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The influence of the pressure and temperature on the light emission of the ZnO
ARTICLE IN PRESS
Physica B 398 (2007) 291–293 www.elsevier.com/locate/physb
The influence of the pressure and temperature on the light emission of the ZnO N.O. Dantas, M.A. Couto dos Santos, F. Cunha, M.A. Maceˆdo Physics Department, Federal University of Sergipe, 49100-000 Sa˜o Cristo´va˜o, SE, Brazil
Abstract A new route for the preparation of zinc oxide powder is described along with its characterization. A proteic sol was prepared dissolving zinc nitrate in filtered coconut water. After calcination at 1000 1C, the powder was compressed to 1.3 108 Pa and ZnO pellets were obtained. The emission spectra were recorded under UV excitation at 325 and 400 nm. The powder showed no spectroscopic response, whereas one peak around 396 nm was observed for the pressed powder (pellet with no heat treatment). The pellets were then annealed for 24 h at 500, 800 and 1000 1C. In the first case, bands at 396 and 440 nm and a structure of narrow peaks around 480 nm (oxygen vacancies) were observed. Increasing the annealing temperature led to a decrease in the intensity of the emissions at 440 and 480 nm. We propose that the high pressure induces a red-shift in the UV region of the ZnO nanopowder emission peaks to 396 nm. This is an indication that the ZnO nanopowder treated under pressure and sintering temperature exhibits the spectroscopic behavior characteristic of the ZnO single crystal. The disappearance of the 440 and 480 nm lines indicate the reduction of oxygen vacancies. The atomic force micrographs suggest a coalescence thermal point. r 2007 Elsevier B.V. All rights reserved. PACS: 78.20. e; 78.40.Fy; 81.20.Fw Keywords: ZnO; Proteic sol–gel process; Light emission
1. Introduction ZnO is a direct semiconductor with a band gap energy of 3.37 eV at 300 K (wurtzite structure). It is a material with unique features such as piezoelectricity, photoelectricity and pyroelectricity; therefore being suitable for a myriad of applications [1]. The physical properties of the ZnO depend strongly on the size and orientation of the nanocrystals as well as in the way they interact with each other. It is possible to produce light emitting devices with wide optical window (ultraviolet, blue, green or white light) and sensor devices, due to its large specific surface area and activity [2–5]. In the related literature, one can find a number of spectroscopic investigations which show that the UV emission of ZnO occurs from 373 to 390 nm [1,6,7]. In the wurtzite structure, the edge of the gap is around Corresponding author. Tel.: +55 79 2105 6810; fax: +55 79 2105 6807.
E-mail address: [email protected] (M.A. Maceˆdo). 0921-4526/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2007.05.017
368 nm. Besides, results in thin film ZnO systems, show that the augmentation of its thickness leads to the suppression of the emission lines attributed to oxygen vacancies. In this paper, we present a new sol–gel route for obtaining high-quality nanosized ZnO powder [8]. This technique uses coconut water as precursor. The photoinduced emission of ZnO nanopowder and pellets was studied and a correlation between the emission, pressure and the sintering temperature was found. 2. Experimental The proteic sol was prepared dissolving zinc nitrate in filtered coconut water (Cocos nucifera) with a concentration of 0.5 mol/dm3. Afterwards, the sol was heated to 100 1C for 24 h and then submitted to calcination at 1000 1C for 24 h in air, in order to completely oxidize the salts. Then, the final product was quenched back to room temperature. The crystalline phase of the powder was
ARTICLE IN PRESS N.O. Dantas et al. / Physica B 398 (2007) 291–293
consistent with standard XRD pattern to the JCPDS 361451. Finally, the powder was compressed by 1.3 108 Pa and a ZnO pellet was obtained. The spectroscopic behavior of the system was obtained in an ISS PC1TM Spectrofluorimeter. The excitation device is equipped with a 300 W xenon lamp and a holographic grating. The emission is collected in a 25-cm monochromator with a resolution of 0.1 nm equipped with a photomultiplier. The excitation and emission slit width were 1 mm, while both monochromators have 1200 grooves/mm. Atomic force microscopy was employed for the topography analysis. The images were acquired in contact mode using a Veeco’s CP-Research AFM and tip. The parameters used were: force constant of 50 nN and scan rate of 1 Hz.
λ exc = 400 nm c x5 d x200
Intensity (a.u.)
