A systematic DFT study of structure and electronic properties of titanium dioxide.

DFT functionals are of paramount importance for an accurate electronic and structural description of transition metal systems. In this work, a systematic analysis using some well-known and commonly used DFT functionals is performed. A comparison of the structural and energetic parameters calculated with the available experimental data is made in order to find the adequate functional for an accurate description of the TiO2 bulk and surface of both anatase and rutile structures. In the absence of experimental data on the surface energy, the theoretical predictions obtained using the high-accuracy HSE06 functional were used as a reference to compare against the surface energy values calculated with the other DFT functionals. A clear improvement in the electronic description of both anatase and rutile was observed by introducing the Hubbard U correction term to PBE, PW91, and OptPBE functionals. The OptPBE-U4 functional was found to offer a good compromise between accurately describing the structural and electronic properties of titania.

Role of magnetization on catalytic pathways of non-oxidative methane activation on neutral iron carbide clusters.

Methane has emerged as a promising fuel due to its abundance and clean combustion properties. It is also a raw material for various value-added chemicals. However, the conversion of methane to other chemicals such as olefins, aromatics, and hydrocarbons is a difficult task. In recent years, ionic iron carbide clusters have been explored as potential catalysts for efficient direct methane conversion. Herein, we have investigated the gas-phase methane conversion process on various neutral iron carbide clusters with different Fe:C ratios using density functional theory. Reaction pathways were studied on mononuclear and trinuclear iron carbide clusters in the three lowest energy spin multiplicity channels. Three descriptors - methane binding energy, the effective energy barrier for C-H bond activation, and the effective energy required for methyl radical evolution - were chosen to identify the best catalyst among the clusters considered. Isomers of Fe3C6 (Fe3C6-iso) and Fe3C9 (Fe3C9-iso) are recognized as being the most promising catalysts among all the clusters considered here because they require the least methyl radical evolution energy, a step that is crucial in methane conversion to higher hydrocarbon but also requires the most energy.

Internal Conversion and Vibrational Energy Redistribution in Chlorophyll A.

We have computationally investigated the role of intramolecular vibrational modes in determining nonradiative relaxation pathways of photoexcited electronic states in isolated chlorophyll A (ChlA) molecules. To simulate the excited state relaxation from the initially excited Soret state to the lowest excited state Qy, the approach of nonadiabatic excited state molecular dynamics has been adopted. The intramolecular vibrational energy relaxation and redistribution that accompany the electronic internal conversion process is followed by analyzing the excited state trajectories in terms of the ground state equilibrium normal modes. The time dependence of the normal mode velocities is determined by projecting instantaneous Cartesian velocities onto the normal mode vectors. Our analysis of the time evolution of the average mode energies uncovers that only a small subset of the medium-to-high frequency normal modes actively participate in the electronic relaxation processes. These active modes are characterized by the highest overlap with the nonadiabatic coupling vectors (NACRs) during the electronic transitions. Further statistical analysis of the nonadiabatic transitions reveals that the electronic and vibrational energy relaxation occurs via two distinct pathways with significantly different time scales on which the hopping from Soret to Qx occurs thereby dictating the overall dynamics. Furthermore, the NACRs corresponding to each of the transitions have been consistently found to be predominantly similar to a set of normal modes that vary depending upon the transition and the identified categories. Each pathway exhibits a differential time scale of energy transfer and also a differential set of predominant active modes. Our present analysis can be considered as a general approach allowing identification of a reduced subset of specific vibrational coordinates associated with nonradiative relaxation pathways. Therefore, it represents an adequate prior strategy that can particularly facilitates mixed quantum-classical approaches.

Non-radiative relaxation of photoexcited chlorophylls: theoretical and experimental study.

Nonradiative relaxation of high-energy excited states to the lowest excited state in chlorophylls marks the first step in the process of photosynthesis. We perform ultrafast transient absorption spectroscopy measurements, that reveal this internal conversion dynamics to be slightly slower in chlorophyll B than in chlorophyll A. Modeling this process with non-adiabatic excited state molecular dynamics simulations uncovers a critical role played by the different side groups in the two molecules in governing the intramolecular redistribution of excited state wavefunction, leading, in turn, to different time-scales. Even given smaller electron-vibrational couplings compared to common organic conjugated chromophores, these molecules are able to efficiently dissipate about 1 eV of electronic energy into heat on the timescale of around 200 fs. This is achieved via selective participation of specific atomic groups and complex global migration of the wavefunction from the outer to inner ring, which may have important implications for biological light-harvesting function.

Elucidating the enhancement in optical properties of low band gap polymers by tuning the structure of alkyl side chains.

We carry out a computational study of optical properties of two novel 5,6-difluorobenzo[c][1,2,5]-thiadiazole-based polymers, PFBT-T20TT and PFBT-T12TT, to elucidate the surprisingly superior performance of polymer solar cells based on the former, when it differs from the latter only in the alkyl side chains. Density Functional Theory (DFT) based geometry optimization at the B3LYP/6-31G(d) level reveals differences in internal coordinates, which are important in tuning the electronic and optical properties. We further calculate the electronic structure at room temperature by employing molecular dynamics (MD) simulations in combination with DFT techniques. The energies of the highest occupied molecular orbital (HOMO) are found to be in reasonable agreement with the available experimental data and the HOMO-lowest unoccupied MO energy gap is found to be similar for both the molecules. The electronic density of the HOMO in PFBT-T20TT is, however, found to be significantly more delocalized along the backbone, which is proposed to be conducive for the formation of charge separated states leading to an improved device performance. Furthermore, via fitting the absorption spectra calculated with the multi-mode Brownian Oscillator model, we have also extracted a weaker exciton-phonon coupling parameter in PFBT-T20TT, consistent with the trends revealed via the DFT-MD results.

