Article pubs.acs.org/IECR
Low Temperature Mn Doped ZnO Nanorod Array: Synthesis and Its Photoluminescence Behavior Ajaya Kumar Singh,*,† Gautam Sheel Thool,† Prakriti Ranjan Bangal,‡ Sunkara Sakunthala Madhavendra,‡ and Surya Prakash Singh*,‡ †
Department of Chemistry, Govt. VYT PG Autonomous College Durg, Durg, Chhattisgarh 491001, India Inorganic and Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology, Uppal Road, Tarnaka, Hyderabad, Andhra Pradesh 500007, India
‡
S Supporting Information *
ABSTRACT: The present study focused on low temperature synthesis of Mn doped ZnO nanorod array film via chemical bath deposition method on glass substrates. Microstructural, morphological, and optical properties of Mn doped ZnO nanorods were investigated. X-ray diffraction patterns showed sharp and intense peaks, indicating the highly crystalline nature of the film. Energy dispersive X-ray (EDAX) results confirmed the presence of Mn ions in ZnO nanorods. Scanning electron microscopy (SEM) pictures suggested Mn doped ZnO nanorods were well aligned and distributed throughout the surface. Vibrational analysis has been carried out by Fourier transform infrared and Raman spectroscopy. Room temperature photoluminescence (PL) exhibited the presence of one broad defects related band in the visible region ranging 440−640 nm. Blue shifting in the absorption edge with Mn doping was observed in absorption spectra.
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INTRODUCTION Synthesis and study of ZnO nanostructures are currently attracting researchers. A variety of ZnO nanostructures have already being synthesized, such as, nanowires, nanobelts, nanotubes, nanoflowers, and nanorods. Among these, ZnO nanorods show unique superiority because of high surface-tovolume ratio, hence, large surface area1 and potential applications in solar cell2 and LEDs.3 ZnO is an n-type semiconductor of hexagonal wurtzite structure and having optical transparency in the visible range.4 The visible light emission of ZnO has great importance for white-light LEDs,5 and it can be achieved through doping of transition metal ions (TM) in ZnO lattice. Doping is the effective way to magically manipulate physical and optical properties of a wide band gap semiconductor. ZnO has a relatively large band gap (3.37 eV) and high exciton binding energy (60 meV) at room temperature which makes it an excellent host material for doping of TM.6 Among TM, incorporation of Mn2+ in ZnO lattice has been studied intensively. Most of the studies of Mn doped ZnO have been focused on its magnetic and spintronic properties, due to its dilute magnetic semiconductor behavior.7 Recently, Prabhakar et al.8 studied its optical properties for multispectral photodetectors and optical switches, but photoluminescence behavior of the Mn doped ZnO nanostructure is still an unfold area which could limit the application of this material. Several synthetic approaches have been made to synthesize Mn doped ZnO nanorods such as plasma-enhanced chemical vapor deposition (PECVD),9 chemical vapor deposition (CVD),10 and solution growth method.11 Chemical bath deposition (CBD) as well as SILAR both are solution growth processes which have attracted special interest for researchers, due to its simplicity, inexpensive equipment, and low deposition temperatures, which altogether results in low cost © XXXX American Chemical Society
processes and large area of scientific and industrial applications. In addition to this, the solution growth method has an advantage of using organic moieties as capping agents to control the size of ZnO nanostructures and can avoid problems associated with high temperature. In spite of the fact that Mn2+ has ∼14% solubility in ZnO lattice, incorporation of Mn in ZnO lattice via a low temperature method is still difficult because of higher bond energy for Mn−O compared to Zn−O, hence more energy is required to replace Zn2+ by Mn2+ in ZnO lattice12 compared to a high temperature method (like chemical vapor deposition), where it is easier to diffuse the Mn2+ from vapor into the host lattice. There are a few studies focused on solution phase synthesis of Mn doped ZnO nanorods and nanowires. Clavel et al.13 synthesized Mn doped ZnO nanowire at high temperature up to 310 °C. Vinod et al.14 and Li et al.15 demonstrated hydrothermal and solvothermal routes to prepare Mn doped ZnO nanorods which require special experimental conditions i.e. autoclave and maintaining the pressure during reaction. A nonaqueous sol−gel method was employed by Djerdj et al.11 which also need autoclave, temperature up to 200 °C, and long reaction time ∼3 days. Moreover these solution phase methods were employed for the synthesis of Mn doped ZnO nanorods or nanowires in powder form. Preparation of materials as films have one major advantage over powder form is that films can directly be applicable for device fabrication. Panigrahy et al.16 synthesized Mn doped ZnO nanorods via a single pot solution growth method, but the resultant nanorods was not well Received: January 7, 2014 Revised: April 4, 2014 Accepted: May 13, 2014
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Figure 1. Diagramatic representation of growth of Mn doped ZnO nanorods.
