Article pubs.acs.org/Langmuir
Understanding Anisotropic Plasma Etching of Two-Dimensional Polystyrene Opals for Advanced Materials Fabrication Eser M. Akinoglu, Anthony J. Morfa, and Michael Giersig* Department of Physics, Freie Universität Berlin, 14195 Berlin, Germany S Supporting Information *
ABSTRACT: Anisotropic deformation of polystyrene particles in an oxygenated (O2/Ar) plasma is observed for radio frequency (rf) plasma and inductively coupled plasma (ICP). A facile model based on a ratio of completely isotropic and completely anisotropic etching is presented to describe the anisotropy of the etching process and is implemented to determine the height of the spheroid-shaped polystyrene particles. In our systems, we find the plasma etching to be 54% isotropic in the rf plasma and 79% isotropic in the ICP. With this model, the maximum material deposition thickness for nanofabrication with plasma-etched nanosphere lithography or colloid lithography can be predicted. Moreover, the etching of polystyrene particles in an oxygenated plasma is investigated versus the etching time, gas flow, gas composition, temperature, substrate material, and particle size. The results of this study allow precise shape tuning during the fabrication of nanostructured surfaces with size-dependent properties for bionic, medical, and photonic applications.
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INTRODUCTION In nature, fascinating macroscopic functions of some biosurfaces are enabled through structures and morphologies on the microscale and nanoscale. For instance, some plant species exhibit water-repellent and self-cleaning surfaces on which millimeter-scale raindrops show poor adhesion, commonly known as the lotus leaf effect,1 whereas the mosquito, C. pipiens, possesses antifogging eyes that prevent the adhesion of microscale water droplets.2 Other impressive examples are abundant, including microscopic grooves on the skin of sharks and dolphins that reduce drag in turbulent flow and needle-shaped microstructures under the legs of water striders that allow them to glide effortlessly on a water surface.3−5 From these examples, biomimetic research has grown, and currently researchers are able to mimic different biosurfaces. Gao et al. attributed the superhydrophobic properties of C. pipiens’ eyes to the hierarchical microstructures and nanostructures composed of hexagonally ordered, nonclose-packed (hncp) “nipples” on the nanoscale and hexagonally close-packed (hcp) ommatidia on the microscale and reproduced an analogous system using soft lithography.2 There have been many other notable attempts to produce hydrophobic surfaces.5,6 However, surfaces with microscale and nanoscale structures are not limited to superhydrophobic applications but are also used for medical applications such as implantable medical bionic devices and regenerative bionics for tissue regeneration where the composition and nanostructure of electrodes determine the properties of the electrode−cellular interface that assembles in response to the implant invasion.7 © 2014 American Chemical Society
A crucial step in developing such applications is the fabrication of large-area 2D nanostructured surfaces at low cost. One approach to nanofabrication that has grown in popularity over the last two decades, in part because of its high throughput in-parallel process enabled by a self-assembling system, is nanosphere lithography (NSL), more broadly known as natural lithography or colloidal lithography, which uses monodisperse colloidal dispersions of polystyrene spheres (PSSs) organized in a hexagonally close-packed monolayer by dip coating, electrophoretic deposition, electrochemical deposition, spin coating, or interface assembly techniques (Figure S1A in the Supporting Information).8−12 These ordered arrays can be further used as a template for advanced functional materials with size-dependent bionic, optical, plasmonic, photonic, magnetic, and catalytic properties13−17 using metal or inorganic material deposition, wet or dry (i.e., plasma) etching, annealing, or a combination of the previous techniques.12,18,19 Hexagonally ordered non-close-packed colloidal arrays of polystyrene particles (PSPs), where the PSPs are arranged in a hexagonal lattice and are spaced out equally, can be obtained by plasma etching in oxygenated atmospheres and are especially desirable for bionic applications but are also useful for optical, photonic, and sensing applications.2,7,16,20−22 Such hncp arrays are obtained by etching PSP arrays with two important properties: a desired particle size with the original Received: January 1, 2014 Revised: February 27, 2014 Published: March 2, 2014 12354
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purchased from Li Jing Ke Ji), float glass (purchased from Laborhandel Krumpholz), sapphire (purchased from CrysTec GmbH), fused silica (purchased from CrysTec GmbH), and ITO (5−15 Ω per square sheet resistance, purchased from Delta Technologies) on float glass surfaces cleaned in acetone, ethanol, and pure water with the aid of an ultrasonic bath for 15 min each, followed by a subsequent piranha etch for 1 h and a pure water rinse. The last two steps were not applied to ITO on float glass. The piranha etching solution was prepared by adding concentrated sulfuric acid (96%) to concentrated hydrogen peroxide (35%) in a ratio of 3:1. Characterization and Metallization. The characterization of PSP arrays was performed on a Hitachi 8030 scanning electron microscope. PSP arrays on silicon support substrates were cleaved with a diamond knife to obtain cross-sectional images. Silver was deposited at 0.3 nm/s in a custom-built e-beam evaporation chamber with a 3 nm titanium adhesion layer deposited at 0.1 nm/s in between. The PSPs were removed by immersing the system sequentially into toluene, acetone, ethanol, and pure water in an ultrasonic bath for 15 min each. The