Time-dependent Photoemission Research Articles

Photocatalyzed oxidation of water on oxygen pretreated rutile TiO2(110)

The oxygen evolution reaction (OER) is the bottleneck in the overall photocatalytic splitting of water. The active sites (terminal titanium or bridging oxygen) and active species (molecular or dissociative water) of the initial step of the photocatalyzed OER on the prototypical photocatalyst TiO2, remain debatable. Herein, the photocatalytic chemistry of monolayer water on oxygen-pretreated TiO2(110) (o-TiO2(110)) and reduced TiO2(110) (r-TiO2(110)) surfaces initiated by 400 nm light illumination was investigated by time-dependent two-photon photoemission spectroscopy (TD-2PPE). The photoinduced reduction of the H2O/o-TiO2(110) interface rather than the H2O/r-TiO2(110) interface was detected by TD-2PPE. The difference in 2PPE originated from the presence of the terminal hydroxyl anions (OHt¯) on H2O/o-TiO2(110), as identified by X-ray photoelectron spectroscopy and temperature-programmed desorption. Therefore, the evolution of the electronic structure of H2O/o-TiO2(110) was attributed to the photocatalyzed oxidation of the terminal hydroxyl anions, which most likely formed gaseous •OH radicals, reducing the interface. This work suggested that the oxidation of hydroxyl anions on top of the terminal titanium ions on TiO2, which were excluded previously in solution, need to be considered in the mechanistic studies of the photocatalyzed OER.

Photoinduced pairing in Mott insulators

Utilizing time-evolution techniques in (infinite) matrix-product-state representation, we study the non-equilibrium dynamics of driven Mott insulators and demonstrate photoinduced \etaη pairing directly in the thermodynamic limit. Analyzing the time evolution of the corresponding pairing correlations, we determine the optimal laser pump parameters for which long-range \etaη-pairing becomes dominant after pulse irradiation. The time-dependent photoemission spectra for this optimal pump parameter set show clear signatures of the photoinduced insulator-to-metal phase transition related to the formation of \etaη pairs.

Photoinduced metallization of excitonic insulators

Utilizing the time-dependent density-matrix renormalization group technique, we numerically prove photoinduced pairing states in the extended Falicov-Kimball model (EFKM) at half filling, both with and without internal SU(2) symmetry. In the time-dependent photoemission spectra an extra band appears above the Fermi energy after pulse irradiation, indicating an insulator-to-metal transition. Even in the absence of the SU(2) structure, the pair correlations are enhanced during the pump, and afterwards decrease over time. This implies the possible metallization of Ta$_2$NiSe$_5$, a strong candidate for an excitonic insulator material, for which the EFKM is considered to be the minimal theoretical model. Simulating the photoemission with optimized pulse parameters, we demonstrate a photoinduced quantum phase transition, in accord with recent findings in time- and angle-resolved photoemission spectroscopy experiments on Ta$_2$NiSe$_5$.

Nonequilibrium dynamics in pumped Mott insulators

We use time-evolution techniques for (infinite) matrix-product-states to calculate, directly in the thermodynamic limit, the time-dependent photoemission spectra and dynamic structure factors of the half-filled Hubbard chain after pulse irradiation. These quantities exhibit clear signatures of the photoinduced phase transition from insulator to metal that occurs because of the formation of so-called $\eta$ pairs. In addition, the spin dynamic structure factor loses spectral weight in the whole momentum space, reflecting the suppression of antiferromagnetic correlations due to the buildup of $\eta$-pairing states. The numerical method demonstrated in this work can be readily applied to other one-dimensional models driven out of equilibrium by optical pumping.

Time-dependent photoemission from droplets: influence of size and charge on the photophysics near the surface.

Photoemission from submicrometer droplets containing a mixture of dioctyl phthalate and dioctyl sebacate was investigated by femtosecond and nanosecond photoionization. Photoelectron spectra recorded after ionization with single 4.7 eV femtosecond or nanosecond laser pulses showed marked differences between the two cases. These differences were attributed to ionization of long-lived states which only occurred within the duration of the nanosecond pulse. The tentative assignment of the long-lived states to dioctyl phthalate triplet states is discussed. A nanosecond–femtosecond pump–probe scheme using 4.7 eV (pump) and 3.1 eV (probe) pulses was used to investigate the decay dynamics of these long-lived states. The dynamics showed an accelerated decay rate at higher dioctyl phthalate concentrations. Furthermore, the dependence of the decay dynamics on droplet size and charge was investigated. The decay of the long-lived states was found to be faster in smaller droplets as well as in neutral droplets compared with both positively and negatively charged droplets. Possible mechanisms to explain these observations and the dominance of contributions from the droplets surface are discussed.

