Temperature-dependent properties of nc-Si thin films synthesized in low-pressure, thermally nonequilibrium, high-density inductively coupled plasmas

Silicon thin films were synthesized simultaneously on single-crystal silicon and glass substrates by lowpressure, thermally nonequilibrium, high-density inductively coupled plasma-assisted chemical vapor deposition from the silane precursor gas without any additional hydrogen dilution in a broad range of substrate temperatures from 100 to 500 °C. The effect of the substrate temperature on the morphological, structural and optical properties of the synthesized silicon thin films is systematically studied by X-ray diffractometry, Raman spectroscopy, UV-vis spectroscopy, and scanning electron microscopy. It is shown that the formation of nanocrystalline silicon (nc-Si) occurs when the substrate temperature is higher than 200 °C and that all the deposited nc-Si films have a preferential growth along the (111) direction. However, the mean grain size of the (111) orientation slightly and gradually decreases while the mean grain size of the (220) orientation shows a monotonous increase with the increased substrate temperature from 200 to 500 °C. It is also found that the crystal volume fraction of the synthesized nc-Si thin films has a maximum value of ∼69.1% at a substrate temperature of 300 rather than 500 °C. This rather unexpected result is interpreted through the interplay of thermokinetic surface diffusion and hydrogen termination effects. Furthermore, we have also shown that with the increased substrate temperature from 100 to 500 °C, the optical bandgap is reduced while the growth rates tend to increase. The maximum rates of change of the optical bandgap and the growth rates occur when the substrate temperature is increased from 400 to 500 °C. These results are highly relevant to the development of photovoltaic thin-film solar cells, thin-film transistors, and flat-panel displays.

Effect of elemental composition and size on electron confinement in self-assembled SiC quantum dots : a combinatorial approach

A high level of control over quantum dot (QD) properties such as size and composition during fabrication is required to precisely tune the eventual electronic properties of the QD. Nanoscale synthesis efforts and theoretical studies of electronic properties are traditionally treated quite separately. In this paper, a combinatorial approach has been taken to relate the process synthesis parameters and the electron confinement properties of the QDs. First, hybrid numerical calculations with different influx parameters for Si1-x Cx QDs were carried out to simulate the changes in carbon content x and size. Second, the ionization energy theory was applied to understand the electronic properties of Si1-x Cx QDs. Third, stoichiometric (x=0.5) silicon carbide QDs were grown by means of inductively coupled plasma-assisted rf magnetron sputtering. Finally, the effect of QD size and elemental composition were then incorporated in the ionization energy theory to explain the evolution of the Si1-x Cx photoluminescence spectra. These results are important for the development of deterministic synthesis approaches of self-assembled nanoscale quantum confinement structures.

Inductively coupled Ar/CH₄/H₂plasmas for low-temperature deposition of ordered carbon nanostructures

The study of inductively coupled Ar/CH 4/H 2 plasmas in the plasma enhanced chemical vapor deposition (PECVD) of self-assembled carbon nanostructures (CN) was presented. A spatially averaged (global) discharge model was developed to study the densities and fluxes of the radical neutrals and charged species, the effective electron temperature, and methane conversion factors under various conditions. It was found that the deposited cation fluxes in the PECVD of CNs generally exceed those of the radical neutrals. The agreement with the optical emission spectroscopy (OES) and quadrupole mass spectrometry (QMS) was also derived through numerical results.

High-rate, low-temperature synthesis of composition controlled hydrogenated amorphous silicon carbide films in low-frequency inductively coupled plasmas

It is commonly believed that in order to synthesize high-quality hydrogenated amorphous silicon carbide (a-Si1-xCx : H) films at competitive deposition rates it is necessary to operate plasma discharges at high power regimes and with heavy hydrogen dilution. Here we report on the fabrication of hydrogenated amorphous silicon carbide films with different carbon contents x (ranging from 0.09 to 0.71) at high deposition rates using inductively coupled plasma (ICP) chemical vapour deposition with no hydrogen dilution and at relatively low power densities (∼0.025 W cm -3) as compared with existing reports. The film growth rate R d peaks at x = 0.09 and x = 0.71, and equals 18 nm min-1 and 17 nm min-1, respectively, which is higher than other existing reports on the fabrication of a-Si1-xCx : H films. The extra carbon atoms for carbon-rich a-Si1-xCx : H samples are incorporated via diamond-like sp3 C-C bonding as deduced by Fourier transform infrared absorption and Raman spectroscopy analyses. The specimens feature a large optical band gap, with the maximum of 3.74 eV obtained at x = 0.71. All the a-Si1-xCx : H samples exhibit low-temperature (77 K) photoluminescence (PL), whereas only the carbon-rich a-Si1-xCx : H samples (x ≥ 0.55) exhibit room-temperature (300 K) PL. Such behaviour is explained by the static disorder model. High film quality in our work can be attributed to the high efficiency of the custom-designed ICP reactor to create reactive radical species required for the film growth. This technique can be used for a broader range of material systems where precise compositional control is required. © 2008 IOP Publishing Ltd.

