Alkylamine capped metal nanoparticle "inks" for printable SERS substrates, electronics and broadband photodetectors.

Abstract

We report a facile and general method for the preparation of alkylamine capped metal (Au and Ag) nanoparticle "ink" with high solubility. Using these metal nanoparticle "inks", we have demonstrated their applications for large scale fabrication of highly efficient surface enhanced Raman scattering (SERS) substrates by a facile solution processing method. These SERS substrates can detect analytes down to a few nM. The flexible plastic SERS substrates have also been demonstrated. The annealing temperature dependent conductivity of the nanoparticle films indicated a transition temperature above which high conductivity was achieved. The transition temperature could be tailored to the plastic compatible temperatures by using proper alkylamine as the capping agent. The ultrafast electron relaxation studies of the nanoparticle films demonstrated that faster electron relaxation was observed at higher annealing temperatures due to stronger electronic coupling between the nanoparticles. The applications of these highly concentrated alkylamine capped metal nanoparticle inks for the printable electronics were demonstrated by printing the oleylamine capped gold nanoparticles ink as source and drain for the graphene field effect transistor. Furthermore, the broadband photoresponse properties of the Au and Ag nanoparticle films have been demonstrated by using visible and near-infrared lasers. These investigations demonstrate that these nanoparticle "inks" are promising for applications in printable SERS substrates, electronics, and broadband photoresponse devices.

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Alkylamine capped metal nanoparticle ‘‘inks’’ for printable SERS substrates,
electronics and broadband photodetectors†
Lakshminarayana Polavarapu, Kiran Kumar Manga, Kuai Yu, Priscilla Kailian Ang, Hanh Duyen Cao,
Janardhan Balapanuru, Kian Ping Loh*and Qing-Hua Xu*
Received 12th December 2010, Accepted 24th March 2011
DOI: 10.1039/c0nr00972e
We report a facile and general method for the preparation of alkylamine capped metal (Au and Ag)
nanoparticle ‘‘ink’’ with high solubility. Using these metal nanoparticle ‘‘inks’’, we have demonstrated
their applications for large scale fabrication of highly efficient surface enhanced Raman scattering
(SERS) substrates by a facile solution processing method. These SERS substrates can detect analytes
down to a few nM. The flexible plastic SERS substrates have also been demonstrated. The annealing
temperature dependent conductivity of the nanoparticle films indicated a transition temperature above
which high conductivity was achieved. The transition temperature could be tailored to the plastic
compatible temperatures by using proper alkylamine as the capping agent. The ultrafast electron
relaxation studies of the nanoparticle films demonstrated that faster electron relaxation was observed at
higher annealing temperatures due to stronger electronic coupling between the nanoparticles.
The applications of these highly concentrated alkylamine capped metal nanoparticle inks for the
printable electronics were demonstrated by printing the oleylamine capped gold nanoparticles ink as
source and drain for the graphene field effect transistor. Furthermore, the broadband photoresponse
properties of the Au and Ag nanoparticle films have been demonstrated by using visible and near-
infrared lasers. These investigations demonstrate that these nanoparticle ‘‘inks’’ are promising for
applications in printable SERS substrates, electronics, and broadband photoresponse devices.
Introduction
Noble metal nanoparticles are of great interest owing to their
unique properties and potential wide applications.1–8These noble
metal nanoparticles display an interesting phenomenon called
surface plasmon resonance (SPR), which arises from the collec-
tive oscillations of conductive electrons induced by incident
electromagnetic radiation.9The SPR depends on the size and
shape of the metal nanoparticles. Various methods have been
developed to prepare metal nanoparticles of different morphol-
ogies in the past two decades.10–13The applications of metal
nanoparticles in plasmonics,14photonics,15electronic devices16–21
and surface enhanced Raman scattering substrates22–28have been
under intensive study.
