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A combined confocal microscopy measurement and first-principle calculation for investigating the spatial distribution of neutral oxygen vacancies on ZnO nanowire surfaces

March 19, 2014 6:18 pm / Leave a comment

Introduction

Zinc oxide (ZnO) is a semiconductor which has generated a great deal of interest in device applications in recent years due to its several useful electrical and optical properties. Density functional theory (DFT) based calculations is a powerful method of investigating the ground state properties of materials. It is well known that the ZnO nanowire (NW) contain a large density of intrinsic defects such as oxygen deficiencies or vacancies (V_O), which influence their electronic and optical properties as well as their performance in device applications. Hence understanding the spatial distribution of the shallow intrinsic surface defects on the nanostructure’s surfaces and deep intrinsic bulk defects plays a crucial role in understanding and tailoring the electronic properties of the NW based device. The ZnO (0001) and (10-10) surfaces are the most important among the different surface structures of ZnO as they occur predominantly due to their relative stability.

Discussion

From multiple arguments along the theoretical considerations of the various intrinsic defects formation energies, the NWs’ synthesis temperature and photoluminescence response, it is likely that the green luminescence emitted by the synthesized NW arrays during the confocal photoluminescence (CPL) microscopy measurements (obtained with a PicoQuant MicroTime 200 confocal microscope) originates from neutral oxygen vacancies. In addition, the luminescence intensity or the magnitude of the measured CPL spectra is also directly correlated with the concentration of V_O in the ZnO NWs. Therefore the CPL measurements can provide a direct and non-destructive qualitative measurement of the directional dependence of the V_O concentration in the ZnO NWs. The spatial profile of the luminescence intensity indicates that the V_O concentration is largest near the edges of the (0001) and (10-10) surfaces of the NWs and the V_O concentrations decreases significantly in the interior parts of the NWs [Figs. 7(a) and 8(a) of J. Appl. Phys. 114, 034901 (2013)].

In order to gain insight into the physical mechanisms that govern the confocal optical properties of the ZnO NWs in the [0001] and [10-10] directions, DFT calculations using the full-potential linearized augmented plane-wave plus local orbitals method was also utilized. N x 1 x 1 supercell-slab (SS) models was utilized in the surface simulations by stacking the ZnO primitive unit cells along the a-axis of the hexagonal lattice which is followed by a vacuum region of several angstroms to decouple the interactions between the SSs. Self-consistency in the SS calculations was achieved by iterative convergence of the minimum total energy and Hellman-Feynman forces to a value below 0.0001 Ry and 1 mRy/a.u. respectively. Comparison with the CPL measurements demonstrates that the N x 1 x 1 SSs was suitable as an approximation of the ZnO NW cross sections where the confocal measurements are taken [Figs. 7(b) and 8(b) of J. Appl. Phys. 114, 034901 (2013)].

The calculations show that the spatial electronic profile of the neutral V_O formation energy in the [0001] and [10-10] directions is lower near the boundary and increases to bulk-like values in the SS center, which explains the behaviour of the spatial profile of the CPL measurements. For further details and discussions on the results as well as the in depth descriptions of the experimental procedures and first-principle calculations, the reader is referred to J. Appl. Phys. 114, 034901 (2013). Using this combined experimental and theoretical approach, a better qualitative understanding of the spatial distribution of the surface and deep V_O in the ZnO NWs was achieved, thus leading to a more efficient functionalization and improved integration of the ZnO NW in future nanodevices for better performance. These results would have important implications for ZnO nanostructures in potential nanoscale applications.

(a) Schematic diagram depicting the (0001) polar and (10-10) non-polar surfaces of a ZnO nanowire. (b) Spatial distribution of the CPL intensity along the blue line on the (0001) surface. (c) Spatial distribution of the oxygen vacancies defect formation energy along a 6 x 1 x 1 SS approximating the cross section of the ZnO NW [K. M. Wong, et al., J. Appl. Phys. 114, 034901 (2013).]

Reference

Wong, Kin Mun, Alay-e-Abbas, S. M., Fang, Y., Shaukat, A., & Lei, Y. Spatial distribution of neutral oxygen vacancies on ZnO nanowire surfaces: An investigation combining confocal microscopy and first principles calculations Journal of Applied Physics, 114 (3), 034901 (2013). DOI: 10.1063/1.4813517

Influence of the oxygen vacancies on the phase transformation of ZnO (0001) nanosheets from graphite-like structure to wurtzite lattice

June 21, 2013 8:45 pm / Leave a comment

Abstract

The all electron full-potential linearized augmented plane wave plus local orbitals method was utilized to study the structural properties of ZnO (0001) ultra-thin films (nanosheets). From the calculations, it was observed that in the presence of oxygen vacancies at the Zn-terminated (0001) surface of the ZnO nanosheets, the structural phase transformation from the graphite-like structure to wurtzite lattice occur even if the thickness of the ZnO nanosheet along the c-axis is less than or equal to 4 atomic graphite-like layers [J. Appl. Phys. 113, 014304 (2013)].

