pubs.acs.org/NanoLett
Octapod-Shaped Colloidal Nanocrystals of Cadmium Chalcogenides via “One-Pot” Cation Exchange and Seeded Growth Sasanka Deka,† Karol Miszta,† Dirk Dorfs,† Alessandro Genovese, Giovanni Bertoni, and Liberato Manna* Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy ABSTRACT The growth behavior of cadmium chalcogenides (CdE ) CdS, CdSe, and CdTe) on sphalerite Cu2-xSe nanocrystals (size range 10-15 nm) is studied. Due to the capability of Cu2-xSe to undergo a fast and quantitative cation exchange reaction in the presence of excessive Cd2+ ions, no Cu2-xSe/CdE heterostructures are obtained and instead branched CdSe/CdE nanocrystals are built which consist of a sphalerite CdSe core and wurtzite CdE arms. While CdTe growth yields multiarmed structures with overall tetrahedral symmetry, CdS and CdSe arm growth leads to octapod-shaped nanocrystals. These results differ significantly from literature findings about the growth of CdE on sphalerite CdSe particles, which until now had always yielded tetrapod-shaped nanocrystals. KEYWORDS Quantum dot, semiconductor, heterostructure, tetrapod, octapod, multipod
B
ranched nanocrystals are a class of nanomaterials that has gained increasing attention in the last years, because of their many potential applications in optics,1 photovoltaics,2 nanomechanics,3 sensing,4 scanning probe microscopy,5 and electronics.3,6,7 There are several synthetic routes that can yield branched nanostructures, for example those exploiting polymorphism,8-10 twinning,11 or the Mullins-Sekerka instability under high supersaturation conditions.12 In the case of nanocrystals prepared in the solution phase, many beautiful examples of nanocrystals with prescribed branching geometries have been reported to date, and a high level of control has been achieved in the case of nanotetrapods, which are nanocrystals composed of four arms branching at tetrahedral angles from a common central region.9,13-18 For these nanotetrapods, the most uniform samples “in solution” in terms of narrowness of distributions of arm lengths and diameters were prepared so far by synthesizing first cubic sphalerite seeds of a II-VI semiconductor, onto which hexagonal wurtzite arms (of the same material of the core or of another II-VI semiconductor) were then grown in a second step.16-18 The latter approach is usually referred to as “seeded growth” synthesis. Nanocrystals with a higher number of branches, like the octapods, are also known for a series of materials, among them CdS,19 CdSe,20 PbS,21 PbSe,22 Pt,23,24 FePt,25 and Cu2O.26 In most of these reported examples, the octapod shape arises as a result of the fast growth along the eight [111] directions of a starting nanocrystal “seed”, when the crystal structure of the seed has octahedral symmetry, and
the final branched nanocrystals are single crystals. In octapods of CdS and CdSe, the arms have instead hexagonal wurtzite structure.20 The formation of these octapods can be understood as originating from the evolution of multiple wurtzite domains on top of a cubic sphalerite seed, each domain growing on one of the eight {111} facets of the seed, with its [0001] direction parallel to one of the [111] directions of the seed (we use henceforth the four-index Miller-Bravais notation when naming crystallographic planes, facets, and directions in hexagonal crystals27). This is however a rather unusual event for II-VI semiconductors, since sphalerite has only tetrahedral symmetry, and it is generally believed that four of these {111} facets are less reactive, namely, the +{111} ones (meaning 11¯1¯, 1¯11¯, 1¯1¯1, and 111), while the -{111} ones (meaning 1¯11, 11¯1, 111¯, and 1¯ 1¯ 1¯) are more reactive and they promote nucleation of wurtzite arms, so that usually a tetrapod shape is finally obtained, as discussed above. For cadmium chalcogenides it was therefore not clear up to now what were the parameters that could govern the preferred formation of octapods over tetrapods, left aside whether the same fine level of control that had been achieved for tetrapods would be likewise possible for octapods. In the present work we report a systematic scheme for the synthesis in solution of octapod-shaped nanocrystals, which are either made of a CdSe central core and CdS arms or are entirely made of CdSe, with controlled arm lengths and diameters, and with a remarkable narrowness in the distribution of arm lengths. The scheme combines: (1) our recently developed ability to grow colloidal Cu2-xSe nanocrystals (x ranging from 0.20 to 0.32) with uniform sizes in the cubic berzelianite phase;28 (2) the property of copper selenide nanocrystals to undergo rapid cation exchange in the presence of an excess of cadmium ions;29 (3) the recently
