Increasing the Room-Temperature Electric Double Layer Retention Time in Two-Dimensional Crystal FETs

Poly(vinyl alcohol) (PVA) and LiClO4, a solid polymer electrolyte with a glass transition temperature (T-g) of 80 degrees C, is used to electrostatically gate graphene field-effect transistors. The ions in PVA:LiC104 are drifted into place by field-effect at T > Tg, providing n- or p-type doping, and when the device is cooled to room temperature, the polymer mobility and, hence ion mobility are arrested and the electric double layer (EDL) is "locked" into place in the absence of a gate bias. Unlike other electrolytes used to gate two-dimensional devices for which the Tg, and therefore the "locking" temperature, is well below room temperature, the electrolyte demonstrated in this work provides a route to achieve room-temperature EDL stability. Specifically, a 6 orders of magnitude increase in the room temperature EDL retention time is demonstrated over the commonly used electrolyte, poly(ethylene oxide) (PEO) and LiClO4. Hall measurements confirm that large sheet carrier densities can be achieved with PVA:LiClO4 at top gate programming voltages of > 2 V (-6.3 +/- 0.03 X 10(13) cm(-2) for electrons and 1.6 +/- 0.3 X 10(14) cm(-2) for holes). Transient drain current measurements show that at least 75% of the EDL is retained after more than 4 h at room temperature. Unlike PEO-based electrolytes, PVA:LiClO4 is compatible with the chemicals used in standard photolithographic processes enabling the direct deposition of patterned, metal contacts on the surface of the electrolyte. A thermal instability in the electrolyte is detected by both I V measurements and differential scanning calorimetry, and FTIR measurements suggest that thermally catalyzed cross-linking may be driving phase separation between the polymer and the salt. Nevertheless, this work highlights how the relationship between polymer and ion mobility can be exploited to tune the state retention time and the charge carrier density of a 2D crystal transistor.