Surface-enhancedRamanscattering(SERS)spectroscopyhave
been demonstrated useful in ultrasensitive chemical and biolog-
ical detections.25Various physical methods such as electron beam
lithography,22sputtering,26chemical vapor deposition,29nano-
spherelithography31andfocusedionbeampatterning32havebeen
used to fabricate SERS substrates by controlling the assembly of
silver or gold nanoparticles into specific structures. Although the
substrates fabricated using above mentioned techniques showed
largeRamanenhancementswithgoodreproducibility,theyareof
high cost and difficult to extend to the large area device fabrica-
tion. Solution-processing methods are attractive because of their
advantages of easy fabrication of large area device, physical
flexibility, and most importantly, low cost.
Themetalnanoparticleshavealsofoundapplicationsinefficient
photodetection owing to plasmon enhanced photoresponse.18,33
Mostof thepreviousworkutilized plasmonresonance toenhance
photocurrent efficiency of semiconductor devices via electric field
enhancement.34However, the photoresponse bandwidths of these
semiconductor devices are usually limited by their band-gap
energies.There areafewreportsonthephotoconductivity ofgold
nanoparticlefilms.8,35Silvernanoparticleshavebeenrarelystudied
inthisregard.Assilverismuchcheaperthangold,itisattractiveto
Department of Chemistry, National University of Singapore, 3 Science
Drive 3, Singapore 117543. E-mail: chmxqh@nus.edu.sg; chmlokp@nus.
edu.sg; Fax: +65 6779-1691; Tel: +65 6516-2847
† Electronic Supplementary Information (ESI) available: Preparation of
nanoparticle ink, SERS using gold nanoparticle films, SERS signals of 1
nM rhodamine6G onoleylamine
Comparison of maximum SERS enhancement for Ag and Au
nanoparticle substrates, surface morphology of gold nanoparticle film
after annealing,ultrafastelectron
nanoparticle films, preparation of graphene oxide, multi-layer graphene
film field-effect transistor (MLG-FET), Electric I–V characteristics of
the Au and Ag nano particle films. See DOI: 10.1039/c0nr00972e/
capped silvernanoparticle,
relaxationproperties ofthe
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use silver nanoparticles films as an alternative for photodetector
and other optoelectronic applications.
Another related important active research area is to explore
simple and cost effective methods to prepare conductive metal
nanoparticles with high stability and solubility.19–21,36–38These
metal nanoparticles could be used as ‘‘ink’’ for solution pro-
cessing printable conductors,21
systems,39electrodes.40The nanoparticle solutions with high
concentrations are usually called as ‘‘inks’’. It should be noted
that nanoparticle ‘‘inks’’ are different from nanopastes that are
not printable due to high viscosity. The ligand protected metal
nanoparticle ‘‘inks’’ can be used to prepare conductive metal
films at ambient temperatures. So far most metal nanoparticle
‘‘inks’’ are prepared by using thiol capping agents,21,36,39which
are toxic and expensive. Moreover, thiols have strong tendency
to etch the nanoparticles, which has negative effects on the long
term stability. The thiol capped metal nanoparticle ‘‘inks’’ were
usually prepared via a two-step process,21,41in which the gold
ions were first phase-transferred from water to toluene using
surfactant followed by reduction of gold ions with NaBH4in the
presence of alkanethiols. Although this phase transfer method
has been demonstrated to be successful in preparing gold nano-
particles, it cannot be utilized to prepare the silver nanoparticles.
