A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy. Johannesburg, 2014. / The synthesis of good crystalline nanomaterials by green methods is one of the means to preventing global warming. Application of microwave thermal methods and the use of green solvents to synthesize nanomaterials contribute to this goal. Further, the low cost synthesis of nanomaterials contributes to their ease of availability in the market at affordable costs. In this study, different NiSx phases and GaN nanomaterials were obtained by microwave-assisted solution phase synthesis. NiS2, GaN, -NiS, ( & ) NiS and Ni3S2 phases were obtained by using different reagents and applying different reaction parameters. These materials were characterized by X-ray diffraction, transmission electron microscopy, scanning electron microscopy and photoluminescence, to evaluate their crystalline phases, morphologies, particle size distribution and optical properties respectively. Hierarchical structures of cubic phase NiS2 and spherical HDA capped nanostructures were synthesized by a MW-assisted hydrothermal technique. The product phase purity was optimized and the effect of precursor concentration and capping agents were discussed. Further, optical properties of bare and HDA capped NiS2 materials are reported. Detailed analysis of the PL properties shown by these materials in the UV-vis range has been given by considering their calculated DOE energy band diagrams. Single phase -NiS nanostructures with uniformly distributed hierarchical networks were synthesized for the first time in this study. The materials were evaluated for thermal stability under an oxidative environment and at temperatures between 150 oC and 600 oC. NiS materials showed stability at 300 oC and NiO formation was observed from 350 oC to 600 oC. The annealing effect on the crystalline size and IR absorption of the annealed samples is reported by XRD and FTIR studied. The EPR properties of the annealed materials were studied and compared to the oxidized materials. The transition temperature of the -NiS was further confirmed by performing electrical measurements on the as-synthesized material. Further, hydrostatic pressure sensing properties, ethanol and tomato VOCs sensing properties of the -NiS/PVA composite based devices were carried out and the results are reported. The ethanol gas sensing properties of the devices prepared showed the highest response when compared to hydrostatic pressure sensing and tomato VOCs gas sensing. UV-blue emitting GaN nanostructures were obtained for the first time by a onestep MW-assisted solvothermal technique. Sensor devices based on the hexagonal wurtzite structures obtained and their PVA composites (GaN/PVA) were prepared with different GaN NPs concentrations. A very high response to hydrostatic pressure was achieved for the devices prepared. The sensitivity of a GaN/PVA composite based device was analyzed for tomato VOCs detection and the results are presented. Binary phase ( & ) synthesis of NiS materials is commonly reported for the synthesis of Ni:S ratio of 1:1 stoichiometry. This is due to the formation of both phases at temperatures lower than 200 oC. Here, the effect of NaOH and the S source was investigated as reaction parameters. It was found that the concentration of OH- ions in solution plays a huge role in the formation of binary phase NiS as well as its morphology distribution in the nanostructures. Hexagonal nanoplatelets, nanorods and nanorodbased flower-like structures were obtained when different reaction parameters were varied in the presence of NaOH. Further, the solubility of different S precursors in the solvent used was studied and found to affect both the morphology and crystalline phase distribution of the products. Preliminary work on the synthesis of Ni3S2 and Se and Te doped Ni3S2 is presented in the last chapter. The crystallite sizes of the materials were determined by use of the Scherrer equation and the elemental composition was confirmed by EDS analysis. The relative humidity gas sensing of the samples materials was determined and sensitivity response of the material to humidity was obtained for the first time.

