Shayla Sawyer

High responsivity, fast ultraviolet photodetector fabricated from ZnO nanoparticle–graphene core–shell structures

We report a simple, efficient and versatile method for assembling metal oxide nanomaterial-graphene core-shell structures. An ultraviolet photodetector fabricated from the ZnO nanoparticle-graphene core-shell structures showed high responsivity and fast transient response, which are attributed to the improved carrier transport efficiency arising from graphene encapsulation.

Organic–Inorganic Heterointerfaces for Ultrasensitive Detection of Ultraviolet Light

The performance of graphene field-effect transistors is limited by the drastically reduced carrier mobility of graphene on silicon dioxide (SiO2) substrates. Here we demonstrate an ultrasensitive ultraviolet (UV) phototransistor featuring an organic self-assembled monolayer (SAM) sandwiched between an inorganic ZnO quantum dots decorated graphene channel and a conventional SiO2/Si substrate. Remarkably, the room-temperature mobility of the chemical-vapor-deposition grown graphene channel on the SAM is an order-of-magnitude higher than on SiO2, thereby drastically reducing electron transit-time in the channel. The resulting recirculation of electrons (in the graphene channel) within the lifetime of the photogenerated holes (in the ZnO) increases the photoresponsivity and gain of the transistor to ∼10(8) A/W and ∼3 × 10(9), respectively with a UV to visible rejection ratio of ∼10(3). Our UV photodetector device manufacturing is also compatible with current semiconductor processing, and suitable for large volume production.

An ultraviolet photodetector fabricated from WO3 nanodiscs/reduced graphene oxide composite material

A high sensitivity, fast ultraviolet (UV) photodetector was fabricated from WO₃ nanodiscs (NDs)/reduced graphene oxide (RGO) composite material. The WO₃ NDs/reduced GO composite material was synthesized using a facile three-step synthesis procedure. First, the Na₂WO₄/GO precursor was synthesized by homogeneous precipitation. Second, the Na₂WO₄/GO precursor was transformed into Na₂WO₄/GO composites by acidification. Finally, the Na₂WO₄/GO composites were reduced to WO₃ NDs/RGO via a hydrothermal reduction process. The UV photodetector showed a fast transient response and high responsivity, which are attributed to the improved carrier transport and collection efficiency through graphene. The excellent material properties of the WO₃ NDs/RGO composite demonstrated in this work may open up new possibilities for using WO₃ NDs/RGO for future optoelectronic applications.

Enhanced Ultraviolet Emission from Poly(vinyl alcohol) ZnO Nanoparticles Using a SiO2–Au Core/Shell Structure

Enhanced near band gap edge (NBE) emissions of PVA-ZnO nanoparticles were achieved by employing SiO(2)-Au core/shell nanostructures whereas the defect-level emission (DLE) is greatly suppressed. A maximum enhancement of nearly 400% was observed using SiO(2)-Au for the emission with optical resonance at 554 nm. SiO(2)-Au core/shell nanostructures also show a superior tunability of resonance energy as compared to that of the pure metal nanoparticles. The enhancement of the NBE emission and suppressed DLE is ascribed to the transfer of the energetic electrons excited by surface plasmon from metal nanoparticles to the conduction band of ZnO nanoparticles.

Cl‐Doped ZnO Nanowire Arrays on 3D Graphene Foam with Highly Efficient Field Emission and Photocatalytic Properties

