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The design and construction of 3D architectures enabled by stimuli-responsive soft materials can yield novel functionalities for next generation soft-bodied actuating devices. Apart from additive manufacturing processes, origami inspired technology offers an alternative approach to fabricate 3D actuators from planar materials. Here we report a class of near-infrared (NIR) responsive 3D active origamis that deploy, actuate and transform between multistable structural equilibria. By exploiting the nonlinear coefficient of thermal expansion (CTE) of graphene oxide (GO), graphene oxide/ethylene cellulose (GO/EC) bilayers are readily fabricated to deliver precise origami structure control, and rapid low-temperature-triggered photothermal actuation. Complexity in 3D shapes is produced through heterogeneously patterning GO domains on 2D EC thin films, which allows us to customize 3D architectures that adapt to various robotic functions. The strategy also enables the construction of material systems possessing naturally inaccessible properties, such as remotely controlled mechanical metamaterials with auxetic behavior and bionic flowers with a rapid blooming rate. Harnessing deformability with multiple degrees of freedom (DOF) upon light irradiation, this work leads to breakthroughs in the design and implementation of shape-morphing functions with soft origamis. National Research Foundation (NRF) Accepted version This work was supported by the Competitive Research Program (NRF-CRP13-2014-02) and NRF Investigatorship (Award No. NRF-NRFI2016-05) under the National Research Foundation, Prime Minister’s Office, Singapore. D. Gao acknowledge the research scholarships awarded by Nanyang Technological University, Singapore.

Smart sensing electronic devices with good transparency, high stretchability, and self-powered sensing characteristics are essential in wearable health monitoring systems. This paper innovatively proposes a stretchable nanocomposite nanogenerator with good transparency that can be conformally attached to the human body to harvest biomechanical energy and monitor physiological signals. The work reports an innovative device that uses sprayed silver nanowires as transparent electrodes and sandwiches a nanocomposite of piezoelectric BaTiO3 and polydimethylsiloxane as the sensing layer, which exhibits good transparency and mechanical transformability with stretchable, foldable, and twistable properties. The highly flexible nanogenerator affords a good input–output linearity under the vertical force and the sensing ability to detect lateral stretching deformation up to 60% strain under piezoelectric mechanisms. Furthermore, the proposed device can effectively harvest touch energies from the human body as a sing...

Solar-blind photodetectors based on Zn2GeO4 (ZGO) nanowires (NWs) with high transparency and good flexibility were successfully fabricated on a polyethylene terephthalate (PET) substrate using a facile all solution-processible method. The electrodes and functional channels are constructed with silver NWs (Ag NWs) and ZGO NWs, respectively, using a spray coating method. The spray-coated all-NW device exhibited high transparency, good mechanical bending stability, high photoresponse, and short cutoff wavelength. The Schottky barrier between the Ag–ZGO NWs together with the junction barrier between interconnecting ZGO NWs contributed to a high photoresponse and improved switching time in the devices. We propose that the unique energy band structure of the ZGO NW networks provides an additional mechanism to further reduce the cutoff wavelength in solar-blind photodetectors.

Electronics with multifunctionalities such as transparency, portability, and flexibility are anticipated for future circuitry development. Flexible memory is one of the indispensable elements in a hybrid electronic integrated circuit as the information storage device. Herein, we demonstrate a transparent, flexible, and transferable hexagonal boron nitride (hBN)-based resistive switching memory with indium tin oxide (ITO) and graphene electrodes on soft polydimethylsiloxane (PDMS) substrate. The ITO/hBN/graphene/PDMS memory device not only exhibits excellent performance in terms of optical transmittance (∼85% in the visible wavelength), ON/OFF ratio (∼480), retention time (∼5 × 104 s) but also shows robust flexibility under bending conditions and stable operation on arbitrary substrates. More importantly, direct observation of indium filaments in an ITO/hBN/graphene device is found via ex situ transmission electron microscopy, which provides critical insight on the complex resistive switching mechanisms.

