Cao Z. (Ed.) Thin Film Growth: Physics, materials science and applications

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268 Thin ﬁ lm growth

The changes in the electrostatic potential due to the modulations in the

polarity can even be quanti ed for the three stacking domains of FeO/

Pt(111) (Rienks et al., 2005). For this purpose, the position of the  rst

six FER was detected with dI/dV spectroscopy performed in a large bias

window (1–10 V) (Fig. 11.9a). In correspondence to the dI/dV maps, the

series of FER starts in the top domain at 4.25 V (see circle in Fig. 11.8a),

whereas the fcc (square) and hcp (triangle) domains follow at 4.7 and 5.1

V, respectively. It should be noted that due to the limited spatial resolution

of dI/dV spectroscopy at high bias, low-lying FER of neighbouring domains

show up in the spectrum of a selected region as well. Consequently, the dI/

dV peaks of fcc and hcp domains are split into two and three maxima with

the lower ones arising from conductance contributions of the adjacent top

and fcc domains, respectively.

The local surface potential F is determined from  tting the experimental

FER positions to a quantum-mechanical model that describes the resonances

as eigenstates in a triangular potential. The well is con ned by the sample

surface and the vacuum barrier that slopes down with the tip-induced electric

 eld F (Fig. 11.9b) (Kolesnychenko et al., 2000). Here, F is assumed to be

constant during spectroscopy due to the enabled feedback loop of the STM.

The position of the nth FER calculates to:



with m being the free electron mass (Kolesnychenko et al., 2000). A  t of

all experimental FER except the  rst one, whose energy position is altered

by other effects, yields the surface potential. The derived F values increase

from 3.5 eV in the top region to 3.65 and 3.85 eV in the fcc and hcp domains,

in good agreement with the results of the dI/dV maps. Conclusively, the

hcp and top domains are the most and least polar ones, corroborating the

predictions from the hard sphere model.

Complementary information on the surface polarity is obtained by measuring

the effective barrier height F

eff

that is experienced by electrons tunnelling

into the FeO  lm (second approach). The barrier height is measured via dI/dz

spectroscopy, detecting the current response I to a change in the tip-sample

distance z (Chen, 1993). The relationship between those quantities can be

derived from a one-dimensional model for electron tunnelling through a

square-shaped barrier:

effef



lndIln

Although the barrier height is not identical to the real surface potential, it is

a monotonous function of F (Olesen et al., 1996). It will thus be largest on

ThinFilm-Zexian-11.indd 268 7/1/11 9:43:28 AM

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269Electronic properties and adsorption behaviour of thin ﬁlms

the most polar oxide region, as the electrons have to overcome a substantial

surface dipole. Figure 11.10 shows two approaches to probe the effective

barrier height of the FeO thin lm. In Fig. 11.10(a), the logarithmic current

response to a linear distance ramp is recorded and F

eff

is extracted from the

slope of the ln(I)-Dz curves. In Fig. 11.10(b), the d(lnI)/dz signal is mapped

directly with lock-in technique by adding a small z-modulation to the STM

feedback loop. In both cases, spatial variations in the effective barrier height

are detected on the FeO surface. Here, the top domains exhibit the smallest

slope in ln(I)-Dz curves and the lowest intensity in d(lnI)/dz maps, indicating

a low barrier height. The hcp domains, on the other hand, are characterized

by a large F

eff

value, in agreement with their more polar nature. The two

techniques discussed above, namely evaluating the FER energies and probing

the effective barrier height, therefore reveal similar modulations in the local

surface potential and provide unmatched insight into the polarity of the three

FeO stacking domains.

