In Situ Atomic-Scale Imaging of Interfacial Water under 3D Nanoscale Confinement

Abstract

Capillary condensation of water from vapor is an everyday phenomenon which has a wide range of scientific and technological implications. Many aspects of capillary condensation are not well understood such as the structure of interfacial water, the existence of distinct properties of confined water, or the validity of the Kelvin equation at nanoscale. We note the absence of high-spatial resolution images inside a meniscus. Here, we develop an AFM-based method to provide in situ atomic-scale resolution maps of the solid–water interface of a nanomeniscus (80–250 nm 3 ). The separation between the first two hydration layers on graphite is 0.30 nm, while on mica it is 0.28 nm. Those values are very close to the ones expected for the same surfaces immersed in bulk water. Thus, the hydration layer structure on a crystalline surface is independent of the water volume.

Hydration Dynamics and the Future of Small-Amplitude AFM Imaging in Air

“…Moreover, the user must cope with and understand the character of some dynamic variables, such as the sharpness of the tip [ 9 , 10 ], otherwise referred to as the tip radius, R, in high-resolution imaging applications. Perhaps counter-intuitively to the newcomer, the field has rapidly advanced in two extremes—in liquid [ 11 , 12 , 13 , 14 ] and UHV environments [ 15 ]—while several complex phenomena have hindered the imaging and quantification of phenomena in air [ 16 ] with similar resolutions, controls, or throughputs [ 3 ]. There has been research in air in terms of capillary interactions [ 17 ], spontaneous capillary condensation [ 16 ], and the way the air environment affects surfaces [ 18 ], molecules on it, and modes of imaging [ 3 , 19 , 20 , 21 ].…”

“…Perhaps counter-intuitively to the newcomer, the field has rapidly advanced in two extremes—in liquid [ 11 , 12 , 13 , 14 ] and UHV environments [ 15 ]—while several complex phenomena have hindered the imaging and quantification of phenomena in air [ 16 ] with similar resolutions, controls, or throughputs [ 3 ]. There has been research in air in terms of capillary interactions [ 17 ], spontaneous capillary condensation [ 16 ], and the way the air environment affects surfaces [ 18 ], molecules on it, and modes of imaging [ 3 , 19 , 20 , 21 ]. In terms of force measurements in air, most have focused on capillary interactions [ 22 ] arising from the formation and rupture of a capillary neck as a function of the relative humidity (RH), either experimentally [ 16 ] or theoretically [ 23 ].…”

“…There has been research in air in terms of capillary interactions [ 17 ], spontaneous capillary condensation [ 16 ], and the way the air environment affects surfaces [ 18 ], molecules on it, and modes of imaging [ 3 , 19 , 20 , 21 ]. In terms of force measurements in air, most have focused on capillary interactions [ 22 ] arising from the formation and rupture of a capillary neck as a function of the relative humidity (RH), either experimentally [ 16 ] or theoretically [ 23 ]. Here, a capillary neck between the tip and the substrate forms and ruptures.…”

“…In dynamic AFM, it is argued that the neck forms and ruptures [ 22 , 24 ] during each oscillation cycle, implying that the process is stable enough to occur at a rate of thousands of times per second. Some models consider the constant volume approximation [ 25 ], while others argue that the condensation from water in the neighborhood of the tip sample junction is also responsible for the volume of the neck [ 16 ]. The constant volume approximation assumes that the water layers on the surfaces contribute to the neck.…”

“…The oscillatory contributions correspond to the surface forces that provide information about the spatial frequencies of the liquid density. The monotonic term, many times neglected by the community developing the 3D mapping force technique [ 16 ], is the component of the force typically studied by force reconstruction methods that rely on averaging. It is worth noting, in the context of the topic under consideration, that even current research tends to focus on [ 26 ] the distinction between dry conditions and high-humidity conditions, i.e., extremes, and might obviate the aging process of the surface [ 27 ].…”