Atomic Force Microscopy (AFM) and Kelvin Probe Microscopy (KPM) are two surface characterization methods suitable for semiconductor nanocrystal applications. In this thesis work, an AFM system with KPM capability was developed and implemented. It was observed that, the effect of electrostatic interaction of the probe cantilever with the sample can be significantly reduced by using higher order resonant modes for Kelvin force detection. Germanium nanocrystals were grown on silicon substrate using different growth conditions. Both characterization methods were applied to the nanocrystal samples. Variation of nanocrystal sizes with varying annealing temperature were observed. Kelvin spectroscopy measurements made on nanocrystal samples using the KPM apparatus displayed charging effects.

Recently extensive efforts have been spent in order to achieve all silicon based photonic devices exploiting the efficient light emission from nanostructured silicon systems. In this thesis, silicon based nanostructures have been investigated for electro-optical and photovoltaic applications. The thesis focused on three application areas of silicon nanostructures: Light emitting diode (LED), light modulation using quantum confined Stark effect (QCSE) and photovoltaic applications. In the context of LED applications, ZnO nanocrystal/silicon heterojunctions were investigated. Contrary to observation of pure ultraviolet photoluminescence (PL) from ZnO nanocrystals that were synthesized through vapor liquid solidification (VLS) method, visible emissions were observed in the electroluminescence (EL) due to defect states of ZnO. The discrepancy between these emissions could be ascribed to both change in excitation mechanisms and the defect formation on ZnO nanocrystals surface during device fabrication steps. EL properties of silicon nanocrystals embedded in SiO2 matrix were also systematically studied with and without Tb doping. Turn-on voltage of the Tb doped LED structures was reduced below 10 V for the first time. Clear observation of QCSE has been demonstrated for the first time in Si nanocrystals embedded in SiO2 through systematic PL measurements under external electric field. Temperature and size dependence of QCSE measurements were consistently supported by our theoretical calculations using linear combination of bulk Bloch bands (LCBB) as the expansion basis. We have managed to modulate the exciton energy as high as 80 meV with field strength below MV/cm. Our study could be a starting point for fabrication of electro-optical modulators in futures for all silicon based photonic applications. In the last part of the thesis, formation kinetics of silicon nanowires arrays using a solution based novel technique called as metal assisted etching (MAE) has been systematically studied over large area silicon wafers. In parametric studies good control over nanowire formation was provided. Silicon nanowires were tested as an antireflective layer for industrial size solar cell applications. It was shown that with further improvements in surface passivation and contact formation, silicon nanowires could be utilized in very efficient silicon solar cells.

As devices and new technologies continue to shrink, nanocrystalline multi-metal compounds are becoming increasingly important for high efficiency and multifunctionality. However, synthetic methods to make desirable nanocrystalline multi-metallics are not yet matured. In response to this deficiency, we have developed several solution-based methods to synthesize nanocrystalline binary alloy and intermetallic compounds. This dissertation describes the processes we have developed, as well as our investigations into the use of lithographically patterned surfaces for template-directed self-assembly of solution dispersible colloids. We used a modified polyol process to synthesize nanocrystalline intermetallics of late transition and main-group metals in the M-Sn, Pt-M', and Co-Sb systems. These compounds are known to have interesting physical properties and as nanocrystalline materials they may be useful for magnetic, thermoelectric, and catalytic applications. While the polyol method is quite general, it is limited to metals that are somewhat easy to reduce. Accordingly, we focused our synthetic efforts on intermetallics comprised of highly electropositive metals. We find that we can react single-metal nanoparticles with zero-valent organometallic Zinc reagents in hot, coordinating amine solvents via a thermal decomposition process to form several intermetallics in the M''-Zn system. Characterization of the single-metal intermediates and final intermetallic products shows a general retention of morphology throughout the reaction, and changes in optical properties are also observed. Following this principle of conversion chemistry, we can employ the high reactivity of nanocrystals to reversibly convert between intermetallic phases within the Pt-Sn system, where PtSn2 ↔ PtSn ↔ Pt3Sn. Our conversion chemistry occurs in solution at temperatures below 300 °C and within 1 hour, highlighting the high reactivity of our nanocrystalline materials compared to the bulk. Some evidence of the generality for this process is also presented. Our nanocrystalline powders are dispersible in solution, and as such are amenable to solution-based processing techniques developed for colloidal dispersions. Accordingly, we have investigated the use of lithographically patterned surfaces to control the self-assembly of colloidal particles. We find that we can rapidly crystallize 2-dimensional building blocks, as well as use epitaxial templates to direct the formation of interesting superlattice structures comprised of a bidisperse population of particles.

