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Topologic connection between 2-D layered structures and 3-D diamond structures for conventional semiconductors.

Wang J, Zhang Y - Sci Rep (2016)

Bottom Line: When coming to identify new 2D materials, our intuition would suggest us to look from layered instead of 3D materials.Each path is found to further split into two branches under tensile strain-low buckled and high buckled structures, which respectively lead to a low and high buckled monolayer structure.Most promising new layered or planar structures identified include BeO, GaN, and ZnO on the tensile strain side, Ge, Si, and GaP on the compressive strain side.

View Article: PubMed Central - PubMed

Affiliation: Department of Electrical and Computer Engineering, The University of North Carolina at Charlotte 9201 University City Boulevard, Charlotte, NC 28223, USA.

ABSTRACT
When coming to identify new 2D materials, our intuition would suggest us to look from layered instead of 3D materials. However, since graphite can be hypothetically derived from diamond by stretching it along its [111] axis, many 3D materials can also potentially be explored as new candidates for 2D materials. Using a density functional theory, we perform a systematic study over the common Group IV, III-V, and II-VI semiconductors along different deformation paths to reveal new structures that are topologically connected to but distinctly different from the 3D parent structure. Specifically, we explore two major phase transition paths, originating respectively from wurtzite and NiAs structure, by applying compressive and tensile strain along the symmetry axis, and calculating the total energy changes to search for potential metastable states, as well as phonon spectra to examine the structural stability. Each path is found to further split into two branches under tensile strain-low buckled and high buckled structures, which respectively lead to a low and high buckled monolayer structure. Most promising new layered or planar structures identified include BeO, GaN, and ZnO on the tensile strain side, Ge, Si, and GaP on the compressive strain side.

No MeSH data available.


Related in: MedlinePlus

Variation of the total energy vs. c/2 (the separation of the bilayer) for three cases.(a) ideal wurtzite structure with a fixed u parameter (blue), (b) hexagonal planar structure (red), and (c) allowing full relaxation but keeping the  stacking (black).
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f2: Variation of the total energy vs. c/2 (the separation of the bilayer) for three cases.(a) ideal wurtzite structure with a fixed u parameter (blue), (b) hexagonal planar structure (red), and (c) allowing full relaxation but keeping the stacking (black).

