Electronic properties of carbon nanotubes

28

Electronic properties of carbon nanotubes John E Fischer* and Alan T Johnsont Single-wall carbon nanotubes exhibit many properties analogous to quantum dots and wires at very low temperatures: Coulomb blockade and single-electron charging. These and other phenomena may be exploited in constructing active electronic devices of unprecedentedly small size. Bulk material may be chemically doped to yield mass-normalized electrical conductivity higher than that of copper. Addresses *Department of Materials Science and Engineering and Laboratory for Research on the Structure of Matter, University of Pennsylvania, Philadelphia, PA 19104-62?2, USA; e-mail: fischer@sol1 .Irsm.upenn.edu tDepartment of Physics and Astronomy and Laboratory for Research on the Structure of Matter, University of Pennsylvania, Philadelphia, PA 19104-62?2, USA *Correspondence: John E Fischer Current Opinion in Solid State & Materials Science 1999, 4:28-33

Electronic identifier: 1359-0286-004-00028 © Elsevier Science Ltd ISSN 1359-0286 Abbreviations AFM atomic force microscope/microscopy FET field effect transistor LL Luttinger liquid MWNT multi-wall carbon nanotube SET single-electron transistor STM scanning tunneling microscope/microscopy SWNT single-wall carbon nanotube

Introduction Scientific and technological interest in carbon nanotubes continues to expand rapidly, driven in part by improvements in synthesis and the availability of commercial samples for testing [1,2]. Among the many possible applications being discussed and explored, these nanometer-diameter cylinders of pure carbon show promise as a new family of electronic materials for active devices, much smaller than can be achieved by conventional lithography. A general overview of the subject appeared in this journal in 1997 [3"']. Our goal here is to provide an update focussing on nanotubes as a new electronic material, surveying the most significant literature since October 1997. Our emphasis is on single-walled nanotubes (SWNTs) with occasional reference to results on multi-walled tubes (MWNTs). T h e electronic properties of SWNTs present scientific challenges and technological opportunities on several size scales. Individual tubes are the easiest to treat theoretically but the most difficult to isolate and measure. Here one expects, and finds, a number of phenomena familiar from work on quantum dots and wires: Coulomb blockade, single electron charging, and so on. Crystalline bundles, or 'ropes', consisting of a few to hundreds of tubes packed on

a 2D triangular lattice, are abundant in pristine material produced either by laser ablation [4] or by arc discharge [5] and become more prevalent with subsequent annealing [6]. Some of the single tube properties are believed to carry over to ropes, while the effects of inter-tube coupling remain to be clarified. Finally, bulk material consists of a low-density tangle of long ropes and tubes with many crossing points, along with several kinds of impurities (principally graphite and graphitic onions/capsules, metallic catalyst residues and amorphous carbon), some of which can be chemically or physically removed [6]. It is believed that ropes and bulk SWNTs will be important in high strength, highly conducting composite materials. Electronic

properties

of isolated tubes

Single-tube and single-rope circuits allow investigations of the intrinsic electronic physics of nanotubes and serve as prototype molecular devices. Making electrical contact is a large obstacle to producing any single-molecule circuit, but for nanotubes this has been achieved using random deposition [7°°]; 'rational design' where tubes are deposited on a substrate, located using optical or atomic force microscopy (AFM) and then intentionally contacted [8°]; and by direct manipulation onto contacts using the AFM [9"]. T h e random approach is the simplest of the three and gives the gentlest treatment of the nanotube sample. T h e second approach is more time-consuming, but has a better success rate when a multi-lead (e.g. four probe) geometry is desired. If it is possible to bring tubes into electrical contact with each other, AFM manipulation will allow the creation of complex all-tube circuitry. For each of these techniques, the gate for field effect circuits (e.g. transistors) is either a third lead near the nanotube circuit, or the substrate itself if the wafer is degenerately doped. In all experiments to date the S W N T is contacted through a tunnel barrier with -0.5 Mf~ resistance. Little is known about the tunnel contact, although it could be due to chemical contamination (perhaps amorphous carbon surrounding the tube), carrier depletion due to the formation of a Schottky barrier at the metal contact, or bends in the tube where it goes over the thick (-20 nm) metal contacts. T h e r e is some evidence that 'rational design' gives stronger (although still tunnel) contact to the tube, but the tube is 'cut' by deposition of the metal leads on top of it [8°]. T h e discovery of a method for making good (ohmic) contact to tubes and ropes will be critical for illuminating the intrinsic conduction properties of this material, particularly in the case of Lnttinger liquid (LL) behavior. Although this remains a goal for the future, some novel approaches have already shown promise, including electron-beam illumination of the contact region [10].

