Microwave-assisted synthesis, characterization and photoluminescence interaction studies of undoped, Zr2+, Rh3+ and Pd2+ doped ZnS quantum dots

Accepted Manuscript Title: Microwave-assisted synthesis, characterization and photoluminescence interaction studies of undoped, Zr2+ , Rh3+ and Pd2+ doped ZnS quantum dots Authors: Dasari Ayodhya, Guttena Veerabhadram PII: DOI: Reference:

S2352-9245(17)30044-3 https://doi.org/10.1016/j.md.2018.08.001 MD 51

To appear in: Received date: Revised date: Accepted date:

14-11-2017 1-8-2018 16-8-2018

Please cite this article as: Ayodhya D, Veerabhadram G, Microwave-assisted synthesis, characterization and photoluminescence interaction studies of undoped, Zr2+ , Rh3+ and Pd2+ doped ZnS quantum dots, Materials Discovery (2018), https://doi.org/10.1016/j.md.2018.08.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Microwave-assisted synthesis, characterization and photoluminescence interaction studies of undoped, Zr2+, Rh3+ and Pd2+ doped ZnS quantum dots Dasari Ayodhya*, Guttena Veerabhadram Department of Chemistry, Osmania University, Hyderabad, Telangana State–500007, India Corresponding author: [email protected]

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Mobile: 91-9010877323

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Microwave-assisted synthesis, characterization and photoluminescence interaction studies of undoped, Zr2+, Rh3+ and Pd2+ doped ZnS quantum dots Graphical abstract

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Abstract

Here, we report the simple and low-cost synthesis of undoped, Zr2+, Rh3+ and Pd2+ doped ZnS quantum dots (QDs) by a microwave-assisted method. We study the compositional,

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structural, and optical properties by XRD, SEM-EDX, TEM, FTIR, UV-vis and PL

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spectroscopy. The quantum confinement effect of the products was confirmed by means of

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spectroscopic measurements. XRD and TEM show that the synthesized ZnS quantum dots have cubic structures with a diameter of about less than 10 nm. The fluorescence interaction

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studies suggest that L-Cysteine was effectively quenched the fluorescence intensity of ZnS

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QDs and form stable Cys-ZnS complex using fluorescence spectroscopy. Keywords

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Zinc sulfide, Quantum dots, Microwave-assisted method, Spectral characterization, LCysteine, Photoluminescence interaction

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1. Introduction

Semiconductor nanoparticles have attracted widespread attention because of their

unique optical and electrical properties suitable for applications in gas sensors, ultraviolet

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detectors, photoelectric devices, photocatalysis and luminescence properties [1-6]. Recently extensive efforts have been devoted towards the synthesis and characterization of metal doped chalcogenide nanoparticles. The nano-sized chalcogenides exhibit excellent electrical, optical and magnetic properties compared to that of bulk material due to quantum confinement and surface effects. In order to get enhanced emissions of luminescent materials with various wavelengths, it is important to synthesize ions doped semiconductor 2

nanoparticles. Recently, ions doped semiconductor nanoparticles with applications in sensors, solar cells, photocatalysts and light emitting diodes, have been attracting special attention [27]. The interesting, unique physical properties and strong application potential of some wide band gap II-VI compound semiconductors have received considerable attention among researchers [8-12]. Among the family of II-VI semiconductor compounds, ZnS has been realized to be

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the potential material for extensive research, possible applications in magneto-optical devices and the potential generation of results to other candidates because of its wide band gap (3.54eV), large exciton binding energy (40 meV), and high index of refraction (2.27 at 1 μm).

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ZnS with cubic zinc blende and hexagonal wurtzite crystal structure has band gap energies of

~3.72 and ~3.77 eV at room temperature, respectively [13-14]. Particularly, this makes it convenient for use as a host lattice for a large variety of dopants. Therefore, during the last

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several decades, there are many studies on II-VI semiconductor nanoparticles doped with different metal ions. Up to date, ZnS has been investigated as a matrix to synthesize doping

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nanostructures for enhanced optical properties. Among them, Cu2+ [15], Ag+ [16], Au3+ [16],

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Mn2+ [15, 17], Fe3+ [18], Ni2+ [18], Mg2+ [19], Pb2+ [20], Gd3+ [21], Cd2+ [22], La3+ [23],

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Eu3+/2+ [24-25], Dy3+ [26], Sn2+ [27], Nd2+ [28], Co2+ [18, 29], Cr2+ [30] and Ru3+ [31] ionsdoped ZnS nanostructures have been obtained. From this point of view, we emphasize a

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microwave-assisted method to prepare Zr2+, Rh3+ and Pd2+ doped ZnS QDs and to investigate the optimized conditions for their photoluminescence emission and quenching properties.

