Femtosecond Time-resolved Two-photon Photoemission Research Articles

Femtosecond time-resolved two-photon photoemission studies of ultrafast carrier relaxation in Cu2O photoelectrodes

Cuprous oxide (Cu2O) is a promising material for solar-driven water splitting to produce hydrogen. However, the relatively small accessible photovoltage limits the development of efficient Cu2O based photocathodes. Here, femtosecond time-resolved two-photon photoemission spectroscopy has been used to probe the electronic structure and dynamics of photoexcited charge carriers at the Cu2O surface as well as the interface between Cu2O and a platinum (Pt) adlayer. By referencing ultrafast energy-resolved surface sensitive spectroscopy to bulk data we identify the full bulk to surface transport dynamics for excited electrons rapidly localized within an intrinsic deep continuous defect band ranging from the whole crystal volume to the surface. No evidence of bulk electrons reaching the surface at the conduction band level is found resulting into a substantial loss of their energy through ultrafast trapping. Our results uncover main factors limiting the energy conversion processes in Cu2O and provide guidance for future material development.

Competition Between Hot-Electron Cooling and Large Polaron Screening in CsPbBr3 Perovskite Single Crystals

Lead halide perovskites (LHPs) are solution processable semiconductors characterized by long carrier lifetimes. Recent studies have suggested that electrons and holes in LHPs interact with phonons to form large polarons on subpicosecond time-scales and polaron formation may also slow down hot carrier cooling. Using femtosecond time-resolved two-photon photoemission (TR-2PPE) and transient reflectance (TR) spectroscopies, we follow the initial electron cooling and polaron formation dynamics in single-crystal CsPbBr3 perovskite. We find that the hot electrons cool down initially (≤0.2 ps) with rates of −0.64 ± 0.06 eV/ps and −0.82 ± 0.08 eV/ps at 300 and 80 K, respectively. This weakly temperature-dependent rate is attributed to the initial relaxation of unscreened hot electrons by the emission of longitudinal optical (LO) phonons. On longer time scales, we observe dynamic changes in the photoemission cross-section and in the red-shift of the optical bandgap. We attribute these dynamic changes to large pola...

Adsorbate-mediated relaxation dynamics of hot electrons at metal/organic interfaces

Hot-electron dynamics at metal surfaces are crucial for charge transport properties and chemical reactions at metal/organic interfaces. In order to study the influence of electron-donating adsorbates on hot-electron lifetimes we performed femtosecond time-resolved two-photon photoemission measurements of tetrathiafulvalene (TTF)-covered Au(111) surfaces. The electron-donating nature of TTF provides density of states near the Fermi energy (${E}_{F}$) increasing the possibility for electron-electron scattering events. This leads to an additional fast electron-electron scattering assisted hot-electron relaxation channel for low intermediate state energies (${E}_{i}\ensuremath{-}{E}_{F}$) at the TTF/Au(111) interface with hot-electron lifetimes up to $340\phantom{\rule{0.28em}{0ex}}\mathrm{fs}$. However, suppressing the electron donation via intercalation of electron-accepting tetracyanoquinodimethane molecules causes quenching of this fast electron relaxation channel. This results in an asymptotic increase of the hot-electron lifetimes for infinitesimal small intermediate state energies up to about $1000\phantom{\rule{0.28em}{0ex}}\mathrm{fs}$ as predicted by the pure Landau theory of Fermi liquids.

Ultrafast electron dynamics at water covered alkali adatoms adsorbed on Cu(111).

Here we report on the ultrafast electron dynamics of the alkalis Na, K, and Cs coadsorbed with D2O on Cu(111) surfaces, which we investigated with femtosecond time-resolved two-photon photoemission. The well known transient electronic binding energy stabilization in bare adsorbed alkalis is enhanced by the presence of water which acts as a solvent and increases the transient energy gain. We observe for all adsorbed alkalis a transient binding energy stabilization of 100-300 meV. The stabilization rates range from 1 to 2 eV ps(-1). Here the heavier alkali exhibits a slower stabilization which we explain by their weaker static alkali-water interaction observed in thermal desorption spectroscopy. The population dynamics at low water coverage is described by a single exponential. With increasing water coverage the behavior becomes non-exponential suggesting an additional excited state due to electron solvation.