Electronic properties of carbon nanotubes Fischer and Johnson 29

Transport experiments verify the basic predictions of band structure calculations [2,3°°], but leave many questions unanswered. Room temperature measurements in our laboratory of randomly fabricated single tube circuits find both metallic and large gap (-0.5 V) semiconductor tubes, with metal tubes making up somewhat less than half the product of pulsed laser vaporization. Low temperature STM measurements of both atomic structure and spectroscopy of individual tubes [11",12 °] agree with the basic predictions that tubes with the index (n,m) conduct if n-m is three times an integer otherwise the tubes are semiconducting. Here n and m refer to the components of the wrapping vector in units of the two-dimensional graphene basis vectors a and b; the wrapping vector defines the diameter and chirality of the resulting graphene cylinder [3°°].To date there has been no experiment that correlates the atomic structure with the conducting properties along a tube. Additionally, there is no experimental evidence for the small (-20 mV) 'curvature gap' expected for (0,mod 3) zig-zag tubes [13,14], and little is known about the electronic coupling of tubes within a rope [13,15°]. Fundamental questions about the nature of the electron system and transport remain open. As the long-range Coulomb interaction between electrons is poorly screened in one dimension, theory [16] predicts that an isolated armchair nanotube should be an L L with measurable deviations from non-interacting (Fermi liquid) behavior at all temperatures below the energy scale of the 1D subband separation (for tubes with 1.4 nm diameter, the energy scale is 0.5 V or 5000K). If an L L is subject to the Coulomb blockade, it will have novel behavior associated with the spin-charge separation characteristic of LLs. At low temperatures, the system is open to a variety of instabilities [17], including charge density wave, spin density wave, superconducting, and Mott insulating transitions. No theoretical guidance exists as to which of these is relevant and what temperature characterizes these transitions, and experiment has yet to probe whether these states of matter exist on tubes or are destroyed by defect scattering or inter-tube coupling. Other questions include whether transport in single tubes is ballistic or diffusive and how this varies as a function of temperature. In the absence of impurities and geometric distortion, the transport should be ballistic, but this has not been observed. STM measurements on ropes show significant elastic distortion of single tubes [18], as well as electron backscattering from chemical and other contaminants (W Clauss et al., unpublished data), both of which should be reflected in conventional transport measurements. Work on MWNTs provides evidence for ballistic transport [19°], in contrast to the STM observations on SWNTs and the proposal that twiston scattering is responsible for the linear temperature dependence of the resistance of bulk S W N T material [14]. Future work will need to illuminate whether these results reflect the physics of single tubes and ropes or that of tube and rope junctions.

Figure I

0.15

0.1 Vg = -0.988 V

-

~

0.05 rv

-0.05 '

~

Vg= -0.9~17 V

-0.1 -0.15 Ii -6

/I -4

I

I

I

I

-2

0

2

4

6

Bias ( m Y )

]

Current Opinion in Solid State & Materials Science

Current-voltage characteristics (I-Vs) for a SET made from a single metallic carbon nanotube. The zero-current gap at low bias voltage is due to the Coulomb blockade by single-electron charging. The size of the gap can be controlled by the voltage on a capacitively-coupled gate electrode, Vg. Small step-like features on the I-Vs are due to the set of discrete electronic energy levels that exists on the tube. There is a step in the I-V each time an energy level is brought into resonance with the Fermi energy of one of the metal leads. From the figure, we infer a charging energy of 2.5 meV and a energy level splitting of 0.45 meV.The I-Vs were taken at 20 mK and zero magnetic field. Data courtesy of Radoslav Antonov (a graduate student at the University of Pennsylvania).

