Emergence and Recent Advances in MXenes for Diverse Sensing Applications

Many scientific fields regard MXene as a ground-breaking 2D material. The metal-like great electrical conductivity and high surface area of MXenes are attractive properties as an innovative sensor material that can go beyond the limitations of current sensor technology. This is especially true in the field of sensors and transducers as a whole. The thorough analysis offers a road map for commercializing MXene-based sensors as well as an extensive overview of current developments in sensor technology. Pressure and strain, electrochemical, biological, gas, capacitive, and photoluminescent sensors are the few broad categories into which the current sensors based on MXenes and their composites are meticulously organized. The performance of each class is improved using exemplary structural, compositional, and production strategies, which are presented and discussed in detail. Finally, obstacles that prevent MXene-based sensors from being commercialized are examined, and potential directions to realize MXene-based sensors that can be viable and marketable are looked upon. This chapter thus unequivocally offers a comprehensive analysis of past and present MXene-based sensor technologies as well as promising foresight for the next generation of multimodal, low-cost, high-performance sensors for electronic applications.

Intertwined crystal structure, magnetic, and charge transport properties in mixed valent A-site ordered manganite NdBaMn2O6

In the present work, we report a correlation between crystal structure, magnetic, and electrical properties in an exotic magnetic compound, NdBaMn2O6 having ∼92% ordering between Nd and Ba, investigated using temperature-dependent synchrotron x-ray diffraction (XRD), dc magnetization and transport measurements. Temperature-dependent XRD data reveals that the compound undergoes various complex crystallographic phase transitions from high-temperature (T > 320 K) P4/mmm phase to intermediate (320 K – 280 K) Cmmm phase to a mixed (Cmmm and P21am) phase (280 K – 220 K) and to low-temperature P21am phase (T < 220 K). It is found that these crystallographic phase compositions play a crucial role in controlling its magnetic and transport properties. Temperature-dependent dc magnetization data show a sharp drop at the onset of mixed phase (Cmmm + P21am) at 280 K followed by a broad hump at ∼ 220 K where mixed phase to P21am transition occurs, thus indicating a correlation between the structural and magnetic properties. The dc magnetization in the mixed phase region is calculated by considering a superposition of the magnetic moments of Cmmm and P21am phases weighted by the fraction of each phase, which exactly follows the experimental magnetization data. Temperature variation of resistivity data shows a jump at ∼260 K, a temperature corresponding to 50–50% phase composition of Cmmm and P21am phases. The compound shows insulating behavior over a whole temperature range as confirmed from the resistivity data. Further, application of magnetic field causes a shift of magnetic and transport transition temperatures which may be due to the magnetic field induced structural transition in the system.

Anti-perovskites for photovoltaics: materials development and challenges

For the next-generation solar cells with excellent device efficiency and stability, designing advanced light absorber materials with exceptional optoelectronic properties is extremely crucial. Perovskites have attracted great attention due to their high-power conversion efficiency, and low fabrication cost. Eventhough perovskites achieved the highest efficiency of 25.7% within a decade, lead (Pb) toxicity is one of the main issues that needs to be addressed. Also, they are susceptible to degradation under ambient conditions. On the other hand, anti-perovskites, which are electronically inverted perovskites, possess structural flexibility, environmentally benign chemical composition, appropriate band gap and hence, have the capability to replace perovskites as the absorber layer for next-generation solar cells. Thus, a thorough assessment is urgently required to spark widespread concern in this family of compounds. Based on the current research progress, the potential of anti-perovskites in solar cell research is compiled in this study. The structural variety, optoelectronic characteristics, and uncharted territory of these compounds are covered in great detail. Finally, we have discussed the future research directions for the development of anti-perovskite materials for the next generation efficient and stable solar cells.

Dynamic Structural Evolution and Dual Emission Behavior in Hybrid Organic Lead Bromide Perovskites

The optoelectronic properties of organic lead halide perovskites (OLHPs) strongly depend on their underlying crystal symmetry and dynamics. Here, we exploit temperature-dependent synchrotron powder X-ray diffraction and temperature-dependent photoluminescence to investigate how the subtle structural changes happening in the pure and mixed A-site cation MA1–xFAxPbBr3 (x = 0, 0.5, and 1) systems influences their optoelectronic properties. Diffraction investigations reveal a cubic structure at high temperatures and tetragonal and orthorhombic structures with octahedral distortion at low temperatures. Steady state photoluminescence and time correlated single photon counting study reveals that the dual emission behavior of these OLHPs is due to the direct-indirect band formation. In the orthorhombic phase of MAPbBr3, the indirect band is dominated by self-trapped exciton (STE) emission due to the higher-order lattice distortions of PbBr6 octahedra. Our findings provide a comprehensive explanation of the dual emission behavior of OLHPs while also providing a rationale for previous experimental observations.

