Exciton polaron formation and hot-carrier relaxation in rigid Dion-Jacobson-type two-dimensional perovskites.

The efficiency of two-dimensional Dion-Jacobson-type materials relies on the complex interplay between electronic and lattice dynamics; however, questions remain about the functional role of exciton-phonon interactions. Here we establish the robust polaronic nature of the excitons in these materials at room temperature by combining ultrafast spectroscopy and electronic structure calculations. We show that polaronic distortion is associated with low-frequency (30-60 cm-1) lead iodide octahedral lattice motions. More importantly, we discover how targeted ligand modification of this two-dimensional perovskite structure manipulates exciton-phonon coupling, exciton polaron population and carrier cooling. At high excitation density, stronger exciton-phonon coupling increases the hot-carrier lifetime, forming a hot-phonon bottleneck. Our study provides detailed insight into the exciton-phonon coupling and its role in carrier cooling in two-dimensional perovskites relevant for developing emerging hybrid semiconductor materials with tailored properties.

Water-Mediated Charge Transfer and Electron Localization in a Co3Fe2 Cyanide-Bridged Trigonal Bipyramidal Complex.

The effects of temperature and chemical environment on a pentanuclear cyanide-bridged, trigonal bipyramidal molecular paramagnet have been investigated. Using element- and oxidation state-specific near-ambient pressure X-ray photoemission spectroscopy (NAP-XPS) to probe charge transfer and second order, nonlinear vibrational spectroscopy, which is sensitive to symmetry changes based on charge (de)localization coupled with DFT, a detailed picture of environmental effects on charge-transfer-induced spin transitions is presented. The molecular cluster, Co3Fe2(tmphen)6(µ-CN)6(t-CN)6, abbrev. Co3Fe2, shows changes in electronic behavior depending on the chemical environment. NAP-XPS shows that temperature changes induce a metal-to-metal charge transfer (MMCT) in Co3Fe2 between a Co and Fe center, while cycling between ultrahigh vacuum and 2 mbar of water at constant temperature causes oxidation state changes not fully captured by the MMCT picture. Sum frequency generation vibrational spectroscopy (SFG-VS) probes the role of the cyanide ligand, which controls the electron (de)localization via the superexchange coupling. Spectral shifts and intensity changes indicate a change from a charge delocalized, Robin-Day class II/III high spin state to a charge-localized, class I low spin state consistent with DFT. In the presence of a H-bonding solvent, the complex adopts a localized electronic structure, while removal of the solvent delocalizes the charges and drives an MMCT. This change in Robin-Day classification of the complex as a function of chemical environment results in reversible switching of the dipole moment, analogous to molecular multiferroics. These results illustrate the important role of the chemical environment and solvation on underlying charge and spin transitions in this and related complexes.

Coherent Two-Dimensional and Broadband Electronic Spectroscopies.

Over the past few decades, coherent broadband spectroscopy has been widely used to improve our understanding of ultrafast processes (e.g., photoinduced electron transfer, proton transfer, and proton-coupled electron transfer reactions) at femtosecond resolution. The advances in femtosecond laser technology along with the development of nonlinear multidimensional spectroscopy enabled further insights into ultrafast energy transfer and carrier relaxation processes in complex biological and material systems. New discoveries and interpretations have led to improved design principles for optimizing the photophysical properties of various artificial systems. In this review, we first provide a detailed theoretical framework of both coherent broadband and two-dimensional electronic spectroscopy (2DES). We then discuss a selection of experimental approaches and considerations of 2DES along with best practices for data processing and analysis. Finally, we review several examples where coherent broadband and 2DES were employed to reveal mechanisms of photoinitiated ultrafast processes in molecular, biological, and material systems. We end the review with a brief perspective on the future of the experimental techniques themselves and their potential to answer an even greater range of scientific questions.

Extreme Ultraviolet Reflection-Absorption Spectroscopy: Probing Dynamics at Surfaces from a Molecular Perspective.

Extreme ultraviolet 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 extreme ultraviolet reflection-absorption (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 govern 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 XUV-RA spectroscopy to study photoinduced 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 photoswitchable 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.

Plasmon-Resonant Vibrational Sum Frequency Generation of Electrochemical Interfaces: Direct Observation of Carbon Dioxide Electroreduction on Gold.

