Rapid Assembly of Colloidal Crystals under Laser Illumination on a

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Rapid assembly of colloidal crystals under laser illumination on GeSbTe substrate Kei Yamaguchi, Eiji Yamamoto, Ryo Soma, Bokusui Nakayama, Masashi Kuwahara, and Toshiharu Saiki Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00176 • Publication Date (Web): 22 Apr 2019 Downloaded from http://pubs.acs.org on April 23, 2019

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Rapid assembly of colloidal crystals under laser illumination on GeSbTe substrate Kei Yamaguchi,† Eiji Yamamoto,∗,‡ Ryo Soma,† Bokusui Nakayama,† Masashi Kuwahara,¶ and Toshiharu Saiki∗,† †Department of Electronics and Electrical Engineering, Keio University, Yokohama-shi, Kanagawa 223-8522, Japan ‡Department of System Design Engineering, Keio University, Yokohama-shi, Kanagawa 223-8522, Japan ¶National Institute of Advanced Industrial Science and Technology, Tsukuba-shi, Ibaraki 305-8560, Japan E-mail: [email protected]; [email protected] Phone: +81-45-563-1151. Fax: +81-45-566-1720 Abstract Optical techniques have been actively studied for manipulating nano- to microsized objects. However, long-range attraction and the rapid transport of particles within thin quasi-two-dimensional systems are difficult due to the weak thermophoretic forces. Here, we introduce an experimental system that can rapidly generate quasi-twodimensional colloidal crystals in deionized water, sandwiched between two hard plates. When a pulsed laser is irradiating on a chalcogenide phase-change material spattered on one side of the plates, the induced Marangoni-like flow causes a colloidal self-assembly of the order of tens of micrometers within the laser spot, with a transport velocity of a few tens of micrometer per second. This is due to the large thermal gradient induced by chalcogenide characteristics, such as high laser absorption and low thermal

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conductivity, and strong hydrodynamic slip flow at the hydrophobic chalcogenide interface. Moreover, the colloidal crystals exhibit various lattice structures, depending on the laser intensity and chamber distance. For a certain range of the chamber distance, the colloidal crystal phases can be alternated by tuning the laser intensity in real time. Our system forms and deforms quasi-two-dimensional colloidal crystals at an on-demand location on a GeSbTe substrate.

INTRODUCTION Optical techniques have been intensively investigated for manipulating nano- to microsized particle structures in nonequilibrium conditions. The rapid and on-demand control of particle-structure formation under a laser-focused spot can lead novel applications, such as in microfluidic devices, photonic devices, and molecular sensors. 1,2 Under laser irradiation, the induced temperature gradient causes the particles to migrate following the linear function of the temperature gradient, in a phenomenon called thermophoresis. 3 By turning on and off the thermal gradient, the reversible formation of twodimensional colloidal crystals can be manipulated on substrates. 4–6 The thermophoretic mobility depends on the interfacial properties of the particles. 7,8 At large thermal gradients, the hydrodynamic slip flow is dominant on the particle surface and subsequently, hydrodynamic interaction occurs between particles. 5 However, thus far, the reported speed of thermophoresis is typically well below 1 µm/s, under a temperature gradients of less than 0.1 K/µm, 1 which is considerably slower than electrophoresis. 9–11 To generate a strong migration force, plasmonic nanotweezer techniques have been used by constructing plasmonic nanoarchitectures on substrates. 12 The enhanced optical forces, and the optically induced thermophoretic and convection flows cause colloidal aggregation. 13,14 A plasmonic nanoantenna in conjunction with an electric field can move particle with a velocity exceeding 10 µm/s. 15 Recently, self-assembly of colloidal particles has been reported at the bubble/water interface. 16 This is attributed to a large surface tension gradient along the gas-liquid interface 2


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by temperature gradient, which is known as the Marangoni effect. 17 The implementation of similar thermal properties on the water/substrate boundary can cause strong and rapid attraction of particles. Here, we report an experimental system in which a pulsed laser focused on a GeSbTe substrate can rapidly generate quasi-two-dimensional colloidal crystals in the order of tens of micrometers, in several tens of seconds. GeSbTe is a phase-change material, 18 which has thermal properties including high laser absorption, low thermal conductivity, and low surface energy. The laser-induced strong thermo-osmotic flow moves the colloidal particles to the laser spots from a long range of several hundred micrometers. The crystallization can be turned on and off with a pulsed laser.