The study of crystal nucleation represents a considerable challenge to both experiment and theory. Crystallisation from solutions is initiated by the association of only a few ions. The resulting aggregates are the embryonic precursors to crystals and exhibit diameters of less than a nanometre. While experimental studies offer a wide variety of insights at the macroscopic scale, the atomistic level of detail often remains elusive. On the other hand, computer simulation approaches may easily achieve microscopic resolution and hence appear particularly suited for analysis of the mechanisms of ion aggregation. On the basis of atomistic models, new insights are obtained into the early steps of ion association and the self-organisation of disordered aggregates into crystalline structures. / Das Studium der Nukleation von Kristallen stellt eine immense Herausforderung sowohl an die Experimentatoren als auch an die Theoretiker dar. Die Bildung eines Kristalls aus einer Lösung beginnt mit dem Zusammenschluss einzelner Ionen zu kleinen Aggregaten. Diese embryonalen Vorstufen von Kristallen umfassen nur einige Teilchen und weisen Durchmesser von weniger als einem Nanometer auf. Experimentelle Untersuchungen sind oftmals auf die makro- und mesoskopische Größenskala beschränkt und können vergleichsweise wenige Informationen über die atomaren Aggregationsprozesse liefern. Molekulare Simulationen verlaufen im Gegensatz dazu unmittelbar auf der atomaren Detailstufe und stellen so eine hervorragende Ergänzung zum Experiment dar. Im Computer werden dabei Modellszenarien entwickelt, die Aufschlüsse über die elementaren Schritte der Aggregation von Ionen geben können und aufzeigen, wie sich zunächst ungeordnete Agglomerate allmählich zu periodisch geordneten Strukturen organisieren.

Thin film copper indium gallium selenide (CIGS) solar cells have exhibited single junction power conversion efficiencies above 20% and have been commercialized. The large scale production of CIGS solar cells, however, is hampered by the relatively high cost and poor stoichiometric control of coevaporating tertiary and quaternary semiconductors in high vacuum. To reduce the overall cost of production, CIGS nanocrystals with predetermined stoichiometry and crystal phase were synthesized in solution. Colloidal nanocrystals of CIGS provide a novel route for production of electronic devices. Colloidal nanocrystals combine the well understood device physics of inorganic crystalline semiconductors with the solution processability of amorphous organic semiconductors. This approach reduces the overall cost of CIGS manufacturing and can be used to fabricate solar cells on flexible and light-weight plastic substrates. As deposited CIGS nanocrystal solar cells were fabricated by ambient spray-deposition. Devices with efficiencies of 3.1% under AM1.5 illumination were fabricated. Examining the external and internal quantum efficiency spectrums of the devices reveal that in nanocrystal devices only the space charge region is actively contributing to the extracted photocurrent. The device efficiency of the as-deposited nanocrystal films is presently limited by the small crystalline grains (≈ 15 nm) in the absorber layer and the relatively large interparticle spacing due to the organic capping ligands on the nanocrystal surfaces. Small grains and large interparticle spacing limits high density extraction of electrons and holes from the nanocrystal film. A Mott-Schottky estimation of the space charge region reveals that only 50 nm depth of the nanocrystalline absorber is effectively contributing to the photogenerated current. One strategy to improve charge collection involves increased space charge region for extraction by vertical stacking of diodes. A much longer absorption path for the photons exists in the space charge region with the stacked devices, increasing the probability that the incident radiation is absorbed and then extracted. This method enables an increase in the collected short circuit current. The overall device efficiency, however, suffers with the increased series resistance and shunt conductance of the device. Growth of nanocrystal grains was deemed necessary to achieve power conversion efficiencies comparable to vapor deposited CIGS films. Simple thermal treatment of the nanocrystal layers did not contribute to the growth of the crystalline grain size. At the same time, because of the loss of selenium and increased trap density in the absorber layer, there was a measurable decrease in device efficiency with thermal processing. For increased grain size, the thermal treatment of the absorber layer took place in presence of compensating amounts of selenium vapor. The process of selenization, as it is called, took place at 500°C in a graphite box and led to an increase of the grain size from 15 nm to several microns in diameter. Devices with the increased grain size yielded efficiencies up to 5.1% under AM1.5 radiation. Mott-Schottky analysis of the selenized films revealed a reduction in doping density and a comparable increase in the space-charge region depth with the increased grain size. The increased collection combined with the much higher carrier mobility in the larger grains led to achieved Jsc values greater than 20 mA/cm2. Light beam induced current microscopy (LBIC) maps of the devices with selenized absorber layers revealed significant heterogeneity in photogenerated current. Distribution of current hotspots in the film corresponded with highly selenized regions of the absorber films. In an effort to improve the overall device efficiency, improvements in the selenization process are necessary. It was determined that the selenization procedure is dependent on the selenization temperature and processing environment. Meanwhile, the reactor geometry and nanocrystal inks composition played important roles in determining selenized film morphology and the resulting device efficiency. Further work is necessary to optimize all the parameters to improve device efficiency even further. / text

