Electrical and Optical Characterization of Graphene Oxide and Reduced Graphene Oxide Thin Films

: Despite a growing interest in graphene, an aspect which is less studied is the electrical and optical characterization of graphene oxide (GO)-based transparent conductors obtained using thermal annealing. In addition, few research works have studied the electrical properties of GO and reduced graphene oxide (RGO) ﬁlms using electrical impedance measurements. In this study, electric impedance measurements are performed on GO and thermally reduced GO ﬁlms dip-coated on glass substrates. The electric resistance of RGO ﬁlms decreases by about two orders of magnitude compared to GO ﬁlms. Moreover, optical microscopy and variable angle spectroscopic ellipsometry (VASE) were carried out on the same samples. Thermal annealing increases the optical conductivity and the absorption coefﬁcient of GO ﬁlms. Such ﬁndings could be used in many optoelectronic applications, improving future GO applicability.

The study of the electric properties of graphene-based materials opens new possibilities for solid state electronics and for composite materials [30].
Although the result of weak disorder on the electrical conductivity of a single graphene sheet has been widely investigated [31,32], there are not a lot of research works regarding the conductivity of a disordered graphene-based material, such as graphene oxide (GO). GO is insulating because of the breaking of the conjugated electronic structure by means of oxidized functional groups, and it contains permanent defects and disorders [33]. Hence, the electrical properties of GO sheets are dissimilar from those of pristine graphene. The insulating GO films have resistance values higher than about 10 10 Ω/sq due to their reduced carrier mobility [34]. The reduction in GO is the key to re-establishing the excellent electrical conductivity of graphene. The attached oxygen-containing groups and lattice defects change the GO electronic structure and are scattering centers that influence electrical transport. Consequently, GO reduction aims to remove the oxygen-containing groups and to recover the conjugated network of the graphitic lattice [35], allowing the retrieval of the electrical conductivity of graphene [36].
Even though most of the oxidized groups are suppressed during the reduction process, the residual ones may restrict reduced graphene oxide (RGO) electron transport properties.
Thus, GO reduction restores the conductivity at orders of magnitude values below that of pristine graphene [37]. RGO conductivity varies from 0.05 S/cm to 500 S/cm based on the degree of reduction, which is associated with the ratio of the graphitic regions (sp 2 ) to the oxidized regions (sp 3 ) [38,39].
Chemical and thermal reduction are two main methods of GO reduction [40]. Another method of GO reduction is the use of broadband visible light [41].
Reducing GO by using chemical reduction is a very scalable method. Unfortunately, this method uses ecologically harmful reagents, such as hydrazine and hydrazine hydrate [42]. Thus, there is a need to study other low-cost and environmentally friendly GO reduction methods.
Thermal reduction is non-toxic, environmental friendly and ideal for large-scale industrial applications [42]. The thermal reduction in GO is typically achieved above 200 • C and is successful at higher temperatures [43].
GO-based transparent conductors have more defects with lower electrical conductivity, but they may be promising for mass production because of their low cost. Nevertheless, many challenges are still ahead, e.g., the relatively low conductivity, before full-scale industrial application of these materials is possible.
The research on the reduction in GO films using thermal annealing may facilitate its application in optoelectronic devices. Indeed, GO and RGO find several applications in optoelectronic devices, such as perovskite solar cells and photodetectors [44][45][46][47].
Various work on the electrical and optical properties of GO and RGO has already been published [48]. However, research on the electrical and optical characterization of RGO films obtained with thermal annealing is less explored [49] in comparison to other reduction methods, such as chemical reduction [50] and UV reduction [51]. Moreover, few research works study the electrical properties of GO and RGO films using electrical impedance measurements, according to our knowledge. Our work wants to fill this gap.
Therefore, the aim of this work is to characterize GO and thermally reduced GO films deposited on glass substrates, using optical microscopy, impedance spectroscopy and variable angle spectroscopic ellipsometry (VASE) measurements [52].

Synthesis and Thermal Reduction in GO Films
Corning glass substrates were washed using a mix of sulfuric acid (H 2 SO 4 ) and hydrogen peroxide (H 2 O 2 ), known as "piranha solution".
The samples were produced using 12 mg of GO (Sigma Aldrich, St. Louis, MO, USA), 3 mL of double distilled H 2 O and 3 mL of methanol (Sigma Aldrich, St. Louis, MO, USA).
The dip-coating process [53] was used to deposit GO thin films on glass substrates with a homemade apparatus at a speed of 1 mm/min. GO films on glass substrates were then heated at 450 • C for 20 min in an Ar atmosphere furnace.

