arXiv subjects

Deepak Kumar Rai

Publications and source records attributed to Deepak Kumar Rai.

7 recordsLinked to original sources

arXiv2021

The rise of Single-Atom Catalysts

In recent years, single-atom catalysts attracted lots of attention because of their high catalytic activity, selectivity, stability, maximum atom utilization, exceptional performance, and low cost. Single-atom catalyst contains isolated individual atom which are coordinated with the surface atoms of support such as a metal oxide or 2d - materials. In this review article, we present the advancement in single-atom catalysis in recent years with a focus on the various synthesis methods and their application in catalytic reactions. We also demonstrate the reaction mechanism of a single-atom catalyst for different catalytic reactions from theoretical aspects using density functional theory.

cond-mat.mtrl-sci↗

arXiv2020

Pariser-Parr-Pople Model based Configuration-Interaction Study of Linear Optical Absorption in Lower-Symmetry Polycyclic Aromatic Hydrocarbon Molecules

The electronic and optical properties of various polycyclic aromatic hydrocarbons (PAHs) with lower symmetry, namely, benzo[ghi]perylene (C$_{22}$H$_{12}$), benzo[a]coronene (C$_{28}$H$_{14}$), naphtho[2,3a]coronene (C$_{32}$H$_{16}$), anthra[2,3a]coronene (C$_{36}$H$_{18}$), and naphtho[8,1,2-abc]coronene (C$_{30}$H$_{14}$) were investigated. For the purpose, we performed electron-correlated calculations using screened, and standard parameters in the $π$-electron Pariser-Parr-Pople (PPP) Hamiltonian, and the correlation effects were included, both for ground and excited states, using the multi-reference singles-doubles configuration-interaction (MRSDCI) methodology. PPP model Hamiltonian includes long-range Coulomb interactions, which increase the accuracy of our calculations. The results of our calculations predict that, with the increasing sizes of the coronene derivatives, optical spectra are red shifted, and the optical gaps decrease. In each spectrum, the first peak representing the optical gap is of moderate intensity, while the more intense peaks appear at higher energies. Our computed spectra are in good agreement with the available experimental data. For the purpose of comparison, we also performed first-principles time-dependent density-functional theory (TDDFT) calculations of the optical gaps of these molecules using Gaussian basis functions, and found that they yielded values lower than our CI results.

physics.chem-ph↗

arXiv2019

Systematic First Principles Configuration-Interaction Calculations of Linear Optical Absorption Spectra in Silicon Hydrides : Si$_2$H$_{2n}$ ($n = 1-3$)

We have performed first principles electron-correlated calculations employing large basis sets to optimize the geometries, and to compute linear optical absorption spectra of various low-lying conformers of silicon hydrides: Si$_{2}$H$_{2n}$, $n=1,2,3$. The geometry optimization for various isomers was carried out at the coupled-cluster singles-doubles-perturbative-triples {[}CCSD(T){]} level of theory, while their excited states and absorption spectra were computed using a large-scale multi-reference singles-doubles configuration-interaction (MRSDCI) approach, which includes electron-correlation effects at a sophisticated level. Our calculated spectra are the first ones for Si$_{2}$H$_{2}$ and Si$_{2}$H$_{4}$ conformers, while for Si$_{2}$H$_{6}$ we obtain excellent agreement with the experimental measurements, suggesting that our computational approach is reliable. Our calculated absorption spectra exhibit a strong structure-property relationship, suggesting the possibility of identifying various conformers based on their optical absorption fingerprints. Furthermore, we have also performed geometry optimization for the selected optically excited states, providing us insights into their character.

physics.atm-clus↗

arXiv2019

Excited States and Optical Properties of Hydrogen-Passivated Rectangular Graphenes: A Computational Study

