physicsworld图.docx
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physicsworld图.docx
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physicsworld图
Graphenesupercapacitorbreaksstoragerecord
Nov26,20108comments
Curvedgraphenesheets
ResearchersintheUShavemadeagraphene-basedsupercapacitorthatcanstoreasmuchenergyperunitmassasnickelmetalhydridebatteries–butunlikebatteries,itcanbechargedordischargedinjustminutesorevenseconds.Thenewdevicehasaspecificenergydensityof85.6Wh/kgatroomtemperatureand136Wh/kgat80°C.Thesearethehighestevervaluesfor"electricdoublelayer"supercapacitorsbasedoncarbonnanomaterials.
Supercapacitors,moreaccuratelyknownaselectricdouble-layercapacitorsorelectrochemicalcapacitors,canstoremuchmorechargethanconventionalcapacitors.Animportantfeatureofsupercapacitorsisthatthereisanextremelynarrowgapbetweentheelectrodes–whichareultrathinlayers.Thismeansthatalargeamountofelectricalchargecanbestoredinatinyvolume.
ThenewdevicewasmadebyBorJangofUS-basedNanotekInstrumentsandcolleagues.Ithaselectrodesmadeofgraphenemixedwith5wt%SuperP(anacetyleneblackthatactsasaconductiveadditive)and10wt%PTFEbinder.Asheetofcarbonjustoneatomthick,grapheneisaverygoodelectricalconductoraswellasbeingextremelystrongandflexible.
Theresearcherscoattheresultingslurryontothesurfaceofacurrentcollectorandassemblecoin-sizedcapacitorsinaglovebox.Theelectrolyte-electrodeinterfaceismadeof"Celguard-3501"andtheelectrolyteisachemicalcalledEMIMBF4.
Fastcharging
WebelievethatthisistrulyabreakthroughinenergytechnologyBorJang,NanotekInstruments
Theenergydensityvaluesofthesupercapacitorarecomparabletothatofnickelmetalhydridebatteries."Thisnewtechnologymakesforanenergystoragedevicethatstoresnearlyasmuchenergyasinabatterybutwhichcanberechargedinsecondsorminutes,"Jangexplained."Webelievethatthisistrulyabreakthroughinenergytechnology."Thedevicemightbeusedtorechargemobilephones,digitalcamerasandmicro-EVs,headds.
Theteam,whichincludesscientistsfromAngstronMaterialsintheUSandDalianUniversityofTechnologyinChina,arenowworkinghardtofurtherimprovetheenergydensityofthedevice."Ourgoalistomakeasupercapacitorthatstoresasmuchenergyasthebestlithium-ionbatteries(forthesameweight)butwhichcanstillberechargedinlessthantwominutes,"saidJang.
Histeamfirstdiscoveredthatgraphenecouldbeusedasasupercapacitorelectrodematerialin2006.Sincethen,scientistsaroundtheworldhavemadegreatstridesinimprovingthespecificcapacitanceoftheseelectrodesbutthedevicesstillfallshortofthetheoreticalcapacitancevaluesof550F/g.
"Despitethetheoreticallyhighspecificsurfaceareaofsingle-layergraphene(whichcanreachupto2.675m2/g),asupercapacitanceof550F/ghasnotbeenreachedinarealdevicebecausethegraphenesheetstendtore-stacktogether,"explainedJang."Wearetryingtoovercomethisproblembydevelopingastrategythatpreventsthegraphenesheetsfromstickingtoeachotherface-to-face.Thiscanbeachievedifcurvedgraphenesheetsareusedinsteadofflatones."
TheworkwasreportedinNanoLetters.
Abouttheauthor
BelleDuméisacontributingeditortonanotechweb.org
BosonsbossedintoBose–Einsteincondensate
Nov24,20106comments
BonnphysicistsandtheirphotonBEC
Manyphysicistsbelieveditcouldnotbedone,butnowateaminGermanyhascreatedaBose–Einsteincondensate(BEC)fromphotons.BECsareformedwhenidenticalbosons–particleswithintegerspin–arecooleduntilallparticlesareinthesamequantumstate.ThismeansthataBECcomprisingtensofthousandsofparticlesbehavesasasinglequantumparticle.
ThefirstBECwasmadein1995bycoolingacloudofrubidiumatomstonearabsolutezeroandtodaysuchcondensatesareroutinelyusedtostudyavarietyofquantumphenomena.However,fewphysicistshadcontemplatedmakingaBECfromthemostcommonbosonintheuniverse–thephoton.Thisisbecausephotonsareeasilycreatedordestroyedwhentheyinteractwithothermatter,whichmakesitverydifficulttocoolafixednumberofphotonssuchthattheyformacondensate.
ButnowMartinWeitzandcolleaguesattheUniversityofBonninGermanyhavedevisedawayofisolatingandcoolingphotons.Althoughtheycannotcaptureafixednumberofphotons,thenumberfluctuatesaroundameanvalue,allowingtheensembletobecharacterizedusingconventionalBECtheory.
Trappedbetweentwomirrors
Theteamtrappeditsphotonsbetweentwoconcavemirrorsthatareseparatedbyamaximumof1.5µm.Thisdistance(towithinanintegernumber)definesthemaximumwavelength–orminimumenergy–ofaphotonthatisconfinedlongitudinallywithinthecavitybetweenthemirrors.Thecavityisfilledwithadyethatisheldatroomtemperature–and,crucially,thethermalenergyofthedyeisabout1%ofthephotonenergy.
