Evaporation-Induced Self-Assembly: Functional ... · This presentation reviews progress on understanding so-called evaporation-induced silica/surfactant self-assembly (EISA) as a

IntroductionIt is certainly a great honor to receive

the MRS Medal and to have the opportu-nity to present some of our research to theMRS audience, which includes a greatnumber of good friends and colleagues.

In his MRS Medal lecture, Ivan K.Schuller emphasized that his research in-corporated few biological concepts orcomponents (see article by Schuller in thisissue of MRS Bulletin). In contrast, our re-search has been largely inspired by biol-ogy. From a materials perspective, biologyserves as an excellent model. Biologicalsystems are composed of nanoscale com-ponents and exhibit complex functionalities

that have evolved over millions of years.We look to such natural systems to see ifwe can emulate some of these proven bio-logical designs and incorporate them intorobust engineering materials. Occasionally,we can be so presumptuous as to considerimproving upon nature’s designs bynanostructuring and using a broader paletteof chemicals. We also want to integratethese nanoscale materials into structuresthat operate on larger length scales thatallow electrical, optical, or fluidic address-ability and interrogation. This has directedour research largely toward making thinfilms.

When we look at natural materials, wesee that nature often combines multiplehard and soft materials in hierarchical de-signs to obtain synergistic and often opti-mized properties or combinations ofproperties. This is an interesting aspect ofbiology that we can try to mimic in engi-neering materials.

In constructing such composite struc-tures, a significant challenge is how tocontrollably organize or define multiplematerials on multiple length scales. To ad-dress this challenge, we have combinedsol-gel chemistry with molecular self-assembly in several evaporation-drivenprocessing procedures collectively re-ferred to as evaporation-induced self-assembly (EISA).

Self-Assembly ProceduresSelf-assembly is defined as the sponta-

neous, reversible organization of preexist-ing components by means of non-covalentinteractions such as hydrogen bondingand van der Waals, electrostatic, or hy-drophobic forces.1 Although the forces orenergies of individual interactions are onthe order of kT (k is the Boltzmann constant;T is temperature), the collective behaviorof the huge number of these (possibly di-rectional) interactions drives the reliableand reproducible organization of compo-nents into assemblies that represent aglobal or local equilibrium. During the lastdecade, self-assembly has emerged as apromising approach to defining materialfeatures on length scales smaller than thoseconveniently accessible by lithography. Assuch, molecular self-assembly has remaineda very active area of research.

Molecular self-assembly employs mole-cules that are encoded or programmed by means of surface properties, shape,charge, and so on to organize in specificarrangements without external interven-tion. Our work uses two-sided moleculescalled surfactants (commonly known asdetergents), composed of hydrophobicand hydrophilic parts.

As understood by detergent phase dia-grams (Figure 1), above a critical concen-tration, surfactants added to water (and,optionally, oil) self-assemble into sphericalmicelles that maintain their hydrophilicparts in contact with water and shield thehydrophobic parts (along with any oil)within the micellar interior. At higherconcentrations, micelles further self-organize into periodic structures calledmesophases—true thermodynamically de-fined phases that obey the Gibbs phase rule.

As recognized by Kresge et al.,2 surfac-tant self-assembly conducted in waterwith added hydrophilic components (e.g.,silicic acid or hydrolyzed metal alkoxides)

MRS BULLETIN/SEPTEMBER 2004 631

Evaporation-InducedSelf-Assembly:FunctionalNanostructuresMade Easy

C. Jeffrey Brinker

AbstractThe following article is an edited transcript based on the MRS Medalist presentation

given by C. Jeffrey Brinker (Sandia National Laboratories and the University of NewMexico) on December 3, 2003, at the Materials Research Society Fall Meeting inBoston. Brinker received the Medal for “his pioneering application of principles of sol-gelchemistry to the self-assembly of functional nanoscale materials.” Nature combines hardand soft materials, often in hierarchical architectures, to obtain synergistic, optimizedproperties with proven, complex functionalities. Emulating natural designs in robustengineering materials using efficient processing approaches represents a fundamentalchallenge to materials chemists. This presentation reviews progress on understandingso-called evaporation-induced silica/surfactant self-assembly (EISA) as a simple, generalmeans of preparing porous thin-film nanostructures. Such porous materials are of interestfor membranes, low-dielectric-constant (low-k) insulators, and even ‘”nano-valves” thatopen and close in response to an external stimulus. EISA can also be used tosimultaneously organize hydrophilic and hydrophobic precursors into hybridnanocomposites that are optically or chemically polymerizable, patternable, oradjustable.

Keywords: azobenzene, functional materials, nanocomposites, nanocrystals,nanostructures, self-assembly.

provides a robust synthetic approach totranscribing the nanoscopic order of amesophase into inorganic materials.Through electrostatic and hydrogen-bonding interactions, the hydrophiliccomponents organize around the hydro-philic micellar exteriors and condense toform oxides—essentially, fossilized repli-cas of the mesophases.

