A novel three dimensional semimetallic MoS2Zhen-Kun Tang, Hui Zhang, Hao Liu, Woon-Ming Lau, and Li-Min Liu

Citation: Journal of Applied Physics 115, 204302 (2014); doi: 10.1063/1.4879241 View online: http://dx.doi.org/10.1063/1.4879241 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/115/20?ver=pdfcov Published by the AIP Publishing Articles you may be interested in On the mobility and contact resistance evaluation for transistors based on MoS2 or two-dimensionalsemiconducting atomic crystals Appl. Phys. Lett. 104, 113504 (2014); 10.1063/1.4868536 Atomistic full-band simulations of monolayer MoS2 transistors Appl. Phys. Lett. 103, 223509 (2013); 10.1063/1.4837455 Work function modulation of bilayer MoS2 nanoflake by backgate electric field effect Appl. Phys. Lett. 103, 033122 (2013); 10.1063/1.4816076 Thermoelectric performance of MX2 (M=Mo,W; X=S,Se) monolayers J. Appl. Phys. 113, 104304 (2013); 10.1063/1.4794363 Electronic structure and optical conductivity of two dimensional (2D) MoS2: Pseudopotential DFT versus fullpotential calculations AIP Conf. Proc. 1447, 1269 (2012); 10.1063/1.4710474

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A novel three dimensional semimetallic MoS2

Zhen-Kun Tang,1,2 Hui Zhang,1 Hao Liu,3 Woon-Ming Lau,1,3 and Li-Min Liu1,a)

1Beijing Computational Science Research Center, Beijing 100084, China2Departments of Physics and Electronics, Hengyang Normal University, Hengyang 421008, China3Chengdu Green Energy and Green Manufacturing Technology R&D Center, Chengdu, Sichuan 610207,China

(Received 3 April 2014; accepted 10 May 2014; published online 22 May 2014)

Transition metal dichalcogenides (TMDs) have many potential applications, while the performances

of TMDs are generally limited by the less surface active sites and the poor electron transport

efficiency. Here, a novel three-dimensional (3D) structure of molybdenum disulfide (MoS2) with

larger surface area was proposed based on first-principle calculations. 3D layered MoS2 structure

contains the basal surface and joint zone between the different nanoribbons, which is

thermodynamically stable at room temperature, as confirmed by first principles molecular dynamics

calculations. Compared the two-dimensional layered structures, the 3D MoS2 not only owns the

large surface areas but also can effectively avoid the aggregation. Interestingly, although the basal

surface remains the property of the intrinsic semiconductor as the bulk MoS2, the joint zone of 3D

MoS2 exhibits semimetallic, which is derived from degenerate 3d orbitals of the Mo atoms. The

high stability, large surface area, and high conductivity make 3D MoS2 have great potentials as high

performance catalyst. VC 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4879241]

I. INTRODUCTION

In the past ten years, tremendous attention has been

paid to graphene, the thinnest material with exceptional me-

chanical, thermal, electronic, magnetic, and optical

properties.1–4 These outstanding properties of graphene are

due to its atomic-layer thickness and two-dimensional (2D)

morphology. Unfortunately, the absence of band gap in

pristine graphene severely limits its uses for electronic de-

vice. To overcome this, more efforts have been devoted to

study other layered structure materials.5–8 In particular,

transition metal dichalcogenides (TMDs) have received sig-

nificant attentions because of their sizable band gaps.9–11

Among dozens of layered TMDs, monolayer MoS2 with a

direct band gap of 1.8 eV (Ref. 12) has attracted particular

interests because of their many potential applications in

electro-catalysts,13–15 transistors,16–18 photolumines-

cence,19 and rechargeable batteries.20

Although the monolayer MoS2 has large surface area and

direct band gap, while the performance of MoS2 as catalyst is

greatly hindered by less surface active sites and the poor elec-

tron transport efficiency. Thus, it is great desire to design the

new structure to further improve the catalytic efficiency of

MoS2, especially for the hydrogen evolution reaction

(HER).21–24 Both theoretical25 and experimental14 works sug-

gest that the edge atoms of the MoS2 play the vital role for

the HER activity, while the atoms of basal surface are catalyt-

ically inert. Thus, the catalytic activity of MoS2 is closely

related to its surface areas, because of the active sites only

appear on the surface. Although the two-dimensional (2D)