* To whom correspondence should be addressed,
[email protected]. † These authors contributed equally to this work. Received for review: 07/20/2010 Published on Web: 08/11/2010
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sively (Figure 1d-b). Most octapods appeared as “crosses” in transmission, as in them the four arms touching the carbon support film on the grid were perfectly eclipsed by the remaining four arms pointing away from the film. The octapod shape of the nanocrystals could be assessed unambiguously by scanning electron microscopy (SEM, see Figure 2a) and by high-resolution transmission electron microscopy (HRTEM) (see Figure 2d-f). We had several proofs that the first stage of the synthesis of these octapods is indeed a cation exchange process that converts the Cu2-xSe nanocrystals into sphalerite CdSe nanocrystals. First, aliquots recovered a few seconds (30-60 s) after the injection (of the Cu2-xSe seeds and the S precursors) were practically indistinguishable from the initial Cu2-xSe nanocrystals from the morphological point of view (i.e., under low resolution transmission electron microscopy, TEM). However both HRTEM and X-ray powder diffraction (XRD) proved that they actually had sphalerite CdSe phase.30 Also, attempts to synthesize octapods having CdS arms, at temperatures around 300 °C or lower, were unsuccessful. The nanocrystals recovered in these cases were again sphalerite CdSe.30 Hence at these temperatures cation exchange could already take place, but the S:TOP complex was not reactive enough toward cadmium phosphonate, so that no growth of CdS arms was observed. Sphalerite CdSe nanocrystals were also obtained when injecting Cu2-xSe nanocrystals, dissolved this time in pure TOP (hence without S-TOP), in the mixture of cadmium alkylphosphonate in TOPO, at temperatures up to 380 °C.30 Cation exchange was also observed when injecting the Cu2-xSe/TOP mixture in a solution of cadmium oleate, oleic acid, and octadecene at 300 °C.30 Octapods with CdS arms could be additionally synthesized in a “two-pot” approach, namely, first large sphalerite CdSe nanocrystals were prepared, following any of the cation exchange approaches described above. They were then isolated from the growth solution, purified and dissolved in S:TOP,30 and this solution was injected in a hot cadmium phosphonate/TOPO mixture. No differences in nanocrystal morphology or in aspect ratio of the octapod arms were seen between the one-pot and the two-pot approaches, provided that all the other experimental parameters were kept constant. This also proves that the copper ions expelled from the lattice (following cation exchange) have no influence on the shape evolution of the nanocrystals in the one-pot approach, since in the two-pot approach (using cleaned CdSe seeds) such ions are not present any more during the growth of the arms. It also suggests that the peculiarity in the octapod growth resides most likely in the size and in the morphology of the CdSe nanocrystals used as seeds, as they were prepared from cuboctahedral Cu2-xSe nanoparticles with all eight {111} facets being well developed. The mutual crystallographic orientations of each crystalline component in the octapods could be assessed by
FIGURE 1. TEM images of Cu2-xSe seeds and of octapods with CdSe core and CdS arms synthesized by seeded growth. (a) Cu2-xSe seeds, the insets show a HRTEM image and a schematic model of the crystal habit. (b-d) Octapods (CdSe core, CdS arms) obtained at growth temperatures of (b) 320 °C, (c) 350 °C, and (d) 380 °C. In each panel the inset shows a higher magnification of a selected area on the grid. The scale bar is 100 nm in all panels and 50 nm in the insets.
