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Water adsorption on electron irradiated NaF(001) surface
Applied Surface Science 130–132 Ž1998. 598–601
Water adsorption on electron irradiated NaF ž001 / surface T. Yamada, K. Miura
)
Department of Physics, Aichi UniÕersity of Education, Hirosawa 1, Igaya-cho, Kariya 448, Japan Received 1 September 1997; accepted 6 January 1998
Abstract We report water adsorption on an electron irradiated NaFŽ001. surface. Although water adsorption on the non-electron irradiated surface continues a complicated feature with water islands over several hours, water adsorption on the electron irradiated surface has a fast formation in 1 h from a water island pattern to a uniform film. This is because the surface irradiated by electrons has surface OH-centers. It is possible, by the electron irradiation technique, to control the wettability of the NaFŽ001. surface. q 1998 Elsevier Science B.V. All rights reserved. PACS: 61.16Ch; 68.45.Da; 82.30.Nr Keywords: Water adsorption; NaFŽ001.; Electron irradiation; Wettability; Hydrogen bonded clusters; Scanning force microscope
1. Introduction In just few years the knowledge on water adsorption at ionic crystal surfaces in air has been rapidly accumulating due to recent techniques w1–4x. The technique using an optical second harmonic generation ŽSHG. has revealed the transient orientation of the adsorbed water of monolayer or submonolayer on the NaClŽ001. surface at room temperature in air w1x. The observation using a non-contact mode of scanning force microscopy ŽSFM. has directly showed the striking differences for water adsorption on alkali halide Ž001. surfaces w4x. These experimental results have concluded that the behaviors of water adsorption on alkali halide Ž001. surfaces are induced by the competition between H 2 O–H 2 O and H 2 O–substrate interactions. In this paper, we focus on the change of water adsorption on the NaFŽ001. )
Corresponding author. Tel.: q81-566-26-2345; fax: q81566-26-2345; e-mail: [email protected].
surface by the electron irradiation, and report that it is possible to control the wettability of the surface by the electron irradiation technique.
2. Experimental Specimens were obtained by irradiating several electrons with an energy of 30 eV per 4 = 4 nm2 area for single crystal cleaved in an argon atmosphere, and within a few minutes were transferred to the SFM apparatus that was also flooded by argon gas. The SFM apparatus is a SPI-3700 ŽSeiko Instruments.. The images from specimens were obtained in both the contact and non-contact monitoring modes in air with a low relative humidity of about 30% at room temperature ŽRT.. The tip mounted on the cantilever due to the non-contact monitoring mode is made of silicon and the cantilever has a spring constant of 1.8 Nrm. The tip mounted on the cantilever due to contact monitoring mode, the so-called
0169-4332r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 1 2 2 - 6
T. Yamada, K. Miurar Applied Surface Science 130–132 (1998) 598–601
599
Fig. 1. Topview images due to non-contact mode from a NaFŽ001. surface vs. elapsed time exposure to an atmosphere with a low relative humidity of about 30%. The specified times written under figures for the NaFŽ001. mean the elapsed time after exposure to an atmosphere. The enlarged versions of the left-handed image and the right-handed one shown by the insets are given below and the cross sections of the parts sandwiched by the arrows are illustrated as a height distribution.
atomic force microscope, AFM, is made of Si 3 N4 and the measurements were made by constant repulsive force mode with the load of 0.2 nN.
trated by the inset in the first image and its height distribution are given below. The mean height and the mean size of the water islands on the NaFŽ001. surface were respectively measured to be about 2 nm and about 60 nm w5x. 1 The water islands seem to
3. Results and discussion Shown in Fig. 1 are topview images due to noncontact mode from a NaFŽ001. surface vs. elapsed time after exposure to air. The image from the NaFŽ001. surface exhibits water island pattern but as discussed below, a complicated feature with water islands develops during continued exposure w4x. The enlarged version of the typical water islands illus-
1
Since a tip–sample interaction depends on the dielectric constants of both the tip and sample Žsee Ref. w6x., it is necessary to know the accurate knowledge on the dielectronic constants to obtain the correct heights of water islands or water films. We actually expect that the mean height of water islands would be very small because the dielectric constant of water Žthe relative dielectric constant: about 80. is much larger than that at the NaFŽ001. surface.
