**5. Compounds:** *h***BN, TMDs, and MXenes**

*h*BN is an interesting 2D material with a band gap of around 6 eV, which makes it an excellent dielectric, and it can be incorporated into different heterostructures for the electrostatic gating (protective cover, substrate, gate dielectric, or tunnel barrier) of other 2D materials due to that it does not require a lattice matching to generate van der Waals structures (electronic bands near the Fermi level (*Ef* ) have graphene‐like linear dispersion) [3]. It forms a 2D‐crystalline structure composed of alternating atoms of boron and nitrogen, with lattice spacing similar to that of graphene [43]. It is commonly used in FETs as ideal substrate and gate dielectric. The doping level can also be varied by applying an external electric field, and it decreases with increasing *h*BN layer thickness and approaches zero for thick layers. It can be partially oxidized (PO‐*h*BN) to decrease its optical transmission (>60%) and band gap (from 3.97 to 5.46 eV) [44].

Transition metal chalcogenides (TMCs) are thin‐layered semiconducting structures of the type *MX*2, where *M* is a transition metal atom (Mo, W, etc.) and *X* is a chalcogen atom (S, Se, or Te) [45]. One layer of *M* atoms is sandwiched between two layers of *X* atoms. TMCs are commonly restricted to chemical elements of the groups IV (Ti, Zr, Hf and so on), group V (for example V, Nb or Ta), or group VI (Mo, W and so on), and *X* is a chalcogen (S, Se or Te) [46]. These materials have layered structures of the form chalcogen‐metal‐chalcogen (*X*‐*M*‐*X*), with the chalcogen atoms in two hexagonal planes separated by a plane of metal atoms. The overall symmetry of TMDs is hexagonal or rhombohedral, and the metal atoms have octahedral or trigonal prismatic coordination [46]. MoS2 is one of the most typical TMDs; it has a direct band gap of 1.8 eV in monolayer, which can be tuned in different ways [47]. A MoS2 monolayer has a thickness of 6.5 Å, and it has been used in chemical and gas sensors [48]. MoS2‐based sensors have been implemented to detect NH3 down of 400 ppb [49]. Besides, MoS2 has applications in fields such as flexible electronics, energy storage and harvesting as well as electrochemical catalysis [47].


**Table 1.** 2D materials with metallic behaviour, that is, band gap = 0.0 eV [50].

In order to compare the electrical properties of different 2D materials, three tables were created for ordering these materials in three groups, as follows: (a) 2D materials with metallic behav‐ iour (**Table 1**), (b) 2D materials with semiconducting behaviour (**Table 2**), and (c) 2D materials with electrical insulator behaviour (**Table 3**).


**Table 2.** 2D materials with semiconducting behaviour (0 < *Eg* < 3 eV) [50].

des), and energy storage (batteries); however, a lot of work is still required to determine its

*h*BN is an interesting 2D material with a band gap of around 6 eV, which makes it an excellent dielectric, and it can be incorporated into different heterostructures for the electrostatic gating (protective cover, substrate, gate dielectric, or tunnel barrier) of other 2D materials due to that it does not require a lattice matching to generate van der Waals structures (electronic bands

structure composed of alternating atoms of boron and nitrogen, with lattice spacing similar to that of graphene [43]. It is commonly used in FETs as ideal substrate and gate dielectric. The doping level can also be varied by applying an external electric field, and it decreases with increasing *h*BN layer thickness and approaches zero for thick layers. It can be partially oxidized (PO‐*h*BN) to decrease its optical transmission (>60%) and band gap (from 3.97 to 5.46 eV) [44].

Transition metal chalcogenides (TMCs) are thin‐layered semiconducting structures of the type *MX*2, where *M* is a transition metal atom (Mo, W, etc.) and *X* is a chalcogen atom (S, Se, or Te) [45]. One layer of *M* atoms is sandwiched between two layers of *X* atoms. TMCs are commonly restricted to chemical elements of the groups IV (Ti, Zr, Hf and so on), group V (for example V, Nb or Ta), or group VI (Mo, W and so on), and *X* is a chalcogen (S, Se or Te) [46]. These materials have layered structures of the form chalcogen‐metal‐chalcogen (*X*‐*M*‐*X*), with the chalcogen atoms in two hexagonal planes separated by a plane of metal atoms. The overall symmetry of TMDs is hexagonal or rhombohedral, and the metal atoms have octahedral or trigonal prismatic coordination [46]. MoS2 is one of the most typical TMDs; it has a direct band gap of 1.8 eV in monolayer, which can be tuned in different ways [47]. A MoS2 monolayer has a thickness of 6.5 Å, and it has been used in chemical and gas sensors [48]. MoS2‐based sensors have been implemented to detect NH3 down of 400 ppb [49]. Besides, MoS2 has applications in fields such as flexible electronics, energy storage and harvesting as well as electrochemical

) have graphene‐like linear dispersion) [3]. It forms a 2D‐crystalline

practical applications.

near the Fermi level (*Ef*

catalysis [47].

**Chemical formula**

TaTe2

**Table 1.** 2D materials with metallic behaviour, that is, band gap = 0.0 eV [50].

**Chemical formula**

C (graphene) NbS2 WTe2 VBr2 VS2 CrS2 FeSe NbSe2 CoTe2 VI2 VSe2 CrSe2 LiFeAs NbTe2 RhTe2 PFeLi VTe2 CrTe2 YSe2 TaS2‐*AB* IrTe2 IYGa Ni2Te2Sb ZnIn2S4 TiSe2 TaS2‐*AA* NiTe2 PTe2Ti2 Cu2S Zn2In2S5 TiTe2 TaSe2‐*AB* SiTe2 FeS VCl2 SbSiNi ZrTe2 TaSe2‐*AA* AlCl2 FeTe HfTe2 Ag2ReCl6

**Chemical formula**

**Chemical formula**

**Chemical formula**

**Chemical formula**

**5. Compounds:** *h***BN, TMDs, and MXenes**

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


**Table 3.** 2D materials with insulating dielectric behaviour (*Eg* > 3.0 eV) [50].

TMCs can be formed with multiple layers bound to each other by van der Waals forces. 2D TMDs are exciting materials for future applications in nanoelectronics, nanophotonics, and sensing [49]. TMDs monolayers such as MoS2, MoSe2, MoTe2, WS2, and WSe2 have a direct band gap and can be used in electronics as FETs and in optoelectronics as emitters (light‐emitting diodes) and optical detectors (photodetectors) [3]. Molybdenum and tungsten dichalcogenides are a family of compounds that is structurally and chemically well defined. WS2 is a layered material consisting of stacked S‐W‐S slabs with a binding energy in *c*‐direction of 0.14 eV. WS2 has optical band gap energy of 1.46 eV; its electrical conductivity is in the order of 10-3 S/ cm and has *n*‐type conductivity [51]. Also, this material can be used in heterogeneous catalysis and electrochemical hydrogen storage. MoS2, MoSe2, WS2, and WSe2 are promising semicon‐ ducting materials for solar energy conversion, since they work as absorber materials due to the weak connection by intermolecular van der Waals forces among monolayers.

The TMD monolayer crystalline structure has no inversion centre, which allows a new degree of freedom of charge carriers called the *k*‐valley index giving place to the new field of physics called valleytronics (technology that controls the valley degree of freedom of certain semicon‐ ductors, that is, a local maximum/minimum on the valence/conduction band, that present multiple valleys inside the first Brillouin zone) [48]. WSe2‐MoSe2 heterostructures can be optically polarized to produce valley‐specific interlayer excitons with lifetimes of 40 ns, which can be exploited in optoelectronic applications [52]. In addition, spin‐orbit coupling among TMDC monolayers allows control of the electron spin through of the tuning the applied photonic energy, due to that a spin splitting in the meV range is presented in conduction and valence bands. TMDs can be combined with other 2D materials such as graphene and *h*BN to generate a new class of van der Waals heterostructure devices. TMDs can behave as electrical conductor, semimetal, semiconductor, and dielectric material, [50, 53–54], as well as, super‐ conductor [55–56]. In the case of semiconductors, they have direct and indirect band gaps with values ranging from ultraviolet range to infrared range through the visible range. TMD monolayers are structurally stable; these have band gaps and show electron mobilities with comparable values to those of silicon. Therefore, they can be used to fabricate FETs [46].

