First-Principles Calculations of Clean and Defected ZnO Surfaces Nunzio Roberto D’Amico, †,‡ Giovanni Cantele,* ,† and Domenico Ninno ‡,† † CNR-SPIN, Complesso Universitario Monte Sant’Angelo, Dipartimento di Scienze Fisiche, Via Cintia, 80126 Napoli, Italy ‡ Universita ̀ degli Studi di Napoli “Federico II”, Dipartimento di Scienze Fisiche, Complesso Universitario Monte Sant’Angelo, Via Cintia, 80126 Napoli, Italy ABSTRACT: We report on a theoretical study of the nonpolar ZnO (101̅0) and (112̅0) surfaces carried out in the framework of density functional theory, aiming to elucidate the thermodynamic and kinetic stability of the clean surface against the formation and diffusion of oxygen vacancies. At variance with other oxide materials and ZnO surfaces with different orientation, we show that, under exposure to molecular oxygen in the gas phase, no significant amounts of oxygen vacancies can be sustained by the surface, in agreement with recent Scanning Tunnelling Microscope (STM) observations. However, our calculations show also that under ultrahigh vacuum and high-temperature conditions the observation of oxygen vacancies might be possible, as reported in earlier experiments of Gö pel and Lampe. 1 We characterize the defected surfaces electronic and structural properties as a function of the position of the defect with respect to the surface and discuss the diffusion paths of such defects both parallel and across the surface. ■ INTRODUCTION The detection of specific gas molecules in mixtures of various gases is increasingly required for the control and monitoring of several industrial and medical processes. 2,3 Solid-state gas sensors based on metal-oxide-semiconductor materials have attracted considerable attention during the past decade 4-10 due to low cost, small dimensions, and high compatibility with microelectronics processing. The conventional sensing mech- anism relies on the charge transfer between the absorbed gas and the metal oxide surface. Depending on the semiconductor type, the charge transfer will either increase or decrease the concentration of the majority carriers, thereby increasing or decreasing the sensor electrical conductance. 9,10 Zinc oxide (ZnO), tin dioxide (SnO 2 ), and titanium dioxide (TiO 2 ) nanostructures have been identified as promising gas- sensitive materials with many well-documented applica- tions. 10-16 In particular, ZnO is interesting because of its mixed covalent/ionic character in the chemical bonding, with peculiar properties such as large exciton binding energy (60 meV) and direct band gap (3.4 eV). The versatility and multifunctionality of this material are testified by the wide range of possible applications in varistors, 17 surface acoustic wave devices, 18 transparent conducting oxide electrodes, 19 solar cells, 20 blue/UV light emitting devices, 21 self-powered (nano)- devices, 22 and, as mentioned, gas sensors. 23,24 Of course, it is expected that photocatalysis and gas sensing applications require a precise control of the surface morphology, chemistry, and composition: usually, nanostructures with different surface facets are employed, the (101̅0) and (112̅0) surfaces/facets being the most stable and abundant ones. 25,26 Since the ZnO properties are highly sensitive to the nature and concentration of lattice imperfections, 27,28 understanding the thermodynamics and kinetics of point defects in ZnO is not only of fundamental but also of significant technological interest. For example, zinc migration, which is believed to proceed through the migration of intrinsic defects in the vicinity of grain boundariesmost likely zinc interstitialshas been discussed in connection with the degradation of varistors devices. 29,30 Moreover, it has been argued that the presence of native surface and subsurface point defects, their concentration, and depth distribution can strongly affect the electronic properties and electrical response of metal-ZnO interfaces and Schottky barriers, as well as the gas adsorption on specific ZnO surfaces. 31 Some ab initio density functional theory (DFT) calculations have been performed in the past to elucidate the behavior of both intrinsic 32-35 and extrinsic point defects. 36,37 It is widely accepted that oxygen vacancies can be considered as the most abundant and the chemically most reactive kind of atomic defects for a large variety of oxides. 38 Previous theoretical studies of the bulk ZnO and its surfaces have proven the impact of oxygen deficiency on the electronic and structural proper- ties. 39-43 Nevertheless, a systematic study of the vacancy diffusion along and across the ZnO nonpolar (101̅0) and (112̅0) surfaces together with the implications on relevant material properties is still lacking. Moreover, the actual Received: July 9, 2012 Revised: September 12, 2012 Published: September 14, 2012 Article pubs.acs.org/JPCC © 2012 American Chemical Society 21391 dx.doi.org/10.1021/jp306785z | J. Phys. Chem. C 2012, 116, 21391-21400