 Proceedings of Engineering and Technology Innovation , vol. 4, 2016, pp. 16 - 18 16 Photoluminescence Study of Hydrothermally Grown ZnO Nanostructures Wei-You Chen 1 , Jiunn-Chyi Lee 2 , Hung-Pin Hsu 1 , Ya-Fen Wu 1,* 1 Department of Electronic Engineering, Ming Chi University of Technology, New Taipei City, Taiwan. 2 Department of Electrica l Engineering, Ta ipei City University of Science and Technology, Ta ipei, Ta iwan . Received 15 January 2016; received in revised form 27 February 2016; accept ed 06 March 2016 Abstract In th is work, the hydrotherma lly gro wn ZnO nanostructures were successfully prepa red using the aqueous solution of zinc nitride d e - hydrate (Zn(NO3)2 ). The effect o f solution mo la rity on the opt ica l and structura l prope rties of ZnO nanostructures were studied by te m- perature dependent photolu minescence (PL) measure ments. The intensity rat io of the u ltra - violet e mission to the visib le e mission is cal- culated. It is found that the calcu lated results are a lso dependent on the solution mo larity and consistent with the c rystal qua lity of Zn O nanostructures . Ke ywor ds : zinc o xide, hydrotherma l, precursor concentration, photoluminescence 1. Introduction Zinc o xide (Zn O) has been e xtensive ly in - vestigated due to its unique propert ies such as wide direct bandgap energy (3.36 e V), la rge e xc iton binding ene rgy (60 me V), and therma l stability. These characte ristics ma ke th is mate - ria l interesting for many app lications such as solar ce lls, optoelect ronic and nanowave de - vices [1-3]. The ro le of the nanostructure size , impu rit ies , and mo rphology is very important to these applicat ions. Various methods have been utilized to synthesize ZnO nanostructures, such as meta l-o rganic che mical vapor depos i- tion (M OCVD), sol-gel methods, and the lo w tempe rature hydrotherma l method [4-5]. The hydrothermal method has been widely used to ZnO structures on Zn O-seeded substrate be- cause of its advantages includ ing lo w-cost, low-te mpe rature , nanoe lectron ic co mpatib le , and suitable for la rge area substrates [6]. The properties o f ZnO produced by the hydrother- mal method a re dependent upon p reparat ion para mete rs such as growth te mperatu re, pre - cursor concentration, and gro wth time [7]. In this work, the hydrotherma lly gro wn Zn O nanostructures were prepared with diffe rent aqueous solution mo la rities and diffe rent dep - osition t ime . The effects of solut ion mo la rity on the optica l and structura l prope rties of Zn O nanostructures were studied by te mperatu re dependent PL measure ments. It is found that the PL e mission intensity inc reases and then decreases with the increas ing solut ion mo la rity. 2. Experiment A thin film of zinc acetate dihydrate (Zn(CH3COO)2∙ 2H2O) was spin coated onto an indiu m-t in-o xide (ITO) glass substrate and an- nealed at 300 °C in o xygen for 20 min. The hydrotherma l method was used to grow ZnO nanostructures on ITO glass using an aqueous solution of zinc nitrate (Zn(NO3)2) and ammo - nium hydro xide (NH3OH). The solution mo lar- ity increases from 10 to 50 mM . PL measure- ments we re performed to e xa mine the structural and optical properties of the ZnO nanostructures. The te mperature dependent PL spectra we re measured using a He-Cd laser operating at a wavelength of 325 n m and the average exc it a- tion intensity was 30 mW. The samples were mounted in a closed-cycle He cryostat where the temperature (T) was varied from 10 to 300 K. 3. Results and Discussion Fig. 1 shows the XRD patterns of Zn O nanostructures fabricated with solution-mola rity 20 mM and deposition time 1 hour. The detected XRD pea ks are characteristic of the ZnO with hexagonal wurt zite type crystalline structure. The strongest (002) pea k indicates that the fab- ricated ZnO is grown along the c-axis direction. * Corresponding aut hor. Email: yfwu@mail.mcut .edu.tw Proceedings of Engineering and Technology Innovation , vol. 4, 2016, pp. 16 - 18 17 Copyright © TAETI Fig. 1 XRD patterns of ZnO nanostructures Fig. 2 PL spectrum of ZnO nanostructures measured at room temperature The room te mperature PL spectrum of this sample is shown in Fig. 2. Observing the meas- ured result, the PL spectrum shows a strong yellow-orange emission peak at ~610 n m in the visible region and a wea k pea k in the ultrav iolet (UV) reg ion at ~390 n m. Room-te mperature PL spectra of Zn O usually e xhib it a broad peak in the visible region. The origin of this e mission peak can be attributed to the defect in ZnO, such as oxygen vacancies located in ZnO surfaces [8]. The UV peak is attributed to free-e xc iton re- combination corresponding to the near- band-edge transition in ZnO. Fig. 3 Visib le e mission of PL spectra for the ZnO nanostructures with various solu- tion molarities Fig. 3 presents the visible e mission of PL spectra for the sa mples prepared at deposition time 3 hours, with various solution mola rit ies. It is clearly seen that the emission intensity in- creases as the solution mo larity increased fro m 10 to 30 mM, and then decreases as the solution mo larity is higher than 30 mM. We suppose that the increase in emission intensity could be as- sociated with the increasing concentration of the oxygen vacancies [9]. The obtained results are similar for the sample with 1-hour deposition time. The relative PL intensity ratio of the emis- sion in the UV region (IUV) to the emission in the visible region (Ivis) is shown in Fig. 4. It can be seen that the ratio increases with the molarit ies for the sa mples, and decreases as solution mo- larity greater than 50 mM. The IUVIvis is one of the main factors that usually used to compare the optical properties of samples [10]. The higher PL intensity ratio means fewer structural defects in the sa mples. Too higher solution mola rity leads to the disorder of ZnO structures , and deteriorate the crystal quality. Fig. 4 Variation of IUVIvis values with different solution molarities 4. Conclusions In this paper, the hydrothermally grown ZnO nanostructures were prepared with different aqueous solution mola rit ies and diffe rent dep o- sition time. The te mperature dependent PL measure ments were carried out to investigate the effects of solution molarit ies on the optical and structural properties of ZnO nanostructures . It is found that the visible emission intensity of PL spectra increases and then decreases with solu- tion mola rit ies. Moreover, the PL intensity ratio of the emission in the UV reg ion to the emission in the visib le region is calculated . The values of ratio increase as the mo larity varying fro m 10 to Proceedings of Engineering and Technology Innovation , vol. 4, 2016, pp. 16 - 18 18 Copyright © TAETI 40 mM , and decrease at mola rity 50 mM. It is concluded that the optical property and crystal quality of ZnO nanostructures are improved by an appropriately increasing solution molarity. However, too higher solution molarity leads to the disorder of ZnO structures, and deteriorate the crystal quality. References [1] W. Water and S. Y. Chu, “Physical and structural properties of ZnO sputtered films ,” Mater. Lett., vol. 55, pp. 67-72, 2002. [2] R. Ferro, J. A. Rodríguez, and P. Bertrand, “Peculia rit ies of nitrogen dio xide detection with sprayed undoped and indium-doped zinc oxide thin films,” Thin So lid Films , vol. 516, pp. 2225-2230, 2008. [3] T. Szabó, J. Németh, and I. 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