Determination of Silica and Germania Film Network Structures on Ru

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Determination of Silica and Germania Film Network Structures on Ru(0001) at the Atomic Scale Adrián Leandro Lewandowski, Philomena Schlexer, Sergio Tosoni, Leonard Gura, Patrik Marschalik, Christin Büchner, Hannah Burrall, Kristen M. Burson, Wolf-Dieter Schneider, Gianfranco Pacchioni, and Markus Heyde J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07110 • Publication Date (Web): 01 Oct 2018 Downloaded from http://pubs.acs.org on October 10, 2018

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The Journal of Physical Chemistry

Determination of Silica and Germania Film Network Structures on Ru(0001) at the Atomic Scale Adrián Leandro Lewandowski,

†

Gura,

Burson,

§

†

Patrik Marschalik,

†

‡

Philomena Schlexer,

¶

Christin Büchner,

Wolf-Dieter Schneider,

†

Sergio Tosoni,

§

Hannah Burrall,

Gianfranco Pacchioni,

‡

‡

Leonard

Kristen M.

∗,†

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, CA 94720, USA

§Taylor Science Center, Hamilton College, 198 College Hill Road, Clinton, NY 13323, USA E-mail: [email protected]

Phone: +49 30 8413 4149. Fax: +49 30 8413 4105

1

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract The detailed structure of silica and germania lms supported on Ru(0001) metal substrates are compared to each other. Surface science techniques together with density functional theory calculations have been used to gain insights into the atomic arrangement of these prominent glass-forming materials. The monolayer lms of these materials both show predominantly crystalline hexagonal lattices with characteristic domain boundary structures. For the germania monolayer lms a large variety of ring elements within domain boundaries have been observed. Density functional calculations predict stronger interaction with the metal substrate for bilayer germania as compared to bilayer silica lms. Scanning tunneling microscopy images with atomically resolved structural features have given access to silica and germania bilayer lm structures. Both bilayer lms form characteristic amorphous ring structures. However, the germania bilayer lms appear to be more corrugated, pointing to a stronger interaction with the metal support thus giving rise to slightly dierent connectivity rules.

Introduction For a long time the eld of surface science was focused almost exclusively on crystalline and well ordered surface structures. Recently, a new class of lm system was discovered that revealed atomic sites of amorphous oxide network structures for the rst time: the two-dimensional (2D) silica bilayer (BL) lms. 13 Subsequently, the characterization of this specic lm system has become a fascinating research topic. Depending on the preparation conditions, the silica bilayer lm can be tuned between its crystalline and amorphous state. So far this is the only oxide network lm system that has shown such unique structural 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 new insight into glassy structure. In a recent attempt, we have successfully grown germania monolayer 2


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(ML) lms on a Ru(0001) metal substrate. 4 Here we present the rst data on amorphous germania bilayers and compare common structural network features for monolayer as well as for bilayer silica and germania lms. We begin this comparison with monolayer lms and structural motifs observed in their domain boundaries and then discuss bilayer lm coverages. When comparing silica and germania lm structures, it is worth looking into previous studies of their bulk phases. There are many dierent polymorphs of bulk silica (SiO2 ) including amorphous phases and at least 40 studied crystalline polymorphs that may form depending on pressure, temperature, and preparation conditions. 5 Many polymorphs consist of a tetrahedral building unit with a central silicon atom which is bonded to four oxygen atoms. Each oxygen atom is bonded to two silicon atoms, connecting adjacent tetrahedra at the corners. Dierent polymorphs are attained through slight variations in the Si-O-Si angle such as α- and β -quartz,

