Ultrafast Charge Research Articles

Revealing the Fundamental Limit of Gate-Controlled Ultrafast Charge Transfer in Graphene–MoS2 Heterostructures

Revealing the Fundamental Limit of Gate-Controlled Ultrafast Charge Transfer in Graphene–MoS₂ Heterostructures

Achieving Ultrafast Charge Transfer by Engineering Multiple Pn Junctions in 3D Hierarchical Cathodes Toward Photo-Chargeable Zinc-Organic Batteries.

Photo-charging zinc ion batteries (PCZIBs) emerge as an innovative approach for the effective utilization and storage of solar energy. However, challenges originating from the suboptimal accessibility of photoexcited charges to electroactive sites severely restrict their practical applications. Herein, a facile methodology to balance light utilization and electrochemical performance by constructing multiple pn junctions in a 3D hierarchical PTCDA-SP/CuZnS photoelectrochemical cathode is reported. DFT calculations reveal that p-type CuZnS can adjust the local electronic environment of n-type PTCDA-SP, facilitating the formation of multiple pn junctions among the interface of this cathode. This unique nanostructure significantly promotes ultrafast charge transfer within 3ps and prevents other undesirable excited state decay pathways, as well as boosts ultrashort photo-response relaxation time less than 20s, resulting in high values of both photo current and voltage of 72.1µA cm-2 and 813mV cm-2 respectively. Additionally, all these carbonyls are responsible for photoelectrochemical Zn2+ storage cascade, ensuring significant improvements in reversible capacities (ca. 32%). This study describes a paradigm of building pn junction on 3D hierarchical cathodes to construct high-performance PCZIBs.

Ultra-fast photoelectron transfer in bimetallic porphyrin optoelectrode for single neuron modulation

Shrinking the size of photoelectrodes into the nanoscale will enable the precise modulation of cellular and subcellular behaviors of a single neuron and neural circuits. However, compared to photovoltaic devices, the reduced size causes the compromised efficiencies. Here, we present a highly efficient nanoelectrode based on bimetallic zinc and gold porphyrin (ZnAuPN). Upon light excitation, we observe ultrafast energy transfer (~66 ps) and charge transfer (~0.5 ps) through the porphyrin ring, enabling 97% efficiency in separating and transferring photoinduced charges to single Au-atom centers. Leveraging these isolated Au atoms as stimulating electrode arrays, we achieve significant photocurrent injection in single neurons, triggering action potential with millisecond light pulses. Notably, Extracranial near-infrared light irradiation of the motor cortex induces neuronal firing and enhances mouse movement. These results show the potential of nanoscale optoelectrodes for high spatiotemporal modulation of neuronal networks without the need for gene transfection in optogenetics.

Fast Li Ion Transportation via Dielectric Nanoislands

Drastic enhancement in the high rate capability of lithium ion battery up to the level of supercapacitors has been required while maintaining its high energy density, towards the next generation power sources. The strategy to incorporate the dielectric nanoisland including benchmarking dielectric compound, BaTiO3 (BTO), into the active materials–electrolyte interface delivers the ultrafast charge transfer pathway via the dialectic layer [1-3]. A series of experimental results and density functional theory and molecular dynamics (DFT-MD) based calculations demonstrated the activated interfacial charge transfer pathway. For instance, in charging, The solvated Li adsorbed onto the dielectric surface and then desolvated at the same surface. The naked Li preferentially intercalates into the electrode bulk via the triple phase interface (TPI): dielectrics-active materials-electrolyte interface.The idea of fast charge transfer architecture via the mentioned TPI, involving activated electrochemical reaction on dielectric surface, has been utilized to Li ion batteries (LIB) and capacitors, to further enhance their cell performances. For instance, the BTO nanocube (NC) decoration onto cathodes for LIB yields to many unequalled advantages that the conventional nanoparticles hardly achieve; most importantly, the NC displays a high dispersibility owing to the steric hindrance effect originating from the bulky oleic acids on the NC surface. In fact, the NC with the cube length, ca. 25 nm decorated LiCoO2 (LCO) displays significantly enhanced high rate capability as the LIB cathode. The pulsed-laser-deposition (PLD) based nanodecoration technique also effectively increased TPI density. 3D nanodecorated LCO (BTO nanodots were decorated onto LCO raw powder prior to its processing to form a working cathode) were found to exhibit notably higher capacity retention value at 10C rate, namely ~47% higher than that of their sol-gel processed cathode. The activated dielectric interface was also utilized to the lithium ion capacitor (LIC) cathode, activated carbon (AC). Optimized capacity of the BTO-AC composite was 35% higher than that of the bare AC. The strengthened LIC capacity is responsible for the enhancement of the Li desolvation activities on the dielectric surface, rather than that in the AC micropores.[1] T. Teranishi et al., Appl. Phys. Lett.,105,143904 (2014). [2] T. Teranishi et al., Adv. Electron. Mater. 4, 1700413 (2018). [3] T. Teranishi et al., J. Power Sources 494, 229710 (2021). [4] T. Teranishi et al., J. Appl. Phys. 131, 124105 (2022). [5] Y. Toyota et al., ACS Appl. Energy Mater. 7, 1440 (2024).