diameter and height of the PSPs were averaged over 50 and 20 data points, respectively, and were measured using Datinf Measure software. The data points were randomly picked from different locations on a 1 cm × 1 cm substrate to be representative of the entirety of the large-area 2D array of PSPs. Plasma Etching. Radio-frequency plasma etching of PSP arrays was carried out in a Plasma Technology MiniFlecto system in the kilohertz regime at a power of 64 W for different amounts of time. The plasma properties were varied by regulating the flow rates of oxygen and argon independently so that the pressure operation range was between 0.1 and 0.2 mbar with a chamber base pressure of 0.06 mbar. To obtain a reproducible plasma atmosphere, we used a stabilization time of 1000 s for gas flows. Plasma processes were performed at flow rates of 2 sccm oxygen and 1 sccm argon if not mentioned otherwise. The plasma chamber temperature was measured with an infrared thermometer prior to the etching process and was kept constant in the range of 30−34 °C. The chamber does not exceed 80 °C under normal conditions so that the glass-transition temperature of polystyrene is not reached.27 ICP plasma etching of PSP arrays was performed in a Fischione Instruments model 1020 plasma cleaner system. Here, the oxygen to argon gas ratio was fixed at 1:3.
position maintained in the hexagonally ordered arrangement, where the former can be tuned by varied plasma etching parameters and the latter by the initial size of the polystyrene spheres (Figure S1B in the Supporting Information).18,23,24 Typically, plasma etching is performed at low pressures below 10 mbar in an evacuated parallel-plate reactor or a quartz tube where the plasma is activated by a radio frequency (rf) source or as an inductively coupled plasma (ICP) through a coil, respectively.25 Sputtering ions as well as activated neutral chemical species enhancing chemical reactions give rise to high etching rates within the plasma. In general, ionic species are directional in the local electromagnetic fields and are associated with anisotropic physical etching. In contrast, reactive neutral species such as free radicals are associated with isotropic chemical etching where the reactant diffuses toward a surface, is adsorbed, chemically reacts, and is followed by the desorption of byproducts. A combination of both is commonly termed as ion enhanced etching.25 Consequently, spherical polystyrene spheres exposed to an oxygenated plasma should undergo an anisotropic shape modification from spheres to oblate spheroids resembling biconvex microlenses that may show a roughened surface as a result of polymer degradation during plasma etching associated with surface cross-linking.20,21,23,24 The latter describes the recombination of dangling bonds of adjacent polymer chains after the breakage of the polymer backbone though ion bombardment during plasma exposure and is associated with increased etch resistance.26 In many cases, it is desirable to remove the aforementioned lithography masks after material deposition, implying geometrical limitations to the thickness of the deposited material. For nonetched polystyrene spheres, the limitations are trivial and fix the maximum deposited material thickness to a theoretical value of half the sphere diameter. However, for etched polystyrene particles the potential anisotropic shape modification necessitates more understanding to define this limit. In this work, we study the influence of an oxygenated rf plasma on PSSs of different size ranging from 167 nm to 1.39 μm in diameter. We discuss the influence of the processing temperature, the PSS support substrate, and gas composition and gas flow in the plasma medium on the etching process and show that the particle diameter is reduced linearly with increasing etching time. Our investigations show that the deformation of PSPs during plasma exposure is partially anisotropic and partially isotropic, for which we demonstrate a simple geometric model. We compare the anisotropy of the plasma etching process in different systems and find that anisotropy is exhibited to different degrees, which we determine to be 54% isotropic in our rf plasma system in comparison to 79% in our ICP system. Finally, we discuss what effect this anisotropy will have on specific examples of different nanostructured surfaces produced by NSL.
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RESULTS AND DISCUSSION The primary investigations of the effects of different plasma processing conditions on the etching of polystyrene particles were carried out on colloidal monolayer arrays of polystyrene spheres with a diameter of 471 nm on a silicon support substrate. PSPs have been shown to shrink in size continuously with increasing plasma exposure time for fixed plasma parameters in the presence of oxygen.18,23,24,28−32 We find that the diameters of PSPs in an oxygenated plasma change linearly over time regardless of the plasma parameters. Previous reports on the plasma etching of PSSs are not consistent and show linear24,31,32 as well as nonlinear trends,18,23,28−30 where the latter may be attributed to changes in other plasma parameters such as an increasing processing temperature. An exemplary trend in the time-dependent etching series at a flow rate of 2 sccm oxygen and 1 sccm argon is shown in Figure 1A. SEM micrographs for PSPs etched for 0, 200, 500, and 800 s are displayed in a gallery in Figure 1B−E, respectively. Increasing surface roughness can be observed for extended exposure times and has been observed before.18,21,26,30,31,33,34 Eventually, the roughness of the particles grows to such an extent that the PSPs lose their spherical form (Figure 1D), as shown in Figure 1A as a plateau at long plasma exposure times. Here, for the last two data points the diameter was determined by fitting the entirety of the deformed PSPs into the smallest possible circle.
