In this letter, we present non-degenerate ultrafast optical pump-probe studies of the carrier recombination dynamics in MoS 2 monolayers. By tuning the probe to wavelengths much longer than the exciton line, we make the probe transmission sensitive to the total population of photoexcited electrons and holes. Our measurement reveals two distinct time scales over which the photoexcited electrons and holes recombine; a fast time scale that lasts ∼2 ps and a slow time scale that lasts longer than ∼100 ps. The temperature and the pump fluence dependence of the observed carrier dynamics are consistent with defect-assisted recombination as being the dominant mechanism for electron-hole recombination in which the electrons and holes are captured by defects via Auger processes. Strong Coulomb interactions in two dimensional atomic materials, together with strong electron and hole correlations in two dimensional metal dichalcogenides, make Auger processes particularly effective for carrier capture by defects. We present a model for carrier recombination dynamics that quantitatively explains all features of our data for different temperatures and pump fluences. The theoretical estimates for the rate constants for Auger carrier capture are in good agreement with the experimentally determined values. Our results underscore the important role played by Auger processes in two dimensional atomic materials. Electron-Hole Recombination Dynamics in Monolayer MoS 2Two-dimensional (2D) transition metal dichalcogenides (TMDs) have emerged as interesting materials both from the perspective of basic science as well as applications [1][2][3][4][5][6][7] . Applications of these materials in electronics and optoelectronics have been extensively explored in recent years 5,6,[8][9][10][11][12][13][14][15][16][17][18] . arXiv:1409.4518v1 [cond-mat.mes-hall] 16 Sep 20142 The bandgaps of most TMDs are in the visible to near-IR wavelength range, making these materials suitable for light emitters, photodetectors, and solar cells 5,10,13,[16][17][18] . In addition, optical control of valley polarization in TMDs has opened opportunities for devices based on the valley degree of freedom 6 . The lifetimes of electrons and holes are critical to all the proposed and demonstrated TMD optoelectronic devices. Despite the recent progress, carrier lifetimes and nonradiative electron-hole recombination mechanisms in TMDs remain poorly understood. Developing a better understanding of the non-radiative electron-hole recombination mechanisms in TMDs is especially important because the reported quantum efficiencies in both TMD light emitters and detectors are extremely poor; in the .0001-.01 range 10,13,[16][17][18] . Similar quantum efficiencies for TMDs have been observed in photoluminescence experiments 2,3 . In contrast, the best reported internal and external quantum efficiencies observed in photoluminescence in III-V semiconductors exceed 0.9 and 0.7, respectively 19 . Therefore, most of the electrons and holes injected either electrically or optically i...

We measure the optical absorption spectra and optical conductivities of excitons and trions in monolayers of metal dichalcogenide MoS2 and compare the results with theoretical models. Our results show that the Wannier-Mott model for excitons with modifications to account for small exciton radii and large exciton relative wavefunction spread in momentum space, phase space blocking due to Pauli exclusion in doped materials, and wavevector dependent dielectric constant gives results that agree well with experiments. The measured exciton optical absorption spectra are used to obtain experimental estimates for the exciton radii that fall in the 7 − 10Å range and agree well with theory. The measured trion optical absorption spectra are used to obtain values for the trion radii that also agree well with theory. The measured values of the exciton and trion radii correspond to binding energies that are in good agreement with values obtained from first principles calculations.

Using optical-pump terahertz-probe spectroscopy, we study the relaxation dynamics of photoexcited carriers in graphene at different substrate temperatures. We find that at lower temperatures the tail of the relaxation transients measured by the differential probe transmission become slower, extending beyond several hundred picoseconds below 50 K. We interpret the observed relaxation transients as resulting from the cooling of the photoexcited carriers via phonon emission. The slow cooling of the photoexcited carriers at low temperatures is attributed to the bulk of the electron and hole energy distributions moving close enough to the Dirac point that both intraband and interband scattering of carriers via optical phonon emission become inefficient for removing heat from the carriers. Our model, which includes intraband carrier scattering and interband carrier recombination and generation, agrees very well with the experimental observations.

We present results on the radiative lifetimes of excitons and trions in a monolayer of metal dichalcogenide MoS2. The small exciton radius and the large exciton optical oscillator strength result in radiative lifetimes in the 0.18-0.30 ps range for excitons that have small in-plane momenta and couple to radiation. Average lifetimes of thermally distributed excitons depend linearly on the exciton temperature and can be in the few picoseconds range at small temperatures and more than a nanosecond near room temperature. Localized excitons exhibit lifetimes in the same range and the lifetime increases as the localization length decreases. The radiative lifetimes of trions are in the hundreds of picosecond range and increase with the increase in the trion momentum. Average lifetimes of thermally distributed trions increase with the trion temperature as the trions acquire thermal energy and larger momenta. We expect our theoretical results to be applicable to most other 2D transition metal dichalcogenides.

We present results on photoexcited carrier lifetimes in few-layer transition metal dichalcogenide MoS 2 using nondegenerate ultrafast optical pump-probe technique. Our results show a sharp increase of the carrier lifetimes with the number of layers in the sample. Carrier lifetimes increase from few tens of picoseconds in monolayer samples to more than a nanosecond in 10-layer samples. The inverse carrier lifetime was found to scale according to the probability of the carriers being present at the surface layers, as given by the carrier wavefunction in few layer samples, which can be treated as quantum wells. The carrier lifetimes were found to be largely independent of the temperature and the inverse carrier lifetimes scaled linearly with the photoexcited carrier density. These observations are consistent with defect-