The Growth of Colloidal Cadmium Telluride Nanocrystal Quantum

Jun 21, 2007 - Viki Kloper,† Ruth Osovsky,† Joanna Kolny-Olesiak,‡ Aldona Sashchiuk,† and Efrat Lifshitz*,†. Schulich Faculty of Chemistry, ...
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10336

J. Phys. Chem. C 2007, 111, 10336-10341

The Growth of Colloidal Cadmium Telluride Nanocrystal Quantum Dots in the Presence of Cd0 Nanoparticles Viki Kloper,† Ruth Osovsky,† Joanna Kolny-Olesiak,‡ Aldona Sashchiuk,† and Efrat Lifshitz*,† Schulich Faculty of Chemistry, Solid State Institute and the Russell Berrie Nanotechnology Institute, Technion, Haifa 32000, Israel, and Energy and Semiconductor Research Laboratory, Department of Physics, Carl Von Ossietzky UniVersity of Oldenburg, 26111 Oldenburg, Germany ReceiVed: February 1, 2007; In Final Form: May 3, 2007

The paper presents a new synthesis route for the formation of spherical-shaped CdTe semiconductor nanocrystal quantum dots (NQDs) using colloidal solutions with oleic acid and trioctylphosphine stabilizers. The synthesis includes the occurrence of in situ precipitation of crystalline Cd0 nanoparticles prior to the formation of CdTe NQDs. The experimental evidence suggests that the existence of Cd0 induces a regulation of the precursor supply during the CdTe NQDs growth, inducing a control of the growth kinetics and determination the final shape of the NQDs. Also, the results suggest that the initial Cd/Te molar ratio controls the NQDs’ quality, with excess Cd precursor inducing the generation of NQDs with exceptionally high emission QE up to 80% at room temperature.

1. Introduction Considerable progress has been made on the fundamental1 and applied research2,3 of semiconductor nanocrystal quantum dots (NQDs). Colloidal CdTe NQDs, in particular, attract increasing interest due to their large exciton Bohr radius (7.3 nm),4 pronounced quantum size effect, optical activity both in the visible and near-infrared (NIR) spectral regimes (with a bulk band gap of 1.475 eV), and chemical flexibility at the NQDs’ surface. These properties make the CdTe NQDs applicable in various optoelectronics and biological devices. For example, CdTe NQDs films are used in photovoltaic cells.5,6 Furthermore, CdTe NQDs are incorporated into light-emitting diodes, either as closely packed films7 or embedded in a polymer matrix (polyaniline, polypyrrole).8,9 A closely packed arrangement of CdTe NQDs has been achieved10 based on either electrostatic or covalent inter-NQDs interactions, using layer-by-layer assembly or solvent-controlled precipitation. In addition, the electrostatic and covalent interactions between CdTe NQDs has been investigated, showing that these inter-NQDs’ couplings are highly efficient for energy-transfer processes between NQDs.11,12 CdTe NQDs were also used as living cells’ fluorescent labels13,14 or were encapsulated in glass beads for their use in encoding combinatorial libraries.15 The increasing demand for high-quality CdTe NQDs has stimulated researchers to develop various synthesis routes. The aqueous procedures included the use of functionalized thiols (e.g., 3-mercapto-1,2 propanediol) as the stabilizing agents16,17 and a careful control of the solutions’ pH.18 The nonaqueous procedures included the use of coordinating ligands with a hightemperature boiling point, such as dodecylamine (DDA) and trioctylphosphine (TOP),19 for the fabrication of CdTe NQDs using Cd(CH3)2 as the Cd precursor, producing NQDs with an emission quantum efficiency (QE) of about 30-65%. Alterna* To whom correspondence should be addressed. E-mail: ssefrat@ tx.technion.ac.il. Tel: +972 4 8293987. Fax: +972 4 8235107. † Technion. ‡ Carl von Ossietzky University of Oldenburg.

