Quantum dots have many applications, but they would be even more useful if the light they emit wasn’t so blinking random. Thanks to researchers at the Massachusetts Institute of Technology (MIT), this goal is now within reach. By finding a means of keeping quantum dots from blinking on and off – a problem that has blighted these nanoscale chunks of semiconducting material since their discovery – the MIT team paved the way for new, precision applications, including single-photon sources for quantum cryptography and biological imaging.
Quantum dots behave much like atoms in the sense that the band gaps between their electron energy levels mean that they can absorb and emit light at discrete wavelengths. Shining ultraviolet light on a quantum dot, for example, excites it to a higher-energy state. When the dot subsequently drops back down to its ground state, it releases the excess energy by emitting a photon in the visible range of the spectrum, allowing the dot to glow with vivid colours. While this property allows quantum dots to be used in high-end display screens and televisions, the fact that their light is intermittent means they are not used for applications like single-photon emitters, which require more precise control.
Making the blinking stop
In the latest work, researchers led by MIT’s Moungi Bawendi demonstrated that they could achieve such control by firing a beam of mid-infrared laser light at the quantum dots for 200 ms. This technique, which builds on advances in ultrafast electric-field pulse technologies, forces the dots to stop blinking for a period five times longer than the duration of the laser pulse.
The technique works because the blinking effect is thought to stem from excess electrical charge that builds up on the quantum dots when extra electrons attach to their surfaces. These extra electrons change the dots’ surface properties such that the dots release energy via processes that do not involve the emission of a photon. When these extra charges are exposed to a flash of mid-infrared light, however, they get “knocked off” the dots’ surface, explains team member Keith Nelson. This allows the dots to emit light in a stable fashion rather than blinking.
Extended applications on the horizon
The newly stabilized quantum dots might be employed in applications like quantum information science, which require a reliable source of single photons without any intermittency. Biomedical research might benefit too, says team member Jiaojian Shi. “There are many biological processes that really require visualization with a steady photoluminescent tag, like tracking applications,” Shi explains. “For example, when we take medicines, you want to visualize how those drug molecules are being internalized in the cell, and where in the subcellular organelles it ends up.” This could lead to more efficient drug-discovery processes, he adds, “but if the quantum dots start blinking a lot, you basically lose track of where the molecule is”.
Quantum dot array could make ultralow-energy switches
The team says the technique might also be used to stabilize other light emitters such as nitrogen-vacancy centres in diamond, which are employed in ultrahigh-resolution microscopy and as sources of single photons in optical quantum technologies. While producing mid-infrared laser light currently requires bulky and costly equipment, Nelson suggests the technique might not be limited to the mid-infrared range. If it could be extended to the terahertz (THz) range, he says, this could allow for much smaller and less expensive devices.
The research is detailed in Nature Nanotechnology.
