From band gap engineering to molecular photochemistry: A computational study on functionalized two-dimensional materials

Functionalization of molecular and low-dimensional materials is a powerful strategy for tailoring their physical, chemical, and electronic characteristics, making them suitable for advanced applications in nanotechnology, optoelectronics, and quantum information devices. Among the possible functional groups, the methyl group (CH3 ) stands out due to its small size and chemical stability. These features make it an ideal candidate for modifying surfaces and nanostructures to achieve tunable electronic and magnetic behaviors. This thesis presents a comprehensive theoretical investigation of methylation processes and photochemical reactivity in two-dimensional (2D) materials and molecular systems. The study focuses on functionalization using methyl-containing precursors such as methyl chloride (CH3 Cl) and methyllithium (CH3Li), which dissociate into neutral or ionic methyl species depending on the reaction conditions. Using a combination of density functional theory (DFT) and ab-initio molecular dynamics (AIMD), the thesis systematically explores the structural, electronic, and vibrational effects of methylation in single layer graphene (SLG) and bilayer graphene (BLG), hexagonal boron nitride (h-BN), bilayer hexagonal boron nitride (BL h-BN), and hexagonal boron nitride/graphene (h-BN/G) heterostructure. In SLG systems, methyl groups are shown to open band gaps in otherwise semimetallic structures and to induce magnetic states, making such modifications promising for electronic and spintronic applications. For h-BN, methylation leads to a notable narrowing of the wide band gap and may generate localized magnetic moments depending on the spatial arrangement of the methyl groups. In h-BN/G heterostructure, methylation provides a means to tune the band alignment and electronic properties with high precision. Vibrational analyses, including infrared spectra, are conducted to facilitate comparison with experimental data and to assist in the characterization of functionalized surfaces. In addition to methyl functionalization, this thesis investigates the effects of hydroxyl (OH) and bulky triphenylmethyl (C(Ph)3) groups on SLG. Adsorption of an OH radical on SLG proceeds via an initial van der Waals (vdW) physisorption step, followed by chemisorption with a very small transition-state barrier (∼0.03 eV). The chemisorbed OH group forms a strong C–OH covalent bond with an adsorption energy of about –2.54 eV, while adsorption on BLG is weaker (–0.75 eV) due to interlayer screening. OH functionalization opens a modest band gap (∼0.14 eV at the Perdew-Burke-Ernzerhof (PBE) level, ∼0.28 eV at the Heyd–Scuseria–Ernzerhof (HSE)06 level) in SLG, whereas BLG–OH remains essentially metallic. Further adsorption of a C(Ph)3 radical on SLG–OH yields a highly stable structure with a combined adsorption energy of about –2.55 eV and a much larger band gap of ∼0.84 eV. By contrast, pristine and functionalized BLG remain gapless. Moreover, AIMD simulations confirm the thermal stability of SLG–OH at room temperature, with OH groups remaining covalently bound throughout the dynamics. Finally, the thesis extends beyond ground-state properties to photochemical reactivity at molecular interfaces. Using surface hopping nonadiabatic molecular dynamics (NAMD), the excited-state dynamics of pyrene–CH3 Cl complexes are analyzed. Upon UV excitation, CH3Cl in a physisorbed state rapidly dissociates into CH3 and Cl radicals with high efficiency on sub-100 femtosecond timescale. In chemisorbed configurations, dissociation still dominates, but occasional recombination events are observed, revealing a reversible photochemical pathway influenced by bonding geometry and excited-state interactions. These simulations provide atomistic, time-resolved insight into how chemical bonding environments at π-conjugated organic surfaces influence photoinduced reactions. Overall, this thesis establishes a detailed theoretical framework for understanding how chemical functionalization—via small substituents like CH3 and OH or bulky groups like C(Ph)3 —alters the properties of molecular and 2D materials. Functionalization with these groups induces local sp2 → sp3 rehybridization, which plays a central role in modifying stability, reactivity, and electronic structure. It offers predictive insights into the design of materials with tunable electronic, magnetic, and reactive behavior, supporting future advances in materials science, optoelectronic device engineering, photocatalytic surface modification, and quantum technologies.