Voltage-controlled room temperature isothermal reversible spin crossover switching of [Fe{H₂B(pz)₂}₂(bipy)] thin films is demonstrated. This isothermal switching is evident in thin film bilayer structures where the molecular spin crossover film is adjacent to a molecular ferroelectric. The adjacent molecular ferroelectric, either polyvinylidene fluoride hexafluoropropylene or croconic acid (C₅H₂O₅), appears to lock the spin crossover [Fe{H₂B(pz)₂}₂(bipy)] molecular complex largely in the low or high spin state depending on the direction of ferroelectric polarization. In both a planar two terminal diode structure and a transistor structure, the voltage controlled isothermal reversible spin crossover switching of [Fe{H₂B(pz)₂}₂(bipy)] is accompanied by a resistance change and is seen to be nonvolatile, i.e., retained in the absence of an applied electric field. The result appears general, as the voltage controlled nonvolatile switching can be made to work with two different molecular ferroelectrics: croconic acid and polyvinylidene fluoride hexafluoropropylene.

Nonvolatile, molecular multiferroic devices have now been demonstrated, but it is worth giving some consideration to the issue of whether such devices could be a competitive alternative for solid-state nonvolatile memory. For the Fe (II) spin crossover complex [Fe{H₂B(pz)₂}₂(bipy)], where pz = tris(pyrazol-1-yl)-borohydride and bipy = 2,2′-bipyridine, voltage-controlled isothermal changes in the electronic structure and spin state have been demonstrated and are accompanied by changes in conductance. Higher conductance is seen with [Fe{H₂B(pz)₂}₂(bipy)] in the high spin state, while lower conductance occurs for the low spin state. Plausibly, there is the potential here for low-cost molecular solid-state memory because the essential molecular thin films are easily fabricated. However, successful device fabrication does not mean a device that has a practical value. Here, we discuss the progress and challenges yet facing the fabrication of molecular multiferroic devices, which could be considered competitive to silicon.

The interfaces of layered trichalcogenide TiS₃(001), with metals Au and Pt, were examined using X-ray photoemission spectroscopy. In spite of the fact that both Au and Pt are large work function metals, no evidence of Schottky barrier formation was found with this n-type semiconductor. Two- and four-terminal field-effect transistor measurements performed on exfoliated few-nm-thick TiS₃ crystals using pure Au contacts indicate that Au forms an Ohmic contact on TiS₃(001), with negligible contact resistance. The absence of appreciable Schottky barrier formation is attributed to strong interactions with sulfur at the metal-semiconductor interface.

The spin crossover (SCO) transitions at both the surface and over the entire volume of the [Fe{H ₂ B(pz) ₂ } ₂ (bipy)] polycrystalline films on Al ₂ O ₃ substrates have been studied, where pz = pyrazol-1-yl and bipy = 2,2′-bipyridine. For [Fe{H ₂ B(pz) ₂ } ₂ (bipy)] films of hundreds of nm thick, magnetometry and x-ray absorption spectroscopy measurements show thermal hysteresis in the SCO transition with temperature, although the transition in bulk [Fe{H ₂ B(pz) ₂ } ₂ (bipy)] occurs in a non-hysteretic fashion at 157 K. While the size of the crystallites in those films are similar, the hysteresis becomes more prominent in thinner films, indicating a significant effect of the [Fe{H ₂ B(pz) ₂ } ₂ (bipy)]/Al ₂ O ₃ interface. Bistability of spin states, which can be inferred from the thermal hysteresis, was directly observed using temperature-dependent x-ray diffraction; the crystallites behave as spin-state domains that coexist during the transition. The difference between the spin state of molecules at the surface of the [Fe{H ₂ B(pz) ₂ } ₂ (bipy)] films and that of the molecules within the films, during the thermal cycle, indicates that both cooperative (intermolecular) effects and coordination are implicated in perturbations to the SCO transition.