| 1. |
- Escobar Steinvall, Simon, et al.
(författare)
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Visualizing the Mechanism Switching in High-Temperature Au-Catalyzed InAs Nanowire Growth
- 2023
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Ingår i: Crystal Growth and Design. - 1528-7483. ; 23:9, s. 6228-6232
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Tidskriftsartikel (refereegranskat)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.
(författare)
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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
-
Ingår i: The Lancet Gastroenterology & Hepatology. - : ELSEVIER INC. - 2468-1253. ; 7:6, s. 513-521
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Tidskriftsartikel (refereegranskat)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.
(författare)
-
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
-
Ingår i: Lancet Gastroenterology & Hepatology. - : Elsevier BV. - 2468-1253. ; 7:6, s. 513-521
-
Tidskriftsartikel (refereegranskat)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.
(författare)
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In situ analysis of catalyst composition during gold catalyzed GaAs nanowire growth
- 2019
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 10:1
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Tidskriftsartikel (refereegranskat)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.
(författare)
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Independent Control of Nucleation and Layer Growth in Nanowires
- 2020
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Ingår i: ACS Nano. - : American Chemical Society (ACS). - 1936-0851 .- 1936-086X. ; 14:4, s. 3868-3875
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Tidskriftsartikel (refereegranskat)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.
(författare)
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Post-nucleation evolution of the liquid-solid interface in nanowire growth
- 2022
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Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 33:10
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Tidskriftsartikel (refereegranskat)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.
(författare)
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Vapor-solid-solid growth dynamics in GaAs nanowires
- 2021
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Ingår i: Nanoscale Advances. - : Royal Society of Chemistry (RSC). - 2516-0230. ; 3:20, s. 5928-5940
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Tidskriftsartikel (refereegranskat)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.
(författare)
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In situ observations of size effects in GaAs nanowire growth
- 2022
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Ingår i: Nanoscale Horizons. - : Royal Society of Chemistry (RSC). - 2055-6756 .- 2055-6764. ; 8:2, s. 291-296
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Tidskriftsartikel (refereegranskat)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.
(författare)
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Time-resolved compositional mapping during in situ TEM studies
- 2021
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Ingår i: Ultramicroscopy. - : Elsevier BV. - 0304-3991 .- 1879-2723. ; 222
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Tidskriftsartikel (refereegranskat)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.
(författare)
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Compositional Correlation between the Nanoparticle and the Growing Au-Assisted InxGa1-xAs Nanowire
- 2021
-
Ingår i: Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 12:31, s. 7590-7595
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Tidskriftsartikel (refereegranskat)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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| 11. |
- Sjökvist, Robin, et al.
(författare)
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Observation of the Multilayer Growth Mode in Ternary InGaAs Nanowires
- 2022
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Ingår i: ACS Nanoscience AU. - : American Chemical Society (ACS). - 2694-2496. ; 2:6, s. 539-548
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Tidskriftsartikel (refereegranskat)abstract
- Au-seeded semiconductor nanowires have classically been considered to only grow in a layer-by-layer growth mode, where individual layers nucleate and grow one at a time with an incubation step in between. Recent in situ investigations have shown that there are circumstances where binary semiconductor nanowires grow in a multilayer fashion, creating a stack of incomplete layers at the interface between a nanoparticle and a nanowire. In the current investigation, the growth behavior in ternary InGaAs nanowires has been analyzed in situ, using environmental transmission electron microscopy. The investigation has revealed that multilayer growth also occurs for ternary nanowires and appears to be more common than in the binary case. In addition, the size of the multilayer stacks observed is much larger than what has been reported previously. The investigation details the implications of multilayers for the overall growth of the nanowires, as well as the surrounding conditions under which it has manifested. We show that multilayer growth is highly dynamic, where the stack of layers regularly changes size by transporting material between the growing layers. Another observation is that multilayer growth can be initiated in conjunction with the formation of crystallographic defects and compositional changes. In addition, the role that multilayers can have in behaviors such as growth failure and kinking, sometimes observed when creating heterostructures between GaAs and InAs ex situ, is discussed. The prevalence of multilayer growth in this ternary material system implies that, in order to fully understand and accurately predict the growth of nanowires of complex composition and structure, multilayer growth has to be considered.
