Transparent, Stretchable, and Conductive SWNT Films Using Supramolecular Functionalization

12 Pages·2017·1.32 MB·English

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Chemistry and Biochemistry Publications Department of Chemistry and Biochemistry
2016
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Tricia B. Carmichael
University of Windsor
Akhil Vohra
University of Windsor
R. Stephen Carmichael
University of Windsor
Patigul Imin
McMaster University
Mokhtar Imit
McMaster University
See next page for additional authors
Follow this and additional works at: https://scholar.uwindsor.ca/chemistrybiochemistrypub
Part of the Materials Chemistry Commons
RReeccoommmmeennddeedd CCiittaattiioonn
Carmichael, Tricia B.; Vohra, Akhil; Carmichael, R. Stephen; Imin, Patigul; Imit, Mokhtar; Meena, Jagan
Singh; and Adronov, Alex. (2016). Transparent, Stretchable, and Conductive SWNT Films Using
Supramolecular Functionalization and Layer-by-Layer Self-Assembly. RSC Advances, 6, 29254-29263.
https://scholar.uwindsor.ca/chemistrybiochemistrypub/85
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AAuutthhoorrss
Tricia B. Carmichael, Akhil Vohra, R. Stephen Carmichael, Patigul Imin, Mokhtar Imit, Jagan Singh Meena,
and Alex Adronov
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Transparent, stretchable, and conductive SWNT
fi
lms using supramolecular functionalization and
52. Citethis:RSCAdv.,2016,6,29254 layer-by-layer self-assembly†
4:
5
7:
1
16 Akhil Vohra,a Patigul Imin,b Mokhtar Imit,b R. Stephen Carmichael,a
0
3/2 Jagan Singh Meena,a Alex Adronov*b and Tricia Breen Carmichael*a
0
2/
n 2 Wedemonstratefilmsofsingle-walledcarbonnanotubes(SWNTs)ontheelastomerpolydimethylsiloxane
o
or (PDMS)thatarestretchable,conductive,andtransparent.Ourfabricationmethodusesthesupramolecular
s
nd functionalizationofSWNTswithconjugatedpolyelectrolytestogenerateaqueousdispersionsofpositively-
Wi
andnegatively-chargedSWNTs,followedbylayer-by-layerself-assemblyontoaPDMSsubstrate.Adding
of
y bilayersofpositively-andnegatively-chargedSWNTstothesurfacecausesthesheetresistanceandthe
ersit %transmittanceofthefilmtobothprogressivelydecrease.Thesheetresistancedecreasessharplyinthe
niv first five bilayers as the layer-by-layer process efficiently establishes the percolation network, whereas
U
y the%transmittancedeclinesmoregradually.Filmswith25bilayersaretransparent(75%at550nm)and
b
d conductive (560 (cid:1) 90 U ,(cid:3)1). The combination of electrostatic and p-stacking forces very effectively
e
ad Received28thJanuary2016 bindtheSWNTswithinthefilm,producingsmoothfilmsurfaces(root-mean-squareroughnessof18nm)
wnlo Accepted16thMarch2016 and enabling the films to remain conductive up to 80% elongation. We demonstrate the use of the
Do DOI:10.1039/c6ra02629j SWNTfilmsastransparentconductiveelectrodesinlight-emittingdevicesandassoftstrainsensorsthat
16. www.rsc.org/advances arebothwearableandtransparent.
0
2
h
c
Mar Introduction camouage skins31,32 and transparent strain sensors.26,27,33 As
7
1 research on stretchable transparent conductors progresses,
n
d o Thetransparentconductorindiumtinoxide(ITO)isanessential thereisanincreasingfocusonpracticalconcernssuchasscal-
she componentofawiderangeoftoday'selectronicdevices,suchas ability, precise control over lm properties, sustainability, and
ubli liquid-crystal displays, touch panels, and solar cells.1–4 These cost.Here,wedemonstrateafabricationmethodthataddresses
P devices are fabricated on rigid glass substrates; however, the these concerns to produce lms of single-walled carbon nano-
ability to create transparent conductors that are so and tubes (SWNTs) on the elastomer polydimethylsiloxane (PDMS)
stretchableopensthewaytoexcitingnewapplicationsthatare that are stretchable, conductive, and transparent. We supra-
lightweight, portable, conformable, and even wearable. ITO is molecularly modify SWNTs with conjugated polyelectrolytes
a brittle ceramic that requires high-temperature annealing to (CPEs) to disperse them in water, thus avoiding the need for
achievelowsheetresistance,whichmakesitincompatiblewith organicsolvents,andthenusethesesolutionsinalayer-by-layer
elastomeric substrates and stretchable devices.5 A major (LbL)depositionprocessontothePDMSsurface.LbLdeposition
researchfocus,therefore,isthedevelopmentofnewtransparent providesnecontrolovertheopticalandelectricalpropertiesof
conductors that can be deposited on elastomers and support thelmbecauseitbuildsthelmsoneSWNTlayeratatime;at
stretchable electronics.6,7 New stretchable transparent conduc- thesametime,thismethodisbothscalableandlowcost.
