Surface plasmon resonance in nanostructured ag incorporated zns films

Surface plasmon resonance in nanostructured Ag incorporated ZnS films S R Chalana, V Ganesan, and V P Mahadevan Pillai , Citation: AIP Advances 5, 107207 (2015); doi: 10.1063/1.4933075 View online: http://dx.doi.org/10.1063/1.4933075 View Table of Contents: http://aip.scitation.org/toc/adv/5/10 Published by the American Institute of Physics AIP ADVANCES 5, 107207 (2015) Surface plasmon resonance in nanostructured Ag incorporated ZnS films S R Chalana,1 V Ganesan,2 and V P Mahadevan Pillai1,a Department of Optoelectronics, University of Kerala, Kariavattom, Thiruvananthapuram– 695581, Kerala, India UGC-DAE Consortium for Scientific Research, Khandwa Road, Indore- 452017, Madhyapradesh, India (Received 23 June 2015; accepted 29 September 2015; published online October 2015) Silver incorporated zinc sulfide thin films are prepared by RF magnetron sputtering technique and the influence of silver incorporation on the structural, optical and luminescence properties is analyzed using techniques like grazing incidence X-Ray diffraction (GIXRD), atomic force microscopy (AFM), field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDS), micro-Raman spectroscopy, UV-Vis spectroscopy and laser photoluminescence spectroscopy XRD analysis presents hexagonal wurtzite structure for the films A reduction of crystallinity of the films is observed due to Ag incorporation The Raman spectral analysis confirms the reduction of crystallinity and increase of strain due to the Ag incorporation AFM analysis reveals a rough surface morphology for the undoped film and Ag incorporation makes the films uniform, dense and smooth A blue shift of band gap energy with increase in Ag incorporation is observed due to quantum confinement effect An absorption band (450-650 nm region) due to surface plasmon resonance of the Ag clusters present in the ZnS matrix is observed for the samples with higher Ag incorporation The complex dielectric constant, loss factor and distribution of volume and surface energy loss of the ZnS thin films are calculated Laser photoluminescence measurements gives an intense bluish green emission from the ZnS films and a quenching of the PL emission is observed which can be due to the metal plasmonic absorption and non-radiative energy transfer due to Ag incorporation C 2015 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License [http://dx.doi.org/10.1063/1.4933075] I INTRODUCTION ZnS is an important II–VI direct band-gap semiconductor that possesses unique properties and has potential applications in numerous areas like optics, electronics, photocatalysis, lasers and electric nanodevices.1 ZnS has a large band gap (3.7 eV), high refractive index (2.35) and a wide wavelength pass band (0.4-13 µm).2 Polycrystalline and nanocrystalline ZnS thin films have received much attention because of its probable important role in the photovoltaic technology and its vast application in optoelectronic devices.3 Zinc sulfide thin films can be prepared by different techniques including sputtering,4,5 metal oraganic chemical vapor deposition,6 molecular beam epitaxy,7 atomic layer epitaxy,8 chemical bath deposition,9 close spaced vacuum sublimation10 and pulsed laser deposition11,12 Among these techniques, radio frequency [RF] magnetron sputtering is relatively costeffective compared with those listed above and has sufficient control over the stoichiometry and uniformity of the film employed to produce ZnS thin films.5 Zinc sulphide nanoparticles doped with metallic elements has a variety of applications.13 When the external electromagnetic field induced by light interacts with the small metal nanocrystallites, a coherent oscillation of the conduction electrons occurs This coherent oscillation of free electrons a Ph No: (Office) 04712308167 (Mob) 9400946909 e-mail- vpmpillai9@gmail.com 2158-3226/2015/5(10)/107207/17 5, 107207-1 © Author(s) 2015 107207-2 Chalana, Ganesan, and Mahadevan Pillai AIP Advances 5, 107207 (2015) mainly within the surfaces is called surface Plasmon resonance (SPR).14 Nanolayers with metallic clusters or crystals in dielectric matrix have potential applicability for photonics, magnetics and electronics, and single electron devices.15 Surface Plasmon absorption bands of Ag and Au are in the visible and near-infrared (NIR) spectral regions, which is more useful for technological applications.16 Noble metal nanoparticles, such as Au and Ag, can effectively enhance the absorption capability, by the surface plasmon resonance and they can also capture the photo-generated electrons, leading to the reduction of the recombination rate.15 As the energy transfer between the excitons in the nanocrystal and plasmons in the metal surface occurs, the optical properties such as enhancement and quenching of photoluminescence (PL) of nanocrystals near the metal surface are modified The mechanism of enhancement or quenching of PL involves the contributions of the modulated excitation process of the plasmon and the emission process influenced by the exciton energy transfer.17 Several studies on the luminescence properties of silver-doped ZnS nanoparticles have been reported in recent years Sun et al., fabricated multilayer assemblies of silver doped ZnS colloid by a self-assembly