An STM uses quantum tunneling and a very fine probe to detect electrical forces and map the surface topography of materials at the atomic level. The probe is brought within angstroms of the sample surface, and a bias voltage allows electrons to tunnel between the probe and surface. Variations in the tunneling current from irregularities in the electron shells of surface atoms are detected and converted into a 3D image with sub-nanometer resolution. The probe is moved using a piezoelectric scanner for precision positioning and maintaining a constant tunneling current through negative feedback.

1. SCANNING TUNNELING
MICROSCOPY

3. A scanning tunneling
microscope (STM) is a
device that obtains
images of the atoms on the
surfaces of materials.
SCANNING
TUNNELING
MICROSCOPE

5. ATOMIC SIZED PROBE
The STM is not an optical
microscope; instead, it works
by detecting electrical
forces with a probe that
tapers down to a point only
a single atom across
DETECTS ELECTRICAL
FORCES

6. STM DETECTS
IRREGULARITIES
ON THE SAMPLE
The electron shells, or clouds,
surrounding the atoms on
the surface produce
irregularities that are
detected by the probe and
mapped by a computer into
an image.

7. Quantum
tunneling
STM works on three key principles .
Piezo electric
effect
Feedback
current
KEY PRINCIPLES

8. • When the tip is brought very near to
the surface to be examined, a bias
voltage applied between the two allows
electrons to tunnel through the vacuum
separating them.
• The resulting tunneling current is a
function of the tip position, applied
voltage, and the local density of states
(LDOS) of the sample
QUANTUM TUNNELING

9. PRINCIPLE OF PIEZO ELEMENT.
THE APPLIED VOLTAGE MAKES
THE ELEMENT LONGER OR
SHORTER
5X10-7-5X10-12M,I.E.,A FRACTION OF ONE
MICROMETER TO ATOMIC
RESOLUTION
TRIPOD SCANNER
THE COMBINATION OF THREE PIEZO
ELEMENTS MAKES IT POSSIBLE TO
MOVE THE STM TIP IN THE X-, Y-, AND
Z-DIRECTIONS
TUBE SCANNER
IN MOST MODERN SCANNING
PROBE MICROSCOPES, ONE
USES A TUBE GEOMETRY.

10. • The tunneling current is amplified
by the current amplifier to
become a voltage.
• Which is compared with a reference
value
• The difference is then amplified
to drive the z peizo.

11. STM POSITIONING
.The phase of the amplifier
is chosen to provide
negative feedback:
If the tunneling current is
larger than the reference
value, then the voltage
applied to the z piezo tends
to withdraw the tip from the
sample surface, and vice
versa.

12. Doesnot work with insulators
Often needs to used under vacume
Able to obtain very high resolution images of conductors and semiconductors
Probe tips can be made out of thin wires
ADVANTAGES
DISADVANTAGES

13. THANKS
FOR YOUR TIME