nanoparticle-based surface templating

Nanoparticle-Based Surface Templating

Kathy Lu*,w and Chase Hammond

Materials Science and Engineering Department, Virginia Polytechnic Institute and State University,Blacksburg, Virginia 24061

This study is focused on making molds with micrometer-sized feature arrays, studying high solids loading nanoparticlesuspension for nanoparticle-based templating, and cast forming of surface-templated materials. Polydimethylsiloxane (PDMS)and silicone molds with micrometer-sized feature arrays are made using mold cores templated by a focused ion beam mi-croscope. Al2O3 nanoparticle suspensions are evaluated based on nanoparticle interaction energy and suspension flowability inorder to obtain stable and high solids loading suspensions. Templated feature transfer ability is compared for the PDMS andsilicon molds under ambient and freeze-drying conditions. This work provides a new approach for surface templating ofnanoparticle-based materials.

Introduction

Particles serve as one of the basic building blocksfor many materials. As the particle size decreases tonanoscale, exciting opportunities arise for nanomaterialdesign. Because nanoparticle sizes are generally two tothree orders of magnitude smaller than micrometer-sized particles, particle-based feature sizes can be corre-

spondingly decreased by using nanoparticles. One im-mediate possibility is creating fine feature arrays in alarge surface area by surface templating.

With the nanoparticle size decrease, the accompa-nying excessive surface area and surface energy need tobe properly controlled. Because of the natural tendencyof nanoparticles in forming agglomerates as a means ofreducing the excessive amount of surface energy, colloi-dal dispersion has become one of the most active re-search areas in nanoparticle-based processing. Ifparticles are well dispersed, a green body with uniformmicrostructure can then be produced from fully stabi-lized colloidal suspension of nanoparticles.

Int. J. Appl. Ceram. Technol., 8 [4] 965–976 (2011)DOI:10.1111/j.1744-7402.2010.02536.x

Ceramic Product Development and Commercialization

r 2010 The American Ceramic Society

This work is supported by the National Science Foundation under grant no. CMMI-

0824741.

*Member, The American Ceramic [email protected]

mailto:[email protected]

Lange and Yu developed a shaping method calledcolloidal isopressing,1–3 in which a preconsolidated sus-pension was injected into an elastometric mold and iso-pressed to convert the suspension into an elastic body.Micrometer-sized surface features, such as 5 mm widechannels with a depth/width ratio of 2, were producedon the surface of Al2O3 powder compacts althoughfracture of thin vertical portions of micrometer-size fea-tures during pressure release and demolding was anissue.4 In addition, this effort was based on micrometer-sized particles and involved pressure filtration and coldisostatic pressing. Sol–gel processing has been used toproduce micrometer-sized components.5 However, ex-tensive edge damage and a low density of the gel posedchallenges for maintaining the integrity of the greencomponents.

Direct casting is an alternative to these methods. Itinvolves pouring a suspension into a mold, convertingthe suspension to solid state, and then demolding thecomponent. Different from a rather dilute suspensionor gel, the suspension for casting should be made muchmore concentrated. Solid formation can be induced bydirect liquid evaporation (here we call it ambient dry-ing) or by freezing and then sublimating the liquid(here we call it freeze drying). The technique can formcomplex geometry parts without requiring an externalpressure. The key requirement for the suspension isbeing stable and free of agglomerates.6–8 Comparedwith other casting techniques such as tape casting, slipcasting, centrifugal casting, and even electrophoreticdeposition, direct casting involves no suspension flowand has the potential to avoid phase or particle segre-gation. The freeze-drying process can also avoid capil-lary forces, thus minimizing cracking and shapedistortion problems.9–12

In this work, a direct casting process is pursued fornanoparticle-based surface templating. First, differenttemplates are produced by focused ion beam (FIB) mill-ing, and polydimethylsiloxane (PDMS) and siliconemold making. Second, Al2O3 nanoparticle interactionenergy is analyzed and suspension flowability is charac-terized because the prerequisite for nanoparticle castforming is obtaining high solids loading suspensions.Finally, different Al2O3 nanoparticle-based samples aremade by casting and then drying at ambient conditionor freezing condition. The templated molds allow thereproduction of micrometer-sized arrays of differentfeatures using the Al2O3 nanoparticle suspensions.This approach opens up numerous opportunities for

direct device fabrication due to the pressureless and largesurface area templating nature.

