ContentslistsavailableatScienceDirect
Applied
Materials
Today
jo u r n al hom e p a g e :w w w . e l s e v i e r . c o m / l o c a t e / a p m t
Molecular
electronic
plasmonics
Tao
Wang
a,
Christian
A.
Nijhuis
a,b,∗aDepartmentofChemistry,NationalUniversityofSingapore,3ScienceDrive3,117543,Singapore
bCentreforAdvanced2DMaterialsandGrapheneResearchCentre,NationalUniversityofSingapore,6ScienceDrive2,117546,Singapore
a
r
t
i
c
l
e
i
n
f
o
Articlehistory: Received12February2016 Accepted1March2016 Keywords: PlasmonicsMoleculartunneljunctions Quantumplasmonics Chargetransferplasmons Plasmonexcitationanddetection Plasmonassistedtunneling
a
b
s
t
r
a
c
t
Molecularelectronicplasmonics(MEP)isanareaofresearchthatutilizestheelectronicpropertiesof moleculestocontrolandmodulatesurfaceplasmonsandholdsthepotentialtodevelopon-chip inte-gratedmolecular-plasmonicdevicesforinformationprocessingandcomputing.Combiningmolecular electronicswithplasmonicsgivestheopportunitytostudybothchargetransportinmolecular elec-tronicdevicesandplasmonicsinthequantumregime.Here,wereviewtherecentprogressinmolecular electronicplasmonicsandmainlyfocusontheareasofquantumplasmonics,andplasmonexcitation anddetection.Thisreviewalsoidentifieschallengesthatneedtoberesolvedtodrivethisfieldforward includingimprovingmodelsaimedtoadvanceourunderstandingofelectron-plasmoninteractionsin thequantumtunnelingregime.Futureprogressescanbeexpectedtowardsincorporatingfunctional moleculestoactivelycontrolMEPdevicesandintegrationofMEPswithothercircuitcomponents.
©2016TheAuthors.PublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).
Contents
1. Molecularelectronicsandsurfaceplasmons...73
2. Molecularelectronicsappliedinquantumplasmonics...75
2.1. Quantumplasmontheory...75
2.2. Quantumplasmonicswithair/vacuumtunnelingbarriers...76
2.3. Quantumplasmonicswithmoleculartunnelingbarriers...77
3. Molecularelectronicsappliedinplasmonexcitation...77
3.1. Mechanismsofplasmonexcitationintunneljunctions...78
3.2. STM-basedmolecularelectronicplasmonsources...78
3.3. On-chipmolecularelectronicplasmonsources...79
4. Molecularelectronicsappliedinplasmondetection...81
4.1. Mechanismofelectricaldetectionofplasmons...81
4.2. Directplasmondetectionwithmoleculartunneljunctions...81
5. Conclusionsandoutlook...82
Acknowledgements ... 82
References...82
1. Molecularelectronicsandsurfaceplasmons
Nano-electroniccircuitsprovidetheabilitytocontrolcharge
transportatthenanoscalebutdataprocessingandtransportation
speedsarelimitedattheirGHzbandwidths[1–3].Photonic
ele-mentscancarryinformationwithacapacityexceeding1000times
(>THz)thatofelectroniccomponents,howevertherelativelylarge
∗ Correspondingauthorat:DepartmentofChemistry,NationalUniversityof Singapore,3ScienceDrive3,117543,Singapore.
E-mailaddress:christian.nijhuis@nus.edu.sg(C.A.Nijhuis).
wavelengthoflightrequiresopticalcomponentstobetoolarge
tocompeteinsizewithmoderndaynanoelectronics[2–5].With
theability of subwavelength confinementand largebandwidth
(>100THz),surfaceplasmonpolaritons(SPPs)inmetallic
nanos-tructureshavethepotentialtobeintegratedwithnano-electronic
circuitsresultinginatruehybridofopticsandelectronicsatthe
nanoscale[2–7].
Molecular electronics utilizes single molecules or
self-assembled monolayers (SAMs) as electronic components in
molecularjunctions consisting of2 or 3 electrodes[8–17].The
mechanismofchargetransportacrosssuchmolecularjunctionsis
http://dx.doi.org/10.1016/j.apmt.2016.03.001
Fig.1.Operationprinciplesofthreedifferenttypesofmolecularelectronicplasmonic(MEP)devices.(a)Molecularelectronicsappliedinquantumplasmonics.Anincident lightinducesatunnelcurrentintheSAM-bridgedplasmonicdimerthatexcitesquantumplasmonresonances(QPR).(b)Molecularelectronicsappliedinplasmonexcitation. Anappliedbias(V)inducesatunnelingcurrentinthemoleculartunneljunctionthatexcitesplasmonemissionintheplasmonicdimer.(c)Molecularelectronicsappliedin plasmondetection.Thelightinducedtunnelingcurrentincreasesthecurrentofthemoleculartunneljunction.Energy-leveldiagramsoftunnelingjunctionswithconjugated (d)andaliphatic(e)moleculesasthetunnelingbarrier.ThealiphaticmoleculeshavelargerHOMO–LUMOgapsandbarrierheightsϕthanconjugatedmolecules.
quantummechanicaltunneling,andthusmetal–molecule–metal
junctionsare often calledmoleculartunnel junctions (MTJs) or
SAM-basedtunnel junctions (STJs).Recentexperimentsin MTJs
andSTJshaveadvancedourunderstandingofthemechanismsof
chargetransport acrosssuchjunctions significantlyand leadto
experimentaldemonstrationsofconductanceswitching[18–21],
rectificationofcurrents[17,22–27],quantuminterference[28–31],
negative differential conductance [32–34], magneto resistance
[35–39],oroptoelectronics[40–45].
Surfaceplasmonpolaritons(SPPs)arecollectiveoscillationsof
electronsatthemetal–dielectricinterface[6].Surfaceplasmons
areable totheconfine light beyondthediffractionlimit [7] at
thenanoscaletoformlocalizedsurfaceplasmons(LSPs)[46–50]
andpropagating SPPs[51–56].Benefittingfromthestrongfield
enhancementatthesurfacesofmetallicstructures,surface
plas-monsaresuitableforsensingatthesinglemoleculelevel[57–64].
Besidessensing,SPPsarealsopromisinginotherareasincluding
sub-diffractionimaging[65–70],energyharvesting[71–76],
non-linearoptics[77–82],andnano-optoelectronics[83–87].Tofurther
promotelocalfieldenhancement,plasmonicresonatorshavebeen
placedincloseproximitywitheachotherdowntoafew
nanome-ters[88–102]andreachthequantummechanicaltunnelingregime
[89,91,93–97,101,102],i.e.,theregimewherechargescantunnel
betweenthe two plasmonicstructures. This regime is alsothe
lengthscalewheremolecularelectronicsoperatesandthusitseems
anaturalchoicetostudytherelationbetweenplasmonsand
molec-ularelectronics.
Here,we reviewtherecent developments andprogresses of
molecularelectronicplasmonics(MEP).Fig.1showsthatin
prin-ciplethreetypesofMEPdevicescanbeidentified.Althoughthis
figuredepictstheelectrodesassphericalplasmonicresonators,one
orbothelectrodescanalsobeplanarmacroscopicelectrodes.
1.Molecularelectronicsappliedinquantumplasmonics(Fig.1a).
TwocloselyspacedplasmonicresonatorsarebridgedbyaSAM
ontowhichplasmonsareexcitedbyincidentlightorbyan
elec-tronbeam.Theplasmonsinduceanelectricfieldinthegapwhich
drives quantum mechanical tunneling betweenthe two
res-onatorsresultinginquantumplasmonresonances(QPRs)such
astheso-calledchargetransferplasmon(CTP)modes[103–112].
2.Molecularelectronicsappliedinplasmon excitation(Fig.1b).
Thesemolecularelectronicjunctionsarebasedontwoelectrodes
ofwhichatleastonesupportsplasmonsseparatedbyaSAMor
evenasinglemolecule.Anappliedbiasacrossthegapinduces
tunnelingbetweentheelectrodeswhichresultintheexcitation
ofplasmonseitherbydirecttunneling orvia
electrolumines-cencefromthemoleculesinsidethejunctions.
3.Molecularelectronicsappliedinplasmondetection(Fig.1c).The
samejunctionshowninFig.1bcanalsobeusedtodetect
plas-mons.Here,plasmonsareexcitedinthejunctionviaanexternal
light source.These plasmonscouple tothe tunneling charge
carriersandincreasethetunnelingcurrentacrossthejunction
makingitpossibletodetectplasmons.
AsimplifiedformoftheSimmonsequation(Eq.(1))[113]is
commonlyusedtoapproximatemoleculartunnel junctionsand
showshowthetunnelingrateJ(A/cm2)relatestothetunneling
barrierwidthd(innm)andheightϕ(ineV).
J=J0e−ˇdwithˇ=2
2meϕ
2 (1)
Here, ˇ (Å−1) is the tunneling decay coefficient, J0 is
pre-exponentialfactor(A/cm2),m
eistheeffectivemassofanelectron
(inkg),andisthereducedPlanck’sconstant.Thevalueofˇ
deter-mineshowquicklythemeasuredvalueofJdecaysasafunction
of dand depends onthe moleculesinside thejunctions which
determine both the values of d and ϕ. In molecular
electron-ics it is well-known that conjugated molecules have relatively
small HOMO–LUMO gaps (HOMO=highest occupied molecular
Fig.2.Quantumplasmontheories.(a)Schematicsofthedielectricsinvolume-basedQCM(left)andboundary-element-basedQCM(right).AdaptedwithpermissionfromRef. [112].Copyright2015AmericanPhysicalSociety(APS).(b)Generalbehavioursoftheclassicallocal,classicalnon-localandQCMmethodsforplasmonicdimers.Adaptedwith permissionfromRef.[110].Copyright2015theRoyalSocietyofChemistry(RSC).(c,d)OpticalresponseofaAunanospheredimercalculatedbyCEMandQCM,respectively. AdaptedwithpermissionfromRef.[106].Copyright2012Nature.PublishingGroup(NPG).(e)GapconductivitymodulatedCTPmodeinaAubowtiedimer.Adaptedwith permissionfromRef.[108].Copyright2013AmericanChemicalSociety(ACS).(f)GapmorphologymodulatedCTPmodeinaAubardimer.Bothspherical-gapandflat-gap casesareshown.AdaptedwithpermissionfromRef.[109].Copyright2015ACS.BDP=bondingdimerplasmon,CTP=chargetransferplasmon,LAP=longitudinalantenna plasmons.
lowtunnelingbarrierheightsasschematicallyindicatedinFig.1d.