Less well recognized is that addedhydrophobic components (e.g., organicmonomers, initiators, etc.) can be se-questered within the hydrophobic micellarinteriors. Surfactant added to water andoil allows the oil to be solubilized withinthe micelles, making it water-soluble. Thisis like our everyday experience of wash-ing dishes using household detergent. Si-multaneous organization of otherwiseimmiscible hydrophilic and hydrophobicprecursors into mesophases by surfactantself-assembly provides an efficient, viableroute to the formation of ordered organic/inorganic nanocomposites.

The first inorganic mesophases wereformed hydrothermally as powders, justlike zeolites. When I first became aware ofthe Mobil work2 on mesoporous silicas, Ithought it would be important to makethese materials as films, because after cal-cination to remove the surfactants, theywould be useful as membranes, sensors, andlow-dielectric-constant (low-k) insulators.From my understanding of evaporation-driven sol-gel thin-film deposition,3 the ideawas to dilute the silica/water/surfactantsystem with ethanol to create a homoge-neous solution and then rely on ethanoland water evaporation during dip-coating(or other coating methods) to progressively

concentrate surfactant and silica in the de-positing film, driving micelle formationand subsequent continuous silica/surfac-tant mesophase self-assembly.

One of the crucial aspects of this proc-ess, in terms of the sol-gel chemistry, is towork under conditions where the conden-sation rate of the hydrophilic silicic acidprecursors [�Si-OH, where � representsbonds to other hydroxyls (OH) or siloxanes(OSi�), and Si-OH is referred to as asilanol group] is minimized. The idea isnot to frustrate the self-assembly processby gelation that would kinetically trap thesystem at an intermediate nonequilibriumstate. We want the structure to self-assemble, then solidify with the additionof a siloxane condensation catalyst or byheating to form the desired mesoporousproduct. Operating at an acidic pH (pH � 2)minimizes the condensation rate of silanolsto form siloxanes, Si–O–Si.3 In addition,hydrogen bonding and electrostatic inter-actions between silanols and hydrophilicsurfactant head groups can further reducethe condensation rate. These combinedfactors maintain the depositing film in afluid state, even beyond the point whereethanol and water are largely evaporated.This allows the deposited film to be self-healing and enables the use of virtuallyany evaporation-driven process (spin-coating, inkjet printing, or aerosol proc-

632 MRS BULLETIN/SEPTEMBER 2004

Evaporation-Induced Self-Assembly: Functional Nanostructures Made Easy

Figure 1. Generic detergent phase diagram (provided by Davis and Scriven, University ofMinnesota.)

Figure 2. Representative transmission electron microscopy images of self-assembled porousand composite nanostructured coatings4,8,17 and particles.7 (a), (b), (d): Porous thin filmsexhibiting ordered three-dimensional networks of nanopores prepared using the cationicsurfactant cetyltrimethylammonium bromide. (c) Three-dimensional nanocrystalline gold/silica cubic array.17(e) Self-assembled nanostructured particle formed by evaporation-inducedself-assembly (EISA) in an aerosol process.7 (f) Plan view of a cubic mesoporous film4 on asilicon substrate (satellite spots result from electron diffraction from the underlying siliconsubstrate). Images in (a) and (d) are courtesy of N. Liu, unpublished; (b) is from Reference 4.

essing) to create ordered nanostructuredfilms, patterns, or particles (Figure 2).4 – 7

Understanding Self-AssemblyTo understand EISA,8 we have taken

advantage of the steady-state nature ofdip-coating. When a substrate is with-drawn vertically from a solution reservoirat a constant rate, a balance of upwardmoving liquid flux and evaporation causesthe film thickness profile to become steadyin the lab frame. This allows the completeEISA process (from the disordered dilutestate near the reservoir surface to the highlyordered dry state) to be spatially resolvedin the coating direction (normal to thereservoir surface).9,10

Figure 3 shows an optical interfer-ence image of a self-assembling silica/surfactant/ethanol/water system duringdip-coating. Above the gravitationalmeniscus, where there is no longer anydownward motion of the entrained liquid,the film thins exclusively due to evapora-tion, allowing us to calculate the evolvingconcentration of the nonvolatile silica andsurfactant components (Figure 3b).