METHODS Experimental setup Our experimental setup is depicted in Fig. 1A. We constructed a microfluidic sample, which contained microsized polystyrene beads (Polysciences, Inc.) dispersed in deionized water, sandwiched between GeSbTe-coated SiO2 and SiO2 substrates. A thin 100 nm GeSbTe film was deposited on the SiO2 substrate by sputtering. GeSbTe is a chalcogenide phasechange material, whose crystal and amorphous phases can be changed reversibly by laser heating, and has considerably low thermal conductivity. We used two types of polystyrene particles, with diameters of 1.0 µm and 2.0 µm (coefficient of variance: 5 %), respectively, which functioned as spacers. Then, we irradiated the sample using a subnanosecond pulsed laser with a wavelength of 532 nm (pulse width of 750 ps, repetition frequency of 19 kHz), and recorded the particle motion in the irradiated area using a CMOS camera (ORCA– Flash4.0 V3) at 20 fps. The laser was defocused in order to illuminate a wide area of the substrate. When a 100x objective lens was used, the diameter of this area was approximately 100 µm. Additionally, we prepared a 30 nm Au-coated substrate, which has high thermal conductivity, for comparison with the GeSbTe substrate. 3


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Numerical simulation of hard-core particles confined between two plates We performed numerical simulations of hard-core particles confined between two hard plates. The particle dynamics were governed by the Langevin equation,

mv˙ i = −mγv i +

N X

p F inter + F laser + 2γkB T ξ(t),

(1)

j6=i

where γ is the friction, m is the mass of the particle, kB is the Boltzmann’s constant, T is the temperature, and ξ(t) is the white Gaussian noise with hξ(t)i = 0 and hξ(t)ξ(t0 )i = δ(t − t0 ). The interaction force, F inter , between particles is short-ranged, and the excluded-volume repulsive force given by the Weeks–Chandler–Andersen (WCA) potential 19 is UW CA = 4[(σ/r)12 − (σ/r)6 + 1/4] if r < 21/6 σ, or zero otherwise, where σ is the particle diameter, and is the energy scale. Here, we set reduced quantities, σ, , m, γ, and kB T = 1. In the experiments, after tuning on the laser, colloids come toward the laser spot. We assumed a laser driven force by the Gaussian beam toward the center spot, F laser = −β exp (−2rc2 /w02 ), where rc is the distance from the center spot, w0 is the spot size, and β is a constant corresponding to the laser intensity. We carried out simulations with 512 particles. The spot size used in simulations was w0 = 100, which is almost ten times larger than the radius of the colloidal crystals in the simulation system. To draw the phase diagram, we evaluated the UW CA of the formed crystal structures.

RESULTS AND DISCUSSION Crystallization of colloidal particles under pulsed laser irradiation In the experimental system, tracer and spacer polystyrene beads with diameters of 1.0 µm and 2.0 µm, respectively, were immersed in deionized water sandwiched between Ge2 Sb2 Te5

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(GeSbTe)-coated and noncoated cover glasses. The chamber distance, ∼2.0 µm, between the two slides varies slightly depending on the focal plane. Although the distance between the two substrates is mainly determined by the diameter of the spacer particles, it varies locally depending on the pressure applied for sandwiching the two substrates, the particle size distribution, and the concentration of the spacer particles at a focal plane. A subnanosecond pulsed laser with a wavelength of 532 nm was focused on the GeSbTe-coated substrate with a spot size of ∼60 µm. GeSbTe is a phase-change material utilized in optical rewritable memory disks, whose crystal and amorphous states can be switched by irradiating with different pulsed laser intensities. GeSbTe was initially in an amorphous phase in our experiment, and after irradiating with a pulsed laser, the GeSbTe within the laser spot was partially crystallized by laser heating (light colored part in Fig. 1B). When the laser was turned on, the particles moved toward the center of the laser spot, floating near the GeSbTe surface, and formed quasi-two-dimensional crystals subsequently (see Fig. 1B and Movie S1). The transition temperature of GeSbTe is about 413 – 453 K. 20,21 Because GeSbTe was crystalized instantaneously in our experiments, the temperature near the surface of the GeSbTe substrate must be higher than the transition temperature. Although these colloidal crystals remained as long the laser was turned on, when it was turned off, the crystals started to melt and the particles exhibited random walk. Figure 1D shows the lateral trajectories of 58 tracked particles and probability density function (PDF) of particle velocities after tuning on the laser at a fluence of 0.78 mJ/cm2 (see Movie S5). In the system, the particles moved toward the laser spot with a velocity of a few tens of micrometer per second, depending on the laser intensity. When the size of the colloidal crystal increased to a certain extent at the laser spot, the velocity decreased below several micrometer per second (see Fig. 1E). Particles that were several hundred micrometers away from the center of the laser spot moved to the laser spot. In addition, our system could form colloidal crystals using 0.5 µm polystyrene beads although the manipulation of colloidal crystals using submicrometer-sized particles is difficult due to the influence of Brownian 5


