- Email: [email protected]
Effect of Mn doping on microstructural and optical behavior of Zn(1−x)MnxO nanorod by simple auto-combustion method
Accepted Manuscript Effect of Mn doping on microstructural and optical behaviour of Zn(1-x)MnxO nanorod by simple auto-combustion method H.S. Maharana, S. Yadav, A.P. Chakraverty, G.D. Verma PII: DOI: Reference:
S0749-6036(15)00060-9 http://dx.doi.org/10.1016/j.spmi.2015.01.025 YSPMI 3583
To appear in:
Superlattices and Microstructures
Received Date: Accepted Date:
8 January 2015 10 January 2015
Please cite this article as: H.S. Maharana, S. Yadav, A.P. Chakraverty, G.D. Verma, Effect of Mn doping on microstructural and optical behaviour of Zn(1-x)MnxO nanorod by simple auto-combustion method, Superlattices and Microstructures (2015), doi: http://dx.doi.org/10.1016/j.spmi.2015.01.025
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of Mn doping on microstructural and optical behaviour of Zn(1-x)MnxO nanorod by simple auto-combustion method H. S. Maharana1, 2*, S. Yadav1, 2, A. P. Chakraverty3, G. D. Verma1, 2 1
Centre of Nanotechnology, Indian Institute of Technology Roorkee, Roorkee, Uttarakhand 247667, India 2 Department of Physics, Indian Institute of Technology Roorkee, Roorkee, Uttarakhand 247667, India 3 Department of Metallurgical and Materials Engineering, National Institute of Technology, Rourkela 769008, Odisha, India
Abstract Mn doped ZnO nanorods with different doping concentrations (0, 0.02, 0.05, 0.1, 0.15, and 0.2) were synthesized by simple auto-combustion method. The structural, morphological and optical behaviors of Zn(1-x)MnxO were investigated by X-ray diffraction (XRD), field-emission microscopy (FE-SEM) and diffuse reflectance spectroscopy (DRS). The structural properties of un-doped and Mn doped ZnO nanorods exhibit hexagonal wurtzite structure with no secondary phase. From scanning electron micrographs results it can be evidenced that, both crystallite size and surface morphology are significantly affected by Mn doping concentrations. From the optical studies, the reflectance in UV region of doped samples was decreased in comparison to undoped one and the optical band gap varies between 3.33 and 2.81 eV. Photoluminescence spectra shows large emission band originated at 388nm and a weak band centered on 495nm. The present study reveals about the influence of Mn2+ ions on morphological and optical properties of Zn(1-x)MnxO.
Keywords: Dilute magnetic semiconductor, Auto-combustion, ZnO, Nanorod * Author for correspondence. Email: [email protected]
1
1. Introduction Dilute magnetic semiconductors (DMSs) based on zinc oxide (ZnO) doped with transition metals, such as Ni, Co, and Mn has been attracted to experimental and theoretical researchers in recent years, since the predicted room temperature ferromagnetism in DMS can be useful in various applications, such as spintronics, light-emitting diodes (LEDs) and photodiodes [1-7]. Direct band gap ZnO semiconductor has received considerable attention as a potential material for optical devices due to its well-known wide band gap energy of 3.37 eV and large exciton binding energy of 60 meV [8-10]. ZnO has also potentially applicable for LEDs, laser diodes (LDs) and ultraviolet (UV) detection devices [10-12]. In recent years exploring one-dimensional DMS materials are of great interest because nanorods, nanowires (NWs) and nanobelts are ideal materials for fabricating electronic devices such as field effect transistors, sensors, optoelectronic devices, logic circuits, and lasers [13-17]. There are various approaches for developing DMS nanorods and NWs. One method is doping the as-synthesized NWs by ion implantation [18]. This is basically a post growth doping via physical method, which generally requires post-annealing to reduce the defects created by ion implantation. Another approach is the incorporation of doping elements in the precursors during growth, which is a methodology for getting uniformly distributed doping element in the matrix [19, 20]. Synthesizing of nanorod or nanowire by the second approach, an important question is if the doping element is incorporated at the surface or into the volume lattice. Establishing an effective approach for synthesis of volume-doped DMS nanorods or NWs is a fundamental importance for investigating spin transport in one dimensional nanomaterials and fabrication of devices. This present communication reports the structural, morphological and optical properties of Zn1-xMnxO (0 < x < 0.20) nanorod/nanowire prepared by simple auto-combustion method.
