Exciton dynamics in monolayer graphene grown on a Cu(111) surface

We have characterized the carrier dynamics of the excitonic emission emerging from a monolayer of graphene grown on a Cu(111) surface. Excitonic emission from the graphene, with strong and sharp peaks both with a full-width at half-maximum of 2.7 meV, was observed near ~3.16 and ~3.18 eV at 4.2 K. The carrier recombination parameters were studied by measuring both temperature-dependent and time-resolved photoluminescence. The intensity variation with temperature of these two peaks shows an opposing trend. The time-resolved emission was modelled using coupled differential equations and the decay time was found to be dominated by carrier trapping and Auger recombination as the temperature increased.

Structural and Optical Properties of ZnSe Nanoparticles

We successfully synthesized ZnSe nanoparticles (NPs) by using a hydrothermal method. Careful analyses of X-ray diffraction pattern and high-resolution transmission electron microscopy images showed that the synthesized ZnSe NPs with the size of ˜100 nm and sphere-like morphology crystallized in the zincblende/cubic structure (the F-43m space group). This was also confirmed based upon characteristic vibration modes recorded by using Raman scattering spectroscopy. The study on room-temperature absorption and photoluminescence (PL) spectra proved ZnSe NPs having high crystal quality with the band gap energy Eg≈2.63 eV at 300 K and excitonic emission peaked at ˜2.64 eV (469 nm). Particularly, as studying temperature-dependent PL spectra, we found the shift of the emission peak towards lower energies while the PL intensity decreased when temperature increased from 15 to 300 K. The PL spectral parameters were analyzed by using the Arrhenius and Varshni laws.

Intrinsic donor-bound excitons in ultraclean monolayer semiconductors

The monolayer transition metal dichalcogenides are an emergent semiconductor platform exhibiting rich excitonic physics with coupled spin-valley degree of freedom and optical addressability. Here, we report a new series of low energy excitonic emission lines in the photoluminescence spectrum of ultraclean monolayer WSe2. These excitonic satellites are composed of three major peaks with energy separations matching known phonons, and appear only with electron doping. They possess homogenous spatial and spectral distribution, strong power saturation, and anomalously long population (>6 µs) and polarization lifetimes (>100 ns). Resonant excitation of the free inter- and intravalley bright trions leads to opposite optical orientation of the satellites, while excitation of the free dark trion resonance suppresses the satellitesʼ photoluminescence. Defect-controlled crystal synthesis and scanning tunneling microscopy measurements provide corroboration that these features are dark excitons bound to dilute donors, along with associated phonon replicas. Our work opens opportunities to engineer homogenous single emitters and explore collective quantum optical phenomena using intrinsic donor-bound excitons in ultraclean 2D semiconductors.