简介概要

Room temperature ferromagnetism of boron-doped ZnO nanoparticles prepared by solvothermal method

来源期刊：Rare Metals2013年第3期

论文作者：M. Hassan Farooq Xiao-Guang Xu Hai-Ling Yang Cong-Jun Ran Jun Miao M. Zubair Iqbal Yong Jiang

文章页码：264 - 268

摘要：In this study, B-doped ZnO nanoparticles were synthesized by template-free solvothermal method. X-ray diffraction analysis reveals that B-doped ZnO nanoparticles have hexagonal wurtzite structure. Field emission scanning electron microscopy observations show that the nanoparticles have a diameter of 50 nm. The room temperature ferromagnetism increases monotonically with increasing B concentration to the ZnO nanoparticles and reaches the maximum value of saturation magnetization 0.0178 A·m2 ·kg-1 for 5 % B-doped ZnO nanoparticles. Moreover, photoluminescence spectra reveal that B doping causes to produce Zn vacancies （VZn ）. Magnetic moment of oxygen atoms nearest to the B-VZn vacancy pairs can be considered as a source of ferromagnetism for B-doped ZnO nanoparticles.

详情信息展示

Rare Metals 2013,32(03),264-268+2

Room temperature ferromagnetism of boron-doped ZnO nanoparticles prepared by solvothermal method

M. Hassan Farooq Xiao-Guang Xu Hai-Ling Yang Cong-Jun Ran Jun Miao M. Zubair Iqbal Yong Jiang

State Key Laboratory for Advanced Metals and Materials, School of Materials Science and Engineering, University of Science and Technology Beijing

Department of Physics, School of Applied Science, University of Science and Technology Beijing

作者简介：Xiao-Guang Xu e-mail:xgxu@ustb.edu.cn;

收稿日期：29 October 2012

基金：financially supported by the National Natural Science Foundation of China (Nos. 50831002, 51271020, 51071022, and 11174031);the Program for Changjiang Scholars and Innovative Research Team in University (No. IRT1106);Beijing Nova Program (No. 2011031);the Beijing Municipal Natural Science Foundation (No. 2102032);the Fundamental Research Funds for the Central Universities;

Room temperature ferromagnetism of boron-doped ZnO nanoparticles prepared by solvothermal method

Abstract：

In this study, B-doped ZnO nanoparticles were synthesized by template-free solvothermal method. X-ray diffraction analysis reveals that B-doped ZnO nanoparticles have hexagonal wurtzite structure. Field emission scanning electron microscopy observations show that the nanoparticles have a diameter of 50 nm. The room temperature ferromagnetism increases monotonically with increasing B concentration to the ZnO nanoparticles and reaches the maximum value of saturation magnetization 0.0178 A·m2 ·kg-1 for 5 % B-doped ZnO nanoparticles. Moreover, photoluminescence spectra reveal that B doping causes to produce Zn vacancies (VZn ). Magnetic moment of oxygen atoms nearest to the B-VZn vacancy pairs can be considered as a source of ferromagnetism for B-doped ZnO nanoparticles.

Keyword：

RT ferromagnetism; Non-TM-doped ZnO; Zn vacancies; Photoluminescence;

Received： 29 October 2012

1 Introduction

Zn O is a wide band-gap material of 3.37 e V.Zn O has attracted immense interest due to its vast applications such as solar cells,optoelectronic devices,piezoelectric transducers,and even spintronics devices[1–5].Dilute magnetic semiconductors(DMSs)require to exhibit ferromagnetism above room temperature for the fabrication of spintronics devices.Dietl et al.[6]have suggested Zn O as an ideal room temperature dilute magnetic semiconductor.Room temperature ferromagnetism(RTFM)has been reported in pure Zn O due to structural defects[7]such as oxygen vacancies[8,9]and zinc vacancies[10,11].Yan et al.[12]have reported the zinc interstitial at the interface as a source of ferromagnetism for pure Zn O.Wang et al.and Kim et al.[10,13]have reported the magnetic moment of oxygen atoms around zinc vacancies as a possible origin of ferromagnetism for pure Zn O.Density of these defects is strongly dependent on the experimental conditions and is difficult to control.The traditional way of producing the ferromagnetism in Zn O is doping of transition elements[14–21].An experimental study of transition metal(TM)-doped Zn O provides inconsistent results for ferromagnetism.Therefore,the mechanism of ferromagnetism for TM-doped Zn O is still unclear.Some researchers suggested the incorporation of magnetic atoms into Zn O,whereas,others argued the ferromagnetism in TM-doped Zn O is due to clusters of magnetic ions or their secondary phases[22].To overcome this difficulty,alternative dopants are required to produce the ferromagnetism in Zn O.Nonmagnetic elements can be considered as suitable dopants for overcoming this difficulty.Pan et al.[23]have reported the ferromagnetism in carbon-doped Zn O thin films grown by pulse laser deposition.Defects play an important role in the production of ferromagnetism in nonmagnetic element-doped Zn O[24].According to Liu et al.[25],donor defects can be considered as a source of ferromagnetism in TM-doped Zn O.Xu et al.[26]have reported the ferromagnetism for B-doped Zn O thin films prepared by pulse laser deposition both experimentally and theoretically.Boron acts as a donor element for producing the ferromagnetism in B-doped Zn O thin films.In this study,we discussed the ferromagnetism for B-doped Zn O nanoparticles prepared by solvothermal method for the first time.

