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OriginalPaperphys. stat. sol. (b) 245, No. 5 (2008) 983Table 1 A comparison of key properties for Cu/BaTiO 3andCu/MgO(001) interfaces; interfacial distances z Cu, adsoption energiesE ads, and charge transfer across the interface ∆q Cu(positivesign means excess of the electron density) for several surfacecoverages θ and different terminations.substrate term. θ, MLBaTiO 3MgOTiO 2BaOExpt. unknown 2.0–2.5[37](o)z , Å ECuads, eV ∆q Cu, e1/8 1.93 1.07 –0.151/4 1.96 0.99 –0.121/2 2.01 0.95 –0.091/8 1.97 0.99 –0.171/4 2.02 0.89 –0.141/2 2.07 0.90 –0.111/4 2.09 0.71 –0.051/2 2.11 0.73 –0.030.75[8]negligible[8]we could qualitatively check our results for Cu/BaTiO 3 assumingthat the ratio of the binding energies for Cu, Pd, Ptis qualitatively similar on ABO 3 and MgO surfaces. Theexperimental study [8] clearly shows that on the MgO(001) E ads (Cu) < E ads (Pd) < E ads (Pt). This is in a line withthe binding energies for Cu, Pd, Pt on MgO available fromrecent ab initio calculations (their ratio was found to be1 : 1.5 : 2.7, respectively [8]). Our estimate of the adsorptionenergy for Cu/BaTiO 3 interface (0.9–1.1 eV in Table1) is in qualitative agreement with this trend when weconsider the values calculated for Pd/SrTiO 3 (001) (2.5 eV[18]) and Pt/BaTiO 3 (001) (3.9 eV [16]) interfaces. Despitethe fact that the adsorption energy in Cu/MgO(001) interfaceis noticeably smaller as compared to that for both theterminations of Cu/BaTiO 3 interface (Table 1), the interfacialdistances are qualitatively similar to those calculatedearlier for various metals upon strontium and barium titanate(1.9–2.1 Å [16–19]).We found that for the same coverage densities, Cu adatomsare bound somewhat more strongly upon the TiO 2 -terminated surfaces than on the BaO termination, mainlydue to a strong influence of surface Ti ions. E ads decreasesslightly as the Cu coverage on TiO 2 termination increases(from 1.07 eV down to 0.95 eV) since the same surface Tiions should share intermetallic interactions with a largernumber of adatoms (Fig. 2a, b). Meanwhile, 1/2 Cu MLupon both BaO-terminated perovskite and MgO(001) substratesis bound slightly stronger as compared to 1/4 CuML, due to the additional attraction of the enhanced electronicdensity between the Cu atoms and the positivelycharged Ba 2+ or Mg 2+ ion below (Fig. 2f) as discussed recently[15].Although the outermost layers of both MgO(001) andBa-terminated BaTiO 3 (001) substrates possess similar geometry(cf. Fig. 1c, d vs. 1e, f, and notice a scaling factor∼ 2 by which the fcc MgO surface lattice parameter exceedsthat for sc BaTiO 3 ), the difference of their underlayersresults in a different mechanism of Cu adsorption: Tiions positioned under surface O ions and forming partlycovalent Ti–O bonds noticeably increase the reactivity ofthe BaO-terminated (001) perovskite surface as comparedto the MgO(001).To understand better the interface nature, we have examinedthe differential charge density plots for 1/4 and 1/2ML for both BaTiO 3 terminations. Figure 2a, b clearlyshow the polarization not only along the Cu–O 2- bond butalso between Cu atoms and surface Ti ions. As a result,each adatom donates up to 0.15e for both interfacial O anionand intermetallic Cu–Ti bonds. For Cu interfaces withBaO-terminated perovskite and MgO, Ba 2+ and Mg 2+ ionsare almost indifferent to the interfacial bonding, especiallyin the case of low (≤1/4 ML) coverage. However, O ionsof the perovskite surface donate up to 0.17e to each Cuadatom vs. 0.05e from magnesia anions (Table 1). Similarqualitative conclusions were recently obtained for Me/BaTiO 3 (001) [16] where only BaO-termination was considered,Pd/SrTiO 3 (001) and Pt/SrTiO 3 (001) interfaces[18–20] although behavior of Pd and Pt adatoms on TiO 2 -terminated surface was found to be quite different.Analysis of the density of states (DOS) for copperoverlayers (Fig. 3) confirms substantial changes in conductionelectron distributions for different Cu coverages. TheO(2p) states are very sensitive to coverage variation. Forboth net-type coverages upon MgO(001) (1/4 and 1/2 ML),a pseudo-gap (the energy difference between the O(2p) andCu(3d) peaks) is well observed in Fig. 3e, f. These resultsare close to those of X-ray emission spectroscopy [39]where the gap between MgO and Cu peaks was found