layered materials have large surface area, they can easily

recover back to the multilayers because of the van der Waals

(vdW) interaction. Besides this, the poor electrical transport

of MoS2 is the another bottleneck of the catalytic activity.26

Thus, tremendous efforts have been made to improve active

sites21,22,24 and higher conductivity of MoS2.23,24

Generally, 2D material, such as graphene, tends to form

irreversible agglomerates or even restack to form graphite

due to their strong p–p stacking and vdW interactions

between the inter-sheet of graphene, resulting in a dramatic

decrease of the surface area. To fully utilize of the high

intrinsic surface area of 2D material, three-dimensional (3D)

layered materials have attracted tremendous attention and

research interest, owing to their exceptional porous structures

in combination with the inherent electronic properties.27 A

number of different approaches have been employed for the

fabrication of 3D graphene assemblies,28–30 such as flow

directed assembly, layer-by-layer deposition, template-

directed method, and leavening strategy to assemble gra-

phene sheets into the layered and porous 3D macroscopic

structures. The obtained 3D graphene structures show out-

standing electrical and mechanical properties, which enable

them applications in high-performance supercapacitor31,32

and high-performance Li batteries.33,34 In principle, 3D gra-

phene structures can be constructed by zigzag or armchair

graphene nanoribbons as building blocks and sp3 carbon

chains as junction nodes.35 More recently, the novel 3D BN

structures with metallic characteristics have also been studied

by the first principle calculations.36 MoS2 is a widely used

industrial catalyst for photocatalytic hydrogen production,

thus 3D MoS2 structure is greatly expected to solve the prob-

lems of the electron transport and surface area.

In this work, a novel 3D MoS2 model is proposed, which

contains nanosheet and nanoribbons as building blocks. The

mechanical and electronic properties are further calculated

based on density functional theory (DFT). The results show

that the distorted ligand field of the Mo atom on the junction

becomes semimetallic, while the basal surface remains thea)Email: limin.liu@csrc.ac.cn

0021-8979/2014/115(20)/204302/6/$30.00 VC 2014 AIP Publishing LLC115, 204302-1

JOURNAL OF APPLIED PHYSICS 115, 204302 (2014)

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semiconductor. The calculated electronic properties suggest

that the origin of the semimetallic properties is due to the dis-

torted crystal field at the joint zone. First principles molecular

dynamics (FPMD) simulation demonstrates that the relative

stability of the 3D MoS2 is greatly affected by the pore size

and some 3D MoS2 with small pore size (about 10–15 A) are

thermodynamically stable at room temperature. Because of

the huge surface areas and inherent semimetallic properties

of the joint zone, the 3D MoS2 is a prominent alternative

photocatalyst.

II. COMPUTATIONAL METHOD

The first-principles structure and energy calculations are

performed using the Vienna Ab Initio Simulation Package

(VASP).37,38 Projector augmented-wave (PAW) pseudopo-

tentials39 were used to account electron-ion interactions. The

generalized gradient approximation (GGA) with the PBE

function40 was used to treat the exchange-correlation interac-

tion between electrons. The energy cutoff was set to 500 eV

and Monkhorst-Pack scheme was used to sample Brillouin

zone.41 The full geometry optimizations are carried out with

the convergence thresholds of 10�6 eV and 5� 10�3 eV/A for

total energy and ionic force, respectively. It is well-known

that vdW interactions are crucial in the determination of the

equilibrium configurations in layered materials such as

MoS2.42 DFT-D2 approach was used in order to take the

effect of the vdW interaction.43

The mixed Gaussian and plane-wave basis set code

CP2K/Quickstep package44 has been used mostly for the

FPMD simulations. The PBE pseudopotentials and the

Gaussian functions with a double-f polarized basis set (DZVP)

were used for the FPMD simulations. For the auxiliary basis

set of plane waves, a 320 Ry cut-off is used. In the FPMD sim-

ulations, the canonical ensemble was employed45,46 with a tar-

get temperature of 300 K, maintained with a Nose-Hoover

chain thermostat.

III. RESULTS AND DISCUSSION

A. The structure of the 3D MoS2

Layered MoS2 has two conventional crystalline phases:

trigonal (1T) and hexagonal (2H).47 The 1T MoS2 is a meta-

stable metallic structure, and it can be transformed into more

stable semiconducting 2H MoS2 during the Li intercala-

tion.48,49 The stable 2H MoS2 consists of two 2D parallel tri-

angular lattices of S atoms, separated by the same lattice of

Mo atoms translated by 1/3 of the unit-cell diagonal.

