Stable, Single-Layer MX
2
Transition-Metal Oxides and
Dichalcogenides in a Honeycomb-Like Structure
C. Ataca,
†,‡,§
H. Sahin,
‡,§
and S. Ciraci*
,†,‡,§
†
Department of Physics,
‡
Institute of Materials Science and Nanotechnology, and
§
UNAM-National Nanotechnology Research
Center, Bilkent University, Ankara 06800, Turkey
ABSTRACT: Recent studies have revealed that single-layer
transition-metal oxides and dichalcogenides (MX
2
) might offer
properties superior to those of graphene. So far, only very few
MX
2
compounds have been synthesized as suspended single
layers, and some of them have been exfoliated as thin sheets.
Using first-principles structure optimization and phonon
calculations based on density functional theory, we predict
that, out of 88 diff erent combinations of MX
2
compounds,
several of them can be stable in free-standing, single-layer
honeycomb-like structures. These materials have two-dimen-
sional hexagonal lattices and have top-view appearances as if they consisted of either honeycombs or centered honeycombs.
However, their bonding is different from that of graphene; they can be viewed as a positively charged plane of transition-metal
atoms sandwiched between two planes of negatively charged oxygen or chalcogen atoms. Electron correlation in transition-metal
oxides was treated by including Coulomb repulsion through LDA + U calculations. Our analysis of stability was extended to
include in-plane stiffness, as well as ab initio, finite-temperature molecular dynamics calculations. Some of these single-layer
structures are direct- or indirect-band-gap semiconductors, only one compound is half-metal, and the rest are either
ferromagnetic or nonmagnetic metals. Because of their surface polarity, band gap, high in-plane stiffness, and suitability for
functionalization by adatoms or vacancies, these single-layer structures can be utilized in a wide range of technological
applications, especially as nanoscale coatings for surfaces contributing crucial functionalities. In particular, the manifold WX
2
heralds exceptional properties promising future nanoscale applications.
■
INTRODUCTION
Three-dimensional (3D) MX
2
(M, transition metal; X,
chalcogen atom) compounds constitute one of the most
interesting classes of materials and display a wide range of
important properties. Their bulk compounds include oxide
superconductors,
1
half-metallic magnets,
2
superlubricants,
3
catalysts in redox-based reactions,
4
and solar converters,
5
among others. Some of these compounds have D
6h
point-
group symmetry and occur in layered structures formed by the
stacking of weakly interacting two-dimensional (2D) MX
2
layers and are specified as 2H-MX
2
. Another type of layered
structure known as the 1T structure has D
3d
point-group
symmetry and is common to several of MX
2
compounds. Only
a few 3D MX
2
compounds can be stable both in 2H and 1T
structures. In addition to 2H and 1T layered structures, some
MX
2
compounds can be stable in one of the 3D structures
known as rutile, 3R, marcasite, anatase, pyrite, and tetragonal
structures. An extensive review of bulk 3D MX
2
compounds
can be found in ref 6.
Specifically, rutile crystal and thin films of CrO
2
have been
investigated because of their spintronic applications. Tunneling
magnetoresistance was initially observed in films of half-metallic
CrO
2
.
7
Electronic and magnetic properties of bulk
8
and
epitaxial
9
CrO
2
have also been investigated theoretically.
10
Single-layer CoO
2
has played an important role in under-
standing the superconducting properties of nickel and cobalt
oxide based compounds. In these structures, single layers of
CoO
2
are separated generally by thick insulating layers of Na
+
ions and H
2
O molecul es.
1
The high-T
C
superconducti ng
properties of these structures arise from the single layer of
the CoO
2
plane. Electronic, magnetic, and optical properties of
the cubic pyrite-type CoS
2
structure have been investigated
theoretically,
2,11−14
focusing on the half-metallic properties of
this magnet.
VO
2
has various allotropes at different temperatures. For
example, at low temperature, a monoclinic (M1) phase occurs,
whereas at high temperatures, the rutile metallic phase is
favored. By varying the temperature of the system, one can
observe the metal-to-insulator transition.
15
The synthesis and
characterization of the layered structure of bulk VSe
2
and its
superconducting properties have also been invest igated
experimentally.
