Self-Assembled Polystyrene Beads for Templated Covalent

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Self-Assembled Polystyrene Beads for Templated Covalent Functionalization of Graphitic Substrates using Diazonium Chemistry Hans van Gorp, Peter Walke, Ana Braganca, John Greenwood, Oleksandr Ivasenko, Brandon E. E. Hirsch, and Steven De Feyter ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18969 • Publication Date (Web): 27 Feb 2018 Downloaded from http://pubs.acs.org on March 1, 2018

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ACS Applied Materials & Interfaces

Self-Assembled Polystyrene Beads for Templated Covalent Functionalization of Graphitic Substrates using Diazonium Chemistry Hans Van Gorp, Peter Walke, Ana M. Bragança, John Greenwood, Oleksandr Ivasenko, Brandon Hirsch*, Steven De Feyter* Division of Molecular Imaging and Photonics, Department of Chemistry, KU Leuven, Celestijnenlaan 200F, B3001, Leuven, Belgium KEYWORDS: self-assembly, colloidal lithography, hierarchical structures, STM, AFM

ABSTRACT A network of self-assembled polystyrene beads was employed as a lithographic mask during covalent functionalization reactions on graphitic surfaces to create nanocorrals for confined molecular self-assembly studies. The beads were initially assembled into hexagonal arrays at the air-liquid interface and then transferred to the substrate surface. Subsequent electrochemical grafting reactions involving aryl diazonium molecules created covalently bound molecular units that were localized in the void space between the nanospheres. Removal of the bead template exposed hexagonally arranged circular nanocorrals separated by regions of chemisorbed molecules. Small molecule self-assembly was then investigated inside the resultant nanocorrals using scanning tunneling microscopy (STM) to highlight localized confinement effects. Overall, this work illustrates the utility of self-assembly principles to transcend length scale gaps in the development of hierarchically patterned molecular materials.

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INTRODUCTION Colloidal lithography represents a high-throughput fabrication technique capable of straightforward periodic patterning of surfaces in the nanometer to micrometer range.1 This process involved the use of a self-assembled network of colloidal particles to mask the deposition of new materials onto substrate surfaces. Periodic arrays of a variety of different materials including molecular films,2-4 polymers,5 proteins,6-8 metals,9-10 and nanoparticles11-12 have been generated using this technique. Sometimes referred to as nanosphere or particle lithography, these methods are well-developed with established procedures for size, shape, geometry, and pattern spacing control.13 Rational variations in particle size and shape, as well as changes to the deposition procedure are commonly utilized to alter the resultant surface pattern.14-16 Manual assembly methods have also been pioneered to increase the assembly area coverage.17-18 Elaborate architectures involving inorganic metal deposition showcase the multifaceted complexity that can be achieved with colloidal lithography.19 By comparison, the use of colloidal lithography to create arrays of organic material is underdeveloped. At the small molecule scale, the covalent deposition of aryldiazoniums has been previously demonstrated on indium tin oxide (ITO),20 gold,21 glassy carbon,22 and graphene23 substrates. Initial work from Stevenson and coworkers employed polystyrene beads to generate void regions in nitrophenyl films.20 Limited resolution from surface potential microscopy provided sensible correlations to the molecular structure and orientation. Colloidal lithography work from Bélanger and co-workers demonstrated iterative processing with metal deposition or subsequent electrochemical (EC) grafting processes carried out inside the void regions to generate hybrid nanopatterns.22 Ragains, Garno, and co-workers advanced these developments through rational thickness modulation in colloidal patterned nitrophenyl films driven by


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photoredox chemistry.21 More recently, colloidal templated aryldiazonium functionalization of graphene surfaces was demonstrated using enhanced reactivity of strained graphene residing on top of a self-assembled bead network.23 These examples conceptually demonstrate the successful application of particle lithography for the templation of aryldiazonium grafting. Unfortunately, limited nanoscale surface characterization hinders a molecular understanding of the colloidal pattern transfer fidelity relative to the covalent functionalization density. Furthermore, the use of nitrophenyl films contributes to structure polydispersity from undesirable branching reactions that result in ill-defined surface structure.24-25 While colloid lithography is a versatile approach for creating periodic arrays on surfaces, the resultant structure at the molecular level remains poorly understood. Alternative methods such as nanoshaving,26 microcontact printing,27 or dippen/nanopipette methods28 fail to reach the extent of area patterning available with colloidal lithography. In this work, we periodically patterned the covalent attachment of aryldiazonium species using a polystyrene masking bead template (Figure 1). This lithographic treatment generates spatially separated nanocorrals capable of confining two-dimensional (2D) molecular selfassembly processes. Covalent deposition was achieved via the EC reduction of 3,5-bis-tertbutylbenzene diazonium (3,5-TBD) molecules to generate aryl radicals capable of grafting onto highly oriented pyrolytic graphite (HOPG) surfaces.24-25 Covalent functionalization occurs in the gap regions between the particle mask. After deposition, an extensive solvent washing procedure was developed to remove the polystyrene templating beads. Raman spectroscopy of the graphite surface provides chemical evidence of covalent attachment of the aryldiazonium molecules. Structural characterization with atomic force microscopy (AFM) revealed hexagonally packed circular nanocorrals measuring 290 nm in diameter. Scanning tunneling microscopy (STM)


