Liqiao Qin

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.

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.

\hbox{In}_{2}\hbox{O}_{3}

An ultraviolet heterojunction photodiode consisting of epitaxially grown p-GaN layers and polyvinyl alcohol coated ZnO colloidal nanoparticles exhibits a lowpass and bandpass alternative property depending on the illumination direction. At 0 V bias, a time response on the order of 10 s of milliseconds was demonstrated with a responsivity on the order of mA/W with about 100 nW of ultraviolet illumination. The rectification ratio at ±5 V was 1000 under dark environment. Deposition of colloidal ZnO nanoparticles on an independent p-GaN substrate introduces a technique to create a heterostructure pn junction photodiode with wavelength selection by back illumination.

Nanoparticles on a GaN Substrate

Zinc oxide (ZnO) nanoparticles created by top-down wet-chemistry synthesis were deposited onto intrinsic epitaxial grown gallium nitride-based substrates. Aluminum (Al) was evaporated through a shadow mask to form photoconductors. Visible-blind ultraviolet (UV) response has been recorded as high as 0.8 A/W at 375 nm. Experiments demonstrated that the photoconductors exhibited either a low-pass or bandpass spectral response depending on the illumination directions.

Wavelength selective p-GaN/ZnO colloidal nanoparticle heterojunction photodiode

ZnO bulk material (bandgap 3.26 eV) was reduced in size via a top-down wet-chemistry synthesis process into colloidal nanoparticles that were then coated with polyvinyl-alcohol (PVA). When the nanoparticles were excited at 340 nm, strong UV emission at 377 nm was observed, and parasitic green emission (510 nm) which normally appears in ZnO nano-material was suppressed. Quantum confinement effects of the nanoparticles were observed by a 5 nm blue shift from ZnO bulk material (380 nm) in the absorption spectrum. The same characteristics held true when the particles were deposited onto substrates to form a photoconductive detector. Point contact current-voltage characteristics of the detector were measured under darkness and 340 nm UV LED. The ratio of the UV photogenerated current to dark current was as high as 105. Spectral response of the detector demonstrates a sharp cut-off wavelength at 375 nm, with a UV responsivity equivalent to twice that of a commercial UV enhanced Si-based photodetector. The time response indicated the rise and fall time were 22s and 11s respectively. The results confirm that top-down wet-chemistry synthesized ZnO nanoparticles are suitable for small signal visible blind UV detection.

Low-Pass and Bandpass Alternative Ultraviolet Photoconductor Based on Zinc Oxide Nanoparticles on Intrinsic Gallium Nitride-Based Substrate

Bio-sensing sensitivity of a spectrally selective nanoparticle based ultraviolet (UV) photodetector is characterized in comparison to a silicon photodiode and a photomultiplier tube (PMT). The nanoparticle based photodetector is comprised of poly-vinyl alcohol (PVA) coated zinc-oxide (ZnO) nanoparticles deposited on an aluminum-gallium-nitride (AlGaN) epitaxially grown substrate. The sensitivity was determined by measuring the fluorescence intensity of the native fluorophore, tryptophan, in Escherichia coli (E-coli, ATCC-25922) cells. Tryptophan intrinsically fluoresces with a peak at 340 nm under 280 nm UV light illumination. It is shown that this detector can sense the concentration of E-coli to 2.5 × 108 cfu/mL while the silicon photodiode cannot detect the intrinsic fluorescence at all. Nevertheless, the PMT outperformed the ZnO nanoparticle-AlGaN substrate based photodetector with the ability to sense E-coli concentrations to 3.91 × 106 cfu/mL. However, because PMT based systems are commonly limited ...

Ultraviolet Photodetection Based on ZnO Colloidal Nanoparticles Made by Top-Down Wet-Chemistry Synthesis Process

Optical properties of In2O3 nanoparticles coated with polyvinyl-alcohol (PVA) are studied. Compared with uncoated In2O3 nanoparticles, PVA coated sample show enhanced UV-blue emission and suppressed parasitic green emission. Ultraviolet (UV)-blue photodetectors were then fabricated by depositing aluminum (Al) as contacts on top of PVA coated and uncoated samples. The photodetector with PVA coating, exhibits lower dark current and higher responsivity than the photodetector without PVA coating. The rise and fall time of the PVA coated photodetector is about 500 s and 1600 s respectively, one half of the uncoated device. These improvements are attributed to surface passivation of In2O3 nanoparticles by PVA, which reduces the surface defects density and increase free carrier concentration of In2O3 nanoparticles.

BIO-SENSING SENSITIVITY OF A NANOPARTICLE BASED ULTRAVIOLET PHOTODETECTOR

Zinc Oxide (ZnO) nanoparticles were created by a top-down wet-chemistry synthesis process (ZnOA) and then coated with polyvinyl-alcohol (PVA) (ZnO-U). In ZnO-U, strong UV emission was apparent while the parasitic green emission, which normally appears in ZnO suspensions, was suppressed. A standard lift-off process via e-beam lithography was used to fabricate a detector by evaporating Aluminum (Al) as ohmic electrodes on the ZnO nanoparticle film. Photoconductivity experiments showed that linear current-voltage response were achieved and the ZnO-U nanoparticles based detector had a ratio of UV photo-generated current more than 5 times better than that of the ZnO-A based detector. In addition, non-linear current-voltage responses were observed when interdigitated finger Gold (Au) contacts were deposited on ZnO-U. The UV generated current to dark current ratios were between 4 and 7 orders of magnitude, showing better performance than the photodetector with Al contacts. ZnO-U were also deposited on Gallium Nitride (GaN) and Aluminum Gallium Nitride (AlGaN) substrates to create spectrally selective photodetectors. The responsivity of detector based on AlGaN is twice that of commercial UV enhanced Silicon photodiodes. These results confirmed that ZnO nanoparticles coating with PVA is a good material for small-signal, visible blind, and wavelength selective UV detection.

Photoresponse comparison of indium oxide (In 2 O 3 ) nanomaterials

Multiple layers of colloidal nanoparticles were used to make large area, visible blind and wavelength selective, ultraviolet (UV) photodetectors [1]. Polyvinyl-alcohol (PVA) coated ZnO nanoparticles were created by a top-down wet-chemical means and deposited on quartz, glass, or GaN-based substrates [1–3]. The advantages of high sensitivity, low power consumption, and the ability of to filter out ambient light, are the result of layered nano-materials on substrates [4]. In separate experiments, these detectors were tested for both bio-fluorescence and alpha particle scintillator detection as a proof-of-concept.