Epitaxially Driven Formation of Intricate
Supported Gold Nanostructures on a
Lattice-Matched Oxide Substrate
Gabriel A. Devenyi,
†,‡,§
Jianfeng Li,
⊥,#
Robert A. Hughes,
‡,¶
An-Chang Shi,
‡,#
Peter Mascher,
†,§
and John S. Preston*
,†,‡
Department of Engineering Physics, McMaster UniVersity, Hamilton Ontario, Canada
L8S 4L7, Brockhouse Institute for Materials Research, McMaster UniVersity, Hamilton
Ontario, Canada L8S 4M1, Center for Emerging DeVice Technologies, McMaster
UniVersity, Hamilton, Ontario, Canada L8S 4L8, Department of Macromolecular
Science, Fudan UniVersity, Shanghai 200433, China, and Department of Physics and
Astronomy, McMaster UniVersity, Hamilton, Ontario, Canada L8S 4M1
Received July 31, 2009; Revised Manuscript Received October 9, 2009
ABSTRACT
A new class of gold nanostructures has been fabricated on the (100), (111), and (110) surfaces of lattice-matched MgAl
2
O
4
substrates. The
nanostructures were fabricated through a synthesis route where a thin gold film dewets, liquefies, and then slowly self-assembles. The supported
nanostructures are intricately shaped, crystalline, and epitaxially aligned. Simulations based on a continuum elastic theory indicate that the
self-assembly is driven by strained epitaxy and minimization of the surface free energy.
Nanostructures derived from noble metals catalytically
enhance chemical processes,
1
exhibit intense localized surface
plasmon resonances,
2
mediate nanowire growth via the
vapor-liquid-solid mechanism,
3
and are easily bioconju-
gated.
4
As a result, they are objects of great interest for
optical, chemical, and biological applications. Such structures
are now routinely fabricated in aqueous solutions using seed-
mediated chemical growth modes.
5,6
Photochemical
7
and
electrochemical
8
methods are also used and follow similar
growth pathways, but with photons or electric fields trig-
gering the reaction mechanism. Particle shapes produced by
such reactions are widely varying, depending on the func-
tionalization of the noble metal atoms, the concentration and
makeup of the components within the solution, and the
catalytic method used. A wide variety of shapes including
nanospheres,
9
rods,
10
triangular nanoprisms,
7
shells,
11
disks,
12
stars,
13
and cubes
14
are now routinely fabricated. Solution-
based plasmonic nanostructures have also received significant
attention in terms of potential applications in fields such as
biological detection, cancer diagnosis, and photothermal
therapy.
4
While solution-based noble metal nanostructures have
shown tremendous potential, it should be recognized that it
is often highly desirable to have these nanostructures placed
on a substrate in a manner which renders them immobile.
Such nanostructures have been used to enhance solar cell
efficiencies,
15
enhance the emission from light-emitting
diodes,
16
detect biological agents,
17
measure biological
distances,
18
achieve nonlinear optical properties
19
and nega-
tive indices of refraction
20
through the formation of metama-
terials, and enhance the Raman signal from surface-adsorbed
species.
21
Numerous routes exist for fabricating these substrate-
based nanostructures, including the attachment of function-
alized solution-based nanoparticles to the surface,
17
growth
off of surfaces seeded with linked nanoparticles,
22
litho-
graphically patterning continuous thin films,
23
and self-
assembly.
24
Among these various techniques, self-assembly
stands out as an attractive means of forming substrate-
supported nanostructures over large areas because there exists
the potential to engineer the shape, size, and crystallographic
orientation of the nanostructures through substrate-imposed
strains, epitaxy, crystallographic symmetries, substrate sur-
face morphology, interface chemistry, and wettability. Other
advantages of these self-assembly methods are that the
nanostructures often become well-bonded to the substrate
and can show a preferential alignment due to epitaxy, a
property which can be exploited to enhance the anisotropic
* To whom correspondence should be addressed. prestonj@mcmaster.ca.
†
Department of Engineering Physics, McMaster University.
‡
Brockhouse Institute for Materials Research, McMaster University.
§
Center for Emerging Device Technologies, McMaster University.
⊥
Fudan University.
#
Department of Physics and Astronomy, McMaster University.
¶
Present address: Department of Mechanical Engineering, Temple
University, 1947 N. 12th St., Philadelphia, PA 19122.
NANO
LETTERS
2009
Vol. 9, No. 12
4258-4263
10.1021/nl902491g CCC: $40.75 2009 American Chemical Society
Published on Web 10/20/2009

optical properties associated with arrays of asymmetric
plasmonic nanostructures.
25,26
Growth of substrate-based metallic nanostructures via self-
assembly has resulted in the formation of a myriad of shapes
including triangular prisms,
27
square and rectangular is-
lands,
28,29
faceted spheres,
30
rings,
31
and horseshoes.
32
Oxide
substrates, such as mica and SrTiO
3
, have been particularly
effective in the production of supported nanostructures due
to their chemical and thermal stability, crystallographic
perfection, and the wide variety of accessible surface
reconstructions.
