Adsorption of H₂O on Pt Field Emitter: Surface Diffusion and Field-Induced Effects

Abstract

Adsorption of an Η2 O layer onto a Pt field emitter tip under the influence of an electric field and the surface dichusioii of water were studied by using the field electron microscopy method. The presence of a negative field (field electron microscopy mode of operation), examined in the range of 33-44 MV/cm, significantly reduced the water coverage on the emitter at temperatures above 120 K. The reduction could also occur upon drawing a high density field emission current when the emitter was kept at 78 K. Surface dichusion of water, which was observed in the temperature range 120-132 K, corresponded to the "unrolling the carpet" mechanism and started from a water multilayer to a surface region water-and/or hydrogeri-submonolayer covered. This was accompanied by the transitioii process from the state of the current-and field-induced redistribution of Η2O to the state of thermal equilibrium. The activatioii energy of the dichusioii was found to be 19 and 25 kJ/mol depending oii the crystallographic direction. A positive electric field of 44 MV/cm, which was applied at temperatures of the substrate in excess of 121 K, decreased the field emission of the system and raised the desorptioii temperature of the layer over 720 K. It is assumed that the negative electric field causes reorientation of Η2 O molecules at the surface of platinum tip. The positive as well as negative electric fields promote the field desorption of water, which is in accordance with the results reported before.