Novel highly conductive and transparent graphene based conductors

Future wearable electronics, displays and photovoltaic devices rely on highly conductive, transparent and yet mechanically flexible materials. 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. Here we report novel transparent conductors based on few layer graphene (FLG) intercalated with ferric chloride (FeCl3) with an outstandingly high electrical conductivity and optical transparency. We show that upon intercalation a record low sheet resistance of 8.8 Ohm/square 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, making these novel transparent conductors the best candidates for future flexible optoelectronics.

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 fi ve-layers (2L to 5L) were obtained by micromechanical cleavage of natural graphite [ 5 ] on glass or SiO 2 /Si. The number of layers composing each FLG was determined by optical contrast and Raman spectroscopy (see Supporting Information). The intercalation process with FeCl 3 was performed in vacuum with the two-zone vapor transport method. [ 15 ] FeCl 3 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 1 a shows the Raman spectra of pristine FLG on SiO 2 /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 FeCl 3 to graphene modifi es 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 multito single-peak structure, respectively (see Figure 1 a). The shift of the G-band to G 1 = 1612 cm − 1 is a signature of a graphene sheet with only one adjacent FeCl 3 layer, whereas the shift to G 2 = 1625 cm − 1 characterizes a graphene sheet sandwiched between two FeCl 3 layers [13][14][15] (see Figure 1 a). The frequencies, linewidths and lineshapes of the G 1 and G 2 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 FeCl 3 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 Potassium [ 20 , 21 ] and Rubidium. [ 21 ] These observations allow us to identify the structure of intercalated 2L samples as one FeCl 3 layer sandwiched between the two graphene sheets. However, the structural determination of thicker FeCl 3 -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 confi rm the fi ndings of Raman and electrical transport measurements, however the small thickness of FeCl 3 -FLG and the substrate effects make it diffi cult to apply such technique to these systems.
To characterize the structure of thicker FeCl 3 -FLG, we study the oscillatory behaviour of the longitudinal magneto-conductance ( G xx ) in a perpendicular magnetic fi eld (i.e. Shubnikov-de Haas oscillations, SdHO) in combination with the Hall resistance ( R xy ). Here we discuss the representative data for an intercalated 5L sample patterned into a Hall bar geometry (see Figure  1 c and Experimental section). Figure 1 b shows SdHO of G xx as a function of perpendicular magnetic fi eld (B) for different temperatures. It is apparent that for T < 10K G xx 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 G xx (1/B) yields peaks at frequencies f SdHO1 = 1100T and f SdHO2 = 55T (see Figure 1 d), corresponding to charge carrier densities n 1 = (1.07 × 10 14 ± 5 × 10 11 )cm − 2 and n 2 = (5.3 × 10 12 ± 4 × 10 11 ) cm − 2 (with n i = 4 ef SdHOi /h [ 6 , 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 1 e 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 (2 π 2 k B Tm c / eB ) (see Figure 1 g), with cyclotron masses m c1 = (0.25 ± 0.05) m e and m c2 = (0.08 ± 0.001) m e for the high-and low-frequency oscillations, respectively. These values correspond to the expected values of cyclotron mass for massless Dirac fermions m c = h 2 n/4πv 2 F = 0.21m e at n 1 and for chiral massive charge carriers of bilayer graphene m c = h 2 v 2 F π n + (γ / 2) 2 /v 2 F = 0.084m e at n 2 (with v F = 10 6 ms − 1 the Fermi velocity [ 6 ] and γ the interlayer hopping energy [ 15 ] ), see Supporting Information. Therefore, intercalation of FeCl 3 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 FeCl 3 -FLG are readily identifi ed by correlating SdHO to the Hall resistance measurements. The linear dependence of R xy (B) with positive slope identifi es charge carriers as holes, with a Hall carrier density n H = B/(eR xy ) = 3 × 10 14 cm − 2 (see Figure 1 c). Since the total carrier density n tot = ∑ i n i should be higher than n H = ( ∑ i n i μ i ) 2 / ∑ i n i μ 2 i (with n i and μ i the carrier density and mobility of each hole gas), [ 23 ] only a minimum