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Self-Assembled Nanostructured Materials

J a nos H . Fendler

Departm ent of Chem istr y and t he W. M . Keck Center for M olecular El ectronics,Syracuse U ni versit y, Syracuse, N ew York 13244-4100

Received F ebru ary 9, 1996. Revised M anuscript Received M ay 17, 1996X

The w et colloid chemica l constru ction of nanosizedor nanostructured materials(i.e., thosein t h e 1-100 nm range) has been inspired by b i o mi n e ra l i za ti o n (the in vivo forma tion ofinorga nic crysta ls a nd/or a morphous part icles in biological systems) a nd h i e ra rch i ca l l y organized self-assembly (sponta neous stepwise a ssembly of functiona l units). Subsequentt o a su mma ry of t h e mo le cu la r level o rg a n iza t ion of su rfa ct a n t mon ola ye rs, La n g mu ir -B lodgett an d self-assembled films emphasis is placed on t he colloid chemistry underlyingthe self-assembly of na nostructured ma terials. Specifically, deta ils ar e provided on (i) thepreparation and stabilization of nanoparticle dispersions, (ii) the adsorption and desorptionof nanoparticles onto solid surfaces, and (iii) the nanoreactors provided by polar liquidsselectively a dsorbed onto solid surfa ces from bina ry polar -apolar liquid mixtures. The wet colloid chemical prepara tion of na nostructured ma terials in our laborat ories is illustrat edby the layer-by-layer self-assembly of (i) polyelectrolyte -semiconductor nanoparticle, (ii)polyelectrolyte-clay platelets-semiconductor na nopart icle, a nd (iii) polyelectr olyte-graphiteoxide (an d reduced gra phite oxide) ultra thin films.

1. Introduction

1.1. Genesis of Wet Colloid Chemical Self-As-sembly. N anosized or n a n o str u ctu r ed ma te r i a l s hav edimensions, as their name implies, in the 1-100 nmr a n ge. I t is a t t h is s iz e r eg im e t h a t m a n y r e cen tadv anc es hav e b een m ade i n b iol ogy, physi cs , andchemistry. Nanostructured materials are synthesizedin nature by a process known as biomineralization (thein vivo forma tion of inorganic cryst a ls a nd/or a morphouspar ticles in biologica l syst ems).1-3 B iomineralizat ion isbelieved to be mediated by proteins acting a s templat es

at m emb rane i nt erfac es . Most nat ura l ly occurringnanostructured materials are hierarchically organizedm a t e r i a l s .4 This term implies a n organizat ion of ma-teria ls in discrete steps, ra nging from the a tomic to thema croscopic sca le, for optima l overa ll performa nce. Thestructure of abalone shell is a good i l lustrat ion of abricks-an d-morta l hierar chial a rchitecture. Ca lciumcarbona te (a ra gonite) bricks a re bonded by soluble acidicproteins and polysaccharides (i.e., the mortal) which arepresent in the aba lone wa ll. This bricks-a nd-morta lconstruction imparts a great deal of structural strengthto the shell while reducing cracking.

The physics of nanosized materials are described in

terms of size quant izat ion. Size quantiza t ion occurs a tdimensions comparable to the length of the de Broglieelectron, the wa velength of phonons, and t he mean freepat h of exci t ons .5-7 Electron-hole confinement inna nosized, spherical semiconductor pa rticles results inthree-dimensional size quantization, i .e. , in the forma-tion of qua ntum d ots, quant um crysta llites, or zero-dimensiona l excitons. Two-dimensiona l confinementof the cha rge carriers results in q uant um w ell wires,qua nt um w ires, or one-dimensional excitons (i.e., theexciton is provided wit h only one-dimensiona l mobility ).Final ly, in one-dimensiona l size q uant izat ion, the ex-

ci ton is permitted to move in two-dimensions (two-dimensional excitons) with the resultant formation ofquant um w ell s . Ba ndgap engineeri ng b y s i ze anddimension quantizat ion is important since i t leads tomechanical, chemical, a nd electrical, optical, ma gnetic,electrooptical , and magnetooptical properties t ha t ar esubsta ntia lly different from those observed for t he bulkm at eri a l .5-7 For example, quantum dots can be tunedby changing their diameters to absorb and emit l ightat any desi red w av elengt h. Thi s propert y m akes i tfeasi b le t o const ruct a f inely t unab l e and eff ici entsemiconductor la ser.8 Semiconductor q uant um dotscan, in pr inciple, be designed to capt ure a single electronat a t i m e.9,10 Exploita tion of this concept w ill open thedoor for the manufacturing of ul trahigh-density inte-grat ed circuits a nd informa tion storage devices w hichare b ased on t he presenc e or ab senc e of i ndi v i dualelectrons.

Colloid chemists have tradi t ional ly deal t with dis-persed particles, usually in the micrometer-to-submi-crom et er range. More recent ly t heir a t t ent ion hasfocused upon na nometer regime. The constr uction ofnanost ruct ured m at eri a l s b y w et col loi d chem icalmethods has been inspired by mother n at ures organi-zational abi l i ty.11 The term self-assembly implies the

sponta neous a dsorption of molecules or of nan opa rt icles,in a m onopart icularly th ick layer, onto a substra te. Self-assembl ed m ult i l ayer fi l msare formed by the adsorptionof subsequent monolay ers of molecules or na nopart icles.The evolution of self-assembled layers of molecularlynanostructured materials can be traced, at least con-ceptually, from simple surfactant monolayers and mul-ti layers, through their more complex part iculat e a na -logues to the sel f-assembly of simple molecules andlar ger pa rticulat es. Only an extremely brief outline ofth is evolution is provided in the present section. Ava il-able books and review articles on surfactant monolayersand m ult i layers , m ono- and m ult i part i c ul at e Lang-muir -B lodgett (LB ) fi lms, sel f-assembled surfactan tsX Abstract published in Advance ACS Abstracts, J uly 15, 1996.

