Field-Effect Doping of MoS2 Using a Solid Polymer Electrolyte

Transition metal dichalcogenides (TMDs) have been studied for their wide range of unique optical, electronic and mechanical properties.[1] Due to weak interlayer attraction, single layers of TMDs can be isolated by mechanical exfoliation. Some properties depend on the number of TMD layers.  For example, as MoS₂ is thinned from bulk to a monolayer, it changes from a 1.2 eV indirect band gap semiconductor to a 1.9 eV direct band gap semiconductor.[2]

DFT calculations have shown that Li⁺ will form an ionic bond with the Mo in MoS₂.[3] By applying an electrolyte containing Li⁺ to the MoS₂ surface, ions can be driven by an applied field to the MoS₂, yielding both electrostatic and electrochemical doping.

Here, we fabricate a MoS₂ FET device and drop-cast a solid polymer electrolyte (SPE) of polyethylene oxide (PEO) and LiClO₄ onto the MoS₂ surface. The device is top-gated by making contact to the surface of the electrolyte with a probe tip (Fig. 1 inset).   

Prior to the deposition of PEO:LiClO₄, all devices show n-channel behavior when modulated by a backgate, regardless of MoS₂ thickness. Devices with 2-3 layers of MoS₂ (thickness measured with Raman spectroscopy) continue to show n-channel behavior with the electrolyte present, with an on/off ratio of 10⁵. (Fig. 1) However, for devices with >10 layers of MoS₂, p-channel behavior is observed, with an on/off ratio of ~100. (Fig. 2)

The p-channel devices also show features at V_g = -8 V and V_g = 22 V, suggestive of electrochemical reactions between the MoS₂ and the polymer electrolyte. Is it possible that these features are associated with the electrochemical reduction and oxidation of Li⁺. Because the features only appear in the data for devices with >10 layers, we propose that Li⁺ may be intercalating between layers of MoS₂ and undergoing reversible redox reactions.

Though both p- and n- channel devices show hysteresis, p-channel devices show two distinct states. After what appears to a forming step where the current increases to ~1 mA (Fig 2, Run 1), the threshold voltage shifts depending on the direction of the sweep. We define the threshold near 40 V as the “0” state and the threshold near 0 V as the “1” state. The state is set by the previous gate bias (i.e., it increases on the forward sweep to state “0” and decreases to state “1” on the reverse). Retention characteristics were tested by applying a bias (±25 V) to the topgate for 100 seconds, then removing the bias and measuring I_d over time. The device retains state “0” for ~30 s and “1” for >1000 s (Fig. 3).

We demonstrate for the first time the hysteretic response of an MoS₂ FET with an SPE gate dielectric. Devices with >10 layers of MoS₂ have a variable threshold that shifts depending on the previous gate bias. These devices may have application in novel ionic memory and steep subthreshold devices.

Acknowledgements:  This work was supported in part by the Center for Low Energy Systems Technology (LEAST), one of the six SRC STARnet Centers, sponsored by MARCO and DARPA.

[1]     Q. Wang, et al., Nature Nanotech, 7, p. 699-712, (2012)

[2]     B. Radisavljevic, et al., Nature Nanotech., 6, p. 147-150, (2011)

[3]     J. Chang, et al., arXiv, 1305.7162 (2013)