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Study of ferromagnetism in Bi2S3 and ZnS nanocrystalline powders
Physica B 406 (2011) 4661–4665
Contents lists available at SciVerse ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Study of ferromagnetism in Bi2S3 and ZnS nanocrystalline powders Yongjia Zhang a, Hongwei Qin a, Yayan Bao b, Jifan Hu a,n a b
School of Physics, State Key Laboratory for Crystal Materials, Shandong University, Jinan 250100, People’s Republic of China School of Chemical Engineering, Shanxi Datong University, Datong 037009, People’s Republic of China
a r t i c l e i n f o
abstract
Article history: Received 30 May 2011 Received in revised form 18 September 2011 Accepted 19 September 2011 Available online 22 September 2011
Results of magnetic measurements suggested that Bi2S3 and ZnS nanocrystalline powders prepared by hydrothermal method could possibly exhibit room temperature ferromagnetism. The measured saturation magnetization of the powders increases with an increase of annealing temperature from 300 to 500 1C. Ab initio calculations suggested that the cation vacancies on the surface of Bi2S3 and ZnS nanograins could be responsible for the observed magnetic moments. Heat-treatment of Bi2S3 or ZnS nanocrystalline powders in Bi or Zn vapor could bring about an enhancement of ferromagnetism. The calculation results indicated that the interstitial Bi or Zn atoms in Bi2S3 (0 0 1) or ZnS (0 0 1) surface could induce magnetic moments. & 2011 Elsevier B.V. All rights reserved.
Keywords: Magnetism Magnetic measurement Ab initio calculations Vacancy Interstitial
1. Introduction Room temperature ferromagnetism (FM) in pure semiconductors or insulators without any ferromagnetic elements has attracted much attention in the recent years. An unexpected magnetism was observed by Venkatesan et al. in undoped HfO2 thin films [1]. It was found that diamagnetic HfO2 powders presented a weak FM after heating at 750 1C in vacuum, which was eliminated by reheating at 750 1C in air, showing that FM in HfO2 powders came from oxygen vacancies [2]. The room temperature FM was also observed by Hong et al. in TiO2, HfO2, In2O3 and SnO2 thin films. The magnetic moments for In2O3, HfO2 and SnO2 thin films decrease greatly after oxygen-annealing, implying that oxygen vacancy was a key factor in introducing FM [3,4]. Sundaresan et al. found room temperature FM in CeO2, Al2O3, ZnO, In2O3 and SnO2 nanograins, and suggested that ferromagnetism might be a universal characteristic of metal oxide nanoparticles [5]. MgO (containing only s and p electrons) nanograins or thin films exhibit room temperature FM and vacuum or Ar/N2 gas annealing reduces the strength of FM, implying that the observed FM is connected with the Mg vacancies [6–9], rather than oxygen vacancies. First-principle calculations demonstrate that cation vacancies are responsible for the magnetic moments in undoped oxides such as CaO, HfO2, TiO2, ZnO, SnO2, ZrO2 and MgO [10–18]. In addition, room temperature FM has also been experimentally found in undoped GaN semiconductor nanoparticles [19]. Ab initio calculations showed that Ga vacancies could introduce
n
Corresponding author. E-mail address: [email protected] (J. Hu).
0921-4526/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2011.09.055
magnetic moments for GaN both in the cubic structure and wurtzite structure [20–23]. Room temperature FM has been observed in undoped CdS nanograins [19]. However, the origin of magnetism in undoped sulfides is not clear. A question remains: which kind of vacancies (cation or anion) can produce magnetic moments in undoped sulfides? The discussion on FM in above literatures was mainly concentrated upon the ideal intrinsic vacancies could induce FM. On the other hand, evidence that interstitial atoms could exist in nanoparticles have been obtained [24–32]. Schwartz and Gamelin firstly found that heat-treatment of Co2þ : ZnO thin film in Zn vapor could bring about interstitial atoms Zn in Co2 þ : ZnO lattice, which strongly influences the magnetism of the thin film [24]. Stimulated by the work of Schwartz and Gamelin, we have one question: whether the interstitial atoms in the lattice could influence the FM of semiconductors or insulators without any ferromagnetic elements? In this work, we experimentally demonstrated that Bi2S3 and ZnS nanocrystalline powders exhibited room temperature FM. Heat-treatment of Bi2S3 or ZnS nanocrystalline powders in Bi or Zn vapor could bring about an enhancement of ferromagnetism. Our calculation results indicated that both cation vacancies and interstitial atoms could bring about magnetic moments, leading to ferromagnetic coupling. The enhancement of FM after heattreatment in Bi or Zn vapor might result from the influence of interstitial atoms on magnetism.
