Artificial atoms

The three-dimensional (3D) spherically symmetric potential around atoms yields degeneracies known as shells, 1s, 2s, 3s, 3p,... Each shell can hold a specific number of electrons. The electronic configuration is particularly stable when these shells are completely filled wih electrons, occurring at 'magic' atomic numbers 2, 10, 18, 36,... In a similar way, the symmetry of a two-dimensional (2D), disk-shaped quantum dot leads to a shell structure with magic numbers 2, 6, 12, 20,... The lower degree of symmetry in 2D results in a different sequence of magic numbers than in 3D.

By measuring electron transport through quantum dots, a periodic table of artificial 2D elements can be obtained. For this purpose, dots are connected via potential barriers to source and drain contacts. If the barriers are thick enough , the number of electrons on the dot, N, is a well defined integer. This number changes when electrons tunnel to and from the dot. However, due to Coulomb repulsion between electrons, the energy of a dot containing N+1 electrons is larger than when it contains N electrons. Extra energy is therefore needed to add an electron to the dot. Consequently, no current can flow which is known as the Coulomb blockade.

The blockade can be lifted by means of a third electrode closeby, known as the gate contact. A negative voltage applied to this gate is used to supply the extra energy and thereby change the number of free electrons on the dot. This makes it possible to record the current flow between source and drain as the number of electrons on the dot, and hence its energy, is varied. The Coulomb blockade leads to a series of sharp peaks in the measured current (see figure below). At any given peak, the number of electrons on the dot alternates between N and N+ 1. Between the peaks, the current is zero and N remains constant. The distance between consecutive peaks is proportional to the so-called addition energy, which is the difference in energy between dots with N+1 and N electrons. The magic numbers can be identified because significantly higher voltages are needed to add the 2nd, 6th and 12th electron.

Quantum dots are 2D analogies for real atoms. But since they have much larger dimensions they are suitable for experiments that can not be carried out in atomic physics. It is especially interesting to observe the effect of a magnetic fieldd, B, on the atom-like properties. A magnetic flux-quantum in an atom requires typically a B-field as high as 10^6 T, whereas for dots this is of the order 1 T, which is experimentally accessible.

http://qt.tn.tudelft.nl/grkouwen/qdotsite.html this website is under contruction and quite interesting to read have a look at it ....

Quantum dots

When I went through the Interview on Quantum dots, programmable matter, and Wellstone (with Author Wil McCarthy by Rocky Rawstern Editor Nanotechnology Now - June 2003) I understood the intricacies of quantum dots with much of clarity and inquest.In the interview when questioned about the quantum dots McCarthy answers as follows

“Can you give our readers a brief explanation of "artificial atoms" and "quantum dots," and how they - when produced en mass - can create "programmable matter"?
A quantum dot is a device which traps electrons in a very small region of space, forcing them to behave like tiny standing waves, just as they do in atoms. An "artificial atom" is a cloud of electrons trapped in this way. Although it has no nucleus of its own, the artificial atom behaves in many ways like a real atom does. Producing large numbers of artificial atoms inside a bulk material, such as a semiconductor, will alter its properties dramatically so that, for example, it can be made to appear and behave like a metal, or an insulator. The material's color, transparency, reflectance, thermal and magnetic properties can also be altered, in real time.”

To go still more I browsed through the internet and could find many more interesting definitions and dimensions in context to quantum dots. From a very basic understanding to applications I would like to share some essences of my reading as well given the concerned urls for your further reading let me start to go further into the basics of quantum dots

“Quantum dots are small devices that contain a tiny droplet of free electrons. They are fabricated in semiconductor materials and have typical dimensions between nanometers to a few microns. The size and shape of these structures and therefore the number of electrons they contain, can be precisely controlled; a quantum dot can have anything from a single electron to a collection of several thousands. The physics of quantum dots shows many parallels with the behavior of naturally occurring quantum systems in atomic and nuclear physics. As in an atom, the energy levels in a quantum dot become quantized due to the confinement of electrons. Unlike atoms however, quantum dots can be easily connected to electrodes and are therefore excellent tools to study atomic-like properties. There is a wealth of interesting phenomena that have been measured in quantum dot structures over the past decade. This page shows a few examples from our group. The next blog will first discuss briefly the parallels between atoms and quantum dots.