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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