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2. Experimental 2.1 Synthesis Undoped and Mn doped ZnO nanorods/nanowire were synthesized by auto-combustion method from the nitric acid solution of Zn and Mn was taken in the appropriate ratio using glycine as the fuel. Zn and Mn metal powders were taken in stoichiometric amounts and dissolved separately in 4N HNO3 and then mixed together. To the above mixed metal nitrate solution, 2 mole of glycine in the form of aqueous solution per mole metal powder was added. The mixed solution was kept over a hot plate for auto-combustion at 473 K for 4 hours. After the complete evaporation of water, a reddish colored thick gel was formed, which was subsequently burnt to give a final brown colored oxide product. Then, the obtained powder samples were characterized by different characterization techniques for further analysis. 2.2 Characterizations X-ray diffraction patterns of synthesized powder was recorded on a Bruker X-8 X-ray diffractometer using Cu Kα line (1.5418Å) with the X-ray source at a speed of 2 degree per minute in a 2θrange between 20 and 70 degrees to examine the phase formation and calculating the crystallite sizes. The surface morphology and the elemental analysis of all samples were analysed by QUANTA-200F field emission scanning electron microscope having maximum accelerating voltage of 30 kV combined with an EDXA accessory equipped with a CCD (charge coupled device) camera. Diffuse reflectance is an excellent sampling tool for powdered materials in the mid-IR and near infrared ranges. It can also be used for the analysis of intractable solid samples. A Shimadzu UV-3600 UV-Vis NIR spectrometer was used and range studied was from 200 to 800 nm. The samples to be analysed by diffuse reflectance were generally mixed with barium sulphate (1:100 wt. ratios) prior to analysis. Photoluminescence measurements were 3
carried out by a Shimadzu RF-5301 PC spectrometer with excitation wavelength 315 nm. The emission spectra were recorded from 325 to 800 nm. The samples were prepared by adding 10mg powder of the sample to 5ml ethanol followed by sonication prior to analysis.
3. Results and discussion 3.1 XRD analysis Powder XRD patterns of Zn1−xMnxO (00 show that the ZnO structure is not disturbed by Mn addition. No reflections due to any secondary phase are detected in the XRD patterns. The widths of the reflections increase with increasing Mn content, indicating decrease in crystallite size. The indexed peaks of asprepared samples with high intensities corresponding to (100), (002), (101) and lower intensities at (102), (110), (103), (200), (112) and (201) planes indicate to a hexagonal wurtzite structure. No additional peaks corresponding to manganese metal or any other binary phase is clearly observed in Fig.1. This may be due to very lower amount of Mn was added to the prepared sample, which is beyond the detection limit of XRD. But the same was detected by energy dispersive spectrometer (EDS) study reported in later section. Expanded XRD patterns of different compositions of Mn in Zn1−xMnxO (0 < x < 0.20) in 30o-38o and 46o-80o are reported in Fig.2. It is clearly seen from Fig.2 that as Mn doping content increases, diffraction peaks shifts to lower 2θ angles, which indicates the increase of the lattice parameters. The increasing trend in lattice parameters with Mn doping content can be explained by the fact that the ionic radius of Mn2+ (0.80 Å) is larger than that of Zn2+ (0.74 Å) [21]. Moreover, it can also be observed that the intensity of the (101) diffraction peak decreased gradually and the width broadened with increasing Mn doping content. The reason behind this can be explained as the increase in the 4
lattice disorder and strain induced by Mn2+ substitution. Hence, Mn incorporation will lead to an expansion of the ZnO lattice. The average crystallite sizes were calculated from X-ray line broadening using the Scherrer formula [22]. D=
0.94 λ β cos θ
(1)
Where D is the crystallite size, β is the full width at half maximum (FWHM) of the diffraction peak, λ is the wavelength of the incidental X-ray (1.54 A°) and θ is the diffraction angle. A decrease in the average crystallite size with increasing Mn content was confirmed, which is similar to results reported in the literature [22]. With the increasing of Mn concentration, the crystallite size decreases gradually from 40 to 15 nm. The average crystallite size verses Mn doping content for different compositions of Zn1-xMnxO is reported in Fig.3.
