Letter pubs.acs.org/NanoLett
Probing and Controlling Liquid Crystal Helical Nanofilaments Chenhui Zhu,*,† Cheng Wang,*,† Anthony Young,† Feng Liu,‡ Ilja Gunkel,† Dong Chen,§ David Walba,∥ Joseph Maclennan,§ Noel Clark,§ and Alexander Hexemer*,† †
Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States § Department of Physics and Liquid Crystal Materials Research Center, University of Colorado, Boulder, Colorado 80309, United States ∥ Department of Chemistry and Biochemistry and Liquid Crystal Materials Research Center, University of Colorado, Boulder, Colorado 80309, United States ‡
S Supporting Information *
ABSTRACT: We report the first in situ measurement of the helical pitch of the helical nanofilament B4 phase of bent-core liquid crystals using linearly polarized, resonant soft X-ray scattering at the carbon K-edge. A strong, anisotropic scattering peak corresponding to the half-pitch of the twisted smectic layer structure was observed. The equilibrium helical half-pitch of NOBOW is found to be 120 nm, essentially independent of temperature. However, the helical pitch can be tuned by mixing guest organic molecules with the bent-core host, followed by thermal annealing.
KEYWORDS: helical pitch, helix, smectic liquid crystal, resonant soft X-ray scattering, carbon edge, nanofilament, B4, bent-core
C
indication that not only molecular chirality but layer chirality drives the formation of helical structures. Recently, a doubletwisted structure of the bulk arrangement of nanofilaments was proposed17 to explain the mysterious properties of the B4, such as the structural color and ambidextrous optical activity. The B4 helical nanofilament (HNF) phase represents a promising selfassembling, nanosegregated architecture with potential as efficient charge transport systems in organic photovoltaics.18 It also forms porous networks that could be used for chiral separation and asymmetric synthesis.19 The twisting power of molecular or layer chirality is difficult to predict but can sometimes be deduced from the helical pitch. Previous attempts to probe the helical structure of the B4 HNF phase with conventional hard X-ray scattering have met limited success,20,21 due to the fact that these techniques are only sensitive to electron density modulation (scalar susceptibility), which is minimal along the helical axis in a continuous helix. Here, we report in situ measurement of the helical pitch of B4 HNFs from a macroscopic volume, using linearly polarized, resonant, soft X-ray scattering (RSoXS) near the carbon K-edge (∼284 eV). In typical RSoXS, the scattering contrast of a multiple component system, such as a triblock copolymer,22 depends on Δδ(E)2 + Δβ(E)2, where δ and β are respectively the real and imaginary parts of the complex refractive index n = 1 − δ + iβ. This contrast is sensitive to X-ray energy especially
hirality, the absence of inversion symmetry, plays an important role in chemistry, biology, and materials science.1 The origin of biological homochirality represents one of the most fundamental questions related to the origins of life.2 The function of fundamental components of the cell, such as actin, myosin, proteins, and lipids, relies upon their being chiral. Liquid crystals (LCs) have properties between those of a conventional liquid and a solid crystal, and their applications are widespread, from LC displays to lasers, photovoltaics, and nonlinear optics to switchable windows and biosensors.3 Introducing molecular chirality to an LC mesophase may lead to the occurrence of a twisting force, which can modify the equilibrium state with uniform orientational order usually observed in LCs, resulting in various helical structures, including cholesteric LC, blue phase,4,5 smectic-C*, and the newly discovered twist−bend nematic.6,7 In the case of the smectic-C* phase, molecular chirality breaks inversion symmetry, leading to the generation of macroscopic spontaneous polarization and ferroelectric behavior.8 The smectic-C-like phases of achiral, banana-shaped molecules are also observed to exhibit ferroelectricity.9 For example, each individual layer in the so-called B2 phase is chiral because molecular tilt and polarity combine to eliminate inversion symmetry.10 Bent-core LCs exhibit rich phase behavior11−15 (B1, B2, ... B7, SmAPF, SmAPFmod) and have potential applications, including displays and fast electro-optic devices. Of particular interest, the B4 phase16 has been proposed to be made of helices formed by twisted smectic layers composed of aromatic and aliphatic sublayers, an © 2015 American Chemical Society
Received: February 24, 2015 Revised: March 30, 2015 Published: April 13, 2015 3420
DOI: 10.1021/acs.nanolett.5b00760 Nano Lett. 2015, 15, 3420−3424
