Vacancy and Doping States in Monolayer and bulk Black Phosphorus.

The atomic geometries and transition levels of point defects and substitutional dopants in few-layer and bulk black phosphorus are calculated. The vacancy is found to reconstruct in monolayer P to leave a single dangling bond, giving a negative U defect with a +/- transition level at 0.24 eV above the valence band edge. The V(-) state forms an unusual 4-fold coordinated site. In few-layer and bulk black P, the defect becomes a positive U site. The divacancy is much more stable than the monovacancy, and it reconstructs to give no deep gap states. Substitutional dopants such as C, Si, O or S do not give rise to shallow donor or acceptor states but instead reconstruct to form non-doping sites analogous to DX or AX centers in GaAs. Impurities on black P adopt the 8-N rule of bonding, as in amorphous semiconductors, rather than simple substitutional geometries seen in tetrahedral semiconductors

Structure of Zinc Phosphate Dental Cement

The zinc phosphate cement structure consists of excess zinc oxide particles in a non-crystalline, amorphous phosphate matrix. Crystals of hopeite (Zn3(PO4) 2·4H2O) can grow from the cement surface if excess water is present. Extensive pores and surface layers of crystals, which are dependent on moisture conditions, affect the strength and adhesive properties of the cement. </jats:p

Adhesive Taglike Extensions into Acid-Etched Tooth Enamel

Adhesive tag like extensions into acid-etched tooth enamel can be directly observed in great detail by means of a modified scanning electron microscope technique. No taglike extensions of the adhesive were observed in tooth enamel that was not acid-etched before bonding of the adhesive. </jats:p