Showing papers on "Photoexcitation published in 2016"

Photon recycling in lead iodide perovskite solar cells

Abstract: Lead-halide perovskites have emerged as high-performance photovoltaic materials. We mapped the propagation of photogenerated luminescence and charges from a local photoexcitation spot in thin films of lead tri-iodide perovskites. We observed light emission at distances of ≥50 micrometers and found that the peak of the internal photon spectrum red-shifts from 765 to ≥800 nanometers. We used a lateral-contact solar cell with selective electron- and hole-collecting contacts and observed that charge extraction for photoexcitation >50 micrometers away from the contacts arose from repeated recycling between photons and electron-hole pairs. Thus, energy transport is not limited by diffusive charge transport but can occur over long distances through multiple absorption-diffusion-emission events. This process creates high excitation densities within the perovskite layer and allows high open-circuit voltages.

Mechanism for Broadband White-Light Emission from Two-Dimensional (110) Hybrid Perovskites

Abstract: The recently discovered phenomenon of broadband white-light emission at room temperature in the (110) two-dimensional organic–inorganic perovskite (N-MEDA)[PbBr4] (N-MEDA = N1-methylethane-1,2-diammonium) is promising for applications in solid-state lighting. However, the spectral broadening mechanism and, in particular, the processes and dynamics associated with the emissive species are still unclear. Herein, we apply a suite of ultrafast spectroscopic probes to measure the primary events directly following photoexcitation, which allows us to resolve the evolution of light-induced emissive states associated with white-light emission at femtosecond resolution. Terahertz spectra show fast free carrier trapping and transient absorption spectra show the formation of self-trapped excitons on femtosecond time-scales. Emission-wavelength-dependent dynamics of the self-trapped exciton luminescence are observed, indicative of an energy distribution of photogenerated emissive states in the perovskite. Our results ...

Photo-Induced Bandgap Renormalization Governs the Ultrafast Response of Single-Layer MoS2

Abstract: Transition metal dichalcogenides (TMDs) are emerging as promising two-dimensional (2D) semiconductors for optoelectronic and flexible devices. However, a microscopic explanation of their photophysics, of pivotal importance for the understanding and optimization of device operation, is still lacking. Here, we use femtosecond transient absorption spectroscopy, with pump pulse tunability and broadband probing, to monitor the relaxation dynamics of single-layer MoS2 over the entire visible range, upon photoexcitation of different excitonic transitions. We find that, irrespective of excitation photon energy, the transient absorption spectrum shows the simultaneous bleaching of all excitonic transitions and corresponding red-shifted photoinduced absorption bands. First-principle modeling of the ultrafast optical response reveals that a transient bandgap renormalization, caused by the presence of photoexcited carriers, is primarily responsible for the observed features. Our results demonstrate the strong impact ...

Charge Generation Pathways in Organic Solar Cells: Assessing the Contribution from the Electron Acceptor

Abstract: Photocurrent generation in organic bulk heterojunction (BHJ) solar cells is most commonly understood as a process which predominantly involves photoexcitation of the lower ionization potential species (donor) followed by electron transfer to the higher electron affinity material (acceptor) [i.e., photoinduced electron transfer (PET), which we term Channel I]. A mirror process also occurs in which photocurrent is generated through photoexcitation of the acceptor followed by hole transfer to the nonexcited donor or photoinduced hole transfer (PHT), which we term Channel II. The role of Channel II photocurrent generation has often been neglected due to overlap of the individual absorption spectra of the donor and acceptor materials that are commonly used. More recently Channel II charge generation has been explored for several reasons. First, many of the new high-efficiency polymeric donors are used as the minority component in bulk heterojunction blends, and therefore, the acceptor absorption is a significa...

Activation Energies of Plasmonic Catalysts.

Abstract: The activation energy of a catalytic reaction serves not only as a metric of the efficacy of a catalyst but also as a potential indicator of mechanistic differences between the catalytic and noncatalytic reaction. However, activation energies are quite underutilized in the field of photocatalysis. We characterize in detail the effect of visible light excitation on the activation enthalpy of an electron transfer reaction photocatalyzed by plasmonic Au nanoparticles. We find that in the presence of visible light photoexcitation, the activation enthalpy of the Au nanoparticle-catalyzed electron transfer reaction is significantly reduced. The reduction in the activation enthalpy depends on the excitation wavelength, the incident laser power, and the strength of a hole scavenger. On the basis of these results, we argue that the activation enthalpy reduction is directly related to the photoelectrochemical potential built-up on the Au nanoparticle under steady-state light excitation, analogous to electrochemical...

