NSF Award
1710063
Non-technical Abstract:<br/>Colloidal nanostructures are emerging as attractive candidates for low-cost processing of next-generation optical and electronic materials. Their characteristic size falls within the unique length scale where inorganic semiconductors exhibit tunable, molecular-like properties while retaining a good thermal and photo-stability. The chemical synthesis of semiconductor nanostructures typically results in a large variety of particle shapes and sizes, which negatively affects the ensuing device performance. To address this issue, the project aims to explore an alternative synthetic strategy for growing uniform, atomically-defined semiconductor nanoparticles. The innovation lies in employing chemical rather than thermal growth, which benefits from reaction-controllable, incremental steps in the particle formation. This strategy enables an unprecedented precision in controlling both the shape and the uniformity of semiconductor nanostructures. The development of a synthetic methodology for growing atomically-defined colloidal nanoparticles can potentially affect a wide range of emerging technologies relying on solution processing of semiconductors. It is expected to result in improved electrical and optical performance of materials for the solar fuel generation and solid state lighting. The educational aspects of this project are planned to focus on elucidating students through research activities and curriculum development.<br/><br/>Technical Abstract:<br/>The project aims to explore a reaction-limited growth of colloidal semiconductor nanocrystals in an effort to achieve a controllable evolution of particle shapes and sizes. Nanocrystal synthesis is typically performed via thermal activation of precursors which benefits from a fast, low-defect crystallization. As a diffusion-limited growth mechanism, however, the hot-injection strategy makes it inherently difficult to control the nanoparticle size dispersion, ligand coverage, and the cation-to-anion ratio across the sample. As a result, distinct optoelectronic properties of individual nanoparticles become inhomogenously broadened in assemblies. The present strategy employs the sequential deposition of fully saturated cationic and anionic monolayers onto small-diameter clusters, which leads to focusing of particle sizes with the increasing diameter. Each ionic layer is grown via a colloidal analog of the atomic layer deposition to keep precursors and nanocrystals in separate phases. As a result, a self-limited monolayer deposition becomes very effective leading to stoichiometrically defined surfaces at each ion growth cycle. It is expected that an improvement in nanoparticle uniformity down to atomically precise structures is attainable through this strategy. An improved control over the nanoparticle size dispersion is likely to advance the development of light-emitting devices, whereas a well-defined surface composition could avail film-based nanocrystal applications (solar cells, transistors).
