Formation of One-Dimensional Capped ZnO Nanoparticle Assemblies

Aug 19, 2010 - Formation of One-Dimensional Capped ZnO Nanoparticle Assemblies ... an air/liquid interface during solvent evaporation.4,5 In order to...
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Formation of One-Dimensional Capped ZnO Nanoparticle Assemblies at the Air/Water Interface Matthew P. Shortell,† Hong-Wei Liu,‡ Huaiyong Zhu,‡ Esa A. Jaatinen,† and Eric R. Waclawik*,‡ †

Discipline of Physics and ‡Discipline of Chemistry, Faculty of Science & Technology, Queensland University of Technology, GPO Box 2434, Brisbane, QLD, 4001, Australia Received May 27, 2010. Revised Manuscript Received August 4, 2010

The self-assembling behavior and microscopic structure of zinc oxide nanoparticle Langmuir-Blodgett monolayer films were investigated for the case of zinc oxide nanoparticles coated with a hydrophobic layer of dodecanethiol. Evolution of nanoparticle film structure as a function of surface pressure (π) at the air-water interface was monitored in situ using Brewster’s angle microscopy, where it was determined that π = 16 mN/m produced near-defect-free monolayer films. Transmission electron micrographs of drop-cast and Langmuir-Schaefer deposited films of the dodecanethiol-coated zinc oxide nanoparticles revealed that the nanoparticle preparation method yielded a microscopic structure that consisted of one-dimensional rodlike assemblies of nanoparticles with typical dimensions of 25  400 nm, encased in the organic dodecanethiol layer. These nanoparticle-containing rodlike micelles were aligned into ordered arrangements of parallel rods using the Langmuir-Blodgett technique.

Introduction Directing the spontaneous self-assembly of molecules, clusters, and particles into ordered nanostructures using chemical forces presents an interesting challenge, which could also prove useful for developing and advancing a range of new technologies. For instance, the organization of nanocrystals into multidimensional superlattices can significantly alter the assembly’s properties compared to isolated nanocrystals and produce remarkable collective properties.1 When nanoparticle assemblies form the building blocks of a microscopic structure, a number of approaches can be employed to etch a preprepared nanoparticle coating into a desired form; however, allowing the structure to assemble itself instead is a very attractive proposition. A variety of metal and semiconductor nanocrystals can now be self-assembled into ordered aggregates and quantum dot superlattices in ambient conditions, where chemical functionalization of the nanoparticle’s surfaces can be modified to either assist or prevent nanoparticle adsorption and binding on specific sites of a solid substrate.1-3 The self-assembly of such ligand-stabilized nanoparticles with sizes of a few nanometers into two- and threedimensionally ordered arrays has been an area of vibrant research activity for over a decade.2 This type of self-assembly is essentially driven by the interactions of the organic ligands rather than by the particle cores. When the nanoparticle’s coating is uniform and nanoparticle interactions are repulsive, aggregation and selfassembly can be directed to form close-packed hexagonal arrays, particularly in the presence of large surface forces experienced at an air/liquid interface during solvent evaporation.4,5 In order to achieve these ordered systems, the nanoparticle size distribution *Corresponding author. E-mail: [email protected].

(1) Kinge, S.; Crego-Calama, M.; Reinhoudt, D. N. ChemPhysChem 2008, 9, 20. (2) Collier, C. P.; Vossmeyer, T.; Heath, J. R. Annu. Rev. Phys. Chem. 1998, 49, 371. (3) Palms, D.; Priest, C.; Sedev, R.; Ralston, J.; Wegner, G. J. Colloid Interface Sci. 2006, 303, 333. (4) Bentzon, M. D.; van Wonterghem, J.; Mørup, S.; Th€olen, A.; Koch, C. J. W. Philos. Mag. B 1989, 60, 169. (5) Bigioni, T. P.; Lin, X. M.; Nguyen, T. T.; Corwin, E. I.; Witten, T. A.; Jaeger, H. M. Nature Mater. 2006, 5, 265.

