Supporting Information

Transcription

1 Supporting Information Macroscale Lateral Alignment of Semiconductor Nanorods into Freestanding Thin Films Tie Wang, 1 Xirui Wang, 1 Derek LaMontagne, 1 Zhongwu Wang, 2 and Y. Charles Cao 1* 1 Department of Chemistry, University of Florida, Gainesville, FL 32611, USA. 2 Cornell High Energy Synchrotron Source, Wilson Laboratory, Cornell University, Ithaca, NY 14853, USA. * To whom correspondence should be addressed: cao@chem.ufl.edu (Y.C.C.) This PDF file includes: Materials and Methods Figures S1 to S5 Tables S1 and S2 References (S1-S6) S1

2 1. Chemicals. Trioctylphosphine oxide (TOPO, 99%), trioctylphosphine (TOP, 97%), tributylphosphine (TBP, 97%), tetraethoxysilane (TEOS 99%), sulfur (99%), octylamine (99%), dodecylamine (99%), 1,12-diaminododecane (98%), poly(vinylpyrrolidone) (PVP, MW=55,000), and ethylene glycol (99%) were purchased from Aldrich. Cadmium oxide (CdO, 99.99%), selenium (Se, 200 mesh, 99,99%), and dodecyl trimethylammonium bromide (DTAB, 97%) were purchased from Alfa Aesar. Octadecylphosphonic acid (ODPA, 99%) and hexylphosphonic acid (HPA, 99%) were purchased from Polycarbon Inc. Polydimethylsiloxane (PDMS, Sylgard 184 silicone elastomer) was purchased from Dow Corning. Dithiol-functionalized Tween-20 (Tween- 20-SH) was synthesized according to a literature method (S1). Nanopure water (18 MΩ cm) was made by a Barnstead Nanopure Diamond system. All other solvents were purchased from Fisher Scientific International, Inc. 2. Synthesis of 77.0-nm long CdSe/CdS core/shell nanorods. CdSe/CdS nanorods were prepared through a seed-growth method according to literature methods (S2,S3). CdSe nanocrystal seeds (2.7 nm) were used to grow CdSe/CdS nanorods (length: 77.0-nm; diameter: 4.5 nm, Figure S1). After growth at 350 o C, reaction solutions were cooled to room temperature, and then nanorods were precipitated using a mixture of toluene and methanol (1:4, volume ratio). The precipitate was redispersed in a toluene solution containing 8% octylamine and then was incubated for 20 min for nanorod surface treatment. Next, the nanorods were purified through three precipitation/redispersion S2

3 cycles using methanol and hexane, repsectively. The resulting nanorods are highly dispersible in nonpolar organic solvents such as hexane and toluene. Figure S1. TEM images of CdSe/CdS nanorods (l = 77.0±2.1 nm, d = 4.5±0.3 nm). 3. Synthesis of superparticles of double-domed cylinders. Double-domed cylinder superparticles were synthesized using a literature method with minor modifications (S4). In a typical synthesis, a chloroform solution containing 77.0 nm CdSe/CdS nanorods was thoroughly mixed with a DTAB solution (20 mg/ml, 1 ml) using a vortex mixer. Afterwards, the chloroform was removed from the mixture by bubbling Ar at 40 C, yielding a clear, yellow nanorod-micelle solution. Under vigorous stirring, this nanorodmicelle solution was injected into a three-neck flask with ethylene glycol (5.0 ml), and the mixture solution was further stirred at room temperature for 10 min. Then an aqueous solution containing Tween 20-SH (0.1 mm, 1 ml) was injected into the flask, and was stirred for 1 h. Colloidal superparticles were isolated from the growth solution using centrifugation (3300 rpm, 5 min). The yellow precipitate was redispersed in ethanol and the superparticles were further purified twice by centrifugation. The resulting S3