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Nano Letters near absorption edges, leading to great success in investigating polymer blends,23 block copolymers,24 organic bulk heterojunction solar cells,25 and polymeric transistors,26 where the refractive indices of the different components have distinct energy and polarization dependence. However, we exploit the coupling between linearly polarized X-rays and the asymmetric electron cloud of the sample, which results in a tensorial atomic scattering factor at the absorption edge, with the scattering dependent on the orientation of the molecule with respect to the polarization direction of the X-ray beam. Similar approaches have played an important role in discovering smectic-C* clock phases, which often requires the incorporation of sulfur27−29 (K-edge 2.47 keV) or selenium (K-edge 12.6 keV)30 atoms in the LC molecular structure. Labeling LC molecules with heavy atoms can affect the phase sequence and, thus, is not ideal. We show the feasibility of using RSoXS at the carbon K-edge to probe helical structures in LC without any chemical labeling. As a result of molecular orientational contrast near the carbon Kedge, a scattering peak at Q = 2π/h is expected from a helical structure (such as continuously twisting smectic layers) of halfpitch h.31 RSoXS measurements were performed at ALS beamline 11.0.1.2 with linearly polarized photons.32 The X-ray energy was tuned between 270 and 290 eV in our experiments. The material used in this study is NOBOW (P-9-OPIMB, 1,3phenylene bis[4-(4-nonyloxyphenyliminomethyl)-benzoate, shown in Figure 1a), which exhibits a phase sequence Iso− [170 °C]−B2−[TC = 142 °C]−B4. The liquid crystal was dissolved in either chloroform (CF) or chlorobenzene (CB) at room temperature (RT) and then drop-cast onto a 100 nm thick nitride membrane (Norcada) for transmission experiments. Although the X-ray attenuation length for a typical liquid crystal like 5CB (4-cyano-4′-pentylbiphenyl) is about 5 μm at 280 eV (pre-edge), it reduces quickly to 0.2 μm beyond 284.2 eV (post-edge) due to strong absorption. We, therefore, chose 283.5 eV for studies of temperature dependence, where it has enhanced resonant contrast in δ while absorption is small. A simplified structural model of HNFs is shown in Figure 1a, where NOBOW molecules form smectic layers (gray surface) that twist into a helix. The existence of smectic layers in NOBOW/CB systems has been confirmed by grazing incidence small-angle X-ray scattering (GISAXS) of drop-casted NOBOW/CB on a glass slide, measured with 10 keV hard Xrays at ALS beamline 7.3.3.33 A scattering peak is seen at Q = 0.13 Å−1 (Supporting Information Figure S1), in agreement with the reported smectic layer spacing (∼5 nm) in the B4 HNF phase. The formation of HNFs (∼30 nm in diameter and with a helical half-pitch of ∼100 nm) in various NOBOW mixtures has been observed using freeze-fracture transmission electron microscopy (FFTEM).19,34,35 The resonant scattering collected at RT is summarized in Figures 1 and 2. No distinct scattering feature was observed at E = 279.5 eV, however, at E = 283.5 eV, a strong peak appears at Q = 0.007 Å−1 corresponding to a periodicity ∼90 nm, which roughly agrees with the helical half-pitch obtained in FFTEM measurements.16 This peak intensity was much reduced when the incident radiation is tuned 4 eV below the carbon K-edge. Interestingly, the peak is anisotropic, with stronger scattering perpendicular to the X-ray polarization direction (red arrows in Figure 1), and this anisotropy does not change when the sample is rotated around the incoming beam, suggesting that there is no preferred in-plane (x−y) orientation of filaments on the nitride membrane. Remarkably, this anisotropy follows the
Figure 1. (a) NOBOW molecular structure, phase sequence, and the simplified structural model of an HNF, where the gray surfaces are the smectic layer surfaces. The smectic layers stack together and twist into an HNF. (b−e) Resonant soft X-ray scattering from drop-cast films of NOBOW in chlorobenzene. Experiments were performed with horizontally and vertically polarized X-rays (red arrows), at two different X-ray energies. The intensities are shown on a log scale. Cleary, the scattering contrast increases dramatically near the carbon K-edge (284.2 eV). The sample was at room temperature.
Figure 2. Energy dependence of scattering intensity along Φ = 180° in Figure 1e. (a) Integrated intensity I(Q) as a function of X-ray energy. The peak intensity is maximal around 284 eV. (b) Integrated scattering intensity along horizontal (180°) and vertical (270°) directions in Figure 1e.
polarization direction of the incoming X-ray beam as shown in Figures 1c,e, indicating that the scattering contrast must be polarization dependent, the details for which will be discussed later. The peak value Q and half-pitch are confirmed to be independent of the X-ray polarization direction (Supporting Information Figure S2). 3421
DOI: 10.1021/acs.nanolett.5b00760 Nano Lett. 2015, 15, 3420−3424
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Nano Letters
before the peak intensity starts to drop on approaching the transition to the B2 phase (Figure 3a). The scattering peak drops to near-zero intensity without shifting its position (Figure 3a), suggesting that B4 helix does not continuously unwind into the flat B2 lamellar phase, which can be considered as having a helix of infinitely long pitch, but rather undergoes melting. To understand the origin of the temperature dependence of the helical pitch, the same sample was heated to the isotropic phase and then cooled again into the B4, while monitoring the helical pitch. Surprisingly, the helical half-pitch of HNFs formed on cooling from the isotropic phase is about h = 120 nm (Q ∼ 0.005 Å−1, Figure 3b), a periodicity much larger than observed at RT before any heat treatment. On all cooling cycles subsequent to the first, the pitch does not change as the sample is slowly cooled to RT. These observations suggest that HNFs after temperature cycling may have adopted the preferred Gaussian curvature of the smectic layers and attained an