Journal of the Korean Physical Society, Vol. 53, No. 5, November 2008, pp. 29432946
Luminescent Properties of ZnO/MgO Nanocrystal/Polymer Composite Structure Panin
Gennady N.
Department of Physics, Quantum-Functional Semiconductor Research Center, Dongguk University, Seoul 100-715 & Institute of Microelectronics Technology, Russian Academy of Sciences, Chernogolovka, Moscow Distr. 142432 Russia
Baranov
Andrey N.
Inorganic Chemistry Department, Moscow State University, Moscow 119899 Russia Irina A.
Khotina
A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Moscow 119991 Russia Tae W.
Kang
Department of Physics, Quantum-Functional Semiconductor Research Center, Dongguk University, Seoul 100-715 (Received 10 September 2007) We report on cathodoluminescence (CL) studies of the composite nanostructure obtained by spin coating of a mixture of chemically-deposited ZnO/MgO nanocrystals and a solution processable polymer (PDPV) on a Si substrate with patterned gold electrodes. The e ect of an electric eld on the CL of the nanostructure was studied at various bias voltages on the electrodes. A positive voltage suppressed the blue-green emission and shifted the emission maximum to the red region. The emission maximum returned to the blue-green region after the eld had been turned o . A model of the electric- eld-induced color switching in the ZnO/polymer-based nanostructure is proposed. The bias-voltage-induced interface band bending and the deactivation of the radiative centers, in particular, singly-ionized oxygen vacancies in ZnO nanocrystals, are suggested to govern the relative changes of the blue-green-red emissions.
PACS numbers: 78.66.Jg, 78.66.Fd, 78.66.Qn, 78.66.Hf, 78.66.Sq Keywords: Nanocrystals, Luminescence, Polymer composites, Multicolor structures I. INTRODUCTION
The combination of a conjugated polymer and inorganic semiconductor nanocrystals is attractive in the fabrication of light-emitting devices. Such nanocomposites combine the ecient and robust luminescence from inorganic crystals with the good mechanical properties of polymer lms [1]. ZnO (Eg = 3:37 eV, Eb = 60 meV) is a promising wide-band-gap semiconductor for fabrication of excitonic laser and light emitting diodes in the UV-visible region [2]. The luminescence eciency of nanocrystals is generally higher than that of lms due to the reduced structural defect density in nano-sized materials. Moreover, the emission of nanocrystals can be controlled by their size and impurity doping. Mg-, W-, Y-, Eu- and V-doped ZnO particles have been well used as multicolor phosphors for eld-emission applica
E-mail:
[email protected]; Fax: +82-2-2260-3950
tions [3]. In this paper, we report on a study of the cathodoluminescence from a ZnO/MgO/polymer structure prepared by using solution techniques. The e ect of an electric eld on the luminescence [4] and the mechanism of the color switching are described.
II. EXPERIMENTS
The nanocrystals were prepared by dissolving a mixture of analytic-grade ZnCO3 and (MgCO3 )4Mg (OH)2 5H2 O (Aldrich) with a molar Zn/Mg ratio of 1/2 in nitric acid and then forming a precipitate by adding a 2M (NH4 )2 CO3 solution [5]. The precipitate was decomposed in a vacuum furnace at 320 C and was nally oxidized at 500 C in air. This procedure led to the formation of ZnO/MgO nanocrystals of 20 50 nm in size (Figure 1). Poly(4,4- diphenylene diphenylvinylene) (PDPV) [6] was used as polymer matrix. The nanocrys-
-2943-
-2944-
Journal of the Korean Physical Society, Vol. 53, No. 5, November 2008
Fig. 1. HRSEM image of ZnO/MgO nanocrystals.
tals were dispersed in chloroform and were mixed with a polymer dissolved in chloroform by treatment in an ultrasonic bath for 30 min. The obtained composite was deposited by spin coating on a Si substrate with gold electrodes separated by 1 mm. The same procedure was used to get separate polymers and nanocrystal lms. The CL spectra of 3 the samples were obtained by using a XL 30S FEG high-resolution scanning electron microscope (HRSEM) with a MonoCL system for CL spectroscopy.
III. RESULTS AND DISCUSSION
Figure 2 shows the CL spectra of the ZnO/MgO nanocrystals and the PDPV lm. As can be seen from these spectra, PDPV has well-pronounced CL with maximum emission in the blue-green region, which is consistent with the results obtained for this polymer earlier [6, 7]. The CL spectrum of the nanocrystals reveals a more complicated structure with several peaks: 3.54, 2.83, 2.02 and 1.72 eV. The intensity of the CL at 1.72 eV is higher with respect to other peaks, which indicates that the red emission from the nanocrystals related to impurities or intrinsic defects [9,10] is dominant. Figure 3 shows the CL spectra of the ZnO/MgO nanocrystal/PDPV composite, obtained at di erent bias voltages on gold electrodes. At a zero voltage, the CL spectrum combines luminescence from all the materials involved, which caused the broadening of the PDPV spectrum with pronounced peaks at 3.50, 2.86 and 1.79 eV. The main maximum of this asymmetric spectrum in the blue-green region (2.86 eV) results from a pronounced blue-green emission from both the PDPV and the ZnO/MgO nanocrystals. Application of a positive bias (+5 V) to the electrodes suppresses the blue-green emission and shifts the emission maximum to the red region. The e ect is found to be reversible with respect to the application of the electric eld; namely, when the eld is turned o , the emission maximum returns to the blue-green region. The mechanism of the color switching of the lumines-
Fig. 2. CL spectra of (a) a PDPV lm and (b) ZnO:Mg nanocrystals.
