Mn-Doped ZnO Nanoparticles Prepared by Solution Combustion Method

In this paper, we have reported the structural, morphological and optical studies of pure and Mn-doped ZnO nanoparticles synthesized by the simple solution combustion method. The structural, morphological and optical studies are carried out by powder XRD, FE-SEM, HR-TEM, Raman, UV-vis absorption and PL spectra. The XRD pattern indicates that the prepared particles are in hexagonal wurtzite structure with the crystalline size of around 30-60 nm. The FE-SEM and HR-TM images are coincide with each other for aggregation of particles nature. The elemental analyses of pure and doped samples are evaluated by EDX. Raman spectra of the samples show remarkable result in the polar and non-polar branches. Room temperature PL shows the near band edge related emission and the results are related several intrinsic defects in the Mn-doped ZnO nanoparticles. Blue shift of the UV emission has occurred in all the doped samples attributable to Burstein-Moss shift. [DOI: 10.1380/ejssnt.2014.283]


I. INTRODUCTION
Zinc Oxide (ZnO) nanostructures are the topic of great interest due to their unique physical, chemical and biological properties with wide bandgap (3.37 eV) and large exciton binding energy (60 meV). The ZnO is a very promising II-VI compound semiconducting material for a wide range of application in optoelectronics and solar cells. Among the II-VI semiconductors, although Cadmium Selenide and Cadmium Telluride have shown to be promising in solar cell applications, they are toxic and harmful to the environment. With an increasing awareness of green and clean energy, ZnO based nanostructures are more suitable candidates for cost-effective and environmentally friendly energy conversion devices [1][2][3][4][5][6][7]. The ZnO nanoparticles (NP) can be synthesized by various approaches including chemical precipitation, ball milling, sol-gel processing, microwave-assisted synthesis, organometallic synthesis, spray pyrolysis, hydrothermal method and mechanochemical synthesis [8][9][10][11][12][13][14]. Physical properties of ZnO NP can be tailored by controlling the dopants and doping concentration [15]. Doping of transition metal (TM) ions in the ZnO has lead to enhancement of bandgap, optical, electrical and magnetic properties. The TM ions have several advantages as a dopant for ZnO that makes easy to incorporate into ZnO crystal structures and induced the magnetic as well as optical properties [16,17]. Especially, the study of the effect of dopants on optical properties of ZnO based nanomaterials is very important for photonic applications. The optical * This paper was presented at the 12th International Conference on Atomically Controlled Surfaces, Interfaces and Nanostructures   properties of ZnO can be improved by incorporating various ions in the crystal lattice.
In this work, we reported the structural, morphological and optical studies of pure and Mn-doped ZnO NP synthesized by the simple solution combustion method. In this method, a mixer of ethanol and ethylene glycol (EG) are used as the solvent, zinc acetate dihydrate as the zinc source, oxygen gas in the atmosphere as the oxygen source and manganese(II) chloride as the doping agent as well as manganese source. The combustion reaction was carried out in air under ambient pressure and ZnO NP (both pure and doped) was obtained in one step. Therefore, the synthetic approaches provide a simple, cost-effective, easy, and convenient route to obtain large quantities of ZnO NP.