Experimental Procedure

Mold Making

Two steps were needed to make the casting molds.The first step was to make a mold core with desiredfeature sizes, shapes, and patterns. The second step wasto make the patterned molds that can confine the Al2O3

nanoparticle suspension and produce samples with de-signed feature arrays. In this study, a dual beam FIBmicroscope (FEI Helios 600 NanoLab, Hillsboro, OR)was used to make the mold core. The mold core ma-terial was polished silicon wafer. The FIB microscopewas composed of a subnanometer resolution electronmicroscope and a Ga1 ion microscope. The Ga1 sourcehad a continuously adjustable energy range from 0.5 to30 kV and an ion current between 1.5 pA and 21 nA.During templating, the silicon wafer was tilted at 521from the electron beam incident direction. The ionbeam spot size was tuned to about 5 nm. The templatedsilicon wafer surface was examined by the electron mi-croscope right after pattern creation.

PDMS (Dow Corning, Midland, MI) molds weremade using the templated silicon wafer as the moldcore. The PDMS base compound and the curing agentwere mixed at 10:1 ratio, poured over the silicon wafer,and allowed to sit for a few minutes. The silicon waferand the PDMS liquid were then placed in a vacuumchamber and subjected to a pressure of 27 Pa. Becausethe PDMS mixture before curing had a low viscosity,this process effectively removed all the air bubbles in-herent in the PDMS mixture. Once all the air had beenremoved, the silicon mold core and the PDMS mixturewere placed into a drying oven at 1001C. After 45 minthe solidified PDMS mold was separated from the sil-icon wafer. Silicone molds (RTV 664, General Electric,Waterford, NY) were produced using a similar pro-cess.13–17 The silicone base compounds and the curingagent were homogeneously mixed at 10:1 ratio beforebeing poured over the silicon wafer. After that the sil-icon wafer and the silicone mixture were placed under40 Pa pressure for air removal. Because the silicone mix-ture had a high viscosity, the vacuuming process wasrepeated three to five times before being kept at 40 Pafor 15 min. Then the mold was cured for 24 h in air andseparated from the silicon template.

966 International Journal of Applied Ceramic Technology—Lu and Hammond Vol. 8, No. 4, 2011

Al2O3 Suspension Preparation and Characterization

Al2O3 nanoparticles with specific surface area of45 m2/g were used in this study (Nanophase Technol-ogies, Romeoville, IL). The average particle size wasaround 38 nm and the particles had a normal size dis-tribution from 10 to 50 nm.13 The TEM image of theAl2O3 nanoparticles was provided before.16 For theAl2O3 nanoparticle suspension preparation, poly(acrylicacid) (PAA, MW 1800, Sigma-Aldrich, St. Louis, MO)with repeating unit [–CH2CH(CO2H)–] was used as adispersant; glycerol (C3H8O3, Fisher Chemicals, Fair-lawn, NJ) with molecular formula CH2OH–CHOH–CH2OH was used as part of the dispersing medium.The hydrodynamic size of the particles was 43 nm.18 Awater–glycerol mixture at a ratio of 9:1 (water:glycerol)was used as the dispersing medium. The mixture washomogenized for 5 min using a ball mill before use.Al2O3 nanoparticles were added into the dispersing me-dium in 10 g increments along with 2.0 wt% of PAAdispersant (on Al2O3 basis). Because low pH promotesPAA dispersant adsorption onto Al2O3 nanoparti-cles,15,16,19 HCl solution (36.5–38.0%, EMD Chemi-cal, Gibbstown, NJ) was added to lower the pH to 1.5.The suspension was ball milled for 12 h with periodicadjustment of pH to 1.5. Suspensions of approximately20 vol% Al2O3 solids loading were made by this proce-dure. After this step, Al2O3 nanoparticles were againadded in 10 g increments, along with 2.0 wt% of PAAdispersant (on Al2O3 basis) to make different solidsloading suspensions. NH4OH was used to adjust thesuspension to pH 9.5.15,16 The suspension was thenmixed for 24 h to ensure complete homogenization.

