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A quantum dot (QD) is a nanoscale, artificially synthesized crystal composed of semiconducting material, with dimensions typically measured in nanometres—ranging from approximately 10 to 50 atoms in size.Quantum dots are called so because of their quantum mechanical properties and their dot-like size and shape.

The word “Quantum” refers to the fact that quantum dots exhibit quantum confinement effects. When a material is made extremely small—typically in the range of 2 to 10 nanometers—the motion of electrons and holes inside it becomes restricted. This confinement leads to discrete energy levels, similar to atoms.“Dot”-This part of the name comes from their nanoscale, nearly spherical shape. They are tiny particles—essentially “dots”—that are small enough to behave like artificial atoms.

Basic Structure of a Quantum Dot

Core:Made of a semiconductor material like CdSe (Cadmium Selenide), PbS (Lead Sulfide), or InP (Indium Phosphide).This is where the quantum confinement occurs, giving the dot its unique optical properties.

Shell (optional but common):A layer of another semiconductor material with a wider bandgap, such as ZnS (Zinc Sulfide).It protects the core and improves photostability and quantum yield.

Ligands / Surface Coating:Organic molecules or polymers that stabilize the quantum dot in solution.They also make the quantum dot biocompatible or soluble in specific solvents.This layer differs according to the specific application of quantum dots (QDs).

How They Work?

Working of quantum dots varies based on its application. Let us check how properties of Quantum Dots are used in different applications.

The most fascinating property of a quantum dot is its ability to emit different colors of light based on its size when light or electricity hits it (a phenomenon called luminescence).

QLED, which stands for Quantum Dot LED, is a type of display technology that enhances a traditional LED-backlit Liquid Crystal Display (LCD) television by incorporating a film of tiny semiconductor particles called quantum dots.

Working Principle of Quantum Dot LED display panels

The QLED display works by using a blue LED backlight to stimulate the quantum dots, which then emit purer, more saturated colors.

Blue LED Backlight: A full array of LEDs behind the screen (or sometimes along the edges) generates blue light.

Quantum Dot Layer: This blue light passes through a thin film containing billions of quantum dots.When the blue light hits them, the quantum dots re-emit light at very precise wavelengths.The size of the quantum dot determines the color it emits: larger dots emit longer wavelengths (red), while smaller dots emit shorter wavelengths (green).

Color Combination: The combination of the original blue light (which passes through the layer) and the new red and green light emitted by the quantum dots forms a pure white light source.

LCD Panel and Color Filter: This pure white light then passes through the remaining layers of the display, including the liquid crystal (LCD) layer and the color filters. The liquid crystals act as shutters, controlling the amount of light that passes through each red, green, or blue sub-pixel, which ultimately creates the final image on the screen.

The use of quantum dots provides a purer source of red, green, and blue light compared to traditional LCDs, resulting in a significantly wider color gamut (more vibrant and accurate colors) and much higher peak brightness.

Quantum Dots as Qbits

Quantum dots are used to create spin qubits, which are a leading technology for building solid-state quantum computers.The process leverages the quantum dot’s ability to precisely confine and control single electrons.

A semiconductor quantum dot acts as an “artificial atom.” It’s a tiny structure can electrostatically trap and hold a single electron. By adjusting the voltage on an electrode, researchers can precisely control the number of electrons trapped in the dot, down to a single electron.

The qubit, or quantum bit, is encoded in the spin of the confined electron:The electron’s spin has two natural quantum states: spin-up and spin-down .These two states are mapped to the qubit’s computational basis states. Applying a magnetic field splits the spin-up and spin-down states into separate energy levels.Quantum logic operations  are performed by manipulating the electron’s spin using electrical or magnetic pulses.

The quantum dot provides the crucial platform—a clean, stable, and electrically addressable environment—to isolate a single electron’s spin and control its quantum state, which is the definition of a spin qubit. This approach is highly scalable because the manufacturing process is compatible with existing semiconductor fabrication techniques (like those used for classical transistors).

Quantum dot solar cells

Quantum Dot Solar Cells use nanoscale semiconductor particles, such as lead sulfide or cadmium selenide, to boost light absorption and energy conversion. These quantum dots have distinct optical and electronic properties compared to larger materials. By layering quantum dots of different sizes, these solar cells efficiently absorb multiple wavelengths of light for improved energy capture over traditional single-material cells.

Quantum Dot Solar Cells can generate multiple excitons from a single absorbed photon. Excitons, which are pairs consisting of an electron and a positively charged hole, are produced when a semiconductor material absorbs a photon. By contrast, traditional solar cells typically generate only one exciton for each absorbed photon.

Quantum Dot Solar Cells address this limitation by incorporating quantum dots. These nanoscale particles, due to quantum confinement effects, have discrete energy levels. When a photon with energy greater than the quantum dot’s bandgap is absorbed, multiple excitons may be produced. This process, referred to as multiple exciton generation (MEG) or carrier multiplication, can increase the efficiency of solar cells. .

Quantum dots in biomedical imaging

The unique combination of small size, bright, size-tunable fluorescence, and durability makes Quantum Dots powerful tools for advancing biological research and medical diagnostics. Quantum Dots are ideal for tracking molecules and imaging tissues due to their unique properties.

Small Size (Nanoscale)- Their size ( to nanometers) is comparable to that of many large biomolecules (like proteins and antibodies). This allows them to be functionalized (coated) and attached to specific targets within the body or cells without significantly interfering with the target’s natural function.They can penetrate tissues and cells more easily than larger probes, making them suitable for in-vivo (within a living organism) imaging and for tracking individual molecules or cellular components.

Brightness (High Quantum Yield): QDs are significantly brighter and emit light more intensely than conventional organic fluorescent dyes. This high signal allows researchers to image targets with greater clarity and use fewer labels, resulting in high-contrast images.

In summary, quantum dots are versatile nanoscale semiconductors with unique optical and electronic properties, enabling breakthroughs in display technology, quantum computing, solar energy, and biomedical imaging. Their ability to emit precise colours and interact at the molecular level continues to drive innovation across various scientific and industrial fields.