Field Emission from a Composite of Graphene Sheets and ZnO

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Field Emission from a Composite of Graphene Sheets and ZnO Nanowires Wei Tao Zheng,* Yong Min Ho, Hong Wei Tian, Mao Wen, Jun Lei Qi, and Ying Aai Li Department of Materials Science, State Key Laboratory of Superhard Materials, and Key Laboratory of Automobile Materials of MOE, Jilin UniVersity, Changchun, 130012, People’s Republic of China ReceiVed: January 29, 2009; ReVised Manuscript ReceiVed: March 25, 2009

There is a problem with field emission from graphene sheets (GSs) because existing deposition methods lead to sheets that have planar morphological features along entire substrates, which limits field enhancement. To overcome this problem, here we grow pyramid-like GSs on Ni-coated ZnO nanowires via a “base growth” mechanism using plasma-enhanced chemical vapor deposition. The surface morphologies of ZnO-GSs can be controlled by the density of the Ni nanoparticles and deposition time. The ZnO-GSs has a lower turn-on field, 1.3 V/µm, compared to that, 2.5 V/µm, of pure ZnO at a current density of 1 µA/cm2, implying avenues for potential applications of graphene and ZnO. 1. Introduction ZnO-based nanostructures, such as nanowires, nanorods, and nanofibers, have been considered to be good candidates for field emission (FE) emitters, due to their high surface-to-volume ratio, thermal stability, and oxidation resistance.1,2 Up to now, in order to improve the FE properties and stability of ZnO nanostructures, several groups have investigated the fabrication of sharp-tipped nanoneedle, amorphous C, CNx, and NiO film coated nanowires and Ge-doped nanofibers using various methods,3-6 such as direct current magnetron sputtering, atomic layer deposition, and vapor-phase transport. Since a great increase in FE property is significantly concerned with the geometrical morphology on the emitter surface,7 it is also important to control their surface morphologies for better ZnO-based emitters. On the other hand, graphene sheets (GSs) are currently of great interest as efficient FE sources, because they have unique electronic properties, large surface areas, and sharp edges.8,9 In general, GSs can be synthesized by chemical vapor deposition, plasma-enhanced chemical vapor deposition (PECVD), and a chemical exfoliation method associated with graphite intercalation compounds.8-10 However, there is a problem with the field emission from GSs, because the existing deposition methods lead to the sheets that have the planar morphological features along the entire substrates, which limits the field enhancement. To overcome this problem, in this paper, we report that GSs with pyramid-like morphologies have been grown on the ZnO nanowires coated with Ni catalyst nanoparticles by PECVD, and a greatly improved FE property for the hybrid ZnO-GSs material has been achieved. 2. Experimental Section ZnO nanowires were grown on a Si(111) substrate via vaporphase deposition. Si substrate and Zn powder (purity 99.999%) in an alumina boat were placed at the same horizontal level in a reaction chamber. Ar gas was introduced with a flow rate of 50 sccm as a carrier gas, and O2 was input at 2 sccm. The pressure during deposition was kept at 150 Pa, and the reaction lasted for 30 min at 650 °C. Ni nanoparticles were then coated on ZnO nanowires for 3-5 s using dc magnetron sputtering. * To whom correspondence should be addressed. Tel/Fax: +86431 5168246. E-mail: [email protected].

