ZnO Nanoneedles Grown Vertically on Si Substrates by Non‐Catalytic Vapor‐Phase Epitaxy

Abstract

There has been considerable interest in the growth of one-dimensional (1D) semiconductor nanostructures including nanowires. [1±3] In addition to nanowires, semiconductor nanoneedles are of particular interest because their tips show a sharp curvature, offering potential applications as probing tips with high spatial resolution in both vertical and horizontal dimensions or field-emission tips due to the increased field-enhancement factor. [4,5] Nevertheless, semiconductor nanoneedles have rarely been studied, while numerous semiconductor nanowires including Si, InP, GaN, GaAs, GaP, and ZnO have been synthesized. [1±3,6] To prepare the nanowires, metal-catalysis-assisted vapor±liquid±solid (VLS) deposition has been widely employed because this method was used for Si microwhisker growth in the 1960s. [7] Since as-grown nanowires prepared by the VLS method yield a metal nanoparticle on their tips, nanoneedles with sharp tips are difficult to prepare using the VLS method. However, we recently developed a metal-catalyst-free growth method for preparing ID nanostructures, [8] which enables the growth of sharp-tipped ZnO nanoneedles.One-dimensional ZnO nanowires and nanorods have been studied recently for optoelectronic nanodevice applications as the material has a wide and direct fundamental bandgap energy of 3.4 eV, a large excitonic binding energy of~60 meV, and high mechanical and thermal stabilities. [9] Both ZnO films and nanostructures have been prepared, typically on sapphire substrates as the substrate shows a good lattice match with ZnO. [10±12] Nevertheless, the use of Si substrates enables the deposition of nanomaterials on large and cheap substrates, offering possible mass production of the nanomaterials. More importantly, the preparation of nanomaterials on Si represents a breakthrough for nanomaterial integration in Si-based electronic devices. Despite the importance of the nanomaterials growth on Si substrates, vertically well-aligned one-dimensional ZnO nanostructure growth on Si has rarely been reported. [6,13] Here, we report metal±organic chemical vapor deposition (MOCVD) of vertically well-aligned ZnO nanoneedles on Si substrates and their structural and optical characteristics.The surface morphology of as-grown ZnO nanoneedles on Si substrates was investigated using scanning electron microscopy (SEM). As shown in Figures 1a±1c, a high density of ZnO nanoneedles is vertically aligned over the entire substrate and they exhibit sharp tips. The diameter and aspect ratio of ZnO nanoneedles depend on growth conditions including growth time. Typically, nanoneedles grown for 1 h exhibit mean lengths of 740 ± 50 nm and mean diameters of 40 ± 5 nm (Figs. 1d and 1e). Normalized standard deviation values (a standard deviation divided by a mean) in nanoneedle diameter and length distributions are as small as 0.16 and 0.06, respectively. These values are comparable to those of ZnO nanorods grown on Al 2 O 3 (0001) substrates and are one or two orders of magnitude smaller than those prepared by the catalyst-as...

Architectured van der Waals epitaxy of ZnO nanostructures on hexagonal BN

“…37 No deep-level emission, typically observed from bulk ZnO materials, was observed. The full width at half maximum of the near-band-edge was as narrow as~100 meV, which is comparable with a previous report, 38 indicating the high quality and purity of the ZnO nanostructures grown on hBN by vdW epitaxy. The insets of Figure 6e show the scanning electron microscopy and corresponding CL monochromatic images of the ZnO nanostructure array.…”

One-dimensional ZnO nanostructures: Solution growth and functional properties

“…It has been demonstrated to have enormous applications in electronic, optoelectronic, electrochemical, and electromechanical devices [3][4][5][6][7][8], such as ultraviolet (UV) lasers [9,10], light-emitting diodes [11], field emission devices [12][13][14], high performance nanosensors [15][16][17], solar cells [18][19][20][21], piezoelectric nanogenerators [22][23][24], and nanopiezotronics [25][26][27]. One-dimensional (1D) ZnO nanostructures have been synthesized by a wide range of techniques, such as wet chemical methods [28][29][30], physical vapor deposition [31][32][33], metal-organic chemical vapor deposition (MOCVD) [34][35][36], molecular beam epitaxy (MBE) [37], pulsed laser deposition [38,39], sputtering [40], flux methods [41], eletrospinning [42][43][44], and even top-down approaches by etching [45]. Among those techniques, physical vapor deposition and flux methods usually require high temperature, and easily incorporate catalysts or impurities into the...…”

Growth mechanism and characterization of zinc oxide microcages

“…A number of nanometer and micrometer ZnO materials of varied geometries have been produced, e.g. nanowires, nanosprings, nanoneedles, nanowalls, coaxial cables, tubes, tetrapods and sheets [4][5][6][7][8][9][10][11][12][13][14][15][16].…”