Visualizing spinon Fermi surfaces with time-dependent spectroscopy

Quantum simulation experiments have started to explore regimes that are not accessible with exact numerical methods. To probe these systems and enable new physical insights, the need for measurement protocols arises that can bridge the gap to solid-state experiments, and at the same time make optimal use of the capabilities of quantum simulation experiments. Here we propose applying time-dependent photoemission spectroscopy, an established tool in solid-state systems, in cold atom quantum simulators. Concretely, we suggest combining the method with large magnetic field gradients, unattainable in experiments on real materials, to drive Bloch oscillations of spinons, the emergent quasiparticles of spin liquids. We show in exact diagonalization simulations of the one-dimensional $t\text{\ensuremath{-}}J$ model with a single hole that the spinons start to populate previously unoccupied states in an effective band structure, thus allowing us to visualize states invisible in the equilibrium spectrum. The dependence of the spectral function on the time after the pump pulse reveals collective interactions among spinons. In numerical simulations of small two-dimensional systems, spectral weight appears at the ground-state energy at momentum $\mathbf{q}=(\ensuremath{\pi},\ensuremath{\pi})$, where the equilibrium spectral response is strongly suppressed up to higher energies, indicating a possible route toward solving the mystery of the Fermi arcs in the cuprate materials.

Space–charge limited current in nanodiodes: Ballistic, collisional, and dynamical effects

This Perspective reviews the fundamental physics of space–charge interactions that are important in various media: vacuum gap, air gap, liquids, and solids including quantum materials. It outlines the critical and recent developments since a previous review paper on diode physics [Zhang et al. Appl. Phys. Rev. 4, 011304 (2017)] with particular emphasis on various theoretical aspects of the space–charge limited current (SCLC) model: physics at the nano-scale, time-dependent, and transient behaviors; higher-dimensional models; and transitions between electron emission mechanisms and material properties. While many studies focus on steady-state SCLC, the increasing importance of fast-rise time electric pulses, high frequency microwave and terahertz sources, and ultrafast lasers has motivated theoretical investigations in time-dependent SCLC. We particularly focus on recent studies in discrete particle effects, temporal phenomena, time-dependent photoemission to SCLC, and AC beam loading. Due to the reduction in the physical size and complicated geometries, we report recent studies in multi-dimensional SCLC, including finite particle effects, protrusive SCLC, novel techniques for exotic geometries, and fractional models. Due to the importance of using SCLC models in determining the mobility of organic materials, this paper shows the transition of the SCLC model between classical bulk solids and recent two-dimensional (2D) Dirac materials. Next, we describe some selected applications of SCLC in nanodiodes, including nanoscale vacuum-channel transistors, microplasma transistors, thermionic energy converters, and multipactor. Finally, we conclude by highlighting future directions in theoretical modeling and applications of SCLC.

Role of matrix elements in the time-resolved photoemission signal

Time- and angle-resolved photoemission spectroscopy (TR-ARPES) provides access to the ultrafast evolution of electrons and many-body interactions in solid-state systems. However, the momentum- and energy-resolved transient photoemission intensity may not be unambiguously described by the intrinsic relaxation dynamics of photoexcited electrons alone. The interpretation of the time-dependent photoemission signal can be affected by the transient evolution of the electronic distribution, and both the one-electron removal spectral function as well as the photoemission matrix elements. Here we investigate the topological insulator Bi1.1Sb0.9Te2S to demonstrate, by means of a detailed probe-polarization dependent study, the transient contribution of matrix elements to TR-ARPES.

Doping dependence and electron–boson coupling in the ultrafast relaxation of hot electron populations in Ba(Fe1–xCox)2As2

Using femtosecond time- and angle-resolved photoemission spectroscopy we investigate the effect of electron doping on the electron dynamics in in a range of at temperatures slightly above the Néel temperature. By analyzing the time-dependent photoemission intensity of the pump laser excited population as a function of energy, we found that the relaxation times at are doping dependent and about 100 fs shorter at optimal doping than for overdoped and parent compounds. Analysis of the relaxation rates also reveals the presence of a pump fluence dependent step in the relaxation time at which we explain by coupling of the excited electronic system to a boson of this energy. We compare our results with static ARPES and transport measurements and find disagreement and agreement concerning the doping-dependence, respectively. We discuss the effect of the electron–boson coupling on the energy-dependent relaxation and assign the origin of the boson to a magnetic excitation.