torssuchasthinmetallms,8–10conductingpolymers,11,12silver TheintrinsicelectronicandmechanicalpropertiesofSWNTs
nanowires(AgNWs),13–16graphene,17,18andcarbonnanotubes19–27 areidealforstretchableandtransparentconductors;however,
haveledtocompellingtechnologydemonstrationsrangingfrom the key to integrating these materials into stretchable elec-
intrinsically stretchable light-emitting devices8,13,21,28 and solar tronics lies in designing a lm formation process that both
cells,29,30 to rather sophisticated conformable dynamic retains and capitalizes on these properties. The high intrinsic
conductivity(104to106Scm(cid:3)1),highcurrent-carryingcapacity
(upto(cid:4)109Acm(cid:3)2),andhighthermalstability(upto3500W
aDepartmentofChemistryandBiochemistry,UniversityofWindsor,Windsor,Ontario, m(cid:3)1 K(cid:3)1) of SWNTs are well suited to electronic applica-
Canada.E-mail:[email protected] tions.6,19,34,35 SWNTs also possess high stiffness and strength
bDepartment of Chemistry and Chemical Biology, McMaster University, Hamilton, (Young's modulus (cid:4)1–2 TPa; tensile strength 50 GPa);19 thus,
Ontario,Canada
individualSWNTsarenotintrinsicallystretchable.Integrating
†Electronicsupplementaryinformation(ESI)available:Tableofsheetresistance
these materials into stretchable electronics may seem
and%transmittance.SeeDOI:10.1039/c6ra02629j
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Paper RSCAdvances
counterintuitive; however, networks of SWNTs can remain chargedfunctionalitiesdirectlytothenanotubesidewall,andthen
conductive with stretching because individual SWNTs slide alternating the deposition of the functionalized nanotubes with
againsteachotherunderstrain.7,19Designingaprocesstoform either polymers or nanotubes functionalized with the opposite
transparent,conductive,andstretchableSWNTlmsthattake charge.47–50 This methodology has been used to prepare sensors
advantageoftheseelectricalandmechanicalpropertiesbegins andthin-lmelectrodesonrigidglasssubstrates.47–49,51Similarto
withchoosingamethodtoeffectivelydisperseSWNTsinsolu- the exfoliation process by ultrasonication, however, the covalent
tion, which is necessary to produce a uniformly distributed modicationprocessdisruptsthep-conjugationofthenanotubes
SWNT network with low sheet resistance. SWNTs, however, and diminishes the intrinsic conductivity.6,7,52 As a result,
haveastrongtendencytoaggregateinsolutionthroughinter- researchers have turned to supramolecular functionalization of
molecularp–pinteractions,leadingtoinhomogeneousSWNT SWNTs as a non-destructive alternative. Supramolecular func-
52. lms.ResearchershaveusedultrasonicationtoexfoliateSWNTs tionalization uses van der Waals and p–p interactions to non-
4:
7:5 inthepresenceofstabilizingsurfactants,7,36anddepositedthe covalently bind molecules or polymers to SWNT surfaces to
6 1 resulting dispersions on PDMS by spray-coating or Meyer rod solubilize SWNTs while preserving the nanotube structure, and
1
0 coating to produce stretchable, transparent, and conductive thus the intrinsic electrical properties.6,7,52,53 Shim et al. paired
2
03/ lms. These lms have been implemented in pressure and supramolecular functionalization with LbL self-assembly by
2/
2 strain sensors, and used as electrodes in stretchable light- wrapping SWNTs with charged aromatic polymers, and then
n
or o emitting devices.21,27,37 Although ultrasonication effectively alternating their deposition with polyelectrolytes to demonstrate
ds dispersesSWNTs,thedownsideisthatitcandisruptp-conju- transparent conductive electrodes on rigid glass substrates and
n
Wi gationoftheSWNTsbyintroducingdefectsanddanglingbonds free-standing lms that were mechanically exible. The process
of to the sidewalls, thus diminishing the intrinsic conductivity.38 was not demonstrated on so elastomeric substrates such as
y
sit Anotherapproachavoidsthedispersionproblemaltogetherby PDMStoenablestretchabletransparentconductors.54,55
ver directly drawing out solid nanotube lms from aligned SWNT Here, we combine supramolecular functionalization and
ni
U ormulti-walledcarbonnanotubes(MWNT)foreststogenerate LbL self-assembly to fabricate transparent and conductive
y
d b stretchable,transparent,andconductivefreestandingnanotube SWNT lms that are also highly stretchable. We use
de lms, which may be transferred to a PDMS substrate.39 These water-soluble CPEs to generate aqueous dispersions of posi-
a
o
nl stretchable conductors have been employed as stretchable tively-andnegatively-chargedSWNTs,andusetheseinanLbL
w
o loudspeakers, wearable strain sensors, and touch panels.22–26 process on PDMS. The resulting lms are transparent and
D
6. UsingSWNTlmsastransparentelectrodesinthin-lmdevices stretchable,andremainconductiveupto80%elongation.This
1
20 suchaslightemittingelectrochemicalcells(LEECs)ororganic studyalsohighlightsauniqueadvantageofLbLself-assembly:
arch light emitting diodes (OLEDs) presents another challenge: for the LbL process creates CPE–SWNT lms that exhibit a low
M these applications, the roughness ofthe transparent electrode root-mean-squareroughness(18nm),makingthemcandidates
7
n 1 surfacemustbe<100nmtopreventpenetrationofprotruding fortransparentelectrodesinthin-lmdevices.Wedemonstrate
d o SWNTs through overlying thin lms, which can cause device the key features of our CPE–SWNT lms by using them as
e
sh shorting.40–42Embedding SWNTlms in a polymer matrix can transparent electrodes in LEECs and as so, wearable strain
ubli reducethesurfaceroughnessfrom>60nmfordrop-castlmsto sensorstodetecthumanmotion.
