Photoinduced Conductivity Research Articles

Photo-programmable hydrogel iontronics for electrically and chromatically rewritable circuits

Hydrogel-based iontronics is emerging as a promising frontier in healthcare and human-machine interfacing (HMI), offering excellent compatibility with biological systems in terms of electrical, chemical, and mechanical properties. However, conventional hydrogel systems have limitations in dynamically regulating their electrical and optical properties, which restricts their use in adaptive electronics and responsive interfaces. In this study, we present a new hydrogel system with UV photochemistry-induced reversible conductivity, enabling reversible changes in conductivity. Unlike typical photo-responsive hydrogels that revert to their original states upon removal of the light source, the new hydrogel can maintain its activated states without continuous light exposure, facilitating practical applications. By leveraging the photobase triphenylmethane leucohydroxide and photoacid n-nitrobenzaldehyde, we achieve a significant increase in photo-induced conductivity compared to existing photo-ionic hydrogels. Combining the effective photo-induced conductivity and the accompanied photochromatic effect, we demonstrate a full hydrogel-based stylus pad capable of tracking motion and strokes, and a soft calculator keypad with programmable conductivity and imprinted patterns. These advancements underscore the importance of actively controlling localized conductivity and processing light inputs in hydrogels, exhibiting their potential for diverse applications in bioelectronics and HMI.

Comparative study of photo-induced electronic transport along ferroelectric domain walls in lithium niobate single crystals

Ferroelectric domain wall conductivity (DWC) is an intriguing and promising functional property that can be elegantly controlled and steered through a variety of external stimuli such as electric and mechanical fields. Optical-field control, as a noninvasive and flexible tool, has rarely been applied so far, but it significantly expands the possibility for both tuning and probing DWC. On the one hand, as known from second-harmonic or Raman micro-spectroscopy, the optical approach provides information on DW distribution and inclination, while simultaneously probing the DW vibrational modes; on the other hand, photons might be applied to directly generate charge carriers, thereby acting as a functional and spectrally tunable probe to deduce the local absorption properties and bandgaps of conductive DWs. Here, we report on investigating the photo-induced DWC (PI-DWC) of three lithium niobate crystals, containing a very different number of DWs, namely: (A) none, (B) one, and (C) many conductive DWs. All three samples are inspected for their current–voltage behavior in darkness and for different illumination wavelengths swept from 500 nm down to 310 nm. All samples show their maximum PI-DWC at 310 nm; moreover, sample (C) reaches PI-DWCs of several microampere. Interestingly, a noticeable PI-DWC is also observed for sub-bandgap illumination, hinting toward the existence and decisive role of electronic in-gap states that contribute to the electronic charge transport along DWs. Finally, complementary conductive atomic force microscopy investigations under illumination proved that the PI-DWC indeed is confined to the DW area and does not originate from photo-induced bulk conductivity.

Photoinduced conductance and carrier switching in homoannulene ester derivatives: A theoretical exploration

Although difficult, recently a neutral homoannulene ester has been derived, which shows photoswitching between two isomers. On calculating the charge-carrier transport properties, we have found a substantial change in conductance as well as current values as we move from one photoswitchable state to another. Furthermore, the homoannulene ester shows carrier switching on DCV (dicyanovinylene, a strong electron withdrawing group) substitution. Additionally, DCV substitution enhances the conductance switching ratio to a huge extent at zero-bias and this large value is also retained at higher biases. This approach can be successfully used in organic field-effect transistors (OFETs) and other optoelectronic devices, where change in current is the essential requirement.

Neuro-inspired optical sensor array for high-accuracy static image recognition and dynamic trace extraction

Neuro-inspired vision systems hold great promise to address the growing demands of mass data processing for edge computing, a distributed framework that brings computation and data storage closer to the sources of data. In addition to the capability of static image sensing and processing, the hardware implementation of a neuro-inspired vision system also requires the fulfilment of detecting and recognizing moving targets. Here, we demonstrated a neuro-inspired optical sensor based on two-dimensional NbS2/MoS2 hybrid films, which featured remarkable photo-induced conductance plasticity and low electrical energy consumption. A neuro-inspired optical sensor array with 10 × 10 NbS2/MoS2 phototransistors enabled highly integrated functions of sensing, memory, and contrast enhancement capabilities for static images, which benefits convolutional neural network (CNN) with a high image recognition accuracy. More importantly, in-sensor trajectory registration of moving light spots was experimentally implemented such that the post-processing could yield a high restoration accuracy. Our neuro-inspired optical sensor array could provide a fascinating platform for the implementation of high-performance artificial vision systems.

