In Situ Phase Transformations of 3D Nanoislands on 2D Materials in the Ti-Graphene System

The epitaxial growth of functional three-dimensional (3D) nanoislands on two-dimensional (2D) materials is important for controlling interfacial properties when optimising the integration of 2D materials into devices. One way to expand the family of materials that can form epitaxial interfaces with 2D materials is to react epitaxial metallic nanoislands grown on 2D materials with a relevant gaseous precursor. For such reactions, it is important to consider how strain in the nanoislands will influence the reaction. Bonding between the metal and 2D material is typically quasi-van der Waals and can be strong enough to cause coherency strain in the metal, as occurs in the Ti-graphene system. Such coherency strains can cause the metal to form in islands that contain a thicker region in the centre with reduced strain, that is often dislocated. Strain and thickness in the Ti are likely to influence the way phase transformations occur locally within these nanoislands. Furthermore, the effect of surfaces and interfaces at the nanoscale may cause differences from the bulk phase diagram. To explore such effects, we study how Ti nanoislands on graphene react at high temperature with oxygen or with disilane under ultra-high vacuum (UHV) conditions. This system is device-relevant because certain silicides of Ti and the anatase form of TiO₂ are good photocatalysts and cocatalysts, with higher photocatalytic activity when combined with 2D materials.<br/>We first characterise the strains present in single crystal titanium nanoislands of varying thicknesses, deposited using slow evaporation rates in UHV conditions on clean suspended graphene. Then, we carry out reactions of UHV-deposited Ti islands with each of the reactive gases <i>in situ </i>in a Hitachi H-9000 UHVTEM connected to the UHV deposition chambers, recording movies to provide an understanding of the mechanism and kinetics of the reactions. Post-growth analysis includes additional imaging via atomic resolution scanning transmission electron microscopy; electron energy loss spectroscopy; and atomic force microscopy. These are used to explore the role of strain in how nanoislands of different thicknesses transform, and to characterise the structure of the interfaces between transformed areas and the original Ti matrix. Finally, we explore the opportunities presented by the unique morphology and strain states in the Ti islands, contrasting this with the results of similar depositions on conventional 3D substrates. This provides the opportunity to control structure and composition within specific regions of the 2D/3D heterostructure.