Study on band gap energy of pure and Tb3+ doped Zno powder

JOURNAL OF SCIENCE OF HNUE 2011, Vol. 56, N◦. 1, pp. 21-26 STUDY ON BAND GAP ENERGY OF PURE AND Tb3+ DOPED ZnO POWDER Ngo Ngoc Hoa, Nguyen Minh Thuy and Nguyen Manh Nghia(∗) Hanoi National University of Education (∗)E-mail: nghiadhsp@gmail.com Abstract. In this paper, we present one method to determine the band gap energy of ZnO and ZnO:Tb3+materials. ZnO:Tb3+ material has pre- pared by forced hydrolysis method. All samples have zinc oxide hexagonal wurzite structure. The obtained results show that the energy of the band gap depends on Tb3+ molar ratio. Keywords: ZnO:Tb3+, forced hydrolysis. 1. Introduction ZnO is an important semiconductor because it has distinct properties as direct transition and wide band gap energy (Eg=3.2 eV at room temperature) [1]. There- fore, then ZnO as a luminescent material has a long history of practical application and has been the object of extensive research over the past century. Nowadays, it is the suitable host materials for the doping of luminescence centers. In terms of energy transfer from ZnO to the rare - earth RE3+ ions, RE3+ ions characteristic lumines- cence will be acquired. If RE3+ ions are incorporated in semiconductor nanocrystals, the optical properties of nanocrystals are expected to be modified remarkably. In this paper, we have prepared ZnO:Tb3+ materials by forced hydrolysis method. X-ray diffraction patterns show that all samples have zinc oxide hexagonal wurzite structure. The results are obtained to show that the band gap energy of the ZnO:Tb3+ materials depend on the Tb3+molar ratio. 2. Content 2.1. Experimental ZnO and ZnO:Tb3+ samples are fabricated by hydrolysis method. To prepare the sol solution, 100 ml ethanol containing 2.195 g Zn(CH3COO).2H2O was stirred for 3h to get Zn-O-Zn precursor. The precursor was hydrolyzed in an ultrasonic bath at 0oC by addition of 0.84 g LiOH.H2O powder into the flask for 20 min. The resulted colloidal suspension was concentrated and precipitated by adding of hexane. The nanoparticles were redispersed in ethanol and then centrifuged, separated from 21 Ngo Ngoc Hoa, Nguyen Minh Thuy and Nguyen Manh Nghia ethanol by precipitation for several times. The sample was obtained by drying the precipitate at room temperature for a day, then annealing at 200oC for 1h. The synthesis of ZnO:Tb was performed similarly, but by adding Tb(CH3COO)3.H2O to Zn(CH3COO)2.H2O solution. Samples with different Tb concentration have been prepared by varying the amount of Tb(CH3COO)3.H2O present in the precursor solution. Samples with different Tb concentration have been prepared by varying the amount of Tb(CH3COO)3.H2O present in the precursor solution (Tb mol %: 2.5, 5, 7.5 and 10). The Zn2+ concentration in each case was reduced to keep constant the total concentration of metal ions in solution. The structure of the products was examined by X-ray diffraction (XRD). Op- tical properties of samples were investigated by using optical absorption. By using optical absorption data, we plot the variation of (αhνt)2with photon energy c and we have determined the energy of the band gap of the materials ZnO and ZnO:Tb3+. The results show that the energy of the band gap depends on the molar ratio Tb3+. 2.2. Results and discussion 2.2.1. The structure of samples The XRD patterns of ZnO powder samples are shown in Figure 1. The diffrac- tion peaks demonstrate that all samples have ZnO hexagonal wurtzite structure. The XRD patterns express that samples are single crystalline and they have the hexagonal wurtzite structure of ZnO. The peak related to Tb3+ ion does not 22 Study on band gap energy of pure and Tb3+ doped ZnO powder Figure 1. XRD patterns of ZnO(a) and ZnO:5%Tb(b) appear in the XRD patterns of the samples ZnO:5%Tb. In fact, Tb3+ ions occupy Zn2+ sites or interstitial sites in ZnO lattice. The