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Nanometer-scale fabrication on graphite surfaces by scanning tunneling microscopy
Ultramicroscopy 42-44 (1992) 1443-1445 North-Holland
Nanometer-scale fabrication on graphite surfaces by scanning tunneling microscopy K. U e s u g i a n d T. Y a o Department of Electrical Engineering, Hiroshima University, Higashi-Hiroshima 724, Japan Received 12 August 1991
Holes and carbon dots on a nanometer scale are formed on highly oriented pyrolytic graphite (HOPG) surfaces by scanning tunneling microscopy (STM). A hole of 25 A in diameter and 7 ,~ in depth at the center is made by applying a voltage pulse ( + 10 V, 0.1 ms) on the graphite surface in air. A carbon dot of 8 A in diameter and 7 ,~ in height is deposited by applying a short voltage pulse ( + 4 V, 50 tzs) in a mixed gas of acetone and hydrogen at 20 Torr. The results indicate the possibility of nanostructure fabrication by an ~ngstr6m-size electron beam emitted from a STM tip presumably through reactions with ambient gas molecules.
1. Introduction Recently, the scanning tunneling microscope (STM) has been utilized not only to observe high-resolution images of surfaces, but also to develop nanometer-scale fabrication techniques. Nanometer-scale selective etching [1-4] and deposition [5,6] on graphite surfaces have been reported. Albrecht et al. [1] have demonstrated the fabrication of holes on a graphite surface by applying a short voltage pulse (3-8 V, 10-100/~s) to the sample in air or in the presence of water vapor. The holes have an average diameter of 40 A with an average resolvable spacing of 60 .~. Mizutani et al. [4] demonstrated the formation of a hole with monolayer depth by applying voltage pulses (3.5 V, 320 /xs, tip is positively biased) along the circumference of a circle in the presence of water vapor. The hole was 10 nm in diameter and 0.35 nm in depth. Foster et al. [5] demonstrated the attachment of molecules on a graphite surface by applying a voltage pulse, where the samples were surrounded by liquid di(2-ethylhexyl)phthalate. Removal of pinned molecules has also been demonstrated. The purpose of our experiment is to develop nanometer-scale material processing through reactions of the electron beam emitted from an
STM tip with controlled ambient gas molecules. This paper describes the formation of holes and the deposition of carbon dots on graphite surfaces by applying a voltage pulse in ambient gas. The tunneling electron beam emitted from an STM tip has high current density and low energy. Furthermore, the size of the electron beam is 4 ,~ [7]; therefore the fabrication of nanometer-scale structures could be achieved.
2. Experiment The instrument used in the present experiments is a two-chamber STM system operated in ultrahigh vacuum (UHV). Tips of platinum wire or platinum iridium (80:20) wire sharpened by mechanical grinding are used. Samples of highly oriented pyrolytic graphite ( H O P G ) are cleaved with an adhesive tape in air before being loaded into an STM. After the system is evacuated by a turbo molecular pump, the gases (air, a mixed gas of acetone and hydrogen, or hydrogen gas) are introduced into the system. During STM measurements, a voltage pulse is applied when the tip is just above the selected point. We have investigated the effects of controlled ambient gases on the modification of the graphite surfaces by ap-
0304-3991/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
1444
K. Uesugi, T. Yao / Nanometer-scale fabrication on graphite surfaces by STM
plying voltage pulses across the tunneling gap at the selected points. All STM images are obtained in a variable-current mode. The tip voltage is - 0 . 3 V and the average tunneling current is 0.9 nA.
3. Results and discussion
Firstly, we have investigated the formation of holes on a graphite surface. Fig. 1 shows a hole made on the graphite surface in air at 1 atm by applying a voltage pulse. The pulse height is - 10 V (tip is negatively biased) with a pulse width of 0.1 ms. The hole is ~ 25 ,~ in diameter and ~ 7 .& in depth at the center. About two layers of the graphite surface are removed. Holes were created by applying a voltage pulse in a range from - 5 to - 1 0 V. Simultaneously with the formation of a hole, the deposition of contamination is observed around the hole. On the other hand, in vacuum ( ~ 1 × 10 -s Torr), no formation of holes occurred. Thus, it is likely that carbon atoms below the tip are removed through the chemical reactions with oxygen or water vapor [4] in air as: C(graphite) + 0 2 ~ CO 2, C(graphite) + H ~ O ~ CO + H 2. The deposition of carbon has been achieved by applying voltage pulses across the tunneling gap
Fig. 1. STM image of a fabricated hole on a H O P G surface. The size of the image is 61 ×61 ,~2. The hole is ~ 2 5 A in diameter and ~ 7 A in depth. The pulse height is - 10 V and the pulse width is 0.1 ms.
