DSpace at VNU: Effect of Elasticity of the MoS2 Surface on Li Atom Bouncing and Migration: Mechanism from Ab Initio Molecular Dynamic Investigations

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Effect of Elasticity of the MoS Surface on Li Atom Bouncing and
Migration: Mechanism from Ab Initio Molecular Dynamic Investigations
Thi Huynh Ho, Hieu Cao Dong, Yoshiyuki Kawazoe, and Hung Minh Le
J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09954 • Publication Date (Web): 19 Dec 2016
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Effect of Elasticity of the MoS2 Surface on Li Atom
Bouncing and Migration: Mechanism from Ab Initio
Molecular Dynamic Investigations
Thi H. Ho1, Hieu C. Dong1, Yoshiyuki Kawazoe2, Hung M. Le3,4,*

1

Faculty of Materials Science, University of Science, Vietnam National University, Ho Chi Minh

City, Vietnam
2

New Industry Creation Hatchery Center, Tohoku University, Sendai, 980-8579 Japan

3

Computational Chemistry Research Group, Ton Duc Thang University, Ho Chi Minh City,

Vietnam
4

Faculty of Applied Sciences, Ton Duc Thang University, Ho Chi Minh City, Vietnam

ABSTRACT: Born-Oppenheimer molecular dynamics has been carried out to investigate the
evolution of Li-atom trapping on the MoS2 surface. A single Li atom is fired with initial kinetic
energy level (0.2 eV or 2.0 eV) and various targeting factor x, which determines the collision
angle. After getting trapped, Li is observed to bounce elastically and glide on the MoS2 surface
thanks to the "breathing" vibration of MoS2. Both firing energy and targeting factor x are shown
to have a significant effect on the trapping and gliding processes. It is found that higher value of
targeting factor x (≥0.6) and initial firing energy (2.0 eV) would enhance Li migration on the
MoS2 surface. Also, analysis from electronic structure calculations of six representative Li-MoS2

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interacting configurations suggests that there is ionic interaction and partial charge transfer
between the absorbed Li atom and MoS2 monolayer during the bouncing and migration process.
The HSE calculations for those structures unveils the metallization of MoS2 due to Li
attachment.

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I. INTRODUCTION
Over past few decades, transition metal sulfides have become an attractive material due to
its considerable properties such as magnetism, superconductivity, fluorescence, and electrical
properties.1-4 Among these compound, molybdenum disulfide (MoS2) has been also studied
extensively for applications such as electrochemical energy storage and conversion material,5,6
catalyst,7-9 and solid lubricant.10,11 Recently, the demand for effective cathode materials of
lithium-ion rechargeable batteries leads a great research interest concerning MoS2-Li
interactions. Like graphite, MoS2 has a hexagonal unit-cell structure and MoS2 nanoparticles can
be classified as an inorganic nanocarbon analogue of structures like plate-like graphene,12 onionlike fullerenes,13 pipe-like nanotubes,14 which exhibits unique properties. In MoS2, the atoms are
covalently bonded to form a sandwich structure with two-dimensional S-Mo-S trilayers stacked
together through weak Van der Waals interactions.15 With high theoretical specific capacity and
good raw material abundance,16 MoS2 has been considered a suitable material for developing
effective electrodes.
The weak interlayer interaction allows guest atoms and molecules to intercalate reversibly
and diffuse through the weakly-bonding stacked layers.17,18 As a result, the intercalation process
leads to two main effects: expansion of interlayer spacing and charge transfer from the guest to
the MoS2 host.19 Because of such effects, MoS2 has been nominated as a reasonable choice for
electrode materials. Li et al.20 investigated the adsorption and diffusion of lithium atom on the
MoS2, and the results showed that the Li mobility could be significantly facilitated in MoS2
nanosheets because Li binding energy decreases. Rastogi et al.21 demonstrated that Li was one of
the most effective adatoms to enhance the n-type mobile carrier density in MoS2 for battery
applications. In a previous study, structural transition between the thermodynamically unstable T