water (90 °C) for 10 s, then ultrasonically cleaning it for 30 s, and finally drying it in air for 30 s; this process completed a single deposition cycle. First dipping in Mn2+ solution, Mn oxide may adsorb on the surface of the substrate which acts as a nucleation center for ZnO.18 We repeated this cycle up to 15 times to get a thin and homogeneous film. As-prepared seed layers were subjected to annealing at 100 °C for 1 h to eliminate impurities of zinc hydroxide. Growth of Mn Doped ZnO Nanorods. Aqueous solutions of 0.1 M [Zn2+], 0.01 M [Mn2+] as dopant, 0.5 mL of 16.7 M triethanolamine as complexing agent, and NH3 were used to prepare the film of Mn doped ZnO nanorods (see Figure 1). First 10 mL [Zn2+] solution and 0, 0.1, 0.5, 0.75, and 1 mL of [Mn2+] solution namely samples ZM0, ZM1, ZM2, ZM3, and ZM4, respectively, were placed in a 50 mL beaker, and the solution was stirred for 10 min to get a clear and homogeneous solution. Thereafter, 0.5 mL of TEA was added with continuously stirring for 15 min. DD was added to make the volume up to 40 mL and then made the solution alkali with NH3 and pH of the bath maintained ≈10. The deposition temperature was varied from 50 to 90 °C with optimized temperature of 65 °C. The substrates were then placed tilted around 60° inside the beaker and heated up to 65 °C for 150 min. The growth of nanorods takes place downward surface of the substrate, and this surface was chosen for characterization. After deposition, the substrates were removed, washed in running tap water, rinsed in DD to remove soluble impurities, and then dried in air. Characterization Techniques. Mn doped ZnO nanorod films were subjected to different characterization techniques.
aligned to the substrate. Well aligned and ordered nanorods array is usually required for solar cell application.17 Compared to all of the above-mentioned methods, we synthesize Mn doped ZnO nanorods film at very low temperature (below 100 °C), less reaction time ∼2.5 h, and no requirement of any special experimental setup. Here we are reporting an effective way to incorporate Mn2+ in ZnO lattice by the modified SILAR assisted CBD method, and systematic evaluation of Mn doped ZnO nanorods were made by structural, morphological, and photoluminescence studies.
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EXPERIMENTAL SECTION Materials Used. Zinc sulfate monohydrate (ZnSO4·H2O, Merck limited, India), manganese(II) sulfate monohydrate (MnSO4·H2O, Molychem, India), triethanolamine (C6H15NO3, Finar Chemicals, India), and ammonia solution 25% (SDFCL, India) were used as precursors. All the chemicals were analytical reagent grade and were used without further purification. The commercial microscopic glass slides with size 1.45 × 75 × 25 mm3 were used as a substrate for the deposition of Mn doped ZnO nanorods. Before deposition, the substrates were boiled 2 h in chromic acid, cleaned with single distilled water (SD), double distilled water (DD), degreased with acetone, ultrasonically cleaned by DD, and finally dried in air. Preparation of Seed Layer. The seed layer of ZnO was prepared by SILAR. First dipping the substrate in 8 × 10−4 M solution of [Mn2+] for 10 s, then immediately immersing the substrate in an ammonium zincate solution, made up of 0.05 M [Zn2+] and NH3 for 10 s followed by dipping in hot distilled B
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The phase purity and microstructure of films was studied by Bruker D-8 Advance X-ray diffractometer with CuKα X-ray radiation (λ = 0.15496 nm). Surface morphology studies and elemental analysis of the nanorods were carried out employing Hitachi S3000N SEM and LINK ISIS-300 Oxford energy dispersive X-ray spectroscopy (EDAX) fitted to SEM HitachiS520. Optical absorption study was carried out at room temperature by Cary 5000 UV−vis NIR spectrophotometer. Photoluminescence (PL) measurements were performed by Cary eclipse fluorescence spectrophotometer at room temperature. FTIR spectra were obtained using Thermo Nicolet Nexus 670 spectrometer with 4 cm−1 resolution. Micro-Raman scattering measurement was carried out on Horiba Jobin-Yvon LABRAM HR to investigate the effect of the Mn dopant on the microstructures and vibration properties. Morphological Analysis. In order to investigate the effect of Mn doping on morphology of the ZnO film, we carried out SEM analysis. The morphology of undoped and Mn doped ZnO nanorod array grown on glass substrates is shown in Figure 2a-d. Vertically well-aligned hexagonal nanorods grown
Mn in ZnO nanorods are much lower than the amount added of Mn source in the deposition bath; such a situation was also noticed by Wu et al.12 The optical images of undoped and Mn doped ZnO nanorods films are shown in Figure 3. Here one can clearly observe that color of the films change from white to brownish yellow, indicated Mn doping in the samples.
Figure 3. Optical images of different samples.