Theoretical Insights into the Origin of Photoluminescence of Au25(SR)18- Nanoparticles

Abstract: Understanding fundamental behavior of luminescent nanomaterials upon photoexcitation is necessary to expand photocatalytic and biological imaging applications. Despite the significant amount of experimental work into the luminescence of Au25(SR)18– clusters, the origin of photoluminescence in these clusters still remains unclear. In this study, the geometric and electronic structural changes of the Au25(SR)18– (R = H, CH3, CH2CH3, CH2CH2CH3) nanoclusters upon photoexcitation are discussed using time-dependent density functional theory (TD-DFT) methods. Geometric relaxations in the optimized excited states of up to 0.33 A impart remarkable effects on the energy levels of the frontier orbitals of Au25(SR)18– nanoclusters. This gives rise to a Stokes shift of 0.49 eV for Au25(SH)18– in agreement with experiments. Even larger Stokes shifts are predicted for longer ligands. Vibrational frequencies in the 75–80 cm–1 range are calculated for the nuclear motion involved in the excited-state nuclear relaxation; th...

Cooperative photoinduced metastable phase control in strained manganite films

Abstract: Strain engineering can ‘hide’ the ordinal ferrometallic state in manganite films, pushing the system to a metastable state, which can then be controlled through photoexcitation.

Charge Separation and Recombination at Polymer–Fullerene Heterojunctions: Delocalization and Hybridization Effects

Abstract: We address charge separation and recombination in polymer/fullerene solar cells with a multiscale modeling built from accurate atomistic inputs and accounting for disorder, interface electrostatics and genuine quantum effects on equal footings. Our results show that bound localized charge transfer states at the interface coexist with a large majority of thermally accessible delocalized space-separated states that can be also reached by direct photoexcitation, thanks to their strong hybridization with singlet polymer excitons. These findings reconcile the recent experimental reports of ultrafast exciton separation ("hot" process) with the evidence that high quantum yields do not require excess electronic or vibrational energy ("cold" process), and show that delocalization, by shifting the density of charge transfer states toward larger effective electron-hole radii, may reduce energy losses through charge recombination.

Femtosecond X-Ray Scattering Study of Ultrafast Photoinduced Structural Dynamics in Solvated[Co(terpy)2]2$

Abstract: We study the structural dynamics of photoexcited [Co(terpy)_{2}]^{2+} in an aqueous solution with ultrafast x-ray diffuse scattering experiments conducted at the Linac Coherent Light Source. Through direct comparisons with density functional theory calculations, our analysis shows that the photoexcitation event leads to elongation of the Co-N bonds, followed by coherent Co-N bond length oscillations arising from the impulsive excitation of a vibrational mode dominated by the symmetrical stretch of all six Co-N bonds. This mode has a period of 0.33 ps and decays on a subpicosecond time scale. We find that the equilibrium bond-elongated structure of the high spin state is established on a single-picosecond time scale and that this state has a lifetime of ∼7 ps.

Panoramic portrait of primary molecular events preceding excited state proton transfer in water

Abstract: Photochemistry powers numerous processes from luminescence and human vision, to light harvesting However, the elucidation of multidimensional photochemical reaction coordinates on molecular timescales remains challenging We developed wavelength-tunable femtosecond stimulated Raman spectroscopy to simultaneously achieve pre-resonance enhancement for transient reactant and product species of the widely used photoacid pyranine undergoing excited-state proton transfer (ESPT) reaction in solution In the low-frequency region, the 280 cm-1 ring deformation mode following 400 nm photoexcitation exhibits pronounced intensity oscillations on the sub-picosecond timescale due to anharmonic vibrational coupling to the 180 cm-1 hydrogen-bond stretching mode only in ESPT-capable solvents, indicating a primary event of functional relevance This leads to the contact ion pair formation on the 3 ps timescale before diffusion-controlled separation The intermolecular 180 cm-1 mode also reveals vibrational cooling time constants, ∼500 fs and 45 ps in both H2O and D2O, which differ from ESPT time constants of ∼3/8 and 90/250 ps in H2O/D2O, respectively Spectral results using H218O further substantiate the functional role of the intermolecular 180 cm-1 mode in modulating the distance between proton donor and acceptor and forming the transient ion pair The direct observation of molecular structural evolution across a wide spectral region during photochemical reactions enriches our fundamental understanding of potential energy surface and holds the key to advancing energy and biological sciences with exceptional atomic and temporal precision

Organic Linker Defines the Excited-State Decay of Photocatalytic MIL-125(Ti)-Type Materials.

