Electronic an Optical Properties of 2D molecular systems - Paola Mocci

This Thesis presents and explores the attractive landscape of the reduced dimensionality systems world, focusing in particular, on 2D materials. We analyze the effects of partial/complete atomic substitution and specific chemical functionalization on the electronic and optical properties of nanometric portions of 2D compounds, that are of interest for both fundamental research and potential use in technological devices and applications. In particular, we considered the class of PAHs, concentrating our attention on the Circumacenes molecules. For these systems we studied the effects induced by different substitutions: insertions of Si atoms, total substitutions of B and N to form the Boron-Nitride (BN) counterpart and total replacement of peripheral H atoms with F atoms on several physical properties. In the first part of this Thesis we report a systematic comparative study of Coronene and Ovalene molecules analyzing the effects of Si-atoms substitutions (single, double and triple insertions) on the ground-state electronic and optical properties of these systems. We performed all electrons density functional theory (DFT) and time-dependent DFT (TDDFT) calculations to quantify the effects of morphology and chemical modifications on different physical observables. For this part of the work we used the hybrid exchange-correlation functional B3LYP in combination with a gaussian localized basis-set. This adopted union functional/basis-set has proven to produce good results for PAHs and their derivatives. Globally, for this first part of the work we have used NWChem computational code. In the second part, using the same theoretical framework (DFT and TDDFT), we study the electronic and optical properties of the BN counterparts of the five first members of the Circumacenes. In addition, we also select the case of the smallest molecule of the cluster to deepen the perfluorination effects on the same observables. We investigate in a comparative approach the molecular properties of these compounds, presenting the trend for different physical observables and comparing with the corresponding results for the C-made original parents, as well as with the available data from the literature. For this part of the Thesis we used an all-electrons gaussian-based computational package, namely Gaussian-16. In the last part of this Thesis, we present a comparative study focused on the C-made linear Acenes and their Si/Ge-based counterparts. We quantified the effects of complete substitution on the electronic and optical properties of these systems, showing the behavior of the above mentioned observables as a function of the molecular size for each cluster and the differences among clusters of different atomic type at fixed molecular size. In this last part of the work we implemented DFT and TDDFT calculations through Gaussian computational code. In conclusion, using state-of-the-art computational techniques, this Thesis presents original results on key molecular properties of different Polycyclic Aromatic Hydrocarbons, considered as nanometric portions of their infinite successful counterparts. We performed a systematic comparative investigation to evaluate, from a quantitative point-of-view, the effects of different chemical modifications (partial/total substitutions of different atomic types or perfluorination) on the electronic, optical and structural properties of different systems which are the building blocks of materials widely employed in molecular electronics. Our findings can be possibly used to select tailored chemical modifications to specific compounds for future electronic and optical applications.