Elucidating Phase Transitions and the Genesis of Broadband Emission in MA1–xFAxPbBr3 Perovskites

​The octahedral distortion plays a pivotal role in influencing various unique electrical and optical properties of organic lead halide perovskites (OLHPs). Unveiling the nature of the response of the local inorganic octahedra to the photophysical properties is a critical step toward understanding the formation of excited-state defects. Here, we report a fundamental understanding of the process of octahedral distortion and its variation with temperature in MA1–xFAxPbBr3 (x = 0, 0.5, 1) perovskites. Further, the origin of trap states which are responsible for the broadband emission has been elucidated with the help of detailed structural and photophysical analysis. We find that the intensity and Stoke shift of the broadband emission peaks and charge carrier dynamics are significantly influenced by the changes in Pb–Br bond lengths and Pb–Br–Pb angles. Our findings highlight the relationship between the octahedral distortion and the formation of trap states and provide further insights into tailoring the broadband emission by regulating the local inorganic octahedra in OLHPs.

Titania Nanoparticle-Stimulated Ultralow Frequency Detection and High-Pass Filter Behavior of a Flexible Piezoelectric Nanogenerator: A Self-Sustaining Energy Harvester for Active Motion Tracking

A significant driving force for the fabrication of IoT-compatible smart health gear integrated with multifunctional sensors is the growing trend in fitness and the overall wellness of the human body. In this work, we present an autonomous motion and activity-sensing device based on the efficacious nucleation of the polar β-phase in an electroactive polymer. Representatively, we investigate the nucleating effect of TiO2 nanoparticles on weight-modulated PVDF-HFP films (PT-5, PT-10, and PT-15) and subsequently prototype a sensing device with the film that demonstrates superior β-phase nucleation. The PT-10 film, with an optimal polar β-phase, shows the highest remnant polarization (2Pr) and energy density of 0.36 μC/cm2 and 22.3 mJ/cm3, respectively, at 60 kV/cm. The films mimic a high pass filter at frequencies above 10 KHz with very low impedance and high ac conductivity values. The frequency-dependent impedance studies reveal an effective interfacial polarization between TiO2 nanoparticles and PVDF-HFP, explicitly observed in the low-frequency region. Consequently, the sensor fabricated with PT-10 as the sensing layer exhibits ultralow frequency detection (25 Hz) resulting from the blood flow muscle oxygenation. The device successfully senses voluntary joint movements of the human body and actively tracks a range of motions, from brisk walking to running. Additionally, through repetitive human finger-tapping motion, the nanogenerator lights up multiple light-emitting diodes in series and charges capacitors of varying magnitudes under 50 s. The real-time human motion sensing and movement tracking modalities of the sensor hold promise in the arena of smart wearables, sports biomechanics, and contact-based medical devices.

Negative Capacitance and Intrinsic Ferroelectric Behavior in α-MoO3 Culminating as a Robust Piezoelectric Energy Harvester

Driven by the notion that the spectrum of ferroelectric applications can be widened and the current subset of materials could be replaced with earth-abundant, lead-free materials, single-phase α-MoO3 (MO) with biaxial van der Waals gap was synthesized, revealing an orthorhombic Pmcn symmetry with a layered ABAB... sequence, with mirrored A and B layers. The force-driven dielectric constant of MO was found to be 12.5 at 0.5 N force level, which showed dielectric saturation behavior with increasing dynamic force. Ferroelectric studies revealed a maximum of 46% efficiency at 10 kV/cm external electric field, and piezoelectric modulus (d33) of 30 pC/N was obtained using the Berlincourt method. Negative capacitance (NC) effects were observed and attributed to the ferroelectric-induced emf and the inductive reactance, in accordance with Lenz’s law. Based on the confluence of the aforementioned features, a piezoelectric energy harvester (PEH) was fabricated, which reached a peak voltage of 4 V under repetitive finger tapping. The power density of the PEH under 100 MΩ resistive load was found to be 7.32 × 10–1 μW/cm2. This enshrines MO as a promising piezoelectric candidate with exotic properties that can be tailored to have multifaceted applications in the realm of sensing and energy harvesting to drive Internet-of-Things and Industry 4.0.