XRD Analysis. The X-ray diffraction patterns of undoped and Mn doped ZnO nanorods have been recorded in the 2θ range of 20−80° (Figure 4a). All the diffraction peaks could be indexed to pure hexagonal wurtzite structure of ZnO nanorods with the most intense (002) diffraction peak revealing the nanorods grown along the c = [0001] axis and perpendicular to the substrate.19 No additional peaks related to Zn(OH)2, ZnO2 were observed, confirming the phase purity of ZnO nanorods. There was no change found in the wurtzite structure as Mn incorporated in ZnO lattice indicated that the presence of Mn does not alter the crystallization of ZnO. XRD data was also used to estimate lattice constants for pure ZnO nanorods and Mn doped ZnO. The lattice parameter ‘c’ has the value 5.16974 Å, calculated from the (002) diffraction plane for ZM0. The substitution of Zn2+ by Mn2+ ion in ZnO lattice leads to expansion in the unit cell, hence an increase in the lattice constant (Figure 4b) due to the fact that Zn2+ ion in tetrahedral coordination (radius = 0.60 Å) has smaller ionic radii as compared to the Mn2+ ion (radius = 0.66 Å). The increase of lattice parameters is nonmonotonic i.e. sample ZM-2 showed smaller value for ‘c’ compared to undoped ZnO nanorods which may be due to the phase impurity of Mn.11 The increment in unit cell volume with Mn doping concentration has been shown in Figure 4c, increasing unit cell volume (except for ZM-2) with the Mn doping percentage can be correlated to the higher ionic radius of Mn2+. To observe the effect of the Mn2+ doping concentration on the Zn−O bond length in the c-axis direction as well as the other three directions, the relation reported in the literature20 was employed and tabulated in Table 1. In general, an increasing trend in bond length with respect to the Mn doping percentage has been observed due to incorporation of Mn in ZnO lattice. We observed XRD peak shifting for Mn doped ZnO nanorods as compared to undoped ZnO nanorods. We have plotted a graph between XRD peak shifting of (002) planes against the Mn doping percentage [Figure 4d]. In general, the peaks shifted to lower angles except sample ZM-2 which may be due to the phase impurity of Mn. The relative intensity of the (002) plane is often used to estimate the degree of texture of ZnO based films. Texture is commonly defined as the distribution of crystallographic orientation of a polycrystalline material. We employed a relation described in the literature21 to
Figure 2. Representation of SEM images (a) ZM0, (b) ZM1, and (c) and (d) different magnification of ZM2.
throughout the substrates can be observed clearly for undoped and Mn doped samples. The diameter of the ZnO nanorod was estimated using ImageJ software. The average diameter for sample ZM0 was 0.26 μm, whereas for samples ZM1 and ZM2 the average diameter was found to be 0.52 and 0.54 μm, respectively. A significant increment in size of nanorods upon Mn doping is due to the fact that Mn2+ ionic size is larger than that of Zn2+. Wu et al.12 also observed such an increase in diameter of Mn doped ZnO rods; they argued that the higher bond energy of Mn−O could assist the coalescence process and lead to formation of larger-diameter nanorods. The EDAX analysis provided precise composition of the elements present in the material. The incorporation of Mn into ZnO nanorods was confirmed by EDAX. The atomic percentage of Mn was found to be 0, 0.015, 0.22, 0.33, and 0.74% calculated for samples ZM0, ZM1, ZM2, ZM3, and ZM4 from EDAX (SI, Figure S1). Here, it can be observed that estimated amounts of C
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Figure 4. (a) XRD patterns of undoped and Mn doped ZnO nanorods films. (b) Variation of lattice constant ‘c’ with the Mn doping percentage. (c) Variation of unit cell volume with the Mn doping percentage. (d) Peak shifting (002 plane) with the Mn doping percentage.
