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Thesis (M. Phil.)--University of Hong Kong, 2008. / Also available in print.

Thesis (M. Phil.)--University of Hong Kong, 2008. / Includes bibliographical references. Also available in print.

Thesis (M.E.E.)--University of Delaware, 2006. / Principal faculty advisors: Olufemi Olowolafe, Dept. of Electrical and Computer Engineering and Robert L. Opila, Dept. of Materials Science. Includes bibliographical references.

One-dimensional ZnO nanostructures have great potential applications in the fields of optoelectronic and sensor devices.  Therefore, it is very important to realize the controllable growth of one-dimensional ZnO nanostructures and investigate their properties. The main points for this thesis are not only to successfully realize the controllable growth of ZnO nanorods (ZNRs), ZnO nanotubes (ZNTs) and ZnMgO/ZnO heterostructures, but also investigate the structure and optical properties in detail by means of scanning electron microscope (SEM), transmission electron microscope (TEM), resonant Raman spectroscopy (RRS), photoluminescence (PL), time resolved PL (TRPL), X-ray photoelectron spectroscopy (XPS) and Secondary ion mass spectrometry (SIMS). For ZNRs, on one hand, ZNRs have been successfully synthesized by a two-step chemical bath deposition method on Si substrates. The diameter of ZNRs can be well controlled from 150 nm to 40 nm through adjusting the diameter and density of the ZnO nanoparticles pretreated on the Si substrates. The experimental results indicated that both diameter and density of ZnO nanoparticles on the substrates determined the diameter of ZNRs. But when the density is higher than the critical value of 2.3×108cm-2, the density will become the dominant factor to determine the diameter of ZNRs. One the other hand, the surface recombination of ZNRs has been investigated in detail. Raman, RRS and PL results help us reveal that the surface defects play a significant role in the as-grown sample. It is the first time to the best of our knowledge that the Raman measurements can be used to monitor the change of surface defects and deep level defects in the CBD grown ZNRs. Then we utilized TRPL technique, for the first time, to investigate the CBD grown ZNRs with different diameters. The results show that the decay time of the excitons in ZNRs strongly depends on the diameter. The altered decay time is mainly due to the surface recombination process. A thermal treatment under 500°C can strongly suppress the surface recombination channel. A simple carrier and exciton diffusion equation is also used to determine the surface recombination velocity, which results in a value between 1.5 and 4.5 nm/ps. Subsequently, we utilized XPS technique to investigate the surface composition of as-grown and annealed ZNRs so that we can identify the surface recombination centers. The experimental results indicated that the OH and H bonds play the dominant role in facilitating surface recombination but specific chemisorbed oxygen also likely affect the surface recombination. Finally, on the basis of results above, we explored an effective way, i.e. sealing the beaker during the growth process, to effectively suppress the surface recombination of ZNRs and the suppression effect is even better than a 500oC post-thermal treatment. For ZNTs, the structural and optical properties have been studied in detail. ZNTs have been successfully evolved from ZNRs by a simple chemical etching process. Both temperature-dependent PL and TRPL results not only further testify the coexistence of spatially indirect and direct transitions due to the surface band bending, but also reveal that less nonradiative contribution to the emission process in ZNTs finally causes their strong enhancement of luminescence intensity. For ZnMgO/ZnO heterostructures, the Zn0.94Mg0.06O/ZnO heterostructures have been deposited on 2 inch sapphire wafer by metal organic chemical vapor deposition (MOCVD) equipment. PL mapping demonstrates that Mg distribution in the entire wafer is quite uniform with average concentration of ~6%. The annealing effects on the Mg diffusion behaviors in Zn0.94Mg0.06O/ZnO heterostructures have been investigated by SIMS in detail. All the SIMS depth profiles of Mg element have been fitted by three Gaussian distribution functions. The Mg diffusion coefficient in the as-grown Zn0.94Mg0.06O layer deposited at 700 oC is two orders of magnitude lower than that of annealing samples, which clearly testifies that the deposited temperature of 700 oC is much more beneficial to grow ZnMgO/ZnO heterostructures or quantum wells. This thesis not only provides the effective way to fabricate ZNRs, ZNTs and ZnMgO/ZnO heterostructures, but also obtains some beneficial results in aspects of their optical properties, which builds theoretical and experimental foundation for much better understanding fundamental physics and broader applications of low-dimensional ZnO and related structures. / Endimensionella nanostrukturer av ZnO har stora potentiella tillämpningar för optoelektroniska komponenter och sensorer. Huvudresultaten för denna avhandling är inte bara att vi framgångsrikt har realiserat med en kontrollerbar metod ZnO nanotrådar (ZNRs), ZnO nanotuber (ZNTs) och ZnMgO/ZnO heterostrukturer, utan vi har också undersökt deras struktur och optiska egenskaper i detalj. För ZNRs har diametern blivit välkontrollerad från 150 nm  ner till 40 nm. Den storlekskontrollerande mekanismen är i huvudsak relaterad till tätheten av ZnO partiklarna som är fördeponerade på substratet. De optiska mätningarna ger upplysning om att ytrekombinationsprocessen spelar en betydande roll för tillväxten av ZNR. En värmebehandling i efterhand  vid 500 grader Celsius eller användande av en förseglad glasbägare under tillväxtprocessen kan starkt hålla nere kanalerna för ytrekombinationen.För ZNT, dokumenterar vi inte bara samexistensen av rumsliga indirekta och direkta  övergångar på grund av bandböjning, men vi konstaterar också att vi har mindre icke-strålande bidrag till den optiska emissionsprocessen i ZNT. För ZnMgO/ZnO heterostrukturer konstaterar vi med hjälp av analys av Mg diffusionen i den växta och den i efterhand uppvärmda Zn(0.94)Mg(0.06)O filmen, att en tillväxt vid 700 grader Celsius är den mest lämpliga för att växa ZnMgO/ZnO heterostrukturer eller kvantbrunnar.   Denna avhandling ger en teoretisk och experimentell grund för bättre förståelse av grundläggande fysik och för tillämpningar av lågdimensionella strukturer. / SSF, VR

