Utpal Sarkar

A first principle study of pristine and BN-doped graphyne family

Based on first principle calculation using generalized gradient approximation, we report electronic properties of graphyne and its related structures (graphdiyne, graphyne-3, graphyne-4). Boron and nitrogen atoms are systematically substituted into the position of carbon atom and the corresponding changes of the properties are reported. All the structures are found to be direct band gap semiconductors with band gap depending on the concentration and position of the doping material. Our band structure calculation clearly shows that the band gap can be tuned by B–N doping and the spin-polarized calculation depicts the nonmagnetic nature of these structures. The possibility of modulating the band gap provides flexibility for its use in nanoelectronic devices. Projected density of state (PDOS) analysis shed insights on the bonding nature of these novel materials, whereas from the view point of Crystal Orbital Hamilton Population (–COHP) analysis, the nature of chemical bonding between neighbouring atoms and the orbital participating in bonding and antibonding have been explored in details.

Hockey-stick-shaped mesogens based on 1,3,4-thiadiazole: synthesis, mesomorphism, photophysical and DFT studies

ABSTRACT New hockey-stick-shaped 2,5-diphenyl-1,3,4-thiadiazole-containing mesogens have been designed and synthesised. These molecules possess 4-n-alkoxy chain at the elongated arm whereas and 4-n-butyloxy chain at the shorter arm of the molecule. The compounds are fluorescent and emission located in the violet to blue region with a large stoke shift observed in polar solvents. No substantial solvatochromic effect was found in the absorption maxima, but in the emission, a red shift was observed with increasing the solvent polarity. In the solid thin film, compounds emitted green light. The compounds having longer chain (n > 8) exhibit SmA and SmC phase, while the compounds with shorter chain at one end of the molecule display nematic and SmC phase. The density functional theory study has been carried out to investigate the stable configuration and the influence of solvent on bond length, bond angle, bending angle, dipole moment, polarisability of the molecule. It also enables us to predict the possible reactive sites and effect of solvent on the chemical reactivity descriptors. Our study concludes that the molecule becomes more reactive in solvent compared to gas. The large voltage holding ratio confirms that this molecule may be used for active matrix display applications. Graphical Abstract

Dirichlet boundary conditions and effect of confinement on chemical reactivity.

To understand the source of discrepancy in the qualitative trends in the reactivity of the spherically confined atoms/ions when the high pressure is generated through the use of a proper Dirichlet boundary condition [J. Chem. Sci. 2005, 117, 379; Phys. Chem. Chem. Phys. 2008, 10, 1406] and of a cutoff function [Chem. Phys. Lett. 2003, 372, 805; J. Phys. Chem. A 2003, 107, 4877], a modified Herman-Skilman program is run. Results obtained from different formulas of reactivity parameters are analyzed. Change in reactivity for different electronic configurations is also reported. It is observed that the use of different formulas is the major source of discrepancy and not the Dirichlet condition, although the latter is highly recommended. As the cutoff radius of the confining spherical box decreases, the energy of the atom/ion increases, the electronegativity decreases, and the hardness increases and ultimately slightly decreases in an ultraconfined situation. For small R(C) values, softness decreases and electrophilicity increases and attains relatively small values. The reactivity of confined atoms/ions is put in a proper perspective.

First principle study of adsorption of boron-halogenated system on pristine graphyne

To ensure the possibility of using graphyne as a gas sensor, we have studied the adsorption of boron-halogenated system on pristine graphyne with the help of density functional theory using generalized gradient approximation. Depending on binding energy the most stable orientation, adsorption strength and optimal distance between the above mention molecules and graphyne surface have been determined. The band gap of graphyne slightly increases with the adsorption of the boron-halogenated system. The graphyne system behaves as n-type semiconductor when it interacts with BI3 and BCl3 molecules, and it behaves as p-type semiconductor when interaction with BF3 molecule takes place. Our result reveals that the electronic properties of pristine graphyne are highly influenced by the adsorption of boron-halogenated molecule. We have observed that pristine graphyne has zero electric dipole moment, but with the interaction of boron-halogenated molecule, a significant change in the electric dipole moment takes place. Hence, by measuring the electric dipole moment change, graphyne-based gas sensor can be design for the detection of above-mentioned molecules.

Structure, vibrational, and optical properties of platinum cluster: a density functional theory approach

AbstractUsing density functional theory, stability, chemical, and optical properties of small platinum clusters, Ptn (n = 2 to 10) have been investigated. An attempt has been made to establish a correlation between stability and chemical reactivity parameters. The calculated geometries are in agreement with the available experimental and theoretical results. The atom addition energy change (ΔE1) and stability function (ΔE2) reveal that Pt7 is more stable than its neighboring clusters. Very good agreement of the calculated electron affinity with the available experimental results has been observed. The polarizability of the Ptn clusters depends almost linearly on the number of atoms. A correlation between the static polarizability and ionization potential is found, paving a way to calculate polarizabilty of larger clusters from their ionization potential. The calculated vibrational frequencies are compared with available experimental and theoretical results and good agreement between them has been established. In general, the prominent peak of molar absorption coefficient is shifting toward the lower energy side when cluster size grows. Our DOS calculation suggests that d orbital is primarily responsible for HOMO position and s orbital is responsible for LUMO position. Graphical AbstractStability and reactivity of platinum cluster

Moving least-squares enhanced Shepard interpolation for the fast marching and string methods.

