A novel AFM technique for simultaneous non-contact imaging of topography and surface potential in solution

Open Access
Author:
Marchand, David Joseph
Graduate Program:
Chemical Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
May 06, 2011
Committee Members:
  • Seong Han Kim, Thesis Advisor
  • Seong H Kim, Thesis Advisor
Keywords:
  • scanning polarization force microscopy
  • non-contact imaging
  • atomic force microscope
  • electrostatic actuation
Abstract:
A novel Atomic Force Microscopy (AFM) technique is presented that can simultaneously image surface topography and local electrical potential in liquid without mechanical contact between the AFM tip and the sample. This new technique is called Liquid Environment Electroactuation Force Microscopy (LEEFM). The operation of LEEFM is similar to Scanning Polarization Force Microscopy (SPFM) in air. An AC voltage at frequency ω is applied to the AFM tip/cantilever, generating an electrical potential difference between the tip and sample. The electrical double layer does not screen out the electrostatic interactions, even in water containing added ions, because the voltage is switching polarity too fast for double layer to fully form. The resulting capacitive coupling causes an attractive force between the cantilever and sample, so the cantilever vibrates at the applied frequency (1ω) and twice this frequency (2ω), with a distinct vibration amplitude at each frequency. Non-contact imaging of topography is accomplished by feedback controlling the amplitude of the 2ω vibration, which depends on the tip-sample separation in the range of 1~10 nm. The 1ω vibration amplitude and phase shift are simultaneously collected to form maps of the properties of the surface. The exact relationships between the surface properties (e.g. charge) and the cantilever vibration behavior are not yet fully understood.