| 1. |
- Al-Abri, Ruqaiya, et al.
(författare)
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Sub-Picosecond Carrier Dynamics Explored using Automated High-Throughput Studies of Doping Inhomogeneity within a Bayesian Framework
- 2023
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Ingår i: Small. - 1613-6810. ; 19:33
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Tidskriftsartikel (refereegranskat)abstract
- Bottom–up production of semiconductor nanomaterials is often accompanied by inhomogeneity resulting in a spread in electronic properties which may be influenced by the nanoparticle geometry, crystal quality, stoichiometry, or doping. Using photoluminescence spectroscopy of a population of more than 11 000 individual zinc-doped gallium arsenide nanowires, inhomogeneity is revealed in, and correlation between doping and nanowire diameter by use of a Bayesian statistical approach. Recombination of hot-carriers is shown to be responsible for the photoluminescence lineshape; by exploiting lifetime variation across the population, hot-carrier dynamics is revealed at the sub-picosecond timescale showing interband electronic dynamics. High-throughput spectroscopy together with a Bayesian approach are shown to provide unique insight in an inhomogeneous nanomaterial population, and can reveal electronic dynamics otherwise requiring complex pump-probe experiments in highly non-equilibrium conditions.
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| 2. |
- Chen, Yang, et al.
(författare)
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Optical far-field extinction of a single GaAs nanowire towards in situ size control of aerotaxy nanowire growth
- 2020
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Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 31:13
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Tidskriftsartikel (refereegranskat)abstract
- A substrate-free approach of semiconductor nanowire growth has been achieved by the aerotaxy technique previously. In this work, we propose an in situ method to monitor the size of nanowires through non-destructive optical-extinction measurements. Our work aims to build a theoretical look-up database of extinction spectra for a single nanowire of varying dimensions. We describe the origin of possible peaks in the spectra, for example due to nanowire-length dependent Fabry-Perot resonances and nanowire-diameter dependent TM and TE mode resonances. Furthermore, we show that the Au catalyst on top of the nanowire can be ignored in the simulations when the volume of the nanowire is an order of magnitude larger than that of the Au catalyst and the diameter is small compared to the incident wavelength. For the calculation of the extinction spectra, we use the finite element method, the discrete dipole approximation and the Mie theory. To compare with experimental measurements of randomly oriented nanowires, we perform an averaging over nanowire orientation for the modeled results. However, in the experiments, nanowires are accumulating on the quartz window of the measurement setup, which leads to increasing uncertainty in the comparison with the experimental extinction spectra. This uncertainty can be eliminated by considering both a sparse and a dense collection of nanowires on the quartz window in the optical simulations. Finally, we create a database of extinction spectra for a GaAs nanowire of varying diameters and lengths. This database can be used to estimate the diameter and the length of the nanowires by comparing the position of a peak and the peak-to-shoulder difference in the extinction spectrum. Possible tapering of nanowires can be monitored through the appearance of an additional peak at a wavelength of 700-800 nm.
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| 3. |
- Deppert, Knut, et al.
(författare)
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Aerotaxy: High throughput gas-phase epitaxy of nanostructures
- 2020
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Konferensbidrag (refereegranskat)abstract
- Aerotaxy is an aerosol-based growth method for semiconductors and we present in detail how aerotaxy can be used to grow nanowires continuously with controlled nanoscale dimensions, with a high degree of crystallinity and remarkable throughput, including process details and our current understading of the growth processes. Catalytic size-selected Au aerosol particles travel through a heated flow-through reactor and mix with III–V precursor flux, which nucleates the growth of nanowires. We demonstrate that the method allows sensitive and reproducible control of the nanowire dimensions and shape. The reported continuous and potentially high-throughput method is expected to substantially reduce the cost of producing high-quality nanowires and may enable the low-cost fabrication of nanowire-based devices on an industrial scale.
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| 4. |
- Deppert, Knut, et al.
