NANO LETTERS
Controlled Placement of CdSe Nanoparticles in Diblock Copolymer Templates by Electrophoretic Deposition
2005 Vol. 5, No. 2 357-361
Qingling Zhang, Ting Xu, David Butterfield, Matthew J. Misner, Du Yoel Ryu, Todd Emrick,* and Thomas P. Russell* Polymer Science and Engineering Department, UniVersity of Massachusetts, 120 GoVernors DriVe, Amherst, Massachusetts 01003 Received November 17, 2004; Revised Manuscript Received December 14, 2004
ABSTRACT An electrophoretic deposition process is shown to be an effective means of placing CdSe nanoparticles into nanopores and nanotrenches in templates prepared from polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymers. CdSe nanoparticles covered with r,ω-thiocarboxylic acid ligands were used in these studies, and an electric field was used to drive the nanoparticle deposition. Photoluminescence of the quantum dots was maintained within the nanostructures.
Semiconductor nanocrystals have unique optical and electronic properties due to a quantum confinement of the exciton to a distance shorter than the Bohr radius.1 Recent advances in the synthesis of CdSe nanocrystals provide samples of controlled size and narrow size distribution.2-4 For many materials and device applications, it is desirable to disperse and assemble the nanoparticles in polymer materials. Excellent, but random, dispersion of nanopartices in polymer matrices can be achieved by using ligands that render the nanoparticles miscible with the polymer matrix.5-9 In applications related to light emitting, sensor, and electronic devices, precise control over the lateral distribution of the nanoparticles would be advantageous. Inorganic and polymer templates are structural platforms that are well suited for particle assembly over a range of size scales. Such templates can be prepared by lithographic processes to give feature sizes on the micron and submicron scale. Yin et al.10 and Cui et al.11 demonstrated that capillary force could be employed to trap micron-sized particles and nanoparticles into lithographically prepared trenches on the micron and submicron size scale. However, the shrinking size of electronic devices requires the use of smaller template features, deep into the nanoscale, to direct assembly of nanometer-sized particles. Templates prepared from thin films of microphase separated block copolymers are ideal, since the typical feature size of block copolymer domains is in the range of 10-20 nm.12-18 For example, using these nanoporous templates prepared from diblock copolymer thin films, Misner et al.19 demonstrated the effective use of * Corresponding authors. E-mail:
[email protected] (T.E.);
[email protected] (T.P.R.). 10.1021/nl048103t CCC: $30.25 Published on Web 01/06/2005
© 2005 American Chemical Society
capillary force to drive tri-n-octylphosphine oxide (TOPO)covered CdSe nanoparticles into the nanopores of cylindrical diblock copolymer templates. In this study, electrophoretic deposition is utilized to drive nanoparticles into templates with nanoscopic features derived from diblock copolymers. The electrophoretic deposition technique has been used widely for coating ceramic surfaces with charged colloidal particles.20,21 Electrophoretic deposition involves the motion of charged particles in solution under the influence of an electric field and the subsequent deposition of the particles onto an electrode surface. The electrophoretic force is controlled by an applied voltage, which, along with the deposition time and nanoparticle concentration, controls the degree of deposition. Electrophoretic deposition is typically performed on a planar conducting surface. Recently, Bailey et al.22 demonstrated the electrophoretic deposition of an aqueous solution of citrate-stabilized colloidal gold nanoparticles into the trenches of a micropatterned indium tin oxide (ITO) surface prepared by microtransfer molding from a 5 µm × 5 µm square relief master. A calculation by Nadal et al.23 showed that a dielectric strip (such as the molded pattern used by Bailey et al.22) in a conducting electrode could significantly change the hydrodynamic flow of an electrolyte solution in regions near the electrode surface, and that charged particles in this solution tend to slip from the dielectric strip and be deposited onto the conducting area. Here, we employ diblock copolymer templates 30-40 nm in thickness with feature sizes on the order of 15 nm. A key component of this study is the contrast provided by this 30-40 nm layer of dielectric material relative to the
Figure 1. Schematic diagram of the setup used to drive negatively charged CdSe nanoparticles into diblock copolymer templates. Templates with nanopores or nanotrenches served used as the anode, and bare gold-coated silicon wafers served as the cathode.