In this work, the calculated lattice constant a of mono-

layer 2H MoS2 is 3.196 A, and the distance between the Mo

and S atom plane is 1.561 A. These structural parameters of

monolayer 2H MoS2 agree well with the other DFT calcula-

tions, which gave the lattice constant of a¼ 3.19 A and the

distance between the Mo and S atom planes is 1.567 A.50,51

As showed in Figure 1, the 3D MoS2 was constructed by

two different basic units. The horizontal layer is the normal

monolayer MoS2, and the vertical layer is the zigzag MoS2

nanoribbon. In principle, many different sizes of 3D MoS2

can be constructed through adjusting the ratio between the

horizontal and vertical units. In order to distinguish the dis-

tinct sizes of 3D structures, m and n were used to represent

the number of horizontal and vertical Mo atoms per unit cell,

respectively. For example, a 3D MoS2 supercell contains 6

horizontal Mo atoms and 4 vertical Mo atoms, and we refer

this 3D structure as 3DM-64 in the following (see Figures

1(a) and 1(b)). The unit cell of 3DM-64 structure contains 10

Mo atoms and 18 S atoms. In order to know the size effect,

several other models were also considered with the different

length of the vertical basic units from 3 to 6. Thus, four rep-

resentative models, 3DM-63, 3DM-64, 3DM-65, and

3DM-66, are examined in this work.

The basal surface of monolayer MoS2 contains Mo

atoms, which are sandwiched between two outside layers of

S atoms. When the basal surface is connected with a zigzag

MoS2 ribbon (see Figure 1(a)), the outer S atoms readily

bonds with the edge Mo atoms of the zigzag MoS2 ribbon to

form the 3D structure. The optimized lattice constants of the

different sizes of 3D MoS2 are listed in Table I. The calcu-

lated lattice constants of the 3D MoS2 apparently indicate

that the layered MoS2 experiences some distortion in order

to bond with the zigzag nanoribbon. The S atoms of the joint

zone have more Mo neighbors, which should affect the bond

length of Mo-S. The relaxed bond lengths of Mo-S at the

joint zone are listed in Table I. The longest Mo-S bond

length is 2.599 A and the shortest is 2.291 A. Compared with

the initial Mo-S bond length of 2.417 A, all the variation of

Mo-S bond length is within 10%.

B. Thermodynamic stability of the new 3D MoS2

In order to know the thermodynamic stability of the 3D

MoS2, several FPMD simulations for the different sizes 3D

MoS2 (3DM-63, 3DM-64, 3DM-65, and 3DM-66) were car-

ried out at the finite temperatures of 300 K. The structure

evolutions from the different initial configurations as a func-

tion of time are shown in Figure 2. The four different sizes

of 3D structures show the different behaviors during the

FPMD simulations. After 1000 fs FPMD simulations, the

Mo-S bond lengths do not change obviously for the 3DM-63

FIG. 1. Top (a) and side (b) views of the 3D MoS2 with 6 horizontal and 4

vertical Mo atoms (3DM-64). In order to distinguish the distinct size of 3D

structures, m and n were used to represent the number of horizontal and ver-

tical Mo atoms per unit cell, respectively. (c) The detailed atomic structure

of the joint zone for the 3D MoS2 structures, and the bond lengths for the

different Mo-S are labeled as B1 to B11, and the detailed bond lengths are

given in Table II. The yellow and glaucous balls represent sulphur atoms

and molybdenum atoms, respectively.

204302-2 Tang et al. J. Appl. Phys. 115, 204302 (2014)

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and 3DM-64 structures, while Mo-S bond lengths of the joint

zone increase significantly for the 3DM-65 and 3DM-66. At

the time of 1000 fs, the largest Mo-S bond lengths of the

3DM-65 and 3DM-66 become 3.695 and 2.907 A, respec-

tively. Compared with the Mo-S bond length of 2.417 A in

monolayer 2H MoS2, the Mo-S bond lengths of the joint

zone for 3DM-65 and 3DM-66 increase by 53% and 20%,

respectively. Thus, FPMD simulations show that the

3DM-65 and 3DM-66 may be unstable, while the 3DM-63

and 3DM-64 should be thermodynamically stable at room

temperature.

