Research Journal of Chemical Sciences ______________________________________________ ISSN 2231-606X Vol. 3(9), 45-50, September (2013) Res. J. Chem. Sci. International Science Congress Association 45 Photoluminescence Studies of H Treated Chemically Synthesized ZnO NanostructuresPatwari G.1* Bodo B.J., Singha R. and Kalita P.K.2 Assam University (Diphu Campus), Diphu - 782 460, INDIA Guwahati College, Guwahati - 781 022, INDIAAvailable online at: www.isca.in Received 8th August 2013, revised 23rd August 2013, accepted 15th September 2013Abstract Spherical ZnO nanoparticles dispersed in PVA matrix were synthesised through chemical bath deposition method. The HRTEM images confirm the formation of wurtzite ZnO quantum dots. The PL spectra obtained were deconvoluted in order to study the defects due to impurities. Peak positions obtained through de-convolution exhibit the energy levels in the long wavelength regions. The defects such as oxygen vacancy, oxygen antisite and Zn interstitial are associated with emission in the visible region. The band edge transition peak is prominent in PL spectra. The ZnO quantum dots exhibit strong UV emissions at 3.20eV and 3.16eV while other emissions in the range 3.00eV -- 1.77eV for the prepared samples. Keywords: Polyvinyl alcohol, photoluminescence, De-convolution. Introduction Zinc oxide is a versatile material which is used extensively in the field of nanoscience research. Its unique property of having high melting point, wide band gap (3.37eV), large exciton binding energy (60meV), etc. is making it suitable for its wider applications1,2. The properties of nanomaterials being different from that of bulk in having larger surface to volume ratio, quantum confinement effect, etc. are drawing increasing research interest. Consequently ZnO nanostructure based LEDs, photo detectors, nano generators are emerging as potential ZnO based nanodevices. Photoluminescence (PL) investigations yield information on the optical properties and the quality of synthesized material. In photoluminescence, particle absorbs photons and then re-radiates photons. Photo excitation of a bulk semiconductor creates exciton, bound by weak columbic interaction. The minimum energy required to generate such an exciton is called the band gap energy. The increase of energy difference between energy states and band gap is due to quantum confinement effect. Spectrum of UV and visible band emissions are observed in ZnO nanoparticles. The recombination between electrons in the conduction band and holes in the valence band causes UV emission. Mid-band gap states are present in the defects of the crystal lattice of the ZnO. The donors and acceptors are the cause for the defect luminescence in ZnO lying in the visible spectrum. The types of defects can be recognised from the wavelength of the emission spectrum. Oxygen vacancy and Zn interstitial are the type of defects that result into visible emission. So far the quality of the prepared material is concerned, the intensity of the PL spectra provides the necessary information4,5. Various methods have been applied till date to investigate the origin of the defects in the emission spectra of nano ZnO but controversies remain till today6,7 In order to study the defect centers, the PL spectrum of ZnO nanoparticle was deconvoluted by Gaussian fitting. The schematic energy levels for ZnO nanostructures treated with and without H2 are estimated. The treatment of ZnO with H may decrease the content of surface oxygen vacancies as reported by other workers and also this carries the potentiality of endowing better electrical properties to ZnO. Material and Methods Synthesis: Chemical bath deposition (CBD) method was employed for necessary synthesis 8,9. A 3% Polyvinyl Alcohol (PVA) solution was added into the salt solution of zinc sulphate (ZnSO) and sodium hydroxide (NaOH) prior to mixing for the necessary capping effect. The temperature of mixture solution was raised to 353K and kept under constant stirring using magnetic stirrer for 3hours to facilitate the reduction reaction to form ZnO nanostructures (sample-1)10. To the milky white solution of ZnO nanoparticles formed, an additional hydrogen peroxide (H) solution (30%) was added. The resultant colloidal solution was then again put under constant stirring at high temperature for another 1 hour and then collected after cooling to RT(sample 2). Zinc oxide (ZnO) nanostructures were dispersed in presence of (PVA) matrix. The structure of polyvinyl alcohol (partially hydrolysed) is shown below. Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 3(9), 45-50, September (2013) Res. J. Chem. Sci. International Science Congress Association 46 Where R = H or COCH: Polyvinyl alcohol is produced from polyvinyl acetate. The two factors i.e. polymerisation and hydrolysis influence its applications. The agglomeration of the nanoparticles is prohibited by PVA. Besides it also acts as a coating agent11. Polymers can act as a good stabilizing agent for they can cover a larger surface area of the nanaoparticle12. Studies reveal that nano crystal of suitable size can be obtained by optimizing capping agent concentration during the growth process. Charaterization: JEM -2100, a high resolution transmission electron microscope (HRTEM) helps revealing the morphology of nanoparticles and their sizes in detail. It has resolution of 1nm or less. Here the transmitted and diffracted electrons generate two dimensional projection of the sample. On viewing at high magnification, contrast in the image is observed in the form of periodic fringes which is referred to as phase contrast. PL of the as-prepared samples were investigated using a He–Cd laser, a 1m Cerny–Turner spectrograph, and a photomultiplier tube [ Thermospectronics, Model: AB2 (Aminco Bowman Series 2)] .] and thereby the luminescent properties of the nanoparticles were characterised. Optical transitions were investigated for the as-prepared samples using excitation wavelength of 325nm. Deconvolution of the PL spectra was done using the software version of OriginPro 7.0. PL data imported into Origin are being deconvoluted using Gaussian function. Results and Discussion The formation of large number of spherical ZnO quantum dots of average sizes 5.77nm have been confirmed by TEM images. In addition, the HRTEM shows the prime lattice spacing obtained in the range 0.22nm which is close to the (002) plane d-spacing of hexagonal ZnO. Figure-1(a) shows the ZnO quantum dots in PVA matrix along with the bigger nanoparticles of ZnO. The outer layers of these bigger nanoparticles contain zinc peroxide (ZnO). This type of formation of zinc peroxides over zinc oxide was also reported by other workers. In the HRTEM image of figure-1(b) the lattice fringes are seen. Thus the nanoparticles formed are crystalline in nature. The selected area electron diffraction (SAED) pattern is used to learn about the crystalline property of the sample. The SAED pattern of single crystalline material has only spot pattern while polycrystalline material has the form of ring pattern13. The figure-1(c) shows SAED pattern of the ZnO nanocrystal clearly corresponding to a single crystalline with some degree of disorder as evident by spot formation instead of rings. UV-Visible spectroscopy on the synthesized product yielded exciton peak (not shown here) at 293nm. The band gap is estimated to be 4.24eV which is 0.87eV higher than that of bulk ZnO thereby indicating blue shifting14. The nanosized ZnO synthesized in aqueous medium was dispersed in PVA matrix and the corresponding photoluminescence spectra were obtained using the excitation wavelength of 325nmwhich is shown infigure-2. Due to the size effect the PL peak position of sample 2 is located at 355nm while that for sample 1 is located at 361nm. From bulk ZnO to nanocrystal of ZnO there is observed increase in the intensity of UV peak. Further increase in intensity of UV peaks is found because of the formation quantum dots of ZnO naoparticle15. The sample 1 shows an emission maximum at 396nm along with others at 360nm and 470nm. The UV emission found at 396nm is due to excitonic recombination corresponding to near band -gap emission. On the other hand oxygen vacancies result into blue emission at 470nm. The desorption of oxygen on the surface of ZnO nanoparticles may cause the enhancement of PL intensity. The electron-hole pairs recombine nonradiatively because the interstitial oxygen ions (O and O2-) reacting with provide surface states to trap photo generated holes. On oxygen being removed from the surface of the ZnO nanoparticles, fewer centers of nonradiative recombination lead to an increase in PL intensity16. The ZnO quantum dots exhibit strong UV emissions at 3.13eV and 3.18eV while near blue emission at 3.66eV for the prepared samples. Figure-1 HRTEM images of (a) ZnO quantum dots and ZnO nanocrystals and (b) lattice spacing of nano ZnO in the background of PVA matrix (c) a SAED pattern of ZnO nanocrystal Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 3(9), 45-50, September (2013) Res. J. Chem. Sci. International Science Congress Association 47 The structural defects like oxygen vacancy, Zn interstitial or impurity may