Characterization of GO and RGO Films
Optical microscopy images of the samples were acquired with a Zeiss Axioskop (Zeiss, Oberkochen, Germany).
The electrical impedance measurements were performed using a 4294 Precision Impedance Analyzer (Agilent, Santa Clara, CA, USA) with a 42941A pin probe kit that permits measurements in the range 40 Hz-30 MHz. The measurements were performed by using two close (about 1 mm) gold-tipped electrodes.
The optical conductivity of the films was studied using the VASE technique. Spectra of the ellipsometric angles ψ and ∆ were obtained using a M2000 F (Woollam Co., Lincoln, NE, USA) ellipsometer (Rotating Compensator Ellipsometer) in the (1.2-4.1) eV photon energy range. The data were obtained at 50 • , 60 • and 70 • incident angles (room temperature).
The optical model and the best fitting values were determined with the WVASE32 (J.A. Woollam) application, which uses the nonlinear Levenberg-Marquardt algorithm [54] to determine the minimum value of the mean squared error (MSE) [55].
In Scheme 1, the schematic diagram of the synthesis process is reported.

Optical Microscopy Images
In Figure 1, the optical microscopy images of GO and RGO films after thermal annealing are reported.
Optical observation is an immediate way to see the modifications in GO films before and after thermal annealing. The color transition of GO films from dark yellow ( Figure  1a) to black in the initial reduction stage (Figure 1b) is confirmed by optical microscopy images [56]. When GO was reduced, the absorption coefficient increased (see Figure 11). This high absorption coefficient is in accordance with the films becoming black, i.e., the films absorbed light at all wavelengths. Additional morphological analysis of the samples is available in the Supplementary Materials.

Optical Microscopy Images
In Figure 1, the optical microscopy images of GO and RGO films after thermal annealing are reported.

Scheme 1.
Schematic diagram of the synthesis process.

Optical Microscopy Images
In Figure 1, the optical microscopy images of GO and RGO films after thermal annealing are reported.
Optical observation is an immediate way to see the modifications in GO films before and after thermal annealing. The color transition of GO films from dark yellow ( Figure  1a) to black in the initial reduction stage (Figure 1b) is confirmed by optical microscopy images [56]. When GO was reduced, the absorption coefficient increased (see Figure 11). This high absorption coefficient is in accordance with the films becoming black, i.e., the films absorbed light at all wavelengths. Additional morphological analysis of the samples is available in the Supplementary Materials.  Optical observation is an immediate way to see the modifications in GO films before and after thermal annealing. The color transition of GO films from dark yellow (Figure 1a) to black in the initial reduction stage (Figure 1b) is confirmed by optical microscopy images [56]. When GO was reduced, the absorption coefficient increased (see Figure 11). This high absorption coefficient is in accordance with the films becoming black, i.e., the films absorbed light at all wavelengths. Additional morphological analysis of the samples is available in the Supplementary Materials.  Figure 2 shows the electrical impedance of GO and RGO films on glass (modulus and phase).  Figure 2 shows the electrical impedance of GO and RGO films on glass (modulus and phase).

Impedance Measurements
We noticed that, after the annealing treatment, the resistance of the film declined by about two orders of magnitude, although not uniformly. As a comparison, the electrical impedance (modulus and phase) of a commercial CVD-grown monolayer graphene on the SiO2/Si substrate (Graphenea, San Sebastián, Spain) is reported in Figure 3, in which it is possible to observe similar behavior as in RGO films. We noticed that, after the annealing treatment, the resistance of the film declined by about two orders of magnitude, although not uniformly.
As a comparison, the electrical impedance (modulus and phase) of a commercial CVDgrown monolayer graphene on the SiO 2 /Si substrate (Graphenea, San Sebastián, Spain) is reported in Figure 3, in which it is possible to observe similar behavior as in RGO films.  Figure 2 shows the electrical impedance of GO and RGO films on glass (modulus and phase).

Impedance Measurements
We noticed that, after the annealing treatment, the resistance of the film declined by about two orders of magnitude, although not uniformly. As a comparison, the electrical impedance (modulus and phase) of a commercial CVD-grown monolayer graphene on the SiO2/Si substrate (Graphenea, San Sebastián, Spain) is reported in Figure 3, in which it is possible to observe similar behavior as in RGO films.

Variable Angle Spectroscopic Ellipsometry Measurements
First, we examined the bare glass substrates by VASE measurements. Glass substrates were described using a Cauchy-Urbach model [55]. GO and RGO films on glass substrates were modeled as the sum of Lorentz oscillators. The complex dielectric function is described with the following relation [55]: where E is the incident photons energy, ε ∞ is the real part of the dielectric function when E → ∞ , A k is the amplitude (eV 2 ), Γ k is the broadening (eV) end and E k (eV) is the central energy of the k-th oscillator. Table 1 shows the parameters obtained from the best fit values of GO and RGO films on glass substrates. Table 1. Lorenz oscillators parameters for GO and RGO films on glass substrates. Amplitude A k has unit of eV 2 , center energy E k and broadening Γ k have units of eV, d is the thickness of film in nm, the frequency dielectric constant ε ∞ is dimensionless.

