Article pubs.acs.org/Langmuir
Crystal Perfection of Particle Monolayer at the Air−Water Interface Kei Shinotsuka,* Yasuhito Kajita, Koki Hongo, and Yoshihisa Hatta Development Center, Innovation Promotion Division, Oji Holdings Corporation, 1-10-6, Shinonome, Koto-ku, Tokyo 135-8558, Japan ABSTRACT: Crystal growth in colloidal particle monolayers fabricated by Langmuir−Blodgett method on 4 in. Si wafers was investigated under the condition of two techniques, that is, ultrasonic annealing at 1.2 to 1.5 MHz and barrier-sway process at 0.2 to 0.5 Hz. Significant increases of the ordered area were obtained by the both techniques and more than 60 times growth was confirmed. The remaining crystal defects after the growth were categorized as grain boundary, vacancy, and line defect. Both techniques exhibited different features regarding the component ratio of the defects, and different mechanisms for the reorientation of particles are discussed. The driving force of these reorientations is thought to be associated with the 2D Ostwald ripening of colloidal crystals.
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INTRODUCTION Nanosphere lithography 1−3 based on the 2D particle monolayer has attracted considerable interest in recent years because of its simpleness of the process, easiness of large area fabrication, and low production cost. Several applications using the particle monolayer of both the nanometer and the micrometer scales have been reported such as antireflection structure,4−6 photonic crystal,7−9 as well as biotemplate purposes.10−12 The particle monolayer can be achieved by self-assembly of colloidal particles, which is driven by lateral capillary forces between the particles.13,14 Deposition of particle monolayer on a substrate has been performed by several coating techniques such as Langmuir−Blodgett method,15−18 convective selfassembly (including dip coating method),19−22 spin coating,23−25 sedimentation,26 and other methods. Langmuir− Blodgett method has been utilized to prepare a highly ordered particle monolayer membrane, in which the particles are closely packed as 2D hexagonal crystal ((111) plane of fcc). Compared with the other coating techniques, Langmuir−Blodgett method has some superior aspects in terms of high packing density, small ratio of defects, and easiness of forming a monolayer. Particle monolayer obtained by Langmuir-Bridgett method, however, still contains a small amount of crystal imperfections mainly consisting of grain boundaries, vacancy defects, and line defects. These crystalline defects are generally formed when the particles are released at the water−air interface and form a selfassembly membrane. Because the nucleation mode of the particles is sporadic, the subsequent crystal growth results in a 2D polycrystalline entity, which inevitably includes many grain boundaries. Because the gap of the grain boundary and line defects is around one period (= diameter of a particle) or less and vacancy is mostly one period, consequential influence caused by © XXXX American Chemical Society
these inherent defects is thought to be very small; however, when the particle monolayer is used for precision purposes such as lithography mask, micro- or nanofabrication will be influenced by the defects. In this work, we exploited crystal perfection techniques of the particle monolayer by means of ultrasonic annealing and barrier-sway process. Both techniques are expected to facilitate crystal growth at the grain boundaries.
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EXPERIMENTAL SECTION
Spherical colloidal silica particles of 3.0 μm in diameter (1.0 C.V. %) were commercially purchased. The surface of the particles was modified to hydrophobic by immersing treatment with aminopropyltriethoxysilane or allyltrimethoxysilane.18 The degree of hydrophobicity was appropriately controlled by varying the treatment time. After the surface treatment, the colloidal particles were filtrated with 10 μm pore-sized PTFE filters to remove secondary particles included in the slurry. Then, the colloidal particles were dripped with chloroform−ethanol mixture (80/20 v/v %)17,18 on the water surface of a Langmuir−Blodgett trough (surface area 400 × 560 mm), and a particle monolayer was formed at the air−water interface by selfassembly process. A sapphire wafer (φ2-in., thickness 0.43 mm, surface Ra < 0.3 nm), which had been set in the trough beforehand, was withdrawn upward at a rate of 5 mm/min from the subphase water and the particle monolayer was transferred from the water surface onto the wafer surface. A specially designed ultrasonic transducer (effective irradiation field 100 × 100 mm) was set at the bottom of the trough, which was powered by an ultrasonic generator (type SD-32CP-M, New Sun Electron Ultrasonic Devices). 1.2 to 1.5 MHz oscillation was irradiated from the transducer head to the water surface at a power of 1.0 to 2.0 W for 30 min at 20◦C, while the colloidal particles were dripped and Received: August 23, 2015 Revised: October 3, 2015
A
DOI: 10.1021/acs.langmuir.5b03151 Langmuir XXXX, XXX, XXX−XXX
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Langmuir allowed spreading on the water surface. The power of irradiation was restricted, avoiding acoustic cavitation, which causes bubbles in the subphase water. In this range of frequency, ultrasonic wave exhibits high directive transmittance in water, such that a limited field (100 × 100 mm) was irradiated by vertically transmitted sonication (Figure 1, setup A).
