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CdSe Quantum Dot Growth on Magnetic Nickel Nanoparticles - Crystal

Apr 10, 2013 - Synthesis and Characterization of Branched fcc/hcp Ruthenium Nanostructures ... Accounts of Chemical Research 2014 47 (10), 3045-3051...
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CdSe Quantum Dot Growth on Magnetic Nickel Nanoparticles Wai Ruu Siah, Alec P. LaGrow, Moritz J. Banholzer, and Richard D. Tilley* School of Chemical and Physical Sciences and the MacDiarmid Institute for Advanced Materials and Nanotechnology, Victoria University of Wellington, Wellington 6012, New Zealand S Supporting Information *

ABSTRACT: This paper reports the formation of Ni−CdSe core−shell structures. The Ni−CdSe core−shell structures were synthesized via a seed-mediated growth method where a CdSe shell was grown on preformed nickel core particles. The types of nickel cores include nickel nanocubes of 13 nm in size and spherical nickel nanoparticles of 11 and 45 nm in size. The coating of 37 nm nickel carbide is also included for comparison. The resultant thickness and morphology of the CdSe shell layer varied depending on the size, shape, and composition of the nickel-based core particles used. Highresolution transmission electron microscopy characterization and analysis of the Ni−CdSe interface led to further understanding of the factors governing the growth between these two phases. Optical and magnetic characterizations were also carried out on selected samples. The use of nickel-based cores with different size, shape, and composition allows the thickness and morphology of the CdSe shell layer to be controlled. This led to the changes in the CdSe optical properties.

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INTRODUCTION Synthesis and characterization of hybrid nanocrystals have received widespread interest and attention recently due to their potential as multifunctional advanced materials.1−3 A variety of hybrid nanocrystals made up of a combination of metallic, semiconductors, and magnetic materials have been reported in the literature. Magnetic-semiconductor4−9 materials are of particular interest due to their bifunctionality and potential for catalytic and biomedical applications.1−3 These nanocrystals can be formed in two major types of morphologies involving those of core−shells6,7,10 and heterodimers.4,11,12 However, while the methods for synthesizing these magneticsemiconductor colloidal hybrids structures have been established, in depth understanding of the factors governing the structural morphology and preferential orientation between the core and the shell material are limited. The key parameters that are thought to affect the formation of hybrid nanocrystals include the crystal structures, lattice constants, and the nature of the chemical bonds of the materials involved.13 Further understanding of these factors can assist in the design and synthesis of colloidal hybrid nanocrystals with tunable properties. In this work, we investigate the factors governing the structural morphology and preferential orientation between the core and the shell material via a Ni−CdSe system. The Ni− CdSe system is a magnetic-semiconductor system which consists of the face-centered-cubic (fcc)-metal nickel as the core material and cadmium selenide (CdSe) as the shell material. With dependence on the size, shape, and composition of the nickel-based cores used, the thickness and morphology of the CdSe shell vary accordingly. As such, the resultant magnetic © XXXX American Chemical Society

and luminescent properties of the core−shell could be tuned effectively.

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RESULTS AND DISCUSSION The nickel seeds were prepared according to literature methods reported by our group for the nanocubes14 and by Luo et al.15 for the spherical nickel cores (for details see the Supporting Information). The growth of the CdSe shell layer was achieved by dropwise injection of Cd and Se precursor solutions into a solution of oleylamine at 240 °C, containing the nickel seeds. The injection rate was approximately 0.02 mL/min. The cadmium precursor solution was prepared by dissolving cadmium acetate in oleylamine; and the selenium precursor was prepared by reacting selenium powder with trioctyphosphine (TOP) to obtain the Se-TOP complex. The resultant core−shell structures were isolated from the reaction mixture through magnetic separation. Each sample was characterized by TEM, electron diffraction, optical, and magnetic measurements. The Ni Nanocube-CdSe Core−Shells. The nickel nanocube cores provided a flat surface for the nucleation and growth of CdSe. The resultant grain boundary between nickel and CdSe can be studied and provide information on the grain boundary structure as well as the growth behavior of the CdSe domains. Figure 1a shows the low magnification TEM image of the nickel cores used for the coating reactions. The nickel nanocubes were synthesized with a previously reported Received: February 1, 2013 Revised: March 25, 2013

