3-Mercaptobutyric Acid as an Effective Capping Agent for Highly

Article pubs.acs.org/JPCC

3-Mercaptobutyric Acid as an Effective Capping Agent for Highly Luminescent CdTe Quantum Dots: New Insight into the Selection of Mercapto Acids Tan Fang,†,§ Kaiguo Ma,†,§ Lili Ma,†,‡ Jinyi Bai,† Xiang Li,† Huihua Song,‡ and Haiqing Guo*,† †

Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China ‡ Department of Chemistry and Material Sciences, Hebei Normal University, Shijiazhuang 050016, P. R. China S Supporting Information *

ABSTRACT: CdTe quantum dots (QDs) were synthesized by a modified hydrothermal method with Na2TeO3 as the Te source. Thioglycolic acid (TGA), thiolactic acid (TLA), 3mercaptopropionic acid (MPA), 3-mercaptobutyric acid (3MBA), 4-mercaptobutyric acid (MBA), and 5-mercaptovaleric acid (MVA) were used. Their effects on the growth and fluorescence of as-synthesized QDs were investigated: Detrimental precursor aggregation when using TGA-like molecules (TGA and L-cysteine) due to their special secondary coordination was observed and further confirmed by comparing with DL-homocysteine and N-acetyl-L-cysteine. The aggregation could be suppressed by using a bulky methyl group as a side chain to confine the carboxyl group, that is, TLA. Moreover, MPA was confirmed as a better stabilizer compared with linear TGA, MBA, and MVA probably because of its suitable balance between hydrophilicity and hydrophobicity. Accordingly, we designed and synthesized 3MBA that has the same main chain length as MPA but with a side methyl group as TLA. 3MBA-modified CdTe QDs exhibited an excellent optical property with a quantum yield of 71%, much higher than that of MPA and TGA. Preliminary results of CdSe QDs confirmed the versatility of 3MBA. Our results uncovered the possible origin of the advantages of mercapto acids with a methyl side chain and a suitable chain length.

1. INTRODUCTION Exhibiting unique optical properties, quantum dots (QDs) such as CdTe QDs have been extensively investigated.1−7 To synthesize water-soluble QDs, direct synthesis in aqueous solution is simple. The use of mercapto acids as capping agents is a key point to obtain highly fluorescent QDs.7 Up to now, various mercapto acids have been used as capping agents and listed elsewhere.6 Among these mercapto acids, those with a single mercapto group and a single carboxyl group connected by an alkyl chain are widely used. Thioglycolic acid (TGA) and 3-mercaptopropionic acid (MPA) are the most commonly used capping agents.1,4−7 Due to different chain lengths, the secondary coordination of TGA is different from that of MPA. TGA allowed the formation of one-dimensional CdTe nanostructures from quasi-spherical CdTe nanocrystals at proper conditions, whereas MPA did not.8−10 The faster growth of TGA-modified QDs than that of MPA-modified QDs in the presence of hydrazine was observed and attributed to the unique secondary coordination of TGA.11 When mercapto acid with different chain lengths, such as MPA, 6-mercaptohexanoic acid (MHA), and 11-mercaptoundecanoic acid (MUA), were used as capping agents to synthesize CdTe QDs, the cappingagent-dependent growth of QDs was observed.12 The strong © 2012 American Chemical Society

hydrophobicity of MHA and MUA was adverse to the synthesis of highly fluorescent QDs. Apart from linear mercapto acids, researchers also tried to use branched mercapto acids as capping agents. The superiority in fluorescence of CdS QDs capped with thiolactic acid (TLA) to that capped with MPA was observed.13 However, the reason of the superiority was not revealed clearly, and the potential function of the side chain was still uncovered. Herein, we report on the effect of a series of mercapto acids on the aqueous growth of CdTe QDs in consideration of the factors of secondary coordination of carboxyl group, steric hindrance of the methyl side chain, and chain length to balance hydrophilicity and hydrophobicity. It was found that both the suitable chain length and the side methyl group of mercapto acids were important to control the growth and to improve the optical property of QDs. Accordingly, we designed and synthesized 3-mercaptobutyric acid (3MBA) that has the same main chain length with MPA but with a side methyl group as TLA. As a result, 3MBA-modified CdTe QDs Received: March 24, 2012 Revised: May 15, 2012 Published: May 15, 2012 12346

