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We present a self-powered, high-performance graphene-enhanced ultraviolet silicon Schottky photodetector. Different from traditional transparent electrodes, such as indium tin oxides or ultra-thin metals, the unique ultraviolet absorption property of graphene leads to long carrier life time of hot electrons that can contribute to the photocurrent or potential carrier-multiplication. Our proposed structure boosts the internal quantum efficiency over 100%, approaching the upper-limit of silicon-based ultraviolet photodetector. In the near-ultraviolet and mid-ultraviolet spectral region, the proposed ultraviolet photodetector exhibits high performance at zero-biasing (self-powered) mode, including high photo-responsivity (0.2 A W −1 ), fast time response (5 ns), high specific detectivity (1.6 × 10 13 Jones), and internal quantum efficiency greater than 100%. Further, the photo-responsivity is larger than 0.14 A W −1 in wavelength range from 200 to 400 nm, comparable to that of state-of-the-art Si, GaN, SiC Schottky photodetectors. The photodetectors exhibit stable operations in the ambient condition even 2 years after fabrication, showing great potential in practical applications, such as wearable devices, communication, and "dissipation-less" remote sensor networks.

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Keywords: PVDF, piezoelectric effect, self-alignment, β-phase, Piezo Force Microscopy Poly(vinylidene fluoride), (PVDF), is one of the most attractive polymers owing to its remarkable pyro-, piezo-and ferro-electroactive properties. [1][2][3][4] These properties stem from its unique polymorphism which also gives rise to its extraordinary mechanical properties, high chemical resistance, good thermal stability and biocompatibility. [5] PVDF shows four significant crystalline phases α, β, δ and γ; [2,6] with the electroactive β-phase being utilized most frequently for the development of sensors, actuators [7,8] and microgenerators. [4,9,10] Owing to its importance, the formation of electroactive β crystalline phase has been intensively investigated through various routes, including melt casting, [15] solution deposition, [11] spin coating [12] and phase inversion. [13,14] While, the films or membranes formed by 2 melting/crystallisation are dominated by α-phase, [15] those obtained by spin-coating and further dried at temperature between 30-60 ºC are largely dominated by β-phase crystals. [12] For the phase inversion technique, PVDF films are formed by quenching the casting films into a nonsolvent bath to induce a series of liquid-solid and liquid-liquid phase separation events. [5,13,14] The microstructure and crystalline phase of the polymer films in phase-inversion can be controlled by adjusting paramteres like composition, [5,12] type of solvent, [16,17] quenching temperature etc. [18,19] Although PVDF films formed by this technique can have high β-phase contents, they are mostly porous and not suitable for electroactive applications. Moreover, the β-phase crystals are randomly oriented in these PVDF films and show no specific directional preference. Polarization under high electric field, [6,20] by Corona poling, [12] or by mechanical stretching [3,6,15] is subsequently carried out to obtain electroactive PVDF films. These operations need to be done at elevated temperatures (>80 o C), requiring high voltage equipment and films with dense structure and high β-phase content. Spontaneous formation of electroactive -phase in PVDF nanofibers formed by drawing was reported to have piezoelectric coefficient up to d33=58.5 pm/V. [25] Similarly, PVDF mesoscale rod arrays, without any poling process, too were also reported to possess good piezoelectric properties due to their high β content. [24] In both these cases, mechanical stretching and stress during the template guiding are believed to be responsible for the piezoelectric effect. Self-polarization of oriented β-phase crystals has also been observed in ultrathin (~20 nm) PVDF copolymer films by spin-coating or by Langmuir-Blodgett (LB) method, and is attributed to the built-in electric field, in-film stress and the strong interaction of PVDF molecules with polar water. [21,22,23] Generally speaking, PVDF films or membrane formed by melt casting, solution deposition, spin coating and phase inversion etc. do not exhibit aligned β-phase crystals without additional t...