tively, others20 used cadmium stearate as a Cd precursor with different surfactants, such as trioctylphosphine oxide (TOPO), hexadecylamine (HDA), and trioctylamine (TOA). The use of tetradecylphosphonic acid/tributylphosphine (TDPA/TBP) stabilizers in an octadecene (ODE) solution led to the formation of CdTe NQDs with an emission QE of 70%.21 Although oleic acid (OA) has been noted as a special ligand that yields stable metal nanoparticles (NPs)22 and high-quality semiconductor NQDs,23,24 its use in the synthesis of CdTe led to the formation of tetrapod-shaped nanocrystals.21 In addition, spontaneous reorganization of CdTe NQDs into nanowires and various anisotropic shapes were reported.25,26 Weller et al.27 used photoelectron spectroscopy to show that exposure of the CdTe NQDs to air induces the formation of surface oxides, reducing the emission QE. CdTe NQDs were also prepared in recent years as core-shell structures. For example, the CdTe/HgTe NQDs, consisting of a CdTe core and a HgTe shell, showed an emission QE of 44%.28 The reported CdTe/CdSe core-shell NQDs showed a type II recombination process with a photoluminescence (PL) QE of 20%.29 More recently, femtosecond dynamics spectroscopy of the CdTe NQDs’ core-shell structure has been investigated.30 Though a tremendous amount of work has been devoted to the synthesis of CdTe NQDs in the past decade, there is still a need to improve the chemical stability, to achieve higher PL QE, and to clarify the NQDs’ growth mechanism. Thus, this paper describes a modified synthesis method for the preparation of spherical CdTe NQDs with high crystalline quality and an extremely bright PL QE of 80%. The NQDs’ growth mechanism is examined and discussed in detail. CdO and Te powders were used as the starting agents, while OA and TOP were used as the stabilizing ligands. The synthesis included a few sequential stages, starting from the formation of a cadmium oleat complex Cd(OA)2 in OA and ODE precursor solutions, followed by a precipitation of crystalline Cd0 NPs when the Cd(OA)2 was heated to 310 °C, and then followed by the growth of CdTe NQDs when the TOP/Te precursor was injected into the hot Cd solution, in the presence of the Cd0 NPs. The morphology,

10.1021/jp0708906 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/21/2007

CdTe NQDs in the Presence of Cd0 Nanoparticles the crystallographic properties, and chemical composition of the Cd0 NPs and of the CdTe NQDs were examined by the use of high-resolution scanning and transmission electron microscopy (HR-SEM, HR-TEM), selected area electron diffraction (SAED), X-ray diffraction (XRD), and energy-dispersive analysis of X-rays (EDAX). The optical properties of the CdTe NQDs were examined by absorption, and PL spectroscopy. The chemical composition of the various CdTe aliquot solutions was analyzed by atomic absorption spectroscopy (AAS), Fourier transform infrared (FTIR), and 31P nuclear magnetic resonance (NMR) techniques. 2. Experimental Section 2.1. Materials. Oleic acid (OA), 1-octadecene (ODE), trioctylphosphine (TOP, technical grade of 90%), cadmium oxide powder (CdO), and tellurium powder (Te, 99.999%, 200 mesh) were purchased from Sigma-Aldrich. All of the chemicals were used without further purification. 2.2. Synthesis. The synthesis of CdTe NQDs was initiated by the preparation of the precursor solutions under inert conditions in a standard glove box. The Te precursor solution was prepared by dissolving 0.0128-0.0512 g of Te (0.1-0.4 mmol) in 0.25-1.00 mL of TOP until the solution attained a clear, yellowish color. The solution was further diluted with ODE to a total amount of 2.5 mL. The Cd precursor solution was prepared by mixing 0.0256 g of CdO with 200 µL of OA in ODE (10 mL) solution. The Cd solution was heated to 100 °C for 30 min under vacuum in a three-neck flask, removing the water content and resulting in the appearance of a homogeneous red mixture. Further on, the system was flushed by dry Ar gas, while the temperature was raised to 300 °C, followed by formation of a homogeneous transparent solution and the generation of (Cd(OA)2). Further heating to 310 °C, for a period of about 30 min, led to the formation of an additional gray-toblack precipitate, which was eventually characterized as crystalline Cd0 NPs (vide infra). The moment the precipitate appears depends on the initial CdO/OA concentration ratio. It should be noted that the concentration of the CdO within the original precursor solution was always the same; however, the concentration of the Te within its precursor solution varied according to the range indicated above. Thus, the influence of the Cd/Te initial molar ratio on the final quality of the product could be investigated (vide infra). The TOP/Te precursor solution was injected into the threeneck flask about 30 s after the first appearance of the gray Cd0 NPs precipitate, initiating the nucleation of the CdTe NQDs, followed by an immediate change of the solution color and a temperature drop to 260 °C, where further growth of the CdTe NQDs took place. The growth of the NQDs occurred during the first 1-5 min, with the solution gradually changing its color from yellow to red (Figure 2). Aliquots of the prepared CdTe NQDs were drawn periodically from the reaction. Cooling the aliquots to room temperature quenched the NQDs growth. These aliquots were then centrifuged in order to precipitate the crystalline Cd0 NPs and separate them from the CdTe NQDs’ colloidal solution. The isolation of the CdTe NQDs from the remaining organic solution was performed by the addition of an ethanol/acetone mixture and by additional centrifugation. Eventually, the CdTe NQDs were isolated as a clean powder. The separated crystalline Cd0 NPs and the purified CdTe NQDs underwent a series of structural and optical characterizations, as indicated in the Results and Discussion section, and for these measurements, the CdTe NQDs had been redissolved in hexane.

