- Email: [email protected]
Single-wall carbon nanotube based devices
PERGAMON
Carbon 38 (2000) 1745–1749
Single-wall carbon nanotube based devices ´ J.F. Lynch, M. Llaguno, J. Lefebvre*, R.D. Antonov, M. Radosavljevic, A.T. Johnson Department of Physics and Astronomy and Laboratory for Research on the Structure of Matter, University of Pennsylvania, Philadelphia, PA 19104, USA
Abstract We have developed a variety of fabrication techniques for single-wall nanotube (SWNT) circuits. Our methods range from variants of electron beam lithography to AFM nanomanipulations. In this talk, we present our most recent data on three different types of SWNT based devices: the SWNT with a local impurity, the tube–tube junction and the SWNT contacted with electrodes whose separation is less than 30 nm. Each has a specific behavior ranging from a rectifying diode to a double quantum dot in series to an ultra short quantum wire. The functionality of each device can be ascribed to specific molecular adsorbates or controlled mechanical deformation. 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon nanotubes; C. Atomic force microscopy (AFM); D. Transport properties
1. Introduction Molecular electronics research is driven by continual miniaturization of electronic devices. Single-wall carbon nanotubes are especially good candidates for molecular device development because they can be either semiconducting or metallic depending entirely on the geometrical arrangement of carbon atoms [1]. Recently, nanotubebased devices such as low temperature quantum wires and single electron transistors (SETs) [2,3] as well as room temperature field effect transistors (FETs) [4,5] have been demonstrated. Preliminary studies also indicate that electronic properties of nanotubes are very sensitive to local distortions due to mechanical deformations and molecular adsorbates. Nanotube quantum wire, for example, shows irregular single electron charging spectra [2,3,6] which can be better understood as several quantum dots in series [7]. The origin of nanotube fragmentation into multiple dots is unclear but it has been suggested that local barriers are created as the nanotube bends over a nanofabricated electrode [8]. STM experiments also provide evidence that impurities adsorbed onto a SWNT can act as strong electron backscatterers [9]. Here we report data on electronics of three different SWNT devices perturbed by a *Corresponding author. Fax: 11-613-952-8701. E-mail address: [email protected] (J. Lefebvre).
combination of mechanical distortions and molecular impurities. First, we observe current rectification in a semiconducting nanotube due to an adsorbate located near the tube. Second, we show that, in the tube–tube junction, the top nanotube creates tunable tunnel barrier for transport along the bottom tube. Finally, we investigate SWNT devices contacted with closely (,30 nm) spaced electrodes and show that an electrode laying on top of a nanotube can create a tunnel barrier as well.
2. Experimental In order to study a variety of SWNT configurations we have developed a host of techniques to construct nanotube circuits. In a typical sample preparation, we disperse asgrown SWNT material [10] in dichloroethane using a low power sonication. The material is then deposited on the SiO 2 surface with [6] or without predefined nanofabricated electrodes. The nanotubes can then be manipulated into the desired position using tapping mode AFM [11]. Connection to the nanotube junctions is made using standard electron beam lithography with liftoff, while individual tubes are contacted using a novel lithography procedure designed to produce electrodes whose separation is less than 30 nm (nanogap) [12]. In all cases, we use degenerately doped Si underlying the insulating SiO 2 layer as a back gate.
0008-6223 / 00 / $ – see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 00 )00050-6
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J. Lefebvre et al. / Carbon 38 (2000) 1745 – 1749
In order to characterize SWNT samples we performed multi-terminal conductance measurements, commonly employed in quantum dot physics. Source–drain current (I) was recorded as a function of source–drain voltage (V ), tuning gate voltages (Vg ) and temperature.
