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Thermodynamically Controlled Self-Assembly of Two-Dimensional Oxide Nanostructures.

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Communications
Cluster Compounds
Thermodynamically Controlled Self-Assembly of
Two-Dimensional Oxide Nanostructures**
Johannes Schoiswohl, Svetlozar Surnev, Michael Sock,
Michael G. Ramsey, Georg Kresse, and Falko P. Netzer*
The self-assembly of molecules or small clusters, that is, the
spontaneous association of atomic or molecular building
blocks under equilibrium conditions, is emerging as a
successful chemical strategy to fabricate well-defined structures of nanometer dimensions, with potential applications in
many areas of nanotechnology.[1] This bottom-up approach of
synthesis is a promising way to design novel nanoscale
functional materials with atomic precision. The construction
of complex supramolecular aggregates from organic molecular building blocks by the self-assembly route has been
successfully demonstrated recently.[2, 3] Herein, we explore the
possibility of fabricating surface-supported nanoscale oxide
materials in low dimensions by a chemically driven selfassembly process with oxide cluster molecules. As opposed to
the usual molecular self-assembly, where the construction
units are deposited directly from the gas phase, the oxide
building blocks with a unique uniform stoichiometry and
structure form spontaneously on a metal surface. These
building blocks can then be organized into different twodimensional (2D) oxide structures by careful adjustment of
the chemical potential of oxygen mO, which allows the
controlled design of oxide nanostructures on a metal surface.
This process is demonstrated with the formation and subsequent aggregation of planar vanadium oxide [V6O12] clusters,
monitored in situ, under ultrahigh vacuum (UHV), on an
Rh(111) surface at the atomic level by scanning tunneling
microscopy (STM). The fabrication of ultrathin layers of
vanadium oxides has potential commercial interest in the socalled monolayer catalysts, in advanced coating systems, and
in optical devices.[4]
The [V6O12] clusters form spontaneously on an Rh(111)
surface after deposition of vanadium atoms at submonolayer
coverages under appropriate temperature and sufficiently
oxidizing conditions (Figure 1 a). They are a unique, novel
form of “oxide molecules” with a planar 2D hexagonal
structure (Figure 1 b), which is stabilized by interaction with
the rhodium surface but is unstable in the gas phase.[5]
Ab initio density functional theory calculations[6–9] have
[*] J. Schoiswohl, Prof. Dr. S. Surnev, M. Sock, Prof. Dr. M. G. Ramsey,
Prof. Dr. F. P. Netzer
Institut f#r Experimentalphysik
Karl-Franzens-Universit*t Graz
8010 Graz (Austria)
Fax: (+ 43) 316-380-9816
E-mail: falko.netzer@uni-graz.at
Prof. Dr. G. Kresse
Institut f#r Materialphysik
Universit*t Wien, 1090 Wien (Austria)
[**] This work was supported by the Austrian Science Fund FWF and the
Austrian Academy of Sciences (DOC).
5546
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200460150
Angew. Chem. Int. Ed. 2004, 43, 5546 –5549
Angewandte
Chemie
Figure 1. a) Single [V6O12] cluster molecules (470 A 470 B2 ; sample bias
U = + 2 V; tunneling current I = 0.2 nA), b) top: high-resolution STM
image of a [V6O12] cluster (22 A 13 B2 ; U = + 0.5 V; I = 0.1 nA), bottom:
DFT simulated STM image, c) DFT model geometry of the planar V6O12
cluster on Rh(111), d–f) Condensation of [V6O12] clusters at elevated
temperature (100–110 8C) into dimers (2) and trimers (3) (d,e) and
larger aggregates (f). Data d): 155 A 140 B2 ; U = + 1 V; I = 0.1 nA, e):
50 A 50 B2 ; U = + 1 V, I = 0.1 nA, f): 60 A 60 B2 ; U = + 1 V, I = 0.1 nA).
revealed the stoichiometry and the geometry of these starlike
structures (Figure 1 c): the stars consist of six vanadium atoms
which are each coordinated to three rhodium atoms of the
surface. The vanadium atoms are linked by bridging oxygen
atoms; each vanadium atom is attached to a further oxygen
atom, which is positioned above a surface rhodium atom, to
give the V6O12 stoichiometry with a formal VIV oxidation
state. The simulated STM image, (Figure 1 b bottom), reflects
exactly the experimental STM image (Figure 1 b, top), and
thus confirms the proposed model.
