In the present work, the structural, spectral, thermal and magnetic properties of the Cr3 ions doped ZnS nanocrystals are systematically characterized to obtain the information about crystal structure, morphology, local site symmetry, bonding nature and magnetic behaviors.
Journal of Science: Advanced Materials and Devices (2019) 260e266 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Structural, optical, magnetic and thermal investigations on Cr3ỵ ions doped ZnS nanocrystals by co-precipitation method Sk Johny Basha a, V Khidhirbrahmendra a, J Madhavi a, U.S Udayachandran Thampy b, Ch Venkata Reddy c, R.V.S.S.N Ravikumar a, * a b c Department of Physics, Acharya Nagarjuna University, Nagarjuna Nagar, Guntur, 522 510, A.P, India Department of Physics, University of Kerala, Kariavattom Campus, Thiruvananthapuram, Kerala, 695581, India School of Mechanical Engineering, Yeungnam University, Gyeongsan, 712-749, Republic of Korea a r t i c l e i n f o a b s t r a c t Article history: Received 15 October 2018 Received in revised form 27 February 2019 Accepted March 2019 Available online 11 March 2019 Cr3ỵ ions doped ZnS nanocrystals were synthesized using the co-precipitation method The as-prepared sample was characterized to investigate the magnetic and thermal properties along with its structural and spectral studies X-ray diffraction (XRD) analysis confirms the cubic zinc blend structure, and the mean crystallite size is evaluated to be 4.7 nm Scanning electron microscope (SEM) and transmission electron microscopy (TEM) images indicate stone-like structures The deposition of Cr3ỵ ions in ZnS nanocrystals is evidenced by energy dispersive spectroscopy (EDS) analysis From the optical absorption data, the bands in the UV-VIS region are recognized as the characteristic bands of Cr3ỵ ions Crystal eld and Racah parameters are calculated as Dq ¼ 1564, B ¼ 567 and C ¼ 3389 cmÀ1 The electron paramagnetic resonance (EPR) spectrum exhibits well resolved paramagnetic resonance signal at g ¼ 1.982 which arises from the exchange interaction between Cr3ỵ ions It reveals from the correlating optical and EPR results that the Cr3ỵ ions occupy distorted octahedral site symmetry in the host lattice Photoluminescence spectrum shows three distinct emission bands in blue, green and orange regions Commission International de I'Eclairage (CIE) chromaticity coordinates are found in the blue region at (x, y) ¼ (0.172, 0.121) In addition, the prepared nanocrystals exhibit a ferromagnetic behavior with a coercive field (Hc) of 513 Oe The thermal steadiness of the prepared samples was characterized by Thermal gravimetric e Differential thermal analysis (TG-DTA) These Cr3ỵ ions doped ZnS nanocrystals find potential applications in LEDs, spintronics and nanoscale quantum devices © 2019 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: Nanocrystals Co-precipitation Chromium ions Powder XRD Spectroscopic studies Photoluminescence Introduction Semiconductors are the substances that play a major role in the development of science and technology Latest advances in nanotechnology could be expected to develop a variety of new intelligent materials and sophisticated devices, which find numerous applications in various fields like spintronics, cathode ray tubes (CRTs), non-linear optic devices, and anti-reflecting layers [1e3] Semiconductor nanocrystals are better detecting components and have attracted a vast scientific and technological environment * Corresponding author Department of Physics, Acharya Nagarjuna University, Nagarjuna Nagar, 522510, A.P, India Fax: ỵ91 863 2293378 E-mail address: rvssn@yahoo.co.in (R.V.S.S.N Ravikumar) Peer review under responsibility of Vietnam National University, Hanoi because of their superior luminescent, magnetic and optoelectronic properties [4] The present research hotspot is going on a novel design