Impact of thermal annealing in forming gas on the optical and electrical properties of MoS2 monolayer.

Technological applications involving 2D MoS2 require transfer of chemical vapor deposition (CVD) grown material from its original substrate and subsequent lithographic processes. Inevitably, those steps contaminate the surface of the 2D material with polymeric residues affecting the electronic and optical properties of the MoS2. Annealing in forming gas is considered an efficient treatment to partially remove such residues. However, hydrogen also interacts with MoS2 creating or saturating sulfur vacancies. Sulfur vacancies are known to be at the origin of n-doping evident in the majority of as-grown MoS2 samples. In this context, investigating the impact of thermal annealing in forming gas on the electronic and optical properties of MoS2 monolayer is technologically important. In order to address this topic, we have systematically studied the evolution of CVD grown MoS2 monolayer using Raman spectroscopy, photoluminescence, x-ray photoelectron spectroscopy and transport measurements through a series of thermal annealing in forming gas at temperatures up to 500 °C. Efficient removal of the polymeric residues is demonstrated at temperatures as low as 200 °C. Above this value, carrier density modulation is identified by photoluminescence, x-ray photoelectron spectroscopy and electrical characterization and is correlated to the creation of sulfur vacancies. Finally, the degradation of the MoS2 single layer is verified with annealing at or above 350 °C through Raman and photocurrent measurements.

Fullerene-based electrochemical buffer layer for ion-selective electrodes.

In this work, C(60) fullerene is used as an electrochemical mediator for the development of an all-solid-state ISE. The unique electrochemical characteristics of the fullerenes allow for the facile ion-to-electron transduction across the ionically active polymeric ion-selective membrane and the electrochemically active glassy carbon transducer. The interfacial ion-to-electron charge transfer was investigated by Electrochemical Impedance Spectroscopy. The study of the analytical characteristics of a model potassium-selective electrode, together with the EIS studies, reveals that, indeed, the interfacial C(60) electrochemically active layer facilitates the ion-to-electron transduction, providing a stable and reversible solid-state ISE system. This finding is a significant contribution to the efforts aiming at overcoming one of the most significant drawbacks of the solid-state ISEs, that is the potential drift observed during continuous measurements, and could lead to the development of both cation- and anion-sensitive systems.

Thick membrane, solid contact ion selective electrode for the detection of lead at picomolar levels.

A new approach for decreasing the lower detection limit of a lead ion selective electrode (ISE) is presented. The ISE is designed using nonfunctionalized porous glassy carbon loaded with ionophore/plasticizer/additive cocktail. This material acts both as the support for the liquid polymeric membrane and as the signal transducer of the ISE. The high purity of the glassy carbon, together with its high conductivity, allows for the development of a thick, low-resistance composite membrane. This sensor element enables the continuous measurement of lead down to picomolar levels, with very small detection limit deterioration due to the lead ion transport within the bulk of the thick membrane.