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phase and the H phase was investigated with the involvement of adsorbing Li atoms. Such a
transition was shown to have a barrier, which might be reduced by increasing the concentration
of Li atoms.22 By employing a first-principles calculations, Ersan et al.23 demonstrated a
diffusion of Li on the MoS2(1-x)Se2x, and suggested that the adsorption of Li atoms might
metallize the dichalcogenide layer. Concerning the tendency of Li clustering on the MoS2
surface, Putungan and co-workers showed the case of two Li atoms sitting close to each other
was energetically unfavorable because Li dimer would dissociate quickly and re-locate on
nearest Mo top-sites.24 In such a study, the overall migration barrier for Li clustering was
estimated as ~0.5 eV. Although there have been several theoretical studies concerning the
interaction between Li atom and single-layer MoS2 based on density functional theory, it is
necessary to find out more about the interacting mechanism during a dynamic process.
Considering the fact that it still has limited data on molecular dynamics (MD) mechanism, we
believe it is worthy to conduct a fundamental MD investigation to examine the behavior of Li
atom on the MoS2 surface.
In this study, we employ direct ab initio molecular dynamic (MD) simulation of lithium
atom collision with MoS2 at two different levels of Li-firing kinetic energy, i.e. 0.2 eV and 2 eV,
while the MoS2 is set to thermally vibrate at room temperature (300 K). During the process, we
investigate the role of elasticity of MoS2 in trapping Li atom, and find out how a Li atom
interacts and diffuses on the MoS2 surface. Subsequently, we choose several representative
configurations to execute highly qualitative self-consistent calculations for studying the resultant
modification on electronic structure properties. We believe our theoretical study provides more
physical insights and disambiguates the attaching process of Li atoms onto the MoS2 surface.

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II. METHODOLOGY
In this study, our main objective is to investigate the progress of Li-atom trapping by the
MoS2 surface when a single Li atom is allowed to move toward and collide with the
semiconducting layer. This objective can be attained by adopting a MD approach. In our
procedure, there are three primary steps:
i. Setting up a randomized configuration of the pure MoS2 surface (without Li) thermally
vibrating at 300 K.
ii. Executing ab initio MD for the Li-MoS2 system.
iii. Selecting several interesting configurations from the trajectories to perform highly
qualitative self-consistent calculations and study the electronic properties.
In the following sub-sections, we will describe in details how we set up a trajectory sample
and what information should be extracted from the trajectory.

1. Setting up a randomized MoS2 configuration at 300 K
In the initial stage, an MoS2 monolayer consisting of 27 atoms (9 Mo and 18 S atoms) in
a (3×3) supercell is allowed to conduct thermal vibration at room temperature for a period of 500
Rydberg time units (Rtu), and a fixed step size of 0.5 Rtu is chosen for integrations. In real time,
such a period equals 24.19 fs. For a (3×3) MoS2 supercell, the a lattice parameter for the twodimensional system is chosen as 9.59 Å, while 15 Å is assumed to be the length of the c lattice
vector to guarantee the vacuum assumption for the Li atom in the system. For simplicity of this

case study, during the later MD investigation process, we make an assumption that the defined
lattice parameters remain constant during the entire dynamic process of atoms.

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The Car-Parrinello MD25 (CPMD) technique implemented in the Quantum Espresso
package26 is employed in this early stage. The cut-off energy is chosen as 35 Rydberg and the
Martin-Troullier norm-conserving pseudo-potentials27 are employed for the involving atoms
(Mo, S, and Li). The MoS2 system is made experimentally realistic when the Mo and S atoms are
allowed to fluctuate at 300 K. After the CPMD process, the geometry and velocity configurations
are stored in the database for later use. A geometry configuration with well-randomized
velocities is generated by simply choosing a configuration from this CPMD database.

2. Executing direct DFT molecular dynamics
After constructing the data for thermally equilibrated configurations, we insert the Li
atom into the system. With the chosen size of unit cell, the distance between two adjacent Li
atoms is 9.59 Å, which guarantees negligible interaction between Li atoms in the periodic
system. As a benchmark calculation, we perform a MD simulation with variable unit cell, in
which the Li atom is migrating from one S-Mo-S potential trap to another, and learn that the unit
cell parameter responses insignificantly during the Li migration process. Therefore, all
investigated cases herein are conducted with a fixed unit cell.
The Li atom is set the move toward MoS2 with two different levels of initial kinetic

energy: 0.2 eV and 2 eV. Such energy levels are considered “low” because they will not cause a
severe deformation to the MoS2 surface. The higher energy level, 2 eV, might be considered as a
hard collision (bombardment) onto the surface.
The Li atom is located 6 Å aside from the MoS2 surface, and its projection lies on top of
a Mo atom. We hereby consider 21 collision cases at each kinetic energy level. In each case, the
angle of striking velocity is varied as described below.

