Direct Observation of Highly Ordered Dendrimer Soft Building Blocks

Oct 5, 2015 - In the described study, a single domain of supramolecular columnar dendrimers was created over large area. ... should serve as the found...
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Direct Observation of Highly Ordered Dendrimer Soft Building Blocks over a Large Area Kiok Kwon,† Jong Min Ok,† Yun Ho Kim,‡ Jong-Seon Kim,† Woo-Bin Jung,† Soo-Yeon Cho,† and Hee-Tae Jung*,† †

Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST) 335 Gwahangno, Yuseong-gu, Daejeon, 305-701, Korea ‡ Division of Advanced Materials, Korea Research Institute of Chemical Technology (KRICT) 141 Gajeongro, Daejeon 305-600, Korea S Supporting Information *

ABSTRACT: Developing large-area, single domain of organic soft-building blocks such as block copolymers, colloids, and supramolecular materials is one of the most important issues in the materials science and nanotechnology. Owing to their small sizes, complex molecular architectures, and high mobility, supramolecular materials are not well-suited for building large area, single domain structures. In the described study, a single domain of supramolecular columnar dendrimers was created over large area. The columnar structures in these domains have smaller (4.5 nm) diameters, higher area densities (ca. 36 Tera-dots/in2) and larger domains (>0.1 × 0.1 mm2) than those of all existing BCP and colloidal assemblies. By simply annealing dendrimer thin films between two flat solid surfaces, single domains of hexagonal columnar structures are created over large macroscopic areas. Observations made in this effort should serve as the foundation for the design of new routes for bottom-up lithography based on supramolecular building blocks. KEYWORDS: Dendrimer, supramolecule, column, self-assembly, single domain

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processes, would be beneficial if these materials could be used as templates for creating large area, single domain ordered structures. In the study described below, we have developed a general procedure to create large area, single domains of cylindrical structured supramolecular dendrimers (Figure 1). As with other structures created from soft building blocks, the vertically aligned cylindrical phases of the supramolecular dendrimers are particularly relevant to various potential applications, such as optoelectronic materials,15,21 selective membranes,22 and novel nanopattern templates.23 This investigation focused primarily on the fluorinated tapered dendrimer 1 (Figure 1a), which was synthesized and characterized by using a previously described method (Figures S1, S2, and S3), self-assembles into hexagonal cylindrical structure (Figure 1d).11,18 However, our approach is not limited to the fluorinated dendrimer. Other supramolecular materials can be also employed provided appropriate architecture of supramolecular materials is used. The studies with the asymmetric tapered dendrimer 2 (Figure 1b) and discshaped dendrimer 3 (Figure 1c) demonstrated the potential generality of the new method for fabricating large area, single domain structures.

nlike in top-down lithographic approaches in which periodic patterns are fabricated by using photo, nanoimprint, and e-beam lithography, it is extremely difficult to create large area ordered structures without defects using organic soft-building blocks. The reason for this lies in the fact that soft-building blocks spontaneously form structures dictated by their molecular architectures and intermolecular interactions. In spite of the great advances that have been made in the field, it is still very difficult to generate large-area, single domain ordering using currently existing organic soft building blocks.1−11 In particular, supramolecular materials is critically limited to the poly domain structures comprised of a number of small grains because of their complex architectures and multiple sites for intermolecular interactions,12−16 which stands as a major hurdle for widespread applications of these materials. This is a very unfortunate circumstance because supramolecular building blocks can be readily prepared by taking advantage of noncovalent interactions inherent in their molecular recognition characteristics. In addition, supramolecular materials have advantageous properties associated with their small feature sizes (sub-10 nm), short stabilization times required to produce highly regularly ordered structures, and flexible functionalities and structures.11,17−20 Thus, creating a large-area single domain is the huge important for the supramolecular materials to be practically used in future applications. Therefore, the smaller features and diverse molecular architectures of supramolecules, along with their potential adaptability to rapid fabrication © 2015 American Chemical Society

Received: August 16, 2015 Revised: September 23, 2015 Published: October 5, 2015 7552

