ARTICLE pubs.acs.org/Langmuir
Controlling Inplane Orientation of a Monolayer Colloidal Crystal by Meniscus Pinning Eric Chin Hong Ng,† Kah Mun Chin,‡ and C. C. Wong*,‡ † ‡
Singapore-MIT Alliance, N3.2-01-36, 65 Nanyang Drive, Singapore 637460 School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798
bS Supporting Information ABSTRACT: We demonstrate the usage of meniscus pinning by surface relief boundaries to control in-plane orientation of monolayer colloidal crystals without the interruption of grain disorientation. By optimizing the pinning boundary and withdrawal speed, a well controlled linear meniscus contact line offers unidirectional growth of a colloidal crystal—densely packed crystal direction Æ11æ and Æ10æ parallel to linear edge—giving rise to a single domain crystal with only twins and vacancies present as residual defects. The pinning effect works by eliminating the wavy contact line induced by fingering instability which is commonly found in liquid wetting film. It is found that surfactants and colloidal particles play significant roles to enhance edge pinning, increasing the distance traveled by receding bulk meniscus (during substrate withdrawal) before liquid depinning or rupturing.
’ INTRODUCTION Various top-down and bottom-up methods like templateassisted assembly,1,2 gravity sedimentation,3,4 physical confinement,5,6 electric field induced self-assembly,7-10 LangmuirBlodgett method,11,12 and capillary forces induced convective self-assembly13,14 have been explored to develop millimeter- or centimeter-scale single or polycrystalline domains of colloidal crystals. Among these, capillary forces induced convective selfassembly is attractive, requiring only a simple and economical setup. However, common nonidealities14-18 like restricted domain size, thickness nonuniformity, and empty bands or voids are frequently reported. A wavy contact line19 due to Rayleigh instability is believed to cause the multidirectional initiation of colloidal crystal growth, giving rise to multiple domains. Colloidal particles are first trapped16 by the thinning meniscus wedge along the wavy contact line, and then the crystal growth front is formed by the subsequent accumulation of particles by convective flow.16 Since domain growth directions tend to be different along the wavy contact line; this eventually limits the final domain size of colloidal crystals obtained. Besides, dynamic change of receding wetting angle during liquid or substrate withdrawal produces colloidal stripes and bands via stick-slip mechanism.20,21 The initial pinning18 of liquid film onto the substrate will allow colloidal particle deposition along the thinning meniscus wedge. Fast substrate withdrawal or receding bulk meniscus relative to colloidal deposition speed will pull the pinned contact line, either causing depinning or contact-line movement in a fingering pattern,22,23 together with the pinned colloidal domains. Often, the slipping distance is long, creating the next pinning point at a region well after the previous crystal growth r 2011 American Chemical Society
front, giving rise to repeated stripes and empty bands.18,20,21 Depinning often occurs at the initial contact line. In this paper, we propose a novel approach to control unidirectional growth to favor the formation of large domains with uniform thickness. This is achieved by fixing meniscus pinning along a straight, prepatterned linear surface relief. A straight initiation edge of monolayer crystal growth front then acts as a crystal growth seed, permitting the most close-packed direction Æ11æ or Æ10æ (as in 2D hexagonal lattice) to assemble along the surface relief. Pinning along surface relief edges is also enhanced to provide extra friction to minimize Rayleigh instability of contact line. We also optimize the substrate withdrawal speeds in few stages, establishing immobilized crystal growth front and compression of multilayer to monolayer formation (see Supporting Information at (2)).
