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Mn:ZnS heterostructures
Materials Letters 92 (2013) 405–408
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Magnetism and photoluminescence of Mn:ZnO/Mn:ZnS heterostructures X.F. Liu a,n, N. Yang a, H. Li a, R.H. Yu a,n, W. Wei b a b
School of Materials Science and Engineering, Beijing University, Beijing 100191, China School of Mechanical Engineering and Automation, Beihang University, Beijing 100191, China
a r t i c l e i n f o
abstract
Article history: Received 21 August 2012 Accepted 5 November 2012 Available online 10 November 2012
Mn:ZnO/Mn:ZnS core–shell heterostructures, in which Mn2 þ ions are incorporated into ZnO and ZnS host lattice by replacing Zn2 þ ion sites, have been synthesized by coating single-crystalline Mn:ZnO nanorods with a thin layer of Mn:ZnS nanocrystals. The heterostructures possess visible/ultraviolet photoluminescences and intrinsic room-temperature ferromagnetism. Photoluminescent spectrum exhibits enhanced ultraviolet emission of Mn:ZnO, defect-related green emission of Mn:ZnS, and strong orange emission due to 4T1(4G)–6A1(6S) transition of Mn2 þ in ZnS. A strong ferromagnetic layer is found to exist on the surface of the Mn:ZnO core, which makes a much larger contribution to the macroscopic ferromagnetism of the heterostructures. & 2012 Elsevier B.V. All rights reserved.
Keywords: Magnetic materials Semiconductors Luminescence
1. Introduction Wide-band-gap ZnS and ZnO have been intensively investigated due to the promising applications in transparent electrodes, ultraviolet (UV) detectors, laser diode, and spintronic devices [1,2]. Nowadays, great efforts have been devoted to incorporating other elements into ZnS and ZnO nanomaterials to improve their performance. Generally the luminescence of ZnS can be tuned by doping with various transition and rare-earth metal ions to obtain novel luminescent characteristics. For example, Mn:ZnS nanomaterials exhibit characteristic yellow–orange emission due to the d–d transition of Mn2 þ [2,3]. Eu:ZnS nanocrystals show a red luminescence, resulting from the energy transfer from band to band excitation in ZnS to Eu3 þ from the 5D0–7FJ transition [4]. As to ZnO, the addition of other elements can tailor the band gap and modify the UV emission characteristic which is desirable in UV nanodevices [5]. Moreover, the incorporation of transition-metal element into semiconductors, which is so-called diluted magnetic semiconductors (DMSs), may create surprising ferromagnetism that are absent in pure semiconductors. The DMS materials are highly desired in potential spintronic devices that operate not only with electron charge but also utilize the electron spin as an additional degree of freedom to produce new functionalities [6,7]. Recently, semiconductor heterostructure with modulated compositions and interfaces have attracted considerable attention since the system can integrate several different functions into one nanostructure. Pure ZnO–ZnS heterostructures have been considered as unique candidates for UV lasers and detectors working in
n
Corresponding authors. Tel.: þ 86 10 82317101. E-mail addresses: [email protected] (X.F. Liu), [email protected] (R.H. Yu).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.11.013
the 320–400 nm wavelength range [8,9]. However, if doped-ZnS could be combined with doped-ZnO, the constructed heterostructures would exhibit excellent visible/UV emissions and ferromagnetism, which might be superior to pure ZnO–ZnS heterostructures. Additionally, the exact origins of magnetism in DMS do not converged to a definite conclusion. Recent studies have found that the microstructure of DMS plays an important role in tuning the magnetism. Fortunately, the construction of heterostructures could change the microstructure at the interface of the host semiconductors, which offers a new approach to mediating ferromagnetism of DMS and exploring related mechanisms. Here, we firstly report the synthesis and characterizations of optical and magnetic properties of Mn:ZnO/Mn:ZnS heterostructures.
