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Effect of photonic band-gap on photoluminescence of ZnO deposited inside the green synthetic opal
Optics Communications 250 (2005) 111–119 www.elsevier.com/locate/optcom
Effect of photonic band-gap on photoluminescence of ZnO deposited inside the green synthetic opal S.M. Abrarov a
a,*
, Sh.U. Yuldashev a, T.W. Kim b, S.B. Lee c, Y.H. Kwon a, T.W. Kang a
Quantum-functional Semiconductor Research Center, Dongguk University, 3-26 Pil-dong, Chung-ku, Seoul 100-715, South Korea b Advanced Semiconductor Research Center, Division of Electrical and Computer Engineering, Hanyang University, Seoul 133-791, South Korea c Nanotechnology Institute, University of Texas at Dallas, Richardson, TX 75083, USA Received 26 August 2004; received in revised form 12 December 2004; accepted 7 February 2005
Abstract The temperature dependent photoluminescence spectra of ZnO embedded in the voids between the fcc packed SiO2 sub-micron spheres by using a spray pyrolysis were studied. In contrast to ZnO powder, the ZnO inside the green synthetic opal exhibit a broadened dominant excitonic band edge emission and rapidly decreasing deep level emission with decreasing temperature. The overlap between photonic and electronic band-gaps of the opal matrix and ZnO prevents the radiative recombinations through native defects resulting to the suppression of the photoluminescence intensity in the green spectrum. The growth of ZnO inside the green synthetic opal may be useful technique to enhance the UV-blue emission. Ó 2005 Elsevier B.V. All rights reserved. PACS: 78.55.m; 78.55.Et; 42.70.Qs; 82.30.Lp Keywords: ZnO; Photoluminescence; Photonic crystal; Synthetic opal; Spray pyrolysis
1. Introduction
* Corresponding author. Tel.: +82 22260 3205; fax: +82 222603945/3205. E-mail addresses: [email protected], [email protected] (S.M. Abrarov).
Zinc oxide, a promising wide band gap semiconductor, recently has received a considerable attention due to its remarkable optical and electrical properties. ZnO is a very versatile material, which is widely applied in industry for many
0030-4018/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2005.02.016
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purposes such as antireflection coatings, transparent electrodes in solar cells, sensors and surface acoustic wave devises. In addition, ZnO has attracted a strong interest due to its unique luminescent properties, which may be useful for potential applications in UV light emitting devices and flat panel displays as a low voltage phosphor [1–3]. Typically the photoluminescence (PL) spectrum of ZnO consists of excitonic near band edge and deep level (DL) emissions in the UV-blue and green spectral regions, respectively. The origins for DL emission are related to native defects like oxygen vacancies and Zn interstitials [3–5]. Various techniques, such as pulsed laser deposition, magnetron sputtering, chemical vapor deposition (CVD) and molecular beam epitaxy (MBE) are used for preparation of ZnO bulk crystals, films and powders [6–9]. These techniques, however, need the highly expensive equipment and sophisticated growth technology. Among all the available techniques, a spray pyrolysis has many advantages over others. In particular, it is a technologically simple, inexpensive, and efficient for the chemical deposition of ZnO [5,10]. It has been reported recently that spray pyrolysis can be successfully used for fabrication of the high quality bright phosphors like blue-emitting thin films of cerium-doped barium chloride hydrate [11], green-, blue-, and yellow-emitting Ba2B5O9Cl thin-films doped with Tb3+, Tm3+, and Mn2+ [12], and blue-emitting alkaline earth chloroborate thin films doped with Eu [13]. Another significant advantage of the spray pyrolysis is the possibility to grow luminescent nanoparticles in the precursor solution, which enables us to fill uniformly the voids inside the volume of the porous material. Photonic crystals (PhCs) proposed by Yablonovich [14] and John [15], can be used to modify the PL spectrum of the luminescent semiconductors. Besides many properties of PhCs, their ability to suppress the emission band [16] may also be useful in fabrications of the light emitting devices. For example, a strong modification of the PL spectra in CdS nanocrystals embedded into synthetic opal due to the inhibition of spontaneous emission has been reported [17]. This remarkable property of the PhCs may be applied to control the bandwidth
and increase an efficiency of the bright phosphors for full-color displays. In the recent studies dedicated to various phosphor materials, much interest attracted the growth of the luminescent semiconductors inside the matrix opal [17] and inverted matrix opal [18,19]. The successful attempt to combine ZnO with PhC has been recently reported by Gruzintsev et al. [20,21]. In these reports, the PL spectra of the ZnO thin (90 nm) film deposited on the surface of opal matrix by electron-beam sputtering have been investigated. The thin ZnO film in such a structure represents a 2D periodic array of quantum dots distributed between 279 nm SiO2 spheres. As a result of the quantum confinement, the ZnO quantum dots in this configuration exhibit a narrow excitonic emission. However, the similar approach can also be extended for 3D periodic structure by growth of ZnO inside PhC in zinc nitrate or zinc acetate precursor solution. In particular, the complete filling of the synthetic opal volume with ZnO by using a precursor solution as a useful technique to modify the PL spectra of zinc oxide has been first time reported [22,23]. The green porous opal was chosen as a matrix for infiltration since its photonic band-gap overlaps the electronic band-gap of ZnO. The overlap between photonic and electronic band-gaps enables us to suppress undesirable spontaneous emission in the spectral region associated with native defects such as oxygen vacancies and zinc ion interstitials. Preliminary PL studies [22–26] of ZnO embedded into green synthetic opal by using the spray pyrolysis have been reported. However the temperature dependent PL spectra have not been investigated yet and the assumption that the suppression in the green PL band is due to the influence of PhC must be verified more strictly. This paper reports the temperature dependent PL spectra of the ZnO embedded into synthetic opal by using the spray pyrolysis method. PL spectroscopy shows two factors supporting an assumption that the suppression of the DL emission is due to the influence of PhC. First, in contrast to the ZnO powder, the ZnO nanocrystals inside the green synthetic opal exhibit a broadened dominant UV-blue emission. Second, their DL emission in
S.M. Abrarov et al. / Optics Communications 250 (2005) 111–119
the green spectrum rapidly decreases with decreasing temperature. Despite the polycrystalline structure, ZnO inside the green synthetic opal exhibit the enhanced UV-blue PL emission.
