pubs.acs.org/Langmuir © 2009 American Chemical Society
Ambient Synthesis and Characterization of High-Quality CdSe Quantum Dots by an Aqueous Route M. N. Kalasad, M. K. Rabinal,* and B. G. Mulimani† Department of Physics, Karnatak University, Dharwad-580003, India. †Present address: Gulbarga University, Gulbarga-585106, India Received May 20, 2009. Revised Manuscript Received July 23, 2009 Herein, we report the ambient synthesis of CdSe nanoparticles of widely tunable particle size by a solution route. The proposed protocol uses hydrazine hydrate to form an air-stable complex of selenium. These nanoparticles are characterized by X-ray diffraction, FTIR, optical absorption, photoluminescence, and transmission electron microcopy measurements. By varying the molarities of Cd2+ and Se2- ions in solution with 3-mercaptopropionic acid as the capping ligand, the method permits us to synthesize nanoparticles of size ranging from 1.58 to 3.42 nm (estimated from optical absorption edge measurements) by controlling the annealing time of the starting colloid at 100 °C. The extracted quantum dots are of high quality (40% photoluminescence quantum yield) and exhibit colors ranging from deep blue to red. The resulting colloids are very stable, and no precipitate is observed over a period of 6 months. Thus, the method is simple and easily scalable to synthesize fluorescent CdSe nanoparticles.
1. Introduction There has been significant development in the fields of nanophysics and nanochemistry of semiconductor nanoparticles for various applications.1,2 Among the reported semiconductor quantum dots (QDs), CdSe stands out as prominent material because of the possibility of wide tuning of its optical gap by particle size.3,4 Over the past decade, much progress has been made in the syntheses of CdSe nanoparticles using solution chemistry as the most viable route.5-10 A survey of the literature shows that in all of these syntheses selenium is usually introduced in two ways-either as an inorganic salt or as an organometallic compound.11 Most of these precursors are reactive and undergo fast oxidative loss of selenium in the open air; as a result, an inert atmosphere becomes inevitable during the synthesis. Although *Corresponding author. E-mail:
[email protected]. (1) Rogach, A. L.; Gaponik, N.; Lupton, J. M.; Bertoni, C.; Gallardo, D. E.; Dunn, S.; Pira, N. L.; Paderi, M.; Repetto, P.; Romanov, S. G.; O’Dwyer, C.; Sotomayor Torres, C. M.; Eychmuller, A. Angew. Chem., Int. Ed. 2008, 47, 6538– 6549. (2) Cingarapu, S.; Yang, Z.; Sorensen, C. M.; Klabunde, K. J. Chem. Mater. 2009, 21, 1248–1252. (3) Fan, H. Chem. Commun. 2008, 1383–1394. (4) Xing, B.; Li, W.; Dou, H.; Zhang, P.; Sun, K. J. Phys. Chem. C 2008, 112, 14318–14323. (5) Chu, M.; Shen, X.; Liu, G. Nanotechnology 2006, 17, 444–449. (6) Aldeek, F.; Balan, L.; Lambert, J.; Schneider, R. Nanotechnology 2008, 19, 475401–09. (7) Williams, J. V.; Adams, C. N.; Kotov, N. A.; Savage, P. E. Ind. Eng. Chem. Res. 2007, 46, 4358–4362. (8) Luan, W.; Yang, H.; Tu, S.; Wang, Z. Nanotechnology 2007, 18, 175603–06. (9) Roy, M. D.; Herzing, A. A.; De Paoli Lacerda, S. H.; Becker, M. Chem. Commun. 2008, 2106–2108. (10) Yamashita, I.; Hayashi, J.; Hara, M. Chem. Lett. 2004, 33, 1158–1159. (11) (a) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706–8715. (b) Peng, X.; Manna, L.; Yang, W.; Wickham, J.; Scher, E. C.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59–61. (c) Peng, A.; Peng, X. J. Am. Chem. Soc. 2002, 124, 3343–3353. (d) Talapin, D. V.; Haubold, S.; Rogach, A. L.; Kornowski, A.; Haase, M.; Weller, H. J. Phys. Chem. B 2001, 105, 2260–2263. (e) Zhong, X.; Han, M.; Dong, Z.; White, T.; Knoll, W. J. Am. Chem. Soc. 2003, 125, 8589–8594. (12) Zou, L.; Gu, Z.; Zhang, N.; Zhang, Y.; Fang, Z.; Zhu, W.; Zhong, X. J. Mater. Chem. 2008, 18, 2807–2815. (13) (a) Uyeda, H. T.; Medintz, I. L.; Jaiswal, J. K.; Simon, S. M.; Mattoussi, H. J. Am. Chem. Soc. 2005, 127, 3870–3878. (b) Petruska, M. A.; Malko, A. V.; Voyles, P. M.; Klimov, V. I. Adv. Mater. 2003, 15, 610–613.