EXPERIMENTAL SECTION
Nanosphere Arrays. For the preparation of NSL masks, we used PSSs with a diameter of 167 nm (5% by weight) and PSSs with diameters of 471 nm, 784 nm, and 1.39 μm dispersed in water (10% by weight) purchased from Microparticles GmbH Berlin, which are further diluted into an ethanol solution containing 1% styrene and 0.1% sulfuric acid in a ratio of 1:1 and subsequently dispersed on a water−air interface (18 MΩ, Milli-Q water) in a Petri dish. The polystyrene particles are self-assembled into an ordered hexagonally close-packed array and are further deposited on a support substrate by water removal through pumping and subsequent evaporation. In this study, we use n-type silicon (⟨100⟩, 0.007−0.008 Ω cm resistance, 12355
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Figure 2. PSP diameter vs etching time for an initial sphere diameter of 471 nm on silicon in an rf plasma and different gas flow ratios of O2 to Ar: 1 sccm O2 to 0 sccm Ar (green circle), 2 sccm O2 to 0 sccm Ar (red square), 1 sccm O2 to 1 sccm Ar (red circle), 3 sccm O2 to 0 sccm Ar (blue triangle), 2 sccm O2 to 1 sccm Ar (blue square), and 1 sccm O2 to 2 sccm Ar (blue circle). The PSP diameter decrease is linear and depends on the total gas flow, that is, 0.70 nm/s for 1 sccm (green), 0.52 nm/s for 2 sccm (red), and 0.34 nm/s for 3 sccm (blue).
aggressive chemical etching such that the polystyrene particles do not resemble spherical particles any longer (Figure S2A− S2C). Introducing argon into the plasma environment results in smoother PSP surfaces as a result of enhanced physical etching (Figure S2D−S2H). PSP exposure to pure argon plasma shows marginal etching after much longer exposure times compared to that of oxygenated plasmas (Figure S2I). We account for the marginal etching with residual oxygen in the system. These observations are consistent with investigations of the roughening of polystyrene surfaces in oxygenated plasmas.26 Ting et al. reported a lower surface roughening formation for increasingly higher ion bombardment energies and related the lower surface roughening formation in oxygen and argon admixture plasmas with respect to that of a pure oxygen plasma to higher ion to radical flux ratios for increasing argon admixtures.26 We find that a gas ratio of 2 sccm oxygen and 1 or 2 sccm argon is the most suitable for achieving homogeneous etching without leaving residual material behind on the sample surface in our rf plasma system. In general, chemical reactions show sensitivity to the environmental temperature, so a constant system temperature is desired for reproducibility reasons. However, the plasma process increasingly heats the chamber environment with processing time so that a constant temperature cannot be maintained in common experimental setups. Hence, it is desirable to control the initial chamber temperature alternatively to obtain reproducible results. To illustrate the situation, in Figure 3 we show the results of a series of identical etching processes (i.e., a 3 sccm oxygen flow rate and 300 s of plasma exposure for PSSs with a 471 nm diameter supported by a silicon substrate) performed in rapid succession without allowing the system to cool to a set temperature. Clearly, the etching is stronger for higher temperatures with the resulting diameter being 14% smaller for an initial temperature difference of 25 °C. Therefore, the impact of the environmental temperature on the reproducibility of polystyrene sphere plasma etching is rather significant and needs to be controlled
Figure 1. (A) Reduction of the PSP diameter vs etching time with an initial sphere size of 471 nm in a rf plasma at flow rates of 2 sccm O2 and 1 sccm Ar. The linear decrease plateaus at approximately 800 s, where the PSPs lose their spherical shape. Exemplary SEM micrographs are shown for (B) 0, (C) 200, (D) 500, and (E) 800 s etching times, respectively.
