フランス国ボルドー大学との共同学位プログラムによる学位 / 京都大学 / 新制・課程博士 / 博士(エネルギー科学) / 甲第23585号 / エネ博第431号 / 新制||エネ||82(附属図書館) / 京都大学大学院エネルギー科学研究科エネルギー基礎科学専攻 / (主査)教授 石原 慶一, 教授 萩原 理加 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DGAM

Semiconductor nanocrystals (NCs) have emerged as promising fluorophores in a plethora of applications including lighting and display technologies. Cd/Pb-chalcogenide-based NCs are by far the most studied classes of semiconductor NCs due to their exemplary luminescence properties. However, their toxicity poses a limit to their widespread application and use in biological systems, nanomedicine, as biomarkers, etc. Therefore, the search for alternatives that can replace Cd/Pb-chalcogenide-based NCs as fluorophores in various applications is a topic of rigorous research. This PhD thesis delves into the development of synthetic strategies for one such class of materials that can potentially replace Cd/Pb-chalcogenide-based NCs in various applications. I-III-VI semiconductor NCs, containing earth abundant metals which are comparatively less toxic than Cd and Pb have emerged as a suitable alternative. In this group, Cu-In-S/Se (CIS/Se) based NCs have gained significant interest due to their nontoxic nature and interesting optical properties. The principal aim of this thesis is to develop synthetic strategies to obtain morphologically vivid CIS/Se NCs and study their optical properties. Due to the multiple reactive species present in ternary /quaternary NCs, direct method of synthesis wherein all precursors are reacted at the same time exhibit problems of inhomogeneous size, shape, and compositions, along with binary byproducts formed in addition to the desired ternary/quaternary NCs. In view of this limitation of direct method of synthesis, a cation exchange (CE) pathway of synthesis has been developed. In this approach, a binary NC is first synthesized using a conventional direct method, which then serves as a host lattice for the incoming third or fourth cation thus leading to the synthesis of ternary or quaternary multicomponent NCs. Employing this route enables the preservation of the morphology and crystal structure of the host NC after the exchange process, leading to better control over size, shape, and composition of the desired NCs. In this thesis, 0D spherical Cu-Zn-In-Se (CZISe) NCs were synthesized using a CE approach starting with binary Cu2-xSe NCs and thereafter the composition dependence of their optical properties was studied. The synthesized quaternary CZISe NCs exhibited intensive tuneable photoluminescence (PL) in the near infrared (NIR) range and narrow PL band widths in comparison to the band widths generally observed in this class of materials. Long-chain organic ligands on the surface of colloidal NCs limit carrier mobility, and hence surface modification of the NCs becomes necessary for applications where carrier mobility is an important aspect, e.g., in solar cell fabrication. Thus, surface modification of the synthesized CZISe NCs was also explored to make the NCs compatible for prospective applications of solar energy harvesting. In addition to 0D NCs, two-dimensional (2D) NCs have gained significant interest due to their unique anisotropic optical properties. For example, extremely narrow PL band widths were exhibited for CdSe nanoplatelets (NPLs) due to the strong confinement of the NPLs in the thickness direction. These 2D NCs have also been utilized in a wide array of applications, particularly in thin film photovoltaics and optoelectronics, and therefore investigation of 2D morphologies of I-III-VI based NCs is also of utmost interest. In this thesis, 2D Cu-Zn-In-S (CZIS) NPLs were synthesized which exhibited rectangular morphology and were unstacked due to the synthetic strategy employed. CIS NPLs were synthesized using a seed-mediated approach and a subsequent CE with Zn enabled the synthesis of CZIS NPLs. Subsequently, a ZnS shell growth leading to the formation of CZIS/ZnS NPLs resulted in the enhancement of PL intensity. As compared to 2D CIS NCs the Se counterpart is less studied and very few reports of 2D CISe-based NCs are present in literature and the reported 2D CISe based NCs have not exhibited any PL. Due to the narrower band gap of CISe than CIS, it is possible to push the PL into the NIR range which unlocks new applications and therefore developing synthetic strategies for 2D CISe based NCs which exhibit PL in the NIR range was also explored in this synthesis. CISe NPLs were synthesized using a similar seed-mediated approach used for CIS NPLs, but the difference in reactivities of S and Se required significant optimization of the synthesis parameters. A subsequent CE with Zn resulted in the synthesis of CZISe NPLs with inherent PL in the NIR range with very narrow PL band widths.