Introduction

Most of the synthesized Zinc oxide (ZnO) nanostructures in different geometric configurations such as nanowires, nanorods, nanobelts and nanosheets (ultra-thin films) are usually in the wurtzite crystal structure. Cutting of the ZnO (0001) thin film perpendicular to the [0001] axis always result in a Zn-terminated (0001) surface and O-terminated (000-1) surface. For these Tasker type III polar surfaces, there are several stabilization mechanisms for the reduction of the divergence of surface energy such as the charge transfer from the anion surface to the cation surface. However, it was found from density functional theory calculations that for ultra-thin films of ZnO, the graphite-like structure was energetically more favourable as compared to the wurtzite structure. The stability of this phase transformation of wurtzite lattice to graphite-like structure of the ZnO nanosheets is only limited to the thickness of about few Zn-O layers (along the c-axis), beyond which they revert back to the wurtzite phase, and this was subsequently verified by ZnO nanosheets grown by pulsed laser deposition. Conversely, the transition from wurtzite to a graphite-like phase is also observed for ZnO nanostructures under tensile strain. Due to the special properties of graphene, these graphite-like ZnO nanosheets have attracted much interest but the influence of oxygen vacancies on the important transition when the ZnO nanosheets revert back from the graphite-like phase to the wurtzite structure had been scarcely reported. Hence in this short note, this important result would be highlighted and was earlier reported in J. Appl. Phys. 113, 014304 (2013) (obtained using first-principle calculations), which will provide useful guidelines for future experimental explorations.

Discussion

The calculations on the ZnO nanosheets are carried out using the density functional theory method implemented in the Wien2k code and utilizing the revised Perdew-Burke-Ernzerholf (PBE) sol generalized gradient approximation (GGA) parametrization scheme as this functional is known to provide good results for solids and their surfaces as compared to hybrid DFT functional which are 2 to 3 orders more expansive computationally. The ZnO (0001) nanosheets are modelled by supercell slab (SS) by stacking the bulk unit cell of wurtzite ZnO in the c-axis of hexagonal lattice, with the Zn (O) atom layer terminating at the basal plane along the [0001] ([000-1]) directions. In addition, there is a vacuum layer of several angstroms to eliminate the neighboring interactions between the periodic SS. The convergence of the optimized SSs are ensured by selecting the appropriate Monkhorst-Pack k-point mesh and are found to be sufficient to achieve a self-consistent minimum total energy below 0.1 mRy. For further computational details, please refer to J. Appl. Phys. 113, 014304 (2013).

The transition from the bulk-like wurtzite structure to the graphite-like structure for the nanosheets of different sizes is due to a number of different factors. One of the important factors is due to the competition between the bonding energy and the electrostatic energy that finally triggers a structural phase transformation from wurtzite structure to graphite-like structure when the thickness of the nanosheet is lesser than a certain number of Zn – O layers along the [0001] direction. The extreme surface Zn and O atoms of the smaller nanosheets have already lost one of the four bonds due to the surface termination as compared to bulk ZnO, a larger surface to volume ratio for these smaller nanosheets ensures that they are unable to compensate for these broken surface bonds together with the stronger Coulomb’s attraction from the interior atomic layers. This then triggers a collapse of surface atomic layers towards the interior of the nanosheets and thus results in flattening of the atomic layers and hence a phase transformation from the wurtzite lattice to graphite-like lattice for the thinner ZnO nanosheets. In addition, the nature of the bond and energy due to the macroscopic electric field in the [0001] direction are also contributory factors to the phase transformation. Importantly, the creation of surface O-vacancy at the Zn-terminated (0001) surface of the ZnO (0001) nanosheets as depicted in Fig. 4 of J. Appl. Phys. 113, 014304 (2013) results in the removal of the stronger Coulomb’s attraction at the Zn-terminated (0001) surface. Furthermore, (after the introduction of oxygen vacancies at the Zn-terminated (0001) surface of the ZnO (0001) nanosheets,) the reverting of the structural phase transformation from the graphite-like structure back to the wurtzite lattice occur even if the thickness of the ZnO nanosheet along the c-axis is less than or equal to 4 atomic graphite-like layers. This can be observed in Fig. 4(a) (top panel corresponding to the perfect ZnO nanosheets) as compared to the defective ZnO nanosheet [bottom panel of Fig. 4(a)] in J. Appl. Phys. 113, 014304 (2013). Alternatively, the phase transformation of the defective ZnO nanosheet could also be observed in the figure below. Therefore the presence of oxygen vacancies results in eliminating the size-dependent graphite-like structural phase transformation for the defective ZnO nanosheets.

Top Panel – Perfect ZnO nanosheet (without oxygen vacancies) in the graphite-like structure and Bottom Panel – Phase Transformation of the defective ZnO nanosheet from the graphite-like structure to the wurtzite structure with oxygen vacancies at the top Zn-terminated (0001) surface [K. M. Wong, et al., J. Appl. Phys. 113, 014304 (2013).]

For further details and discussions on the results as well as the in depth descriptions of the first-principle calculations on the effect of the oxygen vacancies on the defect formation energy, charge density and electronic band structure of the ZnO (0001) nanosheets of different sizes at the Zn-terminated and O-terminated surfaces, the reader is referred to pages 6-8 in J. Appl. Phys. 113, 014304 (2013). One important effect on the creation of the oxygen vacancies is enhancement of the surface metallization of the defective ZnO nanosheets. The graphitic ZnO thin films are structurally similar to the multilayer of graphite and are expected to have interesting mechanical and electronic properties for potential nanoscale applications.

Reference

Wong, Kin Mun, Alay-e-Abbas, S. M., Shaukat, A., Fang, Y., & Lei, Y. First-principles investigation of the size-dependent structural stability and electronic properties of O-vacancies at the ZnO polar and non-polar surfaces Journal of Applied Physics, 113 (1) (2013). DOI: 10.1063/1.4772647