Ultrafast photoinduced conductivity reduction by bonding orbital control in an incommensurate crystal

Ultrafast photoinduced conductivity reduction by bonding orbital control in an incommensurate crystal

Photo-induced conduction behaviour of chemically synthesized copper- doped lead sulphide hexapod

A hexapod-like structure of pure and copper-doped lead sulphide was synthesized by the chemical bath deposition technique. Subsequently, the synthesized structure was analysed through XRD, FESEM, UV–Vis and FTIR spectroscopy. The UV–Vis spectra showed a band gap exceeding 5 eV, a significant variation from the previously reported values. The investigation encompassed the study of photoconductivity switching behaviour of the samples. The results showed that the exposure to UV illumination substantially increases the photo-current. Also, the Ohmic current-voltage characteristic of the pure sample gets transformed to non-linear in the case of the doped sample. This has been attributed to the formation of intermediate states in the forbidden gap as well as the formation of local junctions between host and dopant atom. The optical switching characteristics of samples were also studied and the decay has been well-fitted with second-order exponential decay. This study is a pioneer investigation of photoconductive traits of copper-doped lead sulphide hexapod-like nanostructures.

Ionic liquids with reversible photo-induced conductivity regulation in aqueous solution

Stimulus-responsive ionic liquids have gained significant attention for their applications in various areas. Herein, three kinds of azobenzimidazole ionic liquids with reversible photo-induced conductivity regulation were designed and synthesized. The change of electrical conductivity under UV/visible light irradiation in aqueous solution was studied, and the effect of chemical structure and concentration of ionic liquids containing azobenzene to the regulation of photoresponse conductivity were discussed. The results showed that exposing the ionic liquid aqueous solution to ultraviolet light significantly increased its conductivity. Ionic liquids with longer alkyl chains exhibited an even greater increase in conductivity, up to 75.5%. Then under the irradiation of visible light, the electrical conductivity of the solution returned to its initial value. Further exploration of the mechanism of the reversible photo-induced conductivity regulation of azobenzene ionic liquids aqueous solution indicated that this may attributed to the formation/dissociation of ionic liquids aggregates in aqueous solution induced by the isomerization of azobenzene under UV/visible light irradiation and resulted the reversible conductivity regulation. This work provides a way for the molecular designing and performance regulation of photo-responsive ionic liquid and were expected to be applied in devices with photoconductive switching and micro photocontrol properties.

Impact of tailoring of the defect states and the band gap towards extreme photocatalytic performance and photo-induced conductivity in cobalt doped ZnO QD

Due to the advantages of tuning the electronic structure and reducing charge carrier recombination, metal doping into semiconductor metal oxides has been considered an efficient method for enhancing photocatalytic activity and photo-induced conductivity. In this paper, we focus on the effect of cobalt doping on the photocatalytic performance and photo-induced conductivity of ZnO QD. It was found that after Co doping, the photocatalytic activity of ZnO QD was remarkably higher than that of undoped ZnO QD when measured with methylene blue (MB) dye. The study showed that the complete degradation of the dye using 5 mol% cobalt doped ZnO QD occurred in just 6 min, which is 4 times faster than that of undoped ZnO QD. The extent of dye mineralization was supported by chemical oxygen demand (COD) study, which revealed that the dye was almost entirely mineralized. Furthermore, the photoconductivity and photosensitivity of 5 mol% doped Co doped ZnO QD were increased by 20 and 7 times, respectively, over that of undoped ZnO QD. The outstanding boost in photocatalytic activity and photoconductivity is caused by the tunable band gap mediated photo response, which increases light harvesting and thus the generation of a large number of electron hole pairs. Another possible explanation is that sub-energy levels formed between the conduction and valence bands act as a trap for electrons and holes, promoting charge separation by limiting photogenerated charge carrier recombination.

