Active Adoption of Void Formation in Metal-Oxide for All Transparent Super-Performing Photodetectors

A novel method for ZnO@NiO core-shell nanoparticle synthesis using pulse laser ablation in liquid and plasma jet techniques

Given their versatile nature and wide range of possible applications, core-shell nanoparticles (NPs) have received considerable attention. This paper proposes a novel method for synthesizing ZnO@NiO core-shell nanoparticles using a hybrid technique. The characterization demonstrates the successful formation of ZnO@NiO core-shell nanoparticles, which have an average crystal size of 13.059 nm. The results indicate that the prepared NPs have excellent antibacterial activity against both Gram-negative and Gram-positive bacteria. This behavior is primarily caused by the accumulation of ZnO@NiO NPs on the bacteria's surface, which results in cytotoxic bacteria and a relatively increased ZnO, resulting in cell death. Moreover, the use of a ZnO@NiO core-shell material will prevent the bacteria from nourishing themselves in the culture medium, among many other reasons. Finally, the PLAL is an easily scalable, cost-effective, and environmentally friendly method for the synthesis of NPs, and the prepared core-shell NPs could be used in other biological applications such as drug delivery, cancer treatment, and further biomedical functionalization.

Nanoscale Iron Redistribution during Thermochemical Decomposition of CaTi1-x Fe x O3-δ Alters the Electrical Transport Pathway: Implications for Oxygen-Transport Membranes, Electrocatalysis, and Photocatalysis

Potential applications of the earth-abundant, low-cost, and non-critical perovskite CaTi_1-x Fe _x O_3-δ in electrocatalysis, photocatalysis, and oxygen-transport membranes have motivated research to tune its chemical composition and morphology. However, investigations on the decomposition mechanism(s) of CaTi_1-x Fe _x O_3-δ under thermochemically reducing conditions are limited, and direct evidence of the nano- and atomic-level decomposition process is not available in the literature. In this work, the phase evolution of CaTi_1-x Fe _x O_3-δ (x = 0-0.4) was investigated in a H₂-containing atmosphere after heat treatments up to 600 °C. The results show that CaTi_1-x Fe _x O_3-δ maintained a stable perovskite phase at low Fe contents while exhibiting a phase decomposition to Fe/Fe oxide nanoparticles as the Fe content increases. In CaTi_0.7Fe_0.3O_3-δ and CaTi_0.6Fe_0.4O_3-δ, the phase evolution to Fe/Fe oxide was greatly influenced by the temperature: Only temperatures of 300 °C and greater facilitated phase evolution. Fully coherent Fe-rich and Fe-depleted perovskite nanodomains were observed directly by atomic-resolution scanning transmission electron microscopy. Prior evidence for such nanodomain formation was not found, and it is thought to result from a near-surface Kirkendall-like phenomenon caused by Fe migration in the absence of Ca and Ti co-migration. Density functional theory simulations of Fe-doped bulk models reveal that Fe in an octahedral interstitial site is energetically more favorable than in a tetrahedral site. In addition to coherent nanodomains, agglomerated Fe/Fe oxide nanoparticles formed on the ceramic surface during decomposition, which altered the electrical transport mechanism. From temperature-dependent electrical conductivity measurements, it was found that heat treatment and phase decomposition change the transport mechanism from thermally activated p-type electronic conductivity through the perovskite to electronic conduction through the iron oxide formed by thermochemical decomposition. This understanding will be useful to those who are developing or employing this and similar earth-abundant functional perovskites for use under reducing conditions, at elevated temperatures, and when designing materials syntheses and processes.

