Abstract
Spin crossover complexes are among the most studied classes of molecular switches and have attracted considerable attention for their potential technological use as active units in multifunctional devices. A fundamental step toward their practical implementation is the integration in macroscopic devices adopting hybrid vertical architectures. First, the physical properties of technological interest shown by these materials in the bulk phase have to be retained once they are deposited on a solid surface. Herein, we describe the study of a hybrid molecular inorganic junction embedding the spin crossover complex [Fe(qnal)2] (qnal = quinoline-naphthaldehyde) as an active switchable thin film sandwiched within energy-optimized metallic electrodes. In these junctions, developed and characterized with the support of state of the art techniques including synchrotron Mössbauer source (SMS) spectroscopy and focused-ion beam scanning transmission electron microscopy, we observed that the spin state conversion of the Fe(II)-based spin crossover film is associated with a transition from a space charge-limited current (SCLC) transport mechanism with shallow traps to a SCLC mechanism characterized by the presence of an exponential distribution of traps concomitant with the spin transition temperature.
Highlights
Molecule-based materials represent a good alternative to conventional inorganic semiconductor materials for the development of innovative devices as a consequence of their rich tunability of the molecular properties.[1−3] the processability and the low weight of organic materials permit the production of flexible devices,[4,5] a key aspect for nextgeneration devices
Spin crossover (SCO) complexes are molecular compounds able to reversibly switch their physical properties upon application of external stimuli. This is associated with a switch of the spin state between two magnetic states of a coordinated metal ion. These inorganic complexes have been proposed as active materials in functional devices with reversible magnetic and electrical response.[6−14] To the best of our knowledge, only a few reported studies deal with the integration of SCO materials in electrical and electromechanical devices,[6,15] since the preparation of high-quality thin films and electronically optimized hybrid architectures is critical
We report the development of an Ag// [Fe(qnal)2]//LiF//Au multilayer device to study the mechanism which determines the current flowing across these junctions as a function of temperature, with the support of gasphase DFT calculations, electrochemistry, magnetization measurements, synchrotron Mössbauer source (SMS) spectroscopy, and focused ion beam scanning transmission electron microscopy (FIB-STEM)
Summary
Molecule-based materials represent a good alternative to conventional inorganic semiconductor materials for the development of innovative devices as a consequence of their rich tunability of the molecular properties.[1−3] the processability and the low weight of organic materials permit the production of flexible devices,[4,5] a key aspect for nextgeneration devices. Spin crossover (SCO) complexes are molecular compounds able to reversibly switch their physical properties upon application of external stimuli (temperature, light irradiation, applied magnetic and electric fields, pressure, etc.) This is associated with a switch of the spin state between two magnetic states (low spin, LS, and high spin, HS) of a coordinated metal ion. Entering in the domain of spintronics, interfacial phenomena play a crucial role In this context, it is important to increase the general knowledge on devices based on thinner molecular layers.[16] Some of us have previously studied[12,13] the transport properties of vertical junctions incorporating very thin films of SCO complexes formulated as [Fe(HB(trz)3)2] (HB(trz)3 = tris(1H-1,2,4-triazol-1-yl)borohydride) and [Fe(H2B(pz)2)2(phen)] (H2B(pz)2 = bis(pyrazol-1-yl)borohydride, phen = 1,10-phenanthroline), demonstrating that different transport mechanisms can occur. Analysis of the current density dependence on applied voltage reveals that the electric transport is described by a transition from space charge-limited current (SCLC) with shallow traps[21] at low temperature to SCLC with an exponential trap distribution[21] at high temperature
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