Here we demonstrated a facile and general method to prepare
low cost, highly soluble and stable alkylamine capped gold and
silver nanoparticle ‘‘inks’’ with particle size of 3–5 nm. Our
method uses alkylamine as the capping agent, which is less toxic
and cheaper compared to the typically used thiol-type capping
agents for the preparation of ‘‘inks’’. We have demonstrated the
fabrication of highly efficient SERS substrates by using these
metal nanoparticles via a facile solution processed method, which
isadvantagousover thepreviously
methods.22,26,29–32These substrates can detect analytes down to
a few nM and flexible plastic substrates have been demonstrated
for the first time using metal nanoparticle ‘‘inks’’. These metal
nanoparticle films were found to transform into highly conduc-
tive metallic films under annealing and the transition tempera-
ture was strongly dependent on the capping agent. We have
demonstrated the application of alkylamine capped metal
nanoparticle inks as printable electrodes for graphene based
field-effect transistors (FETs). The metal nanoparticle films
prepared by using these nanoparticle ‘‘inks’’ also displayed
broadband (400–1100 nm) photoresponse.
circuits,20
micromechanical
reportedphysical
Results and discussion
Characterization of nanoparticle inks
In this work, gold or silver nanoparticle ‘‘inks’’ were prepared by
simply mixing AgNO3or HAuCl4with oleylamine in a toluene
solvent, followed by reduction with aqueous NaBH4. The
nanoparticles were purified by precipitating with ethanol
(see experimental section and ESI† for the details). We found
that any alkylamine could be used to dissolve the metal salts in
organic solvents under sonication without the need of phase
transfer from aqueous to organic phase. This method is simpler
and more advantageous compared to those methods using phase
transfer agents.42Fig. 1a–b shows the pictures of gold and silver
nanoparticle inks in toluene with a concentration of 50 mg ml?1,
The extinction spectra of these gold and silver nanoparticles
(Fig. 1c) show plasmon resonances at 530 and 420 nm respec-
tively. The TEM images (Fig. 1d–e) shows that these silver and
gold nanoparticles contain 3–5 nm spherical nanoparticles.
Chen et al.43and Wang et al.44previously reported synthesis of
oleylamine capped gold and silver nanoparticles at higher
temperatures by using oleylamine as the solvent and reducing
agent. Our current method can be extended to rapid and large
scale preparation of any alkylamine capped Au or Ag nano-
particles with high concentrations and high purity at room
temperature, which is essential for solution processed applica-
tions. The prepared nanoparticles are very stable in highly
concentrated solutions or powder form. No degradation was
observed after storing the concentrated solution or powder for
more than one and a half years at room temperature. This
method uses toluene and ethanol as the solvent, which can be
easily recycled. This method is environmentally friendly and the
offers significantly reduced cost. The prepared alkylamine cap-
ped nanoparticle ‘‘inks’’ have high solubility up to 80 mg ml?1in
toluene, which is good for printable electronics and many other
solution processed applications.
Characterization of solution processed nanoparticle films
The prepared metal nanoparticle ‘‘inks’’ were used to fabricate
SERS substrates via solution processing, which have many
Fig. 1
gold and silver ‘‘inks’’; (d–e) TEM images of the silver and gold
nanoparticles.
Pictures (a–b) and extinction spectra (c) of oleylamine capped
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advantages over the conventional physical methods.22,26,29–32The
nanoparticles were first spin coated onto four clean glass
substrates under same conditions followed by annealing the
substrates. Fig. 2a shows the extinction spectra of the spin coated
silver nanoparticle films after annealing the substrates at
different temperatures (See the experimental section for the
detailed preparation procedures). The silver nanoparticles have
an extinction maximum at 420 nm in toluene solution (Fig. 1c).
When the film was annealed at 100?C, the band maximum of the
extinction spectrum shifted to 490 nm. Its AFM image (Fig. 2b)
shows that silver nanoparticles are closely packed and form
a smooth film. The red shift in the extinction maximum of the
nanoparticle film compared to that of pure particles in solution
suggest strong coupling between the nanoparticles on the film.
