Scientific Background
The physical principle behind the STM is the tunnel effect or quantum tunneling. It refers to the quantum mechanical phenomenon in which a particle can tunnel through a barrier with a potential energy higher than the particle's dynamic energy. This phenomenon could not happen classically since the particle cannot surmount a barrier with total mechanical energy higher than its own.
 |
| Quantum tunneling principle |
Quantum mechanics predicts that an elementary particle such as an electron has a nonzero probability of moving from one side of any physical barrier to the other, regardless of the height or width of the barrier, or in other words, regardless of whether the potential energy of the barrier is greater than the kinetic energy of the particle. Quantum tunneling is a consequence of the wave-particle duality of matter and is often explained using the Heisenberg uncertainty principle. Because purely quantum mechanical concepts are central to the phenomenon, quantum tunneling serves as one of the defining features of both quantum mechanics and the particle-wave duality of matter. And, because of its quantum mechanical nature, the experimental observations of tunneling provide strong evidence for quantum mechanical theory itself [1].
If a potential difference Vbias is applied across two conductive electrodes separated by a distance dt, there is a tunneling current proportional to the distance, the potential and the electronic structure of the electrode materials[12]:
Where:
As an example, let us use the free electron mass and a potential barrier of a few electron volts. A mono atomic change (0.2 − 0.5 nm) in the tunneling distance would then produce a change in the current of about three orders of magnitude [1].
An adequate control of the displacement of the sample with respect to the probe (tip) is necessary. This is done by using piezoelectric elements described as Px, Py and Pz in the figure above. Piezoelectrics are materials which present a very small mechanical deformation upon application of a potential difference across two of its faces. This displacement control can deliver a detailed map of iso-currents, which in the case of conductors corresponds to a topographical map of the sample's surface [12].
There are two ways in which this kind of microscopes can work. Firstly, keeping a constant current by the use of a control circuit as described above; and secondly keeping a constant height and recording the change in the tunneling current. The following figure describes the operation of these two modes.
 |
| STM operation modes |
The figure below describes the different components necessary for the proper operation of a STM. Given all the described elements, it is necessary to point out the following general remarks regarding the operation of a STM:
dt ~ 1nm, implying that a very good mechanical isolation is needed to establish a controllable tunneling distance between probe and sample. Furthermore, a very precise positioning system is needed (scanner) if micro scale displacements are required.
It ~ 1nA, thus the circuitry used to measure this current needs to be extremely sensitive and noise-free. The signal has to be properly isolated.
The scanning tip or probe is a key point in the resolution achievable by the instrument. These tips have to be incredibly sharp and made of a conducting material [13].
 |
| Block diagram of the STM-Uniandes |