conducting substrate. Equally important is the ability to tailor the nanoparticles with functionalized ligands. The carboxylate-functionalized quantum dots, approximately 3 nm in diameter, possess the aqueous solubility and charge needed to enable the electrophoretic deposition process. As shown here, the selectivity and coverage of nanoparticles into nanoporous regions of the templates achieved by electrophoretic depositon is much higher than that obtained by capillary force alone. A thin film of cylindrical PS-b-PMMA with a thickness of Lo (a long period of diblock copolymer) was oriented normal to a substrate surface by anchoring a random copolymer of styrene and methyl methacrylate to the substrate to balance the interfacial interactions between the PS and PMMA blocks with the substrate.15-17 Selective removal of the PMMA phase by irradiation at 254 nm produced films with hollow cylinders ∼15 nm in diameter. Exposure of this template to oxygen plasma ensured the entire removal of residual random copolymer from the underlying surface. The templates were floated onto carboncoated copper grids for the electrophoretic deposition. A schematic diagram of the electrophoretic deposition is shown in Figure 1. Templates containing nanopores on copper grids were used as anodes and bare gold-coated silicon wafers were used as cathodes. Solutions of 11-mercaptoundecanoic acid (MUA)-covered CdSe nanoparticles in water and methanol were prepared and used in the deposition bath. A dc voltage was then applied between the two electrodes. Figure 2a shows a transmission electron microscopy (TEM) image of the template before deposition. The bright dots in the image are the empty nanopores within the darker polystyrene matrix. The hexagonal arrangement of the nanopores results from the original hexagonal packing of PMMA cylinders in the microphase separated diblock copolymer.12,18 The electrophoretic deposition experiments were performed at various electric field strengths, nanoparticle concentrations, and deposition times. The optimal field strength for deposition was found to be 0.4∼0.8 V/cm. The concentration of nanoparticles was ∼1013 particles/mL, and a typical deposi358
Figure 2. TEM images of nanoporous template generated from cylindrical PS-b-PMMA diblock copolymer thin film: (a) before deposition; (b) after 1 min deposition time with an electric field strength of 0.4 V/cm; (c) after 5 min deposition time with an electric field strength of 0.4 V/cm (inset: higher magnification).
tion time ranged from 1 to 10 min. Figure 2b shows a typical image of a nanoparticle-filled template produced after a deposition time of 1 min at an electric field strength of 0.4 V/cm (the small black dots are the CdSe nanoparticles). As seen in the image, individual nanoparticles are embedded within some of the nanopores, while a smaller percentage of the nanopores remain empty. By increasing the deposition time to 5 min, and using the same voltage, a nearly complete deposition of CdSe nanoparticles into the nanopores is observed (Figure 2c). Magnification (Figure 2c, inset) shows that some of the pores may contain more than one nanoparticle. Nano Lett., Vol. 5, No. 2, 2005
Templates prepared from lamellar diblock copolymers were also used for electrophoretic deposition. In the lamellar case, removal of PMMA by UV irradiation and acetic acid washing leaves nanotrenches in the template film as shown in Figure 3a. The worm-like structure is a top-view of lamellae oriented normal to the surface; the bright areas are nanotrenches generated after removal of PMMA. This nanotrench-containing template was used for electrophoretic deposition in a fashion similar to the nanoporous case described previously. The solution concentration of CdSe nanoparticles used in these experiments was ∼1013 particles/ mL. The TEM image in Figure 3b shows the successful deposition of CdSe nanoparticles into the nanotrenches after a deposition time of 1 min and an electric field strength of 0.4 V/cm. Individual CdSe nanoparticles (dark dots in the TEM image) are discernible within the nanotrenches. However, the majority of the template area remains