Crystal-Plane Dependence of Critical Concentration for Nucleation on Hydrothermal ZnO Nanowires

Transcription

1 pubs.acs.org/jpcc Crystal-Plane Dependence of Critical Concentration for Nucleation on Hydrothermal ZnO Nanowires Yong He, Takeshi Yanagida,*, Kazuki Nagashima, Fuwei Zhuge, Gang Meng, Bo Xu, Annop Klamchuen, Sakon Rahong, Masaki Kanai, Xiaomin Li, Masaru Suzuki, Shoichi Kai, and Tomoji Kawai*, The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka Ibaraki, Osaka , Japan State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai , P. R. China Department of Applied Quantum Physics and Nuclear Engineering, Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka , Japan *S Supporting Information ABSTRACT: Hydrothermal ZnO nanowires have shown great potential for various nanoscale device applications due to their fascinating properties and lowtemperature processing. A preferential crystal growth of ZnO (0001) polar plane is essential and fundamental to realize the anisotropic nanowire growth. Here we demonstrate that a critical concentration for a nucleation strongly depends on a crystal plane, which plays an important role on an anisotropic growth of hydrothermal ZnO nanowires. We measure a growth rate of each crystal plane when varying a concentration of Zn ionic species by using a regular array structure. Selective anisotropic growth on (0001) plane emerges within a certain concentration range. Above the concentration range, a crystal growth on (101 0) plane tends to simultaneously occur. This strong concentration dependence on the crystal plane is understood in terms of a critical concentration difference between (0001) plane and (101 0) plane, which is related to the surface energy difference between the crystal planes. INTRODUCTION ZnO nanowires formed via a hydrothermal method have attracted much attention due to their fascinating physical properties and the potentials for nanodevice applications, including light-emitting devices, solar cells, power generators, and others. 1 7 One of the reasons why the hydrothermal method has been so attractive even compared with other methods is that whole hydrothermal processes can be performed under a relatively low-temperature range less than 100 C, 8,9 which is hardly attainable to other conventional vapor-phase methods including VLS mechanism This low-temperature synthesis is favorable, especially when integrating nanowires with other components (e.g., polymers) within the devices. 9,28 In the hydrothermal growth of ZnO nanowires, a preferential nucleation at ZnO (0001) polar plane is essential and fundamental to realize the anisotropic nanowire growth. 4,29,30 The effects of various experimental parameters, including ph, temperature, and ionic species, have been examined to understand their roles on the reaction scheme of hydrothermal growth For example, Yamabi and Imai reported the role of ph on the variation of ionic species during hydrothermal ZnO nanowire growth. 36 Xu et al. demonstrated the importance of temperature on the morphology of hydrothermal ZnO nanowires. 37 Joo et al. recently reported a face-selective electrostatic control of hydrothermal ZnO nanowires by intentionally adding positively charged ions during growth. 32 In most previous studies, the origin of anisotropic crystal growth in hydrothermal ZnO nanowires has been interpreted in terms of the variations of ionic species in aqueous solutions and their electrostatic interactions with ZnO crystal planes. 31,32,38,39 Here we demonstrate that a crystal-plane dependence on critical concentration for a nucleation plays an important role on an anisotropic growth of hydrothermal ZnO nanowires. We found the strong concentration dependence on the crystal growth of each ZnO crystal plane when varying only a concentration of Zn species under the same temperature and ph value. This strong concentration dependence on the crystal plane can be understood in terms of a critical concentration difference between (0001) plane and (101 0) plane for a nucleation. Our findings highlight that controlling precisely a concentration during growth is essential to tailor the morphology of hydrothermal ZnO nanowires. Received: November 16, 2012 Revised: December 17, 2012 Published: December 21, American Chemical Society 1197

