| 1. |
- Ahmed, Taha, et al.
(författare)
-
Phonon–phonon and electron–phonon coupling in nano-dimensional ZnO
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Thermal losses through vibrational coupling are critical bottlenecks limiting several materials classes from reaching their full potential. Altering the phonon–phonon and electron–phonon coupling by controlled suppression of vibrational degrees of freedom through low-dimensionality are promising but still largely unexplored approaches. Here we report a detailed study of the first- and second-order Raman processes as a function of size for low-dimensional ZnO. Wurtzite ZnO nanoparticles were synthesised into 3D frameworks of ZnO crystallites, with tailored crystallite diameters from 10 nm to 150 nm and characterised by electron microscopy, X-ray diffraction and non-resonant and resonant Raman spectroscopy.We present a short derivation of how resonance Raman and the relation between the longitudinal optical (LO) phonons can be utilised to quantify the electron–phonon coupling, its merits, and limitations. Theoretical Raman response using density functional theory is corroborating the experimental data in assigning first- and second-order Raman modes. The Lyddane-Sachs-Teller equation was applied to the measured LO–TO split and revealed no change in the ratio between the static and high-frequency dielectric constant with changing ZnO dimension from 10 nm to 150 nm. The second-order Raman revealed a phonon–phonon coupling that generally increased with particle size and markedly so for differential modes. Resonance Raman showed the fundamental LO mode and the 2nd, 3rd, and 4th overtones. The intensity relation between the fundamental LO mode and its overtones enabled the extraction of the change in electron–phonon coupling via the Huang-Rhys parameter as a function of particle size, which showed an increase with particle size.
|
|
| 2. |
- Johansson, Malin B., 1972-, et al.
(författare)
-
Highly crystalline MAPbI3 perovskite grain formation by irreversible poor-solvent diffusion aggregation, for efficient solar cell fabrication
- 2020
-
Ingår i: Nano Energy. - : Elsevier Ltd. - 2211-2855 .- 2211-3282. ; 78
-
Tidskriftsartikel (refereegranskat)abstract
- Energy efficient synthesis providing high quality crystalline thin films are highly desired in many applications. Here we devise a non-toxic solvent approach for production of highly crystalline MAPbI3 perovskite by exploiting diffusion aggregation processes. Isopropanol solution based methylammonium lead triiodide (MAPbI3) is used in this context, where the crystal growth initiation starts in an unstable suspension far from equilibrium and the subsequent crystallization is driven by the solubility parameters. The crystal formation is monitored by scanning transmission electron microscope (STEM), observing small crystallization centers growing as time evolves to large grains with high crystal purity. Energy dispersive X-ray spectroscopy (EDS) in STEM mode revealed a Pb rich core-shell structure in newly formed grains. Nano-beam Electron Diffraction (NBED) scan defined PbI2 crystallites in the Pb rich shell with a single crystal MAPbI3 core in newly formed grains. After a week stirring, the same aggregated suspension exhibited grains with only single crystal MAPbI3 structure. The NBED analysis shows a kinetically slow transition from a core shell structure to a single crystal grain. This research presents an impactful insight on the factors that may cause sub-stoichiometric grain boundary effects which can influence the solar cell performance. In addition, the structure, morphology and optical properties of the perovskite grains have been presented. A powder of highly crystalline particles was subsequently prepared by evaporation of the solvent in a low-vacuum oven. Thin film MAPbI3 solar cells were fabricated by dissolving the powder and applying it in a classical fabrication route. The MAPbI3 solar cells gave a champion efficiency of 20% (19.9%) and an average efficiency at approximately 17% with low hysteresis effects. Here a strategy to manufacture the material structure without toxic solvents is highlighted. The single-crystal growth devised here opens both for shelf storage of materials as well as a more flexible manufacturing of devices. The process can likely be extended to other fields, where the intermediate porous framework and large surface area would be beneficial for battery or super capacitor materials.
|
|
| 3. |
- Johansson, Malin B, 1972-, et al.