2. Experimental details Firstly, Mn:ZnO nanorods with Mn concentration of 3 at% (marked as S1) were synthesized by the solvothermal method. 0.5 M sodium hydroxide solution was dropped into a mixture containing 0.05 M zinc acetate and appropriate amount of manganese acetate. The solution was then transferred into a Teflon autoclave and kept at 120 1C for 15 h. Secondly, the precipitates were cleaned and used as templates to fabricate Mn:ZnO/Mn:ZnS heterostructures (marked as S2). The above precipitates were dispersed into thioacetamide (TAA) solution at 70 1C with stirring for 30 min. Finally, the products were washed for several times and dried in vacuum. For comparison, pure ZnO nanorods were synthesized as Mn:ZnO nanorods; meanwhile, Mn:ZnS nanocrystals with similar composition and structure as those of Mn:ZnS shell in S2 were prepared by sulfidation of Mn:ZnO templates for 4 h. The crystal structure and morphology of the products were
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3. Results and discussion
10–18 nm and 45–80 nm, respectively. The HRTEM image and corresponding fast Fourier transform (FFT) pattern in the inset of Fig. 2(b) illustrate that these Mn:ZnO nanorods are single phase wurtzite structure and grow along c-axis with side surfaces defined by {10-10} planes. After the Mn:ZnO templates are treated with TAA, the surface of the nanorods becomes rough and is coated by a thin shell of Mn:ZnS nanocrystals which form during the sulfidation process by etching Mn:ZnO nanorods
Fig. 1(a) depicts the XRD patterns of the products. All the diffraction peaks of S1 are well indexed to wurtzite ZnO structure. However, the peak position of S1 shifts toward lower angle direction compared with that of pure ZnO nanorods (shown in the inset). This peak shift can be attributed to the larger radius of ˚ versus Zn2 þ (0.74 A), ˚ suggesting that the Mn2 þ is Mn2 þ (0.80 A) likely incorporated into the ZnO lattice by replacing Zn2 þ with Mn2 þ . After sulfidation reaction, a new phase with a weak and broad diffraction peak at 28.61 appears in S2. The position of this new peak is consistent with that of the (002) peak of Mn:ZnS reference nanocrystals. Since the Mn:ZnS reference nanocrystals with cubic ZnS structure are synthesized by sulfidation Mn:ZnO templates for a longer time, the new phase in S2 should also be the Mn:ZnS nanocrystals with smaller sizes. Noticeably, no traces of Mn-related secondary phases are found in S2 within the resolution of the X-ray diffractometer. It implies that the Mn ions may similarly enter into ZnS lattice and substitute for Zn2 þ sites in the process of sulfidation reaction of S1. Fig. 2(b) shows the low-resolution TEM (LRTEM) image of S1, where the Mn:ZnO templates exhibit rod-shape morphology with smooth surface. The diameter and length of the nanorods are
Fig. 2. PL spectra of S1 ( 3) and S2 measured at RT. Inset: PL spectra of pure ZnO nanorods and Mn:ZnS nanocrystals.
characterized using X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). Photoluminescence (PL) spectra were recorded via Raman spectrometer using He–Cd laser. Magnetic measurements were carried out by superconducting quantum interference device.
Fig. 1. (a) XRD patterns of pure ZnO nanorods (curve a), S1 (curve b), S2 (curve c) and Mn:ZnS nanocrystals (curve d), inset: enlarged curve a and curve b. (b) LRTEM image of S1, inset: HRTEM image and corresponding FFT of S1. (c) LRTEM image of S2. (d) HRTEM image of S2, inset: schematic diagram of S2.