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must be narrow, since a large spread of particle diameters disrupts effective packing.6 In mixtures of binary nanoparticles, chemical functionalization and selection of specific narrow particle size distributions can also yield nanoparticle superlattices that possess quasi-crystalline order with low defect frequency. This process can be considered as a simple sphere-packing phenomena governed by entropy-driven crystallization and simple interparticle potentials.7 Structures with Archimedean tiling structural motifs can even be achieved in these systems.8 An alternative to drying-mediated assembly of nanoparticle arrays9,10 is the Langmuir-Blodgett (L-B) approach of interfacial assembly which can be used to deposit large areas of ordered nanoparticle assemblies onto a range of substrates.11 The L-B technique can generate nanoparticle assemblies over multiple length scales where the final superstructures can be finely tuned through control of the film compression process. In the case of silver nanoparticles for instance, the insulator-to-metal transition has been directly observed.12 The L-B method exploits differences in interfacial energies, and typically two approaches have been taken with nanoparticles. Nanoparticles can be capped with hydrophobic molecules that possess long, methyl-terminated alkyl chains and then dispersed in a suitable solvent which is spread directly onto the water subphase of the L-B apparatus. The hydrophobic nanoparticles are then slowly compressed into a L-B film and transferred to the substrate. Otherwise, capped or sometimes even uncapped nanoparticles can be mixed and directly spread at the air/water interface within a conventional amphiphilic Langmuir-Blodgett film.13 Both of these approaches have (6) Heath, J. R.; Knobler, C. M.; Leff, D. V. J. Phys. Chem. B 1997, 101, 189. (7) Talapin, D. V.; Shevchenko, E. V.; Murray, C. B.; Kornowski, A.; F€orster, S.; Weller, H. J. Am. Chem. Soc. 2004, 126, 12984. (8) Talapin, D. V.; Shevchenko, E. V.; Bodnarchuk, M. I.; Ye, X.; Chen, J.; Murray, C. B. Nature 2009, 461, 964. (9) Denkov, N. D.; Velev, O. D.; Kralchevsky, P. A.; Ivanov, I. B.; Yoshimura, H.; Nagayama, K. Nature 1993, 361, 26. (10) Kralchevsky, P.; Nagayama, K. Langmuir 1994, 10, 23. (11) Yang, P.; Kim, F. ChemPhysChem 2002, 3, 503. (12) Collier, C. P.; Saykally, R. J.; Shiang, J. J.; Hendrichs, S. E.; Heath, J. R. Science 1997, 227, 1978. (13) Naszalyi, L.; Deak, A.; Hild, E.; Ayral, A.; Kovacs, A. L.; Horv€olgyi, Z. Thin Solid Films 2006, 515, 2587.

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successfully been used to deposit ordered arrays of gold and silver nanoparticles,2 but deposition of other materials including semiconductors like CdSe and metal oxides is of interest in this context.13 When L-B compression is applied to anisotropic nanostructures such as nanorods and nanowires, complex formations and behaviors can occur such as the formation of nematic and smectic superstructures rather than the hexagonally close-packed structures commonly observed for near-spherical nanoparticles that experience isotropic nearest-neighbor interactions.11 The case of BaWO4 nanorods is notable. Small aspect ratio (e.g., ∼3-5:1) raftlike BaWO4 nanorod aggregates and nematic phases have been produced by L-B deposition, with multilayers forming at high surface pressures (π). With large aspect ratio BaWO4 nanorods (∼150:1), strong compression generates a nematic phase with near perfect side-by-side alignment. The formation of a nematic phase is characteristic for large aspect ratio nanorods.14,15 It should be noted that the BaWO4 nanorods investigated in these experiments possessed an inorganic core surrounded by a surfactant layer. In the absence of ligands, the self-assembly of colloidal particles into secondary aggregates in the micrometer size regime is a common phenomenon. One way to create nanorods would be to assemble small nanoparticles together into one-dimensional structures.1 Kinetic models for this type of growth generally do not consider the process at the atomic level and in particular do not include crystallographic orientation effects.16 Recently, however, the importance of orientation effects in aggregation-based crystal growth was eloquently demonstrated in experiments on anatase and iron oxide nanoparticles by Penn and Banfield, where “oriented attachment” occurred through the alignment of specific crystallographic lattice planes in adjacent nanoparticles or dislocations in the contact areas.17,18 Since growth of ordered nanoparticle structures is so strongly dependent on nanocrystal surface energies, another feasible approach for constructing new architectures might be to exploit the selective functionalization of different nanocrystal facets.19,20 Crystal orientation-selective capping could in principle be used to prepare surface coatings where the crystals are orientated in a particular direction to create new and stable nanoparticle self-assembled structures such as 1-dimensional chains or even more complex arrangements reminiscent of the types of molecular self-assemblies that have been imaged at the solid-liquid interface by scanning tunneling microscopy.21 Nonuniform stabilizer distributions on nanocrystal surfaces have been demonstrated to assist 1-D assembly in CdTe and CdSe and notably TiO2 systems.22 It is thought that in some cases affinity differences in ligand attachment to different nanocrystal facets can even generate charge anisotropy and thus dipole-dipole interactions between nanoparticles that facilitate the 1-dimensional self-organization of the nanoparticles. The 1-D chainlike form of these assemblies is strongly reminiscent of the magnetic maghemite (γ-Fe2O3) nanoparticle arrangements found in magnetotactic bacteria, where the particles self-assemble (14) Kim, F.; Kwan, S.; Arkana, J.; Yang, P. J. Am. Chem. Soc. 2001, 123, 4360. (15) Kwan, S.; Kim, F.; Arkana, J.; Yang, P. Chem. Commun. 2001, 5, 447. (16) Park, J.; Privman, V.; Matijevic, E. J. Phys. Chem. B 2001, 105, 11630. (17) Penn, R. L.; Banfield, J. F. Science 1998, 281, 969. (18) Banfield, J. F.; Welch, S. A.; Zhang, H.; Ebert, T. T.; Penn, R. L. Science 2000, 289, 751. (19) Chen, J.; Wiley, B. J.; Xia, Y. Langmuir 2007, 23, 4120. (20) Norberg, N. S.; Gamelin, D. R. J. Phys. Chem. B 2005, 109, 20810. (21) Pacholski, C.; Kornowski, A.; Weller, H. Angew. Chem., Int. Ed. 2002, 41, 118. (22) Volkov, Y.; Mitchell, S.; Gaponik, N.; Rakovich, Y. P.; Donegan, J. F.; Kelleher, D.; Rogach, A. L. Chem. Phys. Chem. 2004, 5, 1600. (23) Dunin-Borkowski, R. E.; McCartney, M. R.; Posfai, M.; Frankel, R. B.; Bazylinski, D. A.; Buseck, P. R. Eur. J. Mineral. 2001, 13, 671.

