At Least 10-fold Higher Lubricity of Molecularly Thin D2O vs H2O Films at Single-Layer Graphene–Mica Interfaces

Interfacial water is a widespread lubricant down to the nanometer scale. We investigate the lubricities of molecularly thin H2O and D2O films confined between mica and graphene, via the relaxation of initially applied strain in graphene employing Raman spectroscopy. Surprisingly, the D2O films are at least 1 order of magnitude more lubricant than H2O films, despite the similar bulk viscosities of the two liquids. We propose a mechanism based on the known selective permeation of protons vs deuterons through graphene. Permeated protons and left behind hydroxides may form ion pairs clamping across the graphene sheet and thereby hindering the graphene from sliding on the water layer. This explains the lower lubricity but also the hindering diffusivity of the water layer, which yields a high effective viscosity in accordance with findings in dewetting experiments. Our work elucidates an unexpected effect and provides clues to the behavior of graphene on hydrous surfaces.

Raman spectra were acquired with a confocal Raman microscope (XploRa, Horiba Ltd) located inside a glove box. The glove box was initially purged with dry nitrogen, and then it was purged with dry nitrogen bubbling through washing bottles containing either H2O or D2O described above. Single-and bi-layer graphene pieces were optically detected inside the glove box. The samples were first equilibrated in H2O or D2O vapors at about 40% RH for one hour before the experiments with mica bending commenced. RH values inside the glove box were measured with an RH sensor (Testo 625, Testo Inc.) with the calibration fidelity of ± 2.5% RH, as provided by manufacturing company. A 532 nm excitation laser was used with the illumination intensity less than 1.4 mW on the sample surface under 100x objective (Olympus M Plan N 100X/0.90na). A neon lamp was placed close to the sample such that the spectra acquired on the samples contained the Ne-spectral lines. The Ne-spectral lines were used as references for Raman spectra calibration. The calibration was detailed in 3 . The calibration method using neon lamp was used to measure the samples shown in Figure 2 of the main text and in Figure 6SC, D. For all other samples we used a standard instrument calibration implemented by the instrument manufacturer using a silicon line at about 520 cm -1 . Mica substrate was bent in a four-point bending setup inside the glove box. The setup was described in details previously. 4 The mica surface strain was estimated as mica slab thickness divided by two times the bending radius. The uncertainties of graphene strain induced by mica bending were detailed previously. 4 In brief, the uncertainties were propagated from the uncertainties of mica slab bending radius, mica slab thickness and positioning precision of laser spot position. The 2D peak position scattering, for when the laser spot was repositioned, was estimated to be 0.5 cm -1 . Figure 1S shows AFM images of H2O and D2O wetting graphene-mica interface. Wetting with water has been addressed in details before. 5 The initially dry samples (not shown here) were exposed to controlled mixtures of dry nitrogen with nitrogen bubbling through either D2O or H2O. As the relative humidity (RH) increases, islands of a water film start emerging from the graphene edges. The islands grow inwards and form labyrinthine patterns (Fig. 1S). Eventually, if we increased RH to 40%, the labyrinthine patterns combine to form a homogeneous film (Fig. 1S B) and D)). We do not find any difference between wetting behavior with water and heavy water. Wetting with heavy water started within the same RH range. The wetting patterns with heavy water appear to be identical to water ones, i.e., they exhibited labyrinthine shapes with roughly same width (Fig. 1S). Heavy water and normal water both form homogeneous films at around 30 % RH.