Highly Transparent Bidirectional Transparent Photovoltaics for On-Site Power Generators

If we can transparently produce energy, we may apply invisible power generators to residential architectures to supply energy without losing visibility. Transparent photovoltaic cells (TPVs) are a transparent solar technology that transmits visible light while absorbing the invisible short wavelengths, such as ultraviolet. Installing TPVs in buildings provides an on-site energy supply platform as a window-embedded power generator or color-matched solar cell installation on a building surface. The record-high power generation (10.82 mW) and photocurrent value (68.25 mA) were achieved from large-scale TPVs (25 cm²). The metal oxide heterojunction is the fundamental TPV structure. The high-performance TPVs were achieved by adopting a thin Si film between ZnO and NiO as a functional light-absorbing layer. Based on the large energy band gap of metal oxides, TPVs have a clear transmittance (43%) and good color coordinates, which ensure degrees of freedom to adopt TPV power generators in various colored structures or transparent power windows. The bidirectional feature of TPVs is ultimately desirable to maximize light utilization. TPVs can generate electric power from sunlight during the day and can also work from artificial light sources at night. In the near future, humans will acquire electric power without losing visibility with on-site energy supply platforms.

Chiral Mesostructured NiO Films with Spin Polarisation

Spin polarisation is found in the centrosymmetric nonferromagnetic crystals, chiral mesostructured NiO films (CMNFs), fabricated through the symmetry-breaking effect of a chiral molecule. Two levels of chirality were identified: primary nanoflakes with atomically twisted crystal lattices and secondary helical stacking of the nanoflakes. Spin polarisation of the CMNFs was confirmed by chirality-dependent magnetic-tip conducting atomic force microscopy (mc-AFM) and magnetic field-independent magnetic circular dichroism (MCD). Electron transfer in the symmetry-breaking electric field was speculated to create chirality-dependent effective magnetic fields. The asymmetric spin-orbit coupling (SOC) generated by effective magnetic fields selectively modifies the opposite spin motion in the antipodal CMNFs. Our findings provide fundamental insights for directional spin control in unprecedented functional inorganic materials.

Azurin-TiO2 hybrid nanostructure field effect transistor for efficient ultraviolet detection

Hybrid semiconductor nanostructures have attracted tremendous response due to their unique properties and applications in nano-optoelectronics and sensors. Here, we fabricated a back-gated transistor based on 300 nm channel of the Azurin-TiO₂ hybrid nanostructure, whose enhanced performance is attributed to the synergetic effect of the metal oxide and azurin. Surface potential mapping under the dark and light condition using Kelvin probe force microscopy, gives the perfect correlation of band gap estimation for Azurin, TiO₂ and Azurin-TiO₂ nanostructures. The extracted parameters of the transistor exhibit the majority carrier mobility of 2.26 cm² V^-1 s^-1, Schottky barrier height of 133.56 meV and low off current (6 × 10^-10 A). The photodetector showed the high spectral response of 8.7 × 10⁵ A W^-1 and detectivity of 6.4 × 10¹⁴ Jones for 260 nm wavelength, at an applied gate bias of 5 V. The short carrier transit time (3 μs) and large recombination time (0.4 s) with multiple recirculations of photo generated carries facilitate the high gain of 2.6 × 10⁶. A significant rejection ratio (R ₂₆₀/R ₅₃₀) of 56.2 at V _GS = 5 V and the linear dynamic range of 45.75 dB for 260 nm wavelength is achieved. The obtained rise and fall time of the photodetector is 0.52 s, and 0.65 s, respectively. This study suggests the applicability of Azurin-TiO₂ hybrid nanostructures with high performance for the biocompatible optoelectronic devices.

Double-side operable perovskite photodetector using Cu/Cu2O as a hole transport layer

In this study, a perovskite is integrated with an ultra-thin Cu/Cu₂O (CCO) composite film, a transparent material with high mobility, to achieve a double-side and low-voltage operable photodetector. Compared to photodetectors that utilize metal electrode with perovskite, the use of CCO significantly enhances the photocurrent (from nA up to mA). It acts as a large-scale hole transport layer. The photodetector exhibits high responsivities of 6.79 AW^-1 [illuminated via the fluorine-doped tin oxide (FTO) side] and 10.23 AW^-1 (illuminated via CCO side). The detectivities obtained at both illuminated sides are as high as over 10¹¹ Jones. Additionally, the Cu/Cu₂O-covered perovskite effectively prevents the reaction of perovskite in the interface. This work reveals that the perovskite/CCO heterojunction photodetector can be considered a promising candidate for applications in bifacial-illuminated and flexible/wearable optoelectronic technologies.