3. Experimental results
3.1. SWNT diode Fig. 1(a) shows the AFM image of a sample prepared on predefined electrodes. The nanotube spans three electrodes with an impurity [13] laying near electrode A. The room temperature current–voltage characteristics (I–V ) across the two different segments of the tube show remarkably different behavior. On the clean segment (BC), the I–V curve (Fig. 1(b)) is symmetric around zero bias and has a gap of about 0.5 V. The conductance increases continuously with increasingly negative gate voltage, consistent with other measurements on semiconducting nanotube FETs [4,5]. The I–V characteristic of the tube segment with impurity (AB) is highly asymmetric (Fig. 1(b)). Current flows through the nanotube only when lead B is at a higher potential than lead A (forward bias), but it is strongly suppressed at reverse bias. The forward bias conductance of the nanotube device decreases with increasingly positive
gate voltage, which is the evidence of hole dominated transport in the diode.
3.2. SWNT junction In Fig. 2(a) we show an AFM image of two crossed SWNT bundles (also known as ‘ropes’) contacted at each end to gold leads patterned using electron beam lithography. The diameter of the bottom bundle is 2.5 nm, while that of the top one is 7.5 nm. At room temperature two-probe I–Vs of the bundles are linear with resistances of 330 and 50 kV respectively. The junction I–V curve has a 0.2 V gap at room temperature, and a high voltage differential resistance of 10 MV. At present, the origin of the gap is unclear. We can use the junction to study the effect of local gating on transport along the bottom bundle by applying a small (,0.2 V) voltage to the top bundle. Fig. 2(b) is a plot of current in the bottom bundle at 5 K as a function of gate voltage Vg 1 applied to the degenerate Si substrate and Vg 2 applied to the top bundle. Each curve shows long plateaus indicating that the low bias transport in the bottom bundle is mostly suppressed. Sharp current peaks with resistance on the order of 50 MV are observed in between the plateaus. These observations indicate single electron charging in the bottom bundle. Furthermore, Fig. 2(b) shows that a slight change in back gate voltage can have a dramatic effect on position and strength of Coulomb
Fig. 1. (a) AFM image of a single SWNT crossing three leads. An impurity is visible near lead A. (b) Room temperature I–V characteristics of the nanotube segment with impurity (AB) and the clean section of the tube (BC). Curves are offset for clarity.
J. Lefebvre et al. / Carbon 38 (2000) 1745 – 1749
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Fig. 2. (a) AFM image of a junction made up of two ropes of single wall nanotube. The top SWNT bundle is used to electrostatically gate the bottom bundle. (b) Coulomb blockade oscillations taken at 2.5 mV bias on the bottom bundle as a function of the voltage applied to the top nanotube rope. The curves are recorded at 23.065 V, 23.045 V, 23.025 V, 23.005 V and 22.985 V (from the top down) applied to the backgate and are offset for clarity.
blockade oscillations. As we will detail in the discussion, this behavior is typical of two quantum dots connected in series.
3.3. Transport on short segments of SWNTs The AFM image in Fig. 3(a) shows two gold electrodes separated by sub 30 nm gap, which we use to study short segments of SWNTs. Individual nanotubes spanning such nanogaps exhibit two types of behavior at low temperature. Single metallic nanotubes contacted inside the nanogap have room temperature resistance of approximately 1 MV, most of which is attributable to contact resistance. At low temperatures, the I–V curve has a gap of approximately 50 mV which can be tuned with electrostatic gating (see Fig. 3(b)). These observations are typically an indication of
single electron charging. More extensive characterization of this sample reveals Coulomb blockade diamonds of different sizes which are qualitatively similar to those reported in other nanotube SETs [2,3,6]. We extract the charging energy and energy level splitting from the largest diamonds to be 80 meV and 20 meV respectively. The second behavior is reminiscent of the field effect transistor behavior observed in semiconducting nanotubes [4,5]. In Fig. 3(c) we show a that our device has high bias resistance of 1 MV, hole dominated transport, gain in excess of 10,000 and bias dependent onset of conduction which are all typical of nanotube FETs. However, at 4.5 K and gate voltages between 25 V and 115 V current flow is suppressed for bias voltage of up to 1.5 V. Additionally, we do not observe low bias conduction at 4.5 K; a 0.1 V gap persists out to 220 V applied to the gate (not shown).