DOI: 10.1021/acs.nanolett.5b03284 Nano Lett. 2015, 15, 7552−7557

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Nano Letters

surface is then removed, exposing the dendrimer thin film on the bottom surface (Figure 1h), which is then stained with RuO4 to enhance the mass contrast and electron irradiation stability during transmission electron microscope (TEM) analysis. By using this procedure, a single domain hexagonal array of cylindrical structures (ca. 4.5 nm diameter), oriented perpendicular to the bottom surface (see below), is created using the fluorinated, tapered dendrimer 1 (Figure 1a). In this case, the dendrimer thin film was placed between a bottom carbon-supported TEM grid and a PDMS top surface before being heated above the isotropic transition temperature (78 °C) and then cooled to room temperature. In addition, the same technique was applied to a thin film of dendrimer 2 and discotic liquid crystal 3 deposited between carbon bottom and PDMS top surfaces. In this case, annealing by cooling from an isotropic melt (86 °C for 2 and 126 °C for 3) leads to generation of a single domain hexagonal array of cylindrical structures (Figure S4). As described below, these single domain ordering events not only take place using sandwich systems comprised of carbon bottom and PDMS top surfaces, but also when most of the substrate combinations listed above are employed in sandwich systems. We also observed that ordering of the molecular architecture in the film on the bottom surface is not affected by the rates of annealing and cooling of the dendrimer films between 0.1 and 30 °C/min. A TEM image of the structure created from the fluorinated tapered dendrimer 1 (Figure 1a) is displayed in Figure 2a. Inspection of this image demonstrates that the new technique leads to formation of a single domain hexagonal array of cylindrical structures oriented perpendicular to the bottom surface. The cores of the supramolecular cylinders in the TEM image appear dark because the crownether and aromatic moieties in 1 are preferentially complexed with RuO4 and the outer regions of the cylinders, comprised of semifluorinated alkyl tails, appear as bright regions. Inspection of the TEM images demonstrates that perfect homeotropic alignment of supramolecular columns takes place over a large area of the surface of the carbon coated TEM grid. Likewise, in the TEM image (Figure S4b) of the structure created from dendrimer 2, the aromatic moieties that are complexed with RuO4 appear dark and outer regions of the cylinders comprised of aliphatic tails appear bright. Inspection of a higher magnification TEM image (Figure 2b) of the structure created from the fluorinated tapered dendrimer 1 reveals the existence of a highly periodic structure with an average cylinder diameter of ca. 4.5 nm, which is consistent with the results of a grazing incident small-angle X-ray scattering (GISAXS) measurements (Figure 3d). Analysis of the stained dark region of the TEM image shows that the cylinder cores, comprised of aromatic and crown-ether moieties, are ca. 2.5 nm wide. Furthermore, the ordered array of cylinders has a high areal-density of ca. 36 Teradots per square inch (Tdots/in2). Significantly, this is the much higher areal density of molecular assembling structure with large area (0.1 mm × 0.1 mm) ordered structures without defects, compared to those of BCP and colloidal assemblies that reported thus far. Analysis of the [1−10], [120], [−110], [−2− 10], and [−1−20] reflections in the selected area electron diffraction (SAED) pattern further demonstrates the existence of highly ordered hexagonal cylinders oriented perpendicular to the substrate (Figure 2c).

Figure 1. Preparation of large-area single domain columnar structure. (a) Chemical structure of perfluorinated supramolecular dendrimer 1, (b) asymmetric tapered supramolecular dendrimer 2, and (c) discotic liquid crystal 3 used in this study. (d) Self-assembly of tapered monodendron into a supramolecular cylindrical dendrimer and subsequent formation of hexagonal columnar structures. Schematic illustration showing the steps in the procedure for fabricating single domain columnar structures including (e) depositing the dendrimer thin film on the bottom substrate (carbon-supported TEM grid), (f) covering dendrimer thin film with top surface (PDMS), (g) formation of single domain columnar dendrimer via annealing with the assistance of two flat surfaces, and (h) removal of top flat surface after completion of annealing. Phase transition temperatures are supramolecular material 1, Iso 78 °C Colh 15.4 °C Cr, asymmetric tapered dendrimer 2, Iso 86 °C Colh −22 °C Cr, discotic liquid crystal material 3, Iso 126 °C Colh 26 °C Cr.

