Synthesis and Characterization of Co Ions Doped ZnS-CdS Composite Nanopowder

ZnS-CdS composite nano-powder doped with (0.01 mol %) Cobalt has been collected by a co-precipitation process at 300 K. The sample is characterized by structural, combined spectroscopic methods and magnetic studies. The prepared samples were belonging to cubic structure no impurity phases were observed. Doping of cobalt increase the neighborhood strain assessment and a decreases lattice constants decides from x-ray diffraction data. The crystallite size is 10.42nm. From UV-absorption and EPR studies revealed that the energy band gap of Co doped ZnS-CdS composite nanopowder and extension of sp-d exchange interactions and common d-d transitions. The variation in the energy bandgap varies as a function of cobalt concentration is due to structural modification. Photoluminescence spectrum reveals the defect-related emissions and shows the formation of luminescence. FT-IR spectrum confirmed the feature vibrational manner of Zn, Cd, O–H and sulfide ions are in the host lattice. The dopinginduced magnetic properties are studied by vibrating sample magnetometer which matches with the theoretical values besides ferromagnetic nature. Magnetic studies confirm the ferromagnetic nature of the material. Surface morphology and chemical homogeneity studies were carried out by using SEM with EDAX. Transmission electron microscope recommends the crystalline nature of nanoparticles, with average particle size is of the order of 20100nm.


Introduction
Nano materia ls possess treme ndous approach and indication in distinctive areas, together with che mistry, phys ics, optics, electronics, bio medica l scie nces, and materia ls scie nce. A set of II-VI compo unds have been deliberate wide ly because of their greater probable in the fie ld of optoelectronics. One of the important properties of the se micond uctors whic h distinguis h the m fro m the metals and ins ulators is their energy bandgap. The wa ve length of light absorbed or e mitted by the se micond uctor can be constructed us ing displa y devices and laser materia ls. Also, the electronic and optica l properties of this co mpound can be super inte nded by the concentratio n of imp ur ities in the materia ls as well as growth and operating cond itio ns. Amongst all these Zinc (Zn) and Cad mium (Cd) cha lcoge nides are the most important me mbers of this fa mily and the semico nductors ha ving wide band gaps fro m1.5 eV (Cd-Te) to 3.6eV (ZnS), whic h are all direct band gap in nature. In recent times, the comb inatio n of at least one cha lcoge n anio n and at least one more electropositive ele me nt of the dissimilar category ha ve invo lved considerable aware ness due to the ir imp ortant nonlinear, luminesce nt properties, quantum s ize effects and other important phys ical and che mica l properties. Moreover, Se micond uctors such as ZnS, CdS, ZnO, CdTe are the fore most pro mising materia ls and muc h in de mand for optical and optoelectronic applications [1][2][3]. Amo ng these Zinc sulfide (ZnS) is a binary compo und se mico nd uctor and it has traditio na lly shown exceptiona l phys ica l and che mica l properties and a pro mise for no vel miscella neous applicatio ns, such as electroluminescence, sensors, and lasers and so on, recent research ma inly focused on the var ious physica l and che mica l methods to synthes ize and mixed with ZnS nanopartic les. However, physica l and che mica l approaches often utilize toxic che mica ls to limit clinica l applications and cause environme ntal concerns applications and it has wide band-gap ener gy o f 3.68 eV for bulk cub ic phase. Due to the large bandgap, ZnS is an essentia l se mico nductor host lattice mater ial for electroluminescent devices and it has high trans mittance in the vis ib le regio n and a high re fractive index materia l to test several theoretical mode ls in conde nsed matter phys ics. Cad mium Sulp hide (CdS) is a se micond uctor materia l as it finds applications in photovo lta ic cells (or) solar cells, Cad mium Telluride (CdTe), Copper Ind ium Gallium Sulp hide (CIGS) and Copper Ind ium Sulphide (CIS). It has additio nally approached in diverse electro-optic devices and infra-red devices [4]. The adva ntages of che mica l routes over other assortme nt strategies are: (a) Easier control of the oxidatio n states, (b) Ability to make na nostructures of altered sizes and shapes, (c) Relative ly ine xpens ive. Wang et al., [5] suggested the 1-D nano materia l of CdS/ZnS. previo us ly e xplored with the ZnS Synthes ized nanoparticles thro ugh facile CTAB aqueous mice llar solutio n route [6]. It has been observed that nanoco mpos ite with a dopant in the ir crystal lattice can de mo nstrate dissimilar fro m those with ones on their s urface. The process of adding imp ur ities to host materia l is one o f the most importa nt goals for attainting nove l p hysica l properties in Trans ition meta l ( TM) doped nano-sized se micond uctors. Fro m last ten decades, ma ny experimenta l investigatio ns have been carried out to synthes is and characterized differe nt kinds of compos ite structure such as ZnO/ZnS [7], ZnO/CdS [8], GaN/GaP, ZnO/TiO2 [9,10], PbSe/CdSe [11,12], ZnS/CdS [13][14][15][16][17][18], CdS/ZnS [19][20][21] and Ge/Si [22].The better activity of the ZnS/CdS compo und is due to the response of pictures in the vis ible regio n and electron separation. In recent years, TM doped II-VI se micond uctors have been investigated compre hens ive ly because of their wide range applia nces in electroluminescence devices such as LEDs, optical sensors, etc., In this paper, ZnS and CdS are of particular interest because these wide-band-gap se micond ucting substance they have a capable choice of applications, suc h as sensors, photodetectors, solar cell, and electronic devices. The prepared Co 2+ doped ZnS-CdS co mpo und by coprecip itatio n method is to precipitate the che mical fro m scrap substances in the wastewater by adding together a reagent, which for ms a mysterio us co mposite with the to-be-re mo ved matter. Positive ions such as (heavy) metals, but also ne gative ions like phosp hates and sulfates, can be re moved via precipitation in air pressure with 300K of a substrate at source te mperature. The synthesized and Co 2+ doped ZnS-CdS co mposite nanopowder was prepared by us ing an aqueous solution of sodium sulfide, Zinc acetate dehydrates, Cad mium acetate and ethano l, respectively.
The mixture was stirred ma gnetica lly at 80 0 C. In the presence of TM-doping, it is found that the stabilities of cub ic structures. In this artic le, we are going to present the research work in the area, in which, to the high qua lity of our infor mation nobody has suggested earlier. The research work consists of a s ynthes is of Co 2+ doped ZnS-CdS compos ite nano-powder effect of co 2+ concentratio n on their investigatio ns. However, all the depositio n process has a major effect on structura l, electrical, ener gy band gaps, optical and magnetic properties of Co 2+ doped ZnS-CdS composite nanopowder

