Nanoscale patterning of two metals on silicon surfaces using an ABC triblock copolymer template

Article abstract of DOI:10.1021/ja060366x

Patterning technologically important semiconductor interfaces with nanoscale metal films is important for applications such as metallic interconnects and sensing applications. Self-assembling block copolymer templates are utilized to pattern an aqueous metal reduction reaction, galvanic displacement, on silicon surfaces. Utilization of a triblock copolymer monolayer film, polystyrene-block-poly(2-vinylpyridine)-block-poly(ethylene oxide) (PS-b-P2VP-b-PEO), with two blocks capable of selective transport of different metal complexes to the surface (PEO and P2VP), allows for chemical discrimination and nanoscale patterning. Different regions of the self-assembled structure discriminate between metal complexes at the silicon surface, at which time they undergo the spontaneous reaction at the interface. Gold deposition from gold(III) compounds such as HAuCl₄(aq) in the presence of hydrofluoric acid mirrors the parent block copolymer core structure, whereas silver deposition from Ag(I) salts such as AgNO₃(aq) does the opposite, localizing exclusively under the corona. By carrying out gold deposition first and silver second, sub-100-nm gold features surrounded by silver films can be produced. The chemical selectivity was extended to other metals, including copper, palladium, and platinum. The interfaces were characterized by a variety of methods, including scanning electron microscopy, scanning Auger microscopy, X-ray photoelectron spectroscopy, and atomic force microscopy.

Full text of DOI:10.1021/ja060366x

Published on Web 04/05/2006
Nanoscale Patterning of Two Metals on Silicon Surfaces
Using an ABC Triblock Copolymer Template
Masato Aizawa* and Jillian M. Buriak*
Contribution from the National Institute for Nanotechnology and the Department of Chemistry,
UniVersity of Alberta, Edmonton, Alberta, Canada T6G 2G2
Received January 17, 2006; E-mail: maizawa@ualberta.ca; jburiak@ualberta.ca
Abstract: Patterning technologically important semiconductor interfaces with nanoscale metal films is
important for applications such as metallic interconnects and sensing applications. Self-assembling block
copolymer templates are utilized to pattern an aqueous metal reduction reaction, galvanic displacement,
on silicon surfaces. Utilization of a triblock copolymer monolayer film, polystyrene-block-poly(2-vinylpyridine)-
block-poly(ethylene oxide) (PS-b-P2VP-b-PEO), with two blocks capable of selective transport of different
metal complexes to the surface (PEO and P2VP), allows for chemical discrimination and nanoscale
patterning. Different regions of the self-assembled structure discriminate between metal complexes at the
silicon surface, at which time they undergo the spontaneous reaction at the interface. Gold deposition from
gold(III) compounds such as HAuCl
copolymer core structure, whereas silver deposition from Ag(I) salts such as AgNO
4
(aq) in the presence of hydrofluoric acid mirrors the parent block
(aq) does the opposite,
3
localizing exclusively under the corona. By carrying out gold deposition first and silver second, sub-100-
nm gold features surrounded by silver films can be produced. The chemical selectivity was extended to
other metals, including copper, palladium, and platinum. The interfaces were characterized by a variety of
methods, including scanning electron microscopy, scanning Auger microscopy, X-ray photoelectron
spectroscopy, and atomic force microscopy.
Introduction
define metallic nanoscale features on surfaces, among others.
In terms of highly parallel synthetic strategies, nanofabrication
Metallization of semiconductors with micrometer-scale and
nanoscale control plays an important role in the production of
integrated devices. Controlling size, shape, and locations of
metallic nanoparticles on semiconductors in periodic arrange-
ments is considered to be a key for future nanoelectronic
techniques using self-assembled nanostructures are particularly
4
attractive due to their efficiency and high-throughput. Block
copolymer templates represent an exciting group of self-
assembled structures because of their compatibility with existing
5
1
silicon-based fabrication techniques, commercial availability,
applications. Scanning probe tip-mediated approaches such as
and incredible diversity of chemistries and morphologies.6
Since block copolymers contain two or more chemically
differentiated polymer chains, they demonstrate selective solu-
bilization of various chemical reagents; for instance, a polar
block will sequester polar solutes, a block with metal donor
ligands will selectively bind metal ions, etc. Block copolymers
on surfaces can therefore be used to direct chemical reagents
to an interface, at which they will undergo reactions that mirror
the pattern of the polymer template.
-8
2
dip-pen nanolithography (DPN) and other direct writing
methods like electron beam lithography are powerful tools to
3
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J. AM. CHEM. SOC. 2006, 128, 5877-5886
9
5877

A R T I C L E S
Aizawa and Buriak
of P4VP micelles on solid surfaces when spin- or dip-coated
from a nonpolar solvent like toluene.9-11 The nitrogen of the
pyridine units in the P4VP block coordinates to, or is protonated
More challenging is the preparation of interfaces with two,
or perhaps more, different nanoscale metal features, also with
nanoscale spatial and size control. In order to direct two different
chemical reagents sequentially to the surface in a manner
analogous to positive and negative masking, judicious choice
of selective polymer blocks is required. The ability of chemically
differentiated blocks of a block copolymer to selectively
solubilize chemical reagents is becoming established. Previous
work by several groups has demonstrated sequestration of metal
7
,11
by, metal ions and compounds such as auric acid (HAuCl4).
