Harnessing Exciton–Exciton Annihilation in Two-Dimensional Semiconductors

date: 01.07.2020

category: Sporočila za javnost

 

Member of Laboratory for Laser Techniques (LASTEH), Dr. Daniele Vella, in cooperation with experts from Japan and Singapore, participated in the research Harnessing Exciton–Exciton Annihilation in Two-Dimensional Semiconductors. The results of the research were published in the prestigious Nano Letters magazine with the high impact factor of 12,279.


Two-dimensional semiconducting transition metal dichalcogenides (TMDs) represent a new class of nanomaterials for future emergent technologies in nanoelectronics, optoelectronics and photonics. Miniaturization of transistors and optoelectronic devices have shown their limits related to the scaling of the transistor channel length that gives rise to so-called short channel effects. In the last decade researcher worldwide have shown possibility to fabricate nanoscale devices with electron mobility comparable with silicon devices. Moreover, TMDs exhibit strong light matter (10% light absorption) and a crossover from indirect (bulk system) to direct band gap in the monolayer limit, due to the quantum confinement. This concept enabled studies of photoluminescence, electroluminescence and energy harvesting for fundamental science and future applications. Once TMDs are photoexcited, excitons (bound state of an electron and hole attractive by coulomb interactions) are generated into the material. In a traditional semiconductor their binding energy is on the order of 10 meV, however in the monolayer TMDs it can be 500 meV. This strong binding energy, due to the reduced screening of the coulomb interactions in the monolayer, allowed the observation of excitonic optical transition in the absorption spectrum at room temperature. The physics of the TMDs changes with the excitation density (laser fluence) and the doping level showing different many body interactions involving charged particles and multiple excitons. Understanding the photophysics of TMDs based devices becomes relevant for developing more efficient energy harvesting and electro-optical devices.


 In the present paper, they report an unconventional photoresponse of van der Waals heterostructure devices resulting from efficient exciton-exciton annihilation (EEA). For the porpuse they used a metal-isolator-semiconductor heterostructures, which consist of monolayer transition metal dichalcogenide (TMD), hexagonal boron nitride (hBN), and few-layer graphene. They investigate the photocarrier dynamics of the heterostructure by measuring the bias-dependent spectral features in the photocurrent generated by photons of varying energy ranging from 1.65 to 2.91 eV. The device exhibits photocurrent when photoexcited carriers possess sufficient energy to overcome the high energy barrier of hBN. Interestingly, they find that the device exhibits moderate photocurrent quantum efficiency even when the semiconducting TMD layer is excited at its ground exciton resonance despite the high exciton binding energy and large transport barrier. In forward bias, photocurrent spectrum is featureless and the the interlayer charge transport of non-thermalized photocarriers by Fowler−Nordheim tunnelling is responsible for the photocurrent. Under reverse bias, photocurrent exbits similar threshold behavior but with two prominent peaks corresponding to the excitonic absorption resonances. The observation of a finite photocurrent at the excitation energy below the quasiparticle bandgap of TMDs indicates that  exciton dissociates generating hot holes with sufficient excess energy to overcome potential barrier due to hBN. They proved that EEA is responsible for the excitons dissociation and the conseguent hot carriers generation. Their findings highlight the dominant role of EEA in determining the photoresponse of 2D semiconductor optoelectronic devices and envisage the possibility to use an intelligent heterostructure design by material selection and band gap engineering for enableing improved energy-harvesting devices by exploiting EEA processes in 2D semiconductors.

The original scientific paper is freely available under this link

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