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Graphene-edge probes for scanning tunneling microscopy
G Model IJLEO-58437; No. of Pages 5
ARTICLE IN PRESS Optik xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Original research article
Graphene-edge probes for scanning tunneling microscopy Kevin K.W. Chu, Jeng Shiung Chen, Li-Der Chang, Jeff T.H. Tsai ∗ Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung 20224, Taiwan
a r t i c l e
i n f o
Article history: Received 13 September 2016 Accepted 7 November 2016 Keywords: Scanning tunneling microscope Graphene Field emission
a b s t r a c t Freestanding graphene-edge probes for scanning tunneling microscopy were demonstrated. Graphene was prepared on a Cu wire by thermal chemical vapor deposition (CVD) from solid carbon sources. Follow by a mechanical cutting process which was controlled by a micromanipulator and an optical microscope. The freestanding graphene probes were then fabricated. Our previous study of electron emission patterns from a field emission microscope demonstrated the layered structure of the graphene edge. We found that a single-layer of graphene emitted electrons comes from a limited number of atoms only when the graphene probe was conditioning carefully to achieve a stable emission current. In this research, we applied such activated graphene probes for use in scanning tunneling microscopes for surface morphology detection. The preconditioned, multi-layer graphene probe presented well resolution that was comparable to conventional Pt-Ir probes. Our study generated a practical method for applying individual freestanding graphene for surface probe microscopy with a cost effective process. © 2016 Elsevier GmbH. All rights reserved.
1. Introduction The scanning tunneling microscope (STM)—first invented by Binnig and Rohrer [1,2]—opened an experimental frontier for surveying material surface morphology and electronic structure at atomic resolution. The critical STM component making atomic resolution possible is the probe tip. Usually composed of W or a Pt–Ir alloy, the probe is attached to a piezo-drive, which consists of three conjointly vertical piezoelectric transducers oriented in three dimensions. By applying a bias voltage between the tip and the sample, a current is generated while electron tunneling occurs across the tiny gap between the sample and the tip. Variation of the tunneling current is monitored to represent the distance of the gap. Therefore, while scanning the STM tip across a certain area via piezoelectric transducers, the coordinate position versus the tunneling current can be converted into the surface morphology. The technique relies on the dimension of the probe tip. The sharpness of the probe tip dominates the lateral resolution of the STM [3]. Similarity, the field emission electron source also requires a sharp tip to enhance the local electric field, which can help to overcome the vacuum barrier and emit electrons into the vacuum. Several different types of nanomaterials have been investigated for use in field emission applications. Carbon nanotubes were first applied in the electron source and yielded intense brightness with the electron current density compared to conventional electron sources [4]. Graphene—low-dimensional carbon materials—also demonstrated excellent performance in electron field emission [5,6] owing to the strong field enhancement of their nanometer-scale emission sites [7]. More detailed analysis of electron emission from a few layers of individual freestanding graphene has been investigated to understand the electronic structure at the graphene edge [8]. Dong et al. studied the field emission from graphene, which was produced by
∗ Corresponding author. E-mail address: [email protected] (J.T.H. Tsai). http://dx.doi.org/10.1016/j.ijleo.2016.11.022 0030-4026/© 2016 Elsevier GmbH. All rights reserved.
Please cite this article in press as: K.K.W. Chu, et al., Graphene-edge probes for scanning tunneling microscopy, Optik Int. J. Light Electron Opt. (2016), http://dx.doi.org/10.1016/j.ijleo.2016.11.022
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Fig. 1. (a) Raman spectroscopy of 514 nm light source exam the as grown graphene from thermal CVD using camphor for carbon source. 2D signal appears at 2683 cm−1 with G band at 1582 cm−1 Inset: SEM picture of CVD graphene on Cu, scale bar 30 m. (b) Raman spectroscopy at the tip of the graphene probe, small D signal appear at 1354 cm−1 . Inset: OM picture of the tip, scale bar 100 m.
