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Synthesis and characterization of high-quality water-soluble CdMnTe quantum dots capped by N-acetyl-L-cysteine through hydrothermal method
Journal of Luminescence 159 (2015) 32–37
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Synthesis and characterization of high-quality water-soluble CdMnTe quantum dots capped by N-acetyl-L-cysteine through hydrothermal method Fang Gao, Jiaotian Li, Fengxue Wang, Tanming Yang, Dan Zhaon College of Pharmacy, South-Central University for Nationalities, Wuhan 430074, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 17 February 2014 Received in revised form 19 September 2014 Accepted 24 October 2014 Available online 1 November 2014
High-quality water-soluble Mn2 þ doped CdTe quantum dots (QDs) with N-acetyl-L-cysteine (NAC) as capping reagent have been synthesized through hydrothermal route, allowing a rapid preparation time ( o1 h), tunable emitting peaks (from 530 to 646 nm) and excellent quantum yields (approximately 50%). The influences of various experimental variables, including Mn-to-Cd ratio, Te-to-Cd ratio, pH value, and reaction time on the growth rate and luminescent properties of the obtained QDs have been systematically investigated. And the optimum reaction conditions (Cd:Mn:NAC:Te ¼1.0:1.0:2.4:0.2, pH ¼9.5, 35 min, 200 1C) are found out. The optical features and structure of the obtained CdMnTe QDs have been characterized through fluorescence spectroscopy, UV absorption spectroscopy and TEM. In particular, we realized qualitative, semi-quantitative and quantitative studies on the doping of Mn to CdTe QDs through XPS, EDS, and AAS. The actual molar ratio of Mn to Cd in CdMnTe QDs (551 nm) is 1.166:1.00, very close to the feed ratios (1:1). & 2014 Elsevier B.V. All rights reserved.
Keywords: Quantum dots CdMnTe Synthesis Hydrothermal
1. Introduction Because of their unique photophysical properties, watersoluble quantum dots (QDs) have been applied in many fields including biochemical detection [1], bio-labeling [2,3], solar cells [4] and cellular and in vivo imaging [5–7]. Among them, the CdE (E ¼S, Se, Te) quantum dots are one of the most commonly and widely used types in these fields. However, the heavy metal cadmium contained in these QDs is regarded as a hidden danger to the environment and organic system, raising concerns on safety of their biomedical applications [8]. Therefore, further efforts to reduce their toxicity and to improve their biological compatibility and fluorescence qualities have thus become urgent requirements. On this occasion, the appearance of doped semiconductor nanocrystals shows the potential to be a class of mainstream emissive materials due to their apparent merits in overcoming the toxicity and self-quenching problem of undoped QDs through their substantial ensemble stokes shift [9], narrower emission band, broad excitation band, and better photochemical stability [10]. Zhong and his co-workers [11] have successfully prepared highquality ZnxCd1 xSe nanocrystals by incorporating stoichiometric amounts of Zn and Se into pre-prepared CdSe nanocrystals. The
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Corresponding author. Tel.: þ 86-27-67841196. E-mail address: [email protected] (D. Zhao).
http://dx.doi.org/10.1016/j.jlumin.2014.10.057 0022-2313/& 2014 Elsevier B.V. All rights reserved.
photoluminescence (PL) properties (PL efficiency of 70–85%, FWHM ¼22–30 nm) are comparable to those for the best reported CdSe-based QDs, and in particular, the alloy nanocrystals retain high luminescence (PL efficiency reaches over 40%) when dispersed in aqueous solutions. Actually, the doped QDs are not new in the field, and many different doped QDs have already been reported in the literatures, such as ZnxCd1 xSe [11–13], ZnxCd1 xS [14], CdSeTe [15], and CdxZn1 xTe [16,17]. Compared to the ternary alloyed nanocrystals, Mn2 þ doped II-VI semiconductor QDs can not only reduce the dosage of cadmium, but also possess remarkably intense photoluminescence due to the 4T1(4G)-6A1(6S) transition [18]. These doped dots are as efficient as conventional ones, and possess some unique properties, including low cytotoxicity, high photostability, and reduced chemical sensitivity. These merits ensure them as ideal fluorescent labels for biological assays, cells and tissues imaging, and even in vivo investigations [19]. Besides, the long lifetime of ZnS:Mn2 þ nanocrystals (ca.1 ms) makes the luminescence readily distinguishable from the short-lifetimed background luminescence [10]. Moreover, the impurity states of Mn2 þ has also played special role in affecting the electronic energy structures and transient probabilities in the doped QDs [20]. The dilute magnetic properties of Mn2 þ -doped QDs renders Cd1 xMnxTe the most popular dilute magnetic semiconductor compounds owing to their high technological properties, the highest possible magnetic moment of the manganese 3d-shell and favorable
F. Gao et al. / Journal of Luminescence 159 (2015) 32–37
