Abstract
Atomically thin films of layered materials such as molybdenum disulfide (MoS2) are of growing interest for the study of phase transitions in two-dimensions through electrostatic doping. Electrostatic doping techniques giving access to high carrier densities are needed to achieve such phase transitions. Here we develop a method of electrostatic doping which allows us to reach a maximum n-doping density of 4 × 1014 cm−2 in few-layer MoS2 on glass substrates. With increasing carrier density we first induce an insulator to metal transition and subsequently an incomplete metal to superconductor transition in MoS2 with critical temperature ≈10 K. Contrary to earlier reports, after the onset of superconductivity, the superconducting transition temperature does not depend on the carrier density. Our doping method and the results we obtain in MoS2 for samples as thin as bilayers indicates the potential of this approach.
Highlights
Thin films of layered materials such as molybdenum disulfide (MoS2) are of growing interest for the study of phase transitions in two-dimensions through electrostatic doping
We develop a method of electrostatic doping that allows us to reach a maximum n-doping density of 4 Â 1014 cm À 2 in the simplest of devices consisting of few-layer MoS2 on glass
Our ‘space charge doping’ technique[21] is a natural extension of this method for which we start with the fewlayer sample lying on the glass substrate
Summary
Thin films of layered materials such as molybdenum disulfide (MoS2) are of growing interest for the study of phase transitions in two-dimensions through electrostatic doping. We develop a method of electrostatic doping which allows us to reach a maximum n-doping density of 4 Â 1014 cm À 2 in few-layer MoS2 on glass substrates. Using an ionic liquid as a dielectric, densities above 1014 cm À 2 could be obtained with superconductivity appearing in a narrow region between 8 Â 1013 and 3 Â 1014 cm À 2 and a maximum critical temperature of 11 K (refs 14,15) Both these techniques involve delicate device issues and whereas a solid dielectric does not allow for very high doping, the liquid gating method requires great care to avoid possible intercalation, electrochemical reactions[16] and strain during the liquid to solid transformation of the dielectric, which may be avoided by using a solid ionic conductor[17]. Heating of the polarized substrate causes a drift current which tends to nullify the space charge layer and remove electrostatic doping of the sample This process can be accelerated by the application of an appropriate electric field. The very high carrier density accessible with our doping technique allows us to subsequently induce superconductivity
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