Determination of Silica and Germania Film Network Structures onRu(0001) at the Atomic ScaleAdrian Leandro Lewandowski,† Philomena Schlexer,‡ Sergio Tosoni,‡ Leonard Gura,†

Patrik Marschalik,† Christin Buchner,§ Hannah Burrall,∥ Kristen M. Burson,∥ Wolf-Dieter Schneider,†

Gianfranco Pacchioni,‡ and Markus Heyde*,†

†Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany‡Department of Materials Science, Universita di Milano-Bicocca, Via R. Cozzi, 55, Milan, Italy§Lawrence Berkeley National Laboratory, 1 Cyclotron Road Mailstop 2R0300, Berkeley, California 94720, United States∥Taylor Science Center, Hamilton College, 198 College Hill Road, Clinton, New York 13323, United States

*S Supporting Information

ABSTRACT: The detailed structure of silica and germania films supported onRu(0001) metal substrates are compared to each other. Surface sciencetechniques together with density functional theory calculations have been usedto gain insights into the atomic arrangement of these prominent glass-formingmaterials. The monolayer films of these materials both show predominantlycrystalline hexagonal lattices with characteristic domain boundary structures.For the germania monolayer films a large variety of ring elements withindomain boundaries have been observed. Density functional calculations predict stronger interaction with the metal substrate forbilayer germania as compared to bilayer silica films. Scanning tunneling microscopy images with atomically resolved structuralfeatures have given access to silica and germania bilayer film structures. Both bilayer films form characteristic amorphous ringstructures. However, the germania bilayer films appear to be more corrugated, pointing to a stronger interaction with the metalsupport thus giving rise to slightly different connectivity rules.

■ INTRODUCTION

For a long time the field of surface science was focused almostexclusively on crystalline and well ordered surface structures.Recently, a new class of film system was discovered thatrevealed atomic sites of amorphous oxide network structuresfor the first time: the two-dimensional (2D) silica bilayer (BL)films.1−3 Subsequently, the characterization of this specific filmsystem has become a fascinating research topic. Depending onthe preparation conditions, the silica bilayer film can be tunedbetween its crystalline and amorphous state. So far, this is theonly oxide network film system that has shown such uniquestructural properties.However, silica is not the only prominent amorphous oxide

network former. Comparisons with other prominent glass-formers, such as germania or boron oxide, can provide newinsight into glassy structure. In a recent attempt, we havesuccessfully grown germania monolayer (ML) films on aRu(0001) metal substrate.4 Here we present the first data onamorphous germania bilayers and compare common structuralnetwork features for monolayer as well as for bilayer silica andgermania films. We begin this comparison with monolayerfilms and structural motifs observed in their domainboundaries and then discuss bilayer film coverages.When silica and germania film structures are compared, it is

worth looking into previous studies of their bulk phases.There are many different polymorphs of bulk silica (SiO2)

including amorphous phases and at least 40 studied crystalline

polymorphs that may form depending on pressure, temper-ature, and preparation conditions.5 Many polymorphs consistof a tetrahedral building unit with a central silicon atom whichis bonded to four oxygen atoms. Each oxygen atom is bondedto two silicon atoms, connecting adjacent tetrahedra at thecorners. Different polymorphs are attained through slightvariations in the Si−O−Si angle such as α- and β-quartz, α-and β-cristobalite, coesite, trydimite, and even amorphoussilica.6,7 This confirms the early random network theory ofZachariasen,8 which hypothesizes that both crystalline andamorphous networks share the fundamental tetrahedralbuilding unit, differing primarily in the distribution of theintertetrahedral angle. Stishovite, a high pressure polymorph ofsilica with typical formation conditions around 1650 °C and 15GPa, possesses the rutile structure consisting of octahedra witheach oxygen coordinated to three silicon atoms and eachsilicon coordinated to six oxygen atoms.9 In lunar and Martianmeteorites, a shock induced orthorhombic polymorph of silicacalled seifertite has been observed. It is estimated that aminimum equilibrium shock pressure in excess of 35 GPawould be necessary to produce this structure.10