Catalytic probes with nanostructured surface for gas/discharge diagnostics : a study of a probe signal behaviour

Catalytic probes are used for plasma diagnostics in order to quantify the density of neutral atoms. The probe response primarily depends on the probe material and its surface morphology. Here we report on the design, operation and modelling of the response of niobium pentoxide sensors with a flat and nanowire (NW) surfaces. These sensors were used to detect neutral oxygen atoms in the afterglow region of an inductively coupled rf discharge in oxygen. A very different response of the flat-surface and NW probes to the varying densities of oxygen atoms was explained by modelling heat conduction and taking into account the associated temperature gradients. It was found that the nanostructure probe can measure in a broader range than the flat oxide probe due to an increase in the surface to volume ratio, and the presence of nanostructures which act as a thermal barrier against sensor overheating. These results can be used for the development of the new generation of catalytic probes for gas/discharge diagnostics in a range of industrial and environmental applications.

Tailoring the composition of self-assembled Si1−xCx quantum dots : simulation of plasma/ion-related controls

Precise control of composition and internal structure is essential for a variety of novel technological applications which require highly tailored binary quantum dots (QDs) with predictable optoelectronic and mechanical properties. The delicate balancing act between incoming flux and substrate temperature required for the growth of compositionally graded (Si1-xC x; x varies throughout the internal structure), core-multishell (discrete shells of Si and C or combinations thereof) and selected composition (x set) QDs on low-temperature plasma/ion-flux-exposed Si(100) surfaces is investigated via a hybrid numerical simulation. Incident Si and C ions lead to localized substrate heating and a reduction in surface diffusion activation energy. It is shown that by incorporating ions in the influx, a steady-state composition is reached more quickly (for selected composition QDs) and the composition gradient of a Si1-xCx QD may be fine tuned; additionally (with other deposition conditions remaining the same), larger QDs are obtained on average. It is suggested that ionizing a portion of the influx is another way to control the average size of the QDs, and ultimately, their internal structure. Advantages that can be gained by utilizing plasma/ion-related controls to facilitate the growth of highly tailored, compositionally controlled quantum dots are discussed as well.

Suppression of cluster ions during particle formation events in the atmosphere

Cluster ions and charged and neutral nanoparticle concentrations were monitored using a neutral cluster and air ion spectrometer (NAIS) over a period of one year in Brisbane, Australia. The study yielded 242 complete days of usable data, of which particle formation events were observed on 101 days. Small, intermediate and large ion concentrations were evaluated in real time. In the diurnal cycle, small ion concentration was highest during the second half of the night while large ion concentrations were a maximum during the day. The small ion concentration showed a decrease when the large ion concentration increased. Particle formation was generally followed by a peak in the intermediate ion concentration. The rate of increase of intermediate ions was used as the criteria for identifying particle formation events. Such events were followed by a period of growth to larger sizes and usually occurred between 8 am and 2 pm. Particle formation events were found to be related to the wind direction. The gaseous precursors for the production of secondary particles in the urban environment of Brisbane have been shown to be ammonia and sulfuric acid. During these events, the nanoparticle number concentrations in the size range 1.6 to 42 nm, which were normally lower than 1x104 cm-3, often exceeded 5x104 cm-3 with occasional values over 1x105 cm-3. Cluster ions generally occurred in number concentrations between 300 and 600 cm-3 but decreased significantly to about 200 cm-3 during particle formation events. This was accompanied by an increase in the large ion concentration. We calculated the fraction of nanoparticles that were charged and investigated the occurrence of possible overcharging during particle formation events. Overcharging is defined as the condition where the charged fraction of particles is higher than in charge equilibrium. This can occur when cluster ions attach to neutral particles in the atmosphere, giving rise to larger concentrations of charged particles in the short term. Ion-induced nucleation is one of the mechanisms of particle formation in the atmosphere, and overcharging has previously been considered as an indicator of this process. The possible role of ions in particle formation was investigated.