The experimental results obtained on the polar FeO/Pt(111) lm have been

corroborated by DFT calculations (Giordano et al., 2007b). Due to the large

size of the real FeO-Pt coincidence cell, they were performed with a smaller

computational cell that still contains Fe atoms in characteristic top, fcc and

hcp binding congurations (Fig. 11.11a). While the hcp and fcc regions

display similar properties in the simulations, the top domain sticks out in

various aspects. It is characterized by the largest interfacial separation from

top

fcc

hcp

0 0.4 0.8

Dz (Å)

(a) (b)

I (nA)

1.0

0.5

hcp

top

top fcc hcp

F (eV)

3.5

3.3

3.1

11.10 (a) Tunnel current versus tip-sample distance recorded for the

top and hcp domain of the FeO coincidence cell. From the current

slope, the effective barrier height is determined and displayed in the

inset. (b) d(lnI)/dz image of FeO/Pt(111) taken with lock-in technique

and closed feedback loop. (Image size 5.7 ¥ 5.7 nm

, V

= 4.5 V, Dz

rms

= 1 Å.) Reprinted with permission from Rienks et al. (2005), Copyright

(2010) by the American Physical Society.

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270 Thin ﬁlm growth

the Pt(111) surface and therefore mimics the properties of a free-standing

FeO layer. Additionally, the vertical separation between Fe and O planes

(0.5 Å) is 15–20% smaller in the top than in adjacent hcp and fcc domains

(0.8 Å). The top region therefore has the smallest Fe

-O

d–

surface dipole

and hence the lowest surface potential. Its F value is 0.29 and 0.23 eV lower

than those of the hcp and fcc regions, respectively, in good agreement with

the measured potential modulations in the FeO coincidence cell of 0.35

eV.

Based on the computed surface potentials, STM images have been

simulated for bias voltages in the eld emission mode (Giordano et al.,

2007b). As discussed above, the contrast in this regime is governed by the

availability of FER, which in turn depends on the local surface potential. A

small F gives rise to a bright contrast, as low-lying FER with high electron

transmissibility promote the electron transport, while regions with large F

appear dark. Calculated F maps, depicted with inverted contrast, indeed

reproduce the high-bias STM images of the FeO/Pt system (Fig. 11.11b). In

agreement with our earlier interpretation, the brightest region corresponds to

the one with the lowest surface potential and is assigned to the top domain

that has the lowest polarity of all FeO stacking domains.

Measurements of the surface potential with an STM have proven to be

an adequate means to probe the polarity of FeO thin lms (Rienks et al.,

2007). Similar results were obtained for other polar systems, e.g. MgO(111)

(a)

(b)

4500 mV

fcc top hcp

11.11 (a) Top view of a model structure for the FeO/Pt(111) system

that contains all relevant Fe binding conﬁgurations but is much

smaller than the real coincidence cell. (b) STM image of the

polar FeO ﬁlm taken at +4.5 V sample bias (top, 7 ¥ 7 nm

) and

corresponding surface potential map calculated with the model

shown in (a) (bottom). The map depicts the -F signal, and regions

with low surface potential. Reprinted with permission from Giordano

et al. (2007b), Copyright (2010) by the American Physical Society.

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271Electronic properties and adsorption behaviour of thin ﬁlms

lms grown on Au(111) (Myrach et al., 2011). Also here, a considerable

surface dipole develops from the stacking of a surface O

2–

layer on top

of an interfacial Mg

layer. This layer sequence is compatible with the

electronegative character of gold, which promotes a charge transfer out of

the cationic plane to reduce the vertical dipole moment (Fig. 11.5a). Similar

to the FeO/Pt(111) system, the surface polarity is not homogeneous on the

MgO(111) surface, but for a different reason. The oxide polarity quickly

vanishes with lm thickness, as three-dimensional MgO islands with zero

dipole develop on the surface. The initial building blocks that are responsible

for polarity healing are nano-pyramids of a few layers height (Wolf, 1992),

which are readily observed in STM images of the oxide lm (Fig. 11.12a).

