How Excitons Work in 2D Semiconductors and Real-World Applications
Essentially, when an electron and a positive hole (an empty electron particle in valence band) combine and are able to move freely through a non-metallic crystal as a unit, then the combination of these two particles is called an exciton. It shall be noted that the electron and the positive hole carry opposite charges. Thus they cancel each other's charges, and there is no electrical charge in the exciton. Owing to this property, detecting an exciton can be challenging at times. Now there are different characteristics of excitons, and they are generally classified in two limiting cases - first, the one has a small dielectric constant and the other, which has a large dielectric constant.
Frenkel Exciton
Yakov Frenkel was the first one who proposed the concept of exciton when he stated about the excitation of atoms in a lattice of a certain excitonic insulator. The Frenkel exciton has a relatively small dielectric constant, and its binding energy is on the order of 0.1 to 1 eV. This takes place because, at times, the Coulomb interaction between the hole and an electron may be forceful and strong. Owing to this extra force, the exciton tends to be small; thus, they carry less dielectric constant. EEX = -e2/∈crystals are a general source where Frenkel excitons are found. Further, they can also be located in organic molecular crystals like anthracene and tetracene.
Wannier–Mott Exciton
The dielectric constant is generally large in semiconductors; owing to this, the electric field screen reduces the interaction which takes place in Coulomb between the particles. Through this process, a Wannier-Mott exciton is formed. The said exciton has a radius that is way larger than the lattice spacing. Large exciton radii are favoured greatly by small masses of electrons which are typical of semiconductors. Through this, the said lattice potential can be put into the masses of electron and hole, which forms an exciton polariton. The binding energy in these is quite low, and it is generally in the order of 0.01 eV. These excitons are typically found in semiconductor crystals and liquids such as xenon.
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Charge-Transfer Exciton
There can be an intermediate case between Frenkel and Wannier exciton, and this case has been termed as charge transfer exciton (CT). These generally come to existence when the electron and the hole are present in the adjacent molecules. They occur typically in molecular and organic crystals. Unlike the other two, they have a property to show a static electric dipole. CT excitons may also be present in transition metal oxides. Their concept is always in proximity with a transfer of charge, which mainly occurs from one atomic site to another, this transfer of charge aids in spreading the wave-function around lattice sites.
Exciton in 2D Semiconductors
In two dimensional objects and materials, the whole systematic process is quantum-confined, and the direction of the same is perpendicular to the normal plane of the said object or material. This reduction in the dimensions of the object greatly manipulates the energies and radii of Warrier excitons; generally, the exciton b exciton effect amplifies in such restricted systems. In most excitons in 2d materials semiconductors, the Rytova–Keldysh provides a good approximation which is indicative of the exciton interaction. The said equation is given below.
V(r) = \[\frac{\pi}{2r_{0}}\] \[[H_{0}(\frac{kr}{r_{0}})\] - \[Y_{0}(\frac{kr}{r_{0}})]\]
Self-Trapping of Excitons
In crystals, the phonons and excitons interact; there is lattice vibration. If the coupling between the two is not cohesive in a semiconductor, then excitons generally get dissipated by phonos. On the other hand, if the coupling between the two is strong, owing to great cohesion, then, in such a situation, excitons can be self-trapped. In the case where interlayer exciton is self-trapped, they get surrounded by a dense cover of clouds made up of virtual phonos. This dressing greatly hinders the ability of the excitons to move across the crystal itself. Essentially, there is a local deformation of the lattice which surrounds the exciton. Self-trapping is quite similar to forming strong polarons which strongly couple together.
FAQs on Exciton: Meaning, Types, and Significance in Physics
1. What is an exciton in physics?
An exciton is a bound state of an electron and an electron hole, which are attracted to each other by electrostatic Coulomb forces. It is best understood as a quasiparticle, an elementary excitation that exists within a solid material like a semiconductor or an insulator. When a photon strikes the material, it can excite an electron, leaving a 'hole' behind. This electron-hole pair can then move through the material together as a single, electrically neutral entity, which is the exciton.
2. What are the main types of excitons and how do they differ?
The two primary types of excitons are Frenkel and Wannier-Mott excitons. They differ mainly in their size and the types of materials they are found in.
- Frenkel Excitons: These are tightly bound with a small radius, typically on the order of a single atom or molecule. The electron and hole are located on the same molecule. They are commonly found in organic molecular crystals and have a high binding energy (around 1 eV).
- Wannier-Mott Excitons: These are weakly bound with a large radius that spans many lattice sites in a crystal. The electron and hole are separated by a significant distance. They are characteristic of inorganic semiconductors and have a much lower binding energy (around 10 meV).
3. Where are excitons found and what are their applications in modern technology?
Excitons are fundamental to the operation of many modern optoelectronic devices. They are primarily found in semiconductors, insulators, and certain organic materials. Key applications include:
- Light-Emitting Diodes (LEDs) and OLEDs: In these devices, electrons and holes are injected, they combine to form excitons, and the subsequent decay of these excitons releases energy in the form of light.
- Solar Cells: When sunlight hits a solar cell, it creates excitons. The efficiency of the cell depends on successfully separating the electron and hole from this exciton state to generate an electric current.
- Lasers: Certain types of semiconductor lasers rely on the high concentration and recombination of excitons to produce a coherent beam of light.
4. How is an exciton formed in a semiconductor?
The formation of an exciton in a semiconductor is a precise process initiated by energy absorption. It occurs as follows:
- A photon with energy equal to or greater than the semiconductor's band gap strikes the material.
- This energy excites an electron from the filled valence band into the empty conduction band.
- This process leaves behind a positively charged void, or an electron hole, in the valence band.
- The negatively charged electron in the conduction band and the positively charged hole in the valence band experience a Coulombic attraction, binding them together into the neutral quasiparticle known as an exciton.
5. Why is an exciton considered a quasiparticle and not a true particle?
An exciton is called a quasiparticle because it is not a fundamental particle of nature like an electron or a proton. Instead, it is an emergent phenomenon that arises from the complex interactions within a solid. It behaves *like* a particle—it has definite energy, momentum, and can travel through the crystal lattice. However, it is a composite entity made of an electron and a hole and can only exist within the collective environment of the material. It cannot be isolated in a vacuum; it is an excitation *of the system* itself.
6. What is the importance of exciton binding energy?
The exciton binding energy is the amount of energy required to break the exciton apart into a free electron and a free hole. This property is critically important for two main reasons:
- Stability: It determines how stable the exciton is. A higher binding energy means a stronger attraction between the electron and hole, resulting in a more stable exciton that is less likely to be broken apart by lattice vibrations (phonons).
- Device Operation: For a device like an OLED or a solar cell to function effectively at room temperature, the exciton binding energy must be significantly greater than the thermal energy (kT, approx. 25 meV). If not, the excitons would spontaneously dissociate due to heat, preventing them from recombining to produce light or being separated to produce current.
7. How does an exciton's behaviour relate to the colour of light from an LED?
The colour of light emitted by an LED is directly related to the energy of its excitons. When an exciton recombines (i.e., the electron 'falls' back into the hole), it releases its energy as a single photon of light. The energy of this photon determines its colour, according to the equation E = hc/λ. A high-energy exciton will release a high-energy photon (e.g., blue or violet light), while a lower-energy exciton will release a lower-energy photon (e.g., red or green light). Therefore, engineers design the semiconductor material's band gap to control the energy of the excitons formed, thereby controlling the colour of the LED.
