Spin-crossover Behavior And Transport Properties Of Transition Metal-Aromatic Complexes

Recently,nanotechnology has become the hot topic of the global scientific and technological research.Especially,molecular devices with quantum effects have always been the focus of theoretical and experimental researches due to their unique properties of light,electronic and magnetic properties.In addition,the development of molecular electronic devices is of great importance because the size of normal integrated circuits are close to the limit.Metal-organic complexes show unique structural and physical properties.Some of these complexes with magnetic bistablity are often used as information storage materials and optical materials.The spin-crossover?SCO?complex is the mostly studied bistablity system.The spin state of the SCO complexes can be changed between a low-spin state LS to a highspin state HS by means of external stimuli,such as variation of temperature,charge flow,magnetic or electric fields.This phenomena are mostly associated with the orbital electrons of the first-row?3d-?transition mental with a d⁴-d⁷ electron configuration.So far,a large number of experimental and theoretical investigations of SCO complexes are focused on the Fe???or Fe??? SCO complex,and researches on cobalt-based SCO complex are rare.We start with Co?Bz?₂ complex.The molecule properties were calculated using ab-initio method by GAUSSIAN program with density functional theory?DFT?.It was found that both geometric and electronic structure of LS and the HS state are stable,but due to the large energy gap between them,the SCO behavior can hardly be realized.We then replaced one carbon atom in the benzene ligand with a N atom to get the pyridine ring and two carbon atoms in the benzene ligand with two N atoms in the meta-position to get the pyrimidine ring.We successfully narrow the energy barrier between LS and HS state by replacing the benzene ring with pyridine and pyrimidine ring.In this case,the SCO behavior can be trigerred by stretching the ligands of the complex.We then investigated the lead-moleculelead junctions using Atomistix Tool Kit?ATK?program.It is found that a perfect spin-filter effect of the Co-Pyrim₂ complex only in the [HS] state and negative differential resistance phenomena coexist both in the [LS] and [HS] configurations.In the [HS] case,the current is mainly arised by the spin down channel under small bias.Then,the transition metal atoms from Sc to Fe are taken into consideration.We calculated the molecular properties of M?Bz?₂?M=Sc^Fe?and SCO complex M?pyridine?₂?M=Cr^Fe?.It is found that only Mn?Bz?₂,Cr?pyridine?₂ and Mn?pyridine?₂ complex can realize the SCO behavior by stretching the ligands.Moreover,nearly 100% spin-filter efficiency can be observed in V?Bz?₂,Cr?Bz?₂ and Cr?Pyridine?₂ complexes,while in Ti?Bz?₂?Mn?Bz?₂ and Fe?Bz?₂ complexes,low-bias negative differential resistance?NDR?effect appears.Also,great spin-seebeck effect can be observed in V?Bz?₂?Mn?Bz?₂?Fe?Bz?₂?Mn?pyridine?₂ and Fe?pyridine?₂ junctions.In summary,SCO complexes in single molecular level have many interesting electric/magnetic properties and transport properties.These findings indicate that SCO complexes are promising materials for molecular spintronic devices and provide new idea for future researches.