in a mixed gas of acetone and hydrogen at 20 Torr. A carbon dot of 8 ,~ in diameter and ~ 7 in height is deposited by applying a short voltage pulse (fig. 2). The graphite lattice is clearly observed around the dot. The pulse height is - 4 V with a pulse width of 50 p,s. Similarly, the deposition of carbon dots has been performed by de-
Fig. 2. STM image of a carbon dot on a 18 × 18 ~2 surface area. The dot is 8 ,~ in diameter and ~ 7 ,~ in height. The pulse height is - 4 V and the pulse width is 50/.,s.
K. Uesugi, T. Yao / Nanometer-scale fabrication on graphite surfaces by STM
composing c o n t a m i n a t i o n carbon. Fig. 3 shows a carbon dot deposited by applying the voltage pulse ( - 3 V, 5 0 / x s ) in h y d r o g e n gas at 1 atm. T h e dot is 25 A in diameter and ~ 2 0 ,~ in height. A m i n i m u m feature size down to 10 ,A in diameter was created by applying a voltage pulse in h y d r o g e n gas. However, no deposition of carbon o c c u r r e d w h e n the pulse height was less than - 3 V. T h e threshold voltage is about 3 V, which coincides with the threshold voltage for the deposition of c o n t a m i n a t i o n c a r b o n on A u films [8]. Since we did not observe the deposition of carbon dots either in v a c u u m or in air, but in the presence of organic gas molecules, we rejected the possibility that the deposited dots are c o m p o s e d of platinum atoms which are emitted from the platinum tip used. It is tentatively concluded that the deposited dots are c o m p o s e d of carbon. A n electron b e a m from the tip would decompose a c e t o n e molecules to deposit carbon on the graphite surface t h r o u g h reactions as ( C H 3 ) 2 C O 2C + 3 H 2 + C O in a mixed gas of acetone and hydrogen. It is speculated that the carbon dots were also deposited presumably by decomposition of c o n t a m i n a t i o n ( C n H m) t h r o u g h the reaction a s C n H m ~ n C + m / 2 H 2 in hydrogen gas. These facts suggest the possibility of deposition of carbon atoms at selected points.
1445
4. Conclusion In conclusion, holes and carbon dots with n a n o m e t e r - s c a l e s t r u c t u r e s are f o r m e d on graphite surfaces by applying a voltage pulse across the tunneling gap of STM with the tip bias being negative in different ambient gases. T h e m i n i m u m hole realized is 25 A in diameter and 7 ,~ in depth in air. T h e m i n i m u m carbon dot realized is 8 ,~ in diameter and 7 ,~ in height in a mixed gas of acetone and hydrogen. T h e results indicate the possibility of nanostructure fabrication by STM.
References [1] T.R. Albrecht, M.M. Dovek, M.D. Kirk, C.A. Lang, C.F. Quate and D.P.E. Smith, Appl. Phys. Lett. 55 (1989) 1727. [2] K. Terashima, M. Kondoh and T. Yoshida, J. Vac. Sci. Technol. A 8 (1990) 581. [3] J.P. Rabe, S. Buchholz and A.M. Ritcey, J. Vac. Sci. Technol. A 8 (1990) 679. [4] W. Mizutani, J. Inukai and M. Ono, Jpn. J. Appl. Phys. 29 (1990) L815. [5] J.S. Foster, J.E. Frommer and P.C. Arnen, Nature 331 (1988) 324. [6] R.H. Bernhardt, G.C. McGonigal, R. Schneider and D.J. Thomson, J. Vac. Sci. Technol. A 8 (1990) 667. [7] B. Das and J. Mahanty, Phys. Rev. B 36 (1987) 898. [8] M. Baba and S. Matsui, Jpn. J. Appl. Phys. 29 (1990) 2854.
Fig. 3. STM image of a carbon dot on a 53 × 53 ,~2 surface area. The dot is 25 ,A in diameter and ~ 20 ,A in height. The pulse height is - 3 V and the pulse width is 50/.~s.
