Imaging Water Thin Films in Ambient Conditions Using Atomic Force Microscopy

Abstract

All surfaces exposed to ambient conditions are covered by a thin film of water. Other than at high humidity conditions, i.e., relative humidity higher than 80%, those water films have nanoscale thickness. Nevertheless, even the thinnest film can profoundly affect the physical and chemical properties of the substrate. Information on the structure of these water films can be obtained from spectroscopic techniques based on photons, but these usually have poor lateral resolution. When information with nanometer resolution in the three dimensions is needed, for example for surfaces showing heterogeneity in water affinity at the nanoscale, Atomic Force Microscopy (AFM) is the preferred tool since it can provide such resolution while being operated in ambient conditions. A complication in the interpretation of the data arises when using AFM, however, since, in most cases, direct interaction between a solid probe and a solid surface occurs. This induces strong perturbations of the liquid by the probe that should be controlled or avoided. The aim of this review is to provide an overview of different AFM methods developed to overcome this problem, measuring different interactions between the AFM probe and the water films, and to discuss the type of information about the water film that can be obtained from these interactions.

Keywords: adsorption; atomic force microscopy; thin films; water.

Figure 1

SPFM images of structures formed by water on mica. Bright areas correspond to a second water layer and dark areas to the first water layer. The boundaries tend to have polygonal shapes, as shown in the smaller image where a hexagon is drawn for visual reference. The directions are strongly correlated with the mica lattice. The inset in the large image shows a contact AFM image obtained after the SPFM images, which provides a reference for angle measurements. The histogram shows the angles of the water-film boundaries relative to the mica lattice. (Reprinted with permission from references [13,14], published by American Association for the Advancement of Science, 1995 and Materials Research Society Bulletin, 1997).

Figure 2

(a) SPFM images of water films on BaF2(111) at 20% and 60% RH (size of the images 20 μm × 20 μm, color scale is 5nm from darkest to brightest); (b) evolution of the height of the water films measured from SPFM images with increasing RH. (Reprinted with permission from reference [22], published by AIP Publishing LLC, 2008).

Figure 3

(a) SPFM images acquired at different relative humidity (RH) values. Grey scales are 10 nm. The images at the top correspond to RH = 25%. The middle images, at 30% RH, show a moderate enhancement of the step contrast. At 35% RH (bottom images) the step enhancement is large, in the order of 1 nm as observed in the profile in (b). The enhancement is due to solvated ions; (c) SPFM images taken at 34% RH and two different frequencies, 0.4 and 4 KHz. The enhancement disappears at 4KHz due to the slow mobility of the ions. (Reprinted with permission from reference [18], published by AIP Publishing LLC, 2005).

Figure 4

Changes in the contact potential of a mica surface relative to a hydrophobic tip vs. relative humidity (RH) and for different temperatures. At room temperature the potential first decreases by about 400 mV. This change can be explained by the orientation of water in the first monolayer, which has an average dipole moment pointing towards the surface. At ~20%–30% RH, it reaches a plateau and remains approximately constant until about 80% RH. At higher humidity, the potential increases again. The observation is explained by a change in orientation of water in the second layer, where H from dangling H-bonds point upwards to the vapor phase. Below 0 °C, the change in potential above 80% RH is reversed and becomes negative. This suggests that, in that case, the dipole-down orientation of water in the first layer continues in subsequent layers. (Reprinted with permission from reference [24], published by Elsevier Science B.V., 2000).

Figure 5

Contact AFM, SPFM and KPFM 12 μm × 12 μm images of BaF₂(111) and CaF₂(111) surfaces at ambient conditions (RT and ~50% RH). In contact images only the step structure is observed. By subtracting the contact image from the SPFM image, the regions covered by the water film can be identified. In KPFM, wet areas show a bright contrast due to the average dipole orientation of the water molecules. The KPFM contrast is homogeneous with well defined edges on BaF₂(111) and more diffuse on CaF₂(111) indicating a major structuration of the water molecules forming the films on BaF₂(111). (Reprinted with permission from reference [25], published by Elsevier Science B.V., 2011)

Figure 6

(a) Scheme of the three different interaction regimes when water is present on the surface and the AFM probe: W_nc (water non-contact), W_c (water contact) and Rep. (repulsive); (b) Experimental vs. simulation values for the apparent height (h) of water patches on a BaF₂(111) sample displaying both wet and un-wet regions. The y-axis shows the values of apparent height in nm and in the x-axis the different imaging regimes obtained by working at free amplitudes of 3, 10, 30 and 60 nm, respectively. Imaging regimes are defined as a pair of interaction regimes corresponding to the interaction regime on the wet areas and dry areas of the image. The different types of interactions predicted by the simulations (outlined circles) are experimentally (filled squares) observed to follow similar patterns in terms of apparent height. (Reprinted with permission from reference [10], published by IOP Publishing, 2011)

Figure 7

(a) topography and phase image of water films on a stepped BaF2(111) surface; (b) phase image of a water film trapped between two steps. The water film shows a meniscus-like shape with a curvature showing different contact angle for steps along different crystallographic directions, indicating different water affinity. (Reprinted with permission from reference [25], published by AIP Publishing LLC, 2011)

Figure 8

3D-SPM map of a mica-water interface showing variations of the phase shift of the second excited mode. The stripes are associated with the presence of hydration layers close to the mica surface. (Reprinted with permission from reference [46], published by RSC Publishing, 2013).

Figure 9

(a) a schematic of how graphene locks the first water adlayer on mica into fixed patterns and serves as an ultrathin coating for AFM; (b) the structure of ordinary ice (ice I_h). Open balls represent O atoms, and smaller, solid balls represent H atoms. A single puckered bilayer is highlighted with red. Interlayer distance is c/2 = 0.369 nm when close to 0 °C; (c) AFM image of a monolayer graphene sheet deposited on mica at ambient conditions; (d) A close-up of the blue square in (c); (e) Height profiles along the green line in (d) and from a different sample. The dashed line indicates z = 0.37 nm; (f) AFM image of another sample, where the edge of a monolayer graphene sheet is folded underneath itself. The arrow points to an island with multiple 120° corners; (g) The height profile along the red line in (f), crossing the folded region. Scale bars indicate 1 μm for (c) and 200 nm for (d,f). The same height scale (4 nm) is used for all images. (Reprinted with permission from reference [11], published by American Association for the Advancement of Science, 2010).

Figure 10

AFM images of water trapped between graphene and a BaF₂(111) surface. Transference of the graphene was performed at different relative humidity (RH) conditions. From the images, we can observe adsorbed water increasing from a sub-monolayer coverage (7% and 15% RH) to a complete monolayer (30% and 50% RH), multilayers (70% RH) and forming droplets on the surface (90% RH). (Reprinted with permission from reference [56], published by AIP Publishing LLC, 2013).