The evolution of sheet-plasmon behavior in silver monolayers on Si(1 1 1)-()-Ag surface

The dispersion of sheet plasmons in two-dimensional electron gas (2DEG) systems prepared on Si(1 1 1)-( 3 × 3 )-Ag surface was investigated by angle-resolved high-resolution electron-energy-loss spectroscopy (HREELS) in an in-situ ultrahigh vacuum environment. The evolution of dispersion at the low coverage of additional silver adatoms shows a behavior associating with two-dimensional adatom gas phase and the critical coverage to sustain this phase is also determined. Dispersion at higher coverage of additional silver adatoms features a gap opening in the long wavelength limit (momentum q → 0 ), suggesting a new mode of plasmon. A calculation based on the theory of surface plasmons implies this plasmon is 3D-type surface plasmon and reveals a smooth transition from a 2D sheet plasmon to a 3D surface plasmon.