Using spatially resolved grazing inci-dence small-angle x-ray scattering (GISAXS,see Figures 4a–4d), we interrogate the pe-riodic nanometer-scale structure as a func-tion of thickness/composition, ultimatelyallowing the mapping of structure ontothe appropriate ethanol/water/surfactantphase diagram (Figure 4). We find that thestructure forms by an interfacially medi-ated process where, above the critical

micelle concentration, oriented inter-mediate lamellar and correlated micellarstructures (Figures 4b and 4c) form incompositional regions that are expected(on the basis of the ethanol/water/surfac-tant phase diagram, Figure 4a) to beisotropic. We attribute these discrepanciesto the presence of interfaces and the intro-duction of silica, which serves as a secondhydrophilic component. Enhanced solventevaporation at the liquid/vapor interfacepromotes self-assembly there to form incip-ient lamellar silica/surfactant mesostruc-tures that, based on d spacing, largelyexclude ethanol and water. Further evapo-ration results in a correlated structure com-posed of wormlike cylindrical micelles thatorganize and align at the solid and vaporinterfaces in a proto-hexagonal arrange-ment. Finally, an oriented hexagonalmesophase forms (Figure 4d) with its cylin-der axes oriented parallel to the substratesurface. The hexagonal mesophase appearsto be nucleated at the solid/liquid andliquid/vapor interfaces and, based onx-ray reflectivity experiments,11 grows in-ward toward the film interior throughconversion of the correlated wormlikemesostructure (Figure 4e).

Perhaps surprisingly, GISAXS studieson evaporating surfactant/water/ethanolsystems prepared with and withoutoligosilicic acids as secondary hydrophiliccomponents showed that silicic acidpromoted the formation of orderedmesophases. This is explained by the silicicacid oligomers serving as nonvolatile

hydrophilic components. By maintainingfluidity, they avoid kinetic barriers to self-assembly, as experienced in the silica-freesystem. Concerning thermodynamic fac-tors, we found that by considering silicicacid to be equivalent (on a per mole siliconbasis) to water, the first appearance of thehexagonal mesophase upon evaporationoccurred near the isotropic/hexagonalphase boundary of the correspondingsurfactant/water/ethanol system. Thissuggests that under acidic conditions, theoligosilicic acids have similar interactionswith the hydrophilic head groups aswater. To date, the appropriate quater-nary silicic acid/water/ethanol/surfac-tant phase diagrams have not beendetermined experimentally.

Nanocomposite Self-AssemblySelf-assembly of silica/surfactant meso-

phases is normally followed by calcinationor solvent extraction to remove the surfac-tant, creating a mesoporous material. Al-ternatively, we can use EISA to createnanocomposite materials in which organicpolymers or other organic components areuniformly incorporated within a periodicinorganic nanostructure. The nanoscaleorganization of hard and soft materials isof interest for the development of opti-mized mechanical properties similar tothose that occur in natural materials suchas seashells, which are composed of alter-nating layers of crystalline calcium car-bonate and biopolymers.

By using alcohol or tetrahydrofuran(THF) as a cosolvent, homogeneous, com-plex, multicomponent solutions can beprepared containing (in addition to thesilicic acid precursors), organic monomers,cross-linkers, initiators, and couplingagents. During EISA, preferential evapo-ration of alcohol or THF concentrates thesystem in water. Rather than undergoingmacroscopic phase separation, the hydro-phobic components are incorporated intomicelles and mesophases. As shown inFigure 5, this allows hundreds of alter-nating silica/organic layers to be organ-ized in a single step.12 Subsequent in situorganic/inorganic polymerization resultsin a poly(alkyl methacrylate)/silica nano-composite with a covalently bondedpolymer/silica interface. Such structuresbear some relationship to seashells andother natural materials that are simultane-ously hard, tough, and strong.

An alternative nanocomposite self-assembly strategy is to employ polymerizablesurfactants as structure-directing agentsand monomers.13,14 The surfactant organizesthe silicate nanostructure, as describedearlier; then, light, heat, or another treat-ment is used to polymerize the surfactant



Figure 3. (a) Optical interference image of a steady-state film-drying profile used to calculatethe film profile10 (points y, x, w, v, and c represent positions where spatially resolved grazingincidence small-angle x-ray scattering experiments were performed; c also represents theposition of the critical micelle concentration. (b) Calculated film thickness and surfactantconcentration corresponding to the interference image in (a). (Published with permissionfrom the American Chemical Society).10

monomers, uniformly confined within ananometer-scale “reaction vessel.” Anadvantage of this approach is that we ex-pect the monomers to be uniformly organ-ized within the surfactant mesophase,

allowing potentially facile topochemicalpolymerization, where the topography ofthe monomeric diacetylenic (DA) assem-bly is preserved in the polymerized prod-uct, polydiactelyene.

