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Nanotube electronics and optoelectronics Among the many materials that have been proposed to supplement and, in the long run, possibly succeed Si as a basis for nanoelectronics, carbon nanotubes (CNTs) have attracted the most attention. CNTs are quasi-one-dimensional materials with unique properties ideally suited for electronics. We briefly discuss the electrical and optical properties of CNTs and how they can be employed in electronics and optoelectronics. We focus on single CNT transistors, their fabrication, assembly, doping, electrical characteristics, and integration. We also address the possible use of CNTs in optoelectronic devices such as electroluminescent light emitters and photodetectors. Phaedon Avouris* and Jia Chen IBM Research Division, T. J. Watson Research Center, Yorktown Heights, NY 10598, USA *E-mail: [email protected] In the last few decades, we have witnessed exponential advances associated leakage currents and passive power dissipation, short in Si microelectronics. These advances in computing, channel effects, variations in device structure and doping, etc. communications, and automation are affecting just about every 46 The realization of the approaching limits of scaling has inspired a aspect of our lives and are responsible for bringing in the worldwide effort to develop alternative device technologies. Some ‘information age’. To a large extent, these advances have been the involve radical departures from the existing technology and are result of the continuous scaling, i.e. miniaturization, of electronic considered for the long term. A shorter term approach discussed here devices, particularly of the metal-oxide-semiconductor field-effect maintains the field-effect transistor (FET) principle, as well as the transistor (MOSFET), that has led to denser circuitry and faster general current circuit architecture, but replaces the Si channel of the switching1,2. For example, since 1970 the physical length of the FET by a one-dimensional nanostructure with superior electrical gate and thickness of the gate insulator of MOSFETs have been transport properties. In addition to the efforts to develop new scaled by factors of about 500 and 120, respectively. However, the electronic devices, direct bandgap one-dimensional nanostructures are scaling cannot continue forever; a number of fundamental attracting attention because of the desire to base both electronic and scientific as well as technological limitations place lower limits to optoelectronic technologies on the same material. Among the different the size of Si devices. These involve, among others, electron one-dimensional materials, single-walled CNTs3,4 have many highly tunneling through short channels and thin insulator films and the desirable and distinctive device properties5-20. OCTOBER 2006 | VOLUME 9 | NUMBER 10 ISSN:1369 7021 © Elsevier Ltd 2006 Nanotube electronics and optoelectronics We briefly examine the unique electronic and optoelectronic properties of CNTs. We discuss the structure and fabrication of single CNT-FETs, their properties and electrical characteristics, doping and device integration. We also describe simple CNT optoelectronic devices such as electroluminescent light emitters and photodetectors. Electronic structure of carbon nanotubes Single-walled CNTs are extremely strongly bonded, hollow carbon atomic structures with diameters typically in the range of 1-3 nm. Their electronic structure is usually described in terms of the electronic structure of a folded ‘graphene’ sheet (a layer of graphite)5-7. The CNT circumference is then expressed by a chirality vector C connecting two crystallographically equivalent sites of the two-dimensional graphene sheet (Fig. 1): C = na1 + ma2, where a1 and a2 are the unit vectors of the hexagonal honeycomb lattice. The structure of any CNT can REVIEW FEATURE an (n,m) CNT is metallic when n = m; it has a small gap when n – m = 3i, where i is an integer; while CNTs with n – m ≠ 3i are truly semiconducting5-7. Semiconducting CNTs have a diameter-dependent bandgap Eg. A single-particle, tight-binding description of the electronic structure gives Eg = γ (2a/dCNT), where Eg is the bandgap energy, γ is the hopping matrix element, a is the C-C bond distance, and dCNT is the diameter of the nanotube. Inclusion of electron-electron interactions raises the size of Eg significantly, but its 1/dCNT dependence appears to remain valid. While the electrical and optical bandgaps of semiconducting CNTs were initially considered to be identical, based on the single particle model, we now know that the optical gap is smaller because of the attractive electron-hole (e-h) interaction and that the optical excitations of CNTs involve transitions to exciton states, not interband transitions (Fig. 2)16-20. therefore be described by a pair of integers (n,m) that define its chiral Transport properties vector. The interesting electrical properties of CNTs are largely the result