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because their dipole moments are different.23 Even in semiconductor NP systems such as ZnO which is not magnetic, surface defects on semiconductor nanoparticles and crystal lattice anisotropy can generate an electric dipole moment that can produce chainlike 1-D assemblies through dipole-dipole interactions.24,25 The dipole-induced self-alignment of Au,26 Ag,27 and ZnO28 nanoparticles into chainlike structures has been observed, where nanoparticles adopted crystallographic orientations in specific directions. We consider here the possibility of combining the facet selective passivation of nanoparticle surfaces and the L-B technique to create 1-D NP assemblies that can then be ordered into secondary structures using interfacial assembly methods. In this paper, the deposition of zinc oxide nanoparticles by Langmuir-Blodgett techniques was explored. ZnO is an airstable, transparent, wide-band-gap semiconductor (Eg = 3.37 eV) which is a suitable candidate material for UV-optical applications because its high exciton binding energy of 60 meV could be beneficial for room-temperature operation of a single-photon device for instance. It also has a low refractive index compared to other semiconductors, which should facilitate light extraction in these types of applications. ZnO colloids can be produced with narrow particle size distributions.29 Synthesis from zinc acetate dihydrate in alcohol solvent under basic conditions is the most commonly employed method,29-31 where capping or passivation of the colloid by organic molecules can further refine the colloid size distribution and modify the nanoparticle’s properties.20 A range of organic modifiers have been investigated as capping agents with the aim of selective capping of particular ZnO nanocrystal facets in order to direct crystal growth and to modify their photoluminescence properties.20,32 Capping of ZnO nanoparticles using dodecanethiol molecules to quench crystal growth has also been successfully performed by Wong et al.33 Surface science studies have demonstrated that dodecanethiol strongly adsorbs to both zinc- and oxygen-terminated ZnO surfaces, leading to highly ordered and stable coatings that possess high surface coverage.34 Because of this recent success of DDTfunctionalized ZnO, fabrication of monolayer films of DDTcapped ZnO nanoparticles assembled at the air/water interface was investigated here.