Langmuir 2009, 25(21), 12729–12735
significant success has been achieved by an organometallic route in obtaining superior-quality nanoparticles, there are several limitations.12,13 These methods are not cost-effective because of expensive processing conditions (inert atmosphere and elevated temperature above 300 °C), and as prepared, QDs are hydrophobic and cannot be used directly for biological and other applications, where an aqueous dispersion of nanoparticles is essential.12 Though conversion from hydrophobic to hydrophilic is achieved by a phase-transfer process,14 this is at the cost of limited stability, the growth of particle size, and low photoluminescence efficiency.13 Parallel methods based on an aqueous route are also successfully developed using various molecules as capping ligands.15-19 The resulting colloids are more stable, water-soluble, and biocompatible but exhibit a low quantum yield (QY) of photoluminescence (PL) compared to the colloids obtained via organometallic synthesis.12 However, there is growing interest in using amine-chalcogen complexes as a better choice for preparing numerous QDs.20,21 In general, metal halide and elemental chalcogens are not reactive enough to form II-VI QDs by a solvothermal route. However, by the incorporation of an amine into a reaction mixture, nanoparticles can be obtained easily at moderate temperatures (below 140 °C) as a result of the high reactivity of the amine-chalcogen complex.20 Hyeon’s (14) (a) Nikolic, M. S.; Krack, M.; Aleksandrovic, V.; Kornowski, A.; Forster, S.; Weller, H. Angew. Chem., Int. Ed. 2006, 45, 6577–6580. (b) Yu, W. W.; Chang, E.; Falkner, J. C.; Zhang, J.; Al-Somali, A. M.; Sayes, C. M.; Johns, J.; Drezek, R.; Colvin, V. L. J. Am. Chem. Soc. 2007, 129, 2871–2879. (15) (a) Rajh, T.; Micic, O. I.; Nozik, A. J. J. Phys. Chem. 1993, 97, 11999–12003. (b) Murphy, C. J. Anal. Chem. 2002, 520A–526A. (16) Vossmeyer, T.; Katsikas, L.; Giersig, M.; Popvic, I. G.; Diesner, K.; Chemseddine, A.; Eychmuller, A.; Weller, H. J. Phys. Chem. 1994, 98, 7665–7673. (17) Gu, Z.; Zou, L.; Fang, Z.; Zhu, W.; Zhong, X. Nanotechnology 2008, 19, 135604. (18) Li, J.; Hong, X.; Li, D.; Zhao, K.; Wang, L.; Wang, H.; Du, Z.; Li, J.; Bai, Y.; Li, T. Chem. Commun. 2004, 1740–1741. (19) Liu, Y.; Chen, W.; Joly, A. G.; Wang, Y.; Pope, C.; Zhang, Y.; Bovin, J. -O.; Sherwood, P. J. Phys. Chem. B 2006, 110, 16992–17000. (20) Kwon, S. G.; Hyeon, T. Acc. Chem. Res. 2008, 41, 1696–1709. (21) (a) Joo, J.; Na, H. B.; Yu, T.; Yu, J. H.; Kim, Y. W.; Wu, F.; Zhang, J. Z.; Hyeon, T. J. Am. Chem. Soc. 2003, 125, 11100–11105. (b) Joo, J.; Son, J. S.; Kwon, S. G.; Yu, J. H.; Hyeon, T. J. Am. Chem. Soc. 2006, 128, 5632–5633.
Published on Web 08/27/2009
DOI: 10.1021/la901798y
12729
Article
Kalasad et al.
Table 1. Details of Synthesis Conditions in the Preparation of Differently Sized Nanoparticles and Numerical Values of the Critical Absorption Wavelength, Energy Gap, and Particle Size from Optical Absorption Measurements and Particle Size from XRD and TEM Measurementsa,b ion ratio, volume ratio, temperature and time of reaction molar ratio (mM) sample number (color)
Cd2+
Se2-
volume ratio (mL)
Cd2+
Se2-
optical absorption
temp (oC)
time (min)
critical absorption energy particle photolumiwavelength gap size nescence (λ) (nm) (eV) (nm) QY in %
sample 1 (white) 100 5 2 1 room temp --sample 2 (pale yellow) 100 50 4 1 room temp --sample 3 (yellow) 100 50 4 1 100 30 sample 4 (orange) 100 50 4 1 100 110 sample 5 (red) 100 50 1 2 100 90 a Se molarity is prepared in 99% hydrazine hydrate (N2H4H2O). b The dashes (---) for extraction of nanoparticles from the colloidal solution.
group successfully produced uniform nanocrystals of PbS, ZnS, CdS, and MnS using oleylamine21a and free-standing nanoribbons of CdSe using octylamine.21b The latter structure satisfies the 2D quantum confinement and hence exhibits an extremely narrow PL band, but their QY is only 1 to 2%. However, these amines are insoluble in water and hence preclude the aqueous synthesis of nanoparticles. Therefore, it is very desirable to develop low-cost aqueous routes to prepare fluorescent QDs of CdSe. The main focus of the present work is to synthesize small and more fluorescent QDs of CdSe under open-air conditions using a hydrazine hydrate-Se complex as the source of selenium.