Reversible Photomodulation of Two-Dimensional Electron Gas in LaAlO3/SrTiO3 Heterostructures.

Long-lived photoinduced conductance changes in LaAlO3/SrTiO3 (LAO/STO) heterostructures enable their use in optoelectronic memory applications. However, it remains challenging to quench the persistent photoconductivity (PPC) instantly and reproducibly, which limits the reversible optoelectronic switching. Herein, we demonstrate a reversible photomodulation of two-dimensional electron gas (2DEG) in LAO/STO heterostructures with high reproducibility. By irradiating UV pulses, the 2DEG at the LAO/STO interface is gradually transformed to the PPC state. Notably, the PPC can be completely removed by water treatment when two key requirements are met: (1) the moderate oxygen deficiency in STO and (2) the minimal band edge fluctuation at the interface. Through our X-ray photoelectron spectroscopy and electrical noise analysis, we reveal that the reproducible change in the conductivity of 2DEG is directly attributed to the surface-driven electron relaxation in the STO. Our results provide a stepping-stone toward developing optically tunable memristive devices based on oxide 2DEG systems.

Nanoarchitectonics of Metal Atom Cluster‐Based Building Blocks Applied to the Engineering of Photoelectrodes for Solar Cells

This study deals with the nanoarchitectonic concept applied to the design of photoelectrodes built on two types of cluster core building blocks, namely, {Re6Si 8} and {Re6Sei 8}. The effect of the nature of the metal/ligand on photoinduced conductivity properties is thus investigated through an in‐depth photoelectrochemical study and it is rationalized by the establishment of an energy diagram using a set of complementary optical (ultraviolet–vis–near infrared), electrochemical and spectroscopic (X‐ray photoelectron spectroscopy) characterization techniques. The optical and electronic properties of {Re6Qi 8}‐based films (Q = S or Se) are drastically dependent on the composition. The sulfide‐based photoelectrodes exhibit ambipolar behavior with an n‐type domination whereas the selenide‐based photoelectrodes have a p‐type semiconducting behavior. Such electronic properties can be exalted by increasing the interactions between the cluster building blocks by heating. The design of mixed {Re6Qi 8}‐based photoelectrodes combining the two n‐{Re6Si 8} and p‐{Re6Sei 8} cluster core‐based building blocks is explored. The physical properties of the heterostructures can be tuned by controlling the {Re6Si 8}:{Re6Sei 8} ratio and the interaction between the clusters. The creation of such nanoarchitectonic p–n junctions allows the optimization of the photocurrents generated by increasing the separated charge state lifetime that turns out to be attractive for solar cell applications.

Chern-insulating phase in non-Dirac heterostructure: Role of polarized light and spin-orbit interaction

The non-Dirac two level systems are characterized by their non linear spectrum in kx and ky. We show that the topological phases emerge in Cu2S/MnSe and PbC/MnSe, the non-Dirac heterostructures, in the presence of electromagnetic field and SOC. It is observed that the gap can be tuned when polarized light is incident on the heterostructure by interplay of SOC and incident light. We show variation in Chern state as polarization and field strength is varied. The Chern state of Cu2S/MnSe undergoes a transition from -1 to +1 as the gap is tuned. We also calculate the photo-induced anomalous Hall conductivity of the heterostructures.

Light harvesting hexamolybdenum cluster integrated with the 3D ordered semiconductor inverse opals for optoelectronic property

Solar energy-harvesting materials have significantly contributed to the development of energy-saving applications for several decades. We have mainly used a new concept composed of the electrophoretic deposition technique and photonic crystal structural engineering to understand the tunable light-absorption and electronic conduction properties. A hexamolybdenum cluster compound (denoted as the Mo6 cluster) was successfully functionalized on a tin pyrophosphate semiconductor integrated within an inverse opal photonic crystal. The size of the periodical pores, surface modification, and chemical composition of the infiltration material of the inverse opal film have been investigated to control the photonic bandgap in the visible range and the efficiency of the deposited Mo6 cluster. The photoactive Mo6 clusters act as a visible light harvester and generate an efficient photo-induced current upon light absorption that is enhanced by a slow photon effect occurring at the photonic stopband edges. The electron and proton are transferred in the inorganic-organic network via hydrogen bonds by a hopping mechanism to generate a rapid photoconductivity response during light irradiation. Specific attention focused on the role of humidity and temperature regarding the reproducibility of the experiments and the photosensitivity of the nanocomposite. The suitable tunable photo-induced conduction property in organic-inorganic materials opens a new opportunity for the applicability of cluster-based compounds in visible optoelectronic devices.

