Electronic properties of 2D materials and their junctions

Abstract

With an extensive range of distinctive features at nano meter-scale thicknesses, two-dimensional (2D) materials drawn the attention of the scientific community. Despite tremendous advancements in exploratory research on 2D materials, knowledge of 2D electrical transport and carrier dynamics still in its infancy. Thus, here we highlighted the electrical characteristics of 2D materials with electronic band structure, electronic transport, dielectric constant, carriers mobility. The atomic thinness of 2D materials makes substantially scaled field-effect transistors (FETs) with reduced short-channel effects conceivable, even though strong carrier mobility required for high performance, low-voltage device operations. We also discussed here about factors affecting 2D materials which easily enhanced the activity of those materials for various applications. Presently, Those 2D materials used in state-of-the-art electrical and optoelectronic devices because of the extensive nature of their electronic band structure. 2D materials offer unprecedented freedom for the design of novel p-n junction device topologies in contrast to conventional bulk semiconductors. We also, describe the numerous 2D p-n junctions, such as homo junction and hetero junction including mixed dimensional junctions. Finally, we talked about the problems and potential for the future.

Headnote

Keywords: 2D materials Electrical properties p-n junctions Mixed hereto junctions Homo junctions Electrical transport

ABSTRACT

With an extensive range of distinctive features at nano meter-scale thicknesses, two-dimensional (2D) materials drawn the attention of the scientific community. Despite tremendous advancements in exploratory research on 2D materials, knowledge of 2D electrical transport and carrier dynamics still in its infancy. Thus, here we highlighted the electrical characteristics of 2D materials with electronic band structure, electronic transport, dielectric constant, carriers mobility. The atomic thinness of 2D materials makes substantially scaled field-effect transistors (FETs) with reduced short-channel effects conceivable, even though strong carrier mobility required for high performance, low-voltage device operations. We also discussed here about factors affecting 2D materials which easily enhanced the activity of those materials for various applications. Presently, Those 2D materials used in state-of-the-art electrical and optoelectronic devices because of the extensive nature of their electronic band structure. 2D materials offer unprecedented freedom for the design of novel p-n junction device topologies in contrast to conventional bulk semiconductors. We also, describe the numerous 2D p-n junctions, such as homo junction and hetero junction including mixed dimensional junctions. Finally, we talked about the problems and potential for the future.

(ProQuest: ... denotes formulae omitted.)

1. Introduction

From last few decades, after isolation of graphene, various twodimensional (2D) nano material get attention to synthesized and developed through many techniques [1-8]. Recently, researchers get attention to modify 2D materials and optimize their electronic properties for design of emerging electronic devices like solar cells, supercapacitors, field effect transistors (FETs), and gas sensors [9-19]. Among the electronic properties, it could be controlled through energy bands, and achieved as follows: (1) alloying between two-dimensional materials and form 2D ternary materials; (2) vertical stacking of two-dimensional materials which generate 2D heterostructures [7,8,20], called as van der Waals solids [21]; (3) control the thickness of 2D materials via number of layers. For fabrication of high-tech electronic devices, 2D materials like graphene, transition metal dichalcogenides, black phosphorus, hexagonal boron nitride (h-BN), silicene, germanene, stanene, arsenene, aluminene, antimonene, bismuthine, molybdenum disulfide (M0S2), molybdenum diselenide (MoSe2), MXenes, etc used [1-3,5,9] (see Tables 3 and 4).

2D materials exhibited as single layer and few layers of material. It could be classified either as allotropes of unique chemical elements (graphene, silicene, germanene, etc.) and various chemical compound elements (h-BN, dichalcogenides, MXenes, etc.); latter, it consist two or more covalent bonding elements [22]. The layers of 2D materials were stacked together via van der waal interactions, which could be exfoliated into thin 2D layers. That binding of layers called van der Waals heterostructures [23]. Generally, it used in photovoltaic devices, semiconductor devices, electrodes for supercapacitors, and in water purification [5,24,25]. Mainly, 2D materials used in electronics as well as fabrication of electronic devices [26] (Fig. 1) (Table 1) (see Table 2).

Among all 2D materials, graphene used extensively because of high mobility, widely tunable carrier concentration, and easily isolation [27-29]. Whereas, generally graphene synthesized in large scale via CVD (chemical vapour deposition) method and used for fabrication of electronic, photonic devices, semiconductor, photonics and energy storage [30,31]. 2D materials including graphene had great intensity in electronic and optoelectronic applications [32-36] because of their bandgap ranges with high carrier mobility and efficient electrostatic control. These properties, combined with mechanical flexibility [37-39] and tunability of electronic properties and make it promising material in high-performance 2D field effect transistors (FETs). It was operated in emerging future mobile and loT environment [40-42]. Thus, in accurate characterization of 2D FETs and extraction of highly important device parameters, like resistivity, carrier density, mobility, contact resistance, charge trap densities, dielectric permittivity, and anisotropy in carrier transport, were essential for 2D materials, which correlate with 2D FETs [43-46].

Other 2D semiconductor material like transition metal dichalcogenides (TMDCs) compounds, MX2, where M was transition metal (e.g., Mo, W, Re, and Ta) and X was chalcogen (e.g., S, Se, and Te) get attention for electronic devices because of high charge carrier mobilities, high conductivity and low consumption with remarkable bandgaps which enable to switch field-effect transistors (FETs) [32,43,44,47-51]. At room-temperature, electrical conductivity of semiconductor, directly related to charge carrier density. Because of subsequent atomic thin layer, doping did not possible on 2D materials. Charge transfer doping, used to generate electron and hole carriers in 2D materials [52-54]. In few-layer materials, charge density decreased rapidly in semiconductor materials. 2D materials like В (borophene), Si (silicene), P (phosphorene), As (arsenene), Sb (antimonene), Bi (bismuthene), and Sn (stanene) [55,56] were monolayers (e.g. borophene) which grown on metal surfaces in ultra-high vacuum (UHV) and high chemical reactivity in ambient conditions, also had various device applications. However, black phosphorus (BP) [33,57] were most promising electronic materials [58]. BP was generally used because of its high field-effect mobility (-1000 cm2/Vs at room temperature), direct and tunable bandgap at all thicknesses, and large in-plane anisotropy because of specific atomic structure (Fig. 2(h)) [33,57,59]. Further, BP complements get p-type conduction with n-type TMDCs semiconductors in digital logic and used as p-n heterojunction applications. TMDCs and BP used for electronic applications and get attention for charge transfer.

