Excited Spin-State Trapping in Spin Crossover Complexes on Ferroelectric Substrates

Abstract

We have studied thin films of Fe(II) spin crossover complexes deposited on differently poled ferroelectric PMN-PT [Pb(Mg 1/3 Nb 2/3 )O 3 ] 1-x [PbTiO 3 ] x , x = 0.32) substrates by X-ray absorption spectroscopy (XAS). The X-ray spectra reveal complete temperature driven conversion between high-spin and low-spin states without any observable effect of the ferroelectric polarization on the spin state of the molecules down to 100 K. In the soft X-ray induced excited spin-state trapping (SOXIESST) regime at 3 K large differences occur between the two ferroelectric polarizations. The efficiency of the X-rays in promoting the molecules to the high-spin state is more than an order of magnitude larger when the ferroelectric dipoles of the substrate are pointing towards the surface as compared to the opposite polarization. We explain our findings by a modulation of the polarization-dependent efficiency of the scattering of X-ray generated secondary electrons at the molecules. Our results provide deep insight into the SOXIESST mechanism and they suggest that such molecules could be used as detectors for electrons traveling in the substrate at energies lower than the substrate electron affinity.

Influence of Substrate Electronic Properties on the Integrity and Functionality of an Adsorbed Fe(II) Spin-Crossover Compound

“…Because the spin-transition temperature of Fe-pypyr is relatively high (T 1/2 = 390 K for powder samples), mostly LS-to-HS transitions induced by 532 nm laser light for temperatures below 100 K are considered throughout the manuscript. XAS at the Fe L 3,2 absorption edges is particularly suitable to probe the spin state averaged over an ensemble of molecules 12,16,17,19,20,[22][23][24][25]30,[32][33][34][40][41][42][43][44] . Indeed, the fine structure and energy of the Fe L 3,2 edges sensitively depend on the occupation and the ligand field of the Fe d orbitals.…”

“…The conformation and spin-state bistability of spincrossover (SCO) compounds make them attractive for applications [1][2][3][4][5][6] . In that respect, considerable efforts were made to realize thin films of SCO complexes using physical vapor deposition [2][3][4][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] . These films usually exhibit decent SCO properties.…”

Spin crossover in the Prussian blue analogue FePt(CN)6induced by pressure or X-ray irradiation

“…[50][51][52][53] Irreversible spin changes induced by radiation are less documented, but could involve local structural and/or chemical changes in addition to the spin transition. 50,54 In our case, measurements upon decompression would help ascertain the reversibility of the spin change observed from radiation. Given previous experiments that indicate the sensitivity of PBAs to X-ray radiation, e.g.…”

Soft X-ray Absorption Spectroscopy Study of Spin Crossover Fe-Compounds: Persistent High Spin Configurations under Soft X-ray Irradiation

“…To attain a HS configuration, each of the two electrons should overcome the ligand field splitting energies (10 Dq), but at the same time, obtain the energies of Hund exchanges with the other four electrons (−4 J), so that the energy difference between the HS and LS should be ∆E = 20 Dq − 8 J. It has been shown that the ligand field strength (10 Dq) can significantly influence the population ratio [26,27]. Generally, as bond lengths of the metal ions to the ligands are longer, the HS configuration can be favored more because of the lower 10 Dq.…”

“…Such soft X-ray-induced excited spin state trapping (SOXIESST) effects can lead to persistent HS configuration even below T C [21,22]. The SOXIESST effect has been regarded as a result of temporary trapping in the HS due to the cascade of excitations by illumination, similar to the case of light-induced excited spin state trapping (LIESST) [26,27]. For instance, the SOXIESST effect was described in detail on Fe(phen) 2 (NCS) 2 and Fe{[Me 2 Pyrz] 3 BH} 2 by V. Davesne et al [21,36].…”