| 1. |
- Amloy, Supaluck, et al.
(författare)
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Excitons and biexcitons in InGaN quantum dot like localization centers
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Indium segregation in a narrow InGaN single quantum well creates quantum dot (QD) like exciton localization centers. Cross section transmission electron microscopy reveals varying shapes and lateral sizes in the range ~1-5 nm of the QD-like features, while scanning near field optical microscopy demonstrates a highly inhomogeneous spatial distribution of optically active individual localization centers. Microphotoluminescence spectroscopy confirms the spectrally inhomogeneous distribution of localization centers, in which the exciton and the biexciton related emissions from single centers of varying geometry could be identified by means of excitation power dependencies. Interestingly, the biexciton binding energy (Ebxx) was found to vary from center to center, between 3 to -22 meV, in correlation with the exciton emission energy. Negative binding energies justify the three-dimensional quantum confinement, which confirms QD-like properties of the localization centers.! The observed energy correlation is proposed to be understood as variations of the lateral extension of the confinement potential, which would yield smaller values of Ebxx for reduced lateral extension and higher exciton emission energy. The proposed relation between lateral extension and Ebxx is further supported by the exciton and the biexciton recombination lifetimes of a single QD, which suggest a lateral extension of merely ~3 nm for a QD with strongly negative Ebxx = -15.5 meV.
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| 2. |
- Amloy, Supaluck, et al.
(författare)
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Excitons and biexcitons in InGaN quantum dot like localization centers
- 2014
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Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 25:49, s. 495702-
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Tidskriftsartikel (refereegranskat)abstract
- Indium segregation in a narrow InGaN single quantum well creates quantum dot (QD) like exciton localization centers. Cross-section transmission electron microscopy reveals varying shapes and lateral sizes in the range ∼1–5 nm of the QD-like features, while scanning near field optical microscopy demonstrates a highly inhomogeneous spatial distribution of optically active individual localization centers. Microphotoluminescence spectroscopy confirms the spectrally inhomogeneous distribution of localization centers, in which the exciton and the biexciton related emissions from single centers of varying geometry could be identified by means of excitation power dependencies. Interestingly, the biexciton binding energy (Ebxx) was found to vary from center to center, between 3 to −22 meV, in correlation with the exciton emission energy. Negative binding energies are only justified by a three-dimensional quantum confinement, which confirms QD-like properties of the localization centers. The observed energy correlation is proposed to be understood as variations of the lateral extension of the confinement potential, which would yield smaller values of Ebxx for reduced lateral extension and higher exciton emission energy. The proposed relation between lateral extension and Ebxx is further supported by the exciton and the biexciton recombination lifetimes of a single QD, which suggest a lateral extension of merely ∼3 nm for a QD with strongly negative Ebxx = −15.5 meV.
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| 3. |
- Hsu, Chih-Wei, et al.
(författare)
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Controlled Growth of GaN Pyramidal template hosting InGaN Quantum Dots
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The emission properties of InGaN grown on hexagonal GaN pyramids with various pitch distances (PD) are studied. Emissions associated with InGaN quantum wells (QWs) and InGaN quantum dots (QDs) can be identified. The emission energies of InGaN QWs and QDs shift toward opposite directions with increasing PD; red-shift for QWs and blue-shift for QDs. Based on the source supply mechanism in a selective area growth process, the formation of InGaN QDs on GaN pyramids is believed to be a combined effect of Stranski-Krastanow growth mode and spinodal decomposition taking place at the microscopic (0001) surfaces on GaN pyramids.
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| 4. |
- Lundskog, Anders, et al.
(författare)
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InGaN quantum dot formation mechanism on hexagonal GaN/InGaN/GaN pyramids
- 2012
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Ingår i: Nanotechnology. - : Institute of Physics (IOP). - 0957-4484 .- 1361-6528. ; 23:30, s. 305708-
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Tidskriftsartikel (refereegranskat)abstract
- Growing InGaN quantum dots (QDs) at the apex of hexagonal GaN pyramids is an elegant approach to achieve a deterministic positioning of QDs. Despite similar synthesis procedures by metal–organic chemical vapor deposition, the optical properties of the QDs reported in the literature vary drastically. The QDs tend to exhibit either narrow or broad emission lines in the micro-photoluminescence spectra. By coupled microstructural and optical investigations, the QDs giving rise to narrow emission lines were concluded to nucleate in association with a (0001) facet at the apex of the GaN pyramid.