reported seeded-growth approach for the synthesis of cadmium chalcogenide nanocrystals with controlled shapes and chemical composition and with narrow distributions of geometrical parameters.16-18 In a typical synthesis of branched nanocrystals, preformed monodisperse Cu2-xSe nanocrystals (synthesized following a procedure published by us with minor modifications, see Figure 1a) having cuboctahedral habit and 10-15 nm diameter28,30 were mixed with a solution of trioctylphosphine (TOP) chalcogenide (either S, Se, or Te), the latter prepared by dissolving the chalcogenide powder in TOP. The resulting solution was then injected into a flask containing a cadmium alkylphosphonate (a mixture of hexylphosphonate and octadecylphosphonate) in trioctylphosphine oxide (TOPO) heated at a temperature ranging from 280 to 380 °C, under nitrogen.30 The synthesis was allowed to run for 7 min at that temperature. During that time, the Cu2-xSe nanocrystals underwent first a rapid cation exchange with Cd2+ ions and were transformed into sphalerite cadmium selenide nanocrystals. These nanocrystals acted then as seeds for the growth of wurtzite arms, so that ion exchange and growth of the cadmium chalcogenide on top of the cation-exchanged seeds occurred sequentially, in the same reaction flask (i.e., in “one-pot”). Different behaviors were seen when growing branched nanocrystals with arms having CdS, CdSe, or CdTe composition. We examine the CdS case first. CdS arms could be grown on top of the cation exchanged seeds at temperatures around 320 °C or above, up to 380 °C, the upper temperature at which we heated the mixture of surfactants (Figure 1b-d). In this range of temperatures, octapod-shaped nanoparticles were the product that was formed almost exclu© 2010 American Chemical Society
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FIGURE 2. (a) SEM image of octapods with CdSe core and CdS arms recorded at a 30° tilting angle of the substrate. (b) Model of a CdSe(core)/ CdS(arms) octapod with orientation similar to the ones shown in SEM. (c) X-ray diffraction pattern of the CdSe(core)/CdS(arms) octapods together with the positions and intensities of the bulk reflexes for wurtzite CdS and sphalerite CdSe. (d) HRTEM image of an octapod, observed along the [0 1¯ 1] zone axis of the core region, showing the epitaxial relationship CdSefcc(111)//CdShcp(0002) and CdSefcc[21¯1¯]//CdShcp [101¯0], the lattice planes (0002) of the CdS arms (interplanar distance 3.34 Å) and the (111) planes in the CdSe core (interplanar distance 3.51 Å). (e) The same octapod model seen under two different orientations: in the upper orientation six arms are visible because two are hidden underneath owing to the [011] zone axis projection of core; in the lower orientation, the octapod is aligned along the [001] zone axis of the sphalerite core and only four arms are evident; the labeled facets 1, 3, 4, and 5 correspond respectively to cubic {002}, and hexagonal {101¯0}, {303¯1}, and {0002} crystalline forms. (f) HRTEM image of the central region of an octapod, observed along the [001] zone axis of the CdSe core, showing the [011¯3] directions and the (101¯1) lattice planes of CdS wurtzite arms; the insets represent the magnified view of the CdSefcc region with evident (200) and (220) lattice planes and its corresponding calculated electron diffraction pattern. (g) On the left: optical absorption spectrum of Cu2-xSe nanocrystals and optical absorption and emission spectra of CdSe nanocrystals prepared from the Cu2-xSe nanocrystals by cation exchange. The absorption spectrum of the Cu2-xSe nanocrystals exhibits a shoulder at the position of the direct band gap (480 nm) and a broad maximum around 1400 nm, which might be ascribed to the indirect band gap of Cu2-xSe.32 (g) On the right: optical absorption and emission spectra of the CdSe(core)/CdS(arms) octapods. (h) High-angle angular dark field scanning TEM (HAADF-STEM) image of the CdSe(core)/CdS(arms) octapods. (i) STEM-EDS line scan of a single CdSe(core)/CdS(arms) octapod along the line indicated in the inset of panel h.
