Right On Time: Ultrafast Charge Separation Before Hybrid Exciton Formation.

Organic/inorganic hybrid systems offer great potential for novel solar cell design combining the tunability of organic chromophore absorption properties with high charge carrier mobilities of inorganic semiconductors. However, often such material combinations do not show the expected performance: while ZnO, for example, basically exhibits all necessary properties for a successful application in light-harvesting, it was clearly outpaced by TiO2 in terms of charge separation efficiency. The origin of this deficiency has long been debated. This study employs femtosecond time-resolved photoelectron spectroscopy and many-body ab initio calculations to identify and quantify all elementary steps leading to the suppression of charge separation at an exemplary organic/ZnO interface. It is demonstrated that charge separation indeed occurs efficiently on ultrafast (350 fs) timescales, but that electrons are recaptured at the interface on a 100 ps timescale and subsequently trapped in a strongly bound (0.7 eV) hybrid exciton state with a lifetime exceeding 5 µs. Thus, initially successful charge separation is followed by delayed electron capture at the interface, leading to apparently low charge separation efficiencies. This finding provides a sufficiently large time frame for counter-measures in device design to successfully implement specifically ZnO and, moreover, invites material scientists to revisit charge separation in various kinds of previously discarded hybrid systems.

Ultrashort and metastable doping of the ZnO surface by photoexcited defects.

Shallow donors in semiconductors are known to form impurity bands that induce metallic conduction at sufficient doping densities. The perhaps most direct analogy to such doping in optically excited semiconductors is the photoexcitation of deep electronic defects or dopant levels, creating defect excitons (DX) which may act like shallow donors. In this work, we use time- and angle-resolved photoelectron spectroscopy to observe and characterize DX at the surface of ZnO. The DX are created on a femtosecond timescale upon photoexcitation and have a spatial extent of few nanometers that is confined to the ZnO surface. The localized electronic levels lie at 150 meV below the Fermi energy, very similar to the shallow donor states induced by hydrogen doping [Deinert et al., Phys. Rev. B: Condens. Matter Mater. Phys., 2015, 91, 235313]. The transient dopants exhibit a multi-step decay ranging from hundreds of picoseconds to 77 µs and even longer. By enhancing the DX density, a Mott transition occurs, enabling the ultrafast metallization of the ZnO surface, which we have described previously [Gierster et al., Nat. Commun., 2021, 12, 978]. Depending on the defect density, the duration of the photoinduced metallization ranges from picoseconds to µs and longer, corresponding to the decay dynamics of the DX. The metastable lifetime of the DX is consistent with the observation of persistent photoconductivity (PPC) in ZnO reported in the literature [Madel et al., J. Appl. Phys., 2017, 121, 124301]. In agreement with the theory on PPC [Lany and Zunger, Phys. Rev. B: Condens. Matter Mater. Phys., 2005, 72, 035215], the deep defects are attributed to oxygen vacancies due to their energetic position in the band gap and their formation by surface photolysis upon UV illumination. We show that the photoexcitation of these defects is analogous to chemical doping and enables the transient control of material properties, such as the electrical conductivity, from ultrafast to metastable timescales. The same mechanism should be at play in other semiconductor compounds with deep defects.

Correction: Ultrafast evolution of the complex dielectric function of monolayer WS2 after photoexcitation.

Correction for 'Ultrafast evolution of the complex dielectric function of monolayer WS2 after photoexcitation' by Stefano Calati et al., Phys. Chem. Chem. Phys., 2021, 23, 22640-22646, DOI: 10.1039/d1cp03437e.

Ultrafast evolution of the complex dielectric function of monolayer WS2 after photoexcitation.

Transition metal dichalcogenides emerged as ideal materials for the investigation of exciton physics. Retrieving the excitonic signature in optical spectra, and tracking their time evolution upon photoexcitation requires appropriate analysis procedures, particularly when comparing different measurements, experimental techniques, samples, and substrates. In this work, we investigate the ultrafast time evolution of the exciton resonance of a monolayer of WS2 deposited on fused silica and Si/SiO2, and using two different measurement techniques: time-resolved reflectance and transmittance contrast. By modelling the dielectric function of the exciton with a Lorentz oscillator, using a Fresnell equations formalism, we derive analytical expressions of the exciton lineshape in both cases. The 2D linearized model introduced by Li et al. [Y. Li and T. F. Heinz, 2D Mater., 2018, 5, 025021] is used for the transmittance of the transparent substrate and a Fresnel transfer matrix method [O. Stenzel, The Physics of Thin Film Optical Spectra, Springer Series in Surface Science, 2016] is used to derive the reflectance in the case of the layered Si/SiO2 substrate. By fitting two models to the time-dependent optical spectra, we extract and quantify the time evolution of the parameter describing the excitonic resonance. We find a remarkable agreement between the extracted dynamics from both experiments despite the different side conditions, showing the equivalence and reliability of the two analysis methods in use. With this work, we pave the way to the resilient comparison of the exciton dynamics from different samples, measurements technique and substrates.

