- Email: [email protected]
GaN
Chemical Physics Letters 593 (2014) 28–30
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Electrochemical formation and optoelectronic property of hybrid organic/inorganic heterostructure of PPy/GaN Li-Feng Hu a,b, Feng-Xia Wang b, Feng-Xiang Deng a,b, Yu Zhao b, Ge-Bo Pan b,⇑ a b
Xi’an Jiaotong University, 710049 Xi’an, PR China Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, 215123 Suzhou, PR China
a r t i c l e
i n f o
Article history: Received 30 October 2013 In final form 28 December 2013 Available online 7 January 2014
a b s t r a c t A new hybrid organic/inorganic heterostructure of p-type polypyrrole (PPy) and n-type gallium nitride (GaN) was fabricated by means of electrodeposition and characterized. The Raman spectra indicated that the GaN substrate had an obvious enhancement of Raman scattering of the PPy, and the PL spectra revealed that the excitonic emission and recombination were partially quenched at the PPy/GaN interface. Moreover, the prototype devices were fabricated on the basis of the PPy/GaN heterostructures. The current–voltage characteristics of the devices in dark and under ultraviolet light illumination showed obvious photovoltaic response. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Hybrid heterostructure materials have attracted great attention in recent years, due to their unique optoelectronic properties arising from individual components and along with processes occurring at the interfaces between components [1]. To date, a variety of heterostructure materials have thus been fabricated with the controlled junctions of organic/organic, organic/inorganic, and inorganic/inorganic [2–4]. Among them, the junctions from organic/inorganic semiconductors are of particular interest and show superior performance of improved carrier mobility, stability, and lifetime relative to those of organic/organic junctions. They are widely used in hybrid devices such as solar cells [5], electroluminescent devices [6], and photodetectors [7]. On the other hand, conducting polymers can be grown on semiconductors to form heterostructures, whose properties can be easily adjusted by using different polymer and dopant. Polypyrrole (PPy) is one of the most important conducting polymers used in heterostructures. The optoelectronic properties of semiconductors can be changed by varying the dopant in PPy film to form a p–n structure [8]. However, it is noted that inorganic semiconductors in previous studies are mainly Si, II–VI and III–V GaAs, InP. [9–11]. To the best of our knowledge, no work has been devoted to the GaN-based organic/inorganic heterostructures. Herein, we report the fabrication of a new hybrid organic/inorganic heterostructure, in which p-type PPy was electrodeposited onto n-type gallium nitride (GaN) substrate. The synergic effect between PPy and GaN in hybrid heterostructures was explored. The GaN-based materials are used due to their wide direct energy ⇑ Corresponding author. Fax: +86 0512 62872663. E-mail address: [email protected] (G.-B. Pan). 0009-2614/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2013.12.074
gap (Eg = 3.4 eV), high heat capacity, excellent chemical stability, and unique optoelectronic properties. The as-prepared PPy/GaN heterostructure was further used to fabricate the prototype photodetectors of ultraviolet light. Moreover, the p–n junction photovoltaic theory was used to explain the synergic effect in UV measurement. 2. Experiment Pyrrole and NaClO4 were purchased from Sinopharm Chemical Reagent Co., Ltd. They were dissolved into double distilled water with the concentrations of 0.05 and 0.1 M, respectively. Singlecrystal n-GaN(0 0 0 1) substrates were 7 mm 5 mm, Si doped n-type, and grown by hydride vapor phase epitaxy on sapphire. The n-GaN substrate was cleaned sequentially with acetone, ethanol, and deionized water. The experiments were carried out in a conventional three-electrode cell using a CHI660D potentiostat/galvanostat (Shanghai ChenHua Co., Ltd.) at room temperature. GaN was used as the working electrode, whereas Pt wires were used as the counter and reference electrodes. An electrolyte containing 0.05 M pyrrole and 0.1 M NaClO4 was used for the deposition of PPy. Pyrrole and sodium perchlorate were utilized as monomer and dopant, respectively. PPy film was electrodeposited with a chronopotential mode. The current density was 0.1 mA/cm2 and the deposition time was 1.5 h. The final samples were washed with water, dried with N2 gas, and characterized by scanning electron microscope (SEM) energy dispersive X-ray spectroscope (EDX), photoluminescence spectroscope (PL, excited at 325 nm), and Raman spectrometer (excited at 633 nm). The current–voltage (I–V) characteristics were measured with an Agilent B1500A semiconductor parameter analyzer.
