Influence of Solvent on Octadecyltrichlorosilane Nanostructures

Article pubs.acs.org/JPCC

Influence of Solvent on Octadecyltrichlorosilane Nanostructures Fabricated Using Particle Lithography Amy L. Brownfield, Corey P. Causey,* and Thomas J. Mullen* Department of Chemistry, University of North Florida, Jacksonville, Florida 32224, United States ABSTRACT: Numerous strategies have been devised to register organosilane monolayers for applications ranging from lubricants to semiconductor surface resists. Of these patterning techniques, particle lithography offers a straightforward and high-throughput method to create periodic arrays of organosilane nanopatterns. Herein, we describe the influence of solvent on the solutionphase formation of periodic arrays of nanopores within octadecyltrichlorosilane (OTS) monolayers using particle lithography. Our systematic study of various compositions of two miscible solvents, anhydrous toluene and bicyclohexyl, demonstrates control over nanopore size and OTS surface coverage. Smaller nanopores are generated from solutions with higher anhydrous toluene composition, and larger nanopores are generated from solutions with higher bicyclohexyl composition. A study of the effect of deposition time on nanopore formation found that at shorter deposition times (15 min), the size of the nanopore is limited by the mesosphere-substrate contact geometry as seen with anhydrous toluene solutions. This ability to regulate nanopore size and surface coverage, while preserving the interpattern periodicity, demonstrates an additional level of hierarchical control over organosilane nanostructure formation and enables a broader range of nanostructures that can be fabricated.

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INTRODUCTION The study of organosilane self-assembled monolayers (SAMs) has garnered tremendous interest since they were first reported by Sagiv in 1980.1−4 These systems, which are chemically and thermally robust due to covalent bonds formed with the underlying substrate, can also be tailored by varying their terminal functional groups.5 Adding to their utility, densely packed organosilane SAMs can be prepared on a broad range of substrates, including semiconductor oxides, metal oxides, glass, mica, and quartz.6−15 To date, numerous strategies have been devised to form and to register organosilane SAMs for applications ranging from lubricants to semiconductor surface resists.16−28 Particle lithography is one such strategy that offers a straightforward and high-throughput method to fabricate periodic nanopatterns of organosilane SAMs.10,29−32 In particle lithography, monodispersions of silica (or latex) spheres in water are deposited onto flat surfaces, and close-packed structures with regular interpattern geometries spontaneously assemble as the water evaporates. The resulting two-dimensional arrays serve as templates that guide the formation of organosilane SAMs into ordered nanopatterns. This patterning strategy has been employed to create nanostructures that (i) direct the synthesis of metals, rare earth oxide nanocrystals, and polymer brushes, (ii) capture metal nanoparticles and semiconductor quantum dots, and (iii) respond to changes in the chemical environment.20,33−42 Given the potential applications, much work has been done to better understand the key factors that govern the creation of © 2015 American Chemical Society

organosilane nanopatterns using particle lithography. The selfassembly of organosilanes is mediated by a series of hydrolysis, cross-linking, and silination steps; thus, a broad range of factors, including the amount of water present on the surface, temperature, deposition medium, organosilane structure, deposition time, chemical nature of the substrate, sphere size, and material composition of the sphere have been shown to influence the quality and morphology of the organosilane nanopatterns.2,5,12,29,43−50 For example, various types of nanostructure geometries, including nanopores and nanorings, can be generated by controlling mesosphere drying time (and thus the amount and location of water on the mesosphere template) during the vapor deposition of organosilanes.29,31,51 Shorter drying times (∼60 min) have produced arrays of organosilane nanopores resulting from a thin layer of water distributed across the entire surface. Longer drying times (∼12 h) have led to the formation of organosilane ring nanostructures that result from the water that is retained only around the base of the nanoparticle. Further, changes in periodicity, density, and feature size can be modulated by changes in the diameter of the mesospheres used for both solution- and vapor-phase deposition of organosilane nanopatterns.31,32,51,52 In this report, we investigate the influence of anhydrous toluene and bicyclohexyl on the solution-phase formation of nanopores Received: March 17, 2015 Revised: April 30, 2015 Published: May 4, 2015 12455

