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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Effects of state filling and localization on chemical expansion in praseodymium-oxide perovskites
In oxide materials, an increase in oxygen vacancy concentration often results in lattice expansion, a phenomenon known as chemical expansion that can introduce detrimental stresses and lead to potential device failure. One factor often implicated in the chemical expansion of materials is the degree of localization of the multivalent cation electronic states. When an oxygen is removed from the lattice and a vacancy forms, it is believed that the two released electrons reduce multivalent cations and expand the lattice, with more localized cation states resulting in larger expansion. In this work, we computationally and experimentally studied the chemical expansion of two Pr-based perovskites that exhibit ultra-low chemical expansion, PrGa 1− x Mg x O 3− δ and BaPr 1− x Y x O 3− δ , and their parent compounds PrGaO 3− δ and BaPrO 3− δ . Using density functional theory, the degree of localization of the Pr-4f electrons was varied by adjusting the Hubbard U parameter. We find that the relationship between Pr-4f electron localization and chemical expansion exhibits more complexity than previously established. This relationship depends on the nature of the states filled by the two electrons, which may not necessarily involve the reduction of Pr. F ′-center defects can form if the reduction of Pr is unfavorable, leading to smaller chemical expansions. If hole states are present in the material, the states filled by the electrons can be Pr-4f and/or O-2p hole states depending on the degree of Pr-4f localization. The O-2p holes are more delocalized than the Pr-4f holes, resulting in smaller chemical expansions when the O-2p holes are filled. X-ray photoelectron spectroscopy reveals low concentrations of Pr 4+ in PrGa 0.9 Mg 0.1 O 3− δ and BaPr 0.9 Y 0.1 O 3− δ , supporting the possible role of O-2p holes in the low chemical expansions exhibited by these materials. This work highlights the non-trivial effects of electron localization on chemical expansion, particularly when hole states are present, pointing to design strategies to tune the chemical expansion of materials.
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- Award ID(s):
- 1945482
- NSF-PAR ID:
- 10402816
- Date Published:
- Journal Name:
- Journal of Materials Chemistry A
- Volume:
- 11
- Issue:
- 8
- ISSN:
- 2050-7488
- Page Range / eLocation ID:
- 4045 to 4056
- 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/D2TA06756K
