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The Study of Surface Plasmon Enhanced Emission of ZnO Nanorods on Plasmonic Ag Nanorods Array
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 2 (2015) 4407 – 4412
International Conference on Nano Science & Engineering Applications. ICONSEA-2014
The study of surface plasmon enhanced emission of ZnO nanorods on plasmonic Ag nanorods array Anil Kumar Pal, D. Bharathi Mohan* Department of Physics, School of Physical, Chemical and Applies Sciences, Pondicherry University, Pondicherry-605014, India
Abstract Ag islands and nanorods (NRs) array were fabricated through glancing angle deposition (GLAD) method at an angle of 85 qC by using thermal evaporation technique. Subsequently, ZnO NRs were grown along the side of Ag NRs arrays through hydrothermal reaction. The crystal structure of ZnO/Ag hybrid structure was studied by Glancing Incident X-Ray Diffraction (GIXRD). The surface morphologies of Ag NRs arrays and ZnO NRs were studied through AFM and SEM techniques. The presence of different vibrational energy modes of ZnO structure was studied through confocal Raman spectrometer. The surface plasmon enhanced fluorescence of ZnO NRs was probed through fluorescence spectrophotometer. The presence of strong UV emission and the absence of intense visible emission of ZnO NRs are due to the existence of fewer crystal defects which could make the hybrid structure precious for UV LEDs. Keywords: Thermal evaporation; GLAD, Ag NRs; Hydrothermal reaction; ZnO NRs and surface Plasmon enhanced emission.
1. Introduction ZnO is one of the most promising material exhibits a wide direct band gap of 3.37 eV at 300 K, a large exciton binding energy of 60 MeV and excellent chemical and thermal stabilities. Therefore, ZnO based nanomaterials have attracted much interest especially NRs and nanowires due to their remarkable physical and chemical properties for which ZnO is an excellent candidate for the potential application in nanoscale electronic, light emitting diodes, field emission devices, chemical sensors and energy conversion devices [1]. However many researches are being performed to enhance the luminescence of light-emitting to get very bright light. One of the important aspects is to enhance the fluorescence through surface plasmon-exciton coupling which is achievable in semiconductor/metal hybrid structures [1-3]. The emission enhancement from ZnO depends on morphology (size and shape) and roughness of inter layer metal film which is the most significant for the matching of SPs to emission band energies [3]. In this work, glancing angle deposition (GLAD) of Ag thin films of mass thickness of 1 and 10 nm were carried out on glass as well as Si substrates by thermal evaporation technique to fabricate Ag nanorod arrays. Subsequently ZnO NRs were grown on Ag films by hydrothermal process. The crystal structure, surface morphology, chemical structure, optical absorbance and emission of ZnO/Ag nanostructures have been studied. * Corresponding author. Tel.: +91 413 2654 786. E-mail address: [email protected]
2214-7853 © 2015 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the conference committee members of the International conference on Nano Science & Engineering Applications - 2014 doi:10.1016/j.matpr.2015.10.040
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Anil Kumar Pal and D. Bharathi Mohan / Materials Today: Proceedings 2 (2015) 4407 – 4412
2. Experimental details Silver films of mass thickness 1 and 10 nm were deposited over ultrasonically cleaned glass and Si substrates by thermal evaporation of Ag powder (99.9%, sigma Aldrich, USA) at a low pressure of 1.5× 10 -6 mbar using GLAD method. The substrate placed above the source was inclined with an angle of 85q in the direction of vapor flux. Subsequently, ZnO NRs were grown on as deposited Ag films by following hydrothermal reaction. In hydrothermal reaction, Ag film is immersed vertically in an aqueous solution containing equimolar (0.05 M) concentration of zinc nitrate hexahydrate and hexamethylenetetramine (extra pure, FINAR, India) of each 25 ml and then refluxed in a hot air oven at 90 °C for 5 hrs. Finally, the films drawn from the precursor solution and dried at 100 °C after rinsing with copious amount of de-ionized water. The crystal structure of ZnO/Ag nanostructure was studied by X-Ray Diffraction (XRD) (PANalytical, X’Pert PRO). The surface morphology of Ag films and ZnO/Ag hybrid