In the example shown in Figure 6, DAsurfactant monomers were prepared withhydrophilic oligoethylene oxide (EO)head groups. Increasing the number n ofEO units comprising the head group pro-gressively decreased the surfactant criticalpacking parameter g,15,16 allowing the for-mation of lamellar, hexagonal, and cubicmesophases. UV exposure resulted in theformation of a blue polydiacetylene (PDA)/silica nanocomposite that exhibitedthermo-, solvato- and mechanochromism,which are coloration effects occurring inresponse to thermal, mechanical, andchemical stimuli, respectively, to form thered fluorescent form (Figure 6d). A criticalissue in this approach (and in self-assemblyapproaches in general) is the influencethat nanoscale confinement has on poly-merization and the structure/propertiesof the resulting product (e.g., organicpolymers or silica). In this regard, it isnoteworthy that the diacetylenic surfac-tants used to define the nanostructuresshown in Figure 6 did not polymerizewhen assembled as a Langmuir mono-layer on a flat surface. This implies thatthe curvature created by self-assembly (andthen maintained by the self-assembledstructure during polymerization) stronglyinfluences the proximity/orientation ofthe reactive units and consequently theirability to polymerize. Perhaps more im-portant, we anticipate that the polymerproperties resulting from nano-confinedpolymerization will be different fromthose of their bulk counterparts. Under-standing structural differences of suchnano-confined materials and exploitingthese differences to develop new function-alities represents an important and chal-lenging future direction.

For example, in the case of polydiacety-lene, we found that the PDA/silica nano-composite exhibited reversible thermo- andmechanochromism, properties not nor-mally observed in bulk PDA. For conju-gated polymers in general, nanostructuringshould allow much greater control of ori-entation and charge and energy transferthan for bulk materials. Also, the compos-ite nanostructure should increase thechemical, thermal, and mechanical robust-ness of conjugated polymers, fosteringtheir integration into devices.

Nanocrystal Self-AssemblyWe can expand upon this composite

self-assembly strategy by employing otherhydrophobic components in addition tohydrophobic organic monomers/mole-cules. Recently, we investigated the self-assembly of hydrophobic nanocrystal (NC)mesophases within hydrophilic silicamesophases.17 Most nanocrystal synthesis



Figure 4. Bulk ternary H2O/ethanol/surfactant phase diagram.The evaporation-inducedcompositional trajectories of the three cetyltrimethylammonium bromide (CTAB)/silicasystems (0.10, 0.12, and 0.16, denoting the respective CTAB/silica molar ratios) and theWS (without silica) system (a solution prepared like sample 0.12, but without silica) aremapped onto the bulk water/ethanol/CTAB phase diagram, considering the hydrophilic silicicacid precursors to be equivalent to water. Grazing incidence small-angle x-ray scatteringpatterns obtained after a specified time t (in seconds) are presented for sample 0.12,corresponding to the following: (a) the isotropic phase, (b) the lamellar mesophase, (c) thecorrelated micellar mesophase, and (d) the hexagonal mesophase; (e) ordered mesophasenucleates at liquid–vapor and solid–liquid interfaces and grows inward by conversion of thewormlike micellar interior. (Adapted from Reference 9.)

procedures use hydrophobic ligands(alkane thiols, trioctylphosphine oxide,etc.) to stabilize them from aggregation.Mono-sized NCs can be considered asmoderate-sized hydrophobic molecules.We postulated that their incorporation asindividual NCs in surfactant micelleswould allow further self-assembly into or-dered nanocrystalline mesophases, as an-ticipated from Figure 1. Rather than addhydrophobic NCs directly to isotropic sur-factant/solvent/silica systems, as in poly-mer/silica self-assembly, a genericmicroemulsion procedure was developedto create water-soluble nanocrystal mi-celles. As represented schematically inFigure 7a, a concentrated nanocrystal so-lution prepared in organic solvent (chloro-form, hexane, etc.), is added to an aqueoussolution of surfactant under vigorous stir-ring to create an oil-in-water microemul-sion. Organic solvent evaporation thentransfers the NCs into the aqueous phaseby an interfacial process driven by the hy-drophobic van der Waals interactions be-tween the primary alkanes of the NCstabilizing ligands and the secondaryalkane of the surfactant. This results inthermodynamically defined interdigitatedbilayer structures (Figure 7b). Cationic,anionic, and non-ionic surfactants andphospholipids can all form NC micelles,allowing facile control of micelle surfacecharge and functionality. In addition, fluo-

rescent semiconducting CdSe NCs (stabi-lized by trioctylphosphine oxide) havebeen formed into NC micelles with main-tenance of optical properties. This demon-strates the general nature and flexibility ofthis approach.