The [V6O12] “star” clusters become mobile on the surface
in UHV at elevated temperature ( 100 8C) and diffuse over
the Rh(111) surface.[5] They condense into dimers and trimers,
which appear to be metastable but kinetically stabilized for a
certain time, before they are observed to separate again.
Figure 1 d shows an STM snapshot image of an area of the
star-covered rhodium surface taken at 110 8C, where single
stars, dimers (marked 2), trimers (marked 3), and somewhat
larger cluster aggregates are visible. The formation of these
clusters is driven by reduction. The observed dimer, for
instance, is formed by attaching two stars along one common
side in which two vanadium atoms are shared by the two
adjoined hexagons (Figure 1 e). Its formal stoichiometry is
V10O19 corresponding to a lower oxidation state than the
original stars. The STM image of Figure 1 f has been recorded
under slightly more reducing conditions (in a slightly elevated
background pressure of hydrogen). There, the “stars” have
aggregated into a larger cluster, which is the precursor of the
oxide nano-islands (see below).
In a reducing environment (pH2 108 mbar and at 250 8C
substrate temperature) the [V6O12] clusters assemble into
nano-islands with a well-ordered 2D vanadium oxide monolayer structure (Figure 2 a). Between the vanadium–oxide
islands bright dots are still visible, which correspond to star
clusters, which have not yet been incorporated into the
rectangular structure. The STM image of the inset of
Angew. Chem. Int. Ed. 2004, 43, 5546 –5549
p
Figure 2. a) Assembly of [V6O12] clusters into the rectangular (5 A 3 3)
vanadium–oxide structure under reducing conditions (1000 A 1000 B2 ;
U = + 1.5 V; I = 0.1 nA). Inset: Enlarged section of an island boundary,
p
with the (5 A 3 3)-rect unit cell indicated (70 A 70 B2 ; U = + 0.5 V;
p
I = 0.1 nA). b) DFT derived model of the (5 A 3 3)-rect structure, the
=
unit cell (white square) and O4V O structural units (black circle) are
indicated (V green, O red, Rh gray). Inset: Simulated STM image.
p
Figure 2 a displays the rectangular (5 > 3 3)-rect structure at
higher magnification, which shows details of the structure and
the incorporation of the stars in the vanadium–oxide-island
p
boundary. The geometry of the (5 > 3 3)-rect structure has
been resolved with the help of DFT calculations and
measurements of the phonon spectra.[10] The structure
model as established by DFT is shown in Figure 2 b, the
inset displays a simulated STM image according to this model.
There is excellent agreement between the simulated and
experimental STM images. The structure corresponds to a
V13O21 stoichiometry per unit cell, which contains two
hexagonal [V6O12] clusters rotated by 308 and interlinked by
bridging oxygen atoms plus one O4V=O unit (black circles in
Figure 2 b). Note that the O4V=O units are not in conflict with
the maximal VV oxidation state of the V atoms, since the four
bridging oxygen atoms are also shared with the rhodium
p
substrate. The (5 > 3 3)-rect structure is a novel mixed-valent
vanadium oxide phase, which occurs only in ultrathin layer
form and does not exist as a stable bulk phase. The
comparison of the high-resolution STM image of the (5 >
p
3 3)-rect structure in the inset of Figure 2 a with the STM
image of the aggregate in Figure 1 f indicates that the latter
p
already contains the essential elements of the (5 > 3 3)-rect
unit cell. The cluster island of Figure 1 f constitutes therefore
p
a critical nucleus of the (5 > 3 3)-rect structure.