2 Figure 1. (a) Schematic of fabrication process of hydrothermal method for ZnO nanowires. (b) XRD data of ZnO seed layer on Al 2 O 3 substrate. (c) SEM images of fabricated hole array patterns on substrate. EXPERIMENTAL SECTION We utilize a series of ZnO nanowire arrays to measure a growth rate of each crystal plane. We have fabricated the ZnO nanowire arrays on spatially patterned photoresist/zno seed layer/al 2 O 3 substrates by utilizing conventional hydrothermal method. The schematic of fabrication process is shown in Figure 1a. First, a ZnO seed layer of 20 nm thickness is deposited onto Al 2 O 3 (0001) substrate by pulsed laser deposition method. Prior to the deposition, the substrate is heated up to 600 C. The oxygen partial pressure during the deposition is set to be 1 Pa. The fabricated ZnO seed layer is grown along the 0001 orientation, as shown in Figure 1b of XRD data. After the preparation of the seed layer, we coat photoresist (AZ5206/Z5200 2:1) onto ZnO/Al 2 O 3 substrate by spin-coating at 5500 rpm for 60 s and subsequently bake it at 90 C for 3 min. Then, we create the circular hole array patterns with 610 nm diameter and 390 nm interval distances by nanoimprint lithography, as shown in Figure 1c. After making the hole array patterns, the patterned photoresist is then solidified at 120 C for 5 min. The photoresist residuals at the bottom of patterned hole are removed by reactive ionetching process. The remained photoresist thickness on nonpatterned area is 50 nm. After these patterning processes, we perform the hydrothermal growth of ZnO nanowires. A solution for the hydrothermal reaction is prepared by using equimolar zinc nitrate hexahydrate (Zn(NO 3 ) 2 6H 2 O) and hexamethylenetetramine (HMTA, (CH 2 ) 6 N 4 ). All materials are analytical grade and used without further purifications. Aqueous solution is prepared at room temperature, and the concentration is varied from 0.01 to 40 mm to examine the concentration dependence. We measure the ph values by using a ph meter. The measured ph values are ranged from 6.58 to 6.98 when varying the concentration. The patternedphotoresist/zno/al 2 O 3 substrate is then immersed upside down into the growth solution in a sealed beaker and kept at 95 C for a given time. Structural characterizations of fabricated ZnO nanowires are performed by using X-ray diffraction and field emission scanning electron microscopy. RESULTS AND DISCUSSION Figure 2a shows the typical SEM images of ZnO nanowire arrays when varying the concentration range from 0.01 to 40 mm. The nanowires grown for 20 h are shown in the Figures. ZnO nanowires are vertically grown from the substrate. The nanowire morphology drastically changes with varying concentration. Figure 2b shows the XRD data of fabricated ZnO nanowires. No additional peaks are observed in XRD data even after ZnO nanowires growth, indicating the nanowire growth orientation of 0001 direction. For further confirmation, we perform TEM measurements, as shown in Figure S1 in the Supporting Information, which also highlights the 0001 growth orientation. There are several features in regards to the concentration dependence on the nanowire morphology in Figure 2a. For the 0.01 mm case, there is no visible crystal growth due to the diluted environment. Above 0.1 mm, the nanowires start to grow and the nanowire length increases with increasing the concentration. The multinanowires exist within one hole array, which tend to be unified when increasing the concentration above 5 mm. In such high concentration range, the nanowires tend to grow even along lateral direction, resulting in an increase in nanowire diameter and a decrease in the interval between nanowires. Figure 2c shows the aspect ratio data of nanowires, defined as a ratio of length to diameter. The aspect ratio data shows the maximum around between 1 and 5 mm, highlighting the significant role of concentration on the morphology of nanowires. To examine this concentration dependence as a time series data, Figure 3a,b shows the SEM images of nanowires grown under 1 and 20 mm for different growth time (1, 5, and 20 h). There is clearly a significant difference between the two concentrations on the time series data. In the case of 1 mm, the nanowires grow mainly only along the 0001 direction without any significant lateral 1198

