Showing papers on "Photoexcitation published in 2014"
TL;DR: This introductory review covers the fundamental aspects of photocatalytic and photoelectrochemical water splitting and recent advances in the water splitting reaction under visible light will be presented with a focus on non-oxide semiconductor materials to give an overview of the various problems and solutions.
Abstract: Photocatalytic and photoelectrochemical water splitting under irradiation by sunlight has received much attention for production of renewable hydrogen from water on a large scale. Many challenges still remain in improving energy conversion efficiency, such as utilizing longer-wavelength photons for hydrogen production, enhancing the reaction efficiency at any given wavelength, and increasing the lifetime of the semiconductor materials. This introductory review covers the fundamental aspects of photocatalytic and photoelectrochemical water splitting. Controlling the semiconducting properties of photocatalysts and photoelectrode materials is the primary concern in developing materials for solar water splitting, because they determine how much photoexcitation occurs in a semiconductor under solar illumination and how many photoexcited carriers reach the surface where water splitting takes place. Given a specific semiconductor material, surface modifications are important not only to activate the semiconductor for water splitting but also to facilitate charge separation and to upgrade the stability of the material under photoexcitation. In addition, reducing resistance loss and forming p-n junction have a significant impact on the efficiency of photoelectrochemical water splitting. Correct evaluation of the photocatalytic and photoelectrochemical activity for water splitting is becoming more important in enabling an accurate comparison of a number of studies based on different systems. In the latter part, recent advances in the water splitting reaction under visible light will be presented with a focus on non-oxide semiconductor materials to give an overview of the various problems and solutions.
3,470 citations
TL;DR: It is found that photoexcitation in the pristine CH3NH3PbI3-xClx perovskite results in free charge carrier formation within 1 ps and that these free charge carriers undergo bimolecular recombination on time scales of 10s to 100s of ns.
Abstract: The study of the photophysical properties of organic–metallic lead halide perovskites, which demonstrate excellent photovoltaic performance in devices with electron- and hole-accepting layers, helps to understand their charge photogeneration and recombination mechanism and unravels their potential for other optoelectronic applications. We report surprisingly high photoluminescence (PL) quantum efficiencies, up to 70%, in these solution-processed crystalline films. We find that photoexcitation in the pristine CH3NH3PbI3–xClx perovskite results in free charge carrier formation within 1 ps and that these free charge carriers undergo bimolecular recombination on time scales of 10s to 100s of ns. To exemplify the high luminescence yield of the CH3NH3PbI3–xClx perovskite, we construct and demonstrate the operation of an optically pumped vertical cavity laser comprising a layer of perovskite between a dielectric mirror and evaporated gold top mirrors. These long carrier lifetimes together with exceptionally high...
1,527 citations
TL;DR: The status of understanding of the operation of bulk heterojunction (BHJ) solar cells is reviewed and a summary of the problems to be solved to achieve the predicted power conversion efficiencies of >20% for a single cell is concluded.
Abstract: The status of understanding of the operation of bulk heterojunction (BHJ) solar cells is reviewed. Because the carrier photoexcitation recombination lengths are typically 10 nm in these disordered materials, the length scale for self-assembly must be of order 10–20 nm. Experiments have verified the existence of the BHJ nanostructure, but the morphology remains complex and a limiting factor. Three steps are required for generation of electrical power: i) absorption of photons from the sun; ii) photoinduced charge separation and the generation of mobile carriers; iii) collection of electrons and holes at opposite electrodes. The ultrafast charge transfer process arises from fundamental quantum uncertainty; mobile carriers are directly generated (electrons in the acceptor domains and holes in the donor domains) by the ultrafast charge transfer (≈70%) with ≈30% generated by exciton diffusion to a charge separating heterojunction. Sweep-out of the mobile carriers by the internal field prior to recombination is essential for high performance. Bimolecular recombination dominates in materials where the donor and acceptor phases are pure. Impurities degrade performance by introducing Shockly–Read–Hall decay. The review concludes with a summary of the problems to be solved to achieve the predicted power conversion efficiencies of >20% for a single cell.
1,492 citations
TL;DR: Optical spectroscopy is used to estimate the exciton binding energy in the mixed-halide crystal to be in the range of 50 meV, and it is shown that such a value is consistent with almost full ionization of the excitonic population under photovoltaic cell operating conditions.
