Can cheap quantum-dot films detect very long-wave infrared?
Large mercury telluride quantum dots, with tuned surface chemistry, made solution-processed detectors that respond to infrared light out to 18 micrometres.
Source
Very long wave infrared quantum dot photodetector up to 18 μm
Study at a glance
- Design
- Other — Large HgTe colloidal quantum dots grown by a re-growth method, ligand-exchanged and iodine-treated, then made into photoconductors tested at 80 K with a blackbody source; DFT band calculations support the optics.
- N
- No single N: two main device types (VLWIR and LWIR quantum-dot photoconductors), plus low-mobility comparison devices.
- Population
- HgTe colloidal quantum-dot films and photoconductor devices
- Outcome
- Spectral cutoff, carrier mobility and doping, responsivity, quantum efficiency, noise and specific detectivity
Structured fields used in claim comparison tables when every cited study has a complete layer.
What they did
The team grew unusually large HgTe colloidal quantum dots (about 13.9 and 15.6 nm across) that stayed stable in solution, exchanged their surface ligands to pack them closer, and soaked films in dilute iodine to remove excess electron doping. They measured absorption, doping level (by electrochemistry and transistor measurements) and carrier mobility, and calculated band structures with density functional theory. Films about 500 nm thick were made into photoconductors and tested with a blackbody source while cooled to liquid-nitrogen temperature.
What they found
The very-long-wave device responded up to 18 µm and the long-wave device up to 10 µm. Ligand exchange raised carrier mobility to about 10 cm²/Vs, roughly 100 times higher than without it, and iodine treatment made the films nearly intrinsic. At 80 K the 18 µm detector reached a specific detectivity of 6.6 × 10^8 Jones and the 10 µm detector 2.3 × 10^9 Jones. A low-mobility comparison device was about 670 times less responsive, showing that carrier drift length relative to the electrode gap limits performance.
The limits
What it doesn't show
The detectors only work well when cooled to 80 K, and even there the very-long-wave device has more dark current than photocurrent, so it is far from room-temperature imaging. Quantum efficiency stays low (a few percent) because the films absorb weakly, and the authors note carrier lifetimes remain shorter than in established HgCdTe and superlattice detectors. Only single-pixel photoconductors were tested, not a focal-plane array, so cost and uniformity claims for cameras are projections. Carrier lifetime was estimated as a lower bound from a model rather than measured directly.
Key terms
- Colloidal quantum dot
- A semiconductor nanocrystal grown in solution, small enough that its energy levels depend on its size.
- Intraband transition
- An optical transition between two confined levels within the conduction band (e.g. 1Se to 1Pe), rather than across the band gap.
- Responsivity
- Photocurrent produced per watt of incident light (A/W).
- Specific detectivity (D*)
- A detector figure of merit that normalises signal-to-noise by detector area and bandwidth, measured in Jones.
- Carrier drift length
- How far a photogenerated carrier travels under the applied field before recombining; compared with the electrode gap it sets the collection efficiency.
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Quiz yourself
What did the iodine treatment mainly change in the quantum-dot films?
Common questions
Why is it hard to make quantum dots that detect very long wavelengths?
Long wavelengths need tiny energy gaps, which require very large dots near the bulk limit; these tend to be unstable in solution and need precise doping and surface control.
Why does raising mobility improve the detector so much?
Higher mobility lets carriers drift further before recombining, so a larger fraction reaches the electrodes; the response gain was even larger than the mobility gain.
What does the iodine treatment do?
It removes the excess electrons from the as-made dots so the film is nearly intrinsic, which exposes interband absorption and lowers dark current.
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