Insight into the wetting of a graphene-mica slit pore with a monolayer of water

Scanning force microscopy (SFM) and Raman spectroscopy allow the unraveling of charge doping and strain effects upon wetting and dewetting of a graphene-mica slit pore with water. SFM reveals a wetting monolayer of water, slightly thinner than a single layer of graphene. The Raman spectrum of the dry pore exhibits the $D^{\prime }$ peak of graphene, which practically disappears upon wetting, and recurs when the water layer dewets the pore. Based on the $2D$- and $G$-peak positions, the corresponding peak intensities, and the widths, we conclude that graphene on dry mica is charge-doped and variably strained. A monolayer of water in between graphene and mica removes the doping and reduces the strain. We attribute the $D^{\prime }$ peak to direct contact of the graphene with the ionic mica surface in dry conditions, and we conclude that a complete monolayer of water wetting the slit pore decouples the graphene from the mica substrate both mechanically and electronically.

Figure 1

SFM height images taken on mica partially covered with SLG in (a) less than 10 ppm of water and (b) $16\%$ RH. The insets show phase images taken simultaneously with the height images. The dark areas on the phase images are the SLG. (c) SFM height image of a few-layer graphene with a single-layer step height terrace. The slit pore is partially filled with water. Inset shows phase image taken simultaneously with the height one. The black to white scales of all height and phase images are 1 nm and 10°, respectively. (d) Cross sections averaged over the red and blue rectangles in (c) showing that the $\left(n+1\right)$ graphene terrace unfilled with water is higher than the neighboring $n$ graphene terrace filled with water. This implies that the water layer thickness is smaller than the single-layer graphene step height as it is sketched in (e).

Figure 2

(a) Raman spectra acquired on a single-layer graphene in dry (black line) and humidified (RH $50\%$, red line) nitrogen environments. The $G,2D,D$, and $D^{\prime }$ peaks are labeled. (b) Magnified $D$ and $D^{\prime }$ spectral and intensity ranges. (c) Histograms of peak area ratios for $2D$ to $G$ for dry (black) and humidified (red) SLGs. (d) Raman spectra acquired on a single-layer graphene used to test a $D^{\prime }$ reactivation prediction (see Sec. 4) in dry (black line), humidified (RH $50\%$, red line), dried again (RH $2.5\%$, gray line), and rehumidified (RH $50\%$, magenta line) nitrogen environments. The spectra are offset along the $Y$ axis for clarity.

Figure 3

(a) $2D$ versus $G$ peak positions and (b) FWHMs for dry (black symbols) and humidified (red symbols) SLGs. Different symbols show results from different graphene pieces. The pink stars in (a) labeled with 1–6 are the literature values for undoped and possibly unstrained graphenes: 1 [28], 2 [29], 3 [27], 4 [30], 5 [21], and 6 [31] (see Sec. 4 for the details). The blue square in (a) shows the averaged $2D/G$ peak position for the humidified SLGs; the error bars show the sum of SEs and the instrumental error. Green and blue dotted lines have slopes of 0.55 and 0.2 and show the expected shifts of $2D$ and $G$ peak positions for $p$- and $n$-doping, respectively [31]. The dashed lines in (a) have a slope of 2.2 and are guides for the eye. The results for dry samples can be explained with combination of either $p$- or $n$-doping and mostly tensile strain. (c) Cleavage of mica presumably removes half of potassium ions. We suggest that graphenes become strained when exfoliated onto such surface. (d) A monolayer of water molecules filling into the slit pore between graphene and mica, possibly fills also the gaps between the ${\mathrm{K}}^{+}$ ions removing thereby tensile strain in graphene.