α- and β -cristobalite, coesite, trydimite, and even amorphous silica. 6,7 This conrms the early random network theory of Zachariasen, 8 which hypothesizes that both crystalline and amorphous networks share the fundamental tetrahedral building unit, diering primarily in the distribution of the intertetrahedral angle. Stishovite, a high pressure polymorph of silica with typical formation conditions around 1650◦ C and 15 GPa, possesses the rutile structure consisting of octahedra with each oxygen coordinated to three silicon atoms and each silicon coordinated to six oxygen atoms. 9 In lunar and Martian meteorites, a shock induced orthorhombic polymorph of silica called seifertite has been observed. It is estimated that a minimum equilibrium shock pressure in excess of 35 GPa would be necessary to produce this structure. 10 Bulk germania surfaces do not readily form well-dened GeO2 surfaces, but exhibit a mixture of oxidation states upon substrate heating in oxygen. 11 In order to study stoichiometric GeO2 , bulk crystals of rutile-type or quartz-type GeO2 have been investigated in experiment and theory. Cooled melts of these crystals exhibiting amorphous structures have been 3



Table 1: Physical properties of bulk silica and germania take from the following references 7,1315 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 K

m.p.

1996 K

1389 K

studied as well. Micoulaut et al. reviewed the knowledge of GeO2 bulk structures, which we will briey summarize here. 12 At room temperature, bulk GeO2 is stable in a rutile-like polymorph, where each Ge atom is coordinated in an octahedron by six oxygen atoms. At elevated temperatures or pressures, an α-quartz-like polymorph is formed, that consists of tetrahedral GeO4 building blocks. From X-ray diraction (XRD), neutron diraction (ND) and Raman spectroscopy, amorphous GeO2 is concluded to be composed of oxygen-bridged tetrahedra that form a network without long-range order. Thus, for both amorphous silica and germania, the MO4 (M = Ge, Si) tetrahedral building block is 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 the melting point for bulk amorphous silica and germania. Bond distances are comparable for the two materials, but germania has a lower glass transition temperature and melting point. The larger size of the germanium atom compared to the silicon one, allows more O positions surrounding the cation. 12 This is evidenced in a more distorted O-Ge-O intratetrahedral angle. However, the intertetrahedral angle Ge-O-Ge exhibits a narrower distribution than the Si-O-Si angle. Furthermore, the larger amount of 3-membered rings observed in germania with respect to silica is reected in the lower mean of the Ge-O-Ge angle. 13 For silica, a thin-lm strategy has been employed for the identication of local site geometries, tetrahedral building blocks and larger building units, i.e. rings. Studies of monolayer silica lms grown on Ru(0001) and Mo(112) substrates reveal a hexagonal crystalline network of corner-sharing SiO4 tetrahedra. 16,17 Recently, the rst experimental study on the compa4


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rable monolayer structure for germania on Ru(0001) has also been published. 4 For both silica and germania monolayers, domain boundary structures are present in the crystalline lms. Especially for silica bilayer lms details on the structural arrangement, 18 shifts in the metal work function, 19,20 the band gap, 19 the exchange of network formers, 21,22 the transferability from one substrate to another one, 23 the bending rigidity, 24 as well as several theoretical approaches for modeling have been presented in the literature. 25,26 More details can be found in a recent review article on this lm system. 27 Interestingly, depending on the oxygen anity of the metal substrate, silica bilayers can be formed in either crystalline or vitreous congurations. 28 For germania, to the best of our knowledge, only one density functional theory (DFT) prediction for thin lms 26 as well as one experimental study of germania thin lms 4 have been published. Moreover, no comparable 2D network structures as observed in the present work have been realized before.

Experimental Details Silica and germania lms are grown on Ru(0001) in dierent ultrahigh vacuum chambers (base pressure 10−10 mbar range). The Ru(0001) single crystal is cleaned through several cycles of heating up to 1450 K for 1 minute, annealing in presence of 2 x 10−6 mbar oxygen pressure at 1250 K for 20 minutes and bombarding the surface with Ar+ . The cleanliness of the metal substrate is checked by scanning tunneling microscopy (STM) and low-energy electron diraction (LEED). In addition to our results, in this publication we include STM images of silica lms published by Yang et al. and Mathur et al. 29,30 The recipe of the silica lms in all cases consists of the following procedure. First, the clean Ru(0001) is pre-covered with a (2×2)-3O adlayer by annealing the crystal at 1100 K under an oxygen pressure in the 10−6 mbar range. Subsequently, silicon is deposited by physical vapor deposition (PVD) in a 10−6 mbar oxygen background pressure. Finally, the system is oxidized by annealing

