Dilute Magnetic Semiconductor Cu2FeSnS4 Nanocrystals with a Novel Zincblende Structure

Abstract

Diluted magnetic semiconductor Cu₂FeSnS₄ nanocrystals with a novel zincblende structure have been successfully synthesized by a hot-injection approach. Cu⁺, Fe²⁺, and Sn⁴⁺ ions occupy the same position in the zincblende unit cell, and their occupancy possibilities are 1/2, 1/4, and 1/4, respectively. The nanocrystals were characterized by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), energy-dispersive spectroscopy (EDS), and UV-vis-NIR absorption spectroscopy. The nanocrystals have an average size of 7.5 nm and a band gap of 1.1 eV and show a weak ferromagnetic behavior at low temperature.

1. Introduction

During the past three decades, transition metal ion-doped diluted magnetic semiconductors (DMSs) have attracted great interest because of their outstanding the optical, electronic, and magnetic properties [1–5]. Fe²⁺, Ni²⁺, Mn²⁺, and Co²⁺ are commonly used as intentional impurities and incorporated in group II-VI and III-V semiconductors; ZnO, ZnS, ZnSe, CdS, CdSe, and GaN are the frequently used host materials [6–8]. However, the magnetic ion concentration in these doped DMS nanocrystals is generally less than 2 mol% [9–13]. It is well known that the magnetic properties of DMSs are strongly dependent on the magnetic ion concentration. Nonetheless, a high magnetic ion concentration is detrimental to the magnetic properties for binary DMSs due to the strong antiferromagnetic (AFM) interactions between nearest-neighbor-doping ions [14].

The quaternary Cu₂FeSnS₄ with a high magnetic ion concentration is being considered as an ideal material for avoiding large AFM exchange interactions [14]. However, Cu₂FeSnS₄ usually crystallizes in a stannite structure (space group I-42 m, no. 121), and Cu⁺, Fe²⁺, and Sn⁴⁺ cations have a fixed position in the stannite unit cell [15]. In the previously reports, quaternary Cu₂FeSnS₄ magnetic semiconductor with a stannite structure exhibited an antiferromagnetic behavior with a Néel temperature (𝑇_𝑁) of 6~8 K [15–17]. To the best of our knowledge, there are no reports on the synthesis of Cu₂FeSnS₄ nanocrystals with a zincblende structure in the literature. In this paper, we adopted a hot-injection approach to synthesize quaternary Cu₂FeSnS₄ nanocrystals with a metastable zincblende structure. In zincblende structure, all of cations have a random distribution instead of ordered distribution, which may lead peculiar magnetic properties by controlling both metal distribution and metal-metal distance. The magnetic properties of zincblende Cu₂FeSnS₄ nanocrystals were investigated by a superconducting quantum interference device (SQUID).

2. Experimental

2.1. Chemicals

CuCl₂·2H₂O, FeCl₃·6H₂O, SnCl₄·5H₂O, sulfur powder (99.999%), thiourea, and oleylamine (OM, 80% ~ 90%) were purchased from Aladdin Inc. All chemicals were used as received.

2.2. Preparation of Sulfur Precursors

1.0 M S/OM solution was prepared by dissolving 0.64 g (20 mmol) of sulfur powder in 20.0 mL of OM at 120°C.

2.3. Synthesis of Zincblende Cu₂FeSnS₄ Nanocrystals

In a typical synthesis, 17.0 mg (0.1 mmol) of CuCl₂·2H₂O, 17.0 mg (0.05 mmol) of SnCl₄·5H₂O, 13.5 mg (0.05 mmol) of FeCl₃·6H₂O, and 5.0 mL of OM were added to a 50 mL three-neck flask, and the reaction mixture was heated to 120°C. The inside of the flask was degassed by a vacuum pump for 10 min, and argon gas was charged. This procedure was repeated three times. Then the temperature was increased to 270°C, and 0.5 mL of S/OM solution was injected into the flask. After 30 min, the crude solution was cooled to 60°C and then precipitated with 30 mL of ethanol. Finally, the nanocrystals were dispensed in toluene.

