Future wearable electronics, displays and photovoltaic devices require materials which are mechanically flexible, lightweight and low-cost, in addition to being electrically conductive and optically transparent.1–3 Nowadays indium tin oxide (ITO) is the most wide spread transparent conductor in optoelectronic applications, however the mechanical rigidity of this material limits its use for future flexible devices. In the race to find novel transparent conductors, graphene monolayers and multilayers are the leading candidates as they have the potential to satisfy all future requirements. Graphene, one-atom-thick layer of carbon atoms, is transparent,4 conducting,5, 6 bendable7 and yet one of the strongest known materials.8 However, the use of graphene as a truly transparent conductor remains a great challenge because the lowest values of its sheet resistance (Rs) demonstrated so far are above the values of commercially available ITO (i.e. 10 Ω/□ at an optical transmittance Tr = 85%9). Currently many efforts are concentrated on decreasing the Rs of graphene-based materials while maintaining a high Tr, which will allow their potential to be harnessed in optoelectronic applications. To date, the best values of sheet resistance and transmittance found in graphene-based materials are still far from the performances of ITO, with typical values of Rs = 30 Ω/□ at Tr = 90% for graphene multilayers7, 12 and Rs = 125 Ω/□ at Tr = 97.7% for chemically doped graphene.7, 10, 11
Here we report novel graphene-based transparent conductors with a sheet resistance of 8.8 Ω/□ at Tr = 84%, a carrier density as high as 8.9 × 1014cm−2 and a room temperature carrier mean free path as large as ∼0.6 μm. These materials are obtained by intercalating few-layer graphene (FLG) with ferric chloride (FeCl3).13, 14 Through a combined study of electrical transport and optical transmission measurements we demonstrate that FeCl3 enhances the electrical conductivity of FLG while leaving these graphene-based materials highly transparent. We also show that FeCl3-FLGs are stable in air up to one year, which demonstrates the potential of these materials for industrial production of transparent conductors. The unique combination of record low sheet resistance, high optical transparency and macroscopic room temperature mean free path has not been demonstrated so far in any other doped graphene system, and opens new avenues for graphene-based optoelectronics.
Pristine FLG ranging from two- to five-layers (2L to 5L) were obtained by micromechanical cleavage of natural graphite5 on glass or SiO2/Si. The number of layers composing each FLG was determined by optical contrast and Raman spectroscopy (see Supporting Information). The intercalation process with FeCl3 was performed in vacuum with the two-zone vapor transport method.15 FeCl3 is sublimated in the lowest temperature zone and it diffuses to the higher temperature zone where the intercalation of FLG takes place (see Experimental Section). The structural, electrical and optical characterization is carried out by means of three complementary experimental techniques: low-temperature charge transport, optical transmission and Raman spectroscopy.
Figure 1a shows the Raman spectra of pristine FLG on SiO2/Si, with the G-band at 1580 cm−1 and the 2D-band at 2700 cm−1.16, 17 As expected for pristine FLGs, increasing the number of layers results in an increase of the G-band intensity,18 whereas the 2D-band acquires a multi-peak structure.16, 17 The charge transfer from FeCl3 to graphene modifies the Raman spectra of FLGs in two distinctive ways:13, 14, 19 an upshift of the G-band and a change of the 2D-band from multi- to single-peak structure, respectively (see Figure 1a). The shift of the G-band to G1 = 1612 cm−1 is a signature of a graphene sheet with only one adjacent FeCl3 layer, whereas the shift to G2 = 1625 cm−1 characterizes a graphene sheet sandwiched between two FeCl3 layers [13–15] (see Figure 1a). The frequencies, linewidths and lineshapes of the G1 and G2 peaks do not depend on the number of graphene layers which indicates the decoupling of the FLGs into separate monolayers due to the intercalation of FeCl3 between the graphene sheets. This is consistent with the changes in the 2D-band shape and with the Raman studies of other intercalants such as Potassium20, 21 and Rubidium.21 These observations allow us to identify the structure of intercalated 2L samples as one FeCl3 layer sandwiched between the two graphene sheets. However, the structural determination of thicker FeCl3-FLG cannot rely uniquely on the Raman spectra, it requires complementary knowledge from electrical transport experiments. Direct structural determination for example by X-ray diffraction would be valuable to confirm the findings of Raman and electrical transport measurements, however the small thickness of FeCl3-FLG and the substrate effects make it difficult to apply such technique to these systems.