Many organisms and animals have sensing abilities that are different from those of human beings; for example, snakes have strong smell-, vibration-, touch- and heat-sensing abilities. A nature-mimicking sensing platform capable of sensing multiple stimuli, such as strain, pressure, temperature and other uncorrelated conditions, is highly desirable to broaden the applications of sensors. Here, we construct a semitransparent intelligent skin-like sensing platform based on polyaniline (PANI) nanowire arrays that can act as a bionic component by simultaneously sensing tactile stimuli and detecting colorless, odorless gas. Our multifunctional bionic sensing strategy is remarkably adaptive for versatile applications. The strain-sensing performance is superior to that of most conducting polymer-based sensors reported so far and is comparable to or even better than traditional metal and carbon nanowire/nanotube-based strain sensors. The highest gauge factor demonstrated is 149, making our system a remarkable candidate for strain-sensing applications. The sensor can accurately detect a wide range of human motions. We also demonstrate the simultaneous controlled olfaction ability for the detection of methane with high sensitivity and a fast response time. These results enable the realization of multifunctional and uncorrelated sensing capabilities, which will afford a wide range of applications to augment robotics, treatment, simulated skin, health monitoring and bionic systems. A wearable sensor can track full-range human motion and sniff out toxic gases using polymer nanowires with features similar to snakeskin. The device, which was developed by Pooi See Lee and co-workers at Nanyang Technological University, is based on polyaniline, a conducting polymer with natural flexibility and chemical sensitivity. Polyaniline often becomes unworkably rigid when deposited as a thin film, but the Singapore-based team avoided this issue by growing a vertical forest of polymer nanowires on a thin silicone support. Scanning electron microscopy revealed that when stretched, the nanowires formed scale-like islands and cracks that alter the resistance of the thin film. The researchers demonstrated that these electrical changes were sensitive enough to use for forehead sensors of breathing and eye movement and as a ‘smelling skin’ for detecting methane–an odorless and explosive gas. A semitransparent intelligent skin-like sensor platform based on polyaniline nanowire arrays was constructed, which can act as bionic component by simultaneously sensing tactile stimuli and detecting colorless odorless gas. The highest gauge factor demonstrated is 149, making it a remarkable candidate in strain sensing applications. Simultaneously, we demonstrate the controlled olfaction ability of the sensor with the detection of methane with high sensitivity and fast response time. These results enable the realization of multifunctional and uncorrelated sensing capabilities that will have wide range applications to augment robotics, treatment, simulate skin, health and bionic system.

Reversible electrochemical mirror (REM) device can be electrochemically tuned to exhibit dual transmittance and reflectance modulations in a single device. Conventional REM devices reversibly switched between transparent and mirror states. However, it is of great challenge to maintain the mirror state of the REM devices due to the diffusion of anions into the metallic film at the open-circuit state. In this work, we report a Cu-based REM device which offers reversible switching between transparent, blue and mirror states with judicious selection of electrolyte and controllable electrodeposition. The blue state can be obtained through the formation of copper (I) chloride, CuCl, when copper (II) chloride CuCl2 undergoes electrochemical reduction. The polymer host, PVA (poly(vinyl alcohol)) plays an important role in reducing the surface roughness of the electrodeposited mirror film, improving film uniformity and sustaining the mirror state of the device during the voltage-off state. NRF (Natl Research Foundation, S’pore) Accepted version

Few-layered phosphorene is exfoliated through a liquid phase exfoliation in ethanol, acting as an additive to mix with tempo-oxidized cellulose nanofibrils (CNF), forming a transparent, flexible nanogenerator paper. The gas barrier property of CNF protects phosphorene against oxidation in ambient condition. The phosphorene in the hybrid paper is stable over six months even with exposure to ambient condition (23 °C, 70% RH) in air. The hybrid paper is used as an active layer to construct a triboelectric energy nanogenerator (TENG). This hybrid paper shows an open-circuit voltage of 5.2 V with a current density of 1.8 µA cm−2, which is five and nine times higher than that of the pure CNF paper device, respectively. This work reveals the role of phosphorene in acting as a charge carrier in the hybrid paper and demonstrates a new way to attain stable functional phosphorene hybrids in ambient condition. The resultant phosphorene hybrid paper could be used as a building block for TENG with enhanced output performance.

