Materials Science in Semiconductor Processing 121 (2021) 105395 Contents lists available at ScienceDirect Materials Science in Semiconductor Processing journal homepage: http://www.elsevier.com/locate/mssp Tailoring the optical and magnetic properties of ZnS nanoparticles via 3d and 4f elements co-doping B. Poornaprakash a, 1, U. Chalapathi b, Mirgender Kumar a, 1, S.V. Prabhakar Vattikuti c, Beerelli Rajitha d, e, P.T. Poojitha f, Si-Hyun Park a, * a Department of Electronic Engineering, Yeungnam University, Gyeongsan, 38541, South Korea Center for Opto-electronic Materials and Devices, Korea Institute of Science and Technology (KIST), Seongbuk-gu, Seoul, 02792, Republic of Korea School of Mechanical Engineering, Yeungnam University, Gyeongsan, 38541, South Korea d Department of Physics, Jawaharlal Nehru Technological University Ananthapur, Ananathapuramu, 515002, India e BVRIT Hyderabad College of Engineering for Women, Hyderabad, 500090, India f Department of Physics, Siddartha Educational Academy Group of Institutions, Tirupati, 517502, India b c A R T I C L E I N F O A B S T R A C T Keywords: Zinc sulfide Nanoparticles Co-doping Optoelectronic Spintronics Impurity free cobalt 3d and erbium 4f co-doped ZnS nanoparticles (NPs) were synthesized using a chemical reflux technique. The as-synthesized NPs exhibited high crystallinity and a narrow particle-size distribution. X-ray diffraction and optical analysis showed that cobalt (II) and erbium (III) ions substituted the zinc (II) ions in tetrahedral sites. The tunable bandgap for ZnS NPs was attained through mono- and co-doping. The cobalt and erbium co-doped NPs displayed a robust magnetization with an enhanced coercivity at room temperature compared with the cobalt-doped ZnS NPs; this was due to the exchange interaction between the cobalt (II) electrons as well as the localized carriers incited through the erbium (III) co-doping. The sense captured with the regulating bandgap and ferromagnetic property in a single substance opens a new platform for advanced applications in optoelectronics and spintronic devices. 1. Introduction Spintronics has attracted ample focus because of the possibility of finding a novel epoch in information technology [1,2]. In particular, magnetic-ion-doped semiconductor nanostructures have attracted interest owing to their suitability for advanced applications in magneto-optical devices [3,4]. Zinc sulfide (ZnS) is an interesting and promising semiconductor compound, which has a wide bandgap (3.67 eV) and attractive optical properties [5,6]. In the nano form particularly, it has been recognized as an eminent semiconductor host for bracing ferromagnetism at room temperature after doping with suitable 3d or 4f elements [7,8]. However, most of the reported coercivity and magnetization in ZnS-based systems is low and frail [9,10]; hence, it is crucial to enhance the ferromagnetic properties to investigate the definitive storage applications with tunable coercivity, as well as the saturation magnetization. It is well known that the co-doping of double-transition metals (TMs) [11–13] can enrich the saturation magnetization in zinc-chalcogenide systems. In contrast with transition metals, lanthanides may render robust magnetism because of the higher 4f orbital magnetic moments [14]. A few attempts have been made on the two transition metal co-doped ZnS nanostructures, and improved room temperature ferromagnetic properties were obtained [11]. In our previous study [15], we successfully enhanced the ferromagnetism in (Co + Sm) co-doped ZnS nanoparticles compared to the mono-doped ZnS nanoparticles. However, several researchers [16–19] have attained controllable saturation magnetization and coercivity in ZnO systems through transition and rare earth metal co-doping. It is believed that the ZnO host is similar to the ZnS host in many aspects. These results inspired us to strive to synthesize transition and rare earth metal mono-doped and co-doped ZnS quantum dots. The miscibility of lanthanides in ZnS is hardly restricted on the lattice distortion related to bigger ionic radii of lanthanides. To overcome this issue, we chose the solvothermal route for the current study. In this study, we experimentally achieve tunable optical and magnetic properties of ZnS NPs through cobalt and erbium mono-doping and co-doping. The sense captured with the regulating optical and magnetic * Corresponding author. E-mail address: sihyun_park@ynu.ac.kr (S.