2 Experimental

2.1 Sample preparation

B-doped Zn O nanoparticles were prepared by solvothermal process.Zinc acetate dihydrate(Zn(CH₃COO)₂?2H₂O),boron trichloride(BCl₃),and potassium hydroxide(KOH)were used as starting materials.2-methoxyethanol and diethanolamine(DEA)were used as solvent and stabilizer,respectively.The concentration of the solution was maintained at 0.5 mol?L^-1and the molar ratio of DEA to zinc acetate dihydrate was kept at 1.0.The ratio of B:Zn O varied from 0%to 5%.The solution was heated and stirred at 60°C for 2 h to get clear and homogeneous solution.This homogeneous solution was then put into Teflon autoclave.This autoclave was heated at 160°C for 30 h.After that,nanoparticles were separated from the solution by using centrifuge.Separated nanoparticles were dried at 80°C for 8 h to remove organic components mixed in the nanoparticles.

2.2 Characterization

X-ray diffraction(XRD)analysis was conducted on a Rigaku D/Max-RB X-ray diffractometer using Cu Ka radiation(k=0.154056 nm)to characterize the structure of the product.The morphologic details of the samples were analyzed by field emission scanning electron microscopy(FE-SEM)(Zeiss SUPRA55).Magnetic properties of nanoparticles were examined by alternating gradient magnetometer(AGM).Furthermore,photoluminescence(PL)measurements were conducted by using FL-SP920(Edinburgh Instruments)spectrophotometer at room temperature.

3 Results and discussion

Figure 1 represents the XRD pattern for pure Zn O and B-doped Zn O nanoparticles.XRD analysis reveals that pure Zn O and B-doped Zn O have wurtzite hexagonal structure with preferred orientation of(101),and there is no impurity peak observed in the XRD pattern.All the peaks show complete matching with bulk Zn O(JPCD#36-1451).From the XRD pattern,it can be observed that B atoms are completely incorporated into the Zn O lattice sites instead of forming a second phase.The diffraction angle of the(101)peak decreases with the increasing B concentration in the Zn O nanoparticles,which indicates that the lattice parameter increases with increasing the B concentration.It can be considered as the substitution of B atoms to the Zn site.Table 1 shows the variation of lattice parameters(a and c)with B concentration.

Morphology of the nanoparticles is studied by FESEM Figure 2 shows the SEM images of pure and B-doped Zn O nanoparticles.All the nanoparticles are at nearly equal size of*50 nm.These nanoparticles combine to make the microclusters,which can be observed from Fig.2.

Figure 3 shows the M-H loops for pure Zn O and B-doped Zn O nanoparticles for different concentration of boron(1%3%,and 5%).Pure Zn O nanoparticles behave as diamagnetic at room temperature,whereas,all B-doped Zn O nanoparticles exhibit ferromagnetic behavior at room temperature RTFM increases monotonically with B concentration in Zn O nanoparticles.With increasing the B concentration to the Zn O,probability of substitution of B atoms to Zn sites increases and B substituted at Zn site(B_Zn)causes to reduce the formation energy for B-V_Znpairs[26].B-V_Znpair provides stable RTFM,which increases with increasing the dopants concentration.Saturation magnetization and coercivity increase from 0.0075 to 0.0178 A?m²?kg^-1and from 1.97 to5.732 k A?m^-1for 1%–5%B-doped Zn O nanoparticles,respectively.Xu et al.[26]have reported RTFM theoretically and experimentally for B-doped Zn O thin films and suggested that Zn vacancies(V_Zn)played an important role in RTFM,which is inconsistent with our results.There are three possibilities for B-doping to Zn O,i.e.,B at interstitial site(B_In),B substitution at the Zn site(B_Zn),and B substitution at O site(B_O).Formation energy for B_Znis lower than that of others.B atom at Zn site provides a free electron.Liu et al.[25]have reported that donor defects for TM-doped Zn O can be considered as a source of RTFM.First-principle calculations based on the density functional theory(DFT)have been done by using Vienna ab initio simulation package(VASP)for B-doped Zn O and Zn vacancies.Zn vacancy(V_Zn)provides deficiency of two electrons to the Zn O structure.These calculations show that unpaired electrons for B_Znalong V_Zncan be considered as a source of stable RTFM[26].Theoretically it has been predicted that Zn₃₃V_ZnB₂O₃₆has no net magnetic moment due to unavailability of unpaired electrons required for spin polarization.Zn₃₄V_ZnBO₃₆and Zn₃₂V_Zn2B₂O₃₆provide magnetic moments(0.82 and 1.74 l_B)due to the availability of unpaired electrons required for spin polarization of oxygen atoms at the nearest-neighbor sites.This factor is consistent with our experimental results,with increasing the boron concentration to Zn O ferromagnetism increases due to the availability of B-V_Znpairs.

