Researchers have succeeded in chemically synthesizing cadmium selenide nanocrystals in a flask that are as perfect as materials grown at higher temperatures and in very controlled environments. They have also measured the exceptional photoluminescence efficiency of the materials using a new measurement technique called photothermal threshold quantum yield. The crystals could find use in advanced applications such as luminescent solar concentrators (LSCs) and optical refrigerators.
“From the optical perspective, these materials are nearly perfect,” says Alberto Salleo of Stanford University, who co-led this research effort with Paul Alivisatos of the Lawrence Berkeley National Laboratory. “The British theoretical physicist Sir Charles Frank famously stated that ‘crystals are like people, it’s the defects in them that make them interesting’. So, in this sense, these materials are ‘boring’ because they are so perfect, but they do, however, have unprecedented photoluminescence performance.”
Efficient photoluminescence needed for applications
Many optical applications, including solid-state lighting, colour displays and bioimagers, rely on efficient photoluminescence (the absorption and reemission of light), and the quantum yield of this process is extremely important. Indeed, when it approaches 100%, advanced devices like LSCs and spectrum-shifting greenhouses become possible.
Photoluminescence occurs when an absorbed photon excites an electron from a ground state into a higher-energy excited state, leaving behind a hole. Both charge carriers then quickly relax, in a matter of picoseconds, to the band edges by emitting thermal phonons into the crystal lattice. A few nanoseconds later, the thermalized electron and hole recombine, bringing the material back to its ground state, and this final transition can be either radiative or nonradiative (that is, mediated by defects by emitting more heat).
The performance of devices is measured using the photoluminescence quantum yield (PLQY), which is determined by the competition between radiative relaxation of the photoexcited charge carriers and nonradiative loss. The highest values recorded to date are 99.5 and 99.7 for rare-earth-doped high-bandgap single crystals and epitaxially-deposited thin films, respectively.
Thin films good, nanocrystals better
For commercial applications, it would be better to use nanocrystals (quantum dots), however, because they are more stable and cheaper than their bulk/thin-film counterparts and can be easily placed inside a host of composites, fluids, polymers and even biological environments. They can also be processed over large areas and their light absorption and emission can be tuned.
The PLQY for CdSe/CdS, which is the main core-shell quantum dot studied in research labs today can reach 95%. While high, this is not good enough for applications in which only an absolute minimum amount of light energy should be lost as heat. Indeed, optical refrigeration, thermophotovoltaic engines and thermal energy storage in optical cavities all require PLQYs of more than 99%, with negligible nonradiative losses.
Alivisatos’ team says that it has now succeeded in achieving such PLQYs by growing a 4- to 11-monoloyer CdS shell around a CdSe core using a modified version of a technique reported on in 2013 that produced a material with few surface traps while maintaining a high radiative efficiency.
Photothermal threshold quantum yield
The researchers had to overcome a problem first though to measure the photoluminescence efficiency of their materials: existing techniques do not have the accuracy needed to measure very high PLQYs, and often suffer from at least 2 to 5% uncertainty. To overcome this shortfall, they developed a measurement of the PLQY that does not rely on measuring photon flux but instead makes use of the quantization of light in a process analogous to the photoelectric effect.
Quantum dot characteristics prove not so permanent
“The technique is based on accurately measuring the instances where electron-hole recombination actually occurs nonradiatively and produces heat,” Salleo tells Physics World. “In a perfect emitter, electron-hole pairs at the band edges are expected to recombine without emitting any heat. Deviation from this expected ideal behaviour can be detected very sensitively, which makes our method, which we have called the photothermal threshold quantum yield (PTQY), very accurate.”
The team, reporting its work in Science 10.1126/science.aat3803, says it measured a PTQY of as high as 99.6 +/- 0.2%. “This value indicates that only 0.4% of the electron-hole pairs that are photoexcited recombine by giving off heat – that is, there is almost complete suppression of nonradiative PL decay channels. In other words, for every 1000 photons absorbed, the nanocrystals re-emit 996 photons, which can then be exploited in advanced optoelectronic devices.”
The researchers say they are now looking at the quantum yield of other materials recently made in Alivisatos’ lab.