(författare)
-
Porous Fractals of MAPbI3 Perovskite : Characterization of Crystal Grain Formation by Irreversible Diffusion-Limited Aggregation
- 2018
-
Konferensbidrag (refereegranskat)abstract
- Isopropanol solution based methylammonium lead triiodide (MAPbI3) is studied during the crystallization process. The crystal growth starts in an unstable suspension far from equilibrium by forming different dendritic patterns and terminates with aggregation of stable cubic crystalline grains into fractal clusters. Using transmission electron microscopy (TEM), the time evolution of a newly mixed suspension was studied over a period of two weeks at room temperature and a sequence of the morphological changes was observed. The crystallization process started with single dendritic growth exhibiting branches at 90 degrees angles to one another. After 4 hours, a multi-dendritic growth pattern and a transformation into small crystalline quantum dots were observed. After a week, clusters of crystal grains were formed into a fractal pattern and these patterns appear to be stable also during the second week. Electron and x-ray diffraction revealed the crystallinity of the quantum dots and the clusters of micrometer-sized crystals. Scanning transmission electron microscope (STEM) together with energy dispersive X-ray spectroscopy (EDS) showed that newly formed large grains, from a one hour old solution, displayed a core-shell structure with higher percentage of Pb atoms as compared to iodine at the surface. In the inner core of the grains the percentage of iodine was slightly higher. The electron diffraction (ED) scan over the newly formed grains revealed a polycrystalline surface whereas the inner part had a single crystal pattern. The same solution, now one-week-old, contained grains with only single crystal patterns in the ED scan and showed no core-shell character or polycrystalline surface. The measured percentage of iodine atoms compared to lead was 2:1 throughout the cross section, which is a quantitative value within the measurement. It can be concluded from these measurements that the suspension approaches higher crystallinity of the perovskite grains in an irreversible process, where the perovskite grains are insoluble in isopropanol. The perovskite material has also been characterized with scanning electron microscopy (SEM) and photoluminescence (PL) mapping where both techniques showed a very porous crystalline material. The PL mapping revealed two peaks at 730 and 760 nm for a thin film spin coated from a newly mixed solution, while a film deposited from a one week old solution showed three peaks, the last one at 830 nm. Because of the high crystallinity, it is suggested that all three peaks are due to band-to-band transitions and not due to localized states. These data will be analyzed further; however, the results contain information of the content of quantum dots versus larger crystals, as well as displaying emission intensity variations at different positions of the grains. The purpose with this project is to understand these phenomena of crystal growth. A new mesoporous perovskite material has been designed for optoelectronic purposes.
|
|
| 4. |
- Montero Amenedo, José, 1983-, et al.
(författare)
-
Copper-zinc oxide heterojunction catalysts exhibiting enhanced photocatalytic activity prepared by a hybrid deposition method
- 2021
-
Ingår i: RSC Advances. - : Royal Society of Chemistry. - 2046-2069. ; 11:17, s. 10224-10234
-
Tidskriftsartikel (refereegranskat)abstract
- Heterojunction copper-zinc oxide catalysts were prepared by a hybrid two-step methodology comprising hydrothermal growth of ZnO nanorods (ZnO-NR) followed by deposition of Cu2O nanoparticles using an advanced gas deposition technique (AGD). The obtained bicatalysts were characterized by SEM, AFM, XRD, XPS, PL and spectrophotometry and revealed well-dispersed and crystalline Cu2O nanoparticles attached to the ZnO-NR. The adsorption properties and photocatalytic degradation of Orange II dye in water solutions were measured. It was found that the bicatalysts exhibited a conversion rate and quantum yield that both were about 50% higher compared with ZnO-NR alone, which were attributed to the intrinsic electric field created at the p-n junction formed at the Cu2O/ZnO interface facilitating charge separation of electron-hole pairs formed upon interband photon absorption. The interpretation was evidenced by efficient quenching of characteristic deep level ZnO photoluminescence bands and photoelectron core-level energy shifts. By comparisons with known energy levels in Cu2O and ZnO, the effect was found to be most pronounced for the non-polar ZnO-NR side facets, which accounted for about 95% of the exposed surface area of the catalyst and hence the majority of dye adsorption. It was also found that the dye adsorption capacity of the ZnO nanorods increased considerably after Cu2O deposition thereby facilitating the oxidation of the dye. The results imply the possibility of judiciously aligning band edges on structurally controlled and well-connected low-dimensional semiconductor nanostructures using combined two-step synthesis techniques, where in particular vacuum-based techniques such as AGD allow for growth of well-connected nanocrystals with well developed heterojunction interfaces.
|
|
| 5. |
- Montero Amenedo, José, 1983-, et al.