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in a temperature range between 1100 K and 1220 K in 10−6 mbar oxygen pressure. Our STM images of silica lms were taken at 4.2 K with a PtIr tip. More details of the low temperature STM can be found in reference 31. The images from Yang et al. and Mathur et al. were performed at room temperature with PtIr and W tips, respectively. The preparation of germania lms is based on previously reported studies of a germania monolayer lm on Ru(0001). 4 At rst, a (2×2)-3O adlayer is formed as described before. Germanium is then evaporated by PVD in 2 x 10−6 mbar oxygen pressure. The coverage of the lm is controlled by changing the germanium evaporation time and keeping all other parameters xed. For such ultrathin lms, a linear dependency is found between the coverage and the Ge evaporation time. STM images of the germania lms were taken in a room temperature Beetle-type STM using a PtIr tip. The experimental setup can be found in reference 32.

Computational Details Periodic DFT calculations were performed using the Vienna Ab Initio Simulation Package (VASP). 3335 The generalized gradient approximation (GGA) for the exchange-correlation functional was applied within the in the Perdew, Burke, and Ernzerhof (PBE) formulation. 36,37 To verify the results obtained with the PBE functional, we performed calculations with 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 electronic relaxations, the blocked Davidson iteration scheme was used. 42,43 In geometric structure optimizations, all ions were allowed to ◦

relax until ionic forces were smaller than |0.01| eV /A. For unit cell optimizations of the thin lms, plane waves were expanded up to a kinetic energy cuto of 1200 eV and a k-point set of (13×13×1) in the Monkhorst-Pack scheme 44 was used. For all other calculations a cuto energy of 400 eV was used and the k-point set was reduced to (5×5×1). The Ru bulk

6


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structure was obtained as described in reference 45. The Ru(0001) surface was represented by a 5-layer slab. The lowest two layers of Ru were kept xed in the bulk geometry during structure optimization. The silica and germania bilayer exhibit approximately twice the periodicity of the Ru(0001) surface. The Ru(0001)(2×2) surface super cell exhibits surface lattice parameters of a0 =b0 =0.546 nm. The optimized surface unit cell parameters of the free-standing GeO2 bilayer are a0 =b0 =0.547 nm, and the free-standing SiO2 bilayer exhibits unit cell parameters of a0 =b0 =0.531 nm. Upon deposition of the thin lms on the Ru(0001)-(2×2) surface super cell, the Ru surface lattice parameters were kept xed. Here, the k-point mesh was set to (5×5×1). For the GeO2 bilayer, this results in a deviation of the lattice constants by 0.24% with respect to the Ru(0001)-(2×2) surface super cell. For the SiO2 bilayer the deviation is signicantly larger: -2.76%. For the lm-substrate interaction, van-der-Waals (vdW) forces are important. Thus, vdW-forces were included using the DFT+D2 method as developed by Grimme et al. 46 and modied by Tosoni and Sauer. 47 Dipole correction along the z-axis was applied.

Results & Discussion Figure 1 compares the atomic structure of silica and germania monolayers on Ru(0001). Both systems share many similarities. STM images (Figures 1a and 1b) show that three O atoms in the plane of the lm surround each M (M = Ge, Si) atom. Additionally, DFT side-view models (Figures 1e and 1f) show the 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 forms a (2×2) lattice with respect to the Ru(0001) substrate. 4,16 They show the same registry, namely M atoms are linked to the substrate via O atoms on top and facecentered-cubic sites. 4,30 Atomic oxygen adsorbs on the ruthenium metal substrate on the hexagonal-close-packed hollow site below the center of the ring. The presence of this oxygen atom was observed by STM in the case of silica (Figure 1a) and conrmed by a intensity-

