(Bi,Sb)2 Se3 Alloy Thin Film for Short-Wavelength Infrared Photodetector and TFT Monolithic-Integrated Matrix Imaging.

Short-wavelength infrared photodetectors play a significant role in various fields such as autonomous driving, military security, and biological medicine. However, state-of-the-art short-wavelength infrared photodetectors, such as InGaAs, require high-temperature fabrication and heterogenous integration with complementary metal-oxide-semiconductor (CMOS) readout circuits (ROIC), resulting in a high cost and low imaging resolution. Herein, for the first time, a low-cost, high-performance, high-stable, and thin-film transistor (TFT) ROIC monolithic-integrated (Bi,Sb)2 Se3 alloy thin-film short-wavelength infrared photodetector is reported. The (Bi,Sb)2 Se3 alloy thin-film short-wavelength infrared photodetectors demonstrate a high external quantum efficiency (EQE) of 21.1% (light intensity of 0.76 µW cm-2 ) and a fast response time (3.24 µs). The highest EQE is about two magnitudes than that of the extrinsic photoconduction of Sb2 Se3 (0.051%). In addition, the unpackaged devices demonstrate high electric and thermal stability (almost no attenuation at 120 °C for 312 h), showing potential for in-vehicle applications that may experient such a high temperature. Finally, both the (Bi,Sb)2 Se3 alloy thin film and n-type CdSe buffer layer are directly deposited on the TFT ROIC (with a 64 × 64-pixel array) with a low-temperature process and the material identification and imaging applications are presented. This work is a significant breakthrough in ROIC monolithic-integrated short-wavelength infrared imaging chips.

Hot Spot Engineering in Hierarchical Plasmonic Nanostructures.

The controllable nanogap structures offer an effective way to obtain strong and tunable localized surface plasmon resonance (LSPR). A novel hierarchical plasmonic nanostructure (HPN) is created by incorporating a rotating coordinate system into colloidal lithography. In this nanostructure, the hot spot density is increased drastically by the long-range ordered morphology with discrete metal islands filled in the structural units. Based on the Volmer-Weber growth theory, the precise HPN growth model is established, which guides the hot spot engineering for improved LSPR tunability and strong field enhancement. The hot spot engineering strategy is examined by the application of HPNs as the surface-enhanced Raman spectroscopy (SERS) substrate. It is universally suitable for various SERS characterization excited at different wavelengths. Based on the HPN and hot spot engineering strategy, single-molecule level detection and long-range mapping can be realized simultaneously. In that sense, it offers a great platform and guides the future design for various LSPR applications like surface-enhanced spectra, biosensing, and photocatalysis.

Giant Room-Temperature Electrocaloric Effect of Polymer-Ceramic Composites with Orientated BaSrTiO3 Nanofibers.

Cooling based on the electrocaloric effect (ECE) is a promising solution to environmental and energy efficiency problems of vapor-compression refrigeration. Ferroelectric polymer-ceramics nanocomposites, integrating high electric breakdown of organic ferroelectrics and large EC strength of ceramics, are attractive EC materials. Here, we tuned the orientation of Ba0.67Sr0.33TiO3 nanofibers (BST nfs) in the P(VDF-TrFE-CFE) polymer. When the nfs were aligned parallel to the field, a ΔT of 11.3 K with an EC strength of 0.16 K·m/MV was achieved in the blends. The EC strength not only surpasses advanced nanocomposites but also is comparable to ferroelectric ceramics. The simulation indicates that a significantly higher electric field is concentrated in polymer regions around the ends of the orientated nfs, contributing to easier flipping of polymer chains for large ECE. This work provides a new method to obtain large ECE in composites for next-generation refrigeration.

How Far Can We Push the Rigid Oligomers/Polymers toward Ferroelectric Nematic Liquid Crystals?

The emerging ferroelectric nematic (NF) liquid crystal is a novel 3D-ordered liquid exhibiting macroscopic electric polarization. The combination of the ultrahigh dielectric constant, strong nonlinear optical signal, and high sensitivity to the electric field makes NF materials promising for the development of advanced liquid crystal electroopic devices. Previously, all studies focused on the rod-shaped small molecules with limited length (l) range and dipole moment (µ) values. Here, through the precision synthesis, we extend the aromatic rod-shaped mesogen to oligomer/polymer (repeat unit up to 12 with monodisperse molecular-weight dispersion) and increase the µ value over 30 Debye (D). The NF phase has a widespread existence far beyond our expectation and could be observed in all the oligomer/polymer length range. Notably, the NF phase experiences a nontrivial evolution pathway with the traditional apolar nematic phase completely suppressed, i.e., the NF phase nucleates directly from the isotropic liquid phase. The discovery of thte ferroelectric packing of oligomer/polymer rods not only offers the concept of extending the NF state to oligomers/polymers but also provides some previously overlooked insights in oxybenzoate-based liquid crystal polymer materials.