We discovered that in aqueous media,NC micelles organize hydrophilic compo-nents or precursors at the surfactant/waterinterface. This occurs through electrostaticand hydrogen-bonding interactions in amechanism analogous to that of surfactant-directed self-assembly of silica/surfactantmesophases.18 For example, addition oftetraethyl orthosilicate under basic condi-tions results in the formation of hydrophilicoligo-silicic acid species that organize withNC micelles to form a new type of orderedgold NC/silica mesophase. This nano-crystal mesophase has fcc symmetry (spacegroup Fm3m), as shown in Figures 8a and8b. These images are consistent with an fccunit cell with dimension a of �10.2 nm anduniform spacing between NCs of �6 nm.Our work17 appears to be the first exampleof an ordered fcc nanocrystal arrayformed spontaneously by self-assembly inaqueous media rather than by solventevaporation.19–21 Compared with other or-dered NC arrays, the embedding silicamatrix provides for greater chemical,mechanical, and thermal robustness.Furthermore, thermodynamically con-trolled self-assembly provides greater

order and control of NC spacing, as com-pared with other connected nanocrystalsystems such as those prepared by DNAhybridization.22,23

Using acidic conditions designed tominimize the siloxane condensation rateallows the formation of thin films by meansof standard techniques like spin-coating,micromolding, or inkjet printing. By sup-pressing siloxane condensation, and there-by gel formation, solvent evaporationaccompanying coating induces self-assembly of NC micelles into fcc nano-crystal thin-film mesophases. This occursin a manner similar to the evaporation-induced self-assembly of cubic or hexagonalsilica/surfactant thin-film mesophasesdiscussed earlier.4

The metallic NC mesophases we de-scribe appear to be the first examples 17ofhighly ordered, fully three-dimensional(3D) nanocrystal arrays. NC arrays havebeen the subject of considerable interest, asthey serve as model “artificial solids” withtunable electronic, magnetic, and opticalproperties stemming from single-electroncharging and quantum confinementenergies of individual NCs mediated bycoupling interactions with neighboringNCs.

As an initial investigation of chargetransport in an ordered 3D nanocrystalarray, we fabricated planar metal–insula-tor–metal (MIM) devices, incorporating aAu NC/silica array as the insulator layer(see schematic inset in Figure 9a). We thenmeasured their temperature-dependentI–V characteristics. The I–V curves for thegold NC/silica capacitors are plotted inFigure 9a. At room temperature, the I–Vbehavior is linear, with a zero-biased resis-tance of 14 M�, corresponding to a filmresistivity of about 3 � 106 � cm. Nonlin-earity near zero bias is evident at 200 Kand increases with decreasing tempera-ture. At 100 K and below, conduction oc-curs through the gold NC/silica insulatoronly above a minimum threshold voltageVT, indicative of a collective Coulombblockade24 resulting from electrical isola-tion of the NCs. For such arrays ofCoulomb islands, theory predicts that forV � VT, current scales as a power law, I � I0(V/VT – 1)�. The scaling exponent �reflects the dimensionality of the accessi-ble current-conducting pathways; for oneand two dimensions, both modeling25 andexperiment26 show � to approximatelyequal the array dimensionality. Figure 9bplots representative I–V scaling data forour 3D gold NC/silica arrays measured at78 K. We observe power-law scaling with� � 2.7 (negative bias) and � � 3.1 (posi-tive bias). Note that bias here refers to thesign of the voltage in the current–voltage



Figure 5. Evolution of nanolaminate structure during dip coating. (a), top schematic:Continued evaporation simultaneously organizes the inorganic and organic constituents intoperiodic thin-film mesophases. (a), bottom schematic: Combined organic/inorganicpolymerization locks in the nanocomposite architecture. (b) Transmission electron microscopyimages of a poly(alkyl methacrylate)/silica nanolaminate. (Adapted from Reference 12).

plot in Figure 9a. Currently, there are notheoretical predictions or simulations ofthe 3D scaling exponent; however, thegreater value of � observed here, com-pared with previous studies of 2D arrays26

and quasi-3D arrays,27 is consistent withthe greater number of conductive path-ways expected for a fully 3D array.

In general, such highly ordered NC ar-rays should serve as model ideal systemsin which to discover new nanoscale phe-nomena and to develop and test theoriesof collective optical, electronic, and mag-netic behaviors.

Responsive SystemsLiving systems are able to sense and

respond to their environments throughvarious chemical- and physical-basedtransduction mechanisms. From an engi-neering perspective, robust materials thatrespond predictably to pH,28–31 tempera-ture,31,32 light,33–38 biomolecules,39–41 orelectric fields42,43 could find applications incontrolled release of drugs or corrosioninhibitors, sensors, optical storage, andoptomechanical actuation. So the ques-tion arises, how do we impart responsive,lifelike qualities to robust engineering materials?