equilibrium helical pitch. Once HNFs reach equilibrium, their structure is stable, reflected in the nearly constant pitch. Subsequent heating in the B4 phase has no further effect on the helical pitch either. When a small amount of solvent (CF or CB) is mixed in the B4 network during drop-casting, the HNFs form with a shorter helical pitch (∼86 nm), nearly 30% shorter than that obtained after temperature cycling. Upon heating, the added guest molecules are slowly expelled from the system and the helix starts to unwind toward the preferred pitch (h ∼ 120 nm), but stagnates at h = 97 nm and does not fully recover the equilibrium pitch before going to the isotropic phase. We propose that the addition of guest solvent molecules increases the strain in the top and bottom halves of individual smectic layers, which increases the preferred Gaussian curvature of the layers and results in a shorter helical pitch.16 Pure NOBOW was measured without mixing with CB or CF solvents in order to confirm the equilibrium helical pitch. As expected, the helical half-pitch is found to be about 120 nm (Q ∼ 0.005 Å−1) and nearly temperature independent within the B4 phase. Interestingly a second harmonic (at Q ∼ 0.01 Å−1) was also observed, suggesting that the helical structure is well ordered in pure NOBOW before any heat treatment (Supporting Information Figure S4). To understand the scattering anisotropy quantitatively, we assume that the scattering contrast depends on |P·LN| based on bond orientation sensitivity (X-ray linear dichroism), where P is the X-ray polarization direction and LN is the smectic layer normal. Both P and LN are unit vectors. In the case when the helical axis is along the y direction, LN is in the x−z plane (Figure 4b) and rotates along the helix by 180° every half pitch. In this case, |P·LN| changes from the maximum (when the smectic layer normal is along the polarization) to the minimum (when the smectic layer is perpendicular to the polarization) and to the maximum every half-pitch along the helix, leading to high scattering contrast from a single helix. A pair of strong scattering peaks should be expected in the y direction, that is, at Φ = 90° and Φ = 270° (Figure 4d). On the other hand, when the helical axis is along the x direction, LN is in the y−z plane (Figure 4c) and |P·LN| remains constant along the helix due to the helical symmetry, resulting in minimal scattering contrast from the helical structure, that is, twisting layers. The scattering peak, if it exists at all, should be expected in the x direction, that is, Φ = 0° and Φ = 180°. In the general case (Figure 4a), the scattering peak intensity from a single helix depends on its orientation relative to the X-ray polarization direction, and should follow a sinusoidal function, cos2 Φ in the filament
To investigate the energy dependence of the scattering, we select a line cut, I(Q), along Φ = 180° in Figure 1e (green arrow), averaging azimuthally over Φ = [170°, 190°]. The energy dependence of the line profiles is shown in Figure 2a. Clearly, the peak near 0.007 Å−1 becomes stronger as the X-ray energy approaches the carbon resonant edge, reaches a maximum at the edge, and weakens quickly after the edge. The HNFs are fairly robust, forming in a variety of mixtures of NOBOW and organic solvents, such as P3HT, 8CB, and octanol.19 In our experiments, helices of similar pitch are observed in both NOBOW/CF and NOBOW/CB systems. Most of the solvent (CB or CF) should evaporate very quickly, especially since the measurement is carried out in vacuum. If the NOBOW/CB solution is heated to 90 °C to increase the solubility of the NOBOW in CB right before drop-casting, the helical half-pitch becomes shorter, that is, decreasing from 94 nm without heating to 90 nm with heating (Supporting Information Figure S3), suggesting that a small amount of solvent may remain in the HNFs and affect the pitch. The peak of I(Q) is asymmetric (Figure 1c and e, Figure 2a) and cannot be fitted with a single Gaussian or Lorentzian function. One way to interpret such asymmetry is to attribute the shape to two different structures, in which case it can be fitted with two peaks with different peak positions and widths. Another way is to fit the scattering with an exponentially modified Gaussian, which essentially is fitting with multiple peaks. Line cuts in the horizontal (Φ = 180°) and vertical (Φ = 270°) directions (Figure 1e) are presented in Figure 2b. The vertical cut clearly shows a broader peak, at a slightly higher Q, than the peak in the horizontal cut, suggesting that this broad peak may contribute to the asymmetry of the peak in the horizontal cut, thus the first interpretation could be more suitable. The origin of such a broad peak is unclear. In situ temperature-dependent scattering was carried out to investigate how the helix of NOBOW/CF films evolves with temperature. On first heating from RT to the isotropic (Figure 3a), the peak intensity slowly increases with temperature,
Figure 3. Temperature dependence of the scattering profile of the helical half-pitch (a) on heating from RT and (b) on cooling from the isotropic. On heating, the peak becomes stronger and shifts slightly to smaller Q; on subsequent cooling from the isotropic phase, the peak position does not change. However, the peak position is much smaller on cooling (Q ∼ 0.0052 Å−1, larger helical half-pitch h ∼ 120 nm) than on heating (Q ∼ 0.007 Å−1, h ∼ 90 nm).
