1
Spontaneous doping of 2D MoS
2
basal plane by oxygen substitution during
ambient exposure
János Pető
1
, Tamás Ollár
2
,
Péter Vancsó
1,3
, Zakhar I. Popov
4
, Gábor Zsolt Magda
1
, Gergely
Dobrik
1
, Chanyong Hwang
5
, Pavel B. Sorokin
4
, and Levente Tapasztó
1*
1. Hungarian Academy of Sciences, Centre for Energy Research, Institute of Technical
Physics and Materials Science, 2DNanoelectronics Lendület Research Group 1121
Budapest, Hungary
2. Hungarian Academy of Sciences, Centre for Energy Research, Institute for Energy
Security and Environmental Safety, Surface Chemistry and Catalysis Department 1121
Budapest, Hungary
3. University of Namur, Department of Physics, 61 rue de Bruxelles, 5000 Namur, Belgium
4. National University of Science and Technology MISiS, 119049 Moscow, Russia
5. Korea Research Institute for Standards and Science, Daejeon 305340, South Korea
* tapaszto@mfa.kfki.hu
The chemistry of atomically thin crystals can markedly differ from the
conventional surface reactions of their bulk counterparts. Here we provide atomic level
insights into the ambient oxidation reaction of MoS
2
monolayers by scanning tunneling
microscopy measurements. In contrast to the generally accepted view of
environmentally inert basal plane, we found that oxygen atoms spontaneously
incorporate into the basal plane of MoS
2
single layers during ambient exposure. While
the basal plane was found to be stable against commonly investigated O chemisorption
and MoO
3
conversion, an oxygen substitution reaction has been revealed, upon which
individual sulfur atoms are one by one replaced by oxygen, giving rise to a novel 2D
solid solution type MoS
2-x
O
x
crystal. This oxidation process enables the chemical
modification of single atomic sites of 2D crystals opening new routes towards their
efficient defect engineering. An eloquent example is that the O substitution sites present
all over the basal plane substantially increase the catalytic activity of the 2D MoS
2
crystals for electrochemical H
2
evolution reaction.
2
Layered materials display thickness-dependent properties when approaching the single
layer limit. Chemistry is no exception, as evidenced by the oxidation and hydrogenation of
graphene
1
,
2
. Surface chemistry is a particularly promising approach for engineering the
properties of 2D materials, given their fully exposed atomic structure. However, the chemistry
of 2D Transition Metal Dichalcogenide (TMDC) crystals is mainly defined by their edges
3
,
where coordinatively unsaturated sites prevail. Not coincidentally, these reactive edge sites
were also found to be responsible for the catalytic activity of MoS
2
4
,
5
, one of the most widely
studied TMDC materials. However, the chemical modification restricted to edges is only
efficient in nanoscale islands
6
, as the edge to surface ratio drastically decreases in the 2D
limit. Consequently, for 2D crystals it is of particular importance to chemically and
catalytically activate their entire basal plane. Oxidation has widely been studied for graphene
as a promising approach towards its efficient exfoliation
7
. For 2D TMDC crystals the
oxidation reaction is also of particular importance, as for some crystals it can spontaneously
proceed under ambient conditions. Therefore, its study is essential for understanding their
long-term environmental stability, as well as it can open new routes towards chemically
engineering their properties.
While some TMDC crystals, such as HfSe
2
, MoTe
2
or WTe
2
, are known to be air-
sensitive
8
,
9
,
10
as they rapidly degrade under ambient, the most widely investigated members of
the TMDC family (MoS
2
, MoSe
2
, WS
2
, WSe
2
) have generally been considered air-stable
11
,
12
,
based also on decades-long experience with their bulk crystals. Nevertheless, it has been
demonstrated that in single layer form the oxidation of MoS
2
and WS
2
also occurs under
ambient conditions
13
. The detailed investigations revealed that the oxidation induced etching
originates from edges and grain boundaries and proceeds towards the interior of the flakes.
Due to the higher strength of the Mo-O bonds, as compared to Mo-S, the substitutional
oxidation of the 2D MoS
2
basal plane is in principle also thermodynamically favorable
14
,
15
.
3
However, while at under-coordinated atomic sites on edges and grain boundaries, such
oxidation is a fast, low-barrier process
14,15
, the oxidation of the defect-free basal plane has
been predicted to face relatively high kinetic barriers of about 1.6 eV
,
16
, rendering the basal
plane environmentally stable. Although experiments often reveal significantly lower
activation energy values for the MoS
2
oxidation, such as 0.54 eV or 0.98 eV
17
,
18
, no direct
evidence on the ambient oxidation of the pristine MoS
2
basal plane has been reported so far.