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to 26 ± 3 nm. The average thickness of the CdSe nanoparticles grown on the surface of the nickel cores is 5.3 ± 0.9 nm, and the width is 7 ± 1 nm. Hence, the average size of the CdSe domains is about 6 ± 1 nm. HRTEM of the Ni Nanocube−CdSe Core−Shells. Figure 2 (panels a and b) shows the high resolution TEM images of

Figure 1. (a) TEM image of the as-synthesized nickel nanocube cores. (b) TEM image of the CdSe-coated nickel nanocubes. (c) Size distribution plot of CdSe-coated nickel nanocubes.

Figure 2. Orientation relationships between Ni and CdSe phases. Two preferred orientations identified through HRTEM. (a) Ni{100}/wzCdSe{0001}. (b) Ni{100}/zb-CdSe{111}. (c−d) Enlargement of the boxed areas. (e−f) FFT of the corresponding boxed areas.

method.14 The majority of the particles are cube-shaped particles with an average size of 12.9 ± 0.8 nm. Figure 1b shows the TEM images of the nickel cores after CdSe coating. The EDS spectrum (Figure S1a of the Supporting Information) obtained from this sample confirmed the presence of nickel and CdSe. The selected area electron diffraction (SAED) pattern and XRD pattern (Figure S1, panels b and c, of the Supporting Information ) can be indexed to hexagonal wz-CdSe and cubic zb-CdSe phase. Each nickel nanoparticle is shown to be coated with smaller CdSe particles on the surface. Figure 1c shows the size distribution plot comparing the size distributions of the Ni−CdSe core−shells, nickel cores, and CdSe nanoparticles attached to the surface of the nickel cores. The average size of the nickel nanocube cores is 13 ± 1 nm, which is similar to the original seeds. After the coating reaction, the average size of the Ni−CdSe core−shell particles increased

two typical Ni nanocube−CdSe nanoparticles, oriented in the Ni ⟨100⟩ direction. As can be seen in the image, the squareshaped dark nickel cores are encased by lighter contrast individual CdSe nanocrystals. The grain boundary between Ni and CdSe is clearly visible. The CdSe domains are present on both the corner sites and the flat faces of the cubes. As shown by FFT in Figure 2 (panels e and f), the two CdSe domains high-lighted by red boxes can be indexed to the wzCdSe [10−10] and zb-CdSe [110] zones, respectively. Figure 2 (panels c and d) also show the growth direction of the CdSe domains from the surface of the cube core, which are wz-CdSe ⟨0001⟩ and zb-CdSe ⟨111⟩, respectively. Hence, these two orientation relationships can be assigned as Ni{100}/wzCdSe{0001} and Ni{100}/zb-CdSe{111}. This means that growth of the CdSe off the Ni {100} faces is either ABABAB ordering or ABCABC ordering. In either case, the first layer of B

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Figure 3. The TEM image of (a) 11 nm spherical nickel cores and (b) CdSe-coated 11 nm nickel cores. (c) Size distribution plot of CdSe-coated 11 nm nickel cores. (d) HRTEM image of a 11 nm Ni−CdSe core−shell particle. Inset: FFT pattern of the core−shell particle, indexed to wz-CdSe zone [0001].

Figure 4. (a−b) CdSe-coated 45 nm nickel cores and their respective size distribution plot. (c−d) CdSe-coated 37 nm nickel carbide cores and their respective size distribution plots.

their crystal structure, and the atoms have the same spacings (d = 3.69 Å) and hexagonal arrangement.16 In order to further confirm that this type of growth behavior is not unique to CdSe, the coating of the nickel nanocube cores was carried out with CdS for comparison. The reaction conditions under which

atoms can grow further either to form the wurtzite or the zinc blende phase. The facets that directly join to the {100} surface of the nickel cores are wz-CdSe {0001} and zb-CdSe {111} facets. The CdSe facets are equivalent as they are both the close-packed facet of C