The unambiguous assignment of the chemical composition of the core to pure CdSe could not be based solely on lattice spacing or on a composition profiling of a single octapod via spatially resolved energy dispersive X-ray spectroscopy (EDS, see Figure 2i), since in both cases we could not exclude a CdSxSe1-x alloy in the core region. However, XRD spectra of these octapods indicated the presence of both pure sphalerite CdSe and pure wurtzite CdS, suggesting that the structure and the chemical purity of the CdSe seed were both preserved in the central region of each octapod (Figure 2c). An additional proof came from comparing the photoluminescence (PL) spectrum of the CdSe nanoparticles obtained by cation exchange with that of the resulting octapods, when performing a two-pot synthesis (Figure 2g). In both cases, a single narrow peak centered at around 710 nm was observed. This was close to the bulk band gap of CdSe, given the large size of these nanocrystals and thus the weak quantum confinement (the particle radius is of the same order of the Bohr-exciton radius of CdSe, which is 5.3 nm31). In the octapods, the fact that no blue shift was found in the PL was a clear indication that in the core there was no substantial alloying with sulfur, since this would have shifted the band gap toward higher energies.
HRTEM (see Figure 2). Wurtzite CdS arms were grown onto all the eight {111} facets of the sphalerite CdSe seeds, according to the well-defined and distinct crystallographic relationships CdSefcc(111)//CdShcp(0002) and CdSefcc[21¯1¯]// CdShcp [101¯0] (the first relationship defines the planar interface alignment, the second the vector alignment, and together they fully describe the epitaxial growth relationship between the hexagonal arms and the cubic cores of the octapods, see Figure 2b). When an octapod was observed along the [001] zone axis of the sphalerite core, the resulting projection was characterized by a “cross” shape, in which the sphalerite central region exhibited square symmetry and displayed the (200) and (220) reticular planes. In this latter case the (220) sphalerite planes were found to be perpendicular to the [011¯3] crystalline directions of the wurtzite arms, the latter elongated along their [0001] directions (Figure 1c,d and 2f-n). In this orientation, only four arms were seen in projection, as discussed earlier. Another typical orientation of the octapods had the [01¯1] zone axis of the central sphalerite region aligned with the electron beam, and in the projected image only six of the eight arms were seen (Figure 2d). These two frequent orientations are sketched in Figure 2e. © 2010 American Chemical Society
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The absence of a red shift of the PL of the octapods with respect to the spectrum of the starting CdSe nanocrystals on the contrary is another indication that the seeds already had a size where quantum confinement plays a minor role and hence a further increase in size of the crystal has no significant influence on the energy level structure. The occurrence of a band edge emission from the CdSe(core)/ CdS(arms) octapods, which is not seen from “all-CdSe” octapods (see below), can be taken as an indication for a type-I band alignment, which would hinder charge carriers from reaching the particle surface and hence reduce charge carrier trapping.33 The optical absorption spectra of the octapods were similar to those of the “seeded grown” CdSe(spherical core)/CdS (shell) nanorods and of the CdSe(core)/CdS(arms) tetrapods reported by Carbone et al.16 and Talapin et al.17 Indeed they were characterized both by a weak absorption at low energy originating from the core CdSe region and by a stronger absorption at higher energy from the CdS shell. We could additionally synthesize octapods that were entirely made of CdSe, and in this case the most suitable temperature range for their synthesis was between 320 and 350 °C, either via a “one-pot” approach or via a “two-pot” one (Figure 3a,b). For CdSe, the arms were much shorter than those of the CdS octapods. Below this temperature