27-29,33-35
Of considerable relevance to this
work is a series of reports by Silly et al.
27-29,34-37
detailing
the self-assembly of intricate gold, silver, palladium, copper,
and iron nanocrystals on surface-reconstructed [001] SrTiO
3
substrates at varied substrate temperatures. Scanning tun-
neling microscopy (STM) measurements indicate a self-
assembly process which is strongly influenced by the
properties of the underlying substrate.
In this report, we present an approach for the epitaxially
driven formation of self-assembled gold nanostructures on
MgAl
2
O
4
substrates. This substrate allows for excellent gold
epitaxy since its lattice constant of a ) 8.083 Å is
approximately two times the gold lattice constant of a )
4.078 Å (compressive mismatch ) 0.89%). The nanostruc-
tures, formed on [100]-, [111]-, and [110]-oriented substrates,
were intricate, aligned, and have not been previously
observed. The structures have been observed via scanning
electron microscopy (SEM) over several months and are
stable. The shapes of the nanostructures formed were in
excellent agreement with those predicted by modeling based
on a continuum elastic theory.
The gold nanostructures were formed through the deposi-
tion of gold films on MgAl
2
O
4
substrates (MTI Corp.)
followed by an annealing procedure which facilitated film
dewetting and nanostructure formation. The films were
sputter-coated at room temperature to a thickness of 5 Å-15
Å with a GATAN PECS Model 682 ion beam coating/
etching system. The samples were then placed in a tube
furnace with a 100 sccm flow of argon, heated to 1100 °C
in 45 min, and then held at that temperature for 1 h.
Following this treatment, the sample was cooled to 1000 °C
in 30 min, held at that temperature for an additional hour,
and then allowed to cool to room temperature over an interval
of approximately 8 h. Holding the temperature at both 1100
and 1000 °C was crucial to the formation of the nanostruc-
tures described here. Removal of either step results in the
formation of faceted gold spheres sitting directly on the
substrate.
Scanning electron microscopy (SEM) images of the gold
nanostructures formed on the (100), (111), and (110)
MgAl
2
O
4
substrates, obtained using a JEOL-7000F SEM in
secondary electron mode, are shown in Figure 1. For each
substrate orientation, one observes two types of features, (i)
spheres supported by a necking region attached to a geo-
metrically shaped base (Figure 1a-c) and (ii) standalone base
structures (Figure 1d-f). Convergent beam electron diffrac-
tion (CBED) performed using a Phillips CM12 confirmed
that the supported spheres are crystalline. For each case, the
shape of the base structure reflects the underlying symmetry
of the substrate which is four-fold, three-fold, and two-fold
symmetric for the (100), (111), and (110) surfaces, respec-
tively. X-ray diffraction measurements, using a Bruker 6000
CCD detector on a Bruker three circle D8 goniometer with
a Rigaku RU-200 rotating anode Cu K
R
X-ray generator and
parallel-focusing mirror optics, were used to determine the
substrate orientation relative to the edges of the base
structures and are denoted on the three top-down SEM
images (Figure 1d-f). The crystallographic alignment of
these nanostructures is a clear indication of epitaxy and
is strongly suggestive of {111} gold faceting of the base
structures associated with the [100]- and [111]-oriented
substrates. For the (110) surface, the standalone base
structures are ill-defined and show no obvious faceting, while
those formed in combination with a sphere show shapes
consistent with mixed faceting, possibly having {111} and
{100} facets for the short and long dimension, respectively.
The standalone base dimensions are remarkably uniform with
side lengths of 40, 65, and 65 nm × 110 nm for the (100),
(111), and (110) substrates, respectively.
While there are two basic types of nanostructures formed
on each substrate orientation, these structures are found in
various stages of development. For the most part, the bases
are well-developed and show little size variation. The
spherical structures, however, vary dramatically both in their
size and position relative to the base structures. Figure 2
shows a series of top-down SEM images for the case of the
(111) MgAl
2
O
4
substrate showing an evolution of the
nanostructures from a standalone triangular base to bases
supporting spheres of increasing size. Notable is the fact that
the nanostructure, shown in Figure 2b, manifests itself as a
small sphere which is offset from the center of the base while
Figure 1. SEM images showing the gold nanostructures formed
on MgAl
2
O
4
substrates. The three upper images show spheres
supported by a necking region attached to a geometrically shaped
base for the (a) [100]-, (b) [111]-, and (c) [110]-oriented substrates.
Each of these images was taken at a 70° tilt. The aura seen around
nanostructures is an artifact of imaging. The three lower images
show the top-down view of both standalone base structures and
supported spheres for the (d) [100]-, (e) [111]-, and (f) [110]-
oriented substrates. The in-plane Miller indices of the substrate are
denoted on each of these images. For all cases, the samples were
coated with a thin layer of platinum to improve imaging. Imaging
without platinum shows the same structures but is of poor quality
due to substrate charging effects.
Nano Lett.,Vol. 9, No. 12,009 4259