A New Image Sensor Using Quantum Dots Could Replace CMOS

Quantum dots as a tool for next-generation color image sensors

Quantum dots are nanoparticles that are currently most commonly found in displays, but researchers have managed to leverage the tech to detect wavelengths of light on a sensor. The team argues the tech is capable of replacing CMOS sensors.

Quantum Dot Technology

Quantum dot, or QLED, technology uses a panel of phosphorescent crystals that react to light and electricity to enhance the image quality on televisions. These quantum dots are, individually, extremely small — less than 500 nanometers in size — and can glow in a range of colors that is determined by what atoms are found in each. As Viewsonic explains, instead of using pure white backlights, a television with QLED tech will instead emit red or green when struck by a blue light, which means more highly saturated and accurate colors than a LED screen without quantum dots.

“Every pixel on the monitor emits red, green, or blue light, or sometimes a combination of all three,” Viewsonic explains. “The accuracy of the color of each pixel is defined by wavelengths. And quantum dots can be easily tuned to their determined size to release different specific wavelengths for the best color production.”

The image below shows how quantum dots are arranged in a QLED display:

quantum dots arranged in a display

Quantum dots are extremely useful for significantly improving the quality of a display. Researchers have managed to use these exact principles but in reverse: instead of using quantum dots to display colors, the tech is being used to record them.

The Quantum Dot Sensor

CMOS sensors are great, but there are limitations to where they can be implemented and how they record data is also limited by their size.

“Many rising fields of application, such as self-driving cars, flexible electronics, and healthcare and medical imaging, demand even higher resolutions and levels of integration. This is difficult to achieve because of the way each pixel of a color image is captured,” the researchers explain.

“In most image sensors, the red, green, and blue components of a given pixel are captured independently using a dedicated photodetector ‘cell’ for each color. While the three cells of each pixel are arranged laterally and as close to each other as possible to use the available area efficiently, this design takes at least thrice as much space as each individual cell. In addition, the manufacture and processing costs for these photodetector arrays can be high due to their complexity.”

The researchers, led by Professor Sung Kyu Park of Chung-Ang University in Korea, explain that they have developed a new type of sensor that leverages quantum dot technology.

The sensor uses vertically stacked quantum dots that, as mentioned above, are each sensitive to specific light frequencies. As light passes through the layers of differently tuned dots, only the light that would react with the dot of a specific frequency would be triggered, which is how the sensor knows what color to record that information.

The researchers say that the pixel structure uses much less area per pixel than conventional image sensors which means more of them can be placed in a space than with current CMOS technology.

“The device density of our photodetector array is 5,500 devices per square centimeter, which is remarkably larger than that reported for previous solution-processed flexible photodetectors, which reaches up to 1,600 devices.”

Basically, using a low-temperature fabrication method, the researchers managed to fit in a significantly higher number of pixels in the same small area than is currently possible with conventional methods.

Additionally, the researchers say that making the sensors is much simpler and the resulting sensor has high photosensitivity and great durability, meaning the design could be used for high-resolution sensors across a wide variety of applications.

“We think our design is a great advancement towards establishing a low-cost, high-resolution and integrated image sensor system that goes beyond conventional ones,” Park says.

The full research paper can be read in the January issue of Advanced Materials.

Image credits: Header image provided courtesy of research team and depicts quantum dots as a tool for next-generation color image sensors.