Electronic Structure And Spectral Properties Of Boron Fullerenes And Endohedral Metalloborospherenes : A First Principles Study

An all-boron fullerene B₄₀^-was produced in a laser vaporization supersonic source in 2014.Soon after,the all-boron fullerenes B₃₉^-,B₂₈^-and B₂₉^-were also produced via laser vaporization.The discovery of all-boron fullerenes marks the onset of a new class of boron fullerenes and boron-based nanomaterials.The first observation of the boron fullerene has aroused interest in all-boron fullerenes and their derivatives such as structures and electronic properties of boron fullerenes and metalloborospherenes(B₄₁⁺,B₄₂²⁺,B₄₄,Ca@B₄₀,Be&B₄₀,Sc@B₄₀,Li&B₄₀,Na@B₄₀,Ca@B₃₉⁺,Ca@B₃₈,Ca@B₃₇^- and Li₄&B₃₆).In our work,density functional theory?DFT?calculations are carried out to study the electronic structure and spectral properties of boron clusters B_n^0/-?n=38-40,44?,metalloborospherenes MB₄₀^0/-?M=Ag,Au?and MB₄₄^0/-?M=Li,Na,K?,and electronic structure of B₄₀ crystal.Our works may provide valuable results to assist further experimental identifications,and also may provide theoretical guidance for the applications and study of boron fullerenes and boron-based nanomaterials in the future.The main contents are as follows:1.Electronic structure and spectral properties of boron clusters B_n^0/-?n=38-40?Density functional theory?DFT?and time-dependent density functional theory?TD-DFT?calculations are carried out to comparatively study the vibrational frequencies,infrared spectra,Raman spectra and electronic absorption spectra of boron clusters B_n^0/-?n=38-40?.The numerical simulations show that such boron clusters have different and meaningful spectral features.These spectral features are readily compared with future spectroscopy measurements and can be used as fingerprints to distinguish the boron clusters B_n^0/-with different structures?cage structure or quasi-planar structure?and with different sizes?n=38-40?.2.Electronic structure and spectral properties of metalloborospherenes MB₄₀^0/-?M=Cu,Ag,Au?Density functional theory?DFT?and time-dependent density functional theory?TD-DFT?calculations are carried out to study the structures,stabilities,photoelectron spectra,infrared spectra,Raman spectra and electronic absorption spectra of metalloborospherenes MB₄₀^0/-?M=Cu,Ag,and Au?.It is found that Cu,Ag and Au atoms can form stable exohedral metalloborospherenes M&B₄₀0/-?M=Cu,Ag,and Au?and endohedral metalloborospherenes M@B₄₀0/-?M=Cu,Ag,and Au?.In addition,relative energies of these metalloborospherenes suggest that Cu,Ag and Au atoms favor the exohedral configuration.The calculated spectra suggest that doping of metal atom can change the spectral features,such as increasing of first vertical detachment energy,adding of spectra and Raman active modes,redshift of electronic absorption spectra,and weakening or strengthening of characteristic peaks,since the extra metal atom can modify the electronic structure of borospherene B₄₀.The calculated results also show that metalloborospherenes MB₄₀^0/-?M=Cu,Ag,and Au?have different and meaningful spectral features,insight into the spectral properties is important to understand them and find their potential applications.These spectral features are readily compared with future spectroscopy measurements and can be used as fingerprints to identify and distinguish the metalloborospherenes MB₄₀^0/-?M=Cu,Ag,and Au?.3.Electronic structure and spectral properties of boron fullerene B₄₄^-and metalloborospherenes MB₄₄^0/-?M=Li,Na,and K?Density functional theory?DFT?and time-dependent density functional theory?TD-DFT?calculations are carried out to study the stabilities,photoelectron spectra,infrared spectra,Raman spectra and electronic absorption spectra of borospherene B₄₄^-and metalloborospherenes MB₄₄^0/-?M=Li,Na,and K?.It is found that Li,Na and K atoms can form stable exohedral M&B₄₄^0/-?M=Li,Na,and K?,whereas only Na and K atoms can be stably encapsulated inside the B₄₄^0/-cage.In addition,relative energies of these metalloborospherenes suggest that Na and K atoms favor the exohedral configuration.Importantly,doping of metal atom can modify the stabilities of borospherenes B44 with different structures,which provides a possible route?doping of metal atoms?to produce stable boron clusters or metalloborospherenes.The calculated results