parameters of the basic crystal CdTe [21]. Additionally, the CdMnTe crystals are also good candidate to compete with traditional CdZnTe crystal in detector applications [22]. It is believed that the employed synthesis techniques, concentration of the dopants, particle size, and functional groups present on the surface of the nanocrystals can greatly affect the optical, electronic, or magnetic properties in diluted magnetic semiconductor materials [23]. Even though such physical properties of CdMnTe have been studied [23–28], few efforts have been put on the synthesis process of CdMnTe QDs [23,27]. Besides, since the utilization ratio of Mn in reported article [23] is relatively low (nearly 0.002 to 0.031), it is of great importance to adopt new synthesis method to improve it. At present, the synthesis of QDs includes organometallic method and aqueous synthesis method. The aqueous synthesis method is often preferable because of its simpler synthesizing process, lower cost, more environmentally friendliness, and better biocompatibility [29]. As a conventional aqueous method for QDs synthesis, refluxing route's long reaction time during the synthesis caused by low temperature (o 100 1C) leads to low QY and wide emission peak width of prepared QDs, and greatly limits their practical applications [30]. The hydrothermal route has been regarded as an ideal method for direct synthesis of QDs in aqueous solution because the rapid growth rate under higher reaction temperature (4 100 1C) reduces the surface defects and thus ensures high fluorescence properties of as-prepared QDs [17,30]. In terms of ligand selection, since the majority of the biomedical engineering applications require surface functionalization of nanocrystals, the exploration of other safer and more environment-friendly alternative stabilizers is meaningful in the realization of one-step synthesis of Mn-doped CdTe QDs with wide emission wavelength and high fluorescence qualities. In this work, we report for the first time the use of N-Acetyl-Lcysteine (NAC) as the stabilizer in the process of synthesizing a series of high-quality CdMnTe QDs through one-pot hydrothermal route. NAC is known as an antioxidant to protect cells against oxidative stress and QD-induced cytotoxicity [31,32]. Furthermore, as a derivative of L-cysteine, NAC possesses excellent biocompatibility and water-solubility, and is friendly to its users and environment, inexpensive, stable, nonvolatile, and inodorous [32]. Thanks to the merits of NAC, the as-prepared NAC-capped CdMnTe QDs through hydrothermal route exhibit excellent water solubility and stability. The impacts of various experimental parameters (viz. pH, molar ratio of reactants, and reaction time) have been systematically investigated to find out the optimum reaction condition. Furthermore, we have characterized the optical properties, particle diameter, and distribution of the prepared QDs through various means, including fluorescence spectroscopy, UV absorption spectroscopy, transmission electron microscopy (TEM), and photostability experiments. In particular, we realized qualitative, semi-quantitative, and quantitative research on the components of prepared Mn-doped CdTe QDs through X-ray photoelectron spectroscopy (XPS), electron diffraction spectroscopy (EDS), and atomic absorption spectrometry (AAS). NAC-capped CdMnTe QDs meet the current requirements for fluorescence materials in biological studies, and are sure to have extensive and promising applications in future biomedical fields.
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Chemical Reagent. NAC was purchased from Sigma. All chemicals used were of analytical grade or of the highest purity available. All solutions were prepared using Milli-Q water (Millipore) as the solvent.
2.2. Preparation of CdMnTe nanocrystals In a typical synthesis, sodium borohydride was reacted with tellurium powder in deionized water to produce sodium hydrogen telluride (NaHTe). The mixture of Mn2 þ –NAC and Cd2 þ –NAC were prepared by dissolving CdCl2, MnCl2, and NAC in deionized water and adjusting the pH value to 8–11 by the dropwise addition of NaOH solution (1 mol/L). The fresh NaHTe solution was then injected into a N2-saturated mixture of Mn2 þ –NAC and Cd2 þ – NAC precursor solution under vigorous stirring. The typical molar ratio of Cd, Mn, Te and NAC introduced was 1: n (0.2, 0.5, 0.8, 1.0, 1.5): m (0.1, 0.2, 0.3, 0.4, 0.5): 1.2 N (N equal the total molar quantity of Cd2 þ and Mn2 þ ) in a total volume of 40 mL. The concentration of Cd2 þ was fixed at 4 mmol/L. 40 mL of precursor was put into a Teflon-lined stainless steel autoclave with a volume of 45 mL. The autoclaves were maintained at the desired growth temperature (200 1C). The autoclave was cooled to room temperature at regular time intervals after the initial heating. To remove NAC-Cd/Mn complexes at the end of the synthesis, cold 2-propanol was added to the reaction mixture to precipitate NAC-capped CdMnTe QDs. The as-prepared products were dried overnight under vacuum at 40 1C for further experiments.
2.3. Characterization UV-visible absorption spectra were acquired with a Lambda-35 UV/visible spectrophotometer (PerkinElmer Company). Fluorescence spectra were recorded on a LS55 spectrofluorometer (PerkinElmer Company). All optical measurements were performed at room temperature under ambient conditions. The TEM sample was prepared by dropping an aqueous CdMnTe QDs solution onto an Agar carbon-coated copper grid (400 meshes) with the excess solvent evaporated. The TEM image was obtained at 310 K magnification with an FEI Tecnai G220 twin transmission electron microscope. EDS spectra were captured using an FEI Quanta 200 scanning electron microscope equipped with an energy dispersive X-ray spectrometer. XPS measurements were acquired with a Leybold Heraeus SKL 12 X-ray photoelectron spectrometer. AAS measurements were acquired with a PerkinElmer Analyst 80 atomic absorption spectrometer. The QY of CdTe QDs was measured according to the literature [33]. Rhodamine 6G in ethanol was chosen as the reference standard (QY ¼95%).