When the nanoparticle film was annealed at 150?C, the initial
5 nm silver nanoparticle film partially transformed into nano-
particles of ?50–80 nm in size (Fig. 2c). When the nanoparticle
film was heated up to 200?C, the nanoparticle film fully trans-
formed into larger aggregatednanoparticles of ?70–90nm in size
(Fig. 2d). The extinction spectra of the silver nanoparticle films
annealed at 150 and 200?C exhibit extinction maxima at 520 nm,
consistent with the strongly coupled larger particles as observed
by AFM. The extinction spectrum became broadened as the
annealing temperature increased. When the nanoparticle film
was heated to 250?C, the extinction spectrum of the nanoparticle
film became blue shifted instead, resembling that of individual
spherical nanoparticles. The silver surfaces are known to be
easily oxidized when exposed to oxygen. When the silver nano-
particle film was annealed to 250?C, the capping agent will be
removed and the silver nanoparticle surface exposes to the
atmospheric oxygen that results in the oxidation of the silver
surface. The silver oxide on the surface will reduce the plasmonic
coupling between the nanoparticles, which results in an abrupt
blue shift of the SPR. The AFM image (Fig. 2e) shows that the
nanoparticles became less aggregated and the particles appeared
more spherical in shape.
Gold nanoparticles films were also prepared under the same
experimental conditions. The gold nanoparticles were also found
to grow in size as the annealing temperature increased up to
150?C, but further increase of annealing temperature resulted in
formation of smooth films due to the coalescence of gold nano-
particles (see Fig. S1 in ESI†).
SERS measurements
The extinction spectra and AFM results show that the annealed
substrates produce large aggregated Ag nanoparticles, which are
expected to generate hot spots and give huge SERS signals due to
enhanced electric field. The performance of the SERS substrates
was examined by measuring the Raman signal of rhodamine 6G
(Rh6G) on these nanoparticle films using 514 nm laser as the
excitationsource.Thesamplewaspreparedbydropcasting10mL
of1mMRh6Gethanolsolutionontothefournanoparticlecoated
glass substratesandallowing the solventto evaporateunder table
light. Fig. 2f shows the SERS spectra of Rh6G on four nano-
particle films that were annealed at 100, 150, 200, and 250?C,
respectively. The Raman spectra agree well with the previously
reported results.45The band at 1186 cm?1is associated with C–C
stretching vibrations, and bands at 1308, 1360, 1508, 1572, and
1648 cm?1are due to the aromatic C–C stretching vibrations. The
SERS signal is weakest for the nanoparticle film prepared at
annealing temperature of 100?C. The SERS signal intensity
increases as the annealing temperature increases until 200?C,
where the strongest SERS signal was observed. The Ag nano-
particle film for the substrate prepared by annealing at 200?C is
?22timeshigherthanthatforthesubstratepreparedbyannealing
at100?C. Theheating ofthenanoparticlefilm to100,150,200?C
produces aggregated nanoparticles. The increase of the particle
size with the annealing temperature can be seen clearly from the
AFM images shown in Fig. 2. As the particle size increases with
the increasing annealing temperature from 100 to 200?C, the
SERSintensityincreases.However,whenthefilmwasannealedto
250?C, the distance between the nanoparticles increases due to
formationofoxidelayerontheAgsurface.Theplasmoncoupling
will reduce and consequently the SERS intensity decreases. The
SERS substrate annealed at 200?C could be used to detect very
low concentrations of Rh6G. Analytes with concentration as low
asafewnMcouldbedetectedbyusingtheseAgnanoparticlefilms
(Fig. S2 in ESI†). The detection limit of these SERS substrates is
comparable to those of the substrates preapared by complicated
methods such as electron beam lithography,22Au@pNIPAM,23
and electro-spun Ag nanoparticle free standing films.24
The SERS experiments were also performed on the gold
nanoparticlefilmspreparedundersimilar experimental
Fig. 2
films annealed at 100, 150, 200, and 250?C, respectively, and (f) SERS
spectra of Rh6G (1 mM) on these Au nanoparticle films.
(a) Extinction spectra, (b–e) AFM images of the Ag nanoparticle
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conditions (See Fig. S1 in ESI†). The gold nanoparticle film
prepared by annealing at 150?C gives the largest enhancement.