crystalline size of the samples was calculated from the Scherrer formula. The average crystalline size of the samples is about 12 nm. 2.2.2. The band gap of the materials ZnO and ZnO:Tb3+ The transmission of the samples is determined by relation below: T = exp (−αt) (2.1) where α is the absorption coefficient of the samples, t is the thickness of the samples. α strongly depends on photon energy hν. The dependence of α on the photon energy hν is called the absorption. The absorption of the ZnO sample, which is realized on the JASCO spectrometer/data system, is shown in Figure 2. 23 Ngo Ngoc Hoa, Nguyen Minh Thuy and Nguyen Manh Nghia Figure 2. The obsorption of ZnO sample If the photon energies are larger than the band gap energy Eg, we have the following experimental relation [2]. αhν = A (hν −Eg)1/2 (2.2) Where A is a constant and Eg is the energy of band gap. By multiply equation (2.2) with thickness of sample t, we obtain: (αhνt)2 = (At)2 (hν − Eg) (2.3) The variation of (αhνt)2 with hν of the samples ZnO is shown in Figure 3. Figure 3. The diagram of (αhνt)2 versus hν of ZnO sample 24 Study on band gap energy of pure and Tb3+ doped ZnO powder The optical band gap values are obtained by extrapolating the linear portion of the plots of (αhνt)2 versus hν to α = 0. The result shows that the value of band gap energy is 3.20 eV at room temperature. For the ZnO:Tb3+, the variation of (αhνt)2 with photon energy hν is shown in Figure 4. Figure 4. The variation of (αhνt)2 with hν of the samples ZnO:Tb3+ with diference of the molar ratio Tb3+ The optical band gap of the samples ZnO:Tb3+ increase from 3.3 eV to 3.4 eV with increase of the molar of Tb3+. In comparision to the original ZnO crystal, this value of the band gap is larger. The increase of band gap with the increase of the molar ratio of Tb3+ is due to Moss-Burstein shift [3,4]. For more detailed, in the pure crystal ZnO the band gap is the energy difference between the top of the valence band and the bottom of the conduction band of semiconductors in Figure 5 (a). Since the Pauli principle, prevents states from being doubly occupied and the optical transitions only occur vertically, the energy band gap is given at these points. This is shown in Figure 5 (b). Figure 5. Band gap widening due to the Burstein-Moss shift 25 Ngo Ngoc Hoa, Nguyen Minh Thuy and Nguyen Manh Nghia The value of the band gap Eg = Eg0 +∆E BM g (2.4) where Eg0 is the band gap of pure ZnO. The increase of the energy band gap is determined by the following equation: ∆EBMg = ~2k2F 2 ( 1 me + 1 mh ) (2.5) where kF is the Fermi wave vector; me, mh are the effective masses of the electrons and holes in the conduction band. 3. Conclusion Tb3+ doped ZnO nanocrystals were synthesized by forced hydrolysis method. The samples have hexagonal wurtzite structure of ZnO. We present one method to determine the band gap energy of ZnO and ZnO:Tb3+ materials. The sesult shows that the value of band gap energy is 3.24 eV at room temperature. For the samples ZnO:Tb3+, the band gap energy increase from 3.3 eV to 3.4 eV with increase of the molar of Tb3+. In comparition to the original ZnO crystal, this value of the band gap is larger. The increase of band gap with the increase of the molar ratio of Tb3+ is due to Moss-Burstein shift. REFERENCES [1] Xi Chen, Bing Yan, 2007. Induced synthesis of ZnO:Tb3+ green hybrid phosphors by the assembly of polyethylene glycol matrices. Materials Letters Vol. 61, pp. 1707-1710. [2] T.K.Subramanyam, B.Srinivasulu Naidu, S.Uthanna, 1999. Structure and optical properties of dc reactive magnetron sputtered zinc oxide films. Cryst. Res. Technol Vol. 34, pp. 981-988. [3] Z.B.Fang et al, 2005. Transparent conductive Tb-doped ZnO films prepared by rf reactive magnetron sputtering. Materials Letters Vol. 59, pp. 2611-2614. [4] X. 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