Thin Film Architectures Research Articles

Multiferroicity and Semi‐Cylindrical Alignment in Janus Nanofiber Aggregates

Abstract1D multiferroic fibers are known to exhibit attractive characteristics, including enhanced magnetoelectric (ME) coupling compared to thin film and bulk architectures. A comprehensive understanding of composite fibers, however, has been hindered by the complexity of their structure, leading to limited reports. Here, clear and strong ME coupling is experimentally detected in a composite Janus nanofiber aggregate using second harmonic generation (SHG) polarimetry under different magnetic field orientations. The observation of such a pronounced effect using an all‐optical method has not been previously reported in multiferroic fibers. A series of global fits is performed to the SHG polarimetry results to investigate the behavior of nanofibers within an aggregate. We find the magnetically assembled fibers exhibit semi‐cylindrical alignment as well as the expected lengthwise alignment despite variations in size and composition from fiber to fiber. The ME coupling and the semi‐cylindrical alignment seen in SHG are further corroborated via X‐ray diffraction under similar magnetic field conditions. These findings contribute to the development of complex composite and multifunctional devices using multiferroic nanostructures as building blocks, even those with inhomogeneous shapes and geometries.

Tuning Electrostatics to Promote Ordered Monolayers of Phosphole-Lipids.

Heteroatom-doped π-conjugated systems have been exploited for electronic device applications. Phosphole-containing backbones are particularly intriguing with regard to electron delocalization and ultimately control over the photophysical, redox, and charge transfer characteristics. However, practical application of these materials commonly relies on well-structured, thin film architectures. Self-assembly offers a route to generate ordered 2D structures deposited on solid substrates. The orientation of these deposited films can be manipulated by the introduction of alkyl chains of varying length (to form phosphole-based lipids) and chemical modification of the phosphole-derived headgroup. External controls can also be considered to tune these films' properties, e.g., the electrostatic interactions within the film can be controlled by varying the environment with the addition of simple salt counterions. A series of lipids with phosphole-based π-conjugated headgroups have been designed and exhibit intramolecular conformational changes in response to external conditions. Herein the 2D film structure in Langmuir and Langmuir-Blodgett films is reported in the presence and absence of halide salts. The film morphology obtained from Brewster angle and atomic force microscopy shows the formation of a condensed phase but also 3D aggregates, and grazing incidence X-ray diffraction confirmed the presence of untilted, hexagonally packed chains. The size of the counterion influences its ability to intercalate between the phosphole headgroups, which ultimately provides a means to induce the formation of a well-ordered, single monolayer film without aggregates that can be transferred to the solid substrate.

Amplifying bactericidal activity: Surfactant-mediated AgBr thin film coating over two-dimensional vertically aligned ZnO nanorods for dark-light dual mode disinfection

Thin film coatings with potent antibacterial properties find critical applications in diverse domains such as medical devices, frequently touched surfaces, and food packaging for combating microbial proliferation across diverse scenarios. Two-dimensional photocatalytic antimicrobial coatings, offering a substantial actual-to-apparent surface ratio, hold immense potential for achieving this objective. However, realizing antibacterial performance not just under light but also in dark conditions remains a challenge. To address this, we present AgBr-coated vertically aligned ZnO nanorods (NRs) thin film architecture, employing a unique surfactant-mediated solution-phase spin-coating approach for achieving uniform deposition of AgBr onto ZnO NRs. The resulting ZnO NRs/AgBr heterojunction architectures have been characterized for their microstructural, morphological, elemental, optical, and wettability attributes. The studies have ascertained the tunability of AgBr content by modulating the concentration of its surfactant-based precursor solution. Further, valence band (VB) analyses revealed an increase in the electron density near to the VB edge. The dual role of AgBr as an antimicrobial agent and a photosensitizer, effectively enhancing the visible-light photodisinfection efficacy of ZnO NRs, has been evident through the dark-light dual mode antibacterial studies. Electron paramagnetic resonance measurements have shown hydroxyl radicals being majorly responsible for the visible-light photodisinfection performance. Encouragingly, reusability assessments showcase significant promise, while artificial sweat-wiping studies on the structures unveil heightened photodisinfection efficacy. This enhancement could be attributed to components like urea and lactic acid, speculated to augment the photocatalytic efficiency by minimizing charge recombination.