Here, we discussed electronic properties of 2D materials based on semiconducting materials.

2. Electronic properties of 2D materials

2.1. Electronic structure: tunable band structure with thickness

The Electronic properties of 2D materials depends upon electronic band structure [61-63]. Due to vertical quantum confinement, the electronic structure of 2D materials changed identically with the number of layers [64-66]. Size-dependent bandgaps and plasmon frequencies had unique and special features of most low-dimensional materials like quantum dots (QDs) and carbon nanotubes (CNTs) [67]. Whereas Group VI TMDCs for example M0S2, MoSc2, WS2, and WSc2 showed an evolution in bulk (indirect bandgap around -1.3, 1.1, 1.4, 1.2 eV) to monolayer (direct bandgap around -1.9, 1.6, 2.1, 1.7 eV), respectively (6,27, 75). As well as their thickness increased from monolayer to bilayer, direct bandgap at К point of Brillouin zone relatively unchanged because of dominance of d-orbital of transition metals which isolated from interlayer coupling [66,68]. Interestingly, the Г point changed because of increased chalcogen pz orbitals (Fig. 3). In M0S2, due to small indirect band gap between the F and Q in bilayers decreased photoluminescence (PL) quantum yields in M0S2 [64].

Other hand, carbon atoms of 2D Graphene arranged in hexagonal honeycomb lattice. The electronic structure of graphene easily derived from tight binding model, resultant it consist, peculiar Dirac cones at corners of Brillouin zone [69]. Their electron dispersion explained via linear equation

...

Where,

E = electron energy.

P = momentum.vf = Fermi velocity (106 m/s), including plus and minus signs refer to conduction and valence bands, respectively.

Those bands with conical dispersion intersect at Fermi energy, which make it a semi-metal. The conical dispersion of low-energy carriers in graphene was different from parabolic dispersion in bulk semiconductors and could be as effective 2D Hamiltonian for massless Dirac fermions [69]. A semimetal bi-layered graphene with parabolic dispersion at Fermi energy had small band gap around 250 meV which generate an electric field [70,71]. Whereas, after valence and conduction band interaction, intrinsic graphene did not consist free carriers at the Fermi level. Thus, electrostatic chemical doped generate electron or hole carriers [72,73].

Generally, ultrathin 2D materials implies external applied electric and magnetic fields, which enable electronic structure for field-effect devices [68,74]. In bilayer M0S2, because of tunning band gap, PL measurements under vertical gate fields showed quantum-confined Stark effect [74,75]. In situ scanning spectroscopy of gated devices also showed the Fermi level in monolayer M0S2 which could be moved up to 0.6 eV [76]. However, different valley band structures varied differently with field [68]. For example, according to Wannier functions, few-layered TMDCs predict larger field dependence of K-valleys than Г-valleys or Q-valleys, which consistent with gated PL measurements (Fig. 4) [77]. Whereas, external electric field also observed to induce charge density wave phase transitions in metallic NbSe2 and TaS2 [78,79]. In TMDCs, electronic structure also tuned via switching between metastable phases in TMDCs. Generally, their semiconducting 1H/2H phase easily converted to metallic IT phase through Li ion intercalation. And 2H phase recovered via thermal annealing [80].

For electronic and optoelectronic applications, effect of temperature, doping, and defects on band structure of 2D-TMDs were highly important. For bilayer and thick TMDs, had similar values of indirect and direct gaps (Fig. 5) [81].

The variations in temperature could change the optical and electronic properties of TMDs [82]. For example, in few layer of MoSe2, indirect and direct gaps degenerated, and rise in temperature effectively drive system towards 2D regime via thermally decoupled layers because of interlayer thermal expansion [82]. Increase the interlayer spacing reduces the coupling between layers which enhanced indirect gap, while direct gap at К didn't affect. Tongay et al. showed increase the light emission in few-layer MoSe2 of similar magnitude which move from bilayer to monolayer M0S2 [82,83]. The interlayer lattice parameters calculated through DFT experiments. Thus, in summary, 2D materials which had tunable semiconducting band structures with thickness, applied in various electronic devices [32,40,48,50,84,85].

2.2. Electronic transport

Recently, 2D semiconducting materials primarily driven by steadystate dynamics relevant to electronics rather than just light - induced excited state dynamics, we outlined succinctly here. According to Moore's law, scaling down silicon transistors to increase activity which propelled advancements in digital electronics [86,87]. For nanomaterials and their architectures used in next-generation transistors and electronic circuits which driven by quantum effects intrinsically limit the ability of scaling transistor to 1 nm length scale with conventional methods. The ideal nano materials would be able to combine high performance with low cost, simple processing and flexibility.

Carbon nano material like graphene had high carrier mobility (>200,000 cm2/Vs) [88,89] and get attention in use in electronic devices but due to lack of band gap, pure form of graphene limit their potential use in optoelectronics and transistor effect [90]. For enhancing the bandgap value of graphene with chemical doping, nano ribbon structured and applying electric field which crucially deteriorate carrier mobility because of impurity scattering. Sub nano meter thickness, a band gap of 12 ev in the visible frequency range and relatively high carrier mobility (for example in monolayer M0S2 around 200 cm2/Vs) were all conveniently combined in semiconducting 2D material like TMDs [91]. Those materials were highly promising for high on/off ratios in logic circuits with low power dissipation, to maintain high carrier mobilities at room temperature [92]. Radisavljevic et al. described the use of 2D TMDs for nano electronic devices, also explained the first field effect transistor with monolayer M0S2 as conductive channel and НЮ2 as the gate insulator with on/off current ratios of -108 and mobilities around 200 cm2/Vs comparable to silicon technology [91]. He also explained the signal amplification and logic operations in simple integrated circuits used monolayer MoS2 deposited on doped silicon [93].

Using high gate dielectric were crucial first step to achieving high carrier mobility. This outcome had justified through decreased Coulomb scattering and potential alteration of phonon dispersion brought by dielectric material [91]. According to additional experimental data, Coulomb scattering from charged impurities within the monolayer was dominating scattering mechanism at low temperatures up to 200 K, while electron-phonon scattering takes over at temperatures over 200 К [92]. Due to scattering with optical phonons, recent computational work suggested an upper limit for mobility of defect free n-type monolayer M0S2 of 410 cm2/Vs at ambient temperature [94]. Both n-type and p-type conduction were transistors based on intrinsic monolayer M0S2. Their ambipolar property described the effect of trapping of defects and impurities at interface with gate dielectric [95]. Resultant, monolayer materials were thinner than Debye screening length which affected by impurities in proximity of materials. Due to doping of 2D semiconductors through chemisorption of dopants on substrate could be viable strategy for manipulating electronic properties of 2D materials.

2.3. Dielectric constant

The capacitance, charge screening, and energy storage capabilities of electronic devices may all be calculated using the dielectric constant (e) of a material, a fundamental electrostatic property. It was also crucial for describing the energetic interactions between charged particles in the material and provides details on the collective oscillations of the electron gas, plasmons, excitons, and quasiparticle band structures [96,97]. Contrary to traditional isotropic materials like silicon, the distinctive structure of 2D layered materials results in anisotropic physical properties between the in-plane and out-of-plane directions, such as inhomogeneous dielectric strength and Coulomb interaction strength denoted by e. Because the bonds in in-plane and out-of-plane directions have distinct properties, the theoretical dielectric property of 2D materials like graphene and M0S2 is anisotropic [98-100].

By employing the following relation, Chen et al. experimentally determined the of M0S2 from C-V measurements based on vertical MIS capacitor structures:dMoS2

...

Where,

Cmin = minimum capacitance measured at Vgs < O V

dMoS2 = thickness of M0S2

£moS2 = dielectric constant of M0S2

Cg = + ^)-1 = geometric capacitanceWhere,

Cbn = geometric capacitance of hBN.

Cm = interlayer capacitance originating from interlayer spacing between hBN and M0S2 (Fig- 6(a)) [101].Where,

e = dielectric constant of hBN decreases with increase in frequency (Fig. 6(b)) [102].

Long-running discussions about whether the dielectric constant accurately captures the dielectric properties of such low-dimensional systems have been sparked by the restricted character of atomically thin 2D crystals coupled with the anisotropic dielectric screening. The values of e that both theoretical and experimental methods account for differ by more than an order of magnitude [103]. So, it will be necessary for there to be advancements in the future that enable accurate and dependable measurements of e.

2.4. Screening, dispersion, and mobility of carriers

One of the oldest instances of a 2D system, notably the field-effect inversion layer in silicon, has led to a thorough understanding of the general characteristics of free electron screening in 2D materials [67]. Recent research has shown that anomalous screening effects in graphene caused by its linear energy dispersion were crucial for understanding the causes of graphene's remarkable mobility [89,104]. Major sources of carrier scattering in 2D FETs are depicted in Fig. 7(a): Surface optical (SO) phonons (also known as remote interfacial phonons), surface roughness, Coulomb scattering from charged impurities (CI), electron-phonon interactions, and structural flaws are listed in that order [67]. Firstly, Fivaz and Mooserr reported in 1960, Hall mobility with temperature dependent

p = p0(T/300K)-Y

Where, у was electron-phonon scattering at room temperature and for M0S2, MoSe2, WS2 and GaSe it were 2.6, 2.5, 2.4 and 2.1, respectively [105,106]. In a more recent study, Kassbjerg et al. derived a phonon-limited room temperature mobility of about 400 cm2/Vs by studying electron-phonon scattering in monolayer M0S2 from first principles [107]. Inconsistent current density (hot spots) within few-layer M0S2, competing roles of screening and interlayer resistance were further revealed (Fig. 7(b)) [108].

In more recent times, screening of free carriers and Cis has been fully described under the name "Lindhard screening," allowing for the determination of the mobility's dependence on temperature and carrier density [109]. For the majority of Group VI TMDCs and BP, the scattering matrix components and momentum relaxation rates have been calculated using the dielectric functions and temperature-dependent polarizabilities [110-112]. In specifically, the Boltzmann transport equation's relaxation time approximation is used to determine the mobility from separate scattering mechanisms (p¡) which using Matthiessen's rule [67,109].

...

Two types of electron-phonon interactions seem to predominate for TMDCs: (1) the lattice deformation potential (DP), which involves quasielastic scattering by longitudinal and transverse acoustic phonons, and (2) the Fröhlich interaction, which involves inelastic scattering by inplane optical phonons and out-of-plane homopolar phonons. To get the relaxation rate for various scattering processes, the scattering matrix that results from using Fermi's golden rule is divided by the effective dielectric constant [67,109].