range, hyperbranched nanocrystals were formed,30 which were reminiscent more of snowflakes, and the irregular shapes of the arms indicated that a mixed wurtzite-sphalerite growth of the arms was taking place. In syntheses carried out at 380 °C the final samples were instead tetrapods with conical shaped arms (Figure 3d). In this latter synthesis however, TEM investigation indicated that at the early stages of the reaction (1-2 min) octapod particles were formed first (Figure 3c), which then turned into apparently more thermodynamically stable tetrapod particles later in the synthesis (Figure 3d). An arm-to-arm ripening mechanism was observed in this case, in which four of the arms dissolved while the remaining four grew further. A similar behavior was observed for CdSe(core)/CdS(arms) octapods when they were allowed to ripen for several hours at 380 °C.30 The same crystallographic relationships held between the core (sphalerite CdSe) and each arm (wurtzite CdSe) of these octapods as for the CdSe(core)/CdS(arms) octapods discussed earlier. The X-ray diffraction pattern of these structures matched the wurtzite phase of CdSe; however it should be noticed that all the reflexes of the cubic sphalerite phase of CdSe phase perfectly overlap with some reflexes of the hexagonal wurtzite phase of CdSe (see Figure 3e). Thus the concomitant presence of the two phases cannot be deduced directly by XRD without resorting to a modeling of the diffraction pattern. The optical absorption spectra of solutions of these samples in toluene were matching an almost bulklike behavior of CdSe and were not much different from those of the cation exchanged CdSe seeds. No significant PL was detected at room temperature from solutions of these © 2010 American Chemical Society
FIGURE 3. (a-c) TEM images of “all-CdSe” (i.e., both core and arms are made of CdSe) octapods synthesized by seeded growth of CdSe starting from Cu2-xSe. The growth temperature was (a) 320, (b) 350, and (c) 380 °C (reaction time 1 min in the latter case). The inset in each panel shows a higher magnification of a selected area on the grid. The size bar is 100 nm in all survey images and 50 nm in the insets in panels a-c. (d) TEM image of the nanocrystals synthesized at 380 °C for 7 min; basically all nanocrystals have evolved into tetrapods. (e) X-ray diffraction pattern of the “all-CdSe” octapods together with the bulk reflexes for wurtzite CdSe and sphalerite CdSe. (f) HRTEM image displaying an “all-CdSe” octapod observed along the [01¯1] zone axis of the sphalerite core, as confirmed by the Fourier transform (FT) pattern (inset). In the present case the core region is defective. In particular a twinned cubic slab is evident, which is separated from the core by a twin boundary (TB) showing also a linear stacking fault (SF); the magnified region reports the interface zone showing the crystallographic relationship of epitaxially related hexagonal close packed (hcp) (wurtzite) arms and face center cubic (fcc) (sphalerite) core (inset). (g) Octapod model oriented along the [011] zone axis of the sphalerite core; the labeled facets 1, 3, 6, and 5 correspond respectively to cubic {002} and hexagonal {101¯0}, {101¯1}, and {0002} crystalline forms.
samples, most likely due to the large number of surface defects in these structures. When attempting to grow multipods with CdTe arms, in the temperature range comprised between 280 and 340 °C, only nanocrystals with several long arms departing from the central CdSe seed were synthesized (Figure 4). In some cases there appeared to be arms emerging at a multitude of locations on top of the seeds, without any distinguishable pattern. However in most of these crystals four bundles of multiple arms departed at tetrahedral angles from the central seed, and each bundle was composed of several parallel CdTe arms, all apparently nucleated on the same facet of 3773