Under oxidizing conditions (pO2 = 1 > 108 mbar at T 250 8C), the [V6O12] clusters aggregate into a different oxide
structure (Figure 3 a), where nano-islands of a (5 > 5) vanadium–oxide structure are recognized. The stable DFT model
of this structure is shown in Figure 3 b. AV11O23 stoichiometry
per unit cell is derived from the model, that is, this oxide phase
has a somewhat higher overall oxidation state than the
[V6O12] clusters. The structure contains V6O12, O4V=O, and
O3V=O units, the latter two are indicated by the circles in
Figure 3 b. Again, the (5 > 5) vanadium–oxide layer is not a
stable bulk phase, but a novel 2D oxide layer stabilized by the
metal–oxide interface. The simulated STM image based on
the (5 > 5) model structure (inset Figure 3 b) agrees very well
with the experimental high-resolution STM image (inset
Figure 3 a). The crucial difference between the condensation
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2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5547
Communications
Figure 3. a) Aggregation of [V6O12] clusters into the (5 A 5) vanadium
oxide structure under oxidizing conditions (1000 A 1000 B2 ; U = + 2 V;
I = 0.02 nA). Inset: Magnified view of the (5 A 5) island (70 A 70 B2 ;
U = + 1.2 V; I = 0.1 nA). b) DFT derived model of the (5 A 5) structure,
the unit cell (white) and O4V=O and O3V=O units (black circles) are
indicated (V green, O red, Rh gray). Inset: Simulated STM image.
process under oxidizing and under reducing conditions is that
under reducing conditions the stars condense by connecting
mainly directly at the corners, whereas under oxidizing
conditions oxygen-rich tetrahedral and pyramidal units
(O3V=O and O4V=O) are inserted between the star clusters.
The physical origin of the observed self-assembly of
[V6O12] clusters into different 2D oxide structures on the
Rh(111) surface, depending on reducing or oxidizing conditions during production, can be understood by the surface
phase stability diagram of vanadium oxides on Rh(111), as
calculated by DFT. Figure 4 presents an appropriate section
of the V–O/Rh(111) surface phase diagram in form of a plot
of the chemical potential of oxygen mO versus the vanadium
atom concentration cV at the surface, given in monolayers
(ML) of vanadium atoms, where the regions of stability of the
respective oxide phases are indicated by the black bars. The
position on the abscissa of the bars corresponds to the
concentration of vanadium atoms in a full monolayer of the
Figure 4. Section of the thermodynamic DFT phase diagram of vanadium oxide on Rh(111) in equilibrium with an oxygen reservoir which
controls the chemical potential mO. mO is related to the oxygen partial
pressure through the ideal gas equation mO(T,p) = mO(T,p0)-RT/2 ln (p/
p0).[12] Vertical black bars at the nominal vanadium coverage of each
phase correspond to the mO region of stability. The arrows indicate the
condensation of the [V6O12] clusters into the different 2D island structures under more oxidizing or reducing conditions.
5548
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
respective oxide phase. At a vanadium coverage of cV = 0, the
bare rhodium surface is covered by different chemisorbed
oxygen phases under oxidizing conditions (higher mO), or
it is
pffiffiffiffiffi
clean
under
reducing
conditions
(low
m
).
A
(
13
>
O
pffiffiffiffiffi
13)R13.88 vanadium–oxide phase has been observed on
the Rh(111) surface for cV 0.45 ML under highly oxidizing
conditions,[11] however, this structure is not relevant for the
present discussion. The [V6O12] stars form at low vanadium
coverages in the gray region. Their region of stability is
coexistent in terms of mO with the stability regions of both the
p
(5 > 5) and the (5 > 3 3)-rect structures, and in principle the
[V6O12] stars could condense into islands of one or the other
of these structures. However, a change of mO (corresponding
to moving up or down the diagram), provides an additional
driving force for the condensation of stars into one or the
other structure: for increasing mO the (5 > 5) structure
becomes more stable, whereas for decreasing mO the (5 >
p
3 3)-rect structure is more favorable (see arrows in
Figure 4). The application of oxygen or hydrogen from the
gas phase provides the experimental means for changing mO
and selecting the condensation process. In addition, the
elevated temperature applied during the self-assembly
experiments increases the mobility of the stars, thus reducing
kinetic barriers in the self-assembly process.
