A. Vercik

Thermal equilibrium governing the formation of negatively charged excitons in resonant tunneling diodes

The thermal equilibrium of the formation of negatively charged excitons is studied in this work, by measuring the current and quantum-well photoluminescence in biased resonant tunneling double barrier diodes. We observe that the intensity ratio of negatively charged and neutral excitons depends linearly on the current for fixed temperature and illumination conditions. We propose that the results can be interpreted in terms of a mass-action law governing the concentrations of neutral and charged excitons and free electrons. Measurements at different temperatures and bias yield an electron concentration and a dwell time in the well that are in good agreement with the values previously reported in the literature. We also analyze the dependence of the luminescence on excitation intensity.

Synthesis and characterisation of gold nanoparticles using Mentha piperita leaf extract: a green, non-toxic and rapid method

Eco-friendly synthesis of gold nanoparticles (AuNPs) using Mentha piperita leaf extract was addressed in this work. Different extract concentrations (0.5, 1.0 and 1.5% w/v) were used for the synthesis of AuNPs. Reaction rate increased with temperature and leaf extract concentration; however, prolonged heating might affect negatively the AuNPs synthesis. AuNPs in colloid state were synthesised in acid pH (between 2.5 and 3.1), with potentials for charged surface between –21.8 and –24.1 mV, these colloids were highly stable for a storage time of four weeks. Bioreduction has generated AuNPs with spherical and triangular shape and size range between 3–26 nm for all concentrations of leaf extract, indicating a weak dependence of the nanoparticle size on extract concentrations. The present work, thus, offers a simple, rapid and non-toxic process for the synthesis of AuNPs that would potentially be used in different areas such as foods, cosmetic, medicine, electronics and polymer science.

Photophysics of poly(p-phenylene vinylene) films

We report on time resolved photoluminescence (PLRT) measurements in poly(p-phenylene vinylene) (PPV) films irradiated by laser in the presence of air. We observe a PL intensity enhancement and a biexponential decay dynamics of PL signal for all irradiated films. These results can be understood in terms of a chain shortening process due to carbonyl incorporation and formation of an energy profile that extends and migrates into the film and enables efficient spectral diffusion of excited carriers to a non-degraded PPV segments by Forster energy transfer.

Anomalous photoluminescence behavior in GaAs/AlAs superlattice-like double-barrier diodes under bias

Transport and optical properties of a GaAs/AlAs superlattice-like double-barrier diode were studied by continuous-wave photoluminescence spectroscopy. An enhancement of the quantum-well photoluminescence intensity with increasing temperatures is reported in biased diodes, which is related to transport properties, disappearing at zero-applied voltage. The anomalous observed behavior can be explained in terms of the temperature-dependent transport properties of the minority carriers (holes) in the collector spacer layer, which limits the supply of holes available for tunneling.

Modeling tunneling and generation mechanisms governing the nonequilibrium transient in pulsed metal–oxide–semiconductor diodes

The transient behavior of tunnel metal–oxide–semiconductor structures, pulsed into inversion, is quantitatively described. A simple model for the measured transient currents is proposed, based on the integral form of the continuity equation, leading to an uncoupled solution of the Continuity and Poisson equations. Experimental results for structures with p-type or n-type substrates and different oxide thicknesses are fitted. A map showing the different behavior patterns in terms of surface generation velocity and oxide thickness is given.

Transient currents in pulsed metal–oxide–semiconductor tunnel diodes

The effect of tunneling currents on the transient from deep depletion towards equilibrium in thin oxide metal–oxide–semiconductor capacitors was experimentally investigated. Very thin oxide samples exhibit similar transients for both p- and n-type substrates. This symmetry breaks down for oxide thicknesses of more than 3.5 nm. Three qualitatively different behavior patterns, depending on the oxide thickness, can be identified by the current transient curves for the n-type substrate samples. From an analysis of the associated currents, the three patterns correspond to dominance by the minority carrier tunneling, dominance by the majority carrier tunneling, and enhanced generation through impact ionization.

A new conductometric biosensor based on horseradish peroxidase immobilized on chitosan and chitosan/gold nanoparticle films

Abstract A new conductometric biosensor was developed and characterized; the biosensor was based on horseradish peroxidase that was deposited in chitosan and chitosan/AuNPs films. The biosensors were characterized by scanning electron microscopy and current-voltage curves. Current-voltage curves in biosensors showed that the electrical conductivity and bistability in biosensors can be modulated by horseradish peroxidase. Horseradish peroxidase catalyzed the reduction of H2 O2 to H2 O with the oxidation of the prosthetic group (Fe3+) in the enzyme to Fe4+=O. Conductometric signal in the biosensors increased with the gradual increase of H2 O2 concentration, and it was due to the H2 O2 reduction. Linear hydrogen peroxide detection was observed for a concentration between 0 and 15 mm. The results proved that these biosensors could have promising industrial applications, due to its rapid and sensitive H2 O2 detection.

Influence of minority carrier transport in the optical properties of double barrier diodes

The aim of this work is to study the importance of minority carrier transport in double barrier diodes (DBD). We propose a theoretical model capable to describe the photoluminescence properties observed in GaAs-Al0.35Ga0.65As double barrier diodes with the increase of temperature. In this model, we considered that the minority carries (holes) photocreated at the contact of the structure diffuse, drift and then tunnel to the well. To study the influence of previous transport mechanisms, we solved the continuity equation under the influence of an applied bias and an excitation laser light. The theoretical photocurrent and photoluminescence calculations agree quite well with the experimental observations. Within the approaches of our model, we are able to simulate the minority holes mobility from the photocurrent measurements suggesting the use of the experimental technique as a probe of the carrier mobility.

Transport and optical properties of resonant tunneling structures

Transport properties of a GaAs/AlAs superlattice-like double barrier diode are studied in this work as a function of the sample temperature. An activation energy of about 60meV obtained from the Arrhenius plot is in good agreement with the confined level in the central well. Numerical simulations also confirm the importance of bound levels in the Gand X bands for the resonant tunneling process. The enhancement of photoluminescence as the temperature is increased is also studied. This behavior is associated to the transport properties of holes in the collector contact, which control the supply of minority carriers, which tunnel into the well. The description of the observed results requires the modi-cation of simple known models to take into account the two contributions to the pair generation rate in the well, responsible of the photoluminescence at zero and finite bias.

Resonant magnetotunneling of photogenerated holes in double barrier structures

In this work, we report on a technique—namely, the photoinduced magnetotunneling technique—which allows the direct experimental observation of tunneling of holes through valence-band Landau levels in n-i-n double-barrier resonant tunneling structures. Photocurrent–voltage curves exhibit several peaks associated with the complex nature of the dispersion of holes under parallel electric and magnetic fields applied to the sample.