Kinetic Ionic Permeation and Interfacial Doping of Supported Graphene

Due to its outstanding electrical properties and chemical stability, graphene finds widespread use in various electrochemical applications. Although the presence of electrolytes strongly affects its electrical conductivity, the underlying mechanism has remained elusive. Here, we employ terahertz spectroscopy as a contact-free means to investigate the impact of ubiquitous cations (Li+, Na+, K+, and Ca2+) in aqueous solution on the electronic properties of SiO2-supported graphene. We find that, without applying any external potential, cations can shift the Fermi energy of initially hole-doped graphene by ∼200 meV up to the Dirac point, thus counteracting the initial substrate-induced hole doping. Remarkably, the cation concentration and cation hydration complex size determine the kinetics and magnitude of this shift in the Fermi level. Combined with theoretical calculations, we show that the ion-induced Fermi level shift of graphene involves cationic permeation through graphene. The interfacial cations located between graphene and SiO2 electrostatically counteract the substrate-induced hole doping effect in graphene. These insights are crucial for graphene device processing and further developing graphene as an ion-sensing material.


Optical pump-THz probe (OPTP) spectroscopy.
We perform photo-conductivity measurements of graphene using optical pump-THz probe (OPTP) spectroscopy. In the setup, the THz generation is based on the optical rectification process in ZnTe non-linear crystals, using 800 nm pulses with duration of ~ 50 fs. The time-dependent electric field strength of THz pulses (~ 1-2 ps duration) transmitted the sample is detected by a second ZnTe crystal using electrooptic sampling. By fixing the sampling delay time to the peak position of the THz pulse, we monitor the THz absorption change by a third laser pulse with a wavelength of 800 nm. By tuning the time delay between the pump and THz probe, we measure the time-dependent THz absorption and therefore, the photo-conductivity of our sample in the time domain. The OPTP setup is kept in an N2 environment to avoid THz absorption in air.

Graphene transfer.
For transferring CVD grown graphene on the copper foil to fused SiO2 for THz measurements, a layer of PMMA is spin-coated on top of graphene. Subsequently, the copper is first etched in 3 g/100 ml ammonium persulfate ((NH4)2S2O8) aqueous solution which is filtered by a 0.2 μm Nylon membrane filter in advance. After rinsing the PMMA/graphene in Milli-Q water, we transfer the PMMA/graphene (1 cm × 1 cm) to fused silica. Then we keep the transferred PMMA/graphene on fused silica in vacuum at 80 °C for 12 hours. Afterward, the PMMA is removed in an acetone bath. Finally, the graphene on fused silica is annealed at 650 °C to remove the PMMA residue. We carry out all steps in dust-free environments.

Electrolyte contacting.
We control the volume of electrolyte precisely with a micropipette. We sandwich the electrolyte between two pieces of fused silica with the graphene being transferred on the bottom one and exposed to the electrolyte. The layer thickness of the electrolyte is controlled by a Teflon spacer ~ 10 micrometers (μm). In each measurement, we fix the sample in the OPTP setup to the same position to prevent the doping inhomogeneity of graphene controlled by a translational stage. During the changing of each electrolyte, we use a small piece of absorbent paper to absorb the original electrolyte gently from the edge of fused silica to avoid the damage of graphene, following by Milli-Q water rinsing, to remove the permeated ions at the interface.

The computational methods and details.
The energy cutoff for the plane-wave basis set is 520 eV. The convergence criteria for structure relaxation are 10 -5 eV on the energy and 0.01 eV/Å on the residual force of each atom. The Brillouin zone was sampled with a Monkhorst-Pack mesh with 3 × 3 × 1 k-point grids.
We simulate the interaction between graphene and SiO2 surfaces using the repeated-slab model of three layers, and we set the vacuum separation to be 15 Å [1] . To model the amorphous SiO2 (α-SiO2) used in the experiment in our periodic DFT calculation, we use the βcristobalite crystal reported to possess similar structure to α-SiO2 as an approximation, This approach has been adopted by other authors as well [2][3][4] . The cristobalite silica surface contains the (111) Figure S2. Note: We note that, for sufficiently long immersion (up to several hours), the doping level returns to the initial doping level, indicating that the cation doping effect is thermodynamically reversible. The doping recovery, or reverse doping process, however, can be kinetically controlled by the cation size and Milli-Q water washing or immersion time. In order to quantitatively compare the cation size-dependent reverse doping kinetics, we define here a parameter "re-doping rate v" as the relative THz conductivity recovery from the final doping towards the initial doping, after immersing cation treated graphene into Milli-Q water for 10 minutes (see more experiments details in the supplementary information). The re-doping rate v is ranging from 0% (i.e., rinsing is ineffective to remove the cation doping effect) to 100% (i.e., completely back to the initial doping). Remarkably we observe a size-dependent reverse doping kinetics which is inherently connected to the doping kinetics, as discussed in the following: (1) For three small cations K + , Na + , Li + , which give rise to strong electron doping effect, we observe that after 10 minute immersion of graphene into Milli-Q water, none of them go back to the initial doping. Generally, the re-doping rate is very similar as 40-50% for Na + and K + , and dramatically increase with increasing the hydrated ionic size (80% for Li + ) and eventually approached to 100% for the Ca 2+ (the largest solvated ion used here). This observation is in line with the size-dependent cation doping effect: the larger the ion, the less doping in graphene, and thus the faster the de-doping. The size-dependent reverse doping kinetics discussed here, provide further strong evidence that cation doping and reverse doping processes involve cation permeation though defects in graphene, and interfacial intercalation or diffusion along the graphene-SiO2 interfaces.  and Na residing at graphene-SiO2 substrate interfaces for single and double vacancies, respectively. b, Na residing at graphene-SiO2 substrate interfaces after it permeates through a double vacancy defect (highlighted as a yellow polygon). c, Na residing at graphene-SiO2 substrate interfaces after it permeates through a single vacancy defect (highlighted as a yellow polygon).