View of STUDIES ON POLYMER NANOCOMPOSITES CONTAINING SELENIUM AND CADMIUM BASED QUANTUM DOTS

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STUDIES ON POLYMER NANOCOMPOSITES CONTAINING SELENIUM AND CADMIUM BASED QUANTUM DOTS

Rashmi Jaiswal¹, B.K.Sinha², A.K.Bajpai³

1Department of Physics

Govt. Model Science College (Autonomous), Jabalpur (M.P), India St. Aloysius Institute of Technology Gaur Jabalpur M.P

2.Dept. of Physics & Electronoics, Institute for Excellence in Higher Education, Bhopal (M.P.)

3Dept. of Chemistry, Govt. Model Science College (Autonomous) Jabalpur (M.P) Abstract - Polymers when embedded with QDs display unique properties that results due to combination of the inherent features of polymer matrices that act as housing with unique optical and chemical characteristics of the QDs. Nanocomposite thin films with varying concentrations of Cd²⁺ ions in Cadmium sulphide /poly vinyl alcohol (CdS/PVA) and Cadmium selenide / poly vinyl alcohol (CdSe/PVA) have been reported. When the nanocomposite so formed doped with silver and maganese further reduction in size is observed. Ultraviolet-visible (UV-VIS) spectroscopy reveals that the doped CdS/PVA and CdSe/PVA films show enhanced displacement of the spectrum to shorter wavelength side.

Structural, morphological and optical study of the nanocomposite (pure and doped) shows upgrade in effectiveness and productivity. The crystalline nature of the nanocomposite thin films so formed is translucent, uniform with no perforation and find implementation in optoelectronic devices like solar battery, optical device, thin film organic field effect transistors, organic light emitting diodes, and other nano devices.

Key Words: – CdS- Cadmium Sulphide, CdSe- Cadmium Selenide, PVA -Polyvinyl Alcohol, XRD- Xray diffraction, SEM- Scanning electron microscopy, FTIR- Fourier transform infrared spectroscopy.

1 INTRODUCTION

The development of science and technology has always been associated with the growth in material science and the technology or method by which we process it . These processing techniques are either based on assembling small unit of nanoparticles to obtain desired material or it can be based on splitting up of a big unit of material into desired dimension. In the coming future, most of the upcoming devices will be based on the properties of nanomaterials like quantum confinement effect. Due to large surface to volume ratio the no. of atom at the surface is different than that at the bulk counterpart. Also the imperfection that develops in the crystal structure during processing is no longer a hindrance as the surface properties of nanomaterials allows manipulation. A no. of techniques are available by which this drawback can be overcome like by controlling the condition while precipitating the solution, by forming an emulsion between two immicible liquids, by miscelle or reverse miscelle method, through hydrophilic versus hydrophobic interaction. Different factors like thermodynamic or van der Waal’s forces induces growth of particle

and consolidation, that lead to the formation of bigger particles that settles with time. The purpose of using colloidal nanoparticles is that they maintain stability in colloidal suspension.

Stabilization of nanoparticles can be classified as a) stabilization through electrostatics: it involves the creation of a layer on a layer of adsorbed ions over the nanoparticles that result in a coulombic repulsion between nanoparticles that are approaching ; or b) Spatial hindrance:

achieved by adsorption of polymer molecules over the nanoparticles, Osmotic detestation felt by the polymer molecules due to increase in their concentration when polymer coated with nanoparticles approach close to each other, keeps them (along with the nanoparticles) separated.

Nanotechnology is based on the development, study and fashion of nanomaterials and its use to devices and system whose dimension are in the nanometer range which is less than 100nm. Owing to quantum size effect, micro size effect, surface properties, tunnel effect etc nanomaterials display unique physical and chemical features.

