A laser pulse hits a thin film of MoS₂, and inside it a cloud of electrons with excess energy starts to boil. These hot carriers are like runners who took off at high speed but, on the way to the finish line, are bound to lose all their momentum through collisions with the crystal lattice. The question that has long plagued engineers sounds simple: can we pull these runners out of the material before they cool down? Researchers in Nanjing gave an unexpected answer: the speed here is set not by the runner but by how quickly the lattice drains his energy.
The key to the puzzle lies in the crystal structure. In MoS₂, molybdenum atoms are twice as heavy as sulfur atoms, and this mass difference creates a forbidden gap of about 1 THz in the lattice vibration spectrum. Optical phonons, that is, high-frequency vibrations, cannot decay quickly into acoustic, low-frequency ones. Energy gets stuck in a narrow spot, like a crowd at a stadium exit: everyone wants to get through, but there is only one opening. This is the phonon bottleneck. While phonons are jammed at the exit, they cannot take energy away from electrons, and the electrons stay hot longer.
The results were telling. The photocurrent relaxation time was 48.5 picoseconds for MoS₂ and only 14.2 picoseconds for MoSe₂. When the pump spot was shifted a few micrometers away from the contact, the response time barely changed, even though the signal itself decayed exponentially. The applied voltage also did not speed up or slow down the fast component of the response. All of this means that carriers do not drift under the field but spread due to the electron temperature gradient, like water spreading across a hot frying pan. This kind of motion is called superdiffusion.
The most important comparison was made with optical measurements. The electrical extraction time quantitatively matched the scattering time of optical phonons into acoustic phonons for all studied MoS₂ thicknesses: 5, 13, and 30 nanometers. In MoSe₂, where the masses of molybdenum and selenium atoms differ less, the bottleneck is wider, phonons decay faster, and the response is shorter. This is direct experimental proof: device speed is determined not by carrier mobility but by the lifetime of hot phonons.
The discovery brings us back to the image of the runners. Previously it seemed that what mattered most was their own speed, that is, electron mobility. Now it is clear that they cover the distance faster if the right obstacles are placed in their path, obstacles that keep them from cooling down. Engineers can already choose materials with the needed difference in atomic masses to tune this effect. Ultrafast self-powered photodetectors and optical interconnects based on two-dimensional materials are moving closer to reality.
In MoS₂, molybdenum atoms are twice as heavy as sulfur atoms, which creates a band gap in the phonon spectrum near 1 THz. Because of this, optical phonons cannot decay quickly into acoustic ones, and hot carriers remain energetic for longer.
The idea of controlling a material's properties through lattice vibrations echoes the concept of 'phonon computers' from science fiction, where information is carried by thermal waves rather than electrons.