of In general, scattering of carriers by defects or phonons determines the the unusual electronic structure of graphene itself. In going from electrical transport properties of materials and devices. Because of the graphene to a CNT by folding, one has to account for the additional lack of boundaries in the perfect, hollow cylinder structure of CNTs, quantization arising from electron confinement around the CNT there is no boundary scattering, which plagues other nanostructures circumference. This circumferential component of the wave vector kC such as thin body Si MOSFETs, nanowires, or graphene slices. can take only values fulfilling the condition kC.C = 2n, where n is an Furthermore, CNTs are quasi-one-dimensional materials in which only integer5-7. forward and backward scattering is allowed (low-angle scattering is As a result, each graphene band splits into a number of one- dimensional subbands labeled by n. These allowed energy states are suppressed). Yet the fact that they are not truly one-dimensional cuts of the graphene band structure. When these cuts pass through a materials, that the cylinder has a finite size, allows the carriers in them K point (Fermi point) of the graphene Brillouin zone, the tube is metallic. Otherwise the tube is semiconducting. It can be shown that to ‘bypass’ certain defects. Because of the large momentum change involved in backscattering, only scatterers with a strong, short-ranged Fig. 1 Representation of the CNT atomic structure through the folding of a graphene strip. The chirality vector C and the one-dimensional translational vector P of a (5,2) CNT are shown as an example. OCTOBER 2006 | VOLUME 9 | NUMBER 10 47 REVIEW FEATURE Nanotube electronics and optoelectronics Fig. 2 Computed carrier (electron, hole) energy dependence of the electron-phonon scattering rate of a (25,0) CNT at 300 K and 10 K. The phonon modes corresponding to the peaks in the scattering rate are identified. potential are effective. Thus, elastic scattering mean free paths in CNTs are long, typically of the order of micrometers21-24. At low energies (low applied voltages), the carriers in CNTs can interact with the acoustic phonons of the lattice, but the small electron-acoustic phonon (e-ph) coupling leads to long, temperaturedependent mean free paths of the order of a micrometer25. The radial breathing acoustic mode, however, has a stronger e-ph coupling and when resonantly excited does affect transport26,27. In long (many micrometer) CNTs, acoustic phonon scattering leads to diffusive transport, but the mobility can still be extremely high – of the order of 100 000 cm2/Vs28,29. At high bias, however, when the energy of the carriers reaches those of the zone-boundary (~160 meV) and optical phonons (~180 meV), strong inelastic scattering takes place and the mean free path is reduced to about 10-20 nm27,30-33. Fig. 2 shows theoretical results for the phonon scattering rate as a function of the carrier energy27. Carbon nanotube field-effect transistors The first CNT transistors were fabricated back in 199834,35. In the early efforts, an individual CNT was placed on top of two metal electrodes on a thick (100-200 nm) SiO2 film and the heavily doped Si substrate itself was used as a back gate. While functional, these devices had low performance, primarily because of high series contact resistances (≥1 MΩ) resulting from the weak van der Waals bonding between the CNT channel and the source/drain metal electrodes. A more intimate metal-CNT contact was achieved by depositing the metal electrodes on top of the CNT and subsequently annealing the contacts36. This approach increased the drive currents and transconductance by several orders of magnitude and produced switching Ion/Ioff ratios of 106. Fig. 3 shows the output characteristics of such a back-gated CNT-FET37. 48 OCTOBER 2006 | VOLUME 9 | NUMBER 10 CNT-FETs fabricated using thick gate dielectrics (e.g. 100 nm SiO2) and medium-to-high work function metals (e.g. Ti and Pd) are mostly p-type in air. Annealing such a p-type transistor in vacuum can convert the device to ambipolar and even further to n-type38,39. This was one of the first experimental indications that Schottky barriers (SBs) at the metal-CNT interfaces dominate the transistor switching. Unlike conventional bulk-switched Si devices, whose source and drain contacts are engineered to be ohmic1, switching in CNT-FETs may arise in the bulk or at the contacts, depending on the CNT diameter, nature and geometry of metal electrodes, gate dielectrics, and device geometry. In general, charge transfer at the metal-CNT interface leads to the generation of a SB. Experimental and theoretical work has shown that SBs are much thinner in one dimension than those in three dimensions, and carrier tunneling through the SBs becomes important39-42. Adsorbed species on the electrodes change the surface dipole, i.e. the local work function, thereby modifying the energy level line-up at the metal-CNT contacts38,40. Other types of