A Transparent Photonic Artificial Visual Cortex

Mimicking brain-like functionality with an electronic device is an essential step toward the design of future technologies including artificial visual and memory applications. Here, a proof-of-concept all-oxide-based (NiO/TiO₂ ) highly transparent (54%) heterostructure is proposed and demonstrated, which mimics the primitive functions of the visual cortex. Specifically, orientation selectivity and spatiotemporal processing similar to that of the visual cortex are demonstrated using direct optical stimuli under the self-biased condition due to photovoltaic effect, illustrating an energy-efficient approach for neuromorphic computing. The photocurrent of the device can be modulated from zero to 80 µA by simply rotating the slit by 90°. The device shows fast rise and fall times of 3 and 6 ms, respectively. Based on Kelvin probe force measurements, the observed results are attributed to a lateral photovoltaic effect. This highly transparent, self-biased, photonic triggered device paves the way for the advancement of energy-efficient neuromorphic computation.

Thickness-dependent photoelectrochemical properties of a semitransparent Co3O4 photocathode

Co₃O₄ has been widely studied as a catalyst when coupled with a photoactive material during hydrogen production using water splitting. Here, we demonstrate a photoactive spinel Co₃O₄ electrode grown by the Kirkendall diffusion thermal oxidation of Co nanoparticles. The thickness-dependent structural, physical, optical, and electrical properties of Co₃O₄ samples are comprehensively studied. Our analysis shows that two bandgaps of 1.5 eV and 2.1 eV coexist with p-type conductivity in porous and semitransparent Co₃O₄ samples, which exhibit light-induced photocurrent in photoelectrochemical cells (PEC) containing the alkaline electrolyte. The thickness-dependent properties of Co₃O₄ related to its use as a working electrode in PEC cells are extensively studied and show potential for the application in water oxidation and reduction processes. To demonstrate the stability, an alkaline cell was composed for the water splitting system by using two Co₃O₄ photoelectrodes. The oxygen gas generation rate was obtained to be 7.17 mL·h^-1 cm^-1. Meanwhile, hydrogen gas generation rate was almost twice of 14.35 mL·h^-1·cm^-1 indicating the stoichiometric ratio of 1:2. We propose that a semitransparent Co₃O₄ photoactive electrode is a prospective candidate for use in PEC cells via heterojunctions for hydrogen generation.

High-performing ultrafast transparent photodetector governed by the pyro-phototronic effect

In this work we utilized the advantage of the photo-induced pyroelectric effect - known as "Pyro-phototronic" - to design a self-powered, ultrafast, transparent ultraviolet (UV, 365 nm) photodetector. The device architecture contains an UV absorbing pyroelectric ZnO layer sandwiched between hole-selective V2O5 and a bottom ITO electrode. In addition, the device shows a high optical transmittance, >70%, in the entire visible region. The photo current of the device was enhanced from 19 to 42 μA under pulsed UV light illumination (λ = 365 nm, 4 mW cm-2) by exploiting the pyro-phototronic potential. In addition, the photodetector demonstrated ultrafast responses of ∼4 μs for the rise time and ∼16 μs for the fall time. Further, a high photoresponsivity of ∼36.34 mA W-1 and excellent photodetectivity of ∼6.04 × 1014 Jones, with an enhancement of 725% in both due to the pyroelectric potential, were measured. This novel approach will open a new path to design transparent and ultrafast devices, as well as on the flexible substrates, for future optoelectronic applications.