technique exploiting electrostatic interaction and discussed the mechanism of luminescence enhancement.18 Water dispersible ZnS quantum dots were prepared by Jaiswal et al., in an environment friendly method using chitosan as stabilizing agent and investigated the static mechanism of the quenching process They stated that the Coulombic interaction between the positively charged ZnS QDs and negatively charged Ag nanoparticles led to the formation of electrostatic complex that contribute to the quenching process.19 Single crystals of silver and gold doped ZnS are grown by ‘self sealing, self releasing’ technique by Poolton et al., and investigated their luminescence properties.20 They reported silver related blue and red emissions in ZnS due to shallow-donor-deep-centre pair recombination Murugadoss et al., show that fluorescence of ZnS nanoparticles can be tuned by the interaction with silver.21 Ahn et al., studied the photoluminescence enhancement of ZnS, spin coated on Ag films and this enhancement is attributed to enhanced energy transfer between overlapping band energies of ZnS by surface plasmons of Ag.22 Though, there are extensive reports on the luminescence properties of silver doped ZnS films prepared by different methods, the correlation between surface plasmon resonance and photoluminescence quenching in silver incorporated ZnS films prepared by RF magnetron sputtering technique is in general not well-documented This paper reports the preparation of Ag incorporated ZnS thin films by RF magnetron sputtering technique and the effect of Ag incorporation on crystallinity, microstructure and optical properties In addition, the effect of surface plasmons on the quenching of luminescence emissions from the defects sites of ZnS is discussed II EXPERIMENTAL DETAILS Silver incorporated ZnS thin films were deposited on cleaned quartz substrates at a substrate temperature of 200 0C by RF magnetron sputtering technique The target used for sputtering was pressed ZnS powder (Aldrich 99.99 % purity) with various silver contents The weight percentage of silver used for deposition was 0, 1, 2, 5, and 10 wt % The sputter chamber was initially evacuated to a pressure of 3.0 x 10−6 mbar and then pure argon gas was admitted into the chamber and the argon pressure was maintained at 0.1 mbar The target was powered through a magnetron power supply (Advanced Energy, MDX 500) operated at a power of 150 W The films were deposited for duration of 30 minutes on quartz substrates kept at a distance of cm from the target The as-deposited films thus obtained with different silver incorporation were abbreviated as ZS, ZSA1, ZSA2, ZSA5 and ZSA10 respectively The crystalline structure and crystallographic orientations of the films were characterized by grazing incidence X-ray diffraction (GIXRD) measurements using Siemens D5000 Diffractometer in the 2θ range of 200 - 800 using Cu Kα radiation of wavelength 1.5406 Å Micro-Raman spectra of the films were recorded using Labram-HR 800 spectrometer equipped with argon-ion laser at a spectral resolution of about cm−1 The spectra were recorded with an excitation radiation of wavelength of 514.5 nm Surface morphology of the deposited films was investigated by atomic force microscopy (AFM) (Digital Instruments Nanoscope III, Si3N4 100 m cantilever, 0.58 N/m force constant) measurements in contact mode Grain size and root mean square (rms) surface roughness of the deposited films were determined on an area of µm x µm, using the WsXm software The 107207-3 Chalana, Ganesan, and Mahadevan Pillai AIP Advances 5, 107207 (2015) SEM images were recorded with Hitachi SU6600 variable pressure field emission scanning electron microscope (FESEM) The elemental analysis of the films is done by EDAX attachment associated with the FESEM The optical absorption, transmission and reflection spectroscopic measurements were performed for the wavelength range of 200-900 nm using JASCO V-550 UV–Vis double beam spectrophotometer Photoluminescence spectra of the films were recorded using TRIAX 550 spectrometer, excited using a laser radiation of wavelength of 325 nm at a power of 50 mW from a He-Cd laser [Kimmon Koha] III RESULTS AND DISCUSSIONS A GIXRD studies Fig shows the GIXRD patterns of undoped and Ag incorporated ZnS thin films All the films exhibit polycrystalline nature showing peaks corresponding to (1 14), (1 17) and (0 20) lattice reflection planes of hexagonal wurtzite ZnS phase [JCPDS card No: 89-2345] The XRD pattern of the undoped film presents a sharp intense peak corresponding to (1 17) lattice reflection plane and two weak peaks corresponding to lattice reflection planes (1 14) and (0 20) respectively This suggests good crystalline nature of the film with preferred orientation of crystal growth along (1 17) plane In the Ag incorporated films, the XRD peaks are broad and have less intensity This is an indication of deterioration of crystalline quality with Ag incorporation Ag incorporated films present FIG XRD patterns of undoped and Ag incorporated ZnS thin films (as deposited) prepared by RF magnetron sputtering technique on quartz substrate for duration 30 minutes 107207-4 Chalana, Ganesan, and Mahadevan Pillai AIP Advances 5, 107207 (2015) TABLE I Structural parameters of undoped and silver incorporated ZnS thin