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Jan 22, 2010 - ... School of Electronic Science and Engineering, Southeast University, Nanjing ... Department of Chemistry and Biochemistry, Universit...
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Chem. Mater. 2010, 22, 1437–1444 1437 DOI:10.1021/cm902516f

Bright and Stable Purple/Blue Emitting CdS/ZnS Core/Shell Nanocrystals Grown by Thermal Cycling Using a Single-Source Precursor Dingan Chen,†,‡ Fei Zhao,*,‡ Hang Qi,‡ Michael Rutherford,‡ and Xiaogang Peng*,‡ †

Advanced Photonics Center, School of Electronic Science and Engineering, Southeast University, Nanjing 210096, P. R. China, and ‡Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, Arkansas 72701 Received August 16, 2009. Revised Manuscript Received November 5, 2009

A new approach for synthesis of high quality core/shell nanocrystals, “thermal-cycling coupled single precursor” (TC-SP), was developed. It was applied for the synthesis of high efficiency, good color purity, nearly monodisperse, alloying-free, and stable CdS/ZnS core/shell nanocrystals with their emission peak tunable between 375 and 475 nm. A comparison study on TC-SP versus the classic successive ionic layer adsorption and reaction (SILAR), both in one pot, revealed that, although the reaction temperature was significantly reduced and the procedure was simplified for TC-SP, the overall quality of the CdS/ZnS core/shell nanocrystals yielded by TC-SP was found to be superior. A quantitative chemical etching method was developed for determining the radial distribution of elements in bandgap/composition engineered nanocrystals. Introduction Colloidal semiconductor nanocrystals, commonly known as quantum dots (q-dots), have been enthusiastically exploited1,2 mostly because of their unique size dependent optical properties, especially the size-tunable photoluminescence (PL) and electroluminescence (EL) properties. Their unique PL and EL properties make q-dots important for both fundamental research3,4 and industrial applications, such as light-emitting diodes (LEDs),5,6 biomedical labeling,7,8 lasers,9 solar cell,10 etc. If an application is based on their emission properties, it has been established that q-dots must be made in core/ shell forms. Usually, a wide bandgap shell is epitaxially grown onto the targeted core nanocrystals11,12 in order to achieve desired brightness, reproducibility, and stability. *To whom correspondence should be addressed. E-mail: [email protected] (X.P.); [email protected] (F.Z.).

(1) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Annu. Rev. Mater. Sci. 2000, 30, 545–610. (2) Peng, X.; Thessing, J. Struct. Bonding (Berlin) 2005, 118, 79–119. (3) Brus, L. J. Phys. Chem. 1986, 90, 2555–2560. (4) Alivisatos, A. P. Science 1996, 271, 933–937. (5) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370 (6488), 354–357. (6) Coe, S.; Woo, W. K.; Bawendi, M.; Bulovic, V. Nature 2002, 420 (6917), 800–803. (7) Bruchez, M.Jr; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281(5385), 2013–2016. (8) Chan, W. C. W.; Nie, S. Science 1998, 281(5385), 2016–2018. (9) Klimov, V. I.; Mikhailovsky, A. A.; Xu, S.; Malko, A.; Hollingsworth, J. A.; Leatherdale, C. A.; Eisler, H. J.; Bawendi, M. G. Science 2000, 290(5490), 314–317. (10) Greenham, N. C.; Peng, X.; Alivisatos, A. P. Phys. Rev. B 1996, 54 (24), 17628–17637. (11) Hines, M. A.; Guyot-Sionnest, P. J. Phys. Chem. 1996, 100, 468– 471. (12) Peng, X. G.; Schlamp, M. C.; Kadavanich, A. V.; Alivisatos, A. P. J. Am. Chem. Soc. 1997, 119(30), 7019–7029. r 2010 American Chemical Society