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In the first case, the Li atom is set to strike perpendicularly to the MoS2 surface, and aim
to an Mo atom (referred as D case). From cases 2 to 11, Li is set to strike 10 different spots of
destination on a projected Mo-S linkage (denoted as spot C in Figure 1). Let us denote R as the
projected distance of an equilibrium Mo-S bond. A spot of destination is located at the point x2R
(Å) from the Mo atom as described in Figure 1, where x = 0.1, 0.2, …, 1.0. For convenience, we
refer these cases as the C1, C2, ..., C10 cases. From cases 12 to 21, Li is set to strike 10 different
spots of destination resided on the bisecting vector of two Mo-S bonds (denoted as spot B in
Figure 1). Again, those B spots of destination are appointed similarly to the previous cases with
the x2R factor. For convenience, we refer these as the B1, B2, ..., B10 cases.
In the trajectory integration process, we employ the velocity-Verlet method28 with a
standard step size of 0.484 fs. The atomic forces of Mo, S, and Li atoms are extracted directly
from first-principles self-consistent calculations executed by the Vienna Ab Initio Simulation
package.29-32 The well-established Perdew-Burke-Ernzerhof exchange-correlation functional33-35

is employed, while the kinetic-energy cut-off is chosen as 400 eV, which is a standard cut-off
level and suitable for the cost of long-time Born-Oppenheimer MD simulations. The projectoraugmented wave method36,37 is employed to construct the electronic wave-functions for the
participant atoms, which describes the valence shells of 5s, 4d for Mo, 3s, 3p for O, and 2s, 2p
for Li. To save computational expense for the Born-Oppenheimer MD simulations, we only
perform Γ-point calculations at each integration step. A MD trajectory is terminated after 1,000
integration steps (10,000 Rtu). It should be noted that in the first step, we utilize the input
structure produced by PBE calculations within the well-established Quantum Espresso package,
which in principle should be produce analogous structural ground state with respect to the PBE
calculations with VASP in the second step.

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3. Performing self-consistent calculations for the chosen Li-MoS2 complexes
After finishing the MD process, we pay attention to several interacting configurations at
the stage of Li movement toward the MoS2 surface, when there are interactions that may cause
modifications to the band structure of MoS2. Therefore, qualitative spin-polarized self-consistent
calculations with a k-point mesh of (12×12×1) are executed for the chosen structures. To explore
the partial density of states (PDOS), the Gaussian smearing technique is utilized with a spreading
value of 0.01 eV, and the dipole correction is activated. We perform Bader charger analysis38 to
examine the amount of charge transfer between Li and MoS2. Moreover, the hybrid HSE
calculations39 are also employed to investigate the electronic structures of the chosen
configurations. The cut-off energy level is chosen as 400 eV, while the k-point mesh of (3×3×1)

is chosen for the HSE calculations.

III. RESULTS AND DISCUSSION
1. The collision of Li with MoS2 at the firing kinetic energy of 0.2 eV
Initially, the initial kinetic energy of 0.2 eV is chosen because we would like to
examine the slow absorption and diffusion processes. In a previous study, Ersan et al.23
suggested that a single Li atom could find good settlement on the pure MoS2 surface with an
adsorption energy of 1.92 eV as derived from first-principles calculations. Figure 2 and Figure 3
shows the evolution of total kinetic energy terms of the MoS2 single-layer and Li during the
BOMD processes for 21 investigated cases at 0.2 eV. We observe that the MoS2 monolayer
vibrates periodically at the average initialized temperature of 300 K, in which the S atoms tend to
move up and down. This seems more or less like a "breathing" behavior, and the vibrational
period of MoS2 almost remains constant. In the cases presented in this section, before Li collides

