Article pubs.acs.org/cm
Smectic Nanorod Superlattices Assembled on Liquid Subphases: Structure, Orientation, Defects, and Optical Polarization Benjamin T. Diroll,† Nicholas J. Greybush,‡ Cherie R. Kagan,†,‡,§ and Christopher B. Murray*,†,‡ †
Department of Chemistry, University of Pennsylvania, 231 S. 34th Street, Philadelphia, Pennsylvania 19104, United States Department of Materials Science and Engineering, University of Pennsylvania, 3231 Walnut Street, Philadelphia, Pennsylvania 19104, United States § Department of Electrical and Systems Engineering, University of Pennsylvania, 200 S. 33rd Street, Philadelphia, Pennsylvania 19104, United States ‡
S Supporting Information *
ABSTRACT: Directing the orientation of anisotropic nanocrystal assemblies is important for harnessing the shape-dependent properties of nanocrystal solids in devices. We control the orientation of smectic B superlattices of CdSe/CdS dot-in-rod nanocrystals through assembly on different polar interfaces and quantify the superlattice orientation through correlated small- and wide-angle grazing-incidence diffraction. Small-angle scattering is used to determine the phase of the nanorod superlattices and their preferential growth directions from the subphase. Wide-angle diffraction is used to quantify the orientations of nanorods within the superlattices and with respect to the substrate. Not only are the nanorod long axes aligned within the structures, but truncation of the short axes also coaligns the crystal axes of the nanorods with the zone axes in assembled smectic B crystals. Three dimensional orientational alignment of nanocrystals in superlattices is highly desirable in device applications. Depending on the subphase used for self-assembly, the films range from nearly quantitative vertical to horizontal alignment. Controlling for other variables, we find that the surface tension of the subphase is strongly correlated with the orientational ordering of the nanorod superlattices. The microstructure of nanorod superlattices shows many classic defects of atomic and liquid crystalline systems. The nature of defect structures supports a mechanism of nuclei formation on the subphase−solvent interface rather than in solution. Last, we demonstrate the relationship between optical absorption polarization and superlattice structure using correlated optical spectroscopy and electron microscopy.
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large quasi-Stokes shifts exploited in luminescent solar concentrators.43 Assemblies of hard rods have theoretical structures including both nematic and smectic assemblies.44,45 Under different conditions, many different phases have been observed from uncharged colloidal NRs self-assembled from hydrophobic solutions: low density end-on-end46 or side-by-side phases,47,48 a higher-density nematic phase, 7,33,35,49−52 and smectic phases.6,7,35,51,53−60,35,61 Particles self-assembled in droplets also exhibit smectic assembly into superparticles having spherical and needle shapes.62 Here, we show the formation of polycrystalline films of NR superlattices formed from CdSe/ CdS dot-in-rod NCs using self-assembly on polar liquid surfaces with varying dielectric constants, viscosities, and surface tensions. Individual superlattices reach several micrometers in average size, and because the samples are selfassembled membranes on liquid surfaces, they can be placed onto arbitrary substrates.
elf-assembly of anisotropic nanocrystals (NCs) into singlecomponent and binary crystal structures has generated a substantial increase in the variety of accessible crystalline and liquid crystalline structures achievable using hard materials.1−19 Shape-dependent modulation of interactions using fields, block copolymers, rubbing, or epitaxial surfaces expand even further the accessible phase space and possibilities for epitaxial growth.16,20−25 Anisotropic, particularly uniaxial, assemblies have potential advantages as active layers in optoelectronic devices arising from their polarized electronic structure. Nanorods (NRs) offer a new building block for the diverse applications of liquid crystals.26 In excitonic photovoltaics, NRs or nanowires (NWs) bridge the length-scales necessary for efficient exciton separation (tens of nanometers) and charge collection (hundreds of nanometers) combined with reduced reflection losses.27−30 In planar geometries, such as lightemitting applications, polarized emission from NR or NW solids obviates the need for polarizing elements and provides opportunities for enhanced light capture.31−36 The unique electronic structure of CdSe/CdS dot-in-rod NCs used in this study has also been implicated in low-threshold stimulated emission and lasing,37−41 anomalously slow spin-flipping,42 and © XXXX American Chemical Society
Received: January 28, 2015 Revised: March 4, 2015
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DOI: 10.1021/acs.chemmater.5b00355 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials
Figure 1. a) TEM image of NRs self-assembled on DMA. (b) GISAXS and (c) GIWAXS of the same film. (d) TEM, (e) GISAXS, (f) and GIWAXS of NRs self-assembled on DEG. (g) TEM, (h) GISAXS, (i) and GIWAXS of NRs self-assembled on water. GISAXS data is plotted in colors according to a logarithmic scale; GIWAXS is plotted on a linear color scale.
