Femtosecond Broadband Pump-probe Spectroscopy Research Articles

Ultrafast Plasmonic Hot Hole Transfer and Plasmon Dynamics in a Dual Plasmonic Au@p‐Cu2−xSe Heterostructure

AbstractMetal‐semiconductor heterostructures have significant potential in harvesting plasmon induced hot charge carriers for photocatalytic and photovoltaic devices. Despite numerous studies, the field of hot hole transfer in heterostructures remains unexplored. In this work, the plasmon dynamics of a dual plasmonic core–shell Au@p‐Cu2−xSe heterostructure are investigated with the aid of femtosecond broadband pump probe spectroscopy. Ultrafast studies reveal that hot holes generated within the d and sp band after 400 and 800 nm excitations are injected from Au to copper selenide. Hot hole transfer substantially reduces the transient signal intensity and electron phonon relaxation time for Au plasmon. Increment in the transient signal intensity and slower kinetics for the semiconducting plasmon of Au@Cu2−xSe validate the hot hole transfer in the valence band of copper selenide. Fluence dependent studies reveal that d band hot holes extraction is more efficient than that of sp band hot holes.

Temperature-Dependent Ultrafast Charge Carrier Dynamics in Amorphous and Crystalline Sb2Se3 Thin Films

The Earth abundant polycrystalline antimony selenide (Sb2Se3) thin film is considered to be a potential photovoltaic material for their appropriate band gap, higher absorption coefficient, and nontoxicity. Interband gap defect states play a major role for efficient solar devices, which depends on the crystallinity of the materials. In the present investigation, we have synthesized an amorphous Sb2Se3 thin film by following the thermal evaporation technique and have also converted the film to crystalline after vacuum annealing it at 250 °C. Both amorphous and crystalline thin films were characterized by several analytical techniques, XRD, FE-SEM, optical absorption, XPS, and UPS spectroscopy. The interband gap defect density was found to be much higher in the amorphous thin film as compared to that of the crystalline one. To understand the charge carrier dynamics of the photoexcited Sb2Se3 thin film, femtosecond broad-band pump–probe spectroscopy has been employed at cryogenic temperature (5 K) and room temperature (300 K). Carrier trapping and recombination dynamics for both the films were found to be faster at 5 K than at 300 K and were attributed to higher carrier mobility at high temperatures. Understanding carrier dynamics of this photovoltaic materials are immensely useful for designing efficient devices.

Effect of Confinement on the Exciton and Biexciton Dynamics in Perovskite 2D-Nanosheets and 3D-Nanocrystals.

The performance of the high-end optoelectronic devices is essentially influenced by the intrinsic relaxation mechanisms pursued by the hot carriers. Therefore, the key toward achieving progression in such fields lies in developing a complete understanding of the involved carrier cooling dynamics. In this work, an endeavor has been made to highlight the difference in the cooling mechanisms in 2D CsPbBr3 nanosheets (NSs) and their 3D counterpart nanocrystals (NCs) with the aid of femtosecond broad-band pump-probe spectroscopy, varying the excitation energies. The exciton and biexciton dynamics in both systems are found to be retarded upon increasing the excitation energy. However, in contrast to 3D NCs, carrier cooling is found to be faster in the 2D system, regardless of the excitation energy used, attributing this to less efficient charge screening by Fröhlich interaction in low-dielectric medium. A similar trend is replicated in the biexciton formation rate since the formation is also found to be faster in NSs compared to NCs.

Dynamics of exciton energy renormalization in monolayer transition metal disulfides

Fundamental understandings on the dynamics of charge carriers and excitonic quasiparticles in semiconductors are of central importance for both many-body physics and promising optoelectronic and photonic applications. Here, we investigated the carrier dynamics and many-body interactions in two-dimensional (2D) transition metal dichalcogenides (TMDs), using monolayer WS2 as an example, by employing femtosecond broadband pump-probe spectroscopy. Three time regimes for the exciton energy renormalization are unambiguously revealed with a distinct red-blue-red shift upon above-bandgap optical excitations. We attribute the dominant physical process in the three typical regimes to free carrier screening effect, Coulombic exciton–exciton interactions and Auger photocarrier generation, respectively, which show distinct dependence on the optical excitation wavelength, pump fluences and/or lattice temperature. An intrinsic exciton radiative lifetime of about 1.2 picoseconds (ps) in monolayer WS2 is unraveled at low temperature, and surprisingly the efficient Auger nonradiative decay of both bright and dark excitons puts the system in a nonequilibrium state at the nanosecond timescale. In addition, the dynamics of trions at low temperature is observed to be significantly different from that of excitons, e.g., a long radiative lifetime of ~ 108.7 ps at low excitation densities and the evolution of trion energy as a function of delay times. Our findings elucidate the dynamics of excitonic quasiparticles and the intricate many-body physics in 2D semiconductors, underpinning the future development of photonics, valleytronics and optoelectronics based on 2D semiconductors.

