How fast do plasmon hot electrons jump into a 2D semiconductor?
When two kinds of plasmon in a gold nanostructure are strongly coupled, hot electrons pass into an atom-thin semiconductor in about 40 femtoseconds and more efficiently than one plasmon type alone would allow.
Source
Direct observation of ultrafast plasmonic hot electron transfer in the strong coupling regime
Study at a glance
- Design
- Other — Au grating / MoS2 / Al2O3 / Au heterostructures with varied grating period, probed by reflectance spectroscopy, FDTD simulation, coupled-oscillator fits and femtosecond pump-probe spectroscopy
- N
- No sample count; several grating periods and pump fluences/wavelengths were measured
- Population
- Gold grating on monolayer MoS2 above an alumina spacer and gold film
- Outcome
- Rabi splitting in reflectance; hot-electron injection time, density and external quantum yield
Structured fields used in claim comparison tables when every cited study has a complete layer.
What they did
The authors placed a gold grating on a single layer of MoS2, separated from a gold film by a thin alumina spacer, so that localized plasmons on the grating could strongly couple to propagating surface plasmons. They measured and simulated reflectance spectra for different grating periods and fitted them with a coupled-oscillator model. They then pumped the structure with 780 nm light, below the MoS2 bandgap, and probed the MoS2 exciton absorption to detect injected electrons.
What they found
Reflectance showed anticrossings (Rabi splitting), the signature of strong coupling, in good agreement with simulations and the model. A transient signal appeared only when gold grating and MoS2 were both present; injection took about 40 fs and gave an electron density around 3.55 × 10^11 per square centimetre. The injected density peaked at the 700 nm grating period where coupling was strongest, and the external quantum yield peaked at 1.65% near 810 nm, higher than the authors' estimate for localized plasmons alone.
The limits
What it doesn't show
The 'energy recycling' mechanism for why strong coupling helps is a proposed model supported by a rough efficiency estimate, not measured directly. The injection time comes from deconvolving a signal against an instrument response of similar duration, so it is an estimate. Only one material system and a small set of grating periods were studied, and the yield of about one to two percent is still low for practical devices.
Key terms
- Localized surface plasmon
- A collective oscillation of electrons confined to a metal nanostructure that can radiate light or decay into hot carriers.
- Surface plasmon polariton
- A propagating electron-light wave travelling along a metal-dielectric interface, which loses energy mostly non-radiatively.
- Strong coupling
- Interaction between two oscillators strong enough that they form new hybrid modes, seen as split, anticrossing resonances (Rabi splitting).
- Hot electrons
- Electrons given energy well above the Fermi level, for example by plasmon decay, before they cool.
- Schottky barrier
- An energy barrier at a metal-semiconductor junction that electrons must cross to move from metal into semiconductor.
- Pump-probe spectroscopy
- A technique where one ultrashort pulse excites a sample and a delayed pulse measures the change, tracking dynamics in time.
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Quiz yourself
What signature in reflectance spectra indicates strong coupling?
Common questions
How do they know the signal comes from hot electrons and not direct MoS2 absorption?
The pump photon energy is below the MoS2 bandgap, and neither bare gold gratings nor bare MoS2 produced a signal; only the combined structure did.
Why does strong coupling help?
The authors propose that light radiated by localized plasmons is reabsorbed by the propagating plasmons and handed back, recycling energy that would otherwise be lost, while the gold film enhances the field that pushes electrons across the barrier.
Why did the signal saturate at high pump fluence?
Accumulated injected electrons repel further electrons, limiting how many more can enter the MoS2.
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