Chem. Mater. 2010, 22, 2309–2314 2309 DOI:10.1021/cm9032572
Self-Assembly of TGA-Capped CdTe Nanocrystals into Three-Dimensional Luminescent Nanostructures Hongjun Chen,† Vladimir Lesnyak,† Nadja C. Bigall,‡ Nikolai Gaponik,† and Alexander Eychm€ uller*,† †
Physikalische Chemie, TU Dresden, Bergstr. 66 b, 01062 Dresden, Germany, and ‡ Istituto Italiano di Tecnologia, Via Morego, 30, 16163 Genoa, Italy Received October 23, 2009. Revised Manuscript Received January 27, 2010
In this paper, we report on a convenient and quick self-assembly of thioglycolic acid (TGA)-capped CdTe nanocrystals (NCs) into three-dimensional (3D) nanostructures in solution, in which largescale nanowires are found as building blocks. The wet 3D nanostructures can be further dried by critical CO2 to obtain solids with a volume of about 1 cm3 and a density of about 1/2500th of bulk CdTe. By SEM, EDS, and HRTEM characterization, it is found that the nanowires actually are CdTe@Cd-TGA complex hybrid nanostructures in which many well-separated CdTe NCs are uniformly distributed. The hybrid nanowires can reach several micrometers in length and 25 ( 8 nm in width. As a result of the CdTe NCs with their integrity and effective protection by the Cd-TGA complex, the hybrid nanowires and 3D nanostructures still show visually bright luminescence and retain the size-quantized properties of the CdTe NCs. This new kind of QD-based nanostructures may be suitable for subsequent processing into quantum-confined superstructures, materials and devices. Introduction As a result of its simplicity, versatility, and low cost, selfassembly is an important fabrication method in nanochemistry and nanotechnology. Self-assembly shows great potential for directing the organization of nanoparticles and opens new avenues of nanotechnology through the controlled fabrication of one-, two-, and three-dimensional (3D) nanostructures with unique optical, magnetic, and electronic properties.1 Ligand-stabilized colloidal nanoparticles are ideally suited to hierarchically self-assemble, because the nanoparticle core dictates optical, electronic, or magnetic properties, whereas the surface-bound ligands define the particle’s interactions with its surroundings.2 Guided by intermolecular or interparticle interaction force fields, self-assembly has been successfully used to *Corresponding author. E-mail: alexander.eychmueller@chemie. tu-dresden.de.
(1) Colloids and colloid assemblies; Caruso, F., Ed.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2004 (ISBN: 3-527-30660-9). (2) Lin, Y.; Skaff, H.; Emrick, T.; A. Dinsmore, D. T.; Russell, P. Science 2003, 299, 226. (3) (a) Liu, J.; Raveendran, P.; Qin, G.; Ikushima, Y. Chem. Comm. 2005, 2972. (b) Tang, Z.; Kotov, N. A.; Giersig, M. Science 2002, 297, 237. (c) Bigall, N. C.; Reitzig, M.; Naumann, W.; Simon, P.; Pee, K-H; Eychm€ uller, A. Angew. Chem., Int. Ed. 2008, 47, 