Theoretical Studies On Electron Spin Polarization In Low-dimensional Confined Carbon Systems

The low-dimensional structure of carbon materials usually exhibits peculiar physical and chemical properties on the nanometer scale. Especially, p electrons commonly demonstrate various orbital hybridizations and spin polarization characteristic, which have the siginificant application values in magnetic field. In addition, actinide elements are known to have distinctive electronic structures, the interaction between f valence electrons of actinide elements and p valence electrons of carbon materials contains the complex many-body system and relativistic effects in micro system. It is not only an important challenge of basis theory research at atomic level, but also has significant value in physics and material science, life science and other interdisciplinary field, which arouses great interests in the scientific field. Here, using first-principles density functional theory(DFT), we studied the slippages in stacking of graphene nanofragments induced by spin polarization and the stability of rare gases confined in actinide endohedral metallofullerenes(EMFs) involving electron correlation between f valence electrons and p electrons.Spin polarization and stacking are interesting effects in complex molecular systems. Especially, both of them are closely related to p electrons of carbon materials and can be presented in graphene-based materials. Their possible combination may provide a new perspective in understanding the intermolecular interaction. The nanoscale graphene structures with zigzag edges could possess p electrons spin-polarized ground states. Meanwhile, inspired by phenomenon of macro compass, we guess that graphene nanofragments stacking adsorption with spin-polarized effects may reflect the notable difference with non-spin-polarized adsorption. Here we displayed the displacement between two stacked rhombic graphene nanofragments induced by spin polarization, using first-principles density functional methods. We found that, in stacking of two rhombic graphene nanofragments, a spin-polarized stacked conformation with zero total spin is energetically more favorable than the closed shell stacking. The spin-polarized conformation gives a further horizontal interlayer displacement within 1 angstrom compared with the closed-shell structure. This result highlights that, besides the well-known phenomenologically interpreted van der Waals forces, a specific mechanism dependent on the monomeric spin polarization may lead to obvious mechanical effects in some intermolecular interactions.Considering electron correlation mechanism between p electrons of carbon materials and f valence electrons of actinide elements, the typical low-dimensional nanostructure EMFs can be formed. Due to the structural stability and low chemical activity, rare gases such as He usually serve as protective gases for synthesizing fullerene and EMFs. So far, there has not been any theoretical report on the mechanism of rare gases embedded in EMFs. In this work, using first-principles DFT method, we analyzed UNg@C60(Ng=He, Ne, Ar) system formed by typical actinide EMFs U@C60 encapsulating rare gases He, Ne and Ar. Firstly, we studied the interaction between a He atom and uranium metallofullerene U@C60 in an UHe@C60 system. Analysis of the electron density indicates no observable electron clouds overlap between the He atom and U atom/fullerene cage. Molecular orbital composition analysis show that the 1s shell electrons of the He atom account for >1% contribution to only three MOs, namely, HOMOα-99, HOMOα-98 and HOMOα-94. Furthermore, the modulations of the He atom on the UV-Vis, infrared and Raman spectra are almost negligible. Our work highlights the high stability of the He atom in the actinide metallofullerenes, and it can also help to understand the interaction between fission products of actinide elements and the synthesizing process of EMFs. Secondly, we extended the research system that the U@C60 encapsulating the heavier rare gases(Ne, Ar). Density of states analysis shows that, with the increase of atomic number and nuclear charge number of rare gases, the valence electrons are screened by core electrons resulting in weaker bound with the core. Rare gases and U@C60 gradually increase the contribution to molecular orbital of the whole system. These analyses given above not only help to understand the interaction mechanism between rare gases with larger atomic number and actinide elements, but also has reference significance to the related application of materials and nuclear science.