Phaedon Avouris

Computational Study of Exciton Generation in Suspended Carbon Nanotube Transistors

Optical emission from carbon nanotube transistors (CNTFETs) has recently attracted significant attention due to its potential applications. In this paper, we use a self-consistent numerical solution of the Boltzmann transport equation in the presence of both phonon and exciton scattering to present a detailed study of the operation of a partially suspended CNTFET light emitter, which has been discussed in a recent experiment. We determine the energy distribution of hot carriers in the CNTFET and, as reported in the experiment, observe localized generation of excitons near the trench-substrate junction and an exponential increase in emission intensity with a linear increase in current versus gate voltage. We further provide detailed insight into device operation and propose optimization schemes for efficient exciton generation; a deeper trench increases the generation efficiency, and use of high-k substrate oxides could lead to even larger enhancements.

Predictions of Single-Layer Honeycomb Structures from First Principles

Finding a contender for graphene in the field of 2D electronics and in other possible potential applications of nanotechnology has derived active search for graphene like novel structures, which do not exist in nature. As a matter of fact, the types of 3D layered materials, which make the exfoliation of their single-layer (SL) structures possible, are limited only to graphite, 2h-BN, 2h-MoS2, 2h-WS2, black phosphorus etc. However, most of desired electronic and magnetic properties demand materials that do not have layered allotropes. In view of the location of C, B, and N elements in the periodic table, which constitute SL graphene and BN, questions have been raised as to whether other group IV elements, group III–V and II–VI compounds may also form SL structures. The theoretical methods have provided for quick answers to guide further experiments. These methods, based on the quantum theory, have now reached now a level of providing accurate predictions for chemical, mechanical, electronic, magnetic, and optical properties of matter. In our group, we have carried out studies to explore novel materials in SL structure constituted by group IV elements, group III–V and II–VI, group V elements, transition metal oxides, and dichalcogenides, MX2 in hand t-structures. We also consider their functionalization by decoration of ad-atoms, by creation of the mesh of vacancies and voids, by formation of nanoribbons or in-plane heterostructures. Most of the elements which construct SL materials have valence orbitals similar to carbon. These are atoms having s and p valence orbitals, which can allow three folded, planar sp hybrid orbitals to form σ-bonds between two atoms located at the corners of hexagons. This way a three-fold coordinated honeycomb structure can be constructed. Remaining p orbitals form bonding (antibonding) π(π-) bonds with nearest neighbors. While the σ-bonds between atoms maintain the mechanical strength, π–π*-bonds assure the planar geometry and dominate the electronic energy structure near the Fermi level. SL structures including at least one element from the first row of the periodic table, prefer a planar structure such as graphene, h-BN and SiC, since the π-bond is strong enough to maintain the planar geometry. However, the situation is different for SL structures constructed by elements from rows lying below the first one, where nearest-neighbor distance is relatively longer and hence a weaker π-bond cannot maintain the planar geometry. At the end, the structure is stabilized by dehybridization of planar sp