result into deep-level emissions17. In our case here, the emission in the visible region may also be attributable to the ZnO formation. There is blue-green emission of ZnO at interstitial sites that results from the transition with a self activated center18, 19. The self activated centers are formed by a doubly -ionized Zn vacancy Vzn2- and the singly ionized interstitial Zn. The recombination of electrons in oxygen vacancies also leads to green emission band in the sample. The singly ionized oxygen vacancy in ZnO causes weak green band emission. The corresponding emission is due to the recombination of a photogenerated hole with the singly ionized charge state of the particular defect20. The cause of yellow emissions may be due to the radiative recombination of photo-generated holes and electrons in the singly or doubly ionized oxygen interstitials (O-/2- ) and oxygen anti-sites (Ozn0/-) 21. Red luminescence band is attributed to doubly ionized oxygen vacancies. In the PL spectra intense band edge transition peaks are seen while the sub-band transition peaks are found to be suppressed and diffused. Majority workers found UV exictonic absorption and emission in ZnO nanostructures which yielded additional emissions in the blue-green-yellow regions attributing the defects in the samples correspond to the growth conditions22. Figure-2 PL spectra of ZnO (sample 1) and H treated ZnO (sample 2) 4005006007000.00.10.2 Sample1 Intensity (a.u.) (nm) 4005006007000.00.20.40.6 Sample 2 Intensity (a.u.) (nm)Figure-3 Deconvolution of the Photoluminescence peaks (a) without H and (b) with H treated ZnO nano particles Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 3(9), 45-50, September (2013) Res. J. Chem. Sci. International Science Congress Association 48 The PL spectrum obtained for the samples was de-convoluted using Gaussian function as indicated in figure- 3(a-b). The peak positions help to find the energy levels of the intrinsic defects23. The PL peaks for the H2 treated ZnO was de-convoluted into five peaks centered at ~ 394, 449, 498, 593 and 703nm respectively, whereas for sole ZnO nanoparticles, the PL peaks was de-convoluted into four peaks at~ 388, 414, 466 and 573nm respectively. Other workers such as Kim et al reported broad and intense PL peak at 414.6 nm in ZnO quantum dots while Liu et al observed it in bulk ZnO. These features are attributed to the existence of point defects. According to Wei et al the corresponding band is due to radiative transition of electrons from shallow donor levels which is created by oxygen vacancies to the valence band24. The deconvolution of the PL peaks was done employing Gaussian curve fitting. Table-1 displays the peaks of the individual components along with the corresponding estimated energy levels. Nanocrystals grown by chemical methods have many defects. The defects in the crystal lattice of the ZnO, in the mid-band gap states are due to either a donor or an acceptor. The defect level emissions are found in the PL spectra. The higher crystalline quality of the ZnO crystal is ensured by the weaker emission in the visible region. Emission peaks at the UV region corresponding to near band edge (NBE) emission and visible emissions at violet and blue corresponding to shallow level emissions (SLE) and at green and red corresponding to deep level emissions (DLE) were found. Table-1 PL peak positions and energy levels in the sample Peak Sample no.1 (Only ZnO) Sample no.2 (H 2 O 2 treated ZnO) Nos. (nm) Energy (eV) (nm) Energy (eV) I 388 3.20 394 3.16 II 414 3.00 449 2.77 III 466 2.67 498 2.50 IV 573 2.17 593 2.10 V - - 703 1.77 Figure-4 Schematic energy level diagram illustrating the emission mechanism from ZnO nanoparticles where V is oxygen vacancy and Vzn iszinc vacancy and OZn antitsite oxygen Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 3(9), 45-50, September (2013) Res. J. Chem. Sci. International Science Congress Association 49 From the observed photoluminescence spectra, the schematic energy level diagram of ZnO nanoparticles is depicted as shown in figure-4. From the comparison of the peak wavelengths of the two samples red shift can be noticed. Thus after being treated ZnO with H red shifting occurred which is evident from the de-convolution of the photoluminescence peaks of the two samples. ConclusionThe photoluminescence spectra exhibit Strong emission peaks around 360-396 nm. The red shifting in the PL spectra is clearly reflected from the de-convolution of the photoluminescence peaks of the two samples. The reason for this is the addition of hydrogen peroxide (H) solution to the solution