Large-Area Nanoparticle Films by Continuous AutomatedLangmuir−Blodgett Assembly and DepositionXue Li and James F. Gilchrist*

Department of Chemical and Biomolecular Engineering, Lehigh University, Bethlehem, Pennsylvania 18015, United States

ABSTRACT: The operating parameters and resulting surfacemorphology of automated Langmuir−Blodgett deposition ofmonosized micrometer-scale silica colloids from an aqueoussuspension are investigated. This apparatus allows continuousroll-to-roll deposition of particles into well-ordered arrays. Thereproducible deposition of particle monolayers at low tomoderate deposition rates at web speeds of less than 10 mm/sis possible and accurately characterized by a simple massbalance of particles deposited from solution. At fasterdeposition rates, Landau−Levich flow increases the filmthickness such that flow instabilities hinder uniform particledeposition. A simple phase diagram outlines transitions from dispersed to multilayer coatings and from uniform to erraticdeposition patterns. While the threshold of maximum deposition rate is well-defined for these conditions, changing operatingparameters, particle size, and fluid viscosity and evaporation rate, the maximum speed can be increased significantly.

1. INTRODUCTIONThere are significant challenges to fabricating coatings that havewell-defined 2D and 3D nano- and mesoscale structuresthrough continuous, commercial-scale processes. After a long-standing trend in fundamental and applied research examiningmaterials at the nanoscale, the only way to realize commercialbenefits from these efforts is by developing processes that usedirected assembly or self-assembly. The two key limitations toscalability are cost and control. For example, there have beenadvances using the phase behavior of block co-polymersdeposited in thin films to create periodic nanoscale features.1,2

However, except in high-value-added systems, using polymerswith narrow molecular weight and property distributions iscurrently not affordable for most applications. Likewise, thesesystems are limited to features of a narrow size range, oftenmuch smaller than length scales that interplay with light3 andcan be used for macromolecular separations such as bioMEMSdevices for viral separations.4 While fabrication of colloidalcrystals to be used as macroporous and photonic materials isadvantageous, there are few routes for producing large-areacontinuous films commercially.Materials templated by particle assembly at the nanoscale

have impact in numerous fields such as nanodevices,5,6

electronics,7−9 and functional coatings.10−12 The most commonprocesses for fabrication of highly ordered colloidal monolayersand colloidal crystals are Langmuir−Blodgett deposition,13,14

convective deposition,15,16 and spin coating,17,18 all of whichhave limitations regarding scalability and continuous process-ing. Slot coating19 is routinely used for roll-to-roll deposition ofliquid films; however, it has not been used as a mechanism forcoating ordered particle monolayers. Alternatively, Langmuir−Blodgett deposition20 is well-known for its ability to transferamphiphilic molecules from a stagnant liquid interface onto a

solid substrate and has been extended to transfer othermacromolecules and particles trapped at an air−liquid interface.While this process can be scaled to large stagnant troughs, it isnot particularly continuous and the rate of deposition must becoordinated with the surface pressure by moving one or morebarriers of the trough. Convective deposition, which takesadvantage of the evaporative flux of the solvent, can be scaled,but the rate of deposition and assembly is highly limited by thesolvent properties. Brewer and co-workers21 successfullydemonstrated a continuous process in the millimeter persecond range for ordered monolayer deposition that follows thesame principles as convective deposition and utilizes forcedconvection of dry gas to facilitate drying in the thin film.Coatings can succumb to streaks formed by local instabil-ities.22−24 These streaks can be inhibited by using binarycolloidal suspensions25 or vibration-assisted convective deposi-tion.16 For monolayer deposition, convective deposition islimited to hundreds of micrometers per second. Previous workdemonstrated fabrication of larger scale colloidal crystals usingspin coating18,26 and interfacial self-assembly.27,28 However, thearea coated is limited to the size of the wafer or beaker andcannot be translated to a roll-to-roll platform.29 Of course,other processes such as spray coating allow for faster transfer ofparticles to a substrate but with minimal control of the resultingmicrostructure.Automated Langmuir−Blodgett (ALB) deposition, shown in