2. Experiments Bi2S3 nanopowders were prepared by the hydrothermal method. Certain amounts of Bi (NO3)3 5H2O and Na2S 9H2O
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Y. Zhang et al. / Physica B 406 (2011) 4661–4665
and deionized water were put into a Teflon autoclave. The autoclave was heated to 110 1C and maintained for 4 h, and then cooled to room temperature. The precipitates were washed with deionized water, then dried in vacuum at 60 1C for 4 h, and annealed at temperature TA ¼300 1C and 500 1C for 30 min in vacuum, respectively. Part of the samples were treated in Bi vapor (0.5 Pa) at 300 1C for 30 min by heating Bi metal in vacuum. ZnS powders were also prepared by the hydrothermal method. An aqueous solution of ZnCl2 and sulfourea was mixed with thioalcohol, which was then put into a Teflon autoclave. After heated to 160 1C and maintained for 8 h, the precipitates were washed with deionized water, then dried in vacuum, and annealed at temperature TA ¼300 1C and 500 1C for 30 min in vacuum. Part of the samples were treated in Zn vapor (0.5 Pa) at 500 1C for 30 min by heating Zn metal in vacuum. The structures of vacuum annealed Bi2S3 and ZnS powders were investigated by X-ray diffraction with Cu Ka radiation. The magnetic properties of all the samples were measured using alternating gradient magnetometer (AGM) at room temperature.
between neighboring surfaces. All the surfaces were fully relaxed. Fig. 1 shows the slab models of Bi2S3 (0 0 1) and ZnS (0 0 1) surfaces. The concentrations of V 0Bi and V 0Zn vacancies for the Bi2S3 (0 0 1) and ZnS (0 0 1) surfaces were 5% and 4.17%, respectively. The concentrations of V 0S vacancies for the Bi2S3 (0 0 1) and ZnS (0 0 1) surfaces were 3.33% and 4.17%, respectively. Different distributions of two isolated V 0Bi and V 0Zn vacancies have been investigated. Each structure was fully relaxed before calculating the magnetic moments and density of states (DOS). A plane wave cutoff of 400 eV was used for the basis set. The Brillouin zone integrations were performed using 2 2 3 k-points mesh sampling in the calculation process for Bi2S3 (0 0 1) and ZnS (0 0 1) surfaces, respectively, which were generated by the Monkhorst– Pack scheme. Good convergence was obtained with these parameters. The total energy was converged to be 1.0 10 4 eV/atom, while the Hellman–Feynman force was smaller than 0.01 eV/A˚ in the optimized structure.
4. Results and discussion 3. Computational method Our density functional theory (DFT) calculations were performed using the plane-wave pseudopotential method in the Vienna ab initio simulation package (VASP) [33,34]. The PAW potentials were employed, and the GGA PW91 approximation was used to describe the exchange correlation energy [35,36]. A 1 1 3 bulk Bi2S3 supercell was relaxed for cleaving the surface and getting accurate lattice parameters. The Bi2S3 (0 0 1) surface was cut from the relaxed bulk supercell and a slab of atom lays containing 50 atoms was employed, whereas the ZnS (0 0 1) surface cut from the relaxed 2 2 2 bulk ZnS supercell and a slab of atom lays containing 48 atoms employed. Each slab was separated from the other by a vacuum region of 10 A˚ along the c direction, which was wide enough to inhibit the interaction
Fig. 1. (a) Orthorhombic structure for Bi2S3. Violet balls represent the Bi atoms and the yellow balls represent the S atoms (b) Wurtzite structure for ZnS. Gray balls represent the Zn atoms and the yellow balls represent the S atoms (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
The X-ray diffraction (XRD) patterns of Bi2S3 and ZnS nanocrystalline powders prepared at TA ¼300 1C in vacuum for 30 min was shown in Fig. 2, which indicates that all the samples were single phase without any impurities. In the case of Bi2S3, the powders crystallize as an orthorhombic phase with space group of Pbnm. The lattice parameters could be determined as ˚ b¼11.292 A˚ and c ¼3.969 A˚ for TA ¼ 300 1C. Based a¼11.249 A, on the Scherrer’s formula, the grain size is estimated to be 25 nm for the sample (TA ¼300 1C). As for ZnS, XRD pattern shows that ZnS sample is in a wurtzite structure with space group of P63mc. ˚ The lattice parameters are estimated as: a ¼b¼3.803 A, ˚ and the particle size is determined as 21 nm. The c¼6.222 A, room temperature magnetization (M) versus magnetic field (H) curves after diamagnetic correction for Bi2S3 and ZnS nanocrystalline powders annealed at TA ¼300 1C and TA ¼500 1C in vacuum are shown in Fig. 3. The value of saturation magnetization Ms of Bi2S3 vacuum-annealed sample is 4.8 10 3 emu/g at TA ¼300 1C, and 14.9 10 3 emu/g at TA ¼500 1C. The Ms value of ZnS vacuum-annealed powder is 5.29 10 3 emu/g for TA ¼300 1C and 6.24 10 3 emu/g for TA ¼500 1C, respectively. Generally, there is an abundant of defects or vacancies in the surface of nanograins, and the vacancy could induce the
Fig. 2. XRD patterns of (a) Bi2S3 and (b) ZnS nanocrystalline powders annealed at 300 1C in vacuum.
