NANO LETTERS
“Self-Corralling” Nanorods under an Applied Electric Field
2006 Vol. 6, No. 9 2066-2069
Suresh Gupta,† Qingling Zhang,† Todd Emrick,* and Thomas P. Russell* Polymer Science & Engineering Department, UniVersity of Massachusetts, Amherst, Massachusetts 01003 Received June 9, 2006; Revised Manuscript Received July 17, 2006
ABSTRACT Producing densely packed arrays of nanoscopic anisotropic objects, while necessary for applications in photovoltaic and field emission devices, presents considerable challenges. Here, we present findings on the phase separation of ligand-functionalized nanorods in a polymer matrix under an applied electric field. Densely packed hexagonal arrays of nanorods are produced by this method, where the rods are oriented in the direction of the applied field. Minimization of interfacial energy between the array of nanorods and the surrounding polymer serves to corral the nanorods into the densely packed arrays observed. These findings carry implications toward advancing organic−inorganic heterojunction photovoltaic devices that are expected to benefit from the oriented, densely packed ordered arrays of nanorods produced here.
Orienting and packing anisotropic nanoscopic objects, such as conducting and semiconducting nanorods, are highly desirable for fundamental studies in charge transport, as well as photovoltaic and field emission devices.1,2 Organicinorganic hybrid photovoltaic devices, while having advantages of low cost, processibility, and flexibility, suffer from lower efficiency than silicon-based photovoltaics.3 The efficiency of nanorod-based photovoltaics may be enhanced by a controlled orientation, and close packing, of the rods. However, at low concentration, nanorods tend to pack randomly when deposited from solution onto a substrate, while at higher concentration, liquid crystalline packing is achieved.4-6 By addition of a nonsolvent to the nanorod solution, crystalline aggregates of nanorods have been observed.7 The deposition of these nanorod aggregates onto a substrate produces densely packed arrangements of the nanorods, with no preferential orientation. Here we demonstrate that the combined forces of an applied external field and interfacial energy, give a controlled orientation and dense packing of anisotropic nanoparticles, specifically CdSe nanorods. Prior studies have shown that polymer domains in diblock copolymer templates can be oriented in a predetermined direction using an electric field, due to anisotropy in the dielectric properties of the two blocks.8 In this work, the permanent dipole moment in CdSe nanorods, as well as the anisotropy in the dielectric properties of CdSe nanorods in solution, leads to an alignment of the long axis of the nanorods along the field lines of an applied electric field. Moreover, by the addition of a polymer to the * Corresponding authors. E-mail:
[email protected],
[email protected]. † These authors contributed equally to this work. 10.1021/nl061336v CCC: $33.50 Published on Web 08/08/2006
© 2006 American Chemical Society
nanorod solution, the rods can be “corralled”, due to nonfavorable polymer-ligand interactions. Such interactions can be tailored, by varying the polymer matrix and the ligands on the nanorod surface, and used to self-corral nanorods into densely packed arrays with controlled orientation. CdSe nanorods (∼8 nm in diameter and ∼40 nm in length) were prepared according to published procedures9 that afford the nanorods with a surface ligand coverage of tetradecylphosphonic acid (TDPA) and tri-n-octylphosphine oxide (TOPO). Transmission electron microscopy (TEM, JEOL 2000 FX, operated at 200 kV) showed a nanorod length distribution of ∼10%. Polyethylene oxide (PEO molecular weight 2000 g/mol)-covered CdSe nanorods were prepared through a ligand exchange process according to Zhang et al.10 Polystyrene (PS)-covered CdSe nanorods were prepared similarly. Briefly, alkane-covered rods (50 mg) were refluxed in anhydrous pyridine (2 mL) for 24 h and then precipitated into hexane. Then, pyridine-covered nanorods were heated to 60 °C in a thiol-terminated PS (molecular weight 1000 g/mol) toluene solution for 10 h to give a homogeneous solution. The PS-covered nanorods were purified by precipitation in hexane and dissolution in chloroform. A chloroform solution of these CdSe nanorods (0.5 wt %) and poly(methyl methacrylate) (PMMA, 0.5 wt %, molecular weight 25 000 g/mol) was prepared, and a droplet (∼20 µL) of this solution was placed on a silicon oxide coated silicon wafer. An electric field E (107 V/m) was applied while the chloroform evaporated over an ∼8 h period. This was achieved by using a gold-coated glass slide which has a concave well with a depth of 300 µm and diameter ∼1 cm and applying a 3000 V potential across the two
Figure 1. Schematic representation of the experimental setup for application of an electric field during solvent evaporation of nanorod-polymer composites. A silicon wafer serves as one electrode, and gold-coated soda lime glass serves as the second electrode.
