Electrical transport of zig-zag and folded graphene nanoribbons

ABSTRACTIn recent years, there has been much interest in modelling graphene nanoribbons as they have great potential for use in molecular electronics. We have employed the NEGF formalism to determine the conductivity of graphene nanoribbons in various configurations. The electronic structure calculations were performed within the framework of the Extended Huckel Approximation. Both zigzag and armchair nanoribbons have been considered. In addition, we have also computed the transmission and conductance using the non-equilibrium Greens function formalism for these structures. We also investigated the effect of defects by considering a zigzag nanoribbon with six carbon atoms removed. Finally, the effect of embedding boron nitride aromatic molecules in the nanoribbon has been considered. The results of our calculations are compared with that obtained from recent work carried out using tight-binding model Hamiltonians.

Computational studies investigating the effect of sequencing and environment on the conductance of DNA nanowires

ABSTRACTUnderstanding electron transfer in molecular systems is important, especially in the context of molecular electronics. With the desire to incorporate biological molecules in molecular electronic devices, there is a need to establish the relative importance of the various factors like the environment and the molecular structure (DNA sequence) on the electrical conduction. There has been much debate about mechanisms of electron transfer in biological molecules. We have conducted a systematic study of electron conduction across DNA molecular segments using the non-equilibrium Green function (NEGF) method. The Hamiltonian matrix elements were determined within the framework of the Extended Hückel Approximation. In considering (CG) base pair sequences, we find that the conductance decreases with segment length and that the substitution of (AT) base-pairs also reduces the conductance. When the DNA segments are in aqueous solution, the conductance is found to almost double in magnitude.

Qualitative Thinking in the Age of Modern Computational Chemistry, or What Lionel Salem Knows

The achievements of modern computational chemistry are astounding. It is reasonable today to handle billions of configurations, and to achieve chemical accuracy, kilocalories say, in calculating binding energies and geometries, in ground and transition states of reasonably complex molecules. There is no question that the enterprise of computational theoretical chemistry is successful. Now Lionel Salem and I grew up and developed scientifically in the climate of the very same computer which made all this possible. Russ Pitzer taught me to punch cards; I still miss the sound of the key punch. The extended Hückel method, which several of us developed in the Lipscomb group, would have been impossible without modern computers. But I took a different turn, moving from being a calculator in the framework of semiempirical theory, to being an explainer, the builder of simple molecular orbital models. I was and am still doing calculations, but my abiding interest is in the construction of explanations. And also in thinking up moderately unreasonable things for experimentalists to try. In existing as a scientist, meaning that my work was of continuing interest to other chemists, I was helped in that I moved into whatever part of chemistry I did, just a little ahead of the heavy guns of computational chemistry. So I switched from organic to inorganic molecules just when organic molecules became reasonably calculable. Recently I’ve been less fortunate, for when I moved to solids and surfaces I came back into heavy fire—physicists had been doing calculations on these materials for a long time. And they were (are) hardly likely to believe that one-electron calculations and a chemical viewpoint are of value. I want to make some observations on computational quantum chemistry, perforce influenced by my prejudices. Given the advances in the field, any molecule I can calculate (without geometrical optimization), with the simplest extended Hückel approximation, can be done so much better by most computational chemists. So why don’t I feel threatened; why is there a role for people of my ilk? Or for Lionel. Actually, I do feel threatened and bypassed! But that’s just an emotional reaction, and my aging figures in it.