Experimental techniques
The fabricatio n of co mp lex na nopowder is characterized by a powder X-ray diffraction pattern of the produced sa mp le is rec orded on SHI M ADZU XRD-6100 diffract meter. A mater ial band is that the tiny fraction of occurrence e missio n

Preparation of ZnS-CdS composite nanopowder
The for matio n of ZnS and CdS nanopartic les can be noted by var iation in color of the solution fro m color less to milky white to pale ye llow respective ly which confirmed by a two-step chemical method. Step-1 The co mposite nanopowder ZnS -CdS has been successfully prepared via a coprecip itatio n procedure. In the classica l approach, 0.2 mo l% of Zn (CH3COO)2 in 50 mL identical mass of the water-ethano l mixture and equiva le nt mo lar quantity of Na2S aqueous solution were mixed drop by drop. The obtained solutio n was stirred magnetically for a ho mo geneo us solutio n for 80°C. Finally, a milk y white precip itate appears under the roo m te mpe rature cond itio n whic h designates the assembly of ZnS nanoparticles. Step-2 Cadmium acetate (0.09mol %) is added to 50mL equivalent volumes of the water-ethanol matrix with steady stirring. Subsequently, ten min (0.1mol %) 50 mL Na2S of the mixture was added to the above-prepared product dropwise. After 4h constant stirring, the colorless solution changed into yellow-colored precipitate formed which specifies the construction of ZnS-CdS composite nanopowder. Lastly (0.01) mol% of Co 2+ melted in 50 ml identical volumes of a water-ethanol matrix. This is added to the above colloidal suspension and stirred for 3h. The nanocomposite emulsion was purified with deionized water repeatedly to take apart surplus contaminants. The acquired compound was kept in an ultracentrifuge at 10000 rpm for half an hour. The fine precipitate was dried in a hot furnace at for 2h at120°C a nd gra nulates with a gate motor by ha nd milling. The s ynthes ized Co 2+ doped ZnS-C dS composite nano powder was characterized by various techniques .

X-ray diffraction (XRD) study:
X-ray diffractio n present in the for matio n of concerning the arrange ment about crystalline materia ls and synthes ized Co 2+ doped ZnS-CdS co mposite nanopo wder was deter mined by X-ray diffracto meter fro m 10 o -70 o as sho wn in Fig.1.There is a strong and intense peak that occurs at 2θ=27.
The constitutiona l parameters such as microstrain (ɛ) and dislocation dens ity ( ) va lues were considered fro m the fo llowing interrelatio n. The dis locatio n dens ity

SEM with EDX study:
SEM shows the typica l morp holo gy of the grains is distinguis hed to be near-spherica l and is more consiste nt througho ut the ana lyzed regio n in the sa mp le. The present sa mp le can be observed that a spher ical-like na nostructure with unifor m size and also reveal the striking difference in the sur face roughness of the present samp le as sho wn in Fig.3. The mo lecule s ize of the specime n produced via coprecip itatio n method was in synchro nizatio n with that of the x-ray diffractio n and is in the order of nanos ize. Co mpositiona l ana lys is of the Co 2+ io ns doped ZnS-CdS co mposite nanopowder done by EDX and are displa yed in Fig. 4

TEM study:
The microstructures of the arranged sa mple were further studied us ing TEM.  [25]. The loop at 1362 cm -1 matc hes up with C-H bend ing vibrations. [26] The observed band at 1531cm -1 has been give n to the extended mode of the C=O fa mily [27].

