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

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248 Thin ﬁlm growth

2009). Multilayers and the number of layers they contain are readily identied

by electron reectivity as was done for multilayer graphene on SiC (Ohta,

2008). The in-plane morphology of multilayer graphene is much less studied.

The stacking of the multilayers and azimuthal orientation between layers

are, for instance, not reported or discussed. Diffraction experiments, such

as (micro) low-energy electron diffraction (Sutter, 2008), should provide

valuable hints in this respect. sTM was used to show that the second layer

in a bilayer might exhibit nanometre scale ripples at the surface or be at

depending on the location on the sample (Sutter, 2008, 2009c).

At the surface of metals with low carbon solubility such as Ir or Pt,

the formation of multilayer graphene is generally absent, since once the

surface is covered with graphene, the graphene growth reaction stops. at

high temperatures also in these metals carbon can be stored in the bulk,

(a)

(c)

(b)

(d)

10.13 Repeated sequences (a–d) of CVD growth of graphene on

Ir(111) and selective high temperature (856°C) oxygen etching

(at a partial pressure of 5 ¥ 10

–8

mbar), as observed in situ by

photoemission electron microscopy (ﬁeld of view, 102 mm). The

rotational variant with carbon zigzag rows parallel to dense-packed

rows of Ir(111) appear brighter, while the other variant appears in

grey. Ir(111), due to its high work function, appears in black. The

procedure leads to the selective etching of the second variant and to

the prominent formation of the ﬁrst one with a 99% yield (from van

Gastel, 2009, © American Institute of Physics).

ThinFilm-Zexian-10.indd 248 7/1/11 9:42:59 AM



249Epitaxial growth of graphene thin ﬁlms on single crystal metal surfaces

yet in much lower proportions than in Ni or ru. Cooling down leads to the

segregation of this carbon exclusively at the location of defects in the rst

graphene layers (e.g. holes, substrate step bunches), promoting the local

growth of multilayer graphene (Sutter, 2009a). It seems that only small

multilayer domains (with extensions of a few microns), rather than large

surfaces, can be obtained that way (Sutter, 2009a; Starodub, 2010; Meyer

zu heringdorf, private communication).

10.4 Future trends

an important direction for research is the growth of graphene on thin metallic

lms. This is the rst step towards cheap graphene production on a large

scale (Bae, 2009) which may be followed by the transfer of the graphene

onto suitable supports following the etching of the metallic lm (Kim, 2009;

Reina, 2009a). So far polycrystalline thin lms were employed in this context,

yet the electronic properties of the samples are remarkably high. Controlling

the number of graphene layers, ultimately down to a single one, and the

crystallinity of the lattice to further increase the electronic performance will

require the optimization of the growth conditions (Li, 2010) and the use of

metallic thin lms with much better structural quality. Educated choices

for the nature of the metallic lms should also allow better control of the

number of layers (Li, 2009) and reducing the growth temperatures which

are prohibitively high at present (~1000°C). In this latter respect the plasma-

enhanced growth, which provides carbon species in an already active form

at the surface, is a promising approach, which is not only applied on metal

surfaces (Ismach, 2010; Rümmeli, 2010). Graphene growth on metals from

metallic carbon sources is also an interesting alternative route (hofrichter,

2009; Juang, 2009; Xi, 2011).

The experimental investigation of the interaction between graphene and

the metallic surface has made signicant progress in the last few years,

noticeably concerning the charge transfer between the two materials and

the carbon–metal hybridization, thanks to transport measurements (Huard,

2008; Blake, 2009), photoemission spectroscopy (Oshima, 1997; Grüneis,

2008; Varykhalov, 2008; Pletikosic, 2009; Lacovig, 2009; Sutter, 2009b),

and ab initio calculations (Nemec, 2006, 2008; Giovannetti, 2008; Wang,

2009; Khomyakov, 2009; Ran, 2009). Yet many questions remain open. The

effort to include van der Waals interactions in ab initio simulations has just

been initiated (Vanin, 2009) and is mandatory for quantitative understanding

of graphene–metal interaction in the case of weakly bound systems. The

identication of general trends explaining the different interactions, from

chemisorption to physisorption is in progress at the moment (Khomyakov,

2009). The connection between the strength of the graphene–metal/

carbon–metal interaction and the growth and structure of graphene is not

ThinFilm-Zexian-10.indd 249 7/1/11 9:43:00 AM



250 Thin ﬁlm growth

clearly understood so far, and should become more obvious once growth

and structure of graphene have been studied at elementary scales on a wider

variety of substrates.

The moiré lattices between graphene and transition metal surfaces were

shown to be quite efcient patterns for the self-organization of a variety of

nanometer size clusters. It was initially proven on Ir(111) substrates (N’Diaye,

2006, 2009b) and later on Ru(0001) (Donner, 2009; Pan, 2009; Zhang, 2009).

Novel magnetic, catalytic, or optical effects are being considered in these

systems.

10.5 Sources of further information and advice

Few reviews address graphene growth on metals (Wintterlin, 2009; Oshima,

1997), on SiC (de Heer, 2007), and by chemical ways (Park, 2009). The

unconventional electronic properties of graphene were reviewed in a

comprehensive article (Castro Neto, 2009). More accessible reviews on

this aspect also exist (Geim, 2007; Katsnelson, 2007). A point of view on

the prospects of graphene research and applications was proposed by Geim

(2009).

