The
P and AP phases of a GaSe monolayer.
Credit: Japan Advanced Institute of
Science and Technology.
The
gallium selenide monolayer has been recently discovered to have an alternative
crystal structure and has diverse potential applications in electronics.
Understanding its properties is crucial to understand its functions. Now,
scientists from the Japan Advanced Institute of Science and Technology and the
University of Tokyo have explored its structural stability, electronic states
and transformation of crystal phases.
Solid
materials comprise a symmetric arrangement of atoms that confer properties like
conductivity, strength and durability. Changes in size can change this
arrangement, thereby changing the overall properties of the material. For
instance, the electrical, chemical, optical and mechanical properties of
certain materials can change as we move towards the nanoscale. Science now lets
us study the differences in properties across various dimensions right from
monolayer (atomic) level.
Gallium
selenide (GaSe) is a layered metal-chalcogenide, which is known to have
polytypes, which differ in their stacking sequence of layers, but not a
polymorph, which has a different atomic arrangement inside the layer. GaSe has
sparked a great deal of interest in areas of physical and chemical research,
owing to its potential use in photoconduction, far-infrared conversion and
optical applications. Conventionally, a GaSe monolayer is composed of gallium
(Ga) and selenium (Se) atoms bonded covalently, with the Se atoms projecting
outwards, forming a trigonal prism-like structure called the P phase. Part of
the same research group had earlier reported a novel crystal phase of GaSe
using transmission electron microscopy in Surface and Interface Analysis,
wherein the Se atoms are arranged in a trigonal antiprismatic manner to the Ga
atoms, referred to as AP phase, with a symmetry different from the conventional
P phase (see Picture 1). Because of the novelty of this monolayer structure,
very little is known about how it does its shape shifting. Moreover, how do
variations in the intralayer structure of such compounds affect stability?
To answer
this, Mr. Hirokazu Nitta and Prof. Yukiko Yamada-Takamura from the Japan
Advanced Institute of Science and Technology (JAIST) explored the structural
stability and electronic states of phases of GaSe monolayer using
first-principles calculations, in their latest study in Physical Review B.
Hirokazu
Nitta says, "We have found out through first-principles calculations that
this new phase is metastable, and stability against the ground-state
conventional phase reverses upon applying tensile strain, which we think is
strongly related to the fact that we saw this phase formed only at the
film-substrate interface."
To compare
the structural stability of the P and AP phases of GaSe, the researchers first
calculated the total energy at different in-plane lattice constants, which
represent the size of a unit cell in the crystal, given that its structure
comprises a lattice, an organized meshwork of atoms. The lowest energy that
corresponds to the most stable state was computed and at this state, the P
phase, was found to be more stable than the AP phase.
Then, to
investigate if the AP and P phases can transform into each other, they
determined the energy barriers that the material needs to cross to change, and
additionally performed molecular dynamics calculations using a supercomputer
(see Picture 2). They found the energy barrier for phase transition of P-phase
and AP-phase GaSe monolayers is large likely due to the need of breaking and
making new bonds, which prohibits direct transition from P to AP phase. The
calculations also revealed that the relative stability of P-phase and AP-phase
GaSe monolayers can be reversed by applying tensile strain, or a
stretching-type force.
Highlighting
the importance and future prospects of their study, Prof. Yamada-Takamura says,
"Layered chalcogenides are interesting 2-D materials after graphene,
having wide variety and especially bandgap. We have just found out a new
polymorph (not polytype) of a layered monochalcogenide. Its physical as well as
chemical properties are yet to be discovered."
Together,
the findings of this study describe the electronic structure of a less-known
structure of GaSe that can provide insights into the behavior of similar
epitaxially grown monolayers, revealing yet another secret about the unknown
family members of GaSe and related monochalcogenides.