To characterize the structure of thicker FeCl3-FLG, we study the oscillatory behaviour of the longitudinal magneto-conductance (Gxx) in a perpendicular magnetic field (i.e. Shubnikov-de Haas oscillations, SdHO) in combination with the Hall resistance (Rxy). Here we discuss the representative data for an intercalated 5L sample patterned into a Hall bar geometry (see Figure 1c and Experimental section). Figure 1b shows SdHO of Gxx as a function of perpendicular magnetic field (B) for different temperatures. It is apparent that for T<10K Gxx oscillates with two distinct frequencies. For T>10K only the lower frequency oscillations are visible. These observations indicate that electrical conduction takes place through parallel gases of charge carriers with distinct densities. Indeed, the Fourier transform of Gxx(1/B) yields peaks at frequencies fSdHO1 = 1100T and fSdHO2 = 55T (see Figure 1d), corresponding to charge carrier densities n1 = (1.07 × 1014 ± 5 × 1011)cm−2 and n2 = (5.3 × 1012 ± 4 × 1011) cm−2 (with ni = 4efSdHOi/h6, 22).
The temperature dependence of the magneto-conductivity oscillations allows us to determine the cyclotron mass of the charge carriers in these parallel gases. Figure 1e and f show the low- and high-frequency magneto-conductivity oscillations for different temperatures (see Experimental Section). In all cases, the temperature decay of the amplitude is well described by A(T)∝T/sinh (2π2kBTmc/eB) (see Figure 1g), with cyclotron masses mc1 = (0.25 ± 0.05)me and mc2 = (0.08 ± 0.001)me for the high- and low-frequency oscillations, respectively. These values correspond to the expected values of cyclotron mass for massless Dirac fermions at n1 and for chiral massive charge carriers of bilayer graphene at n2 (with vF = 106 ms−1 the Fermi velocity6 and γ the interlayer hopping energy15), see Supporting Information. Therefore, intercalation of FeCl3 decouples the stacked 5L graphene into parallel gases of massless (1L) and massive (2L) charge carriers.
The charge carrier type (electrons or holes) and the number of parallel gases present in FeCl3-FLG are readily identified by correlating SdHO to the Hall resistance measurements. The linear dependence of Rxy(B) with positive slope identifies charge carriers as holes, with a Hall carrier density nH = B/(eRxy) = 3 × 1014cm−2 (see Figure 1c). Since the total carrier density ntot = Σini should be higher than nH = (Σiniμi)2/Σiniμ2i (with ni and μi the carrier density and mobility of each hole gas),23 only a minimum of three parallel hole gases with n1 = 1.07 × 1014cm−2 can explain the estimated value of nH, i.e. 3×n1 +n2 ≥ nH (see Supporting Information). Therefore, the electrical transport characterization demonstrates the presence of four parallel hole gases, of which one with bilayer character (and density n2) and three with monolayer character (each with density n1). These findings are confirmed by the Raman spectra taken after the device fabrication showing the presence of pristine G, G1 and G2 peaks (see Supporting Information). A schematic of this crystal structure is illustrated in Figure 1h. The bilayer gas is likely to be caused by the first two layers of the stacking which have been de-intercalated due to rinsing in acetone during lift-off.19 The bottom part of the stacking has a per-layer doping of n1 = 1.07 × 1014cm−2 and the stoichiometry of stage-1 FeCl3 graphite intercalation compounds (i.e. where each graphene layer is sandwiched by two FeCl3 layers).15 In total we have investigated electrical transport and Raman spectroscopy in more than 10 intercalated 5L samples and in all cases we confirmed the structure reported in Figure 1h.
A remarkable property of FeCl3-FLGs shown by the electrical transport characterization is that intercalated materials thicker than 3L invariably exhibit a very low-sheet resistance, which is essential for their use as electrical conductors. We find a room temperature value of Rs = 8.8Ω/□ in 5L intercalated FLGs. The 4L intercalated FLGs typically exhibit higher sheet resistance values than the 5L intercalated samples with a similar crystal structure (see Supporting Information for a comparison between the electrical properties and Raman spectra of several 4L and 5L intercalated FLGs). Furthermore, the sheet resistance of FeCl3-FLGs thicker than 2L decreases when lowering the temperature as expected for metallic conduction (see Figure 2a). Contrary to intercalated samples, pristine FLGs always have a higher Rs (>120 Ω/□) and they exhibit non-metallic behaviour as a function of temperature5, 24, 25 (see Figure 2b). This suggests that the origin of the low values of Rs and the metallic nature of the conduction are consequences of intercalation with FeCl3. FeCl3-FLGs thinner than 3L suffer of partial de-intercalation during the device fabrication, which results in a non-metallic behaviour similar to pristine FLGs and in higher Rs values (see Figure 2a).