P
<10nm,providingsmooth,transparent,andconductiveSWNT
lms that have been used as electrodes in stretchable
Methods
light-emittingelectrochemicalcells(LEECs).21
Theexcellent electricalandmechanicalqualities ofSWNTs
CoMoCAT SWNTs (CG-200, SouthWest NanoTechnologies, Inc.),
areaccompaniedbydisadvantageousopticalproperties:SWNTs
poly(dimethylsiloxane)(Sylgard184,DowCorning,Midland,MI),
arestrongabsorbersofvisiblelight;infact,theabsorbanceof
and poly(3,4-polyethylenedioxythiophene)-poly(styrenesulfonate)
vertically aligned SWNT forests (0.98–0.99) throughout the
(PEDOT:PSS) (Clevios-P, Heraeus) were used as received. All
visible spectrum approaches that of an ideal black body.43
otherreagentswerepurchasedcommerciallyandusedasreceived.
Materials and methods to fabricate transparent conductors SWNTs modied with poly[2,5-bis(3-sulfonatopropoxy)-1,4-ethy-
alwaysfaceatrade-offbetweenconductivityandtransparency. nylphenylene-alt-1,4-ethynylphenylene] sodium salt (a-PPE–
Typically, increasing the thickness of a lm increases the SWNTs)53,56 and protonated poly[9,9-bis(diethylaminopropyl)-2,7-
conductivity, yet decreases the transparency. The unfavorable uorene-co-1,4-phenylene](PDAFP–SWNTs)57werepreparedusing
optical properties of SWNTs suggest that nanoscale control of
previously published methods. Nanotube concentrations in the
thelmstructureandthicknessiscrucialtoproduceastrong
dispersions were estimated by UV-vis spectroscopic methods
networkforhighconductivitywhilesimultaneouslymaximizing accordingtopreviouslypublishedprocedures,53andwerefoundto
thelmtransparency.Oneapproachtoachievesuchcontrolis beapproximately1.5–2mgmL(cid:3)1.
LbLself-assembly,inwhichthedepositionofalternatinglayers
ofoppositelychargedmaterialsontoasubstratebuildsupthin
PreparationofactivatedPDMSsubstrates
lms one molecular layer at a time.44–46 This method yields
conformal, uniform thin lms with ne control over the lm PDMSsubstrateswerepreparedbycastinga10:1w:wratioof
thicknessandarchitecture.Carbonnanotubeshavebeeninte- prepolymer:curingagentagainstpolystyrenePetridishesand
grated in LbL fabrication schemes by rst covalently bonding curing at 60 (cid:5)C for 1 h, and then treated with air plasma
Thisjournalis©TheRoyalSocietyofChemistry2016 RSCAdv.,2016,6,29254–29263 | 29255
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(HarrickPlasmaPDC32-G)atmediumdischargesettingfor40 was 1.0 keV and a through the lens detector (TLD) was used.
secondsatanairpressureof10psig(owrateof32mLmin(cid:3)1). Transmission spectra were collected using a CARY 50 Conc
Theoxidizedsubstrateswerethensoakedfor10minina1%(v/ UV-vis spectrophotometer. Electrical characterization was per-
v) aqueous solution of 3-aminopropyltriethoxysilane (APTES), formedusingaKeithley2601Asourcemeter.EGaIn((cid:4)0.01mL)
rinsedwithdistilledwater,andthenimmersedin6MHClfor wasrstdepositedbysyringetothecorners(forsheetresistance
1 min. The samples were then rinsed with distilled water and measurements) or ends (for resistance measurements) of the
driedunderastreamofnitrogen. CPE–SWNTsurfacestofacilitateelectricalcontact.Forelectrical
measurementsunderstrain,sampleswereclampedinamicro-
LbLself-assemblyofCPE–SWNTs vice stretcher (S.T. Japan, USA, Inc.) and the resistance was
measured at 5% increments of strain. Data sets consisted of
2. Aqueousdispersionsofa-PPE–SWNTsandPDAFP–SWNTswere
5 a minimum of three samples, and the average was reported.
4: sonicatedinaBransonsonicator(Model3510)for1minpriorto
7:5 LbL self-assembly. 2 mL of the a-PPE–SWNT dispersion was AFM images were obtained using the dynamic force mode of
16 1 depositedontheactivatedPDMSsubstratefor1min,followed aParkSystemsXE-100AFM(SurfaceScienceWestern,London,
0 ON, Canada). A silicon cantilever with a nominal spring
03/2 by rinsing with distilled water and drying under a stream of constant of 40 N m(cid:3)1, resonant frequency of 300 kHz and tip
2/ nitrogen. The surfacewas thentreated withthe aqueoussolu-
2 radius of 10 nm was used. Images were collected from three
n tionofPDAFP–SWNTsusingthesameprocess.Theprocesswas
or o repeated to form lms with up to 25 bilayers of anionic a- spots on each sample in an area of 40 mm (cid:6) 40 mm, and pro-
nds PPE–SWNTsandcationicPDAFP–SWNTs. cessed using WSxM 5.0 Develop 8.0 soware.58 LEEC devices
Wi werecharacterizedusingaKeithley2601source-measureunitto
of apply a dc voltage and measure the current. Radiance was
y LEECfabrication
sit measuredwithacalibratedUDTS470optometerattachedtoan
er
v Devices were fabricated on both ITO-coated glass substrates integratingsphere.