The plasma environment plays a crucial role in the plasma etching process so that the gas composition and the total gas flow are parameters that have to be considered. In Figure 2, we show the linear decrease in the polystyrene particle diameter with increasing plasma exposure duration for different gas compositions and flow rates. We find that the etching rate of PSPs in the oxygenated rf plasma is dependent only on the overall gas flow, whereas the gas composition has no obvious impact. Trends for total gas flows of 1, 2, and 3 sccm with varying gas compositions are shown in Figure 2 with green, red, and blue markers, yielding diameter decreases of 0.70, 0.52, and 0.34 nm/s over time, respectively. The independence of the etching rate on the gas ratio of oxygen to argon, provided that at least 1 sccm oxygen is used, indicates that this amount of oxygen is sufficient to saturate the chemical etching. Though the etching rate is not affected by the gas composition, the latter has an influence on the shape of etched PSPs. A gallery of exemplary SEM micrographs for PSPs etched with different gas compositions during plasma exposure is shown in Figure S2 (Supporting Information). From Figure S2 it is apparent that the plasma etching results in different degrees of surface roughening and different degrees of material removal between PSPs. Severe roughening is observed for pure oxygen plasmas in the absence of argon as a result of very 12356
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etching. The reduced etching rate for PSPs on insulating substrates can be attributed to a lower ion to radical flux ratio due to charging effects. When using hexagonally close-packed arrays of PSSs for lithography, one is limited by geometrically defined boundaries. In particular, the maximum deposited material thickness σmax is fixed at half the pitch (or the PSS diameter d0) to prevent the polystyrene spheres from being buried under the deposited material (because, of course, the pitch and sphere height are equal). Examples of a 3 nm adhesive titanium layer and a 135 or 223 nm plasmon active silver layer are shown in Figure 5A for σ1 = 0.29d0 and in Figure 5B for σ3 = 0.48d0, respectively. The material is deposited in the interstitial space between and on the PSSs (Figure 5A1,B1), yielding triangular islands after PSP removal (Figure 5A2,B2). The triangular islands formed pyramidal islands with increasing deposition thickness that results from interpolystyrene sphere aperture closing during material deposition onto the PSSs. In contrast, the situation changes for plasma-exposed PSPs. Here, the gap in between the PSP apertures increases and yields a film that is hexagonally perforated with spherical holes, also occasionally termed an antidot array or nanoaperture array, as shown in Figure 5C. However, from cross-sectional SEM micrographs in Figure 5C1,D1 it is clear that the PSPs lost their previous spherical shape during plasma exposure so that the horizontal diameter and the vertical diameter, which we term in the following text diameter d and height h, are not equal. The obvious consequence is that the diameter of plasma-etched PSPs cannot be a direct measure to determine a maximum deposition thickness. To illustrate this situation, we have deposited PSPs etched to dimensions of d′ = 362 nm and h′ = 290 nm with 138 nm (3 nm Ti + 135 nm Ag) and 175 nm (3 nm Ti + 172 nm Ag) material corresponding to σ2 = 0.48h′ and σ3 = 0.48d′ as shown in Figure 5C1,D1, respectively. In contrast to Figure 5C, the PSPs in Figure 5D are buried and cannot be removed. Hence, for most applications, it is important to understand the shape modification and find simple means to predict σmax for plasma-etched polystyrene particles. A gallery of cross-sectional SEM micrographs of PSPs etched for subsequently longer plasma exposure times is shown in Figure 6. It is apparent that an anisotropic shape modification occurs during plasma exposure that can be accounted for by an interplay of anisotropic physical etching due to ionic species, isotropic chemical etching due to reactive species, and a shadowing effect causing a lower flux of reactive radicals in the valleys of surface topography with respect to the peaks, which is indicated by strong surface roughness formation at the peaks, whereas surface roughness is not visible in the valleys of surface topography.26 To find a relationship between the height and the diameter of the polystyrene particles, a plot of height versus diameter of PSPs etched under different conditions and for different amounts of time is shown in Figure 7. Clearly, there is a continuous trend regardless of flow rates and gas composition in the ratio of the etched sphere height to its width. To determine the degree of anisotropy (the ratio of the height to the width of the etched spheres), we apply a simple model based on a completely isotropic etching process where the height of an isotropically etched sphere is described by eq 1,
Figure 3. Dependence of the polystyrene particle diameter for PSS with an initial diameter of 471 nm on silicon after 300 s of plasma exposure at a flow rate of 3 sccm O2 on the initial chamber temperature. Etching is more rapid for higher temperatures, yielding a 14% smaller particle diameter.
well. To this end, we chose to use a fixed temperature range of 30−34 °C at the start of each process. Because the nature of the plasma etching involves charged species, the dielectric environment is expected to have an influence on the plasma process itself. This is especially true for small particles on a surface. We observe significant differences in the etching rate of PSPs for silicon, a semiconductor, with an etching rate of 0.34 nm/s and insulating materials such as glass, fused silica, and sapphire with an etching rate of 0.22 nm/s, shown in Figure 4. Indium tin oxide (ITO), a conductive oxide sputtered as a thin film on float glass, follows the same trend as the insulating materials whereas metallic aluminum substrates (not shown) result in very rapid and less-controllable PSP
Figure 4. Influence of substrate material on the oxygen plasma etching of polystyrene particles. Shown is the PSP diameter vs etching time for polystyrene spheres with an initial diameter of 471 nm and flow rates of 2 sccm O2 and 1 sccm Ar. The insulating materials (black filled) show a noticeably smaller diameter decrease of 0.22 nm/s compared to the semiconductor Si (round unfilled) with a diameter decrease of 0.34 nm/s. ITO on float glass falls on the same line as the solely insulating materials because the ITO is not in electrical contact with the conducting walls of the chamber.
