Langmuir-Blodgett nanorod assembly.

Techniques for directing the assembly of metal or semiconductor quantum dots into novel superstructures have been extensively pursued over the past decades.1-3 Recent interest has been drawn toward 1-dimensional nanoscale building blocks such as nanotubes, nanowires, and nanorods.4-14 If these one-dimensional nanoscale building blocks can be ordered and rationally assembled into appropriate 2-dimensional architectures, they will offer fundamental scientific opportunities for investigating the influence of size and dimensionality with respect to their collective optical, magnetic, and electronic properties, as well as many other technologically important applications. Currently, efforts have been focused on the development of new synthetic methodologies for making nanorods with uniform sizes and aspect ratios.4-14 Few studies addressed the organization of these anisotropic building blocks except the 3-dimensional spontaneous superlattice formation of BaCrO4, FeOOH,13 CdSe,4 Ag,6 and Au12 nanorods. Herein, we report 2-dimensional nanorod monolayer assembly using the Langmuir-Blodgett technique. Pressure-induced isotropic-nematic-smectic phase transitions as well as transformation from monolayer to multilayer nanorod assembly were observed. Uniform BaCrO4 nanorods were prepared by using published procedures.7 Briefly, Barium bis(2-ethylhexyl)sulfosuccinate (Ba(AOT)2) reverse micelles were added to sodium chromate (Na2CrO4)-containing NaAOT microemulsion droplets, to give final molar ratios of [Ba]:[CrO4] ) 1 and water content [H2O]: [NaAOT] ) 10. The as-made yellow precipitate consists of ribbonlike and rectangular superstructures made of uniform nanorods. The nanorods were uniform in length (∼20 nm) and diameter (∼5 nm). Energy-dispersive X-ray analysis and electron diffraction patterns indicated that the nanorods were single crystalline BaCrO4 with an orthorhombic unit cell (a ) 0.91 nm, b ) 0.55 nm, c ) 0.73 nm). These as-made nanorods generally are stabilized with AOT surfactant molecules. They were diluted and redispersed into isooctane to make a stable nanorod colloidal suspension, which is used as stock solution for subsequent Langmuir-Blodgett studies. The nanorod colloidal suspension was spread dropwise (typically 1 mL of 2.5 mg/mL concentration) on the water surface of a Langmuir-Blodgett trough (Nima Technology, M611). The nanorod surface layer was then compressed slowly while the surface pressure was monitored with a Wilhelmy plate. Due to the presence of noncovalently bonded surfactant molecules, the compression starts with a nonzero surface pressure. In addition, since AOT is partially soluble in subphase water, the surface pressure decays with the time. In general, it was observed that the surface pressure increases during the compression. At different stages of compression, the nanorod assemblies at the water-air interface were transferred carefully onto transmission electron microscope (TEM) grids covered with continuous carbon thin film using the Langmuir-Schäffer horizontal liftoff procedure. Nanorod assemblies were examined systematically by using TEM. Initially, at low surface pressure, individual nanorods (generally 3 to 5 rods) form raft-like aggregates. These aggregates disperse on the subphase surface in a mostly isotropic state (Figure 1a). The surface pressure remains unchanged until the nanorods start forming a monolayer and when the monolayer was compressed to a surface pressure of ∼30 mN/m.15 During this process, monolayer of nanorods in a nematic arrangement are first obtained where the directors of these nanorods (or nanorod rafts) are qualitatively aligned presumably dictated by the barrier of the trough. Figure 1b shows such a partial nematic region with an orientational order parameter S of 0.83. The regularity of sideby-side inter-rod distance is reflected in the Fourier transform of the region (Figure 1b, inset). This nematic ordering, however, only occurs within a quite narrow pressure range. With further compression (surface pressure about ∼35 mN/m), nanorod (1) Collier, C. P.; Vossmeyer, T.; Heath, J. R. Annu. ReV. Phys. Chem. 1998, 49, 371-404. (2) Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989-1992. (3) Chung, S. W.; Markovich, G.; Heath, J. R. J. Phys. Chem. B 1998, 102, 6685-6687. (4) Peng, X.; Manna, L.; Yang, W.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59-61. (5) Chang, S.; Shih, C.; Chen, C.; Lai, W.; Wang, C. R. C. Langmuir 1999, 15, 701-709. (6) Korgel, B. A.; Fitzmaurice, D. AdV. Mater. 1998, 10, 661-665. (7) Li, M.; Schnablegger, H.; Mann, S. Nature 1999, 402, 393-395. (8) Park, S. J.; Kim, S.; Lee, S.; Khim, Z. G.; Char, K.; Hyeon, T. J. Am. Chem. Soc. 2000, 35, 8581-8582. (9) Tanori, J.; Pileni, M. P. Langmuir 1997, 13, 639-646. (10) Martin, B. R.; Dermody, D. J.; Reiss, B. D.; Fang, M.; Lyon, A.; Natan, M. J.; Mallouk, T. E. AdV. Mater. 1999, 11, 1021-1025. (11) Chen, C.; Chao, C.; Lang, Z. Chem. Mater. 2000, 12, 1516-1519. (12) Nikoobakht, B.; Wang, Z. L.; El-Sayed, M. A. J. Phys. Chem. B 2000, 104, 8635-8640. (13) Maeda, H.; Maeda, Y. Langmuir 1996, 12, 1446-1452. (14) Gabriel, J. C. P.; Davidson, P. AdV. Mater. 2000, 12, 9-20. (15) The free surfactants in the system can form Langmuir films themselves and may interfere with the formation of the nanorod monolayers. Consequently, the actual surface pressure during the compression may differ from the observed value. Figure 1. Transmission electron microscopy images of the nanorod assemblies at the water/air interface at different stages of compression: (a) isotropic distribution at low pressure; (b) monolayer with partial nematic arrangement; (c) monolayer with smectic arrangement; and (d) nanorod multilayer with nematic configuration. Insets in panels b and c are the Fourier transform of the corresponding image. 4360 J. Am. Chem. Soc. 2001, 123, 4360-4361