Type-I Energy Level Alignment at the PTCDA-Monolayer MoS2 Interface Promotes Resonance Energy Transfer and Luminescence Enhancement.

Van der Waals heterostructures consisting of 2D semiconductors and conjugated molecules are of increasing interest because of the prospect of a synergistic enhancement of (opto)electronic properties. In particular, perylenetetracarboxylic dianhydride (PTCDA) on monolayer (ML)-MoS2 has been identified as promising candidate and a staggered type-II energy level alignment and excited state interfacial charge transfer have been proposed. In contrast, it is here found with inverse and direct angle resolved photoelectron spectroscopy that PTCDA/ML-MoS2 supported by insulating sapphire exhibits a straddling type-I level alignment, with PTCDA having the wider energy gap. Photoluminescence (PL) and sub-picosecond transient absorption measurements reveal that resonance energy transfer, i.e., electron-hole pair (exciton) transfer, from PTCDA to ML-MoS2 occurs on a sub-picosecond time scale. This gives rise to an enhanced PL yield from ML-MoS2 in the heterostructure and an according overall modulation of the photoresponse. These results underpin the importance of a precise knowledge of the interfacial electronic structure in order to understand excited state dynamics and to devise reliable design strategies for optimized optoelectronic functionality in van der Waals heterostructures.

Photoexcited organic molecules en route to highly efficient autoionization.

The conversion of optical and electrical energies in novel materials is key to modern optoelectronic and light-harvesting applications. Here, we investigate the equilibration dynamics of photoexcited 2,7-bis(biphenyl-4-yl)-2',7'-ditertbutyl-9,9'-spirobifluorene (SP6) molecules adsorbed on ZnO(10-10) using femtosecond time-resolved two-photon photoelectron and optical spectroscopies. We find that, after initial ultrafast relaxation on femtosecond and picosecond time scales, an optically dark state is populated, likely the SP6 triplet (T) state, that undergoes Dexter-type energy transfer (rDex = 1.3 nm) and exhibits a long decay time of 0.1 s. Because of this long lifetime, a photostationary state with average T-T distances below 2 nm is established at excitation densities in the 1020 cm-2 s-1 range. This large density enables decay by T-T annihilation (TTA) mediating autoionization despite an extremely low TTA rate of kTTA = 4.5 ⋅ 10-26 m3 s-1. The large external quantum efficiency of the autoionization process (up to 15%) and photocurrent densities in the mA cm-2 range offer great potential for light-harvesting applications.

Impact of Electron Solvation on Ice Structures at the Molecular Scale.

Electron attachment and solvation at ice structures are well-known phenomena. The energy liberated in such events is commonly understood to cause temporary changes at such ice structures, but it may also trigger permanent modifications to a yet unknown extent. We determine the impact of electron solvation on D2O structures adsorbed on Cu(111) with low-temperature scanning tunneling microscopy, two-photon photoemission, and ab initio theory. Solvated electrons, generated by ultraviolet photons, lead not only to transient but also to permanent structural changes through the rearrangement of individual molecules. The persistent changes occur near sites with a high density of dangling OD groups that facilitate electron solvation. We conclude that energy dissipation during solvation triggers permanent molecular rearrangement via vibrational excitation.

Revealing the competing contributions of charge carriers, excitons, and defects to the non-equilibrium optical properties of ZnO.

