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The electronic properties of suspended single wall carbon nanotubes
Carbon 42 (2004) 2649–2653 www.elsevier.com/locate/carbon
The electronic properties of suspended single wall carbon nanotubes A. Hassanien a b
a,*
, M. Tokumoto
a,b
Nanotechnology Research Institute, AIST, Tsukuba, Ibaraki 305-8568, Japan CREST, Japan Science and Technology Agency, Kawaguchi 332-0012, Japan Received 6 January 2004; accepted 7 June 2004 Available online 24 July 2004
Abstract We have performed STM measurements on suspended single wall carbon nanotubes at room temperature in order to probe their intrinsic electronic properties. Comparing with supported nanotubes, we find that substrate–nanotube interactions influence the electronic properties. On supported semiconducting nanotubes bands are shifted toward higher energies while they are more symmetric around the zero bias in the unsupported ones. For metallic tubes we observed that interference patterns of electron waves caused by defects and edges are more pronounced on suspended nanotubes than the supported nanotubes. Interestingly, the pattern do not decay on suspended nanotubes indicating that the coherence length is much longer than what was reported earlier on supported nanotubes. These results highlight the role of substrate–nanotubes interactions and shed some light on their influences on nanotube devices. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: A. Carbon nanotubes; C. Scanning tunneling microscopy; D. Defects
1. Introduction Owing to their unique robust structures, carbon nanotubes (CNTs) have displayed remarkable electronic, mechanical and thermal properties, which made them useful for future technological applications. For example; depending on their diameters and chiralities they can be metallic or semiconducting [1]. Moreover, they conduct high-current without dissipation and can survive sever strain. Many of these properties have not been utilized commercially into devices due to material complexities (tubes with different diameters and chiralities). For that it was necessary to carry out measurement on a single molecule level to explore their intrinsic properties. In this regard the high-resolution power of a scanning probe techniques is used to address a single
*
Corresponding author. Tel.: +81 29 861 5381; fax: +81 29 861 5400. E-mail address: [email protected] (A. Hassanien). 0008-6223/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2004.06.009
nanotube molecule where details about their physical and chemical properties could be explored. The early STM experiments on carbon nanotubes provided very valuable information on their structure and electronic properties [2–5]. In these experiments one needs to cast tubes over a metallic substrate ‘‘usually gold or HOPG’’ to allow electrons to tunnel to/from an STM tip. However, due to substrate interaction many of their properties could not be explored. A band asymmetry around the zero bias and a shift and broadening in van Hove singularities were attributed to a signature of nanotube–substrate interaction [2]. Another signature of substrate–nanotube interactions is the short-range modulation of electron waves close to nanotube edges [6] and defects [7,8]. Although transport experiments on SWNT and theoretical calculations verify that the coherence length is of order of micron, STM experiments deduce a short coherence length of less than 3 nm. (This is calculated from scattering on defects of infinite tubes [8].) Lefebvre et al. [9] have shown that a bright luminescence from suspended nanotubes was clearly observed which
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A. Hassanien, M. Tokumoto / Carbon 42 (2004) 2649–2653
they were not able to detect it on supported nanotubes. In this work we examine the role of substrate–nanotube interactions using room temperature STM experiments on suspended nanotubes. The results show clearly less band asymmetry on suspended nanotubes compared with supported ones. Moreover we have observed interference pattern that only visible on the suspended part and missing on the supported tubes. In addition, we did not see any damping of interference oscillation on suspended nanotubes indicating that the coherence length is much longer than what was deduced on supported tubes.
2. Experimental
3. Results and discussion Fig. 1 shows a true atomic resolution on supported SWCNT. The lattice structure is clearly seen where carbon atoms are forming hexagonal rings on a cylindrical surface; nanotube wall. The distance between any two ˚ . The dark vortices, the c–c bond length, is 1.4 ± 0.1 A areas are the center of carbon hexagons; their orientation with respect to the nanotube axis gives the chiral angle. In our case here the carbon hexagons form a spiral conformation with a chiral angle of 10 ± 1°. I/V STS spectra is shown in Fig. 2. The current is nearly zero up to a threshold of 0.5 V then increases gradually,
Fig. 1. Atomic resolution STM image in topographic modes. Dark areas are centers of the carbon hexagons which form a chiral angle of 10° with the tube axis.