Abstract: Recently, MIL-125(Ti) and NH2 -MIL-125(Ti), two titanium-based metal-organic frameworks, have attracted significant research attention in the field of photocatalysis for solar fuel generation. This work reveals that the differences between these structures are not only based on their light absorption range but also on the decay profile and topography of their excited states. In contrast to MIL-125(Ti), NH2 -MIL-125(Ti) shows markedly longer lifetimes of the charge-separated state, which improves photoconversion by the suppression of competing decay mechanisms. We used spectroelectrochemistry and ultrafast spectroscopy to demonstrate that upon photoexcitation in NH2 -MIL-125(Ti) the electron is located in the Ti-oxo clusters and the hole resides on the aminoterephthalate unit, specifically on the amino group. The results highlight the role of the amino group in NH2 -MIL-125(Ti), the electron donation of which extends the lifetime of the photoexcited state substantially.

Photon recycling in Lead-Iodide Perovskite solar cells(Conference Presentation)

Abstract: We mapped the propagation of photogenerated luminescence and charges from a local photoexcitation spot in thin films of lead tri-iodide perovskites using a confocal microscopy setup with independent excitation and collection objectives. We observed regenerated PL emission at distances as far as 50 micrometers away from photoexcitation. We then made a scratch in the film to increase out-scattering and found that the peak of the internal photon spectrum red-shifts from 765 to ≥800 nanometers. This is caused by the sharp decay of the absorption coefficient at the band tail, which allows longer wavelength photons to travel further between emission and absorption events, originating charges far from excitation. We then built a lateral-contact solar cell with selective electron- and hole-collecting contacts, using a combination of photolitography and electrodeposition. We used these devices as a platform to study photocurrent propagation and found that charge extraction can be achieved well beyond 50 micrometers away from the excitation. We connect these two observations by comparing the decay in intensity of the recycled component of the PL (which is around 765 nm) with the decay in photocurrent. Taking into account that PL is proportional to the square of charge density, whilst photocurrent is proportional to charge density. Photon recycling leads to an increase in internal photon densities, which leads to a build-up of excited charges. This increases the split of quasi-Fermi levels and enhances the achievable open circuit voltage in a solar cell.

Generation of Transient Photocurrents in the Topological Surface State of Sb 2 Te 3 by Direct Optical Excitation with Midinfrared Pulses

Abstract: We combine tunable midinfrared (mid-IR) pump pulses with time- and angle-resolved two-photon photoemission to study ultrafast photoexcitation of the topological surface state (TSS) of Sb_{2}Te_{3}. It is revealed that mid-IR pulses permit a direct excitation from the occupied to the unoccupied part of the TSS across the Dirac point. The novel optical coupling induces asymmetric transient populations of the TSS at ±k_{∥}, which reflects a macroscopic photoexcited electric surface current. By observing the decay of the asymmetric population, we directly investigate the dynamics of the long-lived photocurrent in the time domain. Our discovery promises important advantages of photoexcitation by mid-IR pulses for spintronic applications.

Dichotomous Role of Exciting the Donor or the Acceptor on Charge Generation in Organic Solar Cells

Abstract: In organic solar cells, photoexcitation of the donor or acceptor phase can result in different efficiencies for charge generation. We investigate this difference for four different 2-pyridyl diketopyrrolopyrrole (DPP) polymer–fullerene solar cells. By comparing the external quantum efficiency spectra of the polymer solar cells fabricated with either [60]PCBM or [70]PCBM fullerene derivatives as acceptor, the efficiency of charge generation via donor excitation and acceptor excitation can both be quantified. Surprisingly, we find that to make charge transfer efficient, the offset in energy between the HOMO levels of donor and acceptor that govern charge transfer after excitation of the acceptor must be larger by ∼0.3 eV than the offset between the corresponding two LUMO levels when the donor is excited. As a consequence, the driving force required for efficient charge generation is significantly higher for excitation of the acceptor than for excitation of the donor. By comparing charge generation for a tot...