film with different silver incorporation Sample code FWHM of the peak at (1 17) (degree) ZS ZSA1 ZSA2 ZSA5 ZSA10 0.4175 0.4866 0.7635 0.8361 0.9680 Lattice constant (Å) Particle Lattice size (nm) Strain 10−3 a = b 20 17 11 10 2.596 3.030 4.761 5.225 6.053 3.821 3.824 3.829 3.830 3.831 c 75.09 75.16 75.27 75.49 75.56 Average rms surface Band transmittance Refractive roughness gap (eV) (400-900 nm) (%) index (n) (nm) 11.61 8.16 6.13 5.67 1.05 3.3 3.8 3.9 4.3 4.4 52 72 68 59 51 2.64 2.34 2.28 2.26 2.23 no preferred direction of crystalline growth When silver is incorporated into the ZnS lattice, a strain may be induced into the system which can alter the lattice periodicity and decreases the crystallinity.23 In the present study, the substrate temperature is maintained at a fixed value at 200 0C for different Ag incorporation concentrations So the decrease in crystallinity can be related to increase in Ag incorporation Incorporation of Ag atoms may inhibit the nucleation process of ZnS nanoparticles which can affect the formation of host ZnS structure.24 Apart from ZnS characteristic peaks, no phase corresponding to silver or other silver compounds is observed in the XRD patterns even for the highest silver incorporated film This observation indicates that the incorporation of silver neither changes the structure of ZnS nor resulted in the formation of any new compound and hence homogeneously distributed in the ZnS matrix The lattice constants for the hexagonal structure are taken as a= b, c For a hexagonal system, the d-values of the peaks are related to the lattice parameters by the following equation25: ( ) h2 + hk + k l2 = + 2, (1) 2 d a c where h, k and l are the miller indices and a, b and c are the lattice parameters along x, y and z directions The reported lattice constants for the bulk ZnS are a = b = 3.823 A0 and c = 74.976 A0 [JCPDS card No: 89-2345] Due to Ag incorporation, XRD peaks shows a systematic shift towards lower 2θ angles and hence lattice constant shows a systematic increase (TABLE I and Fig 2(a)) compared to the undoped film, which indicates the presence of residual stress in the films Intrinsic stresses due to the lattice mismatch developed during the deposition of ZnS film on the quartz substrate is difficult to determine because of the amorphous nature of the substrates.26 But an isotropic biaxial stress would be developed within the film plane when polycrystalline ZnS films are deposited onto amorphous quartz substrates.27 The thermal expansion coefficient of ZnS (6.7 x10−6/◦C) is larger than that of the quartz substrate (5.5 × 10−7 /◦C), which may cause the substrate to give a tensile stress to the film when the substrate cools from 200 0C to room temperature after the deposition process This can be the reason for the stress developed even in the undoped ZnS film FIG Structural parameters in Ag incorporated ZnS films: (a) lattice constants versus Ag incorporation (b) Variation of FWHM and grain size with Ag incorporation 107207-5 Chalana, Ganesan, and Mahadevan Pillai AIP Advances 5, 107207 (2015) But in the present study, substrate temperature is kept same i.e., at 200 0C for all the films Therefore the stress originating from the thermal expansion coefficient mismatch between the thin films and substrate is expected to be of the same magnitude for all the films So the main factor which causes the tensile stress can be the incorporation of Ag particles into the ZnS matrix The larger ionic radii of Ag ions (Ag1+ - 0.129 nm and Ag2+ ion - 0.108 nm) compared to Zn2+ ion (.074 nm) may results in the development of strain in the films when the Ag atom gets incorporated into the crystal lattice and causes changes in the lattice parameters of the host ZnS lattice.24 The average size of the crystallites Dh k l in the films can be estimated by the following Debye Scherer equation25 and are given in TABLE I; Dhkl = Kλ , βhkl cos(θ hkl) (2) whereλ is the X-ray wavelength, θ hkl is the Bragg diffraction angle and βhkl is the full width at half maximum (FWHM) in radian of the (1 17) peak in the X-ray diffraction pattern The K factor is dimensionless and is a measure of ‘roundness’ of the particle and often has the value of 0.9 or close to unity.28 It was observed that the full width at half maxima (FWHM) of the diffraction peaks increases with increasing silver content in the films, as illustrated in Fig 2(b) Thus incorporation of Ag not only degrades the crystallinity but also takes effect on the size of the nanoparticles With the increase of the Ag content, the crystallite size of zinc sulfide films decreased from 20 nm to nm This indicates that the average size of the crystallites depends on the silver content in the film and the addition of silver in ZnS prohibited the growth of crystalline grains of ZnS Increase in number of Ag atoms would exert drag forces on boundary motion and grain growth which may cause the reduction of grain growth.29 This is contrary to the observation obtained by Bose et al., when Ag is incorporated in WO3 lattice They have observed an enhancement in crystallinity and crystallite size with Ag incorporation in WO3.30 Lattice strain (T) in the films is calculated using the following equation25: λ − βhkl, (3) D cos θ where T is the lattice strain, λ is the wavelength of X-rays used, θ is the