Plasmonic metamaterial absorbers with strong coupling effects for small pixel infrared detectors.

Here we report a metal-insulator-metal (MIM) based infrared plasmonic metamaterial absorber consisting of deep subwavelength meander line nanoantennas. High absorption composed of two-hybrid modes from 11 µm to 14 µm is experimentally demonstrated with a pixel pitch of 1.47 µm corresponding to a compression ratio of 8.57. The physical mechanisms responsible for novelty spectral absorption, including the strong coupling between the plasmon resonances and the phonon vibrations, material loss from the dielectric spacer, localized surface plasmon resonance (LSPR), and Berreman mode excited by oblique incidence, have been systematically analyzed by finite-difference time-domain (FDTD) method, Fabry-Perot resonance model and two-coupled damped oscillator model. At oblique incidence, a spectral splitting related to the strong coupling between LSPR mode and Berreman mode is also observed. The distribution of local electromagnetic fields and ohmic loss are numerically investigated. Moreover, we evaluate the absorption performances with finite-sized arrays. We also show that the absorber can maintain its absorption with a 2 × 2 nanoantenna array. Such a miniaturized absorber can adapt to infrared focal plane arrays with a pixel size smaller than 5 µm, and thermal analysis is also performed. Our approach provides an effective way to minimize the antenna footprint without undermining the absorber performances, paving the way towards its integration with small pixels of infrared focal plane arrays for enhanced performances and expanded functionalities.

Flexible high-temperature dielectric materials from polymer nanocomposites.

Dielectric materials, which store energy electrostatically, are ubiquitous in advanced electronics and electric power systems. Compared to their ceramic counterparts, polymer dielectrics have higher breakdown strengths and greater reliability, are scalable, lightweight and can be shaped into intricate configurations, and are therefore an ideal choice for many power electronics, power conditioning, and pulsed power applications. However, polymer dielectrics are limited to relatively low working temperatures, and thus fail to meet the rising demand for electricity under the extreme conditions present in applications such as hybrid and electric vehicles, aerospace power electronics, and underground oil and gas exploration. Here we describe crosslinked polymer nanocomposites that contain boron nitride nanosheets, the dielectric properties of which are stable over a broad temperature and frequency range. The nanocomposites have outstanding high-voltage capacitive energy storage capabilities at record temperatures (a Weibull breakdown strength of 403 megavolts per metre and a discharged energy density of 1.8 joules per cubic centimetre at 250 degrees Celsius). Their electrical conduction is several orders of magnitude lower than that of existing polymers and their high operating temperatures are attributed to greatly improved thermal conductivity, owing to the presence of the boron nitride nanosheets, which improve heat dissipation compared to pristine polymers (which are inherently susceptible to thermal runaway). Moreover, the polymer nanocomposites are lightweight, photopatternable and mechanically flexible, and have been demonstrated to preserve excellent dielectric and capacitive performance after intensive bending cycles. These findings enable broader applications of organic materials in high-temperature electronics and energy storage devices.

Genetic optimization of plasmonic metamaterial absorber towards dual-band infrared imaging polarimetry.

Mid-infrared imaging detectors are essential tools for many applications because they can visualize the objects in the dark via thermal radiation. However, these detectors have to pair with separate spectral and polarization filters to select the target spectral bands and polarization states, resulting in complicated and bulky imaging systems. One way to mitigate the need for separate spectral filters and polarizers is to use metamaterial absorbers, which are arrays of optical resonators with sub-wavelength dimensions and spacing, to tailor the responses of the detector pixels. Here we report an intelligent program based on the genetic algorithm that automates the design and optimization of a metal-insulator-metal based metamaterial absorber with multi-sized nanostrip antennas as the top layer. The program starts from a randomly generated pattern of the top antenna layer, and it iteratively approaches the optimized designs of two polarization selective MIM absorbers with wideband high absorption in the specified 3-5 (MWIR) band and 8-12 µm (LWIR) band. The measured absorption spectra of the two optimized designs agree well with the simulated results. The influences of the incident angle of light, the finite size of detector pixels, and the air gap between the neighboring pixels on the spectral absorption are numerically evaluated.

Inverse design of multifunctional plasmonic metamaterial absorbers for infrared polarimetric imaging.

