1. Introduction

The self-assembly of metallic nanomaterials into functional superstructures in two- or three-dimensional space by way of the “bottom-up” method is of importance for the nano-fabrication technology of optical devices, such as polarization components, information storage devices [1] and optical metamaterials [2,3]. Several methods have been used to assemble nanoparticles into different scales, such as polymer matrix [4,5], nanostructured surface [6], and DNA conjugation [7]. Liquid crystal (LC) is a fascinating medium that can be utilized to assemble the micro- or nanoparticles into pre-engineered structures by controlling the assembly in one or more dimensions through combining a great variety of mesophase morphologies and giving easy responses to external stimulated field or boundary conditions [8–11]. Different nanostructured materials of a variety of shapes such as metallic nanoparticles [8,12], semiconductor nanorods [13], and carbon nanotubes [14] have been organized into structures to exhibit specific nano- and micro-scale arrangements in their LC host by taking advantage of the orientational order of LCs. Recently, the self-assembly of gold nanorods (GNRs) in nematic, hexagonal and cholesteric mesophases has been demonstrated, and the alignment of GNRs could be achieved by shear stress and magnetic fields [15,16]. However, these methods are limited to either low order parameter of GNRs or complex facilities. A more general method for aligning GNRs on a large scale remains a challenge.

Here we demonstrate that the colloidal GNRs are designed to orient in a pre-defined order on a large scale embedded in a LC matrix. The LC host is aligned via the surface-mediated force of rubbing the surface, which then forces GNRs to align perpendicular to the directors in the bulk. This aligned hybrid GNRs-LC system shows strong polarization-dependent extinction and scattering on a large scale. We characterize the alignment of GNRs and their optical properties by polarized optical microscopy, dark-field microscopy and polarized extinction/scattering spectra. This technique for organizing nanoparticles is compatible with the state-of-the-art LC display technology and is of potential importance in fabrication of large-area optical devices via self-assembled nanomaterials in information display, optical metamaterials and polarized plasmonic components.

2. Experiment

The GNR-LC composites are constituted of GNRs in a N_c phase. GNRs with a mean width of 20 nm and a mean length of 50 nm were synthesized using a seed-mediated method [17]. The concentration of GNRs is 0.3 nM given by the inductively coupled plasma mass spectrometry (ICP-MS) measurement. Thiol-terminated methoxy-poly(ethylene glycol) (mPEG-SH, JemKem Technology) was used to functionalize GNRs in order to stabilize them [18]. First, the dispersion of GNRs was centrifuged at 9000 rpm (corresponding to 7516 × g) for 20 min and then resuspended to 1 mL of deionized water. Then, 250 μL of an aqueous solution with 2 mM of 5 kDa mPEG-SH were added into 5 mL of a 3.5 nM GNRs dispersion. The mixture was left overnight and then purified via centrifugation to eliminate the excess mPEG-SH. The nematic lyomesophase was made by the sodium decylsulfate/decanol/water (SDS/DeOH/H₂O) ternary system according to the phase diagram in [19] and [20]. Briefly, the hybrid GNRs-LC composites were prepared by adding 38.2 wt% SDS, 7 wt% DeOH and 54.8 wt% GNRs aqueous solution to a centrifuge tube successively and mixing them quickly. The mixture was under sonification and centrifuge several times to unify the compounds. The final concentration of GNRs is about 6 wt % (3 × 10⁻⁷ M) calculated by the total weight of GNRs-LC composites and the original concentration of mPEG-GNRs. The GNRs-LC system shows hexagonal and nematic phases (calamitic, discotic and biaxial) when the temperature varies and the phases are confirmed by their optical textures. Several experiments have shown that the building block of this ternary lyomesophase is biaxial micelle [21–23]. The phases differ from each other by orientational fluctuations around different axes of symmetry of these micelles.

The surface-imposed alignment of lyotropic LC in N_c phase is based on the fact that a rubbing surface can induce a preferential orientation of the micelles [24,25]. The surface treatment creates an easy axis for biaxial micelles, and the far-field director N₀ of LC is along the rubbing direction. We spin-coated a polymethyl methacrylate (PMMA) film 500nm thick on a glass plate. The unidirectional alignment of LC was achieved by rubbing the PMMA film with soft velvet or lens paper 20 times. The LC cell was made by two pieces of glass which were covered with such alignment layers in the inner sides. The LC cell was 5-20μm thick separated by SiO₂ spacers. The GNRs-LC composite was then injected into the LC cell and the cell was then sealed by UV glue.

