Ferromagnetism in sphalerite and wurtzite CdS nanostructures
© Yang et al.; licensee Springer. 2013
Received: 27 November 2012
Accepted: 22 December 2012
Published: 7 January 2013
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© Yang et al.; licensee Springer. 2013
Received: 27 November 2012
Accepted: 22 December 2012
Published: 7 January 2013
Room-temperature ferromagnetism is observed in undoped sphalerite and wurtzite CdS nanostructures which are synthesized by hydrothermal methods. Scanning electron microscopy and transmission electron microscopy results indicate that the sphalerite CdS samples show a spherical-like shape and the wurtzite CdS ones show a flower-like shape, both of which are aggregated by lots of smaller particles. The impurity of the samples has been ruled out by the results of X-ray diffraction, selected-area electron diffraction, and X-ray photoelectron spectroscopy. Magnetization measurements indicate that all the samples exhibit room-temperature ferromagnetism and the saturation magnetization decreases with the increased crystal sizes, revealing that the observed ferromagnetism is defect-related, which is also confirmed by the post-annealing processes. This finding in CdS should be the focus of future electronic and spintronic devices.
Since the first discovery of ferromagnetism (FM) in Mn-doped GaAs , great effort has been paid to search for intrinsic dilute magnetic semiconductors (DMSs) with Curie temperatures (Tc) at or above room temperature (RT) by doping semiconductors with transition metals (TMs) [2, 3]. During the past few years, room-temperature ferromagnetism (RTFM) has been reported in TM-doped DMSs experimentally. Nevertheless, the mechanism of the observed FM remains controversial theoretically, which mainly includes experimental artifacts, segregation of secondary ferromagnetic phases, magnetic clusters, and indirect exchange mediated by carriers, electrons, and holes associated with impurities that are related to the observed RTFM [4–7]. Subsequently, RTFM has also been observed in undoped semiconducting or insulating (such as HfO2, In2O3, MgO, ZnO, SnO2, etc.) [8–12], where nominal magnetic ions are not present, and the term ‘d 0 FM’ [13, 14] was suggested to summarize these cases. It is strongly believed that the point defects in semiconductors or insulators have an open-shell electronic configuration, which can indeed confine the compensating charges in molecular orbitals, forming a local magnetic moment. Recently, experiment results show that the size of the lower dimensional systems, such as film thickness or diameter of nanoparticles, has an effect on the vacancy concentration as well as their magnetic behavior [15, 16]. The results are also supported by theoretical works which show the effects of curvature, confinement, and size on various properties of nanocrystals [17, 18]. Obviously, the surface-to-volume atomic ratio will be increased significantly with the decreased size of nanocrystals. Since the surface has a broken atomic symmetry and it often has higher anisotropy, new surface states that differ from their bulk form are established, which play a crucial role in controlling the electronic, optical, and other properties of nanocrystals.
CdS, belonging to the II-VI compound family, has a considerably important application such as in optoelectronic devices, photocatalysts, solar cells, optical detectors, and nonlinear optical materials [19–25]. If RTFM were achieved in CdS, it would be a potential candidate in the fabrication of new-generation magneto-optical and spintronic devices. Remarkably, lots of investigations have demonstrated FM with Tc above room temperature observed in transition metal ion (such as Fe, Co, Cr, Mn, and V)-doped CdS-based low-dimensional materials [26–30]. Recently, Pan et al. demonstrated that FM can be realized in CdS with C doping via substitution of S which can be attributed to the hole-mediated double-exchange interaction . Li et al. also studied a Cu-doped CdS system by first-principles simulation and predicted that the system shows a half-metallic ferromagnetic character and the Tc of the ground state is above RT . Meanwhile, Ren et al. indicated that Pd doping in CdS may lead to a long-range ferromagnetic coupling order, which results from p d exchange coupling interaction . Moreover, Ma et al. studied the magnetic properties of non-transition metal/element (Be, B, C, N, O, and F)-doped CdS and explained the magnetic coupling by p p interaction involving holes . In this paper, we report the observation of size-dependent RTFM in CdS nanostructures (NSs). The CdS NSs in sphalerite and wurtzite structures were synthesized by hydrothermal methods with different sulfur sources. The structure and magnetic properties of the samples were studied.
CdS NSs were synthesized by hydrothermal methods. In a typical procedure for the synthesis of sphalerite CdS samples, 0.15 M cadmium chloride (CdCl2 · 2.5H2O) and 0.15 M sodium thiosulfate (Na2S2O3 · 5H2O) were added into 40 mL deionized water. After stirring for 30 min, the mixed solution was transferred into a Teflon-lined stainless steel autoclave of 50-mL capacity. After being sealed, the solution was maintained at 90°C for 2, 4, 6, and 8 h, which were denoted as S1, S2, S3, and S4, respectively. The resulting solution was filtered to obtain the samples. To eliminate the impurity ions, the products were further washed with deionized water for several times and then dried in air at 60°C. Wurtzite CdS were synthesized with different sulfur sources. In this method, 0.2 M cadmium chloride (CdCl2 · 2.5H2O) and 0.2 M thioacetamide (CH3CSNH2) were added into 40 mL deionized water. After stirring, the cloudy solution was transferred into a Teflon-lined stainless steel autoclave of 50-mL capacity. After being sealed, the solution was maintained at 60°C for 4, 6, 8, and 10 h, which were denoted as S5, S6, S7, and S8, respectively. The as-formed wurtzite CdS NSs were filtered, washed with deionized water, and then dried in air at 40°C.