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and self-assembled polymers and part iculat e systemsshould be consulted for greater details.12-14

Molecular level organization of surfactants has beeninvestigated for more tha n a century. Techniques forhandling monolayers of surfactants were developed byAgnes P ockels in 1891,15 a n d I r v i n g L a n g m u i r a n d

K at herine B l odget t dem onst rat ed, i n t he 1930s, t hatcompressed monolayers of surfactan ts could be t ra ns-ferred, layer-by-layer, onto sol id substrat es to formul t rat hi n s t ab l e f i l m s, w hi c h are now referred t o asLangmuir -B lodgett films.16 I t wa s Hans Kuhn a nd co-workers who showed, in the early 1970s, that LB filmsca n h a v e p rop er t ie s t h a t a r e n ot e xh ib it e d b y t h eindividual monolayers.17 In part icular, these workersconstructed energy-transfer systems, based on function-alized cyanine dyes, which mimicked the spectral sen-sitization of the photographic process.18,19

The r ecognition of the potent ial of molecular ly orga -nized films a s a dvanced mat erials has fueled the rena is-sa nce of this a rea in th e 1980s. An increasing number

of r esear ch groups focused th eir at tention to t he con-struction, characterization, and potential utilization ofm onol ayers and LB f il m s. Fresh i nsight s ha v e b eengained by multidisciplinar y a pproa ches a nd by employ-ing new t heories and experimental t echniques.13 Thesedevelopments have led, in turn, to the construction ofself-a ssembling monolayers (SAMs). SAMs obviat e theneed for surfactant compression in a Langmuir balancebut r equire strong chemical interactions t o an chor theappropriat el y funct i onal i zed surfac t ant t o t he w ell -cleaned substra te surface. The forma tion of a S AM wa sfirst r eported by Sa giv, who immersed scrupulouslyclean substrates into a di lute solution of n-octadecyl-trichlorosi lane (OTS) in an organic solvent . 20 Self-

assembly involved chemisorption of OTS an d subse-quent hydrolysis of the Si-C l b onds a t t he sub st rat esurfa ce to form a Si -OSi network. Alterna tively, thiol-an d disulfide-functiona lized surfa ctant s ha ve been self-assembled onto coinage metal surfaces, primari ly bychemisorption.21 Self-a ssembling of multilayers of sur-factants was accomplished by sequential steps of (i)adsorbing t hiol (or dithiol) surfactant s, termina lly func-tionalized either prior or subsequent t o the ads orption,onto th e coinag e meta l sur faces, (ii) chemica lly linkingthe second layer of functional ized surfactants to theanchored monolayer, a nd (i ii) repeat ing step i i by thedesired number of t imes. Self-assembly of complexsuperlattices have been recently reported.22

The construction of monolayers and self-assembledfi lms ha s not been l imited to surfacta nts. The meth-odology has been extended to larger molecules and,indeed to supramolecular a ssemblies. Thus, La ngmuirfilms ha ve been formed from fullerenes, polyelectrolytes,polystyrene microspheres, si lylated glass beads, and

surfactan t-coat ed metal l ic, semiconductor, magnetic,and ferroelectric nanoparticles.23 The self-a ssembly ofsupramolecular systems can be tra ced to the reportedadsorption of negatively charged col loidal si lica, orpolystyrene lat ex part icles, onto a cat ionic surfactan tmodified surface.24 The adsorbed anionic col loidalpart icles have been shown, in turn, t o adsorb a mono-particulate layer of positively charged colloids which,then adsorbed nega tively charged col loidal part iclesfrom their dispersions. Repeated a dsorptions, rinsing,and drying of negatively and positively charged colloidalparticles resulted in the buildup of films, consisting ofdesired number of monoparticulate layers.24 Subse-quent ly, oppositely char ged polyelectrolytes25,26 a nd

polyelectrolytes and clay plat elets27

hav e b een sel f-assembled onto solid substrates.

1.2. LB and Self-Assembled Films Comparedand Contrasted. I t i s i nst ruct i ve t o c om pare a ndcontrast nanostructured fi lms prepared by the Lang-muir -Bl odget t t echni que and t hose form ed b y sel f-a ssembly. Although both methods yields molecula rly(or supra molecularly) organ ized ultra thin films, the tw o-dimensional compression of monolayers (or monopar-t i c ul at e f i l m s) on t he w at er surfac e perm i t a b et t erorganizational control of the LB films than that obtain-able for the sponta neously self-assembled fi lms (seeTa ble 1). Self-a ssembly is much more versa tile, how-ever. A grea ter va riety of molecules an d supram olecular

assemblies can be self-assembled than compressed tosta ble monolay ers. Furthermore, no special f i lm bal-ance is required for self-assembly; indeed the methodhas been referred to as a molecular beaker epitaxy. 28

Appropriately selected oppositely charged materials areheld together by str ong ionic bonds an d th us form long-lasting and stable self-assembled films which are oftenimpervious to solvents. In contra st , LB fi lms a re lesssta ble since they are ma inta ined by wea k van der Wa alsintera ctions. Some imperfections remain in both ty pesof f i lm s. They are caused b y i m purit i es a nd b y t hepresence of boundaries between the different domainsof t he sub st rat e and of t he f i l m form i ng m at eri a l s .P ropert ies of LB and sel f-assem bl ed f il m s a re sum -