We propose that this is mainly due to the inability of the employed structural characterization
methods to resolve single-atom level structural changes. Harsh oxidation processes, such as
oxygen plasma treatment, UV-ozone exposure, electrochemical exfoliation, or high
temperature (> 300°C) annealing are able to oxidize the basal plane of MoS
2
crystals. Based
primarily on XPS investigations it was shown that such processes can lead either to covalent
oxygen bonding to the top sulfur atoms
14,
19
, or the formation of completely oxidized MoO
3
areas
20
,
21
,
22
that can subsequently volatilize, leading to etching. However, neither process is
ideal for chemically tuning the properties of MoS
2
sheets. While the formation of MoO
3
destroys the original MoS
2
crystal lattice, yielding an overall disordered and fragmented
structure, the chemisorption of oxygen onto chalcogen atoms is predicted to have a relatively
weak influence on band structure
and properties
14,16
. It has also been shown that oxidation can
both enhance and degrade the catalytic activity of MoS
2
, depending on the structural
details
23
,
24
, further emphasizing the importance of investigating and controlling the resulting
oxidized structure. The possibility of a substitutional oxidation reaction of MoS
2
has also been
raised as being thermodynamically preferred to oxygen chemisorption
15,
25
; however, no clear
experimental evidence has been reported so far. The controlled substitutional oxidation of the
MoS
2
basal plane would be a highly desirable reaction that preserves the original MoS
2
crystal structure, while in contrast to O chemisorption, it is also expected to substantially
4
influence the electronic band structure, enabling a more efficient engineering of its
electronic
14,16
and optical
26
properties.
Here we show that the basal plane of the MoS
2
monolayers subjected to long-term
ambient exposure spontaneously undergo such oxygen substitution reactions giving rise to a
novel, highly crystalline two-dimensional molybdenum oxy-sulfide phase.
Results and discussion
Ambient oxidation of the basal plane revealed at single-atom level
We have prepared mechanically exfoliated MoS
2
single layers on atomically flat Au
(111) substrates using a slightly modified version of a recently developed exfoliation
technique
27
yielding single layers with lateral dimension of hundreds of microns (see SI
section I. for details). Such large area samples are characterized by an extremely low edge to
surface ratio, grain boundary concentration, as well as a much smaller concentration and
variety of intrinsic point defects
28
, establishing them as an excellent model system for
studying the intrinsic chemical properties of the pristine basal plane. The exfoliated MoS
2
samples have been stored under ambient conditions (air, room temperature and ambient light)
for periods of up to 1.5 years. We have employed STM measurements to follow the atomic
level structural changes in the basal plane structure of 2D MoS
2
crystals during long-term
ambient exposure. So far mainly optical, scanning electron and atomic force microscopy
measurements have been employed to monitor the structural changes induced by oxidation in
MoS
2
layers. However, the spatial resolution of these methods does not allow detecting single
atom level modifications. Imaging the atomic-scale structure of the MoS
2
basal plane is
possible by high resolution scanning Transmission Electron Microscopy (TEM). However,
detecting light atoms such as oxygen with TEM is challenging due to their low contrast and
easy knock-out
29
,
30
. By contrast, STM measurements can detect the unaltered structure of
such oxygen defects due to its atomic resolution capability and the low energy of the
5
tunneling electrons
31
. Due to the slow oxidation reaction, and the noninvasive nature of the
measurements, the structure and number of oxidation induced defect sites did not change
during the STM imaging, enabling us to acquire atomic resolution snapshots of the oxidation
process.
Atomic resolution STM images of the basal plane of a mechanically exfoliated MoS
2
single layer after 1 month and 1 year of ambient exposure are shown in Fig. 1b and 1c,
respectively. The STM images have been acquired on the same sample, but at different
locations. In contrast to general expectations, STM measurements reveal clear modifications
in the atomic structure of the MoS
2
basal plane during ambient exposure. Freshly prepared 2D
MoS
2
crystals contain a native point defect density in the range of 10
11
-
10
12
cm
-2
(Fig. S2).
After a month of ambient exposure our STM measurements revealed the formation of new
point defects all over the basal plane increasing their concentration into the 3×10
12
-
2×10
13
cm
-2
range (Fig. 1b). The increase of the atomic scale defect concentration (up to
5×10
13
- 10
14
cm
-2
) appears more strikingly after a year exposure (Fig. 1c), when a substantial
area of the sample surface is already covered by such defects, clearly evidencing the ability of
the ambient exposure to create new defect sites in the basal plane structure of 2D MoS
2
crystals. Although the defect density shows some spatial variation, as evidenced by the
concentration ranges provided above, the effect of the ambient exposure time is more
significant. Higher resolution STM images (Fig. 1d) shed light on the atomic structure of the
defects induced by ambient exposure. Dark triangles in the STM images of 2D MoS
2
crystals
are characteristic to sulfur atom vacancies
31
. Within the dark triangles, bright spots can be
clearly detected that have not been observed before. These central bright spots are a
pronounced feature of the experimental STM images, as their contrast is even stronger than
that of S atoms of the MoS
2
lattice. The most straightforward choice is to attribute the bright
spots inside the S vacancies to atoms or molecules saturating the vacancies under ambient