The Role of Electronic and Phononic Excitation in the Optical Response of Monolayer WS2 after Ultrafast Excitation

Transient changes of the optical response of WS2 monolayers are studied by femtosecond broadband pump-probe spectroscopy. Time-dependent absorption spectra are analyzed by tracking the line width broadening, bleaching, and energy shift of the main exciton resonance as a function of time delay after the excitation. Two main sources for the pump-induced changes of the optical response are identified. Specifically, we find an interplay between modifications induced by many-body interactions from photoexcited carriers and by the subsequent transfer of the excitation to the phonon system followed by cooling of the material through the heat transfer to the substrate.

Excited-State Vibrational Coherence in Perylene Bisimide Probed by Femtosecond Broadband Pump-Probe Spectroscopy.

Broadband laser pulses with ultrashort duration are capable of triggering impulsive excitation of the superposition of vibrational eigenstates, giving rise to quantum beating signals originating from coherent wave packet motions along the potential energy surface. In this work, coherent vibrational wave packet dynamics of an N,N'-bis(2,6-dimethylphenyl)perylene bisimide (DMP-PBI) were investigated by femtosecond broadband pump-probe spectroscopy which features fast and balanced data acquisition with a wide spectral coverage of >200 nm. Clear modulations were observed in the envelope of the stimulated emission decay profiles of DMP-PBI with the oscillation frequencies of 140 and 275 cm(-1). Fast Fourier transform analysis of each oscillatory mode revealed characteristic phase jumps near the maxima of the steady-state fluorescence, indicating that the observed vibrational coherence originates from an excited-state wave packet motion. Quantum calculations of the normal modes at the low-frequency region suggest that low-frequency C-C (C═C) stretching motions accompanied by deformation of the dimethylphenyl substituents are responsible for the manifestation of such coherent wave packet dynamics.

Femtosecond OH Bending Dynamics of Water Nanopools Confined In Reverse Micelles

The relaxation of OH bend excitations of H2O confined in reverse micelles is studied by femtosecond broadband pump-probe spectroscopy for the two ionic systems dioctyl sodium sulfosuccinate (AOT) and dioleoylphosphatidylcholine (DOPC) and for the nonionic tetraethylene glycol dodecyl ether (Brij-30). In the ionic AOT/DOPC reverse micelles, the OH bending lifetime T1 decreases from T1 > 615 fs for a 3:1 ratio of water and AOT/DOPC molecules (w0 = 3) to T1 = 345 fs for a 16:1 ratio (w0 = 16). In contrast, H2O in Brij-30 reverse micelles shows a much shorter T1 = 400 fs at w0 = 2 which decreases to T1 = 250 fs at w0 = 8. OH bend relaxation proceeds mainly via librational overtones of the bend-excited water molecules with a rate correlating with the energy mismatch between the v = 1 OH bend state and the librational overtone. In the ionic systems, the lower librational frequencies at small w0 result in a larger mismatch and longer T1 times. In the nonionic case, the w0-independent librational frequencies with a small energy mismatch lead to shorter T1 times. For w0 ≥ 8, the energy flow into the first hydration shell of the bend-excited molecules makes an additional contribution to the relaxation rate in all systems.

Getting to guanine: mechanism and dynamics of charge separation and charge recombination in DNA revisited

The mechanism and dynamics of charge separation and charge recombination in synthetic DNA hairpins possessing a stilbenedicarboxamide linker and a single guanine-cytosine base pair have been reinvestigated. The combination of femtosecond broad-band pump probe spectroscopy, nanosecond transient absorption experiments, and picosecond fluorescence decay measurements permits analysis of the formation and decay of the stilbene anion radical. Reversible hole injection resulting in the formation of the stilbene-adenine contact radical ion pair is found to occur on the picosecond time scale. The mechanism for charge separation across two or more base pairs is revised from single step superexchange to a multi-step process: hole injection followed by hole transport and hole trapping. The mechanism of charge recombination remains assigned to a superexchange process.

Dynamics and Mechanism of Bridge-Dependent Charge Separation in Pyrenylurea−Nitrobenzene π-Stacked Protophanes

Herein are reported the synthesis, structure, and electronic properties of a series of tertiary di- and polyarylureas possessing pyrene and nitrobenzene end groups separated by a variable number of internal phenylenediamine bridging groups. These molecules adopt folded "protophane" structures in which the adjacent arenes are loosely pi-stacked. The behavior of both the pyrene and nitrobenzene singlet states has been investigated by means of femtosecond broadband pump-probe spectroscopy, and the transients have been assigned on the basis of comparison to reference molecules. Femtosecond time resolution permits direct observation of the fast internal conversion process for both the pyrene and nitrobenzene upper singlet states, as well as the intersystem crossing of nitrobenzene. The ultrafast (ca. 100 fs) charge separation of the donor-acceptor urea having no bridging group is attributed to an internal conversion process. The slower charge separation and charge recombination of the donor-acceptor urea having a single bridging group occur via a bridge-mediated superexchange process. Addition of a second bridging unit results in a role reversal for the pyrene singlet state, which now serves as an excited-state acceptor with the bridging units serving as the electron donors. The change in the directionality of electron transfer upon addition of a second bridging phenylenediamine is a consequence of a decrease in the bridge oxidation potential as well as a decrease in the rate constant for single-step superexchange electron transfer.