The low values of Rs characterizing thick FeCl3-FLG are accompanied by an extremely high charge density. Indeed, the Hall coefficient measurements reveal that nH ranges from 3 × 1014cm−2 to 8.9 × 1014cm−2, depending on the number of layers (see Figure 2c and d). These charge densities exceed even the highest values demonstrated so far by liquid electrolyte26 or ionic27 gating. The corresponding charge carrier mobility for the 4L and 5L samples typically ranges from μH = 1540 cm2V−1 s−1 to μH = 3650 cm2V−1 s−1
(μH = 1/(nHeρxx) with ρxx the longitudinal resistivity and e the electron charge). Consequently, the charge carriers in thick FeCl3-FLG have a macroscopic mean free path as high as 0.6 μm in 5L at room temperature. The outstanding electrical properties, e.g. lower Rs than ITO and macroscopic mean free path, found in FeCl3-FLGs thicker than 3L are of fundamental interest for the development of novel electronic applications based on highly conductive materials.
Whether FeCl3-FLGs can replace ITO in optoelectronic applications strongly depends on their optical properties. Surprisingly, our detailed study of the optical transmission in the visible wavelength range shows that while FeCl3 intercalation improves significantly the electrical properties of graphene, it leaves the optical transparency nearly unchanged. Figure 3a and b show a comparison between the transmittance spectra of pristine FLG and FeCl3-FLG. The transmittance values of pristine FLG at the wavelength of 550nm are in agreement with the expected values,4 highlighted in Figure 3a, and with the results reported by other groups.4, 7 Upon intercalation, the transmittance slightly decreases at low wavelengths, but it is still above 80%. In order to measure an accurate value of transmittance we fit it with a linear dependence on the number of layers for a statistical ensemble of flakes (Figure 3d). This results in similar extinction coefficients per layer for pristine FLG (≈2.4 ± 0.1%) and for FeCl3-FLG (≈ 2.6 ± 0.1%), see Figure 3c and d. For wavelengths longer than 550nm we observe an increase in the optical transparency of FeCl3-FLG. This is a significant advantage of our material compared to ITO whose transparency decreases for wavelengths longer than 600nm.2 This property will provide useful applications that require conductive electrodes which are transparent both in visible and near infrared range. For instance, FeCl3-FLG transparent electrodes could be used for solar cells to harvest energy over an extended wavelength range as compared to ITO-based devices, or for electromagnetic shielding in infrared.
The high transparency observed in FeCl3-FLG complemented by their remarkable electrical properties makes these materials valuable candidates for transparent conductors. However, to replace ITO in optoelectronic applications, it is generally agreed that materials must (at least) have the properties of commercially available ITO (Rs = 10 Ω/□ and Tr = 85%9). Figure 3e compares Rs vs. Tr of FeCl3-FLG materials with ITO,28 and other promising carbon-based candidates to replace ITO such as carbon-nanotube films29 and doped graphene materials.7 It is apparent that Rs and Tr of FeCl3-FLGs outperform the current limits of ITO and of the best values reported so far for doped graphene.7 Therefore, the outstandingly high electrical conductivity and optical transparency make FeCl3-FLG materials the best transparent conductors for optoelectronic devices.
Finally, an important characteristic required by a transparent conductor is its stability upon exposure to air. In principle FLGs could be intercalated with a large variety of molecules, similar to the graphite intercalation compounds (GIC).15 However, most of the GIC are unstable in air, with donor compounds being easily oxidized and acceptors being easily desorbed. Therefore we studied the stability in air of FeCl3-FLG by performing Raman measurements as a function of time. We found that the Raman spectra of FeCl3-FLG samples show no appreciable changes on a time scale of up to one year (see Supporting Information). This property has important implications for the utilization of these materials as transparent conductors in practical applications such as displays and photovoltaic devices.
In conclusion, we demonstrate novel transparent conductors based on few layer graphene intercalated with ferric chloride with an outstandingly high electrical conductivity and optical transparency. We show that upon intercalation a record low sheet resistance of 8.8Ω/□ is attained together with an optical transmittance higher than 84% in the visible range. These parameters outperform the best values of ITO and of other carbon-based materials. The FeCl3-FLGs materials are relatively inexpensive to make and they are easily scalable to industrial production of large area electrodes. Contrary to the numerous chemical species that can be intercalated into graphite (more than a hundred15), many of which are unstable in air, we found that FeCl3-FLGs are air stable on a timescale of at least one year. Other air stable graphite intercalated compounds can only by synthesized in the presence of Chlorine gas,15 which is highly toxic. On the contrary, here we demonstrate that the intercalation of FLG with FeCl3 is easily achieved without the need of using Chlorine gas, which ensures an environmentally friendly industrial processing. Furthermore, the low intercalation temperature (360°) required in the processing allows the use of a wide range of transparent flexible substrates which are compatible with existing transparent electronic technologies. These technological advantages combined with the unique electro-optical properties found in FeCl3-FLG make these materials a valuable alternative to ITO in optoelectronics.