Ultrafast Charge Dynamics Research Articles

NiO thin films fabricated using spray-pyrolysis technique: structural and optical characterization and ultrafast charge dynamics studies

Nickel (II) oxide, NiO, is a wide band gap Mott insulator characterized by strong Coulomb repulsion between d-electrons and displays antiferromagnetic order at room temperature. NiO has gained attention in recent years as a very promising candidate for applications in a broad set of areas, including chemistry and metallurgy to spintronics and energy harvesting. Here, we report on the fabrication of polycrystalline NiO using spray-pyrolysis technique, which is a deposition technique able to produce quite uniform films of pure and crystalline materials without the need of high vacuum or inert atmospheres. The composition and structure of the NiO thin films were then studied using x-ray diffraction, and atomic force and scanning electron microscopies (SEM). The phononic and magnonic properties of the NiO thin films were also studied via Raman spectroscopy, and the ultrafast electron dynamics by using optical pump probe spectroscopy. We found that the NiO samples display the same phonon and magnon excitations expected for single crystal NiO at room temperature, and that electron dynamics in our system is like those of previously reported NiO mono- and polycrystalline systems synthesized using different techniques. These results prove that spray-pyrolysis can be used as affordable and large-scale fabrication technique to synthesize strongly correlated materials for a large set of applications.

Mapping Light-Dressed Floquet Bands by Highly Nonlinear Optical Excitations and Valley Polarization.

Ultrafast nonlinear optical phenomena in solids have been attracting a great deal of interest as novel methodologies for the femtosecond spectroscopy of electron dynamics and control of the properties of materials. Here, we theoretically investigate strong-field nonlinear optical transitions in a prototypical two-dimensional material, hBN, and show that the k-resolved conduction band charge occupation patterns induced by an elliptically polarized laser can be understood in a multiphoton resonant picture, but, remarkably, only if using the Floquet light-dressed states instead of the undressed matter states. Our work demonstrates that Floquet dressing affects ultrafast charge dynamics and photoexcitation even from a single pump pulse and establishes a direct measurable signature for band dressing in nonlinear optical processes in solids, opening new paths for ultrafast spectroscopy and valley manipulation.

Watching the Interplay between Photoinduced Ultrafast Charge Dynamics and Nuclear Vibrations.

Here is presented the ultrafast hole-electron dynamics of photoinduced metal to ligand charge-transfer (MLCT) states in a Ru(II) complex, [Ru(dcbpy)2(NCS)2]4- (dcbpy = 4,4'-dicarboxy-2,2'-bipyridine), a photoactive molecule employed in dye sensitized solar cells. Via cutting-edge computational techniques, a tailored computational protocol is here presented and developed to provide a detailed analysis of the electronic manifold coupled with nuclear vibrations to better understand the nonradiative pathways and the resulting overall dye performances in light-harvesting processes (electron injection). Thus, the effects of different vibrational modes were investigated on both the electronic levels and charge transfer dynamics through a theoretical-computational approach. First, the linear response time-dependent density functional (LR-TDDFT) formalism was employed to characterize excitation energies and spacing among electronic levels (the electronic layouts). Then, to understand the ultrafast (femtosecond) charge dynamics on the molecular scale, we relied on the nonperturbative mean-field quantum electronic dynamics via real-time (RT-) TDDFT. Three vibrational modes were selected, representative for collective nuclear movements that can have a significant influence on the electronic structure: two involving NCS- ligands and one involving dcbpy ligands. As main results, we observed that such MLCT states, under vibrational distortions, are strongly affected and a faster interligand electron transfer mechanism is observed along with an increasing MLCT character of the adiabatic electronic states approaching closer in energy due to the vibrations. Such findings can help both in providing a molecular picture of multidimensional vibro-electronic spectroscopic techniques, used to characterize ultrafast coherent and noncoherent dynamics of complex systems, and to improve dye performances with particular attention to the study of energy or charge transport processes and vibronic couplings.

