Photoresponse Mechanism Research Articles

ZnO tetrapod morphology influence on UV sensing properties.

The aim of this work was to investigate how ZnO tetrapod (ZnO-T) morphology, structure, and surface charge properties (i.e. Debye length) influence their UV sensing properties, shedding light on the underlying photoresponse mechanisms. ZnO-Ts were synthesized and centrifuged to obtain three different fractions with tuned morphology, which were characterized by scanning electron microscopy, transmission electron microscopy, and high-resolution transmission electron microscopy microscopies, x-ray diffraction analysis, Brunauer-Emmett-Teller measurements, FTIR and UV-vis spectroscopies. ZnO-T UV sensors were fabricated and tested comparing among ZnO-T fractions and commercial ZnO nanoparticles. ZnO-T photoresponse was mostly influenced by ZnO-T leg diameter, with the optimal value close to the double Debye length. We also demonstrated how fractionating ZnO-Ts for morphology optimization can increased the responsivity by 2 orders of magnitude. Moreover, ZnO-T showed 3 orders of magnitude higher responsivity compared to commercial ZnO nanopowder. These results are beneficial for the engineering of efficient UV sensors and contribute to a deeper understanding the overall mechanism governing UV photoresponse.

Magnetic Nanodevices and Spin-Transport Properties of a Two-Dimensional CrSCl Monolayer

The two-dimensional intrinsic ferromagnet $\mathrm{Cr}\mathrm{S}\mathrm{Cl}$ monolayer has considerable potential for application in the development of spintronic devices because of properties such as robust ferromagnetic ordering, large spin polarization, high Curie temperature, and high carrier mobilities. Here, we investigate the electromagnetic properties of the $\mathrm{Cr}\mathrm{S}\mathrm{Cl}$ monolayer and the spin-transport properties of some conceptual magnetic devices we construct, such as p-n-junction diodes, field-effect transistors, and phototransistors, by means of first-principles calculations. The results indicate that the p-n-junction diodes of the $\mathrm{Cr}\mathrm{S}\mathrm{Cl}$ monolayer exhibit full spin-polarized transport behavior, and both the p-n and p-i-n junctions show excellent spin-filtering behavior. The phototransistor of the $\mathrm{Cr}\mathrm{S}\mathrm{Cl}$ monolayer exhibits spin-resolved photoresponse characteristics for different wavelengths of light. Furthermore, it possesses the ability to generate a fully spin-up polarized current in the visible range. Our results provide key insights into the fundamental physical properties and the underlying transport and photoresponse mechanisms of the $\mathrm{Cr}\mathrm{S}\mathrm{Cl}$ monolayer.

An overview of pyroelectric photodetector: Photoresponse mechanisms and applications

Recently, pyroelectric materials have gained significant interest for energy harvesting, sensing and detection, and so on. For photodetection, pyroelectric photodetectors show great advantages due to high-performance photo-sensing without an external bias voltage. In this review, the pyroelectric coefficients and three types of important figure of merits are introduced. Moreover, the work mechanisms of pyroelectric photodetectors, including light-induced pyroelectric effect, pyroelectric-photoelectric coupled effect, photovoltaic-pyroelectric coupled effect, pyro-phototronic effect, ferro-pyro-phototronic effect, pyro-phototronic and piezo-phototronic coupling effects, and photo-pyroelectric-thermoelectric effect, are systematically summarized. Finally, the survey of the pyroelectric applications for photoresponse is presented, which divided the pyroelectric photoresponse into four major categories consisting ultraviolet detecting, visible detecting, infrared detection, and broadband detection. This review provides a detailed analysis and summarization of the work mechanisms of pyroelectric photodetectors and the photoresponse applications, which will provide significant guidance and inspiration toward designing the improved pyroelectric photodetectors.

