Properties of nanocrystals are extremely sensitive to their sizes when their sizes are smaller or of the order of the excitonic diameter due to the quantum confinement effect. The interest in this field has been concentrated basically in understanding the size-property relations of nanocrystals, for example, the pronounced variation in the bandgap of the material or the fluorescence emission properties, by tuning the sizes of the nanocrystals. Thus, the optical and electronic properties of semiconductor nanocrystals can be tailor-made to suit the needs of the specific application and hence is of immense importance. One of the major aspects necessary for the actual realization of the various applications is the ability to synthesize nanocrystals of the required size with a controlled size distribution. The growing demand to obtain such nanocrystals with the required size and controlled size distribution is met largely by the solution route synthesis of nanocrystals, that constitutes an important class of synthesis methods due to their ease of implementation and the high degree of flexibility. The main difficulty of this method is that the dependence of the average size and the size distribution of the generated particles on parameters of the reaction are not understood in detail and therefore, the optimal reaction conditions are arrived at essentially in an empirical and intuitive manner. From a fundamental point of view, understanding the growth kinetics of various nanocrystals can provide a deeper insight into the phenomena. The study of growth kinetics can be experimentally achieved by measuring the time evolution of diameter using several in-situ techniques like UV-absorption and small angle X-ray scattering. Having understood the mechanism of growth of nanocrystals, it is possible to obtain the required size of the nanocrystal using optimized synthesis conditions. The properties of these high quality nanocrystals can be further tuned by doping with a small percentage of magnetic ions. The optical and magnetic properties of these nanocrystals play an important role in the various technological applications. The first part of the thesis concentrates on the theoretical methods to study the electronic structure of semiconductor nanocrystals. The second part describes the studies performed on growth of various nanocrystal systems, both in the presence and absence of capping agents. The third part of the thesis describes the studies carried out on ZnO and doped ZnO nanocrystals, synthesized using optimal conditions that were obtained in the earlier part of the thesis. The thesis is divided into five chapters which are described below. Chapter 1 provides a brief overall perspective of various interesting properties of semiconductor nanocrystals, including various concepts relevant for the study of such systems. Chapter 2 describes experimental and theoretical methods used for the study of nanocrystals reported in this thesis. In Chapter 3 of this thesis, we report results of theoretical studies carried out on III-V and II-VI semiconductors using the tight-binding (TB) methodology. Chapter 4 presents the investigations on the growth kinetics of several nanocrystal systems. Chapter 5 presents experimental investigations carried out on undoped and various transition metal (TM) doped ZnO nanocrystals. In summary, we have performed electronic structure calculations on various nanocrystal systems, devised a novel method to obtain the size distribution from UV-absorption spectrum and studied the mechanism of growth in the presence and absence of capping agents in various II-VI semiconductors. Using the optimal conditions obtained from the growth studies, we prepare high quality ZnO nanocrystals of required size, both in free-standing and capped states and doped it with small percentages of various transition metals like Mn, Cu and Fe. We have then studied their optical and magnetic properties.

Semiconductor nanocrystals have immense potential to make an impact in consumer products due to their narrow, tunable emission linewidths. One factor limiting their use is the ease and reproducibility of core/shell nanocrystal syntheses. This thesis aims to address this issue by providing chemical control over the formation of core/shell nanostructures by replacing engineering controls with kinetic controls. Chapter 1 contextualizes our study on nanoparticle synthesis with a brief discussion on the physics of quantum confinement and the importance of narrow size dispersities, core/shell band alignments, and low lattice mismatches and strain at core/shell nanocrystal interfaces. Next, the evolution of cadmium chalcogenide nanocrystal reagents is described, ranging from the original organometallic reagents used in the 1980s to modern approaches involving cadmium phosphonates and carboxylates. This is followed by a description of chalcogen precursors, highlighting the recent introduction of molecules whose well-controlled and tunable reaction rates allow for the size tuning of nanocrystals at 100% yield, and accompanying theories on nanocrystal nucleation. Chapter 2 covers work to expand the library of available sulfur precursors to a wider range of molecules relevant for the synthesis of cadmium sulfide nanocrystals. Using thioureas alone, only very fast or very slow precursor conversion rates can be accessed. This limits the accessible sizes of cadmium sulfide nanocrystals using a single hot injection of precursor at standardized reaction conditions. We observe that thiocarbonate and thiocarbamate precursors with varying electronic substituents allow access to intermediate precursor conversion rates and cadmium sulfide nanocrystal sizes. Interestingly, we note that these new precursor classes nucleate particles with higher monodispersity than ones synthesized from thioureas. These results indicate that in addition to precursor structure controlling precursor conversion rate, precursor structure additionally impacts nanocrystal monodispersity. Chapter 3 expands the library of sulfur and selenium precursors to include cyclic thiones and selenones which extends chemical control of precursor conversion kinetics to cover five orders of magnitude. This unprecedented breadth of rate control allows for the simultaneous hot injection of multiple precursors to generate core/shell or alloyed nanoparticles using precursor reactivity. Using this new synthetic strategy, we observe that kinetic control runs into several issues which we partially attribute to differences in cadmium sulfide and cadmium selenide critical concentrations and growth rates. Nevertheless, combined with a syringe pump shelling method, we are able to access core/shell and alloyed nanocrystals with photoluminescence quantum yields of 67-81%. Chapter 4 applies the concept of nanostructure control via precursor conversion kinetics to a better model system: two-dimensional nanoplatelets. Cadmium chalcogenide nanoplatelets are highly desirable materials due to their exceptionally narrow emission full width half max (FWHM) values which make them pure emitters relative to quantum dots or organic dyes. We synthesize 3 monolayer thick nanoplatelets whose lateral dimensions vary from 10 nm x 10 nm to 186 x 100 nm and demonstrate compositional control on the smallest platelet sizes with STEM EELS.