Uni (15–25 U ,(cid:3)1, Delta Technologies) and 25-bilayer CPE–SWNT
y
d b lms on PDMS (25 bilayers). ITO-coated glass substrates were Results and discussion
e
d cleaned by sonication in deionized H O and isopropanol for
a 2
o
nl 15 min each, followed by treatment with UV ozone cleaner WeusedSWNTsfunctionalizedwithanionicandcationicCPEs
w
o (Jelight,Modelno.42A)for5min.CPE–SWNTlmsonPDMS toassembleSWNT–CPElmsontoachargedPDMSsurfaceone
D
6. wereoxidizedfor30sinanairplasma(airpressureof10psig, layeratatime.Wechoseanionica-PPEandcationicPDAFPas
1
20 owrate32mLmin(cid:3)1)atmediumdischargesetting.0.7mLof the CPEs,which interactwithSWNTsvia supramolecular p–p
h
arc the PEDOT:PSS dispersion in water was sonicated for 15 min interactions to produce anionic and cationic supramolecular
M andthendilutedbyadding0.3mLofisopropanol;thissolution polymer-nanotube assemblies that can be easily dispersed in
7
n 1 was then spin-coated on the surface of either the ITO or water. We assembled SWNT–CPE lms from these aqueous
d o CPE–SWNTlmat1000rpmfor1min.ThePEDOT:PSSlayers dispersions onto an activated PDMS substrate according to
e
sh onITOlmsandCPE–SWNTlmswerethenannealedonahot Scheme1.AeroxidizingthePDMSsurfacewithanairplasma
ubli plateat100(cid:5)Cfor20min.Aercoolingtoroomtemperature, to introduce hydroxyl functional groups, treatment with the
P
a Ru(dtb-bpy) (PF ) /PDMS emissive layer8 was deposited by organosilane 3-aminopropyltriethoxysilane (APTES) produces
3 62
spin-coating a 3:1 v:v mixture of a 40 mg mL(cid:3)1 solution of an amine-terminated surface. Protonating the amino groups
Ru(dtb-bpy) (PF ) in dichloromethane and a 25 mg mL(cid:3)1 with HCl produces a surface comprising positively-charged
3 62
solution of PDMS pre-polymer at 1000 rpm for 30 s, and then ammonium groups. This activated surface electrostatically
annealed in an oven at 60 (cid:5)C overnight. A eutectic gallium– bindstheanionica-PPE–SWNTbysimplydepositingthea-PPE–
indium(EGaIn)cathode(50mL)wasdepositedontothesurface SWNT aqueous solution onto the activated PDMS surface for
of the Ru/PDMS lm using a micropipette and then sealed in oneminute,followedbyrinsingwithwater.Thedepositionof
epoxyresin. thisrstCPE–SWNTlayercreatesanegativelychargedsurface;
exposing this surface to the cationic PDAFP–SWNT aqueous
Strainsensorfabrication dispersiondepositsthesecondCPE–SWNTlayerandcompletes
the formation of a CPE–SWNT bilayer. We repeated the alter-
CPE–SWNT lms on PDMS (25 bilayers) were mounted to the
nating deposition of a-PPE–SWNTs and PDAFP–SWNTs to
back of an adhesive bandage using PDMS pre-polymer as the
generateatotalof25bilayersonthePDMSsurface.
adhesive,followedbycuringat60(cid:5)Cforoneh.Connectorswere
AsCPE–SWNTbilayersarebuiltuponthePDMSsurface,the
fabricatedbygluingstripsofaluminumfoiltotheendsofthe
sheet resistance (R) and % transmittance both progressively
CPE–SWNT lms using a conductive silicone paste (Effective s
decrease; however, they do not decrease at the same rate.
Shielding,WestChesterPA).
Scanning electron microscopy (SEM) of a single bilayer
CPE–SWNT lm (Fig. 1) shows a sparse, porous network of
Characterization
CPE–SWNTs on the PDMS surface. The low density of CPE–
Scanning electron microscope (SEM) images were obtained SWNTsinthelmproducesaweakpercolationnetworkwithan
usinganFEIMagellanExtremeHighResolutionFieldEmission R value of 153 (cid:1) 25 kU ,(cid:3)1 and a transmittance (98.3% at
s
ScanningElectronMicroscope(XHR400FE-SEM).Thevoltage 550nm)thatisonlyslightlydecreasedfromthatofnativePDMS
29256 | RSCAdv.,2016,6,29254–29263 Thisjournalis©TheRoyalSocietyofChemistry2016
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Paper RSCAdvances
2.
5
4:
5
7:
1
6
1
0
2
03/ Fig. 1 SEM image of PDMS with a single bilayer of anionic a-PPE–
22/ SWNTsandcationicPDAFP–SWNTs.
n
o
or
s
d
n
Wi
of
y
sit
er
v
ni
U
y
b
d
e
d
a
o
nl
w
o
D
6.