structures were depicted by using Atomic Force Microscope (AFM, tapping mode in air) (Nanoscope) and scanning electron microscope (SEM) (Hitachi: S-3400N) respectively. Elemental analysis was probed through energy dispersive X-ray spectrometer (Thermo Super Dry II) attached with SEM. The optical absorption of Ag films were taken using UV-Visible spectrophotometer (200-1200 nm, Ocean Optics, HR 4000) and the emission spectra of ZnO/Ag films were recorded with an excitation wavelength of 325 nm by using Spectrofluorometer (FLUOROLOG-FL3-11) which is equipped with a source of Xenon lamp-450 W. Raman spectra were recorded by a Confocal Raman spectrometer (Reinshaw inVia Raman Microscope, UK) with an excitation wavelength of 488 nm using Ar+ laser source with an acquisition time of 30 seconds. The laser power was maintained at 1.5 mW. 3. Result and discussions 3.1. Surface morphology of Ag films (AFM image analysis) Atomic force microscopy (AFM) measurement of Ag NRs array film is performed to study the evolution of surface properties. Ag film of 1 nm mass thickness consists of islands with the average size of 35 nm as observed in figure 1(a). However, the film of 10 nm mass thickness shows the formation of Ag NRs type structures as demonstrated in figure 1(b). It is observed that the NRs are not symmetrical along the length whereas the particles are elongated in one direction makes the nanoparticles columnar growth with average diameter of 22 nm. At the initial stage of thermal deposition, the vaporized Ag atoms deposited onto the substrate surface. Once the sufficient amount of Ag atoms reached the substrate, the nucleation process starts with the formation of Ag islands. The GLAD technique provides an advantage of stacking of incoming vaporized Ag atoms on to the Ag islands which happens due to the shadowing effect mainly responsible for the formation of columnar structures [4]. As the deposition thickness increases, the Ag film surface becomes rougher due to the formation of nanorod arrays. Hence the RMS surface roughness of Ag film of 1 nm mass thickness is found to be 4.8 nm which increases to 1.06 nm for 10 nm Ag film.
Fig. 1. AFM images of Ag films (a) 1 nm mass thickness shows Ag islands, (b) 10 nm mass thickness film shows Ag NRs array.
Anil Kumar Pal and D. Bharathi Mohan / Materials Today: Proceedings 2 (2015) 4407 – 4412
3. 2. Optical absorption Surface plasmon resonance absorption spectra from Ag island film and NRs array structures are shown in figure 2. It can be seen that both Ag island film and nanorod arrays film show two resonance peaks. One sharp absorption peak in ultra violate region around 380 nm for both the films without any shift is due to quadrupolar resonances of Ag nanoparticles [5]. Another absorbance peak observed at 432 nm for Ag island film blue shifted to 424 nm for nanorod array film are attributed to the dipole oscillations of free electrons which are parallel to the substrate plane known as in-plane mode of oscillations [5]. The presence of these two resonance peaks suggests that the shape of nanoparticles is not spherical, rather asymmetric. It can be noticed that the short wave length peak is more prominent compared to dipolar absorption peak in case of island film, however it diminishes in case of nanorod array film. The SPR peak supposed to be red shifted in case of Ag nanorod arrays compared to island film due to decrease in surface to volume ratio [6], however there is a small blue shift is observed could be due to the decrease in the interparticle distance [7].
Fig. 2. Optical absorption spectra of Ag islands and NRs array film. Two resonance peaks are observed suggests the asymmetric shape of Ag nanoparticles.
3. 3. Crystal structure analysis Figure 3 shows the XRD patterns of ZnO films grown on Ag NRs array film. The diffraction peaks (100), (002), (101), (102), (103) and (201) are in good agreement with the JCPDS card (No. 36-1451) confirming the formation of wurtzite ZnO crystals. An evolution of a diffraction peak observed at 38.17 in 2θ degree in case of ZnO film grown on Ag NRs array correspond to (111) crystal plane of Ag which is in good agreement with the JCPDS card (no. 04-0783) for cubic structure of silver. However, the XRD spectra exhibit a less intense (002) diffraction peak indicating random orientation in the growth of ZnO nanoparticles. This random orientation is shown in scanning electron micrograph which is discussed section 3.4.