Large increase in photo-induced conductivity of two-dimensional electron gas at SrTiO3 surface with BiFeO3 topping layer

In this work, we study and compare the photo-induced conductivity of a two-dimensional electron gas (2DEG) at the bare surface of SrTiO3 (STO) and in the heterostructure of BiFeO3 (BFO) and STO, where BFO was deposited by radio frequency magnetron sputtering. The photo-induced conductance of the BFO/STO interface shows a large increase which is 20.62 times more than the sum of photo-induced conductance from each individual BFO thin film and STO crystal. Since this photo-induced conductance of the BFO/STO heterostructure can be adjusted to become higher and lower by applying an electric field to the top surface, we attribute this large increase to the strong photo-induced electrical polarization of BFO. With the two-point setup of positive bias and negative bias, the conductivity also exhibits diode-like behavior where the forward and backward resistances are different. This work provides methods to interplay between light irradiation, electric field, and conductivity in all-oxide electronics.

Synthesis of Epitaxial Structures with Two-Dimensional Si Layers Embedded in a CaF2 Dielectric Matrix

The possibility of fabricating two-dimensional Si layers on a CaF2/Si(111) film by molecular beam epitaxy is studied. The growth conditions, under which the regions of two-dimensional Si layers are formed, are found. Raman spectroscopy, transmission electron microscopy, photoluminescence, and electron paramagnetic resonance (EPR) studies have shown that regions of two-dimensional Si layers are formed in epitaxial structures prepared by the deposition of one to three biatomic Si layers on the CaF2/Si(111) film surface at a temperature of 550°С. The Raman spectra of these structures exhibit a narrow peak at 418 cm–1, which is due to light scattering on vibrations of Si atoms in the plane of a two-dimensional Si layer intercalated with calcium. The EPR spectra of multilayer structures with regions of two-dimensional Si layers embedded in CaF2 demonstrate an isotropic signal with an asymmetric Dyson shape and the g-factor g = 1.9992 under illumination. Consequently, this signal can be attributed to photoinduced conduction electrons in extended two-dimensional Si islands. These results may be useful for understanding the mechanisms of the formation of two-dimensional materials on CaF2/Si(111) substrates.

Repeatable Photoinduced Insulator-to-Metal Transition in Yttrium Oxyhydride Epitaxial Thin Films

Currently, no known material retains its photoinduced metallic conductivity over a long time while also exhibiting repeatable metal-to-insulator recovery. Here, we demonstrate such a highly repeatable photoinduced insulator-to-metal transition in yttrium oxyhydride (YOxHy) epitaxial thin films. The temperature (T) dependence of the electrical resistivity (ρ) of the films transforms from the insulating to the metallic state (dρ/dT > 0) under ultraviolet laser illumination. The YOxHy film recovers its original insulating state when heated (125 °C) under an Ar atmosphere and regains metallic conductivity when subsequently subjected to ultraviolet laser illumination again, showing a repeatable photoinduced insulator-to-metal transition. First-principles calculations show that the itinerant carriers originate from the variations in the charge states of the hydrogen atoms that occupy octahedral interstitial sites. This study indicates that tuning the site occupancy (octahedral/tetrahedral) of the hydrogen atoms exerts a significant effect on the photoresponse of metal hydrides.