(författare)
-
Photobleaching of dyes by CuOx-based heterojunction bi-catalysts
- 2019
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- We present the synthesis and characterization of thin film heterojunction, or bi-catalyst, comprising of photocatalytic coatings based on a combination of two different materials exhibiting p-type and n-type conductivity, respectively. Here we show results for the combination of CuOx together with zinc oxide (ZnO) and tungsten oxide (WO3). The purpose of such compounds is to create an intrinsic electrical field at the np-junction that helps to separate electron-hole pairs formed upon interband photon absorption. At the same time desired photochemical properties implies that the constituent catalysts have appropriate bandgap and band edge position. For this purpose, CuOx/WO3 compound films have been prepared onto glass substrates by co-sputtering of tungsten and copper targets in an argon and oxygen atmosphere. CuOx/ZnO bi-catalyst have also been prepared on glass substrates by a two-step process consisting of deposition of CuOx by reactive magnetron sputtering, followed by the ap plication of ZnO particles by drop coating. The obtained bi-catalyst have been characterized by SEM, XRD, XPS, Raman and spectrophotometry. Finally, the photocatalytic activity of the different compound films is assessed by studying their photobleaching rate of methylene blue and orange II in water solution.
|
|
| 6. |
- Rahman, Mohammad Z., et al.
(författare)
-
Surface polarity, water adhesion and wettability behaviors of iron pyrite
- 2020
-
Ingår i: Materials Today. - : Elsevier BV. - 2214-7853. ; 33, s. 2465-2469
-
Tidskriftsartikel (refereegranskat)abstract
- Earth abundant iron pyrite (FeS2) has attracted considerable attention as a potential non-toxic absorber material in lieu of widely used but toxic and relatively rare Indium containing CIGS and CdTe for thin-film solar cells. Research in this regard has been progressed in understanding its fundamental electronic, optical and chemical properties, while its surface polarity is rarely been studied. In this contribution, we have investigated the adhesion of water to determine the wettability (i.e. hydrophilic or hydrophobic) of naturally grown FeS2, and thereof, the polarity of the surface. FeS2 has a cubic crystal structure that grows and cleaves along its cubic faces. Based on the measurements using Sessile drop method, we have found that the surfaces of the pyrite phase of FeS2 crystals are intrinsically super-hydrophilic and highly polar. Our finding is corroborated with X-ray diffraction, Fourier Transform Infrared, Raman Spectroscopy, particle size determination, and contact angle measurements of the pyrite phase. The surface polarity would be a critical factor for choosing surface passivation protocols and type of contact materials as well as for interpreting surface field effects in the system for solar cell application with material heterojunctions.
|
|
| 7. |
- Thyr, Jakob, 1979-, et al.
(författare)
-
Cu2O-Coated Copper Nanopillars For Photocatalytic Water Cleaning
- 2022
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Water pollution is a severe problem in many parts of the world. In developed countries the increased use of chemicals and urban densification has started to cause stress of previously well-functioning water systems. Advanced oxidation processes (AOPs), is a promising method for degradation of artificial organic pollutants, which are challenging to remove by conventional water treatment techniques. In AOPs hydroxyl radicals (OH•) and reactive oxygen species (O2- and O22-) which are strongly oxidizing species are generated and these subsequently react with and degrade the pollutants. To use nanostructures which are optically active in the visible part of the spectrum is attractive because it both creates a large surface area, promoting surface interface reactions, as well as enables the utilization of a large part of the solar spectrum. In this study flat copper surfaces and 3D nanostructured copper pillars are utilized as base structures. These are subjected to thermal oxidation at low temperature, for a controlled amount of time, creating thin copper oxide layers which makes them photoactive in the visible range. The formed copper oxide and its growth is analysed with SEM, XRD and Raman spectroscopy, and show the formation of Cu2O with a slight incorporation of CuO for the thickest oxide layers. Formation of CuO nano needles, protruding from the Cu2O layer, were observed in the SEM imaging. The photocatalytic performance was tested by degradation of methylene blue in aqueous solution and all of the tested systems showed quite effective performance. The highest degradation rate was seen for copper nanopillars annealed for 4 or 8 min, which exhibited 34% faster degradation than the oxidized flat sample. The study shows that simple and inexpensive thermal oxidation processes can be used to create efficient photoactive Cu2O catalysts even on semi-flat surfaces, and that nanostructuring increases the degradation rates.
|
|
| 8. |
- Thyr, Jakob, 1979-, et al.
