Temperature-Dependent Terahertz Spectra of Isonicotinamide in the Form I Studied Using the Quasi-Harmonic Approximation.

Anharmonicity of molecular vibrational motions is closely associated with the thermal property of crystals. However, the origin of anharmonicity is still not fully understood. Low-frequency vibrations, which are usually defined in the terahertz (THz) range, show excellent sensitivity to anharmonicity. In this work, anharmonicity of isonicotinamide in the form I was investigated by using temperature-dependent terahertz time-domain spectroscopy and the quasi-harmonic approximation (QHA) approach at PBE-D3 and PBE-MBD levels. Both DFT calculations suggest the variation of π-π stacking conformation dominates in the thermal expansion of the unit cell. Frequency shifts of the modes in THz range obtained by QHA approach are found to be qualitatively consistent with experimental observations, demonstrating QHA approach is a useful tool for the interpretation of frequency shifts of modes induced by temperature.

Theoretical insights into TM@PHEs as single-atom catalysts for water splitting based on density functional theory.

Single-atom catalysis is the new frontier of heterogeneous catalysis and has attracted considerable attention as it exhibits great potential in hydrogen evolution to mitigate energy crisis and environmental issues. The support materials for single-atom catalysts (SACs) play a significant role in stabilizing the metal atoms, preventing their aggregation, and enhancing the catalytic activity. Two-dimensional sp2 hybridized PHE-graphene might be a real support for SACs due to the potential energy well induced by its enneagon, hexagon and pentagon carbon rings. In this study, eleven transition metal (TM) atoms adsorbed on PHE-graphene (TM@PHEs) are taken into account based on density functional theory (DFT) and PHE-graphene is proved to be an ideal single-atom carrier for water splitting. In particular, the TM@PHEs (TM = Fe, Ni, Ru, and Pd) exhibit high catalytic activity toward the hydrogen evolution reaction (HER). The reaction path of water splitting is also determined. Due to their much lower energy barrier, both Fe@PHE and Ru@PHE are more promising SACs. In addition, the charge density difference, Bader charge analysis and spin projected density of states (PDOS) are investigated.

Terahertz spectroscopic characterizations and DFT calculations of indomethacin cocrystals with nicotinamide and saccharin.

Co-crystallization is an effective strategy to improve the drug properties such as solubility and stability. However, its thermodynamic backgrounds, especially lattice vibration, haven't been fully understood. In this work, indomethacin (IND) cocrystals formed with nicotinamide (NIC) and saccharin (SAC) are successfully characterized by using terahertz spectroscopy. DFT calculations at PBE-D3 level with and without constrained unit cell are performed to predict the absorption peaks at spectral range. The results suggest that the DFT calculations with constrained unit cell achieve a better agreement with experimental observations. Based on the optimized geometries and calculated phonons, the thermodynamic contributions from lattice vibrations to cocrystal formations are further evaluated. The findings reveal that the vibrational energy plays a comparable role with electronic energy, but has an opposite impact on these two cocrystal formations.

Terahertz spectroscopic characterizations and DFT calculations of carbamazepine cocrystals with nicotinamide, saccharin and fumaric acid.

Carbamazepine cocrystals with nicotinamide, saccharin and fumaric acid were synthesized and characterized by time-domain terahertz spectroscopy. Lattice vibrations of cocrystals with their individual constituents were investigated by means of the dispersion-corrected density functional theory with and without cell parameter constraints. The simulated THz spectra successfully reproduce the features of all the crystals in their experimental spectra. A better agreement between experimental and theoretical THz spectra is achieved when the cell parameter constraints are applied in geometry optimization. Some intensive modes of neat carbamazepine and cocrystals were discussed in terms of the motions of hydrogen bonds. The effect of lattice vibration on these cocrystallizations was further examined to gain insights into the thermodynamics. It is found that lattice vibration is favorable for all these cocrystal formations.

A multiporous carbon family with superior stability, tunable electronic structures and amazing hydrogen storage capability.

The traditional view that natural allotropes are more stable than artificially synthesized structures is widely accepted. For instance, graphite and diamond are more energetically favorable than other new carbon allotropes no matter whether they are experimentally prepared or theoretically predicted. Surprisingly, we find that a family of multiporous carbon (N-diaphenes) could be thermodynamically more stable than natural diamond with the increase of its feature size parameter N. Multiporous N-diaphenes exhibit extremely strong anisotropic mechanical properties and their ideal strength linearly depends on the corresponding yield strain. Density functional theory (DFT) calculations reveal that the bandgap hierarchy of N-diaphenes is inherited from their precursors. In addition, N-diaphenes exhibit superior capability for hydrogen storage due to their large specific surface areas.

Multiporous sp2-hybridized boron nitride (d-BN): stability, mechanical properties, lattice thermal conductivity and promising application in energy storage.

All-solid-state lithium-sulfur (Li-S) batteries offer the possibility of high theoretical energy densities and long cycle lifetime. Finding new multiporous insulating materials or wide bandgap semiconductors suitable for the transportation of Li-ions is a key problem for the application of solid state lithium-sulfur (Li-S) batteries. In this study, an sp2-hybridized multiporous boron nitride (d-BN) is found to be able to be used as a solid electrolyte or filter in Li-S batteries due to the lower energy barriers of Li-ion diffusion along its [110] direction. By means of density functional theory (DFT) calculations, it is also found that the d-BN is more stable than most reported BN polymorphs. In addition, the elastic properties, ideal tensile strength, electronic structure, lattice thermal conductivity and phonon lifetime of d-BN are also investigated.

