Ultrafast electron dynamics in thin films of Prussian Blue analogues
Hedley, Luke Benjamin
The potentially complex energy redistribution processes that occur in inorganic transition metal complexes make the interpretation of their relaxation dynamics a significant challenge. Unlike in organic molecules, where rates for processes such as intersystem crossing can take up to and in excess of many nanoseconds, the timescales involved in inorganic materials are of picosecond / femtosecond timescales and therefore require a more advanced experimental setup to study them. The dynamic processes that are of interest are the evolution of the electron population after excitation through external stimuli, via the various charge transfer and decay processes. These processes can also induce changes in the population of the valence orbitals on the metal ion centres causing a change to the overall spin state. This work aims to develop techniques to monitor the redistribution of the electron population along with the magnetisation of the materials of interest. The dynamic processes are much too fast to be captured using standard spectrometers and magnetic techniques, so a technique which can operate on these very fast timescales is required. Ultrafast laser spectroscopy allows study of the electronic dynamics using a technique called transient transmission which involves studying changes in the transmission spectrum as a function of time. Two laser pulses are employed, one to perturb the sample and another to interrogate the sample. By varying the time delay between the two pulses a picture of how the spectrum changes over time can be constructed. This process allows a picture to be built, of how the electronic population redistributes and decays after excitation. In order to study the magnetisation dynamics of such materials the samples are required to exhibit a magnetic signal. To this end, a magnetically ordered family of inorganic compounds known as Prussian Blues were chosen as the system of interest. These materials consist of transition metal ions linked through cyanide bridging ligands in a rock-salt type structure. Laser spectroscopy was used to monitor the changes in the magnetisation of the sample through a technique called magneto-optical Faraday rotation. This involves monitoring the polarisation of the laser pulses after interaction with the sample, again, at various time delays to measure how the magnetisation of the sample changes over time. These techniques were applied to three chromium based Prussian Blues, vanadium-chromium (VCr), iron-chromium (FeCr) and the chromium-chromium (CrCr) analogues. Through the systematic substitution of the metal ion adjacent to the Cr sites, it was found that the rate of intersystem crossing could be influenced by the nature of the metal center. In the case of both VCr and FeCr analogues, the transfer of population to the final excited state occurred incredibly fast, within the temporal profile of the pump pulse. However, in the case of the CrCr analogue, this population transfer was slowed down suffciently that a growth of the final excited state was observed over the course of the initial 0.5 ps after excitation. The synthesis of these materials was carried out to optimise the morphology of the thin films for use in the laser measurements. During this work it was found that some of these materials exhibit electrochromic activity which was explored in isolated films and as part of multilayered heterostructures. This work was also incredibly helpful with understanding and predicting the spectral signatures of redox processes involved after photo-excitation. This line of research offers the potential to gain a deeper understanding of the dynamic processes occurring in these functional materials which will serve as model systems. The information gained can then be applied to a broader range of complexes. Functional materials which possess much higher magnetic ordering temperatures or larger magnetisation would have a greater potential to be used in future practical applications.