3 Figure 2. (a) Concentration dependence on ZnO nanowire morphology. The varied concentration range is mm. (b) XRD data of ZnO nanowires. (c) Aspect ratio data when varying the concentration. Figure 3. (a) Time series data on ZnO nanowire morphology with the concentration of 1 mm. (b) Time series data on ZnO nanowire morphology with the concentration of 20 mm. sidewall growth for all growth time. For 20 mm, the nanowires tend to unify within an array even at short growth time, 1 h, by growing not only a 0001 direction but also a lateral sidewall direction. Similar trends on the time series data are also 1199

4 Figure 4. (a) Concentration dependence on the nanowire morphology data (length and radius). The growth time is 20 h. (b) Concentration dependence on the nanowire morphology data (length and radius). The growth time is 5 h. Figure 5. (a) Morphology time series data of nanowires when exchanging a solution every 5 h for 1 mm. (b) Morphology time series data of nanowires when exchanging a solution every 5 h for 20 mm. Figure 6. (a) ph data variation when varying the concentration. (b) Concentration dependence on the nanowire morphology data (length and radius) under a constant ph experiments. The growth time is 20 h. confirmed for different concentrations, as seen in the Supporting Information, Figure S2. Thus, the variation of concentration seems to affect not only the growth rate but also the growth direction. To specify more quantitatively the above trends in regards to the concentration dependence, we extract the length and radius data of fabricated nanowires as a function of a concentration, and the results are shown in Figure 4a. It is noted that there are several reports on the concentration dependence of hydrothermal ZnO nanowires However, the intrinsic mechanisms in regards to the concentration dependence on the growth direction have not been discussed. Data of nanowires grown for 20 h are shown in the Figure. The length data are measured over 100 nanowires in cross-sectional SEM images. The radius data are measured over 100 nanowires in top-view SEM images. The nanowire growth along the 0001 direction 1200 starts at around between 0.01 and 0.1 mm, and the nanowire length increases up to over 2 μm when increasing the concentration. On the contrary, the nanowire radius remains almost constant below 5 mm and starts to increase above the threshold concentration around 5 mm. The magnified figures for radius data can be found in the Supporting Information, Figure S3, which more clearly shows the increase in radius values above 5 mm. Although these data are taken from nanowires grown for 20 h, the similar trend is also observed even for nanowires grown for shorter time, 5 h, as shown in Figure 4b. Obviously, there are two threshold concentrations for nanowire growth, one (around 0.01 to0.1 mm) for a growth of (0001) plane and the other (around 1 5 mm) for a lateral sidewall direction-mainly (101 0) plane. As for the growth time dependence of concentration in a closed system, we perform experiments by exchanging a solution every 5 h to maintain a