2. Experimental 2.1. Chemicals Tellurium (reagent powder), CdCl2, MnCl2, rhodamine 6G, and sodium borohydride (NaBH4) were obtained from Sinopharm
Fig. 1. PL spectra of NAC-capped CdMnTe QDs grown at different pH values (λex ¼ 380 nm).
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F. Gao et al. / Journal of Luminescence 159 (2015) 32–37
Fig. 2. PL spectra of NAC-capped CdMnTe QDs with different ratios of (a) Mn to Cd (from a to e was 0.2, 0.5, 0.8, 1.0, and 1.5) and (b) Te to Cd (from a to e was 0.1, 0.2, 0.3, 0.4, and 0.5) (λex ¼ 380 nm).
Fig. 3. (a) Corrected PL spectra and (b) UV-visible absorption spectra of NAC-capped CdMnTe QDs prepared at various reaction times (from left to right: 30, 35, 40, 45, and 50 min) (λex ¼ 380 nm). The inset displays the photoluminescence images of the corresponding CdMnTe QDs under UV and visible lamp irradiation.
3. Results and discussion 3.1. Synthesis of NAC-capped CdMnTe QDs and the reaction conditions. 3.1.1. Influence of the pH Since pH, molar ratio of reactants and reaction time are important parameters to the fluorescence properties of the prepared QDs, experimental conditions must be strictly controlled to obtain high-quality QDs. The precursor solutions were operated in alkaline conditions (pH 8 11). Further lowering the pH would lead to luminescence decrease or precipitation of the CdMnTe QDs, possibly arising from the aggregation of the QDs. As shown in Fig. 1, with other reaction factors fixed ([Cd2 þ ]¼ 4 mmol/L, molar ratio of Cd:Mn:Te:NAC at 1.0:1.0:0.2:2.4, reaction temperature at 200 1C, and reaction time at 35 min), the best QY of prepared QDs occurs at pH ¼9.5.
3.1.2. Influence of molar ratio of reactants We found out that the molar ratio of Cd:Mn:Te played an important role in controlling the QY of the as-prepared CdMnTe QDs. We first fixed the Cd:Te molar ratio (1.0:0.2) and changed the addition amount of Mn. With other reaction parameters fixed (pH at 9.5, reaction time at 35 min, reaction temperature at 200 1C, and Cd concentration at 4 mmol/L), as shown in Fig. 2a, the QY of the as-prepared CdMnTe QDs decreases with the increased Mn:Cd
molar ratio. It is observed that the increase of the Mn:Cd molar ratio from 0.2 to 1.5 resulted in a blue shift of emission peaks from 578 to 553 nm and of absorption peaks (λmax) from 546 to 504 nm. This is because the band gap energies of NAC-capped CdMnTe QDs increase with increasing Mn/Cd atomic ratios [34]. In order to get the NAC-CdMnTe QDs with both high fluorescence efficiency and better utilization ratio of Mn, the optimum Mn-to-CdCl2 molar ratio is 1.0. When fixing the Cd:Mn:NAC molar ratio at 1.0:1.0:2.4 and increasing the addition amount of Te, the QY reached the highest when the Te:Cd ratio is 0.2:1.0 (Fig. 2b). The enhancement of the Te:Cd ratio leads to a higher Te content in as-prepared QDs. The existence of a dangling bond of Te on the surface of QDs and the easy proneness of oxidation create many nonradioactive combination pathways, leading to low QY [30]. The increase of the Te:Cd molar ratio from 0.1 to 0.5 did not produce obvious impact on the shift of emission peaks, which maintained near 545 nm, showing no occurance of obvious particle size increase.
3.1.3. Influence of reaction time The optimum reaction conditions are the following: the molar ratio of Cd:Mn:NAC:Te is 1.0:1.0:2.4:0.2, pH is 9.5, and reaction temperature is 200 1C. On changing the reaction time (30–50 min), the emission peaks (λem) of CdMnTe QDs shift from 530 to 646 nm, and the average quantum yields were approximately 50% (the highest QY reached 63.1%). The corresponding full width at
F. Gao et al. / Journal of Luminescence 159 (2015) 32–37
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half-maximum (FWHM) of the band-edge luminescence were from 39 to 57 nm (Fig. 3a), and the absorption peaks (λmax) moved from 487 to 609 nm when the reaction time increased (Fig. 3b). As shown in the inset of Fig. 3, the emission color under UV irradiation turned from green to yellow, orange, and finally red with the increase of reaction time. 3.2. The characterization of NAC-capped CdMnTe QDs 3.2.1. TEM The morphology of the as-prepared NAC-capped CdMnTe QDs (λem ¼551 nm) was studied through TEM. It is quite evident that these nanoparticles are close to spherical with good dispersion and are uniform with an average diameter of 8.5 7 0.4 nm (Fig. 4).
Fig. 4. TEM image of NAC-capped CdMnTe QDs (λem ¼ 551 nm) reacted for 35 min.
Fig. 5. EDS spectrum of NAC-capped CdMnTe QDs (λem ¼551 nm) at the molar ratio of Cd:Te:Mn ¼ 1:0.2:1.
