Light-induced Charge Transfer Research Articles

Enhancing dielectric passivation on monolayer WS2 via a sacrificial graphene oxide seeding layer

The full utilization of two-dimensional transition metal dichalcogenides (2D TMDCs) faces several challenges, among which is realizing uniform material deposition on the 2D surface. Typical strategies to enable material growth lead to a poor interface quality, degrading the 2D TMDC’s properties. In this work, a sacrificial, graphene oxide-based seeding layer is used (1) as passivation layer, protecting the underlying 2D TMDC and (2) as nucleation layer, enabling uniform material growth. Graphene is transferred on monolayer WS2, establishing a high-quality van der Waals interface. After transfer, the polymeric residues on graphene are cleaned via a combination of wet- and dry treatments and functionalized via dry UV/O3 oxidation. The rate of graphene oxidation is shown to be substrate dependent, which is explained by UV light-induced ultrafast charge transfer between the graphene and WS2 monolayer. The carbon-oxygen functionalities serve as nucleation sites in a subsequent HfO2 ALD process, achieving more uniform dielectric growth and faster layer closure compared to direct deposition. The graphene-based nucleation- / passivation approach offers adaptability, allowing for tailored surface chemistry to enable any alternative material growth, while maintaining a prefect van der Waals interface.

Tailoring light-induced charge transfer and intersystem crossing in FeCO using time-dependent spin-orbit configuration interaction.

Real-time (RT) electronic structure methods provide a natural framework for describing light-matter interactions in arbitrary time-dependent electromagnetic fields (EMF). Optically induced excited state transitions are of particular interest, which require tuned EMF to drive population transfer to and from the specific state(s) of interest. Intersystem crossing, or spin-flip, may be driven through shaped EMF or laser pulses. These transitions can result in long-lived "spin-trapped" excited states, which are especially useful for materials requiring charge separation or protracted excited state lifetimes. Time-dependent configuration interaction (TDCI) is unique among RT methods in that it may be implemented in a basis of eigenstates, allowing for rapid propagation of the time-dependent Schrödinger equation. The recent spin-orbit TDCI (TD-SOCI) enables a real-time description of spin-flip dynamics in an arbitrary EMF and, therefore, provides an ideal framework for rational pulse design. The present study explores the mechanism of multiple spin-flip pathways for a model transition metal complex, FeCO, using shaped pulses designed to drive controlled intersystem crossing and charge transfer. These results show that extremely tunable excited state dynamics can be achieved by considering the dipole transition matrix elements between the states of interest.

Infrared Light-Induced Charge Transfer of Prussian Blue Analogs K0.4Co1.3[Fe(CN)6]·nH2O

Infrared Light-Induced Charge Transfer of Prussian Blue Analogs K0.4Co1.3[Fe(CN)6]·nH2O

Visible-light-induced self-propelled nanobots against nanoplastics

The accumulation of plastic debris in aquatic organisms has raised serious concerns about the potential health implications of their incorporation into the food chain. However, conventional water remediation techniques are incapable of effectively removing nanoplastics (NPs) smaller than 200 nm, which can have harmful effect on animal and human health. Herein, we demonstrate the “on-the-fly” capture of NPs through their enlargement (approximately 4,100 times) using self-propelled nanobots composed of a metal-organic framework. Under visible-light irradiation, the iron hexacyanoferrate (FeHCF) nanobot exhibits fuel-free motion by electrostatically adsorbing NPs. This strategy can contribute to reducing plastic pollution in the environment, which is a significant environmental challenge. Light-induced intervalence charge transfer in the FeHCF nanobot lattice induces bipolarity on the nanobot surface, leading to the binding of negatively charged NPs. The local electron density in the lattice then triggers self-propulsion, thereby inducing agglomeration of FeHCF@NP complexes to stabilize their metastable state. The FeHCF nanobot exhibits a maximum removal capacity of 3,060 mg∙g−1 and rate constant of 0.69 min−1, which are higher than those recorded for materials reported in the literature.