experiments, such as the observation of large potential drops at the source and drain contacts by scanning gate microscopy10,43, have provided further evidence for the importance of SBs. In a CNT-FET, Fermi-level pinning at the metal-nanotube interface is weak40. To first order, the magnitudes of SBs at metal-CNT interfaces are determined by the band gap of the CNT material (~1/d) and the local work function of the contacting metal44. The larger the CNT diameter, the smaller the SB will be. Because of the involvement of tunneling and thermionic emission in the injection of carriers at the contacts, the dependence of the on-current of the transistor on CNT diameter (i.e. SB) is very strong (exponential), as shown in Fig. 444. It is this strong dependence that necessitates the precise control of the CNT diameter in electronics. So far, there have been no direct Nanotube electronics and optoelectronics REVIEW FEATURE Fig. 3 Output characteristics (Ids against Vds) of a 600 nm long, 1.8 nm diameter, back-gated CNT-FET. Inset: schematic of the structure of the CNT-FET. (Adapted from37 and reprinted with permission. © 2005 IEEE.) experimental correlations between CNT-FET function and CNT current for Pt CNT-FETs is significantly larger than that of Pd CNT- chirality. However, for CNTs with diameters within the desirable range FETs45. for electronic applications, i.e. d ≥ 1.8 nm, Fig. 4 suggests that chirality does not to play a significant role. For hole transport, the higher the metal work function, the smaller The width of the SB depends critically on the electrostatic environment (screening). By reducing the gate oxide thickness and/or increasing its dielectric constant k, the screening length (the distance the SB (see Fig. 4). Thus, a small diameter CNT contacted with low over which the metal-CNT contact fields penetrate into the CNT work function electrodes, e.g. Al, has large SBs for p-type transport. On channel) decreases, reducing the SB width and increasing its the other hand, a large diameter CNT (d > 2 nm) contacted by certain transparency41. Indeed, thin, high-k materials (HfO2, ZrO2) have been high work function electrodes, e.g. Pd, can produce nearly ohmic used as gate dielectrics to improve device performance42,46. contacts in p-type transistors45. In addition to the work function of the metal, other factors, Unlike the surface of Si, the bonds on a CNT surface are satisfied and there are no dangling bonds that could form interface traps. A such as the adhesion (wetting) of the CNT by the metal, play very efficient coupling of the gate to the channel can be accomplished important roles in the transport properties of CNT-metal contacts. For by using an electrolyte solution as the gate47. The high dielectric example, the work function of Pt is similar to that of Pd, but the on- constant of the electrolyte (~80) and its ultrathin (~0.5 nm) Helmholtz Fig. 4 Experimental CNT-FET on-current (left axis) and computed SB height (right axis) as a function of the CNT diameter for three types of CNT-FETs involving Pd, Ti, and Al source and drain contacts. (Adapted from44 and reprinted with permission. © 2005 American Chemical Society.) OCTOBER 2006 | VOLUME 9 | NUMBER 10 49 REVIEW FEATURE Nanotube electronics and optoelectronics layer lead to a very high transconductance, but also to a slow switching a reasonable Ion/Ioff ratio (≥104 is typically desirable in logic rate. applications). However, the physics that leads to efficient gate SBs in a CNT-FET impact the characteristics of both the on and off states of the transistor. When the gate voltage Vgs is much higher than the threshold voltage Vth, the CNT-FET is turned on and its drive switching (e.g. thinner gate insulators) and high on-currents in small bandgap CNTs also tends to increase the off-current. In a SB-CNT-FET, the injection of carriers is governed by the current increases with increasing CNT diameter: log10Id ∝ -Eg ∝ -1/dCNT transparencies of the contacts (dependent on the local electrical fields). (Fig. 4)44. In addition to the bandgap dependence on dCNT, electron and Thus, the injection of holes is controlled by the gate field and electrons hole effective masses m*, decrease with increasing dCNT. Tunneling by the drain field. When one of the SBs is much smaller than the other through the SBs will be more efficient as a result of the lower m*, (e.g. the source contact), transport through the device is dominated by leading to higher drive currents. This is consistent with reports of the carriers (holes) injected from that contact. Transport under this higher on-currents for CNT-FETs based on large diameter (2-4 nm) condition involves only one type of carrier: it is unipolar. When the CNTs, usually produced by chemical vapor deposition techniques45, gate and drain fields become comparable (the two SBs are rather than smaller diameter (0.8-1.5 nm) CNTs, produced by high- comparable), it is then possible to inject both electrons and holes pressure CO conversion (HipCO) or laser ablation