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Multidimensional helical nanostructures in multiscale nanochannels Sunhee Lee, Hanim Kim, Ethan Tsai, Jacqueline Mae Richardson, Eva D Korblova, David M. Walba, Noel A. Clark, Sang Bok Lee, and Dong Ki Yoon Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b01620 • Publication Date (Web): 02 Jul 2015 Downloaded from http://pubs.acs.org on July 8, 2015

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Multidimensional helical nanostructures in multiscale nanochannels Sunhee Lee,a† Hanim Kim,a† Ethan Tsai,b Jacqueline M. Richardsonc Eva Korblova,c David M. Walba,c Noel A. Clark,d Sang Bok Lee,a,e* and Dong Ki Yoona*

a

Graduate School of Nanoscience and Technology and KINC, Korea Advanced Institute of

Science and Technology (KAIST), Daejeon 305-701, Rep. of Korea. E-mail: [email protected] b

Department of Chemistry, Metropolitan State University of Denver, Denver, CO 80217, USA.

c

Department of Chemistry and Biochemistry and Soft Materials Research Center, University of Colorado, Boulder, CO 80309, USA.

d

Department of Physics and Soft Materials Research Center, University of Colorado, Boulder, CO 80309, USA.

e

Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742, USA. E-mail: [email protected]

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KEYWORDS: Liquid crystals, helical nanofilament phase, multidimensional confinement, physicochemical treatment

ABSTRACT: We have investigated the various morphological changes of helical nanofilament (HNF; B4) phases in multi-scale nanochannels made of porous anodic aluminum oxide (AAO) film. Single or multi-helical structures could be manipulated depending on the AAO pore size and the higher-temperature phase of each molecule. Furthermore, the nanostructures of HNFs affected by the chemical affinity between the molecule and surface were drastically controlled in surface-modified nanochannels. These well-controlled hierarchical helical structures that have multi-dimensions can be a promising tool for the manipulation of chiral pores or the non-linear optical applications.

Introduction Ever since the bent-core liquid-crystal (LC) molecule was introduced in the 1930s,1 it has been the subject of many research studies because of its unusual structural properties such as polarization, chirality, and undulation. Among various kinds of bent-core LC phases, the helical nanofilament (HNF or B4) phase has been one of the most attractive structures owing to its long-range-ordered chiral structures that can be used in potential applications in chiral optics2,3, non-linear optics4,5 and opto-electrical application6,7.The dimensions of a single HNF, such as the number of layers, helical pitch, and width, can be tuned to the nanoscale to satisfy the thermodynamically favorable conditions for minimizing energy density.8 Although a single HNF possesses these specific structural properties, HNFs self-


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organize to form bouquet-like bulk structures because of their radial growth, and controlled growth in a large areas has been incredibly difficult. Many research groups have tried to align HNFs using conventional techniques that are useful for general LCs9-11 but it is difficult to achieve the goal because of the intrinsic complexity of the HNF phase.12-15 Recently, an innovative alignment system for HNFs was introduced to individually manipulate single HNFs in a large area,16 in which the growing direction as well as dimensions of HNFs could be perfectly controlled by using a film of porous anodic aluminium oxide (AAO) as the HNF material. This effort allows HNFs to have a much wider range of nanostructural dimensions with various AAO pore sizes compared to bulk HNFs. Furthermore, depending on the chemical affinity between the LC molecules and surface of the AAO-film nanochannel, a dramatic morphological change from HNFs to a bamboo-like structure was also found in a similar system.17 However, these studies only focused on the standardized system, where the molecular behavior in the fixed dimensions of a porous AAO film was varied with pore size, for example. Thus, the continuous morphological changes of HNFs in multiple dimensions have not been explored ever, and related defect structures such as disclination lines and dislocations have been examined neither experimentally nor theoretically. In this work, we manipulated multidimensional HNFs by controlling the template channel shape, using multiscale nanochannels of a porous AAO film. As a result, two or more different helical dimensions of pitch and width in HNFs could be generated in one multiscale nanochannel, which can suggest a novel method to fabricate multidimensional nanostructures for the opto-electronic applications.18-20