Decreasing the active layer thickness has been recently reported as an alternative way to achieve fully depleted oxide thin-film transistors for the realization of low-voltage operations. However, the correlation between the active layer thickness and device resistivity to environmental changes is still unclear, which is important for the optimized design of oxide thin-film transistors. In this work, the ambient gas stability of IGZO thin-film transistors is found to be strongly correlated to the IGZO thickness. The TFT with the thinnest IGZO layer shows the highest intrinsic electron mobility in a vacuum, which is greatly reduced after exposure to O2/air. The device with a thick IGZO layer shows similar electron mobility in O2/air, whereas the mobility variation measured in the vacuum is absent. The thickness dependent ambient gas stability is attributed to a high-mobility region in the IGZO surface vicinity with less sputtering-induced damage, which will become electron depleted in O2/air due to the electron transfer to adsorbed gas molecules. The O2 adsorption and deduced IGZO surface band bending is demonstrated by the ambient-pressure x-ray photoemission spectroscopy results.

The dependence of oxygen vacancy suppression on dopant species in amorphous indium oxide (a-InOx) thin film transistors (TFTs) is reported. In a-InOx TFTs incorporating equivalent atom densities of Si- and W-dopants, absorption of oxygen in the host a-InOx matrix was found to depend on difference of Gibbs free energy of the dopants for oxidation. For fully oxidized films, the extracted channel conductivity was higher in the a-InOx TFTs containing dopants of small ionic radius. This can be explained by a reduction in the ionic scattering cross sectional area caused by charge screening effects.

The stable operation of transistors under a positive bias stress (PBS) is achieved using Hf incorporated into InOx-based thin films processed at relatively low temperatures (150 to 250 °C). The mobilities of the Hf-InOx thin-film transistors (TFTs) are higher than 8 cm2/Vs. The TFTs not only have negligible degradation in the mobility and a small shift in the threshold voltage under PBS for 60 h, but they are also thermally stable at 85 °C in air, without the need for a passivation layer. The Hf-InOx TFT can be stable even annealed at 150 °C for positive bias temperature stability (PBTS). A higher stability is achieved by annealing the TFTs at 250 °C, originating from a reduction in the trap density at the Hf-InOx/gate insulator interface. The knowledge obtained here will aid in the realization of stable TFTs processed at low temperatures.

To avoid the problem of air sensitive and wet-etched Zn and/or Ga contained amorphous oxide transistors, we propose an alternative amorphous semiconductor of indium silicon tungsten oxide as the channel material for thin film transistors. In this study, we employ the material to reveal the relation between the active thin film and the transistor performance with aid of x-ray reflectivity study. By adjusting the pre-annealing temperature, we find that the film densification and interface flatness between the film and gate insulator are crucial for achieving controllable high-performance transistors. The material and findings in the study are believed helpful for realizing controllable high-performance stable transistors.

Oxide thin film transistor employing copper source/drain electrodes shows a small turn on voltage and reduced hysteresis. Cross-sectional high-resolution transmission electron microscopy image confirmed the formation of ∼4 nm CuOx related interlayer. The lower bond-dissociation energy of Cu-O compared to Si-O and In-O suggests that the interlayer was formed by adsorbing oxygen molecules from surrounding environment instead of getting oxygen atoms from the semiconductor film. The formation of CuOx interlayer acting as an acceptor could suppress the carrier concentration in the transistor channel, which would be utilized to control the turn on voltage shifts in oxide thin film transistors.