techniques36. through the two opposite contacts of the transistor. Transport in this When Vgs < Vth, the transistor is in its off state. An important parameter in this regime is the inverse subthreshold slope S that regime involves two types of carriers: it is ambipolar. CNT-FETs with thin gate dielectrics and/or small bandgap CNTs under high drain bias measures the how efficiently the gate controls conduction though the tend to become ambipolar. I-V characteristics of such an ambipolar channel. It is defined as S = (dlog10Id /dVgs)-1. In a transistor with device are shown in Fig. 548. The drain current is a leakage current that ohmic source and drain contacts (as in a conventional Si MOSFET), S is increases exponentially with increasing Vds , thus reducing the Ion/Ioff limited by thermionic emission over the channel and is ~kBT/q. Thus, at ratio49. This problem can be solved by electrostatically controlling the room temperature, its limiting value is 60 mV/dec. In a transistor with source and drain contacts separately with multiple gates or introducing SBs dominating the transport, S is generally higher and depends on the charge-transfer doping to obtain different injection rates for electrons electrostatics of the device, including the thickness and dielectric and holes48,50. constant of the gate dielectric, geometry of the electrodes, and trapped Most experiments on CNT-FETs have been on back-gated devices charge around the contacts. The early back-gated, bottom-contact because of the simplicity of their fabrication. To build integrated CNT-FETs with thick gate oxides had S as high as 1 V/dec. Currently, circuits, however, it is vital to be able to gate each CNT-FET devices with thin oxides (<10 nm) have S in the range of independently. Wind et al.51 fabricated the first independently top- 100-150 mV/dec. Theoretical modeling has also predicted gated, thin gate oxide, high-performance CNT-FETs. These CNT-FETs improvements in transistor switching by using thinner and higher provide the first evidence that CNT-FETs could compete with state-of- dielectric constant gate insulators and smaller contacts40-42. the-art Si MOSFETs. Later, Lin et al.48 demonstrated a double-gate It is not only the on-current of the transistor that is important, the CNT-FET structure that allows the independent control of the current in the off state is equally important. It is critical to maintain a switching of the SBs and of the CNT channel, and successfully converts low leakage current to keep both the passive power at a minimum and ambipolar CNT-FETs to unipolar ones. These double-gate CNT-FETs Fig. 5 Transfer characteristics of a large diameter CNT-FET (~1.8 nm) at different drain voltages. 50 OCTOBER 2006 | VOLUME 9 | NUMBER 10 Nanotube electronics and optoelectronics REVIEW FEATURE Fig. 6 (a) Device transfer characteristics of a CNT-FET before and after triethyloxonium hexachloroantimonate (C2H5)3O+SbCl6- (OA) doping at Vds = -0.5 V. (b) Transfer characteristics of a CNT-FET before and after n-doping with hydrazine at Vds = 0.5 V. (Part (b) adapted from59 and reprinted with permission. © 2005 American Chemical Society.) produces S values very close to the thermal limit48. Subsequently, by More recently, a more complex and sophisticated circuit, a five- using the same CNT-FETs and filtering the electron energy distribution stage ring-oscillator (RO), was built on a single, long CNT63. ROs are through band-to-band tunneling, S values below the thermal limit can essential for the characterization of the ac properties of CNT-FETs. In be achieved (~40 meV/dec at 300 K)52. CNT-FETs with very long this RO circuit, five pairs of p- and n-type CNT-FETs in CMOS channels (many micrometers) operating in the diffusive regime are configurations are wired along the length of an ambipolar CNT. Pd dominated by the bulk barrier and are, therefore, bulk-switched like metal is used for all source and drain electrodes. Since the threshold MOSFETs29. voltages for the p- and n-branches of the ambipolar CNT are quite different, two different metals are used to construct the top gates: Pd Doping of nanotubes for the p-type and Al for the n-type transistors. The work function For complementary metal-oxide-semiconductor (CMOS)-type difference of the two metals effectively shifts the FET thresholds and applications, one would like to have both p- and n-type CNT-FETs. allows the formation of the CMOS structure. This novel approach However, semiconducting CNTs cannot be doped using the traditional eliminates the need for doping altogether. As gate insulator, a high-k approach for bulk semiconductors of ion implantation. Furthermore, material, AlOx , was used so as to improve the coupling of the CNT and especially in small-diameter CNTs, carbon substitution by a dopant the gate. In addition to the five-stages, two more inverters are added, such as B or N induces high strain and leads to a defective CNT one at the beginning of the chain to determine the ideal operating lattice53,54. conditions