Modulation of photocarrier relaxation dynamics in two-dimensional semiconductors

Two-dimensional (2D) semiconductors can host a rich set of excitonic species due to the large enhanced coulomb interactions. Excitonic states can exhibit large oscillatory power and strong light-matter interactions, and can dominate the optical properties of 2D semiconductors.

Furthermore, due to the low dimensionality, the excitatory dynamics of 2D semiconductors may be more sensitive to various external excitations, thereby enriching the possible stitching methods.

Understanding the factors that can affect the dynamics of alternately generated excited states represents an important aspect of excitonic physics in 2D semiconductors, and is also important for practical application because excited state lifetimes span multiple optoelectronic and photonic. The key figures are linked to the qualification of the equipment.

While some experience has been accumulated for bulk semiconductors, the atomic nature of 2D semiconductors may make these methods less effective or difficult to optimize.

On the other hand, the unique properties of 2D semiconductors, such as strong exonic states, sensitivity to external environmental factors and flexibility in the construction of vdW heterostructures, promise different modulation strategies from conventional materials.

In a new review article published in Prakash: Science and Applications, a team of researchers led by Professor Fengqiu Wang from Nanjing University, China summarizes the knowledge gained so far and on the modulation of photocarrier relaxation dynamics in 2D semiconductors moves forward.

Following a brief summary on photocarrier relaxation dynamics in 2D semiconductors, the authors first discuss the modulation of Coulomb interactions and the resulting effects on transient properties.

Coulomb interactions in 2D semiconductors can be modified by introducing additional screening from an external dielectric environment or injection charge carriers, leading to modification of the quasi-particle bandgap and exciton binding energies.

Then the factors influencing photocoar dynamics and methods of manipulation are discussed along the relaxation path or mechanism to which they are connected.

The first discussed factor is the initial distribution of photocarriers in electronic band structures, which can affect their decay processes by enabling different available relaxation pathways in the energy and momentum space.

The defect-assisted and phonon-assisted exemption are then discussed. While approaches using defect-assisted relaxation such as ion bombardment and encapsulation are similar for bulk semiconductors, modulation on phonon-assisted relaxation may be different for 2D semiconductors.

"On the one hand, coupling between charge carriers and phonons can be enhanced due to the screening of the charged dielectric; on the other hand high surface-to-surface ratios make 2D materials susceptible to the external phononic environment."

In addition, flexibility in the construction of vdW heterostructures and ultrafast charge transfer across interfaces enables band alignment engineering, however tailoring photocarrier dynamics.

Transitions between different particle species also provide an opportunity to organize through varying the ratio between different quasiparticles, which can modify the relative part of different relaxation pathways, and thus the transient optical responses of the entire sample.

Finally, modulation of the dynamics of spin / valley polarization in 2D TMDs is discussed, and the discussion is mainly focusing on methods to extend the lifetime of spin / valley polarization.

Through this review, the authors aim to provide guidance to develop robust methods that reinforce the behavior of photocarrier relaxation and strengthen physical understanding on this fundamental process in 2D semiconductors.

As stated by the authors in the conclusion, "Both fundamental understanding and practical modulation of photocarrier relaxation in 2D semiconductors still require very difficult research efforts."

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