Symmetry forbidden morphologies and domain boundaries in nanoscale graphene islands

The synthesis of graphene nanoislands with tailored quantum properties requires an atomic control of the morphology and crystal structure. As one reduces their size down to the nanometer scale, domain boundary and edge energetics, as well as nucleation and growth mechanisms impose different stability and kinetic landscape from that at the microscale. This offers the possibility to synthesize structures that are exclusive to the nanoscale, but also calls for fundamental growth studies in order to control them. By employing high-resolution scanning tunneling microscopy we elucidate the atomic stacking configurations, domain boundaries, and edge structure of graphene nanoislands grown on Ni(1 1 1) by CVD and post-annealed at different temperatures. We find a non-conventional multistep mechanism that separates the thermal regimes for growth, edge reconstruction, and final stacking configuration, leading to nanoisland morphologies that are incompatible with their stacking symmetry. Whole islands shift their stacking configuration during cooling down, and others present continuous transitions at the edges. A statistical analysis of the domain structures obtained at different annealing temperatures reveals how polycrystalline, ill-defined structures heal into shape-selected islands of a single predominant stacking. The high crystallinity and the control on morphology and edge structure makes these graphene nanoislands ideal for their application in optoelectronics and spintronics

Estudo da estrutura de nanoilhas de grafeno por microscopia de varredura por tunelamento

Exportado OPUSMade available in DSpace on 2019-08-13T19:38:33Z (GMT). No. of bitstreams: 1 tese_sofia_parreiras.pdf: 32769113 bytes, checksum: e53eb619c746f470a25bbe7809334da6 (MD5) Previous issue date: 14O crescimento epitaxial de grafeno sobre superfícies metálicas é um método promissor para o desenvolvimento de aplicações tecnológicas. Devido à pequena diferença entre parâmetros de rede e à forte interação, o substrato de Ni(111) é particularmente interessante para o crescimento controlado de nanoilhas isoladas de grafeno com morfologias bem definidas (triangulares ou hexagonais). A redução do tamanho para a escala nanométrica altera os mecanismos de nucleação estabilizando diferentes tipos de bordas, dependendo do empilhamento atômico, e alterando o equilíbrio de energia do sistema. Com isso é possível preparar estruturas que são restritas à nanoescala motivando estudos de crescimento para controlá-las. Neste trabalho imagens de alta resolução de microscopia de varredura por tunelamento foram utilizadas para estudar nanoilhas de grafeno crescidas sobre Ni(111) por deposição por vapor químico e sujeitas a diferentes temperaturas de recozimento. Foram determinadas as configurações de empilhamento atômico, fronteiras de domínios e a estrutura de bordas das nanoilhas. Foi descoberto um mecanismo não-convencional de preparação que separa os regimes térmicos de crescimento, reconstrução de bordas e configuração final de empilhamento. Isto leva a nanoilhas com morfologias que são incompatíveis com a sua simetria de empilhamento. Ilhas inteiras têm suas configurações de empilhamento deslocadas durante o resfriamento, enquanto que outras apresentam transições contínuas nas bordas. Uma análise estatística da estrutura de domínios revela como ilhas policristalinas com estruturas irregulares podem ser modificadas através de tratamentos térmicos permitindo selecionar a forma de ilhas com um tipo de empilhamento predominante. Adicionalmente, foi estudado o efeito da formação de ligas de superfície nas propriedades estruturais de nanoilhas de grafeno. Ligas Fe-Ni e Co-Ni modificam o equilíbrio de energia das bordas, o que leva a uma alteração na morfologia das ilhas. Foi observado que as ilhas preparadas sobre ligas apresentam, em geral, uma estrutura policristalina, porém a distribuição de domínios é alterada.The epitaxial growth of graphene over metallic surfaces is one of the promising methods for the developing of technological applications. Due to the small mismatch between the two lattices and the strong interaction, the Ni(111) substrate is particularly interesting for the controlled growth of isolated graphene nanoislands with well-defined morphologies (triangular or hexagonal). The size reduction to the nanometer scale modifies the nucleation and growth mechanisms stabilizing different types of edges depending on the atomic stacking. This offers the possibility to synthesize structures that are exclusive to the nanoscale, but also calls for fundamental growth studies in order to control them. High-resolution Scanning Tunneling Microscopy images were used to study graphene nanoislands grown on Ni(111) by chemical vapor deposition and postannealing at different temperatures. The atomic stacking configurations, domain boundaries, and edge structure of the nanoislands were determined. We find a nonconventional multistep mechanism that separates the thermal regimes for growth, edge reconstruction, and final stacking configuration, leading to nanoisland morphologies that are incompatible with their stacking symmetry. Whole islands shift their stacking configuration during cooling down, and others present continuous transitions at the edges. A statistical analysis of the domain structures obtained at different annealing temperatures reveals how polycrystalline, ill-defined structures heal into shape-selected islands of a single predominant stacking. Additionally, the effect of the surface alloying on the structural properties of graphene nanoislands was investigated. Fe-Ni and Co-Ni alloys modify the energy equilibrium of the edges, leading to changes on the islands morphologies. It was observed that the islands prepared on alloys, in general, exhibit a polycrystalline structure, but the distribution of domains is altered