3.2.2. EDS and XPS The EDS technique has been employed to probe a semiquantitative picture of the composition of the NAC-capped CdMnTe QDs. It must be emphasized that, for the nano-scaled QDs, the element composition obtained through EDS (whose penetration depth of the electron beam is more than 100 nm) must be the bulk composition of multilayered samples of QDs [30]. As shown in Fig. 5, for prepared NAC-capped CdMnTe QDs with molar ratio of Cd:Mn:Te¼ 1.0:1.0:0.2 (λem ¼551 nm), the atomic percentages of Cd, Mn, and Te reach 7.38%, 9.72%, and 1.93% respectively. The calculated constituent ratio of Cd to Mn and Te in the prepared QDs is therefore 1:1.32:0.26, which is very close to the feed ratio. Meanwhile, as a quantitative surface analysis tool sensitive to the atomic composition of the outermost 100 Å of the sample surface [35], XPS has been employed to analyze the chemical composition of CdMnTe QDs. Fig. 6 displays the Cd 3d5/2 peak and characteristic Mn 2p3/2 peaks of the prepared CdMnTe QDs with molar ratio of Cd:Mn:Te ¼1.0:1.0:0.2 (λem ¼551 nm) and Cd:Mn: Te¼1.0:0.5:0.2 (λem ¼573 nm). As the concentration of Mn increases, the width of the Cd 3d5/2 peak becomes broader (from 3.32 to 3.36 eV) and its position shifts to a lower energy (from 404.63 to 404.53 eV) (Fig. 6a). Compared to the binding energy of the Cd atoms bonded to the Te atoms, which of the Cd atoms bonded to the Mn atoms would be expected to appear at a lower energy. This is because the electronegativities (χ) of Te (χP ¼2.10), Cd (χP ¼ 1.69), and Mn (χP ¼1.55) are in descending order. Fig. 6b displays the corresponding Mn 2p3/2 peak of CdMnTe QDs. It is obvious that the peak shifts to a lower energy region (from 641.64 to 640.51 eV) with increasing Mn concentration. The explanation might be analogous to that of Cd 3d5/2 peak, which can also be
Fig. 6. (a) Cd 3d XPS spectrum and (b) Mn (2p) XPS spectrum of the NAC-capped CdMnTe QDs with ratios of Mn to Cd were 0.5 and 1.0, respectively.
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F. Gao et al. / Journal of Luminescence 159 (2015) 32–37
Fig. 7. (a) PL spectra of NAC-capped CdMnTe QDs (λem ¼551 nm) at various pH values and (b) photostability of NAC-capped CdMnTe QDs (λem ¼ 551 nm).
explained by the incorporation of the less electronegative Mn (than Cd) and the formation of Mn-Cd bonding structures [36]. 3.2.3. AAS EDS is a semi-quantitative characterization tool, merely exhibiting the composition of QDs surface elements, while AAS aims to measure the concentration of each element in the QDs after being dissolved in strong acid [17]. So the latter is used to examine the accurate quantitative composition of each element of whole CdMnTe QDs. When the feed ratios of Mn to Cd being fixed at 1:1, the actual molar ratio of Mn to Cd in CdMnTe QDs with emission peak at 551 nm is 1.166:1.00, which is in accordance to the feed ratio. 3.3. Physicochemical properties of NAC-capped CdMnTe QDs Fig. 7a displays the pH effect on the emission intensity of NACcapped CdMnTe QDs (λem ¼551 nm). NAC-capped CdMnTe QDs exhibit better stability in neutral or alkaline solution but the intensity dropped dramatically at lower pH. This is due to the protonation of the thiol moiety of both types of QDs under acidic conditions, leading to the detachment of the capping agent from the nanocrystals. As such, the QD-ligand complexes are destroyed and consequently decrease their PL intensity. The carboxylic acid moiety of NAC is deprotonated in neutral and basic aqueous solutions. Especially under high pH conditions, the negative charges of carboxylate groups located on the surface of the QDs repel each other, thus preventing aggregation. This interaction is beneficial to the stabilization of QDs and leads to higher PL efficiency [32]. The fluorescence of NAC-capped CdMnTe QDs reaches a maximum at pH around 9.5 and remains constant up to pH 11.5. To examine the photostability of the prepared QDs, the CdMnTe QDs (λem ¼551 nm) at an extremely low concentration (abs¼ 0.01) in Tris–HCl buffer (0.05 mol/L, pH ¼8) were irradiated continuously by a xenon lamp (16 W). As shown in Fig. 7b, the fluorescence decreased only 17% of the original emission within 120 min, exhibiting excellent photostability of the prepared QDs. 4. Conclusions With NAC as the stabilizer, high-quality water-soluble NACcapped CdMnTe QDs have been synthesized through hydrothermal
route, and the prepared CdMnTe QDs possess high QYs and narrow PL spectra. Through a series of optimizing experiments, the optimal reaction conditions are found out: molar ratio of Cd:Mn: NAC:Te is 1.0:1.0:2.4:0.2, pH is 9.5, and reaction temperature is 200 1C. The emission peak is tunable from 530 to 646 nm by controlling the reaction time of synthesis (30–50 min). We studied their morphology through TEM, and XPS analysis indicated that Mn atoms have been successfully doped into the nanocrystals. In particular, we examined the feed ratio and the actual constituent ratio of the prepared QDs through EDS and AAS.