30
20 15
3
dI/dV [nA/mV]
25
Tunneling Current I [nA]
The SWCNT of this study synthesized using laser ablation method at Rice University. The nanotubes were sonicated in dichloroethane for 20 min prior to being cast on atomically flat gold substrates. Before the substrates dried out, they were dipped briefly in isopropanol to clean residue of Cl2CH4. The samples were then blown dry with pure nitrogen and then loaded into STM setup. The gold substrate is specially fabricated to yield small a crystalline voids into (1 1 1) surface. The sizes of these voids vary between 10 and 100 nm which was suitable to suspend SWCNT as surface tension force is not dominating to collapse nanotubes into the voids. Our attempts to suspended nanotubes on e beam fabricated trenches of size 250 nm or more, were not successful due to surface tension forces. Atomically resolved topographic images for carbon nanotube were obtained by recording the tip height at constant current. Typical bias parameters are 500 mV and 300 pA for bias voltage and tunneling current respectively. The STS measurements were taken by interrupting the feedback loop and recording the current at different bias voltage. The images we present here have not been processed in any way, unless otherwise stated. First we present our results on supported nanotubes then we discuss results relevant to suspended nanotubes.
10 5
2
1
0 -1500 -1000 -500 0 500 1000 1500 Bias Voltage [mV]
0 -5 -10 -1500
-1000
-500
0
500
1000
1500
Bias Voltage [mV]
Fig. 2. I–V spectroscopy on nanotube. Inset is the dI/dV as a function of bias voltage which displays a semiconducting behavior. A large shift of the Fermi level toward band edge is clearly visible which is attributed to substrate–nanotube interactions.
which is typical behavior for a semiconducting SWCNT. Inset of Fig. 2 displays a gap of about 0.8 eV in the differential conductance, which is a measure of the density of states. The gap is not symmetrically positioned around the zero bias voltage. This has been attributed to doping effect from Au(1 1 1) substrate which tends to shift the Fermi energy toward the valence band of the SWCNT. The shift is quite dramatic that the Fermi level is aligned with the valence band edge. This means that, the DOS inside the band gap is zero and give another evidence that the nanotube is a semiconducting one. This is consistent with previous STM study at 5 K [2].
A. Hassanien, M. Tokumoto / Carbon 42 (2004) 2649–2653
To suspend SWCNT, it is necessary to fabricate trenches or voids, with the right size/depth ratio, into metallic substrate. Fig. 3 shows a tapping mode AFM topographic image of plane Au(1 1 1) substrate that was fabricated using two stages of thermal annealing process. Hexagonal voids with sharp edges within the Au(1 1 1) crystal are clearly visible. This type of gold crystal growth is quite unique and to our knowledge the first of its kind. The size varies between 10 and 100 nm in width and can be up to 25 nm in height. We have used this structure to our advantage in suspending nanotubes as surface tension forces are not dominating at this length scale to draw CNT to the substrate. The crystalline edges minimized the bending of nanotube which might have caused unwanted mechanical deformation. The method of fabricating voids into the Au(1 1 1) utilizes two thermal annealing steps. At first we use flame annealing in ambient conditions followed by thermal annealing 800–950 °C in UHV chamber. The size of the depth depends on the annealing time and temperature. Now we examine the structure and the electronic properties of nanotubes that are suspended over voids in the Au(1 1 1). In Fig. 4 we show a STM image of metallic suspended SWCNT. The tube is bridging a void without any bending at the edges. On a high-resolution image we see the atomic lattice of the SWCNT, superimposed on it much slower oscillation. These oscillation results from interference of excited electron waves within 0.05 eV energy window. The oscillations persist along the whole length of the suspended part and very sensi-
Fig. 3. Tapping mode AFM image of A(1 1 1) surface. Voids of different sizes and sharp edges are clearly visible. We use these structures to suspended nanotubes in order to allow STM/STS investigation.
