Luminescent and Magnetic Properties in Semiconductor Nanocrystals

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Luminescent and Magnetic Properties in Semiconductor Nanocrystals with Radial-Position-Controlled Mn2+ Doping Boping Yang, Xingchao Shen, Huichao Zhang, Yiping Cui, and Jiayu Zhang* Advanced Photonics Center, Southeast University, Nanjing 210096, P. R. China S Supporting Information *

ABSTRACT: Colloidal nanocrystals (NCs) with radial-position-controlled doping were synthesized to study the effect of the binding symmetry around Mn2+ dopant. For the four samples ZnSe:Mn/ZnSe, ZnSe/ZnS(2 ML):Mn/ZnS(2 ML), ZnSe/ZnS(1 ML):Mn/ZnS(2 ML), and ZnSe:Mn/ZnS(2 ML), which were in sequence of binding asymmetry around Mn2+ dopant, their photoluminescent (PL) peak showed gradual redshift (579 to 599 nm) and the PL lifetime became monotonously shorter (0.57 to 0.31 ms), while, as indicated in the electronic paramagnetic resonance spectra, the hyperfine splitting constant became larger (67.9 to 68.4 G) and the g factor became smaller (2.0076 to 2.005). The relation between the luminescent and magnetic properties of the Mndoped NCs was discussed.

due to their unpaired electrons.11−17 In Mn-doped ZnSe NCs, the superfine splitting constant (A) of internal Mn was reported to be larger than that of external Mn.14,15 There were similar results in Mn-doped ZnS,12 CdSe,17 and CdS/ZnS NCs.13 The blue-shift of emission that resulted from external Mn mentioned in ref 15 indicated that EPR spectra may reflect the PL mechanism. EPR parameters (hyperfine splitting constant A and g factor) were considered to illuminate the covalency between Mn2+ ions and the anions (Se2− or/and S2−).11,14,17 In a Mn-doped ZnS NC sample reported previously, the A value was changed from 83.3 to 87 G, indicating that the covalency of Mn−S became weaker.11 Analogous results were reported in Mn-doped ZnSe14 and Mndoped CdSe NCs.17 It has been well-known that covalent bonds exist due to the electronegative difference between the Mn2+ ions and the anions (Se2− or S2−). There should be some relationship between the intensity of the crystal field and the covalency in Mn-doped ZnSeS NCs, which is not very clear so far.18 As we know, the luminescent properties of the Mn-doped semiconductor are significantly influenced by the crystal field around the ions,6,7 so there should be some relationship between EPR and PL spectra. For better understanding the influence of the binding symmetry on the luminescent properties and the relation between EPR spectra and PL properties, four Mn-doped NC samples with different binding symmetry were synthesized in this work, and EPR was used to discriminate the small change of local environment around Mn. When the binding symmetry became lower, the PL peak shifted from 579 to 599 nm, the PL

1. INTRODUCTION Semiconductor nanocrystals (NCs) doped with transition metal ions have attracted great attention because they may generate a new emissive material with high efficiency, little self-absorption due to large Stokes shift, and magnetic properties absent in undoped NCs.1−4 For better understanding the luminescent behaviors of Mn-doped NCs, precisely controlled Mn-doping in CdS/ZnS NCs has been synthesized through a layer-by-layer method, and it was found that the properties of Mn-doped NCs could be modulated by the position of Mn2+ ions.5−7 In the Mn-doped core/shell ZnSe NCs, a change of luminescent color from orange-red to orange was observed along with the increase of the ZnSe shell thickness,6 and it was suggested to result from smaller splitting of the first excited state of Mn2+ ions due to a more symmetric lattice field, but in a similar shellovercoated Mn-doped ZnSe NC sample, with increasing shell thickness, a photoluminescence (PL) red-shift from 575 to 595 nm was reported.7 In Mn-doped CdS/ZnS NCs, when Mn2+ ions were at the interface between the CdS core and the ZnS shell, Mn emission showed a red-shift from 621 to 633 nm when shell thickness was increased to 7.5 monolayers (MLs),8 and it was proposed that this red-shift was proportional to the pressure of the ZnSe shell and irrespective to local environment. Compared with the PL lifetime of 1.8 ms in Mn-doped ZnS, that in Mn-doped ZnSe is 0.19−0.29 ms, and this shortening was interpreted as stronger spin−orbit coupling resulted from heavier Se2− than S2− ion.9 In Mn-doped ZnS, the PL lifetime of Mn at/near the surface was shorter than that of internal/substitutional Mn, which was proposed to be resulted from enhanced transition strength due to lower binding symmetry.10 Electron paramagnetic resonance (EPR) spectra have often been used to detect the local environment around Mn2+ ions © XXXX American Chemical Society

