We report the realization and characterization of independently controllable
tunnel barriers within a carbon nanotube. The nanotubes are mechanically bent
or kinked using an atomic force microscope, and top gates are subsequently
placed near each kink. Transport measurements indicate that the kinks form
gate-controlled tunnel barriers, and that gates placed away from the kinks have
little or no effect on conductance. The overall conductance of the nanotube can
be controlled by tuning the transmissions of either the kinks or the
metal-nanotube contacts.Comment: related papers at http://marcuslab.harvard.ed

Biercuk, M. J.

Chow, J. M.

Marcus, C. M.

Mason, N.

English

arXiv

M. J. Biercuk

N. Mason

J. M. Chow

C. M. Marcus

Crossref

Locally Addressable Tunnel Barriers within a Carbon Nanotube

We report the realization and characterization of independently controllable tunnel barriers within a carbon nanotube. The nanotubes are mechanically bent or kinked using an atomic force microscope, and top gates are subsequently placed near each kink. Transport measurements indicate that the kinks form gate-controlled tunnel barriers, and that gates placed away from the kinks have little or no effect on conductance. The overall conductance of the nanotube can be controlled by tuning the transmissions of either the kinks or the metal-nanotube contacts

Biercuk, M.

Chow, J.

M. Marcus, C.

Copenhagen University Research Information System

u n i ve r s i t y  o f  co pe n h ag e n  Københavns UniversitetLocally addressable tunnel barriers within a carbon nanotubeBiercuk, M.; Mason, N.; Chow, J.; M. Marcus, C.Published in:Nano LettersDOI:10.1021/nl0486721Publication date:2003Citation for published version (APA):Biercuk, M., Mason, N., Chow, J., & M. Marcus, C. (2003). Locally addressable tunnel barriers within a carbonnanotube. Nano Letters, 4(12), 2499-2502. https://doi.org/10.1021/nl0486721Download date: 02. Feb. 2020Locally addressable tunnel barriers within a carbon nanotubeM. J. Biercuk, N. Mason, J. M. Chow and C. M. MarcusDepartment of Physics, Harvard University,Cambridge, Massachusetts 02138We report the realization and characterization of independently controllabletunnel barriers within a carbon nanotube. The nanotubes are mechanically bent or kinkedusing an atomic force microscope, and top gates are subsequently placed near each kink.Transport measurements indicate that the kinks form gate-controlled tunnel barriers, andthat gates placed away from the kinks have little or no effect on conductance. The overallconductance of the nanotube can be controlled by tuning the transmissions of either thekinks or the metal-nanotube contacts.Carbon nanotubes are a leading material system for molecular electronic deviceapplications as well as for fundamental studies of the electronic properties of low-dimensional systems.  Single-walled carbon nanotubes can function as nanoscaleanalogues of electronic elements such as field-effect transistors1-5 and interconnects6, 7 inintegrated circuits.  In addition, nanotubes behave as ballistic conductors with largecurrent density capacity, and also display single electron (quantum dot) charging andquantum effects at low temperatures8-13. Conformational changes in nanotubes modifytheir electronic properties: devices such as diodes have been created using intra-tubetunnel barriers formed from mechanically induced defects or “kinks”14-16. Scanned-gatemicroscopy indicates that the kinks serve as scattering centers8, 16, consistent withtheoretical predictions for bending defects in nanotubes17, 18.  The properties of separatetunnel barriers were not independently adjustable in previous studies, however, as only asingle backgate was available.In this Letter we describe the realization of locally addressable tunnel barrierswithin a single carbon nanotube.  