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Simultaneous Formation and Spatial Patterning of ZnO on ITO Surfaces by Local Laser-Induced Generation of Microbubbles in Aqueous Solutions of [Zn(NH3)4]2+ Sho Fujii,*,†,§ Ryuta Fukano,† Yoshihito Hayami,† Hiroaki Ozawa,† Eiro Muneyuki,‡ Noboru Kitamura,§ and Masa-aki Haga*,† †

Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan ‡ Department of Physics, Faculty of Science and Engineering, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan § Department of Chemistry, Faculty of Science, Hokkaido University, Kita-10, Nishi-8, Kita-ku, Sapporo 060-0810, Japan S Supporting Information *

ABSTRACT: We demonstrate the simultaneous formation and spatial patterning of ZnO nanocrystals on an indium−tin oxide (ITO) surface upon local heating using a laser (1064 nm) and subsequent formation of microbubbles. Laser irradiation of an ITO surface in aqueous [Zn(NH3)4]2+ solution (1.0 × 10−2 M at pH 12.0) under an optical microscope produced ZnO nanocrystals, the presence of which was confirmed by X-ray diffraction analysis and Raman microspectroscopy. Scanning the focused laser beam over the ITO surface generated a spatial ZnO pattern (height: ∼60 nm, width: ∼1 μm) in the absence of a template or mask. The Marangoni convection generated in the vicinity of the microbubbles resulted in a rapid concentration/accumulation of [Zn(NH3)4]2+ around the microbubbles, which led to the formation of ZnO at the solid−bubble−solution three-phase contact line around the bubbles and thus afforded ZnO nanocrystals on the ITO surface upon local heating with a laser. KEYWORDS: microbubble, micropatterning, hydrothermal reaction, ZnO thin film, laser local heating

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INTRODUCTION

Laser light is a powerful tool to fabricate ZnO patterns directly on solid substrates through photophysical and photochemical interactions at the solution/solid interface. Laser interference has been applied to the preparation of ZnO patterns in the absence of the photomasks that are usually required in photolithographic processes. Previously, irradiation from UV lasers (355 nm) has been used to generate holographic patterns11 on thin films of ZnO through the partial dissolution of ZnO under irradiation upon photochemical reaction in a NaCl solution,12 which afforded a residual ZnO pattern. In contrast to this top-down approach, a photothermal process using laser light was applied to ZnO patterning in a bottom-up technique. Continuous laser irradiation (e.g., at 532 nm) was applied locally to a thin film of gold (the absorbing layer), which was precoated with ZnO nanoparticles (seeds) in a solution containing Zn ions. Then, a hydrothermal reaction was induced under a microscope, which resulted in the formation of ZnO nanowires within seconds to minutes.13−15 Xie et al. have reported the nucleation of ZnO

Zinc oxide (ZnO) is a fascinating semiconductor material due to its attractive materials properties, which include, e.g., a wide band gap energy, as well as light-emitting and piezoelectric properties. It is therefore hardly surprising that ZnO has been the focus of many studies on the crystal growth/preparation of ZnO nanomaterials, as well as their applications to optical/ electronic devices.1−5 Patterning and/or selective growth of ZnO on substrates based on simple and inexpensive methods is of primary importance for the integration of ZnO into microsystems with different substrate surfaces.2,6 Soft lithographic (microcontact printing) and colloidal lithographic methods have hitherto been applied for the fabrication of spatial ZnO patterns. These techniques generate a spatial pattern of ZnO precursors, and subsequently synthesize ZnO crystals in situ by a hydrothermal reaction of the precursors.7−10 Although such methods are inexpensive compared to other methods such as top-down lithography, they are multistep processes that include the preparation and removal of templates or molds. Therefore, the development of direct fabrication methods that do not require templates or molds is highly desirable for the controlled generation of spatial patterns of ZnO nanocrystals. © XXXX American Chemical Society

Received: December 28, 2016 Accepted: February 20, 2017 Published: February 20, 2017 A

