Porous Networks of CdSe Nanocrystal Chains from Ultrafine Cd(OH

pubs.acs.org/Langmuir © 2009 American Chemical Society

Porous Networks of CdSe Nanocrystal Chains from Ultrafine Cd(OH)2 Nanowires and Their Composite Materials Sungwook Ko,† Jeong Won Kim,‡ Geon Dae Moon,† Hee-Sang Shim,‡ Won Bae Kim,‡ and Unyong Jeong*,† †

Department of Materials Science and Engineering, Yonsei University, 134 Shiinchon-dong, Seoul, Korea and ‡ Department of Materials Science and Engineering and Program for Integrated Molecular System (PIMS), Gwangju Institute of Science and Technology (GIST), Gwangju 500-172, Korea Received September 12, 2009. Revised Manuscript Received November 20, 2009

Long ultrathin Cd(OH)2 nanowires have been selectively grown on silica colloids in a basic aqueous condition. The Cd(OH)2 nanowires could be detached from the surface of the silica colloids by simply applying ultrasonication and then transformed into isolated CdSe nanocrystal chains. When the transformation into CdSe was conducted without detaching the Cd(OH)2 nanowires, nanoporous CdSe shells composed of wire-like nanocrystal chains were produced. The good solubility of the Cd(OH)2 nanowires in both hydrophilic and hydrophobic solvents facilitated the formation of composites with quantum dots, magnetic particles, organic molecules, and polymers. Embedding premade quantum dots possessed broad light absorption range and enhanced photoluminescence. Large amount of superparamagnetic particles endowed a fast magnetic response in addition to the fluorescence. Composites of organic/nanocrystal chains were readily fabricated by employing the electrostatic attraction between the positively charged Cd(OH)2 nanowires and negatively charged polymers or small molecules.

Introduction Closely spaced nanocrystals exhibit a strong electronic coupling between neighboring nanocrystals,1 leading to significant difference in electronic and optical properties from those of individual nanocrystals. When multiple islands are connected in series, electrons pass through them by tunnel coupling one at a time, making possible to precisely control the charge transport2 and electromagnetic energy transfer.3 Especially, one-dimensional (1D) chains of nanocrystals may generate a miniband4 that each energy level of individual nanocrystals gives rise to and may realize the single electron transistors.5 Although the fabrication of two- or three-dimensionally structured patterns of nanocrystals is now straightforward,6 alignment in a straight 1D direction is still very challenging. Connecting premade nanocrystals is one pathway to obtain nanocrystal chains,7 however most *Corresponding author. E-mail: [email protected]. (1) (a) Keller, M. W.; Eichenberger, A. L.; Martinis, J. M.; Zimmerman, N. M. Science 1999, 285, 1706. (b) Bylander, J.; Duty, T.; Delsing, P. Nature 2005, 434, 361. (c) Hong, S. -K.; Nam, S. W. Phys. Rev. B 2007, 76, 035321. (2) (a) Shangguan, W. Z.; Au Yeung, T. C.; Yu, Y. B.; Kam, C. H. Phys. Rev. B 2001, 63, 235323. (b) Wang, J. X.; Kais, S.; Remacle, F.; Levine, R. D. J. Phys. Chem. B 2002, 106, 12847. (c) Novikov, D. S.; Drndic, M.; Levitov, L. S.; Kastner, M. A.; Jarosz, M. V.; Bawendi, M. G. Phys. Rev. B 2005, 72, 075309. (3) (a) Brongersma, M. L.; Hartman, J. W.; Atwater, H. A. Phys. Rev. B 2000, 62, R16356. (b) Maier, S. A.; Kik, P. G.; Atwater, H. A. Phys. Rev. B 2003, 67, 205402. (c) Citrin, D. S. Nano Lett. 2004, 4, 1562. (4) (a) Diaz, J. G.; Planelles, J.; Jaskolski, W.; Aizpurua, J.; Bryant, G. W. J. Chem. Phys. 2003, 119, 7484. (b) Diaz, J. G.; Planelles, J. J. Phys. Chem. B 2004, 108, 2873. (5) (a) Kelvin, D. L.; Roth, R.; Lim, A. K. L.; Alivisatos, A. P.; McEuen, P. L. Nature 1997, 389, 699. (b) Weiss, D. N.; Brokmann, X.; Calvet, L. E.; Kastner, M. A.; Bawendi, M. G. Appl. Phys. Lett. 2006, 88, 143507. (6) (a) Rao, C. N. R.; Kulkarni, G. U.; Thomas, P. J.; Edwards, P. P. Chem. Soc. Rev. 2000, 29, 27. (b) Zhao, S. -Y.; Wang, S.; Kimura, K. Langmuir 2004, 20, 1977. (c) Fan, H.; Leve, E.; Gabaldon, J.; Wright, A.; Haddad, R. E.; Brinker, C. J. Adv. Mater. 2005, 17, 2587. (7) (a) Cho, K. S.; Talapin, D. V.; Gaschler, W.; Murray, C. B. J. Am. Chem. Soc. 2005, 127, 7140. (b) Devries, G. A.; Brunnbauer, M.; Hu, Y.; Jackson, A. M.; Long, B.; Neltner, B. T.; Uzun, O.; Wunsch, B. H.; Stellacci, F. Science 2007, 315, 358. (8) (a) Rao, C. N. R.; Govindaraj, A.; Deepak, F. L.; Gunari, N. A.; Nath, M. Appl. Phys. Lett. 2001, 78, 1853. (b) Lee, E. J. H.; Ribeiro, C.; Longo, E.; Leite, E. R. J. Phys. Chem. B 2005, 109, 20842.

