Molecular-scale devices from first principles

[摘要] (cont.) We show that these properties are well converged for slabs that have between 5 and 10 layers, depending on the property considered and the surface orientation. We then focus on understanding and characterizing devices. Since it is widely proposed that carbon nanotubes (CNTs) could replace Si in future transistor devices, we examine the work function of single-wall CNTs and the effects that covalent functionalization could have in engineering performance. Electrostatic dipoles form due to the charge asymmetries in the functionalized CNT unit cell, and the use of periodic boundary conditions affects our calculations. We correct for these spurious dipole-dipole interactions with a real-space potential derived directly from the solution to Poisson;;s equation in real-space with open boundary conditions. We find that the functionalizations can be clearly labeled as electropositive and electronegative, and that they decrease or increase the work function of the CNT accordingly. Finally, we join metal surfaces and CNTs to study Schottky barrier heights (SBHs) that form at the interface. We take Al(111) and Pd(111) as examples of low- and high-work function metal surfaces and contact them with the semiconducting (8,0) CNT. We find that in all cases a surface dipole forms that shifts the band structure of the CNT locally. In these systems, we investigate the effects of surface roughness and functionalization on SBHs, and find that controlling the electrostatics at the interface (with functionalization, adsorbates, and device geometry) can lead to further engineering of the SBHs.

[发布机构] Massachusetts Institute of Technology