Defects in Nanocrystals: Structural and Physico-Chemical Aspects discusses the nature of semiconductor systems and the effect of the size and shape on their thermodynamic and optoelectronic properties at the mesoscopic and nanoscopic levels. The nanostructures considered in this book are individual nanometric crystallites, nanocrystalline films, and nanowires of which the thermodynamic, structural, and optical properties are discussed in detail. The work: Outlines the influence of growth processes on their morphology and structure Describes the benefits of optical spectroscopies in the understanding of the role and nature of defects in nanostructured semiconductors Considers the limits of nanothermodynamics Details the critical role of interfaces in nanostructural behavior Covers the importance of embedding media in the physico-chemical properties of nanostructured semiconductors Explains the negligible role of core point defects vs. surface and interface defects Written for researchers, engineers, and those working in the physical and physicochemical sciences, this work comprehensively details the chemical, structural, and optical properties of semiconductor nanostructures for the development of more powerful and efficient devices.
Defects in Nanocrystals: Structural and Physico-Chemical Aspects discusses the nature of semiconductor systems and the effect of the size and shape on their thermodynamic and optoelectronic properties at the mesoscopic and nanoscopic levels. The nanostructures considered in this book are individual nanometric crystallites, nanocrystalline films, and nanowires of which the thermodynamic, structural, and optical properties are discussed in detail. The work: Outlines the influence of growth processes on their morphology and structure Describes the benefits of optical spectroscopies in the understanding of the role and nature of defects in nanostructured semiconductors Considers the limits of nanothermodynamics Details the critical role of interfaces in nanostructural behavior Covers the importance of embedding media in the physico-chemical properties of nanostructured semiconductors Explains the negligible role of core point defects vs. surface and interface defects Written for researchers, engineers, and those working in the physical and physicochemical sciences, this work comprehensively details the chemical, structural, and optical properties of semiconductor nanostructures for the development of more powerful and efficient devices.
Solar energy is one of the few renewable, low-carbon sources with both the maturity and accessibility to meet the ever-increasing global demand for energy. There are also accounting for an increasing percentage of our energy output due to increased adoption in both industrial and residential areas. Wafer based silicon photovolatics (PV) technology has dominated the solar market, whereby its price has increased significantly over the last decades. In order to fully capture the solar energy from the sun and extend the flexibility of PV technology, there is a need for constant innovation for new materials. Currently, there is a class pf emerging PV technologies that offer the potential of increased scalability, flexibility and lower prices. They include hybrid organic-inorganic lead halide perovskite PV, organic PV and colloidal quantum dot (CQD) PV. Colloidal quantum dots are semiconducting nanocrystals that exhibit size tunable electronic and optical properties. Owing to their versatility and facile synthesis, they have seen wide application photovoltaics, light emitting diodes, solar concentrators and bio-imaging. In particular, their PV power conversion efficiency has grown rapidly over the last 9 years from 3% to 16.6%. Despite the rapid progress, the search for better PV materials has been carried out almost exclusively through tremendous numbers of trial and error experiments. This is due to the fact that many fundamental aspects of the materials has not been fully understood, especially the role of defects and trap states. Due to the nature of wet chemistry synthesis, vacancies, intersitial and other extended defects inevitably form. These defects often cause in gap states within the semiconductor bandgap, which sensitively impact the performance of the PV devices. In addition, defects are difficult to measure directly using experimental techniques, and we often rely on spectroscopic and imaging to probe their properties indirectly. The core of the work described in this thesis deals with the theoretical understanding of nanocrystals with the goal of achieving a deeper and more fundamental understanding of the material's properties at the atomic scale, focusing on the roles of defects. To this end, we employ a technique of computational electronic structure calculation methods, namely density functional theory (DFT) calculations. In this thesis we will use DFT to investigate and find the role that defects play at controlling the 1) Stokes shift and 2) trap states in PbS quantum dot, as well as the 3) luminescent properties of CuAlS2 nanocrystals. While we show that points defects can cause excessive Stokes shift in single PbS CQDs, and dimer defects are a source of detrimental trap states in PbS CQD solids, the presence of point defects are the source of high luminescence in CuAl2 nanocrystals. We have also provided insights and design guidelines to control defects to design ever more efficient PV devices at an atomic level. This thesis document is organized as follows: Chapter 1 introduce CQD and their applications in PV and other optoelectronic devices. Chapter 2 summarizes the computational techniques employed in this thesis work. Chapter 3 focuses on the origins of the Stokes shift in PbS nanocrystal. Chapter 4 focuses on the PbS superlattice solids, and highlight the origin of trap states in these solids as due to the presence of dimers. Chapter 5 studies the defect physics of CuAlS2, and identifies the defect states responsible for the high photoluminescene.
