Magnetic Properties of Semiconducting Spinel CdCr2S4

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Magnetic Properties of Semiconducting Spinel CdCr2S4 Nanostructured Films Grown by Low-pressure Metal Organic Chemical Vapor Deposition Chao Pang, Abhishek Srivastava, Molly M Lockart, Tim Mewes, Michael K Bowman, Ningzhong Bao, Liming Shen, and Arunava Gupta ACS Appl. Electron. Mater., Just Accepted Manuscript • DOI: 10.1021/acsaelm.9b00245 • Publication Date (Web): 17 Jul 2019 Downloaded from pubs.acs.org on July 21, 2019

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ACS Applied Electronic Materials

Magnetic Properties of Semiconducting Spinel CdCr2S4 Nanostructured Films Grown by Low-pressure Metal Organic Chemical Vapor Deposition Chao Pang, †,‡ Abhishek Srivastava, †,§ Molly M. Lockart, ┴ Tim Mewes, †,§ Michael K. Bowman, ┴

Ningzhong Bao, ‡ Liming Shen, ‡, * and Arunava Gupta †, *

†Center

for Materials for Information Technology (MINT), The University of Alabama,

Tuscaloosa, AL 35487, United States. ‡State

Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University,

Nanjing, Jiangsu 210009, P. R. China. §Department

of Physics, The University of Alabama, Tuscaloosa, AL 35487, United States.

┴Department

of Chemistry and Biochemistry, The University of Alabama, Tuscaloosa, AL 35487,

United States.

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ACS Applied Electronic Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: Magnetic semiconductors are being extensively investigated because of their promise to provide new functionalities and device concepts for spintronic applications. Nevertheless, it remains a challenge to obtain high-quality thin films of these materials for device usage, particularly of chalcogenide-based magnetic semiconductors. The chalcogenide spinel CdCr2S4 is a well-established ferromagnetic semiconductor. In bulk form, it exhibits unique properties such as colossal magnetocapacitance and magnetoresistance effects and is a promising candidate for spintronic applications. Because of its semiconducting properties and band gap in the visible wavelength range, CdCr2S4 offers exciting possibilities for tailoring its optical and magnetic properties. However, applications that utilize its unique combination of optical, electrical, and magnetic properties are limited because of difficulties in growing high-quality films. Herein, we demonstrate a simple low-pressure metal organic chemical vapor deposition (MOCVD) method for growth of high-quality nanostructured CdCr2S4 thin films using metal organic precursors of Cd and Cr, which provides an attractive route for scaling to large areas. The deposited films are uniform with excellent phase-purity, morphology and crystallinity. In addition to structural and optical characterization, the static and dynamic magnetic properties of the MOCVD grown films have been investigated in detail. KEYWORDS: spinel, chalcogenide, magnetic semiconductor, thin films, MOCVD


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INTRODUCTION Over the past few decades, compounds combining semiconducting and ferromagnetic properties have emerged as an important field of materials science.1-5 Magnetic semiconductors hold a very special position in the field of spintronics because they can potentially enable effective manipulation of both charge and spin. This feature is especially important in devices combining logic functions and information storage capability.6-9 Most investigations thus far have focused on doping magnetic elements to obtain magnetic semiconductors or dilute magnetic oxides.10-12 However, the development and understanding of novel magnetic materials that are intrinsically semiconducting remains a challenging research topic in materials science.13-15 The remarkable chromium-based chalcogenide spinels (chalcospinels), ACr2X4 (A = Cu, Cd, Hg, Fe, Co, etc.; X = S, Se, Te), with diverse electronic properties, offer new opportunities to investigate the origin and control of ferromagnetism in magnetic semiconductors. The chalcospinels exhibit unusual magnetic properties resulting from exchange and superexchange interactions between Cr3+ ions with half-filled t2g ground states (S = 3/2) and no charge or orbital degrees of freedom.16-17 Moreover, depending on the nature of A-site cation(s), they display metallic, semiconducting, or insulating characteristics resulting from combined strong electronic correlation and strong coupling of structural and electronic degrees of freedom.16,