Symmetry forbidden morphologies and domain boundaries in nanoscale graphene islands

The synthesis of graphene nanoislands with tailored quantum properties requires an atomic control of the morphology and crystal structure. As one reduces their size down to the nanometer scale, domain boundary and edge energetics, as well as nucleation and growth mechanisms impose different stability and kinetic landscape from that at the microscale. This offers the possibility to synthesize structures that are exclusive to the nanoscale, but also calls for fundamental growth studies in order to control them. By employing high-resolution scanning tunneling microscopy we elucidate the atomic stacking configurations, domain boundaries, and edge structure of graphene nanoislands grown on Ni(1 1 1) by CVD and post-annealed at different temperatures. We find a non-conventional multistep mechanism that separates the thermal regimes for growth, edge reconstruction, and final stacking configuration, leading to nanoisland morphologies that are incompatible with their stacking symmetry. Whole islands shift their stacking configuration during cooling down, and others present continuous transitions at the edges. A statistical analysis of the domain structures obtained at different annealing temperatures reveals how polycrystalline, ill-defined structures heal into shape-selected islands of a single predominant stacking. The high crystallinity and the control on morphology and edge structure makes these graphene nanoislands ideal for their application in optoelectronics and spintronics

The cobalt oxidation state in preferential CO oxidation on CoOx/Pt(111) investigated by operando X-ray photoemission spectroscopy

The combination of a reducible transition metal oxide and a noble metal such as Pt often leads to active low-temperature catalysts for the preferential oxidation of CO in excess H2 gas (PROX reaction). While CO oxidation has been investigated for such systems in model studies, the added influence of hydrogen gas, representative of PROX, remains less explored. Herein, we use ambient pressure scanning tunneling microscopy and ambient pressure X-ray photoelectron spectroscopy on a CoOx/Pt(111) planar model catalyst to analyze the active phase and the adsorbed species at the CoOx/Pt(111) interface under atmospheres of CO and O2 with a varying partial pressure of H2 gas. By following the evolution of the Co oxidation state as the catalyst is brought to a reaction temperature of above 150 °C, we determine that the active state is characterized by the transformation from planar CoO with Co in the 2+ state to a mixed Co2+/Co3+ phase at the temperature where CO2 production is first observed. Furthermore, our spectroscopy observations of the surface species suggest a reaction pathway for CO oxidation, proceeding from CO exclusively adsorbed on Co2+ sites reacting with the lattice O from the oxide. Under steady state CO oxidation conditions (CO/O2), the mixed oxide phase is replenished from oxygen incorporating into cobalt oxide nanoislands. In CO/O2/H2, however, the onset of the active Co2+/Co3+ phase formation is surprisingly sensitive to the H2 pressure, which we explain by the formation of several possible hydroxylated intermediate phases that expose both Co2+ and Co3+. This variation, however, has no influence on the temperature where CO oxidation is observed. Our study points to the general importance of a dynamic reducibility window of cobalt oxide, which is influenced by hydroxylation, and the bonding strength of CO to the reduced oxide phase as important parameters for the activity of the system