SELECTED ZINC CHALCOGENIDE NANOMATERIALS WITH NOVEL STRUCTURE AND FUNCTIONALITY ZHENG MINRUI (B. Appl. Sc. (Hons.), NUS) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF PHYSICS NATIONAL UNIVERSITY OF SINGAPORE 2014 Declaration I hereby declare that this thesis is my original work and it has been written by me in its entirety. I have duly acknowledged all the sources of information which have been used in the thesis. This thesis has also not been submitted for any degree in any university previously. ____________________ Zheng Minrui 19 August 2014   Acknowledgements ACKNOWLEDGEMENTS First and foremost, I would like to express my most sincere appreciation and gratitude to my supervisor, Prof. Sow Chorng Haur, for his encouragement and unwavering support during the course of my PhD study. I have acquired a lot of scientific knowledge, critical thinking and experimental skills in the field of nanomaterials research under his supervision, while obtaining complete freedom in my research work. Apart from that, he has demonstrated and taught me at the same time many other skills and qualities, including communication skills and optimism in the face of failure, which I am sure will be an invaluable treasure leading down my career path. I would like to express my heartfelt gratitude to Prof. Li Baowen at the Department of Physics and Prof. John T. L. Thong at the Department of Electrical and Computer Engineering for evaluating my collaborative research work and providing in-depth discussion and very useful suggestions. My thanks also go to all other collaborators, including but not limited to Dr. Wang Shijie at Institute of Materials Research and Engineering, Assoc. Prof. Tok Eng Soon at the Department of Physics, Dr. Cai Yongqing at Institute of High Performance Computing, Dr. Bui Cong Tinh at the NUS Graduate School for Integrative Sciences and Engineering, Dr. Liu Dan at the Department of Physics, Dr. Liu Hongwei at Institute of Materials Research and Engineering and Prof. Fan Haiming at National University of Ireland Galway. i  Acknowledgements I would like to thank all lab members including Dr. Lu Junpeng, Dr. Lim Xiaodai, Dr. Deng Suzi, Dr. Hu Zhibin, Dr. Bablu Mukherjee, Dr. Binni Varghese, Mr. Lim Kim Yong, Mr. Teoh Hao Fatt, Ms. Gong Lili, Mr. Chang Sheh Lit and Ms. Tao Ye, Mr Yun Tao, Mr. Rajesh Tamang, and Mr. Rajiv Ramanujam for being great companions along my PhD study. I would like to express my appreciation to Mr. Chen Gin Seng, Ms. Foo Eng Tin, Mrs. Tan Teng Jar, Mr. Suradi Bin Sukri and Mr. Ramasamy Dhasaratha Raman for their kind help rendered during my research experiments. All other technical staff at the Department of Physics Workshop are greatly acknowledged for their help in making gadgets and devices used in my research. I would also like to specially thank Ms. Tan Hui Ru for help with TEM analysis. Last but certainly not least, I would like to dedicate this thesis to my parents for their encouragement and unfading support throughout the years. ii  Table of Contents TABLE OF CONTENTS ACKNOWLEDGEMENTS i TABLE OF CONTENTS iii SUMMARY vi LIST OF TABLES . viii LIST OF FIGURES . ix LIST OF SYMBOLS . xix Chapter Introduction 1.1 The material-structure-functionality paradigm in nanomaterials research and development . 1.1.1 A brief history and some terminologies 1.1.2 Nanomaterials research is not all about size reduction . 1.2 Zinc chalcogenide family of nanomaterials . 1.2.1 Crystal structure 1.2.2 Applications of zinc chalcogenide nanomaterials . 1.3 Thermal transport in nanomaterials 1.4 Research motivation and objectives . 11 1.5 Organization of the thesis . 14 References . 16 Chapter Zinc Chalcogenide Nanomaterials Synthesis and Characterization Techniques . 20 2.1 Chemical vapor deposition using a sealed horizontal tube furnace . 20 2.2 Implementing the Vapor-Solid and Vapor-Liquid-Solid nanostructure growth mechanisms . 26 2.2.1 The Vapor-Solid mechanism 26 2.2.2 The Vapor-Liquid-Solid mechanism 29 2.3 Seed layer deposition by pulsed laser deposition . 32 2.4 General characterization techniques . 35 iii Table of Contents 2.5 Use of a micro-electro-thermal system test fixture for exploring thermal transport properties of individual nanostructures 36 2.6 Micro-photoluminescence and micro-Raman spectroscopy 41 References . 