Light-induced Potential Research Articles

Electron Imaging of Nanoscale Charge Distributions Induced by Femtosecond Light Pulses.

Surface charging is ubiquitously observable during in situ transmission electron microscopy of nonconducting specimens as a result of electron beam/sample interactions or optical stimuli and often limits the achievable image stability and spatial or spectral resolution. Here, we report on the electron-optical imaging of surface charging on a nanostructured surface following femtosecond multiphoton photoemission. By quantitatively extracting the light-induced electrostatic potential and studying the charging dynamics on relevant time scales, we gain insights into the details of the multiphoton photoemission process in the presence of an electrostatic background field. We study the interaction of the charge distribution with the high-energy electron beam and secondary electrons and propose a simple model to describe the interplay of electron- and light-induced processes. In addition, we demonstrate how to mitigate sample charging by simultaneously optically illuminating the sample.

Dynamic modulation of transthylakoid electric potential by chloroplast ATP synthases

The light-induced transthylakoid membrane potential (ΔΨm) can function as a driving force to help catalyzing the formation of ATP molecules, proving a tight connection between ΔΨm and the ATP synthase. Naturally, a question can be raised on the effects of altered functioning of ATP synthases on regulating ΔΨm, which is attractive in the area of photosynthetic research. Lots of findings, when making efforts of solving this difficulty, can offer an in-depth understanding into the mechanism behind. However, the functional network on modulating ΔΨm is highly interdependent. It is difficult to comprehend the consequences of altered activity of ATP synthases on adjusting ΔΨm because parameters that have influences on ΔΨm would themselves be affected by ΔΨm. In this work, a computer model was applied to check the kinetic changes in polarization/depolarization across the thylakoid membrane (TM) regulated by the modified action of ATP synthases. The computing data revealed that under the extreme condition by numerically “switching off” the action of the ATP synthase, the complete inactivation of ATP synthase would markedly impede proton translocation at the cytb6f complex. Concurrently, the KEA3 (CLCe) porter, actively pumping protons into the stroma, further contributes to achieving a sustained low level of ΔΨm. Besides, the quantitative consequences on every particular component of ΔΨm adjusted by the modified functioning of ATP synthases were also explored. By employing the model, we bring evidence from the theoretical perspective that the ATP synthase is a key factor in forming a transmembrane proton loop thereby maintaining a propriate steady-state ΔΨm to meet variable environmental conditions.

Broadband Self-Powered CdS ETL-Based MAPbI3 Heterojunction Photodetector Induced by a Photovoltaic-Pyroelectric-Thermoelectric Effect.

CdS, with a noncentrosymmetric structure, is thought as an important electron transport layer (ETL) in perovskite-based devices, but its pyroelectric effect, which can efficiently modulate the optoelectronic processes, is not well explored. In this work, a MAPbI3 heterojunction of CdS/MAPbI3/Spiro-OMeTAD with a c-axis preferred oxygen-doped CdS ETL is developed as a high-performance photodetector (PD). This PD exhibits a stable self-powered property in the spectral range of ∼360-780 nm due to its excellent photovoltaic effect. Moreover, the light-induced pyroelectric potential in the CdS ETL is demonstrated to be an efficient approach for improving the photoresponses, and different effects are observed for different laser irradiations, which can be well understood from their working mechanisms. Upon 450 nm laser irradiation, the photovoltage responsivity (R) is greatly improved from 596.9 to 6383.6 V/W with an increment of 1069.54%. In addition, the response spectrum is also extended outside the bandgap restriction of the MAPbI3 to 1550 nm due to both the pyroelectric and photothermoelectric effects, which is a big breakthrough for the perovskite heterojunction PD. Through turning the external bias voltage and ambient temperature, the coupling mechanisms of the pyroelectric and photovoltaic effects are further analyzed. This work provides an important understanding of designing the CdS ETL-based perovskite heterojunction for broadband high-performance photoelectric devices by introducing the pyroelectric effect.

An Optical Technique to Produce Embedded Quantum Structures in Semiconductors.

