Ligand-assisted fabrication, structure, and luminescence properties of Fe:ZnSe quantum dots

Materials Science and Engineering B 182 (2014) 86–91

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Ligand-assisted fabrication, structure, and luminescence properties of Fe:ZnSe quantum dots Ruishi Xie ∗ , Xingquan Zhang, Haifeng Liu Analytical and Testing Center, Southwest University of Science and Technology, Mianyang 621010, China

a r t i c l e

i n f o

Article history: Received 12 June 2013 Received in revised form 18 November 2013 Accepted 29 November 2013 Available online 17 December 2013 Keywords: Nanostructures Chemical synthesis X-ray diffraction Optical properties

a b s t r a c t Here, we report a synthetic route for highly emissive Fe:ZnSe quantum dots in aqueous media using the mercaptoacetic acid ligand as stabilizing agent. The structural, morphological, componential, and optical properties of the resulting quantum dots were explored by the X-ray diffraction, transmission electron microscopy, energy-dispersive X-ray spectroscopy, inductively coupled plasma mass spectrometry, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, photoluminescence and UV–visible absorption spectroscopies. The average crystallite size was calculated to be about ca., 4.0 nm using the Scherrer equation, which correlates well with the value obtained from the transmission electron microscopy analysis. The obtained water-soluble Fe:ZnSe quantum dots in the so-called “quantum confinement regime” are spherical shaped, possess the cubic sphalerite crystal structure, and exhibit tunable luminescence properties. The presence of mercaptoacetic acid on the surface of Fe:ZnSe quantum dots was confirmed by the Fourier transform infrared spectroscopy measurements. As the ligand/Zn molar ratio increases from 1.3 to 2.8, there is little shift in the absorption peak of the Fe:ZnSe sample, indicating that the particle size of the obtained quantum dots is not changed during the synthetic process. The photoluminescence quantum yield of the as-prepared water-soluble Fe:ZnSe quantum dots can be up to 39%. The molar ratio of ligand-to-Zn plays a crucial role in determining the final luminescence properties of the resulting quantum dots, and the maximum PL intensity appears as the ligand-to-Zn molar ratio is 2.2. In addition, the underlying mechanism for the resulting tunable luminescence properties of the obtained Fe:ZnSe quantum dots was also elucidated. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.

1. Introduction Colloidal semiconductor nanocrystals, which are also considered as quantum dots (QDs), have been the subject of extensive experimental and theoretical investigations over the last three decades because of their unusual optical and electronic properties [1]. These novel properties render them desirable for various technological applications in a wide range of fields, including optoelectronics, catalysis, photovoltaics, and biomedicine [2–7]. The most widely studied QDs include those of cadmium and lead chalcogenide semiconductors owing to their highly efficient and continuously tunable emission, and their well-established synthetic chemistry [8–12]. In contrast, the considerable interest in these materials is diluted by the fact that cadmium and lead are highly toxic metals. The use of QDs with Cd and Pb has thus become a major concern in practical applications as it poses potential risks

∗ Corresponding author at: Analytical and Testing Center, Southwest University of Science and Technology, 59 Middle Qinglong Avenue, Mianyang, China. Tel.: +86 816 6089509; fax: +86 816 6089508. E-mail address: [email protected] (R. Xie).

to human health and the environment. This has motivated the search for alternative semiconducting materials that are not only technologically useful but are also environmentally benign. Binary II–VI semiconductors (II = Zn and VI = O, S, Se;) and their doped nanostructures become the most suitable alternatives to Cd- and Pb-based semiconductors because of their light-emitting and solar-harvesting properties as well as their low toxicity. In this family of semiconductors, considerable attention has been given to the O- and S-based systems (e.g., ZnO, ZnS) and their doped nanostructures due to their potential applications as photocatalysts and active components in light emitting diodes and solar cell devices [13–18]. On the other hand, the Se-based system (ZnSe) is rarely reported. In this work, our attention has been given to ZnSe which belongs to be a directgap semiconductor with a bulk band gap of 2.70 eV [19]. In addition, ZnSe-based nanocrystals have demonstrated remarkable optical properties and have shown great promise in biolabeling and bioimaging applications [20,21]. ZnSe nanocrystals have been previously prepared using a variety of synthetic strategies [22,23]. For example, the modification with transition-metal elements (e.g., Mn doping) is often used to tune the photoluminescence properties of ZnSe by forming

0921-5107/$ – see front matter. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mseb.2013.11.023