Fig.1 XRD patterns for pure and B-doped Zn O nanoparticles:a XRD patterns from 10°to 90°,and b enlarged view of XRD patterns around(100)and(002)peaks

Table 1 Lattice parameters of Zn O nanoparticles at different B concentrations 下载原图

Table 1 Lattice parameters of Zn O nanoparticles at different B concentrations

Figure 4 represents the PL spectra for pure and B-doped Zn O nanoparticles.Excitation wavelength is taken as340 nm for observing PL spectra.Through observing PL spectra for pure Zn O and B-doped Zn O,near band edge(NBE)transition peak is observed at 375 nm(3.3 e V)for wide band-gap of pure Zn O[27,28].Two peaks can be observed for B-doped Zn O nanoparticles from the PL spectra,the first peak near 400 nm(3 e V),and the second peak near 432 nm(2.87 e V).Samanta et al.[29]and Hofmanm et al.[30]reported that shallow acceptor levels existed at 0.3 and 0.4 e V above the top of the valance band due to Zn vacancies(V_Zn)and oxygen interstitials(O_in)in Zn O.It was reported that the band-gap 3.2 e V for B-doped Zn O thin films,suggesting the B substitution to the Zn site(B_Zn)[31].B_Znprovides the donor level below the bottom of the conduction band and reduces the band gap from 3.3to 3.2 e V.Therefore,donor level for B substitution to the Zn site exists 0.1 e V below the bottom of the conduction band.The first peak near 400 nm(3 e V)for the B-doped Zn O can be attributed to the Zn vacancies(V_Zn),and the second peak near 432 nm(2.87 e V)can be attributed to electron–hole recombination from B donor level(3.2 e V)to Zn vacancy acceptor level(0.3 e V)above the top of the valance band.PL spectra for pure Zn O and B-doped Zn O provide strong evidence that B substitution to the Zn site along with Zn vacancies(B-V_Zn)can be considered as a source of RTFM by spin polarization of the nearestneighbor oxygen atoms.

Fig.2 SEM images for pure and B-doped Zn O nanoparticles:a pure Zn O,b 1%B-doped Zn O,c 3%B-doped Zn O,and d 5%B-doped Zn O

Fig.3 M-H loops for pure and B-doped Zn O nanoparticles at different B concentrations

Fig.4 PL spectra for pure and B-doped Zn O nanoparticles at different B concentrations

4 Conclusion

Pure Zn O and B-doped Zn O nanoparticles were prepared by solvothermal method.All the nanoparticles have wurtzite hexagonal structure with preferred(101)orientation.Morphology of the nanoparticles describes that all prepared nanoparticles are at the same size of*50 nm and combine to form clusters.Saturation magnetization increases monotonically with increasing the B concentration to the Zn O,and reaches maximum value of 0.0178 A?m²?kg^-1for5 at%B-doped Zn O nanoparticles.B substitution at Zn site(B_Zn)along Zn vacancies(V_Zn)provides stable RTFM Unpaired electrons of B-V_Znpairs cause to polarize the spin orbital of nearest-neighbor oxygen atoms,which provide net magnetic moment for RTFM.PL spectra show two strong peaks near 400 and 432 nm for B-doped Zn O nanoparticles,due to Zn vacancies and electron–hole recombination from donor level of B substitution to the Zn sites to acceptor levels of Zn vacancies.PL spectra confirm that unpaired electrons of B-V_Znpairs are responsible for the RTFM in B-doped Zn O nanoparticles.

Acknowledgments This study was financially supported by the National Natural Science Foundation of China(Nos.50831002,51271020,51071022,and 11174031),the Program for Changjiang Scholars and Innovative Research Team in University(No.IRT1106),Beijing Nova Program(No.2011031),the Beijing Municipal Natural Science Foundation(No.2102032),and the Fundamental Research Funds for the Central Universities.