7



voltage low-energy electron diraction study for the germania monolayer lm. 4 The main dierence between both lattices is the orientation of the tetrahedral building blocks MO4 . Consecutive tetrahedra forming a ring are marked with red and black triangles in Figure 1a and 1b. While in the silica monolayer lm two adjacent tetrahedra are facing each other producing a Si-O-Si bond that looks straight in the image plane (Figures 1a and 1c), adjacent GeO4 building blocks are rotated 30◦ respect to each other 4 (Figures 1b and 1d). Figure 2 compares domain boundary structures of silica and germania monolayer lms. An overview can be seen in 2a and 2b, connection points in 2c and 2d and boundaries with a Stone-Wales motif in 2e and 2f. The domain boundary connection points in gures 2c and 2d have the same design: a central 6-membered ring is surrounded by three 5-membered rings and three rings of larger ring size (7-membered for silica and 8-membered rings for germania). The larger rings contribute to the structures of the three incoming domain boundaries: 5577 domain boundaries for silica and 48 for germania. Figures 2e and 2f show domain boundaries with Stone-Wales motifs. In the case of silica there is a continuous line of Stone-Wales defects while for germaina there is a single 6-membered ring separating each defect in the boundary. The similarities between the silica and germania defects were expected given that the core element of the structure models, the tetrahedral building unit, is the same. It has also been reported that the grain boundaries in both silica 48 and germania 4 maintain the same registry with the ruthenium substrate for the crystalline regions on either side of the boundary. Table 2 summarizes the various types of domain boundaries found for silica and germania monolayers as well as bilayer structures. Overall, germania monolayer lms exhibit a greater variety of domain boundary structures compared to silica monolayer lms. Rings from 4-membered to 8-membered combine into various structure elements which form domain boundaries on germania monolayers, whereas the known boundary on silica monolayers consist of 5- and 7-membered rings only. STM images of a new domain boundary structure and complex connection point, shown in Figure 3, highlight the range of ring-sizes found in germania monolayers. The dierences between silica and germania monolayer domain 8


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Figure 1: Comparing silica and germania monolayer structures on Ru(0001). (a) is adapted from Ref. 30. The right part is a STM image of a monolayer of silica on Ru(0001): 1.8 nm×2.4 nm, IT =2 nA, VS =0.9 V, room temperature. (b) STM image of a monolayer 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 are overlaid on both images. Both (a) and (b) show oxygen atomic resolution. (c) and (d) DFT top view models for silica and germania monolayers, respectively. Size: 0.8 nm×0.8 nm. (e) and (f) DFT side-view models for both monolayers. The gures on the left-hand column are green-toned and correspond to silica monolayers lms on Ru(0001). The right-hand column is blue-toned and refers to germania monolayers on the same substrate. Green balls represent silicon atoms, the blue ones germanium atoms and oxygen atoms are shown in red. Light gray balls correspond to the ruthenium atoms of the substrate.

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Figure 2: Comparing silica and germania monolayer domain boundary structures on Ru(0001). STM images comparing dierent features of silica monolayers lms (on the left column) and germania lms (on the right) on Ru(0001). (a) STM image of a monolayer silica lm grown on Ru(0001) taken from Ref. 29: 19 nm×19 nm, IT =0.1 nA, VS =2 V, room temperature. (b) STM image of a germania monolayer: 19 nm×19 nm, IT =0.4 nA, VS =2.5 V, room temperature. (c) STM of a domain boundary defect on silica monolayer taken 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 point observed on germania monolayer lms: 5.0 nm×5.0 nm, IT =0.6 nA, VS =1 V, room temperature.(e) STM image of an antiphase domain boundary formed by 7-,7-, 5- and 5-membered rings: 3.0 nm×3.0 nm, IT =2.0 nA, VS =0.9 V, room temperature. The image was taken from Ref. 30. (f) STM image of a 57756 domain boundary: 3.5 nm×3.5 nm, IT =0.5 nA, VS =2.5 V, room temperature. Ring sizes are color-coded from (c) to (f).