2. Experimental Details All of the chemicals used in the present work were of analytical grade, and the reactions were carried out under open-air conditions using triply distilled water. Selenium (99.99%), cadmium chloride (CdCl2.21/2H2O), 3-mercaptopropionic acid (MPA) (C3H6O2S) were obtained from Sigma-Aldrich and hydrazine hydrate (N2H4H2O) (99%) and acetone (AR) were obtained from sd-Fine Chemicals, India. Aqueous solution of CdCl2 was prepared in distilled water to which required amount of MPA (0.24 M) was added under constant stirring, this resulted in a clear solution. In a separate flask, the required amount of Se powder was dissolved in hydrazine hydrate (99%) in open air. Instantly the colorless solution turns dark brown and remains unchanged under atmospheric conditions at least for 2 h. By changing the molar and volume ratios of Cd2+ and Se2- ions, temperature and time of reactions were changed to obtain different size nanoparticles, all these reactions were carried out in open air conditions and the pH of these solutions varied between 9 and 10. Sample 1: 100 mM CdCl2 and 5 mM hydrazine hydrate-Se in a volume ratio of 2:1 were mixed slowly at room temperature; this mixture gives a light yellow colloid. Sample 2: 100 mM CdCl2 and 50 mM hydrazine hydrate-Se in a volume ratio of 4:1 were mixed at room temperature; this mixtures gives a pale-yellow colloid. Sample 3: 100 mM CdCl2 and 50 mM hydrazine hydrate-Se in a volume ratio of 4:1 were mixed at room temperature and then held at 100 °C for 30 min to get a yellow colloid. Sample 4: Once again, 100 mM CdCl2 and 50 mM hydrazine hydrate-Se in a volume ratio of 4:1 were mixed at room temperature and held at 100 °C for 110 min to obtain an orange colloid. Sample 5: 100 mM CdCl2 and 50 mM hydrazine hydrate-Se in a volume ratio of 1:2 were mixed at room temperature and held at 100 °C for 90 min to obtain a dark-red colloid. The preparation conditions are summarized in Table 1. For each sample, after achieving the required size of quantum dots, the colloid was forcibly precipitated by adding acetone as an antisolvent. The precipitate was repeatedly washed with distilled water until its pH became normal (equal to 7). Finally, the powder was collected by drying under open-air conditions and stored in sealed bottles. 12730 DOI: 10.1021/la901798y
384 475 508 561 600 samples 1
particle size (nm)
XRD
TEM
3.23 1.58 35 1.40 2.00 2.61 2.07 40 1.85 2.25 2.44 2.26 43 2.17 2.65 2.21 2.82 39 2.71 4.40 2.06 3.42 12 3.06 4.80 and 2 in the time column indicates immediate
Optical absorption measurements were made using a Hitachi U-3310 spectrophotometer, and PL spectra were recorded using a Hitachi F-7000 fluorescence spectrophotometer. The FTIR measurements on powdered samples were made using a Nicolet 5700 FTIR spectrometer in transmission mode with a KBr window. These measurements were carried out in the spectral range of 400-4000 cm-1 with a linear scan velocity of 0.632 cm/s, and each spectrum is an average of 100 scans. Powder X-ray diffraction (XRD) measurements were carried out using a Philips X’Pert powder diffractometer with Cu KR1 radiation (λ = 1.54056 A˚) with a scan step of 0.016 deg/s. To record the pattern, nanoparticle powder was coated on a glass plate using silicon grease. Transmission electron micrographs (TEM), high-resolution transmission electron microscopy (HRTEM) images, and selected area electron diffraction (SAED) patterns were recorded using a JEOL JEM2000 FX II electron microscope at 300 kV. The colloidal solution of differently sized nanoparticles was coated onto a 200 mesh holey carbon copper grid that was dried completely under ambient conditions before the measurements.
