Role of defects in the nucleation and growth of Au nanoclusters on SiO 2 thin films

Article abstract of DOI:10.1021/jp046519p

Scanning tunneling microscopy (STM), in conjunction with the nucleation and growth of Au clusters, has been used to identify and quantify various types of defects on ordered, SiO₂ thin films grown on Mo(112). On a low-defect surface, Au clusters nucleate and grow at line defects with metal deposition at room temperature, whereas deposition at 850 K leads to cluster decoration primarily at step edges. On a highly defective surface, clusters nucleate and grow at point defects (oxygen vacancies and/or oxygen vacancy complexes) on the terraces, with some clusters grown on oxygen vacancy complexes remaining even after an 850 K anneal. The average cluster density for low Au coverages deposited at room temperature is identical to that obtained for the same Au coverage deposited at 850 K, consistent with complete titration of point defects by the nucleating clusters.

Full text of DOI:10.1021/jp046519p

J. Phys. Chem. B 2004, 108, 16339-16343
16339
Role of Defects in the Nucleation and Growth of Au Nanoclusters on SiO2 Thin Films
B. K. Min, W. T. Wallace, A. K. Santra, and D. W. Goodman*
Department of Chemistry, Texas A&M UniVersity, P.O. Box 30012, College Station, Texas 77842-3012
ReceiVed: August 3, 2004; In Final Form: September 14, 2004
Scanning tunneling microscopy (STM), in conjunction with the nucleation and growth of Au clusters, has
been used to identify and quantify various types of defects on ordered, SiO thin films grown on Mo(112).
2
On a low-defect surface, Au clusters nucleate and grow at line defects with metal deposition at room
temperature, whereas deposition at 850 K leads to cluster decoration primarily at step edges. On a highly
defective surface, clusters nucleate and grow at point defects (oxygen vacancies and/or oxygen vacancy
complexes) on the terraces, with some clusters grown on oxygen vacancy complexes remaining even after an
850 K anneal. The average cluster density for low Au coverages deposited at room temperature is identical
to that obtained for the same Au coverage deposited at 850 K, consistent with complete titration of point
defects by the nucleating clusters.
I. Introduction
II. Experimental Section
All experiments were carried out in an UHV chamber with
The growth of metals on oxide surfaces is important in
numerous technologies including electronic device fabrication
and heterogeneous catalysis. With respect to the latter, the
-10
a base pressure of 5 × 10 Torr; details of the apparatus have
1
0
1
been described elsewhere. Typically, the STM images were
acquired in the constant current mode at 1-2 V and ∼0.1 nA.
Ultrahigh purity (99.999%) oxygen from MG industries and a
Mo(112) crystal oriented to <0.25° (from Matek, Germany)
were used for the study. The Mo(112) crystal was cleaned by
oxygen treatment and high-temperature annealing (2100 K) until
no traces of carbon and oxygen were evident by Auger electron
spectroscopy (AES). The Au coverage was determined using
calibration data of Au/Mo AES ratios versus Au coverage. The
Au coverage on the SiO2 thin films was determined by
measuring the total volume of the Au particles, showing that
the coverage on the films is consistent with the AES-determined
coverage within 6%. A thermocouple (W-5%Re/W-26%Re) was
used to measure the surface temperature and to calibrate an
optical pyrometer (OMEGA OS3700) prior to disconnection for
the STM measurements. The pyrometer was then used to
measure the temperature in subsequent annealing experiments.
activity and selectivity of metal-catalyzed reactions are often
sensitive to the metal particle morphology. For example, the
catalytic activity of Au supported on certain oxides depends
2
critically on the morphology of the clusters. In general, the
morphology of metal particles on oxide surfaces depends to a
large extent on defects, particularly point defects such as oxygen
3
-8
vacancies.
Several studies have addressed the relationship
between metal cluster nucleation/growth on oxide surfaces and
defects; however, many questions remain unanswered. Most
recently, a study by Besenbacher and co-workers using scanning
tunneling microscopy (STM) showed that bridging oxygen
3
vacancies on TiO2 are active nucleation sites for Au clusters.
In addition, these authors suggested that the diffusion of a
vacancy-cluster complex plays an important role in the formation
of larger Au clusters. Using atomic force microscopy (AFM),
Barth and co-workers have quantified point defects on MgO
by estimating the Pd cluster density as a function of the
4
III. Results and Discussion
deposition surface temperature. In a theoretical study, Bogicevic
and Jennison have calculated the binding energies of various
metals to MgO and showed that dimers of noble metals are
more stable at oxygen vacancies. Using STM, Freund and co-
Ordered, ultrathin SiO2 films with a thickness of 0.35 ( 0.05
nm were synthesized by vapor deposition of Si onto a p(2×3)-O
reconstructed surface of Mo(112), oxidation at 800 K, and an
5
workers have demonstrated preferential decoration of Pd and
Rh metal particles on line defects of Al2O3.6 Very recently,
defects on crystalline SiO2 thin films have been studied in our
laboratory using ultraviolet photoelectron spectroscopy (UPS)
11
anneal in oxygen at 1100-1200 K. The resulting SiO2 thin
,7
film shows a very flat, homogeneous structure with STM, a
12
wide band gap with UPS and MIES, and very sharp c(2×2)
13
diffraction features in low energy electron diffraction (LEED).
9
and metastable impact electron spectroscopy techniques (MIES).
The method of preparation and structural characterization of
1
1,13
In this study we demonstrate that Au clusters nucleate and
grow at defect sites on SiO2 thin films. Consequently, the
nucleation and growth of the Au clusters allow assessment of
the defect density and type.
the film are described in detail elsewhere.
Frequently, defects play a key role in the nucleation and
growth of metals on oxide surfaces, and in the electronic and
14
chemical properties of metal-oxide systems. Defects on oxide
surfaces are usually classified as extended defects (e.g., steps),
1
5
line defects (e.g., antiphase domain boundaries ), and point
*
To whom correspondence should be addressed. E-mail: goodman@
1
6
mail.chem.tamu.edu.
defects (e.g., oxygen vacancies ). Defects in the SiO2 film are
1
0.1021/jp046519p CCC: $27.50 © 2004 American Chemical Society
Published on Web 09/25/2004

