Tunable Visible Emission of Ag-Doped CdZnS Alloy Quantum Dots
© to the authors 2009
Received: 15 July 2009
Accepted: 24 September 2009
Published: 13 October 2009
Highly luminescent Ag-ion-doped Cd1−xZnxS (0 ≤ x ≤ 1) alloy nanocrystals were successfully synthesized by a novel wet chemical precipitation method. Influence of dopant concentration and the Zn/Cd stoichiometric variations in doped alloy nanocrystals have been investigated. The samples were characterized by X-ray diffraction (XRD) and high resolution transmission electron microscope (HRTEM) to investigate the size and structure of the as prepared nanocrystals. A shift in LO phonon modes from micro-Raman investigations and the elemental analysis from the energy dispersive X-ray analysis (EDAX) confirms the stoichiometry of the final product. The average crystallite size was found increasing from 1.0 to 1.4 nm with gradual increase in Ag doping. It was observed that photoluminescence (PL) intensity corresponding to Ag impurity (570 nm), relative to the other two bands 480 and 520 nm that originates due to native defects, enhanced and showed slight red shift with increasing silver doping. In addition, decrease in the band gap energy of the doped nanocrystals indicates that the introduction of dopant ion in the host material influence the particle size of the nanocrystals. The composition dependent bandgap engineering in CdZnS:Ag was achieved to attain the deliberate color tunability and demonstrated successfully, which are potentially important for white light generation.
KeywordsAlloy Nanocrystals Photoluminescence Raman spectroscopy
II–VI compound semiconductor nanocrystals have been paid great attention owing to their unique optical properties that can be tuned not only by changing the particle size but also by changing the composition of the alloy [1–3]. Among them, the wide band gap nanocrystalline materials have opened avenue in fundamental studies and tremendous potential applications in diverse areas such as solar cell, photo-catalysis, sensors, photonic and other optoelectronic devices [4–7]. Toward this end, considerable efforts have been devoted on the high temperature synthesis of Cd1−xZnxS (0 ≤ x ≤ 1) nanocrystalline thin films by chemical vapor deposition (CVD), molecular beam epitaxy (MBE), metal organic vapor phase epitaxy (MOVPE) and rf-sputtering method [8–10], but very less attention is paid toward the low temperature chemical synthesis of high quality water soluble alloy nanocrystals. Nanocrystals synthesized by chemical route gives the chance to control their size, distribution and the most important is to improve the crystallinity by altering the concentration of the reagents and their mixing rate at different temperature . These small-size nanostructures have a large surface to volume ratio, which plays a dominant role in optical properties of the nanostructures. For the good quality nanomaterials, various kinds of capping agents and inorganic shell are used to passivate the undesired sites, which results in the enhancement of luminescence intensity. Doping different activator ions (acts as a recombination center) in these stabilized nanostructures not only gives the chance to obtain required emission color, but it also reduces the self quenching by intrinsic defects in the nanocrystals . There are numerous reports on the optical properties of various metal and rare earth ions doped ZnS, CdS, ZnSe, and CdSe nanostructures, but studies on the their doped alloy nanostructures are still very limited [13–16]. ZnS nanocrystals doped with Ag ion has been paid a great devotion by several researches because of its commercial application as a blue emitting phosphor . In best of our knowledge, there is no report except Karar et al. on the optical properties of the Ag ion doped Cd1−xZnxS alloy nanocrystals .
In our previous report, the structural and photoluminescence properties of chemically synthesized undoped Cd1−xZnxS (0, 0.2, 0.4, 0.6, 0.8 and 1.0) alloy nanocrystals were discussed in detail . It was observed from the obtained results that the XRD peaks are shifted toward higher 2θ values, and the excitation and emission spectra are blue shifted with increasing molar concentration of Zn in alloy. In the present study, for the first time, we have successfully synthesized the Ag-ion-doped Cd1−xZnxS (x = 0, 0.2, 0.4, 0.6, 0.8 and 1.0) alloy nanocrystals by wet chemical precipitation method and systematically studied the PL properties of doped alloy nanocrystals over all the compositions of Zn and Cd constituents. In addition, effect of dopant concentration on the PL intensity of alloy nanocrystals has been also studied and explained. Raman investigation and absorption measurements were also performed to confirm the formation of doped alloy nanocrystals.