The film forming process begins with deposition of an approximately 150 nm thick film of the dendrimer on a bottom solid surface by using solution casting (Figure 1e). The film is then covered with a top solid surface, which plays a pivotal role in creating large area single crystalline ordering (Figure 1f). The bottom and top surfaces directly engage in conformal contact with the thin film. Various materials were employed as solid surfaces in either single bottom-top open air or sandwich systems. The materials used for the sandwich systems include carbon bottom-PDMS top, carbon bottom-carbon top, Teflon AF bottom-PDMS top, PEI bottom-PEI top, Teflon AF bottom-Teflon AF top, Teflon AF bottom-PFPE top, and Teflon AF bottom-PEI top. Upon cooling and within a few minutes, the dendrimer in the sandwich system changes from an isotropic melt to a highly ordered alignment of homeotropic cylindrical structures over a large area (Figure 1g). The top 7553

DOI: 10.1021/acs.nanolett.5b03284 Nano Lett. 2015, 15, 7552−7557

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Figure 2. TEM image of supramolecular column on TEM grid and corresponding selected area diffraction (SEAD) pattern. A single domain columnar structure was created by annealing a thin film of dendrimer 1 between carbon bottom (carbon supported TEM grid) and PDMS top surfaces from the isotropic transition temperature (78 °C) to room temperature at a rate of −0.1 °C/min. (a) TEM images of the perfect homeotropic alignment of supramolecular column on carbon coated TEM grid surface over large area, and (b) TEM images taken under higher magnification of same sample. Scale bar, 20 nm. (c) A selected area of the electron diffraction pattern.

consistently generated within each square (0.1 mm × 0.1 mm) of the TEM grid (3 mm × 3 mm), the orientation of each single domain is slightly different in neighboring TEM grid squares owing to different geometric confinements of the squares. Grazing incident small-angle X-ray scattering (GISAXS) and transmission small-angle X-ray scattering (Tr-SAXS) experiments were performed to gain further evidence that large area (>0.1 mm × 0.1 mm) single domain supramolecular dendrimers are generated using sandwich systems comprised of two flat solid surfaces. Inspection of the GISAXS results (Figure 3d) shows that clear in-plane Bragg peaks (Miller index [100], [110], and [200] of a 2D hexagonal column) exist at q ca. 1.61 nm−1 (period ca. 3.9 nm), √3q ca. 2.79 nm−1, and √4q ca. 3.22 nm−1, which suggests that the hexagonal column is oriented perpendicular to the surface. In the Tr-SAXS

In order to gain additional evidence to show that a large area single domain of supramolecular dendrimers are created by using the technique, angle deviations of each six-spot fast Fourier Transform (FFT) patterns from one grid square on a TEM grid were determined. TEM images (Figure S5) from 12 different regions (Figure 3a) of a square (ca. 0.1 mm × 0.1 mm) grid were found to display the same 2D FFT patterns. This finding demonstrates that the cylindrical structure is comprised of a single domain over the entire area (Figure 3b). The rotation angle (Φ) deviation in the 12 FFT patterns, determined by using the angle between cross line (A) and horizon line (B), was found to be 0.056° (Figure 3c), a value that is smaller than the measurement error. This observation provides further evidence to support the conclusion that a single domain exists within one TEM grid square. It is noteworthy that, although a single domain cylinder is 7554

DOI: 10.1021/acs.nanolett.5b03284 Nano Lett. 2015, 15, 7552−7557

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substrates with different solid surface properties were determined. In the sandwich systems used in this study, the top and bottom surfaces were comprised of carbon, PDMS, fluorinated polymer (Teflon AF), polyethylenimine (PEI), and perfluoropolyether (PFPE). The respective water contact angles of Teflon AF, PDMS, carbon, PFPE, and PEI are 122°, 110°, 108°, 103°, and 10° show that the surfaces of the fluoropolymers (Teflon AF, PFPE), carbon, and PDMS are hydrophobic, while that of PEI is strongly hydrophilic (Figure S7). The results show that alignment of supramolecular cylinders created from 1 using a single bottom substrate with tops open to air are strongly influenced by the surface properties of the substrate. As Hudson et al. already showed that alignment of supramolecular column is determined by the specific interaction between dendrimer and supporting surface,11 vertical alignment of the supramolecular cylinders takes place when substrates with neutral surfaces (carbon) are utilized (Figure S8a), as while planar alignment occurs when the bottom substrates have interactive surfaces (Teflon AF) (Figure S8b).24 Moreover, all of the supramolecular cylinders that are created using single bottom substrates with tops open to air have polydomain structures comprised of a number of small grains. Surprisingly, regardless of the nature of the substrates used as tops and bottoms in sandwich systems, cooling thin films of 1 from isotropic melts leads to formation of large area single domains of vertically oriented cylinders (Figure 4a−f). Employing sandwich systems composed of carbon bottom−