Tunable electrocaloric effect in selective ferroelectric bilayers via electrostatics for solid-state refrigeration and microelectronics thermal management

Ferroelectric (FE) electrocaloric materials research has been blossoming worldwide for solid-state refrigeration and potential cooling systems replacing thermoelectric Peltier coolers in microelectronics. In this work, we report the outcomes from a systematic study of combined phase transition (thermodynamics) based on the phenomenological Landau theory and distributed electric field (electrostatics of thin film interfaces) in FE bilayer films. Specifically, the compositional variation of ferroelectric bilayers results in broken spatial inversion symmetry leading to asymmetric thermodynamic potentials due to a combination of normal (first- and second-order phase transition) and relaxor (dispersive dielectric constant) ferroelectric behaviors devised for efficient electrocaloric cooling effects. Extensive theoretical analyses conducted for bilayers consisting of insulating materials highlight modified phase transition temperature behavior and self-poling by effective electric field amplification arising due to bilayers’ electrostatic coupling yielding significant changes in isothermal entropy (ΔS) and adiabatic temperature (ΔT). The theoretical calculation insights supported with experimental results signify, through case studies for a combination of materials experimental parameters, that amplification of the local electric field and materials engineering maximize the number of coexisting phases at or away from the morphotropic phase boundary of constituent layers in bilayer thin film architectures, which can be applicable toward other classes of materials and multilayer systems. These are effective ways for efficient cooling, in general, and for microelectronics thermal management either directly or by developing a thermal switch with phase change materials integrated with thermoelectric coolers for residual heat dissipation, both at the system and on-chip levels.

3D-printed film architecture via automatic micro 3D-printing system: Micro-intersection engineering of V2O5 thin/thick films for ultrafast electrochromic energy storage devices

Existing approaches to develop electrochromic (EC) energy storage devices typically focus on the modification of active materials toward a porous morphology, crystallinity, heteroatom-doping, and hybridization/composites with functional materials; however, these approaches may be unsuitable for the full commercialization of EC energy storage devices owing to their complex and expensive processes and incompatibility with large-scale and mass production. Herein, we fabricate 3D-printed film architectures for ultrafast EC energy storage devices, including micro-intersections of vanadium oxide (VO) thin/thick films, via an automatic micro 3D-printing system without active material modification. The micro-intersection originates from the one-pot 3D-printing of vertical lines, creating VO grid-micropatterns with nanoscale thin/thick films on fluorine-doped tin oxide (FTO)/glass. The micro-intersection density was engineered by adjusting the number of printed-lines per unit area and optimized micro-intersection density (OD-3DVO) allows enhanced behaviors of electrons and Li-ions as follows: (i) the thin film architecture alleviates rate-limiting processes that produce sluggish electron transfer and ionic diffusion kinetics by shortening transport channels between FTO and the electrolyte, thereby accelerating ultrafast charge transport. (ii) Micro-width/nano-depth 3D-wells induce capillary force pumping and deep electrolyte infiltration at the interface, thereby enhancing electrolyte wettability. (iii) grid-micropatterned film morphology provides extensive redox sites and high visible transmissivity; thus, the OD-3DVO active layers in EC energy storage devices featured simultaneously enhanced optical and energy storage performances, especially ultrafast switching speeds (0.7 and 0.9 s for coloration and bleaching) and record-high areal energy density at high areal power density (14.03 µWh/cm2 at 25 mW/cm2).