At present, high performance core/shell q-dots in the green and red optical window have been made available with traditional CdSe-based core/shells.13-18 Most recently, InP/ZnS19 and InAs/ZnSe20 core/shell q-dots were also reported as noncadmium high performance emitters in the green, red, and even near-infrared color regime. However, q-dot emitters in the high-energy visible window,21,22 roughly 480 nm and above, are rarely reported with similar brightness, color purity, and color-tunability. This impacts some applications significantly, such as LEDs and solid-state lighting. It is worth pointing out that, in other types of emitters, such as organic dyes and inorganic phosphors, it also known to be difficult to obtain high performance blue and high-energy emissions. Therefore, high performance q-dot emitters in the short wavelength window are comparatively more interesting than their green and red counterparts. (13) Dabbousi, B. O.; Rodriguez-Viejo, J.; Mikulec, F. V.; Heine, J. R.; Mattoussi, H.; Ober, R.; Jensen, K. F.; Bawendi, M. G. J. Phys. Chem. B 1997, 101, 9463–9475. (14) Reiss, P.; Bleuse, J.; Pron, A. Nano Lett. 2002, 2(7), 781–784. (15) Li, J. J.; Wang, Y. A.; Guo, W. Z.; Keay, J. C.; Mishima, T. D.; Johnson, M. B.; Peng, X. G. J. Am. Chem. Soc. 2003, 125(41), 12567–12575. (16) Talapin, D. V.; Mekis, I.; Gotzinger, S.; Kornowski, A.; Benson, O.; Weller, H. J. Phys. Chem. B 2004, 108(49), 18826–18831. (17) Xie, R. G.; Kolb, U.; Li, J. X.; Basche, T.; Mews, A. J. Am. Chem. Soc. 2005, 127(20), 7480–7488. (18) Battaglia, D.; Blackman, B.; Peng, X. G. J. Am. Chem. Soc. 2005, 127(31), 10889–10897. (19) Xie, R.; Battaglia, D.; Peng, X. J. Am. Chem. Soc. 2007, 129(50), 15432–15433. (20) Xie, R. G.; Peng, X. G. Angew. Chem., Int. Ed. 2008, 47(40), 7677– 7680. (21) Yu, W. W.; Peng, X. G. Angew. Chem., Int. Ed. 2002, 41(13), 2368– 2371. (22) Steckel, J. S.; Zimmer, J. P.; Coe-Sullivan, S.; Stott, N. E.; Bulovic, V.; Bawendi, M. G. Angew. Chem., Int. Ed. 2004, 43(16), 2154–2158.

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The introduction of the successive ionic layer adsorption and reaction (SILAR) method15 and air-stable precursors greatly improved the structural quality of the epitaxial shell of core/shell nanocrystals. By coupling with thermal cycling (TC),23 i.e., addition/adsorption of precursors at a low reaction temperature and epitaxial growth at a high temperature, the quality of the resulting core/shell nanocrystals from the SILAR method has been further improved. With the importance of bandgap/composition engineering of colloidal nanocrystals through epitaxial growth provided, diverse epitaxy methods would be desirable. Single-precursor approaches, meaning containing both anionic and cationic elements in one precursor, have been previously reported for the synthesis of core nanocrystals.24-28 Single precursors were also applied for the overcoating on the surface of semiconductor and oxide nanocrystals.29,30 Recently, a zincsulfide single precursor, zinc ethylxanthate (Zn(ex)2), was used in the synthesis of CdS/ZnS core/shell nanocrystals in combination with another cationic precursor, zinc stearate.31 Although the results on characterization of the resulting core/shell nanocrystals and the growth process of this quasi-single precursor approach were quite brief, the potential of a simplified version, with a single precursor only, seems to be attractive for several reasons. First, such a single precursor might reduce the shell growth temperature because of its known low decomposition temperature, which should help to discriminate homogeneous nucleation from heterogeneous growth. Second, the reaction schemes could be significantly simplified in comparison to the corresponding SILAR method. Finally, because of the fixed decomposition temperature, it should be easy to choose adsorption temperature and epitaxial growth temperature. This report intended to demonstrate these hypotheses using zinc diethyldithiocarbamate (Zn(DDTC)2) as the sole shell precursor.32-34 To fully realize the above advantages, the singleprecursor approach to be developed in this work shall be combined with the thermal-cycling technique to enable separation of surface adsorption and reaction. In a certain sense, this single-precursor approach shall retain (23) Blackman, B.; Battaglia, D. M.; Mishima, T. D.; Johnson, M. B.; Peng, X. G. Chem. Mater. 2007, 19(15), 3815–3821. (24) Trindade, T.; O’Brien, P. Adv. Mater. 1996, 8(2), 161–163. (25) Malik, M. A.; Revaprasadu, N.; O’Brien, P. Chem. Mater. 2001, 13 (3), 913–920. (26) Cumberland, S. L.; Hanif, K. M.; Javier, A.; Khitrov, G. A.; Strouse, G. F.; Woessner, S. M.; Yun, C. S. Chem. Mater. 2002, 14(4), 1576–1584. (27) Barrelet, C. J.; Wu, Y.; Bell, D. C.; Lieber, C. M. J. Am. Chem. Soc. 2003, 125(38), 11498–11499. (28) Sun, J. W.; Buhro, W. E. Angew. Chem., Int. Ed. 2008, 47(17), 3215– 3218. (29) Revaprasadu, N.; Malik, M. A.; O’Brien, P.; Wakefield, G. Chem. Commun. 1999, 16, 1573–1574. (30) Monteiro, O. C.; Neves, M. C.; Trindade, T. J. Nanosci. Nanotechnol. 2004, 4(1-2), 146–150. (31) Protiere, M.; Reiss, P. Nanoscale Res. Lett. 2006, 1(1), 62–67. (32) Xu, S.; Ziegler, J.; Nann, T. J. Mater. Chem. 2008, 18(23), 2653– 2656. (33) Nose, K.; Soma, Y.; Omata, T.; Otsuka-Yao-Matsuo, S. Chem. Mater. 2009, 21(13), 2607–2613. (34) Zhang, W.; Chen, G.; Wang, J.; Ye, B.-C.; Zhong, X. Inorg. Chem. 2009, 48(20), 9723–9731.

