Solvation Effects on the Electrical Properties of a Microfluid-Assisted Solution Field-Effect Transistor with Atomically Thin MoS2 Layers

A microfluid-assisted solution field-effect transistor (FET) with nanoscale channels of atomically thin MoS2 layers was constructed. The source–drain current (I d) vs gate voltage (V g) characteristics (I d–V g) were examined with a focus on the threshold voltage (V th) at the onset of the I d–V g curve. I d–V g changed when the channel contacted the tetracyanoquinodimethane (TCNQ) and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) solutions in isopropyl alcohol (IPA), acetonitrile (ACN), and dimethyl sulfoxide (DMSO). The shift in V th from the pure solvent condition (ΔV th) increased monotonically with the concentration, which was successfully simulated using Langmuir-type adsorption kinetics. We conclude that the TCNQ and F4-TCNQ solutes were partially solvated by the solvent and adsorbed on the MoS2 channel. Simultaneously, the saturated ΔV th value revealed a significant difference between the TCNQ and F4-TCNQ solutes. The ratio of saturated ΔV th of F4-TCNQ compared to that of TCNQ showed a decrease of 4.2, 1.7, and 1.3 for IPA, ACN, and DMSO, respectively. These results coincided with the order of the dielectric constants of these solvents (18.0, 36.0, and 46.6, respectively). The solutes produced the I d–V g curve by both charge transfer and the gating effect, the latter of which was screened by the presence of a solvent. This study demonstrates that a solution FET can be employed in solid–solution interface chemistry.