dx.doi.org/10.1021/jp302820u | J. Phys. Chem. C 2012, 116, 12346−12352

The Journal of Physical Chemistry C

Article

Scheme 1. Mercapto Acids Used in This Study

Transmission electron microscopy (TEM) images were recorded on a JEOL-2100F electron microscope operated at 200 KV. Samples used for X-ray diffraction (XRD) measurement were further purified by dialysis (MWCO 3500). XRD was recorded with Rigaku DMAX-2400 diffractometer. NMR spectra of organic molecules were tested by Bruker-400 MHz NMR. The element analysis was measured with a Vario EL elemental analyzer.

exhibited excellent optical properties with the highest quantum yield (QY) up to 65%, which was much higher than the cases of MPA and TLA.

2. EXPERIMENTAL SECTION 2.1. Materials. Thioglycolic acid (TGA) was purchased from Beijing Chemical Works. Thiolactic acid (TLA) and 3mercaptopropionic acid (MPA) were purchased from Acros. LCysteine (Cys) was purchased from Sinopharm Chemical Reagent Co., Ltd. DL-Homocysteine (HCys) was purchased from Sigma-Aldrich. N-Acetyl-L-cysteine (ACys) was purchased from J&K Chemical. Other chemicals were all of analytical reagent grade or the highest grade which could be purchased. Deionized water was used as the solvent for aqueous solutions. All chemicals were used without further purification. 2.2. Synthesis of Mercapto Acid-Capped CdTe NCs. CdTe QDs were synthesized by the simple method reported by Dong and co-workers with some modifications.35,36 In a typical process, the precursor solution was obtained by sequential addition of CdCl2, Na2TeO3, mercapto acid, and NaBH4 into a buffer (pH = 8.0, adjusted with NaOH) containing borax and citric acid with a mild magnetic stirring at room temperature and an ambient atmosphere. The final volume of the precursor solution was 300 mL. The concentrations of chemicals in the precursor solution were [CdCl2] = 1 mM, [borax] = 15 mM, and [citric acid] = 15 mM, respectively. The molar ratio of Cd/ Te/mercapto acid was 4:1:12. After another 30 min of stirring, 15 mL of the precursor solution was transferred into a 25-mL Teflon-lined stainless steel autoclave and incubated in an oven at 135 °C for a desired period. Afterward, the autoclave was cooled rapidly with an ice bath. Resulting samples were kept at room temperature in the dark. The Supporting Information is available for the synthesis of CdSe QDs and various mercapto acids. 2.3. Characterizations of Mercapto-Capped CdTe NCs. Samples for absorption and fluorescence measurement were carried out immediately after they were prepared by the hydrothermal method. Ultraviolet−visible (UV−vis) absorption spectra were obtained using a Shimadzu 2550 UV−vis spectrophotometer. The fluorescence was measured with Hitachi F4500 spectrofluorimeter. Samples were excited at 450 nm with fluorescein (pure, J&K, QY = 79%) in 0.1 M NaOH as the fluorescent standard reference. The fluorescein was cross-calibrated at 480 nm using Rhodamine 6G (pure, J&K, QY = 95%) in absolute ethanol to ensure that the experimental quantum yields matched their literature counterparts within ±5%. Please see S7 of the Supporting Information for other details for the measurement of QY of CdTe QDs.

3. RESULTS AND DISCUSSION The structure of mercapto acids used in this study is shown in Scheme 1. Their effects on the aqueous synthesis of CdTe QDs including the secondary coordination of a carboxyl group, steric hindrance of methyl side chain, and the balanced chain length between hydrophilicity and hydrophobicity were investigated, respectively. 3.1. Precursor Aggregation of TGA-like Molecules. TGA is one of the earliest and most widely used capping agents for aqueous synthesis of CdTe QDs, and many successful examples were reported.14−17 So, our experiment began with TGA. However, when TGA was used as a capping agent, the color of the precursor solution turned brown after adding NaBH4. Obvious absorption tailing of the precursor solution was observed (Figure 1), indicating the agglomeration of

