Computational studies on electronic structure and optical properties of a few low-dimensional systems

Abstract

Many interesting and counter intuitive properties have been predicted over decades in low-dimensional systems. i.e. systems. confined to less than three dimensions. In this thesis, we have investigated the electronic structure and optical properties of a few low-dimensional systeM.S. viz., graphene, graphene nanoribbons, single-walled carbon nanotubes and chlorophylls. In chapter 1, we gave a brief introduction to low-dimensional systeM.S. and we have mentioned some exotic properties of all the low-dimensional systeM.S. which we investigated in this thesis. In this chapter we also introduced the computational and theoretical methods like DFT, TDDFT etc. which we have used to determine the electronic structure and optical properties of the materials. In chapter 2, we applied density functional theory (DFT) to understand the interaction of halogen/ interhalogen molecules with nanocarbons (graphene and carbon nanotubes). Previous experimental studies on halogen-nanocarbon composites show a charge transfer from nanocarbon to halogens and correspondingly a shift in the Raman G-band of nanocarbons. In our study, we find shifts in the Fermi-level of nanocarbons up on the adsorption of halogen/ interhalogen molecules. Our findings shows that shift in the Fermi-level and the Raman G-band are in accordance with each other and the amount of shift in Fermi-level or Raman G-band depends on the electron affinity character of halogen/ interhalogen molecules. In chapter 3, we investigated the effect of axial ligation on the structural, redox and absorption properties of a newly discovered chlorophyll, named chl f, and explained the reasons for the observed changes. We have also given the reason for the red-shift in the Qy band of chl f compared to other chls, like chl a and chl b, based on the time dependent density functional theory studies. Chapter 4 is the final chapter and it is a presentation of work in progress. In this chapter, we studied the structural dependency of the nonlinear optical (NLO) properties of graphene nanoribbons (GNRs). In particular, we considered zigzag GNRs (ZGNRs) of various sizes, both in length and width, and studied the variation in linear and hyper-polarizability by changing the size of ZGNRs. We find that hyper-polarizability value of ZGNRs increases with an increase in the conjugation and the values are large enough to use ZGNRs as donor-acceptor bridges. Calculations of polarizability and hyper-polarizability of ZGNRs attached with different donors and acceptors are in progress.