1
0
2
h
c
ar
M
7
1
n
o
d
e
h
blis Scheme1 SchematicillustrationofLbLself-assemblyofCPE-func-
u tionalizedSWNTsonPDMS.
P
(100%).ThevariabilityintheR valuemaybeattributedtothe
s
random mechanism of formation of the percolation network.
Increasingthenumberofbilayersonthesurfaceincreasesthe
numberofpossibleSWNT–SWNTconnections,thusproviding
moreparallelpathwaysforcurrent.TheplotinFig.2aanddata
in Table S1† show that R decreases substantially by 88%
s
((cid:4)135 kU ,(cid:3)1) between a single bilayer and ve bilayers, fol-
lowedbyagradualdeclineofonly10%overtheremaining20
bilayers.TheinitialprecipitousdropinR betweenoneandve
s
bilayers contrasts with a relatively modest drop in trans-
mittance of only (cid:4)4%. Furthermore, the transmittance
decreasesbyapproximatelythesameamounteveryvebilayers
Fig.2 ElectricalandopticalpropertiesofCPE–SWNTfilmsonPDMS.
(Fig. 2b). The fact that these two key lm parameters do not (a)PlotofsheetresistanceversusthenumberofCPE–SWNTbilayers
decreaseintandemspeakstotheefficiencyoftheLbLassembly on PDMS. (b) Transmittance spectra. Numbers in italics indicate the
process.Thetransmittancedecreasesregularly,indicatingthat numberofbilayers.
the deposition process is well behaved: at each ve-bilayer
interval, the consistent decrease in transparency implies that
every iteration of the LbL process adds an approximately added CPE–SWNTs form SWNT–SWNT connections that effi-
equivalent amount of matter. However, each bilayer does not cientlyconstructthepercolationnetwork.Eachofthesebilayers
contributeequallytoreducingR.Intheinitialvebilayers,the contributes additional conductors in a random pattern that
s
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may add additional conduction paths or reinforce existing
paths, resulting in this initial dramatic decrease of R. At the
s
sametime,thereareinevitablejunctionresistancesintroduced
with each new SWNT–SWNT connection that become more
relevantasmorebilayersareadded.Thisjunctionresistanceis
due to imperfect SWNT–SWNT contacts, particularly between
metallic and semiconducting tubes,59 which are likely further-
moreinuencedbythedielectricCPEshellaroundtheSWNTs.
Aer ve bilayers, the benet of adding more CPE–SWNTs to
create more conduction pathways is nearly balanced by the
2.
5 penalty of additional junction resistances; thus, R declines
4: s
7:5 marginally. By 25 bilayers, additional bilayers do not measur-
6 1 ablydecreaseRs.Atthispoint,thelmsareanalogoustoabulk
1
0 material in which the conductivity is limited by the intrinsic
2
03/ junctionresistanceaswellasthetypesofSWNTsinthelm(a
2/
2 mixture of chiralities that are both metallic and
n
or o semiconducting).
ds We chose to focus on 25-bilayer lms for our study of
n
Wi the electrical and mechanical properties of CPE–SWNT
of lms (Fig. 3a). 25-Bilayer lms provide the lowest achievable
versity 5R5s0(5n6m0(cid:1).F9u0rtUhe,rm(cid:3)1o)rwe,hwileeemxpaiencttatihneinsegaltmrasntsopharaevnectyhoefh7i7g%heastt
ni
U number of SWNT–SWNT contacts – and thus redundant
y
d b conduction paths – available to preserve conductivity when
e
d thesestructuresarestretched.DespitetheinclusionoftheCPE
a
nlo in these lms, the sheet resistance and % transmission of
w
o 25-bilayer lms compare quite favorably to other stretchable
D
6. transparent carbon nanotube lms recently reported in the
1
20 literature:SWNTlmsembeddedatthesurfaceofPDMS(564U
arch ,(cid:3)1,65%transparent);33SWNTlmsembeddedatthesurface
M of a polyacrylate stretchable above its glass transition temper-
n 17 ature of 70 (cid:5)C (500 U ,(cid:3)1, 87% transparent);21 carbon nano-
d o tubesspray-coatedontothesurfaceofPDMS(328U,(cid:3)1,79%
e
sh transparent);27carbonnanotubelmstransferredtothesurface
ubli ofDragonSkinelastomer(500U,(cid:3)1,90%transparent);26and
P
freestanding carbon nanotube lms not integrated with an
elastomericsubstrate(1kU,(cid:3)1,78%transparent).25
SEM and atomic force microscopy (AFM) images of
25-bilayerCPE–SWNTlmsrevealthattheselmsalsopossess
smoothsurfaces(Fig.3and4),afeaturethatisessentialtotheir
use as stretchable transparent electrodes in thin-lm devices.
SEM images further show a dense, interconnected network of Fig.3 25-BilayerCPE–SWNTfilmsonPDMS.(a)Photograph.(b)SEM
CPE–SWNTs that is (cid:4)200 nm thick with a smooth surface image(topview).(c)SEMimage(cross-section).