7876. (4) (a) Tang, Z.; Zhang, Z.; Wang, Y.; Glotzer, S. C.; Kotov, N. A. Science 2006, 314, 274. (b) Chen, H.; Dong, S. Langmuir 2007, 23, 12503. (5) (a) Mokari, T.; Rothenberg, E.; Popov, I.; Costi, R.; Banin, U. Science 2004, 304, 1787. (b) Salant, A.; Amitay-Sadovsky, E.; Banin, U. J. Am. Chem. Soc. 2006, 128, 10006. (c) Caswell, K. K.; Wilson, J. N. U.; Bunz, H. F.; Murphy, C. J. J. Am. Chem. Soc. 2003, 125, 13914. (d) Chen, H.; Wang, Y.; Jiang, H.; Liu, B.; Dong, S. Cryst. Growth Des. 2007, 7, 1771. (6) (a) Rakovich, Y. P.; Volkov, Y.; Sapra, S.; Susha, A. S.; D€ oblinger, M.; Donegan, J. F.; Rogach, A. L. J. Phys. Chem. C 2007, 111, 18927. (b) Chen, H.; Jia, J.; Dong, S. Nanotechnology 2007, 18, 245601. r 2010 American Chemical Society
construct differently shaped nanostructures such as nanowires,3 nanosheets,4 nanochains,5 2D networks,6 and superlattices7 in solution or at interfaces. The past decade has witnessed a tremendous progress in the wet-chemical preparation of a variety of semiconductor nanocrystals (NCs) known as quantum dots (QDs) with defined size, shape, and surface chemistry. The properties of QDs strongly depend on the type of nanocrystal, size, shape, and capping agent; thus, they can be tuned by a proper choice of the synthetic conditions. As a result of relatively low toxicity and easy fabrication, the aqueous approach to the synthesis of thiol-capped QDs with optical activities covering the spectral region from the UV through the visible into the near IR attracts more and more attention.8 Besides ordinary QDs, 1D semiconductors with a high aspect ratio such as wires,9 tubes,10 or ribbons11 are also reported. In addition, QD-based nanostructures in all (7) (a) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Science 1995, 270, 1335. (b) Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115. (c) Dumestre, F.; Chaudret, B.; Amiens, C.; Renaud, P.; Fejes, P. Science 2004, 303, 821. (8) (a) Gaponik, N.; Talapin, D. V.; Rogach, A. L.; Hoppe, K.; Shevchenko, E. V.; Kornowski, A.; Eychm€ uller, A.; Weller, H. J. Phys. Chem. B 2002, 106, 7177. (b) Shavel, A.; Gaponik, N.; Eychm€uller, A. J. Phys. Chem. B 2004, 108, 5905. (c) Rogach, A. L.; Kornowski, A.; Gao, M.; Eychm€uller, A.; Weller, H. J. Phys. Chem. B 1999, 103, 3065. (d) Rogach, A. L.; Kershaw, S.; Burt, M.; Harrison, M.; Kornowski, A.; Eychm€uller, A.; Weller, H. Adv. Mater. 1999, 11, 552. (e) Rogach, A. L.; Franzl, T.; Klar, T. A.; Feldmann, J.; Gaponik, N.; Lesnyak, V.; Shavel, A.; Eychm€uller, A.; Rakovich, Y. P.; Donegan, J. F. J. Phys. Chem. C 2007, 111, 14628. (9) (a) Niu, H.; Zhang, L.; Gao, M.; Chen, Y. Langmuir 2005, 21, 4205. (b) Zhang, H.; Wang, D.; M€ohwald, H. Angew. Chem., Int. Ed. 2006, 45, 748. (c) Kuno, M.; Ahmad, O.; Protasenko, V.; Bacinello, D.; Kosel, T. H. Chem. Mater. 2006, 18, 5722. (d) Zhang, H.; Wang, D.; M€ohwald, H.; Bai, Y. J. Am. Chem. Soc. 2006, 128, 10171. (e) Kumar, S.; Ade, M.; Nann, T. Chem.;Eur. J. 2005, 11, 2220.
