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Manipulating the localized surface plasmons
in closely spaced metal nanoparticle arrays
Min Han
National Laboratory of Solid State Microstructures
Nanjing University
Oct. 21, 2015
Outline
1. Manipulating LSP with interparticle
spacing controlling
2. Plasmonic modulation in LSP-SPP
coupling systems
3. Directional emission generated with
near filed coupling of closely spaced LSPs
1. Manipulating LSP with interparticle
spacing controlling
Closely spaced nanoparticle arrays
Interparticle spacing ～
(electron) tunneling length
optical near field range
(spin) exchange interaction range
magnetic dipole interaction range
Controlling the interparticle spacing and pattern (~nm scale)
Tunneling/hopping
SPR near field
coupling
Electron transport
Local electromagnetic
field/ plasmonic prop.
Electronic properties
Optical properties
Exchange/dipole
interaction
Spin arrangement
Magnetic properties,
spintronics
QD devices, microsensors, thermoelectric devices,
optical devices, ……
Junction plasmons: near field coupling
Universal scaling behavior of
metal dimers
SPR wavelength shift due to
plasmon coupling.
There is a universal scaling
behavior of surface plasmon
resonance of matal nanoparticle
dimers:
Plasmon ruler equation
Dl
l
 ae

x

Dl:the resonance shift due to plasmon coupling
l: the resonant wavelength of isolated particle
X: gap size/particle diameter ratio
Acimovic S. S. et al, ACSNano (2009) 3,1231
SPR wavelength shift nearexponentially over a normalized
inter-particle spacing.
Junction Plasmons: near field coupling
Single spheres
Aggregates(two spheres)
d
“Hot spot” Coherently driven AC dipoles couple
strongly.
Polarization
charge
localized at the interface
106 Raman enhancement
at resonance
For nanoparticle dimer with s-polarization
1012 enhancement as d→1
Nanoparticle assemblies with nanoscale gaps can generate the largest local field
enhancement owing to the near field coupling of junction plasmons created in the gaps.
The local area with intensely enhanced field is called ‘hot spots’,
Closely spaced nanoparticle array:
Quantum transport
When the interparticle spacing is short enough, electron transport through tunneling
or hopping occurs sufficiently between adjacent particles under appropriate bias.
Tunneling/hopping
Electron transport
Coulomb gap
Coulomb gap related: Metal-nonmetal transition of nanoparticle
array, temperature, spacing and configuration dependent
For very narrow interparticle spacing, conductive overlap is established between the
NPs, quantum mechanical effects start to be important. These are primarily the
electron tunneling across the junction. A new plasmon mode is enabled. This is the
charge transfer plasmon and involves conduction electrons flowing back and forth
between the two nanoparticles.
Gas phase fabrication of closely spaced
nanoparticle arrays
Gas-Phase Cluster
Beam Deposition
Cluster beam
Temperature
Controller
Optical monitoring
Electric monitoring
Template
Mask
Coverage control
The interparticle spacing may be compatible to the
electron tunneling length.
The current pass through the electrode varies with
the deposition time, the nanoparticle coverage can
be controlled by the monitoring of the conductance
changes.
Dense silver nanoparticle arrays
Silver nanoparticle arrays on Formvar film surface
Constant size, increasing number density
Silver nanoparticle array on amorphous carbon film
Increasing size, constant inter-particle spacing
9nm
@20% coverage
30nm
@55% coverage
Deposition mass increases
Control the size or number density of the nanoparticles
control the deposition mass choose appropriate substrate
By choosing appropriate substrates, the changes on the deposition mass can be
developed into the change of size or number density of NPs.
The surface plasmon resonances of the silver nanoparticle based films
can be systemically tuned by controlling the coverage of the deposited
silver nanoparticles.
Extinction cross sections acquired insitu from the silver nanoparticle film
at different deposition time.
DDA calculated extinction cross for Ag
nanoparticle arrays with different interparticle spacings. The diameter of the
nanoparticle is 8 nm.
A very wide SPR wavelength modulation: At the early stage of the deposition, a SPR
band peaked at 396nm. The extinction band can almost be attributed to the SPR of
isolated silver nanoparticles. With the increase of the deposition time, the SPR band
shows a monotonously red-shifts: from 396nm to about 572nm.
Plasmon Near-Field Coupling
396nm-576nm
SPR band vs deposition mass
The fraction of nanoparticle pairs with
shorter inter-particle spacing increases
Wide modulation on the SPR wavelength with the deposition mass. Shorter intercomes from the change on the fractions of particle spacing permits much stronger
intensely near-field coupled nanoparticles near-field coupling, which affect the SPR
with shorter inter-particle spacing.
wavelength significantly.
Fraction of closely-spaced-nanoparticle-pairs (CPS)：
number of nanoparticle pairs with inter-spacing small than a setting gap/total
nanoparticle number
Plasmonic modulation on closely spaced nanoparticle arrays
Extinction cross sections acquired in-situ
from the Ag NP films deposited on
amorphous carbon film surface. No
significant shifts of SPR peak wavelength
can be observed.
