Embedded Oxidized AgminusPdminusCu Ultrathin Metal Alloy Film Preparedat Low Temperature with Excellent Electronic Optical andMechanical PropertiesSeohan Kim Joseacute Montero Janghee Yoon Yunju Choi Sungmin Park Pungkeun Songand Lars Osterlund

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ABSTRACT Most transparent conducting materials are based onSnIn2O3 (ITO) When applied onto flexible substrates ITO can beprepared in an oxideminusmetalminusoxide (OMO) configuration typicallyITOAgITO where the ductility of the embedded metal layer isintended to reduce the mechanical brittleness and improve the electricalconductivity of the OMO multilayer Hitherto the lower limit of thethickness of the Ag layer has been limited by the percolation thresholdwhich limits the Ag layer to be thicker than sim10 nm to avoidagglomeration and to ensure conductivity and structural stability Metallayers of thicknesses below 10 nm are however desirable for obtainingOMO coatings with better optical properties It is known that agglomeration of the metal layer can to some extent be suppressedwhen substituting Ag by an AgminusPdminusCu (APC) alloy APC-based OMO films exhibit excellent optical and electrical properties butstill continuous APC films well below 10 nm thickness cannot be achieved In this work we demonstrate that controlled oxidation ofAPC results in smooth ultrathin APCO continuous coatings (of thickness sim5 nm) on ITO-coated PET substrates Moderateoxidation yields superficial PdOx formation which suppresses Ag agglomeration while still maintaining excellent conductivity Onthe other hand extensive oxidation of APC leads to extensive Pd oxide nucleation deteriorating the conductivity of the film TheITOAPCOITO films exhibit low resistivity attributed to a high Hall mobility associated with suppressed agglomeration goodstability in high humiditytemperature environments superior transmittance in the visible and infrared region and excellentmechanical bending properties thus providing new opportunities for fabricating superior transparent conducting coatings onpolymer substrates

KEYWORDS transparent conducting materials ultrathin metal layers AgminusPdminusCu alloy OMO structure magnetron sputtering

1 INTRODUCTION

Transparent conducting materials (TCM) which as their nameimplies exhibit high transparency in the visible region and highelectrical conductivity present a multitude of technologicalapplications most notably in the fields of optoelectronics(displays and touch screen technology)1minus4 and energy-efficientfenestration5 Most commonly TCMs are based on tin-dopedindium oxide In2O3Sn (ITO) thin films Consequently ITOhas been the subject of intensive research during the pastdecades6minus8 However ITO-based TCMs present some dis-advantages especially when it comes to their implementation inflexible devices9minus11 A single ITO thin film suffers frommechanical brittleness and poor electrical stability whendeposited onto flexible substrates Alternatives to ITO forflexible TCMs include metal nanowires911 carbon nano-tubes12minus14 metal meshes915 conductive polymers16 andnotably oxideminusmetalminusoxide (OMO) multilayered electro-des417minus21 where the latter are considered state-of-the-art inthe field The ductility of the metal layer provides mechanical

stability to the OMO structure especially when compared to anequivalent single oxide layer such as ITO OMOs presentseveral advantages when compared to other alternatives Inparticular OMO coatings can be easily fabricated by scalablemethods onto flexible substrates including in-line magnetronsputtering which enables roll-to-roll production171822 Thetypical OMO structure consists of a thin metal layer (ofthickness ranging between 10 and 15 nm) placed between twotransparent oxide thin films (40minus50 nm thick) The thicknessesof the top and bottom oxide films are selected to achieveoptimum optical and electrical properties whereas the metallayer has to be kept as thin as possible to maintain good

Received December 8 2021Accepted March 8 2022Published March 22 2022

Research Articlewwwacsamiorg

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transparency The latter has to be achieved without imperilingthe formation of a continuous metal film structure otherwisethe electrical conductivity of the OMO structure will beadversely affectedThe most common material used as metal layer in the typical

OMO structure is Ag This is due to the excellent electricalconductivity and relatively low cost of Ag when compared toother noble metal alternatives such as Au or Pt Unfortunatelycontinuous Ag films below 10 nm are typically not achievableExamples of OMO multilayered electrodes including Ag asmetallic layer are TiO2AgTiO2