In recently years, nonvolatile memory with nanocrystals cell have widely applied to overcome the issue of operation and reliability for conventional floating gate memory. The excellent electrical characteristics of memory device need good endurance, long retention time and small operation voltage. Among numerous memory devices with nanocrystals, the memory device with metal nanocrystals was widely researched. It will be new candidate for flash memory. The advantages of metal nanocrystals has have higher density of states around Fermi level, stronger coupling with conduction channel, wide range of available work functions and smaller energy perturbation due to carrier confinement. So metal nanocrystals can reduce operate voltage, and increase write/erase speed and endurance. Most important of all, we can control the sizes of nanocrystals dot and manufacture at low temperature¡CThis advantage can apply to thin film transistor liquid crystal display; it fabricates driving IC and logical IC on panel for diverseness and adds memory beside switch TFT as image storage to reduce power consumption for portability. In this thesis, we will discuss metal nanocrystals as memory storage medium. And we can use high temperature oxidation, low temperature annealing with oxygen to form nanocrystals. Besides we analyze the effect of electron storage at metal nanocrystals by means of material and electrical analysis.

This research emphasizes on the use of seeded growth in synthesis of noble metal nanocrystals with precise control over the size, shape, and composition. In the first part of this work, I have produced Au nanocrystals with single-crystal structure and truly spherical profiles and investigated their optical properties and self-assembly as induced by dilution with water. These Au nanospheres were generated in high yield and purity, together with controllable sizes continually increased from 5 to 150 nm. I also found these Au nanospheres self-assembled into dimers, larger aggregates, and wavy nanowires, respectively, as diluted with water. In the second part of this work, I demonstrate the kinetic control can be implemented to control the shape of mono- and bi-metallic nanocrystals in seeded growth. The as-prepared single-crystal nanospheres of Au were employed as seeds to synthesize of tetrahedral Au nanocrystals and Au@Pd core-shell nanocrystals with six distinct shapes. The success of the two demonstrations relies on manipulation of reaction kinetics to achieve different product shapes. The reaction kinetics was controlled by varying a set of reaction parameters, including the type and concentration of capping agent, the amount of reductant, and the injection rate of metal precursor solution. In the final part of this work, I will discuss an unusual change in crystallinity observed in seeded growth of Au nanocrystals on Au seeds. In particular, single-crystal Au seeds treated with a chemical species could develop twin defects during the seed-mediated growth process to yield multiply twinned products.