Fig. 2. SEM observation of the graphene probe body (left) and the tip (right) with graphene protrusions.
the liquid phase exfoliation of graphite. They determined that the graphene sheets were oriented at an angle with respect to the substrate surface leading to field emission at low threshold fields. Their process describes a method for the deposition of field emitting thin films [9]. Similar research has also shown that sharpness of the graphene edge is the key factor for enhancing the local field to yield better field emission properties compared to the flat surface of the graphene plane [5]. Therefore, to utilize graphene as a STM probe tip, it will be necessary to create a one-end-fixed free standing cantilever structure. Utilizing the edge of the graphene protrusion on the cantilever, the field enhancement of graphene can be realized from the high aspect ratio of the length/thickness of graphene. Our previous research on the field emission of single-layer graphene sheets has demonstrated that the one-atom-thick emission sites exhibit high current density with a low turn-on field [10]. Therefore, graphene as the STM probe tip has great potential. However, to produce such a graphene probe assembly requires a complicated fabrication process that involves installing individual graphene substrates onto the tip of a metal probe. In this paper, we demonstrated a new fabrication process to generate such freestanding graphene-edge probes. We examined the electron emissions from individual multilayer graphene edges, by observing the emission pattern of individual graphene probes. Our new technique involves the Please cite this article in press as: K.K.W. Chu, et al., Graphene-edge probes for scanning tunneling microscopy, Optik Int. J. Light Electron Opt. (2016), http://dx.doi.org/10.1016/j.ijleo.2016.11.022
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Fig. 3. HOPG surfaces measured using (a) Pt-Ir probe (b) Graphene probe and (c) Pure Cu probe at 5 nm scale.
direct growth of graphene on a Cu wire and mechanical cutting through the Cu wire to produce a freestanding 2D graphene edge while monitoring of the structure-building process with an optical microscope. When a low voltages applied on the graphene probe from the STM, it can produced a tunneling current of hundreds of pA. The pre-conditioned graphene probe applied in STM enhanced the resolution in the atomic regime. 2. Experimental A graphene-based micro-probe was fabricated via chemical vapor deposition (CVD) of graphene on Cu wires, which were 0.25 mm in diameter and 8 mm in length. Cu wires were pre-heated to 1050 ◦ C in H2 /Ar at 3 Torr for 3 h in a conventional vacuum furnace. Camphor (3 g, 99.6% purity)—the solid carbon source—was placed in an isolated chamber. The carbon source Please cite this article in press as: K.K.W. Chu, et al., Graphene-edge probes for scanning tunneling microscopy, Optik Int. J. Light Electron Opt. (2016), http://dx.doi.org/10.1016/j.ijleo.2016.11.022
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Fig. 4. The STM topography of (a) monolayer molecular chain structure and (b) magnify image shows arachidonic acid molecular chain image.
