Surface Enhanced Raman Spectroscopy

Plasmon mediated SERS

1. R.Gandhimathi
Plasmon mediated
Surface Enhanced Raman Spectroscopy

2. Ramanspectra
▪Apowerfultoolforstudyingpropertiesofmatter,startingfromsingle
moleculestobulksolids
▪Ithasbeenemployedasafingerprintofamoleculewhichisextremely
importantforsinglemoleculedetection
▪Still,Ramansignalisveryweakandtofinditsexpediencyinsensing
applications,Ramansignalneedtobeenhancedinmanyorders
▪Localizedsurfaceplasmonresonance(LSPR)supportplasmonmediated
Ramanenhancementupto10
8
times
Introduction

3. Raman Shift vs Intensity
Raman spectroscopy
•RamanShift:Frequencydifferenceof
theinelasticallyscatteredradiationfrom
theincidentbeam
•Itgivesvibrationalenergiesofthe
molecule
Whenthelightbeamisdeflectedbyamolecule,wavelengthofincidentlight
changes.i.e.Anopticaltransitionoccurswhenlightisinelasticallyscattered
byatargetmolecule

4. Raman scattering 0
dP
dQ

▪InRamanscattering,themolecularvibration
changesthepolarizabilityuponexcitation.
InduceddipoleP=αE,
α-thepolarizabilityofthemolecule
(Qisthenormalcoordinateofthevibration)
▪Thusitidentifiesvibrational,&rotational
modesofmoleculesviathelight-matter
interaction
One photon decompose
into two photons 0exc s
=+
-vibrational energy of the molecule 0


5. Energyincrease/decreaseisrelatedtothevibrationalenergyspacinginthegroundstate
Stokes & Anti-stokes lines

6. ▪Non-destructivetechniquewhichcombinesmodernlaserspectroscopywiththeexciting
opticalpropertiesofmetallicnanostructures
▪Identifiesthechemicalidentityandstructuralinformationofmoleculesviaamplifyingthe
signalfromtheweakyetstructurallyrichtechniqueofRamanscatteringtogetherwith
ultrasensitivedetectionlimits
Surface-Enhanced Raman Spectroscopy (SERS)
SERS Mechanism
Chemical effect
FormationCharge‐transfercomplex
throughinteractionoftheadsorbed
moleculesonthemetalsurfacewiththe
photonofsubwavelength
Electromagnetic effect
Interactionbetweenthetransitionmomentofan
adsorbedmoleculewiththeelectricfieldofa
surfaceplasmoninducedbytheincominglight
atthemetalsurface

7. ▪LSPRisresponsiblefortheelectromagnetic-fieldenhancementthatleadsto
surface-enhancedRamanscattering(SERS)
▪Enhancementoriginatesfromthelocalizationoflightatthesurfaceof
nanoparticles
First generation hot spots Hot spots
▪Afieldenhancementlocalizedinavery
smallspatialregionofplasmonic
nanoparticlesiscalledhotspot
▪Hotspotsareoftenidentifiedasnanogaps
betweennanoparticles
Localized surface plasmon Resonance(LSPR)

8. EnhancementofRamanscatteringwithplasmonicsystem
▪Localelectromagneticfieldinthegapis
extremelyintensebecauseofthestrong
electromagneticcoupling
▪MutualexcitationfromthemetalNPsystem
viaenhancedinduceddipoleresultsin
enhancedapparentRamanpolarizabilityof
adsorbedmolecule
Thepresenceofthemetallicstructurenearbythemoleculemodifiesthe
efficiencywithwhichthemoleculeradiatesRamanpower

9. ▪Incidentlightexcitesplasmonicsystem,consequentlyplasmonicsystemre-excites
moleculedipolesystem
▪Photonemissionisduetodipoleradiationoftransitiontakenplacefromexcitedstateto
groundstate
▪Eachnanoparticlefeelstheeffectoftheexternalfieldplusthepolarizingeffectofthe
chargesinducedinthenearbynanoparticle
Dipole moment ⊥axis of dimerDipole moment axis of dimer
Hugeenhancementofthefieldinthegap Nofieldenhancementinthegap
NP-1 NP-2
EM SERS enhancement

10. EM enhancement holds two distinct contributions:
▪Excitation of induced dipole
▪emission of a Raman dipole
NumberofabsorbedincomingphotonswhichinresultsgeneratenewStokesphotons,
,
()
excP
exc
excV