suggesting that the helical ordering is improved. In addition, the peak becomes slightly sharper upon heating, indicating that the correlation length of the helical structure becomes longer. The above observations suggest that the helix formed during dropcasting is not the equilibrium state and that thermal annealing leads to improved ordering. Interestingly, the helical half-pitch increases slowly as T increases, from 86 nm at RT to 97 nm 3422
DOI: 10.1021/acs.nanolett.5b00760 Nano Lett. 2015, 15, 3420−3424
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a strong piece of evidence for the twisted smectic layer model. Such a unique probe will have numerous potential applications in probing nanostructures in a variety of organic compounds. In situ helical pitch measurement clearly shows that (1) at the B4−B2 transition, B4 helical nanofilaments melt on heating rather than unwinding into the flat lamellar B2 phase, (2) in pure materials, the helical structure is nearly temperature independent, and (3) the helical pitch can be tuned up to 30% by the addition of small amount of guest molecules into HNFs followed by thermal heating. The ability to control the helical pitch may be important for potential applications of B4 HNF. Experimental Methods. GISAXS experiments were performed at beamline 7.3.3 at the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory; 10 keV Xrays with energy bandwidth E/ΔE = 100 was used. The incoming beam was about 300 μm high and 700 μm wide. A Pilatus 1 M (pixel size ∼172 μm) from Dectris was positioned 3.8 m away from the sample. The incident angle was chosen to be 0.18 degrees. The raw detector images were calibrated to Qspace using a standard silver behenate sample and Igor NIKA software.39 RSoXS measurements were performed in transmission at beamline 11.0.1.2 with linearly polarized photons at ALS. To reduce attenuation, it was essential to keep both sample and CCD detector in a vacuum chamber, at p = 10−6 Torr. The scattering intensity was recorded by a back illuminated Princeton PI-MTE CCD thermoelectrically cooled to −45 °C. The CCD pixel size is 0.027 mm and the detector is positioned 150 mm away from the sample. The exposure time used is 0.2−2 s. The beam center and the sample-to-detector distance were calibrated using scattering peaks of the polystyrene spheres, which has a known diameter of 300 nm. The X-ray beam (300 × 200 μm2) is linearly polarized, with a polarization direction that can be rotated continuously from horizontal to vertical. A modified version of the NIKA software package39 was used for calibration and data reduction. Freeze-fracture transmission electron microscopy (FFTEM) has been a very powerful tool in revealing liquid crystal nanostructures. In FFTEM, the sample is equilibrated in the phase of interest, then is rapidly quenched in liquid nitrogen to preserve the LC structure and fractured. The fracture surface is then coated by layers of platinum and carbon, which form a replica of the fractured interface that can be imaged using TEM. However, this technique has several limitations: (1) FFTEM images are collected from replicas of a fractured interface, providing information about the topography of random surfaces, (2) detailed temperature-dependent studies are not practical because of the quenching step involved, and (3) only local information is accessible, as is the case for all TEM-based techniques.
Figure 4. Model explaining scattering anisotropy of HNFs. (a) The sample is in the x−y plane, whereas the incoming X-ray is along z, out of the paper. The X-ray polarization direction (marked by P = x) is along x axis. (b) Case 1 (Φ = 90°): the filament axis is along y direction. The inset coordinate is a bottom view of the filament, showing the layer normal (LN) in x−z plane. Note that along the helix, LN rotates and γ changes. (c) Case 2 (Φ = 0°): the filament axis is along the x direction. The inset coordinate is a bottom view of the filament. The inset shows that LN in y−z plane, and it rotates as it moves along the helix. (d) Scattering pattern in pixels (CCD image). The X-ray beam center is marked as BC. The scattering peak of the helices (Q ∼ 0.007 Å−1) is marked as blue dots. (e) Sector graph calculated from (d), showing the scattering as a function of azimuth (0−360°) and radius (Q). (f) Azimuthal variation of scattering intensity (blue: integrated between two vertical lines in (e)) is sinusoidal and is fit well by the square of a cosine function (red).
azimuthal orientation Φ. Experimentally, we can convert collected 2D CCD images into a so-called sector graph, plotting the scattering intensity vs azimuthal angle and the radius from the beam center (BC), which is essentially Q, as shown in Figure 4e. The average over the area between two vertical lines in Figure 4e is essentially the azimuthal intensity distribution I(Φ) around the scattering peak of the helical halfpitch (Figure 4f, blue). The experimental I(Φ) is fit very well by a cos2 Φ function (Figure 4f), confirming the structural model for HNFs and that the HNFs are randomly oriented within the X-ray beam illumination, which is about 0.3 × 0.2 mm. Since no labeling of heavy atoms is required with RSoXS at the carbon edge, immediate use of RSoXS in investigating other helical structures in liquid crystals such as the twist grain boundary smectics (TGB),36 where smectic layers breaks into blocks that exhibit discrete angular jumps in layer normal between neighboring blocks, essentially a helix with discrete orientation variation between its subunits, is possible. Similar polarization-dependent scattering patterns with much stronger temperature dependence have recently been confirmed.37 Comparison between B4 HNF and TGB will likely shed light on the complex interplay38 between elastic forces and the orientation and chirality of the constituent molecules or subunits. In conclusion, we have demonstrated that resonant soft X-ray scattering near the carbon K-edge can be used to probe the ensemble-averaged helical half-pitch of B4 HNF liquid crystals,
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ASSOCIATED CONTENT
S Supporting Information *
Additional information on GISAXS measurements, and the scattering profiles of NOBOW/CB, NOBOW/CF, and pure NOBOW. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. 3423
DOI: 10.1021/acs.nanolett.5b00760 Nano Lett. 2015, 15, 3420−3424
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Nano Letters Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Dr. Wim Bras for helpful discussions. We acknowledge use of Beamlines 7.3.3 and 11.0.1.2 of the Advanced Light Source supported by the Director of the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under contract no. DE-AC0205CH11231. We thank the Liquid Crystal Materials Research Center (NSF MRSEC awards DMR-0820579 and DMR1420736) for financial support of this work.
