UC Berkeley

Semiconductor nanomaterials have become an important class of materials with great potential for applications ranging from catalytic to electronic and optoelectronic devices. For next generation catalytic, optoelectronic, and photonic applications, the synthesis of high-qualify nanomaterials with uniform size, well-defined morphology, composition, and surface chemistry is of key importance, because the electrical, optical, and magnetic properties of these nanomaterials are strongly dependent on those parameters. Besides technical interests, access to defined nanoscale structures is also essential for uncovering their intrinsic properties unaffected by sample heterogeneity. Rigorous understanding of the properties of individual nanocrystals will enable us to exploit them, making it possible to better design and build novel electronic, magnetic, and photonic devices and other functional materials based on these nanostructures.

This dissertation explores both direct synthetic methods and post transformation approaches for rational synthesis of new nanomaterial systems, which are potential candidates for applications in areas of photovoltaics, non-noble-metal plasmonics, light emitting diodes, etc. And their structural, optical, and electrical properties have been investigated in detail. Chapter 1 provides an introduction to the current progress and common strategies used in rational control of the size, shape, composition, and surface chemistry of nanomaterials. Chapter 2 examines the Cu+ cation-exchange mechanisms in CdS nanowires. A detailed transformation diagram of cation-exchange chemistry from CdS to Cu2-xS nanowires is reported. By varying the reaction time and the reactants’ concentration ratio, the progression of the cation-exchange process was captured, and tunable crystal phases of the Cu2-xS are achieved. The overall process occurs in three stages: formation of discontinuous Cu2-xS islands, formation of core-shell CdS-Cu2-xS heterostructures, and complete conversion to Cu2-xS nanowires with controllable crystal phases. Detailed structural characterization reveal that the resultant Cu2-xS phases become more stoichiometric with increasing reaction time and copper precursor concentration. This experimental result suggests a kinetically controlled process limited by diffusion. In Chapter 3, a catalyst-free, solution-phase approach has been developed to obtain single crystalline, orthorhombic CsPbX3 NWs with uniform growth direction. The morphological evolution of the CsPbBr3 nanostructures along the reaction has been investigated, and the reaction protocol has been optimized to achieve a high yield of monodispersed nanowires likely due to a soft template mechanism. The direct synthesized CsPbI3 NWs show a room-temperature stable double-chain phase, with weak photoluminescence mainly from the trap states. Anion-exchange reaction by using the monodispersed CsPbBr3 NWs as templates can retain the favorable corner-sharing orthorhombic phase, and independently control the NW compositions, thus access to a wide range of compositions with bright and tunable photoluminescence spanning over nearly the entire visible spectrum. Meanwhile, surface treatment with the original precursors was performed to effectively passivate the surface states, and improve the quantum yield to over 10 times. In Chapter 4, a stepwise purification method has been developed to purify the ultrathin CsPbX3 NWs with a uniform diameter of 2.2±0.2 nm. The structural and optical properties have been discussed. Aberration-corrected high-resolution TEM shows the NWs are single crystalline, absorption and fluorescence spectrum shows that those NWs possess strong two-dimensional quantum confinement effects along with bright emission. The band gap of these ultrathin NWs can be tuned by anion-exchange reaction.