We have demonstrated that this block copolymer template may
be used for spatial direction of surface chemistry exclusively
via the P4VP block of the template. HAuCl4 and Ag+ ions
coordinated within the hydrophilic P4VP block of monolayers
of self-assembled PS-P4VP block copolymers on semiconduc-
tor interfaces such as silicon, germanium, gallium arsenide, and
complexes and other materials, such as metal nanoparticles,
1
2
8,13-16
indium phosphide are reactive toward surface chemistry.
within diverse blocks.
For instance, anionic complexes
such as AuCl4 , PtCl42-, and PdCl42- interact selectively with
the protonated P2VP block of poly(2 (or 4)-vinylpyridine)-block-
poly(ethylene oxide) diblock copolymers (P2VP-b-PEO) at pH
< 5, while with the poly(butadiene)-block-poly(ethylene oxide)
polymer (PB-b-PEO), complexes such as (CH3CN)2PdCl2 and
KPt(C2H4)Cl3 interact with the double bonds of PB via
π-complexes. To the best of our knowledge, there has been
only one report involving block copolymer templating of two
metal nanostructures within the block copolymer. Sohn, Ter-
anishi, and co-workers solubilized FeCl3 in the core of PS-
P4VP, and dodecanethiol-terminated Au nanoparticles in the
PS portion; upon oxygen plasma treatment, γ-Fe2O3 nanopar-
ticles formed in the core, surrounded by a corona of Au
-
Through a galvanic displacement reaction, the gold and silver
ions were spontaneously reduced to metallic Au and Ag by these
materials, resulting in bound, patterned metal nanoparticles
whose size and average separation were dictated by the parent
block copolymer. This technique allows one to produce mono-
metallic, pseudo-hexagonal arrays of Au or Ag nanoparticles
on the surfaces with precise control over particle size and
interparticle distance.
13
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copolymers, polystyrene-block-poly(2-vinylpyridine)-block-
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5878 J. AM. CHEM. SOC.
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VOL. 128, NO. 17, 2006

Nanoscale Patterning of Metals on Silicon Surfaces
A R T I C L E S
undergo reduction exclusively in the PEO corona. It was possible
to form Au-core and Ag-corona nanostructures on the surface
by subsequently depositing Au and Ag. Among other metals
excess amount of water. Following this cleaning procedure, the wafers
were visibly hydrophilic; water on the surface was immediately removed
with a stream of nitrogen before polymer spin-coating.
Polymer Template Preparation. Core-shell-corona micelles were
prepared in pure water from the triblock copolymers using the method
developed by Jer oˆ me and co-workers.20 Briefly, ∼2 wt % polymer
solutions were prepared by dissolving PS-b-P2VP-b-PEO in a 0.9 g
mixture of DMF and benzene, ensuring that the proportion of DMF
was kept to 70-75 wt %. A 0.1 mL volume of water was then added
to the solution under vigorous stirring in order to initiate the polymer
self-assembly, and stirring was maintained for at least 12 h. After the
micelles were kinetically frozen with 1 mL of water, the organic solvent
mixture was dialyzed against water for 5 days to recover the micelles
in pure water. The typical polymer solution used for spin-coating was
diluted to ∼0.5 wt %.
examined, Na2PdCl4 and Na2PtCl4 are coordinated and reduced
under the P2VP block, while Cu2+ is reduced to nanoparticles
under the PEO block.
Experimental Section
Generalities. Unless otherwise noted, all the experiments were
performed under ambient conditions. Si(100) (p-type, B-doped, F )
0
.01-0.02 Ω‚cm) wafers were purchased from Addison Engineering.
KAuCl ‚xH O, AuCl (99%), AgNO (99.99995%), AgClO ‚H
99%), AgCH COO (99%), AgF (98%), Na PtCl ‚xH O, Na PdCl
O (99%), and CuSO
Chemicals, whereas HAuCl
4
2
3
3
4
2
O
(
3
2
4
2
2
4
‚
3H
2
4 2
‚5H O (99.999%) were purchased from Strem
4
(99.9995%) was purchased from Sigma-
Aldrich. The ABC triblock copolymer, polystyrene-block-poly(2-
vinylpyridine)-block-poly(ethylene oxide) (PS-b-P2VP-b-PEO, with
molecular weights of 20 000, 14 000, and 26 000 for the PS, P2VP,
and PEO blocks, respectively), was purchased from Polymer Source
Single-Metal Galvanic Displacement Deposition on Silicon. A
piece of cleaned Si(100) was spin-coated (Laurell, WS-400B-6NPP-
Lite) using 10 µL of the diluted polymer solution at 5000 rpm for 1
min. The polymer-coated wafer was then immersed in a mixture of 1
mL of 1 mM aqueous metal salt and 9 mL of 1% HF(aq) for a given
time [final solution concentration is therefore 0.1 mM aqueous metal
salt/0.9% HF(aq)]. WARNING: Hydrofluoric acid (HF) is extremely
hazardous in both the liquid (aqueous) and Vapor forms and can cause
seVere tissue damage upon contact with skin or Via respiration. After
metal deposition, the sample was thoroughly rinsed with water and
dried under a nitrogen stream. For Au and Ag deposition, the polymer
templates were dissolved in a toluene ultrasound bath (Cole Palmer,
08891-021) for 5 min, and the sample was dried under a nitrogen
(www.polymersource.com). Water was obtained from a Millipore
system (resistivity >18 MΩ). N,N-Dimethylformamide (DMF), ben-
zene, and toluene used in this study were HPLC grade (Sigma-Aldrich),
and the methanol was Optima grade (Fisher). HF(aq) (49%), H
30%), HCl(aq) (36.5-38.0%), and NH OH(aq) (10-35%) were
semiconductor grade, obtained from J. T. Baker. NH F (40%) was
2 2
O (aq)
(
4
4
purchased as a solution from Riedel-deHaen. All reagents listed above
were used without further purification. Teflon beakers and tweezers
were used exclusively during the cleaning and preparation of the Si
wafers and for all metal deposition procedures.
