GaSe has been reported to exist as a layered structure in four crystallographically distinct polytypes, i.e., P-, E-, 6, and y-GaSe. Each polytype results from the stacking of layers in different ways. Specifically, the different coordination symmetry of the metal atoms in a layer and the dissimilar stacking order of each layer in the unit cells are different in each polytype. The P-, 6- and E-types have a layered hexagonal crystal structure, whereas the y-type crystallizes in a rhombohedra1 structure.

As illustrated in Fig. 1, the structure of GaSe consists of layers that can be regarded as a two-dimensional sheet of closely packed and strongly bonded Ga and Se atoms in the sequence Se-Ga-Ga-Se. Each layer consists of four atomic planes. The Ga atoms are paired to form the two atomic planes inside, while the Se atoms form the top and bottom planes. The interatomic bonding within the layers is strong and mainly of the covalent type, whereas the bonding between adjacent layers is weak and of the van der Waals type. Because of such strong anisotropy in bonding, solid crystallites of GaSe can easily be cleaved along the basal planes.

GaSe is a III-VI semiconductor which has many kinds of structures. We focus on the β-GaSe, which has a hexagonal structure and belongs to the P63/mmc group space. For the bulk phase, the lattice constants of a unit cell are ??=3.75 ?,??=3.75 ?,and ??=16 ?, and the corresponding angles are ??= 90°, ??=90°,and ??= 120°. The crystal structure of the bulk phase is shown in Figure 1a. The crystal structures of a mon-olayer GaSe unit cell, which contains two Ga atoms and two Se atoms, are shown in Figure 1b,c. In the following section, we focus on different sizes of monolayer GaSe.

References:

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[2] HSIN-PAN HUANG  Ching R C  Huei Ru Fuh. Enhanced Sensitivity of CO on Two-Dimensional, Strained, and Defective GaSe.[J]. Molecules, 2021. DOI:10.3390/molecules26040812.