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Vol.04, Issue 05, May 2019 Available Online: www.ajeee.co.in/index.php/AJEEE

2 In nanotechnology, a particle is described as a small entity that behaves as a entire unit in terms of its properties. Nanoparticles can be further catagorised according to their size i.e in terms of its diameter, as large grain size particles cover a range between 10,000 and 2,500 nm, small grain size particles are between 2,500 and 100 nm,ultrafine grain size particles, are between 100 and 1 nm.

1.1 Nanomaterials and their Properties In order to observe quantum size effect in nanomaterials the dimension that has been reduced must be smaller than the phase coherent length for electrons in the materials. When its dimension is of the order of wavelength associated with electron or hole, quantum size effect or quantum confinement effect can be seen.

Some of the properties that are affected by particle size are photo luminescence, blue shift in the optical absorption band, linear and nonlinear optical properties , geometrical strength, melting point, magnetic properties etc^.1.

The two main factors that are responsible for change in the properties of nanoparticles are firstly increase in the surface to volume ratio and secondly the changes in the electronic structure of the material due to quantum size effects.

Thus below a particular threshold size the principle properties of a material can become strongly dependent on the size of the material.

A particle where all the three dimensions are nanometer in range is called nanoparticle. It exist in different shapes and size such as sphere, triangle, cubic, pentagon, rod-shaped, shells, ellipsoidal etc. Nanoparticles when used as building blocks to fashion complex nanostructures such as nano-chains, wires, clusters and nano aggregates, found applications in the fields of optoelectronics, chemistry, biotechnology and medicine (targeted drug delivery). For example, electroluminescence and quantum efficiency in organic light emitting diodes get enhanced due to the use of gold nanoparticle ², As a efficient

catalysts, palladium and platinum nanoparticles can be used ³; Silver nanoparticles are used to develop glucose sensors ⁴ ; and in diagnosing cancer in Magnetic Resonance Imaging (MRI) iron oxide nanoparticles are used as contrast agents ^5. Due to large surface to volume ratio nanoparticles differ from the properties inherent in their bulk counterparts. Therefore, properties exhibited by nano particles like electronic, optical, magnetic and chemical properties are very different from both the bulk and the constituent atoms. For example, the striking colors of gold and silver nanoparticle solutions are due to the red shift of the plasmon band to visible frequencies. This red shift occurs due to the quantum confinement of electrons in the nanoparticle, because the mean free path of electrons is larger than the size of nanoparticles ^6,7. The size and shape as well as the dielectric constant of the surrounding medium significantly affects the optical properties of nanoparticles. Surface related properties are drastically affected with slight change of size, shape and surrounding media of nanoparticles as they have large surface to volume ratio. Therefore, the optical properties can be tuned by controlling the size and shape of nano particles depending on application.

1.2 Classification of Nanomaterials All standard materials like metals, semiconductors, glass, ceramic or polymers can in principle be obtained with nano scale range. The band of nano materials covers a huge range from inorganic or organic, crystalline or amorphous, powders or embedded in a matrix, colloids, suspensions emulsions, nanol ayers and films, up to the class of fullerenes and their derivatives. Typically, there are different method for the classification of nanomaterials such as on the basis of dimensions, composition and process or manufacturing process etc.

Dimensionaly, these materials can be classified into different classes depending on the number of dimensions in which the material has reduced (Fig. 1.1).

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Figure 1: Three quantization configuration types in materials depending upon the confinement in different dimensions.

Thus, they can be classified into: layered or lamellar structures, rod-shaped or filamentary structures, equiaxed or crystallite nanostructured materials.The nanostructured materials can be metals, ceramics, polymers, or semiconductors may contain crystalline, quasi crystalline,

or amorphous phases. If the grains are made up of crystals, the material is called nano crystalline. On the other hand, if they are made up of quasi crystalline or amorphous (glassy) phases⁸ they are termed nano quasi crystals or nanoglasses^9.

