Tunable Wettability of Ferroelectric Lithium Niobate Surfaces

Article pubs.acs.org/JPCC

Tunable Wettability of Ferroelectric Lithium Niobate Surfaces: The Role of Engineered Microstructure and Tailored Metallic Nanostructures Rusul M. Al-Shammari,†,‡ Michele Manzo,§ Katia Gallo,§ James H. Rice,*,† and Brian J. Rodriguez*,†,‡ †

School of Physics and ‡Conway Institute of Biomolecular and Biomedical Research, University College Dublin, Belfield, Dublin 4, Ireland § Department of Applied Physics, KTH − Royal Institute of Technology, 106 91 Stockholm, Sweden ABSTRACT: An important aspect of optimizing micro- and optofluidic devices for labon-a-chip systems is the ability to engineer materials properties including surface structure and charge to control wettability. Biocompatible ferroelectric lithium niobate (LN), which is well-known for acoustic and nonlinear optical applications, has recently found potential micro- and optofluidic applications. However, the tunable wettability of such substrates has yet to be explored in detail. Here, we show that the contact angle of LN substrates can be reproducibly tailored between ∼7° and ∼121° by controlling the surface topography and chemistry at the nano- and micrometer scale via ferroelectric domain and polarization engineering and polarization-directed photoassisted deposition of metallic nanostructures.

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INTRODUCTION Micro- and optofluidic devices are of interest for a wide range of applications including chemical synthesis, biological analysis, crystallization of proteins, and cell and particle sorting.1−6 An important aspect of optimizing the materials used in such devices is the ability to tailor the wettability7−10 by controlling the surface energy (chemical composition and surface charge) and topographical structure (surface roughness).11−14 LiNbO3 (LN) is widely used in optoelectronic and photonic applications due in part to its large nonlinear optical coefficients and reversible spontaneous polarization15 and has recently been used in micro- and optofluidic applications, which take advantage also of the piezoelectric, pyroelectric, or photovoltaic properties of the crystals.16−22 Furthermore, LN has been shown to be cytocompatible with MC3T3 osteoblast cells and neurons, providing opportunities to use LN in biological applications.22−24 The reversible spontaneous polarization, e.g., by electric field poling,25,26 of LN also allows the surface charge and reactivity to be patterned, a property that has been exploited to fabricate metallic nanostructure arrays.27,28 Proton exchange, a technique used to fabricate waveguides in LN,15 has also been used to engineer surface reactivity for the fabrication of metallic nanostructure arrays for Raman-based sensing of molecules.29,30 Proton exchange of a LN surface through reactive ion etched mask openings causes additional changes to the surface topography via proton exchange induced swelling and reactive ion etch (RIE) damage.29,31,32 LN crystals are therefore highly tunable, biocompatible substrates that present opportunities to tailor surface roughness, charge, and reactivity, © XXXX American Chemical Society

factors that are expected to influence surface wettability.21,23,33−36 Depending on the cleaning procedure, the contact angle of LN surfaces has been reported to be ∼35°−50°.23,34,35 High temperature furnace annealing35 and UV laser irradiation34 have been shown to result in superhydrophilic LN surfaces. UV irradiation has also been used to switch surface wettability of other materials.37−40 Patterned wettability on LN surfaces has also been established through exposure to octadecyltrichlorosilane through photolithographically defined openings, which made the exposed regions more hydrophobic,33 and modulation of the pyroelectric effect by polarization reversal has been used to create arrays of oil droplets.21 Water adsorption and desorption on LiNbO3 surfaces has been shown to be polarization dependent with a preferential adsorption on the nonpolar and positive LN surface compared to the negative surface.41 Despite this progress, little is known about the role of proton exchange or photodeposited surface enhanced Raman spectroscopy-active nanostructures on the wettability of LN surfaces. Here, the tunable wettability offered by proton exchange and metallic nanostructure photodeposition on LN is investigated, which might allow the applications of LN crystals in micro- and optofluidics to be extended from serving as the bottom channel in an optofluidic Received: December 7, 2016 Revised: March 7, 2017 Published: March 7, 2017 A

DOI: 10.1021/acs.jpcc.6b12336 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C device18 to plasmon-assisted optofluidics5 and high-sensitivity42 analytics6 applications.

