A. L. A. Fonseca

Large-scale synthesis of single-wall carbon nanotubes by catalytic chemical vapor deposition (CCVD) method

The large-scale production of single-wall carbon nanotubes (SWNTs) is reported. Large quantities of SWNTs can be synthesised by catalytic decomposition of methane over well-dispersed metal particles supported on MgO at 1000°C. The thus produced SWNTs can be separated easily from the support by a simple acidic treatment to obtain a product with high yields (70–80%) of SWNTs. Because the typical synthesis time is 10 min, 1 g of SWNTs can be synthesised per day by this method. The SWNTs are characterized by high-resolution transmission electron microscopy and by Raman spectroscopy, showing the quality and the quantity of products.

Unexpected transition from single to double quantum well potential induced by intense laser fields in a semiconductor quantum well

When an electronic system is irradiated by an intense laser field, the potential “seen” by electrons is modified, which affects significantly the bound-state energy levels, a feature that has been observed in transition energy experiments. For lasers for which the dipole approximation applies, a nonperturbative approach based upon the Kramers–Henneberger translation transformation, followed by Floquet series expansions, yields, for sufficiently high frequencies, the so-called “laser-dressed” potential, which is taken for composing a time-independent Schrodinger equation whose solutions are the desired quasistationary states. This approach, developed originally for atoms, has been verified to be useful also for carriers in semiconductor nanostructures under intense laser fields. In quantum wells, analytical expressions for the dressed potential have been proposed in literature for a nonresonant, intense laser field polarized perpendicularly to the interfaces. By noting that they apply only for α0≤L/2, wher...

Intense field effects on hydrogen impurities in quantum dots

Calculations of the binding energy of an on-center donor hydrogenic impurity in a quasizero-dimensional quantum-well system [quantum dot (QD)] placed in an intense, high-frequency laser field are presented. A nonperturbative theory and a variational approach are used as the framework for this calculation. The effect of the intense laser field is to “dress” the impurity potential making it dependent upon the laser field amplitude. A rapid decrease of the binding energy, for different values of the QD radius and for both infinite and finite potential barriers, with increasing field intensity is predicted. An application is made for a spherical QD made of GaAs/Ga1−xAlxAs heterostructures.

Electric field effects on electron mobility in n-AlGaAs/GaAs/AlGaAs single asymmetric quantum wells

We calculated low-temperature electron mobility in n-AlGaAs/GaAs/AlGaAs single asymmetric quantum wells in the presence of a uniform electric field directed perpendicularly to the interfaces. The quantum well asymmetry is due to the doping profile (one-side modulation doping). Following a variational scheme, we solved both Schrodinger and Poisson equations simultaneously and the results were used to calculate the low-temperature (quasielastic) scattering rates. Only relevant scattering mechanisms were taken into account, namely ionized impurity, interface roughness, alloy disorder, and acoustic phonons (deformation potential and piezoelectric coupling). Our results show that both interface roughness and alloy disorder scattering rates are strongly dependent upon the electric field strength. We also show that there are interesting changes in the dominance of the mobility among different scattering processes, which leads to the formation of a maximum in the mobility dependence on the electric field strength...

Dichotomy of the exciton wave function in semiconductors under intense laser fields

We study the behavior of excitons in a semiconductor irradiated by a monochromatic, linearly polarized, intense laser field. By taking the finiteness of the hole effective mass into account and including the radiation field in a semiclassical manner, we solved the two-body quantum problem in the framework of a nonperturbative theory based upon the Kramers-Henneberger translation transformation for the Schrodinger equation. In the Kramers frame, the rapidly oscillating potential is expanded in a Fourier-Floquet series and, for laser frequencies high enough, only the zeroth-order term survives, the so-called “laser-dressed” potential. By applying the Ehlotzky’s approximation, this potential simplifies to a two-center potential that resembles that for the electronic motion in the H2+ molecule ion. The binding energy for an exciton in bulk GaAs under a nonresonant laser field is then computed by following a variational scheme we recently adapted from the linear combination of atomic orbitals-molecular orbital...

