In-solution and on-surface synthesis and characterization of curved graphenic nanoestructures

Abstract

Graphene is known for its remarkable properties derived from its sp2 hybridized carbon atoms and delocalized π-electrons. Traditionally, applications of graphene have leveraged these properties. However, a significant limitation of graphene is its lack of an inherent band gap, which is crucial for semiconducting materials. This absence prevents graphene from effectively switching on and off, a necessary function for transistors and other semiconductor devices. In this regard, different strategies for modifying graphene's band structure have already been described in the literature, encompassing: (i) the electron confinement in smaller graphene nanostructures or graphene nanoribbons, (ii) by subjecting graphene to mechanical strain or (iii), by hydrogenating graphene with certain patterns. This thesis provides an in-depth exploration into the synthesis and characterization of graphene nanostructures on top of a Au(111) surface under ultra high vacuum conditions (UHV), with a particular focus on studying the generated properties in this graphene nanostructures due to the inducement of curvature in them. This research is significant as it delves into the less explored aspect of how curvature influences graphene’s electronic, magnetic, and optical properties, thereby expanding its potential applications in nanoelectronics and optoelectronics. The thesis was developed under the guidance of Juan Manuel Cuerva Carvajal and Carlos Sánchez Sánchez, spanning two laboratories at the University of Granada (FQM367) and at the Materials Science Institute in Madrid (ICMM-CSIC). Additionally, collaborations were established with other groups and laboratories for various measurements and experiments, including the Instituto de Nanociencia y Materiales de Aragón (INMA), the Regional Centre of Advanced Technologies and Materials in Olomouc, Czech Republic, and the Institute of Physics of the Czech Academy of Sciences in Prague, Czech Republic. The manuscript is organized into six chapters: Introduction, Objectives, Results, Experimental Methods, Conclusions, and Annexes. The Introduction delves into the electronic properties of graphene and graphenic nanostructures, as well as their synthetic methodologies. It provides a detailed explanation of how curvature affects the properties of these materials and explores various methods to induce curvature in sp2 graphenic nanostructures. Additionally, the chapter reviews already reported syntheses of other curved nanographenes, graphene nanoribbons, and cycloarenes, highlighting their emerging electronic and magnetic properties. Significant findings from the thesis include the study of the thermal stability of high-membered rings, used to synthesize saddle-shaped graphene nanostructures from various molecular precursors and under different reaction conditions on a gold surface. Mechanistic studies of the rearrangements of these high-membered rings at high temperatures reveal several key factors in the field of on-surface synthesis: the importance of the initial diastereomeric configuration, the role of adatoms in on-surface reactions, and the impact of surface-induced symmetry breakage. The second section of the Results chapter, which focuses on the synthesis of a family of cycloarenes on surfaces, represents a state-of-the-art advancement in both precursor design and comprehensive characterization using scanning probe microscopy methods. This research significantly enhances the understanding of magnetically active molecules. The study investigates the interactions between free radicals and underlying surfaces, explores the relationship between size enlargement and structural curvature, and delves into electronic properties such as global aromaticity. In the final section of the Results the interaction between free radicals in non-hexagonal rings and the surface beneath them is underscored again. It is demonstrated that these interactions may create curvature in the graphenic structures due to a mismatch between the distances of the surface lattice and the distance between radicals in a molecule, modifying the electronic properties, particularly the bandgap. The Experimental Methods chapter is divided in two sections, as the thesis utilized both insolution and on-surface methodologies. The in-solution section involves the description of the chemical reactions for designing and functionalizing the prior graphene precursors of the final curved nanostructures. Every intermediate and every final precursor were fully characterized by current in-solution methods as: nuclear magnetic resonance, infrared spectroscopy, highresolution mass spectrometry or x-ray diffraction. The on-surface section describes the experimental setup to generate the final nanomaterials on surface and under UHV conditions utilized in the laboratory from the Materials Science Institute of Madrid. As well, this section delves into the basics of the main utilized techniques for the characterization of the materials on surface as the scanning tunnelling microscopy, the atomic force microscopy or the x-ray photoemission spectroscopy. In addition, the basic concepts of the molecular manipulation with the probe of the scanning tunnelling microscope are also addressed in this section. The thesis concludes that curvature is a critical factor that can be manipulated to customize the properties of graphene-based materials for specific applications. This chemical ability to control curvature paves the way for designing materials with desired electronic and optical properties, potentially leading to breakthroughs in electronics, spintronics, sensors, and other technological areas. The thesis is a testament to the innovative spirit of research, pushing the boundaries of what is possible with one of the most promising materials of the 21st century. Finally, the Annex chapter provides additional information on different experiments discussed in the Results chapter, relevant bibliographic references, and computational calculations. It also includes a list of publications resulting from the doctoral thesis.