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J. Lefebvre et al. / Carbon 38 (2000) 1745 – 1749
Such model fully describes the observations on nanotube FETs and on the clean segment BC. We suggest that the impurity on segment AB localizes some charge on the nanotube, doping it n-type. In this way, the adsorbate further distorts the barrier in valence band. Since the impurity is located near contact A (see Fig. 1(a)) the spatial symmetry of the device is broken and so is the symmetry of the valence band. The larger barrier near contact A can be used to fully describe the strongly rectifying I–V characteristics. This experiment also constitutes the first known observation of one-dimensional (1D) electronic subbands [1,15,16] in transport along a SWNT [14].
4.2. SWNT junction
Fig. 3. (a) AFM image of two 250 nm wide gold leads separated by a nanogap; (b) I–V characteristics of a metallic nanotube at 4.5 K with different voltages applied to the gate. The tube exhibits single electron charging with large charging energy and energy level splitting; (c) Conductance versus gate voltage curves of a semiconducting tube taken at different biases and 4.5 K. Such SWNTs present field effect behavior similar to that of FETs.
4. Discussion
4.1. SWNT diode We interpret our measurements using a modified band bending model adapted from the model used to explain the action of nanotube FET [4]. The model assumes that, at the points of contact, the valence band of the nanotube is pinned to the Fermi energy of the leads. Such pinning is often ascribed to the work function mismatch of the metal and the nanotube. Away from the electrodes, the nanotube valence band bends down in order to lower its energy. The amount of bending, which presents a potential barrier for transport at low biases, can be tuned by the gate voltage.
Our nanotube junction results can be explained in terms of a well understood system of weakly coupled quantum dots in series [17]. In order for current to flow through the system both dots have to conduct, which happens only when the chemical potential of both dots is equal to that of the leads. Indeed, only when both dots are resonant with the leads do we see peaks in the current spectrum (not shown). However, as we change the substrate voltage Vg 1 we expect to adjust the coupling strength of the dots. In fact, as we tune the backgate (Vg 1 ) to increase carrier (in this case, hole [4]) density in the bottom bundle the barrier formed by the junction becomes better screened. Indeed, increased carrier concentration increases the tunneling strength between the dots. This relaxes the condition for current flow [18] and additional domains of current flow appear as smaller oscillations extending between larger resonant peaks (see Fig. 2(b)). In the limit of complete coupling, the isolated peaks would join into streaks in Vg 1 2Vg 2 plane indicating a single dot formation [17]. Our data in Fig. 2(b) indicates that the junction is responsible for the double dot formation in the SWNT bundle. To support this claim we compare measured and estimated charging energies of the two bundle segments divided by the junction. We can measure the larger of the two charging energies since the current flows only where both dots are able to conduct. The value of 20 to 30 meV is extracted from I–Vs with no voltage applied to the top SWNT rope. We also measure the lengths of the two segments to be 0.35 mm and 1.2 mm. The corresponding charging energies can be estimated from bundle-back gate capacitance (C) which is computed from the measured length (L), bundle radius (r) and bundle-back gate distance (h): U 5 e 2 /C 5 e 2 ln(2 h /r) / 2pe L [6]. The inferred energies of 20 meV and 6 meV, respectively are in approximate agreement with the measured value of 20–30 meV. Thus, we can identify the junction as the origin of the tunnel barrier dividing the bundle into two coupled quantum dots.