Semiconductor nanocrystal-based photovoltaics are an interesting new technology with the potential to achieve high efficiencies at low cost. CuInSe2 nanocrystals have been synthesized in solution using arrested precipitation and dispersed in solvent to form a “solar ink”. The inks have been deposited under ambient conditions to fabricate photovoltaic devices with efficiency up to 3%. Despite the low cost spray coating deposition technique, device efficiencies remain too low for commercialization. Higher efficiencies up to 7% have been achieved using a high temperature selenization process, but this process is too expensive. New nanocrystal film treatment processes are necessary which can improve the device efficiency at low cost. To this end, CuInSe2 nanocrystals were synthesized using a diphenyl phosphine:Se precursor which allows for precise control over the nanocrystal size. The size is controlled by changing the temperature of the reaction. The smallest size nanocrystals demonstrated extremely high device open circuit voltage. Ligand exchange procedures were used to replace the insulating oleylamine capping ligand used during synthesis with more conductive halide ions or inorganic chalcogenidometallate cluster (ChaM) ligands. These ligands led to improved charge transport in the nanocrystal films. A high-intensity pulsed light processing technique known as photonic curing was used which allows for high temperature sintering of nanocrystal films on temperature-sensitive substrates. High energy pulses cause the nanocrystals to sinter into large grains, primarily through melting and resolidification. The choice of metal back contact has a dramatic effect on the final film morphology, with Au and MoSe2 back contacts providing much better adhesion with the CuInSe2 than Mo back contacts. Nanocrystal sintering without melting can be achieved by replacing the oleylamine ligands with ChaM ligands prior to photonic curing. Low energy photonic curing pulses vaporize the oleylamine ligands without inducing sintering or grain growth. This greatly improved nanocrystal coupling and interparticle charge transport. Multiexcitons were successfully extracted from these nanocrystal films and external quantum efficiencies over 100% were observed. Transient absorption spectroscopy was used to study the multiexciton generation process in CuInSe2 nanocrystal films and colloidal suspensions. The multiexciton generation efficiency, threshold, and Auger lifetimes for CuInSe2 compare well with other nanocrystal materials. / text