DOI: 10.1021/acsami.6b16719 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces and the growth of nanowires on thin films of gold without a seed coating prior to irradiation with a laser (405 nm) for several minutes.16 These studies demonstrate a method to create patterns out of colonies of highly crystallized ZnO nanowires grown on surfaces normal to the substrate from ZnO precursor solutions under laser irradiation. To develop these laser-assisted fabrication techniques further, we attempted the direct ZnO lateral patterning on surfaces by a bottom-up method with laser radiation, which should attract wide interest in research areas concerned with microelectronics and electrooptics. So far, we have developed the fluidic manipulation and bottom-up assembly of nanomaterials in solution by the IR (1064 nm) laser-induced formation of microbubbles as a wet process for, e.g., the fabrication of DNA wires,17,18 the synthesis of nanorings from quantum dots,19 and the crystallization of glycine molecules.20 The laser-induced bubbles create Marangoni and capillary flows that accumulate the dispersed nanomaterial along the substrate−bubble−solution three-phase contact line through evaporative mass flux,21−23 similar to that observed for colloidal crystal growth by convective assembling,24 which results in spatial patterns of the nanomaterial. Likewise, the bottom-up method by laser-induced microbubbles has been further applied to fabricate surface patterns of, e.g., polyoxometalates25,26 and carbon nanotubes27 via the concentration/accumulation of material at the three-phase contact line, which is currently known as “bubble-pen lithography”.28 The Marangoni flow around the microbubbles produced by irradiation from IR lasers has also been studied in detail.29,30 These recent studies motivated us to generate a spatial pattern of ZnO nanocrystals on a solid surface through a laser-induced formation of microbubbles in aqueous solutions of [Zn(NH3)4]2+, concentration/accumulation of Zn by the Marangoni convection/capillary flow around the microbubbles, and subsequent in situ hydrothermal conversion of Zn to ZnO (Figure 1a−c). In this paper, we report the simultaneous formation and spatial patterning of ZnO nanocrystals on a solid substrate in aqueous [Zn(NH3)4]2+ solution using our original method

based on IR laser scanning and bubble manipulation (Figure 1), which does not require precoating with ZnO seeds. In our protocol, the generation of microbubbles requires a substrate that is able to absorb in the IR region (1064 nm). Furthermore, the substrate should ideally exhibit low thermal conductivity to induce local heating on the substrate surface upon irradiation with the laser. The substrate also should be transparent in the visible wavelength region, in order to allow monitoring the phenomena occurring on the substrate surface with an optical microscope. We therefore selected indium−tin oxide (ITO) substrates for the present experiments in favor of thin films of Au, which have typically been used in previous studies.17−20

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EXPERIMENTAL SECTION

Materials. All chemicals used in the present study were purchased from commercial suppliers in Japan and used without further purification. An aqueous solution of [Zn(NH3)4]2+ (1.0 × 10−2 M) was prepared by mixing an aqueous solution of NH3 (14 M) with an aqueous solution of Zn(NO3). The pH value of the solution was adjusted to 11.5−12.5 by controlling the volume of NH3 (aq.) added. Upon addition of NH3 (aq.), Zn(OH)2 initially precipitates before it dissolves to form a transparent solution of [Zn(NH3)4]2+ upon addition of an excess of NH3 (aq.). In the present experiments, the pH value of the solution was maintained >11.5. The thus obtained transparent [Zn(NH3)4]2+ solution was used for the local heating experiments with the laser. Optical Setup. A schematic illustration of the experimental setup is shown in Figure 2. The ITO layer (Kuramoto Co., Ltd., 240 nm thickness on glass, 10 Ω) was consecutively washed with acetone and an aqueous solution of NH3 (14 M), 30%

Figure 1. Schematic illustration of the spatial patterning of ZnO based on the laser-induced formation of microbubbles. (a) Laser light is focused on the surface of indium−tin oxide (ITO) in an aqueous [Zn(NH3)4]2+ solution. (b, c) Formation of microbubbles and concentration/accumulation of [Zn(NH3)4]2+ at the ITO−bubble− solution three-phase contact line and subsequent in situ hydrothermal reaction of [Zn(NH3)4]2+ affording ZnO nucleation and growth. (d) Manipulation of the location of the formation of the bubbles by spatial scanning of the laser beam, which results in a spatial patterning of ZnO on the ITO surface.

