A quantum dot is a man-made crystal composed of semiconductor materials, so small it is effectively a single point containing just a few thousand atoms. These nanocrystals are notable for their ability to emit intensely colored light when energized, behaving like tiny, tunable light bulbs. This optical property has positioned them as an important material in science and technology.
The Science of Size and Color
The defining characteristic of a quantum dot is how its size dictates the color of light it produces, a behavior governed by quantum confinement. Because the dot is so small, its electrons become spatially confined. This restriction forces them into discrete, quantized energy levels, similar to how an atom has specific energy shells.
When a quantum dot absorbs energy from a source like UV light, an electron jumps to a higher energy state before returning to its original level, releasing the excess energy as a photon of light. Smaller dots create greater confinement, which increases the energy gap the electron must cross. This results in a higher-energy photon, producing bluer light.
Conversely, larger dots have less confinement and a smaller energy gap, causing them to emit lower-energy, redder light. This precise, size-dependent tunability allows scientists to create a full spectrum of colors from the same base material simply by controlling the crystal’s dimensions.
Materials and Construction
Most quantum dots are built with a core-shell design to optimize their light-emitting capabilities. A central semiconductor nanocrystal is encapsulated by a shell of a different semiconductor material. This outer shell protects the core, passivates its surface to remove defects, and enhances both the brightness and stability of the light emission.
Common materials include a core of cadmium selenide (CdSe) with a zinc sulfide (ZnS) shell, though others like lead sulfide (PbS) are also used. The core material’s bandgap energy is a determining factor in performance. For instance, indium phosphide (InP) has a large bandgap that allows for broad color tuning.
Because materials like cadmium are toxic heavy metals, research has focused on developing cadmium-free alternatives for consumer and biological applications. Indium phosphide has emerged as a prominent, less toxic substitute for creating vibrant colors. This shift addresses environmental and safety concerns, expanding the potential uses for these nanomaterials.
Applications in Electronics
The most widespread commercial application of quantum dots is in QLED television and monitor displays. In these devices, quantum dots are not the primary light source but are used to enhance the color purity of an LED-backlit LCD screen. The technology uses a quantum dot enhancement film (QDEF), a thin sheet embedded with precisely sized red and green quantum dots.
This film is placed between the blue LED backlight and the LCD panel. When blue light from the LEDs passes through the QDEF, it excites the quantum dots. The smaller dots absorb some blue light and re-emit it as pure green light, while the larger dots convert another portion into pure red.
This process creates a high-quality white light source composed of very narrow red, green, and blue peaks, unlike the broad spectrum from standard white LEDs. This purer backlight allows the display’s color filters to produce more saturated colors, resulting in a wider color gamut and a more vibrant image. This principle is also being applied to general lighting for more efficient LED bulbs.
Medical and Biological Uses
In the biomedical field, the optical properties of quantum dots make them useful tools for imaging and diagnostics. Their brightness, resistance to fading, and tunable colors allow them to serve as superior fluorescent labels compared to traditional organic dyes. Scientists can attach quantum dots to specific biomolecules to track their movement and interactions within living cells.
The ability to produce many distinct colors from a single light source makes quantum dots suitable for multiplexed imaging, where multiple targets can be visualized simultaneously. This is useful in cancer research for identifying different biomarkers on tumor cells.
Beyond imaging, quantum dots are being explored for targeted drug delivery. By loading a therapeutic agent onto a quantum dot’s surface, the drug can be delivered directly to diseased cells. This approach could increase treatment efficacy while minimizing side effects on healthy tissue.
Role in Energy and Sustainability
Quantum dots offer avenues for advancing energy generation, particularly in photovoltaics. Solar cells built with quantum dots could surpass the efficiency of conventional silicon-based cells. Their tunable absorption properties allow them to capture wavelengths of light, such as those in the infrared range, that traditional materials miss.
A primary advantage in solar applications is multiple exciton generation (MEG). When a high-energy photon strikes a quantum dot, it can generate more than one electron-hole pair, or exciton. MEG allows this excess energy to be converted into additional electrical current, boosting power conversion efficiency. Theoretical calculations suggest that quantum dot solar cells could reach efficiencies as high as 66%.