Single-element-based 2D materials (Xenes) have garnered tremendous interest. At present, 16 kinds of Xenes (silicene, borophene, germanene, phosphorene, tellurene, etc.) have been explored, mainly distributed in the third, fourth, fifth, and sixth main groups. Although two Xenes (aluminene and indiene) have not been synthesized due to the limitations of synthetic methods and the stability of Xenes, other Xenes have been successfully created via elaborate artificial design and synthesis. Elemental 2D materials show potential applications in various fields, including spintronics, electronics, optoelectronics, superconducting, photovoltaics, sensors, catalysis, and biomedicines.
1. Introduction
Graphene, initially isolated from graphite via mechanical exfoliation, possesses a layered honeycomb structure and exhibits fascinating electrical and thermal properties
[1]. Subsequently, two-dimensional (2D) materials, including elemental monolayers (Xenes)
[2], transition metal chalcogenides (TMCs)
[3][4], oxides
[5], halides
[6][7], and carbides (MXenes)
[8][9], have shown intriguing properties, such as high carrier mobility
[10], layer-dependent band structures and magnetic properties
[6][11][12], nontrivial topology
[13][14][15][16], valleytronics
[17], etc. Therefore, 2D materials have become promising candidates for various applications relating to next-generation technology, including spintronics, superconducting, nanoelectronics, nanosensing, etc.
Over the past few years, many studies have focused on searching for other 2D Xenes with distinctive and exciting properties beyond graphene (
Figure 1a)
[18][19]. The successful experimental realization of non-graphene 2D analogs (silicene and phosphorene) sparked a continuous expansion of the list of elements with atomically thin forms
[20][21]. Sixteen elemental-main-group 2D Xenes have been predicted theoretically or created experimentally to date (
Figure 1b)
[22][23][24][25][26][27][28][29][30][31][32]. To the best of our knowledge, except aluminene and indiene, the other 13 non-graphene Xenes have been experimentally obtained (
Figure 1c). Currently, monolayer or few-layer Xenes can be created using mechanical exfoliation, liquid phase exfoliation, and epitaxial growth methods
[1][12][22][33].
Figure 1. Development of 2D Xenes. (a) Statistics diagram of the number of research articles on non-graphene 2D Xenes from 2010 to 2021. (b) Overview of 2D analogs of main-group elements, explored using either experimental or theoretical routes. The elements with no signs have not been explored to date. (c) The timeline of the experimental creation of 2D Xenes.
2. Classification of 2D Xenes
The material properties (electronic, optical, chemical, etc.) of 2D Xenes are not only determined by their chemical compositions but are also strongly associated with the atom arrangement in the lattice. Due to the preferred orbital hybridization of various elements in the main group, 2D Xenes have been theoretically predicted or experimentally verified to possess allotropes with diverse crystal lattices. The reported 16 Xenes are classified into main groups, including group III (borophene, aluminene, gallenene, indiene, and thallene); group IV (graphene, silicene, germanene, stanene, and plumbene); group V (phosphorene, arsenene, antimonene, and bismuthene); and group VI (selenene, and tellurene).
In group III elements, theoretical calculations have predicted many allotropes of 2D borophene (B
1−ν☐
ν; ν represents the vacancy concentration) with various vacancy concentrations and 2D aluminene with various forms
[34][35][36][37]. However, only two allotropes of 2D gallenene (buckled and planar structures) have been successfully exfoliated
[29]. Similarly, 2D indiene has been predicted to possess buckled, planar, and puckered geometry
[38]. Among group IV elements, the favorable hybridization state is somewhere between sp
2 and sp
3, leading to various 2D allotropic structures, such as planar honeycomb graphene/stanene
[39][40][41], buckled silicene/germanene/stanene/plumbene
[25][30][42][43], pha-/penta-graphene
[44][45], MoS
2-like stanene
[46], and honeycomb dumbbell (HD)/large honeycomb dumbbell (LHD) silicene/germanene
[46]. Different from group III and group IV, all the elements in group V prefer the sp
3 hybridization state to create buckled (α-form) or puckered (β-form) lattices
[24][47][48][49][50][51][52]. Meanwhile, a γ-form and a δ-form of arsenene have also been proposed
[53]. In group VI, four different allotropic forms of tellurene have been predicted
[27][54], while Se can form a chain, a ring, or a square structure
[54]. To verify the atomic structures of various elemental Xenes, epitaxial growth has been attempted on many substrates, analogous to the epitaxial growth of graphene. The suitable substrates for the epitaxial growth of various non-graphene Xenes (
Figure 2) reveal that the substrate choices affect whether the synthesis can succeed in addition to an allotropic lattice structure.
Figure 2. Summary of the successful substrate choices for the epitaxial growth of various non-graphene 2D Xenes. Each color represents one kind of Xenes.
3. Applications of 2D Xenes
2D Xenes possess various physical and chemical properties, such as flexibility, layer-dependent semiconducting, high carrier mobility, molecule and light sensitivity, topologically nontrivial band structures, etc. In order to utilize 2D Xenes more effectively, surface modifications become particularly important to tune the properties of 2D Xenes. 2D Xenes have become promising for applications in different fields, such as spintronics, electronics, optoelectronics, superconducting, photovoltaics, sensors, catalysis, and biomedicines. For instance, the FET of acene-type graphene nanoribbons exhibits excellent semiconductor characteristics with an on/off ratio of 88
[55]. To enhance the mid-infrared (MIR) absorption of graphene, the localized surface plasmon resonance of B-doped Si quantum dots (QDs) results in a QD/graphene hybrid photodetector with ultrahigh responsivity, gain, and specific detectivity in the UV-to-MIR region
[56]. A 2D bismuthene/Si(111) heterostructure exhibits excellent photodetection performance in the Vis-MIR region due to the promoted generation and transportation of charge carriers in the heterojunction
[57]. Solution-exfoliated black phosphorene flakes, as an electron transport layer, can enhance the performance of organic solar cells
[58]. In addition, layered black phosphorene exhibits the selective detection of methanol
[59]. The thermoelectric power (S) in black phosphorene can be effectively controlled with ion-gating. In the hole-depleted state, the S of black phosphorene can reach +510 μV/K at 210 K, much higher than the bulk single crystal value of +340 μV/K at 300 K
[60]. Under the proper electron-doping and biaxial tensile strain, buckled arsenene shows superconductivity with a high T
c of 30.8 K
[61]. Iodine-decorated stanene exhibits a topologically nontrivial band structure with a larger gap of ~320 meV than that of pristine stanene (~100 meV)
[62]. Graphene/Pt(111) surfaces can cause CO adsorption/desorption and CO oxidation surface reactions
[63]. MoS
2/graphene hybrids decorated by CdS nanocrystals can act as high-performance photocatalysts for H
2 evolution under visible light irradiation
[64]. Moreover, 2D Xenes are also regarded as promising agents for biomedical applications
[65]. For example, an ultrathin bismuthene can act as a sensing platform to detect microRNA with a detection limit of 60 PM
[66]. Polyethylene-coated antimonene quantum dots can be used as photothermal agents with a high photothermal conversion efficacy of 45.5% for photothermal therapy in cancer theranostics
[67]. Overall, thanks to surface modifications, 2D Xenes show great potential for applications in plenty of fields.
This entry is adapted from the peer-reviewed paper 10.3390/molecules28010200