3351wileyonlinelibrary.com for the monolayer. Hence, the monolayer, contrary to bi-and multilayers, behaves like a direct gap semiconductor and shows signifi cant fl uorescence. [ 11,12 ] The exciton binding energy for bulk MoS 2 has been determined to be 45 and 130 meV for the A and B excitons, respectively. [ 13 ] Both exciton binding energies increase upon decreasing the sample thickness, with estimates for monolayer [14][15][16] ranging from 0.4 to 0.9 eV. Despite this high exciton binding energy, monolayer MoS 2 shows a strong photovoltaic effect [ 17 ] and potential for high sensitivity photodetectors. [ 18 ] Both these functionalities require effi cient charge carrier photogeneration (CPG), either via direct excitation of mobile carriers or via exciton dissociation.The spectral signature of charge carriers has been identifi ed by absorption and fl uorescence spectroscopy of MoS 2 , where the charge concentration varies either via the gate voltage in an FET geometry [ 19 ] or via adsorption, [ 20 ] or substrate doping. [ 21 ] The absorption peaks of charges are red-shifted by about 40 meV compared with the ground-state absorption into the A and B excitons and have been attributed to optical transitions from a charged ground state to a charged exciton (trion). The possibility of alternative interpretations, such as polarons [ 22,23 ] or Stark effect in the local electric fi eld of the charges, [24][25][26] does The 2D semiconductor MoS 2 in its mono-and few-layer form is expected to have a signifi cant exciton binding energy of several 100 meV, suggesting excitons as the primary photoexcited species. Nevertheless, even single layers show a strong photovoltaic effect and work as the active material in high sensitivity photodetectors, thus indicating effi cient charge carrier photogeneration. Here, modulation spectroscopy in the sub-ps and ms time scales is used to study the photoexcitation dynamics in few-layer MoS 2 . The results suggest that the primary photoexcitations are excitons that effi ciently dissociate into charges with a characteristic time of 700 fs. Based on these fi ndings, simple suggestions for the design of effi cient MoS 2 photovoltaic and photodetector devices are made.

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.

Ferromagnetism and superconductivity are antagonistic phenomena. Their coexistence implies either a modulated ferromagnetic order parameter on a lengthscale shorter than the superconducting coherence length or a weak exchange coupling between the itinerant superconducting electrons and the localized ordered spins. In some iron based pnictide superconductors the coexistence of ferromagnetism and superconductivity has been clearly demonstrated. The nature of the coexistence, however, remains elusive since no clear understanding of the spin structure in the superconducting state has been reached and the reports on the coupling strength are controversial. We show, by a direct optical pump-probe experiment, that the coupling is weak, since the transfer of the excess energy from the itinerant electrons to ordered localized spins is much slower than the electron-phonon relaxation, implying the coexistence without the short-lengthscale ferromagnetic order parameter modulation. Remarkably, the polarization analysis of the coherently excited spin wave response points towards a simple ferromagnetic ordering of spins with two distinct types of ferromagnetic domains.

We systematically investigate temperature-and spectrally-dependent optical reflectivity dynamics in AAs2Fe2, (A=Ba, Sr and Eu), iron-based superconductors parent spin-density-wave (SDW) compounds. Two different relaxation processes are identified. The behavior of the slower process, which is strongly sensitive to the magneto-structural transition, is analyzed in the framework of the relaxation-bottleneck model involving magnons. The results are compared to recent time resolved angular photoemission results (TR-ARPES) and possible alternative assignment of the slower relaxation to the magneto-structural order parameter relaxation is discussed.

MoS monolayer samples were synthesized on a SiO/Si wafer and transferred to Ir(111) for nano-scale characterization. The samples were extensively characterized during every step of the transfer process, and MoS on the final substrate was examined down to the atomic level by scanning tunneling microscopy (STM). The procedures conducted yielded high-quality monolayer MoS of milimeter-scale size with an average defect density of 2 × 10 cm. The lift-off from the growth substrate was followed by a release of the tensile strain, visible in a widening of the optical band gap measured by photoluminescence. Subsequent transfer to the Ir(111) surface led to a strong drop of this optical signal but without further shifts of characteristic peaks. The electronic band gap was measured by scanning tunneling spectroscopy (STS), revealing n-doping and lateral nano-scale variations. The combined use of STM imaging and density functional theory (DFT) calculations allows us to identify the most recurring point-like defects as S vacancies.

Growth of 2D materials under ultrahigh-vacuum (UHV) conditions allows for an in situ characterization of samples with direct spectroscopic insight. Heteroepitaxy of transition-metal dichalcogenides (TMDs) in UHV remains a challenge for integration of several different monolayers into new functional systems. In this work, we epitaxially grow lateral WS 2 −MoS 2 and vertical WS 2 /MoS 2 heterostructures on graphene. By means of scanning tunneling spectroscopy (STS), we first examined the electronic structure of monolayer MoS 2 , WS 2 , and WS 2 /MoS 2 vertical heterostructure. Moreover, we investigate a band bending in the vicinity of the narrow one-dimensional (1D) interface of the WS 2 −MoS 2 lateral heterostructure and mirror twin boundary (MTB) in the WS 2 /MoS 2 vertical heterostructure. Density functional theory (DFT) is used for the calculation of the band structures, as well as for the density of states (DOS) maps at the interfaces. For the WS 2 −MoS 2 lateral heterostructure, we confirm type-II band alignment and determine the corresponding depletion regions, charge densities, and the electric field at the interface. For the MTB, we observe a symmetric upward bend bending and relate it to the dielectric screening of graphene affecting dominantly the MoS 2 layer. Quasi-freestanding heterostructures with sharp interfaces, large built-in electric field, and narrow depletion region widths are proper candidates for future designing of electronic and optoelectronic devices.

The effect of Lithium atoms evaporation on the surface of monolayer MoS 2 grown on SiO 2 /Si substrate is studied using ultra high vacuum (UHV ∼ 10 −11 mbar) Raman and circularly polarized photoluminescence spectroscopies, at low Lithium coverage (up to ∼0.17 monolayer). With increasing Li doping, the dominant E 1 2g and A 1g Raman modes of MoS 2 shift in energy and broaden. Additionally, non zone-center phonon modes become Raman active. This regards in particular to double resonance Raman scattering processes, involving longitudinal acoustic (LA) phonon modes at the M and K points of the Brillouin zone of MoS 2 and defects. It is also accompanied by significant decrease in the overall intensity and the degree of circular polarization of the photoluminescence spectrum. The observed changes in the optical spectra are understood as a result of electron doping by Lithium atoms and disorder-activated intervalley scattering of electrons and holes in the electronic band structure of monolayer MoS 2 .

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