DOI: 10.1021/acs.jpcc.5b02576 J. Phys. Chem. C 2015, 119, 12455−12463

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The Journal of Physical Chemistry C within octadecyltrichlorosilane (OTS) SAMs formed on Si substrates using particle lithography. Although these two solvents are commonly utilized to generate densely packed OTS SAMs and nanostructures, we observe that the size of the nanopores varies as a function of solvent, while the interpattern periodicity remains constant and dependent on the diameter of the silica nanoparticle template. Control over nanopore size is achieved by systematically mixing these two miscible solvents in various ratios. In addition, the effect of deposition time was investigated to gain insight into the role of the water meniscus in the nanopore formation. We find that at shorter deposition times the nanopore size is limited by diffusion of the solvated OTS molecules into the water meniscus. However, at longer deposition times, diffusion of the solvated OTS molecules is no longer limiting, and the nanopore size is defined by the contact geometry between the mesosphere and substrate. This ability to regulate nanopore size and surface coverage, while preserving the interpattern periodicity, demonstrates an additional level of hierarchical control over OTS nanostructure fabrication and thus broadens the range of nanostructures that can be fabricated.

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Figure 1. Key steps for the fabrication of nanopores within an OTS SAM on Si substrates. (A) Silica mesospheres are cast across a Si substrate and allowed to dry for 3−4 h. (B) An OTS monolayer is formed by immersing the Si substrate into an OTS solution for a predetermined amount of time (1−60 min). (C) The silica mesospheres are removed via sonication in ethanol, revealing a periodic arrangement of nanopores.

EXPERIMENTAL METHODS Materials and Reagents. Boron-doped, polished Si(111) substrates were purchased from Ted Pella (Redding, CA). These substrates were precut to lateral dimensions of 5 mm × 5 mm. Aqueous suspensions of 500 nm silica particles (8000 Series) were purchased from Thermo Scientific (Waltham, MA), and 250 nm silica particles were purchased from Fiber Optic Center Inc. (New Bedford, MA). Octadecyltrichlorosilane (OTS, >90%) was purchased from Sigma-Aldrich (St. Louis, MO). Toluene (anhydrous, >98.8%), bicyclohexyl (>99.0%), hydrogen peroxide (30% aqueous solution), sulfuric acid (ACS grade), and ammonium hydroxide (ACS grade) were purchased from VWR International (Randor, PA). Absolute ethanol was purchased from Pharmco-Aaper (Bookfield, CT). All reagents were used as received. Water (18MΩ) was generated using a Milli-Q system (Q-GARD 2, Millipore, Billerica, MA). Preparation of Si Substrates. Si substrates were cleaned by first immersing in piranha solution (3:1 by volume of sulfuric acid/30% hydrogen peroxide) for 1 h to remove organic impurities, rinsing with copious amounts of 18 MΩ water, submersing in a base etch solution (5:1:1 by volume of 18 MΩ water/ammonium hydroxide/30% hydrogen peroxide) with heating in an oven at 70 °C for 1 h to remove organic impurities and trace metals, and finally rinsing with copious amounts of 18 MΩ water.53,54 The Si substrates were dried under a stream of N2 prior to use in OTS nanostructure fabrication. Caution: piranha is a vigorous oxidant and should be used with extreme care! Preparation of Silica Mesospheres. An aliquot of the stock 500 nm silica mesosphere suspension (2% by weight) was pipetted into a microcentrifuge tube and subjected to centrifugation (9200 rpm) for 10 min. The supernatant was removed, and the pellet was resuspended in 18 MΩ water, using a vortex mixer, to a concentration of 4% by weight. This suspension was subjected to centrifugation and resuspention three more times prior to use in OTS nanostructure generation. A stock suspension of 250 nm silica mesospheres (4% by weight) was generated and prepared using the same centrifugation/ resuspension protocol. Fabrication of Nanopores within OTS SAMs Using Particle Lithography. Nanopores within OTS SAMs on Si substrates were fabricated using the particle lithography strategy depicted in Figure 1. Aliquots (10 μL) of a silica mesosphere