Figure 3. Analysis of the formation of large-area single domain supramolecular column. (a) FFT patterns of hexagonal columnar structure formed from a film of dendrimer 1 on carbon-supported TEM grid from 12 different positions over (b) large area (ca. 0.1 mm × 0.1 mm). (c) Rotation angle (Φ), angle between horizon line (A) ,and cross line (B) deviation of 12 FFT patterns is smaller than the measurement error. (d) GISAXS pattern of vertically aligned supramolecular columns on carbon-coated Si wafer substrate and schematic cartoon (inset) of ordered dendrimers. (e) Tr-SAXS pattern of vertically aligned single domain column on carbon-coated thin Si wafer (10 um) and schematic cartoon (inset).

experiment, the X-ray beam propagates in a direction that is perpendicular to bottom substrate along the axis of the columns (inset of Figure 3e). The formation of a six spot diffraction pattern (Figure 3e) indicates that single domain hexagonal columns are vertically aligned with respect to the bottom substrate and that the lattice spacing period (3.9 nm) is same as that arising from the GISXAS experiment. Importantly, considering that the X-ray beam size is ca. 1 mm × 0.3 mm in the Tr-SAXS experiment (Figure 3e and Figure S6), the results demonstrate that a single domain is created over a large area. It is somewhat surprising that arrays of single domain supramolecular cylinders are generated simply by annealing dendrimer films in the region between sandwich substrates. To gain information about this phenomenon, the effects of surface properties and geometry of confinement system on the formation of these structures were evaluated. The order and orientation of the films annealed on various single (solid bottom−open air surface) and between two sandwich

Figure 4. TEM images of supramolecular column created by annealing a film of dendrimer 1 between different solid surface pairs. (a) Carbon bottom−PDMS top (good contact), (b) Teflon AF bottom−PDMS top (good contact), (c) PEI bottom−PEI top (good contact), (d) carbon bottom−carbon top (good contact), (e) Teflon AF bottom− Teflon AF top (good contact), (f) Teflon AF bottom−PEI top (good contact), (g) Teflon AF bottom−PFPE top (good contact), (h) poor contact, (i) good contact and poor contact (red line) in adjacent region. Fast Fourier transform of each TEM image (left bottom inset) and schematic illustration of two solid surfaces (right bottom inset). Scale bars, 50 nm. 7555

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supramolecular dendrimers. There appears to be no limit to the size of the area of the single domain supramolecular cylinders that can be produced, as long as three key requirements are satisfied. These requirements include (i) geometric confinement of two sandwich surfaces, (ii) conformal contact at the interface between supramolecule films and confinement templates, and (iii) the existence of uniform and continuous thin films. In summary, the investigation described above has led to the development of a simple and general method to create ultradense, single domain arrays of supramolecular cylinders over large areas. In this technique, perfectly ordered single domains of cylinders are generated by utilizing sandwich systems comprised of two parallel solid surfaces as the location for cooling dendrimers from their melts. This discovery is significant in terms of the manipulation of organic soft-building blocks. First, the feature sizes of self-assembled supramolecular dendrimers are smaller (ca. 4.5 nm diameter and 2.5 nm wide cylinder cores) than those generated by currently studied block copolymer and colloidal assemblies. It is important to note that supramolecular building blocks with much smaller feature sizes are potentially accessible by using supramolecular dendrimers with different chemical architectures.30,31 Second, it is possible to create very large single domains (>mm scale) by precisely controlling the geometric configuration of two parallel solid surfaces, conformal contact at the interface between supramolecules and confinement substrates, and the uniformity and continuity of the supramolecular thin film. Third, single domain ordered structures are formed with stabilization times in the range of minutes. Finally, numerous readily prepared supramolecular dendrimers can be utilized in this process as the potential building block. We believe that the observations made in this effort should serve as the foundation for the design of new routes for bottom-up lithography based on supramolecular building blocks.

