Dilute Magnetic Semiconductor Cu 2 FeSnS 4 Nanocrystals with a Novel Zincblende Structure

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.


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][2][3][4][5].Fe 2+ , Ni 2+ , Mn 2+ , and Co 2+ 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][7][8].However, the magnetic ion concentration in these doped DMS nanocrystals is generally less than 2 mol% [9][10][11][12][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 Cu 2 FeSnS 4 with a high magnetic ion concentration is being considered as an ideal material for avoiding large AFM exchange interactions [14].However, Cu 2 FeSnS 4 usually crystallizes in a stannite structure (space group I-42 m, no.121), and Cu + , Fe 2+ , and Sn 4+ cations have a fixed position in the stannite unit cell [15].In the previously reports, quaternary Cu 2 FeSnS 4 magnetic semiconductor with a stannite structure exhibited an antiferromagnetic behavior with a Néel temperature (T N ) of 6∼8 K [15][16][17].To the best of our knowledge, there are no reports on the synthesis of Cu 2 FeSnS 4 nanocrystals with a zincblende structure in the literature.In this paper, we adopted a hot-injection approach to synthesize quaternary Cu 2 FeSnS 4 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 Cu 2 FeSnS 4 nanocrystals were investigated by a superconducting quantum interference device (SQUID).

Experimental
sulfur powder (99.999%), thiourea, and oleylamine (OM, 80% ∼ 90%) were purchased from Aladdin Inc.All chemicals were used as received.O, 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.

Preparation of Sulfur
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 Cu 2 FeSnS 4 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.

Results and Discussion
In contrast to the tetragonal stannite structure, the space symmetry of zincblende structure significantly decreases by the random arrangement of Cu + , Fe 2+ , and Sn 4+ cations in the cubic zincblende unit cell of Cu 2 FeSnS 4 .As shown in the inset of Figure 1, the zincblende unit cell of Cu 2 FeSnS 4 is completely the same as that of ZnS (space group F-43 m, no.216).Note that only Zn 2+ position is replaced by 1/2 Cu + , 1/4 Fe 2+ , and 1/4 Sn 4+ .Figure 1 shows the XRD pattern for as-synthesized Cu 2 FeSnS 4 nanocrystals.Obviously, our XRD pattern did not match those reported in the literature [15][16][17] and the standard JCPDS card database (stannite structure, JCPDS no.44-1476).We therefore simulated the diffraction pattern for zincblende Cu 2 FeSnS 4 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 Cu 2 FeSnS 4 nanocrystals can be further confirmed by SAED image Figure 2     of stannite Cu 2 FeSnS 4 , enabling the exchange interaction at low temperature.As a result, Cu 2 FeSnS 4 nanocrystals with a zincblende structure exhibit a ferromagnetic behavior.
In addition, X-ray photoelectron spectroscopy (XPS) was applied to determine the chemical composition and valence states of Cu 2 FeSnS 4 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.

Conclusion
In summary, dilute magnetic semiconductor Cu 2 FeSnS 4 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 + , Fe 2+ , and Sn 4+ 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 Liion batteries.
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.

Figure 1 :Figure 2 :
Figure 1: The simulated (red line) and experimental (black line) XRD patterns of Cu 2 FeSnS 4 nanocrystals with a zincblende structure; the inset is the unit cell of zincblende Cu 2 FeSnS 4 .

Figure 3 (
a) displays UV-vis-NIR absorption spectrum of as-synthesized Cu 2 FeSnS 4 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 Cu 2 FeSnS 4 nanocrystals are around 1.1 eV.As demonstrated in Figure 3(b), the chemical composition of the nanocrystals is Cu 2 FeSnS 4 .The molar ratio of Cu/Fe/Sn/S is close to 2 : 1 : 1 : 4, which is well consistent with stoichiometric composition of Cu 2 FeSnS 4 .The magnetic properties of Cu 2 FeSnS 4 nanocrystals were characterized by a SQUID magnetometer.As shown in Figure4(a), a clear separation between the ZFC and FC curves is observed at low temperature region which indicates

Figure 5 :
Figure 5: The X-ray photoelectron spectroscopy (XPS) spectra of as-synthesized Cu 2 FeSnS 4 nanocrystals; (a) Cu2p; (b) Fe2p; (c) Sn3d; (d) S2p. 2 In the stannite Cu 2 FeSnS 4 , 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 Cu 2 FeSnS 4 nanocrystals, and the electron clouds overlap of Fe-Fe is more larger than that