cence from the ZnO/MgO nanocrystal/polymer structure implies the presence of channels of radiative recombination, which can be controlled by using an electric eld. Figure 5 shows the energy-band diagrams for small (SP) and large (LP) particles in cross-sections. Application of an electric eld causes depletion e ects, which are known to a ect the green luminescence of ZnO crystals [12,13]. A high density of electrically-active surface states enhances the e ect of band bending at a semiconductor surface, which creates an electron depletion region of width W at the particles surfaces. In the part of this region where the Fermi level EF passes below the VO 0 /VO 00 energy level, all oxygen vacancies are in the nonradiative VO 00 state. The CL maximum at 2.86 eV indicates that both radiative recombination mechanisms including oxygen-vacancy-related centers in ZnO and direct the lowest unoccupied molecular orbital (LUMO) to the highest occupied molecular orbital (HOMO) transfers in the polymer coexist at zero electric eld. It should
Luminescent Properties of ZnO/MgO Nanocrystal/Polymer { Gennady N. Panin et
Fig. 3. CL spectra of a ZnO:Mg nanocrystal-PDPV composite lm with electrodes biased on (+5 V) or without (0 V) application of an electric eld.
al.
-2945-
Fig. 5. Energy-band diagrams for the small (SP) and the large (LP) particles.
polymer-nanocrystal interface. Furthermore, the interface band-bending creates an electron depletion region (W ) at the ZnO crystal surfaces and an conversion of the radiative VO 0 centers to the nonradiative VO 00 state. As a result, the red emission becomes dominant. This mechanism is reversible, and when the bias becomes zero, the green radiative recombination channel from the VO 0 related states, as well as the PDPV emissions should become dominant again, in agreement with our experimental data. Note that the band bending in our experiment does not a ect the red emission, which is attributed to deeper levels (Figure 5). The depletion region width (W ) on the surfaces of the ZnO crystals depends on the surface state density and the charge carrier density in nanocrystals, as it can be seen from Eq. (1): Fig. 4. Electron energy-level diagram for the red and the blue-green radiation centers in ZnO.
be noted that band bending might be reduced by both polymer ZnO surface passivation and strong electron beam illumination. This e ect results in an attraction of minority carriers to the surface, where they become trapped, and converts some of the VO 00 centers to the VO 0 state. As for the red emission related to the recombination via complex zinc-vacancy-involved centers, the radiative recombination mechanism also works and can be recognized as the shoulder on the main spectra at 650 nm. Complete band-bending suppression at both interfaces in the case of the Au-PDPV-ZnO/MgO-Au structure can be obtained by applying a bias voltage to the electrodes. PDPV is a p-type semiconductor similar to many other conjugated polymers, which means that application of a positive bias to the Au-PDPV interface increases the barrier, and the green emission from PDPV is reduced because of charge separation at the
W
= (2"ZnO Vbi =eNd )1=2 ;
(1)
where "ZnO is the dielectric constant of ZnO, Vbi the potential barrier, e the electron charge, and Nd the donor density. It should be noted that the electric- eld-induced switching of the luminescence can be adjusted by using the size of the crystals and the donor density. The cathodoluminescence spectrum of the PDPV-ZnO/MgO nanostructure contains peaks in the blue, green, and red regions, and the ability to tune the color intensity by using an electric eld makes the ZnO-based nanostructures promising for the fabrication of exible highly-ecient white emitters. Before starting the analysis of the switching mechanism in the ZnO/MgO-polymer composite structure, we have to note that a similar e ect was found in PPV lms doped with a ruthenium dinuclear complex [10]. The band gap of the ruthenium complex is about 0.5 eV smaller than that of the polymer. Therefore, the mechanism for the formation of the excited state in the polymer involves the ruthenium complex in a stepwise electron transfer process. In the case of the ZnO/MgO-PDPV 4
-2946-
Journal of the Korean Physical Society, Vol. 53, No. 5, November 2008
composite, the band gap of ZnO/MgO is much higher than that of the polymer, and the latter mechanism of the electric- eld switching between blue-green and red emissions does not work as well as the mechanism proposed for the emission from PPV.carbon nanotube composite [12]. There are several models to explain the bluegreen and the red emissions in ZnO. The most acceptable models assume that the defect centers responsible for the green luminescence are the singly-ionized oxygen vacancy center [12,13] or complex donor. acceptor centers [14{16]. The acceptor level (Zn vacancy) is located 2.5 eV below the conduction band edge [14,17] while the donor level (oxygen vacancy) is known as a shallow level at 0.05 0.19 eV, leading to an emission band centered around 508 540 nm. The blue-green emission in ZnO might also be associated with a transition within a selfactivated center formed by a double-ionized zinc vacancy (VZn ) 2 and a single-ionized oxygen vacancy [18] or by a substitution of Zn by extrinsic impurities such as Cu or Mg in the crystal lattice [3,19]. The blue-green emission from our ZnO nanocrystals could be described by recombination of VO 0 electrons with excited holes in the valence band (Figure 4) while the red emission observed in the nanocrystals at 670 720 nm might be attributed to complex defect-related centers such as deep donor and deep acceptor centers related with VZn, whose density can be increased by Mg doping [21{23].