Thislargeenergydifferencemeansthatitishighlyunlikelythatadditionalphotonswillemergefromthedye,orthatthedyewillcompletelyabsorbaphoton.Instead,thephotonscollidewiththedyemolecules,givinguporreceivingsmallamountsofenergy.Theseinteractionscoolthephotonstoroomtemperature–whichiscoldenoughtocreateaphotonBEC–whilepreservingthenumberofphotons.
TheteamcreatedtheBECbyfiringalaserintothecavitytofillitwithphotons.Thelaserwasthenkeptonthroughouttheexperimenttomakeupforphotonsthatwerelostatthemirrorsandimperfectionsinthecavity.Someofthephotonspassthroughoneofthemirrorstoaspectrometer,whichmeasuresthedistributionofphotonenergiesinthecavity.Atlowlaserintensitiesthecavitycontainsabroadrangeofphotonenergieswithasharpcut-offatthecavity'sminimumenergy.
Criticalnumberofphotons
Whenthelaserintensityisincreased,thenumberofphotonsinthecavityrisesandthebroaddistributionenduresuntilthephotonnumberreachesabout60,000.Abovethiscriticalvalue,accordingtoWeitz,thephotongasisdenseenoughforaBECtoform–muchlikealiquiddropcondensinginagas.
TheteamknowsthattheBEChasformedbecausealargepeakinthephotonenergyspectrumemergesjustabovethecut-offenergy.Thispeakcorrespondstoalargenumberofphotonspilingintothelowestenergystateofthecavity.Asthelaserintensityisincreasedfurther,thenumberofphotonsintheBECreachesmillions.
ToconvincethemselvesthatthepeakisrelatedtoaBEC,ratherthanthecavitybehavinglikealaser,theresearchersrepeatedtheexperimentatseveraldifferentseparationdistances.Theyfoundthatthepeakalwaysemergedatthesamephotondensity–somethingthatwouldnotbeseeninalaser,accordingtoWeitz.
Smalleffectivemass
Thecavityhasaplanardesign,whichmeansthatthephotonsareconfinedtotwodimensions.Asaresultofthelongitudinalconfinement,theybehaveasiftheyareparticleswithan"effectivemass"correspondingtothecut-offenergy.Thismassisstillextremelysmall,whichiswhyphotonswillformaBECatroomtemperatureanddon'tneedtobecooledtomicro-Kelvintemperatureslikeatoms.
Interactionsbetweenthephotonsaremuchweakerthanthosebetweenatomsandthismeansthatphotonscanformatrue2DBEC.Atoms,ontheotherhandcanonlyforma3DBEC.
AccordingtoWeitz,creatingthermalizedlightdoesn'tnecessarilyrequirealaseranddevicescouldbe"pumped"byotherlightsources–includingtheSun.Asaresult,hebelievestheycouldbeusedtoshrinkthesizeofsolarcellsbyconcentratinglightwithindevices.HealsobelievesBECcouldbeusedtobuildsourcesofcoherentlightthatdon'tinvolvealaser.
TheresearchisdescribedinNature468545and,writinginthesameissueofthejournal,JamesAnglinoftheTechnicalUniversityofKaiserslauterncallsit"alandmarkachievement".Healsopointsoutthattheexperimentshowshow"physicsistheartofinterchangeable"becauseinadditiontoshowingthewave-likepropertiesofatoms,BECshavenowbeenusedtoshowparticle-likepropertiesoflight.
Abouttheauthor
HamishJohnstoniseditorof
Plasmonicsensordetectsviruses
Nov22,2010
Nanoplasmonicsensorinaction
ThefirstbiosensormadefromplasmonicnanoholearrayshasbeenunveiledbyresearchersintheUS.Thedevice,whichexploits"extraordinaryopticaltransmission",candetectlivevirusesinabiologicalsolution.
Recentyearshaveseenanumberofviraldiseaseoutbreaks,raisingfearsthatsuchvirusescouldrapidlyspreadandturnintoapandemic.Controllingfutureepidemicswillrequirerapidandsensitivediagnostictechniquescapableofdetectinglowconcentrationsofvirusesinbiologicalsolutions.
Plasmonicstotherescue
Plasmonicsisanewbranchofphotonicsthatemployssurfaceplasmonpolaritons(SPPs),whicharisefromtheinteractionoflightwithcollectiveoscillationsofelectronsatametal'ssurface.
ThenewsensorwasmadebyHaticeAltugandcolleaguesatBostonUniversityandexploitsSPPresonancesthatoccurinplasmonicnanoholearrays.Thesearearraysoftinyholesjust200–350nmacrossandspaced500–800nmapartonverythinnoblemetalfilms,suchasthosemadeofgold.
Atcertainwavelengths,thenanoholearrayscantransmitlightmuchmorestronglythanexpectedforsuchacollectionofapertures.Thisphenomenoniscalledextraordinaryopticaltransmission(EOT)anditoccursthankstoSPPresonances.
Measuringred-shifts
TheresonancewavelengthoftheEOTdependsonthedielectricconstantofthemediumsurroundingtheplasmonsensor.Aspathogensbindtothesensorsurface,therefractiveindexofthemediumincreases,incre
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