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Conductive atomic force microscopy on carbon nanowalls
Journal of Non-Crystalline Solids 358 (2012) 2545–2547
Contents lists available at SciVerse ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Conductive atomic force microscopy on carbon nanowalls A. Vetushka a,⁎, T. Itoh b, Y. Nakanishi b, A. Fejfar a, S. Nonomura c, M. Ledinský a, J. Kočka a a b c
Institute of Physics, Academy of Sciences of the Czech Republic v.v.i., Cukrovarnická 10, 162 00, Prague 6, Czech Republic Department of Electrical and Electronic Engineering, Gifu University, 1–1 Yanagido, Gifu 501–1193, Japan Environmental and Renewable Energy Systems Division, Gifu University, 1–1 Yanagido, Gifu 501–1193, Japan
a r t i c l e
i n f o
Article history: Received 21 August 2011 Received in revised form 20 December 2011 Available online 21 April 2012 Keywords: Carbon nanowalls; Conductive atomic force microscopy; Torsion resonance mode; Nanostructures
a b s t r a c t The nanostructure of carbon nanowalls (CNWs) was investigated by Torsion Resonance (TR) Atomic Force Microscopy (AFM). In this dynamic non-contact imaging mode a cantilever oscillates torsionally and the amplitude of oscillations is used for the AFM feedback. The technique includes benefits of the semicontact (tapping) mode and at the same time allows one to measure the local conductivity. Moreover, the phase signal is much stronger and fine structures of the CNWs were observed. We also present a comparison of the results obtained by TR, tapping, and contact modes. © 2012 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Carbon nanowalls (CNWs) are nanostructures composed of sheets (multilayers) of graphene — a zero-gap semiconductor material which has recently become a subject of intense interest. The extremely interesting electronic properties and high carrier mobility make this material a prospective candidate for the replacement of silicon in selected areas of future nanoelectronic and nanoelectromechanical devices [1,2]. At the same time, CNWs have attracted interest for electrochemical devices [3], electron field emitters [4] as well as for a variety of other possible applications [5]. All these applications would benefit from understanding of the local electronic properties of the CNWs. Conductive atomic force microscopy (C-AFM) is one of the main tools for studying the electrical properties of materials at nanoscale. The technique allows simultaneous measurements of topography and local conductivity [6]. The requirement of tip-sample electrical contact in C-AFM makes it necessary to use the contact mode. However, in the case of such a delicate material as CNWs the AFM tip may deform (or even destroy) the structures of samples. Non-contact mode operates at much lower forces, but here the main problem is the selection of a suitable measurement mode which would keep the tip in close proximity with the surface to allow the current tunneling between the tip and the sample. CNWs are nearly an ideal model system for the development of such a non-contact C-AFM, but there are many other delicate nanostructures, e.g. nanowires [7] or nanorods [8,9] for which the near noncontact C-AFM could be used.
