The approach developed by Tang’s team begins with a fundamental question: how can infrared light be detected in a way that preserves information about its wavelength and intensity?
Selective quantum dots enable infrared color detection
Bulk semiconductors commonly used in infrared detectors contain continuous energy bands. As a result, they absorb a broad range of infrared wavelengths without retaining much information about the specific wavelength that reached the detector. The mercury telluride colloidal quantum dots used by the Chinese research team work differently.
These quantum dots are only about 4 nanometers wide. At that scale, quantum confinement divides their energy levels into discrete steps instead of a continuous band. Infrared photons with different wavelengths and intensities can therefore trigger different electronic transitions, moving electrons between specific energy levels inside each quantum dot rather than simply creating a stronger or weaker version of the same signal.
At wavelengths near 2 micrometers—the longest wavelength tested by the researchers—photons carry just enough energy to move an electron across the quantum dot’s fundamental bandgap. This process produces a relatively small number of charge carriers: negatively charged electrons or positively charged holes that can move through the quantum-dot layer.
Shorter infrared wavelengths contain more energy per photon and can activate additional, higher-energy transitions within the quantum dots. In some cases, the extra energy carried by a single photon can generate more than one electron-hole pair. These additional pathways increase the number of charge carriers produced by the detector.
As a result, shorter infrared wavelengths and more intense infrared light drive more positively charged holes from the quantum-dot layer toward the OLED. This wavelength- and intensity-dependent response allows the device to capture more information about incoming infrared light than conventional detectors.
However, generating a variable number of holes based on infrared wavelength and intensity solved only half of the challenge. The researchers also needed a way to convert that electrical signal into a full-color image on the opposite side of the device.
An energy barrier creates full-color infrared images
The team addressed this challenge by constructing the OLED from two separate light-emitting layers stacked on top of each other. The layer closest to the incoming holes was doped with a red-emitting phosphor, while the layer farther away contained a phosphor that emits cyan light.
The crucial element enabling full-color infrared imaging was an energy barrier measuring approximately 0.82 electron volts between the two emissive layers. This barrier controls how the holes move through the OLED, allowing the device to translate differences in infrared wavelength and intensity into visible red and cyan light.
Source: arstechnica.com