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contrast to the usual Al

stoichiometry. The registry of the substrate and oxide is

provided by the interfacial Al atoms, which are bonded to the Ni rows of the NiAl

part of the interface. This leads to interfacial row matching and to the stress along the

[1 1 0] direction of the oxide, which causes the incorporation of line defects and

the observed antiphase domain boundaries [45]. The latter have also been modeled

in terms of an oxygen-deﬁcient defect structure in the recent work of Kresse and

coworkers [48].

The NiAl(1 1 0) surface is not the only Al-containing alloy surface that can support

a highly ordered oxide overlayer. The formation of a well-ordered alumina ﬁlm

on Ni

Al(1 1 1) by high-temperature oxidation has been reported ﬁrst by Rovida

et al. [49, 50]. The Wandelt group has taken up the idea subsequently and has reﬁned

the preparation procedure [51]. Rosenhahn et al. recognized from STM images [52]

that two long-range superstructures in the nanometer range can be identiﬁed: the

so-called network structure with a periodicity of approximately 2.4 nm and the dot

structure with a periodicity of approximately 4.2 nm, the dot structure being a

(H3 H3)R30



subset of the network structure. SPA-LEED and more extensive

STM investigations of Degen et al. [53] revealed that only the dot structure is a real

Figure 6.2 (a) Top and (b) side view of the

DFT- and STM-based model for the ultrathin

aluminum oxide film on NiAl(1 1 0).

(c–e) Experimental STM images of the film at

temperature. Sample bias voltage and tunneling

current values are (c) 2.5 mV/1.4 nA, (d) 0.2 V/

0.9 nA, and (e) 0.5 V/0.3 nA. Two oxide unit cells

are marked by white rectangles, the diagonal

along which the oxide is commensurate is

yellow, and the parallelogram enclosed by the

yellow and white lines delimits the simulation

cell. Green rectangles and squares highlight

oxygen atoms in a square arrangement. Circles

indicate the Al and O positions from (a) and

(b) in the corresponding colors. (f) Close-up of

the structure. (Reproduced with permission

from Ref. [47].)

154

6 STM Imaging of Oxide Nanolayer Model Systems

superstructure of the alumina ﬁlm, and they reﬁned the unit cell of the (H67 H67)

R48



superstructure to 4.16 nm. Since atomic resolution of the alumina ﬁlm could

not be achieved in this latter work, the building blocks of the oxide layer and its

structural relation at the atomic scale to the substrate could not be established. The

full structural complexity of the alumina on Ni

Al(1 1 1) has been unraveled by the

noncontact atomic force microscopy (AFM) study by Gritschneder et al. [54], who

found that the main structural element of the oxide ﬁlm is a lattice of hexagons that is

pinned to the periodicity of the substrate. The surface unit cell is deﬁned by the dot

structure, with the network structure being formed by a honeycomb-like topographic

modulation as a result of the substrate-overlayer pinning. Recently, Kresse and

coworkers [55] have applied their experience with the alumina model on NiAl(1 1 0) to

the determination of the structure of alumina on Ni

Al(1 1 1). Combining exper-

imental STM images with extensive DFTmodeling, they found a structural similarity

between the alumina ﬁlms on the two NiAl alloy substrates, with an AlOAlO

stacking sequence, square and triangular arrangements of atoms at the surface, and a

hole in the unit cell reaching down to the Ni

Al substrate. In simulated STM images,

these holes have been identiﬁed with the bright contrast of dots in the dot

structure [55]. It is these holes that provide the anchoring centers for the growth

of monodisperse metal clusters [56] and make this alumina overlayer an excellent

nanotemplate for growing regular arrays of nanoparticles.

In summary, the alumina nanolayers formed by the high-temperature oxidation

on NiAl alloy surfaces are structurally and chemically very different from the bulk-

terminated surfaces of the various Al

phases, and they thus provide very

prototypical examples of oxide phases with novel emergent properties because of

interfacial bonding and thickness conﬁnement effects.