Viscosities of Al2O3 nanoparticle suspensions werecharacterized using a rheometer with a cone-plate ge-ometry (AR 2000, TA Instruments, New Castle, DE).All the viscosity measurements were performed with acontrolled shear rate in a decreasing shear rate order.

Casting and Surface Feature Characterization

Al2O3 nanoparticle suspension was filled into thePDMS and silicone molds immediately after the sus-pension preparation using a disposable pipette. Care wastaken to completely fill the molds and avoid air bubbles.The filled molds were kept under ambient condition for1 h. After this prerest, some samples were allowed to dryin air for 12 h. The remaining samples were frozen ina freeze dryer (AdVantage El-53, SP Industries, War-

minster, PA) immediately. The freezing rate was0.251C/min and the freezing temperature was �351C.The samples were kept at the freezing temperature for2 h before the chamber pressure was decreased to 1 Pa.The filled molds were kept at �351C and 1 Pa pressurefor 10 h and then heated to room temperature in stages(�201C for 8 h, �101C for 4 h, �51C for 5 h, and 51Cfor 5 h). During the entire process, a pressure of 1 Pawas maintained.

The templated surfaces of the ambient dried andfreeze-dried samples were analyzed using a LEO550field emission scanning electron microscope (SEM)(Carl Zeiss MicroImaging, Thornwood, NY). To illus-trate the three-dimensional nature of the templated fea-tures, all the SEM images were taken at 451 tilt angle.

Results and Discussion

Templated Molds

The feature arrays created on the silicon wafer areshown in Fig. 1. In Fig. 1a, the islands have a round buttapered geometry. The island base diameter is 10 mm;the island center-to-center distance is 15 mm; and theisland height is 5.0 mm. The island side surface haso1 mm roughness. In Fig. 1b, the round islands stillhave tapered geometry, but to a much less extentbecause these alternating diameter islands are only about2 mm high. This height difference is achieved by con-trolling the FIB templating time. The large islands have10 mm base diameter and the small islands have 5 mmbase diameter. The island center-to-center distance isagain 15 mm. In Fig. 1c, the square islands have 10 mmbase edge length; the center-to-center distance is 15 mm;and the island height is 5 mm. As seen throughout Fig.1, the feature sizes and shapes are well defined. It is alsoimportant to point out that well-focused ion beam cancreate smoother feature surface at a faster rate. For ex-ample, at the same 30 kV FIB voltage and 6.5 nA beamcurrent condition, Fig. 1a takes more than 70 min toproduce but still results in rough feature surface; Fig. 1ctakes o30 min but has much smoother surface.

Figure 2 shows the SEM images of the three differ-ent feature arrays on the PDMS mold surface. Figure 3shows the SEM images of the three different feature ar-rays on the silicone mold surface. As shown, the roundand square islands from the silicon wafer have been ac-curately transferred onto the PDMS and silicone moldsurfaces. The new features are the inverse of the features

www.ceramics.org/ACT Nanoparticle-Based Surface Templating 967

on the silicon wafer; circular cavities with 10 mm diam-eter, circular cavities with 10 and 5 mm alternatingdiameters, and square cavities with 10 mm edge length

Fig. 1. Island arrays produced by focused ion beam on siliconwafer: (a) uniform diameter islands, (b) alternating diameterislands, and (c) uniform size square islands.

Fig. 2. Scanning electron microscopic images of polydimethylsiloxanemold with features present: (a) uniform diameter cavities,(b) alternating diameter cavities, and (c) uniform square cavities.


are present. In Figs. 2c and 3c, there exists a flaw for oneof the square features but with a different orientation.This is a result of silicon wafer damage (chip-off) afterthe FIB templating. This demonstrates that a mold canonly be as good as the template to make it; the details ofthe features have a high degree of reproducibility.

While both PDMS and silicone materials can pro-duce the designed feature arrays from the silicon wafer,the two mold materials produce different surface char-acteristics. The PDMS molds have a wrinkled surfacetexture, while the silicone molds have much smoothersurface. These different surface characteristics are a re-sult of inherent composition differences between thetwo materials. PDMS has pure polydimethylsiloxanecomposition. Silicone has a mixture of vinyl-dimethylpolysiloxane, polyalkylhydrogensiloxane, andultramarine blue with some other minor ingredients.In addition, the PDMS liquid mixture has a much lowerviscosity than the silicone mixture before being curedinto solid molds. After the molds are made, the PDMSmolds are compliant, optically clear, and deformablewhile the silicone molds are rigid with blue color. Thesemold surface texture and rigidity differences will in turnaffect the templated Al2O3 nanoparticle feature surfacesand the templated feature reproducibility as discussedlater.