The barriers are due to bending defects, formed bymechanically kinking14 the nanotube. We show that electrostatic gates placed near each ofthe kinks independently tune these tunnel barriers from transparent to opaque, whereasgates placed away from the kinks (over undeformed sections of the nanotube) have littleor no effect.  In addition, a global backgate is used to modify the transparency of themetal-nanotube contacts.Carbon nanotubes were grown via chemical vapor deposition from patterned Fecatalyst islands on a degenerately doped Si/SiO2 wafer, using methane as the carbonsource19.  After locating a target nanotube with an atomic force microscope (inconventional raster mode), the cantilever tip was lowered and then used to push thenanotube laterally under computer control. The resulting kinks had slightly differentshapes, with typical lateral deviations of ~200!nm from their undisturbed (i.e., straight)configuration.  After kinking, the nanotubes were contacted with Ti/Au electrodes,forming devices of length ~ 1-2 mm.  The wafers were then coated (without patterning)with ~!25!nm SiO2 deposited via plasma-enhanced chemical vapor deposition at roomtemperature20. Multiple Cr/Au top gates (~ 200-300!nm across, ~ 40!nm thick), patternedusing electron beam lithography and lift-off, were positioned over each tube, with a gateplaced near each kink and at least one additional gate over an unkinked section of thesame nanotube as a control.  (Additionally, the doped Si substrate was used as a globalbackgate).We first discuss a device with one kink and three top gates, one gate near a kink(G2) and two gates over unkinked sections of the nanotube (G1 and G3), as shown in theinset to Fig. 1a.  Two-terminal conduction is measured from 1.7K to 300K using avoltage bias of 10 mV and measuring dc current using an Ithaco 1211 current amplifierand digital voltmeter. All gates are connected to dc voltage sources, and are set to 0V(relative to tube ground) unless otherwise specified.  Measurements of conductance as afunction of backgate voltage, VBG, at room temperature and ~135 K show field-effectbehavior consistent with p-type doping. Setting VBG = 0V and applying a voltage, VG2, toG2 gives a strong gate response, while applying voltages VG1 and VG3 to gates G1 and G3produces little change in conductance (Fig. 1a).  These results, as well as those below,demonstrate the local effect of the top gates.  The lack of gate response to G1 and G3demonstrates the field-effect behavior observed as a function of VBG is not due to adepletion of the bulk of the nanotube. Rather, the dominant effect of the backgate ispresumably to alter tunneling at the nanotube-metal contacts21.At low temperature, regions of charges confined by one or more kinks give rise toCoulomb blockade oscillations in conductance as a function of various gate voltages.  Asweep of VG2 at 1.7 K reveals widely spaced Coulomb blockade peaks (Fig. 1b) even atlarge bias voltage (10!mV). Similar single-electron charging events are observed up to~135 K. Device conductance below ~30 K is zero whenever  VG2 = 0V regardless of othergate values, suggesting that the single kink acts as a small quantum dot (i.e., a devicewith at least two barriers) with a large charging energy (~20meV). Previous experimentshave established that closely spaced mechanical defects can create such quantum dots15, 16.With VG2 tuned to a conductance peak, Coulomb oscillations are seen as a function ofeither VG3 or VG1 on a gate-voltage scale considerably smaller than that measured for VG2,likely due to a smaller charging energy (Inset Fig. 1b, Fig. 1c).   