13116
J. Phys. Chem. B 2004, 108, 13116-13118
Oxygen Plasma Generated Copper/Copper Oxides Nanoparticles Wenping Hu,†,* Michio Matsumura,‡ Kazuaki Furukawa,§ and Keiichi Torimitsu§ Center of Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China, Research Center for Solar Energy Chemistry, Osaka UniVersity, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan, and NTT Basic Research Laboratories, NTT Corp., 3-1 Morinosato Wakamiya, Atsugi, Kanagawa 243-0198, Japan ReceiVed: March 2, 2004; In Final Form: June 20, 2004
Oxygen-plasma-generated CuOx nanoparticles enhanced the carriers injection from indium-tin oxide (ITO) anode into organic materials. Atomic force microscopy images revealed that the CuOx nanoparticles appeared as high-ordered nanotip arrays on substrates. Those nanoparticles induced strong field-emission current. The analysis by X-ray diffraction pattern and Fourier transform infrared spectrum demonstrated that CuOx was actually of a mixture of copper and copper oxides (CuO, CuO2, CuO3, and CuO4).
1. Introduction To construct efficient organic electroluminescent (EL) devices, it is important to improve carrier injection from the indium-tin oxide (ITO) anode as well as from the metallic cathode.1 Generally, the work function of ITO cannot be aligned with the often used organic EL materials. This requires holes surmount or tunnel through the barrier of the ITO/organic interface, resulting in hole-limited inefficient devices.2-4 To alleviate the problem, oxygen plasma5 and ultraviolet-ozone6 have been introduced as the methods for ITO treatment, and some organic buffers have been introduced into organic lightemitting diodes (OLED)7-13 as a hole-injection layer (HIL) inserted between the ITO and hole-transport layer (HTL). The EL properties of the devices with such HILs are improved greatly. Recently, we also found that ITO covered with an inorganic nanofilm, i.e., oxygen plasma generated CuOx, can also lower the operational voltage of EL devices.14,15 Here, we continue this research, hoping to clear two questions: (1) why can the CuOx improve the carriers injection, and (2) what is CuOx? 2. Experimental Section The CuOx nanofilms preparation and device fabrication were done as described previously.14,15 We obtained atomic force microscope (AFM) images with a Nanoscope III. X-ray diffraction (XRD) and Fourier transform infrared spectrum (FTIR) were measured with X’pert-MPD (Philip) and Bruker Equinox55 instruments, respectively. 3. Results and Discussion The current-voltage (I-V) characteristics of tris(8-quinolinato)aluminum/N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-diphenyl-4,4′-diamine (ALQ/TPD) double-layer devices with different CuOx layers are shown in Figure 1. It is clear that the CuOx layer is effective at low the operational voltage of the EL devices. As a rough comparison, the improvement of the * To whom correspondence should be addressed. E-mail: huwp@ iccas.ac.cn. † Chinese Academy of Sciences. ‡ Osaka University. § NTT.
Figure 1. I-V characteristics of the device with a structure of ITO/ CuOx/TPD (60 nm)/ALQ (60 nm)/Mg:Ag; the thickness of the CuOx layer was 0, 1, 2, 3, 5, and 10 nm.
hole injection of our modified anode is better than that of the devices with the same thickness of CuPc (3 nm). Why can the oxygen plasma generated CuOx improve the hole injection of ITO anodes? The AFM images of Cu films of nanometer thickness on silicon substrates were examined before and after oxygen plasma treatments. The images of the original 10 nm Cu films are shown in Figure 2a,b. The surfaces of the copper nanofilms were very flat, and few tips extended beyond the nanofilm surface. However, after the oxygen plasma treatment, the roughness of the copper nanofilms increased largely. It is much interesting that the arrangement of the copper nanoclusters on the silicon substrate was highly ordered (Figure 2c), and they appeared as sharp nanotips patterned on the silicon surface (Figure 2d), which indicates a potential new method to pattern silicon substrates by copper oxides. With the variation of the oxygen plasma treatment time, the nanotip diameter varied (Figure 2e,f). The diameter decreased near exponentially with the treatment time increasing. When the silicon substrates are substituted with ITO substrates, the sharp nanotips were still observed, although the order of the tips on ITO is not as good as on silicon substrates. Probably, the sharp nanotips penetrate into the organic layer during device fabrication, which changed the local electric-field distribution around the anode surface to induce effective holes emission. Factually, our rough field emission current measure-
10.1021/jp0490610 CCC: $27.50 © 2004 American Chemical Society Published on Web 08/06/2004
Copper/Copper Oxides Nanoparticles
J. Phys. Chem. B, Vol. 108, No. 35, 2004 13117
Figure 2. AFM images of 10 nm of (a and b) newly deposited copper film on silicon, (c and d) after 10 min oxygen plasma treatment, and (e and f) after 30 min oxygen plasma treatment.