Bragg angle, D is the grain size and βhkl is the full width at half maximum The values of lattice strain for the undoped and Ag incorporated films are calculated and are shown in TABLE I An increase of lattice strain is observed with increase in Ag incorporation This increased strain agrees with the observed crystallinity reduction and broadening of XRD peaks in the Ag incorporated films T tan θ = B Micro Raman spectral analysis Wurtzite ZnS belongs to C6v4 (C63mc) with a primitive cell of two formula units where all the atoms occupy the C3v sites The factor group analysis predicts nine optical modes which are distributed as: Γopt = A1 + E1 + 2E2 + 2B1, (4) A1 and E1 modes are polar and both Raman and IR active E2 modes are nonpolar and only Raman active B1 modes are inactive in both the spectra.31,32 The A1 and E1 symmetry can split into two components: transverse optic (TO) and longitudinal optic (LO), and both of them are highly isotropic, A1 (TO) = E1 (TO) and A1 (LO) = E1 (LO), which is a characteristic feature of the wurtzite ZnS.31,33 Fig 3(a) and 3(b) display micro-Raman spectra of bulk ZnS and wurtzite ZnS thin films with different Ag incorporations in the wavenumber region 100-800 cm−1 Raman spectrum of bulk ZnS presents a very intense band at 349 cm−1 and several weak to medium intense bands The Raman spectrum of the undoped ZnS film presents an intense band at 349 cm−1 which can be assigned to A1(LO) and E1(LO) modes The medium intense band around 263 cm−1 can be due to A1(TO) and E1(TO) modes.34,35 The medium intense Raman band at 207 cm−1 can be due to a first-order longitudinal acoustic (LA) mode The weak Raman bands at 150 cm−1 and 178 cm−1 can be assigned as disorder activated second order acoustic phonons The bands near 420 cm−1 and 448 cm−1 can be due 107207-6 Chalana, Ganesan, and Mahadevan Pillai AIP Advances 5, 107207 (2015) FIG Micro Raman spectra of (a) bulk ZnS and (b) Ag incorporated ZnS films prepared by RF magnetron sputtering technique on quartz substrate for duration time 30 minutes to first order and second order zone-boundary (ZB) phonons.34 The mode around 488 cm−1 can be assigned to the spectral contribution from the quartz substrate The weak Raman bands between 600 and 700 cm−1 can be assigned to combination modes and overtones.34,35 The weak band near 677 cm−1 can be associated with the second order LO phonon.35 The Raman band observed around 616 cm−1 can be attributed to the combination mode of LO and TO modes The Raman bands are not well resolved in the Ag incorporated films and the broad nature of the bands can be an indication of reduction of crystallinity and increase of strain in the films due to the 107207-7 Chalana, Ganesan, and Mahadevan Pillai AIP Advances 5, 107207 (2015) Ag incorporation This observation is in agreement with the results obtained from XRD analysis In the Ag incorporated films the LO mode at 349 cm−1 is red shifted to 335 cm−1 and showed a substantial broadening Lin et al., observed a red shift of LO mode from 352 cm−1 to 348 cm−1 and Chong et al., observed a red shift of LO mode from 352 cm−1 to 331 cm−1 for low doping concentrations They assigned such phonon softening and line broadening of Raman bands with phonon confinement effects.33,35 The decrease of wavenumber observed for the LO mode in the Ag incorporated films may be due to such phonon confinement effects The red shift of LO mode observed in the Ag incorporated films indicates that a tensile strain occurs during the Ag incorporation as revealed by the XRD analysis The A1 (LO) phonon mode corresponds to atomic oscillations along the c-axis Thus the peak value of A1 (LO) mode is sensitive to lattice strain along c-axis If we consider that the LO phonon mode has A1 symmetry, then the strain associated with a lattice elongation (contraction) of the c-axis can be calculated using the following equation,36 ∆ω/ω = (1 + 3∆c/c)−γ − 1, (5) where ∆ω is the LO phonon wavenumber shift from the bulk value ω0 and γ is the Gruneisen parameter whose value is 0.95 for ZnS.36 If we neglect size confinement in the ZnS nano thin films, the lattice elongation ∆c/c of the highest Ag incorporated film when compared with that of the undoped ZnS film is calculated to be 1.29 % Thus, the Ag incorporation results in a tensile strain in the films as expected by the decrease in the LO phonon wavenumber C AFM analysis The surface morphology and topography of ZnS films were studied using atomic force microscopy (AFM) Fig 4(a)-4(e) shows the 3D AFM images of the undoped and Ag doped ZnS films The AFM analysis shows that the silver incorporation has a profound effect on the grain size and the rms surface roughness of the ZnS films The tendency of reduction in the size of the grains with increase in silver incorporation can be clearly seen from the AFM analysis The AFM image of the undoped film presents agglomerated grain geometry without well-defined grain boundary The AFM image of wt % silver incorporated ZnS film presents slightly smaller grains and it also shows no definite grain boundary The films with wt.