J. Phys. Chem. C, Vol. 111, No. 28, 2007 10337 2.3. Instrumental. The absorption spectra were recorded using a JASCO V-570 UV-vis-NIR spectrometer. The CdTe NQDs’ aliquots were designed to show an optical density of 0.15-0.3 in the UV-vis spectral regime. The molar extinction coefficients and the NQDs’ average size were estimated from the absorption curves. The PL spectra of the colloidal CdTe NQDs were recorded using a Perkin Elmer luminescence spectrometer LS 50 at optical densities below 0.10 (at the excitation energy). The absolute values of the PL QE were determined by comparison between the PL QE of NQDs and of the rhodamine 6G (Aldrich), having a known QE of 95% in absolute ethanol. The energy corresponding to the intersection point between the dye and the NQDs’ absorption curves was used as the excitation energy of the NQDs’ PL spectra. The morphology and crystal structure of the Cd0 NPs were determined by the use of a HR-SEM (Zeiss Leo 982 instrument operated at 4 kV) and by XRD (X’Pert Philips instrument, employing a Cu KR line), respectively. AAS measurements were used to determine the amount of the gray Cd0 NP precipitate and CdTe NQDs at various stages of the synthesis. For that purpose, the precipitated Cd0 NPs and CdTe NQDs were dissolved in an acid solution (H2SO4 or HNO3). The structural properties and the chemical composition of the CdTe NQDs were examined with a HR-TEM Technai F20 G2 system operated at 300 kV, equipped with SAED detectors, and the EDAX technique. The crystallographic properties of these NQDs were also determined by the use of XRD. The NQDs growth mechanism was investigated by following the chemical composition of the product and the reaction byproducts using 31P NMR and FTIR spectroscopies. The NMR measurements were performed with a Bruker Avance 300 MHz NMR spectrometer with a switchable probe (19F, 31P, 13C, 29Si, 2H, 1H). The FTIR measurements were carried out on a Nicolet Nexxus 8700 infrared spectrometer using 4 cm-1 resolution and 50 scans per spectrum. The discussed measurements were carried out at room temperature. 3. Results and Discussion The special synthetic procedure utilized in this study followed two guiding stages. The first stage involved the preparation of Cd(OA)2 complex molecules at 300 °C. However, further heating to 310 °C led to a partial decomposition of the Cd(OA)2 complexes and to the formation of a Cd0 NPs. Figure 1A shows a HR-SEM image of the precipitate, suggesting the existence of NPs with well-pronounced facets and sizes between 100 and 150 nm. The XRD spectrum of the NPs, shown in Figure 1B, reveals the existence of hexagonal crystalline Cd0 NPs. It should be mentioned that the precipitate kept its metallic color within the solution. However, upon filtration and exposure to air for a period of a few hours, these NPs were coated with an oxide layer with a reddish color. Indeed, the XRD shown in Figure 1B (scanned over 12 h at ambient conditions) contains also a typical diffraction of a CdO component formed only during the XRD measurement, giving the base to assume that it does not play a role in the CdTe NQDs’ formation. The second guiding stage in the synthetic procedure includes the injection of the TOP/Te precursor into the Cd(OA)2 solution, immediately after the appearance of the Cd0 NPs. Figure 1C shows HR-SEM image of the Cd0 NPs isolated from the reaction mixture after the TOP/Te injection. This figure illustrates that the Cd0 NPs lost their well-pronounced facets, suggesting their involvement in the creation of the CdTe NQDs. Representative aliquots, withdrawn from the reaction vessel after the TOP/Te injection, are shown in Figure 2. The aliquots