In examining biological systems, we notethat responsiveness often derives fromhierarchical assemblies that position envi-ronmentally sensitive moieties onto a 3Dframework. In an attempt to emulate such



Figure 6. (a) Molecular structures ofdiacetylenic (DA) surfactants andpolymerization of DA/silicananocomposites. (b) DA surfactantsserve both as amphiphiles to direct theself-assembly of the silicic acidmesostructure and as monomericprecursors of the conjugated polymer,polydiacetylene (PDA). Increasing thenumber n of oligoethylene oxide (EO)subunits comprising the hydrophilicsurfactant head group results in theformation of higher-curvaturemesophases: lamellar (n � 3)hexagonal (n � 5) cubic (n � 10),due to a decreasing value of thesurfactant packing parameter g.(c) Representative transmission electronmicroscopy images of nanocompositethin films and particles. C1: [110]-orientedhexagonally ordered nanocompositefilm prepared using 2.23% DA-EO5. C2and C3: Two orientations ([100] and[111]) of a cubic nanocomposite film.C4: PDA/silica nanocomposite particlesprepared using an aerosol-assistedevaporation-induced self-assemblytechnique. (d) Patterned polymerizationinduced by UV irradiation andthermochromic transition of hexagonalPDA/silica nanocomposite films.(Adapted from Reference 13).

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biological designs, our idea was to organizeoptically33–35 and thermally31 responsiveazobenzene molecules within a robust,ordered 3D nanostructure using self-assembly (see Figure 10). In this synergis-tic design, photo or thermal energy istransduced into a 3D mechanical responsethat can control pore size and transportbehavior.

Azobenzene derivatives were selectedbecause of their well-studied response to light.44 – 47 Trans cis isomerizationi

changes the molecular dimension (molec-ular length of the cis isomer is �3.4 Åshorter than that of the trans isomer) andalso the dipole moment (0.5–3.1 D).Azobenzenes have been used previouslyto achieve switchable properties. How-ever, in order to accurately control poresize, we needed to anchor azobenzene lig-ands to the pore surfaces of a nanostruc-tured scaffold composed of mono-sizedpores. This allows the rigid inorganic scaf-fold to precisely position azobenzene

ligands in 3D configurations, whereswitching results in a well-defined changein pore size (see Figure 11). The 3D com-posite architecture should also enhancemechanical and thermal stability of theswitchable ligands, important for their in-tegration into devices.

Our synthesis33–35 scheme employed an azobenzene-modified silane, 4-(3-triethoxysilylpropylureido)azobenzene(TSUA), designed to serve as an am-phiphilic cosurfactant after hydrolysis ofthe ethoxy groups. During EISA, thehydrophobic propylureidoazobenzenegroups are positioned in the hydrophobicmicellar cores. The hydrophilic headgroups co-organize with added silicic acidoligomers at the hydrophilic micellar exte-riors. After catalytic or thermally promotedsiloxane condensation, azobenzene ligandsare anchored to the surfaces of mono-sized pores, with the azobenzene ligandsoriented toward the pore interiors (seeFigure 11). Subsequent surfactant extrac-tion produces the target nanocomposite.

Figure 12 shows a transmission electronmicroscopy (TEM) cross-sectional imageof an as-prepared photoresponsive nano-composite film that exhibits a highly or-dered cubic mesostructure. The averagecenter-to-center pore spacing is 5.6 nm.Two-dimensional GISAXS data confirmthat the pores are arranged in a (100)-oriented bcc mesostructure (Im m spacegroup), with a � 5.7 nm.

As determined with UV–visible spec-troscopy, UV irradiation of the transisomer causes transformation to the cisisomer. Removal of the UV radiation, irra-diation with a longer wavelength, or heat-ing switches the system back to the transform. As a control experiment, we madefilms with the same sol but prepared with-out the non-ionic Brij56 surfactant. In thiscase, the azobenzene ligands were ran-domly incorporated in a microporous silicamatrix (pore diameter, less than 1 nm) andexhibited no detectable photoisomerization.Similarly, we observed no photoisomeri-zation for the ordered self-assembledfilms prior to solvent extraction of the sur-factant templates. Only upon surfactantremoval did we create the pore volumerequired (estimated as 127 Å3)48 for photo-isomerization. These results unambigu-ously locate the photosensitive azobenzeneligands along with surfactant within theuniform nanopores of the self-assembledfilms. They emphasize the need to accom-modate the steric demands of the photo-isomerization process by engineering thepore size and positioning the photoactivespecies on the pore surfaces.

To demonstrate optical control of masstransport, we performed a chronoamper-ometry experiment using an azobenzene-

3



Figure 7. (a) Process scheme for the synthesis of water-soluble nanocrystal micelles.Theaddition of oil containing gold nanocrystals (NCs) to a surfactant containing aqueous solutionduring vigorous stirring leads to the formation of an oil in H2O microemulsion. Subsequentevaporation of oil transfers the NCs into the aqueous phase by an interfacial process drivenby the hydrophobic van der Waals interactions between the primary alkane of the stabilizingligand and the secondary alkane of the surfactant, resulting in thermodynamically definedinterdigitated bilayer structures that encapsulate the NCs and make them water-soluble (b).