Egg albumen as the dielectric, and dissolvable Mg and W as the top and bottom electrodes are used to fabricate water-soluble memristors. 4 × 4 cross-bar configuration memristor devices show a bipolar resistive switching behavior with a high to low resistance ratio in the range of 1 × 10(2) to 1 × 10(4), higher than most other biomaterial-based memristors, and a retention time over 10(4) s without any sign of deterioration, demonstrating its high stability and reliability. Metal filaments accompanied by hopping conduction are believed to be responsible for the switching behavior of the memory devices. The Mg and W electrodes, and albumen film all can be dissolved in water within 72 h, showing their transient characteristics. This work demonstrates a new way to fabricate biocompatible and dissolvable electronic devices by using cheap, abundant, and 100% natural materials for the forthcoming bioelectronics era as well as for environmental sensors when the Internet of things takes off.

Vertical contact-separation mode triboelectric generator (TEG) based on lead-free perovskite, zinc stannate (ZnSnO 3 )-polyvinylidene fluoride (PVDF) composite and polyamide-6 (PA6) membrane is demonstrated. For the 5wt% PVDF-ZnSnO 3 nanocomposites, the facile phaseinversion method provides a simple route to achieve high crystallinity and β-phase with a piezoelectric coefficient d 33 of -65 pmV -1 , as compared to -44 pmV -1 for pristine PVDF membranes. Consequently, at a cyclic excitation impact of 490 N/3 Hz, the PVDF-ZnSnO 3 /PA6 based TEGs provide a significantly higher voltage of 520 V and a current © 2016. This manuscript version is made available under the Elsevier user license http://www.elsevier.com/open-access/userlicense/1.0/ 2 density of 2.7 mAm -2 (corresponding charge density of 62.0 µCm -2 ), as compared to the pristine PVDF-PA6 TEG which provides up to 300 V with a current density of 0.91 mAm -2(corresponding to a charge density of 55.0 µCm -2 ). This increase in the electrical output can be attributed to not only the enhanced polarisation of PVDF by ZnSnO 3 leading to an increase in the β-phase content, but also to the surface charge density increase by stress induced polarisation of ZnSnO 3 , leading to the generation of stronger piezoelectric potential.The work thus introduces a novel method of enhancing the surface charge density via the addition of suitable high polarization piezoelectric materials thus eliminating the need for prior charge injection for fluoropolymer membranes.

This work uses a finite volume method to investigate three-dimensional acoustic streaming patterns produced by surface acoustic wave (SAW) propagation within microdroplets. A SAW microfluidic interaction has been modelled using a body force acting on elements of the fluid volume within the interaction area between the SAW and fluid. This enables the flow motion to be obtained by solving the laminar incompressible Navier–Stokes equations driven by an effective body force. The velocity of polystyrene particles within droplets during acoustic streaming has been measured and then used to calibrate the amplitudes of the SAW at different RF powers. The numerical prediction of streaming velocities was compared with the experimental results as a function of RF power and a good agreement was observed. This confirmed that the numerical model provides a basic understanding of the nature of 3D SAW/liquid droplet interaction, including SAW mixing and the concentration of particles suspended in water droplets.

Flexible electronics are a very promising technology for various applications. Several types of flexible devices have been developed, but there has been limited research on flexible electromechanical systems (MEMS). Surface acoustic wave (SAW) devices are not only an essential electronic device, but also are the building blocks for sensors and MEMS. Here we report a method of making flexible SAW devices using ZnO nanocrystals deposited on a cheap and bendable plastic film. The flexible SAW devices exhibit two wave modes - the Rayleigh and Lamb waves with resonant frequencies of 198.1 MHz and 447.0 MHz respectively, and signal amplitudes of 18 dB. The flexible devices have a high temperature coefficient of frequency, and are thus useful as sensitive temperature sensors. Moreover, strong acoustic streaming with a velocity of 3.4 cm/s and particle concentration using the SAW have been achieved, demonstrating the great potential for applications in electronics and MEMS.