Theoretical Studies Of The Structural And Physical Properties Of Two-dimensional Diamond And BN Nanofilms

In recent years, the two dimensional (2D) diamond nanosheets have become oneof most interesting topics in the field of diamond. Since in2009, the graphene isfounded and more and more people began to investigate the other two dimensionalmaterials. In comparison with traditional0-dimensional and one-dimensional diamondnanostructures, the structure and properties of2D diamond nanosheets withsignificant surface and quantum size effects are strongly dependent on theirorientation, number of layer and surface functionalization. For this project, thefirst-principles calculations are proposed to deeply and systematically study thestructural stability and evolution of the2D diamond nanosheets related to the factorsof crystalline orientation, layer number, surface modification, etc. By analyzing theband structure, electron and spin density of state (DOS), it is expected to reveal theunique structure-related electronic and magnetic properties of the2D diamondnanosheets. The main conclusions are below as listed:(1) We report the structural evolution and electronic properties oftwo-dimensional (111)-oriented diamond films modulated by the layer number (n)based on first-principles calculations. At n<6, the film is relaxed into a few graphenelayers; whereas for6≤n≤11, a gradient-graphite-diamond-like (GGDL) structure withgradient changes of interplanar spacing and buckling feature is predicted. A thresholdn=12is determined to realize the thinnest two-dimensional (111)-oriented diamond film. The analysis of electronic band structures reveals the transition from semimetalto wide band semiconductor with the increase of n. semimetal to semiconductor. Thistendency is related to the hybridization transition from the dominated sp2to sp3in thematerials and due to the intrinsic bond characteristics in each layer.(2) We further study the effects of semi-hydrogenation (SH) andfull-hydrogenation (FH) on the structural evolution and properties of two-dimensional(2D) diamond nanofilms by first-principles calculations. Both the hydrogenationprocesses play an important role in stabilizing the2D diamond structures. Especiallyfor the SH cases, a ferrimagnetism characteristic is presented determining by theunpaired electrons on the un-hydrogenated side, and the spin-related bandgaps are inan infrared region of0.74―1.17eV, which are strongly dependent on n. Taking intoaccount the structural evolution, PDOSs and spin density, this n-dependent variationin bandgap for SH2D diamond can be explained as follows:(i) For n<4, the C-Cbonds near the un-hydrogenated side are slightly relaxed having hybrid sp2+xcharacteristics with shorter (larger) bond length (interplanar spacing) with respect tothat from the hydrogenated side. With increasing the layer number n, the sp3bondingbecomes dominant, and at the same time, the degree of spin split of the unpairedelectrons will be enhanced leading to the increase of corresponding bandgap.(ii) Withfurther increasing the layer number (n≥4), the structural relaxation near theun-hydrogenated side is negligible and the thickness-related quantum confinementeffect of the2D structures will be weaken. Consequently, the bandgap decrease to asaturated value of1.06eV after following the inverse law of n. It is worth pointing outthat the SH2D FLD with tunable bandgaps localized in an infrared region of0.74―1.17eV and a ferrimagnetic on the un-hydrogenated side would give much morefreedom in designing modern infrared electronic-spin circuits.(3) By first-principles calculations, we study the layer number (n) dependentstructural evolution, electronic and magnetic properties of two-dimensional bare andsemi-hydrogenated (111)-oriented cubic boron nitride (c-BN) nanosheets. There is athreshold of n=7that for maintaining the cubic phase of BN nanosheets, the original2D c-BN nanosheets with n<7are unstabilization and have a tendency of forming FL h-BN-like structure. This conclusion can explain to a certain why FL bucklingc-BN-like nanosheets could not be experimentally observed so far. It is well knownthat surface hydrogenation has become an important route to modulate the structureand properties of the FL nanosheets, after hydrogenation, the c-BN nanosheets remainthe buckling structure with n>4for H-BN and for n>1BN-H. The hydrogenationwould delocalize the distribution of electrons of N or B atoms inducing the formationof sp3hybridization. With the increasing of the layer number, the effect of internal sp3would prevent the outermost layer with N-end has a tendency of forming sp2hybridized structure with less buckling feature.(4) For the2D bare c-BN nanosheets, the structural evolutions suggest that thec-BN nanosheets with n<7prefer to relax and reconstruct to form h-BN phase, then asemiconductor characteristic is presented with wide direct bandgaps in the region of4.51―4.66eV. The total magnetic moments are about1.3μBin1×1unit cell. Withthe increasing of the layer numbers, the total magnetic moments are nearly the samewith n=7. H-BN and BN-H are presented as p-type and n-type semiconductorcharacteristic with the band gap of3.5eV and3.1eV, respectively. The magneticmoments are presents on the unhydroganeted side, which is related to the surfacehybridization index. For the H-BN nanosheets with n=14, the total magneticmomenrs are0.88μB1.02μBand0.72μBfor n>4. As n=1for the BN-H nanosheetswith cubic phase, a small moment of0.69μBappears from the single layer.Simultaneously, the total magnetic moments are independent on the layer numbers forn≥2, the values of magnetic moments are0.97μB. Therefore, the distribution ofmagenetic moments are related to the hybridization index of the surface.Using the first-principle studies, we predicted the structural evolution oftwo-dimensional diamond and BN nanosheets, and further to inverstigated theelectronic and magnetic properties which are dependent on the layer numbers (n),surface modification and correspongding their vancies. It can be put the way forfinding the new materials of low-dimensional diamond and BN-based devices, whichwill be widely proposed as infrared spin devices, optical waveguides and tunneldevices.