Y. Zhang et al. / Physica B 406 (2011) 4661–4665
Fig. 3. Magnetization (M) versus magnetic field (H) curves measured at room temperature for (a) Bi2S3 and (b) ZnS nanocrystalline powders with an annealing at temperature TA ¼ 300 1C and 500 1C for 30 min in vacuum.
magnetism in nano-materials, such as oxides, nitrides and sulfides [1–23]. In order to verify, which kind of vacancies (cation or anion) can produce magnetic moments in Bi2S3 and ZnS nanocrystals, we performed the density functional theory calculation on magnetism. Fig. 4(a) and (b) display the spin-resolved total density of states (total DOS) of stoichiometric Bi2S3 (0 0 1) and ZnS (0 0 1) surfaces. No spin-polarization emerges around the Fermi energy level, indicating that stoichiometric Bi2S3 (0 0 1) and ZnS (0 0 1) surfaces are non-magnetic. Fig. 4 (c) presents the total DOS of the Bi2S3 with 3.33% S vacancy (the S1 was removed, see Fig. 1(a)). No spin-polarization occurs around the Fermi energy level, indicating that the V 0S vacancy does not induce magnetism in Bi2S3 (0 0 1) surface. The total DOS as well as partial DOS (PDOS) of Bi2S3 with 5% V 0Bi vacancy (the Bi1 was removed, see Fig. 1(a)) are shown in Fig. 4(e). A spin-split around the Fermi level can be obviously seen, which is mainly due to the spinpolarizations of S p electrons, producing a magnetic moment of 1.5 mB. Similar situation occurs in the case of ZnS (0 0 1) surface. The ZnS (0 0 1) surface with 4.17 vacancy (the S1 removed, see Fig. 1(b)) is non-magnetic (see Fig. 4(d)), while that with vacancy (the Zn1 was removed, see Fig. 1(b)) is spin polarized, which mainly comes from the polarizations of S p electrons (see Fig. 4(f)), leading to a magnetic moment of 1.1 mB. In order to investigate the magnetic coupling between the two isolated vacancies, we studied the relative energies of the states between ferro- and antiferromagnetic coupling by GGA calculations. For Bi2S3 supercell with 10% V 0Bi vacancies, we investigated three distributions of two isolated neutral bismuth vacancies. The three cases are removing Bi1 and Bi2, Bi1 and Bi3, Bi2 and Bi3 from the system (see Fig. 1(a)). The obtained magnetic moments for three cases are 3.01, 3.6 and 4.29 mB, respectively. For the above three cases, the total energies of the states with ferromagnetic coupling are found to be lower than the energies of the states with antiferromagnetic coupling, respectively. That is to say, ferromagnetic coupling is energetically favorable. As in the
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Fig. 4. Spin resolved total DOSs for (a) Bi2S3 (0 0 1) surface, (b) ZnS (0 0 1) surface, (c) Bi2S3 (0 0 1) surface with one S vacancy (3.33% in supercell) and (d) ZnS (0 0 1) surface with one S vacancy (4.17% in supercell). Total DOSs (solid lines) and partial DOSs (dotted lines) for (e) Bi2S3 (0 0 1) surface with one Bi vacancy (5% in supercell) and (f) ZnS (0 0 1) surface with one Zn vacancy (4.17% in supercell). The red and blue lines represent S, Bi and Zn p states, respectively, (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
case of ZnS supercell with 8.34% V 0Zn vacancy, we also investigated three distributions of two isolated neutral zinc vacancies. The three cases are removing Zn1 and Zn2, Zn1 and Zn3, Zn1 and Zn4 from the system (see Fig. 1(b)), which produce 2.21, 2.62 and 3.04 mB, respectively. And the ferromagnetic coupling states are more favorable. For Zn or Bi vacancies, the magnetic moments located at the S atoms surrounding Zn or Bi vacancies. Fig. 5 shows the magnetization (M) versus magnetic field (H) curves measured at room temperature for (a) ZnS nanocrystalline powders with an annealing at temperature TA ¼ 500 1C for 30 min in vacuum and Zn vapor, respectively; (b) Bi2S3 powders with an annealing at temperature TA ¼300 1C for 30 min in vacuum and Bi vapor, respectively. The Ms value of ZnS sample after heat-treatment in Zn vapor is 8.092 10 3 emu/g, and the value of Bi2S3 nanopowers after Bi heat-treatment is 23.4 10 3 emu/g, both larger than that of ZnS and Bi2S3 nanocrystalline powders before heattreatment in metal vapor. It can be expected that the interstitial atoms Zn or Bi could be introduced in ZnS or Bi2S3 lattice through heat-treatment in vapor of Zn or Bi. It seems that the introduction of interstitial atoms in the lattice of the surface of nanograin could enhance the FM in ZnS or Bi2S3 nanocrystalline powders. In order to give more insight to the above experimental phenomenon, the influence of interstitial atoms Zn or Bi on the magnetism of ZnS or Bi2S3 nanocrystals was investigated with ab initio calculation. For ZnS (0 0 1) surface, there are two distinct types of interstitial sites in the wurtzite structure: the tetrahedral site and octahedral site. After optimization, the tetrahedral site spontaneously relaxed to octahedral site, indicating that the Zn interstitial (Zni ) atom prefers to the octahedral site where the relative energy is more stable. Moreover, in the ideal octahedral site, we observed that the bond length of Zni Zn was 1.15d0 and that of Zni S was 1.03d0, where d0 was the ZnS bond length
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Fig. 7. Total DOSs (solid lines) and partial DOSs (dotted lines) for (a) the ZnS (0 0 1) surface with one interstitial Zn (4.17% in supercell) and (b) the Bi2S3 (0 0 1) surface with one interstitial Bi (5% in supercell). The red and blue lines represent S, Bi and Zn p states, respectively, (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. Magnetization (M) versus magnetic field (H) curves measured at room temperature for (a) ZnS nanocrystalline powders with an annealing at temperature TA ¼ 500 1C for 30 min in vacuum and Zn vapor, respectively, and (b) Bi2S3 powders with an annealing at temperature TA ¼300 1C for 30 min in vacuum and Bi vapor, respectively.
