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Nanoscale
Published on 25 February 2014. Downloaded by Nanyang Technological University on 28/04/2014 03:13:21.
COMMUNICATION
Cite this: DOI: 10.1039/c3nr06341k
Received 29th November 2013
Accepted 7th February 2014
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Quantum dots on vertically aligned gold nanorod
monolayer: plasmon enhanced fluorescence
Bo Peng,a Zhenpeng Li,a Evren Mutlugun,c Pedro Ludwig Herna´ndez Mart´ınez,c
Dehui Li,a Qing Zhang,a Yuan Gao,ac Hilmi Volkan Demir*acd and Qihua Xiong*ab
DOI: 10.1039/c3nr06341k
www.rsc.org/nanoscale
CTAB-coated Au nanorods were directly self-assembled into a vertically aligned monolayer with highly uniform hot spots through a
simple but robust approach. By coupling with CdSe/ZnS quantum
dots, a maximum enhancement of 10.4 is achieved due to: increased
excitation transition rate, radiative rate, and coupling efficiency of
emission to the far field.
Plasmonics exhibit the potential to manipulate optics on the
sub-wavelength scale, promising exciting applications
including negative refractive index media and metamaterials.1–5
The self-assembly of monodispersed nanoparticles into ordered
structures is an effective approach for designing plasmonics.6–9
Spherical Au nanoparticles have been manipulated to generate
a diverse selection of plasmonic topologies such as superlattice
sheets,10,11 chain networks,12 and chiral pyramidal structures.13
However, ordered assemblies of anisotropic nanostructures,
such as Au nanorods, are still very challenging, because the
anisotropic shape results in various assembling behaviours. By
the Langmuir–Blodgett technique,14,15 hydrophobic polymermodied Au nanorods were self-assembled into monolayer
sheets consisting of random horizontal and vertical Au nanorods at the air–water interface.16 Recently, based on the coffeering effect, the evaporation-induced self-assembly has been
developed.17 During evaporation of the solvent, a ow from the
inner region replenishes the liquid that is evaporated at the
edge. As a result, nanorods are transferred to the droplet edge
and self-assembled into ordered multilayer parallel arrays.18,19
a
Division of Physics and Applied Physics, School of Physical and Mathematical
Sciences, Nanyang Technological University, Singapore 637371. E-mail: hvdemir@
ntu.edu.sg; [email protected]
b
NOVITAS, Nanoelectronics Centre of Excellence, School of Electrical and Electronic
Engineering, Nanyang Technological University, Singapore 639798
c
LUMINOUS, Centre of Excellence for Semiconductor Lighting and Displays, School of
Electrical and Electronic Engineering, Nanyang Technological University, Singapore
639798, Singapore
d
Bilkent University, Department of Physics, Department of Electrical and Electronics
Engineering, UNAM – Institute of Materials Science and Nanotechnology, TR-06800,
Ankara, Turkey
This journal is © The Royal Society of Chemistry 2014
Meanwhile, a near-equilibrium status is also formed at the
internal region of the drying droplet, which contributes to the
self-assembly of free anisotropic nanostructures to their lowest
energy state in solution. A few previous studies have reported
the formation of multilayer vertical arrays by decreasing the
electrostatic repulsive force via modifying Au nanorods with
weak polar ligands instead of cetyltrimethyl-ammonium
bromide (CTAB).20,21 Until recently, there have only been a few
reports on the directional self-assembly of CTAB-stabilized Au
nanorods into vertically aligned multilayer arrays.22,23 Recently,
our group has shown that it is possible to directly self-assemble
CTAB-coated Au nanorods into highly organized vertical
monolayer arrays by synergistically controlling the electrostatic
repulsive force and the van der Waals attractive force.24
In the past decade, plasmonics has been proven to enhance
or quench uorescence as a function of the distance between
the surface plasmons and uorophores.25–30 The previous
reports focused their research on the interactions between uorophores and single Au nanoparticles,31 single silver nanoprisms,32 and Au disks or hole arrays.33 However, a handful of
works reported the resonant energy transfer based on the
metallic colloid arrays as a surface plasmonic system.34,35
Moreover, many colloid substrates suffer from poor reproducibility of “hot spots”. Herein, we demonstrate a simple yet