Received: March 1, 2013 Revised: June 28, 2013

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2.3. Characterization. Absorption and PL spectra were measured with a Shimazu UV3600 spectrophotometer and a FLS920 F900 luminescence spectrometer (Edinburgh), respectively. Transmission electron microscopy (TEM) data were measured with a Tecnai G2 Transmission Electron Microscope (FEI). X-ray diffraction (XRD) spectra were recorded on a D/ max 2500VL/PC diffractometer using Cu Kα radiation. EPR spectra were measured with an X-band EMX-10/12 spectrometer (Bruker). The lifetime decays were recorded by a twochannel color digital phosphor oscillograph (Tektronix TDS 3052), and the excitation light was from a Powerlite Precision II 8010 (Continuum) laser, whose wavelength was 355 nm. All the characterizations were carried out at room temperature.

lifetime varied from 0.57 to 0.31 ms, the A value was increased from 67.9 to 68.4 G, and the g factor was decreased from 2.0076 to 2.005. The intrinsic relation between the luminescent and magnetic properties is discussed.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Zinc stearate (ZnSt2, 12.5%∼14% ZnO), 1octadecene (ODE, tech. 90%), and selenium powder (−200 mesh, 99.999%, metal basis) were purchased from Alfa Aesar. Manganese(II) stearate (MnSt2, >95%) was purchased from Wako. Oleylamine (70%) and sulfur powder (99.98%) were purchased from Aldrich Chemistry. Tri-n-butylphosphine (TBP) was purchased from JiaCheng Chemical. All chemicals were not purified further before using. 2.2. Synthesis of the Four Mn-Doped NC Samples. We synthesized the following samples: ZnSe:Mn/ZnSe (sample I), ZnSe/ZnS(2 ML):Mn/ZnS (sample II), ZnSe/ZnS(1 ML):Mn/ZnS (sample III), and ZnSe:Mn/ZnS (sample IV). These samples were synthesized by three steps: core preparation, Mn adsorption, and shell overcoating. First, ZnSe NCs were prepared as the same as our previous report.19 For samples II and III, 2 and 1 monolayer(s) (MLs) of ZnS were respectively coated on ZnSe particles before Mn adsorption using a successive-ion-layer adsorption and reaction (SILAR) method. This ZnS was coated at 270 °C in a threenecked flask. Stoichiometric S/ODE (2.4 M) and ZnSt2/ODE (0.2 M) were dropwise injected into the solution alternately for 2 or 1 ML ZnS coating on ZnSe. The cores of these four samples were ZnSe, ZnSe/ZnS(2 ML), ZnSe/ZnS(1 ML), and ZnSe, respectively. Second, Mn adsorption: 2.4 × 10−4 mmol of cores, 9 mL of ODE, and 0.5 mL of oleylamine were loaded in a three-necked flask. After degassing for 20 min, the solution was heated to 120 °C. 1.6 mL of MnSt2/ODE (0.02 M) was injected into the solution, and the reaction lasted for 10 min under 120 °C. Then, purification was done in order to remove the residual MnSt2 in the solution. The purification was several repetitions of centrifugation with acetone. This could avoid the interfusion of Mn2+ into the shell during the shell overcoating process. In the step of SILAR shell overcoating, for sample I, 0.2 mL of Se/TBP (2.4 M) and 2 mL of ZnSt2/ODE (0.2 M) were dropwise injected into the solution alternately after the temperature rose to 270 °C. For the other three samples, 0.2 mL of S/ODE (2.4 M) and 2 mL of ZnSt2/ODE (0.2 M) were used. The shell overcoating reaction lasted for 10 min. All synthesis reactions were under an argon atmosphere. As determined by TEM images (Figure S3, Supporting Information), the shells were overcoated onto cores approximately monolayer by monolayer. Our synthesis procedure was similar to that of Yang et al.,5 and they suggested that Mn2+ ions were precisely at the location where these ions were adsorbed. Compared with the synthesis strategy of Yang et al., the temperature for the shell growth in our work was a little lower, and the adsorption temperature of our samples was 100 °C (or more) lower, which brought much less diffusion of dopants. Therefore, we suggest that Mn2+ ions were located at the corresponding interface between core and shell, analogous with the samples in ref 5. The binding symmetry of Mn2+ ions was dependent on the anions around them, which became asymmetrical as the following sequence: Samples I, II, III, and IV and the change of binding symmetry resulted from inner Mn inside NCs and external Mn on the surface, which has been mentioned by Jang et al.,10 is eliminated in our system.