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| 12. |
- Tornberg, Marcus, et al.
(författare)
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Branched InAs nanowire growth by droplet confinement
- 2018
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 113:12
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Tidskriftsartikel (refereegranskat)abstract
- Anisotropy in crystal growth of III-V semiconductor nanowires can be enhanced by the assistance of a liquid particle. During the past decades, selected scientific works have reported a controlled change in the nanowire growth direction by manipulation of the assisting droplet. Although these results are interesting from an engineering point of view, a detailed understanding of the process is necessary in order to rationally design complex nanostructures. In this letter, we utilize our understanding of the growth-assisting droplet to control the morphology and direction of gold-assisted wurtzite-phase InAs nanowires, using controlled droplet displacement followed by resumed growth. By confining the droplet to the nanowire sidewall using zincblende inclusions as barriers, epitaxial growth of horizontal branches from existing nanowires is demonstrated. This is done by tailoring droplet wetting of the nanowire and using identical conditions for the nanowire "stem" and branch growth. This work demonstrates the importance of the droplet dynamics and wetting stability, along with the benefits of crystallographic control, for understanding the growth along different directions. Controlled branched growth is one way to achieve designed nanowire networks.
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| 13. |
- Tornberg, Marcus, et al.
(författare)
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Demonstration of Sn-seeded GaSb homo- and GaAs-GaSb heterostructural nanowires
- 2016
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Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 27:17
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Tidskriftsartikel (refereegranskat)abstract
- The particle-assisted epitaxial growth of antimonide-based nanowires has mainly been realized using gold as the seed material. However, the Au-seeded epitaxial growth of antimonide-based nanowires such as GaSb nanowires presents several challenges such as for example direct nucleation issues and crystal structure tuning. Therefore, it is of great importance to understand the role of seed material choice and properties in the growth behavior of antimonide-based nanowires to obtain a deeper understanding and a better control on their formation processes. In this report, we have investigated the epitaxial growth of GaSb and GaAs-GaSb nanowires using in situ-formed tin seeds by means of metalorganic vapor phase epitaxy technique. This comprehensive report covers the growth of in situ-formed tin seeds and Sn-seeded GaSb nanowires on both GaAs and GaSb (111)B substrates, as well as GaAs-GaSb nanowires on GaAs (111)B substrates. The growth behavior and structural properties of the obtained GaSb nanowires are further investigated and compared with the Au-seeded counterparts. The results provided by this study demonstrate that Sn is a promising seed material for the growth of GaSb nanowires.
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| 14. |
- Tornberg, Marcus, et al.
(författare)
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Direct Observations of Twin Formation Dynamics in Binary Semiconductors
- 2022
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Ingår i: ACS Nanoscience AU. - : American Chemical Society (ACS). - 2694-2496. ; 2:1, s. 49-56
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Tidskriftsartikel (refereegranskat)abstract
- With the increased demand for controlled deterministic growth of III–V semiconductors at the nanoscale, the impact and interest of understanding defect formation and crystal structure switching becomes increasingly important. Vapor–liquid–solid (VLS) growth of semiconductor nanocrystals is an important mechanism for controlling and studying the formation of individual crystal layers and stacking defects. Using in situ studies, combining atomic resolution of transmission electron microscopy and controlled VLS crystal growth using metal organic chemical vapor deposition, we investigate the simplest achievable change in atomic layer stacking–single twinned layers formed in GaAs. Using Au-assisted GaAs nanowires of various diameters, we study the formation of individual layers with atomic resolution to reveal the growth difference in forming a twin defect. We determine that the formation of a twinned layer occurs significantly more slowly than that of a normal crystal layer. To understand this, we conduct thermodynamic modeling and determine that the propagation of a twin is limited by the energy cost of forming the twin interface. Finally, we determine that the slower propagation of twinned layers increases the probability of additional layers nucleating, such that multiple layers grow simultaneously. This observation challenges the current understanding that continuous uniform epitaxial growth, especially in the case of liquid-metal assisted nanowires, proceeds one single layer at a time and that its progression is limited by the nucleation rate.