The growth of GSs on Ni-coated ZnO nanowire was achieved in a radio frequency PECVD system (RF power: 13.56 MHz). The reaction chamber was evacuated to a pressure of 15 Pa, and the pressure was maintained at 150 Pa with an Ar gas flow of 70 sccm. After the substrate was heated to 800 °C for 40 min, methane with a gas flow of 30 sccm was introduced into the chamber, and a plasma power of 200 W was utilized to grow GSs at 800 °C for 2-10 min. The substrate was cooled down to room temperature with an Ar gas flow of 20 sccm. Scanning electron microscopy (SEM) (JEOL JSM-6700F), transmission electron microscope (TEM) (Hitachi H-8100 at 175 kV), high resolution TEM (HRTEM) (JEOL TEM-2010 at 300 kV), X-ray diffraction (XRD) (Bruker D8-tools), Jobin-Yvon LabRam Infinity Raman spectroscopy with a 514.5 nm Ar+ laser excitation, and photoluminescence (PL) spectroscopy using a He-Cd laser (325 nm) as the excitation source were used to study the morphology, structure, and optoelectronic property of hybrid ZnO-GSs material. FE measurements were carried out using a diode configuration, a cathode (sample), and a parallel anode plate in a vacuum chamber maintained at a pressure of 2 × 10-7 Pa with an oil-free turbo-molecular pump. 3. Results and Discussion The XRD pattern of ZnO nanowires was taken to examine their crystal structures. As shown in Figure 1, the peaks at 2θ ) 31.4°, 34.4°, 36.3°, 47.4°, 56.6°, and 62.8° originate from the (100), (002), (101), (102), (110), and (103) reflections of ZnO, respectively, which shows that ZnO nanowires grown on a Si(111) substrate have a wurtzite structure with the lattice constants of a ) 0.325 nm and c ) 0.52 nm (JCPDS-ICDD Card No. 79-205). The peak marked by a star may be the reflection from Si substrate. The morphology and structure of the hybrid ZnO-GSs material were analyzed by SEM, TEM, and HRTEM. Figure 2a,b shows SEM images of as-grown ZnO nanowires (diameter 50-70 nm) and ZnO nanowire coated for 5 s with Ni. Parts c, d, and e of Figure 2 show SEM images of ZnO-GSs synthesized for 2, 5, and 10 min after 3 s of coating, respectively, which shows that ZnO nanowires in the sample grown for 2 min are decorated with sharp protrusions, and their sharpness apparently vanishes according to the increase of deposition time. We can also see that the ZnO nanowires have slightly bent

10.1021/jp900881q CCC: $40.75  2009 American Chemical Society Published on Web 05/04/2009

Field Emission from a Composite of ZnO-GSs

Figure 1. XRD pattern of ZnO nanowires grown on a Si(111) substrate. The star indicates the reflection from Si substrate.

Figure 2. (a) SEM image of as-grown ZnO nanowires. (b) SEM image of ZnO nanowire coated for 5 s with Ni. SEM images of ZnO-GSs grown for (c) 2 min, (d) 5 min, and (e) 10 min after 3 s of coating, respectively.

shapes, which can be due to the weights of the GSs aggregated on the ZnO surface during the deposition in PECVD. Parts a, b, and c of Figure 3 show TEM images of ZnO-GSs grown for 2, 5, and 10 min, respectively. Ni nanoparticles with an average size of about 15 nm are mostly attached on ZnO surface and the GSs synthesized on the nanoparticles have pyramid-like shapes, and the number of the GSs increases with deposition time. The TEM image of the GSs for the sample grown for 10 min and the HRTEM image for the Ni nanoparticle, as shown in Figure 3d,e, reveal that the formation of GSs on ZnO nanowires justly occurs on the Ni nanoparticles. Here the lattice spacings of 0.34, 0.20, and 0.28 nm are in good agreement with the interplanar distances of (002) graphite, (111) metallic Ni,11 and (100) ZnO, respectively. From the fact that the active Ni ions are fairly diffused into ZnO nanostructure12,13 and most pure metals do not wet graphite,14 we speculate that in our PECVD system, the GSs grow via a “base growth” mechanism, due to the effective adhesion of Ni nanoparticles on ZnO surface. At the beginning of growth, the activated carbon atoms are concentrically deposited on Ni catalyst nanoparticles to form base graphene layers parallel to the ZnO nanowire surface (marked by dashed lines in Figure 3d). Later, the outermost of these graphene layers begins to grow perpendicularly to the surface, because the direction of the electric field in the plasma is an energetically most favorable orientation

