Ž .Materials Science and Engineering C 7 1999 11–18

Fabrication of multilayer thin films via metal–macromolecular ligandcomplexation

Jaehyun Kim a, Sukant K. Tripathy a,), Jayant Kumar b, Kethinni G. Chittibabu c

a Center for AdÕanced Materials, Department of Chemistry, UniÕersity of Massachusetts Lowell, Lowell, MA 01854, USAb Center for AdÕanced Materials, Department of Physics, UniÕersity of Massachusetts Lowell, Lowell, MA 01854, USA

c Molecular Technologies, Westford, MA 01886, USA

Received 1 March 1998; accepted 18 August 1998

Abstract

Layer-by-layer complexation of macromolecular ligands employing lanthanide metal ions was developed as a new technique toŽ . w xfabricate multilayer thin films. Poly 3-thiophene acetic acid PTAA was prepared as a macromolecular ligand and was successfully used

in the fabrication of self-assembled multilayer films by alternatively dipping desired substrates in aqueous polymer and Eu3q ionsolutions. The multilayer deposition was monitored by the absorbance increase due to polymer using UV–Visible spectrophotometer. Theeffect of process parameters including temperature, concentration and pH of the solutions were studied to understand and to optimize themultilayer thin film characteristics. The pH of the polymer solution has dramatic influence on the thickness of the multilayer film;absorption of film increased with increase in pH of the solution. The effect of process variables on the absorbance of the depositedmultilayer was explained based on the conformational changes of polymer segments. q 1999 Elsevier Science S.A. All rights reserved.

Keywords: Self-assembly multilayer; Layer-by-layer complexation; Macromolecular ligand

1. Introduction

Ž .Self-assembled multilayers SAMs using layer-by-layerdeposition methods have attracted considerable interest inrecent years, because they are some of the most effectiveapproaches to prepare thin film supramolecular structures.Composition, orientation, thickness and other properties ofeach layer could be manipulated using this technique, thusproviding a route for the formation of ordered structures at

w xthe molecular level 1–5 . This manipulation at the molec-ular level offers many potential advantages in device appli-cations, including use as active components in nonlinear

w xoptical devices 2,6 , stable charge-separated assembliesw xfor photovoltaics 7 , employing materials with selective

w xchemical responses for sensor applications 5 , and light-w xemitting diodes 8 .

Self-assembled thin films have been constructed em-ploying several types of intermolecular interactions includ-

w xing electrostatic attraction 8–15 , hydrogen bondingw x w x17,18 , covalent bonding 6,19–23 , charge-transfer inter-

) Corresponding author. Tel.: q1-978-934-3687; Fax: q1-978-458-9571

w x w xaction 24 , and coordination bonding 25–30 , in the pastfew years. A variety of multilayer assemblies have beenreported based on the electrostatic attraction between op-

w x w xposite charges 8–15 , since Decher et al. 9 first reportedthe layer-by-layer deposition of polyions by extending the

w xpioneering work of Iler 16 . Multilayer thin films areconstructed in this process, through alternating sponta-neous adsorption of a positively charged polymer and anegatively charged polymer from dilute aqueous solutions.All kinds of natural and synthetic polyelectrolytes, whichare composed of charged polymer backbones and smallcounterions, are potential candidates for fabricating variousstructures and devices employing this technique.

Hydrogen bonding interactions have been examined tow x w xprepare monolayer 17 and multilayer thin films 18 .

Self-assembled multilayers of polyaniline with nonionic,Ž .water-soluble polymers including poly vinylpyrrolidone ,

Ž . Ž . Žpoly vinylalcohol , poly acrylamide , and poly ethylen-. w xeoxide are also reported 18 .

Multilayers of self-assembled thin films were alsoformed by covalently functionalizing a glass surface, andsubsequently building multilayers by alternative chemical

w xfunctionalization 6,19–23 . A variety of nonlinear opticalŽ .NLO chromophores have been incorporated in thin film

0928-4931r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved.Ž .PII: S0928-4931 98 00075-7

( )J. Kim et al.rMaterials Science and Engineering C 7 1999 11–1812

multilayer structures with acentric chromophore alignment,w xemploying this technique 6,22,23 .

Multilayer thin films were also prepared by consecutiveadsorption of two kinds of nonionic polymers, with elec-tron-accepting and electron-donating groups in the side

w xchains of the polymers forming the alternate layers 24 .Alternative adsorption of these polymers occurs through

Ž .charge-transfer CT interaction between the donor and theacceptor groups of the polymers.