Figure 1. Absorption spectra of precursor solution.

precursors (Figure S1, Supporting Information) similar to the reported phenomenon.18,19 Further heating the precursor solution led to TGA-modified QDs with poor fluorescence (Table 1). In comparison, when MPA was used as the capping agent, QDs with moderate fluorescence could be obtained by further heating (Table 1). Zhang and co-workers reported that 12347

dx.doi.org/10.1021/jp302820u | J. Phys. Chem. C 2012, 116, 12346−12352

The Journal of Physical Chemistry C

Article

not cause prominent secondary coordination between amide group and cadmium.21 As a result, ACys could be regarded as a MPA-like molecule, and the aggregation should not be observed. Indeed, colorless ACys-modified precursor solution was obtained after adding NaBH4 (Figure 1). From these results, aggregation of precursors when TGA-like molecules were used as capping agents was caused by the special secondary coordination of TGA-like molecules, leading to poor fluorescent QDs. 3.2. Confinement Effect of the Side Chain. The aforementioned the commonly used TGA was not good for obtaining fluorescent QDs under our experiment condition probably because the special secondary coordination of its carboxyl group could induce the aggregation of precursors. However, if the flexible carboxyl group of TGA was confined, the agglomeration of precursor by secondary coordination might be suppressed. The two hydrogen atoms at α-carbon position of TGA were unlikely to have remarkable confinement effect. When one of the two hydrogen atoms was substituted by a bulky methyl group, the confinement might be remarkable. Consequently, TLA, the TGA-like molecule with a bulky methyl group, was adopted as a capping agent. When using TLA as a capping agent, an obvious improvement was observed. The pale brown color of precursor solution after adding NaBH4 indicated less serious aggregation (Figure 1). Further heating led to QDs with remarkably better fluorescence (Table 1). From these results, the confinement effect of the bulky methyl group was prominent. It should be noted that the maximum of QY (QYmax) of TLA-modified QDs was even slightly higher than that of MPAmodified QDs (Table 1). This might be associated with better, at least not worse, passivated surface of QDs by TLA than that by MPA. Better fluorescence of CdS QDs capped with TLA than that with MPA was also observed.13 3.3. Effect of Chain Length. Next we used several linear mercapto acids to investigate the effect of chain length. TGAmodified QDs, as mentioned above, showed poor fluorescence. Apart from TGA, MPA is another most widely used capping agents for CdTe QDs.22,23 An attempt to synthesize QDs with linear mercapto acids with very long carbon chains such as MHA and MUA were reported but were not successful.12 In our experiment, these mercapto acids had very poor solubility and were not adopted hereafter. 4-Mercaptobutyric acid (MBA) and 5-mercaptovaleric acid (MVA), which is a linear mercapto acid with a moderate length of carbon chain, were rarely adopted as capping agents in aqueous synthesis of CdTe QDs. The effect of chain length on the aqueous synthesis of QDs was still unclear. Consequently, we focused on MPA, MBA, and MVA to study the effect of chain length in this section. For the system using MPA as a stabilizer (Figure 2A), the absorption peak near 325 nm completely disappeared, and a new absorption peak near 430 nm emerged in the initial 30 min (Figure 2A). The absorption peak near 430 nm shifted to longer wavelengths and broadened in the further heating. For system using MBA and MVA as the stabilizer, similar phenomena were also observed (Figure 2B). From Figure 3, QDs capped with mercapto acids with a shorter chain length showed faster growth rate. MPA-modified QDs grew fastest, whereas MVA-modified QDs most slowly. However, in the initial stage (less than 2 h), the size of QDs capped with MBA was relatively larger than that with MPA (Figure 3). Nonetheless, the size of QDs capped with MVA was not larger

Table 1. Maximum Quantum Yield (QYmax) of QDs Capped with Various Mercapto Acids at [Cd2+] = 1 mM, Cd: Mercapto Acid: Te = 1: 3: 0.25, pH = 8.0, 135 °Ca capping agent

