Introducing organic molecular crystals into ultrafast electron diffraction

Rohwer, Andrea Berenike (2014-12)

Thesis (MSc)--Stellenbosch University, 2014.

Thesis

ENGLISH ABSTRACT: Organic molecular salts have a wide range of physical properties which can be chemically tailored by minor variations of their substituents. These characteristics include high degrees of anisotropy, electrical conductivity ranging from superconducting to insulating, and structural changes in the crystal lattice during first order phase transitions brought about by minimal changes in temperature, effective pressure, and in some cases even light. Hence, these materials are particularly interesting for the development of molecular electronics and also as study materials in solid state physics. The family of copper-dimethyl-dicyanoquinone-diimine (Cu(DMe-DCNQI)2) salts forms part of the radical anion salt subclass of organic molecular crystals and is of particular interest due to its extraordinarily high conductivity compared to other quasi one-dimensional organic conductors. Its metal-to-insulator phase transition is characterised by conductivity jumps across several orders of magnitude within a few kelvin. Over the past three decades the metallic and insulating phases, as well as the transition behaviour have been investigated extensively utilising a broad spectrum of methods amongst others electrical conductivity, electron spin resonance, and re ectivity measurements, x-ray photoelectron and infrared spectroscopy, x-ray diffraction, and dilatometry. Fast light-switching between phases has been observed in partially deuterated forms of Cu(DCNQI)2 on sub-100-ps time scales. Furthermore, the phase transition is believed to be induced by a deformation of the crystalline lattice and a charge density wave formation which are detectable in diffraction images. Therefore we want to investigate this metal-to-insulator phase transition structurally and temporally via ultrafast electron diffraction. The technique of ultrafast electron diffraction employs the fundamentals of pump-probe spectroscopy: One of the two femtosecond pulsed laser beams excites the thin, crystalline sample, while the other - after being converted into a pulsed electron beam via the photoelectric effect - forms a diffraction image of the sample's lattice structure. The arrival time of the two pulses at the sample can be varied by a few femtoseconds with respect to each other enabling the resolution of ultrafast structural dynamics of the crystal's atomic lattice via electron diffraction. During the work presented in this thesis the sample preparation and characterisation leading to a successful introduction of Cu(DCNQI)2 into our ultrafast electron diffraction setup is presented. A diffraction pattern of comparable quality to that of a commercially available transmission electron microscope was recorded of the metallic state of partially deuterated d6 Cu(DCNQI)2. Subsequent analysis of the obtained diffraction data and further studies of the low temperature state { including simulations as well as experiments { have narrowed down the factors still making the diffraction pattern evasive. Possible solutions to experimental challenges are proposed to make the documentation of structural ultrafast dynamics in these organic molecular salts an attainable goal in the future.

AFRIKAANSE OPSOMMING: Organiese molekulêre soute het `n wye verskeidenheid van fisiese eienskappe wat chemies verander kan word deur geringe variasie in die samestelling van die sout. Hierdie eienskappe sluit in `n hoë graad van anisotropie, elektriese geleidingsvermoë wat strek van supergeleiding tot elektriese isolasie, en strukturele veranderinge in die kristalstruktuur tydens eerste orde fase-oorgange wat veroorsaak word deur geringe veranderinge in temperature, effektiewe druk en in sommige gevalle selfs lig. Gevolglik is hierdie material besonder interessant vir die ontwikkeling van molekulêre elektronika en ook as studiemateriaal in vastetoestandfisika. Die familie van koperdimetieldisianokinoondiimien (Cu(DMe-DCNQI)2) soute vorm `n deel van die radikaal-anioon-sout subklas van organiese molekulêre kristalle en is van besondere belang as gevolg van hulle buitengewone hoë elektriese geleidingsvermoë in vergelyking met ander kwasi-eendimensionele organiese geleiers. Die metaal-na-isolator fase-oorgang van hierdie kristal word gekenmerk deur die verandering van die geleidingsvermoë met verskeie ordegroottes binne `n paar kelvin. Gedurende die laaste drie dekades is die metaal en isolator fases, asook die oorgangsgedrag deeglik ondersoek met behulp van `n wye verskeidenheid van metodes wat onder andere elektriese geleidingsvermoë, elektron-spin resonans en reeksiemetings, x-straal fotoelektron en infrarooi spektroskopie, x-straal diffraksie en dilatometrie insluit. Vinnige skakeling tussen fases is waargeneem in gedeeltelik gedeuteerde vorms van Cu(DCNQI)2 op `n sub-100-ps tydskaal. Daar word verder geglo dat die fase-oorgang geïnduseer word deur `n deformasie van die kristalstruktuur en die vorming van `n ladingsdigtheidgolf wat meetbaar is in elektrondiffraksiebeelde. Om hierdie rede wil ons die metaal-na-isolator fase-oorgang se struktuur- en tydafhanklikheid ondersoek deur gebruik te maak van ultra-vinnige elektron diffraksie. Die tegniek van ultra-vinnige elektron diffraksie maak gebruik van die beginsels van pomp-toets spektroskopie: Een van die twee femtosekonde laserpulse wek die dun kristallyne monster op, terwyl die ander na omskakeling in `n elektronpuls via die foto-elektriese effek `n diffraksiebeeld van die monster se kristalstruktuur vorm. Die aankomtyd van die twee pulse by die monster kan met `n paar femtosekondes ten opsigte van mekaar verander word om die tydresolusie van die ultra-vinnige strukturele dinamika van die kristal se atoomstruktuur deur elektrondiffraksie moontlik te maak. In hierdie tesis word die monstervoorbereiding en karakterisering wat gelei het tot suksesvolle eksperimente op Cu(DCNQI)2 in ons ultra-vinnige elektron diffraksie opstelling behandel. `n Diffraksie patroon waarvan die kwaliteit vergelykbaar is met die van `n kommersiëel beskikbare transmissie elektron mikroskoop is gemeet vir die metaalfase van gedeeltelik gedeuteerde d6 Cu(DCNQI)2. Daaropvolgende analiese van die gemete diffraksiedata en verdere studies van die lae temperatuur toestand wat simulasies sowel as eksperimente insluit het `n klein aantal faktore uitgewys wat steeds die deteksie van die isolatorfase se ladingsdigtheidgolf se kenmerkende diffraksiepatroon verhoed. Moontlike oplossings tot eksperimentele uitdagings word voorgestel om die dokumentering van strukturele ultra-vinnige dinamika in hierdie organiese molekulêre soute `n haalbare toekomstige doelwit te maak.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/95819
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