16,17
A recent theoretical study on the electronic
and magnetic properties of monolayers of VS
2
and VSe
2
18
concluded that the magnetic properties of these structures can
be controlled by applying strain.
The interaction of iron with chalcogens, and specifically with
O
2
molecules, is of great interest, because it involves processes
varying from corrosion to oxygen transport in biological
Received: December 29, 2011
Revised: March 18, 2012
Published: March 23, 2012
Article
pubs.acs.org/JPCC
© 2012 American Chemical Society 8983 dx.doi.org/10.1021/jp212558p | J. Phys. Chem. C 2012, 116, 8983−8999
systems. Such materials can also be used as catalysts or catalytic
supports in redox-based reactions.
4
FeS
2
, the most well-known
compound among Fe-based materials, has been studied
extensively. The pyrite structure, the most stable polymorph
of FeS
2
, is a crucial compound in materials research.
19−23
Recently, a new method has been put forward and used to
synthesize 2D nanowire networks of FeS
2
.
24
Powders of FeS
2
showing rodlike morphologies are attracting c onsiderable
interest, because they are promising materials for solar energy
conversion. The shape and thermodynamic stability of FeS
2
powders have been investigated using first-principles methods,
pointing out the differences between nano- and macroscale
properties.
25
FeS
2
nanosheets on iron substrates are also used
as photocathodes from tandem dye-sensitized solar cells.
26
FeS
2
pyrite nanocrystal inks are also used in thin-film photovoltaic
solar cells.
5
The above brief overview intends to show how
comprehensive and diverse the features of MX
2
crystals can be.
On the other hand, advances in nanotechnology have led to
the synthesis of novel 2D nano structures. For example,
exceptional properties, such as high carrier mobility, linearly
crossing bands at the Fermi level contributing massless
Fermion behavior, and perfect electron−hole symmetry that
originates from a strictly 2D honeycomb structure, have made
graphene an attractive material for future applications.
27,28
Group IV elements, such as Si and Ge, have also been shown to
form buckled honeycomb structures with bands linearly
crossing at the Fermi level.
29−32
In addition, suspended 2D
single-layer BN
33
and, more recently, single-layer transition-
metal dichalcogenides MoS
2
34
and WS
2
35
with honeycomb
structure have been synthesized. Single-layer NbSe
2
was
synthesized only on SiO
2
substrate.
36
Theoretical
37−45
and
experimental studies dealing with the electronic structure,
34,40
lattice dynamics, Raman spectrum
46,47
and Born effective
charges indicate that single-layer MoS
2
is a nonmagnetic
semiconductor displaying exceptional properties. These proper-
ties of single-layer MoS
2
and its nanoribbons
41
have been
exploited in diverse fields such as nanotribology,
3,48
hydrogen
production,
49
hydrodesulfurization,
50
and solar energy produc-
tion.
51
Whereas the charged surfaces of MoS
2
attain a water-
repellant character, specific vacancy defects in MoS
2
can split
H
2
O to produce free H
2
molecule as a sustainable energy
resource.
52
Most recently, a transistor fabricated from a single
MoS
2
layer pointed out features of these materials that can be
superior to those of graphene.
53
Whereas graphene is ideal for
fast analog circuits, single-layer MoS
2
appears to be promising
for optoelectronic devices, solar cells, and light-emitting diodes.
The most recent experimental study by Coleman et al.,
54
which reported liquid exfoliation of MoS
2
,WS
2
, MoSe
2
, TaSe
2
,
NbSe
2
, NiTe
2
, and MoTe
2
nanosheets having honeycomb-like
structures, motivated us to engage in an extensive analysis of
stability to address the question of whether other single-layer
transition-metal dioxides or dichalcogenides MX
2
can exist in
honeycomb-like structures. In this work, we examined MX
2
compounds (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Nb, Mo, W; X
= O, S, Se, Te) to reveal which ones can be stable in 2D
suspended, single-layer structure. We took into account two
different single-layer structures, namely, honeycomb (H) and
centered honeycomb (T) structures; both can be viewed as a
positively charged 2D hexagonal lattice of M atoms sandwiched
between two hexagonal lattices of negatively charged X atoms.