TGA

TLA

MPA

MBA

MVA

3MBA

QYmax/%

0.2

14

12

5

5

59

Fluorescein in 0.1 M NaOH (QY = 79%) was used as a fluorescent reference. The fluorescein was cross-calibrated at 480 nm using Rhodamine 6G (pure, J&K, QY = 95%) in absolute ethanol to ensure that the experimental quantum yields matched their literature counterparts within ±5%. a

the poor quality of nuclei caused by aggregation was responsible for poor fluorescence of as-synthesized QDs.19 Meanwhile, a remarkable different ability between TGA and MPA was observed in the synthesis of one-dimensional nanostructures.8−10 This difference was proposed as the different secondary coordination of TGA and MPA. Generally, the secondary coordination between the carboxyl oxygen of mercapto acids and the cadmium was common, and it could provide better surface passivation, leading to improved fluorescence. However, the secondary coordination of TGAlike molecules was special: the carboxyl oxygen of a TGA-like molecule could also absorb cadmium in the surroundings.8,20 Accordingly, we speculated that the aggregation of precursor was caused by the special secondary coordination of TGA, further leading to poor fluorescent QDs. In other words, the carboxyl group of a TGA molecule on the surface of one small CdTe nanocrystal in the precursor solution might coordinate with cadmium on the surface of another to induce the aggregation of precursors. To prove our speculation, L-cysteine (Cys) was adopted because it was a TGA-like molecule due to the secondary coordination ability of the amine group although it appeared to be a MPA-like molecule.8,21 Besides, a higher nucleophilicity of the amine group should lead to stronger secondary coordination,21 and more serious aggregation of Cys-modified precursors should be expected. The brownish black color of the precursors after adding NaBH4 and more serious absorption tailing indicated more serious aggregation (Figure 1). Moreover, further heating the precursor solution did not lead to QDs with detectable fluorescence. It should be noted that a partial of amine group of Cys should be protonated at pH = 8 and the aggregation of Cysmodified precursors might be associated with interparticle electrostatic attraction. So we also used DL-homocysteine (HCys) for comparison. Due to the secondary coordination of the amine group, HCys should be a MPA-like molecule.21 Because the pKa value between Cys and HCys was not significant (Table 2), aggregation of HCys-modified precursors Table 2. pKa Values of Cys and HCys37 mercapto acid

−COOH

−SH

−NH3+

Cys HCys

1.92 2.22

8.35 8.87

10.46 10.86

should be observed if the aggregation of precursors was only induced by interparticle electrostatic attraction. However, the colorless precursor solution and sharp absorption peak indicated no obvious aggregation (Figure 1). We also used N-acetyl-L-cysteine (ACys) as capping agent. The weak nucleophilicity of the amide group of ACys should 12348

dx.doi.org/10.1021/jp302820u | J. Phys. Chem. C 2012, 116, 12346−12352

The Journal of Physical Chemistry C

Article

Figure 2. Temporal evolution (min) of the absorption spectra of CdTe quantum dots (QDs) capped with MPA (a), MBA (b), and MVA (c).

Figure 3. Diameter of synthesized CdTe QDs capped with MPA, MBA, and MVA versus reaction time. The diameter of synthesized QDs was calculated according to the relationship between the position of absorption peak and the size reported elsewhere.4

Figure 4. Temporal evolution of the fluorescent spectra of CdTe QDs capped with MPA (a), MBA (b), and MVA (c).

12349

dx.doi.org/10.1021/jp302820u | J. Phys. Chem. C 2012, 116, 12346−12352

The Journal of Physical Chemistry C

Article

Scheme 2. Synthesis of 3MBA

Figure 5. Powder XRD pattern (a) and high-resolution TEM image (b) of 3MBA-modified CdTe QDs.