and one at the end to avoid interference with the For this reason, charge-transfer doping has been employed. Charge-transfer dopants are used primarily for two reasons in CNTs: measurement setup (spectrum analyzer). The complete circuit is shown (a) to convert ambipolar devices to p-type50, and (b) to convert p-type in Fig. 7. devices to n-type by, for example, depositing electron donors on them such as K atoms39,55 or amine-containing molecules56-59. Figs. 6a and 6b show examples of both p- and n-type CNT doping. Apart from With this first circuit, an oscillation frequency of up to ~70 MHz can be achieved (at VDD = 1 V), which corresponds to a delay of 1.4 ns per stage. Although this frequency is much higher (~105 times) chemical doping, electrostatic doping through multiple gates has been than those achieved by oscillators based on different CNTs61,62, it is used successfully to suppress SBs and produce bulk-switched p-i-p and still relatively low. However, this frequency does not represent an n-i-n structures48. inherent limitation of the CNTs, which is expected to be in the terahertz range64, but is determined by the parasitics of the circuit that Integrated nanotube electronic circuits can be eliminated by improvements in fabrication. Following the optimization of individual transistors, the obvious next step is to attempt to integrate them into logic circuits. The first CNT Self-assembly of nanotube devices logic gate, a NOT gate, was demonstrated by Derycke et al.60. In this In addition to their superior electrical properties, CNTs may offer the case, a single CNT was patterned into a p-type and an n-type (through possibility of simpler, less expensive device fabrication. As a molecular K-doping) FET pair to form a voltage inverter. This work was followed entity, CNTs should be able to self-assemble into desirable structures. by demonstrations of a number of different logic gates, usually built For the most part, two different strategies have been pursued: with FETs involving separate CNTs61,62. (a) prepatterning of the substrate with chemical species to which CNTs OCTOBER 2006 | VOLUME 9 | NUMBER 10 51 REVIEW FEATURE Nanotube electronics and optoelectronics (a) (b) Fig. 7 Scanning electron microscope image of a five-stage CNT ring oscillator circuit. A single, long CNT molecule is patterned to form the oscillator (see upper right insert). p- and n-type FETs are formed by using different gate metals, Pd and Al, respectively. A 10 nm thick AlOx film is used as the high-k gate insulator (bottom left insert). (Adapted from63 and reprinted with permission. © 2006 American Association for the Advancement of Science.) tend to adhere65,66 or be repulsed by; and (b) functionalizing the CNTs released as heat (phonons), but a fraction of the recombination events themselves with groups that selectively adsorb to a particular structure may involve the emission of a photon. This process is termed on the substrate67,68. Assembly of CNTs on insulators, particularly high-k materials such as the basic oxides HfO2 and Al2O3, is of electroluminescence and is widely used to produce solid-state light sources such as light-emitting diodes (LEDs). particular interest in electronics. Acidic groups, such as phosphonic In order to fabricate LEDs or any other electroluminescent device, (-PO2OH) or hydroxamic (-NHOH) groups, attach rather strongly to one must recombine significant populations of electrons and holes. these oxides66. Thus, selective adsorption can be achieved by Conventionally, this is achieved at an interface between a hole-doped patterning such an oxide surface, for example by imprint lithography, and an electron-doped material (e.g. a p-n junction). In ambipolar CNT- with molecules of the type L-R-PO2OH, where L is a group with affinity FETs at an appropriate bias, however, electrons and holes can be for CNTs, e.g. a -NH2 group, or a nonadhering group, e.g. -CH3. simultaneously injected at the opposite ends of the CNT channel. This A typical example of CNT assembly through functionalization is provided by DNA wrapping69-72. Functionalizing the CNTs themselves could provide even better control of their placement. However, the functionalization must be reversible so that it does not degrade the allows electroluminescence to occur73. While the emission mechanism is the same as that in p-n junctions, ambipolar CNT-FETs do not require chemical doping. CNT electroluminescence exhibits a variety of interesting properties. excellent intrinsic electrical properties of the CNT in the final device. The emitted light is strongly polarized along the tube axis. The Covalent functionalization tends to destroy these properties by radiation also has a characteristic energy that depends on the diameter converting the sp2 hybridization of the C atoms to sp3. Recently, an and chirality of the excited CNT, just as the optical bandgap does, and approach was proposed that bypasses this problem. The CNTs are the length of the electroluminescent region