2. Experimental 2.1. Preparation of synthetic opal The sub-micron size spheres of silicon dioxide (SiO2) were synthesized in Stober–Fink–Bohn process [27] via the hydrolysis of tetraethyl orthosilicate (TEOS) in ethanol solution containing ammonium hydroxide and water. The diameter of the SiO2 spheres depends upon many factors. However, the main factors used to control the size of the particles are the temperature of the TEOS hydrolysis and the concentrations of ammonium hydroxide and water. Particularly, an increase of the ammonium hydroxide results to the increase of the diameter of the spherical particles. On the contrary, the increase of the water concentration causes a decrease of the diameter of the spherical particles. Typically, employing the hydrolysis of TEOS in ethanol solution with ammonium hydroxide and water, the spherical particles with diameters between 0.03 and 3.0 lm can be synthesized. Synthetic opal was assembled to fcc packed matrix by natural sedimentation of SiO2 spheres in water suspension. After sedimentation, the water was slowly evaporated at the temperature 80 °C. The self-assembled fcc packed matrix is mechanically fragile, therefore it was thermally hardened for several hours at temperature 1000 °C. The technique described above enables the fabrication the SiO2 spheres with small deviation (less than 10%) from their average size. The more complete description of the opal matrix fabrication can be found elsewhere [28,29]. 2.2. Infiltration of synthetic opal ZnO nanocrystals was deposited from 0.1 M deionized water solution containing a zinc nitrate hexahydrate, Zn(NO3)2 Æ 6H2O, precursor of
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99.999% purity. The chemical synthesis of ZnO is described by the following schematics: 0 1 ZnðOHÞ2 B C ZnðNO3 Þ2 6H2 O ! @ Zn2 OðNO3 Þ2 A ð1Þ ZnðOHÞðNO3 Þ ! ZnO þ yNw Ox þ zO2 Particularly, in the solution, zinc nitrate is decomposed to zinc hydrogenous and nitrogenous compounds such as zinc dihydroxide, dizincoxide nitrate, and zinc hydroxide nitrate, respectively, as shown in the parenthesis of the schematics. The proportions between zinc hydrogenous and nitrogenous compound concentrations depend on temperature of the precursor solution. Under the thermal annealing, zinc compounds are further decomposed to ZnO, nitrogen oxides, and oxygen molecules. The detailed chemical synthesis of zinc oxide and film or powder deposition technique procedures are described elsewhere [5,10]. To prepare a sample, the 1 mm-thick opal slab was kept in 0.1 M zinc nitrate aqueous solution for 48 h. To remove air from the voids and to improve the fluid flow, the opal slab was soaked under ultrasonic vibration for the first 3 h. The long ultrasonic vibration is not desirable since it may destroy the ZnO nanoparticles and damage the sample. After soaking in precursor solution, the opal slab was dried in the drying oven at 90 °C for 30 min and annealed at 500 °C for 1 h. Since the synthetic opal is a porous material, the solution was effectively observed resulting in a uniform distribution of ZnO nanocrystals inside the synthetic opal. For the sufficient infiltration of ZnO inside the volume of the opal slab, these procedures were repeated several times. The filling factor can be controlled by concentration of the precursor and by number of infiltration cycles. However the high concentration of the precursor may deteriorate the uniformity of ZnO inside the matrix. To improve the penetration of the precursor solution inside the sample, the ZnO layers were removed from the opal surfaces by polishing and cleaning after each infiltration cycle. Finally, for the proper PL measurements, ZnO layers were
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Fig. 1. Normalized transmittance of the bare and ZnO embedded opals.
also removed since their contribution may be strong enough to screen the effect of the photonic band gap on the PL spectra. The filling fraction of ZnO inside the opal matrix was estimated using both the gravimetric method and red shift Dk (Fig. 1) in the transmittance spectra. As a reference sample, a ZnO powder was also deposited on a flat surface of the crystalline quartz substrate (a-quartz) under similar conditions. 2.3. Measurements Temperature dependent spectra were obtained using a SPEX spectrometer equipped with 0.75 m grating monochromator. PL measurements were performed under 10 mW He–Cd laser excitation at the wavelength of 325 nm. A broadband-sensitive photomultiplier tube Hamamatsu R943-02 was used as a detector. The transmittance characteristics were obtained applying a xenon lamp as a white light source exposed to the grating monochromator. To enhance a transmittance, the thickness of the opal slab was reduced to 300 lm. The general morphologies of the samples were
analyzed using a scanning electron microscope (SEM) XL-30 PHILIPS. The temperature was controlled between 10 and 300 K using the CRYODINE He displex system.