 ,exc P

Excitation Rate with plasmonic system -
Excitation Rate in Vacuum -,exc V

Enhancement of the excitation rate
EM SERS enhancement

11. ,
,0
RP tot
RV
W
W


= The ratio of the emission rate in the presence of the nano particles to emission rate in vacuum is
defined by the ratio of emitted power with plasmonic system to the power emitted in vacuum
Largerthepower,thenhighertheemissionratewiththemolecule
Emissionhappensduethetransitionoftheelectronfromtheexcitedstatetotheground
state
Emission rate 
-Emission time -Time needed for electron to go down from excited state to ground state1


=
RamanEmissionrateenhancement

12. In Vacuum ,
, , int
radV
V
radV nradV



=
++ ,,
........(1)
RV excVV
=
,
by
Substitute ??????
??????in equation (1)
Quantumyieldistheratioofradiativepartovertheintrinsicandnon-radiativelosses
(??????
??????),
,,
, int
.
rad V
R V exc V
rad V



=
+
Raman Emission rate in vacuum,
0
nrad V
=

13. Substitute ??????
??????in equation (2),
, , int
radP
P
radP nradP



=
++
where
??????
??????-Quantum efficiency with plasmonic system
Radiative, Non Radiative emission rate
,
-Intrinsic losses
Raman Emission rate with the presence of metal nano particle,
, , int
radP
P
radP nradP



=
++
Emission rate is obtained by considering both radiative and non radiative emissions,,
. .......(2)
R P exc P P
  = ,nrad P
 ,rad P
 int


14. �
�=
RamanEmissionratewithplasmonicsystem
RamanEmissionrateinvacuum,
,
RP
R
RV
Q


=
After substitution , , , int
, , , , int
..
excP radP radV
R
excV radV radP nradP
Q


+
=
++
Intrinsic losses for Raman
emission are much larger
than the radiative part,
,
RP
R
RV
Q


=
Raman Enhancement factor

15. ,,
,,
.
excP radP
excV radV


= ,, int
, , int
..
exc P rad P
R
exc V rad V
Q
 
  
= Q
R=An excitation local field enhancement a radiation local field enhancement
Only enhancement of radiation emission and no more quantum yield
i.e. No quenching,,
,,
( ). ( )
exc P rad P
R exc s
exc V rad V
Q



=
SERS Enhancement factor

16. ,2
,
().()
radP
R exc s
radV
QFE



= Average Enhancement factor
ℏ
0<ℏ
??????
h
0is much smaller
than the Stokes or
excitation photon
energy,2
,
()
excP
exc
excV
FE



=
= Excitation enhancement&#3627408441;&#3627408440;
2
(
??????????????????)
field enhancement factor at the Stokes frequency )&#3627408441;&#3627408440;
2
(
??????,2
,
()
radP
s
radV
FE



= s exc
 2 2 4
().()()
R exc s exc
QFEFEFE==
since
▪Fieldenhancementfactorintheorderof10
4
▪AverageSERSintensitiesfromcoupledplasmonicnanostructures four
ordersofmagnitude

17. ▪Ramanspectroscopyisaprevailingvibrationalspectroscopytechnique,providesthe
structuralinformationofmolecules.However,comparedtotheFluorescenceEmission
process,RamanEmissionisaveryweakprocess
▪InSERS,Ramanscatteringgeneratedbymoleculesisstronglyamplifiedbyplacing
theminthevicinityofplasmonicnanostructures.Enhancementisobtainedbycoupling
oftheincidentandRamanEMfieldsonmetallicsurfaceswithlocalizedsurface-plasmon
resonances
▪Intense plasmonic hotspots associated with the metallic nanostructures enhance the
sensing capabilities by a factor up to 10
11
and permit to observe individual molecules
Summary

18. References
1.YukoS.Yamamotoa,b,YukihiroOzakic,∗,TamitakeItohd,ReviewRecentprogressandfrontiersinthe
electromagneticmechanismofsurface-enhancedRamanscattering”,JournalofPhotochemistryand
PhotobiologyC:2014
2.Song-YuanDing1,JunYi1,Jian-FengLi1,2,BinRen1,2,De-YinWu1,RajapandiyanPanneerselvam1
andZhong-QunTian1,“Nanostructure-basedplasmon-enhancedRamanspectroscopyforsurfaceanalysis
ofmaterials“
3.JohnHenson,AnirbanBhattacharyya,TheodoreD.Moustakas,andRobertoPaiella,“Controllingthe
recombinationrateofsemiconductoractivelayersviacouplingtodispersion-engineeredsurface
plasmons”,J.Opt.Soc.Am.B/Vol.25,No.8/August2008
4.GuoXia1,*,CuixiaZhou1,ShiqunJin1,ChanHuang2,3,4,JinyuXing5andZhijianLiu,Sensitivity
EnhancementofTwo-DimensionalMaterialsBasedonGeneticOptimizationinSurfacePlasmon
Resonance,Sensors2019,19,1198;

19. Tak you