Hot-carrier generation and transfer across nanoparticle-molecule interfaces
Typ
Examensarbete för masterexamen
Program
Applied physics (MPAPP), MSc
Publicerad
2020
Författare
Fojt, Jakub
Modellbyggare
Tidskriftstitel
ISSN
Volymtitel
Utgivare
Sammanfattning
Metallic nanoparticles are important materials for emerging sensing and catalysis
technologies. Their special properties stem from the presence of a localized surface
plasmon resonance (LSPR) mode that can couple to visible light. The LSPR causes
the nanoparticle to scatter and absorb more light at frequencies that match the
plasmon energy. The plasmon excitation has a lifetime of a few femtoseconds before it
dephases into electrons and holes with a strongly athermal energy distribution. In this
thesis, time-dependent density functional theory has been employed to study these
phenomena in Ag nanoparticles. In the first part of this thesis, the photoabsorption
spectra were systematically calculated for a series of Ag nanoparticles. The particles
were between N = 13 and 586 atoms in size and included both regular and irregular
shapes. The main findings are that the LSPR peak frequency depends linearly on
N−1/3 for N ≥ 201.
When a plasmon forms in a nanoparticle in the vicinity of a molecule it may
dephase into a transition of an electron from the nanoparticle to the LUMO state
of the molecule, or from the HOMO state of the molecule to an unoccupied state in
the nanoparticle. These processes are termed direct hot-electron transfer and direct
hot-hole transfer, respectively. In the second part of the thesis, a systematic study
was carried out in which a CO molecule was placed at different distances from the
nanoparticle, and the system was excited with a laser pulse. The results indicate
that for this system direct hot-electron transfer happens with a probability of around
1% and is only weakly dependent on the molecule-nanoparticle separation until it
decays to zero at large distances. Meanwhile, the probability of hot-hole transfer is
between 0.2 and 0.3% at a distance of 1.8Å and decays monotonically. Contributing
factors to the differences are that the molecular LUMO state is sufficiently close
to the Fermi level for hot-electron transfer to occur, while only hybridized tails of
the HOMO state satisfy the corresponding requirement for hot-hole transfer. The
most important criterion for transfer to occur is
Beskrivning
Ämne/nyckelord
Hot-carriers , localized surface plasmon resonance , time-dependent density functional theory