R. Xie et al. / Materials Science and Engineering B 182 (2014) 86–91

the doped ZnSe nanostructures is often done to manipulate the photoluminescence properties of ZnSe. For instance, Norris et al. have prepared doped ZnSe QDs with high crystallinity and efficient emission through the injection of dimethylmanganese and dimethylzinc into hot hexadecylamine at 310 ◦ C [24]. The photoluminescence performance of these doped ZnSe QDs could be significantly improved by adding Mn [25]. By controlled overcoating of ZnSe layer onto pre-formed MnSe nanoclusters in hot organic solvents, Pradhan and co-workers obtained highly emissive and stable Mn-doped ZnSe nanocrystals [25]. More recently, Wu et al. reported the hot-injection synthesis of Mn doped ZnSe QDs in the presence of 1-octadecene as reaction solvent, and these Mn doped ZnSe QDs exhibit shape and crystal phase-dependent photoluminescence properties [26]. Yang et al. have synthesized ZnSe and Fe-doped ZnSe QDs by a microemulsion-mediated hydrothermal method; the resulting QDs have a zinc blende crystal structure, optimal spherical shape, nearly monodispersed and controlled in their Fe/Zn ratio, and the band-edge emission peak is systematically blue-shifted with an increase of Fe concentration [27]. Unfortunately, the above-mentioned synthetic methods, involve the traditional high-temperature organic-based approach, resulting in hydrophobic nanocrystals. To effectively use in biological applications, it is necessary to make the nanocrystals water-dispersible and biocompatible. For realizing this objective, the phase transfer from organic to aqueous media is derived by coating the hydrophobic nanocrystals with amphiphilic materials functionalizing the surface with biomolecules to make it suitable for bioconjugation [28]. However, these additional steps make the whole preparation procedure a very tedious process. Furthermore, the luminescence intensity of the surface-modified QDs is notably lower after the phase transfer, that is, the luminescence intensity of the blue-emitting doped ZnSe QDs substantially decreases after the solubilization in water [20]. Very recently, some efforts have been devoted to the aqueous synthesis of doped ZnSe QDs, but they are limited by either low quantum yields or long production times. For example, Aboulaich et al. have described the preparation of water-dispersible 1-thioglycerol capped Mn-doped ZnSe QDs a PL quantum yield of 3.5% in aqueous solution through the nucleation-doping method [29]. Therefore, it is of great importance to develop a simple and efficient aqueous synthesis route for high-quality doped ZnSe QDs. Herein, we report a facile and green synthetic approach in the preparation of photoluminescent water-soluble Fe doped ZnSe (Fe:ZnSe) QDs directly in aqueous media in the presence of mercaptoacetic acid ligand. Highly reproducible synthesis of Fe:ZnSe QDs with tunable PL properties could readily be achieved. The obtained Fe:ZnSe QDs possess good crystallinity, cubic sphalerite crystal structure. The molar ratio of ligand mercaptoacetic acid to Zn plays a crucial role in determining the final luminescence properties of the Fe:ZnSe QDs. In addition, the underlying mechanism for the resulting tailorable luminescence properties of the as-prepared Fe:ZnSe QDs was also addressed. As compared with other fabrication approaches in aqueous phase (e.g., template approach [30], liquid-solid-solution method [31], solvothermal approach [32], microemulsion-mediated hydrothermal method [33], co-precipitation approach [34], microwave-assisted aqueous synthesis method [35]), the protocol demonstrated here offers the following advantages: (1) use of environmental-friendly reagents, (2) cheaper and simpler, (3) lower reaction temperature (100 ◦ C) to obtain QDs with comparable PL QY and tunable fluorescence, (4) the surface functionalization with water-soluble ligand occurs during the synthesis, and (5) a small diameter of the obtained QDs, which is suitable for biological applications.