In both H and T structures, instead of forming covalent sp
2
bonding with three neighboring carbon atoms in graphene,
each M atom has six nearest X atoms, and each X atom has
three nearest M atoms forming p−d hybridized ionic M−X
bonds. Figures 1 and 2 depict 2D single-layer H and T
structures, together with their hexagonal unit cells, contour
plots of the total charge density ρ
T
, and isosurfaces of difference
charge density Δρ (where charges of free atoms situated at the
optimized crystal structure are subtracted from ρ
T
).
Based on extensive stability analysis using first-principles
calculations of structure optimization, phonon frequenc y,
formation energy, elastic properties, and finite-temperature ab
initio molecular dynamics (MD) calculations, we predict that,
out of 88 different MX
2
compounds, 52 different stable 2D
single-layer H and/or T structures can occur as free-standing.
Because of the instability occurring in long-wavelength
acoustical modes, a few of them can be stable only at small
size. Our results are summarized in Figure 3. The series of MX
2
compounds with M = Cr, Mo, and W and X = O, S, Se, and Te
are nonmagnetic semiconductors in the H structure. On the
other hand, the series with M = V, Mn, and Fe are
ferromagnetic metals with a net magnetic moment (ranging
from 0.2 to 3.0 μ
B
per cell). Three-dimensional 2H-NbSe
2
has
metallic and stable H structure in two dimensions, except for
some instability in very-long-wavelength acoustical waves.
Moreover, NbSe
2
also has a free-standing and stable T
structure with a slightly higher energy. Interestingly, NiS
2
and
NiSe
2
are metallic in the H structure, but become semi-
conductor in the T structure. Whereas some of these single-
layer compounds in Figure 3 appear to survive up to high
Figure 1. Atomic structure and charge density analysis of 2D single-
layer MoO
2
presented as a prototype for MX
2
in the H structure. (a)
Top and (b) side views of H structure showing the primitive unit cell
of the 2D hexagonal lattice with Bravais lattice vectors a
and b
(|a
| =
|b
|) and relevant internal structural parameters. Gray and red balls
indicate metal (M) and oxygen (X = O) atoms, respectively. (c)
Contour plots of the total charge density, ρ
T
. (d) Isosurfaces of
difference charge density, Δρ. Turquoise and yellow regions indicate
depletion and accumulation of electrons, respectively. (e) Charge
density isosurfaces showing p−d hybridization in the Mo−O bond.
The isosurface value is taken as 0.01 electron/Å
3
. In the top view in
panel a, unlike in graphene, M and X
2
occupy alternating corners of a
hexagon.
The Journal of Physical Chemistry C Article
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temperatures, some of them are expected to become unstable as
their temperatures are raised. Transition-metal oxides such as
TiO
2
, CoO
2
, and NbO
2
and CoS
2
and CoSe
2
are found to be
unstable in both the H and T structures. However, these single-
layer MX
2
compounds could be stable if they were placed on
specific substrates. All of these structures display interesting
electronic, magnetic, and mechanical properties and have trends
correlated with the electronegativity of constituent elements X
and M.
Although it is relatively easy for 2D single-layer MX
2
structures to be exfoliated from parent 3D layered 2H-MX
2
or 1T-MX
2
structures, it is not obvious whether stable single
layers can form if the parent 3D crystal is not layered.
Following this simple methodology, by searching only the
existence of parent 3D layered 2H-MX
2
structure from the
Landolt−Bornstein database,
55
Ding et al.
56
reported that
MoSe
2
, MoTe
2
, NbSe
2
,WS
2
, WSe
2
, TaS
2
, and TaSe
2
can have
monolayer H structures and reported their electronic proper-
ties. Nonetheless, recent experimental and theoretical studies
indicating that silicene,
30,57−61
III−V compounds,
31
SiC,
62
and
ZnO
63−65
2D single layers all have honeycomb structures even
though their parent 3D bulk materials are not layered provide
strong reasons for pursuing our analysis to search for single-
layer MX
2
compounds. The intention of this work was to
attract interest in the manifold of single-layer MX
2
compounds
by showing how they can provide a wide range of options in
materials research. Accordingly, rather than providing an in-
depth analysis, our discussion is focused on the broad
properties and various classes of MX
2
compounds showing
similar trends.