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provided molecular resolution of the covalently bound species that define the nanocorral perimeters. These periodically patterned surfaces are subsequently employed for confined molecular self-assembly studies involving 5-octadecyloxyisophthalic acid (ISA-OC18) molecules. Overall, this work provides deep structural insight into colloidal lithography masking of diazonium grafting reactions and their utility for confined molecular assembly studies.

Figure 1. Schematic representation of the colloidal lithography workflow to generate nanocorrals for confined molecular self-assembly. (a) Polystyrene nanospheres self-assemble at the air-liquid interface into hexagonal arrays, which are transferred to a graphite substrate. (b) Colloidal lithography is carried out on the template masked graphite via EC covalent grafting


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reactions involving 3,5-bis-tert-butylbenzene diazonium (3,5-TBD) molecules. The molecules form covalent bonds with the surface in the regions between the colloidal mask. (c) Solvent washing reveals circular nanocorrals of 290 nm, separated by barriers of covalently bound aryl units measuring 94 nm. (d) A schematic showing confined molecular self-assembly of 5octadecyloxy-isophthalic acid (ISA-OC18) inside the nanocorrals demonstrates the pristine nature of the surface and highlights local environmental impacts on long-range molecular assembly. EXPERIMENTAL SECTION Polystyrene Bead Template Preparation The polystyrene beads were assembled at the air-liquid interface following a procedure outlined previously by Rybczynski et al.12 The surface plain polystyrene beads (ø = 384 nm) were initially purchased from Kisker Biotech GmbH & Co. KG (Steinfurt, Germany) in pure water at a concentration of 2.5% w/v (weight per volume). The beads were subsequently prepared in a 50:50 mixture of ethanol and water at a concentration of 2.5% w/v. This procedure involved centrifuging the bead solution followed by removal of half the supernatant and addition of an equivolume amount of ethanol. The beads were then sonicated for resuspension. A small volume (5 µL) of this suspended solution, was then pipetted onto a glass slide. The glass slide was subsequently dipped into a beaker containing milli-Q water (1.0 L) to float the beads at the air-liquid interface. This air-liquid interface was then left undisturbed for four hours to allow assembly (Figure S1). It is well-established that polystyrene beads assemble by minimizing the interfacial energy of the system.29 The hexagonally assembled beads were then transferred to the substrate by scooping the monolayer assembly onto the graphite surface.12 Removal of the graphite from the water with a narrow angle prevents the beads from sliding off the hydrophobic graphite surface and avoids


