Time-resolved X-ray Photoemission Research Articles

(Invited) Discovering Bottlenecks and Opportunities in Hybrid Solar Light Harvesting Systems By Ultrafast X-Ray Spectroscopy

A prerequisite for advancing hybrid solar light harvesting systems is a comprehensive understanding of photoinduced electronic dynamics across a wide range of spatial and temporal scales. Time-resolved spectroscopy techniques in the optical domain have proven instrumental in identifying critical timescales of fundamental processes, such as interfacial charge transfer, charge migration, and carrier lifetimes, which are intimately linked to a system's performance. New light source and instrument developments open opportunities to extend such studies into the X-ray regime. Due to their inherent elemental specificity and sensitivity to local electronic environments, time-domain X-ray spectroscopy techniques are uniquely suited to gain a microscopic picture of dynamics from well-defined reporter sites within often complex nanoscale light harvesting architectures. We will present new insights into fundamental photoinduced dynamics in several archetypical heterostructures based on femtosecond and picosecond time-resolved X-ray photoemission spectroscopy (TRXPS).The photoinduced transient charge redistribution in the model hybrid system of nanoporous zinc oxide (ZnO) sensitized by ruthenium bipyridyl chromophores is probed independently from the viewpoints of the molecular electron donor and the semiconductor acceptor.[1] Charge injection from the chromophore into the semiconductor is found to proceed via a transiently populated interfacial charge transfer (ICT) complex on an overall timescale of ~300 ps. Our results show that, even after release from the ICT states into the ZnO conduction band, the injected electrons remain localized within less than 6 nm from the interface, due to enhanced downward band-bending by the photo-injected charge carriers. This spatial confinement suggests that light-induced charge generation and transport in nanoscale ZnO photocatalytic devices proceeds predominantly within the defect-rich surface region, which may lead to enhanced surface recombination and explain their lower performance compared to titanium dioxide (TiO2)-based systems. The quantitative nature and surface sensitivity of TRXPS also provides direct access to the spatial separation of the holes remaining on the bipyridyl chromophores after charge injection from the semiconductor surface with sub-nm precision.Complementary femtosecond and picosecond TRXPS studies of photoinduced dynamics in copper-phthalocyanine(CuPc)-C60 heterojunctions provide a deeper understanding of both exciton migration pathways toward the interface as well as the absolute efficiency by which ICT states dissociate into separate charges.[2] The measurements reveal surprisingly efficient charge generation from triplet excitons and from the lowest lying ICT states, both of which were previously believed to be inactive during light-to-charge conversion.Photoinduced charge transfer dynamics between spherical gold nanoparticles (AuNPs) and a nanoporous film of TiO2 is monitored by picosecond TRXPS, providing direct access to the absolute photon-to-charge conversion efficiency and subsequent electron-hole recombination dynamics.[3] Preliminary results will be presented for the extension of the measurements into the ambient pressure regime, taking a first step toward the study of photoinduced interfacial chemistry by TRXPS in this model system for nanoplasmonic light harvesting applications.[1] S. Neppl et al., “Nanoscale Confinement of Photo-Injected Electrons at Hybrid Interfaces”, J. Phys. Chem. Lett. 12, 11951 (2021).[2] F. Roth et al., “Direct observation of charge separation in an organic light harvesting system by femtosecond time-resolved XPS”, Nat. Commun. 12, 1196 (2021).[3] M. Borgwardt et al., “Photoinduced Charge Carrier Dynamics and Electron Injection Efficiencies in Au Nanoparticle-Sensitized TiO2 Determined with Picosecond Time-Resolved X-ray Photoelectron Spectroscopy”, J. Phys. Chem. Lett. 11, 5476 (2020). Figure 1

Nanoscale Confinement of Photo-Injected Electrons at Hybrid Interfaces.

