Structure-transport correlation reveals anisotropic charge transport in coupled PbS nanocrystal superlattices

Semiconductive nanocrystals (NCs) can be self-assembled into ordered superlattices (SLs) to create artificial solids with emerging collective properties. Computational studies have predicted that properties such as electronic coupling or charge transport are determined not only by the individual NCs but also by the degree of their organization and structure. However, experimental proof for a correlation between structure and charge transport in NC SLs is still pending. Here, we perform X-ray nano-diffraction and apply Angular X-ray Cross-Correlation Analysis (AXCCA) to characterize the structures of coupled PbS NC SLs, which are directly correlated with the electronic properties of the same SL microdomains. We find strong evidence for the effect of SL crystallinity on charge transport and reveal anisotropic charge transport in highly ordered monocrystalline hexagonal close-packed PbS NC SLs, caused by the dominant effect of shortest interparticle distance. This implies that transport anisotropy should be a general feature of weakly coupled NC SLs.Comment: 49 pages, 20 Figure

Mesoscopic materials studied with advanced X-ray scattering methods

Mesoscopic materials form the bridge between the quantum world of atomic systems and the classical world of macroscopic systems. The characteristic length scale of such structures is in the range of hundreds of nanometers which is comparable to visible light wavelengths. This proximity often leads to unusual optical and conducting properties that are not observed in the macroscopic systems. The possibility of using such unique materials in electronics, photonics, and nanotechnology motivates the growing scientific interest to mesoscopic physics.The physical, mechanical and chemical properties of the mesoscopic materials originate in their nanostructure. In-situ probing of the structure and dynamics of mesoscale systems is a challenging experimental problem. Due to short wavelength and high penetration depth X-rays offer a great opportunity for structural studies of mesoscopic objects. New generation of X-ray sources such as synchrotrons or free-electron lasers offers a variety of powerful tools such as X-ray nanodiffraction, grazing-incidence small-angle X-ray scattering, angular X-ray cross-correlation analysis and time-resolved X-ray diffraction. The present Thesis is focused on applying these methods to mesoscopic systems and includes three separate projects.In the first project, X-ray nanodiffraction is used to study domains and domain boundaries in mesocrystalline superlattices of PbS nanocrystals. This method was complemented with novel angular X-ray cross-correlation analysis which unraveled the orientational order inside the domains and near the domain boundaries. The second project focuses on the structural evolution of the polystyrene colloidal crystals under dry sintering conditions studied using in-situ grazing incidence X-ray scattering. The detailed analysis of the Bragg peaks from the colloidal crystal allowed to reveal the colloidal particle shape transformation during heating of the sample. These two experiments were performed at P10 beamline at PETRA III synchrotron radiationsource.The third project is devoted to the studies of the infrared laser-induced dynamics in the colloidal crystal using a pump-probe setup at an X-ray free-electron laser. Colloidal crystals were pumped with infrared laser of varying power and the subsequent dynamics was measured with picosecond time resolution. Depending on the pumplaser intensity two regimes of laser-matter interaction were accessed. For low pump laser intensity, the vibrational modes excited in the colloidal crystal were analyzed using Lamb theory. Higher pump laser intensities resulted in the generation of a unique periodic plasma in the sample. Combined simulation of the femtosecond plasma dynamics and a hydrodynamic shock wave were performed to explain the experimental data

Probing Dynamics in Colloidal Crystals with Pump-Probe Experiments at LCLS : Methodology and Analysis

We present results of the studies of dynamics in colloidal crystals performed by pump-probe experiments using an X-ray free-electron laser (XFEL). Colloidal crystals were pumped with an infrared laser at a wavelength of 800 nm with varying power and probed by XFEL pulses at an energy of 8 keV with a time delay up to 1000 ps. The positions of the Bragg peaks, and their radial and azimuthal widths were analyzed as a function of the time delay. The spectral analysis of the data did not reveal significant enhancement of frequencies expected in this experiment. This allowed us to conclude that the amplitude of vibrational modes excited in colloidal crystals was less than the systematic error caused by the noise level.publishe

In situ characterization of crystallization and melting of soft, thermoresponsive microgels by small-angle X-ray scattering