In this letter we present our results of C-AFM measurement on CNWs which were performed in torsional resonance (TR) mode [10]. In this mode the AFM cantilever tip oscillates in a close proximity (~1–2 nm) of sample surface with amplitudes of a couple of nanometers or less. The samples were measured by Veeco Dimension 3100 AFM equipped with an extended TUNA (Tunneling AFM) module for the current detection in the pA range. Since the tip is so close to the sample, tunneling current can be detected by TUNA module (TRTUNA mode) even with an applied bias b100 mV. For the TR-TUNA mode we used the BudgetSensors ElectriMulti75-G cantilevers with a force constant of about 3 N/m, and resonant frequency of about 75 kHz. For the C-AFM measurements in contact mode we used the BudgetSensors ElectriCont cantilevers with a force constant of about 0.2 N/m and resonant frequency of about 13 kHz. The Cr/Pt coated Si cantilevers have tip radius of about 25 nm. The local current flowing through the grounded cantilever was induced by a dc voltage bias applied to the bottom of the c-Si substrate. The resonant frequency of torsional oscillations was around 400 kHz and the scanning speed was kept constant, mostly at 0.1 Hz. The TR-TUNA measurements were performed at 25 °C and approx. 30% humidity of the ambient air. The results of the AFM measurements were processed with WSxM software [11]. The scanning electron microscope (SEM) image was obtained by Tescan Mira 3 FEG SEM. CNW films were prepared by hot-wire chemical vapor deposition (HWCVD) [13]. Methane (CH4) was used as source gas and the CH4 pressure was 175 Pa. The coiled tungsten (W) filament was at a distance of 5 mm from the substrate holder. The filament temperature, Tf, was kept at about 2050 °C and was measured by a radiation thermometer. Crystalline silicon (c-Si) was used as substrates and the
⁎ Corresponding author. Tel.: + 420 220318467. E-mail address: [email protected] (A. Vetushka). 0022-3093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2011.12.094
2546
A. Vetushka et al. / Journal of Non-Crystalline Solids 358 (2012) 2545–2547
a
b
Fig. 3. C-AFM results measured in contact mode: a) Height signal, range 1.1 μm; b) Local current, range 100 pA. The bias voltage was + 50 mV. The images are fuzzy because of elastic properties of carbon nanowalls.
Fig. 1. Scanning electron microscopy image (1.6 × 1.6 μm) of carbon nanowalls sample.
orientation was (111). The substrate heating temperature was kept at 500 °C during the deposition. The substrates were treated by hydrogen radical using HWCVD technique before the deposition of CNWs. The treatment conditions were treatment period of 30 min, the Tf of 1750 °C, and H2 pressure of 133 Pa. The substrates were not heated during the hydrogen radical treatment.
very fuzzy image of topography and local current respectively. This result does not provide us with information of the local conductivity of CNWs. It only can be used to estimate that the area covered by nanowalls is conductive while the a-C initial layer is not. As opposed to the contact mode, the TR-TUNA mode is noncontact. The distance between the AFM tip and the sample surface as well as the interaction forces could be controlled by the feedback loop of a microscope. Our experience shows that the best quality images
a
3. Results We studied several samples of CNWs with different deposition times (different stages of growth). Fig. 1 is the SEM image and shows the representative morphology of a 15-min deposited sample. It has nanowalls with lateral dimensions about a few hundreds of nanometers. To obtain information about their heights (3D image) we need to use the AFM. AFM imaging of CNWs can be done for example in semicontact (tapping) mode. Fig. 2a and b demonstrates the topography and phase signals respectively of another sample with bigger and thicker CNWs. The representation of narrow nanowalls is affected by the convolution of sample and tip surfaces, therefore the imaging of only the top part of CNWs and a-C initial layer (or substrate) far enough from the CNWs can be considered as real. In addition, the structures are not solid and the phase signal is very noisy. This elasticity of the structures makes imaging of C-AFM in contact mode very difficult and almost unusable. Fig. 3a and b demonstrates
a
b
Fig. 2. Topography of carbon nanowalls measured by tapping mode: a) Height signal, range 1.1 μm; b) Phase signal, range 45 degrees.
b
c
Fig. 4. C-AFM result measured in non-contact torsional resonance mode: a) Height signal, range 400 nm; b) Phase signal, range 60 degrees; c) Local current map, range 10 pA. The sample bias voltage was + 100 mV with respect to the grounded AFM tip.
A. Vetushka et al. / Journal of Non-Crystalline Solids 358 (2012) 2545–2547
were measured at a soft set-point when minimum tip damage occurs. The minimal set-point prevents artifacts related to the conductive layer abrasion [12]. As in the case of common tapping mode, the TRTUNA allows one to measure the phase shift between excitation and response of the cantilever. Fig. 4 demonstrates the topography, phase shift and map of local currents.