Becauseof theshallowtunneling barrier,thevalue ofˇ is low
(0.1–0.3Å−1[10,12,114])andcoherenttunnelingmaybeobserved
overlargedistancesofupto4–5nm[8–10,115–119].Incontrast,
aliphaticmolecules have relatively large HOMO–LUMOgaps of
8–9eVresulting inlargetunneling barrierheights (Fig.1e) and
ˇvaluesof0.8–0.9Å−1.Consequently,thetunnelingratesacross
thesejunctionsaresmallerthanacrossjunctionswithconjugated
moleculesofequivalentlength[11,114,120].Likewise,tunneling
thoughvacuumisinefficient(ˇ=2.9Å−1[11,120])andtunneling
phenomenacanbeignoredinplasmonicssystemsinvolving
dis-tance largerthan 1nm [106,110]. For these reasons, molecular
tunneljunctions[8–45]areinterestingcandidatesforapplications
inMEP.
2. Molecularelectronicsappliedinquantumplasmonics
Quantum plasmonics studies the interaction between light
withquantumproperties(e.g.,singlephotonsources)and
mat-tersupportingplasmons[121–125].Recently,quantumplasmonics
alsostudiesplasmonmodesinducedbynon-local [126–135]or
quantum mechanicaltunneling phenomena [103–112] between
near-touching (e.g., gap<5nm) plasmonic dimers. Plasmonic
dimershaveattractedintenseinterest becauseoftheenhanced
fieldenhancementsduetocoupledplasmonsinthedimer
junc-tions,leadingtosurfaceenhancedspectroscopiesenablingsingle
moleculedetection [88–102]. Classicalcalculations predict that
largechargedensities canbeinducedattheoppositesidesofa
plasmonicdimerwithasmallgap,leadingtolargefield
enhance-mentsatthegapandstronglyred-shiftedbondingdimerplasmon
(BDP)resonances[126,127].However, theseclassiccalculations
donottakenon-localeffects(thespilloutoftheelectroncharge
distribution)andpossibleelectrontunnelingacrosstheplasmonic
dimerintoaccount.Thenonlocaldielectricresponseand
tunnel-ingeffectscauseablue-shift(orlesspronouncedred-shift)ofthe
BDPresonancesandreducefieldenhancementsrelativeto
clas-sicallocalcalculations[103–112,126–135].Electronstunnelingat
subnanometergapsatsufficientlyhighfieldstrengthswillleadto
anewresonancemode,thecharge-transferplasmon(CTP)mode
[103–112].
2.1. Quantumplasmontheory
Mosttheoreticalinvestigationsofplasmonresonancesuse
clas-sicalelectromagneticmodels(CEM)whicharebasedonMaxwell’s
equations with a frequency-dependent local dielectric function
ε()foreachpartofthenanostructure[126,127].TheCEMmodel
usesaclassicallocalapproach,i.e.,ε()attheinterfacebetween
dif-ferentmaterialschangesabruptlyandisvalidformostplasmonic
systemswithlargevaluesofdof>5nm[110].
Whendreducestothenanometerorevensub-nanometerscale
theelectrondensityspill-outorthepossibleinter-structure
elec-trontunnelingcannotbeignoredandthelocaldielectricfunction
ε()willnotchangeabruptlyattheinterface[128–135].Thus,a
fullquantummechanicaltreatmentofsuchplasmonicsystemis
required.However,becauseofthelargenumberofatoms(e.g.,a
20nmgoldsphereconsistsof∼25000atoms)involvedintypical
plasmonicsystems,itiscurrentlynotpossibletoaddressthis
prob-lemwithfirst-principlemethods[128,131].Instead,suchsystems
have beenmodelledwitha classicalnon-localmodel [128–135]
pioneered by Pendry, Maier, and García deAbajo et al. or the
quantum corrected model (QCM) developed by Aizpurua,
Nord-lander, and Borisov et al. [103–112]. In the classical non-local
calculations,themetalresponseisdescribedusingthe
hydrody-namicnon-localapproachbutwithoutconsideringchargetransfer
acrossthedimers[110].WhileintheQCMcalculations,themetal
is described using a classical local dielectric constant and
tak-ingelectrontunneling acrossthedimergapintoaccount[110].
Basedonthewaytodescribetheelectrontunnelingprocess,the
QCMmodelhasbeenfurtherclassifiedintovolume-basedQCM
[103–110]whichusesartificialdielectricmaterialsinthewhole
gapandboundary-element-basedQCM[112]whichusesnonlocal
boundaryconditionsonlyatthesurfacesofthedimergap(Fig.2a).
Fig.2bshowsthegeneralresultsoftheclassicallocal,
classi-calnon-local,andQCMmodels,forplasmonicdimersseparatedby
vacuum[110].Inthelocalregime(d>5nm),allthesemethodsgive
thesameresult.Inthenon-localregime(0.3nm<d<5nm),both
theclassicalnon-localandtheQCMmethodsgivesimilarresults
andpredicttheBDPmodesred-shiftlessthanpredictedbylocal
method.Inthequantummechanicaltunnelingregime(d<0.3nm),
QCMpredictCTPmodesatd≈0.3nmwhere tunnelinghappens
Fig.3.Quantumplasmonicswithanair/vacuumtunnelingbarrier.(a)Plasmonicresonancesofnear-touchingAubowtieantennas.Here,1=brightdipolemode;2=darkdipole mode;3=CTPmode;4=thecombinationofdarkdipolemodeandbrightdipolemode;5=brightquadrupolemode.AdaptedwithpermissionfromRef.[136].Copyright2012 ACS.(b)Quantumplasmonresonanceinsub-nmplasmoniccavitiesformedbetweentwoAu-coatedAFMtips.Here,A=thelowfrequencymode;BandC=higherfrequency modes;D,E=CTPmodes.AdaptedwithpermissionfromRef.[137].Copyright2012NPG.(c)Quantumplasmonresonanceinsub-nmAunanospheredimers.Adaptedwith permissionfromRef.[138].Copyright2013ACS.(Forinterpretationofthereferencestocolourinthetext,thereaderisreferredtothewebversionofthisarticle.)
classicalnon-localmethodsonlyobserveCTPmodesford≤0nm
(i.e.,touchingdimers),heretheCTPmodeissupportedby
conduc-tionthroughthemetalcontactandnottunneling.Inadditionto
blue-shiftedextinctionspectra,QCMalsopredictsadecreasedfield
enhancementinthedimergapwhentheplasmonicdimerreaches
quantumregimeatd<0.3nm.
Itisworthnotingthattheclassicalnon-localmodelingisusually
performedusingthefiniteelementmethod(FEM)withthe
hydro-dynamicDrudemodeltodescribethedielectricsofthemetal.While
thevolume-basedQCMisperformedusingtheboundaryelement
method(BEM) withtheartificialdielectric materials calculated
by thetime-dependent density functional theory (TDDFT). The
boundary-element-basedQCMusesmodifiednonlocalboundary
conditionsinsteadofTDDFTcalculationstosimplifythe
simula-tionsandshowssimilarresultsasthevolume-basedQCM.
Fig.2canddshowsthetypicalplasmonicresonancesofagold
nanoparticledimer(diameter50nmwithanairdimergap)atthe
near-touching regimecalculated by CEMand QCM respectively
[106].Thereareseveralmainobservations.TheQCMcalculations
showasmallernumberofmodesthantheCEMcalculationsbecause
theFabry-PerotresonancesinQCM arelesspronounced dueto
largedampingraisedbyelectrontunnelingintheinter-structure
junction.TheQCMcalculationsshowthatafirstorderCTPmode
(definedastunnelingCTPmodeortCTPmode)andhigherorder
CTPmodeareobservedford<0.3nmwheretheelectrontunneling
becomesimportant(wenotethatthevalueofdatwhichtunneling
ratesarehighenoughtoobserveCTPmodesdependalsoonthe
tunnelingbarrierheight;seeFig.1).Inotherwords,whentheair
gapdecreasestobelow0.3nm,theBDPswitchestoCTPmodewith
thefollowingfeatures:theresonancepeakblueshifts(insteadof
redshifts)andthepeakintensitydecreaseswhenelectron
tunnel-ingbecomemorepronouncedwithdecreasingvaluesofdbecause
theelectrontunnelingreducestheplasmoniccouplingby
screen-ingthesurfacechargesoflocalizedplasmons.Inaddition,thetCTP
modehappensatinfraredfrequencies(<1eV)withalowresonance
intensity.Astheinfraredresonanceisfarfromtheresonance
fre-quencyoftheplasmonicresonator,thetCTPmodeismainlydueto
theoscillationofelectrontunneling.ThesebehaviorsofbothBDP
andCTPmodesaroundthe<0.3nmairgaparetheindicationofthe
quantumplasmonresonances.
AsthetCTPmodeoriginatesfromtheelectrontunnelingacross
thedimer,thetCTPmodedirectlydependsonthegap
conductiv-ity,i.e.,tunnelingcurrentdensity,raisedbytheplasmonicfields
inthegap.Fig.2eshowsthatthetCTPmodeswitchestoalower
frequency resonance when the tunneling reaches the
Fowler-Nordheimregime[108].BecausethetunnelingcurrentandtheCTP
modeareinducedbytheplasmonicfields,theCTPmodesshould
alsobesensitivetothegeometryoftheplasmonicdimer.Fig.2f
showsthatthisisindeedthecaseandthatthereisnoclearCTP
modeinaplasmonicdimerwithaflatgap(modelledusingtwo
cylindricalplasmonicstructures)asthelongitudinalantenna
plas-mons(LAP)andtheCTPhavealmostthesameresonancefrequency
[109].
2.2. Quantumplasmonicswithair/vacuumtunnelingbarriers
Quantumplasmonicshavebeenstudiedwithplasmonicdimers
containingair/vacuumtunnelingbarriers.Asdiscussedabove,to
identifyquantum plasmon resonances, plasmonic dimers
sepa-ratedbyairorvacuumgapof∼0.3nmareneeded.Suchstructures
are extremely difficult to fabricate using top-down
lithogra-phy methods such as electron-beam lithography(EBL). Fig. 3a
shows theplasmonic resonances,measuredby electron energy
lossspectroscopy(EELS)insideatransmissionelectrontunneling
microscope(TEM),ofbowtieantennasfabricatedbyEBLwith
dif-ferentvaluesofd[136].TheCTPmodewasobservedfortouching
structures,butthesestructuresdidnotrevealtheBDPtoCTPmode
transitionorthetCTPmodelikelybecausethevalueofdof∼0.5nm
wastoolargetoenterthequantummechanicaltunnelingregime.