Al2O3 Nanoparticle Suspension

For nanoparticle-based surface templating, one crit-ical aspect is to produce nanoparticle suspensions withhigh solids loading, stability, and flowability. In order tomeet such requirements, different interaction energyterms between Al2O3 nanoparticles should be analyzed.With the understanding that the Al2O3 nanoparticlesuspension is different from many other dilute suspen-sion systems and the Al2O3 nanoparticles have a rangeof size distribution, Deryaguin, Landau, Verwey, andOverbeek theory can be applied with simplification. Thedetails of the approach and analysis have been publishedelsewhere.18 Here, different Al2O3 nanoparticle inter-action energy terms are considered in a much briefermanner. It is well known that van der Waals interactionis always present. Because both PAA dispersant and ionsare used for Al2O3 nanoparticle colloidal stabilization,electrostatic and steric interactions can be expected. Also,free polymer is present in the suspensions at all pH levelsand this creates depletion energy (attraction) betweenAl2O3 nanoparticles.16 Based on these considerations,

Fig. 3. Scanning electron microscopic images of silicone mold withfeatures present: (a) uniform diameter cavities, (b) alternatingdiameter cavities, and (c) uniform square cavities.


there are four interaction energy terms: van der Waalsinteraction energy (Evdw(h)), electrostatic interactionenergy (Ees(h)), steric interaction energy (Ester(h)), anddepletion interaction energy (Edep(h)). Even though ithas been noted that the steric layer and the electricdouble layer are not independent,20 it is assumed herethat different interaction energies are additive in order toestimate the stability of the Al2O3 nanoparticle suspen-sion system.

When the Al2O3 nanoparticle (assuming they arespherical) separation distance h is much larger than theparticle radius a,21,22 the van der Waals attractionenergy can be expressed as

EvdwðhÞ ¼�Aa

12h1� 5:32h

100ln 1þ 100

5:32h

� �� ð1Þ

A is Hamaker constant. The electrostatic repulsionenergy between the Al2O3 nanoparticles is23,24

EesðhÞ ¼ 2pereoax2 ln½1þ expð�khÞ� ð2Þ

er is the dielectric constant of the water–glycerol mixtureand is approximately 78.40,25 eo is the permittivity offree space (8.854� 10�12 F m�1), and x is the suspen-sion zeta-potential. 1/k is electric double layer thickness,which can be expressed as follows for the Al2O3 nano-particle suspension26,27

k�1 ¼ 0:1515ðCCl� þ CNHþ4Þ�

12 ð3Þ

CCl� and CþNH4

are in mol/dm3 and 1/k is in nm. Thethird interaction energy term to consider is the stericrepulsion energy. If the adsorbed PAA layer thickness isL, there is no steric interaction between the two Al2O3

particles when h42L. When Loho2L, the stericenergy is the same as the mixing interaction energyEmix(h) and is given by28,29

EmixðhÞ ¼32pkTa

5nV 2f

L4

1

2� w

� �L� h

2

� �6

ð4Þ

where k is the Boltzmann constant, T is the absolutetemperature, Vf is average volume fraction of the PAAdispersant in the adsorbed layer, n is the molecular vol-ume of the water–glycerol mixture, and w is the Flory–Huggins parameter. At small Al2O3 particle–particleseparation (hoL), both elastic and mixing interactionsneed to be considered for the steric energy term

EmixðhÞ ¼4pakTL2

nV 2f

1

2� w

� �

� h

2L� 1

4� ln

h

L

� �ð5Þ

EelasticðhÞ ¼2pakTL2r

MWV 2f

h

Lln

h

L

3� h=L

2

� �2" #(

�6 ln3� h=L

2

� �þ 3ð1� h=LÞ

�ð6Þ

Eelastic(h) is the PAA chain elastic interaction energy, r isPAA density, and Mw is PAA molecular weight. Vf andw are assumed as 0.15 and 0.485, respectively.29 Theonly factor that substantially affects the steric interactionenergy is the adsorbed PAA layer thickness L, which canbe estimated based on30