EPR study:
EPR Spectroscopy is a very sensitive and enlighte ning techniq ue has been explo ited for the investigation of var ious para magnetic species in solid or liquid states. The spectrum of the present sa mp le is characterized at 300K. Moreover, the octahedral coordination of Co 2+ ions ha ve lo ngitud inal re laxatio n time is e xtre me ly low. However, at high te mperatures, the spectrum becomes broader due to lo w relaxation time [31]. In perfect octahedral symmetry, the lo west of an orbita l quantum state of d 7 Configuratio n is divide by spin-orbit comb inatio n to provide a ground state Kra mer's doublet with g = 4.33 [32]. In the current research, the EPR spectrum o f Co 2+ doped ZnS-CdS compos ite nanopowder exhib its resonance signa l around g = 2.5 at 100K as sho wn in Fig.8 whic h suggests that the Co 2+ io n resona nce signal is due to uns yste matic allotme nt of distortio ns in octahedral symmetry. [33][34][35][36]. By estimating EP R and optica l a ma lga mation spectral data, the covalence constraint (k o ) is determined by [37]

g = + k o -(λ/) (4)
and pure ionic bond ing respectively. The estimated and the calculated value of ko is 0.92 na me ly where the spin-orbit pair ing stable va lue λ (-178c m -1 ) and g is the doped ZnS-CdS co mposite nanopowder. The luminescence peak exhibits fo ur intense wave length emiss io n on 375nm (3.30eV), and 532nm (2.33eV), 680n m (1.87eV) and 735nm (1.68eV). Due to the vis ible broadband regio n of 500-800n m transitio n metal io ns were wide ly used [38]. The e miss ion at 375nm (3.30eV) whic h may possib ly be recognized to a co mbined c harge at the sulfur interstitials and donor le vel with ho les restrained at the Zinc vacanc ies on acceptor leve l [39]. A sharp ye llo w e missio n at 532 nm (2.33eV) is attrib uted to the characteristic of cobalt [40]. A red emiss ion at 680nm (1.87eV) confir ms the CdS peak [41] and the sharp and most inte nse peak at 735nm (1.69eV) gives orange-red emiss io n in the NIR regio n [42,[43][44][45]. In general, the role of surface quality, sur face defects of the compos ite mater ial is ver y important in ter ms of emiss ion characteristics towards the optical applicatio n. The stoic hio metr ic defects at electronic leve ls are influe nced by photoluminescence in II-VI semico nductors. The present strong tip at 532nm is due to the energy transition from 4 A2g (F)  4 T1g (F) for Co 2+ ions.

CIE coordinates
The CIE chro matic ity dia gra m is used to vis ualize the color Ga mut's (i.e. the ranges of producible colors) for various output devices. The

Conclusion
In summary, Co 2+ doped ZnS-CdS co mposite na nopartic les were successfully synthes ized via che mica l precipitation techniq ue. Powder X-ray diffractio n pattern revealed the structure was cubic. Fro m diffractio n data, the average gra in size was estimated fro m Scherer's method and W-H plots which are in the sequence of nanosca le. Our result is evident that the decrease in structura l para meters confir ms a slight increase in crysta llite size after the depositio n of CdS. SEM with EDAX and TEM ima ges appear ho mo ge neous ly dispersed sphere like compos itio n. FT-IR scale showed the c haracteristic resona nce ma nner of Zn, Cd, O-H, sulfide io ns together within-host lattice. Optica l and EP R spectrum establis hed that the doped Co 2+ io n free at defor med octahedral site equilibrium for which crystal fie ld (Dq) and interelectronic repuls ion ( B, C) constants are estimated. The optica l absorptio n spectra (UV-Vis) reveals and established the effic ie ncy of the Co 2+ doped ZnS-CdS compos ite nanopo wder the analyzed, crystal field and bonding para meters show the bonding nature is moderately cova lent. PL spectra of powder samp le suggest that the tough yello w light e missio n with suppressed red color emiss io n is a successful method to impro ve the optical properties. The obtained CIE color chro maticity va lues ind icated that this samp le can be used for a W hite -LED applia nce. The excha nge interactio n between Co 2+ io ns mediated by carriers contrib ute to the ferro ma gnetis m at roo m te mperature exhib its due to the existence of defects suc h as imp urity defects, vacanc ies or hydroge n che misorption gra in bo undar ies and dislocation have been induced magnetism.