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disruption of the template. After transfer, the bead template is visible by eye on the graphite surface (Figure 2a, inset). The freshly masked substrate was subsequently dried with a gentle argon stream to remove any residual water. After ~30 minutes of drying, the bead network is quite stable on the graphite sample and is able to withstand washing with milli-Q water. Covalent modification of the bead templated HOPG substrate was carried out in a similar fashion to a previously reported approach.25 3,5-bis-tert-butylbenzenediazonium (3,5-TBD) was generated in situ from 3,5-bis-tert-butylaniline (98%, Sigma-Aldrich Co. LLC). After dissolving the aniline precursor (2.0 mM) in 5.0 mL HClaq (50 mM) the reaction is activated by adding 0.1 ml of aqueous NaNO2 (1.0 M). The mixture is stirred for 1 minute prior to loading the solution in a home built electrochemical cell. The electrochemical setup involved a three-electrode design: 1) a Pt wire counter electrode, 2) a Ag / AgCl reference stored in a 3.0 M NaCl solution, and 3) a 50.3 mm2 working graphitic electrode (HOPG grade ZYB, Advanced Ceramics Inc. or CVD graphene on copper foil, Graphenea). Cyclic voltammetry was carried out after addition of the activated 3,5-TBD by sweeping the current three times from +0.2 V to –0.4 V using an Autolab PGSTAT101 potentiostat (Metrohm Autolab BV). All aqueous solutions where prepared using high-purity water (Milli-Q, Millipore, 18.2 MΩ cm, TOC < 3 ppb). Substrates were characterized by atomic force microscopy, scanning tunneling microscopy and Raman spectroscopy. AFM images were collected using a Cypher ES (Asylum Research) system at 32 °C. All measurements were performed in tapping mode at the air/solid interface using OMCL-AC160TS-R3 probes (spring constant ~ 26 N/m) at a resonance frequency of 300 kHz (±100). STM imaging was carried out at room temperature (20 – 22 °C) in constant-current mode on a PicoSPM (Keysight) using mechanically cut Pt/Ir (80% / 20%) tips of ø = 0.25 mm. After nanocorral fabrication, the self-assembly of 5-octadecyloxy-isophthalic


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acid was carried out inside the nanocorrals by dropcasting a solution of 50 µM in octanoic acid onto the surface prior to imaging at the liquid/solid interface. All AFM and STM image processing was carried out using Scanning Probe Imaging Processor (SPIP 6.0.2) software from Image Metrology ApS. All Raman spectra were gathered with an OmegaScope 1000 (AIST-NT). The laser light (632.8 nm) as supplied by an HeNe laser was directed to the substrate in reflection mode with a power density of below 800kW/cm2 at the sample surface. RESULTS AND DISCUSSION AFM Characterization of Bead Template Network After preparing the bead assembly on the HOPG surface, the dried bead network covering the graphite surface was characterized using AFM measurements. AFM images of the surface reveal the hexagonal packing of the bead template (Figure 2a). Monolayer coverage of the beads was confirmed by imaging at the edge of the assembly (Figure S2). AFM height profiles correspond well to expected bead dimensions (ø: 384 nm) with an average topography change of 385 ± 3 nm obtained from a particle analysis of more than 120 particles. This observation also supports monolayer assignment of the self-assembled template and confirms no swelling or anisotropy occurred during the template assembly and transfer process.30 Occasionally, bead heterogeneity and individual bead vertical dislocations can be observed (Figure S3). After characterization of the colloidal template, the surface was prepared for covalent functionalization reactions.


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Figure 2. (a) AFM image of self-assembled polystyrene beads (ø: 384 nm) hexagonally crystallized on the HOPG surface. Inset shows a photograph of the HOPG surface after the assembly of the colloidal mask. (b) Cyclic voltammogram (3 cycles, from +0.2 V to –0.4 V, at a rate of 0.1 V/sec) performed in acidic aqueous electrolyte containing 2.0 mM of 3,5-TBD molecules. The first cycle is shown in dark blue with iterative cycles shown in lighter shades of blue (arrows indicate forward and reverse scan directions). The initial reduction wave of a nontemplated substrate is shown in gray for reference. (c) On the left, an optical image shows a Teflon sample mount with integrated magnetic stirrer. The right photo demonstrates the spin washing procedure performed three times with 10 mL of toluene:2-propanol (50:50 vol.%) at a temperature of 110 °C. (d) AFM image of the covalently patterned graphite surface after bead template removal by solvent washing. Circular corrals measuring approximately 290 nm in diameter are observed in a hexagonal arrangement. Several irregular white features can be seen inside the corrals and at the corral boundaries. They are assigned to residual contamination leftover from the bead removal process. (e) Raman spectrum of the sample after solvent washing


