Direct Assembly of Single Nanocrystal Arrays

Abstract

Nanocrystals are a remarkable class of materials, which exhibit size-dependent optical and electronic properties. Numerous applications have been proposed for these materials but they suffer from a key handicap. Nanocrystals are generally made in solution, rendering integration into devices very challenging. Therefore, there is an increasing demand for new fabrication methods to transfer nanocrystals from solution to a solid-state substrate. In this thesis, we develop a new, direct assembly method based on electrophoretic deposition (EPD), which we call Surface Templated Electrophoretic Deposition (STEPD). Our research starts by demonstrating the successful assembly of single gold nanocrystal arrays. The strength of the applied electric field and the electrolyte concentration are key parameters that control the assembly process. The orientation of asymmetric gold nanorods can also be controlled by carefully designing the template. With further experimental and theoretical investigation, we find that the electrically induced dipole moment on asymmetric nanorods plays a major role in orientating the nanorod during EPD assembly. Our STEPD method is not only able to assemble gold nanorods in designed orientation horizontally but also able to assemble gold nanorods vertically with respect to the substrate. The universality of STEPD is also demonstrated by the successful assembly of gold nanocrystals with different sizes and shapes, magnetic Fe$_3$O$_4$ nanocrystals, fluorescent organic nanoparticles and semiconductor quantum dots with different photoluminescence. During our investigation, we find that there is a minimum particle size limit in STEPD. Very small nanocrystals (i.e. < 20 nm in size) are difficult to assemble via EPD due to their smaller total charge and stronger Brownian motion. Finally, we propose three potential applications derived from STEPD assembled arrays including hydrogen gas sensing, single particle addressing and anti-reflective metasurfaces. Through this thesis, we believe the direct STEPD assembly method holds great potential as an efficient and versatile assembly method for large-area, single nanocrystal arrays.