To achieve good bonding interaction and charge transfer, the ACOO residue needs to form ionic bonds with the defected MoS2surface.. In those studies, organic structures, being employed a
Trang 1Research paper
Physical insights from density functional theory investigations
Hung M Lea,b,⇑, Viet Q Buic, Phuong Hoang Trand, Nguyen-Nguyen Pham-Trand, Yoshiyuki Kawazoee,
a
Computational Chemistry Research Group, Ton Duc Thang University, Ho Chi Minh City, Viet Nam
b
Faculty of Applied Sciences, Ton Duc Thang University, Ho Chi Minh City, Viet Nam
c
Department of Applied Chemistry and Institute of Natural Sciences, Kyung Hee University, Gyeonggi-do 446-701, South Korea
d Faculty of Chemistry, University of Science, Vietnam National University, Ho Chi Minh City, Viet Nam
e
New Industry Creation Hatchery Center, Tohoku University, Sendai 980-8579, Japan
f
Theory and Modeling Department, Culham Centre for Fusion Energy, United Kingdom Atomic Energy Authority, Abingdon OX14 3DB, United Kingdom
a r t i c l e i n f o
Article history:
Received 26 September 2016
In final form 4 November 2016
Available online xxxx
Keywords:
Organic dye
MoS 2
DFT
a b s t r a c t
In this theoretical study, we employ first-principles calculations to explore the bonding nature of organic dyes on the semiconducting MoS2surface To achieve good bonding interaction and charge transfer, the ACOO residue needs to form ionic bonds with the defected MoS2surface In the cases of L0 and a newly synthesized dye named as TN1, we observe the manifestation of an in-gap state at1 eV from the Fermi level, which might enhance photon trapping capability of the complex
Ó 2016 Published by Elsevier B.V
1 Introduction
Molybdenum disulfide, a two-dimensional (2D) material with
multi-layer stacking of MoS2, has been known to possess
applica-tions in several aspects In industry, this material is well-known
as a lubricant because of the weak van der Waals’ interactions
between layers, which thereby produces a low friction coefficient
[1] Interestingly enough, such weak van der Waals interactions
have a significant influence on the electronic property of MoS2
angle-resolved photoelectron spectroscopy and first-principles
calcula-tions, Klein et al.[2]showed that in the multi-layer form, the
mate-rial would possess an indirect band gap of around 1.2 eV, while the
standalone single layer establishes a larger direct band gap of
1.8 eV Besides, its novel catalytic capability also guarantees the
hydrodesulfurization for petroleum refinery[3,4]and water
split-ting for hydrogen production [5] In fact, for a period of time,
MoS2had been considered as an inert material This traditional
belief is no longer true until the successful synthesis of
highly-reactive anionic [Mo3S13]2nanoparticles[6]
The single layer form of MoS2finally finds its position in elec-tronics due to the successful synthesis of highly qualitative mono-crystalline layers [7] Not only integrated into functional electronic devices, such a material with a direct band-gap can be employed in phototransistors with high sensitivity and low noise [8] The functionalization of the MoS2layer have attracted much attention from the research community because of its promising applications in electronics, energy storage, sensing, and catalysis [9] The covalent functionalization of MoS2 was previously dis-cussed by Presolski and Pumera [10] Recently, Chen et al [11] demonstrated a functionalization of exfoliated 2H-MoS2with cys-teine, an organic thiol, and the results showed physisorption rather than covalent attachment Using a first-principles approach, Ataca and Ciraci proposed the attachment of adatom and vacancy
on the surface by employing transition metal bridges[13] In the storyline of photo-sensitivity, there have been two remarkable efforts to tailor the performance of MoS2 in photocatalysis [5] and photodetector[14] In those studies, organic structures, being employed as ‘sensitizing dyes’ and possessing compatible photo-sensitivity with the heterogeneous layer, are employed to decorate the surface of MoS2, and dedicate an essential role in ‘trapping’ photoexcitations In the content of this study, we demonstrate a
http://dx.doi.org/10.1016/j.cplett.2016.11.007
0009-2614/Ó 2016 Published by Elsevier B.V.
⇑ Corresponding author.
E-mail address: leminhhung@tdt.edu.vn (H.M Le).
