Today, two-dimensional materials are one of the key research topics for scientists around the world ^{[1][2][3][4][5]}[1,2,3,4,5]. Since 2004, when the first representative of a new class of materials, a monolayer of carbon atoms, was experimentally obtained in the work by Geim and Novoselov [6], the researchers have seen a real boom in publications on this topic. Over the years, there has been a quantitative and qualitative leap both in the study of graphene itself and of other two-dimensional allotropes—transgraphenes, or X-enes (Figure 1a). As a characteristic numerical indicator confirming the above, the researchers can cite data on the growth in the number of publications that include the keyword “2D material” in the international information and analytical systems of scientific citation Scopus and Web of Science (Figure 1b). By 2022, the number of publications crossed a milestone—more than 10,000 publications on the topic of 2D materials per year (30 articles per day or 1 article every hour).

This interest in two-dimensional materials is not surprising, since, thanks to their remarkable mechanical, thermal, electrical, magnetic, and optical properties, they promise to revolutionize electronics. Among the outstanding characteristics of 2D materials, the following can be distinguished: very high mobility of charge carriers, extreme strength of graphene due to strong carbon–carbon bonds, the ability to control energy structure and bandgap by changing the material composition, and a simple defect structure due only to the presence of vacancies and impurities. In addition, under certain conditions, exotic quantum effects can manifest in these materials: they can be topological insulators and high-temperature superconductors. The unique properties of 2D materials make them promising for the creation of completely new types of devices for functional electronics, nanophotonics, and quantum technologies ^{[7][8][9][10][11][12]}[7,8,9,10,11,12].

Nowadays, the following existing two-dimensional allotropic modifications can be distinguished: graphene (C) and its analogs (transgraphenes or X-enes) borophene (B), aluminene (Al), gallenene (Ga), indiene (In), thallene (Tl), silicene (Si), germanene (Ge), stanene (Sn), plumbene (Pb), phosphorene (P), arsenene (As), antimonene (Sb), bismuthene (Bi), selenene (Se), and tellurene (Te) (Figure 1a).

They are very closely related to graphene and transgraphene derivatives functionalized by hydrogen ions (graphane (CH)_n, silicane (SiH)_n, germanane (GeH)_n, and other transgraphanes (or X-anes) with the general formula (GH)_n, where G is one of the elements that form the initial two-dimensional material: C, Si, Sn, Pb, B, P, etc.) or other ligands (for example, metal cations, hydroxyl groups, organic radicals, with the general formula (GR)_n) [13].

