Magnetism in Dopant-Free Hexagonal CdS Nanorods: Experiments

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Magnetism in Dopant-Free Hexagonal CdS Nanorods: Experiments and First-Principles Analysis Donglin Guo, Hao Hua, Qi Yang, Xiaoyan Li, and Chenguo Hu* Department of Applied Physics, Chongqing University, Chongqing 400044, P. R. China S Supporting Information *

ABSTRACT: We report the ferromagnetism of hexagonal CdS nanorods with a Curie temperature of 305 K, which is found in our experiments. Our first-principles calculations suggest that the magnetism is a result of the Cd vacancy. Compared with nonspin-polarized state in bulk CdS with one Cd vacancy, it is found that the ground state is spin-polarized. The Cd vacancy causes 1↓ 3↑ 3↓ 1↑ 1↓ 3↑ 1↓ the electron configuration of S 2p to change from (a1↑ 1 )(a1 )(t2 )(t2 ) to (a1 )(a1 )(t2 )(t2 ), forming a local magnetic moment of 1.67 μB, in agreement with the experimental results. The double exchange interactions of two S atoms should result in the observed magnetism.

1. INTRODUCTION Due to the combination of ferromagnetism and semiconductor properties which could meet the extensive demand for information storage, diluted magnetic semiconductors (DMS) stimulate great interest.1−3 Nowadays, diluted magnetic semiconductors could be obtained by doping certain semiconductors, for example, transition metal (TM)-doped TiO2, GaN, SnO2, and ZnO.4−8 However, the DMS obtained by the doping method is less than satisfactory due to the formation of clusters of magnetic elements and the secondary phases in the semiconductors. Therefore, defect engineering may be a useful route to realize magnetism in semiconductors, such as BaNbO3 and BaTiO3, where the oxygen vacancies bring about the unexpected magnetism.9,10 In a recent report, for II−VI semiconductors, ZnS (hexagonal phase), the Zn vacancy induces intrinsic ferromagnetism.11 CdS, an important II−VI semiconductor, has considerable important optical applications.12−18 For its cubic phase, the CdS nanoparticle presents room-temperature ferromagnetism,19,20 while for its hexagonal phase, the CdS nanorods show diamagnetism.21 However, in our lab, the CdS nanorods (hexagonal phase) show room-temperature ferromagnetism. As the origin of the ferromagnetism in hexagonal CdS nanorods is unclear, it would be worthwhile investigating the ferromagnetism of CdS nanorods in order to fully understand the cause that leads to the ferromagnetism. © 2014 American Chemical Society

In this paper, the room-temperature ferromagnetism of hexagonal CdS nanorods is found. The microstructural, optical, and magnetic properties of the CdS nanorods are investigated by using X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), UV− visible spectrum, and superconducting quantum interference device (SQUIDs) measurements. The density of states and charge distribution for the hexagonal CdS with intrinsic defects are calculated based on first-principles calculations. The cause of the room-temperature ferromagnetism of hexagonal CdS is discussed in detail.

2. EXPERIMENTAL SECTION 2.1. Preparation of Hexagonal CdS. Two millimoles Cd(NO3)2 and 2 mmol Na2S are added into 10 mol/L NaOH aqueous solution, and then the vessel is moved into a furnace preheated to 220 °C, and kept in it for 24 h. The clean products are obtained by washing several times with absolute ethanol. 2.2. Characterization. The structure, morphology, chemical state, and optical and magnetic properties of CdS nanorods are determined by XRD (X’Pert PRO PHILIPS with Cu Kα radiation), SEM (TESCAN, VEGA2), TEM (Tecnai Received: January 7, 2014 Revised: April 30, 2014 Published: April 30, 2014 11426

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TMG2F30, FEI), X-ray photoelectron spectroscopy (XPS), UV−vis−NIR spectrophotometer (Shimadzu UV 3600), and SQUID, respectively. 2.3. Calculation Details. Our theoretical calculations are performed by VASP software with PAW potential.22,23 Plane waves with cutoff energy of 500 eV are used to expand wave function of the valence electrons (4d105s2 for Cd and 3s23p4 for S). Exchange and correlation functions are implemented by the generalized gradient approximation (GGA) 24 with the Perdew−Burke−Ernzerhof (PBE) gradient-corrected functions. The residual forces and convergence threshold converge to 0.01 eV/Å and 10−6 eV, respectively. Based on the optimized supercell (2 × 2 × 2), the density of states (DOS) and magnetic properties are calculated.