Thesis (Ph. D.)--Auburn University, 2008. / Abstract. Vita. Includes bibliographical references (p. 177-194).

Thesis (Ph. D.)--Rutgers University, 2008. / "Graduate Program in Electrical and Computer Engineering." Includes bibliographical references (p. 150-157).

Recently, efforts have been made for self-powering the batteries and portable electronic devices by piezoelectric nanogenerators. The piezoelectric nanogenerators can work as a power source for nano-systems and also as an active sensor. The piezoelectric nanogenerator is a device that converts random mechanical energy into electrical energy by utilizing the semiconducting and piezoelectric properties. Also, the mechanical energy is always available in and around us for powering these nano devices. The aim of the present thesis work is to explore 1D and 2D ZnO nanostructures (nanorods and nanosheets) on metal alloy substrates for the development of piezoelectric nanogenerators in energy harvesting and sensors applications. Hydrothermal synthesis method was adopted for the growth of ZnO nanostructures. The nanogenerators were fabricated by using the optimized synthesis parameters and subsequently studied their performance for power generation and as an active speed sensor. These 1D and 2D nanostructures based nanogenerators have opened up a new window for the energy harvesting applications and sensors development. The thesis is divided into following six chapters. Chapter 1: This chapter gives a general introduction about energy harvesting devices such as nanogenerators, available energy sources, mechanical energy harvesting, ZnO material and the details on hydrothermal synthesis process. A brief literature survey on different applications of piezoelectric nanogenerators is also included. Chapter 2: A novel flexible metal alloy (Phynox) and its properties along with its applications are discussed in this chapter. Details on the synthesis of 1D ZnO nanorods on Phynox alloy substrate by hydrothermal method are presented. Further, the optimization of parameters such as growth temperature, seed layer annealing and substrate temperature effects on the synthesis of ZnO nanorods are discussed in detail. As-synthesized ZnO nanorods have been characterized using XRD, FE-SEM, TEM and XPS. Chapter 3: It reports on the fabrication of piezoelectric nanogenerator on Phynox alloy substrate as power generating device by harvesting the mechanical energy. Initially, the performance of the nanogenerator for power generation due to finger tip impacts was studied and subsequently its switching polarity test was also carried out. Output voltage measurements were carried out using the in-house developed experimental setup. Stability test was also carried out to see the robustness of the nanogenerator. Finally, the output voltage response of the nanogenerator was studied for its use as an active speed sensor. Chapter 4: Synthesis of Al doped 2D ZnO nanorsheets on Aluminum alloy (AA-6061) substrate by hydrothermal method is reported in this chapter. The optimized parameters such as growth temperature and growth time effects on the synthesis of ZnO nanosheets are discussed. As-synthesized ZnO nanosheets were characterized using XRD, FE-SEM, TEM and XPS. The Al doping in ZnO is confirmed by EDXS and XPS analysis. Chapter 5: Cost effective fabrication of Al doped 2D ZnO nanosheets based nanogenerator for direct current (DC) power generation is reported in this chapter. The performance of the nanogenerator for DC power generation due to finger tip impacts was studied and subsequently its switching polarity test was also carried out. Output voltage measurements were carried out using the in-house developed experimental setup. Stability test was also carried out to see the robustness of the nanogenerator. Finally, the DC output voltage response of the nanogenerator was studied for its use as an active speed sensor. Chapter 6: The first section summarizes the significant features of the work presented in this thesis. In the second section the scope for carrying out the further work is given.