5 concentration. The time series data of nanowire morphologies are shown in Figure 5. The growth time is 20 h, and the concentrations are 1 and 20 mm. For comparison, previous data without exchanging a solution are shown. Although there is a quantitative effect of maintaining a concentration on the growth rate, especially for the length data at longer growth time above 15 h, the difference tends to be smaller, <20% below 10 h. Thus the concentration decrease in our closed system can be estimated to be <20% in the early stage below 10 h of reaction time by assuming first-order reaction. In addition, the concentration dependence on the morphology data is almost independent of the growth time (5 and 20 h), as seen in the similarity between Figure 4a,b. These results consistently demonstrate that the major semiquantitative trends of concentration dependence shown in Figure 4 are valid. These morphology data also highlight that the concentration significantly influences not only the growth rate but also the growth direction. The concentration dependence on the growth rate of nanowires is solely a matter of supplied flux. The concentration dependence on the growth direction seems to be not readily interpreted in terms of existing models based on the variations of ionic species in a solution and their electrostatic interactions with ZnO crystal planes. 32,38 Here we discuss what essentially causes the concentration dependence on the growth direction in Figures 3 and 4. First, we examine the effect of ph when varying the concentration of (Zn(NO 3 ) 2 6H 2 O) and HMTA. This is because Zn(NO 3 ) 2 might act as an weak acid and HMTA is a weak base, 29 and the ph value of solution strongly affects a reaction scheme of hydrothermal ZnO growth via changing the concentration of existing ionic species in a solution. 32,36,38 Figure 6a shows the variation of ph values when varying the concentration. As can be seen, even increasing the concentration from 0.01 to 40 mm, the ph value decreased from 6.98 to Additional data in regards to the ph value variation during ZnO nanowire growth can be seen in the Supporting Information, Figure S4. Obviously the ph variation range is quite small, and such ph variation does not seem to impact the reaction scheme of ZnO hydrothermal synthesis as reported in previous works. 32,38 To confirm the effect of ph variation on our observations in regards to the nanowire morphology change, we have performed hydrothermal experiments under a constant ph condition via adjusting the ph value 7 using NaOH. The nanowire data of such ph-controlled experiments are shown in Figure 6b. The nanowires were grown for 20 h. Clearly the concentration dependence on the nanowire morphology data and the growth direction is consistent with those of phuncontrolled experiments in Figure 4. Other possible experimental parameters, which affect a reaction scheme of ZnO hydrothermal synthesis, are typically a temperature and an ionic species. 33,38,45 Because we control these parameters to be constant in our experiments, these contributions do not seem to be major factors to cause the drastic change of the nanowire morphology in our experiments. Therefore, we consider how a change of concentrations affects a crystal nucleation event at different crystal planes, which essentially determines the anisotropy of crystal growth. Here we consider a nucleation of ZnO crystal at a solid liquid interface. In principle, a nucleation in a solution occurs when a surrounding concentration exceeds a critical concentration for a nucleation, which is above a saturated concentration When a nucleation occurs within a solution in the presence of solid surfaces, an interaction between a 1201 nucleus and a solid surface has to be considered to understand the free energy gain of growth system. 48,50 It is well known that the presence of solid surfaces significantly reduces a free energy barrier for a nucleation because a free energy gain due to an interaction between a nucleus and a solid surface is larger than that between a nucleus and a liquid in most cases If there is a surface energy difference between crystal planes, then each crystal plane may have the different free energy barrier for a nucleation. In the case of ZnO crystal, a (0001) polar plane is well-known to have a highest surface energy (2.0 J/m 2 ) compared with other crystal planes, for example, (101 0) plane, 1.16 J/m The crystal plane with larger surface energy should have a lower free energy barrier for a nucleation (i.e., a lower critical concentration for a nucleation), and a crystal growth appears when a concentration exceeds a critical