chamber was maintained at a constant temperature of 70 ◦ C and linked by a tube to the CVD system. After the camphor was evaporated by using an external heater up to 180 ◦ C, the carbon was drawn into the CVD system via the tube. The high density carbon was diffused onto the Cu wire surface, and subsequently precipitated into graphene. The growth process was optimized to generate graphene at 1050 ◦ C and allowed to cool to room temperature. Raman spectroscopy was used characterize the final product and identify the presence of graphene on Cu. High quality graphene was detected on the surface of the Cu wire. Graphene’s electronic structure is uniquely when exam via Raman spectrum. The number of layers of graphene is considered as the Raman fingerprints for single-, bilayer, and few-layer which changes the electronic structure and electron-phonon interactions to identification of graphene layers [11]. A strong I2D /IG ratio of 2.1 was determined by Raman spectroscopy (Fig. 1(a)) which indicated that only one to two layers of graphene were produced. Following mechanical cutting and chemical dip etching of the Cu, the free-standing graphene protruded from the Cu-based substrate. After the fabrication process, the probe tip was examined by Raman spectroscopy to ensure the existence of graphene (Fig. 1(b)). Although reduction in the I2D /IG ratio was observed after tip formation, an adequate value of 1.25 was obtained. The reduction in the I2D /IG ratio may be due to the folding of graphene on the freestanding section or defects occurring during the tip-forming process. Strong evidence is the appealing of ID . As we reported previously [8], the edge of graphene usually exhibits more defects, which enhance the D peak signal in the Raman spectrum. The protrusion of the graphene edge was only a few layers of atoms in thickness. Therefore, superior tunneling properties should be obtained from the graphene film edge. Scanning electron microscope (SEM) images (Fig. 2) revealed the existence of partial freestanding graphene protrusions. These sections of graphene presented with an irregular shape and thin film structure. They also appeared to have well defined straight edges. Thin, sharp edges are believed to aid electron tunneling. To ensure the tunneling resulted from the graphene instead of the Cu, we prepared a control Cu probe tip to observe the effect of oxidation on the Cu. The control Cu tip was subjected to the same fabrication process without graphene growth steps. The control Cu tip was placed in a humid environment for one week to form an oxidized surface. We tested the control samples at a high field of 15 V/m and observed that no field emission current was obtained.
Please cite this article in press as: K.K.W. Chu, et al., Graphene-edge probes for scanning tunneling microscopy, Optik Int. J. Light Electron Opt. (2016), http://dx.doi.org/10.1016/j.ijleo.2016.11.022
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3. Results and discussion Each graphene probe was tested in a field emission high vacuum chamber to record its emission pattern and estimate the maximum emission current. Rather than activated probes that can achieve an emission current up to 100 pA, we choose even higher current probes that can achieve up to 250 pA field emission current to applied in our STM (NanoSurf Easyscan2) system for this experiment. We then use such probes to observe highly oriented pyrolytic graphite (HOPG) surfaces, and have concluded that these probes have compatible resolution to a conventional Pt-Ir probe at room temperature, in air, at atomic resolution. The comparison images from Pt-Ir probe, Graphene Probe and Control Cu probe and are shown in Fig. 3(a)–(c) respectively. Clear graphite surfaces observed at atomic resolution from Figure (a) and (b) indicated the adequate resolution when using the conventional Pt-Ir probe vs. graphene probe. Whereas from Fig. 3(c), the Cu control probe presents no clear image under the same resolution scale. This result also proves the tunneling current mainly from graphene instead of Cu. The enhanced resolution of the STM results for the graphene probe was observed at room temperature in air. As an example, we tested the molecular of arachidonic acid. Such long chain molecular can be found in many food products. The graphene probe was successfully used to generate a well-defined image of the self-linkage molecules of arachidonic acid in air (Fig. 4) showing the graphene