Savage et al. [137]used a conducting atomic forcemicroscope
(AFM)equippedwithtwogold-spheremodified probestoform
plasmonicdimers.Theseauthorscontrolledthetunnelingcurrent
betweentheAFMtipsinairandsimultaneouslymeasuredthedark
fieldscatteringspectraofthejunction(Fig.2candd).Asthevalueof
ddecreased,theBDPmodesfirstredshiftedwithdecreasingdbut
thenblueshiftedford<∼0.3nmwhichdemonstratesthequantum
behavioroftheplasmonicresonance.Scholletal.controlleddof
twogoldnanospheresinsideaTEMandmeasuredtheirresonance
propertieswithEELS[138].TheyalsoobservedtheBDPtoCTP
tran-sitionwhenthegapofthegoldnanoparticledimeris∼0.27nm
whichisaclearsignatureofthequantumplasmonresonance.
How-ever,inbothcases(Fig.3bandc)[132,133],thetCTPmodecould
notbedirectlyobservedlikelyduetothelimitedspectralrangein
2.3. Quantumplasmonicswithmoleculartunnelingbarriers
Quantumplasmonicshavealsobeenstudiedwithplasmonic
dimers containing molecular tunneling barriers. To form
sub-nanometer plasmonic dimers, SAMs are promising candidates
becauseoftheeaseatwithwhichdcanbecontrolledbysimply
changingthemolecularlengthofthemoleculesinsidetheSAM
greatlysimplifyingthefabricationofstructureswithsmallvalues
ofd[130,139–141].
ThequantumplasmonbehaviorofBDPmodewasfirst
inves-tigated using gold nanoparticles(Au NPs) deposited ona SAM
ofamine-alkanethiolates(S(CH2)nNH2)supportedonagoldfilm
(Fig.4a)[130].Bysimplychangingthevalueofn,theauthorscould
controldattheatomicleveloneCH2unitatatimeandtheBDP
moderedshiftedwithdecreasingn.Thisredshiftwasnotas
pro-nouncedaspredictedbyCEMbuttheexperimentaldatafittedwell
usingnonlocaldielectricfunctionsofAu.Inthisexperiment,the
transitionfromtheBDPmodetotheCTPmode—thekeysignature
ofenteringthequantumregime—wasnotobservedperhapsdueto
therelativelylargevalueofn=2–16,andthelargeHOMO–LUMO
gap(∼8–9eV)ofamine-alkanethiolatesresultinginlargetunneling
barrierheights(Fig.1e).Nevertheless,thisexperimentconfirmed
thenonlocalcontributiontosurfaceplasmonscattering.
ThequantumplasmonbehaviorofBDPmodewasconfirmed
withajunctionconsistingoftwogoldnanospheresseparatedby
a SAMofalkanedithiolates(S-(CH2)n-S)shown inFig.4b [139].
ThisAuNPbasedjunctionhasmoreconfinedandpronouncedgap
plasmonmodesthantheAuNP-SAM-Aufilmjunctiondescribed
above(Fig.4a)whichmayfacilitatetheelectrontunnelingdriven
bythelocalplasmonicfields.TheresonancepeakoftheBDPred
shiftedwithdecreasingn,butitblueshiftedford<∼1nm(orn<5)
whichstronglyindicatesthetransitionofBDPtoCTPmode,aclear
signatureofquantumeffectsintheplasmonicresonanceinthese
structures.
The tCTP mode could be directly observed in junctions
consisting of two silver nanocubes separated by SAM of
1,2-ethanedithiolates(EDT) or1,4-benzenedithiolates (BDT;Fig.4c)
[140]. The Ag nanocubes had atomically flat facets and large
cross-sectionalareas(∼103nm2)fortunnelingwhichincreasedthe
numberoftunnelingeventsacrossthejunctionsandenhancedthe
weaktCTPmode.Theplasmonicresonancesofthedimerswere
characterizedusingEELSinsideaTEM,whichprovidedthe
possi-bilitytocorrelatethespectratothegapsize.AsshowninFig.4c,
thetCTPmodewasobservedasanewlowenergypeakat0.5eV.
BecausetheconductanceoftheBDTSAMwashighercomparedto
theEDTSAM,theresonancepeakofthetCTPmodeshiftedfrom
∼0.6eVto∼1.0eVwhilekeepingthevalueofdequal(considering
thesimilarlengthsofBDTandEDT).ThisshiftinthetCTPenergy
wasrelatedtothereductionofthetunnelingbarrierheightasEDT
hasalargerHOMO–LUMOgap(8eV)thanBDT(5eV)asdepictedin
Fig.1.SincetheconjugateBDTSAMssupporthightunnelingrates,
thetCTPmodecouldbeobservedevenforvaluesofdofupto1.3nm.
ThemolecularelectroniccontrolovertheBDPmode(coupled
plas-monresonance)wasconfirmedinAuNP-SAM-Aufilmjunctions
withSAMsofBPDT(biphenyl-4,4-dithiolate)andBPT
(biphenyl-4-thiolate)byBenzetal.(Fig.4d)[141].HeretheBPDTmolecules
havealargerconductivitythanBPTmoleculesandconsequently
theBDPmodewasblueshifted∼50nmbysimplyreplacingtheBPT
SAMbyaBPDTSAM.Inthisexample,dandtherefractiveindices
ofbothSAMswereverysimilarandthusthechangeinthe
tunnel-ingratedefinedbythemoleculecausedthechangeintheoptical
propertiesofthejunctions.
Fig.4. Quantumplasmonicswithmoleculartunnelingbarriers.(a)Nonlocaleffects intheNP-SAM-filmsystemwithSAMsofamine-alkanethiolates(S(CH2)nNH2).The
scatteringpeakpositionredshiftsasddecreasesandfitswelltothemodelthat utilizesnonlocaldielectricfunctionofAu.AdaptedwithpermissionfromRef.[130]. Copyright2012AmericanAssociationfortheAdvancementofScience(AAAS).(b) Scanningelectronmicroscopy(SEM)imageoftheAuNPdimerslinkedwith alka-nedithiolates(S-(CH2)n-S).Thegraphsshowthescatteringpeakpositionofthe
dimersvsd.AdaptedwithpermissionfromRef.[139].Copyright2014ACS.(c) Directobservationofquantumplasmonresonance(tCTP)inAgnanocudedimers linkedbyEDT(1,2-ethanedithiolates)orBDT(1,4-benzenedithiolates).Duetothe higherconductivityoftheBDTmolecules,thetCTPisblueshiftedrelativetothat ofthejunctionswithEDT.AdaptedwithpermissionfromRef.[140].Copyright 2014AAAS.(d)Moleculeconductivityinducedquantumplasmonresonanceshift inAuNP-SAM-AufilmsystemwithSAMsofBPDT(biphenyl-4,4-dithiolate)orBPT
(biphenyl-4-thiolate)molecules.ThemoreconductiveBPDTmoleculescausethe BDPmodetoblueshiftcomparedrelativetothelessconductiveBPTjunctions. AdaptedwithpermissionfromRef.[141].Copyright2014ACS.(Forinterpretation ofthereferencestocolourinthetext,thereaderisreferredtothewebversionof thisarticle.)
3. Molecularelectronicsappliedinplasmonexcitation
Plasmonscanbeelectricallyexcitedbytwoapproacheswhich
wedefineasdirectandindirectplasmonexcitationasshownin
Fig.5.Directplasmonexcitationreliesonplasmonexcitationby
tunnelingelectronsdirectly.Inotherwords,thetunnelingcurrent
directlycouplestoplasmonmodesin,forinstance,
metal-insulator-metaljunctions[142–149].Indirectplasmonexcitationrelieson
twosteps.First,chargecarriersinducetheelectron–holepair
gen-erationinthejunctionwhichthenrecombineandemitaphoton.
Second,these emittedphotons then excite plasmons.Although
many examples of indirect plasmon sources are based on the
electroluminescenceofsemiconductors(e.g.,lightemitteddiodes
[150–153],siliconparticles[154],orcarbonnanotubes[155]),here
weonly focusin indirectand direct plasmonsources basedon
molecules.
3.1. Mechanismsofplasmonexcitationintunneljunctions
Whenatunnelingcurrentflowsthroughajunction,plasmons
areexcitedin theplasmonicelectrodes(usuallymadeof Auor
Ag).Thisplasmonexcitationprocesshasbeenoftenassumedto
proceedviainelastictunneling[142–149,156–158].Theschematic
diagramofinelastictunnelingisshowninFig.5a.Duringthe
tun-nelprocessmostof theelectronstunnel elastically,however, a
smallportionoftheelectronstunnelinelasticallyandexcite
plas-monsinthetunneljunctions.Thefirstexampleofadirectplasmon
sourcewasreportedbyLambeandMcCarthyin1976[142].They
observedbroadbandlightemissionfromplanar
metal-insulator-metal (MIM) tunnel junctions (Al–Al2O3–Ag/Au junction) from
radiativelydecayingplasmons.Thecolorofthelightdependson
theappliedbiasVapplandthehighfrequencycutoffoftheoptical
spectrafollowsthequantumrelationhcutoff=eVappl.Sincetheir
experiments, plasmonshave been excited in different types of
tunneljunctionsincludingMIMtunneljunctionsbasedon
metal-oxideinsulators[144–146],airgaps[147,148],hexagonalboron
nitride(hBN)[149],andscanningtunnelingmicroscopes(STMs)
[156–183].
Often,thephoton-assistedtunnelingmodelhasbeenusedto
explainhowchargecarrierscoupletoplasmons(seeSection4.1).
Thismodel implies that the emitted photon from thejunction
cannotexceedtheelectronenergyoftheappliedbiasVappl and
hcutoff=eVapplapplies(Fig.5c).Schulletal.reportedthatquantum
shotnoisebecomesimportantfortunneljunctionswithhigh
tun-nelingcurrentsclosetothequantumconductanceG0 [162,164].
These junctions emit photons with energies largerthan eVappl
(Fig.5c).Later,Nitzanetal.establishedthetheoreticallinkbetween
thequantumnoiseandtheacconductanceofthetunneljunction,
and theplasmoniclight emission based onthenonequilibrium
Greenfunction formalism[171]. Theshotnoise directlyexcites
theplasmonicresonanceofthetunneljunctionthroughits
spec-traldistributionattheopticalfrequency.Themodelalsoexplains
thatphotonsareemittedwithhigherenergythaneVapplbecauseof
strongelectron-electroncoupling(Fig.5c).
