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Journal of synchrotron radiation 1(1), 242 - 250 (2023). doi:10.1107/S1600577522010347, The PERCIVAL detector is a CMOS imager designed for the soft X-ray regime at photon sources. Although still in its final development phase, it has recently seen its first user experiments: ptychography at a free-electron laser, holographic imaging at a storage ring and preliminary tests on X-ray photon correlation spectroscopy. The detector performed remarkably well in terms of spatial resolution achievable in the sample plane, owing to its small pixel size, large active area and very large dynamic range; but also in terms of its frame rate, which is significantly faster than traditional CCDs. In particular, it is the combination of these features which makes PERCIVAL an attractive option for soft X-ray science., Published by Wiley-Blackwell, [Erscheinungsort nicht ermittelbar]

The journal of physical chemistry letters 14(20), 4719 - 4725 (2023). doi:10.1021/acs.jpclett.3c00631, The dynamics and time scales of higher-order correlations are studied in supercooled colloidal systems. A combination of X-ray photon correlation spectroscopy (XPCS) and X-ray cross-correlation analysis (XCCA) shows the typical slowing of the dynamics of a hard sphere system when approaching the glass transition. The time scales of higher-order correlations are probed using a novel time correlation function g$_C$, tracking the time evolution of cross-correlation function C. With an increasing volume fraction, the ratio of relaxation times of g$_C$ to the standard individual particle relaxation time obtained by XPCS increases from ∼0.4 to ∼0.9. While a value of ∼0.5 is expected for free diffusion, the increasing values suggest that the local orders within the sample are becoming more long-lived for larger volume fractions. Furthermore, the dynamics of local order is more heterogeneous than the individual particle dynamics. These results indicate that not only the presence but also the lifetime of locally favored structures increases close to the glass transition., Published by ACS, Washington, DC

Hamburg : Deutsches Elektronen-Synchrotron DESY 127 pp. (2021)., This White Paper describes the scope of the Centre for Molecular Water Science (CMWS), a collaborative, cross-disciplinary research initiative that shall use a highly dedicated laboratory infrastructure, the most advanced experimental techniques, leading theory/simulation tools and forefront photon facilities to answer the most important questions in molecular water science. The key scientific challenges, preliminary work, objectives, methodologies and infrastructural needs for five major scientific research areas (pillars) have been identified during the past three years by a group of more than 140 scientists from more than 15 countries. The results are compiled and summarised in this document. This task would not have been possible without the continued support from the host laboratory DESY in Hamburg (Germany). Special thanks to the Chairman of the DESY Board of Directors, Prof Helmut Dosch, and the Director in charge of Photon Science, Prof. Edgar Weckert., Published by Deutsches Elektronen-Synchrotron DESY, Hamburg

IUCrJ 8(5), 775 - 783 (2021). doi:10.1107/S2052252521006333, Many soft-matter systems are composed of macromolecules or nanoparticles suspended in water. The characteristic times at intrinsic length scales of a few nanometres fall therefore in the microsecond and sub-microsecond time regimes. With the development of free-electron lasers (FELs) and fourth-generation synchrotron light-sources, time-resolved experiments in such time and length ranges will become routinely accessible in the near future. In the present work we report our findings on prototypical soft-matter systems, composed of charge-stabilized silica nanoparticles dispersed in water, with radii between 12 and 15 nm and volume fractions between 0.005 and 0.2. The sample dynamics were probed by means of X-ray photon correlation spectroscopy, employing the megahertz pulse repetition rate of the European XFEL and the Adaptive Gain Integrating Pixel Detector. We show that it is possible to correctly identify the dynamical properties that determine the diffusion constant, both for stationary samples and for systems driven by XFEL pulses. Remarkably, despite the high photon density the only observable induced effect is the heating of the scattering volume, meaning that all other X-ray induced effects do not influence the structure and the dynamics on the probed timescales. This work also illustrates the potential to control such induced heating and it can be predicted with thermodynamic models., Published by Chester