of solid-state lightening devices and magnetic materials for emergent devices Wide bandgap semiconductor materials are the most suitable components for the fabrication of such devices These semiconductors have extraordinary significance for both scientific and industrial concerns and also used in various coating applications due to less toxicity [5] Among the family of semiconductors, Sulfides (ZnS, CdS, PbS and SnS) and selenides (ZnSe, CdSe, PbSe and SnSe) are the most interesting materials in the recent past owing to their tremendous potential in various fields such as optoelectronic devices, solar energy conversion, light emitting diodes, optical sensors, photovoltaic devices, biological labeling and medical diagnostics [6e9] ZnS is one of the best n-type semiconductors which happen in two crystalline structures (allotropes) with a https://doi.org/10.1016/j.jsamd.2019.03.002 2468-2179/© 2019 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Sk.J Basha et al / Journal of Science: Advanced Materials and Devices (2019) 260e266 slight variation of bandgap energies The initial one is a metastable cubic zinc blend structure with space group F-4m (216) at room temperature and the other is thermodynamically stable hexagonal wurtzite structure with space group P63mc In this, the cubic zinc blend structure is more stable than a hexagonal wurtzite structure at room temperature [10] Due to its better thermochemical stability, ZnS has a great potential for a new generation of electronic nanodevices, nanosensors [11], nanogenerators and anti-reflection coatings [12] Doping of transition metal ions into inorganic semiconductor nanocrystals can open up new opportunities to tune their physical, optical, electronic and magnetic properties [13] Chromium is a transition metal with different oxidation states and a possible high magnetic moment Among these, Cr3ỵ is more stable and plays a key role in the development of lighting and ferromagnetic materials Zeng et al observed a paramagnetic nature for Cr doped ZnS nanocrystallites [14] while Chawla et al reported a superparamagnetic nature for the mentioned nanoparticles [15] After that Kaur et al [16], and Poornaprakash et al [17] observed weak ferromagnetism in chromium doped ZnS nanoparticles Based on these reports, one can understand that ZnS systems doped with chromium ions exhibit a weak magnetic nature But wide bandgap semiconductors with enhanced magnetic properties are required for a new generation of spin based devices In view of this, we have successfully prepared Cr3ỵ ions doped ZnS nanocrystals with enhanced magnetic properties as compared to those in the previous reports In the present work, the structural, spectral, thermal and magnetic properties of the Cr3ỵ ions doped ZnS nanocrystals are systematically characterized to obtain the information about crystal structure, morphology, local site symmetry, bonding nature and magnetic behaviors Experimental 2.1 Synthesis of Cr3ỵ ions doped ZnS nanocrystals Zinc acetate, sodium sulfide and chromium oxide were used as precursors All the chemical reagents used in this experiment were of analytical grade and used without any additional purication Cr3ỵ ions doped ZnS nanocrystals were prepared using the reported method [18] In the conventional method, 0.01 mol% of chromium oxide was added into the ZnS solution After hours of stirring a precipitate was formed Then the precipitate was centrifuged at 10000 rpm for 30 minutes and dried in air at 120 C for hours The synthesized Cr3ỵ ions doped ZnS nanocrystals were characterized by different structural and spectroscopic techniques 261 DTA analyzer was used to study the thermal properties of the prepared sample Results and discussion 3.1 Structural analysis Fig depicts the X-ray diffraction pattern of Cr3ỵ ions doped ZnS nanocrystals All the peaks observed in the pattern