J. Lefebvre et al. / Carbon 38 (2000) 1745 – 1749
4.3. Transport on short segments of SWNTs Our data on SWNTs bridging nanogaps are qualitatively similar to results reported on nanotube SETs [2,3] and FETs [4,5], however there are a several differences worth pointing out here. In metallic samples, we observe charging energies of 80 meV and energy level splitting of 20 meV, which are an order of magnitude larger than any reported in transport along a nanotube. Such large energies correspond to a very short tube segment, as has been show by Venema et al. [19] in STM-STS measurements of cut tubes. This is somewhat surprising, since typical as-grown SWNT used in this experiment is about 0.5 mm in length. We therefore conclude that contact with electrodes breaks the nanotube into (at least) three segments: one underneath each electrode, and one in the nanogap. Complex charging spectra indicate that there is a finite transmission probability between the different nanotube segments. Indeed, the electrodes do not mechanically break the SWNT; instead they create barriers for electron propagation via mechanical distortions or electrostatic screening of the nanotube. Our semiconducting samples show I–V curves with extraordinarily large(.1.5 V) gap at positive gate voltage which narrows to approximately 0.1 V at negative gate voltage. The 0.1 V gap persists virtually unchanged over a 5 V range in gate voltage and is accompanied by steps suggestive of single electron charging. At the moment, the origin of these observations is not clear but we feel confident that they are due to the nanogap. Nanogaps could section the semiconducting SWNT into small islands of charge as shown above, or create large barriers due to stronger band bending than that observed in more traditional nanotube FETs.
5. Conclusions In summary, we have presented three classes of nanotube devices: a SWNT diode, a SWNT junction, and ultra short nanotube SETs and FETs. We have demonstrated a SWNT diode behavior due to local doping by chemical impurity. We also showed that nanotube bundle and a metal electrode laying on top of a SWNT induce a formation of an isolated quantum dot bound by the barriers. We have indicated that, in the case of SWNT junction, the barrier can be tuned away transforming a
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nanotube which consists of two quantum dots into a single quantum wire. We argue that the wealth of functionality described here can be identified with precisely tailored mechanical deformations and chemical impurities outlined above. In the future, predesigned distortions can be used to create multiple devices on a single SWNT.
Acknowledgements The SWNT material used in this work was provided by the Smalley group at Rice University. Support was provided by Fonds FCAR (Quebec) (J.L.), the U.S. NSF (grant number DMR98-02560), and the David and Lucile Packard Foundation (A.T.J.). We thank C. Kane and E. Mele for useful discussions.
References [1] For review, see Dresselhaus MS, Dresselhaus G, Eklund PC. Science of Fullerenes and Carbon Nanotubes. San Diego, CA: Academic, 1996. [2] Tans SJ et al. Nature 1997;386:474. [3] Bockrath M et al. Science 1997;275:1922. [4] Tans SJ, Verschueuren ARM, Dekker C. Nature 1998;393:49. [5] Martel R et al. Appl Phys Lett 1998;73:2447. [6] Antonov RD. PhD thesis. University of Pennsylvania, unpublished, 1999. [7] Wingreen N, Odintsov A. In: Proceeding of the March Meeting of the APS, 1999. [8] Bezryadin A et al. Phys Rev Lett 1998;80:4036. [9] Clauss W et al., to appear in Eur Phys Lett. [10] SWNT material is produced by pulsed laser vaporization as outlined in Thess A et al. Science 1996;273:482. [11] Lefebvre J et al., submitted to Appl Phys Lett. [12] Lefebvre J et al., to be published. [13] The composition of the impurity remains unknown. Asgrown nanotube material contains a significant impurity content, including metal catalyst, graphitic particles and amorphous carbons. [14] Antonov RD, Johnson AT, to appear in Phys Rev Lett. [15] For observation of electronic subbands in STM-STS studies see: Wildoer JW et al. Nature 1998;391:59. [16] For observation of electronic subbands in STM-STS studies see: Odom TW et al. Nature 1998;391:62. [17] Livermore C et al. Science 1996;274:1332. [18] Blick RH et al. Phys Rev Lett 1998;80:4032. [19] Venema LC et al. Science 1999;238:52.