Metallic micron-length single tubes [7 °° ] and small ropes [8 °] behave as 'quantum dots' at low temperature (see Figure 1), with single electron charging effects and discrete 'particle-in-a-box'-like energy levels. Both the charging energy (typically 5-10 meV) and energy level splitting (near 1 meV) are in rough agreement with theory, but it is unclear whether the observed energy splitting agrees with the existence of spin degeneracy and the two-fold Fermi point degeneracy expected for metallic tubes. Although none of, the collective states referred to above have been observed, charge rearrangement on a tube as a function of gate voltage has been found [7°'], and interpreted as characteristic of molecule-like strong coupling between the electrons and tube atomic configurations. While this experiment also suggested that electrons that added to the tube were all of the same spin, data from the Berkeley group [20] supports the notion that electrons are added with alternating spins, so that the tube ground state is a spin singlet. Finally, much of the data shows that a single tube can spontaneously break up into multiple quantum dots at low temperature [7°°], this is perhaps due to electron localization by geometric distortion of the tube on the substrate. This obviously

30

Electronic materials

Figure 2

i

i

i

i

i

i

1.4

1.2

0

~

1.0

0.8

0.6 I

I

I

50

1O0

150

I

200

I

950

I

300

Temperature (K) Current

Opinion

in S o l i d S t a t e & M a t e r i a l s

Science

Four-probe resistance versus temperature, normalized to 300K for SWNT material before and after potassium doping. The pristine sample exhibits a shallow minimum in R, which is typical of all samples prepared by pulsed laser vaporization. Doping with potassium to an approximate composition KC 8 leads to a 40-fold reduction in R(3OOK) and completely supresses the upturn to negative dR/dT at low temperature.

complicates the interpretation of the data, and opens the question of what phenomena are intrinsic to perfect tubes.

Electronic properties of rope bundles Self-organization during the growth of S W N T into crystalline ropes appears to be a common feature of both principal synthesis methods, laser ablation and arc discharge. T h e mechanism by which this occurs remains a mystery. At very low temperatures (< 40K), ropes exhibit the same kind of Coulomb blockade and single electron charging phenomena as are observed in isolated tubes [8"]. A few resistivity measurements have been made on single ropes at higher temperatures, with results qualitatively equivalent to those of high quality bulk material (see below). Important structural and electronic issues remain to be resolved. For example, the nature of inter-tube interactions will depend critically on whether the coupling is coherent or incoherent. Little is known about the distribution of diameters and chiralities within a rope crystallite. A large disparity in tube diameter would be detrimental to crystal formation, as it is in binary metallic alloys. On the other hand, a very large number of different tube symmetries whose diameters fall in a narrow range exist. Clearly a rope consisting of some metallic and some insulating tubes will be a much more complicated object than a rope consisting of a single tube type.

Many of the structural properties of rope crystals can be understood in terms of a simple model of homogeneous cylinders interacting via an averaged van der Waals potential (RS Lee, LA Girifalco, personal communication), such that the specific relative orientations of neighboring tubes is not important. This is especially relevant to the observation that a hypothetical crystal consisting of only (10,10) metallic tubes cannot have long-range order because the rotational symmetry of the tubes is five-fold whereas the coordination of the triangular lattice is six-fold. On the other hand, this frustration of long-range orientational correlations is predicted to break the particle-hole symmetry which is maintained in the folded 2D graphite band structure, such that both electron and hole pockets exist in metallic tubes [13]. It is also worth exploring the possibility of modulating the rope conductance via a gate electrode as in a field-effect transistor structure, because such devices would be much simpler to construct than the single tube analog. Finally, the size and symmetry of the rope lattice implies the existence of large interstitial channels, into which one can introduce donor or acceptor species analogous to intercalation in layer compounds, conjugated polymers and fullerene crystals [21]. Conductivity enhancement and charge transfer in such materials has been established [22°,23°], but the location of the dopants remains unclear [24]. T h e conductivity per unit mass of these materials exceeds that of copper at 300K. T h e most important application of ropes is likely to be as reinforcing fibers in composite materials with unprecedented strength and electrical conductivity. This will be particularly attractive if alignment of the ropes can be achieved within the matrix [25].

