Bowker M., Davies P.R. (Eds.) Scanning Tunneling Microscopy in Surface Science, Nanoscience and Catalysis

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barrier may just be sufﬁcient to control a growth process has been shown in several

cases [26, 27]. Indeed, this leads to a template-controlled growth without speciﬁc

interaction but by spatially conﬁning the movement of the deposited particles. The

nucleation is then homogeneous but controlled by the template. This approach is

quite similar to the one used in lithography where the spatial conﬁnement is provided

by the lithographically produced pattern on the surface. Since surface diffusion in

this case is the crucial parameter, the process will be subject to the same limitations in

terms of temperature as in the case of template-controlled heterogeneous nucleation.

Before we discuss the template-controlled growth of model catalysts in more detail,

we will have to consider a few aspects of STM imaging of these systems. This will be

crucial for the characterization of the model catalyst surfaces.

2.3

STM Imaging of Oxide Films

Even though scanning tunneling microscopy is nowadays a standard tool for surface

characterization, special care has to be taken when using it for the investigation of

thin oxide ﬁlms. On clean metal surfaces, a simple metal–vacuum–metal tunneling

gap is usually found [28]. In this case, the tunneling current can be to a ﬁrst

approximation described by the Tersoff–Hamann model [29], which postulates that

the main contribution to the tunneling current is due to electrons coming from the

Fermi edge. If we introduce a thin oxide ﬁlm, the tunneling gap can be characterized

by a metal–oxide–vacuum–metal junction (Figure 2.5). As a consequence, the band

structure of the oxide ﬁlm – in particular, the gap between the valence band (VB) and

Figure 2.4 Evolution of the island density with growth

temperature for Au/N/Cu(1 0 0). (Reproduced with permission

from Ref. [24].)

2 The Template Route to Nanostructured Model Catalysts

the conduction band (CB) – will play a crucial role in the tunneling process. If we

disregard local effects – such as a local variation of the barrier height – and take only

the average electronic structure of the ﬁlm into account, we can distinguish two

limiting cases. For small bias voltages, the oxide ﬁlm will only weakly perturb the

tunneling process. The current passes through the oxide ﬁlm because no states for

the tunneling electrons are available (Figure 2.5a). It will thus act much like an

additional vacuum barrier. As a consequence, the distance from the tip to the sample

will be the sum of the ﬁlm thickness d

and the distance between tip and ﬁlm d

. This

does not necessarily imply that the oxide ﬁlm is invisible in STM. If the lateral

structure of the oxide ﬁlm is not homogeneous, the tunneling barrier will change as

a function of the lateral position of the tip and consequently the z-position of the tip

will also change, which will result in a contrast in the STM images. If, at small bias

voltage, the tunneling current is increased, the tip will touch and eventually penetrate

the ﬁlm, leading to a direct overlap of the electronic states of the tip and the sample.

This will cause variations in the tunneling current as a function of the lateral tip

position, which can – in favorable cases – be used to image the surface of the oxide

ﬁlm with atomic resolution [30]. Since at moderate tunneling currents (<1 nA) the

ﬁlm behaves like an additional vacuum barrier, it is possible to image the substrate

tipsubstrate oxide

’

Vac

(a)

(b)

Figure 2.5 Schematic representation of the gap of a metal-

oxide–vacuum–metal tunneling junction in the case of a

low (a) and a high (b) sample bias voltage.

2.3 STM Imaging of Oxide Films

surface with atomic resolution through the oxide ﬁlm with STM [31]. This can be very

helpful for the analysis of the structural relation (commensurability) of oxide ﬁlm and

substrate.

In the case of the high bias voltages – depending on the sign of the bias voltage –

either the conduction band or the valence band of the oxide will participate in the

tunneling process (Figure 2.5b), and it is expected that the topography of the ﬁlm in

the STM images is dramatically changed compared to the case of low bias voltages.

At positive sample bias, the main contribution to the tunneling current stems from

electrons tunneling into the conduction band of the oxide ﬁlm, from which they are

further transported into the substrate. If the ﬁlm is sufﬁciently thin, that is, thinner

than the mean free path of electrons (10 nm), a charging of the oxide ﬁlm due to the

electron current is not expected [32]. It should also be mentioned that, as shown in

Figure 2.5, to a ﬁrst approximation no bias voltage drop is encountered in the ﬁlm

if its thickness is only around 0.5 nm.

The signiﬁcant differences in the tunneling process at low and high bias voltage

will result in a strong bias dependence of the apparent topography of the oxide ﬁlm.