Promoting Oxygen Evolution Reaction Induced By Synergetic Geometric and Electronic Effects of Irco Thin-Film Electrocatalysts

The advancement of water electrolysis technology has seemingly plateaued. Further advances require new strategies to address the key limitations of the oxygen evolution reaction: high overpotential and low stability of electrode materials. Herein, we designed a nanoporous Ir3Co-core@IrO2-shell electrocatalyst with 5 and 3 times higher mass activity and 6 and 2 times higher activity–stability factors than conventional IrO2 nanoparticles and Ir nanoporous electrocatalysts, respectively. The origin of the performance enhancement in the Ir3Co-core@IrO2-shell electrocatalyst was revealed in computational density functional theory calculations. The residual Co after the dealloying process resulted in a down-shift of the d-band center position, thus weakening the −OH* adsorption energy. In addition, the electronic conductivity was enhanced by the three-dimensionally interconnected structure. Under high-current-density operating conditions, the Ohmic losses were reduced by the ultrafast charge transfer pathway provided by the metallic IrCo core.

Ultrafast terahertz conductivity in epitaxial graphene nanoribbons: an interplay between photoexcited and secondary hot carriers

Abstract Optical pump-terahertz probe spectroscopy has been used to investigate ultrafast photo-induced charge carrier transport in 3.4 µm wide graphene ribbons upon scaling the optical pump intensity. For low pump fluences, the deposited pump energy is rapidly redistributed through carrier–carrier scattering, producing secondary hot carriers: the picosecond THz photoconductivity then acquires a negative sign and scales linearly with an increasing pump fluence. At higher fluences, there are not enough equilibrium carriers able to accept the deposited energy, directly generated (excess) carriers start to contribute significantly to the photoconductivity with a positive sign leading to its saturation behavior. This leads to a non-monotonic variation of the carrier mobility and plasmonic resonance frequency as a function of the pump fluence and, at high fluences, to a balance between a decreasing carrier scattering time and an increasing Drude weight. In addition, a weak carrier localization observed for the polarization parallel to the ribbons at low pump fluences is progressively lifted upon increasing the pump fluence as a result of the rise of initial carrier temperature.

A superlattice interface and S-scheme heterojunction for ultrafast charge separation and transfer in photocatalytic H2 evolution

The rapid recombination of photoinduced charge carriers in semiconductors fundamentally limits their application in photocatalysis. Herein, we report that a superlattice interface and S-scheme heterojunction based on Mn0.5Cd0.5S nanorods can significantly promote ultrafast charge separation and transfer. Specifically, the axially distributed zinc blende/wurtzite superlattice interfaces in Mn0.5Cd0.5S nanorods can redistribute photoinduced charge carriers more effectively when boosted by homogeneous internal electric fields and promotes bulk separation. Accordingly, S-scheme heterojunctions between the Mn0.5Cd0.5S nanorods and MnWO4 nanoparticles can further accelerate the surface separation of charge carriers via a heterogeneous internal electric field. Subsequent capture of the photoelectrons by adsorbed H2O is as fast as several picoseconds which results in a photocatalytic H2 evolution rate of 54.4 mmol·g−1·h−1 without any cocatalyst under simulated solar irradiation. The yields are increased by a factor of ~5 times relative to control samples and an apparent quantum efficiency of 63.1% at 420 nm is measured. This work provides a protocol for designing synergistic interface structure for efficient photocatalysis.