Fig. 3. X-ray diffraction patterns of ZnO NRs grown on Ag islands and NRs array film.
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Anil Kumar Pal and D. Bharathi Mohan / Materials Today: Proceedings 2 (2015) 4407 – 4412
3. 4. Morphology and elemental composition of ZnO films The scanning electron micrographs (top view) of ZnO NRs grown on Ag anorods array and the corresponding elemental composition are shown in figure 4. Randomly oriented ZnO nanords are observed in case of Ag islands interlayer with average diameter of 370 nm and length of 4.2 Pm (image not shown). However ZnO films grown on Ag NRs array films shows hexagonal shaped and flower like structure with an average diameter and length of 1.2 Pm and 8 Pm respectively (fig. 4(a)). Other than ZnO seed layer for the preferential growth of ZnO NRs [8-9], metal nanoparticles are also being used for the catalytic growth of ZnO NRs by hydrothermal approach where metal nanoparticles direct the growth direction of ZnO nanoparticles [10]. The randomly oriented ZnO NRs observed on Ag 1nm film could be due to the presence of less denser Ag islands having large inter particle gap. However the formation of ZnO nanorod flower like structures obtained on Ag NRs array film suggests that Ag NRs don’t act as catalytic layer for the preferential growth of ZnO NRs rather it would be helpful for the emission enhancement of ZnO which is discussed in section 3.5. The flower structure could be possible due the initial growth of ZnO thin layer between as grown ZnO NRs and the Ag film, which makes the root of NRs fuse together with preformed ZnO NPs leading to have a indirect contact of ZnO NRs with Ag nanoparticles. The elemental composition of ZnO/Ag films obtained from energy-dispersive X-ray Spectroscopy (EDS) exhibiting elements such as Zn, Ag and O confirming the possibility for the formation of ZnO on Ag film (fig. 4(b)).
Fig. 4. (a) The top view scanning electron micrograph of ZnO NRs grown on Ag NRs array , (b) EDS spectra of ZnO NRs grown on Ag NRs array. The presence of Zn, Ag and O confirming the possibility of formation of ZnO structure.
3. 5. Photoluminescence spectra analysis Figure 5 depicts the PL spectra of ZnO/Ag hybrid structures excited at 325 nm showing a strong emission band near UV region centered at 384 nm (3.23 eV) is attributed to the spontaneous emission from free excitons [32 of jpcc] for ZnO film grown on Ag islands. ZnO flower structure grown on Ag nonarods array film shows an enhanced emission with red shifted to 396 (3.13 eV). Along with near UV emission peak, both ZnO/Ag hybrid structures show several low intense side bands arise at 419, 450, 467, 482 and 491nm in visible region are attributed to multiple trap states due to multi phonon and phonon-exciton interactions originated by the creation of new shallow trap states just below the conduction band which don’t allow the photo excited electrons to recombine immediately [11]. The increase in the emission intensity from ZnO flower like structures is due to the presence of large number of ZnO NRs. Another reason could due to the surface plasmon exciton coupling where,
Anil Kumar Pal and D. Bharathi Mohan / Materials Today: Proceedings 2 (2015) 4407 – 4412
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the excited energy in ZnO is transferred to surface plasmons of Ag and radiate to far field due to momentum matching between surface palsmons and excitons [3, 12].
Fig. 5. Photoluminescence spectra of ZnO NRs grown on Ag islands and NRs array.
3. 6. Chemical structure analysis (Raman spectra) Figure 6 shows Raman spectra of as grown ZnO NRs on Ag NRs array film. Two sharp and high intense peaks dominated at 99 and 437 cm-1 for both the films which are attributed to E 2 (low) and E2 (high) frequency modes respectively while a weak Raman mode, A1 (TO) is observed at 378 cm-1. Apart from these, 2nd order vibration mode is observed at a332 cm-1 originating from zone boundary phonons 3E2H-E2L [13]. The presence of optical phonon high frequency E2 mode is related to the vibrations of oxygen atoms and also the characteristics of ZnO crystal structure which confirms that as grown ZnO nanostructure has wurtzite phase with very good crystallinity [13].