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Figure 1. One-pot reaction scheme via TC-SP. The molecular structure of the single-molecular precursor is given as the inset.

some of the useful features of the thermal-cycling coupled SILAR (TC-SILAR) method. For this reason, this new approach would be called “thermal-cycling coupled single precursor”, or TC-SP, for simplicity. To further simplify the reaction scheme, this work explored a onepot approach for the growth of the CdS/ZnS core/shell nanocrystals using the single-precursor Zn(DDTC)2. The resulting nearly monodisperse CdS/ZnS core/shell nanocrystals yielded from the TC-SP approach were high performance emitters, with high PL quantum yield (up to 50%), high color purity (full width at half-maximum being about 18-25 nm), good color tunability in the optical window from 375 to 475 nm, and good chemical/ photochemical stability. The overall quality was found to be superior to those yielded by the quasi-single precursor approach31 and the TC-SILAR method. Though the epitaxial growth was at a significantly reduced temperature and a simplified one-pot synthetic scheme in comparison to the corresponding TC-SILAR method using separated conventional zinc carboxylate salts and elemental sulfur precursors, the CdS/ZnS core/shell q-dots were proven to be of high structural clarity. A quantitative chemical etching method was developed to confirm the core/shell structure, which should be broadly applicable for characterization of other types of complex nanocrystals yielded by bandgap/composition engineering. Results and Discussion The reaction of TC-SP in one pot can be schematically illustrated using CdS/ZnS core/shell nanocrystals as an example (Figure 1). Although the process was carried in one pot, a simple in situ purification step was found to be necessary to remove unreacted cadmium and sulfur precursors from the synthesis of CdS nanocrystals. This was achieved by adding methanol into the reaction mixture from the synthesis of CdS after it cooled down to ∼50 °C. As demonstrated previously, the methanol layer could efficiently remove the relatively polar precursors. Control experiments showed that CdS core nanocrystals purified in this manner were found to be stable and no additional growth upon heating up to the suitable reaction temperatures for the following TC-SP (Figure S1, Supporting Information). The reaction temperature for the key TC-SP portion for the entire reaction process shown in Figure 1 was

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controlled to the range below 200 °C. A central theme of TC-SP was to determine two temperatures for thermal cycling. The TGA measurements (Figure S2, Supporting Information) revealed that the decomposition of Zn(DDTC)2 in its pure solid form occurred between about 200 and 350 °C, which is not desirable for reducing the reaction temperature. However, with the presence of oleylamine in the ODE solution, the starting decomposition temperature of Zn(DDTC)2 could be lowered down to 80 °C (see caption for Figure S2, Supporting Information) because of the existence of amines.35 Experimental results revealed that, below about 120 °C, decomposition was quite slow. Growth of ZnS onto CdS nanocrystals became noticeable at about 150 °C, but the photoluminescence properties of the resulting core/shell nanocrystals were poor (Figure S3, Supporting Information). With these facts, the temperature for addition/adsorption of the precursor was chosen to be 120 °C. The temperature for the shell growth in TC-SP approach could vary from 160 to 200 °C. The higher the temperature, the shorter the growth time required. For example, it took about 20 min for one monolayer of ZnS to grow at 180 °C. Synthesis of the nearly monodisperse CdS core nanocrystals following the previously reported method,21 however, was carried out in the temperature range significantly higher than 200 °C. Although this part is not the focus of this report, it should be pointed out that our recent results indicate that CdS nanocrystals with similar quality could be formed with a temperature at around 180 °C (to be published later). This means the entire onepot TC-SP procedure shown in Figure 1 could potentially be controlled within the temperature range below 200 °C. The reproducibility of the TC-SP in one pot required the determination of the size and concentration CdS core nanocrystals prior to the shell growth. This was performed by measuring the absorption spectrum of an aliquot prior to the addition of the shell precursor solution with known extinction coefficients.36 The amount for growing a monolayer of the ZnS shell was calculated using a different model from the one suggested in the original SILAR method.15 The current method was based on the structural facts of ZnS (see details in Supporting Information), instead of estimating the amount of atoms in a given monolayer of a nanocrystal. Epitaxial growth of core/shell nanocrystals was monitored by UV-vis (Figure 2) and PL (Figure 3) spectra. Figure 2 shows a series of UV-vis absorption spectra of the CdS core and CdS/ZnS core/shell nanocrystals with different targeted shell thicknesses. While a significant red shift of the first exciton absorption peak for the example reaction was observed for the first monolayer of shell, a small but noticeable red shift was visible in each step of the shell growth in the plot (Figure 2). This is qualitatively expected as the energy difference between CdS and ZnS bandgaps is not extremely high. The red shift, in fact, has been regarded as a signature of successful growth of the (35) Reddy, K. H.; Lingappa, Y. J. Chem. Sci. 1993, 105(2), 87–94. (36) Yu, W. W.; Qu, L. H.; Guo, W. Z.; Peng, X. G. Chem. Mater. 2003, 15(14), 2854–2860.