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| 15. |
- Tornberg, Marcus
(författare)
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Dynamics of a Droplet that Assists III-V Nanowire Growth
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Control of the process of crystal growth has for decades been achieved by addressing the growth temperature and material supply of the growth species. Directional control of both crystal growth and etching has, for example, been achieved by utilizing a liquid droplet to assist the process. This is a common approach to achieve crystal growth of nanostructures, as in the case of Au-assisted III-V semiconductor nanowires. Although controlled droplet-assisted growth has been studied in depth, less attention has been given to the droplet composition and how it dynamically interacts with the nanowire. This thesis explores the fundamental limits for one-directional droplet-assisted crystal growth. This is studied first by intentionally displacing the droplet from the facet at which it originally assisted the crystal growth. The process and cause for displacement is studied for Au-assisted GaAs and InAs nanowires by combining experimental observations and theoretical modeling of the droplet wetting. In addition, it is shown that the final position of the droplet can be controlled by tailoring the surfaces of the nanowire, which in turn is used for design of branched structures. Furthermore, this thesis focuses on the droplet dynamics and the formation of a truncation at the droplet-nanowire interface, as the geometry of the droplet wetting of the top facet approaches the fundamental limit. The studies of this thesis are conducted using metal-organic chemical vapor deposition (MOCVD), both in- and outside an environmental transmission electron microscope (ETEM). Ex-situ analysis of droplet displacement allows us to investigate the statistics of the process, to understand trends of the droplet wetting. On the other hand, performing MOCVD inside an ETEM enables real-time studies of the dynamic processes during growth, such as observations of the droplet wetting angle or the facet truncation. Using a combination of theoretical modeling, high-temperature X-ray energy dispersive spectroscopy and direct imaging during growth, we measure and estimate the previously inaccessible gallium and arsenic concentration in the droplet, as well as the surface energies of the Au-Ga droplet and the GaAs nanowire sidewalls. These findings could in turn be used to further improve our understanding of the atomic arrangement at the crystal surfaces and interfaces during growth. Such an understanding could lead to improved control and design of crystal nanostructures.
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| 16. |
- Tornberg, Marcus, et al.
(författare)
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Enabling In Situ Studies of Metal-Organic Chemical Vapor Deposition in a Transmission Electron Microscope
- 2022
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Ingår i: Microscopy and Microanalysis. - 1431-9276. ; 28:5, s. 1484-1492
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Tidskriftsartikel (refereegranskat)abstract
- The world of environmental microscopy provides the possibility to study and analyze transformations and reactions during realistic conditions to understand the processes better. We report on the design and development of a metal-organic chemical vapor deposition (MOCVD) system integrated with an environmental transmission electron microscope intended for real-time investigations of crystal growth. We demonstrate methods for achieving a wide range of precisely controlled concentrations of precursor gas at the sample, as well as for calibrating the sample partial pressure using the pressure measured elsewhere in the microscope column. The influences of elevated temperature and reactive gas within the pole-piece gap are evaluated with respect to imaging and spectroscopy. We show that X-ray energy-dispersive spectroscopy can be strongly affected by temperatures beyond 500C, while the spatial resolution is largely unaffected by heat and microscope pressure for the relevant conditions. Finally, the influence of the electron beam on the investigated processes is discussed. With this work, we aim to provide crucial input in the development of advanced in situ electron microscopy systems for studies of complex reactions in real time under realistic conditions, for instance as used during formation of semiconductor crystals.