Fig. 6. Raman spectra of ZnO NRs grown on Ag films excited at 488 nm.
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Anil Kumar Pal and D. Bharathi Mohan / Materials Today: Proceedings 2 (2015) 4407 – 4412
4. Conclusions Ag films of mass thickness of 1 nm and 10 nm were deposited by thermal evaporation using GLAD process at an glancing angle of 85q. Ag islands were formed by 1 nm thick film where as NRs array were formed by 10 nm thick film. Two Plasmon resonance peaks are observed for both the Ag films confirmed the presence asymmetric shape of Ag nanoparticles. ZnO film grown by hydrothermal approach on 1 nm Ag film showed randomly oriented ZnO NRs and flower like structures were observed on 10 nm Ag film. XRD demonstrates the growth of wurtzite phase of ZnO crystals. ZnO NRs showed intensive UV emission with multiple side bands in the visible region of emission spectra. The absence of intense visible emission suggested that the as grown ZnO NRs acquiring less crystal defects which may be suitable for the application of UV LEDs. Acknowledgements The work is supported by University Grant Commission (UGC) - Major Project (F. No.4151868/2012(Sr)). The author AKP gratefully acknowledges UGC, India for providing the research fellowship. Central Instrumentation Facility of Pondicherry University provided the facilities for characterization studies. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
Okamoto K, Niki I, Shvartser A, Narukawa Y, Mukai T, Scherer A, Surface-plasmon-enhanced light emitters based on InGaN quantum wells. Nat Mater 2004; 3:601-605. Liu K, Wu W, Chen B, Chen X, Zhang N, Continuous growth and improved PL property of ZnO nanoarrays with assistance of polyethylenimine. Nanoscale 2013; 5:5986-5993. You J B, Zhang X W, Fan Y M, Yin, Z G, Cai P F, Chen N F, Effects of the morphology of ZnO/Ag interface on the surfaceplasmon-enhanced emission of ZnO films. J. Phys. D: Applied Physics 2008; 41:205101. Singh D P, Goel P, singh J P, Revisiting the structure zone model for sculptured silver thin films deposited at low substrate temperatures. J. appl. Phys.2012;112:104324. Thouti E, Chander N, Dutta V, Komarala V K, Optical properties of Ag nanoparticle layers deposited on silicon substrates. J. Opt. 2013; 15:035005. Pal A K, Mohan D B, Fabrication of partially oxidized ultra-thin nanocrystalline silver films: effect of surface plasmon resonance on fluorescence quenching and surface enhanced Raman scattering. Materials Research Express 2014; 1: 025014. Wang C, Ruan W, Ji N, Ji W, Lv S, Zhao C, Zhao B, Preparation of Nanoscale Ag Semishell Array with Tunable Interparticle Distance and Its Application in Surface-Enhanced Raman Scattering. J. Phys. Chem. C 2010; 114:2886. Baviskar P K, Tan W, Zhang J, Sankapal B R, Wet chemical synthesis of ZnO thin films and sensitization to light with N3 dye for solar cell application. J. Phys. D: Applied Physics 2009; 42:125108. Downing J M, Ryan M P, McLachlan M A, Hydrothermal growth of ZnO NRs: The role of KCl in controlling rod morphology. Thin Solid Films 2013; 539:18-22. Wen X, Wu W, Ding Y, Wang Z L, Seedless synthesis of patterned ZnO nanowire arrays on metal thin films (Au, Ag, Cu, Sn) and their application for flexible electromechanical sensing. J. Mat. Chem. 2012;22:9469-9476. Kim S Y, Yeon Y S, Park S M, Kim J H, Song J K, Exciton states of quantum confined ZnO NRs. Chem. Phys. Lett. 2008; 462:100-103. Aslan K, Leonenko Z, Lakowicz J, Geddes C, Annealed Silver-Island Films for Applications in Metal-Enhanced Fluorescence: Interpretation in Terms of Radiating Plasmons. J. Fluore. 2005; 15:643-654. Umar A, Karunagaran B, Suh E K, Hahn Y B, Structural and optical properties of single-crystalline ZnO NRs grown on silicon by thermal evaporation. Nanotechnology 2006; 17:4072.
