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STM observation of enhanced atomic migration by Ni tip on Au(1 1 1) surface
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 272–276 (2004) 1205–1206
STM observation of enhanced atomic migration by Ni tip on Au(1 1 1) surface Hironaga Uchida*, Jooyoung Kim, Takuto Yuasa, Kazuto Kashiwagi, Heejoon Kim, Kazuhiro Nishimura, Mitsuteru Inoue Department of Electrical and Electronic Engineering, Toyohashi University of Technology,1-1 Tempaku, Toyohashi, Aichi 441-8580, Japan
Abstract Enhanced atomic migration with a Ni tip on an Au(1 1 1) surface was investigated by using scanning tunneling microscope. By controlling a tunneling condition, we removed Au monolayer from the Au surface. Mechanism of large atomic migration could be due to atomic force between the Ni tip and the Au surface at close separation. The method of monolayer removal by the Ni tip can be applicable in the nanotechnology as a tool, plane for atoms. r 2003 Elsevier B.V. All rights reserved. PACS: 61.16.Ch; 68.35.Bs; 68.35.Fx Keywords: STM; Au(1 1 1) surface; Ni tip; Atomic migration; Monolayer removal; Decanethiol
1. Introduction Scanning tunneling microscope (STM), which is used to probe surface structures and electronic states, provides a unique means to modify a surface in nanometer scale. By using the STM on an Au(1 1 1) surface, deformation of reconstructed structure [1] and formation of fingerlike stripes [2–4] were reported. In this study, we investigated atomic migration enhanced by a Ni tip on the Au(1 1 1) surface. Removal of Au monolayer on the Au surface was achieved in the selected area.
2. Experiments An Au spherical sample with small (1 1 1) facets was made of an Au wire (99.95%) with a diameter of 0.3 mm by the hydrogen flame fusion method. Experiments were done with Ni and Au tips on the small (1 1 1) facet by *Corresponding author. Tel.: +81-532-44-6731; fax: +81532-44-6757. E-mail address: [email protected] (H. Uchida).
using an STM (PicoSPM, Molecular Imaging Corp.) in air at room temperature. To investigate tip–sample separation, we used decanethiol (C10H21SH) self-assembled monolayer (SAM) on the Au sample surface, which was prepared by immersing the Au sample into 1 mM ethyl alcohol solution of decanethiol at room temperature for 24 h.
3. Results and discussion In case of a Ni tip, enhanced large migration of Au atoms was observed on the Au(1 1 1) surface. Magnitude of atomic migration was controllable by the tunneling current and bias voltage. Fig. 1(a) shows an STM image (600 600 nm2) after scanning an area of 300 300 nm2 with a tunneling current of 0.05 nA. As a result, about half of Au atoms migrated from the scanned area; these Au atoms swept by the Ni tip were piled at right and left sides of the area. Fig. 1(b) shows the result of Ni tip scanning with a tunneling current of 0.3 nA. Increasing a tunneling current, the tip was brought close to a sample surface. As seen in Fig. 1(b), we removed Au atomic monolayer in the area of 300 300 nm2. When we
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.681
ARTICLE IN PRESS 1206
H. Uchida et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 1205–1206
Fig. 1. Migration of Au atoms on an Au(1 1 1) surface by a Ni tip. Tunneling conditions for atomic migration are as follows: Sweeping area: 300 300 nm2, image size: 600 600 nm2. Sample bias voltage Vs ¼ 20:1 V, (a) tunneling current It ¼ 0:05 nA, (b) It ¼ 0:3 nA.
increased a tunneling current more than 0.3 nA, several Au atomic layers were removed in the area. However, the large atomic migration occurred with probability about 20% of the Ni tips in our experiments. In case of an Au tip, no change of the Au(1 1 1) surface occurred during tip scanning. However, when we applied a voltage pulse between the Au tip and Au surface, small one-dimensional atomic migration was observed on the Au(1 1 1) surface; that is, fingerlike stripes grew from a terrace edge. This tip induced small atomic migration can be explained by the electric field effect [4]. Of course, the field effect is important in STM configuration even if with the Ni tip. Consequently, the interaction including the field effect with the Ni tip to the Au surface is much larger than that with the Au tip. In order to study the difference between the Ni and Au tips, we used decanethiol (C10H21SH) self-assembled monolayer (SAM) adsorbed on the Au(1 1 1) surface. Recently, we reported that several lengths of alkanethiol molecules were removed by tip contact with controlling the tunneling currents [5]. Because adsorbed molecules play a role as a tip-height indicator, we applied this molecular removal method to investigate the difference of tip heights on the Au surface by tip materials. In case of the Ni tip on the decanethiol SAM, the molecules were removed with a very small tunneling current of 0.001 nA and a sample bias voltage of –0.1 V as shown in Fig. 2(a). Therefore, in this tunneling condition, the Ni tip could penetrate inside the decanethiol SAM. To withdraw the Ni tip from inside the SAM film, a larger bias voltage of –0.6 V was required (Fig. 2(b)). In contrast, we were not able to remove the decanethiol molecules with the Au tip although a tunneling current was increased up to 8 nA. With a current of 9 nA, the decanethiol SAM and its Au substrate surface were seriously damaged by tip contact because of out of tipcontrol in STM feedback loop. Therefore, the tip-sample separation with the Ni tip should be smaller than that with the Au tip. So, the atomic force between Ni tip and Au sample can be stronger than that with Au tip. These
Fig. 2. Removal of molecules from decanethiol (C10H21SH) self-assembled monolayer on the Au(1 1 1) surface by the Ni tip. Image size: 500 500 nm2, It ¼ 0:001 nA.(a) Vs ¼ 0:1 V, (b) Vs ¼ 0:6 V.