Electronic properties of unoriented bulk SWNT Figure 2 shows the normalized temperature-dependent resistivity [26] measured on unoriented, purified bulk material [4] before and after doping with potassium. T h e positive temperature coefficient at above the minimum in R versus T is indeed characteristic of metallic behavior. At a low temperature there is a crossover in the pristine material to negative dR/dT, which in some samples diverges strongly as T--~0. T h e crossover temperature varies from sample to sample in a range from 50K to >300K. T h e high temperature behavior can be explained qualitatively either by backscattering from elastic twist modes of individual tubes [14], or by electron-electron correlations [27], the former giving somewhat better quantitative agreement with the data. Detailed measurements of the temperature and electric field dependence in the low temperature regime are consistent with 3D variable range hopping, the length scale of the localized states being approximately 600 nm [28°]. This suggests that the localization is not associated with tube defects, but rather is controlled by the morphology of the tangled network of ropes. On the other hand, Figure 1 shows that chemicaldoping completely supresses the low-temperature divergence [29], which is difficult to reconcile with the Berkeley proposal. Similar measurements on individual

Electronic properties of carbon nanotubes Fischer and Johnson

31

Figure 3 'TUBEFET' device geometry, electronic structure and device characteristics. (a) AFM image of a single molecule 'TUBEFET' made from a semiconducting nanotube on Pt leads. (b) Circuit schematic. There is an -0.5 MQ tunnel barrier between the Pt and the tube, possibly due to bends in the tube as it crosses the leads. Two Pt leads are used as the source and drain; the gate is the degenerately-doped Si substrate, isolated from the SWNT by SiO 2. (c) Proposed energy diagram for zero source-drain voltage. The vertical black bars are the tunnel barriers between the nanotube and the leads with Fermi energy, EF. Electron transfer from the tube to the leads due to the work function difference pins the SWNT valence band edge at EF in regions A and C, where it lies on top of the leads (see 2B). The bands bend toward lower energy in region 13. With the gate voltage set to Vg -- 0 V, band bending forms a barrier to transport (solid line). Application of Vg ~<~<0 reduces the barrier (upper dashed line), and the conduction rises sharply. In contrast, Vg > 0 increases the barrier and the current drops to zero (lower dashed line). The conductance increases by a factor of 106 as Vg is varied from +9 V to - 6 V (not shown). (d) A source-drain voltage applied to the TUBEFET suppresses the barrier, causing current flow. (e) Device I-V characteristics at 300K. At Vg = - 3 V, the I-V is ohmic, while the current is strongly suppressed at Vg > 0. Adapted with permission from [33].

(a)

(b)

Pt SiO2 Pt SiO2 Pt 1

2

A

B

C

•

=

3

(c)

A

B

C as

(e)

20

Gate'vohage(v):' +I;/'+~ +4 +6 1 -3 o

~°

< =

0

-20 -1.0

-0.5

0

0.5

1.0

1.5

Vbias (V)

ropes are necessary to clarify the origin of this unusual behavior, which has been pointed out to be analogous to what occurs in some conjugated polymers [30].

Active device structures As nanotubes can be semiconductors or metals, a variety of device applications are possible. Room-temperature single molecule FETs have been made from semiconducting SWNTs using a heavily doped Si substrate as the gate ([31°°,32°°]; see Figure 3). Metallic SWNTs act as micron-long quantum wires at low temperatures, and can be integrated into single electron transistors, with properties dictated by electron wave-mechanics. For F E T operation, the energy band alignment of the leads with respect to the tube is critical. Similar to graphite, the work function of a nanotube is smaller than most metals, so where it contacts a metal, electrons transfer from a semiconducting tube to align its valence band with the metal's Fermi energy. Transport through the ' T U B E F E T ' should be dominated by holes, and it is found that the electrical conductance increases by as much as 106 with a 10 V change in gate voltage. T h e F E T gain and