ScienceDirect Materials Today: Proceedings 2 (2015) 4407 – 4412
International Conference on Nano Science & Engineering Applications. ICONSEA-2014
The study of surface plasmon enhanced emission of ZnO nanorods on plasmonic Ag nanorods array Anil Kumar Pal, D. Bharathi Mohan* Department of Physics, School of Physical, Chemical and Applies Sciences, Pondicherry University, Pondicherry-605014, India
Abstract Ag islands and nanorods (NRs) array were fabricated through glancing angle deposition (GLAD) method at an angle of 85 qC by using thermal evaporation technique. Subsequently, ZnO NRs were grown along the side of Ag NRs arrays through hydrothermal reaction. The crystal structure of ZnO/Ag hybrid structure was studied by Glancing Incident X-Ray Diffraction (GIXRD). The surface morphologies of Ag NRs arrays and ZnO NRs were studied through AFM and SEM techniques. The presence of different vibrational energy modes of ZnO structure was studied through confocal Raman spectrometer. The surface plasmon enhanced fluorescence of ZnO NRs was probed through fluorescence spectrophotometer. The presence of strong UV emission and the absence of intense visible emission of ZnO NRs are due to the existence of fewer crystal defects which could make the hybrid structure precious for UV LEDs. Keywords: Thermal evaporation; GLAD, Ag NRs; Hydrothermal reaction; ZnO NRs and surface Plasmon enhanced emission.
1. Introduction ZnO is one of the most promising material exhibits a wide direct band gap of 3.37 eV at 300 K, a large exciton binding energy of 60 MeV and excellent chemical and thermal stabilities. Therefore, ZnO based nanomaterials have attracted much interest especially NRs and nanowires due to their remarkable physical and chemical properties for which ZnO is an excellent candidate for the potential application in nanoscale electronic, light emitting diodes, field emission devices, chemical sensors and energy conversion devices [1]. However many researches are being performed to enhance the luminescence of light-emitting to get very bright light. One of the important aspects is to enhance the fluorescence through surface plasmon-exciton coupling which is achievable in semiconductor/metal hybrid structures [1-3]. The emission enhancement from ZnO depends on morphology (size and shape) and roughness of inter layer metal film which is the most significant for the matching of SPs to emission band energies [3]. In this work, glancing angle deposition (GLAD) of Ag thin films of mass thickness of 1 and 10 nm were carried out on glass as well as Si substrates by thermal evaporation technique to fabricate Ag nanorod arrays. Subsequently ZnO NRs were grown on Ag films by hydrothermal process. The crystal structure, surface morphology, chemical structure, optical absorbance and emission of ZnO/Ag nanostructures have been studied. * Corresponding author. Tel.: +91 413 2654 786. E-mail address: [email protected]
2214-7853 © 2015 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the conference committee members of the International conference on Nano Science & Engineering Applications - 2014 doi:10.1016/j.matpr.2015.10.040
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Anil Kumar Pal and D. Bharathi Mohan / Materials Today: Proceedings 2 (2015) 4407 – 4412
2. Experimental details Silver films of mass thickness 1 and 10 nm were deposited over ultrasonically cleaned glass and Si substrates by thermal evaporation of Ag powder (99.9%, sigma Aldrich, USA) at a low pressure of 1.5× 10 -6 mbar using GLAD method. The substrate placed above the source was inclined with an angle of 85q in the direction of vapor flux. Subsequently, ZnO NRs were grown on as deposited Ag films by following hydrothermal reaction. In hydrothermal reaction, Ag film is immersed vertically in an aqueous solution containing equimolar (0.05 M) concentration of zinc nitrate hexahydrate and hexamethylenetetramine (extra pure, FINAR, India) of each 25 ml and then refluxed in a hot air oven at 90 °C for 5 hrs. Finally, the films drawn from the precursor solution and dried at 100 °C after rinsing with copious amount of de-ionized water. The crystal structure of ZnO/Ag nanostructure was studied by X-Ray Diffraction (XRD) (PANalytical, X’Pert PRO). The surface morphology of Ag films and ZnO/Ag hybrid structures were depicted by using Atomic Force Microscope (AFM, tapping mode