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| 5. |
- Lundskog, Anders, et al.
(författare)
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Unexpected behavior of InGaN quantum dot emission energy located at apices of hexagonal GaN pyramids
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- InGaN quantum dots (QDs) have been grown at the apices of hexagonal GaN pyramids. The pyramids were selectively grown on a (0001) oriented GaN template through circular apertures in a SiN mask positioned in square arrays. The emission of the InGaN QDs was shifted towards higher energies when the center-to-center distance of the pyramids was increased, while the emission from InGaN quantum wells located on the {1101} facets of the pyramids was energetically shifted towards lower energies. No energy shift was observed for (0001) truncated pyramids with truncation diameters larger than 100 nm.
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| 6. |
- Hagvall, Kristoffer, 1985-
(författare)
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Characterization of aluminum in environmental systems using X-ray absorption and vibrational spectroscopy : The importance of organic matter
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The fate and behavior of many metals in the environment are highly dependent on interactions with natural organic matter (NOM), which is abundant in most soils and surface waters. The complexation with NOM can influence the speciation of the metals by affecting their hydrolysis and solubility. This in turn will also have an effect on the mobility and potential toxicity of the metals. For aluminum (Al) these interactions are of high environmental importance since Al have been shown to have negative effects on plant growth, water living organisms, and fish.This thesis will focus on the interactions between Al(III) and NOM in different environments and under varying geochemical conditions. To study this, infrared (IR) spectroscopy and X-ray absorption spectroscopy (XAS) have primarily been used. Due to the difficulties in analyzing Al using XAS, gallium(III), shown to be a suitable analogue for Al(III), was used as a probe to get complementary information from the Ga(III)-NOM system. The combined results from these studies showed that Ga(III) and Al(III) formed strong chelate complexes with carboxylic groups in NOM and that these complexes were strong enough to suppress the hydrolysis and polymerization of the metals. Furthermore, Al in organic soil and stream water samples was also studied using XAS and the results showed a variation in the speciation from a predominance of organically complexed Al(III) in the stream waters to a mixture of Al(III)-NOM complexes and precipitated Al phases (Al-hydroxides and/or Al-silicates) in the organic soils. To further study mineral-NOM interactions the effects of NOM on the dissolution of gibbsite (g-Al(OH)3(s); a common mineral in the environment) were investigated. The results showed that NOM can promote mineral dissolution and presence of inner-sphere Al(III)-NOM species on the gibbsite surface, detected by IR spectroscopy, could indicate a ligand induced dissolution. To further investigate the structure of the complex formed at the surface of the mineral, an EXAFS study was conducted on the ternary Ga(III)-NOM-gibbsite system. The results indicated either formation of inner-sphere complexes with Ga(III) acting like a bridge between NOM and the gibbsite surface, or the presence of two separate species; Ga(III)-NOM complexes in solution and a precipitated Ga(OH)3(s) phase.As a sidetrack to the Al(III)-NOM studies, a new way of characterizing NOM was developed using simultaneous infrared and potentiometric titrations, multivariate data analysis, and chemical equilibrium modeling. An acid/base model for a fulvic acid was constructed, based on spectroscopic information about functional groups and their pKa values, and indicated that the fulvic acid is to be regarded as a tetra carboxylic acid consisting of at least four fractions of carboxylic acids. This demonstrates new possibilities to study the acid/base and metal complexing properties of NOM, in which the presence of carboxylic acid groups predominate, and to design equilibrium models more reliable than presented before.
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| 7. |
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| 8. |
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| 9. |
- Hagvall, Kristoffer, et al.