unoccupied by nanoparticles. When the electric field strength was increased to 0.8 V/cm and deposition time increased to 10 min, the coverage of nanotrenches by nanoparticles increased dramatically, as shown in Figure 3c. In this image, black worm-like structures are CdSe nanoparticles in the nanotrenches, while gray worm-like lines are the polystyrene template material, and scattered bright areas are unfilled space within nanotrenches. It is seen in Figure 3 that most of the nanotrenches are completely filled with CdSe nanoparticles though empty regions of these trenches are observed. It should be noted that the photoluminescence of CdSe nanoparticles was maintained following the electrophoretic deposition. Photoluminescence was monitored before and after electrophoretic deposition. Shown in Figure 4 are the photoluminescence spectra of MUA-covered CdSe nanoparticles in methanol, in the solid state on a silicon wafer, and embedded within the diblock copolymer template (taken from the sample in Figure 2c). An observed red shift in the solidstate spectrum relative to the solution spectrum is consistent with previous studies on dilute vs. concentrated nanoparticle samples in the solid state,24 and also consistent with a report on CdS nanoparticles in bulk polystyrene-block-poly(ethylene oxide) (PS-b-PEO).25 The selective electrophorectic deposition of these carboxylate-functionalized nanoparticles into nanopores and nanotrenches was enabled by the flow pattern of electrolyte solution induced by the electric field modulated by a patterned electrode. Nadal et al.23 described the use of a dielectric strip in a conducting electrode to change the hydrodynamic flow of electrolyte solution near an electrode surface. Charged particles in this solution tend to migrate around the dielectric strip and deposit onto the conducting area. Nadal, et al.23 also calculated the flow pattern induced by a periodic modulation of the surface by dielectric stripes on a conducting surface. In this case, the fluid “slips” from the insulating dielectric stripes into the conducting zones. The deposition templates prepared from the diblock copolymer thin films used here are patterned with dielectric zones of polystyrene, and the area exposed to the carbon film on the copper grid (through nanopores and nanotrenches) served Nano Lett., Vol. 5, No. 2, 2005
Figure 3. TEM images of template with nanotrenches generated from lamellar PS-b-PMMA diblock copolymer thin film: (a) before deposition; (b) after 1 min deposition time with an electric field strength of 0.4 V/cm; (c) after 10 min deposition time with an electric field strength of 0.8 V/cm. 359
Figure 4. Comparative photoluminescence spectra of MUAcovered CdSe nanoparticles in methanol; in the solid state on a silicon wafer; and embedded in a cylindrical diblock copolymer template.
as the conducting zones. Despite the rather thin layer of polystyrene (ca. 40 nm in thickness) in the dielectric zones, sufficient dielectric insulation was provided by the film to allow deposition only onto the conducting areas where nanopores and nanotrenches were located. In summary, electrophoretic deposition proved to be a simple and efficient method to selectively place CdSe nanoparticles into the nanopores and nanotrenches of diblock copolymer templates. The degree of deposition can be conveniently controlled by adjusting time, solution concentration, and the strength of the applied electric field. The selective deposition of CdSe nanoparticles into the nanopores and nanotrenches was due to the patterned polystyrene as a dielectric material, which directs the fluid flow from the polystyrene film to the conducting areas with nanopores and nanotrenches. The photoluminescence of CdSe nanoparticles was maintained after deposition. This method is not limited to carboxylate-covered quantum dots but is general and can be applied to other charged nanoparticles as well as other nanoporous patterns and architectures. Experimental Section. TOPO-covered CdSe nanoparticles with a size of 3.2 nm were prepared as described in the literature.4 Carboxylate-covered CdSe nanoparticles were obtained by ligand exchange