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studies have employed the templating against wire-like soft materials such as surfactants,8 biomolecules,9 and polymers.10 Vapor transport and condensation method was employed to obtain interconnected metal oxide nanocrystals.11 Very recently, Ozin and co-workers demonstrated that Bi2S3 nanowires having diameters of less than 2 nm may be composed of discontinuous nanocrystals linked by amorphous bridges.12 Despite these advances, experimental studies for more reliable process to massively obtain nanocrystal chains are greatly required. In this paper, we focus on the synthesis of wire-like quantum dot chains of CdSe and the formation of nanoporous CdSe shells composed of the quantum dot chains. Our approach employs a simple chemical transformation of Cd(OH)2 ultrathin nanowires into continuous CdSe nanocrystal chains. Cd(OH)2 is a wide bandgap semiconductor (∼2.75 eV) which can be used in numerous applications due to its optical and electrical properties as well as the capability of feasible transformations into its oxide or chalcogenides such as CdO, CdS, and CdSe.13,14 Ichinose et al. (9) (a) Alivisatos, A. P.; Johnsson, K. P.; Peng, X.; Wilson, T. E.; Loweth, C. J.; Bruchez, M. P.; Schultz, P. G. Nature 1996, 382, 609. (b) Torimoto, T.; Yamashita, M.; Kuwabata, S.; Sakata, T.; Mori, H.; Yoneyama, H. J. Phys. Chem. B 1999, 103, 8799. (c) Lee, S.-W.; Mao, C.; Flynn, C. E.; Belcher, A. M. Science 2002, 296, 892. (d) Gao, X.; Djalali, R.; Haboosheh, A.; Samson, J.; Nuraje, N.; Matsu, H. Adv. Mater. 2005, 17, 1753. (h) Padalkar, S.; Hulleman, J. D.; Kim, S. M.; Rochet, J. C.; Stach, E. A.; Stanciu, L. A. Nanotechnology 2008, 19, 275602. (10) (a) Darling, S. B.; Yufa, N. A.; Cisse, A. L.; Bader, S. D.; Siberner, S. J. Adv. Mater. 2005, 17, 2446. (b) Park, M. J.; Char, K. Langmuir 2006, 2, 1375. (c) Geng, B. Y.; Ma, J. Z.; Liu, X. W.; Du, Q. B.; Kong, M. G.; Zhang, L. D. Appl. Phys. Lett. 2007, 90, 043120. (11) (a) Yang, P.; Lieber, C. M. J. Mater. Res. 1997, 12, 2981. (b) Lao, J.; Huang, J.; Wang, D.; Ren, Z. Adv. Mater. 2004, 16, 65. (c) Alina Magdas, D.; Cremades, A.; Piqueras, J. Appl. Phys. Lett. 2006, 88, 113107. (d) Ko, T. S.; Chu, C. P.; Chen, J. R.; Chang, Y. A.; Lu, T. C.; Kuo, H. C.; Wang, S. C. J. Vac. Sci. Technol. A 2007, 25, 1038. (12) Cademartiri, L.; Malakooti, R.; O’Brien, P. G.; Migliori, A.; Petrov, S.; Kherani, N. P.; Ozin, G. A. Angew. Chem., Int. Ed. 2008, 47, 3814. (13) (a) Tang, B.; Zhou, L.; Ge, J.; Niu, J.; Shi, Z. Inorg. Chem. 2005, 44, 2568. (b) Li, X.; Chu, H.; Li, Y. J. Solid State Chem. 2006, 179, 96. (c) Ye, M.; Zhong, H.; Zheng, W.; Li, Y. Li, Langmuir 2007, 27, 9064. (d) Mlonde, S. N.; Andrews, E. M.; Thomas, P. J.; O'Brien, P. Chem. Commun. 2008, 2768. (e) Santamaria, M.; Bocchetta, P.; Quarto, F. D. Electrochem. Commun. 2009, 11, 580. (14) Shim, H. -S.; Shinde, V. R.; Kim, J. W.; Gujar, T. P.; Joo, O.-S.; Kim, H. J.; Kim, W. B. Chem. Mater. 2009, 21, 1875.