AFM images were recorded from different locations before and after silver nanoparticle deposition. A contact angle measuring system (DSA10, Krüss, Germany) was used to measure the contact angles of sessile 2 μL deionized water droplets on each surface (n = 8). AFM and CA measurements are reported as mean ± standard deviation.

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EXPERIMENTAL METHODS The 500 μm thick z-cut optical grade congruent LN (CasTech Inc., China) was used to fabricate periodically proton exchanged LN (PPELN) templates by selective exchange in molten benzoic acid at 200 °C through photolithographically defined RIE openings in a titanium mask.31,32 A mask having a period of 6.09 μm with a 50% duty cycle was used for 1440 min exchange with corresponding ∼2.9 μm PE depth on +z and −z LN surfaces. A mask having a period of 12.17 μm with a 50% duty cycle was also used for exchange times of 52, 210, 480, 852, 1332, and 1920 min with corresponding PE depths of 0.59, 1.21, 1.79, 2.30, 2.72, and 3.10 μm, respectively, on −z LN surfaces.43 During proton exchange, Li+ ions were substituted with H+ ions from the benzoic acid, thereby modifying the optical, ferroelectric, and piezoelectric properties of the crystal.26,32,44,45 Upon removal of the mask by etching in hydrofluoric acid (48% solution in water) for 10 s, the surface comprised alternating stripes of LN and PE regions. The PE regions further comprised a central RIE region (i.e., proton exchange through the mask opening) with lateral diffusion (LD) regions on either side (i.e., proton exchange under the mask). The periodically poled LN (PPLN) sample (Gooch and Housego) was fabricated with domain periods of 30, 30.2, 30.4, 30.6, 30.8, 30.95, and 31.1 μm. The widths of the domain gratings in the crystallographic y-direction were 1.8 mm for 30 μm and 1.3 mm otherwise. Etched PPLN was prepared by dipping the PPLN sample in hydrofluoric acid for 1 h at room temperature, rinsing in deionized water, and drying with compressed nitrogen. The samples ranged in size from ∼0.5 × 1 to ∼1 × 1.6 cm2. Patterning by polarization reversal or proton exchange allows for the spatial separation of photogenerated charge upon illumination with super-bandgap energy light (bandgap of LN ∼ 3.9 eV46), which can be used to drive the photoreduction of silver nanostructure arrays from AgNO3 solution.28−30,43,47,48 Prior to photodeposition, all samples were sonicated for 20 min each in acetone, isopropanol, and deionized water before being dried with compressed nitrogen. The samples were placed on a glass slide, and a certain volume of 0.01 M AgNO3 solution was pipetted onto the sample surface (70 μL for PPELN samples with 12.17 μm period and uniform PE on opposite side; 100 μL for uniform LN and PPLN; 150 μL for PPELN samples with 6.09 μm period and uniform PE on opposite side). The samples were illuminated with a 254 nm UV light source (11SC-2, Spectroline) located 2 cm above the surface for 5 min for uniform LN, PPLN, and PPELN samples with 12.17 μm period and uniform PE on the opposite side and for 10 min for PPELN samples with 6.09 μm period and uniform PE on the opposite side. After illumination, the samples were immersed in deionized water for 1 min and then blown dry with compressed nitrogen. Samples were reused by cleaning them as described above with an additional step of gently rubbing the sample with lens paper soaked in isopropanol in between the acetone and isopropanol sonication steps. Amplitude modulation atomic force microscopy (AFM) (MFP-3D, Asylum Research) was used to image the surface topography and measure the surface roughness from 8 images of each sample before and after photodeposition with cantilevers having a typical resonant frequency of ∼310 kHz and spring constant of ∼40 N/m (PPP-NCH, Nanosensors).