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with MoS2, the average periodic time for the thermal vibration is approximately 444.61 Rtu
(21.51 fs) at 300 K. However, even after Li successfully establishes bonding with MoS2, the
vibrational period of the layer does not seem to be affected significantly. In our MD process,
MoS2 vibrates around its equilibrium position for about 2,200 Rtu (106.43 fs) while Li moves
closer to MoS2. According to our kinetic energy examination (see Figure 2), at the average
distance of 4.78 Å from the surface, Li seems to start getting attracted by the layer as the kinetic

energy of Li increases dramatically. The attracting effect becomes gradually intensive, and
reaches the maximum level of attraction at the distance of 1.78 Å, where we conceive the largest
momentum of Li moving toward MoS2. In more details, the kinetic energy magnitude of Li
increases dramatically from 0.2 eV at the beginning to over 5 eV thanks to the assistance of
MoS2 breathing and strong attractions of MoS2 upon Li. After that, the repulsive force begins to
occur rapidly during the collision. It is true that Li absorbs kinetic energy from MoS2 and there
are effects of strong attractive and repulsive interactions. MoS2 seems to vibrate stronger as
proved by a significant increase of kinetic energy (more than 3.94 eV) at 4,200 Rtu (203.19 fs).
This is due to the establishment of a stable bonding configuration between MoS2 and Li. During
the collision process, we also observe that Li can rebound several times. However, the Li atom is
quickly pulled back and joins the vibration with MoS2. It should be noted that the bouncing
behavior does not provide enough momentum for Li to escape from the great attraction from the
layer. There are two circumstances that can occur then:
i. Li is trapped around the triangular region formulated by three S ions, or
ii. Li glides from a trap created by three nearest neighbor S atoms to the most nearby
triangular trap. We refer this behavior to as “migration”.

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In the D case (where Li is set to move toward to center of the triangular trap as
described in Figure 1), the C1-C5 cases (in which Li moves toward one Mo-S bond), and the B1B6, B8 and B10 cases (where the collision is projected to the bisector of two Mo-S bonds), we
observe that Li is trapped in the triangular valley constituted by three S atoms. Indeed, the Li

atom fluctuates continuously, but it cannot find a way to get out of the valley during the whole
investigating period with a maximum examination time of 10,000 Rtu (483.78 fs). Figure 4
demonstrates this trapping process in the C1 case, in which we show snapshots, x-, y-, and zdirectional kinetic energies, distances of Li to S atoms on the top layer as well as the
approximated distance of Li to the surface. In the snapshots, Li is attracted strongly and
establishes interactions with three S atoms as it moves closer to MoS2. After that, Li rebounds
up-and-down for several times because of its large kinetic energy absorbed from the MoS2 layer
movement. The rebound of Li decreases gradually until almost kinetic energy is transferred to
MoS2. At 6,590 Rtu (318.81 fs), the y-directional kinetic energy increases as shown in Figure 4
(a). The distance of Li-S(1) decreases significantly, which is an evidence that Li is now pulled
dominantly by S(1) and moves off-center. At the same time, Li seems to run out of kinetic energy
and collides obliquely with respect to the S-S-S center. However, there are still attractive
interactions from S(2) and S(5) toward Li as well as the elastic collision with S(1), Li can be
attracted back, trapped, and circulate around triangular position for at least another bouncing
period.
In the remaining cases, the sliding translation is observed after the elastic collisions.
There are two mechanisms that can lead to this behavior. First, Li strongly collides with one of
three S atoms and rebounds elastically to jump out of the trap as observed in the C6-C10 cases.
In the second mechanism, Li elastically collides at a bisectional point of two Mo-S vectors, then

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bounces back to the third S atom in the trap, and finally leaps to another trap, as can be observed

