Rational Design of Functional Heterostructures in Low-Dimensional Nanomaterials

Nano-scale heterostructure interface, in which geometric and electronic structures of nanomaterials are effectively coupled, brings about novel and unique functionalities that cannot be obtained by single constituents. The intensive research impetus on the interfaces formed by dissimilar nanomaterials attracts world-wide attention due to the interest in both fundamental understanding of interfacial charge transfer mechanisms and their potential applications in a variety of fields. This dissertation reports our recent progress in nano-scale heterostructure research including the novel approach development for functional heterostructure formation and a variety of potential applications in different fields such as heterogeneous catalysis, optoelectronics, ionic modulation, etc. Specifically, precise and controllable heterostructure design is comprehensively investigated and the unique physical properties of carefully designed heterostructures are extensively explored. Chapter 1 illustrates a brief introduction to the historical development of low-dimensional heterostructure formation and reveals the motivation to the further exploitation of heterostructure design. Specifically, one- and two-dimensional heterostructure materials are reviewed with different types of formation approaches for a variety of functionalities. The thrust into this field not only provides deep inspiration for the fundamental understanding of the material design and integration but also sheds light on the potential practical applications. In Chapter 2, the fabrication of high-aspect-ratio silicon nanowire arrays with tunable length and diameter has been optimized, initializing an energy-related application about interfacing silicon nanowire light absorber with catalytic-selective metal nanoparticles for high-efficient solar-driven CO2 conversion to value-added products. On the one hand, vertically one-dimensional silicon nanowire arrays provide a structural guide in directing well-dispersed metal nanoparticle assembly, allowing for the controllable nanoparticle coverage for optimal catalytic performance. On the other hand, high mass activity of Au3Cu nanoparticle catalyst requires the use of high-surface-area semiconductor light-harvesting substrates to realize its full potential with the reactive surface spots. Such Au3Cu nanoparticle decorated silicon nanowire arrays can readily serve as effective CO2 reduction photoelectrodes, exhibiting high CO2-to-CO selectivity close to 80% at -0.20 V vs RHE with suppressed hydrogen evolution. A reduction of 120 mV over-potential was achieved compared to the planar silicon counterpart resulting from the optimized spatial arrangement of nanoparticle catalyst on the high surface area nanowire arrays. Chapter 3 presents the p-n junction formation through a localized thermal-driven phase transition in a single-crystalline halide perovskite CsSnI3 nanowire. This material undergoes a phase transition from a double chain yellow phase to an orthorhombic black phase. The formation energies of the cation and anion vacancies in these two phases are significantly different, which leads to n- and p- type electrical characteristics for yellow and black phases respectively. While incompatibility of the conventional electronic device fabrication process is detrimental for moisture- and chemical-sensitive halide perovskites, novel material processing technique that has been developed here produces precisely controlled device geometries with the preservation of high material quality, which allows the further understanding of the charge transport properties through electrical and thermoelectric characterization. Based on this, the interface formation between these two phases and directional interface propagation within a single NW are directly observed, and the current rectifying behavior of the resulting p-n heterostructure originating from the distinctly different charge transport properties of these two phases was expected. Chapter 4 turns to the investigation of the ionic effect in soft and reconfigurable halide perovskite lattices. A CsPbBr3-CsPbCl3 heterostructure nanowire that bridges the pre-patterned parallel Au electrodes was fabricated via a precisely controlled anion exchange approach. Quantitative correlation between halide ion ratios and photoluminescence emission wavelengths in halide perovskites allows us to visualize the halide ion migration across the heterostructure interface under electric fields. More interestingly, the research revealed that the halide ion migration is heavily dependent on the bias polarity across the nanowire, leading to the field-induced halide ion migration rectification in this solid-state material. This ionic modulation behavior opens up new perspectives for realizing the ionic control and ionic coupling implementation in perovskite-based optoelectronic devices