Fig. 6. Local atomic geometry of the zinc interstitial at the stable octahedral site in ZnS (0 0 1) surface. (a) Top view parallel to the c axis (along the (0 0 1) direction), (b) Side view perpendicular to the c axis. Local atomic geometry of the bismuth interstitial at the stable site in Bi2S3 (0 0 1) surface, (c) Top view parallel to the c axis (along the (0 0 1) direction) and (d) Side view perpendicular to the c axis. Violet balls, Gray balls and yellow balls represent Bi atoms, Zn atoms and S atoms, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
˚ (see Fig. 6(a) and (b)). We also chose several types (d0 ¼2.350 A) of Bi interstitial (Bii ) sites in the orthorhombic structure of Bi2S3. The preferential position of bismuth interstitial atom after relaxation is shown in Fig. 6(c) and (d), where the closest S and Bi atoms are pushed away. The magnetism of the ZnS (0 0 1) surface with the Zni atom is investigated. Fig. 7(a) shows the total DOS as well as PDOS projected onto p orbital on S and Zn atoms when one Zni atom
(4.17% in supercell) can be introduced in ZnS (0 0 1) surface. An obvious spin-split is present around the Fermi level in total DOS, indicative of the existence of local magnetic moments. The PDOS indicates that the spin-polarization of p electrons of S and Zn atoms, which locate at the Fermi level should be responsible for the induced magnetic moments. According to our GGA calculations, the system presents a net magnetic moment of 0.582 mB with a concentration of 4.17% Zni atom. The total magnetic moments of the S and Zn atoms nearby Zni atom are 0.321 and 0.144 mB, respectively, and the sum of both takes a percentage of nearly 59.6% in the total magnetic moments. Fig. 7(b) shows the total DOS and PDOS of Bi and S atoms projected onto p orbital with one Bii atom (5%) in Bi2S3 (0 0 1) surface. Bii could bring about magnetic moment due to the spin-polarizations of Bi and S p electrons, producing a magnetic moment of 0.591 mB. The total magnetic moments of the S and Bi atoms nearby Bii atom are 0.029 and 0.355 mB, respectively, and the sum of both takes a percentage of nearly 64.9% in the total magnetic moments. Two interstitial atoms were created in the supercell with various distances, and the total energies of the ferromagnetic and antiferromagnetic states were calculated. The results indicated that the ferromagnetic coupling is more favorable in all cases. For interstitial Zn atoms or interstitial Bi atoms, the magnetic moments mainly located at interstitial atoms.
5. Conclusions In this work, the results of magnetic measurements indicate that Bi2S3 and ZnS nanocrystalline powders prepared by hydrothermal method could possibly exhibit room temperature ferromagnetism. The measured saturation magnetization of the powders increases with an increase of annealing temperature from 300 to 5001C. Ab initio calculations suggested that the cation vacancies on the surface of Bi2S3 and ZnS nanograins could be responsible for the observed magnetic moments. Heat-treatment of Bi2S3 or ZnS nanocrystalline powders in Bi or Zn vapor could bring about an enhancement of ferromagnetism. The calculation
Y. Zhang et al. / Physica B 406 (2011) 4661–4665
results indicated that the interstitial Bi or Zn atoms in Bi2S3 (0 0 1) or ZnS (0 0 1) surface could induce magnetic moments.