16340 J. Phys. Chem. B, Vol. 108, No. 42, 2004
Letters
Figure 1. STM images of a 1200 K annealed SiO
corrugations with a c(2×2) array are represented by black circles.
2
thin film (U
T
) 2.0 V and I ) 0.1 nA) (a) 200 nm × 200 nm, (b) 10 nm × 10 nm, atomic
recently, line defects on an Al2O3 thin film grown on NiAl(110)
arising from antiphase domain boundaries was reported by
1
5
Kulawik and co-workers. A precise structure of this line-type
defect is not possible because of the limited resolution of the
STM images.
The STM images in Figure 3a,b show the morphologies of
SiO2-supported clusters grown at room temperature (RT) with
Au coverages of 0.10 and 0.40 monolayer (ML), respectively.
At 0.10 ML the Au clusters nucleate at terrace and step edge
sites with no apparent orientation of the clusters with respect
to the substrate. However, the Au clusters grown at 0.40 ML
(Figure 3b) are aligned along the [1,-1,0] direction, consistent
with nucleation and growth along line defects. The decoration
of line defects with metal clusters has been observed previously
6
,7
for Pd and Rh clusters grown on Al2O3 thin films. Au cluster
growth was essentially independent of the rate of metal
deposition (see Figure 3c), consistent with heterogeneous cluster
nucleation. Cluster growth at a surface temperature of 850 K
(Figure 3d) shows that step edges, rather than line defects, are
decorated, implying that step edges are more stable sites for
cluster nucleation and growth.
To investigate the influence of oxygen vacancies on cluster
nucleation, a SiO2 film with a relatively high density of defects
was prepared using an anneal temperature of 1100 K. Although
the film annealed at 1100 K yielded very similar LEED, AES,
and STM data as for a 1200 K annealed film, the growth of Au
clusters was dramatically different, as can be seen in Figure
Figure 2. A 10 nm × 10 nm STM image with the c(2×2) structure
highlighted by white circles. Areas A and B indicate two different
domains separated by a line defect.
evident in the STM images of Figure 1. The steps separating
the terraces in the STM image of Figure 1a originate from the
Mo(112) substrate. In the higher resolution STM image of
Figure 1b, the c(2×2) structure of the substrate is indicated by
small, black open circles. The lattice spacing between adjacent
atoms of the c(2×2) atomic array is ∼0.5 nm, essentially the
same as that found by LEED (0.52 nm). In addition to this
c(2×2) atomic corrugation, some dark contrast spots with one
atomic corrugation size are also seen in the STM image.
A line-type feature with sharp contrast along the [1,-1,0]
direction is apparent in the STM image of Figure 2. Close
inspection shows that the white circles, drawn coincident with
the atomic corrugations on both sides of the line-type feature,
are out-of-registry with respect to each other, with a mismatch
of approximately 0.22 nm. This mismatch is equal to one-half
the distance between two Mo atomic rows (0.445 nm) in the
4
a-d. Whereas on a low-defect (1150-1200 K annealing
temperature) SiO2 film the Au clusters align along line defects,
on the high-defect film (<1150 K annealing), the clusters
nucleate primarily on terrace sites. Also on the low-defect film,
clusters nucleate extensively at step edges, in contrast to little
step-edge decoration for the high-defect film. To test further
whether nucleation and growth of Au nanoclusters correlates
with the point defect density, the nucleation and growth of Au
clusters was carried out at a relatively high surface temperature
(850 K). The results are shown in Figure 4d where the number
density of Au clusters is similar to that for nucleation and growth
at room temperature (Figure 4a).
Although the precise nature of the point defects on our films
is uncertain, the most probable species are oxygen vacancies.
Chemical defects such as silanol groups are unlikely because
our films are annealed to at least 1100 K, well above the
[1,-1,0] direction, and therefore likely due to two domains of
SiO2. That line defects in SiO2 thin films on Mo(112) arise due
to antiphase domain boundaries has previously been suggested
1
8,20
temperature at which such species are stable.
In addition,
by Freund and co-workers on the basis of the significant
no hydroxyl features are observed in the as-prepared films by
high-resolution electron energy loss spectroscopy (HREELS).
broadening of the superlattice spots in LEED.1
8,19
Also, very