High-performance self-powered ultraviolet photodetector in SnO2 microwire/p-GaN heterojunction using graphene as charge collection medium

• Using a facile vapor transport deposition method, the samples of SnO 2 microwires were synthesized individually self-catalyzed by a fast and simple CVD method. By combining p-type GaN substrate, a single SnO 2 microwire heterojunction photodetector working in the ultraviolet wavelengths was constructed. • By replacing the top electrodes from ITO to monolayer graphene, enhanced ultraviolet photoresponses were obtained, which exhibiting a peak responsivity 223.7 mA/W, detectivity ∼ 6.9 × 10 12 Jones, high EQE of 77%, and a fast response speed (rising/decaying times ∼ 18/580 μs) at 360 nm light illumination when the device is operated in self-driven mode. Our results suggest that the graphene transparent electrodes can not only allow the majority of the incident light to reach the contact area but also offer an easy carrier transport channel when the electron-holes were separated. • In particular, the microscopic photoresponse mechanism of the fabricated n-SnO 2 microwire/p-GaN heterojunction photodetector that using monolayer graphene as charge collection medium was studied. The Schottky-potential is formed by the built-in electric-field from graphene to the SnO 2 wire. The built-in field is in the same direction as the original SnO 2 /GaN built-in electric field, which was similar to the reverse voltage applied to the heterojunction. Therefore, the net built-in field toward the SnO 2 /GaN heterointerface can be increased, resulting in the increment of the carrier collection efficiency through graphene as a charge collection medium. Graphene monolayer has been extensively applied as a transparency electrode material in photoelectronic devices due to its high transmittance, high carrier mobility, and ultrafast carrier dynamics. In this study, a high-performance self-powered photodetector, which is made of a SnO 2 microwire, p-type GaN film, and monolayer graphene transparent electrode, was proposed and fabricated. The detector is sensitive to ultraviolet light signals and illustrates pronounced detection performances, including a peak responsivity ∼ 223.7 mA W –1 , a detectivity ∼ 6.9 × 10 12 Jones, fast response speed (rising/decaying times ∼ 18/580 μs), and excellent external quantum efficiency ∼ 77% at 360 nm light illumination without external power supply. Compared with the pristine SnO 2 /GaN photodetector using ITO electrode, the device performances of responsivity and detectivity are significantly increased over 6 × 10 3 % and 3 × 10 3 %, respectively. The performance-enhanced characteristics are mainly attributed to the high-quality heterointerface of n-SnO 2 /p-GaN, the highly conductive capacity, and the unique transparency of graphene electrodes. Particularly, the built-in potential formed at the SnO 2 /GaN heterojunction interface could be strengthened by the Schottky potential barrier derived from the graphene electrode and SnO 2 wire, enhancing the carrier collection efficiency through graphene as a charge collection medium. This work is of great importance and significance to developing excellent-performance ultraviolet photodetectors for photovoltaic and optoelectronic applications in a self-powered operation manner.

Highly photosensitive CsPbBr[formula omitted] NCs-graphene phototransistor with memory function

A high-performance Violet-light phototransistor with memory function was fabricated using CsPbBr3 nanocrystals as light absorption material, graphene as fast charge transfer channel, and graphene oxide as charge storage medium. The device demonstrated a responsivity of 2 × 103 A/W and a detectivity of 2.3 × 1012 Jones under illumination of a 405 nm laser. Moreover, the phototransistor showed an excellent photoswitching stability and a high response speed with the rise/decay times of 93/31 ms, respectively. The photoresponse mechanism was discussed using a charge injection model in an energy band diagram of CsPbBr 3 NCs and graphene interface. In addition, the device exhibited charge storage capabilities with a memory window of about 0.4 V attributed to graphene oxide as trapping layer. This work provides a viable route to fabricate phototransistors with storage capabilities.

Highly sensitive UV photodetector based on solution-processed bismuth oxyiodide epitaxial thin films

The as-fabricated photodetector based on high quality bismuth oxyiodide epitaxial thin film exhibited promising optoelectrical performance under UV light illumination and a tunable photoresponse mechanism depending on the film thickness.