Despite the vast knowledge accumulated on silicon, germanium, and their alloys, these materials still demand research, eminently in view of the improvement of knowledge on silicon–germanium alloys and the potentialities of silicon as a substrate for high-efficiency solar cells and for compound semiconductors and the ongoing development of nanodevices based on nanowires and nanodots. Silicon, Germanium, and Their Alloys: Growth, Defects, Impurities, and Nanocrystals covers the entire spectrum of R&D activities in silicon, germanium, and their alloys, presenting the latest achievements in the field of crystal growth, point defects, extended defects, and impurities of silicon and germanium nanocrystals. World-recognized experts are the authors of the book’s chapters, which span bulk, thin film, and nanostructured materials growth and characterization problems, theoretical modeling, crystal defects, diffusion, and issues of key applicative value, including chemical etching as a defect delineation technique, the spectroscopic analysis of impurities, and the use of devices as tools for the measurement of materials quality.
Since the size, shape, and microstructure of nanocrystalline materials strongly impact physical and chemical properties, the development of new synthetic routes to nanocrystals with controlled composition and morphology is a key objective of the nanomaterials community. This objective is dependent on control of the nucleation and growth mechanisms that occur during the synthetic process, which in turn requires a fundamental understanding of both classical nucleation and growth and non-classical growth processes in nanostructured materials. Recently, a novel growth process called Oriented Attachment (OA) was identified which appears to be a fundamental mechanism during the development of nanoscale materials. OA is a special case of aggregation that provides an important route by which nanocrystals grow, defects are formed, and unique—often symmetry-defying—crystal morphologies can be produced. This growth mechanism involves reversible self-assembly of primary nanocrystals followed by reorientation of the assembled nanoparticles to achieve structural accord at the particle-particle interface, the removal of adsorbates and solvent molecules, and, finally, the irreversible formation of chemical bonds to produce new single crystals, twins, and intergrowths. Crystallization and Growth of Colloidal Nanocrystals provides a current understanding of the mechanisms related to nucleation and growth for use in controlling nanocrystal morphology and physical-chemical properties, and is essential reading for any chemist or materials scientist with an interest in using nanocrystals as building blocks for larger structures. This book provides a compendium for the expert reader as well as an excellent introduction for advanced undergraduate and graduate students seeking a gateway into this dynamic area of research.
Local defect structures are significant to determine material properties since defects introduced into host materials would affect the local/average crystal environments and thus lead to a change of macroscopic physicochemical performances. The intentional design of specific local defects not only depends on the selected synthesis method and preparation process but also relies on the selected dopant or co-dopant ions. A deep understanding of the intrinsic relationships between local defect structures, chemical synthesis and associated properties is thought as one major framework of material genome plan. It also pushes the design, development and application of novel multifunctional materials. Based on local defect structural design coupled with new synthesis strategies, indium and niobium co-doped anatase titanium oxide nanocrystals are synthesized. It is experimentally demonstrated that the dual mechanisms of nucleation and diffusion doping are responsible for the synergistic incorporation of indium difficult-dopants and niobium easy-dopants, and theoretically evidenced that the local defect structures created by indium, niobium co-dopants, reduced titanium and oxygen vacancies are composed of defect clusters and defect pairs. These introduced local defect structures act as nucleation centres of baddeleyite- and lead oxide-like metastable polymorphic phases and induce an abnormal trans-regime structural transition of co-doped anatase titanium oxide nanocrystals under high pressure. Furthermore, these small co-doped nanocrystals can be used as raw materials to manufacture titania-based ceramic capacitors designed in terms of electron-pinned defect dipole mechanism. The sintering temperature is thus lowered to 1200°C, which conquers the technological bottleneck using this material. To develop the third generation of high-efficient visible light catalysts, nitrogen and niobium co-doped anatase titania nanocrystals are synthesized. Experimental and theoretical investigations demonstrate that the formation of highly concentrated defect-pairs is key to significantly enhance visible light catalytic efficiency. In further combination of local defect structural design and the exploration of new synthesis strategies, anatase nanocrystals containing nitrogen and reduced titanium ions are synthesized. The formation of local defect clusters is demonstrated to play an important role on the obvious enhancement of Rhodamine B degradation efficiency under only visible light illumination. It is thus unveiled that a fundamental understanding of the functions of local defect structures and a well-controlled synthetic strategy are critical to develop highly efficient visible light catalysts with unprecedented photocatalytic performances. Through these systematic investigations, it is concluded that local defect structures generated by introduced co-dopants are complicated in strong-correlated titania systems and differ from case to case. A major difficulty to efficiently introduce difficult-dopant ions such as nitrogen and indium at high concentrations is solved. Two high-efficient visible light catalysts are achieved for environmental remediation by using the clean and renewable solar energy; and one raw material for manufacturing new ceramic capacitors and new metastable polymorphic phases is provided. The discussion on the doping mechanisms, the defect formation and their associated impacts on material performances will not only benefit the future development of physical chemistry, material science and defect chemistry, but also opens a new route to design novel multifunctional materials based on local defect structure design.
