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Formation and Physical Properties of h-BN Atomic Layers: A First-Principles Density-Functional Study.

1. Introduction

Since the discovery of a single atomic layer of graphite, that is, graphene, other atomic layers have received much attention [1-8]. Hexagonal boron nitride (h-BN) atomic layers, in which alternating boron and nitrogen atoms are arranged on the honeycomb lattices, have also gained a lot of interest because of the similar structural and mechanical properties to graphene [9-11]. On the other hand, electronic properties of h-BN layers are considerably different from those of graphene; graphene is a gapless material with metallic conduction [12], and the h-BN atomic layers possess a wide band gap [13-15]. Such wide band gap nature of h-BN layers is expected to give rise to new physical properties as well as relevant electronic/optoelectronic applications in nanotechnology.

For example, h-BN atomic layers exhibit high optical emission intensities in deep ultraviolet (UV) regions [13,16-18], and the h-BN bilayers can transform from an indirectgap semiconductor to a direct-gap one by applying strains [19]. Thus, h-BN layers are highly important optoelectronics device materials used in deep UV lasers and light-emitting diodes [13, 17, 20-23]. The atomic vacancies in h-BN layers can be introduced using electron beam irradiations and are shown to be useful as a source of magnetism [10,24-27]. The introductions of C atoms and graphene-like flakes into h-BN sheets have been also performed via electron beam irradiations due to analogous structural properties among B, C, and N elements [28-36], which can tune the electronic and the magnetic properties [37-39]. It is predicted theoretically that the energy band gaps are tunable depending on the size of the graphene flakes [37, 38]. The electronic transport properties of the h-BN atomic layers can be changed from insulating to conductive properties when C atoms are doped, suggesting a possibility to fabricate the novel opto/nanoelectronics applications [31, 32, 34-36, 40]. Furthermore, when the C atom is doped, the exotic conduction channel in h-BN monolayers would emerge, which might improve electronic transport properties [41-46]. Thus, h-BN atomic layers are promising materials to be used in nano electronics and optoelectronics applications.

The purpose of this review is to provide the recent progress of the first-principles density-functional calculations that clarify various physical properties of h-BN atomic layers with lattice defects including atomic vacancies and carbon defects. In Section 2, the magnetic properties of atomic vacancies in h-BN layer are discussed. Section 3 is devoted to discussions on the electronic properties of carbon-doped h-BN layers. In Section 4, the behaviors of the magnetic properties and the energy band gaps are shown with the variation of the graphene-like defect size. Finally, Section 5 summarizes this paper together with outlook.

2. Atomic Vacancy

Atomic vacancies in h-BN layers can be created by electron beam irradiation [10, 25-27]. Triangular-shaped vacancies are observed experimentally by using high-resolution transmission electron microscopy and these vacancies in h-BN layers are arranged with various sizes and peculiar orientation [10]. It is therefore surmised that the B atoms are removed more easily than the N atoms from h-BN monolayers by electron beam irradiation [10].

The introductions of the atomic vacancies into the h-BN monolayers are reported to give rise to the magnetic moment [24, 47]. In Figure 1(a), the schematic view of the triangular atomic vacancies in the h-BN monolayer is shown, where the multivacancies [V.sub.T] are defined with T = [square root of [n.sub.B] + [n.sub.N]] and [n.sub.B] and [n.sub.N] are the numbers of removed B and N atoms, respectively. The total magnetic moments of [V.sup.N.sub.1], [V.sup.N.sub.2], and [V.sup.N.sub.3] are calculated to be 1, 0, and 3[[mu].sub.B], respectively. Unlike graphene [48], the behavior of vacancies [V.sup.N.sub.T] in the magnetic properties does not follow Lieb's theorem because of the structural reconstructions with the N-N bond formation [49]. When the N-N bond formation is broken, the total magnetic moment increases. The total magnetic moment of the [V.sup.N.sub.2] defect increases as the N-N distance around the vacancy increases, though the ground state of [V.sup.N.sub.2] exhibits zero magnetic moment (see Figure 1(b)).

3. Carbon Impurity

Substitutional carbon doping to h-BN layers and BN nanotubes has been performed using the electron beam irradiation technique [32, 33] and substitutionally doped C atoms in the h-BN monolayers have been observed via a transmission electron microscopy method [31]. It was found that the substitution with C atoms decomposed from hydrocarbon molecules takes place mostly at B atom sites, and it was surmised that the substitutional doping proceeds by repairing the B atom vacancies with C atoms broken by the knock-on electron beam [32, 34].

Recent first-principles calculations showed that the substitution of B atoms with C atoms needs less energy costs than that of N atoms under N-rich conditions. Moreover, it was shown that positive charging favors the substitution of B atom with C atom (CB), whereas negative charging favors the substitution of N atom with C atom (CN). In addition, it was shown that the substitution of B atom with C atoms may predominately take place even under B-rich conditions [34, 35].

It was reported that the electronic transport properties of h-BN layers can transform from insulating to conductive properties when C atom is doped [32]. The donor-like impurity states appear below the conduction-band minimum (CBM) when C atom is doped at B atom site, whereas the substitution of N atom with C atom gives rise to the acceptor-like impurity states above the valence-band maximum (VBM) [35, 41]. It was shown that ionization energies for acceptor-like as well as donor-like states can be controlled by applying strains (see Figure 2(a)) [41]. The strains can often produce not only the new structural properties but also the novel electronic properties [19, 50, 51]. Interestingly, in the case of the h-BN monolayers, the exotic transport channel will behave as an active state under more than ~1% compressive strains (Figure 2(b)). In addition, the band gaps are also tunable under strains [52-54].