Pulsed laser deposition of lead-free Cs3Cu2Br5 thin films on GaN substrate for ultraviolet photodetector applications

Recently, intensive attention has been paid to the development of copper halide perovskites (CsCuX, X = I, Br, and Cl) due to their suitable band gaps, large light absorption, environmental stability, eco-friendliness, and low cost. Although CsCuBr possesses a much larger band gap than CsCuI perovskites, there have been few reports about CsCuBr due to its poor stability in the ambient environment. In this study, high-quality Cs3Cu2Br5 thin films with good stability were prepared on GaN substrates by pulse laser deposition (PLD) for the first time. The morphology, crystal structure, and photoelectric properties of the Cs3Cu2Br5 thin films were systematically studied. Moreover, a self-powered UV photodetector based on Cs3Cu2Br5/n-GaN heterojunction was fabricated. The heterojunction device exhibits two clear dominant responses in the UV region located at 270 nm and 360 nm, with the corresponding detectivities of 5.29 × 1011 cm·Hz1/2W−1 and 3.35 × 1011 cm·Hz1/2W−1. Besides, the photoresponse mechanism of the Cs3Cu2Br5/GaN photodetector has been studied by Anderson model. Our results suggest that lead-free Cs3Cu2Br5 thin films prepared by PLD could be a potential candidate for stable and environment-friendly optoelectronic devices, such as UV detectors and light emitting devices.

Light conditions affect the growth, chemical composition, antioxidant and antimicrobial activities of the white-rot fungus Lentinus crinitus mycelial biomass.

The mycelial biomass of basidiomycetes is a promising source of compounds and represents an alternative for industrial and biotechnological applications. Fungi use light as information and hold photoresponse mechanisms, in which sensors respond to light wavelengths and regulate various biological processes. Therefore, this study aimed to investigate the effects of blue, green, and red lights on the growth, chemical composition, and antioxidant and antimicrobial activity of Lentinus crinitus mycelial biomass. The chemical composition of the mycelial biomass was determined by chromatographic methods, antioxidant activity was analyzed by in vitro assays, and antimicrobial activity was investigated by the microdilution assay. The highest mycelial biomass yield was observed under blue-light cultivation. Many primordia arose under blue or green light, whereas the stroma was formed under red light. The presence of light altered the primary fungal metabolism, increasing the carbohydrate, tocopherol, fatty acid, and soluble sugar contents, mostly mannitol, and reducing the protein and organic acid concentrations. Cultivation under red light increased the phenol concentration. In contrast, cultivation under blue and green lights decreased phenol concentration. Benzoic and gallic acids were the main phenolic acids in the hydroalcoholic extracts, and the latter acids increased in all cultures under light, especially red light. Mycelial biomass cultivated under red light showed the highest antioxidant activity in the 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay. The ferric reducing antioxidant power (FRAP) method showed that all light wavelengths increased the antioxidant activity of mycelial biomass, with the highest value under red light. Moreover, the β-carotene/linoleic acid co-oxidation (BCLA) assay demonstrated that the antioxidant activity was affected by light cultivation. Mycelial biomass grown under all conditions exhibited antibacterial and antifungal activities. Thus, mycelial biomass cultivation of L. crinitus under light conditions may be a promising strategy for controlling the mycelial chemical composition and biomass yield.

Sub-bandgap photoresponse and leakage current analysis in gold thin film-hyperdoped silicon photodiodes

We present a study of the sub-bandgap photoresponse and leakage current in gold-hyperdoped silicon photodiodes prepared using pulsed laser melting (PLM) of sub-nanometer gold films on n-type silicon substrates. Variable-temperature photo- and dark-current analysis provide insight into the role of PLM conditions on device performance. In general, we find photocurrent activation energies comparable to room temperature, suggesting a weak thermally-assisted optical photoresponse mechanism. Additionally, we establish a connection between repetitive PLM pulsing and increased device leakage current, which originates from electrically-active defects. Finally, we propose an explanation for the limited sub-bandgap external quantum efficiencies reported for hyperdoped silicon devices on the basis that the depletion layer largely does not encompass the hyperdoped layer where absorption occurs.