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Figure 2. UV-vis spectra of CdS core and CdS/ZnS core/shell nanocrystals with different numbers of ZnS monolayers (ML).

shell without alloying12 or at least not complete alloying between the core and shell. Substantial alloying of CdS and ZnS should shift the bandgap of CdS toward that of ZnS, which implies a blue shift of the absorption peak. Another spectroscopic signature for epitaxial growth, instead of alloying, is the substantial increase of the absorbance in the high energy part of the spectrum (roughly >350 nm) relative to the peak position. For example, although the first exciton peak position was about the same for the one-monolayer, three-monolayer, and five-monolayer samples, the intensity at 300 nm increased from 2 times (for one monolayer) of the absorbance at the peak position to 3.5 times (three monolayer) and to 5.5 times (five monolayer). The PL peak of the bandgap emission of the nanocrystals also shifted to red (Figure 3a) upon the growth of the ZnS shell, which is consistent with the red shift of the absorption spectra (Figure 2). Upon the growth of the ZnS shell, the PL quantum yield of the resulting core/shell nanocrystals increased steadily up to four monolayers of ZnS shell (Figure 3b). Detailed analysis of the PL properties of the core/shell nanocrystals are given below by comparing the core/shell nanocrystals grown from the core nanocrystals with the same size using either TC-SP or thermal-cycling coupled SILAR (TC-SILAR). The precursors for TC-SILAR were elemental sulfur and zinc oleate, separately dissolved in octadecene (ODE), that were the typical “greener” precursors used for developing the original SILAR method.14,15 Though the addition of precursors was performed at the same temperature as the TC-SP scheme (Figure 1), the growth of the shell must be at a temperature not lower than about 220 °C to observe a decent growth rate of the shell under the same solution conditions. The amount of red shift in the UV-vis and PL spectra (see Figure 3a as an example) was pretty much the same for the core/shell nanocrystals with the same shell thickness grown by two different methods. However, the emission color purity and brightness of the resulting core/shell nanocrystals showed some significant differences. The CdS/ZnS core/shell nanocrystals grown by TCSILAR showed substantial trap emission, evidenced as a

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Figure 3. PL properties of the CdS/ZnS core/shell nanocrystals in comparison to the initial core nanocrystals. CdS/ZnS core/shell nanocrystals grown by both TC-SP and thermal-cycling coupled SILAR (TC-SILAR) are shown.

long tail on the long wavelength side of the PL spectra, if the shell thickness was less than three monolayers (Figure 3a). Conversely, though from the same batch of core nanocrystals (Figure 3a, bottom plot), the core/shell nanocrystals grown via TC-SP showed complete elimination of the trap emission even if only one monolayer of the shell was in place. It should be pointed out that the previous report,31 using Zn(ex)2 as the main precursor and zinc stearate as supplemental zinc precursor, also noticed that about two to three monolayers of ZnS shell formation can almost eliminate the surface trap emission of the CdS core nanocrystals. The more efficient elimination of the surface trap emission of the CdS core nanocrystals using the TC-SP approach suggests that the emission quality of the resulting CdS/ZnS core/shell nanocrystals should be better than that yielded by the TC-SILAR method. Indeed, quantitative measurements of PL QY of the core/shell nanocrystals showed significant advantage of TC-SP against TC-SILAR for this specific system. Overall, the PL QY of the core/shell nanocrystals from TC-SP were about twice as efficient than those yielded by TC-SILAR (Figure 3). The PL QY measurements used Coumarin 460 as the standard. Also using a Coumarin dye, the previous articles reported the maximum PL QY values for the CdS/ZnS core/shell nanocrystals grown by the mixed precursors of Zn(ex)2 and zinc stearate in the range between 25% and 40%,31,37 which is lower than the maximum values for TC-SP (50%) and somewhat higher than those for TC-SILAR (25-30%). The above results suggest that, as we anticipated, the TC-SP was not only simpler and required a lower reaction temperature but also yielded the CdS/ZnS core/shell nanocrystals with significantly better optical quality in comparison to the corresponding TC-SILAR for this system. For this reason, unless mentioned differently,

(37) Tan, Z. N.; Zhang, F.; Zhu, T.; Xu, J.; Wang, A. Y.; Dixon, D.; Li, L. S.; Zhang, Q.; Mohney, S. E.; Ruzylo, J. Nano Lett. 2007, 7(12), 3803–3807.