Ultrafast Charge Dynamics in Bulk Zinc Oxide under Intense Photoexcitation

The photo-induced charge dynamics of textbook wide-bandgap semiconductor ZnO have been investigated on the picosecond time-scale. We performed optical Pump-THz Probe experiments in order to measure the dielectric constant of the material after high-fluence photo-excitation of charge carriers. The technique allows access to both carrier lifetime and scattering rates, and it provides direct access to the intrinsic dielectric function changes upon excitation. A complex dynamic is unveiled in the high-fluence pumping regime, where the relaxation time is in the hundreds of picoseconds range and increases with increasing Pump fluence, while the onset of photoconductivity takes place in a few picoseconds. The plasma frequency and the relaxation time dependence on the Pump fluence are discussed.

Manipulating Attosecond Charge Migration in Molecules by Optical Cavities.

The ultrafast electronic charge dynamics in molecules upon photoionization while the nuclear motions are frozen is known as charge migration. In a theoretical study of the quantum dynamics of photoionized 5-bromo-1-pentene, we show that the charge migration process can be induced and enhanced by placing the molecule in an optical cavity, and can be monitored by time-resolved photoelectron spectroscopy. The collective nature of the polaritonic charge migration process is investigated. We find that, unlike spectroscopy, molecular charge dynamics in a cavity is local and does not show many-molecule collective effects. The same conclusion applies to cavity polaritonic chemistry.

Effect of the shot-to-shot variation on charge migration induced by sub-fs x-ray free-electron laser pulses

X-ray free-electron lasers (XFELs) are now able to provide tunable pairs of intense sub-fs pulses in the soft x-ray regime, paving the way for time-resolved investigations of attosecond charge migration in molecules. However, the stochastic shot-to-shot variation of the XFEL pulses may degrade and eventually hide the observable features. We show by means of state-of-the-art calculations that the damping of the charge migration induced by 260 eV pulses in p-aminophenol due to the shot-to-shot variation of pulses generated at the Linac Coherent Light Source is negligible in comparison to the natural damping due to the intrinsic fluctuation of the initial molecular geometry. This result gives us confidence in the utility of XFEL sub-fs pulses for the measurement of charge migration and other ultrafast charge dynamics.

Magnetic field control of light-induced spin accumulation in monolayer MoSe2

Semiconductor transition metal dichalcogenides (TMDs) have equivalent dynamics for their two spin/valley species. This arises from their energy-degenerated spin states, connected via time-reversal symmetry. When an out-of-plane magnetic field is applied, time-reversal symmetry is broken and the energies of the spin-polarized bands shift, resulting in different bandgaps and dynamics in the K and K valleys. Here, we use time-resolved Kerr rotation to study the magnetic field dependence of the spin dynamics in monolayer MoSe2. We show that the magnetic field can control the light-induced spin accumulation of the two valley states, with a small effect on the recombination lifetimes. We unveil that the magnetic field-dependent spin accumulation is in agreement with hole spin dynamics at the longer timescales, indicating that the electron spins have faster relaxation rates. We propose a rate equation model that suggests that lifting the energy-degeneracy of the valleys induces an ultrafast spin-flip toward the stabilization of the valley with the higher valence band energy. Our results provide an experimental insight into the ultrafast charge and spin dynamics in TMDs and a way to control it, which will be useful for the development of new spintronic and valleytronic applications.

Manipulation of Magnetization Switching by Ultrafast Spin‐Polarized Hot‐Electron Transport in Synthetic Antiferromagnet

AbstractUncovering the physical mechanisms that govern ultrafast charge and spin dynamics is becoming indispensable both at the fundamental level and to develop future spin‐based electronics. Recently it has been shown that femtosecond pulsed‐laser excitation of magnetic thin films produces intense and ultrafast spin‐polarized hot electrons, thus attracting a lot of attention. While spin‐polarized hot electrons are known to play a pivotal role in the ultrafast laser‐induced demagnetization, their effect on magnetization switching remains an open issue. This study uncovers the effect of spin‐polarized hot electrons generated by laser excitation on magnetization switching in a Co/Pt based perpendicular magnetic anisotropy‐based synthetic antiferromagnet (p‐SAF) using the time‐resolved magneto‐optical Kerr effect. It has been found that, at low pump fluence, the equivalent magnetic field generated by the hot‐electron spin current plays a dominant role in assisting the magnetization switching of the lower layer in the antiferromagnetic configuration, while the strong thermal stability of the Ruderman Kittel Kasuya Yosida exchange interaction inhibits the further weakening of the switching field at high pump fluence. This study provides a viable way to control the magnetization switching of the antiferromagnetically exchange‐coupled systems for spintronic applications with ultrafast control of the information operation.

