Ultrafast Spectroscopy of Photoactive Nanomaterials

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

Time-resolved spectroscopy is a versatile tool to investigate ultrafast dynamics. The photoinduced dynamics of photoactive nanomaterials occur over a range of timescales and can be initiated by femtosecond processes. Understanding these dynamics is paramount to inform rational design of new materials. In this thesis I detail ultrafast spectroscopic measurements of several photoactive nanomaterials.
Ultrafast transient absorption (TA) was used to investigate the dynamics of localised surface plasmon polaritons on hollow gold nanoshells (HGNs) coupled with excitons in J-aggregates. I determined for the first time, a power dependence to the phonon breathing mode period of HGNs, and investigated the transient response of a novel sample of HGNs, with J-aggregates inside as well as on the outer surface, with TA. Through my pump dependent TA measurements, I was able to isolate the transient signatures associated with J-aggregate HGN hybrid system and I propose that the picosecond response is primarily due to hot electrons rather than plexcitons as has been reported for similar systems.
I used two-dimensional infrared spectroscopy to directly measure 470 ± 50 fs and 2.8 ± 0.5 ps time constants associated with the reorientation of formamidinium cations in formamidinium lead iodide perovskite thin films. Molecular dynamics simulations facilitated association of these time constants with the cation agitating about an equilibrium position, with NH2 groups pointing at opposite faces of the inorganic lattice cube, and the cation undergoing 90° flips, respectively. These timescales preclude the existence of stable (anti)ferroelectric domains in formamidinium lead iodide perovskite films which had been theorised to be the source of the unusually high power conversion efficiencies observed in this material. In addition, time-resolved infrared measurements revealed a prominent vibrational transient feature arising from a vibrational Stark shift.
In the final results chapter of my thesis, I describe the ultrafast transient absorption microscopy experiment that I constructed, discuss functionality tests on the apparatus, and consider the associated design principles.
Date of Award29 Sep 2020
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorTom Oliver (Supervisor) & Henkjan Gersen (Supervisor)

Keywords

  • Ultrafast
  • Time-resolved
  • Nanomaterials
  • Femtosecond
  • TA
  • TRIR
  • 2DIR
  • Transient absorption
  • Two-dimensional infrared
  • Localised surface plasmon polaritons
  • Excitons
  • J-aggregates
  • Formamidinium
  • Perovskite
  • Hybrid perovskite
  • Ferroelectric
  • Large polaron
  • Transient absorption microscope
  • Ultrafast microscopy

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