J. Phys. Chem. C, Vol. 113, No. 21, 2009 9165 of carbon nanostructure growth.15 In the other words, the high local electric field induced in the vicinity of catalyst particles via the plasma sheath increases the polarization of surface charges associated with the polarization of free electrons existing in graphene layers, causing the GSs to grow higher rather than thicker. The HRTEM image in Figure 3f shows that the sharp GS grown for 2 min protrudes and has a good crystalline quality, and the growth orientation of the graphene layers is apparently vertical to the surface. We can see that in the vertical growth process, the leading edge of GS is curled with a small radius of curvature that is usually smaller than the size of catalyst particle. This can be associated with the van der Waals force between graphene layers grown on individual nanoparticle, and the twisting between the neighboring nanostructures, as shown in Figure 3d, can also be interpreted as the same reason. The attractive interaction between the catalyst particles and carbon atoms or between the neighboring GSs during vertical growth is dominant at the central vicinity of Ni nanoparticle groups, resulting in the formation of pyramid-like GSs on ZnO nanowires. By the way, for the sample grown for 10 min, the surface of ZnO nanowire is densely decorated with petal-like GSs, not showing the sharp pyramid-like shapes as for the samples grown for 2 and 5 min (Figure 3c). In fact, as deposition time increases, the persistent accretion of carbon ions at the edge of GS permits the edge-face contact to the neighboring GS (marked by arrow in Figure 3d). This causes an increase in GS surface area and a conspicuous bluntness near the sharp pyramid apex. On the other hand, we can see that in Figure 3c, the GSs were continually grown on a long cylindrical branch formed at the dense range of Ni nanoparticles (pointed by arrow), and the Ni nanoparticles with small sizes were attached to the branch. The HRTEM image in Figure 3g shows that this branch was mostly composed of amorphous carbon structure, and the Ni nanoparticle attached on the branch was covered with the graphene layers of about 2 nm. This growth process can be interpreted as the effects of dense catalyst particles and an overincrease of deposition time. The accretion of carbon ions on the nanowire surface more effectively occurred near the denser range of catalyst particles, which causes the formation of a long amorphous carbon branch according to the vertical growth mechanism, as deposition time increases. Simultaneously, an overincrease of deposition time leads to the loosening of catalyst particles attached on the nanowire surface by sufficient plasma ion bombardment, so the catalyst particles with small sizes can migrate along with the walls of the amorphous carbon branch. Such a migration of catalyst particles and a new formation of graphene layers on the particles bring the continuous growth of GSs on the amorphous carbon branch. The HRTEM image in Figure 3h shows that the GS for the sample grown for 10 min has a wider petal-like morphology and larger surface area than the sharp GS protrusion grown for 2 min (seen in Figure 3f), and its thickness is about 0.37 nm. From these interpretations, we can say that the deposition time has a great influence on the variation in surface morphology of ZnO-GSs as well as the density of catalyst particles. Additionally, HRTEM images in Figure 3i,j show that very thin graphene or, mostly, amorphous carbon layers are formed on the ZnO surfaces devoid of Ni nanoparticles in the samples grown for 2 and 5 min. Here, the lattice spacing of 0.25 nm agrees with interplanar distance of (002) ZnO. Other investigations8,9 have reported that the GSs with thickness of 1-100 nm and lateral dimension 2-4 µm were mostly fabricated on Ni plate substrate or without any need of catalyst on several substrates and had the planar morphological features along the entire substrate. In

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Figure 3. (a-c) TEM images of the ZnO-GSs grown for 2, 5, and 10 min, respectively. (d) TEM image of GSs for the sample grown for 10 min. (e) HRTEM image of Ni nanoparticle. (f) HRTEM image of a sharp GSs protrusion for the sample grown for 2 min. (g) HRTEM image of a Ni nanoparticle attached to an amorphous carbon branch. (h) HRTEM image of petal-like GSs for the sample grown for 10 min. (i and j) HRTEM images of ZnO surface uncovered with Ni nanoparticles in the samples grown for 2 and 5 min.

our case, the GSs with less than 1 nm thickness, 20-100 nm lateral dimension, and 50-100 nm height are synthesized with sharp pyramid-like shapes on ZnO nanowires using catalytic Ni nanoparticles, and the geometrical parameters of the pyramids such as size and height can be controlled by the deposition time. Here, the density of Ni nanoparticles can be also considered as a major factor controlling the geometrical parameters of the pyramids. As shown in Figure 2b, for the sample coated for 5 s, coated nanoparticles have an average size of about 30-40 nm, which is much bigger than that for coating 3 s, and even the film layer of about 25 nm thickness is seen. Although this film layer should be divided into small nanoparticles in the PECVD deposition process, the totally dense groups and large average size of catalyst particles would still exist on the surface of ZnO nanowires. By the results of our experiment, for the samples coated for more than 5 s, the walls of ZnO nanowires have completely been coated by a Ni film layer, not showing a remarkable change in the geometrical morphology of GSs according to the deposition time. This implies that the variation in density and average size of catalyst particles by coating time has a great effect on the formation of GSs with sharp pyramidlike shapes on ZnO nanowires. The optoelectronic properties of the obtained GSs were investigated by Raman spectroscopy with 514.5 nm Ar+ laser excitation and room temperature PL spectroscopy using a He-Cd laser (325 nm) as the excitation source. Figure 4 exhibits the Raman spectra of the ZnO nanowires (curve A) and the ZnO-GSs grown for 2, 5, and 10 min (curves B-D), respectively. The strong peaks at 437 and 570 cm-1 correspond to E2 and E1 (LO) modes16,17 for ZnO, respectively, and the peak at 520 cm-1 may be from the Si substrate. For the sample synthesized for 2 min, a G band (∼1592 cm-1) corresponding to sp2-hybridized carbon and a D band (∼1350 cm-1) originating from disordered carbon18-20 are observed with a decrease in