18

Among

chalcospinels, only a few of the known single-phase materials are both ferromagnetic and semiconducting. They exhibit unique and unusual physicochemical properties such as halfmetallicity and dissipationless anomalous Hall current.19-22 The prototypical chalcospinel ferromagnetic semiconductor, CdCr2S4, has been widely studied in the bulk because of its unique properties such as colossal magnetoresistance, magnetic field-induced structural transformation, spin and orbital frustration, and relaxor ferroelectricity



combined with colossal magnetocapacitive coupling.23-26 The colossal magnetocapacitance and magnetoresistance effects makes it an interesting candidate for spintronic applications and a potential multiferroic material.27 CdCr2S4 offers the exciting possibility of tailoring its optical and magnetic properties because of its semiconducting characteristics and its band gap in the visible wavelength range.28 However, applications of CdCr2S4 that utilize its unique combination of optical, electrical and ferromagnetic properties are limited. We have previously reported on the chemical syntheses of different chalcospinel nanocrystals with various shapes and sizes.29-32 However, none of these magnetic chalcospinel nanocrystals are semiconductors. We recently succeeded in synthesizing magnetic semiconducting CdCr2S4 nanocrystals using a novel “seed-mediated” growth method.33 Advancing on this work, we have explored chemical synthetic routes to obtain high-quality nanostructured CdCr2S4 thin film for potential device applications. Thus far, synthetic routes for chalcospinel films mainly include wet chemistry method and multiple-step spin coating method using nanocrystals.33 The reported wet chemistry methods for the growth of CdCr2S4 films, such as chemical bath deposition and electrochemical deposition, result in the poor crystallinity and impurities.34-37 For multiplestep spin coating method, the nanocrystal defects and capped organic ligands negatively influence the electrical properties of the films, and additional annealing results in inevitable grain boundary defects and sulfur deficiency.38 Precursors reported in the literature for the chemical synthesis of bulk CdCr2S4, including elemental Cd/Cr, CdS, Cr2S3, CdCl2, CrCl3, CdO, CrO3, and CdCr2S4 powder, require elevated temperature for volatilization and reaction.33, 39-40 Thus, development of suitable cadmium and chromium precursors with sufficient volatility and stability for the growth of high-quality nanostructured CdCr2S4 films by chemical methods is an important challenge and a bottleneck for development of chalcogenide-based spintronic/electronic devices.


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Herein, we report a simple and efficient low-pressure metal organic chemical vapor deposition (MOCVD) method for the growth of high-quality ferromagnetic semiconducting CdCr2S4 nanostructured films. MOCVD is a desirable film deposition technique for conformal growth of high-quality films with excellent uniformity over large areas. It is widely utilized in the electronics industry for deposition of a variety of semiconducting and oxide films.41-42 Lowpressure MOCVD often enables film growth at lower temperatures than is possible at atmospheric pressure, and with better coverage and homogeneity. In this work, we have explored the use of transition metal alkyldithiocarbamate complexes that have previously been employed as singlesource precursors to deposit II/VI compound semiconductor films.43-44 The dithiocarbamate complexes are generally air-stable with reasonable volatility and can provide clean deposition with minimum carbon incorporation. We developed a gram-scale chemical synthesis methods for obtaining high-purity diethyldithiocarbamate complexes of cadmium(II) and chromium(III) (Cd(S2CNEt2)2 and Cr(S2CNEt2)3). A homogeneous mixture of these precursors is used in the presence of sulfur vapor for low-pressure MOCVD growth of nanostructured CdCr2S4 films on borosilicate glass substrates. The deposited films are uniform with excellent phase-purity, morphology and crystallinity, which have grain sizes from 100 to 200 nm. Besides determining the band gap from optical absorption measurements, we have studied in detail the static and dynamic magnetic properties of the nanostructured CdCr2S4 films. EXPERIMENTAL SECTION Chemicals. All Chemicals were used as received. Sodium diethyldithiocarbamate trihydrate (NaS2CN(C2H5)2·3H2O, ACS reagent) was purchased from Chem-Impex International. Isopropyl alcohol (IPA, ≥98.0%), dichloromethane (CH2Cl2, ACS reagent), sulfur powder (S, 99.5%),