However, the maximum SERS signal given by Au nanoparticle
substrates is ?17 times smaller than that of Ag nanoparticle film
(Fig. S3 in ESI†). Less enhancement by the gold nanoparticle
films compared to Ag nanoparticle films could be due to strongly
damped plasmons in the gold when excited with green laser
because of the coupling to interband transitions.46
Using these metal nanoparticle ‘‘inks’’, we have also fabricated
plastic flexible SERS substrates (Fig. 3), which were prepared by
dipping the plastic substrates into the nanoparticle inks. These
plastic SERS substrates could be used to detect mM concentra-
tion of analytes without annealing the substrates considering the
thermal instability of the plastic substrates. Recently Liz-Marz? an
and co-workers have developed SERS substrates for broadband
excitation wavelengths using silver nanoparticle films prepared
by the layer-by-layer technique27or templated approach.47Bar-
tosz28and co-workers have also demonstrated that the self-
assembled nanotriangles displayed excellent SERS sensitivity
with an order of magnitude enhancement compared to the
individual nanotriangles. However, these methods are not
convenient for large scale fabrication of SERS substrates. Here
we have demonstrated a facile fabrication of SERS substrates
using a simple solution processing method. This method could be
further extended to fabricate SERS substrates by ink-jet printing
using nanoparticle ‘‘inks’’. The detection limit can be improved
to nM if the substrate is thermally treated.
Conductivity measurements
We have also explored the potential applications of these metal
nanoparticle inks in printable electronics by examining the
dependence of the conductivity of these nanoparticle films on the
annealing temperature. The conductivity was measured on
vacuum evaporated four-in-line probe substrates. Fig. 4a shows
the conductivity of the drop-casted oleylamine capped Au and
Ag nanoparticle films annealed at different temperatures at
atmospheric conditions. The Au and Ag nanoparticle films as
deposited without annealing behaved like an insulator with very
low conductivity of 10?3–10?4S cm?1. The insulator behavior
of the nanoparticle films is because the oleylamine monolayer on
the nanoparticle surface hinders the charge tunneling between
the nanoparticles. The conductivity of the metal nanoparticles
started to increase as the annealing temperature increased above
100?C and jumped rapidly at annealing temperatures above
200
105S cm?1for the Au film and 3.3 ? 105S cm?1for the Ag film,
which are quite close to the bulk conductivity of Au (?4.5 ?
105S cm?1) and Ag (?6.3 ? 105S cm?1). When the annealing
temperature reached 250?C, the black color nanoparticle films
transformed into a yellow color Au film and white color Ag film,
accompanied by sublimation of oleylamine in the form of a black
smoke. The inset of Fig. 4a shows the color of the nanoparticle
film after annealing to 250?C. The optical microscopic image,
AFM image and dark field image of the Au nanoparticle films
shows that there were no cracks in the film after annealing at
250?C (Fig. S4 in ESI†). The annealing temperature dependent
conductivities of both Au and Ag films follow a similar trend,
which suggests that the transformation of the nanoparticle film
into a highly conductive metallic film mainly depends on the
capping agent. We have also prepared dodecylamine and octyl-
amine capped gold nanoparticles and measured their annealing
temperature dependent conductivities. Their transition temper-
atures were observed to be 190–200?C and 130–140?C for
dodecylamine and octylamine capped Au nanoparticles respec-
tively (Fig. 4b). The transition temperatures could thus be
tailored to plastics compatible temperatures by using proper
alkylamine as the capping agent.
The transformation of the oleylamine capped gold nano-
particle film into a highly conductive gold film was monitored by
using UV-vis spectroscopy (Fig. S5 in ESI†). The broad extinc-
tion spectra of the annealed nanoparticle film at raised annealing
temperatures suggest formation of 3D-interconnected structures.