% and wt % silver incorporation present a surface morphology showing the presence of well-defined grains ZnS film doped with 10 wt % silver presents a surface morphology showing the uniform distribution of relatively smaller grains The reduction of the grain size with increase in silver incorporation as observed by the AFM analysis is in agreement with the XRD results The undoped ZnS film shows rms surface roughness of 11.62 nm A systematic decrease of rms surface roughness can be seen (Fig 4(f)) with increase in silver incorporation Thus, silver incorporation makes the films smoother than the undoped film AFM analysis showed that highest incorporation of Ag in ZnS films obviously makes the film dense and uniform with very small grains Regulation of the dense particles and rms surface roughness is considered to be important in the manufacture of many optoelectronic devices Such smooth and dense films are particularly desirable for solar cell device.37 D FESEM analysis Fig shows the scanning electron micrographs of undoped and wt % silver incorporated ZnS films FESEM images showed a closely packed morphology for both undoped and Ag incorporated films FESEM images also show that the nanoparticles are evenly dispersed in the films As observed in the XRD and AFM analysis, the FESEM analysis also presents the tendency of reduction of grain size with silver incorporation E EDS analysis The composition analysis of the ZnS films with different Ag incorporation was carried out by EDS analysis and are shown in Fig All the films present the peaks corresponding to Zn and S in the 107207-8 Chalana, Ganesan, and Mahadevan Pillai AIP Advances 5, 107207 (2015) FIG 3D AFM images of undoped and Ag incorporated ZnS thin films (as deposited) (a) ZS, (b) ZSA1, (c) ZSA2 (d) ZSA5, (e) ZSA10 prepared by RF magnetron sputtering technique on quartz substrate, (f) Variation of rms surface roughness with Ag incorporation EDS spectra indicating the formation of ZnS phase in the films The lower concentration of the silver obtained from the EDS analysis also reveals that silver is not fully incorporated in the ZnS lattice The films with higher silver incorporation show higher oxygen content This indicates the oxidation in the films with higher silver incorporation This can be due to the catalytic action of silver nanoparticles on the surface of the films for the oxidation.38,39 FIG FESEM images of undoped (ZS) and wt % of Ag incorporated (ZSA1) zinc sulfide films prepared by RF magnetron sputtering technique on quartz substrate 107207-9 Chalana, Ganesan, and Mahadevan Pillai AIP Advances 5, 107207 (2015) FIG EDS spectra of undoped and Ag incorporated films and the table showing atomic percentage of elements in the films F Optical analysis The optical transmittance spectra of the undoped and Ag incorporated ZnS films are recorded in the wavelength region 200-900 nm and are given in Fig 7(a) The average optical transmittance in the wavelength range 400-900 nm is calculated and is given in TABLE I The undoped ZnS film presents an average transmittance of 52 % where as wt % Ag incorporated ZnS film shows the highest value of 72 % for the average transmittance The film with 10 wt % of Ag incorporation shows the lowest value of average transmittance The AFM analysis shows the highest value of rms surface roughness for the undoped ZnS film, which may lead to enhanced scattering loss, and that can be the reason for low transmittance observed in the undoped ZnS film.40 When the silver incorporation increases beyond wt %, transmittance of the films decreases systematically The decrease in transmittance with enhanced silver incorporation can be due to various reasons The possibility of occurrence of surface plasmon resonance due to the presence of Ag nanoparticles can be one of the reasons for this reduction in transmittance The deterioration of crystalline quality with increase in Ag incorporation, as revealed by XRD analysis can also be one of the reasons for this reduction in transmittance The XRD, AFM and SEM analysis suggest reduction in grain size with increase in Ag incorporation As the size of the grains decreases, the number density of grain boundaries increases and this may lead to increased scattering loss and also can reduce the transmittance in these films The increased scattering of photons by crystal defects, if any created by Ag incorporation may also be attributed to reduction in transmittance.29 107207-10 Chalana, Ganesan, and Mahadevan Pillai AIP Advances 5, 107207 (2015) FIG (a) Optical transmittance (b) absorbance and (c) reflectance spectra of undoped and Ag incorporated ZnS thin films The absorption spectra of the undoped and Ag incorporated films in the wavelength region 200-900 nm is given in Fig 7(b) The absorption spectra of ZnS films having and 10 wt % of Ag incorporation show the presence of an enhanced absorption in the 450-650 nm region The intensity of this band is found to be higher for the 10 wt % Ag incorporated ZnS film (ZSA10) The broad absorption band observed in this region in samples with higher percentage of Ag incorporation can be due to surface plasmon resonance of the Ag clusters present in the ZnS matrix.41 Bose et al., observed that incorporation of silver in WO3 films results in the formation of broad absorption bands centered around 437 nm due to surface plasmon resonance.30 Excitation of surface plasmons in metal nanoparticles placed on a semiconductor might be expected to enhance optical absorption of incident photons within the semiconductor region