Perspectives

Carbon nanotubes have a great potential in the development of electronic devices with diverse functionality since their electronic properties are themselves diverse, depending on the geometry of the nanotube lattice, the contacts used in the devices, and the temperature. At room temperature the transport may be ballistic in samples with high structural perfection. In the cases where the contacts create tunnel junctions, one can expect nonlinear I-V characteristics which are the signature of the Luttinger liquid behavior. At much lower temperatures, quantum interference effects in the propagation of the electrons can be observed in samples with highly transparent contacts while, in the case of very thick ropes, the reduction in the strength of the Coulomb interaction may give rise to the superconductivity of the nanotubes.

The first step toward the use of the carbon nanotubes in molecular electronics requires the integration of several nanotube devices in order to produce the desired functionality. Important progress is already being made in this direction. In [15], the architecture of two perpendicularly crossed arrays of nanotubes has been proposed as a model for a nonvolatile random access memory. Each crossing point for two perpendicular nanotubes constitutes an addressable device element. The junctions have two stable positions, with a different separation between the crossed nanotubes that can be controlled electromechanically. This allows the definition of ON and OFF states at each crossing point, characterized by respective resistances which may differ in general by orders of magnitude. The feasibility of the proposal has been supported by the realization and investigation of junctions made of crossed ropes of nanotubes [15]. The measurement of the I-V characteristics of crossed nanotube junctions has been also accomplished in [94], with different combinations of individual single-walled nanotubes with metallic and semiconducting character.

Another experimental accomplishment which has opened the way for a nanotube-based electronics can be found in [16]. That work has reported the construction of the first circuit based on a single nanotube capable of performing a logic operation. The circuit represents what is called a voltage inverter, by which a logical 1 can be transformed into a logical 0 and vice versa. This is the realization of the NOT logic function, that is combined with the AND and OR logic operations to build the complex structure of modern microprocessors. A new experimental development has been needed in the construction of the logic circuit, since this requires to place in series two field-effect transistors being respectively of «-type (with excess of conduction electrons) and p-type (with conduction achieved by electron holes). While nanotubes are usually found with the latter character, the transformation to «-type has to be accomplished by doping the nanotubes with alkali metals or, in a simpler way, by heating the nanotubes in a vacuum as shown in [95]. An important feature of the circuit is the gain, which relates the strength of the output to that of the input signal and which, in this case, reaches the value of 1.6. This opens the possibility of assembling gates of the kind proposed into more complex circuits.

The operation of several small circuits built from the combination of nanotube field-effect transistors has been also shown in [17]. What is special in the integration of these devices is that each nanotube transistor has its own local gate, so that the effect of doping by varying the corresponding gate voltage can be controlled independently in each nanotube. A very large capacitive coupling has been achieved between the semiconducting nanotube and the nearby gate, making it possible to shift the Fermi level in the nanotube from the valence band (p-doped regime) to the conduction band («-doped regime) under variations in the gate voltage. In this way, the integration of the nano-tube transistors has allowed to realize several logic circuits, like an inverter, a NOR logic element, or a static random access memory element.

An interesting finding has been that the field-effect transistors made of single nanotubes can have better performance than the leading silicon transistor prototypes [96]. This has been realized in the process of building nanotube-based transistors with larger capacitive coupling between the nanotube and the gate electrode, that controls the density of charge carriers in the molecule. The advances in the design of the transistors have come from placing the gate electrode on top of the nanotube and using a thinner dielectric between them [96]. Thus, smaller variations in the voltage of the gate electrode can lead to significant changes in the resistance of the nanotube. This new kind of transistors has led to a high transconductance (the measure of the capability to carry electric current) at low voltages, outperforming in this respect the best silicon transistor prototypes [96].

The route toward the large-scale integration of nanotube devices presents great complexities, but carbon nanotubes have already shown the potential for more straightforward applications. One of them arises from the strong coupling between the electronic properties and mechanical deformations, that may include the twisting [97], bending [98], or stretching [99-102] of the carbon nanotubes. It has been shown that, in the case of semiconducting nanotubes, a semiconductor-metal transition can take place upon application of sufficient uniaxial strain [102]. The reverse trend has been also measured, by pushing a metallic carbon nano-tube with the tip of an atomic force microscope to produce a decrease of nearly two orders of magnitude in the conductance [103-105]. These observations open the way to use carbon nanotubes as nanoscale mechanical sensors.

The carbon nanotubes have also shown the potential for piezoelectric applications. The injection of charge into the nanotubes can alter their structure, due to the fact that the carbon-carbon bonds modify their lengths according to the electrons or holes added [106]. These effects have been investigated in nanotube sheets, which are made of highly entangled mats of nanotube bundles. In the experiments reported in [107], the changes in the length of strips of such kind of nanotube paper have been measured as a function of the applied voltage, carrying the operation within a NaCl electrolyte. Thus, the expansion or contraction of the strips has been the result of the injection of electronic charge from the electrodes to the surface of the nanotube bundles, with the electrolyte ions forming layers of respective opposite charges to balance those in the nanotubes [107]. The electromechanical actuators thus designed have shown good performance, being able to generate higher stresses than those of natural muscles. An important advantage over conventional ferroelectric actuators is that the nanotube sheets can provide large strains with applied voltages of just a few volts. The mechanical performance should be enhanced in the case of nonbundled nanotubes, and it has been estimated that, for the sheets made of separate nanotubes, the actuator strain could be of the order of ~1% [107]. A number of possible uses of the nanotube actuators have been proposed, from biomedical applications to flow control at high temperatures.

The application of carbon nanotubes as chemical sensors has been also suggested. The nanotubes have the tendency to adsorb gas molecules in their surface. In the case of semiconducting nanotubes, this has been shown to lead to significant changes in the conduction properties [108, 109]. The gas molecules give rise to a transfer of charge that makes the nanotubes become p-doped semiconductors. The change in their conductivity can give then a measure of very small concentrations of particles in the chemical environment at room temperature, in a much more sensitive way than existing chemical sensors.

Carbon nanotubes can be used as tips in scanning probe microscopes, which provides several advantages over usual silicon tips [110, 111]. The ability that the nanotube tips have to buckle elastically reduces the damage that can be produced when crashing into the sample. They lead to an improvement of the resolution, as a consequence of their small diameter. Moreover, they can be modified at the ends to enable the manipulation of structures at the molecular scale [112]. The construction of nanoscale tweezers has been also possible by attaching a pair of carbon nanotubes to respective electrodes, and controlling the nanotube arms by the voltage applied between them [113]. Such a device has made possible the manipulation of different structures at the nanometer scale.

Finally, the technological applications of carbon nano-tubes can also have a more direct impact in everyday life. They have been proposed for the construction of superca-pacitors, which may take advantage of the large surface area accessible in nanotube arrays. These can give rise to capacitors with high power and storage capabilities. Anyhow, the carbon nanotubes may find the most interesting commercial application as electron sources in field-emission devices [114, 115]. These can be used in flat panel displays, as well as in lamps and X-ray sources. The emission is produced by applying a voltage between a surface with nanotube fibers, acting as a cathode, and a substrate with phosphor arrays. The high local fields created in the nanotube geometry make the electrons jump toward the anode, where the contact with the phosphor produces the spots of light in the display. The flat panel nanotube displays turn out to save more energy and to have higher brightness than liquid crystal displays. A similar field-emission effect can be applied to the generation of X-rays, when the anode is replaced by a metal surface, which can lead to interesting applications for medical purposes. All these developments stress once more the significance that the phenomena taking place in minute devices can have for the construction of useful engines, tailored for the needs of our time.

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