In molecular tunnel junctions so far, plasmons have been
excited,ormodulated,usingelectrolumiscentmolecules.Fig.5b
showsthatelectronsandholesareinjectedinthemoleculewhich
thenrecombineandemitaphoton[174–183].Plasmonscanalso
beexcitedviadirecttunnelingbetweenthesubstrateandthetip
andthechallengeistodiscriminatebetweenplasmonsexcitedvia
thephotonsemittedfromthemoleculeorviadirecttunneling.The
energylevelsofmolecules(e.g.,HOMOorLUMO)mayparticipate
inthetunnelingprocessand thusaffecttheplasmonexcitation
process.Therefore, insystems withlow lyingenergy levelsthe
mechanismofplasmonexcitationcanbechallengingtodetermine.
3.2. STM-basedmolecularelectronicplasmonsources
ThehighlylocalizedtunnelingcurrentintheSTMmakeit
pos-sibletoexciteplasmonslocallywithasub-nanometerresolution.
PioneeringexperimentsbyCoombs,GimzewskiandBerndtetal.
[156–158]showedbroadbandlightemissionfromSTMjunctions
ondifferent(polycrystallineAg,Au(110), Cu(111),and Ag(111))
metallicsurfaces.Here,inelastictunnelingelectronsexcitedLSPs
betweentheproximityofthetipandthesample;theseLSPsthen
radiativelydecayedandweredetectedopticallybyplacing
objec-tivesnearbythejunction.Sincethen,plasmonexcitationinSTM
junctionshavebeenintensivelystudiedbothexperimentallyand
theoretically[159–173].Thesestudiesshowedthatthegeometry
ofthetipisimportantintheplasmonexcitation[172,173],enabled
sub-nanometerresolutionplasmonmodemapping[159–161],and
improvedtheunderstandingofplasmonexcitationbycouplingthe
localdensity ofelectronicand opticalstates [163].By
integrat-ingtheSTMwithaninvertedopticalmicroscope,SPPsexcitedby
STMjunctionshavebeendirectlyimagedthroughleakageradiation
microscopy[165–170].
Duringthelastdecade,indirectplasmonexcitationviathe
elec-troluminescenceofmoleculeshasbeeninvestigated[174–183].In
theseexperimentsitisimportanttodecouplethemoleculefrom
themetallicsurfacestoavoidquenching.Inpractice,this
decou-pling hasbeenrealizedby isolatingfluorophores frommetallic
substratesusingthin(in)organicinsulatinglayerssuchasmetal
oxides [174,177],or molecular layers [175,176]; thesetypes of
junctions have been recently reviewed in detail elsewhere by
Shamaiaetal.[40],Galperinaetal.[41],andRosseletal.[178].
Insteadof usinginsulating layerstodecouple themolecules
fromthebottom-electrode,recently“molecular”approacheshave
beenusedtominimizequenching.Usually,unmodifiedporphyrins
lie flaton metalsurfaces becauseof strongmolecule-substrate
interactions.Fig.6ashowsaporphyrinmoleculewitharigid
tripo-dalanchor and spacer to effectively place the porphyrin away
fromthebottom-electrode[181].Fig.6bshowstheSTM-induced
luminescencespectrawhicharecharacteristicfortheporphyrin
moiety proving effective decouplingof theporphyrin fromthe
bottom-electrode although electroluminescence intensity from
thesemoleculeswasabout oneorder ofmagnitude lowerthan
thatofthebaresubstrate.Thisreductionintheluminescent
inten-sitymaybeduetothereducedplasmonfieldsduetothelarge
tip-substratedistanceorthenon-radiativedecaychannelsrelated
tochargetransferordipole-dipoleenergytransfer.Althoughthe
decouplingoftheporphyrinmoleculeissuccessful,further
under-standingandtherecoveryoftheluminescenceintensityisneeded.
Anotherdecouplingexperimenthasbeenrealizedbysimply
lift-ingapolythiophenewirewithaSTMtipfromagoldsurface(Fig.6c
andd)[182].Duringtheexperiment,thepolythiophenewirewas
liftedofffromtheAusurfacebyseveralnanometersusingtheSTM
tip,sothewireendsweredirectlyconnectedtotheelectrodes,
whereasa partofthepolymer chainwassuspended insidethe
junctionanddisconnectedfromthesubstrate.TheSTM-induced
luminescencespectrafromthesuspendedwireshowedabroad
res-onancewhosemaximumdidnotshiftwithvoltage.Thesevoltage
independentspectrawerebelievedtobetheelectroluminescence
ofthesuspendedwiresbytherecombinationofelectronsinjected
fromthetipin theLUMOwithholesinjected fromthesample
intheHOMOofthewirejunction.Thisexperimentintroduceda
newexperimentalmethodtodecouplemoleculesfrommetallic
substratesbysimplyliftingupthemolecules,althoughthe
elec-tronicpropertiesofthejunctionsis notcompletelyclearasthe
tipgeometry,molecule-tipandmolecule-substrateinteractions,or
conformationofthemoleculeinsidethejunction,arenotknown.
Besides molecule-specific electroluminescence, individual
Fig.5. Molecularelectronicsappliedinplasmonexcitation.(a,b)Schematicsofdirect(a)andindirect(b)plasmonexcitationintunneljunctions,respectively.Themolecule canmodulatetheinelastictunnelingbytheirelectronictransitionsbetweenHOMO–LUMOorbitals.(c)Theleftpanelshowthejunctionconductanceasafunctionof tip-substratedistance.TherightpanelshowsthelightemissionspectraasafunctionofthedistancebetweentheAutipandAusubstrate.WhenG/G0<0.3,thespectrafallinthe
hv<eV(inelastictunnelingregime),butforG/G0>0.3,photonenergieswithhv>eVareobserved(quantumshotnoisedominatedregime).AdaptfromRef.[162],Copyright
2009APS.
junctionsbymodifyingtheinelastictunnelingprocess.Fig.6eand
fshowtheschematicofanSTMjunctionwithIr(ppy)3molecules
physisorbed on a monolayer of C60 monolayer supported by
Ag(111)substrate.Here,C60monolayerfunctionsasthedielectric
spacertodecoupleIr(ppy)3moleculesfromtheAgsubstrate[183].
FromthisSTM junction,there aretwomain observations.First,
theSTM-induced plasmon map onsingleIr(ppy)3 molecules at
−1.8VhadthesamespatialshapeastheirHOMOorbitalmapat
−0.2V. Second,thecut-off energy of theplasmonspectrafrom
single Ir(ppy)3 molecules was 0.2V lower than that from the
C60 monolayer. These observations suggest that the HOMO of
theIr(ppy)3 moleculesmodulates theplasmonexcitation both
spatially and spectrally. In other words, the plasmon emission
patternandspectracanbecontrolledatthemolecularscale.
3.3. On-chipmolecularelectronicplasmonsources
Electrically-driven on-chipplasmon sources areessential for
integratedplasmoniccircuits.Recenteffortsmainlyreliedon
indi-rect electrically-driven plasmon sources based onminiaturized
light sources that exploit electroluminescence from nanoscale
semiconductorssuchasnano-lightemittingdiodesorcarbon
nano-tubes[150–155].However,theseplasmonsourcesarelimitedin
speed becausetheyrelyonelectron–holerecombination which
typicallyoccursonthenanosecondtime-scales.Directelectrical
excitationsofplasmonsintunneljunctionsoccuratthequantum
mechanicaltunnelingtimescalesandarethusfast[184].In
princi-ple,MIMjunctionsarecompatiblewithon-chipapplications,butso
fartheyhavenotbeencoupledtoplasmonicwaveguides.Recently,
MIMjunctionsbasedonhexagonalboronnitrideorvacuumasthe
insulatorhavebeencombinedwithopticalantennas[147–149].
Werecentlyreportedon-chipdirectplasmonexcitationusing
moleculartunneljunctionsbasedonSAMs[185].Wefoundthat
theplasmonsinthesejunctionsoriginatefromsingle,
diffraction-limitedspots,followpower-lawdistributedphotonstatistics,and
havewell-definedpolarizationorientationswhicharecontrolled
bythetunnelingdirectiondefinedbythetiltangleoftheSAMor
bysimplychangingtheappliedbiasofthejunction.Fig.7ashows
theschematicof theSTJ which consistsofan EGaIn/Ga2O3 top
electrodeconfinedinathrough-holeinatransparentrubber
(poly-dimethylsiloxane;PDMS)andanultra-flattemplate-strippedAu
(AuTS)electrodesupportingSAMsofSC12.Thesejunctionsexcited
bothlocalizedandpropagatingSPPs(Fig.7bandc).Intherealplane
image(Fig.7b),LSPsarecharacterizedbydiffractionlimited
emis-sionspots andtheSPPsby theunidirectionaland un-diffracted
emissionspotsaroundtheboundaryofthejunction.Intheback
focal planeimage(Fig.7c), the SPPsare shown asnarrowarcs
with specificwavevectors kSPP and labelled with modeI (SPPs
alongtheAu/SAM—Airinterface,kSPP=1.01)andII(SPPsalongthe
Au/SAM—PDMSinterface,kSPP=1.47).Fig.7dshowsthedefocused
plasmonemissionimageofFig.7bwhichindicatesthepolarization
orientationoftheplasmonemissionspots.Theoreticalcalculations
confirmthatthepolarizationorientationoftheplasmonemissionis
∼30◦withrespecttothesurfacenormalandequalstothetiltangle
oftheSAMs.Thisrelationimpliesthattheplasmonexcitationinthe
Fig.6.MolecularelectronicsappliedinplasmonexcitationinSTMs.(a,b)ElectroluminescencemodulatedplasmonexcitationinaSTMjunctionwithafluorophorefromthe bottomelectrodebyintramolecularspacer.AdaptedwithpermissionfromRef.[181].Copyright2013ACS.(c,d)Electroluminescencemodulatedplasmonexcitationina STMjunctionwithasuspendedelectroluminescentoligomer.AdaptedwithpermissionfromRef.[182].Copyright2014APS.(e,f)Molecularorbitalgatedplasmonexcitation inaSTMjunctionwithIr(ppy)3adsorbedonamonolayerofC60.AdaptedwithpermissionfromRef.[183].Copyright2013ACS.