2651
Fig. 4. (a) STM in topographic mode of suspended SWCNTs. (b) High-resolution image shows the lattice structure of the nanotube in addition to a much slower modulations. The line profile of (b) is shown in (c). Unlike interference of electron waves on supported SWCNT, the modulation observed here persist with no sign of damping along the suspended nanotube.
tive to the bias window. It originates form the constructive interference of electron waves with constant phase relationship. Increasing the bias window to 0.15 causes modulation to disappear as a result of phase randomization process. Fig. 5(a)–(c) show topographic images of suspended nanotube at different bias voltage; 50, 75 and 120 mV, respectively. The arrows point at their corresponding line profiles. A significant difference in phase and the period of the oscillation can be clearly seen. The reason is that the larger the bias window the more phase randomization occurs due to summing over more electron states that enter the bias window. In our previous STM study on supported tubes, we observe similar oscillation close to the nanotube ends [6], however there is a fundamental difference between this and oscillations on unsupported nanotubes. In the first type there is damping factor, which washes the interference pattern within 6 nm away from the nanotube edge while the second type we see no damping ‘‘on the suspended portion’’.
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A. Hassanien, M. Tokumoto / Carbon 42 (2004) 2649–2653
Fig. 5. High-resolution STM images of suspended SWCNT at different bias voltages. The arrows point at their corresponding line profile. Interference pattern of the charge density is superimposed on the atomic structure. The period and the phase of the oscillation are very sensitive to the bias window.
0.12 0.10
dI/dV [nA/mV]
Similar decay behavior of the interference pattern was observed at defects on SWCNT and was attributed to interaction effects that limit the lifetime of single electron states [7]. At a distance of P 5 nm away from defects, we did not observe any interference patterns on the supported part of the nanotubes. The decay behavior of these oscillations is a manifestation of interaction process that led to decoherence of electron waves. This could be due to electron–electron (e–e) interactions or electron–phonon (e–ph) interaction processes. Hertel and Moos [10] have studied the electron dynamics in SWCNT bucky paper and concluded that e–e interaction is the most dominant mechanism of the phase relaxation even at room temperature (if we consider only few milli electron volt above Fermi level). In principal, e–e interactions can be due to substrate/nanotube and/or intratube e–e interactions as their measurements were carried out on SWCNT ropes within the bucky paper. By suspending nanotubes, we minimize the e(nanotube)– e(substrate) interaction and the phase randomizing mechanism would be mainly due to e–e intratube interaction plus e–ph (at higher energies). Our observation of undamped oscillations on individual suspended SWCNT indicates that the electron decoherence process is relaxed as nanotube/substrate interaction is minimized. It also asserts the claim of Hertel and Moos that e–e interactions is the main mechanism of phase relaxation for states close to Fermi level even at room temperature. The fact that these oscillations exist at room temperature is quite remarkable. However, it is not unusual for them to occur as we consider interference of electron waves just few milli electron volt above the Fermi energy (50 meV). We have carried out STS studies on suspended SWCNT and found remarkable difference compared with the supported tubes. Results are shown in Fig. 6 where STS are measured at two different positions on the suspended nanotube (a) and (b) curves are taken 5 and 10 nm away from the edge, respectively. The Fermi level in (b) moves closer toward zero bias indicating less doping from substrate. The shift in the Fermi energy be-
0.08
(a)
0.06 0.04 0.02 0.00 -1000
(b)
-500
0
500
1000
Bais Voltage [mV]
Fig. 6. dI/dV curves at two different positions (a) and (b); 5 and 10 nm away from the substrate edge. As we move closer to the substrate the Fermi level gets closer to the valence band edge. This shift demonstrate clearly the effect of substrate interaction on electronic properties of SWCNT.
tween the two curves is 0.1 eV. This, indeed provide an evidence that e–e interactions between nanotube and substrate influence the electronic properties.
4. Conclusion We have carried out room temperature STM investigation on suspended nanotubes in order to elucidate the influence of substrate on the electronic properties. Highresolution STM images show interference of electron waves that are persistent on the suspended nanotubes. The oscillation period and phase are sensitive to the bias voltage. I–V spectroscopy on suspended SWCNT shows less shift of in Fermi energy toward higher energies. Further investigation at low temperature would be very helpful to examine the LDOS near Fermi energy in order to clarify other issues like Tomonaga–Luttinger liquid behavior and lifetime of electronic states especially near defects and edges.
