Electronic and vibrational properties of carbon nanostructures

Abstract

In this thesis, first-principles calculations based on density functional theory (DFT) are used to study the electronic and vibrational properties of 2D, 1D, and 0D carbon nanostructures. Newly proposed carbon nanomaterials with tunable properties are potential candidates of sil- icon replacement in nanoelectronic applications. Part of studying new theoretical materials is assessing their dynamical stability, which is verified through performing phonon calculations on different investigated structures. Patterning 2D carbon allotropes into quasi-1D nanorib- bons is a broadly adopted strategy to open an electronic band gap, which is a desirable feature in areas such as field-effect transistors and switching devices. DFT calculations are performed on 1D tripentaphene nanoribbons to show that some of the investigated ribbons have semiconducting properties unlike their corresponding 2D tripentaphenes. Additionally, a DFT study on 2D naphthylene-β structure shows that the system exhibits semiconducting properties in its spin-polarized state due to its bipartition nature, where in a previous study, the system was confirmed to be metallic in its spin-paired state.Raman scattering is a reliable and non-destructive technique for the characterization of materials. As the graphene nanoribbons are transferred from their growth substrate to the device substrate, characterization of the ribbons is needed to identify their lengths and any defects as part of testing the functionality of the device. Raman spectra of 17-armchair graphene nanoribbons (17-AGNRs) are computed to study the effect of the length and width of the ribbons on the frequencies of the vibrational modes. The computed Raman spectra are then compared with experimental data, where the length and edge type of the ribbons measured in the experiment are identified. Finally, a computational method for the calculation of tip-enhanced Raman scatter- ing (TERS) is developed, where the target molecule is placed in the proximity of a noble- metal nanoparticle at a probing tip leading to the enhancement of the Raman signal. This method uses the discrete-dipole approximation (DDA) to compute the optical response of the nanoparticle, while the polarizabilty of the target molecule is calculated by using the bond polarizability model. TERS spectra are calculated for biphenyl and buckminster- fullerene molecules, and then compared with experimental data confirming the reliability of the method. It is shown that as the tip get sharper, and the incident field is in resonance with the nanoparticle, the TERS effect is more pronounced. Finally and more interestingly, 2D and 3D plots showing scans of the molecule are obtained, which provide visualization of the structure in real space as well as its constituents and chemical properties on the molecular level.