Fig.7. On-chipplasmonexcitationwithSTJs.(a)Schematicoftheon-chipSTJs.Here,thesubstrateisglass,OAisanopticaladhesivethatwasusedintemplate-stripping, AuTSistemplatestrippedAu,PDMSispolydimethylsiloxanebasedrubbermoldwithmicrochannelsthatstabilizesthetop-electrode,EGaInstandsforeutecticalloyofGa
andIn,andGa2O3isanative,conductiveoxidesurfacelayerof0.7nmontheEGaIn.(b,c)Realplaneimage(b)andbackfocalplaneimage(c)oftheplasmonemissionwitha
Fig.8. Tunneljunctionsappliedinplasmondetection.(a)Directplasmon detec-tionbyphoto-assistedtunneling(PAT).(b)Indirectplasmondetectionthrough electron–holeseparation.
tunneling.Thisobservationwasfurtherconfirmedbychangingthe
tiltangleto∼10◦ bysimplyreplacingtheAuTSbyaAgTSbottom
electrode.Usingthesejunctions,plasmonscouldalsobeexcitedin
plasmonicwaveguidesandbias-selectiveplasmonexcitationwas
achievedwithmoleculardiodes.
Noteworthyisthattheplasmonsareexcitedindiscretespots
(Fig.7b).Itiswell-knowninelectricalengineeringthatthecurrent
doesnotflowuniformlyacrossmostjunctionsandthatthe
effec-tiveelectricalcontactareamaybe2–4ordersofmagnitudesmaller
thanthegeometricalcontactarea[186].Theeffectiveelectrical
con-tactareainSAM-basedjunctionswithEGaIntop-electrodeswas
recentlyestimatedtobe4ordersofmagnitudesmallerthanthe
geometricalcontactarea[187,188].ThedatashowninFig.7bshows
visuallythatindeedthecurrentflowacrossthesejunctionsishighly
inhomogeneous.
4. Molecularelectronicsappliedinplasmondetection
Theelectricaldetectionofplasmonsisthereverseprocessof
plasmonexcitation,andcanalsobeclassifiedasdirectand
indi-rect plasmon detection strategies as depicted in Fig. 8. Direct
plasmondetectionisrealizedbyplasmonassistedtunneling.
Indi-rect plasmon detectionhastwo steps. Firstly,plasmons induce
electron–holepairgeneration.Secondly,theseelectron–holepairs
areseparatedandcollectedresultinginaphotocurrent.Here,we
reviewthedirectplasmondetectionbasedonthemoleculartunnel
junctions.
4.1. Mechanismofelectricaldetectionofplasmons
Moleculartunneljunctionscanbeusedtodetectplasmonsboth
indirectlyanddirectly.Becauseoftheplasmonicfield,theaverage
tunnelbarrierheightofthejunctionsisreducedandthusthetunnel
currentincreases(Fig.8a).Thisprocessisin-principlethe
photon-assistedtunneling(PAT)introducedbyTienandGordon[189,190],
however,becauseinplasmondetectiontheelectromagneticfield
isprovidedbyplasmons,thiseffectcanalsobenamedas
plasmon-assistedtunneling.InPAT,theplasmonicfieldsproduceanelectric
fieldwhichcanbeapproximatedasaneffectiveacbiasVaccos(ωt)
addedtotheapplieddcbiaswhereVacistheacbiasamplitude,
ωisthefrequencyoftheplasmonandtistime.Thisacbiasthen
modulatesthedctunnelcurrentgivenbyRefs.[85,189,190].
Idc+ac(Vdc+Vaccos(ωt))=
n=∞
n=−∞
Jn2(eVacω )Idc(Vdc+nωe ) (2)
whereisthereducedPlanck’sconstantandeistheelementary
charge.TheincreasedcurrentIdcduetotheacbiasinthelimitof
smallacamplitudes(eVac≤ω)canbeapproximatedby
Idc=Idc+ac(Vdc+Vaccos(ωt))−Idc(Vdc)
= 1
4V
2 ac
Idc(Vdc+ω/e)+Idc(Vdc−ω/e)−2Idc(Vdc)
(/e)2
(3)Ifthevariationofthetunnelingcurrentissmallonthebiasscale
ofω/e,Eq.(3)canbesimplifiedto[85]
Idc= 1 4V 2 ac
d2Idc dVdc2 (4)Onthecontrary,indirectplasmondetectionwithmolecular
tun-neljunctions utilizeselectronically decoupledmoleculeswitha
smallerbandgap(e.g.,conjugatedmolecules,Fig.1d)asthe
tun-nelingbarrier.Similartophotondetectionusingsemiconductors,
when plasmonic electromagnetic fields reach the tunnel
junc-tions,themoleculeswillabsorbtheplasmonenergyandgenerate
electron–holepairswhicharethenseparatedinanapplied
elec-tricfieldresultinginaphotocurrent(Fig.8b).Torealizeeffective
indirectplasmondetectionusingmolecules,thereare
fundamen-tal issues to overcome. Firstly, the intrinsic HOMO–LUMO gap
of the moleculelimits the detection bandwidth. Secondly, the
electron–holegenerationisusuallyonthetime-scaleof
nanosec-ondswhich limitstheresponse speed. Sofar,indirectplasmon
detection has mainly been investigated based on miniaturized
semiconductorsconnectedtoplasmonicwaveguidesinformsof
Au films [191], single crystalline Ag nanowires [192,194,198],
polycrystallineAuwaveguides[195–197],andMIMgapplasmon
waveguides[193].
4.2. Directplasmondetectionwithmoleculartunneljunctions
Directplasmondetectionwithmoleculartunneljunctionshas
beenrealizedusingmoleculeswithlargeHOMO–LUMOgaps(e.g.,
aliphaticmolecules,Fig.1e)asthetunnelbarrierusingjunctions
oftheformofAuNW-SAM-Au(NW=nanowire)sandwichbyNoy
etal.[199,200].Fig.9ashowsthesuspendnanowiremolecular
tun-neljunctionswithSAMsofSCnwithn=8,10,12.SPPswereexcited
usingalaserandpropagatedalongtheAuNWtothejunctions.
TheSPPscausedanincreaseintheobserveddctunnelingcurrent
(Fig.9b).TheinsetofFig.9bshowsafitofEq.(4)toaplotofthe
increaseinthedccurrentasafunctionofVapplandtheauthors
concludedthatPATexplainstheirobservationswell.Fromthe
mag-nitude ofthe plasmonmodulated current, it isalsopossible to
determinethestrengthsoftheplasmonicfieldinthejunctions.The
enhancementbytheplasmonicfieldsshoulddecayexponentially
withthelengthofthemolecule(i.e.,thevalueofn).Theauthors
foundaplasmonicfieldenhancementofafactorof∼550forn=8,
∼250forn=10,and∼100forn=12.Suchanevolutionofplasmonic
fieldenhancementisnormallydifficulttomeasuredirectlyby
opti-calmeansbecauseofthesmalldimensionsofthegap(∼1nm).The
directplasmondetectionofthemoleculartunneljunctionprovides
theopportunitytocombinemolecularelectronicswithplasmonics
andinsitudeterminationofplasmoniceffects.
Directplasmondetectionhasalsobeenperformedwith
junc-tionsusingcontinuousgoldfilmsastheelectrodes.Fig.9cshows
squeezable breakjunctionsoftheformofAufilm-SAM-Aufilm
[201].ThesejunctionssupportSPPsontheircontinuousgoldfilms
whichcanbeexcitedwithap-polarizedlightintheKretschmann
configurationusingaglassprism.Whenthethingoldfilmswere
squeezedtoforma singlemoleculetunnel junction,SPPs were
electro-Fig.9.Directplasmondetectionwithmoleculartunneljunctions.(a,b)PlasmondetectionbyPATinaAuNW-SAM-Aufilmtunneljunctionwherelocalizedplasmonswere excited.Adaptedwithpermissionfrom[200].Copyright2010ACS.(c,d)PlasmondetectionbyPATinaAufilm-SAM-AufilmtunneljunctionwherepropagatingSPPswere excited.AdaptedwithpermissionfromRef.[201].Copyright2013ACS.
magneticfieldsandsignificantincreaseoftheobservedtunneling
current(Fig.9d).
5. Conclusionsandoutlook
ThisReviewhighlightsthegreat opportunitiesformolecular
plasmonicsbytakingadvantageofboththeconceptsofplasmonics
andmolecularelectronics.Judiciouslydesignedmoleculartunnel
junctionsprovideaninterestingexperimentalplatformtoimprove
ourunderstandingofquantumplasmonics(includingcharge
trans-ferplasmons)andtodemonstratepromisingapplicationsinvolving
nanoscaleplasmongenerationanddetection.Althoughthe
feasi-bilityofcombiningmolecularelectronicsandplasmonicshasbeen
clearlydemonstratedinthestudieshighlightedinthisReview,
sev-eralchallengesremaininmolecularelectronicplasmonics.
Modelingquantum plasmon resonances inmolecular tunnel
junctionsis challenging.Usually,quantum plasmonmodelsuse
homogeneouseffectivedielectricscorrectedbyquantumtunneling
asthetunnelbarriers.Inreality,themoleculesinsidethejunctions
arenotstatic,butvibrateasaresultofthermalorinelastic
excita-tions[185,202–205],causingforinstanceblinkingphenomena.For
agivenjunction,thecurrentflowmaynotbeuniformlydistributed
acrossthejunctionareaand,ingeneral,theshapeofthetunneling
barrierisdisputed[27,206,207].Theseuncertaintiescomplicatethe
constructionofrealisticmodels.Ontheotherhand,although
quan-tumplasmonresonanceshavebeendemonstratedusingmolecule
junctions,activemolecularelectroniccontroloftheseresonances
havenotbeendemonstratedyet.
Molecularelectronicon-chip plasmon sources and detectors
havebeeninvestigated withdcbiases,butthesearepotentially
veryfastastheyoperatedonthequantumtunnelingtime-scales.
To realize ultra-fast plasmonic electroniccircuitry, it is
impor-tanttofabricatemoleculartunneling junctionswithsmallareas
(e.g.,<1m2)toensuresmallcapacitancesandminimalRCdelay
times.Finally, experimentaldemonstrationof simple molecular
plasmonicelectroniccircuitsisstilllacking.Oneofthereasonsis
thatreliablelarge-scalefabricationofmoleculartunneling
junc-tionsisstillchallengingalthoughrecentprogresshasbeenmade
[208–210]. Despite these challenges we believe that molecular
electronicplasmonicsisanexcitingplaygroundforboththeorists
andexperimentalistswithpromisingapplicationsinareas
compli-mentarytotraditionalsemiconductorbasedelectronics.