Steady state genuine multipartite entanglement in harmonic oscillator ensembles with a common environment

Steady state entanglement in ensembles of harmonic oscillators with a common squeezed reservoir is studied. Under certain conditions the ensemble features genuine multipartite entanglement in the steady state. Several analytic results regarding the bipartite and multipartite entanglement properties of the system are derived. We also discuss a possible experimental implementation which may exhibit steady state genuine multipartite entanglement.

A coupled SPH-DEM model for micro-scale structural deformations of plant cells during drying

A single plant cell was modeled with smoothed particle hydrodynamics (SPH) and a discrete element method (DEM) to study the basic micromechanics that govern the cellular structural deformations during drying. This two-dimensional particle-based model consists of two components: a cell fluid model and a cell wall model. The cell fluid was approximated to a highly viscous Newtonian fluid and modeled with SPH. The cell wall was treated as a stiff semi-permeable solid membrane with visco-elastic properties and modeled as a neo-Hookean solid material using a DEM. Compared to existing meshfree particle-based plant cell models, we have specifically introduced cell wall–fluid attraction forces and cell wall bending stiffness effects to address the critical shrinkage characteristics of the plant cells during drying. Also, a moisture domain-based novel approach was used to simulate drying mechanisms within the particle scheme. The model performance was found to be mainly influenced by the particle resolution, initial gap between the outermost fluid particles and wall particles and number of particles in the SPH influence domain. A higher order smoothing kernel was used with adaptive smoothing length to improve the stability and accuracy of the model. Cell deformations at different states of cell dryness were qualitatively and quantitatively compared with microscopic experimental findings on apple cells and a fairly good agreement was observed with some exceptions. The wall–fluid attraction forces and cell wall bending stiffness were found to be significantly improving the model predictions. A detailed sensitivity analysis was also done to further investigate the influence of wall–fluid attraction forces, cell wall bending stiffness, cell wall stiffness and the particle resolution. This novel meshfree based modeling approach is highly applicable for cellular level deformation studies of plant food materials during drying, which characterize large deformations.

Optimisation methods for coupled thermodynamic and 1D design of radial-inflow turbines

Optimisation is a fundamental step in the turbine design process, especially in the development of non-classical designs of radial-inflow turbines working with high-density fluids in low-temperature Organic Rankine Cycles (ORCs). The present work discusses the simultaneous optimisation of the thermodynamic cycle and the one-dimensional design of radial-inflow turbines. In particular, the work describes the integration between a 1D meanline preliminary design code adapted to real gases and the performance estimation approach for radial-inflow turbines in an established ORC cycle analysis procedure. The optimisation approach is split in two distinct loops; the inner operates on the 1D design based on the parameters received from the outer loop, which optimises the thermodynamic cycle. The method uses parameters including brine flow rate, temperature and working fluid, shifting assumptions such as head and flow coefficients into the optimisation routine. The discussed design and optimisation method is then validated against published benchmark cases. Finally, using the same conditions, the coupled optimisation procedure is extended to the preliminary design of a radial-inflow turbine with R143a as working fluid in realistic geothermal conditions and compared against results from commercially-available software RITAL from Concepts-NREC.

Comparative behavior of CdS and CdSe quantum dots in poly(3-hexylthiophene) based nanocomposites

CdS and CdSe nanoparticles have been prepared using conducting poly(3-hexylthiophene) (P3HT) matrix with an objective to understand the effect of nanoparticles on the polymer matrix using electrochemical and spectroscopic techniques. The spectroscopic results reveal that the electronic structure of polymer is strongly influenced by the characteristics of embedded semiconducting nanoparticles. SEM and TEM images show the ordered morphology of the CdS and CdSe nanoparticles in presence of the polymer matrix. Cyclic voltammetry performed both in the presence and absence of light enables us to understand the redox changes in P3HT due to CdS and CdSe quantum dots such as the generation of free radical in the excited state and their electrochemical band gaps.

Zinc oxide quantum dot nanostructures

Zinc oxide (ZnO) is one of the most intensely studied wide band gap semiconductors due to its many desirable properties. This project established new techniques for investigating the hydrodynamic properties of ZnO nanoparticles, their assembly into useful photonic structures, and their multiphoton absorption coefficients for excitation with visible or infrared light rather than ultraviolet light. The methods developed are also applicable to a wide range of nanoparticle samples.

High efficiency quantum dot-sensitised solar cells by material science and device architecture

This thesis studied cadmium sulfide and cadmium selenide quantum dots and their performance as light absorbers in quantum dot-sensitised solar cells. This research has made contributions to the understanding of size dependent photodegradation, passivation and particle growth mechanism of cadmium sulfide quantum dots using SILAR method and the role of ZnSe shell coatings on solar cell performance improvement.