of ZnO nanoparticles. The peak positions obtained after de-convolution help in finding the energy levels of the intrinsic defects. Additional small blue-green-orange emissions were also found which are attributed to the inherent native defects created in the samples. The emission in the visible region may also be attributable to the ZnO formation. The emission spectra of photoluminescence are observed to be high-intensity in UV range and very low intensity in the visible range. Thus a good surface morphology of the ZnO nanoparticles with minimum of surface defects is being established in our observation. Acknowledgements The authors express their sincere gratitude to the Indian Institute of Technology, Guwahati and to the Sophisticated Analytic Instrument Facility (North Eastern Hill University), Shillong. Special thanks to Dr. Sidananda Sarma, Scientific Officer, IITG. References1.Klingshirn C., The Luminescence of ZnO under high one- and two-quantum excitation [J], Phys. Status Solid (b). 71, 547-556 (1975) 2.Liang W.Y. and Yoffe A.D., Transmission Spectra of ZnO Single Crystals [J], Phys. Rev. Lett 20(2), 59–62 (1968) 3.Hu J.Q. and Bando Y., Appl. Phys. Lett.,82, 1401 (2003) 4.Shah M.A., Formation of Zinc oxide nanoparticles by the reaction of zinc metal with methanol at very low temperature, African Phys. Rev., ,. 1 (2008) 5.Timothy H., Photoluminescence in Analysis of Surfaces and Interfaces, Encyclopedia of Analytical Chemistry, Meyers R.A. (Ed.), 9209–9231, John Wiley & Sons Ltd, Chichester, (2000) 6.Vanheusden K., Seager C.H., Warren W.L., Tallant D.R. and Voigt J.A., Appl. Phys. Lett., 68, 403 (1996) 7.Ohashi N., Nakata T., Sekiguchi T., Hosono H., Mizuguchi M., Tsurumi T., Tanaka J., and Haneda H., Jpn. J. Appl. Phys. Part 2 , 38, L113 (1999) 8.Kumar Harish, Rani Renu and Salar Raj Kumar, Synthesis of Nickel Hydroxide Nanoparticles by Reverse Micelle Method and its Antimicrobial Activity, Res.J.Chem.Sci.,1(9), 42-48 (2011) 9.Francis Amala Rejula and Masilamai Dhinakaran, Removal of Zinc (II) by Non Living Biomass of Agaricus Bisporus, Res.J.Recent Sci., 1(9), 13-17 (2012) 10.Pandey Bhawana and Fulekar M.H., Nanotechnology: Remediation Technologies to clean up the Environmental pollutants, Res. J. Chem.Sci., 2(2), 90-96 (2012) 11.61st JECFA, Chemical and Technical Assessment (2003) 12.Mudigoudra B.S., Masti S.P., Chougale R.B., Thermal behavior of Poly (vinyl alcohol)/ Poly (vinyl pyrrolidone)/ Chitosan Ternary Polymer Blend Films, Res. J. Recent Sci.Vol. 1(9), 83-86 (2012) 13.Singh Vineet, Sharna P.K., Chauhan Pratima, Synthesis of CdS nanoparticles with enhanced optical properties, Material Characterisation, 62, 43-52 (2011) 14.Okereke N.A. and Ekpunobi A.J., XRD and UV-VIS-IR Studies of Chemically-Synthesized Copper Selenide Thin Films, Res.J.Chem.Sci.,1(6), 64-70 (2011) 15.Vladimir A. Fonoberov, Khan A. Alim and Alexander A. Balandin, Faxian Xiu and Jianlin Liu,Photoluminescence investigation of the carrier recombination processes in ZnO quantum dots and nanocrystals,Phys.Rev.B,73, 165317 (2006) 16.Nargis Bano, Hussain I., Omer Nour, Magnus Willander, Klason P. and Anne Henry; Study of luminescent centers in ZnO nanorods catalytically grown on 4H-p-SiC 2009, Semiconductor science and technology,(24), 12, 125015 (2009) 17.Djuriši A.B., Leung Y.H., Tam K.H., Hsu Y.F., Ding L., Ge W.K., Zhong Y.C., Wong K.S., Chan W.K., Tam H.L., Cheah K.W., Kwok W.M. and Phillips D.L., Defect emissions in ZnO nanostructures/IOP Science Nanotechnology, 18(9), 095702 (2007) 18.Liu Z. W., Ong C.K.., Yu T. and Shen Z. X., Appl. Phys. Lett.,88, 0531110 (2006) 19.Kenanakis G., Androulidaki M., Koudoumas E., Savvakis C. and Katsarakis N., Photoluminescence of ZnO nanostructures grown by the aqueous chemical growth technique, Superlattices and Microstructures,42, 473-478 (2007) 20. Maensiri S., Masingboon C., Promarak V., Seraphin S., Synthesis and optical properties of nanocrystalline V-doped ZnO powders, Optical Materials, 29, 1700–1705 (2007) 21.Srinivasan G. and Kumar J., Crystal Res. Technol., 41, 893-896 (2006) 22.Lu H.Y. et al, J. of Crystal Growth, 274, 506–511 (2005) Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 3(9), 45-50, September (2013) Res. J. Chem. Sci. International Science Congress Association 50 23.Peng W.Q., Cong G.W., Qu S.C. and Wang Z.G., Synthesis and photoluminescence of ZnS:Cu nanoparticles, Science Direct, Optical Materials, 29, 313-317 (2006) 24.Haranath D., Sahai S., Joshi A.G., Gupta B.K. and Shanker V., Investigation of confinement effects in ZnO quantum dots, IOP Publishing, Nanotechnology,20, 425701 (7pp), (2009)