■
METHOD
Our stability analysis and calculation of physical properties were
carried out using first-principles plane-wave calculations within
density functional theory (DFT) and projector-augmented-
wave (PAW) potentials.
66
The exchange correlation potential
was represented by the local density approximation
67
(LDA)
for both spin-polarized and spin-unpolarized cases. In addition,
our results were tested using the generalized gradient
approximation
68
with the van der Waals correction.
69,70
All
structures were treated using periodic boundary conditions.
Because the electrons of some transition-metal oxides are
strongly correlated, they might not be represented properly by
DFT. Thus, to correct for the deficiencies of DFT, we also
carried out LDA + U calculations.
71
All values calculated in this
article were obtained using LDA unless stated otherwise.
Supercell size, kinetic energy cutoff, and Brillouin-zone (BZ)
sampling of the calculations were determined after extensive
convergence analysis. A large spacing of ∼15 Å between 2D
single layers of MX
2
was used to prevent interlayer interactions.
A plane-wave basis set with kinetic energy cutoff of 520 eV was
used. In the self-consistent-field potential and total energy
calculations, the BZ was sampled by special k points.
72
The
numbers of these k points were (37 × 37 × 1) for the primitive
MX
2
unit cell and were scaled according to the size of the
supercells. All atomic positions and lattice constants were
optimized using the conjugate gradient method, where the total
energy and atomic forces were minimized. The convergence for
energy was chosen as 10
−6
eV between two consecutive steps,
and the maximum Hellmann−Feynman forces acting on each
atom were reduced to a value of less than 0.01 eV/Å upon ionic
relaxation. The pressure in the unit cell was kept below 1 kbar.
Bader analysis is used to calculate the charge on atoms.
73
Because LDA is designed to describe systems with slowly
varying electron densities and can fail to model localized
d orbitals, more accurate electronic structure calculations
were carried out using the screened-nonlocal-exchange Heyd−
Scuseria−Ernzerhof (HSE) functional of the g eneralized
Kohn−Sham scheme
74,75
for specific structures. Frequency-
dependent GW
0
calculations
76
were carried out to correct the
LDA band gaps. However, whereas the GW
0
correction was
successful in predicting 3D bulk MoS
2
, it seems to have
overestimated the band gap of 2D MoS
2
in the H structure.
42
This surprising situation, which is discussed later in the text,
requires further analysis. Numerical calculations were per-
formed using VASP.
77,78
The phonon dispersion curves and
Raman-active modes were calculated using the small-displace-
ment method (SDM)
79
with VASP. For critical situations,
phonon calculations based on the plane-wave self-consistent-
field (PWSCF) method
81
were also performed to carry out the
analysis of Raman- and infrared-active phonon modes and
check specific results.
■
STABILITY ANALYSIS
MX
2
compounds have diverse 3D crystal structures in different
space groups, as presented in Table 1. Among these, the 2H
and 1T structures are layered, and hence, like graphite, they are
formed by stacking of speci fic layers. The structures of the
remaining MX
2
compounds are not layered. The focus of our
study was to determine which of the MX
2
compounds can form
stable 2D single-layer structures. Our study considered only the
structures having hexagonal lattices as shown in Figures 1 and
2. These are H structures (honeycombs) with D
3h
point-group
Figure 2. Atomic structure and charge density analysis of 2D single-
layer NiS
2
presented as a prototype for MX
2
in the T structure. (a)
Top and (b) side views of T structure showing the primitive unit cell
of the 2D hexagonal lattice with Bravais lattice vectors a
and b
(|a
| =
|b
|) and relevant internal structural parameters. (c) Contour plots of
the total charge density, ρ
T
. (d) Isosurfaces of difference charge
density, Δρ . Turquoise and yellow regions indicate depletion and
accumulation of electrons, respectively. (e) Charge density isosurfaces
showing Ni−S bonds. The isosurface value is taken as 0.01 electron/
Å
3
. In the top view in panel a, whereas one of the two X atoms
occupies alternating corners of a regular hexagon, the second X atom is
displaced by (a
+ b
)/3 to occupy the centers of the adjacent hexagons.