in the B7 and B9 cases. To some extent, this process is similar to the first mechanism. Li moves
toward the MoS2 layer with larger deflective targeting factor, then collides with S atoms for
several times. Li can bounce around three S atoms and finally jump to another S-S-S trap due to
high kinetic energy. Figure 5 presents snapshots, x-, y-, and z-directional kinetic energy,
distances of Li to S atoms on the top layer and distance of Li to surface for the trapping process
in the B7 case. In this case, Li collides elastically at the bisecting position between S(1) and S(5) at
the beginning of the trapping process. Therefore, the collision direction is changed, which makes
the sliding migration occur easier. In Figure 5 (a), the x-directional kinetic energy line goes up
almost after the Li collision. Also, there is evidence of deflective collisions of Li with the S
atoms. At 6,880 Rtu (332.84 fs), the gliding process occurs when Li begins to move from the
S(1)-S(2)-S(5) trap to the S(1)-S(4)-S(5) trap because the Li-S(2) and Li-S(5) distances are shorter than
the Li-S(4) distance (illustrated in Figure 5 (b) when we see that the Li-S(4) goes down while the
other two lines go up).
In this section, we observe that the effects depend much on the targeting factor x. After
investigating 21 cases at the kinetic energy level of 0.2 eV, we observe the migration of Li
occurs easier if factor x is greater than or equal to 0.6 (i.e. the shooting angle is over 6.3o). In
Figure 2, the total kinetic energy of Li for 11 investigated cases changes gradually when x
increases. This leads to the conclusion that there is a deep valley of potential well, which tends to
pull Li down and establish a stable Li-MoS2 complex. This was also revealed by two previous
theoretical studies. Before Li collides MoS2, the kinetic terms seem to be similar, but the
movement of Li decreases faster with x ≥ 0.6, which proves that Li possesses more bouncing
collisions. For convenience, we summarize the elapsed time before the first Li collision, number

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of rebounds and elapsed time before Li gliding with respect to each shooting angle for seven
gliding cases (C6-C10, B7 and B9) at 0.2 eV in Table 1. In the C cases, the gliding process
occurs more easily with probability of 50% (5/10 cases), while there are just 2/10 gliding cases
in the bisecting collision cases (B cases). For the B cases, this happens due to the fact that the
kinetic energy of Li can decrease significantly after two collisions at the bisecting position and at
the opposite S atom. In the C6-C7 cases as well as B7 case, the gliding process of Li happens
after bouncing three times on the surface, while C8, C9, C10 and B9 cases, the gliding process
occurs rigorously after only one bouncing period. This shows a tendency that the number of
rebounds is lessened when we increase the shooting angles. Li can elastically collide and jump
out of the potential trap directly at high values of the targeting factor x. If the targeting factor is
low, Li can only escape after several interaction collisions with the three surrounding S atoms.
With the above results, we conclude that the glide of Li would occur easier when increasing the
targeting factor. Interestingly, we observe that the kinetic energy of Li is low (0.13-0.26 eV) in
two gliding moments in the B8 and C9 cases. In those trajectories, migration happens when Li
jumps relatively far from the MoS2 surface. In such circumstances, the potential trap will pull Li
back to the surface while gliding transition occurs almost at the same time. In the other gliding
cases where migration occurs with short Li-MoS2 distances, we observe that the required kinetic
energy of Li is very diverse in a wide range (1.3-4.2 eV).

2. The collision of Li with MoS2 at the firing kinetic energy of 2.0 eV
Previously, at the kinetic energy of 0.2 eV, we observe that there are two behaviors
occurring when Li move to the MoS2 surface: trapping and gliding (migrating). However, with
this level of kinetic energy, trapping is still dominant, and gliding is somewhat difficult to occur

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(only 33% of chance in the investigated cases). We now re-investigate those 21 cases with the
initial Li kinetic energy of 2 eV. Considering the same configuration condition applied for 21
new cases in the MD investigation, we will focus on clarifying how the effect of targeting factor
x enhances Li diffusing ability on the surface at the higher kinetic energy.
After finishing the MD investigations for 10,000 Rtu (483.78 fs), we observe that Li
diffusion occurs in most of the cases (18/21 cases, except C5, B5 and B7). Recall that there are
just 7 gliding cases with the firing energy of 0.2 eV. In the three cases with no Li diffusion (C5,
B5 and B7 cases), Li is observed to be trapped in the potential well and does not have sufficient
time to escape the trap at the end of our MD investigation operation (10,000 Rtu). However, Li
still fluctuates strongly on the surface, and we believe that if we extend the MD trajectory time,
diffusions probably occurs in those three cases.
For the cases where Li diffusions clearly occur, the mechanism for Li gliding on the
MoS2 surface is fairly similar to the cases at 0.2 eV:
i. Li first approaches a triangular hole, rebounds for several times and finally glides to the
most neighboring hole after the alternating collisions with three S atoms.
ii. Li collides strongly with one S atom on the surface and leap to another trap very
rapidly. This happens when the firing angle is large (high value of x).
At the higher firing energy level (considered as hard collision), Li rebounds for less
periods than the cases at 0.2 eV as listed in the Table 1 and Table 2. This demonstrates that the
trapping potential well on MoS2 surface has high elastic behavior. Such a potential well keeps Li
bouncing many times due to the resilience of three symmetric S atoms. The S atoms can elevate
and take down Li measuredly until Li loses most of its kinetic energy. Still, it is hard for the