21 ITOAgITO1820 IZOAgIZO19 and ZnOAgZnO23

Apart from limited optical transparency in thicker films theuse of Ag as metal layer in an OMO structure presents someproblems in particular when exposed to humid environmentsRoss24 reported agglomeration of Ag in the ZnOAgZnOstructure after exposure to 95 relative humidity air at roomtemperature Aoshima et al25 reported not only the appearanceof white spots in a ZnOAgZnOmultilayer after exposure to airat 50 degC and 90 relative humidity but also how the formationof these white spots could be reduced by using an Ag alloy ThusAgminusPdminusCu (APC) alloy films has gained interest as anattractive alternative to pure Ag coatings due to their highelectrical stability and resilience toward agglomeration evenwhen subjected to high relative humidity environments426

Jeong et al27 demonstrated that an APC alloy (Pd and Cucontent of 09 and Cu 17 at respectively) exhibited superiorresistance to agglomeration and excellent adhesion to thesubstrate In addition Kim et al28 showed how an APC layerplaced between bottom and top oxide layers can efficientlyreduce diffusion of Ag atoms Even tough APC in OMOstructure has been studied in the past4232829 there are only fewstudies focusing on the threshold thickness ie the thicknesslimit below the percolation threshold or in other words thelower limit at which metal islands rather than a continuousmetallic film is achieved22

In this work we show that controlled addition of oxygen in areactive magnetron process enables the fabrication of smoothultrathin 5 nm thick continuous APCO films Such APCOfilms exhibit both high electrical conductivity and superioroptical properties Using a low-temperature process we fabricateOMO films in the configuration ITOAPCOITO whichcompare favorably with state-of-the-art OMO structuresprepared at high temperatures The ITOAPCOITO filmsexhibit high optical transmittance in both visible and near-infrared regions together with low electrical sheet resistanceand superior mechanical bending performance The sustainedelectrical stability of the APCO-based films was tested in a high-humidityhigh-temperature environment (95 relatively hu-midity and 90 degC) up to 330 h The results indicate that oxygenincorporated in the ultrathin APC layer provides additionalelectrical stability with maintained excellent optical propertieswhich suggests that ITOAPCOITO can be applied as TCMsin a wide range of applications

2 EXPERIMENTAL DETAILS21 OMO Fabrication The ITOAPCOITO multilayer films

were prepared by magnetron sputtering onto glass silicon andpoly(ethylene terephthalate) (PET) flexible substrates following athree-step procedure (i) deposition of ITO film on a substrate (ii)deposition of an APCO layer on the ITO-covered PET substrate andfinally (iii) deposition of the top ITO coating For steps i and iii an ITOceramic target (SnO2 98 wt ) was used while for step ii a target

consisting of an APC alloy (Ag 98 wt Pd 1 wt Cu 1 wt ) wasemployed and presputtering conducted on ITO and APC targets priorto the deposition process The DC power for APC deposition was keptlow at 30 W by using a calibrated deposition rate of 1 nms Beforestarting the deposition sequence the sputtering chamber was evacuatedto a base pressure of 10 times 10minus6 Torr All films (ITOs and APC) weredeposited without applying substrate heating Only Ar gas (gt999999purity) was introduced in the chamber during steps i and iii while forstep ii a fractional mixing Γ of Ar and O2 (gt99999 purity) wasemployed Γ is defined (in unit of percentage) as Γ = [ϕO2

(ϕO2+ϕAr)]

times 100 where ϕO2and ϕAr are the oxygen and argon gas flow introduced

in the chamber respectively The working pressure in steps i ii and iiiwas set to 50 75 and 50 mTorr respectively Different set of sampleswere obtained by varying Γ in step ii from 0 to 15