Applied Sciences 11, 6179 (1-33) (2021). doi:10.3390/app11136179, X-ray photon correlation spectroscopy (XPCS) enables the study of sample dynamics between micrometer and atomic length scales. As a coherent scattering technique, it benefits from the increased brilliance of the next-generation synchrotron radiation and Free-Electron Laser (FEL) sources. In this article, we will introduce the XPCS concepts and review the latest developments of XPCS with special attention on the extension of accessible time scales to sub-μs and the application of XPCS at FELs. Furthermore, we will discuss future opportunities of XPCS and the related technique X-ray speckle visibility spectroscopy (XSVS) at new X-ray sources. Due to its particular signal-to-noise ratio, the time scales accessible by XPCS scale with the square of the coherent flux, allowing to dramatically extend its applications. This will soon enable studies over more than 18 orders of magnitude in time by XPCS and XSVS., Published by MDPI, Basel

IUCrJ 5(3), 354 - 360 (2018). doi:10.1107/S2052252518005407, We report on the self-assembly of gold nanoparticles coated with a soft poly(ethylene glycol) shell studied by X-ray cross-correlation analysis. Depending on the initial concentration of gold nanoparticles used, structurally heterogeneous films were formed. The films feature hot spots of dominating four- and sixfold local order with patch sizes of a few micrometres, containing 10$^4$–10$^5$ particles. The amplitude of the order parameters suggested that a minimum sample amount was necessary to form well ordered local structures. Furthermore, the increasing variation in order parameters with sample thickness demonstrated a high degree of structural heterogeneity. This wealth of information cannot be obtained by the conventional microscopy techniques that are commonly used to study nanocrystal superstructures, as illustrated by complementary scanning electron microscopy measurements., Published by IUCr, Chester

Scientific reports 10, 5054 (2020). doi:10.1038/s41598-020-61926-y, The ability to deliver two coherent X-ray pulses with precise time-delays ranging from a few femtoseconds to nanoseconds enables critical capabilities of probing ultra-fast phenomena in condensed matter systems at X-ray free electron laser (FEL) sources. Recent progress made in the hard X-ray split-and-delay optics developments now brings a very promising prospect for resolving atomic-scale motions that were not accessible by previous time-resolved techniques. Here, we report on characterizing the spatial and temporal coherence properties of the hard X-ray FEL beam after propagating through split-and-delay optics. Speckle contrast analysis of small-angle scattering measurements from nanoparticles reveals well-preserved transverse coherence of the beam. Measuring intensity fluctuations from successive X-ray pulses also reveals that only single or double temporal modes remain in the transmitted beam, corresponding to nearly Fourier transform limited pulses., Published by Macmillan Publishers Limited, part of Springer Nature, [London]

An X-ray cross-correlation study with emphasis on colloidal crystals is presented and demonstrates how to access higher-order structure beyond pair correlations. In this way symmetries of the crystal can be determined that are inaccessible in conventional crystallography., The results of an X-ray cross-correlation analysis (XCCA) study on hard-sphere colloidal crystals and glasses are presented. The article shows that cross-correlation functions can be used to extract structural information beyond the static structure factor in such systems. In particular, the powder average can be overcome by accessing the crystals’ unit-cell structure. In this case, the results suggest that the crystal is of face-centered cubic type. It is demonstrated that XCCA is a valuable tool for X-ray crystallography, in particular for studies on colloidal systems. These are typically characterized by a rather poor crystalline quality due to size polydispersity and limitations in experimental resolution because of the small q values probed. Furthermore, nontrivial correlations are observed that allow a more detailed insight into crystal structures beyond conventional crystallography, especially to extend knowledge in structure formation processes and phase transitions.