3. RESULTS AND DISCUSSION 3.1. TEM Images. Figure 1 and the insets show the typical TEM images, the selective area electron diffraction pattern

Figure 1. The TEM image of sample IV. Insets a and b are the electron diffraction pattern and HRTEM images, respectively.

(SAEDP, shown as inset a), and high-resolution TEM (HRTEM, shown as inset b) images of sample IV. Three concentric rings of SAEDP corresponding to (1 1 1), (2 2 0), and (3 1 1) diffraction can be clearly observed, which are in agreement with the XRD patterns. The other three samples have similar results. The SAEDP demonstrates that the NCs are in a zinc blende structure. The average sizes from samples I−IV are 4.4, 5.7, 5.1, and 4.3 nm, respectively. HRTEM images suggest that the shell growth is the extension of the core lattice and lattice distortion is absent. The symmetric difference that resulted from lattice distortion can be neglectable. The binding symmetry is mainly affected by the anions around Mn. 3.2. XRD Patterns. The XRD spectra of the four samples are shown in Figure 2. The XRD patterns indicate that all samples are zinc blende phase, according with the results of SAEDP in TEM images. No Mn diffraction peaks are observed in all the samples, which indicate that Mn2+ ions had doped into QDs successfully and no phase transformation resulted from the doping. Compared with bulk ZnSe, the XRD peaks of samples II, III, and IV have a shift to larger angle, which is due to a smaller lattice parameter of ZnS.20 This demonstrates the B

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Figure 2. X-ray diffraction patterns for all samples (left) and HR-TEM of sample II (right). The XRD patterns corresponding to bulk cubic ZnSe (solid) and ZnS (short dot) are shown at the bottom.

successful shell growth and no new ZnSe or ZnS formation, agreeing with the TEM results. Using the Scherrer formula and the (1 1 1) peak full width at half-maximum (fwhm), the average sizes of the samples are calculated and the calculation results match well with the values measured from the TEM images. Sample II has the thickest ZnS shell, and its HRTEM images in Figure 2 indicate that the thicker ZnS shell overcoating also keeps the lattice orientation in ZnSe/ZnS(2 ML):Mn/ZnS NCs. To enable the ZnS shell to adapt the lattice parameter of ZnSe, the coherency strain exists, which should induce the shift of XRD peaks.21 The strain may cause a red-shift of PL peaks.8 According to ref 8, compared with sample I, the PL peak shift of the other three samples that resulted from the strain is 15.50, 29.55, and 48.17 meV for samples II, III, and IV, respectively. There is an obvious difference between these values and our experimental data. Therefore, some other factors may affect the PL peak shift, which will be discussed in the following text. 3.3. EPR Spectra. Figure 3 shows the EPR spectra. The well-resolved six-line hyperfine structure, which is originated from interaction between the electronic spin and the nuclear spin (I = 5/2),22 indicates that Mn2+ ions have been doped into NCs successfully. The additional 10 weak transitions between the six lines in the spectra are due to spin forbidden transition.23 Moreover, no broad background line can be observed for all the samples, demonstrating that isolated Mn2+ ions are dominant.11,24 The A values are 67.9, 68.1, 68.2, and 68.4 G for samples I, II, III, and IV, respectively, which are between the values of bulk ZnSe:Mn (66.1 G)25 and ZnS:Mn (68.4 G),26 indicating that Mn is substitutionally incorporated in the host. Additionally, the values of A for surface-bound Mn in NCs have been reported to be about 90 G or more,15−17 which is much larger than the A values in this work. This is evident that the Mn2+ ions in our samples are at the interface and the changes of A value do not result from different Mn locations (interior and exterior). The gradual increasing of the A value from samples I to IV corresponds to the lowering of the binding symmetry around Mn.15,16,23 Unlike the binding symmetry or site symmetry mentioned in the references above, the change of binding symmetry is due to different anions around Mn2+ ions in our work. Another parameter in EPR spectra is the g factor. The g value is 2.0076, 2.0066, 2.0058, and 2.0050 for samples I, II, III, and IV, respectively. The EPR line-width values of our samples are 2.1, 4.8, 6.7, and 8.1 G, respectively. The EPR line-width (ΔH) will be broadened by spin−lattice interaction and spin−spin interaction. The latter is ignored due to low Mn concentration.

