Periodic Reporting for period 1 - SMART THEME (Surface-supported Molecular ARchiTectures: THEory Meets Experiment)
Okres sprawozdawczy: 2019-11-01 do 2022-10-31
The future of nanoelectronics has been proposed to be switching soon from bulk 3D materials to lower dimensionalities. In the last twenty years much attention has been devoted to the study of intrinsic properties and assembling of new building blocks for future electronics such as 2D layered materials or molecular semiconductors. In a very short time this rapidly growing field has moved from fundamental questions on the physics and chemistry of its elemental components to the realization of the first proofs of principle of nano-electronic devices. However, the high atomic level control required by these new architectures can hardly be achieved by top-down approaches conventionally employed in current nano-electronic fabrication. In the last years a very promising and effective way emerged, generally called “on surface synthesis”, based on a novel bottom-up approach where supramolecular organizations and nanostructures can be built up with atomic precision from well-designed molecular precursors self organizing and reacting at metallic surfaces. Up to now, these techniques have been successfully employed to grow a variety of low dimensional systems as well as of ordered metallo-organic frameworks, graphene nanoribbons with defined widths and edge types or extended graphdiyne wires. The design of an effective synthesis protocol is a lengthy and expensive trial-and-error process involving several iterative steps: the synthesis of molecular precursors, the effective nanostructure growth, the characterization of the products by microscopy and spectroscopy techniques. Furthermore, the interaction of the grown architectures with the underlying surface and the chemical reactivity of these complex systems with respect to their elemental components are poorly understood and generally hard to investigate. The need for a precise atomic description of structures and chemical processes is becoming more urgent and its lack is increasingly becoming a barrier to rapid progresses towards scaling up and industrialization of the processes. In a synergy between theory and experiments, research in the field can be improved and sped up thanks to optimization guidelines derived from advanced modeling. Controllable growth parameters can be extracted from a quantum level description of the molecular precursors, of the basic steps of the synthesis process and of the self-organization mechanisms at the surfaces.
The SMART-THEME (Surface-supported Molecular ARchiTectures: THEory Meets Experiments) project aimed to identify, in a synergy between theory and experiments, new routes toward novel organic/inorganic hybrid nanostructured materials with tailored characteristics.
The SMART-THEME (Surface-supported Molecular ARchiTectures: THEory Meets Experiments) project aimed to identify, in a synergy between theory and experiments, new routes toward novel organic/inorganic hybrid nanostructured materials with tailored characteristics.
The growth of poly-p-phenylene (PPP) chains from molecular precursors and their evolution into graphene nanoribbons have been investigated in detail both from an experimental and theoretical point of view over different metal surfaces. While most of studies had previously focused on gold surfaces, within the project we also considered the case of silver, which presents a very different reactivity and growth dynamics. At all steps of the growth process, we have investigated supramolecular structures by scanning tunnelling microscopy (STM), low energy electron diffraction, X-ray photoelectron spectroscopy and near edge X-Ray absorption fine structure. With respect to previous studies, we employed long annealing times to ensure that a steady state was reached at each intermediate state of the growth, allowing us to observe the appearance of metastable organometallic complexes already at room temperature. Experimental observations have been complemented by simulations conducted in the framework of the density functional theory (DFT) that brought an atomic level description of the molecular organization and reshaping at the metal surface. The excellent agreement between the experimental and simulated STM images provided from one side a validation of the computational approach employed and from another side a deeper understanding of the re-conformation of the molecules induced by the substrate. Growth differences between gold and silver substrates have been discussed on the basis of calculated formation energy for various supra-molecular organizations considering the important role played by intermediate organometallic complexes. Finally, a comparative analysis of H and Br desorption paths and activation energies on gold and silver has stressed the importance of these processes in defining the kinetics of formation of molecular chains and graphene ribbons.
Poly-p-phenylene has been proposed as an initial building block for more complex uni-dimensional electronic components. A direct access to the electronic structure of this system was acquired by angular-resolved photoemission spectroscopy (ARPES) over well aligned chains over stepped vicinal surfaces. A rich information brought by this spectroscopy can be extracted by comparing experimental data with first principle simulations. However, in order to minimize spurious stress effects, very large supercells are required, whose electronic structure results in highly folded bands for both the molecule and the substrate, which can be difficult to correlate with the measurements. To overcome this complication, we have employed an unfolding post-processing which provides a primitive cell effective band structure with a great interpretative value since it can be directly related to ARPES experiments. The simulated unfolded band structure provides a very fine description of the role of the metal substrate in defining the ultimate electronic structure of the chains and of the adsorbed bromine atoms in tuning the band alignment between the metal and the molecules.
Within the project the host group has also successfully synthesized graphyne molecular wires on gold surfaces. By first principles simulations, we have studied the organizations and stability of both the molecular precursors and polymeric chains, showing that interactions with the substrate do not induce a substantial re-conformation of the molecules. The combination of first principles and STM image simulations has permitted to correlate high intensity features observed in experiments with the HOMO and LUMO location within the adsorbed molecular precursors and chains. This information has allowed us to provide a precise description of the location of molecules with respect to the underlying metal surface, an information which is not directly accessible with the experiments.
Poly-p-phenylene has been proposed as an initial building block for more complex uni-dimensional electronic components. A direct access to the electronic structure of this system was acquired by angular-resolved photoemission spectroscopy (ARPES) over well aligned chains over stepped vicinal surfaces. A rich information brought by this spectroscopy can be extracted by comparing experimental data with first principle simulations. However, in order to minimize spurious stress effects, very large supercells are required, whose electronic structure results in highly folded bands for both the molecule and the substrate, which can be difficult to correlate with the measurements. To overcome this complication, we have employed an unfolding post-processing which provides a primitive cell effective band structure with a great interpretative value since it can be directly related to ARPES experiments. The simulated unfolded band structure provides a very fine description of the role of the metal substrate in defining the ultimate electronic structure of the chains and of the adsorbed bromine atoms in tuning the band alignment between the metal and the molecules.
Within the project the host group has also successfully synthesized graphyne molecular wires on gold surfaces. By first principles simulations, we have studied the organizations and stability of both the molecular precursors and polymeric chains, showing that interactions with the substrate do not induce a substantial re-conformation of the molecules. The combination of first principles and STM image simulations has permitted to correlate high intensity features observed in experiments with the HOMO and LUMO location within the adsorbed molecular precursors and chains. This information has allowed us to provide a precise description of the location of molecules with respect to the underlying metal surface, an information which is not directly accessible with the experiments.
The theoretical modeling at the core of the project provided a fundamental look at reactions taking place on surfaces and served as an irreplaceable tool to rationalize experimental evidence. The fellow has employed novel unfolding methodologies to the study of 1D nanostructures at metal surfaces. The work provided a more in-depth interpretation of previous results obtained by the host group. Finally, a big part of the project was dedicated to the development of tools that enhance the predictability of molecular precursors behavior in on-surface synthesis, a key feature for the reproducible growth of 2D materials of technological relevance. The ultimate goal of the project to drive experiments toward optimal synthesis routes was achieved. During the course of the fellowship, the researcher has learned and developed a number of skills important for her career development.