Large-Area Epitaxial Mott Insulating 1T-TaSe2 Monolayer on GaP(111)B.

Two-dimensional Mott materials have recently been reported in the dichalcogenide family with high potential for Mottronic applications. Nevertheless, their widespread use as a single or few layers is hampered by their limited device integration resulting from their growth on graphene, a metallic substrate. Here, we report on the fabrication of 1T-TaSe2 monolayers grown by molecular beam epitaxy on semiconducting gallium phosphide substrates. At the nanoscale, the charge density wave reconstruction and a moiré pattern resulting from the monolayer interaction with the substrate are observed by scanning tunneling microscopy. The fully open gap unveiled by tunneling spectroscopy, which can be further manipulated by the proximity of a metal tip, is confirmed by transport measurements from micrometric to millimetric scales, demonstrating a robust Mott insulating phase at up to 400 K.

Assessing the insulating properties of an ultrathin SrTiO3shell grown around GaAs nanowires with molecular beam epitaxy.

We have studied electronic transport in undoped GaAs/SrTiO3core-shell nanowires standing on their Si substrate with two-tip scanning tunneling microscopy in ultrahigh vacuum. The resistance profile along the nanowires is proportional to the tip separation with resistances per unit length of a few GΩ/µm. Examination of the different transport pathways parallel to the nanowire growth axis reveals that the measured resistance is consistent with a conduction along the interfacial states at the GaAs{110} sidewalls, the 2 nm thick SrTiO3shell being as much as resistive, despite oxygen deficient growth conditions. The origin of the shell resistivity is discussed in light of the nanowire analysis with transmission electron microscopy and Raman spectroscopy, providing good grounds for the use of SrTiO3shells as gate insulators.

Thermal activation of non-radiative Auger recombination in charged colloidal nanocrystals.

Applications of semiconductor nanocrystals such as biomarkers and light-emitting optoelectronic devices require that their fluorescence quantum yield be close to 100%. However, such quantum yields have not been obtained yet, in part, because non-radiative Auger recombination in charged nanocrystals could not be suppressed completely. Here, we synthesize colloidal core/thick-shell CdSe/CdS nanocrystals with 100% quantum yield and completely quenched Auger processes at low temperatures, although the nanocrystals are negatively photocharged. Single particle and ensemble spectroscopy in the temperature range 30-300 K shows that the non-radiative Auger recombination is thermally activated around 200 K. Experimental results are well described by a model suggesting a temperature-dependent delocalization of one of the trion electrons from the CdSe core and enhanced Auger recombination at the abrupt CdS outer surface. These results point to a route for the design of core/shell structures with 100% quantum yield at room temperature.

Efficient biexciton emission in elongated CdSe/ZnS nanocrystals.

We use a combination of low-temperature magneto-optical and lifetime spectroscopies to study the band-edge exciton fine structure of highly photostable single CdSe/ZnS nanocrystals (NCs). Neutral NCs displaying multiline emission spectra and multiexponential photoluminescence (PL) decays are studied as a function of temperature and external magnetic fields. Three different fine structure regimes are identified as a function of the NC aspect ratio. In particular, we identify an optically inactive ground exciton state, whose oscillator strength is tuned up under magnetic field coupling to bright exciton states, and attribute it to the zero angular momentum ground exciton state of elongated NCs. We also show evidence for highly efficient biexciton emission in these NCs, with radiative yields approaching unity in some cases.

Band-edge exciton fine structure of single CdSe/ZnS nanocrystals in external magnetic fields.

We report a spectroscopic study of the two lowest-energy exciton levels of individual CdSe/ZnS nanocrystals under applied magnetic fields. Field-induced coupling between the bright and the dark excitonic states is directly observed in the low-temperature photoluminescence spectrum and decay and allows the determination of the angle between the nanocrystal c axis and the field. Orientation-dependent Zeeman splittings of the dark and bright exciton sublevels are measured and provide the corresponding exciton Landé factors, as well as spin-flip relaxation rates between Zeeman sublevels.

Direct observation of the two lowest exciton zero-phonon lines in single CdSe/ZnS nanocrystals.

We report a spectroscopic study of highly photostable individual CdSe/ZnS colloidal nanocrystals. At low temperature, photoluminescence spectra display two sharp zero-phonon lines which we attribute to the radiative recombination from the two lowest levels of the band-edge exciton fine structure. For the first time, resonant photoluminescence excitation spectra of these lines is performed, and spectral diffusion broadening of 10 microeV is measured over integration times of 100 ms, corresponding to an optical coherence lifetime longer than 100 ps.