Space‐Resolved Photoresponse in Quasi‐Two‐Dimensional Ruddlesden–Popper Perovskites

AbstractQuasi‐two‐dimensional (quasi‐2D) organic–inorganic hybrid perovskites have shown excellent ambient stability with decent photovoltaic performance. Further improvement of device level properties requires a comprehensive understanding of the performance‐limiting mechanisms such as phase segregation, ion/electron coupling conduction, and their effects on charge transport at both the micro‐ and macro‐scale. Here, space resolved investigations of Ruddlesden–Popper (RP) phases of the commonquasi‐2D perovskite are probed using two complementary methods, conducting atomic force microscopy (c‐AFM) and Kelvin probe force microscopy (KPFM). Bias‐driven photocurrent mappings obtained by c‐AFM measurements disclose local inhomogeneous conduction and hysteresis currents in quasi‐2D RP perovskites, while relatively uniform conductivity is observed on individual grains. Bias‐driven KPFM reveals that the surface average photovoltage sign is dominated by the band bending at the buried perovskite–substrate interface. The quasi‐2D RP film exhibits substantial variations in the spatial response of the photovoltage across grains and grain boundaries, which is direct evidence of the inherently benign nature of microstructures, and the final device performance. This research elucidates underlying space‐resolved photoresponse mechanisms behind the lower efficiency of quasi‐2D RP perovskites compared with the 3D perovskites, which is necessary for further development of efficient and stable 2D perovskite‐based devices.

Comprehensive analysis of optoelectronic performance of ultraviolet phototransistors based on AlGaN/GaN heterostructure

Optoelectronic performance of ultraviolet phototransistors (UVPTs) based on AlGaN/GaN high-electron-mobility transistor (HEMT) configuration is comprehensively studied under different illumination wavelengths, light power densities, gate biases, and drain voltages. A special photoresponse mechanism combining photovoltaic effect and photoconductive effect is proposed to explain the variation of detection performance with the optical and electrical conditions. By comparing the photoreponse characteristics under typical illumination wavelengths of 310 and 360 nm, the optoelectronic properties of the HEMT-based UVPTs are deeply revealed and summarized. This work can provide suggestions and guidelines for designing of AlGaN/GaN-based UVPTs in III–V integrated photonic systems.

Molecular dynamics simulations and quantitative calculations on photo-responsive behavior of wormlike micelles constructed by gemini surfactant 12–3-12·2Br− and cinnamates with different ortho-substituents

Photo-responsive wormlike micelles constructed by photo-sensitive molecules and surfactants have attracted widespread attention from researchers. The subtle structural changes in photo-sensitive molecules have a significant effect on the photo-responsive behavior of wormlike micelles, while the underlying mechanism of this effect has not been fully investigated. In this work, photo-responsive wormlike micelles were constructed by Gemini surfactant trimethylene-1, 3-bis (dodecyldimethylammonium bromide) and three cinnamates with different ortho-substituents. The effects of ortho-substituents in cinnamates on the photo-responsive behavior of prepared micelles were explored in details by combining the experimental methods with multi-scale modelling. The experimental results have shown that the wormlike micelles containing different cinnamates exhibit different photo-responsive behaviors and the one containing trans-OMCA shows the best photo-responsiveness (viscosity decrease from 1.63 Pa·s to 0.10 Pa·s with morphology transition from wormlike micelles to spherical micelles) due to the highest conversion fraction (0.88) of the trans-OMCA and forward rate constants (1.09) speed under UV irradiation, which have been reconfirmed by quantitative calculation. Furthermore, the effect of UV irradiation time on the morphology of micelles were studied and the conformational transformation behaviors of wormlike micelles under different UV irradiation time were successfully simulated by adjusting the ratio of cis/trans-isomer. The quantitative calculation illustrates that the existence of methoxy and hydroxyl expands the difference in steric hindrance, hydrophilicity and the electrostatic distribution before and after UV irradiation, and increases the reaction equilibrium constant (methoxy is more effective), thereby enhancing photo-responsive ability of wormlike micelles. The present work provides a comprehensive understanding of photo-responsive morphology transition mechanism of wormlike micelles and the effects of structure of photo-sensitive molecule on the photo-responsiveness of wormlike micelles and thus can find a concrete way to optimize the photo-responsive self-assembly system containing surfactants and photo-sensitive molecule, which is of great importance for the design of smart responsive systems and expansion of their applications.

