**4. 2D allotropes of Si, Ge, Sn, P, As, Sb, and B**

exception of the carbon (C), are analyzed in Section 4; the *h*BN, 2D TMDs, and MXenes are discussed in Section 5, and finally, the optoelectronic applications are analyzed in Section 6.

Graphene is a stable 2D material based on carbon (C). It has been very successful, since it is composed of van der Waals type layers which are one or few atoms thick [1]. It has attracted designers due to its exceptional electronic properties. In addition, graphene is one million times thinner than paper, transparent, and the strongest material in the world. Gapless graphene strongly interacts with light from terahertz to ultraviolet (mainly, photodetectors) range [3].

nately, graphene has zero band gap, which disqualifies it due to its low on/off ratio required

For example, graphene has been chemically modified into different versions such as graphane [9], graphone [10], graphyne [10], graphdiyne [10], fluorographene [11], or graphXene [12]. Graphyne is better than graphene in directional electronic properties and charge carriers. Graphone (partially hydrogenated) and graphane (100% hydrogenation of graphene with stoichiometry CH) have applications in nanoelectronics and spintronics due to the presence of band gap and magnetic properties [1]. Graphone with hydrogen coverage of 8% reaches a

a band gap of 1.25 eV for different coverage [13]. Therefore, the size of the band gap depends exclusively on the H/C ratio, where band gaps of up to 1 eV are obtainable. Graphane achieves a band gap of 3.5 eV (insulating behaviour), although theoretically it achieves a value of 4.5

to a band gap of 3.12 eV. Unfortunately, graphane has an optical band gap different from the electronic band gap, which is not expected for direct band gap materials. FETs based on graphane and/or graphone present large *Ion* and *Ion*/*Ioff* ratios [2], reduced band‐to‐band tunnelling, without disadvantages such as lithography and patterning requirements for conventional circuit integration [9]. Graphyne (allotrope of graphene with one atom thick planar sheet of sp and sp2 bonded carbon atoms arranged in a crystal lattice) and graphdiyne (allotrope of graphene containing two acetylenic linkages in each unit cell rather the one linkage as in graphyne), thanks to their crystalline structures, present tunable band gap giving place to semiconducting materials [10]. The band gap is increased with the reduction in the ribbon width and the number of sheets involved. Graphyne ribbons have band gaps in the semiconducting range from 0.59 to 1.25 eV. A tensile strain of 0.15 increases the band gap in 1 eV, while a compressive strain of 0.1 reduces the band gap in 0.3 eV. For strain‐free graphdiyne, its band gap is 0.47 eV, while with strain it varies from 0.28 to 0.71 eV. Graphdiyne with low doping levels of boron nitride (BN) increases its band gap by 1 eV, and the trend is linear with the increasing dopant concentration [15]. When BN replaces all carbon atoms, a new material is formed, which is called BNdiyne with band gap of 4.39 eV (achieving an insulating behav‐ iour) [10]. Fluorographene (FG), which results due to the exposure of graphene to atomic F,

/Vs for suspended graphene) and low losses through the Joule effect. Unfortu‐

to sp3

/Vs for graphene on SiO2 substrate and

, and other authors have reported

to sp3 configuration, should lead

**3. Graphene and its derivatives**

200,000 cm2

Graphene has a very high carrier mobility (15,000 cm2

106 Two-dimensional Materials - Synthesis, Characterization and Potential Applications

band gap of ∼1.0 eV due to the rehybridization from sp2

eV [13–14]. The presence of configuration changes from sp2

by FETs for applications in digital circuits.

Elemental monolayers of the Group III (B), IV (Si, Ge), and V (P, As, Sb) are emerging as promising 2D materials with electronic applications. These materials are denominated as borenene, silicene, silicane, germanene, germanane, phosphorene, arsenene, and antimonene. A brief summary of the applications in the electronics area of the different 2D allotropes of groups III, IV and V, excluding carbon, is presented below.