The Journal of Physical Chemistry C Article
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symmetry and T structures (centered honeycombs) with C
3v
symmetry. The formation of stable, single-layer MX
2
compounds on a sample specific substrate might be relatively
easy, but it is beyond the scope of the present study. While this
study was being performed, Ding et al.
56
investigated electronic
and vibrational properties of single-layer MX
2
(M = Mo, Nb,
W, Ta; X = S, Se, Te) in the H structure. The only reason they
considered the H structure is because these compounds form
stable 2H-MX
2
in 3D. Instead of providing tests of whether
these 2D nanostructures are stable through the full spectrum of
phonons and temperature-dependent ab initio MD calculations,
they calculated the phonon frequencies only at the Γ point to
reveal the Raman-active modes. Phonon dispersion spectrum
analysis is an important indication of the stability of a system
because phonon modes can become imaginary at other points
in the BZ, when symmetry operations throughout DFT
calculations are not taken into account correctly. Computa-
tionally, systems can be forced to have positive frequencies at
the Γ point; however, this might not mean that they are stable.
In our work, we carefully took into account the symmetry of
the structure and calculated the full phonon dispersion of the
structures. In this respect, the work by Ding et al.
56
had a scope
different from that of the present article, and they apparently
considered a small fraction of the manifold we treat here.
Structure Optimization. Our analysis of stability started
by calculating the total energy of MX
2
compounds in the single-
layer H and T structures depicted in Figures 1 and 2.
Optimization was performed by minimizing the total energy
and atomic forces by varying the atomic positions in the
unit cell and the lattice constants. If a structure was stable,
optimization usually converged to the structure. Furthermore,
we calculated the cohesive energy relative to free constituent
atoms. The cohesive energy per unit cell or per MX
2
unit was
calculated using the expression E
C
= E
T
[M] + 2E
T
[X] −
E
T
[MX
2
], in terms of the total energy of MX
2
, E
T
[MX
2
], and
the total energies of free M and X atoms, E
T
[M] and E
T
[X],
respectively. We found that the calculated cohesive energies
were all positive and in the range of 10−20 eV, indicating a
strong cohesion relative to free atoms of the constituents.
Although a positive value of E
C
alone is not sufficient to
indicate whether a given MX
2
structure can form, the formation
energy, E
f
, is usually taken to be a good criterion. The
formation energies E
f
, which were obtained by subtracting the
cohesive energies of the constituent elements in their
equilibrium (bulk, liquid, or gas) phases, E
C
[M] and E
C
[X].
Specifically, E
f
= E
C
− E
C
[M] − 2E
C
[X]. For the sake of
comparison, we calculated E
f
using both experimental cohesive
energies
80
and calculated cohesive energies of the constituent
elements. Calculated formation energies were positive for all H
and T structures that were found to be stable as a result of a
series of stability analyses.
The optimized lattice constants and other structure
parameters of stable structures are also presented in Table 1.
Experimental data on the lattice constants of MX
2
compounds
in the H structure are not available yet. Even though the lattice
constants of MoS
2
in the H structure (which is the most
studied single-layer MX
2
) have not been measured exper-
imentally yet, they can be inferred from the lattice constants of
2H-MoS
2
. Thus, the lattice constants of MoS
2
are expected to
be close to those of 2H-MoS
2
, which were measured to be a = b =
3.16 Å. The LDA is known to predict overbinding and, hence,
lattice constants that are slightly shorter than the experimental
Figure 3. Summary of the results of our stability analysis comprising 44 different MX
2
compounds that can form stable, 2D single-layer H and/or T
structures. Transition-metal atoms indicated by M are divided into 3d, 4d and 5d groups. MX
2
compounds shaded light gray form neither stable H
nor T structure. In each box, the lower-lying structure (H or T) is the ground state. The resulting structures (T or H) can be half-metallic (+),
metallic (*), or semiconducting (**).
