ELSEVIER Thin Solid Films Films366 366$2000) 135-138

Submicron metal oxide structures structuresby a asol-gel process on on patterned patternedsubstrates substrates

Clernens ~echin~er*,Hans Muffler, Claudia ClaudiaSchafle, Olle Sundberg, Paul Leiderer Leiderer

Universtfi of ofK(>pa\ tunz Fach M M676. D-78457 Konsrun~G&mony

Accepted 27Accepted Januaq 2000 2000

Abstract Abstract We Wereport reportabout a acclrnbinatlon of micro-contact-printing micro-contact-printingand the sol-gel sol-geltechnique techniquewhich whichresults in structures in the themlcroo- and submtcmn submtcmnrange. range.This Thistechnrque which is ishere heredemonstrated demonstratedon electrochromic oxide does not rely on vacuum methods and may therefore thereforebe easilybe easilyupscaled to large areas. Q 2000Q Elseviw Sc~ence$.A. All Allnghts nghtsreserved. reserved.

K~vwords:Nanostructures; K~vwords: Nanostructures; oxide; Tungstenoxide; oxide;Tungsten Wetting Wetting

1. Introduction Introduction in contrast to to homogeneous homogeneousfilm deposition thin depositionasthin it has been beenused usedso far - can- canaaIo be employed employed for for the the fabrication fabrication The sol-gel sol-gel techniqueis a widespread technique widespread of synhe-method methodof nanostructures nanostructureson large largescales scales which whichfor are elec- are elec- useful useful sizing sizing materialsand depositing materials depositingthem themin form formof thin thinhomo- homo-tronic, optical opticalor catalytic applications. applications.This is achieved achievedby geneour geneourfilms. films.Accordingly, Accordingly,has gained gained itimportance it with with combining the benefit benefitof the the sol-gel sol-gelwith process micro- process micro- respect to to scientificand scientific application applicationBased purposes. on the purposes. contact-printing (p-CP), (p-CP),the the latter latter comprisingsurfaces comprising surfaces chemistry chemistryof the the synthesis,a alarge synthesis, largevariety of materials, materials,e.g. with withwell-defined well-definedhydrophilic periodic hydrophilicperiodicand hydrophobic hydrophobic oxidic oxidic semiconductors. semiconductors.and even even glasses superconducting glasses superconductingdomains, respectively respectively[5,6].When Whena amedium mediumof low visc- visc- fiIms can canbe obtained obtained[14]. In this this processan inorganicprocess inorganicosity is applied to such such patterned patterned surfaces, surfaces, spontaneous spontaneous or organic organic molecular molecularM(OR),, precursor M(OR),,a asol, sol,is precursor used as as dewetting occurs occursand andthe fluid forms formsa adroplet droplet pattern pattern starting startingmaterial, material,where whereM isM a metal and (OR) (OR)an organic organicwhich whichreflects reflectsthe symmetry of the the underlying underlying substrate. substrate. compound, compound, respectively.A Amacromolecular respectively. macromolecular oxideThe oxide principIe network principIehas network been been demonstrated demonstratedfor simple already simple already is then obtained obtainedin a moista moist environment environment throughliquids. through liquids. polymersand hydrolysis andother polymers materials [7-101 on homoge- homoge- and poIycondensation whichpoIycondensation which transforms the precursor transforms precursorinto a neous substrates. substrates. Additionally,fabrication Additionally, of patterned patterned highly highly viscous(gel-like) viscous hydrated hydratedmetal metaloxide (MO,). (MO,).In a sol-gel sol-gel structuresby means structures meansof ofmicromolding micromoldingis reported reportedin final finalheat heattreatment treatment at at100-300°C typically typicallythe thecoating is the literature literature111,121. In contrast to this, this, herewe we applyhere apply then dried in an oven to tocomplete hydrolysis hydrolysisand condensa- highly highlyreactive liquids, liquids,i.e, i.e,alkoxides, alkoxides,ro chemically chemically tion. In principle principlesol-gel the sol-gelreaction the canreaction canbe written writtenas [I] patterned patternedsubstrates. After Afterthe dewetting process processan anarray of drops is obtained obtainedwhich whichcan becan considered asconsidered microscopic microscopic test test tubes tubesa achemical where where , reaction, namely namely the the sol-gel sol-gel The The basic basic advantageof the the advantagesol-gel sol-gel is itsprocess its ability process ability process to to process place, takes takeswhich which eventually eventuallythe liquid liquid turns turns form form inorganicstructures inorganic (semiconductors, (semiconductors,ceramics and and drops in solid soliddots. glasses) glasses) at at relatively relatively low low temperatures. temperatures. Furthermore, Furthermore, the the process - being- beingvery similar similar to to conventionalcoating tech- conventional tech- iques such assuch dip-, dip-,spin- or orspray spraycoating, does not not require require2. Sample preparation preparation any vacuum vacuumsteps, and andis therefore a cost-effective cost-effectiveway to We demonstrate demonstrate thefor the principlethe semiconductor principletung- semiconductor produce produce thin thin homogeneousinorganic films homogeneous on large scales. sten stenoxide oxide(WO?) which whichcan be prepared preparedfrom a tungsten tungsten In Inthe following wefollowing wewill will demonstrate demonstratetechnique that - -this that this alkoxide (or alcoholate) precursor, precursor, but butshould the the should principle principle hold for many manyother oxidic materials, materials,We choose W03choose since since * Comspond~ng* author. Tel.: + +49-75-31-883-562 49-75-31-883-562tax: + +49-75- 3 188-33 127. it itis well well knownas aknown chrotnogenic chrotnogenicmaterial material which which can can E-mad E-madaddress. clemens.bech~nger@unl-konstanz de (C. Bechrnger)(C. Bechrnger)change its optical and and electrical electrical113,141 properties upon properties