Silicene is a crystalline 2D allotrope of silicon, with a hexagonal honeycomb structure similar to that of graphene [16]. Silicene has a buckled honeycomb atomic arrangement of sp<sup>3</sup> /sp2 ‐like hybridized Si atoms, which produces a non‐trivial electronic structure and a spin‐orbit coupling of 1.55 meV (under pressure it is increased to 2.9 meV), whose value is much larger than that of graphene (10‐2 meV). In addition, silicene is more easily integrated into current Si‐ based electronics compared to graphene. The use of inert substrates such as *h*BN monolayer and SiC (0001) has silicene‐substrate interaction energies range in 0.067–0.089 eV per Si atom, belonging to typical van der Waals interaction [17]. The characteristic Dirac cone is preserved for silicene deposited on substrates as *h*BN monolayer or hydrogenated Si‐terminated SiC (0001) surface. On the other hand, silicene presents a metallic behaviour, when it is deposited on a hydrogenated C‐terminated SiC (0001) surface. Therefore, silicene presents different electronic properties in accordance with the substrate where it is deposited, which allow to tune a different band gap via the substrate [17]. The presence of Stone‐Wales (SW) defects in silicene can be effectively recovered by thermal annealing. The existence of single and double vacancies (SVs and DVs) in silicene induces small band gaps, and SV defects may transform semimetallic silicene into metallic silicene. Dramatic changes in the electronic properties of the silicene are produced by the presence of Si adatoms, for example, a semimetal behaviour is changed into magnetic semiconductor behaviour [18]. Its thin films provide novel electronic properties suitable for semiconductor device applications at room temperature. It has no band gap, but it can be used in the channel of high‐performance FETs when a vertical electric field is applied [19]. Moreover, it has been predicted that silicene will exhibit a quantum spin Hall effect (QSHE) under an accessible temperature regime [20]. This effect is adequate to applica‐ tions where it is desired that electrical conductance cannot be destroyed by magnetic fields applied to the sample. The physicochemical properties of the silicene will allow the design of novel devices in future electronic industry. Its applications are completely similar to those of graphene. Silicene presents excitonic resonance (π→π\* excitation) at 1.23 eV due to its quasi‐ particle excitation and optical absorption spectrum [21]. It is thermodynamically more stable

than graphane. It can be used for digital electronic applications, photonic and spintronic (magnetic semiconductor) devices, and lithium ion battery electrodes [22].

Germanene is a material made up of a single layer of germanium atoms, which is deposited under high vacuum and high temperature on a substrate such as gold (111) [23]. Its high‐ quality thin films provide novel electronic properties suitable for semiconductor device applications. It has no band gap, but it can be used in the channel of high‐performance FETs when a vertical electric field is applied [19]. Germanene presents excitonic resonance (π→π\* excitation) at 1.10 eV due to its quasi‐particle excitation and optical absorption spectrum [21]. The first bright exciton in germanane is located at 1.45 eV with a binding energy of 0.92 V. It is thermodynamically more stable than graphane. It has no band gap, but attaching a hydrogen atom to each germanium atom creates one band gap that can be exploited in diverse electronic applications [24].

Silicane is the hydride of silicene, that is, it is a crystalline single layer composed of silicene with one hydrogen atom bonded in each atom of Silicon. It is a semiconductor with indirect band gap [25]. Hydrogenation produces an indirect band gap in silicene called silicane, in the range of 2 to 4 eV, due to the rehybridization from sp<sup>2</sup> to sp3 . Its applications may include polariton lasers and optical switches for the observation of excitonic effects at high temperature and in the visible optical range [26]. The first bright exciton in silicane is located at 3.00 eV with a binding energy of 1.07 V. It presents strongly bound excitons with considerable binding energies [21].

Germanane is a crystalline single layer composed of germanium with one hydrogen atom bonded in the *z*‐direction for each atom, that is, with a similar structure to graphane. It is a semiconductor with indirect band gap [25]. Hydrogenation opens a direct band gap in germanene called germanane, in the range of 1.5–3 eV, due to the rehybridization from sp<sup>2</sup> to sp3 . This material is potentially interesting for optoelectronic applications in the blue/violet spectral range such as light‐emitting diodes due to the value of its direct band gap [24]. Its optical band gap is smaller than that of graphane to detect lower photon energies [26]. It presents strongly bound excitons with considerable binding energies [21].