Figure 4. (Top) PL spectra of CdS/ZnS core/shell nanocrystals. (Inset) PL image of a blue-emission sample. (Bottom) XRD patterns of the nanocrystals. The standard diffraction patterns of zinc-blende bulk CdS and ZnS are included as references.

all CdS/ZnS core/shell nanocrystals reported here are those yielded by the TC-SP approach. The TC-SP worked for different sized core nanocrystals as well, which resulted in bright and stable CdS/ZnS core/ shell nanocrystals in a quite broad wavelength range. Figure 4 (top) illustrates a few examples of such high

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performance q-dot emitters in the high energy optical window, approximately from 375 to 475 nm. The peak width (full width at half-maximum, fwhm) of the resulting core/shell nanocrystals was typically in the range between 18 and 25 nm. When the emission peak position was 475 nm, the fwfm was exceptionally broad, about 35 nm (Figure 4, top). This phenomena is probably due to this wavelength being very close to the bulk bandgap of CdS and the excitons in such nanocrystals experienced weak quantum confinement. The crystal structure and size and size distribution of the CdS/ZnS core/shell nanocrystals were studied using X-ray powder diffraction (XRD), transmission electron microscope (TEM), selected area electron diffraction (SAED), and high-resolution TEM (HRTEM). The powder XRD patterns in Figure 4 (bottom) reveal that both CdS core and CdS/ZnS core/shell nanocrystals grown by TC-SP possessed zinc-blende crystal structures. It is interesting to notice that the CdS/ZnS core/shell nanocrystals grown by the use of Zn(ex)2 and zinc stearate as the precursors were wurtzite in structure.31 The relatively broad peaks in the diffraction patterns in Figure 4 (bottom) were consistent with their small sizes. As the ZnS shell grew thicker, the XRD peaks shifted gradually from those corresponding to CdS bulk values to those more similar to the ZnS diffraction peaks. All of these crystal structure features are consistent with epitaxial growth of ZnS shell onto CdS core nanocrystals. The size, shape, and size/shape distribution of the CdS/ ZnS core/shell nanocrystals were well controlled using TC-SP (Figure 5). Upon the growth of ZnS shell, the increase of the particle size was observed in TEM images, which was found to be consistent with the numbers of ZnS monolayers targeted by the injections. The core/shell nanocrystals all retained the nearly monodisperse size distribution of the core nanocrystals (Figure 5), indicated by the formation of ordered 2-dimensional superlattices. The shape of the core/shell nanocrystals was close to being spherical. The corresponding HRTEM image of the sample with six monolayers of ZnS shell is shown as the inset in the corresponding low-resolution TEM image. The HRTEM experiments revealed that the CdS/ZnS core/shell nanocrystals were a single crystalline and in zinc-blende lattice structure. Most core/shell nanocrystal samples examined by TEM were also examined by selected area electron diffraction (data not shown), which confirmed the zinc-blende structure of the core/shell nanocrystals. No pure ZnS phase was detected by electron diffraction in the core/shell nanocrystal samples. These results were found to be consistent with the XRD measurements (Figure 4, bottom, and the related text). A quantitative etching procedure was developed for confirming the core/shell structure using benzoyl peroxide solutions (see Experimental Section for details). Different from the reported method,18,23,38 the oxidative, thus irreversible, etching of the core and core/shell nano-

