Richard Leonelli

Photooxidation and quantum confinement effects in exfoliated black phosphorus

Thin layers of black phosphorus have recently raised interest owing to their two-dimensional (2D) semiconducting properties, such as tunable direct bandgap and high carrier mobilities. This lamellar crystal of phosphorus atoms can be exfoliated down to monolayer 2D-phosphane (also called phosphorene) using procedures similar to those used for graphene. Probing the properties has, however, been challenged by a fast degradation of the thinnest layers on exposure to ambient conditions. Herein, we investigate this chemistry using in situ Raman and transmission electron spectroscopies. The results highlight a thickness-dependent photoassisted oxidation reaction with oxygen dissolved in adsorbed water. The oxidation kinetics is consistent with a phenomenological model involving electron transfer and quantum confinement as key parameters. A procedure carried out in a glove box is used to prepare mono-, bi- and multilayer 2D-phosphane in their pristine states for further studies on the effect of layer thickness on the Raman modes. Controlled experiments in ambient conditions are shown to lower the A(g)(1)/A(g)(2) intensity ratio for ultrathin layers, a signature of oxidation.

Carrier thermal escape in families of InAs/InP self-assembled quantum dots

The temperature evolution of the photoluminescence spectra of two samples of single-layer InAs/InP (001) self-assembled quantum dots is measured from 10 to 300 K. To understand the thermal quenching of their multimodal emission, we develop a coupled rate-equation model that includes the effect of carrier thermal escape from a quantum dot to the wetting layer and to the InP matrix, followed by transport, recapture or nonradiative recombination. Our model reproduces the temperature dependence of the emission of each family of quantum dots with a single set of parameters. We find that the main escape mechanism of the carriers confined in the quantum dots is through thermal emission to the wetting layer. The activation energy for this process is found to be close to one half the energy difference between that of a given family of quantum dots and that of the wetting layer as measured by photoluminescence excitation experiments. This indicates that electron and holes exit the InAs quantum dots as correlated pairs.

Misfit strain, relaxation, and band‐gap shift in GaxIn1−xP/InP epitaxial layers

A detailed investigation of the structural and optoelectronic properties of thick GaInP epilayers on sulfur‐doped InP substrates is reported. Significant variations of the optical absorption and photoluminescence transition energies from light‐ and heavy‐hole states are observed as a function of the epilayer composition as well as of the degree of relaxation of the misfit strain. High‐resolution x‐ray measurements were used to determine the Ga concentrations and the strains and indicate significant anisotropic relaxation in several films. Even small relaxations result in a significant increase in the optical linewidths and a rapid drop in the transition intensities. A model with no free parameters based on the strain Hamiltonian of Pikus and Bir provides excellent agreement with the transition energies and serves to identify unambiguously the transitions observed in the optical spectra. Within this model, isotropic in‐plane relaxation produces a shift of both light‐ and heavy‐hole energies whereas anisotr...

Growth and structural properties of epitaxial GaxIn1−xP on InP

The growth of heteroepitaxial GaxIn1−xP on InP for 0<x<0.25 has been carried out by low‐pressure metalorganic chemical vapor deposition and characterized by high‐resolution x‐ray diffraction and low‐temperature photoluminescence measurements. The x‐ray data indicate that the epilayers are under biaxial tensile strain and that, for samples with x<0.05, the lattice mismatch is accommodated almost completely by tetragonal distortions. From photoluminescence measurements, the energy band gap is found to vary monotonically with the Ga concentration; it also shifts linearly with the elastic strain in the layer. The calculated value of 0.99×104 meV per unit strain is in good agreement with that predicted from elasticity theory.

