LET THERE BE (MORE) LIGHT: This graphic depicts a pixel sensor with a layer of QuantumFilm placed between the color filter (green) and metal transistor (gray) and silicon (blue) layers. Light passes through the color filter and is absorbed by the QuantumFilm rather than traveling to the silicon layer. Image: © INVISAGE TECHNOLOGIES, INC.
Semiconductor crystals known as quantum dots have long held the promise of improving solar cells, lasers and lighting fixtures, but the reality is that integrating these fluorescent nanoparticles into existing technologies has proved difficult. One Silicon Valley start-up now aims to change this by the end of next year using quantum dots to vastly improve the picture-taking quality of cell phone cameras.
The secret, according to Menlo Park, Calif.–based InVisage Technologies, Inc., is a new material called QuantumFilm, which the company introduced Monday at the DEMO Spring 2010 conference in Palm Desert, Calif. QuantumFilm is an extremely light absorbent coating, according to InVisage, that will enable pixel sensors to capture about 95 percent of an image, nearly a fourfold increase over current image sensors. QuantumFilm exists today as a working prototype, with InVisage planning to have production-quality samples ready by year's end.
QuantumFilm's dots are only a few nanometers in diameter, about the thickness of a biological cell membrane. Whereas there are a number of different ways to make quantum dots, one of the most common is colloidal synthesis, where they are grown using a combination of chemicals and heat. The dots—whose composition depends on the chemicals used to fabricate them—form in different shapes and sizes, and both of these factors determine their conducting properties. Smaller dots emit colors closer to the blue end of the spectrum. The larger they get, the redder they get.
A typical camera phone pixel sensor consists of several layers, including a base layer of silicon used by the sensor's electronic transistors and photodetectors. The top layer, typically made of colored plastic or glass, acts as an array of color filters. Sandwiched in between are many layers of metal needed to connect the silicon electronic transistors together. In this scenario, each silicon photodetector is like a bucket that collects light, after which the transistors read the information in that bucket to generate each image, says Ted Sargent, InVisage's founder and chief technology officer.
However, because the light coming into the sensor has to pass through several layers of metal before reaching the silicon, which is a weak light absorber, the sensor detects only about 25 percent of the light that makes up the image, says Sargent, who is also a professor of electrical and computer engineering and Canada research chair in Nanotechnology at the University of Toronto. "You want to be completely absorptive of all the light that hits the sensor," he says. "You need a lot of silicon to stop all of the photons, but you can't have enough silicon in the line of sight to absorb all of the light."
InVisage places a layer of QuantumFilm above the silicon and the metal layers so that it is directly under each sensor's color filter array. Light passes through the color filters and is absorbed by the QuantumFilm. "The light no longer needs to reach the silicon wafer, which is now only used for its excellent electronic properties," Sargent says. This leads to greater sensitivity and translates into improved resolution, he adds.
Whereas larger, more expensive digital single-lens reflex cameras use larger pixels that can capture more light, QuantumFilm is designed to enable more compact cameras—such as those found in cell phones—to capture more light via smaller pixels. "We've come to live with our cell phone cameras being not all that terrific, but wouldn't it be great if we didn't have to make that compromise?" Sargent says. "As the pixel gets smaller, silicon is reaching the end of its usefulness in that area. Silicon was not put on this Earth to be a light absorber."
One common application for quantum dots is to use them as cell markers in biomedical research, says Todd Krauss, an associate professor of chemistry and optics at the University of Rochester's Institute of Optics who studies the fundamental properties of quantum dots as well as their optical properties. "That's really by far the biggest market for quantum dots so far," he says. Although Krauss is unfamiliar with InVisage, he contends that silicon has improved as a light detector over time and questions whether quantum dots would significantly improve silicon's performance.
Of course, it remains to be seen whether QuantumFilm will affect digital cameras in the way InVisage claims. After several years of flying under the radar the company is only now making itself more widely known and would not provide details about the composition of QuantumFilm, citing competitive reasons for secrecy.
To date, quantum dots have been an exciting discovery in search of the proper application. Companies have been using these dots to produce more efficient photovoltaic cells as well as to make light-emitting diodes (LEDs) more versatile. Nanomaterials-maker QD Vision, based in Watertown, Mass., manufactures a quantum dot optical film that they claim makes LEDs energy efficient, glow with a warmer incandescence and have an operational lifetime in excess of 50,000 hours. "This is the first sort of commercial electronic application for quantum dots," says David Hwang, a research associate with Lux Research, Inc.
In addition to solar cells and LEDs, some companies have tried to use quantum dots to improve their display technologies, but producing specific colors is challenging and continues to be expensive. "If you need quantum dots to produce a particular wavelength of light, you need to finely tune the size of the dots," Hwang says. However, whereas the size of the quantum dots is extremely important when making color display screens, it may prove to be less of a concern when creating a material used to improve a pixel's ability to capture light in general, something that could work in InVisage's favor.