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gave a ratiometric intensity value of ID/IG = 0.017 ± 0.001 between the D-peak at 1335 cm–1 and G-peak at 1580 cm–1. Electrochemical Covalent Functionalization Covalent functionalization was carried out by electrochemically grafting aryl diazonium molecules. This process occurs inside a homebuilt EC cell made of Teflon with a Viton o-ring that seals the cell to the basal plane of the graphite surface (Figure S4). Graphite operates as the working electrode, while a Pt wire is used as a counter electrode with a Ag / AgCl reference. After assembly, the cell is filled with 5.0 mL of an acidic aqueous solution (50 mM HCl) containing the reactive 3,5-TBD molecules at a concentration of 2.0 mM. Prior to addition to the EC cell, stabile aniline precursors are activated into their diazonium counterparts through the addition of sodium nitrite (100 µL, 0.1 M), which drives the diazotization reaction.31 Three cyclic voltammetry potential sweeps from +0.2 V to –0.4 V at 0.1 V/sec were conducted to reduce the diazonium species and create aryl radicals. The aryl radicals are capable of radical addition reactions with the graphite electrode to result in sp3-hybridized carbon−carbon bonds with the HOPG surface (Figure 1b).32 The three cyclic voltammetry sweeps are shown in Figure 2b, blue lines. The first 3,5-TBD reduction wave is observed at –0.35 V, and carries ∼14 µC of charge. Each subsequent sweep carries iteratively less current and is shifted to more positive voltages, indicating a saturation of the grafting process. For comparison, the first CV sweep of a non-templated sample is shown in gray in Figure 2b. Relative to a non-templated sample, the templated surface experiences a negative potential shift and carries less than 66 % of the current. Additionally, a nucleation loop, where the reverse scan crosses the forward scan, is observed for the bead templated grafting (Figure 2b, dotted arrow).33 This reveals that a large overpotential is required to initiate the reduction process at the working electrode. This


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observation follows intuition that the colloidal templates impede the diazonium reduction process by restricting diffusion (cathodic shifting) and reducing the extent of grafting (less current). After grafting, the surface was prepared for template removal and surface analysis. Template Removal A new washing procedure was developed to remove the bead template and expose the covalently modified surface. Sonication, which is commonly used to remove the beads,14 causes damage to these graphite substrates. The modified basal plane does not survive such a procedure. An alternative approach to dissolve the polystyrene beads by using toluene avoids substrate destruction, however the covalently modified surface is frequently covered with contamination presumed to be residual polystyrene (Figure S5). Here, we effectively removed the beads by spinning the sample at high velocity (900 rpm) in a mixture of isopropyl alcohol and toluene (50/50 volume ratio) while heating at 90 °C. This procedure, which re-suspended the beads, was repeated three times, each for 30 minutes (Figure 2c). Subsequent rinsing steps with pure toluene and milli-Q water (45 mL each) were then carried out before the samples were dried under a gentle argon stream. Raman Spectroscopy Raman spectroscopy was performed on the washed samples to confirm covalent bond formation to the surface. Raman active modes in the graphite surface are indicative of the composition of carbon in the surface, sp2 or sp3 hybridized. Pristine graphite contains only sp2hybrized carbon atoms and therefore only expresses one first-order band, an asymmetric C-C stretching mode at 1580 cm–1 called the G-band. When sp3-hybridized defects are introduced to the pristine lattice, a symmetric breathing mode at 1335 cm–1 called the D-band is activated. A ratiometric analysis of the D-band with the G-band generates a quantitative description of the


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degree of covalent modification.34 Raman analysis after washing revealed a ID/IG = 0.017 ± 0.001 (Figure 2e, SI section 5). By comparison, grafting at this concentration without a template gives a ID/IG = 0.093,25 a value over five times larger than the colloidal templated sample. Naturally, the bead template covering the surface is responsible for some of this reduction in the ID/IG ratio. Assuming the template covers approximately half the surface, the observed ID/IG of 0.017 for the templated sample cannot be completely rationalized by the area covered with the masking colloid template. Rather, such a small ID/IG ratio indicates that that the density of covalent grafting in the void regions between the particles is reduced when compared to the nontemplated sample. It is probable that the particles also obstruct the electrochemical diffusion of the diazoniums to the HOPG surface to result in reduced grafting density. Nevertheless, the appearance of a D-band motivates a structural investigation of the surface pattern using scanning probe microscopies. AFM Characterization of Nanocorrals AFM images collected after the templated covalent grafting and bead removal show hexagonally arranged circular nanocorral features on the surface (Figure 2d). The nanocorrals measure 290 ± 8 nm in diameter. The center to center distance between the corrals is 384 nm, leaving a ∼94 nm border region decorated with covalently bound molecular units. Each individual corral thus has an aerial footprint of 66000 nm2. The unit cell area of the hexagonally arranged corrals is 128000 nm2. Given that a single corral exists inside a single unit cell, the covalently functionalized void regions account for ~50% of the surface, as assumed previously. AFM line profile analysis for these elevated molecular regions yields a height of ∼2 nm for the nanocorral boundaries. Occasionally, irregular features can be seen inside the corrals or at the corral boundaries. These fragments are believed to be residual contamination leftover from the


