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The present work describes the preparation and the investigation of the room temperature electrical and optical properties of a series of liquid nanocomposites (lnC) prepared with different concentrations of multiwalled carbon nanotubes (MWCNT) in a variety of liquid matrices: glycerin, Vaseline, glucose, propylene glycol and silicone oil (SIO). Special attention is deserved to the SIO matrix, owing to its convenient electrical properties for our purposes. We verified that a small percent fraction of MWCNT dispersed along the SIO matrix is capable of improving the electrical conductivity of the matrix by orders of magnitude, indicating that the MWCNT strongly participates in the electrical conduction mechanism. Also, the application of an external electric field to this lnC resulted in large changes in the optical transmittance, that were interpreted as a consequence of the fieldinduced MWCNT alignment into the liquid matrix. The characteristics of such a new category of nanocomposite in the liquid state suggest further studies.

This article shows results on photoluminescence measurements of Si nanocrystals embedded in a SiO2 matrix. Photoluminescence spectra were obtained as functions of both temperature (10–510 K) and excitation power (0.01–100 mW). Optical emission bands with interesting behavior as temperature was raised were observed, and a model to explain these bands considering contributions from interface states associated with SiO x and Si–O–Si units, whose emissions are located around 1.32 eV, 1.41 eV and 1.57 eV, and from quantum confinement states associated with the Si nanocrystals, was proposed.

In this work we investigated the electronic states of InAsP/GaAs QDs using photoluminescence (PL) and photomodulated transmission (PT) experiments. PL measurements were performed as a function of temperature and excitation power. As the recombination energy of the InAsP/GaAs QDs resides in between the extremes InAs/GaAs and InP/GaAs QDs, these two materials were also analyzed. From PL experiments we could identify the recombination from the QDs ground state and the contribution of the InAsP and InP wetting layer (WL) with a macro PL technique. PL measurements of annealed samples allowed us to to clearly identify the WL electronic states. Specifically for the InP/GaAs QDs, the WL and QD recombination overlap as a single PL band. Photomodulated transmission spectroscopy (PT) showed spectral structure consistent with the QD and WL assignments given after the PL analysis.

We have optically and structurally investigated the effect of the barrier alloy composition and number of wells in strain compensated InGaAsP/InP multiquantum well (MQW) samples grown by low-pressure metalorganic vapor phase epitaxy that emit close to 1.55 μm. Low temperature photoluminescence (PL) spectra exhibited two additional anomalous emission lines, below the fundamental QW transition. The PL temperature evolution of such anomalous transitions deviates significantly from the bulk-like band gap behavior expected. Cross-sectional transmission electron microscopy (XTEM) shows that partial elastic relaxation, characterized by localized surface deformations (like surface channels) and/or interfacial undulations occurs for all samples in which anomalous PL lines are observed. TEM results also show that the extra PL lines are correlated to neither Cu–Pt type atomic ordering in the well/barrier layers nor to phase separation. These anomalous transitions are tentatively attributed to the presence of interfa...

We studied the low temperature (77 K) photomodulated reflection and transmission as well as the photoluminescence at 2.2 K of a self-assembled InxGa1−xAs quantum dot layer grown on a (100) GaAs substrate in the vicinity of a two-dimensional electron gas. The dot layer was grown without rotation of the substrate in order to achieve a gradual variation of the In concentration along the wafer diameter. This resulted in an increase in the density of quantum dots along the In concentration gradient, which is reflected in a characteristic dependence of the relative intensities of the spectral lines. A consistent assignment of the optical structure observed in all spectra leads to an estimate of the average value of the Fermi energy in the conduction band of the wetting layer (EF≃13.4 meV). The variation of this Fermi energy along the composition gradient can be obtained from the spectra, and an estimate of the gradient of the density of quantum dots along this direction can be made. A careful comparison of the ...

We discuss recent measurements of photomodulated and electromodulated spectroscopy (77 K) as well as photoexcitation luminescence (2 K) in some In.Ga1−xAs/GaAs superlattices. Our results show clear evidence of Franz Keldysh and Stark-Wannier regimes for different samples and field values. In one of the samples, light and heavy holes are shown to be in different field regimes simultaneously. These studies support the idea that band line-up at the heterojunction is always such that the light holes are essentially unconfined.

Optical and micro-structural properties of ordered/disordered/ordered $\mathrm{InGaAsP}$ quantum wells grown on $\mathrm{GaAs}$ substrates were investigated by photoluminescence spectroscopy, high-resolution transmission electron microscopy and selective area diffraction. Strong evidence of carrier localization effects was obtained from low temperature photoluminescence experiments. Photoluminescence spectra of thinner quantum wells were dominated by a broad emission band located at energies below the bandgap of the well material. The energy peak position of this emission varied considerably with the laser excitation power. Carrier localization was attributed to potential fluctuations in the barrier and well layers, as a result of two coexisting effects: Spontaneously atomic ordering and, in a minor degree, alloy inhomogeneities. We show that a reduction of the ordering degree in the bottom barrier layer resulted in a considerable decrease of localization effects in quaternary quantum well heterostructures.