Such a bias-dependent tunneling for thin Al

islands on NiAl(1 1 0) was ﬁrst

observed by Bertrams et al. [33]. In this case, the islands were hardly visible at a

sample bias voltage of þ0.4 V, which corresponded to an energy in the bandgap of

the alumina islands. At a bias of þ4 V, however, the electrons tunnel into the

conduction band of the Al

islands and these are imaged with an apparent height

of 0.3 nm. For the similar system of Al

islands on Ni

Al(1 1 1), a systematic study

of the apparent height of the islands as a function of the bias voltage allowed the

estimation of the energetic position of the bottom of the Al

conduction band [34].

As can be seen in Figure 2.6a, the measured apparent height of the Al

islands

rapidly increases in the bias voltage ranging from 2.1 to 3.0 V from a value of 0.25 to

0.6 nm, which is close to the actual thickness of 0.5 nm [35]. This increase is due to

Figure 2.6 (a) Variation in the apparent height of alumina islands

on Ni

Al(1 1 1) as a function of sample bias voltage [34].

(b) STS dI/dV spectra taken at different points of the alumina

film surface [36].

2 The Template Route to Nanostructured Model Catalysts

the lowered tunneling resistance caused by the participation of the conduction band

of the oxide in the tunneling process. This situation is similar to the one shown in

Figure 2.5b. The determination of the band edge using this method is, however, not

very precise and based on the average properties (e.g., average height) of the islands.

However, it provides a ﬁrst indication of the variations in the apparent topography of

alumina ﬁlms as a function of the bias voltage encountered in STM. Furthermore,

a combination of measurements like the one shown in Figure 2.6a with UPS valence

band spectra of the respective oxide ﬁlm provides a rough value of the bandgap of the

ﬁlm [34]. The bandgap of the oxide ﬁlm can be determined using tunneling

spectroscopy, which is shown in Figure 2.6b. These dI/dV spectra clearly show the

top of the valence band at a negative sample bias of about 5 V and the bottom of

conduction band at roughly þ3 V [36] indicating a bandgap of approximately 8 eV.

This variation in the electronic structure of the tunneling gap is reﬂected in the

pronounced variation in the apparent topography of Al

ﬁlms on Ni

Al(1 1 1) as

a function of bias voltage (Figure 2.7). Here, two effects are clearly visible: First, a

contrast reversal between images (c) and (d) can be seen. The formerly hexagonal

arrangement of bright dots (2.0 V) is imaged as holes at a bias voltage of 3.2 V.

This important bias dependence of the apparent topography of insulating ﬁlms

has also been found in several other cases, for example, CaF

/Si(1 1 1) [32], CoO/

Ag(0 0 1) [37, 38], NiO/Ag(0 0 1) [39], and ZrO

/Pt(1 1 1) and can be attributed to the

band structure and more precisely to the bandgap of the insulating ﬁlms. Second,

the apparent corrugation of the ﬁlm changes drastically. For a low bias voltage of 0.5 V,

a corrugation on the order of only 0.02 nm is found, which roughly corresponds to

the corrugation found on metal surfaces, whereas at higher bias a corrugation of up to

0.2 nm is visible (Figure 2.8). It is obvious that the sharp increase in the measured

corrugation takes place in the same energy interval where the increase in the average

island height has been found for this type of oxide ﬁlm (Figure 2.6a). It must,

therefore, be correlated with the electronic structure of the ﬁlm in the region close to

the bottom of the conduction band.

Again a closer look at the STS spectra shown in Figure 2.9 reveals that the dots are,

indeed, closely related to the electronic structure of the ﬁlm. They appear in an energy

range between 2.0 and 2.8 V, where a localized electronic state in the bandgap can be

found at the corners of the unit cell (points labeled A). This state, which is found

Figure 2.7 STM images of an alumina film on Ni

Al(1 1 1) at

different sample bias voltages. (a) U

¼0.5 V, (b) 1.5 V, (c) 2.0 V,

(d) 3.2 V, and (e) 4.2 V. The unit cell of the dot structure is marked

in the images.

2.3 STM Imaging of Oxide Films

0.4

0.3

0.2

0.1

0.0

height [nm]

1086420

distance [nm]

= +0.5 V

= +1.5 V

= +2.0 V

= +3.2 V

Figure 2.8 Line profiles of the alumina film on Ni

Al(1 1 1) for different sample bias voltages [36].

Figure 2.9 (a) STM image of Al

/Ni

Al(1 1 1). The

unit cell of the dot structure is shown as dashed line.

(b) STS spectra taken at the points indicated in the

STM image. The feedback loop was opened at a bias

voltage of þ3.5 and þ3.0 V (inset of b).