Contents lists available atScienceDirect
Chemical Physics Letters
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / c p l e t t
Trang 2theoretical investigation of organic dye attached to an MoS2
sur-face to get more insights of the binding nature and in-gap
occupa-tions The organic dyes of interest consist of the well-known L0
structures[15,16]and one in-house-factorized dye However, prior
to investigating the interactions between a large organic molecule
and MoS2surface, it is necessary to get deeper understanding how
a basic carboxylic residue, i.e formic acid (HCOOH), could establish
attachment to MoS2
2 Computational details
First-principles calculations based on density functional theory
(DFT) are employed as the main investigating method in this study
22]is utilized and the projector-augmented wave method[23]is
employed to construct electronic wave-functions for the
partici-pating atoms Grimme’s D3 empirical corrections for long-range
van der Waals’ interactions are activated for all investigated
mod-els[24] For the assumption of lattice circulation in the x and y
directions, a (5 5) super-cell of MoS2consisting of 75 atoms are
employed with the c-axis length chosen as 28 Å to guarantee
sur-face isolation For computational feasibility, the constant volume
optimization scheme is executed with a force convergence
crite-rion of 104eV The cut-off energy level of 400 eV and a k-point
mesh of (3 3 1) are chosen
3 Results and discussion
3.1 Attachment of HCOOH/HCOOon the pure/defected MoS2surface
The basis of binding between a heterogeneous surface and
organic dye structures relies on the terminated carboxylate
residue, in which oxygen atoms can be attached to the surface [25] Before going into the discussion with dye attachments, we first explore the physics and chemistry understanding of binding origin between an MoS2surface and the simplest carboxylic resi-due, HCOOH In the first case, we assume there is neither surface defect nor formic acid reduction, i.e the original structure of formic acid (HCOOH) is in direct contact with MoS2 Because of surface inertness, only van der Waals interaction is formed to keep formic acid quite immobilized By looking at the charge density cloud in Fig 1(a), we observe that the H and O atoms seem to establish weak interactions with those S atoms on the surface Quantita-tively, to justify the statement of stability, we examine binding energy using the following equation:
Ebinding¼ Esurfaceþ Eresidue Ecomplex ð1Þ
where Esurface, Eresidue, Ecomplexdenote the total energies of the MoS2
surface (with S defect or without S defect depending on case study), organic ligand, and the whole binding complex, respectively In Eq (1), the magnitude of positive Ebindingindicates how strongly the residue is stabilized on the MoS2surface As in the very first case, the binding energy is only 0.02 eV, which can be regarded as a very weak physisorption In two previous studies[26,27], the
weak and caused no adjustments on the electronic properties of the 2D layer The eigenstates representing an HCOOH orbital show
up as a non-bonding state, and the electronic structure of the thin film layer remains unaltered Upon analyzing charge distribution (Fig 1(a)), we observe insignificant charge transfer between MoS2 and formic acid
In the second case, we alternatively consider the attachment of the radical formate residue (HCOO) There is a clear improvement
on binding stability (i.e binding energy is elevated up to 0.53 eV)
In this case (Fig 1(b)), both O atoms seem to reside on the surface and enhance van der Waals interactions with the most nearby S
Fig 1 (a) Charge density distribution of HCOOH interacting with the pure MoS2 monolayer, (b) partial DOS of HCOOabsorption on MoS 2 , and (c) partial DOS of HCOO absorption on MoS
Trang 3Fig 2 Molecular structures of L0 and TN1 (2-cyano-3-(N-butyl-3-indolyl) acrylic acid) dyes.
Fig 3 Partial DOS of L0 radical residue absorption on the defected MoS 2 surface In the Bader charge analysis, red contribution corresponds to positive charge, while green contribution depicts negative charge The dye residue is well immobilized with a binding energy of 0.51 eV (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Trang 4atoms Another piece of evidence showing a minor interaction of
charge density between the O and S anions also implies weak
bonding Even though the interacting model in the second case is
more improved than that in the first case, we do not conceive
sig-nificant change in the electronic property of MoS2 In the electronic
structure, the highest-occupied orbital of HCOOleads to the