2D materials were extremely sensitive to surface roughness (AL) due to their ultrathin bodies. As a result of the disruption of quantized energy levels in the 2D electron flow, surface roughness is also a prevalent issue in traditional III-V semiconductor heterojunctions, where the mobility degrades as the "sixth-power law" with surface roughness (AL-6) [67]. Thus, this effect get magnified in high-mobility 2D materials like graphene suggests that intrinsic mobility limits and reached on atomically flat surface of hBN. Since that 2D materials had bigger scattering cross-sections than bulk semiconductors, structural disorder such point defects and grain boundaries also showed more harmful impacts. More specifically, charged defects and grain boundaries produce long-range fields for Coulomb scattering, whereas point defects often create short-range scattering. On the other hand, sulphur vacancies function as dopants which enhance mobility in CVD M0S2 by filling up trap states below the mobility edge [113].

By carefully controlling substrate and contact engineering, the fundamental limitations of mobility in 2D materials have been reached [115]. For instance, few-layer M0S2 sandwiched between hBN films and in contact with graphene (Fig. 8(a)) exhibits low-temperature mobility exceeding 104 cm2/Vs (limited by Cis) and Shubnikov-de Haas oscillations, whereas room-temperature mobility (100 cm2/Vs) is constrained by optical phonons [114]. Comparable to the optical phonon-limited p-T-26 for bulk M0S2, the temperature exponent (p-T-Y) ranges between 1.9 and 2.3. (Fig. 8(b)). Whereas, monolayer of M0S2 devices on S1O2 exhibited comparable room temperature mobilities of 120 cm2/Vs (60 cm2/Vs), but drastically reduced with low temperature mobilities around 400 cm2/Vs (120-300 cm2/Vs), with y = 0.67-1.7 in unencapsulated devices and у = 0.3-0.73 in top-gated devices [47,92, 116-118]. By taking into account homopolar phonons (Aig - 52 meV) and Raman-active optical phonons (E12g - 49 meV), first principles mobility models for monolayer M0S2 provide justification for the measured values of (Fig. 8(c)) [109,119].

Although the bandgap of few-layered BP had nearly identical to that of bulk BP (0.3 eV), which results in a moderate switching ratio (<500), few-layer BP demonstrated the highest mobility values among p-type materials (-1000 cm2/Vs at ambient temperature). Thus, it discovered that some low-mobility M0S2 transistors (p < 10 cm2/Vs) exhibited in variable range [92,120]. Comparisons between the bandgap and mobility of the most promising 2D materials and those of rival technologies, such as thin-film transistors (TFTs), silicon-on-insulator (SOI), and ultra-thin body (UTB) devices, are instructive [40,50]. M0S2 transistor development observed by ITRS because of their high mobility, high switching ratio (>108), and diminished short-channel effects [121]. While polycrystalline Si, organic semiconductors, and metal-oxides like InGaZnO now dominate TFT technology, large-area M0S2 devices perform far worse, suggesting that they would be a better replacement (Fig. 9) [113,122-124]. Compared to UTB technology, multilayer BP and InSe flakes exhibit superior mobility; however, for practical viability, the problems of large-area growth and environmental stability must be addressed. The short-channel effects, which are regarded as one of the key benefits of 2D electronic devices, are compromised by multilayer devices, despite the fact that they exhibit higher mobility and are more tolerant of non-uniform large-area development.

2.5. Instruments with high frequencies and short channels

2D materials provide the maximum channel length (Lch) limit for integrated circuits since all dimensions in short-channel devices should scale proportionally [121]. Field-effect mobility seems to be no longer the most important performance indicator at short Lch since transport is ballistic. The density of states

g2d = gvm"/jth2

where.

gv = valley degeneracy,

h = Plank constant, which affected via current grows with effective mass m· (74).

Due to a lower in-plane dielectric constant (-4) than Si (-11.7) and GaAs (-12.9), few-layer M0S2 anticipated to improved electrostatic control from gate (40, 107). Due to greater m· of M0S2, MoSe2, and MoTe2 (m· ranges from 0.56 to 0.66 mo) than Si (m· -0.29 mo) and GaAs (m· -0.15 m0), the majority of Group VI TMDCs also anticipated to lower drain leakage current than Si and GaAs [66,68,77,129]. Although slightly slower than that of Si (-107 cm/s), GaAs (7.2 x IO6 cm/s), and graphene (5.5 x 107 cm/s), the saturation velocity (vs) of M0S2 (2.8 x IO6 cm/s) is still high enough to compete with alternative UTB technologies [129,130].

...

The critical value of Lc, where,

And it indicates the point at which the gate starts to lose all control over channel [131]. Despite the fact that the overall channel length was -500 nm and the device was not formally operating in short-channel domain, it was recently demonstrated that M0S2 FETs could be turned off by an embedded gate formed by single carbon nanotube (diameter, Lg -1 nm) [132]. Meanwhile, high frequency scaling and speed performance have been outperformed by increased mobility BP. For instance, 20 nm long BP FETs demonstrated on/off ratios of -100 and width-normalized currents greater than 0.1 mA/pm [133]. The realisation of radio frequency BP devices with fT -20 GHz at Lg = 300 nm is similar [134]. Higher mobilities and shorter Lg, preferably using the ultraclean interfaces of hBN sandwiches, will certainly allow for further advancements in amplifier speed. With short-channel devices, the current saturation also needs to be addressed for high output impedance in real circuits.

3. Factors affecting on 2D materials

The ability to dope ordinary semiconductors n-type or p-type without significantly affecting inherent qualities and the ability to alloy them to adjust bandgap were key factors of 2D materials.

3.1. Doping

As previously mentioned, the majority of metals cause n-type doping, in M0S2 with inducing Fermi level pinning around the CB. On the other hand, p-type M0S2 transistors made possible by high-purity MOO3 (VB -6.3 eV) contacts [135]. Monolayer materials also allow for chemical doping because of their all-surface characteristics. For instance, PL measurements have revealed p- and n-type doping in M0S2 after functionalization with TCNQ (7,7,8,8-tetracyanoquinodimethane) and NADH, respectively [53]. By injecting additional electrons, encourage trion formation, TCNQ (NADH) boosted (decreased) the PL intensity of neutral exciton in monolayer M0S2. With various metal contacts, p-type BP transformed into ambipolar or n-type conduction, among other 2D materials [57,136]. Other 2D materials, including BP, which doped with N.