Acknowledgments This research was supported by the National Science Foundation of China (21105130).
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Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Synthesis and characterization of high-quality water-soluble CdMnTe quantum dots capped by N-acetyl-L-cysteine through hydrothermal method Fang Gao, Jiaotian Li, Fengxue Wang, Tanming Yang, Dan Zhaon College of Pharmacy, South-Central University for Nationalities, Wuhan 430074, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 17 February 2014 Received in revised form 19 September 2014 Accepted 24 October 2014 Available online 1 November 2014
High-quality water-soluble Mn2 þ doped CdTe quantum dots (QDs) with N-acetyl-L-cysteine (NAC) as capping reagent have been synthesized through hydrothermal route, allowing a rapid preparation time ( o1 h), tunable emitting peaks (from 530 to 646 nm) and excellent quantum yields (approximately 50%). The influences of various experimental variables, including Mn-to-Cd ratio, Te-to-Cd ratio, pH value, and reaction time on the growth rate and luminescent properties of the obtained QDs have been systematically investigated. And the optimum reaction conditions (Cd:Mn:NAC:Te ¼1.0:1.0:2.4:0.2, pH ¼9.5, 35 min, 200 1C) are found out. The optical features and structure of the obtained CdMnTe QDs have been characterized through fluorescence spectroscopy, UV absorption spectroscopy and TEM. In particular, we realized qualitative, semi-quantitative and quantitative studies on the doping of Mn to CdTe QDs through XPS, EDS, and AAS. The actual molar ratio of Mn to Cd in CdMnTe QDs (551 nm) is 1.166:1.00, very close to the feed ratios (1:1). & 2014 Elsevier B.V. All rights reserved.
Keywords: Quantum dots CdMnTe Synthesis Hydrothermal
1. Introduction Because of their unique photophysical properties, watersoluble quantum dots (QDs) have been applied in many fields including biochemical detection [1], bio-labeling [2,3], solar cells [4] and cellular and in vivo imaging [5–7]. Among them, the CdE (E ¼S, Se, Te) quantum dots are one of the most commonly and widely used types in these fields. However, the heavy metal cadmium contained in these QDs is regarded as a hidden danger to the environment and organic system, raising concerns on safety of their biomedical applications [8]. Therefore, further efforts to reduce their toxicity and to improve their biological compatibility and fluorescence qualities have thus become urgent requirements. On this occasion, the appearance of doped semiconductor nanocrystals shows the potential to be a class of mainstream emissive materials due to their apparent merits in overcoming the toxicity and self-quenching problem of undoped QDs through their substantial ensemble stokes shift [9], narrower emission band, broad excitation band, and better photochemical stability [10]. Zhong and his co-workers [11] have successfully prepared highquality ZnxCd1 xSe nanocrystals by incorporating stoichiometric amounts of Zn and Se into pre-prepared CdSe nanocrystals. The
n
Corresponding author. Tel.: þ 86-27-67841196. E-mail address: [email protected] (D. Zhao).
http://dx.doi.org/10.1016/j.jlumin.2014.10.057 0022-2313/& 2014 Elsevier B.V. All rights reserved.
photoluminescence (PL) properties (PL efficiency of 70–85%, FWHM ¼22–30 nm) are comparable to those for the best reported CdSe-based QDs, and in particular, the alloy nanocrystals retain high luminescence (PL efficiency reaches over 40%) when dispersed in aqueous solutions. Actually, the doped QDs are not new in the field, and many different doped QDs have already been reported in the literatures, such as ZnxCd1 xSe [11–13], ZnxCd1 xS [14], CdSeTe [15], and CdxZn1 xTe [16,17]. Compared to the ternary alloyed nanocrystals, Mn2 þ doped II-VI semiconductor QDs can not only reduce the dosage of cadmium, but also possess remarkably intense photoluminescence due to the 4T1(4G)-6A1(6S) transition [18]. These doped dots are as efficient as conventional ones, and possess some unique properties, including low cytotoxicity, high photostability, and reduced chemical sensitivity. These merits ensure them as ideal fluorescent labels for biological assays, cells and tissues imaging, and even in vivo investigations [19]. Besides, the long lifetime of ZnS:Mn2 þ nanocrystals (ca.1 ms) makes the luminescence readily distinguishable from the short-lifetimed background luminescence [10]. Moreover, the impurity states of Mn2 þ has also played special role in affecting the electronic energy structures and transient probabilities in the doped QDs [20]. The dilute magnetic properties of Mn2 þ -doped QDs renders Cd1 xMnxTe the most popular dilute magnetic semiconductor compounds owing to their high technological properties, the highest possible magnetic moment of the manganese 3d-shell and favorable
F. Gao et al. / Journal of Luminescence 159 (2015) 32–37