Determining the chirality of Weyl fermions from circular dichroism spectra in time-dependent angle-resolved photoemission

We show that the intensity of pumped states near Weyl point is different when pumped with left- and right-handed circular-polarized light, which leads to a special circular dichroism (CD) in time-dependent angle-resolved photoemission spectra (ARPES). We derive the expression for the CD of time-dependent ARPES, which is directly related to the chirality of Weyl fermions. Based on the above derivation, we further propose a method to determine the chirality for a given Weyl point from the CD of time-dependent ARPES. The corresponding CD spectra for TaAs has then been calculated from first principles, which can be compared with future experiments.

Direct observation of Higgs mode oscillations in the pump-probe photoemission spectra of electron-phonon mediated superconductors

Using the non-equilibrium Keldysh formalism, we solve the equations of motion for electron-phonon superconductivity, including an ultrafast pump field. We present results for time-dependent photoemission spectra out of equilibrium which probes the dynamics of the superconducting gap edge. The partial melting of the order by the pump field leads to oscillations at twice the melted gap frequency, a hallmark of the Higgs or amplitude mode. Thus the Higgs mode can be directly excited through the nonlinear effects of an electromagnetic field and detected without any additional symmetry breaking.

Light-Matter Interaction at Surfaces in the Spatiotemporal Limit of Macroscopic Models.

What is the spatiotemporal limit of a macroscopic model that describes the optoelectronic interaction at the interface between different media? This fundamental question has become relevant for time-dependent photoemission from solid surfaces using probes that resolve attosecond electron dynamics on an atomic length scale. We address this fundamental question by investigating how ultrafast electron screening affects the infrared field distribution for a noble metal such as Cu(111) at the solid-vacuum interface. Attosecond photoemission delay measurements performed at different angles of incidence of the light allow us to study the detailed spatiotemporal dependence of the electromagnetic field distribution. Surprisingly, comparison with Monte Carlo semiclassical calculations reveals that the macroscopic Fresnel equations still properly describe the observed phase of the IR field on the Cu(111) surface on an atomic length and an attosecond time scale.

Kinetics and Dynamics of Photocatalyzed Dissociation of Ethanol on TiO2(110)

The kinetics and dynamics of photocatalyzed dissociation of ethanol on TiO2(110) surface have been studied using the time-dependent and time-resolved femtosecond two-photon photoemission spectroscopy respectively, in order to unravel the photochemical properties of ethanol on this prototypical metal oxide surface. By monitoring the time evolution of the photoinduced excited state which is associated with the photocatalyzed dissociation of ethanol on Ti5c sites of TiO2(110), the fractal-like kinetics of this surface photocatalytic reaction has been obtained. The measured photocatalytic dissociation rate on reduced TiO2(110) is faster than that on the oxidized surface. This is attributed to the larger defect density on the reduced surface which lowers the reaction barrier of the photocatalytic reaction at least methodologically. Possible reasons associated with the defect electrons for the acceleration have been discussed. By performing the interferometric two-pulse correlation on ethanol/TiO2(110) interface, the ultrafast electron dynamics of the excited state has been measured. The analyzed lifetime (24 fs) of the excited state is similar to that on methanol/TiO2(110). The appearance of the excited state provides a channel to mediate the electron transfer between the TiO2 substrate and its environment. Therefore studying its ultrafast electron dynamics may lead to the understanding of the microscopic mechanism of photocatalysis and photoelectrochemical energy conversion on TiO2.

Nitrogen-Doped Graphene: Efficient Growth, Structure, and Electronic Properties

A novel strategy for efficient growth of nitrogen-doped graphene (N-graphene) on a large scale from s-triazine molecules is presented. The growth process has been unveiled in situ using time-dependent photoemission. It has been established that a postannealing of N-graphene after gold intercalation causes a conversion of the N environment from pyridinic to graphitic, allowing to obtain more than 80% of all embedded nitrogen in graphitic form, which is essential for the electron doping in graphene. A band gap, a doping level of 300 meV, and a charge-carrier concentration of ∼8×10(12) electrons per cm2, induced by 0.4 atom % of graphitic nitrogen, have been detected by angle-resolved photoemission spectroscopy, which offers great promise for implementation of this system in next generation electronic devices.