hi = d
(1)
where hi is the height in the case of completely isotropic etching and is shown in Figure 8A and is indicated in Figure 7 12357
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Figure 5. Cross-sectional SEM micrographs before ultrasonic treatment (A1−D1) and top-view SEM micrographs after ultrasonic treatment (A2− D2) of nonetched polystyrene spheres (A, B) and etched polystyrene particles/spheroids (C, D). Different amounts of metal were evaporated on top of the PSPs: (A) 138, (B) 226, (C) 138, and (D) 175 nm. (D) The PSPs are buried below the deposited metal.
Figure 7. Anisotropy of polystyrene particles with an initial diameter of 471 nm exposed to an oxygenated plasma shown in height vs diameter. The polystyrene particles were etched mostly in an rf plasma for different times and under different gas flow ratios of O2 to Ar. Exact gas flow ratios are given in the legend in units of standard cubic centimeters (sccm). PSPs etched in an ICP plasma are shown as black squares. The gray dashed−dotted, dashed, and solid lines represent completely isotropic etching, completely anisotropic etching, and the average of these two. PSPs etched in an rf plasma follow a common trend close to the average of isotropic (55%) and anisotropic (45%) etching regardless of the plasma environment. Etching in an ICP system (blue squares) is more isotropic (76%). Figure 6. Gallery of cross-sectional SEM micrographs of polystyrene spheres with an initial diameter of 471 nm etched for (A) 0, (B) 100, (C) 200, (D) 300, (E) 400, and (F) 500 s at 2 sccm O2 and 1 sccm Ar gas flows. The polystyrene spheres undergo an anisotropic shape modification from sphere to spheroid polystyrene particles with the height to width ratio decreasing from 1 to 0.71 over this series.
and indicated in Figure 7 as a gray dashed line. For all cases measured, these two models must be combined to a geometrical weighted average yielding eq 3, h = ζha + (1 − ζ )hi
where ζ is the degree of anisotropy. We find ζ = 0.45 in our rf plasma system, indicated as a dashed black fit line in Figure 7. The geometric average with ζ = 0.5 is indicated as a solid light -gray line as a guide for the eye. For comparison, we studied the etching of PSPs in an ICP plasma where we find a 1 order of magnitude larger etching rate allowing for less control with much shorter etching times and a degree of anisotropy of ζ = 0.21. Thus, an anisotropic shape modification of PSPs is observed for the most common plasma sources and varies only in the degree of anisotropy, which is different from system to system.
as a gray dotted−dashed line. In contrast, a completely anisotropic etching process can be described by a sphere− sphere intersection problem where initially the spheres overlap and one shifts downward at a constant rate with time and can be described by eq 2, ha = d0 − (d0 2 − d 2)0.5
(3)
(2)
where ha is the height in the case of completely anisotropic etching with an initial PSS diameter of d0 sketched in Figure 8B 12358
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Figure 8. Sketch of geometrical models for completely isotopic and completely anisotropic plasma etching of polystyrene spheres. (A) Completely isotropic etching with nondirectional etching. Here the initial polystyrene sphere decreases in size but preserves its spherical shape. di = h with particle diameter di. (B) Completely anisotropic etching with directional etching from the top. Here the initial polystyrene sphere decreases in size and undergoes a shape modification from sphere to spheroid particle, yielding a biconvex particle shape. (da ≥ h) with particle diameter da.