3. RESULTS AND DISCUSSION In Figure 1, all diffraction peaks match that of the pure hexagonal phase CdS (JCPDS: 89-2944) and no other

Figure 3. XPS spectra of hexagonal CdS nanorods, Cd 3d (a) and S 2p (b).

value for CdS.25 The S 2p XPS result (Figure 3b) indicates that the binding energy (BE) value is 161.4 eV, consistent with the published value.26 The band gap (Eg) of the CdS nanorods can be obtained from reflectance spectrum. The reflectance spectrum obtained from the CdS nanorods is illustrated in Figure 4a. The band gap of the CdS nanorods can be calculated by the Kubelka−Munk function,27,28 from which the band energy is obtained as Eg = 2.43 eV (Figure 4a). On the other hand, the band gap and absorption coefficient (α) at the optical absorption edge comply with the relationship (hνα)2 ∝ hν − Eg, from which the direct band gap of 2.42 eV can be obtained by extrapolating the edge of (hνα)2 to zero (Figure 4b), in accordance with the above results. Although the band gap should in principle be the same for the same sample, experimental errors could not be avoided when the test process or test principle is different, and so the results may have a little difference. The magnetic properties of the CdS nanorods are measured, and the related results are shown in Figure 5 with field sweeping from −7500 to +7500 Oe. A clear hysteresis loop is observed, indicating FM ordering in the material. The sample of the CdS nanorods shows ferromagnetism with a saturation magnetization of ∼2.63 × 10−3 emu/g and a coercive field of ∼270 Oe. Temperature dependence of the magnetization for the field cooling (FC) condition is shown in Figure 5b, which demonstrates a well-defined ferromagnetic−paramagnetic transition. The smooth and featureless χ−T curve suggests the absence of any tiny parasitic magnetic phase. The FM transition temperature is found to be 305 K. The inverse susceptibility is linear above TC and obeys the Curie−Weiss law well. Normally, the scenario of Ruderman−Kittel−Kasuya−Yosida (RKKY) indirect exchange interaction,29 the percolation of bound polarons (BMPs),30 and d−d double exchange31 are the possible mechanisms for the occurrence of ferromagnetism in

Figure 1. XRD pattern of hexagonal CdS nanorods.

impurities are detected, suggesting high purity of the obtained sample under our experimental conditions. Figure 2a,b presents

Figure 2. SEM (a,b), TEM (c), and selected area diffraction pattern (SAED) (d) of hexagonal CdS nanorods.

the SEM images of the CdS nanorods, in which many uniform nanorods are found with 100 nm diameter and 1 μm length. Figure 2c,d is the further characterization of a nanorod, indicating that the nanorod is single crystalline and grows along the [0 0 1] direction. The composition of the CdS nanorods is identified by XPS. In the Cd 3d spectrum (Figure 3a), peaks corresponding to 3d5/2 and 3d3/2 at 404.8 and 411.5 eV well match the literature 11427

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Nevertheless, for addressing this issue completely, theoretical guidance is still needed. In this respect, the possible influence of defects on magnetism is studied. The ideal hexagonal phase CdS crystal, as well as defective structures with (a) a Cd vacancy and (b) a S vacancy is discussed. The schematic picture is shown in Figure 6a. In Figure 6a, the supercell (2 × 2 × 2) contains 16 S atoms and 16 Cd

Figure 4. UV−visible reflection spectrum and Kubelka−Munk funtion (a), and UV−vis absorption spectrum and variation of (hνα)2 versus hν for direct band gap calculation (b) of hexagonal CdS nanorods.

Figure 6. Calculated supercell (a), and the possible S (b) atom and Cd (c) atom removed to create vacancy of hexagonal CdS.

atoms. For the bulk hexagonal CdS, the spin-polarized total density of states (TDOS) is plotted in Figure 7a and no spontaneous magnetization is found, suggesting that the bulk hexagonal CdS is nonmagnetic (NM). We first explore the effect of S vacancy on the magnetism of CdS. We have introduced neutral S vacancy by removing the S atom from the (2 × 2 × 2) CdS supercell. The possible position of S atom that is removed to create S vacancy is shown in Figure 6b. With one S atom removed, the total energy is calculated and the results are presented in Table S1 (Supporting Information). From Table S1, among the possible positions, the energy of the supercell with VS2 or VS11 is lowest. In order to check whether S vacancy causes magnetism, the spin-polarization state of VS2 or VS11 is calculated (Table 1), showing that the S vacancy does not create magnetic moment. Then, we have introduced neutral Cd vacancy by removing one Cd atom from the (2 × 2 × 2)

Figure 5. Magnetic hysteresis loop (a) and magnetization as a function of temperature (b) of hexagonal CdS nanorods.

the studied DMSs. However, all these considerations are based on the distribution of TM dopant dispersion in a semiconductor matrix. The observed magnetism is therefore attributed to point defect.32−37 11428

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Figure 7. TDOS of hexagonal CdS without Cd vacancy (a) and with Cd vacancy (b).

Table 1. Calculated Energy (eV) and Magnetism (μB) of CdS with Possible Defects type of defect

energy (eV)

magnetism (μB)

pure VS2 VS11 VCd4

−98.411 553 −91.6147 126 895 −91.6147 127 428 −93.688 315

0 0 0 1.67

Figure 8. PDOS of S 2p of hexagonal CdS without Cd vacancy (a) and with Cd vacancy (b).