Crossover from Superexchange to Hopping as the Mechanism for Photoinduced Charge Transfer in DNA Hairpin Conjugates

The mechanism and dynamics of photoinduced charge separation and charge recombination have been investigated in synthetic DNA hairpins possessing donor and acceptor stilbenes separated by one to seven A:T base pairs. The application of femtosecond broadband pump-probe spectroscopy, nanosecond transient absorption spectroscopy, and picosecond fluorescence decay measurements permits detailed analysis of the formation and decay of the stilbene acceptor singlet state and of the charge-separated intermediates. When the donor and acceptor are separated by a single A:T base pair, charge separation occurs via a single-step superexchange mechanism. However, when the donor and acceptor are separated by two or more A:T base pairs, charge separation occurs via a multistep process consisting of hole injection, hole transport, and hole trapping. In such cases, hole arrival at the electron donor is slower than hole injection into the bridging A-tract. Rate constants for charge separation (hole arrival) and charge recombination are dependent upon the donor-acceptor distance; however, the rate constant for hole injection is independent of the donor-acceptor distance. The observation of crossover from a superexchange to a hopping mechanism provides a "missing link" in the analysis of DNA electron transfer and requires reevaluation of the existing literature for photoinduced electron transfer in DNA.

Photoinduced Charge Transfer Processes along Triarylamine Redox Cascades

In this paper, we describe the synthesis and photophysical properties of a series of acridine-triarylamine redox cascades. These cascades were designed in order to promote photoinduced hole transfer from an acridine fluorophore into an adjacent triarylamine. The excited dipolar state then injects a hole into the triarylamine redox cascade. Subsequently, the hole migrates along the redox gradient which was tuned by the substituents attached to the triarylamine redox centers. The rate of hole migration was determined by fluorescence lifetime measurements and is in the ns regime and depends strongly on the solvent polarity. The photophysical processes were also investigated by femtosecond broadband pump-probe spectroscopy. Our studies reveal different dynamic processes in the cascades depending on the solvent polarity, e.g., direct charge separation after photoexcitation vs a two step hole transfer mechanism.

Femtosecond probing of the excited state absorption and structural relaxation in biphenyl derivatives

The excited state absorption spectra of biphenyl and 2,5-dimethyl biphenyl have been investigated by femtosecond broad band pump–probe spectroscopy in organic solvents. The results show vastly different intensities that were analyzed by calculating transition density surfaces for characteristic S 1→S n absorptions. The analysis revealed strong symmetry-lowering substituent effects in 2,5-dimethyl biphenyl which directly affect the transition dipole moments for excited state absorption. By measuring the temporal evolution of the transient spectra of the biphenyl derivatives we observed a biphasic relaxation mechanism in the excited state characterized by two distinct time scales of 400 fs and 12 ps.

Femtosecond dynamics in directly linked pyrenyl donor–acceptor systems: orbital control of optical charge transfer in the excited state

The ultrafast charge transfer (CT) dynamics in two directly linked pyrenyl donor–acceptor systems has been studied using femtosecond broadband pump–probe spectroscopy. This method allows to capture complex chemical dynamics by measuring the time-resolved spectral evolution of transient absorption and stimulated emission induced by femtosecond optical pumping. After electronic relaxation, an optical CT transition between two excited states was found in both systems. Although the excited state CT occurs adiabatically significant differences in the electronic coupling are deduced from the band intensities. The results are discussed in terms of an adiabatic state model and compared with predictions from orbital topology arguments.

Laser-Induced Nonequilibrium Electron Distribution in Metals on a Femtosecond Time Scale

A nonequilibrium distribution function for electrons in metals excited by ultrashort laser pulses and its relaxation dynamics is analyzed. This distribution function is based on a kinetic differential equation for the nonequilibrium population of excited electrons. The deviation of the distribution function from the equilibrium one can be monitored by femtosecond broadband pump-probe spectroscopy. Transient changes of the dielectric function Δε and the relaxation of nonequilibrium charge carriers in a thin gold film are discussed for different excitation energies ℏωpump that corresponds to intra- or interband excitation or to photoemission of the excited electrons.