ments for the ITO/CuOx (10 nm) have demonstrated much higher current density than pure ITO, as shown in Figure 3. Simultaneously, it is believed that the increased anode roughness16 will lead the contact area between the anode and the organic layer to increase, which results in an increase of hole injection.16 The sharp nanotip arrays of CuOx shown in Figure 2 will certainly increase the roughness of the anode, i.e., the contact area between the ITO and the organic layer. It is reasonable to imagine that under energetic oxygen ion attack, the copper nanoparticles will be oxidized quickly. The formation of oxides on the surface of copper nanoparticles will alleviate the further oxidation in the inner part of the copper nanpoparticles, because of the increasing resistance of copper oxides to oxygen plasma.17,18 And the out shell of the particles will be overoxidized under the continuous attack of the highly energetic oxygen ion bombardment. Therefore, the final nano-
particles after oxygen plasma treatment were probably layerwrapped structures, copper/copper oxides/copper superoxides, i.e., the final product of the oxygen-plasma-treated copper is a mixture of copper and copper oxides. The coexistence of copper and copper oxides was demonstrated by XRD as shown in Figure 4; it is obvious that before oxygen-plasma treatment, the sample consisted of mostly Cu19 together with a trace of oxides20 (ca. 5%). The appearance of copper oxides may be attributed to the oxidation of Cu during device fabrication and measurements. After exposure of the sample to the oxygen plasma atmosphere for 30 min, the product was mostly copper oxides (2θ ) 33.1°) together with a trace of Cu (2θ ) 43.42°). It is difficult to differentiate different copper oxides by X-ray photoelectron spectroscopy, because of the overlapping of their component peaks.14,18,21 However, different copper oxides can
13118 J. Phys. Chem. B, Vol. 108, No. 35, 2004
Hu et al. Cu to CuO, and other peaks at 748, 842, and 1089 cm-1 are ascribed to CuO3, CuO4, and CuO2 respectively.19,22 Therefore, combining the results from Figures 2-5, the oxygen plasma generated CuOx on silicon substrates can be depicted with the model shown in the inset of Figure 5. 4. Conclusion
Figure 3. Field-emission current for ITO/CuOx and ITO electrodes; the experiments were performed at 1 × 10-5 Torr at room temperature, a silver sphere with a diameter of 1 mm was used as cathode (positioned at 100 µm away the anode), and the measured emission area was 4 mm2.
In summary, the oxygen plasma generated CuOx enhanced the hole injection from ITO into organic HTL. AFM images revealed that the CuOx nanodots appeared as sharp nanotip arrays on substrates. Those sharp nanotip arrays induced strong field-emission current from ITO anodes. The analysis by X-ray diffraction pattern and Fourier transform infrared spectrum demonstrated that CuOx was actually a mixture of copper and copper oxides (CuO, CuO2, CuO3, and CuO4). Acknowledgment. The authors acknowledge a Grant-in-aid from National Natural Science Foundational of China, Ministry of Science and Technology of China, Chinese Academy of Science, and Japan Society for the Promotion of Sciences and the Ministry of Education, Culture, Sports, Science and Technology of Japan. References and Notes
Figure 4. XRD results of the 10 nm Cu deposited on Si(100) substrates, with or without oxygen plasma treatment.
