**3. VOLETs with a doped CVD graphene source**

#### **3.1 Fabrication of SLG-based VOLETs**

**Figure 5** presents a schematic illustration of the structure used and the stages of the fabrication of the SLG-based VOLETs (Gr-VOLETs) with an ITO gate separated by an Al2O3 gate dielectric layer, a SLG source, organic channel layers, and an Al drain. The fabrication steps of the Gr-VOLET investigated are described below. To construct the Gr-VOLET, SLG (4 mm by 20 mm) was transferred onto a VOLET substrate, as mentioned above (Steps 1, 2). The source electrode used was FeCl3-doped SLG. Next, organic semiconducting materials were deposited over the SLG source electrode regions; a channel layer of poly(para-phenylene vinylene) copolymer (known as SY, 70-nm-thick) was coated as an emissive channel layer by spin coating (Step 3), after which a 2-nm-thick electron injection layer of CsF and a 80-nm-thick drain electrode of Al were deposited on the top of the SY channel layer in sequence via thermal deposition at a rate of 0.05 nm s<sup>−</sup><sup>1</sup> under a base pressure of less than 2.7 × 10<sup>−</sup><sup>4</sup> Pa (Step 4). Finally, the fabricated device was encapsulated with an epoxy resin in a glove box. The photograph in **Figure 5** shows the microscopic morphology of the device cross section as observed by field emission scanning electron microscopy (SEM).

*Liquid Crystals and Display Technology*

Next, the transport characteristics of the SLG used were observed by assessing a liquid-gated lateral FET with SLG channels, a Gr-FET, as shown in **Figure 3(d)**. The lateral FET substrate was prepared using the VOLET substrate or a heavily doped n-type Si wafer substrate (0.05-ohm cm) with a thermally grown SiO2 layer (300-nm-thick) as the gate dielectric for the OTFT, together with a laterally patterned metal source and drain electrodes consisting of a Cr layer (5.5-nm-thick) and a Au layer (50-nm-thick) formed on the substrate via a vacuum deposition process with a mask. The channel length (*L*) and width (*W*) of the FET were 50 μm and 1600 μm, respectively (see inset in **Figure 3(d)**). Regarding the transport characteristics of the SLG studied, a liquid-gated lateral Gr-FET was prepared using an FET substrate with acetonitrile (ACN) with 100 mM of tetrabutylammonium hexafluorophosphate (TBAPF6). The channel of the studied SLG of the Gr-FET was gated through the ACN electrolyte with an Ag/AgCl (3.5 M KCl) reference electrode by sweeping the gate voltages from −0.8 to 0 V and then to +0.8 V with a sweep rate

5.21 4.89 410 1.20

**[cm2 V<sup>−</sup><sup>1</sup> s −1 ]** **Sheet resistance, [kΩ square<sup>−</sup><sup>1</sup>**

**]**

**Work function [eV] Dirac point energy [eV] Hole mobility** 

*Summary of the electronic properties of the FeCl3-doped SLG used.*

at *V*DS = 100 mV. In general, the liquid-gate Gr-FET has better transfer

(e *V*G,Dirac − *E*1/2(Fc/Fc+)) − 4.8] eV. (2)


, which is a second-order type of vibrational mode caused by the

) [50]. The

,

 V<sup>−</sup><sup>1</sup> s<sup>−</sup><sup>1</sup> .

characteristics than the conventional back-gate Gr-FET because the liquid gate

For the SLG used here, the Gr-FET showed a clear asymmetrical V-shaped *I*DS-*V*<sup>G</sup> curve with a charge-neutral gate voltage (or Dirac point, *V*Dirac) of ~0.54 V/VAg/AgCl. This large positive value of *V*Dirac clearly indicates that the SLG used is p-type (hole) doped graphene due to the chlorination of graphene by FeCl3 [49]. According to the *V*Dirac value of the SLG, the energy level of the Dirac point *E*D, relative to the vacuum level, can be estimated with regard to the redox potential of a probe material of

Here, 4.8 eV is the absolute energy level of the ferrocene and ferrocenium (Fc/Fc+) redox couple below the vacuum energy level, and *E*1/2(Fc/Fc+) = 0.45 eV [51]. From Eq. (2), the *V*Dirac value of ~0.54 V/VAg/AgCl for the SLG gives a Dirac point energy *E*D of approximately ~4.89 eV. Note that the *E*D value of 4.89 eV is higher than that (~4.49 eV) of epitaxial monolayer graphene [52], also confirming the p-type doping of the SLG. From the transfer characteristics, the carrier (hole) mobility *μ* of the SLG was also estimated using the relationship [53] of

*μ* = (*L*/*W C*g *V*DS)(Δ *I*DS/Δ *V*G), (3)

For the SLG studied here, Raman spectroscopy was also carried out using a confocal Raman system with a laser source operating at 514.5 nm (~1 mW on sample surface). As shown in **Figure 4(a)**, the Raman spectra of the SLGs studied here have two strong characteristic peaks, a G band at around ~1580–1600 cm<sup>−</sup><sup>1</sup>

where *C*g is the top-gate capacitance of graphene (~1.9 μF cm<sup>−</sup><sup>2</sup>

estimated hole mobility for the SLG was approximately ~410cm2

exhibits higher capacitance than the back gate [49, 50].

ferrocene via the following relationship [51]:

*E*D = [−

**148**

due to the E2g vibration of sp2

~2644–2665 cm<sup>−</sup><sup>1</sup>

of 30 mV s<sup>−</sup><sup>1</sup>

**Table 1.**

**Figure 5.**

*Schematic illustration of the fabrication steps of a Gr-VOLET and a cross-sectional scanning electron microscopy (SEM) image of the Gr-VOLET with stacked layers of an ITO gate separated with an Al2O3 gate dielectric, a SLG source, organic channel layers, and an Al drain.*