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Table 2: Boundaries and common defects System

Types

Method

ML SiO2 /Ru(0001)

5577 domain boundary

STM

57 triangular loop defect encompassing a 6MR BL SiO2 /Ru(0001)

475 rectangular loop defect encompassing an 8MR 58 antiphase domain boundary

AFM STM

57 rotational domain

29 48 49 49

Stone-Wales defect

49

57 closed-loop defects 84 antiphase domain boundary

STM

57756 antiphase domain boundary

BL GeO2 /Ru(0001)

29

49

48 domain boundary

ML GeO2 /Ru(0001)

Citation 30

4 4

5678 complex boundary

4

57 triangular loop defect encompassing a 6MR

current study

45678 loop defect encompassing three 6MR

current study

855 antiphase domain boundary

current study

buckling

STM

current study

boundary structures may stem from variations in angular arrangements of the tetrahedral units, evidenced in the distorted germania structures observed in STM, and from coupling eects between the lm and the ruthenium substrate. We now turn to a discussion of silica and germania bilayer lms, rst presenting results from DFT and then from STM. Adhesion energies are dened in eqn. 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 eqn. 1, the term adhesion energy typically refers to a net relaxation in the system that results from covering a bare substrate with a thin lm, normalized over the surface area. All energies refer to fully optimized systems.

Eadh = (E(M O2 /Ru) − E(M O2 ) − E(Ru))/S

(1)

Structure optimizations of free-standing (M4 O6 H4 )6 N (M = Ge, Si; N = 4-8) cages were done at the Γ-point without dispersion forces in unit cells of the size (0.28×0.28×0.18) nm. The cage replicas are therewith separated by at least 1.2 nm of vacuum in each Cartesian direction. The relative stability of the cages is dened in eqn. 2 with respect to the most

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Figure 3: STM images of new domain boundary structures on germania monolayer lms on Ru(0001). (a) 855 boundary: 3.6 nm×3.6 nm, IT =0.5 nA, VS =1.0 V, room temperature. (c) connection point formed by 4-, to 8-membered rings: 6.8 nm×6.8 nm, IT =1.0 nA, VS =2.0 V, room temperature. In (b) and (d) the ring sizes are color-coded accordingly. stable 6-membered cages.

Erel = E((M4 O6 H4 )6 )/6 − E((M4 O6 H4 )N )/N

(2)

We dene the Ru-lm distance as average distances in the z-direction between the uppermost Ru layer and the lowest oxygen layer of the thin lms, d(Ru-Olow ) = zav (Olow ) - zav (Ru). Atomic charges Q have been estimated according to the Bader decomposition scheme. 5052 Now we would like to compare the adhesion properties of the silica versus germania bilayers on Ru(0001), respectively, as derived from DFT. Interesting parameters are summarized in Table 3. Whereas the silica bilayer binds onto the Ru substrate mainly via dispersion forces, the germania bilayer shows a chemical binding to the Ru surface. This chemical binding is represented in the strong adhesion energy, more than 3 times larger than that of the silica bilayer, in the distance between the lm and the substrate as well as in the fact

12


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that there is a charge transfer from the Ru substrate to the germania bilayer. We carefully tested this bonding type using the HSE06 hybrid functional and conrm that the strong binding including the charge transfer 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 from bare metal crystal to a (2×2)-3O layer. Thereby, the electronic properties of the lm system can be modied. 20 In detailed DFT calculations we have varied the amount of interfacial oxygen for silica and germania lms. A full set of them is given in the supplemental material. In Figure 4a a side view of silica bilayer lm 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 bilayer seems to be mostly independent from the amount of interfacial oxygen, a stronger dependency on the atomic structure has been observed for the germania bilayer. In Figure 4c and 4d comparable side view and the respectively calculated DOS are shown for the germania bilayer lm, but without interfacial oxygen in order to highlight the dierence in binding mode between the silica bilayer and the germania bilayer. The oxygen states shown in the DOS in Figure 4d are all part of the germania bilayer.