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L.-F. Hu et al. / Chemical Physics Letters 593 (2014) 28–30
(a)
(b)
(c)
Figure 1. (a) Top-view SEM image of PPy film. (b) Cross-section SEM image of the PPy/GaN interface. (c) EDX spectrum of PPy.
Figure 2. (a) Raman spectra of PPy/GaN interface (blue line), PPy (red line), and GaN (black line). (b) Photoluminescence spectra of GaN (red line) and PPy/GaN interface (black line). The inset was the PL spectrum of PPy. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3. Result and discussion Figure 1a illustrates the top-view SEM image of the as-grown PPy on GaN substrate. As seen from the image, the PPy film is well-covered through the whole substrate. Figure 1b shows the cross-sectional SEM image of the interface, confirming the formation of the GaN/PPy heterostructure. The thickness of PPy film is several micron meters. The formation of high quality junction of PPy/GaN might be attributed to a number of grain boundaries of GaN, which can directly attach PPy molecules [12]. Figure 1c shows the EDX mapping of PPy. The carbon and nitrogen elements are attributed to the PPy polymer. The oxygen and chlorine elements are from the electrolyte, which proves the p-type doped pyrrole [13]. In addition, the gallium and partial nitrogen elements come from the GaN substrate. Figure 2a shows the Raman spectra of the PPy and PPy/GaN interface. It is clear that the Raman spectrum of PPy/GaN is similar to the PPy. The four characteristic bands of PPy are observed at 1588, 1240, 1060, and 935 cm1 and in good agreement with the literatures [13,14]. Moreover, the GaN substrate has an obvious enhancement of Raman scattering of the PPy. Similar phenomenon has been observed for the III–V semiconductor quantum dots and deposited silver nanoparticles [15]. Figure 2b shows the PL spectra of the GaN and the PPy/GaN interface. It is seen that the bare GaN is emitted in the ultraviolet region at 365 nm. At the same excitation wavelength, no obvious emission is observed for the PPy (the inset in Figure 2b). Compared with the bare GaN, there is a distinct decrease for the PPy/GaN interface, implying that both the emission and recombination are partially quenched. The above results may be ascribed to the charge transfer and the energy transfer between the GaN and PPy [16]. Figure 3 shows typical I–V characteristics of the devices based on the GaN, PPy, and PPy/GaN in dark and under illumination. No rectification has been observed for both GaN and PPy (Figure 3a and b), indicating that Ohmic contact is formed for In/GaN/In and
Ag/PPy/Ag and allows for efficient carrier collection at this electrode. However, the device based on the PPy/GaN heterostructure exhibits the excellent diode effect (Figure 3c), and the larger switching Ion/Ioff ratio is obtained compared with that of bare GaN. The excellent performance is reasonably attributed to the interface between the high hole mobility p-type PPy and n-type GaN. The J–V behavior is rectifying in nature implying that the PPy/GaN junction act as a rectifier, i.e., forming a diode. The response of a single diode can be modeled using the ideal diode equation [17]:
J ¼ J sat eV=nV t 1
ð1Þ
where Jsat, n, and Vt are the reverse saturation current, ideality factor, and thermal voltage (26 mV), respectively. The reverse saturation density is to measure how many carriers can overcome the energetic barrier created by the p–n junction in the reverse bias direction. Meanwhile, the ideality factor is to depict the recombination behavior and is an indicator of the density and quality of interfaces in hybrid devices. In the case of PPy/GaN device, the reverse saturation current density is 2.5 107 A/cm2 and the calculated ideality factor is 3.82, implying that a recombination current predominates in the junction and a good diode property of PPy/GaN heterostructure. This is because the ideality factor values reported on organic/inorganic heterostructure interfaces were 2 and 6 [18]. More interestingly, the current under illumination is remarkably higher than that obtained in dark for GaN (Figure 3a) and PPy/GaN (Figure 3c). Figure 3d shows the energy level diagram of the PPy/GaN heterostructure, respectively. It is clear that the light can be harvested by the GaN. The loosely bound electron–hole pair reaches the hybrid interface from the GaN side. The energy level diagram would favor charge-separation at the PPy/GaN interfaces with electrons and holes moving towards GaN and PPy layers, respectively. That is a typical photovoltaic response [19]. Therefore, the PPy/GaN heterostructure shows high photogenerated current and an excellent rectifying property under UV illumination.