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form nickel carbide.17 A TEM image of the nickel carbide cores is shown in Figure S6 of the Supporting Information. Injection of Cd and Se precursor solutions resulted in CdSe growth on the surface of the nickel carbide cores (see the Supporting Information for experimental details). Figure 4c shows the low magnification TEM image of the Ni3C−CdSe core−shell particles. The core particles have an average size of 37 ± 5 nm (Figure 4d). The EDS spectrum and SAED pattern (Figure S7 of the Supporting Information) obtained from this sample confirmed the presence of rhombohedral Ni3C, wz-CdSe, and zb-CdSe. As shown by Figure 4c, there are two types of CdSe nanoparticles formed on the surface of the nickel carbide cores. The first type is made up of discrete CdSe nanoparticles similar to those formed on the nickel nanocubes and the 45 nm nickel cores. Another type of CdSe nanoparticles formed a platelike shape on the surface of the nickel carbide cores (Figure 4c). HRTEM Images of Ni (45 nm)−CdSe and Ni3C (37 nm)−CdSe. Figure 5a shows an HRTEM image of the CdSecoated 45 nm nickel cores with two domains indexed. Both

the nickel nanocubes were coated with CdS was similar to that used for CdSe, except that sulfur precursor solution was injected instead of Se precursor solution (for details see the Supporting Information). Figure S2 of the Supporting Information shows the HRTEM image of a representative NiCdS nanoparticle, whereby two CdS nanocrystals are visible. Although stacking faults are present, it is clear that CdS displays the same preferential orientation as CdSe on the nickel nanocube cores. The Ni (11 nm)−CdSe Core−Shells. The second system investigated involved spherical nickel cores of 11 nm in size. These nickel cores were coated with CdSe under similar reaction conditions used for the Ni nanocube−CdSe core− shells (details in the Supporting Information). Figure 3a shows the low magnification TEM of the nickel cores. They have an average size of 11 ± 1 nm. Figure 3b shows the TEM of the core−shell particles formed upon coating with CdSe shell layer (EDS and SAED shown in Figure S3 of the Supporting Information). The core−shell particles are made up of a mixture of spherical or triangularly shaped particles. Each nickel core is observed to be fully encapsulated in a CdSe shell. Figure 3c shows the size distribution of the Ni−CdSe core− shell structures. The average size of the nickel cores is 9 ± 1 nm, which is very similar to the nickel core size before the coating reaction. The average shell thickness is 7 ± 2 nm. The average size of the Ni−CdSe core−shell structures is around 24 ± 5 nm, with a wide size distribution. The large distribution in the shell thickness of the coated nanoparticles is due to variation in the position of the coated particle within the shell. Figure 3d shows the HRTEM image of a Ni−CdSe core−shell particle. The CdSe shell layer is shown to be single crystalline in nature. The FFT pattern obtained from this particle can be indexed to the [0001] zone of wz-CdSe. The Ni (45 nm)−CdSe Core−Shells. The 45 nm Ni cores synthesized are polycrystalline in nature (Figure S4 of the Supporting Information). Figure 4a shows the low magnification TEM image of the Ni−CdSe particles obtained by coating the 45 nm nickel cores. The particles have a core−shell structure and are approximately 55 ± 8 nm in size (Figure 4b). The core−shell particles are approximately spherical in shape. The coating layer is made up of discrete nanoparticles on the surface of the core particles. The EDS spectrum (see Figure S5a of the Supporting Information) obtained from this sample confirmed the presence of nickel and CdSe. The SAED and XRD patterns can be indexed to wz-CdSe and zb-CdSe (see Figure S5, panels b and c, of the Supporting Information). Figure 4b shows the size distribution plots comparing the size distribution of the Ni−CdSe core−shells, the nickel cores, and the CdSe nanoparticles attached to the surface of the nickel cores. After the coating reaction, the nickel cores have an average size of 45 ± 5 nm, which is unchanged from the original seeds. The average size of the coated particles is 55 ± 8 nm. The average thickness of the CdSe nanoparticles grown on the surface of the nickel cores is 5.2 ± 0.9 nm, and the width is 5.3 ± 0.7 nm. Hence, the average size of the CdSe domains is estimated to be 5.3 ± 0.8 nm. The Ni3C (37 nm)−CdSe Core−Shells. Coating of CdSe was also carried out with nickel carbide cores. Nickel carbide cores were formed by adding octadecene in the reaction mixture. Under the current reaction conditions, octadecene generates active carbon species through thermal and metalcatalyzed solvent decomposition which can react with nickel to