Figure 3. EPR spectra of the four Mn-doped samples.

Therefore, the intensity of spin−lattice interaction is increasing from samples I to IV. Furdyna et al. considered that the EPR line of Mn in bulk II−VI semiconductors should be broadened as the atomic number of the anions increased from S2− to Se2− to Te2−.27 The EPR line-width of sample I (ZnSe:Mn/ZnSe) is smaller than the other three, which is not consistent with the results in ref 27. This may suggest that there are some other factors to change the spin−lattice interaction in NCs so that the EPR line-width is broadened in the opposite direction. We can speculate that less binding symmetry results in stronger spin− lattice interaction, broadening the EPR line-width. 3.4. Absorption and PL Spectra. Figure 4 shows the absorption and PL spectra at room temperature. A red-shift of absorption peak from sample IV to I can be observed. The band gap becomes smaller when NCs grow larger, bringing the redshift of absorption edge.28 Sample I has the largest red-shift than the others due to the smaller band gap of ZnSe than ZnS. Obvious Mn emission (from 4T1 to 6A1) can be observed. It results from partially allowed forbidden transition due to the coupling between the sp electrons of the host and the d electrons of Mn.1,4 The PL peaks are 579 nm (sample I), 588 nm (sample II), 593 nm (sample III), and 599 nm (sample IV), respectively. They are around the Mn emission in bulk ZnSe (585 nm)25 and ZnS (585 nm).1 No emission around 420 nm can be observed, indicating efficient energy transfer from the host to Mn2+ ions. Furthermore, the emission at ∼640 nm reported by another group28 is also absent in our samples. This indicates neglectable Mn−Mn interaction in our samples, which agrees with the above discussion of EPR spectra. Compared with sample I (2.1458 ev), the red-shifts of the PL peak of samples II, III, and IV are 32.84, 50.66, and 71.65 meV, respectively. Besides the PL shift resulted from lattice strain C

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Suyver et al. reported the PL lifetime of ZnSe:Mn NCs is shorter than that of ZnS:Mn NCs due to the heavier Se2− in comparison with S2−.9 However, the lifetime in sample I is longer than the other three sulfur-contained samples, indicating that stronger spin−orbit coupling due to the heavier Se2− in comparison with S2− is not the only origin to influence the lifetimes of Mn-doped NCs. This contrary variety of lifetimes can be reasonably attributed to the enhanced transition strength due to lower binding symmetry,3,10 which is well coincident with the binding symmetry of our samples. 3.6. Relationship of Luminescent and Magnetic Properties. Figure 6 shows the intuitionistic change of A, g

Figure 4. Absorption spectra (left) of the ZnSe core and four Mndoped samples and photoluminescence (excited by 350 nm) spectra (right) of four Mn-doped samples measured at room temperature.

(calculated in section 3.2), PL red-shift of 17.34, 21.11, and 23.48 meV existed obviously, which will be discussed in section 3.6. The luminescence efficiency (relative to rhodamine 6G) of the four samples are some higher than our Mn-doped NCs without shells (38−45% vs 13−16%). This means the shell coating increases the luminescence efficiency. However, some other ways also can work, such as encapsulation in zeolites,29 surface plasmon scattering,30 and codoping.31 3.5. PL Lifetime of Mn2+ Ions. Many groups have studied the variety of the PL lifetime of Mn in bulk semiconductor and NCs.2,28,32−41 Neither the blue emission from the host nor the long wavelength emission (∼640 nm) from Mn−Mn pairs is observed in Figure 4, and the ultrafast components of lifetimes that resulted from the two emission9,28 are absent and all the decay curves can be fitted with single exponential fitting, as shown in Figure 5. The PL lifetime of Mn is at the millisecond

Figure 6. The PL peak position, the average lifetimes, the hyperfine constants A, the g factor, and the EPR line-width vs the sequence number of samples.