cre-ation of an additional eigenstate at the Fermi level, which is in
con-junction with an energy occupation of the surface S atoms because
of the van der Waals interaction In this circumstance, such an
elec-tronic state might behave like an ‘‘agent” between the highest
be noted that the electronic structure of MoS2 is very similar to
that of the pure MoS2layer (seeFig S1, Supplementary Material)
In several previous studies, the band gap of pure MoS2was
pre-dicted to be in the range of 1.63–1.78 eV by PBE calculations with
various types of pseudo-potential sets[28–30]
Upon the examination of the two cases above, we observe that
it is not easy for formic acid and even the radical formate residue to
sneak into MoS2 On the other hand, the formate residue can form
strong bonds to the surface if there is a vacancy at the S site In that
case, the oxygen atoms may penetrate into the layer and establish
direct interactions with Mo During the past few years, progressive
steps have been made toward S vacancy creation in the MoS2layer
[31,32] In reality, sulfur vacancies are common and play an
essen-tial role in catalysis[10,33] To our awareness, Ma et al.[34]even
devoted an effort to repair S vacancy by introducing gaseous
mole-cules, such as CO, NO, and NO2 By adopting first-principles
calcu-lations, the formation energy of a vacancy by removing one S atom
reac-tion with an activareac-tion energy of 2.35 eV[35]; at the same time,
the valence and conduction bands were shown to expand to lower
energy levels and thereby reduce the band gap[36]
In fact, our first-principles calculations demonstrate that this is
really the case when the radical O atom connects to two
left-behind Mo atoms due to the absence of S In addition, the van
der Waals interactions between the other atoms in the residue
and the surrounding S atoms should also be taken into account
Analytically, our binding energy calculation using Eq (1) with
the total energy of defected MoS2suggests that the organic residue
is magnificently stabilized (Ebinding= 2.47 eV) compared to the pre-vious two cases As can be seen inFig 1(c), the amount of charge transfer is much more significant from Mo(4d) to O(2p) Adopting Bader charge density analysis with a qualitative isosurface value of 0.001 eV/cell, we really observe a chemisorption behavior In this chemical connection, it is the HCOOradical group that possesses positive charge, while defected MoS2has negative charge The elec-tronic structure seen from the density of states (DOS) inFig 1(c) is different from the previous two cases Due to the strong bonding with OOCH, the electronic structure of MoS2e changes signifi-cantly The highest-occupied state resulted from the Mo(4d)-O (2p) ionic interaction is located around the Fermi level The energy gap between the highest-occupied state at the Fermi level and the next occupied state is 1.2 eV Furthermore, the next occupied state
of the MoS2layer is shifted drastically from the Fermi level In the previous physisorption case shown inFig 1(b), the HO band is con-stituted solely by the radical formate group, which might not be meaningful in electronic applications
3.2 Binding L0 and a newly-synthesized dye to defected MoS2
At this point, we have a clear understanding of carboxylate
from carboxylic acid groups is not removed from the organic dye, the interaction is extremely weak; in addition, neither electronic tuning nor charge transfer can be found Therefore, in the later investigation of organic dye attachments on MoS2, we only con-sider the binding ofACOOto an MoS2surface with vacancy defect
at the S site It should be kept in mind that the chirality of those large dye molecules makes it harder to stabilize the binding sites between S defects andACOOgroups As a result, the binding ener-gies might be lower than the previous case of HCOOadsorption The structural conformations of two investigated organic dyes are provided inFig 2
As the first attempt to present a realistic model, we explore the possibility of decorating the MoS2layer with L0, a well-known dye belonged to the TPA-based class In reality, this dye has been attached to the surface of TiO2for sensitized solar-cell applications [37], and the electronic structure properties have been verified
Fig 4 Partial DOS of bonding Mo(4d) and O(2p) orbitals in (a) MoS -OOCH, (b) MoS -L0, and (c) MoS -TN1.
Trang 5using DFT calculation methods[38] Apart from the traditional TiO2
surface, we believe there is a prospect of this organic structure to
deliver interesting electronic features on MoS2 From the result of