A. Hassanien, M. Tokumoto / Carbon 42 (2004) 2649–2653
Acknowledgment This work is partially supported by NEDO as part of nanocarbon project.
[5]
[6]
References [1] McEuen PL. Single-wall carbon nanotubes. Phys World 2000; 13(6):31–6. [2] Wildoer JWG, Venema LC, Rinzler AG, Smalley RE, Dekker C. Electronic structure of atomically resolved carbon nanotubes. Nature 1998;391:59–62. [3] Odom TW, Huang J-L, Kim P, Lieber CM. Atomic structure and electronic properties of single-walled carbon nanotubes. Nature 1998;391:62–4. [4] Hassanien A, Tokumoto M, Kumazawa Y, Kataura H, Maniwa Y, Suzuki S, et al. Atomic structure and electronic properties of single-wall carbon nanotubes probed by scanning tunneling
[7]
[8]
[9]
[10]
2653
microscope at room temperature. Appl Phys Lett 1998;73: 3839–41. Hassanien A, Tokumoto M. Scanning tunneling microscopy and spectroscopy study of carbon nanotubes. Mol Mater 2000; 13(1–4):51–8. Hassanien A, Tokumoto M, Umek P, Mihailovic D, Mrzel A. Fermi electron wave packet interference images on carbon nanotubes at room temperature. Appl Lett Phys 2001;78:808–10. Hassanien A, Umek P, Vrbanic D, Venturini P, Mihailovic D. Electronic and structural properties of defects and edge states in hydrogen plasma purified single wall carbon nanotubes. In: AIP Conference Proceeding, vol. 633, 2002. p. 271–4. Ouyang M, Huang J-L, Lieber CM. One-dimensional energy dispersion of single-walled carbon nanotubes by resonant electron scattering. Phys Rev Lett 2002;88:066804–7. Lefebvre J, Homma Y, Finnie P. Bright band gap photoluminescence from unprocessed single-walled carbon nanotubes. Phys Rev Lett 2003;90:217401–4. Hertel T, Moos G. Influence of excited electron lifetimes on the electronic structure of carbon nanotubes. Chem Phys Lett 2000; 320:359–64.
The electronic properties of suspended single wall carbon nanotubes A. Hassanien a b
a,*
, M. Tokumoto
a,b
Nanotechnology Research Institute, AIST, Tsukuba, Ibaraki 305-8568, Japan CREST, Japan Science and Technology Agency, Kawaguchi 332-0012, Japan Received 6 January 2004; accepted 7 June 2004 Available online 24 July 2004
Abstract We have performed STM measurements on suspended single wall carbon nanotubes at room temperature in order to probe their intrinsic electronic properties. Comparing with supported nanotubes, we find that substrate–nanotube interactions influence the electronic properties. On supported semiconducting nanotubes bands are shifted toward higher energies while they are more symmetric around the zero bias in the unsupported ones. For metallic tubes we observed that interference patterns of electron waves caused by defects and edges are more pronounced on suspended nanotubes than the supported nanotubes. Interestingly, the pattern do not decay on suspended nanotubes indicating that the coherence length is much longer than what was reported earlier on supported nanotubes. These results highlight the role of substrate–nanotubes interactions and shed some light on their influences on nanotube devices. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: A. Carbon nanotubes; C. Scanning tunneling microscopy; D. Defects
1. Introduction Owing to their unique robust structures, carbon nanotubes (CNTs) have displayed remarkable electronic, mechanical and thermal properties, which made them useful for future technological applications. For example; depending on their diameters and chiralities they can be metallic or semiconducting [1]. Moreover, they conduct high-current without dissipation and can survive sever strain. Many of these properties have not been utilized commercially into devices due to material complexities (tubes with different diameters and chiralities). For that it was necessary to carry out measurement on a single molecule level to explore their intrinsic properties. In this regard the high-resolution power of a scanning probe techniques is used to address a single
*
Corresponding author. Tel.: +81 29 861 5381; fax: +81 29 861 5400. E-mail address: [email protected] (A. Hassanien). 0008-6223/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2004.06.009
