Monolayer group-III monochalcogenides by oxygen
functionalization: a promising class of two-dimensional
, Cheng-Cheng Liu
, Jijun Zhao
and Yugui Yao
Monolayer group-III monochalcogenides (MX, M = Ga, In; X = S, Se, Te), an emerging category of two-dimensional (2D)
semiconductors, hold great promise for electronics, optoelectronics and catalysts. By ﬁrst-principles calculations, we show that the
phonon dispersion and Raman spectra, as well as the electronic and topological properties of monolayer MX can be tuned by
oxygen functionalization. Chemisorption of oxygen atoms on one side or both sides of the MX sheet narrows or even closes the
band gap, enlarges work function, and signiﬁcantly reduces the carrier effective mass. More excitingly, InS, InSe, and InTe
monolayers with double-side oxygen functionalization are 2D topological insulators with sizeable bulk gap up to 0.21 eV. Their low-
energy bands near the Fermi level are dominated by the p
orbitals of atoms, allowing band engineering via in-plane strains.
Our studies provide viable strategy for realizing quantum spin Hall effect in monolayer group-III monochalcogenides at room
temperature, and utilizing these novel 2D materials for high-speed and dissipationless transport devices.
npj Quantum Materials (2018) 3:16 ; doi:10.1038/s41535-018-0089-0
Group-III monochalcogenides (MX, M = Ga, In; X = S, Se, Te) are
layered semiconductors that have been extensively studied for
decades due to their peculiar properties, such as high carrier
mobility, sombrero-shape valence band edges, rare p-type
electronic behaviors, etc.
It is not until recently that their few
or monolayer sheets (GaS, GaSe, and InSe) are realized in
constituting a new family member of two-
dimensional (2D) materials. Monolayer MX has a hexagonal lattice
composed of two planes of metal atoms sandwiched between
two planes of chalcogen atoms. These 2D sheets have band gap of
the gap and electronic transport properties are
layer dependent, rendering these materials promising candidates
for electronic and optoelectronic devices.
Recently, ﬁeld effect
transistors based on monolayer InSe were fabricated with
ultrahigh carrier mobility above 10
and large on/off
ratio of 10
Photodetectors made of GaS, GaSe, GaTe, or InSe
nanosheets exhibit high photoresponsivity and fast response time
for a broad spectral range.
Besides, these novel 2D
compounds were demonstrated to be potential piezoelectric
and thermoelectric materials,
photo-catalysts and electro-
On the other hand, chemical functionalization of 2D materials is
an effective strategy to tailor the material properties and trigger
exotic phenomena. Oxidation and hydrogenation of graphene
and silicene are widely exploited in experiment for band gap
Other 2D honeycomb lattices modiﬁed by H, O,
and halogen atoms are theoretically proposed, showing tunable
and diverse electronic and topological properties. For instance,
oxidation of monolayer blue phosphorus induces quantum phase
transitions and novel emergent fermions,
and ﬂuorination can create σ-character Dirac cones.
spin Hall (QSH) effects were predicted for the functionalized group
IV and group V monolayers, including the oxides of arsenene and
hydrogenated and halogenated germanene,
with sizable bulk gap up to 1.08 eV
that is sufﬁciently large for practical applications at room
temperature. The halogenated bismuthene also exhibit diverse
topological phases, such as quantum valley Hall insulators
time-reversal-invariant topological superconductors.
experimental observations of QSH effects are reported for HgTe/
CdTe and InAs/GaSb quantum wells,
and Bi(111) bilayer on
The exciting properties of these so-called QSH
insulators or topological insulators (TI) are triggering extensive
research in exploring new candidate TIs for 2D electronics,
spintronics, valleytronics, and quantum
Despite the excellent properties of 2D group-III monochalco-
genides themselves, only limited attentions have been paid to
their chemical modiﬁcations so far. Balakrishnan et al. controllably
oxidized the surface of InSe nanolayers by photo-annealing and
thermal-annealing in air, and obtained InSe/In
as p–n junctions with tunable band gap.
Beechem et al. and Del
Pozo-Zamudio et al. showed that oxidation of ultrathin GaSe and
InSe ﬁlms lead to the reduction of photoluminescence.
However, the structures, physical, and chemical properties of the
modiﬁed MX sheets are still unclear and await explorations.
Herein we investigate the electronic and topological properties
of oxygen functionalized group-III monochalcogenides mono-
layers including GaS, GaSe, GaTe, InS, InSe, and InTe. First-
Received: 5 November 2017 Revised: 12 February 2018 Accepted: 21 February 2018
Key Laboratory of Materials Modiﬁcation by Laser, Ion and Electron Beams (Dalian University of Technology) Ministry of Education Dalian 116024, China and
Laboratory of Nanophotonics and Ultraﬁne Optoelectronic Systems, School of Physics, Beijing Institute of Technology, Beijing 100081, China
Correspondence: Jijun Zhao (email@example.com) or Yugui Yao (firstname.lastname@example.org)
These authors contributed equally: Si Zhou and Cheng-Cheng Liu.
Published in partnership with Nanjing University