Three-dimensional epitaxy of single crystalline semiconductors by polarity-selective multistage growth

Abstract

Despite recent advances in three-dimensional (3D) fabrication, the epitaxial growth of finely controlled, single crystalline 3D structures is still constrained. In this study, we demonstrate the step-by-step growth of hierarchical 3D architectures composed of single crystalline semiconductors. To achieve this goal, we first established control of the preferential growth direction of ZnO crystals by polarity-selective crystallization during the low-temperature solution-phase synthesis. The time-dependent analysis showed that the binding of a citrate additive to the positively charged, top (0001) surface of ZnO crystals led to an inversion of the growth anisotropy from predominantly the vertical direction to the lateral direction with more than a 10 times increase in the width-to-height ratio. With further fine optimization of the charge balance, we minimized the interruption of citrate ions for epitaxial and iterative stacking of Zn and O ions, and achieved single-crystalline hexaplates. This feature was further combined with multistage epitaxial growth of the nanocrystal constituents, producing a 3D, single crystalline semiconductor with excellent luminescent characteristics.

Low-Temperature Hydrothermal Synthesis of Colloidal Crystal Templated Nanostructured Single-Crystalline ZnO

“…The reaction solutions contained a small concentration of citrate ions to suppress the normally rapid ZnO crystal growth in the c -direction, increasing the relative growth rate in the lateral direction. ,, The effect of the citrate ions on the 3D epitaxy process is shown in Figure , which compares the surface morphologies of single crystal ZnO layers prepared using templates (600 nm diameter PS colloids) from solutions with and without citrate ions. SEM image of the ZnO structure formed in the absence of citrate ions (Figure a) obtained after the removal of the template reveals that an array of hexagonal rods, whose shape does not correspond to the curved surface of the colloidal spheres of the template, is formed on the top surface.…”

“…When citrate ions are added to the solution, a porous ZnO structure that corresponds more closely to the interstitial volume of the colloidal template is formed (Figure b), confirming that a generally isotropic growth is preferred to fill the complex internal void space of the colloidal template. In the case of ZnO, the negatively charged citrate ions are preferentially adsorbed on the relatively positively charged (0 0 0 1) surface of the ZnO crystal, leading to the suppression of ZnO growth in the c -direction relative to growth in other directions. , …”

“…In the case of ZnO, the negatively charged citrate ions are preferentially adsorbed on the relatively positively charged (0 0 0 1) surface of the ZnO crystal, leading to the suppression of ZnO growth in the c-direction relative to growth in other directions. 14,33 Optical Properties. The optical properties of the porous ZnO formed via hydrothermal 3D epitaxy were examined through reflectance measurements.…”