Neutron Reflectivity Investigations of Self-AssembledConjugated Polyion Multilayers

G. J. Kellogg, A. M. Mayes,* W. B. Stockton,† M. Ferreira, and M. F. Rubner

Department of Materials Science and Engineering, Massachusetts Institute of Technology,Cambridge, Massachusetts 02139-4307

S. K. Satija

Reactor Radiation Division, National Institute of Standards and Technology,Gaithersburg, Maryland 20899

Received March 25, 1996. In Final Form: August 6, 1996X

Neutron reflectivity has been used to study the organization of self-assembledmultilayers of sulfonatedpolyaniline and polyallylamine. Films were prepared by the sequential adsorption of polycations andpolyanions from dilute aqueous solutions. Scattering contrast was achieved by selective deuteration oftheblocks of bilayers at varying intervals along the film. Themultilayer structurewas found tobepreservedover 40 bilayer depositions, but with an internal organization which decays monotonically away from thesubstrate. These results suggest that the observed interfacialwidths are primarily due to theaccumulationof defects as the bilayers are deposited.

IntroductionMolecular-scale processing of polymers holds great

promise for next-generation optoelectronic device fabrica-tion. Waveguides, light emitting diodes, and anisotropicconductors have been prepared via Langmuir-Blodgettand related molecular assembly schemes.1 Major short-comings of thesemethodsare that theyare typically labor-intensive, relatively inefficient, and difficult to scale-up.Analternate self-assemblymethodwasrecentlydevelopedwhereby oppositely charged polyions are sequentiallyadsorbed fromdilute aqueous solutions.2 Themethodhasthe advantage of being easily automated and could bereadily adapted to industrial-scale processing.It has been shown that such sequential bilayer adsorp-

tions result in monotonic growth of the total thickness ofthe adsorbed film and that solution pHand ionic strengthmay be used to manipulate the individual layer thick-nesses.2-6 However, the characterization methods gener-ally usedsX-ray reflectivity and UV-vis adsorptionsusually yield no information about the internal structureof the film: while the overall thickness as a function ofnumber of layers is highly reproducible, these measure-ments do not reveal whether the resultant film containswell-defined molecular layers or to what degree theselayers interpenetrate. (Decher and co-workers haverecently reported that a different system, a repeatingABAC multilayer, exhibits an X-ray reflectivity Braggpeak indicative of a superlattice.) Neutron reflectivitywith selective deuteration of blocks of layers has been

used to probe the internal structure of films.3 For thesystemof sulfonatedpolystyrene (SPS)andpolyallylamine(PAH), a multilayer structure was reported with a rootmean square interfacial width of σ ) 19 ( 1 Å andcomponent layer thicknesses of dSPS ≈ 34 Å and dPAH ≈19Å. These results are consistentwith somechain-chaininterpenetration. Theuniformity inbilayer thicknessandinterfacialwidths, beyond the first fewdepositedbilayers,suggests that no degradation of the multilayer organiza-tion occurswith successive depositions. This is consistentwith the interpenetration of polymer chains being theprimary source of the observed interfacial widths.WhileSPSandPAHprovide agoodmodel system, these

nonconjugatedpolymersarenot electronically active.Realdevices will need to be made from highly-conjugatedpolymers, where the conjugation length and planarity ofthe backbone can be expected to affect the optoelectronicproperties of the films.1 In this study we report theneutron reflectivitymeasurements of self-assembled filmsfabricated through the sequential adsorption of polyal-lylamine and sulfonated polyaniline (SPAn). SPAn is ofinterest because it is a highly-conjugated, amphotericmacromolecule, where the magnitude and sign of chargemay be manipulated through the control of the dopantconcentrations and pH of solutions. PAH is a commonpolycation which provides the opposite charge necessaryforbuilding films. Filmsof this typeareelectricallyactive,having an electrical conductivity of∼10-3 S/cm. We alsonote that the issue of the interpenetration of chains inself-assembled polyelectrolytemultilayers, electronicallyactive or not, is still largely unresolved. The degree ofinterpenetration is critically important forunderstandingtheelectronicproperties of thin filmsbuilt fromconjugatedpolymers.

