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Low-temperature hydrothermal synthesis of colloidal crystal templated nanostructured single-crystalline ZnO Masao Miyake, Makoto Suginohara, Naoto Narahara, Tetsuji Hirato, and Paul V. Braun Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b03466 • Publication Date (Web): 26 Oct 2017 Downloaded from http://pubs.acs.org on October 27, 2017
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Chemistry of Materials
Low-temperature hydrothermal synthesis of colloidal crystal templated nanostructured single-crystalline ZnO Masao Miyake*,†,‡, Makoto Suginohara†, Naoto Narahara†, Tetsuji Hirato†, Paul V. Braun*,‡ †
Graduate School of Energy Science, Kyoto University, Kyoto 606-8501, Japan
‡
Department of Materials Science and Engineering, Frederick Seitz Materials Research Laboratory, and Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA
ABSTRACT: Single crystal semiconductors almost always exhibit better optoelectrical properties than their polycrystalline or amorous counterparts. While three-dimensionally (3D) nanostructured semiconductor devices have been proposed for numerous applications, in the vast majority of reports, the semiconductor is polycrystalline or amorphous, greatly reducing the potential for advanced properties. While technologies for 3D structuring of semiconductors via use of a 3D template have advanced significantly, approaches for epitaxially growing nanostructured single crystal semiconductors within a template remain limited. Here, we demonstrate the epitaxial growth of 3D-structured ZnO through colloidal templates formed from 225 nm and 600 nm diameter colloidal particles via a low-temperature (~80 °C) hydrothermal process using a flow reactor. The effects of the pH of the reaction solution as well as the additive used on the 3D epitaxy process are investigated. The optical and electrical properties of the epitaxially grown nanostructured ZnO are probed by reflectance, photoluminescence, and Hall effect measurements. It is found that the epitaxially grown nanostructured ZnO generally exhibits superior properties than those of polycrystalline ZnO. The demonstrated hydrothermal epitaxy process should be applicable to other chemical solution-based deposition techniques and help extend the range of materials that can be grown into a 3D nanostructured single-crystalline form.
INTRODUCTION Three-dimensionally (3D) nanostructured semiconductors have been intensively investigated for applications including sensors, solar cells, light-emitting diodes (LEDs), energy storage and catalysis.1–7 Of the various techniques developed for the fabrication of 3D structured materials, synthesis using a sacrificial template is one of the most flexible.7–10 Numerous techniques are available for creating and filling 3D templates, allowing for a diverse set of materials to be formed into exquisite structures.7–10 However, most of the techniques result in materials that are either polycrystalline or amorphous. This is particularly problematic for electronic and optoelectronic applications, where grain boundaries, structural disorder, and other defects on the atomic scale generally have a significant negative effects on the important properties. Not surprisingly, most high-performance electronic and optoelecronic applications use single-crystal semiconductors. As we and others have shown, a limited set of singlecrystalline 3D nanostructured materials can already be obtained by growing material epitaxially into 3D templates off single-crystal substrate. Specific examples include the selective-area vapor phase epitaxy of GaAs,6 GaN,11 and GaInP,12 at high temperatures, and in the one
room temperature example, the electrodeposition of Cu2O.13 This is still a very limited set of materials, and as a result, measurements of the properties of epitaxially grown 3D nanostructured materials are also limited. It should be noted that the vapor phase epitaxy methods require high temperatures (greater than 600 °C) which limits the choice of materials that can be used for the 3D template. Electrodeposition has the significant limitation that only conductive materials can be used as the substrate. Here we demonstrate for the first time, the templated epitaxial growth of nanostructured ZnO through use of a low-temperature hydrothermal synthesis in a flow reactor. The optical and electrical properties of the porous single crystalline ZnO layer are investigated, and found to be significantly better than those of polycrystalline ZnO. It is well known that hydrothermal synthesis can be used to produce bulk single crystals and epitaxial films of various materials and hopefully this work will show that the subset of materials that can be formed in a single crystalline nanostructured form can be expanded significantly. Although hydrothermal synthesis is generally performed in a pressurized aqueous solution at high temperatures, there are examples of growth of for example ZnO,14–18 TiO2,19 and BaTiO320 epitaxial films at ambient pressure and low temperatures (