Mentions: Among all the group IV, III–V, and II–VI semiconductors, only two of them (C and BN) are known to also have a layered structure consisting of hexagonal planar (HP) layers. The layered structure can be obtained hypothetically, if not always practically feasible, by stretching WZ structure along the c axis, until the buckled bilayers collapse into HP layers (with the nearest neighbor atoms on the same plane), namely AA stacked graphite or h-BN in stacking. In a total energy calculation, this phase transition process is manifested as the appearance of a secondary total energy minimum at c0HP > c0wz 9101122, where c0wz and c0HP are respectively the c axis lattice constant for the two phases. Such a secondary minimum was not found for other semiconductors that were explored in the past, including Si, GaAs22, and BeO12. However, we have found that except for C, BN, and BeO, the total energy Etot,p(c) curve actually exhibits a secondary minimum with c0HP < c0wz, as shown in Fig. 2, for the rest compounds. The optimized in-plane (a) and out-of plane (c) lattice parameters for both the WZ and HP structure are listed in Table 1. Each panel of Fig. 2 includes three curves, respectively calculated for an ideal WZ structure (i.e., with a fixed u value), an ideal HP structure, and the distorted WZ structurewith no constrain on u. Clearly the third curve Etot(c) represents the lower-bound of the combined total energy paths described by the other two curves. All these materials fall into one of the two scenarios: (i) c0HP > c0wz, and (ii) c0HP <c0wz. Only C, BN and BeO belong to scenario (i), indicating that they can in principle form a graphite-like planar structure by applying tensile strain. Furthermore, there is an energy barrier between WZ and HP phase in the total energy curve for C, BN and BeO, which helps them to stabilize in either of the two phases. However, BeO has a small barrier of merely 6 meV/atom (missed previously12), which is less than 25 meV of room temperature thermal energy. The phonon calculation has revealed that there are imaginary modes for the HP BeO, in contrast to the case for C and BN where no imaginary mode was found. The appearance of imaginary phonon modes indicates that the structure is unstable against distortion that lowers the symmetry. These observations may explain why graphite and h-BN are the only readily available layered materials that are topologically connected to their 3D counterparts. The other six semiconductors of Fig. 2 belong to scenario (ii), implying that a planar phase could in principle appear by applying compressive strain. It may seem counterintuitive that for the material involving large atoms the planar phase would occur at smaller instead of larger c axis layer spacing than that of WZ. In fact, this is because the fact that while some vertical coupling remains a sufficiently large lateral spacing is required to allow the buckled atoms to drop down to the lower plane of the bilayer, depending on the sizes of the atoms involved. Note that the formation of the HP structure on the basal plane does not necessarily mean that the structure has sp2 + π bonding or can be considered as a layered material. However, the vertical coupling is expected to be weakened, because even though the c axis lattice constant is reduced, as shown in Fig. 1(c), the separation of the vertical atomic planes has been increased to c0HP that is greater than the largest vertical plane separation uc ≈3/8 c0wz in the WZ structure. The fact that these planar structures do not produce a secondary minimum on the total energy path Etot(c), although it nearly happens for Si, Ge, and ZnO, suggests that the HP phase cannot be a metastable phase of the system in free standing, which is further confirmed by the phonon calculations that yield imaginary modes for these planar structures. However, this finding does not preclude the possibility of forming such planar structure, if the material can be constrained by a proper substrate that can serve as a template for epitaxial growth and provide a weak bonding to the epitaxial layer (ref. 23 for ZnO, ref. 24 for silicene, ref. 25 for monolayer WS2). The role of strain in stabilizing graphitic films of InN, AlN, GaN, BeO, ZnO, and SiC has also been studied with first principle calculations26.


Topologic connection between 2-D layered structures and 3-D diamond structures for conventional semiconductors.

Wang J, Zhang Y - Sci Rep (2016)

Variation of the total energy vs. c/2 (the separation of the bilayer) for three cases.(a) ideal wurtzite structure with a fixed u parameter (blue), (b) hexagonal planar structure (red), and (c) allowing full relaxation but keeping the  stacking (black).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4835777&req=5

