+ All Categories
Home > Documents > SCANNING TUNNELING MICROSCOPE & ATOMIC FORCE MICROSCOPY

SCANNING TUNNELING MICROSCOPE & ATOMIC FORCE MICROSCOPY

Date post: 30-May-2018
Category:
Upload: karthick-n
View: 229 times
Download: 0 times
Share this document with a friend

of 32

Transcript
  • 8/9/2019 SCANNING TUNNELING MICROSCOPE & ATOMIC FORCE MICROSCOPY

    1/32

  • 8/9/2019 SCANNING TUNNELING MICROSCOPE & ATOMIC FORCE MICROSCOPY

    2/32

    Gerd BinnigJuly 20, 1947

    German IBM Zurich

    scanning tunneling microscope (1981)

    Nobel Prize in Physics (1986) shared withErnst Ruska (born 1906 TEM 1933)

    http://en.wikipedia.org/wiki/File:Gerd_Binnig_sw.jpg
  • 8/9/2019 SCANNING TUNNELING MICROSCOPE & ATOMIC FORCE MICROSCOPY

    3/32

    Invention of the STM, 1981

  • 8/9/2019 SCANNING TUNNELING MICROSCOPE & ATOMIC FORCE MICROSCOPY

    4/32

    SPM Timeline

  • 8/9/2019 SCANNING TUNNELING MICROSCOPE & ATOMIC FORCE MICROSCOPY

    5/32

    Multimode SPM

  • 8/9/2019 SCANNING TUNNELING MICROSCOPE & ATOMIC FORCE MICROSCOPY

    6/32

    PRINCIPLE OF STM The STM is based on the concept of quantum

    tunneling. When a conducting tip is brought very near to the

    surface to be examined, a bias (voltage difference)applied between the two can allow electrons to

    tunnel through the vacuum between them. The resulting tunneling currentis a function of tipposition, applied voltage, and the local density ofstates (LDOS) of the sample

    Information is acquired by monitoring the current as

    the tip's position scans across the surface, and isusually displayed in image form.

  • 8/9/2019 SCANNING TUNNELING MICROSCOPE & ATOMIC FORCE MICROSCOPY

    7/32

    http://upload.wikimedia.org/wikipedia/commons/f/f9/ScanningTunnelingMicroscope_schematic.png
  • 8/9/2019 SCANNING TUNNELING MICROSCOPE & ATOMIC FORCE MICROSCOPY

    8/32

    STM STM can be a challenging technique, as it

    requires extremely clean and stable surfaces,sharp tips, excellent vibration control, andsophisticated electronics.

    Resolution is considered to be 0.1 nm lateralresolution and 0.01 nm depth resolution

    Individual atoms within materials are imagedand manipulated.

    STM can be used in ultra high vacuum, air,water, and various other liquid or gas ambients,and

    at temperatures ranging from near zero kelvinsto a few hundred degrees Celsius

  • 8/9/2019 SCANNING TUNNELING MICROSCOPE & ATOMIC FORCE MICROSCOPY

    9/32

    STM - PROCEDURE First, a voltage bias is applied and the tip is brought

    close to the sample by some coarse sample-to-tipcontrol, which is turned off when the tip and sampleare sufficiently close.

    At close range, fine control of the tip in all threedimensions when near the sample is typically

    piezoelectric, maintaining tip-sample separation Wtypically in the 4-7 range, which is the equilibriumposition between attractive (3

  • 8/9/2019 SCANNING TUNNELING MICROSCOPE & ATOMIC FORCE MICROSCOPY

    10/32

    STM - PROCEDURE

    If the tip is moved across the sample in the x-y plane, thechanges in surface height and density of states causechanges in current. These changes are mapped inimages.

    This change in current with respect to position can bemeasured itself, or the height, z, of the tip correspondingto a constant current can be measured.

    These two modes are called constant height mode andconstant current mode, respectively.

    In constant current mode, feedback electronics adjustthe height by a voltage to the piezoelectric height controlmechanism[

  • 8/9/2019 SCANNING TUNNELING MICROSCOPE & ATOMIC FORCE MICROSCOPY

    11/32

    Atomic force microscopy The precursor to the AFM, the scanning tunneling

    microscope The AFM consists of a cantilever with a sharp tip

    (probe) at its end that is used to scan the specimensurface. The cantilever is typically silicon or siliconnitride with a tip radius of curvature on the order ofnanometers. When the tip is brought into proximity ofa sample surface, forces between the tip and thesample lead to a deflection of the cantilever

    Typically, the deflection is measured using a laser

    spot reflected from the top surface of the cantileverinto an array of photodiodes