is lrec ≤ 1 µm74. In short reacted with derivatized diazonium salts of the type X - N2+-R -L , to give devices (L < lrec , where L is the channel length), the light emission covalently bonded derivatives of the type CNT-R-L, which as discussed encompasses the entire CNT. In long devices (L >> lrec), on the other above, adhere to the appropriate oxide structures through the group L. hand, the emission will be localized wherever the concentrations of In this case, however, the CNT-R bond can be cleaved cleanly after electrons and holes overlap most strongly. This overlap region can be deposition by thermal annealing, and the electrical properties of the physically moved using a gate electrode, since the relative pristine CNT can be recovered68. contributions of electrons and holes to the total current is strongly gate dependent75. Therefore, a CNT-LED is a movable light source. The 52 Optoelectronic devices gate bias Vg can smoothly and continuously position the site of Electron and hole carriers in semiconductors can recombine by a emission75. In Fig. 8, we demonstrate the translation of the emission variety of different mechanisms. In most cases, the energy will be spot between two electrodes by applying different gate voltages. OCTOBER 2006 | VOLUME 9 | NUMBER 10 Nanotube electronics and optoelectronics REVIEW FEATURE Fig. 8 Movement of the emission spot between source and drain electrodes produced by e-h recombination in an ambipolar CNT-FET by varying the applied gate voltage. (Adapted from75 and reprinted with permission. © 2004 American Physical Society.) (a) (b) (c) Fig. 9 (a) Schematic of the structure used to generate localized light emission under unipolar transport conditions in a CNT. A trench is etched in the SiO2 along the channel of a back-gated CNT-FET. (b) Optical microscope image of the trench and infrared emission near its edge. (c) Variation of the emission intensity with gate voltage. (Parts (b) and (c) adapted from77 and reprinted with permission. © 2005 American Association for the Advancement of Science.) In addition to this translatable emission, localized excitation of a CNT generates an electric current, which can be used as electroluminescence is also observed from particular spots on a CNT a nanosized photodetector79 or as a spectroscopic tool80. Alternatively, under unipolar transport conditions75,76. In this case, the current is in the open-circuit configuration, the device generates a carried by only one type of carrier (electrons or holes). Since both types photovoltage79. Thus, a CNT-FET device can be used as a transistor, of carriers are necessary to produce light, these sites must actively generate e-h pairs. This process occurs near defects, trapped charges in the insulator, or any other inhomogeneities that produce voltage drops along the CNT and generate large, local electric fields. The resulting ‘hot’ carriers produce e-h pairs through a most efficient intra-nanotube impact excitation process (Fig. 9a)77,78. The efficiency of this process can be traced in the quasi-one-dimensional confinement and the weak screening of the Coulombic interactions77,78. The monitoring of localized electroluminescence provides a new tool for detecting defects in CNT devices. Artificial structures can also be fabricated that create the conditions locally, i.e. sudden change in the potential, that generate e-h pairs and light emission77. Such a device is shown in Fig. 9b. Unlike the ambipolar device emission, the light intensity of the unipolar devices depends exponentially on the current, a fact that supports the impact mechanism (Fig. 9c)77. Photoconductivity is the reverse of electroluminescence, with optical radiation producing electron and hole carriers. An example of a photoconductivity measurement is shown in Fig. 10. The resonant Fig. 10 Photoconduction current against light energy in the region of the second allowed exciton state, E22, of a CNT. Inset: polarization dependence of the photocurrent. OCTOBER 2006 | VOLUME 9 | NUMBER 10 53 REVIEW FEATURE Nanotube electronics and optoelectronics light emitter, or light detector. Choosing among these different modes technology will be replaced by CNTs or, for that matter, by any other of operation only requires the bias conditions to be changed. type of technology anytime soon. The speed at which CNT technology evolves will depend strongly on breakthroughs, such as the Conclusions development of novel integration processes. CNTs also provide the It is clear that the unique properties of CNTs make them excellent opportunity for a wealth of new types of applications, e.g. in candidates for nanoelectronics and photonics, and the devices already bioelectronics, and most importantly for new, low-cost methods of demonstrated prove this point. However, as is true with any new fabrication. 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Derycke, V., et al., Appl. Phys. Lett. (2002) 80, 2773 80. Qiu, X., et al., Nano Lett. (2005) 5, 749 OCTOBER 2006 | VOLUME 9 | NUMBER 10