our optimizations, L0 is favorably attached to defected MoS2with
a binding energy of 0.51 eV in a bidentate mode More specifically,
both O atoms make connections to the two Mo sites sharing a
com-mon S defect to establish two chemically equivalent Mo-O
link-ages With those two bridges, it is quite surprising that the
binding energy in this case is lower compared to the attachment
of HCOO We believe the hardship of chirality adjustment is due
to the clumsy conformation of the organic structure Recall that
the radical O atom to the layer, but the binding energy is much
higher In a previous study concerning dye-sensitized MoS2, it
was experimentally demonstrated that the Eosin Y organic dye
formed covalent bond with the defected single MoS2 layer [5]
photo-luminescence spectroscopy showed significant electronic transfer
from Eosin Y to the MoS2layer We will see later in our DOS
anal-ysis that the in-gap states induced by the presence of the dye
mole-cules is responsible for such electronic transfer In terms of
covalent bonding, it was also pointed out in another study by
scan-ning tunneling microscopy that the organic thiols established
interactions with MoS2at the vacancy site[39]
revealed inFig 3seems to be more significant At the connection
has negative charge by perceiving electron density, while the
DOS plot, we observe two interesting features The first in-gap band describes electron occupation at the Fermi level, which is a hybridized band of Mo, S, and L0 radical To some physical extent, this state describes a strong bonding nature like the previous case
(a) and (b)), such occupation is originated from the electron exchange of O(2p) and Mo(4d) orbitals We also observe another in-gap band, which is mostly constituted by the molecular orbital
of L0 This second peak is located at around1 eV inFig 3, and the dominant contribution comes from O(2p) Such interesting in-gap occupation features may allow the dye molecule to absorb photon energy and give up to the MoS2surface In general, the presence of HCOO, L0, or the later dye causes the HO bands of MoS2to be drifted away from the Fermi level
At this stage, we urge to design a new dye molecule so that
improved A new dye molecule is first designed by performing
ab initio calculations, then synthesized in our laboratory This new dye molecule is 2-cyano-3-(N-butyl-3-indolyl) acrylic acid
Fig 5 Partial DOS of TN1 radical residue absorption on the defected MoS 2 surface In the Bader charge analysis, red contribution corresponds to positive charge, while green contribution depicts negative charge The dye residue is well immobilized with a binding energy of 0.71 eV (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Trang 6(Fig 2(b)) For simplicity, we denote it as TN1 More detailed
infor-mation regarding experimental synthesis with FT-IR, GC–MS, H
Supplemen-tary Material (Figs S2–S7) Upon the omission of two phenyl rings,
the new structure TN1 seems to settle better on the surface of
defected MoS2 According to our calculations, the binding energy
is reported as 0.79 eV, higher than that in the L0 case The binding
conformation of TN1 is more perpendicular to the MoS2surface,
where we observe sorts of tilting behavior caused by the van der
Waals’ interactions between one aromatic phenyl group and
the two nearby Mo atoms This bonding behavior is different from
that seen in the case of the L0 bidentate attachment
Examining the DOS of defected MoS2(Fig 5for the TN1
adsorp-tion case), we observe that the electronic behavior of the layer is
very similar to that when L0 is attached to MoS2 This observation
makes sense in terms of chemical interaction equivalence For both
organic structures (L0 and TN1), it is theACOOradical residue that
establishes chemical ionic bonding to two Mo sites nearby the S
van der Waals’ interaction to the surrounding S atoms In the
par-tial DOS plot of the MoS2e-TN1 complex, we observe there is a
polar covalent bond formed as a result of Mo and TN1 orbital
inter-actions (the hybridized peak at the Fermi level), which is dominant
by the O(2p) contribution There is also another band (1 eV)
orig-inated from the ligand contribution to the hybridization, which
serves as an intermediate in-gap state Such an in-gap occupation
resides at a quite lower energy level compared to that of the L0
absorption case This result is not surprising, but implies the fact
that the complex with TN1 is more stable because its bonding
orbi-tals tend to reside at lower energy state
4 Summary
In summary, we have demonstrated a theoretical investigation
of two different organic dye structures on the surface of defected