crystals was performed with a series of benzoyl peroxide solutions with different concentrations. Therefore, each reaction stopped at a different reaction time: the higher the peroxide concentration, the longer the reaction time, and the smaller the nanocrystals yielded. The temporal evolution of the UV-vis spectra of the nanocrystals (Figures S4 and S5, Supporting Information) was used as analytical means for monitoring the reactions. To determine the size, structure, and composition of the etched nanocrystals, UV-vis, TEM, and EDX were applied for the analysis of the final product in each reaction solution after proper purification. The etching of pure CdS core nanocrystals (Figure S4, Supporting Information) resulted in a smooth and fast shift of the absorption spectra when an excess amount of etching reagents was used. Under the similar conditions, however, the absorption spectra of the core/shell nanocrystals (Figure S5, Supporting Information) showed a very slow change initially followed by a fast change. These features are quite similar to the typical core and core/shell nanocrystals during the single-solution etching and can be better visualized by plotting the temporal evolution of either the absorbance at the first exciton absorption peak position of a given sample38,39 (Figure 6a) or the shift of the absorption peak position (Figure S6, Supporting Information). Figure 6a illustrates the temporal evolution of the absorbance at the first exciton absorption peak position of the core and core/shell nanocrystal samples. The slow decrease in the first 7 min for the core/shell etching should correspond to the removal of a reasonably uniform ZnS shell according to the existing literature.18 The sudden acceleration of the absorbance decrease at about 7 min was the “phase-shift” point indicating the etching going through the ZnS and CdS interface of the core/shell nanocrystals.38 Below, to be quantitative, we further

(38) Blackman, B.; Battaglia, D.; Peng, X. G. Chem. Mater. 2008, 20(15), 4847–4853.

(39) Ivanov, S. A.; Piryatinski, A.; Nanda, J.; Tretiak, S.; Zavadil, K. R.; Wallace, W. O.; Werder, D.; Klimov, V. I. J. Am. Chem. Soc. 2007, 129(38), 11708–11719.

Figure 5. TEM images of CdS core and CdS/ZnS core/shell (two, four, and six monolayers) nanocrystals. A representative HRTEM image of one core/shell nanocrystal with six monolayers of ZnS shell is given as the inset.

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Figure 6. (a) Temporal evolution of the absorbance at the first exciton absorption peak (429 nm for the core/shell sample and 401 nm for the core sample) with an excess amount of etching reagents. (b) The Zn to Cd molar ratio determined using EDX (red dots) for the etched nanocrystals and the theoretical values (blue triangles) calculated for the core/shell nanocrystals with different sizes. (c) Examples of TEM images of the core/shell nanocrystals (original and etched).

examined the composition-size correlation of the etched nanocrystals generated in the series of peroxide solutions. The etched nanocrystals in the series of etching solutions were examined by energy-dispersive X-ray spectroscopy (EDX) and TEM (Figure 6b,c). Figure 6c illustrates clearly that upon etching the size of the nanocrystals decreased and the size distribution was retained. Figure 6b summarizes the Zn/Cd atomic ratio versus the final sizes of the etched nanocrystals. Evidently, the Zn/ Cd ratio determined experimentally followed well with the theoretical values (Figure 6b). This quantitatively confirms the core/shell structure of the nanocrystals, and within the experimental errors, no alloy was detected. Combining the UV-vis (Figures 6a, S4, S5, and S6, Supporting Information), EDX, and TEM results (Figure 6b,c), one can confidently state that the CdS/ ZnS core/shell nanocrystals grown using the TC-SP approach had complete shell structures without pinholes, uniform growth of the shell down to a monolayer level, and no noticeable alloying in the shell growth. Conclusions A TC-SP method for the growth of CdS/ZnS core/shell nanocrystals in one pot was developed. TC-SP method allowed a uniform and controlled growth of the ZnS shell with precise control down to a single monolayer level, which is similar to that of the well-established SILAR method. Because of the low activation temperature of the single precursor in the solution, this TC-SP method could be carried out with temperatures below 200 °C, thermal cycled between 120 °C (for the precursor addition) and 160-200 °C (for the shell epitaxy). This reduced reaction

temperature probably rendered an excellent optical performance of the resulting CdS/ZnS core/shell nanocrystals, with PL QY as high as 50% in the high energy optical window (approximately between 375 and 475 nm). The thick and controlled ZnS shell made these purple and blue emitters stable under ambient conditions for at least 6 months. These results imply that the TC-SP method compensates the existing SILAR method15 for controlled bandgap engineering in the field of colloidal nanocrystals. These high performance emitters may make them potential candidates for developing purple and blue nanocrystal-based LEDs with desired properties, which might further enable the design and fabrication of efficient white LEDs for solid-state lighting. The quantitative etching method developed by this work should offer a general strategy for determining the radial distribution of elements in a core/shell as well as other complex colloidal nanocrystals. Experimental Section Chemicals. Cadmium oxide (99.99%), sulfur powder (99.98%), zinc oxide (99.9%), and benzylamine (99%) were purchased from Aldrich. Oleic acid (OA, tech. 90%), 1-octadecene (ODE, tech. 90%), zinc acetate dihydrate (ACS, 98.0101.0%), sodium diethyldithiocarbamate trihydrate (98%), and benzoyl peroxide (97%, dry) were purchased from Alf-Aesar. Oleylamine (tech. g70%) was purchased from Fluka. All organic solvents were purchased from EM Sciences. All chemicals were used directly without any further purification unless otherwise stated. Synthesis of Zinc Diethyldithiocarbamate (Zn(DDTC)2). In a typical synthesis, zinc acetate dihydrate (3.359 g, 15 mmol) was dissolved in 100 mL of distilled water in a 400 mL beaker.