DDA calculated extinction cross sections
for Ag NP arrays with different
particle diameters. The NP arrays have a
constant inter-particle spacing of 10 nm.
With size gradient: SPR wavelength varies little.
Combinational nanoparticle array chip
Dense nanoparticle array bands
Graded nanoparticle bands formed with
stepwise substrate rotations
Combine lot of nanoparticle array bands
with finely adjustable size distribution or
number density on a single substrate.
Graded NP array bands as SERS substrates
SERS spectra of Rhodamine 6G molecules
Average enhancement factor up to 106~107
The maximum enhancement factor for Ag NPs on organic film is one
to two order of magnitude higher than that for Ag NP’s on carbon
SERS micro-mapping on a graded nanoparticle array band substrate
homogeneously distributed with R6G molecules. A 473nm exciting laser was used
SERS Intensity maximum appear at band 5
SERS intensity maximum appear at band 5
and 6, with a SPR wavelength of 505 and
515 nm.
There is an optimum particle number
density to realize maximum Raman
scattering enhancement
NP size dependence of SERS intensity
Rayleigh scattering intensity
increase with the particle
number density monotonously.
SERS intensity increases
monotonously along with the size
gradient, has a same dependence as
the Rayleigh scattering:
incident photon–LSPR interaction
dominant the enhancement
Band 8
Bnad7
Band 6
Band 5
Band 4
Band 3
Band 2
Band 1
NP number density dependence of SERS intensity
For RH6G molecules
with 473nm laser probe
■ “hot spots” locate at the gaps
with interparticle spacing of 23nm
■ in such “hot spots”, scattered
photon–LSPR interaction is
the dominant contribution to
the enhancement
Inter-particle spacing statistics: the fraction of
closely-spaced-nanoparticle-pairs with a certain
inter-particle spacing
■ The decrease of the SERS
intensity
at
very
high
nanoparticle number density
may be ascribed to the fact that
for very narrow junctions,
quantum mechanical effects
start to be important.
Quantum Plasmonics
●For very narrow junctions, quantum
mechanical effects start to be important.
●The field enhancement in a coupled
nanoparticle dimer can be strongly affected
by quantum effects.
●An interparticle current resulting from the
strong field photoemission tends to
neutralize the plasmon-induced surface
charge densities on the opposite sides of the
nanoparticle junction.
●Thus, the coupling between the two
nanoparticles and the field enhancement is
reduced for interparticle specing as large as
1 nm and down to the touching limit.
D.C. Marinica et.al., Nano Lett. 12, 1333(2012)
SPR Enhanced Quantum Transport in Closely Spaced
Nanoparticle Arrays
Ag clusters
Deposit Pd clusters to
percolation threshold
Deposit Ag clusters to a
certain SPR wavelength
Laser
illumination
Pd clusters
measure conductance
under laser illumination
SPR Enhanced Quantum Transport in Closely
Spaced Nanoparticle Arrays
The largest conductance enhancement was
measured when illuminated with a 450nm
laser light.
Surface plasmon resonance of the
silver nanoparticle arrays locates at
450nm
SPR Enhanced Quantum Transport in Closely Spaced
Nanoparticle Arrays
(a) I–V curves of the Pd–Ag hybrid nanoparticle arrays measured with and without
light illumination.
(b) In the absence of irradiation, at low temperature (e.g. 10K), the I–V relationship
exhibit threshold behavior and current plateaus. This behavior demonstrates a
characteristic of Coulomb blockade of transport.
(c) Under light illumination, the I–V curve becomes less nonlinear or even switches to
linear at room temperature, indicating that Coulomb blockade in the nanoparticle
arrays vanishes partially or completely. No current switching behavior can be
observed even at 10K.
SPR Enhanced Quantum Transport in
Closely Spaced Nanoparticle Arrays
Mechanism:
SPR enhancement of tunneling/hopping of
electrons
Photon-induced surface plasmons contribute
to the electron transport in the closely spaced
nanoparticle arrays. The conductivity of the
nanoparticle arrays can be amplified by the
enhancement of tunneling or hopping of
electrons between the closely spaced
nanoparticle couples under the surface plasmon
enhanced near field of silver nanoparticles.
“Hot
spots”
2. Plasmonic modulation in LSP-SPP
coupling systems
Plasmon modulation with coupling of NPs to a metallic film
Ag NPs/LiF/Ag film
structure
Ag NP arrays supporting LSPs
Ag film supporting SPPs
LiF insulator layer sandwished
Spacer layer thickness:
18nm
26nm
30nm
34nm
With the increase of the thickness of LiF film，the spectrum of the light reflected from
the Ag NPs/LiF/Ag film areas change significantly
Manipulating the SPR wavelength with spacer thickness
Extinction=log(I0/IR)
I0
IR
Spacer thickness : 20nm to 32nm
●A rapid increase of the resonance
wavelength
● SPR wavelength changes linearly
with the distance between Ag NPs
and Ag Film
● LSP peak shifts 29 nm per 1 nm
change in spacing
For spacer thickness larger 32nm, a large blue-shift of the LSP peak wavelength due
to the weakening of the coupling strength between the LSP and the SPP
For LSPs excited on a single metal NP in
close proximity to a metal film, LSP
resonance undergoes a blue shift as the
distance between the particle and the film is
increased.
