Abstract
The p-type copper sulfides system with unique properties and the corresponding applications have been investigated in the thesis. In first part, Cu2S nanowire arrays have been fabricated via a facile hydrothermal method on copper substrate. The copper substrate could be served as electrode in ReRAM devices, in which the excellent resistive switching (RS) performance (Vset < 0.2 V, Vreset < -0.1 V, retention > 103, on-off ration > 105) could be demonstrated. Then ZnO branched structure has been deposited on Cu2S for the self-rectifying p-n junction, as the selector of 1D1R structure to prevent the leakage issue. The 1D1R structure also exhibited promising performance (Vset < 1.3 V, Vreset < -0.6 V, on-off ration > 105). In the second part, multilevel resistance has been demonstrated for devices based on individual Cu2S nanowire with two inert (W) electrodes. Up to five levels can be achieved by varying the compliance current (C.C.), significantly enhancing the data storage density. Compared to previous works on multilevel memory, the present devices exhibit outstanding performances with lower operating voltage (Vset < 0.6 V at IC.C.= 1 μA), higher on-off ratio (>105) and longer retention time (> 103 min). From in-situ SEM and TEM analysis, the RS behavior of Cu2S nanowires under high C.C. (> 1 μA) has been found to be dominated by Cu ion diffusion inside the Cu2S nanowire. On the other hand, holes and vacant Cu lattice sites control the RS under low C.C. (< 800 nA). The results of temperature-dependent measurements of resistivity also strongly support the proposed mechanisms. The facile fabrication of Cu2S nanowires with the capability of multilevel switching shall facilitate the realization of high density memristor applications. In the third part, localized surface plasmon resonances (LSPR) in near-infrared (NIR) region have been extensively studied for copper chalcogenide nanostructures, not only for the absorption enhancement but also tunable LSPR characteristics with their free carrier concentrations or defects. In the present work, one-step cation exchange method has been used to synthesize Cu2-xS nanowires with x varied between 0 and 1, including Cu2S, Cu7S4 and CuS and so forth. The plasmonic band of Cu2-xS nanowires shifts to a shorter wavelength with the increase in x, as observed in VIS-NIR spectra, which is attributed to the increase in density of copper vacancies. The Cu2-xS nanowires have been used as catalysts towards the photocatalytic generation of H2 from ammonia borane (AB). Among samples with different Cu-S compositions, Sample 3 which is mainly composed by Cu7S4 exhibited the highest activity in terms of H2 evolution rate (25.54 mmol/g.h). Moreover, a marked enhancement of the H2 evolution rate (157.04 mmol/g.h) could be achieved after decorating the Cu2-xS nanowires with Pd nanoparticles to form the hybrid structures. The results of the present investigation may lead to an effective strategy for the design and development of LSPR materials for photocatalytic applications.