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motion (see Fig. S1). Our system can arbitrarily repeat the formation and deformation of quasi-two-dimensional colloidal crystals at an on-demand location on the GeSbTe. By changing the laser intensity, the kinetics of the colloidal crystal growth can also be controlled. Figure 1C shows the dependency of the colloidal-crystal area growth rate on the laser intensity (see Movies S2–S4); the growth rate of the colloidal crystal increases with the laser fluence. In most previous experiments, the important thermodynamic control variable of the colloidal crystal phase was the particle volume fraction; 22,23 in our system, the laser intensity corresponds to an applied external pressure toward the center.

Colloidal crystal phases The self-assembly of colloidal particles has been intensively studied as an ideal experimental system of particles, including the crystallization, structural phase transition, and melting, 22,23 because colloids can be directly viewed using optical microscopes. Next, we demonstrate the various aggregation forms of the colloidal crystals, including the monolayer triangular lattice (Fig. 2A), buckling phase (Fig. 2B), bilayer square lattice (Fig. 2C), and bilayer triangular lattice (Fig. 2D) (see Movies S1, S2, S6, S7). In these figures, the light and dark particles are in the focal and out-of-focal planes of the microscope, respectively. In the monolayer state, the buckling process can be tracked, where increased geometrical frustration induces straight or zigzag stripe buckling in a triangular lattice (see Fig. S2 and Movie S8). The neighbors of a sphere tend to be in the opposite Ising state for more free-volume entropy. 24–26 To evaluate the triangular and square lattice pattern formations, the Lindemann paramPN q 2 1 27 eter was calculated as follows: φL = N −1 j6=i hrij i − hrij i2 /hrij i, where N is the total number of particles, i, and neighboring particles, j, and rij is the distance between particles. In Figure 2E, the particles are overlaid with dots colored as per the value of φL , where φL for the square and triangle lattices include values ranging from 0.15–0.2 and 0.05–0.1, respectively. The highly ordered lattices form small domains and are connected to the grain 6


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boundaries. Such colloidal crystals were observed by previous Monte-Carlo simulation studies on spherical particle system confined between two parallel plates. 28,29 The crystal pattern depends upon the distance between the two plates and the packing fraction. In our experimental system, the different patterns depend upon the thickness of the colloidal solution (∼2 µm) and the laser intensity, which corresponds to lateral pressure. To explain the observed crystal phase under laser irradiation, we consider a Langevin model of colloidal particles sandwiched between two walls. A short-range repulsive force was applied for the interaction of the particles. We assume that there is a migration flow toward the center of the laser spot by laser irradiation, which drives the particles to the laser spot. Figure 3 shows the phase diagram of the simulated colloidal crystals, with the laser intensity versus the distance between the two walls. The crystal phase strongly depends upon the distance between the two walls; for instance, as the distance increases, we can observe the phase change of the colloidal crystals from a monolayer triangle lattice, buckling phase, bilayer square lattice, or bilayer triangle lattice. The colloidal crystal phase also depends upon the laser intensity for certain ranges of the distance between substrates, H/σ. To demonstrate the transition of the crystal phases in our experimental system, we alternate the laser intensity, which is equivalent to lateral pressure in the simulation, when irradiating with a pulsed laser (see Fig. 4A and supplemental Movie S9); the movie begins with a liquid phase (t = 0 s), and the colloids transform into a coexistence of one-dimensional triangle lattice and quasi-buckling pattern, when the laser is turned on (0.35 mJ/cm2 ). When the laser intensity increases to 0.89 mJ/cm2 (t = 60 s), the coexistence of crystal lattices changes into a buckling phase. After turning off the laser, the colloidal crystal starts to melt, and the particles diffuse back into a bulk and locally melt from the outside boundary. This transition is similar to the transition of crystal phase around H/σ = 1.4 in the simulation model. Here, we present a simple model to interpret the experimental observations. We should note that the simulation model needs to be further improved, e.g., 7


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by considering hydrodynamic interactions and the potential for particle-particle and particleplate interactions, etc., for better explanation of the experiments. In addition, the chamber distance between the two slides varies slightly depending on the focal plane in our current system; this could be controlled by constructing micro pillars on the substrate, which would enable better comparison between simulation and experimental results.