2
stream. For the Cu, Pd, and Pt deposition, however, an H /Ar plasma
Surface Characterization. Metal nanostructures on silicon were
characterized by atomic force microscopy (AFM), scanning electron
microscopy (SEM), scanning Auger microcopy (SAM), and X-ray
photoelectron spectroscopy (XPS). SEM, SAM, and XPS were per-
formed under high-vacuum conditions (<10 Torr). The atomic force
microscope used in this study was a Nanoscope IV (Digital Instruments/
Veeco) using commercially available Si cantilevers in tapping mode
under ambient conditions. SEM (Hitachi S-4800 FE-SEM) of metal
nanostructures was typically performed with an electron energy of 10
keV, while a reduced electron energy (5 keV) was employed in order
to image polymer-coated silicon surfaces to minimize damage. SAM
(Harrick Plasma, PDC 32G, 18 W) was employed to remove the
polymers since removal by toluene ultrasonication led to dissociation
of the metal nanoparticles from the surface.
Au/Ag Mixed Structure Galvanic Displacement on Silicon. Two
separate reaction vessels were used to sequentially pattern Au and Ag
nanoparticles on silicon. First, a polymer-coated silicon shard was
-
8
immersed in a mixture of 1 mL of 1 mM HAuCl
HF(aq) [final solution concentration is therefore 0.1 mM HAuCl
4
(aq) and 9 mL of 1%
(aq)/
4
0.9% HF(aq)] and rinsed with copious water to avoid further gold
deposition on the surface. The sample was then immersed in the second
vessel containing 1 mL of 1 mM AgNO
[final solution concentration is therefore 0.1 mM AgNO
3
(aq) and 9 mL of 1% HF(aq)
(aq)/0.9% HF-
(JEOL, JAMP-9500F) was carried out with an electron accelerating
3
voltage and emission current of 25 kV and 6 nA, respectively. The
Auger peaks of Si KL23L23 (1600 eV), Ag M4N45N45 (350 eV),
and Au M5N67N67 (2015 eV) were selected for the mapping. The
Auger mapping for each element was obtained by plotting (P - B)/B,
where P and B are peak and background intensities, respectively. The
intensities were then scaled using the JEOL processing software for
increased contrast. The average escape depths for Au and Ag MNN
(aq)]. The sample was again rinsed with excess water and the polymer
removed in a toluene ultrasound bath for 5 min, followed by drying
under a nitrogen stream.
Electroless Deposition Growth of Au Nanoparticles on Silicon.
The continuous Au nanostructures on silicon (Figure 3e,f) were obtained
on the basis of the seed-mediated, electroless deposition method
developed by Zamborini and co-workers.1 First, Au nanoparticles
with diameters <10 nm were prepared by immersing a polymer-coated
a,21
electrons from SiO
2
are estimated to be ∼4.2 and ∼1.1 nm, respectively,
18
according to the NIST IMFP database. XPS (Kratos Analytical, Axis-
Ultra) was performed using monochromatic Al KR with a photon energy
of 1486.6 eV. The average escape depths for Au 4f and Ag 3d electrons
silicon shard into a mixture of 1 mL of 1 mM HAuCl
of 1% HF(aq) for 1 min [final solution concentration is therefore 0.1
mM HAuCl (aq)/0.9% HF(aq)]. The sample was subsequently im-
4
(aq) and 9 mL
4
from SiO
2
are estimated to be ∼3.2 and ∼2.6 nm, respectively,
mersed into the electroless growth solution consisting of 9 mL of 0.1
M cetyltrimethylammonium bromide (CTAB), 450 µL of 0.01 M
HAuCl (aq), and 0.05 mL of 0.1 M ascorbic acid (AA). In this mixture,
4
the CTAB and AA are shape-directing and weak reducing agents,
respectively. The polymers were removed via a toluene ultrasound bath
for 5 min, followed by drying under a nitrogen stream.
according to the NIST IMFP database. The instrument was calibrated
on the basis of the C 1s. The Ag 3d and Au 4f metallic positions were
also calibrated using sputtered Ag and Au films, respectively.
2
Silicon Cleaning Procedures. Pieces of Si(100) measuring ∼7 mm
were degreased in a methanol ultrasonic bath for 15 min and dried
with a nitrogen stream. The wafers were then cleaned via standard RCA
cleaning procedures.19 The wafers were first immersed in a hot solution
(19) Kern, W. Overview and Evolution of Semiconductor Wafer Contamination
and Cleaning Technology. In Handbook of Semiconductor Wafer Cleaning
Technology; Kern, W., Ed.; Noyes Publications: Park Ridge, NJ, 1993; p
3.
20) (a) Lei, L.; Gohy, J.-F.; Willet, N.; Zhang, J.-X.; Varshney, S.; Jer oˆ me, R.
Macromolecules 2004, 37, 1089. (b) Lei, L.; Gohy, J.-F.; Willet, N.; Zhang,
J.-X.; Varshney, S.; Jer oˆ me, R. Polymer 2004, 45, 4375.