was needed.29−31 Here, monomer was referred to as a series of complexes formed among cadmium, telluride, and mercapto acids.9 As our experiment was carried out under similar conditions, the OR rate might be mainly governed by the ligand layer. The slower OR rate of QDs capped with mercapto acid with longer chain lengths might be associated with the more compact ligand layer. Mercapto acids with longer chain lengths were packed more densely due to their more hydrophobic nature and stronger van der Waals interactions between neighboring chains.32,33 As result of this more compact ligand layer, the diffusion of monomers was slowed down, thus leading to a slower OR growth rate. The OR rate significantly affected the fluorescent properties of as-synthesized QDs. To obtain strong band-edge fluorescence, internal defects created by OA should be eliminated by continuous dissolution and precipitation.24 On the other hand, surface defects were also detrimental and should be removed by surface reconstruction to obtain a perfect surface.34 The better fluorescence of MPA-modified QDs compared to MBA-modified QDs and MVA-modified QDs might be attributed to the suitable chain length of MPA for OR. The strong hydrophobicity of MBA or MVA was not advantageous to keep a proper OR rate to effectively eliminate internal defects created by OA or to efficiently remove surface defects for obtaining a perfect surface. Accordingly, the band-edge fluorescence of MBA-modified or MVA-modified QDs was poor. Note that the emission peak related to defects in the photoluminescent spectra of MBA-modified QDs was obvious (Figure 4B). From these results, MPA had a suitable chain length with balanced hydrophilicity and hydrophobicity, beneficial for achieving relatively defect-free and moderate fluorescent QDs. 3.4. 3MBA: MPA-like Molecule with a Methyl Side Chain. The investigation on the effect of secondary coordination, methyl side chain and chain length revealed the following trend: the methyl side chain of TLA could confine the carboxyl group for better passivation, and the chain length of MPA was favorable for defect-free QDs. However, TLA, the isomer of MPA, exhibited similar QYmax probably because TLA was a TGA-like molecule and the slight agglomeration of

than that with MBA in the initial stage. MPA-modified QDs had the best fluorescence under our experimental conditions in comparison to QDs capped with MBA or MVA (Figure 4 and Table 1). The capping-agent-dependent growth rate and fluorescence of QDs indicated the remarkable effect of chain length on the aqueous growth of QDs. This effect might be associated with the hydrophilicity and hydrophobicity of the mercapto acids on the surface of QDs. The chain length of MPA was suitable to balance the hydrophilicity and hydrophobicity. The growth of CdTe QDs began with nucleation, followed by the rapid growth stage and the slow growth stage.9 In the rapid growth stage, QDs grew mainly via an oriented attachment (OA) mechanism wherein two crystallographically oriented nanoparticles could combine directly to form a larger one.24 QDs with a strong surface adsorption were prone to grow via an OA mechanism.25,26 During this stage, a large number of defects could be generated.24,27 Accordingly, to obtain QDs with strong band-edge fluorescence, the occurrence of OA-based growth should be avoided.24 Probably due to the more hydrophobic nature of MBA than MPA, QDs capped with MBA were more likely to grow via the OA mechanism in the initial stage, thereby leading to a relatively larger size and relatively larger amount of defects. For MVA, its hydrophobicity might be so strong that a high temperature was required to trigger the growth of MVA-modified QDs, which was reflected by the slightly red shift of absorption spectra of MVA-modified QDs (Figure 2C). In the slow growth stage, QDs grew mainly via Ostwald ripening (OR) mechanism wherein larger QDs grew and smaller ones dissolved. In this stage, the capping-agentdependent electric double-layer structures of QDs might play an important role on the capping-agent-dependent growth and fluorescence of QDs. According to colloid stability theory, QDs possessed electric double-layer structures to be stably dispersed in the aqueous solution.28 This structure consisted of an inorganic core, a ligand layer, an adsorbed layer, and a diffuse layer from the core outward.29 The growth of QDs was governed by the ligand layer, adsorbed layer, and the diffusion layer because the diffusion of monomers through these layers 12350

dx.doi.org/10.1021/jp302820u | J. Phys. Chem. C 2012, 116, 12346−12352

The Journal of Physical Chemistry C

Article

out. The QYmax of MPA-modified CdSe QDs and 3MBAmodified CdSe QDs were 4% and 19%, respectively. These results suggested that 3MBA was a promising capping agent for successful aqueous synthesis of highly fluorescent QDs.