600
T. Yamada, K. Miurar Applied Surface Science 130–132 (1998) 598–601
proceed to a uniform film as time passes, but the growth speed is very slow, e.g., the last image of NaFŽ001. is taken at about 9 h after cleaving. As the time passes, the surface consists of the films complicatedly formed with different thickness. The enlarged version of the water films illustrated by the inset in the last enlarged image and its height distribution are given below. The height difference between the higher area and the lower one was measured to be about 0.5 nm. It should be noted that the edges between the higher areas and the lower ones often have square shape as illustrated in the last image. The edges are aligned with w110x crystallographic directions of a NaFŽ001. surface. This was checked by bringing the tip into contact with the substrate. This picture allows us to determine the epitaxial relation between the water film and the
substrate, e.g., it is supposed that their films consist of an ice-Ih bilayer and a monolayer. Shown in Fig. 2 are topview images due to noncontact mode from an electron irradiated Žei.NaFŽ001. surface vs. elapsed time after exposure to air. First we note the striking differences between the ei-NaFŽ001. and the NaFŽ001. surfaces for the adsorption pattern. Compared to the longer continuous changes Ž9 h. of the NaFŽ001. surface the eiNaFŽ001. has continuous changes over the shorter time Žabout 1 h.. The behavior of water islands on the ei-NaFŽ001. surface monotonously proceeds to a uniform film, where the uniform film formed seems not to change even if the relative humidity increases after the uniform film is formed. The enlarged version of the typical water islands illustrated by the inset and its height distribution are given below. The
Fig. 2. Topview images due to non-contact mode from an electron irradiated Žei.-NaFŽ001. surface vs. elapsed time exposure to an atmosphere with a low relative humidity of about 30%. The specified times written under figures for the ei-NaFŽ001. mean the elapsed time after exposure to an atmosphere. The enlarged version of the left-handed image shown by the inset is given below and the cross section of the part sandwiched by the arrows is illustrated as a height distribution.
T. Yamada, K. Miurar Applied Surface Science 130–132 (1998) 598–601
601
why the mean height Žabout 1 nm. of water islands on the ei-NaFŽ001. surface is lower than that Žabout 2 nm. on the NaFŽ001. surface is yet to be answered. One answer to this question may be that the true thickness of water islands, or property changes because its apparent height or image contrast is modified by the dielectric constant.
4. Conclusions
Fig. 3. The growth form of water adsorption on ionic crystal surfaces in air at room temperature ŽRT., which is already given in the paper w4x.
mean height of water islands on the ei-NaFŽ001. surface was measured to be about 1 nm. Moreover, after the completion of Fig. 2, the images due to the contact mode were taken, which still preserved their surface periodic lattice structures. The previous paper w4x showed that in those cases where a H 2 O–H 2 O interaction is stronger than a H 2 O–substrate interaction, hydrogen bonded clusters grow easily ŽLiFŽ001.-type., but in the opposite case, the uniform film with a monolayer or submonolayer is formed ŽNaClŽ001.-type.. The behavior of water adsorption on the NaFŽ001. surface lies halfway between two cases because the H 2 O–NaFŽ001. surface interaction is almost comparable with the H 2 O– H 2 O interaction. We show in Fig. 3 the model of the growth mechanism of water on ionic surfaces. The behavior of water adsorption at the ei-NaFŽ001. surface is noted to be similar to the LiFŽ001.-type. The origin for this growth mechanism is expected to be the existence of surface OH-centers because by the electron irradiation F centers are formed at alkali halide surfaces and the subsequent H 2 O exposure to the surface produces OH-centers resulting in H 2 O dissociation at F centers w7x. However, the problem
By electron irradiation, with a low energy and a low dose, it is possible to change the wettability of the surface keeping a periodic lattice structure. The behavior of water adsorption on an ei-NaFŽ001. surface changes for LiFŽ001.-type originating in surface OH-centers.
Acknowledgements This paper has been completed through the support of Research Fellowship of Japan Society for the Promotion of Science and through the support of a Grant-in-Aid Ž09640393. for Scientific Research from the Ministry of Education, Science and Culture.