Special Issue: Hans-Joachim Freund and Joachim Sauer Festschrift

Received: July 24, 2018Revised: September 21, 2018Published: October 1, 2018

Article

pubs.acs.org/JPCCCite This: J. Phys. Chem. C 2019, 123, 7889−7897

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Bulk germania surfaces do not readily form well-definedGeO2 surfaces but exhibit a mixture of oxidation states uponsubstrate heating in oxygen.11 In order to study stoichiometricGeO2, bulk crystals of rutile-type or quartz-type GeO2 havebeen investigated in experiment and theory. Cooled melts ofthese crystals exhibiting amorphous structures have beenstudied as well. Micoulaut et al. reviewed the knowledge ofGeO2 bulk structures, which we will briefly summarize here.12

At room temperature, bulk GeO2 is stable in a rutile-likepolymorph, where each Ge atom is coordinated in anoctahedron by six oxygen atoms. At elevated temperatures orpressures, an α-quartz-like polymorph is formed, whichconsists of tetrahedral GeO4 building blocks. From X-raydiffraction (XRD), neutron diffraction (ND), and Ramanspectroscopy, amorphous GeO2 is concluded to be composedof oxygen-bridged tetrahedra that form a network withoutlong-range order. Thus, for both amorphous silica andgermania, the MO4 (M = Ge, Si) tetrahedral building blockis a key ingredient.Table 1 provides atomic distances and angles taken from

XRD, ND, and anomalous X-ray scattering (AXS) experiments

together with the temperatures for the glass transition and themelting point for bulk amorphous silica and germania. Bonddistances are comparable for the two materials, but germaniahas a lower glass transition temperature and melting point. Thelarger size of the germanium atom compared to the silicon one,allows more O positions surrounding the cation.12 This isevidenced in a more distorted O−Ge−O intratetrahedralangle. However, the intertetrahedral angle Ge−O−Ge exhibitsa narrower distribution than the Si−O−Si angle. Furthermore,the larger amount of three-membered rings observed ingermania with respect to silica is reflected in the lower mean ofthe Ge−O−Ge angle.13

For silica, a thin-film strategy has been employed for theidentification of local site geometries, tetrahedral buildingblocks and larger building units, i.e., rings. Studies ofmonolayer silica films grown on Ru(0001) and Mo(112)substrates reveal a hexagonal crystalline network of corner-sharing SiO4 tetrahedra.16,17 Recently, the first experimentalstudy on the comparable monolayer structure for germania onRu(0001) has also been published.4 For both silica andgermania monolayers, domain boundary structures are presentin the crystalline films.Especially for silica bilayer films details on the structural

arrangement,18 shifts in the metal work function,19,20 the bandgap,19 the exchange of network formers,21,22 the transferabilityfrom one substrate to another one,23 the bending rigidity,24 aswell as several theoretical approaches for modeling have beenpresented in the literature.25,26 More details can be found in arecent review article on this film system.27 Interestingly,

depending on the oxygen affinity of the metal substrate, silicabilayers can be formed in either crystalline or vitreousconfigurations.28

For germania, to the best of our knowledge, only one densityfunctional theory (DFT) prediction for thin films26 as well asone experimental study of germania thin films4 have beenpublished. Moreover, no comparable 2D network structures asobserved in the present work have been realized before.

■ EXPERIMENTAL DETAILS

Silica and germania films are grown on Ru(0001) in differentultrahigh vacuum chambers (base pressure 10−10 mbar range).The Ru(0001) single crystal is cleaned through several cyclesof heating up to 1450 K for 1 min, annealing in the presence of2 × 10−6 mbar oxygen pressure at 1250 K for 20 min andbombarding the surface with Ar+. The cleanliness of the metalsubstrate is checked by scanning tunneling microscopy (STM)and low-energy electron diffraction (LEED). In addition to ourresults, in this publication, we include STM images of silicafilms published by Yang et al.29 and Mathur et al.30 The recipeof the silica films in all cases consists of the followingprocedure. First, the clean Ru(0001) is precovered with a(2×2)-3O adlayer by annealing the crystal at 1100 K under anoxygen pressure in the 10−6 mbar range. Subsequently, siliconis deposited by physical vapor deposition (PVD) in a 10−6