nanotube molecule where details about their physical and chemical properties could be explored. The early STM experiments on carbon nanotubes provided very valuable information on their structure and electronic properties [2–5]. In these experiments one needs to cast tubes over a metallic substrate ‘‘usually gold or HOPG’’ to allow electrons to tunnel to/from an STM tip. However, due to substrate interaction many of their properties could not be explored. A band asymmetry around the zero bias and a shift and broadening in van Hove singularities were attributed to a signature of nanotube–substrate interaction [2]. Another signature of substrate–nanotube interactions is the short-range modulation of electron waves close to nanotube edges [6] and defects [7,8]. Although transport experiments on SWNT and theoretical calculations verify that the coherence length is of order of micron, STM experiments deduce a short coherence length of less than 3 nm. (This is calculated from scattering on defects of infinite tubes [8].) Lefebvre et al. [9] have shown that a bright luminescence from suspended nanotubes was clearly observed which
2650
A. Hassanien, M. Tokumoto / Carbon 42 (2004) 2649–2653
they were not able to detect it on supported nanotubes. In this work we examine the role of substrate–nanotube interactions using room temperature STM experiments on suspended nanotubes. The results show clearly less band asymmetry on suspended nanotubes compared with supported ones. Moreover we have observed interference pattern that only visible on the suspended part and missing on the supported tubes. In addition, we did not see any damping of interference oscillation on suspended nanotubes indicating that the coherence length is much longer than what was deduced on supported tubes.
2. Experimental
3. Results and discussion Fig. 1 shows a true atomic resolution on supported SWCNT. The lattice structure is clearly seen where carbon atoms are forming hexagonal rings on a cylindrical surface; nanotube wall. The distance between any two ˚ . The dark vortices, the c–c bond length, is 1.4 ± 0.1 A areas are the center of carbon hexagons; their orientation with respect to the nanotube axis gives the chiral angle. In our case here the carbon hexagons form a spiral conformation with a chiral angle of 10 ± 1°. I/V STS spectra is shown in Fig. 2. The current is nearly zero up to a threshold of 0.5 V then increases gradually,
Fig. 1. Atomic resolution STM image in topographic modes. Dark areas are centers of the carbon hexagons which form a chiral angle of 10° with the tube axis.
30
20 15
3
dI/dV [nA/mV]
25
Tunneling Current I [nA]
The SWCNT of this study synthesized using laser ablation method at Rice University. The nanotubes were sonicated in dichloroethane for 20 min prior to being cast on atomically flat gold substrates. Before the substrates dried out, they were dipped briefly in isopropanol to clean residue of Cl2CH4. The samples were then blown dry with pure nitrogen and then loaded into STM setup. The gold substrate is specially fabricated to yield small a crystalline voids into (1 1 1) surface. The sizes of these voids vary between 10 and 100 nm which was suitable to suspend SWCNT as surface tension force is not dominating to collapse nanotubes into the voids. Our attempts to suspended nanotubes on e beam fabricated trenches of size 250 nm or more, were not successful due to surface tension forces. Atomically resolved topographic images for carbon nanotube were obtained by recording the tip height at constant current. Typical bias parameters are 500 mV and 300 pA for bias voltage and tunneling current respectively. The STS measurements were taken by interrupting the feedback loop and recording the current at different bias voltage. The images we present here have not been processed in any way, unless otherwise stated. First we present our results on supported nanotubes then we discuss results relevant to suspended nanotubes.
10 5
2
1
0 -1500 -1000 -500 0 500 1000 1500 Bias Voltage [mV]
0 -5 -10 -1500
-1000
-500
0
500
1000
1500
Bias Voltage [mV]
Fig. 2. I–V spectroscopy on nanotube. Inset is the dI/dV as a function of bias voltage which displays a semiconducting behavior. A large shift of the Fermi level toward band edge is clearly visible which is attributed to substrate–nanotube interactions.
which is typical behavior for a semiconducting SWCNT. Inset of Fig. 2 displays a gap of about 0.8 eV in the differential conductance, which is a measure of the density of states. The gap is not symmetrically positioned around the zero bias voltage. This has been attributed to doping effect from Au(1 1 1) substrate which tends to shift the Fermi energy toward the valence band of the SWCNT. The shift is quite dramatic that the Fermi level is aligned with the valence band edge. This means that, the DOS inside the band gap is zero and give another evidence that the nanotube is a semiconducting one. This is consistent with previous STM study at 5 K [2].