Figure 1, was first developed by researchers at University ofToronto30 allowing a facile and general way to fabricatemacroscopic closely packed nanoparticle films on solid

Received: October 8, 2015Revised: January 1, 2016Published: January 7, 2016

Article

pubs.acs.org/Langmuir

© 2016 American Chemical Society 1220 DOI: 10.1021/acs.langmuir.5b03760Langmuir 2016, 32, 1220−1226

substrate at water−alcohol interfaces in a continuous mannerover a large area. Due to its independence of the particles’chemical nature, this approach can apply to a large variety ofparticles and surfactants for the formation of macroscopicparticle films. Our results suggest there are two regimes inwhich particles are deposited onto the substrate. The first,where particles come in contact directly with the substrate, caneasily be described by a simple particle balance. The secondregime draws particles still afloat atop a liquid layer throughLandau−Levich31 viscous entrainment. Particles are subse-quently deposited as the liquid layer dries. Systematic particledepositions at constant web speed or suspension flow rates areinvestigated to understand the ability to control particlemorphology over large areas.

2. EXPERIMENTAL SECTION2.1. Experimental Setup. A schematic diagram of the

experimental setup is shown in Figure 1. The nanoparticle coating isproduced by the benchtop nanocoating tool: automated Langmuir−Blodgett (ALB) apparatus (Versuflex, LLC), which mainly consists ofinjection system, carrier fluid system, and web handling system. Thephysical image of the ALB apparatus is shown in Figure 1a. Thecoating process is based on the directed self-assembly of desiredmaterials such as nanoparticle or surfactant. The material to be coatedis first well-dispersed in the suspension which is going to be loaded tothe injection system later on, and then the injection system preciselydispenses the material onto the surface of the carrier fluid in a cartridge10 cm wide. This forms a layer of particles at the interface that flowover a weir of adjustable angle, here kept constant at 20°. The carrierfluid lateral movement, arising from the carrier fluid circulation system,drives the floating layer to be self-assembled into a well-orderedstructure while flowing over the ramp, forming a liquid film from thefront edge of the cartridge to transfer the material floating layer to anadvancing substrate or web. The deposition occurs continuously as thefluid carrying particles at its free interface leaves the trough. The carrierfluid flow rate is kept constant near the upper limits of the ability ofthe system at 300 cm3/min, giving the best results for formingcontinuous monolayer coatings under the proper conditions. Thesuspension delivery rate is controlled independently, and one of thekey goals is to understand the proper ratio of suspension flow rate andweb velocity to generate a given morphology.The passing substrate is realized by the web handling system that

involves five rollers. The two big rollers shown on the right side of thescheme in Figure 1 are bare substrate and substrate with coatedmaterial, corresponding to the bottom and top rollers, respectively.The other three smaller rollers help to manage and keep tension onthe moving substrate. Figure 1b displays a sample on the rolling web.The roller can handle substrates, either flexible or rigid ones, such aswafer and glass slides, which can be set to be affixed to the web. Theweb velocity is adjustable under the digital control system and is also a

main parameter in our experiment. The injection system includes twoparts, syringe pump and syringe; the suspension or solution can beloaded into the syringe and dispensed at different flow rates.

2.2. Materials and Suspension Preparation. The colloidsuspension used in this study is prepared by dispersing silicamicrosphere (SiO2, Fiber Optic Center Inc., New Bedford, MA,USA) in pure ethanol (Decon, King of Prussia, PA USA; 200 proof)with volume fraction varying from 0.01 to 0.2. The SiO2 microspheresare of density 1.8 g/cm3 and average diameter 951 ± 22 nm. In orderto get a well-dispersed SiO2 suspension, 15 min ultrasonic (FisherScientific FS20D) treatment was employed. Then, the SiO2 suspensionwas loaded to a syringe for the consequent injection procedure. In ourexperiments, distilled water (Crystal Spring Waters Co.) is employedas the carrier fluid; the coating material SiO2 is dispersed in ethanolbefore adding to the syringe, ensuring the particles can floating at theethanol−water interface later on.

