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1. Atomic force microscope
2. Atomic nanoscope
3. Atom probe
4. Ballistic conduction
5. Bingel reaction
6. Biomimetic
7. Bio-nano generator
8. Bionanotechnology
9. Break junction
10. Brownian motor
11. Bulk micromachining
12. Cantilever
13. Carbon nanotube
14. Carbyne
15. CeNTech
16. Chemical Compound Microarray
17. Cluster
18. Colloid
19. Comb drive
20. Computronium
22. Diamondoids
23. Dielectrophoresis
24. Dip Pen Nanolithography
25. DNA machine
26. Ecophagy
27. Electrochemical scanning tunneling microscope
28. Electron beam lithography
29. Electrospinning
30. Engines of Creation
31. Exponential assembly
32. Femtotechnology
33. Fermi point
34. Fluctuation dissipation theorem
35. Fluorescence interference contrast microscopy
36. Fullerene
37. Fungimol
38. Gas cluster ion beam
39. Grey goo
40. Hacking Matter
41. History of nanotechnology
42. Hydrogen microsensor
43. Inorganic nanotube
44. Ion-beam sculpting
45. Kelvin probe force microscope
46. Lab-on-a-chip
47. Langmuir-Blodgett film
48. LifeChips
49. List of nanoengineering topics
50. List of nanotechnology applications
51. List of nanotechnology topics
52. Lotus effect
53. Magnetic force microscope
54. Magnetic resonance force microscopy
55. Mechanochemistry
56. Mechanosynthesis
57. MEMS thermal actuator
58. Mesotechnology
59. Micro Contact Printing
60. Microelectromechanical systems
61. Microfluidics
62. Micromachinery
63. Molecular assembler
64. Molecular engineering
65. Molecular logic gate
66. Molecular manufacturing
67. Molecular motors
68. Molecular recognition
69. Molecule
70. Nano-abacus
71. Nanoart
72. Nanobiotechnology
73. Nanocar
74. Nanochemistry
75. Nanocomputer
76. Nanocrystal
77. Nanocrystalline silicon
78. Nanocrystal solar cell
79. Nanoelectrochemistry
80. Nanoelectrode
81. Nanoelectromechanical systems
82. Nanoelectronics
83. Nano-emissive display
84. Nanoengineering
85. Nanoethics
86. Nanofactory
87. Nanoimprint lithography
88. Nanoionics
89. Nanolithography
90. Nanomanufacturing
91. Nanomaterial based catalyst
92. Nanomedicine
93. Nanomorph
94. Nanomotor
95. Nano-optics
96. Nanoparticle
97. Nanoparticle tracking analysis
98. Nanophotonics
99. Nanopore
100. Nanoprobe
101. Nanoring
102. Nanorobot
103. Nanorod
104. Nanoscale
105. Nano-Science Center
106. Nanosensor
107. Nanoshell
108. Nanosight
109. Nanosocialism
110. Nanostructure
111. Nanotechnology
112. Nanotechnology education
113. Nanotechnology in fiction
114. Nanotoxicity
115. Nanotube
116. Nanovid microscopy
117. Nanowire
118. National Nanotechnology Initiative
119. Neowater
120. Niemeyer-Dolan technique
121. Ormosil
122. Photolithography
123. Picotechnology
124. Programmable matter
125. Quantum dot
126. Quantum heterostructure
127. Quantum point contact
128. Quantum solvent
129. Quantum well
130. Quantum wire
131. Richard Feynman
132. Royal Society's nanotech report
133. Scanning gate microscopy
134. Scanning probe lithography
135. Scanning probe microscopy
136. Scanning tunneling microscope
137. Scanning voltage microscopy
138. Self-assembled monolayer
139. Self-assembly
140. Self reconfigurable
141. Self-Reconfiguring Modular Robotics
142. Self-replication
143. Smart dust
144. Smart material
145. Soft lithography
146. Spent nuclear fuel
147. Spin polarized scanning tunneling microscopy
148. Stone Wales defect
149. Supramolecular assembly
150. Supramolecular chemistry
151. Supramolecular electronics
152. Surface micromachining
153. Surface plasmon resonance
154. Synthetic molecular motors
155. Synthetic setae
156. Tapping AFM
157. There's Plenty of Room at the Bottom
158. Transfersome
159. Utility fog

NANOTECHNOLOGY
http://en.wikipedia.org/wiki/Kelvin_probe_force_microscope

All text is available under the terms of the GNU Free Documentation License: http://en.wikipedia.org/wiki/Wikipedia:Text_of_the_GNU_Free_Documentation_License

# Kelvin probe force microscope

In Kelvin probe force microscopy, a conducting cantilever is scanned over a surface at a constant height in order to map the work function of the surface.

