A scanning tunneling microscope (STM) works by utilizing quantum tunneling to image surfaces at the atomic level. Here's a breakdown of the process:
1. The Principle of Quantum Tunneling:
- At the heart of STM is the phenomenon of quantum tunneling. In classical physics, a particle doesn't have enough energy to overcome a potential barrier cannot pass through it. However, in quantum mechanics, there's a probability that the particle can penetrate the barrier, even if it doesn't have enough energy. This is tunneling.
- In STM, the "particle" is an electron, and the "barrier" is the gap between a sharp conducting tip and the surface of the sample being studied.
2. Setting Up the STM:
- Tip and Sample: The STM uses an extremely sharp, electrically conductive tip (often made of tungsten or platinum-iridium) that is brought very close to the surface of the sample.
- Bias Voltage: A small voltage (bias voltage) is applied between the tip and the sample. This creates an electric field across the gap.
3. Generating Tunneling Current:
- When the tip is brought within a few angstroms (0.1-1 nm) of the surface, electrons can tunnel across the gap due to quantum mechanical effects.
- This tunneling creates a small electrical current, called the tunneling current. The tunneling current is extremely sensitive to the distance between the tip and the sample. Even a tiny change in distance results in a significant change in the tunneling current.
4. Scanning the Surface:
- Piezoelectric Control: The tip is mounted on a piezoelectric scanner. Piezoelectric materials change their size in response to an applied voltage. By carefully controlling the voltage applied to the piezoelectric scanner, the tip can be precisely moved in three dimensions (x, y, and z).
- Constant Current or Constant Height Mode: STMs operate in one of two modes:
- Constant Current Mode: A feedback loop maintains a constant tunneling current. As the tip scans across the surface, the feedback loop adjusts the vertical position (z-direction) of the tip to keep the tunneling current constant. The variations in the tip's vertical position are then recorded and used to create an image of the surface topography. This mode is useful for rough surfaces.
- Constant Height Mode: The tip is scanned across the surface at a constant height. The tunneling current is measured as the tip moves. Variations in the tunneling current are then used to create the image. This mode is faster but only suitable for relatively flat surfaces.
5. Image Formation:
- The STM image is generated from the data collected during the scanning process.
- In constant current mode, the image represents the topography of the surface – the vertical position of the tip needed to maintain a constant tunneling current at each point.
- In constant height mode, the image represents the variations in the tunneling current as the tip scans across the surface.
In summary, an STM works by exploiting quantum tunneling. A sharp tip is brought very close to a sample, and a voltage is applied. This causes electrons to tunnel across the gap, creating a current sensitive to the tip-sample distance. By scanning the tip across the surface and monitoring the current, a detailed image of the surface's topography can be created.