A prerequisite for advancing hybrid solar light harvesting systems is a comprehensive understanding of the spatiotemporal dynamics of photoinduced interfacial charge separation. Here, we demonstrate access to this transient charge redistribution for a model hybrid system of nanoporous zinc oxide (ZnO) and ruthenium bipyridyl chromophores. The site-selective probing of the molecular electron donor and semiconductor acceptor by time-resolved X-ray photoemission provides direct insight into the depth distribution of the photoinjected electrons and their interaction with the local band structure on a nanometer length scale. Our results show that these electrons remain localized within less than 6 nm from the interface, due to enhanced downward band bending by the photoinjected charge carriers. This spatial confinement suggests that light-induced charge generation and transport in nanoscale ZnO photocatalytic devices proceeds predominantly within the defect-rich surface region, which may lead to enhanced surface recombination and explain their lower performance compared to titanium dioxide (TiO2)-based systems.

Direct observation of charge separation in an organic light harvesting system by femtosecond time-resolved XPS

The ultrafast dynamics of photon-to-charge conversion in an organic light-harvesting system is studied by femtosecond time-resolved X-ray photoemission spectroscopy (TR-XPS) at the free-electron laser FLASH. This novel experimental technique provides site-specific information about charge separation and enables the monitoring of free charge carrier generation dynamics on their natural timescale, here applied to the model donor-acceptor system CuPc:C60. A previously unobserved channel for exciton dissociation into mobile charge carriers is identified, providing the first direct, real-time characterization of the timescale and efficiency of charge generation from low-energy charge-transfer states in an organic heterojunction. The findings give strong support to the emerging realization that charge separation even from energetically disfavored excitonic states is contributing significantly, indicating new options for light harvesting in organic heterojunctions.

Time-Resolved Photoemission to Unveil Electronic Coupling between Absorbing and Transport Layers in a Quantum Dot-Based Solar Cell

Lead sulfide (PbS) colloidal quantum dot-based photodiodes are remarkable structures because of their outstanding optoelectronic performances obtained via colloidal engineering. They combine surface ligand engineering to design a p–n junction with all-solution processability. Here, we investigate the electronic structure of PbS diode combining static and dynamic photoemissions with transport measurements. We show that the n-type nature of the I– capped PbS CQDs shifts the valence band away from the Fermi level compared to the thiol-capped nanocrystals. This change in majority carriers can be probed by time-resolved X-ray photoemission spectroscopy (TRXPS). We also prove that the photoinduced binding energy shift depends on the nanoparticle surface chemistry. Finally, we demonstrate the ability of TRXPS to selectively probe the electronic structure of each side of an interface. We explore the PbS/MoO3 interface used as a hole extractor in a PbS solar cell using this method. We demonstrate that the PbS layer photosensitizes the MoO3 layer and that the two layers have a quasi-rigid electrostatic coupling. We identify the band bending occurring on the PbS (EDT)/MoO3 to be a limiting factor for the device performance and suggest strategies to overcome this limitation.

Photoinduced Charge Carrier Dynamics and Electron Injection Efficiencies in Au Nanoparticle-Sensitized TiO2 Determined with Picosecond Time-Resolved X-ray Photoelectron Spectroscopy.

Progress in the development of plasmon-enabled light-harvesting technologies requires a better understanding of their fundamental operating principles and current limitations. Here, we employ picosecond time-resolved X-ray photoemission spectroscopy to investigate photoinduced electron transfer in a plasmonic model system composed of 20 nm sized gold nanoparticles (NPs) attached to a nanoporous film of TiO2. The measurement provides direct, quantitative access to transient local charge distributions from the perspectives of the electron donor (AuNP) and the electron acceptor (TiO2). On average, approximately two electrons are injected per NP, corresponding to an electron injection yield per absorbed photon of 0.1%. Back electron transfer from the perspective of the electron donor is dominated by a fast recombination channel proceeding on a time scale of 60 ± 10 ps and a minor contribution that is completed after ∼1 ns. The findings provide a detailed picture of photoinduced charge carrier generation in this NP-semiconductor junction, with important implications for understanding achievable overall photon-to-charge conversion efficiencies.