High-quality GeSn thin-film resonant cavities for short-wave infrared applications

High-quality infrared (IR) devices made of group IV materials are highly promising to replace traditional III–V semiconductor-based IR optoelectronics due primarily to their compatibility with mature silicon-based technologies and much lower costs. In this regard, germanium tin (GeSn) has emerged as the only direct bandgap material in the group IV family exhibiting superior electrical and optical characteristics. In the past years, GeSn IR optoelectronics including lasers and photodetectors have been realized, although novel device architectures are still needed to enhance their device performance. Here, we experimentally demonstrate high-performance, large-area (mm2) GeSn thin-film resonant cavities (film thickness resonance operating at short-wave IR wavelengths by employing membrane transfer techniques). The fabricated asymmetric air/GeSn/metal structures exhibit high absorptance (>90%) at designed resonance wavelengths, which are easily tuned by tailoring the GeSn layer thickness. The measured resonance absorption reveals excellent agreement with numerical simulations, which further elucidates the mode characteristics of the observed cavity resonances. The demonstrated thin-film device architectures could greatly facilitate the development of novel GeSn photonic devices with tunable wavelengths and enhanced performance enabled by strain engineering, and could allow for the integration of GeSn into many existing group IV-based devices for next-generation IR optoelectronics where high performance, small footprint, and low cost are all required.

Interfacial Engineering by VOx/m-TiO2 Films for Optimizing Photon-Generated Carrier to Boost Photoelectrochemical N2 Conversion to NH3.

Optimal design of the photocathode is crucial and a meaningful approach for regulating many important photoelectrochemical (PEC) reactions. Interfacial engineering is substantiated as an effective tactic for tuning the direction of the internal carrier flow in thin-film semiconductor solar devices. Yet, so far, the type of PV device architecture involving in the interfacial transport layer is less adopted in photoelectrochemical (PEC) devices. Herein, the coupled VOx/TiO2 interfacial engineering brings in the construction of an integrated p-ZnTe hetero-structured photocathode, which was composed of a PN junction constructed with p-ZnTe and CdS, VOx as the interface layer for hole transport, and m-TiO2 as the scaffold layer. Compared with the simple PN structure, the photocathodes with the assembly of interfacial engineering enable advances in the combination of apparent quantum efficiency (AQE: 0.6%) and better yield (6.23 μg h-1 cm-2) on photoelec-N2 conversion to NH3. Interfacial engineering and heterojunction construction effects synergistically optimize photoexcited carriers and the separation and transformation at the interface. This favors easier migration of holes to the back and the assembly of electrons on the surface, achieving the intensive charge separation and surface charge injection efficiency of photogenerated carriers. Our work represents a new enlightenment for building thin-film photocathode architectures to boost the effectiveness on solar-driven utilization.

Two quasi-interfacial p-n junctions observed by a dual-irradiation system in perovskite solar cells

In general, perovskite solar cells (PSC) with a sensitized or thin-film architecture absorb light from a single-side illumination, and carrier separation and transport only take place inside the active layer of the perovskite film. Herein, we demonstrated a dual-irradiation PSC system in which light passes through both the fluorinated tin oxide (FTO) side and the Au electrode side, resulting in much faster interfacial charge carrier extraction and transportation than that in a single-irradiation system, in which light passes through from either the FTO or semitransparent Au electrode side. This dual-irradiation PSC system with a configuration of FTO/Cl-TiO2/Mp-TiO2/mixed perovskite/spiro-OMeTAD/Au/ITO can form two quasi-interfacial p-n junctions, which occur separately at the interfaces of TiO2/perovskite and perovskite/spiro-OMeTAD. When the PSC device was illuminated simultaneously from both the FTO and Au/ITO sides, the PSC achieved a total power conversion efficiency (PCE) as high as 20.1% under high light intensity (1.4 sun), which is higher than PCE (18.4%) of a single-irradiation system. The time of flight (TOF) photoconductivity, small perturbation transient photovoltaic (TPV), finite-difference time-domain (FDTD) optical simulations, and dual illumination-side-dependent impedance spectroscopy (ISD-IS) were used to authenticate the presence of two quasi-interfacial p-n junctions in the PSC, creating more charge carriers than only one quasi p-n junction, and thus leading to a fast recombination process.

