Letter pubs.acs.org/NanoLett
Photoinduced Micropattern Alignment of Semiconductor Nanorods with Polarized Emission in a Liquid Crystal Polymer Matrix Julian Schneider,† Wanlong Zhang,‡ Abhishek K. Srivastava,*,‡ Vladimir G. Chigrinov,‡ Hoi-Sing Kwok,‡ and Andrey L. Rogach*,† †
Department of Physics and Materials Science and Centre for Functional Photonics (CFP), City University of Hong Kong, Hong Kong SAR ‡ State Key Laboratory on Advanced Displays and Optoelectronics Technologies, Department of Electronic and Computer Engineering, Hong Kong University of Science and Technology, Hong Kong SAR S Supporting Information *
ABSTRACT: Photoalignment technology provides high alignment quality with an exceptional control over the local director of liquid crystals. Because of the reorientation ability of sulfonic azo dye molecules, they offer high azimuthal and polar anchoring energy with a low pretilt angle for the orientation of liquid crystals and liquid crystal composites. In this work, we make use of this approach to align thin film composites of light-emitting semiconductor nanorods dispersed in a liquid crystal polymer into both one-dimensional and two-dimensional microscale patterns. After unidirectional alignment, the patterns are fabricated by a second irradiation with different polarization azimuth and the employment of a photomask. Fluorescence micrographs reveal the nanorod pattern alignment in domain sizes down to 2 μm. Apart from demonstrating the possibility of controlling the orientation of anisotropic nanocrystals with strongly polarized emission on microscopic scale, our results are promising for the fabrication of complex nanostructures for photonic applications. KEYWORDS: Nanorods, polarized emission, photoalignment, liquid crystal polymer, patterns
T
shape with the transition dipole moments oriented along the long geometrical axis.18,19 For photosensitive azo-dye SD1 (Figure 1a), which is used in this work as the alignment layer, molecular rotation is governed by a reorientation diffusion mechanism.20 Irradiation with linearly polarized light (λ = 450 nm) orients the molecular absorption oscillator in a direction perpendicular to the polarization azimuth of the irradiating light and generates an alignment easy axis with the same orientation.20 The resulting azo-dye films are characterized by an almost zero pretilt angle, and a high order parameter, providing large anchoring energy for the planar alignment of liquid crystals and composites of liquid crystals and colloidal nanocrystals.21−24 While guest−host systems of colloidal nanoparticles and liquid crystals have been studied,25 only a few reports focused on the alignment of anisotropic NRs in the liquid crystal matrix.23,24,26−29 Studies on gold NRs, dispersed in liquid crystals, introduced the successful alignment and modulation of the optical properties by optical manipulation of photoresponsive azobenzene-based surface monolayers.23 Fur-
he anisotropic properties of rod-shaped semiconductor nanocrystals (nanorods, NRs) offer a number of advantages over spherical nanocrystals, such as large extinction coefficients and the absorption and emission of linearly polarized light, as shown for cadmium selenide.1−3 Photoluminescence quantum yields (PL QY) of NRs have been reported to be high by the introduction of heterostructured core−shell nanocrystals.4−7 In order to retain the benefits of NRs not only at the single particle level requires their parallel orientation in films, which can be achieved by a number of different techniques.5,6,8−13 While side-by-side self-assembly at the liquid−air interface is the most common alignment strategy for NRs,5,14 other successful approaches at larger scale involve the employment of an external force, induced, for example, by an electric or magnetic field, electrospinning, or mechanical polymer stretching.6,9−13 Yet, most of the reported strategies struggle to achieve alignment simultaneously on small and large scales, which prohibits the fabrication of alignment patterns. Initial attempts to align NRs in patterns have been reported,15,16 however, the techniques still lacked the control over NR orientation in different domains. Photoalignment technology provides a solution to this problem by its precise control over the local orientation of the photosensitive dye molecules.17,18 These molecules usually have an anisotropic © 2017 American Chemical Society
Received: February 10, 2017 Revised: April 4, 2017 Published: April 10, 2017 3133
DOI: 10.1021/acs.nanolett.7b00563 Nano Lett. 2017, 17, 3133−3138
Letter
Nano Letters
Figure 1. Materials characterization (a−c) and a scheme of the two-step micropattern alignment process. (a) Molecular structure of a sulfonic azodye SD1. (b) Absorption (black) and photoluminescence (red) spectra of CdSe/CdS core−shell NRs. (c) TEM and HRTEM (inset) images of NRs. (d) Alignment process, starting with the photoalignment of the azo-dye SD1, followed by a second irradiation and alignment step with the application of a photomask. NR/LCP mixture is spin coated on top of prealigned SD1 and polymerized in the last step under irradiation with UV light. The bidirectional black arrows on the substrates indicate the alignment direction of the SD1, and the white arrows indicate the main axis of the NR orientation.