The electron relaxation dynamics of the nanoparticle film at
different annealing temperatures were also investigated by using
femtosecond pump–probe experiment. (Figure S5 in ESI). The
electron relaxation time was found to become faster as the
annealing temperature increased. When the film was annealed at
250?C, its relaxation time was similar to that of bulk gold.48
These results suggest that there is strong electronic coupling9
within the nanoparticles after annealing the film above the
?C. The conductivity reaches maxima at 250
?C, 3.1 ?
Fig. 3
dipping the substrate into the Ag nanoparticle ink and then air dried;
(b) SERS spectra of Rh6G (1 mM) on a plastic substrate. No signals were
observed from Ag coated plastic substrate without Rh6G.
(a) Ag nanoparticles coated on flexible plastic substrate by
Fig. 4
particle films measured as a function of annealing temperature in a four-
probe configuration. The inset shows the colors of the Au and Ag
nanoparticle films after annealing at 250?C. (b) Conductivity of the
dodecylamine and octylamine capped Au nanoparticle films measured as
a function of annealing temperature. The transition temperatures for
dodecylamine and octylamine are 190–200
respectively.
(a) Conductivity of the oleylamine capped Au and Ag nano-
?C and 130–140
?C,
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transition temperatures, which is also responsible for the
dramatic increase of conductivity of the film.
We have recently demonstrated the promise of chemically
processed graphene in printed electronics.49Here we employed
gold nanoparticles as printable electrodes for source and drain
contacts on a multi-layer graphene film field-effect transistor
(MLG-FET) with layer thickness of ?15 nm (Fig. S7 in ESI†).
The source and grain were prepared by depositing gold nano-
particles onto the GO film using micropipette followed by
annealing the film at 250?C for 5 min. The prepared source and
drain electrodes are uniform, highly conductive. The conduc-
tivity of the graphene films across the source and drain was
2116 S m?1(see Fig. S8† for the I–V curve). P-Type response was
reproducibly observed for the prepared MLG-FET device with
a maximum hole mobility of 26.4 cm2/(V s) (see ESI† for details).
These experiments demonstrated that these gold nanoparticle
inks could act as attractive materials for printable electrodes and
be extended to applications in printable organic electronics.
Photoresponse measurements
The photoresponse of the solution processed metal nanoparticle
films has also been investigated. Broad range of wavelengths of
light can be detected by using these nanoparticle films owing to
the broad extinction spectra of the film as shown in Fig. 5a. The
photoresponses of the Au and Ag films prepared from oleyl-
amine capped Au and Ag nanoparticles were studied by using
different diode lasers with wavelengths at 405, 532, 980, 1064 nm.
Fig. 6A shows the schematic picture of the device fabrication and
photoresponse measurements. The device was fabricated by spin
coating the oleylamine capped gold or silver nanoparticles on the
PSS: PEDOT modified substrate, annealing at 120?C and then
followed by cathode (aluminium) evaporation (Fig. 6A). The
extinction spectra of the prepared Au and Ag nanoparticle films
on the ITO substrate (Fig. 5a) are very broad, spanning from
visible to near-Infrared wavelengths. The AFM images (Fig. 5b
and c) of the nanoparticle films showed that 30–40 nm particles
were closely packed on the entire substrates. The initial 5 nm
nanoparticles transformed into 30–40 nm particles after
annealing the films at 120?C. These larger particles couple each
other on the entire film, which is responsible for the broad
extinction spectra. Fig. 6B and 6C shows the typical photo-
response of Au and Ag nanoparticle films upon alternating on/
off laser illumination at 532 and 1064 nm in the air at room
temperature. It can be seen that the Au and Ag nanoparticle films
respond to both the visible and near-Infrared illumination. The
photocurrent generated from the silver nanoparticle film is
slightly higher than that generated from the gold nanoparticle
film. The higher photoconductivity of silver might be due to the
high conductivity and closely packed film morphology of the
silver nanoparticle film (Fig. S9 in ESI†).8,50The photocurrent
observed in our study is higher than that of the previously
reported gold nanoparticle films prepared by thiol capped gold
nanoparticles.35The photocurrent response showed a similar
Fig. 5
ITO substrates after annealing the substrate at 120
in comparison with the nanoparticles in toluene solution (dashed lines).