Experimental Details

Polished silicon substrates 75 mm in diameter and 2.5 mmthick7 were immersed in a bath of H2SO4/H2O4 for 30 min andsubsequently rinsed with deionized water. The surfaces of thewafers were then hydroxylated by boiling in NaOH/H2O2 for 1h. Thesurfacesweresubsequently treatedwithamonofunctional

† Current address: 3M Center, Austin, Texas 78726-9000.X Abstract published inAdvanceACSAbstracts,October 1, 1996.(1) Rubner, M. F.; Scotheim, T. A. In Conjugated Polymers; Bredas,

J. L., Silvey, J. L., Eds.; Kluwer Academic Publishers: Boston, MA,1991; p 363 and references therein. Rubner, M. F. In Polymers forElectronic and Photonic Applications; Wong, C. P., Ed.; AcademicPress: Boston, MA, 1993; p 601 and references therein.

(2) Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210/211, 831. Lvov, Y.; Haas, H.; Decher, G.; Mohwald, H.; Kaladev, A. J.Phys. Chem. 1993, 97, 12835. Lvov, Y.; Decher, G.; Mohwald, H.Langmuir 1993, 9, 481.

(3) Schmitt, J.; Grunewald, T.; Decher, G.; Pershan, P. S.; Kjaer, K.;Losche, M. Macromolecules 1993, 26, 7058.

(4) Decher, G.; Lvov, Y.; Schmitt, J.ThinSolid Films 1994, 244, 772.(5) Ferreira,M.;Cheung, J.H.; Rubner,M.F.ThinSolidFilms1994,

244, 806.(6) Ferreira, M.; Rubner, M. F.Macromolecules 1995, 28, 7107. Fou,

A. C.; Rubner, M. F. Macromolecules 1995, 28, 7115. (7) Semiconductor Processing Co., Boston, MA.

5109Langmuir 1996, 12, 5109-5113

S0743-7463(96)00285-5 CCC: $12.00 © 1996 American Chemical Society

coupling agent, (4-aminobutyl)dimethylmethoxysilane, for 1 h,to promote the adhesion of a layer of sulfonated polystyrene; thisSPS layer provided the charges necessary to adsorb the firstlayer of PAH.6 The substrates were stored in deionized wateruntil multilayer deposition had begun.Perdeuteratedanilinemonomerwasobtained fromCambridge

Isotopes. The SPAn and deuterated SPAn (SPAn-d-7) with anestimated molecular weight Mw ≈ 30 000 were synthesized in-house following a procedure previously reported.8 These weremaintained in metastable 0.005 M aqueous solutions adjustedto pH ) 3.5 with methanesulfonic acid (CH3SO3H), a dopantwhich partially-protonates the backbone of SPAn and reducesitsnetnegative charge. Theotherpolyelectrolyte, polyallylaminehydrochloride,waspurchased fromAldrichwithMw≈50-65000.This was used in a 0.005 M aqueous solution adjusted to pH )3.5 with HCl. Multilayer fabrication was performed by dippingthe treated substrates in the PAH solution for 15 min, followedby rinsing with deionized water. The PAH-coated substrateswere subsequently dipped in SPAn or SPAn-d-7 solution for 15min and again rinsed with deionized water. This sequence wasrepeated until the desired number of layers had been deposited.To investigate themultilayer organization, eightbilayerblocks

were selectively contrasted at varying depths within a series offilms by substituting SPAn-d-7 for SPAn. Four 40-bilayersystems were investigated with the following labeling schemes,counting from the substrate up: 8D/32H (A), 16H/8D/16H (B),32H/8D (C), and 8D/8H/8D/8H/8D (D), whereDdenotes bilayerscontainingSPAn-d-7 andHdenotesunlabeledbilayers. SamplesA throughCallowus to determine the structure at the interfacesbetween labeledandunlabeledblocks,while sampleD integratesthe labeled regions of the other three samples into a single filmserving as a test of the reproducibility of fabrication and theconsistency of our analysis. The periodic spacing of the labeledblocks in sample D should also give rise to observable Braggreflections if the film is sufficiently well ordered.Neutron reflectivity measurements were performed on the

