Search for: All records

Inspired by the success of graphene, two-dimensional (2D) materials have been at the forefront of advanced (opto-)nanoelectronics and energy-related fields owing to their exotic properties like sizable bandgaps, Dirac fermions, quantum spin Hall states, topological edge states, and ballistic charge carrier transport, which hold promise for various electronic device applications. Emerging main group elemental 2D materials, beyond graphene, are of particular interest due to their unique structural characteristics, ease of synthetic exploration, and superior property tunability. In this review, we present recent advances in atomic-scale studies of elemental 2D materials with an emphasis on synthetic strategies and structural properties. We also discuss the challenges and perspectives regarding the integration of elemental 2D materials into various heterostructures. 

Bilayer (BL) two-dimensional boron (i.e., borophene) emerges very recently and holds promise for fascinating physical properties and a variety of electronic applications. Despite this potential, the fundamental chemical properties of BL borophene which form the critical foundation of practical applications has been unexplored. Here, we present atomic-level chemical studies of BL borophene using ultrahigh vacuum tip-enhanced Raman spectroscopy (UHV-TERS). UHV-TERS identifies the vibrational fingerprint of BL borophene from mixed-dimensional borophene polymorphs with angstrom-scale chemical spatial resolution. The observed Raman mode is directly correlated with the vibrations of interlayer boron-boron bonds, validating the three-dimensional lattice geometry of BL borophene. By virtue of the single-bond sensitivity of UHV-TERS to oxygen adatoms, we demonstrate the enhanced chemical stability of BL borophene compared to its monolayer counterpart by exposure to controlled oxidizing atmospheres under UHV. In addition to revealing fundamental chemical insights into BL borophene, this work establishes UHV-TERS as a powerful tool to probe interlayer bonding and chemical properties of layered materials at the atomic scale. 

The development of new characterization methods has resulted in innovative studies of the properties of two-dimensional (2D) materials. Observations of nanoscale heterogeneity with scanning probe microscopy methods have led to efforts to further understand these systems and observe new local phenomena by coupling light-based measurement methods into the tip-sample junction. Bringing optical spectroscopy into the near-field in ultrahigh vacuum at cryogenic temperatures has led to highly unique studies of molecules and materials, yielding new insight into otherwise unobservable properties nearing the atomic scale. Here, we discuss studies of 2D materials at the subnanoscale where the measurement method relies on the detection of visible light scattered or emitted from the scanning tunneling microscope (STM). We focus on tip-enhanced Raman spectroscopy, a subset of scattering-type scanning near-field optical microscopy, where incident light is confined and enhanced by a plasmonic STM tip. We also mention scanning tunneling microscope induced luminescence, where the STM tip is used as a highly local light source. The measurement of light-matter interactions within the atomic STM cavity is expected to continue to provide a useful platform to study new materials. 

Abstract Surface-bound reactions have become a viable method to develop nanoarchitectures through bottom-up assembly with near atomic precision. However, the bottom-up fabrication of nanostructures on surfaces requires careful consideration of the intrinsic properties of the precursors and substrate as well as the complex interplay of any interactions that arise in the heterogeneous two-dimensional (2D) system. Therefore, it becomes necessary to consider these systems with characterization methods sensitive to such properties with suitable spatial resolution. Here, low temperature ultrahigh vacuum scanning tunneling microscopy (STM) and tip-enhanced Raman spectroscopy (TERS) were used to investigate the formation of 2D covalent networks via coupling reactions of tetra(4-bromophenyl)porphyrin (Br 4 TPP) molecules on a Ag(100) substrate. Through the combination of STM topographic imaging and TERS vibrational fingerprints, the conformation of molecular precursors on the substrate was understood. Following the thermally activated coupling reaction, STM and TERS imaging confirm the covalent nature of the 2D networks and suggest that the apparent disorder arises from molecular flexibility.