Acknowledgments This work was supported by National Natural Science Foundation of China (nos: 51072103, 50872074, and 50872069), and National Found for Fostering Talents of Basic Science (no. J0730318). References [1] M. Venkatesan, C.B. Fitzgerald, J.M.D. Coey, Nature (London) 430 (2004) 630. [2] J.M.D. Coey, M. Venkatesan, P. Stamenov, C.B. Fitzgerald, L.S. Dorneles, Phys. Rev. B 72 (2005) 024450. [3] N.H. Hong, J. Sakai, N. Poirot, V. Brize´, Phys. Rev. B 73 (2006) 132404. [4] N.H. Hong, N. Poirot, J. Sakai, Phys. Rev. B 77 (2008) 033205. [5] A. Sundaresan, R. Bhargavi, N. Rangarajan, U. Siddesh, C.N.R. Rao, Phys. Rev. B 74 (2006) 161306. [6] J.F. Hu, Z.L. Zhang, M. Zhao, H.W. Qin, M.H. Jiang, Appl. Phys. Lett. 93 (2008) 192503. [7] J.I. Beltra´n, C. Monty, Ll. Balcells, C. Martı´nez-Boubeta, Solid State Commun. 149 (2009) 1654. [8] N. Kumar, D. Sanyal, A. Sundaresan, Chem. Phys. Lett. 477 (2009) 360. [9] C.M. Araujo, M. Kapilashrami, X. Jun, O.D. Jayakumar, S. Nagar, Y. Wu, ˚ C. Arhammar, B. Johansson, L. Belova, R. Ahuja, G.A. Gehring, K.V. Rao, Appl. Phys. Lett. 96 (2010) 232505. [10] C. Das Pemmaraju, S. Sanvito, Phys. Rev. Lett. 94 (2005) 217205. [11] Y. Bai, Q. Chen, Phys. Status Solidi RRL 2 (2008) 25.
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Contents lists available at SciVerse ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Study of ferromagnetism in Bi2S3 and ZnS nanocrystalline powders Yongjia Zhang a, Hongwei Qin a, Yayan Bao b, Jifan Hu a,n a b
School of Physics, State Key Laboratory for Crystal Materials, Shandong University, Jinan 250100, People’s Republic of China School of Chemical Engineering, Shanxi Datong University, Datong 037009, People’s Republic of China
a r t i c l e i n f o
abstract
Article history: Received 30 May 2011 Received in revised form 18 September 2011 Accepted 19 September 2011 Available online 22 September 2011
Results of magnetic measurements suggested that Bi2S3 and ZnS nanocrystalline powders prepared by hydrothermal method could possibly exhibit room temperature ferromagnetism. The measured saturation magnetization of the powders increases with an increase of annealing temperature from 300 to 500 1C. Ab initio calculations suggested that the cation vacancies on the surface of Bi2S3 and ZnS nanograins could be responsible for the observed magnetic moments. Heat-treatment of Bi2S3 or ZnS nanocrystalline powders in Bi or Zn vapor could bring about an enhancement of ferromagnetism. The calculation results indicated that the interstitial Bi or Zn atoms in Bi2S3 (0 0 1) or ZnS (0 0 1) surface could induce magnetic moments. & 2011 Elsevier B.V. All rights reserved.
Keywords: Magnetism Magnetic measurement Ab initio calculations Vacancy Interstitial
1. Introduction Room temperature ferromagnetism (FM) in pure semiconductors or insulators without any ferromagnetic elements has attracted much attention in the recent years. An unexpected magnetism was observed by Venkatesan et al. in undoped HfO2 thin films [1]. It was found that diamagnetic HfO2 powders presented a weak FM after heating at 750 1C in vacuum, which was eliminated by reheating at 750 1C in air, showing that FM in HfO2 powders came from oxygen vacancies [2]. The room temperature FM was also observed by Hong et al. in TiO2, HfO2, In2O3 and SnO2 thin films. The magnetic moments for In2O3, HfO2 and SnO2 thin films decrease greatly after oxygen-annealing, implying that oxygen vacancy was a key factor in introducing FM [3,4]. Sundaresan et al. found room temperature FM in CeO2, Al2O3, ZnO, In2O3 and SnO2 nanograins, and suggested that ferromagnetism might be a universal characteristic of metal oxide nanoparticles [5]. MgO (containing only s and p electrons) nanograins or thin films exhibit room temperature FM and vacuum or Ar/N2 gas annealing reduces the strength of FM, implying that the observed FM is connected with the Mg vacancies [6–9], rather than oxygen vacancies. First-principle calculations demonstrate that cation vacancies are responsible for the magnetic moments in undoped oxides such as CaO, HfO2, TiO2, ZnO, SnO2, ZrO2 and MgO [10–18]. In addition, room temperature FM has also been experimentally found in undoped GaN semiconductor nanoparticles [19]. Ab initio calculations showed that Ga vacancies could introduce
n
Corresponding author. E-mail address: [email protected] (J. Hu).
0921-4526/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2011.09.055
magnetic moments for GaN both in the cubic structure and wurtzite structure [20–23]. Room temperature FM has been observed in undoped CdS nanograins [19]. However, the origin of magnetism in undoped sulfides is not clear. A question remains: which kind of vacancies (cation or anion) can produce magnetic moments in undoped sulfides? The discussion on FM in above literatures was mainly concentrated upon the ideal intrinsic vacancies could induce FM. On the other hand, evidence that interstitial atoms could exist in nanoparticles have been obtained [24–32]. Schwartz and Gamelin firstly found that heat-treatment of Co2þ : ZnO thin film in Zn vapor could bring about interstitial atoms Zn in Co2 þ : ZnO lattice, which strongly influences the magnetism of the thin film [24]. Stimulated by the work of Schwartz and Gamelin, we have one question: whether the interstitial atoms in the lattice could influence the FM of semiconductors or insulators without any ferromagnetic elements? In this work, we experimentally demonstrated that Bi2S3 and ZnS nanocrystalline powders exhibited room temperature FM. Heat-treatment of Bi2S3 or ZnS nanocrystalline powders in Bi or Zn vapor could bring about an enhancement of ferromagnetism. Our calculation results indicated that both cation vacancies and interstitial atoms could bring about magnetic moments, leading to ferromagnetic coupling. The enhancement of FM after heattreatment in Bi or Zn vapor might result from the influence of interstitial atoms on magnetism.