BT-7 instrument at the National Institute of Standards andTechnology research reactor in Gaithersburg, MD. The experi-

mental setup and sample alignment procedures are described inprevious publications.9,10 Neutrons of wavelength λ ) 2.37 Åand energy resolution∆λ/λ≈ 0.01 were obtained from reflectionof the white beam from a graphite monochromator. Reflectivityprofiles were obtained by rotating the sample θ and detector 2θdegrees with reference to the incident beam, with angularresolution∆θ /θ≈ 0.02 determined by the incident defining slits.Reflectivity measurements were made to angles of at least θ )0.8° orqz)0.07Å-1 ()4π sinθ/λ) inneutronmomentumtransfer.Background intensitieswere obtainedby offsetting 2θby+0.25°.After background subtraction, the reflectivity was obtained bynormalizing the net intensity by the main beam intensity foridentical slit conditions.To fit the reflectivity data, scattering length density (b/v)

profiles were generated bymodeling the films as adjacent layersof an assumed thickness di and (b/v)i value, with error-functioninterfaces between blocks of differing scattering length. Theinterfaces are characterized by a rootmean square roughness σi,such that σi describes the interface between layers i - 1 and i.The theoretical reflectivity for a model profile was calculatedusing the Parratt formalism11 and evaluated against theexperimental data. Model parameters di, (b/v)i, and σi weremodified iteratively until a best fit to the data was achieved. Inpractice, it was found that simple slabs corresponding todeuterated or nondeuterated blockswere sometimes insufficientto give good fits to the reflectivity. In these cases, “compositeslabs” of two adjacent interdiffuse blocks were used, especiallyat the air/film interface, to model structures not well describedby error-function interfaces. For these films, the quoted rough-nesses are derived from error-function fits to the compositeinterfaces used in the model.

Results and Discussion

Figure 1a shows themeasured reflectivity as a functionof qz from sampleA, which contains an eight bilayer blockof SPAn-d-7/PAH adjacent to the substrate. Oscillationsin the reflectivity data arise from interferences between(8) Yue, J.; Epstein, A. J. J. Am. Chem. Soc. 1990, 112, 2800.

Figure 1. Neutron reflectivities and fits (-) for films with (a) eight deuterated bilayers at the substrate, (b) eight deuteratedbilayers in the middle of the film, (c) eight deuterated bilayers at the interface, and (d) three groups of eight deuterated bilayerslocated at the substrate, interior of the film, and at the air interface. The insets show the profiles used to generate the modeledreflectivities.

5110 Langmuir, Vol. 12, No. 21, 1996 Kellogg et al.

neutrons that are reflected from the surface of the film,the internal interface created by incorporating the iso-topically labeled block, the interfaces between the filmand the substrate’s native SiO2 layer as well as theinterface between the SiO2 layer and silicon. The oscil-lations decay rapidly as a function of increasing angle θ(qz). Qualitatively, this result implies significant surfaceroughness. The solid line in Figure 1a represents thebest fit to the data, corresponding to the scattering lengthdensity profile shown in the inset. Two observations canbe made on examining this profile. First, the deuteratedblock is clearly distinguishable as a region of highscattering length density next to the substrate, showingthat the deuterated material has remained localized inthe portion of the filmwhere it was deposited. (Note thatthe thin SiO2 layer has been incorporated into the profileat the Si surface.) Second, the interface between thisregion and the unlabeled portion of the film is quite broadwhen compared to the typical width of the substrateinterface, σi ) 21 ( 7 Å vs σs ≈ 3-5 Å.12

The reflectivity results for the differently-labeled filmsare summarized in parts b-d of Figure 1, which show theresults for samples B-D. Films B and C, in which onelabeled block is placed either in the middle of the film orat the surface, respectively, show markedly differentreflectivities, illustrating that themajor features are dueto the location of the labeled bilayers. These reflectivitiesalso become smooth rapidly with increasing qz, showingthat the internal interfaces and film/air interface arebroad. FilmD,which is composedofperiodicallyarrangedlabeled blocks, additionally exhibits a first-order Braggpeak at qz ≈ 0.026 Å-1, corresponding to a block-spacingof L ≈ 264 Å; the expected value is L ) 240 Å from ourlabeling scheme. From this we draw the importantconclusion that the multilayer structure induced byalternately depositing labeled and unlabeled blocks haslargely been preserved.The insets of partsb-dofFigure1display the scattering

length density profiles used to generate the modelreflectivities. Anumber of conclusions canbedrawn fromobservation of theseprofiles. First, the films showsurfaceroughnesseswhicharenearly the same, theaveragevaluebeing σs ) 48 ( 9 Å. This is evident in the raw data aswell, since the range in qz over which oscillations decayis about the same for all samples. Second, the filmthicknesses derived from the fits are self-consistent inthe range of 575-628 Å. This translates to a bilayerthickness range of 14.4-15.7 Å, in good agreement withpreviously reported values for this system.6 Observedthicknessdifferencesbetweensamplesmightbeattributedto small variations in the solution pH or polyelectrolyteconcentration during film fabrication. Third, the b/vlevels and widths of the deuterated blocks and undeu-terated blocks are generally consistent from sample tosample. (Some observed variation in the b/v profilesmight arise fromdifferences in salt content.) All samplesappear to show a broadening and flattening of thedeuterated bilayer blocks, combinedwith a broadening ofthe internal interfaces, as the distance from the substrateincreases. These results can be seen best in Figure 2 inwhich the b/v profiles are superposed for all foursamples: sampleD is indeeda composite of samplesA-C,to within the certainty afforded by our fits.Further consistency of the results can be seen by