Bimetal MOF-derived NiFe-P nanocomposites coupled with Cu3P nanoparticles to construct tandem electron transfer channels for photocatalytic hydrogen evolution

Finely modulated light-induced charge separation and transfer is a central challenge to achieve efficient photocatalysis. Although progress has been made in this field, most of the previous research works focused on the separation or migration of photogenerated carriers but did not build a bridge between the two. How to realize the strong driving and precise migration of carriers has become the focus of our work. We report an ingeniously designed ternary heterojunction. Taking NiFe-MOF as the “parent material”, the FeP4/NixPy heterojunction is derived in situ while maintaining the frame structure through gas-solid reaction, and finally the Z-type electron transfer is realized. With Cu3P anchoring spindle matrix, an electron transport tunnel is opened up in Cu3P/FeP4/NixPy ternary heterojunction under the action of p-n heterojunction built-in electric field driving and accurate energy band matching. The strong driving force of the built-in electric field provides an inexhaustible power for the transmission of electrons, and the fine series of electron transmission channels realizes the precise transmission of electrons. The above fine design makes the perfect fit between the built-in electric field and the electron transfer channel, which not only effectively improves the embarrassing situation of insufficient electron driving force of hydrogen evolution reaction in the previous research, but also makes up for the weakening of semiconductor reduction ability caused by the construction of traditional p-n heterostructures. This research work provides a new idea for the construction of multiple heterostructures and the design of fine interface engineering in the future.

Hyperconjugation in light-switched intramolecular charge transfer of anthrapyridinium derivatives: New insight into molecular motion and fluorescent materials

Light-induced intramolecular charge transfer (ICT) is important in chemical and physical processes involving organic reactions and materials. However, because of the fast light-absorption rates and short excited state lifetime, charge-transfer separation and the resulting molecular motion in the ICT states are generally expected to be unidirectional and uncontrollable. Herein, an unexpected hyperconjugation derived light-switched ICT and tunable fluorescence emissions are described, and these can be rapidly and reversibly controlled in anthrapyridinium dyes by irradiation with light of different wavelengths such as 365 nm and 254 nm. Most importantly, the subtle differences between the dyes in terms of the σ–π (C–H and C–C) hyperconjugation between the alkyl chains and the pyridinium, which were confirmed from the 1H NMR spectra for the first time, and which was further proved to be the key factor affecting the light-induced ICT of the dyes.

Dynamics of VOCs degradation and bacterial inactivation at the interface of AgxO/Ag/TiO2 prepared by HiPIMS under indoor light

• Indoor/outdoor air pollution and bacteria were degraded on sputtered Ag x O/Ag/TiO 2 on textile. • Photocatalytic kinetics are carried out in the dark and under visible light. • Ag/AgxO nanoparticles show a significant effect on the activity of TiO 2 . • A possible mechanism of photocatalytic processes is suggested. Ag x O/Ag/TiO 2 photoactive thin films were produced by different modes of magnetron sputtering (namely, DCMS and HiPIMS). The spectral and microstructural properties of the Ag-Ag x O/TiO 2 thin films were studied by Diffuse Reflection Spectroscopy (DRS) and X-Ray Photoemission Spectroscopy (XPS). The atomic deposition of TiO2 and Ag in the thin sputtered film were studied by TEM. Ag x O/Ag/TiO 2 exhibited fast photocatalytic volatile organic compounds (VOCs) degradation and bacterial inactivation under low visible light intensity. The Langmuir Hinshelwood model was applied to highlight the photocatalytic performance of the Ag x O/Ag/TiO 2 catalyst. The oxidative states of Ag-Ag x O/TiO 2 were studied by XPS. Deconvolution of the Ag3d peak of the Ag x O/Ag/TiO 2 thin films showed the presence of AgO and metallic Ag. It is suggested that the mechanism involving the degradation of VOCs on AgO/Ag/TiO 2 catalysts is due to the transfer of holes from AgO to TiO 2 via the AgO/Ag/TiO 2 heterojunction. The electrostatic interaction between both semiconductors allows this charge transfer. In addition, the contribution of reactive oxygen species (ROS) in bacterial inactivation at the interface of AgO/Ag/TiO 2 thin film was also examined. Light-induced interfacial charge transfer (IFCT) between AgO and TiO 2 leading to pollutants oxidation appears to require low photons’ energy and can oxidize pollutants even at high concentrations. The sputtered coatings have also been studied for repetitive reuse, showing the sustainable activity of the prepared materials.