Spatio-temporal ultrafast pulse shaping at the femtosecond-nanometer scale.

Optical pulse shaping is a fundamental tool for coherent control of the light-matter interaction. While such control enables the measurement of ultrafast temporal dynamics, simultaneous spatiotemporal control is required for studying non-local ultrafast charge dynamics at the nanoscale. However, obtaining accurate spatial control at a sub-wavelength resolution with conventional optical elements poses significant difficulty. Here, we use the spatiotemporal coupling naturally arising in a spatial light modulator based pulse shaping apparatus to achieve accurate control with femto-nano spatiotemporal resolution. We experimentally demonstrate spatial steering at the sub-micron scale of second harmonic generation from nanostructures. In addition, we apply an absolute-value spectral phase to achieve controlled double pulses for nanoscale excitation. We introduce a novel, to the best of our knowledge, scheme for accurate tunable spatiotemporal pump-probe experiments. This method offers rich insight into materials with ultrafast transport phenomena at the femtosecond-nanometer regimes.

Ultrafast charge transfer coupled to quantum proton motion at molecule/metal oxide interface.

Understanding how the nuclear quantum effects (NQEs) in the hydrogen bond (H-bond) network influence the photoexcited charge transfer at semiconductor/molecule interface is a challenging problem. By combining two kinds of emerging molecular dynamics methods at the ab initio level, the path integral-based molecular dynamics and time-dependent nonadiabatic molecular dynamics, and choosing CH3OH/TiO2 as a prototypical system to study, we find that the quantum proton motion in the H-bond network is strongly coupled with the ultrafast photoexcited charge dynamics at the interface. The hole trapping ability of the adsorbed methanol molecule is notably enhanced by the NQEs, and thus, it behaves as a hole scavenger on titanium dioxide. The critical role of the H-bond network is confirmed by in situ scanning tunneling microscope measurements with ultraviolet light illumination. It is concluded the quantum proton motion in the H-bond network plays a critical role in influencing the energy conversion efficiency based on photoexcitation.

State-resolved ultrafast charge and spin dynamics in [Co/Pd] multilayers

We use transient absorption spectroscopy with circularly polarized x rays to detect laser-excited hole states below the Fermi level and compare their dynamics with that of unoccupied states above the Fermi level in ferromagnetic [Co/Pd] multilayers. While below the Fermi level, an instantaneous and significantly stronger demagnetization is observed, above the Fermi level, the demagnetization is delayed by 35 ± 10 fs. This provides a direct visualization of how ultrafast demagnetization proceeds via initial spin-flip scattering of laser-excited holes to the subsequent formation of spin waves.

Unravelling the ultrafast charge dynamics in PbS quantum dots through resonant Auger mapping of the sulfur K-edge.

There is a great fundamental interest in charge dynamics of PbS quantum dots, as they are promising for application in photovoltaics and other optoelectronic devices. The ultrafast charge transport is intriguing, offering insight into the mechanism of electron tunneling processes within the material. In this study, we investigated the charge transfer times of PbS quantum dots of different sizes and non-quantized PbS reference materials by comparing the propensity of localized or delocalized decays of sulfur 1s core hole states excited by X-rays. We show that charge transfer times in PbS quantum dots decrease with excitation energy and are similar at high excitation energy for quantum dots and non-quantized PbS. However, at low excitation energies a distinct difference in charge transfer time is observed with the fastest charge transfer in non-quantized PbS and the slowest in the smallest quantum dots. Our observations can be explained by iodide ligands on the quantum dots creating a barrier for charge transfer, which reduces the probability of interparticle transfer at low excitation energies. The probability of intraparticle charge transfer is limited by the density of available states which we describe according to a wave function in a quantum well model. The stronger quantum confinement effect in smaller PbS quantum dots is manifested as longer charge transfer times relative to the larger quantum dots at low excitation energies.

