Self-assembled growth and green emission of gold nanowhiskers

Article abstract of DOI:10.1063/1.2138360

Unique structured Au nanowhiskers were fabricated via electroless metal deposition on Si wafer in KAuCl4 -HF solution. A self-assembled localized microscopic electrochemical cell model and a diffusion-limited aggregation process are associated with the formation of Au nanowhiskers. A green photoluminescence (PL) band was recorded. Spectral analyses suggest the green PL arises from the radiative recombination of sp -band electrons with d -band holes in the Au nanowhiskers and its intensity enhancement is due to local electric field connected with the Au particle plasmons oscillation. A red PL band was also observed from long-time etched Si substrates and proposed to be related to Si nanocrystals in the surface layer of etched Si wafer. Its stability was considered to be owing to the formation of stable Si-Au bonds on the surfaces of Si nanocrystals.

Full text of DOI:10.1063/1.2138360

Self-assembled growth and green emission of gold nanowhiskers
T. Qiu, X. L. Wu, G. G. Siu, and Paul K. Chu
Citation: Appl. Phys. Lett. 87, 223115 (2005); doi: 10.1063/1.2138360
View online: http://dx.doi.org/10.1063/1.2138360
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APPLIED PHYSICS LETTERS 87, 223115 ͑2005͒
Self-assembled growth and green emission of gold nanowhiskers
T. Qiua͒ and X. L. Wub͒
National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing
210093, People’s Republic of China
G. G. Siu and Paul K. Chub͒
Department of Physics and Materials Science, City University of Hong Kong, Kowloon, Hong Kong,
People’s Republic of China
͑Received 8 June 2005; accepted 4 October 2005; published online 22 November 2005͒
Unique structured Au nanowhiskers were fabricated via electroless metal deposition on Si wafer in
KAuCl4–HF solution. A self-assembled localized microscopic electrochemical cell model and a
diffusion-limited aggregation process are associated with the formation of Au nanowhiskers. A
green photoluminescence ͑PL͒ band was recorded. Spectral analyses suggest the green PL arises
from the radiative recombination of sp-band electrons with d-band holes in the Au nanowhiskers
and its intensity enhancement is due to local electric field connected with the Au particle plasmons
oscillation. A red PL band was also observed from long-time etched Si substrates and proposed to
be related to Si nanocrystals in the surface layer of etched Si wafer. Its stability was considered to
be owing to the formation of stable Si–Au bonds on the surfaces of Si nanocrystals. © 2005
American Institute of Physics. ͓DOI: 10.1063/1.2138360͔
Recently, experiments have shown that there are surpris-
ingly strong metal-metal interactions between gold atoms in
molecular complexes,1 and chemists have started to use them
to design new structures with unusual physical properties.2
The photoluminescence ͑PL͒ measurements of gold nano-
structures are of importance in understanding electrolumi-
nescence device operating, where semitransparent gold elec-
trodes are used.3 Moreover, understanding optical processes
in gold nanostructures may give important information re-
garding applications in surface-enhanced Raman scattering,4
nonlinear optics,5 and surface-enhanced fluorescence.6 In this
letter, we present a relatively rapid method of fabricating
unique structured gold nanowhiskers via electroless metal
deposition on Si wafer in KAuCl4–HF solution. As compared
with other strategies for gold nanostructural synthesis,
mainly including photochemical synthesis,7 template synthe-
sis approach,8 wet chemical synthesis,9 electrochemical
synthesis,10 we believe that our methodology provides a
simple and convenient route to a variety of building blocks
for assembling noble metals with novel structure and func-
tion in nanotechnology.11 A green PL band has been recorded
and considered to be due to the radiative recombination of
sp-band electrons with d-band holes in the Au nanowhiskers.
The sample fabrication method is described as follows:
A p-type, B-doped silicon ͑100͒ ͑1–5 ⍀ cm͒ wafer was first
cleaned by acetone to degrease the Si surface, followed by
etching in diluted aqueous HF solution for 10 min. Then the
cleaned silicon wafer was cut into four pieces. They were
samples were examined with a FEG JSM 6335 field emission
scanning electron microscope ͑SEM͒ and a PHI 5600 x-ray
photoelectron spectrometer ͑XPS͒ with monochromatic Al
K␣ source at 14 kV and 350 W. The photoelectron take-off
angle was 45 °. Powder x-ray diffraction ͑XRD͒ spectrom-
eter was used to characterize the gold nanostructures. Data
were collected on a Japan Rigaku D/Max-RA x-ray diffrac-
tometer with Cu K␣ radiation ͑␭=0.1542 nm͒. The PL mea-
surement was performed on a T64000 triple Raman system
͑Jobin-Yvon Company͒ using the 488 and 514 nm lines as
excitation sources. All the measurements were performed at
room temperature.