Experimental Section Dodecanethiol-Capped ZnO Colloid Concentrate. The ZnO colloid was prepared by adapting the method described in Wong et al.35 Typically, 1 mmol of zinc acetate dihydrate (Zn(CH3CO2)2 3 2H2O; Aldrich, Analytical Reagent (AR) grade) was dissolved in 80 mL of 2-propanol (Aldrich, AR grade) under vigorous stirring at 50 °C and subsequently diluted to a total volume of 920 mL followed by chilling to 0 °C within 1 min under constant stirring. An 80 mL aliquot of a 2  10-2 M NaOH (24) Shim, M.; Guyot-Sionnest, P. J. Chem. Phys. 1999, 111, 6955. (25) Rabani, E. J. Chem. Phys. 2001, 115, 1493. (26) Liao, J.; Zhang, Y.; Yu, W.; Xu, L.; Ge, C.; Liu, J.; Gu, N. Colloids. Surf., A 2003, 223, 177. (27) Giersig, M.; Pastoriza-Santos, I.; Liz-Marzan, L. M. J. Mater. Chem. 2004, 14, 607. (28) Pages, C.; Coppel, Y.; Kahn, M. L.; Maisonnat, A.; Chaudret, B. ChemPhysChem 2009, 10, 2334. (29) Meulenkamp, E. A. J. Phys. Chem. B 1998, 102, 5566. (30) Meulenkamp, E. A. J. Phys. Chem. B 1998, 102, 7764. (31) Spanhel, L. J. Sol-Gel Sci. Technol. 2006, 39, 7. (32) Guo, L.; Yang, S.; Yang, C.; Yu, P.; Wang, J.; Ge, W.; Wong, G. K. L. Chem. Mater. 2000, 12, 2268. (33) Wong, E. M.; Hoertz, P. G.; Liang, C. J.; Shi, B. M.; Meyer, G. J.; Searson, P. C. Langmuir 2001, 17, 8362. (34) Sadik, P. W.; Pearton, S. J.; Norton, D. P.; Lambers, E.; Ren, F. J. Appl. Phys. 2007, 101, 104514. (35) Wong, E. M.; Bonevich, J. E.; Searson, P. C. J. Phys. Chem. B 1998, 102, 7770.

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solution (Aldrich AR grade) in 2-propanol was then slowly added to the Zn(OAc)2 solution at 0 °C under constant stirring. The stable and transparent ZnO colloid suspension so formed was stored at ∼2 °C until required. In order to cap the colloid solution, a dodecanethiol in ethanol solution (0.01 M, Aldrich) was freshly prepared and added to 100 mL quantities of ZnO colloid as required. The amount of capping agent added to the colloid was calculated by assuming that all of the original Zn(OAc)2 added in the colloid preparation had reacted to form ZnO NPs, all the capping agent adsorbed onto the ZnO surfaces, the colloid had a narrow particle size distribution, the ZnO nanoparticles were roughly spherical, and the crystal structure was comparable to bulk ZnO to give the same density. With these considerations, the amount of capping agent required ncap, for a given ZnO colloid particle radius r, is given by ncap ¼

3nZAH Vm 1 RNA r

where nZAH is the amount of ZAH added in the preparation of the colloid (0.1 mmol for 100 mL of colloid), Vm is the molar volume of bulk ZnO (14.52 cm3 mol-1), R is the adsorption area of the capping agent (21.4 A˚2 from Sellers et al.36), and NA is Avogadro’s number. To obtain a ZnO particle radius of 1 nm, 3.5 mL of the dodecanethiol solution was added to 100 mL of the ZnO colloid. This solution was then reduced using a rotary evaporator maintained at a temperature of 30 °C until ∼5 mL of cloudy concentrate remained.