Depending on the volume fraction and interparticle interactions, colloidal suspensions can form different phases, ranging from fluids, crystals, and glasses to gels. For soft microgels that are made from thermoresponsive polymers, the volume fraction can be tuned by temperature, making them excellent systems to experimentally study phase transitions in dense colloidal suspensions. However, investigations of phase transitions at high particle concentration and across the volume phase transition temperature in particular, are challenging due to the deformability and possibility for interpenetration between microgels. Here, we investigate the dense phases of composite core-shell microgels that have a small gold core and a thermoresponsive microgel shell. Employing Ultra Small-Angle X-ray Scattering, we make use of the strong scattering signal from the gold cores with respect to the almost negligible signal from the shells. By changing the temperature we study the freezing and melting transitions of the system in situ. Using Bragg peak analysis and the Williamson-Hall method, we characterize the phase transitions in detail. We show that the system crystallizes into an rhcp structure with different degrees of in-plane and out-of-plane stacking disorder that increase upon particle swelling. We further find that the melting process is distinctly different, where the system separates into two different crystal phases with different melting temperatures and interparticle interactions

Unravelling the structural rearrangement of polymer colloidal crystals under dry sintering conditions

The structural rearrangement of polystyrene colloidal crystals under dry sintering conditions has been revealed by in situ grazing incidence X-ray scattering. The measured diffraction patterns were analysed using distorted wave Born approximation (DWBA) theory and the structural parameters of the as-grown colloidal crystals of three different particle sizes were determined for the in-plane and out-of-plane directions in a film. By analysing the temperature evolution of the diffraction peak positions, integrated intensities, and widths, the detailed scenario of the structural rearrangement of crystalline domains at the nanoscale has been revealed, including thermal expansion, particle shape transformation and crystal amorphisation. Based on DWBA analysis, we demonstrate that in the process of dry sintering, the shape of colloidal particles in a crystal transforms from a sphere to a polyhedron. Our results deepen the understanding of the thermal annealing of polymer colloidal crystals as an efficient route for the design of new nano-materials

Diffraction based Hanbury Brown and Twiss interferometry at a hard x-ray free-electron laser

X-ray free-electron lasers (XFELs) provide extremely bright and highly spatially coherent x-ray radiation with femtosecond pulse duration. Currently, they are widely used in biology and material science. Knowledge of the XFEL statistical properties during an experiment may be vitally important for the accurate interpretation of the results. Here, for the first time, we demonstrate Hanbury Brown and Twiss (HBT) interferometry performed in diffraction mode at an XFEL source. It allowed us to determine the XFEL statistical properties directly from the Bragg peaks originating from colloidal crystals. This approach is different from the traditional one when HBT interferometry is performed in the direct beam without a sample. Our analysis has demonstrated nearly full (80%) global spatial coherence of the XFEL pulses and an average pulse duration on the order of ten femtoseconds for the monochromatized beam, which is significantly shorter than expected from the electron bunch measurements.publishe

In situ characterization of crystallization and melting of soft, thermoresponsive microgels by small-angle X-ray scattering

Depending on the volume fraction and interparticle interactions, colloidal suspensions can form different phases, rangingfrom fluids, crystals, and glasses to gels. For soft microgels that are made from thermoresponsive polymers, the volumefraction can be tuned by temperature, making them excellent systems to experimentally study phase transitions in densecolloidal suspensions. However, investigations of phase transitions at high particle concentration and across the volumephase transition temperature in particular, are challenging due to the deformability and possibility for interpenetrationbetween microgels. Here, we investigate the dense phases of composite core-shell microgels that have a small gold coreand a thermoresponsive microgel shell. Employing Ultra Small Angle X-ray Scattering, we make use of the strong scatteringsignal from the gold cores with respect to the almost negligible signal from the shells. By changing the temperature we studythe freezing and melting transitions of the system in-situ. Using Bragg peak analysis and the Williamson-Hall method, wecharacterize the phase transitions in detail. We show that the system crystallizes into an rhcp structure with differentdegrees of in-plane and out-of-plane stacking disorder that increase upon particle swelling. We further find that the meltingprocess is distinctly different, where the system separates into two different crystal phases with different meltingtemperatures and interparticle interactions

Operando reaction cell for high energy surface sensitive x-ray diffraction and reflectometry

A proof of concept is shown for the design of a high pressure heterogeneous catalysis reaction cell suitable for surface sensitive x-ray diffraction and x-ray reflectometry over planar samples using high energy synchrotron radiation in combination with mass spectrometry. This design enables measurements in a pressure range from several tens to hundreds of bars for surface investigations under realistic industrial conditions in heterogeneous catalysis or gaseous corrosion studies