Indium-doped (In-doped) ZnO nanostructures with four different morphologies, namely nanochips, nanotripods, nanorods, and nanodisks, have been successfully synthesized by a simple hydrothermal method. The effects of indium dopant and various morphologies on the structural, optical and field-emission properties of these ZnO nanostructures were investigated. The XRD patterns demonstrated that the In-doped ZnO nanostructures exhibited the hexagonal wurtzite structure with preferential orientation along the (0 0 2) crystal plane, and a slight difference in lattice parameters was detected among the samples with various morphologies. The doped nanostructures were found to be single crystals grown along the c-axis. The composition of the doped ZnO nanostructures was confirmed by X-ray diffraction (XRD), X-ray photospectroscopy (XPS), and energy-dispersive spectroscopy (EDS). The photoluminescence (PL) spectra of doped ZnO nanostructures having a blue-shift in the UV region show a prominent tuning in the optical band gap, without any significant peak relating to intrinsic defects. From Raman spectra, the 437 cm−1 mode corresponds to the E2 mode for wurtzite ZnO crystals with very sharp features which revealed the better crystallinity of samples. The lowest turn-on field of the field-emission was found to be ∼2.5 V μm−1 and the highest emission current density of 1.13 mA cm−2 was also obtained for In-doped ZnO nanochips under a field of 6.3 V μm−1. The field enhancement factor β was estimated to be 10 640 ± 3 in the case of doped nanochips, which was much higher than that of any previous report. Moreover, the doped ZnO nanostructures exhibit good long period emission current stability with a variation of less than 5% during 25 h under a field of 6.3 V μm−1. The superior field-emission properties were attributed to the better morphologies, In-doping and better crystallinity of In-doped ZnO nanostructures.
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