Ultrahigh Photoresponsivity of W/Graphene/ß-Ga2O3 Schottky Barrier Deep Ultraviolet Photodiodes.

The integration of graphene with semiconductor materials has been studied for developing advanced electronic and optoelectronic devices. Here, we propose ultrahigh photoresponsivity of ß-Ga2O3 photodiodes with a graphene monolayer inserted in a W Schottky contact. After inserting the graphene monolayer, we found a reduction in the leakage current and ideality factor. The Schottky barrier height was also shown to be about 0.53 eV, which is close to an ideal value. This was attributed to a decrease in the interfacial state density and the strong suppression of metal Fermi-level pinning. Based on a W/graphene/ß-Ga2O3 structure, the responsivity and external quantum efficiency reached 14.49 A/W and 7044%, respectively. These values were over 100 times greater than those of the W contact alone. The rise and delay times of the W/graphene/ß-Ga2O3 Schottky barrier photodiodes significantly decreased to 139 and 200 ms, respectively, compared to those obtained without a graphene interlayer (2000 and 3000 ms). In addition, the W/graphene/ß-Ga2O3 Schottky barrier photodiode was highly stable, even at 150 °C.

Boosting the Output Performance of the MoS2 Monolayer-Based Piezoelectric Nanogenerator by Artificial Dual Strain Concentration.

Piezoelectric nanogenerators (PENGs) with molybdenum disulfide (MoS2) monolayers have been intensively studied owing to their superior mechanical durability and stability. However, the limited output performance resulting from a small active area and low strain levels continues to pose a significant challenge that should be overcome. Herein, we report a novel strategy for the epoch-making output performance of a PENG with a MoS2 monolayer by adopting the additive strain concentration concept. The simulation study indicates that strain in the MoS2 monolayer can be initially augmented by the wavy structure resulting from the prestretched poly(dimethylsiloxane) (PDMS) and is further increased through flexural deformation (i.e., bending). Based on these studies, we have developed concentrated strain-applied PENGs with MoS2 monolayers. The wavy structures effectively applied strain to the MoS2 monolayer and generated a piezoelectric output voltage and current of around 580 mV and 47.5 nA, respectively. Our innovative approach to enhancing the performance of PENGs with MoS2 monolayers through the artificial dual strain concept has led to groundbreaking results, achieving the highest recorded output voltage and current for PENGs based on two-dimensional (2D) materials, which provides unique opportunities for the 2D-based energy harvesting field and structural insight into how to improve the net strain on 2D materials.

Magnetic Force Enhanced Sustainability and Power of Cam-Based Triboelectric Nanogenerator.

Since the first invention of triboelectric nanogenerators (TENGs) in 2012, many mechanical systems have been applied to operate TENGs, but mechanical contact losses such as friction and noise are still big obstacles for improving their output performance and sustainability. Here, we report on a magnet-assembled cam-based TENG (MC-TENG), which has enhanced output power and sustainability by utilizing the non-contact repulsive force between the magnets. We investigate the theoretical and experimental dynamic behaviors of MC-TENGs according to the effects of the contact modes, contact and separation times, and contact forces (i.e., pushing and repulsive forces). We suggest an optimized arrangement of magnets for the highest output performance, in which the charging time of the capacitor was 2.59 times faster than in a mechanical cam-based TENG (C-TENG). Finally, we design and demonstrate a MC-TENG-based windmill system to effectively harvest low-speed wind energy, ~4 m/s, which produces very low torque. Thus, it is expected that our frictionless MC-TENG system will provide a sustainable solution for effectively harvesting a broadband of wasted mechanical energies.

Extremely Foldable and Highly Porous Reduced Graphene Oxide Films for Shape-Adaptive Triboelectric Nanogenerators.

Here, a thin and foldable porous reduced graphene oxide (rGO) fabricated by a solvent casting method (SC-rGO) is introduced. The SC-rGO is superior to aluminum as a positive triboelectric material in triboelectric nanogenerators (TENGs), significantly enhancing TENG output performance. The film shows extremely foldable features, where it could be folded by 1/16 size. The electrical properties and device performance of SC-rGO are optimized varying thicknesses from 5 to 30 µm. A 30 µm thick TENG with a non-annealed SC-rGO film (STENG) shows the highest output of about 255 µW cm-2 due to its high carrier concentration, low work function, and high surface area. After annealing, STENG performance is optimized with a 10 µm thick SC-rGO because their work functions decreases, while the corresponding carrier concentrations decrease according to the thickness of the SC-rGO films. The SC-rGO films are highly durable and stable, where their output and conductivity show negligible changes after 100 000 cycles of mechanical deformation. A large SC-rGO with a size of 13 × 3 cm2 is fabricated and is attached inside a person's arm to demonstrate the shape-adaptive characteristics. Consequently, 170 V is obtained and it turns on 19 green light emitting diodes by simply touching the STENG.

All-Printed Electronic Skin Based on Deformable and Ionic Mechanotransducer Array.

Mechanoreceptors in human skin possess high sensitivity, wide sensing range, and high sensing resolution for external stimuli. Several attempts have been made to implement electronic skin (e-skin) that can mimic human skin. However, previous attempts are limited by the fundamental resolution problem arising from the use of film-like materials generated through pouring and spinning processes. Here, an all-printed e-skin based on deformable ionic mechanotransducer array (IMA) inspired by the physiological tactile sensing mechanism and the geometric features of mechanoreceptors in human skin is described. First, an ionic mechanotransduction channel is emulated with a piezocapacitive ionic mechanosensory system that engages in ion migration when the polymer matrix is deformed under a mechanical non-equilibrium state. Furthermore, the versatile shapes of the artificial mechanotransducer are tuned by the printing process variables, which results in high sensitivity (2.65 nF kPa-1 ) and high resolution (13.22 cm-2 ) of the device. It is demonstrated that this IMA is fully bio-inspired by the mechanotransduction and papillary structure of the mechanoreceptors. A high-resolution e-skin with a deformable and transparent IMA, which is fabricated by an all-printing methodology, will open up a new market in the field of soft and stretchable sensory platforms.