(21) (a) Wei, Z.; Mieszawska, A. J.; Zamborini, F. P. Langmuir 2004, 20, 4322.
(b) Wei, Z.; Zamborini, F. P. Langmuir 2004, 20, 11301. (c) Gao, J.; Bender,
C. M.; Murphy, C. J. Langmuir 2003, 19, 9065.
of H
with excess water, they were immersed in another hot solution of H
HCl:H (6:1:1) for 15 min. The wafers were again rinsed with an
2
O:NH
4
OH:H
2
O
2
(5:1:1) for 15 min. After the wafers were rinsed
2
O:
(
2
O
2
(
18) Powell, C. J.; Jablonski, A. NIST Electron Inelastic-Mean-Free-Path
Database, Version 1.1.; National Institute of Standards and Technology:
Gaithersburg, MD, 2000.
J. AM. CHEM. SOC.
9
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A R T I C L E S
Aizawa and Buriak
Figure 1. (a) AFM image of a monolayer of PS-b-P2VP-b-PEO triblock copolymers spin-coated onto a native oxide-capped Si(100) wafer. (b) Molecular
structure of PS-b-P2VP-b-PEO. (c) Schematic structures of core-shell-corona micelles of the triblock copolymers in water and on a surface.
Figure 2. (a) Illustrative representation of galvanic displacement on silicon in the presence of hydrofluoric acid. The standard cell potentials shown are with
respect to the normal hydrogen electrode (NHE). (b) Schematic representation of nanoscale patterning of Au and Ag on a silicon surface using the triblock
copolymer as a template.
Results and Discussion
The diameter of the micelles is approximately 80 nm, and the
cylindrical micelles are a few hundred nanometers to a
micrometer in length. These polymer micelles were prepared
as described in the Experimental Section, in a DMF/benzene
solution with dropwise addition of a small amount of water,
followed by kinetically freezing the micelles in water.20 The
as-formed micelles were recovered in pure water using dialysis
and then spin-coated onto a native oxide-capped silicon wafer
at 5000 rpm for 1 min. In order to bring about metal deposition
via galvanic displacement, the polymer-coated silicon shard was
immersed into an aqueous solution of the metal ion, in the
presence of dilute hydrofluoric acid, at room temperature for
the desired time (30 s to 1 h). The reduction process for these
transition metal ions is best described by mixed potential theory,
roughly based upon the half-cell reactions shown in Figure 2a;
the silicon acts as a reducing agent and is oxidized to form
silicon oxides that are dissolved by the dilute HF, enabling the
An ABC triblock copolymer containing three distinct chemi-
cal functionalities, polystyrene-block-poly(2-vinylpyridine)-
block-poly(ethylene oxide) (PS-b-P2VP-b-PEO, with molecular
weights of 20 000, 14 000, and 26 000 for the PS, P2VP, and
PEO blocks, respectively), was chosen as the template. This
polymer self-assembles into a mixture of rod-shaped and
spherical micelles, each with a PS core and a P2VP shell,
surrounded by a PEO corona.2
of a monolayer of self-assembled spherical and cylindrical
micelles spin-coated onto a native oxide-capped silicon wafer,
0,22
Figure 1 shows an AFM image
2
0
as well as their proposed structure, as shown in Figure 1c.
(
22) (a) Gohy, J.-F.; Willet, N.; Varshney, S. K.; Zhang, J.-X.; Jer oˆ me, R.
e-Polymer 2002, Paper No. 35. (b) Gohy, J.-F.; Willet, N.; Varshney, S.;
Zhang, J.-X.; Jer oˆ me, R. Angew. Chem., Int. Ed. 2001, 40, 3214. (c) Khanal,
A.; Li, Y.; Takisawa, N.; Oishi, Y.; Nakashima, K. Langmuir 2004, 20,
4
809. (d) Li, Y.; Khanal, A.; Kawasaki, N.; Oishi, Y.; Nakashima, K. Bull.
Chem. Soc. Jpn. 2005, 78, 529.
5880 J. AM. CHEM. SOC.
9
VOL. 128, NO. 17, 2006

Nanoscale Patterning of Metals on Silicon Surfaces
A R T I C L E S
Figure 3. SEM images of Au nanostructures on SiOx using the triblock copolymer as a template. The depositions were carried out from (a,b) 0.1 mM
KAuCl4/0.9% HF(aq) for 5 min, (c) 0.1 mM HAuCl4/0.9% HF(aq) for 5 min, (d) 0.1 mM AuCl3/0.9% HF(aq) for 5 min, (e) two-step deposition, involving
-
4
-4
0
.1 mM HAuCl4/0.9% HF(aq) galvanic displacement for 1 min and then electroless deposition with 0.095 M HAuCl4/4.7 × 10 M CTAB/5.3 × 10
M
aqueous ascorbic acid for 60 min, and (f) the same conditions as (e) but the electroless deposition was carried out for 100 min.
reaction to continue.23 Following removal of the silicon from
the aqueous metal bath, the polymer was removed via dissolu-
tion in a toluene ultrasound bath for 5 min at room temperature.
Figure 3a-d shows SEM images of gold deposited on SiOx-
capped Si wafers using the PS-b-P2VP-b-PEO triblock as a
template on silicon, from an aqueous gold(III) precursor solution
Beyond 10 min of deposition time under these conditions, the
gold nanoparticles have a tendency to be removed with the block
copolymer due to the weak Si-Au interaction, most likely due
to silicon etching that undercuts the deposited metal, releasing
23c,24
it with the polymer.