Published on Web 12/09/2009

DOI: 10.1021/la903415n

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Ko et al. Scheme 1. Illustration Describing the Growth of Ultrathin Cd(OH)2 Nanowires on the Surface of a Colloidal Silicaa

a The Cd(OH)2 nanowire brushes can be taken off the silica surface by ultrasonication. Chemical transformation produces a wire-like chain of CdSe nanocrystals. The overgrown nanowire bush is transformed into nanoporous CdSe shells consisting of nanocrystal chains. Premade nanoparticles and molecules can be adsorbed in the Cd(OH)2 bushes and trapped in the porous CdSe shells, making hybrid structures.

demonstrated an aqueous synthesis of ultrathin and uniform Cd(OH)2 nanowires by employing the simple olation process in basic conditions.15 Very recently, we applied a similar concept to suggest a template-free method for the Cd(OH)2 nanowire bundles that can be directly and preferentially grown on various oxide substrates.16 It was also demonstrated that the Cd(OH)2 nanowire bundles could be readily converted into CdSe nanotubes for photoelectrochemical application.14 This study further develops the transformation to obtain CdSe nanocrystal chains, and fabricate hybrid materials with other nanostructured materials and polymers. As demonstrated by the Ozin group,12 quantum dot chains can possess both quantum confinement effect of nanocrystals and transportation of electrons through 1D structure, which implies strong excitonic features and fast electron movement. Such dual properties are greatly required in photovoltaic applications, especially in organic solar cells.

Experimental Section Synthesis of Cd(OH)2 on Silica Colloids. Ammonium hydroxide (100 mL, NH4OH, 28%, Aldrich) was mixed with an aqueous solution (100 mL) of 0.1 M cadmium nitrate tetrahydrate (Cd(NO3)2 3 4H2O, 98%, Aldrich) in a 500 mL beaker under magnetic stirring. Sol-gel processed silica particles (0.4464 g) were introduced into the mixture solution. Mild magnetic stirring was applied to the mixture solution at pH = 12 at 60 °C. As the ammonia evaporated, Cd(OH)2 was nucleated and grew into nanowires after about 3 h, at which the pH decreased to less than 10. Transformation into CdSe Nanocrystal Chains. Selenium powder (80 mg, 99.5%, Aldrich) was dissolved in 100 mL of aqueous sodium borohydride solution (20 mM; NaBH4, 98.5%, Aldrich) at 90 °C for 30 min. The reaction of Se with NaBH4 generates aqueous NaHSe solution. The aqueous NaHSe solution (100 mL) was added to the preheated (90 °C) aqueous suspension of silica colloids with Cd(OH)2 nanowires at the surface. The transformation was completed within ∼10 min. The resulting colloids were collected by centrifuge and washed with DI water. The product was soaked in trioctylphosphine (TOP) in order to remove any possible remaining Se, and then centrifuged. Two cycles of centrifuge and dispersion in water or ethanol were conducted for the final product. The CdSe chains are consisting of single crystalline CdSe directly connected each other. They are stable for a long time and under long-time sonication. Ultrasonication can chop down the long chains to less than 100 nmlong nanorods. (15) Ichinose, I.; Kurashima, K.; Kunitake, T. J. Am. Chem. Soc. 2004, 126, 7162. (16) Shinge, V. R.; Shim, H.-S.; Gujar, T. P.; Kim, H. J.; Kim, W. B. Adv. Mater. 2008, 20, 1008.