X.F. Liu et al. / Materials Letters 92 (2013) 405–408
[shown in Fig. 2(c) and (d)]. Thus, the whole structure of S2 can be more clearly illustrated in the inset of Fig. 2(d). More careful observation from Fig. 2(d) finds that a void appears on the end of the nanorod, and the shell on the end is much thicker than the shell on the edge. In initial sulfidation process, S2 released from the decomposition of TAA reacts with the Zn2 þ slowly dissolved from the surface of Mn:ZnO nanorods to produce Mn:ZnS nanocrystals around the Mn:ZnO nanorods [10]. After coating Mn:ZnS, the shell-separated S2 and Zn2 þ diffuse inward and outward through Mn:ZnS thin shell respectively to continue the sulfidation reaction. The faster outward diffusion rate of Zn2 þ would produce a void between the core and shell [10]. Furthermore, it is well known that the Zn atom terminated (0001) plane (i.e. end of the nanorods) is much more reactive than the nonpolar {10-10} plane (i.e. side surface of the nanorods) [11]. As a result, the Mn:ZnS shell on the end is much thicker and a void initially appears on the end. Fig. 2 provides the PL spectra of S1 and S2, along with pure ZnO nanorods and Mn:ZnS nanocrystals. The spectrum of S1 is similar to that of pure ZnO nanorods, which exhibits a narrow UV emission at 380 nm and a broad visible emission band. The UV emission originates from the near-band-edge transition of ZnO, and the broad visible luminescence is commonly related to the oxygen vacancies (Vos) [12]. After coating Mn:ZnS shell, the spectrum of S2 shows an enhanced UV emission likely rising from the passivation of surface electronic states of Mn:ZnO cores by coating wider-band-gap Mn:ZnS shell [13]. Meanwhile, one can see two obvious visible emissions centered around 510 nm and 590 nm, as observed in ZnS/Mn:ZnO nanoparticles [14]. The weak green emission could be interpreted as a donor–acceptor pair emission involving defect states in the ZnS nanocrystals [15]. While the orange emission at 590 nm is assigned to the 4 T1(4G)–6A1(6S) transition within the 3d shell of Mn2 þ ions. As displayed in the inset, only the strong orange emission appears in the PL spectrum after the Mn:ZnO templates completely transform to Mn:ZnS nanocrystals through long-time sulfidation. It has been reported in Ref. [16] that when Mn2 þ ions substitute for Zn2 þ sites in ZnS lattice, the mixing between sp electrons of ZnS and the d electrons of Mn2 þ occurs and thus makes the forbidden transition partially [16]. As a result, the presence of the
407
characteristic emission of Mn2 þ further confirms that the Mn2 þ ions in the shell are incorporated into the host ZnS lattice by replacing Zn2 þ ions and the S2 product actually forms core–shell Mn:ZnO/Mn:ZnS structure. The photoluminescences of the new type heterostructures combine and improve the characteristics of the two materials, which make them more applicable for the fabrication of optoelectronic devices. The magnetization versus magnetic field (M–H) curves for S1 and S2 are shown in Fig. 3. The apparent hysteresis loop of S2 indicates the ferromagnetic state with saturated magnetization (Ms) of 0.0025 emu/g. Although not all Mn-related impurity phases may be detectable in XRD, the possible impurity phases of Mn, MnO, MnO2, and Mn2O3 are antiferromagnetic, and Mn3O4 is ferromagnetic with a curie temperature of 40 K [17]. Even if these phases are present, they can not account for the observed ferromagnetism. Thus, the RT ferromagnetism should be the intrinsic nature of the materials. Recently, the ferromagnetism of DMS is associated with structural defects based on the bound magnetic polaron (BMP) model. On this point of view, the intrinsic ferromagnetism in S2 is likely ascribed to the large population of structural defects such as Vos in Mn:ZnO cores since they can initiate defect-related hybridization at the Fermi level and establish a long-range ferromagnetic ordering [18]. Although the Mn doped/co-doped ZnS could also be ferromagnetic at RT [19,20], the Mn:ZnS shell of S2 is composed of nanocrystals with diameter of 5 nm, which has transformed into superparamagnetic state in such dimension. However, the Ms value of S2 considerably decreases compared with that of S1. Since the Mn:ZnS shell of S2 is very thin, the quality of the shell is negligible relative to Mn:ZnO cores. Hence, the large decrease in Ms of S2 should not result from the diluting effect of the superparamagnetic shell. The PL spectrum of S1 confirms the presence of large amount of Vos in Mn:ZnO nanorods. In previous studies of ZnO nanowires, it is reported that more Vos preferably distribute on the surface of nanowires owing to the lower formation energy of Vos on surface than that inside of the nanowires [21]. Based on BMP model, the high density of Vos on Mn:ZnO surface can yield a great overall volume occupied by BMP clusters, thus increasing their probability of overlapping more Mn ions into the ferromagnetic
Fig. 3. RT M–H curves of S1 and S2.