near each nanoparticle due to the amplification of localized field.42 The observed decrease in transmittance of the films with higher Ag incorporation (ZSA5 and ZSA10) can have a contribution from enhanced absorption in these films due to surface plasmon resonance of silver Zinc sulphide is known to be II-VI semiconductor with a direct band gap nature The optical band gap Eg can be estimated from the Tauc plot43 (αhν) = A(hν − Eg )n , (6) where Eg is the band gap corresponding to a particular transition occurring in the film, A is a constant known as band edge sharpness (related with the order in crystalline structure of deposited films), ν is the transition frequency and the exponent n characterizes the nature of band transition For crystalline semiconductors, n can take values 1/2, 3/2, or depending on whether the transitions are direct allowed, direct forbidden, indirect allowed and indirect forbidden transitions respectively.43,44 The values of band gap were determined by extrapolating the straight line portion of the (αhν)1/n versus hν graphs to the hν axis, where α is the optical absorption coefficient, hν is the incident photon energy and n depends on the kind of optical transition.44 For all the films the best straight line portion is obtained for n = 1/2 indicating a direct allowed transition in the films Fig 8(a) shows the Tauc plots for the undoped and Ag incorporated films The undoped ZnS film presents band gap energy of 3.3 eV A systematic increase of band gap energy with increase of Ag incorporation can be seen from Fig 8(b) and TABLE I The observed blue shift of band gap energy with increase in Ag incorporation can be due to quantum confinement effect due to decrease in the size 107207-11 Chalana, Ganesan, and Mahadevan Pillai AIP Advances 5, 107207 (2015) FIG (a) Tauc plots obtained for nanostructured ZnS films with different Ag incorporation (b) Variation of grain size and band gap with Ag incorporation of the crystallites in the films45,46 as revealed by XRD, AFM and SEM analysis Other subordinate effects such as polaron, strain, size and imperfection may also leads to a widening of the band gap.47 The extinction coefficient κ in the films can be calculated using the following relation43: αλ , (7) 4π where λ is the wavelength and α is the absorption coefficient The variation of extinction coefficient with wavelength is shown in Fig 9(a) The optical reflection of a thin film is directly dependent on the refractive index of the film through the following relation.48  (1 + R) + 4R − (1 − R)2 κ n= , (8) 1−R where R and κ are the reflectance and extinction coefficients respectively The undoped film shows the highest value for refractive index as 2.647 which is in good agreement with the reported values Nadeem et al., reported that the peak value of the refractive index for the ZnS thin films of various thickness vary in the range of 2.61 to 2.64 and Ndukwe et al., reported a refractive index value of 2.62.49,50 Refractive index of the films decreases with the increase in Ag incorporation (TABLE I) The decrease in the refractive index with Ag incorporation can be attributed to an increase in the carrier concentration in the Ag incorporated thin films.51 The refractive index seems to be a significant factor for photocurrent of the photoelectrode; the smaller the value of this refractive index, the better the performance of the photoelectrode is.19 κ= 107207-12 Chalana, Ganesan, and Mahadevan Pillai AIP Advances 5, 107207 (2015) FIG Optical parameters: (a) Variation of extinction coefficient with wavelength, (b) variation of optical conductivity with photon energy The optical response of a material is mainly studied in terms of the optical conductivity (σ) which is given by the relation.52,53 σ = αnc/4π , (9) Fig 9(b) shows the variation of optical conductivity with the incident photon energy where c is the velocity of light, α is the absorption coefficient and n is the refractive index The optical conductivity shows an increase after 2.92 eV for all the samples This increase at high photon energies is due to the high absorbance of ZnS films in that region The complex dielectric constant is a fundamental intrinsic property of the material From the measured data of absorption and refractive index, the frequency dependent complex dielectric constant (ε) ˜ of ZnS are calculated The frequency dispersion of complex dielectric constant (ε) ˜ characterizes the propagation, reflection and loss of light in the prepared films completely and provides information about the electronic structure of the material Therefore, ε˜ is an important quantity for the design of highly efficient optoelectronic devices.54 The complex dielectric constant is described by Ref 54 the following equation: ε(hν) ˜ = ε 1(hν) + iε 2(hν), ε2 tanδ = , ε1 (10) (11) where ε (= n2 − κ 2) is the real and ε (= 2nκ) is the imaginary parts of the dielectric constant, and tanδ is the loss factor The real and imaginary parts of the spectra are called dispersion and absorption curves (Fig 10(a) and 10(b)) The real part of the dielectric constant shows how much it will slows down the speed of light in the material, whereas the imaginary part shows how a dielectric material absorbs energy from an electric field due to dipole motion The knowledge of the real and the imaginary