in air) (Nanoscope) and scanning electron microscope (SEM) (Hitachi: S-3400N) respectively. Elemental analysis was probed through energy dispersive X-ray spectrometer (Thermo Super Dry II) attached with SEM. The optical absorption of Ag films were taken using UV-Visible spectrophotometer (200-1200 nm, Ocean Optics, HR 4000) and the emission spectra of ZnO/Ag films were recorded with an excitation wavelength of 325 nm by using Spectrofluorometer (FLUOROLOG-FL3-11) which is equipped with a source of Xenon lamp-450 W. Raman spectra were recorded by a Confocal Raman spectrometer (Reinshaw inVia Raman Microscope, UK) with an excitation wavelength of 488 nm using Ar+ laser source with an acquisition time of 30 seconds. The laser power was maintained at 1.5 mW. 3. Result and discussions 3.1. Surface morphology of Ag films (AFM image analysis) Atomic force microscopy (AFM) measurement of Ag NRs array film is performed to study the evolution of surface properties. Ag film of 1 nm mass thickness consists of islands with the average size of 35 nm as observed in figure 1(a). However, the film of 10 nm mass thickness shows the formation of Ag NRs type structures as demonstrated in figure 1(b). It is observed that the NRs are not symmetrical along the length whereas the particles are elongated in one direction makes the nanoparticles columnar growth with average diameter of 22 nm. At the initial stage of thermal deposition, the vaporized Ag atoms deposited onto the substrate surface. Once the sufficient amount of Ag atoms reached the substrate, the nucleation process starts with the formation of Ag islands. The GLAD technique provides an advantage of stacking of incoming vaporized Ag atoms on to the Ag islands which happens due to the shadowing effect mainly responsible for the formation of columnar structures [4]. As the deposition thickness increases, the Ag film surface becomes rougher due to the formation of nanorod arrays. Hence the RMS surface roughness of Ag film of 1 nm mass thickness is found to be 4.8 nm which increases to 1.06 nm for 10 nm Ag film.
Fig. 1. AFM images of Ag films (a) 1 nm mass thickness shows Ag islands, (b) 10 nm mass thickness film shows Ag NRs array.
Anil Kumar Pal and D. Bharathi Mohan / Materials Today: Proceedings 2 (2015) 4407 – 4412
3. 2. Optical absorption Surface plasmon resonance absorption spectra from Ag island film and NRs array structures are shown in figure 2. It can be seen that both Ag island film and nanorod arrays film show two resonance peaks. One sharp absorption peak in ultra violate region around 380 nm for both the films without any shift is due to quadrupolar resonances of Ag nanoparticles [5]. Another absorbance peak observed at 432 nm for Ag island film blue shifted to 424 nm for nanorod array film are attributed to the dipole oscillations of free electrons which are parallel to the substrate plane known as in-plane mode of oscillations [5]. The presence of these two resonance peaks suggests that the shape of nanoparticles is not spherical, rather asymmetric. It can be noticed that the short wave length peak is more prominent compared to dipolar absorption peak in case of island film, however it diminishes in case of nanorod array film. The SPR peak supposed to be red shifted in case of Ag nanorod arrays compared to island film due to decrease in surface to volume ratio [6], however there is a small blue shift is observed could be due to the decrease in the interparticle distance [7].
Fig. 2. Optical absorption spectra of Ag islands and NRs array film. Two resonance peaks are observed suggests the asymmetric shape of Ag nanoparticles.
3. 3. Crystal structure analysis Figure 3 shows the XRD patterns of ZnO films grown on Ag NRs array film. The diffraction peaks (100), (002), (101), (102), (103) and (201) are in good agreement with the JCPDS card (No. 36-1451) confirming the formation of wurtzite ZnO crystals. An evolution of a diffraction peak observed at 38.17 in 2θ degree in case of ZnO film grown on Ag NRs array correspond to (111) crystal plane of Ag which is in good agreement with the JCPDS card (no. 04-0783) for cubic structure of silver. However, the XRD spectra exhibit a less intense (002) diffraction peak indicating random orientation in the growth of ZnO nanoparticles. This random orientation is shown in scanning electron micrograph which is discussed section 3.4.
