Ultra-thin insulating layers of hexagonal boron nitride for high- resolution scanning tunneling spectroscopy

Aalto University, P.O. Box 11000, FI-00076 Aalto www.aalto.fi Author Fabian Schulz Name of the doctoral dissertation Ultra-thin insulating layers of hexagonal boron nitride for high-resolution scanning tunneling spectroscopy Publisher School of Science Unit Department of Applied Physics Series Aalto University publication series DOCTORAL DISSERTATIONS 163/2016 Field of research Manuscript submitted 15 April 2016 Date of the defence 16 September 2016 Permission to publish granted (date) 19 August 2016 Language English Monograph Article dissertation Essay dissertation Abstract Scanning tunneling spectroscopy (STS) allows for probing the local density of states of surfaces and adsorbates with atomic spatial resolution. When molecules or other nanostructures are electronically decoupled from the surface, STS can be interpreted in terms of the electronic structure of the isolated adsorbate. Ultra-thin insulating layers of metal oxides or alkali halides are commonly used to decouple single molecules and atoms. This thesis explores the possibilities of an alternative decoupling material: hexagonal boron nitride (h-BN).Scanning tunneling spectroscopy (STS) allows for probing the local density of states of surfaces and adsorbates with atomic spatial resolution. When molecules or other nanostructures are electronically decoupled from the surface, STS can be interpreted in terms of the electronic structure of the isolated adsorbate. Ultra-thin insulating layers of metal oxides or alkali halides are commonly used to decouple single molecules and atoms. This thesis explores the possibilities of an alternative decoupling material: hexagonal boron nitride (h-BN). We start by investigating the atomic-scale structure and electronic properties of an h-BN monolayer on Ir(111) and find that it is characterized by a moiré superstructure with a work function modulation of approx. 0.5 eV. Subsequent STS experiments on molecules deposited onto the h-BN/Ir(111) system indicate their efficient decoupling from the metallic substrate and local charging through the h-BN work function modulation. Comparing molecules in different charge states, we go beyond the prevalent single-particle picture when interpreting STS on molecules and explain the observed resonances as a series of many-body excited states. Finally, we utilize h-BN covalently attached to graphene (G) islands to decouple the G edges from the metallic substrate. This gives rise to an electronic state at the h-BN/G interface, which closely resembles the edge state theoretically predicted for pristine graphene edges. The work presented in this thesis opens new avenues for high-resolution STS on molecular systems using h-BN as an ultra-thin insulating layer.