A slow positron beam with narrow energy spread can be obtained by moderating the β+ decay of a radioactive source. We report here the improvement in the efficiency of the rare-gas solid positron moderators by using a different geometry. The efficiency for slow positrons, ε, was measured for the cylindrical and the cone configurations of the moderator with the latter yielding ε of (4.6±0.2)×10−3 for solid neon deposited on an encapsulated radioactive 22Na source deposited on a 5 μm Cu98/Be2 window. No corrections were applied for the attenuation caused by the window. The ε for the conical configuration is (2.7 ± 0.2) times higher than that for the cylindrical configuration. Other rare-gas solids (e.g., Ar, Xe, Kr) yielded lower ε as compared to that for the solid neon in agreement with the earlier measured efficiencies of Mills and Gullikson [Appl. Phys. Lett. 49, 1121 <usb>(1986)].

The characteristics of a novel 0D/3D heterojunction photodetector fabricated using solution-processed colloidal MoS2 quantum dots (QDs) on GaAs is presented. MoS2 QDs with a dimension of ∼2 nm, synthesized by a standard sono-chemical exfoliation process with 2D layers have been used for the purpose. The microscopic and spectroscopic studies confirmed the formation of semiconducting (2H phase) MoS2 QDs. The photodetectors were fabricated using n-GaAs substrates with two different doping concentrations resulting in n–n heterojunctions between n-type 0D MoS2 QDs and bulk n-GaAs. The devices fabricated using GaAs with a higher doping concentration, showed an increase in the reverse current of the order of ∼102 upon illumination, while the same with a lower doping concentration showed an increase of the order of ∼103. All the heterojunction photodetector devices show a broadband operation over the visible wavelength range of 400–950 nm, with a peak responsivity of the devices being observed at 500 nm. The peak responsivity and detectivity are found to be ∼400 mA W−1 and ∼4 × 1012 Jones, respectively, even without any external applied bias, which are useful for self-powered photodetection. The results indicate that colloidal MoS2/GaAs based hybrid heterostructures provide a platform for fabricating broadband photodetectors by using highly absorbing MoS2 QDs, which may show the pathway towards next-generation optoelectronic devices with superior detection properties.

The effects of low temperature (≤700 °C ) annealing on the thermal dissociation of hydrogen-passivated interface trap centers of a SiO2-Si(100) system is studied using positron annihilation spectroscopy. The Si—H bonds dissociate with an activation energy of 2.60±0.06 eV. Assuming that the anneal generates trap centers with a single charge, positron measurements indicate that ∼4.5×108 trap centers/cm2 are created by a 600 °C anneal.