nanorods is ∼86% for sample ZM1 which is well comparable with the literature.22 Raman Analysis. Micro-Raman scattering technique is one of the most effective methods to probe the crystalline property, disorder, and defects in the nanostructure materials which can influence the photoluminescence properties of Mn doped ZnO nanorods. The room temperature micro-Raman spectra of ZnO and Mn doped ZnO is shown in Figure 5 in the range of 200− 800 cm−1. ZnO and Mn doped ZnO nanorods have a wurtzite structure with space group P63mc (C46ν) with two formula units per primitive cell, where all the atoms occupy the C3ν sites.23 Group theory predicts that two A1, two E1, two E2, and two B1 modes are present in the Raman spectra of ZnO. E2 (high) phonon mode and E2 (low) phonon modes are associated with
Table 1. Calculated Zn−O Bond Lengths and Relative Intensity Ratio for Different Samples samples
bond length in c-axis direction (Å)
bond length in other three direction (Å)
relative intensity ratio
ZM-0 ZM-1 ZM-2 ZM-3 ZM-4
1.963 52 1.963 62 1.962 31 1.977 31 1.983 17
1.963 57 1.963 63 1.962 32 1.977 29 1.983 06
0.5138 0.8572 0.5333 0.7456 0.6667
calculate the preferred orientation of nanorods and summarized in Table 1. The maximum degree of alignment of ZnO D
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the Raman selection rule, only E2 and A1 longitudinal optical modes (LO) are allowed vibrational modes, if incident light is exactly perpendicular to surface.24 The E2 (high) mode corresponding to a wavenumber of 437.8 cm−1 was observed in ZM0. This mode is characteristic of the wurtzite structure of ZnO (consistent with the XRD results) and can be assigned for lattice vibration of oxygen atoms. Venkatesh et al.25 suggested the E2 (high) mode can be used to characterize the stress in ZnO lattice. For ZM1 nanorods E2 (high) modes appeared at 437.1 cm−1; this red shifting of Raman modes can be ascribed to the local stress arising due to incorporation of Mn2+ ions into the Zn2+ lattice sites. This kind of red shift in Raman spectra of Mn doped ZnO has also been observed previously.16 As we increase in Mn doping concentration, the E2 (high) mode slightly shifted to lower energy and the peak broadened asymmetrically (Figure 5). The reason behind this behavior being similar is explained elsewhere in the literature.26 The peak at around 580 cm−1 in sample ZM1 arises due to the oxygen deficiency such as oxygen vacancies;27 this result is well matched with PL analysis. The Raman spectra also depict the presence of Mn in ZnO nanorods. The peaks at ∼415 and ∼576 cm−1 can be assigned for E1 transverse optical modes (TO) and A1 (LO) phonon modes. The origin of the A1 (LO) mode is due to the zinc interstitial defect.28 One silent mode of ZnO, B1 (low), appeared at ∼268 cm−1 due to incorporation of dopants leading to structural disorder which can be disturbing to the translational symmetry of lattice hence activating the silent mode.29 The second order Raman mode at ∼324 cm−1 is arising due to E2 (high)-E2 (low) multiple scattering phenomenons. This result was also found by Gayen et al.28 and Gao et al.30 for Ni doped ZnO nanorods and ZnO nanorods, respectively. One additional mode at 224 cm−1 might be related to the defect induced mode.31 The peak located at 660 cm−1 may be due to oxygen vacancies, zinc interstitial, and
Figure 5. Room temperature micro-Raman spectra of undoped and Mn doped ZnO nanorods.
oxygen atoms and Zn sublattice, respectively. Gaussian multiple peak fitting to Raman spectra gave different Raman bands as follows: ∼224 cm−1, ∼268 cm−1, ∼324 cm−1, ∼415 cm−1, ∼437 cm−1, ∼576 cm−1, ∼660 cm−1, and ∼743 cm−1. According to
Figure 6. (a) and (b) FTIR spectra of undoped ZnO (ZM0) and Mn doped ZnO (ZM1). E
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antisite oxygen defects induced Raman mode, well matched with that reported by Yang et al.32 In the literature, one additional Raman mode at 523−528 cm−1 is usually observed for Mn doped ZnO nanopowder or thin films, but the exact region of origin of this mode is still unclear. Rao et al.33 suggested that the peak at 525 cm−1 originated due to substitutional Mn ions or Mn-doping-induced defects. Yadav et al.34 showed the Raman mode at 524 cm−1 in Mn-doped ZnO is due to the disorder induced activated 2B1 (low) silent mode of ZnO, whereas Cong et al.35 demonstrated that the peak at 524−527 cm−1 might be due to the local vibration of the Mn ions at the Zn sites. However, a computational study using a real-space recursion method36 stated that this vibrational mode does not originate directly from the vibration of local Mn atoms but from the vibration of Zn ions in a Mn-rich local environment, where the O sites around Zn ions are partially occupied by Mn ions. Hu et al.37 described the peak at ∼528 cm−1 in Mn-doped ZnO is the characteristic Raman mode of Mn2O3 which showed the presence of the Mn2O3 precipitate in the Mn doped ZnO sample. On the basis of their observation, we can reasonably conclude that the absence of a peak at 523−528 cm−1 in our sample is due to the absence of the Mn2O3 precipitate in our samples. Panigrahy et al.16 also did not observe an additional mode at 523−524 cm−1 in Mn doped ZnO nanorods. FTIR Analysis. For the detection of various functional groups