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mainly observed (Figure 4c). In all the syntheses involving CdTe growth (between 280 and 340 °C and even above), even early aliquots were characterized by a tetrahedral multiarmed branched geometry; i.e., no intermediate octapods were seen, as opposed to the CdSe case discussed earlier. Except for the tetrapod samples, in which the diameter of the arms was at the borderline for the quantum confinement regime, in all the optical spectra of the multiarmed particles a distinctive excitonic peak was present which was blue-shifted with respect to the bulk band gap (Figure 4d). In all these structures the XRD patterns confirmed the presence of CdSe sphalerite phase, while for CdTe a mixture of wurtzite and sphalerite phase was most likely present (Figure 4f). Indeed, when observed under HRTEM, almost all CdTe arms had a polycrystalline structure, with alternating sections of wurtzite and sphalerite phases, similarly to what was previously observed by us when synthesizing tetrapod-shaped nanocrystals having CdTe arms.18 Several statements could be made by comparing the shape evolutions of the various nanostructures discussed above. First of all, our results are considerably different from what reported so far on seeded-grown, tetrapod-shaped nanocrystals.11-13 The substantial differences with respect to previous works are the size of the “seeds”, which is much bigger in our experiments (they approach 10-15 nm in diameter), and their cuboctahedral morphology, which is rather unusual for CdSe, being obtained via cation exchange (we also note that there is no previous report on the synthesis of sphalerite CdSe nanocrystals in this size range). Therefore, one important conclusion that we could draw is that starting from big sphalerite CdSe seeds, with all eight 111 facets being well developed, each of these facets has the same probability of acting as nucleation site. Furthermore, our TEM analysis shows unequivocally that, at least for CdS and CdSe, the wurtzite arms that grow on these facets have equal growth rates along the 0001 and the 000¯1 directions. This can be understood by considering that any of the four +{111} sphalerite facets, if continued to grow in the wurtzite phase, would correspond to the 0001 wurtzite facet, while any of the other -{111} sphalerite facets, having opposite polarity, would evolve in the 0001¯ wurtzite facet. Therefore, in previous studies, in which smaller CdSe seeds were utilized, it was most likely the morphology of the seeds (often tetrahedral) that dictated the shape evolution to tetrapods, rather than differences in relative growth rates between the two groups of +{111} and -{111}sphalerite facets (and between the 0001 and the 0001¯ wurtzite facets). Even if a difference in the rate constants of nucleation on the two different sets of {111} facets k1 and k2 is assumed, still an octapod can be obtained when arm growth (with rate constant k3) rather than nucleation on one of the two facets (k1, k2) is the rate-determining step (k3 , k1, k2). For CdTe there still appeared to be a significant difference in nucleation rates between the two sets of {111} facets. A
FIGURE 4. (a-c) TEM images of multiarmed CdSe (core)/CdTe (arms) nanocrystals synthesized by seeded growth of CdTe starting from Cu2-xSe seeds. The growth temperature was (a) 280, (b) 320, and (c) 350 °C. The insets show higher magnification views of selected CdSe (core)/CdTe (arms) multiarmed nanocrystals. The size bar is 100 nm in all survey images and 50 nm in the insets. (d) Optical absorption spectrum of CdSe (core)/CdTe (arms) multiarmed nanocrystals. (e) Idealized scheme of multiple CdTe arms growing on top of a single {111} facet of the cubic CdSe core particle. (f) X-ray diffraction pattern of the multipods together with the bulk reflexes for hexagonal CdTe, cubic CdTe, and cubic CdSe.
FIGURE 5. (a) HAADF STEM image of multiarmed CdSe (core)/CdTe (arms) nanocrystals. (b) HAADF STEM image of a nanocrystal (a “monopod”) in which multiple arms had grown only on top of one facet of a CdSe seed. (c) HRTEM image of a “monopod” displaying the epitaxial relationship CdSefcc(111)//CdTehcp(0002) and CdSefcc[220]// CdTehcp[101¯0] between the core and the arms; in particular the sphalerite core is observed along the [ 1¯12] zone axis, as confirmed by the FT pattern (inset). (d) HRTEM image showing a set of parallel CdTe arms of a multiarmed nanocrystal with their [002] crystallographic wurtzite axes as preferential growth directions.