The application and efficiency of optoelectronic devices depends on the ability to control the absorption and emission processes of photons in semiconductors. This thesis looks at three different applications of spectral management across a broad range of optoelectronic devices: photovoltaics (PVs), luminescent solar concentrators (LSCs) and light-emitting diodes (LEDs). Multiple excitation generation (MEG) – a process in which multiple charge-carrier pairs are generated from a single optical excitation - is a promising way to improve the photocurrent in photovoltaic devices and offers the potential to break the Shockley-Queisser limit. Here we present solar cells fabricated from PbSe nanorods which show external quantum efficiencies exceeding 100 %. This demonstrates the potential for substantial improvements in PV device performance due to MEG. Through spatial and spectral concentration, LSCs have the potential to reduce the cost of photovoltaic energy production and are attractive prospects for photobioreactors and building-integrated applications. Here we introduce versatile star-shaped donor-acceptor molecules based on a central BODIPY acceptor with oligofluorene donor side units. We perform comprehensive device measurements and Monte Carlo ray tracing simulations of LSCs. We find that the measured structures permit waveguide propagation lengths on a par with state-of-the-art nanocrystalline emitters, while proposed hypothetical structures can be seen as viable candidates for photobioreactor and energy production roles and should be synthesized. The efficiency of nanocrystal-based LEDs is inherently limited by the types of crystals used. Cesium lead halide perovskite nanocrystals exhibit photoluminescence quantum efficiencies approaching 100%. However, due to the large surface areas and anion mobility halogen exchange between perovskite nanocrystals of different compositions occurs rapidly, limiting applications. Here, we report significantly reduced halide exchange between chloride and iodide CsPbX3 (X= Cl, I) perovskite nanocrystals. We investigate perovskite-based multi-crystal component samples and their resulting optical and electrical interactions in bulk heterojunction LEDs. Efficient photon reabsorption from CsPbCl3 to CsPbI3 nanocrystals was found to improve LED device performance.

The ability to grow materials in the nanometric size regime with controlled shape and size provide a fundamental synthetic challenge, while allowing for evaluation of such unique nanostructures in multiple applications. In this dissertation, colloidal sodium lanthanide fluoride (NaLnF4) nanocrystals are described with an overall emphasis on i) size control, ii) surface chemistry related towards their applications as nano-bioprobes, and iii) the synthesis and fundamental aspects of epitaxial layer growth generally referred as core-shell nanocrystals. Chapter 1 provides a brief overview on the basic aspects of colloidal nanocrystals. In Chapter 2, synthesis and surface modification of colloidal sodium lanthanide fluoride nanocrystals, epitaxial growth, and their applications in optical and magnetic resonance imaging is reviewed. Chapter 3 describes a phase transfer protocol utilizing polyvinylpyrrolidone and subsequent silica coating of initially hydrophobic upconverting nanocrystals. This protocol is extended in Chapter 4 using end-group functionalized polyvinylpyrrolidone and demonstrates tunability of surface charge and functional groups on upconverting nanocrystals for targeted labeling of human prostate cancer cells. The synthesis of size-tunable NaGdF4 nanocrystals below 10 nm is described in Chapter 5. These nanocrystals are evaluated for their efficacy in magnetic resonance imaging (MRI), and a fundamental insight into the effect of surface gadolinium ions in T1 MRI contrast enhancement is presented. Chapter 6 demonstrates the synthesis of tunable, epitaxial layers on upconverting (core) nanocrystals. A novel synthetic strategy is demonstrated, by deliberate defocusing and self-focusing of differently sized nanocrystals driven by the common physical phenomenon of Ostwald ripening. Utilizing the contraction of lanthanide ions along the series, a fundamental investigation on the effect of compressive/tensile strain epitaxial layer growth is presented in Chapter 7. The fundamental rule of minimal lattice mismatch for epitaxial growth takes into account only the magnitude of mismatch and not the sign of mismatch caused by a compressive/tensile strained layer. A strong asymmetric effect between the compressive/tensile layer growth given the same magnitude of lattice mismatch is observed, demonstrating the necessity of including the sign of mismatch to generate isotropic (conformal)/ pseudomorphic (coherent) epitaxial growth. Finally, in Chapter 8 conclusions and possible future work are discussed. / Graduate / 0494

