Environmental Engineering Reference
In-Depth Information
27.1
Bn surfaces
Fourier-transform infrared (FT-Ir) spectrometry analysis of the turbostratic boron nitride (t-Bn) powder surface activity
revealed [8] BHn
2
, B
2
nH, and some other hydrogen-containing species. shrestha et al. [9] also reported the results of
Bn-substrate adsorption studies performed on powder samples subjected various cleaning treatments. The adsorption of H
species on B- versus n-atoms on the hexagonal boron nitride (h-Bn) (001) surface was investigated [10] theoretically within
the density functional theory (dFT) using a cluster approach. Only the B atoms were subjected to a local transformation from
the hexagonal (h-Bn) to cubic (c-Bn) phase. Upon adsorption of the H species, a neighboring surface B atom was substituted
by a C atom (hence, the number of electrons in the system increased): in the presence of surface C impurities on the electron-
deficient B surface, the adsorption of H on these C impurities will yield an embryonic cubic nucleus. The adsorption of H
species resulted in a local transformation from
sp
2
- to
sp
3
-hybridization with a hereby connected negative adsorption energy
(−55 kJ/mol). This adsorption process is therefore highly unlikely to occur in a chemical vapor deposition (CVd) synthesis, that
is, at temperatures in the 600-1000°C range. reactivities of hydrogen atom/ions with different boron nitride, as well as carbon,
phases were studied in detail [11] by means of frontier orbital theory based on Hartree-Fock (HF) calculations. The results
showed that there is a significant difference in the reactivity of atomic hydrogen with the
sp
2
- and the
sp
3
-carbon phases such
that phase selectivity is facilitated during the CVd of diamond. In contrast, these reactivities of atomic hydrogen with Bn
phases are similar, indicating the difficulty in obtaining a pure c-Bn phase via CVd. In addition, hydrogen ions show higher
reactivity than their neutrals, whereas hydrogen anions show similar reactivity with the two carbon or the two Bn phases. CH
3
species were found to be promoters for the preferential etching selectivity of hydrogen in Bn growth. The joining of methyl
species in the etching process of hydrogen over Bn phases would alter the etching preference of the hydrogen.
The electronic properties of 2d hydrogenated and semihydrogenated h-Bn sheets were investigated [12] using first-principles
calculations. It was found that the hydrogenation effects in the Bn sheets are quite different from those in the graphene sheets.
Hydrogenation changes the band character of Bn sheets, which causes the hydrogenated Bn sheet to have a smaller band gap
than the pristine one. While for the semihydrogenated sheet, the stable B-semihydrogenated Bn sheet is a ferromagnetic metal
due to the unpaired 2
p
z
electrons of n atoms. These studies demonstrated that the electronic properties of Bn sheets can be well
tuned by hydrogenation. A control on the H functionalizations of H-Bn structures by carrier doping was revealed [13] using
dFT calculations. When the system is electron-doped, H-adatoms will exclusively bond with B-atoms, resulting in possible mag-
netization of the system, whereas hole-doping favors the adatoms to form insulating orthodimer structures on the Bn structures.
This behavior is caused by a peculiar chemical bond between the n- and H-adatoms, whose strength significantly depends on the
carrier type and level. Moreover, the adatoms' diffusion on these Bn structures can be steered along a designable path by the carrier
doping still attributed to the carrier-dependent bond stability. This carrier control of functionalizations is robust via H-adatom
concentration and the physical conditions of Bn structures, thus offering an easy route to controllably anchor the properties of
functionalized Bn systems for desired applications.
The structural and electronic characteristics of fully hydrogenated Bn layers and zigzag-edged nanoribbons were investi-
gated [14] using dispersion-corrected dFT calculations. In the fully hydrogenated Bn structure, the hydrogen atoms adsorb on
top of the B and n sites, alternating on both sides of the h-Bn plane in a specific periodic manner. Among various low-energy
hydrogenated membranes referred to as chair, boat, twist-boat, and stirrup, the stirrup conformation is the most energetically
favorable one. The zigzag-edged Bn nanoribbon, prominently fabricated in experiments, possesses intrinsic semimetallicity
with full hydrogenation. The semimetallicity can be tuned by applying a transverse electric bias, thereby providing a promising
route for spintronics device applications. Using dFT, a series of calculations of structural and electronic properties of hydrogen
vacancies in a fully hydrogenated Bn layer were conducted [15]. By dehydrogenating the H-Bn structure, B-terminated
vacancies can be created which induce complete spin polarization around the Fermi level, irrespective of the vacancy size.
On the contrary, the H-Bn structure with n-terminated vacancies can be a small-gap semiconductor, a typical spin gapless
semiconductor, or a metal depending on the vacancy size. Utilizing such vacancy-induced band gap and magnetism changes,
possible applications in spintronics can be proposed, and a special H-Bn-based quantum dot device designed.
Using first-principles calculations, ding et al. [16] investigated the structural and electronic properties of monolayer
porous Bn, and also of graphene (C) and BC
2
n sheets, with different hydrogen passivations. All these porous sheets with
one-hydrogen passivation exhibit direct band gap semiconducting behaviors. The porous Bn sheet has a larger band gap
than the porous C sheet, whereas the porous BC
2
n sheets have variable band gaps depending on the atomic arrangements
of B, C, and n atoms. The stablest conformation of porous BC
2
n sheets is composed of C and Bn hexagons, whereas with
two-hydrogen passivation, it becomes a structure containing continuous Bn and interrupted C zigzag lines. Furthermore,
due to the
sp
3
-hybridization of the edge atoms, the two-hydrogen passivation induces the changes of band gaps as well as
direct-to-indirect band gap transitions in all the porous sheets. These studies demonstrated that the porous C, Bn, and BC
2
n
sheets have semiconducting behaviors with practical band engineering by different values of hydrogen passivation. The
