Scanning tunneling microscopy study of water molecules on Pd(110) at cryogenic temperature

We show scanning tunneling microscopy (STM) images of water molecules on Pd(110) at a cryogenic temperature of 4.7 K which enables the observation of isolated water molecules. We observe water-induced features in the STM images in a low-coverage region (∼ 0.02 ML), which can be classified in terms of height; namely, (A) 0.15 Å, (B) 0.3 Å and (C) 0.55 Å, observed at V_s = -100 mV. In a high-coverage region, feature C dominates and forms a local c(2 × 2) structure. Since the ordered structure is identical with the previously reported superstructure of bilayer water on Pd(110), feature C should correspond to a tetramer of water molecules, a unit of bilayer water. Feature A is located on the Pd(110) row. Since feature A is the smallest one, we consider that this corresponds to the monomer of the water molecule whose existence has been observed previously at temperatures below 10 K by electron energy loss spectroscopy (EELS) measurement.