188宝金博页面版

Bull. Mater. Sci., Vol. 31, No. 3, June 2008, pp. 551–559. © Indian Academy of Sciences. 551ZnO 1-D nanostructures: Low temperature synthesis and characterizations APURBA DEV, S CHAUDHURI† and B N DEV* Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India Abstract. ZnO is one of the most important semiconductors having a wide variety of applications in photonic, field emission and sensing devices. In addition, it exhibits a wide variety of morphologies in the nano regime that can be grown by tuning the growth habit of the ZnO crystal. Among various nanostructures, ori-ented 1-D nanoforms are particularly important for applications such as UV laser, sensors, UV LED, field emission displays, piezoelectric nanogenerator etc. We have developed a soft chemical approach to fabricate well-aligned arrays of various 1-D nanoforms like nanonails, nanowires and nanorods. The microstructural and photoluminescence properties of all the structures were investigated and tuned by varying the synthesis parameters. Field emission study from the aligned nanorod arrays exhibited high current density and a low turn-on field. These arrays also exhibited very strong UV emission and week defect emission. These structures can be utilized to fabricate efficient UV LEDs. Keywords. Aligned 1-D ZnO nanostructures; surfactant mediated growth; optical absorption and emission; field emission properties. 1. Introduction Zinc oxide is an outstanding semiconductor having a wide bandgap energy of 3⋅37 eV and a large excitonic binding energy (60 meV) at room temperature. The excitonic binding energy of ZnO is much higher than the thermal energy at room temperature (26 meV), and it is also much higher than those of other prospective materials such as ZnSe (22 meV), ZnS (40 meV), and GaN (25 meV), which make it one of the outstanding semiconductors for lasing. The lack of centre of symmetry in wurtzite crystals and large electrochemical coupling result in strong piezoelec-tric and pyroelectric properties in ZnO which have important applications like mechanical actuators and piezoelectric sensors (Wang 2004). In addition, ZnO exhibits a diverse group of growth morphologies in the nano regime that has made this material a promising candidate in the field of nanotechnology. Among the various nanoforms, one-dimensional oriented nanostructures such as nanorods, nanowires, nanotubes, nanopins etc are particularly im-portant for efficient field emission that has enormous commercial applications like field emission flat panel displays (Baughman et al 2002), X-ray sources (Senda et al 2004), vacuum microwave amplifiers (Saito and Ue-mura 2000; Milne et al 2004) etc. With their high melting point, good thermal and chemical stability and low elec-tron affinity (Fancher et al 1998), ZnO one-dimensional arrays are promising alternatives to carbon nanotubes (CNT) for field emitters with long lifetimes. Moreover, the suc-cessful demonstration of UV lasing action (Huang et al 2001) from ZnO has added a new direction in the field of nanotechnology and motivated subsequent research for the fabrication of one-dimensional ZnO nanostructured arrays with precise control over size, shape and orientations. The ability to build oriented assemblies of 1-D nanostruc-tures is also very attractive for the fabrication of future photonic (Wang et al 2004; Pan et al 2005), field emis-sion (Li et al 2004) and sensing (Kar et al 2006) devices. As a consequence, several synthetic methodologies have been proposed. Gas phase growth techniques like chemi-cal vapour deposition (CVD), physical vapour deposition (PVD), metalorganic vapour phase epitaxy (MOVPE) and vapour liquid–solid (VLS) etc (Park et al 2002; Lyu et al 2003; Bae et al 2004; Gao and Wang 2004) have been successfully employed to grow ZnO nanorods and nano-wires on solid substrates. However, these methods are very expensive and require high temperature. Recently, solution phase approach to fabricate aligned ZnO nanorods was utilized (Guo et al 2002; Greene et al 2003; Li et al 2004; Li Q et al 2005; Yu et al 2005; Dev et al 2006). This technique has many advantages as it is cost-effective and large scale-up production is possible. In addition, oriented seed-initiated synthesis (Greene et al 2005) has been found to be very effective to produce highly aligned ZnO nanorod arrays. This technique (Greene et al 2005) is suitable for preparing long free-standing ZnO nanorods which exhibit efficient field emission, as long as their aspect ratio is high. But the increase in length is generally accompanied by subsequent increase in diameter. So, if a *Author for correspondence (msbnd@iacs.res.in; dev_apurba@yahoo.com) †Since deceased