In this paper, we perform large-scale electron-correlated calculations of optoelectronic properties of rectangular graphene-like polycyclic aromatic hydrocarbon molecules. Theoretical methodology employed in this work is based upon Pariser-Parr-Pople (PPP) $π$-electron model Hamiltonian, which includes long-range electron-electron interactions. Electron-correlation effects were incorporated using multi-reference singles-doubles configuration-interaction (MRSDCI) method, and the ground and excited state wave functions thus obtained were employed to calculate the linear optical absorption spectra of these molecules, within the electric-dipole approximation. As far as the ground state wave functions of these molecules are concerned, we find that with the increasing size, they develop a strong diradical open-shell character. Our results on optical absorption spectra are in very good agreement with the available experimental results, outlining the importance of electron-correlation effects in accurate description of the excited states. In addition to the optical gap, spin gap of each molecule was also computed using the same methodology. Calculated spin gaps exhibit a decreasing trend with the increasing sizes of the molecules, suggesting that the infinite graphene has a vanishing spin gap.

cond-mat.mtrl-sci↗

arXiv2018

A Pariser-Parr-Pople Model Based Study of Optoelectronic Properties of Phenacenes

In this paper we present a computational study of linear optical absorption in phenacene class of polyaromatic hydrocarbons. For the purpose, we have employed a correlated-electron methodology based upon configuration-interaction (CI) approach, and the Pariser-Parr-Pople (PPP) $π$-electron model Hamiltonian. The molecules studied range from the smallest one with three phenyl rings (phenanthrene) to the largest one with nine phenyl rings. These structures can also be seen as finite-sized hydrogen-passivated armchair graphene nanoribbons of increasing lengths. Our CI calculations reveal that the electron-correlation effects have a profound influence not just on the peak locations, but also on the relative intensity profile of the computed spectra. We also compare our phenacene results with isomeric oligo-acenes, and find that in all the cases former have a wider optical gap than the latter. Our results are found to be in very good agreement with the experiments, where available.

cond-mat.mtrl-sci↗

arXiv2018

Tunable Optoelectronic Properties of Triply-Bonded Carbon Molecules with Linear and Graphyne Substructures

In this paper we present a detailed computational study of the electronic structure and optical properties of triply-bonded hydrocarbons with linear, and graphyne substructures, with the aim of identifying their potential in opto-electronic device applications. For the purpose, we employed a correlated electron methodology based upon the Pariser-Parr-Pople model Hamiltonian, coupled with the configuration interaction (CI) approach, and studied structures containing up to 42 carbon atoms. Our calculations, based upon large-scale CI expansions, reveal that the linear structures have intense optical absorption at the HOMO-LUMO gap, while the graphyne ones have those at higher energies. Thus, the opto-electronic properties depend on the topology of the {graphyne substructures, suggesting that they can be tuned by means of structural modifications. Our results are in very good agreement with the available experimental data.

cond-mat.mtrl-sci↗

arXiv2017

Photoabsorption in Sodium Clusters: First Principles Configuration Interaction Calculations

We present systematic and comprehensive correlated-electron calculations of the linear photoabsorption spectra of small neutral closed- and open-shell sodium clusters (Na_{n}, n=2-6), as well as closed-shell cation clusters (Na_{n}^{+}, n=3, 5). We have employed the configuration interaction (CI) methodology at the full CI (FCI) and quadruple CI (QCI) levels to compute the ground, and the low-lying excited states of the clusters. For most clusters, besides the minimum energy structures, we also consider their energetically close isomers. The photoabsorption spectra were computed under the electric-dipole approximation, employing the dipole-matrix elements connecting the ground state with the excited states of each isomer. Our calculations were tested rigorously for convergence with respect to the basis set, as well as with respect to the size of the active orbital space employed in the CI calculations. These calculations reveal that as far as electron-correlation effects are concerned, core excitations play an important role in determining the optimized ground state geometries of various clusters, thereby requiring all-electron correlated calculations. But, when it comes to low-lying optical excitations, only valence electron correlation effects play an important role, and excellent agreement with the experimental results is obtained within the frozen-core approximation. For the case of Na_{6}, the largest cluster studied in this work, we also discuss the possibility of occurrence of plasmonic resonance in the optical absorption spectrum.

physics.atm-clus↗