Fluence-Dependent Photoinduced Charge Transfer Dynamics in Polymer-Wrapped Semiconducting Single-Walled Carbon Nanotubes.

Because an individual single-walled carbon nanotube (SWNT) can absorb multiple photons, the exciton density within a single tube depends upon excitation conditions. In SWNT-based energy conversion systems, interactions between excitons and charges make it possible for multiple types of charge transfer reactions. We exploit a SWNT-molecular donor-acceptor hybrid system (R-PBN(b)-Ph6-PDI-[(6,5) SWNT]) that fixes spatial organization and stoichiometry of perylene diimide (PDI) electron acceptors on the nanotube surface, to elucidate how excitation fluence affects ultrafast charge separation (CS) and the nature of charge recombination (CR) dynamics triggered upon SWNT near-infrared excitation. Pump-probe data characterizing these photoinduced CS and thermal CR reactions were acquired over excitation fluences that produce ∼5-125 excitons per 700 nm long nanotube. These experiments show that optical excitation gives rise to CS states in which PDI radical anions (PDI-•) and SWNT hole polarons (SWNT•+) have geminate and nongeminate spatial relationships. Under low excitation fluences, the observed dynamics reflect CR reactions of these geminate and nongeminate CS states. As excitation fluence increases, persistent excitons, which have not undergone CS, undergo reaction with ([SWNT(•+)n]-(PDI-•)n) CS states to produce lower-energy CS states that are characterized by hole (SWNT•+) and electron (SWNT•-) polarons. When nongeminate SWNT•+ and SWNT•- charge carriers are generated, CR dynamics depend on the time scale required for these oppositely charged solvated SWNT polarons to encounter each other. Because SWNT excitons have substantial excited-state reduction (1E-/*) and excited-state oxidation (1E*/+) potentials, they can drive additional charge transfer reactions involving initially prepared CS states under experimental conditions where excess excitons are present.

Leveraging Phenazine‐Based Ligands for Optimized Perovskite Optoelectronic Performance Through Chelation and Redox Engineering

AbstractPerovskite nanocrystals (PNCs) hold immense potential for optoelectronic and photovoltaic applications. However, their performance is hindered by surface defects that promote non‐radiative recombination and reduce stability. Surface engineering, particularly through defect passivation, is crucial for achieving high‐performing perovskite solar cells. Chelation has been shown to significantly improve the efficiency and stability of perovskite solar cells. In this study, a novel chelation strategy using 1,10‐Phenanthroline (Phen) is presented as a bidentate chelating ligand to effectively target and passivate these detrimental surface defects. By strategically designing a Phenanthroline derivative, dipyrido[3,2‐a:2′,3′‐c]phenazin‐11‐amine (Phen‐derivative) with optimized redox potentials, dual functionality: efficient defect passivation and hole transport is achieved. X‐ray photoelectron spectroscopy (XPS) confirms the superior binding capability of the Phen‐derivative due to chelation. This strong interaction facilitates efficient and ultrafast charge transfer from PNCs and the formation of a long‐lived charge‐separated state, as evidenced by sustained bleaching in transient absorption spectra. A metal‐dipyrido[3,2‐a:2′,3′‐c]phenazin‐11‐amine complex (Ir‐complex) derived from dipyrido[3,2‐a:2′,3′‐c]phenazin‐11‐amine, but lacking a chelation site, hinders desired hole transfer despite similar charge transfer energetics. This work emphasizes the critical role of chelation‐mediated interfacial interactions and energy alignment in designing effective charge shuttle molecules and unlocking the potential of lead‐chelating hole transporters for next‐generation light‐harvesting technologies.

Ultrafast Charge and Exciton Diffusion in Monolayer Films of 9-Armchair Graphene Nanoribbons.

