Mapping the Origins of Luminescence in ZnO Nanowires by STEM-CL.

Oscar W Kennedy,Charlotte K Williams,Ashley Howkins,Edward R White,Milo S P Shaffer,Ian W Boyd,Paul A Warburton

doi:10.1021/acs.jpclett.8b03286

Abstract

In semiconductor nanowires, understanding both the sources of luminescence (excitonic recombination, defects, etc.) and the distribution of luminescent centers (be they uniformly distributed, or concentrated at structural defects or at the surface) is important for synthesis and applications. We develop scanning transmission electron microscopy-cathodoluminescence (STEM-CL) measurements, allowing the structure and cathodoluminescence (CL) of single ZnO nanowires to be mapped at high resolution. Using a CL pixel resolution of 10 nm, variations of the CL spectra within such nanowires in the direction perpendicular to the nanowire growth axis are identified for the first time. By comparing the local CL spectra with the bulk photoluminescence spectra, the CL spectral features are assigned to internal and surface defect structures. Hyperspectral CL maps are deconvolved to enable characteristic spectral features to be spatially correlated with structural features within single nanowires. We have used these maps to show that the spatial distribution of these defects correlates well with regions that show an increased rate of nonradiative transitions.

Highlights

Zinc oxide (ZnO) offers a number of attractive features relevant to application, including a wide band gap, optical transparency, large exciton binding energy, and earth-abundant elemental composition.[1]
Primary luminescence signals arise from transitions across the band gap often mediated by excitons,[12] and transitions are associated with a variety of defects.[13]
Impurity elements often play a crucial role, but even in elementally pure ZnO there are defect states that lie within the band gap and contribute to luminescence, include those arising at 2.0 eV14 and 1.94 eV (O-rich growth conditions or O interstitials).[15,16]

Summary

Introduction

Zinc oxide (ZnO) offers a number of attractive features relevant to application, including a wide band gap, optical transparency, large exciton binding energy, and earth-abundant elemental composition.[1]. An ideal technique for characterizing defect states in nanostructures would combine spatially resolved emission measurements with high-resolution structural imaging.

Results

Conclusion