Introduction to STM, working principle, advantages and disadvantages.

1. 1

2. • A scanning tunneling
microscope (STM) is an
instrument for imaging
surfaces at the atomic level.
• The scanning tunneling
microscope (STM) was
invented by Binnig and
Rohrer and implemented by
Binnig, Rohrer, Gerber, and
Weibel
• Nobel prize (at IBM Zurich)in
Physics in 1986
• Gerber built first STM as well
as first Atomic Force
Microscope.
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Binnig (right, holding flowers) and Heinrich Rohrer (left, holding flowers)
holding a soccer ball is Christoph Gerber

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4. • For an STM, good resolution is
considered to be 0.1 nm lateral
resolution and 0.01 nm depth
resolution
• The STM is based on the concept
of quantum tunneling which falls
under domain of quantum
mechanics
• Quantum mechanics:Matter has
both particle as well as wave nature
• wavelike properties of electrons
permit them to “tunnel” beyond the
surface of a solid into regions of
space that are forbidden to them
under the rules of classical physics
• Smaller the barrier width more
precisely wave can pass through
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5. • The probability of finding such tunneling electrons decreases
exponentially as the distance from the surface increases.
• The STM makes use of this extreme sensitivity to distance. The
sharp tip of a tungsten needle is positioned a few angstroms from
the sample surface.
• A small voltage is applied between the probe tip and the surface,
causing electrons to tunnel across the gap.
• As the probe is scanned over the surface, it registers variations in
the tunneling current, and this information can be processed to
provide a topographical image of the surface.
• In metals, electrons appear to be freely moving particles, but this is
illusory.
• In reality, the electrons move from atom to atom by tunneling
through the potential barrier between two atomic sites.
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6. 6
• In metals, electrons appear to be freely moving particles, but this is illusory.
• In reality, the electrons move from atom to atom by tunneling through the potential
barrier between two atomic sites.
• In a typical case, with the atoms spaced five angstroms apart, there is a finite
probability that the electron will penetrate the barrier and move to the adjacent
atom.
• The electrons are in motion around the nucleus,
and they approach the barrier with a frequency
of 1017 per second. For each approach to the barrier,
the probability of tunneling is 10−4, and the electrons
cross the barrier at the rate of 1013per second.
• When the tip is moved close to the sample, the spacing between the tip and the
surface is reduced to a value comparable to the spacing between neighbouring
atoms in the lattice.
• In this circumstance, the tunneling electron can move either to the adjacent atoms
in the lattice or to the atom on the tip of the probe. The tunneling current to the tip
measures the density of electrons at the surface of the sample, and this information
is displayed in the image

7. • Greek: piezo means to squeeze or press,
and electric or electron , which means amber,
an ancient source of electric charge
• the electric charge that accumulates in certain
solid materials in response to applied
mechanical stress usually by change of
polarisation
• It provides variation in direction with details
about orientation
• A probe tip, usually made of W or Pt–Ir alloy, is
attached to a piezodrive, which consists of
three mutually perpendicular piezoelectric
transducers: x piezo, y piezo, and z piezo.
Upon applying a voltage, a piezoelectric
transducer expands or contracts
• the tip scans on the xy plane
• The electron wavefunctions in the tip overlap
electron wavefunctions in the sample surface.
A finite tunneling conductance is generated.
By applying a bias voltage between the tip and
the sample, a tunneling current is generated
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8. 1. 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.
• It is faster, as the piezoelectric movements require more
time to register the height change in constant current
mode than the current change in constant height mode
2.Constant current mode
• Contrast on the image is due to variations
in charge density. Feedback electronics adjust
the height by a voltage to the piezoelectric
height control mechanism. 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
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9. • An STM specimen needs a substrate that is extremely flat, down to
the atomic level. If the specimen is uneven, the STM probe will have
difficulties in scanning very steep pits or ridges.
Graphite
One of the most commonly used STM substrates is a special form of
graphite (highly oriented pyrolytic graphite or HOPG). It is a
naturally layered material that is easy to prepare and relatively
inert. A fresh surface can be obtained as easily as pressing a piece
of adhesive tape to the surface and peeling away the top layer. The
resulting surface will have large flat areas useful for scanning.
Other Substrates
Other popular materials that provide large, atomically flat surfaces
include mica, quartz, and silicon. These materials are insulators, so
to be used for STM a thin layer of noble metal (mainly gold or
platinum) is deposited on the surface. Annealing (heating and then
slowly cooling) the metal layer helps to smooth the surface and
produce large flat areas.
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10. • Growing Gold
A thin gold film can also be grown while
deposited on a flat mica surface and then
stripped away. The surface of the gold that
was in contact with the mica will be very flat
over large areas.
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Fig: The surface of gold film
grown on quartz.
Fig: The surface of flame annealed
gold film.

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Atomic force microscopy(AFM)
low-energy electron diffraction (LEED)

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Preparation techniques( molecular evaporators, molecular spray and pulse valves)
and analysing techniques (as X-ray photoelectron spectroscopy (XPS), ultraviolet
photoelectron spectroscopy (UPS), Auger electron spectroscopy (AES), low-energy
electron diffraction (LEED) and low-temperature scanning tunneling microscopy (LT-
STM))

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(a) STM image of the self-assembled monolayer
of cysteine on Au(111)(b) proposed model of
Cysteine (c)ball and stick model of cysteine

14. • The STM can be used not only in ultra-high vacuum but
also in air, water, and various other liquid or gas
ambients , and at temperatures ranging from near zero
kelvin to a few hundred degrees Celsius
• STM gives 2D/3D image and better resolution of atoms
• The STM can be cooled to temperatures less than 4 K
(−269 °C, or −452 °F)—the temperature of liquid
helium. It can be heated above 973 K (700 °C, or 1,300
°F). The low temperature is used to investigate the
properties of superconducting materials, while the high
temperature is employed to study the rapid diffusion
of atoms across the surface of metals and their
corrosion
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15. • The strong electric field between tip
and sample has been utilized to move
atoms along the sample surface. It
has been used to enhance the
etching rates in various gases. In one
instance, a voltage of four volts was
applied; the field at the tip was
strong enough to remove atoms from
the tip and deposit them on a
substrate.
• A sequence of STM images showing
the assembly of two Chinese
characters from single Ag atoms on a
Ag(111) surface. The lateral
manipulation technique allows the
exact placing of single atoms on
desired atomic sites. An assembly
involves not only the movement of
single atoms but requires also many
repair and cleaning steps until the
final structure is completed
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16. • STMs use highly specialized equipment that is fragile
and expensive($30000-$1,50,000)
• STMs can be difficult to use effectively. It requires very
stable and clean surfaces , excellent vibration control
and sharp tips
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17. • C. Julian Chen “ Introduction to Scanning Tunneling
Microscopy ”, Second Edition , Department of Applied Physics
and Applied Mathematics , Columbia University, New York
• Tersoff , J.: Hamann , D. R.: Theory of the scanning tunneling
microscope, Physical Review B 31, 1985, p. 805 –
813(https://en.wikipedia.org/wiki/Scanning_tunneling_micro
scope)
• http://www.nobelprize.org/educational/physics/microscopes
/scanning/preparation.html
• https://nanolab.unibas.ch/TemplateReducedQC2/pages/equi
pment.htm
• http://www.britannica.com/technology/scanning-tunneling-
microscope
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