parameters of the basic crystal CdTe [21]. Additionally, the CdMnTe crystals are also good candidate to compete with traditional CdZnTe crystal in detector applications [22]. It is believed that the employed synthesis techniques, concentration of the dopants, particle size, and functional groups present on the surface of the nanocrystals can greatly affect the optical, electronic, or magnetic properties in diluted magnetic semiconductor materials [23]. Even though such physical properties of CdMnTe have been studied [23–28], few efforts have been put on the synthesis process of CdMnTe QDs [23,27]. Besides, since the utilization ratio of Mn in reported article [23] is relatively low (nearly 0.002 to 0.031), it is of great importance to adopt new synthesis method to improve it. At present, the synthesis of QDs includes organometallic method and aqueous synthesis method. The aqueous synthesis method is often preferable because of its simpler synthesizing process, lower cost, more environmentally friendliness, and better biocompatibility [29]. As a conventional aqueous method for QDs synthesis, refluxing route's long reaction time during the synthesis caused by low temperature (o 100 1C) leads to low QY and wide emission peak width of prepared QDs, and greatly limits their practical applications [30]. The hydrothermal route has been regarded as an ideal method for direct synthesis of QDs in aqueous solution because the rapid growth rate under higher reaction temperature (4 100 1C) reduces the surface defects and thus ensures high fluorescence properties of as-prepared QDs [17,30]. In terms of ligand selection, since the majority of the biomedical engineering applications require surface functionalization of nanocrystals, the exploration of other safer and more environment-friendly alternative stabilizers is meaningful in the realization of one-step synthesis of Mn-doped CdTe QDs with wide emission wavelength and high fluorescence qualities. In this work, we report for the first time the use of N-Acetyl-Lcysteine (NAC) as the stabilizer in the process of synthesizing a series of high-quality CdMnTe QDs through one-pot hydrothermal route. NAC is known as an antioxidant to protect cells against oxidative stress and QD-induced cytotoxicity [31,32]. Furthermore, as a derivative of L-cysteine, NAC possesses excellent biocompatibility and water-solubility, and is friendly to its users and environment, inexpensive, stable, nonvolatile, and inodorous [32]. Thanks to the merits of NAC, the as-prepared NAC-capped CdMnTe QDs through hydrothermal route exhibit excellent water solubility and stability. The impacts of various experimental parameters (viz. pH, molar ratio of reactants, and reaction time) have been systematically investigated to find out the optimum reaction condition. Furthermore, we have characterized the optical properties, particle diameter, and distribution of the prepared QDs through various means, including fluorescence spectroscopy, UV absorption spectroscopy, transmission electron microscopy (TEM), and photostability experiments. In particular, we realized qualitative, semi-quantitative, and quantitative research on the components of prepared Mn-doped CdTe QDs through X-ray photoelectron spectroscopy (XPS), electron diffraction spectroscopy (EDS), and atomic absorption spectrometry (AAS). NAC-capped CdMnTe QDs meet the current requirements for fluorescence materials in biological studies, and are sure to have extensive and promising applications in future biomedical fields.
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Chemical Reagent. NAC was purchased from Sigma. All chemicals used were of analytical grade or of the highest purity available. All solutions were prepared using Milli-Q water (Millipore) as the solvent.
2.2. Preparation of CdMnTe nanocrystals In a typical synthesis, sodium borohydride was reacted with tellurium powder in deionized water to produce sodium hydrogen telluride (NaHTe). The mixture of Mn2 þ –NAC and Cd2 þ –NAC were prepared by dissolving CdCl2, MnCl2, and NAC in deionized water and adjusting the pH value to 8–11 by the dropwise addition of NaOH solution (1 mol/L). The fresh NaHTe solution was then injected into a N2-saturated mixture of Mn2 þ –NAC and Cd2 þ – NAC precursor solution under vigorous stirring. The typical molar ratio of Cd, Mn, Te and NAC introduced was 1: n (0.2, 0.5, 0.8, 1.0, 1.5): m (0.1, 0.2, 0.3, 0.4, 0.5): 1.2 N (N equal the total molar quantity of Cd2 þ and Mn2 þ ) in a total volume of 40 mL. The concentration of Cd2 þ was fixed at 4 mmol/L. 40 mL of precursor was put into a Teflon-lined stainless steel autoclave with a volume of 45 mL. The autoclaves were maintained at the desired growth temperature (200 1C). The autoclave was cooled to room temperature at regular time intervals after the initial heating. To remove NAC-Cd/Mn complexes at the end of the synthesis, cold 2-propanol was added to the reaction mixture to precipitate NAC-capped CdMnTe QDs. The as-prepared products were dried overnight under vacuum at 40 1C for further experiments.
2.3. Characterization UV-visible absorption spectra were acquired with a Lambda-35 UV/visible spectrophotometer (PerkinElmer Company). Fluorescence spectra were recorded on a LS55 spectrofluorometer (PerkinElmer Company). All optical measurements were performed at room temperature under ambient conditions. The TEM sample was prepared by dropping an aqueous CdMnTe QDs solution onto an Agar carbon-coated copper grid (400 meshes) with the excess solvent evaporated. The TEM image was obtained at 310 K magnification with an FEI Tecnai G220 twin transmission electron microscope. EDS spectra were captured using an FEI Quanta 200 scanning electron microscope equipped with an energy dispersive X-ray spectrometer. XPS measurements were acquired with a Leybold Heraeus SKL 12 X-ray photoelectron spectrometer. AAS measurements were acquired with a PerkinElmer Analyst 80 atomic absorption spectrometer. The QY of CdTe QDs was measured according to the literature [33]. Rhodamine 6G in ethanol was chosen as the reference standard (QY ¼95%).