3.2. Defects

Wafer-scale development of polycrystalline 2D materials cannot avoid structural disorder like defects and grain boundaries [137]. Quantum yield in optoelectronic devices could be restricted by non-radiative recombination sites for photo excited carriers because of single S vacancies, double S vacancies, and 5|7 defects at grain boundaries in TMDCs due to their significant density of states within the band gap [137,138]. Nevertheless, in ambient settings, S vacancies could sustain excitons through charge transfer from adsorbate like O2 or N2, which boosts PL efficiency. For optimising quantum yield, passivate S vacancies were more efficient due to enhancement mechanism fails in high quantum yield limit [139]. Defects at SİO2-S1N interface, utilised as substitute for floating-gate flash memory, were an example of defect which could be used to store charge in bulk semiconductors for memory-based devices [129]. Defects of TMDCs showed interesting properties which could be exploit various applications. Transmission electron microscopy (ТЕМ) images revealed migration of S vacancies in MoS2 monolayer through formation of complexes out of plane [140]. Moreover, DFT simulations forecast abnormal defect scattering paths in 2D materials [141]. By creating 416 defect, double S vacancy, 517 defect in S-polar grain boundaries could migrate inside the grain (Fig. 6(d)) [60]. This method explained why single S vacancies were found in mechanically exfoliated M0S2 and mostly double S vacancies and 416 defects in CVD-grown M0S2 [137,140]. Devices made of polycrystalline M0S2 exhibit field-driven defect motion that has been used to achieve resistive activity because defect migration is aided by grain boundaries (Fig. 10) [142]. Because thin 2D M0S2, external gates could control this resistive switching and achieve functions which promise for developing brain-like computing and non-Boolean logic architectures.

Ternary PMTC compounds also exhibit extensive bandgap tunability which demonstrated by GaS(i_x)Sex and exhibited PL peak shifts between 2.5 eV and 2.0 eV as Se content was changed from 0 to 1 [143].

3.3. Alloying

Because TMDCs had cohesive family of chemicals with similar structure and also varied characteristics, alloying them is particularly intriguing. For instance, band gap of ternary compound could continually adjusted between the two extremes of their underlying binary compounds. By swapping either transition metal or chalcogen atom, two strategies used [144-146]. Chalcogen mixing in M0X2 (X = S, Se, or Те) follows the Vegard law (i.e., lattice constant varies linearly with mixing fraction). All combinations have a negative free energy of mixing, but the MoS2-MoSe2 alloys are anticipated to be the most stable [144]. It follows that chalcogen mixing in this instance and dis not exhibit a tendency for long-range order or segregation since S-Se nearest neighbours which were preferred from entropy perspective. It discovered that the optical bandgap of CVD-grown MoS(2-x)Sex changes linearly between 1.85 eV and 1.6 eV as the fraction of Se increases from 0 to 2 (Fig. 11) [145].

Similar to this, stable alloys of Mo(i_x)WxS2 have been found, and DFT calculations predict that the bandgap changes parabolically with W composition [144,146]. Band delocalization was nevertheless preserved in TMDCs due to substantial d-character at band extrema, despite stochastic substitution in alloys breaking down crystalline translational symmetry. Surprisingly, mirror symmetry in z-direction could be broken by uneven chalcogen distribution in the top and bottom layers. In the most extreme case, Janus monolayers with S and Se occupying the bottom and top layers, respectively, produced via CVD [147]. Due to high mobility and photoresponsivity of these compounds, alloying between (In, Ga) and (S, Se, Te) was vibrant for research. Whereas, similar monolayer alloys had not developed, TMDC alloys including transition metals from other groups investigated in the bulk [148].

4. Junctions of 2D materials

4.1. p-n junctions developed from 2D materials

Russell Ohl's accidently discover p-n junction about 80 years ago, it grown to be a crucial element in contemporary electronics [149]. That kind of electronic devices could be developed through linking of two different type semiconductors. Where, p-type semiconductor had excess of holes and n-type semiconductor contained excess of electrons. Resultant, an intrinsic electric field generated at the interface between them. It could be used to separate photo generate electron hole pairs or current rectify. In bulk semiconductors, a p-n junction is often made by doping two distinct crystallographic regions with various ions or dopants to produce a three-dimensional (3D) p-n junction. The resulting face to face arrangement constitutes the only possible device architecture considered for bulk semiconducting materials. If one scale down from 3D to 2D material involved, new and exciting properties arise. The design of p-n junctions now offers more options and freedom [25,150]: in the case of a 2D device, p-n junctions can be built using one of two main architectures: a lateral junction, where two 2D materials could be joined at the same plane to form a one-dimensional interface, or a vertical junction, where two 2D materials could be stacked face to face to form a two-dimensional overlap.

In addition, 2D materials exhibit novel features due to their ultra-thin nature as compared to 3D semiconductors [152-155]. As an example, the bandgap for some materials were thickness dependent, which opens up even more options for developing unique p-n junction designs. Eight different types of p-n junctions made of 2D materials, represented schematically in Fig. 12. We distinguish between homojunctions, which made by linking two different 2D materials. Heterojunctions, which formed by uniting three different types of 2D materials. And mixed dimensional junctions, which formed by combining a 2D material with OD, ID, or 3D materials. The eight different types of p-n junctions as follows.

4.1.1. 2D homostructures

1. Thickness: Thickness-based junctions, in which two sections of the same material with varying thicknesses combine to produce the pand n-regions [156].