of three parallel hole gases with n 1 = 1.07 × 10 14 cm − 2 can explain the estimated value of n H , i.e. 3 × n 1 + n 2 ≥ n H (see Supporting Information). Therefore, the electrical transport characterization demonstrates the presence of four parallel hole gases, of which one with bilayer character (and density n 2 ) and three with monolayer character (each with density n 1 ). These fi ndings are confi rmed by the Raman spectra taken after the device fabrication showing the presence of pristine G, G 1 and G 2 peaks (see Supporting Information). A schematic of this crystal structure is illustrated in Figure 1 h. The bilayer gas is likely to be caused by the fi rst two layers of the stacking which have been de-intercalated due to rinsing in acetone during lift-off. [ 19 ] The  Intensity (a.u.) for a statistical ensemble of fl akes (Figure 3 d). This results in similar extinction coeffi cients per layer for pristine FLG ( ≈ 2.4 ± 0.1%) and for FeCl 3 -FLG ( ≈ 2.6 ± 0.1%), see Figure 3 c and d. For wavelengths longer than 550nm we observe an increase in the optical transparency of FeCl 3 -FLG. This is a signifi cant 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, FeCl 3 -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 FeCl 3 -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 (R s = 10 Ω / ᮀ and Tr = 85% [ 9 ] ). Figure 3 e compares R s vs. Tr of FeCl 3 -FLG materials with ITO, [ 28 ] and other promising carbon-based candidates to replace ITO such as carbon-nanotube fi lms [ 29 ] and doped graphene materials. [ 7 ] It is apparent that R s and Tr of FeCl 3 -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 FeCl 3 -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 FeCl 3 -FLG by performing Raman measurements as a function of time. We found that the Raman spectra of FeCl 3 -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 8.8 Ω / ᮀ 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 FeCl 3 -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 hundred [ 15 ] ), many of which are unstable in air, we found that FeCl 3 -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 bottom part of the stacking has a per-layer doping of n 1 = 1.07 × 10 14 cm − 2 and the stoichiometry of stage-1 FeCl 3 graphite intercalation compounds (i.e. where each graphene layer is sandwiched by two FeCl 3 layers). [ 15 ] In total we have investigated electrical transport and Raman spectroscopy in more than 10 intercalated 5L samples and in all cases we confi rmed the structure reported in Figure 1 h. A remarkable property of FeCl 3 -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 fi nd a room temperature value of R s = 8.8 Ω / ᮀ 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 FeCl 3 -FLGs thicker than 2L decreases when lowering the temperature as expected for metallic conduction (see Figure 2 a). Contrary to intercalated samples, pristine FLGs always have a higher R s ( > 120 Ω / ᮀ ) and they exhibit non-metallic behaviour as a function of temperature [ 5 , 24 , 25 ] (see Figure 2 b). This suggests that the origin of the low values of R s and the metallic nature of the conduction are consequences of intercalation with FeCl 3 . FeCl 3 -FLGs thinner than 3L suffer of partial de-intercalation during the device fabrication, which results in a nonmetallic behaviour similar to pristine FLGs and in higher R s values (see Figure 2 a).
The low values of R s characterizing thick FeCl 3 -FLG are accompanied by an extremely high charge density. Indeed, the Hall coeffi cient measurements reveal that n H ranges from 3 × 10 14 cm − 2 to 8.9 × 10 14 cm − 2 , depending on the number of layers (see Figure 2 c and d). These charge densities exceed even the highest values demonstrated so far by liquid electrolyte [ 26 ] or ionic [ 27 ] gating. The corresponding charge carrier mobility for the 4L and 5L samples typically ranges from μ H = 1540 cm 2 V − 1 s − 1 to μ H = 3650 cm 2 V − 1 s − 1 ( μ H = 1/( n H e ρ xx ) with ρ xx the longitudinal resistivity and e the electron charge). Consequently, the charge carriers in thick FeCl 3 -FLG have a macroscopic mean free path as high as 0.6 μ m in 5L at room temperature. The outstanding electrical properties, e.g. lower R s than ITO and macroscopic mean free path, found in FeCl 3 -FLGs thicker than 3L are of fundamental interest for the development of novel electronic applications based on highly conductive materials.