4 superparticles are highly dispersible in polar solvents such as water, ethanol, and ethylene glycol. The average diameter of the double-domed cylinders superparticles is (3.48±0.63) 100 nm, the average length is (4.31±0.76) 100 nm, and the average volume is 4.0± nm Synthesis of needle-like superparticles from CdSe/CdS nanorods. In a typical synthesis, 1,12-diaminododecane (0.2 µl) was added into a chloroform solution containing CdSe/CdS nanorods (10 mg/ml, 1 ml). Then, the resulting nanorod solution was thoroughly mixed with a DTAB solution (20 mg/ml, 1mL) using a vortex mixer, followed by Ar bubbling to evaporate chloroform at 40 C, yielding a clear, yellow nanorod-micelle aqueous solution. Under vigorous stirring, the nanorod-micelle aqueous solution was injected into a three-neck flask with ethylene glycol (5.0 ml). The mixture solution was further stirred at room temperature for 10 min, and then an aqueous solution containing Tween 20-SH (0.1 mm, 1 ml) was injected into the flask, and was stirred for additional 1 h. The resulting colloidal superparticles were isolated from the growth solution using centrifugation (3300 rpm, 5 min). The yellow precipitate was redispersed into ethanol and the superparticles were further purified twice by centrifugation. Asprepared superparticles are highly dispersible in polar solvents, and indefinitely stable in solvents with strong polarity such as water and ethanol. The superparticles exhibit a needle-like shape with an average diameter of (2.0±0.43) 100 nm, and an average length of (1.3±0.49) 1000 nm, an average volume of 4.8± nm 3. The photoluminescence quantum yield of a typical superparticle sample is ~40% determined using Rhodamine 6G as the reference (S5). For the comparison, dodecylamine (0.2 µl) was added into a chloroform solution containing CdSe/CdS nanorods (10 mg/ml, 1 ml) to replace 1,12-diaminododecane. Then, the resulting superparticles solution was washed three times for TEM imaging (Figure S5). S4

5 Figure S2. Schemes of the atomic packing structures of a CdS crystal at (A) the (101 0), (B) (112 0), and (C) (0001) faces, where Cd atoms are labeled as grey balls and S atoms are labeled as green balls. Characteristic interatomic distances are labeled in each image. 5. Small-angle XRD measurements. Small-angle X-ray diffraction spectra were measured inside a diamond anvil cell (DAC) where two well-aligned diamond anvils have a culet size of 0.5 mm. Double-domed cylinders and needle-like superparticles of ~1 mg were used in a typical measurement, and the measurements were performed at room temperature using an angle dispersive synchrotron X-ray source at the B 2 station of Cornell High Energy Synchrotron Source (CHESS). A double-bouncing Ge (111) monochromator converts incident white X-rays to a monochromatic beam at an optimized energy of 20.0 kev (equivalent to a wavelength of Å). Table S1. d-spacing and lattice constants of hexagonal lattice calculated from SAXS pattern (Figure 1g)** Double-domed cylinders Needle like superparticles 10 (hexagonal) d (10) = 5.6 nm a = 6.5 nm d (10) = 5.7 nm a = 6.6 nm 11 (hexagonal) d (11) = 3.3 nm a = 6.6 nm d (11) = 3.4 nm a = 6.8 nm 20 (hexagonal) d (20) = 2.8 nm a = 6.5 nm d (20) = 2.9 nm a = 6.7 nm Average a = 6.5±0.1 nm a = 6.7±0.1 nm ** The exact peak positions were determined via peak fitting (OriginPro 8.5). The constant of the hexagonal lattice (a) was calculated using the equation (S6): S5

6 1 4 hk k 3 h d a Table 2. The constant of the lamellar structure (l) was calculated (d 00m = l/m) from ED data (Figure 1c): Needle-like superparticles 01 d (01) = 80.1 nm l = 80.1 nm 02 d (02) = 40.1nm l = 80.2 nm Average l = 80.2 nm S6