Here we present plasmon-resonant vibrational sum frequency generation spectroscopy for use in electrochemical measurements. Using surface plasmon resonance we couple light through a CaF2 prism to Au films of >50 nm in order to reach the buried Au/electrolyte interface. The approach enables us to use bulk electrolyte, and high current densities (>1 mA/cm2), and therefore is suitable to probe active intermediates under relevant electrochemical reaction conditions. Fresnel factor modeling of the plasmon resonance for a three layer system (CaF2/Au/electrolyte) shows good agreement with experimental data. Off-angle momentum-matching to the surface plasmon resonance allows us to measure functional groups (-CH, -CD, -CN, -NO2) across a wide range of infrared frequencies by simply scanning the infrared wavelength without any angular realignment. Additionally we report a detection limit <1% of a monolayer for the Au/electrolyte interface. Using this method we observe an active intermediate during CO2 reduction on Au at catalytic currents. Consequently, we believe that this method will provide mechanistic understanding of electrochemical reactions.

Iron(III) Speciation Observed at Aqueous and Glycerol Surfaces: Vibrational Sum Frequency and X-ray.

Aqueous solutions of FeCl3 have been widely studied to shed light on a number of processes from dissolution, mineralization, biology, electrocatalysis, corrosion, to microbial biomineralization. Yet there are little to no molecular level studies of the air-liquid FeCl3 interface. Here, both aqueous and glycerol FeCl3 solution surfaces are investigated with polarized vibrational sum frequency generation (SFG) spectroscopy. We also present the first ever extreme ultraviolet reflection-absorption (XUV-RA) spectroscopy measurements of solvated ions and complexes at a solution interface, and observe with both X-ray photoelectron spectroscopy (XPS) and XUV-RA the existence of Fe(III) at the surface and in the near surface regions of glycerol FeCl3 solutions, where glycerol is used as a high vacuum compatible proxy for water. XPS showed Cl- and Fe(III) species with significant Fe(III) interfacial enrichment. In aqueous solutions, an electrical double layer (EDL) of Cl- and Fe(III) species at 0.5 m FeCl3 concentration is observed as evidenced from an enhancement of molecular ordering of water dipoles, consistent with the observed behavior at the glycerol surface. At higher concentrations in water, the EDL appears to be substantially repressed, indicative of further Fe(III) complex enrichment and dominance of a centrosymmetric Fe(III) species that is surface active. In addition, a significant vibrational red-shift of the dangling OH from the water molecules that straddle the air-water interface reveals that the second solvation shell of the surface active Fe(III) complex permeates the topmost layer of the aqueous interface.

Controlling polaron formation at hematite surfaces by molecular functionalization probed by XUV reflection-absorption spectroscopy.

Small polaron formation is known to limit the photocatalytic charge transport efficiency of hematite via ultrafast carrier self-trapping. While small polaron formation is known to occur in bulk hematite, a complete description of surface polaron formation in this material is not fully understood. Theoretical predictions indicate that the kinetics and thermodynamics of surface polaron formation are different than those in bulk. However, to test these predictions requires the ability to experimentally differentiate polaron formation dynamics at the surface. Near grazing angle extreme ultraviolet reflection-absorption (XUV-RA) spectroscopy is surface sensitive and provides element and oxidation state specific information on a femtosecond time scale. Using XUV-RA, we provide a systematic comparison between surface and bulk polaron formation kinetics and energetics in photoexcited hematite. We find that the rate of surface polaron formation (250 ± 40 fs) is about three times slower than bulk polaron formation (90 ± 5 fs) in photoexcited hematite. Additionally, we show that the surface polaron formation rate can be systematically tuned by surface molecular functionalization. Within the framework of a Marcus type model, the kinetics and energetics of polaron formation are discussed. The slower polaron formation rate observed at the surface is found to result from a greater lattice reorganization relative to bulk hematite, while surface functionalization is shown to tune both the lattice reorganization as well as the polaron stabilization energies. The ability to tune the kinetics and energetics of polaron formation and hopping by molecular functionalization provides the opportunity to synthetically control electron transport in hematite.

Highly Localized Charge Transfer Excitons in Metal Oxide Semiconductors.