comparing the fitted scattering length densities to what

is expected based on the known composition of the filmcomponents. The fitted block scattering lengths, (b/v)i,cluster around two values, as expected from our labelingscheme. We may assume that the structure (numberdensity and thickness) of the bilayers is the samewhetherdeuterated or nondeuterated, the only difference beingthe scattering lengths of SPAn and SPan-d-7. If this isthe case, then the difference in the scattering lengthdensities of the largedeuteratedandnondeuteratedblocksshould be

where nSPAn is the number density of SPAn or SPAn-d-7,dSPAn is the thickness of theSPAnorSPAn-d-7 componentof each bilayer, dPAH is the thickness of the PAHcomponent, and bSPAn-D and bSPAn-H are the scatteringlengths of SPAn-d-7 and SPAn, respectively. The ratioof the thicknesses determines the fractional compositionof the bilayers. Assuming equal thicknesses for thecomponents andanumberdensity consistentwithaSPAnmass density of 1 gm/cm3, we expect

The experimental value can be obtained best near thesubstrate, where the model profiles are least smeared. Itcan be readily seen from parts a and d of Figure 1 thatthis value is nearly the same as that calculated above.Though the difference in the scattering length density

between the deuterated and nondeuterated blocks is aswe expected, the overall scattering length density isuniformly higher than what is expected by ∆(b/v) ≈ 1.0× 10-6 cm-2. This may be attributed to the presence ofHCl counterions (from the PAH solutions) and CH3SO3

-

dopant counterions. It is not possible to calculate theconcentrations of multiple unknowns on the basis of asingle offset value.We can attempt to quantify the depth-dependent

structure by plotting the interfacial widths derived fromthe fits as a function of their position within the films.The error bars for theseparameters are large, on the orderof 10-50% of the value, but all of the values at a givendepth in the various films lie well within these error barsof one another. With this in mind, we display in Figure3 the average interfacial width (roughness) obtained byaveraging the values at a given depth and propagatingthe fitted errors. Though the errors are significant, thetrend is nonetheless clear, showing a roughly linearincrease in roughness with increasing distance from thesubstrate σi ) (16 + 0.7n) Å, where n is the number of

(9) Anastasiadis, S. H.; Russell, T. P.; Satija, S. K.; Majkrzak, C. F.J. Chem. Phys. 1990, 92, 5677.

(10) Mayes, A. M.; Russell, T. P.; Satija, S. K.; Majkrzak, C. F.Macromolecules 1992, 25, 6523.

(11) Parratt, L. G. Phys. Rev. 1954, 95, 359.(12) Russell, T. P. Mater. Sci. Rep. 1990, 5, 171.

Figure 2. Superposition of the three model profiles in partsa-c of Figure 1 (- - -). The superposition captures the structureof the film with three groups of deuterated bilayers (s).

(b/v)D - (b/v)H )nSPAn [dSPAn/(dSPAn + dPAH)][bSPAn-D - bSPAn-H] (1)

(b/v)D - (b/v)H ≈ 8.5 × 10-7 cm-2 (2)

Self-Assembled Conjugated Polyion Multilayers Langmuir, Vol. 12, No. 21, 1996 5111

thebilayer interface countedup fromthe substrate. Thus,we see an increase in the apparent roughness of theinternal interfaces from22 to 38Å,moving from8bilayersto 32 bilayers.The surface roughnesses observed are consistent with

the atomic force microscopy (AFM) measurements per-formed on sample D. Figure 4 shows an AFM measure-ment which indicates a top surface roughness of σs ≈ 60Å, in reasonable agreement with the reflectivity profiles.The lateral lengthscaleof thesurface corrugationsappearsto be 100-1000 Å.The surface roughness and the widths of the internal

interfaces can be compared to those observed for the self-assembled multilayer films of sulfonated polystyrene/

