| 1. |
- Escobar Steinvall, Simon, et al.
(author)
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Visualizing the Mechanism Switching in High-Temperature Au-Catalyzed InAs Nanowire Growth
- 2023
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In: Crystal Growth and Design. - 1528-7483. ; 23:9, s. 6228-6232
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Journal article (peer-reviewed)abstract
- We use environmental transmission electron microscopy to observe in situ the switch from an axial vapor-liquid-solid (VLS) growth mechanism in Au-catalyzed InAs nanowires toward a radial vapor-solid (VS) one, dominated by layers nucleating at the triple-phase line. At elevated temperatures, in addition to high V/III ratios, the affinity for In in the Au catalyst will be greater than that of In in InAs, which in turn reduces the driving force and probability for nucleation at the liquid-solid interface. Consequently, with increased temperature, the catalyst particle stops acting as a sink for incoming material and the decomposition of precursors away from the catalyst increases, making radial vapor-solid growth the dominating growth mechanism. It is further observed that the growth proceeds through multistep propagation rather than a layer-by-layer propagation under these conditions.
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| 2. |
- Forsberg, Anna, et al.
(author)
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Once-only colonoscopy or two rounds of faecal immunochemical testing 2 years apart for colorectal cancer screening (SCREESCO): preliminary report of a randomised controlled trial
- 2022
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In: The Lancet Gastroenterology & Hepatology. - : ELSEVIER INC. - 2468-1253. ; 7:6, s. 513-521
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Journal article (peer-reviewed)abstract
- Background Screening for colorectal cancer is done with lower gastrointestinal endoscopy or stool-based tests. There is little evidence from randomised trials to show primary colonoscopy reduces mortality in colorectal cancer We aimed to investigate the effect of screening with once-only colonoscopy or two rounds of faecal immunochemical test screening on colorectal cancer mortality and incidence. Methods We did a randomised controlled trial in Sweden (SCREESCO). Residents in 18 of 21 regions who were age 60 years in the year of randomisation were identified from a population register maintained by the Swedish Tax Agency. A statistician with no further involvement in the trial used a randomised block method to assign individuals to once-only colonoscopy, two rounds of faecal immunochemical testing (OC-Sensor; 2 years apart), or a control group (no intervention; standard diagnostic pathways), in a ratio of 1:6 for colonoscopy versus control and 1:2 for faecal immunochemical testing versus control. Masking was not possible due to the nature of the trial. The primary endpoints of the trial are colorectal cancer mortality and colorectal cancer incidence. Here, we report preliminary participation rates, baseline findings, and adverse events from March, 2014, to December, 2020, in the two intervention groups after completion of recruitment and screening, up to the completion of the second faecal immunochemical testing round. Analyses were done in the intention-to-screen population, defined as all individuals who were randomly assigned to the respective study group. This study is registered with Clinical Trials.gov, NCT02078804. Findings Between March 1, 2014, and Dec 31, 2020, 278 280 people were induded in the study; 31 140 were assigned to the colonoscopy group, 60 300 to the faecal immunochemical test group, and 186 840 to the control group. 10 679 (35.1%) of 30 400 people who received an invitation for colonoscopy participated. 33 383 (55.5%) of 60 137 people who received a postal faecal immunochemical test participated. In the intention-to-screen analysis, colorectal cancer was detected in 49 (0.16%) of 31140 people in the colonoscopy group versus 121 (0. 20%) of 60 300 in the faecal immunochemical test group (relative risk [RR] 0.78, 95% CI 0.56-1.09). Advanced adenomas were detected in 637 (2.05%) people in the colonoscopy group and 968 (1.61%) in the faecal immunochemical test group (RR 1.27, 95% CI 1.15-1.41). Colonoscopy detected more right-sided advanced adenomas than faecal immunochemical testing. There were two perforations and 15 major bleeds in 16 555 colonoscopies. No intervention-related deaths occurred. Interpretation The diagnostic yield and the low number of adverse events indicate that the design from this trial, both for once-only colonoscopy and faecal immunochemical test screening, could be transferred to a population-based screening service if a benefit in disease-specific mortality is subsequently shown. Copyright (C) 2022 Elsevier Ltd. All rights reserved.