(b–c) The corresponding AFM images.
(a) Extinction spectra of the gold and silver nanoparticle films on
?C (solid lines)
Fig. 6
experimental setup. (B,C) Photocurrent response of Au and Ag nano-
particle films to alternating on/off 532 (B) and 1064 nm (C) laser illu-
mination at a power of 30 mW. (D, E) Laser power dependent
photocurrent generation for the Au (D) and Ag (E) nanoparticle films,
respectively.
(A) Schematicrepresentation of the photoresponse device and the
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pattern to that of the carbon nanotube films, which was
explained as due to low mobility of the interconnected nanotube
network.51,52The photocurrent originates from the hot electrons
excited by SPR absorption, which diffuse through the organic
layer barriers on the nanoparticles. It is believed that the heat
generated in the metal nanoparticle film upon laser illumination
will increase the conductivity of the film, which results in an
increase of photocurrent. The slow build-up of the temperature
upon the laser irradiation and subsequent slow cooling lead to
the observed slow rise and decay. Fig. 6D–E show the power
dependent photocurrent generation from the Au and Ag nano-
particle films under irradiation at 405, 532, 980 and 1064 nm,
respectively. The maximum photocurrent was observed for laser
irradiation wavelength of 532 nm. The photocurrent decreased as
the excitation wavelength changed from 532 to 1064 nm. For
both gold and silver films, the photocurrent at different laser
irradiation power displayed a linear relationship, similar to
many photodetection devices.52The device showed very good
stability. The device performances were checked three months
after their initial preparation and the performance remained
unaltered.
Conclusions
Oleylamine capped gold and silver nanoparticles were prepared
by a general and facile method, which can be generalized to
prepare any alkylamine capped nanoparticles. Using these metal
nanoparticle ‘‘inks’’, we have demonstrated their applications in
large scale fabrication of SERS substrates with a detection limit
of nM concentration via solution processing. Silver nanoparticle
films were found to give larger enhancement compared to gold
nanoparticle films. The annealing temperature dependent
conductivity of the nanoparticle films indicated a transition
temperature above which high conductivity was achieved. The
transition temperatures could be tailored to the plastics
compatible temperatures by using proper alkylamine as the
capping agent. The ultrafast electron relaxation studies of the
nanoparticle films demonstrated that faster electron relaxation
was observed at higher annealing temperatures due to stronger
electronic coupling between the nanoparticles. The application of
these alkylamine capped gold nanoparticle inks to printable
electronics has been demonstrated by fabrication of Graphene
FET by printing the source and drain using the oleylamine
capped gold nanoparticles. Furthermore, the broadband pho-
toresponse properties of the Au and Ag nanoparticle films have
also been demonstrated by using visible and near-infrared lasers.
These results demonstrate that these nanoparticle ‘‘inks’’ are
promising for low cost and large scale production of printable
SERS substrates, electronics, and broadband photo-response
devices.
Experimental section
Materials
Silver nitrate (AgNO3, 99%) and gold(III) chloride trihydrate
(HAuCl4$3H2O, 99.9%), oleylamine, dodecylamine, octylamine
were purchased from Aldrich. All chemicals were used as
received without further purification.
Methods
Preparation of alkylamine capped gold and silver nanoparticle
‘‘inks’’. In a typical synthesis of silver nanoparticle ‘‘ink’’, 100 mg
AgNO3was first dissolved in 50 mL of toluene containing 2 mL
of oleylamine in a round bottom flask. The mixture was soni-
cated about 20 min until all AgNO3 was totally dissolved.