Table 1. Properties of Langmuir-Blodgett and Self-Assembled Films

LB film self-a ssem bled film

fi lm-forming materials subphase insoluble (usually surface active)materials which can be compressed two-dimensionally

any cha rged material which has the a ppropriateadsorption -desorption properties

pr epa r a t i on m et hod m onol a yers , com pr es s ed i n t he f il m ba l a nce,are tra nsferred by repeated substrat e dippingan d w ithdra wa l (X-, Y-, or Z-type d eposition)

(i) prepared su bstra te immersed in dispersionconta ining charged materials for a t imeoptimized for a dsorption, (ii) with dra wnsubstrate r insed a nd w ashed, (ii i) i and i irepeated for oppositely char ged ma terials forthe desired num ber of times

t h i ck n es s, a r e a d et e rm in e d t h e t h i ck n es s of t h e m on ol a y erand the number of layers deposited (>300 lay ersreported), area is limited by t he deposition equipment

determined th e size (usua lly diamet er) ofthe mat erial and the number of layersdeposited (50 layers reported), nodeposition equipment is needed th usthere are few practical l imitat ion to a rea

forces w ea k va n der Wa als int era ct ions st rong ionic or coordina tive bondso rd er, s t a bi li t y good l a yer -t o -l a yer s epa r a t i on a nd t w o -d i m ens i ona l

order, pinholes a nd imperfections (decrease w iththicker films), limited long-term mechanica lstabil i ty in air , w at er , and some polar solvents

some intermingling of layers, pinholes andimperfections (decrease wit h t hicker films),good long term mechanical stability in air,water and some polar solvents

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ma rized in Ta ble 1. Films can, of course, be preparedby alternating the LB technique with self-assembly fort he deposi t i on of any s i ngl e l ayers m at eri a l s a t anydesired order.29

1.3. Aims, Scopes, and Limitations. The under-sta nding of colloid an d surfa ce chemistry is a n essentialrequirement for the construction of sel f-assemblednanost ruct ured m at eria l s . The pri mary a i m of t hi sreview is, therefore, to survey th e col loid and surfacechemistry of stable monodisperse nanoparticles, their

self-assembly onto solid substrates, and the propertiesof the liquid a dsorption lay er developed at the interfa ceof the self-as sembled na nopart icles in liquid mixtur es.Detai ls wil l be provided on the preparation and char-ac t eriz at i on of ult ra t hi n f il m s form ed b y t he sel f-assem b ly of a l t ernat i ng l ayers of pol yel ec t rol yt e-semiconductor nanoparticles,30 polyelectrolyte -exfoliatedcla y plat elet -semiconductor na nopart icles,30 and poly-electrolyte-gra phite plat elets.31 Emphasis will be placedon results obta ined in our own laboratories. Relatedself-assembly of nanostructured materials from inor-ganic solids, part icularly zirconium phosphona tes,32

lipid tubules,33 on solid substrates or on silicon,34,35 orbiosurfaces 36 a s w ell as t he use of self-a ssembled films

for molecular recognition,37 will not be d iscussed here.The reader shoul d b e aw are of t he ext ensi v e and

pertinent work published by researchers in marginallydifferent disciplines. Thus, w hile electr odeposition can-not st rictly be called self-a ssembly, man y of the r ecentadva nces in constructing chemical ly modified elec-trodes38 and depositing nanometer-scale layered struc-tures by pulsing the applied potential during deposi-tion 39 a r e cl ea r l y r e le va n t t o t h e p r es en t r e vi ew .Similarly, chemists should keep in mind the spectacularaccomplishments of solid-state bandgap engineering.

2. Colloid and Surface Chemistry ofSelf-Assembly

2.1. Preparation and Stability of NanoparticleDispersions. Developing reproducible preparations ofstable dispersions containing monodispersed nanopar-ticles in desired compositions, sizes, and morphologiesi s t he b asi c requirem ent for sel f-assem bl y . Equa l lyimportant is the long-term stabi l izat ion of the nano-par ticle dispersions.

Meta llic, semiconducting, ma gnetic, a nd ferroelectricnan opar ticles ha ve been prepar ed by a large va riety ofdifferent methods including h ydrolysis an d r eduction ofappropriate precursors, therma l decomposit ion, pho-tolysis, a nd ra diolysis.14 New methods of prepara tionare being continuously discovered. This a ct ivi ty is

fuel ed b y t he fundam ent al i m port anc e a nd pot ent ia lappli cab i li t y of nanost ruct ured m at eri a l s. Unfort u-nately, the mechanism of formation or monodispersenan opar ticles is not entirely understood. Ca reful at-tention to purity, to the concentrations of the reagents,t o t h e r a t e a n d o r d e r o f t h e i r a d d i t i o n , a n d t o t h etemperature is the minimum requirement for the re-producible formation of nanoparticles in desired com-position and size distribution.

van der Waals attraction between unprotected nano-part i cl es resul t s i n t heir coagul at i on and ult i m at esettling out of the particles from solution. Nan opar ticledispersions can be st abilized electrosta tically a nd st eri-cally.40 The electrostat ic st abi l izat ion involves the

formation of an electrical double layer by the additionof counterions (citra te and chloride ions for gold andhexametaphosphate ion for sulfide semiconductor nano-part icles, for example). Agglomeration of the highlycharged nanoparticles is prevented then by the screen-ing of the Columbic repulsions which decays exponen-tially with increasing interpa rticle dista nces. Displace-ment of the adsorbed anions by a more strongly boundneutra l molecule or cat ion w ill reintroduce the va n derWaa ls at t ract ion and t hus i t w i ll result in coagula t ion.