22 OMO Characterization The electrical and optical propertiesof the samples were evaluated in a Hall-effect measurement systemECOPIA HMS-3000 and in a Shimadzu UV-1800 spectrophotometerrespectively The microstructure was determined by X-ray diffractionXRD analysis using a Bruker D8-Advance instrument employing Cu Kαradiation (λ = 15412 Aring) The surface roughness was estimated by AFMmicroscopy employing a JPK Nanowizard II instrument The oxygencontent in the thin film was estimated by time-of-flight secondary ionmass spectrometry (TOF-SIMS ION-TOF GmbH Munster) using apulsed 30 keV Bi+ primary beam with a current of 101 pA analyzing anarea of 200 times 200 μm2 The film morphology was monitored by field-emission transmission electron microscopy (FE-TEM JEOL) operatedat a working voltage of 200 kV The oxidation state of the differentchemical elements was investigated by X-ray photoelectron spectros-copy (XPS Theta Probe Thermo Scientific) The binding energy wascalibrated from the CminusC contribution due to the C 1s adventitiouscarbon signal at 2848 eV Data analysis was made with the CasaXPSsoftware30 The electrical stability was determined by exposure of theITOAPCOITO electrodes to 95 relative humidity and 90 degC for330 h while the film resistivities were measured every 12 h

23 Optical Simulations The optical properties of the OMOcoatings were modeled by using a four-layered model as depicted inFigure 1 The layer stack comprises a thin film of thickness d2 (d2 = 63

plusmn 12 nm) consisting of a mixed ITOmetal layer sandwiched betweentwo thin ITO layers of thicknesses d1 (d1 = 400 plusmn 49 nm) and d3 (d3 =410 plusmn 36 nm) These three thin (optically coherent) layers are placedon a thick (optically incoherent) glass substrate (represented by aconstant refractive index n = 152) The OMO structure thusconstructed was modeled by the Bruggeman effective mediumapproximation (EMA)31 with a filling factor (volume fraction) f f ofthe metallic phase close to 1 ( f f = 085 plusmn 009 nm) The Bruggemanapproximation is used to include the roughness (inhomogeneity) of themetallic particulate layer In the model fitting parameters d1 d2 d3 andf f were not allowed to vary freely but in a fixed interval that wasconsistent with the experimental observations In the case of the fillingfactor f f this interval was established qualitatively from the SEM andAFM observations In the simulations the optical properties of ITOwere represented by a constant dielectric background εinfin = 4 and a

Figure 1 Layer stack used for effective medium approximation (EMA)modeling of the optical properties of the OMO structure

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Drude oscillator model characterized by the plasma and relaxationfrequencies Ωp and Ωτ respectively

32 For ITO Ωp|ITO = 59445 cmminus1

and Ωτ|ITO = 49765 cmminus1 were used and obtained by fitting the modelto a single ITO 70 nm thick film (not shown) This set of Ωp|ITO andΩτ|ITO parameters correspond to a free charge carrier concentration of15 times 1020 cmminus3 and a resistivity of 85 times 10minus4 Ωm in good agreementwith Hall measurement data obtained for the single ITO film Ωp|ITOand Ωτ|ITO corresponding to the ITO phase have been consideredconstant for all samples studied Finally the interband band gapabsorption of ITO was modeled by a TaucminusLorentz oscillator33 Theoptical model was implemented in the commercial software Scout34

and the parameters presented in Table S1 were obtained after fitting themodel to the experimental data by using the downhill simplexmethod34

3 RESULTS AND DISCUSSION

Figure 2 shows depth profiling analysis obtained by TOF-SIMScorresponding to the samples ITOAgITO and ITOAPCO(Γ = 30)ITO The signal intensity corresponding to InOSnO and Ag as a function of film depth is practically the same inboth cases but not surprisingly the O content distribution isdifferent In the case of the Ag-based OMO the intensity of theO signal drops when the Ag layer is reached (Figure 2a) On theother hand for the APC-based OMO the O signal does notdrop when the APC layer is reached (Figure 2b) confirming theincorporation of oxygen in the APC layer The TOF-SIMSresults in Figure 2 further prove the APC composition in theITOAPCOITO structure We note in Figure 2b that small

Figure 2TOF-SIMS depth profiling results of (a) ITOAgITO and (b) ITOAPCOITO at Γ = 3 There is no significant difference in InO SnOand Ag however the oxygen content in the region on metal layer is significantly different

