RESUMEN
The optical properties of colloidal cesium lead halide perovskite (CsPbBr3) nanocrystals are examined by time-resolved and polarization-resolved spectroscopy in high magnetic fields up to 30 T. We unambiguously show that at cryogenic temperatures the emission is dominated by recombination of negatively charged excitons with radiative decay time of 300 ps. The additional long-lived emission, which decay time shortens from 40 down to 8 ns and in which the decay time shortens and relative amplitude increases in high magnetic fields, evidences the presence of a dark exciton. We evaluate g-factors of the bright exciton gX = +2.4, the electron ge = +2.18, and the hole gh = -0.22.
RESUMEN
Surface plasmon polariton (SPP) beams with an in-plane angular spread of 8° are produced by electrically exciting a 2D plasmonic crystal using a scanning tunneling microscope (STM). The plasmonic crystal consists of a gold nanoparticle (NP) array on a thin gold film on a glass substrate and it is the inelastic tunnel electrons (IET) from the STM that provide a localized and spectrally broadband SPP source. Surface waves on the gold film are shown to be essential for the coupling of the local, electrical excitation to the extended NP array, thus leading to the creation of SPP beams. A simple model of the scattering of SPPs by the array is used to explain the origin and direction of the generated SPP beams under certain conditions. In order to take into account the broadband spectrum of the source, calculations realized using finite-difference time-domain (FDTD) methods are obtained, showing that bandgaps for SPP propagation exist for certain wavelengths and indicating how changing the pitch of the NP array may enhance the SPP beaming effect.
RESUMEN
Raman spectroscopy is well-suited for the characterization of semiconductor materials. However, due the weakness of the Raman signal, the studies of thin semiconductor layers in complex environments, such as ultrahigh vacuum, are rather scarce. Here, we have designed a Raman apparatus based on the use of a fiber optic probe, with a lens collecting the backscattered light directly inserted in ultrahigh vacuum. The solution has been tested for the preparation of III-V semiconductor surfaces, which requires the recovery of their atomic reconstruction. The surfaces were either protected with a thin As amorphous layer or covered with a native oxide prior to their treatment. The analysis of the Raman spectra, which was correlated with the study of the surfaces with low temperature scanning tunneling microscopy at the end of the cleaning process, shows the high potential of Raman spectroscopy for monitoring the cleanliness of III-V semiconductor heterostructures in situ.
RESUMEN
Colloidal InP core nanocrystals are taking over CdSe-based nanocrystals, notably in optoelectronic applications. Despite their use in commercial devices, such as display screens, the optical properties of InP nanocrystals and especially their relation to the exciton fine structures remain poorly understood. In this work, we show that the ensemble magneto-optical properties of InP-based core/shell nanocrystals investigated in strong magnetic fields up to 30 T are strikingly different from other colloidal nanostructures. Notably, the mixing of the lowest spin-forbidden dark exciton state with the nearest spin-allowed bright state does not occur up to the highest magnetic fields applied. This lack of mixing in an ensemble of nanocrystals suggests an anisotropy tolerance of InP nanocrystals. This striking property allowed us to unveil the slow spin dynamics between Zeeman sublevels (up to 400 ns at 15 T). Furthermore, we show that the unexpected magnetic-field-induced lengthening of the dark exciton lifetime results from the hyperfine interaction between the spin of the electron in the dark exciton with the nuclear magnetic moments. Our results demonstrate the richness of the spin physics in InP quantum dots and stress the large potential of InP nanostructures for spin-based applications.
RESUMEN
Semiconductor nanocrystalline heterostructures can be produced by the immersion of semiconductor substrates into an aqueous precursor solution, but this approach usually leads to a high density of interfacial traps. In this work, we study the effect of a chemical passivation of the substrate prior to the nanocrystalline growth. PbS nanoplatelets grown on sulfur-treated InP (001) surfaces at temperatures as low as 95 °C exhibit abrupt crystalline interfaces that allow a direct and reproducible electron transfer to the InP substrate through the nanometer-thick nanoplatelets with scanning tunnelling spectroscopy. It is in sharp contrast with the less defined interface and the hysteresis of the current-voltage characteristics found without the passivation step. Based on a tunnelling effect occurring at energies below the bandgap of PbS, we show the formation of a type II, trap-free, epitaxial heterointerface, with a quality comparable to that grown on a nonreactive InP (110) substrate by molecular beam epitaxy. Our scheme offers an attractive alternative to the fabrication of semiconductor heterostructures in the gas phase.