-H. Park). 1 First and third authors are equally contributed. https://doi.org/10.1016/j.mssp.2020.105395 Received 20 November 2019; Received in revised form 10 August 2020; Accepted 13 August 2020 Available online 28 August 2020 1369-8001/© 2020 Elsevier Ltd. All rights reserved. B. Poornaprakash et al. Materials Science in Semiconductor Processing 121 (2021) 105395 Fig. 1. Low resolution TEM images and size distribution histogram (inset) of (a) pristine, (b) cobalt-doped, (c) erbium-doped, and (d) cobalt and erbium co-doped ZnS NPs. properties of a single substance opens a new pathway for advanced applications in optoelectronics and spintronic devices. small amendment, i.e., only a zinc solution for pristine ZnS, only a cobalt solution for cobalt doping, and only an erbium solution for erbium doping were used in the first step; the remaining procedure was same as that of cobalt and erbium co-doping. The characterization methods employed, including high-resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy (STEM), X-ray diffraction (XRD), Raman spectrometry, diffuse reflectance spectrometry (DRS), X-ray-photoelectron spectroscopy (XPS), and vibrating sample magnetometer (VSM), are the same as those employed in previous studies [9,15,17]. 2. Experimental details Cobalt (3 at%) and erbium (3 at%) mono-doped and cobalt (3 at%) + erbium (3 at%) co-doped ZnS NPs were successfully fabricated using the co-precipitation method at 40 ◦ C for 5 h. The preparation procedure for cobalt (3 at%) + erbium (3 at%) co-doped ZnS NPs was as follows: 0.2 M of zinc acetate and 3 at% of cobalt acetate and erbium chloride were dissolved in 50 ml of deionized water and magnetically stirred for 1 h at 50 ◦ C. Then, 0.2 M of sodium sulfide solution (50 ml) was mixed dropwise to the aforementioned suspension under magnetic stirring. Finally, 0.1 g of cetyltrimethylammonium bromide (CTAB) was added to the suspension as a surfactant and continuously stirred for 5 h at 50 ◦ C. The as-fabricated compounds were centrifuged (8000 rpm for 10 min) several times with ethanol and water. Finally, the precipitates were dried at 100 ◦ C for 10 h an oven. Pristine ZnS, and cobalt and erbium mono-doped ZnS NPs were prepared in the same manner, but with a 3. Results and discussion Typical low-resolution TEM images of pristine, cobalt-doped, erbium-doped, and co-doped ZnS NPs are depicted in Fig. 1 (a) and (d), which show the nearly polydispersed (agglomeration) NPs with narrow size distributions. It is well known that, nanoparticles possess a higher relative surface area and higher relative number of surface atoms. These atoms have unsaturated coordinations and each atom has vacant 2 B. Poornaprakash et al. Materials Science in Semiconductor Processing 121 (2021) 105395 Fig. 2. High resolution TEM images and SAED patterns (inset) of (a) pristine, (b) cobalt-doped, (c) erbium-doped, and (d) cobalt and erbium co-doped ZnS NPs. coordinate sites. They try to make bonds and such bonds tend to form between adjacent particles, this causes the agglomeration. In case of very small particles (~5 nm) it may be difficult to see separate particles. The cobalt and erbium mono- and co-doping do not change the morphology of the pristine samples. However, the size was varied with the mono- and co-doping. The sizes of the synthesized NPs were assessed through image j software and are 4.2, 3.1, 5.8, and 4.4 nm for pristine, cobalt-doped, erbium-doped, and co-doped NPs, respectively. Fig. 2 (a) – (d) shows the HRTEM images and the SAED patterns (inset) of pristine, cobalt, and erbium mono-doped and co-doped ZnS NPs. These images reveal the high crystallinity and the distinct lattice fringes, where 0.326 (pristine), 0.325 (cobalt doped), 0.327 (erbium doped), and 0.326 nm (co-doped) corresponds well with those of the d-spacing of the (111) plane of ZnS (zinc blende). The SAED rings could be assigned ideally to similar crests positions to the (111), (220), and (311) planes of zinc blende zinc sulfide. No rings corresponding to foreign phases were recognized, indicating that the major structure of the synthesized NPs is zinc blende ZnS. Fig. 3 (a) displays the STEM