Figure 2. (a) Schematic illustration of the optical experimental setup. (b) Side view of the sample chamber. (c) Absorptance spectrum of the ITO substrate. (d) Optical microscopy image of a microbubble on the ITO surface generated by local laser heating. B


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Figure 3. (a, b) SEM images of a laser pattern obtained from a [Zn(NH3)4]2+ solution (10 mM, pH 12.0) on the ITO surface. Scale bar in (b): 1 μm. (d−f) EDX maps showing the content of (d) Zn, (e) O, and (f) N for the SEM image in panel c.

energy through photothermal conversion effects, i.e., local heating of the ITO surface. When the laser beam was focused on an ITO surface in a chamber filled with water, the formation of a microbubble (diameter: ∼1.5 μm) could be confirmed at P1064 > 50 mW (Figure 2d). Under the present experimental conditions, P1064 = 50 mW was considered a threshold value, above which the laser-induced formation of microbubbles could be confirmed experimentally. The P1064 value necessary for the formation of microbubbles on the ITO surface (∼50 mW) is almost identical to that on thin films of Au on glass substrates (for films of Au with 10 nm thickness: 50 mW).17−19 Under irradiation at 1064 nm, these thin films of Au exhibit %A and thermal conductivity values of 34% and 38.5 W/(m·K), respectively,17,32 while the relevant values for ITO substrates are 14% and ∼5 W/(m·K).33 Remarkably, almost identical P1064 values are required for the formation of microbubbles on both ITO and Au substrates. Reaction Conditions and Characterization of ZnO. Aqueous [Zn(NH3)4]2+ solutions at pH >11.5 were used for the following experiments. Laser irradiation of an ITO chip immersed in a [Zn(NH3)4]2+ solution generated a microbubble at the ITO surface, which resulted in the deposition of a solid on ITO. Depending on the pH value of the solution, the behavior of the deposited solid changed: at pH >12.0, the solid dissolved within a minute (Figure S1a, b), while at pH 12.0, the solid remained on the ITO surface even after turning off the laser beam (Figure S1c, d). Accordingly, pH 12.0 was selected as the optimum pH value for the following experiments. Figure 3 shows the SEM images and EDX maps of a deposited sample prepared by laser scanning of the ITO surface (acceleration voltage: 1−2 kV). The SEM image (Figure 3a) displays a line pattern predetermined by the laser scanning (line spacing: 5 μm). High magnification of the SEM image (Figure 3b) confirmed the presence of small particles (40−150 nm) on the deposition lines. The EDX maps revealed that the deposited lines are composed of zinc and oxygen, but not of nitrogen (Figure 3c−f). These data indicate that the deposited solid is not aggregates of [Zn(NH3)4]2+ but consists of ZnO that was obtained from the solution (vide infra). Consequently, the generation of a bubble results in the formation of ZnO nanoparticles on the ITO surface. The indium and tin content was monitored throughout the observed area (Figure S2a, b), revealing that the ITO layer was not ablated by the laser

H2O2, and water (NH3:H2O2:H2O = 1:2:10, v/v) prior to use. The ITO substrate (thickness 0.7 mm) was cut with a glasscutter to obtain square sample chips (10 mm × 10 mm). A sample chamber for irradiation with the laser was made out of the ITO chips, a cover glass (Matsunami Glass Ind., Ltd.), and Parafilm (Alcan Inc.). Two pieces of extended Parafilm (thickness: ∼ 40 μm) were used as spacers and placed on a glass plate (Matsunami Glass Ind., Ltd.); a square ITO chip (10 mm × 10 mm) was placed with its surface facing inward (Figure 2a). Then, the chamber was placed on the stage of an inverted optical microscope (IX 71, Olympus), equipped with an oil-immersion objective lens (×100/1.30 NA, Olympus). The previously prepared aqueous [Zn(NH3)4]2+ solution was injected into the chamber by capillary action. A continuous wave (CW) light beam (1064 nm) from a Nd:YAG laser (SIGMAKOKI, FLS-1064−2000) was focused on the ITO surface through the objective lens of the microscope. The laser intensity at 1064 nm (P1064) was monitored before passing the beam through the objective lens. The focus point of the laser beam was then scanned over the ITO surface at a rate of 20 μm/s using computer-controlled x-y stage movement of the microscope. Microscopy images were recorded on a CCD camera (IPX-210L, Imperx) mounted on the microscope (Figure 2b). Physical Measurements. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometry (EDX) measurements were performed on Hitachi High-Technologies S-5500 and JEOL JSM-7800F instruments, respectively. X-ray diffraction and Raman microspectroscopy of ZnO were conducted with a Rigaku RINT-2100 spectrometer (Cu Kα line at λ = 1.5406 Å) and a Renishaw Ramascope System 2000 (λex = 632.8 nm), respectively. Atomic force microscopy (AFM) was conducted on an Agilent 5500 system.