mbar oxygen background pressure. Finally, the system isoxidized by annealing in a temperature range between 1100and 1220 K in 10−6 mbar oxygen pressure. Our STM images ofsilica films were taken at 4.2 K with a PtIr tip. More details ofthe low temperature STM can be found in ref 31. The imagesfrom Yang et al. and Mathur et al. were obtained at roomtemperature with PtIr and W tips, respectively.The preparation of germania films is based on previously

reported studies of a germania monolayer film on Ru(0001).4

At first, a (2×2)-3O adlayer is formed as described before.Germanium is then evaporated by PVD in 2 × 10−6 mbaroxygen pressure. The coverage of the film is controlled bychanging the germanium evaporation time and keeping allother parameters fixed. For such ultrathin films, a lineardependency is found between the coverage and the Geevaporation time. STM images of the germania films weretaken in a room temperature Beetle-type STM using a PtIr tip.The experimental setup can be found in ref 32.

■ COMPUTATIONAL DETAILS

Periodic DFT calculations were performed using the ViennaAb Initio Simulation Package (VASP).33−35 The generalizedgradient approximation (GGA) for the exchange−correlationfunctional was applied within the in the Perdew, Burke, andErnzerhof (PBE) formulation.36,37 To verify the resultsobtained with the PBE functional, we performed calculationswith the HSE06 hybrid functional.38,39 To describe electron−ion interactions, the projector augmented wave (PAW)method was used.40,41 O (2s, 2p), Si (3s, 3p), Ge (4s, 4p),and Ru (4d, 5s) states were treated explicitly. For electronicrelaxations, the blocked Davidson iteration scheme wasused.42,43 In geometric structure optimizations, all ions wereallowed to relax until ionic forces were smaller than |0.01| eV/Å. For unit cell optimizations of the thin films, plane waveswere expanded up to a kinetic energy cutoff of 1200 eV and ak-point set of (13 × 13 × 1) in the Monkhorst−Pack scheme44

was used. For all other calculations, a cutoff energy of 400 eV

Table 1. Physical Properties of Bulk Silica and GermaniaTaken from References 7 and 13−15

SiO2 GeO2

M−O 0.16 nm (ND) (XRD) 0.17 nm (ND) (AXS)O−O 0.26 nm (ND) (XRD) 0.28 nm (ND) (AXS)M−M 0.31 nm (ND) (XRD) 0.32 nm (ND) (AXS)O−M−O 106−114° 104−115° (ND)M−O−M 120−180° (XRD), mean 144° 121−147° (XRD), mean 130°Tg 1474 K 786 Kmp 1996 K 1389 K

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was used and the k-point set was reduced to (5 × 5 × 1). TheRu bulk structure was obtained as described in ref 45.The Ru(0001) surface was represented by a five-layer slab.

The lowest two layers of Ru were kept fixed in the bulkgeometry during structure optimization. The silica andgermania bilayer exhibit approximately twice the periodicityof the Ru(0001) surface. The Ru(0001)-(2×2) surface supercell exhibits surface lattice parameters of a0 = b0 = 0.546 nm.The optimized surface unit cell parameters of the free-standingGeO2 bilayer are a0 = b0 = 0.547 nm, and the free-standingSiO2 bilayer exhibits unit cell parameters of a0 = b0 = 0.531 nm.Upon deposition of the thin films on the Ru(0001)-(2×2)surface supercell, the Ru surface lattice parameters were keptfixed. Here, the k-point mesh was set to (5 × 5 × 1). For theGeO2 bilayer, this results in a deviation of the lattice constantsby 0.24% with respect to the Ru(0001)-(2×2) surface supercell. For the SiO2 bilayer the deviation is significantly larger:−2.76%. For the film−substrate interaction, van-der-Waals(vdW) forces are important. Thus, vdW forces were includedusing the DFT+D2 method as developed by Grimme et al.46

and modified by Tosoni and Sauer.47 Dipole correction alongthe z-axis was applied.