Stacking of 2D materials on top of each other in a controlled fashion can create heterostructures with tailored physical properties that offers another promising approach to design and fabricate novel electronic devices [5]. In addition, either the in‐plane heterojunction or the vertical stacking heterostructures can be realized by delicately tuning the composition and stacking sequences among 2D materials [5]. The development of FETs based on MoS2 increased the scientific interest in the research of TMCs monolayers for novel ultrathin and flexible devices applied in electronics and optoelectronics [54]. Tunnel field‐effect transistors (TFETs) based on vertical stacking of 2D materials can be used to build *p*‐*n* junctions of TMC mono‐ layers to design low‐power logic devices [57]—VIB‐MeX2 (Me = W, Mo; X= Te, Se) monolayers as the *n*‐type source and IVB‐MeX2 (Me = Zr, Hf; X= S, Se) as the *p*‐type drain [57]. The two groups of semiconductors have distinct band edge characters, which will generate intervalley scattering during the electron tunnelling process. Strain is highlighted as an effective way to modify the band edge properties of these 2D TMDs [57]. Graphene (contact electrodes and circuit interconnections)/MoS2 (transistor channel) heterostructures offer a technological alternative to design FETs with metallic drains and sources for practical flexible transparent electronics [58]. Graphene/MoS2 heterostructure opens a 1 meV band gap in MoS2‐supported graphene, and the band gap is tunable under different interlayer distances [59]. Moreover, this heterostructure displays an enhanced light response, which allows even photocatalytic applications. 2D heterostructures play a pivotal role in electrochemical energy storage, sensing, hydrogen generation by photochemical water splitting, and electronic device applications such as FETs [60]. Particularly, in the field of energy conversion and storage, these hybrids will be useful as anodes in lithium ion batteries and supercapacitors [60]. MoS2 and WSe2 have potential applications in electronics because they present high *Ion*/*Ioff* current ratios and unique electro‐optical properties [61]. Lateral WSe2‐MoS2 heterojunction is a key component for building monolayer *p*‐*n* rectifying diodes, light‐emitting diodes, photovoltaic devices, and bipolar junction transistors [61].

WS2 has optical band gap energy of 1.46 eV; its electrical conductivity is in the order of 10-3 S/ cm and has *n*‐type conductivity [51]. Also, this material can be used in heterogeneous catalysis and electrochemical hydrogen storage. MoS2, MoSe2, WS2, and WSe2 are promising semicon‐ ducting materials for solar energy conversion, since they work as absorber materials due to

The TMD monolayer crystalline structure has no inversion centre, which allows a new degree of freedom of charge carriers called the *k*‐valley index giving place to the new field of physics called valleytronics (technology that controls the valley degree of freedom of certain semicon‐ ductors, that is, a local maximum/minimum on the valence/conduction band, that present multiple valleys inside the first Brillouin zone) [48]. WSe2‐MoSe2 heterostructures can be optically polarized to produce valley‐specific interlayer excitons with lifetimes of 40 ns, which can be exploited in optoelectronic applications [52]. In addition, spin‐orbit coupling among TMDC monolayers allows control of the electron spin through of the tuning the applied photonic energy, due to that a spin splitting in the meV range is presented in conduction and valence bands. TMDs can be combined with other 2D materials such as graphene and *h*BN to generate a new class of van der Waals heterostructure devices. TMDs can behave as electrical conductor, semimetal, semiconductor, and dielectric material, [50, 53–54], as well as, super‐ conductor [55–56]. In the case of semiconductors, they have direct and indirect band gaps with values ranging from ultraviolet range to infrared range through the visible range. TMD monolayers are structurally stable; these have band gaps and show electron mobilities with comparable values to those of silicon. Therefore, they can be used to fabricate FETs [46].