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DOI: 10.1021/acs.nanolett.5b00760 Nano Lett. 2015, 15, 3420−3424
Probing and Controlling Liquid Crystal Helical Nanofilaments Chenhui Zhu,*,† Cheng Wang,*,† Anthony Young,† Feng Liu,‡ Ilja Gunkel,† Dong Chen,§ David Walba,∥ Joseph Maclennan,§ Noel Clark,§ and Alexander Hexemer*,† †
Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States § Department of Physics and Liquid Crystal Materials Research Center, University of Colorado, Boulder, Colorado 80309, United States ∥ Department of Chemistry and Biochemistry and Liquid Crystal Materials Research Center, University of Colorado, Boulder, Colorado 80309, United States ‡
S Supporting Information *
ABSTRACT: We report the first in situ measurement of the helical pitch of the helical nanofilament B4 phase of bent-core liquid crystals using linearly polarized, resonant soft X-ray scattering at the carbon K-edge. A strong, anisotropic scattering peak corresponding to the half-pitch of the twisted smectic layer structure was observed. The equilibrium helical half-pitch of NOBOW is found to be 120 nm, essentially independent of temperature. However, the helical pitch can be tuned by mixing guest organic molecules with the bent-core host, followed by thermal annealing.
KEYWORDS: helical pitch, helix, smectic liquid crystal, resonant soft X-ray scattering, carbon edge, nanofilament, B4, bent-core
C
indication that not only molecular chirality but layer chirality drives the formation of helical structures. Recently, a doubletwisted structure of the bulk arrangement of nanofilaments was proposed17 to explain the mysterious properties of the B4, such as the structural color and ambidextrous optical activity. The B4 helical nanofilament (HNF) phase represents a promising selfassembling, nanosegregated architecture with potential as efficient charge transport systems in organic photovoltaics.18 It also forms porous networks that could be used for chiral separation and asymmetric synthesis.19 The twisting power of molecular or layer chirality is difficult to predict but can sometimes be deduced from the helical pitch. Previous attempts to probe the helical structure of the B4 HNF phase with conventional hard X-ray scattering have met limited success,20,21 due to the fact that these techniques are only sensitive to electron density modulation (scalar susceptibility), which is minimal along the helical axis in a continuous helix. Here, we report in situ measurement of the helical pitch of B4 HNFs from a macroscopic volume, using linearly polarized, resonant, soft X-ray scattering (RSoXS) near the carbon K-edge (∼284 eV). In typical RSoXS, the scattering contrast of a multiple component system, such as a triblock copolymer,22 depends on Δδ(E)2 + Δβ(E)2, where δ and β are respectively the real and imaginary parts of the complex refractive index n = 1 − δ + iβ. This contrast is sensitive to X-ray energy especially
hirality, the absence of inversion symmetry, plays an important role in chemistry, biology, and materials science.1 The origin of biological homochirality represents one of the most fundamental questions related to the origins of life.2 The function of fundamental components of the cell, such as actin, myosin, proteins, and lipids, relies upon their being chiral. Liquid crystals (LCs) have properties between those of a conventional liquid and a solid crystal, and their applications are widespread, from LC displays to lasers, photovoltaics, and nonlinear optics to switchable windows and biosensors.3 Introducing molecular chirality to an LC mesophase may lead to the occurrence of a twisting force, which can modify the equilibrium state with uniform orientational order usually observed in LCs, resulting in various helical structures, including cholesteric LC, blue phase,4,5 smectic-C*, and the newly discovered twist−bend nematic.6,7 In the case of the smectic-C* phase, molecular chirality breaks inversion symmetry, leading to the generation of macroscopic spontaneous polarization and ferroelectric behavior.8 The smectic-C-like phases of achiral, banana-shaped molecules are also observed to exhibit ferroelectricity.9 For example, each individual layer in the so-called B2 phase is chiral because molecular tilt and polarity combine to eliminate inversion symmetry.10 Bent-core LCs exhibit rich phase behavior11−15 (B1, B2, ... B7, SmAPF, SmAPFmod) and have potential applications, including displays and fast electro-optic devices. Of particular interest, the B4 phase16 has been proposed to be made of helices formed by twisted smectic layers composed of aromatic and aliphatic sublayers, an © 2015 American Chemical Society
Received: February 24, 2015 Revised: March 30, 2015 Published: April 13, 2015 3420
DOI: 10.1021/acs.nanolett.5b00760 Nano Lett. 2015, 15, 3420−3424
Letter