Little difference was noted between
KAuCl4 and HAuCl4 deposition under these conditions, although
the fidelity of the AuCl3 precursor with respect to the parent
P2VP block is not as good as that of KAuCl4/HAuCl4, since
some small-particle deposition between these blocks can be
17
(KAuCl4, HAuCl4, or AuCl3), in the presence of HF. Deposi-
tion of the gold nanoparticles closely follows the pattern of the
PS/P2VP micelle rods and spheres, pointing to interaction of
-
the AuX4 anion with the P2VP block; no significant deposition
2-
observed. Other anionic compound ions, such as PdCl4 and
occurs underneath the PEO corona. This behavior is similar to
2-
PtCl4 , also deposit spontaneously in a similar manner,
previous results obtained using PS-P4VP templates on silicon
mirroring the spatial pattern of the gold precursors (Supporting
Information). The P2VP block is expected to be protonated by
hydrofluoric acid (the pKa of a similar 2-substituted pyridine,
protonated 2-methylpyridine, is 5.97, while the pKa of HF is
3.17), meaning that the anionic metal complexes act as
1
2
as a result of the directing ability of the vinylpyridine block.
(
23) (a) Jones, D. A. Principles and PreVention of Corrosion; Macmillan: New
York, 1992. (b) Okinaka, Y.; Osaka, T. AdV. Electrochem. Sci. Eng. 1994,
3, 55. (c) Magagnin, L.; Maboudian, R.; Carraro, C. J. Phys. Chem. B
002, 106, 401. (d) Chemla, M.; Homma, T.; Bertagna, V.; Erre, R.; Kubo,
2
N.; Osaka, T. J. Electroanal. Chem. 2003, 559, 111. (e) Peng, K.; Zhu, J.
Electrochim. Acta 2004, 49, 2563. (f) Nagahara, L. A.; Ohmori, T.;
Hashimoto, K.; Fujishima, A. J. Vac. Sci. Technol. A 1993, 11, 763. (g)
Oskam, G.; Long, J. G.; Natarajan, A.; Searson, P. C. J. Phys. D: Appl.
Phys. 1998, 31, 1927. (h) Niwa, D.; Homma, T.; Osaka, T. J. Phys. Chem.
B 2004, 108, 9900. (i) Niwa, D.; Homma, T.; Osaka, T. J. Phys. Chem. B
counteranions to the pyridinium group; the nature of their
binding to the P2VP block is therefore electrostatic.2
2a,b
The
calculated pH of an aqueous solution with 1% HF(aq) is 1.7.
More continuous films of larger gold nanoparticles are
produced if the HAuCl4/HF(aq) is supplemented with an
2
004, 108, 9900. (j) Lin, H.; Mock, J.; Smith, D.; Gao, T.; Sailor, M. J. J.
Phys. Chem. B 2004, 108, 11654. (k) Porter, L. A.; Choi, H. C.; Buriak, J.
M. Nano Lett. 2002, 2, 1067. (l) Zambelli, T.; Munford, M. L.; Pillier, F.;
Bernard, M.-C.; Allongue, P. J. Electrochem. Soc. 2001, 148, C614. (m)
Harraz, F. A.; Tsuboi, T.; Sasano, J.; Sakka, T.; Ogata, Y. H. J. Electrochem.
Soc. 2002, 149, C456. (n) Sun, X.-H.; Wong, N.-B.; Li, C.-P.; Lee, S.-T.;
Kim, P.-S. G.; Sham, T.-K. Chem. Mater. 2004, 16, 1143. (o) Takano, N.;
Niwa, D.; Yamada, T.; Osaka, T. Electrochim. Acta 2000, 45, 3263. (p)
Kim, C.; Oikawa, Y.; Shin, J.; Ozaki, H. Microelectron. J. 2003, 34, 607.
electroless reducing agent solution of ascorbic acid (AA) and
1a,21
the surfactant CTAB, as shown in Figure 3e,f.
The surface
is seeded with small gold nanoparticles produced via galvanic
(
q) Gorostiza, P.; Allongue, P.; D ´ı az, R.; Morante, J. R.; Sanz, F. J. Phys.
(24) Nezhad, M. R. H.; Aizawa, M.; Porter, L. A., Jr.; Ribbe, A. E.; Buriak, J.
Chem. B 2003, 107, 6454.
M. Small 2005, 1, 1076.
J. AM. CHEM. SOC.
9
VOL. 128, NO. 17, 2006 5881

A R T I C L E S
Aizawa and Buriak
Figure 4. SEM images of Ag nanostructures on SiOx using the triblock copolymer as a template, using a solution prepared from 0.1 mM AgNO3/0.9%
HF(aq), varying the time: (a,b) 5 min, (c,d) 20 min, (e) 40 min, and (f) 60 min.
displacement [1 min deposition in 0.1 mM HAuCl4/0.9% HF-
aq)], followed by immersion in the AA/CTAB deposition bath
cessful for selective metal deposition, probably due to the
aggressive nature of this etchant.
(
for 60 min (Figure 3e) and 100 min (Figure 3f). The polymers
were removed by a toluene ultrasound bath for 5 min. Further
increases in the deposition time led to the larger quantities of
Au deposition in both the PVP shell and the PEO corona.