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Formation of Composite Materials. For the generation of CdSe nanocrystal chains incorporating premade quantum dots, the aqueous Cd(OH)2 nanowires on the silica particles were first centrifuged and water was decanted, and then redispersed in THF. Premade CdSe quantum dots capped with oleic acid were introduced in the THF suspension of Cd(OH)2 and waited for 10 min under mechanical stirring. Cd(OH)2 nanowires lost their positive charge in THF and were readily dispersed in THF. The premade CdSe quantum dots were capped with oleic acid and also dispersed in THF. The hydrophobic-hydrophobic interaction between such small materials can overwhelm the dispersion factors and forms stable binding. Centrifuge enable the collection of the Cd(OH)2 and quantum dots in the mixed form. The material was redispersed in small amount of water under violent agitation and added to NaHSe aqueous solution for the conversion into CdSe. For the formation of composite with superparamagnetic particles, iron oxide (Fe3O4, EMG 911, Ferrofluids) nanoparticles with average diameter of 10 nm were simply added to the Cd(OH)2 solution in THF and centrifuged. Conversion into CdSe followed the same process. Characterization. SEM images were taken using a JEOL JSM-6700F at an acceleration voltage 5-20 kV. TEM images and electron diffraction pattern were acquired from a JEOL 2100F at an acceleration voltage 200 kV.

Results and Discussion Scheme 1 illustrates the experimental process used in this work. Ultrathin single crystalline Cd(OH)2 nanowires (∼4 nm in diameter) were obtained on the surface of silica colloids by the controlled chemical precipitation from a basic aqueous solution of Cd(NO3)2. Cd(OH)2 is known to grow on the surface of metal oxide substrates, especially on OH-functionalized surfaces.16 Precipitation of solute molecules is thermodynamically preferential on a compatible solid surface when the concentration of the solute is kept diluted. Without any substrate in a solution batch, the nucleation and growth begins in the solution, starting at the evaporation front of the solution because pH value is the lowest at the solution surface. Such process eventually creates large Cd(OH)2 plates instead of fine nanowires. We used highly concentrated silica colloids under mechanical stirring so that heterogeneous nucleation and growth takes place only on the colloidal surfaces in the homogeneous pH environment. The asgrown Cd(OH)2 nanowire brushes on the silica surface could be immediately taken off by ultrasonication and retrieved as individual nanowires. When the short nanowire brushes on the colloidal surface were allowed to grow further, they formed a dense bush of long ultrathin Cd(OH)2 nanowires. Since the chemical Langmuir 2010, 26(6), 4377–4381

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Figure 1. TEM (A, D) and SEM (B, C) images showing the growth of Cd(OH)2 nanowires on the surface of silica colloids according to reaction time. The images were taken after reaction for (A) 180 min, (B) 210 min, and (C, D) 240 min.

transformation of Cd(OH)2 into CdSe creates mechanical stress due to severe volume change and lattice mismatch, the ultrathin nanowires cannot maintain the original shape of the Cd(OH)2 nanowires. Simultaneous transformation along entire length of the Cd(OH)2 nanowires led to the formation of long chains of CdSe nanocrystals. Isolated CdSe nanocrystal chains were prepared from the separately collected Cd(OH)2 nanowires. Directly from the Cd(OH)2 nanowire bushes on the silica colloids, on the other hand, nanoporous CdSe shells were produced. Figure 1 shows TEM and SEM images monitoring the growth of the Cd(OH)2 nanowires on the silica colloids (∼300 nm in diameter) prepared by the St€ober process.17 Cd(OH)2 nanostructures were selectively precipitated on the silica surface in a basic aqueous solution of Cd(NO3)2 (pH = 12) by slowly evaporating NH3 at 60 °C under mild magnetic stirring. In our experiments, the growth rate of the nanowires could be regulated by adjusting the concentration of the silica colloids at fixed evaporation rates. The Cd(OH)2 nanowires rapidly grew at low silica concentrations, while slow growth of the nanowires was monitored at high concentrations. This kinetic difference is attributed to the increased number of nucleation sites with a limited supply of Cd precursor at high silica concentrations. The results in Figure 1 were obtained at a relatively high concentration of silica colloids in the Cd(NO3)2 solution (0.4464 g of silica in 200 mL of solution). At 180 min of reaction time, ultrathin Cd(OH)2 nanowires protruded from the silica surfaces as shown in the TEM image of Figure 1A. It is worthwhile mentioning that the nanowires seemed to instantly erupt on a specific narrow surface region at the early growth stage rather than random growth over the entire surface of silica colloids. It is thought that the ionic products were first accumulated on the silica surface and then nanowires grew on the specific accumulation sites. The average diameter of these ultrathin nanowires was about 4 nm. The length of the nanowires at this stage was several hundreds of nanometers, which was not detectable with SEM (see Figure S1A of the Supporting Information). Any change in diameter of the nanowires was not detected during further reaction up to 6 h; instead, the nanowires grew longer and denser as the reaction proceeded. At the reaction time of 210 min, the nanowires were noticeably identified with SEM, as shown in Figure 1B. At 240 min reaction time, it is clearly seen that the hairy long nanowires were entangled each other and (17) (a) St€ober, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62. (b) Lu, Y.; McLellan, J.; Xia, Y. Langmuir 2004, 20, 3464.