Doping in bulk semiconductors (e.g., n- or p- type doping in silicon) allows for precise control of their properties and forms the basis for the development of electronic and photovoltaic devices. Recently, there have been reports on the successful synthesis of doped semiconductor nanocrystals (or quantum dots) for potential applications in solar cells and spintronics. For example, nanocrystals of ZnSe (with zinc-blende lattice structure) and CdSe and ZnO (with wurtzite lattice structure) have been doped successfully with transition-metal (TM) elements (Mn, Co, or Ni). Despite the recent progress, however, the underlying mechanisms of doping in colloidal nanocrystals are not well understood. This thesis reports a comprehensive theoretical analysis toward a fundamental kinetic and thermodynamic understanding of doping in ZnO, CdSe, and ZnSe quantum dots based on first-principles density-functional theory (DFT) calculations. The theoretical predictions of this thesis are consistent with experimental measurements and provide fundamental interpretations for the experimental observations. The mechanisms of doping of colloidal ZnO nanocrystals with the TM elements Mn, Co, and Ni is investigated. The dopant atoms are found to have high binding energies for adsorption onto the Zn-vacancy site of the (0001) basal surface and the O-vacancy site of the (0001) basal surface of ZnO nanocrystals; therefore, these surface vacancies provide viable sites for substitutional doping, which is consistent with experimental measurements. However, the doping efficiencies are affected by the strong tendencies of the TM dopants to segregate at the nanocrystal surface facets, as indicated by the corresponding computed dopant surface segregation energy profiles. Furthermore, using the Mn doping of CdSe as a case study, the effect of nanocrystal size on doping efficiency is explored. It is shown that Mn adsorption onto small clusters of CdSe is characterized by high binding energies, which, in conjunction with the Mn surface segregation characteristics on CdSe nanocrystals, explains experimental reports of high doping efficiency for small-size CdSe clusters. In addition, this thesis presents a systematic analysis of TM doping in ZnSe nanocrystals. The analysis focuses on the adsorption and surface segregation of Mn dopants on ZnSe nanocrystal surface facets, as well as dopant-induced nanocrystal morphological transitions, and leads to a fundamental understanding of the underlying mechanisms of dopant incorporation into growing nanocrystals. Both surface kinetics (dopant adsorption onto the nanocrystal surface facets) and thermodynamics (dopant surface segregation) are found to have a significant effect on the doping efficiencies in ZnSe nanocrystals. The analysis also elucidates the important role in determining the doping efficiency of ZnSe nanocrystals played by the chemical potentials of the growth precursor species, which determine the surface structure and morphology of the nanocrystals.

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In Chapter 1, I give a basic introduction to the scientific background necessary for understanding the rest of the dissertation. I describe semiconductor nanocrystals and quantum confinement, how nanocrystals grow and a brief description of the various characterization methods. Finally, I provide some of the general considerations and chemical sources for the experiments performed in the thesis as a whole. In Chapter 2, I summarize the journey towards working with molecular precursors and show the advances and challenges in modeling and understanding conversion made a step into nanoplatelets more feasible. The chalcogenourea syntheses are not included and the modeling of the spherical nanocrystals is in a fairly summarized form here. This chapter is intended to give a brief overview of the highlights, key conclusions, and resulting questions upon which I designed my own experiments. In Chapter 3, I discuss applying precursor conversion method to nanoplatelets and focus on 3ML CdE growth. I briefly introduce nanoplatelets, explain the new conditions necessary to adapt the chalcogenourea library, demonstrate my efforts in characterizing the kinetics and growth mechanisms, and finally show the relationship of precursor reactivity and final nanoplatelet size. The “kobs catalogue” which summarizes the kinetics and sizing from STEM is an appendix at the end of the chapter. In this chapter, we put to the test the idea that we can control nanocrystal synthesis through precursor reactivity. The synthesis of nanocrystal heterostructures controlled by precursor conversion was discussed in Chapter 2. In Chapter 4, the same theory is applied to nanoplatelet synthetic conditions, but because nanoplatelet nucleation is fast compared to the total reaction time, the precursors should result in something closer to what is modeled without extraneous products. At the end of the chapter, a nanoplatelet alloy catalogue records many of the modeling and alloy experiments. Chapter 5 attempts to gather the various side projects that working with nanoplatelets has brought about. All these projects come together when thinking about how the solute supply and surface ligands might determine nanoplatelet formation, which I hope to shed some insight on. In the end, I hope to have gathered enough information to provide thoughtful answers for why nanoplatelets form, how they are ideal for studying compositional growth, and how nanocrystal alloying changes the structural and optical properties of these materials.

This thesis explores how the carrier dynamics within semiconductor nanocrystals can be directly engineered through specific core-shell design. Emphasis is placed on how material characteristics, such as strain or alloying at a core-shell interface, can influence the exciton energies and the recombination dynamics within semiconductor nanocrystals. This study synthesises type-II heterojunction ZnTe/ZnSe core-shell nanocrystals via a diethyl zinc-free synthesis method, producing small size distributions and quantum yields as high as 12%. It was found that the 7% lattice mismatch between the core and shell materials places limitations on the range of structures in which coherent growth is achieved. By developing compositional and strained atomistic core-shell models a variety of physical and optical properties could be simulated and has led to a clear picture of the core-shell architecture to be built. This characterisation provides evidence that the low bulk modulus ZnTe cores are compressed by the higher bulk modulus smaller lattice constant ZnSe shells. Further studies show how strain is manifested in structures with 'sharp' core-shell interfaces and how intentional alloying the interface can influence the growth and exciton energies. A (2-6)-band effective mass model was able to distinguish between the as-grown 'sharp' and 'alloyed' interfaces which indicated that strain accentuates the redshift of the excitonic state whilst reduced strain within an alloyed interface sees a reduced redshift. Single nanocrystal spectroscopy investigations of brightly emitting single graded alloyed nanocrystals and of a size series of commercially available CdSe/ZnS nanocrystals showed almost no fluorescence intermittency (nearly 'non-blinking'). These investigations also identified trion recombination as the main mechanism within the blinking 'off' state. Ultimately this thesis adds to the growing understanding of how specific core-shell architectures manipulate the electronic structure and develops techniques to identify specific material characteristics and how these characteristics influence the physical and optical properties within semiconductor nanocrystals.

