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Identifier

000438128

Title

Pulsed laser induced doping of two-dimensional crystals

Alternative Title

Εμπλουτισμός διδιάστατων κρυστάλλων με χρήση παλμικού λέιζερ

Author

Δεμερίδου, Ιωάννα Ε

Thesis advisor

Κομίνης, Ιωάννης
Στρατάκης, Εμμανουήλ

Reviewer

Κιοσέογλου, Γεώργιος
Γεωργακίλας, Αλέξανδρος
Ρακιτζής, Πέτρος
Πελεκάνος, Νικόλαος
Κυμάκης, Ε.

Abstract

Two-dimensional transition metal dichalcogenides (2D-TMDs) attract worldwide interest in the last few decades since the successful isolation and characterization of graphene. TMDs are semiconductors and provide unique optoelectronic properties attributed to their ultrathin nature. As their numbers of layers decreases, turned from an indirect to direct bandgap semiconductor with possible applications in visible range optoelectronic devices. Here, we investigate the optical properties of monolayer WS2, a member of the TMD family, and the effects of the UV pulsed laserinduced doping. We proposed a novel doping method that modulates the electron density in a 1L-WS₂. Chlorinedoped tungsten disulfide monolayer (1L-WS₂) with tunable charge carrier concentration has been realized by pulsed laser irradiation of the atomically thin lattice in a chlorine environment. A systematic shift of the neutral exciton peak, in the photoluminescence spectra, towards lower energies indicates the reduction of the crystal’s electron density. The capability to progressively tune the carrier density upon variation of the exposure time is demonstrated. The Fermi level shift is correlated to the respective electron density modulation due to the chlorine species. At the same time, is found that the effect can be reversed upon continuous-wave laser scanning of the monolayer in the air. Such bidirectional control of the Fermi level, coupled with the capability offered by lasers to process at pre-selected locations, can be advantageously used for spatially resolved doping modulation in 1L-WS₂ with micrometric resolution. This process has boosted interests in controlling valley polarized light emission of doped monolayer WS₂. TMDs have interesting spin/valley properties due to the large spin-orbit coupling and the non-equivalent valleys in their band structure. We demonstrate spin-valley polarization tunability by more than 40% in 1L-WS2 via photochlorination. The reduction of circular polarization, after the photochlorination treatment, was attributed to the reduction of defect sites in the crystal lattice and consequently to the increase of the non-radiative exciton lifetime. Theoretical simulations and ultrafast time-resolved transient absorption measurements support our findings. Furthermore, we show that the degree of valley polarization strongly depends on the relationship between the doping and the supporting layer of TMDs. These results shed light on the significant role of the substrate on valley polarization in monolayer TMDs. These findings make TMDs promising materials for future applications and computing architectures.

Language

English

Subject

2D-materials

Chlorine

Monolayer WS2

Photoluminescence

Ptotochemical doping

Διδιάστατοι κρύσταλλοι

Μονοστρωματικός κρύσταλλος WS2

Φωτοφωταύγεια

Φωτοχημικός εμπλουτισμός