Article Sodium diethyldithiocarbamate trihydrate (6.897 g, 30 mmol) was dissolved in 60 mL of distilled water and added dropwise into the beaker containing zinc acetate solution with vigorous stirring. White precipitates of Zn(DDTC)2 slowly formed. The mixture was stirred for 20 min to ensure the reaction was completed and filtered on a Buckner funnel. The white precipitate was washed three times with distilled water and dried under vacuum. Preparation of S and Zn Precursor Solutions for SILAR. Sulfur precursor solution (0.1 mol/L) was prepared by adding sulfur powder (0.032 g, 1 mmol) and 10 mL of ODE to a 20 mL glass vial. The sulfur solution was sealed with a rubber septum, purged with argon, heated slightly with a heating gun until the solution turned clear, and then stored at room temperature. The Zn precursor solution (0.1 mol/L) was prepared by combining ZnO (0.082 g) and oleic acid (2.511 g) in a 1:8 molar ratio with ODE (5.681 g) inside a 25 mL three-neck round-bottomed flask equipped with a stir bar. The ZnO mixture was capped with a rubber septum, purged with argon, and then heated to 250 °C with constant stirring until the solution turned clear. After being cooled to ∼60 °C, the solution was placed in a 20 mL glass vial capped with a rubber septum and stored at room temperature. The single source precursor solution (0.1 mol/L) for TC-SP was prepared by adding Zn(DDTC)2 (0.362 g, 1 mmol) and 10 mL of oleylamine to a 20 mL glass vial. The Zn(DDTC)2 solution was sealed with a rubber septum, purged with argon, sonicated until the solution turned clear, and then stored at room temperature. It should be pointed out that the Zn(DDTC)2 solution should be freshly prepared before use. Typical One-Pot Synthesis of CdS/ZnS through TC-SILAR. CdS core was prepared using a procedure modified from a previous report.21 Briefly, a mixture of CdO (0.0256 g, 0.2 mmol), oleic acid (0.502 g, 1.6 mmol), and ODE (6 g) was heated to 260 °C in a 25 mL three-neck round-bottomed reaction flask under argon. When the solution turned clear, 1 mL of sulfur solution (0.1 mol/L) was swiftly injected into this hot solution, and the reaction mixture was allowed to cool down to 240 °C for the further growth of CdS nanocrystals. The UV-vis absorption spectra were monitored. The reaction was stopped and cooled down to 50 °C when the CdS core reached a desired size. An in situ purification procedure was performed as below. An excess of methanol (∼10 mL), an extraction solvent, was added to the flask with stirring at 50 °C. When the ODE layer and methanol layer were separated, the upper methanol layer was taken by syringe to remove unreacted precursors and side products. The purification procedure was repeated three times. Trace amounts of methanol left in the flask were removed by the vacuum or bubbling argon. The purified CdS nanocrystals (∼1.2  10-7 mol of nanocrystal with 3.7 nm in size, calculated by the method based on the previous report36) were ready for the growth of the ZnS shell. A solution of oleylamine (2 mL) was added to the flask, and the reaction was further heated to 120 °C. The regular Zn and S precursor solutions (0.48 mL each) were added consecutively via syringe to the reaction flask containing the CdS cores, waiting 5 min between each injection. The temperature was increased immediately to 220 °C and kept for 20 min to allow the growth of the first ZnS monolayer. After that, the temperature was allowed to decrease to 120 °C; 0.64 mL of the Zn and S precursor solutions were injected as before using the same time intervals and temperature changes for growth of the second monolayer. For the third, fourth, fifth, and sixth monolayer, the temperature cycling was continued by injecting 0.83 mL (third), 1.03 mL (fourth), 1.26 mL (fifth), and 1.51 mL (sixth), respectively, of the precursor solutions. Aliquots (∼0.1 mL) were taken in every step to monitor the reaction.