Abstract: Excitonic solar cells, within which bound electron-hole pairs have a central role in energy harvesting, have represented a hot field of research over the last two decades due to the compelling prospect of low-cost solar energy. However, in such cells, exciton dissociation and charge collection occur with significant losses in energy, essentially due to poor charge screening. Organic-inorganic perovskites show promise for overcoming such limitations. Here, we use optical spectroscopy to estimate the exciton binding energy in the mixed-halide crystal to be in the range of 50 meV. We show that such a value is consistent with almost full ionization of the exciton population under photovoltaic cell operating conditions. However, increasing the total photoexcitation density, excitonic species become dominant, widening the perspective of this material for a host of optoelectronic applications.
1,473 citations
TL;DR: Photoluminescence, transient absorption, time-resolved terahertz and microwave conductivity measurements are applied to determine the time scales of generation and recombination of charge carriers as well as their transport properties in solution-processed CH3NH3PbI3 perovskite materials to unravel the remarkable intrinsic properties of the material.
Abstract: Organometal halide perovskite-based solar cells have recently been reported to be highly efficient, giving an overall power conversion efficiency of up to 15%. However, much of the fundamental photophysical properties underlying this performance has remained unknown. Here, we apply photoluminescence, transient absorption, time-resolved terahertz and microwave conductivity measurements to determine the time scales of generation and recombination of charge carriers as well as their transport properties in solution-processed CH3NH3PbI3 perovskite materials. We found that electron–hole pairs are generated almost instantaneously after photoexcitation and dissociate in 2 ps forming highly mobile charges (25 cm2 V–1 s–1) in the neat perovskite and in perovskite/alumina blends; almost balanced electron and hole mobilities remain very high up to the microsecond time scale. When the perovskite is introduced into a TiO2 mesoporous structure, electron injection from perovskite to the metal oxide is efficient in less ...
1,093 citations
TL;DR: The delocalized plAsmon state observed in this study establishes a novel concept for plasmonic photosensitization of wide band gap semiconductors, leading to efficient conversion of photons to charge carriers and to hybrid materials with a wide variety of applications in photocatalysis and photovoltaics.
Abstract: Photoexcitation of the plasmon band in metallic nanoparticles adsorbed on a TiO2 surface initiates many important photovoltaic and photocatalytic processes. The traditional view on the photoinduced charge separation involves excitation of a surface plasmon, its subsequent dephasing into electron-hole pairs, followed by electron transfer (ET) from the metal nanoparticle into TiO2. We use nonadiabatic molecular dynamics combined with time-domain density functional theory to demonstrate that an electron appears inside TiO2 immediately upon photoexcitation with a high probability (~50%), bypassing the intermediate step of electron-hole thermalization inside the nanoparticle. By providing a detailed, atomistic description of the charge separation, energy relaxation, and electron-hole recombination processes, the simulation rationalizes why the experimentally observed ultrafast photoinduced ET in an Au-TiO2 system is possible in spite of the fast energy relaxation. The simulation shows that the photogenerated plasmon is highly delocalized onto TiO2, and thus, it is shared by the electron donor and acceptor materials. In the 50% of the cases remaining after the instantaneous photogeneration of the charge-separated state, the electron injects into TiO2 on a sub-100 fs time scale by the nonadiabatic mechanism due to high density of acceptor states. The electron-phonon relaxation parallels the injection and is slower, resulting in a transient heating of the TiO2 surface by 40 K. Driven by entropy, the electron moves further into TiO2 bulk. If the electron remains trapped at the TiO2 surface, it recombines with the hole on a picosecond time scale. The obtained ET and recombination times are in excellent agreement with the experiment. The delocalized plasmon state observed in our study establishes a novel concept for plasmonic photosensitization of wide band gap semiconductors, leading to efficient conversion of photons to charge carriers and to hybrid materials with a wide variety of applications in photocatalysis and photovoltaics.
220 citations
TL;DR: Activation of targeted adsorbate-metal bonds through direct photoexcitation of hybridized electronic states enabled selectivity control in preferential CO oxidation in H2 rich streams and opens new avenues to drive selective catalytic reactions that cannot be achieved using thermal energy.