electrodes. The glass slide is made of soda lime glass with mobile Na+ ions. This allows a voltage gradient only in the air gap between the glass slide and silicon wafer. This is shown schematically in Figure 1. The thin nanorod/polymer composite film obtained in this process was transferred onto a copper grid by floating the film onto a 5% HF(aq) solution and then examined by TEM. The TEM image of Figure 2a shows a self-corralled nanorod-polymer composite film, using the procedure described above, to obtain a hexagonally close-packed array of nanorods aligned normal to the film surface. The average center-to-center distance between the nanorods is ∼10 nm, with a separation distance between the rods of ∼2 nm, or approximately twice the length of the attached TOPO ligands. In the absence of an applied field, the nanorods were found to aggregate in an orientation parallel to the film surface, as shown in Figure 2b. The noncentrosymmetric, wurtzite lattice of CdSe nanorods leads to a permanent dipole that increases linearly with increasing volume of the nanorods.11 The unscreened dipole moment P of the nanorods used in our experiments is calculated to be ∼1450 D, based on reported dipole moments of the CdSe nanorods,11 assuming that the nanorods comprise a single crystal. There is a torque T exerted on a permanent dipole when placed in an electric field that aligns the dipole in the direction of the applied electric field. If the rods and
electric field lines are orthogonal, the strength of the torque on the nanorods (T ) P × E) is 4.872 × 10-20 Nm, or 10 times the thermal energy at room temperature that would otherwise randomize the orientation of the nanorods. In addition, the difference between the dielectric constants of CdSe nanorods and chloroform, used here as the solvent, gives rise to a polarization charge that exerts a force on the nanorods in the presence of the applied field.11 The sum of these forces causes the alignment of the nanorods in the direction of the applied electric field. A similar approach has been taken by Ryan et al., and the electric field was shown to orient the nanorods along the field lines.12 When attempted self-corralling experiments were performed without polymer in the solution, nanorod alignment normal to the underlying substrate was not observed, even when the solvent was allowed to evaporate in the presence of an electric field. While CdSe nanorods in solution become oriented in the direction of the applied electric field, as shown by Li et al.,11 the orientation is lost when the solvent evaporates and the field is removed. This can be understood by considering that the ligands attached to the nanorods are highly solvent swollen and, upon solvent evaporation, the large volume loss and removal of the field cause the nanorods to collapse onto the substrate and assume an orientation parallel to the substrate. In the presence of a polymer, minimization of the interfacial tension, arising from the highly nonfavorable interactions between the ligands and the polymer matrix, forces a phase separation and dense packing of the nanorods, i.e., a “corralling” that is frozen in by the polymer matrix upon solvent evaporation. It should also be noted that there is a large enthalpic energy cost for isolated nanorods to be dispersed in the polymer matrix, further underscoring the concept of corralling. Here the effect of concentration of nanorods and the rate of solvent evaporation were not considered and the electric field may be able to orient the nanorods in the absence of polymer as shown by Ryan et al.12 With change of the chemical composition of either the polymer matrix or the ligands attached to the nanorods, the driving force for the corralling can be altered or even
Figure 2. TEM images of “self-corralling” of alkane-covered CdSe nanorods in PMMA: (a) after solvent evaporation under an applied electric field; (b) after solvent evaporation without an applied electric field. The scale bar is 100 nm. Nano Lett., Vol. 6, No. 9, 2006
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Figure 3. TEM images of nanorods in the absence of “self-corralling”: (a) PEO-covered CdSe nanorods in a PMMA matrix; (b) PScovered CdSe nanorods in PMMA matrix. All the samples were prepared under an electric field. The scale bar is 100 nm.
removed, though the dipolar properties of the nanorods remain unchanged. For example, alkane ligands on CdSe nanorods were replaced with PEO, keeping PMMA as the corralling polymer matrix. As in the prior case, a chloroform solution of these PEO-functionalized CdSe nanorods and PMMA was prepared, and a droplet of this solution was allowed to dry under an applied electric field. As shown in Figure 3a, the nanorods did not aggregate, due to the miscibility between the PEO ligands and the surrounding PMMA matrix13 that promotes homogeneous dispersion of the nanorods in the polymer matrix. The lack of orientation observed for these nanorods in the applied field direction indicates not only that it is necessary to have an applied field to orient the nanorods but also that an additional force, acting in the plane of the film, is necessary to force a dense packing of oriented nanorods and to retain the desired alignment. With interfacial energy playing a key role in corralling the nanorods, studies were performed to determine if systems with a low interfacial energy would behave similarly. Figure 3b shows a TEM image of a mixture of PMMA with nanorods having PS ligands, in which the film was solution cast under an applied field. As expected, the PS-covered nanorods phase separate from the PMMA matrix. However, the nanorods remain oriented parallel to the surface of the film, normal to the applied field. Consequently, while the nonfavorable interactions between the PS ligands and the PMMA matrix cause a phase separation, the magnitude of the segmental interactions is too weak14,15 to force a close packing of the nanorods. Hence, during solvent evaporation, volume contraction is sufficiently large such that when the field is removed, orientation of the nanorods in the direction of the field is lost. Strong, nonfavorable polymer-ligand interactions are thus necessary to attain a sufficient packing density of the oriented nanorods so that alignment is not lost when the field is removed. Figure 4 shows another example of self-corralling, in this case, arrays of densely packed CdSe nanorods oriented normal to the film surface, where the nanorods are covered with alkane ligands, and the polymer matrix is regioregular poly(3-hexylthiophene) (P3HT). P3HT is a photoactive polymer that has generated considerable interest in photo2068
Figure 4. “Self-corralling” of alkane-covered CdSe nanorods in a P3HT matrix. The nanorods phase separate and align perpendicular to the substrate upon application of an electric field. The scale bar is 100 nm.