Nature Communications 9(1), 1917 (2018). doi:10.1038/s41467-018-04330-5, The dynamics of liquid water feature a variety of time scales, ranging from extremely fast ballistic-like thermal motion, to slower molecular diffusion and hydrogen-bond rearrangements. Here, we utilize coherent X-ray pulses to investigate the sub-100 fs equilibrium dynamics of water from ambient conditions down to supercooled temperatures. This novel approach utilizes the inherent capability of X-ray speckle visibility spectroscopy to measure equilibrium intermolecular dynamics with lengthscale selectivity, by measuring oxygen motion in momentum space. The observed decay of the speckle contrast at the first diffraction peak, which reflects tetrahedral coordination, is attributed to motion on a molecular scale within the first 120 fs. Through comparison with molecular dynamics simulations, we conclude that the slowing down upon cooling from 328 K down to 253 K is not due to simple thermal ballistic-like motion, but that cage effects play an important role even on timescales over 25 fs due to hydrogen-bonding., Published by Nature Publishing Group, London

IUCrJ 5(6), 801 - 807 (2018). doi:10.1107/S2052252518013696, The application of X-ray photon correlation spectroscopy (XPCS) at free-electron laser (FEL) facilities enables, for the first time, the study of dynamics on a (sub-)nanometre scale in an unreached time range between femtoseconds and seconds. For soft-matter materials, radiation damage is a major limitation when going beyond single-shot applications. Here, an XPCS study is presented at a hard X-ray FEL on radiation-sensitive polymeric poly(N-isopropylacrylamide) (PNIPAM) nanoparticles. The dynamics of aqueous suspensions of densely packed silica-PNIPAM core-shell particles and a PNIPAM nanogel below the radiation-damage threshold are determined. The XPCS data indicate non-diffusive behaviour, suggesting ballistic and stress-dominated heterogeneous particle motions. These results demonstrate the feasibility of XPCS experiments on radiation-sensitive soft-matter materials at FEL sources and pave the way for future applications at MHz repetition rates as well as ultrafast modes using split-pulse devices., Published by Wiley, Chester

International audience; Despite intensive studies in the past decades, the local structure of disordered matter remains widely unknown. We show the results of a coherent x-ray scattering study revealing higher-order correlations in dense colloidal hard-sphere systems in the vicinity of their crystallization and glass transition. With increasing volume fraction, we observe a strong increase in correlations at both medium-range and next-neighbor distances in the supercooled state, both invisible to conventional scattering techniques. Next-neighbor correlations are indicative of ordered precursor clusters preceding crystallization. Furthermore, the increase in such correlations is accompanied by a marked slowing down of the dynamics, proving experimentally a direct relation between orientational order and sample dynamics in a soft matter system. In contrast, correlations continuously increase for nonequilibrated, glassy samples, suggesting that orientational order is reached before the sample slows down to reach (quasi-)equilibrium.

We present an X-ray Compton scattering study on aqueous Trimethylamine N-oxide (TMAO) and guanidine hydrochloride solutions (GdnHCl) as a function of temperature. Independent from the concentration of the solvent, Compton profiles almost resemble results for liquid water as a function of temperature. However, The number of hydrogen bonds per water molecule extracted from the Compton profiles suggests a decrease of hydrogen bonds with rising temperatures for all studied samples, the differences between water and the solutions are weak. Nevertheless, the data indicate a reduced bond weakening with rising TMAO concentration up to 5M of 7.2% compared to 8 % for pure water. In contrast, the addition of GdnHCl appears to behave differently for concentrations up to 3.1 M with a weaker impact on the temperature response of the hydrogen bond structure.