(författare)
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Speciation of aluminum in soils and stream waters: The importance of organic matter
- 2015
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Ingår i: Chemical Geology. - : Elsevier BV. - 0009-2541 .- 1872-6836. ; 417, s. 32-43
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Tidskriftsartikel (refereegranskat)abstract
- Aluminum (Al) is the most common metal in the Earth's crust, and exists mainly in the form of silicates, oxides and hydroxides, or in complexes with natural organic matter (NOM) or inorganic ligands. Since speciation is a key factor for understanding the environmental impact of Al, it is important to determine the chemical forms of Al that predominate in different natural media and under varying geochemical conditions. This study investigated how complexation with NOM influenced the speciation of Al(III) in different environmental samples (isolated aquatic NOM, organic soils and stream waters) using X-ray absorption spectroscopy (XAS) and Infrared (IR) spectroscopy. The combined spectroscopic results showed that Al(III) formed mononuclear complexes with carboxylic functional groups in NOM that were sufficiently stable to suppress the hydrolysis and polymerization of Al(III). These organic Al complexes were favored at slightly acidic pH values between pH 3 and 6 and at decreasing Al concentrations. The XAS results of the organic soils and the stream water samples indicated a variation in the speciation from a predominance of organically complexed Al in the stream waters to a mixture of Al-NOM complexes and precipitated Al phases (Al hydroxides and/or Al silicates) in the organic soils. Although the presented XAS results were limited by relatively low signal-to-noise caused by the low K-edge energy of Al, the combined spectroscopic results provided new and useful information about Al in different environmental samples and showed that NOM and pH are important parameters controlling the speciation of Al. (C) 2015 Elsevier B.V. All rights reserved.
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| 10. |
- Hagvall, Kristoffer, et al.
(författare)
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Spectroscopic characterization of the coordination chemistry and hydrolysis of gallium(III) in the presence of aquatic organic matter
- 2014
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Ingår i: Geochimica et Cosmochimica Acta. - : Elsevier BV. - 0016-7037 .- 1872-9533. ; 146, s. 76-89
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Tidskriftsartikel (refereegranskat)abstract
- Interactions between metals and natural organic matter (NOM) are of great environmental importance and one of the key factors influencing hydrolysis, solubility, and speciation of the metals. However, studying geochemically relevant metals like Al, Fe, and Cu is sometimes associated with analytical problems; for example Fe and Cu are both redox active. Gallium (Ga) is a non-redox active metal that usually occurs at very low concentrations in environmental samples and therefore a wide concentration range of metal(III)-NOM species can be explored by adding Ga(III) to such samples. This makes Ga(III) a good probe and analogue for other metal ions, in particular Al. In addition, due to the increased usage of Ga in society, a better understanding of how Ga interacts with NOM is of importance but such studies are scarce. In this work, Ga(III) interactions with two different organic materials (Suwannee River natural organic matter and Suwannee River fulvic acid) were studied using infrared (IR) and extended X-ray absorption fine structure (EXAFS) spectroscopy in a large experimental range (101-84,076 mu g Ga g(-1) dry weight; pH 3-8). Our IR spectroscopic results showed that Ga(III) is bonded mainly to carboxylic functional groups and suggested that only a fraction of the total number of carboxylic sites in the samples was actively involved in the bonding. Modeling of the EXAFS data revealed that Ga(III) formed mononuclear chelate complexes with NOM that strongly suppressed the hydrolysis and polymerization of Ga(III). At low Ga(III) concentrations (1675-16,649 mu g g(-1)) organic complexes, consisting of 1-3 chelate ring structures, were the dominating species in the entire pH range while at higher concentrations (67,673-84,076 mu g g(-1), pH 3.0-7.0) we detected mixtures of mononuclear organic Ga(III) complexes, Ga(III) (hydr) oxide, and free Ga(III) (here defined as the hydrated Ga(III) ion and its soluble hydrolysis products). Moreover, the EXAFS results showed significantly higher contribution from second-shell C atoms (9-11) for the Ga(III)-organic complexes at the lowest concentration (101-125 mu g g(-1), pH 4.9-5.1), indicating formation of cage-like structures similar to Ga(III)-EDTA. Our combined results showed that Ga(III)-NOM interactions can be of importance for the solubility and speciation of Ga in environmental systems. Furthermore, the similarities between Ga(III) and previous Fe(III) results demonstrate that Ga(III) can be utilized as a probe for metal(III)-NOM interactions over an extended experimental range (e. g., pH and metal concentration) and thereby improve our knowledge about these interactions in general. (C) 2014 Elsevier Ltd. All rights reserved.
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