Benchmarking Electrolyte-Gated Monolayer MoS 2 Field-Effect Transistors in Aqueous Environments

Most electrical sensor and biosensor elements require reliable transducing elements to convert small potential changes into easy to read out current signals. Offering inherent signal magnification and being operable in many relevant environments field‐effect transistors (FETs) are the element of choice in many cases. In particular, using electrolyte gating, numerous sensors and biosensors have been realized in aqueous environments. Over the past years, electrolyte‐gated FETs have been fabricated using a variety of semiconducting materials, including graphene, ZnO, as well as conjugated molecules and polymers. Above all, using conducting polymers top‐performing devices have been achieved. Herein, an approach to use a transition metal dichalcogenide (TMDC)‐based monolayer device as a transducing element is presented. Using MoS2 monolayers, it is shown that such electrolyte‐gated devices may be regarded as very promising transducing elements for sensor and biosensor applications, enabled by their high sensitivity for environmental changes and the possibility of using the naturally occurring sulfur vacancies as grafting points of biorecognition layers. Furthermore, the behavior of such a device under prolonged operation in a dilute biologically relevant electrolyte such as phosphate buffered saline solution (PBS) is reported.


Introduction
[4] In particular, field-effect transistor (FET)-based applications benefit from electrolyte gating.Because the high capacitance values (1-100 μF cm À2 ) achieved, [5] low voltage operation (<1 V) becomes possible, which paves the way for FETs as effective transducing elements to convert gate potential variations ΔV G into sourcedrain currents ΔI D in sensors [6,7] and biosensors [5,[8][9][10][11] in aqueous environments.In such applications, the figure of merit that determines the effectiveness of this conversion is the transconductance (g m ), which is defined as g m ¼ ΔI D /ΔV G .Here, conducting polymer-based organic electrochemical transistor (OECT) is currently the top performer with a typical g m of 2.7 mS followed by ZnO and graphene-based devices achieving a g m of 1.4 and 1.8 mS, respectively. [10]nspired by the high-performance values achieved with graphene-based devices, [12,13] 2D semiconductors with their topography-related high sensitivity to the changes at their interfaces seem to constitute the ideal candidates for electrolyte-gated FETs (EGFETs).
One very promising material class of atomically thin semiconductors is the transition metal dichalcogenides (TMDCs).This material class crystalizes in a layered sandwich structure with strong covalent bonds within the MX 2 (M ¼ transitions metal; X ¼ S, Se, Te) layers but comparatively weak interlayer interactions, allowing to cleave off single layers of the bulk crystal.Devices made from monolayers could be described as interface-only devices and applications of TMDCs as gas sensors have already been shown. [14]To be able to use TMDCs in high-performance FET applications, the main challenge to overcome is the defect control of these monolayers. [15]The two main types of defects are grain boundaries and metal or chalcogenide vacancies.[18][19][20] Recently, the operation of a MoS 2 -based FET device utilizing deionized (DI) water has been shown; however, utilizing MoS 2 multilayers. [21,22]These results constitute a promising starting point for benchmarking FETs in DOI: 10.1002/pssr.202100147Most electrical sensor and biosensor elements require reliable transducing elements to convert small potential changes into easy to read out current signals.
Offering inherent signal magnification and being operable in many relevant environments field-effect transistors (FETs) are the element of choice in many cases.In particular, using electrolyte gating, numerous sensors and biosensors have been realized in aqueous environments.Over the past years, electrolytegated FETs have been fabricated using a variety of semiconducting materials, including graphene, ZnO, as well as conjugated molecules and polymers.Above all, using conducting polymers top-performing devices have been achieved.Herein, an approach to use a transition metal dichalcogenide (TMDC)-based monolayer device as a transducing element is presented.Using MoS 2 monolayers, it is shown that such electrolyte-gated devices may be regarded as very promising transducing elements for sensor and biosensor applications, enabled by their high sensitivity for environmental changes and the possibility of using the naturally occurring sulfur vacancies as grafting points of biorecognition layers.Furthermore, the behavior of such a device under prolonged operation in a dilute biologically relevant electrolyte such as phosphate buffered saline solution (PBS) is reported.
biosensor-relevant electrolytes.By fabricating FETs based on MoS 2 monolayers, using aqueous liquids with the required ionic strength and choosing gate electrodes with advantageous properties we will show that one can push the performance of MoS 2 EGFETs to record heights.In addition to basic device characterization, we also perform long-term stability tests and find that MoS 2 EGFETs may be regarded as very promising transducing elements for sensor and biosensor applications.