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Ip Kit Man = 準一維的硒化鋅納米結構的光致發光研究 / 葉潔雯. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2004. / Includes bibliographical references (leaves 68-71). / Text in English; abstracts in English and Chinese. / Ip Kit Man = Zhun yi wei de xi hua xin na mi jie gou de guang zhi fa guang yan jiu / Ye Jiewen. / ACKNOWLEDGEMENTS --- p.i / ABSTRACT --- p.ii / TABLE OF CONTENTS --- p.vi / LIST OF TABLE --- p.vii / LIST OF FIGURES --- p.viii / Chapter Chapter1 --- Introduction / Chapter 1.1 --- Motivation --- p.1 / Chapter 1.2 --- Literature review --- p.2 / Chapter 1.3 --- Our study --- p.4 / Chapter 1.4 --- Growth conditions --- p.4 / Chapter Chapter2 --- Background / Chapter 2.1 --- Basic principle of PL --- p.6 / Chapter 2.2 --- Theories of excitations in semiconductor / Chapter 2.2.1 --- Bandgap and temperature effect of ZnSe --- p.8 / Chapter 2.2.2 --- Structure of ZnSe --- p.13 / Chapter 2.3 --- Surface effects --- p.16 / Chapter 2.4 --- Gas -Surface Interaction --- p.20 / Chapter Chapter 3 --- Experimental Procedures / Chapter 3.1 --- Setup of PL Spectroscopy --- p.21 / Chapter 3.2 --- Passivation and separation of ZnSe nanostructures --- p.26 / Chapter Chapter 4 --- Results and Discussions / Chapter 4.1 --- Analysis of ZnSe/Si nanostructures / Chapter 4.1.1 --- "Morphology, structure and separation of as-grown ZnSe/Si nanostructures" --- p.30 / Chapter 4.1.2 --- Temperature dependent PL of as-grown and very low density ZnSe/Si nanostructures --- p.33 / Chapter 4.1.3 --- Temperature dependent PL of ZnSe/Al203 nanowires --- p.41 / Chapter 4.1.4 --- Growth conditions dependence on PL --- p.44 / Chapter 4.1.4a --- PL result comparison: ZnSe grown on 3 different substrate --- p.45 / Chapter 4.1.4b --- PL result comparison: Silver and gold catalyst --- p.45 / Chapter 4.2 --- Analysis of the ammonium sulfide treated samples / Chapter 4.2.1 --- XPS results of ammonium sulfide treated samples --- p.47 / Chapter 4.2.2 --- Relationship between immersion time and enhancement factors of PL intensity --- p.49 / Chapter 4.3 --- Analysis of effects in PL of ZnSe/Si nanostructures in different gas ambients --- p.53 / Chapter Chapter 5 --- Conclusions --- p.66 / References --- p.68

Kwong Kwan Wai = 氧化鋅基低維納米結構之生長與分析 / 鄺筠慧. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2004. / Includes bibliographical references (leaves 75-79). / Text in English; abstracts in English and Chinese. / Kwong Kwan Wai = Yang hua xin ji di wei na mi jie gou zhi sheng zhang yu fen xi / Guang Yunhui. / Acknowledgement --- p.i / Abstract --- p.ii / 摘要 --- p.iii / Table of Contents --- p.iv / List of Figures --- p.vii / Chapter Chapter 1 --- Introduction --- p.1 / Chapter Chapter 2 --- Background --- p.3 / Chapter Chapter 3 --- Instrumentation --- p.6 / Chapter 3.1 --- Tube furnace system --- p.6 / Chapter 3.2 --- X-ray Diffraction (XRD) --- p.8 / Chapter 3.3 --- Scanning Electron Microscopy (SEM) --- p.9 / Chapter 3.4 --- Transmission Electron Microscopy (TEM) --- p.12 / Chapter 3.4.1 --- General Review --- p.12 / Chapter 3.4.2 --- Low-magnification imaging --- p.14 / Chapter 3.4.3 --- Transmission electron diffraction (TED) --- p.14 / Chapter 3.4.4 --- High-resolution electron microscopy (HREM) --- p.15 / Chapter 3.4.5 --- Experimental --- p.15 / Chapter Chapter 4 --- Oxygen partial pressure effect on the morphology of ZnO nanostructures --- p.17 / Chapter 4.1 --- Introduction --- p.17 / Chapter 4.2 --- Experimental --- p.17 / Chapter 4.3 --- Results --- p.19 / Chapter 4.3.1 --- SEM study for general