participating in the formation of ZnO nanorods, FTIR measurement was carried out in the wavenumber range from 400 to 4000 cm−1 using the KBr method at room temperature shown in Figure 6a and 6b for Mn doped and undoped ZnO nanorods, respectively. A peak appeared in the lower energy region at 419.31 cm−1 showing the Zn−O bond bending vibration.38 The broad peak in higher energy region at 3200− 3400 cm−1 is due to the stretching vibration of the O−H group, and the peak at 1631.89 cm−1 is due to O−H bending vibration. The C−H stretching vibration band arises at around 2924.87 cm−1 and depicts the presence of an alkyl group; these peaks revealed that the presence of an organic moiety in ZnO nanorods which may triethanol amine residues, acting as capping agents during the growth of ZnO nanorods. UV−visible Spectra. Optical absorption spectra of undoped and Mn doped ZnO nanorods were recorded in the wavelength range of 300 to 800 nm (Figure 7). The enlarged view of spectra is represented in the inset (wavelength range 350−410 nm) for the sake of clarity of the absorption edge. The absorption edge showed a trend to shift toward the lower wavelength with increasing doping concentration indicated the band gap enlargement of ZnO with Mn doping concentration. Such a situation was also observed by Phan et al.19 and Hao et al.39 Photoluminescence Spectra. In order to study of doping induced defects on the photoluminescence behavior of ZnO nanorods, emission spectra were recorded. Roughly, PL spectra can be divided into two bands namely band edge emission which arises due to the recombination of free excitons (electron and hole pair) through an exciton−exciton collision process, and defects level emission in the visible region appears due to the recombination of electron−hole related to intrinsic defects, such as oxygen vacancies (VO), zinc vacancy (VZn), interstitial Zn (Zni), interstitial O (Oi), Zn antisite (ZnO), and oxygen antisites defect OZn (oxygen at zinc site).9 Photoluminescence spectra of Mn doped ZnO nanorods at room temperature have been recorded in the wavelength range 350−650 nm with the
Figure 7. Absorption spectra of undoped and Mn doped ZnO nanorods. The enlarged region from 350 to 425 nm is shown in the inset.
340 nm excitation wavelength and are presented in Figure 8a. It consists of an ultraviolet (UV) emission peak, called a band edge emission which is centered at 383.63 (3.23 eV) nm and a broad defect level (DL) band ranging from 440 to 640 nm, which can be readily resolved by Gaussian curve fitting into two bands located at 516.76 nm (2.4 eV) and 582.52 nm (2.13 eV), respectively. The visible emission mechanism in ZnO is still unclear; it was explained that more than one source is responsible for its origin and affected by growth and experimental conditions.40 A representative Gauss fit deconvoluted PL spectra of sample ZM4 is shown in Figure 8b. Visible bands at 2.4 and 2.13 eV can be ascribed as a green band and a yellow band. The defects play an important role in the visible emission of ZnO nanorods. The formation energy of the oxygen vacancy (Vo) is lowest among all point defect in ZnO and is the most probable candidate for the green band found in the literature.41,42 Vanheusden et al.43 found that the green band at 510 nm (2.43 eV) originated by recombination of singly charged oxygen vacancy (Vo+) and holes from the valence band. Egelhaaf et al.44 demonstrated this green band result from transitions between oxygen vacancies and Zn vacancies. Behara et al.45 assigned the peak at 525 nm (2.36 eV) for transition between the VoZni level and a valence band. Lin et al.46 suggested a green band at 521 nm (2.38 eV) appeared due to the electronic transition between the conduction band and the oxygen antisite defects (Ozn) level; this green emission band showed a close agreement with our results (2.4 eV). Another luminescence band at 2.13 eV is probably due to the complex defect level like VoZni rather than the single point defect in ZnO nanorods, which is suggested by Wei et al.47 We also observed that the intensity of visible emission increased for Mn doped samples as compared to an undoped one, revealing that incorporation of Mn disturbed the ZnO lattice, hence increasing the defects concentration, which is clearly visible in the PL spectra. A similar result was also found by Ronning et al.48 in Mn doped ZnO nanobelts. This enhancement in visible emission has not shown a trend with the Mn doping percentage, indicating incorporation of Mn was not directly related to any specific point defects in ZnO nanorods. F
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the Mn ion implanted ZnO nanorod which is probably due to the fact that incorporation of Mn leads to an increase in the point defect concentration in ZnO nanorods.
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ASSOCIATED CONTENT
S Supporting Information *
EDAX data of Mn doped ZnO nanorods films i.e. sample ZM1, ZM2, ZM3, and ZM4. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS A.K.S. extends his gratitude towards University Grants Commission, India for financial assistance in the form of a project [F.No. 40-99/2011(SR)]. The authors thank Dr. V. J. Rao for UV and PL measurements. S.P.S. thanks XII FY CSIRINTELCOAT (CSC0114) for financial support.