Self-assembled monolayer and multilayers ofmonomeric multidentate ligands with transition metal ions

w xhave been recently fabricated 25–31 . Monolayer films ofŽ . Ž .thiols–copper II and alkylamine acetate–Ru II , combin-

ing thermally induced deposition technique and self-assem-bly chemistry via coordination bonding have been studiedw x25,26 . Multilayer films employing coordination reactions,

Ž . w xsuch as bipyridine ligands with Ni II 27 , a long-chainŽ .thiazolyl azo dye containing resorcinol ligands with Fe II ,

Ž . Ž . Ž . Ž . w xCo II , Cu II , Zn II , and Cd II 28 , diisocyanide ligandsŽ . w x Ž . Ž .with Co II 29,30 , and bis hydroxyquinoline with Zn II

w x31 have been reported. Complexation reaction is a power-ful technique for fabricating thin films with well-controlledthickness from insoluble metal–organic complexes.

Our approach to the fabrication of self-assembled multi-layers through the complexation technique involves theformation of coordination bonding between polymer-basedligands and transition metal ions. Layer-by-layer complex-ation of conjugated polymer and metal ions have beenutilized to form thin films for large area light emitting

w xdiodes 32 . This manuscript includes the synthesis ofŽ .poly 3-thiophene acetic acid as the macromolecular lig-

Ž . Žand, and fabrication of luminescent Eu III –poly 3-.thiophene acetic acid complex multilayer films employing

the layer-by-layer complexation approach. Detailed studieson the characterization of multilayer films and the factorsinfluencing the quality of multilayer films are also pre-sented.

2. Experimental

2.1. Materials

2.1.1. ReagentsThe molecular structures of polymers are shown in Fig.

Ž . Ž .1. Poly diallyl dimethyl ammonium chloride PDAC

Fig. 1. Chemical structures of polymers.

Scheme 1. Polymer synthesis.

was purchased from Aldrich and was used as a primingmaterial. Ethyl 3-thiophene acetate, EuCl P6H O and an-3 2

hydrous ferric chloride were obtained from Aldrich andwere used without further purification.

2.1.2. PolymerizationŽ . Ž .Poly 3-thiophene acetic acid PTAA was synthesized

by chemical dehydrogenation method using anhydrous fer-Ž . Žric chloride Scheme 1 . Ethyl 3-thiophene acetate 4.5 g;.25.64 mmol in 20 ml of chloroform was added to anhy-

Ž .drous ferric chloride 19.5 g; 116.61 mmol suspension in250 ml of chloroform taken in a round-bottomed flaskunder nitrogen atmosphere at room temperature. After

Ž .stirring for 12 h, the poly ethyl 3-thiophene acetate wasprecipitated by pouring in 1000 ml of methanol. The

Ž .precipitate was dissolved in tetrahydrofuran THF andadded to 300 ml of 1 N NaOH in a flask equipped with areflux condenser. After refluxing the solution for 3 h, solidferric hydroxide was removed by filtration. The hydro-

Ž .lyzed polymer solution filtrate was neutralized with con-centrated HCl to precipitate the PTAA. The precipitated

Scheme 2. Multilayer deposition.

( )J. Kim et al.rMaterials Science and Engineering C 7 1999 11–18 13

3q Ž .Fig. 2. Infrared spectra of PTAA and Eu –PTAA complex. a PTAA,Ž .b PTAA–Eu film.

polymer was filtered and washed with 0.1 N HCl solutionand subsequently several times with distilled water. Afterdrying in a vacuum oven at 508C for 2 days, the polymer

Ž .was obtained as a dark powder 3.078 g, 68.4% . Number-and weight-average molecular weights and polydispersityof the synthesized polymer were calculated as 4700 grmol,5800 grmol and 1.23, respectively using GPC. FT-IRŽ y1 . Ž . ŽKBr, cm : 3446 s, n , carboxylic acid , 3107 s,O – H

. Ž . Žn , thiophene ring , 2927 s, n , aliphatic , 1726 s,C – H C – H. Ž . Žn , carboxylic acid , 1631 w, n , 837 n outC 5 O C 5 C C – H. 1 Ž .of plane, thiophene ring . H NMR DMSO-d , d in ppm :6

Ž . Ž12.65 broad, s, 1 H, –COOH , 7.31 and 7.20 1 H,. Žthiophene ring , 3.81 and 3.57 2 H, –CH –, respective2

.head-to-tail coupling, and head to head coupling defects .