Switchable Photoresponse Mechanisms Implemented in Single van der Waals Semiconductor/Metal Heterostructure.

van der Waals (vdW) heterostructures based on two-dimensional (2D) semiconducting materials have been extensively studied for functional applications, and most of the reported devices work with sole mechanism. The emerging metallic 2D materials provide us new options for building functional vdW heterostructures via rational band engineering design. Here, we investigate the vdW semiconductor/metal heterostructure built with 2D semiconducting InSe and metallic 1T-phase NbTe2, whose electron affinity χInSe and work function ΦNbTe2 almost exactly align. Electrical characterization verifies exceptional diode-like rectification ratio of >103 for the InSe/NbTe2 heterostructure device. Further photocurrent mappings reveal the switchable photoresponse mechanisms of this heterostructure or, in other words, the alternative roles that metallic NbTe2 plays. Specifically, this heterostructure device works in a photovoltaic manner under reverse bias, whereas it turns to phototransistor with InSe channel and NbTe2 electrode under high forward bias. The switchable photoresponse mechanisms originate from the band alignment at the interface, where the band bending could be readily adjusted by the bias voltage. In addition, a conceptual optoelectronic logic gate is proposed based on the exclusive working mechanisms. Finally, the photodetection performance of this heterostructure is represented by an ultrahigh responsivity of ∼84 A/W to 532 nm laser. Our results demonstrate the valuable application of 2D metals in functional devices, as well as the potential of implementing photovoltaic device and phototransistor with single vdW heterostructure.

PECVD-prepared high-quality GaN films and their photoresponse properties

In this study, the high-quality GaN films are prepared by a simple, green and low-cost plasma enhanced chemical vapor deposition (PECVD) method at 950 ℃, with Ga<sub>2</sub>O<sub>3</sub> and N<sub>2</sub> serving as a gallium source and a nitrogen source, respectively. In order to improve the crystal quality of GaN films and ascertain the photoresponse mechanism of GaN films, the effect of the preparation temperature of GaN buffer layer on the crystal quality and photoelectric properties of GaN thin films are investigated. It is indicated that with the increase of the buffer temperature of GaN films, the crystal quality of GaN films first increases and then decreases, and the highest crystal quality is obtained at 875 ℃. When buffer layer temperature is 875 ℃, the calculated total dislocation density is 9.74 × 10<sup>9</sup> cm<sup>–2</sup>, and the carrier mobility is 0.713 cm<sup>2</sup>·V<sup>–1</sup>·s<sup>–1</sup>. The crystal quality of GaN film after being annealed is improved. The total dislocation density of GaN film decreases to 7.38 × 10<sup>9</sup> cm<sup>–2</sup>, and the carrier mobility increases to 43.5 cm<sup>2</sup>·V<sup>–1</sup>·s<sup>–1</sup>. The UV-Vis absorption spectrum results indicate that the optical band gap of GaN film is 3.35 eV. The scanning electron microscope (SEM) results indicate that GaN film (buffer layer temperature is 875 ℃) has smooth surface and compact structure. The Hall and X-ray photoelectron spectroscopy (XPS) results indicate that there are N vacancies, Ga vacancies or O doping in the GaN film, which act as deep level to capture photogenerated electrons and holes. With the bias increasing, the photoresponsivity of the GaN film photodetector gradually increases and then reaches a saturation value. This is due to the deep levels produced by vacancy or O doping. In addition, photocurrent response and recovery of GaN film are slow, which is also due to the deep levels formed by vacancy or O doping. At 5-V bias, the photoresponsivity of GaN film is 0.2 A/W, rise time is 15.4 s, and fall time is 24 s. Therefore, the high-quality GaN film prepared by the proposed green and low-cost PECVD method present a strong potential application in ultraviolet photodetector. The PECVD method developed by us provides a feasible way of preparing high-quality GaN films, and the understanding of the photoresponse mechanism of GaN films provides a theoretical basis for the wide application of GaN films.