Whether FeCl 3 -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 FeCl 3 intercalation improves signifi cantly the electrical properties of graphene, it leaves the optical transparency nearly unchanged.  [ 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 fi t it with a linear dependence on the number of layers tube is pumped down to 2 × 10 − 4 mbar at room temperature for 1 hour to reduce the contamination by water molecules. Subsequently, the FLG and the powder are heated for 7.5 hours at 360 ° and 310 ° , respectively. A heating rate of 10 ° /min is used during the warming and cooling of the two zones. Ohmic contacts are fabricated on FeCl 3 -FLG by means of electron-beam lithography and lift-off of thermally evaporated chrome/ gold bilayer (5/50 nm). We have fabricated FeCl 3 -FLG on both SiO 2 /Si and glass substrates and we found no signifi cant differences in their transport properties.
Raman measurements : Raman spectra are collected in ambient air and at room temperature with a Renishaw spectrometer. An excitation laser with a wavelength of 532 nm, focused to a spot size of 1.5 μ m diameter and a × 100 objective lens are used. To avoid sample damage or laser induced heating, the incident power is kept at 5 mW.
Electrical measurements : The longitudinal and the Hall resistances are studied in a 4-probe confi guration by applying an a.c. current bias and measuring the resulting longitudinal and transversal voltages with a lock-in amplifi er. The excitation current is varied to ensure that the energy range where electrical transport takes place is smaller than FLG with FeCl 3 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 FeCl 3 -FLG make these materials a valuable alternative to ITO in optoelectronics. the energy range associated to the temperature of the electrons. This prevents heating of the electrons and the occurrence of nonequilibrium effects. Since conductances are additive, the analysis of Shubnikov-de Haas oscillations is performed on the longitudinal conductivity σ xx = ρ xx / [ ρ 2 x x + ρ 2

Experimental Section
x y ] (with ρ xx and ρ xy the longitudinal and transversal resistivity, respectively). The low frequency magneto-conductivity oscillations shown in Figure 2 d are obtained by averaging out the high frequency oscillations, whereas to obtain the high frequency oscillations shown in Figure 2 e we subtract the low frequency oscillations from the longitudinal conductivity.
Transmission measurements : The transmission of pristine FLG and FeCl 3 -FLG is characterized by measuring the bright-fi eld transmission spectra. A system based on an inverted optical microscope (Nikon Eclipse TE2000-U) combined with a spectrometer and charge-coupled device (CCD) camera (Princeton Instruments, SpectraPro 2500i) is used to acquire data. White light from a tungsten fi lament lamp is used to illuminate the samples and, after passing through the sample, is collected by a dry Nikon lens (S Plan Fluor ELWD) × 40 of numerical aperture 0.60. A slit width of 50 μ m is used for the spectrometer, yielding a spectral resolution < 1 nm for the measurements. In the spectrometer the dispersed light is projected onto the 1024 × 256 lines of the CCD camera. Data from the camera are extracted to give the transmission spectra of the fl ake, or part of it, the data being normalized to the signal obtained through a region of bare substrate. For the visually uniform parts of the fl akes, spectra are averaged along several lines of the CCD camera to improve the signal-to-noise ratio.  [ 4 , 7 ] c) Transmittance at 550nm for pristine FLG as a function of the number of layers. The red line is a linear fi t, which gives the extinction coeffi cient of 2.4 ± 0.1% per layer. d) Transmittance at 550nm for fully intercalated FeCl 3 -FLG (FI), partially intercalated FeCl 3 -FLG (PI) and doped FeCl 3 -FLG (D) as a function of the number of layers. The black line is a linear fi t with the extinction coeffi cient of (2.6 ± 0.1)% per layer. e) Square resistance versus transmittance at 550nm for 4L and 5L FeCl 3 -FLG (from these experiments), ITO, [ 28 ] carbon-nanotube fi lms [ 29 ] and doped graphene materials. [ 7 ] FeCl 3 -FLG outperform the current limit of transparent conductors, which is indicated by the grey area.