probes are sensitive to the carbon bonded structure whereas using Pt-Ir cannot revel any clear images in such task. 4. Conclusion Our experiments demonstrated that graphene films supported on Cu probes could be successfully used to produce graphene probe tips for STM. A freestanding edge was created on the probe using CVD of graphene with point etching and mechanical cutting. Compare to conventional Pt-Ir STM probe, the graphene probe is much cost effective. High current graphene probes were also acting as scanning probes for measuring molecular surface morphology in air instead of ultra-high vacuum environment. In the future, the graphene probe fabrication process described here can be scaled-up to commercial use for high resolution STM probe tips and other types of surface probing microscopy. Acknowledgements The authors gratefully acknowledge the Ministry of Science & Technology, Taiwan under the grants MOST 103-2112-M019-002-MY3. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
G. Binnig, H. Rohrer, C. Gerber, E. Weibel, Tunneling through a controllable vacuum gap, Appl. Phys. Lett. 40 (1982) 178–180. G. Binnig, H. Rohrer, C. Gerber, E. Weibel, Surface studies by scanning tunneling microscopy, Phys. Rev. Lett. 49 (1982) 57–61. R. Wiesendanger, Scanning Probe Microscopy and Spectroscopy: Methods and Applications, Cambridge University Press, 1994. N. de Jonge, Y. Lamy, K. Schoots, T.H. Oosterkamp, High brightness electron beam from a multi-walled carbon nanotube, Nature 420 (November) (2002) 393–395. Z.S. Wu, S.F. Pei, W.C. Ren, D.M. Tang, L.B. Gao, B.L. Liu, et al., Field emission of single-layer graphene films prepared by electrophoretic deposition, Adv. Mater. 21 (May) (2009) 1756–1760. G. Eda, H.E. Unalan, N. Rupesinghe, G.A.J. Amaratunga, M. Chhowalla, Field emission from graphene based composite thin films, Appl. Phys. Lett. 93 (2008) 233502. M. Qian, T. Feng, H. Ding, L.F. Lin, H.B. Li, Y.W. Chen, et al., Electron field emission from screen-printed graphene films, Nanotechnology 20 (2009) 425702. J.T.H. Tsai, T.Y.E. Chu, J.Y. Shiu, C.S. Yang, Field emission from an individual freestanding graphene edge, Small 8 (Dec 21 2012) 3739–3745. A. Malesevic, R. Kemps, A. Vanhulsel, M.P. Chowdhury, A. Volodin, C. Van Haesendonck, Field emission from vertically aligned few-layer graphene, J. Appl. Phys. 104 (2008) 084301. J.H. Dong, B.Q. Zeng, Y.C. Lan, S.K. Tian, Y. Shan, X.C. Liu, et al., Field emission from few-layer graphene nanosheets produced by liquid phase exfoliation of graphite, J. Nanosci. Nanotechnol. 10 (2010) 5051–5055. A.C. Ferrari, et al., Raman spectrum of graphene and graphene layers, Phys. Rev. Lett. 97 (2006) 187401.
Please cite this article in press as: K.K.W. Chu, et al., Graphene-edge probes for scanning tunneling microscopy, Optik Int. J. Light Electron Opt. (2016), http://dx.doi.org/10.1016/j.ijleo.2016.11.022
ARTICLE IN PRESS Optik xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Original research article
Graphene-edge probes for scanning tunneling microscopy Kevin K.W. Chu, Jeng Shiung Chen, Li-Der Chang, Jeff T.H. Tsai ∗ Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung 20224, Taiwan
a r t i c l e
i n f o
Article history: Received 13 September 2016 Accepted 7 November 2016 Keywords: Scanning tunneling microscope Graphene Field emission
a b s t r a c t Freestanding graphene-edge probes for scanning tunneling microscopy were demonstrated. Graphene was prepared on a Cu wire by thermal chemical vapor deposition (CVD) from solid carbon sources. Follow by a mechanical cutting process which was controlled by a micromanipulator and an optical microscope. The freestanding graphene probes were then fabricated. Our previous study of electron emission patterns from a field emission microscope demonstrated the layered structure of the graphene edge. We found that a single-layer of graphene emitted electrons comes from a limited number of atoms only when the graphene probe was conditioning carefully to achieve a stable emission current. In this research, we applied such activated graphene probes for use in scanning tunneling microscopes for surface morphology detection. The preconditioned, multi-layer graphene probe presented well resolution that was comparable to conventional Pt-Ir probes. Our study generated a practical method for applying individual freestanding graphene for surface probe microscopy with a cost effective process. © 2016 Elsevier GmbH. All rights reserved.