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REFERENCES
(1) Shalish, I.; Temkin, H.; Narayanamurti, V. Size-Dependent Surface Luminescence in ZnO Nanowires. Phys. Rev. B 2004, 69, 245401. (2) Edri, E.; Cohen, H.; Hodes, G. Band Alignment in Partial and Complete ZnO/ZnS/CdS/CuSCN Extremely Thin Absorber Cells: An X-ray Photoelectron Spectroscopy Study. ACS Appl. Mater. Interfaces 2013, 5, 5156−5164. (3) Park, W. I.; Yi, G.-C. Electroluminescence in n-ZnO Nanorod Arrays Vertically Grown on p-GaN. Adv. Mater. 2004, 16, 87−90. (4) Raoufi, D.; Raoufi, T. The Effect of Heat Treatment on the Physical Properties of Sol−Gel Derived ZnO Thin Films. Appl. Surf. Sci. 2009, 255, 5812−5817. (5) Peng, W. Q.; Qu, S. C.; Cong, G. W.; Wang, Z. G. Structure and Visible Luminescence of ZnO Nanoparticles. Mater. Sci. Semicond. Process. 2006, 9, 156−159. (6) Ozgur, U.; Alivov, Y. I.; Liu, C.; Take, A.; Reshchikov, M. A.; Dogan, S.; Avrutin, V.; Cho, S. J.; Markoc, H. A Comprehensive Review of ZnO Materials and Devices. J. Appl. Phys. 2005, 98, 041301. (7) Liu, J. J.; Yu, M. H.; Zhou, W. L. Fabrication of Mn-Doped ZnO Diluted Magnetic Semiconductor Nanostructures by Chemical Vapor Deposition. J. Appl. Phys. 2006, 99, 08M119. (8) Prabhakarm, R. R.; Mathews, N.; Jinesh, K. B.; Karthik, K. R. G.; Pramana, S. S.; Varghese, B.; Sow, C. H.; Mhaisalkar, S. Efficient Multispectral Photodetection Using Mn Doped ZnO Nanowires. J. Mater. Chem. 2012, 22, 9678−9683. (9) Hu, P.; Han, N.; Zhang, D.; Ho, J. C.; Chen, Y. Highly Formaldehyde-Sensitive, Transition-Metal Doped ZnO Nanorods Prepared by Plasma-Enhanced Chemical Vapor Deposition. Sens. Actuators, B 2012, 69, 74−80. (10) Baik, J. M.; Lee, J.-L. Fabrication of Vertically Well-Aligned (Zn,Mn)O Nanorods with Room Temperature Ferromagnetism. Adv. Mater. 2005, 17, 2745−2748. (11) Djerdj, I.; Garnweitner, G.; Arcon, D.; Pregelj, M.; Jaglicic, Z.; Niederberger, M. Diluted Magnetic Semiconductors: Mn/Co-doped ZnO Nanorods As Case Study. J. Mater. Chem. 2008, 18, 5208−5217. (12) Wu, D.; Huang, Z.; Yin, G.; Yao, Y.; Liao, X.; Han, D.; Huang, X.; Gu, J. Preparation, Structure and Properties of Mn-Doped ZnO Rod Arrays. CrystEngComm 2010, 12, 192−198. (13) Clavel, G.; Pinna, N.; Zitoun, D. Magnetic Properties of Cobalt and Manganese Doped ZnO Nanowires. Phys. Stat. Sol. A 2007, 204, 118−124.
Figure 8. (a) PL spectra of undoped and Mn doped ZnO nanorods. (b) A Gauss fit deconvoluted PL spectra of sample ZM4.
The intensity of a UV emission peak at ∼380 nm became stronger with Mn doping. This can be explained by the fact that MnO has a larger band gap compared to ZnO, hence electron/ holes confine more efficiently in Mn doped ZnO nanorods, and their recombination gives rise to the UV emission peak.12 The enhancement in UV emission intensity did not follow any linear relation with the Mn doping percentage.