  • 8/9/2019 SCANNING TUNNELING MICROSCOPE & ATOMIC FORCE MICROSCOPY

    12/32

    Force

    Surface Atoms

    Tip Atoms

    Photo detector

    Laser Beam

    Tip

    Cantilever

    Line Scan

    Surface

    Working Principle of AFM

    www.shef.ac.uk/~htsl/afm.htm

  • 8/9/2019 SCANNING TUNNELING MICROSCOPE & ATOMIC FORCE MICROSCOPY

    13/32

  • 8/9/2019 SCANNING TUNNELING MICROSCOPE & ATOMIC FORCE MICROSCOPY

    14/32

  • 8/9/2019 SCANNING TUNNELING MICROSCOPE & ATOMIC FORCE MICROSCOPY

    15/32

    Mode of Operation Force of Interaction

    Contact mode Strong (repulsive) - constant force or constant distance

    Non-contact mode Weak (attractive) - vibrating probe

    Intermittent contact mode Strong (repulsive) - vibrating probe

    Lateral force mode Frictional forces exert a torque on the scanning cantilever

    Magnetic force Magnetic field of the surface is imaged

    Thermal scanning Distribution of thermal conductivity is imaged

    Different Modes of AFM

    http://www.mobot.org/jwcross/spm/

  • 8/9/2019 SCANNING TUNNELING MICROSCOPE & ATOMIC FORCE MICROSCOPY

    16/32

    Repulsive

    Attractive

    Contact Region

    Non-ContactRegion

    DistanceF

    or

    ce

    Contact and Non- Contact Modes of AFM

    www.shef.ac.uk/~htsl/afm.htm

  • 8/9/2019 SCANNING TUNNELING MICROSCOPE & ATOMIC FORCE MICROSCOPY

    17/32

    Three common types of AFM tip

    Normal tip

    (3 m tall)

    Supertip Ultralever

    (3 m tall)

    Courtesy: Jean-Paul Revel, Caltech & Park Scientific Instruments

    http://stm2.nrl.navy.mil/how-afm/how-afm.html

  • 8/9/2019 SCANNING TUNNELING MICROSCOPE & ATOMIC FORCE MICROSCOPY

    18/32

    Scanning Modes of AFM

    Constant Height Modes Constant Force Modes

    Force Force

    www.shef.ac.uk/~htsl/afm.htm

  • 8/9/2019 SCANNING TUNNELING MICROSCOPE & ATOMIC FORCE MICROSCOPY

    19/32

    Comparison of AFM & other Microscopy Techniques

    AFM TEM SEM

    Max resolution Atomic Atomic 1s nm 100s nm

    Imaging Environment Air, fluid, Vacuum, Special gas vacuum vacuum Air, Fluid

    In-situ Yes No No Yes

    In fluid Yes No No Yes

    Sample preparation Easy Difficult Easy Easy

    OM

  • 8/9/2019 SCANNING TUNNELING MICROSCOPE & ATOMIC FORCE MICROSCOPY

    20/32

    STM systems

    Measure the quantum tunnelling current between a wire or metal-coated

    silicon tip and the object surface. An electronic feedback system maintains a

    constant current by positioning the tip to exactly contact the surface.

    AFMsystemsDetect the z-displacement of the cantilever by the reflection of a laser beam

    focused on the top surface of the cantilever. The feedback from this sensor

    maintains the probe at a constant force

    AFM Vs STM

  • 8/9/2019 SCANNING TUNNELING MICROSCOPE & ATOMIC FORCE MICROSCOPY

    21/32

    VVv

  • 8/9/2019 SCANNING TUNNELING MICROSCOPE & ATOMIC FORCE MICROSCOPY

    22/32

  • 8/9/2019 SCANNING TUNNELING MICROSCOPE & ATOMIC FORCE MICROSCOPY

    23/32

  • 8/9/2019 SCANNING TUNNELING MICROSCOPE & ATOMIC FORCE MICROSCOPY

    24/32

  • 8/9/2019 SCANNING TUNNELING MICROSCOPE & ATOMIC FORCE MICROSCOPY

    25/32

  • 8/9/2019 SCANNING TUNNELING MICROSCOPE & ATOMIC FORCE MICROSCOPY

    26/32

  • 8/9/2019 SCANNING TUNNELING MICROSCOPE & ATOMIC FORCE MICROSCOPY

    27/32

  • 8/9/2019 SCANNING TUNNELING MICROSCOPE & ATOMIC FORCE MICROSCOPY

    28/32

  • 8/9/2019 SCANNING TUNNELING MICROSCOPE & ATOMIC FORCE MICROSCOPY

    29/32

  • 8/9/2019 SCANNING TUNNELING MICROSCOPE & ATOMIC FORCE MICROSCOPY

    30/32

  • 8/9/2019 SCANNING TUNNELING MICROSCOPE & ATOMIC FORCE MICROSCOPY

    31/32

    Nano Fabrication

  • 8/9/2019 SCANNING TUNNELING MICROSCOPE & ATOMIC FORCE MICROSCOPY

    32/32

    Applications of AFM


Recommended