Due to its wide band gap and high carrier mobility, ZnO is, among other transparent conductive oxides, an attractive material for light-harvesting and optoelectronic applications. Its functional efficiency, however, is strongly affected by defect-related in-gap states that open up extrinsic decay channels and modify relaxation timescales. As a consequence, almost every sample behaves differently, leading to irreproducible or even contradicting observations. Here, a complementary set of time-resolved spectroscopies is applied to two ZnO samples of different defect density to disentangle the competing contributions of charge carriers, excitons, and defects to the nonequilibrium dynamics after photoexcitation: time-resolved photoluminescence, excited state transmission, and electronic sum-frequency generation. Remarkably, defects affect the transient optical properties of ZnO across more than eight orders of magnitude in time, starting with photodepletion of normally occupied defect states on femtosecond timescales, followed by the competition of free exciton emission and exciton trapping at defect sites within picoseconds, photoluminescence of defect-bound and free excitons on nanosecond timescales, and deeply trapped holes with microsecond lifetimes. These findings not only provide the first comprehensive picture of charge and exciton relaxation pathways in ZnO but also uncover the microscopic origin of previous conflicting observations in this challenging material and thereby offer means of overcoming its difficulties. Noteworthy, a similar competition of intrinsic and defect-related dynamics could likely also be utilized in other oxides with marked defect density as, for instance, TiO2 or SrTiO3.

Multistep and multiscale electron transfer and localization dynamics at a model electrolyte/metal interface.

The lifetime, coupling, and localization dynamics of electronic states in molecular films near metal electrodes fundamentally determine their propensity to act as precursors or reactants in chemical reactions, crucial for a detailed understanding of charge transport and degradation mechanisms in batteries. In the current study, we investigate the formation dynamics of small polarons and their role as intermediate electronic states in thin films of dimethyl sulfoxide (DMSO) on Cu(111) using time- and angle-resolved two-photon photoemission spectroscopy. Upon photoexcitation, a delocalized DMSO electronic state is initially populated two monolayers from the Cu surface, becoming a small polaron on a 200 fs time scale, consistent with localization due to vibrational dynamics of the DMSO film. The small polaron is a precursor state for an extremely long-lived and weakly coupled multilayer electronic state, with a lifetime of several seconds, thirteen orders of magnitude longer than the small polaron. Although the small polaron in DMSO has a lifetime of 140 fs, its role as a precursor state for long-lived electronic states could make it an important intermediate in multistep battery reactivity.

Ultrafast Electronic Band Gap Control in an Excitonic Insulator.

We report on the nonequilibrium dynamics of the electronic structure of the layered semiconductor Ta_{2}NiSe_{5} investigated by time- and angle-resolved photoelectron spectroscopy. We show that below the critical excitation density of F_{C}=0.2 mJ cm^{-2}, the band gap narrows transiently, while it is enhanced above F_{C}. Hartree-Fock calculations reveal that this effect can be explained by the presence of the low-temperature excitonic insulator phase of Ta_{2}NiSe_{5}, whose order parameter is connected to the gap size. This work demonstrates the ability to manipulate the band gap of Ta_{2}NiSe_{5} with light on the femtosecond time scale.

Real-time measurement of the vertical binding energy during the birth of a solvated electron.

Using femtosecond time-resolved two-photon photoelectron spectroscopy, we determine (i) the vertical binding energy (VBE = 0.8 eV) of electrons in the conduction band in supported amorphous solid water (ASW) layers, (ii) the time scale of ultrafast trapping at pre-existing sites (22 fs), and (iii) the initial VBE (1.4 eV) of solvated electrons before significant molecular reorganization sets in. Our results suggest that the excess electron dynamics prior to solvation are representative for bulk ASW.

Instantaneous band gap collapse in photoexcited monoclinic VO2 due to photocarrier doping.

Using femtosecond time-resolved photoelectron spectroscopy we demonstrate that photoexcitation transforms monoclinic VO2 quasi-instantaneously into a metal. Thereby, we exclude an 80 fs structural bottleneck for the photoinduced electronic phase transition of VO2. First-principles many-body perturbation theory calculations reveal a high sensitivity of the VO2 band gap to variations of the dynamically screened Coulomb interaction, supporting a fully electronically driven isostructural insulator-to-metal transition. We thus conclude that the ultrafast band structure renormalization is caused by photoexcitation of carriers from localized V 3d valence states, strongly changing the screening before significant hot-carrier relaxation or ionic motion has occurred.

Large work function reduction by adsorption of a molecule with a negative electron affinity: pyridine on ZnO(1010).

Using thermal desorption and photoelectron spectroscopy to study the adsorption of pyridine on ZnO(1010), we find that the work function is significantly reduced from 4.5 eV for the bare ZnO surface to 1.6 eV for one monolayer of adsorbed pyridine. Further insight into the interface morphology and binding mechanism is obtained using density functional theory. Although semilocal density functional theory provides unsatisfactory total work functions, excellent agreement of the work function changes is achieved for all coverages. In a closed monolayer, pyridine is found to bind to every second surface Zn atom. The strong polarity of the Zn-pyridine bond and the molecular dipole moment act cooperatively, leading to the observed strong work function reduction. Based on simple alignment considerations, we illustrate that even larger work function modifications should be achievable using molecules with negative electron affinity. We expect the application of such molecules to significantly reduce the electron injection barriers at ZnO/organic heterostructures.