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L.-F. Hu et al. / Chemical Physics Letters 593 (2014) 28–30
Figure 3. I–V characteristics of prototype devices in dark and under illumination: (a) GaN, (b) PPy, and (c) PPy/GaN heterostructure. The insets were the device configurations. (d) Energy level diagram of the PPy/GaN heterostructure.
Such characteristics are suitable for the fabrication of optoelectronic devices, in particular for UV detectors. 4. Conclusion A new hybrid organic/inorganic heterostructure of p-type PPy and n-type GaN has been fabricated by electrodeposition and characterized. The Raman spectra indicated that the GaN substrate had an obvious enhancement of Raman scattering of the PPy. The PL spectra revealed that the excitonic emission and recombination was partially quenched at the PPy/GaN interface. Moreover, the prototype devices were fabricated on the basis of the PPy/GaN heterostructures. The current–voltage characteristics of the devices in dark and under ultraviolet light illumination were investigated and showed obvious photovoltaic response. Acknowledgements This Letter was supported by the National Natural Science Foundation of China (No. 21273272), the National Basic Research Program of China (Nos. 2010CB934100 & 2012CB619300), and the Chinese Academy of Sciences.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
H. Zheng, Y. Li, H. Liu, X. Yin, Y. Li, Chem. Soc. Rev. 40 (2011) 4506. M. Bognitzki et al., Adv. Mater. 12 (2000) 637. Y. Li, Y. Bando, D. Golberg, Adv. Mater. 16 (2004) 93. Y. Guo et al., J. Am. Chem. Soc. 130 (2008) 9198. C. Goh, S.R. Scully, M.D. McGehee, J. Appl. Phys. 101 (2007) 114503. M. Sessolo, H.J. Bolink, Adv. Mater. 23 (2011) 1829. T.U. Kampen, S. Park, D.R.T. Zahn, J. Vac, Sci. Technol. B21 (2003) 879. D. Baran, A. Balan, B.M. Esteban, H. Neugebauer, Macromol. Chem. Phys. 211 (2010) 2602. I.E. Vermeir, N.Y. Kim, P.E. Laibinis, Appl. Phys. Lett. 74 (1999) 25. M. Dhingra, S. Shrivastava, P.S. Kumar, S. Annapoorni, J. Electron. Mater. 42 (2013) 6. A.G. Kontos, N. Chrysanthakopoulos, M. Calamiotou, T. Kehagias, P. Komninou, U. Pohl, J. Appl. Phys. Lett. 90 (2001) 3301. S. Park, S.W. Chung, C.A. Mirkin, J. Am. Chem. Soc. 126 (2004) 11772. D. Baran, A. Balan, B.M. Esteban, H. Neugebauer, N.S. Sariciftci, L. Toppare, Macromol. Chem. Phys. 211 (2010) 2602. X. Yuan, X. Zeng, H.J. Zhang, Z.F. Ma, C.Y. Wang, J. Am. Chem. Soc. 132 (2010) 1754. L.G. Quagliano, B. Jusserand, D. Orani, J. Raman Spectrosc. 29 (1998) 721. A. Facchetti, Mater. Today 16 (2013) 4. C.J. Novotny, E.T. Yu, P.K.L. Yu, Nano Lett. 8 (2008) 775. H. Wang et al., J. Solid State Chem. 184 (2011) 881. P.S. Smertenko, V.P. Kostylev, V.V. Kislyuk, A.F. Syngaevsky, S.A. Zynio, O.P. Dimitriev, Sol. Energy Mater. Sol. Cells 92 (2008) 976.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Electrochemical formation and optoelectronic property of hybrid organic/inorganic heterostructure of PPy/GaN Li-Feng Hu a,b, Feng-Xia Wang b, Feng-Xiang Deng a,b, Yu Zhao b, Ge-Bo Pan b,⇑ a b
Xi’an Jiaotong University, 710049 Xi’an, PR China Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, 215123 Suzhou, PR China
a r t i c l e
i n f o
Article history: Received 30 October 2013 In final form 28 December 2013 Available online 7 January 2014
