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    Optical and electrical properties of ZnO nanorods used in photodetectors
    (Eesti Maaülikool, 2024) Murel, Taavi; Nagpal, Keshav (advisor); Rauwel, Protima (advisor)
    Nanomaterials have a higher surface to volume ratio when compared to their bulk counterparts and this makes them efficient to use in photodetection. This study explores the UV sensing abilities of ZnO nanorods grown on commercially available ITO substrates. In this study, two types of ZnO nanorod samples are synthesized, with 5 samples in total. The two types of samples are abbreviated as Type I and Type II, with three type I samples to type I-A, type I-B, type I-C and two type II samples to type II-A and type II-B. Type I samples have variations in the amount of zinc precursor in the seeding solution and Type II samples have variations in the amount of zinc precursor in the growth solution, used to grow ZnO nanorods of this study. To evaluate the electrical and photoresponse characteristics of these ZnO nanorods, 3 different tests were performed – test 1:current-voltage (I-V) in dark, test 2: I-V under UV light and test 3: current-time (I-t) under UV on-off cycles. A 365 nm UV source with 20 mW/cm2 irradiation power was used to evaluate the photoresponse characteristics of the ZnO nanorods. The measurements were performed using an OSSILA source meter. It was observed that changes made to seeding and growth solutions cause different electrical and photoresponse behavior in the nanorod samples. Under UV irradiations, it is found that resistance of all the samples decreases compared to the dark. In other words, the overall output current under UV irradiations has been higher for all ZnO nanorods than in dark. The figure of merits of the nanorods were evaluated by calculating their sensitivity, responsivity, and response time. The responsivity of sample Type I-B was highest with value of 22.82 μA/W and second-best sensitivity with value of 1.53. Sample Type II-A had the highest sensitivity with a value of 1.64, but had significantly lower responsivity than sample Type II-B, with a value of 4.17 μA/W. In addition, the response time of sample Type I-B was quickest, with a τdecay of 235.2 s. Therefore, having highest responsivity, a good sensitivity and quickest τdecay, the Type I-B sample is the best sample of this study. In other words, the amount of zinc precursors used for the seeding solution and growth solution of sample type I-B, are the optimal amounts to grow ZnO nanorods for the UV detection. Nevertheless, from the application point of view the response time of the samples is too slow to be used as a photodetector. Further research is needed to increase the responsivity of the samples.