Figure 2: Classification of nanostructures with regard to different parameters On the basis of composition, morphology,

manufacturing process and distribution of the nano crystalline component nano structured materials has been further classified by Gleiter¹⁰. They can be synthesized by gas phase synthesis which involves condensation, CVD, etc., liquid phase synthesis which involves sol-gel, precipitation, hydrothermal processing, etc. or mechanical procedures that includes ball milling, plastic deformation, etc. Fig. 1.2 shows this classification of

nanostructures on the basis of different parameters.

1.3 Objectives

The proposed research work includes achieving the following objectives as being motivated by the unique role of Cadmium and Selenium based quantum dots and the resulting hybrid materials with polymers:

1. Synthesis of polymer nanocomposites containing pure

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4 CdS, CdSe and doped CdS, CdSe quantum dots.

2. Characterization of as prepared nanocomposites by techniques like FTIR, XRD, TEM, SEM techniques so as to gain the structural and morphological feature into the prepared nanostructure materials.

2. NANOMATERIAL SYNTHESIS

Nanotechnology is inter disciplinary field that deals with the development of new methodologies for the synthesis of nanomaterials. Nanocrystalline materials can be synthesized either by arranging or fashion small clusters to obtain bigger unit of desired dimension or breaking down the bulk material into smaller and smaller units.

Thus, the synthesis of nano material has been classified into two catagories , which are called bottom-up approach and top-down approach.

Bottom-up approach involves to collect, consolidate and fashion individual atoms and molecules into the desired structure.

The top-down approach starts with the large chunk of material or pattern and gradually reduces its dimension or dimensions. Attrition or ball milling is a top-down method of preparing nanoparticles, whereas the colloidal dispersion is an example of bottom-up approach. Hybrid approach includes lithography which include both top down as well as bottom up approach, since the growth of thin films is bottom-up whereas

etching is top-down. Both the approach play vital roles in modern industries and most likely in nanotechnology and nanoscience as well. Both the approaches are associated with advantages and disadvantages .

There are numerous techniques available to synthesize a variety of nanomaterial that can be in the form of powder-crystalline or amorphous colloids, cluster, nano tubes, nanorods, nanowire, thin film etc. There are various physical chemical, biological and hybrid method available to synthesize nanomaterial.

Depending on the material of interest, i.e zero dimensional, one dimensional, two dimensional etc. the technique is to be used.

Nano materials have been synthesized by a number methods including inert gas condensation, mechanical alloying, spray conversion processing, milling, electrode deposition, rapid solidification from the melt, PVD,CVD, co-precipitation, wet chemical processing, sliding wear, spark erosion, plasma processing, auto-ignition, laser ablation, hydrothermal pyrolysis, quenching the melt under high pressure, biological templating, sonochemical synthesis. The method capable of producing very fine or ultra fine grain- sized particle can be used to synthesize nano crystalline materials. The topography, morphology and texture can be varied by suitably controlling the variables in the methods^11-12.

2.1 Method of synthesis of nanomaterials Starting

phase Technique Dimensionality of product

Solid Ball milling 3D

Diversification of

amorphous phases 3D

Spark erosion 3D

Sliding wear 3D

Liquid Rapid solidification 3D Chemical bath

deposition 3D

Electrodeposition 1D, 3D Vapour Physical vapour

deposition

-Evaporation and sputtering

1D

Plasma processing 3D Chemical vapour

condensation 3D, 2D

Chemical reactions 3D

lnert gas

condensation 3D

Table 1: Methods to synthesis nanocrystalline materials

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5 3 EXPERIMENTAL METHOD

3.1 Chemicals

The reagents used will be Selenium powder, Poly Vinyl Alcohol (PVA), Cadmium Chloride (CdCl2) ,Sodium Sulphide (Na2S), Manganese chloride (MnCl2), Silver Nitrate (AgNO3), Sodium Hydroxide(NaOH),Sodium Thiosulphate (Na2SO3) All chemicals has been purchased from local suppliers. All reagents will be used without further puriﬁcation. Deionised water will be used through out the experiments.