Mechanism of rapid assembly of colloidal crystals In our system, the colloidal particles diffuse toward the laser spot with a velocity of a few tens of micrometer per second from a long-range of more than hundred micrometers. The typically reported thermophoretic speed is typically well below 1 µm/s, and the range of attraction is less than submicro meters. To understand the mechanism of this rapid selfassembly of colloidal particles using pulsed laser irradiation on a GeSbTe substrate, we conducted experiments using different substrates. On Si and Au substrates, colloidal crystal creation was not observed even for a high laser intensity, which is about ten times higher than the laser intensity used for GST substrate (see Figs. 5A and 5B). Under much higher laser-power irradiation, the microbubble generation was observed at the laser spot, and the colloids migrated toward the bubble/water boundary at high speed. This is reminiscent of the previously reported rapid colloidal self-assembly induced by the Marangoni effect at the boundary of the bubble/water interface. 16 This rapid colloidal assembly may be due to the characteristics of the GeSbTe substrates: The first characteristic is the laser-irradiation-induced high thermal gradient at the interface of the GeSbTe substrate. GeSbTe can substantially absorb pulsed laser at a wavelength of 532 nm. The thermal conductivity of GeSbTe is considerably low, at 0.17 and 0.5 W/(m·K) in the amorphous and crystal phases, respectively; 30 on the other hand, the thermal conductivities of Au, Si, and silica are 318, 31 168, 31 and 1.3 W/(m·K), respectively, at a temperature of 298 K. Thus, the GeSbTe characteristics, such as high laser absorption and low thermal conductivity, cause a high thermal gradient at a local spot. The thermal gradient induces 8


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a thermo-osmotic flow at solid-liquid interfaces. 32,33 The second characteristic is hydrophobic characteristics of GeSbTe substrates. Wetting of substrates can be explained using the spreading parameter, 34 S = γSV − γSL − γLV , where γSV is the solid/vapor interfacial energy, γSL is the solid/liquid interfacial energy, and γLV is the liquid/vapor interfacial energy. If S < 0, the liquid partially wets the substrate, while if S > 0, the liquid wets the substrate completely. The interfacial energy γSV of GeSbTe at 0.1–0.16 J/m2 35,36 is nearly ten times lower than those of Si and Au at ∼1.3 J/m2 , 37 i.e. GeSbTe is more hydrophobic than Si and Au. A molecular dynamics simulation study showed that nonwetting surfaces considerably amplify the thermo-osmotic flow due to hydrodynamic slip on the solid surface. 38 Thus, these characteristics of GeSbTe may induce Marangoni-like strong flow, generating a long-range driving force that moves the particles toward the laser spot. Using an SiO2 -coated GeSbTe substrate, we observed the slow-down of colloidal migration toward the laser spot, and the formation of a colloidal crystal was not observed under pulsed laser irradiation (see Fig. 5C). This is considered to be because of the high thermal gradient at the laser spot and the hydrophobic interaction at the solid-liquid interface is disrupted by the covered SiO2 on the GeSbTe. The mechanism of rapid and long-range colloidal self-assembly on GeSbTe under pulsed laser irradiation should be further studied in future, and can lead to more efficient techniques for controlling nano- to microsized objects.

CONCLUSION In summary, we have introduced an experimental system for the rapid assembly of quasitwo-dimensional colloidal crystals. When a pulsed laser is focused on a chalcogenide phasechange material, the induced thermo-osmotic flow causes the formation of various quasi-twodimensional lattice structures, depending upon the laser intensity and chamber distance. Such a technique for the formation of colloidal crystals can enable rapid and on-demand tuning of nano devices. Moreover, this colloidal system can explore the open questions on

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single-particle dynamics in quasi-two-dimensional colloidal crystals. 22,23

Acknowledgement We thank Dr. Takuma Akimoto, Dr. Masashi Aono, Dr. Makoto Asai, Dr. Koji Fukagata, Dr. Song-Ju Kim, and Dr. Makoto Naruse for the beneficial discussions. This work was supported by the MEXT Grant-in-Aid for the “Building of Consortia for the Development of Human Resources in Science and Technology”, KAKENHI Grant-in-Aid (Nos. 16H03889 and 24226006) from JSPS, the Core-to-Core Program, Advanced Research Networks, and partially by the Advanced Photon Science Alliance Project from MEXT.