43 Chapter Synthesis of Segmented ZnO Nanowires and Investigation of Their Spatially-Resolved Thermal Transport Properties . 44 3.1 Introduction 44 3.2 Synthesis of vertically-oriented segmented ZnO nanowires via a templated homoepitaxial re-growth approach . 49 3.2.1 Results on Si (100) substrates . 52 3.2.2 Results on sapphire substrates 60 3.3 Spatially-resolved single 2-segment ZnO nanowire thermal conductivity studies 72 3.4 Conclusions 84 References . 85 Chapter Robust Nanoscale Bistable Thermal Conduction in a Single Cleaved ZnO Nanowire 87 4.1 Introduction 88 4.2 Device design rationale and working principle 91 4.3 Active component and device fabrication 99 4.4 Device testing procedure 105 4.5 Device thermal cycling behavior 112 4.6 Theoretical considerations 119 4.7 Device switching speed 124 4.8 Conclusions 125 References . 127 Chapter Synthesis and Optical Properties Study of ZnTe Nanoplates with Quasi-Periodic Twinning 129 5.1 Introduction 129 5.2 Synthesis of quasi-periodically twinned ZnTe nanoplates . 133 5.3 Discussion on possible growth mechanism 139 5.4 Optical properties of quasi-periodically twinned ZnTe nanoplates . 147 iv Table of Contents 5.5 Conclusions 151 References . 153 Chapter Concluding remarks and Future Work 156 6.1 Concluding remarks . 156 6.2 Future work 159 LIST OF PUBLICATIONS . 162 LIST OF CONFERENCE PRESENTATONS . 167 LIST OF PATENTS . 168 v Summary SUMMARY In the course of nearly two decades of nanomaterials research, many materials classes in their various well-defined nano-forms have been individually explored. In this thesis, we aim to go beyond simple structural forms and study nanomaterials with various levels of structural complexity. We focus our attention on selected members, namely ZnO and ZnTe, from the zinc chalcogenide nanomaterials family that has currently been receiving intensive research efforts. In the first part we revolve around vertically-aligned multi-segment ZnO nanowire arrays. An epilayer deposited by the pulsed laser deposition technique is shown to enable the synthesis of vertical ZnO nanowire arrays on various substrates. Employing the concept of homoepitaxial re-growth, additional segments were sequentially built up using as-grown single-segment ZnO nanowires as growth templates through repeated simultaneous radial and axial epitaxy in successive single growth cycles. We then implemented a newlydeveloped scanning electron-beam heating technique to generate the spatiallyresolved thermal resistance profiles of single two-segment ZnO nanowires. 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C. unpublished work. 60. Shiang, J. J.; Risbud, S. H.; Alivisatos, A. P. J. Chem. Phys. 1993, 98, 8432. 61. Zhang, Q.; Zhang, J.; Utama, M. I. B.; Peng, B.; de la Mata, M.; Arbiol, J.; Xiong, Q. Phys. Rev. B 2012, 85, 085418. 155 Chapter Conclusions and Future Work Chapter Concluding remarks and Future Work 6.1 Concluding remarks This thesis work is devoted to a detailed and systematic study of nanostructures from selected zinc chalcogenide semiconductor materials, namely ZnO and ZnTe, from the materials synthesis stage through extensive single nanostructure-based characterization to finally finding their functionalities and possible applications. The nanostructures were synthesized with the common synthetic method of chemical vapor deposition (CVD) carried out using a sealed horizontal tube furnace. However, with the development and incorporation of modifications to the synthetic protocol at various stages, nanostructures with various types of complexity, be it morphologically, structurally, or microstructurally, have been created that go beyond the most-commonly studied one-dimensional and twodimensional forms of nanowires and nanobelts. In the first demonstration, we presented the successful preparation of multisegment ZnO nanowires with distinct segment diameters. These nanostructures with complex morphologies were synthesized by implementing a strategy known as homoepitaxial re-growth. With this synthetic protocol, the entire exterior of the as-synthesized ZnO nanowire segments were adopted as growth templates for further epitaxial growth, and in this manner the segments were