3. Results and Discussion Hydrazine hydrate is a weak base having reductive and complexing properties. This amine instantaneously dissolves elemental Se to form a complex. We have studied the open air stability of this solution by recording the optical absorption spectra of fresh solution and solution aged for 2 h (spectra not shown). There is no noticeable change in the intensity and distribution of the two spectra, indicating that the solution is stable under atmospheric conditions for the above duration. However, this spectrum is considerably distinct as compared to the absorption spectrum reported for bulk Se;22 these features indicate that hydrazine hydrate reduces Se to form an amine-chalcogen complex. There are some reports wherein hydrazine hydrate is used as a reducing agent rather than a complexing agent to prepare metal selenide crystals, particularly nanorods and fractal nanocrystals.23,24 It has been well documented that semiconductor nanoparticles prepared by traditional aqueous routes show a low quantum efficiency of PL.12 Among various capping molecules that have been studied by the aqueous route, thioalkyl acids (generally thioglycolic acid (TGA) and MPA) show better quantum efficiencies with the proper selection of the cation/anion ratio.25,26 In (22) Tang, Z. K.; Loy, M. M. T. Appl. Phys. Lett. 1997, 70, 34–36. (23) (a) Peng, Q.; Dong, Y.; Deng, Z.; Li, Y. Inorg. Chem. 2002, 41, 5249–5254. (b) Xi, L.; Lam, Y. M.; Xu, Y. P.; Li, L. J. J. Colloid Interface Sci. 2008, 320, 491–500. (24) Liu, Y.; Cao, J.; Li, C.; Zeng, J.; Tang, K.; Qian, Y.; Zhang, W. J. Cryst. Growth 2004, 261, 508–513. (25) Li, L.; Qian, H.; Fang, N.; Ren, J. J. Lumin. 2005, 116, 59–66. (26) Rogach, A. L.; Franzl, T.; Klar, T. A.; Feldmann, J.; Gaponik, N.; Lesnyak, V.; Shavel, A.; Eychmuller, A.; Rakovich, Y. P.; Donegan, J. F. J. Phys. Chem. C 2007, 111, 14628–14637.
Langmuir 2009, 25(21), 12729–12735
Kalasad et al.
Article
Figure 2. Photoluminescence spectra of a crude colloid and a colloid of extracted particles of sample 3. The curves clearly show that there is a high quenching of photoluminescence in the crude solution.
Figure 1. (a) Temporal evolution of CdSe quantum particles of sample 3, optical absorption curves of the crude colloid as a function of time. (b) Variation of estimated optical absorption edge and particle size (using eq 2) as a function of time.
the case of CdTe nanocrystals capped with TGA molecules, the PL intensity becomes unstable under room lighting condition, which is due to the degradation of capping molecules.27 In recent reports, it is proposed that MPA can be a superior capping molecule for producing stable colloids of nanoparticles having a wide tunability of particle size.28 To develop the most viable synthesis route to prepare CdSe nanoparticles in the open air, we selected the hydrazine hydrate-Se complex as the source of selenium and MPA as the capping molecules. The expected chemical reaction to form CdSe nanocrystals with cadmium salt and the hydrazine hydrate-Se complex can be given as CdCl2 þ ðNH2 NH2 Þ2 þ þ Se2 - þ H2 O f CdSeðVÞ þ N2 ðvÞ þ 2HCl þ H2 ðvÞ þ H2 O
ð1Þ
The hydrazine hydrate-Se complex releases highly reactive Se2ions that further react with Cd2+ ions in solution to form CdSe nanoparticles capped with MPA molecules. The temperature and residence time of nanoparticles in the crude solution are important parameters in tuning the size of nanoparticles.8 In our case, we have studied the effect of these parameters on the present colloids. As mentioned in the Experimental Details, by controlling the annealing time at a given temperature (100 °C) we could tune the particle size more systematically. Figure 1a shows the optical absorption of a crude solution belonging to sample 3 as a function of time at room temperature. With time, there is a gradual increase in the (27) (a) Bao, H.; Gong, Y.; Li, Z.; Gao, M. Y. Chem. Mater. 2004, 16, 3853– 3859. (b) Gaponik, N.; Talapin, D. V.; Rogach, A. L.; Hoppe, K.; Shevchenko, E. V.; Komowski, A.; Eychmuller, A.; Weller, H. J. Phys. Chem. B 2002, 106, 7177–7185. (28) (a) Zhang, H.; Wang, L.; Xiong, H.; Hu, L.; Yang, B.; Li, W. Adv. Mater. 2003, 15, 1712–1715. (b) Guo, J.; Yang, W.; Wang, C. J. Phys. Chem. B 2005, 109, 17467–17473. (c) Li, L.; Qian, H.; Ren, J. Chem. Commun. 2005, 528–530. (d) He, Y.; Lu, H.; Sai, L.; Lai, W.; Fan, Q.; Wang, L.; Huang, W. J. Phys. Chem. B 2006, 110, 13352–13356. (e) He, Y.; Lu, H.; Sai, L.; Lai, W.; Fan, Q.; Wang, L.; Huang, W. J. Phys. Chem. B 2006, 110, 13370–13374.