(författare)
-
Energy Alignment of Quantum-Confined ZnO Particles with Copper Oxides for Heterojunctions with Improved Photocatalytic Performance
- 2022
-
Ingår i: ACS Nanoscience Au. - : American Chemical Society (ACS). - 2694-2496. ; 2:2, s. 128-139
-
Tidskriftsartikel (refereegranskat)abstract
- The ability to control electronic states by utilizing quantum confinement of one of the material components in heterojunctions is a promising approach to perform energy-level matching. In this work, we report the possibility to achieve optimum energy alignment in heterojunctions made from size-controlled quantum dots (Q-dots) of ZnO in combination with three copper oxides: Cu2O, Cu4O3, and CuO. Quantum confinement effects on the ZnO nanoparticles in the diameter range 2.6–7.4 nm showed that the direct optical band gap decreased from 3.99 to 3.41 eV, with a dominating shift occurring in the conduction band (CB) edge, and thus the possibility to obtain close to 0.6 eV CB edge shift by controlling the size of ZnO. The effect was utilized to align the electronic bands in the ZnO Q-dot/copper oxide heterojunctions to allow for charge transfer between the materials and to test the ability to improve the photocatalytic performance for the system, evaluated by the transformation of a dye molecule in water. The catalyst materials were investigated by X-ray diffraction, scanning electron microscopy, ultraviolet–visible (UV–vis), photoluminescence, and Raman spectroscopy. The most promising material combination was found to be the Cu2O copper oxide in combination with an energy aligned ZnO Q-dot system with approximately 7 nm diameter, showing strong synergy effects in good agreement with the energy-level analysis, outperforming the added effect of its individual components, ZnO-Q-dots and Cu2O, by about 140%. The results show that utilization of a heterojunction with controllable energy alignment can provide a drastically improved photocatalytic performance. Apart from increased photocatalytic activity, specific surface states of ZnO are quenched when the heterojunction is created. It is anticipated that the same approach can be utilized in several material combinations with the added benefit of a system with controllable overpotential and thus added specificity for the targeted reduction reaction.
|
|
| 9. |
- Thyr, Jakob, 1979-, et al.
(författare)
-
Evading the Illusions : Identification of False Peaks in Raman Spectroscopy and Guidelines for Appropriate Measurement Protocols.
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Raman spectroscopy is an important analytical tool in materials science. Its ability to characterize transitions between rotational and vibrational states by analysis of inelastically scattered light, enables it to identify chemical bonds. As changes of the rotational and vibrational states in turn depend on secondary effects, the technique is also suitable for studying more detailed phenomena like molecular interactions, material strain, crystallinity, order and bond formations. This versatility has made it a standard tool in a large variety of science disciplines including chemistry, physics, biology, geology and medicine. There are several advantages with Raman spectroscopy including that it in most cases is non-destructive, requires no sample preparation and that almost any non-metallic type of sample can be measured. Raman scattering has however one major weakness: It is a very low probability process unless probed in resonance with an electronic excitation. To detect such a weak signal at a high spectral resolution, a very sensitive detection system is needed which subsequently leads to a high probability of picking up signal from other processes or from other origin than the measured sample. These spurious signals that sometimes occur in Raman spectra are referred to as false Raman peaks and if they are not correctly identified, they complicate the analysis of the results and increase the risk of misinterpreting the data. This work is aimed to give the fundamental principles of Raman scattering and an overview of the sources of other signals occurring in Raman spectra that include; other photon generating processes, cosmic rays, stray light, artefacts from spectrometer components, and signals from other compounds in or surrounding the sample. The origins of the false Raman peaks are explained and means and measures to identify and counteract them are given. Finally, a Raman measurement protocol that can serve as a guideline for practical confocal Raman spectroscopy measurements is presented.
|
|
| 10. |
- Thyr, Jakob, 1979-, et al.
(författare)
-
Evading the Illusions : Identification of False Peaks in Micro-Raman Spectroscopy and Guidelines for Scientific Best Practice
- 2023
-
Ingår i: Angewandte Chemie International Edition. - : John Wiley & Sons. - 1433-7851 .- 1521-3773. ; 62:43
-
Forskningsöversikt (refereegranskat)abstract
- Micro-Raman spectroscopy is an important analytical tool in a large variety of science disciplines. The technique is suitable for both identification of chemical bonds and studying more detailed phenomena like molecular interactions, material strain, crystallinity, defects, and bond formations. Raman scattering has one major weakness however: it is a very low probability process. The weak signals require very sensitive detection systems, which leads to a high probability of picking up signals from origins other than the sample. This complicates the analysis of the results and increases the risk of misinterpreting data. This work provides an overview of the sources of spurious signals occurring in Raman spectra, including photoluminescence, cosmic rays, stray light, artefacts caused by spectrometer components, and signals from other compounds in or surrounding the sample. The origins of these false Raman peaks are explained and means to identify and counteract them are provided. Raman spectroscopy is a great analysis tool but the spectra are sometimes difficult to interpret due to the occurrence of spectral artefacts. This paper dives into the details of many spurious signals and spectral artefacts that occur in Raman spectra, explains their origin, and provides the tools to identify and avoid them.
|
|