AgNO3was reduced by quickly adding 10 mL of 10 mg ml?1
freshly prepared NaBH4 solution under vigorously stirring
condition. Theaddition ofNaBH4resulted in animmediate color
change from colorless to dark yellow. The mixture was kept
stirring for another 5 min. The nanoparticles containing organic
layer was separated from the aqueous layer in the reaction
mixture and then filtered to remove any undissolved impurities.
The nanoparticles were purified by a series of procedures. First,
the toluene solvent was removed by using rotavapour. 100 mL of
ethanol was then added into remaining solution to precipitate
out the nanoparticles. The precipitate was separated by centri-
fugation (5000 rpm for 15 min) and washed with ethanol two
more times to wash off the remaining uncapped oleylamine and
other impurities. The precipitate was then dissolved in toluene
after being dried for ?10 min. The solution was again filtered to
remove any undissolved impurities. Ag nanoparticle ‘‘ink’’
(in toluene solvent) with a concentration of 50 mg ml?1were then
prepared. The same steps were performed to prepare the oleyl-
amine-cappedgoldnanoparticle
a precursor. Unlike silver salt, the gold salt dissolves in toluene in
the presence of oleylamine very easily by sonicating the solution
for 2–3 min. The similar procedures can be used to prepare any
alkylamine capped gold or silver nanoparticle ‘‘inks’’.
ink using HAuCl4
as
Preparation of SERS substrates using gold and silver nano-
particle ‘‘inks’’. The glass substrates were first sonicated in aqua
regia for 30 min followed by rinsing with water and ethanol. The
glass plates were subsequently washed with acetone and dried in
oven. The SERS substrates were prepared by spin coating the
oleylamine capped Ag or Au ink of concentration 30 mg ml?1
with a spin speed of 600 rpm for 60 s. Four substrates were then
annealed at different temperatures (100,150, 200, and250?C) for
10 min. 10 mL of 1mM Rhodamine6G solution was dropped onto
each SERS substrate using micropipette. The SERS measure-
ments were performed by using a Renishaw micro-Raman
spectrometer with an excitation wavelength of 514 nm.
Conductivity measurement using a four-probe configuration.
The conductivities of the nanoparticle ink at different annealing
temperatures were measured by using a four-probe substrate.
The four-probe substrate was prepared by thermal evaporation
of four aluminium electrodes on glass substrate in a line with
a spacing of 4 mm. The nanoparticle ink was dropped onto these
four probe substrates with a thickness of ?200 nm and the
substrate was annealed to different temperatures with an
increasing rate of 5
temperature for 5 min. The conductivity was calculated by
measuring the resistance of the nanoparticle film by using GS 610
source measurement unit (Yokogawa).
?C per minute and maintained at each
Preparation of nanoparticle films for broadband photoresponse
measurements. Oleylamine capped gold or silver nanoparticles
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Page 7

were dissolved in toluene with a concentration of 30 mg ml?1and
spin-coated on ITO/PEDOT electrode with a spin speed of
800 rpm for 60 s and annealed at 120?C for 10 min under N2
atmosphere. Subsequently Al electrode (100 nm thickness) was
prepared by using evaporation method at 1 ? 10?6bar vacuum
to complete the device fabrication. Active layer (Au or Ag
nanoparticles) film thicknesses were measured to be 80 nm using
surface profiler. The device has a structure of ITO/PEDOT:PSS
(40 nm)/Au or Ag nano particles (80 nm)/Al (100 nm). All
photocurrent and electric (I–V) measurements were carried out
by using an Autolab PGSTAT30 potentiostat (EcoChemie, The
Netherlands). The photocurrent response of Au and Ag nano-
particle films were measured by exciting with diode lasers of
different wavelengths (405, 532, 980 and 1064 nm) ranging from
visible to near-infrared range under an ITO voltage of 0 V.
Acknowledgements
This work is supported by the Faculty of Science, National
University of Singapore (R-143-000-341-112), and NRF (Gra-
phene Related Materials and Devices, R-143-000-360-281).
Lakshminarayana Polavarapu and Kiran Kumar Manga
contributed equally to this work.
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