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Experimental This multidimensional and nanoscaled helical structure has not been reported to date, but is expected to broaden fundamental scientific insight into the field of molecular selfassembly and provide a possible route for a brand new concept of multiwavelength nonlinear optics. In order to achieve this goal, two bent-core LC molecules were introduced. Both 1,3-phenylene bis[4-(4-nonanoxyphenyliminomethyl)benzoate] (NOBOW)21 and biphenyl-3,4′-diyl bis-(4′-decanoxybiphenyl-4-carboxylate) (W513)22 exhibit the HNF phase at room temperature but with different thermal phase transitions (Figure 1 a and b), showing classic HNF and modulated HNF (HNF(mod)) phases, respectively, as previously reported.16,17,21,22 Compared to the classic HNF structure of NOBOW, W513 has a similar helical morphology, with the exception that in-layer modulation resulting from a B1 phase at higher temperature during cooling from the isotropic phase. In the previous study, the width of a HNF obtained in the nanochannel could range from 30 to 80 nm in the form of single filaments, although structural instability of HNFs was found in the 80-nm pore size.16 In such a general case, multiple HNFs were observed in nanochannels with channel diameters above 80 nm.8,23 Basically the pore size of AAO is selectively tunable in any costom-designed geometry24-26. With such basis, we designed consecutive multiscale AAO nanochannels for multidimensional HNF fabrication (Figure 1c). When aluminium foil was anodized by two-step anodization under 40 V with 0.3 M oxalic acid at 8 °C, well-ordered nanochannel arrays with hexagonal symmetry and an interpore distance of 100 nm were synthesised.27 Next, a selected channel, nanochannel 1 (diameter: dAAO1 and dAAO1’= 60, 80 nm each, was widened by phosphoric acid, and then nanochannel 2 (diameter: dAAO2= 30 nm) was generated from the bottom of nanochannel


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1 by an additional round of anodization.25 The ratio of diameters of nanochannels 1 and 2 could be adjusted by varying the pore-widening period (with phosphoric acid) under the same conditions. The maximum value of dAAO1 was determined by the interpore distance obtained under the applied anodization condition.

Results & Discussion Three different diameters in a nanochannel of AAO film could also be formed by a stepwise anodization, which can be useful to fabricate multiscale chiral nanostructures (Figure 2). To manipulate the multiscale HNFs, bent-core LC material was simply loaded into the nanochannels at the isotropic-phase temperature by capillary action. Upon controlled cooling using a cooling–heating stage placed under the sample, individually controlled HNFs were generated from the top to bottom along the nanochannel because of the temperature gradient.16 Here, the nanochannel acted as a cold reservoir to transfer the latent heat from the B2 or B1 phase to the B4 phase transition, thus resulting in layeredges of HNFs contacting the surface of the nanochannel. Both NOBOW and W513 molecules showed similar morphologies varied with the pore diameter of the nanochannel. Single HNFs were observed in the channel with pore diameter dAAO1 (dAAO1 = 60 nm) (Figure 3a and c), but complex structures mixed with multihelical and monohelical structures were found in the channel with pore diameter dAAO1′ (80 nm) (Figure 3b, d), although all HNFs in the channel with pore diameter dAAO2 (30nm) exhibited a monohelical form. Note that helical structural evolution of HNFs at the joint or intermediate space between dAAO1 (including dAAO1′) and dAAO2 was continuous, which means that the layered


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structure in the HNFs was continuously changing with varying the width of HNFs. For the bent core molecules that via B2 intermediate state before reaching to B4, the changes in these linearly joint (multidimensional) HNFs can be understood by a general molecular packing model (Figure 4). The bent-core molecules were packed in a layered structure when in a smectic LC (e.g. B2 or B4) phase, in which the sublayers in a molecule were mismatched because of orthogonally twisted aromatic arms (Figure 4a).8,16,21 The molecules are inherently achiral, but the tilted- polar configuration of the molecules arouses the unique inter-molecular packing behavior to deform layers28, which are stabilized with macroscopic helical structures where the elastic-energy cost could be minimized by local saddle-splay deformation of the layers. In particular, the B2 smectic layer could have either left- or right-handedness depending on the molecular-tilt direction (Figure 4b), thus preserving its chirality throughout the multiscale nanochannel despite a transition to a B4, HNF phase (Figure 4c)8. Here the interlayer distance of the HNF (~5 nm) is constantly maintained in the variation of channel dimensions, meaning that the more layers can exist in the larger pore.16 Because the thermal gradient induces the directional growth of HNFs from top to bottom of the channel, the nucleation and growth of HNFs start from dAAO1 to dAAO2 region. At this transition point, the number of layers, N, is also decreased, and generating dislocations that are typically observed in the smectic LC phase29. In details, initially formed layers in dAAO1 region feel a kind of pressure as the pore size is decreased, and this can be released by merging smectic layers to generate the dislocation during this transition behavior as illustrated in the schematic sketch (Figure 4c).


