Stimuli-Responsive Phase Change Materials: Optical and Optoelectronic Applications

Two bits dual-band switchable terahertz absorber enabled by composite graphene and vanadium dioxide metamaterials

This article presents the design of a 2-bit dual-band switchable terahertz absorber using a stacked combination of graphene and vanadium dioxide (VO2) metamaterials. For the first time, the proposed absorber design offers four switchable states by controlling the conductivity of graphene and VO2 metamaterial layers. The lower absorption band is produced by the graphene metamaterial, whereas the upper band is implemented by the VO2 metamaterial pattern. The structure shows two absorption bands (State 11) at 0.745-0.775 THz and 2.3-5.63 THz, when the Fermi graphene level of graphene is 0.2 eV and the VO2 is in the metallic phase. The lower absorption band is turned off, while keeping the upper band (State 01), when the graphene Fermi level is 0 eV and the VO2 layer is in the metallic phase. The upper absorption band is turned off, while preserving the lower absorption band (State 10) by switching the VO2 into the insulator phase and keeping the graphene Fermi level at 0.2 eV. Finally, both of the absorption bands are turned off by setting the graphene Fermi level to 0 eV and switching the VO2 into the insulating phase. Equivalent circuit modelling analysis and full-wave electromagnetic simulations are used to explain the operation principle of the proposed absorber. Very good agreement is obtained between the theoretical analysis and the simulations confirming the presented design principle for the 2-bit switchable absorber.

Influence of sintering temperature on structural and optical properties of Cd0.5Cu0.5CrxFe2-xO4 ferrites

Two ferrite series were synthesized. One series has nanosize samples that have been prepared by the co-precipitation method, and the second series has the corresponding bulk samples that have been sintered at 1000 °C for 6 h. X-ray diffraction has been used to estimate the cubic spinel structure of both series. The crystallite size, theoretical density, and porosity of the nanomaterials are larger than those of the bulk materials. HRTEM analysis demonstrated the aggregation of nanoscale samples, including an average particle size of 9-22.5 nm. However, bulk specimens have a limited surface area. The agglomeration of the nanoparticles was seen in TEM images, in which the mean particle size was within the limit of the crystallite size (R) result and ranged from 14 to 20 nm. The appearance of the spinel phase in the samples was validated through Raman spectroscopy. Different cation occupation ratios in either tetrahedral or octahedral sites have been identified to be associated with an observable systematic shift and asymmetric flattening in Raman spectra with a variation in Cr3+ concentration. The optical characterization was performed using the UV/Vis methodology, and the results reveal that the absorption cutoff frequency declines as the chromium content rises. It was also estimated that the optical bandgap averaged 3.6 eV for nanosamples and 4.6 eV for overall bulk materials. The highest photoluminescence emission was seen at wavelengths between λem = 415 and 460 nm. The photoluminescence emission peaks of both bulk and nanoscale materials were red-shifted. These results accurately reflect the corresponding energy gap values for almost the same ranges. Sintering leads to a rise in photoluminescence.

Bi2O2Se-based integrated multifunctional optoelectronics

The prominent light-matter interaction in 2D materials has become a pivotal research area that involves either an archetypal study of inherent mechanisms to explore such interactions or specific applications to assess the efficacy of such novel phenomena. With scientifically controlled light-matter interactions, various applications have been developed. Here, we report four diverse applications on a single structure utilizing the efficient photoresponse of Bi₂O₂Se with precisely tuned multiple optical wavelengths. First, the Bi₂O₂Se-based device performs the function of optoelectronic memory using UV (λ = 365 nm, 1.1 mW cm^-2) for the write-in process with SiO₂ as the charge trapping medium followed by a +1 V bias for read-out. Second, associative learning is mimicked with wavelengths of 525 nm and 635 nm. Third, using similar optical inputs, functions of logic gates "AND", "OR", "NAND", and "NOR" are realized with response current and resistance as outputs. Fourth is the demonstration of a 4 bit binary to the decimal converter using wavelengths of 740 nm (LSB), 595 nm, 490 nm, and 385 nm (MSB) as binary inputs and output response current regarded as equivalent decimal output. Our demonstration is a paradigm for Bi₂O₂Se-based devices to be an integral part of future advanced multifunctional electronic systems.

Design of vanadium-dioxide-based resonant structures for tunable optical response

Phase change materials are suitable for tunable photonic devices where the optical response can be altered under external stimuli, such as heat, an electrical or an optical signal. In this scenario, we performed numerical simulations to study the optical properties of a flat unpatterned resonant structure and a grating, both coated with a thin film of vanadium dioxide (VO₂). Our results suggest that it is possible to modulate broadband and narrowband reflectance spectra of the resonant structures in the visible to near-infrared range by more than 40 % when the VO₂ undergoes an insulator-to-metal phase transition. Resonant devices with a tunable spectral response may find application in sensors, filters, absorbers, and detectors.

Nanomaterials based on phase change materials for antibacterial application

This review presented the applications of PCM-based nanomaterials in bacterial infections. Firstly, the composition and biotoxicity were outlined. Secondly, various antibacterial tactics were highlighted. Lastly, the perspectives were discussed.

Bionic Polyurethane with a Reversible Core-Sheath for Real-Time On-Demand Performance Adjustment and Fluorescence Self-Reflection

Smart materials that can respond to external stimuli have attracted considerable scientific interest and achieved fruitful results with the advancement of research. However, materials with adjustable performance and which could be intervened on-demand through stimulation are still rarely mentioned. Furthermore, most of these materials published so far usually require high temperature or the assistance of catalysts to change the structure and adjust their performance, and the process is always irreversible. Herein, we proposed an anthracene-functionalized novel polyurethane with adjustable performance and fluorescence self-reflection inspired by shellfish. Anthracene was used as a dynamic group to make the polymer chain structure topologically isomerize after UV exposure, finally constructing a reversible core-sheath in a homogeneous polymer. Moreover, this process is catalyst-free and has strong spatiotemporal controllability. The appearance of the reversible core-sheath structure could achieve the performance adjustment of materials, and the strength can be increased easily in real time and on-demand by UV light exposure. Through selective irradiation, spatial control stiffening of this material can also be realized. In addition, the performance can also be self-reflected through the fluorescence to realize the performance that is visualizable. This work dramatically simplifies the requirements and conditions for material performance adjustment while expanding the versatility and applications in intelligent materials such as artificial muscles, variably flexible electronic devices, heterogeneous materials, 4D printing, and what may be discovered in the future.