Nano Letters near absorption edges, leading to great success in investigating polymer blends,23 block copolymers,24 organic bulk heterojunction solar cells,25 and polymeric transistors,26 where the refractive indices of the different components have distinct energy and polarization dependence. However, we exploit the coupling between linearly polarized X-rays and the asymmetric electron cloud of the sample, which results in a tensorial atomic scattering factor at the absorption edge, with the scattering dependent on the orientation of the molecule with respect to the polarization direction of the X-ray beam. Similar approaches have played an important role in discovering smectic-C* clock phases, which often requires the incorporation of sulfur27−29 (K-edge 2.47 keV) or selenium (K-edge 12.6 keV)30 atoms in the LC molecular structure. Labeling LC molecules with heavy atoms can affect the phase sequence and, thus, is not ideal. We show the feasibility of using RSoXS at the carbon K-edge to probe helical structures in LC without any chemical labeling. As a result of molecular orientational contrast near the carbon Kedge, a scattering peak at Q = 2π/h is expected from a helical structure (such as continuously twisting smectic layers) of halfpitch h.31 RSoXS measurements were performed at ALS beamline 11.0.1.2 with linearly polarized photons.32 The X-ray energy was tuned between 270 and 290 eV in our experiments. The material used in this study is NOBOW (P-9-OPIMB, 1,3phenylene bis[4-(4-nonyloxyphenyliminomethyl)-benzoate, shown in Figure 1a), which exhibits a phase sequence Iso− [170 °C]−B2−[TC = 142 °C]−B4. The liquid crystal was dissolved in either chloroform (CF) or chlorobenzene (CB) at room temperature (RT) and then drop-cast onto a 100 nm thick nitride membrane (Norcada) for transmission experiments. Although the X-ray attenuation length for a typical liquid crystal like 5CB (4-cyano-4′-pentylbiphenyl) is about 5 μm at 280 eV (pre-edge), it reduces quickly to 0.2 μm beyond 284.2 eV (post-edge) due to strong absorption. We, therefore, chose 283.5 eV for studies of temperature dependence, where it has enhanced resonant contrast in δ while absorption is small. A simplified structural model of HNFs is shown in Figure 1a, where NOBOW molecules form smectic layers (gray surface) that twist into a helix. The existence of smectic layers in NOBOW/CB systems has been confirmed by grazing incidence small-angle X-ray scattering (GISAXS) of drop-casted NOBOW/CB on a glass slide, measured with 10 keV hard Xrays at ALS beamline 7.3.3.33 A scattering peak is seen at Q = 0.13 Å−1 (Supporting Information Figure S1), in agreement with the reported smectic layer spacing (∼5 nm) in the B4 HNF phase. The formation of HNFs (∼30 nm in diameter and with a helical half-pitch of ∼100 nm) in various NOBOW mixtures has been observed using freeze-fracture transmission electron microscopy (FFTEM).19,34,35 The resonant scattering collected at RT is summarized in Figures 1 and 2. No distinct scattering feature was observed at E = 279.5 eV, however, at E = 283.5 eV, a strong peak appears at Q = 0.007 Å−1 corresponding to a periodicity ∼90 nm, which roughly agrees with the helical half-pitch obtained in FFTEM measurements.16 This peak intensity was much reduced when the incident radiation is tuned 4 eV below the carbon K-edge. Interestingly, the peak is anisotropic, with stronger scattering perpendicular to the X-ray polarization direction (red arrows in Figure 1), and this anisotropy does not change when the sample is rotated around the incoming beam, suggesting that there is no preferred in-plane (x−y) orientation of filaments on the nitride membrane. Remarkably, this anisotropy follows the
Figure 1. (a) NOBOW molecular structure, phase sequence, and the simplified structural model of an HNF, where the gray surfaces are the smectic layer surfaces. The smectic layers stack together and twist into an HNF. (b−e) Resonant soft X-ray scattering from drop-cast films of NOBOW in chlorobenzene. Experiments were performed with horizontally and vertically polarized X-rays (red arrows), at two different X-ray energies. The intensities are shown on a log scale. Cleary, the scattering contrast increases dramatically near the carbon K-edge (284.2 eV). The sample was at room temperature.
Figure 2. Energy dependence of scattering intensity along Φ = 180° in Figure 1e. (a) Integrated intensity I(Q) as a function of X-ray energy. The peak intensity is maximal around 284 eV. (b) Integrated scattering intensity along horizontal (180°) and vertical (270°) directions in Figure 1e.
polarization direction of the incoming X-ray beam as shown in Figures 1c,e, indicating that the scattering contrast must be polarization dependent, the details for which will be discussed later. The peak value Q and half-pitch are confirmed to be independent of the X-ray polarization direction (Supporting Information Figure S2). 3421
DOI: 10.1021/acs.nanolett.5b00760 Nano Lett. 2015, 15, 3420−3424
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Nano Letters
before the peak intensity starts to drop on approaching the transition to the B2 phase (Figure 3a). The scattering peak drops to near-zero intensity without shifting its position (Figure 3a), suggesting that B4 helix does not continuously unwind into the flat B2 lamellar phase, which can be considered as having a helix of infinitely long pitch, but rather undergoes melting. To understand the origin of the temperature dependence of the helical pitch, the same sample was heated to the isotropic phase and then cooled again into the B4, while monitoring the helical pitch. Surprisingly, the helical half-pitch of HNFs formed on cooling from the isotropic phase is about h = 120 nm (Q ∼ 0.005 Å−1, Figure 3b), a periodicity much larger than observed at RT before any heat treatment. On all cooling cycles subsequent to the first, the pitch does not change as the sample is slowly cooled to RT. These observations suggest that HNFs after temperature cycling may have adopted the preferred Gaussian curvature of the smectic layers and