suspension were deposited across Si substrates and allowed to dry in a humidity controlled environment (63 ± 2%) at room temperature (21 ± 1 °C) for 3−4 h. Note: the humidity was controlled by bubbling nitrogen through water into either a glovebag (Sigma-Aldrich, St. Louis, MO) or an acrylic desiccator cabinet (Thermo Scientific, Waltham, MA). These conditions yielded closed-packed silica mesospheres consistent with previous reports.10,29,37,55−59 The Si substrates with the silica mesospheres were then immersed into 5 mM solutions of OTS in either anhydrous toluene and/or bicyclohexyl for a predetermined time (1−60 min). Subsequently, the Si substrates were rinsed with toluene, dried under a stream of N2, rinsed with absolute ethanol, and dried under a stream of N2. Silica mesospheres were removed from the Si substrates via sonication in absolute ethanol for 10 min, and the Si substrates were rinsed with absolute ethanol, dried under a stream of N2, rinsed with toluene, and dried under a stream of N2. Atomic Force Microscopy Characterization and Analysis of Nanopores within OTS SAMs. Tapping-mode (AC-mode) atomic force microscopy (AFM) images were acquired using an Agilent 5420 scanning probe microscope with OLTESPA Si cantilevers (Bruker AFM probes, Santa Barbra, CA) with nominal force constants of 2 N/m. The drive frequency of the cantilever was offset by 0.1 kHz lower than the cantilever resonance to maintain repulsive probe−surface interactions.60,61 The damping of the amplitude was set at 60−70% of free oscillation, and scan rates were set to 1.25 Hz. All AFM images were acquired at 256 points per line under ambient conditions. Image processing and analysis of the AFM images were performed using Gwyddion (version 2.40, “Means Medians”), which is an open-source software freely available on the Internet and supported by the Czech Metrology Institute.62 The apparent depths and diameters of the nanopores within the OTS SAMs were determined from cursor profiles across at least 50 nanopores from different regions on the substrates. The apparent depths of the nanopores were measured from the top of 12456


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RESULTS AND DISCUSSION Solvent Influences Nanopore Size within OTS SAMs. Perhaps the greatest utility of particle lithography is the ability

to tailor nanostructure sizes by alterations in mesosphere sizes. However, one limitation of this utility is that changes in mesosphere size and feature density. In this present study, we have discovered a method by which feature size and surface coverage can be altered without compromise to the periodicity and packing density. This control of feature size and surface coverage is related to the solvent composition of the OTS solution that is used to form the nanostructures. For the present study, two commonly used solvents, anhydrous toluene and bicyclohexyl, were evaluated. The formation of nanopores using a 5 mM solution of OTS in anhydrous toluene was examined first. As seen in Figure 2, the surface morphology is consistent with a densely packed OTS SAM, which is unsurprising given that deposition times of 5 min have been previously shown to produce densely packed OTS monolayers.5,45 In addition, the nanopores follow a hexagonal symmetry with a periodicity of 511 ± 17 nm that closely matches the dimensions of the 500 nm silica mesosphere template; the OTS surface coverage (97.9 ± 0.1%) and nanopore surface density (3.7 ± 0.2 features per μm2) are also consistent with previous studies.10,32,33,52 The long-range order of the nanopores is illustrated in Figure 2A by the minimal number of defects, such as missing or irregularly shaped nanopores and line defects. Although the nanopore depth (1.3 ± 0.2 nm) is less than expected for a densely packed OTS SAM (2.2−2.8 nm),46,50