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bouncing process to retain for a long time, and migration finally occurs. Overall, the gliding
process of Li can take place much easier than the cases at 0.2 eV. Besides, we also observe the
intensive impact of targeting factor. When x is greater than or equal to 0.6, the number of
rebounds reduces from 5 times to only once and the gliding process of Li changes from indirect
to the direct way. At low targeting factor (0 ≤ x < 0.6), Li only escapes the trap after rebounding
and then colliding elastically with three nearest neighbor S atoms. During this bouncing process,
the kinetic energy of Li is much lower than the kinetic energy at the colliding moment. The
gliding process occurs indirectly depending upon the appropriate collisions with S atoms.
Contradictorily, for cases with high targeting factor (x ≥ 0.6), Li collides directly at one of three
S atoms or bisecting site, and then jump out of the trap with the aide of high kinetic energy.
Moreover, as showed in the Figure 6 and Figure 7, the kinetic energies in the D, C1-C4 and B1B4 cases retain almost similar until 6,200 Rtu (299.94 fs). Li can still rebound for several times
before Li glides away. For the C6-C10, B5, B6 and B8-B10 cases, the kinetic energies change at
the early stage, which shows the gliding process can occur after the first collision with MoS2.
Comparing cases at two kinetic energy levels of 0.2 eV and 2 eV, we still see there is a
threshold of the strong attracting forces when Li moves closer to MoS2. Li seems to be pulled at
the distance of 4.70 Å and the kinetic energy of Li reaches the maximum value of about 6.5 eV
at the Li-S layer distance of 1.65 Å. After that, the repulsive forces increase dramatically while
Li continues to approach closer to collide with the surface. We also observe that the gliding
movement of Li for most cases tends to follow the following path: at the beginning, Li jumps out
of the traps constituted by three S atoms above the Mo atom, then it moves to the hollow site of

MoS2 and finally turns to another trapping site. Actually, there are several theoretical studies
concerning the diffusion path of Li adsorbed on the MoS2 surface. The diffusion process occurs

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due to the migrating of Li from a trapping site to another site by passing through a hollow site.
Xu et al.41 reported that the T site of MoS2 is more energetically stable to bind with Li than the H
site (those adsorption sites are defined in Figure 1). Therefore, Li can be trapped easier in the T
site for a long time than in the H site before jumping out of the trap. According to our
observation from MD trajectories, Li moves to the H site and quickly glides to the T trap by
following the zigzag-like path. Overall, our MD evidence establishes a good agreement with this
study.
In addition to clarifying the impact of firing energy and targeting factor x, we carefully
examine the kinetic energy of Li as well as the elapsed time at the gliding moment, when Li
escapes one trap for another, for all gliding cases as shown in Figure 8. With 2.0 eV kinetic
energy, the gliding process occurs more frequently than the cases of 0.2 eV. Besides, in the cases
with lower targeting factors x (< 0.6), gliding seems to occur after about 320 fs for both firing
kinetic energy levels. During the short period before gliding, the kinetic energy of Li is low due
to the "indirect" gliding of Li, i.e. Li only moves to another potential trap after bouncing and
colliding elastically with the nearby S atoms for several times. When the targeting factor x
increases, the number of rebounds seems to decrease and Li is able to migrate faster with elapsed
time varying from 170 fs to 50 fs, which is much shorter than the above 320 fs. Especially, the

gliding process can also occur directly after Li collides with the MoS2 surface and bounces for
only one time. We also observe that the elapsed time before gliding is less than 100 fs (see
Figure 8). In such circumstances, the kinetic energy of Li is high (3-4.5 eV) because of energy
adsorption from the vibrational movement of MoS2. With the two last cases of B9 and B10, we
observe that the gliding process can even occur before Li collides with MoS2 as Li seems to be
more attracted to the bisecting position. Therefore, in the association of high targeting factor and

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high initial kinetic energy, it is easier for Li to deflect and the gliding process can take place
more spontaneously. Overall, we can conclude at this point that the gliding behavior relies
heavily on the firing energy, beside the effect of targeting factor x.