C oat i ng nanopart i c les b y pol ym ers or surfact ant ssterically sta bilizes them. Steric stabilizat ion origina tesin entropic a nd osmotic effects. The entr opy effect ca nbe understood in t erms of the required reorga nizat ionof t he surfac t ant coat i ng around t he nanopart i c les i fthey ar e to be packed t ighter. Decreasing the distancebetween na noparticles w ould force the sta bilizing poly-mers or surfa ctants into a sma ller an d more restrictedspacesa process which would decrease the entropy ofthe syst em. Decrease of the entr opy renders, of course,a closer approach of the nanoparticles to be thermody-na mically unfa vorable. The osmotic effect sta bilizes thenanoparticles by increased solvation of the polymerchains wh ich a re present in higher concentra tion in the

interpa rticle regions. Size-qua ntized nanopart icles ha vebeen recently stabilized by the covalent or coordinateaddition of phosphine or thiol ligands.41

I t should be mentioned that the monodispersi ty ofnanoparticles have been substantially increased by gelfiltration or size selected precipitation.14

2.2. Adsorption and Desorption of Nanopar-ticles onto Solid Surfaces. Self-assembly of nano-particles to the oppositely charged substrate surface isgoverned by a del icate balance of the adsorption anddesorption equ ilibria. The efficient a dsorption of one(and only one) monoparticulate layer of nanoparticlesonto the oppositely charged substrate surface is the

objective of the immersion st ep. P reventing t he de-sorption of the n a nopart icles during t he rinsing processis of equal importance. The optimization of the sel f-assem bl y i n t erm s of m axi m iz ing t he adsorpt i on ofnanoparticles from their dispersions and minimizingtheir desorption on rinsing requires the judicious selec-tion of sta bilizer(s) a nd t he careful contr ol of the kineticsof t he process . The sel f-assem bl y of C dS and P b Snanoparticles was found to be most efficient , for ex-ample, if the semiconductor particles were coated by a1:3 mixture of thiolactic acid a nd ethyl mercapta ne.30

I t h a s t o b e a d m it t e d t h a t t h is com pos it i on of t h ecapping a gent w as arr i v ed a t b y m a ny t r i a l a nd errorexperiments.

2.3. Nanoreactors Provided by Polar LiquidsSelectively Adsorbed onto Solid Surfaces fromBinary Polar-Apolar L iquid Mixtures. The struc-ture and composition of porous solids profoundly influ-e nce t h e a d s or p t ion of l iq u id s a t t h e ir i nt e r fa c es .Generally, the composition of the adsorption layer at asolid int erface, x1s, i s d if fe re nt t h a n t h a t w ou ld b eexpected ba sed on the equilibrium distr ibution of a giventw o component liquid (x1 a nd x2) in the absence of anyi m mersed or di spersed sol id . St a t i ng i t di fferent ly ,solids immersed or dispersed in multicomponent sol-v ent s m edi at e a sol vent sort i ng i n t heir i m mediat ev ici nit y . Adv ant age can, t herefore, b e t a ken of t headsorption layer to provide a nanophase reactor within

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which size controlled particles can be in situ grown.In bina ry mixtur es the preferentia l adsorption of one

of the two liquids (say, 1) in the interfacial adsorptionlayer is expressed by 42,43

w here n1()n is the adsorption excess a mount of a dsor-bent (expressed as per m grams of adsorbent), no is thenumber of moles of th e liquid mixture in the d ispersedsystem, n1s + n2s ) ns i s t he m a t eria l cont ent of t headsorption layer a nd x1s ) n1s/ns (or x2s ) n2s/ns) is thecomposit ion of the adsorption layer (x1 i s t he m ol efra ction of component 1). P referent ial a dsorption of oneof t he tw o l iquids (say, 1) at the sol id pa rt icle surfaceca n occu r t o t h e e xt e nt t h a t t h e a d s or pt i on l a y erpractically consist of only component 1 (i .e. , x1s . x2s

a nd x1s = 1) even t hough the preferred l iquid is onlythe minor component in the equilibrium mixture (i.e. ,x1 , 1, x2 = 1, and n1(n) = n1s).

The v ol ume of t he adsorpt i on l ayer (Vs) ca n becalculated by the adsorption space filling model:43

w here as is the a vai lable surface area (see Figure 1), tis the thickness of the a dsorption layer, nis is the amountof ith component, and Vm,i is the molar volumes of thead sorbed ith component a t the interfa ce. If component1 is a dsorbed a t the interfa ce (i .e. , x1s . x1), then the

volume of the adsorption layer can be expressed by

Relat ively simple a dsorption excess isotherm mea-surem ent s c an b e used for t he det erm i nat i on of t hevalues for the adsorption capaci ty (i .e. , n1s, n2s, t h emat erial content of the a dsorption layer), the a dsorptionvolume (Vs) and the composition of th e a dsorption la yer(x1s).

Generation of size-quantized semiconductor nanopar-ticles a t dispersed orga noclay complexes43 and l ayeredsilicates44 i llustrat e the use of the na nophase r eactorsprovided by adsorbed bina ry liquids. The bina ry liquid

pairs, etha nol(1)-cyclohexane(2) and methanol(1)-cy -clohexane(2), were selected since the polar componentof the liquid mixture (1) preferentially adsorbed at thesolid interface hence its mole fraction in the bulk (x1)was negligible (i.e., x1s . x1 a nd x2s , x2); an d since thesemiconductor precursors (Cd2+ and Zn 2+) were highlysoluble in the l iquid w hich preferential ly adsorbed a tthe interface (methanol and ethanol) but was insolublein the bulk pha se (predomina ntly cyclohexan e). Theseconditions effectively limited the nucleation and growth

of the semiconductors to the na nophase rea ctor providedb y t h e a d s or pt i on l a y er a t t h e s ol id i nt e rf a ce . B yvarying the mole fraction of the polar liquid (1) it wasfound possible to control the volume of the nanophaserea ctor an d hence the size of the semiconductor part iclesgrown t herein. Extension of these principles t o self-assembled films is the subject of our current researchactivities.31