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CONCLUSION In conclusion, we successfully synthesized high density Mn doped ZnO nanorod array film by a simple and facile low temperature solution growth CBD method. The synthesized materials are polycrystalline in nature having wurtzite structure and preferentially oriented along the c-axis, and the maximum degree of alignment of ZnO nanorods was found to be ∼86% for sample ZM1. SEM images confirmed the presence of ZnO nanorods. Incorporation of Mn2+ into the ZnO crystal lattice is strongly evident from EDAX and Raman analysis. The doping percentage of Mn2+ was varied as 0.015, 0.22, 0.33, and 0.74%. The presence of Mn in ZnO lattice changed the bond length and unit cell volume which has been demonstrated by XRD analysis. Gauss fitted PL spectra exhibited the presence of two visible bands, one green band at 2.4 eV and another at 2.13 eV. The intensity of the visible band was found to be increased for G
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(14) Vinod, R.; Bushiri, M. J.; Sajan, P.; Achary, S. K. R.; MuñozSanjosé, V. Mn2+-Induced Room-Temperature Ferromagnetism and Spin-Glass Behavior in Hydrothermally Grown Mn-Doped ZnO Nanorods. Phys. Stat. Sol. A 2014, DOI: 10.1002/pssa.201330394. (15) Li, J.; Fan, H.; Chen, X.; Cao, Z. Structural and Photoluminescence of Mn-Doped ZnO Single-Crystalline Nanorods Grown via Solvothermal Method. Colloids Surf., A 2009, 349, 202−206. (16) Panigrahy, B.; Aslam, M.; Bahadur, D. Aqueous Synthesis of Mn- and Co-Doped ZnO Nanorods. J. Phys. Chem. C 2010, 114, 11758−11763. (17) Lockett, A. M.; Thomas, P. J.; O’Brien, P. Influence of Seeding Layers on the Morphology, Density, and Critical Dimensions of ZnO Nanostructures Grown by Chemical Bath Deposition. J. Phys. Chem. C 2012, 116, 8089−8094. (18) Kokotov, M.; Biller, A.; Hodes, G. Reproducible Chemical Bath Deposition of ZnO by a One-Step Method: The Importance of “Contaminants” in Nucleation. Chem. Mater. 2008, 20, 4542−4544. (19) Phan, T.-L.; Yu, S. C. Optical and Magnetic Properties of Zn1−xMnxO Nanorods Grown by Chemical Vapor Deposition. J. Phys. Chem. C 2013, 117, 6443−6453. (20) Mote, V. D.; Purushotham, Y.; Dole, B. N. Structural and Morphological Studies on Mn Substituted ZnO Nanometer-Sized Crystals. Cryst. Res. Technol. 2011, 46, 705−710. (21) Thool, G. S.; Singh, A. K.; Singh, R. S.; Gupta, A.; Susan, Md. A. B. H. Facile Synthesis of Flat Crystal ZnO Thin Flms by Solution Growth Method: A Micro-Structural Investigation. J. Saudi Chem. Soc. 2014, http://dx.doi.org/10.1016/j.jscs.2014.02.005. (22) Gurav, K. V.; Patil, U. M.; Pawar, S. M.; Kim, J. H.; Lokhande, C. D. Controlled Crystallite Orientation in ZnO Nanorods Prepared by Chemical Bath Deposition: Effect of H2O2. J. Alloys Compd. 2011, 509, 7723−7728. (23) Vinodkumar, R.; Lethy, K. J.; Arunkumar, P. R.; Krishnan, R. R.; Pillai, N. V.; Pillai, V. P. M.; Philips, R. Effect of Cadmium Oxide Incorporation on the Microstructural and Optical Properties of Pulsed Laser Deposited Nanostructured Zinc Oxide Thin Films. Mater. Chem. Phys. 2010, 121, 406−413. (24) Zhang, Y.; Jia, H. B.; Wang, R. H.; Chen, C. P.; Luo, X. H.; Yu, D. P. Low-Temperature Growth and Raman Scattering Study of Vertically Aligned ZnO Nanowires on Si Substrate. Appl. Phys. Lett. 2003, 83, 4631−4633. (25) Venkatesh, P. S.; Purushothaman, V.; Muthu, S. E.; Arumugam, S.; Ramakrishnan; Jeganathan, K.; Ramamurth, K. Role of Point Defects on the Enhancement of Room Temperature Ferromagnetism in ZnO Nanorods. CrystEngComm 2012, 14, 4713−4718. (26) Phan, T.-L.; Yu, S. C.; Vincent, R.; Bui, H. M.; Thanh, T. D.; Lam, V. D.; Lee, Y. P. Influence of Mn Doping on Structural, Optical, and Magnetic Properties of Zn1−xMnxO Nanorods. J. Appl. Phys. 2010, 108, 044910. (27) Wu, J. J.; Liu, S. C. Catalyst-Free Growth and Characterization of ZnO Nanorods. J. Phys. Chem. B 2002, 106, 9546−9551. (28) Gayen, R. N.; Rajaram, A.; Bhar, R.; Pal, A. K. Ni-Doped Vertically Aligned Zinc Oxide Nanorods Prepared by Hybrid Wet Chemical Route. Thin Solid Films 2010, 518, 1627−1636. (29) Hu, Y. M.; Wang, C. Y.; Lee, S. S.; Han, T. C.; Chou, W. Y.; Chen, G. J. Identification of Mn-Related Raman Modes in Mn-Doped ZnO Thin Films. J. Raman Spectrosc. 2011, 42, 434−437. (30) Gao, H.; Yan, F.; Li, J.; Zeng, Y.; Wang, J. Synthesis and Characterization of ZnO Nanorods and Nanoflowers Grown on GaNBased LED Epiwafer Using a Solution Deposition Method. J. Phys. D. Appl. Phys. 2007, 40, 3654. (31) Manjon, F. J.; Mari, B.; Serrano, J.; Romero, A. H. Silent Raman Modes in Zinc Oxide and Related Nitrides. J. Appl. Phys. 2005, 97, 053516. (32) Yang, L. W.; Wu, X. L.; Huang, G. S.; Qiu, T.; Yang, Y. M. In Situ Synthesis of Mn-Doped ZnO Multileg Nanostructures and MnRelated Raman Vibration. J. Appl. Phys. 2005, 97, 014308. (33) Rao, Y.; Xu, H.; Liang, Y.; Hark, S. Synthesis, micro-structural and magnetic properties of Mn-doped ZnO nanowires. CrystEngComm 2011, 13, 2566−2570.