Langmuir 2009, 25(21), 12729–12735
absorption edge toward lower energy. The particle size is estimated using a simple equation proposed for spherically shaped particle29 (eq 2) ΔEg ¼
h2 1 1 þ 8a2 me mh
ð2Þ
where ΔEg = (Eng - Ebg ) is the difference between the band gaps of nanoparticles (Eng ) and the bulk (Ebg ) (CdSe Ebg = 1.74 eV), h is Planck’s constant, a is the diameter of a particle, and me and mh are the effective masses of electrons and holes, respectively. The variation of the optical absorption edge with time is plotted in Figure 1b, and the estimated particle size (using eq 2) is plotted in the same Figure. It is observed that the particle size increases from 2.20 to 2.47 nm over a period of 191 h. The temporal evolution of CdSe nanoparticles capped with different organic molecules has been studied by a number of authors.8,30 In most of these cases, the reported work is carried out at higher temperature, ranging from 200 to 300 °C. At these temperatures, there is fast growth in the size of the nanoparticles. In the present case, the growth at room temperature is quite slow, only about 13%. This clearly indicates that the growth kinetics in our case is slow; this is due to the fact that MPA acts as an effective capping agent in producing stable colloids, which is an important aspect of the processing of nanoparticles. Nanoparticles of different sizes were isolated by arrest precipitation and repeated washing of the crude solution to conduct further characterizations. Figure 2 shows the PL spectra of sample 3 that were recorded for the crude solution and isolated particles redispersed in water. The isolated particles exhibit high PL compared to the crude solution. There is an almost 2 orders of magnitude increase in the emission intensity, indicating that the surrounding liquid medium has a significant effect on the emission properties. In the literature, there are reports on the effect of (29) Hambrock, J.; Birkner, A.; Fischer, R. A. J. Mater. Chem. 2001, 11, 3197– 3201. (30) (a) Peng, Z. A.; Peng, X. J. Am. Chem. Soc. 2001, 123, 183–184. (b) Mekis, I.; Talapin, D. V.; Kornowski, A.; Haase, M.; Weller, H. J. Phys. Chem. B 2003, 107, 7454–7462. (c) Al-Salim, N.; Young, A. G.; Tilley, R. D.; James McQuillan, A.; Xia, J. Chem. Mater. 2007, 19, 5185–5193.
DOI: 10.1021/la901798y
12731
Article
Kalasad et al.
Figure 4. Optical absorption curves of extracted quantum dots of differently sized CdSe nanoparticles (samples 1-5) redispersed in water.
Figure 3. Colloidal solutions of extracted CdSe nanoparticles (left to right) consisting of samples 1-5 redispersed in water under normal light and under UV light (352 nm tube of 6 W and 8 in.).
amines on the fluorescence properties of quantum dots, but the phenomenon was poorly understood.31 Recently, Scaiano et al. carried out systematic work on the fluorescence quenching of CdSe quantum dot by tertiary amines.32 The fluorescence yield of these particles decreased with increased percentage of amines, and it followed Stern-Volmer behavior without affecting the fluorescence lifetime. It is believed that PL quenching is due to nonradiative recombination of photoexcited holes in the valence band of QDs and electrons of amine molecules that are present in the surrounding. Because we use amines as media in the synthesis, this could lead to a decrease in the PL efficiency of QDs in crude solution. The curve observed for isolated particles shows the overlap of two emission curves, almost of similar intensities, one at 510 nm and another at 535 nm indicating that both excitonic and shallow traps are responsible for the emission process. The dotted curves in the Figure show the Gaussian fit, and from these curves the fwhm is estimated to be 53 nm for excitonic emission and 74 nm for trap emission. The present value of fwhm = 53 nm (for the excitonic curve) is a reasonably good number and is close to the values that are reported in the case of superior nanoparticles (31) (a) Landes, C.; El-Sayed, M. A. J. Phys. Chem. A 2002, 106, 7621–7627. (b) Liang, J. G.; Zhang, S. S.; Ai, X. P.; Ji, X. H.; He, Z. K. Spectrochim. Acta, Part A 2005, 61, 2974–2978. (c) Maurel, V.; Laferriere, M.; Billone, P.; Godin, R.; Scaiano, J. C. J. Phys. Chem. B 2006, 110, 16353–16358. (32) Galian, R. E.; Scaiano, J. C. Photochem. Photobiol. Sci. 2009, 8, 70–74. (33) Wu, D.; Kordesch, M. E.; Van Patten, P. G. Chem. Mater. 2005, 17, 6436– 6441.