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RESULTS AND DISCUSSION Schematic diagrams of cross sections of uniform LN, etched and unetched PPLN, and PPELN surfaces are shown in Figure 1. The surface topography images for +z and −z cut uniform

Figure 1. Schematic diagrams of cross sections of (a) +z and −z uniform LN, (b) PPLN and etched PPLN, and (c) +z and −z PPELN surfaces. Arrows correspond to the spontaneous polarization, P, and corresponding bound polarization charge is indicated by + and − symbols. Dashed lines in (b) indicate domain walls that separate domains having opposite polarization directions. In (c), the polarization in the PE areas is strongly reduced and has the same orientation as the surrounding LN substrate.

LN samples before and after silver photodeposition are shown in Figure 2. Uniform +z and −z LN surfaces were found to have similar surface roughness (∼0.4 nm) before deposition and a slight difference in contact angle with the −z surface being more hydrophobic (Table 1; 43.5 ± 0.5° versus 53.8 ± 0.4°). After photodeposition, both the surface roughness and the contact angle increased, with the −z surface remaining slightly more hydrophobic than the +z surface (85.0 ± 0.2° versus 79.9 ± 0.1°). The appearance of the photodeposited silver also differed (Figure 2c,d). Measurements of the average particle size by AFM are challenging given that the height of the particles could be underestimated when particles are on top of each other and their width overestimated due to tip convolution, the effect of which can increase with tip wear. Measurements of 10 particles from each surface revealed that particles on the −z surface were larger with a greater distribution in particle size (13.2 ± 3.5 nm versus 6.2 ± 0.9 nm in height; 117.7 ± 38.8 nm versus 52.1 ± 26.6 nm in width). The deposition of silver nanoparticles on both +z and −z LN surfaces has been previously observed to be strongly B


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surface and the increase in surface roughness after silver nanoparticle deposition. We have repeated these experiments several times, and the trends were reproducible. For instance, in one experiment, the contact angle increased after photodeposition of silver nanoparticles from 39.2 ± 0.5° to 74.2 ± 1.2° for uniform +z and from 32.0 ± 0.9° to 81.1 ± 1.4° for uniform −z LN. The surface topography of the originally −z cut surface of the unetched PPLN sample is shown in Figure 3a. The contact

Figure 2. AFM topography images of (a) uniform +z and (b) −z LN before silver deposition. AFM topography images of (c) uniform +z and (d) −z LN surfaces after silver deposition (19 nm z-scale).

dependent on polarization-dependent band bending.28,49 The deposition on −z surfaces has been attributed to the photoelectric effect as influenced by low electron affinity and band bending, whereas deposition on +z surfaces has been attributed to the accumulation of photogenerated electrons that screen the bound polarization charge.28,49 The different mechanism of deposition may be related to the different size distributions obtained. The results suggest a correlation between roughness and contact angle, consistent with other studies.50 Whereas the contact angle for the smooth uniform samples is a function of the surface chemistry, the contact angle after photodeposition depends on the surface roughness and the surface chemistry of silver and LN. The contact angle for smooth silver films has been reported to be 110°51 while electrochemically deposited dendritic silver particle films have been reported to be superhydrophobic with a contact angle of 150.9°.52 In the present study, the uniform LN samples became more hydrophobic with the presence of silver nanoparticles, increasing the contact angles by ∼40° from their initial values. The change in contact angle on uniform LN surfaces is attributed to the change in the chemical composition of the

Figure 3. AFM topography images of (a, c) PPLN and (b, d) etched PPLN surfaces before and after silver deposition (20 nm z-scale before etching and 3570 nm z-scale after etching).

