title: Exciton and charge carrier dynamics in few-layer WS2.
abstract: Semiconducting transition metal dichalcogenides (TMDs) have been applied as the active layer in photodetectors and solar cells, displaying substantial charge photogeneration yields. However, their large exciton binding energy, which increases with decreasing thickness (number of layers), as well as the strong resonance peaks in the absorption spectra suggest that excitons are the primary photoexcited states. Detailed time-domain studies of the photoexcitation dynamics in TMDs exist mostly for MoS2. Here, we use femtosecond optical spectroscopy to study the exciton and charge dynamics following impulsive photoexcitation in few-layer WS2. We confirm excitons as the primary photoexcitation species and find that they dissociate into charge pairs with a time constant of about 1.3 ps. The better separation of the spectral features compared to MoS2 allows us to resolve a previously undetected process: these charges diffuse through the samples and get trapped at defects, such as flake edges or grain boundaries, causing an appreciable change of their transient absorption spectra. This finding opens the way to further studies of traps in TMD samples with different defect contents.
authors.affiliation: Department of Complex Matter, Jozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia. victor.vega@ijs.si christoph.gadermaier@ijs.si and Jozef Stefan International Postgraduate School, Jamova 39, 1000 Ljubljana, Slovenia.
authors.forename: Victor
authors.initials: V
authors.lastname: Vega-Mayoral
authors.affiliation: Department of Complex Matter, Jozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia. victor.vega@ijs.si christoph.gadermaier@ijs.si and Jozef Stefan International Postgraduate School, Jamova 39, 1000 Ljubljana, Slovenia.
authors.forename: Daniele
authors.initials: D
authors.lastname: Vella
authors.affiliation: Department of Complex Matter, Jozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia. victor.vega@ijs.si christoph.gadermaier@ijs.si and Jozef Stefan International Postgraduate School, Jamova 39, 1000 Ljubljana, Slovenia.
authors.forename: Tetiana
authors.initials: T
authors.lastname: Borzda
authors.affiliation: Department of Complex Matter, Jozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia. victor.vega@ijs.si christoph.gadermaier@ijs.si and Jozef Stefan International Postgraduate School, Jamova 39, 1000 Ljubljana, Slovenia.
authors.forename: Matej
authors.initials: M
authors.lastname: Prijatelj
authors.affiliation: IFN-CNR, Department of Physics, Politecnico di Milano, P. Leonardo da Vinci 32, 20133 Milan, Italy.
authors.forename: Iacopo
authors.initials: I
authors.lastname: Tempra
authors.affiliation: IFN-CNR, Department of Physics, Politecnico di Milano, P. Leonardo da Vinci 32, 20133 Milan, Italy.
authors.forename: Eva A A
authors.initials: EA
authors.lastname: Pogna
authors.affiliation: IFN-CNR, Department of Physics, Politecnico di Milano, P. Leonardo da Vinci 32, 20133 Milan, Italy.
authors.forename: Stefano
authors.initials: S
authors.lastname: Dal Conte
authors.affiliation: Jozef Stefan International Postgraduate School, Jamova 39, 1000 Ljubljana, Slovenia and Center for Nano Science and Technology, Italian Institute of Technology, Via Pascoli 70/3, 20133 Milano, Italy.
authors.forename: Peter
authors.initials: P
authors.lastname: Topolovsek
authors.affiliation: Institute of Physics, Bijenicka 46, 10000 Zagreb, Croatia and Center of Excellence for Advanced Materials and Sensing Devices, Bijenicka 46, HR-10000, Zagreb, Croatia.
authors.forename: Natasa
authors.initials: N
authors.lastname: Vujicic
authors.affiliation: IFN-CNR, Department of Physics, Politecnico di Milano, P. Leonardo da Vinci 32, 20133 Milan, Italy.
authors.forename: Giulio
authors.initials: G
authors.lastname: Cerullo
authors.affiliation: Department of Complex Matter, Jozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia. victor.vega@ijs.si christoph.gadermaier@ijs.si and Jozef Stefan International Postgraduate School, Jamova 39, 1000 Ljubljana, Slovenia and Center of Excellence in Nanoscience and Nanotechnology, Jamova 39, 1000 Ljubljana, Slovenia.
authors.forename: Dragan
authors.initials: D
authors.lastname: Mihailovic
authors.affiliation: Department of Complex Matter, Jozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia. victor.vega@ijs.si christoph.gadermaier@ijs.si and Jozef Stefan International Postgraduate School, Jamova 39, 1000 Ljubljana, Slovenia.
authors.forename: Christoph
authors.initials: C
authors.lastname: Gadermaier