Sub-Ohmic spin-boson model with off-diagonal coupling: ground state properties.

We have carried out analytical and numerical studies of the spin-boson model in the sub-ohmic regime with the influence of both the diagonal and the off-diagonal coupling accounted for, via the Davydov D1 variational ansatz. While a second-order phase transition is known to be exhibited by this model in the presence of diagonal coupling only, we demonstrate the emergence of a discontinuous first order phase transition upon incorporation of the off-diagonal coupling. A plot of the ground state energy versus magnetization highlights the discontinuous nature of the transition between the isotropic (zero magnetization) state and nematic (finite magnetization) phases. We have also calculated the entanglement entropy and a discontinuity found at a critical coupling strength further supports the discontinuous crossover in the spin-boson model in the presence of off-diagonal coupling. It is further revealed via a canonical transformation approach that for the special case of identical exponents for the spectral densities of the diagonal and the off-diagonal coupling, there exists a continuous crossover from a single localized phase to doubly degenerate localized phase with differing magnetizations.

Disorder influenced absorption line shapes of a chromophore coupled to two-level systems.

We have carried out a theoretical and numerical study of disorder-induced changes in the absorption line shape of a chromophore embedded in a host matrix. The stochastic sudden jump model is employed wherein the host matrix molecules are treated as noninteracting two-level systems (TLSs) occupying points on a three-dimensional lattice with randomly oriented dipole moments. By systematically controlling the degree of positional disorder (α) attributed to them, a perfectly crystalline (α = 0) or a glassy environment (α = 1) or a combination of the two is obtained. The interaction between the chromophore and the TLSs is assumed to be of the dipole-dipole form. With an increase in α, the broadening of the absorption line shape was found to follow a power-law behavior. More importantly, it is revealed in the long-time limit that the resultant line shape is Gaussian in the absence of disorder but transforms to Lorentzian for a completely disordered environment. For an arbitrarily intermediate value of α, the resultant line shape can be approximately fitted by a linear combination of Gaussian and Lorentzian components. The Lorentzian profile for the disordered medium is attributed to the chomophore-TLS pairs with vanishingly small separation between them.

Investigation of the effects of commensurability on friction between concentric carbon nanotubes.

That a commensurate contact usually leads to greater friction than an incommensurate one is a commonly held view in nanotribology. However, this perception seems paradoxical as commensurability is found to have negligible effect on the energy dissipation in double-walled carbon nanotube (DWNT) based oscillators. Using molecular dynamics simulations, we investigate such a paradox from the viewpoint of the atomic origin of friction. It is revealed that the commensurability cannot have a pronounced effect on friction unless the intertube interaction strength and the energy corrugation exceed their critical values. Both the commensurate and incommensurate oscillators constructed from natural DWNTs with an intertube distance of about 3.4 Å, may thus exhibit similar performance.

Thermal-gradient-induced interaction energy ramp and actuation of relative axial motion in short-sleeved double-walled carbon nanotubes.

We investigate the phenomenon of actuation of relative linear motion in double-walled carbon nanotubes (DWNTs) resulting from a temperature gradient. Molecular dynamics simulations of DWNTs with short outer tube reveal that the outer tube is driven towards the cold end of the long inner tube. It is also found that the terminal velocity of the sleeve roughly depends linearly on the applied thermal gradient. We calculate the inter-tube interaction energy surface which is revealed to have a gradient depending upon the applied thermal gradient. Consequently, it is proposed that the origin of the thermophoretic motion of the outer tube may be attributed partially to the existence of such an energy gradient. A simple analytical model is presented accounting for the gradient in energy profile as well as the effect of biased thermal noise. It is shown that the proposed model predicts the dynamical behaviour of the long-time performance reasonably well.

Tight binding description on the band gap opening of pyrene-dispersed graphene.

Opening up a band gap in graphene holds a crucial significance in the realization of graphene-based electronics. Doping with organic molecules to alter the electronic properties of graphene is perceived as an effective band gap engineering approach. Using the tight binding model, we examined the band gap opening of monolayer graphene due to the adsorption of pyrene molecules on both of its sides. It was found that the breakdown of the sublattice symmetry in pyrene-dispersed graphene leads to a band gap of ∼10 meV.

Sustained smooth dynamics in short-sleeved nanobearings based on double-walled carbon nanotubes.

We carry out a molecular dynamics study of nanobearings based on double-walled carbon nanotubes with a short rotating outer tube. A (4, 4)/(9, 9) bearing configuration shows peculiar stabilization of rotational motion at certain values of angular velocities. The observed trend is found at those values of initial angular velocities (in the current context, 0.8-1.5 rad ps(-1)) which denote a transitional regime between nearly frictionless operation at low initial angular speeds and decaying performance at high initial angular velocities. With the use of detailed 'principal components analysis', we find that the energy dissipation occurs mainly due to the excitation of wavy modes in the inner tube of the bearing. It is also proposed that wavy deformation is facilitated by the actuation of axial translation of the outer tube, which acts as an energy channelling mode. Hence, we find that the absence of dissipative wavy modes results in sustained smooth rotational dynamics of the nanobearing at low temperature.