angle of PPLN was determined to be 49.9 ± 0.2°, a value similar to the contact angles of the uniform LN surfaces, despite the processing used to engineer the domains and the reported wafer to wafer variation of LN.35 As the contact angle for PPLN is in between the values for +z and −z LN, it is likely that the contact angle can be tuned slightly by changing the ratio of the areas of +z and −z regions under the drop. The surface topography of the PPLN crystal surface after deposition is shown in Figure 3b. Nanoparticles deposit preferentially on the domain wall, as reported elsewhere,28,53−56 and have an average height of 2.4 ± 0.3 nm based measurements of 10 nanoparticles. The preferential deposition has previously been

Table 1. Surface Roughness and Wettability Characterization of Uniform LN, PPLN, Etched PPLN, PELN, and PPELNa roughness (nm) sample +z LN −z LN PPLN etched PPLN +z PELN −z PELN +z PPELN −z PPELN

before deposition 1.0 0.9 0.2 254.8 0.7 0.5 5.2 6.3

± ± ± ± ± ± ± ±

0.1 0.1 0.1 5.2 0.1 0.1 0.6 0.4

contact angle (deg) after deposition 1.1 5.7 1.1 443.9 1.0 1.3 15.5 23.9

± ± ± ± ± ± ± ±

0.4 1.1 0.2 7.3 0.1 0.4 2.1 3.1

before deposition 43.5 53.8 49.9 42.4 66.8 41.0 75.1 65.0

± ± ± ± ± ± ± ±

0.5 0.4 0.2 5.3 0.9 0.7 0.1 0.1

after deposition 79.9 85.0 54.8 120.6 93.1 75.3 94.0 84.2

± ± ± ± ± ± ± ±

0.1 0.2 0.3 10.2 0.1 0.3 1.2 2.4

Roughness values were calculated from 5 μm × 5 μm areas for uniform LN, 20 μm × 20 and 60 μm × 60 μm areas for PPLN and etched PPLN surfaces, respectively, 5 μm × 5 and 20 μm × 20 μm areas for +z and −z PELN and PPELN surfaces (3 μm PE depth). a

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The Journal of Physical Chemistry C attributed to the presence of an in-plane electric field.28,53 Nanowire formation was also observed in the absence of domain walls and attributed to the presence of defects.49 The surface becomes more hydrophobic with the presence of silver and increase in roughness (Table 1); however, the modest increase of contact angles, to 54.8 ± 0.3°, compared to the uniform samples might reflect the more localized deposition of silver along the domain walls. For three separate experiments the average increase before and after silver nanoparticle deposition on PPLN has been determined to be 6.1 ± 6.6°. The surface topography of the etched PPLN sample is shown in Figure 3c. The preferential etching of the −z domains57 led to the formation of trenches of 650 ± 50 nm depth for the etching conditions used and thereby an increased surface roughness. The contact angle after etching was 42.4 ± 5.3° versus 49.9 ± 0.2° before etching. In this instance, the increased roughness arising from the etching did not increase the contact angle. This finding might depend on the geometry of the etched structures and also on the etching, which may alter the surface chemistry. After silver deposition, the surface became hydrophobic with a contact angle of 120.6 ± 10.2° and the roughness increased from 254.8 ± 5.2 nm before the deposition to 443.9 ± 7.3 nm after the deposition (Table 1). Silver nanoparticles deposited on both positive and negative domains on etched PPLN. Notably, the particles on the etched −z domains are larger than those on +z domains (111.2 ± 88.8 nm versus 24.8 ± 6.3 nm in height; based on 10 measurements each). This unexpected result may be attributed to the change in photoelectric deposition following etching, i.e., a change in the electron affinity; however, further investigation is required to elucidate the mechanism. For three separate experiments, the average increase before and after silver nanoparticle deposition on etched PPLN has been determined to be 73.5 ± 20.6°. The change in contact angle on etched PPLN surfaces after silver nanoparticle deposition is attributed to the change in the chemical composition of the surface and the increase in surface roughness after photodeposition. It has been previously reported that UV laser irradiation leads to superhydrophilic LN surfaces.34 To investigate whether the UV pen lamp used can also affect the wettability of LN surfaces, we measured the contact angle of uniform LN surfaces before and 3 days after UV illumination. We found that the surfaces became superhydrophilic (6.6 ± 1.56°) after exposure and that they fully recovered their moderate hydrophobicity after 3 days. The initial decrease in contact angle results from the formation of electron hole pairs under super-bandgap illumination. The photogenerated charge acts to screen the polarization charge, affecting band bending and surface potential, and thereby the contact angle.58 Similarly, the nanoparticle-coated uniform LN surfaces became more hydrophobic 3 days after the UV illumination used to fabricate the nanoparticles (81.5 ± 0.9° and 90.8 ± 0.1° for +z and −z, respectively). All subsequent contact angle measurements were recorded immediately after photodeposition. AFM topography images of uniform PE areas on +z and −z LN surfaces before and after photodeposition are shown in Figure 4. The values for contact angle and roughness are provided in Table 1 and highlight that the surface roughness remains unchanged after PE and that the PE-induced change in chemical composition has only a moderate effect on wettability. As with the uniform LN samples, the roughness and contact angle increased for the uniform PE samples following the deposition of silver nanoparticles. In this case, the change in