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ZnS nanoparticles with different structures and morphologies including nanotubes, nanosheets and nanowires have been successfully synthesized and studied using a variety of methods. A number of synthesis methods are available for the preparation of ultrafine metal

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doped ZnS NPs such as sol-gel, hydrothermal, microwave, solid state techniques and spray pyrolysis methods etc. [6, 14, 32-34]. Among all these methods the preparation of ZnS NPs via a microwave-assisted chemical route has received great attention because it provides

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favorable factors such as large-scale production, high yield with low cost, high reaction rate, and rapid and homogeneous heating compared to a conventional chemical route which are key parameters to design required tunable properties. The conventional heating sources are very slow to heat the reaction mixture, and also there is a possibility of decomposition of the material at the hot surface of the reaction vessel. The approach utilized in the present work is based on microwave synthesis of nanoparticles from metal salts in solutions. Microwave

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irradiation (MWI) has several advantages over conventional methods, which include short reaction times, small particle sizes, narrow size distribution and high purity [35]. Semiconductor materials show unusual luminescence properties induced by the quantum size effect. Efforts have been made in realizing luminescence tuneable materials simply by changing the particle size and size distribution and great progress have been achieved [36]. These materials not only give luminescence in various regions but also can

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add to the excellent properties of ZnS QDs. Optical properties can be altered through doping [37] and ZnS doped with transition metal ion alters the optical and luminescence properties

which lead to emission in visible region [38]. In doped ZnS QDs, impurity ions occupy the

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ZnS lattice site and behave as a trap site for electrons and holes. The electrons are excited from the ZnS valence band to conduction band by absorbing energy equal to or greater than the band gap energy. Subsequent relaxation of these photoexcited electrons to some surface

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states or levels is followed by radiative decay, enabling luminescence in the visible region. The combination of many desired properties in one material drives great interest to carry out

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research on doped ZnS QDs.

In this paper, we report the successful synthesis of cubic ZnS QDs and ZnS QDs

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doped with Zr2+, Rh3+ and Pd2+, having average grain size of less than 10 nm, by the microwave-assisted method to study the effect of dopant on structural, surface and optical

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properties of ZnS QDs without capping agent. The absorption edges of doped ZnS QDs were blue shifted with respect to the undoped ZnS QDs. The band gap values of doped and

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undoped ZnS QDs were determined from UV-vis spectroscopy and PL emission spectroscopy. A rapid luminescence quenching with increasing Cys concentration was

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observed. The binding parameters were determined by Stern-Volmer relation. 2. Experimental

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2.1. Materials and Methods All the chemicals used for synthesis were of AR grade, purchased from Sigma

Aldrich chemicals, and for spectral analysis, spectral grade solvents were used. Double distilled water was used for preparing the solutions. 2.2. Synthesis of undoped, Zr2+, Rh3+ and Pd2+ doped ZnS QDs

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In a typical experiment, for preparing Zr2+, Rh3+ and Pd2+ doped ZnS QDs, 25 ml of 0.1 M zinc acetate and an equal quantity of 0.1 M sodium sulfide were dissolved separately in double distilled water. The solutions were stirred for 30 min using a magnetic stirrer. In a separate beaker, 10 ml of 0.1 mM of dopant solutions (zirconium chloride, rhodium chloride and palladium chloride solutions for Zr2+, Rh3+ and Pd2+) were separately dissolved in double distilled water and were stirred. To a stirred solution of zinc acetate, a solution of dopants was poured drop by drop. After 30 min, the solution of sodium sulfide was poured drop by

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drop similarly into the above mixture of dopant and zinc acetate solution. Immediately, the precipitated material was obtained and it was stirred to form a highly dispersed sol, which

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was then aged for 40 h and dried in a microwave oven operated at 1000 W and frequency 2450 MHz for 10 min. After formation of a white colored precipitate, the resulting ZnS powder was collected, filtered, washed with double distilled water and absolute ethanol several times to remove the impurities. The product was then dried in vacuum. Crystallization

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of the as-prepared product was carried out by heating at 450 oC for 3 h. A similar procedure

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was adopted to synthesize undoped ZnS QDs. The advantage of microwave-assisted synthesis

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over the conventional heating is the improved kinetics of the reaction generally by one or two orders of magnitude, due to rapid initial heating and the generation of localized high-

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2.3. Characterization techniques

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temperature zones at reaction sites, which produces high purity with high yield.