The Journal of Physical Chemistry C Article
dx.doi.org/10.1021/jp212558p | J. Phys. Chem. C 2012, 116, 8983−89998986
Table 1. Calculated Values of Stable, Free-Standing, 2D Single-Layer MX
2
in the H and T Structures: Lateral Lattice Constants,
|a
| = |b
|; Bond Lengths, d
M−X
and d
X−X
;X−M−X Bond Angles, θ; Cohesive Energies per MX
2
Unit, E
C
; Formation Energies per
MX
2
Unit, E
f
;
a
80
Energy Band Gaps, E
g
; GW
0
-Corrected Energy Band Gaps, E
g
GW
0
;
b
Total Magnetic Moments in the Unit Cell,
μ; Excess Charges on M Atoms, ρ
M
;
c
Excess Charges on X Atoms, ρ
X
;
d
In-Plane Stiffness Values, C; and 3D Bulk Structures of
MX
2
e
−
g
type
a
(Å)
d
M−X
(Å)
d
X−X
(Å)
θ
(deg)
E
C
(eV)
E
f
(eV)
E
g
(eV)
E
g
GW
0
(eV)
μ
(μ
B
)
ρ
M
(electrons)
ρ
X
(electrons)
C
(N/m)
3D bulk
structure
h
ScO
2
H 3.16 2.09 2.04 58.30 20.35 7.83 (11.25) 1.05 − 1.00 1.90 −0.95 76.33 −
T 3.22 2.07 2.61 78.02 20.53 8.01 (11.43) M − 1.00 1.96 −0.98 58.07 −
ScS
2
H 3.70 2.52 2.69 64.42 16.31 3.54 (6.71) 0.44 − 1.00 1.64 −0.82 44.41 −
T 3.62 2.50 3.44 87.05 16.48 3.71 (6.88) M − NM 1.62 −0.81 29.39 −
ScSe
2
H 3.84 2.65 2.90 66.39 15.12 3.23 (6.30) 0.27 − 1.00 1.56 −0.78 39.09 −
T 3.52 2.64 3.94 96.42 15.42 3.54 (6.60) M − NM 1.44 −0.72 18.67 −
ScTe
2
H 3.62 2.89 3.98 87.17 13.67 2.25 (5.39) M − NM 1.34 −0.67 38.28 −
T 3.72 2.85 4.33 98.58 14.05 2.63 (5.77) M − NM 1.33 −0.67 13.89 −
TiS
2
T 3.32 2.39 3.42 91.73 18.36 3.97 (7.81) M − NM 1.60 −0.80 76.33 1T
82
TiSe
2
T 3.43 2.51 3.68 94.04 16.92 3.42 (7.15) M − NM 1.39 −0.70 63.92 1T
83
TiTe
2
H 3.62 2.75 3.57 81.09 14.76 1.72 (5.53) M − NM 1.16 −0.58 9.10 1T
84
T 3.64 2.73 4.06 96.30 15.10 2.06 (5.87) M − NM 1.18 −0.59 41.01
VO
2
H 2.70 1.92 2.24 71.34 21.64 7.20 (11.13) M − 0.52 1.79 −0.90 171.98 P4/ncc,
85
I4/m
85
VS
2
T 3.10 2.31 3.43 95.94 17.46 2.78 (6.45) M − 0.33 1.29 −0.65 104.26 1T
86,87
H 3.09 2.31 2.95 79.14 17.47 2.79 (6.46) M − 0.19 1.18 −0.59 106.03
VSe
2
H 3.24 2.45 3.17 80.49 15.97 2.17 (5.74) M − 0.68 1.05 −0.53 82.90 1T
88
T 3.24 2.44 3.66 97.04 15.99 2.20 (5.76) M − 0.35 1.08 −0.54 80.16
VTe
2
H 3.48 2.66 3.48 81.90 14.17 0.83 (4.48) M − 0.83 0.80 −0.40 49.66 1T
89
T 3.46 2.64 4.00 98.35 14.24 0.90 (4.55) M − NM 0.83 −0.41 54.45