Acknowledgements
WeacknowledgetheNationalResearchFoundation(NRF)for
supporting this research under the Competitive Research
Pro-gramme(CRP)program(awardNRF-CRP8-2011-07).
References
[1]InternationalTechnologyRoadmapforSemiconductors,2013,edn,available onlineathttp://public.itrs.net/.
[2]R.Zia,J.A.Schuller,A.Chandran,M.L.Brongersma,Plasmonics—thewaveof chip-scaledevicetechnologies,Mater.Today9(2006)20–27.
[3]M.L.Brongersma,V.M.Shalaev,Thecaseforplasmonics,Science328(2010) 440–441.
[4]E.Ozbay,Plasmonics:mergingphotonicsandelectronicsatnanoscale dimensions,Science311(2006)189–193.
[5]H.A.Atwater,Thepromiseofplasmonics,Sci.Am.296(2007)56–62. [6]W.L.Barnes,A.Dereux,T.W.Ebbesen,Surfaceplasmonsubwavelength
optics,Nature424(2003)824–830.
[7]D.K.Gramotnev,S.I.Bozhevolnyi,Plasmonicsbeyondthediffractionlimit, Nat.Photon.4(2010)83–91.
[8]C.Joachim,J.K.Gimzewski,A.Aviram,Electronicsusinghybrid-molecular andmono-moleculardevices,Nature408(2000)541–548.
[9]A.Nitzan,M.Ratner,Electrontransportinmolecularwirefunctions,Science 300(2003)1384–1389.
[10]A.Salomon,D.Cahen,S.Lindsay,J.Tomfohr,V.B.Engelkes,C.D.Frisbie, Comparisonofelectronictransportmeasurementsonorganicmolecules, Adv.Mater.15(2003)1881–1890.
[11]R.L.McCreery,Molecularelectronicjunctions,Chem.Mater.16(2004) 4477–4496.
[12]N.J.Tao,Electrontransportinmolecularjunctions,Nat.Nanotechnol.1 (2006)173–181.
[13]H.Haick,D.Cahen,Contactingorganicmoleculesbysoftmethods:towards molecule-basedelectronicdevices,Acc.Chem.Res.41(2008)359–366. [14]M.Ratner,Abriefhistoryofmolecularelectronics,Nat.Nanotechnol.8
(2013)378–381.
[15]L.Sun,Y.A.Diaz-Fernandez,T.A.Gschneidtner,F.Westerlund,S.Lara-Avilab, K.Moth-Poulsen,Single-moleculeelectronics:fromchemicaldesignto functionaldevices,Chem.Soc.Rev.43(2014)7378–7411.
[16]Y.Zhang,Z.Zhao,D.Fracasso,R.C.Chiechi,Bottom-upmoleculartunneling junctionsformedbyself-assembly,Isr.J.Chem.54(2014)513–533. [17]R.M.Metzger,Unimolecularelectronics,Chem.Rev.115(2015)5056–5115. [18]A.S.Blum,J.G.Kushmerick,D.P.Long,C.H.Patterson,J.C.Yang,J.C.
Henderson,Y.Yao,J.M.Tour,R.Shashidhar,B.R.Ratna,Molecularlyinherent voltage-controlledconductanceswitching,Nat.Mater.4(2005)167–172. [19]P.Liljeroth,Peter,J.Repp,G.Meyer,Current-inducedhydrogen
tautomerizationandconductanceswitchingofnaphthalocyanine molecules,Science317(2007)1203–1206.
[20]S.Y.Quek,M.Kamenetska,M.L.Steigerwald,H.J.Choi,S.G.Louie,M.S. Hybertsen,J.B.Neaton,L.Venkataraman,Mechanicallycontrolledbinary conductanceswitchingofasingle-moleculejunction,Nat.Nanotechnol.4 (2009)230–234.
[21]F.Schwarz,G.Kastlunger,F.Lissel,C.Egler-Lucas,S.N.Semenov,K. Venkatesan,H.Berke,R.Stadler,E.Lörtscher,Field-inducedconductance switchingbycharge-statealternationinorganometallicsingle-molecule junctions,Nat.Nanotechnol.11(2016)170–176.
[22]M.Elbing,R.Ochs,M.Koentopp,M.Fischer,C.vonHanisch,F.Weigend,F. Evers,H.B.Weber,M.Mayor,Asingle-moleculediode,Proc.Natl.Acad.Sci. U.S.A.102(2005)8815–8820.
[23]I.Díez-Pérez,J.Hihath,Y.Lee,L.Yu,L.Adamska,M.A.Kozhushner,I.I. Oleynik,N.J.Tao,Rectificationandstabilityofasinglemoleculardiodewith controlledorientation,Nat.Chem.1(2009)635–641.
[24]N.Nerngchamnong,L.Yuan,D.Qi,J.Li,D.Thompson,C.A.Nijhuis,Theroleof vanderWaalsforcesintheperformanceofmoleculardiodes,Nat. Nanotechnol.8(2013)113–118.
[25]H.J.Yoon,K.C.Liao,M.R.Lockett,S.W.Kwok,M.Baghbanzadeh,G.M. Whitesides,Rectificationintunnelingjunctions:2,2-bipyridyl-terminated
n-alkanethiolates,J.Am.Chem.Soc.136(2014)17155–17162. [26]B.Capozzi,J.Xia,O.Adak,E.J.Dell,Z.F.Liu,J.C.Taylor,J.B.Neaton,L.M.
Campos,L.Venkataraman,Single-moleculediodeswithhighrectification ratiosthroughenvironmentalcontrol,Nat.Nanotechnol.10(2015)522–527. [27]L.Yuan,N.Nerngchamnong,L.Cao,H.Hamoudi,E.DelBarco,M.Roemer,R.
Sriramula,D.Thompson,C.A.Nijhuis,Controllingthedirectionof rectificationinamoleculardiode,Nat.Commun.6(2015)6324. [28]D.Fracasso,H.Valkenier,J.C.Hummelen,G.C.Solomon,RyanC.Chiechi,
Evidenceforquantuminterferenceinsamsofarylethynylenethiolatesin tunnelingjunctionswitheutecticGa–In(EGAIN)top-contacts,J.Am.Chem. Soc.133(2011)9556–9563.
[29]C.M.Guédon,H.Valkenier,T.Markussen,K.S.Thygesen,J.C.Hummelen,S.J. vanderMolen,Observationofquantuminterferenceinmolecularcharge transport,Nat.Nanotechnol.7(2012)305–309.
[30]H.Vazquez,R.Skouta,S.Schneebeli,M.Kamenetska,R.Breslow,L. Venkataraman,M.S.Hybertsen,Probingtheconductancesuperpositionlaw insingle-moleculecircuitswithparallelpaths,Nat.Nanotechnol.7(2012) 663–667.
[31]C.R.Arroyo,S.Tarkuc,R.Frisenda,J.S.Seldenthuis,C.H.M.Woerde,R. Eelkema,F.C.Grozema,H.S.J.vanderZant,Signaturesofquantum interferenceeffectsonchargetransportthroughasinglebenzenering, Angew.Chem.Int.Ed.52(2013)3152–3155.
[32]J.He,S.Lindsay,Onthemechanismofnegativedifferentialresistancein ferrocenylundecanethiolself-assembledmonolayers,J.Am.Chem.Soc.127 (2005)11932–11933.
[33]X.W.Tu,G.Mikaelian,W.Ho,Controllingsingle-moleculenegative differentialresistanceinadouble-barriertunneljunction,Phys.Rev.Lett. 100(2008)126807.
[34]M.L.Perrin,M.Koole,J.S.Seldenthuis,J.A.CelisGil,H.Valkenier,J.C. Hummelen,N.Renaud,F.C.Grozema,J.M.Thijssen,D.Dulic,H.S.J.vander Zant,Largenegativedifferentialconductanceinsingle-moleculebreak junctions,Nat.Nanotechnol.9(2014)830–834.
[35]C.M.Ramsey,E.delBarco,S.Hill,S.J.Shah,C.C.Beedle,D.N.Hendrickson, Quantuminterferenceoftunneltrajectoriesbetweenstatesofdifferentspin lengthinadimericmolecularnanomagnet,Nat.Phys.4(2008)277–281. [36]S.Schmaus,A.Bagrets,Y.Nahas,T.K.Yamada,A.Bork,M.Bowen,E.
Beaurepaire,F.Evers,W.Wulfhekel,Giantmagnetoresistancethrougha singlemolecule,Nat.Nanotechnol.6(2011)185–189.
[37]E.Burzurí,A.S.Zyazin,A.Cornia,H.S.J.vanderZant,Directobservationof magneticanisotropyinanindividualFe4single-moleculemagnet,Phys. Rev.Lett.109(2012)147203.
[38]B.Warner,F.ElHallak,H.Prüser,J.Sharp,M.Persson,A.J.Fisher,C.F. Hirjibehedin,Tunablemagnetoresistanceinanasymmetricallycoupled single-moleculejunction,Nat.Nanotechnol.10(2015)259–263.
[39]A.C.Aragonès,D.Aravena,J.I.Cerdá,Z.Acís-Castillo,H.Li,J.A.Real,F.Sanz,J. Hihath,E.Ruiz,I.Díez-Pérez,Largeconductanceswitchingina
single-moleculedevicethroughroomtemperaturespin-dependent transport,NanoLett.16(2016)218–226.
[40]T.Shamaia,Y.Selzer,Spectroscopyofmolecularjunctions,Chem.Soc.Rev. 40(2011)2293–2305.
[41]M.Galperina,A.Nitzan,Molecularoptoelectronics:theinteractionof molecularconductionjunctionswithlight,Phys.Chem.Chem.Phys.14 (2012)9421–9438.
[42]S.V.Aradhya,L.Venkataraman,Single-moleculejunctionsbeyondelectronic transport,Nat.Nanotechnol.8(2013)399–410.
[43]S.Battacharyya,A.Kibel,G.Kodis,P.A.Liddell,M.Gervaldo,D.Gust,S. Lindsay,Opticalmodulationofmolecularconductance,NanoLett.11(2011) 2709–2714.
[44]C.Jia,J.Wang,C.Yao,Y.Cao,Y.W.Zhong,Z.R.Liu,Z.F.Liu,X.F.Guo, Conductanceswitchingandmechanismsinsingle-moleculejunctions, Angew.Chem.Int.Ed.52(2013)8666–8670.