morphology --- p.21 / Chapter 4.3.2 --- TEM study for detailed microstructures of the tetrapods and the nanocombs --- p.28 / Chapter 4.3.2.1 --- Tetrapods --- p.28 / Chapter 4.3.2.2 --- Nanocombs --- p.29 / Chapter 4.4 --- Discussions --- p.34 / Chapter 4.4.1 --- Thermal reduction and oxidation to form ZnO --- p.34 / Chapter 4.4.2 --- Vapor-Solid (VS) growth mechanism of low-dimensional nanostructures --- p.34 / Chapter 4.4.3 --- The oxygen partial pressure effect on the morphology of the ZnO nanostructures --- p.35 / Chapter 4.4.4 --- Transition from tetrapod to nanocomb --- p.36 / Chapter 4.4.5 --- Decreasing size effect --- p.39 / Chapter Chapter 5 --- Self-assembly of periodical ZnO/C multilayers on Zn nanowire --- p.40 / Chapter 5.1 --- Introduction --- p.40 / Chapter 5.2 --- Experimental --- p.42 / Chapter 5.3 --- Results and Discussion --- p.46 / Chapter 5.3.1 --- Freshly synthesized Zn nanowires --- p.46 / Chapter 5.3.2 --- Introducing carbon into the Zn nanowires一Solid phase diffusion --- p.50 / Chapter 5.3.3 --- Introducing carbon into the Zn nanowires一Gas phase reaction --- p.52 / Chapter 5.3.3.1 --- Diffusion of the gas molecules through the ZnO sheath to the Zn/ZnO interface --- p.60 / Chapter 5.3.3.2 --- Chemical reaction(s) --- p.60 / Chapter 5.3.3.3 --- Phase separation of ZnO and C --- p.61 / Chapter 5.3.3.4 --- Self-organized multilayers --- p.62 / Chapter 5.3.3.5 --- Factors affecting SAM formation --- p.64 / Chapter 5.3.3.5.1 --- Crystallinity of original oxide sheath (series A) --- p.64 / Chapter 5.3.3.5.2 --- Temperature (series B) --- p.66 / Chapter 5.3.3.5.3 --- Gas molecules (series C) --- p.69 / Chapter Chapter 6 --- Conclusions --- p.72 / Appendix --- p.74 / References --- p.75

Liu Zhuang = 硒化鋅准一維納米結構的形貌和陰極射線致發光的研究 / 劉壯. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2004. / Includes bibliographical references (leaves 60-63). / Text in English; abstracts in English and Chinese. / Liu Zhuang = Xi hua xin zhun yi wei na mi jie gou de xing mao he yin ji she xian zhi fa guang de yan jiu / Liu Zhuang. / ABSTRACT --- p.i / ACKNOWLEDGMENTS --- p.iii / TABLE OF CONTENTS --- p.iv / LIST OF TABLES --- p.vi / LIST OF FIGURES --- p.vii / Chapter 1. --- Introduction --- p.1 / Chapter 1.1 --- Synthesizing quasi one-dimensional nanostructures of ZnSe using MOCVD --- p.1 / Chapter 1.2 --- Well-aligned nanostructures and heterocrystalline nanorods --- p.2 / Chapter 1.3 --- Advantages of CL in studying nanostructures --- p.3 / Chapter 2. --- Experimental conditions and procedures --- p.7 / Chapter 2.1 --- Information on the samples --- p.7 / Chapter 2.2 --- SEM and CL setting --- p.9 / Chapter 2.3 --- CL calibration --- p.10 / Chapter 3. --- Experimental results and data analysis --- p.14 / Chapter 3.1 --- SEM results and discussion --- p.14 / Chapter 3.1.1 --- SEM studies helping to find the optimal growth condition --- p.14 / Chapter 3.1.2 --- Morphological studies --- p.18 / Chapter 3.1.3 --- The growth direction of well-aligned nanostructure --- p.22 / Chapter 3.1.4 --- Discussion of the growth mechanisms of the nanostructures based on the morphological studies --- p.28 / Chapter 3.2 --- CL results and discussion --- p.34 / Chapter 3.2.1 --- CL spectra of nanostructures of different morphologies --- p.34 / Chapter 3.2.2 --- CL images for studying of distribution of localized radiative centers --- p.40 / Chapter 3.2.3 --- CL studies of a single nanorod --- p.44 / Chapter 3.2.3.1 --- Room temperature CL studies of a single nanorod --- p.44 / Chapter 3.2.3.2 --- Liquid Helium temperature (4.5 K) CL studies of a single nanorod --- p.50 / Chapter 4. --- Conclusions --- p.59 / Reference --- p.60