a b s t r a c t A new hybrid organic/inorganic heterostructure of p-type polypyrrole (PPy) and n-type gallium nitride (GaN) was fabricated by means of electrodeposition and characterized. The Raman spectra indicated that the GaN substrate had an obvious enhancement of Raman scattering of the PPy, and the PL spectra revealed that the excitonic emission and recombination were partially quenched at the PPy/GaN interface. Moreover, the prototype devices were fabricated on the basis of the PPy/GaN heterostructures. The current–voltage characteristics of the devices in dark and under ultraviolet light illumination showed obvious photovoltaic response. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Hybrid heterostructure materials have attracted great attention in recent years, due to their unique optoelectronic properties arising from individual components and along with processes occurring at the interfaces between components [1]. To date, a variety of heterostructure materials have thus been fabricated with the controlled junctions of organic/organic, organic/inorganic, and inorganic/inorganic [2–4]. Among them, the junctions from organic/inorganic semiconductors are of particular interest and show superior performance of improved carrier mobility, stability, and lifetime relative to those of organic/organic junctions. They are widely used in hybrid devices such as solar cells [5], electroluminescent devices [6], and photodetectors [7]. On the other hand, conducting polymers can be grown on semiconductors to form heterostructures, whose properties can be easily adjusted by using different polymer and dopant. Polypyrrole (PPy) is one of the most important conducting polymers used in heterostructures. The optoelectronic properties of semiconductors can be changed by varying the dopant in PPy film to form a p–n structure [8]. However, it is noted that inorganic semiconductors in previous studies are mainly Si, II–VI and III–V GaAs, InP. [9–11]. To the best of our knowledge, no work has been devoted to the GaN-based organic/inorganic heterostructures. Herein, we report the fabrication of a new hybrid organic/inorganic heterostructure, in which p-type PPy was electrodeposited onto n-type gallium nitride (GaN) substrate. The synergic effect between PPy and GaN in hybrid heterostructures was explored. The GaN-based materials are used due to their wide direct energy ⇑ Corresponding author. Fax: +86 0512 62872663. E-mail address: [email protected] (G.-B. Pan). 0009-2614/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2013.12.074
gap (Eg = 3.4 eV), high heat capacity, excellent chemical stability, and unique optoelectronic properties. The as-prepared PPy/GaN heterostructure was further used to fabricate the prototype photodetectors of ultraviolet light. Moreover, the p–n junction photovoltaic theory was used to explain the synergic effect in UV measurement. 2. Experiment Pyrrole and NaClO4 were purchased from Sinopharm Chemical Reagent Co., Ltd. They were dissolved into double distilled water with the concentrations of 0.05 and 0.1 M, respectively. Singlecrystal n-GaN(0 0 0 1) substrates were 7 mm 5 mm, Si doped n-type, and grown by hydride vapor phase epitaxy on sapphire. The n-GaN substrate was cleaned sequentially with acetone, ethanol, and deionized water. The experiments were carried out in a conventional three-electrode cell using a CHI660D potentiostat/galvanostat (Shanghai ChenHua Co., Ltd.) at room temperature. GaN was used as the working electrode, whereas Pt wires were used as the counter and reference electrodes. An electrolyte containing 0.05 M pyrrole and 0.1 M NaClO4 was used for the deposition of PPy. Pyrrole and sodium perchlorate were utilized as monomer and dopant, respectively. PPy film was electrodeposited with a chronopotential mode. The current density was 0.1 mA/cm2 and the deposition time was 1.5 h. The final samples were washed with water, dried with N2 gas, and characterized by scanning electron microscope (SEM) energy dispersive X-ray spectroscope (EDX), photoluminescence spectroscope (PL, excited at 325 nm), and Raman spectrometer (excited at 633 nm). The current–voltage (I–V) characteristics were measured with an Agilent B1500A semiconductor parameter analyzer.
