Three-dimensional binary superlattices of magnetic nanocrystals and semiconductor quantum dots

Transcription

1 Supplementary Information Three-dimensional binary superlattices of magnetic nanocrystals and semiconductor quantum dots F.X. Redl*, K.-S. Cho *, C.B. Murray*, S. O Brien *IBM, T. J. Watson Research Center, Nanoscale Materials and Devices, 111 Kitchawan Road, Route 134, Yorktown Heights, NY 1598, P.O. Box 218 Department of Applied Physics & Applied Mathematics, Columbia University, 2 SW Mudd Building, 5 West 12 th Street, New York, NY 127 Advanced Materials Research Institute (AMRI) in the University of New Orleans. Maghemite nanocrystals were prepared according to Hyeon et al 1. Trioctylamine was used as solvent instead of the reported dioctyl ether for the decomposition of iron pentacarbonyl. Also a fourfold reaction-scale was applied in order to get enough material for the screening process. The use of cheaper trioctylamine allows higher reaction temperatures and gives high yields of 11 nm -Fe 2 O 3 nanocrystals with a size distribution of 5% (RMS deviation) after removal of insoluble particles in hexane by centrifugation and precipitation of the majority of soluble products with ethanol. Solutions in hexane are stable over month and show no aggregation or precipitation. PbSe nanoparticle were produced by reacting lead acetate with selenide dissolved in trioctylphosphine 2. 6 nm particles with a size distribution of about 6% were obtained after one precipitation step. Concentrations were estimated

2 by drying and weighing of the material. For this purpose a weight percentage of 3 % oleic acid (obtained by elemental analysis for maghemite nanoparticles) was assumed for both materials. TEM grids (Ted Pella, 1814-F) were immersed into.1 ml of a diluted dibutyl ether (99.5%, Aldrich) solution of lead selenide and maghemite nanoparticles for the formation of bimodal structures. The ratio of the particles was adjusted to a tenfold surplus of smaller lead selenide particles. The used glass vials had about the threefold diameter of the grids. This arrangement was tilted by 3 and placed into a vacuum oven allowing only reduced exchange with the environment. The evaporation of the solvent at 6 C took place over night. Afterwards material at the backside of the grid was removed by cautious wiping. Finally it was dried in vacuum at 6 C. Images of the particles were taken on a Phillips CM12 Transmission Electron Microscope (TEM) in bright field mode at 12kV. Wide angle and small angel electron diffraction pattern were taken in selected area diffraction mode covering areas of about 6 m 2. UV/Vis/NIR spectra were obtained either from films on a sapphire substrate by drying of hexane/octane solutions or from dispersions in 1 cm quartz cuvettes using a Perkin Elmer lambda9 spectrometer after evaporation of the original solvent and extraction of excess oleic acid with acetone and ethanol and redispersing the material in tetrachloroethylene. ZFC, FC, and hystereses loops were obtained utilizing a Quantum Design MPMS 2 SQUID magnetometer and thin layers of iron oxide particles of a mixture deposited on a silicon-waver by evaporation of the solvent (hexane). Depicted superlattice structures in Figures 1,2, and 4 were generated with CrystalMaker 4.3 and Rasmol

3 Figure 3: UV/Vis/NIR spectra of a) 6 nm PbSe-NCs dispersed in tetrachloroethylene (C 2 Cl 4 ), b) 11 nm -Fe 2 O 3 NCs dispersed in C 2 Cl 4, c) a superlattice of 11 nm -Fe 2 O 3 on sapphire annealed at 2 C in air for 6 h (normalized to match with UV-absorption of b, d at 4 nm) a superlattice grown from a 13:1 mixture of 5 nm PbSe NCs and 11 nm -Fe 2 O 3 NCs on sapphire, e) a superlattice grown from 11 nm -Fe 2 O 3 NCs on sapphire. -values of graphs a and b are related to the formula units PbSe and -Fe 2 O 3. Additional oxidation of the -Fe 2 O 3 can be carried out by annealing the dry sample ( -Fe 2 O 3 and oleic acid) at 2 C in air for several hours, which leads to a stable but insoluble film, or by bubbling air through a solution of -Fe 2 O 3 and a surplus of lauric acid at C for several hours. The solubility of the product in hexane can be increased by addition of oleylamine.