Hung Ngar Chun = 氧化鋅納米棒的生長和表徵 / 洪雅真. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2004. / Includes bibliographical references. / Text in English; abstracts in English and Chinese. / Hung Ngar Chun = Yang hua xin na mi bang de sheng zhang he biao zheng / Hong Yazhen. / Acknowledgements --- p.i / Abstract --- p.ii / 摘要 --- p.iii / List of Tables --- p.iv / List of Figures --- p.v / Table of contents --- p.vii / Chapter Chapter 1 --- Introduction / Chapter 1.1. --- Objectives --- p.1-1 / Chapter 1.2. --- Background --- p.1-1 / Chapter 1.2.1. --- Nanomaterials --- p.1-1 / Chapter 1.2.1.1. --- From three-dimensional to one-dimensional --- p.1-1 / Chapter 1.2.1.2. --- One-dimensional nanomaterials --- p.1-2 / Chapter 1.2.2. --- Characteristics and potential applications --- p.1-2 / Chapter 1.2.3. --- Growth mechanisms --- p.1-4 / Chapter 1.2.3.1. --- The VLS growth --- p.1-4 / Chapter 1.2.3.2. --- The VS growth --- p.1-5 / Chapter 1.2.4. --- ZnO --- p.1-6 / Chapter 1.2.4.1. --- Characteristics and potential applications --- p.1-6 / Chapter 1.2.4.2. --- Recent works performed by the others --- p.1-7 / Chapter 1.2.4.2.1. --- Different fabrication systems --- p.1-7 / Chapter 1.2.4.2.2. --- Different morphologies and shapes --- p.1-8 / Chapter 1.3. --- Our work --- p.1-9 / Chapter 1.3.1. --- Advantages of our fabrication method --- p.1-9 / Chapter 1.4. --- Thesis layout --- p.1-9 / References --- p.1-10 / Figures --- p.1-13 / Tables --- p.1-14 / Chapter Chapter 2 --- Methodology and Experiments / Chapter 2.1. --- Introduction --- p.2-1 / Chapter 2.2. --- The setup --- p.2-1 / Chapter 2.2.1. --- Preparation of substrate --- p.2-1 / Chapter 2.2.2. --- Chamber pressure and gas flow rate --- p.2-2 / Chapter 2.2.3. --- Heating profile --- p.2-2 / Chapter 2.3. --- The two important growth parameters --- p.2-3 / Chapter 2.3.1. --- Substrate compositions --- p.2-3 / Chapter 2.3.2. --- Temperature --- p.2-3 / Chapter 2.4. --- Methods of characterizations --- p.2-4 / Chapter 2.4.1. --- Morphology --- p.2-4 / Chapter 2.4.1.1. --- SEM --- p.2-4 / Chapter 2.4.1.2. --- TEM --- p.2-5 / Chapter 2.4.2. --- Phases and Microstructures --- p.2-5 / Chapter 2.4.2.1. --- XRD --- p.2-5 / Chapter 2.4.2.2. --- HRTEM --- p.2-5 / Chapter 2.4.3. --- Cathodoluminescence --- p.2-6 / References --- p.2-7 / Figures --- p.2-8 / Chapter Chapter 3 --- Results / Chapter 3.1. --- Introduction --- p.3-1 / Chapter 3.2. --- General Morphologies --- p.3-1 / Chapter 3.3. --- Microstructural analysis --- p.3-2 / Chapter 3.4. --- Samples sintered using substrate of different composition --- p.3-2 / Chapter 3.5. --- Samples sintered at different temperatures --- p.3-4 / Chapter 3.6. --- The cathodoluminescence of the ZnO nanorods --- p.3-4 / References --- p.3-6 / Figures --- p.3-7 / Chapter Chapter 4 --- Discussions / Chapter 4.1. --- Introduction --- p.4-1 / Chapter 4.2. --- Proposed growth model --- p.4-1 / Chapter 4.2.1. --- Chemical reactions --- p.4-1 / Chapter 4.2.2. --- Justification on the effect of residue oxygen and leakage --- p.4-2 / Chapter 4.2.3. --- Justification on the possibility of VLS growth --- p.4-3 / Chapter 4.2.4. --- Coarsening mechanisms --- p.4-3 / Chapter 4.2.4.1. --- Oswald Ripening --- p.4-4 / Chapter 4.2.4.2. --- Preferential growth direction --- p.4-4 / Chapter 4.3. --- The effects of substrate composition --- p.4-5 / Chapter 4.3.1. --- Roles of Si and Si02 --- p.4-5 / Chapter 4.4. --- The effects of temperature --- p.4-6 / Chapter 4.4.1. --- Range of sintering temperature --- p.4-6 / Chapter 4.4.2. --- Diameter of the rods --- p.4-7 / Chapter 4.4.3. --- Luminescence behavior --- p.4-7 / References --- p.4-10 / Figures --- p.4-11 / Chapter Chapter 5 --- Conclusions and future works

