3.1. XRD - Structural analysis
The XRD spectra of un-doped ZnS, Mn = 3% doped ZnS and Mn = 3% and Cu 2% dual doped ZnS QDs for the different diffraction angles (2θ) from 20° to 70° are presented in Fig. 2a. The noticed broad peaks indicate that the synthesized samples are in the nano-scale levels. The entire samples display the wide diffraction peaks about 28.5°, 48.7° and 57.5° and they are related to (111), (220), and (311) orientations, respectively. Here, the preferred orientation is along (111) plane due to its highest peak intensity throughout the entire samples. The attained three XRD peaks are matched well with the JCPDS card no: 80 − 0020 which reflected that all the synthesized samples posses the cubic structure of ZnS.
The XRD peak intensity of ZnS is decreased Mn doping and reduced further by Cu, Mn dual doping and the reduced peak intensity designates the size mutation and crystalline deterioration. Throughout this work, (111) orientation with higher XRD intensity is preferred to study the effect of doping (Mn/Cu) on ZnS. The current reduction in crystallinity reflects the disorder induced in the Zn-Mn-S lattice by the incorporation of Cu. The generation of extra or additional impurity / secondary phases such as Mn/Cu metals, Mn/Cu sulphides or Mn/Cu oxides are ruled out because no unwanted additional peaks were noticed in the XRD studies. Therefore, the present XRD patterns confirm the phase singularity without any extra phases of the prepared samples. No secondary or impurity phases found in the XRD analysis recommended that the doping elements Mn and Cu are appropriately substituted into Zn-S lattice [14]. The observed slight turn of XRD peaks along higher 2θ region by Cu addition also signifies that Cu is appropriately substituted into Zn0.97Mn0.03S lattice. The identical shift along higher diffraction angles was reported in Ni added ZnS nanostrucutres by Kaur et al. [15]
Figure 2b shows the variation of FWHM and crystallite size of un-doped ZnS, Zn0.97Mn0.03S and Zn0.95Mn0.03Cu0.02S. The incorporation of Cu elevates the FWHM which reflects the shrinkage of crystallite size. The average crystallite size of the prepared nanostructures have been estimated with the help of Scherrer’s formula [16], 0.9λ / β cos θ, where β is FWHM in radian and θ is the diffraction angle. Figure 2b clearly shows the slight increment in size by Mn doping and the significant reduction in size by Mn and Cu dual doping. The slight increase of size in Mn-doped ZnS is due to replacement of Mn2+ ions (ionic radius ∼ 0.80 Å) in the position of Zn2+ (ionic radius ∼ 0.74 Å) [17]. The same kind of size increment is noticed by Wang et al. [18] in Mn added ZnS.
The decrease of crystallite size at Cu addition is supported by the dissimilarity between the ionic radius between Cu and Zn. Here, the ionic radius of Cu2+ ions (≈0.71 Å) is smaller than the ionic radius of Zn2+ ions as well as Mn2+ ions. The decrease of size was discussed by Reddy et al. [19] at Zn0.97−xCuxCr0.03S QDs with Cu > 3% and they described that the size reduction was due to the generation of lattice disorder by the Cu addition. Wang et al. [20] revealed that the simultaneous substitution of Mn2+ and Cu2+ ions were not homogeneously dispersed in Zn-S nanostructures but the core portion includes more Mn2+ ions and the surface portion contains much Cu2+ ions. It is concluded from the XRD analysis that the incorporation of Cu2+ ions weaken the size as well as produce the defect associated luminescence states.
3.2. Optical absorption and transmittance spectra
Optical absorption spectra of pure ZnS, Mn = 3% doped ZnS and Mn = 3% and Cu = 2% simultaneously doped ZnS within the wavelength ranging between 300 nm and 550 nm are illustrated in Fig. 3a. The absorption edge of all the three samples is well below the visible wavelength i.e., below 400 nm (∼ 320 nm to 400 nm) [21]. Both the increase of absorption intensity as well as shift of absorption edge along long wavelength region are noticed by the substitution of Mn into Zn-S lattice (Zn0.97Mn0.03S). The elevated absorption intensity by Mn addition may be due to the increase of size and also the proper incorporation of Mn2+ ions in the position of Zn2+ ions where the ionic radius of Mn2+ ions is higher than Zn2+ ions. The modulation in crystallite size by Mn addition is the one more probable reasons for the noticed increment in absorption intensity and also the red shift of absorption edge. The same kind of red shift in absorption edge by Mn addition in ZnS was revealed by Li et al. [22]. One more literature [23] confirmed the shift of absorption edge along higher wavelength region by Mn addition in ZnO.