This thesis explores colloidal semiconductor nanocrystal solutions as a feedstock for creating thin film semiconductor materials through printing processes. This thesis will span the synthesis of nanocrystals, ligand exchange chemistry, solution phase characterization methods, thin film device fabrication, thin film characterization methods, and device characteristics. We will focus extensively relating the structure of nanocrystals in solution and in thin films to their chemistry, optical properties and electronic properties. By way of introduction, the origin and nature of semiconductor nanocrystals will be explored. This discussion will place semiconductor nanocrystals in their historical context, namely the oil-shocks of the 1970s. The interest in II-VI semiconductor materials stemmed from a desire find photochemical synthetic routes to reduce the use of fossil fuels. As a result, II-VI semiconductor nanocrystal are far more developed synthetically. Additionally, our understanding of II-VI semiconductor nanocrystals is couched in the language of solid state physics rather than chemistry. This will lead into a discussion of their electronic structure and the iterative nature of nanocrystal synthetic development and our theoretical understanding of nanocrystals. The first chapter will discuss nanocrystal synthetic methods in a broad context, finally narrowing in on the synthesis chosen for this work. Following a description of the synthesis, we will then describe the ligand chemistry and the reactions which may be performed in the ligand shell. The final sections of the chapter will describe the synthetic routes to the three nanocrystal materials used in the rest of this work, namely CdSe-CdCl2/PBu3, CdSe-CdCl2/NH2Bu, and CdSe/NH2Bu. The second chapter will introduce the crystal structure of II-VI semiconductor nanocrystals and describe how the structure is measured. This will lead in to a discussion of pair distribution function analysis of X-ray data and examples of its application to the solution phase structure of semiconductor nanocrystals. Some size dependent structural properties, namely stain, will be demonstrated by PDF. At the end evidence for surface reconstruction in solution as ligands are removed will be presented. In the final chapter, techniques for film formation and ligand dissolution with be presented. Annealing of films produces electronic and structural changes which can be observed in the absorbance spectrum, electron microscopy, and X-ray scattering. I propose a three phase annealing model which includes 1) reversible desorption of the organic ligands, 2) irreversible particle fusion, and 3 ripening of grains. The temperature at which ripening occurs depends sensitively on the sample content, which increase chloride concentration decreasing the temperature at which ripening occurs. The ripening process is found to correlate with a phase transition from zinc blende to wurtzite, which indicates that grain boundary mobility is an important part of the ripening process. Finally thin film transistors are characterized electronically. Fused grains show superior electron mobility as high as 25 cm2/(Vs) and on/off ratios of 10\up5 and less than 0.5 V hysteresis in threshold voltage without the addition of indium. Surprisingly, the ripened grains show poorer transport characteristics. The manuscript concludes by noting the importance of the sintering process in achieving conductivity in thin films and discussing future directions to build upon this work.

This work investigates the electronic properties of very small gold and semiconductor particles using Scanning Tunneling Microscopy/Spectroscopy (STM/STS) and Ballistic Electron Emission Spectroscopy (BEES). Complementary theoretical works were also performed. The first theoretical work was to calculate the quantized states in the CdS/HgS/CdS quantum-well-quantum-dot nanocrystals. An eight-band envelope function method was applied to this system. This method treats exactly the coupling between the conduction bands, the light-hole bands, the heavy-hole bands, and the spin-orbit split bands. The contributions of all other bands were taken into account using second order perturbation theory. Gold nanocrystals with diameters of 1.5 nm have discrete energy levels with energy spacings of about 0.2 eV. These values are comparable to the single electron charging energy, which was about 0.5 eV in our experimental configuration. Since bulk gold doesnt have an energy gap, we expect the electron levels both below and above the Fermi level should be involved in the tunneling. Measured spectroscopy data have rich features. In order to understand and relate these features to the electronic properties of the nanocrystals, we developed a tunneling model. This model includes the effect of excited states that have electron-hole pairs. The relaxation between discrete electron energy levels can also be included in this model. We also considered how the nanocrystals affect the BEES current. In this work an ultra-high vacuum and low-temperature STM was re-designed and rebuilt. The BEEM/BEES capabilities were incorporated into the STM. We used this STM to image gold nanocrystals and semiconductor nanocrystals. STS and BEES spectra of gold nanocrystals were collected and compared with calculations.