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| 3. |
- Forsberg, A., et al.
(author)
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Once-only colonoscopy or two rounds of faecal immunochemical testing 2 years apart for colorectal cancer screening (SCREESCO): preliminary report of a randomised controlled trial
- 2022
-
In: Lancet Gastroenterology & Hepatology. - : Elsevier BV. - 2468-1253. ; 7:6, s. 513-521
-
Journal article (peer-reviewed)abstract
- Background Screening for colorectal cancer is done with lower gastrointestinal endoscopy or stool-based tests. There is little evidence from randomised trials to show primary colonoscopy reduces mortality in colorectal cancer We aimed to investigate the effect of screening with once-only colonoscopy or two rounds of faecal immunochemical test screening on colorectal cancer mortality and incidence. Methods We did a randomised controlled trial in Sweden (SCREESCO). Residents in 18 of 21 regions who were age 60 years in the year of randomisation were identified from a population register maintained by the Swedish Tax Agency. A statistician with no further involvement in the trial used a randomised block method to assign individuals to once-only colonoscopy, two rounds of faecal immunochemical testing (OC-Sensor; 2 years apart), or a control group (no intervention; standard diagnostic pathways), in a ratio of 1:6 for colonoscopy versus control and 1:2 for faecal immunochemical testing versus control. Masking was not possible due to the nature of the trial. The primary endpoints of the trial are colorectal cancer mortality and colorectal cancer incidence. Here, we report preliminary participation rates, baseline findings, and adverse events from March, 2014, to December, 2020, in the two intervention groups after completion of recruitment and screening, up to the completion of the second faecal immunochemical testing round. Analyses were done in the intention-to-screen population, defined as all individuals who were randomly assigned to the respective study group. This study is registered with Clinical Trials.gov, NCT02078804. Findings Between March 1, 2014, and Dec 31, 2020, 278 280 people were induded in the study; 31 140 were assigned to the colonoscopy group, 60 300 to the faecal immunochemical test group, and 186 840 to the control group. 10 679 (35.1%) of 30 400 people who received an invitation for colonoscopy participated. 33 383 (55.5%) of 60 137 people who received a postal faecal immunochemical test participated. In the intention-to-screen analysis, colorectal cancer was detected in 49 (0.16%) of 31140 people in the colonoscopy group versus 121 (0. 20%) of 60 300 in the faecal immunochemical test group (relative risk [RR] 0.78, 95% CI 0.56-1.09). Advanced adenomas were detected in 637 (2.05%) people in the colonoscopy group and 968 (1.61%) in the faecal immunochemical test group (RR 1.27, 95% CI 1.15-1.41). Colonoscopy detected more right-sided advanced adenomas than faecal immunochemical testing. There were two perforations and 15 major bleeds in 16 555 colonoscopies. No intervention-related deaths occurred. Interpretation The diagnostic yield and the low number of adverse events indicate that the design from this trial, both for once-only colonoscopy and faecal immunochemical test screening, could be transferred to a population-based screening service if a benefit in disease-specific mortality is subsequently shown. Copyright (C) 2022 Elsevier Ltd. All rights reserved.
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| 4. |
- Maliakkal, Carina B., et al.
(author)
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In situ analysis of catalyst composition during gold catalyzed GaAs nanowire growth
- 2019
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In: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 10:1
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Journal article (peer-reviewed)abstract
- Semiconductor nanowires offer the opportunity to incorporate novel structures and functionality into electronic and optoelectronic devices. A clear understanding of the nanowire growth mechanism is essential for well-controlled growth of structures with desired properties, but the understanding is currently limited by a lack of empirical measurements of important parameters during growth, such as catalyst particle composition. However, this is difficult to accurately determine by investigating post-growth. We report direct in situ measurement of the catalyst composition during nanowire growth for the first time. We study Au-seeded GaAs nanowires inside an electron microscope as they grow and measure the catalyst composition using X-ray energy dispersive spectroscopy. The Ga content in the catalyst during growth increases with both temperature and Ga precursor flux.