attained an equilibrium helical pitch. Once HNFs reach equilibrium, their structure is stable, reflected in the nearly constant pitch. Subsequent heating in the B4 phase has no further effect on the helical pitch either. When a small amount of solvent (CF or CB) is mixed in the B4 network during drop-casting, the HNFs form with a shorter helical pitch (∼86 nm), nearly 30% shorter than that obtained after temperature cycling. Upon heating, the added guest molecules are slowly expelled from the system and the helix starts to unwind toward the preferred pitch (h ∼ 120 nm), but stagnates at h = 97 nm and does not fully recover the equilibrium pitch before going to the isotropic phase. We propose that the addition of guest solvent molecules increases the strain in the top and bottom halves of individual smectic layers, which increases the preferred Gaussian curvature of the layers and results in a shorter helical pitch.16 Pure NOBOW was measured without mixing with CB or CF solvents in order to confirm the equilibrium helical pitch. As expected, the helical half-pitch is found to be about 120 nm (Q ∼ 0.005 Å−1) and nearly temperature independent within the B4 phase. Interestingly a second harmonic (at Q ∼ 0.01 Å−1) was also observed, suggesting that the helical structure is well ordered in pure NOBOW before any heat treatment (Supporting Information Figure S4). To understand the scattering anisotropy quantitatively, we assume that the scattering contrast depends on |P·LN| based on bond orientation sensitivity (X-ray linear dichroism), where P is the X-ray polarization direction and LN is the smectic layer normal. Both P and LN are unit vectors. In the case when the helical axis is along the y direction, LN is in the x−z plane (Figure 4b) and rotates along the helix by 180° every half pitch. In this case, |P·LN| changes from the maximum (when the smectic layer normal is along the polarization) to the minimum (when the smectic layer is perpendicular to the polarization) and to the maximum every half-pitch along the helix, leading to high scattering contrast from a single helix. A pair of strong scattering peaks should be expected in the y direction, that is, at Φ = 90° and Φ = 270° (Figure 4d). On the other hand, when the helical axis is along the x direction, LN is in the y−z plane (Figure 4c) and |P·LN| remains constant along the helix due to the helical symmetry, resulting in minimal scattering contrast from the helical structure, that is, twisting layers. The scattering peak, if it exists at all, should be expected in the x direction, that is, Φ = 0° and Φ = 180°. In the general case (Figure 4a), the scattering peak intensity from a single helix depends on its orientation relative to the X-ray polarization direction, and should follow a sinusoidal function, cos2 Φ in the filament
To investigate the energy dependence of the scattering, we select a line cut, I(Q), along Φ = 180° in Figure 1e (green arrow), averaging azimuthally over Φ = [170°, 190°]. The energy dependence of the line profiles is shown in Figure 2a. Clearly, the peak near 0.007 Å−1 becomes stronger as the X-ray energy approaches the carbon resonant edge, reaches a maximum at the edge, and weakens quickly after the edge. The HNFs are fairly robust, forming in a variety of mixtures of NOBOW and organic solvents, such as P3HT, 8CB, and octanol.19 In our experiments, helices of similar pitch are observed in both NOBOW/CF and NOBOW/CB systems. Most of the solvent (CB or CF) should evaporate very quickly, especially since the measurement is carried out in vacuum. If the NOBOW/CB solution is heated to 90 °C to increase the solubility of the NOBOW in CB right before drop-casting, the helical half-pitch becomes shorter, that is, decreasing from 94 nm without heating to 90 nm with heating (Supporting Information Figure S3), suggesting that a small amount of solvent may remain in the HNFs and affect the pitch. The peak of I(Q) is asymmetric (Figure 1c and e, Figure 2a) and cannot be fitted with a single Gaussian or Lorentzian function. One way to interpret such asymmetry is to attribute the shape to two different structures, in which case it can be fitted with two peaks with different peak positions and widths. Another way is to fit the scattering with an exponentially modified Gaussian, which essentially is fitting with multiple peaks. Line cuts in the horizontal (Φ = 180°) and vertical (Φ = 270°) directions (Figure 1e) are presented in Figure 2b. The vertical cut clearly shows a broader peak, at a slightly higher Q, than the peak in the horizontal cut, suggesting that this broad peak may contribute to the asymmetry of the peak in the horizontal cut, thus the first interpretation could be more suitable. The origin of such a broad peak is unclear. In situ temperature-dependent scattering was carried out to investigate how the helix of NOBOW/CF films evolves with temperature. On first heating from RT to the isotropic (Figure 3a), the peak intensity slowly increases with temperature,
Figure 3. Temperature dependence of the scattering profile of the helical half-pitch (a) on heating from RT and (b) on cooling from the isotropic. On heating, the peak becomes stronger and shifts slightly to smaller Q; on subsequent cooling from the isotropic phase, the peak position does not change. However, the peak position is much smaller on cooling (Q ∼ 0.0052 Å−1, larger helical half-pitch h ∼ 120 nm) than on heating (Q ∼ 0.007 Å−1, h ∼ 90 nm).