7 Figure S3. (a) Scheme of double-domed cylinders superparticle formation. (i) Embryo formation, (ii) colloidal crystallization at an equilibrium condition, (iii) double-domed cylinder formation. (b) Scheme of needle-like superparticle formation. The bonding of 1,12-dodecanediamine on the side of nanorods causes the top/bottom of the nanorods to have higher solvophobicity than the side faces during nanorod-micelle decomposition. This, in turn, leads to rapid growth of supercrystalline domains along the nanorod long axes and results in the formation of needle-like superparticles. (i) Embryo formation, (ii) colloidal crystallization under non-equilibrium conditions, and (iii) needle-like superparticle formation. 6. Preparation of freestanding polymer thin films with unidirectionally aligned needle-like superparticles. As a template, a line-patterned Si 3 N 4 substrate of 1.0 cm 2.0 cm (groove width = 1 µm, depth = 0.6 µm, gap = 1 µm) or an optical grating of 5.0 cm 5.0 cm (groove width = 1.5 µm, depth = 0.6 µm, gap = 0.5 µm) were cleaned under sonication for 15 min in each of the following solvents: hexane, chloroform, and acetone, S7

8 and then were rinsed with ethanol and dried under Ar flow. Next, an ethanol solution containing tetraethoxysilane (TEOS, 40 % in volume) and needle-like superparticles (100 µl, 2 mg/ml) was dropped onto the surface of a template. The droplet was forced to spread back and forth on the template using a shaker of 1 Hz until the solution was totally dried. The superparticle-containing template was washed using ethanol three times to remove free superparticles that were not inside the template channels, and then was dried using Ar. In the next step, a template was immersed with a solution contain polydimethylsiloxane (PDMS) elastomer and curing agent (with a 10:1 volume ratio) in a 20-mL glass petri dish, and then was annealed in an oven at 100 ºC for 1 h. After cooling to room temperature, a uniform, free-standing PDMS film was peeled off the template, and the PDMS film was characterized using SEM and fluorescence microscopy (a TE Nikon EZ-C1 confocal microscope). 7. Photoluminescence (PL) polarization measurements. The polarized PL emission from a needle-like superparticle embedded PDMS thin film was recorded with a fluorometer (Fluorolog-3, Horiba Jobin-Yvon, Irvine, California, USA). The samples were excited with a circularly polarized light at 380 nm generated via a linear polarizer and a quarter wave plate (Figure S4). The linearly polarized PL emission from the film was measured through a linear polarizer and a quarter wave plate mounted on a stage for polarization rotation, and the resulting circularly polarized light was measured by a fluorometer detector (Figure S4). S8

9 Figure S4. Scheme of the PL measurement setup. Inset: linearly polarized PL emission spectra measured from a typical freestanding PDMS film embedded with unidirectionally aligned CdSe/CdS nanorods at varying polarization angles. 8. Energy down-conversion LED panel. A light panel consisting of 10 LEDs (λ = 380 nm, bandwidth (FWHM) = 12 nm, P = 20 mw, Super Bright LEDs, Inc., St. Louis, Missouri, USA) was built and covered with two pieces of superparticle-embedded PDMS thin films with nanorod-alignment directions at a 90 o angle. Under the excitation of LED lights, orange emission lights can been seen through a 500-nm cutoff filter. Optical images were taken using a Nikon D3000 camera with a polarizer under different angles, and the optical images are shown in Figure 3d-3g in the main text. S9

10 Figure S5. TEM image of nanorod superparticle made from dodecylamine treatment. S10

11 References and Notes S1. H. Wu, H. Zhu, J. Zhuang, S. Yang, C. Liu, Y. C. Cao, Angew. Chem. Int. Ed. 47, 3730 (2008). S2. L. Carbone et al., Nano Lett. 7, 2942 (2007). S3. A. A. Lutich, C. Mauser, E. D. Como, J. Huang, A. Vaneski, D. V. Talapin, A. L. Rogach, J. Feldmann, Nano Lett. 10, 4646 (2010). S4. J. Zhuang, A.D. Shaller, J. Lynch, H. Wu, O. Chen, A. D. Q. Li, Y. C. Cao, J. Am. Chem. Soc. 131, 6084 (2009). S5. J. N. Demas, G. A. Crosby, J. Phys. Chem. 75, 991, (1971). S6. Th. Hahn, International tables for crystallography, Volume A (D. Reidel Publishing: Boston, 1983). S11