Computational Nanoelectronics A. A. Farajian Institute for Materials Research, Tohoku University, Sendai , Japan In collaboration with K. Esfarjani, K. Sasaki, T.M. Briere, R.V. Belosludov, H. Mizuseki, M. Mikami, Y.Kawazoe, and B.I. Yakobson

Overview: Molecular electronics insertion strategy; Active atom wire interconnects  Keeping the initial target application simple, cheap and unsophisticated: passive interconnects  Initial products will be silicon complements with response time of the order of second: sensors  Moving on to active devices, with novel function, form, or cost advantage  Finally; introducing entirely new generation of products: commercial delivery time of more than one decade Molecular Electronics J.M. Tour, World Scientific (2003)

Nanotube molecular quantum wires Credit: C. Dekker

Nanotube nanotransistor Credit: C. Dekker

Nanotube logic nanogate Credit: C. Dekker

Doped nanotube bundle Credit: R. Smalley

Doping with C 60 - and Cs + Credit: G.-H. Jeong 2 nm 4 nm (b) (a)

Formation of junction between empty and Cs + –doped parts Credit: G.-H. Jeong

Conductance of a single benzene molecule Credit: J.M. Tour

DNA conductance along axis D. Porath et al.

Specific systems within the prescribed scheme:  Shielded, passive/active, molecular wires: polythiophene/polyaniline inside cyclodextrines  Building upon the existing silicon base: Bi line on Si surface  Active (rectifying) device: doped nanotube junction  How good is DNA? Cheking DNA’s transport

Doped nanotube junction

Negative differential resistance

Rectifying effect

Doped Nanotube Junctions

Ab initio calculation: inside doping is favored by ~ 0.2 eV

Ab initio calculation: energetics of light and heavy dopings

Ab initio calculation: band structures of light and heavy dopings

Ab initio calculation: density of states of light and heavy dopings

Junction and Bulk Geometries

Surface Green’s Function Matching

Screening charge pattern for doped metallic junction (initial shifts of chemical potentials: 2.5 eV)

Screening charge pattern for doped semiconducting junction ( initial shifts of chemical potentials: 2.5 eV)

Metallic nanotube doped by a charged dopant

Screening charge pattern of (5,5) for an external point charge 1.0 e

Bi line on Si(001): relatively stable

Bi line on Si(001): stable

Hamiltonian and overlap  Using the above-mentioned basis, the Hamiltonian of the system is obtained using Gaussian 98 program  Moreover, as the basis is non-orthogonal, the overlap matrix is also obtained  The Hamiltonian and overlap matrices are then used in calculating the conductance of the system using the Green’s function approach

Reflected and Transmitted Amplitudes; Transmission Matrix

Junction and Bulk Geometries

Conductance

Conductance, alternative derivation  Conductance [2e 2 /h]:  With  Being the Green’s function of the molecule (junction part of the system)

Surface Green’s functions  And  With Σ 1(2) being the surface terms describing the semi-infinite parts attached to the junction part  Finally

PT attached to gold contacts

PT in cross-linked Alpha CD

PT in Beta CD

Molecular wire: transport through shielded polythiophene

HOMO-LUMO energies(Hartree) PT in ACD non- interacting PT in BCD interacting PT in BCD non- interacting PT LUMO HOMO

Density of States

Conductance

Spatial Extension of MOs (n~80; E~0.3) LUMO HOMO LUMO+n

DNA conductance perpendicular to axis in collaboration with T.M. Briere Au(111) STM Tip Au(111) Substrate

AT Base Pair

CG Base Pair

Bulk Gold Contact

Density of States (Fermi energy ~ -0.1)

Conductance

AT: Spatial distribution of HOMO (E ~ )

AT: Spatial distribution of LUMO+n (E ~ 0.570)

Conclusions:  Two stable positions for Cs along diagonal direction  Rectifying effect  New nearly flat bands via doping  Alignment of Frmi energy and van Hofe singularity: possibility of superconductivity  In DNA transport, dominant current-carrying states are localized on the hydrogen bonds  A high density of states does not necesserarily mean high conductance  AT and CG have different conductance due to differently localized states