3. Results and discussion Fig. 1 shows the normalized transmittance of the unfilled opal (bare opal) and ZnO embedded opal. The appearance of the red shift in transmittance is due to the decrease of the refractive index contrast since the index of refraction of ZnO is relatively close to that of SiO2. The filling factor of the ZnO was found by using two methods. In the first method, the relation between the optical wavelength k with the filling factors FSiO2, FZnO and Fair for SiO2, ZnO and air was used [17]: pffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffi k ¼ 2 0:816D eSiO2 F SiO2 þ eZnO F ZnO pffiffiffiffiffiffi þ eair ðF air F ZnO ÞÞ; where D is the average diameter of spheres, eSiO2 , eZnO, and eair are the dielectric constants of SiO2,
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ZnO, and air, respectively. Considering the fact that the filling factors for the SiO2 and air of the bare opal are 74% and 26%, respectively, and using above equation, the filling factor for ZnO was estimated 7%. In the second method, the filling factor was determined gravimetrically, i.e., by weighting sample before and after ZnO infiltration. The filling factor magnitudes for ZnO estimated by using the red shift and determined gravimetrically by increase of the sample weight are in good agreement. It should be mentioned that the filling factor of ZnO could be very high when inverted opal is used. For instance, the 96% filling factor of CdS inside the inverted opal was reported [19]. The ability to suppress the DL emission and the feasibility to achieve the filling factor above 90% in the inverted opal indicate the significance of ZnO infiltrated into PhC for potential applications in optoelectronics. Fig. 2(a) shows SEM image of a bare synthetic opal consisting of the fcc packed SiO2 submicron spheres. The diameters of the SiO2 spheres vary between 220 and 240 nm. The sizes of the ZnO nanoparticles synthesized under ultrasonic vibration from the nitrate precursor solution may be less than 20 nm [30]. The ZnO nanoparticles obtained by means of spray pyrolysis are sufficiently small compared to the void sizes between the SiO2 spheres. Therefore ZnO can be grown inside the synthetic opal uniformly since the fluid flow in the precursor solution is able to penetrate deep inside the volume of the opal matrix. Due to the growth of ZnO on the spherical surfaces, the spherical shapes are transformed to the honeycomb-type cell in the (1 1 1) plane and square-type cell in the (1 0 0) plane. Figs. 2(b) and (c) show the SEM images of ZnO embedded opal normal to (1 0 0) and (1 1 1) planes, respectively. Fig. 3 shows the temperature dependent PL spectra of the pure ZnO powder. A free exciton peak (XA) is located near 3.28 eV at 300 K. The UV-blue emission increases at temperatures below 240 K. The free exciton peak is highest in the UV-blue emission spectra at temperatures between 30 and 240 K. The blue shift due to the quantum confinement in the PL spectra is not
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Fig. 2. SEM images of the: (a) bare opal, (b) ZnO embedded opal in the (1 1 1) plane and (c) ZnO embedded opal in the (1 0 0) plane.
observed since ZnO nanocrystals are not separated, i.e., they are in physical contact with each other. The strongest DL emission peak near 2.38 (510 nm) is related to native defects such as oxygen vacancies and Zn interstitials commonly observed in ZnO bulk crystals, films, and powders [3–5]. The DL peaks in the green spectral
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Fig. 3. Temperature dependent photoluminescence spectra of the pure ZnO powder.
region remain very strong even at low temperatures due to the high crystal defects density in the polycrystalline ZnO powder. Fig. 4 shows the low-temperature PL spectra of the pure ZnO powder in the UV-blue region. Below 30 K the contribution from the free exciton located at 3.374 eV decreases and the emission from the donor bound exciton located at 3.360 eV increases with decreasing temperature.
Fig. 4. Low temperature photoluminescence spectra of the pure ZnO powder in the UV-blue region.
Such a behavior is commonly observed in the PL spectra of ZnO and results from the decomposition of the bound exciton to the free exciton due to increased thermal energy at temperature above 25–30 K [31]. Fig. 5 shows the temperature dependent PL spectra of the ZnO embedded opal. At 300 K the highest peak in the UV-blue region is observed near 3.26 eV, while the free exciton peak (XA) is observed at 3.28 eV. In contrast to ZnO powder, the ZnO nanocrystals inside the synthetic opal exhibit a strong UV-blue emission dominating even above room temperature. Presumably, the PL spectra is broadened and enhanced in the UV-blue region due to Purcell enhancement [32,33]. In particular, the modification of the PL characteristics can be described from the fact that the suppression of the PL band leads to the spectral redistribution (recycling) of the emission energy [16]. Since the radiative recombination rate in the green spectral band is decreased, the spontaneous emission mostly releases through allowed transitions leading to the broadened and enhanced PL spectra in the UV-blue region. The similar observation, the enhancement of the PL intensity in the UV band and its decrease in the green band, for ZnO nanocrystals inside the synthetic opal has been reported by Masalov et al. [24].
