Tailoring oxygen vacancies at ZnO (1100)surface: An ab initio study

KK Korir, A Catellani, G Cicero

Tailoring oxygen vacancies at ZnO (1100)surface: An ab initio study

Citation:

KK Korir, A Catellani, C.G., 2016. Tailoring oxygen vacancies at ZnO (1100)surface: An ab initio study. Journal of Applied Physics, 120(12), p.125301.

Abstract:

Oxygen vacancies in ZnO crystals have significant impacts on its properties and applications. On the basis of ab initio results, we describe the oxygen vacancy distribution and diffusion paths away from the ZnO( 11¯00) surface, aiming to elucidate thermodynamics and kinetic stability of the vacancies and a possible control mechanism. In view of defect engineering and sensor applications, we propose efficient routes to chemically control the equilibrium concentration of the oxygen vacancies at ZnO surfaces by exposure to specific reactive gases: we show that the oxygen vacancy concentration can be increased using sulfur oxide as post-growth treatment, while under exposure to ozone, no significant amount of oxygen vacancies can be sustained on the surface.

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Korir, K.K., et al., 2020. Tuning electronic structure of ZnO nanowires via 3d transition metal dopants for improved photo-electrochemical water splitting: An ab initio study. Materials Today Communication, 26, p.101929. Abstract

ZnO nanowires have been proposed as potential photo-anode materials for photo-electrochemical water splitting due to their low toxicity, simple synthesis and easy modification routes. However, ZnO suffers from low PEC activity and photo-corrosion eﬀ ;ects, and therefore, application of ZnO nanowires in PEC water splitting still awaits development of effective design and synthesis strategies to improve its PEC eﬃciencies to commercially viable levels. Here, we present ab initio Density Functional Theory calculations considering 3d transition metal doping as a potential route towards attainment of ZnO nanowires with superior PEC activity. Our results show that the stability of 3d transition metal dopants in ZnO NWs is dependent on the d character of the transition metal dopant as well as their concentration and doping site, with most transition metal atoms being energetically most favorable at the Zn substitutional site both in O-rich and Zn-rich conditions considered. Specifically, we find all 3d transition metal dopants in ZnO NW under O-rich conditions as well as Sc, Ti and V under Zn-rich conditions have negative formation energies at the considered dopant concentrations of 1−6 atm. %, indicating that these dopants can readily be incorporated into ZnO NWs at thermodynamic equilibrium conditions. The electronic properties of Ti and V at 2% and 4% dopant concentration, respectively, yield a staggered band-structure configuration, while Sc, Cr, Mn, Co, Ni, and Cu dopants in ZnO NWs induce band-edge states. In addition, 3d TM dopants induces significant red-shift of the absorption edge of ZnO NW due to reduction in band gap, and are projected to improve visual light harvesting capabilities. Finally, the band alignment relative to the redox potential of water revealed that the valence band maximum of Sc, V, Ni and Cu doped ZnO NWs remains strongly positive above the oxidation potential of O2/H2O, while their reduction potential remain negative below the reduction potential of H+/H2, favouring PEC applications.

Michele Re Fiorentin, Kiptiemoi Korir Kiprono, F.R., 2020. Substitutional impurities in monolayer hexagonal boron nitride as single-photon emitters. Nanomaterials and Nanotechnology, 10, p.1847980420949349. Abstract

Single-photon emitters in hexagonal boron nitride have attracted great attention over the last few years due to their excellent optoelectronical properties. Despite the vast range of results reported in the literature, studies on substitutional impurities belonging to the 13th and 15th groups have not been reported yet. Here, through theoretical modeling, we provide direct evidence that hexagonal boron nitride can be opportunely modified by introducing impurity atoms such as aluminum or phosphorus that may work as color centers for single-photon emission. By means of density functional theory, we focus on determining the structural stability, induced strain, and charge states of such defects and discuss their electronic properties. Nitrogen substitutions with heteroatoms of group 15 are shown to provide attractive features (e.g. deep defect levels and localized defect states) for single-photon emission. These results may open up new possibilities for employing innovative quantum emitters based on hexagonal boron nitride for emerging applications in nanophotonics and nanoscale sensing devices.

K.K.Korir, & Philemon, K.T., 2020. Tailoring single walled carbon nanotube for improved CO2 gas applications: Insights from ab initio simulations. Materialia, 11, p. 100694. Abstract

Single walled carbon nanotubes has been identified as a potential material for CO2 gas sensing, capture and storage, however, comprehensive understanding of adsorption/desorption mechanisms that drives these application is still lacking yet such knowledge is essential for mainstream application of SWCNT in the identified areas. In this work, we use Density Functional Theory to study CO2 storage and sensing on SWCNTs with the aim of unraveling how such applications can be enhanced with the introduction of dopants with emphasis on Al, B, N and S as potential dopants. It is observed that doping SWCNT with N and B can be easily achieved compared to Al and S, which reported high and positive formation energies thus, can only be achieved under non-equilibrium condition. N doping improves SWCNT interaction with CO2 molecules and when subjected to thermal treatment the adsorbed CO2 is release to the atmosphere at 423 K thus a reusable sensing element can be achieved. It was further observed that the diffusion of molecular CO2 within the proximity of Al and S dopants in SWCNT matrix is not favored, while N and B doped SWCNT tend to have lower barrier energies to CO2, thus can offer better control of carbon storage. Our finding can assist in the design and optimization of SWCNT for energy and environmental applications.

Kiptiemoi Korir Kiprono

PhD, Condensed Matter Physics

Tailoring oxygen vacancies at ZnO (1100)surface: An ab initio study

Citation:

Abstract:

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