Ultrafast Exciton Population, Relaxation, and Decay Dynamics in Thin Oligothiophene Films

Femtosecond time-resolved two-photon photoemission spectroscopy is utilized to determine the electronically excited states dynamics at the α-sexithiophene (6T)/Au(111) interface and within the 6T film. We found that a photoinduced transition between the highest occupied molecular orbital and lowest unoccupied molecular orbital is essential in order to observe exciton population, which occurs within 100 fs. In thin 6T films, the exciton exhibits a lifetime of 650 fs. On a time scale of 400 fs, an energetic stabilization is observed leading to the formation of a polaron or electron trapping at defect states. The lifetime of this state is 6.3 ps. Coverage-dependent measurements show that apart from the excited state decay within the film, a substrate-mediated relaxation channel is operative. The present study demonstrates that two-photon photoemission spectroscopy is a powerful tool to investigate the whole life cycle from creation to decay of excitons in an organic semiconductor.

Polaron dynamics in thin polythiophene films studied with time-resolved photoemission

Femtosecond time-resolved two-photon photoemission spectroscopy is employed to study the dynamics of an excited state in a thin regioregular poly(3-hexylthiophene) (RR-P3HT) film deposited on a conducting polymer poly(3,4-ethylene-dioxythiophene): poly-(styrenesulfonate) (PEDT:PSS) electrode following optical excitation at 2.1 eV. We found that the biexponential decay of this excited state has a fast component (2.6 ps) assigned to bound polaron pairs which recombine quickly or separate to be added to the slow component (7.6 ps). The latter is attributed to polarons generated via charge transfer between adjacent polymer chains.

Direct Mapping of Hot-Electron Relaxation and Multiplication Dynamics in PbSe Quantum Dots

How hot electrons relax in semiconductor quantum dots is of critical importance to many potential applications, such as solar energy conversion, light emission, and photon detection. A quantitative answer to this question has not been possible due in part to limitations of current experimental techniques in probing hot electron populations. Here we use femtosecond time-resolved two-photon photoemission spectroscopy to carry out a complete mapping in time- and energy-domains of hot electron relaxation and multiexciton generation (MEG) dynamics in lead selenide quantum dots functionalized with 1,2-ethanedithiols. We find a linear scaling law between the hot electron relaxation rate and its energy above the conduction band minimum. There is no evidence of MEG from intraband hot electron relaxation for excitation photon energy as high as three times the bandgap (3E(g)). Rather, MEG occurs in this system only from interband hot electron transitions at sufficiently high photon energies (~4E(g)).

Dynamics of optically excited electrons in the conducting polymer PEDT:PSS

Femtosecond time-resolved two-photon photoemission spectroscopy is employed to study the dynamics of the non-equilibrium electron distribution in the conducting polymer poly(3,4-ethylene-dioxythiophene): poly-(styrenesulfonate) (PEDT:PSS) film following optical excitation at 2.1 eV. We found that the electron thermalization occurs on a ultrafast timescale of around 60 fs analogous to the relaxation times of optically excited electrons in Au(111).

