Will, Moritz 
ORCID: 0000-0003-4822-9122
(2020).
Cluster Superlattices on Hexagonal Boron Nitride and Graphene/Ir(111) and Their Embedding with Carbon.
PhD thesis, Universität zu Köln.
Abstract
Cluster and nanoparticle research is at the forefront of nanotechnology owing to the exceptional physicochemical properties of individual nanoparticles which greatly differ from their bulk counterparts. The preparation of similar sized and well ordered clusters is a challenging
task. A remarkable method to fabricate dense, two-dimensional arrays of clusters with a narrow size distribution is ultra-high vacuum growth on surface templates. Such templated cluster superlattices have been successful as model systems for heterogeneous catalysis, magnetism,
optics, and electronics, as they allow the study of cluster properties with conventional surface
science techniques. Their periodic arrangement allows the application of both local probes and integrating methods.
Wide-spread application of supported cluster superlattices is inherently hampered by their tendency to degrade under reaction conditions, i.e. atmospheric pressure and/or temperatures of a few hundred degrees Celsius. Under these circumstances, they lose their advantageous periodic arrangement, narrow size distribution and homogeneous environment, and in some cases even the cluster structure may be altered or destroyed. One way of approaching this issue is to curb the decay of cluster superlattices, another is the exploration of new substrates for templated cluster growth that provide enhanced stability.
Single-layer hexagonal boron nitride on Ir(111) is a new template for the growth of highly stable cluster superlattices. We demonstrate, that it enables the synthesis of highly ordered Ir, Pt, Au, and C cluster superlattices. Characterization with scanning tunneling microscopy reveals high superlattice densities and an unparalleled thermal stability. The results are corroborated by x-ray photoemission spectroscopy and density functional theory calculations, which unravel the binding mechanism of the clusters to the substrate. Furthermore, we explore carbon embedding of cluster superlattices in ultra-high vacuum as a method to suppress sintering of cluster superlattices. Carbon embedding is also an integral step in the formation of a novel free-standing material containing a cluster superlattice sandwiched between a two-dimensional material and a thin matrix of amorphous carbon. The thesis comprises three manuscripts in their entirety, a comprehensive overview of the relevant scientific background, and a detailed discussion of each chapter together with an outlook.
In the first manuscript, we introduce a monolayer of hexagonal boron nitride on Ir(111) as a template for the growth of periodic arrays of clusters. We demonstrate the templating capabilities for Ir, Au and C clusters. In an exemplary case study on Ir clusters, we examine the cluster binding site and epitaxial growth, and compare scanning tunneling microscopy results to ab initio density functional theory calculations. Ir clusters grow epitaxially in the valley regions of the moiré. By varying the Ir deposition, the average cluster size of the arrays can be tuned. Moreover, we find that the thermal stability of the cluster superlattices on h-BN/Ir(111) is exceptional and exceeds all other known substrates for templated growth. The excellent stability is explained by selective rehybridization of the h-BN underneath the clusters with bond formation between B and Ir cluster atoms.
In the second experimental chapter, we expand the range of cluster materials forming superlattices on hexagonal boron nitride on Ir(111) to Pt. We elucidate the growth and structure of the Pt clusters by means of x-ray photoemission spectroscopy and scanning tunneling microscopy. The XPS results provide direct evidence of the binding mechanism by rehybridization and confirms the calculated results obtained for Ir clusters on hexagonal boron nitride on Ir(111). We analyze the thermal stability of Pt clusters and observe cluster superlattice degradation via Smoluchowski ripening and intercalation below the h-BN above 650 K. Notably, one- and three-layer
clusters are favored during growth of Pt clusters on hexagonal boron nitride on Ir(111). Moreover, mild heating to 730 K induces a shape transformation from one- to three-layer clusters for the case of an average cluster size of 63 atoms, indicating two-layer Pt clusters to be metastable. Scanning tunneling spectroscopy reveals the electronic decoupling of the Pt clusters from the substrate, which gives rise to Coulomb blockade effects.
Third, we demonstrate the embedding of Ir cluster superlattices on graphene on Ir(111) with elemental carbon. Embedded clusters have a dramatically increased thermal stability against coalescence compared to bare clusters, as evidenced by scanning tunneling microscopy. Moreover, in controlled cluster pick-up experiments with the tip of the scanning tunneling microscope, we show the improved mechanical stability of embedded clusters. We demonstrate that the deposited carbon adheres first to the clusters, leading to conformal embedding without impeding the structural perfectness of the cluster superlattice. Scanning tunneling spectroscopy reveals that the only path left for cluster degradation is via intercalation below the Gr layer at temperatures above 850 K. This embedding method constitutes a crucial step in the fabrication of a novel material, consisting of a cluster superlattice sandwiched between a 2D material and an embedding matrix of carbon.
The scientific appendix provides additional data on the interaction of further materials with graphene and hexagonal boron nitride on Ir(111). We explore low temperature deposition as a method to create perfect arrays of Au and Fe clusters on hexagonal boron nitride on Ir(111). Furthermore, we investigate Mo, Ta, Nb, Dy, and Tm deposition on graphene and hexagonal boron nitride on Ir(111) in view of cluster superlattice formation. The interaction of CO with Pt clusters on hexagonal boron nitride on Ir(111) is studied in operando by scanning tunneling microscopy. Shape transformations and sintering of Pt clusters are found to occur upon CO adsorption.
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