■ RESULTS AND DISCUSSIONFigure 1 compares the atomic structure of silica and germaniamonolayers on Ru(0001). Both systems share manysimilarities. STM images (Figure 1a,b) show that three Oatoms in the plane of the film surround each M (M = Ge, Si)atom. Additionally, DFT side-view models (Figure 1e,f) showthe fourth oxygen below, forming a bond to the Ru(0001)substrate. Both oxides crystallize in a honeycomb-like structure(6 M atoms per ring) that form a (2 × 2) lattice with respect tothe Ru(0001) substrate.4,16 They show the same registry,namely, M atoms are linked to the substrate via O atoms ontop and face-centered-cubic sites.4,30 Atomic oxygen adsorbson the ruthenium metal substrate on the hexagonal-close-packed hollow site below the center of the ring. The presenceof this oxygen atom was observed by STM in the case of silica(Figure 1a) and confirmed by a intensity−voltage low-energyelectron diffraction study for the germania monolayer film.4

The main difference between both lattices is the orientation ofthe tetrahedral building blocks MO4. Consecutive tetrahedraforming a ring are marked with red and black triangles inFigure 1a,b. While in the silica monolayer film two adjacenttetrahedra are facing each other producing a Si−O−Si bondthat looks straight in the image plane (Figure 1a,c), adjacentGeO4 building blocks are rotated 30° with respect to eachother4 (Figure 1b,d).Figure 2 compares domain boundary structures of silica and

germania monolayer films. An overview can be seen in Figure2a,b, connection points in Figure 2c,d, and boundaries with aStone−Wales motif in Figure 2e,f. The domain boundaryconnection points in Figure 2c,d have the same design: acentral six-membered ring is surrounded by three five-membered rings and three rings of larger ring size (seven-membered for silica and eight-membered rings for germania).The larger rings contribute to the structures of the threeincoming domain boundaries: 5577 domain boundaries forsilica and 48 for germania. Parts e and f of Figure 2 showdomain boundaries with Stone−Wales motifs. In the case ofsilica, there is a continuous line of Stone−Wales defects whilefor germaina there is a single six-membered ring separatingeach defect in the boundary. The similarities between the silica

and germania defects were expected given that the coreelement of the structure models, the tetrahedral building unit,is the same. It has also been reported that the grain boundariesin both silica48 and germania4 maintain the same registry withthe ruthenium substrate for the crystalline regions on eitherside of the boundary.Table 2 summarizes the various types of domain boundaries

found for silica and germania monolayers as well as bilayerstructures. Overall, germania monolayer films exhibit a greatervariety of domain boundary structures compared to silicamonolayer films. Rings from four-membered to eight-membered combine into various structure elements whichform domain boundaries on germania monolayers, whereas theknown boundary on silica monolayers consist of five- andseven-membered rings only. STM images of a new domainboundary structure and complex connection point, shown in

Figure 1. Comparing silica and germania monolayer structures onRu(0001). (a) is adapted with permission from ref 30. The right partis an STM image of a monolayer of silica on Ru(0001): 1.8 nm × 2.4nm, IT = 2 nA, VS = 0.9 V, room temperature. (b) STM image of amonolayer of germania: 2.4 nm × 2.4 nm, IT = 6.6 nA, VS = 0.58 V,room temperature. A model represented by balls and triangles areoverlaid on both images. Both (a) and (b) show oxygen atomicresolution. (c) and (d) DFT top view models for silica and germaniamonolayers, respectively. Size: 0.8 nm × 0.8 nm. (e) and (f) DFTside-view models for both monolayers. The figures on the left-handcolumn are green-toned and correspond to silica monolayers films onRu(0001). The right-hand column is blue-toned and refers togermania monolayers on the same substrate. Green balls representsilicon atoms, the blue ones germanium atoms, and oxygen atoms areshown in red. Light gray balls correspond to the ruthenium atoms ofthe substrate.

