The solid-state field-effect transistor, FET, and its theories were paramount in the discovery and studies of graphene. In the past two decades another transistor based on conducting polymers, called organic electrochemical transistor (ECT), has been developed and largely studied. The main difference between organic ECTs and FETs is the mode and extent of channel doping; while in FETs the channel only has surface doping through dipoles, the mixed ionic–electronic conductivity of the channel material in organic ECTs enables bulk electrochemical doping. As a result, organic ECTs maximize conductance modulation at the expense of speed. To date ECTs have been based on conducting polymers, but here we show that MXenes, a class of 2D materials beyond graphene, enable the realization of electrochemical transistors (ECTs). We show that the formulas for organic ECTs can be applied to these 2D ECTs and used to extract parameters like mobility. These MXene ECTs have high transconductance values but low on–off ratios. We further show that conductance switching data measured using ECT, in combination with other in situ–ex situ electrochemical measurements, is a powerful tool for correlating the change in conductance to that of the redox state, to our knowledge, this is the first report of this important correlation for MXene films. 2D ECTs can draw great inspiration and theoretical tools from the field of organic ECTs and have the potential to considerably extend the capabilities of transistors beyond those of conducting polymer ECTs, with added properties such as extreme heat resistance, tolerance for solvents, and higher conductivity for both electrons and ions than conducting polymers.
1. Introduction
The discovery of the basic electronic behavior of one-atomthick graphene in 2004,gave rise to two important directions in materials science: the field of physics in two-dimensional (2D) materials and the realization of a future roadmap of 2D materials beyond graphene. Additionally, one of the most important technical advances in 2D materials has been the discovery that layered crystals can be exfoliated and colloidally stabilized in liquids, enabling their fabrication from polar solvents, such as water.Computational studies have predicted that thousands of materials can exfoliate into 2D materials;a number that is driven by the contribution of scientific technologies. Among them, transition metal dichalcogenides (TMDs) and transition metal carbides and/or carbonitride labeled MXenes4 currently comprise large classes of 2D materials beyond graphene. TMDs and MXenes introduce many new capabilities not present in graphene, such as tunable bandgaps and conductivity, and high pseudocapcitance.
2.Spectroelectrochemical analysis
We performed in situ UV-vis-NIR spectroelectrochemical analysis to characterize the electrochromic properties of the MXene films either spin-coated or LbL assembled on indium tin oxide (ITO) coated glass using a 100 mg mL−1 poly(vinyl alcohol) (PVA)/6 v/v% H2SO4 gel electrolyte in water (Fig. 1a, see the Experimental section for details). We used an Ag/AgCl pellet as a pseudo-reference electrode to match the electrochemical potential between the spectroelectrochemical and the electrochemical transistor measurements (described in section 2.3), while a platinum coil was used as the counter electrode.
Fig. 1 (a) Schematic diagram of the in situ spectroelectrochemical measurement setup. UV-vis spectroelectrochemical measurements of (b) spincoated (10 layers) and LbL-assembled (c) 3 bilayers and (d) 10 bilayer MXene films at decreasing redox doping voltages from 0 to −0.8 V with steps of 0.1 V.
The UV-Vis-NIR spectra of spin-coated MXene films exhibit a broad absorption spectrum, with a continuous increase of absorbance from the near-IR region to the UV attributed to the inter-band transitions and a shoulder at around 760 nm (Fig. 1b). Decreasing the number of spin-coating steps leads to a minor increase in the absorption at 760 nm with respect to the local maximum at λ < 450 nm (Fig. S1a†). In contrast to the spin-coated films, LbL-assembled MXenes with 3, 10, and 15 bilayers (Fig. 1c, d, and S1b†) exhibit a distinct band with an absorbance maximum at 760 nm attributed to surface plasmons, i.e., the collective oscillations of free electronic charge carriers.Such a maximum is red-shifted to 807 nm for 20 bilayers (Fig. S1c†), indicating changes in MXene organization with the increase in the number of layers. Overall, the increase in the number of LbL-assembled bilayers from 3 to 20 MXene bilayers leads to an enhancement in the magnitude of absorption of the films, consistent with an increase in thickness.
3.Conclusion
The solid-state field-effect transistor, FET, and its theories were paramount in the discovery and studies of graphene. Here, we show that MXenes, a class of 2D materials beyond graphene, have mixed electronic–ionic properties that enable the realization of electrochemical transistors (ECTs). The current parameters of MXene ECTs show good normalized transconductance (12.5Scm−1 ) but low on–off ratios (1.19), these values are already state of the art for use in devices like electrochemical random-access memories (ECRAMs) or biosensors.
