Quantum dots have recently made the leap from the lab to your local Best Buy in QLED TVs. These new TVs boast brighter colors due to a grid of quantum dots—tiny nanoparticles that are really good at emitting one color of light—incorporated into the display. To know how well a quantum dot works, we need to know how efficiently it emits light.
Until recently, we could only measure quantum dot efficiency to about 5% precision. This past summer, researchers at the Photonics at Thermodynamic Limits (PTL) Energy Frontier Research Center (EFRC) developed a new measurement technique that is 100 times more precise. This technique will allow scientists to gain new insight into some of the world’s brightest emitters by distinguishing 95% efficiency from 99.9% efficiency. This may not seem like a big difference, but knowing near-perfect efficiencies so precisely really matters. Ultrahigh efficiencies unlock applications even more exciting than more vibrant TV displays, such as optical refrigerators or optical batteries.
“There are a bunch of different applications that we can imagine, but we’ve never had access to them before, because we didn’t know whether our efficiency was even remotely high enough to achieve them,” commented Noah Bronstein, a former postdoctoral researcher at UC Berkeley in the PTL EFRC.
Quantum dots emit one bright color
Quantum dots are small semiconductor crystals, only 10–50 atoms across. They emit different colors (or energies) of light based on their size. A small quantum dot will emit blue—or high-energy—light, and a large quantum dot will emit red—or low-energy—light.
Typical LED TVs start with white light, and each pixel is given a unique color by adding red, blue, and green color filters. These filters are not very efficient, as they block a lot of the initial light. The new QLED TVs start with blue light, which gets absorbed and then reemitted by green and red quantum dots. Since quantum dots are so efficient, they waste less of the initial light. They also emit red, blue, or green light that is purer than the red, blue, or green generated by color filters. This leads to brighter and more accurate colors on our TVs.
A quantum dot can absorb a photon—or a particle of light—of any energy higher than its emission energy. Then, it can either reemit that light at its characteristic emission energy and the difference in energy between the absorbed and emitted photon is lost as heat, or, it can fail to reemit the light, losing all of the energy of the absorbed photon as heat. Truly efficient quantum dots reemit all of the light that they absorb, losing minimal energy to heat.
To measure the efficiency of a quantum dot, we need to know its quantum yield, or photons emitted per photons absorbed. This is just measuring the light you get out over the light you put in.
Counting photons is hard, measuring energies is easy
Measuring the light out as a percentage of the light in sounds pretty simple, but counting photons is hard. Photons are tiny and fast, and they travel in many directions once a material emits them.
Traditionally, scientists measure quantum yield by trying to keep track of all of the photons that a material emits. As light travels from the material to a detector, photons escape to the environment, getting lost along the way. Scientists have to estimate how many photons are “lost,” and the uncertainty in this estimation limits the quantum yield precision to plus or minus 5%.
The new measurement technique, called photothermal threshold quantum yield, or PTQY, measures energies rather than counting photons. This leads to precisions as good as .05%.
In PTQY, researchers keep track of whether absorbed light is lost as heat or emitted as light. To measure light lost as heat, they take advantage of a way of measuring heat very sensitively. The sample is immersed in a liquid that refracts light at different angles depending on how much it heats up. Small angle changes can lead to large separations at long distances—imagine how far away from your friend you would end up if you walked across the country at paths separated by 1º from each other! So, tiny amounts of heat can be measured using this technique.
In PTQY, scientists excite the quantum dots with different colors, or energies, of light. For each excitation energy, they measure how much light and heat the material gives off. The quantum yield can be extracted from the excitation energy threshold that results in net positive heat generation, divided by the quantum dot’s characteristic emission energy. Since measuring energies is easy, PTQY is extremely precise.
The researchers used this new technique to measure the quantum yield of quantum dots. Before, we only knew that these quantum dots were more than 97% efficient. With this new technique, we can now declare that they are 99.6±0.2% efficient.
Bronstein and David Hanifi, a graduate student at Stanford University in the PTL EFRC, conceived of the measurement technique together on pen and paper. “As a fundamental scientist that’s the most exciting part—to see a pen and paper idea materialize into something real, something that you can use,” said Hanifi.
Near-perfect materials open new applications
Many exciting applications require materials with quantum yields greater than 99%. It is only with such high efficiencies that we can imagine optical refrigerators, where photons take the place of a liquid coolant. At even higher efficiencies, optical batteries become possible, where photons can be stored in a cavity to store energy.
To know if we have created materials that are perfect enough to unlock these applications, we need to be able to measure them. This new PTQY technique—which measures quantum yields to 100 times higher precisions than was previously possible—represents a leap in the measurement of near-perfect materials.