Because the initial galvanic displacement is carried out for only
a short period of time, the films adhere well to the silicon
surface, as subsequent growth is not reliant upon a mechanism
that involves silicon corrosion and undercutting. The cationic
surfactant protects the surface from nonselective deposition
under these conditions, most likely via coordination to the silicon
oxide that has a net negative charge.25
In terms of other metal salts that mirror the selectivity of
+
Ag , copper(II) sulfate was tested since both are water-solvated
+
2+
26
cations, [Ag(H2O)4] and [Cu(H2O)6] under these conditions.
Copper deposition also follows the PEO block of the parent
polymer template (Supporting Information), but the weak Si-
Cu interaction required block copolymer removal with H2/Ar
plasma since toluene ultrasonication resulted only in a pitted
surface with no adhering copper metal. XPS of the copper(II)
sulfate deposition before plasma treatment confirms that the
metal is reduced via galvanic displacement since all the copper
appears as Cu(0) (Supporting Information).
SAM was utilized to obtain compositional maps of a surface
by forming an image from Auger electrons emitted by the
elements of interest. Figures 5 and 6 are comprised of four
images for each metal, gold and silver, prepared from HAuCl4
and AgNO3 precursors, in the presence of HF: (a) the SEM
image taken with the scanning Auger microscope, (b) the
elemental map for silicon (KLL signals), (c) the elemental map
for the metal (MNN signals for Au or Ag), and (d) the combined
silicon/metal elemental map (KLL/MNN). As can be seen from
Figures 5e and 6e, the signals for Au/Ag and Si are opposite:
the deposited metal masks the underlying silicon substrate,
whereas in regions where the silicon is exposed, the signal for
the metal is low. The scanning Auger images support the
Silver nitrate deposition, as opposed to gold, results in silver
deposition under the PEO corona, instead of the P2VP/PS
blocks, leading to the opposite pattern, as shown in Figure 4.
The Ag nanoparticles are larger toward the center of the PEO
corona, up to approximately 30 nm in diameter, and are
surrounded by smaller (<10 nm) silver nanoparticles, for
deposition times of up to 10 min. At longer times (Figure 4c-
f), silver particles up to 100 nm are formed. Other silver salts
such as silver perchlorate and silver acetate, in the presence of
fluoride, produce identical patterns (Supporting Information).
Use of the water-soluble AgF salt, containing a fluoride
counteranion, was tested to determine if the use of HF(aq) could
be avoided, but this silver salt led to little observed deposition.
Replacement of HF(aq) with 1% NH4F(aq) was also unsuc-
(26) (a) Mesaric, S. S.; Hume, D. N. Inorg. Chem. 1963, 2, 1063. (b) Cotton,
F. A.; Willkinson, G.; Murillo, C. A.; Bochman, M. AdVanced Inorganic
Chemistry, 6th ed.; John Wiley & Sons: New York, 1999; pp 868 and
1084.
(25) Chen, Z.; Singh, R. K. J. Electrochem. Soc. 2004, 151, R360.
5
882 J. AM. CHEM. SOC. VOL. 128, NO. 17, 2006
9

Nanoscale Patterning of Metals on Silicon Surfaces
A R T I C L E S
Figure 5. Scanning Auger microscopy (SAM) images of Au nanostructures on SiOx. (a) SEM image, (b) Si KLL SAM, (c) Au MNN SAM, (d) fitting Au
MNN (red) and Si KLL (green) intensities over (a), and (e) SAM line profiles of (top) Au MNN and (bottom) Si KLL. The line positions are shown in red
in the SEM image. The depositions were carried out from 0.1 mM HAuCl4/0.9% HF(aq) for 10 min.
Figure 6. SAM of Ag nanostructures on SiOx-capped silicon surfaces. (a) SEM image, (b) Si KLL SAM, (c) Ag MNN SAM, (d) fitting Ag MNN (red) and
Si KLL (green) intensities over (a), and (e) SAM line profiles of (top) Ag MNN and (bottom) Si KLL. The line positions are shown in red in the SEM image.
The depositions were carried out from 0.1 mM AgNO3/0.9% HF(aq) for 10 min.
conclusions based on SEM images of positive and negative gold
and silver galvanic displacement reactions under different
polymer blocks.
While SEM and SAM point to metal reduction, only XPS
can confirm the spontaneous reduction of the metal ions to M(0)