2.3. Film Deposition. PVC substrate (Grafix Plastics, MapleHeights, OH, USA), a common low-cost substrate for many industrialapplications, is used for all of the samples prepared and for analyzingthe deposition process. It is understood that various surface treatmentsand other materials will have a wide range of surface charge andhydrophobicity, not considered in the current study but which mayhave some effect on the stability of the entrained liquid film and theadhesion of particles to the substrate. The PVC roll is mounted to thebottom big roller so as to dispense the spare PVC substrate during thefilm deposition process. Before deposition, the SiO2 suspension isloaded into the syringe; a syringe pump was used to precisely controlthe amount of SiO2 released to the system. In order to make sure thatthe SiO2 particles can floating onto the air−liquid interface, the needleof the syringe is placed at a soft engagement at the air−liquid interface.The circulation water lateral movement brings forward the floatingSiO2 film, and as the floating particle layer encounters the verticalpassing substrate, the lateral pressure and the liquid film work togetherto deposit the SiO2 particles onto the PVC substrate. At thedownstream of the system, a heated fan was placed so as to facilitatethe drying process.

2.4. Characterization. The morphology of the nanoparticle filmswas characterized by using an Epson (Stylus CX3810) scanner.Samples 12 in. long are placed on the flatbed scanner allowing highlyeven lighting scanned at 1200 dpi for a resolution of about 20 μm/pixel. As described previously,32,24 samples of the film were scannedusing confocal laser scanning microscopy and analyzed to determinethe number of particles per area. Using this information, the intensitywas correlated to the concentration using Beer’s law. This technique isaccurate from low-concentration sub-monolayer samples up to morethan 7 layers of particles in a colloidal crystal. This allowed scanningover roughly 0.3 m2 areas with 20 μm resolution in just minutes,impossible for optical and electron microscopy characterization at theparticle scale.

Figure 1. Schematic illustration of experiment setup by ALB. Images of (a) ALB apparatus, (b) films on the web, and deposited films of threedifferent patterns: (c) discontinuous, (d) monolayer, and (e) multilayer.

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3. RESULTS AND DISCUSSION

The particle coating process has two distinct stages, a Langmuirtrough regime upstream and coating regime downstream. TheLangmuir trough region is where particles are assembled in athin liquid film at or near the air−liquid interface aided by thestress imparted by the flow of the carrier liquid over a weir.Instead of the relatively stagnant fluid typically found in aLangmuir trough, the carrier fluid continuously advects theparticles downstream while spreading out the suspension thinfilm, aiding the motion of particles to get trapped at the air−liquid interface. Coating and deposition occur at the end of theLangmuir trough region where the fluid contacts the verticallypassing substrate dragging the fluid upward into a thin filmresiding on the web. The exact mode of this deposition iscomplex and depends on the flow conditions, as described later.The quality of the resulting particle-based film primarily

depends on web velocity, suspension flow rate, and suspensionvolume fraction. The observed surface morphologies arecategorized as discontinuous, monolayer, and multilayercoatings, shown in Figure 1c−e, respectively. The discontin-uous film coatings are intermittent depositions of domains ofcoated particles separated by uncoated or sparsely coatedregions. At higher coverages, the regions of coated particlesmay be interconnected with islands of bare substrate.Monolayer deposition results in a continuous, uniform coatingspanning the entire width of the trough having opalescenceresulting from the single layer of micrometer-scale particles.Multilayer coatings also result in a single continuous coatingspanning the entire width of the trough; however, the opticalclarity is highly variable due to regions of two or more layers ofparticles as confirmed by optical microscopy.In order to investigate the relationship between all of these

parameters and the resulting film structure, suspension volumefraction, suspension flow rate, and web velocity were alteredindependently. Three different suspension volume fractions of0.01, 0.05, and 0.2 were employed. Under each volume fraction,the suspension flow rate was varied from 0.1 to 3.2 mL/minand the web velocity was varied from 1 to 40 mm/s at a givenSiO2 suspension volumetric flow rate. The various filmmorphologies resulting from these operating conditions areshown in Figure 2. The suspension volume fraction and volumeflow rate work together to determine the particle mass flowrate, independent from the web velocity.In Figure 2, the monolayer deposition follows a linear trend

for lower particle mass flow rate and web speeds. Thisdependence is easily explained by mass conservation of theparticles. Assuming all of the SiO2 particles injected into thecoating system go onto the web substrate, the area of thesubstrate covered by particles is