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bead removal process that become trapped on the surface after drying. This residual material is visualized clearly from a height analysis (Figure S7). This AFM analysis combined with the Raman spectroscopic studies gives strong evidence supporting the covalent nanopatterning of the aryl units to define the hexagonal nanocorrals. Unfortunately, AFM resolution limitations obstruct a molecular understanding of the surface structure. STM Characterization of Nanocorrals Sub-molecular structural resolution afforded by STM is capable of revealing more detailed insight about the covalently nanopatterned surfaces. STM images collected at the airsolid interface show circular regions of bare graphite surrounded by brighter features indicating elevated regions (Figure 3a). These elevated features define the boundaries of the circular nanocorrals and are assigned to the covalently grafted molecules. Similar structural observations of these molecules have been observed in other related works.25-26 STM line profiles from the current image reveal that corrals have a diameter of 291 ± 4 nm, which correlates well with the AFM measurements of the corrals (Figure S8). The apparent STM height of the covalently bound molecules measures only 3.0 ± 1.0 Å (Figure S9). While lower than expected, this is consistent with previous work.25-26 The AFM height profiles by comparison measure ~2 nm. The height differences between the two scanning probe techniques is believed to be grounded in the differing mechanisms of analyzing the surface topography. STM utilizes a quantum mechanical tunneling current to examine the surface topography. However, this current depends on the local density of states often overruling pure topography effects. Alternatively, AFM probes surface topography by leveraging van der Waals forces between the tip and the sample. Thus, it provides a more accurate height analysis. These new covalently patterned surfaces can now be applied for targeted application studies involving confined molecular self-assembly.


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Figure 3. (a) STM topography image after colloidal template removal showing a covalently patterned surface with hexagonally arranged corrals of 384 nm periodicity as defined by the polystyrene template beads. (b) Chemical structure of 5-octadecyloxy-isophthalic acid (ISAOC18). (c) High resolution STM topography image of ISA-OC18 molecular self-assembly on bare HOPG with a superimposed schematic model. (d) STM current image of the confined selfassembly of ISA-OC18 occurring inside the circular nanocorrals created using colloidal lithography. ([ISA-OC18] = 50 µM) in octanoic acid at room temperature. Image parameters: (a) Vb = –0.8 V, It = 70 pA, (b) Vb = –0.675 V, It = 100 pA, and (c) Vb = –0.7 V, It = 80 pA. Confined Molecular Self-Assembly Confined two-dimensional (2D) molecular self-assembly studies can be carried out inside the nanocorrals on the surface. Under suitable conditions, molecular self-assembly occurs spontaneously at liquid-solid interfaces, by depositing a solution containing the analyte of interest on the surface.35-36 This process involves dissolving the target molecules, 5octadecyloxy-isophthalic acid (ISA-OC18, Figure 3b), inside an octanoic acid solvent at a concentration of 50 µM. Dropcast deposition of ∼5 µL of this solution on the covalently nanopatterned graphite surface followed by STM imaging revealed the ISA-OC18 self-assembly inside the circular corrals (Figure 3d). The ISA-OC18 molecules are observed to form linear lamella structures with a similar packing arrangement as is observed on bare graphite surfaces37-


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and on covalently defected surfaces.41 A schematic model demonstrates the packing

arrangement of the ISA-OC18 self-assembly (Figure 3c). The observation of molecular self-assembly inside the corrals demonstrates the pristine nature of the surface in these locations. However, the average domain size of ISA-OC18 inside the nanocorrals is significantly smaller than that of self-assembly formed under identical conditions (concentration, temperature, etc.) on unmodified graphite samples. Within the corrals, the averaged domain size was found to be 2590 nm2 after analyzing more than thirty corrals (Figure S10). Under similar assembly conditions on pristine HOPG, domains exceed ∼42600 nm2, a value nearly 20x larger. The decrease in observed domain size can be rationalized by several possibilities. Confinement created by the nanocorrals directly hinders ripening processes and possibly stimulates new nucleation events to result in smaller domains.26 It is also possible that undesirable grafted molecules inside the corrals may act as kinetic barriers to slow Ostwald ripening.41 Residual contamination from the polystyrene bead template removal may also impact the size of domains. The boundaries between the nanocorrals, which are decorated by the covalently bound molecules, largely lack self-assembly. Previous work from our lab has already shown that self-assembly does not progress on substrates that have been densely modified with covalently bound molecular units.26 Occasionally, regions with relatively lower density of covalently bound molecules trap local pockets of molecular self-assembly (Figure 3d, red boxes). These localized pockets of order demonstrate how the local nanoscale environment can dictate molecular organizational principles. Ongoing studies utilizing these patterned nanocorrals target a deeper elucidation of confined assembly principles. The corrals in this work are classified as ex situ corrals. Self-assembly is carried out inside the corrals only after an initial patterning step. Previously, we investigated confinement


