The precise characterization of local variations in structural properties and interfacial interactions at the sub-nanometric level is critically important to nano-structures and materials for understanding their fundamental characteristics and implementing various functionalities. To this end, ultrahigh-vacuum tip-enhanced Raman spectroscopy (UHV-TERS) has been developed by combining the chemical specificity of Raman spectroscopy with the spatial resolution of a scanning tunneling microscope (STM). As a developing technique, UHV-TERS has been predominantly employed to investigate adsorbates (e.g., single molecules and low-dimensional materials) supported on Raman-silent surfaces, but not yet used to characterize complex heterostructures particularly with Raman-active substrates, which hold richer physics and chemistry with hybrid Raman responses. Here, we present a study via UHV-STM-TERS on an organic-two dimensional vertical heterostructure between tetraphenyldibenzoperiflanthene (DBP) and borophene (i.e., boron monolayer). In addition to identifying the vibrational signatures of DBP molecules on the surface, TERS measurements extend their chemical and conformational sensitivity to underlying vertical interfaces and substrate structures that are inaccessible solely via STM, and reveal delicate ripples and compressive strains of the borophene lattice underneath molecules. More significantly, taking advantage of the angstrom-scale spatial resolution of UHV-TERS allows us to visualize and analyze an extremely localized (in the range of 1-1.5 nm) interfacial strain of borophene induced by molecular adsorption with a magnitude as small as ~0.6%. Remarkably, the strain can be released in a reversible fashion through removal of adsorbed molecules via STM-controlled molecular manipulations. Our work broadens the application of UHV-TERS from surface analyses to sub-surface characterizations, demonstrates its outstanding ability to probe the local nature of interfacial properties in 2D heterostructures, and presents its astonishing versatility by coupling with molecular manipulations.

Angstrom-scale spectroscopic visualization of interfacial interactions in an organic/borophene vertical heterostructure

ACS Spring 2022

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