Evidence for near-infrared photoluminescence of nitrogen vacancy centers in 4 H -SiC

Abstract: We present evidence of near-infrared photoluminescence (PL) signature of nitrogen vacancy centers (NCVSi)− in silicon carbide (SiC). This center exhibits an S=1 ground state spin similar to the NV− center in diamond. We have performed photoluminescence excitation measurements at cryogenic temperature and demonstrated efficient photoexcitation of distinct photoluminescence from (NCVSi)− in 4H-SiC. Furthermore, by correlating the energies of measured zero phonon lines (ZPLs) with theoretical values derived from hybrid density functional theory each of the ZPLs has been associated to the respective occupation of hexagonal (h) and quasicubic (k) lattice sites in close analogy to neutral divacancy centers (VCVSi)0 in the same material. Finally, with the appropriate choice of excitation energy we demonstrated the selective excitation of (NCVSi)− PL with no contamination by (VCVSi)0 PL, thereby opening the way towards the optical detection of (NCVSi)− electron spin resonance.

Photoinduced gap closure in an excitonic insulator

Abstract: We study the dynamical phase transition out of an excitonic insulator phase after photoexcitation using a time-dependent extension of the self-consistent GW method. We connect the evolution of the photoemission spectra to the dynamics of the excitonic order parameter and identify two dynamical phase transition points marked by a slowdown in the relaxation: one critical point is connected with the trapping in a nonthermal state with reduced exciton density and the second corresponds to the thermal phase transition. The transfer of kinetic energy from the photoexcited carriers to the exciton condensate is shown to be the main mechanism for the gap melting. We analyze the low energy dynamics of screening, which strongly depends on the presence of the excitonic gap, and argue that it is difficult to interpret the static component of the screened interaction as the effective interaction of some low energy model. Instead we propose a phenomenological measure for the effective interaction which indicates that screening has minor effects on the low energy dynamics.

Tuning of near- and far-field properties of all-dielectric dimer nanoantennas via ultrafast electron-hole plasma photoexcitation

Abstract: All-optical ultrafast signal modulation and routing by low-loss nanodevices is a crucial step towards an ultracompact optical chip with high performance. Here, we propose a specifically designed silicon dimer nanoantenna, which is tunable via photoexcitation of dense electron-hole plasma with ultrafast relaxation rate. On the basis of this concept, we demonstrate the effect of beam steering by up to 20 degrees through simple variation of the intensity of incident light. The effect, which is suitable for ultrafast light routing in an optical chip, is demonstrated both in the visible and near-IR spectral regions for silicon- and germanium-based nanoantennas. We also reveal the effect of electron-hole plasma photoexcitation on the local density of states (LDOS) in the dimer gap and find that the orientation averaged LDOS can be altered by 50%, whereas modification of the projected LDOS can be even more dramatic, almost five-fold for transverse dipole orientation. Moreover, our analytical model sheds light on the transient dynamics of the studied nonlinear nanoantennas, yielding all temporal characteristics of the suggested ultrafast nanodevice. The proposed concept paves the way to the creation of low-loss, ultrafast, and compact devices for optical signal modulation and routing.

Interfacial Charge-Carrier Trapping in CH3NH3PbI3-Based Heterolayered Structures Revealed by Time-Resolved Photoluminescence Spectroscopy

Abstract: The fast-decaying component of photoluminescence (PL) under very weak pulse photoexcitation is dominated by the rapid relaxation of the photoexcited carriers into a small number of carrier-trapping defect states. Here, we report the subnanosecond decay of the PL under excitation weaker than 1 nJ/cm2 both in CH3NH3PbI3-based heterostructures and bare thin films. The trap-site density at the interface was evaluated on the basis of the fluence-dependent PL decay profiles. It was found that high-density defects determining the PL decay dynamics are formed near the interface between CH3NH3PbI3 and the hole-transporting Spiro-OMeTAD but not at the CH3NH3PbI3/TiO2 interface and the interior regions of CH3NH3PbI3 films. This finding can aid the fabrication of high-quality heterointerfaces, which are required improving the photoconversion efficiency of perovskite-based solar cells.

