Interfaces-are-the-Dominant-Feature-of-Dielectrics

IEEETransactionsonDielectricsandElectricalInsulationVol.11,No.5;October2004739InterfacesaretheDominantFeatureofDielectricsattheNanometricLevelTJLewisSchoolofInformaticsUniversityofWales,BangorDeanStreet,Bangor,Gwynedd,LL571UT,UKABSTRACTItisarguedthatthebehaviorofdielectricparticlesastheyshrinkinsizethroughmicrometrictonanometricscaleswillbeincreasinglydominatedbythepropertiesoftheirinterfaceswiththeenvironment.Thevariousinteratomicandintermolecu-larforcesthatdeterminethestructureoftheseinterfacesarereviewedwithspecialemphasisontheirelectricalnature.Anumberofsituationsinwhichpassiveanddynamicdielectricpropertiesaretraceabletonanometricinterfacialpropertiesareconsidered.Itisalsodemonstratedthatsuchinterfacesarenanometricelectrome-()chanicalNEMsystemswhichactingcollectivelyalsoexplainpiezoelectricityinmacroscopicsystems.Interfacesarenaturallynanometricentitiesandmusthaveamajorroleinthefuturedevelopmentofnanotechnology.Theirubiquitousemploy-mentinlivingsystemsisnotedandcomparisonsuggestssynergisticopportunities.IndexTerms—Nanodielectrics,composites,interfaces,interfacialforces,dou-blelayers,interfacesasdielectricelements,NEM,piezoelectricity.1INTRODUCTIONHEdevelopmentofascientificcultureinwhichTthepropertiesofsystemsandsystemelementsareconsideredonananometricscalegrowsapace.Attheoutset,however,itcanbearguedthatmuch,ifnotthewhole,ofmodernscienceandtechnologyisalreadyfirmlybasedonnanometric,indeedsub-nanometricconcepts.Inparticularchemistryandmuchofbiologyandphysicsarebasedonlexiconsofpropertiesconsideredatthemolecu-larandatomiclevelsofsmallness.Intheseestablishedscienceshowevertheelementalpropertiesareusuallyin-tegratedovermacroscopicvolumestoreachexplanationsformacroscopicproperties.Theclassicaldielectricsitua-tion,forexample,istoconsiderthepropertiesofmolecu-Ž.lardipolesasub-nanometricconceptandtosumthemŽappropriatelytoobtainthedielectricpermittivitya.macroscopicconcept.Thenewdevelopmentsinnanosci-enceandtechnologystopshortofthefinalintegrationandconsiderwhatspecialpropertiesarepresentatthenano-metriclevelandhowtheymightbeexploited.Thisisameso-worldbetweenquantumandcontinuumscience.Inthecaseofdielectricsthisexerciseinvolvesthepropertiesofelementaryunitssuchasmacromolecules,monolayersandmembranesandnanoandmesometricparticles.Mod-Manuscriptrecei®edon4March2004,infinalform14April2004.elingdevicesandsystemelementsofthisorderofsmall-nessmeansthattheybegintotakeonaneverydayrealitybutinfacttheyareremotefromcommonexperience.Thisisstrikinglydemonstratedbytherealizationthat1gof100nmdiametersphericalAlparticleswouldbeenoughtoprovideeveryhumanbeingalivetodaywithabout3
105particlesandthata3nmdiameterclusteris106timessmallerthanafullstopinatypicallineofprint.Neverthe-lessinintegratedcircuitsdevicesofthesedimensionsandevensmallerareroutinelyemployedinmyriadnumbers.Fordielectricsystemsinwhichthelengthscalesarebe-wxlow200nmorsoi.e.nanometricdielectrics1ornanodi-electrics,thesurfacesandinterfacesbetweendielectricel-ements,themselvesofnanometricdimension,becomein-creasinglyprominent.Thenormalscalinglawsofthemi-croandmacroworldscannolongerbeusedtopredictpropertiesandtheforcessuchasoftenseenincolloidalsystemstakeover.Sinceinterfacesdominatedielectricsit-uationsatthislevel,nanodielectricsandinterfacesbecomeinextricable.Afeatureofdielectricinterfacesinequilib-riumisthattheyarelikelytosustainlargeelectricalfields.Infrequentlyacknowledgedhoweveristhepresenceofotherinteractivefieldswithinthemarisingfrommechani-calforcesandfromchemicalandentropicgradients.Thesehavebeenwellemphasizedhoweverinthecaseoftheintraandextracellularinterfacespresentinlivingsystems1070-9878rrrrr04rrrrr$20.00
2004IEEE739Lewis:InterfacesaretheDominantFeatureofDielectricsattheNanometricLe©el740wx2wherethecombinedfieldsconfiguredandorderedbyinterfacesproducethenecessaryfluxesofionsandmoleculesforlife.Themolecularorganizationandnon-centro-symmetry,whicharecommonfeaturesofdielectricinterfacesandtheproperties,whichderivefromthese,makethemat-tractivecandidatesforsensingandtransducingroles.TheadventoftheacronymsMEMSandNEMSforelectrome-chanicaldevicesonthemicroandnanoscaleshighlightstherecognitionofthepotentialforthetechnicalexploita-tionofinterfaces.Inthefollowingthedielectricfeaturesofinterfaceswillbereviewedandtheirdominanceinsys-temsatthenanometricscaleofsmallnesswillbeempha-sized.2THENANOMETRICCONDITIONConsideraninterfacesetupbetweentwootherwiseuniformmaterialphasesAandB.Eachatomormoleculeinthetwo-phasesystemwillequilibratewithitssurround-ingviasetsofshortandlong-rangeforces.Deepineachphasetheforces,althoughfluctuatingoveratomicdis-tances,areessentiallyconstantandcharacteristicofthephase.Astheinterfacewiththeotherphaseisap-proachedhowevertheywillbecomeincreasinglymodified.Therangeoverwhichtheforcesaredifferentfromthe‘bulk’valuesdefinesaninterfaceABacrosswhichthein-tensityIofanypropertyassociatedwiththeforceswillchange.TheextentoftheinterfaceisthendefinedbythechangeinIandwilldependonthepropertyofinterestwx3.Thismightbetheconcentrationofaconstituentofthesystem,suchasamo