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Low temperature aqueous chemical growth, structural, and optical properties of Mn-doped ZnO nanowires
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Low temperature aqueous chemical growth, structural, and optical properties of Mndoped ZnO nanowires Chan Oeurn Chey, Omer Nur, Magnus Willander
www.elsevier.com/locate/jcrysgro
PII: DOI: Reference:
S0022-0248(13)00264-9 http://dx.doi.org/10.1016/j.jcrysgro.2013.04.015 CRYS21516
To appear in:
Journal of Crystal Growth
Received date: 28 February 2013 Accepted date: 7 April 2013 Cite this article as: Chan Oeurn Chey, Omer Nur, Magnus Willander, Low temperature aqueous chemical growth, structural, and optical properties of Mn-doped ZnO nanowires, Journal of Crystal Growth, http://dx.doi.org/10.1016/j. jcrysgro.2013.04.015 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 galley proof before it is published in its final citable 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.
Low temperature aqueous chemical growth, structural, and optical properties of Mn-doped ZnO nanowires Chan Oeurn Chey*, Omer Nur, Magnus Willander Physical electronics and nanotechnology division, department of Science and Technology, campus Norrköping, Linköping University, SE-60174 Norrköping, Sweden *Correspondence author: [email protected] Email: [email protected], [email protected] Tel.: +46 (0)11 36 31 67; Fax: +46 (0)11 36 32 70 Abstract Mn-doped ZnO nanowires were successfully synthesized by using the low temperature aqueous chemical growth (ACG) method. Field emission scanning electron microscopy (FESEM), energy dispersive x-ray (EDX), X-ray diffraction (XRD), X-Ray photoelectron spectroscopy (XPS), and photoluminescence (PL) spectroscopy have been used to characterize the grown Zn1-xMnxO. The FESEM and the XRD measurements revealed that the grown of Mn-doped ZnO had wurtzite structure and the lattice parameters and the size of the crystal changed according to the change of concentration of the dopant. The chemical composition and charge states of the Mn ions doped in the ZnO nanowires was analyzed by the EDX and the XPS, respectively, indicated that the Mn ions is incorporated onto zinc sites in the ZnO nanowires. PL spectroscopy shows a strong ultraviolet (UV) emission peak at 378 nm (3.27 eV) from the Mn-doped ZnO nanowires, which is shifted 6 nm to the lower wavelength compared to ZnO nanowires grown by the same ACG method. The unique feature of our samples were the simple low temperature growth method which provides no clustering and the as-synthesized Mn-doped ZnO nanowires have shown good crystal quality. This capability to fabricate Mn-doped ZnO nanowires is of potential to develop new spintronic, photonic and sensor devices fabrication on any substrates. Keywords: Growth Zinc oxide nanostructures, Low temperature aqueous chemical growth method, Hydrothermal method, Mn-doped ZnO nanowires.
1. Introduction One dimensional (1D) zinc oxide (ZnO) nanostructures are II-VI semiconducting material, possessing unique characteristics such as wide direct band gap of 3.37 eV, relatively large exciton binding energy of 60 meV at room temperature (RT), transparent, conductivity, piezoelectric and pyroelectric properties [1-4]. Due to these unique properties ZnO nanostructures have potential of developing nano-systems with applications in electronics, photonics and energy harvesting such as biophotonics, bio-electronics, and piezoelectric power generation, piezo-phototronic, sensors, UV light-emitting diodes (LEDs), lasers, memory device and also for life-science applications [1-12].
On the other hand, by
introducing transition metals (TMs) impurities into ZnO nanostructures a change of the ZnO structural, electrical, magnetic, and optical properties of ZnO nanostructures can be achieved at room temperature. Moreover, the TM-doped ZnO has the potential to be a multifunctional material with magnetic, semiconducting and optical properties that makes it attractive in spintronic devices and sensor devices such as spin-based light-emitting diodes, UV sensors, spin transistors, non-volatile memory, and ultrafast optical switches [13-18]. Since Mn is one of the TMs, therefore Mn doping in the semiconductors such as ZnO is attracted interests of researchers [19-24]. Recently, there are a wide range of techniques for growing Mn-doped ZnO nanostructures such as chemical vapor deposition (CVD) [25-29], pulsed laser deposition (PLD) [30-35], thermal evaporation [36-40], solid state reaction and sol–gel methods [41-43] and hydrothermal method [44-48]. Among the techniques above, the hydrothermal methods are favorable friendly and attractive techniques due to simplicity, low cost, less hazardous, easy to scale up, and also can possibly be used to grow the nanostructures on flexible substrates. Moreover, the hydrothermal method is a powerful technique for growing 1D ZnO nanostructures [4]. However, even though the growth method is same but the properties of the grown semiconductors are different according to the preparation parameters, growth
mechanism and several controlled parameters. This paper presents the growth of Mn-doped ZnO nanowires by the low temperature aqueous chemical growth (ACG) methods. In principle we have adopted a procedure similar to the one described in [49-50] with modification of the growth chemical compounds to incorporate the Mn ions in the ZnO crystal. Although this ACG method is a low temperature approach ( 100oC) it is a very effective method to control the morphology, structures and properties by varying the different growth conditions such as temperature, growth time, precursor concentration, and growth solution pH value. After growth, the morphology, structure, electronic structure and optical property of the grown Mn-doped ZnO nanowires have been investigated. 2. Experiments 2.1 Material and method Zinc nitrate hexahydrate [Zn(NO)3)26H2O], Zinc acetate hexahydrate (C4H10O6Zn), Manganese (II) chlorides hexahydrate (MnCl26H2O), hexamethylenetetramine (HMT) (C6H12N4), methanol, ethanol and KOH were used for synthesis of the Mn-doped ZnO nanowires in this study. All chemicals were purchased from Sigma-Aldrich and where used without further purification. The low temperature ACG method procedure for growing Mndoped ZnO nanowires is described briefly. Firstly, an aqueous solution A is prepared by mixing of equimolar 0.075 M of Zinc nitrate hexahydrate and HMT in deionized water. A diluted solution B is a solution of different concentrations of the dopant Mn prepared by dissolving a specific percentage of MnCl26H2O in deionized water consisted of 20 percent ethanol. Secondly, solutions A and B were mixed then homogenizing by ultrasonication bath for 1 hour. Then the mixed solution was subsequently stirred with a magnetic stirrer at room temperature for another hour before use. After the growth solution is prepared, p-type Si and glass substrates were spin coated with a seed solution containing zinc acetate at 2000 rpm for 30s respectively and then the samples were annealed at 90oC for 10 minutes. Finally, the
coated substrates or samples were placed horizontally in the growth aqueous solution and kept in a preheated oven for 5 hours at 80oC. When the growth was completed, the samples were washed with deionized water and dried at RT for further characterization. 2.2 Sample characterization The grown Zn1-xMnxO nanowires were characterized by field emission scanning electron microscope (FESEM) in order to investigate the morphology of the grown samples. Elemental analysis of the grown Zn1-xMnxO was done by energy dispersive X-ray (EDX) spectroscopy which is attached to the FESEM and X-ray photoelectron spectroscopy (XPS) used for characterization of the charge states of Mn ions in the ZnO nanowires. Structural characterization was performed with X-ray diffraction (XRD) Philips PW 1729 diffractometer utilizing Cu kD operated at 40 kV and 40 mA, and finally photoluminescence (PL) spectroscopy was carried out at room temperature using a coherent MBD266 laser in order to investigate the optical properties of the Zn1-xMnxO nanowires. 3. Results and Discussions 3.1 Morphological and chemical composition characterization Surface morphology of the Zn1-xMnxO nanostructures with different doping concentration of Mn have been performed by using SEM. Figures 1 (a-c) shown the SEM images of undoped Mn-ZnO nanowires, and 1.0% and 5.0% of Mn concentration doped ZnO nanowires, respectively. The incorporation of different concentration of Mn ions into ZnO nanowires caused change of the surface morphology to a disc like wurtzite nanostructure. For the 1.0% doped Mn concentration the morphology of the doped Mn-ZnO is homogeneous dense growth, well aligned and almost all of the nanowires were perpendicular to the surface of the substrate. For the 5.0% of doped Mn concentration the morphology of the nanowires was changed to a disc like structure. The crystalline nature of the nanostructures was affected due to the increase of dopant concentration. It means that the dopant was introduced into the pure
ZnO nanowires. The chemical composition of the grown Zn1-xMnxO was measured by using EDX is shown in Figure 1 (d). The EDX data revealed that the Mn peaks were at 0.56, 0.63, 5.89 and 6.49 keV. This indicates the Mn ion was incorporated in the ZnO nanowires. 3.2 Structural characterization Figure 2 (a) shows representative XRD patterns of the undoped ZnO nanowires, and 1.0%, and 5.0% Mn-doped ZnO nanowires. All peaks obtained from both undoped and Mndoped ZnO nanocrystals are indexed to hexagonal wurtzite structure of ZnO consistent with the JCPDS No. 36-1451file. Moreover, the high and sharp diffraction peaks of the 002 plain demonstrate that the grown samples have well crystalline nature, providing evidence that the undoped and Mn- doped ZnO grow along the c-axis [20]. In addition, there is no metallic Mn or MnO phase observed in the XRD pattern of Zn1-xMnxO, which indicates that there are no additional crystalline structure exists in the samples, indicating that the Mn ions are substituted in the Zn sites [20, 23]. However, from Figure 2 (b) we observed that the 002 peaks were shifted to the lower 2 values and full width at haft maximum (FWHM) was larger while increasing the doping concentration of the Mn. This shifting is due to the difference in ionic radii of Mn2+ (0.66 Å) and Zn2+ (0.60 Å) which suggests that the Mn ions incorporated into the ZnO crystal lattice. This Mn incorporation caused an expansion of the ZnO lattice. This observation was also observed in similar samples reported in [23, 24, and 52]. The hexagonal lattice parameters and particle sizes of the undoped and Mn-doped ZnO with respect to 002 planes were calculated by using Bragg’s law and the Scherer’s formula [18, 24] are shown in Table 1. From the table we observed that the averages of the particle sizes in Mn-doped ZnO were smaller than the particle sizes in undoped sample while the Mn concentrations were increased and the lattice parameters “a” and “c” were increasing with increasing the concentration of the Mn. In the table, the parameters a, c and d are lattice constants and D is thickness or particle size.
3.3. Local electronic structure X-Ray photoelectron spectroscopy (XPS) was used for studying the charge state and chemical composition of the Mn2+ in the Zn1-xMnxO nanowires. Figure 3 (a) shows the XPS spectrum of the Zn0.95Mn0.005O for different compositions of Zn, O and Mn. Figure 3 (b) shows two strong XPS peaks at binding energies of 1022.5 eV and 1045.5 eV corresponding to the energies of Zn 2p3/2 and Zn 2p1/2, respectively, while the XPS peak at a binding energy of 531.3 eV represents the O 1s peak and the two weak XPS peaks at binding energies of 641.7 and 657.3 eV correspond to the binding energies of Mn 2p3/2 and Mn 2p1/2, respectively. This implies that there are small amount of Mn2+ doped into the ZnO nanowires, indicating the Mn2+ valence state has substituted some of the Zn ions in the Zn0.95Mn0.05O nanowires. This result is very similar to the reports in the literature of samples grown by other techniques [25-26, 51]. 3.4 Optical Properties Photoluminescence (PL) spectroscopy is used to study the optical properties and electron transition from band to band and from donor levels to excited states in the Zn0.95Mn0.005O. Figure 4 shows the PL emission of Zn0.95Mn0.005O nanowires at RT. A wide ultraviolet (UV) emission peak observed at 378 nm (3.28 eV) can be attributed to the exciton transitions. This UV emission peak exhibits a redshift of 6 nm to the lower wavelength in comparison with the PL emission of ZnO nanowires grow by ACG method in [53-54] that usually the UV peak is observed at 385 nm (3.22 eV). The shift is related to the change of the band gap (Eg) due to the presence of Mn ions in the ZnO crystalline. This result is again very similar to other report in the literature of samples grown by other techniques [26]. Furthermore, only very sharp peak at 529 nm (2.34 eV) was observed from the PL of the Zn0.95Mn0.005O that related to the oxygen vacancy in the sample. However, this peak unlike the peaks of undoped ZnO that have a broad emission peaks around 529 nm which attributed to the oxygen vacancy [38]. The
sharp and weak deep level emission (DLE) peaks were observed in the Mn doped ZnO indicates that it has a lower concentration of oxygen vacancies. This result is in strong agreement with the report of [44] that the absent of some oxygen vacancy due to the Mn doping in ZnO causes decrease in the quantity of the defect. 4. Conclusions We have successfully extended the low temperature aqueous chemical growth method used to grow un-doped ZnO nanowires to achieved Mn-doped ZnO nanowires of high quality. This has been achieved by modifying the growth chemicals by adding Mn ion containing compound. The morphological, structural, and optical properties of the Mn-doped ZnO nanowires have been investigated. FESEM micrographs show that the Mn-doped ZnO nanowires have a nature of hexagonal wurtzite ZnO structure. EDX, XRD and XPS measurements indicated that the dopants have uniformly substituted positions onto zinc sites in the wurtzite lattice of the ZnO nanowires. Further, we have observed no second phase of dopant impurities or clustering in the Mn-doped ZnO grown nanowires. Photoluminescence spectroscopy shows a strong near band gap edge emission from the Mn-doped ZnO nanowires which was observed to be shifted 6 nm to lower wavelength, compared to undoped ZnO nanowires. This ACG method provides compatible technology for device fabrication on flexible large area substrates. The present results suggest that the existence of Mn element changes the structural and optical properties of the ZnO nanowires. The incorporation of transition metal ions such as Mn into ZnO nanowires via the low temperature growth method provide potential for the development of new wide range of components e.g. spintronics and sensor devices on any substrates. Moreover, this low temperature growth mechanism is a low cost, environmental friendly, easy to use, and is a low energy consumption method.
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Figure Captions Figure 1 (a) SEM image of ZnO nanowires (b) SEM image of Zn0.99Mn0.01O nanowires (c) SEM image of Zn0.95Mn0.05O nanowires (d) EDX spectrum of Zn0.95Mn0.05O nanowires Figure 2 (a) The XRD patterns of ZnO nanowires, Zn0.99Mn0.01O and Zn0.95Mn0.05O (b) The XRD patterns show the 002 peaks were shift to the lower 2 values Figure 3 (a) The XPS spectra of Zn0.95Mn0.05O nanowires (b) The XPS spectra of Zn2p, O1s and Mn2p Figure 4 Room temperature PL spectrum of Zn0.95Mn0.05O nanowires
Table captions Table 1 Lattice parameters and crystals size at (002) plane of undoped and doped Mn-ZnO nanowires Table 1 a (Å)
c (Å)
D (Å)
2.59754
3.00000
5.19509
632.456
0.15
2.59754
3.00276
5.19509
457.221
0.15
2.60339
3.00614
5.20679
547.849
Samples
FWHM (deg.)
ZnO
0.13
Zn0.99Mn0.01O Zn0.95Mn0.05O
d (Å)
Figure 1 a-d
Figure 2a
Figure 2b
Figure 3a
Figure 3b
Figure 4
Low temperature aqueous chemical growth, structural, and optical properties of Mndoped ZnO nanowires Chan Oeurn Chey, Omer Nur, Magnus Willander
www.elsevier.com/locate/jcrysgro
PII: DOI: Reference:
S0022-0248(13)00264-9 http://dx.doi.org/10.1016/j.jcrysgro.2013.04.015 CRYS21516
To appear in:
Journal of Crystal Growth
Received date: 28 February 2013 Accepted date: 7 April 2013 Cite this article as: Chan Oeurn Chey, Omer Nur, Magnus Willander, Low temperature aqueous chemical growth, structural, and optical properties of Mn-doped ZnO nanowires, Journal of Crystal Growth, http://dx.doi.org/10.1016/j. jcrysgro.2013.04.015 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 galley proof before it is published in its final citable 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.
Low temperature aqueous chemical growth, structural, and optical properties of Mn-doped ZnO nanowires Chan Oeurn Chey*, Omer Nur, Magnus Willander Physical electronics and nanotechnology division, department of Science and Technology, campus Norrköping, Linköping University, SE-60174 Norrköping, Sweden *Correspondence author: [email protected] Email: [email protected], [email protected] Tel.: +46 (0)11 36 31 67; Fax: +46 (0)11 36 32 70 Abstract Mn-doped ZnO nanowires were successfully synthesized by using the low temperature aqueous chemical growth (ACG) method. Field emission scanning electron microscopy (FESEM), energy dispersive x-ray (EDX), X-ray diffraction (XRD), X-Ray photoelectron spectroscopy (XPS), and photoluminescence (PL) spectroscopy have been used to characterize the grown Zn1-xMnxO. The FESEM and the XRD measurements revealed that the grown of Mn-doped ZnO had wurtzite structure and the lattice parameters and the size of the crystal changed according to the change of concentration of the dopant. The chemical composition and charge states of the Mn ions doped in the ZnO nanowires was analyzed by the EDX and the XPS, respectively, indicated that the Mn ions is incorporated onto zinc sites in the ZnO nanowires. PL spectroscopy shows a strong ultraviolet (UV) emission peak at 378 nm (3.27 eV) from the Mn-doped ZnO nanowires, which is shifted 6 nm to the lower wavelength compared to ZnO nanowires grown by the same ACG method. The unique feature of our samples were the simple low temperature growth method which provides no clustering and the as-synthesized Mn-doped ZnO nanowires have shown good crystal quality. This capability to fabricate Mn-doped ZnO nanowires is of potential to develop new spintronic, photonic and sensor devices fabrication on any substrates. Keywords: Growth Zinc oxide nanostructures, Low temperature aqueous chemical growth method, Hydrothermal method, Mn-doped ZnO nanowires.
1. Introduction One dimensional (1D) zinc oxide (ZnO) nanostructures are II-VI semiconducting material, possessing unique characteristics such as wide direct band gap of 3.37 eV, relatively large exciton binding energy of 60 meV at room temperature (RT), transparent, conductivity, piezoelectric and pyroelectric properties [1-4]. Due to these unique properties ZnO nanostructures have potential of developing nano-systems with applications in electronics, photonics and energy harvesting such as biophotonics, bio-electronics, and piezoelectric power generation, piezo-phototronic, sensors, UV light-emitting diodes (LEDs), lasers, memory device and also for life-science applications [1-12].
On the other hand, by
introducing transition metals (TMs) impurities into ZnO nanostructures a change of the ZnO structural, electrical, magnetic, and optical properties of ZnO nanostructures can be achieved at room temperature. Moreover, the TM-doped ZnO has the potential to be a multifunctional material with magnetic, semiconducting and optical properties that makes it attractive in spintronic devices and sensor devices such as spin-based light-emitting diodes, UV sensors, spin transistors, non-volatile memory, and ultrafast optical switches [13-18]. Since Mn is one of the TMs, therefore Mn doping in the semiconductors such as ZnO is attracted interests of researchers [19-24]. Recently, there are a wide range of techniques for growing Mn-doped ZnO nanostructures such as chemical vapor deposition (CVD) [25-29], pulsed laser deposition (PLD) [30-35], thermal evaporation [36-40], solid state reaction and sol–gel methods [41-43] and hydrothermal method [44-48]. Among the techniques above, the hydrothermal methods are favorable friendly and attractive techniques due to simplicity, low cost, less hazardous, easy to scale up, and also can possibly be used to grow the nanostructures on flexible substrates. Moreover, the hydrothermal method is a powerful technique for growing 1D ZnO nanostructures [4]. However, even though the growth method is same but the properties of the grown semiconductors are different according to the preparation parameters, growth
mechanism and several controlled parameters. This paper presents the growth of Mn-doped ZnO nanowires by the low temperature aqueous chemical growth (ACG) methods. In principle we have adopted a procedure similar to the one described in [49-50] with modification of the growth chemical compounds to incorporate the Mn ions in the ZnO crystal. Although this ACG method is a low temperature approach ( 100oC) it is a very effective method to control the morphology, structures and properties by varying the different growth conditions such as temperature, growth time, precursor concentration, and growth solution pH value. After growth, the morphology, structure, electronic structure and optical property of the grown Mn-doped ZnO nanowires have been investigated. 2. Experiments 2.1 Material and method Zinc nitrate hexahydrate [Zn(NO)3)26H2O], Zinc acetate hexahydrate (C4H10O6Zn), Manganese (II) chlorides hexahydrate (MnCl26H2O), hexamethylenetetramine (HMT) (C6H12N4), methanol, ethanol and KOH were used for synthesis of the Mn-doped ZnO nanowires in this study. All chemicals were purchased from Sigma-Aldrich and where used without further purification. The low temperature ACG method procedure for growing Mndoped ZnO nanowires is described briefly. Firstly, an aqueous solution A is prepared by mixing of equimolar 0.075 M of Zinc nitrate hexahydrate and HMT in deionized water. A diluted solution B is a solution of different concentrations of the dopant Mn prepared by dissolving a specific percentage of MnCl26H2O in deionized water consisted of 20 percent ethanol. Secondly, solutions A and B were mixed then homogenizing by ultrasonication bath for 1 hour. Then the mixed solution was subsequently stirred with a magnetic stirrer at room temperature for another hour before use. After the growth solution is prepared, p-type Si and glass substrates were spin coated with a seed solution containing zinc acetate at 2000 rpm for 30s respectively and then the samples were annealed at 90oC for 10 minutes. Finally, the
coated substrates or samples were placed horizontally in the growth aqueous solution and kept in a preheated oven for 5 hours at 80oC. When the growth was completed, the samples were washed with deionized water and dried at RT for further characterization. 2.2 Sample characterization The grown Zn1-xMnxO nanowires were characterized by field emission scanning electron microscope (FESEM) in order to investigate the morphology of the grown samples. Elemental analysis of the grown Zn1-xMnxO was done by energy dispersive X-ray (EDX) spectroscopy which is attached to the FESEM and X-ray photoelectron spectroscopy (XPS) used for characterization of the charge states of Mn ions in the ZnO nanowires. Structural characterization was performed with X-ray diffraction (XRD) Philips PW 1729 diffractometer utilizing Cu kD operated at 40 kV and 40 mA, and finally photoluminescence (PL) spectroscopy was carried out at room temperature using a coherent MBD266 laser in order to investigate the optical properties of the Zn1-xMnxO nanowires. 3. Results and Discussions 3.1 Morphological and chemical composition characterization Surface morphology of the Zn1-xMnxO nanostructures with different doping concentration of Mn have been performed by using SEM. Figures 1 (a-c) shown the SEM images of undoped Mn-ZnO nanowires, and 1.0% and 5.0% of Mn concentration doped ZnO nanowires, respectively. The incorporation of different concentration of Mn ions into ZnO nanowires caused change of the surface morphology to a disc like wurtzite nanostructure. For the 1.0% doped Mn concentration the morphology of the doped Mn-ZnO is homogeneous dense growth, well aligned and almost all of the nanowires were perpendicular to the surface of the substrate. For the 5.0% of doped Mn concentration the morphology of the nanowires was changed to a disc like structure. The crystalline nature of the nanostructures was affected due to the increase of dopant concentration. It means that the dopant was introduced into the pure
ZnO nanowires. The chemical composition of the grown Zn1-xMnxO was measured by using EDX is shown in Figure 1 (d). The EDX data revealed that the Mn peaks were at 0.56, 0.63, 5.89 and 6.49 keV. This indicates the Mn ion was incorporated in the ZnO nanowires. 3.2 Structural characterization Figure 2 (a) shows representative XRD patterns of the undoped ZnO nanowires, and 1.0%, and 5.0% Mn-doped ZnO nanowires. All peaks obtained from both undoped and Mndoped ZnO nanocrystals are indexed to hexagonal wurtzite structure of ZnO consistent with the JCPDS No. 36-1451file. Moreover, the high and sharp diffraction peaks of the 002 plain demonstrate that the grown samples have well crystalline nature, providing evidence that the undoped and Mn- doped ZnO grow along the c-axis [20]. In addition, there is no metallic Mn or MnO phase observed in the XRD pattern of Zn1-xMnxO, which indicates that there are no additional crystalline structure exists in the samples, indicating that the Mn ions are substituted in the Zn sites [20, 23]. However, from Figure 2 (b) we observed that the 002 peaks were shifted to the lower 2 values and full width at haft maximum (FWHM) was larger while increasing the doping concentration of the Mn. This shifting is due to the difference in ionic radii of Mn2+ (0.66 Å) and Zn2+ (0.60 Å) which suggests that the Mn ions incorporated into the ZnO crystal lattice. This Mn incorporation caused an expansion of the ZnO lattice. This observation was also observed in similar samples reported in [23, 24, and 52]. The hexagonal lattice parameters and particle sizes of the undoped and Mn-doped ZnO with respect to 002 planes were calculated by using Bragg’s law and the Scherer’s formula [18, 24] are shown in Table 1. From the table we observed that the averages of the particle sizes in Mn-doped ZnO were smaller than the particle sizes in undoped sample while the Mn concentrations were increased and the lattice parameters “a” and “c” were increasing with increasing the concentration of the Mn. In the table, the parameters a, c and d are lattice constants and D is thickness or particle size.