2 The Template Route to Nanostructured Model Catalysts

only at the corners of the unit cell, increases the tunneling probability at these

locations in the respective bias voltage range resulting in the observed contrast (see

Figure 2.7). STS spectra taken at the points labeled B do not show this state. It is,

thus, clearly localized at the corners of the unit cell.

This particular example shows that special care has to be taken when the

topography of thin oxide layers on metallic substrates is measured by STM. Electronic

effects at or near the band edges can dramatically inﬂuence the tunneling proba-

bilities and, thus, lead to rather large changes in the measured corrugation.

Obviously, this phenomenon can also inﬂuence the imaging of clusters on oxide

ﬁlms, since corrugation changes of about 0.2 nm, which have been encountered

here, are on the order of the interlayer distance in metals and can considerably

inﬂuence the cluster height determination. However, other effects may also be

encountered when imaging clusters on oxide ﬁlms, which will be discussed in the

following section.

2.4

STM Imaging of Metal Particles on Oxide Films

As we have seen in the previous chapter, the apparent topography and corrugation of

thin oxide ﬁlms as imaged by STM may vary drastically as a function of the sample

bias. This will of course play an important role in the determination of cluster sizes

with STM, which will be discussed in the following section. The determination of the

size of the metallic nanoparticles on oxide ﬁlms is a crucial issue in the investigation

of model catalysts since the reactivity of the particles may be closely related to their

size. Therefore, the investigation of reactions on model catalysts calls for a precise

determination of the particle size. If the sizes of the metal particles on an oxidic

support are measured by STM, two different effects, which distort the size mea-

surement, have to be taken into account.

First, convolution effects between the tip and the cluster, both in general of

comparable size, must be considered. This has already been pointed out by Reis

et al. [40] in 1989 in the context of roughness measurement [40] and Barbet et al. in

1993 for imaging colloidal gold beads [41] using STM. The situation one encounters

for small metal particles on an oxide ﬁlm is depicted in Figure 2.10. If we disregard

electronic effects, which will be discussed below, the trajectory of an STM tip above

a cluster will follow the dotted path depicted in Figure 2.10. Two consequences

become immediately obvious: (i) The apparent cluster height h



, which can be

derived, should be identical to the actual cluster height h. Thus, from this point of

view the cluster height can be correctly measured by STM. Since the height is in

general an integer multiple of a step height of the corresponding cluster material,

it is rather easy to judge on the number of atomic layers, which constitute the cluster.

(ii) The measured cluster diameter d



will be larger than the actual cluster diameter d.

Moreover, the actual value of d



will depend not only on the cluster diameter but also

on the morphology and radius of the tip. Even if we deﬁne the diameter of the cluster

at the FWHM in a line scan of the particle (d

), the measurement will still not reﬂect

2.4 STM Imaging of Metal Particles on Oxide Films

the actual diameter of the cluster. To account for this, a detailed analysis of the tip

shape is necessary, as it has been described in detail by Bowker et al. [5] as well as

Klyachko and Chen [42], for the case of C

molecules on Ge(1 0 0) and Si(1 0 0). But

even if we were able to overcome these geometric distortions by estimating the tip

shape and explicitly including it in the particle size calculations, a second and more

important problem will come into play.

The complex electronic structure of the tunneling contact in these systems, which

is schematically depicted in Figure 2.11, renders the tunneling process rather

difﬁcult. We are now facing a junction of metal–oxide–metal–vacuum–metal type

and a number of effects have to be considered. First, the electronic structure of the

clusters will play an important role. Here, we have to consider again two cases: very

small particles, which still have – much like molecules – discrete electronic states [43],

and larger particles, which possess a band structure and can be regarded as metals.

We will conﬁne our discussion to the latter case since most of the arguments found

there are also applicable to the former. For a metallic particle, the tunneling current

Figure 2.10 Trajectory of an STM tip above an oxide-supported metal particle.

tipsubstrate oxide

Vac

cluster

Figure 2.11 Schematic representation of the tunneling gap for an oxide-supported metal particle.

2 The Template Route to Nanostructured Model Catalysts

from the tip can pass into the particle at all bias voltages much as it is the case on

extended metal surfaces because of the continuous band structure of the metal. As a

consequence, the tunneling resistivity will be to a ﬁrst approximation related to the

electronic density of states of the cluster. However, once the tunneling electron has

passed the cluster, the further transport of the electron through the oxide ﬁlm to the

substrate strongly depends on the band structure of the oxide ﬁlm. If the bias voltage

is large, then the energy of the electron corresponds to that of the conduction band of

the oxide and tunneling is facilitated.

If, however, the bias voltage is low, the oxide ﬁlm will act as an additional tunneling

barrier (see also Figure 2.5) and the electron may not as easily relax into the substrate.