voltaic devices, making this type of polymer-nanorod assembly especially encouraging. In future studies we will examine the effect of corralled nanorods where the photoactive polymer is attached directly to the rods,16,17 an appealing structure from the standpoint of potentially improving charge transport efficiency in photovoltaic applications.18 In summary, phase separation phenomena in combination with an applied electric field is shown to effectively corral CdSe nanorods into densely packed arrays that stand perpendicular to the underlying substrate. A coupling of the phase separation and the field alignment is critically important to achieving the desired assemblies. The strength of the force corralling the nanorods is governed by the interfacial energy between the ligands attached to the nanorods and the polymer matrix. If the interfacial energy is not sufficiently Nano Lett., Vol. 6, No. 9, 2006
large, then alignment of the nanorods is not achieved. On extrapolation to the use of photoactive ligands on the nanorods, a viable route to highly efficient photovoltaic devices is conceivable. Acknowledgment. This work was supported by the U.S. Department of Energy (T.P.R.), the NSF supported MRSEC at the University of Massachusetts Amherst, a NSF CAREER Award (T.E.), and the Army Research Office through a MURI award. References (1) Fan, S. S.; Chapline, M. G.; Franklin, N. R.; Tombler, T. W.; Cassell, A. M.; Dai, H. J. Science 1999, 283, 512-514. (2) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425-2427. (3) Coakley, K. M.; McGehee, M. D. Chem. Mater. 2004, 16, 45334542. (4) Kim, F.; Kwan, S.; Akana, J.; Yang, P. D. J. Am. Chem. Soc. 2001, 123, 4360-4361. (5) Li, L. S.; Alivisatos, A. P. AdV. Mater. 2003, 15, 408. (6) Li, L. S.; Walda, J.; Manna, L.; Alivisatos, A. P. Nano Lett. 2002, 2, 557-560.
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(7) Talapin, D. V.; Shevchenko, E. V.; Murray, C. B.; Kornowski, A.; Forster, S.; Weller, H. J. Am. Chem. Soc. 2004, 126, 12984-12988. (8) Morkved, T. L.; Lu, M.; Urbas, A. M.; Ehrichs, E. E.; Jaeger, H. M.; Mansky, P.; Russell, T. P. Science 1996, 273, 931-933. (9) Peng, Z. A.; Peng, X. G. J. Am. Chem. Soc. 2002, 124, 3343-3353. (10) Zhang, Q. L.; Gupta, S.; Emrick, T.; Russell, T. P. J. Am. Chem. Soc. 2006, 128, 3898-3899. (11) Li, L. S.; Alivisatos, A. P. Phys. ReV. Lett. 2003, 90. (12) Ryan, K. M.; Mastroianni, A.; Stancil, K. A.; Liu, H.; Alivisatos, A. P. Nano Lett. 2006, 6, 1479-1482. (13) Ito, H.; Russell, T. P.; Wignall, G. D. Macromolecules 1987, 20, 2213-2220. (14) Russell, T. P.; Hjelm, R. P.; Seeger, P. A. Macromolecules 1990, 23, 890-893. (15) Strobl, G. R. The physics of Polymers: Concepts for Understanding Their Structures and BehaVior, 2nd ed.; Springer-Verlag: Berlin, 1997. (16) Liu, J. S.; Tanaka, T.; Sivula, K.; Alivisatos, A. P.; Frechet, J. M. J. J. Am. Chem. Soc. 2004, 126, 6550-6551. (17) Skaff, H.; Sill, K.; Emrick, T. J. Am. Chem. Soc. 2004, 126, 1132211325. (18) Scher, E. C.; Manna, L.; Alivisatos, A. P. Philos. Trans. R. Soc. London, Ser. A 2003, 361, 241-255.
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