UC Berkeley

One-dimensional inorganic nanowires have many properties different from their bulk form. They have been demonstrated as crucial building blocks in various applications such as electronics, optics, catalysts, and sensors. Colloidal synthesis is a powerful method for the preparation of nanowires, not only because it is low-cost and scalable, but also due to its flexibility in achieving shape and composition control. To fully exploit nanowires’ unique traits in functional devices, the syntheses of materials should be tailored to the particular application. How to rationally design new chemistry to produce nanowires that suffice specific chemical and physical requirements is essential. This dissertation explores the cases where the unique geometry of nanowires is elegantly utilized, in transparent conductors and as catalyst. The efforts that have been made to design, and develop new materials towards better device performance are discussed. Advanced characterizations and carefully contemplated control experiments are also described in detail, aiming to provide new insights into reaction mechanism and structure-function correlation.

Metal (especially copper and silver) nanowire mesh is considered among the best candidates for replacing indium-tin-oxide as the next generation transparent electrode material. Compared to conventional oxide based conductors, nanowire mesh has the advantage of high mechanical flexibility, and low processing cost while maintaining good optical-electrical performance. However, the major challenges of nanowire electrodes are the strong light scattering and low air stability. Large aspect-ratio, small-diameter and surface stabilized metal nanowires are desired for achieving a high-performance transparent conducting film. A new, general synthesis approach which uses heat driven organic radicals as a mild reducing agent to make high-aspect ratio ultrathin copper nanowires (diameter ~ 17nm; length ~ 17 μm) has been developed. The reaction mechanism has been studied in detail using in-situ, temperature dependent electron paramagnetic resonance spectroscopy. These mild radicals slowly reduce copper ion into decahedral nanoseeds in presence of primary amine and then grow further into five-fold twinned nanowires. The transparent conducting films made from the ultrathin nanowires exhibit excellent transparency and conductivity (sheet resistance ~ 28 Ω/sq, transmittance ~ 90%, haze ~ 2%). Moreover, these films show significantly reduced haze factors in comparison with reports thus far, mainly due to the small diameter. Furthermore, to overcome the limitation of copper’s low stability towards oxidation, we developed a solution based approach to coat reduced graphene oxide nanosheets onto the nanowire surface. The core-shell structure greatly enhanced the air-stability of the conducting films. The resulting nanowire electrodes maintained its original conductivity after being stored over 200 days in ambient air and up to 48 hours in high temperature high humidity environment. We also successfully grow a few atomic layers of gold atoms on to the copper surface. These epitaxial Cu@Au core shell nanowire films exhibit even greater stability which can hold up their original conductivity up to 700 hours in harsh environment.

Metal nanowires can also be used as catalyst. Copper is uniquely active for the electrocatalytic reduction of carbon dioxide (CO2) to products beyond carbon monoxide, such as methane and ethylene. Therefore, understanding selectivity trends for CO2 electrocatalysis on copper surfaces is critical for developing more efficient catalysts for CO2 conversion to higher order products. We investigate the electrocatalytic activity of ultrathin (diameter ~20 nm) five-fold twinned copper nanowires for CO2 reduction. These copper nanowire catalysts were found to exhibit high methane selectivity over other carbon products, reaching 55% Faradaic efficiency (F.E.) at -1.25 V vs. RHE while other products were produced with less than 5% F.E. This selectivity was found to be sensitive to morphological changes in the nanowire catalyst observed over the course of electrolysis. Wrapping the wires with graphene oxide was found to be a successful strategy for preserving both the morphology and selectivity of the Cu nanowires. These results suggest that product selectivity on Cu nanowires is highly dependent on morphological features, and that hydrocarbon selectivity can be manipulated by structural evolution.

Semiconductor nanowires are an important class of nanomaterials. Silicon is the most important semiconductor in microelectronics industry. Continuous device miniaturization has led to the emergence of the nanoelectronics industry, in which Si nanowires can play an essential role. Most of the bottom-up synthesis of silicon nanowires are via the vapor-liquid-solid method, which normally requires temperature above 400 °C. In this work, an approach of low-temperature synthesis of silicon nanowires is described. This method uses tris(trimethylsilyl)silane or trisilane as the Si precursor via a Ga-mediated solution-liquid-solid approach to cut down the growth temperature of silicon nanowire to below 200 °C. It is further demonstrated that this Si chemistry can be adopted to incorporate Si atoms into III-V semiconductor lattices, which holds promise to produce new Si-containing alloy semiconductor nanowire.