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

Abstract

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 configurationinteraction (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.

A first-principles study of the electronic, vibrational, and optical properties of planar SiC quantum dots

“…This is similar to the case of planar aromatic hydrocarbons for which σ-π separation holds good when it comes to low-lying optically excited states in the UV-Vis region. Therefore, one can obtain an effective π-electron Hamiltonian for planar SiC-QDs for describing their optical excitations, similar to the PPP model [101] for the aromatic hydrocarbon molecules and H-passivated graphene fragments used extensively in our group [71,72,102]. We plan to develop such an effective model Hamiltonian for SiC and other similar planar π-conjugated systems in future because it will allow us to perform more rigorous electron-correlated calculations due to a drastic reduction in the degrees of freedom achieved in making a transition from the first-principles all-electron approach to the one involving only the π-electrons.…”

“…Therefore, we have passivated the edges with the H atoms to make them chemically inert because of which the QDs manage to retain their symmetric shapes. In previous several works, we have studied the electronic structure and optical properties of graphenederived nanostructures such as graphene nanoribbons as well as QDs with H-passivated edges, employing a Pariser-Parr-Pople (PPP) model based semiempirical approach [70][71][72]. However, in the present work we have employed a DFT-based first-principles approach, and studied the stability, electronic structure, Raman and optical absorption spectra of six different strictly planar structures of SiC-QDs, for which 2D monolayers have been demonstrated to be stable in the previous works [56][57][58].…”

Theoretical insights into dopamine photochemistry adsorbed on graphene-type nanostructures

“…At the same time, theoretical calculations based on TDDFT showed that for a 0D graphene particle with a dimension of a few nanometres, additional peaks of lower intensity can appear in the 350-700 nm interval, which is clearly influenced by the dimension of the graphene model system. 61,62,99 Theoretical UV absorption spectra were computed considering the graphene nanostructure shown in Fig. 1 both in the optimized (GrNP) and fixed (GrS) geometry forms, by applying the TDDFT/oB97X-D3BJ/def2-TZVPP level of theory.…”

Electrochemical reactions catalyzed by carbon dots from computational investigations: functional groups, dopants, and defects

“…Theoretical studies revealed that zigzag CDs exhibit edge magnetism with possible applications in spintronic devices, 24 while armchair CDs are direct bandgap semiconductors with potential optoelectronic applications. 25 The creation of extrinsic defects via the introduction of heteroatoms such as nitrogen, boron, and oxygen is extremely effectively to tune the properties of not only CDs but also other carbon materials in electrochemistry. Due to their similar radius, these heteroatoms can enter the lattice matrix by substituting carbon atoms.…”