the CdSe seed (Figure 4a,b and Figure 5a). This is more evident in Figure 5b,d which reports a peculiar case where only one facet of the seed had promoted growth of multiple parallel CdTe arms. Above 340 °C, CdTe tetrapods were © 2010 American Chemical Society
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possible rationalization of the observed shapes in this case was that the selective growth of CdTe arms on only one set of facets (say the -{111} ones) was so fast that there was little chance for CdTe nucleation on the remaining four facets (i.e., all CdTe monomer was consumed before any arm growth could take place on the +{111} facets). Also, the relatively large surface area of each of the {111} facets of the CdSe seeds (a few nanometers2), coupled with an extremely high growth rate of CdTe on them, favored the occurrence of multiple heterogeneous nucleation events of CdTe on each facet. All these nuclei then grew further and evolved into separate, parallel CdTe arms (see Figure 5). In a simplified kinetic model this could be explained by a difference in k1 and k2, with the nucleation being also the rate-determining step (k3 . (k1, k2)). At high temperature (380 °C) most likely all these branches coalesced into a single arm, so that the final nanocrystal shape in this case was a tetrapod. Thermodynamic factors could be also invoked for the observed shape transformation of both the “all-CdSe” and the CdSe(core)/CdS(arms) octapods into tetrapods at 380 °C. Moreover, in the case of CdSe(core)/CdTe(arms) heterostructures, reshaping of the initial Cu2-xSe seeds or of the cation exchanged CdSe seeds from cuboctahedral habit to tetrahedral habit could be excluded as the reason for formation of particles with tetrapod symmetry. Indeed, in the case of CdSe(core)/CdS(arms) heterostructures, octapods were synthesized even at high temperatures (350-380 °C), which is a strong indication that at these temperatures there was no reshaping of the starting seeds. In conclusion, we have developed a systematic “one-pot” approach for the synthesis of CdSe/CdS and of “all CdSe” octapod-shaped nanocrystals as well as multiarmed CdTe/ CdSe nanocrystals, starting from Cu2-xSe nanocrystals, by a sequential cation exchange and seeded growth mechanism. Cation exchange in nanocrystals is a subject that has gained a lot of attention recently, since it has allowed the development of previously nonaccessible structures29,34 and applications.35-37 In the present case, it enabled us to synthesize sphalerite CdSe nanocrystals with peculiar habit and with diameters larger than 10 nm, which are the key requirements for the synthesis of octapod-shaped nanocrystals. It is worth noticing that, in the one-pot synthesis approach reported by us, even if large amounts of copper ions were released in the growth solution during the cation exchange step, no trace of them was found in the final nanocrystals, neither optically (i.e., no additional PL peaks ascribable to Cu dopant species were identified) nor via elemental analysis.30 Our approach therefore should be extendable to other potentially “sacrificial” nanocrystalline materials, i.e., which could be converted easily into nanocrystals of other materials, while preserving their overall size and shape. In this way unique nanocrystal shapes might be fabricated which could not be directly accessible via more traditional synthesis schemes. Also, these branched nano© 2010 American Chemical Society
crystals should be useful in photovoltaic applications and as building blocks in self-assembled mesoporous structures. Acknowledgment. The authors acknowledge financial support from European Union through the FP7 starting ERC Grant NANO-ARCH (Contract Number 240111). We acknowledge Andrea Falqui, Rosaria Brescia, and Sergio Marras for discussion and for help with the electron microscopy and Mauro Povia for help with the X-ray diffraction experiments. Supporting Information Available. Detailed description of the various syntheses of nanocrystals and additional electron microscope images. This information is available free of charge via the Internet at http://pubs.acs.org/. REFERENCES AND NOTES (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28)
