An atomic force microscopy study on the local surface potential differenceof amorphous solid water formed by water vapor deposition at 15 and 100
KHidaka, H.,1* Tomaru, T.,2 Watanabe, N.2
1hidaka@lowtem.hokudai.ac.jp, Institute of Low Temperature Science, Hokkaido University,Japan
2
Institute of Low Temperature Science, Hokkaido University, Japan

Amorphous solid water (ASW) is well known to abundantly exist as icy dust particles inmolecular clouds. The icy dust particles, which are mineral nanoparticles covered with an ASW
mantle primarily composed of H2O, play an important role in chemical evolution in space. Thisis because the surface of icy dust facilitates a variety of molecular syntheses through physicaland chemical processes such as adsorption, diffusion, reaction and desorption of atoms andmolecules1,2. However, it is difficult to say that our understanding of ASW itself, which serves
as the site for various physical and chemical processes, is sufficiently advanced.
ASW films formed by vapor deposition onto a cold surface are known to have negative surface
potentials due to spontaneous polarization, which varies depending on the temperature and film
thickness3,4. However, the mechanisms underlying this polarization, responsible for the surface
potentials, remain unclear. Although previous studies that used Kelvin probe measurements
have proposed the polarization model of ASW named tilted dipole model, the direct evidence
confirming validity of this model has not been obtained yet. One of the major reasons is the low
spatial resolution in the measurement of contact potential difference by the Kelvin probe
method. To overcome this limitation in spatial resolution, we used FM-AFM to measure
variations in the negative surface voltage of ASW with nanoscale resolution depending on thesurface structures.
We conducted measurements on two types of ASW formed by water vapor deposition at 15 and100K on a Si (111) 7×7 substrate in ultra-high vacuum conditions. The topographic images ofthe surface structure showed temperature dependence, attributed to the differences in diffusion
length of water molecules on the surface until reaching stabilization at the measurement
temperature. We also measured the local contact potential difference between probe tip and 400measurement points set on ASW surface within a 200 nm x 200 nm are, respectively. On both
ASW surfaces, the local contact potential difference varied at each measurement point without
correlation with local thickness (topographic height). This suggests that this polarization
phenomenon cannot simply attributed to the thickness of ASW but depends on the localstructure. Our observations with nanoscale resolution imply that the tilted dipole model is a
plausible representation of the spontaneous polarization of ASW.

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