References w1x P. Tepper, J.C. Zink, H. Schmeiz, B. Wassermann, J. Reif, E. Matthias, J. Vac. Sci. Technol. B 7 Ž1989. 1212. w2x J.C. Zink, J. Reif, E. Matthias, Phys. Rev. Lett. 68 Ž1992. 3595. w3x K. Miura, Phys. Rev. B 52 Ž1995. 7872. w4x K. Miura, K. Maeda, T. Yamada, Mater. Sci. Forum 239–241 Ž1997. 663. w5x J.N. Israelachiviri, Intermolecular and Surface Forces, Academic, London, 1991. w6x C. Girard, D. van Labeke, J.M. Vigoureux, Phys. Rev. B 40 Ž1989. 12133. w7x S. Folsch, M. Henzler, Surf.Sci. 247 Ž1991. 269. ¨
Water adsorption on electron irradiated NaF ž001 / surface T. Yamada, K. Miura
)
Department of Physics, Aichi UniÕersity of Education, Hirosawa 1, Igaya-cho, Kariya 448, Japan Received 1 September 1997; accepted 6 January 1998
Abstract We report water adsorption on an electron irradiated NaFŽ001. surface. Although water adsorption on the non-electron irradiated surface continues a complicated feature with water islands over several hours, water adsorption on the electron irradiated surface has a fast formation in 1 h from a water island pattern to a uniform film. This is because the surface irradiated by electrons has surface OH-centers. It is possible, by the electron irradiation technique, to control the wettability of the NaFŽ001. surface. q 1998 Elsevier Science B.V. All rights reserved. PACS: 61.16Ch; 68.45.Da; 82.30.Nr Keywords: Water adsorption; NaFŽ001.; Electron irradiation; Wettability; Hydrogen bonded clusters; Scanning force microscope
1. Introduction In just few years the knowledge on water adsorption at ionic crystal surfaces in air has been rapidly accumulating due to recent techniques w1–4x. The technique using an optical second harmonic generation ŽSHG. has revealed the transient orientation of the adsorbed water of monolayer or submonolayer on the NaClŽ001. surface at room temperature in air w1x. The observation using a non-contact mode of scanning force microscopy ŽSFM. has directly showed the striking differences for water adsorption on alkali halide Ž001. surfaces w4x. These experimental results have concluded that the behaviors of water adsorption on alkali halide Ž001. surfaces are induced by the competition between H 2 O–H 2 O and H 2 O–substrate interactions. In this paper, we focus on the change of water adsorption on the NaFŽ001. )
Corresponding author. Tel.: q81-566-26-2345; fax: q81566-26-2345; e-mail: [email protected].
surface by the electron irradiation, and report that it is possible to control the wettability of the surface by the electron irradiation technique.
2. Experimental Specimens were obtained by irradiating several electrons with an energy of 30 eV per 4 = 4 nm2 area for single crystal cleaved in an argon atmosphere, and within a few minutes were transferred to the SFM apparatus that was also flooded by argon gas. The SFM apparatus is a SPI-3700 ŽSeiko Instruments.. The images from specimens were obtained in both the contact and non-contact monitoring modes in air with a low relative humidity of about 30% at room temperature ŽRT.. The tip mounted on the cantilever due to the non-contact monitoring mode is made of silicon and the cantilever has a spring constant of 1.8 Nrm. The tip mounted on the cantilever due to contact monitoring mode, the so-called
0169-4332r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 1 2 2 - 6
T. Yamada, K. Miurar Applied Surface Science 130–132 (1998) 598–601
599
Fig. 1. Topview images due to non-contact mode from a NaFŽ001. surface vs. elapsed time exposure to an atmosphere with a low relative humidity of about 30%. The specified times written under figures for the NaFŽ001. mean the elapsed time after exposure to an atmosphere. The enlarged versions of the left-handed image and the right-handed one shown by the insets are given below and the cross sections of the parts sandwiched by the arrows are illustrated as a height distribution.
atomic force microscope, AFM, is made of Si 3 N4 and the measurements were made by constant repulsive force mode with the load of 0.2 nN.
trated by the inset in the first image and its height distribution are given below. The mean height and the mean size of the water islands on the NaFŽ001. surface were respectively measured to be about 2 nm and about 60 nm w5x. 1 The water islands seem to
3. Results and discussion Shown in Fig. 1 are topview images due to noncontact mode from a NaFŽ001. surface vs. elapsed time after exposure to air. The image from the NaFŽ001. surface exhibits water island pattern but as discussed below, a complicated feature with water islands develops during continued exposure w4x. The enlarged version of the typical water islands illus-
1
Since a tip–sample interaction depends on the dielectric constants of both the tip and sample Žsee Ref. w6x., it is necessary to know the accurate knowledge on the dielectronic constants to obtain the correct heights of water islands or water films. We actually expect that the mean height of water islands would be very small because the dielectric constant of water Žthe relative dielectric constant: about 80. is much larger than that at the NaFŽ001. surface.
