Assemblies of Blue-emitting CdSe Nanocrystals in Redox Copper Phenanthroline-based Polymeric Chains

Shu-Juan Fu and Kuan-Jiuh Lin*

* Department of Chemistry, Center of Nanoscience and Nanotechnology, National Chung Hsing University, Taichung 402, Taiwan. E-mail:kjlin@dragon.nchu.edu.tw

ABSTRACT The polymer backbone of 1DOP-Cu(II) consists of

phosphonate groups, [-Cu-O3P(CH2)2-PO3-Cu-], at which acid-functionalized groups can serve as a stabilizing shell that are effect in passiviting the surface of nanoparticles. Like trioctylphosphine oxide (TOPO)/trioctylphospine (TOP) species containing phophine oxides (O=P), CdSe particles would be trapped in 1DOP-Cu surface sites to yield a novel nanocomposite that has a unique 1D structure. Firstly, blue-emitting CdSe-doped micelles, spherical assemblies of CdSe nanoclusters into CTAB-micelles, were prepared at room temperature for the first time. The CdSe-doped nanoclusters, well-dispersed in water for a few weeks, were then self-arranged along the 1DOP-Cu polymeric chains to give rise to one-dimensional heterostructures.

Keywords: composite, energy transfer, hybrid, one-dimensional materials, quantum dots,

1. INTRODUCTION

Assemblies of quantum-confined semiconductor

nanoparticles into a matrix phases such as block copolymer micelles[1, 2] and silica shell[3] have importantly been considered issues for opening the door to “tunable” optical, electrical, and even mechanical properties. Moreover, sharp tuning of 3D spherical morphology in water solution is a fascinating target owing to favourable optical properties in biological delivery, making it an ideal bioavailability for fluorescence labelling and photocatalysis applications.[4 – 8] Most importantly, assemblies of Qdots into 1D array could exhibit polarized emission leading to polarized lasing and was found to be advantageous in polymer nanocrystal-based solar cells because of improvement in charge transport properties.[9] It is therefore of interest to explore the kind of hybrid nanostructures whereby nanopartilces are wrapping around the polymers. Progress in synthetic chemistry has made rational design of a metal nanoparticle array by using macromolecules such as DNA,[10] proteins[11] and polysaccharide[12], giving 1D bioinorganic nanocomposite materials. However, little is known about the interaction between Qdots and coordination polymers in aqueous solution. In this paper we report that redox copper-

based polymer chain structure is capable of assembling CdSe nanoparticles into a one-dimensional array.

2. RESULTS AND DISCUSSIONS

2.1 Preparation of fibrous structures of

copper-based coordination polymer (1DOP-Cu) We previously reported a one-dimensional redox copper

phenanthroline-based polymeric chain structure, namely 1DOP-Cu, which revealed supramolecular machine for the first time.[13] In the viewpoint of crystal structure, 1DOP-Cu adopts a π - π coordination sheet-like framework that are composed of mats of 1D coordination polymeric chains in the solid state. It has been found that solids of 1DOP-Cu are slightly soluble in water, in which the pale yellow solution was stable for a few weeks, and its optical profile of UV-Vis spectrum was similar to that of the solid state. We therefore assume that the 1D chain structure exfoliating from the π - π framework exits in the aqueous solution. Furthermore, the topography of HRTEM for the pale yellow solution reveals the fibrous structures that are indicative of polymeric chains of 1DOP-Cu (Fig. 1). The

Figure 1. Spacing-filling plot of 1DOP-Cu showing the corrugated π-coordination framework constructed from copper-ethylenediphosphate chains incorporating phen molecules.

NSTI-Nanotech 2007, www.nsti.org, ISBN 1420063766 Vol. 4, 2007418

most important intriguing feature is that the backbone of 1DOP-Cu consists of phosphonate groups, [-Cu-O3P(CH2)2-PO3-Cu-], at which acid-functionalized groups can serve as a stabilizing shell that are effect in passiviting the surface of nanoparticles.[14] Like trioctylphosphine oxide (TOPO)/trioctylphospine (TOP) species containing phophine oxides (O=P), CdSe particles would be trapped in 1DOP-Cu surface sites to yield a novel nanocomposite that has a unique 1D structure. Accordingly, schematic approach for the synthesis of CdSe/1DOP-Cu hybrid architectures is illustrated in Figure 2.

