high-Tc superconductor assemblies

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Synthetic

Metals

84 (1997)

407408

Molecular Engineering of Organic Conductor / High-Tc Superconductor Assemblies Loa, Ji-Ping Zhoua, Feng Kub, Chad A. Mirkin*b John T. McDevitt*a, Jason E. Ritchiea, Marvin B. Clevenger a, Rung-Kuang a Department of Chemistry & Biochemistry, The University of Texas at Austin, Austin, TX, 78712, USA b Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston IL 60208, USA Abstract Organic monolayers on high-Tc superconductors have allowed for the precise structural control of composite organic conductor / highTc superconductor assemblies. These hybrid systems display a number of novel electronic and structural properties. This paper discusses the utility of the monolayer self-assembly procedure for the preparation of both conductive polymer and crystalline charge transfer salt layers which are supported directly on top of cuprate superconductor thin film samples. Keywords: Self-Assembly, Superconductor

Composites, Charge Transfer Salts

1. Introduction A number of researchers have begun to prepare polymer / superconductor composites with the hope of improving the processability and properties of these hybrid materials.’ Initial smdies designed to explore conductive polymer systems in the context of high-Tc applications have now been initiated.2 Such polymers offer both the prospects for enhanced processability, as well as a wide range of electrical conductivities. Polymeric systems of this type can be doped reversibly from neutral, non-conducting forms to oxidized states which display high electrical conductivity. In their non-conducting form, the electronic interactions between conductive polymers and superconductors should be minimal. Electrochemically deposited polymers may find utility as passivation layers for the protection of high-Tc structures against corrosion or as an insulating dielectric barriers for active devices. However, a doped polymer in intimate contact with a superconductor is expected to display interesting electronic interactions, such as the proximity effect, in an analogous manner to the well documented behavior that has been observed for metal/superconductor and semiconductor / superconductor systems.‘s’ The important challenge in this area is to define conditions that can be exploited to assemble, in a chemically compatible manner, the hybrid conductive polymer / cuprate superconductor, and organic superconductor / cuprate superconductor structures. Furthermore, it will be necessary to develop techniques which can be utilized to explore the physical and electrical properties of the composite systems. This paper describes two methods for the preparation of hybrid organic conductor I cuprate superconductor assemblies3e4 2. Results 2.1 PolypyrroIeNBa2Cu307 composites The spontaneous adsorption of molecular assemblies onto the surfaces of superconductor films provides an effective and simple method for controlling the interfacial properties of high-Tc structures and devices.3,5,6 Here, we show that electroactive self-assembled monolayers (i.e. SAMs) based on alkylamlne substituted pyrrole functionalities can be used to alter the growth of polypyrrole onto c-axis oriented films of YBa2Cu307 , For these studies,

* Corresponding author. 0379-67791971S17.00 0 1997 Elsevier Science Sk All rights reserved PII SO379-6779(96)03960-4

polypyrrole was electrochemically polymerized using a potential step technique (1 .lV vs. SCE reference) in a O.lM solution of Bu4NBF4 in 1% pyrrole / acetonitrile. Information relevant to the mechanism of polymerization was explored by varying the deposition times and the applied biases. Data presented in Figure 1 was acquired with a 5 second polymerization time so that details related to the nucleation and initial polymerization can be readily ascertained. Interestingly, AFM and SEM images of polypyrrole layers grown under identical conditions onto films of c-axis orientedYBa2Cu307 thin films, with and without the pyrrole self-assembled monolayer, reveal that the SAM modified surface produces both thicker and more uniform layers (Figure 1). Polypyrrole grown onto the modified superconductor is observed to have >98% surface coverage with feature sizes of approximately 0.20 to 0.25um width and a surface roughness of
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tion signalswhich would indicatethe presenceof the crystalline organicmaterial.ThisUnusual effect ishighly reproduciblehaving beenobservedin over 100Carefullypreparedsamples. However, theinclusionof theSAM bufferlayerservesto improvethecrystallinity of the(BEDT-TTF)2I3layer.Figure2B showsa SEM image of atypical(BEDT-lTF)213sample deposited ontotheSAM coated cupratefilm wherecrystallineorganicmaterialis apparent.X-ray diffractionstudies confirmthecrystallinesignatureof suchsamples. Studiesarenowin progress to explorethe electricalandsuperconductingcharacteristics of thesehybridorganicsuperconductor/cuprate superconductor structures. Furthermore, prospects for creatingorganic superconductor tunneljunctionsarecurrentlybeingevaluated.