shown to be capable of stacking up sequentially with repeated independent growth cycles. This 156 Chapter Conclusions and Future Work synthesis strategy offers great prospects of tunability as the exact growth parameters for each growth event could be modified independently. Besides morphological control, we have also worked on other issues such as alignment and density control with which these nanostructures appear on a particular growth substrate. We have shown that a ZnO seed layer deposited on common substrates, such as Si (100) or sapphire substrates, by the pulsed deposition (PLD) technique serves as a great starting point for ZnO nanostructure growth. On Si (100) substrates, perfectly vertical ZnO nanowire arrays could be obtained on such a seed layer after the first stage of seed layer thickening and ZnO nanoridges formation. Nanowire density control could barely be achieved in this case. However, the seed layer deposited on sapphire substrates with similar deposition parameters was shown to be able to offer density-controlled ZnO nanowire formation through density control of surface defects that serve as nucleation sites for subsequent nanowire growth. Using this approach, multisegment ZnO nanowires with extremely low number density was successfully synthesized, which facilitates the detachment and single-nanowire-based characterizations. Subsequently, the spatially-resolved thermal conduction properties of a 2segment ZnO nanowire was explored using an in-house developed scanning focused electron-beam heating technique in a proof-of-concept study. Cumulative thermal resistance profiles in close proximities across the transition junction were obtained, from which thermal conductivity values corresponding to each segment within the same nanowire could be extracted. We obtained thermal conductivity 157 Chapter Conclusions and Future Work values of 11.7 W/m·K and 12.3 W/m·K corresponding to nanowire segments with diameters of 46.6 nm and 69.5 nm respectively, which is a direct consequence of the fact that the thicker segment has a higher surface roughness. In a second study, we have explored the thermal conduction properties of a cleaved single-segment ZnO nanowire connected through van der Waals interactions. Such a situation has been created via in-situ cleaving of an intact single-segment ZnO nanowire that has been bonded to the microscale thermal property test fixture at both ends, and represents a state of structural complexity to the nanowire system. We have shown that the thermomechanical stress developed on the micsroscale test platform as a result of a relatively large pulsed thermal bias could effectively couple to the van der Waals interactions that connects the two nanowire segments, in the process toggling the junction between the open and closed states. As a result, the entire system exhibits bistability in thermal conductance by designating the cleaved ZnO nanowire as the nanoscale thermal conduction channel. The thermal conductance states are well-defined with a difference of 22 times between the on and off states. We have conducted ab-initio calculations, which indicate that the high thermal conductance observed in the on state is closely related to the high effective van der Waals force between the nanowire segments as a result of in-situ cleaving. The demonstration of nanoscale bistable thermal conduction has huge implications on the development of thermal switches and nonvolatile thermal memory technologies. Turning to ZnTe nanostructures, we have discovered that the adoption of Au catalysts with extremely small particle sizes promotes the intriguing growth of 158 Chapter Conclusions and Future Work quasi-periodically twinned ZnTe nanoplates in a typical Vapor-Liquid-Solid (VLS) growth. These microstructurally-complex twinned ZnTe nanoplates constitute an ideal test platform for probing various transport phenomena across the atomically coherent twin boundary. We have investigated their formation and tentatively assigned a growth mechanism combining both VLS growth and Vapor-Solid (VS) growth initiated by the extremely small Au particles. Preliminary singlenanostructure-based optical studies show that these nanostructures exhibit a large redshift in their photoluminescence (PL) of up to 50 nm as a result of both a high number of photoexcited carrier generation as well as an enhanced lattice temperature due to electron-phonon thermalization. Resonance Raman studies have indicated a high electron-phonon coupling inside these structures. This thesis has demonstrated versatility with respect to nanostructure synthesis and highlighted the fact that the resulting nanostructures with derived complexities could have tailored functionalities. It is very important to focus on detailed nanostructure design and synthesis for specific applications in nanomaterials research in the coming time. 6.2 Future work As an extension to the current thesis work, this section points out interesting aspects for further exploration to each project that constitutes the thesis. It also contains a list of recommended research actions for both immediate follow-up as well as future work. 159 Chapter Conclusions and Future Work For thermal transport studies in multi-segment ZnO nanowires, a very intriguing prospect is to study the effect of the presence of segment transition junctions on the scattering of phonons and its implications on possible directionally asymmetric thermal conduction, which is a highly desired phenomenon for the development of a phononic device of thermal diode. Such studies could be implemented from four aspects of interest. Firstly, segmented ZnO nanowires with largely contrasting segments in terms of segment diameters could be used, thereby magnifying the effects at segment transition junctions. Secondly, segmented ZnO nanowires with different appearances of transition junctions could be fabricated and studied to elucidate the effect of exact transition junction shapes on the asymmetric forward and backward phonon scattering rate in thermal transport. Thirdly, segmented ZnO nanowires with more segments could be used, which could cascade the effects over many segment transition junctions. Lastly, the temperature-dependent device performance should be carefully evaluated, especially at low temperatures. For cleaved-nanowire-based thermal switches, an immediate task for further exploration is to extend the idea onto nanowires from a range of materials other than ZnO. The aim of this is to study the prospect of tunability by changing the active component for thermal conduction. We believe that the key performance metrics for the cleaved-nanowire-based thermal switches, such as the magnitude of the switching thermal bias and ON-OFF ratio, are highly correlated with the magnitude of the van der Waals interaction and intrinsic mechanical properties of the active component. Therefore, by selecting nanowires of an appropriate 160 Chapter Conclusions and Future Work material, thermal switches with intended switching characteristics could readily be fabricated. For quasi-periodically twinned ZnTe nanoplates, the first task is to perfect the synthesis procedure so that desired nanostructures with more consistent yields could be synthesized with improved reproducibility. In order to achieve this, a deep experimental exploration into the exact growth mechanism of such intriguing nanostructures is imperative. Atomic-scale microscopy techniques such as high-angle annular dark-field (HAADF) Z-contrast imaging in a scanning transmission electron microscope (STEM) should be utilized to trace the exact location of Au atoms within the grown nanostructures in order to shed more light on their growth mechanism. A better understanding of the growth mechanism would also facilitate the synthesis of ZnTe twin-plane superlattice with identical twin segment lengths in the future. Besides elucidating their exact growth mechanism, another immediate task is to carry out single-nanostructure-based orientation-dependent electrical and thermal transport studies at various temperatures, so as to evaluate any anisotropy along the two principal orientations being perpendicular and parallel to the twin boundary. However, the challenge here is to have a knowledge of the exact twin boundary orientation during device fabrication. Currently, we are looking into the possibility of using resonance Raman spectroscopy as a convenient and fast means of identifying twin boundary orientation. 