The intrinsic charge-induced surface stress of Ni thin films during electrochemical reactions with an alkaline electrolyte is measured in situ. Surface stresses induced by H absorption/desorption, α-Ni(OH)2 formation, capacitive double-layer charging, the α- to β-Ni(OH)2 transformation, and β-Ni(OH)2/β-NiOOH redox reactions are identified, and each provided additive contributions to the overall stress state. Surface stresses are magnified in high-surface-area nanoporous Ni because local stress-relaxation mechanisms are restricted when compared to a smooth Ni film. Ni film reversible tensile/compressive surface stresses correlate with anodic/cathodic potential scanning but with an opposite trend to that of a less reactive Au film. Surface stresses in the Ni films are up to 40 times that of Au films and suggest the possibility of using controlled surface-stress generation for electrochemical actuation.

A high performance diketopyrrolopyrrole (DPP)–based semiconducting polymer is modified with ligands to enable metal coordination, and its subsequent effect as field-effect transistors is investigated. In specific, pyridine-2,6-dicarboxamide (PDCA) units are incorporated in a DPP–based polymer backbone with a content from 0 to 30 mol%, and the resulting polymers are then mixed with Fe(II) ions. The coordination and spontaneous oxidation converts Fe(II) to Fe(III) ions to result in Fe(III)-containing metallopolymers. The resulting metallopolymers are observed to show good solubility in organic solvents and can be easily processed as thin films. The charge transport characteristics are subsequently investigated through the fabrication of field–effect transistor devices, in which an enhanced charge carrier mobility with the Fe(III)-containing metallopolymers is observed. In specific, an almost twofold improvement in the charge carrier mobility is obtained for the 20% PDCA-containing polymer after Fe coordination (from 0.96 to 1.84 cm2 V−1 s−1). Furthermore, the operation stability of the metallopolymer-based devices is found to be significantly improved with low bias stress. Its superior electrical characteristics are attributed to the doping effect of the Fe ions. This study indicates that incorporation of appropriate metallic ions to polymer presents a viable approach to enhance the performance of polymer–based transistor devices.

In this study, the mechanical properties of freestanding membranes made of graphene oxide (GO), titania nanorods (TNRs), and silk fibroin (SF) are investigated and their application is demonstrated as electrostatically driven actuators. Using a stamping process, the membranes are transferred onto substrates with circular apertures or square cavities measuring ∼80 to 245 µm in diameter or edge length, respectively. Afterwards, the membranes are exposed to deep-UV (DUV) radiation in order to photocatalytically convert GO to reduced graphene oxide (rGO). Microbulge tests combined with atomic force microscopy (AFM) measurements reveal enhanced mechanical stability after the DUV treatment, as indicated by an increase of Young's modulus from ∼22 to ∼35 GPa. The toughness of the DUV-treated membranes is up to ∼1.25 MJ m−3, while their ultimate biaxial tensile stress and strain are in the range of ∼377 MPa and ∼0.68%, respectively. Further, by applying voltages of up to ±40 V the membranes are electrostatically actuated and deflected by up to ∼1.7 µm, as determined via in situ AFM measurements. A simple electrostatic model is presented that describes the deflection of the membrane as a function of the applied voltage very well.

An innovative possibility to introduce additional functionality to organic field-effect transistors (OFETs) is to employ photochromic molecules, which undergo reversible isomerization under applied stimuli such as irradiation with specific wavelengths. As a result, the transistors not only can be switched on/off by the applied voltages, they can also be programmed by alternate triggers, such as light. Here, reversible switching of OFETs is presented by blending various dihydroazulene/vinylheptafulvene photoswitches into polythiophene-based conjugated polymers. In result, the transfer characteristics of the transistors are altered significantly through UV irradiation. In contrast to current literature on different photoswitches such as spiropyrans or diarylethenes, the backreaction is induced thermally and not via visible light irradiation and reproducibly yields the pristine transistor characteristics. This reversible switching upon alternating UV irradiation and thermal annealing is quantified by figures of merit such as the magnitude of drain current, threshold voltage, and subthreshold swing. Irradiating the devices with different doses of UV light shows that the magnitude of switching directly depends on the respective UV dose, hence enabling a multi-level electronic system. Furthermore, long-term cyclability over 100 steps of repeated UV light exposure and thermal annealing is demonstrated.

Derivatives of the hexacyano-[3]-radialene anion radical (CN6-CP•−) emerge as a promising new family of p-dopants having a doping strength comparable to that of archetypical dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-quinodimethane (F4TCNQ). Here, mixed solution (MxS) and sequential processing (SqP) doping methods are compared by using a model semiconductor poly(3-hexylthiophene) (P3HT) and the dopant CN6-CP•−NBu4+ (NBu4+ = tetrabutylammonium). MxS films show a moderate yet thickness-independent conductivity of ≈0.1 S cm−1. For the SqP case, the highest conductivity value of ≈6 S cm−1 is achieved for the thinnest (1.5–3 nm) films whereas conductivity drops two orders of magnitudes for 100 times thicker films. These results are explained in terms of an interfacial doping mechanism realized in the SqP films, where only layers close to the P3HT/dopant interface are doped efficiently, whereas internal P3HT layers remain essentially undoped. This structure is in agreement with transmission electron microscopy, atomic force microscopy, and Kelvin probe force microscopy results. The temperature-dependent conductivity measurements reveal a lower activation energy for charge carriers in SqP samples than in MxS films (79 meV vs 110 meV), which could be a reason for their superior conductivity.