Figure 8. Representative transmission electron microscopy (TEM) images of Aunanocrystal/silica mesophases. (a) [100] and (b) [210] orientations of bulk samples.Lower-right inset in (a): High-resolution TEM of sample in (a) showing gold NC latticefringes. Upper-left inset in (a): Selected-area diffraction (SAED) pattern from the image in(a). Inset in (b): SAED pattern from the image in (b). (Adapted from Reference 17.)

functionalized nanocomposite membraneto modify the working electrode in anelectrochemical cell (see Figure 13). Thechronoamperometry experiment uses fer-rocene dimethanol (FDM) as a molecularprobe and provides a measure of masstransport through the nanocompositemembrane by monitoring the steady-stateoxidative current at constant potential forthe reaction taking place on the workingelectrode surface, which is indium tinoxide (ITO). At constant potential, the ef-fective pore size limits the diffusion rate ofprobing molecules to the electrode surfaceduring electrolysis. Under dark condi-

tions, the azobenzene moieties are pre-dominantly in their extended trans form.Upon UV irradiation (� � 360 nm), theazobenzene moieties isomerize to themore compact cis form, which we expectedwould increase the diffusion rate and, cor-respondingly, the oxidative current. Like-wise, exposure to visible light (� � 435 nm)triggers the reverse cis trans isomeriza-tion of the azobenzene moieties, whichshould decrease the current to the pre-UVexposure level.

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Figure 14 shows typical current versustime photoresponse data for the membrane-modified electrode. We observe that afteran initial transient decay, the oxidativecurrent of ferrocene dimethanol reaches asteady state after about 360 s. Upon UV ir-radiation, the current increases progres-sively due to trans cis isomerization,which increases the pore size. It thenreaches a new plateau, corresponding tothe photostationary state of trans cisisomerization. Visible-light exposure de-creases the oxidative current due to cistrans isomerization, which decreases thepore size. After 200 s, the current is re-duced to its pre-UV exposure level. Thiscorresponds to the photostationary stateof cis trans isomerization. We performedthree cycles of alternating UV/visible lightexposure to show the reversibility and re-peatability of this process. In the last cycle,a much weaker visible light (0.5 mW) wasused instead of the stronger visible light(5.6 mW). We observed a much slower de-crease in current, demonstrating a slowerresponse under lower illumination levels.Control experiments were performed on abare ITO electrode and one coated withmesoporous silica. However, no photo-response was observed in either of thesesystems. This demonstrates that the photo-response is not an artifact of the ITO elec-trode or the mesoporous silica film, but atrue effect arising from the isomerization

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Figure 9. (a) Current–voltage ( I–V) curves measured from 300 K to 78 K.The schematic in(a) shows the architecture of a metal–insulator–metal (MIM) device, fabricated with a100-nm-thick Au/silica nanocrystal array as the insulator. (b) At T � 78 K, current displays apower-law dependence for V � VT, with scaling exponent � � 2.7 (negative bias) and � � 3.0(positive bias). VT and � were determined by the Levenberg–Marquardt nonlinearleast-squares method. (Adapted from Reference 17.)

Figure 10. Concept of responsivenanocomposites. Organization ofstimulus-responsive ligands within athree-dimensional porous frameworkimparts new molecular-level functionality,useful for opening and closing ananoscale valve. (Adapted from BruceBunker, unpublished work.)

Figure 11. Schematic illustration of photoresponsive nanocomposites prepared byevaporation-induced self-assembly.The trans or cis conformation of the azobenzene unitwas calculated using Chem3D Pro molecular modeling analysis software. Atomic legend:grey, carbon; red, oxygen; blue, nitrogen; green, silicon; H atoms are omitted. (Adapted fromReference 33).

of azobenzene moieties attached to thenanopore surfaces.

To correlate the observed current changeswith the actual isomerization state, a UV/visible spectroscopy study was performed,where the nanocomposite film used forchronoamperometry was immersed in theelectrolyte solution and illuminated ex-actly as for the first I–t cycle in thechronoamperometry experiment. Asshown in the inset of Figure 14, the ab-sorbance of the nanocomposite film at356 nm, attributed to the �–�* transition ofazobenzene in the trans form, decreasesprogressively under UV irradiation,reaching a plateau corresponding to thephotostationary state of trans cis iso-merization. The following visible-light ex-posure causes the reverse cis transisomerization, increasing the absorbancegradually before reaching the pre-UVstate. These data exactly correlate the con-formational changes of azobenzene moi-eties in the nanocomposite film with theoxidative current changes measured in thechronoamperometry experiment. Thus,we clearly demonstrate control of masstransport on the nanoscale.