A new metallic π-conjugated carbon sheet used for the cathode of Li-S batteries.

Lithium-sulfur (Li-S) batteries are considered as the most promising next generation high density energy storage devices. However, the commercialization of Li-S batteries is hindered by the shuttle effect of polysulfides, the low electronic conductivity of the sulfur cathode and a large volume expansion during lithiation. Herein, we predict a new two dimensional sp2 hybridized carbon allotrope (PHE-graphene) and prove its thermodynamic and kinetic stability. If it is utilized to encapsulate the cathode of Li-S batteries, not only will the shuttle effect be avoided but also the electronic conductivity of the sulfur cathode will be improved significantly owing to its metallic electronic band structure. The thermal conductivity of PHE-graphene was found to be very high and even comparable with graphene, which is helpful for the heat dissipation of cathodes. In addition, PHE-graphene also exhibited superior mechanical properties including ideal tensile strength and in-plane stiffness.

Encapsulation of cathode in lithium-sulfur batteries with a novel two-dimensional carbon allotrope: DHP-graphene.

Sulfur cathodes in lithium-sulfur (Li-S) batteries still suffer from their low electronic conductivity, undesired dissolution of lithium polysulfide (Li2S n , 3 ≤ n ≤ 8) species into the electrolyte, and large degree volume change during the cycle. To overcome these problems, an effective encapsulation for the sulfur cathode is necessary. By means of particle swarm optimization (PSO) and density functional theory (DFT), we have predicted a stable metallic two-dimensional sp 2-hybridized carbon allotrope (DHP-graphene). This carbon sheet can prevent S atoms from cathode entering electrolyte. However, Li-ions can shuttle freely due to the increasing difference in Li-ions concentration between electrolyte and cathode along with the potential difference between cathode and anode during charge-discharge cycles. In addition, versatile electronic band structures and linear dispersion are found in DHP-graphene nanoribbons but only metallic band structure occurs for DHP-graphene nanotubes.

Chirality control of nonplanar lead phthalocyanine (PbPc) and its potential application in high-density storage: a theoretical investigation.

On single-crystal surfaces, achiral molecules may become chiral owing to confinement in two dimensions (2D). Metal phthalocyanines (MPcs) on Cu(001) and Ag(100) surfaces have exhibited a chiral electronic state. However, the chirality is not always desirable since crystal defects (grain boundaries) inevitably occur between two different chiral domains during the self-assembly of single layers. In this theoretical study, we propose to utilize metal(001) substrates with different electron configurations to mediate the azimuthal orientations of nonplanar PbPc. The results show that PbPc is chiral on Cu(001) with a partially filled s orbital (3d(10)4s(1)) but achiral on Pd(001) with a completely filled d orbital (4d(10)). The mechanism that PbPc prefers achiral azimuthal orientation rather than chiral orientation on Pd(001) is clarified. In addition, we predict that PbPc can form a (3 × 4) surface reconstruction. While it is used for data storage, the capacity is almost three orders of magnitude higher than the present storage materials.

Positioning and switching phthalocyanine molecules on a Cu(100) surface at room temperature.

Reversible molecular switches with molecular orientation as the information carrier have been achieved on individual phthalocyanine (H2Pc) molecules adsorbed on a Cu(100) surface at room temperature. Scanning tunneling microscopy (STM) imaging directly demonstrates that H2Pc molecules can be controlled to move along the [011] or [011̅] surface direction of the Cu(100) surface, and the orientation of H2Pc molecules can also be switched between two angles of ±28° with respect to the [011] surface direction by a lateral manipulation. Owing to the highly efficient control over the adsorption site and orientation of H2Pc adsorbed on the Cu(100) surface by lateral manipulation, a pyramidal array formed by 10 H2Pc molecules has been constructed on the Cu surface as a prototype of binary memory, and every molecule within such a molecular array can be individually and reversibly controlled by a STM tip.

Switching molecular orientation of individual fullerene at room temperature.

Reversible molecular switches with molecular orientation as the information carrier have been achieved on individual fullerene molecules adsorbed on Si (111) surface at room temperature. Scanning tunneling microscopy imaging directly demonstrates that the orientation of individual fullerene with an adsorption geometry of 5-6 bond is rotated by integral times as 30 degree after a pulse bias is applied between the STM tip and the molecule. Dependences of the molecular rotation probability on the voltage and the process of applied bias reveal that the rotation of a fullerene molecule takes place in two successive steps: the bonding between the fullerene and the Si surface is firstly weakened via electronic excitation and then low energy electron bombardment causes the molecule to rotate by certain degree.

Role of Thermal Process on Self-Assembled Structures of 4'-([2,2':6',2''-Terpyridin]-4'-Yl)-[1,1'-Biphenyl]-4-Carboxylic Acid on Au(III).

The role of dynamic processes on self-assembled structures of 4'-([2,2':6', 2''-terpyridin]-4'-yl)-[1,1'-biphenyl]-4-carboxylic acid (l) molecules on Au(III) has been studied by scanning tunneling microscopy. The as-deposited monolayer is closed-packed and periodic in a short-range due to dipole forces. A thermal annealing process at 110 degrees drives such disordered monolayer into ordered chain-like structures, determined by the combination of the dipole forces and hydrogen bonding. Further annealing at 130 degrees turns the whole monolayer into a bowknot-like structure in which hydrogen bonding plays the dominant role in the formation of assembled structures. Such dependence of an assembled structure on the process demonstrates that an assembled structure can be regulated and controlled not only by the molecular structure but also by the thermal process to form the assembled structure.