Ferromagnetism in sphalerite and wurtzite CdS nanostructures

Zhaolong Yang 0 Daqiang Gao 0 Zhonghua Zhu 0 Jing Zhang 0 Zhenhua Shi 0 Zhipeng Zhang 0 Desheng Xue 0 0 Key Laboratory for Magnetism and Magnetic Materials of MOE, Lanzhou University , Lanzhou 730000, People's Republic of China 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. - Background Since the first discovery of ferromagnetism (FM) in Mndoped GaAs [1], 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 d0 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 CdSbased 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 [18]. Li et al. also studied a Cu-doped CdS system by first-principles simulation and predicted that the system shows a halfmetallic ferromagnetic character and the Tc of the ground state is above RT [31]. 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 [32]. 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 [33]. 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. Methods 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 90C 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 60C. 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 60C for 4, 6, 8, and 10 h, which were denoted as S5, S6, S7, and S8, respectively. The asformed wurtzite CdS NSs were filtered, washed with deionized water, and then dried in air at 40C. X-ray diffraction (XRD; XPert PRO PHILIPS with Cu K 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 Sc (...truncated)