ConclusionI hope I have convinced the reader of

the power of self-assembly as a means toprecisely organize complex (and often dis-parate) materials on the nanoscale. Our

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variations on the theme of evaporation-induced self-assembly allow nanostructuresto be integrated easily into devices and, insome cases, provide a pathway to materialsthat have no bulk nanostructured counter-parts (e.g., organic/inorganic nanocom-posites). Many challenges remain, however.Our self-assembled nanostructures, al-though beautiful, are boringly repetitive.To better emulate biology, we need to breaksymmetry and develop structure and func-tion on a broader range of length scales. Iexpect there to be much future progress inso-called directed assembly, where activeintervention such as the application of anexternal field can be used to perturb ormodulate the assembly process. Currently,there is much interest in structure forma-tion by energy-dissipating systems, wherelarge-scale patterns often emerge. Com-bined with smaller-scale molecular self-assembly, this may be a means of

developing hierarchical structures. Finally,we should consider compartmentalizedsynthesis, for example, within membrane-bound molds, where selective ion chan-nels are used to spatially control pH, ionicstrength, and the introduction of precursorions. Such an approach could allow theformation of synthetic analogues to naturalstructures like diatoms and spicules.

AcknowledgmentsThis work has benefited tremendously

from the research of creative students andpostdoctoral fellows including YunfengLu, Hongyou Fan, Dhaval Doshi, BerndSmarsly, Ralf Koehn, Yi Yang, NanguoLiu, and Alan Sellinger. I also have profitedfrom collaborations with my colleagues atSandia and the University of New Mexico,including Alan Hurd (now at LosAlamos), Frank van Swol, Kevin Malloy,Darren Dunphy, Darryl Sasaki, AlainBurns, Gabriel Lopez, and Abhaya Datye,and visiting professor Alain Gibaud (Uni-versité du Maine, Le Mans, France). Thiswork was partially supported by the U.S.Department of Energy Basic Energy Sci-ences Program, the Air Force Office ofScientific Research, the National ScienceFoundation (NIRT, NNIN, and the CeramicComposite Materials Center programs), theArmy Research Office, Sandia NationalLaboratory’s laboratory-directed R&Dprogram and the Center for IntegratedNanotechnologies, and DARPA. TEM in-vestigations were performed in the De-partment of Earth and Planetary Sciencesat the University of New Mexico. Sandia is



Figure 12. Cross-sectional transmissionelectron microscopy image of aphotoresponsive nanoporous film.34

Inset:Two-dimensional grazing incidencesmall-angle x-ray scattering pattern,shown in reverse gray scale.The lowerhalf-plane is in the shadow of the siliconsubstrate, and the scattering spots areattenuated. (Published with permissionof the American Chemical Society.)34

Figure 13. Schematic drawing of (top)an electrochemical cell and (bottom)mass transport of probing moleculesthrough the photoresponsivenanocomposite membrane integratedon an indium tin oxide (ITO) electrode.34

A: Diffusion through smaller pores withazobenzene ligands in their transconfiguration. B: Diffusion throughlarger pores with azobenzene ligands intheir cis configuration. Red ovals areFDM (ferrocene dimethanol); greenovals are FDM++; small orange ovals areazobenzene in cis form; small elongatedorange ovals are azobenzene in transform. CE, RE, and WE refer to thecounter electrode, reference electrode,and working electrode, respectively.(Published with permission from theAmerican Chemical Society.)34

Figure 14. Current–time ( I– t) behaviorof a photoresponsive nanocompositefilm under alternate exposure to UV(360 nm) and visible light (435 nm). Lastcycle uses room light, 400–700 nm.Inset shows absorbance at 356 nm(�–�* transition of the trans isomer) ofthe same film immersed in a buffersolution containing 1 mM FDM(ferrocene dimethanol).The time scaleof the UV/vis data corresponds to thatof the first cycle in the I– t responsecurve using FDM as the molecularprobe. (Adapted from Reference 34.)

a multiprogram laboratory operated bySandia Corporation, a Lockheed MartinCompany, for the U.S. Department ofEnergy’s National Nuclear Security Ad-ministration under contract DE-AC04-94ALB5000.

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C. Jeffrey Brinker re-ceived his BS, MS, andPhD degrees in ceramicscience and engineering atRutgers University. Hejoined Sandia NationalLaboratories (SNL) as amember of the technicalstaff in 1979. He was

promoted to Distinguished Member of theTechnical Staff at SNL and appointed Distin-guished National Laboratory Professor ofChemistry and Chemical Engineering at theUniversity of New Mexico (UNM) in 1991.Since 1999, he has been jointly employed atSNL, where he is a Sandia Fellow, and atUNM, where he is a professor of Chemicaland Nuclear Engineering and co-director ofthe Center for Micro-Engineered Materials.

Several of Brinker’s 250-plus publicationsare among the highest cited in materials re-search. Among Brinker’s most recent honorsare election to the National Academy of Engi-neering (2002) and recipient of the Departmentof Energy’s Ernest O. Lawrence MemorialAward in Materials Science (2002).

Brinker can be reached by e-mail [email protected].



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