12732 DOI: 10.1021/la901798y
prepared by nonaqueous routes.33 It is interesting that both pure excitonic and mixed (excitonic and trap) emission are routinely observed in the PL spectra of QDs.6,34 There is a strong current scientific priority to use QDs to develop chemical/biological sensors, and this is purely based on the effect induced by surface-bonded chemical/biological analytes on the PL properties. Therefore, the emission associated with traps could be an important parameter in the quantitative assessment of these analytes. Further work in this direction is necessary to produce nanoparticles whose excitonic and trap emissions are well resolved. Figure 3 shows the solutions of isolated CdSe nanoparticles of different sizes redispersed in water under normal light and UV light (352 nm tube of 6 W power and 8 in length). The solution of sample 1 is light yellow in color. The present picture clearly shows that our protocol of synthesis allows the complete tuning of the size of nanoparticles; as a result, there is emission over the entire region of the visible spectrum. These colloids are very stable with time, and there was no visible precipitation even up to 6 months. Figure 4 is a plot of optical absorption curves of differently sized quantum dots redispersed in water. The critical absorption edge, a tangent drawn at the onset of absorption, ranges from 384 nm for sample 1 to 600 nm for sample 5. There is a reasonable shift of these curves to the higher-energy side as compared to the bulk absorption edge of CdSe (713 nm and Ebg = 1.74 eV). There is more than a 300 nm shift in the optical absorption edge. Using these values, the particle size is estimated for each absorption curve using eq 2, and the numerical values are given in Table 1; this size ranges from 1.58 to 3.42 nm. In the past, many reports have been devoted to prepare small, monodisperse CdSe QDs having high PLs. In order to achieve these goals, a variety of chemical methods have been developed.5-9 In most of these cases, the lowest absorption edge begins from 450 nm (the corresponding particle size would be 1.92 nm). As mentioned above, TOP/ TOPO-capped CdSe nanoparticles exhibit superior properties. In (34) (a) Sharma, H.; Sharma, S. N.; Kumar, U.; Singh, V. N.; Mehta, B. R.; Singh, G.; Shivaprasad, S. M.; Kakkar, R. J Mater Sci: Mater Med 2008 (b) Xing, R.; Wang, X.; Yan, L.; Zhang, C.; Yang, Z.; Wang, X.; Guo, Z. Dalton Trans. 2009, 1710–1713. (c) Zhang, S.; Yu, J.; Li, X.; Tian, W. Nanotechnology 2004, 15, 1108– 1112.
Langmuir 2009, 25(21), 12729–12735
Kalasad et al.
Figure 5. Photoluminescence spectra of extracted quantum dots redispersed in water for samples 1-5. Symbol S stands for sample.
recent work, Patten’s group tried to substitute a new class of capping molecules in place of TOPO because of the unknown chemical severity of these molecules.33 It was observed that hexadecyl hexadecanoate and benzophenone molecules appear to be excellent replacements for TOPO but the smallest particle size that was achieved with optimum synthetic parameters was 2.6 nm. However, these particles exhibit superior emission. Only in a few reports a small particle size is achieved. The capping of various thiols on CdSe quantum dots has been studied under identical conditions of preparation, with particle size in the range of 1.4 to 2.2 nm observed for thioalcohols and 2.1 to 3.2 nm observed for TGA.35 It is important to note that these values are estimated using the first excitonic peak in the optical absorption curve, which always occurs at higher energy than the optical absorption edge. Hence, this estimation of size is always smaller than the size taken from the optical absorption edge. The present synthesis gives a very small particle size (1.58 nm estimated from the optical absorption edge) that can be procured in the form of powder and stored for a longer time without the agglomeration of particles. In Figure 5, the normalized PL spectra of various nanoparticles are given. The PL QY of different particles is estimated using rhodamine B as a standard by dispersing both nanoparticles and dye molecules in ethyl alcohol using the procedure given in ref 36 (QY of the dye is taken as 89%37). The estimated QY for different nanoparticles is given in Table 1, and a high value of around 40% is observed. Only in the case of sample 5 is a low QY observed, but its emission is sharp (fwhm = 16 nm) compared to the emission of other samples. In this case, emission is purely excitonic, and the surface states make a negligible contribution. This could be due to the fact that as particles grow the surface states become less important in the emission process. Generally, a small QY (0.3 to 16%) is observed for CdSe QDs synthesized by the aqueous route.7,38 In a recent report involving the conversion of hydrophobic to hydrophilic species using a gemini surfactant as the capping agent, (35) Rogach, A. L.; Kornowski, A.; Gao, M.; Eychmuller, A.; Weller, H. J. Phys. Chem. B 1999, 103, 3065–3069. (36) Kloepfer, J. A.; Cohen, N.; Nadeau, J. L. J. Phys. Chem. B 2004, 108, 17042–17049. (37) Li, H.; Wang, X.; Gao, Z.; He, Z. Nanotechnology 2007, 18, 205603–06. (38) Sondi, I.; Siiman, O.; Matijevic, E. J. Colloid Interface Sci. 2004, 275, 503– 507.