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three dimensions are in the focus of current research activities because of their potential applications for bioimaging,12 light-emitting diodes (LEDs),13 and nanophotonic devices.14 Self-assembly with its great advantages is being used to construct such nanostructures. By virtue of different functional molecules or polymers as templates, CdTe NCs have been assembled into wires,9a branched wires,15 and tubes,10 respectively. Self-assembly can also take place without any templates. For example, it has been shown that thioglycolic acid (TGA)-capped CdTe NCs can self-assemble into 1D brightly emitting nanowires driven by anisotropic dipolar interparticle forces through a gentle destabilization of the colloidal solution by partial removal of the capping agent.3b By the addition of a specific buffer, TGA-capped CdTe NCs can also self-assemble into 1D nanowires16 and even 2D networks6a with bright luminescence. CdTe NCs capped with 2-(dimethylamino)ethanethiol can form 2D free-floating sheets and possess a certain mechanical robustness, showing remarkable stability during gentle stirring and drying.4a 3D QD-based nanostructures are rarely reported. Brock’s group recently used CdS,17 CdSe, and CdSe/ZnS NCs18 synthesized in organic media to form 3D aerogels. Following this work, our group reported on the controlled self-assembly of thiol-capped CdTe NCs as well as noble metal nanoparticles into 3D aerogels, extending the range of the building blocks from NCs synthesized in organic media to those synthesized in aqueous media.19 In this paper, we demonstrate a new kind of QD-based 3D luminescent nanostructures, namely, CdTe@CdTGA complex hybrid nanowires constructed by TGAcapped CdTe NCs through a convenient and quick (10) Niu, H.; Gao, M. Angew. Chem., Int. Ed. 2006, 45, 6462. (11) Cao, X.; Lan, X.; Guo, Y.; Zhao, C. Cryst. Growth Des. 2008, 8, 575. (12) (a) Lovric, J.; Bazzi, H. S.; Cuie, Y.; Fortin, G. R. A.; Winnik, F. M.; Maysinger, D. J. Mol. Med. 2005, 83, 377. (b) Gao, X.; Cui, Y.; Levenson, R. M.; Chung, L. W. K.; Nie, S. Nat. Biotechnol. 2004, 22, 969. (c) Ruan, G.; Agrawal, A.; Marcus, A. I.; Nie, S. J. Am. Chem. Soc. 2007, 129, 14759. (13) (a) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354. (b) Gao, M.; Lesser, C.; Kirstein, S.; M€ohwald, H.; Rogach, A. L.; Weller, H. J. Appl. Phys. 2000, 87, 2297. (c) Gao, M.; Sun, J.; Dulkeith, E.; Gaponik, N.; Lemmer, U.; Feldmann, J. Langmuir 2002, 18, 4098. (d) Gallardo, D. E.; Bertoni, C.; Dunn, S.; Gaponik, N.; Eychm€ uller, A. Adv. Mater. 2007, 19, 3364. (e) Bertoni, C.; Gallardo, D.; Dunn, S.; Gaponik, N.; Eychm€uller, A. Appl. Phys. Lett. 2007, 90, 034107. (f) Caruge, J. M.; Halpert, J. E.; Wood, V.; Bulovic, V.; Bawendi, M. G. Nat. Photonics 2008, 2, 247. (14) (a) Rakovich, Y. P.; Donegan, J. F.; Gerlach, M.; Bradley, A. L.; Connolly, T. M.; Boland, J. J.; Gaponik, N.; Rogach, A. Phys. Rev. A 2004, 70, 051801. (b) Solovyev, V. G.; Romanov, S. G.; Sotomayor Torres, C. M.; M€uller, M.; Zentel, R.; Gaponik, N.; Eychm€uller, A.; Rogach, A. L. J. Appl. Phys. 2003, 94, 1205. (15) Zhang, L.; Gaponik, N.; M€ uller, J.; Plate, U.; Weller, H.; Erker, G.; Fuchs, H.; Rogach, A. L.; Chi, L. Small 2005, 1, 524. (16) Volkov, Y.; Mitchell, S.; Gaponik, N.; Rakovich, Y. P.; Donegan, J. F.; Kelleher, D.; Rogach, A. L. ChemPhysChem 2004, 5, 1600. (17) Mohanan, J. L.; Arachchige, I. U.; Brock, S. L. Science 2005, 307, 397. (18) (a) Arachchige, I. U.; Brock, S. L. J. Am. Chem. Soc. 2006, 128, 7964. (b) Arachchige, I. U.; Brock, S. L. J. Am. Chem. Soc. 2007, 129, 1840. (19) (a) Gaponik, N.; Wolf, A.; Marx, R.; Lesnyak, V.; Schilling, K.; Eychm€ uller, A. Adv. Mater. 2008, 20, 4257. (b) Bigall, N. C.; Herrmann, A.-K.; Vogel, M.; Rose, M.; Simon, P.; Carrillo-Cabrera, W.; Dorfs, D.; Kaskel, S.; Gaponik, N.; Eychm€uller, A. Angew. Chem., Int. Ed. 2009, 48, 9731.