Stanene is a crystalline allotrope of tin arranged in a single layer of atoms as graphene, whose behaviour is of a topological insulator with the capacity of displaying dissipationless currents at its edges near room temperature. It has an inverted band gap which can be tuned by compressive strain [27]. When magnetic doping and electrical gating are applied in stanene deposited on InSb substrate, then properties such as quantum anomalous Hall effect, Chern half metallicity, and topological superconductivity can be implemented. The spin‐orbit interaction opens a 70 meV energy band gap in the *k* point of first Brillouin zone, but by applying strain the energy band gap in the band structure is closed [28]. It has a behaviour of topological insulator and phonon‐mediated superconductor (doping with calcium and/or lithium) at a very low transition temperature (*Tc*∼ 1.3 K) [29]. 2D stanene with hydrogenation (Stanane) has a band gap that can be modulated by electric field and strain, and also, it has room‐temperature ferromagnetic behaviour. These qualities are dependent on the arrange‐ ment of hydrogen atoms in the stanene [30].

Phosphorene is the 2D crystalline allotrope of phosphorus consisting of a single layer of atoms as graphene, which possesses a non‐zero band gap (∼0.1 eV) while displaying high electron mobility. These qualities make it a better electronic material than graphene [31]. Zigzag phosphorene nanoribbons have metallic behaviour regardless of the ribbon width, whereas armchair phosphorene has semiconducting behaviour with indirect band gap which decreases with increasing ribbon width [32]. A compressive (or tensile) strain can reduce (or enlarge) the band gap of phosphorene, while in‐plane electric field can reduce the band gap of phosphor‐ ene. Phosphorene is a *p*‐type semiconducting material with a band gap value of 0.31–0.36 eV, carrier mobility of 286 cm<sup>2</sup> /Vs, and presents photoluminescence in the visible optical range [33]. Besides, phosphorene has a high specific capacity, superb stability, and high electrical conductivity [34]. These capacities allow its use in FETs, batteries, radio receivers, and gas sensors.

than graphane. It can be used for digital electronic applications, photonic and spintronic

Germanene is a material made up of a single layer of germanium atoms, which is deposited under high vacuum and high temperature on a substrate such as gold (111) [23]. Its high‐ quality thin films provide novel electronic properties suitable for semiconductor device applications. It has no band gap, but it can be used in the channel of high‐performance FETs when a vertical electric field is applied [19]. Germanene presents excitonic resonance (π→π\* excitation) at 1.10 eV due to its quasi‐particle excitation and optical absorption spectrum [21]. The first bright exciton in germanane is located at 1.45 eV with a binding energy of 0.92 V. It is thermodynamically more stable than graphane. It has no band gap, but attaching a hydrogen atom to each germanium atom creates one band gap that can be exploited in diverse electronic

Silicane is the hydride of silicene, that is, it is a crystalline single layer composed of silicene with one hydrogen atom bonded in each atom of Silicon. It is a semiconductor with indirect band gap [25]. Hydrogenation produces an indirect band gap in silicene called silicane, in the

polariton lasers and optical switches for the observation of excitonic effects at high temperature and in the visible optical range [26]. The first bright exciton in silicane is located at 3.00 eV with a binding energy of 1.07 V. It presents strongly bound excitons with considerable binding

Germanane is a crystalline single layer composed of germanium with one hydrogen atom bonded in the *z*‐direction for each atom, that is, with a similar structure to graphane. It is a semiconductor with indirect band gap [25]. Hydrogenation opens a direct band gap in germanene called germanane, in the range of 1.5–3 eV, due to the rehybridization from sp<sup>2</sup>

. This material is potentially interesting for optoelectronic applications in the blue/violet spectral range such as light‐emitting diodes due to the value of its direct band gap [24]. Its optical band gap is smaller than that of graphane to detect lower photon energies [26]. It