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Figure 1. (A) HR-SEM image of crystalline Cd0 NPs before the TOP/ Te injection. (B) XRD of the crystalline Cd0 NPs before the TOP/Te injection. (C) HR-SEM image of crystalline Cd0 NPs after the TOP/Te injection.

are characterized by their orange-red color, confirming the formation of CdTe NQDs with various average sizes. The gray precipitated Cd0 NPs are also seen at the bottom of the vessel, as marked by the blue circles. An illustrative HR-TEM image of CdTe NQDs with 4.0 nm diameter (prepared with initial Cd/ Te molar ratio of 1:1) is shown in Figure 3A. The results reveal the formation of spherical-shaped NQDs with a ∼5% size distribution. Figure 3B shows the corresponding SAED image of the sample shown in Figure 3A, and Figure 3C shows the relevant XRD pattern of the same CdTe NQD sample. The SAED pattern and the XRD bands correspond to the {111}, {220}, and {311} crystallographic planes, confirming the existence of a cubic zinc blende structure of a CdTe semiconductor. It should be indicated that the injection of the TOP/Te at 300 °C without the formation of Cd0 NPs led to the creation of CdTe tetrapods, as discussed in ref 21. The presented paper emphasizes explicitly the role of the Cd0 NPs in the growth of spherical-shaped CdTe NQDs with exceptionally high emission QE (vide infra). By following the concentrations of the Cd(OA)2 precursor molecules, the Cd0 NPs, and the CdTe NQDs

Kloper et al. during the growth reaction, the growth mechanism was clearly understood. Figure 4A shows a plot of the percentage of Cd atoms (Cd%) that were transformed from Cd(OA)2 precursor molecules to Cd0 NPs, [Cd% ) (Cd0/Cd(OA)2) × 100], versus the time elapsed after the appearance of the gray precipitate. This Cd% was determined by the AAS, as indicated in the Experimental Section. The crossed circles (X) and the triangles (2, 4) in Figure 4A designate the Cd% before and after the TOP/Te injection, respectively, using an initial Cd/Te molar ratio of 1.0:0.8 (bold symbol) and 1.0:1.25 (open symbol). The arrow in Figure 4A marks the moment of the TOP/Te injection. It can be seen from Figure 4A that the Cd% increases gradually as long as the injection of TOP/Te solution is avoided. However, after the Te injection, the Cd0 precipitation is discontinued, and the Cd% is even slightly reduced during the CdTe growth. Figure 4B and C represents plots of the percentage of Cd2+ ions (Cd2+ %) retained as a precursor (squares) or formed CdTe NQDs (circles) versus the time elapsed after the TOP/Te injection, as determined by an AAS with the initial Cd/Te molar ratios of 1.0:1.25 (open symbols in Figure 4B) and 1.0:0.8 (bold symbols in Figure 4C). These plots reveal that the use of a Cd/ Te ratio of 1.0:1.25 led to a chemical efficiency of 90% in the formation of CdTe NQDs, after a loss of 10% of the Cd(OA)2 molecules in their precipitation to Cd0 NPs (see Figure 4A). The growth mechanism of the CdTe NQDs was further investigated by following the FTIR and 31P NMR spectra of the various aliquots. Figure 5A shows the FTIR spectra of pure OA (dark gray) and ODE (black) solvents. Figure 5B represents the FTIR spectra of the reaction solution before (black) and after (dark gray) the appearance of Cd0 NPs and upon the injection of the TOP/Te and the initiation of the CdTe NQDs growth (gray). The inset in Figure 5B shows a magnification of the spectral range between 1500 and 1600 cm-1. Comparison of the curves in Figure 5A and B reveals the disappearance of the typical broad O-H stretching (at ∼3000 cm-1) and outof-plane bending (at 934 cm-1) modes of the OA in the FTIR spectra of the reaction aliquots (Figure 5B). In addition, the spectra in Figure 5B showed a few new bands corresponding to a C-O stretching at 1530 cm-1 after the Cd0 precipitation (green curve) and at 1554 cm-1 after the appearance of the CdTe NQDs (yellow curve). Those C-O stretching modes suggest the formation of peroxides or anhydrides as byproducts of the reaction, as shown in Figure 5B. The FTIR band at 1554 cm-1 corresponds to the CdO stretching mode, existing in OA, peroxide, and anhydride.31 Figure 6 shows 31P NMR spectra of a pure TOP/Te precursor solution (black) and of an aliquot taken from the CdTe NQDs’ reaction solution (dark gray). The chemical shift at -13 ppm is attributed to free TOP molecules. The chemical shift at 46.5 ppm (47 ppm) corresponds to a TOP/Te bond. However, the chemical shift at 51 ppm of the CdTe NQDs sample is attributed to a TOPO molecule, presumably formed as a reaction byproduct. On the basis of the FTIR and 31P NMR investigations, we propose the following reaction mechanism (given in Scheme 1), leading to the formation of Cd0 NPs and to the growth of the CdTe NQDs in the presence of the indicated Cd0 NPs. The

