Browsing by Author "Wang, Junfeng"

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    Synthesis, magnetism and organisation of molecular coordination compounds for molecular spintronics
    (2016) Wang, Junfeng; Dressel, Martin (Prof. Dr.)
    Magnets have fascinated humans for millennia, playing a prominent role in the development of society, science and technology. When approaching the nanoscale, magnets show unconventional properties. As far as the development of molecular nanomagnets (also called Single-Molecule Magnets, SMMs), the interest in lanthanides based SMMs has been boosted since 2003. In view of rational design of SMMs with controllable magnetic exchange interaction, molecular LEGO-like approach allows assembling the magnetic blocks, the shielding blocks and the linker blocks to produce SMMs. Mononuclear and polynuclear dysprosium(III)-based SMMs were prepared from hexafluoroacetylacetonate (hfac) and different linkers using this method, namely Dy(hfac)3bpy (bpy = 2,2-bipyridine), 1; Dy(hfac)3tBu- bpy (tBu-bpy = 4,4-di-tert-butyl-2,2-bipyridine), 2; [Dy(hfac)3(EtOAc)]2bpm (EtOAc = ethyl acetate, bpm = 2,2-bipyrimidine), 3 and [Dy(hfac)3]3HAT (HAT = 1,4,5,8,9,12- hexaazatriphenylene), 4. Only compound 3 exhibits distinct SMM behaviour under zero direct current (DC) field. Quantum tunnelling of the magnetisation (QTM) for all compounds was suppressed by an external field of 4 kOe, and frequency-dependent out-of-phase alternating current (AC) susceptibilities for all were observed, confirming SMMs behaviour. The biggest effective energy barrier to reverse the spin is obtained for 4 with the value of 84 K. In the case of fine-tuning of magnetic properties of SMMs, we studied a series of dinuclear dysprosium(III)-based SMMs by substituting coordinated solvent molecule, sharing the general formula [Dy(hfac)3(R)]2bpm, in which R = nothing (5), water (H2O, 6), ethyl acetate (EtOAc, 3), and methyl propionate (MP, 7). Systematic comparison and analysis indicate that coordinated solvent molecule can influence the local geometry of dysprosium(III) ions. Nevertheless, the magnetic properties of these dimer systems were little influenced by these substitutions and the effective anisotropy barriers for all compounds were slightly increased. Thanks to hexafluoroacetylacetonate group with enhanced volatility, the dinuclear dysprosium(III)-based SMM (complex 3) can be deposited on various surfaces by means of molecular evaporation methodology in high vacuum. Surface technique results provide clear evidence of structural and electronic integrity of the molecules grafted on surfaces. Controllable morphology and thickness of molecules films when deposited on surfaces were achieved and characterised by AFM. Magnetic property measurements prove that the properties of deposited molecules remain unvaried. More specific, AC susceptibility measurements show that the multi-layer system maintains the same magnetic dynamics as that found in bulk samples. The successful deposition of intact SMMs on surfaces represents the first step towards the fabrication of spintronic and optoelectronic devices based on thin films of lanthanide based SMMs. At last, the successful experience of thermally depositing intact complex 3 on surfaces allows grafting SMMs onto graphene. The solubility of complex 3 in organic solvents makes it possible to decorate single-walled carbon nanotubes (SWCNTs) with magnetic molecules. In the case of exfoliated graphene, AFM imaging of surface-decorated graphene reveals that organisation of individual magnetic molecules can be fine designed by controlling the evaporation process. In another case of SWCNTs, combining characterisation of high resolution tunnelling electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and Raman techniques on the hybrids SMMs–SWCNTs clearly shows 3 were grafted onto SWCNTs. Magnetic properties measurements show the paramagnetic response of the hybrid materials compared to the diamagnetic response of pure SWCNTs. All these achievements are helpful for introducing magnetic molecules into carbon nanostructure-based devices in the viewpoint of spintronic devices.