precursors affected corresponding QDs. Therefore, the effect of methyl side chain might be further revealed when a MPA-like molecule with a methyl side chain as TLA was used as a capping agent. Thus, we designed and synthesized 3MBA, the MPA-like molecule with a methyl side chain as TLA. The schematic illustration of the synthesis of 3MBA is shown in Scheme 2. 3MBA-modified QDs indeed exhibited remarkably improved fluorescence (QYmax = 59%) than MPA-modified QDs and MBA-modified QDs (Table 1). The aqueous synthesis of QDs using 3MBA as a capping agent was robust and reproducible. These results strongly suggested that the methyl side groups in mercapto acids play a significant role in the aqueous synthesis of QDs on improving their photoluminescence. It was also suggested that the combination of a suitable chain length and a methyl side group are important for designing an efficient mercapto acid as a capping agent. The powder XRD pattern and high-resolution tranmission electron microscopy (HRTEM) image (Figure 5) confirmed the formation of 3MBA-modified CdTe QDs. After 1.5 years in the darkness, no observable precipitates could be observed. The QY of 3MBAmodified CdTe QDs could still reach 45% after 1.5 years of storage. Precipitates could be observed for CdTe QDs capped with other mercapto acids (TGA, TLA, MPA, MBA, and MVA) after 1.5 years of storage at the same conditions, indicating poor stability. These data indicate the excellent stability of 3MBAmodified CdTe QDs, which was not worse than the stability reported by other researchers.6 To further reveal the 3MBA as the promising capping agent, we optimized reaction conditions for CdTe QDs capped with TGA (Figure S2, Supporting Information), TLA (Figure S3), MPA (Figure S4), MBA (Figure S5), and 3MBA (Figure S6). The phenomena were in accordance with that in literature.1,16,22,29,35 The data of QYmax of CdTe QDs capped with these mercapto acids at the optimized conditions were listed in Table 3. From Table 3, CdTe QDs capped with 3MBA still exhibited higher QYmax (65%) among that capped with TGA, TLA, MPA, and MBA.

4. CONCLUSION We investigated the effect of mercapto acids on the aqueous synthesis of CdTe QDs. Three effects of mercapto acids, namely, secondary coordination of carboxyl group, steric hindrance of methyl side chain, and chain length for the hydrophilicity−hydrophobicity balance were proved to be the main factors in aqueous synthesis to control the growth and fluorescence of QDs. It was confirmed that the secondary coordination of TGA-like molecules (TGA and Cys) could induce the aggregation of precursors, which was unfavorable for achieving highly fluorescent QDs. Nonetheless, the secondary coordination could be confined by introducing a methyl group as a side chain as in the case of TLA. It was also confirmed that MPA was a better capping agent in comparison to other linear mercapto acids. By a comprehensive consideration of effects from both side methyl group and chain length of mercapto acids, we designed and synthesized 3MBA that has the same main chain length as MPA and methyl side group as TLA. As a result, 3MBA-modified CdTe QDs exhibited an excellent optical property with a quantum yield as high as 71%. Preliminary aqueous synthesis of CdSe QDs confirmed the versatility of 3MBA. These results might be helpful for synthesis of highly photoluminescent nanocrystals by selecting and designing appropriate capping agents.

■

S Supporting Information *

High-resolution TEM image of precursor solution of TGAmodified CdTe QDs (Figure S1); QY of as-synthesized QDs as a function of reaction time at different experiment conditions when TGA (Figure S2), TLA (Figure S3), MPA (Figure S4), MBA (Figure S5), and 3MBA (Figure S6) were used as capping agents; measurement of QY of CdTe QDs (S7); synthesis of MBA (S8), 3MBA (S9), MVA (S10), and CdSe QDs (S11); aqueous synthesis of CdTe QDs using NaHTe (S12). This material is available free of charge via the Internet at http:// pubs.acs.org.

