Scanning Electron Microscopes and Tunnelling Microscopes
Transcript of Scanning Electron Microscopes and Tunnelling Microscopes
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Scanning electron microscopes and tunnelling microscopes
Scanning electron microscopes
A Scanning electron microscope allows us to see things
that are smaller than a wavelength of light. It scans an
object using a high energy beam of electrons which are
fired from above and down onto the object below. To
prevent the electrons from bouncing off air particles
and altering their direction, the process is done in a
vacuum. Circular electromagnets are used to focus the
electrons at a specific spot on the object. These electrons hit the object and cause the
object to give off electrons of its own. The electrons emitted by the object are attracted to
the collector. Some electrons pass through the collector and land on the fluorescent screen
which gives off light as the electrons hit. To ensure the electrons are emitted, the object is
often plated with a fine coating of metal. The angle of fire can be changed to hit other areas
of the object.
Scanning tunnelling microscope
A scanning tunnelling microscope is allows us to see and
manipulate individual atoms. It
a voltage bias is applied and the tip is brought close to the
sample by some coarse sample-to-tip control, which is turnedoff when the tip and sample are sufficiently close. At close
range, fine control of the tip in all three dimensions when near
the sample is typicallypiezoelectric, maintaining tip-sample
separation W typically in the 4-7range, which is the equilibrium position between attractive
(3
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If the tip is moved across the sample in the x-y plane, the changes in surface height and density of
states cause changes in current. These changes are mapped in images. This change in current with
respect to position can be measured itself, or the height, z, of the tip corresponding to a constant
current can be measured.[4]These two modes are called constant height mode and constant current
mode, respectively. In constant current mode, feedback electronics adjust the height by a voltage to
the piezoelectric height control mechanism.[5]This leads to a height variation and thus the image
comes from the tip topography across the sample and gives a constant charge density surface; this
means contrast on the image is due to variations in charge density.[6]In constant height mode, the
voltage and height are both held constant while the current changes to keep the voltage from
changing; this leads to an image made of current changes over the surface, which can be related to
charge density.[6]The benefit to using a constant height mode is that it is faster, as the piezoelectric
movements require more time to register the height change in constant current mode, than the
voltage change in constant height mode.[6]All images produced by STM are grayscale, with color
optionally added in post-processing in order to visually emphasize important features.
In addition to scanning across the sample, information on the electronic structure at a given location in
the sample can be obtained by sweeping voltage and measuring current at a specific location.[3]This
type of measurement is calledscanning tunneling spectroscopy(STS) and typically results in a plot of
the localdensity of statesas a function of energy within the sample. The advantage of STM over
other measurements of the density of states lies in its ability to make extremely local measurements:
for example, the density of states at animpuritysite can be compared to the density of states far from
impurities.[7]
Framerates of at least 1 Hz enable so called Video-STM (up to 50 Hz is possible).[8][9]This can be
used to scan surfacediffusion.[10]
[edit]Instrumentation
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