Determining the electronic transport properties of graphene nanoribbons is crucial for assessing their suitability for applications. So far, this has been highly challenging both through experimental and theoretical approaches. This is particularly the case for graphene nanoribbons that are prepared by chemical vapor deposition, which is a scalable and industry-compatible bottom-up growth method that results in closely packed arrays of ribbons with relatively short lengths of a few tens of nanometers. In this study, the experimental technique of spatiotemporal microscopy is applied to study monolayer films of 9-armchair graphene nanoribbons prepared using this growth method, and combined with linear-scaling quantum transport calculations of arrays of thousands of nanoribbons. Both approaches directly resolve electronic spreading in space and time through diffusion and give an initial diffusivity approaching 200 cm2 s-1 during the first picosecond after photoexcitation. This corresponds to a mobility up to 550 cm2 V-1 s-1. The quasi-free carriers then form excitons, which spread with a diffusivity of tens of cm2 s-1. The results indicate that this relatively large charge carrier mobility is the result of electronic transport not being hindered by defects nor inter-ribbon hopping. This confirms their suitability for applications that require efficient electronictransport.

Ultrafast Charge Transfer-Induced Unusual Nonlinear Optical Response in ReSe2/ReS2 Heterostructure.

Ultrafast charge transfer in van der Waals heterostructures can effectively engineer the optical and electrical properties of two-dimensional semiconductors for designing photonic and optoelectronic devices. However, the nonlinear absorption conversion dynamics with the pump intensity and the underlying physical mechanisms in a type-II heterostructure remain largely unexplored, yet hold considerable potential for all-optical logic gates. Herein, two-dimensional ReSe2/ReS2 heterostructure is designed to realize an unusual transition from reverse saturable absorption to saturable absorption (SA) with a conversion pump intensity threshold of approximately 170 GW/cm2. Such an intriguing phenomenon is attributed to the decrease of two-photon absorption (TPA) of ReS2 and the increase of SA of ReSe2 with the pump intensity. Based on the characterization results of X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, femtosecond transient absorption spectrum, Kelvin probe force microscopy, and density functional theory calculation, a type-II charge-transfer-energy level model is proposed combined with the TPA of ReS2 and SA of ReSe2 processes. The results reveal the critical role of ultrafast interfacial charge transfer in tuning the unusual nonlinear absorption and improving the SA of ReSe2/ReS2 under different excitation wavelengths. Our finding deepens the understanding of nonlinear absorption physical mechanisms in two-dimensional heterostructure materials, which may further diversify the nonlinear optical materials and photonic devices.

Quantifying relaxation time constants in MoS2xSe2(1-x) alloys: Impact of Stoichiometry and Si/SiO2 interference

Despite a decade of extensive research on two-dimensional transition metal dichalcogenides, there are still some gaps in our understanding of their remarkable properties, limiting their potential applications. One such gap relates to the properties of these alloys, which offer tunable band gaps and can serve as the basis for various optoelectronic and nonlinear optical devices. A comprehensive understanding of the ultrafast charge carrier excitation and relaxation dynamics as well as their characteristic relaxation time constants, is crucial for the further use of these materials in specific applications. In this work, we experimentally investigate the dynamics of ultrafast relaxation processes in monolayer MoS2xSe2(1-x) alloys on a Si/SiO2 substrate using time-resolved spectroscopy. We determine the characteristic relaxation time constants for structures with different stoichiometric compositions and refine these constants by accounting for interference effects arising from the substrate.

Linker‐Mediated Delocalized Excited States in Dimeric Acceptors Enable Efficient Exciton Dissociation with Negligible Energy‐level Offsets

Efficient exciton dissociation at low energy offsets is key to overcoming voltage losses in organic solar cells. In this work, we developed two dimeric acceptors, i‐YT and o‐YT, by precisely controlling the position of an asymmetric electron‐donating linker. It induced the foldamer conformation of i‐YT with a para linkage (relative to the dicyano groups), while retaining the unfold conformation for o‐YT. This subtle structural modification influenced the molecular assembly properties, enabled near‐zero energy offset exciton dissociation and power conversion efficiencies exceeding 18% for i‐YT based organic solar cells. Detailed excitonic dynamics further revealed that the linker position critically influences three processes: the formation of delocalized singlet excited states, ultrafast charge transfer (~5 ps) in solid blends, and the suppression of exciton recombination. Additionally, devices based on i‐YT demonstrated outstanding long‐term stability, retaining over 85% of their initial efficiency after 1,400 hours of continuous illumination. These findings introduce a new class of dimeric acceptors that combine high efficiency with exceptional stability, offering a promising pathway toward low‐energy‐loss organic photovoltaics.