Figure 1. Schematic illustrations of the setup of Langmuir-Brodgett trough. Setup A: ultrasonic irradiation to the particle monolayer. Setup B: barrier-sway process to the particle monolayer. Another physical treatment was given to the particle monolayer by manipulating the barrier of Langmuir−Blodgett trough to sway forward and backward on the water alternately at a frequency rate of 0.2 to 0.5 Hz with an amplitude of 25−35 mm for 30 min at 20 °C. Owing to the barrier-sway process, the particle monolayer experienced expansion and contraction of the interparticle spacing periodically in the lateral direction (Figure 1, setup B). In this particular case, the process does not produce “wave” because the manipulation by expansion and contraction of the water surface area does not conduct energy to a direction. The coating of particle monolayer onto a wafer by Langmuir− Blodgett method was performed after these physical treatment techniques were completed. The crystallinity of the particle monolayer was examined by optical microscopy images (NIKON Optiphot-200) measuring the length of grain boundary, number of vacancy, and length of line defect. The image analysis was carried out with imageprocessing software of A-zou-kun. A typical feature of interfacing two crystal domains over a grain boundary was captured by SEM (JEOL JSM-7800F) magnified at 15 000×.
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Figure 2. Particle monolayer coated on 4 in. sapphire wafers. (A) Control, (B) with ultrasonic treatment, (C) with barrier-sway treatment, and (D) their approximate grain area of the top 10 largest crystals.
RESULTS AND DISCUSSION The appearances of the sapphire wafers coated with the particle monolayer are shown in Figure 2. Figure 2A is the control coating consisting of polycrystalline domains. These crystal domains each have crystal orientation that differs from the surrounding domains. Ultrasonic annealing, however, brought about distinct enlargement of the area of each domain as shown in Figure 2B. The crystal growth was caused by the kinetic energy supplied by ultrasonic irradiation. Furthermore, the barrier-sway process resulted in the production of extraordinary large single-crystalline domains, as shown in Figure 2C. Compared with the ultrasonic, the barrier-sway exhibited higher efficiency for reorientation of the particles. Figure 2D shows the approximate crystal area of the top 10 largest domains of the three methods of coatings. It is notable that the top two areas of the barrier-sway recorded significant increase, reaching 62 times larger than that of control and 2.6 times larger than that of ultrasonic.