by galvanic displacement. The Au 4f XPS spectra of Figure
7a,b correspond to Figure 3c, showing gold deposited onto the
SiOx wafer in the presence of HF and HAuCl4. The peak position
indicates that the majority of Au ions are spontaneously
reduced to metallic Au on the surface, although the shoulders
3
+
J. AM. CHEM. SOC.
9
VOL. 128, NO. 17, 2006 5883

A R T I C L E S
Aizawa and Buriak
eV, and therefore the oxidation state of the silver cannot be
29
precisely determined. Since nanoparticles are not observed on
the surface by SEM, however, it is doubtful that deposition
occurs in the absence of a fluoride source. Like the case with
HAuCl4, the block copolymer clearly is not responsible for much
+
or any of the Ag reduction through PEO oxidation. Because
silver ions can dissolve in and interact with PEO, it is assumed
that transport of silver ions through the thin PEO coating allows
30
for growth of the silver metal nanostructures. The Au(III) salt
utilized was either anionic or neutral, and so has less affinity
for PEO and does not cross this barrier to the silicon interface.27
We assume that the lack of deposition of silver under the
P2VP/PS blocks is due to two factors: (i) competition with
simple protonation by the HF in the solution (vide supra) for
coordination of the silver cation to the pyridine groups, and (ii)
the poor coordination ability of the P2VP block as compared
to that of a P4VP block. It is expected that the pyridine units in
the P2VP shell are predominately protonated under these
conditions, and thus the low concentration of silver cations
interact little with this block. Even if the silver ion binds to the
pyridine, the binding is direct, involving a coordinative Ag-N
bond to the sterically hindered 2-pyridine group, whereas the
Figure 7. XPS of Au 4f (left) and Ag 3d (right). (a) HAuCl4 deposition
on a polymer-coated SiOx wafer shard in the presence of 0.9% HF(aq). (b)
Removal of the polymer via toluene ultrasonication resulted in strengthening
of Au signals due to exposure of Au nanoparticles. (c) Same conditions as
(a) except reaction carried out in the absence of HF(aq). (d) Polymer removal
with toluene ultrasonication led to a decrease of the Au(III) signals. (e)
H2/Ar plasma treatment inducing the reduction of Au(III) and simultaneous
polymer removal. (f) AgNO3 deposition on a polymer-coated SiOx silicon
wafer in the presence of 0.9% HF. (g) Removal of the polymer with toluene
ultrasonication has no effect on the Ag intensity since Ag nanoparticles
are deposited underneath a thin PEO film. (h) Same conditions as (f) except
the reaction carried out in the absence of HF. (i) Sample from (h) after
polymer removal with toluene ultrasonication. (j) Sample from (h) after
H2/Ar plasma treatment.
8
square planar, d Au(III) complex is merely a counteranion of
a cationic pyridinium group, and thus is not constrained in this
manner. Indeed, Zn(II) salts have been shown to interact to a
considerably greater extent with P4VP as opposed to P2VP due
to the steric hindrance imposed upon the metal cation when
3
1
binding directly to a 2-pyridine group. These two factors
together can explain the observed spatial directing ability of
this block copolymer monolayer.
on the main peak at higher binding energy suggest a small
Since the gold and silver deposit in different blocks of the
PS-b-P2VP-b-PEO polymer, it is therefore possible to generate
metallic patterns that consist of a gold center, surrounded by
silver shell, via sequential dipping of the polymer-coated silicon
wafer in aqueous metal ion solutions. The order in which the
dipping is carried out is critical due to the different oxidation/
reduction potentials of the metals and metal ions. Because Au-
12
fraction of species with oxidation states higher than 0. Polymer
removal intensified the Au signal due to the exposure of the
nanoparticles (Figure 7b). To ensure that metal reduction and
deposition is not mediated by PEO oxidation,1
6,27,28
the same
reaction was carried out in the absence of hydrofluoric acid,
the essential ingredient that exposes the underlying silicon
through SiOx dissolution. As shown in Figure 7c, all observable
(III) has a higher oxidation potential compared with Ag(0), gold
6q,r
gold is present exclusively as Au(III), and thus no polymer-
induced reduction occurs under these conditions. Toluene
ultrasonication largely removes most of the Au(III) ions (Figure
was intentionally deposited first. The triblock copolymer-coated
silicon wafer was immersed into the 0.1 mM HAuCl4/0.9% HF-
(aq) solution for 1 min, followed by rinsing with water. The
7
d) as the polymer dissolves in the solvent, taking the bound
sample was then immersed into 0.1 mM AgNO3/0.9% HF(aq)
for 1 min. Figure 8a,b shows SEM images of Au/Ag nano-
structures after removal of the polymer with toluene ultrasoni-
cation. The Au nanoparticles ∼5 nm in diameter are surrounded
by larger Ag nanoparticles >10 nm in diameter, forming the
Au core and Ag shell nanostructured films. Figure 8c,d shows
SEM images of the Au/Ag nanostructures obtained by the same
deposition procedure described above, but the Au and Ag
deposition times were increased to 5 and 20 min, respectively.
Under these conditions, there is increased gold deposition
gold ions with it. In contrast to H2/Ar plasma, reduction of the
Au(III)-loaded polymer from Figure 7c leads to gold metal
deposition, as would be expected (Figure 7e) on the basis of
7
,11
earlier results with P2VP-containing block copolymers.
Similar XPS results were obtained for silver deposition as
well (Figure 7f-j), although several significant differences were
noted; the XPS spectra correspond to the samples imaged in
Figure 4a,b. First, polymer removal (Figure 7f,g) following silver
galvanic displacement did not result in an obvious increase in
intensity of XPS signals due to the fact that the PEO block is
thin and does not coat the deposited silver metal uniformly.
Second, immersion of the polymer-coated SiOx wafer into
aqueous AgNO3 in the absence of HF(aq) shows low levels of
silver, certainly due to the thinness of the PEO layer (Figure
(sufficient for visualization by SAM), and Ag nanoparticles
nucleate and grow selectively at the boundary regions between
the P2VP and PEO. Silver nanoparticle nucleation on the silicon
(
29) Wagner, C. D.; Naumkin, A. V.; Kraut-Vass, A.; Allison, J. W.; Powell,
C. J.; Rumble, J. R., Jr. NIST X-ray Photoelectron Spectroscopy Database,
Version 3.4 (Web Version), 2003.
7
h-j) that prevents significant polymer loading with silver ion.
+
The binding energy difference between Ag and Ag(0) is ∼0.4
(
30) (a) Rao, S. S.; Rao, K. V. S.; Shareefuddin, M.; Rao, U. V. S.; Chandra,
S. Solid State Ionics 1994, 67, 331. (b) Sunderrajan, S.; Freeman, B. D.;
Hall, C. K.; Pinnau, I. J. Membr. Sci. 2001, 182, 1. (c) Liu, L.; Feng, X.
