Molecular Memory with Atomically-Smooth Graphene Contacts

We report the use of bilayer graphene as an atomically-smooth contact for nanoscale devices. A two-terminal Bucky ball (C60) based molecular memory is fabricated with bilayer graphene as a contact on the polycrystalline nickel electrode. Graphene provides an atomically-smooth covering over an otherwise rough metal surface. The use of graphene additionally prohibits the electromigration of nickel atoms into the C60 layer. The devices exhibit a low-resistance state in the first sweep cycle and irreversibly switch to a high resistance state at 0.8-1.2 V bias. The reverse sweep has a hysteresis behavior as well. In the subsequent cycles, the devices retain the high-resistance state, thus making it write-once read-many memory (WORM). The ratio of current in low-resistance to high-resistance state is lying in 20-40 range for various devices with excellent retention characteristics. Control sample without the bilayer graphene shows random hysteresis and switching.Comment: 13 pages and 4 figure

Collective Modes of Massive Dirac Fermions in Armchair Graphene Nanoribbons

We report the plasmon dispersion characteristics of intrinsic and extrinsic armchair graphene nanoribbons of atomic width N = 5 using a p_z-orbital tight binding model with third-nearest-neighbor (3nn) coupling. The coupling parameters are obtained by fitting the 3nn dispersions to that of an extended Huckel theory. The resultant massive Dirac Fermion system has a band gap E_g \approx 64 meV. The extrinsic plasmon dispersion relation is found to approach a common dispersion curve as the chemical potential $\mu$ increases, whereas the intrinsic plasmon dispersion relation is found to have both energy and momentum thresholds. We also report an analytical model for the extrinsic plasmon group velocity in the q \rightarrow 0 limit

Theoretical study of isolated dangling bonds, dangling bond wires, and dangling bond clusters on a H : Si(001)-(2x1) surface

We theoretically study the electronic band structure of isolated unpaired and paired dangling bonds (DBs), DB wires, and DB clusters on H:Si(001)-(2x1) surface using extended Huckel theory and report their effect on the Si band gap. We show that an isolated unpaired DB introduces a near-midgap state, whereas a paired DB leads to pi and pi(*) states, similar to those introduced by an unpassivated asymmetric dimer (AD) Si(001)-(2x1) surface. On the other hand, the surface state induced due to an unpaired DB wire in the direction along the dimer row (referred to as [(1) over bar 10]) has a large dispersion due to the strong coupling between the adjacent DBs, being 3.84 A apart. However, in the direction perpendicular to the dimer row (referred to as [110]), the DBs are 7.68 A apart and there is a reduced coupling between them due to exponential dependence of the wave function, leading to a small dispersion. Moreover, a paired DB wire in the [(1) over bar 10] direction introduces pi and pi(*) states similar to those of an AD surface, but with a large dispersion, and a paired DB wire in the [110] direction exhibits surface states with a smaller dispersion, as expected. Besides this, we report the electronic structure of different DB clusters, which exhibit states inside the band gap that can be interpreted as superpositions of states due to unpaired and paired DBs

Plasmon dispersion in semimetallic armchair graphene nanoribbons

The dispersion relations for plasmons in intrinsic and extrinsic semimetallic armchair graphene nanoribbons (acGNR) are calculated in the random phase approximation using the orthogonal p_z-orbital tight binding method. Our model predicts new plasmons for acGNR of odd atomic widths N=5,11,17,... Our model further predicts plasmons in acGNR of even atomic width N=2,8,14,... related to those found using a Dirac continuum model, but with different quantitative dispersion characteristics. We find that the dispersion of all plasmons in semimetallic acGNR depends strongly on the localization of the p_z electronic wavefunctions. We also find that overlap integrals for acGNR behave in a more complex way than predicted by the Dirac continuum model, suggesting that these plasmons will experience a small damping for all q not equal to 0. Plasmons in extrinsic semimetallic acGNR with the chemical potential in the lowest (highest) conduction (valence) band are found to have dispersion characteristics nearly identical to their intrinsic counterparts, with negligible differencs in dispersion arising from the slight differences in overlap integrals for the interband and intraband transitions.Comment: 8 pages, 9 figure