficiency increases to 18% and the emission associated with surface states is eliminated from the PL spectrum. Time-resolved photoluminescence (TRPL) experiments conducted at room temperature on the as-grown and HF-etched nanocrystals show that before etching the PL intensity decay is multi-exponential, with a fast (3ns) component independent of wavelength, associated with the non-radiative recombination processes. The etching process effectively eliminates the non-radiative component and the post-etching PL decay can be fitted with a single exponential decay characterized by long (45ns) lifetimes. We tentatively associate these long lifetimes with the recombination of carriers from spin-forbidden states. This assignment is supported by the observation of a significant redshift of the feature associated with band-edge recombination in the PL spectrum with respect to the lowest energy feature in the photoluminescence excitation (PLE) spectrum.

We have demonstrated single dot spectroscopy of InAs/GaAs self-assembled quantum dots embedded in a bias controlled Schottky diode. The photoluminescence spectra exhibit discrete lines depending on bias, which we attribute to the recombination of positively charged, neutral, and negatively charged confined excitons. With excitation directly into the dot, large circular polarization memory is exhibited by the two charged exciton (trion) lines. This indicates long spin lifetimes for both the electron and the heavy hole in the quantum dots.

Photoluminescence (PL) of nanocrystalline Si (nc-Si) assemblies formed by thermal crystallization of amorphous Si/SiO2 and SiO/SiO2 superlattices (SLs) has been investigated at different temperatures and excitation conditions. The low temperature resonant PL spectroscopy reveals phonon-assisted excitonic recombination. At room temperature the samples formed from a-SiO/SiO2 SLs possess relatively high PL quantum yield (∼ 1%). The PL transients have non-exponential decay, which indicates the exciton energy transfer in nc-Si ensembles. The excitonic energy of Er-doped nc-Si SL structures can be almost completely transferred to Er ions incorporated in SiO2 matrix that results in a strong emission line at 0.81 eV.

We have found that traps can dominate important relaxation processes in PbS nanoparticles prepared by precipitation in borosilicate glass. The primary photoluminescence peak energy falls about 50 meV below the lowest absorption peak energy and has a long decay time of ∼4 μs. Photoinduced bleaching of the lowest absorption peak also has a long decay lifetime of ∼2 μs. Photoinduced bleaching also exhibits a surprisingly long rise time of hundreds of nanoseconds. Such long-lifetime effects must be attributed to multiple traps.

We seek to use electrically conducting polymers, such as those commonly utilized in polymeric LEDs, as hosts for silicon nanoparticles. The proper design of multilayered devices based on these materials will yield efficient light-emitters in which charge carriers localize and recombine within the nanoparticles. Furthermore, these may combine the flexibility and processability of polymeric LEDs with the reliability of inorganic materials. We have synthesized luminescent silicon nanoparticles and have characterized their photoluminescence (PL) using continuous-wave and time-resolved spectroscopy. These particles have been incorporated into a variety of transparent solid hosts. The photoluminescence obtained from particle-containing poly(methyl methacrylate) (PMMA) matrices is very similar to that of the particles in solution, both in spectral content and PL decay characteristics. However, when incorporated into a variety of conducting polymers, such as poly(N-vinylcarbazole) (PVK), the nanoparticles do not retain their photoluminescence properties. A variety of chemical species have been reported as effective PL quenchers for porous silicon. We believe that these polymers quench the luminescence through similar mechanisms. Protective passivation of the nanoparticle surface is suggested as a strategy for overcoming this quenching.

The UV-photoluminescence (PL) properties of GaN and ZnO nanocrystallites and nanocrystallite ensembles were studied utilizing micro-photoluminescence. We address the origin of the light emissions of the nanocrystallite as to whether it is due a bandgap or excitonic recombination process. The other topic presented here focuses on the interaction of the laser with a collective of crystallites; we address the phenomena of intensity saturation at a high density of laser excitations as well as the impact of the vacuum state on the PL characteristics. Our analysis indicates that the PL of both GaN and ZnO nanocrysallites is excitonic-like and very similar to the behavior of the free exciton in bulk materials. Additionally, we attribute the intensity saturation of GaN and ZnO to the laser heating and heat trapping which takes place in the enclosure of the nanocrystallite ensemble. In vacuum the PL energy was found to exhibit a strong PL energy redshift relative to the PL in air. We attribute the observed shift to a thermal effect and analyze it in terms of the conditions enabling a convective cooling in the ensemble: the mean free path of air in atmospheric pressure and in vacuum relative to the interparticle separation inside the ensemble.

A scalable, room-temperature technique for controlled synthesis of luminescent II-VI nanocrystals has been developed by using the dispersed phase of stable, well-characterized microemulsions as templates for nanoparticle synthesis. The microemulsions were formed by self-assembly of poly (ethylene oxide)-poly (propylene oxide)-poly (ethylene oxide) (PEO-PPO-PEO) amphiphilic block copolymer and heptane in formamide. By adjusting the surfactant to dispersed phase ratio, stable microemulsions were obtained with droplet diameter of ∼40nm. These microemulsions avoid problems or rapid droplet-droplet coalescence that hamper reverse micelles and lead to polydisperse particle populations. Luminescent ZnSe quantum dots were synthesized by reacting diethylzinc (dissolved in the heptane dispersed phase) with hydrogen selenide gas (diluted in hydrogen). The gas was bubbled through the microemulsion, dissolved in the formamide, and diffused to the nanodroplet interfaces to react with diethylzinc. The experiments indicate that a single nanocrystal is formed in each nanodroplet by coalescence of clusters (nuclei) and smaller crystals. The energy released during coalescence is sufficient to anneal the clusters into high-quality crystals. The process allows precise control of nanocrystal size by adjusting the initial concentration of diethylzinc in heptane. The “as grown” nanocrystals exhibit size-dependent luminescence, narrow and symmetric emission, good monodispersity (confirmed by TEM analysis), and excellent photochemical stability. The technique is currently being extended to the synthesis of CdSe nanocrystals with promising preliminary results.