Hyoung Joon Choi

Amine−Gold Linked Single-Molecule Circuits: Experiment and Theory

A combination of theory and experiment is used to quantitatively understand the conductance of single-molecule benzenediamine-gold junctions. A newly developed analysis is applied to a measured junction conductance distribution, based on 59 000 individual conductance traces, which has a clear peak at 0.0064 G0 and a width of +/-47%. This analysis establishes that the distribution width originates predominantly from variations in conductance across different junctions rather than variations in conductance during junction elongation. Conductance calculations based on density functional theory (DFT) for 15 distinct junction geometries show a similar spread. We show explicitly that differences in local structure have a limited influence on conductance because the amine-Au bonding motif is well-defined and flexible, explaining the narrow distributions seen in the experiments. The minimal impact of junction structure on conductance permits an unambiguous comparison of calculated and measured conductance values and a direct assessment of the widely used DFT theoretical framework. The average calculated conductance (0.046 G0) is found to be seven times larger than experiment. This discrepancy is explained quantitatively in terms of electron correlation effects to the molecular level alignments in the junction.

Broken symmetry and pseudogaps in ropes of carbon nanotubes

Since the discovery of carbon nanotubes, it has been speculated that these materials should behave like nanoscale wires with unusual electronic properties and exceptional strength. Recently, ‘ropes’ of close-packed single-wall nanotubes have been synthesized in high yield. The tubes in these ropes are mainly of the (10,10) type, which is predicted to be metallic. Experiments on individual nanotubes and ropes, indicate that these systems indeed have transport properties that qualify them to be viewed as nanoscale quantum wires at low temperature. It has been expected that the close-packing of individual nanotubes into ropes does not change their electronic properties significantly. Here, however, we present first-principles calculations which show that a broken symmetry of the (10,10) tube caused by interactions between tubes in a rope induces a pseudogap of about 0.1 eV at the Fermi level. This pseudogap strongly modifies many of the fundamental electronic properties: we predict a semimetal-like temperature dependence of the electrical conductivity and a finite gap in the infrared absorption spectrum. The existence of both electron and hole charge carriers will lead to qualitatively different thermopower and Hall-effect behaviours from those expected for a normal metal.

Observation of tunable band gap and anisotropic Dirac semimetal state in black phosphorus

Tuning the band gap of black phosphorus Most materials used in electronics are semiconductors. The sizable energy gap in their electronic structure makes it easy to turn the conduction of electricity on and off. Graphene naturally lacks this band gap unless it undergoes certain modifications. Kim et al. studied the electronic structure of black phosphorus—a related two-dimensional material. By sprinkling potassium atoms on top of single layers of black phosphorus, the material changed from being a semiconductor to having a gapless linear dispersion similar to that of graphene. Science, this issue p. 723 Surface doping with potassium is used to tune black phosphorus from a semiconducting to a graphene-like electronic structure. Black phosphorus consists of stacked layers of phosphorene, a two-dimensional semiconductor with promising device characteristics. We report the realization of a widely tunable band gap in few-layer black phosphorus doped with potassium using an in situ surface doping technique. Through band structure measurements and calculations, we demonstrate that a vertical electric field from dopants modulates the band gap, owing to the giant Stark effect, and tunes the material from a moderate-gap semiconductor to a band-inverted semimetal. At the critical field of this band inversion, the material becomes a Dirac semimetal with anisotropic dispersion, linear in armchair and quadratic in zigzag directions. The tunable band structure of black phosphorus may allow great flexibility in design and optimization of electronic and optoelectronic devices.

Origin of anomalous electronic structures of epitaxial graphene on silicon carbide.

On the basis of first-principles calculations, we report that a novel interfacial atomic structure occurs between graphene and the surface of silicon carbide, destroying the Dirac point of graphene and opening a substantial energy gap there. In the calculated atomic structures, a quasiperiodic 6x6 domain pattern emerges out of a larger commensurate 6 sqrt [3] x 6 sqrt [3]R30 degrees periodic interfacial reconstruction, resolving a long standing experimental controversy on the periodicity of the interfacial superstructures. Our theoretical energy spectrum shows a gap and midgap states at the Dirac point of graphene, which are in excellent agreement with the recently observed anomalous angle-resolved photoemission spectra. Beyond solving unexplained issues in epitaxial graphene, our atomistic study may provide a way to engineer the energy gaps of graphene on substrates.