4 METHOD

1. Method of synthesis of pure CdS/PVA and doped CdS/PVA nanocomposite

CdS nanoparticles embedded in PVA matrix (PVA-CdS) will be deposited on glass substrates by the solution casting method. The deposition is carried out with the mixture of Cd²⁺ion coordinated matrix solutions (PVA-Cd²⁺) and Sodium Sulfide (Na2S·9H2O). Matrix solutions containing different concentration of Cadmium ions are prepared to investigate the effects of Cadmium concentrations on the characteristics of deposited PVA/CdS nanoparticles, which is accomplished by adding different molar concentration of Cadmium Chloride (CdCl2) 5Mm,7Mm and 9Mm (marked as S1,S2,S3 ) to aqueous solution of PVA prepared by adding 10g in 100ml of DI water.

Initially, PVA is dissolved separately in DI water and kept overnight.

These solutions are constantly stirred by magnetic stirrers until the solution becomes transparent. When different concentration of CdCl2 were added to the prepared PVA solution, then the mixture (PVA and CdCl2) was stirred for 2 hr to 3 hr at 60 -70 °C. When CdCl2 is mixed with PVA aqueous solutions, the -OH groups in PVA act as the coordinate sites for Cadmium ion aggregation and PVA-Cd²⁺

composites are formed. The mixtures of Cadmium Chloride and PVA are stirred, separately for the formation of PVA-Cd²⁺

matrix solutions. In matrix solution, the Cd²⁺ ions substituted for -OH groups in PVA and the Cd²⁺ ions were then coordinated with PVA to form the PVA- Cd⁺² composites in the solution.

For the formation of PVA-CdS nanopartilces, S²⁻ solution is prepared by Sodium Sulfide, the pH of the solution was maintained by adding measured amount of NaOH to the solution. The Na2

S solution is added dropwise and was continuously stirred for 4 h ,the colors of the composites will change to yellow that indicates the formation of the CdS nanoparticles.These CdS/PVA nanocomposite is then casted on glass substrates which is homogenously distributed, uniform films were obtained after drying at room temperature for 2 days in order to remove residual de ionized water.

Method of synthesis of doped Mn:CdS/PVA and Ag:CdS/PVA nanocomposite thin films

For preparation of Mn:CdS/PVA 1.5g of MnCl2 is added in 25ml of DI water which is then added to the above prepared PVA- CdS nanocomposite before the formation of the matrix and then lead to the formation of film. Similarly for the preparation of Ag:CdS/PVA 20 microlitre of 0.6 M silver nitrate solution is added to the as prepared CdS/PVA nanocomposite before the formation of the matrix and that lead to formation of the thin film.

2. Method of synthesis of pure PVA/CdSe and doped PVA/CdSe nanocomposite

The CdSe/PVA thin films were deposited on a glass substrate by reacting Cd²⁺ embedded PVA with sodium selenosulphite. For the preparation of sodium selenosulphite solution (1M) (Na2SeSO3), powedered selenium (0.05mol) is added to 100 ml solution of sodium sulphite (Na2SO3)(1M) The mixture that results was then refluxed at 70-80 °C for 3-4 hr with continous stirring. After this, the final solution was filtered with a filter paper and was stored in the dark place at 60-70 °C to prevent its decomposition

In a reaction, a matrix solution was prepared by adding 1 mL of cadmium chloride into 20 mL aqueous solution of PVA and stirred continuously for 15–25 minutes. 1 mL of diluted sodium seleno sulphite (0.1 M) was then added drop wise

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6 into this matrix solution, and the mixture were stirred continuously for another 15- 20 minutes. Thus the resulting solution becomes transparent, and suddenly the colour changes to orange the colour of the final solution that contains Cd²⁺ and Se²⁻ ions in the polymer matrix changes from transparent to brown indicating the formation of CdSe nanocrystals in the PVA matrix. These PVA-CdSe nanocomposite is than casted on a chemically clean glass substrates, homogenous and uniform films were obtained after drying it at room temperature for two days in order to remove residual deionised water.