Electron Dynamics at the ZnO (101̅0) Surface

We use femtosecond time-resolved two-photon photoemission spectroscopy (TR-2PPE) to study the dynamics of electrons excited at the ZnO (101̅0) surface. Efficient relaxation of hot electrons within the Γ valley of the bulk conduction band results in sub-30 fs lifetimes for electron energies greater than 0.1 eV above the conduction band minimum (CBM). These relaxation rates, which are among the fastest observed in any semiconductor over the same energy range, are consistent with the emission of longitudinal optical phonons resulting from strong Fröhlich coupling. For energy at or below the CBM, the excited electron lifetime increases exponentially with decreasing energy to as long as 1 ps. Dynamics in this region can be described by electronic relaxation within a quasi-continuum of defect-derived surface states whose density decreases exponentially into the band gap. Deliberately increasing defects on the ZnO surface drastically decreases the lifetime of electrons in this energy region. Existence of these states is consistent with observed upward band-bending and Fermi level pinning at the (101̅0) surface.

Ultrafast electron dynamics in Pb/Si(111) investigated by two-photon photoemission

Ultrafast electron dynamics in Pb/Si(111) quantum well structures have been investigated by femtosecond time-resolved two-photon photoemission spectroscopy. Three unoccupied quantum well states (or subbands) as well as an image potential state are identified and analyzed as a function of Pb thickness. Using 1.9 eV photon energy for optical excitation electron-hole pairs are formed in the Pb film and in the Si substrate. The hot electron population in the quantum well states exhibits a biexponential decay and both decay times increase when the energy difference to the Fermi level is reduced. The slower decay component with decay times of 130 and 900 fs is attributed to delayed filling of the states in the adlayer due to electronic relaxation in the Si(111) substrate. The faster decay (30--140 fs) is caused by electron-electron scattering in the Pb film. At a first glance, the extracted decay times increase toward the Fermi level as predicted by the Fermi-liquid theory. However, by investigation of the quantized electronic structure more detailed insight into scattering among defined subbands has been gained. The rise of the excited electron population in the lowest unoccupied quantum well state is delayed by 70 fs which occurs concomitantly with a decay in the second lowest state. We describe this simultaneous decay and rise in the two adjacent subbands by coupled rate equations and attribute it to intersubband scattering. Thereby, we demonstrate the importance of intersubband decay in low-dimensional metallic systems in addition to intraband scattering processes considered in Fermi-liquid theory and derived descriptions.

Interferometric two-photon photoemission correlation technique and femtosecond wet-electron dynamics at the TiO2(110) surface

The femtosecond time-resolved two-photon photoemission (TR-2PP) and the ultra high vacuum (UHV) surface science techniques are integrated to investigate the electronic structures and the interfacial electron transfer dynamics at the atomically ordered adsorbate overlayers on TiO2 singlecrystalline surfaces. Our research into the CH3OH/TiO2 system exhibits complex dynamics, providing abundant information with regard to electron transport and solvation processes in the interfacial solvent structures. These represent the fundamentally physical, photochemical, and photocatalytic reactions of protic chemicals covered with metal-oxides.

A two-photon photoemission study of spin-dependent electron dynamics

We describe a spin- and femtosecond time-resolved two-photon photoemission experiment with an overall energy resolution of 90 meV and an angular resolution of ±2.5° at an electron count rate of a few kHz. As a model system we have studied image-potential states in front of a three monolayer iron film on Cu(1 0 0). With our setup an exchange splitting of both, the n = 1 and n = 2 image-potential state is clearly discernible. A strong dependence of the spin polarization in the image-potential states on the polarization of the pump light can be observed. Time-resolved measurements reveal that the lifetime of excited majority-spin electrons exceeds that of minority-spin electrons by a factor of 1.4.

Using image resonances to probe molecular conduction at the n-heptane∕Au(111) interface

The binding energies and lifetimes of the n=1 image resonance on Au(111) are measured as a function of n-heptane layer thickness by femtosecond time-resolved two-photon photoemission (TR-2PPE) spectroscopy. The lifetime of the image resonance dramatically increases from approximately 4 fs on clean Au(111) to 1.6 ps with three layers of n-heptane. Because the image resonance is above the L1 band edge of Au, this increase in lifetime is attributed to the tunneling barrier presented by the sigma-sigma* band gap of the n-heptane film. We use the one-dimensional dielectric continuum model (DCM) to approximate the surface potential and to determine the binding energies and the lifetimes of the image resonances. The exact solution of the DCM potential is determined in two ways: the first by wave-packet propagation and the second by using a tight-binding Green's function approach. The first approach allows band-edge effects to be treated. The latter approach is particularly useful in illustrating the similarity between TR-2PPE and conductance measurements.