Their effect on the oxide polarity has been deduced from the barrier height

images again (Fig. 11.12b). As expected, the F

eff

value turns minimal above

the surface protrusions, demonstrating the effective quenching of the surface

dipole due to the island growth. It should be noted that this morphology-driven

removal of the polarity bears similarities to the octopolar reconstruction that

is the dominant healing mechanism on bulk MgO(111) (Barbier and Renaud,

1997; Barbier et al., 2000).

In general, the STM is able to provide spatially resolved information on

the distinct properties of polar surfaces. The technique is not only suited to

characterize the various polarity healing mechanisms from a topographic

point of view, but enables a local determination of the surface potential and

its correlation to the oxide polarity. Although STM-based approaches do not

reach the same quantitative accuracy in determining surface potentials as,

(Å)

0 20 40 60 80

Position (Å)

Low d(lnI)/dz High

(a) (b)

11.12 (a) Topographic image of a bilayer MgO(111) island grown on

Au(111) (–0.25 V, 9 ¥ 9 nm

). A height proﬁle taken across the island

is shown in the inset. (b) Effective barrier height (d(lnI)/dz) image

of the same surface region. Particularly low values of the barrier

height are revealed on the ad-structures that cover the oxide island,

indicating their importance in quenching the surface dipole.

ThinFilm-Zexian-11.indd 271 7/1/11 9:43:29 AM

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272 Thin ﬁ lm growth

for instance, Kelvin–Probe spectroscopy, they feature an unmatched spatial

resolution with respect to alternative techniques.

11.4 Adsorption properties of polar ﬁ lms

Residual polarity gives rise to unusual adsorption and chemical properties of

thin oxide  lms (Goniakowski et al., 2008; Sun et al., 2009). The difference

to non-polar systems lies in the electrostatic contribution to the surface free

energy that originates from the uncompensated surface dipole (see discussion

in Section 11.1). The binding behaviour of adsorbates is therefore not only

governed by the usual physisorption and chemisorption effects, but includes

changes in the electrostatic energy of the system as adsorbates might reduce its

polarity. Given the magnitude of the energies involved, the binding potential

of polar surfaces can be substantially higher compared to non-polar ones. A

direct manifestation of this effect is the wetting growth of metals on polar

surfaces, whereas mainly three-dimensional deposits form on non-polar oxide

materials (Goniakowski and Noguera, 2002; Meyer and Marx, 2004).

Two mechanisms have to be considered in conjunction with polarity

healing via adsorbates. In a  rst scenario, the ad-species become charged

upon adsorption and alter the electron density on the surface. As discussed

in Section 11.1, depolarization of the system takes place when the surface

charge density equals the bulk density times the ratio between interlayer

distance d and unit cell height D:

[11.4]

This condition can now be ful lled by adsorbing the required number of

charged species to the surface. The most prominent example of this mechanism

is the attachment of protons (H

) that often originate from the heterolytic

splitting of water to polar surfaces (hydroxylation). For rocksalt (111), every

surface site needs to be occupied by a hydroxyl group in order to quench

the polarity, although d/D = 0.5. The reason is that each H

carries only half

the charge of an oxide ion (Mg

, O

2–

). In the case of wurzite (0001), on the

other hand, 50% surface coverage would be suf cient. The hydroxylation

of polar oxide surfaces has been intensively studied with infrared re ection

absorption and high resolution electron energy loss spectroscopy (HREELS),

as discussed for instance in Rohr et al. (1994), Poon et al., (2006) and Wang

(2008). At the local scale, proton attachment was investigated with the STM

on Cr

/Cr(110)  lms (Maurice et al., 2001) and more recently on FeO/

Pt(111) (Merte et al., 2009; Knudsen et al., 2010). Although dipole removal

via hydroxylation is observed most frequently, any other adsorbate that is

easily ionized or polarized can be used instead. Examples for the adsorption

of charged ad-species on polar oxide  lms are given in Section 11.4.1.