Figure 4. Raman spectra of (A) ZnO nanowires and ZnO-GSs grown for (B) 2 min, (C) 5 min, and (D) 10 min, respectively.

peak intensities for ZnO. The origin of the D band in the GSs can result from the structural defects such as corrugation, twisting, and edges.8,21 Especially, for the sample grown for 10 min, the intensity ratio (1.09) of D over G band (ID/IG) is higher than those (0.95, 0.92) for 2 and 5 min, and the peak at 2699 cm-1 corresponding to the overtone of the D band and the peak at 2941 cm-1 associating with (D + G) band also appeared, showing a substantial increase in the degree of disorder in GSs. Indeed, the growth of GSs on the nanowire surface by the intensive interaction between carbon ions and Ni catalyst particles is mostly terminated after forming the sharp GSs protrusion seen in Figure 3f. Over the duration of deposition time, the accretion of carbon ions occurs at the edge or top vicinity of GSs kept apart from the catalyst particles. In this range, the persistent accretion of carbon ions is obtained in a

Field Emission from a Composite of ZnO-GSs

Figure 5. PL spectra of (A) ZnO nanowires and ZnO-GSs grown for (B) 2 min, (C) 5 min, and (D) 10 min, respectively.

way that contacts the edge-face of neighboring GSs or increases the surface area of petal-like GSs. As a result, a self-organized mesoporous arrangement containing relatively more defects rather than ordered graphite structure is formed due to a greatly decreased interaction between carbon ions and catalyst particles. Even in the sample grown for 2 min (seen in Figure 3f), we can see the apparent formation of these crystal defects at the edge and, especially, in the vicinity of the top of the sharp GSs protrusion. This shows that an appropriate control of deposition time has also an effect on the synthesis of better-quality ZnO-GSs structures. Figure 5 shows the room temperature PL spectra for (A) ZnO nanowires and (B-D) ZnO-GSs grown for 2, 5, and 10 min, respectively, in which the ultraviolet (UV) emission peak at about 380 nm corresponds to the near band-edge emission of ZnO crystal.22 In addition to the UV emission, we also find a broad blue-emission peak at about 488 nm, originating from the recombination of photoexcited holes with singly ionized oxygen vacancies.23 The intensity of the UV emission peak is remarkably decreased as the deposition time increases, which can be due to the limited sensitivity of the PL detector due to an increase in the amount of GSs covered on the ZnO surface. Besides, the UV emission peak in the sample grown for 2 min shows an apparent blue shift compared with that for ZnO nanowires, which can be attributed to Ni nanoparticles covered on the ZnO surface. The diffusion interaction of Ni ions in the ZnO surface leads to the presence of trivalent Ni3+ cations or the substitution of zinc atoms in ZnO crystal, which may increase the interstitial defects, similar to interstitial zinc. Since the blue-emission peak concerned with interstitial zinc or trivalent Ni3+ cations defects appears at 440-470 nm,23,24 we guess that the simultaneous decrease of emission peak at 380 and 488 nm due to the GSs covered on ZnO surface and the increase of the blue-emission peak at 440-470 nm result in UV emission peak at 389 nm with full wide of half-maximum (fwhm) of about 70 nm for the sample grown for 2 min. In the FE experiments, an indium-tin oxide (ITO) coated glass was used as an anode, and the silicon substrate with ZnO-GSs nanostructure that acted as cathode was mounted on a stainless steel plate. During our experiments, the anode-cathode distance was kept constant by two Teflon spacers with a thickness of 300 µm and the measured emission area was 10 × 10 mm2. Figure 6 displays the current density (J) as a function of electric field strength (E) for (curve A) ZnO nanowires and (curve B) ZnO-GSs grown for 2 min. A hybrid ZnO-GSs

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Figure 6. FE current density as a function of electric field for (A) ZnO and (B) ZnO-GSs grown for 2 min, in which the inset exhibits the Fowler-Nordheim plots corresponding to (A) and (B), respectively.

