Abstract
Luminescent low-dimensional carbon materials, especially graphene quantum dots (GQDs) and graphene nanoribbons (GNRs), have emerged in recent years as superior light emitters for a variety of optical applications, such as sensoring, imaging, and photonics. However, despite substantial achievements have been made for these materials, their luminescence mechanisms and the tuning of their optical properties are still far from fully understood, significantly hindering the advancement of their applications and especially their applications in high-tech areas, such as quantum computing and quantum communication. In this thesis, we study the static and dynamic features of the excited states in a variety of nanostructures of GQDs and GNRs using density functional theory (DFT) and time-dependent DFT (TD-DFT). This work provides fundamental knowledge on the exciton characteristics of these graphene-based nanomaterials, clarifies some inconsistent viewpoints regarding their optical properties, and offers guidance for further experiments on tuning and engineering the luminescent properties of related materials.In Chapter 3, we investigate the influence of several kinds of chemical groups on the optical properties of graphene nanostructures when these chemical groups are covalently bonded onto the graphene surface, revealing a strong functionalization pattern dependence of the absorption and emission energies. Specifically, given that the optical gap of the studied nanographene model is 2.68 eV, the optical gap difference between two of its oxide isomers can be as large as 2.39 eV. It is predicted that chemical species on graphene surface shall be the primary origin of the broad absorption and emission spectra experimentally observed for the GQD samples.
In Chapter 4, we study the influence of surface chemical groups on the exciton distributions in graphene nanostructures in order to understand the mechanism of trap emission, which is an essential phenomenon in relevant materials but its mechanism has yet to be elucidated. We find that in some of the studied structural configurations, surface functional groups can localize the excitons in the lowest singlet excited (S1) state and meanwhile significantly increase the oscillator strengths for the transition from the S1 state to the ground (S0) state, which signifies the emission center role of these groups. In contrast, the surface groups in the other investigated structures only show negligible influence on the transition possibility.
In Chapter 5, we report an exciton self-trapping phenomenon, which is induced by edge-bonded ether groups in graphene nanosheets after structural relaxation in the S1 state. The significant structural deformation associated with the self-trapped exciton causes a great deal of Stokes shift (ST) energy loss (0.53-1.16 eV). This exciton self-trapping phenomenon is very likely to be common in GQDs and carbon dots (CDs) and is potentially a very plausible explanation for the huge redshift in emission that has been observed in experiments for these materials.
In Chapter 6, nonadiabatic excited-state dynamics simulations are performed to ascertain the role of epoxy groups in inducing excited-state nonradiative decay. It is found that breaking of one C-O bond at the epoxy moiety facilitates the formation of a conical intersection (CI), at which the S1-S0 energy difference is smaller than 0.2 eV. During the simulation, 33.3% of the excited structures quickly relaxed to the CI point within 300 fs. With the findings in the above chapters considered together, it is concluded that the photoluminescent capabilities of graphene epoxides are determined by the competition between the radiative and nonradiative decay channels of the excited states. These results highlight the potential role of individual chemical groups in facilitating excited-state radiative and nonradiative decays which collectively govern the emission properties of relevant materials.
| Date of Award | 11 Feb 2019 |
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| Original language | English |
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| Supervisor | Ruiqin ZHANG (Supervisor) |