indicate cubic phase of ZnS and it is well matched with the standard diffraction pattern (JCPDS file No 05-0566) The observed diffraction peaks are corresponding to the planes , and which have lattice constant a ¼ 0.5316 nm The average crystallite size of Cr3ỵ ions doped ZnS nanocrystals is calculated using Debye Scherrer's formula D ¼ (kl/bCosq) where k, l, b and q have their usual meanings The average crystallite size of Cr3ỵ ions doped ZnS nanocrystals is found as 4.7 nm The comparison of crystallite sizes of Cr3ỵ ions doped ZnS nanoparticles with/without capping agents are listed in Table From Table 1, it is hard to synthesize chromium doped ZnS systems without capping agent below nm But in the present investigation, Cr3ỵ doped ZnS nanocrystals are successfully prepared below nm without any stabilizing ligand The dislocation density (d) of the prepared sample is calculated using the equation d ¼ 1/D2 and the evaluated value is 4.526 Â 1016 lines/m2 Texture coefficient (TC) is determined from the XRD peak intensities using the formula TC(hkl) ¼ [I(hkl)/I0(hkl)] / [(1/N) P (I(hkl)/I0(hkl))] where I, Io and N are the observed, standard intensities of (h k l) plane and the total number of diffraction peaks taken into account respectively TC > implies the preferentially oriented sample [26] The calculated TC is 1.28 which indicates the well crystalline nature of the Cr3ỵ ions doped ZnS nanocrystals 3.2 Morphological studies The SEM micrographs of Cr3ỵ ions doped ZnS nanocrystals at different magnifications are shown in Fig The magnifications of mm and 100 nm consist of tangled stone-like structures EDS 2.2 Characterization Shimadzu LABX XRD-6100 diffractometer was used to investigate the structure, phase and crystallite size of prepared nanocrystals Zeisse VO18 scanning electron microscope (SEM) with EDS attachment was used to carry out the surface morphology and chemical analysis Hitachi H-7600 and CCD camera system AMTV-600 was used to capture transmission electron microscopy (TEM) images Jasco V-670 spectrophotometer was utilized to record the optical absorption spectrum in the UV-Vis-NIR region (200-1400 nm) Room temperature electron paramagnetic resonance (EPR) spectrum was recorded on JES-FA200 ESR spectrometer Horiba Fluromax- was used to obtain the photoluminescence spectrum at room temperature Room temperature ferromagnetism was observed using Lake Shore 7410 vibrating sample magnetometer (VSM) Shimadzu DTG-60H TG- Fig Powder X-ray diffraction pattern and identied lattice indices of Cr3ỵ ions doped ZnS nanocrystals 262 Sk.J Basha et al / Journal of Science: Advanced Materials and Devices (2019) 260e266 Table Comparison chart for the synthesis of Chromium ions doped ZnS with or without capping agents S.No Chromium doped ZnS: with or without capping agent synthesis temperature ( C) Crystallite Size (nm) Ref e e EDTA PVP Thioglycerol 2-mercaptoethanol EDTA Thioglycolic Acid e 70 70 50 100 60 60 100e700 60e70 120 39e49 10 6e10 3e7 3e5 3e6 3e60 3e5 4.7 [19] [20] [21] [14] [22] [23] [24] [25] This work analysis is carried out to analyze the chemical composition and constituent elements of the synthesized product The elemental mapping analysis of Cr3ỵ ions doped ZnS nanocrystals is shown in Fig The observed EDS spectrum of the prepared sample confirms the presence of target elements (Sulphur, Chromium and Zinc species) The stoichiometric ratio of the prepared sample is Zn:S:Cr ¼ 64.28:35.07:0.65 This clearly shows that the prepared sample does not contain any foreign elements, which is also confirmed by XRD studies TEM micrographs of the Cr3ỵ ions doped ZnS nanocrystals are depicted in Fig 4, show irregular stone-like structures 3.3 UV-VIS absorption studies The UV-VIS absorption spectrum of Cr3ỵ ions doped ZnS nanocrystals is recorded in the range of 200e800 nm as shown in Fig The spectrum exhibits two strong