[en] We report on a theoretical framework for magnetic hyperthermia where the amount of heat generated by nanoparticles can be understood when both the physical and hydrodynamic size distributions are known accurately. The model is validated by studying the magnetic, colloidal and heating properties of magnetite/maghemite nanoparticles of different sizes dispersed in solvents of varying viscosity. We show that heating arising due to susceptibility losses can be neglected with hysteresis loss being the dominant mechanism. We show that it is crucial to measure the specific absorption rate of samples only when embedded in a solid matrix to avoid heating by stirring. However the data shows that distributions of both size and anisotropy must be included in theoretical models. (fast track communication)

[en] Ordered ferroelectric neodymium (Nd)-substituted bismuth titanate (Bi_4−xNd_xTi₃O₁₂) is fabricated on silicon microchannel plates (Si-MCPs). A significantly enhanced photoluminescence peak at about 890 nm (∼1.4 eV) corresponding to the 4 f³ − 4 f³ forced electric dipole transition from the ⁴I_9/2 ground state to different excited states of Nd³⁺ is observed. Moreover, the near-infrared emission intensity can be tuned by adjusting the Nd composition. The effects can be ascribed to the difference in the host lattice between the Nd-doped titanate with a simple perovskite structure and the result rendered by the regular cubic array of the Si-MCPs. (paper)

[en] In this work we study the properties of wetting layers in InAs/InGaAs/GaAs quantum dot (QD) structures suitable for single photon emission; the mandatory low density of QDs is obtained by an molecular beam epitaxy (MBE) approach based on the deposition of sub-critical InAs coverages followed by post-growth annealing. Such a growth regime is fundamentally different from the Stranski–Krastanow (SK) one commonly used for the deposition of QDs. By fitting x-ray diffraction (XRD) spectra, ten-steps composition profiles were derived and used as inputs of model calculations of the two-dimensional quantum energy system: model results were validated by comparison with photoluminescence spectra. A strong reduction of In molar fraction in wetting layers in comparison with SK structures was found, causing a larger wavefunction delocalization for carriers that populate the wetting layer energy levels. Moreover, by considering the limits for strain relaxation when In_xGa_1−xAs capping layers are used, we grew structures with the highest possible values of x to study the modifications of the energy system. When x = 0.20 the electron–heavy hole overlap is almost halved and the carriers' probability of being in the first nanometre of the wetting layer is reduced by 60%. These results will be useful for advanced design of QD nanostructures for single photon sources. (paper)

[en] Using an in situ Kelvin probe method to investigate the development of interface dipole layers in HfO₂/Si structures during interfacial Si oxidation, we found that the dipole strength is greatest when about one monolayer of Si–O bonds is formed at the HfO₂/Si interfaces and that further Si–O bond formation reduces the dipole strength. We discuss the difference between the potential profiles of HfO₂/one-monolayer SiO/Si and HfO₂/two-monolayer SiO/Si structures by using atomistic interface models taking into account dielectric constant screening. (paper)

[en] With a probe of atomic dimensions, the aberration-corrected scanning transmission electron microscope (STEM) has become one of the most advanced analytical tools for materials research. However, electron-beam damage limits its applications to materials that are insensitive to damage, and these are a small fraction of the materials that are important to the future of material science. One important group of materials which are susceptible to the electron beam is electrically insulating inorganic materials, including both crystals and glasses. In this paper, damage phenomena for these materials under the illumination of a finely focused electron beam in STEM are summarized. The leading mechanism for damage is massive atom displacement driven by an electric field, which is induced by the excitations and ionizations of atomic electrons. The electric field induced by the STEM probe, and thus the damage, is spatially delocalized and dependent on beam current density (dose rate) and exposure time but approximately independent of specimen thickness. This mechanism causes either phase separation, including precipitation and formation of gas bubbles, or phase transformation. From a microscopic analysis point of view, higher beam currents and longer exposures should therefore be avoided if the induced electric field is the main cause for damage. Methods of minimizing damage are reviewed. (paper)

[en] We investigate the effect of post-growth rapid thermal annealing (RTA) on the transport and lasing characteristics of terahertz quantum-cascade lasers (THz QCLs) operating in a frequency range between 4.88 and 4.94 THz. The emission frequencies are blue shifted by about 80 GHz after RTA, which is attributed to a shift of the gain maximum to higher frequencies due to composition grading at the interfaces between the quantum wells and barriers of the annealed wafer pieces. The optical output power of the annealed THz QCLs is reduced, which is explained by a broadening of the levels due to the annealing process. (paper)

[en] In this work two-photon absorption laser-induced fluorescence was used to measure oxygen atom (O) concentrations in streamer discharge afterglow in a variety of fuel/air mixtures in order to account for the O reaction pathways in transient plasma ignition. It is demonstrated that O atoms are generated in high concentration (∼5 × 10¹⁷ cm⁻³) directly below the high-voltage anode in a point-to-plane geometry. The corresponding lifetimes in air were on the order of hundreds of microseconds. Fuel chemistry provides consumption pathways via chain branching reactions even without sustained combustion, and the corresponding O-atom lifetimes were much shorter than in air and dependent on the fuel concentration. At the richest conditions, corresponding to a fuel–air equivalence ratio of 2.4, O lifetimes were on the order a few microseconds or less. These experimental results are compared to modelling estimates in order to better understand the role of atomic oxygen in the chemical processes leading to ignition. (paper)

[en] The temperature dependence features of a typical intrinsic semiconductor resistance were used to study the resistances in single-walled carbon nanotube (SWNT) networks formed on gold nanoparticle (AuNP) templates, using the resistance–temperature relations in the temperature range from 440 to 570 K. The SWNTs were synthesized using the chemical vapour deposition method, with ethanol as a carbon source and cobalt particles as the catalyst. The synthesis was performed at 1073 K, for a duration of 60 min. It was found that the negative temperature coefficients and the energy gaps, which indicated the degree of semiconductor behaviour, increased with increasing sample starting resistance. However, at the same sample starting resistance, the degree of semiconductor behaviour increased with increasing AuNP number density, resulted from the change in the effective diameter of the semiconducting SWNTs of the sample, which occurred because of the effects of the AuNP number density. (paper)

[en] In recent years, magnetic nanoparticles (MNPs) have gained increased attention due to their superparamagnetic properties. These properties allow the development of innovative biomedical applications such as targeted drug delivery and tumour heating. However, these modalities lack effective operation arising from the inaccurate quantification of the spatial MNP distribution. This paper proposes an approach for assessing the one-dimensional (1D) MNP distribution using electron paramagnetic resonance (EPR). EPR is able to accurately determine the MNP concentration in a single volume but not the MNP distribution throughout this volume. A new approach that exploits the solution of inverse problems for the correct interpretation of the measured EPR signals, is investigated. We achieve reconstruction of the 1D distribution of MNPs using EPR. Furthermore, the impact of temperature control on the reconstructed distributions is analysed by comparing two EPR setups where the latter setup is temperature controlled. Reconstruction quality for the temperature-controlled setup increases with an average of 5% and with a maximum increase of 13% for distributions with relatively lower iron concentrations and higher resolutions. However, these measurements are only a validation of our new method and form no hard limits. (paper)

[en] We report radial, p–n junction, sub-micrometre, pillar array textured solar cells, fabricated on an n-type Czochralski silicon wafer. Relatively simple processing schemes such as metal-assisted chemical etching and spin on dopant techniques were employed for the fabrication of the proposed solar cells. Atomic layer deposition (ALD) grown aluminum oxide (Al₂O₃) was employed as a surface passivation layer on the B-doped emitter surface. In spite of the fact that the sub-micrometre pillar array textured surface has a relatively high surface-to-volume ratio, we observed an open circuit voltage (V_OC) and a short circuit current density (J_SC) as high as 572 mV and 29.9 mA cm⁻², respectively, which leads to a power conversion efficiency in excess of 11.30%, for the optimized structure of the solar cell described herein. Broadband omnidirectional antireflection effects along with the light trapping property of the sub-micrometre, pillar array textured surface and the excellent passivation quality of the ALD-grown Al₂O₃ on the B-doped emitter surface were responsible for the enhanced electrical performance of the proposed solar cells. (paper)