chromium chloride hexahydrate (CrCl3·6H2O, 98%), and cadmium chloride (CdCl2, 99%) were obtained from Alfa Aesar. Synthesis and purification of Cr(S2CNEt2)3. In a typical reaction, NaS2CN(C2H5)2·3H2O (2.253 g, 10 mmol) was dissolved in 30 mL IPA, and the solution was sonicated for 20 min. At the same time, CrCl3·6H2O (0.89 g, 3.4 mmol) was dissolved in 20 mL IPA and magnetically stirred for 20 min at 70 °C. The sodium diethyldithiocarbamate solution then was added to the CrCl3·6H2O solution with stirring at 90 ºC and refluxed for 5 h. After reaction, a blue-green solid was collected by centrifugation and dissolved in dichloromethane. The Cr(S2CNEt2)3 solution and insoluble white impurities were separated by centrifugation at 8000 rpm for 6 min. The resulting solution was transferred to a beaker with 400 mL IPA added. After recrystallization for 3 hours, the dark blue precipitate was isolated via centrifugation at 6500 rpm for 5 min and washed once before drying. Synthesis and purification of Cd(S2CNEt2)2. In a typical reaction, NaS2CN(C2H5)2·3H2O (2.253 g, 10 mmol) was dissolved in 30 mL IPA, and the solution was sonicated for 20 min. At the same time, CdCl2 (0.92 g, 5 mmol) was dissolved in 20 mL deionized water and magnetically stirred for 20 min. The CdCl2 solution was added to sodium diethyldithiocarbamate solution with stirring at room temperature for 1 h. The resulting white precipitate was collected by centrifugation at 6500 rpm for 5 min and washed three times using IPA before drying. Deposition of CdCr2S4 films. Borosilicate glass substrates (0.5 cm×0.5 cm, VWR International) were cleaned using detergent followed by repeated washing with deionized water prior to ultrasonic cleaning in acetone and isopropanol, each for 30 min. The low-pressure MOCVD system consists of a quartz tube reactor with a nominal diameter of 1.5” and length of 30”, housed


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in a tubular furnace. The pressure inside the quartz tube was maintained at ~0.2 Torr using a roughing pump. The sulfur powder and mixture of the synthesized Cd(S2CNEt2)2 and Cr(S2CNEt2)3 powders were placed upstream in separate porcelain boats in lower temperature zones of the tubular furnace. After pumping for 10 min, the sulfur powder was heated to 180 °C, while the mixture of Cd(S2CNEt2)2 and Cr(S2CNEt2)3 powder was simultaneously heated to 400 °C. Ultrahigh purity argon gas, the flow rate of which was maintained at 300 sccm using a Brooks Instrument Quantum gas mass flow controller (MFC), was used as a carrier gas for the sublimed precursors to be transported to the reaction zone, where it decomposed on the glass substrates (500 °C) to form polycrystalline CdCr2S4 thin films. After deposition for 60 min, the furnace was cooled down and the CdCr2S4 thin films taken out for characterization. Structural and magnetic characterization. XRD patterns were recorded on a Bruker D8 X-ray diffractometer equipped with Cu Kα radiation source. Scanning electron microscopy (SEM) analysis was performed using a JEOL 7000 FESEM equipped with EDX detector. Surface roughness was determined by atomic force microscopy (AFM, Asylum Research). The absorption characteristics of the films were recorded with an UV−Vis spectrophotometer (Agilent, Cary 500, USA). Band gap data processing was done using OriginPro 2018b and MATLAB R2018b software. X-ray photoelectron spectroscopy (XPS) analysis was performed on a Thermo Scientific K-Alpha spectrometer. Magnetic measurements were carried out using a Quantum Design Dynacool Physical Properties Measurement System (PPMS) equipped with a Vibrating Sample Magnetometer (VSM). Continuous wave (CW) EPR spectra were recorded in the temperature range of 20-290 K on an ELEXSYS E680 EPR spectrometer with a Flexline ER 4118 CF Cryostat and an ER 4118X-MD4 ENDOR resonator (Bruker-Biospin, Billerica, MA). CW EPR measurements



were performed at a nominal microwave frequency of 9.75 GHz with microwave power of 0.02 mW, modulation frequency of 100 kHz, and modulation amplitude of 5.0 G. Measurements were made to check for power saturation at various temperatures. CW data processing was done with Bruker’s Xepr software (Bruker-Biospin, Billerica, MA) and Origin (OriginPro, 2018b). We evaluated the double integrals (DI) by subtracting the baseline and doubly integrating the CW spectrum over a window that encompassed the full first-derivative peak and sufficient baseline on either side of the peak. The DI could not be calculated for temperatures below 55 K for the in-plane measurements and 65 K for the out-of-plane measurements because the first-derivative peaks were too broad and extended past the maximum magnetic field sweep width. EPR linewidths were calculated as the peak-to-peak linewidth. The ferromagnetic resonance properties were measured using a custom designed broadband ferromagnetic resonance setup using coplanar waveguide for microwave transmission from the microwave source and operating in 1-40 GHz range. The external magnetic field was swept through the resonance field of the sample at a fixed frequency. The transmitted power through the sample was detected by a rf diode. Lock-in technique by modulating the amplitude of the external magnetic field at 800 Hz was used to enhance the signal to noise ratio (SNR). Temperature-dependent measurements were carried out in a closed-cycle cryostat.