Figure 2. Representative aliquots of CdTe NQDs drawn from the reaction solution at time intervals of 20 s. The blue circles emphasize the existence of gray crystalline Cd0 NPs.

CdTe NQDs in the Presence of Cd0 Nanoparticles

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Figure 5. (A) FTIR spectra of the pure OA (gray) and ODE (black) solvents. (B) FTIR spectra of the reaction byproducts, Cd(OA)2 in ODE (black curve), Cd(OA)2 and Cd0 NPs (dark gray), and of the CdTe NQD reaction solution (gray). The inset in B shows a magnification of the spectral range between 1500 and 1600 cm-1.

Figure 3. (A) HR-TEM image of CdTe NQDs with an average diameter of 4 nm and a size distribution of 5%. (B) SAED pattern of the sample shown in Figure 3A. (C) XRD of the sample shown in Figure 3A. Figure 6. 31P NMR spectra of a TOP/Te precursor solution (black) and of a CdTe NQD reaction solution (gray).

Figure 4. (A) Plots of Cd% appearing as Cd0 NPs versus the time elapsed after the first appearance (at t ) 0) of the gray Cd0 NPs precipitate. The crossed circles (X) and the triangles (2, 4) designate the percent of Cd before and after the TOP/Te injection, respectively, using initial Cd/Te molar ratios of 1.0:0.8 (bold symbol) and 1.0:1.25 (open symbol). The arrow in Figure 4A marks the moment of the TOP/ Te injection. (B) and (C) Plots of the Cd2+ % ions versus the reaction time in CdTe NQDs (O, b) and in Cd(OA)2 precursors (0, 9), with initial Cd/Te molar ratios of 1.0:1.25 (open symbols) and 1.0:0.8 (bold symbols).

Figure 7. Absorbance spectra (dashed line) and PL spectra (solid line) of CdTe NQDs with diameters between 3.1 and 3.8 nm.

formation of Cd(OA)2 at 300 °C was generated in OA and ODE solvents (step A in the scheme). Further heating to 310 °C induced an oxidation-reduction reaction within the Cd(OA)2 precursor, resulting in a partial (∼10%) conversion of this complex into Cd0 and an oleat peroxide (ROO-OOR) byproduct, as shown in step B. When the TOP/Te precursor solution was injected into the reaction solution at 310 °C, eventually the CdTe NQDs were formed, capped with OA/TOP surfactants (step C). Also, the released TOP molecules reacted with the

oleat peroxide to form TOPO and oleat anhydride (RO-OOR) byproducts (step D), in agreement with FTIR and 31P NMR observations, shown in Figures 5 and 6. The reaction sequence given in Scheme 1 reveals that the Cd0 NPs have a direct involvement in the growth of the CdTe NQDs when the initial precipitation of Cd0 reduces the free-standing Cd ion concentration and, consequently, avoids a fast kinetic growth of pods on typical energetic facets, as reported before.21 However, at the point when the Cd monomers supply in the solution is reduced,

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Kloper et al.