Polarized optical microscopy (POM) and polarized dark-field microscopy were used to characterize the alignment of LC and GNRs, respectively. An Olympus BX-51 mounted with a CCD camera (company) with 10 × , 20 × , 50 × and 100 × (oil immersion) objectives were used. A microscope mounted with a microspectrometer (USB2000, Ocean) was used to measure the polarization-dependent extinction spectra. The experimental setup for the transmittance measurements consists of an incident light from a halogen tungsten lamp, crossing a bright-field condenser, the GNRs-LC sample, a polarizer and finally a spectrometer. The scattering spectra were measured with a similar method but with a dark-field condenser instead of a bright-field condenser.

3. Results and discussion

The uniform alignment of calamitic nematic (N_c) lyotropic LC phase by surface-induced force was characterized by Polarized optical microscopy (POM) observation. The texture of LC appears to be totally dark when the rubbing direction k is parallel or perpendicular to the polarization of incident light P and brightest when k is oriented at 45°with respect to P, in Figs. 1(a) and 1(b). The orientation of the director of LC (N) is confirmed by inserting a phase compensator (530nm λ−plate) into the POM microscope (Fig. 1(c)). Because the N_c phase is optically negative ($n_e<n_o$) [20], the blue interference color shows that N is along k in the laboratory coordination, in Fig. 1(c). Thus, in the presence of micro-grooves by rubbing the surface, N is parallel to k, while without any treatment of the surface layer N only tends to orient in the plane parallel to the substrate surface under the influence of the surface.

 

Fig. 1 (a, b) Polarized optical microscopy images of GNR-doped LC for two different rubbing directions (indicated by k) between crossed polarizers (indicated by (P) and (A), showing high-quality long-range alignment. (c) Polarized optical microscopy images of GNR-doped LC with a phase compensator (530nm λ−plate, slow axis γ) inserted to the POM, showing the alignment of nematic LC is along the rubbing direction. The scale bars are 50μm.

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The orientation of N_c near the surface is elastically transferred to the bulk of the N_c phase and the whole LC sample is oriented at N along k. Because of the alignment of GNRs in the LC matrix, the collective surface plasmon resonance (SPR) is excited and the sample shows strong polarization and wavelength dependent extinction. These extinction spectra were measured by micro-spectrometer mounted in the microscope with a analyzer (see Fig. 2(b)). The GNRs-LC composite shows a red color when P‖N (or k), while it becomes green when P⊥k, in Figs. 2(c)-2(d). By means of this method, GNRs in LC can be aligned in a device-scale (on the order of inches) and is shown in the insets of Figs. 2(c) and 2(d). Based on localized SPR theory of nanorods, when the polarization of incident light is parallel to the long axis of a nanorod, the longitudinal SPR mode is excited and the nanorod absorbs and scatters light strongly, while the transverse SPR mode is excited when the polarization of incident light is perpendicular to the long axis of nanorod. Surprisingly, the long axis of GNRs in the N_c matrix is perpendicular to N (or k) in the bulk. The optical observation along with rotational symmetry reveal that GNRs are oriented in the plane perpendicularly to N there is no preferred orientation of GNRs in the plane, as illustrated in Fig. 2(a). This alignment is also observed in the cholesteric phase of nematic lyomesophase and can be explained by the phenomenological model that has been established [16]. This alignment can be understood based on the absorption and anchoring of rod-like building-block of LC by the flexible polymer chains capped on GNRs [26]. The nematic lyotropic LC is composed of biaxial micelles and represented by three symmetry axes. When the micelles form N_c phase, the orthorhombic micelles fluctuate around their longest axis and degenerate to rod-like shape [21–23]. The flexible chains of mPEG radiate from the surface of GNRs in the aqueous solution because water is good solvent for mPEG. Then the rod-like micelles will absorb and insert into this polymer chains, which leads to the rod-like micelles orienting perpendicular to the surface of GNRs. Because there are more polymer chains on the side face of GNRs than the ends, this results in the long axis of GNRs orienting perpendicular to the director of N_c. Combining with the flexibility of the polymer chains, and the minimization of the anchoring energy of micelles on the GNRs surface along with the uniform alignment of the far field director N₀ lead to the long axis of GNRs being perpendicular to N₀ (or k).

 

Fig. 2 (a) Schematic illustration of GNR alignment with a negative order parameter in the nematic LC. Red lines indicate the uniform director field of LC host. (b) The experiment setup used to measure the polarization-dependent extinction spectra of aligned GNR-LC composites. (c,d) The transmission optical microscopy images obtained for unpolarized light passing first through the cell with aligned GNRs and then through a linear polarizer (P) either perpendicular (c) or parallel (d) to the rubbing direction k. The insets of (c) and (d) show the images of the whole sample (in a centimeter scale) in two orthogonal cases. The scale bars are 50μm.