’ EXPERIMENTAL SECTION Substrates. Silicon wafers (4 in. in diameter) were first pretreated with piranha solution (Caution!), with a ratio of three volumes of concentrated H2SO4 to one volume of 30% H2O2. This step will remove most of the organic residues and hydroxylate the silicon surface. The substrates were then patterned with optical lithography to produce numerous straight lines of surface relief, made of hydrophobic resist (SU 8 2000.5) of 200 nm thickness. The length of the surface relief is designed at 7 mm, such that the growth disturbances at the edges can be neglected. After the hard-bake process, the patterned wafers were treated Received: October 24, 2010 Revised: December 3, 2010 Published: February 11, 2011 2244
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Figure 1. (a) Laboratory setup used to grow colloidal crystal array with predetermined domain orientation. (b) Illustration of pinned meniscus wedge at patterned surface relief. d is defined as pinning distance as shown. with UV ozone for 2 min, making the surface relief slightly hydrophilic.24 This is to improve the adhesion and pinning of liquid film along the surface relief edges. Final contact angles of surface relief before and after UV ozone treatment are 78.69 and 62.50, respectively. UV ozone treatment was executed using Novascan PSD-UV. Materials. Aqueous suspensions of highly monodisperse polystyrene (PS) latex spheres with sulfate groups on the surface (1 wt % aqueous dispersions, 800 ( 9 nm in diameter, surfactant-free) were obtained from Duke Scientific. It was then diluted to 0.25 wt %. In order to reduce the wetting angle of suspension, 0.05 mg/mL of sodium dodecyl sulfate (SDS) was added into the suspensions. Other concentrations of SDS were also prepared to study its effect on pinning capability. Instrumentation. We adopt similar laboratory setup (Figure 1) used by Dimitrov et al,19 with wider chamber wall (25 mm) relative to the surface relief length of 7 mm. The substrate was mounted such that the linear surface relief is parallel to the bulk meniscus. A rotation stage is attached to the vertical motorized stage to assist this alignment step. A small gap of 2 mm between chamber wall and substrate was fixed to minimize solvent evaporation from the meniscus and confine the evaporation to the edge of pinned meniscus. Zaber motorized linear stages (T-LS Series) were used to withdraw the substrate with programmed speed. A horizontal long-focus microscope was used to monitor the pinning behavior (5 objective lens) and array growth (50 objective lens). Pinning Enhancement. Prior to colloidal crystal growth, pinning of pure water film and colloidal suspension film (with varying surfactant concentration) along surface relief was studied. It is desired to have maximum pinning strength to ensure uniform and straight contact line of colloidal suspension film, minimizing fingering instability.19,22,23 The effects of physical surface relief, surfactant concentration, and particle accumulation on pinning distance are compared. We define the pinning distance as maximum distance traveled by bulk meniscus before depinning occurs along the surface relief, as shown in Figure 1b. First the chamber was filled with respective solutions slowly up to the edge of surface relief. Equilibrium time of 2 min was given for pinning. Care must be taken to prevent prestretching and precompression of the pinned liquid film. Then the lower stage was moved vertically downward at 1 μm/s to stretch the pinned film. Once depinning occurred along the desired contact line, the distance traveled was recorded. At least three readings were recorded for each set of solution. Colloidal Crystal Growth. The chamber was first filled with colloidal suspension slowly up to the edge of surface relief. Lower stage can also be moved vertically up at slow speed (1-20 μm/s) to achieve this pinning. Once the meniscus was pinned along the surface relief, 1 min of equilibrium time was given for particle accumulation since the convective flow is slow. Then the substrate was withdrawn at a speed of 1.5 μm/s for 1 min, and gradually slowed to 0.5 μm/s for subsequent
growth. The assembly was done at room temperature (25 C), to minimize the evaporation rate and particle flux. Characterization. The quality of monolayer colloidal crystal was analyzed using scanning electron microscope (JEOL JSM 6360). Consecutive SEM photos along surface relief and growth direction were recorded to quantify its domain orientation and size. A wide scan up to 100 μm 1 mm is presented in the Supporting Information for evidence.
