Overview

Nanotechnology does not only implies working at ever available smaller dimensions but working at the nanoscale which helps scientists to make use of the unique chemical, physical, optical, and mechanical properties of materials that occur naturally at smaller scales. The recent giant leaps in nanoscale particles areas of study such as microscopy gave scientists new tools to understand and take good use of these phenomena that occur naturally when matter is organized at the nanoscale. Most importantly, these phenomena are based on "quantum tunneling effects" and other simple physical effects such as expanded surface area.

Quantum tunneling can be described as a quantum mechanical or a nanoscopic phenomenon where a particle tunnels through a barrier that it could not surmount as explained classically. This property of a particle is significant in some physical phenomena like the nuclear fusion that occurs in the Sun.

Its mode of operation is applicable in modern devices such as the tunnel diode, the scanning tunneling microscope, and quantum computing. Quantum tunneling is projected to create physical limits to how small transistors can be made, due to electrons being able to tunnel past them if they are too small. It is being regarded as one of the novel implications of quantum mechanics and it is basic and fundamental to nanotechnology.

Quantum tunneling effect was accepted as a general physical phenomenon came mid-century after its prediction in the early 20th century.
Heisenberg uncertainty principle and the wave–particle duality of matter are often used to explain Quantum tunneling.

Quantum Tunneling Explained

From classical non-relativistic point of view, the total energy of a particle can be expressed

Where M is the mass of a particle, hk is a function of momentum moving with a potential U.

Considering a case which is classically allowed when the total energy of a particle is greater than the potential energy (E > U), whereas the condition in which E < U is considered to be classically forbidden. To understand these conditions better, let us imagine a particle rolling on a potential surface represented by U(x), it can only exist in a region of space in which U(x) < E, at the total energy E and once U(x) ≥ E, the particle must observe a classical turning point, that is, the particle must classically turn around .

However, in quantum physics, t is possible for the particle to be found in this classically forbidden region. Quantum tunneling explains how a particle can penetrate a barrier and emerge on the other side of the barrier. The barrier can be a vacuum, an insulator or a region of high potential energy. Due to the wave-like behaviour of particles, and the possibility of explaining an object by means of a probability wave, quantum physics predicts that without enough energy to overcome the barrier, there exist a finite probability that an object trapped behind a barrier could at times appear on the other side of the barrier, without actually breaking it down.

For example, if an electron in an electric field is repelled, there is nevertheless some probability that, no matter how small, some parts of the electron will find itself on the other side of the field.

Applications of Quantum Tunneling

Scanning Tunneling Microscopy (STM)

At an atomic level, scanning tunneling microscope is a sensitive device used to map the topography of materials. With the tip at a higher voltage than the material, STM functions by running an extremely sharp tip of only a single atom thick over the surface of the material.

A non-negligible tunneling current flows from electrons that tunnel from the surface of the material due to the high voltage allowed and through the potential barrier represented by the air, to the tip of the microscope, makes a complete circuit. The microscope can resolve where the atoms are on the surface of the material by measuring the amount of current that flows at a given distance. STMs are accurate to 0.001 nm, or about 1% of an atomic diameter.

Tunnel Diodes
A tunnel diode, also known as Esaki diode is a semiconductor comprising of a thin insulator sandwiched between two semiconductors, it is capable of high speed operation. This diode is capable of frequencies well into the microwave range. Tunnel diodes are a two terminal device containing a heavily doped p-n junction whereby the electric current through the diode is caused by quantum tunneling. Consequently, the current across the tunnel diode will decrease as the voltage increases meaning it will exhibit a negative resistance.

A tunnel diode has a highly concentrated impurities which is 1000 times greater than that of normal p-n junction diode, resulting in p-n junction of a tunnel diode contains an extremely narrow depletion region in the order of nanometres ( as required in nanotechnology). In ordinary diodes, current can be produced when an applied voltage is more than the built in voltage of the depletion region. In contrary, a small voltage less than the built-in voltage of the depletion region can produce a sufficient electric current due to quantum tunneling between the n and p region in tunnel diodes.

The narrow depletion region is necessary to lower the barrier thickness for proper tunneling. Tunnel diodes are useful in construction of a logic memory storage devices, ultra-high-speed switches and relaxation oscillator circuits. They are commonly used in the nuclear industry due to the high resistance to radiation.

Quantum Biology

Electron tunneling is functional to many biochemical reduction-oxidation (redox) reactions such as cellular respiration, photosynthesis as well as enzymatic catalysis while proton tunneling is very key to spontaneous mutation of DNA. Quantum tunneling effects inform of electron tunneling and proton tunneling make quantum tunneling a central non-trivial quantum effects in quantum biology.

Quantum biology can be explained in relation to DNA when there is a spontaneous mutation and normal DNA replication takes place after a significant proton has defied the odds in quantum tunneling known as proton tunneling. In DNA, a hydrogen bond joins normal base pairs and along the hydrogen bond separated by a potential energy barrier, there exists a double well potential.

The double well potential is asymmetric with one well deeper than the other so the proton could rest in the deeper well. For a mutation to occur, there is tunneling of proton into the shallower of the two potential wells for a mutation to occur. Tautomeric transition is the movement of the proton from its regular position. Per-Olov Lowdin was the first to develop this theory of spontaneous mutation within the double helix of quantum biology.

Learn more about nanotechnology in the attached video below

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