Abstract: Engineering heterogeneous metal catalysts for high selectivity in thermal driven reactions typically involves the synthesis of nanostructures with well-controlled geometries and compositions. However, inherent relationships between the energetics of elementary steps limit the control of catalytic selectivity through these approaches. Photon excitation of metal catalysts can induce chemical reactivity channels that cannot be accessed using thermal energy, although the potential for targeted activation of adsorbate–metal bonds is limited because the processes of photon absorption and adsorbate–metal bond photoexcitation are typically separated spatially. Here, we show that the use of sub-5-nanometer metal particles as photocatalysts enables direct photoexcitation of hybridized adsorbate–metal states as the dominant mechanism driving photochemistry. Activation of targeted adsorbate–metal bonds through direct photoexcitation of hybridized electronic states enabled selectivity control in preferential CO oxidat...
209 citations
TL;DR: It is concluded 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.
Abstract: Using femtosecond time-resolved photoelectron spectroscopy we demonstrate that photoexcitation transforms monoclinic ${\mathrm{VO}}_{2}$ quasi-instantaneously into a metal. Thereby, we exclude an 80 fs structural bottleneck for the photoinduced electronic phase transition of ${\mathrm{VO}}_{2}$. First-principles many-body perturbation theory calculations reveal a high sensitivity of the ${\mathrm{VO}}_{2}$ 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.
201 citations
TL;DR: The IrO2-loaded SrTiO3 doped with rhodium and antimony synthesized by a conventional solid-state reaction splits water under visible light and simulated sunlight irradiation giving 0.1% of the apparent quantum yield at 420 nm is the longest among achieved photocatalytic water splitting with one-step photoexcitation.
178 citations
TL;DR: It is observed that the photoinduced terahertz absorption increases in charge neutral graphene but decreases in highly doped graphene, and it is shown that this transition from semiconductor-like to metal-like response is unique for zero bandgap materials such as graphene.
Abstract: We investigate the ultrafast terahertz response of electrostatically gated graphene upon optical excitation. We observe that the photoinduced terahertz absorption increases in charge neutral graphene but decreases in highly doped graphene. We show that this transition from semiconductor-like to metal-like response is unique for zero bandgap materials such as graphene. In charge neutral graphene photoexcited hot carriers effectively increase electron and hole densities and increase the conductivity. In highly doped graphene, however, photoexcitation does not change net conducting carrier concentration. Instead, it mainly increases electron scattering rate and reduce the conductivity.
152 citations
TL;DR: It is inferred from the data that photoexcitation initially leads to formation of bound electron-hole pairs in the form of neutral excitons, which can be understood by distinguishing nanoplatelets with and without exciton quenching site, which are present in the sample with close to equal amounts.
Abstract: The nature and decay dynamics of photoexcited states in CdSe core-only and CdSe/CdS core/shell nanoplatelets was studied. The photophysical species produced after ultrafast photoexcitation are studied using a combination of time-resolved photoluminescence (PL), transient absorption (TA), and terahertz (THz) conductivity measurements. The PL, TA, and THz exhibit very different decay kinetics, which leads to the immediate conclusion that photoexcitation produces different photophysical species. It is inferred from the data that photoexcitation initially leads to formation of bound electron–hole pairs in the form of neutral excitons. The decay dynamics of these excitons can be understood by distinguishing nanoplatelets with and without exciton quenching site, which are present in the sample with close to equal amounts. In absence of a quenching site, the excitons undergo PL decay to the ground state. In nanoplatelets with a quenching site, part of the initially produced excitons decays by hole trapping at a ...
TL;DR: The results indicate that the dynamics of a structural symmetry-breaking transition are determined by a high-symmetry excited state potential energy surface distinct from that of the initial low-temperature state.
Abstract: Using femtosecond time-resolved x-ray diffraction, we directly monitor the coherent lattice dynamics through an ultrafast charge-density-wave-to-metal transition in the prototypical Peierls system K(0.3)MoO(3) over a wide range of relevant excitation fluences. While in the low fluence regime we directly follow the structural dynamics associated with the collective amplitude mode; for fluences above the melting threshold of the electronic density modulation we observe a transient recovery of the periodic lattice distortion. We can describe these structural dynamics as a motion along the coordinate of the Peierls distortion triggered by the prompt collapse of electronic order after photoexcitation. The results indicate that the dynamics of a structural symmetry-breaking transition are determined by a high-symmetry excited state potential energy surface distinct from that of the initial low-temperature state.