2. Electrostatic doping: Electrostatically doped junctions, where local electrostatic gates regulate the doping in various parts of the same 2D material [157,158].

3. Chemical doping: Chemical doping, in which molecules, nanoparticles, or quantum dots adhere to the surface of a 2D material to change the doping of a specific region [159,160].

4. Stacked doping: The process of stacking two flakes of a given 2D material with various levels of doping on top of one another to create an out-of-plane junction [161].

4.1.2. 2D heterostructures

5. Vertical heterojunctions: Vertical heterojunctions, in which a junction is produced in an out-of-plane direction by stacking two different 2D materials on top of one another [162,163].

6. Lateral heterojunctions: Where two two-dimensional materials are connected over a one-dimensional interface in the same plane [164-166].

4.1.3. Mixed-dimensional

7. 2D-0D and 2D-1D p-n junctions: Structures where a molecular crystal or a nanotube film would be in contact with a 2D substance [167].

8. 2D-3D p-n junctions: Structures in which a bulk 3D semiconductor contact with a 2D material [168,169].

4. 2. 2D material-based p-n junctions

(i) Homojunction

Homojunction devices were related to p-n junctions, which generated through single 2D material [170]. Fig. 13(a-c) showed three examples of junctions: (i) quantum confinement effect; (ii) electrostatic gating; (iii) chemical doping.

1. Modulation of thickness

Fig. 14 showed thickness modulation of 2D homojunction. In ultrathin nano materials, quantum confinement effects lead to the bandgap energy becoming a thickness-dependent variable. Resultant, a p-n junction could be created in which the p and n regions are composed of the same material but have different thicknesses, for example, single WSe2 flakes based p-n junction (Fig. 14) [171]. Xu and co-workers, fabricate bilayer WSe2 flakes based p-n junction, which partially separated into monolayer by Ar plasma. The exfoliation process itself used to create these homojunctions which were already made up of various thickness regions [172].

2. Electric charge doping

The higher electric field-effect tunability of 2D materials typically goes hand in hand with their lower thickness. An electrostatically defined 2D p-n junction conceptually depicted in Fig. 15. The optical representation of the actual device also showed in Fig. 15. Due to ambipolar nature of WSe2, prefer to use in it [173-177]. Pospischil and co-workers described SİO2/S1 substrate that used to define two metallic gates which spaced apart by 460 nm. A 100 nm thick Sİ3N4 gate dielectric was placed on top of the gates. After that, source and drain electrodes were defined, a flake of mechanically exfoliated WSe2 was deterministically applied, partially covering both of the prepatterned gates electrodes. The two areas of WSe2 flake positioned above localized gates which regulated by doping of two split gates. The fabrication of similar devices using BP [178] and graphene [179,180] was based on this split-gate architecture.

3. Synthetic chemical doping

The low thickness of 2D materials make them highly sensitive to environment and surround their surface with addition of external electric fields to making them more sensitive. By introducing doping effects, for instance, molecules that have physiosorbed or chemisorbed onto the surface can affect transport in the 2D material [53,181-183] and by chemically doping the material, a p-n junction could be created. Graphene [184,185], TMDCs, and black phosphorous [186,187] were all employed in the fabrication of chemically doped p-n junctions using this principle. Choi and co-workers described chemical doped M0S2 p-n junction (Fig. 16) [54]. Li and co-workers described BV and AUCI3 created out-of-plane p-n junction. Where, few layer of M0S2 flake was p-doped with AUCI3 and other one with BV [188].

4. Substance doping

Substance or elemental doping was a different method of doping of 2D materials. For example, bulk TMDCs doped with Nb, Fe and Re used as substitutional p-type or n-type dopant which replace with Mo, W metallic atoms [189]. Suh and co-workers described, 2D layer material via this method [190]. Jin and co-workers described, p-n homojunction via stacking undoped MoSe2 onto doped MoSe2 with Nb atoms. A similar device was created by stacking p-type M0S2 doped with Nb onto n-type M0S2 doped with Fe atoms (Fig. 17) [191,192].

(ii) Heterostructures

Because the wide range of bandgap energies and various doped 2D materials, combining two different 2D materials to create a heterostructure, one of the most promising approaches [197,198]. The possibility of stacking 2D materials in atomically abrupt heterojunctions held together exclusively by van der Waals (vdW) interactions [21,199]. It had one of the most intriguing future possibilities for these materials [200]. In contrast to bulk semiconductor heterojunctions which were covalently bonded, vdW heterojunctions prevent dangling bonds and trapped charges [201,202]. Graphene was first 2D material to be employed in van der Waals heterojunctions as a FET with hBN and a transistor with bulk Si [203,204]. However various graphene-based vdW heterojunctions depend on electrostatic doping to adjust the graphene Fermi level on practically defect-free hBN substrates. With many semiconductor heterojunction applications, like light emitters and solar cells, due to zero bandgap, graphene was incompatible. On the other hand, vdW heterojunctions from 2D semiconductors were employed in a variety of cutting-edge devices, including Esaki diodes, tunnel transistors, gate-tunable solar cells, light-emitting diodes (LEDs), anti-ambipolar rectifiers, and photodetectors [36,177,205-209]. Here, rather than evaluating specific devices, we concentrate on the more general ideas of band alignment, device architecture, and bottleneck problems. Here we divided it into two sections: vertical heterojunction and lateral heterojunction (Fig. 18).