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2. Experimental Mercaptoacetic acid (MAA), selenium powder (Se), zinc acetate dihydrate (Zn (CH3 COO)2 ·2H2 O), ferrous sulphate heptahydrate (FeSO4 ·7H2 O), and sodium borohydride (NaBH4 ) were analytical grade and used without further purification. Double distilled water was prepared using an UPH-IV-10T UP water purification system. Preparation of NaHSe: Se powder (100 mmol) was mixed with deionized water (10 mL) in a 100 mL three-neck flask. A 20 mL aqueous solution of NaBH4 (10 M) was carefully added to this mixture and the flask was immediately purged with nitrogen. After 1 h at room temperature, the black Se powder disappeared and the virtually colorless NaHSe solution was obtained. In a typical experiment, 25 mL FeSO4 solution (0.2 M) was loaded into a 250 mL three-neck flask and degassed for 0.5 h by bubbling with nitrogen. Freshly prepared NaHSe solution was added to the N2 -saturated FeSO4 solution at pH 10 in the presence of MAA as stabilizing reagent. The reaction was then switched from nitrogen bubbling to nitrogen flow and subjected to a reflux at 110 ◦ C for 1 h. 40 mL of Zn (CH3 COO)2 stock solution (5 M) was added and the reaction was refluxed for 5 h. The feeding ratios of Fe-to-Se-to-Zn and Zn-to-MAA were 1:25:50 and 1:2.5, respectively. Finally, the reaction mixture was allowed to cool down to room temperature. The resulting Fe:ZnSe QDs were precipitated and washed several times with 2-propanol, and dried in vacuum. X-ray diffraction (XRD) measurement of the QDs was carried out using a PANalytical X’Pert PRO X-ray diffractometer with Cu K␣ ˚ operating at 40 kV and 40 mA. To prepare radiation ( = 1.54187 A) a powder XRD sample, a highly concentrated nanocrystal solution was deposited on a glass substrate and dried in a vacuum desiccator at room temperature. The size and morphology observations were performed using a JEOL JEM-2010 transmission electron microscope (TEM) operating at 200 kV. Samples for TEM characterization were prepared by dispersing powders on a carbon-coated copper grid. The compositional analysis for the as-prepared sample was performed using an energy-dispersive X-ray (EDX) spectroscopy system, of an accessory of FEI Inspect F scanning electron microscope. X-ray photoelectron spectroscopy (XPS) analysis was carried out using a Kratos XSAM 800 X-ray photoelectron spectrometer with Mg K␣ radiation (1253.6 eV). Quantitative elemental analysis for the amount of Fe in doped ZnSe QDs was performed using an Agilent 7700x inductively coupled plasma mass spectrometer (ICPMS). FTIR spectrum was recorded with a Thermo Scientific Nicolet 6700 Fourier transform infrared spectrometer. The FTIR sample was prepared by pressing the mixture of Fe:ZnSe QDs and potassium bromide (KBr) into a thin pellet. UV–vis absorption and PL spectra of QDs were obtained with a Unico UV-2101PC ultraviolet–visible spectrometer and a Hitachi F-7000 fluorescence spectrometer, respectively. Excitation wavelength was set at 280 nm. All of the optical measurements were performed under ambient conditions. The samples were diluted with deionized water and placed directly in quartz cuvettes (1 cm path length) for characterization, without any size sorting. The photoluminescence quantum yields (PL QYs) of QDs were estimated using quinine sulfate in aqueous 0.05 M H2 SO4 as the PL reference.

3. Results and discussion To confirm the crystal structure, phase purity and crystallite size of the resulting QDs, the XRD analysis was conducted. Fig. 1 illustrates the XRD patterns of the synthesized Fe:ZnSe and undoped ZnSe QDs. The positions of three main peaks respectively correspond to (1 1 1), (2 2 0), and (3 1 1) planes of cubic sphalerite crystal structure, matching the standard card (JCPDS no. 80-0021). No other impurity phases are observed in the XRD pattern of Fe:ZnSe

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Fig. 1. XRD patterns of the as-fabricated (a) Fe:ZnSe and (b) undoped ZnSe QDs.