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Fig. 5. Temperature dependent photoluminescence spectra of the ZnO embedded opal.
In the spectral region related to the crystal defects, the strongest DL emission peak around 2.38 eV (510 nm) is observed. However this peak rapidly decreases with decreasing temperature and almost disappears below 200 K. Fig. 6 shows the PL spectra of pure ZnO powder and ZnO embedded opal at 10 K. For the ZnO powder, the free exciton and donor bound exciton are located at 3.374 and 3.360 eV, respectively. The emission peaks at 3.315 and 3.235 eV are attributed to donor–acceptor pairs [34,35]. The FWHM values of these peaks are around 3–10 meV. Four distinct peaks at 3.367, 3.327, 3.295, and 3.210 eV are observed in the low temperature spectra. Presumably, the peaks at 3.367 and 3.327 eV are attributed to free exciton and donor bound exciton, respectively, while the peaks located at 3.295 and 3.210 eV are attributed to donor–acceptor pairs. The FWHM values for these peaks are approximately twice the values for the lines in ZnO powder spectrum. The effect of photonic band-gap on PL emission can be seen from comparison between the spectra of the reference sample and ZnO embedded opal, shown in the Figs. 3 and 5, respectively. Due to
Fig. 6. Low temperature spectra of the: (a) ZnO powder and (b) ZnO embedded opal.
the high density of crystal defects, the UV-blue emission of ZnO powder increases insignificantly, while the DL emission strongly dominates even
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at very low temperatures (Fig. 3). On the contrary, the ZnO embadded into opal exhibit the broadened dominant UV-blue emission and rapidly decreasing DL emission with decreasing temperature (Fig. 5). Furthermore, the band edge exciton emission sustains the highest peak above room temperature. The photonic band-gap overlaps the green spectral region, associated with radiative recombinations through native defects like oxygen vacancies and Zn interstitials. The overlap between photonic and electronic band-gaps prevents the radiative recombinations through native defects ultimately leading to suppression of the PL intensity in the green spectral region.
4. Summary The temperature dependent PL spectra of ZnO embedded into synthetic opal were studied. To fill the voids between the SiO2 spheres with ZnO nanocrystals, the technologically simple and inexpensive spray pyrolysis was used. For the formation of ZnO, the opal matrix was soaked in the deionized aqueous solution with zinc nitrate precursor and thermally annealed. The green PL is suppressed due to the inhibition of spontaneous emission since the photonic band-gap of the opal matrix overlaps the DL recombination spectrum of ZnO. In contrast to the ZnO powder, the ZnO nanoparticles embedded into synthetic opal exhibit, first, the broadened dominant UV-blue emission and, second, the rapid decrease of the DL emission with decreasing temperature. These two factors support the assumption that the green PL suppression in ZnO is due to the influence of PhC. The infiltration of luminescent ZnO into PhC may be promising in the fabrication of efficient bright phosphors for the full-color displays [26].
Acknowledgements This work is supported by the Korea Science and Engineering Foundation through the Quantum-functional Semiconductor Research Center,