Figure 2. Nanopores deposited from a 5 mM OTS solution of toluene for 5 min using 500 nm silica mesospheres. (A) A representative 8 μm × 8 μm topographic AFM image of an array of nanopores. (B) A representative 4 μm × 4 μm topographic AFM image of an array of nanopores and (D) corresponding cursor profile across three nanopores. (C) A representative 800 nm × 800 nm topographic AFM image of three nanopores and (E) corresponding cursor profile across a single nanopore. Both AFM images were acquired in tapping mode using an OLTESPA Si cantilever.

Figure 3. Nanopores deposited from a 5 mM OTS solution of bicyclohexyl for 5 min using 500 nm silica mesospheres. (A) A representative 8 μm × 8 μm topographic AFM image of an array of nanopores. (B) A representative 4 μm × 4 μm topographic AFM image of an array of nanopores and (D) corresponding cursor profile across three nanopores. (C) A representative 800 nm × 800 nm topographic AFM image of three nanopores and (E) corresponding cursor profile across a single nanopore. Both AFM images were acquired in tapping mode using an OLTESPA Si cantilever.

the OTS SAMs to the exposed Si substrates. Although this measurement maybe more consistent with respect to comparing cursor profiles, it may underestimate the peak-to-peak fluctuations of nanopores and the OTS SAM. The nanopore diameters were calculated as the full-width-at-half-heights of the individual nanopores. The nanopore periodicities were determined from at least 50 cursor profiles measuring edge-to-edge distances of adjacent nanopores. The OTS surface coverages were calculated by counting the number of pixels above a threshold and dividing by the total number of pixels in a set of at least three 4 μm × 4 μm AFM images for each deposition condition. The threshold values were determined by using the full-width-at-half-height across several nanopores within the AFM image. The nanopore surface densities were determined by counting the number of nanopores in a set of at least three 4 μm × 4 μm AFM images for each deposition condition. The RMS roughness for the nanopores and the OTS SAMs were calculated using a set of at least three 2 μm × 2 μm AFM images for each deposition condition. AFM images were acquired from different regions across the substrates, and the resulting fractions, nanopore surface densities, and RMS roughnesses were averaged.

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Figure 4. Comparison of nanopores deposited for 5 min from 5 mM OTS solutions of 3:1, 1:1, and 1:3 mixtures by volume of toluene and bicyclohexyl using 500 nm silica mesospheres. (A−C) Representative 4 μm × 4 μm topographic AFM images of nanopores deposited from OTS solutions of varying mixtures by volume of toluene and bicyclohexyl. (D−F) Representative 800 nm × 800 nm topographic AFM images of three nanopores deposited from OTS solutions of varying mixtures by volume of toluene and bicyclohexyl. (G−I) Corresponding cursor profiles across single nanopores as indicated in (D−F), respectively. All AFM images were acquired in tapping mode using an OLTESPA Si cantilever.

Table 1. Surface Characteristics of Nanopores Deposited from 5 mM OTS Solutions for 5 min Using 500 nm Mesosphere Templates as a Function of Solvent Composition solvent composition (toluene:bicyclohexyl) 1:0 0:1 1:3 1:1 3:1