TL;DR: In this paper, the authors used ultrafast optical-pump terahertz-probe spectroscopy to study the surface-scattering dynamics of hot Dirac electrons.
Abstract: Two-dimensional surface-scattering dynamics are central in the physics of topological insulators. Numerous electrical and optical measurements have evidenced that the origins of novel optoelectronic response can be traced back to Dirac surface-electron dynamics. Intrinsic surface dynamics, however, remain elusive because these experiments cannot access the frequencies of the surface-scattering rate. Time-resolved terahertz spectroscopy is the only apparatus for directly probing the collective response of low-energy electronic transitions. Here, by utilizing ultrafast optical-pump terahertz-probe spectroscopy, we discovered anomalous characteristics of the surface-scattering dynamics. Upon photoexcitation, the surface-scattering rate is increased and results in negative dynamic conductance at low temperature. Surprisingly, the differential changes of the surface-scattering rate are strongly reduced by photoexcited electrons at elevated temperature. We find that this nontrivial surface-electron dynamics is due to opening a carrier-relaxation channel from bulk to the surface state---one distinct characteristic of topological insulators. Our observations reveal unexpected surface dynamics of hot Dirac electrons, providing experimental a priori knowledge toward ultrafast optoelectronic operations.
TL;DR: This work uses picosecond X-ray absorption spectroscopy at the Ti K-edge and the Ru L3-edge to address the nature and lifetime of electron traps at room temperature for photoexcited bare and N719-dye-sensitized anatase and amorphous TiO2 nanoparticles and shows that 100 ps after photoexcitation, the electrons are trapped deep in the defect-rich surface shell in the case of anataseTiO2.
Abstract: Titanium dioxide (TiO2) is the most popular material for applications in solar-energy conversion and photocatalysis, both of which rely on the creation, transport, and trapping of charges (holes and electrons). The nature and lifetime of electron traps at room temperature have so far not been elucidated. Herein, we use picosecond X-ray absorption spectroscopy at the Ti K-edge and the Ru L3-edge to address this issue for photoexcited bare and N719-dye-sensitized anatase and amorphous TiO2 nanoparticles. Our results show that 100 ps after photoexcitation, the electrons are trapped deep in the defect-rich surface shell in the case of anatase TiO2, whereas they are inside the bulk in the case of amorphous TiO2. In the case of dye-sensitized anatase or amorphous TiO2, the electrons are trapped at the outer surface. Only two traps were identified in all cases, with lifetimes in the range of nanoseconds to tens of nanoseconds.
TL;DR: Ultrafast x-ray diffraction with femtosecond temporal resolution is applied to monitor the lattice dynamics in a thin film of multiferroic BiFeO3 after above-band-gap photoexcitation to indicate a quasi-instantaneous photoinduced stress which decays on a nanosecond time scale.
Abstract: We apply ultrafast x-ray diffraction with femtosecond temporal resolution to monitor the lattice dynamics in a thin film of multiferroic BiFeO3 after above-band-gap photoexcitation. The sound-velocity limited evolution of the observed lattice strains indicates a quasi-instantaneous photoinduced stress which decays on a nanosecond time scale. This stress exhibits an inhomogeneous spatial profile evidenced by the broadening of the Bragg peak. These new data require substantial modification of existing models of photogenerated stresses in BiFeO3: the relevant excited charge carriers must remain localized to be consistent with the data.
TL;DR: There is very different partitioning of the photon momentum in one-photon ionization (the photoelectric effect) as compared to multiphoton processes, suggesting that there is a rich, unexplored physics to be studied between these two limits which can be generated with current ultrafast laser technology.