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RESULTS AND DISCUSSION Formation of Microbubbles on the ITO Surface. Initially, we confirmed the generation of microbubbles induced by laser local heating on the ITO surface experimentally. The vis-NIR spectrum of the ITO substrate in Figure 2c indicates that the percentage of light absorptance (%A) of the substrate at 1064 nm is 14%, which is in good agreement with previously reported values.31 These results demonstrate that the photoenergy absorbed by the ITO layer is converted into thermal C


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ITO surface, a broad main peak was observed at 439 cm−1, together with shoulder peaks at 331 and 410 cm−1. These Raman signals confirm that the deposited solid is ZnO. The Raman spectral band shape and intensity depend on the crystallinity of the compound under examination. In the deposited solid, the peak at 383 cm−1 (A1T mode) is not clearly defined, which is indicative of a disordered structure for the deposited ZnO considering that the low peak intensity suggests the presence of order−disorder structures in the lattice.36,37 The relatively wide bandwidth (400−450 cm−1) of the E1T mode band (410 cm−1) observed for the deposited ZnO also indicates the presence of several other configurations in the ZnO crystal.36 On the basis of the spectrum in Figure 5 and the data hitherto reported, the deposited lines in Figure 3 can be confidently assigned to ZnO. Based on a comparison with previous reports,31 the deposited ZnO should be partly oriented on the ITO substrate, although the Raman spectrum suggests several configurations within the ZnO crystals. Pattern Formation. Upon scanning a focused laser beam (20 μm/s) over an ITO surface in aqueous [Zn(NH3)4]2+ solution (1.0 × 10−2 M at pH 12.0), a grid line pattern of ZnO (interval: 5 μm) was successfully fabricated (see Movie S1 in the ESI). Figure 6 shows the AFM image of the line of solid ZnO on ITO and the relevant height profiles along the lines. The average height and width of the lines are 72 ± 3 nm and 1.1 ± 0.1 μm, respectively. A three-dimensional (3D) image of the AFM data is shown in Figure 6c. The yellow areas, corresponding to the most elevated sections, are mainly located

irradiation. At high acceleration voltages (10−15 kV), the electron beam deformed the ZnO lines during the SEM measurements (Figure S2c); however, neither ablation nor deformation of the ITO surface below the ZnO lines were observed during the SEM measurements. The X-ray diffraction (XRD) pattern of the deposited solid on ITO is shown in Figure 4, together with that of ITO as a

Figure 4. XRD pattern for ITO (red) and for the material deposited on ITO (blue).

reference. The XRD pattern of the deposited solid shows a weak but clearly identifiable peak at 34.5°, in addition to intense peaks at 21.4, 30.4, 35.3, and 37.5°, which originate from the ITO substrate. The peak at 34.5° indicated by the red arrow in Figure 4 was ascribed to the (002) orientation of ZnO (wurtzite structure). Fortunato et al. have previously reported that highly oriented ZnO films (thickness: 130 nm) with the caxis perpendicular to the surface of an Al2O3/TiO2 layer on an ITO substrate exhibited a peak at 34.1°, which was assigned to the (002) plane.31,34 On the basis of these reports, we conclude that the solid deposited on the ITO substrate is ZnO. The mean crystal size (Dm) of the deposited ZnO (∼30 nm) was 0.9λ calculated using the Scherrer equation:34 Dm = β cos θ , whereby

β refers to the full-width at half-maximum of the peak (rad) and θ to the Bragg angle. The Dm value observed indicates that the ZnO produced by laser irradiation consists of aggregates of ZnO nanocrystals on the ITO surface. Raman microspectroscopy was used to characterize the deposited solid using ZnO powder as a reference (Figure 5).