The morphologies of the gold nanostructures are
etching-time dependent, as shown in Fig. 1. At the initial
etched in
a
5.0 mol/L HF solution containing
0.02 mol/L KAuCl4 at 50 °C for 30, 10, 1 min, and 5 s,
respectively. The container is a conventional Teflon-lined
stainless steel vessel. After the etching process, the four sili-
con wafers were rinsed with de-ionized water and blown dry
in air. The morphology and chemical composition of the
a͒
Also at: Department of Physics and Materials Science, City University of
FIG. 1. SEM images of the gold nanostructures grown on silicon wafers via
Hong Kong, Kowloon, Hong Kong, People’s Republic of China.
electroless metal deposition in a 5.0 mol/L HF solution containing
b͒
Authors to whom correspondence should be addressed; electronic mail:
0.02 mol/L KAuCl4 at 50 C. The etching time is ͑a͒ 5 s, ͑b͒ 1, ͑c͒ 10, and
͑d͒ 30 min.
hkxlwu@nju.edu.cn, paul.chu@cityu.edu.hk
0003-6951/2005/87͑22͒/223115/3/$22.50
87, 223115-1
© 2005 American Institute of Physics
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223115-2
Qiu et al.
Appl. Phys. Lett. 87, 223115 ͑2005͒
FIG. 3. SEM images ͑scale bar: 1 ␮m͒ of residual silicon substrates etched
in HF–KAuCl4 solution with different times: ͑a͒ 5 s, ͑b͒ 1, ͑c͒ 10, and ͑d͒ 30
min.
FIG. 2. XRD pattern of the as-prepared gold nanowhiskers shown in Fig.
1͑d͒. The inset shows the binding energy position of Au 4f7/2 at 84.2 eV.
could be regarded as the template that is something like
Xiao’s work about ultrasonically assisted template synthesis
of palladium and silver dendritic nanostructures.17 During
the initial stage, high concentration of the gold salt and re-
duction agent lead to reduction-nucleation growth of gold
nanoclusters at a lot of positions to form chainlike network.
With prolonging reaction duration, the concentration of gold
salt and reduction agent greatly decrease, the growth is
mainly driven by decreasing surface energy and thus the
whisker-like gold nanostructures are formed.
The PL spectra of the gold nanostructures, taken under
excitation with the 488 nm line of an Ar+ laser, are presented
in Fig. 4͑a͒. The emission peaks at ϳ550 nm are centered
near the interband absorption edge of bulk gold.18 Optically
excited metal surfaces show no or very little luminescence.
stage, the deposited gold atoms first form nuclei and then
form nanoclusters which distribute on the surface of the sili-
con wafer to form a continuous film ͓Figs. 1͑a͒ and 1͑b͔͒.
After etching for 10 min, change has obviously taken place
on the surface of the silicon wafer ͓Fig. 1͑c͔͒. Many gold
nanoclusters diffuse and stick together to form whisker-like
nanostructures. This can be more clearly seen in the 30 min
etched Si wafer ͓Fig. 1͑d͔͒. The diameters of the nanowhis-
kers are nonuniform and in the range of 0.1–1 ␮m. Their
maximal lengths can reach about 10 ␮m. Typical powder
XRD patterns of the nanowhiskers are shown in Fig. 2. The
four diffraction peaks can respectively be indexed to the
͑111͒, ͑200͒, ͑220͒, and ͑311͒ planes of fcc gold ͑JCPDS
04-0784͒. The corresponding XPS spectrum of the gold
nanowhiskers with Au 4f7/2 signal at 84.2 eV, shown in the
inset of Fig. 2, displays additional proof.12
The formation of the gold nanowhiskers can be under-
stood on the basis of self-assembled localized microscopic
electrochemical cell model and diffusion-limited aggregation
process ͑DLA͒.13,14 At the beginning, the silicon etching and
gold deposition occur simultaneously at the silicon surface.
The deposited gold atoms first form nuclei and then form
nanoclusters which uniformly distribute on the surface of the
silicon wafer ͓Fig. 1͑a͔͒. These gold nanoclusters and the Si
areas surrounding these gold nuclei could, respectively, act
as local cathodes and anodes in the electrochemical redox
reaction process.15 We believe that the electrochemical cell
model is similar to that of forming Si nanowire arrays in the
case of Ag deposition.15,16 That is to say, many small flat
honeycombs form around one tiny deposited gold nanoclus-
ter. Numerous nanosized honeycomb-like anodes and one
gold nanocluster acting as local cathode form an electro-
chemical cell.16 These cells could be self-assembled on the
surface of the silicon wafer. To prove this process and give
direct evidence of the cell model, the etched Si wafers were
treated in an ultrasound water bath to remove the deposited
gold nanostructures and only leave the residual Si substrates,
as shown in Fig. 3. It can be seen that the honeycombs be-
come larger and deeper with increasing the etching time.
