Near-field optical spectroscopy of two-dimensional materials

• Main Author: Mignuzzi, Sandro
• Other Authors: Bonini, Nicola ; Richards, David Robert
• Published: King's College London (University of London) 2017
• Subjects: 530
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.733347

Real materials contain structural defects which significantly affect their properties. Defects, in a general sense, are ubiquitous and encompass the diverse variety of elements capable of disrupting the continuity and translational symmetry of a crystalline lattice, both in terms of its structural morphology, and in terms of local modulation of its electrical and optical properties. In this perspective, atomic vacancies, line vacancies, atomic rearrangements, local doping inhomogeneity, chemically adsorbed adatoms, all fall within the broad category of defects. Thus, the nanoscale details of surface structure plays a pivotal role in understanding the impact defects may have on the overall properties of the material, and this is particularly true for "all-surface" materials such as two-dimensional (2-D) crystals. Even the interface between two atomically thin layers has a strong impact on the electronic and optical properties of few-layered stacks; therefore, also the interface associated with stacking and layer orientation can be viewed as an extend defect in two dimensions. While macroscopic morphological characterization methods can provide averaged information over a lateral extent defined by their spatial resolution, high resolution (i.e. nanoscale) imaging has the potential to unveil important insights into the role of defects that dominate several aspects of surface chemistry and physics. On the one hand, defects in 2-D materials can be seen as deleterious as they may alter their electrical, chemical, magnetic and mechanical properties. On the other hand, the intentional creation of nanoscale defects may offer an additional degree of freedom for engineering their properties. In this perspective, having structural defects can be either detrimental or beneficial, depending on the targeted application. Despite the ever expanding literature on the study of the interplay between defects and the optical, electrical and mechanical properties of two dimensional materials, direct and non-destructive imaging of defect formation at the nanoscale remains a significant challenge. Although techniques such as electron microscopies or scanning tunnelling microscopy can be used to resolve individual lattice defects, they may be destructive or restricted to specific (e.g. conductive) substrates. This thesis presents a nanoscale optical investigation of 2-D materials, such as graphene and single-layer MoS2, with a particular focus on the characterisation of defects. The field enhancement at the tip-apex of a metal-coated atomic force microscopy (AFM) tip is used to decrease the spatial resolution beyond the diffraction limit. In the case of the investigation of Raman scattering, this near-field optical technique is known as tip-enhanced Raman spectroscopy (TERS). TERS is here demonstrated to be a valid technique to probe the distribution of point-like defects at the nanoscale, especially in the case of barely defective graphene. An analytical model to describe near-field imaging of pointlike Raman scatterers, which is of general applicability to zero-dimensional scatterers such as molecules, is presented. The near-field image, constructed from the Raman intensity, is found to depend on the Raman tensor and the orientation of the scatterer. The model can be also used to explain the different values of near-field Raman enhancement observed for different Raman bands. Motivated by the successful optical characterization of defects in graphene by means of Raman spectroscopy, it is now timely to expand the study of structural defects to other 2-D materials, such as semiconducting transition metal dichalcogenides. MoS2 is one of the most prominent members of this newly discovered category of chalcogenide monolayers. Defect-induced Raman scattering of single-layer MoS2 is studied by means of a controlled introduction of defects using ion-bombardment. Phonon confinement is used to explain the evolution of peak widths and shifts, and a metric based on Raman intensities is proposed to quantify defects. To gain insight into the defect-induced Raman processes, polarised and resonance Raman spectroscopy are employed.