factors, EPR line-width, PL peak, and PL lifetime of the four samples. In Mn-doped ZnSe NCs, the A values of internal and external Mn2+ ions have been reported to be obviously different (65 vs 95 G).16 Analogous results were reported in Mn-doped ZnS NCs,12 Mn-doped CdSe,17 and Mn-doped CdS/ZnS.13 Different from the large difference mentioned in the references above, the A values of our four samples have very small changes (only 0.5 G totally). This means that the binding symmetry has less effect on luminescent and magnetic properties than the case of the external and internal Mn2+ ions. The Mn2+ ions in our samples are at the interface of cubic NCs, and the symmetry change merely results from different anions around Mn. The increase of the A value and the decrease of the g factor could reflect the weakening of covalency between the Mn2+ ions and the anions.11,14,17 The covalency is weakening from sample I to IV, and this may result from the decrease of electron cloud superposition due to lower binding symmetry. The effect of the configurational symmetry of the NCs on the first excited state of Mn2+ ions can be ignored because all the samples are of cubic structure. The PL red-shift results from bigger splitting of the first excited state of Mn due to enhanced crystal field.40 Therefore, the strength of the crystal field increases from sample I to IV. Subsequently, an intuitionistic relation between covalency and the strength of crystal field can be seen: in low concentration Mn-doped ZnSexS1−x NCs, the covalent bond between Mn2+ ions and the anions is weaker and the crystal field is stronger. This may be described as follows: because the crystal theory is based on the ionic electrostatic

Figure 5. Lifetime decay curves of the four samples. All the lifetime decays are at the millisecond scale.

region, which is analogous with many previous reports,6,9,36,38,40 and the lifetime value is 0.57, 0.49, 0.39, and 0.31 ms for samples I, II, III, and IV, respectively. The PL lifetime of Mn in bulk ZnSe was reported at 0.03−0.8 ms,32−34 while the value in bulk ZnS was reported as about 1.6 ms41 or 1.8 ms.9 Meanwhile, the PL lifetime of Mn in ZnSe NCs was 0.3− 0.62 ms,6,32,41 and the value in ZnS NCs was 0.11−2 ms.10,35,37,39 D

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field, weaker covalency (stronger ionicity) enhances the crystal field. The decreasing of PL lifetime from sample I to IV may be due to the following two factors: (1) the lower symmetry enhances the transition strength;10 (2) stronger spin−lattice interaction, which is confirmed by the broadening of EPR linewidth, results in phonon increasing transition process.

