Large-area functionalized CVD graphene for work function matched transparent electrodes

The efficiency of flexible photovoltaic and organic light emitting devices is heavily dependent on the availability of flexible and transparent conductors with at least a similar workfunction to that of Indium Tin Oxide. Here we present the first study of the work function of large area (up to 9 cm2) FeCl3 intercalated graphene grown by chemical vapour deposition on Nickel, and demonstrate values as large as 5.1 eV. Upon intercalation, a charge density per graphene layer of 5 ⋅ 1013 ± 5 ⋅ 1012 cm−2 is attained, making this material an attractive platform for the study of plasmonic excitations in the infrared wavelength spectrum of interest to the telecommunication industry. Finally, we demonstrate the potential of this material for flexible electronics in a transparent circuit on a polyethylene naphthalate substrate.


Spatial dependence of the work function
To ascertain the spatial distribution of the work function () we have conducted a statistical study of the estimated values of  in three representative areas of about 20 µm 2 highlighted in the micrograph picture of Figure S1. The most commonly occurring values of work function recorded at 5.1, 5 and 4.9 eV with the majority of the surface area of the sample about 60% characterized by a work function value of 5.1 eV (see Table S1). The observed three dominant values of work function are consistent with three distinct cases: graphene sandwiched between FeCl3, graphene with FeCl3 only on one side and graphene with no direct contact to FeCl3.  Table S1. Percentage coverage of the most commonly observed sample in the three studied areas indicated in Figure S1. Area  = 4.9 eV  = 5 eV

Characterization of Ni-grown CVD graphene
Nickel-grown CVD graphene purchased from Graphene-supermarket (Wafer of 100mm Graphene Film on Nickel) was used in this work since this growth method gives multilayer graphene on large areas (up to 100 cm 2 ) which is needed for the intercalation of FeCl3 [3]. The product datasheet specify that this is a continuous film of multilayers (up to 7-10 layers). The characterization of the multilayer graphene transferred on Si/SiO2 (with 300nm thick SiO2) substrate is shown in Figure  S2. More specifically, Figure S2a shows a false-colour map of an optical micrograph picture (see Figure S2b) of transferred multilayer graphene on Si/SiO2 after wet etching Ni in FeCl3 solution. Different layer thicknesses and different domains are clearly visible and their sizes are consistent with previous studies [3]. A statistical analysis of the grain size shows that ~35% of the sample is 4 layers thick with and average domain area of ~150 µm 2 , see Figure S2c. Figure S2d shows an optical micrograph of a representative area of the same substrate after intercalation with FeCl3, and the statistical analysis of this micrograph image shows no significant change of the average domain area upon intercalation which has to be expected since FeCl3 is not etching graphene [1, 4 and 5], see Figure S2e. Scanning electron microscope (SEM) images of Ni-CVD graphene on SiO2 before and after FeCl3 intercalation are shown in panels f) and g) respectively. After intercalation we observe a better contrast on the image, given by the higher conductivity of the graphene, and FeCl3 residues (bright spots, Fe has a higher atomic number than carbon) on the surface and at the boundaries of the islands. shows an optical micrograph of a representative area after FeCl3 intercalation on which a statistical study to determine the average grain area has been performed, panel e). Panels f) and g) show SEM images of Ni-CVD graphene before and after FeCl3 intercalation respectively. Scale-bars are 20µm in panels a), b), d) f) and g) and 2µm in the insets of panels f) and g).

Comparison of the charge density estimated from Raman spectroscopy and from quantum oscillations in the magneto-conductance
To assess the accuracy with which the stiffening of the E2g phonon mode can be reliably used to estimate the charge density in FeCl3-few-layer intercalated graphene (FeCl3-FLG) we conduct a comparative study of the charge density obtained from the Raman G-peak shift and the period of the Shubnikov-de Haas oscillations (SdHO) presented in the work of ref. [1]. Figure S3 shows the Raman spectrum of FeCl3-FLG and the G-peak shift. Using the theory in ref. [2] the G2 peak at 2 = (1623.24 ± 0.02) −1 gives a charge density of = (9.0 ± 0.5) • 10 13 −2 and the SdHO measurements in ref. [1] report a value of = (10.700 ± 0.005) • 10 13 −2 . Hence the difference in charge density estimated from Raman and SdHO is 1 • 10 13 −2 . This discrepancy can simply originate from the fact that Raman spectroscopy is a local probe (the laser spot-size is typically <1µm) whereas SdHO is probing the charge density on a macroscopic scale corresponding to the distance between source and drain contacts. Given the small discrepancy between the two measurements we can conclude that Raman spectroscopy is a valuable, non-destructive, tool to estimate the charge concentration in highly doped graphene, in particular over large areas. Figure S3. Raman spectrum of a FeCl3-intercalated trilayer graphene taken from ref. [1] where stage-2 (G1 peak) and stage-1 (G2 peak) intercalation of graphene are present.

Comparison of Raman maps of as-transferred Ni-CVD graphene and FeCl3-intercalated
Ni-CVD graphene.
FeCl3 is used at two separate steps in the fabrication. Firstly, the Ni substrate of the CVD-multilayer graphene is etched in 1 mol of FeCl3 dissolved in de-ionised water. Secondly, the multilayer graphene transferred onto a glass substrate is exposed to vapours of FeCl3 during the intercalation process. To elucidate the doping induced by the two separate exposures of the multilayer to FeCl3 we present estimates of the charge density from the Raman measurements after each FeCl3 exposure for a representative area of 100x100μm ( Figure S4a). After the etching of Ni in FeCl3 solution, we find experimentally an average doping of just 1 • 10 13 −2 and maximum values which do not exceed 2 • 10 13 −2 , see Figure S4b. After intercalation of FeCl3 the carrier concentration in the same area of multilayer graphene is as high as 5.5 • 10 13 −2 with an average value of 2.8 • 10 13 −2 , see Fig.S4c. Indeed, the direct comparison of the Raman spectra before and after intercalation acquired at the same location clearly shows the shift of the G-peak, as expected for high charge density, see Figure S4d. Figure S4: a) A grayscale optical microscope image of few-layer CVD graphene on glass (scale bar = 25μm). Carrier concentration maps over a 100x100um area (dashed line) are shown before, b), and after intercalation (main text Figure 2b). Panel c) shows a statistical analysis of the carrier concentration over the 100x100μm area highlighted in (a) before and after intercalation. d) Raman