Charge-transfer states in photosynthesis and organic solar cells

Light-induced charge-transfer mechanisms are at the heart of both photosynthesis and photovoltaics. The underlying photophysical mechanisms occurring within photosynthesis and organic photovoltaics in particular show striking similarities. However, they are studied by distinct research communities, often using different terminology. This contribution aims to provide an introductory review and comparison of the light-induced charge-transfer mechanisms occurring in natural photosynthesis and synthetic organic photovoltaics, with a particular focus on the role of so-called charge-transfer complexes characterized by an excited state in which there is charge-transfer from an electron-donating to an electron-accepting molecular entity. From light absorption to fully separated charges, it is important to understand how a charge-transfer complex is excited, forming a charge-transfer state, which can decay to the ground state or provide free charge carries in the case of photovoltaics, or radicals for photochemistry in photosynthetic complexes. Our motivation originates from an ambiguity in the interpretation of charge-transfer states. This review attempts to standardize terminology between both research fields with the general aim of initiating a cross-fertilization between the insights and methodologies of these two worlds regarding the role of charge-transfer complexes, inspiring the cross-disciplinary development of next-generation solar cells. Likewise, we hope to encourage photosynthesis researchers to collaborate with the photovoltaics field, thereby gaining further knowledge of the charge-transfer process in natural light-harvesting systems.

Steering the Ultrafast Opening and Closure Dynamics of a Photochromic Coordination Cage by Guest Molecules.

Photochemical studies on supramolecular hosts that can encapsulate small guest molecules commonly focus on three aspects: photoswitching the cage to release or trap the guest, the effect of the confining environment on the guest, and light-induced exciton or charge transfer within the cage structure. Here, we exploit ultrafast spectroscopy to address how the guest alters the photoswitching characteristics of the cage. For this, the impacts of three disparate guest compounds on ring-opening or ring-closure of a dithienylethene (DTE) ligand in a photoswitchable DTE-based coordination cage are juxtaposed. The guest modulates both outcome and timescale of the cage's photodynamics, by an interplay of structural strain, heavy-atom effect, and enhancement of charge-transfer processes exercised by the guest on the photo-excited cage. The approach might prove beneficial for attuning the applicability of photoswitchable nanocontainers and desired guest compounds.

Polarity-Dependent Twisted Intramolecular Charge Transfer in Diethylamino Coumarin Revealed by Ultrafast Spectroscopy

Twisting intramolecular charge transfer (TICT) is a common nonradiative relaxation pathway for a molecule with a flexible substituent, effectively reducing the fluorescence quantum yield (FQY) by swift twisting motions. In this work, we investigate coumarin 481 (C481) that contains a diethylamino group in solution by femtosecond transient absorption (fs-TA), femtosecond stimulated Raman spectroscopy (FSRS), and theoretical calculations, aided by coumarin 153 with conformational locking of the alkyl arms as a control sample. In different solvents with decreasing polarity, the transition energy barrier between the fluorescent state and TICT state increases, leading to an increase of the FQY. Correlating the fluorescence decay time constant with solvent polarity and viscosity parameters, the multivariable linear regression analysis indicates that the chromophore’s nonradiative relaxation pathway is affected by both hydrogen (H)-bond donating and accepting capabilities as well as dipolarity of the solvent. Results from the ground- and excited-state FSRS shed important light on structural dynamics of C481 undergoing prompt light-induced intramolecular charge transfer from the diethylamino group toward –C=O and –CF3 groups, while the excited-state C=O stretch marker band tracks initial solvation and vibrational cooling dynamics in aprotic and protic solvents (regardless of polarity) as well as H-bonding dynamics in the fluorescent state for C481 in high-polarity protic solvents like methanol. The uncovered mechanistic insights into the molecular origin for the fluorogenicity of C481 as an environment-polarity sensor substantiate the generality of ultrafast TICT state formation of flexible molecules in solution, and the site-dependent substituent(s) as an effective route to modulate the fluorescence properties for such compact, engineerable, and versatile chemosensors.