material has the lower turn-on field (E required for extracting a J of 1 µA/cm2) of 1.3 V/µm than that (2.5 V/µm) for ZnO nanowires and the lower threshold field (E required for producing a J of 1 mA/cm2) of 5.7 V/µm. This indicates that the FE property for the obtained hybrid ZnO-GSs has been improved significantly compared to that of pure ZnO nanowires. The Fowler-Nordheim (F-N) plots shown in the inset of Figure 6 fit well to the linear relationship given by ln(J/E2) ) ln(Aβ2/ φ) - Bφ3/2/βE, which indicates that the measured currents are mostly due to vacuum tunneling from ZnO or ZnO-GSs, where A ) 1.54 × 10-6 A eV V-2, B ) 6.83 × 103 eV-3/2 V µm-1, β is the field enhancement factor, and φ is the work function of emitters. From the slopes of F-N plots, the field enhancement factor for ZnO-GSs (φGS ∼ 5.0 eV) is estimated to be about 1.5 × 104, which is much higher than that (7.4 × 103) for ZnO nanowires (φZnO ) 5.3 eV). In general, a significant increase in field enhancement factor β is greatly attributed to the geometrical factors such as small curvature radius and a high density of emitters,25 and the relatively large diameter of ZnO nanorod or nanowire is considered to be a limitation to further improve the FE property of ZnO nanostructures.3 Furthermore, the more effective FE can be realized from the emitter bodies oriented parallel to the substrate if some emission sources contributing to emission current density exist on the emitter surface.26 In our case, the root sizes of pyramid-like GSs are nearly comparable to the diameters of ZnO nanowires, and their apexs are much sharper than the ZnO tips. A number of sharp edges with a small radii of curvature existing in pyramid-like GS can act as independent emitters on the ZnO surface, in addition to ZnO tips, thus causing greatly enhanced FE property under the given electric field. In addition, since the work function of carbon material is commonly lower than that of ZnO, there is no Schottky barrier limiting the transfer of the electrons in ZnO-GS junction, which can be considered as another reason for improvement of the FE property of ZnO-GSs. 4. Conclusion In conclusion, the pyramid-like GSs on the ZnO nanowires have been fabricated by radio frequency PECVD. The Ni nanoparticles coated on the ZnO nanowires play a catalytic role in growing GSs, and the geometrical factors of GSs can be controlled by the density of coated Ni nanoparticles and

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deposition time. The sharp edges of GSs with small radii and a high density act as additional emission sites, leading to the enhanced local field on ZnO nanowire surface, which results in a significantly improved FE property compared to pure ZnO nanowires. Therefore, it is reasonable to expect that the hybrid ZnO-GSs material can be used as a promising candidate for the field emission devices. References and Notes (1) Tseng, Y. K.; Huang, C. J.; Cheng, H. M.; Lin, I. N.; Liu, K. S.; Chen, I. C. AdV. Funct. Mater. 2003, 13, 811. (2) Lee, C. J.; Lee, T. J.; Lyu, S. C.; Zhang, Y.; Ruh, H.; Lee, H. J. Appl. Phys. Lett. 2002, 81, 3648. (3) Zhu, Y. W.; Zhang, H. Z.; Sun, X. C.; Feng, S. Q.; Xu, J.; Zhao, Q.; Xiang, B.; Wang, R. M.; Yu, D. P. Appl. Phys. Lett. 2003, 83, 144. (4) Liao, L.; Li, J. C.; Wang, D. F.; Liu, C.; Liu, C. S.; Fu, Q.; Fan, L. X. Nanotechnology 2005, 16, 985. (5) Yang, J. H.; Lee, S. Y.; Song, W. S.; Shin, Y. S.; Park, C. Y. J. Vac. Sci. Technol. B 2008, 26, 1021. (6) Xu, C. X.; Sun, X. W.; Chen, B. J. Appl. Phys. Lett. 2004, 84, 1540. (7) Green, J. M.; Dong, L. F.; Gutu, T.; Jiao, J.; Conely, J. F., Jr.; Ono, Y. J. Appl. Phys. 2006, 99, 094308. (8) Wang, J. J.; Zhu, M. Y.; Outlaw, R. A.; Zhao, X.; Manos, D. M.; Holloway, B. C.; Mammana, V. P. Appl. Phys. Lett. 2004, 85, 1265. (9) Obraztsov, A. N.; Tyurnina, A. V.; Obraztsova, E. A.; Zolotukhin, A. A.; Liu, B. H.; Chin, K. C.; Wee, A. T. S. Carbon 2008, 46, 963. (10) Viculis, L. M.; Mack, J. J.; Kaner, R. B. Science 2003, 299, 1361.

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