Worth mentioning in the context of this chapter, which is devoted to

graphene on metals, are spintronics effects in this system. Efcient spin-

polarization and ltering were predicted (Karpan, 2007, 2008; Yazyev, 2009)

and have started to be explored at the experimental level (Dedkov, 2008a).

Rashba effects developing at the graphene–metal interface were recently

the matter of noticeable interest (Dedkov, 2008b; Varykhalov, 2008; Rader,

2009; Rashba, 2009; Kuemmeth, 2009).

10.6 Acknowledgements

Johann Coraux acknowledges the alexander von humboldt Foundation for

a research grant.

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256

Electronic properties and adsorption

behaviour of thin films with polar character

N. Nilius, Fritz-Haber-institut der MPG, Germany

Abstract: Thin lms of polar nature are distinctively different from their

non-polar counterparts. They feature characteristic properties in their surface

morphology, electronic structure and adsorption behaviour, which originate

from the effort of such systems to lower the polarity-induced electrostatic

energy. This chapter provides insight into the general concepts of polarity

and typical polarity healing mechanisms. A main focus is the polarity of thin

oxide lms and the chapter deals with the various observable consequences

of uncompensated surface dipoles in such systems. Furthermore, the distinct

adsorption characteristic of polar lms is discussed, putting special emphasis

on the peculiar growth modes, self-assembly phenomena and charging

effects that accompany residual polarity. Finally, brief reference is made to

potential applications of polar materials.

Key words: polar thin lms, adsorption behaviour, charge transfer, self-

assembly effects, scanning tunnelling microscopy, density functional theory.

11.1 Introduction to oxide polarity

Thin lms with polar character form a particularly interesting class of materials,

as they possess a number of unusual structural and electronic properties

and a unique adsorption and reaction behaviour with respect to non-polar

systems (Noguera, 1996; Goniakowski et al., 2008). The decisive property

of a polar system is a macroscopic surface dipole that orients perpendicular

to the surface and originates from a distinct crystallographic structure of the

material. Surface polarity usually arises from the presence of elementary

cells in the atomic lattice that carry an uncompensated dipole moment. As

electrostatic dipoles are related to the occurrence of charged species in the

lattice, surface polarity is restricted to highly ionic materials and does not

occur in covalently bound or metallic systems. Prototypical polar materials

are therefore suldes, oxides and halides that are characterized by a high

level of ionicity. In this chapter, we will concentrate exclusively on polar

oxide lms, whereas information on halides and suldes can be found in

the literature.

P. W. Tasker introduced a simple three-level scheme to classify polar

systems (Tasker, 1979). Type I materials consist neither of charged atomic

layers parallel to the surface nor of unit cells with a dipole moment and are

ThinFilm-Zexian-11.indd 256 7/1/11 9:43:25 AM

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257Electronic properties and adsorption behaviour of thin ﬁlms

consequently non-polar (Fig. 11.1a). Type II systems are built of atomic planes

that carry a net charge. However those charges add up to a vanishing dipole

moment within each unit cell, rendering the ideal system non-polar again.

One recognizes immediately, however, that polarity can be introduced to such

systems if the top-most surface plane does not coincide with the boundaries

of the unit cell and a layer of uncompensated charges resides at the surface

(Fig. 11.1b, dashed line). The polarity of type II systems is therefore not

intrinsic to a certain crystallographic orientation, but depends on the surface

termination. A typical example is the Al

(0001) surface, which is non-

polar for the Al-termination as a charge-compensated unit cell that includes

the surface can be constructed, but polar for the O-termination. The same

consideration holds for the O- (non-polar) versus the Ti-terminated (polar)

surface of TiO

(110). Type III systems are intrinsically polar according to

Tasker’s scheme, because they comprise stacks of oppositely-charged atomic

planes as well as elementary cells that have a non-vanishing dipole moment

along the surface normal (Fig. 11.1c). Prominent examples are the rocksalt

crystals with (111) orientation, e.g. MgO(111), NiO(111) and FeO(111), and

the (0001) planes of wurzite, for instance ZnO(0001).

The specic properties of polar systems become evident in thicker lms.

Each unit perpendicular to the surface carries a dipole moment m of:

m = s · d [11.1]

with s being the charge density in the layer and d the layer distance. For

polar lms consisting of thousands of charge-separated planes, those dipole

moments add up to a macroscopic quantity that nally diverges in the bulk

material (Fig. 11.2). The electrostatic energy V

total

to stabilize the dipole

Type I

(a)

Type II

(b)

Type III

(c)

11.1 Classiﬁcation of ionic systems according to the Tasker scheme.

(a) and (c) are intrinsically non-polar and polar surfaces, respectively.

The polarity of (b) depends on the termination. A non-polar

surface is obtained when cutting the crystal between two dipole-

compensated unit cells, whereas polarity arises for a cut through the

unit cell (dashed line)

ThinFilm-Zexian-11.indd 257 7/1/11 9:43:25 AM

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