Dynamics and reactivity of trapped electrons on supported ice crystallites.

The solvation dynamics and reactivity of localized excess electrons in aqueous environments have attracted great attention in many areas of physics, chemistry, and biology. This manifold attraction results from the importance of water as a solvent in nature as well as from the key role of low-energy electrons in many chemical reactions. One prominent example is the electron-induced dissociation of chlorofluorocarbons (CFCs). Low-energy electrons are also critical in the radiation chemistry that occurs in nuclear reactors. Excess electrons in an aqueous environment are localized and stabilized by the local rearrangement of the surrounding water dipoles. Such solvated or hydrated electrons are known to play an important role in systems such as biochemical reactions and atmospheric chemistry. Despite numerous studies over many years, little is known about the microscopic details of these electron-induced chemical processes, and interest in the fundamental processes involved in the reactivity of trapped electrons continues. In this Account, we present a surface science study of the dynamics and reactivity of such localized low-energy electrons at D(2)O crystallites that are supported by a Ru(001) single crystal metal surface. This approach enables us to investigate the generation and relaxation dynamics as well as dissociative electron attachment (DEA) reaction of excess electrons under well-defined conditions. They are generated by photoexcitation in the metal template and transferred to trapping sites at the vacuum interface of crystalline D(2)O islands. In these traps, the electrons are effectively decoupled from the electronic states of the metal template, leading to extraordinarily long excited state lifetimes on the order of minutes. Using these long-lived, low-energy electrons, we study the DEA to CFCl(3) that is coadsorbed at very low concentrations (∼10(12) cm(-2)). Using rate equations and direct measurement of the change of surface dipole moment, we estimated the electron surface density for DEA, yielding cross sections that are orders of magnitude higher than the electron density measured in the gas phase.

Reactivity of water-electron complexes on crystalline ice surfaces.

The interactions between long-living electrons trapped in defects of crystalline D2O and electronegative molecules have been investigated using two-photon photoemission spectroscopy. When covered by a Xe adlayer, the spectroscopic signature of the trapped electrons vanishes, which provides evidence that the trapping sites are located on the surface of the crystalline ice. The reactive character of these surface-trapped electrons with molecules has been studied. In the case of CFCl3 adsorbed on top of the ice, we show that the trapped electrons induce the dissociation of the molecules, via a dissociative electron attachment process, resulting in *CFCl2 and Cl(-) formation. The latter species are responsible for the observed increase of the work function and presumably for the deactivation of the surface trapping sites with respect to subsequent light-induced population by excited electrons. This process is thought to be of high efficiency since it is observed for a very low CFCl3 coverage of only approximately 0.004 monolayer (ML). In the case of exposure of the crystalline ice to a partial pressure of gaseous O2, the deactivation of the trapping site has also been observed. The mechanism is thought to involve the formation of the O2*(-) transient anion by electron attachment, followed by its reactive interaction with the water molecules of the defect. In both cases, the mechanisms are triggered by negative ion resonances which are known from experiments using a primary electron beam to be effective for isolated molecules for ballistic electrons of approximately 0 eV. We thereby demonstrate a similarity between the processes induced by these primary, very low kinetic-energy electrons and by the long-living surface electrons on the crystalline ice surface. These results suggest that the photoexcited trapped electrons can play an important role in the heterogeneous chemical processes on condensed water surfaces and could be relevant in the polar stratosphere chemistry.

A surface science approach to ultrafast electron transfer and solvation dynamics at interfaces.