[45]J.A.Fereiro,R.L.McCreery,A.J.Bergren,Directopticaldeterminationof interfacialtransportbarriersinmoleculartunneljunctions,J.Am.Chem. Soc.135(2013)9584–9587.
[46]S.A.Maier,H.A.Atwater,Plasmonics:localizationandguidingof electromagneticenergyinmetal/dielectricstructures,J.Appl.Phys.98 (2005)011101.
[47]E.Hutter,J.H.Fendler,Exploitationoflocalizedsurfaceplasmonresonance, Adv.Mater.16(2004)1685–1706.
[48]K.A.Willets,R.P.VanDuyne,Localizedsurfaceplasmonresonance spectroscopyandsensing,Annu.Rev.Phys.Chem.58(2007)267–297. [49]M.E.Stewart,C.R.Anderton,L.B.Thompson,J.Maria,S.K.Gray,J.A.Rogers,
R.G.Nuzzo,Nanostructuredplasmonicsensors,Chem.Rev.108(2008) 494–521.
[50]K.M.Mayer,J.H.Hafner,Localizedsurfaceplasmonresonancesensors, Chem.Rev.111(2011)3828–3857.
[51]R.M.Dickson,L.A.Lyon,Unidirectionalplasmonpropagationinmetallic nanowires,J.Phys.Chem.B104(2000)6095–6098.
[52]A.V.Zayats,I.I.Smolyaninovb,A.A.Maradudinc,Nano-opticsofsurface plasmonpolaritons,Phys.Rep.408(2005)131–314.
[53]E.Moreno,S.G.Rodrigo,S.I.Bozhevolnyi,L.Martin-Moreno,F.J.Garcia-Vidal, Guidingandfocusingofelectromagneticfieldswithwedgeplasmon polaritons,Phys.Rev.Lett.100(2008)023901.
[54]S.I.Bozhevolnyi,V.S.Volkov,E.Devaux,J.Y.Laluet,T.W.Ebbesen,Channel plasmonsubwavelengthwaveguidecomponentsincludinginterferometers andringresonators,Nature440(2006)508–511.
[55]R.Zia,M.L.Brongersma,Surfaceplasmonpolaritonanaloguetoyoung’s double-slitexperiment,Nat.Nanotechnol.2(2007)426–429.
[56]P.Berini,I.DeLeon,Surfaceplasmon–polaritonamplifiersandlasers,Nat. Photon.6(2012)16–24.
[57]P.Anger,P.Bharadwaj,L.Novotny,Enhancementandquenchingof single-moleculefluorescence,Phys.Rev.Lett.96(2006)113002. [58]S.Kühn,U.Håkanson,L.Rogobete,V.Sandoghdar,Enhancementof
single-moleculefluorescenceusingagoldnanoparticleasanoptical nanoantenna,Phys.Rev.Lett.97(2006)017402.
[59]S.Lal,S.Link,N.J.Halas,Nano-opticsfromsensingtowaveguiding,Nat. Photon.1(2007)641–648.
[60]X.M.Qian,S.M.Nie,Single-moleculeandsingle-nanoparticleSERS:from fundamentalmechanismstobiomedicalapplications,Chem.Soc.Rev.37 (2008)912–920.
[61]A.Kinkhabwala,Z.Yu,S.Fan,Y.Avlasevich,K.Müllen,W.E.Moerner,Large single-moleculefluorescenceenhancementsproducedbyabowtie nanoantenna,Nat.Photon.3(2009)654–657.
[62]P.Zijlstra,P.M.Paulo,M.Orrit,Opticaldetectionofsinglenon-absorbing moleculesusingthesurfaceplasmonresonanceofagoldnanorod,Nat. Nanotechnol.7(2012)379–382.
[63]D.Punj,M.Mivelle,S.B.Moparthi,T.S.vanZanten,H.Rigneault,N.F.van Hulst,M.F.García-Parajó,JérômeWenger,Aplasmonic‘antenna-in-box’ platformforenhancedsingle-moleculeanalysisatmicromolar concentrations,Nat.Nanotechnol.8(2013)512–516.
[64]M.D.Sonntag,J.M.Klingsporn,A.B.Zrimsek,B.Sharma,L.K.Ruvunaa,R.P. VanDuyne,Molecularplasmonicsfornanoscalespectroscopy,Chem.Soc. Rev.43(2014)1230–1247.
[65]N.Fang,H.Lee,C.Sun,X.Zhang,Sub-diffraction-limitedopticalimaging withasilversuperlens,Science308(2005)534–537.
[66]Z.W.Liu,H.Lee,Y.Xiong,C.Sun,X.Zhang,Far-fieldopticalhyperlens magnifyingsub-diffraction-limitedobjects,Science315(2007)1686. [67]X.Zhang,Z.Liu,Superlensestoovercomethediffractionlimit,Nat.Mater.7
(2008)435–441.
[68]S.Kawata,A.Ono,P.Verma,Subwavelengthcolourimagingwithametallic nanolens,Nat.Photon.2(2008)438–442.
[69]S.Kawata,Y.Inouye,P.Verma,Plasmonicsfornear-fieldnano-imagingand superlensing,Nat.Photon.3(2009)388–394.
[70]P.Li,T.Wang,H.B ¨ockmann,T.Taubner,Graphene-enhancedInfrared near-fieldmicroscopy,NanoLett.14(2014)4400–4405.
[71]S.Pillai,K.R.Catchpole,T.Trupke,M.A.Green,Surfaceplasmonenhanced siliconsolarcells,J.Appl.Phys.101(2007)093105.
[72]K.R.Catchpole,A.Polman,Plasmonicsolarcells,Opt.Express16(2008) 21793–21800.
[73]H.A.Atwater,A.Polman,Plasmonicsforimprovedphotovoltaicdevices,Nat. Mater.9(2010)205–213.
[74]C.Cesar,Plasmon-inducedhot-electrongenerationat
nanoparticle/metal-oxideinterfacesforphotovoltaicandphotocatalytic devices,Nat.Photon.8(2014)95–103.
[75]I.Thomann,B.A.Pinaud,Z.Chen,B.M.Clemens,T.F.Jaramillo,M.L. Brongersma,Plasmonenhancedsolar-to-fuelenergyconversion,NanoLett. 11(2011)3440–3446.
[76]J.L.Wu,F.C.Chen,Y.S.Hsiao,F.C.Chien,P.L.Chen,C.H.Kuo,M.H.Huang,C.S. Hsu,Surfaceplasmoniceffectsofmetallicnanoparticlesontheperformance ofpolymerbulkheterojunctionsolarcells,ACSNano5(2011)959–967. [77]S.C.Kim,J.H.Jin,Y.J.Kim,I.Y.Park,Y.S.Kim,S.W.Kim,High-harmonic
generationbyresonantplasmonfieldenhancement,Nature453(2008) 757–760.
[78]J.Renger,R.Quidant,N.VanHulst,L.Novotny,Surface-enhancednonlinear four-wavemixing,Phys.Rev.Lett.104(2010)046803.
[79]M.Kauranen,A.V.Zayats,Nonlinearplasmonics,Nat.Photon.6(2012) 737–748.
[80]S.M.Chen,G.X.Li,F.Zeuner,W.H.Wong,E.Y.B.Pun,T.Zentgraf,K.W.Cheah, S.Zhang,Symmetry-selectivethird-harmonicgenerationfromplasmonic metacrystals,Phys.Rev.Lett.113(2014)033901.
[81]K.O’Brien,H.Suchowski,J.Rho,A.Salandrino,B.Kante,X.Yin,X.Zhang, Predictingnonlinearpropertiesofmetamaterialsfromthelinearresponse, Nat.Mater.14(2015)379–383.
[82]M.Celebrano,X.F.Wu,M.Baselli,S.Großmann,P.Biagioni,A.Locatelli,C.De Angelis,G.Cerullo,R.Osellame,B.Hecht,L.Duò,F.Ciccacci,M.Finazzi, Modematchinginmultiresonantplasmonicnanoantennasforenhanced secondharmonicgeneration,Nat.Nanotechnol.10(2015)412–417. [83]L.Tang,S.E.Kocabas,S.Latif,A.K.Okyay,D.S.Ly-Gagnon,K.C.Saraswat,
D.A.B.Miller,Nanometre-scalegermaniumphotodetectorenhancedbya near-infrareddipoleantenna,Nat.Photon.2(2008)226–229.
[84]J.A.Dionne,K.Diest,L.A.Sweatlock,H.A.Atwater,PlasMOStor:a metal–oxide–Sifieldeffectplasmonicmodulator,NanoLett.9(2009) 897–902.
[85]D.R.Ward,F.Hüser,F.Pauly,J.C.Cuevas,D.Natelson,Opticalrectification andfieldenhancementinaplasmonicnanogap,Nat.Nanotechnol.5(2010) 732–736.
[86]W.Cai,A.P.Vasudev,M.L.Brongersma,Electricallycontrollednonlinear generationoflightwithplasmonics,Science333(2010)1720–1723. [87]M.W.Knight,H.Sobhani,P.Nordlander,N.J.Halas,Photodetectionwith
activeopticalantennas,Science332(2011)702–704.
[88]H.X.Xu,E.J.Bjerneld,M.Kall,L.Borjesson,Spectroscopyofsingle hemoglobinmoleculesbysurfaceenhancedRamanscattering,Phys.Rev. Lett.83(1999)4357.
[89]C.Sönnichsen,B.M.Reinhard,J.Liphardt,A.P.Alivisatos,Amolecularruler basedonplasmoncouplingofsinglegoldandsilvernanoparticles,Nat. Biotechnol.23(2005)741–745.
[90]C.E.Talley,J.B.Jackson,C.Oubre,N.K.Grady,C.W.Hollars,S.M.Lane,T.R. Huser,P.Nordlander,N.J.Halas,Surface-enhancedRamanscatteringfrom individualAunanoparticlesandnanoparticledimersubstrates,NanoLett.5 (2005)1569–1574.
[91]D.R.Ward,N.K.Grady,C.S.Levin,N.J.Halas,Y.Wu,P.Nordlander,D. Natelson,Electromigratednanoscalegapsforsurface-enhancedRaman spectroscopy,NanoLett.7(2007)1396–1400.
[92]P.K.Jain,W.Huang,M.A.El-Sayed,Ontheuniversalscalingbehaviorofthe distancedecayofplasmoncouplinginmetalnanoparticlepairs:aplasmon rulerequation,NanoLett.7(2007)2080–2088.