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self-assembly route. Compared to the reports referenced above, our approach has some obvious advantages: (i) the self-assembly process is very quick, taking only about 8 min; (ii) the perspective of up-scaling is due to the simplicity of the synthesis and handling of the QDs in water; (iii) the absence of any oxidant like H2O2, tetranitromethane, or hν/O218,19 in the self-assembly process guarantees the QDs to still retain their physical integrity and luminescence; and (iv) unlike most 1D single-crystal semiconductors which exhibit low or no luminescence owing to the weaker quantum-confinement effect, the nanowires and 3D nanostructures synthesized by our route show visually bright luminescence due to a good separation of the QDs in the nanostructure. Experimental Section Chemicals. Al2Te3 was purchased from Cerac Inc. Cd(ClO4) 3 6H2O, H2SO4, NaAc, NaOH, ethanol, and acetone were purchased from Merck. All reagents were used as received without further purification. The water used was purified through a Millipore system. Synthesis of TGA-Capped CdTe NCs and 3D Nanostructures. TGA-capped CdTe NCs were prepared according to the literature.8a,e,21 After filtration the crude CdTe NCs colloids were directly used without any postpreparative treatment. The Cd and Te containing compounds should be handled carefully due to their toxicity. In a typical synthesis of 3D CdTe@Cd-TGA complex hybrid nanostructures, 0.6 mL of CdTe NCs colloid, 3 mL of ethanol, and 1 mL of 0.5 M sodium acetate were mixed with subsequent heating at 70 C for 8 min. During this process, first floccules of the hybrid nanostructures appeared, and they gradually grew into bigger objects. After decantation, the wet 3D hybrid nanostructures were obtained. After complete solvent exchange with water-free acetone,19 the wet 3D hybrid structures were further dried by a supercritical drying technique with liquid CO2 in a critical point drier. Instrumentation. UV-vis absorption spectra were recorded with a Cary 50 spectrophotometer (Varian). UV-vis reflection spectra from the dry 3D hybrid nanostructures were recorded on a Cary 5000 spectrophotometer (Varian) equipped with an integrating sphere (Labsphere Varian) for measurements with scattering samples. Photoluminescence (PL) measurements were performed at room temperature using a FluoroMax-2 spectrofluorimeter (Instruments SA). The X-ray diffraction (XRD) patterns were collected on a D5000 diffractometer (Siemens, Cu KR radiation). A critical point drier 13200J-AB (Spi Supplies) was used. Energy-dispersive X-ray spectroscopy (EDS) and scanning electron microscopy (SEM) were performed on a Zeiss DSM 982 Gemini instrument. The samples for SEM and EDS characterization were prepared by transferring the wet 3D hybrid nanostructures onto silicon slides with subsequent natural drying. For better conductivity, a thin layer of gold was sputtered onto the samples. Transmission electron micrographs (TEM) and high-resolution transmission electron (20) (a) Kagan, C. R.; Murray, C. B.; Nirmal, M.; Bawendi, M. G. Phys. Rev. Lett. 1996, 76, 1517. (b) Cicek, N.; Nizamoglu, S.; Ozel, T.; Mutlugun, E.; Karatay, D. U.; Lesnyak, V.; Otto, T.; Gaponik, N.; Eychm€uller, A.; Demir, H. V. Appl. Phys. Lett. 2009, 94, 061105. (c) Tang, Z.; Ozturk, B.; Wang, Y.; Kotov, N. A. J. Phys. Chem. B 2004, 108, 6927. (21) Shavel, A.; Gaponik, N.; Eychm€ uller, A. J. Phys. Chem. B 2006, 110, 19280.
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Figure 2. Typical absorption spectra of the aqueous solutions of the CdTe NCs (solid lines) and the reflection spectra of the dry 3D CdTe@Cd-TGA complex hybrid nanostructures (dashed lines).
Figure 1. Typical photographs of the wet (upper) and dry (below) 3D CdTe@Cd-TGA complex hybrid nanostructures under day light (left) and under UV lamp excitation (λex = 365 nm) (right).
micrographs (HRTEM) were made on a Tecnai T20 microscope operating at 200 kV (FEI). The samples for TEM and HRTEM characterization were prepared by sonication of the wet 3D hybrid nanostructures in acetone for 2 min and transferring a drop of solution onto a carbon-coated copper grid.