Further etching of intermediate honeycombs will lead to the
formation of special shaped Si nanowires.16
The synchronous growth of gold nanowhiskers should
be considered within the framework of a DLA model, which
involves in cluster formation by the adhesion of a particle
with random path to a selected seed on contact and allows
the particle to diffuse and stick to the growing structure.14 In
situ prepared honeycombs around the gold nanoclusters
FIG. 4. PL spectra of the gold nanostructured films on silicon wafers etched
in HF–KAuCl4 solution ͑a͒ with different times ͑␭ex=488 nm͒ and ͑b͒ dif-
ferent illumination positions of laser beam ͑␭ex=514 nm͒. The inset shows a
schematic diagram of the illumination points.
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223115-3
Qiu et al.
Appl. Phys. Lett. 87, 223115 ͑2005͒
For instance, smooth gold films show a PL with very low
efficiencies of ϳ10−10 following excitation of electron tran-
sitions from the 5d to the 6sp bands.18 In other words, only
one photon is emitted under excitation of per 1010 electron-
hole pairs. One likely reason for this low PL efficiency is that
nonradiative energy relaxation processes of photoexcited car-
riers in metals, such as Coulomb carrier-carrier scattering,
are much faster than radiative electron-hole recombination,
thus quenching the PL.19 A unique exception from the rule of
low PL yields in metals is noble-metal nanoparticles. For
example, PL efficiencies on the order of 10−4 have recently
been observed in gold nanorods.20 The origin of this effect is
being argued.19–22 Conceivably, the enhancement is caused
by an acceleration of the radiative process in the nanopar-
ticles as compared to the bulk, due to size-dependent screen-
ing effects which accelerate electron-electron scattering23
and lead to emission by hot carriers of particle plasmons, i.e.,
collective oscillations of the conduction electrons.24,25 In our
experiments, intensity of the green PL is enhanced in rougher
gold nanostructured films. Thus, we suggest that the intensity
enhancement of the green emission from the radiative re-
combination of sp-band electrons with d-band holes is due to
local electric field associated with the gold particle plasmons
oscillation.
Here, it should be mentioned that the PL from long-time
etched Si substrates themselves cannot be ignored. A broad
emission band centered at ϳ680 nm can clearly be seen in
the sample etched for 10 min, as shown in Fig. 4͑a͒. Chen et
al.26 have reported that the PL intensity from Au-passivated
porous Si nanocrystals can be enhanced due to the formation
of stable Au–Si bonds at the surface of Si nanocrystals. Our
PL spectrum is similar to that in the literature. Therefore, we
believe that the red PL band arises from Si nanocrystals with
surface Si–Au bonds. To identify the PL mechanism, we
made the following experiments: Fine adjust the position of
the sample using microscopic monitor and let laser beam
illuminate a gold particle ͑ϳ50 ␮mϫ50 ␮m͒ from center to
edge found on the surface of 30 min etched Si wafer, as
schematically shown in the inset of Fig. 4͑b͒ by points 1–4,
and then collect the PL signal. Figure 4͑b͒ shows the corre-
sponding PL spectra, taken under excitation with the 514.5
nm line of Ar+ laser. It can be seen that the intensity of the
680 nm emission is position dependent. The PL from point 4
is the strongest and the PL from point 1 has vanished. If we
consider the limited penetration depth of laser beam, the red
PL can be understood to be from Si nanocrystals and its
intensity increase from points 1 to 4 is due to the increased
density of luminescent Si nanocrystal. Meantime, the exis-
tence of stable Si–Au bonds also leads to stability of the Si
nanocrystal surface.26 As a result, the red PL peak position
remains unchanged with the sample storage history in air.
In summary, a rapid, inexpensive method of fabricating
gold nanowhiskers has been described on the basis of elec-
troless metal deposition technique. The formation of the gold
nanowhiskers can be interpreted on the basis of self-
assembled localized microscopic electrochemical cell model
and DLA process. A green PL band is observed at 550 nm.
PL spectral analyses suggest that the green emission arises
from the radiative recombination of sp-band electrons with
d-band holes and its intensity enhancement is due to local
electric field associated with the gold particle plasmons os-
cillation. A red PL band was also observed from long-time
etched Si substrates and proposed to be related to Si nano-
crystals in the surface layer of Si wafer. Its stability was
considered to be owing to the formation of stable Si–Au
bonds on the surfaces of Si nanocrystals.
This work was supported by the Grants ͑Nos. 10225416
and 60476038͒ from the Natural Science Foundations of
China and the LAPEM. Partial support was also from the
Major State Basic Research Project No. G001CB3095 of
China and Hong Kong Research Grants Council ͑RGC͒
Competitive Earmarked Research Grants ͑CERG͒ Nos.
CityU 1137/03E and CityU 1120/04E, and City University of
Hong Kong Strategic Research Grant ͑SRG͒ No. 7001642.
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