**1. Introduction**

Strongly correlated insulators at commensurate band fillings, such as the Mott insulator and the charge-ordered insulator, exhibit metal-insulator transitions by shifting the band filling [1]. Intriguing phenomena, such as high-temperature superconductivity and colossal magnetoresistance, emerge in the vicinity of the transition. Band filling control is generally accomplished by chemical substitution. Although this technique can change the band filling over a wide range, it cannot avoid the disorder caused by the introduction of dopants (impurities) and requires different samples for each band filling. Recently, electrostatic doping, which is based on the principle of field-effect transistors (FETs) and can avoid serious impurity effects that may occur in chemical doping, has found widespread use in the study of physical properties [2]. In particular, electric double layer transistors (EDLTs), which use ionic liquids as the gate electrolyte, have been widely adopted in the past decade because they allow a wider range of band filling control than FETs [3,4].

Molecular conductors are a suitable platform for the electrostatic doping of strongly correlated insulators. They contain various strongly correlated insulators and generally form single crystals with clean surfaces. Their lattice constants are generally larger than those of inorganic compounds, so that low electric fields can change large band fillings.

**Citation:** Kawasugi, Y.; Masuda, H.; Pu, J.; Takenobu, T.; Yamamoto, H.M.; Kato, R.; Tajima, N. Electric Double Layer Doping of Charge-Ordered Insulators α-(BEDT-TTF)2I3 and α-(BETS)2I3. *Crystals* **2021**, *11*, 791. https://doi.org/10.3390/ cryst11070791

Academic Editor: Toshio Naito

Received: 17 June 2021 Accepted: 4 July 2021 Published: 7 July 2021

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FETs and EDLTs based on molecular Mott insulators have been developed, and fieldinduced phase transitions have been investigated [5,6]. Studies of the doping effect on charge-ordered insulators have been limited. Yamamoto et al. [7,8] and Kimata et al. [9,10] fabricated FET devices using the charge-ordered insulator α-(BEDT-TTF)2I3 and observed decreases in the two-probe resistance between the source and drain electrodes by several tens of percent. However, the charge-ordered state was robust to field-effect doping, and the activation energy was essentially independent of the gate voltage. In this study, we fabricated EDLTs based on α-(BEDT-TTF)2I3 and α-(BETS)2I3 to investigate dense doping effects on the charge-ordered insulators compared to those in the FET measurements.

#### **2. Materials and Methods**

α-(BEDT-TTF)2I3 is a quasi-two-dimensional molecular conductor in which the conducting BEDT-TTF layer and the insulating I3 layer are stacked alternately. Although α-(BEDT-TTF)2I3 is a semimetal according to band calculations, and the resistance decreases by cooling, it shows a metal-insulator transition at 135 K [11]. The insulating state is a charge-ordered state in which horizontal charge stripes are formed along the crystallographic *b* axis. The transition temperature is lowered when pressure is applied, and disappears at 1.5 GPa; at this pressure, the Dirac fermion phase emerges [12]. α-(BETS)2I3 is the selenium analog of α-(BEDT-TTF)2I3. It also shows metal–insulator transition but at a lower transition temperature (∼ 50 K) [13]. Therefore, it is considered that the electronic state of α-(BETS)2I3 is similar to that of moderately pressurized α-(BEDT-TTF)2I3. However, recent X-ray diffraction and 13C nuclear magnetic resonance experiments revealed that α-(BETS)2I3 maintains inversion symmetry below the transition temperature [14]. These results imply a different insulating mechanism from simple charge ordering (the spin-orbit interaction may play an important role). The mechanism is still under debate [14–16].