Table 3: Comparison of the adhesion properties of the bilayers. Eadh (eV/nm ) d(Ru-Olow ) (nm) q(Ru-slab) (|e|) 2

SiO2 /Ru(0001) -1.76 0.265 0.09

GeO2 /Ru(0001) -6.78 0.217 0.60

To characterize the binding in terms of atomic and electronic structure in more detail, we show the adsorption geometries, along with the DOS in Figure 4. We see that the strong interaction of the germania bilayer results in a structural distortion, which is in good agreement with the experimental observations. The distortion that presents the bilayer of germania with respect to the silica brings with it a decrease in the symmetry of the system. While the free-standing silica bilayer shows a D6h symmetry, the free-standing germania bilayer breaks this symmetry due to the relative rotation of the top and bottom tetrahedra 13



which eliminates the horizontal mirror plane and the C6 rotational axis, leaving the system in D3 symmetry, which is further reduced when adsorbed to the Ru support. The DOS reveals the charge transfer from the substrate to the germania bilayer, giving rise to Ge and O states right above and below the Fermi level. The free-standing germania bilayer has a band gap of around 3 eV with the PBE functional.

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) Germania bilayer model supported on pristinie Ru(0001). (d) DOS of the germania bilayer on Ru(0001). In Figure 5 we compare stabilities of dierently sized silica and germania cages. We nd that the relative energy of ring sizes other than 6 are lower for silica than for germania, indicating that the silica bilayer more readily forms a broader ring size distribution. The more strongly bound germania bilayer may counteract the strain introduced by ring sizes other than 6 by forming more complex patterns of dierently sized rings that may involve dierent Ge coordination numbers. Figure 6 shows STM images of coexisting monolayer and bilayer structures for silica and germania. Both images have the same size: 12.0 nm x 7.0 nm. The specic tunneling 14


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Figure 5: Comparing the stability of bilayer silica and germania ring structures. (a) Relative energy dierences of dierently sized rings with respect to the lowest energy 6-membered ring. (b) and (c) top and side views of the DFT models of the hexagonal silica and germania bilayers, respectively (see computational details). conditions sense atomic oxygen positions. 4 The crystalline silica monolayer in Figure 6a appears darker in the STM image. The periodic hexagonal structure of the monolayer is disturbed by a domain boundary that contains 5- to 7-membered rings, similar to the one shown in Figure 2e . In the supplement the spatial ring size distribution along this domain boundary is illustrated. It bridges the monolayer patch completely and separates the crystalline domains, which are commensurate to the underlying Ru(0001) substrate and show an oset by a single atomic position in the [1100] direction. The amorphous silica bilayer appears brighter in the image and exhibits a broad ring size distribution that involves 4- to 9-membered rings. A similar coexistence of two phases is found in the germania lm in Figure 6b. It consists of a crystalline hexagonal monolayer and a bilayer with less structural order that appears brighter in the image. The apparent heights of the silica and germania ◦

◦

bilayer respect to the monolayer are around 1.4 A and 1.3 A, respectively, which is in line with previous measurements in silica lms. 53 In particular, the ratio of root mean square roughness for BL to ML silica is 2.0±0.4, while for germania it is 5.5±0.3. Because roughness analysis can be signicantly aected by the condition of the STM tip, care has been taken to compare monolayer and bilayer regions with the same tip condition (e.g. within the same image) for the purpose of calculating these

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Figure 6: Comparing ultrathin lms of silica and germania on Ru(0001), probed by STM. Both lms consist of coexisting crystalline monolayer and amorphous bilayer phases. On half of the images red dots are used to mark distinguishable oxygen positions. Figure (a) was adapted from reference 53. (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 =0.4 nA, VS =-0.5 V, room temperature. ratios. The monolayer regions of silica and germania are very similar, consequently dierences in roughness ratios are attributable primarily to dierences in the bilayer structures and are likely related to the underlying bonding congurations for silica and germania bilayers. Silica bilayer structures consist of two planar layers of ring structures which are bridged by oxygen atoms as shown in the atomic model in Figure 4a. 45,54 The top and bottom layer exhibit the identical network structure, while a sideview reveals only four-membered rings that result from pairs of Si-O-Si connected with two oxygen bridges. The bonding structure is fully saturated with no bonds to the substrate. Given that the germania bilayer lms were grown and annealed in an excess of oxygen, we expect that like the silica bilayers, germania bilayers will have a fully saturated bonding structure. The buckled germania bilayer may result from dierent ring-sizes which connect the rst with the second layer of the lm and may include bonds between the germania lm and the substrate, an interpretation supported by the aforementioned DFT results. Nevertheless, some patches in the germania bilayer suggest