6.3.2

Titanium Oxide Nanolayers

There is considerable interest in titanium oxides owing to their importance in

many areas, such as photo-assisted oxidation, heterogeneous catalysis and gas

sensors [57–59]. Titanium dioxide (TiO

), in particular, is one of the most prominent

materials in the industrial catalysis used for the selective reduction of NOx [60, 61],

photocatalysis for pollutant elimination [62] or organic synthesis [63], photovoltaic

devices [64], sensors [65], and paints [66]. Additional applications include its use as a

food additive [67], in cosmetics [68], and as a potential tool in cancer treatment [69].

Titaniumoxide is one of the best-characterized model systems in surface science [70].

Most of the studies were performed on single-crystal surfaces, but at present,

ultrathin Ti-oxide ﬁlms grown on metallic substrates are a subject of intensive

investigation. Self-assembled, low-dimensional Ti-oxide nanostructures could be of

potential interest for applications in electronic devices, nanocatalysts, and gas

sensors. Controlling the oxidation state and the dimensions of these nanostructures

may allow the production of a new class of technologically important materials.

Ultrathin Ti-oxide ﬁlms have been grown on various metal surfaces, Pt(1 1 1)

[71–75], Pt(1 0 0) [33, 34], Pt(1 1 0) [76], Mo(1 0 0) [77, 78], Mo(1 1 0) [79, 80],

6.3 Case Studies: Selected Oxide–Metal Systems

155

Mo(1 1 2) [81], W(1 0 0) [82], Ni(1 1 0) [83–86], and Ru(0 0 0 1) [87]. The group of

Somorjai was ﬁrst to report the growth of ordered TiOx overlayers on a Pt(1 1 1)

substrate [71]. The as-grown ﬁlms were disordered but annealing between 500 and

700



C in an oxygen background caused the formation of a hexagonal (H43 H43)

R7.6



superstructure, which was observed in LEED and STM for coverages ranging

between 1 and 5 ML, and was tentatively ascribed to a reconstructed TiO

(1 1 1)

surface. Annealing at higher temperatures in UHV produced a second ordered

structure, with a reduced Ti

stoichiometry, according to XPS data [71]. Ten years

after this work, Granozzi and coworkers [72] reexamined this system, focusing their

investigation on the submonolayer to monolayer coverage range and revealed the

complexity of the TiOx-Pt(1 1 1) phase diagram in great detail. These researchers

evaporated Ti reactively onto a clean Pt(1 1 1) surface, kept at room temperature,

in an oxygen pressure of 1 10

6

mbar. By varying the latter in a postoxidation step

between UHV and 1 10

6

mbar, they observed in LEED and STM six different

oxide structures in the coverage range up to 1.2 ML; these are summarized in

Figure 6.3. At low Ti coverage (0.4 ML) and after annealing at 400



Cin10

7

mbar

, a hexagonal overlayer with a lattice constant of 6 Å has been obtained, which is

incommensurate with the Pt(1 1 1) substrate (Figure 6.3a). For this kagom



e-phase

(k-TiOx), the authors have tentatively proposed a structure model with a formal

stoichiometry, where the Ti and O ions are arranged in a honeycomb

lattice, identical to that of surface-V

layers on Pd(1 1 1) [88]. At higher Ti

coverages, between 0.8 and 2 ML, and annealing temperature of 700



C and oxygen

pressure of 5 10

6

mbar, a rectangular structure (Figure 6.3b) grows in the form

of rectangular-shaped islands. On the basis of XPS results and because of the

similarity of the STM images to those reported for a rect-VO

phase on Pd(1 1 1) by

Surnev et al. [89], this rectangular structure has been ascribed to an interface-

stabilized phase with a TiO

stoichiometry. Striking similarities to the structure of

V-oxide overlayers on Pd(1 1 1) [15] and Rh(1 1 1) [18, 90] surfaces have also been

recognized for Ti-oxide phases with a zigzag- (Figure 6.3c and d) and wagon-

wheel-like appearance (Figure 6.3e and f) in the STM images [72]. More recent

work of the same group, combined with DFT calculations [74, 75], has yielded

structural models for the zigzag TiOx phases in terms of layers with Pt–Ti–O

stacking and stoichiometries varying between TiO

1.33

(z-TiOx) and TiO

1.2

-TiOx).