Figure 2. Schematic representation of synthesis of one-dimensional arrangement of CdSe Qdots by copper phen-based superstructure (1DOP-Cu). The present work can be divided into three distinct steps: (a) Room temperature spherical assemblies of CdSe nanoparticles in CTAB matrix, so-called the CdSe-doped micelles. (b) Undirectional arrangement of CdSe-doped micelles along polymeric chains of 1DOP-Cu, giving a new hybrid composite, namely, CdSe@1DOP-Cu. (c) Preparation of a one-dimensional CdSe Qdots array after recovering through ultracentrifugation with deionized water.

2.2 Synthesis of CdSe-doped micelles and their optical properties

The key step is to form quantum-confined Qdots by means of judicious choices of encapsulating materials. Cetyltrimethylammonium bromide (CTAB) is a well-know cationic surfactant that has been used as a functional ligands or structure-directing agent in the synthesis of Au,[15] Ag,[16] and Pt[17] nanoparticles of various size and particles. Moreover, CTAB has also been used a suitable micelle to synthesize 3D dendritic-type Pt nanoclusters.[18,

19] To achieve the spherical architectures, we developed a room temperature method to encapsulate CdSe Qdots into a nanospherical CTAB matrix to form 3D dendritic-type nanoclusters, so called the CdSe-doped micelles. Our synthesis approach is quit simple: First, a mixture of Cd(NO3).4H2O (0.473 g, 2 mmol), 2-mercaptoethanol acid (as stabilizer, 0.335 mL, 4.8 mmol), CTAB (as surfactant, 0.1895 g, 0.52 mmole ), and H2O (100 mL) was loaded in a three-neck flask. After stirring for 30 mins, oxygen-free NaHSe aqueous solution (6 mL, 2 mmol) was then swiftly

injected into the mixture solution during rapid stirring at room temperature. After the injection, the mixed solution was changing the color of the solution immediately to yellow precipitation. The yellow products were collected through careful decantation and washed several times with deionic water.

Microscopic observation indicated that the yellow products were well-dispersed in water after sonication for 3 – 5 min. We investigated the dimension of CdSe particles by using high resolution transmission electron microscopy (HRTEM). A drop from each diluted aqueous suspension was placed on a copper TEM grid and then the water was left to evaporate by using vacuum pumping. Figure 3a clearly displayed well-dispersed micelles with the size range of 80 – 100 nm, which indicated that the CTAB molecules in the form of reverse micelles control the seeding growth. The lattice structure of the trapped CdSe Qdots cannot be defined due to the presence of the surfactant. However, most of the encapsulated CdSe Qdots are located within the CTAB-stabilized micelles. HRTEM images (Fig. 3b,c) of CdSe-doped micelles reveal CdSe nanocrystals having high degree of monodispersity. The CdSe Qdots have a diameter range of 1.6 - 2.1 nm. According to the mass approximation model proposed by Brus,[20, 21] CdSe-doped micelles with particle size of 1.6 - 2.1 nm are expected to have fascinating property on blue-light emission. Owing to the fact that the CdSe Qdots were encapsulated in the nanospheres, it was reasonable to expect that the optical properties of the entrapped CdSe would be more chemically stable than those of free

Figure 3. HRTEM images for CdSe-doped micelles, showing the quantum-confined CdSe nanoparticles dispersed throughout a spherical CTAB matrix in water. CdSe Qdots. Evidence for optical stability for CdSe-doped micelles has been studied through UV-Vis and emission spectroscopy. As shown in Figure 4 it was found that the CdSe-doped micelles reflect the first excitonic transition band at 415 nm in which absorption profile was remained

NSTI-Nanotech 2007, www.nsti.org, ISBN 1420063766 Vol. 4, 2007 419

up for 3 weeks. This suggested that there is no aggregation on storage within spherical micelles. Furthermore, its corresponding emission spectrum shows an emission band centered at 480 nm with bandwidth of 50 nm as well as a shoulder envelope around 585 nm. The blue-emission band of CdSe-doped micelles is well preserved under CdSe Qdots encapsulation in the CTAB-stabilized micelles. To the best of our knowledge, this blue-emitting was for the first time investigated in luminescent CdSe nanocrystals.