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Figure 2. TEM images showing the transformed CdSe nanocrystal chains and nanoporous shells. (A) Nanocrystal chains transformed from the isolated Cd(OH)2 nanowires obtained by applying ultrasonication. The upper inset is the electron diffraction image of the zinc blende nanocrystals. The bottom inset is a HRTEM image of the crystalline lattice indicated with an arrow. (B) Core-shell structures composed of silica core and the porous CdSe shells transformed directly from Cd(OH)2 bushes on the silica colloids. (C) Broken CdSe shell indicating the core is movable. (D) High magnification of the square box in part C, showing the nanoporous structure of the shell. The scale bars in parts C and D are 100 and 5 nm, respectively.

formed a nanowire bush (Figure 1, parts C and D). When the nanowires were allowed to grow longer than 6 h, they started to aggregate into bundles (see Figure S1B in the Supporting Information). The diameter of the nanowire bundles ranged from 50 to 100 nm. The aggregation to make bundles would take place when the free volume for each growing nanowire becomes decreased. Indeed, the aggregation on the surfaces of silica colloids in this study occurred much later comparing with the results on flat oxide layers previously reported in ref 15. This is because the convex curvature of the silica colloids with a substantially larger surface area can accommodate large volume enough for the axially growing nanowires, which enables the nanowires keep isolated for longer time. On flat surfaces with much less nucleation sites, the volume is limited for growing nanowires, therefore, the nanowires readily aggregate at the very early stage of their growth. The as-synthesized ultrathin Cd(OH)2 nanowires were transformed into CdSe by reacting with NaHSe aqueous solution at 90 °C for 10 min. NaHSe solution was prepared by dissolving Se powder in a NaBH4 aqueous solution.14,18 The transformed CdSe nanocrystal chains are displayed in the TEM images (Figure 2). The CdSe in Figure 2A was obtained from the isolated Cd(OH)2 nanowires taken off from the silica surface. The transformation took place at any place along the Cd(OH)2 nanowires. We believe the reaction is not an anion exchange reaction, but chemical reaction between Cd2þ and Se2- ions within the Cd(OH)2 templates. We can find the explicit reason in the conversion reaction from Cd(OH)2 bundles to hollow CdSe tubes.14 Cd2þ ions diffuse outward from Cd(OH)2 to react with Se2- and form CdSe layer at the surface of the initial Cd(OH)2 bundles. Because of the large lattice mismatch (a = 6.08 for zinc blende CdSe, and a = 3.50, c = 4.71 for hexagonal Cd(OH)2) and volume shrinkage (ΔV/Vo = 0.437), deformation in nanocrystal chains was not (18) Yamamoto, H.; Oshima, K. Main Group Metals in Organic Synthesis; Wiley-VCH: Weinheim, Germany, 2006; p 816.