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domains and establishing a strong ferromagnetic surface. From the surface to the inside of Mn:ZnO nanorods, the number of BMPs gradually reduces because of the decrease in the number of Vos. Consequently, the surface of the Mn:ZnO nanorods makes a much larger contribution to the macroscopic ferromagnetism. In sulfidation process, a shell of Mn:ZnS nanocrystals form around the Mn:ZnO nanorods by etching Mn:ZnO surface. Thus, the strong ferromagnetic Mn:ZnO nanorod surface is gradually destroyed, which induces the large decrease of Ms in the heterostructures. 4. Conclusions Mn:ZnO/Mn:ZnS heterostructures have been prepared via wet chemical methods. The photoluminescences of the heterostructures combine and improve the characteristics of Mn:ZnO and Mn:ZnS nanomaterials. Simultaneously, the heterostructures exhibit intrinsic RT ferromagnetism which is closely correlated to the structural defects and mainly originates from the strong ferromagnetic surface of the Mn:ZnO cores. The excellent photoluminescence of this new type heterostructures makes them attractive for the fabrication of optoelectronic devices. In addition, the correlation between the structure and the ferromagnetism of the heterostructures is helpful for elucidating the ferromagnetism of DMS.
Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51102006) and Fundamental Research Funds for the Central Universities.
References [1] Huang Y, Cai Y, Liu H. Particuology 2011;9:533–6. [2] Fang XS, Zhai TY, Gautam UK, Li L, Wu LM, Yoshio B, et al. Prog Mater Sci 2011;56:175–287. [3] Fang YC, Chu SY, Chen HC, Kao PC, Chen IG, Hwang CS. J Electrochem Soc 2009;156:K55–8. [4] Sun LD, Yan CH, Liu CH, Liao CS, Li D, Yu JQ. J Alloys Compd 1998;275:234–7. [5] Iqbal J, Liu XF, Majid A, Yu RH. J Superconductivity Novel Magn 2011;24:699–704. [6] Mohapatra J, Mishra DK, Singh SK. Mater Lett 2012;75:91–4. [7] Sebastian KC, Chawda M, Jonny L, Bodas D. Mater Lett 2010;64:2269–72. [8] Yan J, Fang XS, Zhang LD, Bando Y, Gautam UK, Dierre B, et al. Nano Lett 2008;8:2794–9. [9] Hu LF, Yan J, Liao MY, Xiang HJ, Gong XG, Zhang LD, et al. Adv Mater 2012;24:2305–9. [10] Shuai XM, Shen WZ. J Phys Chem C 2011;115:6415–22. [11] Wu D, Xiao B, Liu N, Xiao Y, Jiang K. Mater Sci Eng B-Adv Funct Solid-State Mater 2010;175:195–200. [12] Yuan K, Yu QX, Gao QQ, Wang J, Zhang XT. Appl Surf Sci 2012;258:3350–3. [13] Datta A, Panda SK, Chaudhuri S. J Phys Chem C 2007;111:17260–4. [14] Limaye MV, Singh SB, Date SK, Gholap RS, Kulkarni SK. Mater Res Bull 2009;44:339–44. [15] Kar S, Chaudhuri S. J Phys Chem B 2005;109:3298–302. [16] Sooklal K, Cullum BM, Angel SM, Murphy CJ. The photophysics of Mn2 þ on ZnS nanoclusters. Dordrecht: Kluwer Academic; 1996 p. 455–65. [17] Droubay TC, Keavney DJ, Kaspar TC, Heald SM, Wang CM, Johnson CA, et al. Phys Rev B 2009;79:155203. [18] Gu H, Zhang W, Xu Y, Yan M. Appl Phys Lett 2012;100:202401. [19] Xie JM. J Magn Magn Mater 2010;322:L37–41. [20] Chen HX. J Magn Magn Mater 2012;324:2086–90. [21] Wang Q, Sun Q, Chen G, Kawazoe Y, Jena P. Phys Rev B 2008;77:205411.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Magnetism and photoluminescence of Mn:ZnO/Mn:ZnS heterostructures X.F. Liu a,n, N. Yang a, H. Li a, R.H. Yu a,n, W. Wei b a b
School of Materials Science and Engineering, Beijing University, Beijing 100191, China School of Mechanical Engineering and Automation, Beihang University, Beijing 100191, China
a r t i c l e i n f o
abstract
Article history: Received 21 August 2012 Accepted 5 November 2012 Available online 10 November 2012
Mn:ZnO/Mn:ZnS core–shell heterostructures, in which Mn2 þ ions are incorporated into ZnO and ZnS host lattice by replacing Zn2 þ ion sites, have been synthesized by coating single-crystalline Mn:ZnO nanorods with a thin layer of Mn:ZnS nanocrystals. The heterostructures possess visible/ultraviolet photoluminescences and intrinsic room-temperature ferromagnetism. Photoluminescent spectrum exhibits enhanced ultraviolet emission of Mn:ZnO, defect-related green emission of Mn:ZnS, and strong orange emission due to 4T1(4G)–6A1(6S) transition of Mn2 þ in ZnS. A strong ferromagnetic layer is found to exist on the surface of the Mn:ZnO core, which makes a much larger contribution to the macroscopic ferromagnetism of the heterostructures. & 2012 Elsevier B.V. All rights reserved.