π θ =N a U W2p (1)

while the amount of particles delivered upstream is given by

ϕ π = V N a43S S

3

(2)

where N is the deposition rate of the number of particles, θ isthe particle packing fraction on the substrate with a maximumvalue of θ = π/√12 for a well-ordered hexagonally packedmonolayer,a is the particle radius, Up is the velocity of particlestranslated from the trough to the web, W is the coating width,ϕS is the delivered suspension particle volume fraction, and VS

is the suspension volume flow rate. The particle mass flow rateis m = ρpϕSVS, where ρp is the particle density.We hypothesize there are two distinct modes of deposition,

as shown in Figure 3. The first mode is almost direct particledeposition onto the substrate. In this case, at slower webvelocity (UW) particles are pinned to the substrate almostimmediately due to the thickness of the thin film, depositedthrough the force imparted by surface tension of the meniscusbetween adjacent particles. Thus, the particles are movingdirectly at UW; that is, Up = UW, where UW is the web velocity

Figure 2. Phase diagram of the resulting morphology of the particlefilm as a function of particle mass flow rate (m). The solid lineindicates the theoretical monolayer deposition trend line (β = 0.35; θ= π/√12), while above and below the line at moderate m and UWroughly outline the discontinuous and multilayer deposition regions,respectively. The inset on the left is a full range of samples preparedunder extended particle mass flow rate (m) and higher web velocity(UW). The inset on the right is a typical confocal microscope image ofthe monolayer microstructure.

Figure 3. Hypothesized deposition modes under (a) low web velocitywhere particles come into contact with the surface because of localevaporation and (b) high web velocity where particles float on the fluidinterface far downstream.

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of the substrate rolling vertically past the trough. In such case,combining eqs 1 and 2, we get

βθ

ϕ= UaW

V3

41

W S S (3)

The parameter β is added to adjust for the fact that not allparticles injected upstream reside at the fluid interface. Toestimate β, the suspension delivery system was used to depositparticles into a static reservoir of carrier fluid and after manyhours the fraction of particles that remained on the surface wasβ ≈ 0.35. It is assumed these particles resided at the air−liquidinterface due to capillary forces. The surface energy associatedwith a 1 μm silica particle is much greater than 10 kT,suggesting it is stable at the interface and much stronger thangravity or Brownian motion. The result from the static reservoiragreed with the trend line fit of our monolayer data. While theflow near the suspension delivery mechanism and delivery ofparticles to the interface is a highly complex process, it operatesat steady state in this system. Changing various aspects of thedelivery mechanism may result in changes to the fraction ofparticles that eventually reside on the surface. This equationsuggests that, for a given surface coverage, UW is linearlydependent on the particle mass flow rate, m. Although themonolayer structure in these films are likely less ordered thanmaximum hexagonal packing, the linear trend is a strong fit forthe experimental data where UW ≤ 10 mm/s. For UW = 10mm/s, delivering the suspension having ϕS = 0.2 results in ahigh-quality monolayer deposition. At this same speed, lowervolume fraction suspensions delivered at correspondinglyhigher VS result in multilayer depositions or in discontinuousmultilayer regions, no longer following the trend of monolayerdeposition.To further elucidate this result, experiments operated under