robust approach to directionally assemble CTAB-coated Au
nanorods into a vertically aligned monolayer and investigate the
energy transfer between plasmonic array and monolayer CdSe/
ZnS quantum dots (QDs). Vertically aligned Au nanorod
monolayers exhibit a strong, reproducible, and highly homogeneous distribution of hot spots. Our experiments show that
Au nanorods with an aspect ratio from 2.1 to 3.2 can be selfassembled into a vertically aligned monolayer and the electrostatic force and van der Waals force predominate in the selfassembly. The SiO2 lms were coated on the vertically aligned
Au nanorod monolayer by sputter deposition as a spacer to tune
the distance between the plasmonic monolayer and QDs. A
distinct plasmonic enhancement property was uncovered and a
maximum uorescence enhancement of 10.4 was achieved at a
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20 nm SiO2 spacer. Fluorescence-lifetime imaging microscopy
(FLIM) was used to measure the lifetime of QDs, which
decreased from 4.1 ns to minimum, 0.9 ns, due to the
increased radiative decay rate. Therefore, the plasmonic
monolayer arrays, due to its enhanced optical density of states,
can serve as 2D optical materials such as directionality
controller, light enhancers, colour lters, and ultrasensitive
SERS sensors.36–38
Based on the evaporation-induced strategy, CTAB-coated Au
nanorods with an aspect ratio of 2.7, which were synthesized
by the seeded growth method,39 were self-assembled into a
vertically aligned monolayer on Si substrates when the Debye
length was 3.0 nm (Fig. 1a). The Au nanorods in the monolayer
are aligned to a hexagonal close-packed structure. The inset in
Fig. 1a shows a 3-D schematic diagram of the arrays. The interior gap distance between two adjacent Au nanorods is
7.7 nm. Fig. 1b exhibits the monolayer structure in a vivid
fashion. Surprisingly, the vertically aligned hexagonal Au
nanorod bi-layer was formed in the case when the Debye length
decreased to 1.3 nm by adding NaCl (Fig. 1c) and the interior
gap distance is 3.4 nm, which corresponds to the length of a
CTAB bi-layer.17,40 Our approach can be extended to other Au
nanorods with different aspect ratios. Fig. 1d shows the
absorption spectra of different Au nanorods. The longitudinal
plasmon bands are 656, 680, 717, 755 and 780 nm, where the
aspect ratios are 2.1, 2.3, 2.7, 2.9 and 3.2, respectively. And the
diameters of the corresponding Au nanorods are 45.4, 42.6,
39.8, 32.3 and 27.3 nm, while the lengths are 94.2, 96, 100.5,
94.1, and 88.2 nm, respectively.
During the self-assembly of Au nanorods, there are three
forces: van der Waals force, depletion force and electrostatic
force, whose energy is dened by EvdW, Edep, Eele, respectively. The
van der Waals and the depletion force are attractive forces and
the electrostatic force is a repulsive force. The attractive forces
Fig. 1 SEM image of a vertically aligned Au nanorod monolayer: (a) top
view, (b) view from the edge. The insets in (a) show the schematic
picture of a vertically aligned self-assembled Au nanorod monolayer.
(c) The side-view SEM image of vertically aligned hexagonal Au
nanorod bi-layer. (d) The absorption spectra of Au nanorod aqueous
solution. The longitudinal plasmon bands are 656, 680, 717, 755 and
780 nm, respectively.
Nanoscale
Communication
push Au nanorods to approach each other, whereas the repulsive
forces make the nanorods moving away from each other.
Therefore, the synergy between the repulsive force and attractive
force ensures the alignment of Au nanorods at the equilibrium
status, rather than a random aggregation. We have calculated
EvdW, Edep and Eele for ve kinds of Au nanorods with 656, 680,
717, 755 and 780 nm plasmon band as a function of edge-to-edge
gap size in the case that the Debye length was 3.0 nm. EvdW and
Edep can be obtained from previous reports.20,41 For all Au nanorods, EvdW is much larger than Edep. Both EvdW and Edep increase
as the gap size decreases (Fig. 2a). But EvdW and Edep decrease as
the plasmon band increases from 656 to 780 nm. To calculate Eele
between two adjacent Au nanorods, Derjaguin's approximation is
used.42 We assume the Au nanorod consists of many slices of
parallel thin plates, which contribute to Eele.40 Fig. 2c shows that
Eele decreases with the increasing gap size and also decreases as
the plasmon band of Au nanorods increases from 656 to 780 nm.