[93]M.Danckwerts,L.Novotny,Opticalfrequencymixingatcoupledgold nanoparticles,Phys.Rev.Lett.98(2007)026104.
[94]M.W.Chu,V.Myroshnychenko,C.H.Chen,J.P.Deng,C.Y.Mou,F.J.Garcíade Abajo,Probingbrightanddarksurface-plasmonmodesinindividualand couplednoblemetalnanoparticlesusinganelectronbeam,NanoLett.9 (2009)399–404.
[95]A.M.Funston,C.Novo,T.J.Davis,P.Mulvaney,Plasmoncouplingofgold nanorodsatshortdistancesandindifferentgeometries,NanoLett.9(2009) 1651–1658.
[96]S.S.A ´cimovi ´c,M.P.Kreuzer,M.U.González,R.Quidant,Plasmonnear-field couplinginmetaldimersasasteptowardsingle-moleculesensing,ACS Nano3(2009)1231–1237.
[97]A.L.Koh,K.Bao,I.Khan,W.E.Smith,G.Kothleitner,P.Nordlander,S.A. Maier,D.W.McComb,Electronenergy-lossspectroscopy(EELS)ofsurface plasmonsinsinglesilvernanoparticlesanddimers:influenceofbeam damageandmappingofdarkmodes,ACSNano3(2009)3015–3022. [98]F.M.Huang,J.J.Baumberg,Activelytunedplasmonsonelastomerically
drivenaunanoparticledimers,NanoLett.10(2010)1787–1792.
[99]K.L.Wustholz,A.I.Henry,J.M.McMahon,R.G.Freeman,N.Valley,M.E.Piotti, M.J.Natan,G.C.Schatz,R.P.VanDuyne,Structure-activityrelationshipsin goldnanoparticledimersandtrimersforsurface-enhancedRaman spectroscopy,J.Am.Chem.Soc.132(2010)10903–10910.
[100]N.J.Halas,S.Lal,W.S.Chang,S.Link,P.Nordlander,Plasmonsinstrongly coupledmetallicnanostructures,Chem.Rev.111(2011)3913–3961. [101]J.M.Romo-Herrera,R.A.Alvarez-Puebla,LuisM.Liz-Marzán,Controlled
assemblyofplasmoniccolloidalnanoparticleclusters,Nanoscale3(2011) 1304–1315.
[102]D.Punj,R.Regmi,A.Devilez,R.Plauchu,S.B.Moparthi,B.Stout,N.Bonod,H. Rigneault,J.Wenger,Self-assemblednanoparticledimerantennasfor plasmonic-enhancedsingle-moleculefluorescencedetectionatmicromolar concentrations,ACSPhotonics2(2015)1099–1107.
[103]J.B.Lassiter,J.Aizpurua,L.I.Hernandez,D.W.Brandl,I.Romero,S.Lal,J.H. Hafner,P.Nordlander,N.J.Halas,Closeencountersbetweentwonanoshells, NanoLett.8(2008)1212–1218.
[104] J.Zuloaga,E.Prodan,P.Nordlander,Quantumdescriptionoftheplasmon resonancesofananoparticledimer,NanoLett.9(2009)887–891. [105] O.Pérez-González,N.Zabala,A.G.Borisov,N.J.Halas,P.Nordlander,J.
Aizpurua,Opticalspectroscopyofconductivejunctionsinplasmonic cavities,NanoLett.10(2010)3090–3095.
[106] R.Esteban,A.G.Borisov,P.Nordlander,J.Aizpurua,Bridgingquantumand classicalplasmonicswithaquantum-correctedmodel,Nat.Commun.3 (2012)825.
[107] D.C.Marinica,A.K.Kazansky,P.Nordlander,J.Aizpurua,A.G.Borisov, Quantumplasmonics:nonlineareffectsinthefieldenhancementofa plasmonicnanoparticledimer,NanoLett.12(2012)1333–1339. [108] L.Wu,H.Duan,P.Bai,M.Bosman,J.K.W.Yang,E.Li,Fowler–Nordheim
tunnelinginducedchargetransferplasmonsbetweennearlytouching nanoparticles,ACSNano7(2013)707–716.
[109] R.Esteban,G.Aguirregabiria,A.G.Borisov,Y.M.Wang,P.Nordlander,G.W. Bryant,J.Aizpurua,Themorphologyofnarrowgapsmodifiestheplasmonic response,ACSPhotonics2(2015)295–305.
[110] R.Esteban,A.Zugarramurdi,P.Zhang,P.Nordlander,F.J.Garcia-Vidal,A.G. Borisov,J.Aizpurua,Aclassicaltreatmentofopticaltunnelinginplasmonic gaps:extendingthequantumcorrectedmodeltopracticalsituations, FaradayDiscuss.178(2015)151–183.
[111] F.Wen,Y.Zhang,S.Gottheim,N.S.King,Y.Zhang,P.Nordlander,N.J.Halas, Chargetransferplasmons:opticalfrequencyconductancesandtunable infraredresonances,ACSNano9(2015)6428–6435.
[112] U.Hohenester,Quantumcorrectedmodelforplasmonicnanoparticles:a boundaryelementmethodimplementation,Phys.Rev.B91(2015)205436. [113] J.G.Simmons,Generalizedformulafortheelectrictunneleffectbetween
similarelectrodesseparatedbyathininsulatingfilm,J.Appl.Phys.34 (1963)1793–1803.
[114] H.B.Akkerman,B.deBoer,Electricalconductionthroughsinglemolecules andself-assembledmonolayers,J.Phys.Condens.Matter20(2008)013001. [115] L.Lafferentz,F.Ample,H.Yu,S.Hecht,C.Joachim,L.Grill,Conductanceofa
singleconjugatedpolymerasacontinuousfunctionofitslength,Science 323(2009)1193–1197.
[116] L.Sepunaru,I.Pecht,M.Sheves,D.Cahen,Solid-stateelectrontransport acrossazurin:fromatemperature-independenttoatemperature-activated mechanism,J.Am.Chem.Soc.133(2011)2421–2423.
[117] H.Yan,A.J.Bergren,R.L.McCreery,M.L.DellaRocca,P.Martin,P.Lafarge,J.C. Lacroix,Activationlesschargetransportacross4.5–22nminmolecular electronicjunctions,Proc.Nat.Acad.Sci.110(2013)5326–5330.
[118] N.Amdursky,D.Rchak,L.Sepunaru,I.Pecht,M.Sheves,D.Cahen,Electronic transportviaproteins,Adv.Mater.26(2014)7142–7161.
[119] K.S.Kumar,R.R.Pasula,S.Lim,C.A.Nijhuis,Long-rangetunnelingprocesses acrossferritin-basedjunctions,Adv.Mater.28(2016)1824–1830. [120] C.A.Nijhuis,W.F.Reus,J.R.Barber,G.M.Whitesides,Comparisonof
SAM-basedjunctionswithGa2O3/EGaIntopelectrodestootherlarge-area
tunnelingjunctions,J.Phys.Chem.C116(2012)14139–14150.
[121] A.V.Akimov,A.Mukherjee,C.L.Yu,D.E.Chang,A.S.Zibrov,P.R.Hemmer,H. Park,M.D.Lukin,Generationofsingleopticalplasmonsinmetallic nanowirescoupledtoquantumdots,Nature450(2007)402–406. [122] R.Kolesov,B.Grotz,G.Balasubramanian,R.J.Stöhr,A.A.L.Nicolet,P.R.
Hemmer,F.Jelezko,J.Wrachtrup,Wave-particledualityofsinglesurface plasmonpolaritons,Nat.Phys.5(2009)470–474.
[123] R.W.Heeres,L.P.Kouwenhoven,V.Zwiller,Quantuminterferencein plasmoniccircuits,Nat.Nanotechnol.8(2013)719–722.
[124] M.S.Tame,K.R.McEnery,S.K.Özdemir,J.Lee,S.A.Maier,M.S.Kim,Quantum plasmonics,Nat.Phys.9(2013)329–340.
[125] J.S.Fakonas,H.Lee,Y.A.Kelaita,H.A.Atwater,Two-plasmonquantum interference,Nat.Photon.8(2014)317–320.
[126] P.Nordlander,C.Oubre,E.Prodan,K.Li,M.I.Stockman,Plasmon hybridizationinnanoparticledimers,NanoLett.4(2004)899–903. [127] I.Romero,J.Aizpurua,G.W.Bryant,F.J.GarcíadeAbajo,Plasmonsinnearly
touchingmetallicnanoparticles:singularresponseinthelimitoftouching dimers,Opt.Express14(2006)9988–9999.
[128] F.J.GarcíadeAbajo,Nonlocaleffectsintheplasmonsofstronglyinteracting nanoparticles,dimers,andwaveguides,J.Phys.Chem.C112(2008) 17983–17987.
[129] C.David,F.J.GarcíadeAbajo,Spatialnonlocalityintheopticalresponseof metalnanoparticles,J.Phys.Chem.C115(2011)19470–19475. [130] C.Ciraci,R.T.Hill,Y.Urzhumov,A.I.Fernandez-Dominguez,S.A.Maier,J.B.
Pendry,A.Chilkoti,D.R.Smith,Probingtheultimatelimitsofplasmonic enhancement,Science337(2012)1072–1074.
[131] T.V.Teperik,P.Nordlander,J.Aizpurua,A.G.Borisov,Robustsubnanometric plasmonrulerbyrescalingofthenonlocalopticalresponse,Phys.Rev.Lett. 110(2013)263901.
[132] Y.Luo,A.I.Fernandez-Dominguez,A.Wiener,S.A.Maier,J.B.Pendry,Surface plasmonsandnonlocality:asimplemodel,Phys.Rev.Lett.111(2013) 093901.
[133] N.A.Mortensen,S.Raza,M.Wubs,T.Sondergaard,S.I.Bozhevolnyi,A generalizednon-localopticalresponsetheoryforplasmonicnanostructures, Nat.Commun.5(2014)3809.
[134] C.David,F.J.GarcíadeAbajo,Surfaceplasmondependenceontheelectron densityprofileatmetalsurfaces,ACSNano8(2014)9558.
[135] G.Toscano,C.Rockstuhl,F.Evers,H.Xu,N.A.Mortensen,M.Wubs, Resonanceshiftsandspill-outeffectsinself-consistenthydrodynamic nanoplasmonics,Nat.Commun.6(2015)7132.