LB Film Preparation and Spectroscopic Characterization of Samples. LB films were fabricated in a KSV minitrough with a pair of moveable barriers. All measurements were carried out using double-barrier compression on the surface of 18.5 MΩ water purified by a Millipore Milli-Q system. Particles were prepared for isotherm measurements and deposition in the following manner. The capped ZnO QD concentrate was spread by carefully casting small droplets from a glass 100 μL syringe onto the deionized water in the LB trough after which 30 min was allowed for the isopropanol to evaporate. Prior to LangmuirSchaefer deposition, the barriers were compressed at a rate of 5 mm/min until the deposition pressure of 16 mN/m was reached; this π was maintained for 1 h by automatic, incremental adjustments of the barriers by the KSV instrument. A quartz slide upon which the TEM grids were placed was used as the substrate for the LB film deposition. The TEM grids were lifted through the Langmuir film at the air/water interface at a rate of 2 mm/min. One layer was deposited in an upward scan. UV absorption spectroscopy of the ZnO QD colloid and ZnO QD capped concentrate was performed on a Cary 50 spectrometer. Diffuse reflectance UV absorption spectroscopy for the LB Film was performed on films deposited at a surface pressure of 30 mN/m using a Cary 5000 spectrometer due to the low signal of the thin film. UV-vis fluorescence spectrometry for both solutions and film was performed on a Cary Eclipse spectrometer using an excitation wavelength of 320 nm. Brewster’s angle microsopy (BAM) images were generated with a 5 mW, 633 nm helium-neon laser beam with a TEM00 Gaussian spatial profile that was collimated and had a diameter of 10 mm. The laser beam was linearly polarized with a polarization purity of 500:1, which was further improved by propagating the beam through two linear polarizers, each having a polarization extinction ratio that was better than 1000:1. The reflection from the film-water interface was demagnified by a factor of 2 and imaged on a 1280  1024 resolution CCD array that had an active area of 4.5 mm3.6 mm. To prepare drop-cast films, a small droplet of DDT-ZnO NPs in 2-propanol was dropped onto the TEM grids using a microsyringe and allowed to evaporate in air. (36) Sellers, H.; Ulman, A.; Schnidman, Y.; Eilers, J. E. J. Am. Chem. Soc. 1993, 115, 9389.

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Raman spectra of dodecanethiol, DDT-modified ZnO colloid, and pure ZnO colloid were obtained using a Renishaw System 1000 Raman microscope (Renishaw plc Wotton-under-Edge Gloucestershire, England) under a 50 objective with a HeNe laser giving 10 mW of 633 nm radiation at the sample. Resolution of better than 4 cm-1 was obtained in all cases. The system was first calibrated to the 520.5 cm-1 band of a silicon wafer, and spectra were taken over four accumulations of scans from 4000 to 100 cm-1 using a 40 s time constant.

Results When spread on the air/water interface, DDT-ZnO NPs will thus form “nontraditional” Langmuir-Blodgett monolayers, in the sense that rather than being amphiphilic, the overlayer’s nanoparticle surfaces will present only the hydrophobic end groups of the dodecanethiol ligands toward the water surface. We note that the ZnO NP solutions were capped with an excess of dodecanethiol; therefore, in addition to hydrophobic coated ZnO NPs, amphiphilic dodecanethiol molecules were also present at the air/water interface. It is therefore interesting to compare Langmuir monolayer isotherms of our DDT-ZnO NP solutions compared to those composed by amphiphiles whose hydrophilic head groups expose to the water surface and whose hydrophobic tails orient toward the air. When such monolayers are compressed, they pass through a highly compressible gaseous phase, to a less compressible liquid phase, and then finally a solid phase in which the molecules closely pack. The surface-area/pressure isotherm of the dodecanethiol-capped ZnO NPs is given in Figure 1. As might be expected, in compression we observed only a small gradient in π up to an area of 180 cm2, which was consistent with a two-dimensional gaseous and liquid-type phase at the air/water interface. All the films studied displayed this behavior at low coverage. Above 180 cm2, the compression isotherm gradient increased, but the gradient increase was not nearly as sharp as the solid-phase isotherm of a typical organic molecule amphiphile such as palmitic acid for instance. Within the range tested we could not detect a film collapse using the L-B trough’s Wilhelmy plate. It is quite possible then that above π = 20 mN/m the monolayer films readily folded or collapsed to form multilayered structures, making identification of overcompression and the switch from monolayer to a multilayer film at the air/ water interface difficult to detect. In expansion, following compression, the capped nanoparticle-based isotherm clearly possessed a large hysteresis. It is therefore possible that the films passed from L-B monolayer to a collapsed or multilayer structure at π values higher than 20 mN/m. In a number of cases, above π=20 mN/m visible ripples at the edges of the L-B trough were observed, which indicated that the optical density of the films in these areas may have been modified by multilayer formation. Notably for small nanoparticle systems,37 similar “wrinkles” have been noted by others as with DDT-coated Au monolayer films at high compression, where the wrinkles were interpreted as folding of the nanoparticle films back over themselves.38,39 To maintain a NP monolayer during deposition, a suitable π that generated dense NP-packing but below which monolayer folding was absent needed to be established. The evolution of the Langmuir films was further examined in situ during compressions and expansions at different surface pressures using BAM (Figure 2). BAM employs a p-polarized (37) Vella, D.; Aussillous, P.; Mahadevan, L. Europhys. Lett. 2004, 68, 212. (38) Huang, S.; Minami, K.; Sakaue, H.; Shingubara, S.; Takahagi, T. Langmuir 2004, 20, 2274. (39) Schultz, D. G.; Lin, X. M.; Li, F.; Gebhardt, J.; Meron, M.; Viccaro, P. J.; Lin, B. J. Phys. Chem. B 2006, 110, 24522.