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avoidable. The size of the CdSe nanocrystals had narrow size distribution (5-10 nm). It is because the uniform diameter of the ultrathin Cd(OH)2 nanowires limited the dimension of nanocrystals during the transformation. The electron diffraction peak in the inset matched with zinc blende crystal structure. The inset HRTEM image of a nanocrystal indicated by an arrow revealed interplanar distance of 0.351 nm for the spacing in (111) plane of the zinc blende. Each bead part of the chain was single-domain crystals. Such single domain crystals directly connected each other can endow interesting electrical and optical properties.19 When the transformation was executed from the Cd(OH)2 bushes on the silica surfaces, spherical shells fashioned of CdSe nanocrystal chains were created as exhibited in Figure 2B. It is thought that the decreased solubility of CdSe in water led to collapse of the chains toward the silica surface. Since the Cd(OH)2 nanowires were entangled each other, the collapsed CdSe chains naturally had a porous shell structure. The silica colloids within the CdSe shells were not centered (Figure 2B) and possessed clean surfaces (Figure 2C), which indicates the colloids were not bound to the CdSe shells. On the basis of such observations, the Cd(OH)2 nanowires are considered detached from the silica surfaces during the transformation, while the entangled structure of the pristine Cd(OH)2 nanowires created mechanically stable nanoporous CdSe layers. Figure 2D is a higher magnification of the white box in Figure 2C, displaying a highly nanoporous CdSe shell. The silica colloids could be etched in a dilute HF solution (2 wt %). The spherical hollow CdSe shell structure was maintained in most cases, but the shells on large silica colloids collapsed during the etching process (see Figure S2, parts A and B, in the Supporting Information). The atomic composition did not change after the acid treatment, as indicated in the EDX analysis of Figure S2C (Supporting Information). The silica colloids with Cd(OH)2 nanowire bush could be dispersed in both hydrophilic and hydrophobic solvents such as water, alcohols, chloroform, THF, etc. The good solubility of Cd(OH)2 to various organic solvents greatly facilitate further functionalization via composite formation in solution phases. Parts A-E of Figure 3 exhibit attachment of premade quantum dots (A, B) and superparamagnetic iron oxide nanoparticles (D) to the Cd(OH)2 nanowire surfaces, and display the optical (C) and magnetic characteristics (E) of the transformed CdSe chains. For the preparation, the nanoparticles and the silica colloids with Cd(OH)2 nanowires were dispersed in THF together, and retrieved by centrifuge. Once the nanoparticles penetrate through the flexible Cd(OH)2 nanowire bush, the outward diffusion is limited and the retention is enhanced. Parts A and B of Figure 3 clearly demonstrate the quantum dots decorating the surface of Cd(OH)2 nanowires. Once transformed into CdSe porous shells, the nanoparticles could not leak out through the nanopores. Figure 3C displays the emission spectra of the premade quantum dots (green), nanocrystal chains (red), and nanocrystal chains including premade quantum dots (black). The spectra were recorded in THF because organic solvent can increase the quantum efficiency of CdSe nanocrystals.20 The as-transformed CdSe porous shells emitted light centered at ∼540 nm and small amount of emission at lower wavelengths (Figure 3C). When the premade quantum dots (emission at 510 nm) were embedded in the porous CdSe shells, the emission ranged from 450 to 600 nm. Although the spectrum from the CdSe shells containing QDs has the maximum at the identical wavelength with that of QDs, it (19) Peng, L.; Manna, X.; Yang, W.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59. (20) Yu, K.; Zaman, B.; Singh, S.; Wang, D.; Ripmeester, J. A. Chem. Mater. 2005, 17, 2552.

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Figure 3. (A, B) TEM images showing attachment of premade quantum dots to the dense brush Cd(OH)2 nanowires. (C) Photoluminescence spectra of quantum dots (green), porous CdSe shells transformed without quantum dots (red), and porous CdSe shells in the presence of the quantum dots (black). (D) TEM image exhibiting heavy loading of iron oxide nanoparticles to the Cd(OH)2 nanowires and (E) a camera image showing the high response of the colloids in part D.

Figure 4. Fluorescence microscope images under illumination with filtered wavelength (510-550 nm) of (A) CdSe shells transformed without premade quantum dots and (B) CdSe shells transformed in the presence of premade green-fluorescence quantum dots.

exhibited somewhat broader emission than those from pure QDs or CdSe shells without QDs. The longer wavelength is attributed to aggregation of premade QDs and as-transformed CdSe shells. The emission at shorter wavelength is not currently clear, but we suppose it was because the transformations of Cd(OH)2 into CdSe with and without premade QDs had been carried out separately, although the initial Cd(OH)2 was the same. Therefore, the emission from the CdSe shells itself may be different in cases with and without QDs. Such broader emission was repeatedly observed. The as-transformed CdSe porous shells optically emitted bright red color when they were illuminated with filtered wavelength (510-550 nm), while their emission was weak with shorter wavelengths. The CdSe porous shells with premade QDs emitted yellowish light under the same wavelength illumination, as shown in Figure 4. They emitted light with shorter wavelengths. The color change is consistent with the additional emission at the shorter wavelengths from the premade QDs. Colloids containing both photoluminescence and superparamagnetism have attracted wide interest due to their multiple functions useful in sensing, imaging, and separation of biomolecules. The easy process to embed iron oxide nanoparticles in the porous CdSe shells can be another synthetic pathway for such functional colloids. Figure 3D Langmuir 2010, 26(6), 4377–4381

