Probabilistic CTL: Difference between revisions
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'''Electrostatic force microscopy''' (EFM) is a type of dynamic non-contact [[atomic force microscopy]] where the electrostatic force is probed. ("Dynamic" here means that the [[cantilever]] is [[oscillating]] and does not make contact with the sample). This force arises due to the attraction or [[repulsion]] of separated [[charge (physics)|charge]]s. It is a long-ranged force and can be detected 100 [[nanometer|nm]] from the sample. For example, consider a conductive cantilever tip and sample which are separated a distance ''z'' usually by a vacuum. A bias voltage between tip and sample is applied by an external battery forming a capacitor, ''C'', between the two. The capacitance of the system depends on the geometry of the tip and sample. The total energy stored in that capacitor is ''U = ½ C⋅ΔV<sup>2</sup>''. The work done by the battery to maintain a constant voltage, ''ΔV'', between the capacitor plates (tip and sample) is ''-2U''. By definition, taking the negative gradient of the total energy ''U<sub>total</sub> = -U'' gives the force. The ''z'' component of the force (the force along the axis connecting the tip and sample) is thus: | |||
<math>F_{electrostatic} = \frac{1}{2} \frac{\partial C}{\partial z} \Delta V^2 </math>. | |||
Since ''{{frac|∂C|∂z}} < 0'' this force is always attractive. The electrostatic force can be probed by changing the voltage, and that force is parabolic with respect to the voltage. One note to make is that ''ΔV'' is not simply the voltage difference between the tip and sample. Since the tip and sample are often not the same material, and furthermore can be subject to trapped charges, debris, etc., there is a difference between the [[work function]]s of the two. This difference, when expressed in terms of a voltage, is called the contact potential difference, ''V<sub>CPD</sub>'' This causes the apex of the parabola to rest at ''ΔV = V<sub>tip</sub> − V<sub>sample</sub> − V<sub>CPD</sub> = 0''. Typically, the value of ''V<sub>CPD</sub>'' is on the order of a few hundred [[millivolt]]s. Forces as small as [[piconewton]]s can routinely be detected with this method. | |||
With an electrostatic force microscope, like the [[atomic force microscope]] it is based on, the sample can be immersed in liquid. | |||
== See also == | |||
* [[Kelvin probe force microscopy]] — a scanning probe microscopy technique very similar to EFM, except with emphasis on the measurement of ''V<sub>CPD</sub>''. | |||
== References == | |||
L. Kantorovich, A. Livshits, and M. Stoneham, J. Phys.:Condens. Matter 12, 795 (2000) | |||
* [http://www.pnas.org/cgi/content/abstract/92/22/10384 "Electrostatic Force Microscope for Probing Surface Charges in Aqueous Solutions"] by S. Xu and M.F. Arnsdorf | |||
{{SPM2}} | |||
[[Category:Scanning probe microscopy]] | |||
Latest revision as of 14:53, 21 June 2013
Electrostatic force microscopy (EFM) is a type of dynamic non-contact atomic force microscopy where the electrostatic force is probed. ("Dynamic" here means that the cantilever is oscillating and does not make contact with the sample). This force arises due to the attraction or repulsion of separated charges. It is a long-ranged force and can be detected 100 nm from the sample. For example, consider a conductive cantilever tip and sample which are separated a distance z usually by a vacuum. A bias voltage between tip and sample is applied by an external battery forming a capacitor, C, between the two. The capacitance of the system depends on the geometry of the tip and sample. The total energy stored in that capacitor is U = ½ C⋅ΔV2. The work done by the battery to maintain a constant voltage, ΔV, between the capacitor plates (tip and sample) is -2U. By definition, taking the negative gradient of the total energy Utotal = -U gives the force. The z component of the force (the force along the axis connecting the tip and sample) is thus:
.
Since Template:Frac < 0 this force is always attractive. The electrostatic force can be probed by changing the voltage, and that force is parabolic with respect to the voltage. One note to make is that ΔV is not simply the voltage difference between the tip and sample. Since the tip and sample are often not the same material, and furthermore can be subject to trapped charges, debris, etc., there is a difference between the work functions of the two. This difference, when expressed in terms of a voltage, is called the contact potential difference, VCPD This causes the apex of the parabola to rest at ΔV = Vtip − Vsample − VCPD = 0. Typically, the value of VCPD is on the order of a few hundred millivolts. Forces as small as piconewtons can routinely be detected with this method.
With an electrostatic force microscope, like the atomic force microscope it is based on, the sample can be immersed in liquid.
See also
- Kelvin probe force microscopy — a scanning probe microscopy technique very similar to EFM, except with emphasis on the measurement of VCPD.
References
L. Kantorovich, A. Livshits, and M. Stoneham, J. Phys.:Condens. Matter 12, 795 (2000)
- "Electrostatic Force Microscope for Probing Surface Charges in Aqueous Solutions" by S. Xu and M.F. Arnsdorf