Can voltage make a single quantum dot emit photons faster?
Pumping extra electrons into a single quantum dot with a voltage made it emit light up to about 210 times faster while it still emitted photons one at a time.
Source
Electrical control of single-photon emission in highly charged individual colloidal quantum dots
Study at a glance
- Design
- Other — Lab experiment: individual giant-shell CdSe/CdS quantum dots on an ITO electrode in an electrochemical cell, with time-resolved confocal photoluminescence and photon-correlation measurements while the bias was varied from 0 to -2 V.
- N
- N=37 · 37 individual quantum dots from two batches were tested; 13 showed charging beyond the doubly negative exciton. Headline numbers come from single representative dots.
- Population
- Giant-shell CdSe/CdS colloidal quantum dots (4 nm core; two shell thicknesses)
- Outcome
- Fluorescence lifetime (decay rate), emission intensity, blinking and second-order photon correlation g(2)(0) versus applied voltage
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What they did
The researchers made two batches of quantum dots with the same small cadmium-selenide core but different thicknesses of cadmium-sulfide shell. They placed single dots on a transparent electrode inside a liquid electrochemical cell, excited them with pulsed blue laser light, and recorded how bright each dot was and how long it took to emit while stepping or sweeping the voltage to negative values. They compared the lifetime-intensity pairs with a statistical scaling model of charged excitons and checked single-photon emission with a Hanbury Brown and Twiss setup.
What they found
At zero volts a typical dot blinked between a neutral exciton (lifetime about 125 ns) and a charged trion (about 42 ns). Making the voltage more negative suppressed blinking and progressively shortened the lifetime, down to 0.9 ns at -2 V for one dot, a 140-fold speed-up; for the thicker-shell batch the decay rate rose up to 210-fold with only a 12-fold drop in intensity. The dot remained a single-photon emitter above -1.8 V, the charging was reversible over 540 cycles, and the simple scaling model fitted only up to about 4 extra electrons (thin shell) or 12 (thick shell), beyond which many-body effects seem to matter.
The limits
What it doesn't show
Only 13 of the 37 dots tested (about 30%) charged beyond the doubly charged state, so the dramatic effect is not universal and the reason some dots do not respond is unexplained. The headline enhancement factors come from individual dots, and the number of electrons is inferred from a model fit rather than measured directly, with the model failing at the highest charges. The voltage range was capped at -2 V to avoid damaging the electrode, and the work was done in liquid electrolyte, which may not transfer directly to solid-state devices.
Key terms
- Colloidal quantum dot
- A semiconductor nanocrystal a few nanometres across whose small size confines electrons, giving size-tunable light emission.
- Exciton and trion
- An exciton is a bound electron-hole pair; a trion is an exciton with one extra charge, here an extra electron.
- Auger recombination
- A non-radiative process where an electron-hole pair gives its energy to another carrier instead of emitting a photon, dimming the emission.
- Fluorescence lifetime
- The average time between excitation and photon emission; a shorter lifetime means a faster emission rate.
- g(2)(0) photon antibunching
- A photon-correlation measure; values below 0.5 show that the source emits single photons one at a time.
- Statistical scaling model
- A simple model in which radiative and Auger rates grow with the number of electrons according to counting of recombination pathways.
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What was the largest increase in decay rate reported?
Common questions
Why does adding electrons make the dot emit faster?
Each extra electron adds another way for the hole to recombine radiatively, so the radiative rate grows roughly in proportion to the number of electrons.
Why doesn't the dot just go dark when highly charged?
The thick shell slows Auger recombination, so the extra non-radiative losses grow more slowly than the radiative speed-up, keeping intensity reasonably high.
Why is it important that g(2)(0) stayed below 0.5?
It shows the dot still emits one photon at a time, which is what quantum light sources need.
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