GO/Glass RGO/Glass
7.6 ± 2.5 9.9 ± 3.8 The models provide outstanding data fittings (MSE~8). As we can see from Table 1, we found the π* peak of plasma excitation at 4.7 eV [50] for the GO films.
After the annealing treatment, the quantity of oxygen decreased, a transition from sp 3 to sp 2 hybridization happened and the absorbance peak showed a red shift [57]. The reduction process caused a variation in the coverage of functional groups, thus changing the RGO [50] electronic density of states.
The red shift of the absorption peak from 5.1 eV (for GO films) to 4.6 eV (for RGO films) was due to an excitonic effect [50].
The complex dielectric constants for GO and RGO films on glass derived from the analysis of VASE measurement are shown in Figures 4 and 5, respectively.      Moreover, the refractive index and the extinction coefficient of GO and RGO films are reported in Figures 6 and 7, respectively.     These measurements show an increase in the values of the index of refraction and the extinction coefficient of GO films after the annealing, in accordance with the work of Jung at al. [49].
The complex optical conductivity = 1 + 2 is connected to the complex dielectric constant = 1 + 2 by the following relations [58]: These measurements show an increase in the values of the index of refraction and the extinction coefficient of GO films after the annealing, in accordance with the work of Jung et al. [49].
The complex optical conductivity σ = σ 1 + σ 2 is connected to the complex dielectric constant ε = ε 1 + ε 2 by the following relations [58]: where is the angular frequency and ε 0 is the free space dielectric constant. The optical conductivities of GO and RGO films on glass substrates are reported in Figures 8 and 9, respectively. Thermal annealing increased the optical conductivity of GO by about one order of magnitude.
where is the angular frequency and 0 is the free space dielectric constant. The optical conductivities of GO and RGO films on glass substrates are reported in Figures 8 and 9, respectively. Thermal annealing increased the optical conductivity of GO by about one order of magnitude.   The absorption coefficient is calculated from the estimated extinction coefficient (ℎ ): where ℎ and are the energy and wavelength of the light, respectively. The absorption coefficient is calculated from the estimated extinction coefficient k(hv): where hν and λ are the energy and wavelength of the light, respectively. Figures 10 and 11 show the absorption coefficient against hν photon energy for GO and RGO films, respectively. Thermal annealing increases the absorption coefficient of GO films, as is visible in Figure 11.  11 show the absorption coefficient against ℎ photon energy for GO and RGO films, respectively. Thermal annealing increases the absorption coefficient of GO films, as is visible in Figure 11.

Conclusions
Herein, we investigated the properties of GO and thermally reduced GO (RGO) thin films dip-coated on glass substrates. The electrical and optical properties were studied by optical microscopy, impedance and VASE measurements.
From optical microscopy pictures of the samples, it is possible to observe that a color transition from dark yellow to black occurs as a sign of GO reduction.
We found from impedance measurements that the electric resistance of RGO films decreased by about two orders of magnitude in comparison to GO films.
We found from VASE measurements that thermal annealing increased the optical conductivity of GO by about one order of magnitude. In addition, thermal annealing increased the absorption coefficient of GO films.
Hence, the electrical and optical properties of GO films are significantly enhanced using a thermal annealing treatment.
RGO films on glass substrates, obtained using thermal annealing, could thus find use in several fields, such as solar cells and other optoelectronic applications.

Conclusions
Herein, we investigated the properties of GO and thermally reduced GO (RGO) thin films dip-coated on glass substrates. The electrical and optical properties were studied by optical microscopy, impedance and VASE measurements.
From optical microscopy pictures of the samples, it is possible to observe that a color transition from dark yellow to black occurs as a sign of GO reduction.
We found from impedance measurements that the electric resistance of RGO films decreased by about two orders of magnitude in comparison to GO films.
We found from VASE measurements that thermal annealing increased the optical conductivity of GO by about one order of magnitude. In addition, thermal annealing increased the absorption coefficient of GO films.
Hence, the electrical and optical properties of GO films are significantly enhanced using a thermal annealing treatment.
RGO films on glass substrates, obtained using thermal annealing, could thus find use in several fields, such as solar cells and other optoelectronic applications.
Author Contributions: Investigation, G.G.P. and C.V.; Supervision, C.V.; Writing-original draft, G.G.P. and C.V.; Writing-review and editing, G.G.P. and C.V. All authors have read and agreed to the published version of the manuscript.

Conflicts of Interest:
The authors declare no conflict of interest.