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J. Phys. Chem. B, Vol. 108, No. 42, 2004 16341
Figure 3. STM images (200 nm × 200 nm, U
T
2
) 1.8 V and I ) 0.08 nA) of Au clusters on a 1200 K annealed SiO film after deposition of (a)
0
.10 ML of Au at 0.010 ML/min, surface temperature of 300 K; (b) 0.40 ML of Au at 0.010 ML/min, surface temperature of 300 K; (c) 0.33 ML
of Au at 0.033 ML/min, surface temperature of 300 K; and (d) ∼0.4 ML at 0.010 ML/min, surface temperature of 850 K.
3
To further test the stability of the various defect sites, different
coverages of Au were deposited on a highly defective SiO2
surface followed by an anneal to 850 K. The number densities
of Au clusters obtained at each state are shown in Figure 5.
For the 0.2, 0.7, and 1.3 ML Au coverages, the number densities
after an anneal to 850 K show constant values irrespective of
Au coverage. Additionally, the cluster number density present
upon deposition of 0.6 ML Au at a surface temperature of
require several oxygen vacancies. According to this model, the
number of Au atoms in a 1.4 nm cluster is ∼20 and would
require ∼7 oxygen vacancies.
As seen in Figure 6, the density of Au clusters above 0.1
ML increases and saturates between 0.3 and 0.6 ML. The
increase in cluster density from 0.1 to 0.3 ML indicates that
there are nucleation sites in addition to the point defects
described above. After reaching saturation, the Au cluster density
is essentially constant whereas the average cluster size increases.
This suggests that deposited adatoms are captured by existing
Au clusters within the saturation coverage range up to 1.3 ML.
850 K is indicated at the right of the figure. As can be seen, the
density at this deposition temperature is similar to that after
the other coverages have been annealed. These data indicate
the saturation of especially stable nucleation points on the
terraces.
Finally, on the basis of our STM observations, the relative
stability of Au nanoclusters on various defect sites can be
estimated. As seen in Figure 4d, Au clusters do not preferentially
decorate step edge sites even during deposition at a high surface
temperature. Therefore, point defects with several oxygen
The average cluster density as a function of Au coverage,
estimated from the STM images of several scanned areas, is
shown in Figure 6. This figure shows three distinct regions
corresponding to Au coverages of <0.17, 0.17-0.33, and >0.33
ML. Up to ∼0.17 ML the cluster density is essentially
independent of the Au coverage. The cluster density rapidly
increases between ∼0.17 and 0.33 ML, after which the cluster
density remains essentially constant.
21
vacancies (oxygen vacancy complexes) are more stable for
cluster growth than step edges. In addition, as mentioned
previously, step edges are more stable sites than line defects
for cluster growth (Figure 3d). It should be noted that cluster
growth related to point defects disappears upon an anneal to
1150-1200 K in an oxygen environment and cluster nucleation
changes to decoration of line defects, although oxygen vacancies
are still expected to be present. However, most vacancies are
single oxygen vacancies and therefore do not play a role as
active nucleation and growth sites for Au clusters. These do,
however, compete with line defects, which are more stable sites
for Au nanoclusters. On the basis of these results, the stability
These results suggest that the low coverage region with a
constant density of Au clusters correlates with the number of
point defects (oxygen vacancies). However, the precise number
of neighboring oxygen vacancies required for nucleation and
growth of clusters is difficult to assess. According to Besen-
bacher and co-workers, single oxygen vacancies on TiO2 bind
approximately three Au atoms, whereas larger Au clusters