161 List of Publications LIST OF PUBLICATIONS 1. Minrui Zheng, Rongguo Xie, Yongqing Cai, Cong Tinh Bui, Qingxin Zhang, Chorng Haur Sow, Baowen Li, John T. L. Thong Robust bistable thermal conduction in a single cleaved nanowire (Submitted) 2. Cong Tinh Bui, Rongguo Xie, Minrui Zheng, Qingxin Zhang, Chorng Haur Sow, Baowen Li, John T. L. Thong Diameter-dependent thermal transport in individual ZnO nanowires and its correlation with surface coating and defects Small, 8, (2012), 738 - 745 + cover 3. Cheng Han, Du Xiang, Minrui Zheng, Jiadan Lin, Jianqiang Zhong, Chorng Haur Sow, Wei Chen Tuning the electronic properties of ZnO nanowire field effect transistors via surface functionalization (Submitted) 4. Hongwei Liu, Junpeng Lu, Minrui Zheng, Sing Hai Tang, Chorng Haur Sow, Xinhai Zhang, Lin Ke Size effects on metal-insulator phase transition in individual vanadium dioxide nanowires (accepted for publication in Optics Express) 5. Minggang Xia, Zhaofang Cheng, Jinyun Han, Minrui Zheng, Chorng Haur Sow, John T. L. Thong, Shengli Zhang, Baowen Li Gallium ion implantation greatly reduces thermal conductivity and enhances electronic one of ZnO nanowires AIP Advances, 4, (2014), 057128 6. Junpeng Lu, Hongwei Liu, Minrui Zheng, Hongji Zhang, Sharon Xiaodai Lim, Eng Soon Tok, Chorng Haur Sow Laser modified ZnO/CdSSe core-shell nanowire arrays for microsteganography and improved photoconduction Scientific Reports, 4, (2014), 6350 162 List of Publications 7. Yan Ru Choi, Minrui Zheng, Fan Bai, Junjun Liu, Eng Soon Tok, Zhifeng Huang, Chorng Haur Sow Laser-induced greenish-blue photoluminescence of mesoporous silicon nanowires Scientific Reports, 4, (2014), 4940 8. Junpeng Lu, Hongwei Liu, Suzi Deng, Minrui Zheng, Yinghui Wang, Jeroen A. van Kan, Sing Hai Tang, Xinhai Zhang, Chorng Haur Sow, Subodh G. Mhaisalkar Highly sensitive and multispectral responsive phototransistor using tungstendoped VO2 nanwires Nanoscale, 6, (2014), 7619 – 7627 9. Bablu Mukherjee, Govinda Murali, Sharon Xiaodai Lim, Minrui Zheng, Eng Soon Tok, Chorng Haur Sow Direct laser micropatterning of GeSe2 nanostructures film with controlled optoelectrical properties RSC Advances, 4, (2014), 10013 - 10021 10. Xiaoli Liu, Minrui Zheng, Yunbo Lv, Jie Fang, Chorng Haur Sow, Haiming Fan, Jun Ding Large-scale synthesis of high-content Fe nanotubes/nanorings with high magnetization by H2 reduction process Materials Research Bulletin, 48, (2013), 5003 - 5007 11. Hongwei Liu, Junpeng Lu, Minrui Zheng, Sing Hai Tang, Xinhai Zhang, Chorng Haur Sow Composition-dependent ultra-high photoconductivity in ternary CdSxSe1-x nanobelts as measured by optical pump-terahertz probe spectroscopy Nano Research, 6, (2013), 808 - 821 12. Christie T. Cherian, Minrui Zheng, M. V. Reddy, B. V. R. Chowdari, Chorng Haur Sow Zn2SnO4 nanowires versus nanoplates: electrochemical performance and morphological evolution during Li-cycling ACS Applied Materials & Interfaces, 5, (2013), 6054 - 6060 163 List of Publications 13. Zheng Zhang, Junpeng Lu, Tao Yun, Minrui Zheng, Jisheng Pan, Chorng Haur Sow, Eng Soon Tok Desorption of ambient gas molecules and phase transformation of α-Fe2O3 nanostructures during ultrahigh vacuum annealing The Journal of Physical Chemistry C, 117, (2013), 1509 - 1517 14. Bablu Mukherjee, Zhibin Hu, Minrui Zheng, Yongqing Cai, Yuan Ping Feng, Eng Soon Tok, Chorng Haur Sow Stepped-surfaced GeSe2 nanobelts with high-gain photoconductivity Journal of Materials Chemistry, 22, (2012), 24882 - 24888 15. Junpeng Lu, Cheng Sun, Minrui Zheng, Yinghui Wang, Nripan Mathews, Jeroen A. van Kan, Subodh G. Mhaisalkar, Chorng Haur Sow Ultrasensitive phototransistor based on K-enriched MoO3 single nanowires The Journal of Physical Chemistry C, 116, (2012), 22015 - 22020 16. Wei Jiang Goh, Venkata S. Makam, Jun Hu, Lifeng Kang, Minrui Zheng, Sia Lee Yoong, Chammika N. B. Udalagama, Giorgia Pastorin Iron oxide filled magnetic carbon nanotube-enzyme conjugates for recycling of amyloglucosidase: toward useful applications in biofuel production process Langmuir, 28, (2012), 16864 - 16873 17. Junpeng Lu, Xiaodai Lim, Minrui Zheng, Subodh G. Mhaisalkar, Chorng Haur Sow Direct laser pruning of CdSxSe1-x nanobelts en route to a multicolored pattern with controlled functionalities ACS