0040-6090/001$ - see- front ma1tt.r Q 2000Q Elsevier ScienceElsevier S.A. Science All Allrights resewed.rights Pfl: 50040-6090(00)00865-R 136 136 C'. B~clri~r#c,r-cr oLB~clri~r#c,r- / ThL1/ Solid FiJprrs JhO (2000) I.j.5-1.38

technique which which has has becomeas nanospherebecome known lithogra-nanosphere known phy, and which whichworks in principle principleas follows follows[22-241: colloidal particles particlesof equal equalwhich size size are aresuspended suspendedin a a solvent solvent(e.g. water) are deposited depositedon a aglass glasssurface. The particles usually usuallyarange themselves themselves randomly. randomly. However, However, during the evaporationthe of the solvent, solvent,capilIary lateral lateral forcescapilIary occur which whichmake the the colloids colloidsform formhexagonally to to hexagonally arranged, close-packed close-packedarrays inarrays a aself-assembly self-assembly process. process. We prepared prepared colloidal colloidalwith polystyrene monolayers polystyreneIPS) monolayers microspheres of 3 3Frn and and 840nm diameter,840 diameter,respectively. The typical typicalsize of the themonolayers monolayerswas wasin the theorder orderof several mm' up to to1 1 cm'. From those thosemonolayers monolayersstamps were casted and andused for p-CP. Afterp-CP. werethiols were transferred transferred to toa a gold goldsubstrate we obtained obtained hexagonalsurface surfacedomains hexagonal as asconfirmed confirmedby lateral lateralforce microscopy. microscopy.The Thebright brightand dark areas in Fig. 1 1correspond correspond to toand hydrophilic hydro- hydrophilic 9.2 urn phobic phobic regions.respectively. regions.