S.; Chakma, A. Sep. Purif. Technol. 2004, 38, 255.
(
27) (a) Sakai, T.; Alexandridis, P. Langmuir 2005, 21, 8019. (b) Sakai, T.;
Alexandridis, P. Langmuir 2004, 20, 8426.
(
28) Liz-Marzan, L.; Lado-Tourino, I. Langmuir 1996, 12, 3585.
(31) Kuo, S.-W.; Wu, C.-H.; Chang, F.-C. Macromolecules 2004, 37, 192.
5884 J. AM. CHEM. SOC.
9
VOL. 128, NO. 17, 2006

Nanoscale Patterning of Metals on Silicon Surfaces
A R T I C L E S
Figure 8. SEM images of AuAg nanostructures on SiOx-capped silicon wafer shards. The Au and Ag galvanic displacement depositions were carried out
in 0.1 mM HAuCl4/0.9% HF(aq) and 0.1 mM AgNO3/0.9% HF(aq), respectively. (a) Au deposition (1 min) followed by Ag (5 min), (b) Au (1 min) followed
by Ag (10 min), (c,d) Au (5 min) followed by Ag (20 min), and (e,f) Ag (5 min) followed by Au (10 min) deposition.
most likely becomes disfavored when more gold is deposited
due to cathodic protection, and so it occurs preferentially at
formation is cathodic protection of the silicon: the electrone-
gativity of the metal deposits (silver initially, followed by
subsequent gold growth) ensures that electron flow occurs
exclusively via the existing metal on the surface, preventing
defect sites on or near the Au/Si interface, or on the gold surface
itself.2
3i,32
At the edge of sensitivity limits for SAM, it is clear
2
3i,32
that silver deposition takes place around the micelle core and
not within, while for gold it is the opposite (Supporting
Information). XPS of these interfaces indicate that, as the Ag
deposition time increases, both the Au 4f and Ag 3d X-ray
photoelectron spectra shift toward lower binding energy (Sup-
porting Information). The broad Au 4f XPS feature becomes
narrower and keeps shifting toward lower binding energy as
the Ag deposition proceeds. This is likely due to partial electron
transfer from Ag to Au nanoparticles at the boundaries; such
intermetallic electron transfer has been reported for AuAg
nucleation in surrounding areas.
AuCl4 anion is selectively transported through the protonated
P2VP block, and so growth is limited to the interface between
Just as importantly, the
-
-
the P2VP and PEO blocks since AuCl4 cannot pass through
the PEO-covered surface.
Conclusions
The chemical selectivity of block copolymers can be utilized
to direct spatially defined surface chemistry on semiconductors.
In this case, an ABC triblock copolymer, PS-b-P2VP-b-PEO,
can act as a self-assembled template to deposit metallic
nanostructured films on silicon surfaces by an aqueous galvanic
displacement reaction. The P2VP and PEO blocks discriminate
between anionic compounds and water-solvated cations, leading
to positive and negative patterning, with respect to the block
copolymer micelle cores. The silver and gold ions interact with
the P2VP and PEO blocks in different manners, resulting in
the observed selectivity of deposition: the reaction of silicon
and Au(III) occurs only in the region of the P2VP block, and
silicon and silver(I) react only in the PEO corona. Through
sequential dipping procedures, it is possible to form metallic
core/shell films of gold and silver, if gold is deposited first. If
silver is deposited initially, only gold rings form due to a
galvanic displacement between Au(III) and Ag(0), due to the
higher oxidizing ability of gold with respect to silver. To
3
3
δ+
-
alloys. The resulting Ag can react with OH or O2 in
34
solution to form Ag2O or AgO. These oxide peaks are known
to shift toward lower binding energies by 0.1-0.9 eV compared
to the metallic peak.29 When the order is reversed and silver is
deposited first, the Au(III) ions first undergo a galvanic
displacement reaction with the silver metal, dissolving the silver
structures. Simultaneously, gold deposition takes place on the
silicon but only at the interface between the PEO and P2VP
blocks, leading to ring structures of gold (Figure 8e,f). SAM
reveals very weak Ag MNN signals and an intense Au pattern
(Supporting Information). The reason for the ring structure
(
32) Aizawa, M.; Cooper, A. M.; Malac, M.; Buriak, J. M. Nano Lett. 2005, 5,
8
15.
(33) Yamamoto, M.; Nakamoto, M. Chem. Lett. 2004, 33, 1340.
34) Qian, L.; Yang, X. Colloids Surf. A 2005, 260, 79.
(
J. AM. CHEM. SOC.
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A R T I C L E S
Aizawa and Buriak
summarize, the chemical selectivity of ABC triblock copolymers
can be harnessed to direct the reaction of reagents to the surface
in a spatially defined manner. This work is presently being
extended toward the chemical selectivity of aligned triblock
copolymer structures for both metal and organic deposition
chemistry on semiconductor interfaces.
Dimitre Karpuzov and his colleagues at the Alberta Centre for
Surface Engineering and Science (ACSES) are thanked for their
considerable assistance with scanning Auger microscopy (SAM),
XPS, and other surface science techniques.
Supporting Information Available: SEM images referred to
in the text, higher resolution SAM images, AFM images, and
XPS spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
Acknowledgment. We thank the National Research Council
(NRC) of Canada, the University of Alberta, NSERC, and the
Canadian Foundation for Innovation (CRC to J.M.B.). Dr.
JA060366X
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