Scanning tunneling microscopy (STM) measurements are one of the effective tools to observe the surface electronic structures at atomic level [55-61]. The B atoms and N atoms in graphene can be clearly identified experimentally and theoretically [62-66]. The STM image of the h-BN monolayer has a large bright triangular shape around the C defect surrounded by six bright spots above each N atom, since the C defect state consists of the triangular-shaped spatial distributions of local density of states (LDOS), and in addition to this defect state, the n states in the valence bands contribute the STM image (Figure 3(a)). On the other hand, for the C substitution of the B atom, the STM image has a small dark spot above the C defect which is surrounded by six bright spots above each B atom (Figure 3(b)) [41]. The C defects in the h-BN monolayers might be identified by using STM methods.

4. Carbon Flake

The h-BN monolayers embedded with triangular graphene flakes can modify the electronic properties as well as the magnetic properties [34, 37]. The magnetic moments of the graphene-like flake embedded h-BN monolayer can be controlled depending on the number of the substituted C atoms in the h-BN monolayers (Figure 4), where [alpha] and [beta] are the numbers of the B atoms and the N atoms replaced with the C atoms and positive and negative ([alpha] - [beta]) values are used for the T1-[C.sub.[alpha]][C.sub.[beta]] and T2- [C.sub.[alpha]][C.sub.[beta]] structures, respectively. T1-[C.sub.[alpha]][C.sub.[beta]] and T2-[C.sub.[alpha]][C.sub.[beta]] structures give rise to [absolute value of [alpha] - [beta]][[mu].sub.B] magnetic moments per unit cell. By tuning the sizes of the triangle graphene flakes, the magnetic moments of T1- and T2-graphene-embedded BN sheet are controllable.

The energy band gap values of h-BN monolayers are shown to be tunable [37-39]. By replacing B and N atoms with graphene quantum dots (QD), the energy band gaps of h-BN monolayers decrease. As the size d of the graphene QD increases, it was shown that the band gaps diminish from ~3.6 eV to ~1.6 eV (Figure 5).

5. Concluding Remarks

We have reviewed the recent works of the first-principles density-functional study of h-BN atomic layers. The atomic vacancies can be created in the h-BN layers by electron beam irradiation techniques. The triangular-shaped atomic vacancies in h-BN layers can give rise to the magnetic moment depending on the vacancy sizes, and moreover the magnetic moment is shown to be tunable under strain.

The C atom can be doped to the h-BN layers by using electron beam irradiation combined with introducing the hydrocarbon molecules, and it is deduced that C atoms are doped more easily at B atom sites than at N atom sites. The first-principles density-functional calculations have revealed that the substitution of B atoms with C atoms becomes more favorable in energy than that of N atoms under Nrich conditions. The substitutions of the B atom and the N atom with the C atom induce the donor-like state below the CBM and the acceptor-like state above the VBM, respectively. The ionization energies are controllable for the donor and the acceptor states by applying strain, and furthermore the exotic electronic state can be opened as an active conduction channel under strain.

The graphene-like flake in the h-BN layers can modify the magnetic and the electronic properties. By substituting B atoms and N atoms at the two different sublattices with C atoms, the magnetic moment of the h-BN layers can be tuned. In addition, the variation of the size of the QD can tune the energy band gaps.

By tuning not only the sizes of the atomic vacancies and the C atoms flake but also strains, new magnetic and electronic properties of the h-BN layers would emerge, which might provide the novel nanoelectronics, optoelectronics, and spintronics devices.

https://doi.org/10.1155/2017/2676432

Conflicts of Interest

The author declares that there are no conflicts of interest regarding the publication of this paper.

Acknowledgments

This work was partly supported by MEXT Elements Strategy Initiative to Form Core Research Center through Tokodai Institute for Element Strategy and JSPS KAKENHI (Grants nos. JP17K05053 and JP26390062). Computations were partly done at Institute for Solid State Physics, the University of Tokyo, and at Cybermedia Center of Osaka University.

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Yoshitaka Fujimoto

Department of Physics, Tokyo Institute of Technology, Tokyo, Japan

Correspondence should be addressed to Yoshitaka Fujimoto; fujimoto@stat.phys.titech.ac.jp

Received 15 June 2017; Accepted 20 July 2017; Published 22 August 2017

Academic Editor: Achim Trampert

Caption: Figure 1: (a) Schematic view of h-BN monolayers for atomic vacancies. (b) Total energy and N-N distance near [V.sup.N.sub.2] defect as a function of imposed magnetic moment. Reproduced with permission from [24]; copyright 2012, the American Institute of Physics.

Caption: Figure 2: (a) Relative ionization energy ([DELTA]IE) for acceptor and donor states in C-doped h-BN monolayers as a function of applied strain. (b) Contour plot of electron density of C-doped h-BN monolayer at the r point of the CBM, where B atom is replaced by C atom. Reproduced with permission from [41]; copyright 2016, the American Physical Society.

Caption: Figure 3: Calculated STM images of C-doped h-BN monolayers, where C atom is replaced at (a) the N site and (b) the B site. Reproduced with permission from [41]; copyright 2016, the American Physical Society.

Caption: Figure 4: Variation of magnetic moments of T1- [C.sub.[alpha]][C.sub.[beta]] and T2- [C.sub.[alpha]][C.sub.[beta]] with ([alpha] - [beta]). Reproduced with permission from [37]; copyright 2011, the American Physical Society.

Caption: Figure 5: Energy band gap value [E.sub.g] as a function of QD diameter d. Reproduced with permission from [38]; copyright 2011, the American Institute of Physics.
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