Results and Discussion Two different sizes of TGA-capped CdTe NCs were used: NCs with a green emission at 531 nm (∼2.2 nm in diameter) and NCs with a red emission at 634 nm (∼3.4 nm in diameter). After the quick self-assembly of the NCs and supercritical CO2 drying, finally the corresponding 3D nanostructures were obtained as shown in Figure 1. It can be seen that the CdTe NCs are assembled into yellow and pink monoliths in either the wet (Figure 1, top) or the dry (Figure 1, bottom) state. The dry 3D nanostructures shown have a volume of approximately 1 cm3 and a weight of approximately 2.6 mg which combines for about 1/2500th of the density of bulk CdTe. Under UV lamp excitation (λex = 365 nm), an intense luminescence of both the wet and the dry structures is seen. The emissions being green or red depending on the initial NC size did not subsequently differ between the wet and the dry states. Despite these monoliths exhibiting 3D nanostructures, they nevertheless retain the optical features of their corresponding NC building blocks. As illustrated in Figure 2, the dry 3D nanostructures with different emissions exhibit absorption onsets which are similar to the corresponding NCs precursors, demonstrating the retention of the electronic properties in the dry 3D nanostructures. Figure 3 displays the typical PL spectra of the original CdTe NC solutions, the wet and the dry 3D nanostructures, respectively. The emissions are strong and the bands are narrow, reflecting the narrow particle size distribution in both wet and dry 3D nanostructures.
Figure 3. Typical PL spectra of the aqueous solutions of the CdTe NCs (solid lines), wet (dashed lines), and dry (dotted lines) 3D CdTe@CdTGA complex hybrid nanostructures.
The observed red-shift of the PL in the 3D nanostructures compared to the PL of their original NC solutions is a common phenomenon observed from densely packed NC ensembles,20a in particular CdTe NC aerogels,19a specially designed CdTe NC layer-by-layer nanostructures,20b or nanowires.20c This red-shift may be the result of energy transfer from relatively smaller (with a higher exciton energy) to relatively larger (with a lower exciton energy) NCs assembled in the proximity of each other. Statistically such energy transfer should result in the relative enhancement of the lower energy part of the PL spectrum. Alternatively, this red-shift may be explained by reabsorption of the high-energy part of the emission in the optically dense nanostructures. At the same time, the retention of the quantum-confinement in the 3D nanostructures suggests that the CdTe NCs remain effectively isolated, which can be attributed to the presence of spacers between the NCs.17 Hence, in terms of the distance between the optically active entities, the nanostructures are in balance between the conditions facilitating and those preventing energy transfer, which is a key point in high quality light management (white balance, color rendering index, etc.).20b
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Figure 4. XRD patterns of CdTe NCs and resulting 3D CdTe@CdTGA complex hybrid nanostructures.
The powder XRD pattern shown in Figure 4 is characteristic for zinc blende CdTe (JCPDS file, No.75-2086) illustrating that the nanostructure formation has no apparent impact on the crystallinity of the primary CdTe NCs. The large width of the diffraction peaks is related to the small size of the CdTe NCs in the 3D nanostructures. Weak signal and noisy appearance of the powder XRD spectrum may be explained by relatively high content (see discussion below) of amorphous Cd thiolate complexes in the nanostructures formed. Typical SEM, TEM, and HRTEM images of the 3D nanostructures are shown in Figure 5. From the SEM image (Figure 5A), it can be seen that many curved nanowires with high aspect ratio (containing also about 5% of big spherical particles) are stacked together. The nanowires can reach several micrometers in length and 25 ( 8 nm in width. From Figure 5B, it can be seen that the produced nanowires exhibit smooth surfaces and that they are curved or bent. Therefore, these nanowires are easily entangled and pile up together to form 3D nanostructures. Figure 5C shows a representative HRTEM image of the nanowires. It is clearly discerned that the nanowires are not single crystalline but rather composed of well-separated CdTe NCs with differently oriented lattice planes. This is in contrast to CdTe nanowires organized by CdTe NCs through partial removal of the capping agent in which the CdTe NCs recrystallized into single-crystalline wurtzite-phase nanowires on the much larger time scale of several days.3b Owing to the retention of single crystal lattices these novel QD-based nanostructures still keep the optical and electronic properties of their building blocks;along the line of the above shown absorption and PL spectra (Figures 2 and 3). The influence of sodium acetate, ethanol, and temperature on the morphology of the 3D nanostructures was investigated. As seen in Figure 6A, without the addition of sodium acetate, no separate nanowires are obtained. In the same way, if no ethanol is added, only clear CdTe QD solutions and no 3D nanostructures are produced. Conducting this experiment at room temperature, it needs
Figure 5. Typical SEM (A), TEM (B), and HRTEM (C) images of the 3D CdTe@Cd-TGA complex hybrid nanostructures.
