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Atmospheric and long-term aging effects on electrical properties of WSe 2 transistors with various thicknesses are examined. Although countless published studies report electrical properties of transition-metal dichalcogenide materials, many are not attentive to testing environment or to age of samples, which we have found significantly impacts results. Our as-fabricated exfoliated WSe 2 pristine devices are predominantly n-type, which is attributed to selenium vacancies. Transfer characteristics of as-fabricated devices measured in air then vacuum reveal physisorbed atmospheric molecules significantly reduced n-type conduction in air. First-principles calculations suggest this short-term reversible atmospheric effect can be attributed primarily to physisorbed H 2 O on pristine WSe 2 , which is easily removed from the pristine surface in vacuum due to the low adsorption energy. Devices aged in air for over 300 h demonstrate irreversibly increased p-type conduction and decreased n-type conduction. Additionally, they develop an extended time constant for recovery of the atmospheric adsorbents effect. Short-term atmospheric aging (up to approximately 900 h) is attributed to O 2 and H 2 O molecules physisorbed to selenium vacancies where electron transfer from the bulk and adsorbed binding energies are higher than the H 2 O-pristine WSe 2 . The residual/permanent aging component is attributed to electron trapping molecular O 2 and isoelectronic O chemisorption at selenium vacancies, which also passivates the near-conduction band gap state, p-doping the material, with very high binding energy. All effects demonstrated have the expected thickness dependence, namely, thinner devices are more sensitive to atmospheric and long-term aging effects.

The formation of an electric double layer in ionic liquid (IL) can electrostatically induce charge carriers and/or intercalate ions in and out of the lattice which can trigger a large change of the electronic, optical, and magnetic properties of materials and even modify the crystal structure. We present a systematic study of ionic liquid gating of exfoliated 2D molybdenum trioxide (MoO 3 ) devices and correlate the resultant electrical properties to the electrochemical doping via ion migration during the IL biasing process. A nearly 9 orders of magnitude modulation of the MoO 3 conductivity is obtained for the two types of ionic liquids that are investigated. In addition, notably rapid on/off switching was realized through a lithium-containing ionic liquid whereas much slower modulation was induced via oxygen extraction/intercalation. Time of flight-secondary ion mass spectrometry confirms the Li intercalation. Density functional theory (DFT) calculations have been carried out to examine the underlying metallization mechanism. Results of short-pulse tests show the potential of these MoO 3 devices as neuromorphic computing elements due to their synaptic plasticity.