Synthesis and assembly of binary, ternary and quaternary semiconductor nanorods

Herein is presented novel routes towards synthesis and assembly of colloidal semiconductor nanorods. A versatile colloidal approach was developed to synthesise complex multicomponent copper chalcogenide (Cu2ZnSnS4) nanorods with tight control in shape, size and crystal phase. Assembly protocols to control the position and orientation of semiconductor nanorods were significantly progressed in this work leading to controlled approaches for assembly of binary (CdS, CdSe) to ternary (CuInS2) and quaternary (CuInGaS2, Cu2ZnSnS4) nanorods into close packed superstructures. Specifically, the preparation of highly monodispersed, anisotropic Cu2ZnSnS4 (CZTS) colloidal nanocrystals is described in chapter 2. The reaction conditions used (selectivity of precursors, addition of anionic precursor at low temperature (150-160 °C), and surfactant/solvent composition) resulted in a tuneable shape from spherical to rod CZTS nanocrystals. The formation of rod shaped CZTS nanocrystal is only possible by stabilising the nuclei of nanocrystal in wurtzite phase by using strong coordination ligands such as thiols, followed by controlled growth in (002) direction. These monodispersed nanorods are further assembled into 2D sheets, consisting of each rod vertically aligned by using drying mediated assembly approach. A charge based assembly method is introduced in chapter 3 and 4 where a precise control of nanorod orientation (1D, 2D, 3D) in the assembled superstructure is achieved by manipulating the surface charge of nanorods. By optimising the initial concentration of nanorods in solution, it is possible to nucleate the assembly as 2D sheets where each rod is close-packed in the side by side fashion. These 2D sheets are further deposited on the substrate by gravity sedimentation and give interesting Moiré pattern when analysed with TEM/STEM as discussed in chapter 5. These Moiré interference patterns arise from rotational offsets between respective monolayer 2D sheets and the exact degree of rotational offsets can be measured by SAED and FFT. In Chapter 6, the reversible assembly de-assembly method is shown for ternary and quaternary nanorod (CuInS2 and CuIn1-xGaxS2) in solution. The monodispersed nanorods are self-assembled into micron size 3D superstructure over time. These superstructures attain the perfect hexagonal close-packing which is predominantly entropy driven. These superstructures were de-assembled by a facile ligand exchange; reverting the solution to a random nanorod dispersion. As these nanorods have a modified ligand environment which influence the net charge, their vertical assembly were then controlled by either optimizing the solution concentration to form micron size free floating 2D sheets or on the conductive substrate by electrophoresis. Chapter 7 discussed the influence of external field such as DC field on the nanorod assembly. The use of DC field in the nanorod solution enables the uniform vertical alignment of nanorods on a square-centimetre conductive substrate. The precise control on the number of layers is shown. The research work summarized in this thesis describes both synthetic and assembly techniques that enable a fine control on shape/size morphology and composition at nanoscale level that is extendable to the device-scale.