Figure 8. Plots of the CdTe NQDs’ diameter (A), their molar concentration (B), the fwhm of the 1S exciton absorption band (C), and their emission QE (D) versus the reaction time. The CdTe NQDs were prepared with various initial Cd/Te molar ratios, as indicated in the Figure 8A inset.

SCHEME 1: Proposed Synthesis Mechanism

it is backed up by the availability of the Cd0 to be oxidized and participate in the continuation of the CdTe NQDs’ growth, according to the reverse reaction indicated in step B of Scheme 1. This regulates the rate of the reaction and predominately permits the generation of spherical CdTe NQDs. In addition, the existence of the crystalline Cd0 NPs precipitate may induce an inhomogeneous medium, which is constantly and vigorously stirred in a turbulent fashion. Then, the growth of the CdTe occurs in a small volume confined by the crystalline Cd0 NPs’ turbulent stream, in a similar fashion to the growth of CdTe NQDs within a microfluid constrained environment,32,33 inducing the generation of spherically shaped CdTe NQDs. It should be mentioned that a catalysis stimulation of the CdTe nucleation at the Cd0 NPs surfaces was excluded due to the absence of any evidence of CdTe-Cd0-coupled structures in the TEM and

XRD investigations. Similar interpretations for synthesis mechanisms of PbSe and CdSe NQDs were reported in references 34 and 35. The optical properties of the prepared CdTe NQDs are reflected in their absorption and PL spectra. The dashed lines in Figure 7 show the absorbance curves of various aliquots drawn from the CdTe reaction solution, while the solid lines represent the corresponding nonresonantly excited PL spectra. The reaction, with an initial Cd/Te molar ratio of 1:1, was carried out at a growth temperature of 260 °C with a reaction duration of 5 min. The absorption spectra consist of three to four exciton bands, with the lower 1S exciton energy band varying between 2.31 and 1.98 eV, corresponding to NQDs’ sizes between 3.1 and 3.9 nm (diameter). These 1S exciton absorption bands have a full-width at half-maximum (fwhm) of ∼30 nm. The 1S exciton absorption bands are blue-shifted with respect to the

