A laser pulse hits a thin film of MoS₂, and electrons with extra energy appear inside it. These hot carriers are like sprinters who start off fast but have to shed all their momentum on the way to the finish line through collisions with the crystal lattice. The question that has long bothered engineers sounds simple: can we pull these runners out of the material before they cool down? Researchers in Nanjing gave an unexpected answer: what sets the pace here is not the runner, but how quickly the lattice drains his energy.
The key to the puzzle lies in the structure of the crystal. In MoS₂, molybdenum atoms are twice as heavy as sulfur atoms, and this mass difference creates a forbidden gap around 1 THz in the lattice vibration spectrum. Optical phonons, that is, high-frequency vibrations, cannot rapidly decay 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 the phonons crowd at the exit, they cannot take energy away from the 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 neither sped up nor slowed down the fast component of the response. All 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 optical-to-acoustic phonon scattering time for all the MoS₂ thicknesses studied: 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.
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.