II. EXPERIMENTAL
The synthesis method is similar to our previous report [18], 0.05 M and 0.1 M of zinc acetate dihydrate is dissolved in 100 ml of mixed solvent of ethanol and EG with the volume ratio of 60/40 ml, respectively. Then, different millimole of (1, 2, 5 and 10) manganese(II) chloride is introduced into the above solution under constant magnetic stirring. The solution is transferred into a spirit lamp with an absorbent cotton lamp wick and then the spirit lamp is fired. After the lamp wick is extinguished, the samples are repeatedly dispersed into distilled water to wash and remove the impurity by ultrasonic process. Finally, the sample is dried in hot air oven.
Powder X-ray diffraction (XRD) of the samples are carried out by a Rigaku-rint 2100 with Cu-Kα radiation (λ = 0.1540598 nm) at the scanning rate of 0.02 • /sec from 20 • to 70 • . The field emission-scanning electron microscopy (FE-SEM) and energy dispersive Xray spectrometry (EDX) images are analyzed through Quanta-200F SEM with AMETEK EDX. High resolution-   structure. The XRD peak broadening is produced by both size and strain which are compared from the bulk ZnO. Hence the full-width at half-maximum (FWHM) of XRD peaks have been deconvoluted by pseudo Voigt curve fitting. The Lorentzian part of the peak is used to estimate crystalline size and the Gaussian part of the peak is used to find the strain in the nanocrystals [18]. The crystalline sizes have been calculated by Scherrer equation and listed in Table I  The presence of some bigger particles might be attributed to the aggregation of smaller particles. The particle morphology changes significantly with respect to the incorporation of doping agent. Figure 4(a) shows the TEM images of pure ZnO NP for 0.1 M of zinc precursor. It clearly indicates that the particles are aggregating or overlapping of smaller particles to each other. The selected area electron diffraction (SAED) of pure ZnO NP is shown in Fig. 4(b). The diffraction rings indicates that the prepared samples are polycrystalline nature of ZnO.  Table II. The EDX spectra of pure ZnO NP have Zn and O elements. Also, we found week amount of Carbon element due to the environment and experimental setup. The atomic concentration of Mn ions in 10 mM of Mn-doped ZnO NP (0.05 M) sample was 9.6% whereas the Mn ions has been reduced as 4.9% when increase the precursor concentration as 0.1 M for the same amount of doping concentration. The stoichiometric ratio is comparatively good in the Mn-doped samples. Also, it clearly indicates that the Mn ions incorporated into Zn lattice. The XRD and EDX are in good agreement with each other.
The Raman spectra are more sensitive to crystallization, structural disorder and defects in micro and nanostructures. The vibration properties of the hexagonal shaped ZnO NP were investigated by Raman scattering  [19,20]. The Raman spectra of pure and Mn-doped ZnO NP are presented in Fig. 6 [21]. The broad peak at about 1145 cm −1 should belong to the multi phonon process [22]. The doping of Mn ions in ZnO matrix has a remarkable effect on the polar and nonpolar branches. The high frequency branch of E 2H mode involves oxygen motion as well as sensitive to internal stress and is characteristic of wurtzite structure of ZnO nanostructures [23][24][25]. The mode has a drastic reduction in the intensity of all doped samples. It might be due to the breakdown of translational crystal symmetry by the incorporated defects and impurity. The A 1 (TO) and A 1 (LO)/E 1 (LO) polar branches are appeared at about 380 and 570 cm −1 , respectively for the doped samples. On the incorporation of Mn ions in ZnO, the A 1 (LO)/E 1 (LO) peak has been broadened and shifted towards lower energy. Such a shift and broadening in the A 1 (LO)/E 1 (LO) phonon mode can be attributed to scattering contribution of the A 1 (LO)/E 1 (LO) branch outside the BZ center [26] and this phonon mode is commonly assigned to the defect complexes containing oxygen vacancy (V O ) and zinc interstitial (Zn i ) in ZnO. A characteristic broad peak at about 665 cm −1 was obtained for Mn-doped ZnO samples. It might be exhibiting the Hausmannite complex oxide of Manganese (Mn 2+ Mn 3+ 2 O 4 ) [27] to the incorporation of ZnO matrix or it might be related to intrinsic host lattice defects, which become activated as vibrating complex [28]. These host lattice defects in ZnO are activated and amplified, which will soften the related phonon mode by reducing the force constant of atom vibration. The broad Raman peak between 1050 and 1180 cm −1 has been presented in all the doped samples and it shifts to the lower frequency side compared with the pure ZnO NP.
The optical properties of the prepared samples were studied by UV-vis absorption and PL spectroscopies. The UV-vis absorption spectra of the pure and Mn-doped ZnO NP are presented in Fig. 7  The Mn-doped ZnO NP shows the blue shift of the NBE (obtained in UV region) for all the samples due to Burstein-Moss effect. According to the Burstein-Moss shift at metal doped ZnO, the Fermi level shifts into the conduction band (CB). It exhibits the absorption band transition from the valence band (VB) to the Fermi level in the CB instead of from the VB to the bottom of the CB [29]. Hence, the changes of transition levels lead to the energy gap broadening and result in the blue shift of UV emission. Also, we expect the blue shift of Mn-doped ZnO samples due to band gap of MnO (4.2 eV) [30].
The blue (430, 460 and 487 nm) and green (511, 535, 567 and 595 nm) emission are presented on the broad PL spectrum. The strong blue emission in the ZnO NP is usually attributed by two defects levels, either transition from Zn i to VB or transition from the bottom of the CB to O i level [31]. The other week blue emission which is around 460 nm (2.69 eV) can be assigned to the energy of transition of electron from Zn i to V Zn [32]. The blue-green emission of 487 nm (2.55 eV) is possible due to surface defects in the ZnO Nanocrystals. It can be attributed to the transition between V O to O i [33]. The green emission also represents the transition of photogenerated electron from the CB edge to a trap level [34]. The V O on the surface has been assumed to be the most likely candidate for the recombination centers. The 535 nm (2.32 eV) emission peak corresponds to the singly ionized V O in ZnO. The green emission peaks 567 nm is originated mainly from O Zn defects [35,36]. The weak green band (595 nm) emission corresponds to the singly ionized oxygen vacancy and this emission results from the recombination of a photogenerated hole with the single ionized charge state of specific defects.

IV. CONCLUSIONS
In summary, we have studied the structural, morphological and optical properties of pure and various amounts Mn-doped ZnO NP synthesized by simple solution combustion method. The XRD pattern indicates that the prepared particles are in hexagonal wurtzite structure with the average particles size of around 30-60 nm. Raman scattering of ZnO NP has first and second orders optical modes which are the characteristic bonds of the wurtzite ZnO. The XRD, EDX and Raman results are correlating to each other for both pure and Mn-doped ZnO NP. The PL results are related to several intrinsic defects such as V O , V Zn , O i and etc. in the Mn-doped ZnO NP. The blue shift of the UV emission has been occurred in all the doped samples due to Burstein-Moss effect.