2.2. Layer-by-layer complexation

2.2.1. Surface cleaning and hydrophilizationŽ 2 .Slide glass 25=75 mm was hydrophilized using 1%

Chem-solv w solution treatment and used as the substratew x w14 . The slides were washed with 1% Chem-solv solu-

Ž .tion in deionized water D.I. water under ultrasonication,to generate negative charges on the surface by partial

Fig. 3. Absorption spectra of Eu3q –PTAA multilayer thin films with anŽ .increasing number of bilayers from four bilayers to 52 bilayers .

Fig. 4. Absorption increase according to the number of bilayers.

hydrolysis. After 180 min of this treatment, slides wererinsed with D.I. water in ultrasonicator for 30 min. Rinsingwas repeated twice. D.I. water from a Milli-Q system wasused in all the experiments. The resistivity of the D.I.water used was higher than 18.2 MV cm and the totalorganic content was less than 10 ppb.

2.2.2. Alternate adsorptionSimplified procedure for depositing polymer–Eu3q

Žcomplexes is shown in Scheme 2. The polymers PTAA

Fig. 5. Absorbance change with an increasing dipping time.

( )J. Kim et al.rMaterials Science and Engineering C 7 1999 11–1814

.and PDAC were dissolved in D.I. water, and the concen-tration of each polymer solution was calculated based onthe repeat unit. The priming step was used for depositingPTAA molecules, which act as polymer ligands, by elec-trostatic interaction.

Hydrophilized glass slides were immersed in 1 mMŽ .PDAC solution pH 6.5 for 10 min at room temperature

Ž .Step 2 in Scheme 2 followed by washing with buffered

Ž .D.I. Water pH 6.5 . After removing the slides from thewash solution, a stream of nitrogen was blown over thefilm surface until the adhering water layer was completelyremoved. PTAA polymer molecules were adsorbed on theprepared positively charged surface, by immersion of sub-

Ž .strates into 1 mM PTAA solution pH 6.5 for 10 min atŽ .room temperature Step 3 . This was followed by the

washing and drying procedure described earlier. This bi-

Ž . 3q Ž . 3q Ž . 3q Ž . 3q Ž .Fig. 6. pH effect on multilayer deposition. a pH 3.5 Eu solution, b pH 4.5 Eu solution, c pH 5.5 Eu solution, d pH 6.5 Eu solution, e pH7.5 Eu3q solution. – l – pH 4.5 PTAA; – B – pH 5.5 PTAA; – ' – pH 6.5 PTAA; – = – pH 7.5 PTAA; – U – pH 8.5 PTAA.

( )J. Kim et al.rMaterials Science and Engineering C 7 1999 11–18 15

layer from the polycation and polyanion forms the priminglayer for the subsequent layer-by-layer complexation pro-cess. Europium ion–PTAA complex multilayers were as-sembled onto this substrate by alternatively dipping the

Žprimed substrates into europium and PTAA solutions Steps.4 and 5 following the appropriate experimental condi-

tions. The substrates were cleaned by proper washing anddrying procedures, before immersion in each solution.

2.2.3. CharacterizationUV–Visible absorption spectra were recorded using a

GBC UVrVIS 916 spectrophotometer. Infrared spectrawere obtained from a Perkin-Elmer 1760X FT-IR spec-trometer using KBr pellet. 1H NMR spectra were taken ona Bruker ARX-200 spectrometer operating at 200 MHz.The molecular weights of the polymer were determined by

Ž . Žgel permeation chromatography GPC instrument WatersModel 510 pump and Waters 410 refractive index detec-

.tor with styragel columns relative to polystyrene standardsŽ .using dimethylformamide DMF as the eluent.

3. Results and discussion

3.1. Formation of Eu3q-conjugated polymer complex asmultilayer thin films

Polyacids are widely studied polymer ligands for form-w xing macromolecular complexes 33 . Ionization of the

polymer acids is generally carried out before the complex-ation reaction, because carboxalates are better ligands forlanthanide metal ions compared to carboxylic acids. HencePTAA, was first dissolved in a 0.1 N NaOH aqueoussolution. The pH of the solution was subsequently adjustedby using 0.1 and 0.01 N solutions of HCl and NaOH.FT-IR was used for confirming the Eu3q complex forma-tion during multilayer deposition. Powdered samples of thecomplex were used for FT-IR measurements by peelingseveral films from the glass substrates, due to the difficultyin acquiring reliable spectra by attenuation method fromultrathin films. FT-IR spectra of PTAA and multilayers of

3q Ž .PTAArEu complex Fig. 2 show that carbonyl stretch-ing frequency shifted from 1726 cmy1 to 1665 cmy1 dueto the decreased bond energy of the coordinated carbonylgroups. The strong absorption band between 1580 cmy1

and 1426 cmy1 corresponds to the asymmetric and sym-metric stretching vibrations of the coordinated carboxylate

w xgroups 34 .