The performance of a semiconductor quantum-electronic device ultimately depends on the quality of the semiconductor materials it is made of and on how well the device is isolated from electrostatic fluctuations caused by unavoidable surface charges and other sources of electric noise. Current technology to fabricate quantum semiconductor devices relies on surface gates which impose strong limitations on the maximum distance from the surface where the confining electrostatic potentials can be engineered. Surface gates also introduce strain fields which cause imperfections in the semiconductor crystal structure. Another way to create confining electrostatic potentials inside semiconductors is by means of light and photosensitive dopants. Light can be structured in the form of perfectly parallel sheets of high and low intensity which can penetrate deep into a semiconductor and, importantly, light does not deteriorate the quality of the semiconductor crystal. In this work, we employ these important properties of structured light to form metastable states of photo-sensitive impurities inside a GaAs/AlGaAs quantum well structure in order to create persistent periodic electrostatic potentials at large predetermined distances from the sample surface. The amplitude of the light-induced potential is controlled by gradually increasing the light fluence at the sample surface and simultaneously measuring the amplitude of Weiss commensurability oscillations in the magnetoresistivity.

Size-Independent Reconfigurable Logic Gate with Bismuth Oxide Based Photoelectrochemical Device.

XOR gate, an important building block in computational circuits, is often constructed by combining other basic logic gates, and the hybridity inevitably leads to its complexity. A photoelectrochemical device could realize XOR function based on the current change of the photoelectrode; however, such signal is highly sensitive to photoelectrode size and therefore requires precise manufacturing at a high cost. Herein we developed a novel XOR gate based on the light-induced open-circuit potential (OCP) of the Bi2O3 photoelectrode. Surprisingly, the OCP of Bi2O3 does not increase with light intensity according to the traditional logarithmic relationship. Instead, an unusual decrease in OCP is observed at high light intensity, which is attributed to the dramatic light-induced increase in surface states that can be easily regulated by varying the oxygen partial pressure during reactive magnetron sputtering. Based on such a nonmonotonic variation of OCP, a facile Bi2O3-based gate is designed to realize the XOR function. Unlike the commonly used current signal, OCP is size independent, and therefore, the Bi2O3-based gate does not require high manufacturing accuracy. Moreover, in addition to XOR, the Bi2O3-based PEC gate also demonstrates great versatility in realizing other logic functions including AND, OR, NOT, NIH, NAND, and NOR. The strategy of modulating and applying nonmonotonic OCP signal opens a new avenue for designing size-independent reconfigurable logic gates at low manufacturing cost.

Effect of ion fluxes on regulating the light-induced transthylakoid electric potential difference

The light-induced transthylakoid membrane potential (ΔΨ) can not only drive the ATP synthesis through the ATP-synthase in chloroplasts but serve as an essential modifier in the acclimation of photosynthesis to fluctuating light conditions. It has been manifested that during photosynthesis, the light-induced ΔΨ is responsive to multiple factors among which the ion channels/transporters (e.g., V–K+, VCCN1, and KEA3) are key to adjust the ion distribution on the two sides of the thylakoid membrane and hence shape the kinetics of ΔΨ. However, an in-depth mechanistic understanding of ion fluxes on adjusting the transthylakoid electric potentials is still unclear. This lack of a mechanistic understanding is due to the experimental difficulty of closely observing ion fluxes in vivo and also hacking the evolution of parameters in a highly intertwined photosynthetic network. In this work, a computer model was applied to investigate the roles of ion fluxes on adjusting transthylakoid electric potentials upon a temporal cycle of a period of high illumination followed by a dark-adapted phase. The computing data revealed that, firstly, upon illumination, the dissipation of the steady-ΔΨ by ∼10 mV is contributed from the V–K+-driven K+ flux whilst ∼8 mV of the steady-ΔΨ is dissipated by the VCCN1-pumped Cl− flux, but there were no appreciable KEA3-evoked variations on ΔΨ; secondly, on transition from high light to darkness, V–K+ and KEA3 are serving as major contributors whereas VCCN1 taking a counterbalancing part in shaping a standard trace of ECS (electrochromic shift), which commonly shows a sharp fall to a minimum before returning to the baseline in darkness. Besides, the functional consequences on components of ΔΨ adjusted by every particular ion channel/transporter were also explored. By employing the model, we bring evidence that particular thylakoid-harbored proteins, namely V–K+, VCCN1, and KEA3, function by distinct mechanisms in the dynamic adjustment of electric potential, which might be mainly importnat under fluctuating light conditions.