The number of the potential energy calculations required by the quadratic string method (QSM), and the fast marching method (FMM) is significantly reduced by using Shepard interpolation, with a moving least squares to fit the higher-order derivatives of the potential. The derivatives of the potential are fitted up to fifth order. With an error estimate for the interpolated values, this moving least squares enhanced Shepard interpolation scheme drastically reduces the number of potential energy calculations in FMM, often by up 80%. Fitting up through the highest order tested here (fifth order) gave the best results for all grid spacings. For QSM, using enhanced Shepard interpolation gave slightly better results than using the usual second order approximate, damped Broyden-Fletcher-Goldfarb-Shanno updated Hessian to approximate the surface. To test these methods we examined two analytic potentials, the rotational dihedral potential of alanine dipeptide and the S(N)2 reaction of methyl chloride with fluoride.

A computational study on structure, stability and bonding in Noble Gas bound metal Nitrates, Sulfates and Carbonates (Metal = Cu, Ag, Au)

AbstractA density functional theory based study is performed to investigate the noble gas (Ng = Ar-Rn) binding ability of nitrates, sulfates and carbonates of noble metal (M). Their ability to bind Ng atoms is assessed through bond dissociation energy and thermochemical parameters like dissociation enthalpy and dissociation free energy change corresponding to the dissociation of Ng bound compound producing Ng and the respective salt. The zero-point energy corrected dissociation energy values per Ng atom for the dissociation process producing Ng atom(s) and the corresponding salts range within 6.0–13.1 kcal/mol in NgCuNO3, 3.1–9.8 kcal/mol in NgAgNO3, 6.0–13.2 kcal/mol in NgCuSO4, 3.2–10.1 kcal/mol in NgAgSO4, 5.1–11.7 kcal/mol in Ng2Cu2SO4, 2.5–8.6 kcal/mol in Ng2Ag2SO4, 8.1–19.9 kcal/mol in Ng2Au2SO4, 5.7–12.4 kcal/mol in NgCuCO3, 2.3–8.0 kcal/mol in Ng2Ag2CO3 and 7.3–18.2 kcal/mol in Ng2Au2CO3, with a gradual increase in moving from Ar to Rn. For a given type of system, the stability of Ng bound analogues follows the order as Au > Cu > Ag. All dissociation processes are endothermic in nature whereas they become endergonic as well in most of the cases of Kr-Rn bound analogues at 298 K. Natural population analysis along with the computation of Wiberg bond indices, and electron density analyses provide insights into the nature of the Ng-M bonds. The Ng-M bonds can be represented as partial covalent bonds as supported by the different electron density descriptors. Graphical AbstractDifferent nitrates, sulfates and carbonates of noble metals (M = Cu, Ag, Au) can bind noble gas (Ng) atoms quite effectively. The M-Ng bonds in these compounds are found to be partially covalent in nature.

A density functional study of chemical, magnetic and thermodynamic properties of small palladium clusters

Structural, chemical, magnetic and thermodynamic properties of palladium clusters Pdn with n = 2–11 are studied using density functional methods. The average bond length, entropy, enthalpy and polarisability are observed to increase as the cluster grows in size. The binding energy per atom also increases with cluster size. Stability function and atom addition energy change predict that Pd4, Pd6 and Pd9 are relatively more stable than their neighbouring clusters. Electron affinity, electronegativity and electrophilicity values suggest that larger clusters have stronger tendency to accept electrons, thereby supporting the relative stability of Pd4 and Pd6. Chemical hardness is also seen to decrease with cluster size, which suggests that large clusters are more prone to changes in their electronic structure. The magnetic properties of these clusters are reported.

Geometry, chemical reactivity and Raman spectra of gold clusters

Abstract Structures, stability, and chemical reactivity of Aun (n = 2-10) clusters are investigated using density functional theory (DFT). We have studied the reactivity parameters of the clusters in terms of relevant electronic structure principles. It is observed that stability and properties are strongly dependent on the cluster size. Clusters with an even number of atoms are found to be energetically and chemically more stable than odd-numbered clusters. Electronic structure of clusters has been investigated using partial density of states (PDOS). PDOS analysis clearly shows that energy states of highest occupied molecular orbital and lowest unoccupied molecular orbital are predominantly contributed by s orbital. From time-dependent DFT calculations, it is shown that absorption spectra of even-numbered clusters are more intense and are observed at lower wavelength region than the odd-sized gold clusters.

Nickel cluster functionalised carbon nanotube for CO molecule detection: a theoretical study

ABSTRACT Using spin-polarised density functional theory calculation single-walled carbon nanotube (SWCNT) whose sidewall is functionalised with nickel cluster is studied for its possible application in CO molecule sensing. We have chosen (6,0) SWCNT functionalised with Ni13 cluster as the model for nanotube-cluster system. Changes in the properties of nanotube-cluster system brought by the CO molecule are reported. The CO molecule binding is energetically more favourable to the nanotube-cluster system than the pristine nanotube. The electronic properties are investigated in terms of density of states and bandstructure calculations. Pristine carbon nanotubes are intrinsically non-magnetic but nanotubes functionalised with nickel cluster are observed to have a huge magnetic moment which reduced on adsorbing CO molecule. The change in magnetisation upon CO adsorption may be detected using a suitable magnetometer. This result suggests the possibility of using carbon nanotube-cluster system to detect CO molecules. Bader charge analysis shows that CO molecule withdraws electronic charge from the cluster atoms. Nature of chemical bonding is studied with crystal orbital Hamilton population (–COHP) analysis.