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effects on molecular self-assembly occurring inside nanocorrals created by in situ STM nanoshaving of densely modified surfaces directly in the presence of the molecular self-assembly solution.26 Such a situation allows self-assembly to occur as the nanocorral is being formed, which affords kinetic controls to the nanoshaving process. Here, these processing steps (corral creation and molecular assembly) are separated in an attempt to capture an unbiased examination of molecular self-assembly occurring inside nanoconfined spaces. Unfortunately, we find that corral boundary effects and stray, undesirable grafted molecules cloud a clear deduction of confinement impacts on the nucleation and growth processes. Pattern fidelity improvements are likely to emerge from additional template curing procedures that rigidify the mask and improve surface contact.16 Alternatively, new electrochemical grafting procedures including sequential grafting treatments, chronoamperometry, or changes to the diazonium activation and diffusion conditions may provide improvements to template fidelity transfer. Such fidelity improvements are necessary for potential application transfer to novel 2D materials. The discovery of 2D materials (graphene, boron nitride, black phosphorous, etc.) has opened new fields of research targeting the exploitation of their remarkable electrical, thermal and mechanical properties.42 Central to these properties, is the dimensional confinement of electrons inside the single atomic sheet. Use of covalent functionalization practices that foster the formation of nanoscale superlattices offers unique opportunities to tailor these materials for optoelectronic and electronic applications.43 In particular, the periodic covalent modification of graphene, a single atomic sheet of sp2-hybridized carbon atoms, has been widely reported.44-48 Unfortunately, limited structural resolution from these studies hinders a deeper understanding of the structure/property relationships. Attempts to transfer this colloidal lithography processing to graphene on copper foil resulted in ill-defined patterning with significantly lower pattern transfer


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fidelity when compared to graphite samples (Figure S11). Practical challenges of substrate flatness and rigidity arose when working with graphene on metal foils. Colloid transfer to the flexible foils frequently resulted in damaged, unreliable substrates. Problems also arose in applying our washing procedure to graphene. Rigid supports are thus expected to yield improved colloidal lithography transfer to graphene materials. Nevertheless, this work highlights the potential utility of covalent functionalization alongside colloidal lithography methods to pattern 2D materials at the nanoscale. Conclusion In summary, colloidal lithography was performed using electrochemical covalent deposition of aryl diazonium molecules on graphite substrates to create spatially separated nanocorrals measuring 290 nm in diameter. These nanocorrals were then employed for confined self-assembly studies involving isophthalic acid molecules. Attempts to transfer the patterning methods to graphene substrates resulted in ill-defined corrals. While colloidal lithography provides a high-throughput method for nanostructuring novel 2D materials, the fidelity of pattern transfer must be optimized prior to impactful application. STM provides detailed molecular level insight into these periodically functionalized surfaces to unveil the unique localized effects that impact the resultant properties. Rational extensions of this work to isolate other assemblies or entrap reactive components may assist the fabrication of well-defined architectures. Such hierarchically organized materials help establish confinement principles that can be used to advance rational bottom-up material design by manipulating crystallization processes and reaction pathways for small molecules49 and polymeric materials.50-51 ASSOCIATED CONTENT Supporting Information


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Supporting Information regarding sample preparation and characterization by AFM, STM, and Raman spectroscopy is available free of charge via the Internet at http:// pubs.acs.org. AUTHOR INFORMATION Corresponding Author * Email addresses: [email protected] (B.E.H.) and [email protected] (S.DF) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interests. ACKNOWLEDGMENTS This work is supported by the Fund of Scientific Research Flanders (FWO), KU Leuven Internal Funds, Belgian Federal Science Policy Office (IAP-7/05), and the Hercules Foundation. The research leading to these results has also received funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ERC Grant Agreement No. 340324. B.E.H. acknowledges the FWO for a postdoctoral fellowship. The authors acknowledge Jasper Deckers for imaging assistance.


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Self-Assembled Polystyrene Beads for Templated Covalent

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