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Mauser, C.; Limmer, T.; Da Como, E.; Becker, K.; Rogach, A. L.; Feldmann, J.; Talapin, D. V. Phys. Rev. B 2008, 77, 153303. Zhou, Y.; Li, Y. C.; Zhong, H. Z.; Hou, J. H.; Ding, Y. Q.; Yang, C. H.; Li, Y. F. Nanotechnology 2006, 17 (16), 4041–4047. Fang, L.; Park, J. Y.; Cui, Y.; Alivisatos, P.; Shcrier, J.; Lee, B.; Wang, L. W.; Salmeron, M. J. Chem. Phys. 2007, 127, 184704. Gao, T.; Wang, T. H. Appl. Phys. A: Mater. Sci. Process. 2005, 80 (7), 1451–1454. Nobile, C.; Ashby, P. D.; Schuck, P. J.; Fiore, A.; Mastria, R.; Cingolani, R.; Manna, L.; Krahne, R. Small 2008, 4 (12), 2123– 2126. Cui, Y.; Banin, U.; Bjork, M. T.; Alivisatos, A. P. Nano Lett. 2005, 5 (7), 1519–1523. Newton, M. C.; Firth, S.; Warburton, P. A. Appl. Phys. Lett. 2006, 89, 7. art. n. 072104. Manna, L.; Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc. 2000, 122 (51), 12700–12706. Manna, L.; Milliron, D. J.; Meisel, A.; Scher, E. C.; Alivisatos, A. P. Nat. Mater. 2003, 2 (6), 382–385. Kanaras, A. G.; Sonnichsen, C.; Liu, H. T.; Alivisatos, A. P. Nano Lett. 2005, 5 (11), 2164–2167. Carbone, L.; Kudera, S.; Carlino, E.; Parak, W. J.; Giannini, C.; Cingolani, R.; Manna, L. J. Am. Chem. Soc. 2006, 128 (3), 748– 755. Mullins, W. W.; Sekerka, R. F. J. Appl. Phys. 1964, 35, 444–451. Asokan, S.; Krueger, K. M.; Colvin, V. L.; Wong, M. S. Small 2007, 3 (7), 1164–1169. Newton, M. C.; Warburton, P. A. Mater. Today 2007, 10 (5), 50– 54. Xu, H. Y.; Liang, Y.; Liu, Z. A.; Zhang, X. T.; Hark, S. K. Adv. Mater. 2008, 20 (17), 3294–3297. Carbone, L.; et al. Nano Lett. 2007, 7 (10), 2942–2950. Talapin, D. V.; Nelson, J. H.; Shevchenko, E. V.; Aloni, S.; Sadtler, B.; Alivisatos, A. P. Nano Lett. 2007, 7 (10), 2951–2959. Fiore, A.; et al. J. Am. Chem. Soc. 2009, 131 (6), 2274–2282. He, X. B.; Gao, L. J. Phys. Chem. C 2009, 113 (25), 10981–10989. Cao, X. B.; Zhao, C.; Lan, X. M.; Yao, D.; Shen, W. J. J. Alloys Compd. 2009, 474 (1-2), 61–67. Bashouti, M.; Lifshitz, E. Inorg. Chem. 2008, 47 (2), 678–682. Wang, X. Q.; Xi, G. C.; Liu, Y. K.; Qian, Y. T. Cryst. Growth Des. 2008, 8 (4), 1406–1411. Ren, J. T.; Tilley, R. D. J. Am. Chem. Soc. 2007, 129 (11), 3287– 3291. Cheong, S.; Watt, J.; Ingham, B.; Toney, M. F.; Tilley, R. D. J. Am. Chem. Soc. 2009, 131 (40), 14590–14595. Chou, S. W.; Zhu, C. L.; Neeleshwar, S.; Chen, C. L.; Chen, Y. Y.; Chen, C. C. Chem. Mater. 2009, 21 (20), 4955–4961. Chang, Y.; Zeng, H. C. Cryst. Growth Des. 2004, 4 (2), 273–278. Williams, D. B.; Carter, C. B. Transmission Electron Microscopy: A Textbook for Materials Science: Springer: Berlin, 2004. Deka, S.; Genovese, A.; Yang, Z.; Miszta, K.; Bertoni, G.; Krahne, R.; Giannini, C.; Manna, L. J. Am. Chem. Soc. 2010, 132 (26), 8912– 8914. DOI: 10.1021/nl102539a | Nano Lett. 2010, 10, 3770-–3776
(29) Jain, P. K.; Amirav, L.; Aloni, S.; Alivisatos, A. P. J. Am. Chem. Soc. 2010, 132 (29), 9997–9999. (30) Additional details on materials and methods are available in the Supporting Information. (31) Gaponenko, S. V. Optical Properties of Semiconductor Nanocrystals; Cambridge University Press: Cambridge, 1998. (32) Statkute, G.; Tomasiunas, R. J. Appl. Phys. 2007, 101, 113715. (33) Steiner, D.; Dorfs, D.; Banin, U.; Della Sala, F.; Manna, L.; Millo, O. Nano Lett. 2008, 8 (9), 2954–2958.
© 2010 American Chemical Society
(34) Son, D. H.; Hughes, S. M.; Yin, Y. D.; Alivisatos, A. P. Science 2004, 306 (5698), 1009–1012. (35) Robinson, R. D.; Sadtler, B.; Demchenko, D. O.; Erdonmez, C. K.; Wang, L. W.; Alivisatos, A. P. Science 2007, 317 (5836), 355–358. (36) Li, J. S.; Zhang, T. R.; Ge, H. P.; Yin, Y. D.; Zhong, W. W. Angew. Chem., Int. Ed. 2009, 48 (9), 1588–1591. (37) Li, J. S.; Schachermeyer, S.; Wang, Y.; Yin, Y. D.; Zhong, W. W. Anal. Chem. 2009, 81 (23), 9723–9729.
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