Carrier recombination dynamics in In x Ga 1 − x N / GaN multiple quantum wells

We have mesured the carrier recombination dynamics in InGaN/GaN multiple quantum wells over an unprecedented range in intensity. We find that at times shorter than 30\,ns, they follow an exponential form, and a power law at times longer than 1\,

Structural and optical characterization of monolayer interfaces in Ga0.47In0.53As/InP multiple quantum wells grown by chemical beam epitaxy

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Structural and optical properties of strain‐relaxed InAsP/InP heterostructures grown by metalorganic vapor phase epitaxy on InP(001) using tertiarybutylarsine

s. To explain these biphasic dynamics, we propose a simple three-level model where a charge-separated state interplays with the radiative state through charge transfer following a tunneling mechanism. We show how the distribution of distances in charge-separated states controls the dynamics at long time. Our results imply that charge recombination happens on nearly-isolated clusters of localization centers.

Autofluorescence of Condensed Heme Aggregates in Malaria Pigment and Its Synthetic Equivalent Hematin Anhydride (β-Hematin)

We have carried out a detailed structural and optical characterization of Ga0.47In0.53As/InP multiple quantum wells grown by chemical beam epitaxy using a well‐defined sequence of growth interruption times between successive layers. These growth interruption times result in the formation of interfacial layers which drastically alter the structural properties of Ga0.47In0.53As/InP multiple quantum wells. An analysis of double‐crystal x‐ray diffraction data reveals that exposure of InP to arsine for 2 s is sufficient to create approximately 3 monolayers of InAs0.55P0.45 ternary under biaxial compressive strain at the InP/Ga047In0.53As interface. Moreover, exposure of Ga0.47In0.53As to phosphine for 2 s results in the formation of approximately 2 monolayers of Ga0.48In0.52As0.21P0.79 quaternary under biaxial tensile strain at the Ga0.47In0.53As/InP interface. We find that long exposures to hydrides (over 5 s) rather than short ones give rise to interfacial layers with less compositional disorder and/or thick...

Organometallic vapor phase epitaxy of GaAs1−xNx alloy layers on GaAs(001): Nitrogen incorporation and lattice parameter variation

A combination of transmission electron microscopy and high‐resolution x‐ray diffraction analyses has been used to determine the exact strain in each layer of InAsP/InP multiple‐quantum‐well structures grown by metalorganic vapor phase epitaxy on InP(001) using trimethylindium, tertiarybutylarsine, and phosphine as precursors. The strain‐relaxed structures are characterized by misfit dislocations located exclusively at (i) the interface between the buffer layer and the multilayer, and (ii) the interface between the multilayer and the cap layer. The low‐temperature optical absorption spectra show well resolved excitonic transitions that are significantly shifted by strain relaxation. The spectra are analyzed with a solution to the Schrodinger equation in the envelope function formalism using the Bastard–Marzin model. The energies for the major transitions involving light‐ and heavy‐holes are predicted accurately for all samples, allowing the determination of the heterojunction band offset. The heavy‐ and li...

Optical properties of submonolayer InAs/InP quantum dots on vicinal surfaces

The condensed crystalline phase of iron(III) protoporphyrin IX either isolated from parasite culture as malaria pigment (hemozoin) or synthetic equivalent hematin anhydride exhibits a solid-state autofluorescence characterized by an excitation maximum of 555 nm and an emission maximum of 577 nm. The excitation spectrum maximum at 555 nm corresponds to the Q(0,0) band in the absorption spectrum which represents the lowest singlet of the material. This suggests that the fluorescent emission is due to the heme condensed phase. The photoluminescence lifetime of tau(f) = 2.7 +/- 0.8 ns as measured at four wavelengths between 550 and 600 nm is in the range of Frankel exciton in porphyrinic condensed phases. The material is shown to have an optical band gap of 2.04 eV characteristic of a semiconductor. Luminescence is markedly dependent upon the degree of hydration and the emission does not seem to be caused by presence of zinc(II) protoporphyrin IX or free-base protoporphyrin IX in the lattice. The autofluorescence can be used for in vivo tracking of hemozoin, for determination of parasitemia levels, and for infection monitoring and possibly for drug screening studies.