Photoexcitation Circular Dichroism in Chiral Molecules

Abstract: Chirality is ubiquitous in nature and fundamental in science, from particle physics to metamaterials.The most established technique of chiral discrimination - photoabsorption circular dichroism - relies on the magnetic properties of a chiral medium and yields an extremely weak chiral response. We propose and demonstrate a new, orders of magnitude more sensitive type of circular dichroism in neutral molecules: photoexitation circular dichroism. It does not rely on weak magnetic effects, but takes advantage of the coherent helical motion of bound electrons excited by ultrashort circularly polarized light. It results in an ultrafast chiral response and the efficient excitation of a macroscopic chiral density in an initially isotropic ensemble of randomly oriented chiral molecules. We probe this excitation without the aid of further chiral interactions using linearly polarized laser pulses. Our time-resolved study of vibronic chiral dynamics opens a way to the efficient initiation, control and monitoring of chiral chemical change in neutral molecules at the level of electrons.

Exciton and charge carrier dynamics in few-layer WS2

Abstract: Semiconducting transition metal dichalcogenides (TMDs) have been applied as the active layer in photodetectors and solar cells, displaying substantial charge photogeneration yields. However, their large exciton binding energy, which increases with decreasing thickness (number of layers), as well as the strong resonance peaks in the absorption spectra suggest that excitons are the primary photoexcited states. Detailed time-domain studies of the photoexcitation dynamics in TMDs exist mostly for MoS2. Here, we use femtosecond optical spectroscopy to study the exciton and charge dynamics following impulsive photoexcitation in few-layer WS2. We confirm excitons as the primary photoexcitation species and find that they dissociate into charge pairs with a time constant of about 1.3 ps. The better separation of the spectral features compared to MoS2 allows us to resolve a previously undetected process: these charges diffuse through the samples and get trapped at defects, such as flake edges or grain boundaries, causing an appreciable change of their transient absorption spectra. This finding opens the way to further studies of traps in TMD samples with different defect contents.

Ultrafast Dynamics of Metal Plasmons Induced by 2D Semiconductor Excitons in Hybrid Nanostructure Arrays

Abstract: With the advanced progress achieved in the field of nanotechnology, localized surface plasmon resonances are actively considered to improve the efficiency of metal-based photocatalysis, photodetection, and photovoltaics. Here, we report on the exchange of energy and electric charges in a hybrid composed of a two-dimensional tungsten disulfide (2D-WS2) monolayer and an array of aluminum (Al) nanodisks. Femtosecond pump–probe spectroscopy results indicate that within ∼830 fs after photoexcitation of the 2D-WS2 semiconductor energy transfer from the 2D-WS2 excitons excites the plasmons of the Al array. Then, upon the radiative and/or nonradiative damping of these excited plasmons, energy and/or electron transfer back to the 2D-WS2 semiconductor takes place as indicated by an increase in the reflected probe at the 2D-exciton transition energies at later time delays. This simultaneous exchange of energy and charges between the metal and the 2D-WS2 semiconductor resulted in an extension of the average lifetime ...

ZnO Luminescence and scintillation studied via photoexcitation, X-ray excitation, and gamma-induced positron spectroscopy.

Abstract: The luminescence and scintillation properties of ZnO single crystals were studied by photoluminescence and X-ray-induced luminescence (XRIL) techniques. XRIL allowed a direct comparison to be made between the near-band emission (NBE) and trap emissions providing insight into the carrier recombination efficiency in the ZnO crystals. It also provided bulk luminescence measurements that were not affected by surface states. The origin of a green emission, the dominant trap emission in ZnO, was then investigated by gamma-induced positron spectroscopy (GIPS) - a unique defect spectroscopy method that enables positron lifetime measurements to be made for a sample without contributions from positron annihilation in the source materials. The measurements showed a single positron decay curve with a 175 ps lifetime component that was attributed to Zn vacancies passivated by hydrogen. Both oxygen vacancies and hydrogen-decorated Zn vacancies were suggested to contribute to the green emission. By combining scintillation measurements with XRIL, the fast scintillation in ZnO crystals was found to be strongly correlated with the ratio between the defect luminescence and NBE. This study reports the first application of GIPS to semiconductors, and it reveals the great benefits of the XRIL technique for the study of emission and scintillation properties of materials.