The ability to observe charge localization in photocatalytic materials on the ultrafast time scale promises to reveal important correlations between excited state electronic structure and photochemical energy conversion. Of particular interest is the ability to determine hole localization in the hybridized valence band of transition metal oxide semiconductors. Using femtosecond extreme ultraviolet reflection absorption (XUV-RA) spectroscopy we directly observe the formation of photoexcited electrons and holes in Fe2O3, Co3O4, and NiO occurring within the 100 fs instrument response. In each material, holes localize to the O 2p valence band states as probed at the O L1-edge, while electrons localize to metal 3d conduction band states on this same time scale as probed at the metal M2,3-edge. Chemical shifts at the O L1-edge enable unambiguous comparison of metal-oxygen (M-O) bond covalency. Pump flux dependent measurements show that the exciton radius is on the order of a single M-O bond length, revealing a highly localized nature of exciton in each metal oxide studied.

Identifying the acceptor state in NiO hole collection layers: direct observation of exciton dissociation and interfacial hole transfer across a Fe2O3/NiO heterojunction.

NiO is widely utilized as a hole transport layer in solar energy devices where light absorption in a photoactive layer is followed by charge separation and hole injection into a NiO collection layer. Due to the complex electronic structure of the hybridized valence band in NiO, the chemical nature of the hole acceptor state has remained an open question, despite the fact that hole localization in this material significantly influences device efficiency. To comment on this, we present results of ultrafast charge carrier dynamics in a NiO based model heterojunction (Fe2O3/NiO) using extreme ultraviolet reflection-absorption (XUV-RA) spectroscopy. Element specific XUV-RA spectroscopy demonstrates the formation of transient Ni3+ within 10 ps following selective photoexcitation of the underlying Fe2O3 substrate. This indicates that hole transfer in this system occurs to NiO valence band states composed of significant Ni 3d character. Additionally, we show that this hole injection process proceeds via a two-step sequential mechanism where fast, field-driven exciton dissociation occurs in Fe2O3 in 680 ± 60 fs, followed by subsequent hole injection to NiO in 9.2 ± 2.9 ps. These results reveal the chemical nature of the hole acceptor state in widely used NiO hole transport layers and provides a direct observation of exciton dissociation and interfacial hole transfer in this model system.

All Lit Up: Exploring the Photophysical Properties of Protein Polymers.

Microtubules and actin filaments are protein polymers that play a variety of energy conversion roles in the biological cell. While these polymers are being increasingly harnessed for mechanochemical roles both inside and outside physiological conditions, their capabilities for photonic energy conversion are not well understood. In this Perspective, we first introduce the reader to the photophysical properties of protein polymers, examining light harvesting by their constituent aromatic residues. We then discuss both the opportunities and the challenges in interfacing protein biochemistry with photophysics. We also review the literature reporting the response of microtubules and actin filaments to infrared light, illustrating the potential of these polymers to these polymers serve as targets for photobiomodulation. Finally, we present broad challenges and questions in the field of protein biophotonics. Understanding how protein polymers interact with light will pioneer both biohybrid device fabrication and light-based therapeutics.

Direct Observation of Carbon Dioxide Electroreduction on Gold: Site Blocking by the Stern Layer Controls CO2 Adsorption Kinetics.

Directly observing active surface intermediates represents a major challenge in electrocatalysis, especially for CO2 electroreduction on Au. We use in-situ, plasmon-enhanced vibrational sum frequency generation spectroscopy, which has detection limits of <1% of a monolayer and can access the Au/electrolyte interface during active electrocatalysis in the absence of mass transport limitations. Measuring the potential-dependent surface coverage of atop CO confirms that the rate-determining step for this reaction is CO2 adsorption. An analysis of the interfacial electric field reveals the formation of a dense cation layer at the electrode surface, which is correlated to the onset of CO production. The Tafel slope increases in conjunction with the field saturation due to active site blocking by adsorbed cations. These findings show that CO2 reduction is extremely sensitive to the potential-dependent structure of the electrochemical double layer and provides direct observation of the interfacial processes that govern these kinetics.

Elucidating ultrafast electron dynamics at surfaces using extreme ultraviolet (XUV) reflection-absorption spectroscopy.