Photo‐Induced Ultrafast Charge Transfer and Air‐Stable Radical Formation in Tetraphenylpyrene Derivatives

AbstractStable organic radicals generated by photo‐excitation hold applications in molecular switching devices and information storage. It remains challenging to develop photo‐generated radical materials with rapid response and air stability in the solid state. Here, we report a structure based on 1,3,6,8‐tetraphenylpyrene derivative (Py‐TTAc) displaying photo‐induced radicals with air stability in the solid state. Photo‐induced electron transfer, exposed to a 365 nm ultraviolet lamp for 1 minute, affords radicals in Py‐TTAc powder as confirmed by electron paramagnetic resonance (EPR) spectroscopy. The maximum radical concentration reaches 2.21 % after continuous irradiation for 1 hour and recurs more than 10 times without any chemical degradation. The mechanistic study according to the femtosecond transient absorption (fsTA) and X‐ray technology suggests that the radicals are derived from photo‐induced symmetry‐breaking charge separation (SB‐CS) and stabilized through non‐covalent interactions. The photo‐generated stable radical system is employed in anti‐counterfeiting paper and optoelectronic device applications. This study will provide insights into the development of photoactive organic radical materials.

Linker-Mediated Delocalized Excited States in Dimeric Acceptors Enable Efficient Exciton Dissociation with Negligible Energy-Level Offsets.

Efficient exciton dissociation at low energy offsets is key to overcoming voltage losses in organic solar cells. In this work, we developed two dimeric acceptors, i-YT and o-YT, by precisely controlling the position of an asymmetric electron-donating linker. It induced the foldamer conformation of i-YT with a para linkage (relative to the dicyano groups), while retaining the unfold conformation for o-YT. This subtle structural modification influenced the molecular assembly properties, enabled near-zero energy offset exciton dissociation and power conversion efficiencies exceeding 18 % for i-YT based organic solar cells. Detailed excitonic dynamics further revealed that the linker position critically influences three processes: the formation of delocalized singlet excited states, ultrafast charge transfer (~5 ps) in solid blends, and the suppression of exciton recombination. Additionally, devices based on i-YT demonstrated outstanding long-term stability, retaining 85 % of their initial efficiency after 1,400 hours of continuous illumination. These findings introduce a new class of dimeric acceptors that combine high efficiency with exceptional stability, offering a promising pathway toward low-energy-loss organic photovoltaics.

Photo-Induced Ultrafast Charge Transfer and Air-Stable Radical Formation in Tetraphenylpyrene Derivatives.

Stable organic radicals generated by photo-excitation hold applications in molecular switching devices and information storage. It remains challenging to develop photo-generated radical materials with rapid response and air stability in the solid state. Here, we report a structure based on 1,3,6,8-tetraphenylpyrene derivative (Py-TTAc) displaying photo-induced radicals with air stability in the solid state. Photo-induced electron transfer, exposed to a 365 nm ultraviolet lamp for 1 minute, affords radicals in Py-TTAc powder as confirmed by electron paramagnetic resonance (EPR) spectroscopy. The maximum radical concentration reaches 2.21 % after continuous irradiation for 1 hour and recurs more than 10 times without any chemical degradation. The mechanistic study according to the femtosecond transient absorption (fsTA) and X-ray technology suggests that the radicals are derived from photo-induced symmetry-breaking charge separation (SB-CS) and stabilized through non-covalent interactions. The photo-generated stable radical system is employed in anti-counterfeiting paper and optoelectronic device applications. This study will provide insights into the development of photoactive organic radical materials.

Ultrafast Charge Separation Driven by Solvation-Coupled Intramolecular Torsion.

Photoinduced intramolecular charge separation in a pyrene- and triarylamine-based donor-acceptor dyad was studied by polarization-dependent femtosecond time-resolved transient absorption (TA) spectroscopy in polar solvents. Photoexcitation forms an excited state with charge transfer (CT) character due to the intrinsic electronic coupling between the triarylamine and pyrene groups, resulting in ultrafast charge separation (CS) in polar solvents. TA measurements reveal a correlation between the rate of CS and solvation dynamics, which implies that solvation is involved in the CS reaction. In addition, polarization-dependent TA spectroscopy was devoted to tracking the ultrafast anisotropy evolution of the cationic absorption band, which is attributed to intramolecular torsional motion and is proposed to be coupled to diffusive orientational solvent modes. The results therefore reveal that the evolution of the CT state in the condensed phase is driven by solvation-coupled excited-state structural relaxation. In other words, intramolecular torsional motion is directly confirmed to be involved in the reaction coordinate of the CS reaction in a strongly coupled donor-acceptor dyad.