image of cobalt and erbium co-doped ZnS NPs. It is known that cobalt and erbium are well co-doped in the ZnS NPs. The zinc, sulfur, cobalt, and erbium K-edge mappings are illustrated in Fig. 3 (b)–(e), respectively. It can be seen that the yellow color (assigned to Zn), red color (assigned to S), orange color (assigned to Co), and green color (assigned to Er) are randomly distributed in the synthesized sample, which confirms the formation of uniform NPs. Fig. 3 (f) shows the EDAX spectrum of cobalt and erbium co-doped ZnS NPs, exhibiting Zn, S, Co, and Er signals. The chemical composition of cobalt and erbium co-doped ZnS was found to be Zn: S: Co: Er = 47.18: 47.23: 2.48: 3.11, which is nearer to the initial value. Fig. 4 exhibits the XRD patterns of pristine, cobalt, and erbium monodoped and co-doped ZnS QDs. All the NPs display a routine zinblende ZnS phase (JCPDS No. 05–0566), exhibiting only (111), (200), and (311) 3 B. Poornaprakash et al. Materials Science in Semiconductor Processing 121 (2021) 105395 Fig. 3. (a) STEM image and the spatial distributions of the (b) Zn, (c) S, (d) Co, (e) Er and (f) EDAX spectrum of cobalt and erbium co-doped ZnS NPs. 4 B. Poornaprakash et al. Materials Science in Semiconductor Processing 121 (2021) 105395 Fig. 6. DRS spectra of (a) pristine, (b) cobalt-doped, (c) erbium-doped, and (d) cobalt and erbium co-doped ZnS NPs. Fig. 4. XRD patterns of (a) pristine, (b) cobalt-doped, (c) erbium-doped, and (d) cobalt and erbium co-doped ZnS NPs. the wavelength of the X-ray radiation, β is the full width at half maximum and θ is the angle of diffraction. The average size of the crystallite was found to be in the range of 3–6 nm. The Raman spectra of pristine, cobalt, erbium mono-doped, and codoped ZnS NPs in the spectral range from 200 to 400 cm−1 are presented in Fig. 5. All the synthesized NPs show two Raman modes at approximately 261 and 343 cm−1. Brafman et al. [20] noticed the E2 modes of wurtzite type ZnS at 72 and 286 cm−1 and the transverse optical (TO) and longitudinal optical (LO) modes of cubic ZnS at 276 and 351 cm−1, respectively. Hence, the absence of Raman modes at 72 and 286 cm−1 in the spectra of the synthesized QDs indicate the cubic phase of the synthesized NPs. Based on the selection rule for the zone center phonons corresponding to the zinc blende crystal, only TO is allowed for scattering by the (110) face, only LO mode is allowed for scattering by the (100) face, and the TO and LO modes are allowed for scattering by the (111) face. However, in the present sample in which the NPs are randomly oriented, all Raman peaks may be detectable. The Raman peaks noticed at approximately 261 and 343 cm−1 can be assigned to the TO and LO modes of the cubic ZnS [21]. The Raman modes of the synthesized samples moved towards the lower frequency side compared to the bulk ZnS values as a result of the quantum confinement effect. The absence of foreign phases, and the defect-related Raman modes, indicate the existence of a single cubic phase of the samples. Fig. 6 exhibits the DRS spectra of the pristine, cobalt, erbium monodoped, and co-doped ZnS NPs, in the spectral range from 200 nm to 800 cm−1, are presented in Fig. 5. As anticipated, the pristine ZnS NPs exhibited the characteristic spectrum of ZnS with its sharp absorption edge rising at 354 nm. The cobalt doped ZnS NPs shows the absorption edge at 397 nm and the intensity of the reflectance spectrum decreased compared to that of pristine ZnS NPs, indicating that the cobalt ions penetrated the crystal lattice occupied sites with a different local crystal field. Compared to the pristine ZnS NPs, the erbium doped ZnS NPs exhibited eight absorption bands at 378, 406, 442, 450, 487, 520, 541, and 652 nm, which indicates the existence of erbium (III) ions in the doped ZnS NPs. These bands could be ascribed to the 4f-4f transition of Er (III) ions from the ground state (4I15/2) to different excited states (4G11/2, (2G, 4F, 2H)9/2, 4F3/2, 4F5/2, 4F7/2, 2H11/2, 4S3/2, and 4F9/2) [22]. In co-doped ZnS NPs, it can be seen that, the intensity of the reflectance spectra smeared out, designated that additional cobalt or erbium ion penetrated into ZnS occupied sites with a different local crystal field [23]. The obtained red shift of the prepared samples may be due to the sp-d