Fig. 3. X-ray diffraction patterns of ZnO NRs grown on Ag islands and NRs array film.
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Anil Kumar Pal and D. Bharathi Mohan / Materials Today: Proceedings 2 (2015) 4407 – 4412
3. 4. Morphology and elemental composition of ZnO films The scanning electron micrographs (top view) of ZnO NRs grown on Ag anorods array and the corresponding elemental composition are shown in figure 4. Randomly oriented ZnO nanords are observed in case of Ag islands interlayer with average diameter of 370 nm and length of 4.2 Pm (image not shown). However ZnO films grown on Ag NRs array films shows hexagonal shaped and flower like structure with an average diameter and length of 1.2 Pm and 8 Pm respectively (fig. 4(a)). Other than ZnO seed layer for the preferential growth of ZnO NRs [8-9], metal nanoparticles are also being used for the catalytic growth of ZnO NRs by hydrothermal approach where metal nanoparticles direct the growth direction of ZnO nanoparticles [10]. The randomly oriented ZnO NRs observed on Ag 1nm film could be due to the presence of less denser Ag islands having large inter particle gap. However the formation of ZnO nanorod flower like structures obtained on Ag NRs array film suggests that Ag NRs don’t act as catalytic layer for the preferential growth of ZnO NRs rather it would be helpful for the emission enhancement of ZnO which is discussed in section 3.5. The flower structure could be possible due the initial growth of ZnO thin layer between as grown ZnO NRs and the Ag film, which makes the root of NRs fuse together with preformed ZnO NPs leading to have a indirect contact of ZnO NRs with Ag nanoparticles. The elemental composition of ZnO/Ag films obtained from energy-dispersive X-ray Spectroscopy (EDS) exhibiting elements such as Zn, Ag and O confirming the possibility for the formation of ZnO on Ag film (fig. 4(b)).
Fig. 4. (a) The top view scanning electron micrograph of ZnO NRs grown on Ag NRs array , (b) EDS spectra of ZnO NRs grown on Ag NRs array. The presence of Zn, Ag and O confirming the possibility of formation of ZnO structure.
3. 5. Photoluminescence spectra analysis Figure 5 depicts the PL spectra of ZnO/Ag hybrid structures excited at 325 nm showing a strong emission band near UV region centered at 384 nm (3.23 eV) is attributed to the spontaneous emission from free excitons [32 of jpcc] for ZnO film grown on Ag islands. ZnO flower structure grown on Ag nonarods array film shows an enhanced emission with red shifted to 396 (3.13 eV). Along with near UV emission peak, both ZnO/Ag hybrid structures show several low intense side bands arise at 419, 450, 467, 482 and 491nm in visible region are attributed to multiple trap states due to multi phonon and phonon-exciton interactions originated by the creation of new shallow trap states just below the conduction band which don’t allow the photo excited electrons to recombine immediately [11]. The increase in the emission intensity from ZnO flower like structures is due to the presence of large number of ZnO NRs. Another reason could due to the surface plasmon exciton coupling where,
Anil Kumar Pal and D. Bharathi Mohan / Materials Today: Proceedings 2 (2015) 4407 – 4412
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the excited energy in ZnO is transferred to surface plasmons of Ag and radiate to far field due to momentum matching between surface palsmons and excitons [3, 12].
Fig. 5. Photoluminescence spectra of ZnO NRs grown on Ag islands and NRs array.
3. 6. Chemical structure analysis (Raman spectra) Figure 6 shows Raman spectra of as grown ZnO NRs on Ag NRs array film. Two sharp and high intense peaks dominated at 99 and 437 cm-1 for both the films which are attributed to E 2 (low) and E2 (high) frequency modes respectively while a weak Raman mode, A1 (TO) is observed at 378 cm-1. Apart from these, 2nd order vibration mode is observed at a332 cm-1 originating from zone boundary phonons 3E2H-E2L [13]. The presence of optical phonon high frequency E2 mode is related to the vibrations of oxygen atoms and also the characteristics of ZnO crystal structure which confirms that as grown ZnO nanostructure has wurtzite phase with very good crystallinity [13].