Nanostructured materials especially, one-dimensional (1D) nanostructures have unique physical, chemical, mechanical properties and are the building blocks for a range of nanoscale devices. The procedure employed for the synthesis of nanostructures involves the use of sophisticated instruments or rigorous chemical reactions. The motivation of our work is to develop a strategy that is simple, cost effective and applicable to a host of oxide materials. Nanostructures of various oxides have been grown from the metal as the source material. 1D ZnO nanostructures have been obtained by simply heating Zn metal in ambient air at temperatures below 600 °C. The nanostructures grow on the surface of the source material and the morphology is controlled by monitoring the curvature of the source material. This technique has an added advantage that neither any catalyst nor any gas flow is required. Tetrapods of ZnO are obtained when Zn is heated above 700 °C in ambient air. It has been shown that the morphology and the aspect ratio (length-to-diameter ratio) of the tetrapods depend on the temperature and the temperature gradient. Photoluminescence studies reveal good optical quality ZnO nanostructures. The technique employed to synthesize 1D ZnO nanostructures has been checked for other oxides. The temperature required for the synthesis of Ga2O3 nanostructures is 1200 °C. Many researchers have shown that Ga2O3 emits in the blue-green region. A red emission is required to get the impression of white light which has been seen for nitrogen doped Ga2O3. As the temperature is very high and Ga is heated in ambient air, unintentional nitrogen doping of 1D Ga2O3 nanostructures is obtained which is the reason for white light emission. The morphology of Ga2O3 nanostructures has been controlled by monitoring the curvature of the starting material as is the case of ZnO. Similar technique has also been employed for the synthesis of CuO nanostructures. The morphology is temperature dependent and 1D CuO nanostructures are obtained when the synthesis temperature is between 400 and 600 C. Possible growth mechanisms have been proposed for all these oxide materials. The entire thesis is based on the results discussed above. It has been organized as follows: Chapter 1 deals with the introduction to nanostructures, importance of 1D nanostructures, the specific applications of different morphologies, materials that are widely explored in the synthesis of nanostructures and different approaches to the synthesis of nanostructures. Growth mechanisms like VLS, VS and SLS are briefly discussed. A brief review on the basic physical properties, applications and different morphologies of ZnO, Ga2O3 and CuO is outlined with emphasis to the various synthesis techniques. Finally the aim and scope of the present work is discussed. Chapter 2 describes the experimental setup used for the synthesis and the basic principles of characterization techniques like x-ray diffraction (XRD), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), energy dispersive spectrum (EDS), electron energy loss spectroscopy (EELS), photoluminescence (PL), Raman spectroscopy, x-ray photoelectron spectroscopy (XPS), UV-Visible spectroscopy, Fourier Transform Infrared Spectroscopy (FTIR) and thermogravimetric analysis (TGA). Chapter 3 deals with the synthesis of 1D ZnO nanostructures with different morphologies such as nanoneedles, nanorods, nanobelts from Zn powder/granule. The growth process is found to be different from the conventional VS mechanism. The advantage and the versatility of the method is emphasized. In this method, neither a catalyst nor any gas flow is required for the synthesis of oxide nanostructures. Depending upon the Zn powder or Zn granules as the starting material different nanostructures of ZnO have been synthesized. The as-synthesized materials are characterized by XRD, SEM, HRTEM, EDS, TGA and Raman spectroscopy and the results are discussed. Chapter 4 describes the controlled growth of ZnO tetrapods and the influence of temperature and temperature gradient on the growth process. Though there are several methods to synthesize ZnO tetrapods and it has been established that ZnO tetrapods can be synthesized by heating Zn in air, it is advantageous to grow tetrapods of different morphologies with different lengths. The large scale synthesis of ZnO tetrapods by heating Zn in air ambient is discussed in this chapter. The key parameters that control the diameter, length, and morphology of tetrapods are identified. It is shown that the morphology and dimensions of the tetrapods depend not only on the vaporization temperature but also on the temperature gradient of the furnace. The influence of vaporization temperature and growth temperature on the morphology of the tetrapods is discussed elaborately. Chapter 5 explains the one-step synthesis of nitrogen doped Ga2O3 nanostructures of different morphologies and the different growth mechanisms. The experimental method employed for the synthesis of nanostructures is simple and is different from the other reported methods. Neither any catalyst/substrate preparation nor any gas flow is required for the synthesis of Ga2O3 nanostructures. The synthesis involves the heating of molten Ga at high temperatures. Single crystalline monoclinic phase of nitrogen-doped Ga2O3 nanorods, nanobelts and nanoneedles are obtained by this method. The morphology is controlled by monitoring the curvature of the Ga droplet which is achieved by using different substrates. Possible growth processes of different morphology have been proposed. Chapter 6 includes some surprising results on the white light emission of Ga2O3 nanorods. High synthesis temperature generates a high vapor pressure suitable for the growth of Ga2O3 nanorods, creates oxygen vacancy and incorporates nitrogen from the ambient. The oxygen vacancy is responsible for the bluish-green emission, while nitrogen is responsible for the red emission. As a consequence, white light emission is observed from Ga2O3 nanorods when irradiated with UV light. The interesting point is that neither post-treatment of the nanorods nor size control is required for white light emission. Chapter 7 describes the synthesis of CuO nanostructures by heating Cu foil in air ambient. This is an attempt to check whether the synthesis technique employed for ZnO and Ga2O3 is applicable to other oxides. The as-synthesized CuO nanostructures are characterized by XRD, SEM, HRTEM, EDS, TGA, UV-visible, FTIR and the results are discussed. Chapter 8 gives the conclusions and the overall summary of the thesis.