concentration for each crystal plane. On the basis of this scenario, when increasing a concentration, first a nucleation at a (0001) plane only emerges without any nucleation at other crystal planes, and such <0001> oriented crystal growth in principle can continue until the concentration exceeds next lower critical concentration of other crystal planes, for example, (101 0) plane. Above such concentration, the anisotropic crystal growth along 0001 no longer exists and the crystal growth along lateral directions can coexist. Figure 7 shows the Figure 7. Schematic of a concentration dependence on a crystal growth of hydrothermal ZnO nanowires as a crystal-plane dependence of a critical concentration for nucleation. schematic of the above implications to interpret the concentration dependence on the morphology of hydrothermal ZnO nanowires. As clearly seen in the schematic image, to maintain the anisotropic crystal growth along a 0001 direction, controlling a concentration to be within between the critical concentration for (0001) plane and the next lowest critical concentration for (101 0) plane is essential. Thus the present concept based on a crystal-plane dependence of a critical concentration for a nucleation can rigorously explain why we have observed the concentration dependence on the growth direction in hydrothermal ZnO nanowires. SUMMARY In summary, we show that a critical concentration for a nucleation strongly depends on a crystal plane, which plays a critical role in the anisotropic growth of hydrothermal ZnO nanowires. We measure the crystal growth of each crystal plane when varying the concentration of Zn ionic species by using a regular array structure. Selective anisotropic growth on (0001) plane emerges within a certain concentration range. Above the

6 concentration range, a crystal growth on (101 0) plane tends to simultaneously occur. This strong concentration dependence on the crystal plane is understood in terms of a difference between (0001) plane and (101 0) plane at a critical concentration for a nucleation. Considering the universality of the present design concept, controlling a concentration during the nanowire growth process would impact not only hydrothermal ZnO nanowires but also any anisotropic 1D nanowire growths. ASSOCIATED CONTENT *S Supporting Information Low-magnification TEM image of ZnO nanowire, highmagnification TEM image of ZnO nanowire, selected area electron diffraction pattern, indicating the [0001] growth orientation, and EDS pattern of ZnO nanowire. Time series SEM images of morphology change of nanowires when varying a concentration ranged from 1 to 40 mm. Magnified data of the concentration dependence on the radius data, and ph value change as a function of nanowire growth time. This material is available free of charge via the Internet at AUTHOR INFORMATION Corresponding Author * yanagi32@sanken.osaka-u.ac.jp (T.Y.); kawai@sanken.osaka-u.ac.jp (T.K). Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This study was partially supported by a Grant-in-Aid for Scientific Research on Innovative Areas [ ] from the Ministry of Education, Culture Sports, Science and Technology (MEXT) of Japan. F.Z., B.X., G.M., and Y.H. were supported by NEXT. K.N. was supported by TEPCO Memorial Foundation. T.K. was supported by FIRST program. REFERENCES (1) Huang, M. H.; Mao, S.; Feick, H.; Yan, H. Q.; Wu, Y. Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. D. Science 2001, 292, (2) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. D. Nat. Mater. 2005, 4, (3) Qin, Y.; Wang, X.; Wang, Z. L. Nature 2008, 451, (4) Xu, S.; Wang, Z. L. Nano Res. 2011, 4, (5) Park, W. I.; Yi, G. C. Adv. Mater. 2004, 16, (6) Wei, Y.; Xu, C.; Xu, S.; Li, C.; Wu, W.; Wang, Z. L. Nano Lett. 2010, 10, (7) Yang, R.; Qin, Y.; Dai, L.; Wang, Z. L. Nat. Nanotechnol. 2009, 4, (8) Vayssieres, L. Adv. Mater. 2003, 15, (9) Greene, L. E.; Law, M.; Goldberger, J.; Kim, F.; Johnson, J. C.; Zhang, Y. F.; Saykally, R. J.; Yang, P. D. Angew. Chem., Int. Ed. 2003, 42, (10) Heo, Y. W.; Varadarajan, V.; Kaufman, M.; Kim, K.; Norton, D. P.; Ren, F.; Fleming, P. H. Appl. Phys. Lett. 2002, 81, (11) Yao, B. D.; Chan, Y. F.; Wang, N. Appl. Phys. Lett. 2002, 81, (12) Huang, M. H.; Wu, Y. Y.; Feick, H.; Tran, N.; Weber, E.; Yang, P. D. Adv. Mater. 