80% of the sensory information we take in relies on our vision. It helps us recognize the forms, brightness, and colors of objects. Quick question: What makes it possible for us to distinguish different colors?
The retina in the human eye has three types of cones*. Each responds to different wavelengths of light. Generally, visible light ranges between 400-700 nanometers (nm), in which a wavelength of 700 nm is perceived as red and 400 nm as purple. In the middle, somewhere around 550 nm, light is perceived as green.
* A type of photoreceptors involved in sight
This means that what we see as a specific color on an object is an interpretation of light wavelength. A leaf is green because our eyes detect a wavelength of 550 nm, while an apple looks red because we see light reflected with a 700 nm wavelength. If we were to reflect light of all wavelengths, we see white like the clouds in the sky. On the contrary, when an object absorbs light of all wavelengths such as graphite, it looks black.
But there are other ways that light is emitted and perceived by us: Think about fire, lamps, stars and the sun. They’re not reflecting light but rather creating light from within. A simple experiment can demonstrate this characteristic as heating up a pair of metal chopsticks above the stove flame will show how their color changes. At first, the chopsticks will look the same, but as they continue to be heated, metal turns red. Fiery red as we call it. But if we were to push it further with higher temperature, red will become orange, then yellow, and finally white. In the end, all we’ll see is a bluish-white remnant of the metal chopsticks.
Colors change when the wavelength of most of the light emitted by an object becomes shorter as the temperature rises. At lower temperatures, an object emits red light, which has a relatively long wavelength, but as the temperature increases, the color transforms to orange and then eventually to blue, which has a shorter wavelength. When the object gives off light in varying wavelengths, the object is seen as white. When the temperature rises even further, the object emits far more light in the range of wavelengths for blue, eventually being perceived as bluish-white in the human eye.
Size Determines Color
As described so far, objects we normally see are perceived through reflective light or self-emitting light. Objects that are minuscule in size, however, are a different story. Take gold as an example: Its golden yellow color stays consistent even when it’s big like a gold bar or small like a ring. However, its color changes once its size is reduced to a few nanometers.
When gold is reduced to a particle that is 7 nm, it becomes red; when reduced to 5 nm, green; and when reduced to 3 nm, blue. This is attributed to a phenomenon called “localized surface plasmon resonance,” which occurs when the particle size of an object is smaller than the wavelength of light. Metals such as gold and silver carry free electrons on the surface that are not held by the atomic nucleus. These free electrons come in contact with light. Light has properties of both particles and waves, but in this context, its wave-like characteristics are important. Light creates waves through the interaction between the electric and magnetic fields. When electrons resonate with the waves of light in the visible spectrum, electrons collectively oscillate. This collective oscillation is referred to as plasmons. The waves of light caused by the oscillation of electrons result in different colors of the nanoscale particles of a metal compared to when it is in a bigger size.
When the nanoparticles increase in size, the resonance also grows, broadening the waves of light. Particles that are 3 nm are blue in color, but as the particle size increases, it appears red.
Such color changes can be seen not only in metals, but also in semiconductors. Semiconductors consist of silicon and one of the following two elements: elements from group 13 such as zinc or cadmium, which tend to lose electrons, or elements from group 15 such as sulfur or selenium, which tend to gain electrons. When a light-emitting device made of semiconductors is connected to an electric circuit where electric current flows, the electrons become excited by absorbing the electrical energy. Once excited electrons fall back to a state of lower energy and stabilize, they emit light. The energy gap (or often dubbed the “band gap”) between the excited and ground state determines the color of light being emitted. Properties and structure of the compounds that comprise the semiconductor shape the band gap. Reducing the size of the semiconductor to nanoscale results in a change in the band gap. In other words, color changes.
What If Light-Emitting Devices in Displays are Colored?
Displays for TVs, monitors, and smartphones express a wide variety of colors using the primary colors of red, blue, and green. It’s been that way with the cathode-ray tube TVs from the past, liquid-crystal displays (LCDs), and light-emitting diode (LED) displays as well. Equipped with a separate light source, cathode-ray tube TVs and LCDs expressed colors by using a color filter for the three primary colors. On the other hand, LEDs (short for “light emitting diode”), which were introduced in the 21st century, have light-emitting structures with three primary colors even without color filters. This slims down the thickness of a display and enables production of various curvatures. Color expression also becomes more detailed. Reduced power consumption is another notable advantage.
However, enhancing the level of color expression by decreasing the size of LEDs leads to a reduction in the amount of light emitted, which needs to be made up for. To make things even more challenging, reducing the gap between each individual light-emitting diode is not an easy task.
As a solution to the challenges previously mentioned, Samsung Display chose to utilize quantum dot (QD) technology, which leverages the band gap created by the nanoscale size of semiconductors. The nanoscale light-emitting semiconductors emit blue light at 2 nm, green light at 3 nm, and red light at 7 nm. A quantum dot is structured with a core that emits light in varying colors wrapped in a shell with ligands (or lipids) attached to the outer surface of the shell. As the size of QDs is extremely minuscule, color expression can be much more detailed. QDs offer reproduction of richer and more accurate colors in each contrast level, drawing the attention as the next-generation material for large displays thanks to its efficient use of light and simple structure.
Aren’t you mesmerized by the reality unfolded by the nanoscale world where colors are realized in ways beyond our common sense?
※This article holds the opinions of the author and does not reflect the stance nor strategies of Samsung Display Newsroom.