2. Experimental 2.1. Chemicals Tellurium (reagent powder), CdCl2, MnCl2, rhodamine 6G, and sodium borohydride (NaBH4) were obtained from Sinopharm
Fig. 1. PL spectra of NAC-capped CdMnTe QDs grown at different pH values (λex ¼ 380 nm).
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Fig. 2. PL spectra of NAC-capped CdMnTe QDs with different ratios of (a) Mn to Cd (from a to e was 0.2, 0.5, 0.8, 1.0, and 1.5) and (b) Te to Cd (from a to e was 0.1, 0.2, 0.3, 0.4, and 0.5) (λex ¼ 380 nm).
Fig. 3. (a) Corrected PL spectra and (b) UV-visible absorption spectra of NAC-capped CdMnTe QDs prepared at various reaction times (from left to right: 30, 35, 40, 45, and 50 min) (λex ¼ 380 nm). The inset displays the photoluminescence images of the corresponding CdMnTe QDs under UV and visible lamp irradiation.
3. Results and discussion 3.1. Synthesis of NAC-capped CdMnTe QDs and the reaction conditions. 3.1.1. Influence of the pH Since pH, molar ratio of reactants and reaction time are important parameters to the fluorescence properties of the prepared QDs, experimental conditions must be strictly controlled to obtain high-quality QDs. The precursor solutions were operated in alkaline conditions (pH 8 11). Further lowering the pH would lead to luminescence decrease or precipitation of the CdMnTe QDs, possibly arising from the aggregation of the QDs. As shown in Fig. 1, with other reaction factors fixed ([Cd2 þ ]¼ 4 mmol/L, molar ratio of Cd:Mn:Te:NAC at 1.0:1.0:0.2:2.4, reaction temperature at 200 1C, and reaction time at 35 min), the best QY of prepared QDs occurs at pH ¼9.5.
3.1.2. Influence of molar ratio of reactants We found out that the molar ratio of Cd:Mn:Te played an important role in controlling the QY of the as-prepared CdMnTe QDs. We first fixed the Cd:Te molar ratio (1.0:0.2) and changed the addition amount of Mn. With other reaction parameters fixed (pH at 9.5, reaction time at 35 min, reaction temperature at 200 1C, and Cd concentration at 4 mmol/L), as shown in Fig. 2a, the QY of the as-prepared CdMnTe QDs decreases with the increased Mn:Cd
molar ratio. It is observed that the increase of the Mn:Cd molar ratio from 0.2 to 1.5 resulted in a blue shift of emission peaks from 578 to 553 nm and of absorption peaks (λmax) from 546 to 504 nm. This is because the band gap energies of NAC-capped CdMnTe QDs increase with increasing Mn/Cd atomic ratios [34]. In order to get the NAC-CdMnTe QDs with both high fluorescence efficiency and better utilization ratio of Mn, the optimum Mn-to-CdCl2 molar ratio is 1.0. When fixing the Cd:Mn:NAC molar ratio at 1.0:1.0:2.4 and increasing the addition amount of Te, the QY reached the highest when the Te:Cd ratio is 0.2:1.0 (Fig. 2b). The enhancement of the Te:Cd ratio leads to a higher Te content in as-prepared QDs. The existence of a dangling bond of Te on the surface of QDs and the easy proneness of oxidation create many nonradioactive combination pathways, leading to low QY [30]. The increase of the Te:Cd molar ratio from 0.1 to 0.5 did not produce obvious impact on the shift of emission peaks, which maintained near 545 nm, showing no occurance of obvious particle size increase.
3.1.3. Influence of reaction time The optimum reaction conditions are the following: the molar ratio of Cd:Mn:NAC:Te is 1.0:1.0:2.4:0.2, pH is 9.5, and reaction temperature is 200 1C. On changing the reaction time (30–50 min), the emission peaks (λem) of CdMnTe QDs shift from 530 to 646 nm, and the average quantum yields were approximately 50% (the highest QY reached 63.1%). The corresponding full width at
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half-maximum (FWHM) of the band-edge luminescence were from 39 to 57 nm (Fig. 3a), and the absorption peaks (λmax) moved from 487 to 609 nm when the reaction time increased (Fig. 3b). As shown in the inset of Fig. 3, the emission color under UV irradiation turned from green to yellow, orange, and finally red with the increase of reaction time. 3.2. The characterization of NAC-capped CdMnTe QDs 3.2.1. TEM The morphology of the as-prepared NAC-capped CdMnTe QDs (λem ¼551 nm) was studied through TEM. It is quite evident that these nanoparticles are close to spherical with good dispersion and are uniform with an average diameter of 8.5 7 0.4 nm (Fig. 4).
Fig. 4. TEM image of NAC-capped CdMnTe QDs (λem ¼ 551 nm) reacted for 35 min.
Fig. 5. EDS spectrum of NAC-capped CdMnTe QDs (λem ¼551 nm) at the molar ratio of Cd:Te:Mn ¼ 1:0.2:1.