The most reduced TiOx phases on Pt(1 1 1) comprise the commensurate wagon-

wheel structures, w-TiOx (corresponding to the hexagonal (H43 H43)R7.6



superstructure, reported initially by Boffa et al. [71]) and w

-TiOx (with a (7 7)

R21.8



periodicity), which have been interpreted as TiO(1 1 1) bilayers because of

their characteristic contrast in the STM images, in close similarity to bilayers of

FeO(1 1 1) on Pt(1 1 1) [91] and VO(1 1 1) on Rh(1 1 1) [18] surfaces.

Ultrathin Ti-oxide ﬁlms grown on Pt(1 0 0) by Matsumoto et al. [33, 34] show also

a rich phase diagram of interface-stabilized oxide structures as a function of the Ti

coverage and the oxide preparation conditions. An interesting contrast to this general

growth behavior of oxide nanolayers display the titania bilayers on a (1 2)-Pt(1 1 0)

surface, investigated by Sambi and coworkers [76]. Here, reactive evaporation of

approximately 2 MLTi in an oxygen atmosphere of 1 10

6

mbar was found to result

156

6 STM Imaging of Oxide Nanolayer Model Systems

Figure 6.3 STM images of various TiOx structures on Pt(1 1 1):

(a) k-TiOx (30 Å 30 Å, 0.4 V, 1.06 nA); (b) rect-TiO

(90 Å 90 Å, þ0.8 V, 1.8 nA); (c) z-TiOx (60 Å 60 Å, þ0.1 V,

1.5 nA); (d) z

-TiOx (90 Å 90 Å, þ0.8 V, 1.5 nA); (e) w-TiOx

(75 Å 75 Å, þ1.3 V, 1.9 nA); (f) w

-TiOx (126 Å 126 Å, þ0.2 V,

1.0 nA). (Reproduced with permission from Ref. [72].)

6.3 Case Studies: Selected Oxide–Metal Systems

157

in the formation of a wetting Ti-oxide layer on the Pt(1 1 0) surface, which exhibits a

regular array of dark stripes in STM images, corresponding to a (14 4) superstruc-

ture (Figure 6.4a, c, and d). The high-resolution image (Figure 6.4b) reveals that a

rectangular overlayer unit cell with dimensions 3.0 Å 3.9 Å is at the root of this

coincidence structure, and it is compatible with the unit cell of 2D titania nanosheets

with the lepidocrocite structure, which can be obtained by delamination of layered

titanates (for a model see the inset in Figure 6.4b). DFTcalculations performed in the

same work have revealed that such a lepidocrocite nanosheet is the most stable 2D

titania phase, irrespective of the presence of the substrate, and it can be considered

as a distorted anatase (0 0 1) bilayer obtained by sliding the upper atomic layer

with respect to the lower one by half a unit cell in the [1 0 0] direction. By this

transformation, the resulting lepidocrocite nanosheet is stabilized by 1.24 eV with

respect to the undistorted anatase (0 0 1) bilayer. The authors have concluded that

it is the reduced dimensionality of the oxide overlayer, rather than the interaction

Figure 6.4 (a) Large-area STM image

(620 Å 620 Å, þ0.48 V, 1.4 nA) of a single-

domain lepidocrocite nanosheet on (1 2)-

Pt(1 1 0). The central brighter area is separated

from the lower terrace by a substrate mono-

atomic step. Inset: (14 4) LEED pattern.

(b) High-resolution STM image (137 Å 137 Å ,

þ0.28 V, 1.65 nA) of the nanosheet. The

overlayer (superstructure) unit cell is indicated

by the small (large) rectangle. Inset:

Tersoff–Hamann simulation of the STM image

with superimposed solid sphere model (oxygen:

dark gray/red; titanium: light gray/blue). Dark

stripes along [0 0 1] are due to overlayer bending

toward the substrate (see the interface side view

in the [0 0 1] plane, lower left corner; Pt: large,

gray). (c),(d) High-resolution STM images

of the nanosheet (136 Å 136 Å, þ0.42 V,

0.9 nA and 180 Å 180 Å, þ1.80 V, 1.0 nA).

(Reproduced with permission from Ref. [76].)