C. The electronic structure of the 3D MoS2

As shown above, the 3D MoS2 should be stable at the

room temperature, and such 3D MoS2 not only owns rela-

tively large surface area but also it can avoid the aggregation

between the layered structures. It is well-known that mono-

layer MoS2 is direct semiconductor with a band gap of

1.8 eV, thus it is necessary to know whether the 3D MoS2

owns the unique electronic structure. In order to know the

detailed electronic structures of the 3D MoS2, the band struc-

tures for the four different sizes of 3D MoS2 are investigated,

and the calculated results are shown in Figure 3. The band

structure of either 3D MoS2 is obviously different from that

of the monolayer MoS2. There is no any overlap between the

bottom of the conduction band and the top of the valence

band for either 3D MoS2, and thus all the four considered

sizes of 3D MoS2 structures are semimetal. The band struc-

tures of the four sizes of 3D MoS2 near the Fermi level are

quite similar, that is, to say, all 3D MoS2 exhibits the same

kind of electronic properties of semimetallicity. In the fol-

lowing, we mainly discuss the electronic structures of

3DM-64, except we noted.

The band structure of MoS2 is sensitive to the crystal

structure symmetry. 2H-MoS2 is a semiconductor with a

band gap of 1.8 eV between the filled dz2 and empty dx2�y2;xy

bands, while the 1T phase is metallic with the Fermi level

lying in the middle of degenerate dxy;yz;xz single band.52 More

importantly, edges in 2D MoS2 are vital for the band struc-

tures.53,54 Considering the distorted ligand field of the Mo

atoms of the joint zone, the semimetallic properties should

come from the Mo atoms of the joint zone. To confirm it, the

corresponding electronic densities near the Fermi level for

the different 3D MoS2 are plotted in Figure 4. In all consid-

ered sizes of the 3D MoS2 structures, the electronic densities

near the Fermi level are mainly distributed on the Mo atoms

of the joint zone. It should be noted that no electronic den-

sities are distributed on the Mo atoms of basal surface for all

3D MoS2, which indicates that the basal surface remains the

intrinsic feature of semiconductor. Considering the semime-

tallic properties of 3D MoS2 structures around the joint zone,

electron transport along the joint zone should be rather

quick.

Based on the atom positions and the distributions of

electronic densities for the four 3D MoS2, the Mo atoms can

be easily divided into two types (see Figure 4): Mo of the

joint zone (labeled as Mo-J1, Mo-J2, and so on) and Mo of

the others zone (labeled as Mo-O1, Mo-O2, and so on). In

order to know the detailed difference of Mo atoms between

these two groups, Bader charge density analysis is used. In

this work, Bader charge density analysis employed the

so-called zero flux surfaces to divide atoms, and the Bader

charge density analysis is only dependent on charge density

distribution, which can be used to quantify the cost of

removing charge from an atom.55 The charge enclosed

within the Bader volume is a good approximation to the total

electronic charge of an atom.56,57

Because of the similar structures and electronic density

distributions of different sizes of 3D MoS2, the typical

3DM-64 is selected as a representative structure for Bader

FIG. 2. The structure revolution of the different sizes of 3D MoS2 as a func-

tion of the FPMD simulation times: (a) 3DM-63; (b) 3DM-64; (c) 3DM-65;

(d) 3DM-66. The changes of Mo-S bond lengths for the joint zone are shown

for the typical structures. The unit is A.

TABLE I. The optimized lattice constants and the representative bond

lengths of the four typical 3D MoS2 structures. Here, the 3DM-63 represents

that the system contains 6 horizontal and 3 vertical Mo atoms per unit cell.

Parameters 3DM-63 3DM-64 3DM-65 3DM-66

a (A) 3.200 3.200 3.200 3.200

b (A) 16.910 16.907 16.904 16.906

c (A) 11.824 14.576 17.351 20.113

a (deg) 95.623 95.333 88.674 94.307

b (deg) 94.169 92.903 92.807 92.096

c (deg) 89.506 90.504 91.172 90.515

B1 (A) 2.411 2.443 2.291 2.415

B2 (A) 2.425 2.471 2.590 2.420

B3 (A) 2.599 2.425 2.509 2.474

B4 (A) 2.465 2.410 2.440 2.463

B5 (A) 2.471 2.556 2.432 2.550

B6 (A) 2.374 2.445 2,431 2.373

B7 (A) 2.567 2.500 2.505 2.569

B8 (A) 2.499 2.565 2.576 2.499

B9 (A) 2.446 2.373 2.559 2.445

B10 (A) 2.555 2.549 2.428 2.554

B11 (A) 2.426 2.353 2.313 2.424

B12 (A) 2.353 2.425 2.516 2.349

204302-3 Tang et al. J. Appl. Phys. 115, 204302 (2014)

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charge analysis. The calculated results are shown in Table II.

Mo atoms of the joint zone (Mo-J) have relatively larger

Bader charges and Bader volume than those of basal surface,

indicating that more electrons are localized on the joint zone.