Covalent Functionalization of CdSe Quantum Dot Films with Molecular [FeFe] Hydrogenase Mimics for Light-Driven Hydrogen Evolution.

CdSe quantum dots (QDs) combined with [FeFe] hydrogenase mimics as molecular catalytic reaction centers based on earth-abundant elements have demonstrated promising activity for photocatalytic hydrogen generation. Direct linking of the [FeFe] hydrogenase mimics to the QD surface is expected to establish a close contact between the [FeFe] hydrogenase mimics and the light-harvesting QDs, supporting the transfer and accumulation of several electrons needed to drive hydrogen evolution. In this work, we report on the functionalization of QDs immobilized in a thin-film architecture on a substrate with [FeFe] hydrogenase mimics by covalent linking via carboxylate groups as the anchoring functionality. The functionalization was monitored via UV/vis, photoluminescence, IR, and X-ray photoelectron spectroscopy and quantified via micro-X-ray fluorescence spectrometry. The activity of the functionalized thin film was demonstrated, and turn-over numbers in the range of 360-580 (short linkers) and 130-160 (long linkers) were achieved. This work presents a proof-of-concept study, showing the potential of thin-film architectures of immobilized QDs as a platform for light-driven hydrogen evolution without the need for intricate surface modifications to ensure colloidal stability in aqueous environments.

Effect of aluminum doping on the structural, optical and electrical properties of ZnO thin films processed under thermal shock conditions

The structural, morphological, optical, and electrical properties of Al-doped ZnO thin films annealed under thermal shock conditions were studied. The films were prepared using the sol–gel method and deposited on glass substrates by dip-coating technique. The effects of varying the concentration of aluminum (Al) were analyzed using X-ray Diffraction (XRD), scanning electron microscope (SEM), UV–Vis, and the 4-point probe method. The XRD data demonstrated a degradation in the crystalline structure of the films and the emergence of other preferred directions (100) and (101), which supported the creation of a hexagonal wurtzite structure. The SEM images supported the findings of the XRD analysis. The best electrical and optical performance was obtained for an Al concentration of 1 at%, where the resistivity value was 0.191 Ω.cm and the band gap was 3.26 eV, with a high transmittance greater than 90%.The findings of this study render the developed thin-film architecture a viable candidate for high-performance optoelectronic applications.

Epitaxial tin selenide thin film thermoelectrics

To enable the realization of miniaturized thermoelectric energy generation (TEG) devices for autonomous wireless sensors, high-quality thin film architectures are required. Although tin selenide (SnSe) has been identified as a promising thermoelectric material exhibiting ZT values up to 2.6 at 923 K for single crystals, most thin film studies evaluated polycrystalline or textured SnSe samples. Here, we have explored for the first time the impact of epitaxial alignment of the orthorhombic SnSe crystal structure on an orthorhombic DyScO3 substrate, in strong contrast to the few previous studies on cubic substrates. The achieved (100)-oriented single crystalline SnSe thin films exhibit the formation of two SnSe domain types. The in-plane electrical conductivity along the (b,c)-plane shows an abrupt increase above 400 K instead of the typical steady increase. The in-plane Seebeck coefficients exhibit very similar values as single crystals, leading to a maximum power factor of about 6.0 μW·K−2·cm−1. For these SnSe thin films, exhibiting two domain variants with in-plane alignment of the (b,c)-plane, a typical low thermal conductivity is expected, demonstrating the effectiveness of epitaxial alignment to enhance thermoelectric performance and to enable the realization of miniaturized thermoelectric energy generation (TEG) devices for autonomous wireless sensors.