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RESULTS AND DISSCUSSION Crystal Structure, Crystallinity, and Morphology. Figure 1a shows a schematic diagram of the home-built low-pressure MOCVD system. The possible reaction mechanism, crystal structure, and thermogravimetric analysis of Cd(S2CNEt2)2 and Cr(S2CNEt2)3 are provided in the Supporting Information (Figure S1 and S2). The phase purity and crystallinity of the deposited nanostructured CdCr2S4 films are confirmed by X-ray diffraction (XRD), as shown in Figurer 1b. The films exhibit a face-centered cubic spinel structure with Fd3𝑚 space group, and the major diffraction peaks are indexed as (220), (311), (400), (331), (422), (511), (440), and (620) planes that match very well with the standard bulk JCPDS CdCr2S4 pattern no. 16-0506. Based on peak positions, the lattice parameter is calculated to be 10.20 Å, which is close to the reported bulk value of 10.24 Å for CdCr2S4.25 In exploring the grown process of nanostructured CdCr2S4 films, we find that nonoptimal growth conditions, such as lower deposition temperatures, principally favor formation of wurtzite phase CdS film growth. On the other hand, increasing the deposition temperature results in sulfur deficiency in the deposited films. With changing the amount of precursors or deposition time, the thickness of nanostructured CdCr2S4 films can be controlled from ~300 to ~600 nm with grain sizes from ~100 to ~200 nm. A 580 nm-thick CdCr2S4 film, which is representative, has been used for structural, optical, and magnetic properties characterization details discussed below.



Figure 1. Growth of nanostructured CdCr2S4 film and its crystal structure and morphology characterization. (a) Schematic diagram of the home-built low-pressure MOCVD setup. (b) XRD pattern of polycrystalline CdCr2S4 film deposited on borosilicate glass substrate. Inset displays the crystal structure of spinel CdCr2S4 (Cd, pink; Cr, blue; S, yellow). (c) Low- and (d) highmagnification planar SEM images. (e) high-magnification cross-sectional SEM images. (f) 2D AFM image of the CdCr2S4 film. High-quality nanostructured CdCr2S4 films are obtained using the low-pressure MOCVD method under optimal deposition conditions, which have smooth morphology as verified by SEM and AFM, as shown in Figures 1c-f. From the low- and high-magnification SEM images in Figurers 1c and 1d, one can clearly observe that the uniform and homogeneous CdCr2S4 layer fully covers the substrate with a grain size of ~150 nm. The CdCr2S4 layer adheres strongly to borosilicate glass substrates and forms a smooth film surface, which is shiny and reflective. Unlike most previous reported results showing non-homogenous film growth, the MOCVD grown films are dense with uniform grain size and crystallinity, over a relatively large area (Figure 2a). Cross-


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section SEM image of the nanostructured CdCr2S4 film (Figure 1e) further reveals a compact morphology without pinholes. From the two-dimensional (2D) AFM height image in Figure 1f, the root-mean-square roughness of the film is 18.6 nm for a scan area of 5 x 5 μm2. Chemical Valence and Film Composition. Figure 2a presents X-ray energy dispersive spectroscopy (EDS) elemental mapping of the deposited nanostructured CdCr2S4 film. Based on the EDS result shown in Figure 2b, the Cd/Cr/S ratio is 1 : 2.05 : 4.25, which is close to the desired stoichiometry. X-ray photoelectron spectroscopy (XPS) studies have also been carried out to investigate the chemical states of the ions in the CdCr2S4 films. A survey spectrum of the film identified the presence of Cd, Cr, S, O and C, and high-resolution spectra of Cd 3d, Cr 2p, and S 2p are measured to determine the oxidation states of the constituent elements (Figure 2c). In the Cd 3d spectrum, peaks at 403.1 and 409.6 eV are ascribed to Cd 3d5/2 and Cd 3d3/2, which can be assigned to Cd (II) with peak splitting of 6.5 eV.45 The Cr 2p spectrum exhibits a spin-orbit separation of 9.6 eV between the Cr 2p3/2 and Cr 2p1/2 states, indicative of Cr(III).46 The peaks identified at 161.8 and 162.6 eV in the high-resolution spectrum of S are generally attributed to S 2p3/2 and 2p1/2, respectively.47 These results further confirm that the as-deposited films are primarily CdCr2S4. In brief, the nanostructured CdCr2S4 films grown by low-pressure MOCVD method exhibit significantly improved quality than previous chemically synthesized CdCr2S4 films.



