Diffraction Symmetry in Crystalline, Close-Packed C60

Reentrant semiconducting behavior in polymerized fullerite structures with increasing sp3-carbon content

The electronic behavior of polymerized fullerite structures, ranging from one-dimensional to three-dimensional polymers, was studied using density functional theory with the hybrid Heyd-Scuseria-Ernzerhof functional and a 6-31G(d,p) basis set. The bandgap across these structures decreases with the rise of sp3-carbon content until metallic behavior is observed. A further increase induces a reopening of the bandgap, revealing a reentrant semiconducting behavior in this class of materials. This behavior is understood in terms of the new electronic states originated by polymeric bonding and the effect of the volume reduction on the dispersion of sp2-states. This study highlights the fullerite polymers as a magnificent platform to tune electronic properties.

Skyrmion lattice structural transition in MnSi

Magnetic skyrmions exhibit particle-like properties owing to the topology of their swirling spin texture, providing opportunities to study crystallization of topological particles. However, they mostly end up with a triangular lattice, and thus, the packing degree of freedom in the skyrmion particles has been overlooked so far. We report a structural transition of the skyrmion lattice in MnSi. By use of small-angle neutron scattering, we explore a metastable skyrmion state spreading over a wide temperature and magnetic field region, after thermal quenching. The quenched skyrmions undergo a triangular-to-square lattice transition with decreasing magnetic field at low temperatures. Our study suggests that various skyrmion lattices can emerge at low temperatures, where the skyrmions exhibit distinct topological nature and high sensitivity to the local magnetic anisotropy arising from the underlying chemical lattice.

Theoretical Study of the Charge-Transfer State Separation within Marcus Theory: The C60-Anthracene Case Study

We study, within Marcus theory, the possibility of the charge-transfer (CT) state splitting at organic interfaces and a subsequent transport of the free charge carriers to the electrodes. As a case study we analyze model anthracene-C60 interfaces. Kinetic Monte Carlo (KMC) simulations on the cold CT state were performed at a range of applied electric fields, and with the fields applied at a range of angles to the interface to simulate the action of the electric field in a bulk heterojunction (BHJ) interface. The results show that the inclusion of polarization in our model increases CT state dissociation and charge collection. The effect of the electric field on CT state splitting and free charge carrier conduction is analyzed in detail with and without polarization. Also, depending on the relative orientation of the anthracene and C60 molecules at the interface, CT state splitting shows different behavior with respect to both applied field strength and applied field angle. The importance of the hot CT in helping the charge carrier dissociation is also analyzed in our scheme.

Coronene-based charge-transfer complexes

Recent developments in the arena of charge-transfer complexes composed of the D 6h-symmetric polycyclic aromatic hydrocarbon, coronene, are highlighted with emphasis on the structural and physical properties of these complexes. Because of the dual electron-donating and -accepting abilities of coronene, this group involves structurally-defined four cation salts and three anion salts. The Jahn-Teller distortions and in-plane motion of coronene molecules in the solids, both of which are closely associated with the high symmetry of coronene molecules, and syntheses of clathrate-type complexes are also presented.

Nanoscale Control of Morphology in Fullerene-Based Electron-Conducting Buffers via Organic Vapor Phase Deposition

Small molecular weight organic thin film mixtures of the electron-conducting C60 in a wide energy gap matrix, 3,5,3',5'-tetra(m-pyrid-3-yl)phenyl[1,1']biphenyl (BP4mPy) forms a high efficiency electron filtering buffer in organic photovoltaics (OPV). Electrons are conducted via percolating paths of C60 whereas excitons are blocked by the BP4mPy. We find that the conductivity and exciton blocking efficiency of the blends are strongly dependent on film morphology that can be precisely controlled by the conditions used in the organic vapor phase deposition (OVPD). Specifically, we find that a background carrier gas pressure of 0.28 Torr leads to extended and highly conductive crystalline C60 domains. Furthermore, the structure is strongly influenced by carrier gas pressure. Via a combination of morphological measurements and molecular dynamics simulations, we find that this dependence is due to kinetically induced structural annealing at the growth interface. The highest electron mobility of (6.1 ± 0.5) × 10(-3) (cm(2)/V·s) is obtained at 0.28 Torr, which is approximately 2 orders of magnitude higher than for amorphous C60 films. The fill factors and power conversion efficiencies of vacuum deposited tetraphenyldibenzoperiflanthene (DBP):C70 planar mixed heterojunction OPVs using an OVPD-grown buffer layer are (8.0 ± 0.2)% compared to (6.6 ± 0.2)% using amorphous buffers grown by vacuum thermal evaporation.

Orientational ordering in solid C60 fullerene-cubane

We study the structure and phase behavior of fullerene-cubane C(60) x C(8)H(8) by Monte Carlo simulation. Using a simple potential model capturing the icosahedral and cubic symmetries of its molecular constituents, we reproduce the experimentally observed phase transition from a cubic to an orthorhombic crystal lattice and the accompanying rotational freezing of the C(60) molecules. We elaborate a scheme to identify the low-temperature orientations of individual molecules and to detect a pattern of orientational ordering similar to the arrangement of C(60) molecules in solid C(60). Our configuration of orientations supports a doubled periodicity along one of the crystal axes.

Gas-phase methylation of [60]fullerene

Direct methylation of [60]fullerene via a gas-phase reaction in a CH4/H2 atmosphere was performed using a modified hot filament chemical vapor deposition method. Pressures were varied from 10 to 60 mbar and the substrate was maintained at 690 degrees C. High-resolution matrix-assisted laser desorption ionization (MALDI) mass spectrometry analysis showed signals corresponding to C60H18-2n(H,CH3)n. Collision-induced dissociation experiments confirmed a maximum of 18 ligands possible to the [60]fullerene cage.

Electron crystallography of organic and biological molecules--techniques and comparison to X-ray crystallographic results

In this review, it is shown how ab initio analyses of molecular crystal structures can be carried out with electron diffraction intensities and high-resolution, low-dose, electron micrographs, dispelling the long-held myth that such determinations are not possible. The results for small organics, polymethylene compounds, various polymers, and even intregral membrane proteins, are in good agreement with independent X-ray crystal structure determinations, which, however, were not needed to solve the crystallographic phase problem from the electron scattering data. The concept of electron crystallography benefits from the utility of electron micrographs for providing phase information, in addition to the standard 'direct' methods used nowadays by all crystallographers.