suggest that B44 tends to get electrons from the doped metal.Metalloborospherenes MB₄₄^- are characterized as charge-transfer complexes(M+B₄₄^2-),where B₄₄ tends to get two electrons from the extra electron and the doped metal,resulting in similar features with anionic B₄₄^2-.In addition,doping of metal atom can change the spectral features,such as blueshift or redshift and weakening or strengthening of characteristic peaks,since the extra metal atom can modify the electronic structure.The calculated results show that B₄₄^-and metalloborospherenes MB₄₄^0/-?M=Li,Na,and K?have different and meaningful spectral features,insight into the spectral properties is important to understand them and find their potential applications.These spectral features are readily compared with future spectroscopy measurements and can be used as fingerprints to identify and distinguish the borospherene B₄₄^-and metalloborospherenes MB₄₄^0/-?M=Li,Na,and K?.4.Electronic structure and spectral properties of borospherene B₄₀ under different external electric fieldsExternal electric field can influence the structure and property of molecule.It is necessary to understand the electrostatic field effects in the boron fullerene B₄₀.Density functional theory method at the PBE0 level with the 6-31G* basis set is carried out to investigate the ground state structures,mulliken atomic charges,the highest occupied molecular orbital?HOMO?energy levels,the lowest unoccupied molecular orbital?LUMO?energy levels,energy gaps,electric dipole moments,infrared spectra and Raman spectra of boron fullerene B₄₀ under the external electric field within the range of values F= 0-0.06 a.u..The electronic spectra?the first 18 excited states containing excited energies,excited wavelengths and oscillator strengths?of boron fullerene B₄₀ are calculated by the time-dependent density functional theory method?TD-PBE0?with the 6-31G* basis set under the same external electric field.The results show that boron fullerene B₄₀ can be elongated in the direction of electric field and B₄₀ molecule is polarized under the external electric field.Meanwhile,the addition of external electric field results in lower symmetry?C2v?,however,electronic state of boron fullerene B₄₀ is not changed under the external electric field.Moreover,the calculated results show that electric dipole moment is proved to be increasing with the increase of the external field intensity,but the total energy and energy gap are proved to decrease with the increase of external field intensity.The addition of external electric field can modify the infrared and Raman spectra,such as shift of vibrational frequencies and strengthening of infrared and Raman peaks.Furthermore,the calculated results indicate that the external electric field has significant effects on the electronic spectra of boron fullerene B₄₀.The increase of the electric field intensity can lead to the redshift of electronic spectra.With the change of the electric field intensity,the strongest excited state?with the biggest oscillator strength?can become very weak?with the small oscillator strength?or optically inactive?with the oscillator strength of zero?.Meanwhile,the weak excited state can become the strongest excited state by the external field.The ground state properties and spectral properties of boron fullerene B₄₀ can be modified by the external electric field.Our works can provide theoretical guidance for the application of boron fullerene B₄₀ in the future.5.Electronic structure of B₄₀ crystalTotal energy and the electron structure of B₄₀ crystal are calculated using the density-functional theory method.For the exchange-correlation potential,we have used generalized gradient approximation?GGA?with the Perdew-Burke-Ernzerhof?PBE?formulation.We find that fcc and bcc B₄₀ are molecular crystals condensed by van der Waals force,and that the resulting fcc and bcc B₄₀ are semiconductor.The calculated results indicate that the fcc B₄₀ crystal is a direct-gap semiconductor with an energy gap of 1.33 eV.Both the valence-band top and the conduction-band bottom are located at the G point.However,the calculated results indicate that the bcc B₄₀ crystal is an indirect-gap semiconductor with an energy gap of 1.30 eV.The valence-band top is located at H point and the conduction-band bottom is located at the P point.