APPLIED PHYSICS LETTERS 98, 011912 共2011兲

Formation of metal nanoparticles in silicon nanopores: Plasmon
resonance studies
S. Polisski,1,a兲 B. Goller,1 S. C. Heck,2 S. A. Maier,2 M. Fujii,3 and D. Kovalev1
1Department of Physics, University of Bath, Bath BA2 7AY, United Kingdom
Department of Physics, Imperial College London, London SW7 2AZ, United Kingdom
3Department of Electrical and Electronics Engineering, Kobe University, Rokkodai, Nada,
Kobe 657-8501, Japan
2共Received 24 November 2010; accepted 20 December 2010; published online 6 January 2011兲We present a method for the formation of noble metal nanoparticle ensembles in nanostructured
silicon. The key idea is based on the unique property of the large reduction potential of extended
internal hydrogen-terminated porous silicon surfaces. The process of metal nanoparticle formationin porous silicon was experimentally traced using their optical plasmon resonance response. We also
demonstrate that bimetallic compounds can be formed in porous silicon and that their composition
can be controlled using this technique. Experimental results were found to contradict partially withconsiderations based on Mie theory. © 2011 American Institute of Physics. 关doi:10.1063/1.3537811兴
Noble metal nanostructures have very specific opticalproperties due to localized surface plasmon resonances
共LSPRs兲, light-induced collective oscillations of their conduction electrons.1 The optical properties of gold 共Au兲 andsilver 共Ag兲 nanoparticles 共NPs兲 have been extensively studied, as their LSPR lies in the visible spectrum as long as theparticles are ⬃100 nm or less.1 Plasmonics is a growing
field with applications in biology using the effect of surface
enhanced Raman scattering 共SERS兲,2 photovoltaics,3 andcatalysis,4,5 among others. Metal NPs can be deposited on
various supporting substrates or chemically synthesized by
the reduction in metal salt.6 In contrast to common chemical
methods, which require the presence of reducing agents andstabilizers, in porous silicon 共PSi兲 metal NPs are formed
without their addition.
In this paper we demonstrate that NPs of Au, platinum
共Pt兲, and a mixture of both can be synthesized in PSi in acontrollable way. This technique employs the large reduction
potential of extended hydrogen-terminated surface of PSi toreduce the metal salt, resulting in the formation of metal NPs
inside the pores. NP growth can be controlled by the PSi
matrix 共selection of pore size distribution兲, metal salt concentration, and exposure time.
Freestanding PSi layers have been prepared in HFethanol solutions via standard electrochemical etching7 of
B-doped 共100兲 Si substrates with a typical resistivity of
2 – 5 ⍀ cm 共p− type Si兲, resulting in PSi layers with
⬃67– 73% porosity and 7 nm mean pore diameter. Si substrates with a resistivity of 20– 30 m⍀ cm 共p+ type Si兲 wereused to etch freestanding PSi layers having 15 nm mean pore
diameter and ⬃64% porosity. The porosity was measured
gravimetricly. The pore size distribution was derived via
standard BJH method.8 To form metal NPs, PSi was immersed in alcoholic metal salt solutions as they can uniformly wet the H-terminated hydrophobic PSi surface. The
metal salt is reduced by hydrogen on the PSi surface which
results in metal NP formation and HCl synthesis during thereaction.9,10 Optical transmission spectra of PSi were measured to detect the spectral position of the peak of the dipolara兲Electronic mail: s.polisski@bath.ac.uk.

0003-6951/2011/98共1兲/011912/3/$30.00

LSPR. Extinction spectra were obtained by normalizing thetransmission spectra of sample layers after formation of
metal NPs to those of the initial ones.
We first used PSi layers prepared from p− type Si as hostfor the growth of Au NPs. A low concentration 共0.1 mM兲 ofthe Au precursor 共HAuCl4兲 in ethanol is used so that Au NPs
grow slowly. Their growth can be monitored in situ by measuring the extinction spectra 关see Fig. 1共a兲兴.
In order to estimate the size of Au NPs formed, transmission electron microscope 共TEM兲 images of the Au/PSi
samples were acquired. Figure 1共b兲 shows an example of Au
NPs formed in a PSi matrix after 40 min of exposure to the0.1 mM HAuCl4 solution in ethanol. It shows that Au NPs
formed in PSi matrix have diameters ⬍10 nm, which correlates well with the ⬃7 nm mean pore size. Additional
atomic resolution studies demonstrate that the size of major
fraction of formed metal nanoparticles is indeed below 10
nm.
As the Au NPs in Fig. 1共b兲 are spherical, we used Mietheory11 to gain more detailed information about the NPs
embedded in the matrix. Mie theory is an analytical solution
to Maxwell’s equations used to describe the interaction ofFIG. 1. 共Color online兲 PSi layers exposed to 0.1 mM HAuCl4 solution. 共a兲
Extinction spectra of Au NPs vs exposure time. 共b兲 TEM image of a Au/PSi
sample after 40 min of exposure. 共c兲 Integrated area under extinction spectravs exposure time.