8a). When one Cd vacancy is introduced, the PDOSs of S atom around Cd vacancy is changed (Figure 8b). It shows that the Cd vacancy introduces some impurity states, and these states mainly originate from S 2p orbitals. The Fermi level is located in the middle of these band gap states, implying that the bulk CdS with one Cd vacancy exhibits typical half-metallic character. In Figure 8b, the up-spin and down-spin S 2pz orbitals are occupied, and the up-spin S 2px and S 2py are also occupied. However, the Fermi level is located in the middle of the down-spin S 2px and S 2py orbitals, indicating that the down-spin S 2px and S 2py are empty. The related electron configuration of S 2p around Cd vacancy is px↑, py↑, and pz↑↓, as is shown in Figure 8b. In hexagonal CdS, each Cd cation is surrounded by a tetrahedron of four S anions with four sp3 orbitals. The symmetry of this tetrahedral crystal field is Td. When a Cd vacancy is created in the pure bulk CdS, two holes are introduced. According to the molecular orbital theory, the four sp3 orbitals of neighbors of the Cd vacancy combine into a singlet a1 state and a higher energy triplet t2. The a1 state is doubly occupied and does not contribute to the spin polarization, while t↑2 and t↓2 are occupied with three and one electrons, respectively. Hence the system has a high spin 1↓ 3↑ 1↓ configuration via (a1↑ 1 )(a1 )(t2 )(t2 ) with the magnetic moment of 1.67 μB, in agreement with electronic structure analysis above. The spin density distribution around Cd vacancy is

CdS supercell. The possible position of Cd vacancy is shown in Figure 6c. The total energy with one Cd atom removed is calculated, as shown in Table S2 (Supporting Information). From Table S2, among the possible positions, the energy of the supercell with VCd4 is lowest. In order to check whether Cd vacancy leads to magnetism, the spin-polarization state of VCd4 is calculated and shown in Table 1. It is found that the Cd vacancy can induce magnetism (1.67 μB) in the bulk CdS, which is evidenced by the asymmetric TDOS in Figure 7b. In addition, the non-spin-polarized calculation for the bulk CdS with one Cd vacancy shows that the spin-polarized state (Table 1) is more stable (about 75 meV energy lower) with respect to the non-spin-polarized state (Table S2), which reveals its ground state is spin-polarized. The existence of Cd vacancy in the nanorods may be produced in the growth process, where the CdS will grow toward the energy minimum assembly with the best stability. In our growth process, the Cd vacancy makes the CdS nanorods more stable. To get a further understanding into the electronic structure and magnetic properties, we calculate the projected density of states (PDOSs) and spin density distribution for the (2 × 2 × 2) CdS supercell with one Cd vacancy, as shown in Figures 8 and 9. For the bulk CdS, the p orbital of S atom has six electrons, meaning that the px, py, and pz orbital has a pair of spin-up and spin-down electrons, px↑↓, py↑↓, and pz↑↓ (Figure 11429

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ASSOCIATED CONTENT

* Supporting Information S

Calculated energy (eV) of CdS with possible S vacancy or Cd vacancy. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 23 65678362. Fax: +86 23 65678362. E-mail: hucg@ cqu.edu.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the NSFCQ (cstc2012jjB0006), SRFDP (20110191110034, 20120191120039), NSFC (11204388), and the large-scale equipment sharing fund of Chongqing University.

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REFERENCES

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Figure 9. Spin density distribution of hexagonal CdS.

presented in Figure 9 to further analyze spin polarization from Cd vacancy. As shown in Figure 9a, the distribution of spin density around the Cd vacancy indicates that the main contribution stems from 3p of the four nearest tetrahedral S atoms surrounding the vacancy site, while the magnetic moment on Cd atoms is almost negligible. The defect states are strongly localized on S atom around Cd vacancy with 0.37 μB/S. In Figure 9b, the distribution also demonstrates an isotropic character of spin polarization in the hexagonal phase CdS. The carrier-mediated mechanism38 could be used to explain the magnetism caused by the Cd vacancy. From Figure 7b, the position of impurity level is located in the band gap. Because the bandwidth is smaller than the exchange splitting, incomplete filling of the bands exists. According to these features described above, we could explain the observed ferromagnetism of the hexagonal CdS by the double exchange interaction.38

4. CONCLUSIONS In summary, the ferromagnetism (Curie temperature 305 K) is found in the hexagonal CdS nanorods via experiments. Our first-principles calculations for the bulk CdS suggest that the Cd vacancy introduces spin-polarized impurity level in the band gap with half-metallic properties, generating a magnetic moment 1.67 μB. The substantial magnetic interaction between the Cd vacancy and the nearest S atom leads to strong ferromagnetic coupling; meanwhile, the ground state is spinpolarized. The distribution of spin density around the Cd vacancy indicates that the main contribution stems from 3p of the four nearest tetrahedral S atoms surrounding the vacancy site, but the magnetic moment on Cd atoms is almost negligible. As the Cd vacancy makes the electron configuration 1↓ 3↑ 3↓ 1↑ 1↓ 3↑ of S 2p to change from (a1↑ 1 )(a1 )(t2 )(t2 ) to (a1 )(a1 )(t2 )1↓ (t2 ), a local magnetic moment of 1.67 μB is found, in agreement with the experimental results. 11430

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