Figure 3 (aminusg) AFM images of Ag and APCO films prepared at different oxygen flow ratios and (h) corresponding plots of root-mean-square (rms)surface roughness obtained from lines scans of the AFM images The decreased rms as a function of oxygen flow ratio is associated with suppression ofAg island agglomeration and formation of continuous APCO layers

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amounts of Cu migrate into adjacent ITO regions thus furtherdiluting the Cu concentration Importantly and as we shall seebelow the small amounts of Cu present in ITO films do notdeteriorate the optical or electrical properties of the ITOAPCOITO filmIn Figure S1 XRD patterns of ITOAgITO and ITO

APCOITO at various oxygen flow ratios Γ are presented It isevident that the OMO films are amorphous In general foroxideminusmetalminusoxide (OMO) structures the top ITO layerreadily crystallizes when it is deposited on a metal layer Incontrast our metal layers with very small thicknesses lt10 nmdo not promote crystallization in agreement with previousstudies35

Figures 3aminusg depict AFM topographical images of 5 nm thick(nominal) Ag and APCO single films deposited at differentvalues of Γ In addition the average root-mean-square (rms)surface roughness as obtained by AFM measurements ispresented for each set of samples in Figure 3h As can beobserved in Figure 3a the Ag layer forms a particulate filmconsisting of Ag islands ie the thickness of the Ag layer isbelow the percolation threshold above which a continuous filmcan be achieved This is not an unexpected result the thresholdthickness for achieving a continuous Ag thin film is known to bearound 10 nm Figure 3b shows that substitution of Ag by anAPC alloy results in the formation of smoother more compactfilm while still exhibiting particulate features in AFM Incontrast the APCO films exhibit smooth morphologies with

reduced rms Still as evident in Figure 3b APC films obtained atΓ = 0 present a relatively high surface roughness and does notform continuous films although the APC islands becomeconsiderable flatter than in the case of Ag suggesting a relativelystronger support interaction Irrespectively of Γ APCO filmsexhibit considerably lower surface roughness than pure Agcoatings The formation of a continuous 5 nm thick film takesplace above about Γ = 3 (Figure 3cminusg) Increasing Γ up to 3results in a dramatic drop of the surface roughness of the APCOfilms down to rms = 026 nm (Figure 3h) We note theimportance of oxygen for obtaining a continuous APCO filmWhen Γ gt 3 a slightly increased surface roughness is observedin Figure 3eminusg which can be attributed to formation of Pd oxidenuclei as seen in the XPS data (Figure 4) Thus in the APC alloythin film Pd suppresses Ag agglomeration due to superficial Pdoxide formation indicating decrease of the interface energy ofthe Ag alloy and stronger substrate interaction thus promotingcontinuous layer growth At too high oxygen loading howeverextensive oxide formation occurs leading to not only someroughening but also deteriorating conductivity as discussedbelowFigure 4 shows high-resolution XPS spectra of APCO thin

films prepared at different oxygen flow ratios Γ No significantdifferences are observed in the Ag 3d Cu 2p and C 1s spectra asa function of O concentration in the films However accordingto the TOF-SIMS results presented in Figure 2 Cu partiallydiffuses into neighboring ITO regions This together with the

Figure 4High-resolution XPS coreminuselectron spectra of (a) Ag 3d (b) Pd 3d (c) Cu 2p (d) O 1s and (e) C 1s for APCO films prepared at differentoxygen flow ratios from top to bottom 0 25 3 5 and 10 The dashed curves show peak deconvoluted bands There is no significant difference in Ag3d Cu 2p and C 1s However the O 1s orbital peak clearly shows decrease of adsorbed oxygen (Oads = 5321 eV) and increase of lattice oxygen (Olatt =5313 eV) which indicates oxide formation Pd oxidation related peak (PdOx52 = 3387 eV PdO52 = 3371 eV PdOx32 = 3438 eV and PdO32 = 3417eV) increased as a function of oxygen flow during the deposition process PdOx (x gt 1) peaks appear at higher binding energy compared to PdO due tohighly oxidized Pd atoms in nonstoichiometric Pd oxide