Figure 5. FTIR results for the 10 nm copper films deposited on glass substrate after oxygen plasma treatment for 30 min. (Inset) The possible model of CuOx particles on substrates.
be identified by FTIR. The FTIR of the oxygen plasma generated CuOx is shown in Figure 5. In Figure 5, it is obvious that the main peak at 660 cm-1 is because of the oxidation of
(1) Huang, S. L.; Tang, C. W. Appl. Phys. Lett. 1999, 74, 3209. (2) Milliron, D. J.; Hill, I. G.; Shen, C.; Kahn, A.; Schwartz, J. J. Appl. Phys. 2000, 87, 572. (3) Burrows, P. E.; Forrest, S. R. Appl. Phys. Lett. 1994, 64, 2285. (4) Scott, J. C.; Malliaras, G. G.; Chen, W. D.; Breach, J. C.; Salem, J. R.; Brock, P. J.; Sachs, S. B.; Chidsey, C. E. D. Appl. Phys. Lett. 1999, 74, 1510. (5) Steuber, F.; Staudigel, J.; Sto¨ssel, M.; Simmerer, J.; Winacker, A. Appl. Phys. Lett. 1999, 74, 3558. (6) Sugiyama, K.; Ishii, H.; Ouchi, Y.; Seki, K. J. Appl. Phys. 2000, 87, 295. (7) Van Slyke, S. A.; Chen, C. H.; Tang, C. W. Appl. Phys. Lett. 1996, 69, 2160. (8) Chkoda, L.; Heske, C.; Sokolowski, M.; Umbach, E. Appl. Phys. Lett. 2000, 77, 1093. (9) Shirota, Y.; Kuwabara, Y.; Inada, H.; Wakimoto, T.; Nakada, H.; Yonemoto, Y.; Kawami, S.; Imai, K. Appl. Phys. Lett. 1994, 65, 807. (10) Nu¨esch, F.; Rothberg, L. J.; Forsythe, E. W.; Toan, Le, Q.; Gao, Y. L. Appl. Phys. Lett. 1999, 74, 880. (11) Yang, Y.; Heeger, A. J. Appl. Phys. Lett. 1994, 64, 1245. (12) Ho, P. K. H.; Kim, J. S.; Burroughes, J. H.; Becker, H.; Li, S. F. Y.; Brown, T. M.; Cacialli, F.; Friend, R. H. Nature 2000, 404, 481. (13) Gross, M.; Mu¨ller, D. C.; Nothofer, H. G.; Scherf, U.; Neher, D.; Bra¨uchle C.; Meerholz, K. Nature 2000, 405, 661. (14) Hu, W.; Manabe, K.; Furukawa, T.; Matsumura, M. Appl. Phys. Lett. 2002, 80, 2640. (15) Hu, W.; Matsumura, M. Appl. Phys. Lett. 2002, 81, 806. (16) Li, F.; Tang, H.; Shinar, J.; Resto, O.; Weisz, S. Z. Appl. Phys. Lett. 1997, 70, 2741. (17) Hsiao, H.; Miller, D.; Kellock, A. J. Vac. Sci. Technol. A 1996, 14, 1028. (18) Chusuei, C. C.; Brookshier, M. A.; Goodman, D. W. Langmuir 1999, 15, 2806. (19) Luzeau, P.; Xu, X. Z.; Lague¨s, M.; Hess, N.; Contour, J. P.; Nanot, M.; Queyroux, F.; Touzeau M.; Pagnon, D. J. Vac. Sci. Technol. A 1990, 8, 3938. (20) Ogale, S. B.; Bilurkar, P. G.; Mate, N.; Kanetkar, S. M. J. Appl. Phys. 1992, 72, 3765. (21) Wong, A. S.; Krishnan, R. G.; Sarkar, G. J. Vac. Sci. Technol. A 2000, 18, 1619. (22) Chertihin, G. V.; Andrews, L.; Bauschlicher, C. W., Jr. J. Phys. Chem. A 1997, 101, 4026.