Keywords: Magnetic materials Semiconductors Luminescence
1. Introduction Wide-band-gap ZnS and ZnO have been intensively investigated due to the promising applications in transparent electrodes, ultraviolet (UV) detectors, laser diode, and spintronic devices [1,2]. Nowadays, great efforts have been devoted to incorporating other elements into ZnS and ZnO nanomaterials to improve their performance. Generally the luminescence of ZnS can be tuned by doping with various transition and rare-earth metal ions to obtain novel luminescent characteristics. For example, Mn:ZnS nanomaterials exhibit characteristic yellow–orange emission due to the d–d transition of Mn2 þ [2,3]. Eu:ZnS nanocrystals show a red luminescence, resulting from the energy transfer from band to band excitation in ZnS to Eu3 þ from the 5D0–7FJ transition [4]. As to ZnO, the addition of other elements can tailor the band gap and modify the UV emission characteristic which is desirable in UV nanodevices [5]. Moreover, the incorporation of transition-metal element into semiconductors, which is so-called diluted magnetic semiconductors (DMSs), may create surprising ferromagnetism that are absent in pure semiconductors. The DMS materials are highly desired in potential spintronic devices that operate not only with electron charge but also utilize the electron spin as an additional degree of freedom to produce new functionalities [6,7]. Recently, semiconductor heterostructure with modulated compositions and interfaces have attracted considerable attention since the system can integrate several different functions into one nanostructure. Pure ZnO–ZnS heterostructures have been considered as unique candidates for UV lasers and detectors working in
n
Corresponding authors. Tel.: þ 86 10 82317101. E-mail addresses: [email protected] (X.F. Liu), [email protected] (R.H. Yu).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.11.013
the 320–400 nm wavelength range [8,9]. However, if doped-ZnS could be combined with doped-ZnO, the constructed heterostructures would exhibit excellent visible/UV emissions and ferromagnetism, which might be superior to pure ZnO–ZnS heterostructures. Additionally, the exact origins of magnetism in DMS do not converged to a definite conclusion. Recent studies have found that the microstructure of DMS plays an important role in tuning the magnetism. Fortunately, the construction of heterostructures could change the microstructure at the interface of the host semiconductors, which offers a new approach to mediating ferromagnetism of DMS and exploring related mechanisms. Here, we firstly report the synthesis and characterizations of optical and magnetic properties of Mn:ZnO/Mn:ZnS heterostructures.
2. Experimental details Firstly, Mn:ZnO nanorods with Mn concentration of 3 at% (marked as S1) were synthesized by the solvothermal method. 0.5 M sodium hydroxide solution was dropped into a mixture containing 0.05 M zinc acetate and appropriate amount of manganese acetate. The solution was then transferred into a Teflon autoclave and kept at 120 1C for 15 h. Secondly, the precipitates were cleaned and used as templates to fabricate Mn:ZnO/Mn:ZnS heterostructures (marked as S2). The above precipitates were dispersed into thioacetamide (TAA) solution at 70 1C with stirring for 30 min. Finally, the products were washed for several times and dried in vacuum. For comparison, pure ZnO nanorods were synthesized as Mn:ZnO nanorods; meanwhile, Mn:ZnS nanocrystals with similar composition and structure as those of Mn:ZnS shell in S2 were prepared by sulfidation of Mn:ZnO templates for 4 h. The crystal structure and morphology of the products were
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X.F. Liu et al. / Materials Letters 92 (2013) 405–408
3. Results and discussion
10–18 nm and 45–80 nm, respectively. The HRTEM image and corresponding fast Fourier transform (FFT) pattern in the inset of Fig. 2(b) illustrate that these Mn:ZnO nanorods are single phase wurtzite structure and grow along c-axis with side surfaces defined by {10-10} planes. After the Mn:ZnO templates are treated with TAA, the surface of the nanorods becomes rough and is coated by a thin shell of Mn:ZnS nanocrystals which form during the sulfidation process by etching Mn:ZnO nanorods