polyallylamine (SPS/PAH) on silicon using X-ray andneutron reflectivity.3 In this system, the widths of theinternal interfaces were found to be about 19 Åwith littlechange as a function of the distance from the substrate.The free surfaces of the films were reported to have aroughness σs ≈ 13-15 Å for a 48 bilayer film, consistentwith the observation of up to 25 thickness oscillations(Kiessig fringes) in theX-ray reflectivity. OurSPAn/PAHfilms showmuchbroader internal interfaces andagreatersurface roughness; preliminary X-ray reflectivity mea-surements show only one to two Kiessig fringes.At least two interpretations canbemade fromthe larger

observed internal interfacial andsurface roughnesses seeninSPAn/PAH. If interpenetration of chains is responsiblefor the widths of these interfaces, this interpenetrationwould have to occur over more than a single bilayer:bilayer thicknesses are≈15Åwhile all interfacial widthsare>20 Å. It is difficult to imagine how interpenetrationordiffusionwould result inbroader interfaces farther fromthesubstrate. Analternative interpretationwouldbe thatthe interfaces are actually very rough, or corrugated,rather thanbeingdiffuse. Since themicroscopic roughnessat the silicon surface is expected to be no greater than 5Å,12 this internal roughness would have to be introducedduring the deposition of the bilayers. The surface rough-ness of the films (σs ) 48 ( 9 Å) supports this latterinterpretation. This interpretation still leaves open theissue of interpenetration. If the linear fit inFigure 3wereto be literally believed, itwould suggest an intrinsicwidthon the order of 16Å,which is approximately the thicknessof a bilayer. This would lead to the conclusion thatinterpenetration over the bilayer length scale is probable.Given themagnitudeof the errorbars in the figure though,this interpretation is highly speculative. Whether thebasic structural unit of the film is individual layers, an

Figure 3. Average widths of interfaces between deuteratedblocksandnondeuteratedblocks of bilayers or film/air interface(value at 40 bilayers). The solid line is a linear fit showing thatthe width of the internal interfaces increases with number ofadsorbed bilayers and that this width accurately reflects theroughness at the film/air interface.

Figure 4. AFM image of Film D showing a corrugated surface with roughness of σs ≈ 60 Å.

5112 Langmuir, Vol. 12, No. 21, 1996 Kellogg et al.

intimately mixed bilayer, or bilayers with some interdif-fusion cannot be determined conclusively from theseresults.If the interfacial widths are dominated by roughness,

wemust askwhy this system is somuch rougher than theSPS/PAH system described above. We suspect that twofactors could be responsible. First, highly-conjugatedSPAn is relatively stiff; hence, this system lacks therelaxationmechanisms inherent to flexiblepolymerchainswhich could smooth any irregularities from uneven layerdeposition. Second, SPAn solutions are metastable andprone to aggregation. Aggregate particles could providelarger irregularities which then accumulate with theincreasing number of depositions. Such aggregate par-ticles could explain the highly-corrugated surface seenwith AFM.In conclusion, we have shown through neutron reflec-

tivity that the system SPAn/PAH self-assembles in sucha way as to maintain significant multilayer organizationfor films as thick as 40 bilayers. The internal interfacesof these films are quite broad with widths increasingapproximately linearly with distance from the substrate.We interpret the observed interfacial widths and surfaceroughness as the result of defects introduced during

deposition; not only would the roughness due to defectsbe propagated through the film as new layers aredeposited, but new defects with each deposition wouldincrease the roughnesswith increasing thickness. Similarconformal roughness phenomena have been observed inmultilayered structures prepared by vapor depositionmethods.13 Off-specular scattering measurements, notperformed in this study, offer a means to further inves-tigate the degree of roughness conformality exhibited bythese systems.14 With improved control over the filmdeposition process to reduce defect incorporation, thisstudy suggests that reproducible manufacture of thin-film heterostructures containing electronically-activepolyelectrolytes is feasible.

Acknowledgment. This work was supported prima-rily by the MRSEC Program of the National ScienceFoundation under Award Number DMR-9400334.

LA960285M

(13) Savage,D.E.;Kleiner, J.; Schimke,N.;Phang,Y.-H.; Jankowski,T.; Jacobs, J.; Kariotis, R.; Lagally, M. G. J. Appl. Phys. 1991, 69, 1411.Kortright, J. B. J. Appl. Phys. 1991, 70, 3620.

(14) Sinha, S. K.; Sirota, E. B.; Garoff, S.; Stanley, H. B. Phys. Rev.B 1988, 38, 2297. Pynn, R. Phys. Rev. B 1992, 45, 602.

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