Stacking of 2D materials on top of each other in a controlled fashion can create heterostructures with tailored physical properties that offers another promising approach to design and fabricate novel electronic devices [5]. In addition, either the in‐plane heterojunction or the vertical stacking heterostructures can be realized by delicately tuning the composition and stacking sequences among 2D materials [5]. The development of FETs based on MoS2 increased the scientific interest in the research of TMCs monolayers for novel ultrathin and flexible devices applied in electronics and optoelectronics [54]. Tunnel field‐effect transistors (TFETs) based on vertical stacking of 2D materials can be used to build *p*‐*n* junctions of TMC mono‐ layers to design low‐power logic devices [57]—VIB‐MeX2 (Me = W, Mo; X= Te, Se) monolayers as the *n*‐type source and IVB‐MeX2 (Me = Zr, Hf; X= S, Se) as the *p*‐type drain [57]. The two groups of semiconductors have distinct band edge characters, which will generate intervalley scattering during the electron tunnelling process. Strain is highlighted as an effective way to modify the band edge properties of these 2D TMDs [57]. Graphene (contact electrodes and circuit interconnections)/MoS2 (transistor channel) heterostructures offer a technological alternative to design FETs with metallic drains and sources for practical flexible transparent electronics [58]. Graphene/MoS2 heterostructure opens a 1 meV band gap in MoS2‐supported graphene, and the band gap is tunable under different interlayer distances [59]. Moreover, this heterostructure displays an enhanced light response, which allows even photocatalytic applications. 2D heterostructures play a pivotal role in electrochemical energy storage, sensing, hydrogen generation by photochemical water splitting, and electronic device applications such as FETs [60]. Particularly, in the field of energy conversion and storage, these hybrids will be

the weak connection by intermolecular van der Waals forces among monolayers.

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

2D TMDs under out‐of‐plane pressure decrease their band gap with increasing pressure and it can be closed too, which implies a semiconductor‐metal transition [62]. The critical pressure for the semiconductor‐metal transition is larger for the thinner nanoribbons, and the band gap closes faster for the Mo‐containing nanoribbons than the W‐containing ones. In addition, the physical mechanism of the band gap variation relates to the charge accumulation and deloc‐ alization in the interlayer region [62].

Layered TMDs are susceptible to electronic instability because its charge density wave (CDW) phases are diverse and commensurate, which leads to a strong electron‐phonon and electron‐ electron interactions due to the Mott insulating phase presented [55]. Therefore, it is necessary find an adequate ordering of atoms to produce star‐of‐David clusters that allow boost superconductivity in a layered chalcogenide such as 1*T*‐TaS2*‐x*Se*x* to achieve different electronic degrees of freedom leading to tunable band gaps. Because the Ta 5*d* orbital allows supercon‐ ductivity and Se 4*p* orbital leads to metallic phase, an unusual Se/S ordering creates super‐ conductivity stable states [55]. Superconductivity can be induced through electrostatic gating using ionic liquid in materials such as MoSe2, while electrochemical gating can be induced using KClO4/polyethylene glycol (PEG) using a crossover from surface doping to bulk doping in materials such as MoTe2 and WS2 [56]. Moreover, the discovery of superconductors based on Mo and W allow to affirm that superconductivity is a common property for semiconducting TMDs. In bulk materials, the Zeeman effect is detrimental to superconductivity, however, in nanomaterials such as MoS2 monolayers, it allows the development of ionic‐gated transistors capable of realizing magnetotransport at coupling the spin‐orbit with the spins of Cooper pairs in a direction orthogonal to the magnetic field [63]. This discovery leads to the Ising super‐ conductor with a critical magnetic field *BC*2 far beyond the Pauli paramagnetic limit, consistent with Zeeman‐protected superconductivity [63].

In 2D TMDs with 1T‐*MX*2 structure, where *M* = (W or Mo) and *X* = (Te, Se, or S), it is possible that a structural distortion causes an intrinsic band inversion between chalcogenide‐*p* and metal‐*d* bands [64]. Moreover, spin‐orbit coupling opens a band gap that is tunable by vertical electrical fields and/or strains. It is feasible to develop a topological FET made of van der Waals heterostructures of 1T‐*MX*2 and 2D dielectric layers that can be switched off by electric field through a topological phase transition instead of carrier depletion [64].

MXenes can be defined as layered transition metal carbides and carbonitrides with general formula of *Mn*+1*AXn* (e.g., Ti2AlC, Ti3AlC2, Ta4AlC3), where *M* stands for early transition metal, *A* is mainly a group IIA or IVA (i.e., groups 13 or 14) element, *X* stands for carbon and/or nitrogen with *n* = 1, 2 or 3 [65]. MXenes have high electric conductivity (1500 S/cm, due to the metallic conductivity of transition metal carbides) combined with hydrophilic surfaces (due to their hydroxyl or oxygen terminated surfaces) [66], that is, they behave as conductive clays. These materials show to be promising in energy storage applications such as Li‐ion batteries and supercapacitors [65–66].