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When the layered structures intercalated, the process can be assumed to be gradual because the molecular configuration such as tilt or polar direction in the layer was not changed. This hypothesis is supported by the chirality preservation in the resulting narrowed HNFs (Figure 4b, d). A direct view of the multidimensional HNF supported by the line-defect generation is shown in the transmission electron microscope (TEM) image in Figure 3d. As reported in the previous study16, the local elastic energy of the layer is minimized by two major factors: the internal saddle splay layer distortion and the force against surface anchoring from the confining geometries, which determine the width and pitch variations of the final HNFs. Consistently, each half-pitch (hp) is linearly changed as a function of pore size, e.g. ~100 nm in a 30-nm pore and ~115 nm in a 60-nm pore.16 Especially, the HNF(mod) that has the B1 phase at higher temperature is relatively unstable compared to the HNF having B2 layer structures at higher temperature phase, this is why HNFs(mod) look irregular in the larger pore. Generally, a single HNF was not stable in the nanochannel with a pore diameter of 80nm owing to the elastic-energy cost of saddle-splay deformation of the layer (Figure 3b).8,16 In order to improve the stability of HNF(mod), chemical treatment was applied to our nanochannel system.17 As previously reported, the direction of HNF growth can be controlled globally even in bulk HNFs,30 in which the long axis of the HNF is aligned parallel to the LC-phobic treated substrate while the short axis of the HNF is set perpendicular to a LC-philic treated surface. Based on this strategy, a LC-phobic selfassembled monolayer (SAM) of octadecyltrimethoxysilane (OTS) was covalently bonded to the inner-surface of AAO multiscale nanochannels17,30, which results in the final


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HNF(mod) showing highly regular multidimensional nanostructures, even in the 80-nmdiameter region (Figure 5a). On the LC-phobic surface, the lowered surface energy helped the W513 molecules in the HNF(mod) to strongly associated with each other rather than with the inner-surface of the nanochannels, resulting in a strongly coiled HNF(mod) that was not transformed to multihelical structures. This result is also well correlated with the molecular packing model in Figure 4. The pre-aligned W513 molecules in the B1 phase (higher-temperature phase) were spontaneously twisted to form the HNF(mod) while they had little interaction with the surrounding nanochannels. By taking into consideration the physical and chemical properties of bent-core molecules, a complicated HNF(mod) that overcomes physical limitation in dimensions can be obtained. Additionally, the stability of the HNF(mod) in the AAO nanochannels was also perturbed by the surface treatment using LC-philic SAMs.17,30 For instance, the LC molecules were strongly held and aligned parallel to the surface that was treated with a LC-philic SAM of 2-(methoxy(polyethyleneoxy) propyl) trimethoxysilane (PEG 6/9). The resulting HNF(mod) was deformed to multihelical structures due to the intermolecule interaction being relatively weak compared to the interaction with the SAM-treated surface. Thus, the HNF(mod) of W513 in PEG 6/9, which was a LC-philic organosilane-coated nanochannel, changed to a disordered structure (Figure 5b). The chemical affinity between various organosilanes and W513 was investigated by contact-angle measurements (Figure 5c). On the OTS-treated substrate, the contact angle of W513 was relatively large, ~48°, compared to the contact angle of ~19° for W513 on the PEG 6/9-treated surface.


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This trend was confirmed by varying the cooling rate, in which the slower cooling rate effectively improve surface anchoring between bent-core molecules and the inner surface of nanopore, while the LC molecules are aggregated together to form HNFs with less interruption from the surface. As shown in Figure 6a and 6c, resultant morphologies of nanoconfined HNFs are susceptibly affected by the pore size and the surface anchoring with a slow cooling rate of -5 °C/min. On the other hand, such effects are remarkably suppressed to from normal HNFs under the fast cooling condition, -10 °C/min as shown in Figure 6b and 6d. This is more drastically observed in the HNF(mod) case compared to the normal HNFs due to the structural stability of higher temperature phase (Figure 6c and 6d). These results show that the combination of physical nanoconfinement and chemical treatment can control the helical nanostructure, and it did so perfectly for HNF(mod), thus providing essential information for practical application of complex helical nanostructures in optics.

Conclusions We demonstrated morphological control of multidimensional helical nanostructures, including HNF and HNF(mod), using multiscale nanochannels. Our results showed that two or more helical dimensions of a HNF could be precisely controlled in a linearly joined single HNF in multiscale nanochannels. The molecular packing model of multidimensional HNFs based on the polar and chiral layered structure is also proposed in this paper. Additionally, it was demonstrated that the additional chemical treatment using various SAMs yielded HNF(mod) with very stable structures in nanochannels with 80-nm pores, which was not possible with conventional physical


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confinement. Our efforts in this work are important as this method of hierarchical assembling of helical nanostructures can become an entirely new approach for fabrication of electro-optic materials.