Thin-film transistors (TFTs) with a high stability and a high field-effect mobility have been achieved using W-doped indium oxide semiconductors in a low-temperature process (∼150 °C). By incorporating WO3 into indium oxide, TFTs that were highly stable under a negative bias stress were reproducibly achieved without high-temperature annealing, and the degradation of the field-effect mobility was not pronounced. This may be due to the efficient suppression of the excess oxygen vacancies in the film by the high dissociation energy of the bond between oxygen and W atoms and to the different charge states of W ions.

PVDF-g-HEMA [poly(vinylidene fluoride)-graft-poly(2-hydroxyethylmethacrylate)]/Barium Titanate (BaTiO3) nanocomposites were prepared successfully via an in situ synthesis method without any catalyst or initiator. The in situ synthesis approach enables the formation of oxide nanoparticles in the presence of the grafted polymer with a hydroxyl functionalization group for direct coupling with oxide nanofillers. This elegant in situ nanoparticle synthesis method provides a facile, cost-effective and void-free dispersion of the nanoparticles in the matrix. The dielectric nanoparticle (BaTiO3) is well-attached onto the insulating polymer (PVDF) surface due to the surface anchoring linkage through hydrogen bonding between the two components, leading to the reduced aggregation in the resultant nanocomposites. The novel PVDF-g-HEMA/BaTiO3 nanocomposites are investigated as high energy density capacitor materials, achieving highest dielectric constant reaching up to 333, and a dielectric loss of 0.73 at 30 wt% BaTiO3 at 1 kHz.

Boron nitride nanotubes (BNNTs) are excellent nanofillers to enhance the mechanical and thermal properties of polymer nanocomposites. Despite the rapid progress in the effective syntheses of BNNTs, the ease of processability and solubility are major roadblocks for their widespread applications. The present work reports for the first time: a facile and environment-friendly green approach for aqueous bioinspired functionalization of boron nitride nanotubes through the use of dopamine, a synthetic mimic of mussel adhesive proteins. This approach is based on the affinity of the amino group of dopamine molecules for the boron atoms in BNNTs and π–π interactions as well as van der Waals interactions between the BNNTs network and dopamine molecules. Functionalization of the boron nitride nanotubes was evidently found to be associated with the existing π–π bonding and van der Waals interactions at the BNNT surfaces, involving the aromatic core structures of the dopamine molecules based on various characterizations. The resultant functionalized BNNTs are highly dispersible in water and a number of solvents. Polymer nanocomposites were prepared using pristine and dopamine functionalized BNNTs as reinforcement in a poly(vinyl difluoroethylene) matrix and tested for the thermal and mechanical properties. The functionalized BNNT reinforced polymer nanocomposites exhibit superior properties as compared to nanocomposites based on pristine BNNTs. This comprehensive study indicates that dopamine functionalized BNNTs are promising materials for various applications and are expected to form the basis of a new class of chemically reactive nanostructures.

Electroactive materials are of great interest for high performance capacitive behavior due to their relaxed ferroelectric properties. In this work, we studied the dielectric properties of poly(vinylidene fluoride) (PVDF) grafted with polystyrene (PS) by electron beam radiation induced free radical graft copolymerization reaction in solution. The dielectric constant of polystyrene grafted PVDF copolymers reaches about 90 at 100 Hz at room temperature representing more than seven times increment compared with the pristine PVDF matrix. The dielectric loss of 0.005 at 1 KHz can be achieved. Correlation of the dielectric properties with the graft copolymerization reaction mechanism was discussed. This route represents one of the most effective techniques to synthesize potential graft copolymers with desirable dielectric properties in a wide frequency range for energy storage applications.

Surface functionalization of BaTiO3 nanoparticles with dopamine was carried out using a reflux method to strongly bind dopamine on the BaTiO3 nanoparticle surface and improve its compatibility with the polyvinylidene fluoride (PVDF) polymer matrix. Fourier transform infrared spectra confirm the successful surface functionalization of BaTiO3 nanoparticles after immobilization with dopamine. Electrical properties of the resultant nanocomposite show that the dielectric constant can be enhanced up to 56.8 with low dielectric loss.