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(a)
(b)
(c)
Figure 1. (a) Top-view SEM image of PPy film. (b) Cross-section SEM image of the PPy/GaN interface. (c) EDX spectrum of PPy.
Figure 2. (a) Raman spectra of PPy/GaN interface (blue line), PPy (red line), and GaN (black line). (b) Photoluminescence spectra of GaN (red line) and PPy/GaN interface (black line). The inset was the PL spectrum of PPy. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3. Result and discussion Figure 1a illustrates the top-view SEM image of the as-grown PPy on GaN substrate. As seen from the image, the PPy film is well-covered through the whole substrate. Figure 1b shows the cross-sectional SEM image of the interface, confirming the formation of the GaN/PPy heterostructure. The thickness of PPy film is several micron meters. The formation of high quality junction of PPy/GaN might be attributed to a number of grain boundaries of GaN, which can directly attach PPy molecules [12]. Figure 1c shows the EDX mapping of PPy. The carbon and nitrogen elements are attributed to the PPy polymer. The oxygen and chlorine elements are from the electrolyte, which proves the p-type doped pyrrole [13]. In addition, the gallium and partial nitrogen elements come from the GaN substrate. Figure 2a shows the Raman spectra of the PPy and PPy/GaN interface. It is clear that the Raman spectrum of PPy/GaN is similar to the PPy. The four characteristic bands of PPy are observed at 1588, 1240, 1060, and 935 cm1 and in good agreement with the literatures [13,14]. Moreover, the GaN substrate has an obvious enhancement of Raman scattering of the PPy. Similar phenomenon has been observed for the III–V semiconductor quantum dots and deposited silver nanoparticles [15]. Figure 2b shows the PL spectra of the GaN and the PPy/GaN interface. It is seen that the bare GaN is emitted in the ultraviolet region at 365 nm. At the same excitation wavelength, no obvious emission is observed for the PPy (the inset in Figure 2b). Compared with the bare GaN, there is a distinct decrease for the PPy/GaN interface, implying that both the emission and recombination are partially quenched. The above results may be ascribed to the charge transfer and the energy transfer between the GaN and PPy [16]. Figure 3 shows typical I–V characteristics of the devices based on the GaN, PPy, and PPy/GaN in dark and under illumination. No rectification has been observed for both GaN and PPy (Figure 3a and b), indicating that Ohmic contact is formed for In/GaN/In and
Ag/PPy/Ag and allows for efficient carrier collection at this electrode. However, the device based on the PPy/GaN heterostructure exhibits the excellent diode effect (Figure 3c), and the larger switching Ion/Ioff ratio is obtained compared with that of bare GaN. The excellent performance is reasonably attributed to the interface between the high hole mobility p-type PPy and n-type GaN. The J–V behavior is rectifying in nature implying that the PPy/GaN junction act as a rectifier, i.e., forming a diode. The response of a single diode can be modeled using the ideal diode equation [17]:
J ¼ J sat eV=nV t 1
ð1Þ
where Jsat, n, and Vt are the reverse saturation current, ideality factor, and thermal voltage (26 mV), respectively. The reverse saturation density is to measure how many carriers can overcome the energetic barrier created by the p–n junction in the reverse bias direction. Meanwhile, the ideality factor is to depict the recombination behavior and is an indicator of the density and quality of interfaces in hybrid devices. In the case of PPy/GaN device, the reverse saturation current density is 2.5 107 A/cm2 and the calculated ideality factor is 3.82, implying that a recombination current predominates in the junction and a good diode property of PPy/GaN heterostructure. This is because the ideality factor values reported on organic/inorganic heterostructure interfaces were 2 and 6 [18]. More interestingly, the current under illumination is remarkably higher than that obtained in dark for GaN (Figure 3a) and PPy/GaN (Figure 3c). Figure 3d shows the energy level diagram of the PPy/GaN heterostructure, respectively. It is clear that the light can be harvested by the GaN. The loosely bound electron–hole pair reaches the hybrid interface from the GaN side. The energy level diagram would favor charge-separation at the PPy/GaN interfaces with electrons and holes moving towards GaN and PPy layers, respectively. That is a typical photovoltaic response [19]. Therefore, the PPy/GaN heterostructure shows high photogenerated current and an excellent rectifying property under UV illumination.