Efficient charge generation from triplet excitons in metal-organic heterojunctions

The success of many emerging molecular electronics concepts hinges on an atomistic understanding of the underlying electronic dynamics. We employ picosecond time-resolved x-ray photoemission spectroscopy (tr-XPS) to elucidate the roles of singlet and triplet excitons for photoinduced charge generation at a copper-phthalocyanine--${\mathrm{C}}_{60}$ heterojunction. Contrary to common belief, fast intersystem crossing to triplet excitons after photoexcitation is not a loss channel but contributes to a significantly larger extent to the time-integrated interfacial charge generation than the initially excited singlet excitons. The tr-XPS data provide direct access to the diffusivity of the triplet excitons ${D}_{\mathrm{CuPc}}=(1.8\ifmmode\pm\else\textpm\fi{}1.2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}/\mathrm{s}$ (where CuPc is copper-phthalocyanine) and their diffusion length ${L}_{\mathrm{diff}}=(8\ifmmode\pm\else\textpm\fi{}3)\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$.

Direct observation of Oersted-field-induced magnetization dynamics in magnetic nanostripes

We have used time-resolved x-ray photoemission electron microscopy to investigate the magnetization dynamics induced by nanosecond current pulses in NiFe/Cu/Co nanostripes. A large tilt of the NiFe magnetization in the direction transverse to the stripe is observed during the pulses. We show that this effect cannot be quantitatively understood from the amplitude of the Oersted field and the shape anisotropy. High-frequency oscillations observed at the onset of the pulses are attributed to precessional motion of the NiFe magnetization about the effective field. We discuss the possible origins of the large magnetization tilt and the potential implications of the static and dynamic effects of the Oersted field on current-induced domain-wall motion in such stripes.

Photoemission microscopy study of picosecond magnetodynamics in spin-valve-type thin film elements

Exploring ultimate time scales of magnetic switching processes is an important issue in spin electronics. In spin valves or magnetic tunnelling junctions magnetic anisotropies and coupling phenomena alter the magnetodynamic response of the entire system. Understanding the role of these interactions is a key to the design of optimized devices. We have employed time-resolved X-ray photoemission microscopy to address the magnetodynamics in spin-valve-type model systems in the ns- and ps-regime. In Co/Cr/Fe(0 0 1) single crystal elements we find a strong influence of the magnetocrystalline anisotropy, which tends to suppress rotation processes. In addition, we observe a dynamic “decoupling” of the layers. In low-anisotropy FeNi/Cr/FeCo trilayers, the interlayer coupling character determines the dynamic response. Particularly, rotational processes in the FeNi and FeCo layers are temporarily shifted to each other, which can be related to different coercivities of the individual layers. By contrast, the domain wall motion in both layers closely agrees, caused by an enhanced coupling due to the domain wall stray fields. Our examples demonstrate that the detailed magnetodynamics in coupled magnetic layers is quite complex and depends strongly on the timescale under consideration.

Polarity reversal of a magnetic vortex core by a unipolar, nonresonant in-plane pulsed magnetic field

We report the polarity reversal of a magnetic vortex core using a nonresonant in-plane pulsed magnetic field of arbitrary waveform studied using time-resolved x-ray photoemission electron microscopy and micromagnetic simulations. The imaging and simulations show that a 5 mT pulse, higher than the critical field for nonlinear effects, effectively leads to the randomization of the vortex core polarity. The micromagnetic simulations further show that the onset of stochastic core polarity randomization does not necessarily coincide with the critical reversal field, leading to a field window for predictable core reversal.

Magnetization dynamics in microscopic spin-valve elements: Shortcomings of the macrospin picture

We have studied ultrafast magnetodynamics in micropatterned spin-valve structures using time-resolved x-ray photoemission electron microscopy combined with x-ray magnetic circular dichroism. Exciting the system with ultrafast field pulses of $250\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$ width, we find the dynamic response of the free layer to fall into two distinctly different contributions. On the one hand, it exhibits localized spin wave modes that strongly depend on the shape of the micropattern. A field pulse applied perpendicular to the exchange bias field along the diagonal of a square pattern leads to the excitation of a standing spin wave mode with two nodes along the field direction. This mode is strongly suppressed for a pattern of elliptical shape. On the other hand, the integrated response of the free layer roughly follows a single-spin model with a damping constant of $\ensuremath{\alpha}=0.025$ independent of the shape and resembles the response of a critically damped forced oscillator.