Supporting Information Available The following files are available free of charge. • Supplemental figures (PDF). • Movie S1 (Figure1B_and_Figure2D.mpg. The movie is accelerated 16×.) • Movie S2 (Figure1C_0.32mJ/cm2_and_Figure2A.mpg.mpg. The movie is accelerated 16×.) • Movie S3 (Figure1C_0.41mJ/cm2.mpg.mpg. The movie is accelerated 16×.) • Movie S4 (Figure1C_0.51mJ/cm2.mpg. The movie is accelerated 10×.) • Movie S5 (Figure1D_velocity.mpg. Real time movie.) • Movie S6 (Figure2B.mpg. The movie is accelerated 10×.) • Movie S7 (Figure2C.mpg.mpg. The movie is accelerated 16×.) • Movie S8 (Figure_buckling.mpg. The movie is accelerated 14×.) 10


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• Movie S9 (Figure4.mpg. The movie is accelerated 18×.)

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Graphical TOC Entry Laser irradiation

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0.06 0.04

0.1

0.4

0.1 0

20

25

30

Velocity [μm/s]

0.4

PDF

0.2

15

0.5

0.3

Time [s]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 18

0.3 0.2 0.1 0

4

6

8

Velocity [μm/s]

10

12

Figure 1: Migration of the polystyrene beads near the GeSbTe thin-film under irradiation with a laser beam in the experimental setup. (A) Schematic illustration of experiment. (B) Time lapsed images of the colloidal crystal growth under pulsed laser irradiation at a fluence of 0.51 mJ/cm2 . The amorphous state of GeSbTe (black-colored region) is switched to a crystalline state (light-colored region), after laser irradiation. (C) Growth of the colloidal crystal area within the laser spot. The different colored lines indicate different laser fluence. (D) Lateral trajectories and PDF of velocities of 58 tracked particles after tuning on the laser with a fluence of 0.78 mJ/cm2 . The green solid line in the PDF represents a Gaussian distribution. (E) for 60 tracked particles 9 s after tuning on the laser.

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Page 17 of 18

A

E

C

0.2

0.15 0.1

0.05

5 µm

10 µm 0

B

D

10 µm

10 µm

Figure 2: Various crystal lattices. The colloidal crystal structures show four patterns. (A) Monolayer triangular lattice, (B) buckling phase, (C) bilayer square lattice, and (D) bilayer triangular lattice with a laser fluence of 0.32, 0.65, 0.93, and 0.82 mJ/cm2 , respectively. The light and dark particles are on the GeSbTe and glass slides, respectively. (E) Colored images of the bilayer square lattice (C) and triangular lattice (D) using the Lindemann parameter.

A

B

2 σ

1.8 1△ B B and 2□ 2□ and 2△ 2□ 2△ Disorder

1.6

H/σ

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

y

x

1.4 1.2

z

1

B

2△

2□

0

5

10

15

Laser intensity β

20

Figure 3: Simulation results of hard-core particles (diameter: σ) confined between two parallel plates with a separation distance, H. (A) Snapshots of the buckling phase (B), bilayer square lattice (2), and bilayer triangle lattice (24). The different colors indicate the particles in different layers. (B) Phase diagram of the quasi-two-dimensional colloidal crystals. The M and symbols indicate triangle and square lattices, respectively.

17


H

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Laser off

10 μm

0.35 mJ/cm2

Page 18 of 18

Laser on

t=0s

0.89 mJ/cm2

t = 56 s

Laser on

t = 80 s

Figure 4: Transition of the colloidal crystal from a coexistence of one-dimensional triangle lattice and quasi-buckling pattern to buckling phase by turning the laser intensity.

A

5 µm

Laser off

Laser on

Laser on

Laser on

t=0s

t = 15 s

t = 30 s

t = 45 s

Laser off

Laser on

Laser on

Laser on

t=0s

t = 10 s

t = 20 s

t = 30 s

Laser off

Laser on

Laser on

Laser on

t=0s

t=8s

t = 16 s

t = 24 s

B

10 µm

C

5 µm

Figure 5: Self-assembly of colloidal particles on different substrates. (A) Si substrate with 7.8 mJ/cm2 , (B) Au substrate with 4.7 mJ/cm2 , (C) SiO2 coated GST substrate with 1.9 mJ/cm2 .

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Rapid Assembly of Colloidal Crystals under Laser Illumination on a

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