suggesting that the helical ordering is improved. In addition, the peak becomes slightly sharper upon heating, indicating that the correlation length of the helical structure becomes longer. The above observations suggest that the helix formed during dropcasting is not the equilibrium state and that thermal annealing leads to improved ordering. Interestingly, the helical half-pitch increases slowly as T increases, from 86 nm at RT to 97 nm 3422
DOI: 10.1021/acs.nanolett.5b00760 Nano Lett. 2015, 15, 3420−3424
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a strong piece of evidence for the twisted smectic layer model. Such a unique probe will have numerous potential applications in probing nanostructures in a variety of organic compounds. In situ helical pitch measurement clearly shows that (1) at the B4−B2 transition, B4 helical nanofilaments melt on heating rather than unwinding into the flat lamellar B2 phase, (2) in pure materials, the helical structure is nearly temperature independent, and (3) the helical pitch can be tuned up to 30% by the addition of small amount of guest molecules into HNFs followed by thermal heating. The ability to control the helical pitch may be important for potential applications of B4 HNF. Experimental Methods. GISAXS experiments were performed at beamline 7.3.3 at the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory; 10 keV Xrays with energy bandwidth E/ΔE = 100 was used. The incoming beam was about 300 μm high and 700 μm wide. A Pilatus 1 M (pixel size ∼172 μm) from Dectris was positioned 3.8 m away from the sample. The incident angle was chosen to be 0.18 degrees. The raw detector images were calibrated to Qspace using a standard silver behenate sample and Igor NIKA software.39 RSoXS measurements were performed in transmission at beamline 11.0.1.2 with linearly polarized photons at ALS. To reduce attenuation, it was essential to keep both sample and CCD detector in a vacuum chamber, at p = 10−6 Torr. The scattering intensity was recorded by a back illuminated Princeton PI-MTE CCD thermoelectrically cooled to −45 °C. The CCD pixel size is 0.027 mm and the detector is positioned 150 mm away from the sample. The exposure time used is 0.2−2 s. The beam center and the sample-to-detector distance were calibrated using scattering peaks of the polystyrene spheres, which has a known diameter of 300 nm. The X-ray beam (300 × 200 μm2) is linearly polarized, with a polarization direction that can be rotated continuously from horizontal to vertical. A modified version of the NIKA software package39 was used for calibration and data reduction. Freeze-fracture transmission electron microscopy (FFTEM) has been a very powerful tool in revealing liquid crystal nanostructures. In FFTEM, the sample is equilibrated in the phase of interest, then is rapidly quenched in liquid nitrogen to preserve the LC structure and fractured. The fracture surface is then coated by layers of platinum and carbon, which form a replica of the fractured interface that can be imaged using TEM. However, this technique has several limitations: (1) FFTEM images are collected from replicas of a fractured interface, providing information about the topography of random surfaces, (2) detailed temperature-dependent studies are not practical because of the quenching step involved, and (3) only local information is accessible, as is the case for all TEM-based techniques.
Figure 4. Model explaining scattering anisotropy of HNFs. (a) The sample is in the x−y plane, whereas the incoming X-ray is along z, out of the paper. The X-ray polarization direction (marked by P = x) is along x axis. (b) Case 1 (Φ = 90°): the filament axis is along y direction. The inset coordinate is a bottom view of the filament, showing the layer normal (LN) in x−z plane. Note that along the helix, LN rotates and γ changes. (c) Case 2 (Φ = 0°): the filament axis is along the x direction. The inset coordinate is a bottom view of the filament. The inset shows that LN in y−z plane, and it rotates as it moves along the helix. (d) Scattering pattern in pixels (CCD image). The X-ray beam center is marked as BC. The scattering peak of the helices (Q ∼ 0.007 Å−1) is marked as blue dots. (e) Sector graph calculated from (d), showing the scattering as a function of azimuth (0−360°) and radius (Q). (f) Azimuthal variation of scattering intensity (blue: integrated between two vertical lines in (e)) is sinusoidal and is fit well by the square of a cosine function (red).
azimuthal orientation Φ. Experimentally, we can convert collected 2D CCD images into a so-called sector graph, plotting the scattering intensity vs azimuthal angle and the radius from the beam center (BC), which is essentially Q, as shown in Figure 4e. The average over the area between two vertical lines in Figure 4e is essentially the azimuthal intensity distribution I(Φ) around the scattering peak of the helical halfpitch (Figure 4f, blue). The experimental I(Φ) is fit very well by a cos2 Φ function (Figure 4f), confirming the structural model for HNFs and that the HNFs are randomly oriented within the X-ray beam illumination, which is about 0.3 × 0.2 mm. Since no labeling of heavy atoms is required with RSoXS at the carbon edge, immediate use of RSoXS in investigating other helical structures in liquid crystals such as the twist grain boundary smectics (TGB),36 where smectic layers breaks into blocks that exhibit discrete angular jumps in layer normal between neighboring blocks, essentially a helix with discrete orientation variation between its subunits, is possible. Similar polarization-dependent scattering patterns with much stronger temperature dependence have recently been confirmed.37 Comparison between B4 HNF and TGB will likely shed light on the complex interplay38 between elastic forces and the orientation and chirality of the constituent molecules or subunits. In conclusion, we have demonstrated that resonant soft X-ray scattering near the carbon K-edge can be used to probe the ensemble-averaged helical half-pitch of B4 HNF liquid crystals,
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ASSOCIATED CONTENT
S Supporting Information *
Additional information on GISAXS measurements, and the scattering profiles of NOBOW/CB, NOBOW/CF, and pure NOBOW. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. 3423
DOI: 10.1021/acs.nanolett.5b00760 Nano Lett. 2015, 15, 3420−3424
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Nano Letters Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Dr. Wim Bras for helpful discussions. We acknowledge use of Beamlines 7.3.3 and 11.0.1.2 of the Advanced Light Source supported by the Director of the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under contract no. DE-AC0205CH11231. We thank the Liquid Crystal Materials Research Center (NSF MRSEC awards DMR-0820579 and DMR1420736) for financial support of this work.
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