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Figure 2. Composition and chemical states characterization of the nanostructured CdCr2S4 film. (a) FESEM image, mixed and individual elemental mapping of the deposited film. (b) EDS spectrum and elemental composition of the film. (c) XPS survey spectrum, high-resolution XPS spectra of Cd 3d, Cr 2p, and S 2p of the CdCr2S4 film. Optical Properties. The optical absorption characteristics of the nanostructured CdCr2S4 films are measured by UV-Vis spectrophotometry, as shown in Figure 3a. The band gap of CdCr2S4 is determined to be ~2.5 eV for nanoparticles, while thin films and bulk are reported to have a band gap of 2.4-2.6 eV.33-34, 48 In Figure 3b, Tauc plot is obtained from the absorption spectrum of a nanostructured CdCr2S4 film (thickness of 580 nm) using the following equation49: (𝛼ℎ𝜈)𝑛 = 𝐴(ℎ𝜈 ― 𝐸𝑔)

(1)

where α is the optical absorption coefficient, h is Planck's constant, υ is frequency, Eg is band gap of the material, A is a constant, and n = 2 or 0.5 for a direct and indirect band gap semiconductor, respectively. Extrapolation of the (αhν)2 vs. hυ plot to the photon energy axis gives a direct band


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gap value of ~2.21 eV for our CdCr2S4 film, where Eupper is the highest photon energy in Tauc plot and Elower is the other end of photon energy for liner fit.

Figure 3. Optical properties characterization of the nanostructured CdCr2S4 film (thickness of 580 nm). (a) UV-Vis absorption spectrum of the CdCr2S4 film grown on borosilicate glass substrate. Inset shows a photograph of the film. (b) Tauc plot of (αhυ)2 vs. hυ to determine the band gap of the films. The inset shows the band gap vs. lower end of photon energy for fitting.



Magnetic Properties. The static magnetic properties of the nanostructured CdCr2S4 films have been measured using a vibrating sample magnetometer (VSM). A series of M-T scans are shown in Figure 4a for the 580 nm nanostructured film, where the magnetization is plotted as a function of temperature for several different applied magnetic fields. We have also carried out isothermal M-H measurements of the magnetization versus applied magnetic field for more accurate determination of the transition temperatures. Representative isothermal plots for the CdCr2S4 film are given in Figure 4b. Based on the isothermal measurements near the transition temperature, we have determined the Curie temperature (TC) by using the Arrott plot method (isothermal plots of M2 vs. H/M),50 as shown in Figure 4c. The TC determined using this method is ~82 K, which is in good agreement with the reported bulk value of 83 K.25 Figure 4d shows plots of the magnetization as a function of applied magnetic field at 5 and 300 K, respectively. As indicated, the nanostructured CdCr2S4 film exhibits ferromagnetic behavior at 5 K with a fairly low coercivity value of ~170 Oe. The magnetization is ~340 emu/cm3 at a field of 50 kOe, which is close to reported bulk value.51 As expected, the film is paramagnetic at room temperature, as confirmed by the linear field dependence of the magnetization shown in Figure 4d.


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Figure 4. Magnetic characterization of the nanostructured CdCr2S4 film (thickness of 580 nm). (a) Magnetization (M) versus temperature (T) of the film measured at different magnetic fields. Inset shows plots of the slope, dM/dT, calculated from (a). (b) Representative isothermal M-H measurements and (c) Arrott plots from 58 to 109 K with temperature steps of 3 K. (d) Magnetization as a function of field measured at 5 and 300 K. The inset displays an enlarged portion of the hysteresis loop measured at 5 K. Dynamic Magnetic Properties and Magnetic Anisotropy. The dynamic magnetic properties of the CdCr2S4 films have been investigated by continuous-wave electron paramagnetic resonance (CW EPR, X-band). The EPR spectra are measured in two orientations, one where the film is parallel to the external applied magnetic field (in-plane) and one where it is perpendicular to the