LðnmÞ ¼ 0:06ðMwÞ0:5 ð7Þ

For the Al2O3 nanoparticle system, the steric re-pulsion energy can be calculated based on Emix(h) andEelastic(h) at different particle–particle separation dis-tance h. According to the pragmatic theory, the deple-tion energy (attraction) Edep(h) is given by31

EdepðhÞ ¼ 2pam1 � mo1

nD� h

2þ L� q þ p

� �2

ð8Þ

when Doa, hoa. D is the depletion layer thickness, p isthe degree of interpenetration between the free and ad-sorbed PAA chains, q is the compression of the graftedPAA layer, m1 is the chemical potential of the water–glycerol mixture at jp volume fraction of free PAA, andm1

O is the corresponding value at jp 5 0.0. p and q aresmall and assumed to cancel out with each other, D canbe assumed approximately as 1.0 nm, m1�m1

O can becalculated based on the projected length of PAA repeat-ing unit and potentiometirc titration. Therefore, thedepletion interaction energy Edep(h) can be obtained.

The total interaction energy ET, a sum of all theabove energy terms, as a function of Al2O3 particle–particle separation distance h has been calculated asshown in Fig. 4. As it shows, the total interaction energyis a strong function of the PAA adsorption layer thick-ness. As the polymer adsorption layer thickness increasesfrom 1.0 to 3.5 nm, the total interaction energy changesfrom attraction (�) to repulsion (1). Most strikingly,


the total interaction energy is much higher for the sus-pension with 3.5 nm PAA adsorption layer thickness.Because peak ET should be several times of kT to resistcoagulation, the Al2O3 nanoparticle suspension will notbe stable unless the PAA adsorption layer thickness is2.5 nm or higher. From a different aspect, the PAA ad-sorption layer thickness and molecular weight cannot betoo high, such as 3.5 nm or higher. Otherwise, the re-pulsion energy between Al2O3 nanoparticles, such as30 kT, will be too high for obtaining high solids loadingsuspensions. If the solids loading of the Al2O3 nano-particle suspension is too low, it will lead to low densityand integrity for the templated nanoparticle samples. Inthis study, the PAA molecular weight is 1800, whichprovides about 2.5 nm adsorption layer thickness, themost desired condition to maintain Al2O3 nanoparticlesuspension stability while achieving high solids loading.

Suspension flowability reflects the magnitude ofAl2O3 nanoparticle interaction macroscopically and isan important aspect for producing the templated andmicrometer-sized features. The Al2O3 nanoparticle sus-pension should have good flowability so that the roundand square cavities can be filled completely during cast-ing. Flowability can be evaluated by the viscosity of theAl2O3 nanoparticle suspensions. With the preferredPAA dispersant at MW 1800, Fig. 5 shows that viscos-ities are a strong function of Al2O3 nanoparticle solidsloading. When the Al2O3 nanoparticle solids loadingincreases from 30 to 45 vol%, the viscosity increases atall shear rates, by approximately two orders of magni-tude. As the shear rate decreases, the viscosities increasemonotonically but the Al2O3 nanoparticle suspensiondoes not obey Newtonian flow. Instead, the suspensiondisplays a shear-thinning behavior, reflecting the poly-meric link breakage between the Al2O3 nanoparticles

under shear. At 30 vol% solids loading, the viscositychanges from 0.09 to 0.28 Pa s when the shear rate de-creases from 200 to 10 s�1. At 45 vol% solids loading,the viscosity changes from 1.59 to 9.44 Pa s when theshear rate decreases from 200 to 10 s�1. As the Al2O3

suspension solids loading increases, the viscosity differ-ence becomes much larger. This indicates that highersolids loading Al2O3 suspensions have more drastic‘‘linkage’’ and nanoparticle interaction, and the disper-sion conditions should be more rigorously controlledand optimized.