Abstract: We investigate photon-momentum sharing between an electron and an ion following different photoionization regimes. We find very different partitioning of the photon momentum in one-photon ionization (the photoelectric effect) as compared to multiphoton processes. In the photoelectric effect, the electron acquires a momentum that is much greater than the single photon momentum ℏω/c [up to (8/5) ℏω/c] whereas in the strong-field ionization regime, the photoelectron only acquires the momentum corresponding to the photons absorbed above the field-free ionization threshold plus a momentum corresponding to a fraction (3/10) of the ionization potential Ip. In both cases, due to the smallness of the electron-ion mass ratio, the ion takes nearly the entire momentum of all absorbed N photons (via the electron-ion center of mass). Additionally, the ion takes, as a recoil, the photoelectron momentum resulting from mutual electron-ion interaction in the electromagnetic field. Consequently, the momentum partitioning of the photofragments is very different in both regimes. This suggests that there is a rich, unexplored physics to be studied between these two limits which can be generated with current ultrafast laser technology.
TL;DR: Strong indications for the formation of an interfacial charge-transfer state are presented, providing direct insight into a transient electronic configuration that may limit the efficiency of photoinduced free charge-carrier generation.
Abstract: Understanding interfacial charge-transfer processes on the atomic level is crucial to support the rational design of energy-challenge relevant systems such as solar cells, batteries, and photocatalysts. A femtosecond time-resolved core-level photoelectron spectroscopy study is performed that probes the electronic structure of the interface between ruthenium-based N3 dye molecules and ZnO nanocrystals within the first picosecond after photoexcitation and from the unique perspective of the Ru reporter atom at the center of the dye. A transient chemical shift of the Ru 3d inner-shell photolines by (2.3 ± 0.2) eV to higher binding energies is observed 500 fs after photoexcitation of the dye. The experimental results are interpreted with the aid of ab initio calculations using constrained density functional theory. Strong indications for the formation of an interfacial charge-transfer state are presented, providing direct insight into a transient electronic configuration that may limit the efficiency of photoinduced free charge-carrier generation.
TL;DR: It is shown that several model high-efficiency organic solar cell blends exhibit flat IQEs across the visible spectrum, suggesting that charge generation is occurring either via a dominant single channel or via both channels but with comparable efficiencies.
Abstract: The conventional picture of photocurrent generation in organic solar cells involves photoexcitation of the electron donor, followed by electron transfer to the acceptor via an interfacial charge-transfer state (Channel I). It has been shown that the mirror-image process of acceptor photoexcitation leading to hole transfer to the donor is also an efficient means to generate photocurrent (Channel II). The donor and acceptor components may have overlapping or distinct absorption characteristics. Hence, different excitation wavelengths may preferentially activate one channel or the other, or indeed both. As such, the internal quantum efficiency (IQE) of the solar cell may likewise depend on the excitation wavelength. We show that several model high-efficiency organic solar cell blends, notably PCDTBT:PC70BM and PCPDTBT:PC60/70BM, exhibit flat IQEs across the visible spectrum, suggesting that charge generation is occurring either via a dominant single channel or via both channels but with comparable efficienci...
TL;DR: Two-dimensional electronic spectroscopy can be adapted to detect chiral signals and that these signals reveal how excitations delocalize and contract following excitation, which will provide an incisive tool to probe ultrafast transient molecular fluctuations that are obscured in non-chiral experiments.
Abstract: Nonlinear chiral optical activity is difficult to measure because of weak signal amidst strong achiral background. Here, the authors perform a nonlinear chiral two-dimensional spectroscopic mapping of light-harvesting complex 2 during photoexcitation and observe exciton delocalization.
TL;DR: These first-principles quantum dynamics simulations, in conjunction with recent experiments, allow us to clearly resolve the mechanistic details of the ultrafast dynamics within [Cu(dmp)2](+), which have been disputed in the literature.
Abstract: The ultrafast nonadiabatic dynamics of a prototypical Cu(I)–phenanthroline complex, [Cu(dmp)2]+ (dmp = 2,9-dimethyl-1,10-phenanthroline), initiated after photoexcitation into the optically bright metal-to-ligand charge-transfer (MLCT) state (S3) is investigated using quantum nuclear dynamics. In agreement with recent experimental conclusions, we find that the system undergoes rapid (∼100 fs) internal conversion from S3 into the S2 and S1 states at or near the Franck–Condon (FC) geometry. This is preceded by a dynamic component with a time constant of ∼400 fs, which corresponds to the flattening of the ligands associated with the pseudo Jahn–Teller distortion. Importantly, our simulations demonstrate that this latter aspect is in competition with subpicosecond intersystem crossing (ISC). The mechanism for ISC is shown to be a dynamic effect, in the sense that it arises from the system traversing the pseudo Jahn–Teller coordinate where the singlet and triplet states become degenerate, leading to efficient c...