During the addition of Cu in Zn-Mn-S lattice, the absorption intensity suppressed to lower value and also the absorption edge moved along the lower wavelength region i.e., blue shift of energy gap. The existing blue shift of absorption edge was supported by the size reduction which promote the energy gap to higher value [24]. In addition, the Cu-doping stimulates more defect and creates the irregularity or imperfections through Zn-Mn-S lattice owing to the ionic radius variation among Cu2+, Mn2+ and Zn2+ ions which may also the probable reason for the reduction in absorption intensity [25]. The light scattering by grain boundaries and the reflection of incident light by Cu clusters in Zn0.95Mn0.03Cu0.02S may also reduce the absorption intensity [19].
Figure 3b presents the transmittance spectra of pure ZnS, Mn = 3% doped ZnS and Mn = 3% and Cu = 2% simultaneously doped ZnS within the wavelength ranging between 300 nm and 550 nm. The whole samples reveal the semitransparent character with highest transmittance in the visible wavelength region. The observed rapid increase of transmittance along UV wavelength region throughout the samples is due to the inter-band transition commencing from valence band level to conduction band. The percentage of transmittance in the visible region gets down by Mn-doping (single doping) and further diminished by double (Mn and Cu doping) doping where it varies between 50–75%.
3.3. Energy gap estimation
The inclusion of Mn and Cu into ZnS not only affect the absorption and transmittance and also tailored the energy gap of the synthesized samples. Therefore, to investigate the influence of Mn/ Cu through band structure as well as electronic transition, the correlation among the absorption coefficient and the photon energy was examined using Tauc's relation [26], hυ = A(hυ - Eg)n, where, Eg is energy gap and the exponent 'n' is taken as 0.5. The linear part of ‘hυ’ designates the direct band gap of the samples.
The band gap of the synthesized materials is acquired from the plot between (αhυ)2 and hυ as revealed in Fig. 4a. The band gap of pure ZnS is found to be 3.51 eV. The steep decrease in band gap from 3.51 eV (ZnS) to 3.28 eV (Zn0.97Mn0.03S, ΔEg ∼ 0.23 eV) was noticed during the addition of Mn2+ in Zn-S lattice where Mn2+ may occupies either interstitial sites or substitutional positions of Zn2+ ions. The substitution of Mn2+ ions generates the dislocated or irregular atoms and induces the imperfections in the structural bonding of ZnS which are made by the 3d electrons in Mn atoms [21]. The stimulated sp-d exchange interaction by Mn between the s and p electrons from ZnS and 3d electrons from Mn2+ ions is responsible for the reduction in energy gap [27]. The addition of Mn re-generate the newer energy levels closer to the valence band of ZnS which is also the another possible reason for current shrinkage in energy gap [28].
The abrupt increase of energy gap from 3.28 eV (Zn0.97Mn0.03S) to 3.94 eV ((Zn0.95Mn0.03Cu0.02S, ΔEg ∼ 0.66 eV) was obtained by Cu doping where Cu2+ ions with lower ionic radius may be replace Zn2+ ions at substitutional sites. The addition of Cu liberates more free charge carriers in Zn-Mn-S lattice which modify the Fermi level and move closer to the conduction band and broadens the energy gap. The current enhancement in energy gap i.e., blue shift of energy gap by Cu doping is the Burstein-Moss shift [29]. The existence of poor crystallinity by Cu addition also play a major role in the increase of energy gap [30]. Figure 4b illustrates the change in average crystallite size and energy gap of un-doped ZnS, Zn0.97Mn0.03S and Zn0.95Mn0.03Cu0.02O nanostructures. When Mn is introduced into ZnS energy gap reduced to lower value at the same time size of the nanoparticles enhanced slightly to higher value i.e., it obey the size effect [31]. Moreover Cu addition into Zn-Mn-S increases the energy gap which is in good agreement to the quantum confinement effect of the nanoparticles [32].