3.3. Local electronic structure X-Ray photoelectron spectroscopy (XPS) was used for studying the charge state and chemical composition of the Mn2+ in the Zn1-xMnxO nanowires. Figure 3 (a) shows the XPS spectrum of the Zn0.95Mn0.005O for different compositions of Zn, O and Mn. Figure 3 (b) shows two strong XPS peaks at binding energies of 1022.5 eV and 1045.5 eV corresponding to the energies of Zn 2p3/2 and Zn 2p1/2, respectively, while the XPS peak at a binding energy of 531.3 eV represents the O 1s peak and the two weak XPS peaks at binding energies of 641.7 and 657.3 eV correspond to the binding energies of Mn 2p3/2 and Mn 2p1/2, respectively. This implies that there are small amount of Mn2+ doped into the ZnO nanowires, indicating the Mn2+ valence state has substituted some of the Zn ions in the Zn0.95Mn0.05O nanowires. This result is very similar to the reports in the literature of samples grown by other techniques [25-26, 51]. 3.4 Optical Properties Photoluminescence (PL) spectroscopy is used to study the optical properties and electron transition from band to band and from donor levels to excited states in the Zn0.95Mn0.005O. Figure 4 shows the PL emission of Zn0.95Mn0.005O nanowires at RT. A wide ultraviolet (UV) emission peak observed at 378 nm (3.28 eV) can be attributed to the exciton transitions. This UV emission peak exhibits a redshift of 6 nm to the lower wavelength in comparison with the PL emission of ZnO nanowires grow by ACG method in [53-54] that usually the UV peak is observed at 385 nm (3.22 eV). The shift is related to the change of the band gap (Eg) due to the presence of Mn ions in the ZnO crystalline. This result is again very similar to other report in the literature of samples grown by other techniques [26]. Furthermore, only very sharp peak at 529 nm (2.34 eV) was observed from the PL of the Zn0.95Mn0.005O that related to the oxygen vacancy in the sample. However, this peak unlike the peaks of undoped ZnO that have a broad emission peaks around 529 nm which attributed to the oxygen vacancy [38]. The
sharp and weak deep level emission (DLE) peaks were observed in the Mn doped ZnO indicates that it has a lower concentration of oxygen vacancies. This result is in strong agreement with the report of [44] that the absent of some oxygen vacancy due to the Mn doping in ZnO causes decrease in the quantity of the defect. 4. Conclusions We have successfully extended the low temperature aqueous chemical growth method used to grow un-doped ZnO nanowires to achieved Mn-doped ZnO nanowires of high quality. This has been achieved by modifying the growth chemicals by adding Mn ion containing compound. The morphological, structural, and optical properties of the Mn-doped ZnO nanowires have been investigated. FESEM micrographs show that the Mn-doped ZnO nanowires have a nature of hexagonal wurtzite ZnO structure. EDX, XRD and XPS measurements indicated that the dopants have uniformly substituted positions onto zinc sites in the wurtzite lattice of the ZnO nanowires. Further, we have observed no second phase of dopant impurities or clustering in the Mn-doped ZnO grown nanowires. Photoluminescence spectroscopy shows a strong near band gap edge emission from the Mn-doped ZnO nanowires which was observed to be shifted 6 nm to lower wavelength, compared to undoped ZnO nanowires. This ACG method provides compatible technology for device fabrication on flexible large area substrates. The present results suggest that the existence of Mn element changes the structural and optical properties of the ZnO nanowires. The incorporation of transition metal ions such as Mn into ZnO nanowires via the low temperature growth method provide potential for the development of new wide range of components e.g. spintronics and sensor devices on any substrates. Moreover, this low temperature growth mechanism is a low cost, environmental friendly, easy to use, and is a low energy consumption method.
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Figure Captions Figure 1 (a) SEM image of ZnO nanowires (b) SEM image of Zn0.99Mn0.01O nanowires (c) SEM image of Zn0.95Mn0.05O nanowires (d) EDX spectrum of Zn0.95Mn0.05O nanowires Figure 2 (a) The XRD patterns of ZnO nanowires, Zn0.99Mn0.01O and Zn0.95Mn0.05O (b) The XRD patterns show the 002 peaks were shift to the lower 2 values Figure 3 (a) The XPS spectra of Zn0.95Mn0.05O nanowires (b) The XPS spectra of Zn2p, O1s and Mn2p Figure 4 Room temperature PL spectrum of Zn0.95Mn0.05O nanowires
Table captions Table 1 Lattice parameters and crystals size at (002) plane of undoped and doped Mn-ZnO nanowires Table 1 a (Å)
c (Å)
D (Å)
2.59754
3.00000
5.19509
632.456
0.15
2.59754
3.00276
5.19509
457.221
0.15
2.60339
3.00614
5.20679
547.849
Samples
FWHM (deg.)
ZnO
0.13
Zn0.99Mn0.01O Zn0.95Mn0.05O
d (Å)
Figure 1 a-d
Figure 2a
Figure 2b
Figure 3a
Figure 3b
Figure 4