Sushmitha Veeralingam

Multifunctional wearable sensors have gained significant popularity in recent years for point of care diagnosis, tackling the myriad of obstacles faced in coping with health-related issues. However, complex fabrication, lack of biocompatibility, non-reusability, and accuracy limit their widespread use. In this work, we report a clean-room-free fabrication of molybdenum diselenide (MoSe₂) interspersed with polyvinyl alcohol (PVA) based multifunctional device for in situ and non-invasive high-fidelity human gesture recognition, pulse rate monitoring, and skin hydration sensing. Detailed morphological characterization studies reveal the formation of a rhombohedral structure for MoSe₂ nanoflakes stacked vertically to form a micro flower structure. Group synaptic activity of neurons results in a subtle electrical impulse, which, in turn, generates an electric field that is detected by the as-fabricated MoSe₂/PVA device when attached to the forehead and interfaced to Open Brain-Computer Interface platform-based Cyton biosensing board. The device is also used as an ultrasensitive pressure sensor for arterial pulse pressure monitoring. This detection mechanism of the multifunctional sensor can be attributed to the piezoresistive effect of MoSe₂ nanoparticles, wherein the dipoles reorient to form an internal polarization upon detection of physiological information. The strategy employed here paves the way toward replacing wet electrodes in conventional electroencephalogram (EEG)/electrocardiogram (ECG) measurements that result in skin abrasion and signal quality degradation with low-cost, reliable, skin-friendly, wearable MoSe₂/PVA dry electrodes for rapid assessment.

Herein, a lead-free nickel–iron layered-double hydroxide (NiFe LDH)-based piezoelectric nanomaterial is used to fabricate a self-powered unit based on a piezoelectric nanogenerator and high-performance asymmetric supercapacitor (ASC). XRD analysis of the NiFe LDH nanostructure confirms the distortion of the octahedral structural of (Ni,Fe)(OH)²⁺ cationic layers with respect to anionic layers of (CO₃)²⁻, which is responsible for the piezoelectric effect in the NiFe LDH nanostructures. A piezoelectric force microscope (PFM) study of the NiFe LDH nanostructure is performed and a superior piezoelectric charge coefficient of 274 pm V⁻¹ is obtained. A flexible, semi-transparent NiFe LDH:PDMS-based piezoelectric nanogenerator is fabricated with the device configuration of Pt-coated PET/(NiFe LDH:PDMS)/ITO-coated PET. A peak output voltage was obtained as 53 V and a peak current density was obtained as 0.79 μA cm⁻² under a compression force of 0.15 kgf. The high value of the piezoelectric charge coefficient of the NiFe LDH and the low surface energy of the PDMS polymer is accountable for the generation of this high output. The NiFe LDH:PDMS-based piezoelectric nanogenerator (PENG) possesses a high energy-conversion efficiency of 34 ± 0.2% with a fast response time of 40 ms. Furthermore, to explore the practical utility of the nanogenerator, an asymmetric supercapacitor using the as-prepared NiFe LDH and biomass-derived activated carbon (AC) as the positive and negative electrode, respectively, is fabricated resulting in a capacitance of 1340 F g⁻¹ at 1 A g⁻¹. The supercapacitor is charged using the NiFe LDH nanogenerator and an efficient charging curve of up to 0.8 V·s is observed under continuous tapping for 100 s. The integrated self-powered system that works by charging the ASC using a piezoelectric hybrid nanogenerator is demonstrated, which provides a promising direction in powering smart wearable electronic devices.

Conventional heterojunction photodetectors rely on planar junction architecture which suffer from low interfacial contact area, inferior light absorption characteristics and complex fabrication schemes. Heterojunctions based on mixed dimensional nanostructures such as 0D-1D, 1D-2D, 1D-3D etc have recently garnered exceptional research interest owing to their atomically sharp interfaces, tunable junction properties such as enhanced light absorption cross-section. In this work, a flexible broadband UV–vis photodetector employing mixed dimensional heterostructure of 1D NiO nanofibers and 3D Fe ₂ O ₃ nanoparticles is fabricated. NiO nanofibers were synthesized via economical and scalable electro-spinning technique and made composite with Fe ₂ O ₃ nanoclusters for hetero-structure fabrication. The optical absorption spectra of NiO nanofibers and Fe ₂ O ₃ nanoparticles exhibit peak absorption in UV and visible spectra, respectively. The as-fabricated photodetector displays quick response times of 0.09 s and 0.18 s and responsivities of 5.7 mA W ⁻¹ (0.03 mW cm ⁻² ) and 5.2 mA W ⁻¹ (0.01 mW cm ⁻² ) for UV and visible spectra, respectively. The fabricated NiO–Fe ₂ O ₃ device also exhibits excellent detectivity in the order of 10 ¹² jones. The superior performance of the device is ascribed to the type-II heterojunction between NiO–Fe ₂ O ₃ nanostructures, which results in the localized built-in potential at their interface, that aids in the effective carrier separation and transportation. Further, the flexible photodetector displays excellent robustness when bent over ∼1000 cycles thereby proving its potential towards developing reliable, diverse functional opto-electronic devices.