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fact that the initial (before diffusion) Cu content in the APC filmis already small can hinder the observation of Cu oxide speciesby XPS In contrast the Pd 3d spectrum clearly shows anincreasing degree of Pd oxidation as Γ increases In Figure 4 it isseen that PdOx peaks appear at higher binding energiescompared to PdO (Table S2) which we attribute to highlyoxidized Pd atoms This is similar to what has previously beenreported for Pd4+ species in very small oxidized Pd nano-particles36 thus indicating the formation of a PdOx phase with xgt 1 containing mixed Pd2+ and Pd4+ in the ultrathin APCOfilms Notably at small O concentration (Γ lt 5) mainly highlyoxidized Pd4+ is observed whereas at highO concentration Pd2+associated with stoichiometric PdO oxide starts to dominateThese results suggest that the nonstoichiometric PdOx phase inthe APCO structure is responsible for the suppression of APCagglomeration and as it will be discussed later the goodelectrical properties observed in these samples Confirming thisinterpretation the O 1s orbital peak shows a relative decrease ofadsorbed oxygen (Oads = 5321 eV) and increase of latticeoxygen (Olatt = 5313 eV) demonstrating increased oxidation ofPd as a function of oxygen flow during the deposition processThis suggests that superficial PdOx formation decreases thesurface energy of the alloy and strengthen the substrate

interaction thus promoting the growth of a continuousAPCO single-layer thin filmFigure 5 shows FE-TEM cross-sectional images of ITOAg

ITO ITOAPC (Γ = 0)ITO ITOAPCO (Γ = 3)ITOand ITOAPCO (Γ = 10)ITO structures of nominalthickness 5 nm deduced from the calibrated deposition growthrates Each FE-TEM image in Figure 5 is accompanied by aschematic drawing for their easier interpretation (Figure 5 lowerpanels) The FE-TEM images confirm that the Ag layer consistsof agglomerated Ag islands forming a particulate layer withthickness of about 12 nm The same result is apparent for theAPC alloy film (Γ = 0) but as expected from the AFM analysisof the single film in this case the particles are flattened outforming an 8 nm thick particulate layer In contrast in the case ofAPCO (Γ = 3) little or no coalescence is observed and a 5nm thick continuous film is formed showing that the oxygen-modified APC alloy suppresses metal particle agglomerationFigure 5d shows the results for an APCO (Γ = 10) film Thisresult further shows that above about Γ = 3 agglomeration ofAPCO is avoided due to Pd oxidation (Figure 4) yielding aboutthe same film thickness (sim5 nm) Above 3 XPS data show thatan excessive oxide layer forms at high oxygen concentration

Figure 5 FE-TEM cross-sectional images of (a) Ag (b) APC (c) APCO (Γ = 3) and (d) APCO (Γ = 10) showing suppressed Ag agglomerationin the APCO film structure Lower figures depict schematic 3D and cross-sectional illustration of the FE-TEM images

Figure 6 Electrical transport properties of (a) single Ag and APC metal layers with thickness of 5 nm and (b) ITO (40 nm)Ag or APCOITO (40nm) as a function of oxygen flow ratio Γ (see text)

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during synthesis which also results in a slightly increased surfaceroughness as shown by the AFM data (Figure 3)Figure 6a shows the resistivity ρ carrier density n and Hall