QDs, indicating the formation of a pure cubic phase of ZnSe only. In fact, no diffraction peaks corresponding to Fe precipitates or Fe-related impurity phase are observed, which may indicate the formation of Fe:ZnSe solid solution instead of Fe precipitation or second phase, to some extent indicating that the Fe ions has been doped into the lattice of ZnSe host. In addition, the broadening of diffraction peaks shows the formation of QDs. Using the full width at half maximum (FWHM) of the three main XRD peaks and Scherrer equation, the mean crystallite size of Fe:ZnSe QDs was estimated to be about 4 nm, which is much smaller than the Bohr exciton diameter (9.0 nm) of bulk ZnSe [36], showing that these QDs of this work are in the strong quantum confinement regime. The inter˚ for Fe:ZnSe QDs, and the planar spacing (d111 ) is about 3.245 A, ˚ Therefore, interplanar spacing (d111 ) of undoped ZnSe is 3.244 A. ˚ with the smaller radius the substitution of Zn2+ ions (rZn 2+ = 0.74 A) ˚ results in a small increase in the interplanar Fe2+ ions (rFe 2+ = 0.72 A) spacing of Fe:ZnSe QDs. To provide insight into the size and morphology of the asprepared QDs, the TEM analysis was carried out. Fig. 2 presents the typical TEM image and the corresponding size distribution diagram of the Fe:ZnSe QDs. We can see that the average particle diameter of Fe:ZnSe QDs is 4.4 ± 0.5 nm, which is consistent with the particle size determined by the XRD analysis. Moreover, the TEM observation of this work also confirms that the obtained QDs are spherical-shaped, crystalline, and nearly monodispersed. In order to confirm the composition of the Fe:ZnSe sample, the EDX spectroscopy analysis was conducted, as shown in Fig. 3. The very strong peaks for Zn and Se are found in the spectrum. Also, a detectable amount of Fe in the spectrum indicates that the Fe impurity was incorporated into the ZnSe matrix. Furthermore, the peaks for O, C and S are ascribed to the presence of MAA. The doping concentration of iron ions in the Fe:ZnSe sample was calculated to be 1.28 at.%, which is less than the initial Fe-doping concentration of the reaction solution. To determine the doping concentration of Fe ions more precisely, the ICP-MS analysis was performed. Three trials of the sample measurements reveal that the average elemental proportion of the Fe ions relative to ZnSe parent material was 1.57 at.%, matching the result obtained from the EDX analysis. To further confirm whether or not the Fe element has been doped into the ZnSe lattice, the surface composition and chemical state were explored by XPS analysis, as delineated in Fig. 4. The characteristic peaks of Fe element are not as strong as those of the Zn and Se elements, which could be attributed to the addition of Fe ions into the interior of particles. In addition, the binding energies of 709.3 and 723.1 eV correspond to the peaks of Fe 2p3/2 and Fe

Fig. 2. (a) TEM image of the as-prepared Fe:ZnSe QDs, the inset presents the magnified image of a particle. (b) The corresponding size distribution diagram of the Fe:ZnSe QDs.

2p1/2 [37], respectively, indicating the Fe was successfully doped into the ZnSe host lattice as a divalent ion. Recently, protocols have been explored for surface functionalization of QDs with organic substances or polymers in order to make QDs resistance to agglomeration or make them bio-compatible [38,39]. In the present work, we used the MAA as the capping agent and the stabilizer to control the growth of the Fe:ZnSe QDs during the synthesis process. The FTIR spectroscopy gives qualitative information about the manner in which the adsorbed ligand

Fig. 3. EDX spectrum of the as-synthesized Fe:ZnSe QDs.

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Fig. 6. UV–vis absorption spectra of the Fe:ZnSe QDs synthesized at different ligandto-Zn ratios.

Fig. 4. XPS survey profile of the obtained Fe:ZnSe QDs (bottom), high-resolution binding energy diagram of Fe 2p for the Fe:ZnSe QDs (top).

molecules are bound to the surface of the Fe:ZnSe QDs. Fig. 5 represents the FTIR spectra of MAA and the resulting Fe:ZnSe QDs. One can see a number of characteristic spectral bands such as the O H stretching vibration at 3397 cm−1 , the O H deformation vibration at 925 cm−1 , the COO– asymmetric vibration at 1572 cm−1 , the COO– symmetric vibration at 1386 cm−1 , the C O stretching vibration at 1226 cm−1 , and the C S stretching vibration at 657 cm−1 . Nevertheless, the vibration peaks of S H (ca., 2670 and 2568 cm−1 ) are absent in the FTIR spectrum of the obtained Fe:ZnSe QDs, resulting from the covalent bonds between sulfydryls ( SH) and Zn atoms on the surface of Fe:ZnSe QDs, which indicates that MAA as the stabilizer capped the Fe:ZnSe QDs. Meanwhile, the free carboxylic acid group exists as carboxylate ion and makes the capped Fe:ZnSe QDs soluble in water. The UV–vis absorption spectroscopy is a useful technique to examine the optical properties of the quantum-sized particles. Normally, the wavelength at the maximum exciton absorption decreases as the size of nanoparticles decreases, because of a consequence of quantum confinement of the photogenerated electron-hole carriers. The UV–vis absorption spectra of

