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Investigating the photo-induced ultrafast insulator-metal phase transition in organic Cu(DCNQI)2 salts by ultrafast electron diffraction

Smit, Albert Bart (2018-03)

Thesis (PhD)--Stellenbosch University, 2018.

Thesis

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.

AFRIKAANSE OPSOMMING: Hierdie werk bied die strukturele dinamika van die organiese Cu(DCNQI)2 (DCNQI: dicyanoquinquinediimine) kristal aan, wat ’n foto-ge¨ınduseerde isolator-tot-metaal (IM) oorgang ondergaan. Cu(DCNQI)2, ’n radikale ioon sout, is ’n besonderse voorbeeld van ’n groot p-orbitaal stelsel wat verantwoordelik is vir anisotropiese metaal geleidingsvermo¨e. Sulke molekulˆere vastestowwe met gedelokaliseerde p-elektrone is van belang vanwe¨e hul dinamiese, optiese, elektriese, magnetiese en elektro-optiese eienskappe. Menige het al reeds toepassings gevind in byvoorbeeld organiese liguitstralende diodes (OLEDs), veld-effek transistors (OFETs) en lig-oestende materiale. In die geval van Cu(DCNQI)2, kan ’n indrukwekkende diskontinue ineenstorting van die eendimensionele geleidende fase plaasvind tydens verkoeling. Dit is as gevolg van ’n eerste-orde Peierls oorgang in die teenwoordigheid van sterk elektron-fononkoppeling, wat verband hou met die trimerisering van kristal lae langs die geleidende c-as in die (mikroskopiese) kristalstruktuur. Ons demonstreer dat hierdie I-M-oorgang, hoogs afstelbaar deur chemiese modifikasies van die ligand van die molekule, binne een miljoenste van ’n miljoenste sekonde met lig oorgeskakel kan word. Dit maak die materiaal geskik vir toepassings in ho¨espoed optiese sensors met uitstekende seinrespons. Ons monitor die ultravinnige molekulˆere bewegings wat verantwoordelik is vir die sub-pikosekonde eliminasie van die trimerisasie (en dus die vernietiging van die isolasie fase) in 50 nm dik Cu(Me,Br-DCNQI)2 enkelkristalle met ultravinnige elektron diffraksie (UED). Om al die kristaldinamika op te vang, word ultra kort elektron pulse (t = 1 ps) – wat gegenereer is in ’n 30 kV DC elektron-geweer – gebruik om elektrondiffraksie patrone te versamel vir verskillende vertragingstye relatief tot die ultra kort laserpomppulse (t = 150 fs, l = 620 nm), wat die oorgang begin. Om betekenisvolle strukturele inligting uit ons UED-data te versamel, is die effek van klein veranderinge aan die bekende kristalstruktuur op die elektrondiffraksiepatrone gesimuleer. Deur hierdie berekeninge te vergelyk met die dinamika van eksperimentele diffraksie seine, is die translasie beweging van cyano groepe verantwoordelik gevind vir die inwerkingtreding van die I-M-fase-oorgang. Hierdie translasie is onstabiel en die isolerende fase word herstel met ’n ontspanningstyd van ongeveer 6.5 ps, maar wanneer die oorgang naby die M-I-fase grens ge-fotostimuleer word, word ’n addisionele translasie van die metiel- en broomgroepe – weg van die aromatiese ring – waargeneem. Hierdie toename in ligandgrootte, wat ’n interne drukvermindering veroorsaak, omsluit die metaalfase opties vir tydskale groter as 100 ps. Dit toon aan dat ’n ultravinnige, foto-ge¨ınduseerde, effektiewe interne drukvermindering nodig is om die metaalgeleidings-eienskappe heeltemal te verander en opties te sluit. Hierdie waarnemings openbaar die unieke paaie wat die ultravinnige molekulˆere bewegings in Cu(DCNQI)2 volg tydens die I-M-oorgang.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/103598
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