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Cd(OH)2 nanowires. Similar process can be applied to polymerbased composites. As an example, we demonstrate composite formation with poly(acrylic acid)(PAA) in Figures 5. Because of the low pKa value of PAA (4.5-4.7), the polymer has negative charge at neutral water or basic conditions. Simple addition of the polymer in the Cd(OH)2 nanowire suspension in water immediately forms a gel-like composite (Figure 5A). The SEM image in Figure 5B shows the Cd(OH)2 nanowires were surrounded the PAA chains. Once the composite was formed, it was irreversible. The transformation into CdSe gave a polymeric solid composite (Figure 5C). The TEM image in Figure 5D exhibits a lot of CdSe nanocrystal chains in PAA matrix. We expect the process can be further extended to fabricate functionalized composite patterns on various substrates.

Conclusions Figure 5. (A, B) A camera image and a SEM image showing a hybrid of poly(acrylic acid)(PAA) and Cd(OH)2 nanowires, taken right after adding the polymer into the nanowire suspension in water. (C, D) SEM and TEM image of a hybrid of PAA/CdSe nanocrystal chains.

displays iron oxide nanoparticles heavily loaded in the CdSe shells. Figure 3E demonstrates the strong magnetic response of the colloids prepared in this study. Such response comes from the high density of iron oxide nanoparticles in the shells. The samples showed the same photo luminescence as the bare CdSe shells. Another simple way to make composite structures is molecular adsorption to the Cd(OH)2 nanowire surfaces. Ichinose et al. demonstrated that Cd(OH)2 nanowires prepared from Cd(NO3)2 have positive charges at the surface.15 They suggested that the Cd ions at surface exist in the form of [Cd(OH)(H2O)p-1]þ and may condense each other to form divalent [Cd(H2O)p-1O-Cd(H2O)p-1]2þ ions. The ζ potentials of the silica colloids with the Cd(OH)2 nanowires varied with the length of the nanowires. The ζ potential of bare silica colloids was -27. When the Cd(OH)2 nanowires were short (3 h reaction time), the ζ potential was -14.2. But, when the nanowires grew long enough (4 h reaction time) the value increased to 7.95, indicating the surface charge turned positive. The positive charge can be utilized to form composites with negatively charged organic or inorganic materials. Figure S3 in the Supporting Information shows fluoresceinloaded Cd(OH)2 nanowires obtained in a basic aqueous solution. The organic dyes kept adsorbed even after several washing process with neutral water. The strong fluorescence from the composite tells the existence of positive charge at the surface of the

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We have obtained ultrathin long Cd(OH)2 nanowires on silica colloids and transformed them into CdSe nanocrystal chains. Isolated Cd(OH)2 nanowires taken off the silica colloids produced individual CdSe nanocrystal chains, while the hairy Cd(OH)2 nanowire bushes generated nanoporous CdSe shells composed of wire-like nanocrystal chains. The good solubility of Cd(OH)2 nanowires to both hydrophilic and hydrophobic solvents enabled easy formation of composite structures in the CdSe shells. The photoluminescence intensity of CdSe shells embedding premade QDs was enhanced compared with those without containing premade QDs. The chemically connected structure may provide good charge transport capability, which may be useful in solar cell applications. Composite with superparmagnetic iron oxide nanoparticles could provide optical/magnetic multiple functions to the colloids. The positive charge of Cd(OH)2 nanowires facilitated the adsorption of negatively charged molecules at the surface, resulting in composites with small molecules or polymeric materials. The simple process and high throughput may offer new class of composites with multiple functions. Acknowledgment. We would like to acknowledge the financial support from the Pioneer Research Program of KOSEF (00805103) and Korea Research Foundation (KRF-2007-311D00367). Supporting Information Available: Figures showing SEM, TEM, optical images, and EDS analysis of the Cd(OH)2 nanowires, CdSe shells, and composites. This material is available free of charge via the Internet at http://pubs.acs.org.

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