Theoretical Study On The Ground State Structure And Physical Properties Of TM-doped Boron And Iron-nitrogen Clusters

The spatial scale of the cluster ranges from a few angstroms to a few hundred angstroms.When the size of the materials is reduced to the nanometer dimension,their physical and chemical properties are significantly different from those of the corresponding bulk materials.Clusters act as an intermediate bridge between microstructure and macroscopic properties,whose structure and properties can be tailored by adjusting their size and chemical composition,thus providing an opportunity to discover new mechanisms and design new materials with customized properties and opening the door to new functional materials composed of clusters.With the development of cluster research,abundant research achievements in this field have not only made people have a profound understanding of the evolution from cluster to bulk material,but also revealed many new rules and phenomena.At the same time,the research results in this field have gradually attracted more attention.Cluster is increasingly closely related to other disciplines,such as inorganic chemistry,materials science,biomedicine,surface physics and so on.In this paper,medium-sized boron clusters doped with transition metals(TM)and Fe-N clusters are investigated theoretically.Firstly,the CALYPSO structure prediction program was used to search for the local optimal structure on the potential energy surface of the cluster.Then,the geometric configuration,chemical bond,electrical structures and magnetic properties of TM-B and Fe-N cluster were calculated and analyzed under the framework of DFT.The specific main research contents,conclusions and significance are summarized as follows:(1)The size-dependent effect of the geometric configuration and its ability to easily form multi-center delocalized bonds have attracted the attention of materials scientists.Experimental and theoretical studies have shown that the TM doped with boron cluster exhibits rich and interesting geometric configurations and chemical bondings.For example,Ta doped with B10-cluster is a wheel structure with high symmetry,while Ta doped with B20-cluster is a tubular molecular rotor with two boron atoms at its top.However,it remains to be explored whether there are other structures with high stability in the intermediate size between the planar wheel-like molecules and 3D tubular rotar molecules.The ground state structures of TaBn0/-(n=10-20)clusters were obtained by using CALYPSO structure prediction method and DFT calculations.We found that the ground state structures of neutral and anionic are basically the same except for the size of n=10,and follow the structural evolution from half-sandwich-like to drum-like and finally to tubular molecules.Then,the photoelectron spectroscopy(PES)of the ground state of TaBn-clusters were simulated and compared with the experimental spectrum to confirm the correctness of the obtained ground state and the reliability of the calculation method.In addition,we calculated the binding energy,the second order difference energy and HOMO-LUMO energy gap and found that the anionic TaB12-clusters has relatively strong stability.Therefore,we further analyzed the chemical bonding patterns of TaB12-,and found that there is a strong interaction between the B-2p orbital and Ta-5d orbital,which enhances the stability of TaB12-cluster structure.Our work reveals the structural evolution of Ta doped medium sized boron clusters and analyzes in detail the bonding pattern of TaB12-clusters with high stability.(2)Determining the geometric configuration and growth behavior of the ground states of clusters has always been the basis of cluster research,and it is also a challenging work,because the number of isomers on the potential energy surface of clusters has an exponential relationship with the size of cluster.The combination of photoelectron spectroscopy and DFT is an effective technique for studying boron based clusters in recent years.In this work,we obtained the ground state and two metastable structures of neutral and anionic RuBn0/-(n=9-20)clusters,and found that the structure evolution pattern and structural transformation point:planar structure to half-sandwich structure occurs at n=12,half-sandwich structure to drum-like structure occurs at n=14,drum-like to cage-like structure occurs n=19.By comparing the structure of Fe-doped boron clusters,we found that the size of doped atoms is an important factor for the formation of intriguing structures for designing new nanomaterials.Then,we simulated the PES of the anionic structure,in order to provide detailed reference information for future experimental characterization.In view of the existing experimental spectra RuB9-and RuB10-clusters,we also simulated the PES of other two metastable structures and compared with the experiment results,which validate the accuracy of ground state and the current theoretical method.Our results show that Ru atom doping enriches the inherent planar structure properties of boron clusters in the medium size range,and new halfsandwich,drum-like and cage-like structures appear.The simulated PES of RuBnclusters provide detailed electronic structure information,and hopefully provide reference information for future theoretical and experimental work related to the system.(3)The magnetic behavior of iron clusters is quite complex.Most of the iron clusters are high spin states with a total magnetic moment of about 3n μB(n is the number of iron atoms).However,the maximum average magnetic moment of iron atom in bulk iron material is only 2.15μB.Therefore,the theoretical study of Fe16N2 clusters is very helpful to understand the giant magnetoresistance effect of Fe16N2 films,and improve our understanding for the microscopic nature behind it.We are committed to a comprehensive theoretical study of the magnetic properties and chemical pattern bonding in neutral and anionic Fe8N0/-clusters,and to comparing with bare Fe80/-clusters.The ground state structures of neutral and anionic Fe8N0/clusters were obtained by extensive structure search using CALYPSO method and DFT calculations.The results show that the shape of pure Fe8 clusters is distorted after N atom doping,and the HOMO-LUMO gap is greatly affected.The analysis of thermodynamic stability shows that Fe8N0/-clusters have better thermal stability than their corresponding pure Fe80/-clusters.Total spin magnetic moments of the neutral Fe8N clusters decreased by 0.5 μB compared with that of Fe8 clusters,while the total spin magnetic moments of the anionic Fe8N-clusters increased by 0.5 μB compared with Fe8-clusters.The chemical bonding analysis for Fe80/-and Fe8N0/-clusters show that the bonding orbitals mainly came from β orbitals.The 3d orbitals of Fe atoms are almost not involved in bonding,which is the main reason for the high spin states of Fe80/-and Fe8N0/-clusters.This work reveals the ground state structures of Fe80/-and Fe8N0/-clusters and the reason for their high spin multiplicities,and further explores their magnetic properties and chemical bonding patterns.(4)The coalescence process of two Fe8N clusters and the structure of Fe16N2 cluster are explored.The results show that the combination may proceed out without an energy barrier,and the combined Fe16N2 geometry strongly depends on the initial mutual orientation of the two parts.In the lowest energy structure of Fe16N2,two N atoms are connected to Fe atoms,leading a 6 μB reducation of total spin magnetic moment of the ground state of Fe16.In order to gain insight into the dependence of the properties on the charge state and estimate the binding energy of two N atoms,we optimized the neutral and single charged Fe16N2 and Fe16N clusters and found that the adsorption of N atoms has no significant effect on the electronic properties of Fe16N2 clusters,such as electron affinity and ionization energy,but the average binding energy per atom is significantly changed.The IR and Raman spectra of the simulated Fe16N2 clusters and its anion and cation clusters show that the position of the strongest peak in the IR spectrum strongly depends on the charge,so it present fingerprints of the charged states.The chemical bonds of the ground state structure of Fe16N20/±1 clusters were described by localized molecular orbitals.