Nano, 6, (2012), 8298 - 8307 18. Hongwei Liu, Cheng Sun, Junpeng Lu, Minrui Zheng, Kim Yong Lim, Nripan Mathews, Subodh G. Mhaisalkar, Sing Hai Tang, Xinhai Zhang, Chorng Haur Sow Improved electrical property of Sb-doped SnO2 nanonets as measured by contact and non-contact approaches RSC Advances, 2, (2012), 9590 - 9595 19. Bablu Mukherjee, Binni Varghese, Minrui Zheng, K. R. G. Karthik, Nripan Mathews, Subodh G. Mhaisalkar, Eng Soon Tok, Chorng Haur Sow Synthesis, characterization and electrical properties of hybrid Zn2GeO4-ZnO beaded nanowire arrays Journal of Crystal Growth, 346, (2012), 32 - 39 164 List of Publications 20. Zhibin Hu, Chenggang Zhou, Minrui Zheng, Junpeng Lu, Binni Varghese, Hansong Cheng, Chorng Haur Sow K-enriched MoO3 nanobundles: a layered structure with high electric conductivity The Journal of Physics Chemistry C, 116, (2012), 3962 - 3967 21. Tao Wen, Jia Hua Shi, Minrui Zheng, Binni Varghese, Chorng Haur Sow, Hardy Sze On Chan Morphology evolution of polyaniline microstructures via reverse micelles and their hydrophobicity Journal of Nanoscience and Nanotechnology, 12, (2012), 91 - 98 22. Junpeng Lu, Hongwei Liu, Cheng Sun, Minrui Zheng, Nripan Mathews, Gin Seng Chen, Subodh G. Mhaisalkar, Xinhai Zhang, Chorng Haur Sow Optical and electrical applications of ZnSxSe1-x nanowires-network with uniform and controllable stoichiometry Nanoscale, 4, (2012), 976 - 981 23. Junpeng Lu, Cheng Sun, Minrui Zheng, Nripan Mathews, Hongwei Liu, Gin Seng Chen, Xinhai Zhang, Subodh G. Mhaisalkar, Chorng Haur Sow Facile one-step synthesis of CdSxSe1-x nanobelts with uniform and controllable stoichiometry The Journal of Physical Chemistry C, 115, (2011), 19538 - 19545 24. Junpeng Lu, Tao Yun, Minrui Zheng, Chorng Haur Sow Enhanced field emission properties of α-Fe2O3 nanostructures with the removal of adsorbed gas molecules The Journal of Physical Chemistry C, 115, (2011), 8816 - 8824 25. Jia Hua Shi, Tao Wen, Binni Varghese, Minrui Zheng, Chorng Haur Sow, Hardy Sze On Chan One-pot synthesis and electrical characterization of monodisperse solid polyaniline sub-microspheres Advanced Science Letters, (2011), 449 - 457 26. Cheng Sun, Nripan Mathews, Minrui Zheng, Chorng Haur Sow, Lydia Helena Wong, Subodh G. Mhaisalkar Aligned tin oxide nanonets for high-performance transistors The Journal of Physical Chemistry C, 114 (2010), 1331 - 1336 165 List of Publications 27. Su Zi Deng, Hai Ming Fan, Miao Wang, Minrui Zheng, Jia Bao Yi, Rong Qin Wu, Hui Ru Tan, Chorng Haur Sow, Jun Ding, Yuan Ping Feng, Kian Ping Loh Thiol-capped ZnO nanowire/nanotube arrays with tunable magnetic properties at room temperature ACS Nano, 4, (2010), 495 - 505 28. Su Zi Deng, Kian Ping Loh, Jia Bao Yi, Jun Ding, Hui Ru Tan, Ming Lin, Yong Lim Foo, Minrui Zheng, Chorng Haur Sow Room temperature ferromagnetism at self-assembled monolayer modified Ag nanocluster-ZnO nanowire interface Applied Physics Letters, 93, (2008), 193111 166 List of Conference Presentations LIST OF CONFERENCE PRESENTATONS 1. Minrui Zheng, Rongguo Xie, Yongqing Cai, Cong Tinh Bui, Qingxin Zhang, Chorng Haur Sow, Baowen Li, John T. L. Thong Robust bistable thermal conduction in a single cleaved nanowire Workshop on the Chemistry of Energy Conversion: From Molecular Design to Advanced Materials & 6th MRS-S Conference on Advanced Materials, Singapore (2014) 2. Minrui Zheng, Guozhang Dai, Rong Liu, Yunhao Lu, Qinlin Zhang, Bingsuo Zou, Yuan Ping Feng, Haiming Fan, Chorng Haur Sow Synthesis of rotationally-twinned Au-doped ZnTe nanobelts and excitation power dependent redshift in their photoluminescence ICMAT 2011, International Conference on Materials for Advanced Technologies, Singapore (2011) 167 List of Patents LIST OF PATENTS 1. Han Junginger, Giorgia Pastorin, Minrui Zheng, Chengkuo Lee Apparatus that includes nano-sized projections and a method for manufacturing thereof (2011) 168 [...]