Carbon 38 (2000) 1745–1749
Single-wall carbon nanotube based devices ´ J.F. Lynch, M. Llaguno, J. Lefebvre*, R.D. Antonov, M. Radosavljevic, A.T. Johnson Department of Physics and Astronomy and Laboratory for Research on the Structure of Matter, University of Pennsylvania, Philadelphia, PA 19104, USA
Abstract We have developed a variety of fabrication techniques for single-wall nanotube (SWNT) circuits. Our methods range from variants of electron beam lithography to AFM nanomanipulations. In this talk, we present our most recent data on three different types of SWNT based devices: the SWNT with a local impurity, the tube–tube junction and the SWNT contacted with electrodes whose separation is less than 30 nm. Each has a specific behavior ranging from a rectifying diode to a double quantum dot in series to an ultra short quantum wire. The functionality of each device can be ascribed to specific molecular adsorbates or controlled mechanical deformation. 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon nanotubes; C. Atomic force microscopy (AFM); D. Transport properties
1. Introduction Molecular electronics research is driven by continual miniaturization of electronic devices. Single-wall carbon nanotubes are especially good candidates for molecular device development because they can be either semiconducting or metallic depending entirely on the geometrical arrangement of carbon atoms [1]. Recently, nanotubebased devices such as low temperature quantum wires and single electron transistors (SETs) [2,3] as well as room temperature field effect transistors (FETs) [4,5] have been demonstrated. Preliminary studies also indicate that electronic properties of nanotubes are very sensitive to local distortions due to mechanical deformations and molecular adsorbates. Nanotube quantum wire, for example, shows irregular single electron charging spectra [2,3,6] which can be better understood as several quantum dots in series [7]. The origin of nanotube fragmentation into multiple dots is unclear but it has been suggested that local barriers are created as the nanotube bends over a nanofabricated electrode [8]. STM experiments also provide evidence that impurities adsorbed onto a SWNT can act as strong electron backscatterers [9]. Here we report data on electronics of three different SWNT devices perturbed by a *Corresponding author. Fax: 11-613-952-8701. E-mail address: [email protected] (J. Lefebvre).
combination of mechanical distortions and molecular impurities. First, we observe current rectification in a semiconducting nanotube due to an adsorbate located near the tube. Second, we show that, in the tube–tube junction, the top nanotube creates tunable tunnel barrier for transport along the bottom tube. Finally, we investigate SWNT devices contacted with closely (,30 nm) spaced electrodes and show that an electrode laying on top of a nanotube can create a tunnel barrier as well.
2. Experimental In order to study a variety of SWNT configurations we have developed a host of techniques to construct nanotube circuits. In a typical sample preparation, we disperse asgrown SWNT material [10] in dichloroethane using a low power sonication. The material is then deposited on the SiO 2 surface with [6] or without predefined nanofabricated electrodes. The nanotubes can then be manipulated into the desired position using tapping mode AFM [11]. Connection to the nanotube junctions is made using standard electron beam lithography with liftoff, while individual tubes are contacted using a novel lithography procedure designed to produce electrodes whose separation is less than 30 nm (nanogap) [12]. In all cases, we use degenerately doped Si underlying the insulating SiO 2 layer as a back gate.
0008-6223 / 00 / $ – see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 00 )00050-6
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J. Lefebvre et al. / Carbon 38 (2000) 1745 – 1749
In order to characterize SWNT samples we performed multi-terminal conductance measurements, commonly employed in quantum dot physics. Source–drain current (I) was recorded as a function of source–drain voltage (V ), tuning gate voltages (Vg ) and temperature.