Excess electrons in polar media, such as water or ice, are screened by reorientation of the surrounding molecular dipoles. This process of electron solvation is of vital importance for various fields of physical chemistry and biology as, for instance, in electrochemistry or photosynthesis. Generation of such excess electrons in bulk water involves either photoionization of solvent molecules or doping with e.g. alkali atoms, involving possibly perturbing interactions of the system with the parent-cation. Such effects are avoided when using a surface science approach to electron solvation: in the case of polar adsorbate layers on metal surfaces, the substrate acts as an electron source from where photoexcited carriers are injected into the adlayer. Besides the investigation of electron solvation at such interfaces, this approach allows for the investigation of heterogeneous electron transfer, as the excited solvated electron population continuously decays back to the metal substrate. In this manner, electron transfer and solvation processes are intimately connected at any polar adsorbate-metal interface. In this tutorial review, we discuss recent experiments on the ultrafast dynamics of photoinduced electron transfer and solvation processes at amorphous ice-metal interfaces. Femtosecond time-resolved two-photon photoelectron spectroscopy is employed as a direct probe of the electron dynamics, which enables the analysis of all elementary processes: the charge injection across the interface, the subsequent electron localization and solvation, and the dynamics of electron transfer back to the substrate. Using surface science techniques to grow and characterize various well-defined ice structures, we gain detailed insight into the correlation between adsorbate structure and electron solvation dynamics, the location (bulk versus surface) of the solvation site, and the role of the electronic structure of the underlying metal substrate on the electron transfer rate.

Determination of the electron's solvation site on D2O/Cu(111) using Xe overlayers and femtosecond photoelectron spectroscopy.

We investigate the binding site of solvated electrons in amorphous D(2)O clusters and D(2)O wetting layers adsorbed on Cu(111) by means of two-photon photoelectron (2PPE) spectroscopy. On the basis of different interactions of bulk- or surface-bound solvated electrons with rare gas atoms, titration experiments using Xe overlayers reveal the location of the electron solvation sites. In the case of flat clusters with a height of 2-4 bilayers adsorbed on Cu(111), solvated electrons are found to reside at the ice-vacuum interface, whereas a bulk character is found for solvated electrons in wetting layers. Furthermore, time-resolved experiments are performed to determine the origin of the transition between these different solvation sites with increasing D(2)O coverage. We employ an empirical model calculation to analyse the rate of electron transfer back to the substrate and the energetic stabilization of the solvated electrons, which allows further insight into the binding site for clusters. We find that the solvated electrons reside at the edges of the clusters. Therefore, we attribute the transition from surface- to bulk-solvation to the coalescence of the clusters to a closed ice film occurring at a nominal coverage of 2-3 BL, while the distance of the binding sites to the metal-ice interface is maintained.

Ultrafast electron transfer dynamics at NH3/Cu(111) interfaces: determination of the transient tunneling barrier.

Electron transfer (ET) dynamics at molecule-metal interfaces plays a key role in various fields as surface photochemistry or the development of molecular electronic devices. The bare transfer process is often described in terms of tunneling through an interfacial barrier that depends on the distance of the excited electron to the metal substrate. However, a quantitative characterization of such potential barriers is still lacking. In the present time-resolved two-photon photoemission (2PPE) study of amorphous NH 3 layers on Cu(111) we show that photoinjection of electrons is followed by charge solvation leading to the formation of a transient potential barrier at the interface that determines the ET to the substrate. We demonstrate that the electrons are localized at the ammonia-vacuum interface and that the ET rate depends exponentially on the NH 3 layer thickness with inverse range parameters beta between 1.8 and 2.7 nm (-1). Systematic analysis of this time-resolved and layer thickness-dependent data finally enables the determination of the temporal evolution of the interfacial potential barrier using a simple model description. We find that the tunneling barrier forms after tau E = 180 fs and subsequently rises more than three times faster than the binding energy gain of the solvated electrons.

Ultrafast electron dynamics at ice-metal interfaces: competition between heterogeneous electron transfer and solvation.

Microscopic insight into heterogeneous electron transfer requires an understanding of the participating donor and acceptor states and of their respective interaction. In the regime of strong electronic coupling, two limits have been discussed where either the states overlap directly or the states are separated by a potential barrier. In both situations, the transfer probability is determined by the magnitude of the wave function overlap, whereby in the case of the potential barrier, its width and height are rate limiting. In our study, we observe a dynamical crossover between these two regimes by investigating the electron-transfer dynamics of localized, solvated electrons at ice-metal interfaces. Employing femtosecond time-resolved two-photon photoelectron spectroscopy, we analyze the population dynamics of excess electrons in the ice layer, which experience the competing processes of transfer to the metal electrode and energetic stabilization in the ice by molecular reorientation. Comparing the dynamics of D(2)O on Cu(111) and Ru(001), we observe an early regime at t < 300 fs, where the transfer time is determined by wave-function overlap with the metal and a second regime (t > 300 fs), where the transfer proceeds nearly independent of the substrate. The assignment of these two regimes to the established mechanisms of electron transfer is backed by an empirical model calculation that reproduces the experimental data in an excellent manner.