AVS 57th International Symposium & Exhibition
    Surface Science Thursday Sessions
       Session SS-ThA

Paper SS-ThA3
Ion Segregation and Deliquescence of Alkali Halide Nanocrystals on SiO2 Revealed by Scanning Polarization Force Microscopy

Thursday, October 21, 2010, 2:40 pm, Room Picuris

Session: Environmental Interfaces
Presenter: K. Arima, Osaka University, Japan
Authors: K. Arima, Osaka University, Japan
P. Jiang, Lawrence Berkeley National Laboratory
D.-S. Lin, National Tsing Hua University, Taiwan, Republic of China
A. Verdaguer, CSIC-ICN, Spain
H. Bluhm, Lawrence Berkeley National Laboratory
M. Salmeron, Lawrence Berkeley National Laboratory
Correspondent: Click to Email
The adsorption of water on alkali halide (KBr, KCl, KF, NaCl) nanocrystals on SiO2 and their deliquescence was investigated as a function of relative humidity (RH) from 8% to near saturation by scanning polarization force microscopy (SPFM). SPFM is a noncontact AFM operation mode based on electrostatic forces, which gives two images (topographic image modulated by a local dielectric constant and surface potential image) simultaneously. At low humidity water adsorption solvates ions at the surface of the crystals and increases their mobility. This results in a large increase in the dielectric constant, which is manifested in an increase in the electrostatic force and in an increase in the apparent height of the nanocrystals. Above a critical RH (58% RH in the case of KBr nanocrystals) the diffusion of ions leads to Ostwald ripening, where larger nanocrystals grow at the expense of the smaller ones. At the deliquescence point droplets were formed. For KBr, KCl and NaCl, the droplets exhibit a negative surface potential relative to the surrounding region, indicative of the preferential segregation of anions to the air-solution interface. We will also show spectra obtained by ambient pressure X-ray photoelectron spectroscopy to complement the SPFM results.