Analyzing the effect of ion binding to the membrane-surface on regulating the light-induced transthylakoid electric potential (ΔΨm).

The transthylakoid membrane potential (ΔΨm) is essential because it can drive the ATP synthesis through the CF0–CF1 type of ATP-synthase in chloroplasts as an energetic equivalent similar to ΔpH. In addition, a high fraction of proton motive force (PMF) stored as the ΔΨm component is physiologically important in the acclimation of photosynthesis to environmental stresses. It has been shown that ΔΨm is the sum of the Donnan potential difference (ΔΨdn) and the diffusion potential difference (ΔΨd). Specifically, ΔΨdn, ΔΨd, and ΔΨm are strongly associated with the ionic activities near the membrane surface, particularly, the extent of ion binding to the charged/neutral sites adjacent to the membrane surface. However, an in-depth analysis of the effect of altered cationic binding to the membrane surface on adjusting the transthylakoid electric potentials (ΔΨdn, ΔΨd, and ΔΨm) is still missing. This lack of a mechanistic understanding is due to the experimental difficulty of closely observing cations binding to the membrane surface in vivo. In this work, a computer model was proposed to investigate the transthylakoid electric phenomena in the chloroplast focusing on the interaction between cations and the negative charges close to the membrane surface. By employing the model, we simulated the membrane potential and consequently, the measured ECS traces, proxing the ΔΨm, were well described by the computing results on continuous illumination followed by a dark-adapted period. Moreover, the computing data clarified the components of transthylakoid membrane potential, unraveled the functional consequences of altered cationic attachment to the membrane surface on adjusting the transthylakoid electric potential, and further revealed the key role played by Donnan potential in regulating the energization of the thylakoid membrane. The current model for calculating electric potentials can function as a preliminary network for the further development into a more detailed theoretical model by which multiple important variables involved in photosynthesis can be explored.

Measurements of the light-induced steady state electric potential generation by photosynthetic pigment-protein complexes.

In this minireview, we consider the methods of measurements of the light-induced steady state transmembrane electric potential (Δψ) generation by photosynthetic systems, e.g. photosystem I (PS I). The microelectrode technique and the detection of electrochromic bandshifts of carotenoid pigments are most appropriate for Δψ measurements in situ and in vivo. Direct electrometrical method and Δψ measurements in the photovoltaic system based on membrane filter (MF) sandwiched between semiconductor indium tin oxide electrodes (ITO) are suitable for studies of isolated pigment-protein complexes and small natural vesicles-chromatophores. In the presence of trehalose, ITO|PS I-MF|ITO system allows to keep a steady state level of ∆ψ after 1h of illumination. According to preliminary experiments, this system is capable of providing steady state light-induced ∆ψ after several months of storage in the dark at room temperature under controlled humidity in the presence of trehalose. The long-term generation of light-induced ∆ψ in relatively simple system may serve as a source of the solar-to-electric energy conversion.