constant particle mass flow rate and constant web velocity arehighlighted. For each set of experiments at relatively low UW, asingle particle mass flow rate and web velocity exists forming anideal monolayer deposition, separating discontinuous deposi-tion from multilayer deposition. Figure 4 displays images ofsamples prepared under constant particle mass flow rate, m =2.4 mg/s with a suspension ϕS = 0.05 at VS = 1.6 mL/min,while varying UW. Figure 4c shows a continuous monolayerstructure where the film is fully covered throughout the coatingwidth and particles are uniformly distributed. Starting from themonolayer structure, deposition at higher UW results in adiscontinuous structure shown in Figure 4a,b and deposition atlower UW shown in Figure 4d,e result in a multilayer structure.Optical microscopy suggests that the individually coateddomains in the discontinuous coating region have roughly thesame surface packing density, and the reduction of coatedmaterial primarily results from a patchwise coating of particles.Alternatively, in the multilayer region, the coating is continuousand the mass balance is satisfied by deposition of multiplelayers of particles. If the assumption that deposited particleswere trapped at the interface in the Langmuir trough region,the formation of this multilayer region is either a result ofsurface pressure removing particles from the interface or surfacebuckling33−35 at the particle scale during deposition. Theappearance of lines of varying optical density suggests thatlocally the multilayer is changing in surface coverage and theselines represent stress across the film during deposition,suggesting long-range surface buckling may be the mechanismforming the multilayer structure. This also suggests that the

assumption that the particles are deposited directly onto theweb may be valid.Likewise, for constant UW = 5 mm/s while varying m gives

similar results. The primary difference is each particle mass flowrate can be obtained by varying either the suspension volumefraction or the delivery rate. Figure 5 shows the morphologychange as m is increased under a fixed web velocity. Parts a andb of Figure 5 display the discontinuous regime of coating wherethe amount of particles added to the surface is insufficient tocoat the web at this speed while parts d and e of Figure 5 showconditions that result in multilayer deposition, again demon-strating features of multiple scales all much larger than theconstituent size, suggesting the multilayer results from stressimparted on the surface.

Figure 4. Images of films from discontinuous to multilayer at constantflow rate (m = 2.4 mg/s, ϕS = 0.05, and VS = 1.6 mL/min). The webvelocities are UW = 10, 7, 5, 4, and 2 mm/s, corresponding to panelsa−e, respectively.

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The breakdown of the linear trend at higher web velocity islikely a result of coating in the Landau−Levich regime, asshown in Figure 3b. As shown in the inset of Figure 2, at higherparticle mass flow rate, no apparent continuous monolayermorphology occurs at any web velocity. In order to interpretsuch a phenomenon, consider the behavior of the suspensionwhile translating upward with the substrate. In Figure 3a, atlower UW the apparatus is essentially pushing the monolayeronto the substrate with minimal fluid entrained. As shown inFigure 3b, the web essentially entrains a thin film of fluid alongwith the interfacial colloidal layer. According to the analysisperformed by Landau and Levich,31 the height of the thin film,

h, of fluid dictated by the balance of viscous drag and capillaryforces related to the radius of curvature of the liquid interface, isgiven as

=h l Ca0.945 c2/3

(4)

where lc = (γ/ρg)1/2 is the capillary length and Ca = μUW/γ isthe capillary number based on the surface tension γ. In thelaminar regime, the velocity profile can be calculated by theStokes equation:

ρ μ= ∇g u2(5)

where u is the vertical velocity field (note that y is orientedupward). Assuming no slip between the fluid and web, and freesurface shear stress free boundary condition, the interfacialvelocity (UI) is given by

ρμ

= −U h Ug

h( )2I W

2

(6)

ρμ

ρμ = − = +u U

gh U

gh

3 6W2

I2

(7)

u is the average velocity of the thin liquid film, related to UWand UI. Gravity-driven flow draws this fluid downward. Itshould be noted that much more sophisticated scalingapproaches have been considered where stress imposed bythe particle layer at the free surface is included as an elasticcomponent.36 Because the web is simply in contact with theliquid film and not pulled through the suspension, contact linedynamics at the wetting line make the analysis much morecomplicated than the simple approach taken here; however thescaling arguments in previous work may hold if one measuresthe thin film thickness profile.For large UW the liquid thin film is thicker, resulting in

particles residing at the air−liquid interface after beingtransferred to the web only after drying downstream. A simplemass balance gives

ρμ

βθ

ϕ− = Ug

haW

V2

34

1W

2S S

(8)