TL;DR: A novel method to prepare silicon quantum dots (Si-QDs) having excellent stability in water without organic-ligands by simultaneously doping phosphorus and boron is reported.
Abstract: We report a novel method to prepare silicon quantum dots (Si-QDs) having excellent stability in water without organic-ligands by simultaneously doping phosphorus and boron. The codoped Si-QDs in water exhibit bright size-tunable luminescence in a biological window. The luminescence of codoped Si-QDs is very stable under continuous photoexcitation in water.
TL;DR: Ab initio time-domain simulations of the charge separation and energy relaxation across an interface formed by poly(3-hexylthiophene) (P3HT) and a single-walled carbon nanotube (CNT) showed strong asymmetry, which can be utilized to improve the performance of solar cells by optimizing simultaneously light harvesting, charge separation, andenergy relaxation.
Abstract: To achieve a high photon-to-charge conversion efficiency, the electron–hole pair generated by photon absorption in organic photovoltaic systems must overcome the Coulomb attraction, which often results in voltage loss. Bearing this in mind, we performed ab initio time-domain simulations of the charge separation and energy relaxation across an interface formed by poly(3-hexylthiophene) (P3HT) and a single-walled carbon nanotube (CNT). The dynamics of the positive and negative charges showed strong asymmetry. Photoexcitation of the polymer leads to a 100 fs electron transfer, in agreement with the experiment, followed by a loss of 0.6 eV of energy within 0.5 ps. Photoexcitation of the CNT leads to hole transfer, which requires nearly 2 ps, but loses only 0.3 eV of energy. The strong disparity arises due to the differences in the localization of the photoexcited donor states, the number densities of the acceptor states, and the phonon modes involved. Used as a chromophore, P3HT produces faster charge separat...
TL;DR: In this article, a photo-thermoelectric photocurrent was generated at the graphene-metal interface, focusing on the time-resolved photocurrent, the effects of photon energy, Fermi energy and light polarization.
Abstract: Photoexcitation of graphene leads to an interesting sequence of phenomena, some of which can be exploited in optoelectronic devices based on graphene. In particular, the efficient and ultrafast generation of an electron distribution with an elevated electron temperature and the concomitant generation of a photo-thermoelectric voltage at symmetry-breaking interfaces is of interest for photosensing and light harvesting. Here, we experimentally study the generated photocurrent at the graphene-metal interface, focusing on the time-resolved photocurrent, the effects of photon energy, Fermi energy and light polarization. We show that a single framework based on photo-thermoelectric photocurrent generation explains all experimental results.
TL;DR: In this article, the radiative recombination of photoexcited carriers bound at native donors and acceptors in exfoliated nanoflakes of nominally undoped rhombohedral γ-polytype InSe was studied.
Abstract: We report on the radiative recombination of photo-excited carriers bound at native donors and acceptors in exfoliated nanoflakes of nominally undoped rhombohedral γ-polytype InSe. The binding energies of these states are found to increase with the decrease in flake thickness, L. We model their dependence on L using a two-dimensional hydrogenic model for impurities and show that they are strongly sensitive to the position of the impurities within the nanolayer.
TL;DR: This optical freezing of charges, which is the reverse of the photoinduced melting of electronic orders, is attributed to the ~10% reduction of t driven by the strong, high-frequency (ω ≧ t/ħ) electric field.
Abstract: In strongly correlated systems, the material properties can be drastically altered through subtle external perturbations. Here, the authors show that photoexcitation of the organic conductor α-(ET)2I3 with ultrashort pulses leads to a counter-intuitive freezing of the electron motion.
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TL;DR: In this paper, the authors use continuous wave photomodulation spectroscopy to identify the optical signature of long-lived charge carriers and femtosecond pump-probe spectrograms to follow the CPG dynamics.