4. Discussion The result of TR-TUNA measurement has many advantages over other modes. The height signal (see Fig. 4a) is qualitative as in the case of tapping mode (see Fig. 2a). At the same time, the phase signal (see Fig. 4b) is stronger than the tapping mode (see Fig. 2b). Particularly, it shows that the edges of CNWs consist of sections which are separated by something like boundaries. The change in phase signal could be caused by many factors like viscoelasticity, adhesion and also contact area. But we observed this “boundaries” on every sample of CNWs of different sizes and depositions at different stages of growth. It has to be noted that one can find similar structure in the results measured at tapping mode, although they are nearly masked by noise. Actually, we noticed these structures in the tapping mode phase image only after the observation at TR mode. In addition to the mechanical properties the TR-TUNA mode provided us with information on local conductivity. The comparison of Figs. 3b and 4c shows the different range of measured current at comparable bias voltages. The local currents of TRTUNA image are smaller because of the additional distance between the sample surface and the AFM tip which the electrons overcome by tunneling. In ambient conditions the gap between the tip and the sample may be filled by condensed water which is present on the surface because of air humidity. Furthermore, the value of the effective local barrier height for electron tunneling grows with the tip-surface distance.
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5. Conclusions We have used torsional resonance tunneling AFM (TR-TUNA) to measure the local conductivity of the carbon nanowalls, a self-organized nanostructured material with such a low stiffness that the use of normal contact mode conductive AFM is impossible. The strong phase signal of the technique has allowed us to observe fine structures of the nanowalls, which may reflect the properties of the nanowall edges. Surprisingly high conductivity on top of the nanowalls (~10 pA at only 100 mV applied bias) may be related to the in-plane conductivity of graphene sheets of which the walls are composed. Acknowledgements This research was supported by AV0Z 10100521, PolySiMode FP7 240826, LC06040, LC510, KAN400100701 and MEB 061012 projects. References [1] Z. Chen, Y.-M. Lin, M.J. Rooks, P. Avouris, Physica E 40 (2007) 228–232. [2] O. Frank, G. Tsoukleri, J. Parthenios, K. Papagelis, I. Riaz, R. Jalil, K.S. Novoselov, C. Galiotis, ACS Nano 4 (2010) 3131–3138. [3] G. Che, B.B. Lakshmi, E.R. Fisher, C.R. Martin, Nature 393 (1998) 346–349. [4] W. Zhu, C. Bower, G.P. Kochanski, S. Jin, Diam. Relat. Mater. 10 (2001) 1709–1713. [5] M. Hiramatsu, M. Hori, Carbon Nanowalls, Springer Vienna, Vienna, 2010. [6] R.A. Oliver, Rep. Prog. Phys. 71 (2008) 076501. [7] J. Červenka, M. Ledinský, J. Stuchlík, H. Stuchlíková, S. Bakardjieva, K. Hruška, A. Fejfar, J. Kočka, Nanotechnology 21 (2010) 415604. [8] M. Vanecek, O. Babchenko, A. Purkrt, J. Holovsky, N. Neykova, A. Poruba, Z. Remes, J. Meier, U. Kroll, Appl. Phys. Lett. 98 (2011) 163503. [9] C. Teichert, I. Beinik, in: B. Bhushan (Ed.), Scanning Probe Microscopy in Nanoscience and Nanotechnology 2, Springer Berlin Heidelberg, Berlin, Heidelberg, 2011, pp. 691–721. [10] L. Huang, C. Su, Ultramicroscopy 100 (2004) 277–285. [11] I. Horcas, R. Fernández, J.M. Gómez-Rodríguez, J. Colchero, J. Gómez-Herrero, A.M. Baro, Rev. Sci. Instrum. 78 (2007) 013705. [12] D. Cavalcoli, M. Rossi, A. Tomasi, A. Cavallini, Nanotechnology 20 (2009) 045702. [13] T. Itoh, Y. Nakanishi, T. Ito, A. Vetushka, M. Ledinsky, A. Fejfar, J. Kocka, S. Nonomura, J. Non-Cryst. Solids (2011), doi:10.1016/j.jnoncrysol.2012.01.062.