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| 5. |
- Maliakkal, Carina B., et al.
(author)
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Independent Control of Nucleation and Layer Growth in Nanowires
- 2020
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In: ACS Nano. - : American Chemical Society (ACS). - 1936-0851 .- 1936-086X. ; 14:4, s. 3868-3875
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Journal article (peer-reviewed)abstract
- Control of the crystallization process is central to developing nanomaterials with atomic precision to meet the demands of electronic and quantum technology applications. Semiconductor nanowires grown by the vapor-liquid-solid process are a promising material system in which the ability to form components with structure and composition not achievable in bulk is well-established. Here, we use in situ TEM imaging of Au-catalyzed GaAs nanowire growth to understand the processes by which the growth dynamics are connected to the experimental parameters. We find that two sequential steps in the crystallization process - nucleation and layer growth - can occur on similar time scales and can be controlled independently using different growth parameters. Importantly, the layer growth process contributes significantly to the growth time for all conditions and will play a major role in determining material properties such as compositional uniformity, dopant density, and impurity incorporation. The results are understood through theoretical simulations correlating the growth dynamics, liquid droplet, and experimental parameters. The key insights discussed here are not restricted to Au-catalyzed GaAs nanowire growth but can be extended to most compound nanowire growths in which the different growth species has very different solubility in the catalyst particle.
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| 6. |
- Maliakkal, Carina B., et al.
(author)
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Post-nucleation evolution of the liquid-solid interface in nanowire growth
- 2022
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In: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 33:10
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Journal article (peer-reviewed)abstract
- We study using in situ transmission electron microscopy the birth of GaAs nanowires from liquid Au-Ga catalysts on amorphous substrates. Lattice-resolved observations of the starting stages of growth are reported here for the first time. It reveals how the initial nanostructure evolves into a nanowire growing in a zincblende 111 or the equivalent wurtzite0001 direction. This growth direction(s) is what is typically observed in most III-V and II-VI nanowires. However, the reason for this preferential nanowire growth along this direction is still a dilemma. Based on the videos recorded shortly after the nucleation of nanowires, we argue that the lower catalyst droplet-nanowire interface energy of the {111} facet when zincblende (or the equivalent {0001} facet in wurtzite) is the reason for this direction selectivity in nanowires.
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| 7. |
- Maliakkal, Carina B., et al.
(author)
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Vapor-solid-solid growth dynamics in GaAs nanowires
- 2021
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In: Nanoscale Advances. - : Royal Society of Chemistry (RSC). - 2516-0230. ; 3:20, s. 5928-5940
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Journal article (peer-reviewed)abstract
- Semiconductor nanowires are promising material systems for coming-of-age nanotechnology. The usage of the vapor-solid-solid (VSS) route, where the catalyst used for promoting axial growth of nanowires is a solid, offers certain advantages compared to the common vapor-liquid-solid (VLS) route (using a liquid catalyst). The VSS growth of group-IV elemental nanowires has been investigated by other groupsin situduring growth in a transmission electron microscope (TEM). Though it is known that compound nanowire growth has different dynamics compared to elemental semiconductors, the layer growth dynamics of VSS growth of compound nanowires have not been studied yet. Here we investigate for the first time controlled VSS growth of compound nanowires byin situmicroscopy, using Au-seeded GaAs as a model system. The ledge-flow growth kinetics and dynamics at the wire-catalyst interface are studied and compared for liquid and solid catalysts under similar growth conditions. Here the temperature and thermal history of the system are manipulated to control the catalyst phase. In the first experiment discussed here we reduce the growth temperature in steps to solidify the initially liquid catalyst, and compare the dynamics between VLS and VSS growth observed at slightly different temperatures. In the second experiment we exploit thermal hysteresis of the system to obtain both VLS and VSS at the same temperature. The VSS growth rate is comparable or slightly slower than the VLS growth rate. Unlike in the VLS case, during VSS growth we frequently observe that a new layer starts before the previous layer is completely grown,i.e., ‘multilayer growth’. Understanding the VSS growth mode enables better control of nanowire properties by widening the range of usable nanowire growth parameters.