The LSP wavelength blue shift can be
explained by treating the NP as a dipole
placed above a conducting plane, result in the
creation of an image dipole in the metal. The
spectral shift comes from the interaction
between the closely spaced NP and its image
be polarized normal to the surface.
For closely spaced NP array, the in-plane (parallel to the film) dipole moments of the
images are opposite to those of the NPs. Complex near field coupling among the
dipole moments of the images as well as the NPs are included and generate
broadening and red-shift of the LSP resonance peak.
3. Directional emission generated with
near filed couping of closely spaced LSPs
Increase the light extraction efficiency of LEDs
Light Emission in a GaN LED
Most of the generated photons from
the active layer remain inside the
LEDs due to the total internal
reflection (TIR) caused by the large
difference in refractive index of
semiconductor and air
Conventional GaN based LEDs: only
4% of the generated light can escape
out of the LEDs
Light extraction enhancement
Scattering of evanescent field near the
dielectric
medium/air
interface
induced by the total internal reflection
of the light by nanoparticle arrays
can effectively extract the light out
the dielectric medium.
Extract TIR light with plasmonic nanoparticle layers :
scattering the evanescent wave into far field
Distribution of evanescent field near the GaN
layer-air interface under TIR.
Simulated with FDTD.
Nanoparticles
Air
GaN
FDTD calculation results demonstrate that with
Ag NPs coating, significant amount of TIR light
can be extracted and emitted as free-propagating
radiation
Extract TIR light with plasmonic nanoparticle layers
Experimental setup for the analysis of the extraction of light incident
beyond the critical angle of total internal reflection with Ag NP layers .
A scanning stage was used, the incident angles of the illumination can
be varied to generate a TIR geometry.
Extract TIR light with plasmonic nanoparticle layers
The intensity of the extracted light
increase with the size of Ag NPs.
The presence of the Ag NPs enables the extraction of TIR light
The transmission spectra measured beyond the critical angle is sensitively dependent
on the wavelength as well as the size and density of the Ag nanoparticles. Significant
light extraction appears at the plasmon resonance wavelength.
(a)
(b)
(c)
Light output from
bare LED
Light output from
Ag NP-covered LED
Due to the extinction of the Ag NP arrays, light emitted from LED undergo strong
inherent losses when it passes through the Ag NP layer.
Actually, introducing Ag NPs layers to the light output plane of LED generally
results in light output reduction.
FDTD calculations demonstrated:
●Large Ag nanoparticle arrays can effectively scatter
evanescent wave into far-field radiation with high
directionality.
●The extracted light propagates mainly along the
direction perpendicular to the substrate surface that the
Ag NPs located in both along the forward direction and
backward direction.
Angular radiation profile measurement of the
extracted light
Far field distribution of the extracted
TIR light from FDTD calculation
● Angle distribution of the extracted radiation is rather narrow
● Most of light extracted from the prism by Ag NPs propagates along the direction
perpendicular to the prism surface
● The observed radiation profile is in good agreement with the FDTD simulation
results
Angular radiation profile measurement
Spectral transmittance of forward and backward extracted light
dove prism
A quantitative measurement of the forward and backward extraction efficiency of light
trapped beyond the TIR critical angle
●The backward extraction seems more effective
●More than 50% trapped light can be extracted out from the backward direction at the
resonance wavelength
Extract TIR light with plasmonic nanoparticle layers :
scattering the evanescent wave into far field
The directionality of scattered light was ascribed to the NP array antennas effect (an
analogue of Yagi-Uda antennas).
Under resonant condition, the electric fields of Ag NPs couple in the substrate surface
plane, the far-field radiations propagate along the direction perpendicular to the coupling
plane
●The
directional
scattering
processes on the sapphire substrate
eliminate the TIR conditions when
the scattered light incident again on
the light output surface, so that they
can escape from the semiconductor to
air.
A back scattering scheme of light output enhancement of GaN LEDs
with Ag NP arrays
PL spectra of the GaN-LED wafers
A 122% enhancement of PL emission was observed
Resonant backscattering of the PL emission on the Ag NPs coated on
the sapphire substrate surface plays a critical role
LOP(mW)
Conventional LED
LED with Ag NPs
/350mA
117.3
213.5
increased
—
82.0%
LOP(mW)
/700mA
182.5
355.1
increased
—
94.6%
In conclusion:
We have shown the manipulating on the resonant wavelength
and direction of the light scattered from the near field
coupled LSPs.
The plasmonic properties of the closely spaced Ag nanoparticlebased nanostructures can be seriously tuned by varying the
nanoparticle array configurations.