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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Atomic force microscopy | 6/9 | https://en.wikipedia.org/wiki/Atomic_force_microscopy | reference | science, encyclopedia | 2026-05-05T10:03:47.482245+00:00 | kb-cron |
== Probe == An AFM probe has a sharp tip on the free-swinging end of a cantilever that protrudes from a holder. The dimensions of the cantilever are in the scale of micrometers. The radius of the tip is usually on the scale of a few nanometers to a few tens of nanometers. (Specialized probes exist with much larger end radii, for example probes for indentation of soft materials.) The cantilever holder, also called the holder chip—often 1.6 mm by 3.4 mm in size—allows the operator to hold the AFM cantilever/probe assembly with tweezers and fit it into the corresponding holder clips on the scanning head of the atomic force microscope. This device is most commonly called an "AFM probe", but other names include "AFM tip" and "cantilever" (employing the name of a single part as the name of the whole device). An AFM probe is a particular type of SPM probe. AFM probes are manufactured with MEMS technology. Most AFM probes used are made from silicon (Si), but borosilicate glass and silicon nitride are also in use. AFM probes are considered consumables as they are often replaced when the tip apex becomes dull or contaminated or when the cantilever is broken. They can cost from a couple of tens of dollars up to hundreds of dollars per cantilever for the most specialized cantilever/probe combinations. To use the device, the tip is brought very close to the surface of the object under investigation, and the cantilever is deflected by the interaction between the tip and the surface, which is what the AFM is designed to measure. A spatial map of the interaction can be made by measuring the deflection at many points on a 2D surface. Several types of interaction can be detected. Depending on the interaction under investigation, the surface of the tip of the AFM probe needs to be modified with a coating. Among the coatings used are gold – for covalent bonding of biological molecules and the detection of their interaction with a surface, diamond for increased wear resistance and magnetic coatings for detecting the magnetic properties of the investigated surface. Another solution exists to achieve high resolution magnetic imaging: equipping the probe with a microSQUID. The AFM tips are fabricated using silicon micro machining and the precise positioning of the microSQUID loop is achieved using electron beam lithography. The additional attachment of a quantum dot to the tip apex of a conductive probe enables surface potential imaging with high lateral resolution, scanning quantum dot microscopy. The surface of the cantilevers can also be modified. These coatings are mostly applied in order to increase the reflectance of the cantilever and to improve the deflection signal.
== Forces as a function of tip geometry == The forces between the tip and the sample strongly depend on the geometry of the tip. Various studies were exploited in the past years to write the forces as a function of the tip parameters. Among the different forces between the tip and the sample, the water meniscus forces are highly interesting, both in air and liquid environment. Other forces must be considered, like the Coulomb force, van der Waals forces, double layer interactions, solvation forces, hydration and hydrophobic forces.
=== Water meniscus === Water meniscus forces are highly interesting for AFM measurements in air. Due to the ambient humidity, a thin layer of water is formed between the tip and the sample during air measurements. The resulting capillary force gives rise to a strong attractive force that pulls the tip onto the surface. In fact, the adhesion force measured between tip and sample in ambient air of finite humidity is usually dominated by capillary forces. As a consequence, it is difficult to pull the tip away from the surface. For soft samples including many polymers and in particular biological materials, the strong adhesive capillary force gives rise to sample degradation and destruction upon imaging in contact mode. Historically, these problems were an important motivation for the development of dynamic imaging in air (e.g. "tapping mode"). During tapping mode imaging in air, capillary bridges still form. Yet, for suitable imaging conditions, the capillary bridges are formed and broken in every oscillation cycle of the cantilever normal to the surface, as can be inferred from an analysis of cantilever amplitude and phase vs. distance curves. As a consequence, destructive shear forces are largely reduced and soft samples can be investigated. In order to quantify the equilibrium capillary force, it is necessary to start from the Laplace equation for pressure:
P
=
γ
L
(
1
r
1
+
1
r
0
)
≃
γ
L
r
e
f
f
{\displaystyle P=\gamma _{L}\left({\frac {1}{r}}_{1}+{\frac {1}{r}}_{0}\right)\simeq {\frac {\gamma _{L}}{r_{eff}}}}
where γL, is the surface energy and r0 and r1 are defined in the figure. The pressure is applied on an area of
A
≃
2
π
R
≃
[
r
e
f
f
(
1
+
cos
θ
)
+
h
]
{\displaystyle A\simeq 2\pi R\simeq [r_{eff}(1+\cos \theta )+h]}
where θ is the angle between the tip's surface and the liquid's surface while h is the height difference between the surrounding liquid and the top of the miniscus. The force that pulls together the two surfaces is
F
=
2
π
R
γ
L
(
1
+
cos
θ
+
h
r
e
f
f
)
{\displaystyle F=2\pi R\gamma _{L}\left(1+\cos \theta +{\frac {h}{r_{eff}}}\right)}
The same formula could also be calculated as a function of relative humidity. Gao calculated formulas for different tip geometries. As an example, the force decreases by 20% for a conical tip with respect to a spherical tip. When these forces are calculated, a difference must be made between the wet on dry situation and the wet on wet situation. For a spherical tip, the force is: