Structural and Optical Properties of Mn Doped Zinc Oxide Shahid Husain, Lila A. Alkhtaby, Irshad Ahmad Bhat and M. Wasi Khan1 Department of Physics, Aligarh Muslim University, Aligarh 1 Department of Applied Physics, Aligarh Muslim University, Aligarh E-mail:
[email protected]
ABSTRACT Nanostructured semiconductors are of great interest due to their fascinating physiochemical properties, contrary to their bulk counterparts. Zinc oxide is a potential material for spintronic devices. That can also be used for fabrication of gas sensors, piezoelectric transducers and solar cell windows. We have synthesized the Mn doped zinc oxide nanoparticles using sol gel technique for Mn concentration x = 0.01, 0.02, 0.03 and 0.05. The samples are characterized with powder x-ray diffraction for phase purity. All the samples are found in single phase with wurtzite lattice structure. The UV/V spectra of nanoparticles indicate a decrease in the band gap from 3.08 eV to 3.05 eV with 1 % change in Mn concentration. But further increase in Mn concentration results in the increase of band gap to 3.12 eV. The Photoluminescence spectra are recorded in order to observe the effect of Mn doping on emission bands of ZnO.
INTRODUTION Zinc oxide (ZnO) based dilute magnetic semiconductors (DMS) have attracted great research interest for having a great potential of making various room temperature electromagnetic devices [1, 2]. ZnO based DMS have some advantages over other due to large band gap (~3.4 eV), large exciton binding energy at room temperature (~ 60meV) very short luminescence life time etc. Later on it has been identified as an excellent candidate for high Curie temperature magnetism when doped with transition metal ions, particularly Mn [3-5]. Some reports[6-7] on the observation of room temperature ferromagnetism in Mn doped ZnO attracted the attention of scientific community. These features make it a potential candidate for fabricating various optoelectronic and magneto-optical devices. The optical properties of nanostructured ZnO doped with transition metals are currently the subject of numerous investigations in response to a strong demand for nanostructured optoelectronic devices in the future. In view of above we have synthesized the Mn doped ZnO nanoparticles and studied their structural and optical properties.
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EXPERIMENTAL The samples of manganese doped zinc oxide (Zn1-x MnxO) are prepared by the sol-gel method. Zinc acetate dehydrate (CH3 COO)2 Zn.2H2 O and manganese nitrate hydrate have been dissolved in distilled water separately, then mixed together to get 200 mL volume with molarities of 0.6 M in different values of x = 0.01, 0.02, and 0.03 gm, the solution is stirred with a magnetic stirrer adding ammonia to protect the reaction group, the ammonia is added drop by drop until we get pH value 9 at room temperature to yield clear and homogenous gel then centrifuge it with 17300 rpm for a period of 10 s and then kept in a oven at 100°C .Finally we grind the samples and kept in a furnace at 400°C for 12 hours. The XRD patterns of Zn1-X Mn X O (x = 0.01, 0.02, and 0.03) samples are recorded with Rigaku Minifex x-ray diffractometer in 2θ range of 20 – 80 degrees. The absorption spectra are recorded using a Perkin Elmer LAMBDA 35 UV/Vis/ NIR system with main and second sample compartment so the spectrum was recorded by taking the main sample as reference and hence transmission due to the second sample only was obtained ,the sample was run using SBW 2nm for wavelength range from 800 to 250 nm with data interval of 1 nm ,the spectra are recorded as absorbance versus wavelength plots. RESULTS AND DISCUSSION The XRD patterns of Zn1-X MnXO (x = 0.01, 0.02, and 0.03) samples are shown in Fig.1(a),(b) and (c). All the patterns are found to have hexagonal wurtzite structure without any additional impurity phases, It means that the wurtzite structure is not affected by Mn doping . Moreover, as no extra peaks are detected, indicating that all the constituent precursors have been completely decomposed. Further, the 2θ values of the most intense peak are not altered with the increase in the Mn concentration. Lattice parameters are determined using PowderX software. The average crystallite size of the samples is estimated using a (101) peak broadening technique and are formed to be in the range of 20–25 nm as tabulated in Table 1.
(a)
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(b)
(c) Fig. 1. XRD patterns of Zn1-xMn xO( x= 0.01(a), 0.02(b),0.03(c))
Table 1. Some important parameters of Mn doped ZnO obtained from x-ray diffraction and UV/V spectroscopy Concentartion Concentration Crystallite Size (nm) 0.01 25.72 0.02 21.81 0.03 21.27
a ( Å)
c(Å)
3.32 3.32 3.32
5.26 5.30 5.31
Unit cell Volume (Å)3 68.72 71.58 71.75
Bandgap (eV) 3.08 3.05 3.11
We have recorded the absorption spectra of Zn1-xMnxO (x =0.01, 0.02, 0.03 and 0.05) in the ultraviolet/visible (UV/V) region of wavelength as shown in Fig.2. The absorption is found to decrease with the Mn concentration except for x = 0.03 sample that shows highest value of absorption for the whole measurement range.
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In the semi-conducting materials, the optical band gap (Eg) is defined as the energy where the absorption coefficient has a value >104 cm-1 .In order to find the value of Eg for oue samples we make use of the Tauc relation. αhν = A (hν - Eg )m where α is absorption coefficient given by α = 2.303 log (T/d)(d is the thickness of the sample and T is the transmission), hν is the photon energy. 0.40
Zn1-xMnxO
x = 0.03
0.35 x = 0.01
Absorption(A.U.)
0.30 x = 0.02
0.25 0.20 0.15 0.10 x = 0.05
0.05 200
300
400
500
600
700
800
Wavelength(nm)
Fig. 2. Absorption versus wavelength plots for Zn1-xMn xO (x =0.01, 0.02, 0.03 and 0.05).
Fig. 3. shows the plots of (αhν)2 versus hν for all the samples under investigation. Linearity of the plots indicates that the material is of direct bandgap nature.The values of Eg have been estimated by taking the intercept of the extrapolation to zero absorption with photon energy axis i.e. (αhν )2 → 0 . 20
x=0.01, Eg=3.08 eV x=0.02, Eg=3.05 eV
(αhν)
2
x=0.03, Eg=3.11 eV
10
0
2
3
4
5
hν(eV)
Fig. 3. Plots of (αhν)2 versus photon energy (hν) for Zn1-xMn xO (x =0.01, 0.02 and 0.03).
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The Eg is found to be 3.08 eV for 1% Mn doping and decreases to 3.05 for 2%. But further increase in Mn concentration results in increase of band gap. This change in the value of Eg depends on several factors such as grain size, carrier concentration, lattice strain etc. But for these samples we assume the lattice strain changes with the Mn doping and hence reflected as the variation of energy gap. It is well known that photoluminescence (PL) of ZnO shows a strong defect related mid-gap green emission. The most widely accepted origin of this emission is oxygen vacancies. The room temperature PL spectra for Zn1-xMnxO (x =0.01, 0.02, 0.03 and 0.05) are shown in Fig.4. The luminescence intensity decreases with increase in Mn doping from 1% to 2% but further increase of Mn concentration to 3% results in the increase of luminescence intensity but still remains lower as compared to 1% doped sample. On further increase of Mn concentration luminescence intensity falls lower but still remains more than 2% sample. For all the samples a sharp ultraviolet emission of the band edge luminescence is clearly visible at about 3.23 eV corresponding to a wavelength of approximately 382 nm. Another emission band is observed at about 3.41 eV for all the samples except for x = 0.02 concentration. While an emission band at about 2.63 eV corresponding to blue region of spectrum is also visible in the spectra. This band is more prominent for x = 0.03 and 0.05 samples. 22 20
Zn1-xMnxO
18 16 x = 0.01
Intensity(A.U.)
14 12 10 8
x = 0.03
6 x = 0.05
4 x = 0.02
2 0 340
360
380
400
420
440
460
480
500
520
wavelength(nm)
Fig. 4. Photoluminescence spectra of Zn1-xMnxO (x =0.01, 0.02, 0.03 and 0.05). CONCLUSIONS We have studied the structural and optical properties of Zn1-xMnxO (x =0.01, 0.02, 0.03 and 0.05). All the samples are found to be formed in single phase wurtzite structure. The particle size is in the range of 20-25 nm. The energy band gap determined using UV/V spectroscopy shows variation with the Mn concentration. The PL spectra show sharp ultraviolet bands at about 3.23 and 3.42 eV and not so strong band in the blue region of spectrum at about 2.63 eV.
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