(Digital Presentation) Resistive Switching and Brain-Inspired Computing in Perovskite Nanowires and Quantum Wires

In the past decade, halide perovskites (HPs) have shot to fame in the genre of optoelectronics and photovoltaics owing to their large absorption co-effcient, high color purity, tunable bandgap and long charge diffusion lengths. Besides these traits, HPs also possess innumerable charge transport pathways, inherent hysteresis, high charge-carrier and ionic mobilities which render them as ideal candidates for resistive random access switching memories (RRAMs). However owing to material and electrical instability associated with HP thin-film devices, the figures-of-merits (FOMs) namely retention, endurance and switching speed were not up to the state of-the-art standard until recently. In order to revolutionize HP Re-RAMs we devised a unique device structure where we replaced the thin-film architecture with vertically aligned high density HP nanowires and quantum wires embedded in a porous alumina membrane (PAM) sandwiched between metallic silver and aluminum contacts. The excellent passivation provided by the PAM imparted the requisite electrical and material stability to the environmentally delicate HPs by drastically reducing the surface diffusion pathways and thereby thwarting the moisture induced attacks. Extrapolated retention time as high as 28.3 years and measured device endurance of a million cycles were obtained. Utilizing the single crystalline HP nanowires and quantum wires and their associated high ionic and electronic mobilities, switching speed as fast as 100 ps was also obtained. These FOMs represent record values for HP RRAMs ever reported. Furthermore a 14 nm lateral size HP quantum wire RRAM cell was fabricated and a cross-bar device architecture with a unique sneaky path mitigation scheme were developed, which successfully exhibited the scalability potential of our devices. We further coupled the optoelectronic and switching behaviors and were able to obtain optical programmability among the low resistance states. Besides data storage, the HP nanowires and quantum wires were employed in developing neuromorphic devices enabled with low power and high precision computing capabilities. Specifically, we obtained robust multi-level states in two types of brain-inspired devices capable of performing analog processing tasks by using silver as the top electrode and precisely controlling the current injection in the monocrytalline switching medium and by using indium doped tin oxide as the top electrode and inducing a novel valence change mechanism in the HP nanowires triggering the gradual conductance change. All in all, our nanowire and quantum wire devices propel HP RRAMs to the state-of-the-art standard in multifarious applications concerning future data storage and neuromorphic computing.

Trench-Structured High-Current-Driving Aluminum-Doped Indium–Tin–Zinc Oxide Semiconductor Thin-Film Transistor

This letter presents a thin-film transistor architecture, in which a “trench” is introduced between the source and drain electrode to enhance current flow. The top-gate top-contact oxide Trench thin-film transistor has a superior on-current per width of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$27.7~\mu \text{A}/ \mu \text{m}$ </tex-math></inline-formula> at a drain voltage of 4.1 V. It also has a good subthreshold swing of 0.122 V/dec and turn-on voltage of −0.4 V. This study explores the operating mechanism of the high-current-driving Trench oxide thin-film transistor.

High-performance thin-film transistor device architecture for flexible and printed electronics

A device design paradigm for thin-film transistors (TFTs) suitable for fabrication using methods available for flexible and printed electronics devices and circuits is described. The TFT architecture utilizes an array of nanospike-shaped electrodes as the source and drain electrodes. This results in improved carrier injection, greater gate control of the drain current, and lower threshold and operating voltage. The on-currents are also higher in comparison with standard flat edge electrode TFTs with equivalent channel dimensions. Importantly, the design is very tolerant of thick gate insulators. The proposed architecture requires one level of relatively high resolution patterning of the source and drain contacts, which can be potentially realized with methods that have been previously employed in flexible electronics such as nanoimprint lithography or roll-to-roll photolithography. The experimental data presented in this paper were obtained from TFTs fabricated using conventional fabrication methods, as the emphasis in this paper is on the device design and in demonstrating the advantageous features of the new architecture in future flexible systems.