to be∼1 eV. Projections of Mg(3p) and Ba(6p) states showrather negligible contribution to the interaction on theCu/MgO(001) and Cu/BaTiO 3 (001) interfaces, respectively.Quite to the contrary, Ti(3d) states (which are not shownhere due to their absence on MgO substrate but whichcould be easily estimated as the difference between the totalDOS and DOS projected on O and Cu states) are veryactive in the interfacial interactions in the vicinity of theband gap; one may observe their different role at TiO 2 - andBaO-terminated substrates in Fig. 3a, d. For the TiO 2perovskite termination, a quite complicated DOS structureis observed where the Cu–O pseudo-gap grows with increasingCu coverage. On the BaO-terminated substrate,both O and Cu states are substantially shifted towardslower energies and, thus, the pseudo-gap is negative forboth coatings. This could result from the opposite chargetransfer across the interface discussed above (Table 1).Summing up, the conclusion can be drawn that DOS forboth Cu/BaTiO 3 (001) interfaces clearly shows a strong interfacialinteraction which reflects not only the polarization effects(physisorption) as in the case of a Cu/MgO(001) interfacebut also the chemisorption contribution.4 Conclusions The atomic and electronic structure ofordered Cu submonolayers with different coverages uponthe partly covalent BaTiO 3 (001) substrate (with BaO andTiO 2 terminations) as well as on the purely ionic MgO(001) substrate have been examined in this study usingwww.pss-b.com© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

physicapssstatussolidib984 Y. F. Zhukovski et al.: A comparative ab inito study of Cu overlayers-10 -50a) 1.7 eVεeVb) -10 2.4 eV -50εeVεεc) -10 -50-0.5 eV eV d)-10 -5-1.4 eV0eVTotal DOSεεPDOS of interfacial O (2p)PDOS od interfacial Cu (3d)e)-10 -51.8 eV0eVf)-10 -51.6 eV0eVTotal DOS for pure substrate slabPDOS of interfacial O (2p) for puresubstrate slabFigure 3 (online colour at: www.pss-b.com) DOS, both total and projected on O (2p) and Cu (3d) energy states, in a comparison withsimilar DOS for perfect oxide substrates for six configurations of Cu over BaTiO 3and MgO as shown in Fig. 1: 1/4 and 1/2 Cu ML adsorptionover regular O 2− ions upon both TiO 2-terminated (a, b) and BaO-terminated (c, d) BaTiO 3(001) substrate and MgO(001) substrate(e, f). The largest peaks were normalized to the same value whereas a convolution of individual ε ienergy levels was plotted usinga Gaussian line shape with a half-width ∆ε = 0.5 eV.the formalism of hybrid B3PW DFT-LCAO calculations.The Cu–O interaction is found to be a driving force for astrong Cu adsorption on both defect-less oxide substrates.The contributions to the interfacial bonding from Ba andMg ions are almost negligible, even when Cu adatoms arelocated directly above these cations.In contrast, surface and subsurface Ti ions on the perovskitesubstrate substantially affect the Cu adsorption.On the TiO 2 -terminated BaTiO 3 surface, the partial intermetallicbonding between Cu adatoms and surface Ti ionsresults in a partial transfer of the electron density from theCu atoms towards the substrate (Fig. 1a, b and Table 1),unlike the other four configurations where Cu adatoms behaveas electron acceptors. (For Cu coatings >1/4 MLthese effects are less pronounced.)The analysis of DOS spectra shows that if the interactionbetween Cu and MgO could be still considered as physisorption(a strong polarization of interfacial Cu adatomsand O ions), the Cu adsorption on BaTiO 3 surface ratherreveals the chemisorption contribution, especially at aTiO 2 -terminated substrate. This conclusion is confirmed bythe magnitude of charge transfer across the Cu/BaTiO 3 interface,which exceeds that for Cu/MgO by a factor ofabout three.Acknowledgements This study was partially supported byboth the MRSEC program of the National Science Foundation(DMR-0520513) and the Latvian National Program on FunctionalMaterials and Technologies for Microelectronics and Photonics(05.0005.1.1). DEE acknowledges support of DOE, throughGrant No. DE-FG02-05ER46255. The authors kindly thankD. Fuks, E. Heifets, and A. M. Stoneham for numerous fruitfuldiscussions. The technical assistance of S. Piskunov and Yu.Mastrikov was most valuable.© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-b.com

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