Andrew J. Mayne

Control over the optical properties of atomically thin two-dimensional (2D) layers, including those of transition metal dichalcogenides (TMDs), is needed for future optoelectronic applications. Here, the near-field coupling between TMDs and graphene/graphite is used to engineer the exciton line shape and charge state. Fano-like asymmetric spectral features are produced in WS₂, MoSe₂, and WSe₂ van der Waals heterostructures combined with graphene, graphite, or jointly with hexagonal boron nitride (h-BN) as supporting or encapsulating layers. Furthermore, trion emission is suppressed in h-BN encapsulated WSe₂/graphene with a neutral exciton red shift (44 meV) and binding energy reduction (30 meV). The response of these systems to electron beam and light probes is well-described in terms of 2D optical conductivities of the involved materials. Beyond fundamental insights into the interaction of TMD excitons with structured environments, this study opens an unexplored avenue toward shaping the spectral profile of narrow optical modes for application in nanophotonic devices.

Silicene is a two-dimensional (2D) material with very promising electronic properties for applications in silicon modern technology. However, the first experimental synthesis of silicene on metallic surfaces shows strong interactions between the silicene and its substrate, which can alter its electronic properties. Here, we report on the first steps of silicene growth on an insulating surface (NaCl) using scanning tunneling microscopy (STM), low energy electron diffraction (LEED), Auger electron spectroscopy (AES), and angle-resolved photoemission spectroscopy (ARPES). We demonstrate the importance of temperature annealing in the growth of silicene on NaCl. Indeed, after deposition of silicon on the NaCl/Ag(110) surface, we observe the following stages: i) at room temperature, the silicon atoms accumulate on top of the NaCl layer without any given order. ii) At 60°C, silicon dimers start to grow on the NaCl. iii) At 140°C, these dimers form a 2D silicon chains on the surface. iv) After a post-annealing at 200°C, evident 2D silicon nanoribbons with a honeycomb-like structure were observed. Our results of the first silicene growth stages on an insulating surface are a necessary step for exploring its growth mechanism further.

The emergence of peculiar phenomena in 1D phosphorene chains (P chains) has been proposed in theoretical studies, notably the Stark and Seebeck effects, room temperature magnetism, and topological phase transitions. Attempts so far to fabricate P chains, using the top-down approach starting from a few layers of bulk black phosphorus, have failed to produce reliably precise control of P chains. We show that molecular beam epitaxy gives a controllable bottom-up approach to grow atomically thin, crystalline 1D flat P chains on a Ag(111) substrate. Scanning tunneling microscopy, angle-resolved photoemission spectroscopy, and density functional theory calculations reveal that the armchair-shaped chains are semiconducting with an intrinsic 1.80 ± 0.20 eV band gap. This could make these P chains an ideal material for opto-electronic devices.

The synthesis of blue phosphorene by molecular beam epitaxy (MBE) has recently come under the spotlight due to its potential applications in electronic and optoelectronic devices. However, this synthesis remains a significant challenge. The surface reactivity between the P atoms and the Au atoms should be considered for the P/Au(111) system. In the MBE process, the temperature of the substrate is a key parameter for the growth of blue phosphorene. During the initial growth stage, irregularly shaped Phosphorus clusters grow on top of Au(111) surface at room temperature. When the substrate temperature is increased, these clusters transform into a phosphorene-like structure with a honeycomb lattice. An atom exchange reaction is observed between the P and first layer Au atoms under thermal activation at higher temperature, where the P atoms replace Au atoms to form a blue phosphorene structure within the top Au layer and at the step edges.

We report on the oxidation of self-assembled silicene nanoribbons grown on the Ag (110) surface using scanning tunneling microscopy and high-resolution photoemission spectroscopy.

In this study, the self-assembled molecular network and electronic properties of Ni-phthalocyanine (NiPc) molecules on monolayer graphene (MLG)/6H-SiC(0001) were studied by room temperature Scanning Tunnelling Microscopy (STM) and Density Functional Theory (DFT) calculations. In this study, a very weak electronic coupling between the graphene and the NiPc molecules is found. This is due to the very small charge transfer of only 0.035e⁻ per molecule. The weak molecule–graphene interaction has two observable consequences: sub-molecular resolution was obtained in the STM spectroscopy at room-temperature with the molecules adsorbed directly on the graphene, and the occupied and unoccupied molecular resonance peaks were observed to shift their position in energy as a function of the tip–surface distance. This is due to the temporary local charging (either positive or negative) that is achieved by decreasing the surface voltage under the STM tip. This may have important consequences for future studies of the opto-electronic properties of such hybrid graphene–molecule systems.

Room-temperature light emission from single chemisorbed perylene based molecules adsorbed on silicon carbide (SiC) is probed by scanning tunneling microscopy (STM). A new approach to STM-induced luminescence of a single molecule is explored using a wide-band-gap semiconductor to decouple electronically the molecule from the surface. After molecular adsorption, the lowest unoccupied molecular orbital and the highest occupied molecular orbital (HOMO) both lie within the bulk band gap and below the Fermi level of the substrate. The maximum photon energy of the light emission from the molecule shows a fixed shift of 1.5 eV relative to the maximum energy of the tunnel electrons. This is consistent with the photons being generated by inelastic electron tunneling between the HOMO and the unoccupied electronic states of the STM tip.

Scanning tunneling microscopy (STM) studies of the fullerene C${}_{60}$ molecule adsorbed on the silicon carbide SiC(0001)-$3\ifmmode\times\else\texttimes\fi{}3$ surface, combined with density functional theory (DFT) calculations, show that chemisorption of individual C${}_{60}$ molecules occurs through the formation of one bond to one silicon adatom only in contrast to multiple bond formation on other semiconducting surfaces. We observe three stable adsorption sites with respect to the Si adatoms of the surface unit cell. Comprehensive DFT calculations give different adsorption energies for the three most abundant sites showing that van der Waals forces between the C${}_{60}$ molecule and the neighboring surface atoms need to be considered. The C${}_{60}$ molecules are observed to form small clusters even at low coverage indicating the presence of a mobile molecular precursor state and nonnegligible intermolecular interactions.

Scanning Tunneling Microscopy (STM), Scanning Tunneling Spectroscopy (STS), and manipulation studies were performed on an ordered self-assembled monolayer (SAM) of N,N′-bis(1-hexylheptyl)perylene-3,4:9,10-bis(dicarboximide) molecules on epitaxial graphene on hexagonal silicon carbide – SiC(0001). Four novel aspects of the molecular SAM on graphene are presented. Molecules adsorb in both armchair and zig-zag configurations, giving rise to six orientations of the molecular layer with respect to the underlying substrate. The interaction between the molecules and the graphene surface shifts the LUMO towards the Fermi level, inducing a charge transfer and the opening of a band gap in the graphene, with the LUMO inside. This decouples the LUMO from the surface rendering it invisible in the dI/dV spectroscopy. The HOMO only becomes visible at short tip-surface distances, as its energy lies within the band gap of the SiC substrate. Finally, the observed molecular defects are very particular, being composed exclusively of molecular dimers. These molecular dimers have a stronger interaction with the graphene than other molecules.