1. Introduction The scanning tunneling microscope (STM)—first invented by Binnig and Rohrer [1,2]—opened an experimental frontier for surveying material surface morphology and electronic structure at atomic resolution. The critical STM component making atomic resolution possible is the probe tip. Usually composed of W or a Pt–Ir alloy, the probe is attached to a piezo-drive, which consists of three conjointly vertical piezoelectric transducers oriented in three dimensions. By applying a bias voltage between the tip and the sample, a current is generated while electron tunneling occurs across the tiny gap between the sample and the tip. Variation of the tunneling current is monitored to represent the distance of the gap. Therefore, while scanning the STM tip across a certain area via piezoelectric transducers, the coordinate position versus the tunneling current can be converted into the surface morphology. The technique relies on the dimension of the probe tip. The sharpness of the probe tip dominates the lateral resolution of the STM [3]. Similarity, the field emission electron source also requires a sharp tip to enhance the local electric field, which can help to overcome the vacuum barrier and emit electrons into the vacuum. Several different types of nanomaterials have been investigated for use in field emission applications. Carbon nanotubes were first applied in the electron source and yielded intense brightness with the electron current density compared to conventional electron sources [4]. Graphene—low-dimensional carbon materials—also demonstrated excellent performance in electron field emission [5,6] owing to the strong field enhancement of their nanometer-scale emission sites [7]. More detailed analysis of electron emission from a few layers of individual freestanding graphene has been investigated to understand the electronic structure at the graphene edge [8]. Dong et al. studied the field emission from graphene, which was produced by
∗ Corresponding author. E-mail address: [email protected] (J.T.H. Tsai). http://dx.doi.org/10.1016/j.ijleo.2016.11.022 0030-4026/© 2016 Elsevier GmbH. All rights reserved.
Please cite this article in press as: K.K.W. Chu, et al., Graphene-edge probes for scanning tunneling microscopy, Optik Int. J. Light Electron Opt. (2016), http://dx.doi.org/10.1016/j.ijleo.2016.11.022
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Fig. 1. (a) Raman spectroscopy of 514 nm light source exam the as grown graphene from thermal CVD using camphor for carbon source. 2D signal appears at 2683 cm−1 with G band at 1582 cm−1 Inset: SEM picture of CVD graphene on Cu, scale bar 30 m. (b) Raman spectroscopy at the tip of the graphene probe, small D signal appear at 1354 cm−1 . Inset: OM picture of the tip, scale bar 100 m.
Fig. 2. SEM observation of the graphene probe body (left) and the tip (right) with graphene protrusions.
the liquid phase exfoliation of graphite. They determined that the graphene sheets were oriented at an angle with respect to the substrate surface leading to field emission at low threshold fields. Their process describes a method for the deposition of field emitting thin films [9]. Similar research has also shown that sharpness of the graphene edge is the key factor for enhancing the local field to yield better field emission properties compared to the flat surface of the graphene plane [5]. Therefore, to utilize graphene as a STM probe tip, it will be necessary to create a one-end-fixed free standing cantilever structure. Utilizing the edge of the graphene protrusion on the cantilever, the field enhancement of graphene can be realized from the high aspect ratio of the length/thickness of graphene. Our previous research on the field emission of single-layer graphene sheets has demonstrated that the one-atom-thick emission sites exhibit high current density with a low turn-on field [10]. Therefore, graphene as the STM probe tip has great potential. However, to produce such a graphene probe assembly requires a complicated fabrication process that involves installing individual graphene substrates onto the tip of a metal probe. In this paper, we demonstrated a new fabrication process to generate such freestanding graphene-edge probes. We examined the electron emissions from individual multilayer graphene edges, by observing the emission pattern of individual graphene probes. Our new technique involves the Please cite this article in press as: K.K.W. Chu, et al., Graphene-edge probes for scanning tunneling microscopy, Optik Int. J. Light Electron Opt. (2016), http://dx.doi.org/10.1016/j.ijleo.2016.11.022
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Fig. 3. HOPG surfaces measured using (a) Pt-Ir probe (b) Graphene probe and (c) Pure Cu probe at 5 nm scale.