Here we review the recent development of extreme ultraviolet reflection-absorption (XUV-RA) spectroscopy. This method combines the benefits of X-ray absorption spectroscopy, such as element, oxidation, and spin state specificity, with surface sensitivity and ultrafast time resolution, having a probe depth of only a few nm and an instrument response less than 100 fs. Using this technique we investigated the ultrafast electron dynamics at a hematite (α-Fe2O3) surface. Surface electron trapping and small polaron formation both occur in 660 fs following photoexcitation. These kinetics are independent of surface morphology indicating that electron trapping is not mediated by defects. Instead, small polaron formation is proposed as the likely driving force for surface electron trapping. We also show that in Fe2O3, Co3O4, and NiO, band gap excitation promotes electron transfer from O 2p valence band states to metal 3d conduction band states. In addition to detecting the photoexcited electron at the metal M2,3-edge, the valence band hole is directly observed as transient signal at the O L1-edge. The size of the resulting charge transfer exciton is on the order of a single metal-oxygen bond length. Spectral shifts at the O L1-edge correlate with metal-oxygen bond covalency, confirming the relationship between valence band hybridization and the overpotential for water oxidation. These examples demonstrate the unique ability to measure ultrafast electron dynamics with element and chemical state resolution using XUV-RA spectroscopy. Accordingly, this method is poised to play an important role to reveal chemical details of previously unseen surface electron dynamics.

Ultrafast Electron Trapping and Defect-Mediated Recombination in NiO Probed by Femtosecond Extreme Ultraviolet Reflection-Absorption Spectroscopy.

Understanding the chemical nature of defect sites as well as the mechanism of defect-mediated recombination is critical for the rational design of energy conversion materials with improved efficiency. Using femtosecond extreme ultraviolet (XUV) spectroscopy in conjunction with X-ray photoelectron spectroscopy (XPS), we present results on the ultrafast electron dynamics in NiO prepared with varying concentrations of defect states. We find that oxygen vacancy defects do not serve as the primary recombination center, but rather the recombination rate scales linearly with the density of Ni metal defects. This suggests that grain boundaries between Ni metal and NiO are responsible for fast carrier recombination in partially reduced NiO. Our kinetic model shows that the photoexcited electrons self-trap via small polaron formation on the subpicosecond time scale. Additionally, we estimate an absolute measurement of small polaron formation rates, direct versus defect-mediated recombination rates, and the small polaron diffusion coefficient in NiO. This study provides important parameters for engineering NiO based materials for solar energy harvesting applications.

Surface electron dynamics in hematite (α-Fe2O3): correlation between ultrafast surface electron trapping and small polaron formation.

Spectroscopically following charge carrier dynamics in catalytic materials has proven to be a difficult task due to the ultrafast timescales involved in charge trapping and the lack of spectroscopic tools available to selectively probe surface electronic structure. Transient extreme ultraviolet reflection-absorption (XUV-RA) spectroscopy is able to follow surface electron dynamics due to its element, oxidation-state, and surface specificity, as well as the ultrafast time-resolution which can be achieved with XUV pulses produced by high harmonic generation. Here, we use ultrafast XUV-RA spectroscopy to show that charge localization and small polaron formation in Fe2O3 occur within ∼660 fs. The photoexcitation of hematite at 400 nm initially leads to an electronically-delocalized ligand-to-metal charge transfer (LMCT) state, which subsequently evolves into a surface localized LMCT state. Comparison of the charge carrier dynamics for single and polycrystalline samples shows that the observed dynamics are negligibly influenced by grain boundaries and surface defects. Rather, correlation between experimental results and spectral simulations reveals that the lattice expansion during small polaron formation occurs on the identical time scale as surface trapping and represents the probable driving force for sub-ps electron localization to the hematite surface.

Real Time Quantification of Ultrafast Photoinduced Bimolecular Electron Transfer Rate: Direct Probing of the Transient Intermediate.

Fluorescence quenching studies through steady-state and time-resolved measurements are inadequate to quantify the bimolecular electron transfer rate in bulk homogeneous solution due to constraints from diffusion. To nullify the effect of diffusion, direct evaluation of the rate of formation of a transient intermediate produced upon the electron transfer is essential. Methyl viologen, a well-known electron acceptor, produces a radical cation after accepting an electron, which has a characteristic strong and broad absorption band centered at 600 nm. Hence it is a good choice to evaluate the rate of photoinduced electron transfer reaction employing femtosecond broadband transient absorption spectroscopy. The time constant of the aforementioned process between pyrene and methyl viologen in methanol has been estimated to be 2.5 ± 0.4 ps using the same technique. The time constant for the backward reaction was found to be 14 ± 1 ps. These values did not change with variation of concentration of quencher, i.e., methyl viologen. Hence, we can infer that diffusion has no contribution in the estimation of rate constants. However, on changing the solvent from methanol to ethanol, the time constant of the electron transfer reaction has been found to increase and has accounted for the change in solvent reorganization energy.