(34) Yadav, H. K.; Sreenivas, K.; Katiyar, R. S.; Gupta, V. Defect Induced Activation of Raman Silent Modes in RF Co-Sputtered Mn Doped ZnO Thin Films. J. Phys. D Appl. Phys. 2007, 40, 6005. (35) Cong, C. J.; Liao, L.; Liu, Q. Y.; Li, J. C.; Zhang, K. L. Effect of Temperature on the Ferromagnetism of Mn-doped ZnO Nanoparticles and Mn-Ralated Raman Vibration. Nanotechnology 2006, 17, 1520. (36) Zhong, H. M.; Wang, J. B.; Chen, X. S.; Li, Z. F.; Xu, W. L.; Lu, W. J. Effect of Mn+ ion Implantation on the Raman Spectra of ZnO. Appl. Phys. 2006, 99, 103905. (37) Hu, Y. M.; Wang, C. Y.; Lee, S. S.; Han, T. C.; Chou, W. Y.; Chen, G. J. Identification of Mn-Related Raman Modes in Mn-Doped ZnO Thin Films. J. Raman Spectrosc. 2011, 42, 434−437. (38) Wei, X. Q.; Zhang, Z. G.; Liu, M.; Chen, C. S.; Sun, G.; Xue, C. S.; Huang, H. Z.; Man, B. Y. Annealing Effect on the Microstructure and Photoluminescence of ZnO Thin Films. Mater. Chem. Phys. 2007, 101, 285−290. (39) Hao, Y.-M.; Lou, S.-Y.; Zhou, S.-M.; Yuan, R.-J.; Zhu, G.-Y.; Li, N. Structural, Optical, and Magnetic Studies of Manganese-Doped Zinc Oxide Hierarchical Microspheres by Self-Assembly of Nanoparticles. Nanoscale Res. Lett. 2012, 7, 100. (40) Lv, J.; Li, C.; BelBruno, J. J. Defect Evolution on the Optical Properties of H+-Implanted ZnO Whiskers. CrystEngComm 2013, 15, 5620−5625. (41) Tay, Y. Y.; Tan, T. T.; Boey, F.; Liang, M. H.; Ye, J.; Zhao, Y.; Norby, T.; Li, S. Correlation Between the Characteristic Green Emissions and Specific Defects of ZnO. Phys. Chem. Chem. Phys. 2010, 12, 2373. (42) Gong, Y.; Andelman, T.; Neumark, G. F.; O’Brien, S.; Kuskovsky, I. L. Origin of Defect-Related Green Emission from ZnO Nanoparticles: Effect of Surface Modification. Nanoscale Res. Lett. 2007, 2, 297−302. (43) Vanheusden, K.; Warren, W. L.; Seager, C. H.; Tallant, D. R.; Voiget, J. A.; Gnade, B. E. Mechanisms Behind Green Photoluminescence in ZnO Phosphor Powders. J. Appl. Phys. 1996, 79, 7983. (44) Egelhaaf, H. J.; Oelkrug, D. Luminescence and Nonradiative Deactivation of Excited States Involving Oxygen Defect Centers in Polycrystalline ZnO. J. Cryst. Growth 1996, 161, 190−195. (45) Behera, D.; Acharya, B. S. Nano-Star Formation in Al-doped ZnO Thin Film Deposited by Dip-Dry Method and its Characterization using Atomic Force Microscopy, Electron Probe Microscopy, Photoluminescence and Laser Raman Spectroscopy. J. Lumin. 2008, 128, 1577−1586. (46) Lin, B.; Fu, Z.; Jia, Y. Green Luminescent Center in Undoped Zinc Oxide Flms Deposited on Silicon Substrates. Appl. Phys. Lett. 2001, 79, 943−945. (47) Wei, S.; Lian, J.; Wu, H. Annealing Effect on the Photoluminescence Properties of ZnO Nanorod Array Prepared by a PLDAssistant Wet Chemical Method. Mater. Charact. 2010, 61, 1239− 1244. (48) Ronning, C.; Gao, P. X.; Ding, Y.; Wang, Z. L. ManganeseDoped ZnO Nanobelts for Spintronics. Appl. Phys. Lett. 2004, 84, 783−785.
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dx.doi.org/10.1021/ie500077v | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