and by the research program and fund of Dongguk University, 2004. References [1] Y. Nakanishi, A. Miyake, H. Kominami, T. Aoki, Y. Hatanaka, G. Shimaoka, Appl. Surf. Sci. 142 (1999) 233. [2] C.H. Lin, Bi-Shiou Chiou, C.H. Chang, J.D. Lin, Mater. Chem. Phys. 77 (2002) 647. [3] K. Vanheusden, C.H. Seager, W.L. Warren, D.R. Tallant, J.A. Voigt, Appl. Phys. Lett. 68 (1996) 403. [4] K. Vanheusden, W.L. Warren, C.H. Seager, D.R. Tallant, J.A. Voigt, B.E. Gnade, J. Appl. Phys. 79 (1996) 7983. [5] S.A. Studenikin, N. Golero, M. Cocivera, J. Appl. Phys. 84 (1998) 2287. [6] J. Narayan, K. Dovidenko, A.K. Sharma, S. Oktyabrsky, J. Appl. Phys. 84 (1998) 2597. [7] Y. Chen, D.M. Bangall, H. Koh, K. Park, K. Hiraga, Z. Zhu, T. Yao, J. Appl. Phys. 84 (1998) 3912. [8] C.R. Gorla, N.W. Emanetoglu, S. Liang, W.E. Mayo, Y. Lu, M. Wraback, H. Shen, J. Appl. Phys. 85 (1999) 2595. [9] C. Agashe, O. Kluth, J. Hupkes, U. Zastrow, B. Rech, M. Wuttig, J. Appl. Phys. 95 (2004) 1911. [10] S.A. Studenikin, N. Golero, M. Cocivera, J. Appl. Phys. 83 (1998) 2104. [11] J. Hao, Z. Lou, M. Cocivera, Appl. Phys. Lett. 82 (2003) 1404. [12] J. Hao, J. Gao, M. Cocivera, Appl. Phys. Lett. 82 (2003) 2224. [13] J. Hao, J. Gao, M. Cocivera, Appl. Phys. Lett. 82 (2003) 2778. [14] E. Yablonovitch, Phys. Rev. Lett. 58 (1987) 2059. [15] S. John, Phys. Rev. Lett. 58 (1987) 2486. [16] J.G. Fleming, S.Y. Lin, I. El-Kady, R. Biswas, K.M. Ho, Nature 417 (2002) 52. [17] A. Blanco, C. Lopez, R. Mayoral, H. Miguez, F. Meseguer, A. Mifsud, J. Herrero, Appl. Phys. Lett. 73 (1998). [18] S.G. Romanov, T. Maka, C.M. Sotomayor Torres, M. Muller and, R. Zentel, Appl. Phys. Lett. 79 (2001) 731. [19] A. Blanco, H. Miguez, F. Meseguer, C. Lopez, F. Lopez-Tejeira, J. Sanchez-Dehesa, Appl. Phys. Lett. 78 (2001). [20] A.N. Gruzintsev, V.T. Volkov, G.A. Emelchenko, I.A. Karpov, V.M. Masalov, G.M. Mikhailov, E.E. Yakimov, Semiconductors 37 (2003) 314. [21] A.N. Gruzintsev, V.T. Volkov, G.A. Emelchenko, I.A. Karpov, W.M. Maslov, G.M. Michailov, E.E. Yakimov, Thin Solid Films 459 (2004) 111. [22] S.M. Abrarov, S.U. Yuldashev, V. Yu. Sokolov, T.W. Kang, in: The 12th Seoul International Symposium on the Physics of Semiconductors and Applications, Abstract, 14 March, 2004, pp. A3–P11. [23] S.M. Abrarov, S.U. Yuldashev, S.B. Lee, T.W. Kang, J. Lumin. 109 (2004) 25.
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Effect of photonic band-gap on photoluminescence of ZnO deposited inside the green synthetic opal S.M. Abrarov a
a,*
, Sh.U. Yuldashev a, T.W. Kim b, S.B. Lee c, Y.H. Kwon a, T.W. Kang a
Quantum-functional Semiconductor Research Center, Dongguk University, 3-26 Pil-dong, Chung-ku, Seoul 100-715, South Korea b Advanced Semiconductor Research Center, Division of Electrical and Computer Engineering, Hanyang University, Seoul 133-791, South Korea c Nanotechnology Institute, University of Texas at Dallas, Richardson, TX 75083, USA Received 26 August 2004; received in revised form 12 December 2004; accepted 7 February 2005
Abstract The temperature dependent photoluminescence spectra of ZnO embedded in the voids between the fcc packed SiO2 sub-micron spheres by using a spray pyrolysis were studied. In contrast to ZnO powder, the ZnO inside the green synthetic opal exhibit a broadened dominant excitonic band edge emission and rapidly decreasing deep level emission with decreasing temperature. The overlap between photonic and electronic band-gaps of the opal matrix and ZnO prevents the radiative recombinations through native defects resulting to the suppression of the photoluminescence intensity in the green spectrum. The growth of ZnO inside the green synthetic opal may be useful technique to enhance the UV-blue emission. Ó 2005 Elsevier B.V. All rights reserved. PACS: 78.55.m; 78.55.Et; 42.70.Qs; 82.30.Lp Keywords: ZnO; Photoluminescence; Photonic crystal; Synthetic opal; Spray pyrolysis
1. Introduction
* Corresponding author. Tel.: +82 22260 3205; fax: +82 222603945/3205. E-mail addresses: [email protected], [email protected] (S.M. Abrarov).
