Nanophysics is the branch of physics that deals with the production and study of small systems, typically having a scale of a few nanometres (1 nm=10−9m). (A human hair is about 80 000 nm in diameter.) This means that in nanophysics one is dealing with physical systems that are formed of parts consisting of a relatively small number of atoms, typically less than 100 nanometres in size. Consequently, quantum and surface effects are important features of nanophysics.
The subject can be traced back to a remarkably visionary and subsequently famous talk given by the US physicist Richard Feynman at a 1959 meeting of the American Physical Society at Caltech with the title: There’s Plenty of Room at the Bottom. In this talk, Feynman envisaged the manipulation and control of matter down to the level of individual atoms. He speculated on its use in a number of ways. Since then a great deal of progress has been made in realizing Feynman’s vision.
The rapidly emerging technology associated with nanophysics is called nanotechnology, and it has involved an interdisciplinary approach from a number of branches of science and technology. The possibility of manipulating individual atoms has many potential applications in chemistry, electronics, engineering, materials science, molecular biology, medicine, and computer technology. The subject has led to the coinage of a vast number of ‘nano words’—nanochemistry, nanoengineering, nanomedicine, nanomaterials, nanomanipulation, nanostructures, etc.
Techniques in nanophysics
One of the main experimental techniques used for investigating nanostructures is scanning probe microscopy, in which a fine probe is moved across a surface in a raster pattern, and a measurement is made of some interaction between the probe and the surface. A large number of different types of scanning probe microscopy have developed, depending on the kind of interaction used. The original form was scanning tunnelling microscopy invented at IBM in 1981 by the German physicist Gerd Binnig (1947– ) and the Swiss Heinrich Rohrer (1933–2013). Another type is atomic force microscopy, which was invented in 1986. Instruments of this kind can detect the position of individual surface atoms.
In 1989, Don Eigler at IBM demonstrated that a scanning tunnelling microscope could be used to move individual atoms around. Working at liquid helium temperatures, he was able to move individual xenon atoms on a nickel surface. Eigler positioned 35 atoms to spell out the letters IBM, with each letter about 5nm high, and then used the STM to produce a famous photograph of this logo. Further work involving moving individual atoms has subsequently been done at room temperatures.
Other techniques in nanophysics include molecular beam epitaxy, ion-beam etching and implantation, and vapour deposition, all of which have been used extensively in semiconductor technology.
Nanoelectronics
An example of nanophysics is given by a system known as a quantum dot, i.e. an electrical circuit at the nanometre scale in which electrons are confined to a small region. The smaller the region within which the electron is confined, the more conspicuous are the effects of quantum mechanics. This means that quantum dots have quantized energy levels that can readily be measured and that a quantum dot can be regarded as an ‘artificial atom’. Since the energy levels of a quantum dot can be altered by altering the size, materials can be created for specific purposes. The confinement of an electron is achieved by creating a potential well that traps the electron.
One technique for making quantum dots, starts with gallium arsenide (GaAs) combined with aluminium gallium arsenide (AlGaAs) in which some gallium atoms are replaced by aluminium atoms. It is possible to produce a crystal in which layers of AlGaAs only a few atoms thick can be deposited onto the gallium arsenide. This results in electrons being trapped in a thin layer at the interfaces due to the energy gap between the valence band and the conduction band being different in gallium arsenide and AlGaAs. In this system, electrons are confined in one direction but are free to move parallel to the interface. This is an example of a quantum well.
It is possible to create a quantum dot from a quantum well by attaching metal strips to the surface of this well, with this metal strip shaped so as to confine electrons within a small region. This is done by applying a negative charge to the metal. This results in electrons in the quantum well being repelled from the metal and confined to a small region, the dimensions of which can be controlled.
Single electron transistors
Another nanoelectronic device is the single electron transistor (SET). In a SET the aim is to have an electric current that can be controlled so as to flow one electron at a time. Rather like a mini-capacitor, the structure in a SET consists of two metal plates separated by an insulator. It is cooled to a temperature of about a 1 K above absolute zero, so that the system is in the ground state. If an electron tunnels from one plate across the insulator to the other plate then, as with large capacitors, energy is stored. However, electric charge is not a continuous quantity but occurs in discrete amounts, with the minimum amount being the charge of an electron. As a consequence of this, it is energetically favourable for electron tunnelling to occur. This prohibition on electron tunnelling is called a Coulomb blockade. A Coulomb blockade is not a quantum-mechanical phenomenon but occurs because there is a minimum amount of electric charge, with a relatively large amount of energy being required to transfer an electron from one plate to the other plate in the capacitor.
If the plates of the mini-capacitor are connected to a current then opposite charges accumulate on the plates, thus making it energetically favourable for electron tunnelling to occur, and this can be done in such a way that electrons can tunnel through one at a time. Devices based on the combination of single electron tunnelling and the Coulomb blockade have many potential practical applications. For example, in a digital logic electronic circuit the presence of an electron would correspond to 0.
A single electron transistor can be constructed using quantum dots. When a quantum dot is part of an electrical circuit it can act as the insulator in a minicapacitor. In such a set-up it is not possible for an electron to go through the quantum dot unless it can tunnel through—i.e. the energy of the electron corresponds to an allowed energy for electrons in the quantum dot. If a sufficiently high voltage is applied then electrons are accelerated to energies that enable them to overcome the Coulomb blockade, thus increasing the probability of electrons tunnelling through and hence increasing the current that can flow. Transistor action can be obtained by having a bias to control tunnelling, with the transistor action being at the level of single electrons. It is hoped that single electron transistors will enable further miniaturization of electrical components in the future.