2.4. Characterization

X-ray diffraction (XRD) patterns were recorded by a Bruker D8 FOUCS X-ray diffractometer using Cu Kα radiation, and the accelerating voltage and current were 40 kV and 40 mA, respectively. UV-vis-NIR absorption spectrum was measured by Shimadzu UV-3600. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) images were taken on a FEI Tecnai G2 F20 with an accelerating voltage of 200 kV. Energy disperse X-ray spectroscopy (EDS) spectrum was obtained by using a scanning electron microscope (Hitachi S-4800) equipped with a Bruker AXS XFlash detector 4010. The magnetization of Cu₂FeSnS₄ nanocrystals was obtained by SQUID (MPMS-XL-7, Quantum Design, Ltd.) between 2 and 100 K using zero-field-cooled (ZFC) and field-cooling (FC) procedures in an applied field of 50 Oe.

3. Results and Discussion

In contrast to the tetragonal stannite structure, the space symmetry of zincblende structure significantly decreases by the random arrangement of Cu⁺, Fe²⁺, and Sn⁴⁺ cations in the cubic zincblende unit cell of Cu₂FeSnS₄. As shown in the inset of Figure 1, the zincblende unit cell of Cu₂FeSnS₄ is completely the same as that of ZnS (space group F-43 m, no. 216). Note that only Zn²⁺ position is replaced by 1/2 Cu⁺, 1/4 Fe²⁺, and 1/4 Sn⁴⁺. Figure 1 shows the XRD pattern for as-synthesized Cu₂FeSnS₄ nanocrystals. Obviously, our XRD pattern did not match those reported in the literature [15–17] and the standard JCPDS card database (stannite structure, JCPDS no. 44-1476). We therefore simulated the diffraction pattern for zincblende Cu₂FeSnS₄ and compared it with the experimental pattern. The simulated and experimental patterns match very well, signifying that these nanocrystals possess a zincblende structure with a space group F-43 m and unit cell parameter 𝑎=5.429 Å. In addition, the zincblende structure of Cu₂FeSnS₄ nanocrystals can be further confirmed by SAED image Figure 2(b). It should be noted that the lattice parameters of zincblende Cu₂FeSnS₄ nanocrystals measured from SAED images are very close to those calculated from the XRD patterns.

Figure 1 

The simulated (red line) and experimental (black line) XRD patterns of Cu2FeSnS4 nanocrystals with a zincblende structure; the inset is the unit cell of zincblende Cu2FeSnS4.

Figure 2 

TEM (a and c) and SAED (b) images of Cu2FeSnS4 nanocrystals.

Low-resolution TEM image of zincblende Cu₂FeSnS₄ nanocrystals is shown in Figure 2(c). The Cu₂FeSnS₄ nanocrystals are nearly spherical in shape and have the average diameter of 7.5 nm. In addition, the nanocrystals exhibit a very narrow size distribution and have a standard deviation of 7.4%. The high-resolution TEM image clearly revealed the continuous lattice fringes, and the calculated 𝑑 spacing corresponding to (111) plane is 3.138 Å, which is in good agreement with that value determined by XRD pattern (3.136 Å) or SAED image (3.140 Å).

Figure 3(a) displays UV-vis-NIR absorption spectrum of as-synthesized Cu₂FeSnS₄ nanocrystals. We calculated the optical band gap (Eg) by extrapolating the linear portion of the absorption spectrum to hν axis, and the calculated optical band gaps for Cu₂FeSnS₄ nanocrystals are around 1.1 eV. As demonstrated in Figure 3(b), the chemical composition of the nanocrystals is Cu₂FeSnS₄.The molar ratio of Cu/Fe/Sn/S is close to 2 : 1 : 1 : 4, which is well consistent with stoichiometric composition of Cu₂FeSnS₄.

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Figure 3 

UV-vis-NIR absorption (a) and EDS (b) spectra of Cu2FeSnS4 nanocrystals.