Table 3. QYmax of QDs Capped with Various Mercapto Acids at the Optimized Conditionsa capping agent

TGAb

TLAc

MPAd

MBAe

3MBAf

QYmax/%

19

45

31

7

65

ASSOCIATED CONTENT

■

Fluorescein in 0.1 M NaOH (QY = 79%) was used as a fluorescent reference. The fluorescein was cross-calibrated at 480 nm using Rhodamine 6G (pure, J&K, QY = 95%) in absolute ethanol to ensure that the experimental quantum yields matched their literature counterparts within ±5%. bpH = 5.5, Cd:TGA = 1:3, Cd:Te = 4:1, T = 135 °C, [Cd] = 1 mM. cpH = 5.5, Cd:TLA = 1:3, Cd:Te = 8:1, T = 135 °C, [Cd] = 1 mM. dpH = 7.0, Cd:MPA = 1:3, Cd:Te = 4:1, T = 135 °C, [Cd] = 1 mM. epH = 7.0, Cd:MBA = 1:3, Cd:Te = 4:1, T = 135 °C, [Cd] = 1 mM. fpH = 6.0, Cd:3MBA = 1:3, Cd:Te = 4:1, T = 135 °C, [Cd] = 1 mM. a

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: 86-10-62755702. Author Contributions §

These two authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was partially supported by 973 Program (No. 2011CB933300) of China.

We also prepared CdTe QDs using the commonly adopted NaHTe method reported by Zhang and co-workers.36 The QY of CdTe QDs capped with either TGA (30%) or MPA (51%) was similar to the reported results,36 whereas the QY of 3MBAmodifed QDs was 71% at the same conditions (Figure S12, Supporting Information). The preliminary experiment of aqueous synthesis of CdSe QDs using 3MBA and MPA as a capping agent was also carried

■

REFERENCES

(1) Gaponik, N.; Talapin, D. V.; Rogach, A. L.; Hoppe, K.; Shevchenko, E. V.; Kornowski, A.; Eychmuller, A.; Weller, H. J. Phys. Chem. B 2002, 106, 7177−7185. (2) Medintz, I. L.; Uyeda, H. T.; Goldman, E. R.; Mattoussi, H. Nat. Mater. 2005, 4, 435−446.