nanopore periodicity (nm) 511 500 504 506 510

± ± ± ± ±

17 23 24 23 21

nanopore depth (nm) 1.3 1.4 1.3 1.4 1.3

± ± ± ± ±

0.2 0.1 0.1 0.1 0.1

nanopore diameter (nm) 85 184 114 132 158

± ± ± ± ±

8 11 6 7 7

nanopore RMS roughness (nm) 0.24 0.24 0.22 0.17 0.28

± ± ± ± ±

0.05 0.03 0.03 0.01 0.03

OTS surface coverage (%) 97.9 91.7 96.8 94.6 93.1

± ± ± ± ±

0.4 0.8 0.9 0.8 1.0

nanopore surface density (features/μm2) 3.7 3.9 3.6 3.8 3.9

± ± ± ± ±

0.2 0.2 0.4 0.4 0.5

Interestingly, nanopores formed from an OTS solution of bicyclohexyl show a marked increase in diameter but no appreciable change in the nanopore depth, periodicity, RMS roughness, or surface density. Figure 3 shows representative AFM topographic images (A, B, and C) and corresponding cursor profiles (D and E) of nanopores deposited from a 5 mM OTS solution in bicyclohexyl for 5 min. The nanopores have similar hexagonal symmetry, periodicity (500 ± 23 nm), and surface density (3.9 ± 0.2 features per μm2) to the nanopores fabricated from the anhydrous toluene solution. Further, the types and numbers of defects over a large area of the surface are consistent with the nanopores fabricated from the anhydrous toluene solution and previous studies.32,33 This indicates that the defects in nanopore arrays are independent of the solvent and are mainly due to the assembly of silica mesospheres. The depth of these nanopores (1.4 ± 0.1 nm) and the RMS

it is consistent with previous studies that employed silica mesospheres using similar deposition conditions.31,33,36 The substrate, solvent, concentration, temperature, humidity, and deposition conditions all have a considerable influence on the thickness of OTS SAMs.5,31,46 Figure 2C shows a representative high-resolution 800 nm × 800 nm AFM image of three individual nanopores, and Figure 2E displays a cursor profile across one of the nanopores as indicated by the red line in Figure 2C. The diameter of the nanopores measures 85 ± 8 nm, which is consistent with previous studies.32,33 The RMS roughness of the nanopores measures 0.24 ± 0.05 nm, which is similar to the RMS roughness of the OTS SAM (0.26 ± 0.07 nm). This suggests that some OTS molecules deposit into the nanopore either during OTS deposition, removal of the mesospheres, and/or AFM imaging, which is to be expected for reactive species such as organosilanes. 12458


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as well as the surface coverage of the organosilane SAMs have been controlled by the size of the mesosphere templates and the amount of water present on the substrate.10,30−32,42,52 Herein, we demonstrate feature size and surface coverage control through changes in solvent rather than changes in mesosphere size. Tailoring Solvent Composition Enables Control of Nanopore Size and Surface Coverage. To ascertain the effects of solvent composition on the nanopore formation, OTS SAMs were fabricated with 500 nm silica mesosphere templates from solutions of varying proportions of anhydrous toluene and bicyclohexyl and imaged with tapping-mode AFM. Figure 4 shows representative AFM topographic images and corresponding cursor profiles of nanopores formed during 5 min deposition times in 5 mM OTS dissolved in 3:1, 1:1, or 1:3 by volume of anhydrous toluene and bicyclohexyl (Table 1). The hexagonal symmetry and periodicity of the nanopores for all three films closely match the dimensions of the 500 nm silica mesosphere templates and the OTS films deposited from single-component solutions. Further, the nanopore depth remains relatively constant (ranging from 1.3 to 1.4 nm) as the percentage of bicyclohexyl of the solvent mixture was increased. Although the periodicity and depth of the nanopores are independent of the composition of solvent mixture, the nanopore diameter increases, and the OTS surface coverage decreases as the percentage of bicyclohexyl increases (Figure 5). In total, these data show that the nanopore diameter and surface coverage vary as a function of solvent. Therefore, the size

Figure 5. Nanopore diameter and OTS surface coverage plotted as a function of bicyclohexyl solution composition used to generate the nanopores with OTS SAMs.

roughness of the nanopores (0.24 ± 0.03 nm) are consistent with the depth and roughness of those formed from the anhydrous toluene solution. The diameter of the nanopores is considerably larger, measuring 184 ± 11 nm. Additionally, the OTS surface coverage for the substrates formed in bicyclohexyl (91.7 ± 0.8%) is lower than the OTS surface coverages for nanopores formed in anhydrous toluene, further highlighting that the nanopores are increasing in diameter while the periodicity remains unaltered. Until now, the size and structure of individual organosilane nanostructures (rings, nanopores, etc.)