3. Electronic properties for representative Li-MoS2 interacting configurations
In this section, qualitative self-consistent calculations are executed for six
configurations to examine the electronic structure when Li moves and collides with MoS2. The
configurations that we choose for the investigation include: (a) a starting configuration at the
beginning of MD simulation, (b) the configuration at the moment Li starts to get attracted by
MoS2 (kinetic energy of Li starts to increase) (c) the configuration with highest kinetic energy
(repulsion begins to occur), and (d), (e), (f) three collision configurations in the D, C8, and B8
cases with the initial firing energy of 2.0 eV. The density of states (DOS) of these configurations
is shown in the Figure 9. At the beginning, there is an evidence of a weak Li-MoS2 interaction

due to the insignificant hybridization of Li and MoS2 orbitals near the Fermi level (Figure 9 (a)).
At the approximate distance of 4.70 Å from the surface, Li seem to be pulled by MoS2 and the
Li-2s state becomes delocalized and a hybridized eigenstate shows up quite below the Fermi
level as showed in Figure 9(b), and indicates partial charge is transferred to the MoS2. Looking at
the DOS of pure MoS2 form given by HSE calculations (Figure 10), we observe that the
conduction band maximum remains almost unaltered with respect to the Fermi level even when
Li is introduced into the system. For the cases where Li is approaching closely to the surface,
there is a significant change in the valence band as seen in Figure 9(c)-(f). In terms of Bader
charge, we also notice a significant increase of charge of Li from +0.37 to +0.86 as listed in
Table 3. Then, Li is pulled stronger and stronger by the surface; as a result, the kinetic energy of

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Li begins to increase and finally reaches its maximum. At the highest kinetic energy, the
projected DOS shows Li-2s overlapping with the orbitals of MoS2, which features for the ionic
hybridization interactions. Meanwhile, the Bader charge now increases to +0.86 and seems to get
quite larger than the values at the collision moment. In the three later cases, the DOS
distributions of MoS2 also occupy energy states below the Fermi level. In these situations, Li
approaches very closely to the surface S atoms, and it can get a small amount of electronic
charge back, thereby becomes quite less positive (see Table 3). The positive charge of Li is
lowest for the D case (+0.77), while the C8 case gives the highest positive charge (+0.84).
The results of spin-polarized PBE calculations show that all investigated systems do not

exhibit magnetic moments. However, when we perform the HSE calculations for those
configurations, magnetism is found. The starting configuration (a) has a magnetic moment of
0.29 µB, which mainly arises from the 2s orbital of Li (35%) and 4d orbitals of Mo (56%), and
the electronic contribution from Li seems not to affect the overall band gap of MoS2. When Li
begins to get attracted (configuration (b)), the spin polarization term is stronger (0.58 µB) with
the major contribution from the 4d orbitals of Mo (>80%). In this configuration, an in-gap state
occurs at the Fermi level, which is not observed by the PBE calculations. In configuration (c), Li
approaches very close to the MoS2 layer, and a magnetic moment of 0.87 µB is found (90%
contributed by 4d orbitals of Mo), and the in-gap state is now very clear due to the strong
interaction between Li and MoS2. In last three cases of configurations (d), (e), and (f), the
magnetic moments are reported as 0.54 µB, 0.88 µB, and 0.56 µB, respectively. In configuration
(e), we obtain the highest magnetic moment as Li is set to strike at an S atom. Also, the in-gap
states show up very clearly in configurations (d) and (e), which reveals metallicity of the
structure. This observation is in good agreement with the metallization of MoS2 discussed by

