National Laboratory of Solid State MicrostructuresNanjing University

Oct. 21, 2015

Manipulating the localized surface plasmons in closely spaced metal nanoparticle arrays

Min Han

Outline

1. Manipulating LSP with interparticle spacing controlling

1. Manipulating LSP with interparticle spacing controlling

2. Plasmonic modulation in LSP-SPP coupling systems

2. Plasmonic modulation in LSP-SPP coupling systems

3. Directional emission generated with near filed coupling of closely spaced LSPs

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)

Electronic properties

Electron transport

Optical properties

Local electromagnetic field/ plasmonic prop.

Spin arrangement

Magnetic properties, spintronics

Tunneling/hopping

SPR near field coupling

Exchange/dipole interaction

QD devices, microsensors, thermoelectric devices, optical devices, ……

Junction plasmons: near field coupling

the resonance shift due to plasmon coupling the resonant wavelength of isolated particleX: gap size/particle diameter ratio

Acimovic S. S. et al, ACSNano (2009) 3,1231

Plasmon ruler equation

Universal scaling behavior of metal dimers

x

ea

SPR wavelength shift due to plasmon coupling.

There is a universal scaling behavior of surface plasmon resonance of matal nanoparticle dimers:

SPR wavelength shift near-exponentially over a normalized inter-particle spacing.

Junction Plasmons: near field coupling

Aggregates(two spheres)Single spheres

106 Raman enhancement

at resonance

Coherently driven AC dipoles couple strongly. Polarization charge localized at the interface

For nanoparticle dimer with s-polarization1012 enhancement as d→1

d“Hot spot”

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

Temperature Controller

To TOFMonochromator

Light source

Photo detector

Photo detector

Gas-Phase Cluster Beam Deposition

Cluster beam

Mask

Template

Optical monitoringElectric monitoring

Coverage control

The current pass through the electrode varies with the deposition time, the nanoparticle coverage can be controlled by the monitoring of the conductance changes.

The interparticle spacing may be compatible to the electron tunneling length.

Dense silver nanoparticle arrays

Deposition mass increases

Control the size or number density of the nanoparticles control the deposition mass choose appropriate substrate

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

By choosing appropriate substrates, the changes on the deposition mass can be developed into the change of size or number density of NPs.

Extinction cross sections acquired in-situ from the silver nanoparticle film at different deposition time.

DDA calculated extinction cross for Ag nanoparticle arrays with different inter- particle 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.

The surface plasmon resonances of the silver nanoparticle based films can be systemically tuned by controlling the coverage of the deposited silver nanoparticles.

SPR band vs deposition mass

396nm-576nm

Plasmon Near-Field Coupling

The fraction of nanoparticle pairs with shorter inter-particle spacing increases with the deposition mass. Shorter inter-particle spacing permits much stronger near-field coupling, which affect the SPR wavelength significantly.

Wide modulation on the SPR wavelength comes from the change on the fractions of intensely near-field coupled nanoparticles with shorter inter-particle spacing.

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

With size gradient: SPR wavelength varies little.

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.

Combinational nanoparticle array chip

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.

Dense nanoparticle array bands

Average enhancement factor up to 106~107

Graded NP array bands as SERS substrates

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 spectra of Rhodamine 6G molecules

SERS Intensity maximum appear at band 5

SERS micro-mapping on a graded nanoparticle array band substratehomogeneously distributed with R6G molecules. A 473nm exciting laser was used

There is an optimum particle number density to realize maximum Raman scattering enhancement

SERS intensity maximum appear at band 5 and 6, with a SPR wavelength of 505 and 515 nm.

Rayleigh scattering intensity increase with the particle number density monotonously.

Band 8Bnad7Band 6Band 5Band 4Band 3Band 2Band 1

SERS intensity increases monotonously along with the size gradient, has a same dependence as the Rayleigh scattering: incident photon–LSPR interaction dominant the enhancement

NP size dependence of SERS intensity

NP number density dependence of SERS intensity

Inter-particle spacing statistics: the fraction of closely-spaced-nanoparticle-pairs with a certain inter-particle spacing

For RH6G molecules

with 473nm laser probe

■ “hot spots” locate at the gaps with interparticle spacing of 2-3nm

■ in such “hot spots”, scattered photon–LSPR interaction is

the dominant contribution to the enhancement

■ 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

Pd clusters Ag clusters

Deposit Pd clusters to percolation threshold

Deposit Ag clusters to a certain SPR wavelength

measure conductance under laser illumination

Laser

illum

ination

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

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

Spacer layer thickness: 18nm 26nm 30nm 34nm

I0

IR

Extinction=log(I0/IR)

Manipulating the SPR wavelength with spacer thickness

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

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 airConventional 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.

Increase the light extraction efficiency of LEDs

Distribution of evanescent field near the GaN layer-air interface under TIR. Simulated with FDTD.

Extract TIR light with plasmonic nanoparticle layers :scattering the evanescent wave into far field

GaN

Nanoparticles

Air

FDTD calculation results demonstrate that with Ag NPs coating, significant amount of TIR light can be extracted and emitted as free-propagating radiation

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.

Extract TIR light with plasmonic nanoparticle layers

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.

The presence of the Ag NPs enables the extraction of TIR light

(a) (b)(c)

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.

Light output from bare LED

Light output from Ag NP-covered LED

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

● 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

Far field distribution of the extracted TIR light from FDTD calculation

Angular radiation profile measurement

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

dove prism

Spectral transmittance of forward and backward extracted light

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

A back scattering scheme of light output enhancement of GaN LEDs with Ag NP arrays

●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 122% enhancement of PL emission was observedResonant backscattering of the PL emission on the Ag NPs coated on the sapphire substrate surface plays a critical role

PL spectra of the GaN-LED wafers

 LOP(mW)

/350mAincreased  

LOP(mW)

/700mAincreased

Conventional LED

117.3 —  

182.5 —

LED with Ag NPs 213.5 82.0%   355.1 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 nanoparticle-based nanostructures can be seriously tuned by varying the nanoparticle array configurations.