Figure 4. Absoption (a) and emission (b) spectra for CdSe-doped micelles. Arrow shows the first exciton peak for CdSe Qdots. The excitation wavelength was 382 nm.

2.3 Assemblies of CdSe-doped micelles along 1DOP-Cu polymer chains (CdSe/1DOP-Cu)

Next, we want to focus on the study of the interaction between CdSe-dopped micelles and 1DOP-Cu polymeric chains. An aqueous of dispersed 1DOP-Cu chains was then added to the CdSe-doped micelles solution. The mixture was centrifuge (3000 rpm) for 10 mins, and the supernatant was removed with a pipette. The CdSe/1DOP-Cu composite was separated and then dispersed into fresh water. By repeating this process eight times, any excess Qdots was removed. To obtain further evidence that 1DOP-Cu interacts with CdSe Qdots, the morphology of the composite was directly observed by AFM and HRTEM (Fig. 5). The diameter of spherical CdSe-doped micelles was found to be contracted when the CdSe/1DOP-Cu composite was recovered through ultracentrifugation with deionized water. From the AFM images shown in Figure 5a, we can see that CdSe-doped micelles with average diameters of 20 - 30 nm are closely packed along the one-dimensional polymeric chains on the glass surface. Apparently, HRTEM images (Fig. 5b) also show that the diameter of spherical cluster of CdSe-doped micelles was contracted to 20 nm. Energy-dispersive X-ray line-scan profiles for CdSe/1DOP-Cu nanocomposite reveal C, O, S, Cu, Se, Cd, and S bands when the electron beam is focused inside one-dimensional array, which are in good qualitative agreement with the element analysis of CdSe/1DOP-Cu

hybrid heterostructures. When the hybrid solution was maintained at steady state for 1 h, we observed the presence of fibrous structures of the composite, with diameters of around 2 – 3 nm (Fig. 5c, d), indicating that the quantum-confined CdSe nanopartciles appear to be a one-dimensional array in the presence of 1DOP-Cu. The promising result strongly supports the fact that 1DOP-Cu can enwrap CdSe-doped micelles without any gaps and arrange Qdots in a one-dimensional array. This facile technology opens up a new avenue for assemblies of inorganic nanoparticles in one-dimensional nanoparterning structure, which is indispensable for nanotechnologies.

Figure 5. AFM (a) and HRTEM (b – c) images for CdSe@1DOP-Cu composites, showing the assembling of CdSe/1DOP-Cu hybrid architecture into a one-dimensional fashion.

3. CONCLUSION

We have successfully aligned stable blue-emitting CdSe

nanocrystals in 1D fashion by copper phen-based polymeric chains. This 1D nanocomposite would be readily applicable to the design of new heterogeneous photocatalysts, homogenous assays of phototoxic drugs, and superoxide dismutase (well-known O2-activiating sites in biology). [22,

23]

REFERENCES

[1] Y. Chen, Z. Rosenzweig, Nano Lett. 2002, 2(11), 1299. [2] N. Duxin, F. Liu, H. Vali, A. Eisenberg, J. Am. Chem.

Soc., 2005, 127, 10063. [3] Z. Zhelev, H. Ohba, R.Bakalova, J. Am. Chem. Soc.