Rg. 1 1 Lateralforce Lateral Image Imageof ofa achelnlcally patterned patterned old surface old surfacewherc hnght and anddark mas indicate indicatehydrophillc and andhydrophobic hydrophobicdumalns, dumalns, respectively respectivelyThe structure structurewas obtained obtainedhy hynucrecontact printing uslng 3. Experimental Experimentalresults results a amontdayw nP polyclyrene spherec orspherec 840 nm dlameternm dlameteras asa amaster. When Whenthe sol is applied appliedby bydip-coating dip-coatinga agold surface to to applying applying electricalfields. irradiation electrical irradiation with with ultravioletpatterned ultraviolet patterned lighta stamp with light with made from fromspheres of 3 3pm in [IS] or exposure exposureto hydrogen containinggases 1161. containing There- There- diameter, a periodica periodicstructure is formed asformed wggested wggestedalready fore forethe materia1 is interesting interestingfor potential potentialapplications in by the theoccurrence of interference interferencewhiclr colors whiclrare visible colors displays, imaging devices devicesand and"smart" "smart"windows windows[17,18]. [17,18].with withthe bare bareeye and and confirmedwith confirmedan optical microc;cope. Since the the presented presenteddoes approach not not approachrequire require or vacuumFig. vacuum Fig.2 2shows an atomic force microscope microscopeimage imageof the etching etching techniquesit might techniques mightbe a apromising promisingmethod methodfor the sample sampleafter it it was was annealed120°C annealed for about at at about 30 min. min. fatincation fatincationof oxidic nanostructures nanostructureson large largescales. Obviously, the sol dewetted dewettedcompletely the hydrophobic hydrophobic Tungsten Tungsten alkoxide alkoxidewere prepared precursors preparedby precursorsbydissol- dissol- sites (see also alsoFig. I), thus thus producing producing hexa-monodisperse, monodisperse, ving 3 3g gtungsten tungsten hexachloride(WC16) inhexachloride 10 ml . ethanol.gonally gonally arrangeddrops where arranged where duringthe subsequent during anneal-subsequent First Firsta violent violent reactionoccurs reactionwhich whichturns the yellow yellow solu-ing solu- ingstep a chemical reaction, reaction,i.e. thei.e. sol-gel tmnsition, take?tmnsition, tion tion rapidly rapidlyblue. blue.To into Toavoid into the gelation gelationto totake place place. place.As Asa result, result,one obtains obtainsW03 dots which strcjngly already alreadyduring the the deposition depositionof the processsol, we process stabi- adhere to the thesurface and and withstandan ultrasonic withstand bath treat- lized itlized against against hydrolysisby byadding hydrolysis adding10-20 rnl acetylacetone ment mentof 1 M1 hydrochloricM hydrochloricIn orcler .to demonsrratc acid. thatdemonsrratc that as a asurfactant surfactant(C5H8O2) [19]. [19].The final mixture mixturecontained these thesedroplets exhibit chromogenic chromogenicpropel-ties wc fil.c;t fil.c;t also 7also rnl7 dilute dilute(I M) hydrochloric hydrochloricin order toacid enhance acid evaporated evaporateda athin Iayer Iayerof palladium palladiumand then exposed exposedthe the dewetting dewetting properties.When Whenthis properties.this liquidis appIied liquid appIiedby dip dip structure to hydrogen hydrogengas. gas.We observed an immediate or spin coatingspin coatingto toa ahomogeneous homogeneoussubstrate and annealed annealedat about about12UoC, a thina thinfilm of tungsten tungsten(WO;) oxide (WO;)is oxide formed formed which whichexhibits a ahighly highly reversible reversible electrochromic electrochromic behavior behavior comparable comparablethat thatof evaporated to to evaporatedor sputtered sputtered material material [3,20,21 [3,20,21. Chemically Chemically patterned patternedwere fabricated substrates fabricated substrates with with self-assembled self-assembled(SAMs) monolayers on gold monolayers goldcoated glass glass sheets sheetsby using using combinationsof combinationsoctadecanethiol octadecanethiol (CH~(CH~)I~SH) and 11 1-rnercaptoundecanoI -rnercaptoundecanoI (HS(CHI)I,OH). resulting resultingin hydrophobic hydrophobic and and hydrophilic hydrophilic domains, respectively. domains,To transfer respectively. transfer the thethe thiolssurface thiols surfacea to to stamp ofstamp an elastomerican elastomericmaterial materialis required. required.can Stamps becan be Stamps ohtained ohtainedby bycasting castingpoly(dimethylsi1oxane) poly(dimethylsi1oxane)(PDMS) (PDMS)onto an appropriate appropriatemaster. A A negative negativeimage of the master is Fig. Fig.2. Atomic Atomicforce microscopy microscopy(AFM)image imageot m marmy armyot tungcten produced producedin the the cured cured Conventional Conventionalas methods methods PDMS. PDMS. oxide dots obtained by dip coating coatingof tungqten alkoxide alkoxidetn a achemicalFy chemicalFy photolithography photolithographyor X-ray lithography lithographybe used can so can patterned surface. Aftersurface. an annealing step at 12WC. the llquld llqulddrop< aredrop< produce produce the the masters. masters.we weused Here, aHere, different however, different transformedhowever, into solid dots. C. Bechinger et al. /Thin Solid Films 366 (2000) 135-138 137 the substrate for about 15 s before it was removed by tilting the substrate. Although exactly the same recipe for the sol was used, now the 'inverse' structure of Fig. 2 i1s obtaineci. While the details of this process are not understo od in deta il -. :- A*.- 4 yet, we believe that the formation of this -geometl v la UUC; LO micro-phase separation between the SI~rfactant :ind the so1. When the sol rests at the substrate, the: acetylace:tone migllt preferentially wet the hydrophilic site:s, thus le:aving on1 Y the hydrophobic areas for the tungsten alkoxide. Thus, just by a the details how the sol is applied to the surface a completely different surface structure may be obtained.