In the latter case, charging effects of the particle may be encountered, which will

in turn inﬂuence the tunneling current and thus the apparent topography of the

particle. This will, however, depend on the thickness of the oxide ﬁlm and thus the

second tunneling gap. We expect therefore that much like the apparent topography

of the oxide ﬁlm, the apparent height and size of clusters on oxide ﬁlms will show

strong bias dependence. This is shown in Figure 2.12 for a single Fe cluster on

/Ni

Al(1 1 1).

In Figure 2.12a, which was taken at a bias of 3.2 V, that is, in the range of the oxide

conduction band, the cluster appears rather large and the surrounding oxide ﬁlm is

also imaged with large corrugation. At small bias voltage (Figure 2.12b), however,

the cluster appears to be much smaller and the oxide ﬁlm is not visible any more.

The hexagonal pattern surrounding the cluster is due to the atomic structure of the

Al(1 1 1) substrate, which can be resolved in this case through the oxide ﬁlm. By

comparing these images, the whole problem of particle size measurements on such

complex systems becomes very apparent.

To systematically investigate this phenomenon, a collection of 12 Fe clusters on

/Ni

Al(1 1 1) has been imaged by STM in the bias voltage range from 0 to 5 V.

In Figure 2.13, it is clearly visible that not only the apparent structure of the oxide ﬁlm

undergoes a strong bias dependence, as it has already been shown in the previous

section, but also the appearance of the clusters changes dramatically as a function of

bias voltage. A thorough analysis of the cluster heights and diameters as a function of

the bias voltages leads to the curves presented in Figure 2.14. It is visible at ﬁrst glance

Figure 2.12 STM images of a single Fe cluster on Al

Al(1 1 1): (a) 3.2 V and (b) 0.19 V, 6.2 nm 6.2 nm [44].

2.4 STM Imaging of Metal Particles on Oxide Films

Figure 2.13 STM images of Fe clusters on Al

/Ni

Al(1 1 1)

taken for different sample bias voltages. The image size is

48 nm 48 nm. The labeled particles were used for the evaluation

of the particle height and diameter [44].

2 The Template Route to Nanostructured Model Catalysts

that the variations of cluster height and size are signiﬁcant in the investigated range

of bias voltages. There are, however, three distinct regions, which show a particular

behavior. The range from 0 to 1 V is characterized only by a slight variation in the

cluster height and diameter. The same is true for the bias range above 3 V. However,

in between (1–3 V), an increase in the apparent height and diameter is encountered.

To explain this, we turn back to Figure 2.11. Region I is clearly in the bandgap of the

oxideso that aluminadoesnotcontributetothetunnelingprocess.RegionIIIisrelated

to the conduction band of the oxide so that the oxide is taking part in the tunneling

process, but the contribution of the oxide can be regarded as constant to a ﬁrst

approximation. This implies that in these two regions theappearance of the clusters is

largely determined by the electronic structure of the cluster itself. Apparently, this

structure is not a strong function of the bias voltage in the present case. In region II,

however, we ﬁnd a steep increase in both values. This must be due to the electronic

structureofthetunneling gap.Toexplainthisvariation,itisnecessarytohaveadetailed

look at electronic states, which participate in the tunneling process. Here, contribu-

tions of both the cluster and the oxide ﬁlm have to be considered. First of all we note

that above 2 Va contrast reversal and an increase in the measured corrugation in the

STMimages of the pureoxideﬁlm was found,whichhas been discussedabove. This is

correlated with the appearance of the dots in the STM images exactly at the positions

where the clusters are located in the present case. This contrast reversal, which is due

to the electronic structure of the oxide ﬁlm, can therefore explain the increased

values of the cluster height and diameter in the range from 2 to 3 V. For bias voltages

between 1 and 2 V, however, the increase in cluster height and diameter cannot be

attributed to thealumina band structure.Thus, itmustbe relatedto the bandstructure

of the Fe clusters themselves. Theoretical data from literature can shed some light on

the question whether or not the electronic structure of the clusters is responsible for

this sudden increase in Figure 2.14. The calculated density of states for an Fe cluster

containing 7 Fe atoms shows an intense empty sp-state that is localized in the range

from 1 to 2 eV [45]. This state will facilitate tunneling into the cluster and, thus, lead to

a decrease in the tunneling resistance. This in turn will, under constant current

Figure 2.14 Apparent cluster height (a) and diameter (b) as a

function of bias voltage. Data were obtained from the Fe clusters

shown in Figure 2.13 [44].

2.4 STM Imaging of Metal Particles on Oxide Films