Rapid Charge Transfer Endowed by Hollow‐structured Z‐Scheme Heterojunction for Coupling Benzylamine Oxidation With CO2 Reduction

AbstractIn the field of photocatalysis, ultrafast electron transfer at the interface is the key factor affecting photocatalytic activity. Herein, ultrafast carrier transport is achieved through constructing a Z‐scheme heterojunction of CoSe@Zn0.5Cd0.5S (CoSe@ZCS), which is prepared by in situ growth of ZCS on the ZIF‐67‐derived hollow CoSe. The ultrafast charge transfer at the Z‐scheme heterojunction interface is verified by advanced fs‐transient absorption, which provides vital evidence for the specific mechanism of photocatalytic charge transfer. In addition, the presence of key intermediates (*COOH and C═N) is detected by in‐situ FTIR spectroscopy, which further clarified the mechanism of coupling benzylamine oxidation with CO2 photoconversion. DFT calculations also confirm that the Z‐scheme heterojunction effectively reduces the energy barrier of the rate‐limiting step of *COOH formation, facilitating the photocatalytic CO2 reduction process of ZCS. Benefiting from the ultrafast electron transfer at the interface of the Z‐scheme heterojunction, CoSe@ZCS exhibits excellent bifunctional photocatalytic performance. This work lays the foundation for further exploration of the charge transfer mode at the heterojunction interface to facilitate solar‐driven energy conversion.

Protein Medium Facilitates Electron Transfer in Photosynthetic Heliobacterial Reaction Center.

This computational study addresses the question of how large membrane-bound proteins of electron transport chains facilitate fast vector-based charge transport. We study electron transfer reactions following ultrafast initial charge separation induced by absorption of light by P800 primary pair and leading to the electron localization at the A0 cofactor. Two subsequent, much slower reactions, electron transfer to the iron-sulfur cluster Fx and reduction of the menaquinone (MQ) cofactor, are studied by combining molecular dynamics simulations, electronic structure calculations, and theoretical modeling. The low value of the electronic coupling between A0 and Fx brings this reaction to the microsecond time scale even at the zero activation barrier. In contrast, A0-MQ electron transfer occurs on a subnanosecond time scale and might become the preferred route for charge transport. We elucidate mechanistic properties of the protein medium allowing fast, vectorial charge transfer. The electric field is high and inhomogeneous inside the protein and is coupled to high polarizabilities of cofactors to significantly lower the reaction barrier. The A0-MQ separation puts this reaction at the edge between the plateau characterizing the reaction dynamical control and exponential falloff due to electronic tunneling. A strong separation in relaxation times of the medium dynamics for the forward and backward reactions promotes vectorial charge transfer.

Construction of an S-scheme heterojunction with ultra-fast charge transfer at the BP/MOF-808 interface for efficient visible light degradation of metronidazole in the environment

Emerging S-scheme heterojunctions provide catalysts with superior electron transfer pathways and strong redox capacities. Constructing S-scheme heterojunction with two-dimensional nanosemiconductors (with very narrow bandgaps) as interfaces to activate the redox capacity of organic semiconductors (with wide bandgaps) represents a novel method for achieving efficient visible-light photocatalytic degradation of antibiotics. In this study, a BP/MOF-808 S-scheme heterojunction was synthesized through an hydrothermal method. Under visible light excitation (λ = 460 nm), it was demonstrated that photogenerated electrons are transferred from MOF-808 to BP, thereby retaining strong redox capability within the catalytic system. The 3-BP/MOF-808 (300 mg BP) heterojunction exhibited optimal photocatalytic activity, approximately 27.77 times and 27.34 times higher than those of BP and MOF-808, respectively. The results of this work are expected to guide the design and transformation of S-scheme photocatalysts for efficient environmental degradation of antibiotics.