Blue and white light emission from zinc oxide nanoforests.

Nafisa Noor,Luca Lucera,Venkata Manthina,Helena Silva,Thomas Capuano,Ali Gokirmak,Alexander G Agrios

doi:10.3762/bjnano.6.255

Abstract

Blue and white light emission is observed when high voltage stress is applied using micrometer-separated tungsten probes across a nanoforest formed of ZnO nanorods. The optical spectrum of the emitted light consistently shows three fine peaks with very high amplitude in the 465–485 nm (blue) range, corresponding to atomic transitions of zinc. Additional peaks with smaller amplitudes in the 330–650 nm range and broad spectrum white light is observed depending on the excitation conditions. The spatial and spectral distribution of the emitted light, with pink–orange regions identifying percolation paths in some cases and high intensity blue and white light with center to edge variations in others, indicate that multiple mechanisms lead to light emission. Under certain conditions, the tungsten probe tips used to make electrical contact with the ZnO structures melt during the excitation, indicating that the local temperature can exceed 3422 °C, which is the melting temperature of tungsten. The distinct and narrow peaks in the optical spectra and the abrupt increase in current at high electric fields suggest that a plasma is formed by application of the electrical bias, giving rise to light emission via atomic transitions in gaseous zinc and oxygen. The broad spectrum, white light emission is possibly due to the free electron transitions in the plasma and blackbody radiation from molten silicon. The white light may also arise from the recombination through multiple defect levels in ZnO or due to the optical excitation from solid ZnO. The electrical measurements performed at different ambient pressures result in light emission with distinguishable differences in the emission properties and I–V curves, which also indicate that the dielectric breakdown of ZnO, sublimation, and plasma formation processes are the underlying mechanisms.

Highlights

The interest in environmentally friendly semiconductors, biocompatible [1], functional nanostructures [2], nanoscale electronic devices and large area electronics has led to significant research efforts in metal-oxide semiconductors such as ZnO
Blue and white light emission is observed when high voltage stress is applied using micrometer-separated tungsten probes across a nanoforest formed of ZnO nanorods
The distinct and narrow peaks in the optical spectra and the abrupt increase in current at high electric fields suggest that a plasma is formed by application of the electrical bias, giving rise to light emission via atomic transitions in gaseous zinc and oxygen

Summary

Introduction

The interest in environmentally friendly semiconductors, biocompatible [1], functional nanostructures [2], nanoscale electronic devices and large area electronics has led to significant research efforts in metal-oxide semiconductors such as ZnO. ZnO is a common, low-cost, antibacterial [3] material that forms various nanostructures depending on the process conditions. It is a direct and wide band gap semiconductor (≈3.4 eV) with large exciton binding energy (60 meV). ZnO, with its interesting electronic and optical properties [14] and possibility of synthesis using relatively simple approaches, can become a low-cost alternative to GaN [8]. It is one of the substances that sublimate congruently at atmospheric pressure [15,16,17]:. The process yields 2–2.5 μm long ZnO nanorods homogeneously grown along the c-direction of the wurtzite structure with 50–250 nm diameter (Figure 1) on top of a very thin layer of ZnO film (≈2–3 nm)

Experimental setup

Results and Discussion

Conclusion