Langmuir 2009, 25(21), 12729–12735
Article
a high QY close to 40% is observed. These are core-shell CdSe/ ZnS quantum dots that are expected to show superior PL.37 Generally, it is observed that the binding of thiols, OH-, and certain other ions on the surface of nanoparticles enhance the PL.15 It is known that thiols effectively passivate the strong nonradiative surface recombination states of nanoparticles, which leads to an increase in PL efficiency. In the present case, the short-chain linear capping molecules are expected to passivate the CdSe surface states more effectively to give a high QY. The photoluminescence stability of sample 3 was studied by dispersing the nanoparticles in ethyl alcohol, and the colloid was aged under room light. The PL spectra recorded on fresh and aged (8 days) solutions are given as Supporting Information. It is observed that the initial QY of fresh solution decreases slightly from 43% to 40% without affecting the nature of the spectrum. The effect of organic ligands on the emission properties of nanoparticles is an important aspect, which has been studied by a number of workers.39 Parameters such as aging, the temperature, and the pH of the colloid can affect the emission intensity; with aging, both increases40 and decreases41 in PL intensity have been observed and attributed to various reasons. In the present case, the slight decrease in QY could be related to the slow photodegradation of capping molecules in the colloidal state. However, further study is required to ascertain the effect and to draw final conclusions. Powders of isolated quantum dots were used to record FTIR spectra to confirm the surface binding of capping molecules. Spectra of different samples are shown in Figure 6. A broad absorption peak at around 3430 cm-1 can be assigned to the O-H vibration, and strong bands at 2923 and 2855 cm-1 are due to C-H stretching vibrations of the alkyl chains of MPA molecules in all spectra. The absence of the S-H stretching mode around 2560 cm-1 in these spectra clearly indicates that thiol group of MPA are bound to surface atoms of quantum dots through the Cd-S bond. The peak at 3278 cm-1 in the higher particle size (sample 5) is due to the O-H vibrations of carboxylic acid. The band at 1654 cm-1 is due to the vibrational mode of free carboxylic acid. The sharp peak at 1552 cm-1 can be assigned to the vibration of the carboxylate anion of MPA molecules. The band at 1406 cm-1 is due to the symmetric stretching vibration of C-O. The vibrational mode of carboxylic acid becomes more prominent with larger particles (sample 5) and is even comparable to the mode associated with the carboxylate anion (COO-). It is interesting that all of the particles show a sharp, prominent peak due to carboxylate anions, and this imparts a net negative charge on the outer surface of nanoparticles, which essentially helps nanoparticles not to coagulate and agglomerate. The existence of the carboxylate anion peak in CdSe capped with thioalkyl acids has been clearly observed by other groups.42 X-ray powder patterns were recorded on different powders of nanoparticles, and the patterns are shown in Figure 7. The three broad, distinct peaks at 2θ = 25, 43, and 50°, which are (39) (a) Liu, L.; Peng, Q.; Li, Y. Inorg. Chem. 2008, 47, 3182–3187. (b) Kalyuzhny, G.; Murray, R. W. J. Phys. Chem. B 2005, 109, 7012–7021. (c) Jeong, S.; Achermann, M.; Nanda, J.; Ivanov, S.; Klimov, V. I.; Hollingsworth, J. A. J. Am. Chem. Soc. 2005, 127, 10126–1027. (40) (a) Zhang, H.; Wang, D.; Hartmann, J.; Mohwald, H. J. Phys. Chem. C 2007, 111, 9678–9683. (b) Torimoto, T.; Murakami, S.; Sakuraoka, M.; Iwasaki, K.; Okazaki, K; Shibayama, T.; Ohtani, B. J. Phys. Chem. B 2006, 110, 13314–13318. (c) Wang, Y.; Tang, Z.; Correa-Duarte, M. A.; Pastoriza-Santos, I.; Giersig, M.; Kotov, N. A.; Liz-Marzan, L. M. J. Phys. Chem. B 2004, 108, 15461–15469. (41) (a) Li, H.; Shih, W. Y.; Shih, W. Ind. Ing. Chem. Res. 2007, 46, 2013–2019. (b) Bullen, C.; Mulvaney, P. Langmuir 2006, 22, 3007–3013. (42) (a) Iwasaki, K.; Torimoto, T.; Shibayama, T.; Takahashi, H.; Ohtani, B. J. Phys. Chem. B 2004, 108, 11946–11952. (b) Hao, E.; Lian, T. Langmuir 2000, 16, 7879–7881. (c) Jiang, H.; Ju, H. Anal. Chem. 2007, 79, 6690–6696.
