Doping of MoTe2 via surface charge-transfer in ambient air
Gheorghe Stan, Cristian Ciobanu, Sri Ranga Jai Likith, Asha Rani, Sergiy Krylyuk, Albert Davydov
Doping is a key process that facilitates the use of semiconductors for electronic and optoelectronic devices, by which the concentration and type of majority carriers (electrons or holes) can be modified controllably to achieve desired conduction properties. We found that changes in the surface potential of MoTe2 are determined by air adsorbates and the number of layers of exfoliated material. Changes measured in surface potential and work function could be attributed to the permanent dipole of the adsorbed molecule as well as to the charge transfer between the surface and the adsorbates (induced dipoles). Through density functional theory calculations we are able to remove this ambiguity, showing that (non-polar) oxygen molecules physisorb and drain electrons from the surface, thereby making it p-type and increasing its work function. This is in agreement with our Kelvin probe microscopy measurements; other adsorbates, such as water or hydroxyl, do not lead to agreement with the experimentally measured variations of the work function. Surface charge transfer doping (SCTD) driven by adsorbates can be easily adjusted through thermal annealing of the entire sample, leading to an effective modulation of the surface potential of MoTe2. Furthermore, we also illustrate local control of the surface charge doping by contact electrification using the probe of an electrostatic force microscope as a floating gate. After annealing or contact electrification, the air-exposed MoTe2 surfaces exhibit a slow reversal processes of re-adsorption of oxygen, restoring of the p- type doping. These investigations can complement typical field effect transistor measurements used to extract electronic transport properties of devices based on 2D materials. As a reversible and controllable nanoscale physisroption process, SCTD can thus open new avenues for the emerging field of 2D-TMDC electronics.