absorption bands at 466 (21453 cmÀ1), 639 nm (15645 cmÀ1) and two weak bonds at 356 (28082 cmÀ1), 692 nm (14508 cmÀ1) According to their positions, the spin allowed bands at 466 and 639 nm are assigned to transitions from 4A2g(F) to 4T1g(F) and 4T2g(F) The other two weak spin forbidden bands at 356 and 692 nm are assigned to the transitions from 4A2g(F) to 2A1g(G) and 2Eg(G), respectively with the help of TanabeeSugano diagram (1954) These absorption bands indicate that the doped Cr3ỵ ions are at the distorted octahedral environment in the ZnS host lattice The band position (n1 ¼ 15,645 cmÀ1) corresponding to the transition 4A2g(F) / 4T2g(F) gives 10 Dq value The wavenumber of the band corresponding to 4A2g(F) / 4T1g(F) is 21453 cmÀ1 (n2) By the following equations, Racah parameters B and C values are evaluated [27] B ẳ (2n21 ỵ n22 e 3n1n2)/(15n2 e 27n1) Fig EDS pattern of the Cr3ỵ ions doped ZnS nanocrystals where n1 and n2 represents the energies of 4A2g(F) / 4T2g(F) and A2g(F) / 4T1g(F), respectively C/B ¼ 1/3.05[E(2E)/B e 7.9 ỵ 1.8 (B/Dq)] The evaluated values are B ẳ 567 and C ¼ 3389 cmÀ1 The obtained B value is lower than that of free ion value (Bfree ¼ 918 cmÀ1) The energy matrices for different values of Dq, B and C are evaluated for d3 configuration and a good fit is observed at Dq ¼ 1564, B ¼ 567 and C ẳ 3389 cm1 for Cr3ỵ ions doped ZnS nanocrystals These values are well tuned with Cr3ỵ ions containing nanopowders [28] The band head data of Cr3ỵ ions doped ZnS nanocrystals and their assignments are given in Table In the present study, the value of Dq/B is 2.7583 which is greater than the intermediate crystal field (2.3) The higher value of Dq/B indicates the strong crystal field between Cr3ỵ ions and crystal lattice The crystal eld parameters of Cr3ỵ ions doped ZnS nanocrystals are compared with different Cr3ỵ ions doped frameworks in Table The optical band gap energy (Eg) for the prepared nanocrystals is determined using the equation Eg ¼ 1240/lc, where lc is the absorption edge or cut-off wavelength in nm and Eg is the band gap energy [31] The calculated band gap energy for the prepared sample is 3.70 eV (335 nm) The band gap energy is also calculated from Tauc relation (ahy) ¼ A (hy À Eg)n, where a, hy, Eg and A have their usual meanings, and n ¼ 1/2 for a direct allowed transition The bandgap value is estimated by plotting (ahy)2 against hy Tauc plot for optical band gap energy of Cr3ỵ ions doped ZnS nanocrystals is shown in Fig The evaluated bandgap energy is 3.72 eV Both experimental and calculated bandgap energies are nearly equal This type of wide bandgap semiconductors may be used in optoelectronic devices Similarly D Samrma et al synthesized Cr doped ZnS quantum dots using capping agent thioglycolic acid with chemical co-precipitation method and observed bandgap energy of Fig SEM images of the Cr3ỵ ions doped ZnS nanocrystals Sk.J Basha et al / Journal of Science: Advanced Materials and Devices (2019) 260e266 263 Fig TEM images of the Cr3ỵ ions doped ZnS nanocrystals Fig UV-VIS absorption spectrum of the Cr3ỵ ions doped ZnS nanocrystals 4.1 eV [25] But in the present work, the observed bandgap energy is blue shifted which may be due to the quantum confinement effects 3.4 EPR analysis 3ỵ Fig shows the EPR spectrum of Cr ions doped ZnS nanocrystals The characteristic resonance signal at g ẳ 1.982 is observed for Cr3ỵ ions in the ZnS nanocrystals and arises due to the exchange Table Assignments of various transitions for the Cr3ỵ ions doped ZnS nanocrystals Transitions from 4A2g (F) Wavelength (nm) Wavenumber (cmÀ1) Observed Calculated 356 466 639 692 28082 21453 15645 14446 28075 21449 15640 14437 A1g(G) T1g(F) T2g(F) Eg(G) Dq (cmÀ1) B (cmÀ1) C (cmÀ1) 1564 567 3389 Table Comparison of crystal eld parameters of Cr3ỵ ions doped ZnS nanocrystals with Cr3ỵ ions doped nanopowders S No Compound