This is consistent withlarger quantum dots forming between the defect and tunnel barriers at the contacts22, 23,with energy levels modulated by VG1 and VG3. In this case, resonant transport occurs onlywhen the energy levels between all dots (source-kink, kink, kink-drain) are resonantlyaligned. At VG1=0V the source-kink dot is transparent; the resulting double-dot system isevident in a two-dimensional plot of dc current as a function of VG2 and VG3 (inset toFigure 1b). The pattern of alternating high and low conductance regions is due to energylevels moving in and out of resonant alignment between the two dots24.We next discuss a device with multiple kinks, each with a nearby gate. The insetto Figure 2 shows a realistic schematic of a device with two gated kinks (under G4 andG6) and an additional top gate (G5) over the section of the nanotube between the kinks.Room temperature measurements show a strong response to VBG (Fig. 2a).  When VBG isswept at 70!K, the field-effect behavior is superposed with irregular Coulomboscillations. A two-dimensional plot of dc current (VSD=10mV) as a function of VBG andVG5 shows the negligible effect of VG5 (Fig. 2b).  In contrast, measurements of dc currentas a function of VBG and VG4 show a strong gate response to VG4.  The nearly horizontalfeatures visible in Fig. 2c correspond to values of VG4 at which transport is suppressed byreducing the transparency of the underlying kink. The dashed line points out one suchfeature, which has a slight slope because of capacitive coupling between the backgate andthe kink. Oscillatory features as a function of VBG persist, again with a small slope (dottedline) due to coupling between VG4 and VBG.Local control over individual kinks is demonstrated in Figure 3a, where dc current(VSD=10mV) is plotted as a function of VG4 and VG6. To maximize conductance we setVBG=-10V.  When both VG4 and VG6 are large and negative, current through the device islarge, indicating that both kinks are transparent.  Beyond this corner of the plot,conductance is strongly suppressed by either of the two top-gate voltages.  The sharpturn-on of the double-kink device in response to multiple input voltages is equivalent toan AND logic element.  The appearance of perpendicular horizontal and vertical bandsindicates that G4 and G6 have very little cross-capacitance.  Measurements at smallernegative backgate voltages produce similar features with a suppressed current flow.Measurements of dc current as a function of VG4 and VG5 produce the same features inresponse to VG4, but show no response to VG5 (Fig. 3b).  Despite differences in the shapesof the three kinks studied here, we note that all respond to gate voltages on approximatelythe same scale.In conclusion, we have fabricated and investigated nanotube devices withintentional bend and kinks created using an atomic force microscope, and electrostatictop gates near the kinks. These kinks behave as controllable tunnel barriers with localgate addressability.  In contrast, gates placed away from the kinks on the same nanotubeshad little or no effect on conductance. Future devices aim to use gate-tunable kinks tomanipulate electron hybridization between different sections of a nanotube in thequantum regime.This work was supported by funding from the NSF under EIA-0210736, theArmy Research Office, under DAAD19-02-1-0039 and DAAD19-02-1-0191, and theHarvard MRSEC. M.J.B. acknowledges support from an NSF Graduate ResearchFellowship and from an ARO Quantum Computing Graduate Research Fellowship. N.M.acknowledges support from the Harvard Society of Fellows.References:1 S. J. Tans, R. M. Verschueren, and C. Dekker, Nature 393, 49 (1998).2 R. Martel, T. Schmidt, H. R. Shea, et al., App. Phys. Lett. 73, 2447 (1998).3 A. Bachtold, P. Hadley, T. Nakanishi, et al., Science 294, 1317 (2001).4 S. J. Wind, J. Appenzeller, R. Martel, et al., App. Phys. Lett. 80, 3817 (2002).5 S. J. Wind, J. Appenzeller, and P. Avouris, Phys. Rev. Lett. 91, 058301/1 (2003).6 W. Hoenlein, Jap. J. of