Zinc acetate (Zn(CH3COO)2·2H2O), cadmium acetate (Cd(CH3COO)2·2H2O), urea (NH2CONH2), silver nitrate (AgNO3), triethylamine (N(CH2CH3)3) were purchased from Merck India Limited and thiourea (NH2CSNH2) was purchased from RENKEM. All chemicals were of analytical grade and used as received without any further purification. Milli-Q water was used as a solvent for all chemical reactions. Nanocrystals were prepared using a simple chemical co-precipitation method as reported by Chawla et al. with minor modifications in the reaction conditions . The detailed description of the experimental procedure is described as follows: firstly appropriate amount of zinc acetate and cadmium acetate (according to their molar ratio as required for the particular composition) was dissolved in 25 ml of water to make 0.5 M solution named solution A. To make the sulfur solution, 1 M thiourea, 1 M urea and 2 ml triethylamine (TEA) were dissolved in 25 ml water named solution B. Both the solutions were stirred until a colorless transparent solution obtained. TEA was used in the chemical reaction because it makes a complex with Zn and Cd ion and reduces their solubility product difference and urea to control the pH of solution. Doping of Ag ion was achieved by adding the calculated amount of Ag(NO3)2 (0, 2, 4, 8, 12 and 15 M%) in the initial solution A. Solution B was kept at hot plate magnetic stirrer at 70°C temperature and then solution A was added drop wise in solution B @ 1 ml/min with keeping the stirring on. The color of obtained precipitate changes from white to deep yellow by varying the ratio of Cd and Zn in the starting solution. Finally the precipitate was centrifuged, washed with water and ethanol several times and then dried in vacuum oven at 100°C to obtain the powder nanocrystals for the characterization purpose.
To calculate the particle size and structure of the nanocrystals, X-Ray diffraction was carried out on Rigaku D/max-2200 PC diffractometer operated at 40 kV/20 mA using CuKα radiation with wavelength of 1.54 Å in the wide angle 2θ range from 10 to 60°. Technai 30 G2S-Twin high resolution transmission electron microscope (HRTEM) operated at 300 KV was used to obtain the particle size and electron diffraction image. For TEM, sample was prepared by suspending the Cd0.4Zn0.6S:Ag (8 M % of Ag ion) powder in ethanol and depositing a drop of this ethanolic solution onto the carbon coated copper grid and then dried it at room temperature. Photoluminescence (PL) measurement was carried out using Perkin Elmer LS-55 Luminescence spectrophotometer, while Perkin Elmer Lambda-35 was used to measure UV–vis absorption spectra. A Renishaw micro-Raman spectrometer (Model-2000, λ = 514 nm) integrated with nanonics atomic force microscopy (AFM) is used to investigate the optical phonon modes of the synthesized nanopowder. Raman spectrum is recorded by selecting 20X objective of integrated Lica microscope with the spot size 20 μm.
Results and Discussion
Synthesis of high quality luminescent and free standing Cd1-xZnxS (0 ≤ x ≤ 1) nanocrystals doped with Ag ion is first time reported by chemical precipitation method. We have systematically examined the photoluminescence properties for a fixed composition of alloy with varying amount of Ag doping concentration. Doping concentration significantly improved the emission intensity corresponding to the dopant ion up to an optimum concentration. X-ray diffraction and UV–Vis absorption spectra show the increase in particle size with increasing doping concentration. HRTEM image reveals the crystalline nature of the particle having cubic structure with average grain size less than 5 nm. Absorption and Raman investigation confirms the formation of alloy nanocrystals. For the fixed Ag ion doping concentration, the PL spectra of the samples show the emission tunability in full visible range with the change in composition of the alloys and can be used for the white light generation.
We are grateful to Professor R. N. Bhargava, Nano Crystal Technology, New York (USA) for continuous encouragements and scientific discussions. One of the authors Ruchi Sethi would like to thank, Mr. Ashish K. Keshari Nanophosphor Application Centre, Physics Department, University of Allahabad, India for XRD measurements. Department of Science and Technology, New Delhi, India is thankfully acknowledged for financial support to “Nanophosphor Application Centre” project under ‘IRHPA’scheme.
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