3.2 Microstructural and EDS Study The field emission scanning electron micrographs of the synthesized Zn(1-x)MnxO samples are shown in Fig. 4. Transformation of mixed structure of nanocrystal and nanorod to nano-needle then to nanowire like structure with increasing amount of Mn doping was confirmed from microstructures, which makes it interesting for discussion. The change in morphology and size depending on the increase in Mn doping concentration can clearly be seen. The undoped sample Zn1−xMnxO (x =0) (Fig. 4(a)) shows the mixed kind of structure. The microstructure of Fig. 4(a) reveals the mixture of nanocrystals with some nanorod like structures in its initial growth stage. Fig.4 (b and c) show more uniform growth of nanorods with increasing concentration of Mn doping. But in case of Zn1−xMnxO (x = 0.10) transformation of nanorods to more dense with lesser diameter nano-needle like structures were observed from fig.4 (d). As the Mn doping 5
concentration increases, we found more uniform growth of nano-needle like structures gradually transforms to ultrathin nanowire. In case of fig. 4(e and f) it can be observed ultrathin and long nanowire branches created from hexagonal shaped nanorods. From above results it can be concluded that, both crystallite size and surface morphology are significantly affected by Mn doping concentrations. That may be explained as with the increment in Mn doping content, the Mn atoms occupied vacant sites in ZnO to form excess zinc ions. These zinc ions initially diffused to the surface of the ZnO through grain boundaries, and then re-reacted with oxygen to form new ZnO nuclei on the ZnO grains or at the ZnO grain boundaries, and finally the ZnO nano-rod structures are formed on these nuclei along the optimal orientation [23]. Top view image of nanorod arrays of Zn1−xMnxO (x = 0.105 and 0.20) shown in fig.5. At highest loading level of Mn in Zn1−xMnxO (x = 0.20), the structure of the nano-rod growth in fig.5(b) looks ordered periodic honeycomb like pattern, which is completely different from other Mn loading level shown in Fig.5(a). EDS of Zn1−xMnxO at x=0.02 and 0.2 was shown in Fig.6 (a and b). In both cases the spectrum confirms the presence of Zn along with Mn and O.
3.3 DRS study The room temperature optical absorption spectra of the various compositions of Zn1-xMnxO are shown in Fig.7. Apart from the band gap transition, other optical reflectance features are clearly seen for the doped samples at lower energies. The reflectance regions of doped samples were decreased in comparison to undoped one with addition of Mn content. UV-vis diffuse reflectance spectrum of the prepared powder was also used to determine the band gap of respective samples with increasing Mn concentration. The band gap values for the undoped and doped samples are obtained as 3.33, 2.98, 2.99, 2.81, 2.86 and 2.84eV for x=0, 0.02, 0.05, 0.10, 0.15 and 0.20
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respectively, which also confirmed lower bandgap of doped samples compared to undoped one. The blue shift of the band edge indicates the incorporation of Mn ions inside ZnO lattice. It can be said as the above changes in parameters are some indicators of the incorporation of Mn ions inside the ZnO lattice.
3.4 Photoluminescence (PL) Spectra PL spectroscopy is an effective method for investigation of defects in semiconductors. Fig. 8 shows the characteristic PL spectra of undoped and Mn-doped ZnO samples at room temperature. The room temperature PL spectra of the developed nanorods excited by 315 nm UV light using a He-Cd laser as the source. There are obvious changes found in the spectrum of the doped samples as compared with the undoped. Firstly, the PL spectrum of the un-doped consists of a strong UV band peak at 388 nm (~3.192eV) originating from the band to band emission of ZnO nanocrystals. Besides this, a very weak blue-green emission band centered at about 495 nm. The green-blue emission peak around 2.6 eV (495 nm) is commonly referred to as a deeplevel emission. The second weak UV emission is originated from excitonic recombination corresponding to the near band-gap emission of ZnO, while the emission bands in the visible range are due to the recombination of photo-generated holes with singly ionized charge states in intrinsic defects such as oxygen vacancies, Zn interstitials or impurities [24]. In other words it can be explained as the excess exciton impurity and crystalline defect scattering, there exists a deep-level emission around 2.6 eV (495 nm) in ZnO nanorods. The almost negligible deep level emission suggests good optical quality of the ZnO nanorods.
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4. Conclusions Mn doped ZnO dilute magnetic semiconductor nanorod was successfully prepared by simple auto-combustion method. A comprehensive investigation was aimed to accomplish the optimization of Mn doping concentrations on structural, morphological and optical studies. The above results have been pointed out for the prepared nanorod/naowire samples. The results of the present work can be summarized as follows (i) XRD data showed that all the samples having single phase ZnO orientations; (ii) FE-SEM results indicated the gradually transformation of nanorod to nano-needle then to nanowire like structure with increasing amount of Mn doping; (iii) the optical band gap of Mn doped ZnO nanorods decreased in compare to undoped ZnO; (iv) Optical measurements confirmed the significant enhancement of the deep level emission of ZnO by Mn doping.
Acknowledgement Authors are very much grateful to Institute of Instrumentation Centre (IIC) of Indian Institute of Technology Roorkee for providing various characterizations facilities for analyzing the project.
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[3] R. Fiederling, M. Keim, G. Reuscher, W. Ossau, G. Schmidt, A. Waag and L. W. Molenkamp, Injection and detection of a spin-polarized current in a light-emitting diode, Nature 402 (1999) 787-790. [4] C. H. Li, G. Kioseoglou, O. M. J. van’t Erve, A. T. Hanbicki, B. T. Jonker, R. Mallory, M. Yasar and A. Petrou, Spin injection across (110) interfaces: Fe⁄GaAs(110) spin-light-emitting diodes, Applied Physics Letters 85 (2004) 1544-1546. [5] M. H. Ham, S. Yoon, Y. Park, L. Bian, M. Ramsteiner and J. M. Myoung, Electrical spin injection from room-temperature ferromagnetic (Ga, Mn)N in nitride-based spin-polarized lightemitting diodes, Journal of Physics: Condensed Matter 18 (2006) 7703-7708. [6] L. C. Chen, C. H. Tien, Y. M. Luo and C. S. Mu, Photoluminescence and spin relaxation of MnZnO/GaN-based light-emitting diodes, Thin Solid Films 519 (2011) 2516-2519. [7] L. C. Chen, C. H. Tien and Y. Y. Hsu, Optoelectronic Properties of the p-MnZnO/n-Si Structure Photodiodes in a Strong Magnetic Field, Japanese Journal of Applied Physics 49 (2010) 063002. [8] X. H. Zhang, S. J. Chua, A. M. Yong, H. Y. Yang, S. P. Lau, S. F. Yu, X. W. Sun, L. Miao, M. Tanemura and S. Tanemura, Exciton radiative lifetime in ZnO nanorods fabricated by vapor phase transport method,Applied Physics Letters 90 (2007) 013107 (1-3). [9] Y. Danhara, T. Hirai, Y. Harada and N. Ohno, Exciton luminescence of ZnO fine particles, Physica Status Solidi (C) 3 (2006) 3565-3568. [10] J. H. Lim, C. K. Kang, K. K. Kim, I. K. Park, D. K. Hwang and S. J. Park, UV Electroluminescence Emission from ZnO Light-Emitting Diodes Grown by High-Temperature Radiofrequency Sputtering, Advanced Materials 18 (2006) 2720-2724.
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[11] Z. P. Wei, Y. M. Lu, D. Z. Shen, Z. Z. Zhang, B. Yao, B. H. Li, J. Y. Zhang, D. X. Zhao, X. W. Fan and Z. K. Tang, Room temperature p-n ZnO blue-violet light-emitting diodes, Applied Physics Letters 90 (2007) 042113(1-3). [12] E. S. P. Leong, S. F. Yu and S. P. Lau, Directional edge-emitting UV random laser diodes, Applied Physics Letters 89 (2006) 221109 (1-3). [13] Y. Nakayama, P. J. Pauzauskie, A. Radenovic, R. M. Onorato, R. J. Saykally, J. Liphardt, and P. Yang, Tunable nanowire nonlinear optical probe, Nature (London) 447 (2007) 1098-1101. [14] S. J. Pearton, D. P. Norton, K. Ip, Y. W. Heo, and T. Steiner, Recent progress in processing and properties of ZnO, Prog. Mater. Sci. 50 (2005) 293-340. [15] S. N. Cha, J. E. Jang, Y. Choi, G. A. J. Amaratunga, G. W. Ho, M. E. Welland, D. G. Hasko, D. J. Kang, and J. M. Kim, High performance ZnO nanowire field effect transistor using self-aligned nanogap gate electrodes, Applied Physics Letters 89 (2006) 263102 (1-3). [16] Z. L. Wang, Nanopiezotronics, Adv. Mater. 19 (2007) 889-892. [17] R. W. Miles, G. Zoppi, I. Forbes, Inorganic photovoltaic cells, Mater. Today 10 (2007) 2027. [18] C. Ronning, P. X. Gao, Y. Ding, Z. L. Wang, and D. Schwen, Manganese-doped ZnO nanobelts for spintronics, Applied Physics Letters 84 (2004) 783-785. [19] J. J. Liu, M. H. Yu, and W. L. Zhou, Well-aligned Mn-doped ZnO nanowires synthesized by a chemical vapor deposition method, Applied Physics Letters 87 (2005) 172505(1-3). [20] H. L. Yan, X. L. Zhong, J. B. Wang, G. J. Huang, S. L. Ding, G. C. Zhou, and Y. C. Zhou, Cathodoluminescence and room temperature ferromagnetism of Mn-doped ZnO nanorod arrays grown by chemical vapor deposition, Applied Physics Letters 90 (2007) 082503(1-3).
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[21] J. Anghel, A. Thurber, D. A. Tenne, C. B. Hanna and A. Punnoose, Correlation between saturation magnetization, bandgap, and lattice volume of transition metal (M=Cr, Mn, Fe, Co, or Ni) doped Zn1−xMxO nanoparticles, J. Appl. Phys. 107 (2010) 09E314. [22] K. Omri, J. E. Ghoul, O. M. Lemine, M. Bououdina, B. Zhang and L. E. Mira, Magnetic and optical properties of manganese doped ZnO nanoparticles synthesized by sol-gel technique, Superlattices and Microstructures 60 (2013) 139-147. [23] K. J. Chen, F. Y. Hung, S. J. Chang and Z. S. Hu, The crystallized mechanism and optical properties of sol-gel synthesized ZnO nanowires, J. Electrochem. Soc. 157 (3) (2010) H241H245. [24] G. Kenanakisa, M. Androulidakic, E. Koudoumasa, C. Savvakisa, N. Katsarakisa, Photoluminescence of ZnO nanostructures grown by the aqueous chemical growth technique, Superlattices and Microstructures 42 (2007) 473-478.
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Figure captions
Fig. 1 XRD patterns of different compositions of Zn1−xMnxO (0 < x < 0.20). Fig. 2 Expanded XRD patterns of different compositions of Zn1−xMnxO (0 < x < 0.20) in (a) 30o38o and (b) 46o-80o. Fig. 3 Crystallite size verses Mn doping concentrations. Fig. 4 FE-SEM micrographs of different compositions of Zn1−xMnxO (0 < x < 0.20) (a) x=0.00, (b) x= 0.02, (c) x= 0.05, (d) x= 0.10, (e) x= 0.15 and (f) x= 0.2. Fig. 5 Top-view SEM micrographs of different compositions of Zn1−xMnxO (a) x=0.10 and (b) x= 0.20. Fig.6 EDS of Zn1−xMnxO at (a) x= 0.02 and (b) x= 0.20. Fig.7 DRS spectra of different compositions of Zn1−xMnxO (0 < x < 0.20). Fig. 8 Photoluminescence spectrum of the as-synthesized Mn doped ZnO nanorods.
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Highlights •
Mn doped ZnO nanorod was successfully prepared by simple auto-combustion method.
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Structural and morphological properties of prepared nanorod are significantly influenced by Mn doping.
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Optical results suggest good quality optical properties.
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S0749-6036(15)00060-9 http://dx.doi.org/10.1016/j.spmi.2015.01.025 YSPMI 3583
To appear in:
Superlattices and Microstructures
Received Date: Accepted Date:
8 January 2015 10 January 2015
Please cite this article as: H.S. Maharana, S. Yadav, A.P. Chakraverty, G.D. Verma, Effect of Mn doping on microstructural and optical behaviour of Zn(1-x)MnxO nanorod by simple auto-combustion method, Superlattices and Microstructures (2015), doi: http://dx.doi.org/10.1016/j.spmi.2015.01.025
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of Mn doping on microstructural and optical behaviour of Zn(1-x)MnxO nanorod by simple auto-combustion method H. S. Maharana1, 2*, S. Yadav1, 2, A. P. Chakraverty3, G. D. Verma1, 2 1
Centre of Nanotechnology, Indian Institute of Technology Roorkee, Roorkee, Uttarakhand 247667, India 2 Department of Physics, Indian Institute of Technology Roorkee, Roorkee, Uttarakhand 247667, India 3 Department of Metallurgical and Materials Engineering, National Institute of Technology, Rourkela 769008, Odisha, India
Abstract Mn doped ZnO nanorods with different doping concentrations (0, 0.02, 0.05, 0.1, 0.15, and 0.2) were synthesized by simple auto-combustion method. The structural, morphological and optical behaviors of Zn(1-x)MnxO were investigated by X-ray diffraction (XRD), field-emission microscopy (FE-SEM) and diffuse reflectance spectroscopy (DRS). The structural properties of un-doped and Mn doped ZnO nanorods exhibit hexagonal wurtzite structure with no secondary phase. From scanning electron micrographs results it can be evidenced that, both crystallite size and surface morphology are significantly affected by Mn doping concentrations. From the optical studies, the reflectance in UV region of doped samples was decreased in comparison to undoped one and the optical band gap varies between 3.33 and 2.81 eV. Photoluminescence spectra shows large emission band originated at 388nm and a weak band centered on 495nm. The present study reveals about the influence of Mn2+ ions on morphological and optical properties of Zn(1-x)MnxO.
Keywords: Dilute magnetic semiconductor, Auto-combustion, ZnO, Nanorod * Author for correspondence. Email: [email protected]
1
1. Introduction Dilute magnetic semiconductors (DMSs) based on zinc oxide (ZnO) doped with transition metals, such as Ni, Co, and Mn has been attracted to experimental and theoretical researchers in recent years, since the predicted room temperature ferromagnetism in DMS can be useful in various applications, such as spintronics, light-emitting diodes (LEDs) and photodiodes [1-7]. Direct band gap ZnO semiconductor has received considerable attention as a potential material for optical devices due to its well-known wide band gap energy of 3.37 eV and large exciton binding energy of 60 meV [8-10]. ZnO has also potentially applicable for LEDs, laser diodes (LDs) and ultraviolet (UV) detection devices [10-12]. In recent years exploring one-dimensional DMS materials are of great interest because nanorods, nanowires (NWs) and nanobelts are ideal materials for fabricating electronic devices such as field effect transistors, sensors, optoelectronic devices, logic circuits, and lasers [13-17]. There are various approaches for developing DMS nanorods and NWs. One method is doping the as-synthesized NWs by ion implantation [18]. This is basically a post growth doping via physical method, which generally requires post-annealing to reduce the defects created by ion implantation. Another approach is the incorporation of doping elements in the precursors during growth, which is a methodology for getting uniformly distributed doping element in the matrix [19, 20]. Synthesizing of nanorod or nanowire by the second approach, an important question is if the doping element is incorporated at the surface or into the volume lattice. Establishing an effective approach for synthesis of volume-doped DMS nanorods or NWs is a fundamental importance for investigating spin transport in one dimensional nanomaterials and fabrication of devices. This present communication reports the structural, morphological and optical properties of Zn1-xMnxO (0 < x < 0.20) nanorod/nanowire prepared by simple auto-combustion method.
2
2. Experimental 2.1 Synthesis Undoped and Mn doped ZnO nanorods/nanowire were synthesized by auto-combustion method from the nitric acid solution of Zn and Mn was taken in the appropriate ratio using glycine as the fuel. Zn and Mn metal powders were taken in stoichiometric amounts and dissolved separately in 4N HNO3 and then mixed together. To the above mixed metal nitrate solution, 2 mole of glycine in the form of aqueous solution per mole metal powder was added. The mixed solution was kept over a hot plate for auto-combustion at 473 K for 4 hours. After the complete evaporation of water, a reddish colored thick gel was formed, which was subsequently burnt to give a final brown colored oxide product. Then, the obtained powder samples were characterized by different characterization techniques for further analysis. 2.2 Characterizations X-ray diffraction patterns of synthesized powder was recorded on a Bruker X-8 X-ray diffractometer using Cu Kα line (1.5418Å) with the X-ray source at a speed of 2 degree per minute in a 2θrange between 20 and 70 degrees to examine the phase formation and calculating the crystallite sizes. The surface morphology and the elemental analysis of all samples were analysed by QUANTA-200F field emission scanning electron microscope having maximum accelerating voltage of 30 kV combined with an EDXA accessory equipped with a CCD (charge coupled device) camera. Diffuse reflectance is an excellent sampling tool for powdered materials in the mid-IR and near infrared ranges. It can also be used for the analysis of intractable solid samples. A Shimadzu UV-3600 UV-Vis NIR spectrometer was used and range studied was from 200 to 800 nm. The samples to be analysed by diffuse reflectance were generally mixed with barium sulphate (1:100 wt. ratios) prior to analysis. Photoluminescence measurements were 3
carried out by a Shimadzu RF-5301 PC spectrometer with excitation wavelength 315 nm. The emission spectra were recorded from 325 to 800 nm. The samples were prepared by adding 10mg powder of the sample to 5ml ethanol followed by sonication prior to analysis.
3. Results and discussion 3.1 XRD analysis Powder XRD patterns of Zn1−xMnxO (0
lattice disorder and strain induced by Mn2+ substitution. Hence, Mn incorporation will lead to an expansion of the ZnO lattice. The average crystallite sizes were calculated from X-ray line broadening using the Scherrer formula [22]. D=
0.94 λ β cos θ
(1)
Where D is the crystallite size, β is the full width at half maximum (FWHM) of the diffraction peak, λ is the wavelength of the incidental X-ray (1.54 A°) and θ is the diffraction angle. A decrease in the average crystallite size with increasing Mn content was confirmed, which is similar to results reported in the literature [22]. With the increasing of Mn concentration, the crystallite size decreases gradually from 40 to 15 nm. The average crystallite size verses Mn doping content for different compositions of Zn1-xMnxO is reported in Fig.3.
3.2 Microstructural and EDS Study The field emission scanning electron micrographs of the synthesized Zn(1-x)MnxO samples are shown in Fig. 4. Transformation of mixed structure of nanocrystal and nanorod to nano-needle then to nanowire like structure with increasing amount of Mn doping was confirmed from microstructures, which makes it interesting for discussion. The change in morphology and size depending on the increase in Mn doping concentration can clearly be seen. The undoped sample Zn1−xMnxO (x =0) (Fig. 4(a)) shows the mixed kind of structure. The microstructure of Fig. 4(a) reveals the mixture of nanocrystals with some nanorod like structures in its initial growth stage. Fig.4 (b and c) show more uniform growth of nanorods with increasing concentration of Mn doping. But in case of Zn1−xMnxO (x = 0.10) transformation of nanorods to more dense with lesser diameter nano-needle like structures were observed from fig.4 (d). As the Mn doping 5
concentration increases, we found more uniform growth of nano-needle like structures gradually transforms to ultrathin nanowire. In case of fig. 4(e and f) it can be observed ultrathin and long nanowire branches created from hexagonal shaped nanorods. From above results it can be concluded that, both crystallite size and surface morphology are significantly affected by Mn doping concentrations. That may be explained as with the increment in Mn doping content, the Mn atoms occupied vacant sites in ZnO to form excess zinc ions. These zinc ions initially diffused to the surface of the ZnO through grain boundaries, and then re-reacted with oxygen to form new ZnO nuclei on the ZnO grains or at the ZnO grain boundaries, and finally the ZnO nano-rod structures are formed on these nuclei along the optimal orientation [23]. Top view image of nanorod arrays of Zn1−xMnxO (x = 0.105 and 0.20) shown in fig.5. At highest loading level of Mn in Zn1−xMnxO (x = 0.20), the structure of the nano-rod growth in fig.5(b) looks ordered periodic honeycomb like pattern, which is completely different from other Mn loading level shown in Fig.5(a). EDS of Zn1−xMnxO at x=0.02 and 0.2 was shown in Fig.6 (a and b). In both cases the spectrum confirms the presence of Zn along with Mn and O.
3.3 DRS study The room temperature optical absorption spectra of the various compositions of Zn1-xMnxO are shown in Fig.7. Apart from the band gap transition, other optical reflectance features are clearly seen for the doped samples at lower energies. The reflectance regions of doped samples were decreased in comparison to undoped one with addition of Mn content. UV-vis diffuse reflectance spectrum of the prepared powder was also used to determine the band gap of respective samples with increasing Mn concentration. The band gap values for the undoped and doped samples are obtained as 3.33, 2.98, 2.99, 2.81, 2.86 and 2.84eV for x=0, 0.02, 0.05, 0.10, 0.15 and 0.20
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respectively, which also confirmed lower bandgap of doped samples compared to undoped one. The blue shift of the band edge indicates the incorporation of Mn ions inside ZnO lattice. It can be said as the above changes in parameters are some indicators of the incorporation of Mn ions inside the ZnO lattice.
3.4 Photoluminescence (PL) Spectra PL spectroscopy is an effective method for investigation of defects in semiconductors. Fig. 8 shows the characteristic PL spectra of undoped and Mn-doped ZnO samples at room temperature. The room temperature PL spectra of the developed nanorods excited by 315 nm UV light using a He-Cd laser as the source. There are obvious changes found in the spectrum of the doped samples as compared with the undoped. Firstly, the PL spectrum of the un-doped consists of a strong UV band peak at 388 nm (~3.192eV) originating from the band to band emission of ZnO nanocrystals. Besides this, a very weak blue-green emission band centered at about 495 nm. The green-blue emission peak around 2.6 eV (495 nm) is commonly referred to as a deeplevel emission. The second weak UV emission is originated from excitonic recombination corresponding to the near band-gap emission of ZnO, while the emission bands in the visible range are due to the recombination of photo-generated holes with singly ionized charge states in intrinsic defects such as oxygen vacancies, Zn interstitials or impurities [24]. In other words it can be explained as the excess exciton impurity and crystalline defect scattering, there exists a deep-level emission around 2.6 eV (495 nm) in ZnO nanorods. The almost negligible deep level emission suggests good optical quality of the ZnO nanorods.
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4. Conclusions Mn doped ZnO dilute magnetic semiconductor nanorod was successfully prepared by simple auto-combustion method. A comprehensive investigation was aimed to accomplish the optimization of Mn doping concentrations on structural, morphological and optical studies. The above results have been pointed out for the prepared nanorod/naowire samples. The results of the present work can be summarized as follows (i) XRD data showed that all the samples having single phase ZnO orientations; (ii) FE-SEM results indicated the gradually transformation of nanorod to nano-needle then to nanowire like structure with increasing amount of Mn doping; (iii) the optical band gap of Mn doped ZnO nanorods decreased in compare to undoped ZnO; (iv) Optical measurements confirmed the significant enhancement of the deep level emission of ZnO by Mn doping.
Acknowledgement Authors are very much grateful to Institute of Instrumentation Centre (IIC) of Indian Institute of Technology Roorkee for providing various characterizations facilities for analyzing the project.
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Figure captions
Fig. 1 XRD patterns of different compositions of Zn1−xMnxO (0 < x < 0.20). Fig. 2 Expanded XRD patterns of different compositions of Zn1−xMnxO (0 < x < 0.20) in (a) 30o38o and (b) 46o-80o. Fig. 3 Crystallite size verses Mn doping concentrations. Fig. 4 FE-SEM micrographs of different compositions of Zn1−xMnxO (0 < x < 0.20) (a) x=0.00, (b) x= 0.02, (c) x= 0.05, (d) x= 0.10, (e) x= 0.15 and (f) x= 0.2. Fig. 5 Top-view SEM micrographs of different compositions of Zn1−xMnxO (a) x=0.10 and (b) x= 0.20. Fig.6 EDS of Zn1−xMnxO at (a) x= 0.02 and (b) x= 0.20. Fig.7 DRS spectra of different compositions of Zn1−xMnxO (0 < x < 0.20). Fig. 8 Photoluminescence spectrum of the as-synthesized Mn doped ZnO nanorods.
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Highlights •
Mn doped ZnO nanorod was successfully prepared by simple auto-combustion method.
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Structural and morphological properties of prepared nanorod are significantly influenced by Mn doping.
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Optical results suggest good quality optical properties.
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