CdTe NQDs in the Presence of Cd0 Nanoparticles 1S bulk exciton absorption situated at 1.475 eV. The nonresonantly excited PL excitonic band of the 3.1 nm NQD sample is Stokes-shifted with respect to the corresponding 1S exciton absorption band by ∼75 meV, while that of the 3.8 nm NQD sample is Stokes-shifted by ∼40 meV. Figure 8 presents a few plots showing the dependence of the average diameter (D, in units of nm) (Figure 8A), the produced NQDs’ molar concentration (Figure 8B), the fwhm of the 1S exciton absorption band (Figure 8C), and the CdTe NQDs’ emission QE on the NQDs’ growth time of various CdTe NQD aliquots. These aliquots were taken from reactions initiated with different Cd/Te molar ratios, as indexed in the inset of Figure 8A. The average diameter and molar concentrations of the produced CdTe NQDs in each aliquot were extracted from the absorption spectra using the relation given in reference 21. The variety of plots in Figure 8 suggests that the initial precursor’s molar ratio plays a major role in the control of the quality of the generated CdTe NQDs. Excess Te (referring to the open symbols in Figure 8) led to a fast increase of the NQDs’ diameter (Figure 8A), with a relatively narrow fwhm of the exciton absorption (Figure 8C) and a nearly constant concentration of the NQDs produced within an aliquot (Figure 8B). Excess Cd (bold symbols in the figure) induced slower size growth (Figure 8A) and partial decomposition of the created NQDs back to the solution at a longer reaction duration followed by an increase of the absorption band fwhm (Figure 8C). The simultaneous decomposition of the small NQDs and the increase of the size distribution (or the absorption band fwhm) revealed the occurrence of an Ostwald ripening process in the presence of excess Cd precursor, leading to a decrease of the surface/ volume ratio and predicting an improvement of the surface quality. Furthermore, the integration of excess Cd atoms in the NQDs was confirmed by the EDAX measurements (not shown), implying that NQDs that were prepared with excess Cd precursor showed a final Cd/Te molar ratio larger than 1. Since the SAED pattern (shown in Figure 3B) confirms the existence of a cubic zinc blende crystallographic structure at the core of the NQDs, we presume that any excess Cd should be located at the external surfaces of the NQDs. It is presumed that these excess Cd at the surface is further stabilized by the OA ligands, long and narrow molecular chains, which can be closely packed around the NQDs’ exterior surfaces, while excess Te should be stabilized by the TOP ligands, bulky molecules that may have a steric hindrance, thus avoiding close-packing at the NQDs’ surface. Therefore, the excess Cd at the NQDs’ surface is covered efficiently by the OA, giving optimal chemical passivation that reduces carrier trapping sites and consequently increases the emission QE36 up to 80%. 4. Conclusions The results described in this paper present the occurrence of in situ precipitation of crystalline Cd0 NPs prior to the formation of CdTe NQDs and examine the influence of these NPs on the CdTe NQDs’ growth. The experimental evidence suggests that the existence of Cd0 induces a regulation of the precursor supply during the CdTe NQDs’ generation, avoiding fast growth on top of certain energetic facets (001) into a pod and, instead, creating the formation of spherically shaped CdTe NQDs. Thus, the existence of the Cd0 NPs induces a control of the growth kinetics, determining the final shape of the NQDs. Also, the results above suggest that the initial Cd/Te molar ratio controls the NQDs’ quality, with excess Cd precursor inducing the generation of NQDs with exceptionally high emission QE up to 80%.