1. Vertical heterojunction:

In 2D materials, vertical heterostructures consist of a common architecture. The ability to stack 2D materials on top of one another without being constrained by the lattice constants made possible by surface devoid of interlayer van der Waals interactions and dangling bonds [201,210-212]. For example Dean and co-workers firstly explained stacked graphene on top of h-BN. And described that atomically thin p-n junctions might be produced by stacking an n-type 2D material onto a p-type 2D material utilising the deterministic transfer approach [213,214]. Another example described in Fig. 19(a), on single layer M0S2 and WSe2 based p-n junction [215]. They also fabricate the heterojunction via individual mechanically exfoliated monolayer of SİO2/S1 substrate and deposited Pd get in contact to inject electrons and holes into n-MoS2 and p-WSe2 layers, respectively. Lee and co-workers described attachment of graphene with M0S2 and WSe2 monolayers and improve the collection of charges, realizing a graphene sandwiched 2D p-n junction [216].

2. Lateral junctions

In this situation, bottom-up fabrication techniques used. A lateral junction device formed by Duan and co-workers which displayed in Fig. 19(b) [217], where WSe2 and WS2 are contacted separately. The 50 nm-thick AI2O3 layer which deposited over the WSe2 to shield the WS2 contact electrodes (Fig. 19(c)). It depicted a different device which formed by Li and co-workers [207]. In it M0S2 and WSe2 generated flakes were made contact with other metals, Ti and Pd, respectively.

(iii) Hybrid and mixed dimensional

In addition, pure 2D architecture, create hybrid p-n junctions, where 2D material comes into contact with material which had high or low dimensional structures, like in bulk semiconductor used Si. Fig. 20 (a) and (b) showed a schematic view of 2D-0D and 2D-3D p-n junction. Surfaces of molecular organic crystals and nanotube films exhibit saturated bonds, which make them free of dangling bonds. These lowdimensional materials could be combined with 2D materials in van der Waals heterostructures because they often interact via van der Waals forces [155,232].

A 1D-2D junction made by depositing a layer of sorted single walled carbon nanotubes (SWCNTs) onto a single-layer M0S2 flake which demonstrated in Fig. 21(a). The combining of a single nano wire and a 2D material demonstrated a distinct type of 1D-2D junction. We found junctions between ZnO nanowire, WSe2, BP, and M0S2 [233-235]. As in 0D-2D junction, bilayer M0S2 flake and a thin Cu-phthalocyanine (CuPC) film that was thermally evaporated through a window opened in a PMMA layer (Fig. 21(b)). Lopez-Sanchez and co-workers described a heterojunction which composed of p-type silicon and n-type monolayer M0S2 and form M0S2-Sİ device (Fig. 22). It was created by depositing a M0S2 monolayer flake onto a highly doped, p-type Si substrate, which was previously designed and coated in S1O2, with underlying Si. As like M0S2 and Si-based devices [35,236,237]. Gehring and co-workers described 2D-3D junction which composed of few layers of nano meter thick black phosphorus flake on top of highly n-doped GaAs substrate [238].

5. Characterization methods

Due to demand for 2D material junction devices to be used in various applications seems currently great, but development in that direction seriously affected by critical scientific advancements required to create one type of semiconductor above another. The development of diverse growth techniques such as molecular beam epitaxy (MBE) [248,249], solid/liquid-phase epitaxy (LPE) [250,251], pulsed laser deposition [252], metal organic chemical vapour deposition (MOCVD) [253,254], hydrothermal [255,256], solvothermal [257,258], ultrasonic calcination [259], in situ deposition [260] and several wet chemical methodologies [261] contribute significantly in the fabrication of novel 2D heterojunction devices. Essentially, the transition metal dichalcogenides TMDs have amazing electrical, physical, and chemical properties, as well as 2D ultra-thin thickness and morphologies, which had enormous potential for a wide range of applications [262-264]. The reduction in layer numbers and thicknesses has a significant impact on the properties of heterojunctions. The improvements in fabrication methods actually provide effective tweaking of various microscopic interface properties. Resonant tunnelling effects, one of the intriguing phenomena that occur from manifestation of discrete energy levels, p-n heterojunction device technology has a number of features, including reverse saturation current, diffusion potential, energy bandgap, forbidden gap, bias voltage, breakdown voltage, and nature of the recombination centres that could be well described by the I-V characteristics [265]. The simplest I-V characterization method could be used to understand the specific type and quality of semiconductor p-n junction [266,267]. Variety of complementary approaches, nevertheless, that offer the most important understanding of flaws or impurity (extrinsic) recombination states in the p-n junction. Some of the more sophisticated characterization techniques, including transmission electron microscopy (ТЕМ), electron beam-induced current (EBIC), scanning electron microscopy (SEM), scanning near field optical microscopy (SNOM), deep-level transient spectroscopy (DLTS), secondary ionmass spectrometry (SIMS), and cathodoluminescence (CL) [268, 269].

Electron beams used in strong, expensive, and time-consuming methods called cathodoluminescence and electroluminescence to investigate the characteristics of optoelectronic materials. The first stimulation of electron-hole pairs by the electron beam helps local characteristics in the area detect luminescence radiations. The distribution of associated defect structure in semiconductor junction showed on the spatial distribution variation luminescence map. It's possible to link the luminescence emission wavelengths with various resistance distributions and types of defects. The non-radioactive p-n junctions, however, render the approach useless. For examining flaws in a p-n junction semiconductor, the electron beam-induced current mapping technique is used in conjunction with scanning electron microscopy (SEM) or scanning tunnelling microscopy (STEM) [270]. The current variations via the p-n junction are measured by scanning electron beams across the sample; the defect states trap the electrons, lowering the current value. Both radiative and non-radiative transitions can be effectively handled by this strategy. Aside from that, NSOM uses optical excitation to scan defect map signals sent over optical fibres in order to express different contrast processes. Similar to other very sensitive techniques, DLTS provides detailed information about electrically active faults concealed within the prohibited gap. The scanner operates at a fixed capacitance value, while the defect charge state retrieval roots the capacitance transient in accordance with the density of the defect population. With the use of atomic-level resolution defect pictures, the p-n junctions were directly examined using the influential microscopic method known as ТЕМ. Because ТЕМ imaging uses a small quantity of material and does not properly validate the faults throughout the full sample, its ability to confirm defects is undeniable [271].