We dene the total interaction energy by Etotal ¼ EvdW + Edep + Eele,
which rst decreases to a minimum and then increases as the
gap size increases. However, for all Au nanorods with different
plasmon bands, the total energy is minimized at 7.5 nm gap size,
which indicates that all Au nanorods can be self-assembled into a
vertically aligned monolayer. When the Debye length is decreased
to 1.3 nm by adding NaCl, the calculated minimum Etotal is
achieved at a gap size of 3.9 nm. However, the lowest limit we can
achieve in an aligned Au nanorod monolayer is 6.7 nm.
Therefore, a vertically aligned Au nanorod bi-layer is formed to
achieve the lowest energy state. In our experiments, the edge-toedge gap size of bi-layer arrays is 3.4 nm in the case that the
Debye length is 1.3 nm. We suggest that the CTAB molecules are
inter-digitated when they approach to touch each other, which
leads to a strong depletion force.17 Therefore, the experimental
gap size is a little smaller than the calculated data.
We focused on the monolayer arrays consisting of Au
nanorods with a longitudinal plasmon band at 717 nm and
Fig. 2 Interaction free energy as a function of the gap size in the case
when the Debye length k1 is 3 nm: (a) van der Waals force, EvdW, (b)
depletion force, Edep, (c) electrostatic repulsive force, Eele, (d) total
interaction energy, defined by ETotal ¼ EvdW + Edep + Eele.
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investigated the energy transfer between plasmonic arrays and
monolayer CdSe/ZnS QDs. Sputtered SiO2 layer was used as a
spacer to control the distance between the plasmonic monolayer and QDs (Fig. 3a). 10 mL CdSe/ZnS QDs hexane dispersion
was drop-cast onto the surface of acetonitrile in a Teon well
(2 2 2 cm3).6 A monolayer quantum dot (QD) selfassembled lm (a typical TEM image is shown in Fig. 3b) was
formed within 5 s and then transferred onto the vertically
aligned Au nanorod monolayer. The absorption peak of the
vertically aligned monolayer consisting of Au nanorod with
717 nm plasmon band is 610 nm checked by CRAIC 20
microspectrophotometer (Fig. 3c). The polarization of white
light was parallel to the interparticle distance. Therefore, the
transverse plasmon modes interact attractively and these
underwent a red shi from 520 to 610 nm.43 The photoluminescence (PL) of CdSe/ZnS QD monolayer with and without
plasmonic coupling was measured by a home-built system. A
532 nm laser was used as the excitation, whose polarization
direction was along the side of Au nanorod hexagon in the
vertically aligned monolayer. Fig. 3d shows room-temperature
steady-state PL spectra of CdSe/ZnS QD monolayer on Si
substrates and the vertically aligned Au nanorod monolayer,
respectively. The PL peak is at 614 nm, which is close to the
absorption peak of Au rod arrays. Therefore, a maximum
coupling between PL and the scattering of the vertically aligned
Au nanorod monolayer can be achieved, which inuences the
enhancement of QD emission.44 A pronounced SiO2 spacer
Nanoscale
dependence is spotted from Fig. 3d. We evaluate the enhancement factor (EF) using EF ¼ I/I0, where I and I0 are the PL
intensities of CdSe/ZnS QD monolayer with and without the
vertically aligned Au nanorod monolayer, respectively. The PL
intensities are extracted and EF is plotted versus SiO2 spacer in
Fig. 3e. When the SiO2 spacer thickness is 5 nm, the PL of the
QD monolayer on vertically aligned Au nanorod monolayer is
quenched.22 When we systematically change the separation
between QDs and the plasmonic array, we have clearly observed
the distance dependent PL enhancement. The PL intensity with
plasmonic coupling at a 10 nm SiO2 spacer is slightly larger
than that of QD monolayer without plasmonic coupling. When
the SiO2 spacer is further increased to 50 nm, the PL intensity
of QD monolayer on the vertically aligned Au nanorod monolayer is comparable to the PL signal of the QD monolayer on Si
substrates within the experimental error, which indicates that
the plasmonic coupling has barely any effect to the PL when the
spacer is larger than 50 nm. In our experiments, PL exhibits a
local maximum at 20 nm SiO2 spacer as also in agreement
with the previous reports.35,45 Generally speaking, the incident
light is conned into a small spatial space by the surface plasmon, resulting in a strong local electromagnetic eld, |E|. The
coupling of the excited dipole with plasmons not only has a
great effect on both the nonradiative and radiative mode of the
transition dipole, but also increases the absorption of incident
light.46 The competition of the nonradiative and radiative mode
determines PL quenching and enhancement. In the case of PL
Fig. 3 (a) Schematic picture of the coupling of vertically aligned Au nanorod monolayer and CdSe/ZnS QDs monolayer films. SiO2 layer is used as
a spacer to control the distance between the plasmonic monolayer and QDs. (b) TEM image of monolayer films of CdSe/ZnS QDs (QDs) with PL
emission at 614 nm. (c) Normalized absorption spectrum of a vertically aligned monolayer consisting of Au nanorods with 717 nm plasmon band.
