First-principles study of the interfacial structures of Au/ MgO„001… D. Chen, X. L. Ma,* and Y. M. Wang Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China Received 22 March 2006; revised manuscript received 19 December 2006; published 8 March 2007 By using a first-principles method, the theoretical analysis of six probable Au/ Mg001 interface models points out that two of them with oxygen or magnesium vacancies in the interface are most stable. It is found in this work that besides O or Mg vacancies having to exist in this interface, the stability of an interface depends deeply on the atomic configuration of interface. Such a configuration in which each Au atom in the upper Au layer of interface bonds with each O atom in the MgO under layer is theoretically considered to be most stable. Nevertheless, these results need to be confirmed by experiments. DOI: 10.1103/PhysRevB.75.125409 PACS numbers: 73.20.-r, 68.35.Ct, 31.15.Ar, 31.15.Ew I. INTRODUCTION The interface characteristic of metal oxides is often the controlling factor limiting their practical applications in ca- talysis, adhesion, corrosion, and microelectronics. Thus, de- termining it and understanding its influence are of great im- portance. For instance, although gold is one of the noblest metals and has little activity for chemisorption, 1 it was found that nanosize particles of gold supported on various oxides exhibit an enhanced catalytic activity, 2–4 which would be of great significance in environmental and chemical technolo- gies, for example, to be used as a sensor. 5 Such a question of common interest stimulates many researchers to theoretically explain the interfacial behavior of metal oxides. 6–15 Many theoretical and experimental attempts have been done on the deposition and growth of Au clusters on MgO, 9,16–19 and some focused on adatoms and dimmers 6–8 and the adsorption characteristics on a regular MgO001 surface; 9–11 however, they only concerned the adhesion of the isolated adatoms and adsorbed dimmers on the surface of oxide. A minority of them, such as the papers of Goniakowski 12 and Herschend et al., 13 concentrated on the interfacial characteristics of metal coverages. To our best knowledge, the electronic structures of Au/ MgO interfaces, which are physically and technologi- cally important, remain unclear. Therefore, a systematic study of the Au/ MgO interfaces is still needed. This is just our goal in the present paper. II. COMPUTATIONAL DETAILS As we know, it is possible to use first-principles calcula- tions to study interface electronic structures and to acquire the results that agreed with experiments if the interface mod- els used are reasonable. 20 In the present study of Au/MgO interfaces, the plane-wave pseudopotential PWPP method in the CASTEP code 21 has been employed. In the calculations, the energy cutoff was set at 350 eV 25.7 Ry, which is a better choice with respect to the com- putational efficiency and convergence, 22 and the number of k points within the irreducible wedge of the Brillouin zone was determined by the Monkhorst-Pack scheme. 23 A k-point mesh of 6  6  1 was used to sample each computational cell. All the atoms in each computational cell were relaxed until its total energy difference between two steps is smaller than 1  10 -5 eV/ atom. The total energy was minimized by means of a conjugate gradient technique. 24 The ultrasoft pseudopotentials 25 of gold, magnesium, and oxygen were taken and optimized to determine the appropriate plane-wave basis set. The density-mixing scheme based on the Pulay algorithm 26 was used for self-consistent-field SCF calcula- tion. The SCF tolerance was set at 1  10 -6 eV/atom. The atomic calculations and the formation of the pseudopoten- tials were treated self-consistently through the local-density approximation LDA. The LDA exchange-correlation func- tional has been used in this study, because recent studies about Au on oxides indicated that it can provide a better Au / MgO description. 27–30 By using the Gasteiger method, 31 the spatial distributions of electronic charge and electrostatic potential were calculated for a whole system or a selection of atoms. Partial density of states PDOS gives a qualitative treatment on the nature of electron hybridization in the sys- tem and can be produced for certain angular momenta on selected atoms. If more than one atom is selected, the con- tributions in each angular momentum channel from all se- lected atoms are added together. Experimentally, 32 vacancies have been found on the sur- faces of Au nanoparticles embedded in MgO. Even in MgO crystals, oxygen and magnesium vacancies still exist, 33 and recent experiments have directly observed these defects on the 001 surface of ultrahigh-vacuum cleaved single MgO crystals. 34 Thus, the interaction of Au with oxygen and mag- nesium vacancies must be taken into consideration in mod- eling and calculating of Au/ MgO interface. Practically, the most often observed defects in MgO are oxygen vacancies. They are in different charge states de- pending on the condition in which they are prepared. Once the vacancies are formed, their electronic structures could be changed by adding or removing electrons in them; 35 there- fore, the oxygen and magnesium vacancies can be classified as positive, negative, and neutral by their charges. The neu- tral oxygen vacancies are generally stable in bulk MgO. 36 In what follows, we address ourselves firstly to calculating the formation energies of neutral oxygen and magnesium vacan- cies in bulk MgO and comparing them with the experimental and previous computational results to give a feasible evi- dence of our method. PHYSICAL REVIEW B 75, 125409 2007 1098-0121/2007/7512/1254097 ©2007 The American Physical Society 125409-1