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Infomat: double sided modification to stabilize the electronic structure of boron alkenes

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The discovery of

graphene its excellent properties have aroused unprecedented interest in two-dimensional (2D) materials. In the past decade scientists have proposed synthesized a large number of hexagonal two-dimensional materials. However there are only a few single element two-dimensional materials including silylene germanene stanylene plumbene phosphorylene antimony bismuth. Different from the atomic level flattening pure SP2 hybrid of graphene these single element two-dimensional materials are mostly mixed sp2-sp3 hybrid due to the increased bond length resulting in out of plane folds of the lattice. In addition due to the unsaturated PZ orbitals most single element two-dimensional materials face serious structural chemical instability problems in experiments. Generally there are two strategies to stabilize these structures: first the materials are placed on a specific substrate the electrons transferred from the substrate can improve the structural stability. However the strong interaction between material substrate will destroy the unique electronic properties of single element two-dimensional materials such as the Dirac cone in Silene greatly limit its application as an independent material. 2、 Functional groups are used to passivate the surface of the material to saturate the PZ orbital electron filling such as the topological properties in functionalized bismuth alkenes. In addition to the single element monolayers mentioned above the crystalline atoms of boron are monolayers borophenes have recently attracted extensive research interest. Due to the small covalent radius of boron atoms many kinds of bonding boron alkenes have a variety of planar or quasi planar allotropic structures. Theoretical predictions of α β χ phase borones have been made some of which have been successfully synthesized. Focusing on hydrogen storage batteries spintronic devices these studies provide a wide range of applications. Because the boron atom of

is less than one electron of carbon atom in the allotrope of bornene the stability of honeycomb hexagonal borate with the same atomic framework as graphene is questioned. Early studies have shown that the planar honeycomb like boron alkene exists as a subatomic lattice in bulk MgB2 the superconducting transport may occur in the boron plane. Hexagonal lattice may also bring other interesting properties such as the Dirac cone in graphene. However because the in-plane σ b out of plane π b are only partially occupied the honeycomb borones are considered to be unstable. One possible way to stabilize the honeycomb boron alkenes is to combine them with the metal atoms that provide electrons similar to the embedding process in the bulk MgB2.

in order to explore other possible methods to stabilize boron alkenes Professor Li Kou of Queensl University of science technology in Australia Professor Zhong Fang Chen of University of Puerto Rico in the United States through first principles calculation it is predicted that in addition to H atom monovalent functional group x (x = f Cl Br I Oh or NH2) can be used to stabilize honeycomb boron monolayer. Accordingly due to the change of electron transfer chemical environment the electronic properties of boron alkenes can be adjusted by functionalization. Contrary to the metal properties of the original boron alkenes all these functionalized boron alkenes exhibit semiconductor properties the b gap height depends on the choice of functional groups. The electronic properties including b gap can be further adjusted by external strain. In addition the carrier mobility of these surface modified boron alkenes also depends on the functional groups which can be equal to or even higher than that of MoS2 black phosphorus. This work is expected to extend the potential applications of boronenes to the electronic field stimulate more research on the use of surface modification to stabilize other unstable 2D materials. This work published

online on infomat with the title of "double ‐ sided surface functionalization: an effective approach to stabilize modulate the electronic structure of graph ‐ like borophene". We extracted several parts of the article focused on the introduction of

top view side view of

is shown in Fig. 1 where a) x = f; b) x = CL; c) x = br; d) x = I; E) x = Oh; F) x = NH2. Structural stability stability mechanism of

the authors systematically evaluated the dynamic stability of these functionalized borones by calculating the phonon dispersion. No obvious virtual mode was observed in the first Brillouin zone which confirmed their dynamic stability.