Colloidal nanocrystals show much promise as an optoelectronics architecture due to facile control over electronic properties afforded by chemical control of size, shape, and heterostructure. Unfortunately, realizing practical devices has been forestalled by the ubiquitous presence of charge "trap" states which compete with band-edge excitons and result in limited device efficiencies. Little is known about the defining characteristics of these traps, making engineered strategies for their removal difficult. This thesis outlines pulsed optically detected magnetic resonance as a powerful spectroscopy of the chemical and electronic nature of these deleterious states. Counterintuitive for such heavy atom materials, some trap species possess very long spin coherence lifetimes (up to 1.6 μs). This quality allows use of the trapped charge's magnetic moment as a local probe of the trap state itself and its local environment. Beyond state characterization, this spectroscopy can demonstrate novel effects in heterostructured nanocrystals, such as spatially-remote readout of spin information and the coherent control of light harvesting yield.
Dopants and defects play an important role in the luminescence of semiconductors, from classic bulk phosphors to more recently developed colloidal nanocrystals. This thesis describes several studies on the photophysics of defect-related luminescence in semiconductor nanocrystals, with a particular focus on copper and silver charge-transfer luminescence. The combination of density functional theory (DFT) and spectroscopy can provide deeper insight into the photophysics of these materials and help resolve unanswered questions about their luminescence. In chapter 2, DFT is used to investigate various aspects of the electronic structure and photoluminescence mechanism of copper-doped CdSe nanocrystals, including the copper oxidation state, the origin of the broad luminescence line width, and the excited-state singlet−triplet splitting. These calculations support and expand upon previous experimental results. Chapter 3 extends these experimental and computational studies to silver-doped CdSe nanocrystals. These materials have very similar photoluminescence properties to their copper-doped analogues, but they have significant electronic-structure differences due to inverted bonding between the silver dopant and its neighboring anions. Chapter 4 addresses the similarities between the photoluminescence of copper-doped and copper indium sulfide nanocrystals. DFT calculations demonstrate a significant tendency for the hole to localize in the valence band of copper indium sulfide, which has significant copper character and resembles the copper impurity level in doped nanocrystals. Other dopants and defects can also influence nanocrystal luminescence in different ways. Chapter 5 addresses the effects of slow electron trapping and detrapping on the intensity and dynamics of delayed luminescence in nanocrystals by varying the excitation pulse width. Chapter 6 is an investigation of photoinduced magnetization in manganese-doped CdSe nanocrystals; the formation of excitonic magnetic polarons is studied by continuous-wave and time-resolved magneto-optical spectroscopies. Together, these studies demonstrate a variety of ways that dopants and defects can affect the photoluminescence of semiconductor nanocrystals.
We focused on cutting-edge science and technology of Nanocrystals in this book. "Nanocrystal" is expected to lead to the creation of new materials with revolutionary properties and functions. It will open up fresh possibilities for the solution to the environmental problems and energy problems. We wish that this book contributes to bequeath a beautiful environment and valuable resources to subsequent generations.
This textbook provides students with a complete working knowledge of the properties of imperfections in crystalline solids. Readers will learn how to apply the fundamental principles of mechanics and thermodynamics to defect properties in materials science, gaining all the knowledge and tools needed to put this into practice in their own research. Beginning with an introduction to defects and a brief review of basic elasticity theory and statistical thermodynamics, the authors go on to guide the reader in a step-by-step way through point, line, and planar defects, with an emphasis on their structural, thermodynamic, and kinetic properties. Numerous end-of-chapter exercises enable students to put their knowledge into practice, and with solutions for instructors and MATLAB® programs available online, this is an essential text for advanced undergraduate and introductory graduate courses in crystal defects, as well as being ideal for self-study.