The self-assembly process of inorganic vanadium–oxide
clusters into metal-supported 2D vanadium oxide nanostructures is different in two respects from the reported
aggregation of supramolecular networks of organic molecules
on surfaces.[2, 3] 1) the “molecular” building blocks of the
aggregation, that is, the planar 2D [V6O12] clusters, do not
exist as stable entities in the gas phase but form spontaneously
at the Rh(111) surface under the appropriate thermodynamic
conditions; they constitute a novel kind of cluster material,
that is stabilized by the metal-cluster interface, 2) the driving
force for the self-assembly is provided by the chemical
potential of oxygen, and the precise control of mO allows us to
steer the aggregation into different patterns, that is, into
different 2D vanadium–oxide structures. The assembly process occurs by reductive or oxidative condensation, thus
chemical bonding interactions with partly covalent and ionic
character are involved. This situation is in contrast to the
noncovalent interactions, which are responsible for the
attachment of organic molecules into supramolecular networks.[2, 3] Despite the much stronger bonding interactions, the
herein reported vanadium–oxide-cluster aggregation is reversible: by careful adjustment of the chemical potential of
oxygen and the surface temperature, the [V6O12] star molecules can be regenerated at the Rh(111) surface by 2D rep
evaporation from both (5 > 5) and (5 > 3 3)-rect vanadium–
oxide phases. The self-assembly of oxide cluster “molecules”
and their controlled aggregation into 2D oxide structures has
been observed herein for the one particular vanadium–oxide/
Rh system. It is likely that this occurs also on other oxide–
metal surfaces: the described approach may open up a new
route for the fabrication of novel oxide materials with
nanoscale dimensions.
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Angew. Chem. Int. Ed. 2004, 43, 5546 –5549
Angewandte
Chemie
Experimental Section
STM imaging has been performed in a variable-temperature STM
system at room temperature or at elevated temperature. The [V6O12]
clusters were prepared by depositing 0.05 monolayer (ML) of
vanadium atoms at RT onto the oxygen-precovered Rh(111)2 > 1
surface (O precoverage: 0.5 ML). The surface was heated subsequently to 300–350 8C for 30 seconds and then cooled to RT. The DFT
calculations were performed using the program VASP[6–8] and a plane
wave cutoff of 250 eV. The STM simulations are based on the Tersoff–
Hamann approach.[9] The charge iso-surfaces were evaluated at a
value selected so that the brightest spots are located 4 C above the
core of the topmost atom in the oxide. The experimental bias
conditions were mimicked in the simulations.
Received: March 29, 2004
.
Keywords: cluster compounds · density functional calculations ·
oxides · self-assembly · vanadium
Angew. Chem. Int. Ed. 2004, 43, 5546 –5549
[1] G. M. Whitesides, J. P. Mathias, C. T. Seto, Science 1991, 254,
1312.
[2] T. Yokoyama, S. Yokoyama, T. Kamikado, Y. Okuno, S.
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[3] A. Dmitriev, H. Spillmann, N. Lin, J. V. Barth, K. Kern, Angew.
Chem. 2003, 115, 2774; Angew. Chem. Int. Ed. 2003, 42, 2670.
[4] S. Surnev, M. G. Ramsey, F. P. Netzer, Prog. Surf. Sci. 2003, 33,
117.
[5] J. Schoiswohl, G. Kresse, S. Surnev, M. Sock, M. G. Ramsey, F. P.
Netzer, Phys. Rev. Lett. 2004, 92, 206 103.
[6] G. Kresse, J. FurthmJller, Comput. Mater. Sci. 1996, 6, 15.
[7] Y. Wang, J. P. Perdew, Phys. Rev. B 1991, 44, 13 298.
[8] G. Kresse, D. Joubert, Phys. Rev. B 1999, 59, 1758.
[9] J. Tersoff, D. R. Hamann, Phys. Rev. B 1985, 31, 805.
p
[10] The phonon spectra of the (5 > 3 3)-rect structure were
measured in high-resolution electron energy loss experiments
and are in excellent agreement to the calculated phonon spectra
according to the proposed DFT structure model.
[11] J. Schoiswohl, S. Surnev, M. Sock, S. Eck, G. Kresse, M. G.
Ramsey, F. P. Netzer, Phys. Rev. B 2004, 69, 155 403.
[12] K. Reuter, M. Scheffler, Phys. Rev. B 2002, 65, 0354 061.
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2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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