Slava V. Rotkin

High-performance electronics using dense, perfectly aligned arrays of single-walled carbon nanotubes

Single-walled carbon nanotubes (SWNTs) have many exceptional electronic properties. Realizing the full potential of SWNTs in realistic electronic systems requires a scalable approach to device and circuit integration. We report the use of dense, perfectly aligned arrays of long, perfectly linear SWNTs as an effective thin-film semiconductor suitable for integration into transistors and other classes of electronic devices. The large number of SWNTs enable excellent device-level performance characteristics and good device-to-device uniformity, even with SWNTs that are electronically heterogeneous. Measurements on p- and n-channel transistors that involve as many as approximately 2,100 SWNTs reveal device-level mobilities and scaled transconductances approaching approximately 1,000 cm(2) V(-1) s(-1) and approximately 3,000 S m(-1), respectively, and with current outputs of up to approximately 1 A in devices that use interdigitated electrodes. PMOS and CMOS logic gates and mechanically flexible transistors on plastic provide examples of devices that can be formed with this approach. Collectively, these results may represent a route to large-scale integrated nanotube electronics.

Calculation of pull-in voltages for carbon-nanotube-based nanoelectromechanical switches

We study the pull-in voltage characteristics of several nanotube electromechanical switches, such as double-wall carbon nanotubes suspended over a graphitic ground electrode. We propose parametrized continuum models for three coupled energy domains: the elastostatic energy domain, the electrostatic energy domain and the van der Waals energy domain. We compare the accuracy of the continuum models with atomistic simulations. Numerical simulations based on continuum models closely match the experimental data reported for carbon-nanotube-based nanotweezers. An analytical expression, based on a lumped model, is derived to compute the pull-in voltage of cantilever and fixed-fixed switches. We investigate the significance of van der Waals interactions in the design of nanoelectromechanical switches.

Gate capacitance coupling of singled-walled carbon nanotube thin-film transistors

The electrostatic coupling between singled-walled carbon nanotube (SWCNT) networks/arrays and planar gate electrodes in thin-film transistors (TFTs) is analyzed both in the quantum limit with an analytical model and in the classical limit with finite-element modeling. The computed capacitance depends on both the thickness of the gate dielectric and the average spacing between the tubes, with some dependence on the distribution of these spacings. Experiments on transistors that use submonolayer, random networks of SWCNTs verify certain aspects of these calculations. The results are important for the development of networks or arrays of nanotubes as active layers in TFTs and other electronic devices.

High-Frequency Performance of Submicrometer Transistors That Use Aligned Arrays of Single-Walled Carbon Nanotubes

The unique electronic properties of single-walled carbon nanotubes (SWNTs) make them promising candidates for next generation electronics, particularly in systems that demand high frequency (e.g., radio frequency, RF) operation. Transistors that incorporate perfectly aligned, parallel arrays of SWNTs avoid the practical limitations of devices that use individual tubes, and they also enable comprehensive experimental and theoretical evaluation of the intrinsic properties. Thus, devices consisting of arrays represent a practical route to use of SWNTs for RF devices and circuits. The results presented here reveal many aspects of device operation in such array layouts, including full compatibility with conventional small signal models of RF response. Submicrometer channel length devices show unity current gain (f(t)) and unity power gain frequencies (f(max)) as high as approximately 5 and approximately 9 GHz, respectively, with measured scattering parameters (S-parameters) that agree quantitatively with calculation. The small signal models of the devices provide the essential intrinsic parameters: saturation velocities of 1.2 x 10(7) cm/s and intrinsic values of f(t) of approximately 30 GHz for a gate length of 700 nm, increasing with decreasing length. The results provide clear insights into the challenges and opportunities of SWNT arrays for applications in RF electronics.

Raman spectroscopy as probe of nanometre-scale strain variations in graphene.

Confocal Raman spectroscopy has emerged as a major, versatile workhorse for the non-invasive characterization of graphene. Although it is successfully used to determine the number of layers, the quality of edges, and the effects of strain, doping and disorder, the nature of the experimentally observed broadening of the most prominent Raman 2D line has remained unclear. Here we show that the observed 2D line width contains valuable information on strain variations in graphene on length scales far below the laser spot size, that is, on the nanometre-scale. This finding is highly relevant as it has been shown recently that such nanometre-scaled strain variations limit the carrier mobility in high-quality graphene devices. Consequently, the 2D line width is a good and easily accessible quantity for classifying the crystalline quality, nanometre-scale flatness as well as local electronic properties of graphene, all important for future scientific and industrial applications.

Electronic Response and Bandstructure Modulation of Carbon Nanotubes in a Transverse Electrical Field

The electronic properties of carbon nanotubes in a uniform transverse field are investigated within a single orbital tight-binding model. For doped nanotubes, the dielectric function is found to depend not only on symmetry of the tube, but also on radius and Fermi level position. Bandgap opening/closing is predicted for zigzag tubes, while it is found that armchair tubes always remain metallic, which is explained by the symmetry in their configuration. The bandstructures for both types are considerably modified when the field strength is large enough to mix neighboring subbands.

Analysis of non-planar graphitic structures: from arched edge planes of graphite crystals to nanotubes

Abstract A unified approach to the analysis of the mechanisms that lead to the edge reconstruction of graphite and growth of a variety of non-planar graphitic structures, such as nanotubes, is suggested. Transmission electron microscopy (TEM) shows that nano-arches are formed on the edge planes of natural and synthetic graphite, as well as graphite polyhedral crystals, which are built of graphene sheets; this makes the edge reconstruction of graphite different from the surface reconstruction of other crystals. A theoretical study of edge zipping in graphite and formation of tubular carbon structures has been performed using an integrated approach combining molecular dynamics simulation and analytical continual energetics modeling. The suggested theoretical framework describes the formation of curved surfaces in a wide range of dimensions, which is a general feature of the growth of layered materials. Layered materials isostructural to graphite, such as hexagonal BN, demonstrate similar edge structures and also form nanotubes. Thus, the ability of materials to form arches as a result of edge reconstruction points out to their ability to form nanotubes and vice versa. TEM studies of graphite and hexagonal boron nitride provide experimental verification of our analytical model.

An Essential Mechanism of Heat Dissipation in Carbon Nanotube Electronics

Excess heat generated in integrated circuits is one of the major problems of modern electronics. Surface phonon-polariton scattering is shown here to be the dominant mechanism for hot charge carrier energy dissipation in a nanotube device fabricated on a polar substrate, such as SiO(2). By use of microscopic quantum models, the Joule losses were calculated for the various energy dissipation channels as a function of the electric field, doping, and temperature. The polariton mechanism must be taken into account to obtain an accurate estimate of the effective thermal coupling of the nonsuspended nanotube to the substrate, which was found to be 0.1-0.2 W/(m x K) even in the absence of the bare phononic thermal coupling.

Modeling hysteresis phenomena in nanotube field-effect transistors

A model is developed to explain a hysteresis observed experimentally in nanotube field-effect transistors. The model explains the hysteresis through trapping of electrons in an oxide layer. The Fowler-Nordheim tunneling mechanism is held responsible for the electron injection. The influence of different parameters such as the sweeping rate or the range of the gate voltage on the hysteresis is studied and compared with experimental results.

Universal description of channel conductivity for nanotube and nanowire transistors

A theory of drift-diffusion transport in a low-dimensional field-effect transistor is developed. Two cases of a semiconductor nanowire and a single-wall nanotube are considered using self-consistent electrostatics to obtain a general expression for the transconductance. This quantum-wire channel device description is shown to differ from a classical device theory because of the specific nanowire charge density distribution.