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

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.

Photoluminescence of ZnO Nanowires: A Review

“…These results are consistent with the spatial distribution of green emission centers seen in the SEM-CL images reported in [70,76], where it can be seen that the most intense green emission occurs near the surface of the nanostructures or at grain boundaries.…”

“…Corroborations of both oxygen vacancy and zinc vacancy models have been presented in the work of Fabbri. et al [70] and Kennedy [76]. In these works, the spatial distribution of NBE of ZnO nanostructures by scanning electron microscopy cathodoluminescence techniques (SEM-CL) (Figure 9) is presented.…”

“…In these works, the spatial distribution of NBE of ZnO nanostructures by scanning electron microscopy cathodoluminescence techniques (SEM-CL) (Figure 9) is presented. Even though these authors also attribute the green emission to distinct point defects ( [70] to V Zn and [76] V O , there is a compromise that the defect responsible for the green emission is mainly located at the surface [136]. The layer responsible for the green emission was estimated to be 30 nm thick (Hsu et al [158]) in polarized photoluminescent measurements of aligned ZnO nanorods.…”

“…(d) STEM image of nanowire amplitude maps of (e) near-band-edge emission, (f) surface emission, (g) defect emission 1, and (h) defect emission 2 deconvolved components. (i) Counts integrated across all energies; for further permissions related to the material (d-i), please refer to ACS https://pubs.acs.org/doi/10.1021/acs.jpclett.8b03286 [76].…”

Native Point Defect Measurement and Manipulation in ZnO Nanostructures

“…Thus, extended defects such as dislocations and stacking faults are observable and correlated to structural features using spatial imaging and cathodoluminescence spectroscopy (CLS) on a nanoscale [27,28,29,30,31]. Electron microscope studies have described how ZnO properties vary within interfaces, micro- and nano- structures and the effect of growth and processing methods [32,33]. For example, Figure 4 illustrates CL spectra of n-ZnO nanorod/GaN heterostructures grown in a solution of zinc (II) acetate dehydrate (Zn(Ac)) and KOH in methanol [33].…”