The magnetic properties of Cu₂FeSnS₄ nanocrystals were characterized by a SQUID magnetometer. As shown in Figure 4(a), a clear separation between the ZFC and FC curves is observed at low temperature region which indicates the absence of a magnetic ordering transition. The plot of 𝜒^{(−1)−𝑇} (blue line in Figure 4(a)) indicates classical Curie-Weiss behavior of this sample at high temperature region. The magnetic susceptibility, 𝜒(𝑇), at the high-temperature limit can be represented by the Curie-Weiss law, 𝜒(𝑇)=𝐶/(𝑇−Θ), where 𝐶 is the Curie constant, 𝑇 is the temperature in Kelvin, and Θ is the Curie temperature. The isothermal magnetization curves of Cu₂FeSnS₄ nanocrystals at 2 k and 300 k in magnetic fields up to ±50 kOe are shown in Figure 4(b). The “s” shape hysteresis loop at 2 K is shown in the inset of Figure 4(b), with a coercive force of 56 Oe and relatively large residual magnetization. It can be concluded that these Cu₂FeSnS₄ nanocrystals exhibit ferromagnetic behavior at this temperature. The hysteresis loop obtained at 300 K does not show hysteresis behavior and is weakly field dependent and linear, indicating that the nanocrystals become a paramagnetic material at 300 K. Note that the stannite Cu₂FeSnS₄ usually exhibited an antiferromagnetic behavior in the literatures [15–17]. It is well known that the unit cell parameter is 2.91 Å in iron which shows ferromagnetic behavior. In the stannite Cu₂FeSnS₄, the interaction Fe-Fe could be super-exchange interaction with the aid of nonmagnetic ions due to the large nearest distance of Fe-Fe (5.45 Å). However, the nearest distance of Fe-Fe is only 3.86 Å in the zincblende Cu₂FeSnS₄ nanocrystals, and the electron clouds overlap of Fe-Fe is more larger than that of stannite Cu₂FeSnS₄, enabling the exchange interaction at low temperature. As a result, Cu₂FeSnS₄ nanocrystals with a zincblende structure exhibit a ferromagnetic behavior.

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Figure 4 

(a) Temperature dependence of the magnetization for Cu2FeSnS4 nanocrystals. (b) The field dependence of the magnetization for Cu2FeSnS4 nanocrystals at 2 k and 300 k; inset: the magnification of the hysteresis loop at 2 K.

In addition, X-ray photoelectron spectroscopy (XPS) was applied to determine the chemical composition and valence states of Cu₂FeSnS₄ nanocrystals. In Figure 5(a), the Cu 2p core splits into 2p3/2 (931.6 eV) and 2p1/2 (951.6 eV) peaks, which are characteristic of Cu⁺. Two peaks of Fe2p and Sn3d, located at 716.0 eV and 725.7 eV, 486.4 eV and 494.7 eV, suggesting that the valence states of Fe and Sn ions in the nanocrystals are +2 and +4, respectively.

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Figure 5 

The X-ray photoelectron spectroscopy (XPS) spectra of as-synthesized Cu2FeSnS4 nanocrystals; (a) Cu2p; (b) Fe2p; (c) Sn3d; (d) S2p.

4. Conclusion

In summary, dilute magnetic semiconductor Cu₂FeSnS₄ nanocrystals with a novel zincblende structure have been successfully synthesized. The optical and magnetic properties were characterized, and the nanocrystals have a band gap of 1.1 eV and exhibited a ferromagnetic behavior at low temperature. The ferromagnetic properties may be attributed to the novel zincblende structure; that is, Cu⁺, Fe²⁺, and Sn⁴⁺ ions occupy the same site in the unit cell and have a random distribution. Moreover, these dispersible and low-cost DMS nanocrystals have a high potential for thin film solar cells, spintronics, magnetic switching, magnetic recording, and Li-ion batteries.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grant nos. 21071142 and 51172229), the Fund for Creative Research Groups (Grant no. 20921002), and the Natural Science Foundation for Young Scientists of Jilin Province (20100105).