(författare)
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Pseudo-particle continuum modelling of nanowire growth in aerotaxy
- 2020
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Konferensbidrag (refereegranskat)abstract
- Aerotaxy growth of GaAs nanowires has shown to be capable of making GaAs wires, GaAsxP1-xwires (Metaferia, 2016) and even pn-junctions with phovovoltaic properties. With a goal of better understanding the aerotaxy process, we have developed a model for nanowire growth in Aerotaxy, based on a surface diffusion model for substrate-based growth. Our model considers the growth conditions, varying in time and space, as the seed particles and growing nanowires pass through the reactor. The growth is modeled as a chemical reaction, where gases but also particles are described as chemical species. By interpreting the continuous Au and GaAs species as wires with a length corresponding to the amount of material at that point, the particulate nature can be taken into account, without the arduous task of explicit particle modeling. Pseudo-particle modelling can potentially be used as a more general strategy for gas phase particle growth.
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| 5. |
- Gunnarsson, Stefán Bragi, et al.
(författare)
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Dual topography of laminin corona on gallium arsenide nanowires
- 2020
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Ingår i: Biointerphases. - : American Vacuum Society. - 1934-8630 .- 1559-4106. ; 15:5
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Tidskriftsartikel (refereegranskat)abstract
- Nanowires (NWs) are novel nanomaterials with applications in everything from medical implants to solar cells. With increasing number of applications, it is increasingly likely that organisms are exposed to these materials either intentionally or by accident. It is, therefore, important to study their interactions with biological systems and biomolecules. Upon exposure to biological fluids, nanostructure surfaces are quickly covered by a biomolecule corona. The composition of the corona determines the nanostructure's biological fate. Furthermore, upon adsorption, the protein structure can be affected. In order to study the corona morphology, we used two model proteins, laminin of the extracellular matrix and the immune system enzyme myeloperoxidase. We image the protein corona directly by cryo-TEM and enhance resolution by labeling the corona with activated gold nanoparticles. Three-dimensional imaging of the protein corona further increases the resolution and reveals irregularities in corona topography. By doing so, we identified bimodal distribution of spacing between gold nanoparticles and the NW surface for laminin corona at 58 and 85 nm distance from the NWs' surface. The dual topography of the corona is adding a new complexity of the protein corona surface and its interactions with the surrounding biology.
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| 6. |
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| 7. |
- Isaxon, Christina, et al.
(författare)
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Workplace Emissions and Exposures During Semiconductor Nanowire Production, Post-production, and Maintenance Work
- 2020
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Ingår i: Annals of Work Exposures and Health. - : Oxford University Press (OUP). - 2398-7308 .- 2398-7316. ; 64:1, s. 38-54
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Tidskriftsartikel (refereegranskat)abstract
- BackgroundNanowires are a high-aspect-ratio material of increasing interest for a wide range of applications. A new and promising method to produce nanowires is by aerotaxy, where the wires are grown in a continuous stream of gas. The aerotaxy method can grow nanowires much faster than by more conventional methods. Nanowires have important properties in common with asbestos fibers, which indicate that there can be potential health effects if exposure occurs. No conclusive exposure (or emission) data from aerotaxy-production of nanowires has so far been published.MethodsDifferent work tasks during semiconductor nanowire production, post-production, and maintenance were studied. A combination of direct-reading instruments for number concentration (0.007–20 µm) and filter sampling was used to assess the emissions (a couple of centimeter from the emission sources), the exposure in the personal breathing zone (max 30 cm from nose–mouth), and the concentrations in the background zone (at least 3 m from any emission source). The filters were analyzed for metal dust composition and number concentration of nanowires. Various surfaces were sampled for nanowire contamination.ResultsThe particle concentrations in the emission zone (measured with direct-reading instruments) were elevated during cleaning of arc discharge, manual reactor cleaning, exchange of nanowire outflow filters, and sonication of substrates with nanowires. In the case of cleaning of the arc discharge and manual reactor cleaning, the emissions affected the concentrations in the personal breathing zone and were high enough to also affect the concentrations in the background. Filter analysis with electron microscopy could confirm the presence of nanowires in some of the air samples.ConclusionsOur results show that a major part of the potential for exposure occurs not during the actual manufacturing, but during the cleaning and maintenance procedures. The exposures and emissions were evaluated pre- and post-upscaling the production and showed that some work tasks (e.g. exchange of nanowire outflow filters and sonication of substrates with nanowires) increased the emissions post-upscaling.