ZnO nanowires and nanorods are a new class of one-dimensional nanomaterials with a wide range of applications in NEMS. The motivation for this work stems from the lack of understanding and characterization of their thermomechanical behaviors essential for their incorporation in nanosystems. The overall goal of this work is to develop a fundamental understanding of the mechanisms controlling the responses of these nanostructures with focus on: (1) development of a molecular dynamics based framework for analyzing thermomechanical behaviors, (2) characterization of the thermal and mechanical behaviors in ZnO nanowires and (3) development of models for pseudoelasticity and thermal conductivity. The thermal response analyses show that the values of thermal conductivity are one order of magnitude lower than that for bulk ZnO due to surface scattering of phonons. A modified equation for phonon radiative transport incorporating the effects of surface scattering is used to model the thermal conductivity as a function of wire size and temperature. Quasistatic tensile loading of wires show that the elastic moduli values are 68.2-27.8% higher than that for bulk ZnO. Previously unknown phase transformations from the initial wurtzite (WZ) structure to graphitic (HX) and body-centered-tetragonal (BCT-4) phases are discovered in nanowires which lead to a more complete understanding of the extent of polymorphism in ZnO and its dependence on load triaxiality. The reversibility of the WZ-to-HX transform gives rise to a novel pseudoelastic behavior with recoverable strains up to 16%. A micromechanical continuum model is developed to capture the major characteristics of the pseudoelastic behavior accounting for size and temperature effects. The effect of the phase transformations on the thermal properties is characterized. Results obtained show that the WZ→HX phase transformation causes a novel transition in thermal response with the conductivity of HX wires being 20.5-28.5% higher than that of the initial WZ-structured wires. The results obtained here can provide guidance and criteria for the design and fabrication of a range of new building blocks for nanometer-scale devices that rely on thermomechanical responses.