Christian Schüller

Low-temperature photocarrier dynamics in monolayer MoS2

The band structure of MoS2 strongly depends on the number of layers, and a transition from indirect to direct-gap semiconductor has been observed recently for a single layer of MoS2. Single-layer MoS2 therefore becomes an efficient emitter of photoluminescence even at room temperature. Here, we report on scanning Raman and on temperature-dependent, as well as time-resolved photoluminescence measurements on single-layer MoS2 flakes prepared by exfoliation. We observe the emergence of two distinct photoluminescence peaks at low temperatures. The photocarrier recombination at low temperatures occurs on the few-picosecond timescale, but with increasing temperatures, a biexponential photoluminescence decay with a longer-lived component is observed.

Raman spectroscopy of the interlayer shear mode in few-layer MoS2 flakes

Single- and few-layer MoS2 has recently gained attention as an interesting material system for opto-electronics. Here, we report on scanning Raman measurements on few-layer MoS2 flakes prepared by exfoliation. We observe a Raman mode corresponding to a rigid shearing oscillation of adjacent layers. This mode appears at very low Raman shifts between 20 and 30 cm−1. Its position strongly depends on the number of layers, which we independently determine using atomic force microscopy and investigation of the other characteristic Raman modes. Raman spectroscopy of the shear mode, therefore, is a useful tool to determine the number of layers for few-layer MoS2 flakes.

Resonant internal quantum transitions and femtosecond radiative decay of excitons in monolayer WSe2

Atomically thin two-dimensional crystals have revolutionized materials science. In particular, monolayer transition metal dichalcogenides promise novel optoelectronic applications, owing to their direct energy gaps in the optical range. Their electronic and optical properties are dominated by Coulomb-bound electron-hole pairs called excitons, whose unusual internal structure, symmetry, many-body effects and dynamics have been vividly discussed. Here we report the first direct experimental access to all 1s A excitons, regardless of momentum--inside and outside the radiative cone--in single-layer WSe2. Phase-locked mid-infrared pulses reveal the internal orbital 1s-2p resonance, which is highly sensitive to the shape of the excitonic envelope functions and provides accurate transition energies, oscillator strengths, densities and linewidths. Remarkably, the observed decay dynamics indicates an ultrafast radiative annihilation of small-momentum excitons within 150 fs, whereas Auger recombination prevails for optically dark states. The results provide a comprehensive view of excitons and introduce a new degree of freedom for quantum control, optoelectronics and valleytronics of dichalcogenide monolayers.

Identification of excitons, trions and biexcitons in single-layer WS2

Single-layer WS

Low-temperature photoluminescence of oxide-covered single-layer MoS2

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Excitonic linewidth and coherence lifetime in monolayer transition metal dichalcogenides

is a direct-gap semiconductor showing strong excitonic photoluminescence features in the visible spectral range. Here, we present temperature-dependent photoluminescence measurements on mechanically exfoliated single-layer WS

Scanning Raman spectroscopy of graphene antidot lattices: Evidence for systematic p-type doping

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Trion fine structure and coupled spin–valley dynamics in monolayer tungsten disulfide

, revealing the existence of neutral and charged excitons at low temperatures as well as at room temperature. By applying a gate voltage, we can electrically control the ratio of excitons and trions and assert a residual n-type doping of our samples. At high excitation densities and low temperatures, an additional peak at energies below the trion dominates the photoluminescence, which we identify as biexciton emission.

Coulomb engineering of the bandgap and excitons in two-dimensional materials

We present a photoluminescence study of single-layer MoS2 flakes on SiO2 surfaces. We demonstrate that the luminescence peak position of flakes prepared from natural MoS2, which varies by up to 25 meV between individual flakes, can be homogenized by annealing in vacuum. We use HfO2 and Al2O3 layers prepared by atomic layer deposition to cover some of our flakes. In these flakes, we observe a suppression of the low-energy luminescence peak which appears in asprepared flakes at low temperatures. We infer that this peak originates from excitons bound to surface adsorbates. We also observe different temperature-induced shifts of the luminescence peaks for the oxide-covered flakes. This effect stems from the different thermal expansion coefficients of the oxide layers and the MoS2 flakes. It indicates that the single-layer MoS2 flakes strongly adhere to the oxide layers and are therefore strained. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A direct comparison of CVD-grown and exfoliated MoS2 using optical spectroscopy

Atomically thin transition metal dichalcogenides are direct-gap semiconductors with strong light–matter and Coulomb interactions. The latter accounts for tightly bound excitons, which dominate their optical properties. Besides the optically accessible bright excitons, these systems exhibit a variety of dark excitonic states. They are not visible in the optical spectra, but can strongly influence the coherence lifetime and the linewidth of the emission from bright exciton states. Here, we investigate the microscopic origin of the excitonic coherence lifetime in two representative materials (WS2 and MoSe2) through a study combining microscopic theory with spectroscopic measurements. We show that the excitonic coherence lifetime is determined by phonon-induced intravalley scattering and intervalley scattering into dark excitonic states. In particular, in WS2, we identify exciton relaxation processes involving phonon emission into lower-lying dark states that are operative at all temperatures.