12351

dx.doi.org/10.1021/jp302820u | J. Phys. Chem. C 2012, 116, 12346−12352

The Journal of Physical Chemistry C

Article

(3) Michalet, X.; Pinaud, F. F.; Bentolila, L. A.; Tsay, J. M.; Doose, S.; Li, J. J.; Sundaresan, G.; Wu, A. M.; Gambhir, S. S.; Weiss, S. Science 2005, 307, 538−544. (4) 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. (5) Gaponik, N.; Hickey, S. G.; Dorfs, D.; Rogach, A. L.; Eychmuller, A. Small 2010, 6, 1364−1378. (6) Gaponik, N.; Rogach, A. L. Phys. Chem. Chem. Phys. 2010, 12, 8685−8693. (7) Li, Y. L.; Jing, L. H.; Qiao, R. R.; Gao, M. Y. Chem. Commun. 2011, 47, 9293−9311. (8) Zhang, H.; Wang, D. Y.; Mohwald, H. Angew. Chem., Int. Ed. 2006, 45, 748−751. (9) Zhang, H.; Wang, D. Y.; Yang, B.; Mohwald, H. J. Am. Chem. Soc. 2006, 128, 10171−10180. (10) Zhang, H.; Wang, D.; Hartmann, J.; Mohwald, H. J. Phys. Chem. C 2007, 111, 9678−9683. (11) Han, J. S.; Luo, X. T.; Zhou, D.; Sun, H. Z.; Zhang, H.; Yang, B. J. Phys. Chem. C 2010, 114, 6418−6425. (12) Aldeek, F.; Balan, L.; Lambert, J.; Schneider, R. Nanotechnology 2008, 19, 475401. (13) Acar, H. Y.; Kas, R.; Yurtsever, E.; Ozen, C.; Lieberwirth, I. J. Phys. Chem. C 2009, 113, 10005−10012. (14) Gao, M. Y.; Kirstein, S.; Mohwald, H.; Rogach, A. L.; Kornowski, A.; Eychmuller, A.; Weller, H. J. Phys. Chem. B 1998, 102, 8360−8363. (15) Bao, H. B.; Gong, Y. J.; Li, Z.; Gao, M. Y. Chem. Mater. 2004, 16, 3853−3859. (16) Guo, J.; Yang, W. L.; Wang, C. C. J. Phys. Chem. B 2005, 109, 17467−17473. (17) He, Y.; Sai, L. M.; Lu, H. T.; Hu, M.; Lai, W. Y.; Fan, Q. L.; Wang, L. H.; Huang, W. Chem. Mater. 2007, 19, 359−365. (18) Zhang, H.; Liu, Y.; Zhang, J. H.; Wang, C. L.; Li, M. J.; Yang, B. J. Phys. Chem. C 2008, 112, 1885−1889. (19) Zhou, D.; Han, J. S.; Liu, Y.; Liu, M.; Zhang, X.; Zhang, H.; Yang, B. J. Phys. Chem. C 2010, 114, 22487−22492. (20) Niu, H. J.; Gao, M. Y. Angew. Chem., Int. Ed. 2006, 45, 6462− 6466. (21) Park, Y. S.; Dmytruk, A.; Dmitruk, I.; Kasuya, A.; Takeda, M.; Ohuchi, N.; Okamoto, Y.; Kaji, N.; Tokeshi, M.; Baba, Y. ACS Nano 2010, 4, 121−128. (22) Zou, L.; Gu, Z. Y.; Zhang, N.; Zhang, Y. L.; Fang, Z.; Zhu, W. H.; Zhong, X. H. J. Mater. Chem. 2008, 18, 2807−2815. (23) Duan, J. L.; Song, L. X.; Zhan, J. H. Nano Res. 2009, 2, 61−68. (24) Zheng, J. S.; Huang, F.; Yin, S. G.; Wang, Y. J.; Lin, Z.; Wu, X. L.; Zhao, Y. B. J. Am. Chem. Soc. 2010, 132, 9528−9530. (25) Huang, F.; Zhang, H. Z.; Banfield, J. F. Nano Lett. 2003, 3, 373− 378. (26) Zhang, J.; Lin, Z.; Lan, Y. Z.; Ren, G. Q.; Chen, D. G.; Huang, F.; Hong, M. C. J. Am. Chem. Soc. 2006, 128, 12981−12987. (27) Yin, S. G.; Huang, F.; Zhang, J.; Zheng, J. S.; Lin, Z. J. Phys. Chem. C 2011, 115, 10357−10364. (28) Laaksonen, T.; Ahonen, P.; Johans, C.; Kontturi, K. ChemPhysChem 2006, 7, 2143−2149. (29) Zhang, H.; Liu, Y.; Wang, C. L.; Zhang, J. H.; Sun, H. Z.; Li, M. J.; Yang, B. ChemPhysChem 2008, 9, 1309−1316. (30) Wang, C. L.; Zhang, H.; Zhang, J. H.; Lv, N.; Li, M. J.; Sun, H. Z.; Yang, B. J. Phys. Chem. C 2008, 112, 6330−6336. (31) Wang, C. L.; Zhang, H.; Xu, S. H.; Lv, N.; Liu, Y.; Li, M. J.; Sun, H. Z.; Zhang, J. H.; Yang, B. J. Phys. Chem. C 2009, 113, 827−833. (32) Durig, U.; Zuger, O.; Michel, B.; Haussling, L.; Ringsdorf, H. Phys. Rev. B 1993, 48, 1711−1717. (33) Blum, A. S.; Moore, M. H.; Ratna, B. R. Langmuir 2008, 24, 9194−9197. (34) Talapin, D. V.; Rogach, A. L.; Shevchenko, E. V.; Kornowski, A.; Haase, M.; Weller, H. J. Am. Chem. Soc. 2002, 124, 5782−5790. (35) Ying, E.; Li, D.; Guo, S.; Dong, S.; Wang, J. PLoS One 2008, 3, e2222.

(36) Zhang, H.; Wang, L. P.; Xiong, H. M.; Hu, L. H.; Yang, B.; Li, W. Adv. Mater. 2003, 15, 1712−1715. (37) Pitman, I. H.; Morris, I. J. Aust. J. Chem. 1979, 32, 1567−1573.

12352

dx.doi.org/10.1021/jp302820u | J. Phys. Chem. C 2012, 116, 12346−12352