Relaxation dynamics of magic clusters

Using femtosecond time-resolved two-photon photoemission spectroscopy, relaxation rates of the optically excited states in Al13 ,S i4 , and Au6 clusters are estimated to be 250 fs, 1.2 ps, and about 1 ns, respectively. These clusters show closed electronic shell structures and large gaps between the highest occupied molecular orbital and the lowest unoccupied molecular orbital in the anionic sAl 13 d or neutral (Si4 and Au6) states. Thus the different behaviors of these clusters for the relaxation dynamics cannot be rationalized within a relaxation mechanism dominated by Auger-like electron-electron coupling. We suggest that for these small clusters, the major decay mechanism might be electron-phonon coupling.

Time-Resolved Two-Photon Photoemission Study of Dodecanethiolate-Passivated Ag Nanoparticles

Metallic nanoparticles are attracting much interest from the viewpoints of both fundamental and device physics, since they show the distinctive optical, physical, and chemical properties found in neither bulk nor molecular/ atomic systems. Among the nanoparticle systems, the surface-passivated nanoparticles are very stable and exhibit the closed-packed nanoparticle self-assemblies on the singlecrystalline substrates. Therefore, it is considered that they could be important constituents of future nanostructured devices, such as single electron device, catalyst, and ultra highly density memory. On the other hand, the transient dynamics of hot-electrons created by photoexcitation or inelastic scattering of other energenic particles play an important role in the various physical and chemical phenomena. The excited hot-electrons relax through a number of decay processes, such as electron–electron scattering, electron-phonon scattering, electron-impurity/ defect scattering, and scattering with other elementary excitations. The detailed understanding of hot-electron dynamics will provide not only information about the fundamental interactions in the many-body systems but also knowledge of the various optical, physical and chemical processes. In order to elucidate the intriguing properties of metallic nanoparticles and to develop the future devices, it is indispensable to understand the excited-electron dynamics as well as their electronic structures. In this work, we have carried out a femtosecond time-resolved two-photon photoemission (TR-2PPE) study of dodecanethiolate (DT)-passivated Ag nanoparticles on the highly oriented pyrolytic graphite (HOPG) substrates. Among the time-resolved spectroscopic methods, TR-2PPE spectroscopy is a unique method to observe directly the temporal evolution of excited-electrons that may not be easily accessible through optical techniques. The DT-passivated Ag nanoparticles were synthesized by the two-phase (water–toluene) reduction method. The detailed procedure is described elsewhere. The size and shape of the synthesized nanoparticles were characterized by ex-situ observations with transmission electron microscope (TEM). Figure 1 shows the TEM micrograph and the size distribution in diameter of DT-passivated Ag nanoparticles used in this work. As shown in Fig. 1, the mean diameter and standard deviation are dm 1⁄4 5:3 nm and 1⁄4 0:47 nm, respectively. The present samples have an extremely narrow size distribution in diameter ( 3:0 eV). The total energy resolution was about 150meV, and time resolution was about 10 fs. All measurements were performed at room temperature. Figure 2 shows the two-photon photoemission (2PPE) spectrum of the DT-passivated Ag nanoparticles on the HOPG substrate with dm 1⁄4 5:3 nm measured with photon energy of 3.3 eV at zero pump-probe delay, compared with that of HOPG substrate. These spectra are plotted as a function of intermediate-state energy relative to the Fermilevel. In Fig. 2, the intermediate-state energy of 3.3 eV corresponds to the highest intermediate-state, with onephoton energy (h 1⁄4 3:3 eV) above the Fermi-level. As shown in Fig. 2, the 2PPE spectral features are definitely different among the HOPG substrate and DT-passivated Ag nanoparticles on HOPG substrate. The 2PPE spectrum of Fig. 1. TEM micrograph and size distribution in diameter of dodecanethiolate-passivated Ag nanoparticles with mean diameter of dm 1⁄4 5:3 nm and standard deviation of 1⁄4 0:47 nm.