Zinc oxide, a promising wide band gap semiconductor, recently has received a considerable attention due to its remarkable optical and electrical properties. ZnO is a very versatile material, which is widely applied in industry for many
0030-4018/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2005.02.016
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purposes such as antireflection coatings, transparent electrodes in solar cells, sensors and surface acoustic wave devises. In addition, ZnO has attracted a strong interest due to its unique luminescent properties, which may be useful for potential applications in UV light emitting devices and flat panel displays as a low voltage phosphor [1–3]. Typically the photoluminescence (PL) spectrum of ZnO consists of excitonic near band edge and deep level (DL) emissions in the UV-blue and green spectral regions, respectively. The origins for DL emission are related to native defects like oxygen vacancies and Zn interstitials [3–5]. Various techniques, such as pulsed laser deposition, magnetron sputtering, chemical vapor deposition (CVD) and molecular beam epitaxy (MBE) are used for preparation of ZnO bulk crystals, films and powders [6–9]. These techniques, however, need the highly expensive equipment and sophisticated growth technology. Among all the available techniques, a spray pyrolysis has many advantages over others. In particular, it is a technologically simple, inexpensive, and efficient for the chemical deposition of ZnO [5,10]. It has been reported recently that spray pyrolysis can be successfully used for fabrication of the high quality bright phosphors like blue-emitting thin films of cerium-doped barium chloride hydrate [11], green-, blue-, and yellow-emitting Ba2B5O9Cl thin-films doped with Tb3+, Tm3+, and Mn2+ [12], and blue-emitting alkaline earth chloroborate thin films doped with Eu [13]. Another significant advantage of the spray pyrolysis is the possibility to grow luminescent nanoparticles in the precursor solution, which enables us to fill uniformly the voids inside the volume of the porous material. Photonic crystals (PhCs) proposed by Yablonovich [14] and John [15], can be used to modify the PL spectrum of the luminescent semiconductors. Besides many properties of PhCs, their ability to suppress the emission band [16] may also be useful in fabrications of the light emitting devices. For example, a strong modification of the PL spectra in CdS nanocrystals embedded into synthetic opal due to the inhibition of spontaneous emission has been reported [17]. This remarkable property of the PhCs may be applied to control the bandwidth
and increase an efficiency of the bright phosphors for full-color displays. In the recent studies dedicated to various phosphor materials, much interest attracted the growth of the luminescent semiconductors inside the matrix opal [17] and inverted matrix opal [18,19]. The successful attempt to combine ZnO with PhC has been recently reported by Gruzintsev et al. [20,21]. In these reports, the PL spectra of the ZnO thin (90 nm) film deposited on the surface of opal matrix by electron-beam sputtering have been investigated. The thin ZnO film in such a structure represents a 2D periodic array of quantum dots distributed between 279 nm SiO2 spheres. As a result of the quantum confinement, the ZnO quantum dots in this configuration exhibit a narrow excitonic emission. However, the similar approach can also be extended for 3D periodic structure by growth of ZnO inside PhC in zinc nitrate or zinc acetate precursor solution. In particular, the complete filling of the synthetic opal volume with ZnO by using a precursor solution as a useful technique to modify the PL spectra of zinc oxide has been first time reported [22,23]. The green porous opal was chosen as a matrix for infiltration since its photonic band-gap overlaps the electronic band-gap of ZnO. The overlap between photonic and electronic band-gaps enables us to suppress undesirable spontaneous emission in the spectral region associated with native defects such as oxygen vacancies and zinc ion interstitials. Preliminary PL studies [22–26] of ZnO embedded into green synthetic opal by using the spray pyrolysis have been reported. However the temperature dependent PL spectra have not been investigated yet and the assumption that the suppression in the green PL band is due to the influence of PhC must be verified more strictly. This paper reports the temperature dependent PL spectra of the ZnO embedded into synthetic opal by using the spray pyrolysis method. PL spectroscopy shows two factors supporting an assumption that the suppression of the DL emission is due to the influence of PhC. First, in contrast to the ZnO powder, the ZnO nanocrystals inside the green synthetic opal exhibit a broadened dominant UV-blue emission. Second, their DL emission in
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the green spectrum rapidly decreases with decreasing temperature. Despite the polycrystalline structure, ZnO inside the green synthetic opal exhibit the enhanced UV-blue PL emission.
2. Experimental 2.1. Preparation of synthetic opal The sub-micron size spheres of silicon dioxide (SiO2) were synthesized in Stober–Fink–Bohn process [27] via the hydrolysis of tetraethyl orthosilicate (TEOS) in ethanol solution containing ammonium hydroxide and water. The diameter of the SiO2 spheres depends upon many factors. However, the main factors used to control the size of the particles are the temperature of the TEOS hydrolysis and the concentrations of ammonium hydroxide and water. Particularly, an increase of the ammonium hydroxide results to the increase of the diameter of the spherical particles. On the contrary, the increase of the water concentration causes a decrease of the diameter of the spherical particles. Typically, employing the hydrolysis of TEOS in ethanol solution with ammonium hydroxide and water, the spherical particles with diameters between 0.03 and 3.0 lm can be synthesized. Synthetic opal was assembled to fcc packed matrix by natural sedimentation of SiO2 spheres in water suspension. After sedimentation, the water was slowly evaporated at the temperature 80 °C. The self-assembled fcc packed matrix is mechanically fragile, therefore it was thermally hardened for several hours at temperature 1000 °C. The technique described above enables the fabrication the SiO2 spheres with small deviation (less than 10%) from their average size. The more complete description of the opal matrix fabrication can be found elsewhere [28,29]. 2.2. Infiltration of synthetic opal ZnO nanocrystals was deposited from 0.1 M deionized water solution containing a zinc nitrate hexahydrate, Zn(NO3)2 Æ 6H2O, precursor of
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99.999% purity. The chemical synthesis of ZnO is described by the following schematics: 0 1 ZnðOHÞ2 B C ZnðNO3 Þ2 6H2 O ! @ Zn2 OðNO3 Þ2 A ð1Þ ZnðOHÞðNO3 Þ ! ZnO þ yNw Ox þ zO2 Particularly, in the solution, zinc nitrate is decomposed to zinc hydrogenous and nitrogenous compounds such as zinc dihydroxide, dizincoxide nitrate, and zinc hydroxide nitrate, respectively, as shown in the parenthesis of the schematics. The proportions between zinc hydrogenous and nitrogenous compound concentrations depend on temperature of the precursor solution. Under the thermal annealing, zinc compounds are further decomposed to ZnO, nitrogen oxides, and oxygen molecules. The detailed chemical synthesis of zinc oxide and film or powder deposition technique procedures are described elsewhere [5,10]. To prepare a sample, the 1 mm-thick opal slab was kept in 0.1 M zinc nitrate aqueous solution for 48 h. To remove air from the voids and to improve the fluid flow, the opal slab was soaked under ultrasonic vibration for the first 3 h. The long ultrasonic vibration is not desirable since it may destroy the ZnO nanoparticles and damage the sample. After soaking in precursor solution, the opal slab was dried in the drying oven at 90 °C for 30 min and annealed at 500 °C for 1 h. Since the synthetic opal is a porous material, the solution was effectively observed resulting in a uniform distribution of ZnO nanocrystals inside the synthetic opal. For the sufficient infiltration of ZnO inside the volume of the opal slab, these procedures were repeated several times. The filling factor can be controlled by concentration of the precursor and by number of infiltration cycles. However the high concentration of the precursor may deteriorate the uniformity of ZnO inside the matrix. To improve the penetration of the precursor solution inside the sample, the ZnO layers were removed from the opal surfaces by polishing and cleaning after each infiltration cycle. Finally, for the proper PL measurements, ZnO layers were
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Fig. 1. Normalized transmittance of the bare and ZnO embedded opals.