and sp-f exchange interaction between the cobalt, erbium, and ZnS host, and designates a decrease in optical bandgap of ZnS after mono-doping and co-doping. This is a very common phenomenon in transition metal and rare earth ion doped II–VI semiconductor compounds [15,17]. Fig. 7 depicts the Kubelk-Munk plots of the prepared Fig. 5. Raman spectra of (a) pristine, (b) cobalt-doped, (c) erbium-doped, and (d) cobalt and erbium co-doped ZnS NPs. diffraction peaks. No foreign phases related to cobalt or erbium-related oxides or clusters were found in the synthesized NPs within the instrument diagnosis limitation. The introduction of cobalt ions into the ZnS lattice causes the (111) peak to shift towards the higher-angle side, indicating lattice reduction due to the small ionic radius of cobalt (0.058 nm) compared to that of zinc (0.074 nm). In contrast, the (111) peak in erbium-doped ZnS shifts towards the lower diffraction angle in comparison with that of the pristine ZnS, indicating lattice expansion due to the bigger ionic radius of erbium (0.088 nm) compared to that of zinc (0.074 nm). The introduction of both cobalt and erbium ions in the ZnS crystal lattice causes the ZnS (111) peak to shift to the lower angle, causing a trivial expansion of the lattice. The lattice parameters of the prepared samples are calculated via the formula 1/d2 = 1/a2 (h2+ k2+ l2), where’d’ is the interplanar separation rendered by the miller indices, viz. the h, k and l values and ‘a’ is the lattice constant. The lattice estimated parameters are 5.39, 5.36, 5.43, and 5.40 Å for pristine, cobaltdoped, erbium-doped, and co-doped NPs, respectively. The average crystallite size was assessed by using Debye-Scherrer equation, D = kλ/ βcosθ, where D is the crystallite size, K is a constant taken to be 0.94, λ is 5 B. Poornaprakash et al. Materials Science in Semiconductor Processing 121 (2021) 105395 the Er 4d level; thus, the trivalent (+3) valence state of erbium in the NPs is confirmed [24]. The elemental composition is Zn: S: Co: Er = 46.66: 48.10: 2.33: 2.91, which is very close to the initial value. Moreover, we detected no impurities or foreign phases in the XPS spectrum, which implied the impurity free nature of the synthesized samples. Fig. 9 illustrates the M-H curves for pristine, cobalt, erbium monodoped, and co-doped ZnS NPs. The pristine ZnS QDs displayed the anticipated diamagnetic feature, which is a typical phenomenon in pure ZnS systems [8,9,15] and is because of the lack of unpaired electrons. The cobalt-doped ZnS NPs showed well-defined ferromagnetic features, whereas the erbium-doped ZnS NPs displayed super paramagnetic features at room temperature. However, the cobalt and erbium co-doped ZnS displayed robust ferromagnetism at room temperature. It is believed that pristine ZnS can also exhibit ferromagnetism if it contains sufficient intrinsic defects. To confirm this, pristine ZnS was first measured by VSM and the results displayed only the diamagnetic feature. The saturation moments (Ms) for cobalt-doped and co-doped ZnS were 0.025 and 0.062 emu/g, respectively. The obtained ferromagnetism of cobalt-doped and co-doped ZnS NPs and super paramagnetic nature of the erbium doped-ZnS NPs did not form any other metal clusters or impurity phases as we did not detect any foreign phases or clusters in the XRD, Raman, and XPS data. Therefore, the ferromagnetism and superparamagnetism obtained in this study could be the intrinsic behavior of the NPs. Moreover, the saturation magnetizations of co-doped NPs were higher than those of the cobalt doped ZnS NPs. The coercivity (Hc) of cobalt-doped and co-doped ZnS NPs were 120 and 286 Oe, respectively. The co-doped NPs with 3d and 4f elements display a much higher coercivity than cobalt -doped ZnS NPs. In general, two fundamental theories have been developed since Néel published the basic study on the coercivity of magnetic substances [25]. The domain wall bowing theory [26] assumes the domain wall to be pliable in one dimension, and the potential theory [27] deals with rigid domain walls of a finite length. In the first case, the interaction between localized pinning centers and the domain wall are studied. In the second case, the coercive force is determined through spatial fluctuations of the defect content, and it Fig. 