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Typical One-Pot Synthesis of CdS/ZnS through TC-SP. For a typical synthesis, the purified CdS cores (∼1.2  10-7 mol of nanocrystals with 3.7 nm in size) were prepared as described above. The Zn(DDTC)2 precursor solution (0.48 mL) was added via syringe to the reaction flask at 50 °C. The system was heated immediately to 180 °C for 20 min to allow the growth of the first ZnS monolayer. Next, the temperature of the reaction solution was allowed to decrease to 120 °C, and then, 0.64 mL of Zn(DDTC)2 precursor solution was added to the reaction flask for the growth of the second monolayer of ZnS shell. For the third, fourth, fifth, and sixth monolayer, the temperature cycling was continued by injecting 0.83 mL (third), 1.03 mL (fourth), 1.26 mL (fifth), and 1.51 mL (sixth), respectively, of the single precursor Zn(DDTC)2 solutions. Aliquots (∼0.1 mL) were taken to monitor the reaction. When the desired synthesis was completed, the reaction was cooled down to room temperature. An extraction procedure (hexane and methanol in volume ratio being 1:3 as the extraction solvent) followed by precipitation using acetone was applied to the reaction mixture. A centrifuge procedure followed by decantation was used to isolate the nanocrystal solid. The resulting nanocrystals could be redispersed in toluene, hexane, or other nonpolar solvents. Quantitative Etching of the CdS and CdS/ZnS Nanocrystals. Etching of the CdS and CdS/ZnS nanocrystals was carried out using a modified scheme from our previous reports.23,38 In a typical process, the purified CdS or CdS/ZnS nanocrystal solution (0.5 mL) from the TC-SILAR synthesis was added to 1.0 mL of benzylamine. The resulting mixture was sonicated for 20 min to allow an adequate exchange of the original surface ligands to benzylamine. Subsequently, 0.1 mL of sonicated CdS or CdS/ZnS nanocrystal solution was transferred to a 2.5 mL quartz cuvette by microsyringe, followed by 0.8 mL of methanol and 1.2 mL of toluene. The optical density of the first exciton absorption peak of this mixture was around 0.3. To initiate the etching process, 0.2 mL of a 0.2 mol/L benzoyl peroxide solution in a toluene/methanol mixture (2:1 in volume ratio) was added to the cuvette by microsyringe. This process was monitored by UV-vis spectrometer. To control the etching process of CdS/ ZnS nanocrystals, a series of solutions of benzoyl peroxide with different concentrations were used. The reaction solutions in different glass vials were stirred overnight to ensure the etching process was completed. The UV-vis spectra were measured for each solution. The resulting solutions were purified as in the following procedure. The mixture was added to 3 mL of distilled water and centrifuged for 5 min until the toluene and water layers were separated. This extraction procedure was repeated three times to remove polar side products, such as ions generated and the oxidant. An excess of methanol was added to the toluene solution until the solution became cloudy. The precipitate was spun down in a centrifuge at 2000 rpm for 10 min. The purified precipitate was ready for the EDX measurement. For TEM measurement, a small portion of the purified precipitate was redissolved in toluene. For the CdS/ZnS nanocrystals yielded from TC-SP, 2 mL of their toluene solution was loaded in a 25 mL three-neck flask; 5 mL of benzylamine was added into the flask, and the mixture was heated to 50 °C with stirring overnight. The solution was transferred to a 20 mL centrifuge tube; an excess of acetone was added to the tube until precipitate appeared. The mixture was centrifuged under 3000 rpm for 10 min. The resulting nanocrystals were redissolved in 2 mL of toluene for etching using the same procedure described in the above paragraph.

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Transmission Electron Microscopy (TEM) and High-Resolution TEM (HR-TEM). The low-resolution TEM images were taken on a JEOL 100CX transmission electron microscope with an acceleration voltage of 100 kV using a Formvar coated copper grid. HRTEM pictures were taken on a FEI Titan microscope with an acceleration voltage of 300 kV. The nanocrystals were deposited onto ultrathin carbon film, attached to TEM mesh grids. Optical Measurements. UV-vis absorption and photoluminescence (PL) spectra were measured on a HP 8453 diode array spectrophotometer and a Spex Fluorolog-3 spectrofluorometer, respectively. Photoluminescence quantum yields of samples were determined using Coumarin 460 dissolved in ethanol as standards. X-ray Powder Diffraction (XRD). XRD patterns were obtained on a Philips PW1830 X-ray powder diffractometer using Cu KR line (λ) 1.5418 A˚. Energy-Dispersive X-ray Spectroscopy (EDX). EDX was investigated under a Philips ESEM XL30 scanning electron

Chen et al. microscope equipped with a field emission gun and operated at 10 kV.

Acknowledgment. This work was partially supported by the NSF and NIH. We thank Dr. Huajun Zhou for assistance on EDX measurement and Dr. Mourad Benamara for HRTEM measurements. D.C. is grateful for the State Scholarship (No. [2007]3020) provided by China Scholarship Council. Supporting Information Available: The stability of CdS core nanocrystals, the decomposition of Zn(DDTC)2, UV-Vis and PL spectra of CdS/ZnS nanocrystals at 150 °C, evolution of absorption spectra of CdS and CdS/ZnS nanocrystal during etching, and calculation for the injection for the shell growth (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.