The room temperature X-ray diffractograms of the synthesized samples were recorded using a Philips X-ray diffractometer (Cu-Kα, λ = 1.5406 Å) in the 2θ range 10–80°, step size

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(2θ) is 0.02° and scan step time 0.15 s for phase confirmation. The UV-vis diffuse reflectance spectra (UV-vis DRS) were recorded using a Shimadzu 3600 double beam UV-vis

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spectrophotometer in the spectral range of 200-800 nm. Transmission electron microscopy (TEM) was performed on Tecnai G2 microscope operated at 200 kV. Samples were prepared by dispersing the powder in water. Imaging was carried out by depositing a few drops of

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suspension on a carbon coated 400 mesh Cu grid. The solvent was left to evaporate before imaging. Scanning electron microscopy (SEM) images of fabricated ZnS QDs were obtained using ZEISS EVO18 electron microscope with Energy-dispersive X-ray spectroscopy (EDX). Fourier transform infrared (FTIR) spectra on KBr pellet were measured on a Shimadzu spectrophotometer in the range of 4000-400 cm−1. The photoluminescence (PL) spectrum was measured with an RF-5301PC spectrofluorometer (Shimadzu, Japan).

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2.4. Photoluminescence interactions study Photoluminescence spectroscopy gives further information about the interactions between undoped, Zr2+, Rh3+, and Pd2+ doped ZnS QDs and Cys as a quencher. In a typical procedure, the measurement of emission wavelength and quenching study of undoped, Zr2+, Rh3+, and Pd2+ doped ZnS QDs were conducted at room temperature. The emission spectra of doped and undoped ZnS QDs have been measured in the absence and presence of Cys. The

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fluorescence spectra of the samples were obtained at wavelengths between 350 and 650 nm with excitation at 340 nm. All measurements were carried out at room temperature using a

quartz cuvette with four polished faces and 10 mm optical path length. The quenching

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experiments may give further information about the binding ability of nanoparticles with Cys.

Subsequently, for quenching measurements, the appropriate volume of undoped, Zr2+, Rh3+, and Pd2+ doped ZnS QDs and a series of Cys standard solutions were taken in a 10 ml test

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tube separately, diluted with water and homogenized for determination. Here, we report that the fluorescence intensity of ZnS QDs was quenched by the addition of Cys solution to

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spectofluorometrically study the interaction between the ZnS QDs and Cys.

3.1. UV-vis absorption analysis

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3. Results and discussion

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The UV-vis absorption spectra of undoped, Zr2+, Rh3+ and Pd2+ doped ZnS QDs by the microwave-assisted method are shown in Fig. 1. The effects of Zr2+, Rh3+ and Pd2+ dopants on the UV-vis absorption spectrum of ZnS QDs were studied. An absorption edge

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around 320-340 nm (Zr2+, Rh3+ and Pd2+ doped ZnS QDs) and 370 nm (undoped ZnS QDs) was observed, which does not appreciably change with the variation of the dopants. The

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calculated band gap values of Zr2+, Rh3+ and Pd2+ doped and undoped ZnS QDs are in the range of 3.88-4.2 eV. The band gap values of doped and undoped ZnS QDs were higher than that of the bulk ZnS (3.54 eV). A blue-shift in the absorption of bulk ZnS may be accounted

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for by the quantum confinement effect in nanoparticles [39], and it gives the relationship between particles size and its band gap energy. According to this relationship, band gap increases as the size of the particles becomes smaller. From the comparison of UV-vis absorption spectra of microwave-assisted synthesized ZnS QDs with the other conventional methods it is seen that there is a blue-shift with decreasing size of the QDs. This may be due to the fact that effect of Zr2+, Rh3+ and Pd2+ ions incorporated into the lattice of ZnS QDs are small and do not change the particle size under similar synthesis conditions. 6