Dq (cmÀ1) B (cmÀ1) C (cmÀ1) Dq/B References CdO CdO ZnO ZnS 1465 1540 1700 1564 765 619 636 567 2950 3327 3161 3389 1.915 2.487 2.672 2.758 [29] [28] [30] Present work Fig Tauc plot of the Cr3ỵ ions doped ZnS nanocrystals interaction of Cr3ỵ ions Similar results were observed in the previously reported Cr3ỵ doped nanopowders [28e30] reported in Table This clearly indicates the effective incorporation of Cr3ỵ ions into the ZnS nanocrystals The chemical bonding parameter (a) is calculated from the optical and EPR data by the relation, g ¼ gee (8la/D), where l (91 cmÀ1), ge (2.0023) and D (15645 cmÀ1) have their usual meanings The value of a (0 a 1) is the characteristic of bonding nature of Cr3ỵ ions with the surrounding ligands Lower a value indicates greater covalent nature and higher value of a represents the strong ionic bonding nature In the present investigation, the evaluated a value is 0.43 which indicates that the bonding nature between Cr3ỵ ions and its surrounding ligands is nearer to covalent Optical and EPR results reveal that the Cr3ỵ ions occupy distorted octahedral site symmetry in the host lattice 3.5 Photoluminescence (PL) studies Fig depicts the room temperature PL spectrum of Cr3ỵ ions doped ZnS nanocrystals Under the excitation wavelength of 325 nm, the spectrum exhibits three emission bands at 422, 524 and 609 nm The broad emission peak in the blue region monitored at 422 nm arises from surface states [32] The weak shoulder peak 264 Sk.J Basha et al / Journal of Science: Advanced Materials and Devices (2019) 260e266 Fig EPR spectrum of the Cr3ỵ ions doped ZnS nanocrystals Table Comparison of g, a and nature of Cr3ỵ ions doped ZnS nanocrystals with Cr3ỵ ions doped nanopowders Sample with Cr3ỵ g value a Nature References CdO CdO ZnO ZnS 1.973 1.973 1.986 1.982 0.596 0.44 0.38 0.436 covalent covalent covalent covalent [29] [28] [30] This work Fig CIE chromaticity diagram of the Cr3ỵ ions doped ZnS nanocrystals 3.6 Magnetic characterization observed at 524 nm in the green region is attributed to the defective states arises due to the doped Cr3ỵ ions [22] The other weak orange peak at 609 nm is ascribed to the recombination of electron hole pair of surface state and with sulphur vacancy centers [17] To bitterly understand the better luminescent properties, Commission International de I'Eclairage (CIE) and Color correlation temperature (CCT) values are required indeed Fig shows Commission International de I'Eclairage (CIE) 1931 chromaticity diagram for 0.01 mol % of Cr3ỵ ions doped ZnS nanocrystals The chromaticity coordinates are evaluated to be (x, y) ¼ (0.172, 0.121) using PL data From the gure, the CIE chromaticity coordinates of Cr3ỵ ions doped ZnS nanocrystals are located in blue region The corresponding CCT value can be calculated using McCamy equation [33] and is 3402 K From the CIE and CCT values, the prepared material emitts blue color which may be used in UV lamps and other blue LEDs Fig Photoluminescence spectrum of the Cr3ỵ ions doped ZnS nanocrystals at room temperature The room temperature ferromagnetic behavior of Cr3ỵ ions doped ZnS nanocrystals is carried out using VSM analysis Fig 10 shows the magnetic hysteresis (MeH) curve of Cr3ỵ ions doped ZnS nanocrystals at room temperature In accordance with the MeH data, it is clear that 0.01 mol% of Cr3ỵ ions doped ZnS nanocrystals exhibit room temperature ferromagnetism Reddy et al reported ferromagnetic nature for low concentration of Cr doped ZnS nanoparticles while at higher concentration of Cr doping, diminished magnetic nature is observed This may be due to the interaction between neighboring Cr-Cr ions [21] Similarly, ferromagnetic nature is observed for low concentration of transition metal ions (Cr3ỵ, Fe3ỵ, Co2ỵ and Ni2ỵ) doped ZnO nanopowders by Babu et al [34e36] The obtained values of the coercive field (Hc), saturation