3. Experimental results
3.1. SWNT diode Fig. 1(a) shows the AFM image of a sample prepared on predefined electrodes. The nanotube spans three electrodes with an impurity [13] laying near electrode A. The room temperature current–voltage characteristics (I–V ) across the two different segments of the tube show remarkably different behavior. On the clean segment (BC), the I–V curve (Fig. 1(b)) is symmetric around zero bias and has a gap of about 0.5 V. The conductance increases continuously with increasingly negative gate voltage, consistent with other measurements on semiconducting nanotube FETs [4,5]. The I–V characteristic of the tube segment with impurity (AB) is highly asymmetric (Fig. 1(b)). Current flows through the nanotube only when lead B is at a higher potential than lead A (forward bias), but it is strongly suppressed at reverse bias. The forward bias conductance of the nanotube device decreases with increasingly positive
gate voltage, which is the evidence of hole dominated transport in the diode.
3.2. SWNT junction In Fig. 2(a) we show an AFM image of two crossed SWNT bundles (also known as ‘ropes’) contacted at each end to gold leads patterned using electron beam lithography. The diameter of the bottom bundle is 2.5 nm, while that of the top one is 7.5 nm. At room temperature two-probe I–Vs of the bundles are linear with resistances of 330 and 50 kV respectively. The junction I–V curve has a 0.2 V gap at room temperature, and a high voltage differential resistance of 10 MV. At present, the origin of the gap is unclear. We can use the junction to study the effect of local gating on transport along the bottom bundle by applying a small (,0.2 V) voltage to the top bundle. Fig. 2(b) is a plot of current in the bottom bundle at 5 K as a function of gate voltage Vg 1 applied to the degenerate Si substrate and Vg 2 applied to the top bundle. Each curve shows long plateaus indicating that the low bias transport in the bottom bundle is mostly suppressed. Sharp current peaks with resistance on the order of 50 MV are observed in between the plateaus. These observations indicate single electron charging in the bottom bundle. Furthermore, Fig. 2(b) shows that a slight change in back gate voltage can have a dramatic effect on position and strength of Coulomb
Fig. 1. (a) AFM image of a single SWNT crossing three leads. An impurity is visible near lead A. (b) Room temperature I–V characteristics of the nanotube segment with impurity (AB) and the clean section of the tube (BC). Curves are offset for clarity.
J. Lefebvre et al. / Carbon 38 (2000) 1745 – 1749
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Fig. 2. (a) AFM image of a junction made up of two ropes of single wall nanotube. The top SWNT bundle is used to electrostatically gate the bottom bundle. (b) Coulomb blockade oscillations taken at 2.5 mV bias on the bottom bundle as a function of the voltage applied to the top nanotube rope. The curves are recorded at 23.065 V, 23.045 V, 23.025 V, 23.005 V and 22.985 V (from the top down) applied to the backgate and are offset for clarity.
blockade oscillations. As we will detail in the discussion, this behavior is typical of two quantum dots connected in series.
3.3. Transport on short segments of SWNTs The AFM image in Fig. 3(a) shows two gold electrodes separated by sub 30 nm gap, which we use to study short segments of SWNTs. Individual nanotubes spanning such nanogaps exhibit two types of behavior at low temperature. Single metallic nanotubes contacted inside the nanogap have room temperature resistance of approximately 1 MV, most of which is attributable to contact resistance. At low temperatures, the I–V curve has a gap of approximately 50 mV which can be tuned with electrostatic gating (see Fig. 3(b)). These observations are typically an indication of
single electron charging. More extensive characterization of this sample reveals Coulomb blockade diamonds of different sizes which are qualitatively similar to those reported in other nanotube SETs [2,3,6]. We extract the charging energy and energy level splitting from the largest diamonds to be 80 meV and 20 meV respectively. The second behavior is reminiscent of the field effect transistor behavior observed in semiconducting nanotubes [4,5]. In Fig. 3(c) we show a that our device has high bias resistance of 1 MV, hole dominated transport, gain in excess of 10,000 and bias dependent onset of conduction which are all typical of nanotube FETs. However, at 4.5 K and gate voltages between 25 V and 115 V current flow is suppressed for bias voltage of up to 1.5 V. Additionally, we do not observe low bias conduction at 4.5 K; a 0.1 V gap persists out to 220 V applied to the gate (not shown).
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J. Lefebvre et al. / Carbon 38 (2000) 1745 – 1749
Such model fully describes the observations on nanotube FETs and on the clean segment BC. We suggest that the impurity on segment AB localizes some charge on the nanotube, doping it n-type. In this way, the adsorbate further distorts the barrier in valence band. Since the impurity is located near contact A (see Fig. 1(a)) the spatial symmetry of the device is broken and so is the symmetry of the valence band. The larger barrier near contact A can be used to fully describe the strongly rectifying I–V characteristics. This experiment also constitutes the first known observation of one-dimensional (1D) electronic subbands [1,15,16] in transport along a SWNT [14].
4.2. SWNT junction
Fig. 3. (a) AFM image of two 250 nm wide gold leads separated by a nanogap; (b) I–V characteristics of a metallic nanotube at 4.5 K with different voltages applied to the gate. The tube exhibits single electron charging with large charging energy and energy level splitting; (c) Conductance versus gate voltage curves of a semiconducting tube taken at different biases and 4.5 K. Such SWNTs present field effect behavior similar to that of FETs.
4. Discussion
4.1. SWNT diode We interpret our measurements using a modified band bending model adapted from the model used to explain the action of nanotube FET [4]. The model assumes that, at the points of contact, the valence band of the nanotube is pinned to the Fermi energy of the leads. Such pinning is often ascribed to the work function mismatch of the metal and the nanotube. Away from the electrodes, the nanotube valence band bends down in order to lower its energy. The amount of bending, which presents a potential barrier for transport at low biases, can be tuned by the gate voltage.
Our nanotube junction results can be explained in terms of a well understood system of weakly coupled quantum dots in series [17]. In order for current to flow through the system both dots have to conduct, which happens only when the chemical potential of both dots is equal to that of the leads. Indeed, only when both dots are resonant with the leads do we see peaks in the current spectrum (not shown). However, as we change the substrate voltage Vg 1 we expect to adjust the coupling strength of the dots. In fact, as we tune the backgate (Vg 1 ) to increase carrier (in this case, hole [4]) density in the bottom bundle the barrier formed by the junction becomes better screened. Indeed, increased carrier concentration increases the tunneling strength between the dots. This relaxes the condition for current flow [18] and additional domains of current flow appear as smaller oscillations extending between larger resonant peaks (see Fig. 2(b)). In the limit of complete coupling, the isolated peaks would join into streaks in Vg 1 2Vg 2 plane indicating a single dot formation [17]. Our data in Fig. 2(b) indicates that the junction is responsible for the double dot formation in the SWNT bundle. To support this claim we compare measured and estimated charging energies of the two bundle segments divided by the junction. We can measure the larger of the two charging energies since the current flows only where both dots are able to conduct. The value of 20 to 30 meV is extracted from I–Vs with no voltage applied to the top SWNT rope. We also measure the lengths of the two segments to be 0.35 mm and 1.2 mm. The corresponding charging energies can be estimated from bundle-back gate capacitance (C) which is computed from the measured length (L), bundle radius (r) and bundle-back gate distance (h): U 5 e 2 /C 5 e 2 ln(2 h /r) / 2pe L [6]. The inferred energies of 20 meV and 6 meV, respectively are in approximate agreement with the measured value of 20–30 meV. Thus, we can identify the junction as the origin of the tunnel barrier dividing the bundle into two coupled quantum dots.
