J. Phys. Chem. B 2003, 107, 13597-13600
13597
Two-Dimensional Au and Au-Cu Alloy Nanocrystals with Orientation in (111) Plane
Embedded in Glassy Silica Films‡
†
Goutam De and C. N. R. Rao*
Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for AdVanced Scientific Research,
Jakkur P.O., Bangalore 560 064, India
ReceiVed: August 22, 2003
Triangular Au and disk-shaped Au-Cu alloy nanocrystals have been generated in thin SiO
2
film matrixes,
by sol-gel spin-coating followed by annealing at 800 °C in 10%H -90%Ar. Transmission electron microscope
2
images confirm the shapes of the embedded nanocrystals. X-ray diffraction patterns reveal that the Au and
Au-Cu nanocrystals have a (111) orientation. The disk-shaped alloy nanocrystals exhibit surface plasmon
absorption at wavelengths intermediate between those of Au and Cu.
Metal nanocrystals embedded in appropriate glassy hosts are
known to exhibit enhanced nonlinear optical properties, with a
large intensity-dependent refractive index, which is related to
a solution of TEOS in n-propanol (50% of the total) of the
appropriate concentration, an aqueous-n-propanol solution con-
taining HAuCl4‚3H2O or/and CuCl2‚H2O was added under
stirring, and the stirring continued for 2 h at 22 ( 2 °C. After
this period, i-butanol was added and the sol was stirred for
another 1 h. The total H2O/TEOS molar ratio was maintained
at 8. A catalytic amount of HCl was added (HCl/TEOS ) 0.01)
in the case of the pure Cu sol. The total equivalent SiO2 and
metal content of the sols was about 4.5 wt % in all cases. Films
were deposited on clean silica glass slides (type II, Heraeus) or
silicon wafers by spin-coating, employing a spinning rate of
1000-1500 rpm. The resulting films were dried at 75 °C in air
followed by heat-treatment at 450 °C for 30 min to remove the
organic matter. The films were heated in 10%H2-90%Ar (gas
flow was controlled by mass flow controller) at 700 °C for 1 h
and then at 800 °C for 1 h. The film thickness, estimated by
cross-sectional scanning electron microscopy (SEM), was in the
150-180 nm range. On annealing in air at 450 °C, Au meta7l
nanocrystals were formed while Cu remained as an oxide.
Annealing in a 10%H2/90%Ar atmosphere (450°-800 °C)
reduced the Cu ions to metallic Cu and spontaneously formed
Au-Cu alloys.
(3) 1
the real part of the third-order susceptibility, ø . Such materials
have potential optoelectronic applications in optical switching
and optical limiting devices because of the ultrafast nonlinear
response.1 Optical properties of these composites are greatly
influenced by the interface between the nanocrystals and the
,2
3
matrix. Accordingly, the shape, size, and composition of the
nanocrystals would play an important role in modulating the
properties.4 There is considerable interest today in synthesizing
-7
8
shape-controlled gold nanocrystals from solution. A few other
metal nanocrystals of different shapes, such as prism-shaped
9
10
11
12
Ag, disk-shaped Ag and Co, and triangular Ag, have also
been reported recently. For real applications, however, the
nanocrystals have to be embedded in a suitable matrix which
can withstand high-intensity laser light. Metal nanocrystals
1e,3c,7,13
incorporated in thin films or glasses prepared by sol-gel,
ion-exchange, ion-implantation,
fusion are generally spherical in shape. Sol-gel-derived metal
nanocrystals incorporated in SiO2 films are known to exhibit
14
1c,15
16
sputtering, and glass
17
(3)
1e,f
high ø values with good reproducibility. In this communica-
tion, we report, for the first time, a sol-gel synthesis of highly
oriented triangular Au nanoplates as well as of Au-Cu alloy
nanodisks embedded in thin SiO2 films, the alloy composition
being in the range commonly employed for jewelry. Such shaped
and oriented nanocrystals belonging to the quantum-size regime
embedded in thin SiO2 film might show interesting nonlinear
optical responses.
Powder X-ray diffraction (XRD) patterns of the thin film
samples were recorded with a diffractometer operating at 40
kV and 30 mA using Ni-filtered Cu KR radiation. In Figure 1,
we show the XRD pattern of the SiO2 film containing Au
nanocrystals deposited on silica glass substrates. The pattern
1
8
shows only the (111) Bragg reflection, indicating a highly
oriented growth of the Au nanocrystals. The (200) and (220)
Bragg reflections are hardly visible (see inset of Figure 1). The
XRD patterns of the films containing the Au-Cu alloy
nanocrystals also show only the (111) reflection, indicating
oriented growth (Figure 2). The d(111) value decreases with
the increasing Cu content, showing the formation of good solid
solutions (Table 1). Thus, in the case of Au5Cu1, the (111)
reflection is 2.32 Å corresponding to a ) 0.4018 nm while the
a value calculated from Vegard’s law is 0.4018 nm.1 In Au4-
Cu2 and Au3Cu3, the d values are slightly higher, indicating a
lower Cu concentration than indicated by the nominal composi-
tion (Table 1). So in the latter cases a very small amount of the
Cu atoms do not form the alloy. The compositions of the alloys
derived from the respective d values are listed in Table 1. It is
Au and Au-Cu alloy nanocrystals embedded in silica hosts
were prepared from sols starting with tetraethyl orthosilicate
(
TEOS), HAuCl4‚3H2O, CuCl2‚2H2O, n-propanol, i-butanol, and
water, by the sol-gel spin coating technique. The compositions
of the films are listed in Table 1. The molar ratio of the metal
(M ) Au, Au-Cu alloy, or Cu) to SiO2 was kept constant in
all the films, being 6 equivalent mol % M:94% SiO2. The
general method of preparation of the films was as follows. To
9,20
*
Corresponding author. Phone: +91 80 8563075. Fax: +91 80
8
462760. E-mail: cnrrao@jncasr.ac.in.
†
Permanent address: Sol-Gel Division, Central Glass and Ceramic
Research Institute, Jadavpur, Kolkata 700 032, India.
‡
This manuscript is in honor of Professor Henglein’s 65th birthday.
1
0.1021/jp0310091 CCC: $25.00 © 2003 American Chemical Society
Published on Web 11/14/2003
De, Goutam
