| 1. |
- Rein, Christian, et al.
(författare)
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Element-specific investigations of ultrafast dynamics in photoexcited Cu2ZnSnS4 nanoparticles in solution
- 2021
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Ingår i: Structural Dynamics. - : AIP Publishing. - 2329-7778. ; 8:2
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Tidskriftsartikel (refereegranskat)abstract
- Ultrafast, light-induced dynamics in copper-zinc-tin-sulfide (CZTS) photovoltaic nanoparticles are investigated through a combination of optical and x-ray transient absorption spectroscopy. Laser-pump, x-ray-probe spectroscopy on a colloidal CZTS nanoparticle ink yields element-specificity, which reveals a rapid photo-induced shift of electron density away from Cu-sites, affecting the molecular orbital occupation and structure of CZTS. We observe the formation of a stable charge-separated and thermally excited structure, which persists for nanoseconds and involves an increased charge density at the Zn sites. Combined with density functional theory calculations, the results provide new insight into the structural and electronic dynamics of CZTS absorbers for solar cells.
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| 2. |
- Späh, Alexander, 1989-
(författare)
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X-Ray Investigations of the Liquid-Liquid Critical Point Hypothesis in Supercooled Water
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis presents experimental x-ray scattering studies on supercooled liquid water. A liquid-liquid transition between two structurally distinct configurations has been found in deeply supercooled water, indicating the existence of a liquid- liquid critical point. The experiments were performed at large-scale x-ray facilities, mostly using free electron x-ray lasers including PAL-XFEL in Korea, SACLA in Japan, LCLS in the USA, SwissFEL in Switzerland and European XFEL in Germany, as well as using synchrotrons including APS in the USA, PETRA III in Germany and ESRF in France.Two conceptually different experimental approaches have been used to investigate the metastable phase of supercooled water. The first approach is based on rapid evaporative cooling of μm-sized water droplets that are injected into a vacuum chamber. Using this method, supercooled liquid water samples with temperatures down to approximately 227 K have been obtained, with the lowest temperature limited by homogeneous ice crystallization occurring after just a few milliseconds. In a second approach, structurally arrested high-pressure and therefore high-density amorphous ice samples are heated by an ultrafast infrared laser pulse. The fast heating melts the ice into a corresponding high-density liquid. At short time delays between the heating laser pulse and a subsequent x-ray probe pulse, the supercooled liquefied sample still experiences the high internal pressure of the initial state. At longer pump-probe delay times the supercooled water sample releases its internal pressure through structural relaxation. Hence, varying the pump-probe delay allows to probe the sample at different pressures.Together, these two approaches have been used to access a region within the metastable phase diagram of supercooled water that has previously been inaccessible. Using elastic x-ray scattering measurements as a structural probe of the liquid, we identified the existence of a liquid-liquid phase transition in deeply supercooled water. The observed phase transition is interpreted as the transition between a high-density and a low-density liquid phase. At high pressure this phase transition is discontinuous or first-order like, featuring a characteristic double-peak feature in the observed x-ray scattering intensity of the first diffraction maxima. At ambient pressure, however, we observe a continuous shift of the first diffraction maxima that is consistent with a continuous or second-order phase transition between the two liquids. Further evidence of a continuous phase transition at ambient pressure is seen in the temperature dependent maxima of the measured correlation length, isothermal compressibility and heat capacity, which indicate the existence of a Widom line.In summary, the experiments support the existence of a liquid-liquid critical point where the experimentally observed Widom line and phase coexistence line would both meet. The main result, however, is the first experimental observation of a liquid-liquid transition within a pure liquid.
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| 3. |
- van Driel, Tim Brandt, et al.
(författare)
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Disentangling detector data in XFEL studies of temporally resolved solution state chemistry
- 2015
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Ingår i: Faraday Discussions. - : Royal Society of Chemistry (RSC). - 1364-5498 .- 1359-6640. ; 177, s. 443-465
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Tidskriftsartikel (refereegranskat)abstract
- With the arrival of X-ray Free Electron Lasers (XFELs), 2D area detectors with a large dynamic range for detection of hard X-rays with fast readout rates are required for many types of experiments. Extracting the desired information from these detectors has been challenging due to unpredicted fluctuations in the measured images. For techniques such as time-resolved X-ray Diffuse Scattering (XDS), small differences in signal intensity are the starting point for analysis. Fluctuations in the total detected signal remain in the differences under investigation, obfuscating the signal. To correct such artefacts, Singular Value Decomposition (SVD) can be used to identify and characterize the observed detector fluctuations and assist in assigning some of them to variations in physical parameters such as X-ray energy and X-ray intensity. This paper presents a methodology for robustly identifying, separating and correcting fluctuations on area detectors based on XFEL beam characteristics, to enable the study of temporally resolved solution state chemistry on the femtosecond timescale.
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