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Robust Direct Bandgap Characteristics of One- and Two-Dimensional ReS2.

Yu ZG, Cai Y, Zhang YW - Sci Rep (2015)

Bottom Line: Two-dimensional (2D) transition-metal dichalcogenides (TMDs), most notably, MoS2 and WS2, have attracted significant attention due to their sizable and direct bandgap characteristics.In addition, the direct bandgap of ReS2 nanoribbons is only weakly dependent on their width.These robust characteristics strongly suggest that ReS2 has great potential for applications in optoelectronic nanodevices.

View Article: PubMed Central - PubMed

Affiliation: Institute of High Performance Computing, Singapore 138632, Singapore.

ABSTRACT
Two-dimensional (2D) transition-metal dichalcogenides (TMDs), most notably, MoS2 and WS2, have attracted significant attention due to their sizable and direct bandgap characteristics. Although several interesting MoS2 and WS2-based optoelectronic devices have been reported, their processability and reproducibility are limited since their electrical properties are strongly dependent of the number of layers, strain and sample sizes. It is highly desirable to have a robust direct bandgap TMD, which is insensitive to those factors. In this work, using density functional theory, we explore the effects of layer number, strain and ribbon width on the electronic properties of ReS2, a new member in the TMD family. The calculation results reveal that for monolayer ReS2, the nature (direct versus indirect) and magnitude of its bandgap are insensitive to strain. Importantly, the predicted bandgap and also charge carrier mobilities are nearly independent of the number of layers. In addition, the direct bandgap of ReS2 nanoribbons is only weakly dependent on their width. These robust characteristics strongly suggest that ReS2 has great potential for applications in optoelectronic nanodevices.

No MeSH data available.


Related in: MedlinePlus

(a) Calculated band structures of ML (black) and bulk ReS2 (red). (b) Variation of electron (red) and hole (black) effective masses with biaxial strain in ML ReS2. (c) Variation of relative energy with biaxial strain in ML ReS2. Here, we set the relative energy of the free strain system is zero. (d) Band edge shift as a function of biaxial strain in ML ReS2.
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f2: (a) Calculated band structures of ML (black) and bulk ReS2 (red). (b) Variation of electron (red) and hole (black) effective masses with biaxial strain in ML ReS2. (c) Variation of relative energy with biaxial strain in ML ReS2. Here, we set the relative energy of the free strain system is zero. (d) Band edge shift as a function of biaxial strain in ML ReS2.

Mentions: The optimized ReS2 unitcell, which exhibits in a distorted octahedral layer structure with triclinic symmetry, is shown in Fig. 1. The calculated lattice constants are a = 6.51 Å, b = 6.41 Å, and c = 6.46 Å, respectively. These calculated lattice constants are in good agreement with experimental values (a = 6.45 Å, b = 6.39 Å, and c = 6.40)17. They are also exactly the same as the theoretical results reported by Tongay et al. (a = 6.51 Å, b = 6.41 Å)12. The S-Re bond length, Re-Re distance and S-S distance in one layer are 2.43/2.37 Å, 2.81 Å, and 2.88/3.25 Å, respectively. Note that the two values of S-Re bond length and S-S distance are due to the slight lattice distortion of S atoms in bulk ReS2. Hence, ReS2 has a unique crystal structure, which is distinctively different from other TMDs, in which their graphene-like hexagonal crystal structure is composed of layers of metal atoms sandwiched between layers of chalcogen atoms. From Fig. 2a, it is seen that bulk ReS2 is a direct gap semiconductor with a bandgap of 1.30 eV, which is close to the experimental value of 1.32 eV18. Based on the optimized lattice constants of bulk ReS2, we increase the interlayer spacing c to 20 Å to build a ML ReS2 model. It is found that the optimized lattice constants of ML ReS2 are a = 6.51 Å and b = 6.41 Å, which are exactly the same as the bulk. The same optimized lattice constants of bulk and ML ReS2 indicate that the interlayer coupling is negligible. This issue will be discussed in details later. The calculated band structure of ML ReS2 is shown in Fig. 2a. It is seen that ML ReS2 also shows a direct gap semiconductor characteristic with a bandgap of 1.43 eV, which is exactly the same as recently reported value12. It is worth noting that the nature of band structure of ReS2 is independent of the number of layer, which is in strong contrast to other TMDs, such as MoS2 and WS2, in which the nature of their band structures is strongly dependent on the number of layers6.