Lifetimes of image-potential states on Cu(100) and Ag(100) measured by femtosecond time-resolved two-photon photoemission

We report time-resolved studies of image-potential states on Cu(100) and Ag(100). Femtosecond ultraviolet-pump and infrared-probe techniques combined with two-photon photoemission have been used to measure relaxation dynamics of $n=1,$ 2, and 3 image-potential states. We found the lifetimes of these states on Cu(100) at room temperature to be $40\ifmmode\pm\else\textpm\fi{}6,120\ifmmode\pm\else\textpm\fi{}15,$ and $300\ifmmode\pm\else\textpm\fi{}20\mathrm{fs},$ respectively. The corresponding values for Ag(100) are $55\ifmmode\pm\else\textpm\fi{}5,160\ifmmode\pm\else\textpm\fi{}10,$ and $360\ifmmode\pm\else\textpm\fi{}15\mathrm{fs}.$

Can we control lifetimes of electronic states at surfaces by adsorbate resonances?

Femtosecond time-resolved two-photon photoemission is used to study the excited electronic states of a Cu(111) surface covered with a monolayer of physisorbed O 2 on top of one to five Xe spacer layers. The spectra are dominated by a state 4.0 eV above E F whose lifetime depends strongly on the number of Xe spacer layers. The lifetime decreases with increasing number of spacer layers from (650±30) fs for one Xe layer to (90±10) fs for five layers. To explain this trend we propose a model where the image potential states couple to a negative-ion resonance of the O 2 molecule.

Dynamics of photoexcited electrons in metals studied with time-resolved two-photon photoemission

Femtosecond time-resolved two-photon photoemission (2PPE) is used to investigate the hot electron dynamics of Cu(111), Ag(110) and Ta(poly). The experimentally derived relaxation rates of photoexcited electrons scale with the available phase space for scattering with electrons below the Fermi level and are compared with predictions from Fermi liquid theory. A simulation employing rate equations to account for hot electron lifetimes, transport effects and the creation of secondary electrons demonstrates that the relaxation dynamics is strongly influenced by secondary electron cascades. With Ag(110) a pronounced temperature dependence of the photoemission yield is observed, indicating the importance of electron-phonon scattering for momentum conservation in the 2PPE process.

Femtosecond studies of electron tunneling at metal-dielectric interfaces

Femtosecond time-resolved two-photon photoemission spectrocopy (TPPE) has been used to measure the lifetimes of image potential electrons at alkane mono- and bilayers on Ag(111). The n = 1 lifetimes for zero, one, and two layers of n-heptane on Ag(111) are 32 ± 10 fs, 155 ± 20 fs, and 1580 ± 200 fs, respectively. This approximately exponential increase in lifetime is consistent with a tunneling picture in which the adlayer forms a barrier that slows the decay of an image potential electron back into the metal. The existence of the tunneling barrier is consistent with the repulsive electron affinity of the longer chain n-alkanes in the condensed phase. The lifetimes of the higher quantum states indicate that the presence of the monolayer significantly reduces coupling of the image states to the bulk band structure, so that further changes in lifetime are determined by the adlayer barrier and an attempt rate related to the classical oscillation time in the modified image potential well. These results are compared with quantitative predictions of a model by Cole which considers the tunneling barrier presented by the layer and the effect of the layer on the attempt rate. These results are considered in the context of previous TPPE studies of metal-insulator interfaces.