ARTICLE pubs.acs.org/Langmuir
Digital Selective Growth of ZnO Nanowire Arrays from Inkjet-Printed Nanoparticle Seeds on a Flexible Substrate Seung Hwan Ko,*,†,‡ Daeho Lee,‡ Nico Hotz,‡,§ Junyeob Yeo,† Sukjoon Hong,† Koo Hyun Nam,† and Costas P. Grigoropoulos*,‡ †
Applied Nano Technology and Science Laboratory, Department of Mechanical Engineering, KAIST, 291 Daehak-ro, Yusong-Gu, Daejeon 305-701, Korea ‡ Laser Thermal Laboratory, Department of Mechanical Engineering, University of California, Berkeley, California 94720-1740, United States § Department of Mechanical Engineering and Materials Science, Thermodynamics and Sustainable Energy Laboratory, Duke University, 303 Hudson Hall, Durham, North Carolina 27708-0300, United States
bS Supporting Information ABSTRACT: In this article, we introduce fully digital selective ZnO nanowire array growth on inkjet-printed seed patterning. Through proper natural convection suppression during hydrothermal growth, successful ZnO nanowire local growth can be achieved. Without any need for photolithographic processing or stamp preparation, the nanowire growth location can be easily modified when the inkjet printing process is integrated with a CAD (computer-aided design) system to allow a high degree of freedom when the design needs to be changed. The current proposed process is very fast, low-cost, environmentally benign, and low-temperature. Therefore, it can be applied to a flexible plastic substrate and scaled up for larger substrates for mass production or roll-to-roll processing.
1. INTRODUCTION Extended and oriented nanostructures are desirable for many applications, including microelectronic devices, chemical and biological sensing and diagnosis, light-emitting displays, energy conversion and storage, catalysis, drug delivery, separation, and optical storage.1 5 Among the nanorods, ZnO nanowires (NW) have attracted much attention because of their direct band gap of 3.37 eV and a large exciton binding energy of 60 meV, which make them promising candidates for applications in various optoelectronics. The synthesis of nano/microstructures of ZnO has been carried out using various methods, such as evaporation and condensation,6 physical vapor deposition,7 chemical vapor deposition,8 and solvothermal9 and hydrothermal methods.10 Among them, the hydrothermal method is one of the most attractive candidates for industrial use because of their rapid, low-cost, low-temperature, and environmentally friendly processing nature.1 The ability to build functional nanostructured assemblies at predetermined locations requires both control of their growth structure and morphology and their placement at a specific location.4 The usual approach entails a series of multisteps for the growth, harvesting, and placement of nanowires, which are very time-consuming, expensive, and low-yield. To overcome the difficulties encountered in multistep serial processes, new approaches use direct growth at a specific location with the desired r 2011 American Chemical Society
nanowire morphology. For site-selective patterned nanowire growth, a photolithographic method is usually applied to pattern the catalyst layer for local growth.1,5,11 13 Other approaches use the microcontact printing method for surface modification by patterned self-assembled monolayers with hydrophobic and hydrophilic end groups on silver or silicon substrates3,4,14 or the microcontact printing of seed nanoparticles.15 However, these approaches are subject to practical limitations because they need a photomask for photolithography or a mold for contact printing and cannot be modified once fabrication is complete. Furthermore, the photolithographic method cannot be applied to heat- and corrosive-chemical-sensitive plastic substrates. In this research, we introduce fully digitally selective ZnO nanowire growth using inkjet-printed seed patterning and hydrothermal ZnO local growth. The proposed process is very fast, low-cost, environmentally benign, and low-temperature. Therefore, it can be applied to flexible plastic substrates and scaled up for larger substrates for mass production or roll-to-roll processing. Most of all, the nanowire growth location can be easily modified when the inkjet printing process is integrated with a Received: September 27, 2011 Revised: November 27, 2011 Published: November 30, 2011 4787
dx.doi.org/10.1021/la203781x | Langmuir 2012, 28, 4787–4792
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ARTICLE
Figure 1. (a) Process schematics of the directly patterned ZnO NW growth from the inkjet-printed ZnO NPs. The inset shows the TEM image of the ZnO seed NPs prepared by a modified Pacholski method. (b) Time-resolved image of the ZnO seed NP inkjet printing by a piezo electrically driven drop-on-demand inkjet printing system.
Figure 2. Selective ZnO NW growth (a d) without natural convection suppression and (e h) with natural convection control. Note the secondary growth in a d and the absence of the secondary growth in e h.
CAD (computer-aided design) system to allow many degrees of freedom in the development stage.
2. EXPERIMENTAL SECTION Directly patterned ZnO NW arrays were selectively grown from the inkjet-printed ZnO NP nuclei through the hydrothermal decomposition of a zinc complex. The process is mainly composed of two simple steps as
shown in Figure 1a; (1) ZnO NP inkjet printing on a substrate and (2) subsequent selective ZnO NW hydrothermal growth on the inkjetprinted ZnO NP seeds. All of the processes were carried out at a plasticcompatible low temperature (