Tam, Yiu Ho = 納米線在液晶中的取向研究 / 譚耀豪. / "November 2009." / Accompanying CD-ROM contains data files. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2010. / Includes bibliographical references (leaves 79-83). / Abstracts in English and Chinese. / Library's copy: lacks accompanying CD-ROM. / Tam, Yiu Ho = Na mi xian zai ye jing zhong de qu xiang yan jiu / Tan Yaohao. / Abstract --- p.1 / 摘要 --- p.2 / Acknowledgements --- p.3 / Table of contents --- p.4 / Chapter Chapter 1 --- Introduction --- p.8 / Chapter 1.1 --- Background --- p.8 / Chapter 1.2 --- Review of Liquid crystals --- p.8 / Chapter 1.2.1 --- Molecular structure --- p.8 / Chapter 1.2.2 --- Phases --- p.10 / Chapter 1.2.3 --- Classification of liquid crystalline phases --- p.11 / Chapter 1.2.4 --- Elastic property --- p.12 / Chapter 1.2.5 --- Electrical property --- p.13 / Chapter 1.2.6 --- Optical property --- p.14 / Chapter 1.2.7 --- Alignment methods of liquid crystals --- p.14 / Chapter 1.2.8 --- Dynamics of the director of liquid crystals --- p.15 / Chapter 1.2.9 --- Transmittance calculations --- p.17 / Chapter 1.3 --- Interactions between liquid crystal and nanorods --- p.19 / Chapter 1.3.1 --- Longitudinal anchoring --- p.19 / Chapter 1.3.2 --- Theoretical model of the elastic interaction --- p.20 / Chapter 1.4 --- Motivations --- p.22 / Chapter 1.5 --- Our work --- p.23 / Chapter 1.6 --- Overview of the thesis --- p.23 / Chapter Chapter 2 --- Experiments --- p.24 / Chapter 2.1 --- Cell fabrications --- p.24 / Chapter 2.2 --- Filling of liquid crystal and dispersing nanorods --- p.26 / Chapter 2.3 --- Measurements --- p.27 / Chapter 2.3.1 --- Measurements of alignment and deduction of the orientation of nanorods --- p.27 / Chapter 2.3.2 --- Measurements of the optical transmittance of liquid crystal cells --- p.27 / Chapter 2.4 --- Experimental procedures --- p.29 / Chapter 2.4.1 --- Study of the statics of nanorods and liquid crystal --- p.29 / Chapter 2.4.2 --- Study of the dynamics of nanorods and liquid crystal --- p.30 / Chapter Chapter 3 --- Numerical solutions of the equations of motions --- p.31 / Chapter 3.1 --- Solving procedures --- p.31 / Chapter 3.1.1 --- Conversion of differential equations into difference equations --- p.31 / Chapter 3.1.2 --- Solving algorithm --- p.33 / Chapter 3.2 --- Comparison of numerical and analytical solutions --- p.35 / Chapter 3.2.1 --- Analytic solution of a simplified equation of motion --- p.35 / Chapter 3.2.2 --- Checking for the agreement between the numerical and analytic results --- p.36 / Chapter 3.2.3 --- Checking for the accuracy of the numerical results --- p.39 / Chapter Chapter 4 --- Method of analysis --- p.41 / Chapter 4.1 --- Deduction of the orientation of nanorods --- p.41 / Chapter 4.2 --- Method of curves fitting to experimental data --- p.43 / Chapter 4.2.1 --- Procedures of fitting the transmission curves --- p.43 / Chapter 4.2.2 --- Procedures of fitting the curves of angle ß --- p.44 / Chapter Chapter 5 --- Results and discussion --- p.48 / Chapter 5.1 --- Study of the statics of nanorods and liquid crystal --- p.48 / Chapter 5.1.1 --- Study of the alignment ability of liquid crystal on the nanorods --- p.48 / Chapter 5.1.2 --- Study of the longitudinal anchoring effect of liquid crystal on the nanorods --- p.54 / Chapter 5.2 --- Study of the dynamics of nanorods and the liquid crystal --- p.57 / Chapter 5.2.1 --- Dependence on the length of nanorods --- p.57 / Chapter 5.2.1 --- Temperature dependence on the LC-nanorod interaction --- p.66 / Chapter Chapter 6 --- Conclusions --- p.76 / References --- p.79 / Appendices --- p.84