Figure 5. (a) HRTEM image of the 45 nm Ni−CdSe core−shell structures, displaying hemisphere-like growth behavior of CdSe on the surface of the nickel cores. (b−c) Enlarged version of areas enclosed in red squares and their FFT patterns. D

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CdSe particles are viewed down the zb-CdSe [110] zone (Figure 5, panels b and c). The (111) CdSe plane is directly in contact with the Ni surface. This indicates that the zb-CdSe crystal grows in the ⟨111⟩ direction from the nickel surface. In Figure 6a, the HRTEM image of a Ni3C−CdSe core−shell nanoparticle is shown. Figure 6b shows the enlargement of the

nickel nanocube cores, the presence of faceted surfaces enables the growth of larger CdSe domains. Finally, the use of nickel carbide cores led to the formation of even larger CdSe nanoplate domains due to the larger size of the nickel carbide cores. The CdSe domain size was seen to be controlled by the size of the nanoparticle core and the proportion and size of crystal facets on the seeds surface. In particular, the comparison of CdSe growth for 11 nm spherical nickel cores and 13 nm nickel nanocubes reveals that although the particle size is similar, the CdSe growth is different. While the 11 nm spherical nickel cores were encapsulated in a single-crystalline CdSe shell, the 13 nm nickel nanocubes were coated with individual and discrete CdSe domains. Both shape and size of the spherical nickel particles is likely to have played a role. Due to their small size, the lattice strain that arises from lattice mismatch can be relieved through processes such as surface relaxation.18 The Geometry of the Ni−CdSe Inorganic Interface. A single characteristic common across the systems investigated in this study is that the crystallographic facet of CdSe attached to the surface of the core particles. They are either wz-CdSe {0001} facets or zb-CdSe {111} facets. These two facets are completely equivalent, both displaying 6-fold hexagonal symmetry. From this observation, it can be inferred that these two CdSe facets are the most favored at the Ni−CdSe interface. In particular, when the Ni−CdSe interface is planar, the preferred orientations of the wz-CdSe {0001} facets or zbCdSe {111} facets are clearly shown. However, this preference is less obvious for corner sites or for curved Ni surfaces of the spherical nickel cores. It can be inferred that when the Ni− CdSe interface is planar, the preferred Ni/wz-CdSe{0001} and Ni/zb-CdSe{111} orientation gives rise to maximum Ni−Se bonding. Hence, these orientations are preferred. Another potential factor leading to the observed Ni−CdSe preferential orientation could be that of epitaxial relationship between the two materials. As shown by the previous work of Figuerola and co-workers,16 preferential orientation between CdSe{0001} and Au{111} facets have been observed. This preferential orientation was ascribed to the epitaxial relationship between these two facets. Two reasons led the authors to believe that the Au−CdSe bonding relationship was epitaxial in nature. First, the CdSe{0001} and Au{111} facets have the same six-fold geometry. Second, the estimated lattice mismatch between these two facets was small. The similarity in geometry and small lattice mismatch (3.2%) gives rise to a low-energy interface. Hence, this CdSe{0001}/Au{111} relationship is preferred. In our system, especially the experiments with the nickel nanocube−CdSe interface, the Ni and CdSe is highly dissimilar and the lattice mismatch is large. At the interface, the Ni{100} facet has a four-fold cubic symmetry, while the CdSe{0001} facet has six-fold symmetry. Remarkably, despite being energetically unfavorable, the Ni-CdSe system displays the same preferential orientation as the Au−CdSe system. This result shows that while in some cases, epitaxial bonding plays a role in determining preferential orientation; in this case, the strong bonding between Ni−CdSe dominates over its tendency to form epitaxial bonding. The Effect of Surface Defects on the Nickel and Nickel Carbide Cores. Another possible factor that can affect the growth of the CdSe shell layers is the presence of surface defects on the nickel cores.18 These defects are in the form of discontinuities of the nickel and nickel carbide crystal lattice