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273Electronic properties and adsorption behaviour of thin ﬁlms

A second mechanism for dipole quenching is the metallization of polar

surfaces. In this case, the adsorbates create electron bands that cross the

Fermi energy of the system and can be lled with the required number

of electrons to satisfy Eq. 11.4. Those electronic states might be intrinsic

to the ad-species or induced into the polar material upon adsorption. The

rst case is typically realized with metallic adsorbates that possess states

around the Fermi level. For instance, a single Pd layer attached to a Mg-

terminated MgO(111) slab was found to charge up negatively by lling a

Pd-like interface state (Goniakowski and Noguera, 2002). The computed

charge density in the Pd layer hereby adopts 50% of the MgO bulk value

in accordance with Eq. 11.4. Further thickening of the metal lm does not

change this situation, indicating that a single layer is sufcient to cancel the

oxide polarity. Experimentally, the wetting growth of metals on polar oxide

materials has been studied for Cu on Zn(0001) (Koplitz et al., 2003; Dulub

et al., 2005; Kroll and Köhler, 2007). Especially, on the O-terminated surface,

a layer-by-layer growth of Cu is observed that reects, however, not only

the impact of oxide polarity but also the generally higher adhesion for this

surface termination. A similar result was obtained for the polar FeO/Pt(111)

lm, where extended 2D islands form upon Pd deposition (Fig. 11.13a). It

should be noted that metals usually develop 3D particles when deposited

0.4 nm

H-covered

Clean

–3 –2 –1 0 1 2 3

Sample bias (V)

Log I (nA)

0.1

0.01

1e–3

(b)(a)

11.13 (a) STM image of a planar Pd island grown on a FeO/Pt(111)

ﬁlm (9.0 ¥ 9.0 nm

). The two-dimensional growth mode is triggered

by the polarity of the system. (b) STM images of a clean (left)

and a hydrogen covered ZnO(10-10) surface (right). The dashed

rectangle marks the (1¥1) surface unit cell. H adsorption leads to a

metallization of the ZnO(10-10) surface, as shown by the vanishing

band gap in I-V curves taken on the H-covered surface. Reprinted

with permission from Wang et al. (2005), Copyright (2010) by the

American Physical Society.

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274 Thin ﬁlm growth

on non-polar materials in order to minimize the contact area with the inert

oxide surface (Bäumer and Freund, 1999).

Induction of a metallic surface state in the substrate itself has been

demonstrated with STM conductance spectroscopy and HREELS for the

ZnO(10-10) surface (Wang et al., 2005; Yin et al., 2006). In those experiments,

the oxide band gap was found to disappear after adsorbing one H atom per

unit cell of the ZnO surface (Fig. 11.13b). This result was explained with

the transfer of the hydrogen electron into the initially empty 4s-band of zinc

that subsequently becomes half-lled. Increasing the dosage to two H atoms

per unit cell renders the surface non-metallic again, as a second electron is

added to the surface band that consequently shifts below the Fermi level.

A similar metallization effect has so far not been revealed for the polar

ZnO(0001) surface.

The detailed adsorption properties of polar systems are discussed for the

FeO/Pt(111) lm in the following section. This particular system is selected

again because it has already been introduced in the previous sections.

Furthermore, the FeO lm exhibits spatial modulations of the surface polarity,

which induces a template effect in the arrangement of the adsorbates. It is

therefore an example where uncompensated polarity induces self-assembly

phenomena on an oxide surface.

11.4.1 Adsorption of metal atoms on polar FeO ﬁlms

As discussed in Section 11.3, the polarity of the FeO lm arises from the

interplay between an Fe

interface and an O

d–

surface plane (Vurens et al.,

1988; Kim et al., 1997). The Fe-O layer separation is not constant, but varies

between 0.52 and 0.78 Å due to different Fe-O stacking congurations on

the Pt(111) surface. Large interlayer distances occur for the fcc and hcp

domains, while small separations are found in the top region of the FeO-Pt

coincidence cell (see Fig. 11.7a for a structure model). As discussed above,

such modulations in the interlayer distance affect the vertical dipole strength

and therefore the polarity of the lm, whereby the hcp and top regions turn

out to be the most and least polar ones, respectively (Rienks et al., 2005).