2006, 128, 6324. [4] L. Rogach, A. Kornowski, M. Gao, A. Eychmüller, H,

Weller, J. Phys. Chem. B 1999, 103, 3065. [5] Z. A. Peng, X. Peng, J. Am. Chem. Soc. 2001, 123(1),

183. [6] D. V. Talapin, A. L. Rogach, A Kornowski.; M. Haase;

H. Weller, Nano Lett. 2001, 1(4), 207.

NSTI-Nanotech 2007, www.nsti.org, ISBN 1420063766 Vol. 4, 2007420

[7] M. Jr, Bruchez, M. Moronne, P. Gin, S. Weiss, A. P. Alivisators, Science 1998, 281, 2013

[8] L. Li, J. Hu, W. Yang, A. P. Alivisatos, Nano Lett. 2001, 1(7), 349.

[9] V. L. Colvin, M. C. Schlamp, A. P. Alivisators, Nature, 1994, 370, 354 – 356.

[10] a) A. P. Alivisatos, K. P. Johnsson, X. Peng, T. E. Wilson, C. J. Loweth, M. P. J. Bruchez, P. G. Schultz, Nature 1996, 382, 609 –611; b) J. J. Storhoff, A. A. Lazarides, R. C. Mucic, C. A. Mirkin, R. L. Letsinger, G. C. Schatz, J. Am. Chem. Soc. 2000, 122, 4640 –4650.

[11] S. Connolly, S. N. Rao, D. Fitzmaurice, J. Phys. Chem. B 2000, 104, 4765 – 4776.

[12] a) M. Brust, D. Bethell, C. J. Kiely, D. J. Schiffrin, Langmuir 1998, 14, 5425 – 5429; b) Y. Kuwahara, T. Akiyama, S. Yamada, Thin Solid Films 2001, 393, 273 – 277; c) T. Sagara, T. Kato, N. Nakashima, J. Phys. Chem. B 2002, 106, 1205 – 1212.

[13] K. J. Lin, S. J. Fu, C. Y. Cheng, W. H. Chen, H. M. Kao, Angew. Chem. Int. Ed, 2004, 43, 4186.

[14] a) X. Michalet, F. F. Pinaud, L. A. Bentolila, J. M. Tsay, S. Doose, J. J. Li, G. Sundaresan, A. M. Wu, S. S. Gambhir, S. Weiss, Science, 2005, 307, 538. b) A. Bae, M. Tumata, T. Hasegawa, C. Li, K. Kaneko, K. Sakurai, S. Shinkai, Angew. Chem. Int. Ed, 2005, 44, 2033.c) E. Oh, M.-Y. Hong, D. Lee, S.-H. Nam, H. C. Yoon, H.-S. Kim, J. Am. Chem. Soc., 2005, 127, 3270..d) S. J. Clarke, C. A. Hollmann, Z. Zhang, D. Suffern, S. E. Bradforth. N. M. Dimitrijevic, W. G. minarik, J. L. Nadeau, Nature materials 2006, 5, 409.

[15] B. Nikoobakht, M. A . El-Sayed, Chem. Mater. 2003, 15, 1957.

[16] N. R. Jana, L. Gearheart, C. J. Murphy, Chem. Commun. 2001, 617.

[17] T. Zheng, N. Nishiyama, Y. Egashira, K. Ueyama, Colloids Surf. A 2005, 262, 52.

[18] M. H. Ullah, W. S. Chung, I. Kim, C. S. Ha, Small 2006, 2, 870.

[19] K. J. Lin, L. J. Chen, M. R. Parasad, C. Y. Cheng, Adv. Mater. 2004, 16(20), 1845.

[20] L. E. Brus, J. Phys. Chem. 1986, 90, 2555. [21] K. Hiratani, M. Nomoto, H. Sugihara, T. Okada,

Analyst 1992, 117, 1491. [22] W. Kaim, J. Rall, Angew. Chem. Int. Ed. Engl. 1996,

35, 43 - 60.K. [23] Tanaka, A. Tengeiji, T. Kato, N. Toyama, M.

Shionoya, Science, 2003, 299, 1212 - 1213.

NSTI-Nanotech 2007, www.nsti.org, ISBN 1420063766 Vol. 4, 2007 421