We fabricated EDLTs based on α-(BEDT-TTF)2I3 and α-(BETS)2I3 by laminating thin single crystals onto polyethylene terephthalate (PET) substrates where Au electrodes were pre-evaporated (Figure 1). The source, drain, and gate electrodes (18 nm thick Au) were patterned on the substrate using photolithography. We electrochemically synthesized thin (~100 nm) single crystals of α-(BEDT-TTF)2I3 (α-(BETS)2I3) from a chlorobenzene solution of BEDT-TTF (BETS) and tetrabutylammonium triiodide by applying 5 μA for 20 h. The thin crystal was transferred into 2-propanol with a pipette and guided onto the substrate. After the substrate was removed from the 2-propanol and dried, the crystal adhered to the substrate. Although the X-ray diffraction measurement was difficult, we were able to see through a polarizer that the crystal is a single crystal in which the two-dimensional conducting plane is parallel to the substrate (due to the polarizing property of I3 −). As the gate electrolyte, ionic liquid 1-ethyl-3-methylimidazolium 2-(2-methoxyethoxy)ethyl sulfate was dropped to cover both the sample and the gate electrode. Lastly, the EDLT was covered by a 1.2 μm thick polyethylene naphthalate (PEN) film to reduce thermal stress at low temperatures by thinning the gate electrolyte. After completion, the EDLT was immediately cooled down in a cryostat, and the gate voltage at 220 K was changed to suppress the chemical reaction between the compounds and the ionic liquids. Charge displacement current measurements were performed by sweeping the gate voltage between ±0.5 V and measuring the gate current using a source-measure unit (KEITHLEY 2636B, Keithley Instruments, Cleveland, OH, USA). We derived the accumulated surface charge density from

$$p = \frac{\mathcal{Q}}{eA} = \frac{\int I\_{\mathcal{G}}dV\_{\mathcal{G}}}{r\_{VeA}} \tag{1}$$

where *I*G, *V*G, *rV*, *e*, and *A* denote the gate current, gate voltage, sweep rate of the gate voltage, elementary charge, and area of the sample, respectively [17]. As for the temperature dependence of the resistance, we employed the standard four-probe method using a DC source (KEITHLEY 2400, Keithley Instruments, Cleveland, OH, USA) and a nano voltmeter (Agilent 34420A, Agilent Technologies, Santa Clara, CA, USA) under various gate voltages. The gate voltage was applied at 220 K in descending order from +0.4 V to −0.4 V in α-

(BEDT-TTF)2I3 and from +0.8 V to −0.6 V V in α-(BETS)2I3. Beyond those voltage ranges, the sample resistances tended to increase instead, probably because of degradation by the chemical reactions between the ionic liquid and the molecular conductors. The cooling rate was 0.75 K/min, and the data shown were captured during cooling. For α-(BETS)2I3, we also measured the Hall effect using a superconducting magnet that generates up to 8 T (TeslatronPT, Oxford Instruments, Abingdon, UK).

**Figure 1.** (**a**) BEDT-TTF and BETS molecules, and crystal structures of α-(BEDT-TTF)2I3 and α-(BETS)2I3 (I3 is not shown). (**b**) Schematic side view and (**c**) optical top view of an EDLT device. The α-(BETS)2I3 crystal in (**c**) is laser-shaped into the Hall bar. The gate electrode (area: 800 <sup>×</sup> 800 cm2) is patterned on the substrate, a few hundred micrometers away from the crystal in (**c**).

#### **3. Results**

#### *3.1. Charge Displacement Current Measurement*

First, we performed charge displacement current measurements at 220 K to confirm the accumulated charge density by electric double layer doping. Figure 2 shows the gate voltage *V*<sup>G</sup> dependence of the charge density *p*, estimated from Equation (1). To the BEDT-TTF salt, the gate voltage of 0.5 V corresponds to approximately 10% doping (100% doping = 1 electron or hole per 2 BEDT-TTF molecules). Electrons are slightly more likely to be doped than holes at the same magnitude of gate voltage. In the case of the BETS salt, 0.5 V corresponds to only ~3% (hole) and ~5% (electron) doping. For reference, 10% doping corresponds to the gate voltage of approximately 250 V in an FET with a 300 nm thick SiO2 film, which generally exceeds the withstand voltage of the SiO2 film. In our previous study on the electric double layer doping of κ-(BEDT-TTF)2Cu[N(CN)2]Cl, the doping concentration at 0.5 V reached 20%, and the *p*–*V*<sup>G</sup> curve was more linear [6].