Figure 6. (a) HRTEM image of a CdSe-coated nickel carbide particle. (b−c) Enlarged version of areas enclosed in red squares, along with their corresponding FFT patterns.

nickel domain enclosed in a red box. The corresponding FFT pattern can be indexed to nickel carbide zone [100]. This means that the CdSe domain is growing off the (006) facet of the nickel carbide core. The enlargement of the CdSe domain enclosed in a red box is shown in Figure 6c. On the basis of the corresponding FFT pattern, zb-CdSe (111) planes were shown to be parallel to the surface of the nickel carbide core. This indicates that the CdSe crystal facet attached to the nickel carbide surface is in the zb-CdSe ⟨111⟩ direction. Effect of the Ni Cores. The effect of the size, shape, and composition of the nickel-based cores is clearly demonstrated in this study. When small 11 nm spherical nickel cores were used, single-crystalline CdSe shells encapsulating the whole core were formed. When larger 45 nm spherical nickel cores were used, growth of discrete CdSe particles resulted. With E

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due to the polycrystalline nature of these cores. The influence of defects on the nickel and nickel carbide cores is likely to be small. Defects on the core particles can cause the Ni−CdSe interface to be defective and irregular.18 Results show that the Ni−CdSe interface is very well-defined, where the Ni and CdSe phases are clearly distinguishable. Hence, it can be concluded that the primary factor governing the growth of the CdSe lies in the nature, size, shape, and composition of the core particles. Magnetic Properties of the Core−Shell Structures. The magnetic properties of the core−shell structures formed were investigated by measuring their magnetization profiles with a SQUID (superconducting quantum interference device). The resultant hysteresis loops are shown in Figure 7 (panels a− c).

The 45 nm nickel cores (Figure 7a) show ferromagnetic hysteresis with both coercivity of 0.11 kOe and remnant magnetization of 11.5 emu/g present. The saturation magnetization (Ms) is 42.5 emu/g, which is similar to previously reported studies of Ni nanoparticles.15,19,20 Once coated with the CdSe shell, the magnetic saturation decreases by 23% to 32.7 emu/g. This decrease in the mass magnetization can be explained by the increase in total mass due to the addition of a diamagnetic shell layer. In accordance with this mass increase, it is shown that the magnetic properties of the Ni cores are not altered. Similar observations have been reported by previous works, where the diamagnetic shell layer leads to lower magnetic response of the magnetic cores.7,10,12,21,22 For the 11 and 13 nm Ni core systems, a similar magnetic behavior is observed. The 11 and 13 nm Ni core systems have lower Ms values compared to the 45 nm Ni core system due to their smaller size.23 These smaller Ni cores show superparamagnetic behavior as shown in the insets, with both remnance and coercivity absent. This shows that at this size range, the 11 and 13 nm nickel cores have reached the critical single domain size, a condition for displaying superparamagnetic behavior.23 A similar decrease in mass saturation magnetization is observed for both smaller Ni core sizes upon CdSe coating (from 7.7 to 4.8 emu/g for the 11 nm Ni− CdSe system and from 8.4 to 5.5 emu/g for the 13 nm Ni− CdSe system), which is again in accordance with the weight increase due to the shell layer. Both systems show similar Ms values due to similar core sizes. Optical Properties of the Core−Shell Structures. The optical properties of the core−shell structures synthesized have been studied. The core−shell structures synthesized in this study have an estimated quantum yield of 1−2%. This value is low compared to unattached CdSe nanoparticles, which have a quantum yield up to 20−30%.24 This decrease in quantum yield is believed to be caused by interface electron transition between the CdSe shell and the nickel core, which leads to the quenching of the CdSe photoluminescence.6,10,25,26 While the quantum yield is not satisfactory, further work is needed in order to improve this characteristic. Figure 7 shows the photoluminescence (PL) spectra of the Ni nanocube−CdSe, 11 nm Ni−CdSe, 45 nm Ni−CdSe, and 37 nm Ni3C−CdSe core−shell structures. For the 45 nm Ni− CdSe (CdSe size = 5.3 ± 0.8 nm) and Ni nanocube−CdSe (CdSe size = 6 ± 1 nm) core−shell structures, the PL peak is located at 638 and 645 nm, respectively. For the 11 nm Ni− CdSe core−shell (CdSe thickness = 7 ± 2 nm) structures, the photoluminescence peak is located at 665 nm. These results show that the peak position of the photoluminescence signal is dependent on the size of the CdSe domains, where increasing the CdSe particle size leads to photoluminescence signals at longer wavelengths.10,24 For the 37 nm Ni3C−CdSe structures, a broadband photoluminescence signal that extends from about 550 nm to beyond 750 nm is observed. This broadband photoluminescence is likely a result of the large size distribution of the CdSe domains (thickness of 3.5 ± 0.4 nm and width of 16 ± 4 nm) grown on the nickel carbide cores. These results illustrate that the size and shape of the core can control the CdSe shell and the optical properties of the nanoparticles.