Nanometer-scale fabrication on graphite surfaces by scanning tunneling microscopy K. U e s u g i a n d T. Y a o Department of Electrical Engineering, Hiroshima University, Higashi-Hiroshima 724, Japan Received 12 August 1991
Holes and carbon dots on a nanometer scale are formed on highly oriented pyrolytic graphite (HOPG) surfaces by scanning tunneling microscopy (STM). A hole of 25 A in diameter and 7 ,~ in depth at the center is made by applying a voltage pulse ( + 10 V, 0.1 ms) on the graphite surface in air. A carbon dot of 8 A in diameter and 7 ,~ in height is deposited by applying a short voltage pulse ( + 4 V, 50 tzs) in a mixed gas of acetone and hydrogen at 20 Torr. The results indicate the possibility of nanostructure fabrication by an ~ngstr6m-size electron beam emitted from a STM tip presumably through reactions with ambient gas molecules.
1. Introduction Recently, the scanning tunneling microscope (STM) has been utilized not only to observe high-resolution images of surfaces, but also to develop nanometer-scale fabrication techniques. Nanometer-scale selective etching [1-4] and deposition [5,6] on graphite surfaces have been reported. Albrecht et al. [1] have demonstrated the fabrication of holes on a graphite surface by applying a short voltage pulse (3-8 V, 10-100/~s) to the sample in air or in the presence of water vapor. The holes have an average diameter of 40 A with an average resolvable spacing of 60 .~. Mizutani et al. [4] demonstrated the formation of a hole with monolayer depth by applying voltage pulses (3.5 V, 320 /xs, tip is positively biased) along the circumference of a circle in the presence of water vapor. The hole was 10 nm in diameter and 0.35 nm in depth. Foster et al. [5] demonstrated the attachment of molecules on a graphite surface by applying a voltage pulse, where the samples were surrounded by liquid di(2-ethylhexyl)phthalate. Removal of pinned molecules has also been demonstrated. The purpose of our experiment is to develop nanometer-scale material processing through reactions of the electron beam emitted from an
STM tip with controlled ambient gas molecules. This paper describes the formation of holes and the deposition of carbon dots on graphite surfaces by applying a voltage pulse in ambient gas. The tunneling electron beam emitted from an STM tip has high current density and low energy. Furthermore, the size of the electron beam is 4 ,~ [7]; therefore the fabrication of nanometer-scale structures could be achieved.
2. Experiment The instrument used in the present experiments is a two-chamber STM system operated in ultrahigh vacuum (UHV). Tips of platinum wire or platinum iridium (80:20) wire sharpened by mechanical grinding are used. Samples of highly oriented pyrolytic graphite ( H O P G ) are cleaved with an adhesive tape in air before being loaded into an STM. After the system is evacuated by a turbo molecular pump, the gases (air, a mixed gas of acetone and hydrogen, or hydrogen gas) are introduced into the system. During STM measurements, a voltage pulse is applied when the tip is just above the selected point. We have investigated the effects of controlled ambient gases on the modification of the graphite surfaces by ap-
0304-3991/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
1444
K. Uesugi, T. Yao / Nanometer-scale fabrication on graphite surfaces by STM
plying voltage pulses across the tunneling gap at the selected points. All STM images are obtained in a variable-current mode. The tip voltage is - 0 . 3 V and the average tunneling current is 0.9 nA.
3. Results and discussion
Firstly, we have investigated the formation of holes on a graphite surface. Fig. 1 shows a hole made on the graphite surface in air at 1 atm by applying a voltage pulse. The pulse height is - 10 V (tip is negatively biased) with a pulse width of 0.1 ms. The hole is ~ 25 ,~ in diameter and ~ 7 .& in depth at the center. About two layers of the graphite surface are removed. Holes were created by applying a voltage pulse in a range from - 5 to - 1 0 V. Simultaneously with the formation of a hole, the deposition of contamination is observed around the hole. On the other hand, in vacuum ( ~ 1 × 10 -s Torr), no formation of holes occurred. Thus, it is likely that carbon atoms below the tip are removed through the chemical reactions with oxygen or water vapor [4] in air as: C(graphite) + 0 2 ~ CO 2, C(graphite) + H ~ O ~ CO + H 2. The deposition of carbon has been achieved by applying voltage pulses across the tunneling gap
Fig. 1. STM image of a fabricated hole on a H O P G surface. The size of the image is 61 ×61 ,~2. The hole is ~ 2 5 A in diameter and ~ 7 A in depth. The pulse height is - 10 V and the pulse width is 0.1 ms.