mobility μ corresponding to a single Ag layer as well as forvarious APCO layers with different oxygen compositionssputtered at different O2 partial pressures (different Γ values)All these films are expected to be 5 nm thick based on thecalibrated sputtering growth rate The Ag and APC films (theAPCO films deposited at Γ = 0) exhibit similar electricalproperties as shown in Figure 6a As Γ increases ρ decreasesreaching a minimum at Γ = 3 coinciding with a remarkableincrease of μ As it will be demonstrated below metallic layersdeposited at Γ lt 3 are still discontinuous and hence themaximum value of μmeasured when Γ = 3 is attributed to theformation of an APCO film transforming the structure to aconnected network of grains However a further increase of Γabove 3 results in deteriorating electrical properties of theAPCO layer which as elaborated above can be attributed tothe formation of extensive Pd oxide and the concomitant drop ofn and μ Figure 6b shows ρ n and μ for the analogous OMOmultilayered film structures ITO (40 nm)Ag (5 nm)ITO (40nm) and ITO (40 nm)APCO (5 nm)ITO (40 nm) with theAPCO layer deposited at different values of Γ Resultspresented in Figure 6b and are consistent with the resultsshown in Figure 6a The use of APCO deposited at Γ = 30results in a minimum of ρ Again this is attributed to anenhanced μ due to formation of a continuous metal filmSimilarly increasing Γ above 3 has a detrimental effect in ρwhich can be explained by oxidation and drop of n and μ asobserved in Figure 5b The increased resistance and lowermobility at Γ gt 3 coincide with increased rms seen in Figure3h further supporting that bulk oxidation occurs at the highestΓvalues However compared to a single metal layer the OMOstructure shows higher resilience of electrical transport proper-ties toward oxidation which can be attributed the protectiveembedding ITOThe results above can be understood by oxide formation at the

surface of APC The thin film nucleation and growthmechanisms which determine whether the thin film micro-structure is particulate or continuous depend on the cohesiveenergy of the metal atoms their surface energy and the bindingenergy to the supporting ITO surface3738 It is known that Agfilms become rough at small thicknesses due to the largeinterface energy of Ag favoring VolmerminusWeber growth Both Pd

and Cu can alloy with bulk Ag and it is known that Pd alloyedwith Ag (as in APC) can suppress agglomeration By use of smallamounts of Pd addition in Ag thin Ag coatings have beenachieved in the past29273940 Our results show that purposefullyadding small amounts of oxygen in the in the plasma duringsputtering of APC up to about Γ = 3 promotes formation asuperficial PdOx phase that prevents particle agglomeration in acontrolled way leading to ultrathin continuous electricallyconducting APCO films In contrast too high Γ values lead toextensive PdO formation evident in Figure 4 with an increasedsurface roughness (Figure 3) and deteriorating electricalproperties (Figure 6) As shown below APCO also showsresilience to oxidation by ambient humidity and elevatedtemperaturesFigure 7a shows experimental and calculated optical trans-

mittance T corresponding to ITOAgITO and ITOAPCOITO prepared at different Γ ratios Figure 7b shows amagnification of the visible part of the spectrum of the curvespresented in Figure 7a Calculated curves were obtained by usingthe three-layered model described in section 22 Transmittancecurves were found to be in good agreement with theexperimental data for both ITOAgITO and ITOAPC (Γ =0)ITO (Figure S2) The metallic layer was here modeled byusing the optical constants of Ag reported by Johnson andChristy41 This confirms that Ag thin films retain practically thesame refractive index of Ag bulk even for very low thicknesses42

However using the refractive index of Ag fails when modelingITOAPCOITO coatings deposited at Γ gt 0 Instead themetallic layer must be represented by a Drude oscillator modelwith fitting parametersΩp|Metal andΩτ|Metal as specified for eachsample in Table S1 The parameters in Table S1 give results thatare in excellent agreement with the experimental data as shownin Figure 7 Note that for Γ = 0 the parameters Ωp|Metal andΩτ|Metal presented in Table S1 also show agreement with thosereported for Ag41 As shown in Figure 6b T decreases for Γ gt3 According to the data presented in Table S1 the lowertransparency in the visible region of the OMO coatingsdeposited at Γ gt 3 can be attributed to a decreased mobilityof the free electrons in the metallic layer ie an increase ofΩτ|Metal This may be attributed to scattering by impuritiesassociated with the incipient formation of a disordered oxidephase43 We note that despite the success of our optical model inreproducing the optical properties of the different OMOcoatings the conclusions drawn from the modeling should be

Figure 7 (a) Measured transmittance spectra for ITOAgITO and ITOAPCOITO deposited under different oxygen to Ar ratio Γ (color lines)and calculated transmittance spectra using the Bruggeman effective medium approximation (see text) (b) Magnification of (a) in the region between500 and 800 nm (c) Figure of merit (FoM T10Rsheet) of ITOAgITO and ITOAPCOITO as a function of Γ is plotted

