Boron fullerenes: FromB80to hole doped boron sheets

Abstract

We demonstrate the existence of a family of stable boron fullerenes containing 80n 2 atoms that is related to the family of 60n 2 carbon icosahedral fullerene series and is compatible with the recently proposed stable boron sheets composed of triangular and hexagonal motifs. All electron density-functional calculations on the B 320 , B 720 , B 1280 , and B 2000 confirm their stability and show that the quantum size effects open up electronic band gaps in the boron fullerenes at B 1280 . Boron fullerenes below B 2000 have valence electronic structure identical to their corresponding carbon cousins from 60n 2 family.Since the discovery of carbon fullerenes and nanotubes, several studies have examined the possible existence of hollow cage structures for the neighboring element, boron. 1-5 Boron, with three valence electrons in 2s and 2p orbitals, can form sp 2 hybridized orbitals. The crystalline forms of boron contain B 12 icosahedral units, but the most stable structure of bulk boron is controversial. 6-8 In cluster form, for very small sizes containing up to 20 atoms, it prefers planar or quasiplanar structures. The planar to three-dimensional transition in boron clusters seems to occur around B 20 . 9,10 Around this size the double ring tubular structures are more favorable. The extended planar honeycomb sheet, i.e., graphene equivalent of boron, is electron deficient and unstable. 11,12 The addition of a boron atom at the center of each hexagon results in a triangular sheet which is electron rich. This sheet is compatible with Aufbau principle of Boustani, 4 which suggests that the most stable boron nanostructures would be based on buckled triangular motifs. 13 The flat triangular sheet buckles when optimized and is more stable than the planar honeycomb sheet. The nanotubes formed by curling up the buckled triangular sheets are metallic. 11 During the last year, a number of interesting works on boron nanostructures have been reported ͑cf. Fig. 1͒. [14][15][16][17][18][19][20] In the first of these, a stable hollow cage structure of 80 boron atoms was reported by Szwacki et al. 14 The most interesting aspect of the B 80 is its structural resemblance with C 60 fullerene. The basic framework of the B 80 fullerene is similar to that of C 60 with 12 pentagonal and 20 hexagonal rings ͑see Fig. 1͒. The B 60 , an exact analog of C 60 , is electron deficient and unstable. However, an addition of 20 boron atoms at the center of each hexagonal ring stabilizes the electron deficient B 60 to give the stable B 80 fullerene. The B 80 fullerene is isovalent ͑equal number of valence electrons͒ with C 60 . Szwacki et al. 14 explained the stability of B 80 fullerene in terms of its structure of six interwoven double rings.About the same time, Tang and Ismail-Beigi, 15 and Yang et al. 16 proposed novel boron sheets that consist of combinations of triangular and hexagonal rings ͑cf. Fig. 1͒. These boron sheets are obtained by removing atoms from flat triangular sheets and can be viewed as hole doped triangular sheets. This arrangem...

Dispersion corrections in the boron buckyball and nanotubes

“…1). 8 Subsequent investigations [10][11][12][13][14][15][16] indicated that the atomic buckling are particularly sensitive to the basis set employed and the level of theory used in describing exchange-correlation effects. However, recent investigation by Szwacki and Tymczak with the use of computational intensive ab-initio calculations up to second-order Møller-Plesset perturbation (MP2) theory reveals that the I h boron buckyball is the lowest energy conformation owing to the crucial importance of correlation effects.…”

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“…Studies focused on searching the lowest energy configurations has yielded a variety of stable boron structures due to electron deficient nature of boron. Such structures involve molecular wheels [1,2], boron sheets composed of triangular and hexagonal motifs [3][4][5][6][7][8][9][10][11][12], cage architectures based on empty polygons [13], crossing double rings [14,15], snub structures [8], filled hexagons [16] and filled pentagons [17,18]. Boron based forms are known for their complex chemistry.…”