Kwok Siong Teh, PhD, San Francisco State University
Research on energy materials is growing in importance as traditional sources of energy are becoming increasingly difficult and costly to obtain. Among the various types of energy materials, transition metal oxide nanostructures -- especially zinc oxide (ZnO) nanostructures -- have been heavily researched because they exhibit dual semiconducting and piezoelectric properties, and are bio-safe. These properties make ZnO nanostructures a suitable candidate for energy generation in applications such as bulk heterojunction photovoltaic cells and piezoelectric energy scavengers. While ZnO nanostructures hold tremendous potential to be a practical energy source, little is known about how its conversion efficiency and long-term reliability are influenced by fundamental properties such as dimensional and mechanical properties. In addition, time-consuming and costly fabrication techniques that often necessitate the use of catalysts are a critical obstacle that inhibits the broad applications of ZnO for energy usage. In this project, we (1) develop a catalyst-free, rapid synthesis method to fabricate ZnO nanostructures for energy generation, and (2) investigate how the dimensional and mechanical properties of ZnO nanostructures affect their energy conversion efficiencies.
During the reporting time period, our work focused on the development of an ambient-pressure, catalyst-free rapid thermal method to synthesize (1) non-porous, conformal nanocrystalline ZnO films (2) template-less, aligned ZnO nanostructures, and (3) ZnO nanorods by high-temperature thermal annealing in an inert environment. We have characterize the quality and crystallinity of these ZnO nanostructures and nanocrystalline films.
2.1 Catalyst-free, rapid synthesis of ZnO nanostructures on silicon
Using a low-power (between 90-110W) rapid thermal plasma chemical vapor deposition (CVD) technique developed in-house, we have achieved highly controllable deposition of conformal ZnO nanocrystalline thin films on non-lattice matched crystalline surfaces such as Si(100), sapphire, and muscovite, indium tin oxide; amorphous surfaces such as borosilicate glass, fused quartz, titanium, gold; and more importantly, polymer (polyimide). With a previously developed close-looped temperature control system, we have achieved good repeatability of a well-controlled deposition process for dense, non-porous ZnO nanocrystalline thin films at substrate temperatures between 160C to 330C. Importantly, such repeatable, fine temperature control allowed us to deposit ZnO films on flexible substrates such as polymer with no observable damage to the substrate. Based on scanning electron microscope (SEM) images, energy dispersive x-ray (EDX), and x-ray diffraction (XRD) analyses of ZnO deposited on various substrates, we observed the crystallographic orientations of the as-deposited nanocrystalline films to be predominantly c plane-oriented. This points to the fact that influence of the substrate minimal and the growth of the films is independent of substrate type. This result has been reported in a journal article and several conference proceedings manuscripts we have published.
2.2 Template-less, Aligned ZnO Nanowires Synthesis
Using the rapid thermal plasma CVD process developed, we were successful at synthesizing aligned ZnO nanowires without the use of a template. These nanowires are of uniform diameters at approximately 200 nm each, and are spaced 400 nm apart, in an orderly fashion. One of the key findings was that at relatively medium substrate temperatures (around 500C), a ZnO film consisting of predominantly highly textured, c-oriented ZnO crystals were observed at the initial stage of the crystal growth. The surface uniformity of such c-oriented ZnO “mesas” on a lattice-mismatched Si substrate allows the mesas to act as a seed layer for the subsequent growth of c-oriented ZnO nanowires. Hence, by controlling substrate temperature in the aforementioned temperature range, we were able to synthesize highly aligned ZnO nanowires with good repeatability.
2.3 ZnO Nanostructures via High Temperature Annealing
While aligned nanowires tend to form at medium temperatures (around 500C), we discovered that at relatively high annealing temperature (above 800C) in an inert (argon) environment, ZnO crystals could restructure and produce secondary growth of c-oriented nanorods on ZnO grains. This secondary growth of ZnO nanorods on select ZnO grains occur at the expense of grains adjacent to said grains. In short, at elevated temperature, a dominant grain would continue to grow in the c-direction by consuming materials (Zn and O) from nearby grains. We hypothesize this to be due to a combination of grain boundary and bulk diffusion associated with the energy of the atoms. This may explain the more porous nature of the annealed sample.
During the second/third (extension) year of this grant, the PI was able to (1) establish additional control hardware and apparatus essential to the success of the project, and gather more important research data, and (2) involved 5 more engineering undergraduate students, including two Afro-American students and one woman, in the research of ZnO. Results made possible with the support of this research grant has been published in the Nanoscale Research Letters in October 2011 (DOI: 10.1186/1556-276X-6-568) and numerous IEEE conferences including IEEE NEMS 2012 and IEEE NANO 2011, as well as Materials Research Society Spring Meetings (2011, 2012), with two former undergraduate researchers, Joachim Pedersen and Heather Esposito, as student co-authors. In addition, this research grant has been instrumental in two former undergraduate students, Joachim Pedersen and Mark Brunson, gaining admission into prestigious doctorate programs at UC Davis and University of Washington (UW), Seattle. Joachim, in particular, was awarded a 3-year NSF Graduate Research Fellowship for the ZnO work he carried out under the sponsorship of this ACS PRF Grant. Mark was first awarded a 2011 ASM-SAMPE Materials Technology Scholarship in November 2011, and subsequently, he was awarded a 3-year ARCS Fellowship to attend the doctorate program in materials science at UW Seattle for his part of the ZnO work performed in the lab. We are very grateful for ACS’s support of this work and the students.