Gradual Morphological Change in PEDOT:PSS Thin Films Immersed in an Aqueous Solution.

The poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) film is a promising material for electrodes, biomolecular sensor channels, and probes for physiological signals because the electrical conduction of PEDOT:PSS is tuned simply through the electrochemical reaction with the target analyte. However, forming a specific morphology or nanostructure on PEDOT:PSS thin films immersed in an aqueous solution is still a challenge. Herein, we report the mechanism for the stepwise morphological change in the highly conductive PEDOT:PSS layer that successfully explains the electrical and structural modulations that occur after a soaking test in various pH conditions. The change in PEDOT:PSS begins with the rapid swelling and dissolution of PSS-rich domains and the simultaneous structural rearrangement of the remaining PEDOT chains within 1 s of dipping. Analysis confirms that the pH conditions of an aqueous solution govern the oxidation state and the form of the PEDOT chains. After removing the water molecules, additional PEDOT-rich grains were generated and accumulated on the surface of the film, which exhibited hydrophobic barrier characteristics. With the help of this intrinsic barrier on the PEDOT:PSS surface, the sheet resistance slightly increased from 72 to 144 Ω/sq even after dipping in a water bath for 350 h. We also demonstrate the usability of the proposed approach on a sensor to detect vitamin C in an aqueous medium. Utilizing the electrochemical reaction of PEDOT:PSS films, the simple resistor sensor showed a response time of less than 150 s, which is 10 times faster than that observed in a previous report. The soaked samples also showed a more reliable linear correlation between the current change and the amount of ascorbic acid compared with pristine PEDOT:PSS. Both the proposed mechanism and the role of accumulated PEDOT-rich regions illustrate the versatile potential of highly conductive PEDOT:PSS films in the field of bioelectronic applications, owing to the increased design architecture.

Neuro-Transistor Based on UV-Treated Charge Trapping in MoTe2 for Artificial Synaptic Features.

The diversity of brain functions depend on the release of neurotransmitters in chemical synapses. The back gated three terminal field effect transistors (FETs) are auspicious candidates for the emulation of biological functions to recognize the proficient neuromorphic computing systems. In order to encourage the hysteresis loops, we treated the bottom side of MoTe2 flake with deep ultraviolet light in ambient conditions. Here, we modulate the short-term and long-term memory effects due to the trapping and de-trapping of electron events in few layers of a MoTe2 transistor. However, MoTe2 FETs are investigated to reveal the time constants of electron trapping/de-trapping while applying the gate-voltage pulses. Our devices exploit the hysteresis effect in the transfer curves of MoTe2 FETs to explore the excitatory/inhibitory post-synaptic currents (EPSC/IPSC), long-term potentiation (LTP), long-term depression (LTD), spike timing/amplitude-dependent plasticity (STDP/SADP), and paired pulse facilitation (PPF). Further, the time constants for potentiation and depression is found to be 0.6 and 0.9 s, respectively which seems plausible for biological synapses. In addition, the change of synaptic weight in MoTe2 conductance is found to be 41% at negative gate pulse and 38% for positive gate pulse, respectively. Our findings can provide an essential role in the advancement of smart neuromorphic electronics.

Effect of Laser-Induced Direct Micropatterning on Polymer Optoelectronic Devices.

Solution-processed polymer devices have been studied as a low-cost alternative to the conventional vacuum-processed organic devices. However, forming a specific pattern on polymer semiconductor films without costly lithography is still challenging. Herein, we report a low-cost direct patterning method for polymer optoelectronic devices, which can successfully engrave designated patterns on the polymer semiconductor layer regardless of its size and even after device encapsulation. Irradiation of a 100 ns pulse laser forms high-resolution patterns on devices such as polymer light-emitting diodes and polymer memory devices. The biggest advantage of the proposed patterning method is that it does not produce any physical damage in the device, such as leakage current or degraded light-emitting efficiency. Analysis confirms that the laser-induced heat alters the solid or crystal structure of the polymer semiconducting layers so that the designated areas of the polymer devices can be selectively and deliberately deactivated. We demonstrate the usability of the developed laser-induced direct-patterning method on the polymer devices by engraving a digital image onto "ON-state" light-emitting devices and by generating multiple states onto a 4 × 4 matrix polymer nonvolatile memory.

Simultaneously enhanced optical, electrical, and mechanical properties of highly stretchable transparent silver nanowire electrodes using organic surface modifier.

We report on a new surface modifier which simultaneously improves electrical, optical, and mechanical properties of silver nanowire-based stretchable transparent electrodes. The transparent electrodes treated with 11-aminoundecanoic acid achieve a low sheet resistance of 26.0 ohm/sq and a high transmittance of 90% with an excellent stretchability. These improvements are attributed to the effective formation of a strong chemical bond between silver nanowire networks and elastomeric substrates by 11-aminoundecanoic acid treatment. The resistance change of the optimized silver nanowire/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) thin-films is only about 10% when the film is stretched by 120%. In addition, the chemical stability of stretchable silver nanowire films is significantly improved by the introduction of conductive PEDOT:PSS overcoat film. The optimized electrodes are utilized as high-performance stretchable transparent heaters, successfully illustrating its feasibility for future wearable electronics.