Figure 6. Comparison of nanopores deposited from 5 mM OTS solutions of bicyclohexyl at the deposition times of 1, 15, 30, and 60 min using 500 nm silica mesospheres. (A−D) Representative 4 μm × 4 μm topographic AFM images of nanopores deposited from OTS solutions at varying deposition times. (E−H) Representative 800 nm × 800 nm topographic AFM images of three nanopores deposited from OTS solutions at varying deposition times. (I−L) Corresponding cursor profiles across single nanopores as indicated in (E−H), respectively. All AFM images were acquired in tapping mode using an OLTESPA Si cantilever. 12459


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Table 2. Surface Characteristics of Nanopores Deposited from 5 mM OTS Solutions of Bicyclohexyl Using 500 nm Mesosphere Templates as a Function of Deposition Time deposition time (min) 1 5 15 30 60

nanopore periodicity (nm) 510 500 511 511 515

± ± ± ± ±

24 23 19 24 17

nanopore depth (nm) 1.3 1.4 1.2 1.2 1.3

± ± ± ± ±

0.1 0.1 0.1 0.1 0.1

nanopore diameter (nm) 185 184 109 85 85

± ± ± ± ±

11 11 10 10 8

nanopore RMS roughness (nm) 0.22 0.24 0.16 0.20 0.33

of the nanopore can be tailored by changing the solvent composition, while the periodicity and density remain constant. Deposition Time Influences Nanopore Structure and Surface Coverage. To gain insight into the formation of the nanopores from bicyclohexyl solutions, 500 nm silica mesosphere templates were treated with OTS at deposition times ranging from 1 to 60 min and imaged with tapping-mode AFM. Figures 6A−D show representative AFM topographic images of 4 μm × 4 μm regions of nanopores on Si substrates formed from 5 mM OTS solutions in bicyclohexyl for deposition times of 1, 15, 30, and 60 min, respectively. Nanopores with hexagonal symmetry, periodicity, RMS roughness, and surface density are consistent with the dimensions of the 500 nm silica mesosphere templates, and nanopores assembled from other solvent mixtures are observed for all of the deposition times (Table 2). The OTS SAM formed during a 1 min deposition time shows two domain types with a height difference of 0.5 ± 0.1 nm (Figure 6A). The taller domains are attributed to more densely packed regions of OTS, and the shorter domains are attributed to less densely packed and incomplete regions of OTS. These results are consistent with previous observations that OTS SAM formation requires several minutes for a closely packed monolayer, and therefore incomplete regions of OTS are expected at immersion times of less than two minutes.6,46 Note that the taller domains, which are attributed to the densely packed regions of OTS, tend to surround the nanopores and thus were used in the nanopore depth measurements. Figures 6E−H show high-resolution 800 nm × 800 nm AFM images of nanopores formed from various OTS depositions times, and Figures 6I−L display the corresponding cursor profiles. As the deposition time increases, the diameter of nanopores decreases. For example, the diameter of the nanopore formed at a 1 min deposition time (185 ± 11 nm) is comparable with the diameter of the nanopores formed at a 5 min deposition time (184 ± 11 nm). The diameter of the nanopore formed from a 15 min deposition time (109 ± 10 nm) is smaller than the diameter of the nanopores at shorter deposition times (1 and 5 min) but larger than the diameter of the nanopores formed from longer deposition times (30 and 60 min). The diameters observed from 30 and 60 min deposition times (85 ± 10 nm and 85 ± 8 nm, respectively) are similar to the diameters of the nanopores formed from the anhydrous toluene solution (Figure 2C) for 5 min and previous studies with deposition times of 1 h.32,33 The dependence of nanopore diameter on deposition time suggests that diffusion of the solvated OTS molecules into the meniscus surrounding each mesosphere determines the final diameter of the nanopore. The larger diameters observed from short deposition times (5 min or less, Figure 7A) suggest that the bicyclohexyl-solvated OTS molecules diffuse more slowly into the water meniscus, while longer deposition times (15 min or greater, Figure 7B) allow the bicyclohexyl-solvated OTS molecules to diffuse more completely into the water meniscus