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| 8. |
- Johansson, Jonas, et al.
(författare)
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Calculation of hole concentrations in Zn doped GaAs nanowires
- 2020
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Ingår i: Nanomaterials. - : MDPI AG. - 2079-4991. ; 10:12
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Tidskriftsartikel (refereegranskat)abstract
- We have previously demonstrated that we can grow p-type GaAs nanowires using Zn doping during gold catalyzed growth with aerotaxy. In this investigation, we show how to calculate the hole concentrations in such nanowires. We base the calculations on the Zhang–Northrup defect formation energy. Using density functional theory, we calculate the energy of the defect, a Zn atom on a Ga site, using a supercell approach. The chemical potentials of Zn and Ga in the liquid catalyst particle are calculated from a thermodynamically assessed database including Au, Zn, Ga, and As. These quantities together with the chemical potential of the carriers enable us to calculate the hole concentration in the nanowires self-consistently. We validate our theoretical results against aerotaxy grown GaAs nanowires where we have varied the hole concentration by varying the Zn/Ga ratio in the aerotaxy growth.
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| 9. |
- Manchester, The Univ., et al.
(författare)
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High-throughput spectroscopy of semiconductor nanowires in the presence of inhomogeneity
- 2021
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Ingår i: - : SPIE. ; , s. 20-20
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Konferensbidrag (refereegranskat)abstract
- Controllable doping in semiconductor nanowires is essential for development of optoelectronic devices. Despite great progress, a fundamental challenge remains in controlling the uniformity of doping, particularly in the presence of relatively high levels of geometrical inhomogeneity in bottom-up growth. A relatively high doping level of 1E18 cm-3 corresponds to just ~1000 activated dopants in a 2µm long, 50nm diameter nanowire. High-throughput photoluminescence spectroscopy enables the collection of doping distributions across many (>10k) nanowires, but geometric variation adds additional uncertainty to the modelling. We present an approach that uses large datasets of doping and emission intensity to infer both doping and diameter across a growth, and apply Bayesian methods to study the underlying distributions in Zn-doped aerotaxy-grown GaAs nanowires. This new big-data enabled approach provides a route to exploit inherent inhomogeneity to reveal fundamental recombination mechanisms.
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| 10. |
- Metaferia, Wondwosen, et al.
(författare)
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N-type doping and morphology of GaAs nanowires in Aerotaxy
- 2018
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Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 29:28
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Tidskriftsartikel (refereegranskat)abstract
- Controlled doping in semiconductor nanowires modifies their electrical and optical properties, which are important for high efficiency optoelectronic devices. We have grown n-type (Sn) doped GaAs nanowires in Aerotaxy, a new continuous gas phase mass production technique. The morphology of Sn doped nanowires is found to be a strong function of dopant, tetraethyltin to trimethylgallium flow ratio, Au-Ga-Sn alloying, and nanowire growth temperatures. High temperature and high flow ratios result in low morphological quality nanowires and in parasitic growth on the wire base and surface. Alloying and growth temperatures of 400 °C and 530 °C, respectively, resulted in good morphological quality nanowires for a flow ratio of TESn to TMGa up to 2.25 ×10-3. The wires are pure zinc-blende for all investigated growth conditions, whereas nanowires grown by metal-organic vapor phase epitaxy with the same growth conditions are usually mainly Wurtzite. The growth rate of the doped wires is found to be dependent more on the TESn flow fraction than on alloying and nanowire growth temperatures. Our photoluminescence measurements, supported by four-point probe resistivity measurements, reveal that the carrier concentration in the doped wires varies only slightly (1-3) ×1019 cm-3 with TESn flow fraction and both alloying and growth temperatures, indicating that good morphological quality wires with high carrier density can be grown with low TESn flow. Carrier concentrations lower than 1019 cm-3 can be grown by further reducing the flow fraction of TESn, which may give better morphology wires.
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