The Bader charge of each Mo atom for basal surface (Mo-O)

is very close to the Bader charge (10.827 e) of Mo atoms for

monolayer 2H MoS2.

To further explore the origin of semimetallicity, both

density of states (DOS) and projected DOS of representative

3DM-64 structure are plotted. As shown in Figure 5(a), only

a negligible state at the Fermi level exists, which is typical

characteristic of semimetal. The DOS at the Fermi level is

mainly contributed by the Mo atoms of joint zone. It is well-

known that the band gap of 2H-MoS2 origins from the

splitting of Mo 3d bands.52 In the following, we will also

consider the 3d orbitals of Mo atoms.

Both the 3d DOS of Mo-J and Mo-O for 3DM-64 struc-

ture are shown in Figure 5(b). It is obvious that these two dif-

ferent Mo atoms exhibit distinct electronic properties. The

former (Mo-J) is semimetallic, while the latter (Mo-O)

shows the semiconducting behavior as the monolayer MoS2

(see Figure 5(b)). In the joint zone, the distorted ligand field

dramatically changes Mo 3d splitting, which makes the Mo

atoms of the joint zone becomes semimetallic; while the Mo

atoms of the basal surface remains the semiconducting. Such

feature should be the main reason that the 3D MoS2 exhibits

the semimetallic properties for the joint zone, while the basal

surface remains semiconducting. The calculated electronic

structures for the 3D MoS2 clearly suggest that the distorted

Mo-S ligand field at the joint zone results in the degenerate

Mo 3d band across the Fermi level. The 3D MoS2 with high

stability, large surface areas, and excellent conductivity

should be a highly efficient and more inexpensive catalyst

for HER.

FIG. 3. The band structures of the four

considered 3D MoS2. (a) 3DM-63; (b)

3DM-64; (c) 3DM-65; (d) 3DM-66.

The high symmetry K point path in the

Brillouin Zone is chosen as: G (0, 0, 0)

! X (1/2, 0, 0)!M (1/2, 1/2, 0)! G

(0, 0, 0) ! Z (0, 0, 1/2) ! R (1/2, 0,

1/2) !A (1/2, 1/2, 1/2) ! Z (0, 0,

1/2).

FIG. 4. The electronic densities near the Fermi level (�0.2–0 eV) of the four

different sizes 3D MoS2. (a) 3DM-63; (b) 3DM-64; (c) 3DM-65; (d) 3DM-

66. The Mo atoms of the joint zone are labeled as Mo-J1, Mo-J2, and so on,

and the Mo atoms of the other zone are labeled as Mo-O1 and Mo-O2 and

so on. Deep blue isosurfaces correspond to the electronic densities (the iso-

value is 0.015 e/A3).

TABLE II. The calculated Bader charge and volume of different Mo atoms

for the 3DM-64 structure. The Mo pseudopotential used in the calculations

containing twelve valence electrons (4p64d55s1).

Atom

Bader

charge (e)

Bader

volume (A3) Atom

Bader

charge (e)

Bader

volume (A3)

Mo-J1 10.944 13.432 Mo-O1 10.812 13.038

Mo-J2 10.911 13.382 Mo-O2 10.810 13.008

Mo-J3 10.853 13.976 Mo-O3 10.813 13.012

Mo-J4 10.833 13.560 Mo-O4 10.810 12.972

Mo-J5 11.072 14.394 Mo-O5 10.809 13.066

204302-4 Tang et al. J. Appl. Phys. 115, 204302 (2014)

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IV. CONCLUSION

Based on the first principle calculations, a novel 3D

MoS2 structure was proposed. The calculations reveal that

the 3D MoS2 not only owns large surface area but also it can

effectively avoid the aggregation as the layered structures.

First principles molecular dynamics simulations show that

the 3D MoS2 structures with suitable aperture (3DM-63 and

3DM-64) are thermodynamically stable at room temperature.

Interestingly, the basal surface of 3D MoS2 remains the in-

herent semiconducting feature of 2H MoS2, while the joint

zone exhibits semimetallic because of the distorted Mo-S

ligand fields across the Fermi level. Considering the unique

electronic property, high thermodynamic stability and large

surface area, the 3D MoS2 has great potential to be high per-

formance photocatalysts.

ACKNOWLEDGMENTS

This work was supported by the National Natural Science

Foundation of China (No. 51222212), the CAEP foundation

(Grant No. 2012B0302052), the Science Foundation

of Hengyang Normal University (No. 13B41), the MOST

of China (973 Project, Grant No. 2011CB922200), and

the Construct Program for Key Disciplines in Hunan

Province.

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