Fig 1 - APM imageacquired for YB%Cu,O, (c-axisoriented) thintilms

supported onMgO(100)substrates. Dataisprovidedfor (A) baresuperconductor aswellas(B) superconductor ontowhicha monolayer of N-(3aminopropyl)pyrrole wascoated.Polypyrrole waspolymerized ontoboth filmsbyapotential stepprocedure inwhichthesamples werebroughtfrom arestingpotential ofO.OVvs.SCEtoavalueof l.lV vs.SCEforaperiodof 5 seconds. Crystalline organic metals, such as those based on bis(ethylenedithio) tetrathiafulvalene (BEDT-TI’F),arelesssusceptible to theseeffects.Our calculationshaveshownthat crystalline conductorsof thistype areexpectedto exhibit largesuperconducting coherencelengths.*Basedon this expectation,recentefforts havebeendevotedto searchfor methodsthat canbe usedto preparewell orderedBEDT-TTF/ cupratesuperconductor structures. Although therearenow a largenumberof conductorsandsuperconductorsbasedonBEDT-TTFsalts,themajorityof thesematerialshavebeenpreparedonly assinglecrystalsvia electrochemical methods.In orderto makefunctionalsystems whichcanbereadily interfacedwith high-Tc structures,it is necessaryto preparethin films of theseorganicconductors.Recently,methodshave been developedfor the vaporphasedepositionof thin films of (BEDTTl-F)213.’ Wehavedevelopedandrefinedmethodsfor the preparationof well ordered,singlephasethin film layersof a, @J-, andp phases of (BEDT-‘ITF)213usingsublimation andthermalannealmethods. A variety of substrates suchasAl203, MgO, LaA103,andglasshave beenusedto supportthe ordereddepositionof thesefilms. High quality x-ray diffractionsignalsdocumentthe phasepurity andexclusiveorderedc-axisorientationfor such(BEDTTI’F)213 films. Interestingly,organicsuperconductor layersdeposited directlyonto YBa2Cu307 lead to non-crystallineCBEDT-‘ITF)213layersby XRD. In aneffort to improvethe crystallinity of the organiclayer,a self-assembled monolayercomposed of alinearalkylaminereagent wascreatedonthesurfaceof YBa2Cu307 prior to the vaporphase processingstep.Thus, trilayer assemblies incorporatinga SAM buffer layer wereprepared.Herea threestepprocesswasusedto createthedesiredstructure.First, YBa2Cu307wasdeposited onto MgO (100)via thelaserablationmethod.Second,aself-assembled monolayerof dodecylamine or octadecylamine wasformedonthe surfaceof thecupratecompound usingourpreviouslyreportedprocedure.315 Third, the (BEDT-TTF)213 was sublimedonto the derivatizedsuperconductor. The importanceof theinclusionof theSAM buffer layerisevidentfrom thedatapresented in Figure2. Herethedirectdeposition of (BEDT-TTF)213onto YBa2Cu307 leadsto a non-crystalline layer of theorganicsuperconductor (Figure2A). Evidencethatthe (BEDT-TTF)213 iscoatingtheYBa2Cu307comesfromx-ray fluorescence, AFM, andopticalreflectivity studies, Althoughthe(BEDTTIF)213 is presentwith the expectedelementalcomposition,the sampleis lackingthesharpcrystalfacetsandx-ray powderdiffrac-

Fig 2 - SEMof (A) (BEDT-TTF&deposited directlyontoYBa$u,O,.

(B)(BEDT-TTF&deposited ontooctadecylamine modified YE?a$u,O, 3. Conclusions

In summary,thispaperdescribes theconstructionandstudyof hybrid assemblies basedon conductivepolymer and crystalline chargetransfersaltswhicharesupported on cupratesuperconductor platforms.In the conductingpolymerarea,a monolayerof N(3-aminopropyl)pyrrole is usedas a templatelayer for the controlledgrowthof polypyrroleonYBa2Cu307. Largeincreases in the polymergrowth ratesandradically smootherpoIymerlayers are obtainedusingthis adsorbedmonolayer.Moreover,dramatic improvements in thepolymer/ superconductor adhesion properties arenoted.In the chargetransfersaltarea,spontaneous adsorption of an alkylaminemonolayeronto the cupratesuperconductor surfaceis usedto controlthe crystallinepropertiesof thereondeposited(BEDT-lTF)213.Work in thesecondarearepresents animportant stepin the directionof developingorganicsuperconductor I insuiator/ high-Tc superconductor structures. 4. Acknowledgments

JTM andCAM boththanktheAFOSRandONB for supportof thiswork. CAM wishesto thankthe ScienceandTechnologyCenter for SuperconductivityandJTM wishesto thank the NSF and Welchfoundationfor financialsupport. References (1) McDdtt,

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