several hours to completely settle the CdTe QDs, and the CdTe QDs stably stick to the walls of the flask. As shown in Figure 6B, in this case no 3D nanostructures and only a QD film is formed demonstrating that the temperature is a very important parameter for this self-assembly process. To get further insight into the underlying formation mechanism of the nanowires, SEM images of the samples at earlier stages of the reaction, namely, after 1 and 5 min, are presented in Figure 7. As shown in Figure 7A (sample after 1 min of reaction), at the beginning a merely structured, thin, and flat material can be observed. After a reaction time of 5 min (Figure 7B), the final nanowires (cf. Figure 5A) have almost formed and the sheet-like material has disappeared. EDS was used to analyze the time-dependent elemental compositions of the nanowires from the CdTe NC precursors during the self-assembly process, as listed in Table 1. From this table, the following
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Figure 6. Typical SEM images of the products synthesized without sodium acetate (A) or at room temperature (B) and otherwise identical experimental conditions as in Figure 5.
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Figure 7. Typical SEM images of the earlier stages of the 3D CdTe@Cd-TGA complex hybrid nanostructure formation after a reaction time of 1 (A) and 5 (B) minutes.
trends can be deduced: the Cd component remains almost unaltered in the course of time; the S component is increased, while the Te component is gradually reduced. It is also found that the changes of the S and Te contents are large at the initial self-assembly time (from 0 to 1 min) and gradually decrease (from 1 to 5 min), and finally (from 5 to 8 min) almost no changes can be observed. Niu and Gao reported that TGA can react with Cd2þ ions to form 1D nanowires even without CdTe nanocrystals.10 In this formation process most probably the Cd-TGA complexes adopt a similar coordination structure as the linear chain structures of [CdII(μ-SCH2COOCH2CH3)2]¥ reported by Dance et al.22 in which the carboxylic groups of TGA align along the R axis, while the single Cd-TGA polymeric chains undergo anisotropic aggregation along the R and b axes, which leads to the formation of the wirelike structure.10 Therefore, we assume that the formation of 1D Cd-TGA complex nanowires evolving readily through the reaction of TGA with Cd2þ ions plays a crucial role in the reaction mechanism. We consider our wire-like objects to be amorphous Cd-TGA complex nanowires (seen in Figure 5B) acting as glue for the CdTe NCs distributed both on the surface and inside. Almost all of the CdTe NCs are incorporated in the final product as evidenced also by almost complete loss of characteristic CdTe absorption and emission in the supernatant. From the EDS analysis, it can also be seen that the
amount of the Cd component remains almost constant in the whole self-assembly process. These two observations rule out the possibility of a loss of CdTe NCs causing the small ratio of the Te component in 3D nanostructures. We can also exclude the possibility of decomposition of the CdTe NCs causing the seemingly small Te content in the 3D nanostructures. If the CdTe NCs were decomposed, the Te2- ions should be very easily oxidized by the oxygen of air, and as a result the black color of the elementary Te should appear.23 This is, however, not the case. Moreover, the diffraction peaks of elementary Te are also not present in the XRD data (see Figure 4). Thus, a proper explanation for the elemental ratios observed is the formation of CdTe@Cd-TGA complex hybrid nanowires in conjunction with the limited detection depth of EDS. If we consider many of the CdTe NCs to be encapsulated by Cd-TGA complex
(22) Dance, I. G.; Scudder, M. L.; Secomb, R. Inorg. Chem. 1983, 22, 1794.
(23) Tang, Z.; Wang, Y.; Sun, K.; Kotov, N. A. Adv. Mater. 2005, 17, 358.