(d) Photoluminescence spectra of CdSe/ZnS QDs monolayer films on vertically aligned Au nanorod monolayer. The silica spacer thickness is 5,
10, 15, 20, 25, 30, 40, and 50 nm, respectively. Non-monotonous behaviour is observed. (e) Plot of calculation (black line + circle, the red circle
show the theoretical data at the experimental spacer thickness) and experimental (red line + star) enhancement factor as a function of silica
spacer thickness.
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quenching, the nonradiative energy transfer from uorophores
to plasmonic monolayer predominates. Both the small distance
and large spectral overlap between the PL spectra and plasmon
band lead to a fast nonradiative energy transfer from
the excited dipole to the plasmon. In the case of the PL
enhancement, rst of all, absorption increases due to the
increase of |E|, gexc f |p$E|2, where p is the transition dipole
moment of the uorophores.25 Second, the radiative rate is
enhanced signicantly due to the Purcell effect.29,44,47,48 Because
2 0
the radiative rate is related to |E|2, rsp
rad f |E| rrad, where the
radiative decay rates with and without plasmonic coupling are
0 33
rsp
Therefore, the enhancement factors are large
rad and rrad.
enough to exhibit the increase of PL intensity.31,35,46 In our case,
the plasmon resonance frequency (610 nm) corresponds to
the emission frequency (614 nm) of CdSe/ZnS QDs. Therefore,
the coupling of QD emission to the far eld is increased by the
scattering of Au nanorod monolayer arrays, which also induces
PL enhancement.44
To theoretically estimate the enhancement factor, we
consider two main assumptions. First, we calculate the
normalized rate of energy dissipation due to a radiative dipole
on top of a metallic surface. Second, we estimate the electric
eld enhancement due to surface plasmon of the metallic
nanorod where the cylinder surface plasmon is approximated to
a disk. The total effective electric eld of one unit in the vertically aligned monolayer is assumed to be 6 times stronger than
one disk because the Au nanorods are aligned in a hexagonal
close packed structure. Therefore, the intensity enhancement
factor is dened as
1
Pðr0 ; uemission Þ
h r; r0 ; uexcitation ; uemission ¼ Aðr; uexcitation Þ
P0 ðr0 ; uemission Þ
where P/P0 is the normalized rate of energy dissipation of a
radiative dipole dened as49,50
ðN 3
h
pffiffiffiffiffiffiffiffiffiffiffiffi i
P
1
s ds
pffiffiffiffiffiffiffiffiffiffiffiffi rðpÞ ðsÞexp 2ik1 1 s2 h þ
¼ 1 þ Re
P0
2
1 s2
0
ðN
pffiffiffiffiffiffiffiffiffiffiffiffi i
h
1
sds pffiffiffiffiffiffiffiffiffiffiffiffi rðsÞ ðsÞ 1 s2 rðpÞ ðsÞ exp 2ik1 1 s2 h
Re
2
1 s2
0
where r(s)(s) and r(p)(s) are the reection coefficients for s- and ppolarized waves, respectively, dened as
pffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
k1 1 s 2 k2 2 s 2 k 1 2
rðsÞ ðsÞ ¼ pffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
k 1 1 s 2 þ k2 2 s 2 k 1 2
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
pffiffiffiffiffiffiffiffiffiffiffiffi
32 k1 1 s2 31 k2 2 s2 k1 2
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
r ðsÞ ¼
pffiffiffiffiffiffiffiffiffiffiffiffi
32 k1 1 s2 þ 31 k2 2 s2 k1 2
ðpÞ
where 3i and ki are the dielectric constant and wave vector of the
medium 1 (SiO2 spacer) and 2 (Au rod). And the electric eld
enhancement factor due to the presence of the metallic nanostructure is dened as51
ð
jEmetal ðr; uÞj2 dV
VQD
Aðr; uÞ ¼ ð
2
jE0 ðr; uÞj dV
VQD
Nanoscale
where E0(r,u) and Emetal(r,u) are the electric eld without and
with metallic nanostructure, respectively. The electric eld for
the cylinder with radius “a” is
8
1
>
>
2 z r ¼ 0; z . 0
>
>
z
>
>
>
1þ
>
>
a
>
>
>
1
0
>
>
<
r
z
!