2. Experiments Bi2S3 nanopowders were prepared by the hydrothermal method. Certain amounts of Bi (NO3)3 5H2O and Na2S 9H2O
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Y. Zhang et al. / Physica B 406 (2011) 4661–4665
and deionized water were put into a Teflon autoclave. The autoclave was heated to 110 1C and maintained for 4 h, and then cooled to room temperature. The precipitates were washed with deionized water, then dried in vacuum at 60 1C for 4 h, and annealed at temperature TA ¼300 1C and 500 1C for 30 min in vacuum, respectively. Part of the samples were treated in Bi vapor (0.5 Pa) at 300 1C for 30 min by heating Bi metal in vacuum. ZnS powders were also prepared by the hydrothermal method. An aqueous solution of ZnCl2 and sulfourea was mixed with thioalcohol, which was then put into a Teflon autoclave. After heated to 160 1C and maintained for 8 h, the precipitates were washed with deionized water, then dried in vacuum, and annealed at temperature TA ¼300 1C and 500 1C for 30 min in vacuum. Part of the samples were treated in Zn vapor (0.5 Pa) at 500 1C for 30 min by heating Zn metal in vacuum. The structures of vacuum annealed Bi2S3 and ZnS powders were investigated by X-ray diffraction with Cu Ka radiation. The magnetic properties of all the samples were measured using alternating gradient magnetometer (AGM) at room temperature.
between neighboring surfaces. All the surfaces were fully relaxed. Fig. 1 shows the slab models of Bi2S3 (0 0 1) and ZnS (0 0 1) surfaces. The concentrations of V 0Bi and V 0Zn vacancies for the Bi2S3 (0 0 1) and ZnS (0 0 1) surfaces were 5% and 4.17%, respectively. The concentrations of V 0S vacancies for the Bi2S3 (0 0 1) and ZnS (0 0 1) surfaces were 3.33% and 4.17%, respectively. Different distributions of two isolated V 0Bi and V 0Zn vacancies have been investigated. Each structure was fully relaxed before calculating the magnetic moments and density of states (DOS). A plane wave cutoff of 400 eV was used for the basis set. The Brillouin zone integrations were performed using 2 2 3 k-points mesh sampling in the calculation process for Bi2S3 (0 0 1) and ZnS (0 0 1) surfaces, respectively, which were generated by the Monkhorst– Pack scheme. Good convergence was obtained with these parameters. The total energy was converged to be 1.0 10 4 eV/atom, while the Hellman–Feynman force was smaller than 0.01 eV/A˚ in the optimized structure.
4. Results and discussion 3. Computational method Our density functional theory (DFT) calculations were performed using the plane-wave pseudopotential method in the Vienna ab initio simulation package (VASP) [33,34]. The PAW potentials were employed, and the GGA PW91 approximation was used to describe the exchange correlation energy [35,36]. A 1 1 3 bulk Bi2S3 supercell was relaxed for cleaving the surface and getting accurate lattice parameters. The Bi2S3 (0 0 1) surface was cut from the relaxed bulk supercell and a slab of atom lays containing 50 atoms was employed, whereas the ZnS (0 0 1) surface cut from the relaxed 2 2 2 bulk ZnS supercell and a slab of atom lays containing 48 atoms employed. Each slab was separated from the other by a vacuum region of 10 A˚ along the c direction, which was wide enough to inhibit the interaction
Fig. 1. (a) Orthorhombic structure for Bi2S3. Violet balls represent the Bi atoms and the yellow balls represent the S atoms (b) Wurtzite structure for ZnS. Gray balls represent the Zn atoms and the yellow balls represent the S atoms (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
The X-ray diffraction (XRD) patterns of Bi2S3 and ZnS nanocrystalline powders prepared at TA ¼300 1C in vacuum for 30 min was shown in Fig. 2, which indicates that all the samples were single phase without any impurities. In the case of Bi2S3, the powders crystallize as an orthorhombic phase with space group of Pbnm. The lattice parameters could be determined as ˚ b¼11.292 A˚ and c ¼3.969 A˚ for TA ¼ 300 1C. Based a¼11.249 A, on the Scherrer’s formula, the grain size is estimated to be 25 nm for the sample (TA ¼300 1C). As for ZnS, XRD pattern shows that ZnS sample is in a wurtzite structure with space group of P63mc. ˚ The lattice parameters are estimated as: a ¼b¼3.803 A, ˚ and the particle size is determined as 21 nm. The c¼6.222 A, room temperature magnetization (M) versus magnetic field (H) curves after diamagnetic correction for Bi2S3 and ZnS nanocrystalline powders annealed at TA ¼300 1C and TA ¼500 1C in vacuum are shown in Fig. 3. The value of saturation magnetization Ms of Bi2S3 vacuum-annealed sample is 4.8 10 3 emu/g at TA ¼300 1C, and 14.9 10 3 emu/g at TA ¼500 1C. The Ms value of ZnS vacuum-annealed powder is 5.29 10 3 emu/g for TA ¼300 1C and 6.24 10 3 emu/g for TA ¼500 1C, respectively. Generally, there is an abundant of defects or vacancies in the surface of nanograins, and the vacancy could induce the
Fig. 2. XRD patterns of (a) Bi2S3 and (b) ZnS nanocrystalline powders annealed at 300 1C in vacuum.