X-ray diffraction (XRD; X’Pert PRO PHILIPS with Cu Kα radiation, Almelo, The Netherlands) was employed to study the structure of the samples. The morphologies of the samples were obtained using a scanning electron microscope (SEM; Hitachi S-4800, Chiyoda-ku, Japan). Microstructures of the samples were characterized using a transmission electron microscope (TEM; Tecnai TMG2F30, FEI, Hillsboro, OR, USA) and high-resolution TEM (HRTEM) equipped with selected-area electron diffraction (SAED) and energy-dispersive X-ray spectrum (EDS). The measurements of static magnetic properties were made using a Quantum Design MPMS magnetometer based on a superconducting quantum interference device (SQUID; San Diego, CA, USA). Electron spin resonance (ESR; JEOL, JES-FA300, microwave frequency is 8.984 GHz, Akishima-shi, Japan) spectra were recorded to study the dynamic magnetic properties of the samples. The chemical bonding state and the compositions of the samples were determined by X-ray photoelectron spectroscopy (XPS; VG Scientific ESCALAB-210 spectrometer, East Grinstead, UK) with monochromatic Mg Kα X-rays (1,253.6 eV). The thermogravimetric and differential thermal analysis (TG-DTA; DuPont Instruments 1090B, Parkersburg, VA, USA) was employed to obtain the variation of mass and phase transition details of the samples during argon annealing.
ESR was performed to further characterize the magnetic properties of the sphalerite CdS NSs. Figure 5c depicts the ESR results measured at different temperatures from 90 to 300 K for sample S1. It can be seen that the sample shows resonance signals with applied magnetic field from 0 to 500 mT. The center magnetic fields (Hcenter) for the sample are far from 321 mT which characterize a free electron (g = 2.0023), indicating that the sample has obvious FM , and the ferromagnetic coupling between the moments increase with the decreasing temperature. According to the theory of ferromagnetic resonance , the relationship between resonance field and microwave frequency in the ferromagnetic resonance is hν = gμB · H, where h, ν, g, μB, and H are the Planck constant, frequency of the applied microwave magnetic field, g-factor, Bohr magnetron, and resonance magnetic field, respectively. In FM materials, the orbital angular momentum quenching in the crystal field and g-factor is 2.0023; the resonance field is made up of applied field Ha and magnetocrystalline anisotropy field Hk: H = Ha + Hk. If we define Ha as H and attribute the change of Hk to the g-factor, which is defined as an effective g-factor (geff), then the ferromagnetic resonance relationship changes to hν = geffμB · Ha. Hk will increase with the decreasing temperature, and then geff will get higher. In sample S1, the geff increases from 2.54 to 2.74 as the temperatures decrease from RT to 90 K. The results in Figure 5d indicate that the variation of ΔH (=321 mT − Hcenter, which represents the position of the resonance peak) measured at different temperatures is consistent with that of Ms in the samples.
The M H curves for the post-annealing samples and the variation of their Ms are shown in Figure 8c,d, respectively. It can be seen that the Ms of the samples decrease continuously after post-annealing at 200°C and 400°C. However, the Ms increases with the increasing annealing temperature when the annealing temperature exceeds 400°C. The chemical composition calculated from the XPS result shows that Cd and S have an atomic ratio of 76.7:23.3 for sample S1 after being annealed at 800°C, which indicates that the density of sulfur vacancies gets higher than that of the as-prepared sample. As the analysis of the above annealing progresses, it can be understood that argon annealing at a temperature lower than 400°C results in crystal grain reconstruction and growth which compensates the sulfur vacancies. However, when the annealing temperature gets higher, the sample begins to decompose and promotes large amount of vacancies. Subsequently, the exchange interaction between these different concentrations of sulfur vacancies changes the Ms. Note that changes of Ms for the wurtzite-structure samples after post-annealing processes have the same variation as those for the sphalerite ones above. The post-annealing results further clarify the role of sulfur vacancies in triggering the RTFM in undoped CdS [34, 41].
In summary, well-crystalline CdS NSs both in sphalerite and wurtzite were synthesized by simple hydrothermal methods. The NSs were self-aggregated into spherical and flower shapes, respectively. Intrinsic FM is observed in the samples by the magnetic hysteresis loops and prominent ferromagnetic resonance signals. The mechanism of RTFM from sulfur vacancies is proposed. Moreover, the magnetization value can be tuned by changing the concentration of sulfur vacancies, which is affected by the particle size and annealing condition. These findings not only demonstrate that pure CdS shows tunable RTFM, but also suggest that introduction of sulfur vacancies can be a significant way to mediate the d 0 FM.
This work is supported by the National Basic Research Program of China (grant no. 2012CB933101), the NSFC (grant nos. 11034004 and 51202101), the National Science Fund for Distinguished Young Scholars (grant no. 50925103), and the Fundamental Research Funds for the Central Universities (no. lzujbky-2012-28).
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