Fig. 1(a) depicts the XRD patterns of the products. All the diffraction peaks of S1 are well indexed to wurtzite ZnO structure. However, the peak position of S1 shifts toward lower angle direction compared with that of pure ZnO nanorods (shown in the inset). This peak shift can be attributed to the larger radius of ˚ versus Zn2 þ (0.74 A), ˚ suggesting that the Mn2 þ is Mn2 þ (0.80 A) likely incorporated into the ZnO lattice by replacing Zn2 þ with Mn2 þ . After sulfidation reaction, a new phase with a weak and broad diffraction peak at 28.61 appears in S2. The position of this new peak is consistent with that of the (002) peak of Mn:ZnS reference nanocrystals. Since the Mn:ZnS reference nanocrystals with cubic ZnS structure are synthesized by sulfidation Mn:ZnO templates for a longer time, the new phase in S2 should also be the Mn:ZnS nanocrystals with smaller sizes. Noticeably, no traces of Mn-related secondary phases are found in S2 within the resolution of the X-ray diffractometer. It implies that the Mn ions may similarly enter into ZnS lattice and substitute for Zn2 þ sites in the process of sulfidation reaction of S1. Fig. 2(b) shows the low-resolution TEM (LRTEM) image of S1, where the Mn:ZnO templates exhibit rod-shape morphology with smooth surface. The diameter and length of the nanorods are
Fig. 2. PL spectra of S1 ( 3) and S2 measured at RT. Inset: PL spectra of pure ZnO nanorods and Mn:ZnS nanocrystals.
characterized using X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). Photoluminescence (PL) spectra were recorded via Raman spectrometer using He–Cd laser. Magnetic measurements were carried out by superconducting quantum interference device.
Fig. 1. (a) XRD patterns of pure ZnO nanorods (curve a), S1 (curve b), S2 (curve c) and Mn:ZnS nanocrystals (curve d), inset: enlarged curve a and curve b. (b) LRTEM image of S1, inset: HRTEM image and corresponding FFT of S1. (c) LRTEM image of S2. (d) HRTEM image of S2, inset: schematic diagram of S2.
X.F. Liu et al. / Materials Letters 92 (2013) 405–408
[shown in Fig. 2(c) and (d)]. Thus, the whole structure of S2 can be more clearly illustrated in the inset of Fig. 2(d). More careful observation from Fig. 2(d) finds that a void appears on the end of the nanorod, and the shell on the end is much thicker than the shell on the edge. In initial sulfidation process, S2 released from the decomposition of TAA reacts with the Zn2 þ slowly dissolved from the surface of Mn:ZnO nanorods to produce Mn:ZnS nanocrystals around the Mn:ZnO nanorods [10]. After coating Mn:ZnS, the shell-separated S2 and Zn2 þ diffuse inward and outward through Mn:ZnS thin shell respectively to continue the sulfidation reaction. The faster outward diffusion rate of Zn2 þ would produce a void between the core and shell [10]. Furthermore, it is well known that the Zn atom terminated (0001) plane (i.e. end of the nanorods) is much more reactive than the nonpolar {10-10} plane (i.e. side surface of the nanorods) [11]. As a result, the Mn:ZnS shell on the end is much thicker and a void initially appears on the end. Fig. 2 provides the PL spectra of S1 and S2, along with pure ZnO nanorods and Mn:ZnS nanocrystals. The spectrum of S1 is similar to that of pure ZnO nanorods, which exhibits a narrow UV emission at 380 nm and a broad visible emission band. The UV emission originates from the near-band-edge transition of ZnO, and the broad visible luminescence is commonly related to the oxygen vacancies (Vos) [12]. After coating Mn:ZnS shell, the spectrum of S2 shows an enhanced UV emission likely rising from the passivation of surface electronic states of Mn:ZnO cores by coating wider-band-gap Mn:ZnS shell [13]. Meanwhile, one can see two obvious visible emissions centered around 510 nm and 590 nm, as observed in ZnS/Mn:ZnO nanoparticles [14]. The weak green emission could be interpreted as a donor–acceptor pair emission involving defect states in the ZnS nanocrystals [15]. While the orange emission at 590 nm is assigned to the 4 T1(4G)–6A1(6S) transition within the 3d shell of Mn2 þ ions. As displayed in the inset, only the strong orange emission appears in the PL spectrum after the Mn:ZnO templates completely transform to Mn:ZnS nanocrystals through long-time sulfidation. It has been reported in Ref. [16] that when Mn2 þ ions substitute for Zn2 þ sites in ZnS lattice, the mixing between sp electrons of ZnS and the d electrons of Mn2 þ occurs and thus makes the forbidden transition partially [16]. As a result, the presence of the