3. Layer-by-Layer Self-Assembly ofPolyelectrolyte-Inorganic Nanoparticle

Sandwich Films

3.1. Alternating Layers of Polyelectrolyte-

Semiconductor Nanoparticles. The layer-by-layerself-assembly of polyelectrolyte-semiconductor nano-par ticles onto substra tes is deceptively simple. A well-cl eaned subst rat e i s pri med b y adsorbi ng a l ayer ofsurfac t ant or pol yel ect rolyt e ont o i t s surfac e. Theprimed substrate is then immersed into a dilute aqueoussolution of a cationic polyelectrolyte, for a time opti-mized for a dsorption of a monolayer, rinsed, a nd dried.The next step is the immersion of the polyelectrolytemonolay er covered substra te into a dilute dispersion ofsurfacta nt-coat ed negatively charged semiconductornanoparticles, also for a time optimized for adsorptionof a monoparticulat e layer, rinsing, a nd drying. Theseoperations complete the self-assembly of a polyelectro-

lyte monolayersmonopa rt icula te la yer of semiconductorna nopar ticle sandw ich onto the primed substrat e. Sub-sequent san dwich units are deposited ana logously. Themethod is i l lustrated by the sel f-assembly of a poly-(diallylmethylammonium chloride), P, CdS nanoparticlefilm onto different substr a tes, S (gold, silver, platinum ,qua rtz sl ide, highly ordered pyrolytic graphite, HOPG ,mica, a nd Teflon).30 The procedure involved the follow-ing st eps: (i) immersion of th e well clean ed (soa ked ina concentrated sulfuric acid solution of Nocromix for0.5 -3 hours, repeatedly rinsed by ample amounts ofdeionized wa ter , an d dr ied) S in to a 1.0%, w/v, a queoussolution of P, kept at pH ) 8.5 (no buffer), for 15 min;(i i) rinsing with a stream of deionized dist i lled wa ter,

kept a t pH ) 8.5; (iii) immersion int o an a queous CdSnanoparticle dispersion, kept at pH ) 9-10, for 24 h;an d (iv) wa shing with a st ream of deionized wa ter, kepta t p H ) 8.5. Ea ch washing wa s fol lowed with dryingby a stream of N 2 for 30 s. Thus , performin g steps i-ivled to the self-assembly of a one layer of polycation -one l ayer of C dS nanopart i c l e sandw i c h uni t on t hesubstrate, for which we adopted the S -(P /Cd S) short -han d notat ion. Subsequent P -CdS na nopar ticle sand-wich units were self-assembled by repeating steps i -vn t imes to produce fi lms comprised of n number ofsandw i ch unit s , S -(P /C dS )n (see Fi gure 2 for sel f-assembly of an S -(P/Cd S )n film). Similar met hodologies(st eps i-iv) were employed for t he self-a ssembly of many

Figure 1. Schematics of the absorption layer, formed uponthe immersion of a self-assembled na noparticle film in a tw o-component (x1 a nd x2) l iq u id . S ) solid phase of the na nopar-ticles, Vs ) volume of the adsorption layer, t ) thickness ofthe adsorption layer, an d x1s ) mole fra ction of the component1 present in the adsorption layer.

n1(n)

) no(x1s

- x1)/m ) n1s

- (n1s

+ n2s)x1 )

ns(x1

s- x1) (1)

Vs

) ast ) n1

sVm,1 + n2

sVm,2 + n3Vm,3 + .. .

(i ) 1, 2, 3, ...) (2)

Vs

) n1sVm,1 (3)


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other polyelectrolyte -semiconductor nanoparticle sand-wich films.

Absorption a nd emission spectrophotometry, sur faceplasmon spectroscopy, X-ray diffractometry, scanningforce microscopy, a nd t ra nsmiss ion electron microscopyhave been used for monitoring the sel f-assembly andchara cterizing the str uctures of self-a ssembled films.30

Optimization of the self-assembly of the initial layerof P is of part icular importance. The thickness of the

initia l layer of P, self-a ssembled on a gold electr ode, wa sfound t o b e dependent on t he concent rat i on of t hepolyelectrolyte a nd on the immersion t ime. This isi l lustrated by the surface plasmon spectroscopy of aseries of deposit ions in di fferent concentrat ions of P(Figure 3). Furthermore, soaking in an aq ueous solu-t i on for 4 h di d not rem ov e P , as ev i denc ed b y t heunaltered surface plasmon spectrum. AFM ima ges ofthe polyelectrolyte film revealed featureless wavelikes t r uct u r es w i t h h ei gh t v a r ia t i on s of a b ou t 0. 2 n m(Figure 4). Apparent ly, multiple va n der Waa ls forcesmaintain P on a variety of substrate surfaces, even int he ab sence of cov al ent b onding. Assumi ng P t o b ecompletely str etched on th e surfa ce of S, t he mean a rea

occupied by a positive charge on P can be estimated tobe 10 2. I t is this surface charge and relat ively highcharge density which provide the electrostatic drivingforce for t he self-assembly of the negatively charged,surfactant-stabilized CdS, PbS, and TiO2 nanoparticlesonto the positive surface of P . The a bsorption spectraof the semiconductor na nopart icle dispersions in a que-ous solutions are retained in the self-assembled films(see Figure 5 for the spectra of CdS nanoparticles intheir dispersions and in the self-assembled S -(P/C dS )20

film). Fur therm ore, the a bsorption edges (ca. 430 nmfor CdS and ca. 500 nm for PbS) correspond to meanparticle diameters of 35 and 100 .