also removed since their contribution may be strong enough to screen the effect of the photonic band gap on the PL spectra. The filling fraction of ZnO inside the opal matrix was estimated using both the gravimetric method and red shift Dk (Fig. 1) in the transmittance spectra. As a reference sample, a ZnO powder was also deposited on a flat surface of the crystalline quartz substrate (a-quartz) under similar conditions. 2.3. Measurements Temperature dependent spectra were obtained using a SPEX spectrometer equipped with 0.75 m grating monochromator. PL measurements were performed under 10 mW He–Cd laser excitation at the wavelength of 325 nm. A broadband-sensitive photomultiplier tube Hamamatsu R943-02 was used as a detector. The transmittance characteristics were obtained applying a xenon lamp as a white light source exposed to the grating monochromator. To enhance a transmittance, the thickness of the opal slab was reduced to 300 lm. The general morphologies of the samples were
analyzed using a scanning electron microscope (SEM) XL-30 PHILIPS. The temperature was controlled between 10 and 300 K using the CRYODINE He displex system.
3. Results and discussion Fig. 1 shows the normalized transmittance of the unfilled opal (bare opal) and ZnO embedded opal. The appearance of the red shift in transmittance is due to the decrease of the refractive index contrast since the index of refraction of ZnO is relatively close to that of SiO2. The filling factor of the ZnO was found by using two methods. In the first method, the relation between the optical wavelength k with the filling factors FSiO2, FZnO and Fair for SiO2, ZnO and air was used [17]: pffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffi k ¼ 2 0:816D eSiO2 F SiO2 þ eZnO F ZnO pffiffiffiffiffiffi þ eair ðF air F ZnO ÞÞ; where D is the average diameter of spheres, eSiO2 , eZnO, and eair are the dielectric constants of SiO2,
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ZnO, and air, respectively. Considering the fact that the filling factors for the SiO2 and air of the bare opal are 74% and 26%, respectively, and using above equation, the filling factor for ZnO was estimated 7%. In the second method, the filling factor was determined gravimetrically, i.e., by weighting sample before and after ZnO infiltration. The filling factor magnitudes for ZnO estimated by using the red shift and determined gravimetrically by increase of the sample weight are in good agreement. It should be mentioned that the filling factor of ZnO could be very high when inverted opal is used. For instance, the 96% filling factor of CdS inside the inverted opal was reported [19]. The ability to suppress the DL emission and the feasibility to achieve the filling factor above 90% in the inverted opal indicate the significance of ZnO infiltrated into PhC for potential applications in optoelectronics. Fig. 2(a) shows SEM image of a bare synthetic opal consisting of the fcc packed SiO2 submicron spheres. The diameters of the SiO2 spheres vary between 220 and 240 nm. The sizes of the ZnO nanoparticles synthesized under ultrasonic vibration from the nitrate precursor solution may be less than 20 nm [30]. The ZnO nanoparticles obtained by means of spray pyrolysis are sufficiently small compared to the void sizes between the SiO2 spheres. Therefore ZnO can be grown inside the synthetic opal uniformly since the fluid flow in the precursor solution is able to penetrate deep inside the volume of the opal matrix. Due to the growth of ZnO on the spherical surfaces, the spherical shapes are transformed to the honeycomb-type cell in the (1 1 1) plane and square-type cell in the (1 0 0) plane. Figs. 2(b) and (c) show the SEM images of ZnO embedded opal normal to (1 0 0) and (1 1 1) planes, respectively. Fig. 3 shows the temperature dependent PL spectra of the pure ZnO powder. A free exciton peak (XA) is located near 3.28 eV at 300 K. The UV-blue emission increases at temperatures below 240 K. The free exciton peak is highest in the UV-blue emission spectra at temperatures between 30 and 240 K. The blue shift due to the quantum confinement in the PL spectra is not
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Fig. 2. SEM images of the: (a) bare opal, (b) ZnO embedded opal in the (1 1 1) plane and (c) ZnO embedded opal in the (1 0 0) plane.