For Al2O3 nanoparticle suspension casting, espe-cially when producing the templated features shown inFig. 1, the suspension should be kept at suitable solidsloading levels. When the solids loading is too low, thecast samples will have low green density and integrity,and present a challenge for handling and sintering dens-ification. When the solids loading is too high, the sus-pension will be too viscous to flow into the templatedcavities. Obviously, the solids loading should be chosenbelow the jamming point, the suspension transitionstate from being flowable to being infinitely viscous.Our prior study has identified that 45% solids loading isclose to the jamming point.32 Macroscopic samples havebeen made with 40 vol% solids loading alumina nano-particle suspension.13,14 In consideration of the finefeature arrays to be templated, the suspension of35 vol% Al2O3 solids loading is used in this study.

Nanoparticle-Based Templating

Ambient Drying: Based on the simplified quantifica-tion of Al2O3 nanoparticle interaction energy and mea-surement of the suspension flowability, the Al2O3

Fig. 5. Viscosity change versus shear rate for different Al2O3 solidsloading suspensions. Each curve is an average of three measurementsand the standard deviation is much smaller than the markers andcannot be shown on the figure.

Fig. 4. Total interaction energy change versus particle–particleseparation distance h for the studied Al2O3 nanoparticle systems.


nanoparticle suspension of 35 vol% solids loading hasbeen templated. The round and square feature arrays areshown in Fig. 6 for the PDMS mold under the ambientdrying condition. The PDMS mold produces smoothfeature surfaces. Comparing Figs. 1a and b, and 6, how-ever, the PDMS mold produces features with a lessdefined edge profile. The round islands turn into hemi-spheres for both uniform and alternating diameter is-land cases. Another more interesting observation is thatthe square island features have different heights. Someare only around 1 mm high while others are about 5 mmhigh as designed. This result is a direct consequence ofthe less than desired PDMS mold rigidity. Duringtemplating, the PDMS mold cavities distort under theweight of the Al2O3 nanoparticle suspension and lead tofeature shape deviation. It should also be mentionedthat the chip-off from the silicon wafer is reproduced asa missing corner for the Al2O3 nanoparticle square is-lands at the lower middle location of Fig. 6c, a dem-onstration of the feature reproducing capability of thetechnique.

For the silicone mold, different mold feature arraysproduce different results (Fig. 7). The round islandsproduced from the round cavities, both uniform andalternating diameters, are relatively defect free. Thealternating diameter island arrays in Fig. 7b are abouthalf the height of the uniform diameter ones in Fig. 7aas designed. However, the square islands on the Al2O3

nanoparticle sample surface show different results. Asshown in Fig. 7c, the surfaces of the square features aresmooth, but the edges and corners of the features are notwell defined. Fractures and pores appear at those loca-tions. The broken edges and corners are believed to re-sult from the stress concentration at these sharptransition locations. Also, pores in the submicrometerrange are more likely to be trapped at these locations.Fundamentally, these issues mirror feature geometryeffect on sample integrity at a macroscopic scale.Unless it is necessary, sharp edges and abrupt transi-tions should be avoided.

Overall, the silicone mold produces more definedfeatures than the PDMS mold at ambient drying con-ditions. This is because the silicone molds are more rigidat ambient condition and can preserve the cavity shapeswhen filled with the Al2O3 nanoparticle suspension.Also, the island feature surfaces from the silicone moldare smoother. This can be traced back to the smoothsilicone mold surface texture versus the wrinkled PDMSmold surface texture. One desirable result from the

Fig. 6. Different features made from PDMS molds by ambientdrying: (a) features produced from uniform diameter cavities,(b) features produced from alternating diameter cavities, and(c) features produced from uniform square cavities.


PDMS mold is that the square features show fewercracks and trapped pores because of its more compliantnature.

Freeze Drying: Freeze drying the Al2O3 nanoparticlesuspension to form the round and square islands hasproved more complicated than ambient drying. This isbecause the integrity of the dried samples is sensitive tofreeze-drying conditions. However, this process is stillpreferred in the long term because (1) drying conditioncan be precisely controlled and (2) capillary forces fromthe liquid suspension can be avoided. These advantageswill increase the reliability and reproducibility of theprocess and avoid defect formation for the templatedsamples.