Wetting of graphene-mica interface with D2O and H2O films
The formation of labyrinthine patterns by water during the initial intercalation into the graphene-mica interface was explained by an equilibrium between electrostatic repulsion of water dipoles ordered normally to the interface, and the line tension at the boundaries between dry and wetted areas. 5 Dipoles of D2O and H2O molecules are rather similar, 6 therefore it is reasonable to expect similar electrostatic repulsions between the molecules and correspondingly similar labyrinthine patterns. Figure 1S. AFM height and phase images (the insets) of SLGs exposed to A) H2O and C) D2O vapor at about 17% and 16% RH, respectively. The phase images show no significant contrast difference between wetted and dry areas. This indicates that both liquids intercalate into the graphene-mica interface. Both liquids form similar labyrinthine wetting patterns. B) and D) are the height images of the same areas acquired at higher RH, i.e., 40% for H2O and 30% for D2O. Figure 2S shows the images of Figure 3 of the main text without coloring of the dewetting patterns. Figure 2S. The figure is identical to the figure 3 in the main text, yet with all coloring, labelling and highlighting removed. The coloring possibly hides important features. For instance, with no coloring, one can see from the cross section in B), that the bottom of the dewetting patterns within double layer graphene (purple area in the cross section) is higher to compare with the neighboring top of the water filled single layer graphene (area I). We conclude from this that the height of the dewetting layer is smaller than the single layer graphene height, i.e., it is smaller than roughly 3.4 angstrom. 7 Figure 3S shows dewetting patterns formed during relatively slow dewetting of mono-, biand few-layer graphene -mica interfaces filled with H2O, i.e., RH was decreased from 40% to 13% in 2 minutes, and then it was kept constant. An overview image (Fig. 3S.A) shows both single-and double-layer graphene covered areas. The single layer graphene stripe is rather narrow, therefore also dewetting patterns are rather small laterally. Nevertheless, one can clearly recognize the fractal shape of the dewetting pattern (Fig. 3S.C). The dimension of the fractal is roughly 1.7. The dewetting patterns in bilayers (Fig. 3S.A and B) are more compact with fractal dimension around 1.85. Figure 3S. AFM height images of single-and few-layer graphene flakes on mica filled initially with H2O and then slowly dried with N2 gas. RH was decreased from 40% to 13% in 2 minutes, and then it was kept constant for at least an hour before the images were made. A) An overview image. B) Magnification of graphene bilayer covered area. C) Magnification of SLG covered area.

Fast dewetting of single layer graphene-mica interface initially filled D2O and H2O films
Instantaneous drying from 40% to ~ 0% within less than a minute by purging the AFM sample cell with dry nitrogen caused growth of dewetting patterns that are highly ramified fractal structures for both D2O and H2O filled single layer graphene-mica interfaces (Fig. 4S). Figure 4S. AFM topography images of single layer graphenes on mica. Similar fractal patterns formed in about one minute during fast dewetting of A) H2O and B) D2O intercalated single layer graphene -mica interfaces. Here RH was decreased from 40% to roughly 0% within less than a minute by direct purging of the AFM sample cell with dry N2.

AFM topography images of single-and bilayer-graphenes after strain measurements
All graphene pieces used for the strain relaxation measurements were imaged with AFM under ambient conditions to determine their lateral dimensions. Figure 5S shows a few example images. The single layer graphenes in A) and B) were exposed to D2O and H2O vapors respectively during strain relaxation measurements. The lateral size of SLG in A) is larger to compare with the one in B) (note the different image scale); nevertheless, the SLG piece in A) releaxed the strain much faster to compare with the one in B), see the main text for the details.

More examples of strain relaxation in graphene with intercalated H2O and D2O layer
Here we show Raman results from another 4 SLG samples. Two of them were measured in H2O vapor while the other two were measured in D2O vapor. The experiments were performed in the similar way as we did on samples shown in Figure 2. In all cases the RH was high enough to form molecular layers at the graphene-mica interface. We can find significant difference of strain relaxation behaviour in samples measured with H2O and D2O. The results are always similar to the data shown in Figure 2.

Strain relaxation in graphene on layers of H2O and D2O mixtures
We performed the strain transfer measurements on graphene samples exposed to mixed vapors with different ratio of H2O and D2O. For mixing ratios of D2O: H2O of 7:3 and 3:7 we found the same behaviour of the 2D peak position as for pure D2O. Only for mixing ratio of 1:9 D2O:H2O we observed 2D peak changes with time due to strain relaxation. However, in that case the relaxation time was lowered (2 Minutes) compared to pure water (8 Minutes). Figure 7S. strain relaxations in a SLG exposed to vapor with mixing ratio D2O:H2O of 1:9.
Bilayer graphene spectra on flat and bent mica Figure 9S. Raman spectra from the bilayer graphene samples in Figure 2C, exposed to H2O vapor acquired before and after bending of mica. Figure 10S shows the evolution of D2O and H2O dewetting patterns of two SLG samples. The RH was decreased within 2 minutes from 40% to about 13% and then kept constant. Figure  10S A) and E) are the first images acquired when RH reached 13%. Then AFM imaging under tapping mode was repeated on the zoomed areas on both SLGs ( Figure 10S, areas marked by green boxes). Figure 10S. development of the A-D) D2O and E-H) H2O dewetting patterns at two graphene-mica interfaces. The time interval between each two adjacent images are about 30 minutes.