direct growth of graphene on a Cu wire and mechanical cutting through the Cu wire to produce a freestanding 2D graphene edge while monitoring of the structure-building process with an optical microscope. When a low voltages applied on the graphene probe from the STM, it can produced a tunneling current of hundreds of pA. The pre-conditioned graphene probe applied in STM enhanced the resolution in the atomic regime. 2. Experimental A graphene-based micro-probe was fabricated via chemical vapor deposition (CVD) of graphene on Cu wires, which were 0.25 mm in diameter and 8 mm in length. Cu wires were pre-heated to 1050 ◦ C in H2 /Ar at 3 Torr for 3 h in a conventional vacuum furnace. Camphor (3 g, 99.6% purity)—the solid carbon source—was placed in an isolated chamber. The carbon source Please cite this article in press as: K.K.W. Chu, et al., Graphene-edge probes for scanning tunneling microscopy, Optik Int. J. Light Electron Opt. (2016), http://dx.doi.org/10.1016/j.ijleo.2016.11.022
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Fig. 4. The STM topography of (a) monolayer molecular chain structure and (b) magnify image shows arachidonic acid molecular chain image.
chamber was maintained at a constant temperature of 70 ◦ C and linked by a tube to the CVD system. After the camphor was evaporated by using an external heater up to 180 ◦ C, the carbon was drawn into the CVD system via the tube. The high density carbon was diffused onto the Cu wire surface, and subsequently precipitated into graphene. The growth process was optimized to generate graphene at 1050 ◦ C and allowed to cool to room temperature. Raman spectroscopy was used characterize the final product and identify the presence of graphene on Cu. High quality graphene was detected on the surface of the Cu wire. Graphene’s electronic structure is uniquely when exam via Raman spectrum. The number of layers of graphene is considered as the Raman fingerprints for single-, bilayer, and few-layer which changes the electronic structure and electron-phonon interactions to identification of graphene layers [11]. A strong I2D /IG ratio of 2.1 was determined by Raman spectroscopy (Fig. 1(a)) which indicated that only one to two layers of graphene were produced. Following mechanical cutting and chemical dip etching of the Cu, the free-standing graphene protruded from the Cu-based substrate. After the fabrication process, the probe tip was examined by Raman spectroscopy to ensure the existence of graphene (Fig. 1(b)). Although reduction in the I2D /IG ratio was observed after tip formation, an adequate value of 1.25 was obtained. The reduction in the I2D /IG ratio may be due to the folding of graphene on the freestanding section or defects occurring during the tip-forming process. Strong evidence is the appealing of ID . As we reported previously [8], the edge of graphene usually exhibits more defects, which enhance the D peak signal in the Raman spectrum. The protrusion of the graphene edge was only a few layers of atoms in thickness. Therefore, superior tunneling properties should be obtained from the graphene film edge. Scanning electron microscope (SEM) images (Fig. 2) revealed the existence of partial freestanding graphene protrusions. These sections of graphene presented with an irregular shape and thin film structure. They also appeared to have well defined straight edges. Thin, sharp edges are believed to aid electron tunneling. To ensure the tunneling resulted from the graphene instead of the Cu, we prepared a control Cu probe tip to observe the effect of oxidation on the Cu. The control Cu tip was subjected to the same fabrication process without graphene growth steps. The control Cu tip was placed in a humid environment for one week to form an oxidized surface. We tested the control samples at a high field of 15 V/m and observed that no field emission current was obtained.