Proceedings of the National Academy of Sciences of the United States of America 114(31), 8193 – 8198 (2017). doi:10.1073/pnas.1705303114, Water exists in high- and low-density amorphous ice forms (HDA and LDA), which could correspond to the glassy states of high- (HDL) and low-density liquid (LDL) in the metastable part of the phase diagram. However, the nature of both the glass transition and the high-to-low-density transition are debated and new experimental evidence is needed. Here we combine wide-angle X-ray scattering (WAXS) with X-ray photon-correlation spectroscopy (XPCS) in the small-angle X-ray scattering (SAXS) geometry to probe both the structural and dynamical properties during the high-to-low-density transition in amorphous ice at 1 bar. By analyzing the structure factor and the radial distribution function, the coexistence of two structurally distinct domains is observed at T = 125 K. XPCS probes the dynamics in momentum space, which in the SAXS geometry reflects structural relaxation on the nanometer length scale. The dynamics of HDA are characterized by a slow component with a large time constant, arising from viscoelastic relaxation and stress release from nanometer-sized heterogeneities. Above 110 K a faster, strongly temperature-dependent component appears, with momentum transfer dependence pointing toward nanoscale diffusion. This dynamical component slows down after transition into the low-density form at 130 K, but remains diffusive. The diffusive character of both the high- and low-density forms is discussed among different interpretations and the results are most consistent with the hypothesis of a liquid–liquid transition in the ultraviscous regime., Published by PNAS, Washington, DC

Journal of applied crystallography 49(6), 2046 - 2052(2016). doi:10.1107/S1600576716017313, The results of an X-ray cross-correlation analysis (XCCA) study on hard-spherecolloidal crystals and glasses are presented. The article shows that cross-correlation functions can be used to extract structural information beyond thestatic structure factor in such systems. In particular, the powder average can beovercome by accessing the crystals’ unit-cell structure. In this case, the resultssuggest that the crystal is of face-centered cubic type. It is demonstrated thatXCCA is a valuable tool for X-ray crystallography, in particular for studies oncolloidal systems. These are typically characterized by a rather poor crystallinequality due to size polydispersity and limitations in experimental resolutionbecause of the small $q$ values probed. Furthermore, nontrivial correlations areobserved that allow a more detailed insight into crystal structures beyondconventional crystallography, especially to extend knowledge in structureformation processes and phase transitions., Published by Munksgaard, Copenhagen

Hamburg : Deutsches Elektronen-Synchrotron DESY 1 online resource (125 pages) : illustrations (2021)., Published by Deutsches Elektronen-Synchrotron DESY, Hamburg

In this thesis, the structure and dynamics of highly concentrated core-shell nanogel particlescomposed of a silica core and a poly(N-isoproylacrylamide) (PNIPAm) shell suspendedin water are studied. The particles were synthesised with dierent core-to-shell size ratiosand prepared at various concentrations with volume fractions e between approximately0:004 and 2. X-ray photon correlation spectroscopy (XPCS) enables to follow the dynamical,small-angle X-ray scattering (SAXS) the structural changes of the thermo-responsivenanogel particles which undergo a volume phase transition at a so-called lower criticalsolution temperature (LCST) of 32C. All samples were studied in a broad temperaturerange around the LCST with approximately 15C < T < 45C and the main focus is ondensely-packed systems.For a system with e = 0:55, the structural relaxation time c was found to decreasewith increasing temperature. Above the LCST, the structural relaxation time initiallycontinues to decrease. The eect is accompanied by a transition from stretched to compressedexponential behaviour of the intensity autocorrelation functions, which could bedetermined by analysing the intensity autocorrelation functions. Upon further heating asudden slowing down at a critical temperature Tc 37C. In parallel, the q-dependenceof the relaxation time shows an anomalous change from c / q􀀀3 to c / q􀀀1. Theresults reveal a temperature-induced transition from swollen repulsive particles with polymerdriven dynamics at lower temperatures T < Tc towards a colloidal gel composed ofcollapsed attractive particles. The transition of structure and dynamics shows a distinctdependency on the eective particle volume fraction. Below a critical concentration notransition upon heating was found.The de-swelling behaviour of the PNIPam nanogel was inuenced with the additionof Trimethylamine N-oxide (TMAO) to the concentrated particle suspension. With increasingTMAO concentration, a shift of the LCST towards lower temperatures and apreferential hydrophobic state could be determined.With all results a full phase diagram of a silica-PNIPAm core-shell system is presentedas a function of temperature and concentration. It spans from dilute suspensions toan eective volume fraction of more than ninety percent covering two liquid phases, astructural rearrangement phase and a colloidal gel phase.