Nicolas Bruque

Conductance switching in diarylethenes bridging carbon nanotubes

The recently reported photoswitching of diarylethene derivative molecules bridging carbon nanotube (CNT) contacts is theoretically analyzed. The short lifetime of the lowest unoccupied molecular orbital (LUMO) indicates that neither the open nor closed form of the molecule can be photoexcited into a charge-neutral excited state for any appreciable length of time preventing photochromic ring opening. Analysis of the highest occupied molecular orbital (HOMO) and LUMO lifetimes also suggests that photoexcitation results in oxidation of the molecules. This either reduces the quantum yield of photochromic ring closing, or it gives rise to the possibility of oxidative ring closing. Analysis of the resistance values and energy levels indicates that the HOMO energy levels of the closed isomers relevant for transport must lie within a few k(B)T of the CNT Fermi level. For armchair contacts, the change in resistance with isomer or substituent group is the result of shifts in the energy level of the molecular HOMO. The coupling of the molecular HOMO to the CNT contacts is insensitive to the isomer type or substituent group. For zigzag CNTs, the conductance is dominated by surface states at the Fermi level on the cut ends of the CNTs so that the conductance is relatively insensitive to the isomer type, and the conductance switching ratio is low. Multiple bridging molecules can interact coherently, resulting in energy splitting, shifting, and interference that cause a nonlinear change in conductance with increasing numbers of molecules. Instead of a factor of 3 increase in conductance expected for three independent channels, a factor of 10(3) increase in conductance is obtained for three bridging molecules.

Electronic transport through a CNT-Pseudopeptide-CNT hybrid material

We present electron transmission studies on a pseudopeptide fragment (P) linking two (10,0) semi-infinite carbon nanotubes (CNTs). Calculations are performed using the non-equilibrium Green function formalism (NEGF) implemented within the tight-binding molecular dynamics density functional theory code FIREBALL. Results are compared with the transmission for an ideal (10,0) infinite CNT and a hydrogen passivated CNT-pseudopeptide-CNT (CNT–P–CNT) structure. The transmission spectrum indicates that the pseudopeptide fragment acts as a good bridge for hole transfer and strongly suppresses electron transfer.

Modeling and Design of Beyond the Roadmap Materials and Devices: Nanowires, Nanotubes, and Molecules

As device dimensions shrink to the molecular scale, theory and modeling assume an ever greater role. The analytical ability to experimentally determine the chemistry and geometry for individual molecular devices does not yet exist. Heroic experiments can be required to fabricate devices at this scale. Theory and modeling can relatively quickly explore the effect of the microscopic chemistry and geometry determining the electron and hole transport. The performance metrics of extremely scaled, difficult-to-fabricate designs can be compared. Theory and modeling can identify promising directions and provide physical understanding of experimental results. We apply theory and modeling to understand, analyze, design and optimize CNT, nanowire, and molecular based devices. The theory, modeling and design of chemically and biologically assembled carbon nanotubes (CNTs) is described. A CNT-molecular-resonant tunneling device is analyzed. Interface geometry is shown to have a large effect on the electron and hole transport. The intrinsic performance metrics of a CNT field effect transistor on insulator (COIFET) are determined.