’ RESULTS AND DISCUSSION Pinning Enhancement. We first compare the pinning capability obtained between bare substrate and substrate with surface relief. Pure water film pinning along surface relief was observed to be 70% higher than that of bare substrate, recorded at 1000 μm pinning distance on average (star icon in Figure 2). Better pinning along surface relief may arise from OH groups created along the edges of surface relief during UV ozone treatment. Figure 2 shows that the existence of colloidal particles in the suspension can enhance pinning for the entire tested range of surfactant concentration used. This is in accordance to earlier studies18 that particles trapped under the thin meniscus wedge provide friction to the movement of contact line. Pure surfactant solution does not help pinning (Figure 3a), but the presence of colloidal particles in suspension were observed to enhance pinning strength up to 1850 μm (pink line in Figure 2). It is worth mentioning that no depinning was observed in this combination (SDS and colloids). Instead, rupturing of stretched liquid film was observed at a region located after the colloidal crystal growth front (Figure 3b). This is due to faster speed of receding bulk meniscus compared to speed of colloidal deposition under constant withdrawal at 1 μm/s. Particle protrusion out of the stretched film could also contribute to the nucleation of such rupturing. As the crystal growth front remains wet after rupturing, this may also indicate that the pinning strength along surface relief or on the assembled colloidal array is stronger than the surface tension holding the solvent film together. The accumulation of particles along surface relief has a width of about 5-10 μm. Comparing the mixture of surfactants (0.05-0.1 mg/mL) and particle solution with only pure particle solution, the presence of surfactants with particles does enhance the pinning by 20%. This could be explained by the cooperative interaction of hydrophobic surfactant tails with hydrophobic region of SU-8 resist and hydrogen bonding between water molecules with hydroxyl groups created by UV ozone treatment. 2245
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Langmuir Colloidal Crystal Growth. From the understanding gained from Figure 2, 0.05 mg/mL of SDS concentration is chosen for domain-oriented growth of colloidal crystal. The initial pinning of colloidal suspension leads to accumulation of particles by convective flow. As the evaporation is minimized and confined to the edge region, particle flux is slow and equilibrium time is required for pinning enhancement. In this case, particles will be trapped under confined meniscus. Accumulation of large area of particles will serve as a pinning base, giving extra friction18 to prevent contact line movement (and depinning) away from surface relief upon stretching. Subsequent substrate withdrawal at 1.5 μm/s causes the film to be stretched, pressing bilayer or multilayer of particles down to monolayer formation (see sequence of photos in Supporting Information at (2)). Then the gradual reduction of withdrawal speed to 0.5 μm/s allows continuous monolayer colloidal crystal growth after the capillary compression. 0.5 μm/s is the speed of monolayer crystal growth rate observed under the experimental conditions. From the microscopic observation, straight pinning of contact line along surface relief sets a straight initiation line for colloidal crystal nucleation and growth (Figure 4a). In contrast (Figure 4b), wavy contact line allows different growing directions from
Figure 2. Plot illustrating the distance a liquid film was stretched before depinning along surface relief (with pure SDS concentration) and the distance stretched before rupturing near colloidal crystal growth front, under various surfactant concentrations with the presence of particles.
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nonstraight initiation edge, leading to distinct domain boundaries. Although fingering effects of wetting solvent are also observed above the assembled colloidal crystal, as indicated in Figure 4, parts a and b, they do not bring significant disturbance to the inplane-oriented growth of colloidal crystal from straight surface relief. The assembled colloidal crystal remains wet, pinned at respective contact lines which nucleate the growth. Hence, it can be concluded that engineered surface relief is able to minimize or eliminate Rayleigh-instability induced wavy contact line prior to colloidal crystal nucleation and growth. Close-packed arrays formed along surface relief will subsequently serve as a crystal seed for in-plane oriented growth. This primary step is critical and the crystal seed must be perfect, or else domain boundaries will be created due to nonuniform growth direction. Regions away from the pinned contact lines have slightly higher meniscus thickness and show vivid fingering effects above the assembled crystal. Continuous pulling of bulk meniscus and drying of wet colloidal crystal may allow part of the assembled colloidal particles to protrude out of the meniscus, creating many microscopic pinning lines surrounding partly submerged particles. In this case, more surface area of water film is created and the local pinning strength may not be sufficient to resist dewetting at the particle level. Rayleigh instability may then prevail to minimize surface area, creating fingering effects over assembled crystal. Figure 5a illustrates the monolayer colloidal crystal packed along the surface relief. Particles are lined up in the close-packed direction Æ11æ or Æ10æ, along the surface relief. As a result, straight contact line creates a straight crystal growth front, giving rise to unidirectional domain growth. As evaporation is confined only to the meniscus edge, directional convective flow of particles maintains the array growth by replenishing the free particles near the growth front. Speed of withdrawal is critical to ensure no interruption of colloidal deposition which gives rise to empty bands, and no multilayer formation at large particle flux.18 Continuous replenishment of particles to ordered and oriented growth front at slow and controllable speed results in large monolayer crystal domain of (see Supporting Information at (3)). This process is rather slow compared to spin coating,25 but it allows control over domain orientation relative to the predetermined surface relief. Red lines indicated in Figure 5a and (3) at Supporting Information indicate the excellent control over intended domain growth direction and uniformity of domain orientation. Intensity line-plot
Figure 3. (a) Depinning occurs along the surface relief, indicated by red circle. The absence of colloidal particles in suspension causes earlier depinning with smaller pinning distance. (b) Liquid film ruptures near colloidal crystal growth front, leaving behind wet colloidal crystal along surface relief. Colloidal deposition is interrupted due to faster speed (1 μm/s) of film stretching, relative to slower flux of particles to the growth front. The scale bar is 10 μm. 2246
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Figure 4. (a) Straight pinning along surface relief, setting a straight initiation line for colloidal crystal nucleation and growth. (b) Colloidal assembly on bare wafer substrate only allows wavy contact line to dominate the nucleation and growth of colloidal crystal, leading to polydomain growth. Fingering effect of wetting solvent is observed above the assembled colloidal crystal as indicated. The growth directions are indicated by yellow arrows. The scale bar is 8 μm.
