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SCANNING TUNNELING MICROSCOPE • A scanning tunneling microscope (STM) is a device that obtains images of the atoms on the surfaces of materials.
ATOMIC SIZED PROBE DETECTS ELECTRICAL FORCES 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
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.
KEY PRINCIPLES STM works on three key principles . Feedback current Quantum tunneling Piezo electric effect
QUANTUM TUNNELING • 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
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.
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.
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.
ADVANTAGES • Probe tips can be made out of thin wires • Able to obtain very high resolution images of conductors and semiconductors • DISADVANTAGES • Doesnot work with insulators • Often needs to used under vacume
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