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| 17. |
- Tornberg, Marcus, et al.
(författare)
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Kinetics of Au-Ga Droplet Mediated Decomposition of GaAs Nanowires
- 2019
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; , s. 3498-3504
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Tidskriftsartikel (refereegranskat)abstract
- Particle-assisted III-V semiconductor nanowire growth and applications thereof have been studied extensively. However, the stability of nanowires in contact with the particle and the particle chemical composition as a function of temperature remain largely unknown. In this work, we use in situ transmission electron microscopy to investigate the interface between a Au-Ga particle and the top facet of an ?1 1 1 ?-oriented GaAs nanowire grown via the vapor-liquid-solid process. We observed a thermally activated bilayer-by-bilayer removal of the GaAs facet in contact with the liquid particle during annealing between 300 and 420 °C in vacuum. Interestingly, the GaAs-removal rates initially depend on the thermal history of the sample and are time-invariant at later times. In situ X-ray energy dispersive spectroscopy was also used to determine that the Ga content in the particle at any given temperature remains constant over extended periods of time and increases with increasing temperature from 300 to 400 °C. We attribute the observed phenomena to droplet-assisted decomposition of GaAs at a rate that is controlled by the amount of Ga in the droplet. We suggest that the observed transients in removal rates are a direct consequence of time-dependent changes in the Ga content. Our results provide new insights into the role of droplet composition on the thermal stability of GaAs nanowires and complement the existing knowledge on the factors influencing nanowire growth. Moreover, understanding the nanowire stability and decomposition is important for improving processing protocols for the successful fabrication and sustained operation of nanowire-based devices.
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| 18. |
- Tornberg, Marcus, et al.
(författare)
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Limits of III-V Nanowire Growth Based on Droplet Dynamics
- 2020
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Ingår i: Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 11:8, s. 2949-2954
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Tidskriftsartikel (refereegranskat)abstract
- Crystal growth of semiconductor nanowires from a liquid droplet depends on the stability of this droplet's liquid-solid interface. Because of the assisting property of the droplet, growth will be hindered if the droplet is displaced onto the nanowire sidewalls. Using real-time observation of such growth by in situ transmission electron microscopy combined with theoretical analysis of the surface energies involved, we observe a reoccurring truncation at the edge of the droplet-nanowire interface. We demonstrate that creating a truncation widens the parameter range for having a droplet on the top facet, which allows continued nanowire growth. Combining experiment and theory provides an explanation for the previously reported truncation phenomenon of the growth interface based only on droplet wetting dynamics. In addition to determining the fundamental limits of droplet-assisted nanowire growth, this allows experimental estimation of the surface tension and the surface energies of the nanowire such as the otherwise metastable wurtzite GaAs {101¯ 0} facet.
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| 19. |
- Tornberg, Marcus, et al.
(författare)
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Thermodynamic stability of gold-assisted InAs nanowire growth
- 2017
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 121:39, s. 21678-21684
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Tidskriftsartikel (refereegranskat)abstract
- Growth of III-V semiconductor nanowires is generally assisted by a liquid particle in order to get a highly anisotropic crystallization. The thermodynamic stability of the particle is therefore of importance for control and understanding of the nanowire growth process. In this report we explore the particle stability by manipulating its properties, specifically its surface tension and volume, by accumulating indium in the particle during nanowire growth. We demonstrate a droplet displacement, from the top to one of the nanowire side facets, when exceeding the stability limit for a gold particle wetting an [0001]-oriented InAs nanowire. This particle displacement is attributed to a lowered surface tension and a truncation of the top facet. In addition, our results indicate reversibility of the displacement, showing that the (111/0001) facet is the most favorable for a droplet to wet during common growth conditions. The stability condition for InAs growth is determined experimentally, and the understanding developed can easily be applied to other III-V nanowires.
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