Dibyajyoti Ghosh

Computational studies on magnetism and the optical properties of transition metal embedded graphitic carbon nitride sheets

Using density functional theory (DFT), we have explored the structural, electronic, magnetic and optical properties of two-dimensional 3d-transition metal (TM)-embedded graphitic carbon nitride (g-C3N4) sheets. g-C3N4 sheets are structurally modified in different ways depending upon the radius of embedded-TM atoms and the crystal field stabilization energy gained by the corresponding geometry. Bare g-C3N4, which is a wide-gap semiconductor, becomes metallic upon TM inclusion. The d-orbitals of TMs hybridize with the pπ-orbitals of the g-C3N4 framework and close the band gap in TM-embedded g-C3N4 (TM-g-C3N4). Interestingly, for V, Cr and Fe embedded g-C3N4, the TM atoms interact ferromagnetically to each other and result in a ferromagnetic ground state. However, Mn couples antiferromagnetically and Cu and Zn are nonmagnetic in the ground state of their corresponding TM-g-C3N4 sheets. Because of structural distortion, Co- and Ni-g-C3N4 do not have a well-ordered magnetic orientation. Performing Heisenberg-model-based Monte Carlo simulations, we predict that V-, Cr- and Fe-g-C3N4 would possess Curie temperatures (Tc) of 205 K, 170.5 K, and 115 K, respectively. Furthermore, these modified g-C3N4 sheets also show prominent absorption at low energy, which evidently confirms their efficient photoabsorption capacity. The present study demonstrates the multifunctional behavior of TM-g-C3N4, which shows significant promise for application in various fields such as in memory devices or for photocatalysis.

Line defects at the heterojunction of hybrid boron nitride–graphene nanoribbons

Using ab initio molecular dynamics (AIMD) simulations, we have explored the structural reconstruction of a special kind of line defect, which is constructed from tetragonal rings and is implanted at the heterojunction of hybrid boron nitride–graphene (BN–C) nanoribbons. It appears that nanoribbons get reconstructed in various ways to form different kinds of line defect depending upon the nature of the atoms at the heterojunction. Along with 5–8–5, we also report two new kinds of line defects, 8–8–8 and 7–4–7, at the heterojunction. The electronic and magnetic properties of the reconstructed nanoribbons are calculated using density functional theory (DFT). These nanoribbons show a wide range of electronic structures ranging from semiconducting to spin polarized metallic behaviour.

Spin-crossover molecule based thermoelectric junction

Using ab-initio numerical methods, we explore the spin-dependent transport and thermoelectric properties of a spin-crossover molecule (i.e., iron complex of 2-(1H-pyrazol-1-yl)-6-(1H-tetrazole-5-yl)pyridine) based nano-junction. We demonstrate a large magnetoresistance, efficient conductance-switching, and spin-filter activity in this molecule-based two-terminal device. The spin-crossover process also modulates the thermoelectric entities. It can efficiently switch the magnitude as well as spin-polarization of the thermocurrent. We find that thermocurrent is changed by ∼4 orders of magnitude upon spin-crossover. Moreover, it also substantially affects the thermopower and consequently, the device shows extremely efficient spin-crossover magnetothermopower generation. Furthermore, by tuning the chemical potential of electrodes into a certain range, a pure spin-thermopower can be achieved for the high-spin state. Finally, the reasonably large values of figure-of-merit in the presence and absence of phonon demonstrate a large heat-to-voltage conversion efficiency of the device. We believe that our study will pave an alternative way of tuning the transport and thermoelectric properties through the spin-crossover process and can have potential applications in generation of spin-dependent current, information storage, and processing.

Shining Light on New-Generation Two-Dimensional Materials from a Computational Viewpoint

Energy- and sensing-related applications using two-dimensional (2D) materials with tunable optoelectronic properties have been a hot topic of research. The genres of 2D materials grow every day, leading to new possibilities in optoelectronic devices. In this Perspective, we have discussed in a nutshell several impacts of light-matter interactions in new-generation 2D materials. Using reliable computational approaches, in-depth understanding about the fundamental optical absorption and emission character as well as further prediction of the potential applications for these materials in the field of photovoltaics and sensing have been explored. Various modifications of the parent 2D materials by computational designing with enhanced performance have been investigated to guide the experimental efforts. The major computational challenges and their probable solutions for 2D-material-based optoelectronic research have also been briefly outlined.