Introducing organic molecular crystals into ultrafast electron diffraction

[摘要] ENGLISH ABSTRACT: Organic molecular salts have a wide range of physical properties which can be chemically tailoredby minor variations of their substituents. These characteristics include high degrees of anisotropy,electrical conductivity ranging from superconducting to insulating, and structural changes in thecrystal 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 interestingfor 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 theradical anion salt subclass of organic molecular crystals and is of particular interest due to its extraordinarilyhigh conductivity compared to other quasi one-dimensional organic conductors. Itsmetal-to-insulator phase transition is characterised by conductivity jumps across several orders ofmagnitude 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 spectrumof methods amongst others electrical conductivity, electron spin resonance, and reectivity measurements,x-ray photoelectron and infrared spectroscopy, x-ray diffraction, and dilatometry. Fastlight-switching between phases has been observed in partially deuterated forms of Cu(DCNQI)2 onsub-100-ps time scales. Furthermore, the phase transition is believed to be induced by a deformationof the crystalline lattice and a charge density wave formation which are detectable in diffractionimages. Therefore we want to investigate this metal-to-insulator phase transition structurally andtemporally via ultrafast electron diffraction. The technique of ultrafast electron diffraction employsthe fundamentals of pump-probe spectroscopy: One of the two femtosecond pulsed laser beamsexcites the thin, crystalline sample, while the other - after being converted into a pulsed electronbeam via the photoelectric effect - forms a diffraction image of the sample's lattice structure. Thearrival time of the two pulses at the sample can be varied by a few femtoseconds with respect toeach 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 characterisationleading to a successful introduction of Cu(DCNQI)2 into our ultrafast electron diffractionsetup is presented. A diffraction pattern of comparable quality to that of a commercially availabletransmission electron microscope was recorded of the metallic state of partially deuterated d6Cu(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 factorsstill making the diffraction patternevasive. Possible solutions to experimental challenges are proposed to make the documentation ofstructural ultrafast dynamics in these organic molecular salts an attainable goal in the future.

[发布机构] Stellenbosch University