Holger Naggert

Spin-Crossover Complex on Au(111): Structural and Electronic Differences Between Mono- and Multilayers

Submono-, mono- and multilayers of the Fe(II) spin-crossover (SCO) complex [Fe(bpz)2 (phen)] (bpz=dihydrobis(pyrazolyl)borate, phen=1,10-phenanthroline) have beenprepared by vacuum deposition on Au(111) substrates and investigated with near edge X-ray absorption fine structure (NEXAFS) spectroscopy and scanning tunneling microscopy (STM). As evidenced by NEXAFS, molecules of the second layer exhibit a thermal spin crossover transition, although with a more gradual characteristics than in the bulk. For mono- and submonolayers of [Fe(bpz)2 (phen)] deposited on Au(111) substrates at room temperature both NEXAFS and STM indicate a dissociation of [Fe(bpz)2 (phen)] on Au(111) into four-coordinate complexes, [Fe(bpz)2 ], and phen molecules. Keeping the gold substrate at elevated temperatures ordered monolayers of intact molecules of [Fe(bpz)2 (phen)] are formed which can be spin-switched by electron-induced excited spin-state trapping (ELIESST).

Highly Efficient Thermal and Light-Induced Spin-State Switching of an Fe(II) Complex in Direct Contact with a Solid Surface

Spin crossover (SCO) complexes possess a bistable spin state that reacts sensitively to changes in temperature or excitation with light. These effects have been well investigated in solids and solutions, while technological applications require the immobilization and contacting of the molecules at surfaces, which often results in the suppression of the SCO. We report on the thermal and light-induced SCO of [Fe(bpz)2phen] molecules in direct contact with a highly oriented pyrolytic graphite surface. We are able to switch on the magnetic moment of the molecules by illumination with green light at T = 6 K, and off by increasing the temperature to 65 K. The light-induced switching process is highly efficient leading to a complete spin conversion from the low-spin to the high-spin state within a submonolayer of molecules. [Fe(bpz)2phen] complexes immobilized on weakly interacting graphite substrates are thus promising candidates to realize the vision of an optically controlled molecular logic unit for spintronic devices.

Iron(II) Spin‐Crossover Complexes in Ultrathin Films: Electronic Structure and Spin‐State Switching by Visible and Vacuum‐UV Light

The electronic structure of the iron(II) spin crossover complex [Fe(H2bpz)2(phen)] deposited as an ultrathin film on Au(111) is determined by means of UV-photoelectron spectroscopy (UPS) in the high-spin and in the low-spin state. This also allows monitoring the thermal as well as photoinduced spin transition in this system. Moreover, the complex is excited to the metastable high-spin state by irradiation with vacuum-UV light. Relaxation rates after photoexcitation are determined as a function of temperature. They exhibit a transition from thermally activated to tunneling behavior and are two orders of magnitude higher than in the bulk material.

Vacuum-evaporable spin-crossover complexes: physicochemical properties in the crystalline bulk and in thin films deposited from the gas phase

Four analogues of the spin-crossover complex [Fe(H2Bpz2)2(phen)] (H2Bpz2 = dihydrobis(pyrazolyl)borate; 2) containing functionalized 1,10-phenanthroline (phen) ligands have been prepared; i.e., [Fe(H2Bpz2)2(L)], L = 4-methyl-1,10-phenanthroline (3), 5-chloro-1,10-phenanthroline (4), 4,7-dichloro-1,10-phenanthroline (5), and 4,7-dimethyl-1,10-phenanthroline (6). The systems are investigated by magnetic susceptibility measurements and a range of spectroscopies in the solid state and in thin films obtained by physical vapour deposition (PVD). Thermal as well as light-induced SCO behaviour is observed for 3–6 in the films. By contrast, thermal SCO in the solid state occurs only for 3 and 4 but is absent for 5 and 6. These findings are discussed in the light of cooperative and intermolecular interactions.

Soft-x-ray-induced spin-state switching of an adsorbed Fe(II) spin-crossover complex

For probing the nature of spin-state switching in spin-crossover molecules adsorbed on surfaces, x-ray absorption spectroscopy has emerged as a powerful tool due to its high sensitivity and element selectivity in tracing even subtle electronic, magnetic, or chemical changes. However, the x-rays itself can induce a spin transition and might have unwanted influence while investigating the effect of other stimuli such as temperature or light, or of the surface, on the spin switching behaviour. Herein, we present the spin switching of an Fe(II) complex adsorbed on a highly oriented pyrolytic graphite surface with particular emphasis on the x-ray-induced switching. For a submonolayer coverage, the complex undergoes a complete and reversible temperature- and light-induced spin transition. The spin states are switched both ways by x-rays at 5 K, i.e. from the high-spin state to the low-spin state or vice versa, depending on the relative amount of each species. Furthermore, we quantify the fraction of molecules undergoing soft x-ray-induced photochemistry, a process which results in an irreversible low-spin state component, for a particular exposure time. This can be greatly suppressed by reducing the beam intensity.

Evolution of cooperativity in the spin transition of an iron(II) complex on a graphite surface

Cooperative effects determine the spin-state bistability of spin-crossover molecules (SCMs). Herein, the ultimate scale limit at which cooperative spin switching becomes effective is investigated in a complex [Fe(H2B(pz)2)2(bipy)] deposited on a highly oriented pyrolytic graphite surface, using x-ray absorption spectroscopy. This system exhibits a complete thermal- and light-induced spin transition at thicknesses ranging from submonolayers to multilayers. On increasing the coverage from 0.35(4) to 10(1) monolayers, the width of the temperature-induced spin transition curve narrows significantly, evidencing the buildup of cooperative effects. While the molecules at the submonolayers exhibit an apparent anticooperative behavior, the multilayers starting from a double-layer exhibit a distinctly cooperative spin switching, with a free-molecule-like behavior indicated at around a monolayer. These observations will serve as useful guidelines in designing SCM-based devices.Spin-crossover molecules offer a potential route towards molecular spintronics, but retaining the bistability of the spin state upon surface deposition is challenging. Here, the authors study the spin-crossover behaviours of an Fe(II) complex deposited on graphite, determining the scale limit at which cooperative spin switching becomes effective.