f2: Variation of the total energy vs. c/2 (the separation of the bilayer) for three cases.(a) ideal wurtzite structure with a fixed u parameter (blue), (b) hexagonal planar structure (red), and (c) allowing full relaxation but keeping the stacking (black).
Mentions: Among all the group IV, III–V, and II–VI semiconductors, only two of them (C and BN) are known to also have a layered structure consisting of hexagonal planar (HP) layers. The layered structure can be obtained hypothetically, if not always practically feasible, by stretching WZ structure along the c axis, until the buckled bilayers collapse into HP layers (with the nearest neighbor atoms on the same plane), namely AA stacked graphite or h-BN in stacking. In a total energy calculation, this phase transition process is manifested as the appearance of a secondary total energy minimum at c0HP > c0wz 9101122, where c0wz and c0HP are respectively the c axis lattice constant for the two phases. Such a secondary minimum was not found for other semiconductors that were explored in the past, including Si, GaAs22, and BeO12. However, we have found that except for C, BN, and BeO, the total energy Etot,p(c) curve actually exhibits a secondary minimum with c0HP < c0wz, as shown in Fig. 2, for the rest compounds. The optimized in-plane (a) and out-of plane (c) lattice parameters for both the WZ and HP structure are listed in Table 1. Each panel of Fig. 2 includes three curves, respectively calculated for an ideal WZ structure (i.e., with a fixed u value), an ideal HP structure, and the distorted WZ structurewith no constrain on u. Clearly the third curve Etot(c) represents the lower-bound of the combined total energy paths described by the other two curves. All these materials fall into one of the two scenarios: (i) c0HP > c0wz, and (ii) c0HP <c0wz. Only C, BN and BeO belong to scenario (i), indicating that they can in principle form a graphite-like planar structure by applying tensile strain. Furthermore, there is an energy barrier between WZ and HP phase in the total energy curve for C, BN and BeO, which helps them to stabilize in either of the two phases. However, BeO has a small barrier of merely 6 meV/atom (missed previously12), which is less than 25 meV of room temperature thermal energy. The phonon calculation has revealed that there are imaginary modes for the HP BeO, in contrast to the case for C and BN where no imaginary mode was found. The appearance of imaginary phonon modes indicates that the structure is unstable against distortion that lowers the symmetry. These observations may explain why graphite and h-BN are the only readily available layered materials that are topologically connected to their 3D counterparts. The other six semiconductors of Fig. 2 belong to scenario (ii), implying that a planar phase could in principle appear by applying compressive strain. It may seem counterintuitive that for the material involving large atoms the planar phase would occur at smaller instead of larger c axis layer spacing than that of WZ. In fact, this is because the fact that while some vertical coupling remains a sufficiently large lateral spacing is required to allow the buckled atoms to drop down to the lower plane of the bilayer, depending on the sizes of the atoms involved. Note that the formation of the HP structure on the basal plane does not necessarily mean that the structure has sp2 + π bonding or can be considered as a layered material. However, the vertical coupling is expected to be weakened, because even though the c axis lattice constant is reduced, as shown in Fig. 1(c), the separation of the vertical atomic planes has been increased to c0HP that is greater than the largest vertical plane separation uc ≈3/8 c0wz in the WZ structure. The fact that these planar structures do not produce a secondary minimum on the total energy path Etot(c), although it nearly happens for Si, Ge, and ZnO, suggests that the HP phase cannot be a metastable phase of the system in free standing, which is further confirmed by the phonon calculations that yield imaginary modes for these planar structures. However, this finding does not preclude the possibility of forming such planar structure, if the material can be constrained by a proper substrate that can serve as a template for epitaxial growth and provide a weak bonding to the epitaxial layer (ref. 23 for ZnO, ref. 24 for silicene, ref. 25 for monolayer WS2). The role of strain in stabilizing graphitic films of InN, AlN, GaN, BeO, ZnO, and SiC has also been studied with first principle calculations26.

Bottom Line: When coming to identify new 2D materials, our intuition would suggest us to look from layered instead of 3D materials.Each path is found to further split into two branches under tensile strain-low buckled and high buckled structures, which respectively lead to a low and high buckled monolayer structure.Most promising new layered or planar structures identified include BeO, GaN, and ZnO on the tensile strain side, Ge, Si, and GaP on the compressive strain side.

View Article: PubMed Central - PubMed

Affiliation: Department of Electrical and Computer Engineering, The University of North Carolina at Charlotte 9201 University City Boulevard, Charlotte, NC 28223, USA.

ABSTRACT
When coming to identify new 2D materials, our intuition would suggest us to look from layered instead of 3D materials. However, since graphite can be hypothetically derived from diamond by stretching it along its [111] axis, many 3D materials can also potentially be explored as new candidates for 2D materials. Using a density functional theory, we perform a systematic study over the common Group IV, III-V, and II-VI semiconductors along different deformation paths to reveal new structures that are topologically connected to but distinctly different from the 3D parent structure. Specifically, we explore two major phase transition paths, originating respectively from wurtzite and NiAs structure, by applying compressive and tensile strain along the symmetry axis, and calculating the total energy changes to search for potential metastable states, as well as phonon spectra to examine the structural stability. Each path is found to further split into two branches under tensile strain-low buckled and high buckled structures, which respectively lead to a low and high buckled monolayer structure. Most promising new layered or planar structures identified include BeO, GaN, and ZnO on the tensile strain side, Ge, Si, and GaP on the compressive strain side.

No MeSH data available.


Related in: MedlinePlus