Self-Trapping of Magnetic Oscillation Modes in Landau Flux-Closure Structures

We investigated the magnetodynamics in rectangular Permalloy platelets by means of time-resolved x-ray photoemission microscopy. 10 nm thick platelets of size 16 x 32 microm were excited by an oscillatory field along the short side of the sample with a fundamental frequency of 500 MHz and considerable contributions of higher harmonics. Under the influence of the oscillatory field, the Néel wall in the initial classical Landau pattern shifts away from the center, corresponding to an induced magnetic moment perpendicular to the exciting field. This phenomenon is explained by a self-trapping effect of the dominating spin-wave mode when the system is excited just below the resonance frequency. The basic driving mechanism is the maximization of entropy.

Time-resolved X-ray photoemission electron microscopy: imaging magnetodynamics on the 100 ps scale and below

We present recent results of time-resolved X-ray photoemission electron microscopy (TR-XPEEM) investigations on magnetic systems. Our studies of microstructured permalloy particles employ a magnetic pump XPEEM probe approach. The stroboscopic experiments feature a time resolution of Δ τ ≤ 130 ps and yield magnetic domain images with a surprising richness of details. We observe a strong influence of incoherent magnetization rotation processes, which lead to complicated transient domain structures with a blocked relaxation behavior.

Incoherent magnetization rotation observed in subnanosecond time-resolving x-ray photoemission electron microscopy

We present recent results of time-resolved x-ray photoemission electron microscopy on permalloy microstructures. The stroboscopic experiments feature a time-resolution of Δτ⩽130ps. We observe a strong influence of incoherent magnetization rotation processes, leading to a significant transient stray-field formation at the edges of the microstructure.

Investigating magnetization dynamics in permalloy microstructures using time-resolved x-ray photoemission electron microscope

We present results of a direct imaging approach to visualize the dynamics of magnetic domains on the nanosecond scale. The experiments are carried out by means of an x-ray photoemission electron microscope (X-PEEM) in a stroboscopic mode and exploit the intrinsic time structure of the synchrotron radiation delivered by the storage ring facility ESRF (Grenoble). In this way we combine the high lateral resolution of a PEEM with a subnanosecond time resolution.

XPS, NEXAFS and theoretical study of phenylacetylene adsorbed on Cu(100)

Abstract The adsorption dynamics and geometry of phenylacetylene (PA) on Cu(1 0 0) was investigated by means of X-ray photoemission (XPS), near edge X-ray absorption (NEXAFS) spectroscopies and by quantum chemical calculations. The clean metal surface was exposed to PA vapors and the C1s core level spectrum evolution was monitored by means of time-resolved XPS measurements. Photoemission (C1s) and absorption (C K-edge) spectra were recorded both in monolayer and multilayer regimes; polarization-dependent NEXAFS investigations were carried out on the PA monolayer at room temperature. The experimental results have been interpreted with the help of quantum chemical calculations: density functional theory (DFT) plane-wave based calculations for the optimization of the interaction geometry of PA on a two layered (1 0 0)-slab of Cu atoms using periodic boundary conditions and ab initio simulation of the XPS spectrum for a small cluster including PA in the optimized adsorbate geometry and the two Cu atoms more directly involved in the adsorption.

Real-time XPS investigation of the impact-energy dependence ofC2H4adsorption on Ag(100)

We investigate ethylene adsorption on clean and oxygen-covered Ag(100) dosing gas-phase molecules at selected kinetic energies ${E}_{i}$ by a supersonic molecular beam and by performing time-resolved x-ray photoemission measurements at the SuperESCA beamline of Elettra. In accord with a previous study [Chem. Phys. Lett. 331, 177 (2000)] we find that at low crystal temperatures T ethylene can access a variety of adsorption states. In particular, metastable physisorption is observed for all ${E}_{i}.$ Chemisorption into the \ensuremath{\pi}-bonded state occurs only for ethylene seeded in He ${(E}_{i}=0.36\mathrm{eV})$ and for $T<110\mathrm{K}$ for the clean surface, while it occurs readily when the surface is precovered with oxygen, the barrier to adsorption being removed. Apart from these species the data show the existence of a further, very weakly bound, state desorbing at $T=110\mathrm{K}$ which was not reported previously, to our knowledge.