Optical Properties and Light-Induced Charge Transfer in Selected Aromatic C60 Fullerene Derivatives and in Their Bulk Heterojunctions with Poly(3-Hexylthiophene).

Fullerene derivatives offer great scope for modification of the basic molecule, often called a buckyball. In recent years, they have been the subject of numerous studies, in particular in terms of their applications, including in solar cells. Here, the properties of four recently synthesized fullerene C60 derivatives were examined regarding their optical properties and the efficiency of the charge transfer process, both in fullerene derivatives themselves and in their heterojunctions with poly (3-hexylthiophene). Optical absorption, electron spin resonance (ESR), and time-resolved photoluminescence (TRPL) techniques were applied to study the synthesized molecules. It was shown that the absorption processes in fullerene derivatives are dominated by absorption of the fullerene cage and do not significantly depend on the type of the derivative. It was also found by ESR and TRPL studies that asymmetrical, dipole-like derivatives exhibit stronger light-induced charge transfer properties than their symmetrical counterparts. The observed inhomogeneous broadening of the ESR lines indicated a large disorder of all polymer–fullerene derivative blends. The density functional theory was applied to explain the results of the optical absorption experiments.

Light-Induced Charge Transfer to Achieve Deep-Red Emission in SrSc2O4:Bi toward Multiple Optical Applications

Bismuth (Bi) is used for luminescent materials due to its unique optical performance, but deep-red light from Bi-doped materials is rarely reported. In particular, establishing a design principle for Bi-doped red materials is considered to be a significant challenge. Herein, using a deep-red SrSc2O4:Bi material featuring Bi–Bi pair emission, light-induced charge-transfer from BiSc3+–BiSr3+ to BiSc4+–BiSr2+ enables the realization of Bi2+2P3/2(1) → 2S1/2 deep-red emission. Intriguingly, SrSc2O4:Bi displays an excellent zero-thermal-quenching performance from 298 to 423 K, with a peak intensity that retains 98% of the intensity at 298 K and an integrated intensity at 423 K that even reaches 110% of the initial intensity. The intriguing spectroscopic characteristics of SrSc2O4:Bi make it a promising candidate in the agricultural field, night-vision security, and the medical treatment area. This work advances the understanding of red luminescence in Bi-activated luminescent materials and thus can initiate more exploitation of red materials for emerging applications.

2D Multiferroicity with Ferroelectric Switching Induced Spin-Constrained Photoelectricity.

Multiferroic materials with tunable magnetoelectric orders enable the integration of sensing, data storage, and processing into one single device. The scarcity of single-phase multiferroics spurs extensive research in pursuit of composite systems combining different types of ferroic materials. In this work, spin-constrained photoelectric memory is proposed in two-dimensional (2D) layered magnetic/ferroelectric heterostructures, holding the possibility of low-power electrical write operation and nondestructive optical read operation. The ground state of ferromagnetic (FM) and antiferromagnetic (AFM) orderings in the magnetic layer is altered by polarization direction of the ferroelectric layer. Specifically, the FM heterostructure exhibits a type-II band alignment. Due to the light-induced charge transfer, spin-polarized/unpolarized current arises from the FM/AFM state, which can be recorded as the "1"/"0" state and served for logic processing and memory applications. Our first-principles calculations demonstrate that the NiI2/In2Se3 heterobilayer is an ideal candidate to realize such a spin-dependent photoelectric memory. The reversible FM state (easy-axis magnetic anisotropy) and AFM state (easy-plane magnetic orientation) in the NiI2 layer originate from interfacial charge transfer and effective electric field due to the proximity effect. This work offers considerable potential in the integration of memory processing capability into one single device with 2D layered multiferroic heterostructures.