CrO
2
H 2.58 1.88 2.29 75.21 19.55 6.25 (10.25) 0.50 1.80 NM 1.54 −0.77 220.94 R
90
CrS
2
H 2.97 2.25 2.92 80.86 15.89 2.35 (6.09) 1.07 1.84 NM 0.92 − 0.46 129.00 1T*
91
CrSe
2
H 3.13 2.38 3.11 81.54 14.32 1.65 (5.30) 0.86 1.51 NM 0.77 − 0.38 104.58 1T*
91
CrTe
2
H 3.39 2.58 3.38 81.56 12.52 0.32 (4.04) 0.60 1.12 NM 0.46 − 0.23 77.37 −
MnO
2
H 2.61 1.87 2.22 72.70 17.71 4.57 (9.59) M − 0.69 1.31 −0.65 134.07 R
92
T 2.82 1.88 2.50 83.07 18.43 5.28 (10.31) 0.28 − 3.00 1.64 −0.82 157.12
MnS
2
T 3.12 2.27 3.29 93.08 14.82 1.43 (6.20) M − 2.38 0.92 −0.46 66.87 P
93−95
MnSe
2
T 3.27 2.39 3.50 93.78 13.61 1.11 (5.77) M − 2.35 0.74 −0.37 56.61 P
96,97
MnTe
2
T 3.54 2.59 3.77 93.56 12.27 0.22 (4.97) M − 2.29 0.41 −0.20 44.77 P
98,96
FeO
2
H 2.62 1.88 2.24 73.08 17.37 3.25 (7.89) M − 1.82 1.38 −0.69 131.99 M
4
FeS
2
H 3.06 2.22 2.68 74.20 15.50 1.14 (5.52) M − 1.12 0.57 −0.29 59.20 P,
19−23
Ma
6
FeSe
2
H 3.22 2.35 2.87 75.36 14.93 1.45 (5.73) M − 1.18 0.42 −0.21 49.89 P,
99
Ma
100
FeTe
2
H 3.48 2.53 3.08 74.98 13.21 0.19 (4.55) M − 1.08 0.06 −0.03 37.71 Ma
101
CoTe
2
H 3.52 2.51 2.96 72.16 13.44 0.29 (4.67) M − NM −0.19 0.10 56.15 Ma
6,102
NiO
2
T 2.77 1.84 2.44 82.82 16.76 3.10 (7.12) 1.38 − NM 1.34 −0.67 146.64 1T
103
NiS
2
H 3.40 2.24 2.14 57.16 14.35 0.45 (4.21) M − NM 0.42 −0.21 39.51 P
104
T 3.28 2.12 2.97 84.46 14.91 1.00 (4.77) 0.51 − NM 0.49 −0.24 86.23
NiSe
2
H 3.33 2.35 2.71 70.29 13.49 0.47 (4.13) M − NM 0.25 −0.12 35.92 P
104
T 3.46 2.34 3.15 84.59 13.97 0.95 (4.61) 0.10 − NM 0.27 −0.13 62.73
NiTe
2
H 3.59 2.54 2.93 70.55 12.92 0.36 (4.10) M − NM −0.12 0.06 41.00 1T
6
T 3.64 2.52 3.47 87.33 13.19 0.63 (4.37) M − NM −0.12 0.06 43.65
NbS
2
T 3.30 2.45 3.62 95.25 19.64 3.17 (6.37) M − NM 1.52 −0.76 96.60 1T,
105
2H
106
NbSe
2
T 3.39 2.57 3.87 97.48 18.13 2.56 (5.64) M − NM 1.27 −0.64 70.47 2H,
107
4H,
108
1T
6
H 3.40 2.57 3.33 80.68 18.23 2.65 (5.74) M − NM 1.23 −0.62 87.24
NbTe
2
T 3.56 2.77 4.24 100.05 16.38 1.26 (4.43) M − NM 0.90 −0.45 64.08 1T
+
109
MoO
2
H 2.78 2.00 2.42 73.92 22.65 6.79 (10.63) 0.97 2.42 NM 1.84 −0.92 223.93 R
+
,
110,111
Mcl
110,111
The Journal of Physical Chemistry C Article
dx.doi.org/10.1021/jp212558p | J. Phys. Chem. C 2012, 116, 8983−89998987