MAA-stabilized Fe:ZnSe QDs synthesized at different ligand-to-Zn ratios are exhibited in Fig. 6. As the ligand/Zn molar ratio increase from 1.3 to 2.8, the absorption peak shows little shift, thus indicating that the particle size of the obtained QDs is not changed during the synthetic process. The absorption peaks of the samples are blueshifted as compared with the corresponding band gap (2.70 eV) of bulk ZnSe. This large blue shift could be attributed to the quantum confinement effect. The properties of nanocrystalline materials deviate from the corresponding bulk properties when the sizes of the crystallites become smaller than the excitonic Bohr radius (aB ),

aB =

4εh2 e2



1 1 + ∗ m∗e mh

 (1)

where, ␧ is the dielectric constant, and me * and mh * are the effective masses of the electron and hole, respectively. From Eq. (1), the excitonic Bohr radius of ZnSe can be determined using the following values of ␧ = 9.1, me * = 0.15 m0 and mh * = 0.66 m0 . The excitonic Bohr radius of ZnSe is ca., 4.5 nm (when the particle size is smaller than the excitonic Bohr radius, a large percentage of the atoms are present in the particle surface, thus modifying the optical properties of the particles). In the present work, the particle size was calculated using the effective mass approximation,

 Egn = Egb +

2h2 Egb (/R) m∗

2

1/2 (2)

where, R is the radius of the particles, Egn and Egb are the band gaps of nano and bulk systems, respectively, and m* is the effective electron and hole mass (m∗ = 1/m∗e + 1/m∗h ). The fundamental absorption, which corresponds to the transition from the valence band to the conduction band, can be used to determine the band gap of the material. According to the Tauc relation [40], the relation between absorption coefficient and band gap energy can be written as follow ˛=

Fig. 5. FTIR spectra of (a) MAA and (b) the resulting Fe:ZnSe QDs.

A(h − Egn ) h

n

(3)

where, h is the photon energy, ˛ is the absorption coefficient, A is a constant, Egn is the band gap of the nanoparticles, and the exponent n depends on the type of transition. The n may have values 1/2, 2, 3/2, and 3 corresponding to the allowed direct, allowed indirect, forbidden direct and forbidden indirect transitions, respectively. The shape of the absorption edge is solely due to the electronic

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Fig. 7. Plot of (˛h)2 versus h␯ for the as-synthesized Fe:ZnSe QDs.

transition from the top of the valence band to the bottom of the conduction band [41]. For ZnSe, n = 1/2. By applying the above equation to the absorption spectrum, a relationship curve of (˛h)2 against h is displayed in Fig. 7, the band gap value (Egn ) for Fe:ZnSe QDs was calculated to be 3.10 eV, which is higher than that of bulk ZnSe (2.70 eV). This obvious blue-shift is due to the strong influence of the quantum confinement of the QDs. Based on the effective mass approximation, the particle diameter was calculated to be 4.1 nm for MAA-stabilized Fe:ZnSe QDs. The particle size is smaller than the excitonic Bohr diameter of bulk ZnSe. Consequently, a large blue shift was observed in the absorption peaks for Fe:ZnSe QDs. In recent years, some researchers have manipulated the optical properties of nanoparticles. Calandra synthesized stable nickel nanoparticles by a route based on the reduction of NiCl2 ionic clusters in the confined space of reversed micelles, the author tuned the optical properties of nickel nanoparticles by exploiting different ligand (alkyl-mercaptane)-to-metal ratio, the synthesized lipophilic Ni nanoparticles could be solubilised in any apolar solvent and at a high concentration, and the Ni nanoparticles displayed novel UV–vis extinction peaks [42]. The molar ratio of ligand MAA to Zn should have a significant effect on the optical properties the Fe:ZnSe QDs by applying the stabilizing agent for the resulting QDs. To manipulate the PL properties of the obtained QDs, we varied the molar ratios of ligand-to-Zn from 1.3 to 2.8, while kept the other experimental variables fixed. Since MAA serves as a stabilizer for the Fe:ZnSe QDs, Fe and Zn precursors, when the MAA/Zn ratio approaches 1 (in our experiment less than 1.3), the Zn precursor solution turns turbid at room temperature. Hence we gave up the experiment of MAA/Zn ratio less than 1.3. We can observe from Fig. 8 that the molar ratios of ligand-to-Zn in precursor solution has a dramatic effect on the PL intensity of Fe:ZnSe QDs. From Fig. 8, the PL intensity of the resulting QDs increases as the molar ratio of ligand-to-Zn in precursors decreases from 1.3 to 2.2, while the PL intensity will decrease when the molar ratio exceeds 2.2. By forming the Zn-thiol surface layers at the QD surface, the stabilizer MAA plays an important role in the passivation, such as preventing nanoparticles aggregation and precipitation. The occupation of surface sites by stabilizer molecules instead of Se atoms conduce to not only the formation of a favorable structure of QD for removing the dangling bonds of Se atoms from the surface, but also the prevention of the oxidation of Se atoms. For aqueous synthesis of thiol-stabilizing Fe:ZnSe QDs, the concentration of monothio-Zn complex in precursor solution was believed to have an important effect on the PL intensity of the obtained QDs. When the molar ratio between ligand and Zn in