292
420
480
540
600
660
720
λ (nm)
3. Results and discussion The emission measurements of the ZnO nanopowder and pressed powder were recorded with no annealing treatment. For the pellets, the emission spectra were recorded for 500, 800 and 1000 1C annealing temperatures. The excitation wavelength were at 325 and 400 nm in all cases. This is shown in Figs. 1 and 2. In both cases, it is clear that the spectroscopic behavior of the ZnO nanoparticles is altered by the pressure. The emission line at 396 nm, which appears in all spectra of the pellets, is interpreted as a transition from the edge of the conduction band to the valence band. As the annealing temperature grows, the peaks due to oxygen vacancies vanish. The increase in the annealing temperature has the same effect in the ZnO nanoparticles as observed in Ref. [6]. Because the emission band around 396 nm appears just after submitting the powder to
emission (a.u.)
λ exc =325 nm d x100 e x100 a
b d
c
e
420
480
540 λ (nm)
600
660
Fig. 1. Emission measurements with excitation at 325 nm of the pellet annealed for (a) 500 1C, (b) 800 1C, (c) 1000 1C, (d) pressed powder and (e) powder with no annealing.
Fig. 2. Emission measurements with excitation at 400 nm of the pellet annealed for (a) 500 1C, (b) 800 1C, (c) 1000 1C, (d) pressed powder and (e) powder with no annealing.
pressure in order to prepare the pellets, the sample seems to start the process towards a spectroscopic bulk behavior from a spectroscopic powder behavior of formation of the monocrystal. From the spectra after the heating treatment from 500 to 1000 1C, one can clearly observe the vanishing of the emission lines around 480 nm and the maintenance of the transition at 396 nm, interpreted as the disappearance of defect levels due to the oxygen vacancies. This is an indication that heating the sample with a temperature above a coalescence thermal point (thermal point from which the neighbor particles coalesce to form the crystal) leads to the depletion of oxygen ions from the particle surface. The thermal agitation leads to friction between particles. Above this temperature, here called coalescence thermal point, the particles coalesce to form the single crystal. The topographic structure was analyzed using atomic force microscopy (AFM) in contact mode. The grain size of the samples treated at different temperatures had small variations up to 500 1C, but an accurate measurement of the grain dimensions was severely hampered due to their poor stability when probed by the scanning AFM tip. Using the software provided by Veeco, we measured the roughness of the three samples in several different spots. The values obtained varied slightly in each spot of the same sample, but a clear tendency towards a lower value as the cure temperature was increased was clearly noticeable. The increase in temperature led to clearly larger grain sizes, but the stability only reached acceptable behavior above 900 1C. In Fig. 3, a picture taken of the surface of the sample treated to 1000 1C is shown. A large terrace can be visualized in the center which extends over at least 300 nm in diameter. Although not widespread, this kind of feature could be easily found in several spots and indicates that a strong coalescence process took place.
ARTICLE IN PRESS N.O. Dantas et al. / Physica B 398 (2007) 291–293
293
as a function of the temperature indicates that there may exist a coalescence thermal point. The atomic force micrographs support this explanation.
Acknowledgments The authors deeply acknowledge the NANOFOTON/ CNPq Brazilian network for financial support.
References Fig. 3. AFM image of the ZnO pellet sintered at 1000 1C.
4. Conclusion ZnO nanopowder was prepared by proteic sol–gel method and ZnO pellets were compressed and thermally treated. The spectroscopic behavior of the ZnO powder as prepared did not show any spectroscopic response. A preliminary interpretation is that ZnO nanopowder behaves as a ZnO-isolated molecule, with no band structure or gap, but levels in the UV region above 325 nm. A similar result was observed in Ag nanoparticles embedded in fluoroborate glass [9]. The vanishing of the emission lines
[1] Z.L. Wang, Mater. Today 7 (2004) 26. [2] M. Yang, D. Wang, Y. Lin, Z. Li, Q. Zhang, Mater. Chem. Phys. 88 (2004) 333. [3] G.S. Trivikrama Rao, D. Tarakarama Rao, Sensors Actuators B 55 (1999) 166. [4] P. Mitra, A.P. Chatterjee, H.S. Maiti, Mater. Lett. 35 (1998) 33. [5] N. Golego, S.A. Studenikin, M. Cocivera, J. Electrochem. Soc. 147 (4) (2000) 1592. [6] C.M. Hou, K.K. Huang, Z.M. Gao, X.S. Li, S.H. Feng, Y.T. Zhang, G.T. Du, Chem. Res. Chin. Univ. 22 (2006) 552. [7] L. Jing, Y. Qu, B. Wang, S. Li, B. Jiang, L. Yang, W. Fu, H. Fu, J. Sun, Sol. Energy Mater. Sol. Cell 90 (2006) 1773. [8] M. Maceˆdo, J. M. Sasaki, Patent Pending 0203876-5/BR. [9] M.A. Couto dos Santos, Contribution to the study of quantum yield of 4f–4f transitions in amorphous media containing metallic particles, Ph.D. Thesis, 1992.