4. Conclusions

In summary we have shown that the dewetting of tungsten alcoholates on surfaces which are prepatterned via micro- contact printing- leads to the formation of small drops. Due to the high reactivity of this material a subsequent chemical Fig. 3. AFM image (3.73 X 3.75 pm2) of tungsten oxide dots fabricated by dip coating on a gold surface prepatterned with a stamp made from a reaction, i.e. a sol-gel process, occurs and finally leads to the monolayer of 600 nm microspheres. The inset shows a cross section of a formation of solid tungsten oxide dots. We also demon- dot after the annealing process which can be fitted to a spherical cap (solid strated that by simply changing the process how the &ox- line). The typical size (FWHM) of the dots is about 150 nm. ide is applied to the surface either the 'positive' or the 'inverse' structure is obtained. The results presented here change in the reflected intensity which is typical for tung- should also apply to many other materials which can be sten oxide thin films [25]. produced by the sol-gel technique. Since the approach An array of W03 dots formed on a substrate prepattemd presented here does not rely on vacuum techniques it with a stamp molded from a monolayer of 600 nm PS might be a convenient way of nanostructure fabrication. spheres is shown in Fig. 3. After the annealing process, the typical full width half maximum (FWHM) of the dots is in the order of 150 nm as can be seen from a cross section Acknowledgements in the inset. The profile of the solid dots fits very well to a spherical cap function (solid line) which is typically This work was supported by the Deutsche Forschungsge- obtained for liquid drops. Obviously the polycondensation meinschaft, SF33 5 13. does not alter the shape of the dot which is in agreement with the fact that tungsten oxide forms an amorphous network at these temperatures [20,26]. Only above 350°C where crystallization occurs, deviations from a spherical References shape are observed. Fig. 4 shows an example where sol was allowed to rest on [I] C.J. Brinker, G.W. Scherer, Sol-gel Science, Academic Press, San Diego, CA, 1990. [2] S. Katayama, M. Sekine, H. Fudouzi, M. Kuwabara, J. Appl. Phys. 71 (1992) 2795. [3] C. Sanchez, J. Livage, M. Henry, F. Babonneau, J. Non-Cryst. Sol. 100 (1988) 65. [4] C.J. Brinker, A.J. Hurd, G.C. Frye, K.J. Ward, C.S. Ashley, J. Non- Cryst. Sol. 121 (1990) 294. [5] Y. Xia, J. Tien, D. Qin, G.M. Whitesides, Langmuir 12 (1996) 4033. [6] G.P. Lopez, H.A. Biebuyck, C.D. Frisbie, G.M. Whitesides, Science 260 (1993) 647. [7] A. Kumar, G. Whitesides, Science 263 (1994) 60. [El Z. Huang, P.-C. Wang, A.G. MacDiamuid, Y. Xia, G. Whitesides, Langmuir 13 (1997) 6480. [9] M. Boltau, S. Walheim, J. Mlynek, G. Krausch, U. Steiner, Nature 391 (1998) 877. [lo] H. Yang, N. Coombs, G.A. Ozin, Adv. Mater. 9 (1997) 811. [ll] C. Marzolin, S.P. Smith, M. Prentiss, G.M. Whitesides, Adv. Mater. Fig. 4. Inverse structure, which is obtained when the sol is not applied by 10 (1998) 571. dip coating but is allowed to rest on the chemically patterned substrate. The [12] P. Yang, T. Deng, D. Zhao, et al., Science 282 (1998) 2244. substrate prepattern was obtained by a colloidal monolayer of polystyrene [13] S.K. Deb, Philos. Mag. 27 (1973) 801. spheres of 3 pm diameter. [14] R.S. Crandall, B.W. Faughnan, Phys. Rev. Lett. 39 (1977) 232. 138 C CBechingtr er 01. / Thin/ Solid Frlnls 366 366(2000) 135-138

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