98, 011912-1© 2011 American Institute of Physics

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Polisski et al.

light with spheres of arbitrary radius and composition, which
can be used to model the LSPR of metal NPs. PSi can bedescribed as an effective medium, where the spatial variationin the refractive index of the material is much smaller than
the wavelength of light. For PSi this is generally true, but
may no longer be the case once PSi is doped with metal NPs
due to the confined nature of the electromagnetic fieldaround a metal NP. Therefore, the local refractive index
needs to be considered.
The Bruggemann approximation11 is used to calculate alocal effective refractive index n ⬃1.9– 2.1 for PSi, being ina good agreement with nonlocal measured values. Using Mietheory and n, the LSPR of small Au NPs is calculated to beat ⬃570– 590 nm 关compare with experimental data in Fig.
1共a兲兴.
The area under the extinction spectrum shows a linear
increase with time during the reaction 关line in Fig. 1共c兲兴.
Using Mie theory a linear increase in the area underneathextinction spectra can be due to an increase in the number ofparticles in an ensemble or due to an increase in particle size.
We mainly attribute the experimentally observed linear increase in to the formation of a large number of small particles rather than to the continuous growth of a constantnumber of NPs.
If high concentrations of salt solution are used 共10 mM兲
we observe after ⬃3 min that the extinction spectra do notchange significantly with time 共saturation behavior兲. Thisphenomenon can be attributed to the fact that after the consumption of surface-bonded hydrogen no more Au salt canbe reduced in PSi. The time frame for low resolution measurements was intentionally selected, as in this case the difference in saturation between low and high salt concentration
regimes is clearly seen. The saturation, in case of low concentrated solutions, will occur after several hours, allowing
controlling the growth of metal NPs.
In order to understand the effect of solution concentration and pore size distribution on the formation of Au NPs
we exposed freestanding PSi layers, made from p+ type Si
substrates, for 20 min to different concentrations of HAuCl4solutions 共0.1–1 mM兲 to achieve various loading levels of Auin PSi. After Au deposition, half of the PSi samples wereoxidized in air at 1100 ° C for 1 h, to form a PSiO2 host,which is transparent in the spectral range of interest. The
maxima of the LSPR spectra in oxidized PSi layers are blueshifted compared to those incorporated in PSi due to achange in the local refractive index of the host 共reduction ofn to ⬃1.4兲. Since the LSPR of Au NPs can be clearly seen in
the oxidized samples, they remain stable during a high temperature treatment.
Bimetallic Au–Pt NPs have recently been demonstrated
for use in a number of applications including SERS 共Ref. 12兲
and catalysis.13 In this paper we show that bimetallic NPs
can also be synthesized by the reduction in mixtures of metalsalts in PSi. PSi, prepared from p+ type Si substrates, wasexposed for 20 min to a mix of HAuCl4 and PtCl4 in ethanol共different relative concentrations, in total 1 mM兲. The PSi
layers were impregnated with three different molar ratios of
Au: Pt 共1:1, 1: 2, and 1: 3兲 before being annealed to obtain
PSiO2. The extinction spectra plotted in Fig. 2共a兲 represent
samples with 3 wt % metal loading with respect to Si mass
and appear as a broad, but single peak, which shifts toshorter wavelength with increased Pt concentration.

FIG. 2. 共Color online兲 共a兲 Extinction spectra for metal NPs in PSiO2
共formed in PSi, exposed to a Au/Pt salt mixture, and then annealed兲. 共b兲
Calculated normalized extinction cross sections for Pt and Au NPs as well ascore shell particles.

