Structure and Transport in Nanocrystalline Cadmium Selenide Thin Films
Creators: Norman, Zachariah Mitchell
Publisher: name: Columbia University
Year: 2015
Subjects: Semiconductor nanocrystals
Semiconductor nanoparticles
Thin films
Thin film devices--Design and construction
Nanostructured materials
Cadmium selenide
Nanofluids
Chemistry
Materials science

description: 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.