The AFM images of sel f-assembled CdS part iclesrevealed the formation of a well-packed monoparticlelay er (Figur e 6). The successful self-a ssembly of a la rgenum b er of repeat i ng sandw i c h uni t s of S -(P /C dS )n,S -(P/P bS )n, and S -(P /TiO2)n wa s monitored by absorp-tion an d emission spectroscopic measurements. Forexample, the observed good linearities in the plots ofabsorbances vs n in S -(P /Cd S )n and S -(P/P bS )n indicatet he uni form i t y of t he sandw i c h uni t s t hat w ere sel f-assem bl ed (see i nsert i n Fi gure 5). Act i vat ed C dSnanoparticle dispersions showed a strong fluorescence

w i t h a n e m i s s i o n m a x i m u m a t 4 9 0 n m , w h i c h w a sa t t r i b u t e d t o t h e 1 s e-1s h exitonic tra nsi t ion. Verysimilar emission spectra were obtained for the self-a ssembled S -(P /C dS )20 films (Figure 6). Fur therm ore,t he rel a t i v e i nt ensi t y of t he em i ssi on w as found t oincrease l inearly with increasing P /CdS units, sel f-assembled on the substra te, up to n ) 15 (see insert inFigu re 6). The good linear ity of the fluorescence int en-sit y v s n plot i ndi cat es t he uni form i t y of t he sel f-assem bl ed f il m . Thi s w as a l so c onf irm ed b y X -raydiffraction of a S -(P /C dS )20 sa mple. The observed 2) 1.70 peak (not shown) corresponds to a 6.5 ( 0.8 nmrepeat ing unit in the (P /CdS)20 f il m . Si nce t he C dSparticle diameters were determined to be 4.0 ( 0.5 nm

by dyna mic l ight scat tering, th e di fference (6.5 ( 0.8nm - 4.0 ( 0.5 nm ) 2.5 ( 0.5 nm) a grees well withtha t determined for the thickness of a polyelectrolytelayer by surface plasmon spectroscopy (2.0 ( 0.5 nm,vide supra).

3.2. AlternatingLayers of Polyelectrolyte-ClayPlatelets-Semiconductor-Nanoparticles. Smecticclays, such a s Na +-montmorillonite, have been exfoli-a t e d i nt o (10 ( 2)--thick lay ered silica tes, oftenreferred to as clay organocomplexes, by ion-exchangereaction with cationic surfactants.30,31 The high lat eralb ond s t rengt h and aspec t ra t i os hav e rendered c l ayorganocomplexes to be eminently suita ble mat erials for

na noconstruction. Indeed, the nanocomposites prepa redby m ixing polymers a nd clay organocomplexes ha dsuperior mechanical properties.45 However, a layer-by-layer assembly of nanocomposites is required for thepreparation of ultrathin films for advanced optical andelectrooptical device construction. This ha s promptedthe sprea ding of clay organ ocomplexes on wa ter sur facesa n d t h ei r s u bs eq u en t t r a n s fe r t o s u b st r a t e s b y t h eLangmuir -B l od g et t t e ch n iq u e a s w e ll a s t h e s el f-a ssembly of polyelectrolyte-clay platelet films.46

Alternating layers of polyelectrolyte (P)-clay platelet(M) sandw ich fi lms ha ve sel f-assembled in a mann erana l ogous t o t hat show n for S -(P /C dS )n (Figure 2).30

Significant ly, the thickness of a given multila yer S -(P /

Figure 2. Schemat ics of the self-assembly of a n S -(P/Cd S )nfilm.

Figure 3. Su r fac e p lasmon sp ect r oscopic c ur ve s for t hesequential deposit ion of self-assembled layers: 1 ) bare Au

film, 2 ) P on gold DP l ) 18 , 3 ) P /G O on gold D G r ) 25 ,4 ) (P/G O)P on gold DP l ) 16 , 5 ) (P /G O)2 on gold DG r ) 20, 5 ) (P /G O)2P o n g o l d D P l ) 14 . F it t ing p ar ame t e r s :polymer ) 2.25, G O ) 2.24 for all layers.


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M)n f i lm was found to depend on the potential whichwa s a pplied during th e deposition(s) of M. This behav-i or w a s est ab li shed b y elect rochem ical and surfaceplasmon spectroscopic meas urements. In t he electro-chemi cal m easurem ent s t he w orki ng elect rode w a sprepared by the sequential self-assembly of the requirednum b er (n) of (P /M) sandw i ch uni t s on a pla t i numsubstra te (S). AgCl wa s used a s the r eference electrode.Cyclic voltammograms of 10 mM K 3Fe(CN)6 in aq ueous

solutions provided useful information on the propertiesof the S(P/M)n f i lm self-assembled onto t he workingelectrode. Oxidat ions a nd r eductions occurred a t themetal electrode interface as a result of the di ffusion-contr olled penetra tion of the Fe(CN)63- an d Fe(CN)64-

ions through t he sel f-assembled fi lm. Increasing cur-rent drops upon the deposition of successive layers of

Figure 4. AFM ima ges of a la yer P (a), P/P bS (b), P /CdS (c), and P /TiO2 (d) on mica.

Figure 5. Absorption spectra of CdS nanoparticles in theirdispersions (1) and in the self-assembled S -(P /Cd S )20 film.Insert shows a plot of absorbances at 244 nm vs the numberof P\CdS sandwich unit self-assembled.