Metamaterial absorbers, consisting of assembling arrays of optical resonators with subwavelength dimensions and spacing, allow efficiently absorption electromagnetic radiation by leveraging the strong electrical and magnetic resonances. Beyond the enhanced absorption, there is a growing interest to realize multi-functional absorbers, for example, absorbers with extended bandwidth, strong polarization extinction ratio, to name a few. Traditionally, designing multi-functional absorbers require complex brute-force optimizations with sizable parameter space, which turn out to be rather inefficient. Here, using the particle swarm optimization algorithm, we design and experimentally demonstrate broadband and highly polarization selective mid-IR metal-insulator-metal absorbers, covering the technologically important 3-5 µm atmospheric transparency band. With spectrally averaged absorption exceeding 70%, a high polarization extinction ratio of 40.6 is concurrently achieved by the algorithm. We also investigate the incident angle dependence of the spectral absorption and clarify the origin of optical losses. By integrating with the growing range of mid-IR detectors and imagers, our devices can enable new applications such as mid-IR full Stokes imaging polarimetry for remote sensing.

Sandwich-structured polymer nanocomposites with high energy density and great charge-discharge efficiency at elevated temperatures.

The demand for a new generation of high-temperature dielectric materials toward capacitive energy storage has been driven by the rise of high-power applications such as electric vehicles, aircraft, and pulsed power systems where the power electronics are exposed to elevated temperatures. Polymer dielectrics are characterized by being lightweight, and their scalability, mechanical flexibility, high dielectric strength, and great reliability, but they are limited to relatively low operating temperatures. The existing polymer nanocomposite-based dielectrics with a limited energy density at high temperatures also present a major barrier to achieving significant reductions in size and weight of energy devices. Here we report the sandwich structures as an efficient route to high-temperature dielectric polymer nanocomposites that simultaneously possess high dielectric constant and low dielectric loss. In contrast to the conventional single-layer configuration, the rationally designed sandwich-structured polymer nanocomposites are capable of integrating the complementary properties of spatially organized multicomponents in a synergistic fashion to raise dielectric constant, and subsequently greatly improve discharged energy densities while retaining low loss and high charge-discharge efficiency at elevated temperatures. At 150 °C and 200 MV m(-1), an operating condition toward electric vehicle applications, the sandwich-structured polymer nanocomposites outperform the state-of-the-art polymer-based dielectrics in terms of energy density, power density, charge-discharge efficiency, and cyclability. The excellent dielectric and capacitive properties of the polymer nanocomposites may pave a way for widespread applications in modern electronics and power modules where harsh operating conditions are present.

Analysis of the acoustoelectric response of SAW gas sensors using a COM model.

Surface acoustic wave (SAW) gas sensors based on the acoustoelectric effect exhibit wide application prospects for in situ gas detection. However, establishing accurate models for calculating the scattering parameters of SAW gas sensors remains a challenge. Here, we present a coupling of modes (COM) model that includes the acoustoelectric effect and specifically explains the nonmonotonic variation in the center frequency with respect to the sensing film's sheet conductivity. Several sensing parameters of the gas sensors, including the center frequency, insertion loss, and phase, were experimentally compared for accuracy and practicality. Finally, the frequency of the phase extremum (FPE) shift was determined to vary monotonically, and the range of selectable test points was wide, making the FPE an appropriate response parameter for leveraging in SAW gas sensors. The simulation results of the COM model were highly consistent with the experimental results. Our study is proposed to provide theoretical guidance for the future development of gas SAW sensors.

Exploring the Mpemba effect: a universal ice pressing enables porous ceramics.

Piezoceramics with global porosity and local compaction are highly desired to exploit the combination of mechanical and electrical properties. However, achieving such a functional combination is challenging because of the lack of techniques for applying uniform pressure inside porous ceramic green parts. Nature provides many examples of generating strong forces inside the macro and micro channels via the state transformation of water. Inspired by these phenomena, we present a technique of "ice and fire", that is, water freezing (ice pressing) and high-temperature sintering (fire), to produce ideal porous piezoceramics. We introduce a new compaction method called the "ice pressing method", which manipulates liquid phase transition for compaction. This method has several advantages, including uniform pressure distribution, a wide pressure range, high effectiveness, and selective freezing. It can generate an ultrahigh pressure of up to 180 MPa on the piezoceramic green skeletons in minutes while retaining their functional pore structures. By exploiting the Mpemba phenomenon, we further accelerate the compaction procedure by 11%. The first ice-pressed and second fire-consolidated lead zirconate titanate (PZT) ceramics are highly densified and exhibit an outstanding piezoelectric response (d33 = 531 pC N-1), comparable to conventional pressed bulk counterparts and 10-20 times higher than those of unpressed materials. The novel ice pressing method breaks the limitation of lacking a compaction technique for porous ceramics. The versatile and effective ice pressing method is a green and low-cost route promoting applications in sensors, acoustics, water filtration, catalyst substrates, and energy harvesting.