of TOPO-covered CdSe nanoparticles with 11-mercaptoundecanoic acid (MUA).26 In a typical ligand exchange procedure, about 20 mg of TOPOcovered CdSe nanoparticles was added to a methanol (2 mL) solution of 40 mg MUA. To this mixture was added tetramethylammonium hydroxide. An optically clear red solution formed immediately, indicating a rapid replacement of the hydrophobic TOPO ligands with MUA. This mixture was stirred at 70 °C under N2 atmosphere for 10 h, and the nanoparticles were precipitated in diethyl ether. Excess MUA was removed by repeated dissolution in methanol and precipitation into ether. Diblock copolymers of polystyrene and poly(methyl methacrylate) were synthesized by anionic polymerization using sec-butyl lithium as the initiator. Two diblock copolymer samples were used. One sample had a number average molecular weight of 66 000, a polydispersity of 1.09, and a PMMA volume fraction of 0.25. This diblock copolymer 360
self-assembles into a cylindrical phase, where the cylinders are composed of PMMA. The other diblock copolymer had a number average of molecular weight of 93 000, a polydispersity of 1.09, and a PMMA volume fraction of 0.50. This diblock copolymer forms alternating lamellae upon phase separation.7 A hydroxyl terminated random copolymer of styrene and methyl methacrylate containing 58% styrene was anchored to the surface of silicon substrates.12 Thin films of PS-b-PMMA with a thickness of ∼Lo were prepared by spin-casting toluene solutions onto these substrates. Thin films were annealed at 170 °C under vacuum for 48 h, exposed to deep UV irradiation (λ ) 254 nm) for 30 min, then washed with acetic acid for 5 min. This produced a film of polystyrene with either nanopores or nanotrenches, depending upon the initial morphology of the diblock copolymer used (nanoporous templates were generated from cylindrical diblock copolymers, and templates with nanotrenches were prepared from lamellar diblock copolymers). The resulting template film was exposed to oxygen plasma for 10 s to remove the underlying random copolymer. The template film was then floated onto a 5 wt % aqueous hydrofluoric acid solution and retrieved with a carbonreinforced copper grid. This copper grid was attached to a silicon wafer coated with gold by a conducting tape, and constituted one anode for the deposition. The cathode was a bare gold-coated silicon wafer. The two electrodes were immersed in a dilute water/methanol solution (∼1013 particles/ mL; water/methanol 80/20 by volume) of MUA-covered CdSe nanoparticles. To drive CdSe nanoparticles into nanopores or nanotrenches in the film, a direct current voltage was applied between the two electrodes. The pH of the aqueous MUA-covered CdSe nanoparticle solution was adjusted to ∼8 by addition of tetramethylammonium hydroxide. After deposition, the template on the copper grid was examined under a JEOL transmission electron microscope (TEM) operated at 100 kV. Solution and solid-state photoluminescence spectra were measured using a PerkinElmer LS 50 spectrometer. Acknowledgment. This work was funded by the National Science Foundation through a CAREER award (CHE0239486), the National Science Foundation supported Materials Research Science and Engineering Center (MRSEC) and the associated Research Experience for Undergraduates (REU) program at UMass Amherst, and the Department of Energy and Office of Basic Energy Sciences. D.B. acknowledges with thanks a Research Experience for Undergraduates Fellowship from NSF-MRSEC. References (1) Alivisatos, A. P. Science 1996, 271, 933. (2) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (3) Peng, X. G.; Manna, L.; Yang, W. D.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature (London) 2000, 404, 59. (4) Peng, Z. A.; Peng, X. G. J. Am. Chem. Soc. 2001, 123, 183. (5) Skaff, H.; Ilker, M. F.; Coughlin, E. B.; Emrick, T. J. Am. Chem. Soc. 2002, 124, 5729. (6) Sill, K.; Emrick, T. J. Am. Chem. Soc. 2004, 16, 1240. (7) Corbierre, M. K.; Cameron, N. S.; Sutton, M.; Mochrie, S. G. J.; Lurio, L. B.; Ruhm, A.; Lennox, R. B. J. Am. Chem. Soc. 2001, 123, 10411.
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