Physica B 398 (2007) 291–293 www.elsevier.com/locate/physb
The influence of the pressure and temperature on the light emission of the ZnO N.O. Dantas, M.A. Couto dos Santos, F. Cunha, M.A. Maceˆdo Physics Department, Federal University of Sergipe, 49100-000 Sa˜o Cristo´va˜o, SE, Brazil
Abstract A new route for the preparation of zinc oxide powder is described along with its characterization. A proteic sol was prepared dissolving zinc nitrate in filtered coconut water. After calcination at 1000 1C, the powder was compressed to 1.3 108 Pa and ZnO pellets were obtained. The emission spectra were recorded under UV excitation at 325 and 400 nm. The powder showed no spectroscopic response, whereas one peak around 396 nm was observed for the pressed powder (pellet with no heat treatment). The pellets were then annealed for 24 h at 500, 800 and 1000 1C. In the first case, bands at 396 and 440 nm and a structure of narrow peaks around 480 nm (oxygen vacancies) were observed. Increasing the annealing temperature led to a decrease in the intensity of the emissions at 440 and 480 nm. We propose that the high pressure induces a red-shift in the UV region of the ZnO nanopowder emission peaks to 396 nm. This is an indication that the ZnO nanopowder treated under pressure and sintering temperature exhibits the spectroscopic behavior characteristic of the ZnO single crystal. The disappearance of the 440 and 480 nm lines indicate the reduction of oxygen vacancies. The atomic force micrographs suggest a coalescence thermal point. r 2007 Elsevier B.V. All rights reserved. PACS: 78.20. e; 78.40.Fy; 81.20.Fw Keywords: ZnO; Proteic sol–gel process; Light emission
1. Introduction ZnO is a direct semiconductor with a band gap energy of 3.37 eV at 300 K (wurtzite structure). It is a material with unique features such as piezoelectricity, photoelectricity and pyroelectricity; therefore being suitable for a myriad of applications [1]. The physical properties of the ZnO depend strongly on the size and orientation of the nanocrystals as well as in the way they interact with each other. It is possible to produce light emitting devices with wide optical window (ultraviolet, blue, green or white light) and sensor devices, due to its large specific surface area and activity [2–5]. In the related literature, one can find a number of spectroscopic investigations which show that the UV emission of ZnO occurs from 373 to 390 nm [1,6,7]. In the wurtzite structure, the edge of the gap is around Corresponding author. Tel.: +55 79 2105 6810; fax: +55 79 2105 6807.
E-mail address: [email protected] (M.A. Maceˆdo). 0921-4526/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2007.05.017
368 nm. Besides, results in thin film ZnO systems, show that the augmentation of its thickness leads to the suppression of the emission lines attributed to oxygen vacancies. In this paper, we present a new sol–gel route for obtaining high-quality nanosized ZnO powder [8]. This technique uses coconut water as precursor. The photoinduced emission of ZnO nanopowder and pellets was studied and a correlation between the emission, pressure and the sintering temperature was found. 2. Experimental The proteic sol was prepared dissolving zinc nitrate in filtered coconut water (Cocos nucifera) with a concentration of 0.5 mol/dm3. Afterwards, the sol was heated to 100 1C for 24 h and then submitted to calcination at 1000 1C for 24 h in air, in order to completely oxidize the salts. Then, the final product was quenched back to room temperature. The crystalline phase of the powder was
ARTICLE IN PRESS N.O. Dantas et al. / Physica B 398 (2007) 291–293
consistent with standard XRD pattern to the JCPDS 361451. Finally, the powder was compressed by 1.3 108 Pa and a ZnO pellet was obtained. The spectroscopic behavior of the system was obtained in an ISS PC1TM Spectrofluorimeter. The excitation device is equipped with a 300 W xenon lamp and a holographic grating. The emission is collected in a 25-cm monochromator with a resolution of 0.1 nm equipped with a photomultiplier. The excitation and emission slit width were 1 mm, while both monochromators have 1200 grooves/mm. Atomic force microscopy was employed for the topography analysis. The images were acquired in contact mode using a Veeco’s CP-Research AFM and tip. The parameters used were: force constant of 50 nN and scan rate of 1 Hz.
λ exc = 400 nm c x5 d x200
Intensity (a.u.)
