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Author: search.filters.author.Rao, V. RamgopalSubject: search.filters.subject.Microscopy

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    A non-volatile resistive memory effect in 2,2′,6,6′-tetraphenyl-dipyranylidene thin films as observed in field-effect transistors and by conductive atomic force microscopy
    (RSC, 2017) Rao, V. Ramgopal
    The charge transport properties of 2,2′,6,6′-tetraphenyldipyranylidene (DIPO-Ph4), a large planar quinoïd π-conjugated heterocycle, are investigated in field-effect transistor (FET) configuration and by conductive atomic force microscopy (c-AFM). The FET properties show a clear p-type behavior with a hole mobility up to 2 × 10−2 cm2 V−1 s−1 and on/off ratio of 104. The transfer characteristics Id/Vg present a clear hysteresis typical of a resistive memory effect. This memory effect is again observed by means of c-AFM in lateral mode using a nearby gold top-contact as the counter-electrode. The c-AFM current response recorded for variable distances d = 0.5–9.0 μm between the AFM tip and the top electrode shows a resistive switching behavior in the low-voltage 0.0–3.0 V region. Repeated “write-read-erase-read” cycles performed at low frequency reveal a non-volatile memory effect in the form of high-resistance and low-resistance states with a stable on/off ratio of 102 during cycling operation.
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    Organic passivation of Al0.5Ga0.5N epilayers using self-assembled monolayer of Zn(II) porphyrin for improved solar-blind photodetector performance
    (IOP, 2021-03) Rao, V. Ramgopal
    We report on the passivation of surface states of Al0.5Ga0.5N epilayers by employing self-assembled monolayers (SAMs) of organic molecules, which led to a significant improvement in the performance of Al0.5Ga0.5N based solar-blind photodetector. The formation of SAM of meso-(5-hydroxyphenyl)-10,15,20-tri(p-tolyl) porphyrin (ZnTPP(OH)) on the surface of Al0.5Ga0.5N was probed by contact angle measurement, x-ray photoelectron spectroscopy, and atomic force microscopy. The successful passivation of surface states was confirmed by Kelvin probe force microscopy as a significant decrease in the surface potential of Al0.5Ga0.5N by ∼280 mV was observed. The inference was supported by a four-fold increase in the photoluminescence intensity of the near-band edge emission peak upon passivation. As a result, the dark current of the as-fabricated solar-blind photodetector reduced by two orders of magnitude, without compromising with the magnitude of the photo current at 270 nm. The role of SAM was evident in improving the performance of the photodetector as the peak value of photo-to-dark current ratio enhanced by ∼36 times. The peak responsivity of the photodetector increased from 1.6 to 2.2 mA W−1 at 10 V. The significant reduction in the dark current and enhancement in the responsivity led to an improvement in the specific detectivity by ∼10 times. Additionally, the response speed of the photodetector was found to improve significantly from 4 to 0.5 s.