Single-layer black phosphorus (BP), or phosphorene, is a highly anisotropic two-dimensional elemental material possessing promising semiconductor properties for flexible electronics. However, the direct bandgap of single-layer black phosphorus predicted theoretically has not been directly measured, and the properties of its edges have not been considered in detail. Here we report atomic scale electronic variation related to strain-induced anisotropic deformation of the puckered honeycomb structure of freshly cleaved black phosphorus using a high-resolution scanning tunneling spectroscopy (STS) survey along the light (x) and heavy (y) effective mass directions. Through a combination of STS measurements and first-principles calculations, a model for edge reconstruction is also determined. The reconstruction is shown to self-passivate most dangling bonds by switching the coordination number of phosphorus from 3 to 5 or 3 to 4.

Electronic Bandgap and Edge Reconstruction in Phosphorene Materials

Structures and electronic transport on silicon surfaces

http://www-surface.phys.s.u-tokyo.ac.jp/papers/1999/AizawaSS_99.pdf

Asymmetric structure of the Si(111)-3×3-Ag surface

A method for fabricating microscopic four-point probes is presented The method uses silicon-based microfabrication technology involving only two patterning steps The last step in the fabrication process is an unmasked deposition of the conducting probe material, and it is thus possible to select the conducting material either for a silicon wafer or a single probe unit Using shadow masking photolithography an electrode spacing (pitch) down to 11 μm was obtained, with cantilever separation down to 200 nm Characterisation measurements have shown the microscopic probes to be mechanically very flexible and robust Repeated conductivity measurements on polythiophene films in the same surface area are reproduced within an accuracy of 3% Automated nanoresolution position control allows scanning across millimetre sized areas, in order to create high spatial resolution maps of the in-plane conductivity

Scanning microscopic four-point conductivity probes

Surface phases on silicon

Photoemission spectroscopy has shown that each Ag atom in its two-dimensional adatom gas (2DAG) phase deposited on the Si(111)-$\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$-Ag surface at room temperature donates one electron into an antibonding surface-state band of this substrate, resulting in a steep increase in electrical conductance through the band. The surface space-charge layer makes no contribution to the conductance increase by the 2DAG adsorption, estimated from the band-bending measurements. When the 2DAG nucleates into three-dimensional Ag microcrystals by further deposition beyond a critical supersaturation coverage, the carrier-doping effect vanishes, returning to a lower conductance. These results reveal that the surface state acts as a surface conduction band. The electron mobility in this band is estimated to be on the order of 10 cm${}^{2}$/V s.

Surface electrical conduction due to carrier doping into a surface-state band on Si(111)-3×3-Ag

Electrical conduction via surface-state bands

When Ag adatoms were deposited on top of the Si(111)-\ensuremath{\surd}3\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}3-Ag surface at room temperature with less than a critical coverage ${\mathrm{\ensuremath{\Theta}}}_{\mathit{C}}$ (\ensuremath{\sim}0.03 atomic layer), they were found to exist as a supersaturated metastable two-dimensional gas phase, which made the surface electrical conductance extremely high. With the coverage beyond ${\mathrm{\ensuremath{\Theta}}}_{\mathit{C}}$, the gas phase began to nucleate into three-dimensional microcrystals, leading to a steep reduction in the electrical conductance. A kinetic overshoot beyond the critical supersaturation for nucleation was also detected in in situ conductance measurements during the Ag deposition. \textcopyright{} 1996 The American Physical Society.

Two-dimensional adatom gas on the Si(111)-(√3×√3)-Ag surface detected through changes in electrical conduction

By utilizing a variety of surface super structures formed on silicon surfaces, we have clarified close correlations between the atomic-scale structures and surface-electrical-conduction phenomena. In paricular, we have succeeded for the first time in experimentally confirming the electrical conduction via surface-state bands that are inherent in the surface supertructures. Also, an important phenomenon has been found: atoms adsorbed on the surface donate carriers to the surface-state band, resulting in a remarkable enhancement of conductivity. The ultimate two-dimensional electron systems composed of surface-state bands, which are made close up in our study, are expected to provide a new stage insurface physics.

/pdf/surface-electrical-conduction-correlated-with-surface-48smud296l.pdf

Yuji Nakajima

Papers

Surface electrical conduction due to carrier doping into a surface-state band on Si(111)-3×3-Ag

Electrical conduction via surface-state bands

Two-dimensional adatom gas on the Si(111)-(√3×√3)-Ag surface detected through changes in electrical conduction

Surface Electrical Conduction Correlated with Surface Structures and Atom Dynamics

Electrical functional properties of surface superstructures on semiconductors