We have to consider three possibilities when two metalsalts are simultaneously reduced in the PSi matrix: formationof 共i兲 pure metal NPs, 共ii兲 core-shell NPs, or 共iii兲 alloy NPs.
To interpret the experimental results in Fig. 2共a兲, we use Mietheory and a local refractive index n ⬃1.4 for PSiO2. Using
published data for Au 共Ref. 14兲 and Pt,15 pure Pt and pure AuNPs, whose size is of the order of the pore size 共⬃15 nm兲,
have a resonance at ⬃280 nm and ⬃525 nm, respectively
关see Fig. 2共b兲兴. For a pure Pt NP to have a resonance at⬃350 nm, its diameter should be ⬃70 nm, which is muchlarger than the mean pore size of PSi and the NP size observed by TEM. Having confirmed that the resonance at⬃350 nm cannot be due to pure metal NPs, we consider theoptical properties of core/shell NPs, whose LSPR peak position is governed by the shell 关see Fig. 2共b兲兴. We focus on Au
core/Pt shell NPs, as a Au coated Pt NP cannot have a resonance at ⬃350 nm. As with pure metal NPs, a Au NP with aPt shell would have to be much larger than the mean poresize of PSi to have a peak at ⬃350 nm. Having exhausted 共i兲
and 共ii兲, we have to consider the formation of Au–Pt alloy
NPs.
Although the optical constants of pure metals have been
published,14,15 there is no data in the literature for the opticalproperties of Au–Pt alloys. However, the optical propertiesof Au–Ag alloys have been widely studied.6,16,17 We therefore begin by outlining the results for Au–Ag NPs using Mietheory and a linear effective dielectric function for the metal:⑀共x兲 = 共1 − x兲⑀y + x⑀Au ,共1兲
16,17

where x is the percentage of Au and y is either Ag or Pt.
We use published data for Au,14 Ag,14 and Pt.15
Using Mie theory and varying ⑀共x兲 for Au–Ag from
100% Ag 共black兲 to 100% Au 共cyan兲 in Fig. 3共a兲 in steps of
20% the optical properties of Au–Ag bimetallic NPs varylinearly. The linear change in the calculated LSPR peak position of Au–Ag bimetallic NPs is in agreement withexperiments.6,16,17 Applying the same approach to Au–PtNPs, the calculated LSPR peak positions, shown in Fig. 3共b兲,
do not shift linearly from 100% Pt 共black兲 to 100% Au
共cyan兲 when an effective dielectric function is used for themetal. In order to understand why the calculated LSPR peak
positions of Pt–Au bimetallic particles do not vary linearly,
we calculated the optical properties of other bimetallic combination of noble metals, such as Ag–Pt, Pt–Pd, Pt–Cu, andAu–Cu using published data for pure metals14,15 and Eq. 共1兲.
We found that the calculated optical properties of bimetallic
NPs only vary linearly when the real part of the dielectric

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Appl. Phys. Lett. 98, 011912 共2011兲

Polisski et al.

extended hydrogen-terminated surface of PSi layers. We
showed that PSi matrix defines the size of formed metal NPs.
Their size can also be controlled by varying the exposure
time to the salt solutions and the solution concentration. We
demonstrated that optical measurements enable the monitoring of noble metal NPs formation in situ. It was demonstrated that the formation mechanism of bimetallic NPs is thesame as for the monometallic NPs and that their composition
can be controlled by appropriate choice of metal salt concentration and their ratios.
FIG. 3. 共Color online兲 Calculated normalized extinction cross sections for
共a兲 Ag–Au NPs and 共b兲 Pt–Au NPs, where the dielectric function of themetal was modeled as an effective medium. Black curves are: 共a兲 100% Agand 共b兲 100% Pt with Au concentration increasing in 20% steps to 100% Au
共cyan兲. Inset: real part of the dielectric function for Au, Pt, and Ag 共Refs.
14兲.

functions of the two metals do not cross. Considering theinset in Fig. 3共b兲 the real part of the dielectric functions of Ptand Au cross in the wavelength region close to the LSPR ofAu–Pt alloy, leading to nonlinear behavior in the calculated
optical properties plotted in Fig. 3共b兲. In the inset in Fig. 3共b兲the real part of the dielectric functions of Au and Ag areparallel at long wavelengths but overlap at ⬃370 nm. A linear change in the calculated LSPR above 370 nm is calculated, but nonlinear behavior at and below this wavelength isobserved 关Fig. 3共a兲兴. Therefore, although the optical properties of bimetallic Au–Ag NPs calculated using an effective
index for the metal appears to match experiment,6,16,17 this
only occurs as the real part of the dielectric for Au and Ag donot cross in the wavelength region close to the LSPR. We
therefore conclude that effective dielectrics should not be
used to model the optical properties of bimetallic alloy NPs,
as previously suggested.16 It was not possible to model theLSPR measured at ⬃350 nm in Fig. 2共a兲 using an effective
dielectric function for Au–Pt. Since this peak cannot be related to 共i兲 or 共ii兲, we conclude that this LSPR is due to theformation of Au/Pt alloy NPs.
We have presented a method of growing Au and Au–Pt
bimetallic NPs by using the chemical reductive potential ofThis work was supported by the Engineering andPhysical Sciences Research Council 共EPSRC兲 Grant No. EP/F018622/1.
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