Untitled Quiz

Scanning Tunneling Microscopy (STM)

• Invented by Gerd Binnig and Heinrich Rohrer in 1986, receiving a Nobel Prize.
• Based on quantum tunneling, where electrons penetrate regions classically forbidden.
• Substances under investigation must be electrically conductive (e.g., copper).
• Inner shell electrons are tightly bound.
• Outer shell (valence) electrons are loosely bound, forming an electron gas.
• Atomic nuclei and inner shell electrons have a net positive charge and arrange in a lattice.
• Classical physics: Electrons as particles need sufficient energy to overcome the barrier.
• Quantum tunneling: Electrons as waves can tunnel through the barrier.
• STM construction: Metal probe with a fine tip (single atom), positively charged, above the surface.
• Gap between probe tip and surface is finely adjusted (a few tenths of a nanometer).
• Tunneling current (electric current) is measured as electrons tunnel across the gap.
• Current stays constant as tip scans across the surface.
• Topographic map is created from the probe tip's up-and-down motion as it scans.
• This map reveals individual atoms and molecules.
• Atomic-scale imaging enables studying surface structures and properties.
• Surface manipulation is possible by moving atoms and molecules.

STM Applications

• Enables the study of surface reactions and catalytic processes.
• Used to study materials properties (conductivity, magnetism, superconductivity).
• Crucial for building and characterizing nanostructures (nanotubes, nanowires, and nanoparticles).
• Can image biological samples (DNA, proteins, and cells) in their natural environment.

STM History

• Invented in 1981 at IBM Zurich.
• First commercial STMs released in 1986.
• Widely used in research by the 1990s.
• Advancements in resolution and speed continued into the 2000s.
• New techniques (like nc-AFM) were developed to complement STM.

STM Operation

• Current generated is nano-sized (few tenths of a nanoampere).
• Probability of tunneling decreases rapidly with increasing tip-to-specimen distance.
• Even 0.01 nm separation causes noticeable changes in tunneling current.
• Scanning across a surface causes continuous changes in tunneling current.
• Current is higher when the tip is above a bump (feature) and lower when it's in a valley (feature).
• Constant-current mode (CCM): Computer constantly adjusts tip height/position to maintain constant tunneling current.
• Tip moves up/down to maintain constant current throughout the scan.

Other STM achievements and features

• Imaging individual atoms (1982).
• Manipulating individual atoms (1990).
• Creating artificial molecules (2000).
• Observing chemical reactions at atomic level (2007).
• Control is done by mounting the probe on 3 tiny ceramic posts (which change position/stretch when voltage changes).
• Piezoelectric materials respond to voltage by changing shape (or to deformation by generating voltage).
• Recording height at different points creates a topographic map.
• Used for imaging and creation of nanostructures.