± ± ± ± ±

0.04 0.03 0.01 0.02 0.04

OTS surface coverage (%) 92.2 92.2 97.2 97.0 97.1

± ± ± ± ±

0.3 0.1 0.2 0.4 0.5

nanopore surface density (features/μm2) 4.0 3.9 4.0 3.7 3.7

± ± ± ± ±

0.3 0.2 0.2 0.5 0.1

Figure 7. Schematic of the proposed hypothesis for the dependence of nanopore diameter on the deposition time. (A) At short deposition times (5 min or less), the bicyclohexyl-solvated OTS molecules diffuse more slowly into the water meniscus, which limits the diameter of the nanopore. (B) At long deposition times (15 min or greater), the bicyclohexyl-solvated OTS molecules diffuse into the water meniscus more completely, which results in the diameter of the nanopores being limited by the mesosphere−substrate contact geometry.

where the diameter of the nanopores is limited by the mesosphere−substrate contact geometry. Although anhydrous toluene and bicyclohexyl are commonly utilized to form densely packed OTS monolayers, the estimated water solubilities of anhydrous toluene (573.1 mg/L) and bicyclohexyl (0.183 mg/L) suggest the diffusion of the solvated OTS molecules may be significantly different.63 This hypothesis is consistent with the observation that the diffusion of toluenesolvated OTS molecules into the water meniscus surrounding a mesosphere seems to occur more rapidly than the diffusion of bicyclohexyl-solvated OTS molecules. Thus, the diameter of the nanopores deposited from the anhydrous toluene solution for 5 min (Figure 2) and previous studies with 500 nm silica mesospheres with deposition times of 1 h are also defined by the mesosphere−substrate contact geometry.32,33 It is important to note that the extent of diffusion of the bicyclohexylsolvated OTS molecules at short deposition times is difficult to determine because the size of the water meniscus is highly dependent on the humidity and drying times. Effect of Solvent Control Over Nanopore Size and Surface Coverage Is Independent of the Silica Mesosphere Template. To illustrate the applicability of solvent control over feature size, similar studies were conducted using smaller silica mesosphere templates. Figures 8A and B display representative AFM topographic images of 4 μm × 4 μm regions of nanopores in OTS SAMs on Si substrates deposited for 5 min from 5 mM OTS solutions in anhydrous toluene and bicyclohexyl, respectively, using 250 nm silica mesosphere templates. The nanopores within the OTS SAM formed from both solvents exhibit hexagonal symmetry with periodicities and surface densities that correlate with the dimensions of the 250 nm silica mesosphere templates (Table 3). Few nanopore 12460


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measures 77 ± 6 nm, which is larger when compared to the diameter of the nanopores formed from the toluene solution. However, the difference of the nanopore diameters between the anhydrous toluene and bicyclohexyl for the 250 nm silica mesospheres is not as pronounced as with the 500 nm silica mesospheres, which is likely due to the differences of the mesosphere−contact geometry and the size of the water meniscus surrounding each of the mesospheres.