Figure 4. AFM topography images of (a) +z and (b) −z PE surfaces before deposition and (c) +z and (d) −z PE surfaces after silver deposition (20 nm z-scale).

contact angle is attributable primarily to the change in roughness and chemical composition provided by the silver nanoparticles. The uniform PE regions analyzed have been exposed to both RIE and proton exchange. For another set of samples, it was possible to measure the contact angle for regions that were only uniformly PE (+z, 61.5 ± 0.9°; −z, 62.8 ± 0.8°), as opposed to those which underwent RIE and PE (+z, 65.2 ± 0.1°; −z, 64.8 ± 0.1°). Given the similarities between these values and to allow comparison between uniform PE and PPELN regions on the same crystal surface, in the remaining text, we discuss only the case of areas that have been exposed to both RIE and proton exchange when we refer to uniform PE areas. AFM topography images, highlighting the RIE, LD, and unexchanged LN regions of +z and −z PPELN surfaces before deposition, are provided in Figure 5a,b. The PE region comprises the dark stripe resulting from the overetching of the surface by RIE during mask fabrication and the adjacent LD regions associated with surface swelling during proton exchange. The PPELN period was 6.09 μm. The contact angles of +z and −z PPELN surfaces were measured to be 75.1 ± 0.1° and 65.0 ± 0.1°, respectively (Table 1). No significant differences were observed when the camera used for contact angle measurements was oriented perpendicular or, alternatively, parallel to the 1D PPELN stripes. The spatially selective proton exchange process affects both the chemical composition and the surface topography,29 thereby altering the surface wettability. AFM topography images of the same +z and −z PPELN surfaces after deposition are provided in Figure 5c,d. Silver nanoparticle deposition occurred in the LD region where the electrons accumulated, as reported elsewhere,29,30,59 leading to an increase in surface roughness after silver nanoparticle deposition for both +z and −z PPELN (Table 1). After deposition, the contact angles were measured to be 94.0 ± 1.2° for +z PPELN and 84.2 ± 2.4° for −z PPELN. The silver D


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Figure 5. AFM topography images of (a) +z and (b) −z PPELN surfaces before deposition and (c) +z and (d) −z PPELN surfaces after silver deposition (65 nm z-scale).

