We utilize a high-mobility double-gated graphene field-effect transistor to measure the accumulated charge created by positron annihilation in its back-gate. The device consists of an exfoliated graphene flake stacked between two hexagonal boron nitride flakes placed on a 1 cm2 substrate of 500 μm thick conducting p-doped Si capped by 285 nm-thick SiO2. The device is placed in close proximity to a 780 kBq 22Na positron source emitting a constant flux of positrons. During the measurement, positrons annihilate within the back-gate, kept floating using a low-capacitance relay. The accumulated positive charge capacitively couples to the graphene device and builds a positive voltage, detectable through a shift in the top-gate dependent graphene resistance characteristic. The shift in the position of the top-gate Dirac peak is then used for extracting the exact voltage buildup and quantitative evaluation of the accumulated charge. Reaching a positron current sensitivity of ∼1.2 fA/Hz, detected over 20 min, our results demonstrate the utility of two-dimensional layered materials as probes for charging dynamics of positrons in solids.
Bibliographical noteFunding Information:
Devices for this study were fabricated at the Center for Nanoscience and Nanotechnology, the Hebrew University. Funding for this work was provided by the IAEC through Pazy Grant No. 328/20 and by Israel Science Foundation Grant No. 861/19. P.O. was supported by a fellowship by the Ministry of Science and Technology, Israel. T.R.D. acknowledges support from the Lady Davis Postdoctoral Fellowship program. K.W. and T.T. acknowledge support from the Elemental Strategy Initiative conducted by the MEXT, Japan (Grant No. JPMXP0112101001) and JSPS KAKENHI (Grant Nos. JP19H05790 and JP20H00354).
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