Hidden surface photovoltages revealed by pump probe KPFM

In this work, we use pump-probe Kelvin probe force microscopy (pp-KPFM) in combination with non-contact atomic force microscopy (nc-AFM) under ultrahigh vacuum, to investigate the nature of the light-induced surface potential dynamics in alumina-passivated crystalline silicon, and in an organic bulk heterojunction thin film based on the PTB7-PC71BM tandem. In both cases, we demonstrate that it is possible to identify and separate the contributions of two different kinds of photo-induced charge distributions that give rise to potential shifts with opposite polarities, each characterized by different dynamics. The data acquired on the passivated crystalline silicon are shown to be fully consistent with the band-bending at the silicon-oxide interface, and with electron trapping processes in acceptors states and in the passivation layer. The full sequence of events that follow the electron–hole generation can be observed on the pp-KPFM curves, i.e. the carriers spatial separation and hole accumulation in the space charge area, the electron trapping, the electron–hole recombination, and finally the electron trap-release. Two dimensional dynamical maps of the organic blend photo-response are obtained by recording the pump-probe KPFM curves in data cube mode, and by implementing a specific batch processing protocol. Sample areas displaying an extra positive SPV component characterized by decay time-constants of a few tens of microseconds are thus revealed, and are tentatively attributed to specific interfaces formed between a polymer-enriched skin layer and recessed acceptor aggregates. Decay time constant images of the negative SPV component confirm that the acceptor clusters act as electron-trapping centres. Whatever the photovoltaic technology, our results exemplify how some of the SPV components may remain completely hidden to conventional SPV imaging by KPFM, with possible consequences in terms of photo-response misinterpretation. This work furthermore highlights the need of implementing time-resolved techniques that can provide a quantitative measurement of the time-resolved potential.

Light-induced osteogenic differentiation of BMSCs with graphene/TiO2 composite coating on Ti implant

Light-induced surface potential have been demonstrated as an effective bone marrow mesenchymal stem cells (BMSCs) osteogenic differentiation regulator. However, traditional bone repair implants almost were weak or no light-responsive. Fortunately, surface modification was a feasible strategy to realize its light functionalization for bone implants. Herein, a graphene oxide (GO)/titanium dioxide (TiO2) nanodots composite coating on the surface of titanium (Ti) implant was constructed, and GO was reduced to reduced graphene oxide (rGO) with the method of UV-assisted photocatalytic reduction. After rGO deposited on the surface of TiO2, a heterojunction formed at the interface of rGO and TiO2. With visible light illumination, positive charges accumulated on the surface of rGO/TiO2 film, and performed as a positive surface potential change. The light-induced surface potential which was generated under proper light intensity is harmless to the cell adhesion and proliferation behavior, but presented a good BMSCs osteogenic differentiation promoting effect, and the activation of the voltage-gated calcium channels through surface potential and the promotion of the adsorption of osteogenic growth factors could be the reason. This work given a new insight of the modification for Ti implant with a light-induced surface potential, and shows potential application for bone regeneration on the clinical practice through light stimulation.

Use of Cotton Textiles Coated by Ir(III) Tetrazole Complexes within Ceramic Silica Nanophases for Photo-Induced Self-Marker and Antibacterial Application.

This study was aimed at the production and characterization of coated cotton textiles with luminescent ceramic nanophases doped with cationic Ir(III) tetrazole complexes. We confirmed that SiO2 nanoparticles (NPs) do not affect the phosphorescent properties of the complexes that maintain their emission (610 and 490 nm). For the first time we transferred the luminescence feature from nanosol to textile surface, highlighting the advantages of using nanosilica as an encapsulating and stabilizing matrix. The optimized Ir@SiO2 suspensions were homogenously applied onto the cotton surface by dip-pad-dry-cure technique, as proved by the 2p-fluorescence microscope analysis. Once we verified the self-marker properties of the Ir(III) complex, we observed an excellent washing fastness of the coating with a very limited release. SiO2 in the washing water was quantified at maximum around 1.5 wt% and Ir below the inductively coupled plasma optical emission spectrometry (ICP-OES) detection limit of 1 ppm. A Franz cell test was used to evaluate any possible ex-vivo uptake of Ir@SiO2 nanoparticles across human skin tissues, showing that epidermis and dermis stop over 99% of Ir, implying a reduced impact on human health. The light-induced antimicrobial potential of the Ir@SiO2 were assessed toward both Gram(−) and Gram(+) bacteria. The results encouraged further developments of such functional textiles coated by self-markers and antibacterial active nanophases.