nanostructures hence resulted in an ∼20° increase in contact angle on the PPELN surfaces. To investigate whether the contact angle can be further tuned by controlling the width of the LD regions (where silver nanoparticles typically deposit29,43), we prepared surfaces that have been periodically proton exchanged for different times.43 Both the depth of the PE regions and the width of the LD regions increased with increasing exchange times.43 AFM topography images of some of the samples with different PE depths are shown in Figure 6. The surface roughness generally increased with exchange time; however, the influence of the latter on the contact angle is less clear, with values ranging from ∼36° to ∼71° (Table 2). For shallow depths, the buried PE:LN interface can influence the patterns of silver photodeposition, as shown previously.43 For the PE depth of 0.59 μm, silver nanoparticles deposit on the entire PE region (Figure 6d) leading to a moderate increase in surface roughness and contact angle. For PE depths ≥1.21 μm (Figure 6e,f), silver nanostructures form only on the LD regions, as previously reported,29,43 allowing the roughness of the surface and the resulting contact angles to be tailored, with values ranging from ∼90° to ∼104° (Table 2). The presence of silver nanoparticles increased the contact angles in all cases.

Figure 6. AFM topography images prior to deposition on −z PPELN surfaces with PE depths of (a) 0.59, (b) 1.79, and (c) 3.10 μm. AFM topography images after silver deposition on −z PPELN surfaces with PE depths of (d) 0.59, (e) 1.79, and (f) 3.10 μm (24 nm z-scale).

Table 2. Surface Roughness and Wettability Characterization of −z PPELN Surfaces with Different PE Depthsb roughness (nm) PE depth (μm) 0.59 1.21 1.79 2.30 2.72 3.10

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b

CONCLUSIONS The contact angle of lithium niobate surfaces (typically ∼44° for +z and ∼54° for −z) can be reproducibly tailored by controlling the surface topography and chemistry within a range of ∼7° following UV exposure to ∼121° through periodic patterning and silver nanoparticle deposition. In all cases, compared to the surface prior to photodeposition, the presence of silver nanoparticles increased the contact angle of the surface investigated. By controlling the proton exposure time and thereby the depth of the proton exchanged region and the width of the lateral diffusion regions where nanoparticles typically deposit, it was possible to control the area decorated with nanoparticles and also the contact angle in the range of

before deposition 1.7 1.2 2.2 2.6 4.0 4.1

± ± ± ± ± ±

0.2 0.1 0.1 0.3 0.5 0.7

contact angle (deg)

after deposition 2.1 6.5 5.2 6.7 19.1 10.7

± ± ± ± ± ±

1.0 2.4 1.2 0.4 2.5 1.4

before deposition 63.8 66.9 35.8 60.9 71.1 45.4

± ± ± ± ± ±

0.1 0.1 0.1 0.5 0.7 0.7

after deposition 68.9 92.3 104.0 102.0 95.8 90.0

± ± ± ± ± ±

0.2 1.3 1.1 0.7 1.5 0.3

Roughness values were calculated from 14 μm × 14 μm areas.

∼69° to ∼104°. The demonstrated tunable wettability offered by etching, proton exchange, and metallic nanostructure photodeposition provides a route for extending the use of lithium niobate in plasmon-assisted optofluidic applications.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (B.J.R.). *E-mail [email protected] (J.H.R.). ORCID

Rusul M. Al-Shammari: 0000-0002-7214-1457 James H. Rice: 0000-0002-1035-5708 E


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Brian J. Rodriguez: 0000-0001-9419-2717 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This publication has emanated from research conducted with the financial support of the UCD School of Physics. This work was also supported by the Swedish Scientific Research Council (VR 622-2010-526 and 621-2011-4040) and the ADOPT Linnaeus Centre for Advanced Optics and Photonics in Stockholm. The AFM used for this research work was funded by Science Foundation Ireland (SFI07/IN1/B931). The authors are grateful to Peng Li and Gil Lee for access to the contact angle measurement system and for assistance with etching. The authors are thankful to Craig Carville, Sabine Neumayer, and Ilia Ivanov for insightful discussions.

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Tunable Wettability of Ferroelectric Lithium Niobate Surfaces

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