Table 1. Time-Dependent Elemental Composition of the 3D CdTe@Cd-TGA Complex Hybrid Nanostructures from the CdTe NCs Precursors during the Self-Assembly Process elemental composition reaction time (min)
Cd (%)
S (%)
Te (%)
0 1 5 8
49.5 45.6 46.5 47.8
22.3 43.8 46.9 46.7
28.2 10.6 6.6 5.7
Cd/S/Te ratio 1:0.45:0.57 1:0.96:0.23 1:1:0.14 1:0.98:0.12
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nanowires, those will not be detected by EDS which results in a lowered Te component in the final 3D nanostructures while the newly formed Cd-TGA nanowires still sustain the unaltered Cd component. This can also explain why the colors of the 3D nanostructures are blanched in comparison to those of the initial CdTe NCs solution (cf. Figure 1). Regarding now the underlying formation mechanism, we suggest that the presence of a large quantity of ethanol results in a partial depletion of TGA molecules on the surface of the CdTe NCs3b which enables them to react with the extra Cd2þ ions in solution to quickly form the 1D Cd-TGA nanowires.10 On the other side, the CdTe NCs become unstable due to the partial removal of the TGA molecules. Furthermore, the residual TGA molecules on the surface of the CdTe NCs may also be involved in the reaction with Cd2þ ions in solution. Therefore, it is reasonable to presume that the CdTe NCs will be easily encapsulated into the 1D Cd-TGA nanowires to form the CdTe@Cd-TGA complex hybrid nanowires during the self-assembly process. Moreover, if the Cd:Te ratio before the reaction (Table 1) is taken as a reference corresponding to the originally present CdTe NC phase, the molar ratio of the amorphous (Cd-TGA) and crystalline phases in the final nanostructure is estimated approximately as 4:1. The experimental conditions of high temperature (70 C) and high salt concentration (0.5 M sodium acetate) facilitate the quick formation of the CdTe@Cd-TGA complex hybrid nanowires (8 min). To further verify the formation mechanism, we purified the CdTe NCs by precipitation and redispersion and repeated the experiment. In this case, only by centrifugation of the sample could the formation of a small precipitate be achieved, which was characterized by SEM. As shown in Figure 8, the morphology of the product is significantly different to that of the samples obtained without washing the nanoparticles. Although some nanowires can still be seen, a large amount of spheres ranging from 180 nm to 1.3 μm occurred, similar to the colloidal ZnSe spheres reported by Zhong et al.24 It is reasonable to deduce that the loss of a large quantity of Cd2þ ions leads to a severe change in the shapes of the product, demonstrating that the excess of Cd2þ ions plays a vital role in this self-assembly process. It is noted that the CdTe@Cd-TGA complex hybrid nanowires are completely different from the (24) Zhong, H.; Wei, Z.; Ye, M.; Yan, Y.; Zhou, Y.; Ding, Y.; Yang, C.; Li, Y. Langmuir 2007, 23, 9008.
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Figure 8. Typical SEM image of the product synthesized using the purified CdTe QDs and otherwise identical experimental conditions as in Figure 5.
reported single-crystalline wurtzite-phase CdTe nanowires3b,9a,11 and that they are also different from the 2D CdTe nanosheets organized by close-packed CdTe NCs through some cooperative driving forces.4a In the selfassembly process reported here, the complex interaction between the Cd2þ ions and TGA molecules (including the dissociative TGA molecules and the residual TGA molecules on the surface of the CdTe NCs) is the main reason for the formation of the CdTe@Cd-TGA complex hybrid nanowires. The in situ formed Cd-TGA complexes play a role as mechanical support for the CdTe NCs and also act as spacers between the inner CdTe NCs limiting their interactions. In addition, the 1D Cd-TGA complex nanowires also provide an effective protection for the CdTe NCs inside. Conclusion In summary, we report on a new kind of QD-based nanostructure through a quick and convenient self-assembly route from CdTe NCs. Because of the well-separated CdTe NCs distributed in the CdTe@Cd-TGA complex hybrid nanowires, the 3D nanostructures fabricated maintain quantum confinement and luminescence, which may be suitable for the manufacturing of LEDs (e.g., as light-conversion layers), optical sensors, and optical gain applications. Acknowledgment. We thank Ellen Kern for the SEM measurements and H.C. appreciates the support from the Alexander von Humboldt (AvH) Foundation. This research was supported by the EU project INNOVASOL and the DFG project EY16/10-1.