external magnetic field (out-of-plane). The CW EPR signal originates from the localized 3d electrons of Cr3+ ions.52-53 Figure 5a and b show the in-plane and out-of-plane CW EPR spectra at temperatures from 20 to 290 K. At high temperatures, the first-derivative EPR peaks for both inplane and out-of-plane measurements are symmetrical. At the TC around ∼80 K the first-derivative EPR peak is the sharpest. As the temperature decreases below TC, the first derivative peaks broaden and the peak-to-peak amplitudes decrease, even though the total integrated EPR absorption signal increases. When the magnetic field is in-plane with respect to the film, the peak shifts to lower magnetic field and when the magnetic field is out-of-plane, the peak shifts to higher magnetic field. The peak-to-peak linewidth (ΔH) are plotted in Figure 5c as a function of temperature for both magnetic field orientations. The EPR spectra of the film are similar regardless of its orientation with respect to the magnetic field in the paramagnetic region but the linewidth starts to increase below 100 K, indicating the onset of a phase transition. This signifies that the film is magnetized at low temperatures, and that the film’s internal magnetic field influences its resonance as a function of the external applied field. The temperature dependence of the EPR resonance field (Hres) is shown in Figure 5d. The resonance field is constant above the phase transition temperature (paramagnetic region), but below 100 K the Hres decreases when the external field is parallel to the film plane and increases when the external field is perpendicular to the film. This behavior also indicates a phase transition from paramagnetic to ferromagnetic. The difference in Hres for in-plane and out-of-plane fields arises from the anisotropy of the film due to dipole-dipole interaction which renders the film normal as hard axis. To further investigate the magnetic anisotropy for the films, a series of angle- and temperature-dependent EPR spectra have been taken. As shown in Figure 5e, the variation of Hres with angle becomes more pronounced as the temperature decreases, indicating a strong anisotropy. The transition from a paramagnetic to ferromagnetic state onset is


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at ~100 K, which is about 17 K above the reported ferromagnetic transition temperature (~83 K).25 This indicates that the ferromagnetic phase is stabilized by the presence of a magnetic field, which is also evident in the M vs T plot (Figure 4a). The temperature dependence of the inverse spin susceptibility, which is proportional to the 1/double integral (DI) of the EPR spectra, is shown in Figure 5f. The inverse of spin susceptibility decreases as temperature drops but ceases to change significantly around TC and remains constant at lower temperatures.

Figure 5. CW EPR characterization of the nanostructured CdCr2S4 film (thickness of 580 nm). (a) EPR spectra as a function of temperature for the film aligned parallel to the external applied magnetic field and (b) perpendicular to the external magnetic field. (c) The linewidth as a function of temperature for both magnetic field orientations. (d) Resonance field as a function of temperature for both magnetic field orientations. (e) Resonance field as a function of direction of external field (0° perpendicular to film plane and 90° parallel to film plane) at different temperatures. (f) Inverse double integral of CW EPR spectra characterizing the spin susceptibility of the CdCr2S4 film. The inset displays an enlarged portion around TC.



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To further investigate the magnetization dynamics and observed anisotropy of the nanostructured CdCr2S4 films, broadband ferromagnetic resonance (FMR) measurements have been performed from 10 to 40 GHz at different temperatures, for both in-plane and out-of-plane orientations. By fitting the FMR spectra to the derivative of a Lorentzian function having both absorptive and dispersive terms, we determine values of the resonance field (Hres) and peak-topeak linewidth (ΔH).54 In Figure 6a, we show representative Hres as a function of FMR frequency (f) at 70 K for both magnetic field orientations. Figures. 6b and 6c present ΔH as a function of f at different temperatures for both magnetic field orientations, showing a clear increase in ΔH between 80 K and 90 K. We fit both in-plane and out-of-plane data to Kittel equations simultaneously with 𝛾′and 𝑀𝑒𝑓𝑓,0 as shared parameters, 𝑓𝑖𝑛 ― 𝑝𝑙𝑎𝑛𝑒 = |𝛾′| (𝐻𝑟𝑒𝑠) ∙ (𝐻𝑟𝑒𝑠 + 4𝜋(𝑀𝑒𝑓𝑓,0 + 𝜒𝐻𝑟𝑒𝑠)) 𝑓𝑜𝑢𝑡 ― 𝑜𝑓 ― 𝑝𝑙𝑎𝑛𝑒 = |𝛾′|(𝐻𝑟𝑒𝑠 ― 4𝜋(𝑀𝑒𝑓𝑓,0 + 𝜒𝐻𝑟𝑒𝑠))