X-ray crystallography is a form of microscopy that allows the three-dimensional arrangement of atoms belonging to molecules within crystals to be determined. In this method, a crystal is illuminated with a beam of X-rays and the diffracted amplitudes resulting from the illumination are measured and computationally processed to enable the electron density of the unit molecule, or the unit cell, constituting the crystal to be calculated. The recent development of the X-ray free-electron laser (XFEL) provides new routes for determining molecular structures via its ability to generate intense but brief X-ray pulses. These new instruments enable diffraction measurements to be obtained from crystals that have a small number of unit cells, referred to as nanocrystals, and molecular structure determination via this technique is known as serial femtosecond nanocrystallography (SFX). This thesis is concerned with the characterisation of diffraction data obtained from SFX experiments and the techniques for reconstructing the electron density of the molecule from such data. The noise characteristics of diffraction measurements from nanocrystals is developed. Methods for directly inverting nanocrystal diffraction to obtain the electron density of the molecule are analysed and an approach to ameliorate the effect of noise is proposed and evaluated by simulation. A model for diffraction by nanocrystals that include the effects of different unit cell arrangements and incomplete unit cells on the crystal surface is also developed and explored by simulation. The diffraction by finite crystals is shown to be equal to the incoherent average over a set of unit cells that contain different molecular arrangements related to the symmetry of the crystal at hand. The problem of image reconstruction under this circumstance is investigated. The more general problem of reconstructing multiple, unrelated, objects from their averaged diffraction is also explored and uniqueness properties along with reconstruction algorithms developed. The problem of reconstructing multiple, related, unit cells is studied and preliminary results are obtained. These results show that iterative phase retrieval algorithms can in principle be adapted to reconstruct the electron density of a crystalline specimen from the data obtained in SFX and the retrieval of phases from the diffracted intensity averaged over multiple objects is feasible.

In this study, we used ion implantation technique to synthesize semiconductor (Ge, Si) nanocrystals in SiO2 matrix. Ge and Si nanocrystals have been successfully formed by Ge and Si implantation and post annealing. Implanted samples were examined by characterization techniques such as TEM, XPS, EDS, SAD, SIMS, PL, Raman and FTIR spectroscopy and the presence of Ge and Si nanocrystals in the SiO2 matrix has been evidenced by these measurements. It was shown that implantation dose, implantation energy, annealing temperature, annealing time and annealing ambient are important parameters for the formation and evolution of semiconductor nanocrystals embedded in SiO2 matrix. The size and size distribution of Ge and Si nanocrystals were estimated successfully by fitting Raman and PL spectra obtained from Ge and Si implanted samples, respectively. It was demonstrated that Si implanted and post annealed samples exhibit two broad PL peaks at &amp / #8764 / 625 and 850 nm, even at room temperature. Origin of these peaks was investigated by temperature, excitation power and excitation wavelength dependence of PL spectrum and etch-measure experiments and it was shown that the peak observed at &amp / #8764 / 625 nm is related with defects (clusters or chain of Si located near the surface) while the other is related to the Si nanocrystals. As an expected effect of quantum size phenomenon, the peak observed at &amp / #8764 / 850 nm was found to depend on the nanocrystal size. Finally, the formation and evolution of Ge and Si nanocrystals were monitored by FTIR spectroscopy and it was shown that the deformation in SiO2 matrix caused by ion implantation tends to recover itself much quicker in the case of the Ge implantation. This is a result of effective segregation of Ge atoms at relatively low temperatures.

The evolution of the MOSFET technology has been driven by the aggressive shrinkage of the device size to improve the device performance and to increase the circuit density. Currently, many research demonstrated that the continuous polycrystalline silicon film in the floating-gate dielectric could be replaced with nanocrystal (nc) embedded high-k thin film to minimize the charge loss due to the defective thin tunnel dielectric layer. This research deals with both the statistical aspect of reliability and electrical aspect of reliability characterization as well. In this study, the Zr-doped HfO2 (ZrHfO) high-k MOS capacitors, which separately contain the nanocrystalline zinc oxide (nc-ZnO), silicon (nc-Si), Indium Tin Oxide (nc-ITO) and ruthenium (nc-Ru) are studied on their memory properties, charge transportation mechanism, ramp-relax test, accelerated life tests, failure rate estimation and thermal effect on the above reliability properties. C-V hysteresis result show that the amount of charges trapped in nanocrystal embedded films is in the order of nc-ZnO>nc-Ru>nc-Si~nc-ITO, which might probably be influenced by the EOT of each sample. In addition, all the results show that the nc-ZnO embedded ZrHfO non-volatile memory capacitor has the best memory property and reliability. In this study, the optimal burn-in time for this kind of device has been also investigated with nonparametric Bayesian analysis. The results show the optimal burn-in period for nc-ZnO embedded high-k device is 5470s with the maximum one-year mission reliability.