Comprehensive analysis of photon dynamics in thin-film GaAs solar cells with planar and textured rear mirrors

This study describes a comprehensive analysis of light trapping and photon recycling (PR) in thin-film GaAs solar cells, explicitly considering the effects of (non-perfect) light scattering at a textured rear mirror. We calculate the probabilities of escape, reabsorption in the absorber and parasitic absorption for internally generated photons, as well as the reflectance, absorptance and parasitic absorptance of externally incident photons. Combined with the internal luminescent efficiency to account for non-radiative recombination non-explicitly, but in a phenomenological manner, device performance metrics such as the short-circuit current density ( J s c ), open-circuit voltage ( V o c ) and its radiative limit, the external luminescent efficiency, the contribution of PR to the V o c and the power conversion efficiency ( P C E ) are derived. We analyze a typical thin-film GaAs solar cell architecture, consisting of an n-GaAs/p-InGaP heterojunction structure cladded by commonly employed front- and rear-side layer stacks. The probabilities of escape, reabsorption and parasitic absorption and the resulting performance metrics are presented as a function of absorber thickness, rear-mirror reflectivity, haze factor, internal luminescent efficiency, front grid coverage and band tailing. The simulations show that the implementation of a textured rear mirror reduces PR and its importance for device performance. In these textured cells, a high rear-mirror reflectivity and, thereby, efficient PR, is less crucial to achieve V o c values close to the radiative limit. The increased J s c in textured cells renders light trapping favorable for the P C E , regardless of absorber thickness and material quality and despite reduced PR. Furthermore, the impact of an absorbing front grid and lateral transport of internal luminescence on PR is estimated, emphasizing the importance of transparent (non-absorbing) contact layers, at the rear, but importantly, also at the front of the device. To conclude, P C E limits of 300-nm-thick GaAs solar cells are deduced in the context of an experimentally realized light-trapping scheme based on a randomly textured rear mirror. P C E s exceeding 25.5% are deemed realistic. • Photon recycling and light trapping are studied in textured thin-film GaAs cells • Ray-optical modeling yields probabilities of escape and (parasitic) absorption • Textured rear mirrors reduce photon recycling and its impact on cell performance • Light trapping improves efficiency for any absorber thickness and material quality • Efficiency limits of 300-nm GaAs cells are currently 25.5% and ultimately ¿28%

Large freestanding 2D covalent organic framework nanofilms exhibiting high strength and stiffness

Two-dimensional covalent organic frameworks (2D COFs) represent an ideal platform to develop novel technological applications. The integration of 2D COFs into thin-film device architectures requires a deep knowledge of their mechanical performance, especially as large-area ultrathin films. Here, we report the synthesis, transfer, and mechanical characterization of large-area freestanding 2D COF films with nanometer thicknesses. Imine-linked COF nanofilms are prepared by condensation reaction at air–water interface, which provides freestanding, uniform centimeter-scale 2D COF films with controlled thickness. The developed procedure enables the direct transfer of the synthetized large-area COF nanofilm onto patterned substrates for mechanical characterization. Tensile tests are performed on freestanding 2D COF films with 85 nm thickness and with a testing area as large as 0.3 mm2. The measured strength of the COF nanofilms is 188 ± 57 MPa, while the Young's modulus is 37 ± 15 GPa. Our findings not only demonstrate the high stiffness and strength of COF nanofilms over a large-area, which make them suitable for applications where high mechanical performance is required, but also pave the way for a fundamental understanding of the relationship between the structures and macroscopic mechanical properties of 2D COFs. Thus, the method that we developed herein will enable a broad exploration of the properties of large-area 2D COFs that will guide their engineering design toward the development of novel COF-based devices.

Gold Nanoparticle-Crystalline rubrene hybrid nanocomposite via plasma processing and realization of Plasmon-enhanced organic thin film transistor with high responsivity