2. Structural Characteristics of Single-Element 2D Materials

The concept of a two-dimensional allotropic modification of silicon was proposed in 1994 [14]. Subsequently, the new material was actively studied by theorists and was named “silicene” by analogy with graphene [15]. All graphene-like materials of elements of group IVA are characterized by mixed sp²–sp³ hybridization [16]. Unlike graphene (Figure 2a), the lattice of silicene, germanene, stanene, and plumbene is not absolutely flat, but curved. Their structure can be represented as two sublattices displaced relative to each other in the vertical direction (Figure 2b). Such curved honeycomb structure and ordered buckling of the surface lead to exceptional stability and increased flexibility compared with graphene. This geometry defines the wide spectrum of their potential applications in electronics ^[17][18][19][17,18,19]. Such a buckled hexagonal structure was also observed in all further works on the synthesis of transgraphenes of group IVA ^{[20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57][58]}[20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58]. The data on the lattice parameters of group IVA graphene-like 2D materials (silicene, germanene, stanene, and plumbene) such as the distance l between nearest atoms in the honeycomb structure, the lattice constant a, and the displacement parameter between the upper and lower atoms δ are reviewed in Table 1. Borophene, aluminene, gallenene, indiene, and thallene—representatives of group IIIA transgraphenes—can have various modifications of the crystal lattice ^{[59][60][61][62][63][64][65][66][67][68][69][70][71][72][73]}[59,60,61,62,63,64,65,66,67,68,69,70,71,72,73]. Borophene has a flat or curved structure, consisting of many equilateral triangles, forming a two-dimensional network (Figure 3). In this case, its structure can be considered as a solid solution of the composition B_1−vV_v, where v is the concentration of vacancies V of boron atoms. When there are no vacancies of boron atoms, borophene has a curved lattice with the parameter δ ≈ 0.08 nm (Figure 3a), and in the presence of vacancies with the concentration v = 1/6, its structure becomes flat (Figure 3b). Borophene structures with a different v are also possible. The distance between boron atoms in the borophene structure is about 0.17 nm ^[2][59][2,59]. Gallenene has a flat honeycomb structure similar to graphene (Figure 2a), or a slightly curved configuration such as group IVA transgraphenes (Figure 2b). The crystal lattice parameters of gallenene have the following values: the distance between the gallium atoms is l = 0.25 nm, the lattice constant a is about 0.39 nm, and the buckling parameter δ = 0–0.08 nm ^[63][64][63,64]. Aluminene has not yet been synthesized experimentally, but theoretical calculations using the density functional theory predict the possibility of its existence with a crystal lattice such as graphene, borophene, or even phosphorene (Figure 4). Thus, aluminene can have a flat, buckled, or puckered structure ^[67][68][67,68]. For the graphene-like modification of the aluminene crystal lattice, the distance between aluminum atoms is predicted to be l ≈ 0.26 nm [65]. The two-dimensional allotropic modification of indium (indiene) is also predicted only theoretically so far. Presumably, it can have three variants of the crystal structure: flat (like in graphene) (Figure 2a), buckled (like in transgraphenes of group IVA) (Figure 2b), or puckered (like in phosphorene) (Figure 4a). For all modifications of indiene, the calculated distance between indium atoms is l ≈ 0.29 nm, and the lattice constant is a ≈ 0.5 nm for the flat geometry and a ≈ 0.425 nm for the buckled one at δ ≈ 0.15 nm ^{[65][70][71][72]}[65,70,71,72]. The last representative of group IIIA transgraphenes, thallene, was synthesized in 2020 on the NiSi₂/Si(111) surface [73] in the form of a flat honeycomb structure similar to graphene (Figure 2a), with parameter l ≈ 0.38 nm. Density functional theory calculations predict for a free-standing thallene l = 0.3 nm (Table 2). The first representative of transgraphenes of group VA, phosphorene can be in two modifications: puckered (or α-phase) (Figure 4a) and buckled (or β-phase) (Figure 4b). The lattice parameters of phosphorene have the following values: the distance between phosphorus atoms is l = 0.23 nm, the lattice constant a is about 0.33 nm, and the buckling parameter δ = 0.12 nm ^[72][74][72,74]. For the two-dimensional modification of arsenic (arsenene), theoretical calculations predict the following lattice parameters: the distance between arsenic atoms is l = 0.25 nm, the lattice constant a is about 0.36 nm, and the buckling parameter δ = 0.14 nm ^[75][76][75,76]. Experimental studies of the synthesis of two-dimensional antimony (antimonene) turned out to be quite successful, and today this material has already been fabricated on a large number of substrates ^{[77][78][79][80][81][82]}[77,78,79,80,81,82]. The distance between antimony atoms ranges from 0.29 nm and more, depending on the substrate; the lattice constant is slightly more than 0.4 nm and δ ≈ 0.17 nm ^[75][78][75,78]. For bismuthene, a two-dimensional allotropic modification of bismuth, the lattice constant of a curved graphene-like structure was calculated to be a = 0.434 nm [72]. However, its actual value may increase depending on the substrate [83] (Table 3). ]. Then, their electrical, thermoelectric, and thermal properties were theoretically investigated ^{[85][86][87][88]}[85,86,87,88]. Two modifications of the crystal lattice are predicted for selenene and tellurene (Figure 5). According to the theoretical calculations, the distance between atoms in selenene should be l ≈ 0.27 nm [88]. The two-dimensional tellurium layer (tellurene) has the same crystal structure modifications as selenene (Figure 5), and its lattice parameter l is about 0.3 nm ^[88][89][90][88,89,90] (Table 4). Finally, let us take a look at the structural properties of single-element 2D materials from the group VIA. The stable states of the two-dimensional modification of selenium (selenene) were first calculated within the framework of the density functional theory in 2017 [84 Thus, numerous experimental studies confirmed the curved structure of transgraphenes and also showed that such a structure is more flexible than that of graphene, which makes it possible to control the energy spectra by adjusting the synthesis conditions.

3. Epitaxial Fabrication of Single-Element 2D Materials

3.1. Group IIIA Single-Element 2D Materials

Borophene, the first representative of 2D materials of group IIIA, was first synthesized experimentally on Ag(111) substrate in 2015 ^[59][60][59,60]. Earlier attempts at heteroepitaxial fabrication of borophene were complicated by a low barrier of the formation of three-dimensional boron islands [91]. Therefore, it was necessary to carefully select a suitable substrate in order to overcome this unwanted 3D island formation. A low threshold for nucleation of two-dimensional clusters was found on a number of metal substrates, which facilitated the formation of extended layers of large-area single-crystal borophene. Among them are Al(111) [92], Au(111) [93], Cu(111) [94], and Ir(111) [95] (Figure 6). In this case the nearest-neighbor distance of the potential substrate for the fabrication of 2D material should match with the lattice parameter a of the honeycomb lattice. In addition to metal substrates, the possibility of obtaining layers of two-dimensional boron modification on surfaces such as transition metal diborides ^[96][97][96,97] with the (0001) orientation is predicted.