Growth of Wafer-Scale Standing Layers of WS2 for Self-Biased High-Speed UV-Visible-NIR Optoelectronic Devices

This work describes the wafer-scale standing growth of (002)-plane-oriented layers of WS₂ and their suitability for use in self-biased broad-band high-speed photodetection. The WS₂ layers are grown using large-scale sputtering, and the effects of the processing parameters such as the deposition temperature, deposition time, and sputtering power are studied. The structural, physical, chemical, optical, and electrical properties of the WS₂ samples are also investigated. On the basis of the broad-band light absorption and high-speed in-plane carrier transport characteristics of the WS₂ layers, a self-biased broad-band high-speed photodetector is fabricated by forming a type-II heterojunction. This WS₂/Si heterojunction is sensitive to ultraviolet, visible, and near-infrared photons and shows an ultrafast photoresponse (1.1 μs) along with an excellent responsivity (4 mA/W) and a specific detectivity (∼1.5 × 10¹⁰ Jones). A comprehensive Mott-Schottky analysis is performed to evaluate the parameters of the device, such as the frequency-dependent flat-band potential and carrier concentration. Further, the photodetection parameters of the device, such as its linear dynamic range, rising time, and falling time, are evaluated to elucidate its spectral and transient characteristics. The device exhibits remarkably improved transient and spectral photodetection performances as compared to those of photodetectors based on atomically thin WS₂ and two-dimensional materials. These results suggest that the proposed method is feasible for the manipulation of vertically standing WS₂ layers that exhibit high in-plane carrier mobility and allow for high-performance broad-band photodetection and energy device applications.

Wafer-scale production of vertical SnS multilayers for high-performing photoelectric devices

This study achieved wafer-scale, high quality tin monosulfide (SnS) layers. By using a solid-state reaction, the vertically aligned SnS layers spontaneously grew with sulphur reduction from the sputtered SnS₂ particles without any post processes. The quality of the SnS vertical layers was observed by high resolution transmission electron microscopy, which confirmed an interlayer space of 0.56 nm for a perfect match to the theoretical value. The phase purity of SnS was confirmed by Raman spectroscopy. The intrinsic energy band gap value (1.6 eV) of SnS is attractive for photoelectric devices. To form a heterojunction, the vertical SnS layers were grown on a n-type Si substrate. Due to the nanoscale size and vertical standing features of the SnS layers, a significantly low reflection (<5%) was realized for the SnS/n-Si heterojunction device. As a photovoltaic cell, the device provides a higher open circuit voltage (>300 mV). For photodetection, the response speed is faster than 15 μs for near infrared wavelength photons, which is a 1000 times improvement over the horizontally shaped device. The vertically standing SnS layers show high photoreactive performance, which confirms that the functional design of 2D materials is an effective route to achieve enhanced photoelectric devices, such as photodetectors and solar cells.

Three-dimensional nano-heterojunction networks: a highly performing structure for fast visible-blind UV photodetectors

Visible-blind ultraviolet photodetectors are a promising emerging technology for the development of wide bandgap optoelectronic devices with greatly reduced power consumption and size requirements. A standing challenge is to improve the slow response time of these nanostructured devices. Here, we present a three-dimensional nanoscale heterojunction architecture for fast-responsive visible-blind UV photodetectors. The device layout consists of p-type NiO clusters densely packed on the surface of an ultraporous network of electron-depleted n-type ZnO nanoparticles. This 3D structure can detect very low UV light densities while operating with a near-zero power consumption of ca. 4 × 10^-11 watts and a low bias of 0.2 mV. Most notably, heterojunction formation decreases the device rise and decay times by 26 and 20 times, respectively. These drastic enhancements in photoresponse dynamics are attributed to the stronger surface band bending and improved electron-hole separation of the nanoscale NiO/ZnO interface. These findings demonstrate a superior structural design and a simple, low-cost CMOS-compatible process for the engineering of high-performance wearable photodetectors.