Core-shell ZnO/ZnMnO nanowires on a-Al2O3 and GaN (buffer layer)/Si (111) substrates were fabricated by pulsed laser deposition using a Au catalyst. Two ZnO targets with a Mn content of 10% were sintered at 1150 and 550 °C in order to achieve the domination in them of paramagnetic MnO2 and ferromagnetic Mn2O3 phases, respectively. Cluster mechanism of laser ablation as a source of possible incorporation of secondary phases to the wire shell is discussed. Raman spectroscopy under excitation by an Ar+ laser revealed a broad peak related to the Mn-induced disorder and a redshift in the A1-LO phonon. Resonant Raman measurements revealed an increase in the multiphonon scattering caused by disorder in ZnO upon doping by Mn. Besides the UV emission, a vibronic green emission band assisted by a ∼ 71 meV LO phonon is also observed in the photoluminescence spectra. Core-shell structures with smooth shells show a high exciton to green band intensity ratio ( ∼ 10) even at room temperature. © 2009 American Institute of Physics

We report the observation of slow emission bands overlapped with the near-band-edge steady-state luminescence of nominally undoped ZnO crystals. At low temperatures the time-resolved spectra are dominated by the emission of several high-energy bound exciton lines and the two-electron satellite spectral region. Furthermore, two donor-acceptor pair transitions at 3.22 and 3.238 eV are clearly identified in temperature-dependent time-resolved spectroscopy. These donor-acceptor pairs involve a common shallow donor at 67 meV and deep acceptor levels at 250 and 232 meV.

We present structural, optical and transport data on GaN samples grown by hybrid, two-step low temperature pulsed laser deposition. The band gap of samples with good crystallinity has been deduced from optical spectra. Large below gap band tails were observed. In samples with the lowest crystalline quality the PL spectra are quite dependent on spot laser incidence. The most intense PL lines can be attributed to excitons bounded to stacking faults. When the crystalline quality of the samples is increased the ubiquitous yellow emission band can be detected following a quenching process described by a similar activation energy to that one found in MOCVD grown samples. The samples with the highest quality present, besides the yellow band, show a large near band edge emission which peaked at 3.47 eV and could be observed up to room temperature. The large width of the NBE is attributed to effect of a wide distribution of band tail states on the excitons. Photoconductivity data supports this interpretation

The similarities between ZnO and GaN samples reveal that the semiconductor oxide is a potential material as a promising material for optoelectronic devices. In this work, we study by spectroscopic and RBS techniques the characteristics of bulk ZnO. A comparison between the energy separation of the several groups of near band edge photoluminescence emission bands and the energy separation between the free exciton resonances observed in reflectivity is made. We report on the luminescence dependence from temperature and the variation of the relative intensities of the lines. From the data, an assignment of recombination centers is discussed. © 2001 Elsevier Science B.V. All rights reserved.

We report the spectroscopic observation of self-trapped polaron exciton signatures in colloidally synthesized, ligand-assisted CsPbBr3single bondCs4PbBr6 nanocrystals. Temperature dependence of photoluminescence spectroscopy show broad shoulder emissions below the exciton signatures in visible wavelength region, which are attributed to the formation of self-trapped polaron excitons corresponding to the relaxation of electrons and holes localized at Pb23+, and Pb3+ sites, respectively. While self-trapped polaron exciton emission peaks shifted to lower energies side, its intensity substantially enhanced as the temperature decreases from 180 to 14 K. The polaron-excitons band splitting is attributed to the changes in self-trapped energy levels driven by the strong exchange field of Pb-6p orbitals during relaxation process, coupled with specific lattice distortions arising from phonon anharmonicity; those self-trapped carriers act like non-radiative centers for deteriorating exciton emission intensity.

Collective excitations of charged particles under the influence of an electromagnetic field give rise to a rich variety of hybrid light-matter quasiparticles with unique properties. In metals, intraband collective response manifested by negative permittivity leads to plasmon polaritons with extreme field confinement, wavelength “squeezing,” and potentially low propagation losses. In contrast, photons in semiconductors commonly couple to interband collective response in the form of exciton polaritons, which give rise to completely different polaritonic properties, described by a superposition of the photon and exciton and an anti-crossing of the eigenstates. In this work, we identify the existence of plasmon-like collective excitations originating from the interband excitonic response of biased bilayer and trilayer graphene, in the form of graphene exciton polaritons (GEPs). We find that GEPs possess electrically tunable polaritonic properties and discover that such excitations follow a universal dispersion law for all surface polaritons in 2D excitonic systems. Accounting for nonlocal corrections to the excitonic response, we find that the GEPs exhibit confinement factors that can exceed those of graphene plasmons, and with moderate losses that would enable their observation in cryo-SNOM experiments. These predictions of plasmon-like interband collective excitations in biased graphene systems open up new research avenues for tunable plasmonic phenomena based on excitonic systems, and the ability to control and manipulate such phenomena at the atomic scale.

With the recent advances in two-dimensional (2D) materials fabrication techniques, the variety of these materials that became available for experimental studies has increased substantially over the past few years. The possibility of manipulating the number of layers in these materials also brings new perspectives to explore the physical properties of multilayer 2D materials. This is interesting because the physical properties of some 2D materials, such as the band alignment between layers, quasi-particle effective masses, and optical band gaps, can be tuned by the number of layers. One can explore such a multilayer semiconductor system in two ways: with a stack of layers of the same material, or combining layers of different materials. In this work, we consider the first case, where the material is black phosphorus (BP). Motivated by recent experimental observations of light absorption in few-layer BP, we develop a theoretical model to explain the origin of a series of high-intensity peaks interspersed by pairs of low-intensity peaks, in the experimental absortion spectra of BP. The model consists in describing excitonic states in a few-layer of BP through the effective mass approximation, which deals with the electron and hole in-plane coordinates, along with a unidimensional tight-binding approximation that accounts for inter-layer couplings. This yields excitonics transitions between different combinations of the sub-bands created by the coupled BP layers, which leads to a series of high and low-intensity peaks that are related to the oscillator strength of the excitonic states involved. These high and low-intensity states are referred in the literature as bright and dark excitons states, respectively. The energy and oscillator strength of the sub-bands exciton states can be controlled through a perpendicular electric field and the number of layers. The results obtained by this model are consistent with what is observed in the experiment and, in addition, they help elucidating aspects that the experiment did not consider about subband excitonic states. Among them, are the hybridization between excitonic states and the dependency of diamagnetic shift with an electric field for different states.