App. Phys. Part 1 41, 4370 (2002).7 J. Li, Q. Ye, A. Cassell, et al., App. Phys. Lett. 82, 2491 (2003).8 M. Bockrath, W. Liang, D. Bozovic, et al., Science 291, 283 (2001).9 L. Chico, M. P. Lopez Sancho, and M. C. Munoz, Phys. Rev. Lett. 81, 1278 (1998).10 K. Ishibashi, T. Ida, M. Suzuki, et al., Jap. J. of App. Phys. Part 1 39, 7053 (2000).11 H. Mehrez, H. Guo, J. Wang, et al., Phys. Rev. B 63, 245410/1 (2001).12 J. Nygard and D. H. Cobden, App. Phys. Lett. 79, 4216 (2001).13 M. Suzuki, K. Ishibashi, T. Ida, et al., Jap. J. of App. Phys. Part 1 40, 1915 (2001).14 H. W. C. Postma, M. de Jonge, and C. Dekker, Phys. Rev. B 62, R10653 (2000).15 H. W. C. Postma, T. Teepen, Z. Yao, et al., Science 293, 76 (2001).16 D. Bozovic, M. Bockrath, J. H. Hafner, et al., App. Phys. Lett. 78, 3693 (2001).17 L. F. Chibotaru, S. A. Bovin, and A. Ceulemans, Phys. Rev. B 66, 161401 (2002).18 V. Meunier, L. Henrard, and P. Lambin, Phys. Rev. B 57, 2586 (1998).19 M. J. Biercuk, N. Mason and C. M. Marcus, Nano Lett., in press (2003).20 N. Mason, M. J. Biercuk, C. M. Marcus, in press (2003).21 S. Heinze, J. Tersoff, R. Martel, et al., Phys. Rev. Lett. 89, 106801/1 (2002).22 S. J. Tans, M. H. Devoret, H. Dai, et al., Nature 386, 474 (1997).23 P. L. McEuen, M. Bockrath, D. H. Cobden, et al., Phys. Rev. Lett. 83, 5098 (1999).24 W. G. van der Wiel, S. De Franceschi, J. M. Elzerman, et al., Reviews of ModernPhysics 75, 1 (2003).Figure Captions:Fig. 1 (a) Gate response of single-kink device.  Plot shows dc current as a function ofthree gate voltages, VG1, VG2 and VG3 over different sections of the nanotube,measured at T=135K (bottom, left axes) and of VBG at room temperature (top,right axes).  Inset:  SEM image of the device with small arrows indicating theposition of the nanotube under the SiO2 insulating layer and large arrowheadindicating the location of the kink.Fig. 1(b) Gate response of kinked device to VG2 (over kink) at low temperature showingCoulomb charging.  Inset: Grayscale plot of dc current as a function of VG2 andVG3 displaying resonant transport through multiple dots in series.Fig. 1(c) Horizontal slices through grayscale plot in Fig. 1(b) at on- and off-peak valuesfor VG2.  On peak shows coulomb blockade as a function of VG3 while off-peakshows total suppression of current.Fig. 2(a)  Room temperature conductance of double-kink device displaying approximatep-type field-effect behavior. Inset: Schematic of double kink device, showingkinks of different shapes and bending angles.Fig. 2(b) Grayscale plot of dc current as a function of VBG and VG5.  Coulomb oscillationsappear in VBG, but no effect of VG5 is evident.Fig. 2(c) dc current as a function of VBG and VG4 showing the ability to tune transportthrough gate control of a kink.  Two scales of voltage additivity are present,indicated by dotted and dashed lines (see text).Fig. 3(a) dc current as a function of VG4 and VG6, both over kinks in the nanotube,demonstrating the ability to inhibit transport through the device by appropriatelytuning either kink.Fig. 3(b) dc current as a function of VG5 and VG4. VG5 has essentially no effect ontransport through the device.T=1.7K120I DC (pA)-900 -800VG3 (mV)VG2=-849mVVG2=-836mVT=135K(a)(b) (c)1086420I DC (nA)-2 -1 0 1 2Top Gate Voltage (V)302520151050IDC  (nA)-10 -5 0 5 10VBG (V) G2 G1 G3BG (RT)6420I DC (pA)-865 -855 -845 -835VG2 (mV)VG3 (mV)VG2 (mV)Figure 186420I DC (nA)-10 -5 0 5 10VBG (V)-10 -5 0 5 10VBG (V)-202VG4 (V)40200IDC  (pA)-202VG5 (V)100500IDC  (pA)G4 G5 G6RTT = 70KT = 70K(a)(c)(b)Figure 2-2-1012VG5 (V)-2 -1 0 1 2VG4 (V)-2-1012VG6 (V)4003002001000IDC (pA)VBG = -10VVBG = -10VT=70KT=70K(a)(b)Figure 3

Locally addressable tunnel barriers within a carbon nanotube

Locally addressable tunnel barriers within a carbon nanotube

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