The Photoluminescent and Magnetic Properties of Mn2+ Ions at the Interface of Core/Shell Mn-Doped Nanocrystals

Citation: Yang B (2017) The Photoluminescent and Magnetic Properties of Mn2+ Ions at the Interface of Core/Shell MnDoped Nanocrystals. Int J Nanoparticles Nanotech 3:009 Copyright: © 2017 Yang B. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. *Corresponding author: Boping Yang, Yancheng Institute of Technology, Yancheng 224051, P. R. China, Tel: 860515-88168187, E-mail: bpyang023@163.com Yang. Int J Nanoparticles Nanotech 2017, 3:009

It is well known that the emission of Mn 2+ in NCs is attributed to the 4 T 1 to 6 A 1 transition [7,13,14]. This emission has been found to be effected by shell thickness in core/shell ZnSe and CdS/ZnS NCs [15][16][17]. The change of luminescent color was suggested to be resulted from change of the splitting of the 6 A 1 state of Mn 2+ ions due to different shell thickness. Additionally, enhanced crystal Due to unpaired electrons, the local environment of Mn 2+ ions in Mn-doped NCs has been explored by EPR spectra [9,11,12,13]. The values of superfine splitting constant (A) of EPR spectra can evidence the position of Mn 2+ ions [12,19,20]. Additionally, the EPR spectra also demonstrate the covalency between the anion and cation [8,21]. Larger value of A suggested weaker covalency [12,20,21]. In Mn-CdSe NCs, larger A value was suggested to be resulted from the Mn aggregation at the surface of the NC, which implied reduced covalency [14]. Similar report was shown in core/shell CdS/ZnS NCs [22].
Our previous work has investigated the relationship between crystal filed and covalency by Mn-doped NCs with different binding symmetry [8]. To a certain degree, this structure-dependent result clarified the relationship mentioned above. In this work, both the structure-dependent and temperature-dependent properties of Mndoped NCs were considered for further understanding of this relationship.

Experimental Details
All Mn-doped NCs in this work were synthesized through the method as shown in our previous work [8]. The process includes core fabrication, elimination of residual Mn 2+ in the solution and shell growing. The synthesis procedure in this work contained the elimination of the residual Mn in the solution, which suggested there were no Mn 2+ ions in the ZnS shell. The synthesis process are as follows: (1) ZnSe cores preparation and purification; (2) Mn adsorption on ZnSe at 120 °C and the elimination of the residual Mn in the solution; (3) ZnSe shell growth at 270 °C. This could demonstrated that the Mn 2+ ions were at the interface of the core/shell NCs and no Mn 2+ ions were at the surface. Furthermore, EPR spectra also suggest that the Mn 2+ ions were at the interface of core and shell of the NCs. The NCs are ZnSe:Mn/ ZnS (sample I), ZnSe/ZnS(1ML):Mn/ZnS (sample II), ZnSe/ZnS(2ML):Mn/ZnS (sample II) and ZnSe:Mn/ ZnSe (sample IV), respectively. Location of Mn 2+ ions at the interface of the core/shell NCs results in increasing binding symmetry from sample I to IV [8]. Transmission Electron Microscopy (TEM) images were gained from 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. PL spectra were gained on a FLS920 F900 luminescence spectrometer (Edinburgh). EPR spectra were measured with an X-band EMX-10/12 spectrometer (Bruker). ZnSe:Mn/ZnSe NCs were spin coated on quartz glass and mounted in a vacuum liquid nitrogen cryostat for the temperature-dependent measurement. The temperature range was from 77 to 297 K.   red shift of the luminescence [23]. In this work, the PL peak shifts reflected by temperature-dependent and structure-dependent properties demonstrate that the strength of crystal field decreased.

Results and Discussion
Some previous reports have demonstrated the covalency between Mn 2+ ions and the anions around them by the EPR spectra, which showed that increasing A value or/and decreasing g value correspond to weakening covalency [8,21,22,24]. Mn 2+ ions have been doped into the NCs, which is indicated by the well-resolved hyperfine structure in Figure 5. No Mn-Mn pairs formed during the synthesis because there are no broad background lines in the spectra under different temperature [25]. Additional weak transitions between the six hyperfine lines due to spin forbidden transition are also observed. From the two inserts in Figure 5, the horizontal narrowing of the spectra was demonstrated with increasing temperature. It means that A becomes smaller in higher temperature. From Figure 6, decreasing A value can be observed both in ZnSe:Mn/ZnSe NCs under elevating temperature and NCs with more symmetrical binding structure. Meanwhile, all the values of A are 66.5~69.5, which indicates no Mn 2+ ions are at the surface of NCs [19]. This agrees with the synthesis strategy and ensures that this work is based on the Mn 2+ ions at the interface of the core/ shell NCs. Figure 7 shows the g values of EPR spectra. In ZnSe:Mn/ZnSe NCs under elevating temperature and NCs with more symmetrical binding structure, g values become bigger. According to references [8,21,22,24] elevating temperature can resulted in stronger covalency in ZnSe:Mn/ZnSe NCs and more symmetrical binding structure also did this. The decreasing crystal field reflected by PL shift is interrelated to stronger covalency suggested by EPR spectra. This relationship between into QDs successfully and no phase transformation was resulted from the doping. The ZnSe and ZnS patterns in Figure 2 demonstrate that sample II, III and IV have ZnS shells. The shift of (1 1 1) peak is due to the oppression of ZnS shell.
Typical PL spectra of ZnSe:Mn/ZnSe NCs under different temperature are shown in Figure 3. All the peaks are around 585 nm due to the transition from 4 T 1 to 6 A 1 [7,13,14]. The PL peaks of ZnSe:Mn/ZnSe NCs under different temperature and NCs with different binding symmetry are shown in Figure 4. The PL peak of ZnSe:Mn/ZnSe NCs shift to higher energy under elevating temperature (Figure 4a). This temperature-dependent trend demonstrates that increasing temperature results in smaller splitting of the 6 A 1 state of Mn 2+ ions in ZnSe:Mn/ZnSe NCs. Figure 4b shows the structure-dependent PL peak of the four samples. Except the PL shift resulted from the pressure of shell in core/shell NCs [17], the PL peak of NCs has a blue shift with decreasing binding symmetry. This also demonstrates smaller splitting of the 6 A 1 state Mn 2+ ions. Stronger crystal field results in bigger splitting of 6 A 1 state of Mn 2+ , which brings about   crystal field and covalency is demonstrated by temperature-dependent and structure-dependent properties of Mn-doped NCs. This relationship can be interpreted as: the crystal theory is based on the ionic electrostatic field, so stronger covalency (weaker ionicity) weakens the crystal field.

Conclusion
In summary, PL and magnetic properties of NCs with different binding symmetry and ZnSe:Mn/ZnSe NCs under different temperature were discussed. Mn 2+ ions were at the interface of the cubic core/shell NCs. Increasing crystal field and weakening covalency was associat-ed through PL peaks and g/A values of the EPR spectra. Both temperature-dependent and structure-dependent results clarified this relationship.