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L.-F. Hu et al. / Chemical Physics Letters 593 (2014) 28–30
Figure 3. I–V characteristics of prototype devices in dark and under illumination: (a) GaN, (b) PPy, and (c) PPy/GaN heterostructure. The insets were the device configurations. (d) Energy level diagram of the PPy/GaN heterostructure.
Such characteristics are suitable for the fabrication of optoelectronic devices, in particular for UV detectors. 4. Conclusion A new hybrid organic/inorganic heterostructure of p-type PPy and n-type GaN has been fabricated by electrodeposition and characterized. The Raman spectra indicated that the GaN substrate had an obvious enhancement of Raman scattering of the PPy. The PL spectra revealed that the excitonic emission and recombination was partially quenched at the PPy/GaN interface. Moreover, the prototype devices were fabricated on the basis of the PPy/GaN heterostructures. The current–voltage characteristics of the devices in dark and under ultraviolet light illumination were investigated and showed obvious photovoltaic response. Acknowledgements This Letter was supported by the National Natural Science Foundation of China (No. 21273272), the National Basic Research Program of China (Nos. 2010CB934100 & 2012CB619300), and the Chinese Academy of Sciences.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
H. Zheng, Y. Li, H. Liu, X. Yin, Y. Li, Chem. Soc. Rev. 40 (2011) 4506. M. Bognitzki et al., Adv. Mater. 12 (2000) 637. Y. Li, Y. Bando, D. Golberg, Adv. Mater. 16 (2004) 93. Y. Guo et al., J. Am. Chem. Soc. 130 (2008) 9198. C. Goh, S.R. Scully, M.D. McGehee, J. Appl. Phys. 101 (2007) 114503. M. Sessolo, H.J. Bolink, Adv. Mater. 23 (2011) 1829. T.U. Kampen, S. Park, D.R.T. Zahn, J. Vac, Sci. Technol. B21 (2003) 879. D. Baran, A. Balan, B.M. Esteban, H. Neugebauer, Macromol. Chem. Phys. 211 (2010) 2602. I.E. Vermeir, N.Y. Kim, P.E. Laibinis, Appl. Phys. Lett. 74 (1999) 25. M. Dhingra, S. Shrivastava, P.S. Kumar, S. Annapoorni, J. Electron. Mater. 42 (2013) 6. A.G. Kontos, N. Chrysanthakopoulos, M. Calamiotou, T. Kehagias, P. Komninou, U. Pohl, J. Appl. Phys. Lett. 90 (2001) 3301. S. Park, S.W. Chung, C.A. Mirkin, J. Am. Chem. Soc. 126 (2004) 11772. D. Baran, A. Balan, B.M. Esteban, H. Neugebauer, N.S. Sariciftci, L. Toppare, Macromol. Chem. Phys. 211 (2010) 2602. X. Yuan, X. Zeng, H.J. Zhang, Z.F. Ma, C.Y. Wang, J. Am. Chem. Soc. 132 (2010) 1754. L.G. Quagliano, B. Jusserand, D. Orani, J. Raman Spectrosc. 29 (1998) 721. A. Facchetti, Mater. Today 16 (2013) 4. C.J. Novotny, E.T. Yu, P.K.L. Yu, Nano Lett. 8 (2008) 775. H. Wang et al., J. Solid State Chem. 184 (2011) 881. P.S. Smertenko, V.P. Kostylev, V.V. Kislyuk, A.F. Syngaevsky, S.A. Zynio, O.P. Dimitriev, Sol. Energy Mater. Sol. Cells 92 (2008) 976.