3.2. PL analysis The photoluminescence (PL) spectra of the microwave-assisted synthesized undoped, Zr2+, Rh3+ and Pd2+ doped ZnS QDs were recorded using a spectrophotometer at an excitation wavelength of 340 nm at room temperature. From these spectra, it is seen that all the samples exhibited broad and asymmetric emission peaks. The PL spectra of undoped, Zr2+, Rh3+ and Pd2+ doped ZnS QDs show an emission at 420-450 nm, which is shown in Fig. 2. Pure ZnS

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QDs show emissions at 421 and 483 nm. However, the PL spectra of the Zr2+, Rh3+ and Pd2+ doped ZnS QDs contain different peaks at around 431, 438 and 435 nm, while undoped ZnS

QDs also exhibited a sharp peak at 440 nm. As can be seen, the excitonic emission of the

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undoped ZnS QDs experiences a complete disappearance while the dopant emission appears. As a transition metal ion, these ions can make their individual states within the forbidden band gap of ZnS and the carriers can recombine through these levels. The blue emission at

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421 nm corresponds to a sulfur vacancy in the lattice sites [40]. From the other conventional methods, the synthesis of ZnS QDs formed due to the fact that the particles grew with time

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and that longer reaction time can also promote the particle aggregation to form larger

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particles, leading to a red shift of the absorption peak. Therefore, the observed

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photoluminescence of the aqueous-based microwave-assisted ZnS QDs can be attributed to a recombination of electrons at the sulfur vacancy donor level or in the conduction band with

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holes trapped at the zinc vacancy acceptor level or in the valence band. There are many possible recombination paths and thereby many trap-state emissions with different energies,

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causing the relatively wide emission peak. 3.3. FTIR analysis

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To study the functional groups present in a compound, molecular geometry, and interor intra-molecular interactions of the microwave-assisted synthesized undoped and Zr2+, Rh3+ and Pd2+ doped ZnS QDs, FTIR spectra of the samples were recorded at room temperature. Fig. 3 shows the FTIR spectra in the 500 to 4000 cm-1 wavenumber range of undoped and

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Zr2+, Rh3+ and Pd2+ doped ZnS QDs. Since the FTIR spectra of doped samples are nearly similar to that of undoped ZnS samples, it indicates that the dopant metal ions are substituted within the lattice of ZnS. The strong and broadened absorption band near 656 and 1006 cm−1 are assigned to Zn-S vibrations [41]. The strong band around 1130 cm-1 corresponds to the characteristic frequency of inorganic ions, corresponding to ZnS QDs. The broad band at 3000-3600 cm−1 is attributed to -OH stretching vibrations and the bands at 2961 cm−1 are 7

assigned to stretching vibrations of C-H. The band at 2354 cm−1 is assigned to CO2 peak in moisture. The peaks observed at 1550 cm−1 and 1428 cm−1 are attributed to stretching vibration of C=O. With the presence of Zr2+, Rh3+ and Pd2+ doped ZnS QDs samples, the bands at 1428 cm−1 were shifted to the bands at 1397, 1412 and 1418 cm−1, respectively, due to the presence of Zr2+, Rh3+ and Pd2+ ions in ZnS lattice site. This difference in peak position occurs also due to the size distribution of the nanoparticles present in the microwave-assisted

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synthesized undoped and doped ZnS QDs.

3.4. Powder XRD analysis

The powder XRD patterns of the undoped, Zr2+, Rh3+ and Pd2+ doped ZnS QDs in the

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range of 2θ is 10-80o are shown in Fig. 4. Three strong diffraction peaks at 2θ values of

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28.4°, 47.2° and 56.5° corresponding to reflections from (111), (220) and (311) planes of the ZnS cubic phase (JCPDS No. 05-0566) can be identified for all samples. Broadening of the

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ZnS XRD characteristic peaks can be observed due to the polycrystalline nature led by the

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self-assembly of small granules [42]. The average crystallite size of the present samples was estimated from Debye-Scherrer’s formula. The calculated crystallite size of the synthesized

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undoped, Zr2+, Rh3+ and Pd2+ doped ZnS QDs are in the range of 2-10 nm but the exact size of the grains is not calculated due to the grains having the very small size in nature. In

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addition, the rapid microwave heating provides uniform temperature and concentration conditions for the nucleation and growth. It is well-known that, when nucleation and growth can be separated, particles with a narrow size distribution are obtained. From the uniform

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heating results in a narrow particle size distribution, this increases the activity of the ZnS

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QDs. The lattice parameters of cubic ZnS are calculated using the expression: 1/d2 = 1/a2 (h2 + k2 + l2)