The observed increase in the area of crystalline domain is associated with the Ostwald ripening theory, whereby the growth of larger size crystals is favor at the expense of smaller ones, resulting in an increase in the 3D crystal volume as a linear function of time. 2D Ostwald ripening has been also reported on the observation of molecular quasi-monolayer films,27−29 and it is characterized by reduction in the perimeterto-area ratio, resulting in the decrease in the interfacial energy between the crystals.29 The classic Ostwald ripening theory was established as Lifshitz−Slyozov−Wagner (LSW) theory30 ⟨R ⟩3 = ⟨R 0⟩3 + B
8γc∞v 2D t 9R gT
(1) DOI: 10.1021/acs.langmuir.5b03151 Langmuir XXXX, XXX, XXX−XXX
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Langmuir
additive physical mobility to the particles drastically prompted Ostwald ripening. In the case of 2-D molecular polycrystal, the available space for the reorientation of the crystal growth is liquid-like zones at the grain boundaries, and coarsening progresses as a function of time.29 On the contrary, the reorientation of the colloidal polycrystal in a static state becomes difficult because the gap at the grain boundary is not sufficient for a particle to diffuse and reorient. (Figure 3). Indeed, if the system is left without any physical vibratory forces, no coarsening of crystal domains was observed even after several hours. In this aspect, the Ostwald ripening of colloidal crystal is different from that of molecular crystal. A typical optical microscopy image of the particle monolayer coated as control is shown in Figure 4. There are three main defects in the image, that is, (A) grain boundaries, (B) vacancies, and (C) line defects. Line defect should be distinguished from edge dislocation because there is no Burger’s vector observed in line defects in this work.31 Figure 5 exhibits the total amount of these three defects measured in 9 areas (one area is 300 × 300 μm) on the φ2-in. sapphire wafers. The nine areas are apart from one another by 10 mm. As shown in Figure 5A, the average of the total length of grain boundary in a measured area became drastically small in the barrier-sway (14.4 μm) and the ultrasonic (112.5 μm), while the control included plentiful grain boundaries (689.9 μm). This micrographic difference is consistent with the apparent differences in the area of colloidal crystals in Figure 2. It is thought that the barrier-sway gives opportunity to all of the
Figure 3. SEM image of a typical grain boundary in between crystal domains 1 and 2. The gap of the grain boundary is around one diameter of a particle or less.
where ⟨R⟩ is average radius of grains, γ is surface energy of grain, c∞ is solubility of the grain, v is molar volume of grains, D is diffusion coefficient of the particles, Rg is ideal gas constant, T is absolute temperature, and t is time. The ultrasonic annealing or barrier-sway process is thought to enhance the diffusivity of the particles, such that the D in the eq 1 is increased, resulting in the higher crystal growth rate. Thus, it is thought that the
Figure 4. Example of an optical microscopy image of particle monolayer coated as control (upper) and processed image of the same area (bottom). There are (A) grain boundaries, (B) vacancies, and (C) line defects, shown by arrows. C
DOI: 10.1021/acs.langmuir.5b03151 Langmuir XXXX, XXX, XXX−XXX
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Figure 6. Schematic models of reorientation processes at the grain boundary. A1: Ultrasonic annealing enhances kinetic mobility of a particle, A2: coupled oscillation during the transition state, A3: reorientation to a stable site, B1−B2: barrier-sway causes expansion of interparticle spacing while particles translate to the grain boundary, and B2: when the spacing contracts the translating particles settle into stable sites.
energy for reorientation, which performs similar role as the thermal fluctuations in the molecular Ostwald ripening. Under the vibratory forces a particle behaves as an oscillator with spring constant k. When the surface distance of two particles is >10 nm, the electrostatic force and the van der Waals force are negligible small, and hence k is approximately determined only from the gradient of capillary force versus surface distance plot, which is 5.67 × 10−3 N/m.32 Although the particles at the grain boundaries are thought to behave as 2D multiple coupled oscillator (Figure 6A), the estimation here is focused on one-directional translation from smaller to larger crystalline domain, which is simplified to 1D single oscillator with two springs. The natural frequency of the single oscillator f n = 1/2π(2k/m)0.5 gives 0.95 MHz, where m is mass (3.11 × 10−14 kg: weight of one 3.0 μm silica particle). Therefore, the irradiated ultrasonic frequency of 1.2 to 1.5 MHz is higher than the natural frequency of a particle to maximize its amplitude. Even though the oscillation given in this study was not the normal mode, the kinetic mobility of particles was distinctively enhanced, which means if it is appropriately adjusted to the natural frequency there is further capability to realize a higher mobility to translate. On the contrary, the frequency of barrier-sway with 0.2 to 0.5 Hz totally differs from the range of normal mode of the particles. In this specific case, the particles do not oscillate around the original position but experience regular periodic motion in which interparticle spacing expand and contract periodically, facilitating reorientation of the particles (Figure
Figure 5. Comparison of defects in the optical microscopy images. (A) Total length of grain boundaries, (B) number of vacancy defects, and (C) total length of line defects.