Comb-structured triboelectric nanogenerators for multi-directional energy scavenging from human movements.

A triboelectric nanogenerator (TENG) is an emerging energy harvesting technology utilizing multi-directional, wasted mechanical energies stemming from vibrations, winds, waves, body movements, etc. In this study, we report a comb-structured TENG (CTENG) capable of effectively scavenging multi-directional motions from human movements, which include walking, jumping, and running. By attaching CTENG to a person's calf, we obtain a root-mean-square (RMS) power value of 5.28 µW (i.e. 13.12 V and 0.4 µA) for 1 s during mild running action (~5 m/s), which is sufficient for powering 10 light emitting diodes (LEDs). We integrate a CTENG with a simple hand-held pendulum (HHP) system with a natural frequency of 5.5 Hz. The natural frequency and input energy of our HHP system can be easily controlled by changing the holder mass and initial bending displacement, thus producing different output behaviors for the CTENG. Under the optimal HHP-based CTENG system design, the maximum output reaches 116 V at 6.5 µA under 0.1 kg mass and 4 cm bending displacement conditions. The corresponding output energy is 52.7 µJ for an operation time of 10.8 s. Our HHP-CTENG system can sufficiently power 45 LEDs and shows different output performances by varying the driving velocity of a vehicle, thus demonstrating the possibility for a self-powered velocity monitoring system.

Continuous scavenging of broadband vibrations via omnipotent tandem triboelectric nanogenerators with cascade impact structure.

Ambient vibration energy is highly irregular in force and frequency. Triboelectric nanogenerators (TENG) can convert ambient mechanical energy into useable electricity. In order to effectively convert irregular ambient vibrations into electricity, the TENG should be capable of reliably continuous operation despite variability in input forces and frequencies. In this study, we propose a tandem triboelectric nanogenerator with cascade impact structure (CIT-TENG) for continuously scavenging input vibrations with broadband frequencies. Based on resonance theory, four TENGs were explicitly designed to operate in tandem and cover a targeted frequency range of 0-40 Hz. However, due to the cascade impact structure of CIT-TENG, each TENG could produce output even under non-resonant conditions. We systematically studied the cascade impact dynamics of the CIT-TENG using finite element simulations and experiments to show how it enables continuous scavenging from 0-40 Hz even under low input accelerations of 0.2 G-0.5 G m/s2. Finally, we demonstrated that the CIT-TENG could not only scavenge broadband vibrations from a single source such as a car dashboard, but it could also scavenge very low frequency vibrations from water waves and very high frequency vibrations from air compressor machines. Thus, we showed that the CIT-TENG can be used in multiple applications without any need for redesign validating its use as an omnipotent vibration energy scavenger.

Mesoporous Highly-Deformable Composite Polymer for a Gapless Triboelectric Nanogenerator via a One-Step Metal Oxidation Process.

The oxidation of metal microparticles (MPs) in a polymer film yields a mesoporous highly-deformable composite polymer for enhancing performance and creating a gapless structure of triboelectric nanogenerators (TENGs). This is a one-step scalable synthesis for developing large-scale, cost-effective, and light-weight mesoporous polymer composites. We demonstrate mesoporous aluminum oxide (Al2O3) polydimethylsiloxane (PDMS) composites with a nano-flake structure on the surface of Al2O3 MPs in pores. The porosity of mesoporous Al2O3-PDMS films reaches 71.35% as the concentration of Al MPs increases to 15%. As a result, the film capacitance is enhanced 1.8 times, and TENG output performance is 6.67-times greater at 33.3 kPa and 4 Hz. The pressure sensitivity of 6.71 V/kPa and 0.18 µA/kPa is determined under the pressure range of 5.5⁻33.3 kPa. Based on these structures, we apply mesoporous Al2O3-PDMS film to a gapless TENG structure and obtain a linear pressure sensitivity of 1.00 V/kPa and 0.02 µA/kPa, respectively. Finally, we demonstrate self-powered safety cushion sensors for monitoring human sitting position by using gapless TENGs, which are developed with a large-scale and highly-deformable mesoporous Al2O3-PDMS film with dimensions of 6 × 5 pixels (33 × 27 cm²).

Effects of Embedded TiO2-x Nanoparticles on Triboelectric Nanogenerator Performance.

Triboelectric nanogenerators (TENGs) are used as self-power sources for various types of devices by converting external waves, wind, or other mechanical energies into electric power. However, obtaining a high-output performance is still of major concern for many applications. In this study, to enhance the output performance of polydimethylsiloxane (PDMS)-based TENGs, highly dielectric TiO2-x nanoparticles (NPs) were embedded as a function of weight ratio. TiO2-x NPs embedded in PDMS at 5% showed the highest output voltage and current. The improved output performance at 5% is strongly related to the change of oxygen vacancies on the PDMS surface, as well as the increased dielectric constant. Specifically, oxygen vacancies in the oxide nanoparticles are electrically positive charges, which is an important factor that can contribute to the exchange and trapping of electrons when driving a TENG. However, in TiO2-x NPs containing over 5%, the output performance was significantly degraded because of the increased leakage characteristics of the PDMS layer due to TiO2-x NPs aggregation, which formed an electron path.