K. H. Khoo

Negative differential resistance in carbon atomic wire-carbon nanotube junctions.

Negative differential resistance (NDR) was recently observed in carbon nanotube junctions just before breaking and hypothesized to arise from the formation of monatomic carbon wires in the junction. Motivated by these results, a first-principles scattering-state approach, based on density functional theory, is used to study the transport properties of carbon chains covalently connecting metallic carbon nanotube leads at finite bias. The I- V characteristics of short carbon chains are predicted to exhibit even-odd behavior, and NDR is found for both even and odd chain junctions in our calculations.

Novel Orientational Ordering and Reentrant Metallicity in KxC60 Monolayers for 3 x 5

STM studies on K(x)C(60) monolayers reveal new behavior over a wide range of the phase diagram. As x increases from 3 to 5 K(x)C(60) monolayers undergo metal-insulator-metal reentrant phase transitions and exhibit a variety of novel orientational orderings, including a complex 7-molecule, pinwheel-like structure. The proposed driving mechanism for the orientational ordering is the lowering of electron kinetic energy by maximizing the overlap of neighboring molecular orbitals. In insulating (metallic) K(x)C(60) this gives rise to orbital versions of the superexchange (double-exchange) interaction.

Electron transport and optical properties of carbon nanostructures from first principles

Abstract Recent developments in ab initio studies of the nonlinear electron transport and optical properties of nanostructures are discussed. As examples of applications, results are presented for carbon atomic wires and single-walled carbon nanotubes. For the carbon atomic wires, strong nonlinearities in the I – V characteristics and conductance are obtained, and the role of interface chemistry and lead composition is demonstrated to be extremely important in determining its transport properties. For single-walled carbon nanotubes, explicit treatment of many-electron interactions shows that excitonic effects are dominant in these quasi-one dimensional systems and thus essential to explain the observed optical absorption spectra.

van der Waals Bonded Co/h-BN Contacts to Ultrathin Black Phosphorus Devices

Because of the chemical inertness of two dimensional (2D) hexagonal-boron nitride (h-BN), few atomic-layer h-BN is often used to encapsulate air-sensitive 2D crystals such as black phosphorus (BP). However, the effects of h-BN on Schottky barrier height, doping, and contact resistance are not well-known. Here, we investigate these effects by fabricating h-BN encapsulated BP transistors with cobalt (Co) contacts. In sharp contrast to directly Co contacted p-type BP devices, we observe strong n-type conduction upon insertion of the h-BN at the Co/BP interface. First-principles calculations show that this difference arises from the much larger interface dipole at the Co/h-BN interface compared to the Co/BP interface, which reduces the work function of the Co/h-BN contact. The Co/h-BN contacts exhibit low contact resistances (∼4.5 kΩ) and are Schottky barrier-free. This allows us to probe high electron mobilities (4,200 cm2/(V s)) and observe insulator-metal transitions even under two-terminal measurement geometry.

Energy Resolved Imaging of Fullerene Molecular Orbitals

We have used scanning tunneling spectroscopy to spatially map the energy‐resolved local density of states of C60 molecules in the monomer and monolayer regimes on the Ag(001) surface. Spectral maps were obtained for molecular states derived from the C60 HOMO, LUMO, and LUMO+1 orbitals. An unexpected density inversion is observed between the spectral maps of the LUMO and LUMO+1 states. The observed spatial inhomogeneities and density inversion are explained using ab initio pseudopotential density functional calculations. These calculations demonstrate the need for explicitly including the STM tip trajectory when interpreting STM images of electronically inhomogeneous systems.

Spatially Dependent Inelastic Tunneling in Gd@C82

We have measured the elastic and inelastic tunneling properties of isolated Gd@C82 molecules on the Ag(001) using cryogenic scanning tunneling microcopy and spectroscopy. We observe several inelastic excitations in our tunneling spectra, and the dominant inelastic channel is strongly spatially localized. Density functional theory calculations show that the observed localization in inelastic tunneling arises from localization in the electron‐phonon coupling to a C82 cage phonon mode.