J. Phys. Chem. C, Vol. 111, No. 28, 2007 10341 Acknowledgment. This project was supported by the German Israel Foundation (GIF) Contract No. 156/03-12.6, by the German-Israel Program (DIP) Project No. D 3.2, and by the Niedersachsen Foundation, supported by the State of Lower Saxony and the Volkswagen Foundation, Hannover, Germany. Dr. A. Sashchiuk expresses her deep gratitude to the Ministry of Absorption of the State of Israel for the KAMEA Fellowship. References and Notes (1) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025. (2) Nozik, A. J. Physica E 2002, 14, 115. (3) Shavel, A.; Gaponik, N.; Eychmu¨ller, A. ChemPhysChem 2005, 6, 449. (4) Esch, V.; Fluegel, B.; Khitrova, G.; Gibbs, H. M.; Jiajin, X.; Kang, K.; Koc, S. W.; Liu, L. C.; Risbud, S. H.; Peyghambarian, N. Phys. ReV. B 1990, 42, 7450. (5) Mathew, X.; Enriquez, J. P.; Romeo, A.; Tiwari, A. N. Sol. Energy 2004, 77, 831. (6) Ferekides, C. S.; Balasubramanian, U.; Mamazza, R.; Viswanathan, V.; Zhao, H.; Morel, D. L. Sol. Energy 2004, 77, 823. (7) Porter, V. J.; Mentzel, T.; Charpentier, S.; Kastner, M. A.; Bawendi, M. G. Phys. ReV. B 2006, 73, 155303. (8) Gaponik, N. P.; Talapin, D. V.; Rogach, A. L. Phys. Chem. Chem. Phys. 1999, 1, 1787. (9) Gaponik, N. P.; Talapin, D. V.; Rogach, A. L.; Eychmu¨ller, A. J. Mater. Chem. 2000, 10, 2163. (10) Shavel, A.; Gaponik, N.; Eychmu¨ller, A. Eur. J. Inorg. Chem. 2005, 2005, 3613. (11) Osovsky, R.; Shavel, A.; Gaponik, N.; Amirav, L.; Eychmu¨ller, A.; Weller, H.; Lifshitz, E. J. Phys. Chem. B 2005, 109, 20244. (12) Franzl, T.; Shavel, A.; Rogach, A. L.; Gaponik, N.; Klar, T. A.; Eychmu¨ller, A.; Feldmann, J. Small 2005, 1, 392. (13) Li, J.; Zhao, K.; Hong, X.; Yuan, H.; Ma, L.; Li, J.; Bai, Y.; Li, T. Colloids Surf., B 2005, 40, 179. (14) Zhang, H.; Wang, L.; Xiong, H.; Hu, L.; Yang, B.; Li, W. AdV. Mater. 2003, 15, 1712. (15) Gaponik, N.; Radtchenko, I. L.; Sukhorukov, G. B.; Weller, H.; Rogach, A. L. AdV. Mater. 2002, 14, 879. (16) Rajh, T.; Micic, O. I.; Nozik, A. J. J. Phys. Chem. 1993, 97, 11999. (17) Guo, J.; Yang, W.; Wang, C. J. Phys. Chem. B 2005, 109, 17467. (18) Li, L.; Qian, H.; Fang, N.; Ren, J. J. Lumin. 2006, 116, 59. (19) Talapin, D. V.; Rogach, A. L.; Mekis, I.; Haubold, S.; Kornowski, A.; Haase, M.; Weller, H. Colloids Surf., A 2002, 202, 145. (20) Kumar, S.; Nann, T. Chem. Commun. 2003, 19, 2478. (21) Yu, W. W.; Wang, Y. A.; Peng, X. Chem. Mater. 2003, 15, 4300. (22) Puntes, V. F.; Krishnan, K. M.; Alivisatos, P. Appl. Phys. Lett. 2001, 78, 2187. (23) Murray, C. B.; Sun, S.; Gaschler, W.; Doyle, H.; Betley, T. A.; Kagan, C. R. IBM J. Res. DeV. 2001, 45, 47. (24) Brumer, M.; Kigel, A.; Amirav, L.; Sashchiuk, A.; Solomesch, O.; Tessler, N.; Lifshitz, E. AdV. Funct. Mater. 2005, 15, 1111. (25) Tang, Z.; Kotov, N. A.; Giersig, M. Science 2002, 297, 237. (26) Tang, Z.; Wang, Y.; Shanbhag, S.; Giersig, M.; Kotov, N. A. J. Am. Chem. Soc. 2006, 128, 6730. (27) Lobo, A.; Borchert, H.; Talapin, D. V.; Weller, H.; Moller, T. Colloids Surf., A 2006, 286, 1. (28) Kershaw, S. V.; Burt, M.; Harrison, M.; Rogach, A.; Weller, H.; Eychmuller, A. Appl. Phys. Lett. 1999, 75, 1694. (29) Kim, S.; Fisher, B.; Eisler, H. J.; Bawendi, M. J. Am. Chem. Soc. 2003, 125, 11466. (30) Chou, P. T.; Chen, C. Y.; Cheng, C. T.; Pu, S. C.; Wu, K. C.; Cheng, Y. M.; Lai, C. W.; Chou, Y. H.; Chiu, H. T. ChemPhysChem 2006, 7, 222. (31) Wu, N.; Fu, L.; Su, M.; Aslam, M.; Wong, K. C.; Dravid, V. P. Nano Lett. 2004, 4, 383. (32) Chan, E. M.; Mathies, R. A.; Alivisatos, A. P. Nano Lett. 2003, 3, 199. (33) Nakamura, H.; Yamaguchi, Y.; Miyazaki, M.; Uehara, M.; Maeda, H.; Mulvaney, P. Chem. Lett. 2002, 1072. (34) Steckel, J. S.; Yen, B. K. H.; Oertel, D. C.; Bawendi, M. G. J. Am. Chem. Soc. 2006, 128, 13032. (35) Liu, H.; Owen, J. S.; Alivisatos, P. J. Am. Chem. Soc. 2007, 129, 305. (36) Borchert, H.; Talapin, D. V.; Gaponik, N.; McGinley, C.; Adam, S.; Lobo, A.; Moller, T.; Weller, H. J. Phys. Chem. B 2003, 107, 9662.