Stanene is a crystalline allotrope of tin arranged in a single layer of atoms as graphene, whose behaviour is of a topological insulator with the capacity of displaying dissipationless currents at its edges near room temperature. It has an inverted band gap which can be tuned by compressive strain [27]. When magnetic doping and electrical gating are applied in stanene deposited on InSb substrate, then properties such as quantum anomalous Hall effect, Chern half metallicity, and topological superconductivity can be implemented. The spin‐orbit interaction opens a 70 meV energy band gap in the *k* point of first Brillouin zone, but by applying strain the energy band gap in the band structure is closed [28]. It has a behaviour of topological insulator and phonon‐mediated superconductor (doping with calcium and/or lithium) at a very low transition temperature (*Tc*∼ 1.3 K) [29]. 2D stanene with hydrogenation (Stanane) has a band gap that can be modulated by electric field and strain, and also, it has room‐temperature ferromagnetic behaviour. These qualities are dependent on the arrange‐

presents strongly bound excitons with considerable binding energies [21].

to sp3

. Its applications may include

to

(magnetic semiconductor) devices, and lithium ion battery electrodes [22].

108 Two-dimensional Materials - Synthesis, Characterization and Potential Applications

range of 2 to 4 eV, due to the rehybridization from sp<sup>2</sup>

ment of hydrogen atoms in the stanene [30].

applications [24].

energies [21].

sp3

Arsenene is a 2D crystalline allotrope of arsenic consisting of a single layer of atoms as the graphene, which can absorb light atoms to develop an effective method to functionalize it with B, C, N, and/or F to induce magnetism or N and/or F to induce *n*‐type doping [35]. Its electronic band structure is dependent on edge shapes; armchair nanoribbons have large indirect band gap (due to stronger quantum confinement), while zigzag nanoribbons have small direct band gap [36]. In addition, arsenene has small carrier mobilities in the orders of magnitude of 0.5– 1.2×103 cm2 /Vs for potential applications in nanoelectronics and nanodevices. A tensile strain (6%) applied to the monolayers provides an indirect‐direct band gap transition (whose value is reduced with the strain), which allows these materials to be applied in light‐emitting diodes and solar cells [37]. Under compressive strains, the band gap of monolayer and bilayer arsenenes initially increases and then rapidly decreases [38].

Antimonene is a 2D crystalline of antimony consisting of a single layer of atoms as the graphene, which has two stable and semiconducting allotropes (*α* and *β*), and with indirect band gap. *α*‐Sb has a puckered structure with two atomic sub‐layers, and *β*‐Sb has a buckled hexagonal lattice. A moderate tensile strain applied to the monolayers provides an indirect‐ direct band gap transition, which allows these materials to be applied in optoelectronics [39]. In addition, antimonene can be tuned as a topological insulator to achieve QSHE at high temperatures using large tensile strain up to 18% and a band gap of 270 meV and whose characteristics meet the requirements of future electronic devices with low power consumption [40]. It has the same behaviour as that of arsenene with respect to the electronic band structure.

Borophene is a crystalline allotrope of boron consisting of icosahedral B<sup>12</sup> units fused as supericosahedra arranged in a 2D sheet with a hexagonal hole in the middle on silver surfaces under ultrahigh vacuum conditions [41]. Boron is non‐metallic by nature as a bulk material, but it shows metallic and semiconducting behaviour at the nanoscale and has atomic thickness. Thanks to the hexagonal holes, various chemical modifications are possible to tune the electronic, optical, and chemical properties of borophenes [42]. It has a higher tensile strength than any other known material. The stability of these sheets is enhanced by vacancies or out‐ of‐plane distortions. Borophene has a highly anisotropic metallic behaviour, where electrical conductivity is confined along the chains [41]. It is maintained as a promising material for applications ranging from electronics (sensors and electronic devices), photovoltaics (electro‐ des), and energy storage (batteries); however, a lot of work is still required to determine its practical applications.