Please cite this article in press as: K.K.W. Chu, et al., Graphene-edge probes for scanning tunneling microscopy, Optik Int. J. Light Electron Opt. (2016), http://dx.doi.org/10.1016/j.ijleo.2016.11.022
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3. Results and discussion Each graphene probe was tested in a field emission high vacuum chamber to record its emission pattern and estimate the maximum emission current. Rather than activated probes that can achieve an emission current up to 100 pA, we choose even higher current probes that can achieve up to 250 pA field emission current to applied in our STM (NanoSurf Easyscan2) system for this experiment. We then use such probes to observe highly oriented pyrolytic graphite (HOPG) surfaces, and have concluded that these probes have compatible resolution to a conventional Pt-Ir probe at room temperature, in air, at atomic resolution. The comparison images from Pt-Ir probe, Graphene Probe and Control Cu probe and are shown in Fig. 3(a)–(c) respectively. Clear graphite surfaces observed at atomic resolution from Figure (a) and (b) indicated the adequate resolution when using the conventional Pt-Ir probe vs. graphene probe. Whereas from Fig. 3(c), the Cu control probe presents no clear image under the same resolution scale. This result also proves the tunneling current mainly from graphene instead of Cu. The enhanced resolution of the STM results for the graphene probe was observed at room temperature in air. As an example, we tested the molecular of arachidonic acid. Such long chain molecular can be found in many food products. The graphene probe was successfully used to generate a well-defined image of the self-linkage molecules of arachidonic acid in air (Fig. 4) showing the graphene probes are sensitive to the carbon bonded structure whereas using Pt-Ir cannot revel any clear images in such task. 4. Conclusion Our experiments demonstrated that graphene films supported on Cu probes could be successfully used to produce graphene probe tips for STM. A freestanding edge was created on the probe using CVD of graphene with point etching and mechanical cutting. Compare to conventional Pt-Ir STM probe, the graphene probe is much cost effective. High current graphene probes were also acting as scanning probes for measuring molecular surface morphology in air instead of ultra-high vacuum environment. In the future, the graphene probe fabrication process described here can be scaled-up to commercial use for high resolution STM probe tips and other types of surface probing microscopy. Acknowledgements The authors gratefully acknowledge the Ministry of Science & Technology, Taiwan under the grants MOST 103-2112-M019-002-MY3. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
G. Binnig, H. Rohrer, C. Gerber, E. Weibel, Tunneling through a controllable vacuum gap, Appl. Phys. Lett. 40 (1982) 178–180. G. Binnig, H. Rohrer, C. Gerber, E. Weibel, Surface studies by scanning tunneling microscopy, Phys. Rev. Lett. 49 (1982) 57–61. R. Wiesendanger, Scanning Probe Microscopy and Spectroscopy: Methods and Applications, Cambridge University Press, 1994. N. de Jonge, Y. Lamy, K. Schoots, T.H. Oosterkamp, High brightness electron beam from a multi-walled carbon nanotube, Nature 420 (November) (2002) 393–395. Z.S. Wu, S.F. Pei, W.C. Ren, D.M. Tang, L.B. Gao, B.L. Liu, et al., Field emission of single-layer graphene films prepared by electrophoretic deposition, Adv. Mater. 21 (May) (2009) 1756–1760. G. Eda, H.E. Unalan, N. Rupesinghe, G.A.J. Amaratunga, M. Chhowalla, Field emission from graphene based composite thin films, Appl. Phys. Lett. 93 (2008) 233502. M. Qian, T. Feng, H. Ding, L.F. Lin, H.B. Li, Y.W. Chen, et al., Electron field emission from screen-printed graphene films, Nanotechnology 20 (2009) 425702. J.T.H. Tsai, T.Y.E. Chu, J.Y. Shiu, C.S. Yang, Field emission from an individual freestanding graphene edge, Small 8 (Dec 21 2012) 3739–3745. A. Malesevic, R. Kemps, A. Vanhulsel, M.P. Chowdhury, A. Volodin, C. Van Haesendonck, Field emission from vertically aligned few-layer graphene, J. Appl. Phys. 104 (2008) 084301. J.H. Dong, B.Q. Zeng, Y.C. Lan, S.K. Tian, Y. Shan, X.C. Liu, et al., Field emission from few-layer graphene nanosheets produced by liquid phase exfoliation of graphite, J. Nanosci. Nanotechnol. 10 (2010) 5051–5055. A.C. Ferrari, et al., Raman spectrum of graphene and graphene layers, Phys. Rev. Lett. 97 (2006) 187401.
Please cite this article in press as: K.K.W. Chu, et al., Graphene-edge probes for scanning tunneling microscopy, Optik Int. J. Light Electron Opt. (2016), http://dx.doi.org/10.1016/j.ijleo.2016.11.022