Ultrafast charge dynamics in molecular thin films

Conjugated molecular thin films have potential for novel optoelectronic devices such as flexible solar cells and display devices. However, a detailed understanding of their excited state dynamics is crucial for their technological applications. Tetrapyrrole complexes are an important class of conjugated organic molecules for optoelectronic applications due to their high light gathering ability and thermal stability. In this review, we summarize excited state dynamics of porphyrin and phthalocyanine molecular thin films investigated by pump–probe techniques. The relaxation dynamics for metallo-phthalocyanine molecules are observed to be ultrafast in nature and are in femtosecond time scale while that for metal free phthalocyanine are moderately fast. In case of metallo-phthalocyanines exciton lifetime is much longer than for metal free phthalocyanines. The central metal atom plays an important role in the excited state dynamics of porphyrin molecular thin films thus providing additional excitation channels. Time-resolved photoemission spectroscopy reveals about the evolution of charge transfer excitons at donor–acceptor interfaces and their dissociation mechanism. In case of metallo-phthalocyanines the exciton transport is dominated by hopping mechanism rather than spatial coherent length of excitons. The pump–probe studies provide a basis for appropriate selection of molecules for technological applications.

Controlled ultrafast \u03c0\u03c0*-\u03c0\u03c3* dynamics in tryptophan-based peptides with tailored micro-environment

Ultrafast charge, energy and structural dynamics in molecules are driven by the topology of the multidimensional potential energy surfaces that determines the coordinated electronic and nuclear motion. These processes are also strongly influenced by the interaction with the molecular environment, making very challenging a general understanding of these dynamics on a microscopic level. Here we use electrospray and mass spectrometry technologies to produce isolated molecular ions with a controlled micro-environment. We measure ultrafast photo-induced ππ*-πσ* dynamics in tryptophan species in the presence of a single, charged adduct. A striking increase of the timescale by more than one order of magnitude is observed when changing the added adduct atom. A model is proposed to rationalize the results, based on the localized and delocalized effects of the adduct on the electronic structure of the molecule. These results offer perspectives to control ultrafast molecular processes by designing the micro-environment on the Angström length scale.

Giant Photoluminescence Enhancement and Resonant Charge Transfer in Atomically Thin Two-Dimensional Cr2Ge2Te6/WS2 Heterostructures.

Hybridization of two-dimensional (2D) magnetic semiconductors with transition-metal dichalcogenides (TMDC) monolayers can significantly engineer the light-matter interactions and provide a promising platform for enhanced excitonic systems with artificially tailored band alignments. Here, we report the fabrication of heterostructures with monolayer WS2 on 2D Cr2Ge2Te6 (CGT), which displayed giant photoluminescence enhancement at specific CGT layer numbers. The highly enhanced quantum yield obtained can be explained by novel photoexcited carrier dynamics, facilitated by alternate relaxation channels, resulting in resonance charge transfer at the heterointerface. 2D CGT revealed a strongly layer-dependent work function (up to ∼750 meV), which greatly modulates the band positioning in the heterostructure. These heterostructures conceived both type I and type II band alignments, which are verified by Kelvin probe force microscopy and PL measurements. In addition to layer modulation, we uncover temperature and power dependence of the resonance charge transfer in the multilayer heterostructure. Our findings provide further insights into the ultrafast charge dynamics occurring at the atomic interfaces. The results may pave the way for novel optoelectronics based on van der Waals heterostructures.

Ultrafast charge dynamics and photoluminescence in bilayer MoS2

Our examination of the interplay of ultrafast charge dynamics and electron–phonon interaction in the AA′ stacked bilayer MoS2 provides a microscopic basis for understanding the features (two peaks) in the emission spectrum. We demonstrate that while the initial accumulation of excited charge occurs at and near the Q point of the two-dimensional Brillioun zone, emission takes place predominantly through two pathways: direct charge recombination at the K point and indirect phonon-assisted recombination of electrons at the valley and holes at the hill of the Brillouin zone. Analysis of the wave vector dependencies of the electron–phonon interaction traces the higher energy peak to phonon-assisted relaxation of the excited electrons from the Q to the K valley in the conduction band. Our results thus reveal the importance of ultrafast charge dynamics in understanding photoemissive properties of a few-layer transition-metal dichalcogenide. These calculations are based on time dependent density functional theory in the density matrix formulation.

Ultrafast Charge Transfer Coupled to Quantum Proton Motion at Molecule/Metal Oxide Interface

Understanding the ultrafast dynamics of photoexcited carriers at the semiconductor/molecule interface is essential for improving the solar energy conversion efficiency. At such interfaces, the hydrogen bond (H-bond) network is often formed, and the quantum proton motion is expected. However, how the nuclear quantum effects (NQEs) will influence the photoexcited charge transfer is still unknown. By combining two kinds of emerging molecular dynamics methods at the ab initio level - the path-integral based molecular dynamics and time-dependent nonadiabatic molecular dynamics, and choosing CH3OH/TiO as a prototypical system to study, we find that the quantum proton motion in the H-bond network is strongly coupled with the ultrafast photoexcited charge dynamics at the interface. The hole trapping ability of adsorbed CH3OH molecule is then notably enhanced by the NQEs, and thus, it behaves as a hole scavenger on TiO2. It is concluded that the quantum proton motion in the H-bond network may play a critical role in enhancing the energy conversion efficiency based on photoexcitation.

THz spintronic emitters: a review on achievements and future challenges

Abstract The field of THz spintronics is a novel direction in the research field of nanomagnetism and spintronics that combines magnetism with optical physics and ultrafast photonics. The experimental scheme of the field involves the use of femtosecond laser pulses to trigger ultrafast spin and charge dynamics in thin films composed of ferromagnetic and nonmagnetic thin layers, where the nonmagnetic layer features a strong spin–orbit coupling. The technological and scientific key challenges of THz spintronic emitters are to increase their intensity and to shape the frequency bandwidth. To achieve the control of the source of the radiation, namely the transport of the ultrafast spin current is required. In this review, we address the generation, detection, efficiency and the future perspectives of THz emitters. We present the state-of-the-art of efficient emission in terms of materials, geometrical stack, interface quality and patterning. The impressive so far results hold the promise for new generation of THz physics based on spintronic emitters.

Subfemtosecond charge dynamics in vertically stacked bilayer silicene

AbstractUltrafast charge dynamics in two‐dimensional materials have attracted great interest from both fundamental and applied perspectives due to its significance in the next‐generation battery and solar cells. In this work, using the realistic quantum mechanical real‐time propagation time‐dependent density functional theory (rt‐TDDFT) simulations, we disclose the ultrafast charge dynamics in vertically stacked bilayer silicene subjected to an external bias. Among other insights, our study indicates that even modest changes in interlayer distance produce significant modulations in the ultrafast charge dynamics at the subfemtosecond time scale. The findings in this work may open new pathways for the use of bilayer silicene in novel optoelectronic and photovoltaic devices.

Dual Coordination of Ti and Pb Using Bilinkable Ligands Improves Perovskite Solar Cell Performance and Stability

AbstractCharge recombination due to interfacial defects is an important source of loss in perovskite solar cells. Here, a two‐sided passivation strategy is implemented by incorporating a bilinker molecule, thiophene‐based carboxylic acid (TCA), which passivates defects on both the perovskite side and the TiO2 side of the electron‐extracting heterojunction in perovskite solar cells. Density functional theory and ultrafast charge dynamics reveal a 50% reduction in charge recombination at this interface. Perovskite solar cells made using TCA‐passivated heterojunctions achieve a power conversion efficiency of 21.2% compared to 19.8% for control cells. The TCA‐containing cells retain 96% of initial efficiency following 50 h of UV‐filtered MPP testing.