7. Kubelka-Munk plots of (a) pristine, (b) cobalt-doped, (c) erbium-doped, and (d) cobalt and erbium co-doped ZnS NPs. samples and the optical band gap values are 3.50, 3.12, 3.45, and 2.56 eV for pristine, cobalt-doped, erbium-doped, and co-doped ZnS QDs, respectively. The tunable optical properties may be more useful for optoelectronic applications. XPS was utilized to investigate the local structure of cobalt and erbium co-doped ZnS NPs. The typical XPS survey spectrum for cobalt and erbium co-doped ZnS NPs is exhibited in Fig. 8, in which only zinc, sulfur, cobalt, and erbium elements are observed. The binding energies of Co 2p3/2 and Co 2p1/2 are 779.4 and 795.53 eV, respectively, and the energy variation between these two crests is 16.13 eV. The spectra of the Co 2p core level of cobalt and erbium co-doped ZnS NPs exhibited only the Co2+ valence state in the samples. Moreover, the signals of CoO and Co3+ were undetectable within the resolution of the technique, excluding any potential metallic cobalt clusters existence, which was consistent with both the XRD results and the Raman experimental results. The peak at 168.8 eV in the spectrum indicates the existence of erbium in the synthesized NPs and this binding energy corresponds to Fig. 8. XPS survey scan and cobalt and erbium narrow scans (inset) of cobalt and erbium co-doped ZnS NPs. 6 B. Poornaprakash et al. Materials Science in Semiconductor Processing 121 (2021) 105395 Fig. 9. M-H curves of (a) pristine, (b) cobalt-doped, (c) erbium-doped, and (d) cobalt and erbium co-doped ZnS NPs. CRediT authorship contribution statement plays the role of the rigid domain wall pinning centers. The enhanced coercivity of the co-doped ZnS NPs may be related to both the stress anisotropy originating from the defects, and the domain wall pinning effects. The intrinsic defects induce energy levels within the ZnS band gap, which probably amalgamate with the electrons located at the erbium 4f and cobalt 3d shells, hence provoking the rise in magnetic orderings with higher reversal field energies. In addition, the lattice distortion in ZnS because of the different ionic radii of erbium and zinc can result a substantial strain, which forms the pinning centers, hence hindering the rotation of the magnetization. The enhanced magnetization and coercivity make the cobalt and erbium co-doped ZnS NPs a desirable candidate for practical spintronic applications. B. Poornaprakash: Conceptualization, Methodology, Writing original draft. U. Chalapathi: Investigation. Mirgender Kumar: Methodology, Investigation. S.V. Prabhakar Vattikuti: Investigation. Beerelli Rajitha: Investigation. P.T. Poojitha: Investigation. Si-Hyun Park: Writing - review & editing, Supervision, Project administration, Funding acquisition. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 4. Conclusions Acknowledgements In conclusion, our experimental results confirmed that the optical and magnetic property modulation in cluster-free cobalt and erbium codoped ZnS NPs can be attained via co-doping, which causes an exchange interaction between cobalt 3d electrons as well as the localized carriers induced through erbium (III) ion co-doping. Hence, these results confirmed that the capabilities of the cobalt (3d) and erbium (4f) element co-doping method for tailoring both optical and ferromagnetic properties in ZnS NPs for optoelectronic and spintronic applications. This work was supported by the National Research Foundation of Korea funded by the Ministry of Science. (NRF-20201G1A1014959) and this work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2019R1A2C1089080). 7 B. Poornaprakash et al. Materials Science in Semiconductor Processing 121 (2021) 105395 References [14] M. Subramanian, P. Thakur, M. Tanemura, T. Hihara, V. Ganesan, T. Soga, K. H. Chae, R. Jayavel, T. Jimbo, Intrinsic ferromagnetism and magnetic anisotropy in Gd-doped ZnO thin films synthesized by pulsed spray pyrolysis method, J. Appl. Phys. 108 (2010) 053904. [15] B. Poornaprakash, P.T. Poojitha, U. Chalapathi, K. Subramanyam, Si-Hyun Park, Synthesis, structural, optical, and magnetic properties of Co doped, Sm doped and Co+ Sm co-doped ZnS nanoparticles, Physica 83 (2016) 180. [16] J.J. Lee, G.Z. Xing, J.B. Yi, T. Chen, M. Ionescu, S. 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