Results and Discussion
Figure 1a,b shows a schematic of the bottom contact EGFETs with typical gate electrodes to bias the transistors in an aqueous environment with a polarizable Pt and a nonpolarizable Ag/AgCl electrode, respectively.Pt is an example of a polarizable electrode, meaning that charge separation occurs at the electrode/electrolyte interface and only little to no current passes which results in the electrode acting as a capacitor, also represented in the equivalent circuit below Figure 1a.In contrast to that the Ag/AgCl electrode shown in Figure 1b is an example for a nonpolarizable electrode, meaning that no EDL is formed at the electrode/electrolyte interface as described in detail in the literature. [23]While using a polarizable electrode, the EDL formed at the semiconductor/electrolyte is typically limited by its surface capacitance.This can be overcome by using a nonpolarizable electrode such as an Ag/AgCl electrode, [5] where the maximum possible EDL capacitance and therefore gating effect can act polarizing on the semiconductor.Figure 1c shows an optical image of an EGFET using an MoS 2 monolayer with a 30 μm long and 1 mm wide channel with bottom contact source-drain electrodes.For a description of the exfoliation and transfer process, see previous studies. [24,25]he outline of the monolayer is accentuated by a dotted red line to highlight the faint contrast between the region covered by monolayer and the bare substrate/electrode.To avoid hydrogen evolution reactions, which may happen at sulfur edge sites in MoS 2 , [26] all relevant measurements have been done within a 500 mV potential window.Outside this window, a sudden increase in gate currents can be seen in Figure S1 and S4, Supporting Information, which points toward catalytic activity.
To demonstrate the importance of choosing the proper gate electrode, the device was measured with two different electrodes.To work in a biosensor-relevant environment, PBS (PBS Â 10) was chosen as the electrolyte, which was diluted to a NaCl concentration of 10 À2 м. Figure 2a shows the output characteristic of a device measured by using the Pt wire as gate electrode (the architecture is shown in Figure 1a).This device already shows very promising output characteristics when compared with the state of the art, [21,22] showing transport across the channel for gate voltages (V GS ) V GS ≥ 300 mV and reaching saturation drain currents (I D ) of about 2.5 μA for V GS ¼ 500 mV.The device shows a second rise in I D for larger drain source voltages (V DS ).The corresponding gate currents for output and transfer characteristics shown in Figure 2a,b utilizing a Pt gate are shown in S3 and when comparing them to Figure S1, Supporting Information, the influence of the different working principles of the electrodes on the gate current can be seen.The device performance can be significantly improved by exchanging the Pt gate electrode to an Ag/AgCl electrode (see Figure 1b).Figure 2c shows the output characteristic using the Ag/AgCl gate with an increase in the saturation current to about 470 μA.This corresponds to almost a 200-fold increase compared with the device using the Pt electrode.When compared, the shape of the output characteristics (Figure 2a,c) also slightly changes as a function of the used electrode.The Pt electrode, when compared with the Ag/AgCl electrode reaches saturation for smaller V DS (<150 mV), shows linear current increase for a smaller V DS range and the pinch off happens over a smaller region.This is an effect of the shifting threshold voltages of different gate electrodes.This shift is as large as the electrochemical potentials of the electrodes are shifted with respect to each other.The transfer measurements for a device with an Ag/AgCl electrode are shown in Figure 2d.This device reaches peak currents of almost 1 mA in saturation, a mobility μ of 26 cm 2 V À1 s À1 , a subthreshold swing (SS) of 70 mV dec À1 and a threshold voltage V Th of 110 mV.