PDMS top (Figure 4a), Teflon AF bottom−PDMS top (Figure 4b), PEI bottom−PEI top (Figure 4c), carbon bottom−carbon top (Figure 4d), Teflon AF bottom−Teflon AF top (Figure 4e), and Teflon AF bottom−PEI top (Figure 4f), the single domain supramolecular cylinder structures created under the same experimental conditions do not display any significant differences in the degree of ordering (Figure 4a−f). Also, in each case the 2D FFT contains clear six spot patterns of hexagonal closely packed columns. Conformal contact between the dendrimers and the solid substrates is an important factor in governing formation of single domain, large area columnar structures (Figure 4a−f). Specifically, the surfaces of carbon, Teflon AF, and PEI all have good contact with dendrimer films, but owing to its highly flexible characteristics, the surface of PDMS has the best conformal contact with the dendrimer films.25 In contrast, it is difficult to produce a large-area single domain using PFPE as the top surface because of the lack of full contact, which causes different orientations of the cylinders (Figure 4h) and polydomain formation. Even when pressure is applied to the PFPE top substrate, good contact to the dendrimer thin film does not take place over a large area. Inspection of the 2D FFT pattern (inset of Figure 4h) shows that films generated by using substrates that have surfaces with relatively poor contact have polydomain structures with many different orientations of the supramolecular cylinders. In addition, both poor and good contact regions exist in the Teflon AF bottom−PFPE top sandwich system, a phenomenon that leads to different alignments of the cylinders in each region (Figure 4i). Finally, the formation of uniform and continuous dendrimer films is an important requirement for fabricating large-area, single domain of the supramolecular structures. Because they have great tendencies to self-assemble and large mesophase stabilities, dendrimers are inclined to form perfect structures by restoring the original order around defects (Figure S9a). However, when their heights are high, wrinkles in a continuous film cause generation of a polydomain structure (Figure S9b). When single substrates (solid bottom−top open air) are employed, alignment of the supramolecular cylinders is determined by specific interactions taking place at the interface between the dendrimer film and the surface of the bottom substrate.11 In contrast, in sandwich two substrate systems, single domain columnar structure is spontaneously created over large area. Although the homeotropic alignment of certain discotic liquid crystals26,27 and specific dendronized polymer28 is generated between two sandwich systems, the ordering is dependent on the types of substrate and specific interaction. Furthermore, there has been no report to show direct visualization of the large-area single domain of columnar structure, which is created spontaneous in sandwiched system with a fast stabilization times. In our method, although the relationship between the molecular structure and monodomain formation is not entirely understood, it is likely that a tendency exists for the planar aromatic moieties of the dendrimer to adopt a face-on orientation with both the flat top and bottom substrate surfaces.29 These interactions enable a high degree of ordering of the orientation of the dendrimer in a single domain over a large area. In addition, the strong self-assembly properties of each cylinders and slice of the dendrimers cause formation of a single domain with homeotropic alignment during growth between the top and bottom solid surfaces.26−29 As a result, the method developed in this effort should serve as a universal route for fabrication of large area single domains of

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b03284. Detailed experimental procedure; details of preparation of the dendrimer 1 and analysis of thermal and optical; TEM image of single domain hexagonal columnar structure of dendrimer; detailed TEM images taken from position 1 to position 12 of Figure 4b; Tr-SAXS pattern of vertically aligned hexagonal columnar structure on bottom substrate; water contact angles of different substrates; alignment of supramolecular dendrimer 1 between single bottom surface and air surface; alignment of supramolecular dendrimer 1 under the existence of small defects and ripples (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 7556

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(29) Jung, H.-T.; Kim, S. O.; Ko, Y. K.; Yoon, D. K.; Hudson, S. D.; Percec, V.; Holerca, M. N.; Cho, W.-D.; Mosier, P. E. Macromolecules 2002, 35, 3717−3721. (30) Rosen, B. M.; Wilson, D. A.; Wilson, C. J.; Peterca, M.; Won, B. C.; Huang, C.; Lipski, L. R.; Zeng, X.; Ungar, G.; Heiney, P. A.; Percec, V. J. Am. Chem. Soc. 2009, 131, 17500−17521. (31) Percec, V.; Cho, W.-D.; Mosier, P. E.; Ungar, G.; Yeardley, D. J. P. J. Am. Chem. Soc. 1998, 120, 11061−11061.

ACKNOWLEDGMENTS This research was technically supported by Pohang Accelerator Laboratory (PAL), small-angle X-ray scattering (3C1 SAXS) beamline.

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REFERENCES

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DOI: 10.1021/acs.nanolett.5b03284 Nano Lett. 2015, 15, 7552−7557