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Figure 7. (a) Hysteresis loops of 45 nm Ni cores and Ni−CdSe core− shells. Inset: Enlargement of the region close to the zero field, showing the presence of coercivity. (b) Hysteresis loops of 11 nm Ni cores and Ni−CdSe core−shells. (c) Hysteresis loops of Ni nanocube cores and Ni nanocube−CdSe core−shells.

CONCLUSION In conclusion, this study has led to further understanding of the growth behavior of CdSe on fcc nickel and the effect of the CdSe shell on the optical and magnetic properties of the core− F

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(3) Choi, S.-H.; Na, H. B.; Park, Y. I.; An, K.; Kwon, S. G.; Jang, Y.; Park, M.-h.; Moon, J.; Son, J. S.; Song, I. C.; Moon, W. K.; Hyeon, T. J. Am. Chem. Soc. 2008, 130, 15573. (4) Kwon, K.-W.; Shim, M. J. Am. Chem. Soc. 2005, 127, 10269. (5) Buonsanti, R.; Grillo, V.; Carlino, E.; Giannini, C.; Gozzo, F.; Garcia-Hernandez, M.; Garcia, M. A.; Cingolani, R.; Cozzoli, P. D. J. Am. Chem. Soc. 2010, 132, 2437. (6) Kim, H.; Achermann, M.; Balet, L. P.; Hollingsworth, J. A.; Klimov, V. I. J. Am. Chem. Soc. 2005, 127, 544. (7) Gao, J.; Zhang, B.; Gao, Y.; Pan, Y.; Zhang, X.; Xu, B. J. Am. Chem. Soc. 2007, 129, 11928. (8) McDaniel, H.; Shim, M. ACS Nano 2009, 3, 434. (9) Yuhas, B. D.; Habas, S. E.; Fakra, S. C.; Mokari, T. ACS Nano 2009, 3, 3369. (10) Tian, Z.-Q.; Zhang, Z.-L.; Gao, J.; Huang, B.-H.; Xie, H.-Y.; Xie, M.; Abruna, H. D.; Pang, D.-W. Chem. Commun. (Cambridge, U.K.) 2009, 4025. (11) Gao, J.; Zhang, W.; Huang, P.; Zhang, B.; Zhang, X.; Xu, B. J. Am. Chem. Soc. 2008, 130, 3710. (12) Gu, H.; Zheng, R.; Zhang, X.; Xu, B. J. Am. Chem. Soc. 2004, 126, 5664. (13) Wu, H.; Chen, O.; Zhuang, J.; Lynch, J.; LaMontagne, D.; Nagaoka, Y.; Cao, Y. C. J. Am. Chem. Soc. 2011, 133, 14327. (14) LaGrow, A. P.; Ingham, B.; Cheong, S.; Williams, G. V. M; Dotzler, C.; Toney, M. F.; Jefferson, D. A.; Corbos, E. C.; Bishop, P. T.; Cookson, J.; Tilley, R. D. J. Am. Chem. Soc. 2012, 134, 855. (15) Chen, Y.; Peng, D.-L.; Lin, D.; Luo, X. Nanotechnol. 2007, 18, 505703/1. (16) Figuerola, A.; Huis, M. v.; Zanella, M.; Genovese, A.; Marras, S.; Falqui, A.; Zandbergen, H. W.; Cingolani, R.; Manna, L. Nano Lett. 2010, 10, 3028. (17) Schaefer, Z. L.; Weeber, K. M.; Misra, R.; Schiffer, P.; Schaak, R. E. Chem. Mater. 2011, 23, 2475. (18) Golan, Y.; Alperson, B.; Hutchison, J. L.; Hodes, G.; Rubinstein, I. Adv. Mater. 1997, 9, 236. (19) Zhang, D.; Li, G.; Yu, J. C. Cryst. Growth Des. 2009, 9, 2812. (20) LaGrow, A. P.; Cheong, S.; Watt, J.; Ingham, B.; Toney, M. F.; Jefferson, D. A.; Tilley, R. D. Adv. Mater. 2013, 25, 1552−1556. (21) He, S.; Zhang, H.; Delikanli, S.; Qin, Y.; Swihart, M. T.; Zeng, H. J. Phys. Chem. C 2009, 113, 87. (22) Peng, S.; Lei, C.; Ren, Y.; Cook, R. E.; Sun, Y. Angew. Chem., Int. Ed. 2011, 50, 3158. (23) Chen, D.-H.; Hsieh, C.-H. J. Mater. Chem. 2002, 12, 2412. (24) Washington, A. L.; Foley, M. E.; Cheong, S.; Quffa, L.; Breshike, C.; Watt, J.; Tilley, R. D.; Strouse, G. F. J. Am. Chem. Soc. 2012, 134, 17046. (25) Saunders, A. E.; Popov, I.; Banin, U. J. Phys. Chem. B 2006, 110, 25421. (26) Maynadie, J.; Salant, A.; Falqui, A.; Respaud, M.; Shaviv, E.; Banin, U.; Soulantica, K.; Chaudret, B. Angew. Chem., Int. Ed. 2009, 48, 1814.