J. Lefebvre et al. / Carbon 38 (2000) 1745 – 1749
4.3. Transport on short segments of SWNTs Our data on SWNTs bridging nanogaps are qualitatively similar to results reported on nanotube SETs [2,3] and FETs [4,5], however there are a several differences worth pointing out here. In metallic samples, we observe charging energies of 80 meV and energy level splitting of 20 meV, which are an order of magnitude larger than any reported in transport along a nanotube. Such large energies correspond to a very short tube segment, as has been show by Venema et al. [19] in STM-STS measurements of cut tubes. This is somewhat surprising, since typical as-grown SWNT used in this experiment is about 0.5 mm in length. We therefore conclude that contact with electrodes breaks the nanotube into (at least) three segments: one underneath each electrode, and one in the nanogap. Complex charging spectra indicate that there is a finite transmission probability between the different nanotube segments. Indeed, the electrodes do not mechanically break the SWNT; instead they create barriers for electron propagation via mechanical distortions or electrostatic screening of the nanotube. Our semiconducting samples show I–V curves with extraordinarily large(.1.5 V) gap at positive gate voltage which narrows to approximately 0.1 V at negative gate voltage. The 0.1 V gap persists virtually unchanged over a 5 V range in gate voltage and is accompanied by steps suggestive of single electron charging. At the moment, the origin of these observations is not clear but we feel confident that they are due to the nanogap. Nanogaps could section the semiconducting SWNT into small islands of charge as shown above, or create large barriers due to stronger band bending than that observed in more traditional nanotube FETs.
5. Conclusions In summary, we have presented three classes of nanotube devices: a SWNT diode, a SWNT junction, and ultra short nanotube SETs and FETs. We have demonstrated a SWNT diode behavior due to local doping by chemical impurity. We also showed that nanotube bundle and a metal electrode laying on top of a SWNT induce a formation of an isolated quantum dot bound by the barriers. We have indicated that, in the case of SWNT junction, the barrier can be tuned away transforming a
1749
nanotube which consists of two quantum dots into a single quantum wire. We argue that the wealth of functionality described here can be identified with precisely tailored mechanical deformations and chemical impurities outlined above. In the future, predesigned distortions can be used to create multiple devices on a single SWNT.
Acknowledgements The SWNT material used in this work was provided by the Smalley group at Rice University. Support was provided by Fonds FCAR (Quebec) (J.L.), the U.S. NSF (grant number DMR98-02560), and the David and Lucile Packard Foundation (A.T.J.). We thank C. Kane and E. Mele for useful discussions.
References [1] For review, see Dresselhaus MS, Dresselhaus G, Eklund PC. Science of Fullerenes and Carbon Nanotubes. San Diego, CA: Academic, 1996. [2] Tans SJ et al. Nature 1997;386:474. [3] Bockrath M et al. Science 1997;275:1922. [4] Tans SJ, Verschueuren ARM, Dekker C. Nature 1998;393:49. [5] Martel R et al. Appl Phys Lett 1998;73:2447. [6] Antonov RD. PhD thesis. University of Pennsylvania, unpublished, 1999. [7] Wingreen N, Odintsov A. In: Proceeding of the March Meeting of the APS, 1999. [8] Bezryadin A et al. Phys Rev Lett 1998;80:4036. [9] Clauss W et al., to appear in Eur Phys Lett. [10] SWNT material is produced by pulsed laser vaporization as outlined in Thess A et al. Science 1996;273:482. [11] Lefebvre J et al., submitted to Appl Phys Lett. [12] Lefebvre J et al., to be published. [13] The composition of the impurity remains unknown. Asgrown nanotube material contains a significant impurity content, including metal catalyst, graphitic particles and amorphous carbons. [14] Antonov RD, Johnson AT, to appear in Phys Rev Lett. [15] For observation of electronic subbands in STM-STS studies see: Wildoer JW et al. Nature 1998;391:59. [16] For observation of electronic subbands in STM-STS studies see: Odom TW et al. Nature 1998;391:62. [17] Livermore C et al. Science 1996;274:1332. [18] Blick RH et al. Phys Rev Lett 1998;80:4032. [19] Venema LC et al. Science 1999;238:52.