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Figure 3. TEM images of DDT-capped ZnO NP monolayers deposited by the L-S technique. Figure 1. Compression and expansion isotherms of dodecanethiolcapped ZnO NPs. The letters indicate notable regions of monolayer structure imaged by Brewster’s angle microscopy in Figure 2.

Figure 2. BAM images of dodecanethiol-capped ZnO Langmuir monolayers.

laser adjusted to strike air/water interface at the Brewster’s angle (∼53.1° here) so that all the light is transmitted when no interfacial film is present.40,41 When nanoparticles are present at the air/water interface, the effective refractive index at the surface changes; this partially reflects the laser beam that was detected by a CCD camera in our case. Figure 2 shows BAM images for selected barrier positions and hence surface pressures, π. Black spots inside the Gaussian laser beam spot were frequently observed. Since the BAM signal in the absence of any film was set to a faint red, completely black regions corresponded to areas with high film roughness, multilayer NP aggregates for instance. Thus, it could be concluded that completely compressed films formed at surface pressures larger than 25 mN/m (Figures 1c and 2c) possessed a large number of rough and/or multilayer DDTZnO NP regions in the BAM viewing window. BAM images in Figure 2e,f taken during expansion showed evidence for monolayer films that had broken apart or partial monolayer films. From these and many other in situ BAM experiments it was deduced that an appropriate surface pressure for deposition of monolayer films was 16 mN/m. Above π = 16 mN/m, large rough (40) Henon, S.; Meunier, J. Rev. Sci. Instrum. 1991, 62, 936. (41) Hoenig, D.; Moebius, D. J. Phys. Chem. 1991, 95, 4590.

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spots and/or multilayers in the films were detectable. Below this pressure, in both compression and expansion, the monolayer films consisted of micrometer-size domains or patches separated by large voids which were also deemed unsuitable for film deposition. All Langmuir-Schaefer (L-S) depositions onto TEM grids were performed at a surface pressure of 16 mN/m. A sample of TEM images of ZnO NP-coated grids is shown in Figure 3, and the results were quite interesting and unexpected. Following the prevalent literature on capped-gold nanoparticles, it was assumed that the dodecanethiol molecules would form inverse micelles around the ZnO NPs here. From the TEM images it would appear that the thiol molecules self-assembled into rodlike micelles that frequently encased clusters of ZnO NPs held together in long chains. Their dimensions typically appeared to be between approximately 100-800 nm in length, with average diameters of 25 nm and a standard deviation of 2.5 nm. Higher resolution images of the films were attempted, but due to the large quantities of organics present (presumably the DDT), charging of the film and damage occurred at higher TEM accelerating voltages.42 Interestingly, the DDT rods aligned parallel to each other, perpendicular to the direction of barrier movement of the LangmuirBlodgett apparatus. It is proposed that the compression and incremental barrier movements that were used to maintain the constant 16 mN/m surface pressure during the monolayer film’s stabilization process may have assisted the ordering of DDT-ZnO NP structures into these parallel arrays. L-B compression of hydrophobic nanorod aggregates has previously yielded very similar nematic phases.11 Treating these rods as a fluid of long thin rods, it has been proposed that the ordering of the selfassembled structures with increasing π occurred by minimizing the excluded volume per particle to drive the formation of an ordered anisotropic phase.11,43 Alternatively, the anisotropy of interaction between the nanorods could also favor the alignment of the rods at moderate-to-high film compression, where attractive van der Waals and directional capillary forces might assist the anisotropic assembly process in these systems.11 To see if formation of one-dimensional DDT-capped ZnO structures in the Langmuir films was a general phenomenon under ambient conditions, films of the nanoparticle concentrate and a 100 diluted solution were drop-cast onto TEM grids and allowed to dry in air (Figure 4). The TEM results of these films revealed that identical rods self-assembled from these solutions as previously occurred with the L-S deposited films. Some degree of (42) Due to the large quantity of dodecanethiol coating present at the surfaces of the ZnO NPs and in the monolayer aggregates themselves, only snapshot images (