speed are presently limited by the high-resistance contacts; if the contact resistance can be reduced to near the quantum limit of 6 K~2, a switching speed of 10 T H z may be possible. Tans etal. [31 °°] have argued that tube-based FETs are similar to a conventional barrier injection transit time (BARRITT) diode and can be understood as quasi-ballistic devices with semi-classical ideas of band bending and space charge buildup. In contrast, Martel et al. [32 °°] have attributed its function to diffusive transport of a hole gas on the tube caused by acceptor doping induced by S W N T processing. T h e y find a relatively low hole mobility of 20 cmZ/Vs-1. S W N T transistor circuits below 30 nm in size may experience strong electron wave-mechanics effects with unknown impacts on performance. T h e answer may lie in ultra-miniaturized circuits that rely on quantum effects (e.g. single electron transistors or SETs), currently an active research area. SETs consist of a source and drain connected to a small conducting island through tunnel barriers, and have recently been made using metallic SWNTs [7"',8°]. A S E T function is based on quantum effects, so it

39

Electronic materials

improves with decreasing size instead of degrading. If ways to self-assemble complex, nanoscale S E T circuits can be found, they may be the next step in device miniaturization. Multi-SET circuits have been suggested as the basis of novel logic circuits that use single electrons as bits. A predictable circuit function will require unprecedented control of impurities or new defect-tolerant computing architectures. Only F E T and S E T devices have been realized by experiment, but other devices, such as rectifiers, have been considered theoretically. As S W N T electronic properties are intimately linked to geometry, they are profoundly affected by atomic defects [33] or joints [34]. One intriguing possibility is that a 'device on a tube' results if tubes with different wrapping vectors are joined by pairs of pentagon-heptagon defects [33]. Metal-semiconductor junctions could be made along a single tube, and it is found theoretically that multiple defects joining two semiconducting tubes generate states in the energy gap near the joint, similar to dopants in conventional semiconductors. Such a nanotube heterojunction can only be created during growth or by joining two tubes after growth, but rotating a single bond on an otherwise perfect tube also induces changes in the electronic structure, and may prove feasible using electron beam or scanning probe modification. To date there is only indirect evidence that such singletube device structures exist. In one experiment [35] an STM tip was pulled along a S W N T material. Occasionally the S T M I-V curve changed from ohmic to nonlinear, consistent with the presence of on-tube junctions. In a second experiment in our laboratory, a single tube linking three electrodes had an obvious impurity lying on it between electrodes two and three. T h e I-V characteristic of the clean tube section was that of a semiconducting tube. In contrast, the tube section with the contaminant had the I-V characteristic of a diode, implying that the impurity dramatically altered the S W N T electronic structure. In the future AFM modification might be used to create circuits by bending or twisting a single tube, introducing defects along its length, or even joining tubes by 'nano-welding' with a current pulse.

Conclusions T h e electronic properties of SWNTs offer excellent prospects for extremely small novel devices. Large-scale exploitation of quantum size effects in circuits will require new approaches to self-organization and assembly. This new family of electronic materials continues to provide surprises, for example the existence of C60 molecules, probably charged, within the confines of a 1.4 nm diameter S W N T [36]. A related strategy utilizes S W N T or M W N T as probe tips in AFMs or STMs to perform ultrafine lithography on traditional substrates [37•]. We look forward to continued explosive growth in this field.

References and recommended reading Papers of particular interest, published within the annual period of review, have been highlighted as: • of special interest • ° of outstanding interest 1.

On the World Wide Web URL: http://cnst.rice.edu/tubes/

2.

On the World Wide Web URL: http://carbolex.com/

3. An

Bernholc J, Roland C, Yakobsen BI: Nanotubes. Curr Opin Solid StateMater Sci 1997, 2:706-715. extensive overview of the electronic and mechanical properties of nanotubes. 4.