Fig.2 phonon spectra of single-layer borates with b4x4 structure a) b4f4; b) b4cl4; c) b4br4; d) b4i4; E) B4 (OH) 4; F) B4 (NH2) 4

are important for explaining the stability origin of functionalized borates on hexagonal honeycomb surface. The local geometry of the molecule is similar to that of 2D b4h4. The stability of b4h4 monolayer can be explained by studying the molecule B2H6. The stability of ethylborone is obtained due to two typical three center two electron (3c-2e) b-h-b bonds. The same 3c-2e bond is found in two-dimensional b4h4 by ssadndp calculation. Specifically the unit cell of b4h4 has four boron atoms four hydrogen atoms providing 16 valence electrons or 8 chemical bond / lone pairs. According to ssp analysis four of the chemical bonds are assigned to the classical two center two electron (2c-2e) B-B covalent bond (Fig. 3b) while the other four are the same as the 3c-2e b-h-b bond in the borolene molecule (Fig. 3C).

Fig. 3 A) schematic diagram of 3c-2e chemical bond in the molecule of ethylene boron; B c) b4h4; D-H) b4f4; I ‐ m) B4 (OH) 4 monolayer chemical bond diagram. The electronic structure of

the effect of stress on the energy b structure the corresponding density of states (DOS) of

changed greatly when both sides of hexagonal honeycomb boron were functionalized. The surface passivation of boron alkenes with functional groups greatly changes the electronic properties of the original boron alkenes which is caused by electron transfer PZ orbital transfer near Fermi. The strong in-plane bonds (sum of S PX py) of boron alkenes are not fully occupied which leads to instability. However these bonds are fully occupied after functionalization which stabilizes the two-dimensional boron alkenes. On the other h all the Fermi energy levels in the vicinity of the original metal structure are quite large. Except for b4i4 all the functionalized boron alkenes are direct b gap semiconductors at Γ point with b gap values in the range of 0.20-1.30 ev. For b4f4 b4cl4 b4br4 b-px F / CVBM is determined by L / BR PX orbital which indicates the strong hybridization between B F / CL / BR atoms. CBM is contributed by b-pz orbit. For b4x4 (x = I oh NH2) VBM state is mainly derived from px. The CBM of B4 (OH) 4 is also controlled by PZ orbit. But the main contribution of b4i4's CBM comes from b-py. The energy b structures of a) b4f4; b) b4cl4; c) b4br4; d) b4i4; E) B4 (OH) 4 F) B4 (NH2) 4 as well as the corresponding DOS are calculated by

Figure 4. The Fermi level is set to zero. The VBM CBM states highlight

in pink. In the case of B4 (OH) 4 the b gap increases by ~ 89.2% under 3% tensile strain in a direction the compression strain in the same direction decreases the b gap. Taking b4i4 as an example the b gap decreases by about 87.9% at - 3% strain. Corresponding to the structural anisotropy of boron alkenes the trend is opposite when the strain is applied along the B direction: for all these functionalized monolayers the b gap decreases monotonically with strain from - 3% to 3%. Except for the molecules functionalized by OH group all the other functionalized bornene monolayers (x = f Cl Br I NH2) retain semiconductor properties regardless of whether the b gap increases or decreases (- 3% to 3%) in the studied strain range. This kind of b structure modulation is mainly caused by the simultaneous shift of VBM CBM. Taking b4f4 as an example under the tensile strain along B direction VBM moves up CBM moves down resulting in the decrease of b gap. Under compressive strain VBM CBM move down up respectively resulting in the increase of b gap. Except for the borodiene functionalized by OH group it can transform from the semiconductor phase under - 2% compressive strain in a direction to the metal phase under ~ 2% tensile stress in B direction. The strain dependent b structure provides an effective means to regulate the electronic properties of functionalized boron alkenes.

Fig. 5 a) b gap strain relationship along different directions; B c) b structure of b4f4 B4 (OH) 4 near Γ point under different strain deformation along B direction. The Fermi level is set to zero the black dotted line shows the VBM CBM

of b4f4 B4 (OH) 4 without strain
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