Scanning Tunneling Microscope
As with many other technologies, the components of a scanning tunneling microscope are now far more affordable than they were only a few years ago. Accordingly, the LIA Engineering Club has embarked on a project to build such a microscope at the LIA STEM Centre.
Components of the Scanning Tunneling Microscope
- Vibration-Damping Mechanism
- Scan Platform
- Piezo-electric device
- Scan Tip
A scanning tunneling microscope is built to produce images of atoms in specially selected material samples. Due to the extremely small scale involved, this requires special isolation from everyday vibrations. The scanning tunneling microscope is accordingly mounted on special springs and weights in order to minimize these vibrations which would otherwise make scanning impossible.
Scan Platform - Coarse Distance Adjustment
A mechanism of unusual precision is required in order to bring the scan tip sufficiently close to the sample. An extremely small current must be made to jump across a narrow gap between the atoms in the sample and the atoms in the scan tip. Accordingly, precision screws are used to vary the distance between two pieces of aluminum. The scan tip is mounted to one of the pieces of aluminum and the sample to the other in such a way that a change in distance caused by adjusting a screw on one side of the scan platform results in a much smaller change in distance between the scan tip and the sample. This mechanism is also known as a coarse adjustment as it is only designed to bring the scan tip to the neighborhood of the sample. During scanning, variations of distance on the atomic scale are achieved by a piezo-electric device attached to the scan tip.
Piezo-Electric Device - Fine Distance Adjustment
Since atomic distances are involved in scanning for atoms, a mechanism that can produce variations on the atomic scale is required. Amazingly, this can be achieved with a common piezo-electric buzzer, a small, inexpensive device that produces a buzzing sound in electronic circuits. By cutting the circular crystal disc of buzzer in four equal sections and connecting each to a voltage source independently, a means of adjusting the x, y and z position of the scan head can be achieved. The thickness of the disc is proportional to the voltage applied. Small variations in voltage happen to produce minute variations of thickness that correspond to the atomic scale.
A fine tungsten wire is cut at an angle and then treated with a chemical process in order to produce sharpness on the atomic scale. Ideally, the end of the tip will consist of a single atom. With this level of precision, the STM will be able to discern the proximity of other atoms.
In the STM, a voltage potential is applied across the gap between the scan head and the sample. This causes some electrons to "tunnel," even though they do not posess enough energy to jump across the gap, a phenomenon explained by Quantum Mechanics. Naturally, as the STM device is dealing with electrons jumping out of individual atoms, this involves an incredibly small current on the order of picoamperes. (10^-12 Amperes). Electrical components exist, however, that can amplify such unimaginably insignificant currents to levels that can be detected by regular electronic circuits. Further, the electronics of the STM needs to take analog signals and convert them to digital signals for processing in the computer. This is the overall concept of the electronic circuit in the Scanning Tunneling microscope.
The final component of the Scanning Tunneling microscope is the software. This is designed to send signals to the microscope to scan systematically over the sample and take the signals from the scanning apparatus, analysing them and converting them into meaningful information such as an image of the atoms in the sample.