Thess A, Lee RS, Nikolaev P, Dai H, Petit P, Robert J, Xu C, Lee H, Kim SG, Colbert DT et aL: Crystalline ropes of metallic carbon nanotubes. Science 1996, 273:483-487.

5.

,lournet C, Maser WK, Bernier P, Loiseau A, Lamy de la Chapelle M, Lefrant S, Deniard P, Lee RS, Fischer ,IE: Large scale production of single-wall carbon nanotubes by the electric arc technique. Nature 1997, 388:756-758.

6.

RinzlerAG, Liu J, Nikolaev P, Huffman GB, Rodriguez-Macias F,I, Boul P,I, Lu AH, Heymann D, Colbert DT, Lee RS eta/.: Large scale purification of single wall carbon nanotubes: process, product and characterization. App/Phys A 1998, 67:29-36.

Z eo

TansSJ, Devoret MH, Dai H, Thess A, Smalley RE, Geerligs UI, Dekker C: Individual single-wall carbon nanotubes as quantum wires. Nature 1997, 386:474-477. First demonstration of quantum size effects in individual SWNT.

8. •

Bockrath M, Cobden DH, McEuen PL, Chopra N, Zettl A, Thess A, Smalley RE: Single electron transport in ropes of carbon nanotubes. Science 1997, 275:1922-1925. First demonstration of quantum size effects in SWNT rope crystals.

9. •

Hertel T, Martel R, Avouris P: Manipulation of individual carbon nanotubes and their interaction with surfaces..I Phys Chem B 1998, 102:910-915. An important prerequisite to rational design of active devices based on nanotubes. 10. Bachtold A, Henny M, Tarrier C, Strunk C, Schonenberger C, Salvetet ,i P, Conard JM, Forro L: Contacting carbon nanotubes selectively with low-ohmic contacts for four-probe electric measurements. Appl Phys Lett 1998, 73:274-276. 11. Wildoer JWG, Venema LC, Rinzler AG, Smalley RE, Dekker C: • Electronic structure of atomically resolved carbon nanotubes. Nature 1998, 391:59-62. Simultaneous atomically-resolved imaging and tunneling spectra confirm theoretical predictions for SWNT of different (n,m) symmetries. 12. Odom TW, Huang 3L, Kim P, Lieber CM: Atomic structure and • electronic properties of single-walled carbon nanotubes. Nature 1998, 391:62-64. See annotation to [11 °]. 13. Delaney P, Choi HJ, Ihm .i, Louie SG, Cohen ML: Broken symmetry and pseudogaps in ropes of carbon nanotubes. Nature 1998, 391:466-468. 14. Kane CL, Mele F-J:Size, shape, and low-energy electronic structure of carbon nanotubes. Phys Rev Lett 1997, 78:1932-1935. 15. Kane CL, Mele EJ, Lee RS, Fischer JE, Petit P, Dai H, Thess A, • Smalley RE: Temperature dependent resistivity of single wall carbon nanotubes. Europhys Lett 1998, 41:683-688. Combined theory and experiment point to twist modes as the dominant scattering mechanism at high temperature. 16. Kane CL, Balents L, Fisher MPA: Coulomb interactions and mesoscopic effects in carbon nanotubes. Phys Rev Lett 1997, 79:5086-5089. 17. Krotov YA, Lee DH, Louie SG: Low energy properties of (n,n) carbon nanotubes. Phys Rev Lett 1997, 78:4245-4248. 18. Clauss W, Bergeron DJ, Johnson AT: Atomic resolution STM imaging of a twisted single-wall carbon nanotube. Phys Rev B 1998, 58:R4266-R4269. 19. Frank S, Poncharal P, Wang ZL, De Heer WA: Carbon nanotube ~'he quantum resistors. Science 1998, 280:1744-1747. only experiment to date confirming ballistic transport, done here for MWNT at room temperature.