600
T. Yamada, K. Miurar Applied Surface Science 130–132 (1998) 598–601
proceed to a uniform film as time passes, but the growth speed is very slow, e.g., the last image of NaFŽ001. is taken at about 9 h after cleaving. As the time passes, the surface consists of the films complicatedly formed with different thickness. The enlarged version of the water films illustrated by the inset in the last enlarged image and its height distribution are given below. The height difference between the higher area and the lower one was measured to be about 0.5 nm. It should be noted that the edges between the higher areas and the lower ones often have square shape as illustrated in the last image. The edges are aligned with w110x crystallographic directions of a NaFŽ001. surface. This was checked by bringing the tip into contact with the substrate. This picture allows us to determine the epitaxial relation between the water film and the
substrate, e.g., it is supposed that their films consist of an ice-Ih bilayer and a monolayer. Shown in Fig. 2 are topview images due to noncontact mode from an electron irradiated Žei.NaFŽ001. surface vs. elapsed time after exposure to air. First we note the striking differences between the ei-NaFŽ001. and the NaFŽ001. surfaces for the adsorption pattern. Compared to the longer continuous changes Ž9 h. of the NaFŽ001. surface the eiNaFŽ001. has continuous changes over the shorter time Žabout 1 h.. The behavior of water islands on the ei-NaFŽ001. surface monotonously proceeds to a uniform film, where the uniform film formed seems not to change even if the relative humidity increases after the uniform film is formed. The enlarged version of the typical water islands illustrated by the inset and its height distribution are given below. The
Fig. 2. Topview images due to non-contact mode from an electron irradiated Žei.-NaFŽ001. surface vs. elapsed time exposure to an atmosphere with a low relative humidity of about 30%. The specified times written under figures for the ei-NaFŽ001. mean the elapsed time after exposure to an atmosphere. The enlarged version of the left-handed image shown by the inset is given below and the cross section of the part sandwiched by the arrows is illustrated as a height distribution.
T. Yamada, K. Miurar Applied Surface Science 130–132 (1998) 598–601
601
why the mean height Žabout 1 nm. of water islands on the ei-NaFŽ001. surface is lower than that Žabout 2 nm. on the NaFŽ001. surface is yet to be answered. One answer to this question may be that the true thickness of water islands, or property changes because its apparent height or image contrast is modified by the dielectric constant.
4. Conclusions
Fig. 3. The growth form of water adsorption on ionic crystal surfaces in air at room temperature ŽRT., which is already given in the paper w4x.
mean height of water islands on the ei-NaFŽ001. surface was measured to be about 1 nm. Moreover, after the completion of Fig. 2, the images due to the contact mode were taken, which still preserved their surface periodic lattice structures. The previous paper w4x showed that in those cases where a H 2 O–H 2 O interaction is stronger than a H 2 O–substrate interaction, hydrogen bonded clusters grow easily ŽLiFŽ001.-type., but in the opposite case, the uniform film with a monolayer or submonolayer is formed ŽNaClŽ001.-type.. The behavior of water adsorption on the NaFŽ001. surface lies halfway between two cases because the H 2 O–NaFŽ001. surface interaction is almost comparable with the H 2 O– H 2 O interaction. We show in Fig. 3 the model of the growth mechanism of water on ionic surfaces. The behavior of water adsorption at the ei-NaFŽ001. surface is noted to be similar to the LiFŽ001.-type. The origin for this growth mechanism is expected to be the existence of surface OH-centers because by the electron irradiation F centers are formed at alkali halide surfaces and the subsequent H 2 O exposure to the surface produces OH-centers resulting in H 2 O dissociation at F centers w7x. However, the problem
By electron irradiation, with a low energy and a low dose, it is possible to change the wettability of the surface keeping a periodic lattice structure. The behavior of water adsorption on an ei-NaFŽ001. surface changes for LiFŽ001.-type originating in surface OH-centers.
Acknowledgements This paper has been completed through the support of Research Fellowship of Japan Society for the Promotion of Science and through the support of a Grant-in-Aid Ž09640393. for Scientific Research from the Ministry of Education, Science and Culture.
References w1x P. Tepper, J.C. Zink, H. Schmeiz, B. Wassermann, J. Reif, E. Matthias, J. Vac. Sci. Technol. B 7 Ž1989. 1212. w2x J.C. Zink, J. Reif, E. Matthias, Phys. Rev. Lett. 68 Ž1992. 3595. w3x K. Miura, Phys. Rev. B 52 Ž1995. 7872. w4x K. Miura, K. Maeda, T. Yamada, Mater. Sci. Forum 239–241 Ž1997. 663. w5x J.N. Israelachiviri, Intermolecular and Surface Forces, Academic, London, 1991. w6x C. Girard, D. van Labeke, J.M. Vigoureux, Phys. Rev. B 40 Ž1989. 12133. w7x S. Folsch, M. Henzler, Surf.Sci. 247 Ž1991. 269. ¨