Although vertical and horizontal alignment have often been addressed as if they were separate phases, rather than distinct orientations, diffraction data demonstrates that these projections are indeed the same phase. We also found that specific zone axes of the two-dimensional smectic B layers oriented preferentially with respect to the subphase plane. In addition to characterizing the average assemblies formed by NRs using Xray methods, we also analyzed the microstructure and common defects observed in NR superlattices which include defects common in both crystalline and liquid crystalline systems. Complementary to diffraction data, defects provide important information about the pathway and energy landscape of NR self-assembly and suggest that self-assembly nucleates at the liquid−liquid interface. Finally, we performed correlated optical and electron microscopy measurements of individual NR superlattices that show structure-dependent polarized optical properties.
We performed transmission electron microscopy (TEM) and grazing-incidence small- and wide-angle X-ray scattering (GISAXS and GIWAXS, respectively) to describe the structure of the NR superlattice films. In particular, we used correlated measurements of small- and wide-angle scattering to ascertain the interparticle structure and NR orientation within the superlattice structure. In addition to the expected coalignment of the liquid crystal director with the long NR axis, we found that side-wall truncation of the NRs restricts the rotational freedom of NRs within the hexagonal monolayers of smectic B lamellae. That is, NR superlattices have three-dimensional order along all axes of the underlying NR building blocks, which is a crucial order parameter in theoretical treatments of excitonic or electronic transport.63,64 Quantitative determination of NR orientation was achieved by carefully correcting GIWAXS data for the intersection of the flat detector with the Ewald sphere.65,66 Depending on the subphase used in selfassembly, NR films varied between nearly quantitative vertical (homeotropic) and horizontal (heterogeneous) alignment. B
DOI: 10.1021/acs.chemmater.5b00355 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials
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RESULTS AND DISCUSSION Structure and Orientation of Nanorod Films. The NRs used in this study have an inorganic size of 28.4 nm in length and 5.8 nm in diameter, and they have an organic capping layer of octadecylphosphinic acid. (See Supporting Information Figure S1.) Films of self-assembled NRs were made by casting concentrated hexanes solutions (30 mg/mL) onto polar liquids and restricting the evaporation rate of the solution following previous work from our group.67 Similar work was previously shown to generate large-area liquid-crystalline assemblies of colloidal NRs and LEDs.35,61In this work, we explored selfassembly on polar liquid surfaces including glycerol, water, ethylene glycol (EG), diethylene glycol (DEG), triethylene glycol (TEG), tetraethylene glycol (TetraEG), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), dimethylacetimide (DMA), formamide (FA), and acetonitrile (ACN). Although all of these liquids are polar, they have variable dielectric constants, surface tensions, and viscosities, which are catalogued in Supporting Information Table S1. Other factors which affect the quality and morphology of self-assembled structures, including temperature, dispersing solvent, concentration, monodispersity, aspect ratio, and free ligand concentration, were controlled by casting all samples from the same stock dispersion at room temperature. Differences, if any, in the selfassembly properties of the films are thus understood to arise from the subphase. In contrast, self-assembly at the liquid−air interface or in the solution volume should be insensitive to subphase.56,68,69 Deposition and restricted evaporation produced ordered NR assemblies which can be classified into several structures (Supporting Information Figure S2), but primarily the smectic B structure. Ordering of the NRs in the film is characterized using three measurements: real-space imaging by TEM, GISAXS, and GIWAXS. TEM images are useful but highly local guides to the morphology of thin films. TEM also suffers from the fact that two-dimensional projections are not always simple to reconstruct into the three-dimensional solid. The small-angle scattering data measures the inter-NR structure (q < 0.25 Å−1) and is a powerful tool to distinguish ordering of NR superlattices in glassy, smectic A, smectic B, smectic C, and hcp-type assembly. GIWAXS data, covering q from 0.5 Å−1 to 2.5 Å−1 in our experiments, examines the NR orientations, but raw data is subject to a number of distortions due to the greater curvature of the Ewald sphere at wide detection angles.65 (This phenomenon is discussed in more detail below and shown in Supporting Information Figure S3.) Carefully corrected, GIWAXS is a powerful tool for determining the orientation of samples with respect to the substrate plane and, if correlated with GISAXS, the superlattice structure. Depending on the subphase material that was used for the self-assembly process, we observed different degrees of crystallinity and orientation. Figure 1 shows the three extreme scenarios observed: short-range ordering and weak orientation preference; strong vertical orientation of hexagonal superlattices; and strong horizontal orientation of smectic superlattices. These orientations are demonstrated in TEM, GISAXS, and GIWAXS data from films self-assembled on DMA (Figure 1a−c), DEG (Figure 1d−f), and water (Figure 1g−i). In the case of DMA, hexanes solutions rapidly mix into the subphase, resulting in kinetically trapped NR glasses51 with paracrystalline ordering of NR bundles, evidenced from the TEM micrograph (Figure 1a). These bundles have no strong orientation with
respect to the plane of assembly. Figure 1b,c shows smooth rings of intensity in the GISAXS and GIWAXS data, respectively. Even in this low-crystallinity sample, Figure 1b shows weak two-dimensional hexagonal packing from NR bundles evidenced from the (10), (11), and (20) rings at q = 0.091 Å−1, 0.156 Å−1, and 0.182 Å−1, respectively. The organization of NRs is quite different for those subphases in which the partition of hexanes into the subphase is much slower than the rate of evaporation. Figure 1d shows a vertical, hexagonally packed, multilayer array of NRs selfassembled on a DEG subphase. GISAXS data in Figure 1e shows intense spots along the reflected beam horizon representing the (10), (11), and (20) planes of a twodimensional hexagonal sheet with a unit cell parameter a = 7.9 nm. This corresponds to a spacing of approximately 2 nm between the particles, consistent with the octadecylphosphonic acid surface-terminating ligands. Evidence of multilayer end-onend stacking is observed from diffracted intensity close to the beam stop of spacings at integer values of q ≈ 0.02 Å−1, consistent with a smectic B lamellar spacing of 31 nm. This suggests that regions of hcp packing, for which the (001) and (003) reflections are forbidden, do not dominate vertical ordering although they are occasionally observed by TEM. Although diffraction from vertically close-packed NRs is observed along qz, this represents a much smaller fraction of the sample than the scattering along qx, which is obscured under the horizon. Transmission SAXS measurements of the same film show in-plane hexagonal ordering and no evidence of interlamellar reflections in the sample plane (Supporting Information Figure S4). This strong orientation preference is confirmed from GIWAXS data. Figure 1f shows a strong (002) reflection from CdS at q = 1.86 Å−1, which corresponds to the long-axis of the NR, near χ = 0° (defined along qz), as expected for vertically aligned rods. The complementary CdS (100) reflections at q = 1.75 Å−1 appear strongest near χ = 90°. The CdS (101) reflection at q = 1.98 Å−1 maximizes at χ ≈ 62°, and the CdS (102) reflection at q = 2.56 Å−1 maximizes at χ ≈ 43° consistent with the angle between the (100), (002), (101), and (102) planes of wurtzite CdS. Although the CdSe/CdS dot-inrods contain CdSe, no diffraction is observed from the CdSe cores because they represent