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where ‘a’ is the lattice constant and ‘d’ is the interplanar separation. The estimated

lattice parameter for the undoped ZnS is 5.418 Å and those for the samples containing Zr2+, Rh3+ and Pd2+ doped ZnS QDs are 5.401, 5.394 and 5.382 Å, respectively. These results suggested that the substitutional doping does not affect the crystal structure significantly. 3.5. SEM analysis 8

Scanning electron microscopy is a suitable technique to study the surface morphology of the sample. The morphologies of the undoped, Zr2+, Rh3+ and Pd2+ doped ZnS QDs were also investigated by high-resolution SEM images, as shown in Figs. 5(a-d). From SEM analysis, it is observed that all the samples are agglomerated and no particular dopant dependent morphological features are noticed, which is the most common problem in nanoparticles [26]. Nanoparticles have a higher relative surface area and higher relative number of surface atoms, these atoms have unsaturated coordination and each atom has

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vacant coordinate sites. They endeavor to make bonds and such bonds tend to form between

adjacent particles, which cause the agglomeration of nanoparticles. The different shapes with

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sizes like microstructure can be observed in SEM images (Figs. 5(a-d)). The diameters of the

undoped and doped ZnS QDs exhibited a range of sizes with an average of 200 nm that can be found on the surface of the sample. This indicates that the advantages of the microwave treatment resulting from removing moisture and generating clean surfaces outweigh the

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disadvantage of increasing particle size and subsequently decreasing surface area. The EDX

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spectra of undoped and Zr2+, Rh3+ and Pd2+ doped ZnS QDs show the presence of Zn, Zr, Rh,

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Pd and S signals and no other impurities were noticed within the detection limit of the technique. The estimated atomic percentages of Zn, Zr, Rh, Pd and S were very close to the

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nominal values.

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3.6. TEM analysis

TEM is a versatile technique for visualizing the grain size and gives authentic

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information about the particle size distribution and structural information. The method allows the study of incorporation of one or more types of active metals such as Zr, Rh, and Pd, we are currently exploring the effects of microwave radiation on the morphology, particle size

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and size distribution of the QDs. Fig. 6(a) shows the high-resolution TEM image of ZnS QDs, which reveals that the prepared nanoparticles possess nearly spherical morphology with narrow size distribution. We can see the size of synthesized ZnS QDs from the TEM images

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which approximates between 2 and 10 nm with large aggregation effect observed in it. Fig. 6(b) shows the corresponding selected area electron diffraction (SAED) pattern for the synthesized ZnS QDs. The SAED ring pattern confirms the polycrystalline nature of the nanoparticles. The corresponding SAED pattern of the ZnS QDs, which the diffraction rings are associated with, shows polycrystalline ZnS with a cubic structure. No impurity phase related to dopants could be identified. In addition, all of these are properly matched with the XRD results; it is the incorporation of dopant ions into the ZnS host lattice as substitutes. 9

3.7. Photoluminescence study 3.7.1. Photoluminescence quenching study of undoped, Zr2+, Rh3+, and Pd2+ doped ZnS QDs by Cysteine Photoluminescence properties of doped ZnS QDs are very sensitive to the surface defects and size. Thus, the technique can be used to study the presence of various defects and energy states in the samples. In case of wet chemical synthesis, many of the defect centers are

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the result of some un-reacted precursors or intermediate complexes that remain attached to the surfaces of the nanoparticles by means of dangling bonds [43]. It is expected that dopant

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materials play a significant role for the host in specific applications, which can effectively

impose a material's microstructure, properties and also function. Room temperature PL emission spectra of undoped and Zr2+, Rh3+ and Pd2+ doped ZnS QDs prepared by the microwave-assisted method are shown in Fig. 2. The transition metallic ions create excellent

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luminescence centers in the phosphor lattice which give rise to characteristic emissions due to

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different d-d transitions. The narrow PL emission bands (which are slightly blue-shifted) are

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assigned as the near-band-edge excitonic emission as the energy corresponding to these peaks is almost equal to the energy band gap of cubic ZnS (estimated from the UV-vis analysis).