parts of the dielectric constant provides information about the loss factor (Fig 10(c)) which is the ratio of the imaginary part to the real part of the dielectric constant.52,55 The variation of the real part (ε 1) of the dielectric constant shows the spectral behavior of refractive index because of the smaller value of κ compared to the values of n2 In the undoped film, the real part of the dielectric constant shows an oscillatory nature for the lower photon energy values When photon energy increases, this oscillatory behavior decreases and ε decreases ZnS films with lower incorporation of silver (1 wt % and wt %) shows almost constant behavior for the real part of dielectric constant with increase in photon energy In ZnS films doped with and 10 wt % of silver, real part of the dielectric constant first increases and then decreases with increase in photon energy Imaginary part (ε 2) of the dielectric constant is mainly depends on the κ values, which are related to the variation of absorption coefficient The imaginary part of the dielectric constant is oscillatory for the lower photon energy values and gradually increases with increase in photon energy for the undoped ZnS film ZnS films doped with and wt % of silver shows almost constant behaviors for the imaginary part of dielectric constant with increase in photon energy ZnS films of higher silver incorporation (5 and 10 wt %) shows rise and fall in the imaginary part of the dielectric constant 107207-13 Chalana, Ganesan, and Mahadevan Pillai AIP Advances 5, 107207 (2015) FIG 10 (a)Dispersion curve, (b) absorption curve, (c) loss factor of undoped and Ag incorporated ZnS thin films The volume (bulk) energy loss function, VELF, and the surface energy loss function, SELF, are related to the real and imaginary parts of the dielectric constants by the following relations.43 VELF = SELF = ε 22 ε 21 − ε 22 , ε 22 ((ε + 1)2 + ε 22) (12) , (13) The distribution of the volume and surface energy loss of the as-deposited films as a function of the photon energy are shown in Fig 11 and Fig 12 respectively It can be seen that the volume energy loss function for the undoped film is found to be lesser compared to that of the silver doped films whereas the value of surface energy loss function is higher in the undoped film compared to the silver doped films G Photoluminescence analysis Room-temperature photoluminescence (PL) spectra of the undoped and Ag-doped ZnS thin films at a laser excitation wavelength of 325 nm are shown in Fig 13 It is well established that in semiconductor nanoparticles the photoemission occur by the recombination of electrons and holes, which are the photo excited carriers, via direct band-band recombination, recombination via shallow trap states or via deep trapped states of different kind.21 The undoped and Ag incorporated films show a broad emission extending from near-ultraviolet to visible spectral region (350 nm - 650 nm) This bluish green emission band centered on 494 nm can have originated from the vacancies and interstitial defects on the surface of ZnS such as the recombination of trapped electrons on the sulfur vacancy donor level (Vs) to interstitial sulfur states (Is).56,57 With the increase in Ag incorporation from wt % to 10 wt %, intensity of this emission band decreases Three main mechanisms that can account for the photoluminescence quenching are surface plasmon resonance absorption of metal, non-radiative energy transfer from semiconductor to metal nanoclusters and the electron transfer from semiconductors to metal nanoparticles.58 Observed gradual 107207-14 Chalana, Ganesan, and Mahadevan Pillai AIP Advances 5, 107207 (2015) FIG 11 The distribution of the volume energy loss for undoped and Ag incorporated ZnS films as a function of photon energy quenching of visible emission in the present work can be related to the metal plasmonic absorption since the ZnS films with higher Ag incorporation have a broad plasmonic absorption peak covering the visible range (450 nm - 650 nm) This broad plasmonic absorption results in the absorption of emissions from ZnS nanoparticles by the Ag particles in the higher Ag incorporated ZnS films.59 Guan et al., confirmed that the noble metal nanoparticles such as gold and silver could quench the emitted light of a luminophor and this quenching effect was stronger with the increase in concentration of the metal nanoparticles.60 When the semiconductor nanoparticles and the metal nanoparticles are at a moderate distance between them, photoluminescence can be improved by the field enhancement of the metal nanoparticles which can affect the photon flux to the semiconductor nanoparticles.61 This type of PL enhancement is observed by Jung et al.,61 in their study of examining the optimum nanostructures for the PL enhancement of CdS where the CdS nanoparticles were drop-coated on to Au/SiO2 nanocomposites prepared by RF magnetron sputtering But in the present investigation, Ag nanoparticles were mixed with ZnS nanopowder during grinding process and this mixture is used for deposition So nanoparticles of Ag and ZnS contained in the plasma plume get deposited during the coating process and Ag atoms may be distributed more uniformly in the prepared films Thus ZnS nanoparticles can be in close proximity to the Ag nanoparticles