Frontispiece: Elucidating Inner Workings of Naturally Sourced Organic Optoelectronic Materials with Ultrafast Spectroscopy

Ultrafast spectroscopic techniques in the electronic and vibrational domains have enabled the real-time tracking of molecular motions from electrons to atoms in naturally sourced organic chromophores, powering their intrinsic photoresponse and excited-state mechanisms that could be rationally tuned and designed for versatile optoelectronic applications. An interplay between charge/proton/energy transfer across singlet and triplet states has been highlighted as these organic optoelectronic materials represent a sustainable and efficient route to meet the growing energy and technological demands. Read more in the Review by T. D. Krueger and C. Fang on page 17736 ff.

Multiple mechanisms of the low temperature photoresponse in niobium diselenide

Niobium diselenide (NbSe2) is a layered transition metal dichalcogenide with novel quantum phases at low temperatures (T) such as superconductivity and charge density wave order. While its electronic correlations and the interaction between electrons and other collective modes have been explored extensively, a detailed study of the transport behavior of photo-excited charge carriers still remains elusive. Here, we report a systematic investigation of the photoresponse generated in homogenous NbSe2 nano-flakes near the superconducting critical temperature (Tc). By combining scanning photocurrent microscopy and classic photoconductivity measurements, three distinctive mechanisms of the photoresponse are established, including the band bending at the NbSe2–metal junction, the perturbation of the superconducting state, and the photo-bolometric effect. Among them, the photo-induced phase transition from the superconducting to normal state results in an extremely large photocurrent, which is tunable by the bias voltage and is consistent with the observation via the electrical transport characterization. The photoresponsivity of our device reaches 42.3 A/W, and the response time is less than 2 μs at T = 3.8 K for an excitation in the visible wavelength, whose performance could be further improved by optimizing the device design and the experimental condition. Our result sheds light on ultrasensitive broadband photodetection with atomically thin NbSe2 and points to a potential means of probing the correlated electronic phases by exploring light–matter interactions.

Bilateral Photoresponse of a Graphene-Oxide-Semiconductor Heterostructure Diode

We report on the photodetection properties of a graphene-oxide-semiconductor (GOS) diode by measuring its current-voltage characteristics under illumination with light-emitting diodes (LEDs). We demonstrate that a GOS structure, with graphene used as a transparent gate electrode to form an inversion layer at the oxide-semiconductor interface, can function as a GOS field-effect transistor operable at low temperatures down to 1.5 K. By investigating the gate tunneling current in a GOS diode with a transistor structure, we find that the dark current is below approximately 0.1 nA at $T=150$ K, which is almost two orders of magnitude lower than that in a graphene-semiconductor (GS) Schottky-diode photodetector. Notably, the GOS diode shows a bilateral photoresponse in both forward- and reverse-bias regimes under LED illumination. The photocurrent responsivity $R$ reaches approximately 100 mA/W at $T=150$ K with a low bias voltage of approximately $\ensuremath{-}0.6$ V, which is one order of magnitude lower than that applied in a GS Schottky-diode photodetector. We propose that a GOS heterostructure can be made to behave as a $p$-$i$-$n$ or an $n$-$i$-$p$ diode by manipulating the polarity of the bias voltage applied to the graphene gate. In addition, we quantitatively simulate the key features of the dark current by taking into account the associated bipolar current in graphene along with band-to-trap tunneling and trap-assisted tunneling processes. Our theoretical model sheds light on the mechanism of the bilateral photoresponse of the GOS photodetector. Our work paves the way to engineering hybrid devices made of two-dimensional materials and conventional semiconductors with CMOS integrability.

