Spatial distribution of neutral oxygen vacancies on ZnO nanowire surfaces: An investigation combining confocal microscopy and first principles calculations

Abstract

A qualitative approach using room-temperature confocal microscopy is employed to investigate the spatial distribution of shallow and deep oxygen vacancy (VO) concentrations on the polar (0001) and non-polar (101¯0) surfaces of zinc oxide (ZnO) nanowires (NWs). Using the spectral intensity variation of the confocal photoluminescence of the green emission at different spatial locations on the surface, the VO concentrations of an individual ZnO NW can be obtained. The green emission at different spatial locations on the ZnO NW polar (0001) and non-polar (101¯0) surfaces is found to have maximum intensity near the NW edges, decreasing to a minimum near the NW center. First-principles calculations using simple supercell-slab (SS) models are employed to approximate/model the defects on the ZnO NW (101¯0) and (0001) surfaces. These calculations give increased insight into the physical mechanism behind the green emission spectral intensity and the characteristics of an individual ZnO NW. The highly accurate density functional theory (DFT)-based full-potential linearized augmented plane-wave plus local orbitals (FP-LAPW + lo) method is used to compute the defect formation energy (DFE) of the SSs. Previously, using these SS models, it was demonstrated through the FP-LAPW + lo method that in the presence of oxygen vacancies at the (0001) surface, the phase transformation of the SSs in the graphite-like structure to the wurtzite lattice structure will occur even if the thickness of the graphite-like SSs are equal to or less than 4 atomic graphite-like layers [Wong et al., J. Appl. Phys. 113, 014304 (2013)]. The spatial profile of the neutral VO DFEs from the DFT calculations along the ZnO [0001] and [101¯0] directions is found to reasonably explain the spatial profile of the measured confocal luminescence intensity on these surfaces, leading to the conclusion that the green emission spectra of the NWs likely originate from neutral oxygen vacancies. Another significant result is that the variation in the calculated DFE along the ZnO [0001] and [101¯0] directions shows different behaviors owing to the non-polar and polar nature of these SSs. These results are important for tuning and understanding the variations in the optical response of ZnO NW-based devices in different geometric configurations.

Electronic Structure of In3–xSe4 Electron Transport Layer for Chalcogenide/p-Si Heterojunction Solar Cells

“…The electronic properties of a compound are revealed by the band structure, partial density of states (PDOS), and total density of state (TDOS), which are also closely related to charge density distribution and Fermi surface. 44 The results of band structure calculations along the high-symmetry directions within the Brillouin zone for In 3– x Se 4 are presented in Figure 2a–d. The horizontal dashed line is the signature of Fermi level, E F , while the valence and conduction bands are indicated by blue and red, respectively.…”

Defect spectroscopy of single ZnO microwires

“…5 The study of the spatial distribution of the defects on the surfaces, interfaces, and bulk of nano and micro-structures is an important issue for tailoring the optoelectronic properties of devices. 6 Moreover, it is well known from mechanical properties of whiskers that the number of defects decreases with size in these structures. 7 The latter correlation is explained considering that as diameter decreases, the surface approaches the bulk where defects and impurities can segregate during the growth of the nanostructures, leaving the bulk with a relatively small concentration of defects.…”

Surface modification of highly porous 3D networks via atmospheric plasma treatment

“…This is confirmed in the filtered surface scans (Figure b and d), as only those parts of the tetrapods have an increased intensity of the E 1 (LO) which are parallel to the substrate. Moreover, the presence of the E 1 (LO) mode in the Raman spectrum indicates an oxygen deficiency in the ZnO micro‐ and nanostructures . On taking a closer look at the peak‐to‐peak ratios between the E 2 (high) and E 1 (LO) modes for the plasma‐treated ZnO network, a clear difference in the intensity can be observed when compared to the peaks of the untreated T‐ZnO.…”