Facile synthesis of silver nanoparticles useful for fabrication of high-conductivity elements for printed electronics

Article abstract of DOI:10.1021/ja043425k

A facile synthesis of stable silver nanoparticles having a particle size of <10 nm is described. The synthesis involved reduction of silver acetate with a substituted hydrazine, such as PhNHNH₂, in the presence of a 1-alkylamine, such as C₁₆H₃₃NH₂, in toluene at 25-60 °C. Spin-coated thin films or printed electronic features of alkylamine-stabilized silver nanoparticles could be easily converted at 120-160 °C into highly conductive films or elements with conductivity of 2-4 × 10⁴ S cm^-1. Organic thin-film transistors with printed silver source/drain electrodes of this nature exhibited field-effect transistor properties which are similar to those of the devices using vacuum-deposited silver electrodes. Copyright

Full text of DOI:10.1021/ja043425k

Published on Web 02/18/2005
Facile Synthesis of Silver Nanoparticles Useful for Fabrication of
High-Conductivity Elements for Printed Electronics
Yuning Li, Yiliang Wu, and Beng S. Ong*
Materials Design and Integration Laboratory, Xerox Research Centre of Canada,
Mississauga, Ontario, Canada L5K 2L1
Received October 30, 2004; E-mail: Beng.Ong@xrcc.xeroxlabs.com
Printed transistor circuits using liquid-based printing techniques
Scheme 1
for patterning and deposition are of great interest1 as they
represent potentially low-cost alternatives to amorphous silicon
technologies for electronics. Potential applications for these circuits
are diverse, ranging from large-area electronics (e.g., active-matrix
LCDs, organic light emitting diodes, e-paper), where fast switching
speeds are not essential, to low-end devices (e.g., wearable
electronics, smart labels, radio frequency identification tags), where
mechanical flexibility is required or where high cost of silicon chip
packaging becomes prohibitive. While great strides have recently
been achieved in designing liquid processable semiconductors for
printed electronics, little progress has been made in printable
conductive materials, despite their importance as electrodes, pixel
pads, conductive lines, and tracks in enabling low-cost electronics.
Earlier work on printable conductors focused on organic materials,
-5
and polymer stabilizers,16 are not particularly useful as they are
difficult to remove at low annealing temperatures. We elected to
study alkylamines as stabilizers for silver nanoparticles since they
17
have weaker interactions with silver. These weak interactions can
potentially be broken up at significantly lower temperatures, thus
enabling the “stripped” silver nanoparticles to coalesce to form a
continuous conductive layer.
6
5b,7
such as polyaniline and PEDOT/PSS.
conductivity materials (<3 S cm ), aside from their potential
These are very low-
Oleylamine-stabilized silver nanoparticles were prepared from
silver acetate and oleylamine in refluxing toluene according to
-
1
chemical, thermal, and electrical instabilities. Noble metals, such
17
Hiramatsu and Osterloh’s procedure, but no significant conductiv-
4
5
as gold and silver, which possess high conductivity (∼10 -10 S
ity was observed when a thin film prepared from these nanoparticles
was annealed at 160 °C; this was likely due to the difficulty in
removing the high-boiling oleylamine. Synthesis of silver nano-
particles with shorter-chain alkylamine stabilizers, such as 1-hexa-
decylamine or 1-dodecylamine, at refluxing toluene temperature
led to silver precipitation, and at lower reaction temperatures (∼60
-
1
cm ) and operational stability, require high temperature and high
vacuum deposition. Gold nanoparticles have been successfully
8
explored as a low-temperature, high-conductivity alternative, but
the high cost of gold may negate its merits. Liquid-processed silver
materials accordingly represent an appealing approach if both the
high conductivity and low-temperature processing requirements can
be met.
°
C), insignificant reaction.
Nonetheless, we found that the reduction of silver acetate at lower
Electroless deposition of silver has been used with microcontact
temperatures could be accelerated if an appropriate reducing agent
was added. While reducing agents, such as NaBH , hydrazine, and
9
printing to fabricate thin-film transistor (TFT) electrodes, but this
4
approach is too complex to be practical. Commercial silver pastes
and inks generally provide thin-film conductivity up to only 2000
aldehyde, provided rapid acceleration leading to large silver
aggregates, substituted hydrazines, such as phenylhydrazine, were
effective in modulating the reduction to an extent which gave stable