3.2. Multilayer deposition

PTAA was chosen as the macromolecular ligand to beused as a monitor for the multilayer deposition due to itsstrong absorption in the visible region. The deposited filmsin fact can be observed by naked eye due to the strongabsorption by the polymer in the visible region. Fig. 3

shows the visible absorption spectra of the multilayer filmsup to 52 bilayers. Absorption was found to increase lin-

Ž .early with the number of deposited bilayers Fig. 4 .

3.3. Determination of optimum dipping time

The optimum dipping time for each solution was deter-3q Ž .mined using a 5 mM Eu ion solution pH 5.5 and a 10

Ž .mM PTAA solution pH 6.5 at 508C. Four bilayer filmswere prepared with different dipping times and the opti-mum dipping time was determined as the time required forthe largest absorption among these films. The depositiontime for the Eu3q ion solution was initially changed from

Ž .1 to 10 min with fixed PTAA dipping time 10 min .Subsequently the dipping time in PTAA solution wasoptimized by fixing the optimized Eu3q ion solution dip-ping time. The results are shown in Fig. 5. The establishedoptimum dipping times were 6 min for the Eu3q and 4 minfor the PTAA solutions. These dipping times were consis-tently used for the rest of the experiments.

3.4. Effect of pH of the Eu3q and PTAA polymer solutionson the thickness

The effect of pH of the polymer and Eu3q metal ionsolutions on the multilayer deposition was studied byfixing the other process parameters. The concentration ofeuropium ion solution and PTAA solution used were 5 and10 mM, respectively. The temperature was maintainedwithin 50–558C. Eu3q ion solutions at five different pH

Ž .conditions pH 3.5, 4.5, 5.5, 6.5, and 7.5 and PTAAŽsolutions at five pH conditions pH 4.5, 5.5, 6.5, 7.5, and

.8.5 were prepared using 0.1 and 0.01 N solutions of HCland NaOH. Perkin-Elmer Metron IV pH meter was cali-

Ž .brated with standard pH buffer solutions Orion Researchand was used to prepare and test the pH of the experimen-tal solutions.

Fig. 7. Absorbance per bilayer according to solution pH.

( )J. Kim et al.rMaterials Science and Engineering C 7 1999 11–1816

The net absorption and consequently the thickness ofŽthe films increased Net absorbance s observed ab-

.sorbance y absorbance of priming layer with decrease inthe pH of PTAA solution as shown in Fig. 6. This can beexplained based on the conformational changes of thepolymer molecules. PTAA is a conjugated polymers andyet its backbone possess sufficient conformational flexibil-

w xity in solution 35,36 . Since PTAA has carboxylic acidgroup in its side chain, at the higher pH values employedthe polymer becomes an almost completely chargedpolyanion and has a rigid rod conformation. In contrast, inlower pH solutions, charge density along the polymerchains is reduced and thus the conformation of the poly-mer molecule is expected to be significantly coiled.

When polymer molecules are adsorbed on a surface onwhich Eu3q ions are spread uniformly, almost fully-charged polymer chains will be deposited as a very thinmonomolecular film, due to their stretched conformation.In contrast, coiled polymer chains will be adsorbed throughonly a few complexation sites because of their coiled stateat lower pH. Variation in the thickness of the adsorbedpolymer layer occurs as a result of this effect and conse-

w xquently the absorbance of the films is affected 37 .The effect of pH of the Eu3q ion solution on the

thickness of the multilayer film was much weaker and nosignificant trend of visible absorption change was ob-served. In low pH range of PTAA solutions, absorptionincreased with increase in pH of Eu3q ion solution. How-

Ž . 3q Ž . 3q Ž . 3q Ž . 3qFig. 8. Concentration effect on multilayer deposition. a 1 mM Eu solution, b 2 mM Eu solution, c 5 mM Eu solution, d 10 mM Eusolution. – l – PTAA 1 mM; – B – PTAA 2 mM; – ' – PTAA 5 mM; – = – PTAA 10 mM.

( )J. Kim et al.rMaterials Science and Engineering C 7 1999 11–18 17

ever, when pH of PTAA solution exceeded 6.5, the ab-sorbance change is negligible with increase in pH of Eu3q

Ž .solution. Absorbance per bilayer data Fig. 7 clearlyshows the effect of pH on the thickness of multilayer thinfilms.