An environmentally friendly, low-cost, and large-scale method is developed for fabrication of Cl-doped ZnO nanowire arrays (NWAs) on 3D graphene foam (Cl-ZnO NWAs/GF), and investigates its applications as a highly efficient field emitter and photocatalyst. The introduction of Cl-dopant in ZnO increases free electrons in the conduction band of ZnO and also leads to the rough surface of ZnO NWAs, which greatly improves the field emission properties of the Cl-ZnO NWAs/GF. The Cl-ZnO NWAs/GF demonstrates a low turn-on field (≈1.6 V μm(-1)), a high field enhancement factor (≈12844), and excellent field emission stability. Also, the Cl-ZnO NWAs/GF shows high photocatalytic efficiency under UV irradiation, enabling photodegradation of organic dyes such as RhB within ≈75 min, with excellent recyclability. The excellent photocatalytic performance of the Cl-ZnO NWAs/GF originates from the highly efficient charge separation efficiency at the heterointerface of Cl-ZnO and GF, as well as improved electron transport efficiency due to the doping of Cl. These results open up new possibilities of using Cl-ZnO and graphene-based hybrid nanostructures for various functional devices.

Heterojunction photodiode fabricated from hydrogen treated ZnO nanowires grown on p-silicon substrate

A heterojunction photodiode was fabricated from ZnO nanowires (NWs) grown on a p-type Si (100) substrate using a hydrothermal method. Post growth hydrogen treatment was used to improve the conductivity of the ZnO NWs. The heterojunction photodiode showed diode characteristics with low reverse saturation current (5.58 × 10(-7) A), relatively fast transient response, and high responsivity (22 A/W at 363 nm). Experiments show that the photoresponsivity of the photodiode is dependent on the polarity of the voltages. The photoresponsivity of the device was discussed in terms of the band diagrams of the heterojunction and the carrier diffusion process.

Current and optical noise of GaN/AlGaN light emitting diodes

Low frequency noise of current and light intensity of ultraviolet light emitting diodes (LED) with wavelength from 265to340nm are the superposition of the 1∕f and generation-recombination noise. The dependence of generation-recombination noise on the LED current has a maximum caused by a relatively shallow trap level in the quantum well. The upper bound of this trap level concentration is estimated to be Nt=7×1015cm−3. The relative spectral noise density of the light intensity fluctuations decreased with an increase of the LED forward current. At high currents, the difference in the noise level for LEDs with different wavelength is small and is of the same order of magnitude or even smaller than for visible LEDs.

Metal–Semiconductor–Metal Ultraviolet Photodetectors Based on Zinc-Oxide Colloidal Nanoparticles

Metal-semiconductor-metal ultraviolet (UV) photodetectors were created with zinc-oxide colloidal nanoparticles coated with polyvinyl-alcohol. Gold-interdigitated finger contacts with different parameters were patterned on the nanoparticles by optical lithography. The photodetectors exhibited UV-photogenerated current to dark current ratios ranging from 3.85 × 106 to 1.34 × 108, depending on the finger parameters. The spectral responses demonstrate a 375 nm cutoff wavelength, with a peak responsivity of 731.42 A/W at 345 nm.

Heterojunction photodiode fabricated from multiwalled carbon nanotube/ZnO nanowire/p-silicon composite structure

A heterojunction photodiode was fabricated from multiwalled carbon nanotubes (MWCNTs)/ZnO nanowires/p-Si (100) substrate composite structure. The heterojunction photodiode demonstrated a faster transient response and higher responsivity than the reference sample without deposition of MWCNTs, which is attributed to improved carrier collection and transport efficiency through the MWCNTs network. The high photoresponsivities of the devices are explained in terms of operation as a hybrid of photodiode and photoconductor modes. The spectral response of the devices showed dependence on voltage polarity and is attributed to the high valance band offset in the interfacial region of ZnO and p-Si substrate.

High Responsivity, Bandpass Near-UV Photodetector Fabricated From PVA-

High photoresponsivity in the near-ultraviolet (UV) range was observed for a photodetector fabricated from polyvinyl-alcohol (PVA)-coated In2O3 nanoparticles. The high responsivity is due to large depletion region introduced by ion-adsorbed oxygen on the surface of In2O3 nanoparticles. Experiments demonstrated that the photodetector exhibited either a low-pass or bandpass spectral response, depending on the illumination directions. Transient characteristics of photoconductivity have been studied, which shows a rise time of 700 s and a fall time of 1350 s.