First-principles Investigations Of The Structures And Properties Of Low-dimenisonal Boron

Boron, the neighbor of carbon in the elemental table, has many similarities to carbon.Low-dimensional carbon materials, including carbon fullerenes, graphene, carbon nanotubes,and carbon nanoribbons, have attracted considerable attentions because of their unique structuresand properties. Therefore, it can be questionable whether low-dimensional boron materials couldhave unique structures and properties? Accompanied with the study of low-dimensional boronmaterials, great progress has been made.Some novel two-dimensional boron sheets have been found by using first-principlescalculations with particle swarm algorithm. One-dimensional boron nanoribbons and boronnanotubes have been constructed based on these planar boron sheets with relatively lower totalenergy. The structures and electronic properties of boron sheets, boron nanoribbons, and boronnanotubes have been investigated by first-principles method. The discovered various electronicproperties of low-dimensional boron materials make them have various potential applications innanodevices and spin electronics.Over7000two-dimensional boron sheets have been explored and40boron sheets withrelatively lower total energies of them have been selected for the detailed analysis. We found anew boron sheet (named as struc-a sheet) has relatively lower total energy. The energy differencein comparison with the well known-sheet is less than0.01eV. The reported boron sheets canbe divided into several types according to the different hexagon hole density.(i)When thenumerator of hexagon hole density equals to1, the most stable boron sheets have the verticalarrangements of the hexagonal lattices and the triangular lattices for≥1/7, except for thestruc-1/4. For the≤1/8, the most stable boron sheet has the uniform distribution of a singlehexagonal lattice in the triangular lattice.(ii)When the numerator of hexagon hole density notequals to1, the most stable structure has the distribution of three connected hexagonal latticewhen≥3/19and the most stable structure has the distribution of two connected hexagonalstructure when≤2/13. The calculationed band structures show that these40two-dimensionalborn sheets are all metallic. Electronic localization function has been used to analyze the chemical bonds of the five boron sheets with lower energy (α-sheet,-sheet, struc-a, struc-1/8,and struc-2/15).The boron nanoribbons are built by cutting five two-dimensional boron sheets with lowertotal energy. These boron nanoribbons can be divided into armchair, straight and zigzag typesaccording to the edge of boron nanoribbons. The boron nanoribbons have rather high totalenergy when the width of nanoribbon is small. With the increase of width of boron nanoribbons,the total energy gradually decreased and finally approached to a fixed value. The most stableboron nanoribbons have the type of double straight line edges, both in α-sheet and struc-ananoribbons. The calculated electronic properties show that the investigated boron nanoribbonsare all metallic.The five boron planar sheets with lower total energy have been used to construct boronnanotubes, either with armchair or zigzag types. The geometry optimization results show that thestructural deformation is relatively large when the diameter of boron nanotubes is small. Thestructural deformation is getting smaller and smaller when the width of diameter of nanotubes isincreasing. There are some total energy difference between the armchair and zigzag types ofboron nanotubes. The total energy of boron nanotubes is lowered with the increasing of thenanotubes diameters, and finally approaches a fixed value. The boron nanotubes constructedfrom struc-a have lowest total energy among all the investigate boron nanotubes when thediameter is small. When the diameter of boron nanotubes is large than2nm, the struc-1/8has thelowest total energy. The calculated electronic properties indicate that most of them are metallic.