Correction: Exploring the Mpemba effect: a universal ice pressing enables porous ceramics.

Correction for 'Exploring the Mpemba effect: a universal ice pressing enables porous ceramics' by Xiaodan Yang et al., Mater. Horiz., 2024, DOI: https://doi.org/10.1039/d3mh01869e.

Mobile energy storage technologies for boosting carbon neutrality.

Carbon neutrality calls for renewable energies, and the efficient use of renewable energies requires energy storage mediums that enable the storage of excess energy and reuse after spatiotemporal reallocation. Compared with traditional energy storage technologies, mobile energy storage technologies have the merits of low cost and high energy conversion efficiency, can be flexibly located, and cover a large range from miniature to large systems and from high energy density to high power density, although most of them still face challenges or technical bottlenecks. In this review, we provide an overview of the opportunities and challenges of these emerging energy storage technologies (including rechargeable batteries, fuel cells, and electrochemical and dielectric capacitors). Innovative materials, strategies, and technologies are highlighted. Finally, the future directions are envisioned. We hope this review will advance the development of mobile energy storage technologies and boost carbon neutrality.

Substantially Enhanced Electrocaloric Effect in Ba(Zr0.2Ti0.8)O3 Lead-Free Ferroelectric Ceramics via Lattice Stress Engineering.

As an alternative to conventional vapor-compression refrigeration, cooling devices based on electrocaloric (EC) materials are environmentally friendly and highly efficient, which are promising in realizing solid-state cooling. Lead-free ferroelectric ceramics with competitive EC performance are urgently desirable for EC cooling devices. In the past few decades, constructing phase coexistence and high polarizability have been two crucial factors in optimizing the EC performance. Different from the external stress generated through heavy equipment and inner interface stress caused by complex interface structures, the internal lattice stress induced by ion substitution engineering is a relatively simple and efficient means to tune the phase structure and polarizability. In this work, we introduce low-radius Li+ into BaZr0.2Ti0.8O3 (BZT) to form a particular A-site substituted cell structure, leading to a change of the internal lattice stress. With the increase of lattice stress, the fraction of the rhombohedral phase in the rhombohedral-cubic (R-C) coexisting system and ferroelectricity are all pronouncedly enhanced for the Li2CO3-doped sample, resulting in the significant enhancement of saturated polarization (Ps) as well as EC performance [e.g., adiabatic temperature change (ΔT) and isothermal entropy change (ΔS)]. Under the same conditions (i.e., 333 K and 70 kV cm-1), the ΔT of 5.7 mol % Li2CO3-doped BZT is 1.37 K, which is larger than that of the pure BZT ceramics (0.61 K). Consequently, in cooperation with the great improvement of electric field breakdown strength (Eb) from 70 to 150 kV cm-1, 5.7 mol % Li2CO3-doped BZT achieved a large ΔT of 2.26 K at a temperature of 333 K, which is a competitive performance in the field of electrocaloric effect (ECE). This work provides a simple but effective approach to designing high-performance electrocaloric materials for next-generation refrigeration.

Highly Sensitive Chemiresistive H2S Detection at Subzero Temperature over the Sb-Doped SnO2@g-C3N4 Heterojunctions under UV Illumination.

NASA has detected H2S in the persistently shadowed region of the lunar South Pole through NIR and UV/vis spectroscopy remotely, but in situ detection is generally considered to be more accurate and convincing. However, subzero temperatures in space drastically reduce chemisorbed oxygen ions for gas sensing reactions, making gas sensing at subzero temperature something that has rarely been attempted. Herein, we report an in situ semiconductor H2S gas sensor assisted by UV illumination at subzero temperature. We constructed a g-C3N4 network to wrap the porous Sb doped SnO2 microspheres to form type II heterojunctions, which facilitate the separation and transport of photoinduced charge carriers under UV irradiation. This UV-driven technique affords the gas sensor a fast response time of 14 s and a response value of 20.1 toward 2 ppm H2S at -20 °C, realizing the sensitive response of the semiconductor gas sensor at subzero temperature for the first time. Both the experimental observations and theoretical calculation results provide evidence that UV irradiation and the formation of type II heterojunctions together promote the performance at subzero temperature. This work fills the gap of semiconductor gas sensors working at subzero temperature and suggests a feasible method for deep space gas detection.