Figure 5. SEM of domain-oriented growth of colloidal crystal from the edge of straight surface relief, producing high degree of directionality. Particles are lined up in the close-packed direction Æ10æ or Æ11æ, along the surface relief. Red lines drawn serve as a guide to illustrate the perfect orientation under the straight pinning effect. (b) For comparison, colloidal assembly on bare substrate is shown, explaining the effect of wavy lines which result in domain growth of various directions. The desired growth directions are from left to right. (c and d) Line-plot profile generated using ImageJ along middle red line in part a and across central region in part b, respectively.
profiles generated using ImageJ26 for both Figure 5a and 5b are compared in 5c and 5d respectively, showing high degree of
directionality for engineered pinning assembly. SEM photos shown in Supporting Information are consecutive scans over large area, 2247
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Æ10æ (or Æ11æ) close-packed direction. This linear nucleation of Æ10æ colloidal lines serves as crystal seed for subsequent growth, giving rise to a large single domain, which can be potentially used in applications requiring high crystallinity. Furthermore, we provide an insight on how engineered pinning allows excellent control over growth direction of particle arrays, giving rise to desired domain orientation. However, careful attention is still required in this method to prevent increased flux of particles upon long period of evaporation. Dilute suspension is always preferred to allow better control over array growth, and to prevent accumulation of extra particles which would result in multilayer formation. Last, contamination-free substrate and uniform liquid film thickness under pinning is critically required.
’ ASSOCIATED CONTENT Figure 6. Meniscus thickness is altered around the dust particle, giving rise to multilayer formation in the vicinity. The scale bar is 8 μm.
demonstrating predetermined domain orientations, with respect to the surface reliefs. Domain Size and Quality. Conservatively, the domain crystal size observed is as large as 1 mm2, despite existence of small misoriented domains found within this large crystal domain. Faster flux of colloidal particles to the growth front is observed after growing 1 mm distance from the edge. This could be caused by the increase of particle concentration due to solvent evaporation. The increase of particle flux renders the growth uncontrollable at large area, leading to multilayer formation. From the SEM photos in Supporting Information, the domain orientation of large crystal domain remains similar despite the creation of small misoriented domains within. The existence of these small misoriented domains can be attributed to unclean colloidal suspension or nonuniformity of particle size. Besides, particle vacancies and twinning are two other defects observed during the characterization. Vacancy formation can be explained thermodynamically while twinning can be attributed to the crystal strain created within large domain of colloidal crystal. From observation, formation of vacancy and twinning do not bring much disturbance to the direction of crystal growth front. The occasional occurrence of multilayer domain seems correlated to dust and dirt particles (Figure 6). This is due to the altered meniscus thickness around the contaminant particles. Also, this multilayer domain may disturb the unidirectional growth of colloidal crystal, creating domain boundaries and mismatches. Size of Particles. High monodispersity of particle sizes is preferred to prevent domain mismatch and other defects. 800 nm colloidal particles are chosen in this study due to its visual clarity under the optical microscope. Smaller particles under the current thickness of meniscus may give rise to bilayer or multilayer formation. It is important to engineer a uniform thickness of meniscus relative to the particle diameter used. Smaller heights of surface relief or SAMs can be coated to engineer the pinning line with suitable contact angle or confined meniscus wedge of the right thickness, enabling smaller particle assembly under inplane oriented growth. Fingering instability of contact line must be greatly reduced by using the right pinning conditions, prior to colloidal crystal nucleation and growth.
’ CONCLUSION Domain-oriented growth of colloidal crystal is successfully demonstrated using engineered pinning of straight contact line, providing unidirectional growth of colloidal deposition perpendicular to
bS
Supporting Information. Tables containing the data of pinning distance traveled under the effect of surfactant concentration and existence of colloidal particles and figures of a photo sequence showing the capillary compression of bilayer into monolayer during substrate withdrawal and film stretching and consecutive scanning of monolayer colloidal crystal over large area (about 100 μm 1 mm). This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
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