The same modulations should govern the spatial distribution of adsorbates

on the FeO surface as well.

A rst demonstration of this effect was provided in a low-temperature

STM experiment, where single Au atoms were deposited onto the lm at 10

K sample temperature (Nilius et al., 2005). The incoming atoms perform a

transient diffusion on the FeO surface due to their initial thermal energy, and

are able to reach their preferred adsorption sites. After thermalization, they

can be imaged in the STM as 1.0 Å high, circular protrusions. The adatoms

are not randomly distributed on the surface, but exhibit a large tendency to

attach to the hcp domains of the coincidence cell. From the ve adatoms

ThinFilm-Zexian-11.indd 274 7/1/11 9:43:30 AM

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275Electronic properties and adsorption behaviour of thin ﬁlms

shown in Fig. 11.14(a), only the uppermost one binds to a top region while

the four lower ones sit in hcp domains. At a nominal Au coverage of 0.01

ML, which is close to one atom per coincidence cell, more than 70% of

the hcp domains are occupied with Au atoms, whereas adjacent fcc and top

regions remain nearly adsorbate-free (occupancy below 10%) (Fig. 11.14c).

The site-specic adsorption behaviour of Au therefore leads to the formation

of a hexagonal adatom array, the lattice parameter of which matches the size

of the FeO/Pt(111) coincidence cell. The long-range order in the adatom

arrangement becomes evident in a 2D pair-distribution function n(x, y),

calculated from more than 700 atom positions (Fig. 11.15). The highest

probability to nd a neighbouring Au atom is in the hcp domain of the

next FeO/Pt coincidence cell; however, also second and third neighbouring

hcp domains show substantial occupancy due to the perfect ordering of the

adsorbates. A distinct atom distribution is discernible even within each hcp

region, as marked by seven, hexagonally-arranged spots with 3.1 Å mutual

distance. This ne structure reects the few atomic binding sites in the

centre of each hcp domain that are actually populated by Au. Based on the

pair distribution function, the modulation of Au binding energies within the

coincidence cell DE can be estimated by assuming a Boltzmann distribution

for the occupation probability: n(x, y)µ exp(

–DE

) (Silly et al., 2004; Kulawik

top

fcc

hcp

Occupancy

0.6

0.4

0.2

0.0

(a) (b)

(c)

11.14 (a) STM topographic image of Au atoms on FeO/Pt(111) (0.5 V,

13 ¥ 13 nm

). Four out of ﬁve adatoms occupy hcp domains in the

FeO coincidence cell. (b) Overview STM image demonstrating the

ordering of Au atoms into a hexagonal lattice (0.5 V, 60 ¥ 60 nm

different FeO stacking domains. The plot is based on the evaluation

of ~700 atom positions. Reprinted with permission from Nilius et al.

ThinFilm-Zexian-11.indd 275 7/1/11 9:43:30 AM

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276 Thin ﬁlm growth

et al., 2005). As the self-assembly is observed for temperatures as high as 50

K, this temperature is used to calculate a lower bound for DE. The analysis

yields an energetic preference of (10 ± 2) meV for Au binding to the hcp

domains with respect to adjacent fcc and top regions. It is interesting to

note that Au atoms remain essentially monomeric upon adsorption onto the

oxide lm even at relatively high coverage. No aggregation sets in before

the majority of hcp domains is lled, which suggests a repulsive, most likely

Coulomb-type interaction between the adatoms that inhibits cluster formation

(Nilius et al., 2005).

