Gopinathan Anoop

Enhanced thermoelectric performance of PEDOT:PSS/PANI–CSA polymer multilayer structures

A layer-by-layer deposition of two conducting polymers, each layer of which is a few tenths of nanometer thick, has been successfully performed to enhance the thermoelectric power factor of organic thin films, which are critical components of flexible thermoelectric energy harvesting devices. The multilayer films were deposited via multiple solution processes, which exhibit enhanced electrical conductivity without any significant degradation of the Seebeck coefficient, in contrast to a coupling behavior between the electrical conductivity and the Seebeck coefficient in bulk materials. The electrical conductivity and power factor—proportional to the electrical conductivity—of 5(PEDOT:PSS/PANI–CSA) multilayer films are 1.3 and 2 times higher than those of a single PEDOT:PSS layer. Transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) reveal distinct interfaces through which an enhanced electrical conductivity and power factor have been achieved in our multilayer films. From the TEM, EELS, and Raman analyses, a model for the enhancement of the electrical conductivity has been proposed. The enhancement of electrical conductivity occurs via stretching of PEDOT and PANI chains and hole diffusion from the PANI–CSA layer to the PEDOT:PSS layer. The band alignment in the multilayer structure not only enhances electrical conductivity but also maintains the Seebeck coefficient at an optimum value. Our study suggests that the layer-by-layer deposition of polymer thin films is a promising technique for manipulating the thermoelectric properties of each polymer component to enhance thermoelectric performance.

Low voltage actuator using ionic polymer metal nanocomposites based on a miscible polymer blend

Bio-compatible actuators are required to exhibit a large actuation displacement and force at a low voltage for various applications in liquid environments, including swimming robots, biomedical catheters, biomimetic sensory-actuators, and drug delivery micro-pumps. Recently, ionic polymer metal nanocomposites (IPMNCs) based on Nafion have been widely used for bio-compatible actuators; however, they have been demonstrated to operate only at high voltages in the range of 2 to 5 V, resulting in water hydrolysis problems which are accompanied by a degradation of actuation performance. Here, we show that IPMNC actuators based on a poly(vinylidenefluoride-co-trifluoroethylene) [P(VDF-TrFE)]/polyvinylpyrrolidone (PVP)/polystyrene sulfonic acid (PSSA) polymer blend membrane can exhibit a large actuation displacement and force at a low voltage of 1 V. Due to the ferroelectric nature of P(VDF-TrFE), the large dipole moment of P(VDF-TrFE) can cause strong intermolecular bonding, causing the P(VDF-TrFE)/PVP/PSSA blend membrane to be miscible. We found that the P(VDF-TrFE)/PVP/PSSA blend membrane with a blending ratio of 30/15/55 can produce the highest proton conductivity (0.0065 S cm−1) and ion exchange capacity (2.95 meq g−1) as compared to those of the commercial Nafion membrane, due to its miscible nature. Our IPMNC exhibits both an enhanced actuation displacement and force by up to 2 times in comparison with those of the IPMNC based on the commercial Nafion-based ionic membrane. Our P(VDF-TrFE)/PVP/PSSA IPMNC shows a stable actuation performance for up to 2200 cycles in hydrated conditions.

Enhanced sensing performance of carboxyl graphene–ionic liquid attached ionic polymer–metal nanocomposite based polymer strain sensors

Soft bendable polymer sensors have been widely used to monitor prosthetics, heartbeat, joint pain, and several other medical conditions because of their flexible nature. Recently, sensors based on piezoelectric inorganic materials, conducting polymers, and commercial Nafion based ionic polymer–metal nanocomposites (IPMNCs), have been extensively studied for sensor applications; however, existing inorganic and polymer materials exhibit low sensing currents due to weak interfacial bonding between the electrode and sensing material. Here, we show that biocompatible IPMNC sensors based on a carboxyl graphene (COG)–acidic ionic liquid (IL) (1-butyl-3-methylimidazolium-hydrogen sulfate)–poly(vinylidene fluoride–trifluoroethylene–chlorotrifluoroethylene) [P(VDF–TrFE–CTFE)]–polyvinylpyrrolidone (PVP)–polystyrene sulfonic acid (PSSA) ionic blend membrane can generate a high sensing current (6 mA cm−2) with a bending strain of 0.009. The ionic exchange capacity (IEC) (1.36 times), proton conductivity (3.4 times), and Youngs modulus (176 times) of P(VDF–TrFE–CTFE)/PVP/PSSA/COG ionic blend membranes are enhanced compared to those of P(VDF–TrFE–CTFE)/PVP/PSSA. In comparison to a commercial Nafion membrane, enhanced values of water uptake (WUP) (5.61 times), IEC (3.26 times), and Youngs modulus (6 times) were achieved by our P(VDF–TrFE–CTFE)/PVP/PSSA/COG/IL ionic blend membrane. Polymer sensors based on (PVDF–TrFE–CTFE)/PVP/PSSA/COG/IL IPMNC exhibit stable sensing currents in dry conditions for up to 6000 cycles. Our proposed blend fabricated through attaching COG and IL will find applications in several other devices such as supercapacitors due to its high capacitance (3.92 mF).

Structural, electrical, and luminescence characteristics of vacuum-annealed epitaxial (Ba,La)SnO3 thin films

AbstractThe correlation between the structural, electrical, and luminescence behaviors of La-doped BaSnO3 (LBSO) epitaxial films was intensively studied. We found that Sn2+ defects and oxygen vacancies control the electrical properties of epitaxial LBSO films that are grown on (001)-oriented SrTiO3 substrates using pulsed laser deposition. Under optimized deposition condition, the films exhibit room temperature resistivity of 16 mΩ·cm, with a mobility of 1.62 cm2V−1s−1. To further reduce the resistivity, the films were vacuum-annealed at various temperatures in the range from 600℃ to 900℃ and the film annealed at 600℃ exhibited the lowest room temperature resistivity of 5 mΩ·cm with the highest mobility of 3.09 cm2V−1s−1. The decrease of resistivity in the film vacuum-annealed at 600℃ originates from the higher concentration of Sn2+ ions and oxygen vacancies, which was also confirmed from photoluminescence studies, in which emission peaks associated with Sn2+ defects were observed at 710 and 910 nm. Raman analysis revealed the presence of defect states related to octahedral tilting in vacuum-annealed LBSO films. Our studies show that the electrical properties of epitaxial films could be controlled by the Sn2+ defects generated with oxygen vacancies during the vacuum-annealing of the films.

Direct growth of nano-crystalline graphite films using pulsed laser deposition with in-situ monitoring based on reflection high-energy electron diffraction technique

We report an experimental method to overcome the long processing time required for fabricating graphite films by a transfer process from a catalytic layer to a substrate, as well as our study of the growth process of graphite films using a pulsed laser deposition combined with in-situ monitoring based on reflection high-energy electron diffraction technique. We monitored the structural evolution of nano-crystalline graphite films directly grown on AlN-coated Si substrates without any catalytic layer. We found that the carbon films grown for less than 600 s cannot manifest the graphite structure due to a high defect density arising from grain boundaries; however, the carbon film can gradually become a nano-crystalline graphite film with a thickness of approximately up to 5 nm. The Raman spectra and electrical properties of carbon films indicate that the nano-crystalline graphite films can be fabricated, even at the growth temperature as low as 850 °C within 600 s.