Article Cite This: Chem. Mater. 2017, 29, 9734-9741
pubs.acs.org/cm
Low-Temperature Hydrothermal Synthesis of Colloidal Crystal Templated Nanostructured Single-Crystalline ZnO Masao Miyake,*,†,‡ Makoto Suginohara,† Naoto Narahara,† Tetsuji Hirato,† and 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 UrbanaChampaign, Urbana, Illinois 61801, United States
‡
S Supporting Information *
ABSTRACT: Single crystal semiconductors almost always exhibit better optoelectrical properties than their polycrystalline or amorphous 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 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 properties superior to 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.
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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 significant negative effects on the important properties. Not surprisingly, most high-performance electronic and optoelectronic 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 singlecrystal 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 © 2017 American Chemical Society
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 lowtemperature hydrothermal synthesis in a flow reactor. The optical and electrical properties of the porous single crystalline ZnO layer were 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 epitaxial films, Received: August 16, 2017 Revised: October 24, 2017 Published: October 26, 2017 9734
DOI: 10.1021/acs.chemmater.7b03466 Chem. Mater. 2017, 29, 9734−9741
Article
Chemistry of Materials
Figure 1. (a−d) Schematic illustrations of the steps for the epitaxial growth of nanostructured single crystal ZnO. (a) Sputtering deposition of epitaxial ZnO seed layer on A-plane sapphire substrate. (b) Self-assembly of PS colloidal crystal on ZnO seed layer. (c) Hydrothermal epitaxial growth of ZnO through and within PS colloidal template. (d) ZnO porous layer after removal of PS colloidal template. (e) Schematic of flow reactor employed for hydrothermal growth of ZnO. induced self-assembly.29 Sulfate PS colloids (Invitrogen) (225 and 600 nm in diameter) were dispersed in water as a ∼0.14 wt % suspension. The substrate was attached to a glass plate and placed at a ∼20° angle in a 15 mL plastic vial containing the suspension at 50 °C until the water had evaporated, leaving behind a colloidal crystal on the substrate. The colloidal crystals were sintered at 95 °C for 1 h. The colloidal crystal formed from 225 nm colloids was further exposed to oxygen plasma at 500 mTorr and 60 W for 20 s (Nordson MARCH CS-1701) to enhance the robustness of the colloidal template. Hydrothermal Growth of ZnO. The reaction solution for the hydrothermal growth of ZnO was prepared by dispersing an excess amount of ZnO powder in an aqueous solution containing the indicated concentration of NH4OH, NH4NO3, and sodium citrate tribasic dehydrate, followed by filtering through a 0.2 μm membrane (Sartorius), forming a saturated solution. The pH of the solutions was adjusted by varying the ratio of the concentrations of NH4OH and NH4NO3 while keeping their total concentration constant at 1 M. The pH of the solutions with NH4OH/NH4NO3 concentration ratios of 0.8/0.2 and 0.6/0.4 were 10.4 and 9.9, respectively. The hydrothermal growth was performed using a custom-built flow reactor.22 The details of the growth procedure using the flow reactor are described elsewhere.22 In brief, the substrate was attached to the upper wall of the 20 mm-wide and 1 mm-deep channel of the reactor; the PS colloidal crystal template was made to face downward. To facilitate infilling of the PS colloidal crystal template with the reaction solution, the template was first wetted by 20 μL of isopropanol. Next, the reaction solution was made to flow through the channel using a syringe pump at a constant flow rate of 30 μL min−1. To prevent the formation of bubbles on the substrate surface, a 20 psi inline backpressure regulator (Upchurch Scientific) was installed at the outlet of the channel. The substrate was heated to 80 °C at a rate of 4 °C min−1 using a ceramic heater affixed to the reactor and then kept at this temperature for 3 h, leading to ZnO crystal growth through the template. After the completion of the ZnO growth process, the PS colloids were removed by etching in tetrahydrofuran and heating in air at 275 °C for 2 h. As a control sample, polycrystalline porous ZnO was prepared in the same manner, but using a glass substrate instead of the sapphire substrate. Furthermore, a dense epitaxial ZnO film and a dense polycrystalline ZnO film were prepared under the same conditions but without the 3D template. All the control samples were heated in air at 275 °C for 2 h to match the processing conditions of the templated ZnO. Characterization. The reflectance spectra of the samples were measured using a silicon photodiode array spectrometer coupled to an optical microscope.30 A 20× objective with a numerical aperture of
for example ZnO,14−18 TiO2,19 and BaTiO3,20 at ambient pressure and low temperatures (