Freezing rate is the most significant parameter thataffects the integrity of freeze-cast samples15 and tem-plated feature reproducibility. For the Al2O3 nanopar-ticle suspension, the following trends have beenobserved based on pure experimental efforts. Fast freez-ing rate (40.51C/min) results in high porosity for thesolid-state samples. Slow freezing rate (r0.251C/min)reduces sample porosity at the solid state and producesdesired microstructures. This is contrary to the estab-lished understanding on ice phase transformation: fastcooling rate should facilitate amorphous phase forma-tion and thus fewer pores from ice crystal evaporation.The fundamental cause is still unclear. The mold ma-terial also has a drastic effect on the freeze-dried sam-ples. Mold surface–suspension interaction dictates if thefreeze-dried sample can be successfully separated fromthe mold or not.33 Also, mold rigidity at freezing con-ditions needs to be evaluated in order to avoid featureshape distortion. These issues will be discussed in futurestudies. In this work, the focus is to compare the tem-plated feature characteristics from the PDMS and sili-cone molds.

Figure 8 shows the SEM images of the freeze-driedisland arrays produced with the PDMS mold. Referringback to Fig. 6, the freeze-drying process has resulted inmore defined features than the ambient drying process.The features in Fig. 8 are similar to the ones producedunder ambient drying condition from the silicone molds(Fig. 7). Freeze drying apparently improves the rigidityof the PDMS mold and its feature shape reproducibil-ity. The debris-like appearance on some of the featuresis due to the dirt falling on the mold surface. This meansmaintaining clean mold surfaces is important whenthe feature is micrometer-sized or smaller. The more

Fig. 7. Different features made from silicone molds by ambientdrying: (a) features produced from uniform diameter cavities,(b) features produced from alternating diameter cavities, and(c) features produced from square cavities.


Fig. 8. Different features made from PDMS molds by freezedrying: (a) features produced from uniform diameter cavities,(b) features produced from alternating diameter cavities, and(c) features produced from uniform square cavities.

Fig. 9. Different features made from silicone molds by freezedrying: (a) features produced from uniform diameter cavities,(b) features produced from alternating diameter cavities, and(c) features produced from uniform square cavities.


challenging issue is the broken features shown in Fig. 8aand c. This is a smaller problem for Fig. 8b, probablybecause of the shorter island height designed. The bro-ken features reflect higher than the desired affinity be-tween the PDMS mold surface and the templated Al2O3

nanoparticle surface. During the templated Al2O3 fea-ture-PDMS mold separation, strong adhesion betweenthe two surfaces causes the breakage of the island fea-tures.33

The island arrays produced from the silicone moldsunder the freeze-drying conditions are shown in Fig. 9.The templated feature shapes are severely distorted. Thefreeze-drying process appears to cause the Al2O3 nano-particle suspension to adhere to the mold surface. As aresult, some of the templated Al2O3 features are stuckon the silicone mold during the mold release. The tem-plated feature shape distortion also indicates that thesilicone mold might have deformed during freezing. Forthe freeze-drying conditions, the PDMS mold is pre-ferred for feature shape reproduction.

So far, the discussion has focused on feature shape re-producibility. Another consistent observation is featuresize change. As shown in our previous work, freeze-driedsamples from 40 vol% solid loading Al2O3 nanoparticlesuspension have 52% relative density.14 In this work, thetemplated feature sizes are about 10% smaller than thedesigned sizes. The only case that cannot be compared isthe samples from the silicone mold under the freeze-dry-ing condition because of severe feature shape distortion.This means both ambient drying and freeze drying inducesubstantial shrinkage during water–glycerol removal. Thisaspect has not been studied in the freeze-drying area andwill be examined in the future.

Conclusions

A nanoparticle-based surface templating process hasbeen studied using Al2O3 nanoparticle suspension un-der ambient and freeze-drying conditions. With a tem-plated silicon core to make PDMS and silicone moldsand high solids loading Al2O3 nanoparticle suspensionsof good flowability to fill the molds, micrometer-sizedarrays of different size and shape islands have been cre-ated on Al2O3 nanoparticle material surfaces. The sili-cone molds produce more desirable features at theambient drying condition while the PDMS molds pro-duce more desirable features under the freeze-dryingcondition. Mold surface texture and affinity as well as

mold material rigidity greatly affect the feature shapereproducibility. This templating process can be appliedto large surface area nanoparticle-based materials andpresents great potentials for direct device fabrication.

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