observed since ZnO nanocrystals are not separated, i.e., they are in physical contact with each other. The strongest DL emission peak near 2.38 (510 nm) is related to native defects such as oxygen vacancies and Zn interstitials commonly observed in ZnO bulk crystals, films, and powders [3–5]. The DL peaks in the green spectral
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Fig. 3. Temperature dependent photoluminescence spectra of the pure ZnO powder.
region remain very strong even at low temperatures due to the high crystal defects density in the polycrystalline ZnO powder. Fig. 4 shows the low-temperature PL spectra of the pure ZnO powder in the UV-blue region. Below 30 K the contribution from the free exciton located at 3.374 eV decreases and the emission from the donor bound exciton located at 3.360 eV increases with decreasing temperature.
Fig. 4. Low temperature photoluminescence spectra of the pure ZnO powder in the UV-blue region.
Such a behavior is commonly observed in the PL spectra of ZnO and results from the decomposition of the bound exciton to the free exciton due to increased thermal energy at temperature above 25–30 K [31]. Fig. 5 shows the temperature dependent PL spectra of the ZnO embedded opal. At 300 K the highest peak in the UV-blue region is observed near 3.26 eV, while the free exciton peak (XA) is observed at 3.28 eV. In contrast to ZnO powder, the ZnO nanocrystals inside the synthetic opal exhibit a strong UV-blue emission dominating even above room temperature. Presumably, the PL spectra is broadened and enhanced in the UV-blue region due to Purcell enhancement [32,33]. In particular, the modification of the PL characteristics can be described from the fact that the suppression of the PL band leads to the spectral redistribution (recycling) of the emission energy [16]. Since the radiative recombination rate in the green spectral band is decreased, the spontaneous emission mostly releases through allowed transitions leading to the broadened and enhanced PL spectra in the UV-blue region. The similar observation, the enhancement of the PL intensity in the UV band and its decrease in the green band, for ZnO nanocrystals inside the synthetic opal has been reported by Masalov et al. [24].
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Fig. 5. Temperature dependent photoluminescence spectra of the ZnO embedded opal.
In the spectral region related to the crystal defects, the strongest DL emission peak around 2.38 eV (510 nm) is observed. However this peak rapidly decreases with decreasing temperature and almost disappears below 200 K. Fig. 6 shows the PL spectra of pure ZnO powder and ZnO embedded opal at 10 K. For the ZnO powder, the free exciton and donor bound exciton are located at 3.374 and 3.360 eV, respectively. The emission peaks at 3.315 and 3.235 eV are attributed to donor–acceptor pairs [34,35]. The FWHM values of these peaks are around 3–10 meV. Four distinct peaks at 3.367, 3.327, 3.295, and 3.210 eV are observed in the low temperature spectra. Presumably, the peaks at 3.367 and 3.327 eV are attributed to free exciton and donor bound exciton, respectively, while the peaks located at 3.295 and 3.210 eV are attributed to donor–acceptor pairs. The FWHM values for these peaks are approximately twice the values for the lines in ZnO powder spectrum. The effect of photonic band-gap on PL emission can be seen from comparison between the spectra of the reference sample and ZnO embedded opal, shown in the Figs. 3 and 5, respectively. Due to
Fig. 6. Low temperature spectra of the: (a) ZnO powder and (b) ZnO embedded opal.
the high density of crystal defects, the UV-blue emission of ZnO powder increases insignificantly, while the DL emission strongly dominates even
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at very low temperatures (Fig. 3). On the contrary, the ZnO embadded into opal exhibit the broadened dominant UV-blue emission and rapidly decreasing DL emission with decreasing temperature (Fig. 5). Furthermore, the band edge exciton emission sustains the highest peak above room temperature. The photonic band-gap overlaps the green spectral region, associated with radiative recombinations through native defects like oxygen vacancies and Zn interstitials. The overlap between photonic and electronic band-gaps prevents the radiative recombinations through native defects ultimately leading to suppression of the PL intensity in the green spectral region.
4. Summary The temperature dependent PL spectra of ZnO embedded into synthetic opal were studied. To fill the voids between the SiO2 spheres with ZnO nanocrystals, the technologically simple and inexpensive spray pyrolysis was used. For the formation of ZnO, the opal matrix was soaked in the deionized aqueous solution with zinc nitrate precursor and thermally annealed. The green PL is suppressed due to the inhibition of spontaneous emission since the photonic band-gap of the opal matrix overlaps the DL recombination spectrum of ZnO. In contrast to the ZnO powder, the ZnO nanoparticles embedded into synthetic opal exhibit, first, the broadened dominant UV-blue emission and, second, the rapid decrease of the DL emission with decreasing temperature. These two factors support the assumption that the green PL suppression in ZnO is due to the influence of PhC. The infiltration of luminescent ZnO into PhC may be promising in the fabrication of efficient bright phosphors for the full-color displays [26].
Acknowledgements This work is supported by the Korea Science and Engineering Foundation through the Quantum-functional Semiconductor Research Center,
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