X-Ray Investigation of Ultra-Thin Spin-Crossover Molecular Films

Abstract

This thesis concerns with the investigation of two closely-related spin-crossover molecules (SCMs)—[Fe(H2B(pz)2)bipy] (pz = pyrazole; bipy = 2,2’-bipyridine) and [Fe(H2B(pz)2)phen] (phen = 1,10-phenanthroline)—and their derivatives in submonolayers and in ultra-thin films deposited mainly on a highly-oriented graphite (HOPG) substrate (apart from Au(111) and Bi(111) substrates) with the aim of gaining insight into the fundamental processes governing the spin switching in such systems, using x-ray absorption spectroscopy (XAS). In a submonolayer of the SCM [Fe(H2B(pz)2)bipy] deposited on an HOPG substrate, x-ray-induced HS→LS transition, termed as reverse-SOXIESST, is observed at 5K for the first time—apart from the observation of soft x-ray-induced excited spin-state trapping (SOXIESST) already reported in the literature for the bulk material. The switching rates are found to be highly dependent upon the photon fluxes. This observation is rationalized as the spin switching processes being essentially caused by the interaction between the x-ray-induced secondary electrons and the molecules. The observed SOXIESST and reverse-SOXIESST phenomenon is analogous to light- and electron-induced spin switching reported in the literature. Both thermal- and light-induced spin transition of [Fe(H2B(pz)2)bipy] deposited on an HOPG substrate in coverage ranging from submonolayers to multilayers (of up to 10 monolayers (ML)) is systematically probed for cooperative effects and its evolution. In the thermal-induced spin transitions, the submonolayers exhibit an apparent anticooperativity, while a free-molecule-like behaviour is indicated at the monolayer; yet, the multilayers, starting from the double-layer, evidenced cooperativity in their spin-transition processes, with the interaction energy increasing to about 60% of the reported bulk value for the 10-ML sample. The light-induced spin transitions—albeit highly efficient—are free from cooperative effects. The photo-induced metastable HS state of the submonolayers at low temperatures is highly unstable, and showed a clear departure in the mode of spin relaxation from the bulk material. The SCM [Fe(H2B(pz)2)phen] is methylated with two ([Fe(H2B(pz)2)2(phen–me2)]) and four ([Fe(H2B(pz)2)2(phen–me4)]) methyl compounds at the phen ligand. The daughter molecules exhibit spin-crossover behaviour entirely different from the parent molecule upon contact with an HOPG surface; while [Fe(H2B(pz)2)phen] is found to undergo complete spin transition with light and temperature, about 50% of [Fe(H2B(pz)2)2(phen–me2)] molecules are trapped in the HS state within the accessible temperature range; [Fe(H2B(pz)2)2(phen–me4)] molecules lose their spin-crossover behaviour altogether upon contact with the HOPG surface. Similar spin-crossover behaviour is exhibited by the parent and the daughter molecule [Fe(H2B(pz)2)2(phen–me2)] on Au(111) and Bi(111) substrates: both the molecules undergo decomposition, resulting in the loss of spin-crossover on Au(111), while just about 50% of both types of molecules retain their spin-crossover on Bi(111) substrate.