Electronic properties of carbon nanotubes Fischer and Johnson

20. Cobden DH, Bockrath M, McEuen PL, Rinzler AG, Smalley RE: Spin splitting and even-odd effects in carbon nanotubes. Phys Rev Lett 1998, 81:681-684. 21. Fischer JE, Bernier P: Les cristaux de fullerl~ne. La Recherche 1993, 24:46-53. [Title translation: Crystals of fullerene.] 22. Lee RS, Kim HJ, Fischer JE, Thess A, Smalley RE: Conductivity • enhancement in K- and Br-doped single-wall carbon nanotube bundles. Nature 1997, 388:255-257. Donor or acceptor 'intercalation' yields a twenty to forty-fold increase in electrical conductivity at 300K. 23. Rao AM, Eklund PC, Bando S, Thess A, Smalley RE: Evidence for • charge transfer in doped carbon nanotube bundles from Raman scattering. Nature 1997, 388:257-259. The radial breathing mode is softened by adding charge to the li orbitals, as in intercalated graphite and solid C60. 24. Bower C, Kleinhammes A, Wu Y, Zhou O: Intercalation and partial exfoliation of single-walled carbon nanotubes by nitric acid. Chem Phys Lett 1998, 288:481-486. 25. Lin J, Bower C, Zhou O: Alignment of carbon nanotubes in a polymer matrix by mechanical stretching. Appl Phys Lett 1998, 73:1197-1200. 26. FischerJE, Dai H, Thess A, Lee RS, Hanjani NM, DeHaas D, Smalley RE: Metallic resistivity in crystalline ropes of single-wall carbon nanotubes. Phys Rev B 199"7, 55:R4921-R4924. 27. Balents L, Fisher MPA: Correlation effects in carbon nanotubes. Phys Rev B 1997, 55:11973-11976. 28. Fuhrer MS, Varadarajan U, Holmes W, Richards PL, Delaney P, • Louie SG, Zettl A: Transport and localization in single-walled carbon nanotubes. In Progress in Molecular Nanostructures. Edited by Kuzmany H, Fink J, Mehring M, Roth S. AlP Conf Proc 1998, 442:6-73.

33

Large electric field supresses the low temperature divergence in resistivity, and the temperature dependence fits a 3D variable range hopping model. 29. FischerJE, Lee RS, Kim HJ, RinzlerAG, Smalley RE, Yagushinski SL, Bozhko AD, Slovsky DE, Nalimova VA: Bulk properties of crystalline single wall carbon nanotubes: purification, pressure effects and transport. In Progress in Mo/ecu/ar Nanostructures. Edited by Kuzmany H, Fink J, Mehring M, Roth S. AlP Conf Proc 1998, 442:34-38. 30. Kaiser AB, Dusberg G, Roth S: Heterogeneous model for conduction in carbon nanotubes. Phys Rev B 1998, 57:141-142. 31. Tans SJ, Verschueren ARM, Dekker C: Room-temperature transistor ee based on a single carbon nanotube. Nature 1998, 393:49-52. Nonlinear I-V characteristics similar to BARII-r (barrier injection transit time) diode achieved at 300K on an individual SWNT. 32. Martel R, Schmidt T, Shea HR, Hertel T, Avouris P: Single- and multi • e wall carbon nanotube field-effect transistors. App/Phys Lett 1998, 73:2447-2449. Same observations as in [31 "°] but for MWNT. 33. Chico L, Crespi VH, Benedict I_X, Louie SG, Cohen ML: Pure carbon nanoscale devices: nanotube heterojunctions. Phys Rev Lett 1996, 76:971-974. 34. Menon M, Strivastava D: Carbon nanotube T-junctions: nanoscale metal-semiconductor-metal contact devices. Phys Rev Lett 1997, 79:4453-4456. 35. Collins PG, Zettl A, Bando H, Thess A, Smalley RE: Nanotube nanodevice. Science 1997, 278:100-103. 36. Smith BW, Monthioux M, Luzzi DE: Encapsulated C60 in carbon nanotubes. Nature 1998, 396:323-324. 37. Dai H, Franklin N, Han J: Exploiting the properties of carbon • nanotubes for nanolithography. App/Phys Lett 1998, 73:15081510. Field-induced anodization of oxidized Si(100) using a MWNT tip on an AFM.