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Figure 3, highlight the range of ring-sizes found in germaniamonolayers. The differences between silica and germaniamonolayer domain boundary structures may stem fromvariations in angular arrangements of the tetrahedral units,evidenced in the distorted germania structures observed inSTM, and from coupling effects between the film and theruthenium substrate.We now turn to a discussion of silica and germania bilayer

films, first presenting results from DFT and then from STM.Adhesion energies are defined in eq 1 where E(X) is the

total energy of X and S is the surface area of the Ru(0001)-(2×2) super cell. As seen from eq 1, the term adhesion energy

typically refers to a net relaxation in the system that resultsfrom covering a bare substrate with a thin film, normalizedover the surface area. All energies refer to fully optimizedsystems.

E E E E S( (MO /Ru) (MO ) (Ru))/adh 2 2= − − (1)

Structure optimizations of free-standing (M4O6H4)6N (M =Ge, Si; N = 4−8) cages were done at the Γ-point withoutdispersion forces in unit cells of the size (2.8 × 2.8 × 1.8) nm.

Figure 2. Comparing silica and germania monolayer domainboundary structures on Ru(0001). STM images comparing differentfeatures of silica monolayers films (on the left column) and germaniafilms (on the right) on Ru(0001). (a) STM image of a monolayersilica film grown on Ru(0001) reprinted from ref 29: 19 nm × 19 nm,IT = 0.1 nA, VS = 2 V, room temperature. (b) STM image of agermania monolayer: 19 nm × 19 nm, IT = 0.4 nA, VS = 2.5 V, roomtemperature. (c) STM of a domain boundary defect on silicamonolayer reprinted from ref 29: 3.8 nm × 3.8 nm, IT = 0.15 nA, VS =1.2 V, room temperature. (d) STM of an analog connection pointobserved on germania monolayer films: 5.0 nm × 5.0 nm, IT = 0.6 nA,VS = 1 V, room temperature. (e) STM image of an antiphase domainboundary formed by seven-, seven-, five-, and five-membered rings:3.0 nm × 3.0 nm, IT = 2.0 nA, VS = 0.9 V, room temperature.Background image was reprinted with permission from ref 30.Copyright 2015 APS. (f) STM image of a 57756 domain boundary:3.5 nm × 3.5 nm, IT = 0.5 nA, VS = 2.5 V, room temperature. Ringsizes are color-coded from (c) to (f).

Table 2. Boundaries and Common Defects

system types method citation

ML SiO2/Ru(0001) 5577 domain boundary STM 3057 triangular loop defectencompassing a 6MR

29

475 rectangular loop defectencompassing an 8MR

29

BL SiO2/Ru(0001) 58 antiphase domainboundary

AFMSTM

48

57 rotational domain 4948 domain boundary 49Stone−Wales defect 4957 closed-loop defects 49

ML GeO2/Ru(0001) 84 antiphase domainboundary

STM 4

57756 antiphase domainboundary

4

5678 complex boundary 457 triangular loop defectencompassing a 6MR

currentstudy

45678 loop defectencompassing three 6MR

currentstudy

855 antiphase domainboundary

currentstudy

BL GeO2/Ru(0001) buckling STM currentstudy

Figure 3. STM images of new domain boundary structures ongermania monolayer films on Ru(0001). (a) 855 boundary: 3.6 nm ×3.6 nm, IT = 0.5 nA, VS = 1.0 V, room temperature. (c) Connectionpoint formed by four- to eight-membered rings: 6.8 nm × 6.8 nm, IT= 1.0 nA, VS = 2.0 V, room temperature. In (b) and (d) the ring sizesare color-coded accordingly.

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The cage replicas are therewith separated by at least 1.2 nm ofvacuum in each Cartesian direction. The relative stability of thecages is defined in eq 2 with respect to the most stable six-membered cages.