Enhanced Energy Storage Performance by A-B Site Ambipolar Co-Doping in Antiferroelectrics.

Antiferroelectric materials are promising to be used for power capacitive devices. To improve the energy storage performance, solid-solution and defect engineering are widely used to suppress the long-range order by introducing local heterogeneities. However, both methods generally deteriorate either the maximum polarization or breakdown electric field due to damaged intrinsic polarization or increased leakage. Here, we show that forming defect-dipole clusters by A-B site acceptor-donor co-doping in antiferroelectrics can comprehensively enhance the energy storage performance. We took the La-Mn co-doped (Pb0.9Ba0.04La0.04)(Zr0.65Sn0.3Ti0.05)O3 (PBLZST) as an example. For co-doping with unequal amounts, high dielectric loss, impurity phase, and decreased polarization were observed. By contrast, La and Mn in an equal amount of co-doping can significantly improve the overall energy storage performance. An over 48% increasement in both the maximum polarization (62.7 µC/cm2) and breakdown electric field (242.6 kV/cm) was obtained in 1 mol % La and 1 mol % Mn equally co-doped PBLZST, followed by a nearly two-time enhancement in Wrec (6.52 J/cm3) compared with that of the pure matrix. Moreover, a high energy storage efficiency of 86.3% with an enhanced temperature stability over a wide temperature range can be achieved. The defect-dipole clusters associated with charge-compensated co-doping are suggested to contribute to an enhanced dielectric permittivity, linear polarization behavior, and maximum polarization strength compared with that of the unequal co-doping cases. The defect-dipole clusters are suggested to couple with the host, leading to a high energy storage performance. The proposed strategy is believed to be applicable to modify the energy storage behavior of antiferroelectrics.

Thermoelectric coupling effect in BNT-BZT-xGaN pyroelectric ceramics for low-grade temperature-driven energy harvesting.

Pyroelectric energy harvesting has received increasing attention due to its ability to convert low-grade waste heat into electricity. However, the low output energy density driven by low-grade temperature limits its practical applications. Here, we show a high-performance hybrid BNT-BZT-xGaN thermal energy harvesting system with environmentally friendly lead-free BNT-BZT pyroelectric matrix and high thermal conductivity GaN as dopant. The theoretical analysis of BNT-BZT and BNT-BZT-xGaN with x = 0.1 wt% suggests that the introduction of GaN facilitates the resonance vibration between Ga and Ti, O atoms, which not only contributes to the enhancement of the lattice heat conduction, but also improves the vibration of TiO6 octahedra, resulting in simultaneous improvement of thermal conductivity and pyroelectric coefficient. Therefore, a thermoelectric coupling enhanced energy harvesting density of 80 µJ cm-3 has been achieved in BNT-BZT-xGaN ceramics with x = 0.1 wt% driven by a temperature variation of 2 oC, at the optical load resistance of 600 MΩ.

Molecular Ferroelectric Crystals with Superior Pyroelectricity, Plasticity, and Recyclability.

The pyroelectric effect is used in a wide range of applications such as infrared (IR) detection and thermal energy harvesting, which require the pyroelectric materials to simultaneously have a high pyroelectric coefficient and a low dielectric constant for high figures of merit. However, in conventional proper ferroelectrics, the positive correlation between the pyroelectric coefficient and the dielectric constant imposes an insurmountable challenge in upgrading the figures of merit. Here, we explored superior pyroelectricity in [(CH3)4N][FeCl4] (TMA-FC) and [(CH3)4N][FeCl3Br] (TMA-FCB) molecular ferroelectric plastic crystals, which could decouple this positive correlation due to the nature of improper polarization behavior. Therefore, TMA-FC and TMA-FCB derive a high pyroelectric coefficient and a low dielectric constant simultaneously, yielding record-high figures of merit around room temperature. Furthermore, the favorable plasticity enables ferroelectric crystals to attach surfaces with different shapes for device design and integration. More interestingly, the molecular ferroelectrics could be softened and reshaped at elevated temperatures without decay in pyroelectricity, making them recyclable for cost savings and e-waste reduction. Combined with the facile fabrication process, the findings of this work would open avenues for employing molecular ferroelectric plastic crystals in the manufacture of high-performance pyroelectric devices.

Response to Comment on "Improper molecular ferroelectrics with simultaneous ultrahigh pyroelectricity and figures of merit".

Szafranski and Katrusiak stated that [Hdabco]ClO4 and [Hdabco]BF4 are proper ferroelectrics and exhibit much smaller pyroelectric coefficients than our results. We disagree with the arguments and provide a detailed answer highlighting misunderstandings in their interpretation.