Concerning a hole trap in α-Al2O3:C,Mg

We report the existence of a hole trap in α-Al2O3:C,Mg as determined using thermoluminescence (TL) and optically stimulated luminescence (OSL). The associated experiments are based on the hypotheses that if a TL glow peak is associated with a hole trap in the glow curve of α-Al2O3:C,Mg, that hole trap will not participate in any light-induced electron-transfer process and its removal will not cause the OSL intensity to decrease. To examine the first hypothesis, a TL glow curve was recorded at 1 °C/s to establish the position of glow peaks. There is a high intensity peak at 184 °C (labeled as peak IV) and eight secondary peaks at 48, 80, 108, 228, 288, 320, 386, and 426 °C (peaks I, II, III, and V–IX). The light-induced charge transfer between various electron traps associated with the peaks was studied. This study reveals that all peaks except the one at 228 °C (peak V) participate in the charge-transfer process. A test of the second hypothesis shows that peak V is also not reproduced by illumination (phototransfer), and the depletion of this peak does not influence the charge-transfer process. An additional study on the effect of temperature on the TL and OSL intensity was carried out to further assess the nature of the charge traps associated with the peaks. The depletion of peak V does not affect the OSL intensity. The results from both experiments are clear evidence consistent with the hypothesis that the charge trap associated with peak V is a hole trap.

Dynamics of light-induced charge transfer between carbon nanotube and CdSe/CdS core/shell nanocrystals

The integration of semiconducting colloidal nanocrystals (NCs) with carbon nanotubes (CNTs) in a single device presents a unique platform that combines optical flexibility with high charge carrying capability. These qualities are desirable in many applications such as photovoltaic cells, photocatalysis, and light sensors. Here, we present hybrid devices that incorporate various CdSe/CdS core/shell NCs, such as seeded quantum dots and asymmetric seeded nanorods (a-sNRs), with a single-wall CNT in a field-effect transistor geometry. We used electrical measurements to probe a light-induced charge transfer (LICT) between the CdSe/CdS NCs and the CNT. We investigate the effect of gate voltage on the LICT magnitude and temporal characteristics. Surprisingly, the measured photo-response depends on the gate voltage, and we observe both electrons and holes transfer from the a-sNRs to the CNT. Furthermore, a comparison between LICT measurements on different devices with different CNTs and NC types reveals that the charge transfer time is directly proportional to the shell-thickness around the CdSe core and inversely correlated with the NCs size. The recovery of the charge trapped inside the CdSe/CdS NCs is characterized by two distinct fast and slow relaxation times, which depend on the NCs size and CNT type. Although, the charge relaxation time is similar between the symmetric QDs and the asymmetric sNRs, the overall percentage of the remaining charge in the QDs is significantly larger than in the sNRs. Understanding both gate voltage and NCs size effect on the LICT processes can optimize the performance of optoelectronic devices.

Interfacial self-assembly of ultrathin polydiacetylene/graphene oxide nanocomposites: A new method for synergetic enhancement of surface charge transfer without doping

A new method for interfacial self-assembly of graphene oxide (GO) and 10,12-pentacosadiynoic acid into nanometer-thick light-absorbing films exhibiting synergetic enhancement of conductive properties is developed. The blue form of polydiacetylene (PDA) nanopatches with a thickness of 5–10 nm is stabilized through the interactions between GO and PDA. The hole-transferring polymer decreases contact resistance in the GO sublayer through “healing” the gaps between the GO nanosheets. The separation of electron-hole pairs at the PDA/GO interface under applied electric field leads to a doping-free enhancement of the in-plane surface conductivity by 6 orders of magnitude up to 46.4 S·cm−1 with respect to those of individual components. These PDA/GO nanocomposites were applied as ultrathin diodes/photodiodes in a solar cell by creating a PDA/C60 heterojunction and a light-induced cross-layer charge transfer. The method can be adapted to a variety of polymerizable light-absorbing surfactants for fabricating GO-based nanohybrids for photoelectronics.