Probing multiphoton light-induced molecular potentials

The strong coupling between intense laser fields and valence electrons in molecules causes distortions of the potential energy hypersurfaces which determine the motion of the nuclei and influence possible reaction pathways. The coupling strength varies with the angle between the light electric field and valence orbital, and thereby adds another dimension to the effective molecular potential energy surface, leading to the emergence of light-induced conical intersections. Here, we demonstrate that multiphoton couplings can give rise to complex light-induced potential energy surfaces that govern molecular behavior. In the laser-induced dissociation of H2+, the simplest of molecules, we measure a strongly modulated angular distribution of protons which has escaped prior observation. Using two-color Floquet theory, we show that the modulations result from ultrafast dynamics on light-induced molecular potentials. These potentials are shaped by the amplitude, duration and phase of the dressing fields, allowing for manipulating the dissociation dynamics of small molecules.

Signatures of Light-Induced Potential Energy Surfaces in

SynopsisUsing theory and Cold Target Recoil Ion Momentum Spectroscopy we find signatures of light-induced molecular potential energy surfaces in the 3-dimensional proton momentum distributions of dissociating .

Near-ultraviolet photodetector based on hexagonal TmFeO3 ferroelectric semiconductor thin film with photovoltaic and pyroelectric effects

The switchable ferroelectric photovoltaic (FPV) effect facilitates application of multifunctional photoelectric devices. The drawback of the FPV effect is that it generates a very low photocurrent in highly insulated ferroelectric materials. In contrast, the light-induced pyroelectric effect enhances photoelectric performance. Both effects strongly depend on the ferroelectric polarization of the material. In this study, we fabricated and characterized a near-ultraviolet photodetector consisting of a Pt/hexagonal TmFeO3/Pt heterojunction. The switchable FPV and light-induced pyroelectric effects are both observed in a hexagonal TmFeO3 ferroelectric semiconductor film. An additional potential arises from the light-induced pyroelectric effect, which strongly depends on the light intensity. The Schottky barrier height can be modulated by both the poling electric field and light-induced pyroelectric potential. Increasing the power density above the threshold leads to switchable polarization via the light-induced pyroelectric potential. The coexistence of photovoltaic and pyroelectric effects in the hexagonal TmFeO3 ferroelectric semiconductor makes it possible to develop electronic, thermal, and optical sensors as well as energy conversion devices.

Topological properties in ABA trilayer graphene under the irradiation of light**Project supported by the National Natural Science Foundation of China (Grant No. 61604106) and Shandong Provincial Natural Science Foundation, China (Grant No. ZR2014FL025).

We study ABA trilayer graphene under irradiation of a circularly polarized light. In high-frequency regime, the effective low-energy Hamiltonian is obtained based on the Floquet theory. With increasing circularly polarized light intensity, the band structure shows a band gap closing and reopening, which happen twice. The process of the band gap closing and reopening is accompanied with a topological phase transition. We investigate the Chern numbers and the anomalous Hall conductivities to confirm the topological phase transition. The interplay between light-induced circularity-dependent effective potential and effective sublattice potential is discussed.

Self-Powered Si/CdS Flexible Photodetector with Broadband Response from 325 to 1550 nm Based on Pyro-phototronic Effect: An Approach for Photosensing below Bandgap Energy.

Cadmium sulfide (CdS) has received widespread attention as the building block of optoelectronic devices due to its extraordinary optoelectronic properties, low work function, and excellent thermal and chemical stability. Here, a self-powered flexible photodetector (PD) based on p-Si/n-CdS nanowires heterostructure is fabricated. By introducing the pyro-phototronic effect derived from wurtzite structured CdS, the self-powered PD shows a broadband response range, even beyond the bandgap limitation, from UV (325 nm) to near infrared (1550 nm) under zero bias with fast response speed. The light-induced pyroelectric potential is utilized to modulate the optoelectronic processes and thus improve the photoresponse performance. Lasers with different wavelengths have different effects on the self-powered PDs and corresponding working mechanisms are carefully investigated. Upon 325 nm laser illumination, the rise time and fall time of the self-powered PD are 245 and 277 µs, respectively, which are faster than those of most previously reported CdS-based nanostructure PDs. Meanwhile, the photoresponsivity R and specific detectivity D* regarding to the relative peak-to-peak current are both enhanced by 67.8 times, compared with those only based on the photovoltaic effect-induced photocurrent. The self-powered flexible PD with fast speed, stable, and broadband response is expected to have extensive applications in various environments.