158

6 STM Imaging of Oxide Nanolayer Model Systems

with the substrate, that stabilizes the lepidocrocite structure, in contrast to what has

been typically observed for other ultrathin oxide phases on metal substrates, reviewed

in the present work. These ﬁndings are also supported by results reported for TiO

layers on Pt(1 1 1) [72] and Ni(1 1 0) [85], which exhibit a structure very similar to that

of the lepidocrocite nanosheets.

The group of Thornton has used a Ni(1 1 0) substrate with the aim to grow

epitaxially TiO

layers, which can mimic the behavior of rutile TiO

(1 1 0) single-

crystal surfaces. Postoxidation in an oxygen pressure of 1 10

7

mbar at 800 K of Ti

ﬁlms with a coverage exceeding 1 ML was reported to result in 3D Ti-oxide islands

with a structure resembling the surface of the bulk rutile TiO

(1 1 0) surface [83–85].

For a Ti coverage between 1 and 2 ML, the unit cell dimensions of the rutile-like

islands were 2.9 Å 6.2 Å, that is, slightly contracted from single-crystal TiO

(1 1 0)

(2.96 Å 6.5 Å), but thicker ﬁlms display the relaxed rutile surface lattice constant.

The question to what extent such titania layers can resemble bulk samples is of

signiﬁcant relevance for many technological applications and was treated in a recent

study by this group [86]. Here, titania ﬁlms with a thickness of up to 4 ML have been

subjected to reductive treatments at elevated temperatures and an oxygen pressure

of 1 10

7

mbar. Annealing at temperature between 873 and 1023 K causes a 1 2

reconstruction of the surface of the rutile TiO

(1 1 0) islands, in close similarity to

the well-documented reduction behavior of bulk TiO

(1 1 0) crystals [92–94]. Higher

annealing temperatures of up to 1100 K result in a completely changed ﬁlm

morphology, with the original rutile islands being replaced by a continuous ﬁlm

displaying step edges and facets in STM images (Figure 6.5). Here, regularly spaced

lines with a periodicity of approximately 34–35 Å can be seen, which correspond to

the intersections of {1 3 2} and {1 2 1} families of crystallographic shear planes with

the (1 1 0) surface and the facets, as previously reported for single-crystal titania

surfaces [95]. As the crystallographic shear planes are bulk defects rather than surface

defects, the authors conclude that ultrathin TiO

ﬁlms on Ni(1 1 0) behave at least

structurally in a bulk-like manner. The question to what extent this also holds for

other physical and chemical properties of TiO

nanolayers remains to be answered by

future experiments.

6.3.3

Vanadium Oxide Nanolayers

In the bulk form, vanadium oxides display different oxidation states and VO

coordination spheres and exhibit a broad variety of electronic, magnetic, and

structural properties [96, 97], which make these materials attractive for many

industrial applications. Prominent examples range from the area of catalysis, where

V-oxides are used as components of important industrial catalysts for oxidation

reactions [98] and environment pollution control [99], to optoelectronics, for the

construction of light-induced electrical switching devices [100] and smart thermo-

chromic windows. In view of the importance of vanadium oxides in different

technological applications, the fabrication of this material in nanostructured form

is a particularly attractive goal.

6.3 Case Studies: Selected Oxide–Metal Systems

159

We refrain here from giving an extensive overview of studies on the surface

structure of vanadium oxide nanolayers, as this has already been done for up to year

2003 in our recent review [97]. Instead, we would like to focus on prototypical

examples, selected from the V-oxide–Rh(1 1 1) phase diagram, which demonstrate

the power of STM measurements, when combined with state-of-the-art DFT calcula-

tions, to resolve complex oxide nanostructures. Other examples will highlight the

usefulness of combining STMand STS data on a local scale, as well as data from STM

measurements, and sample area-averaging spectroscopic techniques, such as XPS

and NEXAFS, to derive as complete a picture as possible of the investigated system.

