Picosecond X-ray Diffraction Studies of Bulk and Nanostructure Materials
Sammanfattning: Fast phenomena occurring after laser excitation were studied using time-resolved X-ray diffraction (TXRD). In most experiments, a femtosecond laser pulse was used to excite the sample, and X-rays were used as a probe. The X-ray diffraction technique was used to study bulk semiconductor samples, molten liquids, ferro-electric domain switching in potassium dihydrogen phosphate (KDP), and strain propagation in graphite and semiconductor nanowires. When a laser pulse is absorbed by a solid, a wide range of phase transitions and phenomena can be induced. If the laser fluence is high enough to melt the material, repetitive illumination will create periodic structures on the surface of the sample. This effect was studied using static X-ray diffraction, and it was shown that the effect is important if liquid scattering experiments are carried out on molten samples using the laser in repetitive mode. When the laser fluence is too low to cause sample melting, coherent acoustic phonons can be excited, and this effect was studied in semiconductor nanowires. The time resolution of the synchrotron light source is defined by the length of the electron bunch in the storage ring, and is typically 50-300 ps. In order to achieve higher time resolution, short X-ray pulses, such as those at the SLS, or fast detectors, such as the streak cameras available at MAX-lab can be used. X-ray diffraction is a very sensitive technique for the study of structures, since X-ray photons scatter from all the electrons in the sample. Scattered X-rays can be used to recreate the atomic structure in the sample. In TXRD the sample is perturbed and subsequently probed after a certain delay, giving a snapshot of the structure at a given time. Several images can be merged providing a real-time movie of the structural changes. This was achieved with nanosecond time resolution at MAX-lab, when a laser-created liquid was studied. The development of a sub-picosecond, hard X-ray streak camera was one of the prerequisites for many of the studies presented in this thesis. This detector was used to study the acoustic vibrations in InSb nanowires. Oscillations with a period of 30-70 ps were recorded, and were attributed to acoustic phonons in the semiconductor nanowire. A dramatic decrease in the velocity of acoustic waves was also observed in these structures.
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