Synthesis and Rheology of Intercalated Polystyrene/Na+-Montmorillonite Nanocomposites

Polystyrene (PS)/clay nanocomposites were synthesized by the emulsion polymerization of styrene in the presence of sodium ion-exchanged montmorillonite (Na+-MMT), demonstrating that the strongly hydrophobic PS was intercalated into the hydrophilic silicate layers. The nanocomposites were examined by means of X-ray diffraction, transmission electron microscopy, thermogravimetric analysis. The rheological properties of the PS/Na+-MMT nanocomposites were also studied to exhibit more pronounced shear thinning behavior with increasing clay content.

Graphyne: Hexagonal network of carbon with versatile Dirac cones

FIG. 1. (Color online) Schematic representations of (a) graphene, and (b) α, (c) β, and (d) γ graphyne. Red quadrangles indicate unit cells. (e) The hopping matrix elements along a carbon triple bond in graphyne. The carbon atoms 1 and 4 are at vertices of a hexagon in graphyne. (f) Effective direct hopping matrix element between the two carbon atoms 1 and 4 in (e).

First-principles calculation of the superconducting transition in MgB2 within the anisotropic Eliashberg formalism

We present a study of the superconducting transition in MgB2 using the ab initio pseudopotential density-functional method, a fully anisotropic Eliashberg equation, and a conventional estimate for mu*. Our study shows that the anisotropic Eliashberg equation, constructed with ab initio calculated momentum-dependent electron-phonon interaction and anharmonic phonon frequencies, yields an average electron-phonon coupling constant lambda = 0.61, a transition temperature T-sub c = 39 K, and a boron isotope-effect exponent alpha-sub B = 0.32. The calculated values for T-sub c, lambda, and alpha-sub B are in excellent agreement with transport, specific-heat,and isotope-effect measurements, respectively. The individual values of the electron-phonon coupling lambda (k,k-prime) on the various pieces of the Fermi surface, however, vary from 0.1 to 2.5. The observed T-sub c is a result of both the raising effect of anisotropy in the electron-phonon couplings and the lowering effect of anharmonicity in the relevant phonon modes.

Preparation and Rheological Characteristics of Solvent-Cast Poly(ethylene oxide)/Montmorillonite Nanocomposites

The organophilic montmorillonite clay and poly(ethylene oxide) (PEO) nanocomposites were intercalated by a solvent casting method using chloroform as the cosolvent. The prepared nanocomposites were characterized by an X-ray diffraction method to examine their microstructure. Rheological properties of both the PEO/clay nanocomposites and the immiscible PEO/clay blends were investigated via a rotational rheometer in steady shear mode with a parallel plate geometry. The shear thinning viscosity data were fitted with the Carreau model, which showed that steady shear viscosity increases with increasing clay loading. The hysteresis phenomenon is observed to be enhanced with clay loading. PEO/clay nanocomposites exhibit higher zero-shear-rate viscosity and sharper shear thinning behaviors than immiscible PEO/clay blends.

Conductance and geometry of pyridine-linked single-molecule junctions

We have measured the conductance and characterized molecule-electrode binding geometries of four pyridine-terminated molecules by elongating and then compressing gold point contacts in a solution of molecules. We have found that all pyridine-terminated molecules exhibit bistable conductance signatures, signifying that the nature of the pyridine-gold bond allows two distinct conductance states that are accessed as the gold-molecule-gold junction is elongated. We have identified the low-conductance state as corresponding to a molecule fully stretched out between the gold electrodes, where the distance between contacts correlates with the length of the molecule; the high-conductance state is due to a molecule bound at an angle. For all molecules, we have found that the distribution of junction elongations in the low-conductance state is the same, while in the high-conductance state, the most likely elongation length increases linearly with molecule length. The results of first-principles conductance calculations for the four molecules in the low-conductance geometry agree well with the experimental results and show that the dominant conducting channel in the conjugated pyridine-linked molecules is through the pi* orbital.

Length Dependence of Conductance in Aromatic Single-Molecule Junctions

Using a scattering-state approach incorporating self-energy corrections to the junction level alignment, the conductance G of oligophenyldiamine-Au junctions is calculated and elucidated. In agreement with experiment, we find G decays exponentially with the number of phenyls with decay constant beta = 1.7. A straightforward, parameter-free self-energy correction, including electronic exchange and correlations beyond density functional theory (DFT), is found to be essential for understanding the measured values of both G and beta. Importantly, our results confirm quantitatively the picture of off-resonant tunneling in these systems and show that exchange and correlation effects absent from standard DFT calculations contribute significantly to beta.