E E E N((M O H ) )/6 ((M O H ) )/Nrel 4 6 4 6 4 6 4= − (2)

We define the Ru-film distance as average distances in the z-direction between the uppermost Ru layer and the lowestoxygen layer of the thin films, d(Ru−Olow) = zav(Olow) −zav(Ru). Atomic charges Q have been estimated according tothe Bader decomposition scheme.50−52

Now we compare the adhesion properties of the silica versusgermania bilayers on Ru(0001), respectively, as derived fromDFT. Interesting parameters are summarized in Table 3.

Whereas the silica bilayer binds onto the Ru substrate mainlyvia dispersion forces, the germania bilayer shows a chemicalbinding to the Ru surface. This chemical binding is representedin the strong adhesion energy, more than 3 times larger thanthat of the silica bilayer, in the distance between the film andthe substrate as well as in the fact that there is a charge transferfrom the Ru substrate to the germania bilayer. We carefullytested this bonding type using the HSE06 hybrid functionaland confirm that the strong binding including the chargetransfer persists.For silica, the presence of the (2×2)-3O oxygen interlayer

constitutes the most stable phase after the synthesis process.However, the amount of interfacial oxygen can be tuned frombare metal crystal to a (2×2)-3O layer. Thereby, the electronicproperties of the film system can be modified.20 In detailedDFT calculations we have varied the amount of interfacialoxygen for silica and germania films. A full set of them is givenin the Supporting Information. In Figure 4a a side view of silicabilayer film with a (2×2)-3O oxygen layer on the Ru(0001)

together with density of states (DOS) in Figure 4b is given.While the structural arrangement inside of the silica bilayerseems to be mostly independent from the amount of interfacialoxygen, a stronger dependency on the atomic structure hasbeen observed for the germania bilayer. In Figure 4c,dcomparable side view and the respectively calculated DOSare shown for the germania bilayer film, but without interfacialoxygen in order to highlight the difference in binding modebetween the silica bilayer and the germania bilayer. The oxygenstates shown in the DOS in Figure 4d are all part of thegermania bilayer.To characterize the binding in terms of atomic and

electronic structure in more detail, we show the adsorptiongeometries, along with the DOS in Figure 4. We see that thestrong interaction of the germania bilayer results in a structuraldistortion, which is in good agreement with the experimentalobservations. The distortion that presents the bilayer ofgermania with respect to the silica brings with it a decrease inthe symmetry of the system. While the free-standing silicabilayer shows a D6h symmetry, the free-standing germaniabilayer breaks this symmetry due to the relative rotation of thetop and bottom tetrahedra which eliminates the horizontalmirror plane and the C6 rotational axis, leaving the system inD3 symmetry, which is further reduced when adsorbed to theRu support. The DOS reveals the charge transfer from thesubstrate to the germania bilayer, giving rise to Ge and O statesright above and below the Fermi level. The free-standinggermania bilayer has a band gap of around 3 eV with the PBEfunctional.In Figure 5 we compare stabilities of differently sized silica

and germania cages. We find that the relative energy of ringsizes other than 6 are lower for silica than for germania,indicating that the silica bilayer more readily forms a broaderring size distribution. The more strongly bound germaniabilayer may counteract the strain introduced by ring sizes otherthan 6 by forming more complex patterns of differently sizedrings that may involve different Ge coordination numbers.Figure 6 shows STM images of coexisting monolayer and

bilayer structures for silica and germania. Both images have thesame size: 12.0 nm × 7.0 nm. The specific tunnelingconditions sense atomic oxygen positions.4 The crystallinesilica monolayer in Figure 6a appears darker in the STM image.The periodic hexagonal structure of the monolayer is disturbedby a domain boundary that contains five- to seven-memberedrings, similar to the one shown in Figure 2e. In the SupportingInformation the spatial ring size distribution along this domainboundary is illustrated. It bridges the monolayer patchcompletely and separates the crystalline domains, which arecommensurate to the underlying Ru(0001) substrate and showan offset by a single atomic position in the [1100] direction.The amorphous silica bilayer appears brighter in the image andexhibits a broad ring size distribution that involves four- tonine-membered rings. A similar coexistence of two phases isfound in the germania film in Figure 6b. It consists of acrystalline hexagonal monolayer and a bilayer with lessstructural order that appears brighter in the image. Theapparent heights of the silica and germania bilayer respect tothe monolayer are around 1.4 and 1.3 Å, respectively, which isin line with previous measurements in silica films.53