r0 ^
r þ z^ C
B
EðrÞ ¼ E0 ð 1
1
a
a
C
B
>
>
p
ffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ffi
r dr
B
>
2 3=2 C
2
>
2
A 0 0
@
>
1 r0
0
r
z
>
>
>
þ
r
0
>
>
a
a
>
>
>
>
:
r . 0; z . 0
where r is the radial component of the electric eld perpendicular to the cylinder axis. Z is the z component of the electric
eld parallel to the cylinder axis. As shown in Fig. 3e, the
numerical calculation results are in good agreement with our
experimental data.
To understand the coupling of plasmonic monolayer and
QDs, uorescence-lifetime imaging microscopy was used to
obtain time-resolved uorescence spectroscopy, which is critically dependent on the distance between the plasmonic
monolayer and QDs. Fig. 4a shows a microscopic image of the
vertically aligned Au nanorod monolayer (yellow color) coated
by CdSe/ZnS QD monolayer lm. The SiO2 spacer is 10 nm.
From the crack band of QD monolayer lms, it is clearly shown
that the QD monolayer lms are darker than Si substrates and
cover the vertically aligned Au nanorod monolayer. Fig. 4b
shows a FLIM image of QD monolayer corresponding to Fig. 4a.
The crack band of QD monolayer lm is vivid. The lifetime of
CdSe/ZnS QD monolayer on a vertically aligned Au nanorod
monolayer is much shorter than that on Si substrates, which
indicates that the lifetime of QDs decreases due to plasmonic
interaction. Fig. 4c shows time-resolved PL spectra of CdSe/ZnS
QDs. The SiO2 spacer thicknesses are 5, 10, 15, 20, 25, 30, 40 and
50 nm, respectively. All the time-resolved PL decay curve can be
well tted as a bi-exponential function, I(t) ¼ A1 exp(t/s1) +
A2 exp(t/s2), with two time constants: a fast decay (s1) accompanied by long-lasting emission (s2), where I(t) is PL intensity.52,53 The results are consistent with previous studies. The
fast decay s1 is associated with recombination of carriers from
the delocalized states in the core region, where the slow decay s2
originates from recombination of carriers from the localized
states at the hetero-interface.54,55 The average lifetime is dened
by s ¼ s1 a1 + s2 a2, where a1 and a2 are the fraction of s1
ands2, respectively. For CdSe/ZnS QDs on Si substrates, the
lifetime is s ¼ 4.1 0.1 ns (s1 ¼ 2.3 0.3 ns and s2 ¼ 10.3 2.1
ns). Fig. 4d shows the lifetime s as a function of SiO2 spacer
thickness. The lifetime was obtained by measurement and
statistical analysis of many CdSe/ZnS QD monolayers on the
vertically aligned Au nanorod monolayer (>10). The decay lifetimes s are 2.4 0.14, 2.0 0.2, 1.8 0.15, 0.9 0.1, 1.9 0.25,
2.6 0.16, 3.5 0.21, and 3.7 0.2 ns in the case that SiO2
spacer thickness is 5, 10, 15, 20, 25, 30, 40 and 50 nm, respectively. The lifetime rst achieves a minimum at a 20 nm spacer
and then increases, which corresponds to the PL enhancement
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Nanoscale
0
new
sp
0
rsp
rad ¼ rrad + rrad , where rrad and rrad are the radiative decay rate
with and without plasmonic coupling, respectively.34 Furthermore, the correspondence between plasmon resonance
frequency (610 nm) and emission frequency (614 nm)
enhances the coupling efficiency of emission to the far eld.
Therefore, the PL of CdSe/ZnS QDs is enhanced by plasmons.