Here, UW is not linearly proportional to m due to the existenceof liquid thin film. In fact at high enough UW, where UW ≥ 23γ/0.9456μ for water, there is no solution to this equation. Moreimportantly, at a speed much slower than this threshold, theflow down the web becomes unstable.37 This instability of theflow is apparent as ripples form in the flow that translatesdownward. The final coating morphology, which likely is aresult of both flow instabilities and Marangoni flows that occurduring drying, are shown in the inset of Figure 2, where acritical UW is reached and flow no longer results in a monolayerdeposition.As discussed previously about the inset diagram in Figure 2, a

discontinuous multilayer structure was observed, indicating notonly the apparent nonlinear relationship between web velocityand particle concentration but also the film instability at higherparticle mass flow rate. Figure 6 shows various films withdiscontinuous multilayer morphology. We believe suchinstability comes from both the capillary-driven flow and theMarangoni effect during the drying process. The depositionoccurs during the evaporation of the solvent from the liquidfilm, but the free surface in our experiment is dynamic due tothe liquid drainage downward of the vertical passing substrate.At higher m and UW, the thickness of the liquid film increases.

Figure 5. Morphology of films from discontinuous to multilayer atconstant web velocity (UW = 5 mm/s). The SiO2 mass flow rate isincreased from a to e: (a) m = 0.6 mg/s, ϕS = 0.05, and VS = 0.4 mL/min; (b) m = 1.2 mg/s, ϕS = 0.05, and VS = 0.8 mL/min; (c) m = 2.4mg/s, ϕS = 0.05, and VS = 1.6 mL/min; (d) m = 4.8 mg/s, ϕS = 0.2, VS= 0.8 mL/min; (e) m = 9.6 mg/s, ϕS = 0.2, and VS = 1.6 mL/min.

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Once the particles are deposited onto the substrate, theirsurrounding liquid thins and the deposited particles distort thedynamic free surface. Within the thicker suspension film, it isclear that capillary attractions between particles entrapped inthe suspension film act to form the particle aggregates shown inFigure 6 as different white features. On the other hand, as thesubstrate passed to the horizontal region, the Marangoni effectacts to develop instability and needs to be taken into accountdue to the temperature gradient as well as the evaporation ofthe two miscible liquids (water/ethanol).38 Furthermore, thehydrophobic nature of the substrate poly(vinyl chloride)(PVC) also contributes to the discontinuous pattern.

4. CONCLUSIONSIn this systematic study, we investigate continuous roll-to-rolldeposition of particles floating at an interface. At low speedsand mass flow rates, a simple mass balance gives the idealdeposition parameters for monolayer coverage, separatingincomplete particle deposition across the substrate frommultilayer depositions. At higher speeds, instabilities associatedwith the thin film carried from the trough onto the web ruin theuniformity of the deposited layer at all conditions. Of course,

there are ways to reduce the effects of these flow instabilities byvarying viscosity, the angle at which the substrate passes thetrough effectively reducing gravity, and the drying rate andwettability of the solvent. Addition of surfactant also will havesome effect on the film formation and resulting instabilities, andit is likely that the presence of particles at the interfacethemselves may be reducing the onset of these thin filminstabilities. Of course, the ability to increase deposition speedand tune the microstructure will increase the robustness of thissystem for a wider variety of applications.

■ AUTHOR INFORMATIONCorresponding Author*Tel.: +1 (610) 758-4781. Fax: +1 (610) 758-5057. E-mail:gilchrist@lehigh.edu.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis material is based upon work supported by the NationalScience Foundation Scalable Nanomanufacturing Programunder Grant No. 1120399. We thank Dr. Eric Daniels for aidin setup and optimization of the instrument.

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Figure 6. Film instability images for (a) m = 19.2 mg/s and UW = 40mm/s, (b) m = 19.2 mg/s and UW = 30 mm/s, (c) m = 19.2 mg/s andUW = 20 mm/s, and (d) m = 9.6 mg/s and UW = 20 mm/s.

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DOI: 10.1021/acs.langmuir.5b03760Langmuir 2016, 32, 1220−1226

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