For the linear regime, these values are μ ¼ 34 cm 2 V À1 s À1 , SS ¼ 78 mV dec À1 , and V Th ¼ 190 mV.A constant capacitance of 5.2 μF cm À2 was assumed for mobility calculations which was measured via impedance spectroscopy.For the on/off ratio, we find 10 4 , which is not as good as values found in the literature. [21]This decrease is attributed to an increase in the off-state current caused by the ionic strength of the electrolyte.In S5 and S6, we show the corresponding device characteristics using DI water with Pt and an Ag/AgCl electrode.The reproducibility is addressed in Table S1, Supporting Information, where an average over 17 devices and the resulting standard deviation of the figures of merit is given.Essentially all device parameters are superior when using PBS as compared with DI water due to the reduced ionic strength of the latter.When operated at negative V DS , I D does not show a saturating behavior which is accompanied by rising gate currents, an indication for faradaic currents being passed, meaning that there are electrochemical reactions taking place as shown in Figure S1 and S4, Supporting Information.To extract the figure of merit for FETs sensor applications, the transconductance g m is calculated for the linear and saturation region depending on the applied gate bias (see S2).For the linear region, we find a peak of g m at V DS ¼ 50 mV and V GS ¼ 360 mV with g m ¼ 0.35 mS, while in saturation for V D ¼ 400 mV and V G ¼ 530 mV, a top value of g m ¼ 1.98 mS is found.To put these results into perspective, we compared the performance of our device to a literature study on OECTs by Khodagholy et al. [10] When compared with the typical performance of a poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)-based OECT with g m ¼ 2.7 mS, the MoS 2 monolayer device shows comparable top performance.When further putting the transconductance in relation to V DS , our devices outperform the literature example with 7 mS V À1 compared with 6.7 mS V À1 , although it has to be mentioned that this advantage in performance stems from the device architecture and when the transconductance is put into relation to the channel width (1000 vs 10 μm), the MoS 2 -based devices still fall behind the OECTs.The conductivity values reached are 90 S m À1 for MoS 2 and 270 S m À1 for a typical PEDOT:PSS OECT.OECTs experience what is called a 3D capacitance as ions can migrate into the active layer. [23]For an OECT with a 130 nm-thick PEDOT:PSS layer, the equivalent 2D capacitance would be in the range of 500 μF cm À1 , while the presented Ag/AgCl device experiences an capacitance reduced by about two orders of magnitude. [8]This highlights that MoS 2 is performing above its 2D dimensionality and just a single layer exhibits performance almost on-par with the 3D polymeric competitors while still providing the surface-only character-enhancing susceptibility to interfacial sensing events.
To test the device for stability in Figure 3, we show the constant operation of the device in 30 consecutive 1 h-long constant measurement cycles.The last data points from the experiments were extracted and plotted versus the measurement time to show the behavior under constant bias stress.We see a steady increase in current until run 17, after which a steady state starts to develop which has fully developed after run 25 and lasting for the next 5 h of measurements after which the experiment was stopped.This behavior shows that the device needs some conditioning before it has reached its peak performance and a constant current.
The exact reason for the necessary conditioning, which was observed in all devices, is not fully understood and is subject to further investigation.One way to avoid the conditioning might be found in modifying the contacts with a thin an insulating polymer layer to avoid electrochemical reactions at the contacts as shown to help for similar devices. [21,24]This would also avoid   a modulation of the contact resistance at the source and drain electrodes has it has been shown in the literature. [27,28]lternatively we are testing other members from the TMDC material family such as WSe 2 which are known for a more balanced transport and a lower defect density. [29,30]

