Shape, size and functionalization dependent raman characterization of graphene quantum dots by DFT method

Abstract

Graphene Quantum Dots (GQDs), unique nanoscale fragments of graphene, are the focus of this study. Their distinct optical, electronic, and chemical properties, a result of quantum confinement and edge effects, are examined through Raman characterization using Density Functional Theory (DFT). The study delves into the influence of variations in shape, size, and functionalization on GQD properties, and also explores band gap values to understand their electronic behavior. The ultimate goal is to provide a theoretical framework for predicting and optimizing the performance of GQDs in a wide range of applications. The research utilizes quantum mechanical methods, particularly HF and DFT, to investigate the electronic structure and Raman spectra of GQDs as a function of shapes such as diamond, square, and hexagonal with varying sizes. The study also explores the effects of different functional groups, including carboxylic acid, hydroxyl, and epoxy, on the electronic properties of GQDs. It is observed that in the diamond-shaped GQDs, the bandgap decreases with increasing size. Square-shaped GQDs have mixed zigzag and armchair edges, leading to unique electronic properties. The bandgap shows a sudden increase, followed by a decreasing trend as the size increases. For the hexagonal-shaped GQDs, the bandgap consistently decreases with increasing size, indicating a stable trend in electronic properties with size. After thoroughly exploring the size and shape effects, we turned our attention to functional groups. The presence and position of these groups significantly impact the electronic properties of GQDs, a crucial finding with direct implications for practical applications. For instance, the bandgap varies slightly depending on the position of the carboxylic acid group, a detail that could be instrumental in designing GQDs for specific uses. The Raman spectrum of GQDs, a key tool for characterizing these changes, is highly dependent on their size and functional groups. The intensity increases with the size of the GQD, but the Raman shift remains around ~1300 cm⁻¹ for all GQDs. Peaks around ~3300 cm⁻¹, corresponding to C-H bonds, diminish with increasing GQD size due to these bonds being edge-localized. Pristine GQDs exhibit intense D bands due to E2g breathing motion from a single center. Functionalized GQDs show new bands and shoulders in the Raman spectrum due to distorted breathing motion caused by functional groups, which also reduce the peak intensity. Functional groups like carboxylic acid, hydroxyl, and epoxy alter the electronic properties, with their effects depending on the position and type of the group. Raman spectroscopy emerges as a useful tool for characterizing these changes, with the Raman spectrum intensity increasing with GQD size, while functionalization introduces new spectral features and reduces peak intensity, a practical insight for future research and development. Understanding these relationships is crucial for optimizing GQDs for specific applications such as bioimaging, sensing, and optoelectronics. This thesis's findings provide a comprehensive theoretical framework for the controlled synthesis and functionalization of GQDs, paving the way for their effective utilization in various technological domains.