for all sphere sizes, in contrast to 0.34 nm/s and ζ = 0.45 when considering only PSSs with a diameter of 471 nm. The data points shown in Figure 9A were fit such that each line has the same slope whereas the data points in Figure 9B were fit such that all measured values contributed equally to ζ. Finally, we show how careful control over the etching process can be used to produce several nanostructured surfaces for bionic, photonic, plasmonic, and medical applications using NSL in Figure 10. Hexagonally ordered and perpendicular aligned multiwalled carbon nanotubes grown by plasmaenhanced chemical vapor deposition after the deposition of nickel catalyst islands (Figure 10A) have been shown to have photonic as well as superhydrophobic properties.35,36 Similar structures were used for cell seeding and growth.37 Figure 10B shows SiO2 microcavities that may find applications as bioreactors.38,39 Triangular gold arrays (Figure 10D) and perforated hole arrays (Figure 10C) are two distinct but related large-area photonic and plasmonic nanostructured surfaces that have been investigated separately14,40 and together as a percolation problem,16 with unique optical properties. The transmission spectra of these two particular structures are shown in Figure 10E and underline their vastly different optical response on either side of the percolation threshold. However, applications do not always necessitate the removal of the PSPs. For instance, this was demonstrated by López-Garciá et al., who used hcp and hncp arrays of PSPs on a surface plasmon resonance supporting metallic film to successfully tune hybrid photonic− plasmonic modes on the metal−dielectric interface by reducing the PSP size while keeping the lattice parameter constant.41
Of course, differently sized polystyrene spheres are produced in large batches, with different properties from batch to batch. To extend our study, different PSS sizes ranging from 167 nm to 1.39 μm were investigated. We find that the etching rate (Figure 9A) as well as the degree of anisotropy (Figure 9B) do not show a significant size dependency. In particular, we find an etching rate of 0.35 nm/s and a degree of anisotropy ζ = 0.46
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CONCLUSIONS
We have investigated the etching of polystyrene spheres in an oxygenated plasma, specifically, the dependence of etching time, gas flow, gas composition, temperature, substrate material, and particle size. Under typical conditions, the anisotropic nature of the plasma etching process is shown that is independent of the type of plasma source and changes only in its degree of anisotropy. Furthermore, we present a model to predict the height of etched spheroid-shaped polystyrene particles only from their diameter that can be measured easily in an SEM. Our results facilitate the shape tuning in the fabrication of nanostructured materials fabricated by any etched nanosphere technique such as NSL, especially with respect to
Figure 9. (A) Diameter decrease over etching time for different PSP sizes at 2 sccm O2 and 1 sccm Ar gas flow. (B) Anisotropy of PSPs of different sizes, normalized to the height and plotted vs the diameter. 12359
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ACKNOWLEDGMENTS We acknowledge Dr. K. Ellmer of HZB for assistance with metal evaporation. A.J.M. acknowledges the AvH Stiftung for financial support. M.G. acknowledges financial support from the National Science Centre allocated on the basis of decision number DEC-2013/06/A/ST4/00373. E.M.A. acknowledges financial support from the European Union under project CosmoPHOS-nano (no. 310337).
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Figure 10. Gallery of bionic and photonic applications. (A) Hexagonally ordered and perpendicular aligned multiwalled carbon nanotubes. (B) Biocompatible SiO2 microcavities. (C) Gold film with hexagonally ordered perforations. (D) Quasi-triangular gold island arrays. (E) Transmission spectra of the structures in panels C (black) and D (gray).
the maximum deposition thickness of materials on the mask. Thus, these findings allow for more accurate control in the tuning of size-dependent properties of functional nanostructured surfaces for many applications, including bionic, medical, and photonic applications.
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ASSOCIATED CONTENT
S Supporting Information *