Abstract: The two-dimensional semiconductor MoS2 in its mono- and few-layer form is expected to have a significant exciton binding energy of several 100 meV, leading to the consensus that excitons are the primary photoexcited species. Nevertheless, even single layers show a strong photovoltaic effect and work as the active material in high sensitivity photodetectors, thus indicating efficient charge carrier photogeneration (CPG). Here we use continuous wave photomodulation spectroscopy to identify the optical signature of long-lived charge carriers and femtosecond pump-probe spectroscopy to follow the CPG dynamics. We find that intitial photoexcitation yields a branching between excitons and charge carriers, followed by excitation energy dependent hot exciton dissociation as an additional CPG mechanism. Based on these findings, we make simple suggestions for the design of more efficient MoS2 photovoltaic and photodetector devices.
TL;DR: In this article, the electron-induced adsorbate dynamics on the metal surface was modeled using a nonadiabatic, first-principles based inelastic electron scattering model.
Abstract: Visible light driven catalysis on metal surfaces and nanoparticles has attracted significant attention in recent years as a potential route for driving selective chemical reactions that are difficult to achieve with thermal energy. It is most often assumed that photochemistry on metal surfaces occurs through a substrate-mediated process of adsorbate-metal bond photoexcitation, although crucial underlying phenomena controlling the efficiency of this process are still poorly understood. In this work, substrate-mediated photochemistry on metal surfaces was analyzed by combining dynamical models associated with the metal substrate photoexcitation and electron-mediated bond-activation processes. An extended version of two-temperature model was utilized to treat temporal evolution of photoexcited charge carriers in the metal substrate. The electron-induced adsorbate dynamics on the metal surface was modeled using a nonadiabatic, first-principles based inelastic electron scattering model. Photoactivation of thre...
TL;DR: In this paper, the authors used optical pump-THz probe spectroscopy at low temperatures to study the hot carrier response in thin Bi$2$Se$_3$ films of several thicknesses, allowing them to separate the bulk from the surface transient response.
Abstract: We use optical pump--THz probe spectroscopy at low temperatures to study the hot carrier response in thin Bi$_2$Se$_3$ films of several thicknesses, allowing us to separate the bulk from the surface transient response. We find that for thinner films the photoexcitation changes the transport scattering rate and reduces the THz conductivity, which relaxes within 10 picoseconds (ps). For thicker films, the conductivity increases upon photoexcitation and scales with increasing both the film thickness and the optical fluence, with a decay time of approximately 5 ps as well as a much higher scattering rate. These different dynamics are attributed to the surface and bulk electrons, respectively, and demonstrate that long-lived mobile surface photo-carriers can be accessed independently below certain film thicknesses for possible optoelectronic applications.
TL;DR: In this article, the effect of femtosecond laser irradiation on bulk and single-layer MoS2 on silicon oxide was studied using optical, field emission scanning electron microscopy and Raman microscopy.
Abstract: The effect of femtosecond laser irradiation on bulk and single-layer MoS2 on silicon oxide is studied. Optical, field emission scanning electron microscopy and Raman microscopy were used to quantify the damage. The intensity of A1g and E2g1 vibrational modes was recorded as a function of the number of irradiation pulses. The observed behavior was attributed to laser-induced bond breaking and subsequent atoms removal due to electronic excitations. The single-pulse optical damage threshold was determined for the monolayer and bulk under 800 nm and 1030 nm pulsed laser irradiation, and the role of two-photon versus one photon absorption effects is discussed.
TL;DR: The electronic structure of the iron(II) spin crossover complex deposited as an ultrathin film on Au(111) is determined by means of UV-photoelectron spectroscopy (UPS) in the high-spin and in the low-spin state to monitor the thermal as well as photoinduced spin transition in this system.
Abstract: The electronic structure of the iron(II) spin crossover complex [Fe(H2bpz)2(phen)] deposited as an ultrathin film on Au(111) is determined by means of UV-photoelectron spectroscopy (UPS) in the high-spin and in the low-spin state. This also allows monitoring the thermal as well as photoinduced spin transition in this system. Moreover, the complex is excited to the metastable high-spin state by irradiation with vacuum-UV light. Relaxation rates after photoexcitation are determined as a function of temperature. They exhibit a transition from thermally activated to tunneling behavior and are two orders of magnitude higher than in the bulk material.