Transport of Polarons in Graphene Nanoribbons

The field-induced dynamics of polarons in armchair graphene nanoribbons (GNRs) is theoretically investigated in the framework of a two-dimensional tight-binding model with lattice relaxation. Our findings show that the semiconductor behavior, fundamental to polaron transport to take place, depends upon of a suitable balance between the GNR width and the electron-phonon (e-ph) coupling strength. In a similar way, we found that the parameter space for which the polaron is dynamically stable is limited to an even narrower region of the GNR width and the e-ph coupling strength. Interestingly, the interplay between the external electric field and the e-ph coupling plays the role to define a phase transition from subsonic to supersonic velocities for polarons in GNRs.

Magnetic field effect on the laser-driven density of states for electrons in a cylindrical quantum wire: transition from one-dimensional to zero-dimensional behavior

The influence of a uniform magnetic field on the density of states (DoS) for carriers confined in a cylindrical semiconductor quantum wire irradiated by a monochromatic, linearly polarized, intense laser field is computed here non-perturbatively, following the Greens function scheme introduced by some of the authors in a recent work (Lima et al 2009 Solid State Commun. 149 678). Besides the known changes in the DoS provoked by an intense terahertz laser field—namely, a significant reduction and the appearance of Franz–Keldysh-like oscillations—our model reveals that the inclusion of a longitudinal magnetic field induces additional blueshifts on the energy levels of the allowed states. Our results show that the increase of the blueshifts with the magnitude of the magnetic field depends only on the azimuthal quantum number m (m=0, 1, 2, ...), being more pronounced for states with higher values of m, which leads to some energy crossovers. For all states, we have obtained, even in the absence of a magnetic field, a localization effect that leads to a transition in the DoS from the usual profile of quasi-1D systems to a peaked profile typical of quasi-0D systems, as e.g. those found for electrons confined in a quantum dot.

Optical transitions involving impurities in semiconductors under additional infrared laser radiation

Abstract The influence of an intense infrared (IR) laser field on the optical absorption edge associated to acceptor impurities of a direct-gap semiconductor is discussed. It is shown that as the IR laser field intensity increases the absorption coefficient is modified so as to give rise to an absorption tail below the free-field forbidden gap. An application is made for the case of the GaAs irradiated by an intense CO2 laser.

Electron Mobility in One (Two)-Side Modulation-Doped GaAs/AlxGa1—xAs Asymmetric Quantum Wells

We have calculated the electron mobility in one (two)-side modulation-doped GaAs/AlxGa1—xAs asymmetric quantum wells. Coupled Schrodinger and Poisson equations were solved through the extended Fang-Howard variational method by considering the effective mass approximation, finite well barriers, the exchange-correlation correction to the effective potential and electrons in the quantum limit. The results were used to determine the electron mobility where only the relevant scattering processes, namely acoustic-phonon, ionized impurity, alloy disorder and interface roughness were considered. The partials and total mobility dependences on temperature were found and from the acoustic-phonon scattering contribution we have determined the temperature coefficient α. It is shown that the ionized impurity scattering is the dominant mechanism and that alloy disorder scattering overcomes acoustic-phonon scattering at very low temperatures. The ionized impurity and total mobilities were also related to the spacer width and the two-dimensional electron gas density. We have also considered the inclusion of a second modulation-doping spike on the reverse side of the quantum well and its effect on mobility has been discussed.

Fullerene formation in acetylene/oxygen/argon/chlorine flames

The impact of Cl-2 addition on the fullerene formation in acetylene/oxygen/argon flat flames burning at low pressure was investigated. The quantities of C-60 and C-70 in the soot were determined by HPLC after extraction with toluene. Negative ion mass spectra were measured using chemical ionization by means of a CH4/N2O mixture. The chlorine containing species were identified by means of a precursor ion mass spectrum. The addition of 25% of Cl-2 led to a dramatic increase of fullerene formation; soot containing 7.7% of (C-60+C-70) was deposited in the combustion chamber. The mass spectra showed signals at the masses of C10H2, C11H8 and C12H10 for soot formed in flames with and without chlorine addition while peaks corresponding to the polycyclic aromatic hydrocarbons (PAH) C18H10, C22H12 and C24H12 could be observed only without chlorine addition. A positive role of thermally and photochemically unstable chlorinated intermediate species is suggested for the increase of fullerene formation. Copyright (C) 1996 Elsevier Science Ltd