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This near band-edge excitonic emission originates from the radiative extinction of free excitons and suggests that the synthesized nanoparticles are well crystallized. The origin of

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broad emission band in the visible region is due to the superposition of several peaks. The photoluminescent property of the semiconductor nanoparticles confirms the

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formation of nanoparticles and occurrence of quantum confinement effect at the nanoscale. PL quenching of the undoped and Zr2+, Rh3+ and Pd2+ doped ZnS QDs is also strongly

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affected by the concentration of Cys molecule. This phenomenon arises from the binding of Cys of interest to the surface of NPs as an acceptor and changing the surface state of QDs, which can be described clearly by the well-known Stern-Volmer equation. The quenching analysis of synthesized undoped and Zr2+, Rh3+ and Pd2+ doped ZnS QDs by different

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concentrations of Cys (1×10‒6 to 10×10‒6 M) was recorded using fluorescence spectra. It indicates that the increase in the concentration of Cys decreases the fluorescence intensity of ZnS QDs as shown in Fig. 7(a). The following Stern–Volmer equation was further employed to describe the fluorescence quenching efficiency of Cys. Fig. 7(b) represents the relative fluorescence response (F0/F) of undoped and Zr2+, Rh3+ and Pd2+ doped ZnS QDs as a function of Cys concentration. 10

𝐹𝑜

= 𝐾𝑆𝑉 [𝑄] + 1

𝐹

(2)

where F0 and F are the fluorescence intensity of undoped, Zr2+, Rh3+ and Pd2+ doped ZnS QDs with and without the quencher [Q = Cys], and KSV is Stern-Volmer quenching constant. The fluorescence quenching data followed the Stern-Volmer equation and a good linear correlation (R2 = 0.98) was observed. The equation for the same can be represented as: 𝐹𝑜

= 𝐾𝑠𝑣 [𝐶𝑦𝑠] + 1.001

(3)

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𝐹

On the basis of the above equation, quenching constant KSV (slope of the linear fit)

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was calculated for undoped, Zr2+, Rh3+ and Pd2+ doped ZnS QDs to be 2.355×104, 2.587×104, 2.618×104 and 2.743×104 mol‒1 dm3, respectively. The linear relationship of the SternVolmer plot suggests that only one type of quencher is available and affects the fluorophore. According to Stern-Volmer equation, when all other variables are held constant, the higher

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the KSV, the lower is the concentration of Cys required to quench the emission intensity.

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These results of the fluorescence study indicate that the quenching effect of Cys on the

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fluorescence emission of undoped and Zr2+, Rh3+ and Pd2+ doped ZnS QDs is found to be concentration dependent. The quenching data were analyzed by the modified Stern–Volmer

𝐹0

= −𝐹

1 𝑓𝑎 𝑘𝑎

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+ [𝑄]

1 𝑓𝑎

(4)

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𝐹0

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equation:

where fa is the fraction of the initial fluorescence and ka is the effective quenching

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constant. A plot of (F0/ F0−F) versus 1/[Q] (Fig. 7(c)) gives a straight line and the value of ka of undoped, Zr2+, Rh3+ and Pd2+ doped ZnS QDs, calculated from the slope, was found to be

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1.0508×107, 1.214×107, 1.261×107 and 1.293×107 M−1, and the fa were found to be 1.02, 1.17, 1.24, and 1.32 from the intercept of the plot, respectively. The PL quenching is observed in the Zr2+, Rh3+ and Pd2+ doped ZnS QDs samples compared to undoped ZnS and

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is attributed to the dopant ions, which act as quenching centers of fluorescence. 3.7.2. Photoluminescence binding study of Cysteine with undoped, Zr2+, Rh3+ and Pd2+ doped ZnS QDs As mentioned in previous sections, the fluorescence quenching of different concentrations of Cys on undoped, Zr2+, Rh3+ and Pd2+ doped ZnS QDs occurred via complex formation. Thus, the binding constant (Ka) and a number of binding sites (n) can be deduced 11

as follows. For the quenching interaction, if it is assumed that there are similar and independent binding sites in the Cys-ZnS complex, the Ka and ‘n’ at room temperature can be evaluated from the double logarithm regression curve of log [(F0−F)/F] versus log [Q] as shown in the Fig. 7(d), based on the following equation: log

𝐹𝑜 −𝐹 𝐹

= log Ka + n log [Q]

(5)

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The values of Ka and ‘n’ for the interaction between Cys and undoped, Zr2+, Rh3+ and Pd2+ doped ZnS QDs at room temperature are calculated from this relative equation. The

average number of binding sites (n) for undoped, Zr2+, Rh3+ and Pd2+ doped ZnS QDs per Cys