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Fig. 3. Magnetization (M) versus magnetic field (H) curves measured at room temperature for (a) Bi2S3 and (b) ZnS nanocrystalline powders with an annealing at temperature TA ¼ 300 1C and 500 1C for 30 min in vacuum.
magnetism in nano-materials, such as oxides, nitrides and sulfides [1–23]. In order to verify, which kind of vacancies (cation or anion) can produce magnetic moments in Bi2S3 and ZnS nanocrystals, we performed the density functional theory calculation on magnetism. Fig. 4(a) and (b) display the spin-resolved total density of states (total DOS) of stoichiometric Bi2S3 (0 0 1) and ZnS (0 0 1) surfaces. No spin-polarization emerges around the Fermi energy level, indicating that stoichiometric Bi2S3 (0 0 1) and ZnS (0 0 1) surfaces are non-magnetic. Fig. 4 (c) presents the total DOS of the Bi2S3 with 3.33% S vacancy (the S1 was removed, see Fig. 1(a)). No spin-polarization occurs around the Fermi energy level, indicating that the V 0S vacancy does not induce magnetism in Bi2S3 (0 0 1) surface. The total DOS as well as partial DOS (PDOS) of Bi2S3 with 5% V 0Bi vacancy (the Bi1 was removed, see Fig. 1(a)) are shown in Fig. 4(e). A spin-split around the Fermi level can be obviously seen, which is mainly due to the spinpolarizations of S p electrons, producing a magnetic moment of 1.5 mB. Similar situation occurs in the case of ZnS (0 0 1) surface. The ZnS (0 0 1) surface with 4.17 vacancy (the S1 removed, see Fig. 1(b)) is non-magnetic (see Fig. 4(d)), while that with vacancy (the Zn1 was removed, see Fig. 1(b)) is spin polarized, which mainly comes from the polarizations of S p electrons (see Fig. 4(f)), leading to a magnetic moment of 1.1 mB. In order to investigate the magnetic coupling between the two isolated vacancies, we studied the relative energies of the states between ferro- and antiferromagnetic coupling by GGA calculations. For Bi2S3 supercell with 10% V 0Bi vacancies, we investigated three distributions of two isolated neutral bismuth vacancies. The three cases are removing Bi1 and Bi2, Bi1 and Bi3, Bi2 and Bi3 from the system (see Fig. 1(a)). The obtained magnetic moments for three cases are 3.01, 3.6 and 4.29 mB, respectively. For the above three cases, the total energies of the states with ferromagnetic coupling are found to be lower than the energies of the states with antiferromagnetic coupling, respectively. That is to say, ferromagnetic coupling is energetically favorable. As in the
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Fig. 4. Spin resolved total DOSs for (a) Bi2S3 (0 0 1) surface, (b) ZnS (0 0 1) surface, (c) Bi2S3 (0 0 1) surface with one S vacancy (3.33% in supercell) and (d) ZnS (0 0 1) surface with one S vacancy (4.17% in supercell). Total DOSs (solid lines) and partial DOSs (dotted lines) for (e) Bi2S3 (0 0 1) surface with one Bi vacancy (5% in supercell) and (f) ZnS (0 0 1) surface with one Zn vacancy (4.17% in supercell). The red and blue lines represent S, Bi and Zn p states, respectively, (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
case of ZnS supercell with 8.34% V 0Zn vacancy, we also investigated three distributions of two isolated neutral zinc vacancies. The three cases are removing Zn1 and Zn2, Zn1 and Zn3, Zn1 and Zn4 from the system (see Fig. 1(b)), which produce 2.21, 2.62 and 3.04 mB, respectively. And the ferromagnetic coupling states are more favorable. For Zn or Bi vacancies, the magnetic moments located at the S atoms surrounding Zn or Bi vacancies. Fig. 5 shows the magnetization (M) versus magnetic field (H) curves measured at room temperature for (a) ZnS nanocrystalline powders with an annealing at temperature TA ¼ 500 1C for 30 min in vacuum and Zn vapor, respectively; (b) Bi2S3 powders with an annealing at temperature TA ¼300 1C for 30 min in vacuum and Bi vapor, respectively. The Ms value of ZnS sample after heat-treatment in Zn vapor is 8.092 10 3 emu/g, and the value of Bi2S3 nanopowers after Bi heat-treatment is 23.4 10 3 emu/g, both larger than that of ZnS and Bi2S3 nanocrystalline powders before heattreatment in metal vapor. It can be expected that the interstitial atoms Zn or Bi could be introduced in ZnS or Bi2S3 lattice through heat-treatment in vapor of Zn or Bi. It seems that the introduction of interstitial atoms in the lattice of the surface of nanograin could enhance the FM in ZnS or Bi2S3 nanocrystalline powders. In order to give more insight to the above experimental phenomenon, the influence of interstitial atoms Zn or Bi on the magnetism of ZnS or Bi2S3 nanocrystals was investigated with ab initio calculation. For ZnS (0 0 1) surface, there are two distinct types of interstitial sites in the wurtzite structure: the tetrahedral site and octahedral site. After optimization, the tetrahedral site spontaneously relaxed to octahedral site, indicating that the Zn interstitial (Zni ) atom prefers to the octahedral site where the relative energy is more stable. Moreover, in the ideal octahedral site, we observed that the bond length of Zni Zn was 1.15d0 and that of Zni S was 1.03d0, where d0 was the ZnS bond length