Photoinduced conductivity during sub-bandgap illumination in periodically poled MgO:LiNbO3 with charged domain walls

We report observations of a spectral dependence of the conductivity in 5 mol.% MgO doped LiNbO3 single crystals with periodical domain structure (PDS) of stripe domains with charged domain walls during sub-bandgap illumination by probing laser beams. The photoinduced conductivity is manifested through a comparatively slow relaxation of the first-order Bragg electrooptical diffraction efficiency after application of an external electric field to the crystal. The relaxation rate connected with screening of the external field due to redistribution of the space charge over the conductive charged domain walls linearly increased in the initial stage with the probe beam intensity. The parameters of the effective photoinduced conductivity averaged over the PDS period that describe its dependence on the intensity decreased from 10.9⋅10−14 to 1.79⋅10−16 Ω−1W−1m as the wavelength of the probe beam increased from 457 to 980 nm.

Direct Observation of Photo-Induced Reversible Sol-Gel Transition in Block Copolymer Self-Assembly Containing an Azobenzene Ionic Liquid.

Using atomic force microscopy, the photo-induced reversible changes in a block copolymer self-assembly containing an azobenzene ionic liquid, which undergoes sol-gel transition is directly observed. This is the first report on the sol-gel transition of an ABA-type block copolymer consisting of upper critical solution temperature (UCST)-type A blocks in a photoresponsive ionic liquid mixture. The sol-gel transition is accompanied by an order-to-disorder structural change, which subsequently induces a change in the ionic conductivity. Surprisingly, the photo-induced ionic conductivity and rheological changes occurs rapidly (≈30 s) despite the dense (≈80 wt%) polymeric system. The rapid structural change is probably attributable to the fast diffusion of the ionic liquid.

Photo-induced conductivity transient in n-type β-(Al0.16Ga0.84)2O3 and β-Ga2O3

Photo-induced conductivity transients are reported for unintentionally doped (UID) β-Ga2O3 and n-type β-(Al0.16Ga0.84)2O3. The illumination of (UID) β-Ga2O3 and β-(Al0.16Ga0.84)2O3/Ga2O3 heterojunction with a sub-bandgap light ranging from 400 to 1000 nm (1.2–3.1 eV) increases their conductivity. The increase in the conductivity still remains after the light is turned off, and then slowly exhausts. From the transient photoconductivity, the optical cross-sections of the photo-ionized defects have been measured as a function of the photon energy, and the optical absorption peaks of the ionized defects have been calculated. Thus, the measured photoconductivity in both β-Ga2O3 and (Al0.16Ga0.84)2O3 are induced by broad optical absorption peaks that have been estimated to be 2.52–2.88 eV and 2.61–3.11 eV.

Supramolecular Non-Helical One-Dimensional Channels and Microtubes Assembled from Enantiomers of Difluorenol.

The design and assembly of photoelectro-active molecular channel structures is of great importance because of their advantages in charge mobility, photo-induced electron transfer, proton conduction, and exciton transport. Herein, we report the use of racemic 9,9'-diphenyl-[2,2'-bifluorene]-9,9'-diol (DPFOH) enantiomers to produce non-helical 1D channel structures. Although the individual molecule does not present any molecular symmetry, two pairs of racemic DPFOH enantiomers can form a C2 -symmetric closed loop via the stereoscopic herringbone assembly. Thanks to the special symmetry derived from the enantiomer pairs, the multiple supramolecular interactions, and the padding from solvent molecules, this conventionally unstable topological structure is achieved. The etching of solvent in 1D channels leads to the formation of microtubes, which exhibit a significant lithium-ion conductivity of 1.77×10-4 S cm, indicating the potential research value of this novel 1D channel structure.

Observation of the Photoinduced Conductivity in a Regular Domain Structure with Tilted Walls in MgO:LiNbO3 at a Wavelength of 632.8 nm at Bragg Diffraction

A 632.8-nm radiation-induced change in the conductivity of a regular domain structure (RDS) formed in a 5% MgO:LiNbO3 crystal has been detected for the first time. As a result, the relaxation rate for the Bragg diffraction efficiency on the RDS, which is observed after the application of an external electric field, increases with the intensity of a probe beam. This dependence is linear in the initial stage of relaxation caused by the screening of the external field because of the redistribution of charges over tilted conductive domain walls of the RDS. For the probe beam with an intensity of 49 mW/mm2, the induced effective conductivity of the RDS, which is estimated as σeff = 3.5×10−9Ω−1m−1, is more than four orders of magnitude higher than the dark conductivity of the single-domain MgO:LiNbO3 sample.