Conclusion
We were able to successfully show the reliable operation of an MoS 2 monolayer-based EGFET in a biologically relevant liquid, showing decent mobility, very low SS, and an exceptional transconductance of %2 mS.Although the device does show an increasing current under constant bias at first, after some conditioning of the device the current reaches a saturation level during prolonged operation.The performance may be further improved by modifying the interface to the dielectric substrate to avoid the possibilities of polaron scattering and the introduction of trap states through the oxide interface, as well as by eliminating sulfur and molybdenum vacancies and grain boundaries in the active MoS 2 layer as both are known to degrade electrical performance.Yet given the high transconductance, this MoS 2 -based device is an ideal candidate for biosensing, as monolayer materials with their "no-bulk-only-surface" character come with an inherent sensitivity to interfacial changes.Furthermore, the sulfur vacancies may lend themselves as anchoring points for molecular finetuning of biorecognition layers.

Experimental Section
MoS 2 Exfoliation and Transfer: For the MoS 2 exfoliation, ultraflat template-stripped Au was used as exfoliation membrane.A MoS 2 crystal (2D semiconductors, synthetic MoS 2 crystal) is cleaved with the help of Kapton tape and the freshly exposed MoS 2 surface is pressed onto the ultraflat gold membrane.After heating at 200 C for 1 min, the MoS 2 carrying Kapton tape can be removed, leaving monolayers on gold behind.For a detailed description of the process and a full MoS 2 characterization, see Heyl et al. [24] For the transfer, polystyrene (PS, Sigma-Aldrich, average M w % 280 000) in toluene (Roth, ROTISOLV HPLC) (80 mg mL À1 ) was spin-coated onto the exfoliated gold substrates (3500 rpm, 60 s), annealed for 10 min at 80 C. Finally, the PS/MoS 2 / Au membrane was etched in KI/I 2 solution (Sigma-Aldrich, "Au etchant, standard") to remove the Au layer.The final PS/MoS 2 was repeatedly washed in DI water to remove any residual etchant solution and transferred on the final substrate.
FET Preparation: Silicon substrates (1.5 Â 2 cm) were ultrasonicated in Hellmanex II and IPA (Roth, ROTISOLV HPLC) and treated in an oxygen plasma for 5 min.FET drain and source electrodes were fabricated by depositing 5 nm of chromium (0.5 Å s À1 ) and 55 nm of gold (%1 Å s À1 ) deposited at a base pressure of 10 À7 mbar.After transfer of the MoS 2 carrying PS membranes samples were dried, annealed at 150 C (30 min), and the PS was removed by the help of hot toluene.
Electrical Characterizations: All electrical FET characterization was done with a Keithley 4200A-SCS Parameter Analyzer using an Ossila OFET test board using a polydimethylsiloxane (PDMS) pool to hold the diluted electrolyte (Alfa Aesar, PBS 10Â, pH 7.4), which was equipped with a glass cover with a small hole to account for the electrode.

Figure 1 .
Figure 1.a,b) Schematics of an MoS 2 -based EGFET utilizing a polarizable electrode such as Pt (a) and a nonpolarizable electrode (b) as a gate, including the equivalent circuit diagram below.c) Optical microscopy image of an MoS 2 -based EGFET.Dotted red line marks the monolayer edge.Inset monolayer step height from atomic force microscopy measurements.

Figure 3 .
Figure 3. Thirty cycles of 1 h-long measurements have been done consecutively.Electrolyte was changed after the first 15 measurements cycles.Relative current measured after 1 h plotted versus the measurement cycle.Measurements done in dilute PBS ([NaCl] ¼ 10 À2 M) with V DS ¼ 50 mV and V GS ¼ 400 mV.

Figure 2 .
Figure 2. a,c) Monolayer MoS 2 EGFET output characteristic using Pt (a) and Ag/AgCl (c) electrodes (3 M inner KCl filling) as a gate in dilute PBS (c(NaCl) ¼ 10 À2 M). b,d) Corresponding transfer characteristic for the Pt and Ag/AgCl gate, respectively, on a semilogarithmic (top) and linear (bottom) scale.