Study of electrical properties of bistable molecular materials : towards nanoelectronic devices

Abstract : The central theme of this thesis is the evaluation of potential interest and applicability of molecular spin crossover (SCO) complexes for nanoelectronic applications. The electrical properties of the [Fe(Htrz)2(trz)](BF4) complex and its Zn substituted analogues were analyzed first in the bulk powder form using broadband dielectric spectroscopy. It has been shown that the ac and dc conductivities as well as the dielectric constant and the dielectric relaxation frequency exhibit an important drop when going from the low spin (LS) to the high spin (HS) state. The iron ions kept their spin transition properties in the Zn diluted samples, but the SCO curves were significantly altered. The Zn substitution of active iron centers led to an important decrease of the electrical conductivity of ca. 6 orders of magnitude (for Zn/Fe = 0.75). We concluded from these results that the ferrous ions directly participate to the charge transport process, which was analyzed in the frame of an activated hopping conductivity model. Micrometric particles of [Fe(Htrz)2(trz)](BF4) were then integrated by dielectrophoresis between interdigitated gold electrodes leading to a device exhibiting bistability in the I-V,T characteristics. The stability of the starting material and the electronic device were carefully controlled and the concomitant effect of temperature changes, light irradiation and voltage bias on the current intensity were analyzed in detail. We showed that the device can be preferentially addressed by light stimulation according to its spin state and the switching from the metastable HS to the stable LS state was also demonstrated by applying an electric field step inside the hysteresis loop. The field effects were discussed in the frame of static and dynamic Ising-like models, while the photo-induced phenomena were tentatively attributed to surface phenomena. The [Fe(bpz)2(phen)] complex was also investigated by dielectric spectroscopy in the bulk powder form and then integrated by high vacuum thermal evaporation into a large-area vertical device with Al (top) and ITO (bottom) electrodes. This approach allowed us to probe the spin-state switching in the SCO layer by optical means while detecting the associated resistance changes both in the tunneling (10 nm junction) and injection-limited (30 and 100 nm junctions) regimes. The tunneling current in the thin SCO junctions showed a drop when going from the LS to the HS state, while the rectifying behavior of the 'thick' junctions did not reveal any significant spin-state dependence. The ensemble of these results provides guidance with new perspectives for the construction of electronic and spintronic devices incorporating SCO molecular materials.
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Constantin Lefter. Study of electrical properties of bistable molecular materials : towards nanoelectronic devices. Mechanics of materials [physics.class-ph]. Université Paul Sabatier - Toulouse III, 2016. English. ⟨NNT : 2016TOU30003⟩. ⟨tel-01426619⟩

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