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molecule is 1.108, 1.121, 1.128 and 1.131, respectively. The calculated value of Ka for the

Cys with undoped, Zr2+, Rh3+ and Pd2+ doped ZnS QDs is 0.244×106, 0.312×106, 0.324×106, and 0.331×106. Moreover, the high values of ‘n’ and Ka, not only result in the exposition of the electrostatic interactions but also provide easier access of the QDs to the binding sites of

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Cys. Different parameters including binding constants (Ka), a number of binding sites (n),

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and quenching constants (KSV) were calculated and the results revealed the formation of very

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stable complexes between Cys and undoped, Zr2+, Rh3+ and Pd2+ doped ZnS QDs in an aqueous solution. Here we also observed the quenching of PL intensity with doped ZnS QDs,

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from this quenching of PL intensity occurs as the defect level increased with an effect of dopant as a function. This can be correlated with the decreased nucleation of ZnS QDs with

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doping. Reports on Co, Mn and Ni doped ZnS NPs also show quenching of PL intensity [4445]. Recently, Kumar et al. [46] also reported blue and green emissions in doped ZnS NPs,

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which were assigned to recombination between the shallow donor level and the t2 level of doped ions that replaced Zn2+ in the ZnS host matrix. The enhanced PL intensities of the Zr,

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Rh, and Pd doped ZnS NPs may be more useful for optoelectronic devices. 4. Conclusions

In this paper, we describe a simple microwave-assisted method to fabricate and

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stabilize undoped and Zr2+, Rh3+ and Pd2+ doped ZnS QDs without using any capping agent. TEM analysis showed that the synthesized NPs were less than 10 nm in size. The cubic phase of synthesized undoped and doped ZnS QDs was observed from the XRD. The optical properties of the undoped and doped ZnS QDs were investigated by UV-vis and PL spectroscopy. The obtained undoped and doped ZnS QDs showed a band gap of 3.88-4.2 eV and particle size in the range of 2-10 nm. The structure obtained from XRD is in agreement 12

with published literature for cubic zinc blende structure. Thus, the synthesized undoped, Zr2+, Rh3+, and Pd2+ doped ZnS QDs showed enhanced fluorescence quenching and binding interactions with the Cys molecules at room temperature. The output of this study clearly suggests that synthesized undoped, Zr2+, Rh3+, and Pd2+ doped ZnS QDs can be used as the promising nanomaterials for efficient fluorescent material for quenching and binding applications. Finally, the high activity and stability of the undoped and doped ZnS QDs prepared using the microwave-assisted method in the presence of dopants are remarkable and

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imply that determination of fluorescence interactions can be designed and tested using this approach.

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Financial support

No financial support was provided to any of the authors in the creation/writing of this

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manuscript.

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Conflict of interest

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The authors declare that they have no competing interests.

Acknowledgments

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The authors would like to acknowledge the Head, Department of Chemistry, Osmania University for providing the necessary facilities. One of the authors, D. Ayodhya wishes to

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thank the UGC, New Delhi for the award of SRF which supported this work. The authors would like to thank DST–FIST, New Delhi, India for providing necessary analytical facilities

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in the department.

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Fig. 1 UV-vis absorption spectra of undoped, Zr2+, Rh3+, and Pd2+ doped ZnS QDs

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Fig. 2 Photoluminescence spectra of undoped, Zr2+, Rh3+, and Pd2+ doped ZnS QDs

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Fig. 3 FTIR spectra of undoped, Zr2+, Rh3+, and Pd2+ doped ZnS QDs

Fig. 4 Powder XRD spectra of undoped, Zr2+, Rh3+, and Pd2+ doped ZnS QDs

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Fig. 5 The high-resolution SEM images of (a) undoped ZnS QDs and (b-d) Zr2+, Rh3+, and

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Pd2+ doped ZnS QDs

Fig. 6 (a) TEM image and (b) SAED pattern of ZnS QDs

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Fig. 7 (a) Fluorescence quenching spectra of ZnS QDs quenched by Cys in the concentration range of 0‒10×10-6 M, (b) Stern‒Volmer plot of undoped, Zr2+, Rh3+, and Pd2+ doped ZnS

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QDs, (c) modified Stern‒Volmer plot of undoped, Zr2+, Rh3+, and Pd2+ doped ZnS QDs and

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(d) the plot of log [F0‒F/F0] vs. log [Q] for binding constant of Cys-ZnS QDs

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