(S1) Large area SEM micrographs of nonetched and etched PSP arrays. (S2) Gallery of SEM micrographs for PSPs etched with different gas compositions and for different amounts of time. This material is available free of charge via the Internet at http://pubs.acs.org
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REFERENCES
(1) Neinhuis, C.; Barthlott, W. Characterization and Distribution of Water-Repellent, Self-Cleaning Plant Surfaces. Ann. Bot. 1997, 79, 667−677. (2) Gao, X.; Yan, X.; Yao, X.; Xu, L.; Zhang, K.; Zhang, J.; Yang, B.; Jiang, L. The Dry-Style Antifogging Properties of Mosquito Compound Eyes and Artificial Analogues Prepared by Soft Lithography. Adv. Mater. 2007, 19, 2213−2217. (3) Bushnell, D. M.; Moore, K. J. Drag Reduction in Nature. Annu. Rev. Fluid Mech. 1991, 23, 65−79. (4) Gao, X.; Jiang, L. Biophysics: Water-Repellent Legs of Water Striders. Nature 2004, 432, 36. (5) Lee, Y.; Yoo, Y.; Kim, J.; Widhiarini, S.; Park, B.; Park, H. C.; Yoon, K. J.; Byun, D. Mimicking a Superhydrophobic Insect Wing by Argon and Oxygen Ion Beam Treatment on Polytetrafluoroethylene Film. J. Bionic Eng. 2009, 6, 365−370. (6) Li, Y.; Huang, X. J.; Heo, S. H.; Li, C. C.; Choi, Y. K.; Cai, W. P.; Cho, S. O. Superhydrophobic Bionic Surfaces with Hierarchical Microsphere/SWCNT Composite Arrays. Langmuir 2007, 23, 2169− 2174. (7) Wallace, G. G.; Higgins, M. J.; Moulton, S. E.; Wang, C. Nanobionics: The Impact of Nanotechnology on Implantable Medical Bionic Devices. Nanoscale 2012, 4, 4327−4347. (8) Deckman, H. W.; Dunsmuir, J. H. Natural Lithography. Appl. Phys. Lett. 1982, 41, 377−379. (9) Denkov, N. D.; Velev, O. D.; Kralchevski, P. A.; Ivanov, I. B.; Yoshimura, H.; Nagayama, K. Mechanism of Formation of TwoDimensional Crystals from Latex Particles on Substrates. Langmuir 1992, 8, 3183−3190. (10) Giersig, M.; Mulvaney, P. Preparation of Ordered Colloid Monolayers by Electrophoretic Deposition. Langmuir 1993, 9, 3408− 3413. (11) Hulteen, J. C.; Van Duyne, R. P. Nanosphere lithography: A materials general fabrication process for periodic particle array surfaces. J. Vac. Sci. Technol., A 1995, 13, 1553−1558. (12) Kosiorek, A.; Kandulski, W.; Chudzinski, P.; Kempa, K.; Giersig, M. Shadow Nanosphere Lithography: Simulation and Experiment. Nano Lett. 2004, 4, 1359−1363. (13) Jensen, T. R.; Malinsky, M. D.; Haynes, C. L.; Van Duyne, R. P. Nanosphere Lithography: Tunable Localized Surface Plasmon Resonance Spectra of Silver Nanoparticles. J. Phys. Chem. B 2000, 104, 10549−10556. (14) Peng, Y.; Marcoux, C.; Patoka, P.; Hilgendorff, M.; Giersig, M.; Kempa, K. Plasmonics of Thin Film Quasitriangular Nanoparticles. Appl. Phys. Lett. 2010, 96, 133104. (15) Ctistis, G.; Papaioannou, E.; Patoka, P.; Gutek, J.; Fumagalli, P.; Giersig, M. Optical and Magnetic Properties of Hexagonal Arrays of Subwavelength Holes in Optically Thin Cobalt Films. Nano Lett. 2009, 9, 1−6. (16) Akinoglu, E. M.; Sun, T.; Gao, J.; Giersig, M.; Ren, Z.; Kempa, K. Evidence for Critical Scaling of Plasmonic Modes at the Percolation Threshold in Metallic Nanostuructures. Appl. Phys. Lett. 2013, 103, 171106. (17) Wood, M. A. Colloidal lithography and current fabrication techniques producing in-plane nanotopography for biological applications. J. R. Soc., Interface 2007, 4, 1−17. (18) Haginoya, C.; Ishibashi, M.; Koike, K. Nanostructure Array Fabrication with a Size-Controllable Natural Lithography. Appl. Phys. Lett. 1997, 71, 2934−2936.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. 12360
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of Carbon Nanotube-Based 3D Networks as Scaffolds for Cell Seeding and Growth. Nano Lett. 2004, 4, 2233−2236. (38) Grayson, W. L.; Martens, T. P.; Eng, G. M.; Radisic, M.; VunjakNovakovic, G. Biomimetic Approach to Tissue Engineering. Sem. Cell Dev. Biol. 2009, 20, 665−673. (39) Retterer, S. T.; Siuti, P.; Choi, C.-K.; Thomas, D. K.; Doktycz, M. J. Development and Fabrication of Nanoporous Silicon-Based Bioreactors within a Microfluidic Chip. Lab Chip. 2010, 10, 1174− 1181. (40) Ctistis, G.; Patoka, P.; Wang, X.; Kempa, K.; Giersig, M. Optical Transmission through Hexagonal Arrays of Subwavelength Holes in Thin Metal Films. Nano Lett. 2007, 7, 2926−2930. (41) López-García, M.; Galisteo-López, J. F.; Blanco, Á ; López, C.; García-Martín, A. High Degree of Optical Tunability of SelfAssembled Photonic-Plasmonic Crystals by Filling Fraction Modification. Adv. Funct. Mater. 2010, 20, 4338−4343.
