Introduction

A Brief History of the Scanning Tunneling Microscope

• Invented, 1981, by Gerd Bennig and Heinrich Rohrer at IBM's Zurich Research Laboratories.
• 1983, first images revealing atomic details were made of gold surfaces, Au(110).
• 1986, Bennig and Rohrer win a Nobel Prize for their invention.
• 1993, Davidson College Physics Department purchases a Burleigh Instructional STM for the mere price of $11,000.

Tunneling and Other Quantum Mechanical Words

• Quantum mechanically speaking, electrons have both wave properties and particle properties.
• The wave nature of an electron is governed by the Schrodinger equation as follows:




• For a well of finite potential, the Schrodinger equation shows that there is a probability of the electron existing beyond the boundaries of the well.
  
• When two finite wells are positioned adjacent to each other, the Schrodinger equation shows a probability of an electron moving from one well to the other without possessing enough energy to escape classically from either well. This effect is known as tunneling. When tunneling occurs, the particle never really exists in the space between the two wells.
  
• Tunneling is not a rare phenomenon that only occurs in laboratires with the "Oak Ridge" in the title. Tunneling explains many common physical phenomenon, such as alpha decay and the Scanning Tunneling Microscope.

The Scanning Tunneling Microscope

• A metal tip scans a microscopic area of the sample surface at a distance of around 10 Angstroms-- just close enough for the electron clouds to overlap slightly.
  
• The tip, made of PtIr or electrochemically etched Tungsten, usually has one atom which protrudes further than the rest of the atoms. This one atom tip facilitates a tunneling current on the order of one nanoamp.

A one atom tip	Energy diagram showing tunneling between the tip and the sample surface

• A bias voltage is applied to facilitate tunneling in one direction in order to measure the tunneling current.
• The needle is scanned across the surface of the sample by applying a voltage to different areas of a cylindrically shaped piezzo-electric device.
• The tip can operate in one of two modes:
• Constant Current Mode- The piezzo-electic device moves the tip toward and away from the surface to maintain a constant reference current
• Constant Height Mode- The tip is maintained at a constant height and variations in in the tunneling current are measured, thus mapping the surface
• Since the Schrodinger equation in the space between adjacent wells is an exponentially decaying function, the probability of encoutering electrons and thus the tunneling current fall of exponentially with increasing seperation between the surface and the tip.
• This exponentially changing function allows the STM to give very precise measurements within 0.1 Angstroms.