Multifunctional Polyoxometalate Platforms for Supramolecular Light-Driven Hydrogen Evolution*.

Multifunctional supramolecular systems are a central research topic in light‐driven solar energy conversion. Here, we report a polyoxometalate (POM)‐based supramolecular dyad, where two platinum‐complex hydrogen evolution catalysts are covalently anchored to an Anderson polyoxomolybdate anion. Supramolecular electrostatic coupling of the system to an iridium photosensitizer enables visible light‐driven hydrogen evolution. Combined theory and experiment demonstrate the multifunctionality of the POM, which acts as photosensitizer/catalyst‐binding‐site[1] and facilitates light‐induced charge‐transfer and catalytic turnover. Chemical modification of the Pt‐catalyst site leads to increased hydrogen evolution reactivity. Mechanistic studies shed light on the role of the individual components and provide a molecular understanding of the interactions which govern stability and reactivity. The system could serve as a blueprint for multifunctional polyoxometalates in energy conversion and storage.

Defective Black TiO2: Effects of Annealing Atmospheres and Urea Addition on the Properties and Photocatalytic Activities.

A series of black TiO2 with and without the addition of urea were successfully prepared using a simple one-step synthetic method by calcination under different atmospheres (vacuum, He, or N2). The physicochemical, optical, and light-induced charge transfer properties of the as-prepared samples were characterized by various techniques. It was found that a vacuum atmosphere was more beneficial for the formation of oxygen vacancies (OVs) than the inert gases (He and N2) and the addition of urea-inhibited OVs formation. The samples annealed in the vacuum condition exhibited better visible-light adsorption abilities, narrower bandgaps, higher photo-induced charge separation efficiency, and lower recombination rates. Hydroxyl radicals (·OH) were the dominant oxidative species in the samples annealed under a vacuum. Finally, the samples annealed under vacuum conditions displayed higher photocatalytic activity for methylene blue (MB) degradation than the samples annealed under He or N2. Based on the above, this study provides new insights into the effects of annealing atmospheres and urea addition on the properties of black TiO2.

Electric Field-Modulated Charge Transfer in Geometrically Tailored MoX2/WX2 (X = S, Se) Heterostructures

Light-induced interlayer charge transfer in staggered-type heterostructures (HSs) in transition-metal dichalcogenides provides the opportunity to improve the performance of optoelectronic applications. Herein, we employ density functional theory to investigate the vertical electric-field-controlled interlayer charge transfer in stacked MoX2/WX2 (X=S, Se) HSs. Upon application of electric field from −3 to 3 V/nm, we observe the band-alignment transition, band inversion, and offset variations in these HSs. Furthermore, these electric fields are found to modulate charge localization/delocalization across the layers, which provides insight into charge transfer. The positive electric field is supposed to localize the charges in WS2, whereas the charges are localized in MoS2 at negative electric field. Based on charge localization/delocalization, our study suggests that the interlayer hole transfer upon MoS2 photoexcitation can be suppressed at higher positive electric fields, whereas electron transfer can be blocked by excitation of WS2. In contrast, negative electric fields (of −3 V/nm) can induce interlayer hole and electron transfer. Owing to the tunability of interlayer charge transfer by means of a vertical electric field, our findings bear paramount importance in modulating electron–hole recombination and charge-transfer time, which is beneficial for future optoelectronic devices.

Light-Induced Charge Transfer in Two-Dimensional Hybrid Lead Halide Perovskites

Light-Induced Charge Transfer in Two-Dimensional Hybrid Lead Halide Perovskites