Tuning the Charge Transfer Dynamics of the Nanostructured GaN Photoelectrodes for Efficient Photoelectrochemical Detection in the Ultraviolet Band

AbstractThe intriguing surface sensitivity of the single‐crystalline semiconductor nanowires offers tremendous opportunity in tuning the physical properties of nanophotonic and nanoelectronic devices for versatile applications. Particularly, in the pursuit of emerging photoelectrochemical (PEC)‐type devices, significant efforts have been devoted to understanding the charge transfer dynamics between the nanowires and the electrolyte. Here, a PEC‐type ultraviolet photodetector consisting of GaN p‐n junction nanowires as photoelectrodes is constructed. It is found that two competing charge transport processes at the nanowires’ surface as well as in the p‐n junction co‐determine the photoresponsive behavior of the device. Furthermore, the surface platinum (Pt) decoration has successfully tuned the charge transfer dynamics by enhancing the charge transport efficiency at the surface, resulting in a twenty‐fold increase of the photocurrent compared to the pristine GaN nanowires. Theoretical calculations reveal that the newly formed electronic states at the Pt/GaN interface account for the improved charge transfer at the surface, and the optimal hydrogen adsorption energy contributes to the boosted PEC reaction rate. The synergy of these two effects uncover the underlying mechanism of the high photoresponse of the constructed Pt/GaN‐nanowires‐based PEC photodetectors.

Nano-imaging photoresponse in a moir\xe9 unit cell of minimally twisted bilayer graphene

Graphene-based moiré superlattices have recently emerged as a unique class of tuneable solid-state systems that exhibit significant optoelectronic activity. Local probing at length scales of the superlattice should provide deeper insight into the microscopic mechanisms of photoresponse and the exact role of the moiré lattice. Here, we employ a nanoscale probe to study photoresponse within a single moiré unit cell of minimally twisted bilayer graphene. Our measurements reveal a spatially rich photoresponse, whose sign and magnitude are governed by the fine structure of the moiré lattice and its orientation with respect to measurement contacts. This results in a strong directional effect and a striking spatial dependence of the gate-voltage response within the moiré domains. The spatial profile and carrier-density dependence of the measured photocurrent point towards a photo-thermoelectric induced response that is further corroborated by good agreement with numerical simulations. Our work shows sub-diffraction photocurrent spectroscopy is an exceptional tool for uncovering the optoelectronic properties of moiré superlattices.

Semi-convertible Hydrogel Enabled Photoresponsive Lubrication

Summary A semi-convertible hydrogel with reversible photoresponsive supramolecular lubricating ability is demonstrated by integrating a responsive supramolecular system of α-cyclodextrins/polyethylene glycol (α-CD/PEG) and competitive guest 1-[p-(phenylazo)benzyl]pyridinium bromide (AzoPB) within a poly(vinyl alcohol) (PVA) and polyacrylamide (PAAm) framework. The competitive host-guest interaction between α-CD/PEG supramolecular network and AzoPB after UV and visible-light irradiation enables the appearance and disappearance of the sol-state layer on the hydrogel surface while the PVA and PAAm consistently keeps the framework. This semi-convertible process realizes the reversible photoresponsive lubricating ability and variable toughness. We envision that this finding will offer a possible way for fabricating novel smart devices far beyond lubrication, leading to an emerging branch of interdisciplinary materials science that integrates supramolecular chemistry, hydrogel, and surface science.