(7) Pradhan, N.; Goorskey, D.; Thessing, J.; Peng, X. An Alternative of CdSe Nanocrystal Emitters: Pure and Tunable Impurity Emissions in ZnSe Nanocrystals. J. Am. Chem. Soc. 2005, 127, 17586−17587. (8) Ithurria, S.; Guyot-Sionnest, P.; Mahler, B.; Dubertret, B. Mn2+ as a Ridial Pressure Gauge in Colloidal Core/Shell Nanocrystals. Phys. Rev. Lett. 2007, 99, 265501. (9) Suyver, J. F.; Wuister, S. F.; Kelly, J. J.; Meijerink, A. Luminescence of Nanocrystalline ZnSe:Mn2+. Phys. Chem. Chem. Phys. 2000, 2, 5445−5448. (10) Kim, M. R.; Chung, J. H.; Jang, D.-J. Spectroscopy and Dynamics of Mn2+ in ZnS Nanoparticles. Phys. Chem. Chem. Phys. 2009, 11, 1003−1006. (11) Liu, J.; Liu, C.; Zheng, Y.; Li, D.; Xu, W.; Yu, J. Three Types of Site of Mn2+ in ZnS:Mn2+ Nanocrystal/Pyrex Glass Composites. J. Phys.: Condens. Matter 1999, 11, 5377−5384. (12) Norman, T. J., Jr.; Magana, D.; Wilson, T.; Burns, C.; Zhang, J. Z.; Cao, D.; Bridges, F. Optical and Surface Structural Properties of Mn2+-Doped ZnSe Nanoparticles. J. Phys. Chem. B 2003, 107, 6309− 6317. (13) Yang, Y.; Chen, O.; Angerhofer, A.; Charles Cao, Y. On Doping CdS/ZnS Core/Shell Nanocrystals with Mn. J. Am. Chem. Soc. 2008, 130, 15649−15661. (14) Bhattacharayya, S.; Perelshtein, I.; Moshe, O.; Rich, D. H.; Gedanken, A. One-Step Solvent-Free Synthesis and Characterization of Zn1‑xMnxSe@C Nanorods and Nanowires. Adv. Funct. Mater. 2008, 18, 1641−1653. (15) Zu, L.; Wills, A. W.; Kennedy, T. A.; Glaser, E. R.; Norris, D. J. Effect of Different Manganese Precursors on the Doping Efficiency in ZnSe Nanocrystals. J. Phys. Chem. C 2010, 114, 21969−21975. (16) Acharya, S.; Sarma, D. D.; Jana, N. R.; Pradhan, N. An Alternate Route to High-Quality ZnSe and Mn-Doped ZnSe Nanocrystals. J. Phys. Chem. Lett. 2010, 1, 485−488. (17) Zheng, W.; Wang, Z.; Wright, J.; Goundie, B.; Dalal, N. S.; Meulen-berg, R. W.; Strouse, G. F. Probing the Local Site Enviroments in Mn-CdSe Quantum Dots. J. Phys. Chem. C 2011, 115, 23305− 23314. (18) Owen, J.; Thornley, J. H. M. Covalent Bonding and Magnetic Properties of Transition Metal Ions. Rep. Prog. Phys. 1966, 29, 675− 728. (19) Yang, B.; Zhang, J.; Cui, Y.; Wang, K. White Light-Emitting Diode Coated with ZnSe:Mn/ZnSe Nanocrystals Films Enveloped by SiO2. Appl. Opt. 2011, 50, G137−G141. (20) Zheng, F.; Ping, W.; Xinhua, Z.; Yang, Y.-J. Synthesis of Highly Luminescent Mn:ZnSe/ZnS Nanocrystals in Aqueous Media. Nanotechnology 2010, 21, 305604. (21) Zhang, J.; Zhang, X.; Zhang, J. Y. Dependence of Microstructure and Luminescence on Shell Layers in Colloidal CdSe/CdS Core/Shell Nanocrystals. J. Phys. Chem. C 2010, 114, 3904−3908. (22) Kennedy, T. A.; Glaser, E. r; Klein, P. B.; Bhargava, R. N. Symmetry and Electronic Structure of the Mn Impurity in ZnS Nanocrystals. Phys. Rev. B 1995, 52, R14356. (23) Gonzalez Beermann, P. A.; McGarvey, B. R.; Muralidharan, S.; Sung, R. C. W. EPR Spectra of Mn2+-Doped ZnS Quantum dots. Chem. Mater. 2004, 16, 915−918. (24) Ji, T.; Jian, W.-B.; Fang, J.e The First Synthesis of Pb1‑xMnxSe Nanocrystals. J. Am. Chem. Soc. 2003, 125, 8448−8449. (25) Norris, D. J.; Yao, N.; T Charnock, F.; Kennedy, T. A. HighQuality Manganese-Doped ZnSe Nanocrystals. Nano Lett. 2001, 1, 3− 7. (26) Park, B. J.; Im, W. B.; Chung, W. J.; Seo, H. S.; Ahn, J. T. Internal Pressure Effect on Cathodoluminescence Enhancement of ZnS:Mn2+ Synthesized by a Sealed Vessel. J. Mater. Res. 2007, 22, 2838−2844. (27) Samarth, N.; Furdyna, J. K. Electron Paramagnetic Resonance in Cd1‑xMnxS, Cd1‑xMnxSe, and Cd1‑xMnxTe. Phys. Rev. B 1988, 37, 9227−9239. (28) Gan, C.; Zhang, Y.; Battaglia, D.; Peng, X.; Xiao, M. Fluorescence Lifetime of Mn-Doped ZnSe Quantum Dots with Size Dependence. Appl. Phys. Lett. 2008, 92, 241111.

4. CONCLUSION Four colloidal Mn-doped NC samples with different binding symmetry were synthesized by the radial-position-controlled method. Experimental data showed a PL peak red-shift and PL lifetime shortening from sample I to IV. EPR parameters reflected the covalent bond between Mn2+ ions and the anions became weaker. The PL peak red-shift indicated the enhancement of the crystal field. An intuitionistic relation between covalency and the strength of crystal field was suggested. The shortening PL lifetime possibly resulted from stronger spin− lattice interaction, confirmed by the line-width of EPR spectra. A straightforward relation between the luminescent and magnetic properties of Mn-doped NCs may be of some use to their applications.

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ASSOCIATED CONTENT

S Supporting Information *

Description of the claim of the position of Mn in nanocrystals (NCs) and the monolayer. By NC etching, TEM images, and coupled plasma (ICP) measurement for element content, we can speculate that Mn2+ ions are at the interface of the core and shell. Layer-by-layer synthesis is demonstrated by the TEM images and sizes of each layer coating the NCs. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding support from the National Basic Research Program of China (973 Program, 2012CB921801) and the Science and Technology Department of JiangSu Province (BE2012163) is acknowledged.

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REFERENCES

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp402129k | J. Phys. Chem. C XXXX, XXX, XXX−XXX