Assembly of noble metal nanocrystals gives rise to extraordinary plasmonic properties that are distinct from those of isolated ones. We have prepared clusters that are composed of two-dimensionally-ordered gold nanocubes on flat substrates and investigated their plasmonic properties. It is found that the plasmon resonances of the nanocube clusters are highly dependent on both the number and ordering of the nanocubes in the clusters. FDTD calculations reveal that the rich plasmon modes in the clusters originate from the interparticle couplings in the cluster and the couplings between the entire clusters and the substrate. / I believe that my research work has provided an in-depth fundamental understanding of the localized surface plasmon resonances of gold nanocrystals and will have a number of implications for the applications of metallic nanostructures in optics, optoelectronics, and biotechnology. / Noble metal nanocrystals have attracted much interest due to their rich optical properties, which arise from the localized surface plasmon resonances, the collective oscillations of free electrons confined on the nanoscale. Under resonant excitation by light, noble metal nanocrystals exhibit extremely large light scattering and absorption, as well as large near-field enhancements. These fascinating properties bring about a variety of applications, including plasmonic sensing, plasmonic waveguiding, surface-enhanced Raman scattering, plasmon-enhanced fluorescence, photothermal cancer therapy, and plasmonic-enhanced energy harvesting. Among various noble metal nanocrystals, gold nanocrystals exhibit high chemical stability and large biological compatibility. Moreover, their plasmon resonance wavelengths can be synthetically tuned from the visible to near infrared spectral regions. In this thesis, a systematic study on the localized surface plasmon resonances of gold nanocrystals is presented, both experimentally and theoretically. / Photothermal conversion of gold nanocrystals can be applied in the areas of photothermal polymerization, photothermal imaging, drug release from capsules, and photothermal therapy. We have investigated the photothermal conversion properties of different gold nanocrystals and their composites. The studies show that the plasmon wavelength, particle volume, shell coating, and assembly of gold nanocrystals all play important roles in their photothermal conversion efficiency. / The refractive index sensitivity of gold nanocrystals is a key factor in their practical sensing applications. I will first introduce the systematic studies on the dependence of the index sensitivity on the shapes and sizes of gold nanocrystals that have varying plasmon resonance wavelengths. The index sensitivity has been found to generally increase as the plasmon resonance wavelength for a fixed nanocrystal shape becomes longer and as the curvature of the nanocrystals gets larger. I have further studied the dependence of the index sensitivity on the different shapes of gold nanocrystals that have the same longitudinal plasmon resonance wavelength. The refractive index sensitivities have been found to vary with the nanocrystal shape. Finite-difference time-domain (FDTD) calculations have been performed on these nanocrystals to reveal the origin of this dependence. A linear relationship is found between the index sensitivity and the product of the electric polarizability with the curvature. On the basis of these studies, a novel plasmonic optical fiber device has further been fabricated to detect small changes in the local dielectric environment. / When fabricating plasmonic devices, such as waveguides, optical switches, plasmonic sensors, and plasmon-enhanced solar cells, one needs to attach metal nanocrystals onto different substrates. The interactions between gold nanocrystals and the substrates can strongly modify the plasmonic responses of the nanocrystals and therefore need to be taken into account when designing of various plasmonic devices. We have further investigated the coupling between gold nanocrystals and substrates with different dielectric properties, including insulating, semiconducting, and metallic ones. It is found that the substrates play an important role in both the scattering patterns and scattering spectra of the supported gold nanocrystals. Specifically, Fano-type resonances can be observed for large nanocrystals sitting on silicon substrates that have a large dielectric constant. / Chen, Huanjun. / Adviser: Jianfang Wang. / Source: Dissertation Abstracts International, Volume: 72-04, Section: B, page: . / Thesis (Ph.D.)--Chinese University of Hong Kong, 2010. / Includes bibliographical references. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. Ann Arbor, MI : ProQuest Information and Learning Company, [200-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract also in Chinese.

Silicon nanocrystals embedded in silicon nitride have received attention as promising materials for optoelectronic applications. More specifically, band gap engineering of novel materials based on silicon nanocrystals has been proposed for possible application in an all-silicon tandem solar cell within the field of `third generation' photovoltaics. Such an application would require nanocrystals to exhibit quantum confinement whereby the optical and electrical properties of a film could be tuned by controlling the size of these `quantum dots'. This thesis investigates the correlation between the structural and optical properties of silicon nanocrystals grown in silicon nitride multilayer structures via solid phase crystallisation, as part of an experimental investigation into quantum confinement. A study of the relevant processing parameters for the solid phase crystallization of silicon nanocrystals in amorphous silicon nitride is presented and the effectiveness of the multilayer approach for controlling nanocrystal size is demonstrated. Structural characterisation using transmission electron microscopy and glancing incidence x-ray diffraction is complemented with a new application of Fourier transform infrared spectroscopy for the detection of silicon nanocrystals. A case study on the effects of annealing temperature on the photoluminescence from silicon nitride multilayers is presented. While a clear correlation between the structural, molecular and optical properties is demonstrated, evidence of quantum confinement remains ambiguous. The investigation into the limits of parameter space for the formation of silicon nanocrystals in silicon nitride multilayers also leads to the formation of a novel Si-Si3N4 nanocomposite material. A comprehensive study of the photoluminescence from silicon nanocrystals embedded in nitride is presented in the context of homogeneous and multilayer nitride films. Size dependent PL and absorption is demonstrated for silicon nitride multilayers with silicon-rich silicon nitride layer thicknesses varying from 1 to 4.5 nm, indicating the formation of quantum wells. These same structures are annealed to form arrays of silicon nanocrystals. Although the PL and absorption spectra suggest quantum effects, inherent ambiguities remain. The findings in this thesis provide greater insight into the nature of confinement and indicate the need for further research if the successful implementation of these structures into an all silicon tandem cell is to be achieved.