different separations might be induced differences of decays of wave functions between Ni and Au tips. The Ni tip could provide a function of tool ‘‘plane’’ for atomic modification in nanotechnology. On the other hand, although the Ni tip surface may be covered by an oxide layer or water in air, the layer must be very thin because the Au surface was not destroyed during scanning, and separation between the Ni tip and Au surface was controllable. 4. Conclusions We observed the enhanced atomic migration with Ni tip on the Au(1 1 1) surface in air. By controlling the tunneling condition, we removed one Au atomic layer from the Au(1 1 1) surface. The Ni tip acts as a tool ‘‘plane’’ on the Au surface. The mechanism of enhanced atomic migration could be due to atomic force between the Ni tip and the Au surface at close separation. Acknowledgements We would like to thank Prof. O.A. Aktsipetrov, Dr. S. Watanabe and Dr. H. Kuramochi for valuable discussions. This work was supported in The 21st Century COE Program ‘‘Intelligent Human Sensing’’, from the ministry of Education, Culture, Sports, Science and Technology. References [1] Y. Hasegawa, Ph. Avouris, Science 258 (1992) 1763. [2] R. Emch, J. Nogami, M.M. Dovek, C.A. Lang, C.F. Quate, J. Appl. Phys. 65 (1989) 79. [3] Z. Wang, M. Moskovits, J. Appl. Phys. 71 (1992) 5401. [4] J. Kim, H. Uchida, K. Yoshida, H.J. Kim, K. Nishimura, M. Inoue, Jpn. J. Appl. Phys. 42 (2003) 3616. [5] J. Kim, H. Uchida, N. Honda, N. Hashizume, Y. Hashimoto, H.J. Kim, K. Nishimura, M. Inoue, Jpn. J. Appl. Phys. 42 (2003) 4770.
Journal of Magnetism and Magnetic Materials 272–276 (2004) 1205–1206
STM observation of enhanced atomic migration by Ni tip on Au(1 1 1) surface Hironaga Uchida*, Jooyoung Kim, Takuto Yuasa, Kazuto Kashiwagi, Heejoon Kim, Kazuhiro Nishimura, Mitsuteru Inoue Department of Electrical and Electronic Engineering, Toyohashi University of Technology,1-1 Tempaku, Toyohashi, Aichi 441-8580, Japan
Abstract Enhanced atomic migration with a Ni tip on an Au(1 1 1) surface was investigated by using scanning tunneling microscope. By controlling a tunneling condition, we removed Au monolayer from the Au surface. Mechanism of large atomic migration could be due to atomic force between the Ni tip and the Au surface at close separation. The method of monolayer removal by the Ni tip can be applicable in the nanotechnology as a tool, plane for atoms. r 2003 Elsevier B.V. All rights reserved. PACS: 61.16.Ch; 68.35.Bs; 68.35.Fx Keywords: STM; Au(1 1 1) surface; Ni tip; Atomic migration; Monolayer removal; Decanethiol
1. Introduction Scanning tunneling microscope (STM), which is used to probe surface structures and electronic states, provides a unique means to modify a surface in nanometer scale. By using the STM on an Au(1 1 1) surface, deformation of reconstructed structure [1] and formation of fingerlike stripes [2–4] were reported. In this study, we investigated atomic migration enhanced by a Ni tip on the Au(1 1 1) surface. Removal of Au monolayer on the Au surface was achieved in the selected area.
2. Experiments An Au spherical sample with small (1 1 1) facets was made of an Au wire (99.95%) with a diameter of 0.3 mm by the hydrogen flame fusion method. Experiments were done with Ni and Au tips on the small (1 1 1) facet by *Corresponding author. Tel.: +81-532-44-6731; fax: +81532-44-6757. E-mail address: [email protected] (H. Uchida).
using an STM (PicoSPM, Molecular Imaging Corp.) in air at room temperature. To investigate tip–sample separation, we used decanethiol (C10H21SH) self-assembled monolayer (SAM) on the Au sample surface, which was prepared by immersing the Au sample into 1 mM ethyl alcohol solution of decanethiol at room temperature for 24 h.