(2) (3)

where |𝛾′| is the absolute value of the gyromagnetic ratio and 𝑀𝑒𝑓𝑓,0 is the effective magnetization extrapolated to zero external field.55 The above equations are modified Kittel equations, which takes into account that the sample magnetization is not constant in the field range used for FMR, compare Figure 4b. In a first order approximation we can assume that the effective magnetization depends linearly on the applied field (Equation 4) 𝑀𝑒𝑓𝑓(𝐻) = 𝑀𝑒𝑓𝑓,0 +𝜒𝐻

(4)

where 𝜒 is the magnetic susceptibility. The Kittel plots at different temperatures for both magnetic field orientations are shown in Figures 6d and 6e. The difference between FMR in in-plane and out-of-plane orientation is due to dipole-dipole interaction and the thin film geometry of the sample,


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which is also known as shape anisotropy. However, for paramagnetic samples there is no shape anisotropy, which is why there is no difference between of the resonance fields for in in-plane and out-of-plane orientation measurements. We have not found evidence of any additional perpendicular anisotropy in the ferromagnetic phase, which may originate for example from interfacial strain. In Table 1, we summarize the values of the effective magnetization Meff,0 and 1

gyromagnetic ratio 𝛾′ = 𝑔𝜇𝐵ℎ obtained from broadband FMR measurements for the film at different temperatures. Table 1. Values of effective magnetization Meff, 0, gyromagnetic ratio ɣ’ obtained from broadband FMR measurements. The errors reported for all the FMR ﬁt parameters are the statistical error margins of the fitting. T (K)

300

2.799 ɣ’ ± [𝑀𝐻𝑧/𝑂𝑒 0.000 ] 1 Meff, 0 [𝑒𝑚𝑢/ 𝑐𝑚3]

0 ± 0.1

200

150

120

110

100

90

80

70

50

30

2.805 ± 0.000 2

2.801 ± 0.000 1

2.803 ± 0.000 4

2.811 ± 0.000 3

2.817 ± 0.000 4

2.825 ± 0.001

2.835 ± 0.003

2.836 ± 0.003

2.839 ± 0.003

2.848 ± 0.005

0 ± 0.4

0 ± 0.7

0 ± 2

0 ± 3

1.4 ± 0.4

82 ± 5

112 ± 3

182 ± 4

269 ± 6

311 ± 2

The plot of Meff,0 vs T from FMR is plotted together with M vs T from VSM in Figure 4a, showing Meff,0 is essentially zero for temperatures above 90 K, and there is sharp increase in Meff,0 below 90 K consistent with the transition temperature determined using the Arrott plot. The M vs T data has also been evaluated from the EPR spectra, which are similar to FMR and VSM measurements (Figure S3, Supporting Information). The g factor obtained from fitting with the Kittel equations gives a value of ~2.00 above 100 K in the paramagnetic region. Below 100 K, the g factor starts to increase as the temperature decreases, further supporting evidence for phase



transition to ferromagnetic order. The measured g factors in Figure 6f are similar to the values reported in bulk and nanocrystal samples.42, 47

Figure 6. Broadband FMR characterization of the CdCr2S4 film. (a) Representative resonance field as a function of microwave frequency at 70 K for both magnetic field orientations. The continuous line is fit to the Kittel’s equation. A typical spectrum is shown in the inset. (b) Linewidth as a function of frequency at different temperatures for external magnetic field parallel to film plane and (c) perpendicular to film plane. (d) Kittel plots at different temperatures for external magnetic field parallel to film plane and (e) perpendicular to film. (f) Plot of the extracted g factor as a function of temperature. CONCLUSION In summary, we have developed a simple and novel route to grow high-quality spinel CdCr2S4 nanostructured films via low-pressure metal organic chemical vapor deposition


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using synthesized diethyldithiocarbamate complexes as precursors. High-quality ferromagnetic semiconducting CdCr2S4 films are obtained by the reaction of Cd(S2CNEt2)2 and Cr(S2CNEt2)3 in the presence of sulfur vapor under relatively mild deposition conditions. Phase-pure nanostructured CdCr2S4 films are grown on borosilicate glass substrates, with uniform and homogeneous morphology. The optical properties, static magnetic properties, dynamic magnetic properties, and magnetic anisotropy of the nanostructured CdCr2S4 films have been systematically investigated. This work offers a novel strategy and a guiding chemical deposition route for the growth of high-quality chalcogenide spinel films. We believe that the MOCVD method using metal dithiocarbamate and other related precursors can be extended to the growth of a variety of other chalcogenide-based magnetic semiconductors for different applications.



ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

Reaction mechanism, crystal structure, and thermal decomposition of Cd(S2CNEt2)2 and Cr(S2CNEt2)3; magnetization as a function of temperature obtained from EPR data. AUTHOR INFORMATION Corresponding Author [email protected] (A. Gupta) [email protected] (L. Shen) Notes The authors declare no competing financial interests. Acknowledgements This research was supported by the National Science Foundation of U.S under Grant No. CHE-1508259, the Natural Science Foundation of China (No. 51425202), the Natural Science Foundation of Jiangsu Province (BK20160093), and the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). As a joint Ph.D. student, Chao Pang was partially supported by the China Scholarship Council. The authors acknowledge the help received from Dr. Chan-Woong Na during the initial phase of the work.


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Table of Contents Graphic


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Table of Contents Graphic



Figure 1. Growth of nanostructured CdCr2S4 film and its crystal structure and morphology characterization. (a) Schematic diagram of the home-built low-pressure MOCVD setup. (b) XRD pattern of polycrystalline CdCr2S4 film deposited on borosilicate glass substrate. Inset displays the crystal structure of spinel CdCr2S4 (Cd, pink; Cr, blue; S, yellow). (c) Low- and (d) high-magnification planar SEM images. (e) highmagnification cross-sectional SEM images. (f) 2D AFM image of the CdCr2S4 film.


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Figure 2. Composition and chemical states characterization of the nanostructured CdCr2S4 film. (a) FESEM image, mixed and individual elemental mapping of the deposited film. (b) EDS spectrum and elemental composition of the film. (c) XPS survey spectrum, high-resolution XPS spectra of Cd 3d, Cr 2p, and S 2p of the CdCr2S4 film.



Figure 3. Optical properties characterization of the nanostructured CdCr2S4 film (thickness of 580 nm). (a) UV-Vis absorption spectrum of the CdCr2S4 film grown on borosilicate glass substrate. Inset shows a photograph of the film. (b) Tauc plot of (αhυ)2 vs. hυ to determine the band gap of the films. The inset shows the band gap vs. lower end of photon energy for fitting.


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Figure 4. Magnetic characterization of the nanostructured CdCr2S4 film (thickness of 580 nm). (a) Magnetization (M) versus temperature (T) of the film measured at different magnetic fields. Inset shows plots of the slope, dM/dT, calculated from (a). (b) Representative isothermal M-H measurements and (c) Arrott plots from 58 to 109 K with temperature steps of 3 K. (d) Magnetization as a function of field measured at 5 and 300 K. The inset displays an enlarged portion of the hysteresis loop measured at 5 K.



Figure 5. CW EPR characterization of the nanostructured CdCr2S4 film (thickness of 580 nm). (a) EPR spectra as a function of temperature for the film aligned parallel to the external applied magnetic field and (b) perpendicular to the external magnetic field. (c) The linewidth as a function of temperature for both magnetic field orientations. (d) Resonance field as a function of temperature for both magnetic field orientations. (e) Resonance field as a function of direction of external field (0° perpendicular to film plane and 90° parallel to film plane) at different temperatures. (f) Inverse double integral of CW EPR spectra characterizing the spin susceptibility of the CdCr2S4 film. The inset displays an enlarged portion around TC.


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Figure 6. Broadband FMR characterization of the CdCr2S4 film. (a) Representative resonance field as a function of microwave frequency at 70 K for both magnetic field orientations. The continuous line is fit to the Kittel’s equation. A typical spectrum is shown in the inset. (b) Linewidth as a function of frequency at different temperatures for external magnetic field parallel to film plane and (c) perpendicular to film plane. (d) Kittel plots at different temperatures for external magnetic field parallel to film plane and (e) perpendicular to film. (f) Plot of the extracted g factor as a function of temperature.


Magnetic Properties of Semiconducting Spinel CdCr2S4

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