Contents lists available at SciVerse ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Conductive atomic force microscopy on carbon nanowalls A. Vetushka a,⁎, T. Itoh b, Y. Nakanishi b, A. Fejfar a, S. Nonomura c, M. Ledinský a, J. Kočka a a b c
Institute of Physics, Academy of Sciences of the Czech Republic v.v.i., Cukrovarnická 10, 162 00, Prague 6, Czech Republic Department of Electrical and Electronic Engineering, Gifu University, 1–1 Yanagido, Gifu 501–1193, Japan Environmental and Renewable Energy Systems Division, Gifu University, 1–1 Yanagido, Gifu 501–1193, Japan
a r t i c l e
i n f o
Article history: Received 21 August 2011 Received in revised form 20 December 2011 Available online 21 April 2012 Keywords: Carbon nanowalls; Conductive atomic force microscopy; Torsion resonance mode; Nanostructures
a b s t r a c t The nanostructure of carbon nanowalls (CNWs) was investigated by Torsion Resonance (TR) Atomic Force Microscopy (AFM). In this dynamic non-contact imaging mode a cantilever oscillates torsionally and the amplitude of oscillations is used for the AFM feedback. The technique includes benefits of the semicontact (tapping) mode and at the same time allows one to measure the local conductivity. Moreover, the phase signal is much stronger and fine structures of the CNWs were observed. We also present a comparison of the results obtained by TR, tapping, and contact modes. © 2012 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Carbon nanowalls (CNWs) are nanostructures composed of sheets (multilayers) of graphene — a zero-gap semiconductor material which has recently become a subject of intense interest. The extremely interesting electronic properties and high carrier mobility make this material a prospective candidate for the replacement of silicon in selected areas of future nanoelectronic and nanoelectromechanical devices [1,2]. At the same time, CNWs have attracted interest for electrochemical devices [3], electron field emitters [4] as well as for a variety of other possible applications [5]. All these applications would benefit from understanding of the local electronic properties of the CNWs. Conductive atomic force microscopy (C-AFM) is one of the main tools for studying the electrical properties of materials at nanoscale. The technique allows simultaneous measurements of topography and local conductivity [6]. The requirement of tip-sample electrical contact in C-AFM makes it necessary to use the contact mode. However, in the case of such a delicate material as CNWs the AFM tip may deform (or even destroy) the structures of samples. Non-contact mode operates at much lower forces, but here the main problem is the selection of a suitable measurement mode which would keep the tip in close proximity with the surface to allow the current tunneling between the tip and the sample. CNWs are nearly an ideal model system for the development of such a non-contact C-AFM, but there are many other delicate nanostructures, e.g. nanowires [7] or nanorods [8,9] for which the near noncontact C-AFM could be used.