(Fig.3bandc).AnalysisofAFMimages(Fig.4)yieldsanRMS
roughnessof18(cid:1)1nmandamaximumpeak-to-valleydistance
Surfaceroughnessisafundamentally importantparameter
of70nm.TheabsenceofCPE–SWNTsprotrudingfromthelm
of electrodes in thin-lm devices. Protrusions from nano-
surface in the SEM and AFM images is consistent with a self-
structured electrodes can cause device shorting by producing
assembly process in which CPE–SWNTs in solution assemble
localizedregionsofhighelectriceldsorpenetratingthrough
in a “lying down” conguration to maximize favorable inter-
an overlying device thin lm.40–42 Most reports of stretchable
molecular electrostatic forces and p-stacking interactions. As
and transparent carbon nanotube lms in the literature have
aresult,theRMSroughnessofCPE–SWNTlmsis70%lower
not included surface roughness characterization. These lms
than that of SWNT lms formed by drop-casting (RMS rough-
aretypicallystudiedaspressure,strain,andtouchsensorsfor
ness¼60nm).Theelectrostaticandp-stackinginteractionsof
whichsurfaceroughnessisnotanimportantparameter.There
CPE–SWNT lms are also strong enough to render them
is only one report in the literature of a stretchable and trans-
durabletophysicalmarring.TherewasanegligiblechangeinR
(620(cid:1)140U,(cid:3)1)of25-bilayerCPE–SWNTlmsaervigorouss parent carbon nanotube lm employed in a thin-lm
light-emitting device, in which carbon nanotube lms with
agitationwithaglovedngerfor10s.
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resulted in the emission of light (Fig. 5a), which we recorded
over a 30 minute testing period. The evolution of current and
radiance (Fig. 5a) and external quantum efficiency (EQE)
(Fig. 5b) show that the devices reach their maximum EQE of
0.004%at116s.Thecontinuousoperationofthesedevicesfor
30minutes–withnoevidenceofelectricalshorts–veriesthat
CPE–SWNTlmsonPDMSareviabletransparentelectrodesfor
thin-lmdevices.However,theEQEofLEECswithaCPE–SWNT
electrodeislowerthanthatofanalogousdevicesinwhichthe
CPE–SWNT electrode is replaced by a standard ITO electrode
52. and coated with PEDOT:PSS (EQE ¼ 0.01 (cid:1) 0.002). We expect
7:54: that the device properties of CPE–SWNT LEECs can be
6 1 improved by making the electric eld at the anode more
01 uniform. Uniformity of the electric elds at the electrodes is
2
03/ essential for optimal device operation and good device effi-
2/
2 ciencies due to the electrochemical doping process that is
n
ndsor o ccaentitornalotof athveooltpaegrea,titnhgemPeFc6h(cid:3)acnoisumnteorfioLnEsECmsi.6g1r–a65teUpuonndearppthlie-
Wi inuence of the electric eld to the anode interface, leaving
of uncompensatedmetalionsnearthecathode.Theaccumulation
ersity anddepletionofPF6(cid:3)counterionsproducehighelectricelds
v near the electrode interfaces, lowering the barrier to charge
ni
U injection. Charge injection reduces Ru2+ ions adjacent to the
y
d b cathode and oxidizes Ru2+ ions adjacent to the anode; subse-
de quentchargehoppingcreatesanemissivezoneofadjacentRu+
a
nlo and Ru3+ ions, which undergo a recombinative process with
w
o
D
6.
1
0
2
h
c
ar
M
7 Fig.4 Surfacetopographyof25-bilayerCPE–SWNTfilmonPDMS.(a)
1
n AFMimage.(b)AFMprofilecorrespondingtothesectiondenotedby
o
d theblacklineinpart(a).
e
h
s
bli
u
P
low surface roughness were fabricated by embedding nano-
tubesintoapolyacrylateandusedastransparentelectrodesin
polymer light-emitting electrochemical cells (PLECs). Heating
the PLECs above the glass transition temperature of the poly-
acrylate (70 (cid:5)C) produced an intrinsically stretchable thin-lm
device.21 We used a proof-of-concept demonstration to show
that the low surface roughness of 25-bilayer CPE–SWNT lms
on PDMS makes them viable as transparent electrodes by
implementing them in thin-lm LEEC devices. LEECs are
simple devices that consist of an anode and cathode sand-
wichedaroundanemissivelayer.60,61Weusedtheionictransi-
tion metal complex, Ru(dtb-bpy) (PF ) as the emissive
3 62
material. Ru(dtb-bpy) (PF ) supports charge injection and
3 62
transport in the device, as well as emissive recombination to
produce 630 nm light.60 We fabricated LEECs by spin-coating
a layer of PEDOT:PSS as a hole transport layer on the
CPE–SWNTlmonPDMStoconstructthetransparentanode,
followedbyspin-coatingalayerofRu(dtb-bpy) (PF ) dispersed
3 62
inaPDMSpolymermatrixtoimprovetheuniformityofthelm. Fig. 5 Demonstration of 25-bilayer CPE–SWNT films on PDMS as
WeusedadropofliquidEGaInonthesurfaceasthecathodeto transparentanodesinLEECs.Evolutionof(a)current(solidline)and
completethedevice.Applyingadcvoltageof40Vtothedevices radiance (dotted line) and (b) EQE over a 30 minute testing period.
Insetin(a)isaphotographofanoperatingLEEC.
