Researchers investigated the carrier diffusion and dissociation dynamics of MoS2 edge states

Data:2022-07-15  |  【 A  A  A 】  |  【Print】 【Close

Recently, a research team led by Prof. LIU Xinfeng from the National Center for Nanoscience and Technology (NCNST) of the Chinese Academy of Sciences (CAS), reported ultrafast internal exciton dissociation through edge states in MoS2 nanosheets with diffusion blocking. The research is published on Nano Letters.

The surface states of materials have special physical and chemical properties, and the surface states of condensed matter materials have always been the focus of research. Among them, the edge states of two-dimensional transition metal dichalcogenides (TMDCs) have also been extensively studied, which have been found to possess properties such as edge conduction, edge plasmons, spin-protected transport, and high catalytic activity. These excellent performances deepen the basic research of TMDC and enrich its application prospects. Some features such as sub bandgap distribution, metallic bands, and spin degeneracy of edge electronic states have been studied. However, their dynamics are not fully understood, how and how fast are edge states filled and how do they contribute to overall charge transport and recombination? Addressing these issues is crucial for optimizing and scaling TMDC applications, however, weak edge signals are often drowned out by internal backgrounds or interfered by edge impurities, requiring extremely exposed and clean edge structures, so research remains challenging.

Electronic states on edges of TMDCs have been partially revealed with characteristics containing sub bandgap distributions, metallic bands, spin non-degeneracy, terminal localization. However, their dynamics have not been fully understood, especially how and how fast edge state populate, how they contribute to overall charge transport and recombination. Addressing these questions is critical for optimizing and extending TMDCs applications, but remains a challenge, for that weak edge signals is usually drowned out by interior backgrounds or disturbed by edge impurities. Extremely exposed and clean edge structures are highly desired. Recent research efforts including thermal annealing, vapor-liquid-solid growth, and nanowire embedding in different layered materials bring out chances in enhancing edge signal for experimental detections.

What exactly is the study of the excitation and relaxation dynamics of the electronic states of the edge states? Can we study these processes using ultrafast pump-probe methods? Researcher LIU Xinfeng, the leader of the team, said.

In this work, they reveal edge state population dynamics and population competition with exciton using ball-milling produced 10 nm MoS2 nanosheets with high zigzag edges exposed. Combined with electron energy loss spectroscopy (EELS) and first principles calculations, we identify the as predicted sub bandgap edge state absorption covering form 1.23 eV to 1.78 eV. Due to nanometer size, these nanosheets have highly exposed edges, accounting for ~11% sheet area, facilitating the direct transient absorption (TA) study for edge states. With above bandgap excitations, photoexcited exciton first populate and soon dissociate into edge electronic states within ~0.40 ps via interband transitions. With below bandgap excitations, edge state has a longer population time up to 1.0 ps, which is due to reduced intra band orbital coupling through nonadiabatic coupling (NAC) analysis. They find that edge state introduces the external potential gradient on boundary, terminating exciton diffusive transport of exciton from interior. We extract the exciton diffusion coefficient of 86.7 cm2/s through measuring exciton dissociation lifetime as a function of nanosheet lateral size.

These results expand the understanding of edge state dynamics in TMDC semiconductors from energy, temporal, and spatial perspectives, providing pathway for edge state manipulation and device optimization.

The research is supported by the Strategic Priority Research Program of Chinese Academy of Sciences and the Ministry of Science and Technology, the CAS Instrument Development Project, National Natural Science Foundation of China and the Support by the DNL Cooperation Fund CAS.

 


Figure 1. EELS measurement of edge states in single MoS2 nanosheet. (Image by LIU Xinfeng et al)

 


Figure 2. Edge state dynamics with band edge resonant excitation. (Image by LIU Xinfeng et al)

 


Figure 3. Energy dependent edge state dynamics. (Image by LIU Xinfeng et al)

 


Figure 4. Diffusion limited edge state formation. (Image by LIU Xinfeng et al)

 


Contact:

LIU Xinfeng

National Center for Nanoscience and Technology

E-mail: liuxf@nanoctr.cn

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