1c reveal the uniform shape of the NRs with an average dimension of 20 × 4 nm. Figure 1d introduces the process flow of the pattern alignment for composite NR/LCP films. Sulfonic azo-dye (SD1, DIC Japan), which is deposited on a glass substrate by spin coating, is horizontally aligned by illumination with linearly polarized light (here denoted as θ = 0°, λ = 450 nm). Subsequent irradiation with polarized light of different polarization azimuth (here θ = 90°) realigns the easy axis of SD1, in-plane.31,32 By restricting the exposed area through a photomask, separate domains with different alignment directions are fabricated. Note that the difference of θ (Δθ) between the first and second irradiation defines the angular difference in the easy axis within different alignment domains of the pattern. While there is no restriction to any specific angle, Δθ of 90° is used in these experiments in order to obtain the maximum contrast between adjacent domains. After alignment of the azo-dye layer, a mixture of NRs and LCP in toluene (120 nM/5 wt %) is deposited onto the SD1 layer by spin coating (1500 rpm, 30 s). LCP molecules immediately align parallel to the alignment easy axis and simultaneously induce the alignment of the NRs.18,33 The composite films show planar alignment with zero pretilt angle, which is of special significance for patterns, as a tilt in the NR orientation directly affects their emission characteristics. As discussed in our previous work,22 the orientation of the NRs occurs perpendicular, in-plane to the alignment direction of the LCP and azo-dye. This was attributed to the repulsive interaction at the interface of the
thermore, it was shown that the same approach can be employed to guide the alignment of plasmonic nanoparticles in well-defined patterns.23,24 In our recent work, we reported the alignment of luminescent CdSe/CdS core−shell NRs in a liquid crystal polymer (LCP) by combining photoalignment technology with the effect of NR side-by-side assembly, governed by the interaction of surface ligands and the surrounding LCP.22 Employment of LCP allowed us to solidify the films and freeze the alignment, resulting in composites with high polarization ratios and an NR order parameter of 0.87. In this work, we demonstrate the lateral size control of this method by fabricating one- and two-dimensional fluorescent grating patterns with defined domain sizes down to 2 μm. This underlines the precise control of photoalignment technology and introduces microscopically defined polarization-sensitive composites that enable the development of promising photonic materials. Highly luminescent CdSe/CdS core−shell NRs (dot-in-rod structures30) were synthesized according to a literature procedure employing a mixture of octadecylphosphonic and hexylphosphonic acids as surface ligands.5 The absorption spectrum (black line) in Figure 1b shows the absorption peak from the CdSe core at 575 nm and a more intense peak from the elongated CdS shell at 450 nm. The PL emission spectrum (red line) demonstrates their excitonic emission with a PL QY of 70%, located at 590 nm. TEM and HRTEM images in Figure 3134
DOI: 10.1021/acs.nanolett.7b00563 Nano Lett. 2017, 17, 3133−3138
Letter
Nano Letters
Figure 2. Characterization of one-dimensional fluorescent grating patterns. (a) Substrate containing six grating patterns with differing pitch/domain sizes in ambient light. (b) Optical microscope image of the same substrate, presenting two neighboring grating patterns separated by a dashed line with domain sizes of 50 (left) and 40 μm (right). (c) Diffraction image of a green laser (543.5 nm) after passing through the diffraction grating, showing zero, first and second order diffraction. (d) Schematic, highlighting the origin of dark (Imin) and bright (Imax) states, according to the orientation of the NRs in regard to the polarizer main axis. (e) Micrographs of fluorescent grating patterns with domain sizes of 40, 30, 20, 10, 5, and 2 μm. (f) Representative intensity profile, recorded from the 30 μm pattern, following Malus’ law. (g) Normalized polarization ratios (from 2.5−1), determined from the intensity profiles (panel f) of each grating pattern, shown in (e). Format is chosen to illustrate the relative drops in the polarization ratios. The films were excited with a 12 V/100 W halogen lamp through a 546/12 nm bandpass filter.
In order to confirm the NR alignment, we used an inverted fluorescence microscope (Leica DMI6000B) with a film polarizer in the optical pathway. Such a setup does not allow us to demonstrate the two phenomena of the absorption and emission anisotropy due to the simultaneous polarization of the excitation and emission. Nevertheless, the patterns are visible by both means. According to previous studies on CdSe/CdS core−shell NRs, the absorption of these materials is highly anisotropic.3,37 When excited with linearly polarized light at a defined θ, NRs which are oriented parallel to θ show maximum absorption and emission, whereas perpendicularly oriented NRs show only weak absorption and emission. This results in a fluorescence intensity difference between the NRs of different orientations, which is visible without any additional polarizers. When excited with nonpolarized light, the patterns are visible by the presence of a polarizer between the sample and the observer, due to the polarized emission (Figure 2d). The light intensity is described by Malus’ law, which is given by I = I0 cos2 θ + Imin, with I0 being the initial intensity and θ being the angle between the polarization of the emitted light and the axis of the polarizer (Figure 2d). Imin is included as an additional term because the emitted light is not perfectly polarized.
hydrophobic NR ligands (long alkyl chain phosphonic acid) and the more hydrophilic LCP units.22,34 In order to preserve the alignment, the composite film is finally exposed for 5 min to UV light to polymerize the LCP. The resulting solid film shows patterned domains with orthogonal NR orientations. Excitation of these films with nonpolarized light gives uniform emission intensities distribution with microscopically different polarization azimuths, as will be demonstrated later. Similar to previously reported photoaligned liquid crystal gratings,35 the NR/LCP thin films are transparent under ambient light (Figure 2a) with minimal haze (