Robust Direct Bandgap Characteristics of One- and Two-Dimensional ReS2.

Yu ZG, Cai Y, Zhang YW - Sci Rep (2015)

(a) Calculated band structures of ML (black) and bulk ReS2 (red). (b) Variation of electron (red) and hole (black) effective masses with biaxial strain in ML ReS2. (c) Variation of relative energy with biaxial strain in ML ReS2. Here, we set the relative energy of the free strain system is zero. (d) Band edge shift as a function of biaxial strain in ML ReS2.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4562306&req=5

f2: (a) Calculated band structures of ML (black) and bulk ReS2 (red). (b) Variation of electron (red) and hole (black) effective masses with biaxial strain in ML ReS2. (c) Variation of relative energy with biaxial strain in ML ReS2. Here, we set the relative energy of the free strain system is zero. (d) Band edge shift as a function of biaxial strain in ML ReS2.
Mentions: The optimized ReS2 unitcell, which exhibits in a distorted octahedral layer structure with triclinic symmetry, is shown in Fig. 1. The calculated lattice constants are a = 6.51 Å, b = 6.41 Å, and c = 6.46 Å, respectively. These calculated lattice constants are in good agreement with experimental values (a = 6.45 Å, b = 6.39 Å, and c = 6.40)17. They are also exactly the same as the theoretical results reported by Tongay et al. (a = 6.51 Å, b = 6.41 Å)12. The S-Re bond length, Re-Re distance and S-S distance in one layer are 2.43/2.37 Å, 2.81 Å, and 2.88/3.25 Å, respectively. Note that the two values of S-Re bond length and S-S distance are due to the slight lattice distortion of S atoms in bulk ReS2. Hence, ReS2 has a unique crystal structure, which is distinctively different from other TMDs, in which their graphene-like hexagonal crystal structure is composed of layers of metal atoms sandwiched between layers of chalcogen atoms. From Fig. 2a, it is seen that bulk ReS2 is a direct gap semiconductor with a bandgap of 1.30 eV, which is close to the experimental value of 1.32 eV18. Based on the optimized lattice constants of bulk ReS2, we increase the interlayer spacing c to 20 Å to build a ML ReS2 model. It is found that the optimized lattice constants of ML ReS2 are a = 6.51 Å and b = 6.41 Å, which are exactly the same as the bulk. The same optimized lattice constants of bulk and ML ReS2 indicate that the interlayer coupling is negligible. This issue will be discussed in details later. The calculated band structure of ML ReS2 is shown in Fig. 2a. It is seen that ML ReS2 also shows a direct gap semiconductor characteristic with a bandgap of 1.43 eV, which is exactly the same as recently reported value12. It is worth noting that the nature of band structure of ReS2 is independent of the number of layer, which is in strong contrast to other TMDs, such as MoS2 and WS2, in which the nature of their band structures is strongly dependent on the number of layers6.

Bottom Line: Two-dimensional (2D) transition-metal dichalcogenides (TMDs), most notably, MoS2 and WS2, have attracted significant attention due to their sizable and direct bandgap characteristics.In addition, the direct bandgap of ReS2 nanoribbons is only weakly dependent on their width.These robust characteristics strongly suggest that ReS2 has great potential for applications in optoelectronic nanodevices.

View Article: PubMed Central - PubMed

Affiliation: Institute of High Performance Computing, Singapore 138632, Singapore.

ABSTRACT
Two-dimensional (2D) transition-metal dichalcogenides (TMDs), most notably, MoS2 and WS2, have attracted significant attention due to their sizable and direct bandgap characteristics. Although several interesting MoS2 and WS2-based optoelectronic devices have been reported, their processability and reproducibility are limited since their electrical properties are strongly dependent of the number of layers, strain and sample sizes. It is highly desirable to have a robust direct bandgap TMD, which is insensitive to those factors. In this work, using density functional theory, we explore the effects of layer number, strain and ribbon width on the electronic properties of ReS2, a new member in the TMD family. The calculation results reveal that for monolayer ReS2, the nature (direct versus indirect) and magnitude of its bandgap are insensitive to strain. Importantly, the predicted bandgap and also charge carrier mobilities are nearly independent of the number of layers. In addition, the direct bandgap of ReS2 nanoribbons is only weakly dependent on their width. These robust characteristics strongly suggest that ReS2 has great potential for applications in optoelectronic nanodevices.

No MeSH data available.


Related in: MedlinePlus