407
characteristic emission of Mn2 þ further confirms that the Mn2 þ ions in the shell are incorporated into the host ZnS lattice by replacing Zn2 þ ions and the S2 product actually forms core–shell Mn:ZnO/Mn:ZnS structure. The photoluminescences of the new type heterostructures combine and improve the characteristics of the two materials, which make them more applicable for the fabrication of optoelectronic devices. The magnetization versus magnetic field (M–H) curves for S1 and S2 are shown in Fig. 3. The apparent hysteresis loop of S2 indicates the ferromagnetic state with saturated magnetization (Ms) of 0.0025 emu/g. Although not all Mn-related impurity phases may be detectable in XRD, the possible impurity phases of Mn, MnO, MnO2, and Mn2O3 are antiferromagnetic, and Mn3O4 is ferromagnetic with a curie temperature of 40 K [17]. Even if these phases are present, they can not account for the observed ferromagnetism. Thus, the RT ferromagnetism should be the intrinsic nature of the materials. Recently, the ferromagnetism of DMS is associated with structural defects based on the bound magnetic polaron (BMP) model. On this point of view, the intrinsic ferromagnetism in S2 is likely ascribed to the large population of structural defects such as Vos in Mn:ZnO cores since they can initiate defect-related hybridization at the Fermi level and establish a long-range ferromagnetic ordering [18]. Although the Mn doped/co-doped ZnS could also be ferromagnetic at RT [19,20], the Mn:ZnS shell of S2 is composed of nanocrystals with diameter of 5 nm, which has transformed into superparamagnetic state in such dimension. However, the Ms value of S2 considerably decreases compared with that of S1. Since the Mn:ZnS shell of S2 is very thin, the quality of the shell is negligible relative to Mn:ZnO cores. Hence, the large decrease in Ms of S2 should not result from the diluting effect of the superparamagnetic shell. The PL spectrum of S1 confirms the presence of large amount of Vos in Mn:ZnO nanorods. In previous studies of ZnO nanowires, it is reported that more Vos preferably distribute on the surface of nanowires owing to the lower formation energy of Vos on surface than that inside of the nanowires [21]. Based on BMP model, the high density of Vos on Mn:ZnO surface can yield a great overall volume occupied by BMP clusters, thus increasing their probability of overlapping more Mn ions into the ferromagnetic
Fig. 3. RT M–H curves of S1 and S2.
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X.F. Liu et al. / Materials Letters 92 (2013) 405–408
domains and establishing a strong ferromagnetic surface. From the surface to the inside of Mn:ZnO nanorods, the number of BMPs gradually reduces because of the decrease in the number of Vos. Consequently, the surface of the Mn:ZnO nanorods makes a much larger contribution to the macroscopic ferromagnetism. In sulfidation process, a shell of Mn:ZnS nanocrystals form around the Mn:ZnO nanorods by etching Mn:ZnO surface. Thus, the strong ferromagnetic Mn:ZnO nanorod surface is gradually destroyed, which induces the large decrease of Ms in the heterostructures. 4. Conclusions Mn:ZnO/Mn:ZnS heterostructures have been prepared via wet chemical methods. The photoluminescences of the heterostructures combine and improve the characteristics of Mn:ZnO and Mn:ZnS nanomaterials. Simultaneously, the heterostructures exhibit intrinsic RT ferromagnetism which is closely correlated to the structural defects and mainly originates from the strong ferromagnetic surface of the Mn:ZnO cores. The excellent photoluminescence of this new type heterostructures makes them attractive for the fabrication of optoelectronic devices. In addition, the correlation between the structure and the ferromagnetism of the heterostructures is helpful for elucidating the ferromagnetism of DMS.
Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51102006) and Fundamental Research Funds for the Central Universities.
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