Light Absorption by Crystalline and Amorphous Silicon Quantum Dots with Silver Adsorbates and Dopants

Recent work on light absorption by model surfaces of Si has shown that Ag adsorbates increase the intensity of photoinduced electronic transitions at lower photon energies. Furthermore, another set of recent results for Si quantum dots (QDs) has shown that P and Al dopants shift the light absorbance toward lower photon energies. In this report, the optical absorbance of Si QDs with P and Al dopants and either one or three Ag adsorbed atoms has been calculated with TD-DFT using the PW91/PW91 density functionals to compare with our previous results. In general, the presence of Ag adsorbates shows both a decrease in the HOMO-LUMO gap and a drastic increase in the absorbance below 4 eV. The addition of dopants leads to a combined effect where the energy gap is further decreased to values below 2 eV. The molecular orbitals for the initial and final states involved in transitions with large oscillator strengths were also calculated, which qualitatively show the excited electrons moving toward the Ag during exci...