Multiple Charge Transfer Dynamics in Colloidal CsPbBr3 Perovskite Quantum Dots Sensitized Molecular Adsorbate

Abstract: Ultrafast charge-transfer (CT) dynamics has been verified in CsPbBr3 (CPB) quantum dot (QD)–4,5-dibromofluorescein (DBF) composite materials, which form a strong CT complex in the ground state and can absorb more photons in the red region of the solar spectrum. Cyclic voltammetry and steady state luminescence studies suggest that the conduction (CB) and valence bands (VB) of CPB lie, respectively, below the LUMO and the HOMO of the DBF molecule. Steady state and time-resolved luminescence measurements with selective photoexcitation reveal the photoexcited hole transfer from CPB QDs to the DBF molecule, which is thermodynamically viable. Additionally, a red-shifted PL band was detected upon excitation of the CT complex that has been attributed to CT luminescence. Femtosecond transient absorption measurements have been performed to measure the hole transfer and direct electron transfer processes in the composite system and have been measured to be 1–1.25 ps and <100 fs, respectively. Dual behavior of the DB...

Photoinduced Enhancement of the Charge Density Wave Amplitude.

Abstract: Symmetry breaking and the emergence of order is one of the most fascinating phenomena in condensed matter physics. It leads to a plethora of intriguing ground states found in antiferromagnets, Mott insulators, superconductors, and density-wave systems. Exploiting states of matter far from equilibrium can provide even more striking routes to symmetry-lowered, ordered states. Here, we demonstrate for the case of elemental chromium that moderate ultrafast photoexcitation can transiently enhance the charge-density-wave (CDW) amplitude by up to 30% above its equilibrium value, while strong excitations lead to an oscillating, large-amplitude CDW state that persists above the equilibrium transition temperature. Both effects result from dynamic electron-phonon interactions, providing an efficient mechanism to selectively transform a broad excitation of the electronic order into a well-defined, long-lived coherent lattice vibration. This mechanism may be exploited to transiently enhance order parameters in other systems with coupled degrees of freedom.

Sub-10 fs Time-Resolved Vibronic Optical Microscopy

Abstract: We introduce femtosecond wide-field transient absorption microscopy combining sub-10 fs pump and probe pulses covering the complete visible (500–650 nm) and near-infrared (650–950 nm) spectrum with diffraction-limited optical resolution. We demonstrate the capabilities of our system by reporting the spatially- and spectrally-resolved transient electronic response of MAPbI3–xClx perovskite films and reveal significant quenching of the transient bleach signal at grain boundaries. The unprecedented temporal resolution enables us to directly observe the formation of band-gap renormalization, completed in 25 fs after photoexcitation. In addition, we acquire hyperspectral Raman maps of TIPS pentacene films with sub-400 nm spatial and sub-15 cm–1 spectral resolution covering the 100–2000 cm–1 window. Our approach opens up the possibility of studying ultrafast dynamics on nanometer length and femtosecond time scales in a variety of two-dimensional and nanoscopic systems.

Large range modification of exciton species in monolayer WS 2 .

Abstract: Unconventional emissions from excitons and trions in monolayer WS2 are studied by photoexcitation. When excited by a 532 nm laser beam, the carrier species in the monolayer WS2 are affected by the excess electrons escaping from photoionization of donor impurity, the concentration of which varies with different locations of the sample. Simply by increasing the excitation power at room temperature, the excess electrons and, thus, the intensity ratio of excited trions and excitons can be continuously tuned over a large range from 0.1 to 7.7. Furthermore, this intensity ratio can also be manipulated by varying temperature. However, in this way, the resonance energy of the excitons and trions shows redshifts with increasing temperature due to electron–phonon coupling. The binding energy of the trion is determined to be ∼26 meV and independent of temperature, indicating strong Coulomb interaction of carriers in such 2D materials.