Figure 5. Raman spectra of ZnO powder (red) and the solid deposited on the ITO (blue) substrate.

The Raman spectrum of ZnO powder exhibits sharp peaks at 331, 383, and 438 cm−1, which correspond to the A1 (acoustic overtone), A1T, and E2H modes, respectively.35 The intense peak at 438 cm−1 originates from one of the two E2 modes (E2L and E2H) that predominantly involve vibrational motions in the wurtzite phase of ZnO. In the case of the deposited solid on the

Figure 6. (a) AFM image of a ZnO/ITO surface (narrow area scan). (b) Height profiles of the solid line 1 and the dashed line 2 in panel a. (c) 3D image for the dotted pale blue square in panel a. D


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which attracts the solute in the solution toward the bubble− substrate−solution three-phase line (Figure 1). We have furthermore reported that, e.g., rhodamine B in solution accumulates at the three-phase contact line of such microbubbles by Marangoni convection flow.20 On the basis of this study and our experimental observations in the context of the present study, we would like to propose the following plausible mechanism for the deposition of ZnO on ITO: (i) upon exposure of the ITO surface to irradiation from the laser beam (1064 nm), a microbubble should be generated through photothermal effects, which would lead to a concentration/ accumulation of [Zn(NH3)4]2+ at the three-phase contact line by convection flow (Figure 1c); (ii) the hydrothermal reactions shown in eqs 1 and 239 should occur at the three-phase contact line upon laser local heating, leading to the deposition of ZnO on the ITO surface. The deposition process, including the nucleation and growth of ZnO nanocrystals, can be rationalized in terms of a convective assembly at the three-phase contact line.21−24 Solvent evaporation from the three-phase contact line of the bubble accelerates the nucleation and growth of ZnO at the interface. The hydrothermal reactions in eqs 1 and 2 that describe the formation of ZnO require relatively high temperatures (T > 85 °C).39,40 Unfortunately, the temperature at the three-phase contact line upon irradiation with a 1064 nm laser is not known at this stage of our investigation. Nevertheless, the formation of vapor microbubbles requires T > 100 °C in the irradiation area of the ITO substrate. A recent report by Roy et al. has demonstrated that the temperature around the bubbles (diameter: 2.0 μm) produced by laser local heating in water is theoretically as high as 107−127 °C.41 As the aqueous solutions used in our experiments contain salts, their boiling points should be higher than that of pure water. Consequently, the temperature around the bubbles at the threephase contact line has to be much higher than 85 °C for the chemical reactions in eqs 1 and 2 to occur. We therefore conclude that the following chemical reactions should proceed at the laser beam focus on the ITO surface, resulting in the formation of ZnO:

on the edges of the ZnO lines, which indicates that the shape of the surface of the ZnO lines is concave and not, as previously reported, tapered.13 The formation of such a concave shape would not be possible, if ZnO had formed via a simple hydrothermal reaction such as conventional optothermal heating. The generation of a concave surface should be caused by an assembly at the three-phase contact line along the interface of the bubbles (Figure 1b,c), and the detail is discussed later. A close inspection of the crossing points of the grid lines demonstrated that the lines overlap and that the height of the crossing point is almost double that of the individual lines (average height: 138 ± 8 nm; Figure 6b). These results indicate that microbubbles are also generated on the solid ZnO. Given that ZnO absorbs IR light,38 laser irradiation on the surface of the deposited ZnO should also induce the formation of microbubbles, which should lead to a deposit of an additional surface layer of ZnO on top of the already deposited ZnO. The translation of the laser-induced bubble on the substrate, in the current study, made nearly uniform lines. In contrast, we have reported the ring-shape accumulation of a solute, e.g., Q dots or molecules took place under stationary bubble on the substrate.19,20 This difference arises from the translation of the bubble on the substrate. During the translation of the bubble, continuous depositions of rings would leave a line of ZnO materials behind, in which the line consisted of a flatten center part with sharp edge. The edge-enhanced shape should be caused by several factors. One possibility is that the front of the translated bubble forces out accumulated materials to alongside of the bubble and then the concave shape is formed, a degree of which may depend on the bubble size, translation fluidity of the bubble, and/or properties of the deposited materials, e.g., adhesive strength to the substrate and stiffness. For another possible factor, distribution of flow velocity around the translated bubble is nonuniform; the velocity of the side area is faster than that of the front/back-side area. Roy et al. have demonstrated continuous depositions and a line patterning of polyoxometalate molecules using the translated bubble (diameter >∼6 μm).25 The shape of the reported line is highly edge-elevated with no empty region in the line but different from uniform lines like our data. However, they have noted a width between edge peaks tends to decrease with decreasing the bubble diameter (the width: 440 μm). These data indicate a smaller bubble provides uniform lines. Accordingly, the ZnO line obtained in this study (width: ca. 1 μm) seems to be the near uniform one without any empty region at the center of the line. For simplicity, we assumed the formation of cubic ZnO particles with dimensions of 1 μm (width) × 20 μm (length) × 70 nm (height) on ITO. Considering a wurtzite structure, the number of moles of ZnO per cube can then be roughly estimated (9.6 × 10−14 mol). In an aqueous solution of [Zn(NH3)4]2+ (1.0 × 10−2 M), the same cubic volume contains 1.4 × 10−17 mol of Zn2+ ions. If ZnO was produced by a simple hydrothermal reaction on ITO, the number of moles of Zn2+ ions in the same volume of solution (1.4 × 10−17 mol) is insufficient to afford ZnO with the aforementioned dimensions. Therefore, the concentration/accumulation of Zn2+ ions in the vicinity of the laser beam focus/microbubbles is indispensable for the formation of ZnO on ITO. Reaction Mechanism. We have previously reported that the generation of microbubbles by laser-induced local heating also produces a Marangoni convection flow around the bubbles,

[Zn(NH3)4 ]2 + + 2OH− → Zn(OH)2 + 4NH3

(1)

Zn(OH)2 → ZnO + H 2O

(2)

Regarding the formation of the pattern of ZnO nanocrystals, the nucleation and growth of ZnO should be separate processes.5 Even though the formation of critical nuclei is rare, their formation rate may be rapid under appropriate conditions (as in the present case),42 while crystal growth of ZnO is usually slow during the hydrothermal reaction(s). For example, it has been reported that the growth of ZnO nanowires prepared by laser heating (405 nm) without ZnO nanocrystalline seeds requires several minutes.16 In contrast, the present process affords ZnO nanoparticles within 1 s, i.e., in the present study, only the ZnO nucleation step is observed. Zainelabdin et al. have rationalized the thermodynamics of the nucleation of ZnO in terms of classical homogeneous nucleation theory.42,43 According to this theory, the free energy change of nucleation, ΔG, is described by the following equations: ΔG = E

4 3 πrp ΔGv + 4πrp2γ 3

(3) DOI: 10.1021/acsami.6b16719 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces ΔGv = − S=

kBT ln S V

C C0

reaction of [Zn(NH3)4]2+ at the area where the laser beam is focused. ZnO is then obtained through a nucleation process. It is worth emphasizing that the present method is able to fabricate controlled spatial patterns of a functional material on a solid substrate, whereby photomasks or templates are not required. The present method based on laser-induced formation of microbubbles thus represents a promising technique for mask-/template-free surface patterning.

(4)

(5)

where rp is the radius of the cluster (nucleus) particle, ΔGv is the change in Gibbs free energy per unit volume of the solid phase, γ is the surface free energy of the cluster, kB is the Boltzmann constant, V is the molecular volume, S is the supersaturation ratio, C is the concentration of zinc, and C0 is the saturation concentration of zinc. At S > 1, the critical energy of nucleation, ΔG*, and the critical nucleus radius, rp*, are obtained from ΔG* = rp* =

16πγ 3 3(ΔGv )2

2γV kBT ln S

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b16719. Optical microscope images of the lines deposited on the ITO surfaces, EDX mappings showing In and Sn, SEM image of a deformed line, and 3D height image of AFM data for a ZnO grid pattern (broader scan). (PDF) Movie of the formation of a ZnO grid pattern under an optical microscope. (AVI)

(6)

(7)

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The nucleation rate, R, is given by (−ΔG * /kBT )

R = ρZje

f (θ ) =

⎛3⎞ ⎛1⎞ 1 − ⎜ ⎟cos θ + ⎜ ⎟cos3 θ ⎝4⎠ ⎝4⎠ 2

AUTHOR INFORMATION

Corresponding Authors

(8)