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regarded with some caution In particular multipole interactionswith the surrounding ITO although not a metallic conductorare disregarded31 A more complete treatment requires detailsabout the particular geometry of the ITOmetal interface andexplicit treatment of their mutual field-induced polarizationFigure 7c shows the figure of merit FoM calculated according totheHaackesrsquos formula FoM= [T(λ = 550 nm)]10Rsheet

44 whereRsheet is the sheet resistance of the OMO coating presented inFigure 6 The OMO coating ITOAPCO (Γ = 30)ITOexhibits the highest FoM thanks to its excellent Hall mobilitytypically for a continuous APC thin film layer withoutpercolation combined with high T values in the visible regionFigure 8 shows the sheet resistance as a function of time for

ITOAgITO and for ITOAPCOITO samples deposited at

Γ values ranging from 0 to 15 The samples were subjected tohigh humidity (95) and temperature (90 degC) conditions for330 h and their sheet resistance was intermittently measuredWhen Ag is used in the OMO the sheet resistance increasesrapidly as a function of time On the other hand when the APCalloy is used the OMO shows much better electrical stabilityAccording to these results it is possible to assume that highhumidity and temperature cause agglomeration of Ag whichreduces the electrical conductivity On the other hand the use ofAPC results in an excellent electrical stability which can beattributed to the suppression of agglomeration by the presenceof Pd and Cu and the formation of superficial Pd oxide Table S3shows comparisons of the electrical and optical properties of theAPCO films reported here and previously published dataFinally in Figure 9 we present the results from dynamic

bending test results employing a bending radius of 5 mm Theduration for each cycle is 2 s with 05 s interval between eachmeasuring The tests were conducted for 5000 cycles Thechange in resistanceΔR was determined to beΔR = Rn+1 minus RnFigure 9a shows ΔRRn as a function of cycling number Figure9b shows the average resistance changes in Figure 9a as afunction of Γ It is evident that the well-known OMO structureITOAPC(Γ = 0)ITO only shows slightly improvedmechanical stability compared to ITOAgITO However theITOAPCOITO obtained by controlled addition of oxygen inthe film synthesis exhibit dramatically improved mechanicalstability with an optimum coinciding with the continuous flatand electrically well-conducting APCO film at Γ = 3 Above Γgt 3 ΔRRn slightly increases because of oxide formationmaking it brittle In Figure S3 photographs of ITOAgITO andITOAPCOITO thin films on PET are shown All samplesexhibited larger than 80 transmittance in visible light region

Figure 8 Change of sheet resistance as a function of time for ITOAgITO and ITOAPCOITO (Γ = 3) in relative humidity 95 and 90degC The inset shows an enlargement of the ordinate axis to facilitatecomparisons of the ITOAPCOITO films

Figure 9 Dynamic bending test results employing a bending radius of 5 mm of ITOAgITO and ITOAPCOITO deposited under differentoxygen-to-Ar ratio Γ (a) shows ΔRRn as a function of cycling number and (b) shows the average resistance change in (a) The duration for eachcycle is 2 s with 05 s interval between each measuring ΔR is determined to be Rn+1 minus Rn