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CONCLUSIONS Particle lithography has been employed to generate arrays of ordered nanopores within OTS SAMs on Si substrates from toluene and bicyclohexyl solutions. While the interpattern geometries and nanopore surface densities correlate to the 500 nm silica mesosphere templates, the diameter of the nanopores increases from 85 ± 8 nm to 184 ± 11 nm when deposited from anhydrous toluene and bicyclohexyl solutions, respectively. These differences in nanopore diameters are also reflected in the OTS surface coverages which decrease from 97.9 ± 0.4% to 91.7 ± 0.8%. This study demonstrates that control over nanopore size and OTS surface coverage can be achieved through variations in solvent composition. Smaller nanopores are generated from solutions with higher anhydrous toluene composition, and larger nanopores are generated from solutions with higher bicyclohexyl composition. The effect of deposition times on nanopore formation found that at shorter deposition times (less than 5 min) the nanopore size is limited by diffusion into the water meniscus around the base of the nanoparticle, and at longer deposition times (greater than 15 min) the size of the nanopore is limited by the mesosphere− substrate contact geometry as seen with anhydrous toluene solutions. This ability to regulate nanopore size, while preserving the interpattern periodicity, demonstrates an additional level of hierarchical control over OTS nanostructure formation and enables a broader range of nanostructures that can be created. Efforts to explore the effects of humidity, drying times, other molecule−solvent combinations, and mesosphere sizes and materials are ongoing.

Figure 8. Nanopores deposited from 5 mM OTS solutions of toluene and bicyclohexyl for 5 min using 250 nm silica mesospheres. (A,B) Representative 4 μm × 4 μm topographic AFM images of arrays of nanopores deposited from OTS solutions of toluene and bicyclohexyl, respectively. (C,D) Representative 800 nm × 800 nm topographic AFM images of nanopores deposited from OTS solutions of toluene and bicyclohexyl, respectively. (E,F) Corresponding cursor profiles across two nanopores as indicated in (C,D), respectively. All AFM images were acquired in tapping mode using an OLTESPA Si cantilever.

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defects are observed in the AFM images, which illustrates the long-range order of the arrays of nanopores. Figures 8C and D show high-resolution 800 nm × 800 nm AFM images of nanopores formed from 5 mM OTS anhydrous toluene and bicyclohexyl solutions, respectively; Figures 8E and F display corresponding cursor profiles across two of the nanopores in Figures 8C and D, respectively. The nanopore depths for the two solvents are consistent with the nanopores formed using the 500 nm mesosphere templates. The diameter of the nanopores formed from the 5 mM anhydrous toluene solution measures 56 ± 5 nm, which is consistent with previous studies using 250 nm silica mesosphere templates.31,36,59 As expected, the nanopore diameter for the 250 nm silica mesospheres is smaller than the diameter of nanopores corresponding to the 500 nm silica mesospheres due to differences in the mesosphere−substrate contact geometry. The diameter of the nanopores formed from the 5 mM bicyclohexyl solution

AUTHOR INFORMATION

Corresponding Authors

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Prof. Bryan Knuckley, Mr. Chad Drexler, and Ms. Jennifer Roshong for helpful and insightful discussions. We thank the University of North Florida for financial support.

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REFERENCES

(1) Pujari, S. P.; Scheres, L.; Marcelis, A. T. M.; Zuilhof, H. Covalent Surface Modification of Oxide Surfaces. Angew. Chem., Int. Ed. 2014, 53, 6322−6356.

Table 3. Surface Characteristics of Nanopores Deposited from 5 mM OTS Solutions for 5 min Using 250 nm Mesosphere Templates as a Function of Solvent Composition solvent composition

nanopore periodicity (nm)

nanopore depth (nm)

nanopore diameter (nm)

nanopore RMS roughness (nm)

OTS surface coverage (%)

nanopore surface density (features/μm2)

toluene bicyclohexyl

269 ± 7 264 ± 11

1.2 ± 0.1 1.3 ± 0.1

56 ± 5 77 ± 6

0.21 ± 0.01 0.18 ± 0.02

96.7 ± 0.1 94.6 ± 0.7

14.1 ± 1.8 13.2 ± 0.7

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Influence of Solvent on Octadecyltrichlorosilane Nanostructures

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