In this letter we present our results of C-AFM measurement on CNWs which were performed in torsional resonance (TR) mode [10]. In this mode the AFM cantilever tip oscillates in a close proximity (~1–2 nm) of sample surface with amplitudes of a couple of nanometers or less. The samples were measured by Veeco Dimension 3100 AFM equipped with an extended TUNA (Tunneling AFM) module for the current detection in the pA range. Since the tip is so close to the sample, tunneling current can be detected by TUNA module (TRTUNA mode) even with an applied bias b100 mV. For the TR-TUNA mode we used the BudgetSensors ElectriMulti75-G cantilevers with a force constant of about 3 N/m, and resonant frequency of about 75 kHz. For the C-AFM measurements in contact mode we used the BudgetSensors ElectriCont cantilevers with a force constant of about 0.2 N/m and resonant frequency of about 13 kHz. The Cr/Pt coated Si cantilevers have tip radius of about 25 nm. The local current flowing through the grounded cantilever was induced by a dc voltage bias applied to the bottom of the c-Si substrate. The resonant frequency of torsional oscillations was around 400 kHz and the scanning speed was kept constant, mostly at 0.1 Hz. The TR-TUNA measurements were performed at 25 °C and approx. 30% humidity of the ambient air. The results of the AFM measurements were processed with WSxM software [11]. The scanning electron microscope (SEM) image was obtained by Tescan Mira 3 FEG SEM. CNW films were prepared by hot-wire chemical vapor deposition (HWCVD) [13]. Methane (CH4) was used as source gas and the CH4 pressure was 175 Pa. The coiled tungsten (W) filament was at a distance of 5 mm from the substrate holder. The filament temperature, Tf, was kept at about 2050 °C and was measured by a radiation thermometer. Crystalline silicon (c-Si) was used as substrates and the
⁎ Corresponding author. Tel.: + 420 220318467. E-mail address: [email protected] (A. Vetushka). 0022-3093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2011.12.094
2546
A. Vetushka et al. / Journal of Non-Crystalline Solids 358 (2012) 2545–2547
a
b
Fig. 3. C-AFM results measured in contact mode: a) Height signal, range 1.1 μm; b) Local current, range 100 pA. The bias voltage was + 50 mV. The images are fuzzy because of elastic properties of carbon nanowalls.
Fig. 1. Scanning electron microscopy image (1.6 × 1.6 μm) of carbon nanowalls sample.
orientation was (111). The substrate heating temperature was kept at 500 °C during the deposition. The substrates were treated by hydrogen radical using HWCVD technique before the deposition of CNWs. The treatment conditions were treatment period of 30 min, the Tf of 1750 °C, and H2 pressure of 133 Pa. The substrates were not heated during the hydrogen radical treatment.
very fuzzy image of topography and local current respectively. This result does not provide us with information of the local conductivity of CNWs. It only can be used to estimate that the area covered by nanowalls is conductive while the a-C initial layer is not. As opposed to the contact mode, the TR-TUNA mode is noncontact. The distance between the AFM tip and the sample surface as well as the interaction forces could be controlled by the feedback loop of a microscope. Our experience shows that the best quality images
a
3. Results We studied several samples of CNWs with different deposition times (different stages of growth). Fig. 1 is the SEM image and shows the representative morphology of a 15-min deposited sample. It has nanowalls with lateral dimensions about a few hundreds of nanometers. To obtain information about their heights (3D image) we need to use the AFM. AFM imaging of CNWs can be done for example in semicontact (tapping) mode. Fig. 2a and b demonstrates the topography and phase signals respectively of another sample with bigger and thicker CNWs. The representation of narrow nanowalls is affected by the convolution of sample and tip surfaces, therefore the imaging of only the top part of CNWs and a-C initial layer (or substrate) far enough from the CNWs can be considered as real. In addition, the structures are not solid and the phase signal is very noisy. This elasticity of the structures makes imaging of C-AFM in contact mode very difficult and almost unusable. Fig. 3a and b demonstrates
a
b
Fig. 2. Topography of carbon nanowalls measured by tapping mode: a) Height signal, range 1.1 μm; b) Phase signal, range 45 degrees.
b
c
Fig. 4. C-AFM result measured in non-contact torsional resonance mode: a) Height signal, range 400 nm; b) Phase signal, range 60 degrees; c) Local current map, range 10 pA. The sample bias voltage was + 100 mV with respect to the grounded AFM tip.
A. Vetushka et al. / Journal of Non-Crystalline Solids 358 (2012) 2545–2547
were measured at a soft set-point when minimum tip damage occurs. The minimal set-point prevents artifacts related to the conductive layer abrasion [12]. As in the case of common tapping mode, the TRTUNA allows one to measure the phase shift between excitation and response of the cantilever. Fig. 4 demonstrates the topography, phase shift and map of local currents.