silver nanoparticles (Scheme 1). Specifically, with just 0.5 molar
equiv of phenylhydrazine in the presence of an alkylamine in
toluene, the reduction of silver acetate at 25-60 °C proceeded
smoothly and was completed in less than 1 h, yielding a dark-
brown solution without precipitation. A black solid of silver
nanoparticles was quantitatively obtained upon simple precipitation
from a nonsolvent, such as a mixture of acetone and methanol.
Silver nanoparticles prepared in this manner were soluble in
common organic solvents (e.g., hexane, toluene, THF, etc.) and
were stable under ambient conditions. Transmission electron
microscopic (TEM) measurements showed that the particle size was
<10 nm (Figure 1a). UV-vis absorption spectrum exhibited a
plasmon band at ∼415 nm, indicative of metallic silver. The present
synthesis is facile and offers the following advantages: (i) one-
phase reaction without additional surfactants; (ii) low reaction
temperature and short reaction time; (iii) less stabilizer (as low as
2.2 molar equiv to silver acetate) and high concentration of silver
nanoparticle product (∼0.5 M); and (iv) relatively inexpensive
starting materials.
S cm , even with annealing temperatures as high as >200 °C.10
Recently, conductive elements have been generated from silver
nanoparticles, albeit at high annealing temperatures (>300 °C).11
Since the melting points of metal particles drop drastically in the
extreme nanometer regime (<10 nm),12 it would thus be possible
to significantly lower the annealing temperature of silver nanopar-
ticles by reducing their particle size. However, the preparation of
stabilized silver nanoparticles in the sub-10 nm range necessary to
enable low-temperature coalescence to conductive elements remains
a synthetic challenge. We report here a facile synthesis of stabilized
silver nanoparticles with a particle size of <10 nm and demonstrate
that they can be transformed at relatively low temperatures to highly
conductive elements suitable for low-cost printed electronics.
A thin film spin cast from alkanethiol-stabilized silver nano-
-1
13
particles (∼10 nm) prepared by published procedures yielded little
conductivity upon annealing around 160 °C, a temperature which
is still considered to be compatible with plastic substrates for
flexible electronics. This was probably due to the strong bonding
of alkanethiol to silver. Silver nanoparticles stabilized with other
stabilizing agents, such as metal carboxylate,14 ammonium salts,
15
3266
9
J. AM. CHEM. SOC. 2005, 127, 3266-3267
10.1021/ja043425k CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
semiconductor layer and the source/drain silver electrodes printed
from NanoAg-C16 (see Supporting Information). All of the TFTs
exhibited good field-effect transistor characteristics, which con-
formed to the conventional gradual channel model in both the linear
and saturated regimes (Figure 2). The devices gave average mobility
2
-1 -1
6
7
of 0.05-0.08 cm V
s , current on/off ratio of 10 -10 , and
threshold voltage of -8 V. These are similar to those of reference
2
TFTs with vacuum-deposited silver electrodes (mobility ∼0.06 cm
-
1
-1
6
7
V
s
; on/off ratio ∼10 -10 ).
In summary, we have demonstrated a facile synthesis of stable
Figure 1. (a) TEM image of NanoAg-C16 nanoparticles on a grid; (b)
SEM image of NanoAg-C16 film after annealing at 140 °C for 30 s.
silver nanoparticles with a particle size of <10 nm. These
nanoparticles were stabilized with easily detachable alkylamines,
thus permitting their ready conversion at low temperatures to highly
conductive silver elements suitable for low-cost, printed electronic
applications. OTFTs with the printed silver source/drain electrodes
of this nature exhibited TFT properties similar to those using
vacuum-deposited silver electrodes.
Acknowledgment. The assistance of our colleague, Ms. Sandra
Gardner, in obtaining the SEM and TEM images is gratefully
acknowledged. Partial financial support of this work is provided
by the National Institute of Standards and Technology through an
Advanced Technology Grant (70NANB0H3033).
Figure 2. (a) Drain current (ID) versus source-drain voltage (VD) as a
function of gate voltage (VG) for a TFT with printed source/drain electrodes
from NanoAg-C16 (channel length ) 90 µm; channel width ) 2250 µm).
Supporting Information Available: Materials synthesis and
characterization, OTFT fabrication. This material is available free of
charge via the Internet at http://pubs.acs.org.
1
/2
(
b) ID and (-ID) versus VG at a constant VD) -40 V used for calculation
of the mobility and current on/off ratio.
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