... material -structure- functionality research cycle and tailor every aspect in achieving the final desired functionality in nanomaterials research 1.2 1.2.1 Zinc chalcogenide family of nanomaterials Crystal structure The common zinc chalcogenide materials family contains four members, namely ZnO, ZnS, ZnSe and ZnTe Their nanostructures constitute one of the most studied family of semiconductor nanomaterials. .. specific 11 Chapter 1 Introduction structures, and finally demonstrating their novel functionalities and potential applications Specifically, we have been choosing to work with complex nanostructures of ZnO and ZnTe from the zinc chalcogenide family This is primarily due to the fact that being the first and last member of the series, ZnO and ZnTe exhibit welldefined distinctive and distinguished materials... low ionicity value and very large Te2- anion size, crystallize undoubtedly in the ZB structure 1.2.2 Applications of zinc chalcogenide nanomaterials Within the family of zinc chalcogenide nanomaterials, ZnO remains to be the star member that has attracted most intensive and unwavering research attention over more than one decade.43 Over the years, many interesting forms of ZnO nanostructures have been... Therefore, materials with high ionicity values and low steric hindrance would in general favor the formation of WZ structure, whereas those with relatively low ionicity and large steric hindrance would favor the ZB structure The ionicity values, the anion radii, and the most stable bulk crystal structure at ambient conditions, together with other information are shown in Table 1.1 for the zinc chalcogenide. .. intriguing and abnormal thermal transport properties in graphene and other two-dimensional materials systems constitute yet another pillar of thermal transport research in nanomaterials nowadays.83,84 1.4 Research motivation and objectives Motivated by the above-mentioned series of key fundamental properties associated with the zinc chalcogenide family of materials, and the fact that each nanostructure... Blue data points are averaged responses with standard deviation Red curve is a best-fit exponential curve with a time constant of 9.8 ms 125 Figure 5.1 (a) Ball -and- stick atomic structure showing the atomic stacking sequence for the zinc- blende (ZB) crystal phase for a binary compound with a slightly rotated view from the direction (b) Ball -and- stick atomic structure showing the atomic stacking... their sizes enter the nanoscale range On the other hand, various materials properties and novel functionalities are also highly correlated with the appearance of their exact structures Here the term "structures" is used in a very broad sense; it in fact includes both morphological/structural layouts at a single -structure level as well as collective structures from an ensemble of materials entities spatially... arising from its specific structure, this thesis sets out to explore complex single-materialcomponent-based nanostructures of selected zinc chalcogenides Efforts would be firstly devoted to developing their synthetic strategies, followed by developing and conducting single-nanostructure-based characterizations that enable the establishment of correlation between the observed properties and their specific... ambient conditions, the zinc chalcogenide materials crystallize in one of the two main crystal structures, 5 Chapter 1 Introduction hexagonal wurtzite (WZ) or cubic zinc blende (ZB) The WZ structure is illustrated in Figure 1.1(a), while the ZB structure unit cell is presented in Figure 1.1(b) The two crystal structures are in fact close variants of each other, in the sense that both structures have tetrahedral... capital letters A, B and C The WZ structure involves the repeated stacking pattern of two types of stacking fashions, and could be coded as having the ABAB stacking sequence The ZB structure, on the other hand, involves the repeated stacking of all three stacking fashions, and could therefore be coded as ABCABC to describe the stacking sequence 6 Chapter 1 Introduction Figure 1.1 (a) Ball -and- stick atomic .  SELECTED ZINC CHALCOGENIDE NANOMATERIALS WITH NOVEL STRUCTURE AND FUNCTIONALITY ZHENG MINRUI (B. Appl. Sc. (Hons.),. size reduction 2 1.2 Zinc chalcogenide family of nanomaterials 5 1.2.1 Crystal structure 5 1.2.2 Applications of zinc chalcogenide nanomaterials 8 1.3 Thermal transport in nanomaterials 9 1.4. structural forms and study nanomaterials with various levels of structural complexity. We focus our attention on selected members, namely ZnO and ZnTe, from the zinc chalcogenide nanomaterials family