Extreme ultraviolet (XUV) light sources based on high
harmonic generation are enabling the development of novel
spectroscopic methods to help advance the frontiers of ultrafast
science and technology. In this account we discuss
the development of XUV-RA spectroscopy at near grazing
incident reflection geometry and highlight recent applications
of this method to study ultrafast electron dynamics at
surfaces. Measuring core-to-valence transitions with broadband,
femtosecond pulses of XUV light extends the benefits
of x-ray absorption spectroscopy to a laboratory tabletop by
providing a chemical fingerprint of materials, including the
ability to resolve individual elements with sensitivity to oxidation
state, spin state, carrier polarity, and coordination
geometry. Combining this chemical state sensitivity with
femtosecond time resolution provides new insight into the
material properties that govern charge carrier dynamics in
complex materials. It is well known that surface dynamics
differ significantly from equivalent processes in bulk materials,
and that charge separation, trapping, transport, and
recombination occurring uniquely at surfaces governs the efficiency
of numerous technologically relevant processes spanning
photocatalysis, photovoltaics, and information storage
and processing. Importantly, XUV-RA spectroscopy at near
grazing angle is also surface sensitive with a probe depth of
3 nm, providing a new window into electronic and structural
dynamics at surfaces and interfaces. Here we highlight
the unique capabilities and recent applications of XUVRA
spectroscopy to study photo-induced surface dynamics
in metal oxide semiconductors, including photocatalytic oxides
(Fe2O3, Co3O4 NiO, and CuFeO2) as well as photoswitchable
magnetic oxide (CoFe2O4). We first compare the
ultrafast electron self-trapping rates via small polaron formation
at the surface and bulk of Fe2O3 where we note that
the energetics and kinetics of this process differ significantly
at the surface. Additionally, we demonstrate the ability to
systematically tune this kinetics by molecular functionalization,
thereby, providing a route to control carrier transport
at surfaces. We also measure the spectral signatures
of charge transfer excitons with site specific localization of
both electrons and holes in a series of transition metal oxide
semiconductors (Fe2O3, NiO, Co3O4). The presence of
valence band holes probed at the oxygen L1-edge confirms
a direct relationship between the metal-oxygen bond covalency
and water oxidation efficiency. For a mixed metal oxide
CuFeO2 in the layered delafossite structure, XUV-RA
reveals that the sub-picosecond hole thermalization from O
2p to Cu 3d states of CuFeO2 leads to the spatial separation
of electrons and holes, resulting in exceptional photocatalytic
performance for H2 evolution and CO2 reduction
of this material. Finally, we provide an example to show the
ability of XUV-RA to probe spin state specific dynamics in a
the photo-switchable ferrimagnet, cobalt ferrite (CoFe2O4).
This study provides a detailed understating of ultrafast spin
switching in a complex magnetic material with site-specific
resolution. In summary, the applications of XUV-RA spectroscopy
demonstrated here illustrate the current abilities
and future promise of this method to extend molecule-level
understanding from well-defined photochemical complexes
to complex materials so that charge and spin dynamics at
surfaces can be tuned with the precision of molecular photochemistry.
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Extracting molecular responses from ultrafast charge dynamics at material interfaces
We introduce a new data analysis method, which can be applied to transient vibrational sum-frequency generation spectroscopy to reveal hidden molecular dynamics of charge transfer at molecular heterojunction interfaces. After validating the method, we used it to extract molecular dynamics at organic semiconductor/metal interfaces, which was otherwise dominated by electronic dynamics. Such an ability can advance the understanding of the roles of molecules in interfacial charge transfer dynamics.
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- Award ID(s):
- 1808111
- NSF-PAR ID:
- 10282070
- Date Published:
- Journal Name:
- Journal of Materials Chemistry C
- Volume:
- 8
- Issue:
- 35
- ISSN:
- 2050-7526
- Page Range / eLocation ID:
- 12062 to 12067
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/d0tc01819h