Figure 1. Molecular structure of bent-core liquid crystals and thermal phase transitions of (a) NOBOW and (b) W513. (c) Schematic drawing of experimental setup. A one-dimensional nanochannel is divided into two hierarchical channels with multiple diameters. Depending on the diameter of the upper channel, a single filament with different helical dimensions or a multiplefilament complex can be formed. LS is defined by layer surface area as well as LE represents layer edge area.


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Figure 2. Multiscale AAO nanochannel templates for linearly joint nanomaterials. 2-step anodization process produced hexagonal close packed pore arrays that have pore to pore distance of 100 nm under 40V with 0.3M oxalic acid at 8 °C.24,26 After the formation of well-ordered pores of AAO film, pore widening procedure was followed to control the pore-size as a function of etching time with phosphoric acid. The expansion of the pore diameter and pore-forming process is repeated, and then the multi-scale nanochannels could be produced. (a) 60 nm to 30 nm, (b) 80 nm to 30 nm junction of the linear nanochannels. (c) In such sequential steps, the multiscale nanopores with three different diameters of 80 nm, 60 nm and 30 nm can be generated in a single nanochannel.


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Figure 3. HNFs and HNFs(mod) in multiscale nanochannels. HNFs of NOBOW in nanochannels measuring (a) 60–30 nm and (b) 80–30 nm in diameter. HNFs(mod) of W513 in nanochannels measuring (c) 60–30 nm and (d) 80–60 nm in diameter.


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Figure 4. (a) Model sketch for the molecular organisation of bent-core molecules; n, s and P represent the molecular direction, the layer normal vector and the polarization direction respectively. (b) Left- and right-handed saddle-splay deformation of bent-core B2 smectic layer structures.8 Top and bottom aromatic groups are positioned orthogonal to each other. To minimise the stress, the layers spontaneously twist to form saddle-splay structures. (c) Chiralitypreserving, multidimensional HNF structure. During B2-to-B4 phase transition in the nanochannels, bent-core molecules form helical structures from top to bottom along the zdirection and meet the bottleneck at the transitional site between dAAO1 and dAAO2. (d) The TEM image shows the linearly joint multidimensional HNFs with different half-pitch (hp) values.


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Figure 5. Combined effect of physical confinement of the nanochannel and chemical confinement by the SAM. (a) SEM image revealing that LC-phobic organosilane helped HNF(mod) remain as a single filament in a 80-nm pore. (b) In contrast, this SEM image shows complex nanostructures in LC-philic coated nanochannels with a pore diameter of 80 nm. (c) Chemical affinity between organosilanes and W513.17


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Figure 6. Different thermal stabilities of NOBOW and W513 on their final morphologies in PEG6/9-treated channels under the different cooling rate control: (a) 5 ℃/min (b) 10 ℃/min for NOBOW, (c) 5 ℃/min, (d) 10 ℃/min for W513. These results show the consistent tendency with the case of non-functionalized channels that is more susceptible for W513 on its morphological stability.


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ASSOCIATED CONTENT Sample preparation for multidimensional AAO, morphological observation on the nanoconfined HNFs and HNFs(mod) in various of surface-modifined AAO

AUTHOR INFORMATION Corresponding Author * S.B.L. E-mail: [email protected] and *D.K.Y. E-mail: [email protected] *To whom correspondence should be addressed. †These authors contributed equally to this work. Author Contributions S.L., H.K. and D.K.Y. designed research, S.L., E.T., J.M.R., and E.K. synthesized materials, H.K., S.L. performed microscopy experiments, H.K., S.L., D.M.W., N.A.C. and D.K.Y. analyzed data, S.L., H.K., S.B.L. and D.K.Y. wrote the article. S.B.L. and D.K.Y. directed the research. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT


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This study was supported by the grant from the National Research Foundation (NRF), funded by

the

Korean

Government

(2012M3A7B4049802,

2012R1A2A2A06046931,

2014M3C1A3052567 and 2014S1A2A2027911), and by the MRSEC Program (NSF DMR1420736).

ABBREVIATIONS HNFs, Helical nanofilaments; AAO, Anodic Aluminum Oxide; HNF(mod), modulated HNF; LC, Liquid crystals; SAMs, Self-assembled monolayers; OTS, octadecyltrimethoxysilane; PEG 6/9, 2[(methoxy (polyethylenoxy) propyl) trimethoxysilane; SEM, Scanning electron microscopy.

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Multidimensional Helical Nanostructures in ... - ACS Publications

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