In the experiment to see the change three different concentrations of CdCl2 ( 0.5 M, 1 M, and 1.2 M) and a fixed concentration of Na2SeSO3 (1 M) has been taken. Further, the films were heated at 100°C for 6 - 7 hours. The samples were labelled as Se1, Se2, andSe3 for CdCl2 concentrations of 0.05 M, 1 M, and 1.2 M, respectively.

Method of synthesis of doped Mn:CdSe/PVA and Ag:CdSe/PVA nanocomposite thin films

For the preparation of Mn: CdSe/PVA 1.5g of MnCl2 is added in 25ml of DI water which is then added to the above prepared PVA-CdSe nanocomposite before the formation of the matrix and lead to form the film. Similarly For the preparation of Ag:CdSe/PVA 20 microlitre of 0.6 M silver nitrate solution is added to the above prepared CdSe/PVA nanocomposite before the formation of the matrix which ultimately lead to the formation of film.

4.1 Characterisation

1. X-ray diffracto meter(XRD)

X-ray diffraction (XRD) is a non-destructive technique that operates on the nanometer scale regime and is based on the scattering of X- rays from crystal lattice that have long range order. In order to characterize diverse range of materials, such as metals, minerals, polymers, plastics, pharmaceuticals, proteins, thin- film, ceramics semi conductors etc it can be used. XRD are classified into two catagories they are X-ray crystallography and X-ray powder diffraction method. X-ray crystallography, is a technique that can be used to study the entire structure of a

crystal. When a single pure crystal cannot be accomplished, X- ray powder diffraction can be used in its place. It can also yield important information about its crystalline structure, such as size, purity level and texture etc.

2. Scanning electron microscopy (SEM) The scanning electron microscope (SEM) forms images of the electrons that are reflected from different lattice sites of a sample. For studying surface morphology or measuring particle sizes these images are useful. Scanning electron microscopy gives the following qualitative informations topography i.e the features at the surface of an object and their texture, morphology shape, size and pattern of the particles making up the object that are on the surface of the sample and composition.

3.UV-Vis Spectrophotometry (UV-Vis) In order to measure the amount of light that can be absorbed by sample a spectrophotometer is employed. It passes a beam of light through a sample and the intensity of light reaching a detector is measured. The absorption, transmission and reflectivity of a thin film can be characterized using this technique.

4. Fourier transforms infrared spectroscopy (FTIR)

It is a technique which is used to collects spectral data in a wide spectral range, it can also reveal infrared spectrum of absorption, emission, photoconductivity or raman scattering of a solid, liquid or gas.

5 RESULT AND DISCUSSION

Morphological and Structural study For pure CdS/PVA and doped CdS/PVA nanocomposite

In order to identify the structural details and phase, the diffraction patterns of as prepared sample x ray diffraction technique is used. Besides the single phase in nano CdS, lattice contraction is observed suggesting the X-ray peaks get shifted to higher diffraction angle with decreasing crystallite size. Due to higher surface to volume ratio the lattice contraction is expected to occur. By using Sherrer formula; the average grain sizes of the crystallite are calculated, β being FWHM and θ is the Braggs angle and K=

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7 0.9. With the increase in temperature the value of pH decreases and due to the common ion effect the rate of reaction becomes very slow and particles formation becomes large. The calculated size is found between 4.8-7.3 nm. The optimum range of temperature is found between (500C-700C).

From XRD, the crystallite size can be calculated by using the Scherer’s formula ,

P = 0. 9 λ/ β cos θ

Where P is the crystallite size, λ is the wavelength (1.54A°), β is the full width at half maximum, and θ is the diffraction angle.

X-ray diffraction curves of Ag:CdS/PVA nanocomposite thin films deposited 100 °C. From the micrographs it is observed that the particle size in CdS/PVA thin film decreases after doping with silver. Fig.3 (c) shows the X-ray diffraction curves of undoped and Ag- doped films. There is observed a large peak that demonstrates the major nanostructure of the films, while small

narrow peaks are a proof for minor microcrystalline phases formed in the film^14-17.