A. Hassanien, M. Tokumoto / Carbon 42 (2004) 2649–2653
To suspend SWCNT, it is necessary to fabricate trenches or voids, with the right size/depth ratio, into metallic substrate. Fig. 3 shows a tapping mode AFM topographic image of plane Au(1 1 1) substrate that was fabricated using two stages of thermal annealing process. Hexagonal voids with sharp edges within the Au(1 1 1) crystal are clearly visible. This type of gold crystal growth is quite unique and to our knowledge the first of its kind. The size varies between 10 and 100 nm in width and can be up to 25 nm in height. We have used this structure to our advantage in suspending nanotubes as surface tension forces are not dominating at this length scale to draw CNT to the substrate. The crystalline edges minimized the bending of nanotube which might have caused unwanted mechanical deformation. The method of fabricating voids into the Au(1 1 1) utilizes two thermal annealing steps. At first we use flame annealing in ambient conditions followed by thermal annealing 800–950 °C in UHV chamber. The size of the depth depends on the annealing time and temperature. Now we examine the structure and the electronic properties of nanotubes that are suspended over voids in the Au(1 1 1). In Fig. 4 we show a STM image of metallic suspended SWCNT. The tube is bridging a void without any bending at the edges. On a high-resolution image we see the atomic lattice of the SWCNT, superimposed on it much slower oscillation. These oscillation results from interference of excited electron waves within 0.05 eV energy window. The oscillations persist along the whole length of the suspended part and very sensi-
Fig. 3. Tapping mode AFM image of A(1 1 1) surface. Voids of different sizes and sharp edges are clearly visible. We use these structures to suspended nanotubes in order to allow STM/STS investigation.
2651
Fig. 4. (a) STM in topographic mode of suspended SWCNTs. (b) High-resolution image shows the lattice structure of the nanotube in addition to a much slower modulations. The line profile of (b) is shown in (c). Unlike interference of electron waves on supported SWCNT, the modulation observed here persist with no sign of damping along the suspended nanotube.
tive to the bias window. It originates form the constructive interference of electron waves with constant phase relationship. Increasing the bias window to 0.15 causes modulation to disappear as a result of phase randomization process. Fig. 5(a)–(c) show topographic images of suspended nanotube at different bias voltage; 50, 75 and 120 mV, respectively. The arrows point at their corresponding line profiles. A significant difference in phase and the period of the oscillation can be clearly seen. The reason is that the larger the bias window the more phase randomization occurs due to summing over more electron states that enter the bias window. In our previous STM study on supported tubes, we observe similar oscillation close to the nanotube ends [6], however there is a fundamental difference between this and oscillations on unsupported nanotubes. In the first type there is damping factor, which washes the interference pattern within 6 nm away from the nanotube edge while the second type we see no damping ‘‘on the suspended portion’’.
2652
A. Hassanien, M. Tokumoto / Carbon 42 (2004) 2649–2653
Fig. 5. High-resolution STM images of suspended SWCNT at different bias voltages. The arrows point at their corresponding line profile. Interference pattern of the charge density is superimposed on the atomic structure. The period and the phase of the oscillation are very sensitive to the bias window.