Fig. 6. Raman spectra of ZnO NRs grown on Ag films excited at 488 nm.
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Anil Kumar Pal and D. Bharathi Mohan / Materials Today: Proceedings 2 (2015) 4407 – 4412
4. Conclusions Ag films of mass thickness of 1 nm and 10 nm were deposited by thermal evaporation using GLAD process at an glancing angle of 85q. Ag islands were formed by 1 nm thick film where as NRs array were formed by 10 nm thick film. Two Plasmon resonance peaks are observed for both the Ag films confirmed the presence asymmetric shape of Ag nanoparticles. ZnO film grown by hydrothermal approach on 1 nm Ag film showed randomly oriented ZnO NRs and flower like structures were observed on 10 nm Ag film. XRD demonstrates the growth of wurtzite phase of ZnO crystals. ZnO NRs showed intensive UV emission with multiple side bands in the visible region of emission spectra. The absence of intense visible emission suggested that the as grown ZnO NRs acquiring less crystal defects which may be suitable for the application of UV LEDs. Acknowledgements The work is supported by University Grant Commission (UGC) - Major Project (F. No.4151868/2012(Sr)). The author AKP gratefully acknowledges UGC, India for providing the research fellowship. Central Instrumentation Facility of Pondicherry University provided the facilities for characterization studies. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
Okamoto K, Niki I, Shvartser A, Narukawa Y, Mukai T, Scherer A, Surface-plasmon-enhanced light emitters based on InGaN quantum wells. Nat Mater 2004; 3:601-605. Liu K, Wu W, Chen B, Chen X, Zhang N, Continuous growth and improved PL property of ZnO nanoarrays with assistance of polyethylenimine. Nanoscale 2013; 5:5986-5993. You J B, Zhang X W, Fan Y M, Yin, Z G, Cai P F, Chen N F, Effects of the morphology of ZnO/Ag interface on the surfaceplasmon-enhanced emission of ZnO films. J. Phys. D: Applied Physics 2008; 41:205101. Singh D P, Goel P, singh J P, Revisiting the structure zone model for sculptured silver thin films deposited at low substrate temperatures. J. appl. Phys.2012;112:104324. Thouti E, Chander N, Dutta V, Komarala V K, Optical properties of Ag nanoparticle layers deposited on silicon substrates. J. Opt. 2013; 15:035005. Pal A K, Mohan D B, Fabrication of partially oxidized ultra-thin nanocrystalline silver films: effect of surface plasmon resonance on fluorescence quenching and surface enhanced Raman scattering. Materials Research Express 2014; 1: 025014. Wang C, Ruan W, Ji N, Ji W, Lv S, Zhao C, Zhao B, Preparation of Nanoscale Ag Semishell Array with Tunable Interparticle Distance and Its Application in Surface-Enhanced Raman Scattering. J. Phys. Chem. C 2010; 114:2886. Baviskar P K, Tan W, Zhang J, Sankapal B R, Wet chemical synthesis of ZnO thin films and sensitization to light with N3 dye for solar cell application. J. Phys. D: Applied Physics 2009; 42:125108. Downing J M, Ryan M P, McLachlan M A, Hydrothermal growth of ZnO NRs: The role of KCl in controlling rod morphology. Thin Solid Films 2013; 539:18-22. Wen X, Wu W, Ding Y, Wang Z L, Seedless synthesis of patterned ZnO nanowire arrays on metal thin films (Au, Ag, Cu, Sn) and their application for flexible electromechanical sensing. J. Mat. Chem. 2012;22:9469-9476. Kim S Y, Yeon Y S, Park S M, Kim J H, Song J K, Exciton states of quantum confined ZnO NRs. Chem. Phys. Lett. 2008; 462:100-103. Aslan K, Leonenko Z, Lakowicz J, Geddes C, Annealed Silver-Island Films for Applications in Metal-Enhanced Fluorescence: Interpretation in Terms of Radiating Plasmons. J. Fluore. 2005; 15:643-654. Umar A, Karunagaran B, Suh E K, Hahn Y B, Structural and optical properties of single-crystalline ZnO NRs grown on silicon by thermal evaporation. Nanotechnology 2006; 17:4072.