The self-assembly of Au atoms on the FeO/Pt(111) surface can be traced

back to the polar nature of the lm. The adatoms preferentially attach to the

hcp domains having the largest surface dipole (Giordano et al., 2007b). The

uncompensated polarity in this region enables an efcient polarization of the

Au, which in turn leads to an electrostatic coupling between the adatoms and

the dipole eld of the polar lm. This binding contribution will be smaller

on the adjacent fcc and top domains, where the dipole strength is reduced.

A more detailed picture of the Au ordering effect on the FeO is obtained

from DFT calculations (Giordano et al., 2008). In this study, the Au interaction

with O-top, Fe-top, bridge and hollow sites has been analysed for all three

stacking domains of the coincidence cell. The largest adsorption energy of

0.6 eV was computed for the O-top sites in hcp domains, where the binding

y Displacement (Å)

–40

–20

fcc

hcp

top

–40 –20 0 20 40

x Displacement (Å)

Energy (meV) Counts

11.15 Two-dimensional pair-distribution function of Au atoms on

the FeO ﬁlm (top). Most events occur in the hcp domains; the sharp

maxima within those regions reﬂect the atomic binding sites of the

adsorbates. The bottom part depicts the potential landscape for Au

adsorption, being calculated by assuming a Boltzmann distribution

according to the Au binding energy. Reprinted with permission

Society.

ThinFilm-Zexian-11.indd 276 7/1/11 9:43:30 AM



277Electronic properties and adsorption behaviour of thin ﬁlms

is 30% stronger than on Fe sites in the hcp and O sites in the fcc and top

domains. This binding preference explains the observed self-assembly of Au

adatoms on the FeO lm. It should be noted that theory nds an even more

favourable adsorption conguration, in which an interfacial Fe atom ips

above the O-plane (Fig. 11.16). The hence under-coordinated Fe atom is able

to bind the surface Au species with ~1.5 eV, much stronger than in the regular

geometry. Due to a concomitant negative charging of the Au adatom, also

the lm polarity is locally quenched in this conguration, as the resulting

d–

-Fe

-Au

d–

stacking carries no dipole moment (Fig. 11.16b). A similar

binding conguration has been predicted for other electronegative metals,

such as Pt (Goniakowski et al., 2009). However, the inverted adsorption

geometry could not be conrmed experimentally, as neither the height nor

the electronic properties of a potential Au-Fe ad-species was compatible

with the STM data. Apparently, the energy barrier to break a Pt-Fe bond at

the interface and swap the local Fe-O stacking sequence is not overcome at

the low temperature of the experiment (Nilius et al., 2005).

The experimentally conrmed Au binding to O atoms in the hcp domains

leads to a 20% increase of the Fe-O layer distance around the adsorption site,

which enhances the surface dipole and therewith the electrostatic Au–FeO

interaction (Giordano et al., 2008). The local lattice distortion is initiated

by a charge transfer from the Au atom into the Pt crystal. The Coulomb

interaction of the Au

lifts the O

d–

ion underneath above the surface plane,

but repels the three adjacent Fe

ions, thereby inducing a polaronic distortion

of the oxide lattice. The formation of charged adsorbates combined with a

polaronic distortion of the surrounding lattice is a common binding scenario

on ionic oxide lms (Pacchioni et al., 2005; Giordano et al., 2007a). The

cationic nature of Au atoms on the FeO lm is further supported by state

density calculations (Fig. 11.17b). The Au 6s orbital of hcp-bound adatoms

is located at +0.3 eV above the Fermi level and thus empty, in contrast to

its half-lled nature in gas-phase gold. This nding indicates that the 6s

electron has been transferred into the support, rendering the Au positively

– Au

d–

– Fe

Pt(111)

(a) (b)

11.16 (a) Experimentally observed and (b) energetically favoured

binding conﬁguration of Au on FeO/Pt(111). In the optimum

geometry, a Fe atom ﬂips above the oxide plane in order to increase

the interaction with the Au. The energy barrier for this restructuring

is not known.

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