MoS2 In the initial attempt, we perform three testing cases for
establish weak van der Waals interactions with the layer The
removal of an S atom actually prevails The HCOOresidue is shown
to bind strongly to the Mo atom with a binding energy of 2.47 eV
When considering actual large dye molecules such as the L0 and
newly-synthesized TN1 structures, we find the binding energies
to be lower due to chirality adjustment of the organic ligands
Hybridized occupation states and charge transfer clearly indicate
strong ionic connections, while there is also one in-gap state
show-ing up at around1 eV from the Fermi level, which might be
sup-portive in photon trapping
Acknowledgments
We are grateful for a research fund from Ton Duc Thang
Univer-sity and the supercomputing support from the High Performance
Computing Infrastructure Office (project hp150037) and the
Insti-tute for Material Research, Tohoku University, Japan Pham-Tran
thanks a financial support from Vietnam National University under
grant HS-2014-18-01
Appendix A Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cplett.2016.11
007 References
[1] T Bartels, W Bock, J Braun, C Busch, W Buss, W Dresel, C Freiler, M Harperscheid, R.-P Heckler, D Hörner, F Kubicki, G Lingg, A Losch, R Luther,
T Mang, S Noll, J Omeis, Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag GmbH & Co KGaA, 2000
[2] A Klein, S Tiefenbacher, V Eyert, C Pettenkofer, W Jaegermann, Phys Rev B
64 (2001) 205416 [3] W Han, P Yuan, Y Fan, G Shi, H Liu, D Bai, X Bao, J Mater Chem 22 (2012)
25340 [4] J.-F Paul, E Payen, J Phys Chem B 107 (2003) 4057 [5] T Jia, M.M.J Li, L Ye, S Wiseman, G Liu, J Qu, K Nakagawa, S.C.E Tsang, Chem Commun 51 (2015) 13496
[6] J Kibsgaard, T.F Jaramillo, F Besenbacher, Nat Chem 6 (2014) 248 [7] J Shi, D Ma, G.-F Han, Y Zhang, Q Ji, T Gao, J Sun, X Song, C Li, Y Zhang, X.-Y Lang, Y Zhang, Z Liu, ACS Nano 8 (2014) 10196
[8] D Lembke, S Bertolazzi, A Kis, Acc Chem Res 48 (2015) 100 [9] X Chen, A.R McDonald, Adv Mater 28 (2016) 5738 [10] S Presolski, M Pumera, Mater Today 19 (2016) 140 [11] X Chen, N.C Berner, C Backes, G.S Duesberg, A.R McDonald, Angew Chem Int Ed 55 (2016) 5803
[12] C Ataca, S Ciraci, J Phys Chem C 115 (2011) 13303 [13] C Backes, N.C Berner, X Chen, P Lafargue, P LaPlace, M Freeley, G.S Duesberg, J.N Coleman, A.R McDonald, Angew Chem Int Ed 54 (2015) 2638 [14] S.H Yu, Y Lee, S.K Jang, J Kang, J Jeon, C Lee, J.Y Lee, H Kim, E Hwang, S Lee, J.H Cho, ACS Nano 8 (2014) 8285
[15] T Kitamura, M Ikeda, K Shigaki, T Inoue, N.A Anderson, X Ai, T Lian, S Yanagida, Chem Mater 16 (2004) 1806
[16] D.P Hagberg, T Marinado, K.M Karlsson, K Nonomura, P Qin, G Boschloo, T Brinck, A Hagfeldt, L Sun, J Org Chem 72 (2007) 9550
[17] J.P Perdew, J.A Chevary, S.H Vosko, K.A Jackson, M.R Pederson, D.J Singh, C Fiolhais, Phys Rev B 46 (1992) 6671
[18] J.P Perdew, J.A Chevary, S.H Vosko, K.A Jackson, M.R Pederson, D.J Singh, C Fiolhais, Phys Rev B 48 (1993) 4978
[19] J.P Perdew, K Burke, M Ernzerhof, Phys Rev Lett 77 (1996) 3865 [20] G Kresse, J Hafner, Phys Rev B 47 (1993) 558
[21] G Kresse, J Hafner, Phys Rev B 49 (1994) 14251 [22] G Kresse, J Furthmüller, Comput Mater Sci 6 (1996) 15 [23] P.E Blöchl, Phys Rev B 50 (1994) 17953
[24] S Grimme, J Antony, S Ehrlich, H Krieg, J Chem Phys 132 (2010) 154104 [25] J Chen, R.E Ruther, Y Tan, L.M Bishop, R.J Hamers, Langmuir 28 (2012)
10437 [26] Q Yue, Z Shao, S Chang, J Li, Nanoscale Res Lett 8 (2013) 1 [27] V.Q Bui, T.T Pham, D.A Le, C.M Thi, H.M Le, J Phys.: Condens Matter 27 (2015) 305005
[28] C Ataca, S Ciraci, Phys Rev B 85 (2012) 195410 [29] T Li, Phys Rev B 85 (2012) 235407
[30] E Scalise, M Houssa, G Pourtois, V Afanas’ev, A Stesmans, Nano Res 5 (2011)
43 [31] H Li, C Tsai, A.L Koh, L Cai, A.W Contryman, A.H Fragapane, J Zhao, H.S Han, H.C Manoharan, F Abild-Pedersen, J.K Norskov, X Zheng, Nat Mater 15 (2016) 48
[32] B.H Kim, M Park, M Lee, S.J Baek, H.Y Jeong, M Choi, S.J Chang, W.G Hong, T.
K Kim, H.R Moon, Y.W Park, N Park, Y Jun, RSC Adv 3 (2013) 18424 [33] R.R Chianelli, M.H Siadati, M.P De la Rosa, G Berhault, J.P Wilcoxon, R Bearden, B.L Abrams, Catal Rev 48 (2006) 1
[34] D Ma, Q Wang, T Li, C He, B Ma, Y Tang, Z Lu, Z Yang, J Mater Chem C 4 (2016) 7093
[35] D Liu, Y Guo, L Fang, J Robertson, Appl Phys Lett 103 (2013) 183113 [36] L.-P Feng, J Su, S Chen, Z.-T Liu, Mater Chem Phys 148 (2014) 5 [37] M Pastore, E Mosconi, F De Angelis, M Grätzel, J Phys Chem C 114 (2010)
7205 [38] M Pastore, F De Angelis, Phys Chem Chem Phys 14 (2012) 920 [39] M Makarova, Y Okawa, M Aono, J Phys Chem C 116 (2012) 22411