Xiao-Qian Wang

Structural and Electronic Properties of Fluorographene

The structural and electronic characteristics of fluorinated graphene are investigated based on first-principles density-functional calculations. A detailed analysis of the energy order for stoichiometric fluorographene membranes indicates that there exists prominent chair and stirrup conformations, which correlate with the experimentally observed in-plane lattice expansion contrary to a contraction in graphane. The optical response of fluorographene is investigated using the GW-Bethe-Salpeter equation approach. The results are in good conformity with the experimentally observed optical gap and reveal predominant charge-transfer excitations arising from strong electron-hole interactions. The appearance of bounded excitons in the ultraviolet region can result in an excitonic Bose-Einstein condensate in fluorographene.

Tunable Band Gap in Hydrogenated Bilayer Graphene

We have studied the electronic structural characteristics of hydrogenated bilayer graphene under a perpendicular electric bias using first-principles density functional calculations. The bias voltage applied between the two hydrogenated graphene layers allows continuous tuning of the band gap and leads to transition from semiconducting to metallic state. Desorption of hydrogen from one layer in the chair conformation yields a ferromagnetic semiconductor with a tunable band gap. The implications of tailoring the band structure of biased system for future graphene-based device applications are discussed.

Chair and Twist-Boat Membranes in Hydrogenated Graphene

Graphane is a two-dimensional system consisting of a single planar layer of fully saturated carbon atoms, which has recently been realized experimentally through hydrogenation of graphene membranes. We have studied the stability of chair, boat, and twist-boat graphane structures using first-principles density functional calculations. Our results indicate that locally stable twist-boat membranes significantly contribute to the experimentally observed lattice contraction. The band gaps of graphane nanoribbons decrease monotonically with the increase of the ribbon width and are insensitive to the edge structure. The implications of these results for future hydrogenated graphene applications are discussed.

Intrinsic half-metallicity in hydrogenated boron-nitride nanoribbons

We have investigated the structural and electronic characteristics of fully hydrogenated boron-nitride (BN) layer and zigzag-edged nanoribbons using dispersion-corrected density-functional calculations. In the fully hydrogenated BN structure, the hydrogen atoms adsorb on top of the B and N sites, alternating on both sides of the hexagonal BN-plane in a specific periodic manner. Among various low-energy hydrogenated membranes referred to as chair, boat, twist-boat, and stirrup, the stirrup conformation is the most energetically favorable one. The zigzag-edged BN nanoribbon, prominently fabricated in experiments, possesses intrinsic half-metallicity with full hydrogenation. The half-metallicity can be tuned by applying a transverse electric bias, thereby providing a promising route for spintronics device applications.

Highly Selective Dispersion of Carbon Nanotubes by Using Poly(phenyleneethynylene)-Guided Supermolecular Assembly

Isolation of single-walled carbon nanotubes (SWNTs) with specific chirality and diameters is critical for achieving optimum performance of SWNTs in various applications. A water-soluble π-conjugated polymer, poly[(m-phenyleneethynylene)-alt-(p-phenyleneethynylene)], 3, is found to exhibit high selectivity in dispersing SWNT (6,5). The polymers ability to sort out SWNT (6,5) appears to be related to the carbon-carbon triple bond, whose free rotation allows a unique assembly of chromophores in a helical conformation. The observation is consistently supported by fluorescence, Raman, and UV-vis-NIR absorption spectra. The intriguing selectivity of 3 to SWNT (6,5), however, is not observed for the vinylene analogue polymer 1, showing that the carbon-carbon triple bond could play a unique role in sorting out a specific SWNT. The observed selectivity from 3 could be attributed to a combination of the helical cavity size restrain and electronic interaction associated with the local chromophore arrangement. This strategy could be expanded for efficient SWNT sorting when the helical conformation is further finely tuned.

Dispersion corrections in the boron buckyball and nanotubes

We have investigated structural and electronic properties of the B80 buckyball and boron nanotubes by means of dispersion-corrected density-functional calculations. Our analysis reveals the vibrational stability for the icosahedral B80 with the inclusion of dispersion corrections, in contrast to the instability to a tetrahedral B80 with puckered capping atoms from preceding density-functional theory calculations. Similarly, the dispersion-corrected density-functional calculations yield non-puckered boron nanotube conformations and an associated metallic state for zigzag tubes. Our study indicates that the incorporation of long-range dispersive interactions is particularly important to the structural and electronic properties of boron fullerenes and nanotubes.

Selective dispersion of single-walled carbon nanotubes by a cationic surfactant

Three single-walled carbon nanotubes (SWNTs), (8,4), (7,5) and (7,6), are significantly enriched via selective dispersion of SWNTs using a cationic surfactant, where the electronic donor–acceptor interaction with the nanotube is assumed to play an important role in the observed selectivity.

Tunable electron and hole doping in FeCl3 intercalated graphene

We have studied the electronic characteristics of FeCl3 intercalated bilayer graphene under a perpendicularly applied electric bias. Evolution of the electronic structure of FeCl3 intercalated bilayer graphene as a function of the applied electric bias is performed using first-principles density-functional theory including interlayer van der Waals interactions. The calculation results demonstrate that the hole-doped graphene layers associated with the high electronegativity of FeCl3 transform into electron-doped layers tuned by the applied bias. The implications of controllable electronic structure of intercalated graphene for future device applications are discussed.

Effect of polymer stereoregularity on polystyrene/single-walled carbon nanotube interactions

We use a combination of computational and experimental studies to elucidate the effect of polymer stereoregularity on the capability of polystyrene interacting with single-walled carbon nanotube (SWNT) surfaces. Calculated binding energies on complexes of slightly oxidized SWNT with isotactic and atactic polystyrene favor the former, which suggests that the isotactic polymer interacts more effectively with the SWNT. The glass transition temperature (Tg) of the isotactic polystyrene/SWNT matrix increases from 90.9 to 100.5 °C as the SWNT content is increased to 0.5%, whereas the glass transition temperature of the atactic polystyrene/SWNT matrix is invariant with increasing SWNT content. Rotating frame 13C T1ρ relaxation rates for the isotactic polymer/SWNT matrix increases from 2.15 to 2.43 ms as the SWNT content is increased from 0.25 to 1.0%. However, the rotating frame 13C T1ρ relaxation rates for the atactic polymer/SWNT matrix decreases from 2.50 to 1.60 ms as SWNT content is increased from 0.25 to 1.0%. Our results demonstrate that the SWNTs are better dispersed within the isotactic polystyrene and the better dispersion is associated with a more effective interaction of the isotactic polymer with the SWNT surface.

A trigonal planar network in hydrogenated epitaxial graphene: a ferromagnetic semiconductor

Hydrogenated epitaxial graphene has distinctive electronic properties compared to the two-sided hydrogenated graphene referred to as graphane. Of particular interest is the experimentally observed room-temperature ferromagnetic semiconducting property, which has remained elusive to theoretical interpretation. Here, we present results of a density functional theory investigation into various hydrogenation patterns. Our results indicate that the stability of a given hydrogenation pattern is strongly influenced by the amount of sp2-hybridized bonding in the structures. A hydrogenation pattern with a trigonal planar network is identified as an intrinsic ferromagnetic semiconductor, which is in very good conformity with experimental observations. Our results provide insight into the structural, electronic, and magnetic properties of hydrogenated epitaxial graphene.