Mak, Chun Chung. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2007. / Includes bibliographical references (leaves 83-85). / Abstracts in English and Chinese. / ABSTRACT --- p.iv / DECLARATION --- p.vi / ACKNOWLEDGEMENT --- p.vii / TABLE OF CONTENTS --- p.ix / LIST OF TABLES --- p.xii / LIST OF FIGURES --- p.xiii / Chapter Chapter One: --- Overview --- p.1 / Introduction to Cerium Oxide --- p.1 / Defects --- p.2 / Effect of dopants --- p.5 / Nano-structures of ceria --- p.8 / Loading metals in ceria supports --- p.10 / Summary --- p.11 / Scope of work --- p.12 / References --- p.14 / Chapter Chapter Two: --- Synthesis of Ceria Nano-feathers / Introduction --- p.17 / Experimental Section (Preparation of nano feathers) --- p.19 / Experimental Section (Preparation of conventional ceria spheres) --- p.19 / Experimental Section (Encapsulation the feather with gold) --- p.19 / Experimental Section (Instrumentation) --- p.20 / Results and Discussion (XRD) --- p.22 / Results and Discussion (SEM) --- p.23 / Results and Discussion (TEM) --- p.24 / Formation Mechanism --- p.30 / Results and Discussion (Nano-beam diffraction) --- p.33 / Results and Discussion (HREM) --- p.35 / Results and Discussion (TEM of gold encapsulated ceria) --- p.36 / Results and Discussion (XPS) --- p.39 / Summary --- p.41 / References --- p.41 / Chapter Chapter Three: --- Preparation of Homogenous Cerium Mixed Oxide Nano-feathers --- p.43 / Introduction --- p.43 / Experimental Section (Preparation of nano-feathers) --- p.44 / Experimental Section (Instrumentation) --- p.45 / Results and Discussion (XRD) --- p.47 / Results and Discussion (XPS) --- p.49 / Results and Discussion (TEM and EELS) --- p.56 / Summary --- p.64 / References --- p.64 / Chapter Chapter Four: --- Engineering Ceria Nano-feathers with Noble Metals: A Sono-chemistry approach --- p.66 / Introduction --- p.66 / Experimental Section (Sample Preparation) --- p.68 / Experimental Section (Instrumentation) --- p.69 / Results and Discussion (XRD) --- p.70 / Results and Discussion (XPS) --- p.71 / Results and Discussion (TEM and HREM) --- p.76 / Summary --- p.82 / References --- p.84 / Conclusions --- p.86

MSc., Faculty of Science, University of the Witwatersrand, 2011 / This dissertation discusses the synthesis method known as laser pyrolysis. The theory on laser pyrolysis has been inferred since 1975, but it is insufficient in predicting the products that can be formed. This is due to the use of a laser, which leads to indecisive reaction pathways from precursor to product. In this work, the laser wavelength and power are varied to initiate a starting point in understanding the complex nature of the laser–precursor interaction, in addition to studying the resulting nanomaterial that is formed by the corresponding laser pyrolysis parameters. The results are justified based on linear and nonlinear optical processes, as well as photophysical and photochemical processes. Experiments to produce tungsten trioxide nanowires were conducted, but similar products could not be achieved, due to the difficulty in emulating ‘sensitive’ variables such as gas pressure and flow rates. However, it was discovered for the first time using this method that six-sided tungsten oxide “stars” can be grown.