3. Results and discussion In case of a Ni tip, enhanced large migration of Au atoms was observed on the Au(1 1 1) surface. Magnitude of atomic migration was controllable by the tunneling current and bias voltage. Fig. 1(a) shows an STM image (600 600 nm2) after scanning an area of 300 300 nm2 with a tunneling current of 0.05 nA. As a result, about half of Au atoms migrated from the scanned area; these Au atoms swept by the Ni tip were piled at right and left sides of the area. Fig. 1(b) shows the result of Ni tip scanning with a tunneling current of 0.3 nA. Increasing a tunneling current, the tip was brought close to a sample surface. As seen in Fig. 1(b), we removed Au atomic monolayer in the area of 300 300 nm2. When we
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.681
ARTICLE IN PRESS 1206
H. Uchida et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 1205–1206
Fig. 1. Migration of Au atoms on an Au(1 1 1) surface by a Ni tip. Tunneling conditions for atomic migration are as follows: Sweeping area: 300 300 nm2, image size: 600 600 nm2. Sample bias voltage Vs ¼ 20:1 V, (a) tunneling current It ¼ 0:05 nA, (b) It ¼ 0:3 nA.
increased a tunneling current more than 0.3 nA, several Au atomic layers were removed in the area. However, the large atomic migration occurred with probability about 20% of the Ni tips in our experiments. In case of an Au tip, no change of the Au(1 1 1) surface occurred during tip scanning. However, when we applied a voltage pulse between the Au tip and Au surface, small one-dimensional atomic migration was observed on the Au(1 1 1) surface; that is, fingerlike stripes grew from a terrace edge. This tip induced small atomic migration can be explained by the electric field effect [4]. Of course, the field effect is important in STM configuration even if with the Ni tip. Consequently, the interaction including the field effect with the Ni tip to the Au surface is much larger than that with the Au tip. In order to study the difference between the Ni and Au tips, we used decanethiol (C10H21SH) self-assembled monolayer (SAM) adsorbed on the Au(1 1 1) surface. Recently, we reported that several lengths of alkanethiol molecules were removed by tip contact with controlling the tunneling currents [5]. Because adsorbed molecules play a role as a tip-height indicator, we applied this molecular removal method to investigate the difference of tip heights on the Au surface by tip materials. In case of the Ni tip on the decanethiol SAM, the molecules were removed with a very small tunneling current of 0.001 nA and a sample bias voltage of –0.1 V as shown in Fig. 2(a). Therefore, in this tunneling condition, the Ni tip could penetrate inside the decanethiol SAM. To withdraw the Ni tip from inside the SAM film, a larger bias voltage of –0.6 V was required (Fig. 2(b)). In contrast, we were not able to remove the decanethiol molecules with the Au tip although a tunneling current was increased up to 8 nA. With a current of 9 nA, the decanethiol SAM and its Au substrate surface were seriously damaged by tip contact because of out of tipcontrol in STM feedback loop. Therefore, the tip-sample separation with the Ni tip should be smaller than that with the Au tip. So, the atomic force between Ni tip and Au sample can be stronger than that with Au tip. These
Fig. 2. Removal of molecules from decanethiol (C10H21SH) self-assembled monolayer on the Au(1 1 1) surface by the Ni tip. Image size: 500 500 nm2, It ¼ 0:001 nA.(a) Vs ¼ 0:1 V, (b) Vs ¼ 0:6 V.
different separations might be induced differences of decays of wave functions between Ni and Au tips. The Ni tip could provide a function of tool ‘‘plane’’ for atomic modification in nanotechnology. On the other hand, although the Ni tip surface may be covered by an oxide layer or water in air, the layer must be very thin because the Au surface was not destroyed during scanning, and separation between the Ni tip and Au surface was controllable. 4. Conclusions We observed the enhanced atomic migration with Ni tip on the Au(1 1 1) surface in air. By controlling the tunneling condition, we removed one Au atomic layer from the Au(1 1 1) surface. The Ni tip acts as a tool ‘‘plane’’ on the Au surface. The mechanism of enhanced atomic migration could be due to atomic force between the Ni tip and the Au surface at close separation. Acknowledgements We would like to thank Prof. O.A. Aktsipetrov, Dr. S. Watanabe and Dr. H. Kuramochi for valuable discussions. This work was supported in The 21st Century COE Program ‘‘Intelligent Human Sensing’’, from the ministry of Education, Culture, Sports, Science and Technology. References [1] Y. Hasegawa, Ph. Avouris, Science 258 (1992) 1763. [2] R. Emch, J. Nogami, M.M. Dovek, C.A. Lang, C.F. Quate, J. Appl. Phys. 65 (1989) 79. [3] Z. Wang, M. Moskovits, J. Appl. Phys. 71 (1992) 5401. [4] J. Kim, H. Uchida, K. Yoshida, H.J. Kim, K. Nishimura, M. Inoue, Jpn. J. Appl. Phys. 42 (2003) 3616. [5] J. Kim, H. Uchida, N. Honda, N. Hashizume, Y. Hashimoto, H.J. Kim, K. Nishimura, M. Inoue, Jpn. J. Appl. Phys. 42 (2003) 4770.