Ultrafast dynamics of formation and autodetachment of a dipole-bound state in an open-shell π-stacked dimer anion

Abstract: Isolated π-stacked dimer radical anions present the simplest model of an excess electron in a π-stacked environment. Here, frequency-, angle-, and time-resolved photoelectron imaging together with electronic structure calculations have been used to characterise the π-stacked coenzyme Q0 dimer radical anion and its exited state dynamics. In the ground electronic state, the excess electron is localised on one monomer with a planar para-quinone ring, which is solvated by the second monomer in which carbonyl groups are bent out of the para-quinone ring plane. Through the π-stacking interaction, the dimer anion exhibits a number of charge-transfer (intermolecular) valence-localised resonances situated in the detachment continuum that undergo efficient internal conversion to a cluster dipole-bound state (DBS) on a ∼60 fs timescale. In turn, the DBS undergoes vibration-mediated autodetachment on a 2.0 ± 0.2 ps timescale. Experimental vibrational structure and supporting calculations assign the intermolecular dynamics to be facilitated by vibrational wagging modes of the carbonyl groups on the non-planar monomer. At photon energies ∼0.6–1.0 eV above the detachment threshold, a competition between photoexcitation of an intermolecular resonance leading to the DBS, and photoexcitation of an intramolecular resonance leading to monomer-like dynamics further illustrates the π-stacking specific dynamics. Overall, this study provides the first direct observation of both internal conversion of resonances into a DBS, and characterisation of a vibration-mediated autodetachment in real-time.

Large Range Manipulation of Exciton Species in Monolayer WS2

Abstract: Unconventional emissions from exciton and trion in monolayer WS2 are studied by photoexcitation. Excited by 532nm laser beam, the carrier species in the monolayer WS2 are affected by the excess electrons escaping from photoionization of donor impurity, the concentration of which varies with different locations of the sample. Simply increasing the excitation power at room temperature, the excess electron and thus the intensity ratio of excited trion and exciton can be continuously tuned over a large range from 0.1 to 7.7. Furthermore, this intensity ratio can also be manipulated by varying temperature. However, in this way the resonance energy of the exciton and trion show red-shifts with increasing temperature due to electron-phonon coupling. The binding energy of the trion is determined to be ~23meV and independent to temperature, indicating strong Coulomb interaction of carriers in such 2D materials.

Photogeneration and Mobility of Charge Carriers in Atomically Thin Colloidal InSe Nanosheets Probed by Ultrafast Terahertz Spectroscopy

Abstract: The implementation of next generation ultrathin electronics by applying highly promising dimensionality-dependent physical properties of two-dimensional (2D) semiconductors is ever increasing. In this context, the van der Waals layered semiconductor InSe has proven its potential as photodetecting material with high charge carrier mobility. We have determined the photogeneration charge carrier quantum yield and mobility in atomically thin colloidal InSe nanosheets (inorganic layer thickness 0.8–1.7 nm, mono/double-layers, ≤ 5 nm including ligands) by ultrafast transient terahertz (THz) spectroscopy. A near unity quantum yield of free charge carriers is determined for low photoexcitation density. The charge carrier quantum yield decreases at higher excitation density due to recombination of electrons and holes, leading to the formation of neutral excitons. In the THz frequency domain, we probe a charge mobility as high as 20 ± 2 cm2/(V s). The THz mobility is similar to field-effect transistor mobilities ex...

Tuning of Near- and Far-Field Properties of All-dielectric Dimer Nanoantennas via Ultrafast Electron-Hole Plasma Photoexcitation

Abstract: Achievement of all-optical ultrafast signal modulation and routing by a low-loss nanodevice is a crucial step towards an ultracompact optical chip with high performance. Here, we propose a specifically designed silicon dimer nanoantenna, which is tunable via photoexcitation of dense electron-hole plasma with ultrafast relaxation rate. Basing on this concept, we demonstrate the effect of beam steering up to 20 degrees via simple variation of incident intensity, being suitable for ultrafast light routing in an optical chip. The effect is demonstrated both in the visible and near-IR spectral regions for silicon and germanium based nanoantennas. We also reveal the effect of electron-hole plasma photoexcitation on local density of states (LDOS) in the dimer gap and find that the orientation averaged LDOS can be altered by 50\%, whereas modification of the projected LDOS can be even more dramatic: almost 500\% for transverse dipole orientation. Moreover, our analytical model sheds light on transient dynamics of the studied nonlinear nanoantennas, yielding all temporal characteristics of the proposed ultrafast nanodevice. The proposed concept paves the ways to creation of low-loss, ultrafast, and compact devices for optical signal modulation and routing.