From CdS to Cu7 S4 Nanorods via a Cation Exchange Route and Their Applications: Environmental Pollution Removal, Photothermal Conversion and Light-Induced Water Evaporation

A detailed transformation process of cation exchange (CE) chemistry from hexagonal phase CdS to CdS-Cu7S4 core-shell heterostructures nanorods and eventually to monoclinic phase Cu7S4 nanorods is reported. Applications of the nanorods in environmental pollution removal, photothermal conversion and light-induced water evaporation were investigated. The influence of the reaction time, morphology, and crystal phase on the photocatalysis, photothermal conversion and light-induced water evaporation was discussed. The results showed maximum removal value of rhodamine B by 97.8% at 1 h timepoint. Aqueous dispersion of the core-shell CdS-Cu7S4 heterostructures nanorods prepared with 2 h (1.0 g/L) exhibited a good photothermal conversion performance as it saw increase in sample temperature by 37.8 oC in 300 s under the irradiation of 980 nm laser. The exposure of as-obtained sample film to simulated sunlight irradiation at 2 h (8 mg) increased the weight loss of pure water by 0.62 g in 30 min with the water evaporation efficiency up to 48.4%. We are reporting an excellent light-induced water evaporation performance potential of the nanorods discussed.

Single-laser-induced quantum interference in photofragmentation reaction of D+2

ABSTRACTWe theoretically study quantum interference phenomena in the photofragmentation reaction of breaking up the diatomic molecular ion D+2 into an atom D and an ionic D+ by a single ultrashort laser pulse. The numerical results show that the occurrence of quantum interference in kinetic energy release spectra is sensitive to the angle between the molecular axis and the laser field polarisation direction. Such sensitivity is shown to be dependent on the initial vibrational quantum state of the system and the pulse intensity. The underlying mechanism for the single-pulse-induced quantum interference is analysed by using a light-induced potential and light-induced conical intersection concept.

Physiological Responses to Cadmium, Nickel and their Interaction in the Seedlings of Two Maize (Zea mays L.) Cultivars

Abstract In the leaves of maize seedlings, cultivars Premia and Blitz, the relatively low 2 μmol/L concentration of cadmium (Cd), nickel (Ni), or both metals acting simultaneously (Cd +Ni) for 72 h, induced a significant metal accumulation, decrease in total K+ content, reduction of light-induced membrane electrical potential (EM) repolarisation in mesophyll cells and changes of ascorbic acid (AsA), dehydroascorbic acid (DHA) and glutathione (GSH) content. Shoot growth and the values of resting EM did not change significantly. Increased K+ leakage, from the leaves, and lipid peroxidation accompanied by increase of TBA-reactive substances (TBARS) were found only in cv. Blitz exposed to Cd + Ni. This indicates a capability of high leaf-cell anti-oxidant pool to ameliorate the toxic effects on plasma membrane of single ions in both cultivars, and of Cd + Ni only in cv. Premia. The decreased total content of K+ in leaves in all variants indicated repressing the K+ uptake and/or distribution to the shoots. Under anoxia, the magnitude of the repolarisation obtained after switching on the light was smaller in Cd-treated cultivar Premia than in the controls, and this also occurred in Ni- and Cd + Ni-treated cultivar Blitz.

Nonresonant electronic transitions induced by vibrational motion in light-induced potentials.

We find a new mechanism of electronic population inversion using strong femtosecond pulses, where the transfer is mediated by vibrational motion on a light-induced potential. The process can be achieved with a single pulse tuning its frequency to the red of the Franck-Condon window. We show the determinant role that the gradient of the transition dipole moment can play on the dynamics, and extend the method to multiphoton processes with odd number of pulses. As an example, we show how the scheme can be applied to population inversion in Na2.