Thisjournalis©TheRoyalSocietyofChemistry2016 RSCAdv.,2016,6,29254–29263 | 29259
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visiblelightemission.60OurCPE–SWNTlmsarecomposedof
amixtureofsemiconductingandmetallicnanotubes;therefore,
theelectriceldgeneratedatthelmsurfaceisinherentlynon-
uniform due to the differing conductivities of the SWNTs
compared to the electric eld generated at the surface of
a uniformly conductive electrode such as ITO. As a result, the
emissive zone of CPE–SWNT devices will be less coherent. To
improve the properties of LEECs formed using CPE–SWNT
electrodes,itwillthusbenecessarytoimprovethehomogeneity
of the electric eld generated at the electrode surface by
52. creating CPE–SWNT lms from a single type of nanotube –
7:54: either purely metallic or purely semiconducting – rather than
6 1 amixtureofmetallicandsemiconductingSWNTs.Anattractive
01 option would be to combine semiconductor-enriched CPE–
2
03/ SWNT lms with redox doping. The uniformly conductive
22/ SWNTscomprisingtheselmswouldproduceahomogeneous
n
or o electriceldtoimprovethedevicepropertiesofLEECs,andmay
ds give the added benet of improving the conductivity of
n
Wi CPE–SWNT electrodes: Blackburn et al. reported the main
of resistanceinsemiconductor-enrichedSWNTlms–thecontact
y
sit resistance of the intertube junctions – can successfully be
er
v reducedusingredoxdopingtoincreasethedelocalizedcarrier
ni
U density and transmission probability through SWNT–SWNT
y
d b junctions. This process is more effective for semiconductor-
de enrichedlmsthanformetal-enrichedlms.66
a
nlo In general, the resistance of SWNT lms increases with
w
o stretching, which can be attributed to the separation of
D
6. SWNT–SWNT junctions as a result of elongation. The magni-
1
20 tudeoftheresistanceincreaseandtherecoveryofconductivity Fig.6 25-BilayerCPE–SWNTfilmsonPDMSsubjectedtostretching.
March uplmon. SreWleNaTsinglmthsedsetrpaoinsitdeedpeonndtoonptohfeeplraosptoemrtieerssomftahyeeSxWpeNrTi- (rae)siCsthaanncgeeasinarfeusniscttaionnceofasthaefnuunmctbioernooff15e%lonstgraatiinonc.yc(ble)sC. hange in
7
n 1 ence delamination, which can lead to irreversible loss of
o
ed conductivity; embedding SWNTs into polymers is regarded as strain,andmeasuredtheresistanceintherelaxedstate(at0%
Publish a25-wbailyayteor CavPoEi–dSWthNisTcrlimticsaalrpernoobtlemme.chDaensipciatellythsteabfialicztedthbayt sthtreainnu)mabeerreovfersytr1a0incyccylcelse.sP(lFoitgs.o6fbth)eshreoswisttahnacteRc/Rh0anregmevaeirnseuds
embedment in a polymer matrix, the combination of electro-
relatively constant (<2) throughout the testing range of 100
staticandp-stackinginteractionsarestrongenoughtostabilize
cycles, although the slight gradual increase in R/R suggests
these lms tostretching, enabling themtoremainconductive somelossofSWNT–SWNTjunctions. 0
tohighelongations(upto80%)andlargelyrecoverconductivity Strainsensors fabricated usingso, conformable materials
when the strain is released. The normalized change in resis-
can be worn on the body to quantify human motion through
tance(R/R )asafunctionofelongationof25-bilayerCPE–SWNT
0 changesinelectricalresistance.Thecharacteristicsofanideal
isplottedinFig.6a.Theresistanceincreasesrelativelylinearly
human motion strain sensor include high stretchability, high
withstrain,andreaches12.3timestheinitialvalue(R )at80%
0 sensitivity, excellent stability, and tolerance to repetitive
elongation. Fracture of the samples occurs beyond 80% elon- strain.67 Hard sensors such as metal foils or inorganic semi-
gation,suggestingthatitmaybepossibletoprepareconductive
conductors are widely used to sense structural changes or
CPE–SWNTlmsthatfunctionatevenhigherstrainsbyusing deformation in stiff materials, such as buildings or aircra.
an elastomer with a higher tensile strength than PDMS. Elon-
Thesesensorsprovidearangeofsensitivities,whicharequan-
gatingCPE–SWNTlmsto80%andthenreturningto0%strain tiedbythegaugefactor,theratioofthefractionalchangein
restorestheconductivityofthelmtoanR/R0recoveryvalueof resistance(DR/R )tomechanicalstrain3.
1.6 (cid:1) 0.2. We attribute this recovery to the ability of the elec- 0
DR=R
trostatic and p-stacking interactions to mechanically stabilize GF¼ 0
3
the CPE–SWNTs in the lm and allow them to reconnect,
althoughthisvalueisalsoconsistentwithasmall,irrecoverable
lossofSWNT–SWNTjunctions.Theelectrostaticandp-stacking Metalstraingauges,withtheirlowgaugefactorsof2–5,are
forces of 25-bilayer CPE–SWNT lms are also sufficient to used to detect gross deformations, whereas semiconductor
stabilize these lms to repetitive stretch-release cycles. We strain gauges are useful for precision measurements due to
subjected25-bilayerCPE–SWNTlmsonPDMStocyclesof15% their high gauge factors (up to 200).68–70 These useful
29260 | RSCAdv.,2016,6,29254–29263 Thisjournalis©TheRoyalSocietyofChemistry2016
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Bending the thumb caused a resistance change of (cid:4)2(cid:6) the
initialvalue,whichcorrespondsto(cid:4)15%strainonthelineof
besttoftheR/R vs.elongationplotinFig.6a.Straightening
0
thethumbrecoveredtheresistancetowithin8%oftheinitial
value.BasedonFig.6a,CPE–SWNTlmsonPDMSarecapable
ofdetectingupto80%strain,suggestingthepotentialtodetect
abroadrangeofhumanorroboticmotion.
