A novel quantum dot method for generating infrared light opens the door to mid-infrared lasers and cost-effective sensors.
A team of researchers led by Philippe Guyot Sionnest, a professor of physics and chemistry at the University of Chicago, recently discovered a way to generate infrared light through colloidal quantum dots that opens the door to the possibility of redefining the mid-infrared range (3 to 5 µm), as the dots they obtained on their first attempt were almost as effective as existing conventional methods.
Colloidal quantum dots are semiconductor nanocrystals/particles with a diameter of about 5 to 20 nm, usually made of cadmium selenide (CdSe), cadmium sulfide (CdS), lead sulfide (PbS), zinc oxide (ZnO), and indium phosphide (InP), which have unique optical and electronic properties. Electron waves resonate inside these particles, like sound or light waves in a cavity, and it creates steady states that can be spectrally tuned to the size of the nanocrystals.
Quantum dots that produce visible light have been found in commercial products such as light-emitting diodes (LEDs) and televisions. But so far, if one wants quantum dots that can produce mid-infrared light, it is usually difficult to achieve.
While organic molecules are made of light atoms, which are ideal for dyes and fluorescence in the visible range, they don't perform as well in the mid-infrared range, where the molecules vibrate also in the mid-infrared region and rapidly suppress electronic excitation.
Inorganic semiconductor quantum dot materials are soluble like dye molecules and have tunable electronic excitations in the mid-infrared band, but they are composed of heavy atoms that vibrate at much lower frequencies, which makes them good infrared and solution-processable materials," says Guyot Sionnest. That's what gave us the idea to study infrared semiconductor quantum dots - it started 25 years ago."
Infrared lasers are currently fabricated by a molecular epitaxy process, which, while effective, is labor-intensive and expensive. Therefore, the researchers wanted to create a better way to realize infrared lasers based on quantum dots.
Quantum mechanics and the cascade effect
The team decided to explore a "cascade" technique that is widely used to make lasers. To do this, they made a black ink made from trillions of tiny core/shell HgSe/CdSe nanocrystals, coated it with a conductive electrode, evaporated a second conductive electrode on top, and energized it.
Their method involves running an electric current through the device, sending millions of electrons to the device. If successful, the electrons will pass through a series of different energy levels, similar to falling down a series of waterfalls. Each time an electron drops an energy level, it gets a chance to emit energy in the form of light. It works thanks to quantum mechanics.
Guyot Sionnest explains, "In a cascade LED, we deal with two states of the quantum dot: the lowest ground state, which is analogous to the s state of the hydrogen atom, and the first excited state, which is analogous to the p state." When an electron relaxes from the p-state to the s-state, it emits mid-infrared light. The bias between the points allows the electron to tunnel from this s state to the p state at the next point, and so on."
To the team's surprise, they saw light on their first attempt to generate infrared light through colloidal quantum dots.Guyot Sionnest said, "The first attempts at our new method of generating infrared light were very effective, and once the efficiency of generating light within the quantum dots is increased, their performance will improve by several orders of magnitude. These light sources will then be able to achieve unprecedented efficiency and low cost."
Guyot Sionnest explains, "The preferred tunneling from the s-state of one quantum dot to the p-state of the next quantum dot is far from obvious, as it is also possible to simply go from the s-state of one dot to the s-state of the next. We initially thought that this preference would require resonance at a finely tuned bias, but in some as yet unknown way the electrons are arranged in a cascade rather than flowing downward, so the bias does not matter."
There are no major challenges involved in this work, as it is an application of the team's previous work on making fluorescent infrared quantum dots in the lab, and they already have experience of making the first mid-infrared LEDs with quantum dots, and measuring their output light.
"But it does require an unusual combination of skills at the chemical and physical interfaces." Guyot Sionnest says, "thanks to Xinygyu Shen and Ananth Kamath. very few teams have been able to combine the chemical skills to make quantum dots, the fabrication tools to make the devices, and the mid-infrared instrumentation to characterize them."
Optical gas sensors and lasers
The most obvious and likely application of infrared light generated through quantum dots is optical gas sensors, says Guyot Sionnest: "The mass production of fast and efficient quantum dot LEDs, and similarly fast and efficient quantum dot detectors, will make optical gas sensing much cheaper than current semiconductor technology. It will also provide better sensitivity than low-cost technologies based on heat sources and thermoelectric detectors."
Lasers are a possible extension of this work, but it is not certain that they will be realized. Beyond that, commercial applications may require the use of quantum dots that are free of toxic and regulated elements such as mercury, cadmium and lead.
Xingyu Shen, a graduate student at Guyot Sionnest, said, "A cost-effective and easy-to-use method of creating infrared light from quantum dots could be very useful."
