Nonvolatile voltage controlled molecular spin state switching

Abstract

Voltage-controlled room temperature isothermal reversible spin crossover switching of [Fe{H 2 B(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 (C 5 H 2 O 5), appears to lock the spin crossover [Fe{H 2 B(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{H 2 B(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.

Probing the unpaired Fe spins across the spin crossover of a coordination polymer

“…This corresponds in the XAS spectra at the Fe L 3 (2p 3/2 ) edge to a decrease of the peak intensity around 710 to 711 eV and an increase of the corresponding t 2g shoulder at around 707 to 708 eV and a peak at around 709 eV (Figure 1b, red). This is very similar to what is observed across the spin transition in [Fe(H 2 B(pz) 2 ) 2 (2,2'-bipy)] (where H 2 B(pz) 2 = bis(hydrido)-bis- 2,2'-bipy = 2,2'-bipyridine), [25][26][27][29][30][31][32][33] [Fe(phen) 2 (NCS) 2 ] (where phen = 1,10-phenanthroline), [34][35][36][37] [Fe(H 2 B(pz) 2 ) 2 (phen)] 38 (where H 2 B(pz) 2 = bis(hydrido)-bis-(1Hpyrazol-1-yl)borate, and phen = a diarylethene-functionalized phenanthroline ligand) and [Fe( 39 particularly when the fact that the XAS spectra taken here are taken at a very rapid scan speed and thus lower resolution. As has been done elsewhere, 25,26,[30][31][32][34][35][36][37][38][39] the XAS spectra of the low spin state (Figure 1b, blue) and the high spin state ( Figure 1b, red), are good "fingerprints" of the spin state and can be used to infer the percentage of high spin and low spin state occupancy at the temperatures where the spin state occupancy is between clearly high spin or low spin.…”

“…The total electron yield mode was used to measure the absorption across the Fe 2p 3/2 (L 3 ) edge, as was done in previous studies. 25,[30][31][32][33][34][35][36][37][38][39] The individual X-ray absorption data were acquired rapidly, so as to avoid sample degradation affected by the measurement process. We acquired an L 3 -edge spectrum less than a minute.…”

“…This system exhibits a wide thermal hysteresis in the spin transition, above room temperature. 18 While X-ray absorption spectroscopy (XAS) is frequently used to probe changes in electronic structure, 17,[25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40] X-ray core level photoemission at the Fe 2p core has also been used to probe the molecular spin crossover, 27,[41][42][43][44][45] but only in a limited manner. So far, data from X-ray photoemission spectroscopy (XPS) has not been adequate to make a detailed comparison with magnetometry or XAS, nor has the XPS data been sufficient to abstract a critical spin crossover transition temperature.…”

Tunable spin-state bistability in a spin crossover molecular complex

“…As we previously showed, that for the ferroelectric polyvinylidene fluoride (PVDF), the interface interactions favor the HS state in [Fe{H2B(pz)2}2(bipy)] when the ferroelectric is poled towards the [Fe{H2B(pz)2}2(bipy)] molecular film. 45,46 This interface with PVDF, unlike Al2O3, should favor the HS state in the bulk of the film, and the LS state at the surface, where coordination is low. As shown in Figure 8, the X-ray absorption spectra, taken in the PLY mode and in the TEY mode for 20 molecular layers of [Fe{H2B(pz)2}2(bipy)] thin film on PVDF, taken at 150 K, indicate the presence of more HS state occupancy in the interior of the film (PLY) than at the surface (TEY).…”

Spin-Crossover Complexes in Direct Contact with Surfaces

“…Spin-crossover (SCO) complexes can be switched between two states characterized by different electronic, optical, magnetic and geometric properties [1,2] making these complexes attractive for a wide range of applications [3][4][5][6], including smart pigments, actuators [7,8], and data storage [9][10][11][12][13][14]. In turn, these compounds also hold great promise as sensors [15][16][17][18][19][20] since the switching may be induced by various parameters such as temperature, light, pressure and current and to a certain extent magnetic field [21].…”