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| 8. |
- Marnauza, Mikelis, et al.
(author)
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In situ observations of size effects in GaAs nanowire growth
- 2022
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In: Nanoscale Horizons. - : Royal Society of Chemistry (RSC). - 2055-6756 .- 2055-6764. ; 8:2, s. 291-296
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Journal article (peer-reviewed)abstract
- Lateral dimensions of III-V nanowires are known to affect the growth dynamics and crystal structure. Investigations into size effects have in the past relied on theoretical models and post growth observations, which only give a limited insight into the growth dynamics. Here we show the first experimental investigation into how nanowire diameter affects the growth dynamics by growing Au-seeded GaAs nanowires in an environmental transmission electron microscope. This was done by recording videos of nanowires during growth and analysing the Ga-limited incubation time and As-limited step-flow time. Our data show that the incubation time is stable across the investigated diameter range aside from a sharp increase for the smallest diameter, whereas the step-flow time is observed to steadily increase across the diameter range. We show using a simple model that this can be explained by the increasing vapour pressure in the droplet. In addition to the existing understanding of nanowire growth at small dimensions being limited by nucleation this work provides experimental evidence that growth is also limited by the inability to finish the step-flow process.
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| 9. |
- Persson, Axel R., et al.
(author)
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Time-resolved compositional mapping during in situ TEM studies
- 2021
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In: Ultramicroscopy. - : Elsevier BV. - 0304-3991 .- 1879-2723. ; 222
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Journal article (peer-reviewed)abstract
- In situ studies using transmission electron microscopy (TEM) can provide insights to how properties, structures and compositions of nanostructures are affected and evolving when exerted to heat or chemical exposure. While high-resolved imaging can be obtained continuously, at video-framerates of hundreds of frames per second (fps), compositional analysis struggles with time resolution due to the long acquisition times for a reliable analysis. This especially holds true when performing mapping (correlated spatial and compositional information). Hence, transient changes are difficult to resolve using mapping. In this work, the time-resolution of sequential mapping using scanning TEM (STEM) and energy dispersive spectroscopy (EDS) is improved by acquiring spectrum images during short times and filtering the spectroscopic data. The suggested algorithm uses regularization to smooth and prevent overfitting (known from compressed sensing) to fit model spectra to the data. The algorithm is applied on simulations as well as acquisitions of catalyzed crystal growth (nanowires), performed in situ in a specialized environmental TEM (ETEM). The results show the improved temporal resolution, where the compositional progression of the different regions of the nanostructure is revealed, here with a time-resolution as low as 16 s compared to the minutes usually needed for similar analysis.
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| 10. |
- Sjökvist, Robin, et al.
(author)
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Compositional Correlation between the Nanoparticle and the Growing Au-Assisted InxGa1-xAs Nanowire
- 2021
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In: Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 12:31, s. 7590-7595
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Journal article (peer-reviewed)abstract
- The nanowire geometry is favorable for the growth of ternary semiconductor materials, because the composition and properties can be tuned freely without substrate lattice matching. To achieve precise control of the composition in ternary semiconductor nanowires, a deeper understanding of the growth is required. One unknown aspect of seeded nanowire growth is how the composition of the catalyst nanoparticle affects the resulting composition of the growing nanowire. We report the first in situ measurements of the nanoparticle and InxGa1-xAs nanowire compositional relationship using an environmental transmission electron microscopy setup. The compositions were measured and correlated during growth, via X-ray energy dispersive spectroscopy. Contrary to predictions from thermodynamic models, the experimental results do not show a miscibility gap. Therefore, we construct a kinetic model that better predicts the compositional trends by suppressing the miscibility gap. The findings imply that compositional control of InxGa1-xAs nanowires is possible across the entire compositional range.
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