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When the polarization of incident light is in the intermediate angles between the long and short axes of GNRs, the aligned GNRs show angular dependence of extinction, in Fig. 3(a). The intensity of extinction varies from the minimum when P‖k to the maximum when P⊥k. The imperfect suppression of the longitudinal peak when P‖k derives from the limited order parameter of GNRs in N_c. The angular dependent extinction coefficient is shown in Fig. 3(b). The scalar order parameter of GNRs is defined as $S_{GNR}=〈P_2(\cos \theta )〉$, where $P_2(\cos \theta )$ is the second Legendre polynomial and θ is an angle between a nanorod and N. In this case GNRs show a negative order parameter with respect to N. The alignment of GNRs could be characterized by its scalar order parameter, which is calculated as , where ${\alpha }_{\parallel }^{ext}$ and ${\alpha }_{\perp }^{ext}$ are the extinction coefficients of the GNRs-LC composites when P‖k and P⊥k, respectively [27]. By using and at the wavelength of the longitudinal SPR peak, we obtain S_GNR = −0.35 ± 0.01, while the order parameter of GNRs with a perfect negative order is −0.5. The order parameter of GNRs could be further improved to approach −0.5 by decreasing the thickness of LC cell or using GNRs with a larger aspect ratio.

 

Fig. 3 (a) The measured polarization-sensitive extinction spectra of GNRs for different angles between the linear polarizer and the rubbing direction. (b) The measured extinction value at the longitudinal SPR peak as a function of the angle between the linear polarizer and the rubbing direction.

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The dark-field microscopy possesses the ability to directly visualize the orientation of single GNR by detecting the polarization-dependent scattering of GNR. The intensity of scattering of GNRs is maximized when the polarization of incident light is parallel to the long axis of GNRs, while it becomes minimal when the polarization of incident light is parallel to the short axis of GNR. In the dark-field microscopy observation of aligned GNRs-LC composites, the intensity of the scattering of single GNR is at a maximum when P⊥k, while it is at a minimum when P‖k, in Figs. 4(a) and 4(b) The dark-field microscopy observation confirms the orientation of GNR perpendicular to the director of LC. The angular-dependent scattering spectra in Fig. 4(c) are obtained by averaging signals over the individual GNRs. The intensity of scattering varies from the minimum (when P‖k) to the maximum (when P⊥k). The scattering coefficient with respect to the angle of P and k are shown in Fig. 4(d). The dark-field scattering spectra of individual GNRs verify the negative order parameter of GNRs in LC host.

 

Fig. 4 The dark-field scattering optical microscopy images obtained for unpolarized light passing first through the cell with aligned individual GNRs in LC and then through a linear polarizer (P) either (a) perpendicular or (b) parallel to the rubbing direction k. (c) The measured polarization-sensitive scattering spectra of GNRs for different angles between the linear polarizer and the rubbing direction. (d) The scattering at the longitudinal SPR peak as a function of the angle between (P) and k. The scale bars are 10μm.

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4. Conclusions

In conclusion, in this work we achieve a bulk alignment of GNRs in nematic lyotropic LC via surface treatment. The aligned GNRs with considerable high concentration (about 6 wt%) show a strong polarization-dependent extinction and scattering. The alignment of colloidal GNRs in a controllable way on a large scale is achieved for the first time and is of interest for fundamental research and technological application. This method, together with applied external magnetic field, is potentially useful for further study of the detailed interaction of nanoparticles with the micelles and can be applied to align various types of nanoparticles in LC hosts. This useful and rather simple method of alignment is compatible with current liquid crystal display technology and potentially useful in electro-optical devices. For example, the GNRs-LC composite can be used as an E-type polarizer, which transmits electric field component parallel to the direction of alignment and absorbs electric field component perpendicular to the direction of alignment [28]. This method of surface-mediated alignment can be extended to fabricate more refined plasmonic devices range from the nanoscale to microscale by more sophisticated methods, such as changing the polymer surface morphology using atomic force microscope [29,30], switching photoresponsive molecular surface monolayer by polarized laser light [31,32], etc. In addition, based on different optical constants and aligned patterns of GNRs derived from the SPR effect, this method can potentially be used to fabricate optical metamaterials, optical cloaking, aligned nano-antenna arrays and plasmonic gratings.