Browsing by Author "Smit, Albert Bart"

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    Investigating the photo-induced ultrafast insulator-metal phase transition in organic Cu(DCNQI)2 salts by ultrafast electron diffraction
    (Stellenbosch : Stellenbosch University, 2018-03) Smit, Albert Bart; Schwoerer, Heinrich; Muller-Nedebock, Kristian; Stellenbosch University. Faculty of Science. Dept. of Physics.
    ENGLISH SUMMARY: This work presents the structural dynamics of the organic crystal Cu(DCNQI)2 (DCNQI: dicyanoquinonediimine) as it undergoes a photo-induced insulator-to-metal (IM) transition. Cu(DCNQI)2, a radical ion salt, is a special case of a large p orbital system, which is responsible for anisotropic metal-like conductivity. Such organic molecular solids with delocalised p electrons are materials of interest due to their dynamic, optical, electrical, magnetic, and electro-optical properties; many have already found applications in, for example, organic light emitting diodes (OLEDs), organic field-effect transistors (OFETs), and as future light harvesting materials. In case of Cu(DCNQI)2, an impressive discontinuous collapse of the onedimensional conductive phase can occur upon cooling. This is due to a first-order Peierls transition in the presence of strong electron-phonon coupling, which is associated with the trimerisation of crystal layers along the conductive c-axis in the (microscopic) lattice structure. We demonstrate that this I-M transition, which is highly tunable by chemical alteration of the molecules’ ligands, can be photoswitched within a millionth of a millionth of a second. This makes the material suitable for applications in high-speed optical sensors with outstanding signal response. We monitor the ultrafast molecular motions responsible for the sub-picosecond lifting of the trimerisation (and therefore the destruction of the insulating phase) in 50 nm thick Cu(Me,Br-DCNQI)2 single crystals using ultrafast electron diffraction (UED). To capture all lattice dynamics, ultrashort electron probe pulses (t 1 ps) generated from a 30 kV DC gun are employed to obtain electron diffraction snapshots for different delay times with respect to the ultrashort laser pump pulses (t 150 fs, l = 620 nm), which initiate the transition. To extract meaningful real-space structural information from our UED data, the effect of small alterations of the known crystalline structure on the electron diffraction patterns are simulated. By comparing these calculations with the dynamics of experimental diffraction signals, the translational movement of the cyano groups was found responsible for the initiation of the I-M phase transition. This translation is unstable, and the insulating phase is restored with a relaxation time of about 6.5 ps. However, when photoexcited close to the M-I phase boundary, an additional translation of the methyl and bromine groups – away from the aromatic ring – is observed. This increase in ligand bulkiness, which causes an internal pressure relief, optically locks the metallic state for timescales greater than 100 ps. We thereby show that an ultrafast, photo-induced, effective internal pressure decrease is required to fully photoswitch and optically lock the metallic conductivity properties. These observations disclose the distinct pathways that ultrafast molecular motions in Cu(DCNQI)2 follow during the I-M transition.
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    A new femtosecond electron diffractometer for structural dynamics experiments at cryogenic temperatures
    (Stellenbosch : Stellenbosch University, 2014-12) Smit, Albert Bart; Schwoerer, Heinrich; Stellenbosch University. Faculty of Science. Dept. of Physics.
    ENGLISH ABSTRACT: In this thesis, a femtosecond electron diffraction (FED) set-up that is capable of investigating the photo-induced switching of Cu(DCNQI)2 from being an insulator to being a conductor is presented. Movies of atomic structural changes with temporal resolution within the typical photo-switching transition timescales (sub-picoseconds) are obtainable with this set-up by employing a femtosecond laser. The experimental technique and the design of a crucial instrument of the machine, the electron gun, are extensively described and characterised both numerically and experimentally. The interest in observing atomic structural changes of Cu(DCNQI)2 in real time is because of the rich variety of the radical salts available that show alloy-specific Charge Density Wave (CDW) transitions. Valuable insights about the driving mechanisms behind these structural changes that are responsible for a change in conductivity are obtainable, as well as the relation between crystal alloys and their transition characteristics. Electron diffraction patterns of crystals in their metallic phase (room temperature) are shown in this thesis, but diffraction patterns of cryo-cooled Cu(DCNQI)2 in its insulating phase are still to be acquired. The temporal resolution of the atomic movie can be improved by recompression of electron pulses that are debunched due to Coulomb repulsion and electron energy spread within a pulse. Numerical and preliminary experimental results presented in this work expose the potential of a simple compression technique. In this way, more electrons in a single electron pulse can be afforded which allows to perform experiments at shorter integration time or lower repetition rate.