IV. CONCLUSION
The cathodoluminescence of the ZnO/MgO polymer nanocomposite structure was investigated at various bias conditions. A positive voltage on an electrode suppressed the blue-green emission and shifted the emission maximum to the red region. The emission maximum returned to the blue-green region after the eld had been turned o . A mechanism for the control of radiative transitions in the ZnO/MgO polymer nanocomposite structure by using an electric eld was considered. Bias-voltageinduced interface band bending and deactivation of radiative centers, in particular, singly ionized oxygen vacancies in ZnO nanocrystals, are suggested to govern the relative changes in the blue-green-red emissions.
ACKNOWLEDGMENTS
The authors gratefully acknowledge support from the Korean Science and Engineering Foundation through the Quantum-functional Semiconductor Research Center at Dongguk University for G.N.P. and T.W.K.
REFERENCES
[1] R. H. Friend, R. W. Gymer, A. B. Holmes, J. H. Burroughes, R. N. Marks, C. Taliani, D. D. C. Bradley, D. A. Dos Santos, J. L. Bredas, M. Logdlund and W. R. Salaneck, Nature 397, 121 (1999). [2] T. W. Kang, Sh. Yuldashev and G. N. Panin, Handbook of Semiconductor Nanostructures and Nanodevices, edited by A. A. Balandin and K. L. Wang (American Scienti c Publishers, California, 2006). [3] V. Z. Mordkovich, H. Hayashi, M. Haemori, T. Fukumura and M. Kawasaki, Adv. Funct. Mater. 13, 519 (2003). [4] G. N. Panin, T. W. Kang, A. N. Aleshin, A. N. Baranov, Y.-J. Oh and I. A. Khotina, Appl. Phys. Lett. 86, 113114 (2005). [5] G. N. Panin, A. N. Baranov, Y.-J. Oh, T. W. Kang and T. W. Kim, J. Crystal Growth 279, 494 (2005). [6] W. J. Feast, I. S. Millichamp, R. H. Friend, M. E. Horton, D. Phillips, S. D. D. V. Rughooputh and G. Rumbles, Synth. Metals 10, 181 (1985). [7] F. Cacialli, R. H. Friend, N. Haylett, R. Daik, W. J. Feast, D. A. Dos Santos and J. L. Bredas, Synth. Metals 84, 643 (1997). [8] C. Falcony, A. Ortiz, M. Garcia and J. S. Helman, J. Appl. Phys. 63, 7 (1998). [9] S. A. Studenikin, N. Golego and M. Cocivera, J. Appl. Phys. 83, 2104 (1998). [10] S. Welter, K. Brunner, J. W. Hofstraat and L. De Cola, Nature 421, 54 (2003). [11] H. S. Woo, R. Czerw, S. Webster, D. L. Carroll, J. Ballato, A. E. Strevens, D. OBrien and W. J. Blau, Appl. Phys. Lett. 77, 1393 (2000). [12] K. Vanheusden, W. L. Warren, C. H. Seager, D. R. Tallant, J. A. Voigt and B. E. Gnade, J. Appl. Phys. 79, 7983 (1996). [13] K. Vanheusden, C. H. Seager, W. L. Warren, D. R. Tallant and J. A. Voigt, Appl. Phys. Lett. 68, 403 (1996). [14] H. J. Egelhaaf and D. Oelkrug, J. Cryst. Growth 161, 190 (1996). [15] S. A. Studenikin and M. Cocivera, J. Appl. Phys. 91, 5060(2002). [16] D. C. Reynolds, D. C. Look and B. Jogai, J. Appl. Phys. 89, 6189 (2001). [17] E. G. Bylander, J. Appl. Phys. 49, 1188 (1978). [18] S. A. Studenikin and M. Cocivera, J. Appl. Phys. 91, 5060 (2002). [19] R. Dingle, Phys. Rev. Lett. 23, 579 (1969). [20] S. A. Studenikin, N. Golego and M. Cocivera, J. Appl. Phys. 84, 2287 (1998). [21] A. Mitra, R. K. Thareja, V. Ganesan, A. Gupa, P. K. Sahoo and V. N. Kulkarni, Appl. Surf. Sci. 174, 232 (2001). [22] A. Ohtomo, R. Shiroki, I. Ohkubo, H. Koinuma and M. Kawasaki, Appl. Phys. Lett. 75, 4088 (1999). [23] N. C. Greenham, X. Peng and A. P. Alivisatos, Phys. Rev. B 54, 17628 (1996).