16342 J. Phys. Chem. B, Vol. 108, No. 42, 2004
Letters
Figure 4. STM images (200 nm × 200 nm, U
T
) 1.8 V and I ) 0.08 nA) of Au clusters on a 1100 K annealed SiO
2
film after deposition of (a)
0
.17 ML of Au at 0.033 ML/min, surface temperature of 300 K; (b) 0.33 ML of Au at 0.033 ML/min, surface temperature of 300 K; (c) 1.32 ML
of Au at 0.033 ML/min, surface temperature of 300 K; and (d) ∼0.3 ML of Au at 0.033 ML/min, surface temperature of 850 K.
Figure 5. Bar graph representation of the average cluster number
density on a highly defective SiO surface under different Au coverages.
For 0.2, 0.7, and 1.3 ML Au coverages, the cluster density both before
and after an anneal to 850 K are shown. For 0.6 ML Au coverage, the
cluster density obtained upon deposition at a surface temperature of
2
Figure 6. Plot showing the Au cluster densities obtained as a function
of Au coverage on a highly defective SiO film upon deposition at
room temperature. Three distinct regions can be seen, which correspond
to Au coverages of less than 0.17 ML, between 0.17 and 0.33 ML,
and greater than 0.33 ML.
2
8
50 K is shown. The constant cluster densities obtained by annealing
to 850 K or depositing at 850 K indicate the saturation of especially
stable nucleation sites on the terraces.
However, higher densities of Au nanoclusters are observed on
the terrace sites of a SiO2 thin film prepared at a lower annealing
temperature. A plot of the number density of Au clusters with
respect to Au coverage shows that the nucleation and growth
of Au clusters is initially dominated by oxygen vacancy
complexes.
of Au nanoclusters on various defect sites is anticipated to be
(
>
from high to low): oxygen vacancy complexes > step edges
line defects > single oxygen vacancies.
IV. Conclusions
On the basis of our STM studies, Au nanoclusters can be
used to highlight defects on SiO2 thin films with cluster growth
related to a specific defect feature. On a well-prepared SiO2
thin film, Au nanoclusters preferentially decorate line defects.
Acknowledgment. We acknowledge with pleasure the
support of this work by U.S. Department of Energy, Office of
Basic Energy Science, Division of Chemical Sciences, the

Letters
J. Phys. Chem. B, Vol. 108, No. 42, 2004 16343
Robert A. Welch Foundation, and the Texas Advanced Tech-
nology Program under Grant no. 010366-0022-2001.
(11) Ozensoy, E.; Min, B. K.; Santra, A. K.; Goodman, D. W. J. Phys.
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(12) Wendt, S.; Kim, Y. D.; Goodman, D. W. Prog. Surf. Sci. 2003,
7
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