5.1 For pure PVA/ CdSe and doped PVA/ CdSe nanocomposite

X-ray diffraction pattern of CdSe/PVA nanocomposite thin film (sample S4) The XRD pattern shows several peaks at 2θ values of 22.1°, 31.1°, 35°, 40.5°, 45.36°, 55.1°, and 65.4° which is due to the diffraction lines formed by the (002), (101), (102), (110), (103), (202), and (210) planes of hexagonal structure of CdSe, respectively The appearance of the (102) and (103) reflection planes at diffraction angles 40° and 45.36° is an indication of the hexagonal (wurtzite) structure of CdSe thin film . The resultant peaks in the XRD curve is an indication of polycrystalline nature of CdSe thin film. From the intensity peaks of XRD the crystallite size in CdSe thin film is evaluated by a Gaussian fit, using Debye-Scherrer formula: where is the full width at half maximum, is the wavelength for X-ray used, and is Bragg’s angle.

Figure 3 XRD curve of CdS/PVA for three different concentrations

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Figure 4 XRD curve Pure CdS/PVA,Mn:CdS/PVA and Ag:CdS/PVA

Figure 5 XRD curve of CdSe/PVA for three different concentration CdSe thin film (S4) so formed by the

action of heat at 300°C, its XRD study reveals reflections from (111), (220) and (311) planes. The crystalline structure of CdSe thin film prepared by CBD technique on glass plate at 0, 43.250 and 51.400 can be ascribed respectively. It has been studied that the deposited CdSe thin films have cubic structure and are polycrystalline in nature. The extended

and low intensity peaks may be due to the small size of nanoparticles in CdSe thin film. From the image it is observed that the size of particle in CdSe/PVA thin film decreases after doping it with Mn and Ag.

The calculation gives the size Mn and Ag doped CdSe/PVA which is found between 2 nm – 4 nm. Ag doping induces change in crystal structure of CdS thin film.

Figure 6 XRD curve of pure CdSe/PVA, Mn: CdSe/PVA and Ag: CdSe/PVA The morphological study of surface of

CdS/PVA thin films casted on the glass substrates have been studied by scanning

electron microscopy (SEM) that operates with an voltage of about 20kV. It gives the following topography The SEM image

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9 of the deposited CdS/PVA, Ag doped CdS/PVA and Mn doped CdS/PVA nanocomposite thin films are shown in Figure. The morphological study of surface clearly indicates that the film is completely homogeneous, without any

non uniformity or cracks and covered the substrate uniformly. From the film it is observed that the particle size in CdS/PVA thin film decreases after doping with silver and manganese ¹³.

Figure 7 SEM micrograph of pure CdS/PVA nanocomposite

Figure 8 SEM micrograph of Ag:CdS/pva nanocomposite

Figure 9 SEM micrograph of pure Mn:CdS/PVA nanocomposite

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10 The SEM image of the deposited CdSe/PVA, Ag doped CdSe/PVA and Mn doped CdSe/PVA nanocomposite thin films are shown in Figure. It is observed that the surfaces are compacted and well covered with, irregular shaped grains that are random in sizes that are inter connected with each other to form large clusters. Consolidation of small

crystallites in the thin films are also clear from the photographs. Such collection of nano grains makes it difficult to evaluate the grain size from SEM images. The estimated grain sizes of the thin films are found to vary from 20 nm to 50 nm which is larger than the XRD results. This larger value of grain sizes may be due to the accumulation of smaller grains.

Figure 10 SEM micrograph of pure CdSe/PVA,Ag: CdSe/PVA and Mn:CdSe/PVA The optical property of semiconducting

material depends mainly upon band gap.