3.4. Fourier transform infrared (FTIR) studies
FTIR is the best technique to examine the chemical bonding in the substance and also to identify the organic group of the materials. The room temperature FTIR spectra of pure ZnS, Zn0.97Mn0.03S and Zn0.95Mn0.03Cu0.02S are shown in Fig. 5 between 400 cm− 1 and 4000 cm− 1. The wide absorption bands for all the samples within the range of 3000–3600 cm− 1 related to OH stretching vibration which designates the survival of water absorbed in the surface of nanoparticles [33]. The observed bands between 1592–1602 cm− 1 ascribed to C = O stretching modes induced from the existing atmospheric CO2 in the sample [34, 35]. The absorption bands centered at 1121 cm− 1 are related to the partial substitution of Mn or Cu atoms into Zn position in Zn-S lattice [1]. The feeble and weak absorption band centered around 928 cm− 1 corresponds to the defect states induced by Cu-substitution in Mn-Zn-S lattice which absent in other two samples. The strong bands about 670–676 cm− 1 and 511–520 cm− 1 are responsible for Zn-S stretching vibrations [36]. The bands corresponding to Zn-S show the better agreement with the earlier studies [37].
3.5. Photoluminescence (PL) spectra
In order to investigate the electronic inter-band transition and influence of doping on emission properties of pure ZnS, Zn0.97Mn0.03S and Cu-doped Zn0.97Mn0.03S samples, PL spectra have been recorded. Figure 6 shows the PL spectra of all the samples in the wavelength ranges from 350 nm to 650 nm at room temperature. Pure ZnS exhibits a wide and broad PL band within the wavelength range from 370 nm to 550 nm which is similar to the literature reported by Faita et al. [38]. The noticed wide emission band represents the existence of lower crystallite size (35 Ǻ) where self activated defect states have been generated by different defects like oxygen defects, Zn interstitials, sulfur vacancies and point defects. Moreover, the presence of asymmetrical peak in ZnS proposes the involvement of many emission bands combined as broad peak with centre along violet and bluish-violet region [39].
Generally, the emission in the visible region arises from impurity or native defect levels. The intensity of the broad band corresponding to violet and bluish-violet emissions in ZnS is decreased sharply by Mn/Cu doping. During Mn-doping into ZnS, the violet emission shifted to higher wavelength side with reduced intensity. One of the reason to decrease the intensity and the red shift of wavelength is the increase of crystallite size which emits the longer wavelengths compared to lower size [40]. Nasser et al. reported that the reducing intensity by Mn addition is due to the diminishing rate of band to band recombination of charge carriers [41]. The observed diminishing emission intensity and the blue band emission at 486 nm by Cu-doping is due to the formation of luminescence centers that trap the electrons and holes and enhance the non-radiative recombination process [42].
Figure 7 shows the energy level diagram of the different synthesized samples to explain the various emissions like violet, blue and yellowish orange. In un-doped ZnS, the UV emission originated from band to band transitions and the other visible emissions induced by defect states related transitions [43]. When Mn is incorporated into ZnS, Mn2+ ions behaves as luminescence centers and make a strong inter-relation with s - p states of the host lattice [44]. Thus, Mn2+ substitution induces a strong characteristic emission along yellow to orange emission [15] as shown in Fig. 7. The strong and prominent band appeared at 587 nm (corresponding to orange emission) in Mn-doped ZnS (which is absent in ZnS) is associated with the electronic transition from 4T1 to 6A1 in the 3d5 intra-configurational of Mn2+ ions [45, 46]. The identical consequences have been described by Kole et al. [47] and Karar et al. [48] in Mn doped ZnS.
The elevated blue emission at 486 nm is the feature of Mn, Cu dual doping which arises from the transition of the electrons from the (surface states) conduction band of ZnS to the ‘t2’ levels of Cu impurities [48]. It is understood from Fig. 7 that the electrons are excited from ground state lower energy state to the conduction band and the electrons may relax to the defect states, from which they recombine through the d-orbital of Cu2+ ions or be transferred into the electronic levels of Mn2+ ions that guide to the characteristic Cu and Mn dopant emissions [20]. The acquired high intensity yellowish-orange emission in Mn/Cu-doped ZnS emphasizes their significance in the application of light emitting diodes and photonic applications.