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that the structural model looks like the silica bilayer one. In order to analyze the short range order of the amorphous silica and germania bilayer lms, radial O-O distances are measured and compared. The oxygen positions are assigned with red dots as shown partially in Figure 6. The higher corrugation of the germania bilayer lm aects the resolution of the STM image and results in an incomplete detection of oxygen positions compared to silica. Figure 7 shows the pair distance histograms (PDHs) for silica and germania. They exhibit similar features up to a radial distance of 1.1 nm, which indicates a similar short range order for both systems. The rst peak of the O-O radial distances is in good agreement with XRD data for bulk silica 15 and germania. 14

Figure 7: Comparing oxygen-oxygen pair distance histograms of the silica and germania bilayer. Vertical red dashed lines show the O-O distance determined by XRD on the bulk materials. In addition to the similar radial atomic distances, similar ring sizes and congurations in both lms are identied and outlined in Figure 8. In order to visualize the ring structure of the germania bilayer, a lter with a scaled structuring element for erosion was applied to the images of Figure 6 and shown in Figure 8. The basis of this lter and its results are presented in the supplementary notes. As reference, the hexagonal network of the monolayers is marked with white outlines. Some rings of the germania bilayer resemble the ones of the silica bilayer. Those are marked in black on the bilayer phases in Figure 8. 4- to 9-membered rings were identied on both bilayer lms, which are color-coded as it is indicated at the bottom of the gure. The size of these rings for both systems is comparable. Besides the similarities, the germania bilayer lm shows also larger loop-like structures. 17



Figure 8: Looking at specic ring structures by ltering the STM images of Figure 6 for better visualization. In the bilayer, similar ring congurations can be identied, however the germania bilayer presents higher roughness than the silica bilayer. The monolayer network is marked in white, while the bilayer one is black. All ring sizes are color-coded as in previous gures. Three loop structures present in the germania bilayer lm 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 =0.4 nA, VS =-0.5 V, room temperature.

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Three examples are highlighted in red in Figure 8b. The larger loops might follow dierent connectivity rules, which could involve dierently coordinated Ge or substrate bonding, as discussed previously.

Conclusion We have presented a side-by-side comparison between silica and germania monolayer and bilayer lms on Ru(0001). Both monolayers present a similar atomic structure. They consist of MO4 (M=Si or Ge) unit blocks strongly coupled to the substrate that form a purely crystalline hexagonal phase. A wider set of ring size combinations is observed for germania monolayer lms forming domain boundaries. DFT calculations predict a strong interaction between the germania bilayer and the substrate, while the silica bilayer is farther apart from the substrate and interacts only weakly with it through van der Waals interactions. This factor plays a key role in the observed structural dierences. The amorphous germania bilayer, presented here for the rst time, compensates for the strain by forming a buckled structure that could involve dierent Ge coordination numbers, direct bonds to the metallic substrate, dierent ring sizes in the direction perpendicular to the substrate, or others. In order to assign all the ring sizes in the germania bilayer lm, a complete atomic model would be needed, which is dicult to establish directly by scanning probe methods due to the corrugated lm and the aected resolution. Therefore, future experiments aim to prepare atter bilayer germania lms by diminishing the lm-support coupling by, e.g., changing the substrate. Our successful fabrication of germania bilayers on Ru(0001) represents the rst step in this direction.

Acknowledgement The authors would like to thank specically Hajo Freund for his decisive scientic input, for his constant motivation, and for numerous fruitful discussions. KMB gratefully acknowledges 19



the support of the Alexander von Humboldt foundation. CB is grateful to the CRC 1109, funded by the Deutsche Forschungsgemeinschaft for nancial support. PS and GP gratefully acknowledge support from the European Marie Curie Project CATSENSE (grant agreement number: 607417).

Supporting Information Available The following les are available free of charge. SupportingInformation.pdf: Details on the domain boundary of the silica monolayer, supplements on the DFT calculations, description of the image lter for ring visualization, as well as the oxygen coordinates of the bilayer phases.

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Graphical TOC Entry

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Determination of Silica and Germania Film Network Structures on Ru

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