Modulation of photocarrier relaxation dynamics in two-dimensional semiconductors

Abstract

Due to strong Coulomb interactions, two-dimensional (2D) semiconductors can support excitons with large binding energies and complex many-particle states. Their strong light-matter coupling and emerging excitonic phenomena make them potential candidates for next-generation optoelectronic and valleytronic devices. The relaxation dynamics of optically excited states are a key ingredient of excitonic physics and directly impact the quantum efficiency and operating bandwidth of most photonic devices. Here, we summarize recent efforts in probing and modulating the photocarrier relaxation dynamics in 2D semiconductors. We classify these results according to the relaxation pathways or mechanisms they are associated with. The approaches discussed include both tailoring sample properties, such as the defect distribution and band structure, and applying external stimuli such as electric fields and mechanical strain. Particular emphasis is placed on discussing how the unique features of 2D semiconductors, including enhanced Coulomb interactions, sensitivity to the surrounding environment, flexible van der Waals (vdW) heterostructure construction, and non-degenerate valley/spin index of 2D transition metal dichalcogenides (TMDs), manifest themselves during photocarrier relaxation and how they can be manipulated. The extensive physical mechanisms that can be used to modulate photocarrier relaxation dynamics are instrumental for understanding and utilizing excitonic states in 2D semiconductors.

Stability Improvement of 2D Perovskite by Fluorinated‐Insulator

“…It is well-known that the exciton binding energy in a 2D system is larger than that in 3D counterpart due to enhanced Coulomb interactions. [56] Here, the introduction of fluorine leads to the subtle changes of the dielectric constant, denoted ε 1 for PEPI and ε 2 for FPEPI, shown in the left panel of Figure 2b. In the linear, nonmagnetic, and phenomenological framework, the dielectric constant can be expressed as ε r = 1+Nα e /ε 0 , where N is atomic density, α e is atomic/molecular polarizability, and ε 0 is dielectric constant in vacuum.…”

Исследование ван-дер-ваальсовых кристаллов GaSe и GaS-=SUB=-x-=/SUB=-Se-=SUB=-1-x-=/SUB=- методом фотоотражения

“…Исследование графеноподобных полупроводниковых кристаллов является одним из актуальных направлений в области современных материалов электронной техники [1][2][3][4][5]. К числу таких полупроводников относится моноселенид галлия (GaSe), состоящий из тетраслоев, внутри которых атомные слои в порядке Se-Ga-Ga-Se связаны ковалентно, в то время как отдельные тетраслои между собой связаны слабыми силами ван-дер-Ваальса.…”

Identifying the Intermediate Free-Carrier Dynamics Across the Charge Separation in Monolayer MoS2/ReSe2 Heterostructures

“…33−36 The understanding of carrier dynamics is essential for laying a foundation for the development of optoelectronic devices. 25,37,38 Compared with individual 2D-TMDs, out-of-plane charge separation adds the degree of freedom of photocarrier dynamics in vertically stacked TMDs heterostructures. [23][24][25][26][27]39,40 Because of the difference of band alignment between different components, the electrons in the conduction band or the holes in the valence band will transfer to another conduction (valence) band with lower energy, and therefore the interlayer excitons can be formed.…”

“…39−47 Since the ultrafast time scale (∼50 fs) of charge transfer (CT) in MoS 2 /WS 2 heterostructures has been experimentally revealed, 39 the CT and interlayer exciton dynamics in vdW heterostructures, such as MoS 2 /WS 2 and MoS 2 /MoSe 2 systems, have been extensively investigated with time-resolved optical spectroscopy. 25,32,37,38,40,41 The obtained consensus is that the CT generally takes place with high efficiency and a high rate within a subpicosecond time scale, 38,40,41,48−53 although it stands as a puzzle considering the large mismatch of parallel momentum of electron immigrating between the adjacent layers. 53,54 Accordingly, different CT mechanisms, including the collective motion of excitons, 55 donor−acceptor delocalization, 56 phonon excitation and assistance, 57−59 energy band couplings, 60 interlayer hopping, 61 and adiabatic transfer, 62 have been proposed theoretically.…”

“…The understanding of carrier dynamics is essential for laying a foundation for the development of optoelectronic devices. ,, Compared with individual 2D-TMDs, out-of-plane charge separation adds the degree of freedom of photocarrier dynamics in vertically stacked TMDs heterostructures. − ,, Because of the difference of band alignment between different components, the electrons in the conduction band or the holes in the valence band will transfer to another conduction (valence) band with lower energy, and therefore the interlayer excitons can be formed. − Since the ultrafast time scale (∼50 fs) of charge transfer (CT) in MoS 2 /WS 2 heterostructures has been experimentally revealed, the CT and interlayer exciton dynamics in vdW heterostructures, such as MoS 2 /WS 2 and MoS 2 /MoSe 2 systems, have been extensively investigated with time-resolved optical spectroscopy. ,,,,, The obtained consensus is that the CT generally takes place with high efficiency and a high rate within a subpicosecond time scale, ,,,− although it stands as a puzzle considering the large mismatch of parallel momentum of electron immigrating between the adjacent layers. , Accordingly, different CT mechanisms, including the collective motion of excitons, donor–acceptor delocalization, phonon excitation and assistance, − energy band couplings, interlayer hopping, and adiabatic transfer, have been proposed theoretically. The identifications for different excitation species or intermediate states, such as intralayer excitons, interlayer excitons, free electron–hole plasmas, and delocalized states, as well as the corresponding formation and relaxation, are prerequisites for the establishment of a complete photocarrier dynamics scenario.…”