Figure 8. Photoluminescence spectrum of the Ni nanocube−CdSe, 11 nm Ni−CdSe, 45 nm Ni−CdSe, and 37 nm Ni3C−CdSe core−shell structures, showing how CdSe domain sizes affect the photoluminescent properties.

shell structures. The CdSe displayed three types of growth behaviors: (1) single-crystal CdSe encapsulation of the 11 nm spherical Ni cores, (2) discrete CdSe growth on spherical nickel surfaces, and (3) extended or platelike CdSe domain growth that covers the highly faceted surfaces of the nickel nanocube and nickel carbide cores. As shown by these results, the growth of the CdSe was dominated by the strong Ni−Se bonds, which enabled CdSe growth to take place on the surface of the nickel cores. Also, the growth of the CdSe domains was controlled by the size and shape of the nickel-based cores. Therefore, the optical properties of hybrid nanostructures can be tuned based on the underlying morphology of the seed crystal.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details for synthesis and additional TEM, energy dispersive spectroscopy (EDS), and selected area electron diffraction (SAED). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address †

School of Chemical and Physical Sciences and the MacDiarmid Institute for Advanced Materials and Nanotechnology, Victoria University of Wellington, Wellington 6012, New Zealand. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the MacDiarmid Institute for funding and MBIE for funding through PROP-29690-HVMSSI. REFERENCES

(1) Pellegrino, T.; Kudera, S.; Liedl, T.; Javier, A. M.; Manna, L.; Parak, W. J. Small 2005, 1, 48. (2) Henning, A. M.; Watt, J.; Miedziak, P/ J.; Cheong, S.; Santonastaso, M.; Song, M.; Takeda, Y.; Kirkland, A. I.; Taylor, S. H.; Tilley, R. D. Angew. Chem., Int. Ed. 2013, 52, 1477. G

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