High performing and operationally stable thin film transistor device is successfully fabricated with an organic frame-work implemented by synthesis of rubrene crystals with gold nanoparticles (Au NPs). A novel synthesis route for the fabrication of the active material in the form of crystalline rubrene composite (CRC) is adopted via plasma processing approach. This report highlights two important features: a) report of a one-step process to synthesize crystalline rubrene gold nanocomposite in a thin film architecture and b) demonstration of a plasmon transistor for plasmon resonance energy detection. The fabricated device is a low-light responsive device showing the typical p-type characteristics. Excellent p-channel characteristics is shown by the device with a maximum field effect mobility of 0.215 cm2/V.s. At the peak wavelength of plasmonic absorption a very high responsivity of 25 A/W is achieved. Hot carriers are collected by the transistor from the nanostructures of Au NPs that contributes to the increase in drain current by modulating the channel conductivity. Due to the versatile nature of the plasmonic nanostructures in terms of wavelength tunability for the CRC material, this architecture has the potential to offer a wide range of spectrum that can be used in various applications.

Sensitive and selective CO2 gas sensor based on CuO/ZnO bilayer thin-film architecture

A CuO/ZnO (C/Z) bilayer thin film was fabricated with a porous top CuO layer to facilitate a sensitive and selective response towards CO2 gas. Such a sensor architecture allowed optimum oxygen and CO2 gas adsorption in the interfacial region. The C/Z thin-film sensor exhibited a good response (47%) for 2500 ppm CO2 at 375 °C as opposed to CuO (15%) at 300 °C and ZnO (16%) at 350 °C. The sensor was selective to CO2 in respect of CO and CH4 gases at 375 °C with selectivity factor κCO2 ∼ 5 and ∼ 8 for CO and CH4 respectively. By analyzing the conductance-time transients for the gas, the adsorption behavior of CO2 on the heterogenous C/Z bilayer thin-film sensor was established. CO2 obeyed an extended Freundlich model of adsorption. Theoretical analysis of the said adsorption model was performed through which the activation energy (EA) and heat of adsorption (Q) of CO2 gas were estimated. A complementary relationship between EA and Q was established. It was shown that EA decreases with increasing concentration from 123.95 to 108.36 kJ/mol for 1000–2500 ppm CO2 for energetically heterogeneous surfaces. Alternatively, Q values increase with increasing concentration from 59.73 to 71.65 kJ/mol for 500–2500 ppm CO2. The CO2 sensing mechanism was elucidated based on surface defects for CuO and ZnO. CO2 sensing in the C/Z bilayer thin-film sensor was controlled by the adsorption of oxygen forming a space charge layer at the surface and interface of the p-n heterojunction and by band-bending as a result of the change of electron concentration across the junction.

Stretchable Thin Film Mechanical‐Strain‐Gated Switches and Logic Gate Functions Based on a Soft Tunneling Barrier (Adv. Mater. 41/2021)

Strain-Gated Switching Mechanical-strain-gated electric switches play a key role in sensing, soft robotics, and stretchable electronics. They typically operate with an “off-switch” function, and exhibit a decrease in conductivity upon an applied strain. In article number 2104769, Soosang Chae, Andreas Fery, and co-workers demonstrate a thin-film architecture that features the inverse behavior: “on-switch”: a strain-induced transition from insulating to metallic conductivity, spanning nine orders of magnitude in conductivity.

Electrocaloric temperature changes in epitaxial Ba1−xSrxTiO3 films

The basic aim of our study is to investigate the correlation between structural parameters and the electrocaloric effect in lead-free epitaxial Ba1−xSrxTiO3 (BSTO) based thin film architectures. Therefore, BSTO thin films with Sr contents of x = 0 to x = 0.3 were grown on SrRuO3 buffered SrTiO3 single crystalline substrates by pulsed laser deposition. Structural characterization verified an epitaxial growth for all Sr contents with an additional tetragonal distortion compared to bulk material. Temperature and frequency-dependent measurements of dielectric properties revealed increased permittivity values for thicker films with broad maxima indicating a diffuse phase transition. The temperature of maximum permittivity decreases with increasing Sr content, whereas polarization measurements indicate a relaxor-like behaviour in particular above room temperature. Adiabatic temperature changes were determined with the indirect method resulting in |ΔT| values of up to 2.9 K for a 680 nm thick BSTO layer with x = 0.3 at an applied electric field of 750 kV cm−1.