How does quantum dot display work?

Last Updated on September 8, 2024 by Narayan

Quantum dot (QD) displays represent a breakthrough in display technology, providing vivid colors, high brightness, and energy efficiency.

In this article, we will discuss the fundamental physics behind quantum dot display technology, covering the properties of quantum dots, their mechanisms of light emission, and how they are used in modern displays.

What are Quantum Dots?

Quantum dots are nanometer-sized semiconductor particles that exhibit unique electronic and optical properties due to quantum mechanical effects.

Typically, they range from 2 to 10 nanometers in size, about the width of 10 to 50 atoms. The small size of quantum dots leads to quantum confinement, significantly altering their electronic and optical characteristics compared to bulk materials.

The History of Quantum Dots

The concept of quantum dots (QDs) emerged in the 1980s, driven by studies on quantum confinement in nanoscale semiconductor particles. Alexei Ekimov and Louis Brus were pioneers in demonstrating the size-dependent properties of QDs, with Ekimov working on QDs in glass matrices and Brus studying them in colloidal solutions. In the 1990s, Moungi Bawendi at MIT advanced the synthesis techniques, enabling precise control over QD size, shape, and surface chemistry, which made them suitable for practical applications such as displays, imaging, and solar cells.

The 2023 Nobel Prize in Chemistry was awarded to Bawendi, Brus, and Ekimov for their foundational contributions to QD research, which have revolutionized nanotechnology and material sciences. Their work laid the groundwork for the use of quantum dots in various technologies, highlighting the importance of quantum mechanics in developing innovative materials with unique optical and electronic properties.

How to Make a Quantum Dot

Quantum dots are made using top-down or bottom-up methods. The top-down approach breaks down bulk materials into nanoscale particles using techniques like lithography and etching, but it lacks control over the size and surface properties. The bottom-up approach, which is more common, builds quantum dots atom by atom, allowing better control over their features.

The most popular bottom-up method is colloidal synthesis, where semiconductor materials react in a solvent with stabilizing agents to control their size and optical properties. Other methods include Molecular Beam Epitaxy (MBE), Chemical Vapor Deposition (CVD), hot-injection, and sol-gel synthesis.

After making the quantum dots, their surfaces are modified with special chemicals (ligands) to improve stability and keep their light-emitting properties. These processes ensure the desired properties of quantum dots for use in displays, medical imaging, and quantum technologies.

Electronic and Optical Properties of Quantum Dots

The distinctive properties of quantum dots are primarily due to their quantum confinement effect. In bulk materials, the electrons and holes (the absence of an electron) are free to move throughout the material.

However, in quantum dots, the movement of these charge carriers is restricted in all three spatial dimensions, confining them to a very small volume.

This confinement leads to the following key effects,

• Discrete Energy Levels: Unlike bulk materials, where energy levels form continuous bands, quantum dots have discrete energy levels due to the limited space available for the electrons. This results in distinct energy states similar to those in atoms, often referred to as “artificial atoms.”
• Size-Dependent Emission: The energy difference between the confined states (the bandgap) depends on the size of the quantum dot. Smaller dots have a larger bandgap and emit shorter wavelength light (towards the blue end of the spectrum), while larger dots have a smaller bandgap and emit longer wavelength light (towards the red end). This phenomenon is governed by the equation,
  \[E = \frac{h^2}{8R^2} \left( \frac{1}{m_e} + \frac{1}{m_h} \right)\]
  where, \( E \) represents the energy of the emitted photon, \( h \) denotes Planck’s constant, \( R \) is the radius of the quantum dot, and \( m_e \) and \( m_h \) are the effective masses of the electron and hole, respectively.
• Photoluminescence: When quantum dots absorb photons, they excite electrons from the valence band to the conduction band, creating electron-hole pairs called excitons. When these excitons recombine, the energy is released in the form of light. The wavelength (and hence color) of the emitted light is determined by the size of the quantum dot due to the quantum confinement effect.

Mechanism of Light Emission and Color Generation in Quantum Dots

The light emission process in quantum dots involves several steps:

1. Photon Absorption: A quantum dot absorbs a photon, causing an electron to jump from the valence band to the conduction band, leaving behind a hole.
2. Exciton Formation: The electron and hole pair up to form an exciton, bound together by Coulombic attraction.
3. Recombination and Emission: The electron eventually recombines with the hole, and the energy released during this recombination is emitted as a photon.

