Growth of Epitaxial Nanowires at the Junctions of Nanowalls

• DOI: 10.1126/science.1082542
• Publication Date: 2003-05-22T20:33:57Z
• Authors: Hou T. Ng; Jun Li; Michael K. Smith; Pho Nguyen; Alan Cassell; Jie Han; M. Meyyappan
• Keywords: 02 engineering and technology; 0210 nano-technology

Interfacing semiconducting nanostructures with conducting or insulating substrates to attain a three-dimensional (3D) integrated platform is highly desirable for advanced nanoscale electronics and optoelectronics applications ( [1][1] ). As such, the assembly and synthesis of these nanostructures, which demonstrate multiple dimensionality, using a bottom-up approach would be useful. Taking ZnO as an example, we show here that high crystallinity 1D and 2D nanostructures can be epitaxially and vertically grown into 3D architectures on both conducting and insulating single crystalline substrates. Our objective was to grow vertically well-aligned 2D and 1D semiconducting nanostructures for nanoelectronics and nanosensor applications. Using a carbothermal reduction process and gold (Au)-catalyzed epitaxial growth ( [2][2] – [4][3] ), we have been able to reproducibly grow hierachically well-ordered and vertically aligned 2D and 1D ZnO nanostructures on electrically conducting, highly oriented pyrolytic graphite (HOPG) and on insulating (1120) sapphire substrates. We achieve these by specifically controlling either the spacing between the catalyst spots or the thickness of the catalyst thin film. When the Au film thickness is maintained above the threshold limit of 15 A, we obtained highly intricate quasi-3D nanostructures ([Fig. 1A][4] and inset), which are composed of an array of vertical 1D nanowires on top of a 2D network of intricate “nanowalls.” The scanning electron microscope (SEM) (top view) further reveals a random honeycomb-like pattern, resembling typical structures observed from anodization of thin-film aluminum. The nanowires are observed to grow from the “nodes” of the nanowalls. The average diameters of the nanowires and the thickness of the nanowalls are of the same order of magnitude (∼80 nm). The hexagonal facets (0001) of the nanowires ( [4][3] ) are clearly visible in the SEM images. This suggests that epitaxial growth of the nanowires occurs at the nodes of the nanowalls, which are themselves assembled epitaxially on the substrates and presumably also have their c -plane parallel to the substrates. The nanowalls have essentially vertical side wall profiles. By controlling the growth time, 2D nanowalls ([Fig. 1B][4]) could be obtained before onset of the 1D nanowires. When the catalyst thickness is kept below the threshold limit of 15 A, discrete individual free-standing 1D nanowires are obtained ([Fig. 1C][4]). ![ Fig. 1. ][5] Fig. 1. Zinc oxide nanowalls and nanowires. ( A ) SEM image of quasi-3D ZnO nanostructures grown on a sapphire using ∼40 to 50 A Au thin film as the catalyst. The inset shows a SEM perspective view. ( B ) 2D ZnO nanowalls on a sapphire with a height ∼5 μm. ( C ) An array of free-standing 1D ZnO nanowires on a HOPG substrate using ∼15 A Au ultrathin film as the catalyst. ( D ) Schematic illustration showing the growth mechanism of ZnO nanowalls and nanowires. The first four sketches are top views; the last, a perspective view. Vertical growth of the ZnO nanostructures are observed as a result of the good epitaxial lattice match of the c plane of ZnO with the hexagonal basal plane of HOPG and (\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(11{\bar{2}}0\) \end{document}) plane of sapphire. Unlike the governing mechanism in the growth of carbon nanowalls ( [5][6] ), the observed growth phenomena appear to be a result of the interplay between the dynamic wetting behavior and thermal-enhanced surface diffusion of gold adatoms on the substrates. The limited wettability between the substrate and Au film facilitates the formation of Au network patterns at an elevated temperature, in contrast to totally isolated droplets that form due to a total incompatibility or a continuous film that forms due to a complete wetting. Control experiments with different growth durations but without ZnO and carbon feed-stock have been performed and examined under an atomic force microscope (AFM). Consistency in the AFM granulometry results has suggested aggregation of the Au nanograins forming larger particulates with narrow grain boundaries within complex channeled Au networks that resemble the 2D nanowall patterns, as depicted in [Fig. 1D][4]. We believe that, with improved wetting to the substrate, ZnO epitaxial growth is initiated along the grain boundaries because these are the most thermodynamically active sites for saturation and precipitation of the Zn atoms (green arrows). As the ZnO nanowalls continue to grow via vapor-liquid-solid (VLS) mechanism, the surface energy at the nodes increases with resultant Au atoms diffusing and accumulating at the nodes (red arrows) for overall energy compensation. At a critical saturation point, 1D nanowires begin to grow from the nodes via the same mechanism. In the 1D individual nanowire growth, the Au nanoparticles are typically well-separated and the surface diffusion length may not be extended enough to allow a continuous network formation. Though it is well known that nanowires are useful in many electronics and optoelectronics applications, the nanowalls and high surface-to-volume ratio nanostructures with multiple dimensionality may be useful in applications in energy storage or conversion and data storage and memory devices. Supporting Online Material [www.sciencemag.org/cgi/content/full/300/5623/1249/DC1][7] Materials and Methods Fig. S1 1. [↵][8] International Technology Roadmap for Semiconductors 2001 (Semiconductor Industry Association, San Jose, CA, 2001); available at 2. [↵][9] H. T. Ng et al. , Appl. Phys. Lett. 82, 2023 (2003). [OpenUrl][10][CrossRef][11] 3. Materials and methods are available as supporting material on Science Online. 4. [↵][12] M. Huang et al ., Science 292, 1897 (2001). [OpenUrl][13][Abstract/FREE Full Text][14] 5. [↵][15] Y. Wu, P. Qiao, T. Chong, Z. Shen, Adv. Mater. 14, 64 (2002). 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