magnetization (Ms) and the remnant magnetization (Mr) of Cr3ỵ ions doped ZnS nanocrystals are 513.35 Oe, 54.47 Â 10À3 emu$gÀ1 and 11.35 Â 10À3 emu$gÀ1 respectively The observed ferromagnetic behavior of Cr3ỵ ions doped ZnS nanocrystals ascribed to the exchange couple interaction between localized d- Fig 10 M-H curve of the Cr3ỵ ions doped ZnS nanocrystals Sk.J Basha et al / Journal of Science: Advanced Materials and Devices (2019) 260e266 265 financial assistance to the Dept of Physics, Acharya Nagarjuna University Authors would like to thank the Director, Centralized Laboratory, ANU and Dept of Physics, University of Kerala for providing Ultracentrifuge and Photoluminescence facilities Authors are also thankful to The Head, SAIF, IIT Madras for providing EPR facility References Fig 11 TGA and DTA curves of the Cr3ỵ ions doped ZnS nanocrystals spins on the Cr ions and the free delocalized carriers The observed ferromagnetic behavior is not obtained due to the secondary phase which is confirmed by XRD studies These results suggest that the prepared material may be used in optical and spintronic quantum devices 3.7 TGA-DTA studies In Cr3ỵ ions doped ZnS nanocrystals, there are three stages of weight loss from TGA and four endothermic peaks are observed in DTA as shown in Fig 11 In the TGA curve, a weight loss of 9.5% (0.903 mg) occurring at 64 C may be due to the evaporation of physically absorbed water from the host matrix On DTA curve four endothermic peaks are observed before 700 C The decomposition of chromium ions at 338 C occurs in the range 320e375 C with a weight loss of 9.071% (À0.854 mg) from the ZnS matrix The endothermic peak observed at 432 C indicates the melting of material and its weight loss is 12.130% (À1.142 mg) The other broad endothermic peak at 697 C may be attributed to the gradual loss of residual sulphur ions from the sample The total weight loss is observed to be 51.34% from room temperature to 1100 C Conclusion In summary, the Cr3ỵ ions doped ZnS nanocrystals were prepared successfully by the co-precipitation method with an average particle size below nm without using any capping agents Morphological studies exhibited unevenly dispersed stone-like constructions The optical and EPR results reveal that the Cr3ỵ ions occupy distorted octahedral site symmetry in the host lattice and the nature of bonding is nearer to covalent The PL spectrum demonstrated strong blue, weak green and orange emissions The evaluated CCT value is 3402 K, indicating that the material may be used in blue LEDs and solid state lighting devices These nanocrystals exhibited also the ferromagnetic nature at room temperature TG-DTA evaluation dictated the steadiness of Cr3ỵ ions doped ZnS nanocrystals The obtained results will give a guidance on exploring such Cr3ỵ ions doped ZnS nanocrystals in spintronics and nanoscale quantum devices Acknowledgements One of the author, Sk Johny Basha is thankful to UGC for providing financial support through RGNFD scheme (No F./201517/RGNF-2015-17-AND-1273) to carry out the research work Authors are thankful to UGC-DRS and DST-FIST, New Delhi for [1] S Gorer, G Hodes, in: P.V Kamat, D Meisel (Eds.), Semiconductor Nanoclusters-Physical, Chemical and Catalytic Aspects, Elsevier, Amsterdam, 1997, pp 297e320 [2] L.B Chandrasekhar, R Chandramohan, R Vijayalakshmi, S Chandrasekaran, Preparation and characterization of Mn-doped ZnS 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Materials and Devices (2019) 260e266 Fig EPR spectrum of the Cr3? ?? ions doped ZnS nanocrystals Table Comparison of g, a and nature of Cr3? ?? ions doped ZnS nanocrystals with Cr3? ?? ions doped nanopowders... the Cr3? ?? ions doped ZnS nanocrystals 3.2 Morphological studies The SEM micrographs of Cr3? ?? ions doped ZnS nanocrystals at different magnications are shown in Fig The magnifications of mm and 100