Bulk CdS with Eg =2.42 eV is direct band gap semiconductor. The absorption spectra of pure CdS/PVA for three different concentration marked as S1,S2

and S3(5Mm,7Mm and 9Mm). From the graph it is observed that with increase in the concentration of CdCl2 the absorption spectra is found to shift towards the blue end of the spectrum. When the pure CdS/PVA nanocomposite is doped with manganese and silver (Mn-doped CdS/PVA and Ag-doped CdS/PVA nanocomposite thin films) prepared at 100°C as shown , from the spectra it is evident that the blue shift in the absorbance edges are with respect to the bulk CdS and pure CdS/PVA, indicating quantum size effect in nanoparticles.

Maxima at 420 nm indicates the absorption spectra of Ag-doped CdS/PVA thin film maxima at around 410 nm

shows absorption spectra of Mn-doped CdS/PVA thin film as compared with undoped CdS/PVA thin film absorption spectra at 450 nm, 30 nm and 40nm blue shift was observed. This shift in spectra might be due to the formation of nanoparticles with smaller size when silver and manganese impurity was added into the pure CdS/PVA thin film. Fig.

shows undoped CdS/PVA for three different concentration of CdCl2 and fig shows undoped CdS/PVA Mn CdS/PVA and Ag-doped CdS/PVA film. The band gap was found to change from 2.562 eV for pure CdS/PVA film to 2.82 eV for Ag- doped and 2.96eV for Mn doped CdS/PVA film^18-20.

The position, shape, and sharpness of the absorption edge are not the same, they change due to the increased CdCl2 molar concentration, which shows that the peak acuteness is directly proportional to CdCl2

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11 concentration that may be credit due to the quantum internment effect in CdSe nanoparticles. From the spectra, it is noticed that there is sharp edged rise in optical density near the elemental band.

The UV-Vis absorption spectra of the CdSe/PVA nanocomposite thin films for three different concentration marked as Se1, Se2, Se3 are shown in Figure. The optical density rises gradually as the concentration of CdCl2 is increased from 0.5 M to 1.2 M. It is evident that at a lower concentration of CdCl2 the CdSe/PVA thin films indicates low absorbance, while those set down at higher concentration of

CdCl2 indicates high optical density and sharp peaks were noticed in the scale of 4000 nm–500 nm The absorption edges in the spectra of CdSe thin films are found to be shifted towards the shorter wavelength of the spectrum as compared to the bulk CdSe band edge of 713 nm, The band gaps of the of the silver and manganese doped CdSe nanocomposite thin films were found to be between 2 eV and 2.5 eV, respectively, which is larger than that of the bulk CdSe band gap of 1.74 eV. The formation of small size CdSe NPs is due to increase in the band gap

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Figure 11 UV Vis spectra of CdS/PVA for three different concentration marked as S1,S2 and S3

Figure 12 UV VIS spectra for CdSe/PVA for three different concentration marked as Se1,Se2 and Se3

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Figure 13 UV Vis spectra of pure CdS/PVA, Ag:CdS/PVA and Mn:CdS/PVA

Figure 14 UV VIS spectra for pure CdSe/PVA,Ag: CdSe/PVA and Mn: CdSe/PVA 5. CONCLUSION

The development of “green” approaches which is a new low-cost method for a synthesis of colloidal semiconductor quantum dots (QDs) in eco-friendly polymers is highly encouraged by their potential application in opto-electronics and photonics, biology and in the field of medicine etc. The embedded QDs in polymers matrix show unique and distinguishible properties owing to combination of unique optical and chemical features of the QDs with the inherent characteristics of polymer

matrices. The latter includes size-tuned spectral properties, high photoluminescence (PL) and electroluminescence quantum yield, broad band absorption spectrum and narrow exciton band emission , high chemical stability and resistance to photochemical- and metabolic abasement, etc. The transport properties of QDs embedded in a conductive polymer can improve its efficiency polymer-based solar cells and light-emittingdiodes (LEDs) Colloidal QD-based LEDs have emerged as a highly efficient choice in thin film

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13 displays with improved color contrast and saturation^21-22 .

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