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Ersan et al.23 In terms of Bader charge, the average charge for each atom type obtained HSE
calculations is in good agreement with the Bader charge derived from PBE calculations (see
Table 3). For comparison of charge analysis, we also conduct PBE calculations for the hydrogenbound isolated Li-MoS2 systems, which resemble the above structures, using the Gaussian 09
package42 with the 6-31G basis set43 for H, Li, S atoms and the LanL2DZ basis set44 for Mo.
Only the H atoms are optimized, while we keep the other atoms frozen to resemble the above

structure of interest obtained from MD simulations. In general, the charge of Li given by
Mulliken analysis is higher than the Bader charge as shown in Table 3. We find a good
agreement between Bader charge and Mulliken charge in predicting the trend of charge with
respect to different Li-MoS2 interacting configuration. When Li is still far away from the MoS2
surface (configurations (a) and (b)), the charge of Li is relatively low (0.97-0.99 proton charge).
As it approaches MoS2, Li tends to give more electrons and becomes more positive (1.04-1.15
proton charge). However, in configuration (d) where Li approaches closely to the Mo atom (and
far away from the surrounding S atoms), the Mulliken charge of Li is almost neural. The
Mulliken charge in this case is contradicting to that observed in Bader charge analysis.

IV. SUMMARY
In this study, we perform Born-Oppenheimer MD to investigate the evolution of Li-atom
trapping on the MoS2 surface. The single Li atom is allowed to move toward and collide with
MoS2 with variable targeting factors x and two firing kinetic energy levels of 0.2 and 2.0 eV.
Interestingly, as we investigate the trapping mechanism during the MD processes, we also
observe a gliding (migration) behavior of the Li atom on the MoS2 surface. Such an interesting

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feature is delivered due to the elastic "breathing" vibration of the semiconducting monolayer.
The removal of S atom from the surface is not observed in our study.
The trapping and gliding behaviors are observed to rely heavily on the firing energy and

targeting factor x. Higher value of targeting factor x (≥0.6) as well as initial firing energy (2.0
eV) would enhance the gliding probability; per contra, Li can be trapped in the potential hole
created by three nearest S atoms for a longer period before Li can finally escape to another. More
specifically, with an initial kinetic energy of 2.0 eV, Li is more probable to translate from one
triangular trap to another in 18/21 cases (85.7%), while there are just 7 gliding cases in the 0.2
eV case (33.3%). Besides, we observe a kinetic energy threshold for the Li movement when Li
moves closer to MoS2 at both investigated levels of firing energy (0.2 eV and 2 eV). Even
though the introduced firing energy does play a decisive role in Li gliding, it seems that such
initial energy is significantly lower than the kinetic energy at the later stage as Li absorbs heat
from MoS2. It should be noted that in all investigated cases herein, we do not observe a bounceoff behavior of Li from the surface.
In the last section, the electronic structure examination for six representative
configurations is performed by PBE and HSE calculations. The PDOS and Bader charge are
analyzed to examine the interactions and electron transfer between Li and MoS2, which can be
employed to clarify the change of electronic behaviors of the Li-MoS2 system. The electronic
result reveals that Li is mostly attracted when it comes closer to the MoS2 surface due to ionic
interactions. At the same time, Li transfers most of its electronic charge to MoS2 and Li
consequently becomes cationic. The DOS evidence given by HSE calculations show that when
Li approaches closer to the MoS2 surface, there exist in-gap states. Such eigenstates indicate the
metallization of the layer, which is in good agreement with a previous study.15 Overall, our MD

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trajectories offer two basic mechanisms for trapping and gliding when firing a single Li atom
onto the single-layer MoS2 surface. We believe that further computational studies are necessary
to supplement how firing multiple Li atoms at the same time (atomic beam) would affect ion
trapping and migration.

SUPPORTING INFORMATION
The trajectory configuration data for all collision cases are provided in the associated
supplementary material. Those files are in the Xcrysden structural format and compressed as
axsf.zip. A trajectory video file is made for illustration of the Li migration. This material is
available free of charge via the Internet at .

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AUTHOR INFORMATION
Corresponding Author
*E-mail:
Notes
The authors declare no competing financial interest.

ACKNOWLEDGMENT
The authors thank high-performance computing assistance from the Institute for Materials
Research, Tohoku University during the course of this research. This work is funded by the

National Foundation for Science and Technology Developments (NAFOSTED) under grant
103.01-2016.53.

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