Neutral gas-phase metal-carbide clusters are generated by laser ablation and are detected in the constructed time-of-flight mass-spectrometer by laser ionisation. Photo-ionisation efficiency (PIE) experiments are performed on the metal-carbide clusters to determine their ionisation potentials (IPs). Complimentary density functional theory (DFT) calculations are performed on the energetically favorable structural isomers of the metalcarbide clusters. Comparison between the calculated IPs of the isomers and the experimental IP allows the carrier of the observed ionisation onset for a metal-carbide cluster to be assigned. The niobium-carbide clusters Nb₃Cy (y = 0–4), Nb₄Cy (y = 0–6) and Nb₅Cy (y = 0–6) are examined by PIE experiments and DFT calculations. The IPs of the niobium-carbide clusters are found to be either left reasonably unchanged from the IPs of the bare metal clusters or moderately reduced. The clusters Nb₃C₂, Nb₄C₄, Nb₅C₂ and Nb₅C₃ display the largest IP reductions for their corresponding cluster series. The structures assigned to the IPs of the Nb₃Cy (y = 1–3) clusters are based on the carbon atoms attaching to the niobium faces and/or niobium-niobium edges of the triangular Nb₃ cluster. However, for Nb₃C₄ the ionisation onset is assigned to a low-lying isomer, which contains a molecular C₂ unit, rather than the lowest energy isomer, a niobium atom deficient 2×2×2 face-centred cubic (fcc) nanocrystal structure. The structures assigned to the IPs of the Nb₄Cy (y = 1–4) clusters are based on the carbon atoms attaching in turn to the niobium faces of the tetrahedral Nb₄ cluster, developing a 2×2×2 fcc nanocrystal structure for Nb₄C₄. For Nb₄C₃ two ionisation onsets are observed; one weak onset at low energy and another more intense onset at high energy. It is proposed that the two onsets are due to ionisation from both a metastable ³A₁ state and the ground ¹A₁ state of the lowest energy isomer. The ionisation onsets of Nb₄C₅ and Nb₄C₆ are also proposed to originate from metastable triplet states of the lowest energy isomers, with the transitions from the ground singlet states calculated to be greater than the highest achievable photon energy in the laboratory. The structures of Nb₄C₅ and Nb₄C₆ have one and two carbon atoms in a 2×2×2 fcc nanocrystal substituted with molecular C₂ units, respectively. The structures assigned to the IPs of the Nb₅Cy (y = 1–6) clusters are based on the underlying Nb₅ cluster being in either a “prolate” or “oblate” trigonal bipyramid geometry; the former has six niobium faces available for carbon addition, while the latter has two niobium butterfly motifs and two niobium faces available for carbon addition. Both the structures of Nb₅C₅ and Nb₅C₆ have the underlying Nb₅ cluster in the oblate trigonal bipyramid geometry and contain one and two molecular C₂ units, respectively. The tantalum-carbide clusters Ta₃Cy (y = 0–3), Ta₄Cy (y = 0–4) and Ta₅Cy (y = 0–6) are examined by PIE experiments and DFT calculations. The IPs of the tantalum-carbide clusters in each series show trends that are very similar to the corresponding iso-valent niobium-carbide cluster series, although the IP reductions upon carbon addition are smaller for the former. For the vast majority of tantalum-carbide clusters, the same structural isomer is assigned to the ionisation onset as that assigned for the corresponding niobium-carbide cluster. Bimetallic tantalum-zirconium-carbide clusters are generated using a constructed double ablation cluster source. The Ta₃ZrCy (y = 0–4) clusters are examined by PIE experiments and DFT calculations. The IP trend for the Ta₃ZrCy cluster series is reasonably similar to that of the Ta₄Cy cluster series, although the IP reductions upon carbon addition are greater for the former. The structures assigned to the IPs of the Ta₃ZrCy (y = 1–4) clusters are based on the carbon atoms attaching in turn to the metal faces of the tetrahedral Ta₃Zr cluster. In summary, the work presented in this thesis demonstrates that the structures of metalcarbide clusters can be inferred by the determination of their IPs through PIE experiments in combination with DFT calculations on candidate structural isomers. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1347219 / Thesis (Ph.D.) - University of Adelaide, School of Chemistry and Physics, 2008