In particular, the ratio of root-mean-square roughness for BLto ML silica is 2.0 ± 0.4, while for germania it is 5.5 ± 0.3.Because roughness analysis can be significantly affected by thecondition of the STM tip, care has been taken to compare

Table 3. Comparison of the Adhesion Properties of theBilayers

SiO2/Ru(0001) GeO2/Ru(0001)

Eadh (eV/nm2) −1.76 −6.78

d(Ru−Olow) (nm) 0.265 0.217q(Ru−slab) (|e|) 0.09 0.60

Figure 4. (a) Silica bilayer model supported on Ru(0001)-(2×2)-3O,which is the most stable phase under experimental conditions. (b)DOS of the silica bilayer on Ru(0001)-(2×2)-3O. (c) Germaniabilayer model supported on pristinie Ru(0001). (d) DOS of thegermania bilayer on Ru(0001).

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monolayer and bilayer regions with the same tip condition(e.g., within the same image) for the purpose of calculatingthese ratios. The monolayer regions of silica and germania arevery similar, consequently differences in roughness ratios areattributable primarily to differences in the bilayer structuresand are likely related to the underlying bonding configurationsfor silica and germania bilayers. Silica bilayer structures consistof two planar layers of ring structures that are bridged byoxygen atoms, as shown in the atomic model in Figure 4a.45,54

The top and bottom layer exhibit the identical networkstructure, while a sideview reveals only four-membered ringsthat result from pairs of Si−O−Si connected with two oxygenbridges. The bonding structure is fully saturated with no bondsto the substrate. Given that the germania bilayer films weregrown and annealed in an excess of oxygen, we expect that likethe silica bilayers, germania bilayers will have a fully saturated

bonding structure. The buckled germania bilayer may resultfrom different ring-sizes which connect the first with thesecond layer of the film and may include bonds between thegermania film and the substrate, an interpretation supported bythe aforementioned DFT results. Nevertheless, some patchesin the germania bilayer suggest that the structural model lookslike the silica bilayer one.In order to analyze the short-range order of the amorphous

silica and germania bilayer films, radial O−O distances aremeasured and compared. The oxygen positions are assignedwith red dots, as shown partially in Figure 6. The highercorrugation of the germania bilayer film affects the resolutionof the STM image and results in an incomplete detection ofoxygen positions compared to silica. Figure 7 shows the pair

distance histograms (PDHs) for silica and germania. Theyexhibit similar features up to a radial distance of 1.1 nm, whichindicates a similar short-range order for both systems. The firstpeak of the O−O radial distances is in good agreement withXRD data for bulk silica15 and germania.14

In addition to the similar radial atomic distances, similar ringsizes and configurations in both films are identified andoutlined in Figure 8. In order to visualize the ring structure ofthe germania bilayer, a filter with a scaled structuring elementfor erosion was applied to the images of Figure 6 and shown inFigure 8. The basis of this filter and its results are presented inthe Supporting Information. As reference, the hexagonalnetwork of the monolayers is marked with white outlines.Some rings of the germania bilayer resemble the ones of thesilica bilayer. Those are marked in black on the bilayer phasesin Figure 8. Four- to nine-membered rings were identified on

Figure 5. Comparing the stability of bilayer silica and germania ring structures. (a) Relative energy differences of differently sized rings with respectto the lowest energy six-membered ring. (b) and (c) top and side views of the DFT models of the hexagonal silica and germania bilayers,respectively (see Computational Details).