2001, 13, (13) Nagashima, K.; Yanagida, T.; Tanaka, H.; Seki, S.; Saeki, A.; Tagawa, S.; Kawai, T. J. Am. Chem. Soc. 2008, 130, (14) Marcu, A.; Yanagida, T.; Nagashima, K.; Oka, K.; Tanaka, H.; Kawai, T. Appl. Phys. Lett. 2008, 92, (15) Oka, K.; Yanagida, T.; Nagashima, K.; Tanaka, H.; Kawai, T. J. Am. Chem. Soc. 2009, 131, (16) Klamchuen, A.; Yanagida, T.; Nagashima, K.; Seki, S.; Oka, K.; Taniguchi, M.; Kawai, T. Appl. Phys. Lett. 2009, 95, (17) Oka, K.; Yanagida, T.; Nagashima, K.; Tanaka, H.; Seki, S.; Honsho, Y.; Ishimaru, M.; Hirata, A.; Kawai, T. Appl. Phys. Lett. 2009, 95, (18) Nagashima, K.; Yanagida, T.; Oka, K.; Taniguchi, M.; Kawai, T.; Kim, J. S.; Park, B. H. Nano Lett. 2010, 10, (19) Oka, K.; Yanagida, T.; Nagashima, K.; Kawai, T.; Kim, J. S.; Park, B.H. J. Am. Chem. Soc. 2010, 132, (20) Nagashima, K.; Yanagida, T.; Oka, K.; Kanai, M.; Klamchuen, A.; Kim, J. S.; Park, B. H.; Kawai, T. Nano Lett. 2011, 11, (21) Nagashima, K.; Yanagida, T.; Tanaka, H.; Kawai, T. Appl. Phys. Lett. 2007, 90, (22) Yanagida, T.; Nagashima, K.; Tanaka, H.; Kawai, T. Appl. Phys. Lett. 2007, 91, (23) Nagashima, K.; Yanagida, T.; Oka, K.; Tanaka, H.; Kawai, T. Appl. Phys. Lett. 2008, 93, (24) Yanagida, T.; Marcu, A.; Matsui, H.; Nagashima, K.; Oka, K.; Yokota, K.; Taniguchi, M.; Kawai, T. J. Phys. Chem. C 2008, 112, (25) Klamchuen, A.; Yanagida, T.; Kanai, M.; Nagashima, K.; Oka, K.; Kawai, T.; Suzuki, M.; Hidaka, Y.; Kai, S. Appl. Phys. Lett. 2010, 97, (26) Nagashima, K.; Yanagida, T.; Tanaka, H.; Kawai, T. J. Appl. Phys. 2007, 101, (27) Zhuge, F.; Yanagida, T.; Nagashima, K.; Yoshida, H.; Kanai, M.; Xu, B.; Klamchuen, A.; Meng, G.; He, Y.; Rahong, S.; Li, X.; Suzuki, M.; Kai, S.; Takeda, S.; Kawai, T. J. Phys. Chem. C 2012, 116, (28) Zhang, S.; Shen, Y.; Fang, H.; Xu, S.; Song, J.; Wang, Z. L. J. Mater. Chem. 2010, 20, (29) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2003, 125, (30) Greene, L. E.; Yuhas, B. D.; Law, M.; Zitoun, D.; Yang, P. D. Inorg. Chem. 2006, 45, (31) Wu, W.; Hu, G.; Cui, S.; Zhou, Y.; Wu, H. Cryst. Growth Des. 2008, 8, (32) Joo, J.; Chow, B. Y.; Prakash, M.; Boyden, E. S.; Jacobson, J. M. Nat. Mater. 2011, 10, (33) Xu, S.; Adiga, N.; Ba, S.; Dasgupta, T.; Wu, C. F. J.; Wang, Z. L. ACS Nano 2009, 3, (34) Lockett, A. M.; Thomas, P. J.; O Brien, P. J. Phys. Chem. C 2012, 116, (35) Lincot, D. MRS Bull. 2010, 35, (36) Yamabi, S.; Imai, H. J. Mater. Chem. 2002, 12, (37) Xu, S.; Wei, Y.; Kirkham, M.; Liu, J.; Mai, W.; Davidovic, D.; Snyder, R. L.; Wang, Z. L. J. Am. Chem. Soc. 2008, 130, (38) Richardson, J. J.; Lange, F. F. Cryst. Growth Des. 2009, 9, (39) Demianets, L. N.; Kostomarov, D. V.; Kuz mina, I. P.; Pushko, S. V. Crystallogr. Rep. 2002, 47, S86 S98. (40) Coltrin, M. E.; Hsu, J. W. P.; Scymgeour, D. A.; Creighton, J. R.; Simmons, N. C.; Matzke, C. M. J. Cryst. Growth 2008, 310, (41) Wei, Y.; Wu, W.; Guo, R.; Yuan, D.; Das, S.; Wang, Z. L. Nano Lett. 2010, 10, (42) Xu, S.; Lao, C.; Weintraub, B.; Wang, Z. L. J. Mater. Res. 2008, 23, (43) Zhang, D.-B.; Wang, S.-J.; Cheng, K.; Dai, S.-X.; Hu, B.-B.; Han, X.; Shi, Q.; Du, Z.-L. ACS Appl. Mater. Interfaces 2012, 4, (44) Govender, K.; Boyle, D. S.; Kenway, P. B.; O Brien, P. J. Mater. Chem. 2004, 14, (45) Richardson, J. J.; Lange, F. F. Cryst. Growth Des. 2009, 9, (46) Anisimov, M. P. Usp. Khim. 2003, 72, (47) Glynn, P. D.; Reardon, E. J. Am. J. Sci. 1990, 290, (48) Mullin, J. W. Crystallization, 4th ed.; Butterworth Heinemann: Oxford, U.K., 2001; pp (49) Vayssieres, L.; Keis, K.; Lindquist, S. E.; Hagfeldt, A. J. Phys. Chem. B 2001, 105,

7 (50) De Yoreo, J. J.; Vekilov, P. G. Rev. Mineral. Geochem. 2003, 54, (51) Na, S. H.; Park, C. H. J. Korean Phys. Soc. 2009, 54, (52) Wander, A.; Schedin, F.; Steadman, P.; Norris, A.; McGrath, R.; Turner, T. S.; Thornton, G.; Harrison, N. M. Phys. Rev. Lett. 2001, 86, (53) Wander, A.; Harrison, N. M. Surf. Sci. 2000, 457, L342 L