3.2.2. EDS and XPS The EDS technique has been employed to probe a semiquantitative picture of the composition of the NAC-capped CdMnTe QDs. It must be emphasized that, for the nano-scaled QDs, the element composition obtained through EDS (whose penetration depth of the electron beam is more than 100 nm) must be the bulk composition of multilayered samples of QDs [30]. As shown in Fig. 5, for prepared NAC-capped CdMnTe QDs with molar ratio of Cd:Mn:Te¼ 1.0:1.0:0.2 (λem ¼551 nm), the atomic percentages of Cd, Mn, and Te reach 7.38%, 9.72%, and 1.93% respectively. The calculated constituent ratio of Cd to Mn and Te in the prepared QDs is therefore 1:1.32:0.26, which is very close to the feed ratio. Meanwhile, as a quantitative surface analysis tool sensitive to the atomic composition of the outermost 100 Å of the sample surface [35], XPS has been employed to analyze the chemical composition of CdMnTe QDs. Fig. 6 displays the Cd 3d5/2 peak and characteristic Mn 2p3/2 peaks of the prepared CdMnTe QDs with molar ratio of Cd:Mn:Te ¼1.0:1.0:0.2 (λem ¼551 nm) and Cd:Mn: Te¼1.0:0.5:0.2 (λem ¼573 nm). As the concentration of Mn increases, the width of the Cd 3d5/2 peak becomes broader (from 3.32 to 3.36 eV) and its position shifts to a lower energy (from 404.63 to 404.53 eV) (Fig. 6a). Compared to the binding energy of the Cd atoms bonded to the Te atoms, which of the Cd atoms bonded to the Mn atoms would be expected to appear at a lower energy. This is because the electronegativities (χ) of Te (χP ¼2.10), Cd (χP ¼ 1.69), and Mn (χP ¼1.55) are in descending order. Fig. 6b displays the corresponding Mn 2p3/2 peak of CdMnTe QDs. It is obvious that the peak shifts to a lower energy region (from 641.64 to 640.51 eV) with increasing Mn concentration. The explanation might be analogous to that of Cd 3d5/2 peak, which can also be
Fig. 6. (a) Cd 3d XPS spectrum and (b) Mn (2p) XPS spectrum of the NAC-capped CdMnTe QDs with ratios of Mn to Cd were 0.5 and 1.0, respectively.
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Fig. 7. (a) PL spectra of NAC-capped CdMnTe QDs (λem ¼551 nm) at various pH values and (b) photostability of NAC-capped CdMnTe QDs (λem ¼ 551 nm).
explained by the incorporation of the less electronegative Mn (than Cd) and the formation of Mn-Cd bonding structures [36]. 3.2.3. AAS EDS is a semi-quantitative characterization tool, merely exhibiting the composition of QDs surface elements, while AAS aims to measure the concentration of each element in the QDs after being dissolved in strong acid [17]. So the latter is used to examine the accurate quantitative composition of each element of whole CdMnTe QDs. When the feed ratios of Mn to Cd being fixed at 1:1, the actual molar ratio of Mn to Cd in CdMnTe QDs with emission peak at 551 nm is 1.166:1.00, which is in accordance to the feed ratio. 3.3. Physicochemical properties of NAC-capped CdMnTe QDs Fig. 7a displays the pH effect on the emission intensity of NACcapped CdMnTe QDs (λem ¼551 nm). NAC-capped CdMnTe QDs exhibit better stability in neutral or alkaline solution but the intensity dropped dramatically at lower pH. This is due to the protonation of the thiol moiety of both types of QDs under acidic conditions, leading to the detachment of the capping agent from the nanocrystals. As such, the QD-ligand complexes are destroyed and consequently decrease their PL intensity. The carboxylic acid moiety of NAC is deprotonated in neutral and basic aqueous solutions. Especially under high pH conditions, the negative charges of carboxylate groups located on the surface of the QDs repel each other, thus preventing aggregation. This interaction is beneficial to the stabilization of QDs and leads to higher PL efficiency [32]. The fluorescence of NAC-capped CdMnTe QDs reaches a maximum at pH around 9.5 and remains constant up to pH 11.5. To examine the photostability of the prepared QDs, the CdMnTe QDs (λem ¼551 nm) at an extremely low concentration (abs¼ 0.01) in Tris–HCl buffer (0.05 mol/L, pH ¼8) were irradiated continuously by a xenon lamp (16 W). As shown in Fig. 7b, the fluorescence decreased only 17% of the original emission within 120 min, exhibiting excellent photostability of the prepared QDs. 4. Conclusions With NAC as the stabilizer, high-quality water-soluble NACcapped CdMnTe QDs have been synthesized through hydrothermal
route, and the prepared CdMnTe QDs possess high QYs and narrow PL spectra. Through a series of optimizing experiments, the optimal reaction conditions are found out: molar ratio of Cd:Mn: NAC:Te is 1.0:1.0:2.4:0.2, pH is 9.5, and reaction temperature is 200 1C. The emission peak is tunable from 530 to 646 nm by controlling the reaction time of synthesis (30–50 min). We studied their morphology through TEM, and XPS analysis indicated that Mn atoms have been successfully doped into the nanocrystals. In particular, we examined the feed ratio and the actual constituent ratio of the prepared QDs through EDS and AAS.
Acknowledgments This research was supported by the National Science Foundation of China (21105130).
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