Core ConceptsThis article discusses quantum dots and their unique and often tuneable optical properties. You will learn where quantum dots tuneability comes from, and how they are made.What Are Quantum Dots?Quantum dots are nanoscale semiconductors, with semiconductor being defined as a material that has electrical conductivity capabilities in between that of a conductor and an insulator. They are generally characterized by their incredibly small radius, measuring in at 1-5 nanometers (nm). When illuminated by UV light, an electron is excited within the particle, causing the electron to jump up to a higher energy state. When the electron drops back down from its excited state, light is produced as the energy is released. The light emitted is dependent on the size of the transition between energy states. However, since the particles are so small, the motion of the electrons is restricted This causes the energy levels to be quantized, resulting in quantum confinement. Due to this, optical properties are exhibited by the quantum dots that are unlike those of bulk materials.Conventional SemiconductorsTo fully comprehend what is so unique about quantum dots, a basic understanding of conventional semiconductors is essential. Materials used in everyday electronics, such as silicon, are considered to be conventional semiconductors. The materials have continuous energy bands: the valence band, filled with electrons, and the conduction band, where electrons can freely move around and conduct electricity. Between these bands is a gap, referred to as the bandgap. To jump from the valence band to the conduction band, the electrons have to gain enough energy, allowing the material to conduct electricity. The bandgap is the determinant of electrical and optical properties and is a fixed property.Quantum dots are unique because the bandgap is not fixed. The nanoscale size of them results in discrete energy levels, which we refer to as quantum confinement. This effect allows for the tunability of the electronic properties, done simply by changing the size of the quantum dot. The smaller the dot is, the larger the bandgap is, thus emitting higher energy light. Larger dots have smaller bandgaps and emit more reddish, low energy light.Properties of Quantum DotsSize-DependencyThe color of light that a quantum dot emits can be altered through the variation of particle size. Smaller particles give off higher energy light, such as blue, while larger particles emit lower energy light, such as red. Since the color can be changed so easily, quantum dots with specific wavelengths can be created.Brightness & StabilityQuantum dots are known for their high photoluminescence. This is a result of the quantum mechanical properties of the particles, including quantum confinement and many others. They are also very stable, meaning that the color doesn’t degrade as fast as other dues when exposed to light.Synthesis of Quantum DotsOne common method of synthesis involves the chemical reduction of metal precursors in solution. This is commonly referred to as colloidal synthesis. The size of the quantum dots can be controlled by changing the reaction conditions such as time, temperature, and concentration of reactants. Another methos involves atoms being deposited onto a substrate in a controlled manner. This is called molecular beam epitaxy and allows for specific size and compositions to be obtained. Materials such as cadmium selenide indium phosphide, and lead sulfide are common materials used to make quantum dots.Applications of Quantum DotsMedical ImagingSince quantum dots are both bright and stable, the possibilities for their use in imaging cells and tissues is seemingly endless. Through conjugation with larger biomolecules, specific cancer cells can be targeted and detected easily due to the fluorescence, allowing for quality imaging to occur.Solar CellsDue to the broad absorption spectrum exhibited by quantum dots, and the ability to vary the size of the energy level gaps, there is great potential for applications in solar cells. Using these same techniques, solar cells could be designed and optimized to more efficiently convert the sunlight captured into electricity. There is active research in this area, as more cost effective and more efficient solutions to solar energy are proposed.QLEDs and QLED TVsQLEDs (quantum dot light-emitting diodes) are one of the most well-known applications of quantum dots. These TVs use the unique properties of quantum dots to produce more vibrant displays than conventional semiconductors are capable of doing. The quantum dots are located in the panels of display, where they work to convert the backlight into primary colors. This enhances the picture quality greatly. This also improves the efficiency of the TV from an energy standpoint.ChallengesMany quantum dots are made from materials like cadmium, which is toxic. This poses a challenge for both environmental and biomedical applications. The manufacturing of process also poses a challenge, as producing them with proper quality and size is difficult on a large scale. These are all challenges that are identified, and researchers are actively working to resolve.ConclusionQuantum dots are a remarkable way of displaying how quantum mechanics play a role in aspects of cutting-edge applications and technology. Continued research and development of these nanoparticles has the potential to open up possibilities to lots of advancements in many aspects.
