On characteristics of PVA/CdS and PVA/CdS:Cu nanocomposites for applications as LED
Highlights
► We synthesized CdS and CdS:Cu nanocrystals by chemical route method. ► PL and EL are found in the same position. ► Emission is only for CdS and CdS:Cu nanocrystals. ► Quantum efficiency is more in doped nanocrystals. ► We achieve low operating and low saturation voltages in case of doped nanocrystals without losing efficiency.
Introduction
Recently significant attention has been paid to the investigation of the hybrid nanostructures combining different types of nanoparticles (NCs) such as nanowires, semiconductor quantum dots (QDs), metal nanoparticles (MP) and so on [1], [2], [3]. Superstructures of semiconductor quantum dots and metal nanoparticles (NCs) have remarkable capability, holding promising potential for application in optoelectronic devices, quantum computation, nanosensors, electrical DNA switches, nanodevices and so on [1], [2], [3]. Singh et al. demonstrated that in a hybrid semiconductor quantum dot—metallic NC system doped in a photonic crystal, the photonic crystal allows to switch the absorption of the metal particle at a given frequency range when the excitonic frequencies lie near the band edge [2].
Electroluminescence, meaning the generation of light by electrical excitation, is a phenomenon which has been observed in a wide range of semiconductors and in organic semiconductors. In 1960, for the first time electroluminescence in organic semiconductors was reported [4]. In 1987 Tang et al. demonstrated efficient electroluminescence in two-layer sublimed molecular film devices [5]. Various II–IV semiconductor NCs can be used as light emitting material in electroluminescent layered structures [4], [5], [6]. The use of different “capping materials” is highly important to the synthesis of monodisperse and smaller nanocrystallites. Organic and biological materials have been found to exert control in the particle size. Thus, molecules such as cysteine, glutathione, oligonucleotides (DNA) and a wide range of polymers have been used to synthesize metal-sulfides and oxides (Mex-S, Mex-Ox) [7]. Polyvinyl alcohol (PVA) is a water-soluble polymer frequently used as a colloid stabilizer. Properties of PVA like the transparency over the whole visible spectrum, good adhesion to hydrophilic surfaces and formation of good oxygen resistant films make PVA a good choice for the fabrication of optical devices [8]. PVA and its related products appear as one very interesting choice for preparing colloidal suspensions due to their biocompatibility and biodegradability aiming at medicine, biology and pharmaceutics applications. PVA is a hydrophilic semi-crystalline polymer produced by polymerization of vinyl acetate to poly (vinyl acetate) (PVAc), and successive hydrolysis to PVA. This reaction is incomplete resulting in polymer with different degrees of hydrolysis (DH). So, it is a copolymer of poly(vinyl alcohol) and poly(vinyl acetate) referred as poly(vinyl alcohol-co-vinyl acetate). PVA is commercially available in highly hydrolyzed grades (DH>98.5%) and partially hydrolyzed ones (DH from 80.0 to 98.5%). The degree of hydrolysis or the molar content of acetate groups in PVA affects its physical and chemical properties, such as solubility, hydrophilic/hydrophobic interactions, pH-sensitivity and viscosity [9]. Many reports on the synthesis and characterization of cadmium sulfide NCs and other II–VI materials, embedded into these polymeric matrices are found in the literature [7], [8], [9], [10], [11], [12]. The EL properties of these structures can be controlled by the methods of preparation and materials used and the tuning of the emission spectra by varying the size of the NCs [5]. Materials such as CdS, CdTe, CdSe, ZnS, ZnO, GaN, InSnO, Ag and Au nanocomposites have been used to fabricate many novel materials and devices that exploit various physical effects, such as quantum confinement occurring at the nanoscale [13], [14]. In nanostructures, narrow PL provides sharp electroluminescence [15]. NCs along with organic semiconductors are used as charge transport material among which N,N′-bis(3 methylphenyl)–N,N′-bis(phenyl)benzidine (TPD) and tris(8-hydroxyquinolinato)aluminum (AlQ3) are mostly used as hole and electron transporting layers, respectively [9]. In the quantum dot LED device, NC layer is sandwiched in between the electron and hole transport layer [4]. In such devices excitons get formed either in the NCl layer or in the organic layer [4], [15].
Syntheses of CdS and ZnS quantum dots by using molecular beam epitaxy as well as by chemical route and their applications have been found in many literatures [16], [17], [18], [19], [20], [21], [22], [23], [24], [25].
In this study, we would like to demonstrate the synthesis of CdS and CdS:Cu (copper doped CdS) quantum dots in PVA matrix by using chemical method at room temperature. The synthesized samples have been characterized by different techniques to reveal their nanostructures. For preliminary study, voltage was applied in the liquid sample of CdS and CdS:Cu and the light emitted from the samples were measured with photodetector. A nano device ITO/TPD/CdS (PVA/CdS) or CdS:Cu (PVA/CdS:Cu) nanocomposites/Al is fabricated to study EL for possible applications in electronics as nano-light-emitting device (LED).
Section snippets
Experimental
To prepare CdS/PVA nanocomposites, PVA solutions are prepared taking 4 g of PVA in 100 ml of distilled water and stirred in a magnetic stirrer [using 2MLH Magnetic stirrer REMI] for certain time [14]. CdCl2, Na2S solutions are prepared taking 2 g of each in 100 ml of distilled water and the solutions are stirred at 60 °C for half an hour. The as-prepared PVA and CdCl2 solutions are taken in the 2:1 ratio and as-prepared Na2S solution is added dropwise till the mixture turns orange, and stirred till
Results and discussion
The prepared undoped and doped samples have been characterized for optical absorption in the range of 300 nm to 600 nm. Results are given in Fig. 1. Band gap has been estimated using Tauc's relation [27]where, α is the absorption coefficient in cm−1, h is the Planck's constant, ν is the frequency of incident light, K is a constant, Eg is the band gap of the material. Band gap is found to be 2.62 eV for undoped and 2.6 eV for the doped one which reveals the blue shift in the band
Acknowledgement
The authors would like to acknowledge the department of Chemistry, G.U, for providing PL and UV–vis facilities, IIT Guwahati for providing XRD, TEM, SEM, HRTEM, SAED and EDS facilities.
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