1. High Resolution Energy Loss Spectroscopy of “Sheet” Plasmons Roy F. Willis, Department of Physics, The Pennsylvania State University University Park, PA 16802 ‘Sheet plasmon’ are collective oscillations of two-dimensional charge sheets, produced in this work, by the metallization of a silicon-wafer surface: Si (111) √m×√n-M, where M=Ag, Au, In; m, n are integers describing a superlattice. Low-energy electrons scatter & excite sheet plasmons. High- resolution EELS measures dispersion behavior, angular frequency, ω vs. wavevector q // Anything that alters the surface charge density (interaction with adsorbed molecules) or spatial distribution of the charge density, will alter the dispersion behavior The dispersion behavior is dependent on the surface free- carrier charge density, n (cm -2 ) Conclusion: 1.Sheet plasmons are sensitive to charge-transfer processes at surface 2.New modes of collective excitation observed Superlatticek F ( Å -1 )n 2D (cm -2 ) Si (111) 7×70.011.5 × 10 11 Si (111) √3×√3-Ag0.081.0 × 10 13 Si (111) √21×√21-Au0.154.0 × 10 14 Si (111) √7×√3-In1.53.6 × 10 15

2. k F (n 1 ) k F (n 2 ) 2D Fermi Surface Superlatticek F ( Å -1 )n 2D (cm -2 ) Si (111) 7×7~ 0.011.5 × 10 11 Si (111) √3×√3-Ag~ 0.081.0 × 10 13 Si (111) √21×√21-Au~ 0.154.0 × 10 14 Si (111) √7×√3-In~ 1.53.6 × 10 15 Si 2DEG C 60

3. q // koko koko ksks θiθi θsθs Semiconductor n=2×10 ¹³ cm -2 n=8 ×10 ¹³ cm - ² Wavevector q (Å -1 ) 0.050.100.15 Energy loss (meV) 800

4. Fig 1 (a) (b) Surface plasmon polariton of wavevector q=2π/λ; (a) surface change density distribution; (b) electric field lines wavevector q λ

5. 3.4 x 10 13 cm -2 3.0 x 10 13 cm -2 5.4 x 10 13 cm -2