Sean R. Wagner

Interfacial Coupling and Electronic Structure of Two-Dimensional Silicon Grown on the Ag(111) Surface at High Temperature

Freestanding silicene, a monolayer of Si arranged in a honeycomb structure, has been predicted to give rise to massless Dirac fermions, akin to graphene. However, Si structures grown on a supporting substrate can show properties that strongly deviate from the freestanding case. Here, combining scanning tunneling microscopy/spectroscopy and differential conductance mapping, we show that the electrical properties of the phase of few-layer Si grown on Ag(111) strongly depend on film thickness, where the electron phase coherence length decreases and the free-electron-like surface state gradually diminishes when approaching the interface. These features are presumably attributable to the inelastic inter-band electron-electron scattering originating from the overlap between the surface state, interface state and the bulk state of the substrate. We further demonstrate that the intrinsic electronic structure of the as grown phase is identical to that of the R30° reconstructed Ag on Si(111), both of which exhibit the parabolic energy-momentum dispersion relation with comparable electron effective masses. These findings highlight the essential role of interfacial coupling on the properties of two-dimensional Si structures grown on supporting substrates, which should be thoroughly scrutinized in pursuit of silicene.

Efficient zinc sulfide cathode layers for organic photovoltaic applications via n-type doping

We demonstrate efficient zinc sulfide cathode window layers in thin-film organic photovoltaics enabled by n-type doping zinc sulfide (ZnS) with aluminum sulfide (Al2S3) directly through co-deposition. By optimizing the Al2S3 concentration, the power conversion efficiency is improved from 0.6% ± 0.2% in undoped ZnS window layer devices to 1.8% ± 0.1%, identical to control devices. The mechanism for this performance enhancement is shown to stem from the enhanced conductivity and interface energetics of ZnS upon n-type doping. This work expands the catalog of efficient, inorganic, non-toxic, cathode side window layers that could be effective in a range of thin-film photovoltaic technologies.

Charge modulation and structural transformation in TaTe2 studied by scanning tunneling microscopy/spectroscopy

We show that the (3 × 1) stripe structure observed in TaTe2 at room temperature arises from the formation of Ta4+–Ta4+ dimer chains along with a separate chain of Ta3+. More importantly, we reveal an intriguing lattice distortion and charge modulation at low temperature, which suggests an interplay and competition between the triple-axis (3 × 3) charge density wave-like modulation and the single-axis (3 × 1) stripe configuration. This work highlights the importance of TaTe2 as an alternative platform with rich structural and electrical phases to explore charge-lattice coupling.

Self-assembly of F16ZnPc thin films and F16ZnPc-ZnPc heterostructures on deactivated Si surfaces studied by scanning tunneling microscopy

Using scanning tunneling microscopy, we show that hexadecafluorinated zinc phthalocyanine (F16ZnPc) molecules form two morphologically different self-assembled structures that both display point-on-line coincident epitaxial registration with the deactivated Si(111)-B 3×3 surface. The packing motif in these structures suggests that fluorination of conjugated organic molecules can lead to stronger molecule-substrate interaction, π-π intermolecular interaction, and side-to-side intermolecular repulsion. The delicate balance and interplay between these interactions determine the self-assembly behavior of fluorinated molecules. Furthermore, we demonstrate the formation of vertically and laterally stacked F16ZnPc-ZnPc heterojunctions, allowing for future spectroscopy investigation of molecular electronic structures and charge transfer behavior at organic-organic hetero-interfaces.