Atomic Force Microscopy-Based Force Spectroscopy …
595
Here, when approaching the sample, the viscous forces pull the cantilever upwards
and then cause it to twist downwards as the tip is removed from the sample.
5.2 Various Theories of Non-Contact Region
The force-distance curve comprises of two separate sections, as explained earlier
in this thread: an approach curve and a retraction curve. The approach curve, also
known as jump-to-contact, happens only if the tip-sample force gradient surpasses the
cantilever’s elastic constant. This jump-to-contact, is a most essential characteristic
of the approach curve, can arise from a region of repulsive forces (double-layer,
steric, hydration, and Van der Waals force in liquids) or attractive forces (Coulomb
or Van der Waals) [20]. We may also conclude that this area provides essential and
fundamental knowledge about the attractive forces between the tip and the sample.
This attractive force’s maximal value is equal to cantilever deflection multiplied
by the cantilever constant at jump-to-contact. Any theory that takes into account
attractive forces, like JKR or Maugis, can evaluate this force. The intrinsic stiffness of
the tip and sample materials causes instability at Jump-to-contact. This phenomena
have been successfully demonstrated by the Pethica and Sutton [24]. Molecular
Dynamics (MD) simulations or Lennard–Jones potentials can also be employed to
predict this instability. This happens only, at a slight separation (1–2 Å), when the
gradient of the surface forces surpasses the gradient of the restored elastic force of the
body [20]. This instability was also observed in MD simulations, when the specimen
is 4.2 Å away from the tip. The distance decreases from 4.2 to 2.1 Å when the tip
jumps to the surface, followed by partial wetting of the bottom of the tip by gold
substrate atoms. This partial wetting of tip is induced by adhesion [20]. Contrary
to jump to contact, jump-off contact occurs during retraction of the tip when the
cantilever’s elastic constant exceeds the gradient of tip-sample adhesive forces. This
jump-off contact is directly related to the tip and the sample surface energies in terms
of the material’s dimensions, stiffness, and adhesion. The phenomena related to jumpoff contact like deflection and distance are always greater than the jump-to-contact
deflection as well as distance. This occurs due to: (1) certain chemical, or adhesive
bonds that can cause emergence of non-conservative forces during the contact, (2)
contact area increases owing to elastic deformation of the specimen around the tip,
and (3) forces imposed by meniscus layers of liquid impurities act in opposition to
the pull-off [20]. Scientists have already measured the pull-off force and adhesion
energy both in air and in water and shown its dependencies on the loading force [20].
In addition to using it as a high-resolution imaging instrument, AFM forcedistance curves may be utilized to investigate useful mechanical property of the
specimen by measuring binding differences between the AFM tip and the sample.
595
Here, when approaching the sample, the viscous forces pull the cantilever upwards
and then cause it to twist downwards as the tip is removed from the sample.
5.2 Various Theories of Non-Contact Region
The force-distance curve comprises of two separate sections, as explained earlier
in this thread: an approach curve and a retraction curve. The approach curve, also
known as jump-to-contact, happens only if the tip-sample force gradient surpasses the
cantilever’s elastic constant. This jump-to-contact, is a most essential characteristic
of the approach curve, can arise from a region of repulsive forces (double-layer,
steric, hydration, and Van der Waals force in liquids) or attractive forces (Coulomb
or Van der Waals) [20]. We may also conclude that this area provides essential and
fundamental knowledge about the attractive forces between the tip and the sample.
This attractive force’s maximal value is equal to cantilever deflection multiplied
by the cantilever constant at jump-to-contact. Any theory that takes into account
attractive forces, like JKR or Maugis, can evaluate this force. The intrinsic stiffness of
the tip and sample materials causes instability at Jump-to-contact. This phenomena
have been successfully demonstrated by the Pethica and Sutton [24]. Molecular
Dynamics (MD) simulations or Lennard–Jones potentials can also be employed to
predict this instability. This happens only, at a slight separation (1–2 Å), when the
gradient of the surface forces surpasses the gradient of the restored elastic force of the
body [20]. This instability was also observed in MD simulations, when the specimen
is 4.2 Å away from the tip. The distance decreases from 4.2 to 2.1 Å when the tip
jumps to the surface, followed by partial wetting of the bottom of the tip by gold
substrate atoms. This partial wetting of tip is induced by adhesion [20]. Contrary
to jump to contact, jump-off contact occurs during retraction of the tip when the
cantilever’s elastic constant exceeds the gradient of tip-sample adhesive forces. This
jump-off contact is directly related to the tip and the sample surface energies in terms
of the material’s dimensions, stiffness, and adhesion. The phenomena related to jumpoff contact like deflection and distance are always greater than the jump-to-contact
deflection as well as distance. This occurs due to: (1) certain chemical, or adhesive
bonds that can cause emergence of non-conservative forces during the contact, (2)
contact area increases owing to elastic deformation of the specimen around the tip,
and (3) forces imposed by meniscus layers of liquid impurities act in opposition to
the pull-off [20]. Scientists have already measured the pull-off force and adhesion
energy both in air and in water and shown its dependencies on the loading force [20].
In addition to using it as a high-resolution imaging instrument, AFM forcedistance curves may be utilized to investigate useful mechanical property of the
specimen by measuring binding differences between the AFM tip and the sample.