Fig. 8. PL spectra of the Fe:ZnSe QDs synthesized at different ligand-to-Zn ratios.

precursor solution is close to 2, there is less monothio-Zn complex than dithio-Zn complex in precursor solution and the PL intensity of Fe:ZnSe QDs becomes weaker. As for our results in Fig. 8, the complex of monothio-Zn increases and the PL intensity of Fe:ZnSe QDs enhances with the decrease of the molar ratio of ligand-to-Zn in precursors. Simultaneously, the tendency has a natural limit at a very low value of the ligand/Zn molar ratio (approximate 1) because of the insufficient amount of MAA stabilizer in the system. In fact, a competition of at least two different factors exists during the QDs growth process. On the one hand, decreasing the molar ratio of ligand-to-Zn will promote the formation of monothio-Zn complex for getting high quality Fe:ZnSe QDs. On the other hand, MAA must be ensured sufficient to provide stability and surface passivation of growing QDs during the synthesis process. The abovementioned two requirements can be satisfied by controlling the molar ratio of ligand-to-Zn at 2.2. As a result, the maximum PL intensity appears as the ligand-to-Zn molar ratio is 2.2 of this work. We also investigated the influence of the molar ratio of ligandto-Zn on the PL QYs of Fe:ZnSe QDs. As shown in Fig. 9, the ligand-to-Zn molar ratio plays an important role in PL QY of the Fe:ZnSe QDs. Although Fe:ZnSe QDs with high QY (ca., 11%) can be obtained when the ratio of ligand-to-Zn is 1.3, the product will precipitate in a few days owing to the lack of sufficient repulsive force among Fe:ZnSe QDs provided by the COO− groups of the ligands

Fig. 9. PL QYs of the Fe:ZnSe QDs synthesized at different ligand-to-Zn ratios.

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absorbed on the Fe:ZnSe QD surface. When the ratio of ligand-to-Zn is 2.2, the Fe:ZnSe QDs not only have a high QY of 39%, but also can be preserved for dozens of days with no weakening in fluorescence intensity. However, the QYs of the resulting QDs decreased dramatically with the further increase of the ligand-to-Zn molar ratio. Because that the excessive increase of the ligand-to-Zn molar ratio weakens the PL intensity. Although MAA ligands can act as useful stabilizers, the photoluminescence is also quenched for the Fe:ZnSe QDs. Because of the conflict between colloidal stability and quenching of photoluminescence, the optimized ligand-to-Zn molar ratio is 2.2, at which the PL intensity is relatively high. 4. Conclusions We report here a facile and green strategy for the synthesis of highly fluorescent and water-soluble Fe:ZnSe QDs in aqueous media in the presence of MAA. The resulting Fe:ZnSe QDs are ultrafine with an average diameter of about 4 nm, possessing cubic sphalerite crystal structure. The band gap of MAA-stabilized Fe:ZnSe QDs was estimated to be 3.10 eV, manifesting the influence of strong quantum confinement. In addition, the PL intensity of Fe:ZnSe QDs can be remarkablely manipulate when utilizing different ligand-to-Zn molar ratios, and the maximum fluorescent emission intensity appears as the ligand-to-Zn molar ratio reaches 2.2. The PL QY of the as-prepared water-soluble Fe:ZnSe quantum dots can be up to 39%. The current investigation provides a useful synthetic route for producing water-soluble and fluorescent Fe:ZnSe QDs, which can be applied in biology, optical coding, or optoelectronic devices. The further work is under progress to broaden the synthetic protocol for additional QDs classes, such as alloyed QDs, and Cd-free doped QDs like Co-doped ZnSe. Acknowledgments The authors gratefully acknowledge financial support from the Doctoral Foundation of Southwest University of Science and Technology (no. 11zx7137). The authors are also grateful to Associate Prof. Jiagang Wu and six anonymous reviewers for their very constructive suggestions to improve the original manuscript. References [1] D.V. Talapin, J.S. Lee, M.V. Kovalenko, E.V. Shevchenko, Chem. Rev. 110 (2010) 389–458.

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