Conclusions
2.
5 WehavecombinedsupramolecularfunctionalizationofSWNTs
4:
7:5 with LbL self-assembly to fabricate transparent, stretchable,
6 1 andconductiveCPE–SWNTlmsonPDMS.TheLbLprocessis
01 green, scalable, and low cost, and it efficiently establishes the
2
03/ percolation network largely within the rst ve bilayers. We
22/ credit the smoothness and ability of our lms to remain
n
or o conductive with stretching to the combination of electrostatic
ds and p-stacking forces, which very effectively bind the SWNTs
n
Wi withinthelm.Thecombinationofpropertiesexhibitedbyour
of CPE–SWNT lms – conductivity, transparency, low surface
y
sit roughness, and stretchability with low resistance changes – is
niver Fig. 7 Demonstration of 25-bilayer CPE–SWNTs on PDMS as whatdistinguishesourlmsfromothersreportedintheliter-
U a wearable strain sensor. Photograph of the sensor mounted on ature.Wehaveexploitedthiscombinationofpropertiestoshow
y
d b ahumanthumbin(a)straightened(unstrained)and(b)bent(strained) that CPE–SWNT lms are viable as transparent conductive
de positions.(c)Plotofnormalizedresistanceasafunctionoftimecor- electrodes in LEECs, and can also be applied as so strain
oa respondingtobending/straighteningcycles.
nl sensors that are both wearable and transparent. Our
w
o CPE–SWNTlmsaremade froma mixtureofsemiconducting
D
6. andmetallicSWNTs,whichmayadverselyaffecttheuniformity
1
20 instruments are, however, entirely unsuitable to monitor oftheelectriceldgeneratedatthesurfaceduetothediffering
h
arc human or even robotic motion, because they lack conform- conductivities of the semiconducting and metallic SWNTs in
M ability and cannot function at high strains typical of, for the lm. This non-uniform electric eld is problematic when
7
n 1 example,thebendingofanelbowjoint.Intrinsicallystretchable theselmsareemployedastransparentelectrodesinthin-lm
o
d conductive materials, such as elastomers loaded with conduc- devices such as LEECs. Therefore, the next step in the devel-
e
sh tive particles or stretchable conductive lms on elastomers, opmentofCPE–SWNTlmswillbetoimprovetheuniformityof
ubli providethesonessandstretchabilityforthese“so”applica- theelectriceldgeneratedatthesurface.Weaimtoaccomplish
P
tions.26,27,37,67,71–73 We fabricated wearable strain sensors from this improvement through redox doping of semiconductor-
25-bilayer CPE–SWNT lms on PDMS for four reasons: rst, enriched CPE–SWNT lms, which may also bring the added
CPE–SWNT lms are so and conformable, exhibit resistance benetofimprovingthelmconductivity.Wewillreportdetails
changes with stretching to high elongations, and tolerate ofsuchoptimizedCPE–SWNTlmsandtheiruseinstretchable
repeated deformation. Second, the gauge factor of these devicesinduecourse.
lmsdeterminedfromtheslopeofthelineofbesttoftheplot
ofR/R vs.elongationinFig.6ais15.0forstrainsof0–80%.This Acknowledgements
0
valueishigherthanthegaugefactorofalignedSWNTlmson
PDMS,whichdecreasesfrom0.82forstrainsof0–40%to0.06 This research was supported by the National Sciences and
forstrains>40.37Third,theprocesstomanufactureCPE–SWNT Engineering Research Council of Canada (NSERC). The EM
lms on PDMS is low-cost, green (due to the aqueous researchdescribedinthispaperwasperformedattheCanadian
CPE–SWNT dispersions), and scalable. Fourth, CPE–SWNT CentreforElectron Microscopy atMcMaster University,which
lms are transparent. Adding transparency to strain sensors is supported by NSERC and other government agencies. We
bringsaddedfunctionalitytomotiondetection,particularlyin thank Glynis de Silviera at the Brockhouse Institute for Mate-
robotics,becausethesesensorscanbeintegratedwithon-board rials Research, McMaster University, for scanning electron
optoelectronic devices and can enable direct observation microscopy,andDrHeng-YongNieatSurfaceScienceWestern
through the sensors.26,27,33 To fabricate CPE–SWNT strain foratomicforcemicroscopy.
sensors,weglueda25-bilayerCPE–SWNT/PDMSstructureonto
an adhesive bandage, and then simply adhered the bandage References
alongthelengthofahumanthumb.Wethenappliedavoltage
of 20 V and measured the change in resistance as the thumb 1 U.Betz,M.K.Olsson,J.Marthy,M.F.Escol´aandF.Atammy,
wasrepeatedlybentandstraightenedat10sintervals(Fig.7). Surf.Coat.Technol.,2006,200,5751–5759.
Thisjournalis©TheRoyalSocietyofChemistry2016 RSCAdv.,2016,6,29254–29263 | 29261