0.12 0.10
dI/dV [nA/mV]
Similar decay behavior of the interference pattern was observed at defects on SWCNT and was attributed to interaction effects that limit the lifetime of single electron states [7]. At a distance of P 5 nm away from defects, we did not observe any interference patterns on the supported part of the nanotubes. The decay behavior of these oscillations is a manifestation of interaction process that led to decoherence of electron waves. This could be due to electron–electron (e–e) interactions or electron–phonon (e–ph) interaction processes. Hertel and Moos [10] have studied the electron dynamics in SWCNT bucky paper and concluded that e–e interaction is the most dominant mechanism of the phase relaxation even at room temperature (if we consider only few milli electron volt above Fermi level). In principal, e–e interactions can be due to substrate/nanotube and/or intratube e–e interactions as their measurements were carried out on SWCNT ropes within the bucky paper. By suspending nanotubes, we minimize the e(nanotube)– e(substrate) interaction and the phase randomizing mechanism would be mainly due to e–e intratube interaction plus e–ph (at higher energies). Our observation of undamped oscillations on individual suspended SWCNT indicates that the electron decoherence process is relaxed as nanotube/substrate interaction is minimized. It also asserts the claim of Hertel and Moos that e–e interactions is the main mechanism of phase relaxation for states close to Fermi level even at room temperature. The fact that these oscillations exist at room temperature is quite remarkable. However, it is not unusual for them to occur as we consider interference of electron waves just few milli electron volt above the Fermi energy (50 meV). We have carried out STS studies on suspended SWCNT and found remarkable difference compared with the supported tubes. Results are shown in Fig. 6 where STS are measured at two different positions on the suspended nanotube (a) and (b) curves are taken 5 and 10 nm away from the edge, respectively. The Fermi level in (b) moves closer toward zero bias indicating less doping from substrate. The shift in the Fermi energy be-
0.08
(a)
0.06 0.04 0.02 0.00 -1000
(b)
-500
0
500
1000
Bais Voltage [mV]
Fig. 6. dI/dV curves at two different positions (a) and (b); 5 and 10 nm away from the substrate edge. As we move closer to the substrate the Fermi level gets closer to the valence band edge. This shift demonstrate clearly the effect of substrate interaction on electronic properties of SWCNT.
tween the two curves is 0.1 eV. This, indeed provide an evidence that e–e interactions between nanotube and substrate influence the electronic properties.
4. Conclusion We have carried out room temperature STM investigation on suspended nanotubes in order to elucidate the influence of substrate on the electronic properties. Highresolution STM images show interference of electron waves that are persistent on the suspended nanotubes. The oscillation period and phase are sensitive to the bias voltage. I–V spectroscopy on suspended SWCNT shows less shift of in Fermi energy toward higher energies. Further investigation at low temperature would be very helpful to examine the LDOS near Fermi energy in order to clarify other issues like Tomonaga–Luttinger liquid behavior and lifetime of electronic states especially near defects and edges.
A. Hassanien, M. Tokumoto / Carbon 42 (2004) 2649–2653
Acknowledgment This work is partially supported by NEDO as part of nanocarbon project.
[5]
[6]
References [1] McEuen PL. Single-wall carbon nanotubes. Phys World 2000; 13(6):31–6. [2] Wildoer JWG, Venema LC, Rinzler AG, Smalley RE, Dekker C. Electronic structure of atomically resolved carbon nanotubes. Nature 1998;391:59–62. [3] Odom TW, Huang J-L, Kim P, Lieber CM. Atomic structure and electronic properties of single-walled carbon nanotubes. Nature 1998;391:62–4. [4] Hassanien A, Tokumoto M, Kumazawa Y, Kataura H, Maniwa Y, Suzuki S, et al. Atomic structure and electronic properties of single-wall carbon nanotubes probed by scanning tunneling
[7]
[8]
[9]
[10]
2653
microscope at room temperature. Appl Phys Lett 1998;73: 3839–41. Hassanien A, Tokumoto M. Scanning tunneling microscopy and spectroscopy study of carbon nanotubes. Mol Mater 2000; 13(1–4):51–8. Hassanien A, Tokumoto M, Umek P, Mihailovic D, Mrzel A. Fermi electron wave packet interference images on carbon nanotubes at room temperature. Appl Lett Phys 2001;78:808–10. Hassanien A, Umek P, Vrbanic D, Venturini P, Mihailovic D. Electronic and structural properties of defects and edge states in hydrogen plasma purified single wall carbon nanotubes. In: AIP Conference Proceeding, vol. 633, 2002. p. 271–4. Ouyang M, Huang J-L, Lieber CM. One-dimensional energy dispersion of single-walled carbon nanotubes by resonant electron scattering. Phys Rev Lett 2002;88:066804–7. Lefebvre J, Homma Y, Finnie P. Bright band gap photoluminescence from unprocessed single-walled carbon nanotubes. Phys Rev Lett 2003;90:217401–4. Hertel T, Moos G. Influence of excited electron lifetimes on the electronic structure of carbon nanotubes. Chem Phys Lett 2000; 320:359–64.