particles at the grain boundaries to translate and reorient themselves, whereas the ultrasonic provides sufficient mobility to the system but allows limited particles to reorient at the same time due to the steric hindrance in the gap of the grain boundaries. Assuming that all colloidal particles have same diameter, the translational motion of a particle obeys the 2D Langevin equation32,33
∂v = F − ξv + R (2) ∂t where m is mass, v is translational velocity vector of the particle, F is vector of external force, ξ is the coefficient of Stokes’ drag force, and R is vector of random Brownian force. In particular, the external force F in this study is thought to consist of capillary force, electrostatic force, and van der Waals force. m
F = F cap + F est + F vdw
(3)
The given vibratory forces by the ultrasonic or barrier-sway are not included in the eq 3 because these forces simply supply the particles with cyclic sinusoidal motion and do not contribute directly for attractive or repulsive forces. Instead, the vibratory kinetic motion of the particles provides activation D
DOI: 10.1021/acs.langmuir.5b03151 Langmuir XXXX, XXX, XXX−XXX
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6B). It is possible to increase and decrease the interparticle distance of all the particles approximately at the same rate, and through this process the particles crystallize repeatedly growing more stable domains depending on Ostwald ripening. Figure 5B shows the average of the total number of vacancy in a measured area regarding barrier-sway, ultrasonic, and control samples. It is intriguing that the number of vacancy is doubled by the barrier-sway (60.1) compared with the control (30.6). The ultrasonic (32.9) did not increase vacancy substantially. It has been reported that Ostwald ripening accompanies diffusion and coarsening of vacancy,34−36 but neither growth nor decrease in vacancy number was observed in this study. The reason for this discrepancy is not clear enough but might be related to the difference of the lattice rigidity between the molecular crystals and colloidal crystals; that is, the former has free volume that enables diffusion of vacancy, whereas the latter has little degree of freedom to fluctuate in the lattice. In the case of the ultrasonic, the number of vacancy of the control and the ultrasonic is almost same, implying that the vacancy sites are conserved while the coalescence of the system progresses. This means the vacancies are not extracted to the grain boundaries by Ostwald ripening and microscopic observations are consistent with it. The number of vacancies in the barrier-sway, however, became two times higher than the ultrasonic and the control. One possible assumption is that if a particle slips out of the original lattice position while the barrier-sway process, a new vacancy might be easily introduced. Figure 5C shows the average of the total length of line defect in a measured area. The line defect of barrier-sway (167.0 μm) increased from the original control (139.3 μm). Because the emergence of line defect is related to the shear failure or slip of a plane (line), the expansion and contraction process of the barrier-sway might damage the crystalline ordering. On the contrary, the ultrasonic annealing decreased the incidence of line defect (79.5 μm), suggesting that the crystal regularity was restored by experiencing the reorientation of colloidal particles.
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CONCLUSIONS It was demonstrated that the ultrasonic annealing and the barrier-sway process increased the area of crystal domains by more than 20 and 60 times, respectively. Because the colloidal crystal is expected for plentiful applications, these techniques may suggest a possibility of industrial utilization. There are still remaining problems to be solved such as the vacancy and line defect in the lattice as well as long processing time. Thus, Ostwald ripening process should be optimized to improve the crystal quality. At the same time, it was shown that Ostwald ripening exerts reorientation drive even on the colloidal crystals, which is thought to behave different from the molecular crystals. A further investigation is required to elucidate the specific growth mechanism.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected], vzv06360@nifty. com. Notes
The authors declare no competing financial interest. E
DOI: 10.1021/acs.langmuir.5b03151 Langmuir XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.langmuir.5b03151 Langmuir XXXX, XXX, XXX−XXX