*E-mail: [email protected]. *E-mail: [email protected].

where ρ is the density of zinc molecules, Z is the Zeldovich factor, and j is the rate at which molecules attach to the nucleus causing it to grow. Equations 6−8 are able to explain well the previous experimental studies on the preparation of ZnO nanostructures at different concentrations and temperatures:5,43−45 the particle size or critical radius of nucleation decreases at temperatures >90 °C, while the nucleation rate increases at high concentrations and temperatures. In our study, the concentration of Zn ions increases at the three-phase contact line due to the bubble, providing the condition S > 1 and thus shifting the chemical equilibrium in eqs 1 and 2 toward the products side (Le Châtelier’s principle). The temperature increase upon laser local heating reduces the values for ΔG* and rp*. Equation 8 allows an estimation of the conditions around the bubbles, e.g., that S > 1, and an increase of T and a reduction of ΔG* increase the nucleation rate. In the present study, heterogeneous nucleation should also be considered, due to the phenomena occurring at the threephase contact line. The free energy change in heterogeneous nucleation, ΔGhetero, is defined by42 ΔG hetero = ΔGf (θ )

ASSOCIATED CONTENT

S Supporting Information *

ORCID

Masa-aki Haga: 0000-0002-1230-3848 Author Contributions

All authors contributed during the preparation of the manuscript. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.H. gratefully acknowledges financial support from the Institute of Science and Engineering at Chuo University. This work was partially supported by JSPS KAKENHI Grant JP16H00811 (to E.M.).

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REFERENCES

(1) Wang, Z. L. Zinc Oxide Nanostructures: Growth, Properties and Applications. J. Phys.: Condens. Matter 2004, 16, R829−R858. (2) Ö zgür, Ü .; Alivov, Y. I.; Liu, C.; Teke, A.; Reshchikov, M. A.; Doğan, S.; Avrutin, V.; Cho, S.-J.; Morkoç, H. A Comprehensive Review of ZnO Materials and Devices. J. Appl. Phys. 2005, 98, 041301. (3) Ö zgür, Ü .; Hofstetter, D.; Morkoç, H. ZnO Devices and Applications: A Review of Current Status and Future Prospects. Proc. IEEE 2010, 98, 1255−1268. (4) Kozuka, Y.; Tsukazaki, A.; Kawasaki, M. Challenges and Opportunities of ZnO-Related Single Crystalline Heterostructures. Appl. Phys. Rev. 2014, 1, 011303. (5) Willander, M.; Hasan, K. u.; Nur, O.; Zainelabdin, A.; Zaman, S.; Amin, G. Recent Progress on Growth and Device Development of ZnO and CuO Nanostructures and Graphene Nanosheets. J. Mater. Chem. 2012, 22, 2337−2350. (6) Kumar, R.; Kumar, G.; Al-Dossary, O.; Umar, A. ZnO Nanostructured Thin Films: Depositions, Properties and Applications - A Review. Mater. Express 2015, 5, 3−23. (7) Göbel, O. F.; Blank, D. H. A.; Elshof, J. E. t. Thin Films of Conductive ZnO Patterned by Micromolding Resulting in Nearly Isolated Features. ACS Appl. Mater. Interfaces 2010, 2, 536−543. (8) Dong, J. J.; Zhang, X. W.; Yin, Z. G.; Zhang, S. G.; Wang, J. X.; Tan, H. R.; Gao, Y.; Si, F. T.; Gao, H. L. Controllable Growth of

(9)

(10)

where θ is the contact angle of the nucleus particle on the substrate. Equation 9 demonstrates how the energy barrier of nucleation is reduced on the substrate surface (ΔGhetero * < ΔG*). Thus, the bubble induced upon laser irradiation provides a nanometer-sized reaction space that satisfies the conditions to rapidly create nucleus particles around the threephase contact line.

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CONCLUSION We have successfully demonstrated the simultaneous formation and spatial patterning of ZnO nanocrystals on an ITO surface, without the need for a ZnO nanocrystalline precoating, based on the laser-induced photothermal formation of microbubbles and a subsequent concentration/accumulation/hydrothermal F


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