The color of the emitted light depends on the energy difference between the electron’s initial and final states, which is size-dependent due to quantum confinement. This size-dependent property allows quantum dots to emit light of specific wavelengths (colors) with high precision, making them ideal for displays.

Quantum Confinement and Energy Band Structure

The behavior of quantum dots can be described using the concept of quantum confinement, which modifies the energy band structure of the material. In quantum dots, the energy of the electron-hole pair (exciton) is quantized due to confinement, and the solutions to the Schrödinger equation give the energy levels for a particle in a potential well.

The effective bandgap energy \(E_g\) of a quantum dot can be approximated by

\[E_g(R) = E_{g, \infty} + \frac{\hbar^2 \pi^2}{2R^2} \left( \frac{1}{m_e} + \frac{1}{m_h} \right)\]

where, \( E_{g, \infty} \) is the bulk bandgap energy, \( \hbar \) represents the reduced Planck’s constant, \( R \) denotes the radius of the quantum dot, and \( m_e \) and \( m_h \) are the effective masses of the electron and hole, respectively.

This equation shows that as the size of the quantum dot decreases, the bandgap energy increases, shifting the emission towards the blue end of the spectrum.

Working Principles of Quantum Dot Displays

Quantum dot displays, such as Quantum Dot Light Emitting Diodes (QLEDs), utilize the unique properties of quantum dots to produce vibrant and accurate colors. Here’s how they work:

1. Structure of QLEDs: QLED displays consist of a backlight unit (typically blue LEDs), a quantum dot enhancement layer (QDEL), and color filters. The QDEL contains quantum dots of different sizes that convert the blue LED light into red and green light through photoluminescence.
2. Role of Quantum Dots in Color Conversion: When blue light from the backlight strikes the quantum dots, they absorb the energy and re-emit it at a specific wavelength (color) based on their size. Larger quantum dots emit red light, and smaller ones emit green light, while the unabsorbed blue light passes through, creating the full color spectrum.
3. Advantages Over Traditional Displays:

• Wide Color Gamut: Quantum dots can emit pure and highly saturated colors, resulting in a broader color range than traditional displays.
• Higher Brightness and Energy Efficiency: Quantum dot displays require less energy to produce the same brightness levels because they emit light directly without needing color filters or polarizers.
• Better Color Accuracy: Quantum dots provide precise control over color output, leading to more accurate and consistent colors.

Mathematical Description of Quantum Dot Emission

The recombination of excitons in quantum dots is central to their light-emitting properties. The rate of radiative recombination (R_r) is given by,

\[R_r = B n_e n_h,\]

where, \( B \) is the radiative recombination coefficient, while \( n_e \) and \( n_h \) represent the densities of electrons and holes, respectively.

The emission spectrum of a quantum dot can be modeled using the Gaussian distribution, which reflects the size distribution of the quantum dots in the ensemble,

\[I(E) = I_0 \exp \left( – \frac{(E – E_0)^2}{2\sigma^2} \right),\]

where, \( I(E) \) represents the intensity of emitted light at energy \( E \), \( I_0 \) is the peak intensity, \( E_0 \) denotes the central emission energy, and \( \sigma \) represents the standard deviation of the energy distribution, which is related to the size distribution of quantum dots.

Practical Applications in Modern Display Devices

Quantum dot displays are now widely used in televisions, monitors, and other devices where color accuracy and efficiency are crucial. The following features highlight their advantages:

• Enhanced Color Reproduction: QLED displays offer superior color performance with more vibrant and accurate color reproduction.
• Improved Brightness and Contrast: The high brightness levels of quantum dot displays provide better visibility in bright environments and enhance the overall viewing experience.
• Energy Efficiency: Quantum dots emit light directly, reducing power consumption and increasing the display’s energy efficiency.

Conclusion

Quantum dot display technology is an exciting use of quantum mechanics and nanotechnology. Quantum dots have unique electronic and optical properties, like size-dependent emission and photoluminescence, which are used to create displays with better color accuracy, brightness, and energy efficiency. With the advancement of this technology, we can expect further improvements in display quality and performance, making quantum dot displays a key part of next-generation visual devices. Understanding the science behind quantum dots and their use in displays helps us see the innovative ways they enhance the visual quality of modern digital displays.