Effect of defects on photocatalytic dissociation of methanol on TiO2(110)

Photocatalytic dissociation of deuterated methanol (CD3OD) on both stoichiometric and reduced TiO2(110) surfaces was investigated using the time-dependent two-photon photoemission (2PPE) method, in order to understand the effect of defects on the kinetics of methanol dissociation on TiO2(110). By monitoring the time evolution of the photoinduced excited state on the methanol covered surface, the photocatalytic dissociation kinetics of methanol on the TiO2 surface were observed. The measured photodissociation rate on the reduced TiO2(110) surface is more than an order of magnitude faster than that on the stoichiometric surface. Since the reduced TiO2(110) surface has considerably more surface and subsurface defects than the stoichiometric surface, the experimental observation suggests that one or both of them could accelerate the photocatalysis process of methanol on the TiO2(110) surface in a significant way.

Photoinduced charge-trapping phenomena in metal/high-k gate stack structures studied by synchrotron radiation photoemission spectroscopy

We have demonstrated photoinduced charge-trapping phenomena in metal/high-k gate stack structures using time-dependent photoemission spectroscopy with synchrotron radiation. Pt metal gate electrode with a large work function releases trapped negative charges near the surface of the HfSiON film while TiN metal gate electrode with a lower work function keeps negative charges in the HfSiON film. The release of negative trapped charges reveals a possibility of positive charge trapping at the interface in the HfSiON film. The location of energy level for negative charges is concluded to be between Pt and TiN Fermi-level in the band gap of the HfSiON film.

Dynamics of graphene growth on a metal surface: a time-dependent photoemission study

Applying time-dependent photoemission we unravel the graphene growth process on a metallic surface by chemical vapor deposition (CVD). Graphene CVD growth is in stark contrast to the standard growth process of two-dimensional films because it is self-limiting and stops as soon as a monolayer of graphene has been synthesized. Most importantly, a novel phase of metastable graphene was discovered that is characterized by permanent and simultaneous construction and deconstruction. The high quality and large area graphene flakes are characterized by angle-resolved photoemission, proving that they are indeed monolayer and cover the whole 1×1 cm Ni(111) substrate. These findings are of high relevance to the intensive search for reliable synthesis methods for large graphene flakes of controlled layer number.

Effects of thermal annealing on charge density and N chemical states in HfSiON films

We have investigated the charge density and N chemical states in HfSiON films annealed at various oxygen gas pressures by time-dependent photoemission spectroscopy. The Si 2p core-level spectra and the sample current reveal that annealing these films at low oxygen partial pressures affects the number of inherent fixed charges and annealing at high oxygen partial pressures results in a decrease in the number of trapped charges. The N 1s spectra of the annealed HfSiON samples indicate a decrease in Hf–N bonds due to the substitution of N by O along with a decrease in the number of trapped charges.

Measuring correlated electron dynamics with time-resolved photoemission spectroscopy

Time-resolved photoemission experiments can reveal fascinating quantum dynamics of correlated electrons. However, the thermalization of the electronic system is typically so fast that very short probe pulses are necessary to resolve the time evolution of the quantum state, and this leads to poor energy resolution due to the energy-time uncertainty relation. Although the photoemission intensity can be calculated from the nonequilibrium electronic Green's functions, the converse procedure is therefore difficult. We analyze a hypothetical time-resolved photoemission experiment on a correlated electronic system, described by the Falicov-Kimball model in dynamical mean-field theory, which relaxes between metallic and insulating phases. We find that the real-time Green's function which describes the transient behavior during the buildup of the metallic state cannot be determined directly from the photoemission signal. On the other hand, the characteristic collapse-and-revival oscillations of an excited Mott insulator can be observed as oscillating weight in the center of the Mott gap in the time-dependent photoemission spectrum.

Analysis of x‐ray irradiation effect in high‐k gate dielectrics by time‐dependent photoemission spectroscopy using synchrotron radiation

AbstractWe have developed a method to precisely determine the band offsets for hafnium oxide (HfO2), hafnium silicate (HfSiO) and nitrided hafnium silicate (HfSiON) films on Si by elimination of x‐ray irradiation effects with time‐dependent photoemission spectroscopy (PES) and x‐ray absorption spectroscopy. The core‐level PES spectra shift during PES measurements, which is explained as the dielectric films charging due to photoemission by the x‐ray irradiation. For elimination of the x‐ray irradiation effect on the determination of the band offsets, time‐dependent photoemission spectroscopy was performed. The precisely determined valence band offsets for HfO2, HfSiO and HfSiON are 3.2, 3.4, and 1.9 eV, respectively. On the other hand, the conduction band offsets are almost the same values of about 1.3 eV. Thus, the elimination of x‐ray irradiation effect enables to precisely determine the band offsets, leading to the accurate estimation of gate leakage current. Copyright © 2008 John Wiley & Sons, Ltd.