Kelvin probe force microscopy (KPFM), also known as surface potential microscopy, is a noncontact variant of atomic force microscopy (AFM) that was invented in 1991. With KPFM, the work function of surfaces can be observed at atomic or molecular scales. The work function relates to many surface phenomena, including catalytic activity, reconstruction of surfaces, doping and band-bending of semiconductors, charge trapping in dielectrics and corrosion. The map of the work function produced by KPFM gives information about the composition and electronic state of the local structures on the surface of a solid.

KPFM is a scanned probe method where the potential offset between a probe tip and a surface can be measured using the same principle as a macroscopic Kelvin probe. The cantilever in the AFM is a reference electrode that forms a capacitor with the surface, over which it is scanned laterally at a constant separation. The cantilever is not piezoelectrically driven at its mechanical resonance frequency ω0 as in normal AFM although an alternating current (AC) voltage is applied at this frequency.

When there is a direct-current (DC) potential difference between the tip and the surface, the AC+DC voltage offset will cause the cantilever to vibrate. The origin of the force can be understood by considering that the energy of the capacitor formed by the cantilever and the surface is

$E = \frac{1}{2}C[V_{dc} + V_{ac}sin(\omega_0 t)]^2 = \frac{1}{2}C[2V_{dc}V_{ac}sin(\omega_0 t) - \frac{1}{2}V_{ac}^2 cos(2\omega_0 t)]$

plus terms at dc. Only the cross-term proportional to the $V_{dc} \cdot V_{ac}$ product is at the resonant frequency ω0. The resulting vibration of the cantilever is detected using usual scanned-probe microscopy methods (typically involving a diode laser and a four-quadrant detector). A null circuit is used to drive the DC potential of the tip to a value which minimizes the vibration. A map of this nulling DC potential versus the lateral position coordinate therefore produces an image of the work function of the surface.

A related technique, electrostatic force microscopy (EFM), directly measures the force produced on a charged tip by the electric field emanating from the surface. EFM operates much like magnetic force microscopy in that the frequency shift or amplitude change of the cantilever oscillation is used to detect the electric field. However, EFM is much more sensitive to topographic artifacts than KFPM and has not proven as useful. Both EFM and KPFM require the use of conductive cantilevers, typically metal-coated silicon or silicon nitride.

## Working Principle

The Kelvin probe force microscope or Kelvin force microscope (KFM) is based on an AFM set-up and the determination of the work function is based on the measurement of the electrostatic forces between the small AFM tip and the sample. The conducting tip and the sample are characterised by (in general) different work functions. When both elements are brought in contact, a net electric current will flow between them until the Fermi levels are aligned. The potential is called the contact potential (difference) denoted generally with VCPD. An electrostatic force between tip and sample builds up, resulting from the net charge transfer. For the measurement a voltage is applied between tip and sample, consisting of a DC-bias VDC and an AC-voltage VAC = sin(ω2t) of frequency ω2 at the second resonance frequency of the AFM cantilever

V = (VDCVCPD) + VACsin(ω2t)

Tuning the AC-frequency to the second resonance frequency of the cantilever results in an improved sensitivity and allows the independent and simultaneous imaging of topography and the contact potential. As a result of these biasing conditions, an oscillating electrostatic force appears, inducing an additional oscillation of the cantilever with the characteristic frequency ω2. The general expression of such electrostatic force not considering coulomb forces due to charges can be written as

$F = \frac{1}{2} \frac{dC}{dz} V^2$

The electrostatic force can be split up into three contributions, as the total electrostatic force F acting on the tip has spectral components at the frequencies ω2 and 2.

$F = F_{DC} + F_{\omega_2} + F_{2 \omega_2}$

The DC component, FDC, contributes to the topographical signal, the term $F_{\omega_2}$ at the characteristic frequency ω2 is used to measure the contact potential and the contribution $F_{2\omega_{2}}$ can be used for capacitance microscopy.

$F_{DC} = - \frac{dC}{dz} [\frac{1}{2}(V_{DC} - V_{CPD})^2 + \frac{1}{4} V^2_{AC}]$

$F_{\omega_2} = - \frac{dC}{dz} [V_{DC} - V_{CPD}] V_{AC} \sin(\omega_2 t)$

$F_{2 \omega_2} = + \frac{1}{4} \frac{dC}{dz} V^2_{AC} \cos(2 \omega_2 t)$

For contact potential measurements a lock-in amplifier is used to detect the cantilever oscillation at ω2. During the scan VDC will be adjusted so that the electrostatic forces between the tip and the sample become zero and thus the oscillation amplitude of the cantilever at the frequency ω2 becomes zero. Since the electrostatic force at ω2 depends on VDCVCPD, VDC corresponds to the contact potential. Absolute values of the sample work function can be obtained if the tip is first calibrated against a reference sample of known work function. Apart from this, one can use the normal topographic scan methods at the resonance frequency ω independently of the above. Thus, in one scan, the topography and the contact potential of the sample are determined simultaneously.