The surface phase diagram of vanadium oxides on Rh(1 1 1) has been investigated

in a series of papers of our group [4, 18, 19, 90, 101–103]. It is characterized by

pronounced polymorphism and many different oxide structures have been detected

as a function of coverage and growth temperature. The vanadium oxide structures

for coverages up to the completion of the ﬁrst monolayer formed on Rh(1 1 1) under

the different preparation conditions may be subdivided into highly oxidized phases

Figure 6.5 STM images of the crystallo-

graphically sheared film: (a) 1700 Å 2100 Å,

þ0.30 V, 0.90 nA; (b) 1500 Å 1800 Å,

þ0.15 V, 0.80 nA. The arrows indicate both the

principal azimuths of the TiO

(1 1 0) surface and

the directions of the CS plane intersections at

the (1 1 0) surface. (c) Line profile along the line

indicated in (a). (d) Line profile along the line

indicated in (b). The image in (b) has a shadow

effect applied, and the line profile in (d) is taken

from an image without the shadow applied.

(Reproduced with permission from Ref. [86].)

160

6 STM Imaging of Oxide Nanolayer Model Systems

containing vanadium atoms in formal oxidation states of around þ5, into more

reduced oxides with V

2 þ

or V

3 þ

species, and into mixed valency vanadium

oxides [18, 90]. The latter are a peculiarity of the vanadium oxide – Rh(1 1 1) phase

diagram – and occur only in a narrow range of the chemical potential of oxygen. The

various vanadium oxide structures have been investigated with a combination of

experimental techniques, including STM, LEED, high-resolution photoelectron

spectroscopy (HR-XPS, UPS) with the use of synchrotron radiation, and HREELS

(vibrational spectroscopy of phonon modes). The results have been combined with

ab initio DFT calculations to derive the structural models for the vanadium oxide

surface phases. We emphasize that the synergy of the different experimental

techniques in combination with a high-level theory was absolutely necessary to

achieve the success obtained in the analysis of these very complex oxide structures.

In the following section, we illustrate the complexity of the structures observed with

just a few examples.

Under highly oxidative conditions, that is, p(O

) 2 10

7

mbar, T

substrate

400



C, a V-oxide layer with a (H7 H7)R19.1



structure, here called simply H7,

grows at low submonolayer coverages (Q<0.3 MLE) as two-dimensional islands,

as seen in the STM image in Figure 6.6a [4, 18]. The high-resolution STM image in

Figure 6.6b reveals the structural details of the H7 phase at the atomic scale, with

a hexagonal lattice that consists of three bright protrusions per unit cell. The DFT

calculations [18] have established a model of the (H7 H7)R19.1



structure

(Figure 6.6c and d): it corresponds to a V

oxide phase that contains identical

pyramidal O

V¼O building blocks (marked squares in Figure 6.6c). Each pyramid

(inset of Figure 6.6d) has the V atom (in green) in the center, four bridging O atoms

(in red) in the basal plane, and a vanadyl-type O atom at the apex. The presence of the

latter O species in the H7 overlayer has been inferred from HREELS spectra [18],

which contain a characteristic loss at approximately 130 meV because of the stretch-

ing vibrations of vanadyl (V¼O) groups. The pyramids are linked together via the four

bridging O atoms at the interface, which results in three VO

units per H7 unit cell,

or in an overall V

stoichiometry. Owing to the fact that the basal O atoms are

shared with the Rh substrate, the formal V

stoichiometry is not incompatible with

the maximum oxidation state of þ5 of the V atoms in the bulk V

structure. The

structure model of the H7-V

phase is conﬁrmed by the good agreement between

the experimental and simulated STM image (inset of Figure 6.6b) and the calculated

phonon spectrum, which reproduces the vanadyl stretching vibrations observed in

the experiment [18].

Within the inverse model catalyst approach, the H7-V

–Rh(1 1 1) nanostruc-

tures have been used to visualize surface processes in the STM with atomic-level

precision [104]. The promoting effect of the V-oxide boundary regions on the

oxidation of CO on Rh(1 1 1) has been established by STM and XPS by comparing

the reaction on two differently prepared H7-V

–Rh(1 1 1) inverse catalyst surfaces,

which consist of large and small two-dimensional oxide islands and bare Rh areas in

between [105]. A reduction of the V-oxide islands at their perimeter by CO has been

observed, which has been suggested to be the reason for the promotion of the CO

oxidation near the metal-oxide phase boundary.

6.3 Case Studies: Selected Oxide–Metal Systems

161

Exposing the H7-phase to a reducing environment, for example, by annealing in

UHV or in a hydrogen atmosphere, results in the formation of various reduced 2D

V-oxide phases [18, 101]. In particular, annealing at 650–750 K in vacuum produces

an oxide structure with a rectangular (5 3H3) unit cell, presented in Figure 6.7.

The large-scale STM image of Figure 6.7a shows well-ordered oxide islands with

rectangular shapes, elongated along the principal h110i azimuthal directions of the

Rh(1 1 1) substrate. The inset of Figure 6.7a displays the (5 3H3)-rect structure at

a higher magniﬁcation, also revealing details on the island boundary; the latter will be

discussed in the next paragraph. The synergy of various experimental and theoretical

methods has allowed us to derive the structure model for the (5 3H3)-rect

phase [18], represented in Figure 6.7b. Accordingly, the structure consists of two

types of building units, the O

V¼O tetragonal pyramid and a planar V

hexagon

(indicated on the ﬁgure), which are linked together to give the observed rectangular

Figure 6.6 The (H7 H7)R19.1



oxide phase:

(a) large-scale STM image (1000 Å 1000 Å,

þ2.0 V, 0.1 nA) of 2D oxide islands on the

Rh(1 1 1) surface; (b) high-resolution STM

image (5 Å 5Å, þ0.75 V, 0.2 nA). The

(H7 H7)R19.1



unit cell and the Rh(1 1 1)

substrate direction are indicated. The inset

shows a DFT-simulated STM image; (c) DFT

model of the H7-V

phase. Unit cell and

structural units are indicated (V green, O red,

Rh gray); (d) Side view of the V

model. The

inset shows a detailed view of the pyramidal

O

V¼O unit. (Reproduced with permission

from Ref. [4].)

162

6 STM Imaging of Oxide Nanolayer Model Systems

structure. The stoichiometry per unit cell is V

, and thus the (5 3H3)-rect

structure corresponds to a novel mixed-valence vanadium oxide phase, which occurs

only in ultrathin layer form. The simulated STM image (inset of Figure 6.7b) is in

excellent agreement with the experimental one and thus supports the structure

model.

Further annealing the (5 3H3)-rect structure at 750 –800 K leads to the transfor-

mation into a (9 9) phase [4, 18], which grows in the form of compact branched 2D

islands with rounded boundary shapes, as shown in the large-scale STM image in

Figure 6.8a. The high-resolution STM image (inset of Figure 6.8a) reveals a complex

contrast pattern of this phase, with the (9 9) unit cell indicated. In the center of the

unit cell, seven hexagonally arranged depressions are visible, showing a local (2 2)

periodicity. The structure model for the (9 9) phase, derived from the DFT

calculations is presented in Figure 6.8b and involves only one type of building unit,

the planar V

hexagons (encircled). The latter are connected in a complex way,

with eightfold and ﬁvefold rings in between, to give the observed (9 9) periodicity.

Each unit cell contains 36 vanadium atoms and 54 oxygen atoms, yielding basically

stoichiometry; the structure thus contains only V atoms in a formal þ3

oxidation state. The DFT model is strongly supported by the simulated STM image,

shown in the inset of Figure 6.8b; note that every single detail is reproduced in

the simulation. Interestingly, the presence of locally ordered (2 2) areas within the

(9 9) unit cell and the overall V

stoichiometry bear a strong similarity to the

(2 2) surface-V

phase observed on the Pd(1 1 1) surface [88]. We recently

suggested that the absence of a long-range ordered (2 2) structure on Rh(1 1 1) is

due to interfacial strain [23]; while the (2 2)-V

superstructure ﬁts perfectly on

the Pd(1 1 1) lattice, there isa 2.3% lattice mismatch between the relaxed (2 2)-V

Figure 6.7 (a) STM image of (5 H3)-rect vanadium oxide islands

on Rh(1 1 1) (1000 Å 1000 Å, þ1.5 V, 0.1 nA). Inset: enlarged

section of an (5 Ö3)-rect island (70 Å 70 Å, þ0.5 V, 0.1 nA);

(b) DFT-derived model of the (5 H3)-rect structure, unit cell

and structural units are indicated (V green, O red, Rh gray).

Inset: simulated STM image. (Reproduced with permission

from Refs [18, 101].)

6.3 Case Studies: Selected Oxide–Metal Systems

163