The band structure of MoS$_2$ 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 MoS$_2$. Single-layer MoS$_2$ therefore becomes an efficient emitter o
f 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 MoS$_2$ 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.
The initialization of spin polarization in localized hole states is investigated using time-resolved Kerr rotation. We find that the sign of the polarization depends on the magnetic field, and the power and the wavelength of the circularly polarized
pump pulse. An analysis of the spin dynamics and the spin-initialization process shows that two mechanisms are responsible for spin polarization with opposite sign: The difference of the g factor between the localized holes and the trions, as well as the capturing process of dark excitons by the localized hole states.
We have investigated antidot lattices, which were prepared on exfoliated graphene single layers via electron-beam lithography and ion etching, by means of scanning Raman spectroscopy. The peak positions, peak widths and intensities of the characteris
tic phonon modes of the carbon lattice have been studied systematically in a series of samples. In the patterned samples, we found a systematic stiffening of the G band mode, accompanied by a line narrowing, while the 2D mode energies are found to be linearly correlated with the G mode energies. We interpret this as evidence for p-type doping of the nanostructured graphene.