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Fig. 7. Total DOSs (solid lines) and partial DOSs (dotted lines) for (a) the ZnS (0 0 1) surface with one interstitial Zn (4.17% in supercell) and (b) the Bi2S3 (0 0 1) surface with one interstitial Bi (5% in supercell). The red and blue lines represent S, Bi and Zn p states, respectively, (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. Magnetization (M) versus magnetic field (H) curves measured at room temperature for (a) ZnS nanocrystalline powders with an annealing at temperature TA ¼ 500 1C for 30 min in vacuum and Zn vapor, respectively, and (b) Bi2S3 powders with an annealing at temperature TA ¼300 1C for 30 min in vacuum and Bi vapor, respectively.
Fig. 6. Local atomic geometry of the zinc interstitial at the stable octahedral site in ZnS (0 0 1) surface. (a) Top view parallel to the c axis (along the (0 0 1) direction), (b) Side view perpendicular to the c axis. Local atomic geometry of the bismuth interstitial at the stable site in Bi2S3 (0 0 1) surface, (c) Top view parallel to the c axis (along the (0 0 1) direction) and (d) Side view perpendicular to the c axis. Violet balls, Gray balls and yellow balls represent Bi atoms, Zn atoms and S atoms, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
˚ (see Fig. 6(a) and (b)). We also chose several types (d0 ¼2.350 A) of Bi interstitial (Bii ) sites in the orthorhombic structure of Bi2S3. The preferential position of bismuth interstitial atom after relaxation is shown in Fig. 6(c) and (d), where the closest S and Bi atoms are pushed away. The magnetism of the ZnS (0 0 1) surface with the Zni atom is investigated. Fig. 7(a) shows the total DOS as well as PDOS projected onto p orbital on S and Zn atoms when one Zni atom
(4.17% in supercell) can be introduced in ZnS (0 0 1) surface. An obvious spin-split is present around the Fermi level in total DOS, indicative of the existence of local magnetic moments. The PDOS indicates that the spin-polarization of p electrons of S and Zn atoms, which locate at the Fermi level should be responsible for the induced magnetic moments. According to our GGA calculations, the system presents a net magnetic moment of 0.582 mB with a concentration of 4.17% Zni atom. The total magnetic moments of the S and Zn atoms nearby Zni atom are 0.321 and 0.144 mB, respectively, and the sum of both takes a percentage of nearly 59.6% in the total magnetic moments. Fig. 7(b) shows the total DOS and PDOS of Bi and S atoms projected onto p orbital with one Bii atom (5%) in Bi2S3 (0 0 1) surface. Bii could bring about magnetic moment due to the spin-polarizations of Bi and S p electrons, producing a magnetic moment of 0.591 mB. The total magnetic moments of the S and Bi atoms nearby Bii atom are 0.029 and 0.355 mB, respectively, and the sum of both takes a percentage of nearly 64.9% in the total magnetic moments. Two interstitial atoms were created in the supercell with various distances, and the total energies of the ferromagnetic and antiferromagnetic states were calculated. The results indicated that the ferromagnetic coupling is more favorable in all cases. For interstitial Zn atoms or interstitial Bi atoms, the magnetic moments mainly located at interstitial atoms.
5. Conclusions In this work, the results of magnetic measurements indicate that Bi2S3 and ZnS nanocrystalline powders prepared by hydrothermal method could possibly exhibit room temperature ferromagnetism. The measured saturation magnetization of the powders increases with an increase of annealing temperature from 300 to 5001C. Ab initio calculations suggested that the cation vacancies on the surface of Bi2S3 and ZnS nanograins could be responsible for the observed magnetic moments. Heat-treatment of Bi2S3 or ZnS nanocrystalline powders in Bi or Zn vapor could bring about an enhancement of ferromagnetism. The calculation
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results indicated that the interstitial Bi or Zn atoms in Bi2S3 (0 0 1) or ZnS (0 0 1) surface could induce magnetic moments.
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