3.5. Effect of concentration of polymer and Eu3q solutionson the thickness

ŽFour different concentration solutions 1, 2, 5 and 10. 3qmM of Eu ion and PTAA were prepared. The results of

the effect of concentration of polymer and Eu3q solutionson the absorbance of multilayer films are summarized inFigs. 8 and 9. There is an optimum concentration ratio

3q Žbetween PTTA and Eu solutions PTAArEu ratio of.2–2.5 for depositing thick multilayer films, as inferred

from the measurement of absorbance per layer. It might beexplained based on the nature of Eu3q ions, as they tend toaggregate at high concentration of Eu3q ion. This occursbecause not all the coordination sites of Eu3q ions are

Žcoordinated by polymer ligands carboxalates in the case.of PTAA . The excess adsorbed metal ions, will have a

tendency to aggregate with other free Eu3q ions in themore concentrated solutions. This may be facilitatedthrough assistance of free chloride or hydroxyl ions. Thisaggregation of metal ions interrupts coordination bondingin the next step and lowers the absorbance increment forthe next layer. At lower concentration of Eu3q ion, thenumber of Eu3q ions are not enough to react with allavailable ligands and thus insufficient number of reactionsites are available for the next layer of PTAA molecules tobe coordinated. This results in a low absorption increment.

3.6. Effect of temperature on the thickness of the multi-layer thin films

Two sets of solutions were used in this experiment tounderstand the nature of attractive forces between the

Fig. 9. Absorbance per bilayer according to the concentrations of solu-tions.

Fig. 10. Absorbance per bilayer according to the temperature of solutions.

Eu3q ions and polymeric carboxalates during multilayerdeposition employing alternating deposition process. Eachset was designed in order to vary the charge density alongthe polymer backbone by varying the pH of the solutions.First set of multilayer films were fabricated from pH 5.5Eu3q solution and pH 7.5 PTAA solution, while thesecond set of multilayers were prepared from pH 4.5 Eu3q

solution and pH 4.5 PTAA solution. Results obtained fromthese experiments are shown as absorbance per bilayer inFig. 10.

The polymer has a higher charge density in the firstcase and at higher charge densities of the polymer chain,the effect of temperature was negligible. Absorbance val-ues per layer of fabricated films at 26, 50 and 658C aresimilar. At high pH, there is little variation in chainconformation as a function of temperature since electro-static repulsion of charged COOy groups plays the domi-nant role. Therefore, change of temperature has no signifi-cant influence on the thickness of deposited layers.

In the lower charge density set, due to relatively fewernumbers of charges along the polymer chain, the mainchain conformation will be strongly affected by tempera-ture. As temperature of the system increases, the macro-molecular coils contract and more hoops and loops areexpected to form while coordinate bonds with Eu3q areformed. Consequently, thickness and optical density of thelayers increase. Rate of formation of coordination bondingis also accelerated by external energy, further adding to theadsorption of polymer molecules during the fixed time ofthe experiment as temperature increases. As a result, ab-sorbance per bilayer is larger at higher temperatures.

4. Conclusion

The fabrication of multilayer thin films via layer-by-layer complexation between lanthanide ions and macro-molecular ligands is reported for the first time. Good

( )J. Kim et al.rMaterials Science and Engineering C 7 1999 11–1818

Žoptical quality multilayer films employing poly 3-thiophene. 3qacetic acid and Eu were assembled using this technique

taking advantage of the bidentate ligands in the polymermolecules. The formation of the multilayer complex wasconfirmed using infrared spectroscopy. Several importantprocess parameters were studied. The difference of absorp-tion increase, consequently, thickness increase in variousprocess conditions could be explained by conformationalchanges of the polymer molecules.

This novel technique can be quite versatile and ex-tended to a variety of polymers containing organic ligandsin their side or main chain and to various metal ions. Itmay enable us not only to combine unique polymer proper-ties to metal complex thin films, but to prepare intermedi-ate thin films for further applications through the incorpo-ration of functional groups into ligand-containing polymermolecules. We are, especially, interested in the applicationof this technique to the fabrication of polymeric light

Ž . w xemitting diodes PLEDs 32 . The fabrication of bright,stable, and color tunable multilayer thin PLEDs may beavailable by employing appropriate pairs of metal andpolymer-bound ligands under optimum condition.

Acknowledgements

The authors thank Drs. Dong-Yu Kim and SrinivasanBalasubramanian for helpful discussions and Steve Kum-iega and Arvind Viswanthan for GPC measurements. J.K.acknowledges support from Samsung Central ResearchInstitute of Chemical Technology. KGC acknowledgesfinancial support from NSF through SBIR program. Sup-port from ONR MURI program is gratefully acknowl-edged.

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