4 Figure 4: TEM micrographs and sketches of AB 5 superlattices (isostructural with intermetallic phase CaCu 5, SG 191) of 11 nm -Fe 2 O 3 and 6.3 nm PbSe NCs. a) Depiction of the CaCu 5 structure as trigonal face-centred prism or layers. b) Hexagonal AB 5 unit cell. c) Depiction of the (1) plane. d) Projection of the (1) SL plane. e) Projection of the (1) SL plane at high magnification. f) Small angle electron diffraction pattern from a 6 m 2 (1) SL area.

5 Figure 5: Collection of low resolution TEM images showing phase-separated regions, random mixtures, phase-separated and binary superlattices of a mixture of 11 nm -Fe 2 O 3 nanocrystals and 6 nm PbSe nanocrystals (the scale bar applies for all facets).

6 11 nm -Fe 2 O 3 particles 5 (22) (311) 5 maghemite ( -Fe 2 O 3 ) reference magnetite (Fe 3 O 4 ) reference c) rel. intensity a) b) (111) (21) (211) (4) 1 (422) (511) (44) Si (1) (533) (731) (642) (8) sin( ) Figure 6: a) X-ray diffraction pattern of 11 nm -Fe 2 O 3 nanocrystals; b) scan with higher resolution and accumulation time; c) enlarged region showing the (311) reflection with matching maghemite reference. Maghemite and magnetite references are included. (Indexing according to JCPDS , Co )

7 arbitrary units a) b) M / (A m 2 kg -1 ) c) 4 d) c) d) T / K H / Oersted Figure 7: a) ZFC and FC of 11 nm -Fe 2 O 3 particle and b) of a random 13:1 ratio mixture of 11 nm -Fe 2 O 3 and 4 nm PbSe NCs (both at 5 Oersted); hystereses loops at 5 K of c) 11 nm -Fe 2 O 3 NCs and d) of a random 13:1 particle ratio mixture of 11 nm -Fe 2 O 3 and 4 nm lead selenide NCs. Inset: hystereses loops at 3 K (films on silicon substrate; corrected for 3 weight% of oleic acid based on elemental analysis; all particles are covered with oleic acid). The difference between the magnetic properties of the mixture and the pure maghemite nanocrystals were determined with Squid measurements (see Figure 7). Because of the currently small dimensions of the bimodal structure, random mixtures of 11 nm maghemite and 4 nm lead selenide particle were investigated. In order to prevent phase separation a concentrated hexane solution was used for fast film formation on a silicon waver (.5 cm 2 ). ZFC and FC measurements show that the blocking temperature of the iron oxide particle shifts from 15 K to 11 K and the ZFC gets narrower at the same time. This is due to an increased average distance between iron oxide nanocrystals caused by the dilution with lead selenide nanocrystals. Therefore magnetic dipole coupling has strong influence on the magnetic properties of the undiluted system 3. Additionally it rationalizes that a random mixture is obtained as opposed to phase

8 separation during the fast film casting. The mixture shows a slightly higher coercivity (37 Oersted) than the pure iron oxide (3 Oersted). 1. Hyeon, T., Lee, S. S., Park, J., Chung, Y. & Na, H. B. Synthesis of highly crystalline and monodisperse maghemite nanocrystallites without a size-selection process. J. Am. Chem. Soc. 123, (21). 2. Murray, C. B. et al. Colloidal synthesis of nanocrystals and nanocrystal superlattices. IBM J. Res. Dev. 45, (21). 3. Luo, W., Nagel, S. R., Rosenbaum, T. F. & Rosensweig, R. E. Dipole Interactions with Random Anisotropy in a Frozen Ferrofluid. Phys. Rev. Lett. 67, (1991).