which may results nonradiative energy transfer from ZnS to the Ag nanoparticles This can also reduce the intensity of visible emission in the Ag incorporated ZnS thin films Increasing the amount of Ag, causes the nanoparticles to become so close to each other that undesired nonradiative recombination becomes dominant due to the destructive interference of the 107207-15 Chalana, Ganesan, and Mahadevan Pillai AIP Advances 5, 107207 (2015) FIG 12 The distribution of the surface energy loss for undoped and Ag incorporated ZnS films as a function of photon energy surface plasmons This results in the quenching of emission with increase in Ag incorporation.61,62 As the Ag doping concentrations increases, the adsorption increases than scattering and suppressed the radiations as photons into free space due to nonradiative dissipation of surface plasmon.62 It may also attenuate the visible PL emission intensity with increase in Ag incorporation Thus Ag acts as electron trapping center, which leads to non-radiative recombinations and acts as a quencher impurity in the host ZnS nanoparticles FIG 13 Photoluminescence spectra of undoped and Ag incorporated ZnS thin films at a laser excitation of 325 nm 107207-16 Chalana, Ganesan, and Mahadevan Pillai AIP Advances 5, 107207 (2015) The decreased PL intensity of the Ag incorporated films may also be attributed to the decrease in crystalline size with Ag incorporation as revealed by the GIXRD, AFM and SEM analysis The decrease in crystallite size can increase the grain boundary density, which may act as a source of dissipation, adsorption and scattering of light generated inside the film that resulted in lower PL brightness.63 In addition to the deterioration of crystalline quality, Ag incorporation results in the reduction of surface roughness The decreased surface roughness of the Ag incorporated films may increase the loss of emitted light due to internal reflections within the film and results in the reduction of PL brightness.63 At the interface between Ag nanoparticles and the rough surface of ZnS films, localized surface plasmons can be created due to the resonant interaction between the electron-charge near the surface of the Ag nanoparticles and the electromagnetic field of the incident light of 325 nm If the interface is sufficiently smooth, the coupled energy resulting from the emission energy of semiconductor and the surface plasmon resonance energy could not be transferred into free space photons due to the less scattering of surface plasmons From AFM images (Fig 4), it can be seen that the average roughness for Ag incorporated samples is smaller than the undoped samples A larger roughness can more efficiently enhance emission by surface plasmon coupling Comparatively very small surface roughness observed for the Ag incorporated ZnS films can quench the emission due to less surface plasmon coupling.64 In the undoped and Ag doped films the near band-edge emission was absent but only a defect related peak was present The suppression of the band edge emission in the films can be due to the significant amount of defects which trap the photo-generated electrons/holes leading to significant reduction in the rate of radiative exciton recombination.65 CONCLUSION Influence of silver incorporation in the structural, optical and luminescence properties of ZnS thin films were studied The XRD analysis reveals the presence of hexagonal wurtzite zinc sulfide in the films XRD analysis also shows that silver incorporation deteriorates the crystalline quality and act as grain growth inhibitor Micro-Raman spectral analysis confirms the presence of hexagonal wurtzite phase in the films AFM and SEM analysis support the reduction of grain size with increase in silver incorporation AFM analysis reveals a gradual decrease of rms surface roughness with Ag incorporation The EDS analysis shows the formation of ZnS phase in the films and also reveals incorporation of silver in the films The observed blue shift of band gap with increase in silver content is due to the quantum confinement effect due to the reduction of crystalline size in the Ag incorporated films An absorption band in the 450-650 nm region observed in the ZnS films with higher silver incorporation can be due to the surface plasmon resonance of the silver clusters Bluish green emission band centered at 494 nm in the laser photoluminescence (PL) spectra, originates from the vacancies and interstitial defects on the surface of ZnS The reduction in intensity of this PL band in the Ag incorporated films can be 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8735 (2009) 65 L Kumar, R Medwal, P Sen, and S Annapoorni, Mater Res Express 1, 015045 (2014) ... the increase in Ag incorporation (TABLE I) The decrease in the refractive index with Ag incorporation can be attributed to an increase in the carrier concentration in the Ag incorporated thin films. 51... and Ag incorporated ZnS films as a function of photon energy surface plasmons This results in the quenching of emission with increase in Ag incorporation.61,62 As the Ag doping concentrations increases,... strain for the undoped and Ag incorporated films are calculated and are shown in TABLE I An increase of lattice strain is observed with increase in Ag incorporation This increased strain agrees