Figure 6. Comparing ultrathin films of silica and germania onRu(0001), probed by STM. Both films consist of coexisting crystallinemonolayer and amorphous bilayer phases. On half of the images reddots are used to mark distinguishable oxygen positions. (a) wasadapted with permission from ref 53. (a) 12.0 nm × 7.0 nm, IT = 10pA, VS = 1.0 V, 4.2 K. (b) 12.0 nm × 7.0 nm, IT = 0.4 nA, VS = −0.5V, room temperature.

Figure 7. Comparing oxygen−oxygen pair distance histograms of thesilica and germania bilayer. Vertical red dashed lines show the O−Odistance determined by XRD on the bulk materials.

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both bilayer films, which are color-coded as indicated at thebottom of the figure. The sizes of these rings for both systemsare comparable.Besides the similarities, the germania bilayer film shows also

larger looplike structures. Three examples are highlighted inred in Figure 8b. The larger loops might follow differentconnectivity rules, which could involve differently coordinatedGe or substrate bonding, as discussed previously.

■ CONCLUSIONWe have presented a side-by-side comparison between silicaand germania monolayer and bilayer films on Ru(0001). Bothmonolayers present a similar atomic structure. They consist ofMO4 (M = Si or Ge) unit blocks strongly coupled to thesubstrate that form a purely crystalline hexagonal phase. Awider set of ring size combinations is observed for germaniamonolayer films forming domain boundaries. DFT calculationspredict a strong interaction between the germania bilayer andthe substrate, while the silica bilayer is farther apart from thesubstrate and interacts only weakly with it through van derWaals interactions. This factor plays a key role in the observedstructural differences. The amorphous germania bilayer,presented here for the first time, compensates for the strainby forming a buckled structure that could involve different Gecoordination numbers, direct bonds to the metallic substrate,different ring sizes in the direction perpendicular to thesubstrate, or others. In order to assign all the ring sizes in thegermania bilayer film, a complete atomic model would beneeded, which is difficult to establish directly by scanning

probe methods due to the corrugated film and the affectedresolution. Therefore, future experiments aim to prepare flatterbilayer germania films by diminishing the film-supportcoupling by, e.g., changing the substrate. Our successfulfabrication of germania bilayers on Ru(0001) represents thefirst step in this direction.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.jpcc.8b07110.

Details on the domain boundary of the silica monolayer,supplements on the DFT calculations, description of theimage filter for ring visualization, and the oxygencoordinates of the bilayer phases.

(PDF)

■ AUTHOR INFORMATIONCorresponding Author*M. Heyde. E-mail: heyde@fhi-berlin.mpg.de. Phone: +49 308413 4149. Fax: +49 30 8413 4105.ORCIDSergio Tosoni: 0000-0001-5700-4086Gianfranco Pacchioni: 0000-0002-4749-0751Markus Heyde: 0000-0002-7049-0485NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors thank specifically Hajo Freund for his decisivescientific input, for his constant motivation, and for numerousfruitful discussions. K.M.B. gratefully acknowledges thesupport of the Alexander von Humboldt foundation. C.B. isgrateful to the CRC 1109, funded by the DeutscheForschungsgemeinschaft for financial support. P.S. and G.P.gratefully acknowledge support from the European MarieCurie Project CATSENSE (grant agreement number:607417).

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Figure 8. Looking at specific ring structures by filtering the STMimages of Figure 6 for better visualization. In the bilayer, similar ringconfigurations can be identified; however, the germania bilayerpresents higher roughness than the silica bilayer. The monolayernetwork is marked in white, while the bilayer one is black. All ringsizes are color-coded as in previous figures. Three loop structurespresent in the germania bilayer film are marked in red. (a) 12.0 nm ×7.0 nm, IT = 10 pA, VS = 1.0 V, 4.2 K. (b) 12.0 nm × 7.0 nm, IT = 0.4nA, VS = −0.5 V, room temperature.

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