D. A. W. Soares

Resistive‐type humidity sensors based on PVP–Co and PVP–I2 complexes

Poly(vinylpyrrolidone) films containing cobalt chloride or iodine were investigated to obtain information on their possible use as a humidity sensor element. FTIR and UV-VIS spectroscopies were used to characterize the PVP-I 2 and PVP-Co complexes. Infrared spectroscopy revealed a structural change of both shape and intensity of the carbonyl and lactam bands, indicating the formation of an ion-coordination polymer. The J-E curves for pure PVP, PVP-I 2 , and PVP-Co films obey ohms law at low voltages, deviate from the linear response at higher voltages, and finally display breakdown behavior. An increase in current density of the PVP matrix with iodine or cobalt doping is attributed to the formation of charge transfer complexes. The observed hysteresis of the I-V characteristics implies that there was some standing voltage in the film, which could be attributed to a disorientation of polar side groups of PVP. The electrical conductivities of the polymeric complexes were very sensitive to environmental humidity. An explanation of the humidity-sensing behavior of the PVP-I 2 and PVP-Co complexes is presented.

Low potential stable glucose detection at dendrimers modified polyaniline nanotubes

The utilization of nanostructured materials for development of biosensors is a growing field in medical diagnostics. In this work a glucose biosensor based on bioactive polyglycerol (PGLD) and chitosan dendrimers (CHD) was developed. PGLD and CHD were bioconjugated with the enzyme glucose oxidase (GOx) to obtain dendrimers with glucose sensing properties. Polyaniline nanotubes (PANINT´s) were used as electron mediator due to their high ability to promote electron-transfer reactions involving GOx. The PGLD-GOx and CHD-GOx were entrapped in PANINT´s during template electrochemical polymerization of aniline. The prepared PGLD-GOx/PANINT´s and CHD-GOx/PANINT´s biosensors exhibit a strong and stable amperometric response to glucose even at a low potential of +100 mV. The based PGLD-GOx/PANINT´s and CHD-GOx/PANINT´s biosensors showed a good performance in glucose concentrations range in human blood. A comparison of the sensitivities to glucose showed that both biosensors have a linearity range between 0.02 and 10 mM, though PGLD-GOx/PANINT´s is more sensitive (10.41 vs. 7.04 nA.mM-1). The difference in the biosensor behavior and the high sensitivity of the PGLD-GOx/PANINT´s may be due to the specific organization of GOx layer at surface of the modifier macromolecule PGLD and their distribution in PANINT´s. The enzyme affinity for the substrate, KMapp remains quite good after GOx immobilization on PGLD and CHD dendrimers and entrapment of the bioconjugates in PANINT´s.

Electro- and photoconductive properties of the PVA/PAN-I2 blends

The electrical conductivity of poly(vinylalcohol)/polyaniline-iodine blend (PVA/PAN-I 2 ) prepared by solution process was investigated. The FTIR spectroscopy revealed a structural change of both shape and intensity of the polyaniline (PAN) bands after doping with iodine, indicating the formation of a charge transfer complex. The J-V curves for pure PAN; PAN-I 2 and PVA/PAN-I 2 film obey the ohm law at lower voltages, deviate from the linear response at higher voltages and finally display the breakdown behavior. The PVA/PAN-I 2 exhibit photoconductivity by UV/visible irradiation as well as oscillations that may be attributed to a nonlinear behavior of the blend.

Systematic study of doping dependence on linear magnetoresistance in p-PbTe

We report on a large linear magnetoresistance effect observed in doped p-PbTe films. While undoped p-PbTe reveals a sublinear magnetoresistance, p-PbTe films doped with BaF2 exhibit a transition to a nearly perfect linear magnetoresistance behaviour that is persistent up to 30 T. The linear magnetoresistance slope ΔR/ΔB is to a good approximation, independent of temperature. This is in agreement with the theory of Quantum Linear Magnetoresistance. We also performed magnetoresistance simulations using a classical model of linear magnetoresistance. We found that this model fails to explain the experimental data. A systematic study of the doping dependence reveals that the linear magnetoresistance response has a maximum for small BaF2 doping levels and diminishes rapidly for increasing doping levels. Exploiting the huge impact of doping on the linear magnetoresistance signal could lead to new classes of devices with giant magnetoresistance behavior.

Room temperature persistent photoconductivity in p-PbTe and p-PbTe:BaF2

We investigated the persistent photoconductivity effect observed in p-PbTe:BaF2 and undoped p-PbTe films in the temperature range of T = 100–300 K. It was observed that the PPC effect scales with temperature and that there is a transition in the relaxation time behavior around ∼150 K. We found that the transition is caused by the particular dynamics of the hole carries between the energy barriers that characterize the traps originated from disorder present in the samples. The analysis was performed by comparing the theory of the random potential with the experimental data and revealed the presence of both random local potential fluctuations and localized states, which can be attributed to the presence of disorder due BaF2 doping and Te vacancies.

Investigation of negative photoconductivity in p-type Pb1-xSnxTe film

We investigated the negative photoconductivity (NPC) effect that was observed in a p-type Pb1-xSnxTe film for temperatures varying from 300 K down to 85 K. We found that this effect is a consequence of defect states located in the bandgap which act as trapping levels, changing the relation between generation and recombination rates. Theoretical calculations predict contributions to the NPC from both conduction and valence bands, which are in accordance with the experimental observations.

Systematic study of transport via surface and bulk states in Bi2Te3 topological insulator

We performed magnetoresistance measurements on Bi2Te3 thin film in the temperature range of T = 1.2–4.0 K and for magnetic fields up to 2 T. The curves exhibited anomalous behavior for temperatures below 4.0 K. Different temperature intervals revealed electrical transport through different conductive channels with clear signatures of weak antilocalization. The magnetoresistance curves were explained using the Hikami–Larkin–Nagaoka model and the 2D Dirac modified model. The comparison between the parameters obtained from the two models revealed the transport via topological surface states and bulk states. In addition, a superconductive like transition is observed for the lowest temperatures and we suggest that this effect can be originated from the misfit dislocations caused by strain, giving rise to a superconductive channel between the interface of the film and the substrate.

Fast photoresponse and high parallel transport in n-type PbTe/PbEuTe quantum wells

We investigated the photoconductivity effect in n-type PbTe/Pb0.88Eu0.12Te quantum wells for a temperature range of 300–10 K using infrared light. The measurements revealed that at high temperatures, the photoresponse has small amplitude. As temperature decreases to T ∼ 75 K, however, the photoconductivity amplitude increases reaching a maximum value 10 times higher than the original value before illumination. From Hall measurements performed under dark and light conditions, we show that this effect is a result of the carrier concentration increase under illumination. Unexpectedly, for further reduction of temperature, the amplitude starts to decrease again. The electrical resistance profiles indicate that the transport occurs through barriers and the well that behave as two parallel channels. For temperatures below 75 K, transport is more effective in the quantum well, where the signal reduction can be associated with the electron-electron scattering due to the increase in the carrier concentration that ...