4. Discussion The result of TR-TUNA measurement has many advantages over other modes. The height signal (see Fig. 4a) is qualitative as in the case of tapping mode (see Fig. 2a). At the same time, the phase signal (see Fig. 4b) is stronger than the tapping mode (see Fig. 2b). Particularly, it shows that the edges of CNWs consist of sections which are separated by something like boundaries. The change in phase signal could be caused by many factors like viscoelasticity, adhesion and also contact area. But we observed this “boundaries” on every sample of CNWs of different sizes and depositions at different stages of growth. It has to be noted that one can find similar structure in the results measured at tapping mode, although they are nearly masked by noise. Actually, we noticed these structures in the tapping mode phase image only after the observation at TR mode. In addition to the mechanical properties the TR-TUNA mode provided us with information on local conductivity. The comparison of Figs. 3b and 4c shows the different range of measured current at comparable bias voltages. The local currents of TRTUNA image are smaller because of the additional distance between the sample surface and the AFM tip which the electrons overcome by tunneling. In ambient conditions the gap between the tip and the sample may be filled by condensed water which is present on the surface because of air humidity. Furthermore, the value of the effective local barrier height for electron tunneling grows with the tip-surface distance.
2547
5. Conclusions We have used torsional resonance tunneling AFM (TR-TUNA) to measure the local conductivity of the carbon nanowalls, a self-organized nanostructured material with such a low stiffness that the use of normal contact mode conductive AFM is impossible. The strong phase signal of the technique has allowed us to observe fine structures of the nanowalls, which may reflect the properties of the nanowall edges. Surprisingly high conductivity on top of the nanowalls (~10 pA at only 100 mV applied bias) may be related to the in-plane conductivity of graphene sheets of which the walls are composed. Acknowledgements This research was supported by AV0Z 10100521, PolySiMode FP7 240826, LC06040, LC510, KAN400100701 and MEB 061012 projects. References [1] Z. Chen, Y.-M. Lin, M.J. Rooks, P. Avouris, Physica E 40 (2007) 228–232. [2] O. Frank, G. Tsoukleri, J. Parthenios, K. Papagelis, I. Riaz, R. Jalil, K.S. Novoselov, C. Galiotis, ACS Nano 4 (2010) 3131–3138. [3] G. Che, B.B. Lakshmi, E.R. Fisher, C.R. Martin, Nature 393 (1998) 346–349. [4] W. Zhu, C. Bower, G.P. Kochanski, S. Jin, Diam. Relat. Mater. 10 (2001) 1709–1713. [5] M. Hiramatsu, M. Hori, Carbon Nanowalls, Springer Vienna, Vienna, 2010. [6] R.A. Oliver, Rep. Prog. Phys. 71 (2008) 076501. [7] J. Červenka, M. Ledinský, J. Stuchlík, H. Stuchlíková, S. Bakardjieva, K. Hruška, A. Fejfar, J. Kočka, Nanotechnology 21 (2010) 415604. [8] M. Vanecek, O. Babchenko, A. Purkrt, J. Holovsky, N. Neykova, A. Poruba, Z. Remes, J. Meier, U. Kroll, Appl. Phys. Lett. 98 (2011) 163503. [9] C. Teichert, I. Beinik, in: B. Bhushan (Ed.), Scanning Probe Microscopy in Nanoscience and Nanotechnology 2, Springer Berlin Heidelberg, Berlin, Heidelberg, 2011, pp. 691–721. [10] L. Huang, C. Su, Ultramicroscopy 100 (2004) 277–285. [11] I. Horcas, R. Fernández, J.M. Gómez-Rodríguez, J. Colchero, J. Gómez-Herrero, A.M. Baro, Rev. Sci. Instrum. 78 (2007) 013705. [12] D. Cavalcoli, M. Rossi, A. Tomasi, A. Cavallini, Nanotechnology 20 (2009) 045702. [13] T. Itoh, Y. Nakanishi, T. Ito, A. Vetushka, M. Ledinsky, A. Fejfar, J. Kocka, S. Nonomura, J. Non-Cryst. Solids (2011), doi:10.1016/j.jnoncrysol.2012.01.062.