Figure 6. Emission spectra of CdS in their dispersions (1)and in the self-assembled S -(P/C dS )20 f i lm. I nse r t shows aplot of emission a t 495 nm vs t he number of P\ CdS san dwichunit self-assembled.


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P /M a re clea rly seen in th e cyclic volta mmogra ms. Thethickness of a g iven multilayer film wa s found to depend

on the potential which w as applied during t he deposi-t ion(s) of M. P osi t ive potentials increased the fi lmthickness, while negat ive ones decreased it. Not unex-pectedly, negatively charged M is at tracted to a posi-tively charged electrode more efficiently than it is to anegat i v el y c harged one. Si m il ar b ehav i or has b eennoted in t he electr ophoretic deposition of meta l par ticlesonto conductive substrates.30,31

The effect of an applied potential during the sel f-assembly also manifested i tsel f in the changes of thesurface plasmon spectra of th e self-assembled fi lms.Sel f- assem b l y of M w as a l so ob serv ed b y usi ng t hetopography, internal sensor, and lateral force modes ofthe AFM (Figure 7). The lat era l force mode wa s found

to be least informat ive due to the small height var iat ionalong the surfa ce. The height var iat ions did not exceed5 nm in a 2700 nm interval as seen in the topographicimages (see Figure 8a). The primary source of t hisheight var iat ion is the sta cking of the cla y platelets andadsorption of not-ful ly-exfoliated aggrega tes. Other-wise, the film is rather uniform and the planar orienta-t ion of the plat elets is readily observable. The plana rorientat ion of t he montmoril lonite plat elets in self-assembled films is substantiated by their transmissionelectron micrograms (Figure 8).

X-ray diffraction measurements showed sharp peaksfor S -(P /M)20 a t 2 ) 2.1 which corresponded to planarperiodicities of 4.3 ( 0.5 nm for t he P /M repeat ing unit s.

Since the thickness of the polyelectrolyte layer wasdetermined by surface plasmon spectroscopy to be 2.0( 0.5 nm, each self-absorbed M must conta in a t leasttw o la yers of clay pla telets (each wit h a thickness of 1.0( 0. 1 n m ). Th e s econ d b r oa d p ea k i n t h e X-r a ydiffraction of S -(P /M)20 films corresponds to the basalspacing between t he clay plat elets w ithin one layer.

Versatility is the major advantage of the self-assemblydescribed here. The method a llows t he constr uction ofcomposite films comprising different nanoparticles whichcan be lay ered in a ny desired order. Indeed the sel f-

Figure 7. AFM ima ges of one layer of M self-assembled on mica. a ) topography, b ) int e r nal se nsor , an d c ) lat eral forces.Set point ) -10 nA, scan rate ) 200 nm /s, r esolut ion ) 200 lines.

Figure 8. Transmission electron micrographs of one layer of

M self-assembled onto one layer of polycation-coated substrate.The scale ba r is 200 nm.


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as sembly of a va riety of P /M/semiconductor na nopar-ticles have been reported. 30,31

3.3. Alternating Layers of Polyelectrolyte-Graphite Platelets. G ra phite (G ), l ike si licat es, ha sa h ighly ordered layered structure. Furth ermore, ul-tra th in G/P composites ha ve many potent ially beneficialproperties, including controllable conductivity and mag-netoresistance.47 Since G cannot be dispersed in wat er,

a viable approach to form ultrathin S-

(P /G )n films isto prepare exfoliated graphite oxide (GO) platelets, self-assem bl e t hem ont o P cov ered ( S), and t hen i n s i t ureduce the S -(P /G O)n films. This approach has, in fact,been realized in our laboratories.31

The const ruct ion of a P /G O self-a ssembled sa ndw ichunit involved the following st eps: (i) immersion of S intoa 2.0%(w/v) aq ueous P solution, kept a t pH ) 8.5 (nobuffer), for 15 min, (ii) rinsing w ith a str eam of deionizeddist i l led water, ( i i i) immersion into the aqueous GOdispersion, for 15 min, and (iv) washing with a streamof deioni zed w a t er . Ea ch w a shing w as fol low ed w i t hdryi ng b y a s t ream of N2 for 30 s. Thus, performingsteps i-iv led to the sel f-assembly of a one layer of

polycation -one la yer of graphite oxide platelet sa ndw ichunit on the substrate, S -(P /G O).31 Subsequ ent P /G Osandwich units were self-assembled by repeating stepsi-v n times to produce films comprised of n number ofsandwich units, S -(P /G O)n.

Self-a ssembly of t he su ccessive P /G O la yers onto aqua rtz sl ide w as monitored by absorption spectropho-tometry. The thickness of one sandw ich unit of P/G Owas determined to be 3.8 ( 0.7 nm by surfa ce plasmonspectroscopy.

X-ra y diffra ction spectra of S-(P /G O)30 is i l lustra tedi n Fi gure 9. The b asa l spac ing of 0. 93 ( 0.05 nmagrees w ell w i t h t he v al ue det erm i ned for a l ayer ofgraphite by surface plasmon spectroscopy.

Reduction of GO in the S -(P /G O)n films to graphite,G wa s a ccomplished either chemically or electrochemi-cally. The chemica l reduction involved the immersionof the sel f-assembled fi lm into an aqueous hydrazinehy dr ide (50%, w /v) solut ion f or 1 -24 h or into a 0.10 MHCl solution which contained Zn pellets (i .e. , into anascent hydrogen generator) for 2-3 h. The electro-chemical reduction of GO (in the S -(P /G O)n film, self-assembled on metal or glassy carbon electrodes) was

performed by scanning the potential from -0.5 to -1.5V vs SCE for 20 min.The extent of GO reduction was monitored by X-ray

diffract ion measurements and by absorption spectro-photometr y. The reduction of G O to G w a s accompan iedby a slight diminishing of the interlayer spacing (from45.2 to 43.7 for the h ydra zine reduction an d from 56to 45 for the electrochemica l reduction. The sma llpeak a t 9.04 (corresponding to the int erplatelet ba sa lspacing)does not appear in t he X-ra y diffraction pa tternof the reduced G O. However in reduced G O there is ashoulder at 5.7 (15.5 ) corresponding to the basa lspacing betw een the r educed G O platelets.