DOI: 10.1021/la901798y
12733
Article
Kalasad et al.
Figure 6. FTIR spectra of different CdSe quantum dots (samples 1-5) loaded into KBr pellets. The symbol S stands for the sample.
Figure 8. TEM and HRTEM pictures of CdSe nanoparticles (sample 3). The inset shows its selected area electron diffraction (SAED) picture.
Figure 7. X-ray powder patterns of differently sized CdSe nanoparticles, where the sample number and estimated particle size using the Debye-Scherrer formula are given.
prominent features in larger particles, correspond to (111), (220), and (311) Miller indices of cubic zinc blende type structure.8,43 The clear absence of reflections at 2θ = 35 and 46° due to (102) and (103) is an indication that the wurtzite CdSe structure is absent and the nanoparticles possess purely zinc blende structure. In certain cases wherein the growth temperature is higher, the thermodynamically stable wurtzite phase is observed.44 As the particle size decreases from sample 5 to sample 1, the width of the (111) diffraction peak is broadened considerably. The particle size in each case is estimated using the Debye-Scherrer’s formula (d = 0.89λ/β cos θ, where d is the particle diameter, λ is the X-ray wavelength, β is the fwhm, and θ is the scattering angle). The estimated particle size for different quantum dots is given in the (43) Iancu, N.; Sharma, R.; Seo, D. Chem. Commun. 2004, 2298–2299. (44) Talapin, D. V.; Shevchenko, E. V.; Kornowski, A.; Gaponik, N.; Haase, M.; Rogach, A. L.; Weller, H. Adv. Mater. 2001, 13, 1868–1871.
12734 DOI: 10.1021/la901798y
Figure and also in Table 1. These values are quite close to the sizes that are estimated using optical absorption edge measurements on their colloidal solutions. TEM measurements were carried out on various colloids to study the size and morphology of CdSe nanoparticles. Figure 8a gives such picture for sample 3 as a representative of other samples. In all cases, the particles are spherical in shape and there is good homogeneity in the particle size distribution. Figure 8b is a typical HRTEM of this sample. The existence of clear lattice planes in the picture confirm that the nanoparticles have good crystallinity, which is further supported by the selected area electron diffraction (SAED) pattern (given in the same Figure as an inset). The average size of particles is estimated and is given in Table 1 for different powders. It varies from 2.00 nm for sample 1 to around 4.80 nm for sample 5. It is obvious that the particle size estimated from HRTEM is higher than those estimated from other measurements such as optical absorption edge and X-ray powder diffraction. Such a difference is commonly observed in the case of nanoparticles.35 It is important to remember that the Debye-Scherrer formula estimates the size of single-crystalline domains (having periodic lattice) but not the actual particle size and hence it is expected that the particle size estimated from Langmuir 2009, 25(21), 12729–12735
Kalasad et al.
Article
the X-ray pattern is always smaller than the size estimated from TEM measurements.
synthesis protocol is simple and cost-effective in the preparation of high-quality CdSe nanoparticles.
4. Conclusions
Acknowledgment. We acknowledge the Department of Science and Technology, Government of India, for financial support. We thank Mrs. Sumana, Department of Physics, National Instruments Facilities, Indian Institute of Science, Bangalore, for recoding the X-ray patterns. M.N.K. is grateful to the SDM College of Engineering and Technology, Dharwad, Karnataka State, India, for their encouragement in carrying out this work.
In summary, stable, fluorescent colloids of CdSe QDs capped with 3-mercaptopropionic acid are prepared by an aqueous route under ambient conditions. A new precursor of selenium ions, that is, a hydrazine hydrate-Se complex, is proposed. The method is simple and the most viable solution route to prepare CdSe quantum dots. The size of nanoparticles is conveniently tuned by controlling the concentration of ions, volume of solutions, annealing temperature, and time of reaction. Colloids of different size are characterized by various experimental techniques such as optical absorption, photoluminescence, FTIR, X-ray powder diffraction and TEM measurements. It is found that these QDs are homogeneous, spherical, and small. Thus, the present
Langmuir 2009, 25(21), 12729–12735
Supporting Information Available: Time-dependent photoluminescence spectra of a colloidal solution of sample 3 under room light conditions. This material is available free of charge via the Internet at http://pubs.acs.org.
DOI: 10.1021/la901798y
12735
