Shakeup processes in the recombination spectra of negatively charged excitons

Trion Emission Dominates the Low-Temperature Photoluminescence of CdSe Nanoplatelets

Colloidal nanoplatelets (NPLs) are atomically flat, quasi-two-dimensional particles of a semiconductor. Despite intense interest in their optical properties, several observations concerning the emission of CdSe NPLs remain puzzling. While their ensemble photoluminescence spectrum consists of a single narrow peak at room temperature, two distinct emission features appear at temperatures below ∼160 K. Several competing explanations for the origin of this two-color emission have been proposed. Here, we present temperature- and time-dependent experiments demonstrating that the two emission colors are due to two subpopulations of uncharged and charged NPLs. We study dilute films of isolated NPLs, thus excluding any explanation relying on collective effects due to NPL stacking. Temperature-dependent measurements explain that trion emission from charged NPLs is bright at cryogenic temperatures, while temperature activation of nonradiative Auger recombination quenches the trion emission above 160 K. Our findings clarify many of the questions surrounding the photoluminescence of CdSe NPLs.

Observation of Electron Shakeup in CdSe/CdS Core/Shell Nanoplatelets

While ensembles of CdSe nanoplatelets (NPLs) show remarkably narrow photoluminescence line widths at room temperature, adding a CdS shell to increase their fluorescence efficiency and photostability causes line width broadening. Moreover, ensemble emission spectra of CdSe/CdS core/shell NPLs become strongly asymmetric at cryogenic temperatures. If the origin of these effects were understood, this could potentially lead to stable core/shell NPLs with narrower emission, which would be advantageous for applications. To move in this direction, we report time-resolved emission spectra of individual CdSe/CdS core/shell NPLs at 4 K. We observe surprisingly complex emission spectra that contain multiple spectrally narrow emission features that change during the experiment. With machine-learning algorithms, we can extract characteristic peak energy differences in these spectra. We show that they are consistent with electron "shakeup lines" from negatively charged trions. In this process, an electron-hole pair recombines radiatively but gives part of its energy to the remaining electron by exciting it into a higher single-electron level. This "shakeup" mechanism is enabled in our NPLs due to strong exciton binding and weak lateral confinement of the charge carriers. Time-resolved single-photon-counting measurements and numerical calculations suggest that spectral jumps in the emission features originate from fluctuations in the confinement potential caused by microscopic structural changes on the NPL surface (e.g., due to mobile surface charges). Our results provide valuable insights into line width broadening mechanisms in colloidal NPLs.

Shake-up effect in photoluminescence of integer quantum Hall system formed in InGaAs/InP quantum wells

Polarization-resolved magneto-photoluminescence is studied in InGaAs/InP single quantum wells. In the range of the filling factor ν ≥ 4 the number of populated Landau levels contributing to the photoluminescence is found to be equal to the corresponding filling factor, while at ν ≤ 3 the number of emitting Landau levels is larger than the filling factor, which implies an occupancy of the Landau levels above the Fermi level. Such partial occupancy of the Landau levels with energies higher than the Fermi energy is due to shake-up processes caused by electron-electron interaction. In accordance to the theory, at the filling factor around ν = 2 the shake-up process was found to manifest itself in the downshifted cusp of the energy of the σ- polarized emission from the excited Landau levels, while no change was observed in the energy of the σ+ polarized emission. The different energies of differently spin-polarized excited Landau levels cause the magnetic field induced polarization of the emission from excited Landau levels. In addition, the bound electron-hole excitonic complexes (trions) associated with different Landau levels were observed.

Optical electron spin pumping in n-doped quantum wells

A theoretical model for optical spin pumping of electrons in a quantum well with low intrinsic electron density is presented. A system of electrons under continuous-wave illumination by circularly polarized light tuned to the electron-trion resonance is considered. The simultaneous off-resonant creation of excitons is also taken into account. The spin flip of trions and their radiative decay as the basic processes which allow the electronic spin pumping, as well as other processes, such as the formation of trions from excitons and electrons, are accounted for in the appropriate kinetic equations. The results obtained for CdTe and GaAs quantum wells indicate that significant electron spin polarization can be achieved in a time range of a few nanoseconds.

Evidence of a transition from nonlinear to linear screening of a two-dimensional electron system detected by photoluminescence spectroscopy

We clearly identify single-electron-localization (SEL), nonlinear screening (NLS), and linear screening (LS) regimes of gate induced electrons in a GaAs quantum well from photoluminescence spectra and intergate capacitance. Neutral and charged excitons observed in the SEL regime rapidly lose their oscillator strength when electron puddles are formed, which mark the onset of NLS. A further increase in the density of the electrons induces the transition from the NLS to LS, where the emission of a charged exciton changes to the recombination of two-dimensional electron gas and a hole.

Observation of charge transport by negatively charged excitons

We report transport of electron-hole complexes in semiconductor quantum wells under applied electric fields. Negatively charged excitons (X-), created by laser excitation of a high electron mobility transistor, are observed to drift upon applying a voltage between the source and drain. In contrast, neutral excitons do not drift under similar conditions. The X- mobility is found to be as high as 6.5 x 10(4) cm2 V-1 s-1. The results demonstrate that X- exists as a free particle in the best-quality samples and suggest that light emission from opto-electronic devices can be manipulated through exciton drift under applied electric fields.

Charged magnetoexcitons in two-dimensions: magnetic translations and families of dark states

We show that optical transitions of charged excitons in semiconductor heterostructures are governed in magnetic fields by a novel exact selection rule, a manifestation of magnetic translations. It is shown that the spin-triplet ground state of the quasi-two-dimensional charged exciton X--a bound state of two electrons and one hole-is optically inactive in photoluminescence at finite magnetic fields. Internal bound-to-bound X- triplet transition has a specific spectral position, below the electron cyclotron resonance, and is strictly prohibited in a translationally invariant system. These results allow one to discriminate between free and disorder-affected charged excitons.