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4 CONCLUSIONSIt is shown that ultrathin (5 nm thick) and continuous oxygen-containing AgminusPdminusCu metallic films (APCO) can be made byreactive magnetron sputtering deposition on flexible PETsubstrates using a single AgminusPdminusCu target and an oxygen-containing argon plasma Such films can be embedded betweentwo ITO films to obtain oxideminusmetalminusoxide (OMO) TCMcoatings in the configuration ITOAPCOITO which exhibitsuperior electrical optical corrosion stability and mechanicalbending properties The reason for the improved optoelectronicproperties is the suppression of metal particle agglomeration inthe APCO layer due to superficial Pd oxidation in the APCalloy While Ag thin films with nominal thickness of 5 nmagglomerate APCO films yield smooth continuous ultrathinfilms in the 5 nm range approaching single-layer films TheAPCO films exhibit low resistivity high Hall mobility andexcellent optical transparency Optimal APCO films wereobtained for argon-to-oxygen ratio Γasymp 3 Above Γ gt 50 theproperties of the optical and electrical properties of the APCOfilms deteriorate due to extensive PdO nucleation yieldingcoarser films The combined AFM FE-TEM and electricaltransport data show that oxygen incorporation in the APC alloysuppress agglomeration suggesting a synergetic effect of thedifferent elements present in the APC alloy as well as thecontrolled surface Pd oxidation Both ITOAgITO and ITOAPCOITO structures were subjected to high relativehumidityhigh temperature and mechanical bending testsWhile the Ag-based OMO coatings subjected to humiditytests suffered from dramatic increase of sheet resistanceattributed to oxidation and agglomeration of Ag the ITOAPCITO structures showed remarkable stable sheet resistanceThe same results were obtained from the bending tests In allcases ITOAPCOITO films with Γ = 30 showed the bestresults demonstrating a balance between deep oxidation andsuperficial surface oxidation We conjecture that ITOAPCOITO coatings may find important applications as TCMs inoptoelectronic applications in particular where mechanicalflexibility is required

ASSOCIATED CONTENTsı Supporting InformationThe Supporting Information is available free of charge athttpspubsacsorgdoi101021acsami1c23766

Drude oscillator parameters Ωp|Metal and Ωτ|Metal for theoptical modeling of the metallic layer binding energy andpeak fitting area of the Pd 3d region from XPS analysiscomparisons of electrical and optical performances withour results and previously published data XRD diffracto-grams of ITOAgITO and ITOAPCOITO as afunction of oxygen flow ratio measured and modeledtransmittance curves for ITOAgITO and ITOAPCITO photographs of ITOAPCOITO thin films on aPET substrate (PDF)

AUTHOR INFORMATIONCorresponding AuthorsPungkeun Song minus Department of Materials Science andEngineering Pusan National University Busan 46241 KoreaEmail pksongpusanackr

Lars Osterlund minus Department of Materials Science andEngineering The Aringngstroumlm Laboratory Uppsala University

SE-75103 Uppsala Sweden orcidorg0000-0003-0296-5247 Email larsosterlundangstromuuse

AuthorsSeohan Kim minus Material Technology Research Institute PusanNational University Busan 46241 Korea Department ofMaterials Science and Engineering The Aringngstroumlm LaboratoryUppsala University SE-75103 Uppsala Sweden

Joseacute Montero minus Department of Materials Science andEngineering The Aringngstroumlm Laboratory Uppsala UniversitySE-75103 Uppsala Sweden orcidorg0000-0003-2917-8569

Janghee Yoon minus Busan Center Korea Basic Science InstituteBusan 46742 Korea

Yunju Choi minus Busan Center Korea Basic Science InstituteBusan 46742 Korea

Sungmin Park minus Department of Materials Science andEngineering Pusan National University Busan 46241 Korea

Complete contact information is available athttpspubsacsorg101021acsami1c23766

Author ContributionsLO and PS contributed equally to this work SKconceptualization methodology investigation data curationvisualization and writing-original draft JM optics simulationwritingminusrevision and editing JY ToF-SIMs TEM analysisYC TEM analysis SP thin film fabrication LOinvestigation supervision funding acquisition writingminusrevisionand editing PS supervision funding acquisition writingminusrevision and editing

NotesThe authors declare no competing financial interest

ACKNOWLEDGMENTS

This research was supported by Basic Science Research Programthrough the National Research Foundation of Korea (NRF)f u n d e d b y t h e M i n i s t r y o f E d u c a t i o n(2019R1A6A3A01091664) and the STINT Mobility Grantfor Internationalization program ldquoNano-templated chromicmaterialsrdquo (Grant MG2020-8871) This work was partlysupported by RampD Platform Establishment of Eco-FriendlyHydrogen Propulsion Ship Program (No 20006644) andMinistry of Environment (G232019012551) and the KoreaIndustrial Complex Corporation (HRBS2116)

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