(19) Kosiorek, A.; Kandulski, W.; Glaczynska, H.; Giersig, M. Fabrication of Nanoscale Rings, Dots, and Rods by Combining Shadow Nanosphere Lithography and Annealed Polystyrene Nanosphere Masks. Small 2005, 1, 439−444. (20) Han, S.; Hao, Z.; Wang, J.; Luo, Y. Controllable TwoDimensional Photonic Crystal Patterns Fabricated by Nanosphere Lithography. J. Vac. Sci. Technol., B 2005, 23, 1585−1588. (21) Fujimura, T.; Tamura, T.; Itoh, T.; Haginoya, C.; Komori, Y.; Koda, T. Morphology and Photonic Band Structure Modification of Polystyrene Particle Layers by Reactive Ion Etching. Appl. Phys. Lett. 2001, 78, 1478−1480. (22) Sun, T.; Akinoglu, E. M.; Guo, C.; Paudel, T.; Gao, J.; Wang, Y.; Giersig, M.; Ren, Z.; Kempa, K. Enhanced Broad-Band Extraordinary Optical Transmission through Subwavelength Perforated Metallic Films on Strongly Polarizable Substrates. Appl. Phys. Lett. 2013, 102, 101114. (23) Tan, B. J.-Y.; Sow, C.-H.; Lim, K.-Y.; Cheong, F.-C.; Chong, G.L.; Wee, A. T.-S.; Ong, C.-K. Fabrication of a Two-Dimensional Periodic Non-Close-Packed Array of Polystyrene Particles. J. Phys. Chem. B 2004, 108, 18575−18579. (24) Plettl, A.; Enderle, F.; Saitner, M.; Manzke, A.; Pfahler, C.; Wiedemann, S.; Ziemann, P. Non-Close-Packed Crystals from SelfAssembled Polystyrene Spheres by Isotropic Plasma Etching: Adding Flexibility to Colloid Lithography. Adv. Funct. Mater. 2009, 19, 3279− 3284. (25) Bellan, P. M. Fundamentals of Plasma Physics; Cambridge University Press: Cambridge, MA, 2006. (26) Ting, Y.-H.; Liu, C.-C.; Park, S.-M.; Jiang, H.; Nealey, P. F.; Wendt, A. E. Surface Roughening of Polystyrene and Poly(methyl methacrylate) in Ar/O2 Plasma Etching. Polymers 2010, 2, 649−663. (27) Keddie, J. L.; Jones, R. A. L.; Cory, R. A. Size-Dependent Depression of the Glass Transition Temperature in Polymer Films. Europhys. Lett. 1994, 27, 59−64. (28) Stoffels, W. W.; Stoffels, E.; Swinkels, G. H. P. M.; Boufnichel, M.; Kroesen, G. M. W. Etching a Single Micrometer-Size Particle in a Plasma. Phys. Rev. E 1999, 59, 2302−2304. (29) Hanarp, P.; Sutherland, D. S.; Gold, J.; Kasemo, B. Control of Nanoparticle Film Structure for Colloidal Lithography. Colloids Surf., A 2003, 214, 23−36. (30) Yan, L.; Wang, K.; Wu, J.; Ye, L. Hydrophobicity of Model Surfaces with Loosely Packed Polystyrene Spheres after Plasma Etching. J. Phys. Chem. B 2006, 110, 11241−11246. (31) Cheung, C. L.; Nikolić, R. J.; Reinhardt, C. E.; Wang, T. F. Fabrication of Nanopillars by Nanosphere Lithography. Nanotechnology 2006, 17, 1339−1343. (32) Brombacher, C.; Saitner, M.; Pfahler, C.; Plettl, A.; Ziemann, P.; Makarov, D.; Assmann, D.; Siekman, M. H.; Abelmann, L.; Albrecht, M. Tailoring Particle Arrays by Isotropic Plasma Etching: An Approach Towards Percolated Perpendicular Media. Nanotechnology 2009, 20, 105304. (33) Valsesia, A.; Meziani, T.; Bretagnol, F.; Colpo, P.; Ceccone, G.; Rossi, F. Plasma Assisted Production of Chemical Nano-Patterns by Nano-Sphere Lithography: Application to Bio-interfaces. J. Phys. D: Appl. Phys. 2007, 40, 2341−2347. (34) Xia, D.; Ku, Z.; Li, D.; Brueck, S. R. J. Formation of Hierarchical Nanoparticle Pattern Arrays Using Colloidal Lithography and TwoStep Self-Assembly: Microspheres Atop Nanospheres. Chem. Mater. 2008, 20, 1847−1854. (35) Kempa, K.; Kimball, B.; Rybczynski, J.; Huang, Z. P.; Wu, P. F.; Steeves, D.; Sennett, M.; Giersig, M.; Rao, D. V. G. L. N.; Carnahan, D. L.; Wang, D. Z.; Lao, J. Y.; Li, W. Z.; Ren, Z. F. Photonic Crystals Based on Periodic Arrays of Aligned Carbon Nanotubes. Nano Lett. 2003, 3, 13−18. (36) Zhu, L.; Xiu, Y.; Xu, J.; Tamirisa, P. A.; Hess, D. W.; Wong, C.P. Superhydrophobicity on Two-Tier Rough Surfaces Fabricated by Controlled Growth of Aligned Carbon Nanotube Arrays Coated with Fluorocarbon. Langmuir 2005, 21, 11208−11212. (37) Correa-Duarte, M. A.; Wagner, N.; Rojas-Chapana, J.; Morsczeck, C.; Thie, M.; Giersig, M. Fabrication and Biocompatibility 12361
dx.doi.org/10.1021/la500003u | Langmuir 2014, 30, 12354−12361