in a mixed gas of acetone and hydrogen at 20 Torr. A carbon dot of 8 ,~ in diameter and ~ 7 in height is deposited by applying a short voltage pulse (fig. 2). The graphite lattice is clearly observed around the dot. The pulse height is - 4 V with a pulse width of 50 p,s. Similarly, the deposition of carbon dots has been performed by de-
Fig. 2. STM image of a carbon dot on a 18 × 18 ~2 surface area. The dot is 8 ,~ in diameter and ~ 7 ,~ in height. The pulse height is - 4 V and the pulse width is 50/.,s.
K. Uesugi, T. Yao / Nanometer-scale fabrication on graphite surfaces by STM
composing c o n t a m i n a t i o n carbon. Fig. 3 shows a carbon dot deposited by applying the voltage pulse ( - 3 V, 5 0 / x s ) in h y d r o g e n gas at 1 atm. T h e dot is 25 A in diameter and ~ 2 0 ,~ in height. A m i n i m u m feature size down to 10 ,A in diameter was created by applying a voltage pulse in h y d r o g e n gas. However, no deposition of carbon o c c u r r e d w h e n the pulse height was less than - 3 V. T h e threshold voltage is about 3 V, which coincides with the threshold voltage for the deposition of c o n t a m i n a t i o n c a r b o n on A u films [8]. Since we did not observe the deposition of carbon dots either in v a c u u m or in air, but in the presence of organic gas molecules, we rejected the possibility that the deposited dots are c o m p o s e d of platinum atoms which are emitted from the platinum tip used. It is tentatively concluded that the deposited dots are c o m p o s e d of carbon. A n electron b e a m from the tip would decompose a c e t o n e molecules to deposit carbon on the graphite surface t h r o u g h reactions as ( C H 3 ) 2 C O 2C + 3 H 2 + C O in a mixed gas of acetone and hydrogen. It is speculated that the carbon dots were also deposited presumably by decomposition of c o n t a m i n a t i o n ( C n H m) t h r o u g h the reaction a s C n H m ~ n C + m / 2 H 2 in hydrogen gas. These facts suggest the possibility of deposition of carbon atoms at selected points.
1445
4. Conclusion In conclusion, holes and carbon dots with n a n o m e t e r - s c a l e s t r u c t u r e s are f o r m e d on graphite surfaces by applying a voltage pulse across the tunneling gap of STM with the tip bias being negative in different ambient gases. T h e m i n i m u m hole realized is 25 A in diameter and 7 ,~ in depth in air. T h e m i n i m u m carbon dot realized is 8 ,~ in diameter and 7 ,~ in height in a mixed gas of acetone and hydrogen. T h e results indicate the possibility of nanostructure fabrication by STM.
References [1] T.R. Albrecht, M.M. Dovek, M.D. Kirk, C.A. Lang, C.F. Quate and D.P.E. Smith, Appl. Phys. Lett. 55 (1989) 1727. [2] K. Terashima, M. Kondoh and T. Yoshida, J. Vac. Sci. Technol. A 8 (1990) 581. [3] J.P. Rabe, S. Buchholz and A.M. Ritcey, J. Vac. Sci. Technol. A 8 (1990) 679. [4] W. Mizutani, J. Inukai and M. Ono, Jpn. J. Appl. Phys. 29 (1990) L815. [5] J.S. Foster, J.E. Frommer and P.C. Arnen, Nature 331 (1988) 324. [6] R.H. Bernhardt, G.C. McGonigal, R. Schneider and D.J. Thomson, J. Vac. Sci. Technol. A 8 (1990) 667. [7] B. Das and J. Mahanty, Phys. Rev. B 36 (1987) 898. [8] M. Baba and S. Matsui, Jpn. J. Appl. Phys. 29 (1990) 2854.
Fig. 3. STM image of a carbon dot on a 53 × 53 ,~2 surface area. The dot is 25 ,A in diameter and ~ 20 ,A in height. The pulse height is - 3 V and the pulse width is 50/.~s.