Defect-engineered graphene functionalization via cycloaddition reaction – towards a versatile platform for nanoscale devices and 3D heterostructures

In recent years it has been shown that the outstanding properties of graphene, a direct consequence of its unique 2D structure, could be further tailored by surface functionalization with suitable materials, towards a fine tuning of the system's physical and chemical properties. In particular, the covalent functionalization of graphene using organic functional groups has been explored as a pivotal step towards the formation of graphene composites at the nanoscale. Alongside the commonly diffused approach with diazonium salts (abundant and quick but hard to control), a more selective and controlled method has been shown as very promising: 1,3-dipolar cycloaddition (1,3-DC) of azomethine ylide has been investigated for the chemical modification of graphene-like systems. However, while graphene's high specific surface area of 2630 m^2/g provides numerous possible binding sites, its chemical inertness makes it difficult to modify graphene's structure without disrupting it or introducing excessive disorder. Thus, to finely control or intentionally design the binding sites of functionalizing molecules on the surface of graphene while preserving the high quality of its unique structure remains an open challenge. A promising route in order to locally improve the reactivity of graphene is to introduce beneficial structural defects. For example, due to the defect-induced electron charge redistribution, defective graphene shows increased chemical reactivity towards addition reactions. At the same time, the precise control in defect formation would allow a fine tailoring of the surface chemistry of graphene, fundamental for the engineering of its electronic properties or for sensing applications. The most versatile approach that satisfies the requirements for a controlled introduction of structural defects in graphene is based on particle irradiation techniques. Indeed, effective defect modulations can be patterned over a large area via electron beam irradiation (EBI), utilizing scanning electron microscopy (SEM), in a very flexible way. The PhD research presented here builds upon this idea. Covalent functionalization of different graphene-based systems has been achieved, allowing to explore various parameters of the functionalization process, including EBI defect-engineering. Firstly, the functionalization procedure is optimized utilizing graphene nanosheets (GNS) and reduced graphene oxide (rGO) dispersed in the liquid phase, and, for the first time, a comparison of the efficiency of 1,3-DC of azomethine ylide in different dispersant solvents (NMP and DMF) is reported. The functionalization is confirmed with electron energy loss spectroscopy (EELS) and energy-dispersive X-ray spectroscopy (EDX) measurements, and new Raman features arising from the functionalization with azomethine ylide are detected. Density functional theory (DFT) models for pristine and functionalized rGO are built and characterized by evaluating the restrained electrostatic potential (RESP)-derived partial atomic charges, which highlighted the localization of the charges in the pristine rGO induced by the presence of defects (epoxy groups) in the initial structure. Furthermore, the computation of the power spectrum (PS) helps with the assignment of characteristic Raman peaks to the functionalization with the azomethine ylide. Finally, the elemental composition of pristine and functionalized graphene is investigated via x--ray photoelectron spectroscopy (XPS) measurements, allowing to confirm the stability of the functionalization (up to 180 °C) and to estimate the efficiency of the 1,3-DC on graphene. Thanks to the local inhomogeneity of the partial charges, due to the presence of oxygen functional groups in the initial structure, a higher functionalization is achieved on rGO (~ 2 times higher than on GNS). The enhancement of the chemical reactivity measured in our defected graphene validates the interest in further exploring the possibility to control the position of defects on higher quality graphene systems. Defect patterns are designed on micromechanically exfoliated graphene flakes on silica substrates by EBI. Their distribution is analyzed with Raman spectroscopy, revealing that surface treatments of the graphene-supporting substrate have strong impact on the lateral resolution that can be achieved on the final defective pattern. Unintentional defects-rich zones are revealed in the adjacent parts of the irradiated areas, and Monte Carlo simulations of primary electrons scattering events demonstrate that these transition zones originate within the area where back-scattered electrons (BSEs) and secondary electrons (SEs) generated near the substrate surface by BSEs (by interaction with organic impurities adsorbed on the Si/SiO_2 substrate) escape from the substrate surface. These results can be exploited in order to design high spatially-resolved defect patterns on monolayer graphene flakes, introducing a selectively enhanced chemical reactivity towards the organic functionalization. To explore this route, defect patterns are designed on exfoliated graphene flakes via low-energy (30 keV) EBI. Raman spectroscopy maps show the appearance of the characteristic D peak only in the patterned area, while AFM images confirm the spatial resolution of the pattern (~ 100 nm). The 1,3-DC of azomethine ylide in-situ involves the localization of a C=C bond of the graphene structure, which is favorable in presence of the defects, hence introducing a selective control of the chemical modification of graphene. The Raman analysis on functionalized graphene flakes exhibits new features only in the patterned area, while the unexposed area still presents the spectrum of pristine graphene, confirming the selectivity introduced via defect patterning. Moreover, AFM images of patterned graphene show an improved adhesion on the silica substrate, allowing to avoid detachment issues during the functionalization procedure in the organic solvent. DFT allows to identify the vibrational contributions of the functional groups of the azomethine ylide grafted on the graphene surface and of the modified vibrational modes of the graphene lattice in the experimental Raman spectrum. Furthermore, under laser irradiation (up to 1.6 mW) the Raman spectrum recovers towards the spectrum of non-functionalized patterned graphene, indicating the desorption of the ylide and the reversibility of the functionalization. Then, the functionalization of epitaxial graphene (EG) on SiC is investigated, benefiting from the valuable addition of scanning tunneling microscopy (STM) and spectroscopy (STS). STM images of functionalized EG reveal the appearance of new structures, randomly arranged over the flat terraces (with lower density) and along the edges (with higher density), with an average height in the range 2 - 15 A, and a graphene surface coverage of ~ 14 %. The graphene structure is preserved after the functionalization procedure, as confirmed by atomically resolved STM images of its hexagonal lattice. STS spectra acquired on functionalization EG indicate the opening of a bandgap (of 0.13 - 0.20 eV) in the local density of states (LDOS) of these structures, in contrast with the zero-gap linear behavior measured on graphene. The Raman analysis of functionalized EG exhibit new features, together with a downshift of the G and 2D peaks. These results indicate the grafting of azomethine ylides on graphene. Finally, to increase the efficiency of the covalent functionalization of EG and, in particular, to be able to spatially design the functionalization of EG, defect patterning via EBI is explored. After patterning, Peak Force - Quantitative NanoMechanical (PF-QNM) measurements allow to identify the designed defect pattern, confirming the spatial resolution of the technique (with different electron doses and e-beam scan step sizes). Moreover, the analysis of the adhesion forces reveals that the patterning results in an enhancement of the adhesion of the graphene with the substrate, as already seen in previous experiments. Although incomplete, these are valuable results in the outlook of a deterministic and controlled chemical functionalization of EG on SiC, which would be extremely beneficial for the fabrication of high quality devices at the nanoscale. In fact, EG on SiC eliminates the need for transfer procedures and presents favorable characteristics for large-scale graphene electronics. The results discussed here open the route for a controlled functionalization of different graphene-based systems with designed molecules, which could act both as active functional groups or passive spacers towards multi-functional sensing devices or multilayered spaced graphene systems optimized for hydrogen storage or gas sensing.