| 1. |
- Hannesdottir, Holmfridur, et al.
(författare)
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On singlet metastable states, ion flux and ion energy in single and dual frequency capacitively coupled oxygen discharges
- 2017
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Ingår i: Journal of Physics D. - : Institute of Physics (IOP). - 0022-3727 .- 1361-6463. ; 50:17
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Tidskriftsartikel (refereegranskat)abstract
- We apply particle-in-cell simulations with Monte Carlo collisions to study the influence of the singlet metastable states on the ion energy distribution in single and dual frequency capacitively coupled oxygen discharges. For this purpose, the one-dimensional object-oriented particle-in-cell Monte Carlo collision code oopd1 is used, in which the discharge model includes the following nine species: electrons, the neutrals O(3P) and O), the negative ions O−, the positive ions O+ and O, and the metastables O(1D), O and O2(b). Earlier, we have explored the effects of adding the species O) and O2(b), and an energy-dependent secondary electron emission yield for oxygen ions and neutrals, to the discharge model. We found that including the two molecular singlet metastable states decreases the ohmic heating and the effective electron temperature in the bulk region (the electronegative core). Here we explore how these metastable states influence dual frequency discharges consisting of a fundamental frequency and the lowest even harmonics. Including or excluding the detachment reactions of the metastables O) and O2(b) can shift the peak electron temperature from the grounded to the powered electrode or vice versa, depending on the phase difference of the two applied frequencies. These metastable states can furthermore significantly influence the peak of the ion energy distribution for O-ions bombarding the powered electrode, and hence the average ion energy upon bombardment of the electrode, and lower the ion flux.
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| 2. |
- Dalarsson, Mariana, et al.
(författare)
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Absorption and optimal plasmonic resonances for small ellipsoidal particles in lossy media
- 2017
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Ingår i: Journal of Physics D. - : Institute of Physics (IOP). - 0022-3727 .- 1361-6463. ; 50:34
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Tidskriftsartikel (refereegranskat)abstract
- A new simplified formula is derived for the absorption cross section of small dielectric ellipsoidal particles embedded in lossy media. The new expression leads directly to a closed form solution for the optimal conjugate match with respect to the surrounding medium, i.e. the optimal permittivity of the ellipsoidal particle that maximizes the absorption at any given frequency. This defines the optimal plasmonic resonance for the ellipsoid. The optimal conjugate match represents a metamaterial in the sense that the corresponding optimal permittivity function may have negative real part (inductive properties), and can not in general be implemented as a passive material over a given bandwidth. A necessary and sufficient condition is derived for the feasibility of tuning the Drude model to the optimal conjugate match at a single frequency, and it is found that all the prolate spheroids and some of the (not too flat) oblate spheroids can be tuned into optimal plasmonic resonance at any desired center frequency. Numerical examples are given to illustrate the analysis. Except for the general understanding of plasmonic resonances in lossy media, it is also anticipated that the new results can be useful for feasibility studies with e.g. the radiotherapeutic hyperthermia based methods to treat cancer based on electrophoretic heating in gold nanoparticle suspensions using microwave radiation.
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| 3. |
- Ghasemi, Masoomeh, et al.
(författare)
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Phase diagrams for understanding gold-seeded growth of GaAs and InAs nanowires
- 2017
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Ingår i: Journal of Physics D: Applied Physics. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 50:13
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Tidskriftsartikel (refereegranskat)abstract
- Phase diagrams are useful tools to study the phase equilibria of nanowire materials systems because the growth of nanowires is accompanied by phase formation and phase transition. We have modeled the phase equilibria of the As-Au-Ga ternary system by means of the CALPHAD method. This method is a well-established semi-empirical technique for thermodynamic modeling in which Gibbs energy functions with free parameters are defined for all phases in a system followed by adjusting these parameters to the experimental data. Using the resulting As-Au-Ga thermodynamic database, four vertical cuts of this ternary system are calculated and all show good agreement with experiments. This ternary system is particularly useful for predicting the state of the Au seed alloys when growing GaAs nanowires and we discuss such predictions. Similar calculations are performed for Au-seeded InAs nanowires. We show that the vapor-liquid-solid (VLS) growth fails for InAs nanowires, while GaAs nanowires can grow from a liquid particle. Our calculations are in agreement with experimental data on the growth of Au-seeded GaAs and InAs nanowires.
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| 4. |
- Larsson, Kajsa, et al.
(författare)
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Instantaneous imaging of ozone in a gliding arc discharge using photofragmentation laser-induced fluorescence
- 2018
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Ingår i: Journal of Physics D: Applied Physics. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 51:13
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Tidskriftsartikel (refereegranskat)abstract
- Ozone vapor, O3, is here visualized in a gliding arc discharge using photofragmentation laser-induced fluorescence. Ozone is imaged by first photodissociating the O3 molecule into an O radical and a vibrationally hot O2 fragment by a pump photon. Thereafter, the vibrationally excited O2 molecule absorbs a second (probe) photon that further transits the O2-molecule to an excited electronic state, and hence, fluorescence from the deexcitation process in the molecule can be detected. Both the photodissociation and excitation processes are achieved within one 248 nm KrF excimer laser pulse that is formed into a laser sheet and the fluorescence is imaged using an intensified CCD camera. The laser-induced signal in the vicinity of the plasma column formed by the gliding arc is confirmed to stem from O3 rather than plasma produced vibrationally hot O2. While both these products can be produced in plasmas a second laser pulse at 266 nm was utilized to separate the pump- from the probe-processes. Such arrangement allowed lifetime studies of vibrationally hot O2, which under these conditions were several orders of magnitude shorter than the lifetime of plasma-produced ozone.
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| 5. |
- Li, Bo, et al.
(författare)
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Femtosecond laser-induced cyano chemiluminescence in methane-seeded nitrogen gas flows for near-wall velocimetry
- 2018
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Ingår i: Journal of Physics D: Applied Physics. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 51:29
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate a velocimetric technique based on femtosecond laser-induced cyano (CN) chemiluminescence (FLICC). High intensity emission originated from CN(B-X) fluorescence was observed in filaments generated by focusing a femtosecond laser in methane-seeded nitrogen gas flows. The emission is strong and can last for hundreds of microseconds with a proper methane concentration. FLICC was adopted for velocity measurements, and promising results were obtained for near-wall measurements.
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| 6. |
- Nordebo, Sven, 1963-, et al.
(författare)
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A parametric model for the changes in the complex valued conductivity of a lung during tidal breathing
- 2018
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Ingår i: Journal of Physics D. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 51:20
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Tidskriftsartikel (refereegranskat)abstract
- Classical homogenization theory based on the Hashin-Shtrikman coated ellipsoids is used to model the changes in the complex valued conductivity (or admittivity) of a lung during tidal breathing. Here, the lung is modeled as a two-phase composite material where the alveolar air-filling corresponds to the inclusion phase. The theory predicts a linear relationship between the real and the imaginary parts of the change in the complex valued conductivity of a lung during tidal breathing, and where the loss cotangent of the change is approximately the same as of the effective background conductivity and hence easy to estimate. The theory is illustrated with numerical examples based on realistic parameter values and frequency ranges used with electrical impedance tomography (EIT). The theory may be potentially useful for imaging and clinical evaluations in connection with lung EIT for respiratory management and control.
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| 7. |
- Nordebo, Sven, 1963-, et al.
(författare)
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On the physical limitations for radio frequency absorption in gold nanoparticle suspensions
- 2017
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Ingår i: Journal of Physics D. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 50:15
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents a study of the physical limitations for radio frequency absorption in gold nanoparticle (GNP) suspensions. A spherical geometry is considered consisting of a spherical suspension of colloidal GNPs characterized as an arbitrary passive dielectric material which is immersed in an arbitrary lossy medium. A relative heating coefficient and a corresponding optimal near field excitation are defined, taking the skin effect of the surrounding medium into account. The classical Mie theory for lossy media is also revisited, and it is shown that the optimal permittivity function yielding a maximal absorption inside the spherical suspension is a conjugate match with respect to the surrounding lossy material. A convex optimization approach is used to investigate the broadband realizability of an arbitrary passive material to approximate the desired conjugate match over a finite bandwidth, similar to the approximation of a metamaterial. A narrowband realizability study shows that for a surrounding medium consisting of a weak electrolyte solution, the electromagnetic heating, due to the electrophoretic (plasmonic) resonance phenomena inside the spherical GNP suspension, can be significant in the microwave regime, provided that the related Drude parameters can be tuned into (or near to) resonance. As a demonstration, some realistic Drude parameters are investigated concerning the volume fraction, mass, and friction constant of the GNPs. The amount of charge that can be accommodated by the GNPs is identified as one of the most important design parameters. However, the problem of reliably modelling, measuring and controlling the charge number of coated GNPs is not yet fully understood, and is still an open research issue in this field. The presented theory and related physical limitations provide a useful framework for further research in this direction. Future research is also aimed at an expansion towards arbitrary suspension geometries and the inclusion of thermodynamical analysis.
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| 8. |
- Pfaff, Sebastian, et al.
(författare)
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Temperature characterization of an operando flow reactor for heterogeneous catalysis
- 2019
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Ingår i: Journal of Physics D: Applied Physics. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 52:32
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Tidskriftsartikel (refereegranskat)abstract
- In this work, we present several methods to determine the temperature of a catalyst sample, as well as the gas surrounding it, in a typical flow reactor used for operando research on heterogeneous catalysis. To determine the sample temperature, we present an approach using calibrated IR-camera imagery, as well as thermographic phosphors. For the gas temperature, we present methods to extract temperature information from Planar Laser Induced Fluorescence (PLIF) measurements, one of which can be used during operando studies with an active catalyst in place.
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| 9. |
- Van Der Velde, Jasper H.M., et al.
(författare)
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Self-healing dyes for super-resolution fluorescence microscopy
- 2019
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Ingår i: Journal of Physics D: Applied Physics. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 52:3
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Tidskriftsartikel (refereegranskat)abstract
- In recent years, optical microscopy techniques have emerged that allow optical imaging at unprecedented resolution beyond the diffraction limit. These techniques exploit photostabilizing buffers to enable photoswitching and/or the enhancement of fluorophore brightness and stability. A major drawback with the use of photostabilizing buffers, however, is that they cannot be used in live cell imaging. In this paper, we tested the performance of self-healing organic fluorophores, which undergo intramolecular photostabilization, in super-resolution microscopy examining both targeted (stimulated emission depletion (STED) microscopy) and stochastic readout (stochastic optical reconstruction microscopy (STORM)). The overall goal of the study was to identify dyes and conditions that lead to improved spatial and temporal resolution of both techniques without the need for mixtures of photostabilizing agents in the imaging buffer. As a result of previously shown superior performance, we identified an ATTO647N-photostabilizer conjugate as a potential candidate for STED microscopy. We have here characterized the photostability and resulting performance of this nitrophenylalanine (NPA) conjugate of ATTO647N on oligonucleotides in STED microscopy. We found that the superior photophysical performance resulted in optimal STED imaging and demonstrated that single-molecule fluorescent transients of individual fluorophores can be obtained with both the excitation- and STED-laser. In similar experiments, we also tested a nitrophenylacetic acid conjugate of STAR635P, another frequently used dye in STED microscopy, and present a characterization of its photophysical properties. Finally, we performed an analysis of the photoswitching kinetics of self-healing Cy5 dyes (containing trolox, cyclooctatetraene and NPA-based stabilizers) in the presence of Tris(2-carboxyethyl)phosphine and cysteamine, which are typically used in STORM microscopy. In line with previous work, we found that intramolecular photostabilization strongly influences photoswitching kinetics and requires careful attention when designing STORM-experiments. In summary, this contribution explores the possibilities and limitations of self-healing dyes in super-resolution microscopy of differing modalities.
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| 10. |
- Zhang, Z. P., et al.
(författare)
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Growth mode of tensile-strained Ge quantum dots grown by molecular beam epitaxy
- 2017
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Ingår i: Journal of Physics D: Applied Physics. - : IOP Publishing. - 1361-6463 .- 0022-3727. ; 50:46
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Tidskriftsartikel (refereegranskat)abstract
- Growth mode of tensile-strained Ge quantum dots on different III-V buffers by molecular beam epitaxy is studied by a combination of reflection high-energy electron diffraction, atomic force microscopy and transmission electron microscopy. The Ge-QDs growth on the InAlAs buffer lattice matched to InP and on InAs buffer on GaSb follows the Volmer-Weber growth mode with round Ge QDs and no Ge wetting layer, while it obeys the Stranski-Krastanov growth mode on GaSb, AlSb and AlGaSb on GaSb substrates, showing rectangular shaped platelets and a clear Ge wetting layer. The discovery of the Volmer-Weber growth mode is essential to avoid forming a wetting layer and the subsequent antiphase-domain defects when capping III-Vs on Ge-QDs, important for potential optoelectronic applications.
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