This thesis presents results on the applications and limitations of plasma enhanced chemical vapour deposition for silicon-based photonics, with an emphasis on optical microcavities for the control of light emission from silicon nanocrystals. ¶ Silicon nanocrystals were formed by precipitation and growth within Si-rich oxide layers (SiOx) deposited by plasma enhanced chemical vapour deposition. The films were found to exhibit strong room temperature photoluminescence, with the optimum emission depending on the composition and processing of the films. The strongest emission was achieved for films with a silicon content of ~40%, following hydrogen passivation. Hydrogen was introduced into the samples by two different methods: by annealing in forming gas (95% N2: 5% H2) or by annealing with a hydrogenated silicon nitride capping layer. Both methods caused an increase in photoluminescence intensity due to the passivation of defects. In contrast, the presence of low levels of iron and gold were shown to reduce the concentration of luminescent nanocrystals due to the creation of non-radiative centres. ¶ Optical microcavity structures containing silicon nanocrystals were also fabricated by Plasma enhanced chemical vapour deposition, using silicon dioxide, silicon nitride and silicon-rich oxide layers. The microcavities consisted of a silicon-rich oxide layer between two distributed Bragg reflectors formed of alternating silicon dioxide/nitride layers. The optical emission from these and related structures were examined and compared with that from individual layers in the structure. This revealed a complex interplay between defect and nanocrystal luminescence, hydrogen passivation and materials structure. The resulting microcavity structures were shown to be suitable for producing a stop-band over the wavelength range of interest for nanocrystal emission, 500-1000nm, and to produce significant intensity enhancement and spectral narrowing. Quality factors of 50-200 were demonstrated. ¶ The application of plasma deposited films was shown to be limited by stress-induced failure that resulted in cracking and delamination of the films during annealing. The SiOx films thicker than about 600nm failed predominantly by cracking. This was shown to be caused by tensile stress in the film caused by hydrogen desorption during high temperature annealing. The resulting cracks showed preferred alignment depending on the crystallographic orientation of the silicon substrate. For films deposited on (100) silicon, two modes of crack propagation were observed, straight cracks aligned along < 100> directions, and wavy cracks aligned along < 110> directions. For films deposited on (110) silicon, straight cracks were observed along [-1 10] directions, with a lesser number aligned along [001] directions. Cracks were also observed for films on (111) silicon. These showed 3-fold symmetry consistent with crack propagation along < 211> directions due to plastic deformation. Details of these crack geometries and their dependencies are discussed.

The conventional floating gate NVSM will suffer some limitations for continued scaling of the device structure. Therefore, two approaches, the silicon-oxide-nitride-oxide-silicon (SONOS) and the nanocrystal nonvolatile memory devices, have been investigated to overcome the limit of the conventional floating gate NVSM. In this thesis, the SONOS-TFT with multiple nanowires structure was proposed and fabricated for memory applications. The memory characteristic of standard SONOS-TFT, channel width of the device is 1£gm, was compared with the nanowires SONOS-TFT, each channel width of the device is 65nm. The SONOS-TFT with multiple nanowires structure (NW SONOS-TFT) has good program/erase efficiency, retention and transfer characteristics due to the larger electric field at the corner region and more number of corners. The NW SONOS-TFTs can be treated as high performance devices and also as high program/erase efficiency nonvolatile memory under adequate voltage range operation. The fabrication of SONOS-TFTs with nano-wire channels is quite easy and involves no additional processes. Such a SONOS-TFT is thereby highly promising for application in the future system-on-panel display applications. In addition, the metal nanocrystals nonvolatile memory fabricated at low temperature is also studied in this thesis. The Ni-silicide nanocrystals memory was successfully fabricated at low temperature. The rapid thermal oxidation at low temperature was executed to make the metal nanocrystals aggregate. The device has superior memory characteristics, such as program/erase efficiency, retention time and endurance. The nonvolatile metal nanocrystals memory fabricated at low temperature processes is very promising for the application on the portable products and panel displays.