Therefore, we suggest that the increased excitation transition
rate, radiative rate of excition recombination for emission, and
coupling efficiency of emission to the far eld lead to
enhancement of CdSe/ZnS QD monolayer lms, which has been
proved by previous reports in theory and experiment.33,44,56,57
Conclusions
Fig. 4 (a) Optical microscopy image of vertically aligned Au nanorod
monolayer covered by CdSe/ZnS QD monolayer films. The SiO2
spacer is 10 nm. (b) Fluorescence-lifetime microscopy image of CdSe/
ZnS QD monolayer films corresponding to (a). The corresponding
vertically aligned Au nanorod monolayer covered by QD monolayer in
(a) are shown in the white dotted circles. The bare CdSe/ZnS QD
monolayer is outside of the golden yellow in (a). The lifetime of CdSe/
ZnS QDs is shortened vividly on the vertically aligned Au nanorod
monolayer. (c) Time-resolved fluorescence spectra of CdSe/ZnS QD
monolayer films on vertically aligned Au nanorod monolayer. The silica
spacer thickness is 5, 10, 15, 20, 25, 30, 40, and 50 nm, respectively.
The solid lines are fits to the data using an exponential decay function
as described in the text. (d) The average lifetime s as a numerical
function of the silica spacer thickness. The lifetime was obtained by
measurement and statistical analysis of (b). The insets show s1 and
s2 versus the silica spacer thickness, respectively.
results, where the enhancement of 10.4 times is maximized at a
20 nm spacer. It is important to note that the trends of s1 and
s2 as a function of the silica spacer thickness are the same with
s, as shown in the insets in Fig. 4d. Both s1 and s2 are minimized
in the case of a 20 nm silica spacer. The decrease of lifetime
corresponds to the pronounced enhancement of PL. Therefore,
the increased radiative rate is large enough and contributes to
the PL enhancement, although plasmon coupling enhances
both the radiative and nonradiative rate.32,46 Generally speaking,
three factors determine the overall PL enhancement: increased
transition rate of electrons, increased radiative rate due to the
coupling of excitons with plasmons, and enhanced coupling
efficiency of the uorescence emission to the far eld.44 When
CdSe/ZnS QDs are excited by incident photons, the electrons in
the valence band transfer into the conduction band. The transition rate can be enhanced by surface plasmons, because the
photon absorption of QDs is increased by plasmonic coupling.25
Meanwhile, the vertically aligned Au nanorod monolayer
reects the incident photons to excite CdSe/ZnS QDs again,
which also contributes to enhancement of the transition rate.34
When exciton recombination induces PL emission, a new
modied radiative decay rate is induced due to plasmonic
coupling, rnew
rad . Therefore, the radiative decay rate is accelerated,
This journal is © The Royal Society of Chemistry 2014
In summary, vertically aligned Au nanorod monolayers with
7.7 nm edge-to-edge gap size were prepared by an evaporationinduced self-assembly strategy in the internal region of a drying
droplet, where a near-equilibrium state is achieved. This
approach is extended to Au nanorods with an aspect ratio of 2.1,
2.3, 2.7, 2.9 and 3.2, where the longitudinal plasmon bands are
656, 680, 717, 755 and 780 nm, respectively. A vertically aligned
Au nanorod bi-layer is formed by decreasing the Debye length to
1.3 nm. During the self-assembly of Au nanorods, the electrostatic force and van der Waals force predominate, determining
the self-assembling behavior of Au nanorods. Based on the
vertically aligned monolayer consisting of Au nanorods with a
longitudinal plasmon band at 717 nm, we investigate the energy
transfer between the plasmonic monolayer and CdSe/ZnS QDs.
The SiO2 lms deposited by sputtering are used as the spacer to
control the distance between the plasmonic array and QDs. The
CdSe/ZnS QD monolayer is coated on the vertically aligned Au
nanorod monolayer. Maximum enhancement of 10.4 times is
achieved at the 20 nm spacer, where the lifetime of CdSe/ZnS
QDs is minimized to 0.9 ns. The PL enhancement is due to three
components: increased excitation transition rate of electrons
from the valence band to the conduction band, increased
radiative rate in the case of exciton recombination for emission,
and increased coupling efficiency of PL to the far eld through
scattering of the vertically aligned Au nanorod monolayer.
Acknowledgements
Q.X. and H.V.D. gratefully acknowledge nancial support from
the Singapore National Research Foundation via a Competitive
Research Program (NRF-CRP-6-2010-2). Q.X. also thanks for
support from the Singapore National Research Foundation
through a fellowship grant (NRF-RF-2009-06), and the Singapore Ministry of Education via two Tier 2 grants (MOE2011-T2-2051 and MOE2011-T2-2-085). Additionally, H.V.D thankfully
acknowledges the Singapore National Research Foundation
Fellowship Program (NRF-RF-2009-09).
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