Band Gap Tailoring, Size Modulation and Photoluminescence Properties of Mn/Cu doped ZnS Nanostructures

ZnS, Mn added ZnS (Zn 0.97 Mn 0.03 S) and Mn, Cu dual doped ZnS (Zn 0.95 Mn 0.03 Cu 0.02 S) QDs have been prepared using co-precipitation technique. The inuence of Mn and Cu addition on the morphology, structure and photoluminescence properties of Mn/Cu incorporated ZnS have been examined. Cubic structure of the synthesized samples was conrmed by X-ray diffraction patterns. The incorporation of Cu in Zn-Mn-S lattice not only decreased the particle/grain size and also generates more defect based luminescent activation centres. The reduced energy gap by Mn addition was explained by sp-d exchange interaction and the elevated energy gap in Cu, Mn dual doped ZnS was expalined by Burstein–Moss effect. The tuning phenomenon of size as well as the energy gap in ZnS by Mn/Cu addition promote these materials for nano-electronic applications. FTIR spectra conrmed the presence of Mn/Cr-Zn-S bondings. The substitution of Mn /Cu provides an effective control over tuning of different emission colours which signies their applications like light emitting diodes.


Introduction
ZnS is one of the most signi cant and extensively considered metal sulphides in the last several years due to its enhanced optical nature, broad energy gap and better luminescence behaviour at higher wavelength region [1,2]. The two common structures of ZnS are wurtzite as well as zinc-blende and they have the band gap between 3.72 eV to 3.77 eV [3,4]. The unique properties aroused from its size and structures leads them for diverse applications such as luminescence devices, photo-voltaics, ultraviolet LEDs, e cient solar cells and different lasers [5].
The addition of transition metal (TM) ions made an extraordinary variation in optical, structural, electrical and transition probabilities in the Zn-S lattice [6,7]. Within the different TMs, Mn was chosen as the rst doping material since it improves the emission properties, enhances the electrical, microstructural and optical characters and also possesses better photo and thermal stability [8]. Chandrasekar et al. [9] discussed the variation in Urbach energy, Stokes's shift and steepness values in Mn added ZnS synthesized by co-precipitation technique. Ashkarran et al. [10] revealed the decrease of photo-catalytic removal e ciency with the help of Bromocresol Green, Rhodamine B, and Bromochlorophenol Blue dyes in Mn added ZnS than un-doped ZnS. In order to avoid the impurity and secondary phase generation, Mn level is restricted as 3%.
It is noticed from the literature that the addition of Cu in ZnS induces the visible luminescence involving blue and green wavelengths whereas the addition of Mn ne-tune the emissions between blue and orange [11]. The simultaneous addition of Mn and Cu is useful to attain the dual emissions [12]. Moreover, the dual doping of Mn and Cu into ZnS enhanced its structural and optical properties by its unusual recombination patterns. Since the luminescence nature of ZnS is signi cantly modi ed by adjusting the small level of TM ions [13], the Mn and Cu addition into ZnS plays considerable role in the ne-tuning of emission properties in a well-controlled way.
The optical and structural evaluation of Mn or Cu doped ZnS [9][10][11] is available in the literature but the complete studies on Mn and Cu dual doped ZnS is nearly scanty. Therefore, Mn and Cu dual doped ZnS have been synthesized using co-precipitation method. In addition, the comprehensive structural and photoluminescence behaviour of Mn and Cu dual doped ZnS have been examined using the appropriate instruments. The deviation in size as a result of Mn/Cu doping was interconnected with the band gap of the materials. The effect of Mn as well as Cu on ZnS and the in uence of defect sites in the photoluminescence properties have been discussed in detail. Aqueous ammonia solution was used to change the pH of the solution. All the chemicals used in the present were in analytical grade (AR) with 99.99% purity purchased from M/s. Merck. Ultra pure deionized water was used as solvent throughout the synthesis process.

Experimental
For the preparation of Zn 0.95 Cu 0.02 Mn 0.03 S, 0.95M zinc acetate, 0.03 M manganese acetate 0.02M copper acetate were gradually dissolved (one by one) in 50 ml double distilled water and kept at constant stirring ( at rate of 1000 rpm) at 80°C until a clear and homogeneous solution is obtained. Then, 50 ml 1 M sodium sul de solution was added drop by drop to the above mixture solution under continuous stirring for 6 h. Aqueous ammonia solution was used to maintain the pH Value of the mixer solution as 9.5. After 6 h, a light gray to dark gray precipitate was deposited at the bottom of the ask. The wet precipitate was ltered out separately and washed with de-ionized water and methanol to remove unwanted impurities formed during the preparation process. Finally, obtained product was dried using micro-oven at 50-80 ̊ C for 5h. The nal precipitates were grounded using an agate mortar. The ow chart for the preparation of Mn, Cu doped ZnS is as shown in Fig. 1.
For the preparation of pure ZnS, 1M zinc acetate alone used without manganese acetate and copper acetate. Similarly, for Zn 0.97 Mn 0.03 S preparation, 0.97M zinc acetate and 0.03M manganese acetate were used without copper acetate. 0.95M zinc acetate, 0.03M manganese acetate and 0.02M copper acetate were used for the preparation of Zn 0.95 Mn 0.03 Cu 0.02 S nanostructures. The preparation procedure of the samples is same as discussed above.

