양자점

AFM 증착 Cr 나노도트. 제임스 E. 모리스 & 후이 서(2004)

양자점

If you read popular science articles, you probably have come across the term “quantum dot.” It pops up everywhere – “quantum dots” for drug delivery, “quantum dots” for multicolored display, “quantum dot lasers” – the list seems endless. So, what is a quantum dot? Roughly speaking, it is a tiny structure made of a solid material. It is so tiny (typically consisting of 1,000 -10,000 atoms) that an electron inside it is severely restricted in its movement. When an electron’s motion is so severely constrained, its kinetic energy can assume only certain allowed values that are determined by the size and shape of the dot as well as the material making up the dot.

Gallium arsenide (GaAs) is a popular material used to make quantum dots because the electron behavior in this material produces useful material properties at room temperature. Note that 1 nanometer = 10-9 meter, which is roughly 100,000 times smaller than the thickness of human hair. Since atoms in GaAs are spaced approximately 0.5 nm apart, a quantum dot contains about 25,000 atoms.

One important property of quantum dots is their exceptionally large surface area-to-volume ratios. In the case of a cube of edge W, this ratio is 6/W; in the case of a sphere of diameter D, the ratio is 6/D.  The ratio increases as the dimension of the dot (W or D) becomes smaller. For a basketball of diameter 0.1 m, the ratio is 60 m-1, but for a quantum dot of diameter 10 nanometers (10-8 m), the ratio is 6×108 m-1, which is ten million times larger. The large surface area-to-volume ratio of the quantum dot is used in chemical and biological sensors. The sensing activity typically takes place only on the surface and not in the interior; therefore, it pays to have a large surface area-to-volume ratio. It may be beneficial to replace a large sensing particle with several tiny particles or quantum dots.

To illustrate this idea further, note that the volume of the basketball is 1021 times larger than that of the quantum dot we just described. The basketball can be broken up into 1021quantum dots. The combined surface area of all these dots is 107 times larger than that of the basketball. Therefore, breaking up the basketball into quantum dots, results in a sensor that is ten million times more effective. That is why quantum dots are so important in applications such as sensing, targeted drug delivery, and catalysis – all of which require a large surface area-to-volume ratio.

Quantum dots are also useful in multi-colored displays. They emit light when an electron inside the dot undergoes a transition from a higher energy (excited) state to a lower energy (ground) state. The color of the emitted light depends on the difference in the energies of the final and initial states which can be related to the size of the dot. Therefore, quantum dots of different sizes will emit different colored light. This is the basis of multicolored displays.

Fabricating quantum dots with good control over size, material purity, and placement on a given surface is not an easy task. Two approaches are common: the “top-down approach” where a large piece of material is chiseled down to a small quantum dot using the process of lithography and etching. A slight variation of this approach is electrostatic delineation of quantum dots where metal pads are placed on a thin layer of material. A negative potential is applied to the pads which drives away the electrons from underneath, leaving a small puddle of electrons in the center; these form a quantum dot.

The second approach is “bottom up” and is known as self-assembly. Here, chemistry causes aggregation of atoms into structures of well defined size (of a few nanometers) and forms quantum dots.

Directed self assembly is a refinement of the process where the spontaneous formation of structures occurs on a patterned substrate that offers preferred sites for nucleation of quantum dots. This is also referred to as template-based self assembly since the patterned substrate acts as a template for spatially ordering the quantum dots.

Quantum dots are fascinating entities that are increasingly making their way into a large assortment of commercial and defense related products. Rapid advancements are being made in perfecting the synthesis methods and you can expect to see quantum dots in many products you use within the next decade or so.

AFM 증착 Cr 나노도트. 제임스 E. 모리스 & 후이 서(2004)

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