Cycl ic vol tammetry of an S -(P /G O)16 film, self-a s-

sembled on meta l or glassy carbon electrodes, revealeda peak positioned at -0.95 ( 0.05 V which correspondto t he r eduction of G O. This process is completelyirreversible a s evidenced by the absence of a peak ont he rev erse scan. Moreov er t hi s peak c an onl y b eob serv ed i n t he f irs t C V sc an. Aft er t hat t he peakdisa ppears ind ica ting t he complete reduction of G O. Thedependence of the peak h eight vs the number of GO/Player s is linear up t o the self-a ssembly of 8-10 sandwichunits.

The lat era l resistivit y of a (P /G O)10 film wa s measuredbetween two 3 mm wide gold stripes evaporated on aglass slide at a d ista nce of 2 mm betw een them (Figure10). Self-a ssembled gra phite oxide films were deposited

on top of them over the w hole surfa ce of the slide. Thea s-deposit ed (P /G O)10 f i lm had an R value of 32 M a tthe 2 mm gap. After reduction by hydrogen, evolvingin situ, the R value dropped to 12 K, which amountsto a 27 000-fold decrea se in the overall resist a nce. Thiscorresponds to t he cha nge in volume conductivity from1.2 104 -1 m -1 to 3.1 107 -1 m -1. Rougheningthe subst ra te sur face (by multiple diamond knife groovesa long the electrode direction) p r i o r t o the deposition ofmulti layers rendered the resist ivi ty decrease (whicha ccompan ied G O reduct ion) to be less pronounced (onlya 370-fold increa se in condu ctivity w a s observed). Thiswas the consequence of imperfect self-assembly on adisturbed surface which resulted in broken fi lms and

Figure9. X-ra y diffraction patt erns an d schematics of S-(P /G O)30. D1 ) r e p e at ing P \GO u nit s , D2 ) b asal sp ac ing , d1a nd d2 dista nces determined by surfa ce plasm on spectroscopy.

Figure 10. Schematics of the arrangements used for resistiv-ity measurements.


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less tha n uniform in-plane conta cts of the GO plat elets.Self-assembled composite (P/G O) films w ere found to

b e rem arkab l y s t a b le i n concent rat ed ac id or b asi csolutions and resisted photodegra da tion. The demon-strated lateral conductivi t ies and the relat ive ease oftransition between nonconductive GO state to conduc-tive G state render these fi lms inherently interestingan d potentially useful a s components of adva nced opti-cal a nd electronic devices.

4. Potential ApplicationsThe relative ease of preparation and the high degree

of versat i l ity render self-assembled ul trat hin fi lmsamena ble to a lar ge variety of diverse applicat ions. Self-as sembled films ca n function as membra nes (bar riers),w i t h cont roll ab le l ev el s of perm eab il it y , for gases ,liquids, covalent molecules, ions, and, indeed, electrons(tunn eling ba rriers). These properties ha ve been ex-ploited for the construction of insulators, passivators,sensors, and modified electrodes. Self-a ssembled na no-structured films are eminently suitable for the construc-tion of devices ba sed on molecula r r ecognition. Mol-ecules or supramolecular assemblies (na nopar ticles)within a self-assembled layer can be aligned spontane-ously, or by changing the temperature, pressure, andpH, or, alternat ively an d a dditiona lly, by th e applicationof a n electric or magn etic field. These cha ra cteristicsperm i t t he form at i on of superlat t i ces w i t h desi redsymmetr y (or dissymmetry) and , hence, the fabricat ionof a host of photonic, electr onic, ma gnetic, an d nonlinearoptical devices. The la yer-by-layer self-as sembly ofinsulat ors (appropriate hydrocar bon surfacta nts, forexample), conductors (meta llic na nopart icles, gra phite,conduct ing polymers, for exa mple), ma gnet ic, ferr oelec-tric, and semiconductors nan opar ticulate f i lms, in an ydesired order, a llows th e constr uction of molecula rly (orsupra molecula rly) resolved heter ostru ctures. These, in

turn, a fford t he optimization of charge separ at ion a ndcarrier transport to the level necessary for the bandgapengineering of qua ntum devices. Control of th e sizesand interparticle distances of the monodispersed nano-par ticles within t he self-a ssembled film ca n be exploitedfor such optica l applica tions as t he const ruction of Br a ggdiffract ion devices for Raman spectrometers and forenhanc i ng t he Ram an i nt ensi t y b y surfac e pl asm oninteractions. 48 Ot her appli cat i ons are l ikel y t o b edeveloped.

The outlook of self-a ssembled na nostructu red ma teri-als is bright. We have a pplied only a sma ll par t of ourchemical know-how to these inherently fa scina ting a ndhighly rewa rding field of investigat ions. I a m confident

tha t t he next few years w ill witness a n explosive growt hof chemical r esear ch focused upon self-a ssembled na no-structured materials.

Acknowledgment. S u pp or t of t h is w or k b y t h eN at i onal Sc ience Foundat i on (U. S. A. ) i s grat efull yacknowledged. True credit is du e, of course, to my co-workers (whose names appear in the cited joint primarypublications) for crea tive, dedicat ed, an d skillful work.

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