Characterization techniques
The diffracted patterns were recorded by RigaKuC/max-2500 diffractometer using CuKa radiation (λ = 1.5406 Å) at 40 kV and 30 mA from 2θ = 20 to 70°. The UV-visible optical absorption and transmittance studies of ZnS, Zn 0.97 Mn 0.03 S and Zn 0.95 Mn 0.03 Cu 0.02 S nanoparticles were carried out to investigate their optical properties using UV-visible spectrometer (Model: lambda 35, Make: Perkin Elmer) from 300 to 550 nm at ambient temperature. Halogen and deuterium lamps were used as sources for visible and UV radiations, respectively. The presence of chemical bonding was studied by Fourier transform infra-red (FTIR) spectrometer (Model: Perkin Elmer, Make: Spectrum RXI) from 400 to 4000 cm − 1 . Pellets were prepared by mixing the nanoparticles with KBr at 1 wt%. Room-temperature photoluminescence (PL) behaviour have been taken out by a uorescence spectrophotometer (Model: Hitachi, Make: F-2500) at excitation wavelength of 330 nm to study the emission properties of the materials.

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 re ected 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 re ects 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 con rm 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 signi es that Cu is appropriately substituted into Zn 0.97 Mn 0.03 S lattice. The identical shift along higher diffraction angles was reported in Ni added ZnS nanostrucutres by Kaur et al. [15]   Mn and Cu dual doping. The slight increase of size in Mn-doped ZnS is due to replacement of Mn 2+ ions (ionic radius ∼ 0.80 Å) in the position of Zn 2+ (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 Cu 2+ ions (≈0.71 Å) is smaller than the ionic radius of Zn 2+ ions as well as Mn 2+ ions. The decrease of size was discussed by Reddy et al. [19] at Zn 0.97−x Cu x Cr 0.03 S 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 Mn 2+ and Cu 2+ ions were not homogeneously dispersed in Zn-S nanostructures but the core portion includes more Mn 2+ ions and the surface portion contains much Cu 2+ ions. It is concluded from the XRD analysis that the incorporation of Cu 2+ ions weaken the size as well as produce the defect associated luminescence states.

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 (Zn 0.97 Mn 0.03 S). The elevated absorption intensity by Mn addition may be due to the increase of size and also the proper incorporation of Mn 2+ ions in the position of Zn 2+ ions where the ionic radius of Mn 2+ ions is higher than Zn 2+ 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] con rmed 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 Cu 2+ , Mn 2+ and Zn 2+ ions which may also the probable reason for the reduction in absorption intensity [25]. The light scattering by grain boundaries and the re ection of incident light by Cu clusters in Zn 0.95 Mn 0.03 Cu 0.02 S may also reduce the absorption intensity [19]. 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%.

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 in uence of Mn/ Cu through band structure as well as electronic transition, the correlation among the absorption coe cient and the photon energy was examined using Tauc's relation [26], hυ = A(hυ -E g ) n , where, E g 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 [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, Zn 0.97 Mn 0.03 S and Zn 0.95 Mn 0.03 Cu 0.02 O 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 con nement effect of the nanoparticles [32].

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, Zn 0.97 Mn 0.03 S and Zn 0.95 Mn 0.03 Cu 0.02 S 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 CO 2 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  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]. Mn is incorporated into ZnS, Mn 2+ ions behaves as luminescence centers and make a strong inter-relation with s -p states of the host lattice [44]. Thus, Mn 2+ 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 4 T 1 to 6 A 1 in the 3d 5 intra-con gurational of Mn 2+ 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 't 2 ' 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 Cu 2+ ions or be transferred into the electronic levels of Mn 2+ 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 signi cance in the application of light emitting diodes and photonic applications. Cubic structure of the synthesized samples was con rmed by X-ray diffraction patterns.

Conclusions
The incorporation of Cu in Zn-Mn-S lattice not only decreased the particle/grain size and also generates more defect based luminescent activation centres. The tuning phenomenon of size as well as the energy gap in ZnS by Mn/Cu addition promote these materials for nano-electronic applications. FTIR spectra con rmed the presence of Mn/Cr-Zn-S bondings.
The substitution of Mn /Cu provides an effective control over tuning of different emission colours which signi es their applications like light emitting diodes.

Declarations
Due to technical limitations, Declarations section is not available for this version.