292
420
480
540
600
660
720
λ (nm)
3. Results and discussion The emission measurements of the ZnO nanopowder and pressed powder were recorded with no annealing treatment. For the pellets, the emission spectra were recorded for 500, 800 and 1000 1C annealing temperatures. The excitation wavelength were at 325 and 400 nm in all cases. This is shown in Figs. 1 and 2. In both cases, it is clear that the spectroscopic behavior of the ZnO nanoparticles is altered by the pressure. The emission line at 396 nm, which appears in all spectra of the pellets, is interpreted as a transition from the edge of the conduction band to the valence band. As the annealing temperature grows, the peaks due to oxygen vacancies vanish. The increase in the annealing temperature has the same effect in the ZnO nanoparticles as observed in Ref. [6]. Because the emission band around 396 nm appears just after submitting the powder to
emission (a.u.)
λ exc =325 nm d x100 e x100 a
b d
c
e
420
480
540 λ (nm)
600
660
Fig. 1. Emission measurements with excitation at 325 nm of the pellet annealed for (a) 500 1C, (b) 800 1C, (c) 1000 1C, (d) pressed powder and (e) powder with no annealing.
Fig. 2. Emission measurements with excitation at 400 nm of the pellet annealed for (a) 500 1C, (b) 800 1C, (c) 1000 1C, (d) pressed powder and (e) powder with no annealing.
pressure in order to prepare the pellets, the sample seems to start the process towards a spectroscopic bulk behavior from a spectroscopic powder behavior of formation of the monocrystal. From the spectra after the heating treatment from 500 to 1000 1C, one can clearly observe the vanishing of the emission lines around 480 nm and the maintenance of the transition at 396 nm, interpreted as the disappearance of defect levels due to the oxygen vacancies. This is an indication that heating the sample with a temperature above a coalescence thermal point (thermal point from which the neighbor particles coalesce to form the crystal) leads to the depletion of oxygen ions from the particle surface. The thermal agitation leads to friction between particles. Above this temperature, here called coalescence thermal point, the particles coalesce to form the single crystal. The topographic structure was analyzed using atomic force microscopy (AFM) in contact mode. The grain size of the samples treated at different temperatures had small variations up to 500 1C, but an accurate measurement of the grain dimensions was severely hampered due to their poor stability when probed by the scanning AFM tip. Using the software provided by Veeco, we measured the roughness of the three samples in several different spots. The values obtained varied slightly in each spot of the same sample, but a clear tendency towards a lower value as the cure temperature was increased was clearly noticeable. The increase in temperature led to clearly larger grain sizes, but the stability only reached acceptable behavior above 900 1C. In Fig. 3, a picture taken of the surface of the sample treated to 1000 1C is shown. A large terrace can be visualized in the center which extends over at least 300 nm in diameter. Although not widespread, this kind of feature could be easily found in several spots and indicates that a strong coalescence process took place.
ARTICLE IN PRESS N.O. Dantas et al. / Physica B 398 (2007) 291–293
293
as a function of the temperature indicates that there may exist a coalescence thermal point. The atomic force micrographs support this explanation.
Acknowledgments The authors deeply acknowledge the NANOFOTON/ CNPq Brazilian network for financial support.
References Fig. 3. AFM image of the ZnO pellet sintered at 1000 1C.
4. Conclusion ZnO nanopowder was prepared by proteic sol–gel method and ZnO pellets were compressed and thermally treated. The spectroscopic behavior of the ZnO powder as prepared did not show any spectroscopic response. A preliminary interpretation is that ZnO nanopowder behaves as a ZnO-isolated molecule, with no band structure or gap, but levels in the UV region above 325 nm. A similar result was observed in Ag nanoparticles embedded in fluoroborate glass [9]. The vanishing of the emission lines
[1] Z.L. Wang, Mater. Today 7 (2004) 26. [2] M. Yang, D. Wang, Y. Lin, Z. Li, Q. Zhang, Mater. Chem. Phys. 88 (2004) 333. [3] G.S. Trivikrama Rao, D. Tarakarama Rao, Sensors Actuators B 55 (1999) 166. [4] P. Mitra, A.P. Chatterjee, H.S. Maiti, Mater. Lett. 35 (1998) 33. [5] N. Golego, S.A. Studenikin, M. Cocivera, J. Electrochem. Soc. 147 (4) (2000) 1592. [6] C.M. Hou, K.K. Huang, Z.M. Gao, X.S. Li, S.H. Feng, Y.T. Zhang, G.T. Du, Chem. Res. Chin. Univ. 22 (2006) 552. [7] L. Jing, Y. Qu, B. Wang, S. Li, B. Jiang, L. Yang, W. Fu, H. Fu, J. Sun, Sol. Energy Mater. Sol. Cell 90 (2006) 1773. [8] M. Maceˆdo, J. M. Sasaki, Patent Pending 0203876-5/BR. [9] M.A. Couto dos Santos, Contribution to the study of quantum yield of 4f–4f transitions in amorphous media containing metallic particles, Ph.D. Thesis, 1992.