The primary ions used in SIMS which used to sputter the sample layer; in addition, the secondary ion emission from the top surface contains information about elemental, crystallographic defects as well as the diffusion-broadening characteristics of p-n junctions. Capacitancevoltage (C-V) calculations also used to determine breakdown voltage and doping density. Unfortunately, this type of methodology makes it difficult to interpret and analyse data. Above all other metrics, it is challenging to determine the precise and actual gadget properties. The pn n I-V method is a suitable technique that offers significantly more material data for the actual device construction and functioning. All semiconductor crystals, whether in perfect and imperfect systems, exhibit significant recombination. Impurities and imperfections in an imperfect semiconductor cause excessive recombination (defects) [271].

5.1. Methods and characterization of band alignment

Characterizing and analysing the alignment of n- and p-type semiconductor bands used in the creation of heterojunctions equally crucial. Anderson model, computational approaches, or experimental methods like ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) all used to quantify band offsets in heterojunctions [272-274]. Thus, using Anderson's model and experimental methods like XPS and UPS, Chiu et al. predicted the bond alignment of heterojunctions based on transition metal dichalcogenides (TMDs) [274].

Anderson model: The Anderson model, commonly referred to as the electron affinity rule (EAR), is based on the idea that the two materials required to form a heterojunction possess different electron affinities. The differences in bandgap between the two materials can be used to calculate the conduction band effect (CBO) and valence bond effect (VBO) of heterojunctions. For instance, the binding energy of each TMDs film is measured using UPS to determine the VBM in order to learn about the band alignment of 2D materials like M0S2, WSe2, and WS2 (Fig. 23(a)). The UPS data, which begins from the Г point known as the VBM, extrapolated to produce the VBM of the materials with regard to Fermi level (EF) as zero points (F). VBM (r)-EF calculated in the second stage, and the results determined to be 1.44, 1.29, and 1.75 eV, respectively (Fig. 23(b)). In order to learn more about the band locations and alignment, the known work function values (WSe2- = 4.00 eV, M0S2 = 4.20 eV, and WS2 = 4.24 eV), which represent the energy difference between the vacuum level and EF, also used. Expected band alignment for the heterojunction may be determined as follows by knowing the work function and measuring VBM (r)-EF values for TMDs by UPS method [271].

Finding the Fermi level differences in the third stage, used to forecast the band orientations (Fig. 23(c)). The Fermi level differences for MoS2-WSe2 and M0S2-WS2 heterojunctions determined to be 0.02 and 0.04 eV. Hence, it's envisaged that electron charge transfer occurred from WSe2 to M0S2 and from M0S2 to WS2. Also, the p-doping and n-doping confirmed the predicted band alignment by Anderson model, which was supported by the red and blue shifting of the metal core levels of the heterojunctions (Fig. 23(d and e)) [271].

To determine the binding energy differences between the core levels (Mo3d5/2, W4fy/2), and VBM (Г) of individual TMDs and heterojunctions, the XPS technique used (Fig. 23(f and g)). Calculating the difference between Mo3d5/2 and W4f7/2 for various heterojunctions yields the band offsets of TMDs heterojunctions [274]. The resultant VBM offsets, depicted in Fig. 2(g),. Here, the band alignments from the XPS analysis (AEv(MoS2 - WSe2) and AEv(MoS2-WS2)) are 0.02 eV and -0.38 eV, respectively, and outcomes from Anderson model. As a result, it was stated that the band alignment for the 2D material-based p-n heterojunctions could be calculated and understood effectively using both Anderson model and experimental methods like XPS and UPS [271].'

6. The p-n junction as a rectifier

The first application of a p-n junction was a current rectifier. At p-n interface, the inherent electric field permits the movement of charge carriers in one direction (forward bias) while obstructing the current in the opposing direction (reverse bias). Several 2D-based p-n junction rectifiers were used, which showed great performances and innovative functionality. The rectification ratio can be controlled using a gate field as an example. Deng and co-workers described gate tunable rectifying current voltage characteristics (IVs) in monolayer M0S2-BP p-n junction [219]. The IVs measured at various gate voltages between -30 V and 50 V (Fig. 24(a)). The forward current (p-type connected to positive voltage and n-type to negative voltage) and the reverse current (p-type connected to negative voltage and n-type to positive voltage) both observed to decrease at negative gate voltages. Both the forward and the reverse current in the device grow as the gate voltage rises. As can be observed in Fig. 24(b), the forward/reverse current ratio-also known as the rectification ratio-increases as the back gate voltage lowers. A gate voltage of - 30 V produces a rectification ratio of 105. The band alignment between M0S2 and black phosphorus at p-n junction interface could be controlled by gate voltage, which allow for this modulation. Moreover, the contact resistance of the BP and M0S2 sheets, respectively, modulated by the gate voltage. Another type of device, known as a "reconfigurable diode," used as electrostatically doped p-n junctions with a split gate arrangement to enable localised hole and electron-doping in various channel regions, due to 2D nature of devices [175,176]. Groenendijk and co-workers described the IV characteristics of double gated monolayer WSe2 homojunction [275]. Due to WSe2's ambipolarity, its simple to access both hole- and electron-doping by adjusting the Fermi energy. The gate bias polarity used to influence the direction of the rectifying behaviour which displayed by two highly non-linear (PN and NP configuration) and liner (NN and PP configuration) IVs (Fig. 24(c)), respectively. Fig. 24(d) showed a condensed band diagram of the device along with voltages applied to local gates to achieve these four configurations.

7. The physical process of electrical transport

Although the shape and IV characteristics of 2D p-n junctions resemble those of a normal p-n junction, the underlying physical process of rectification might vary significantly. Fig. 25(a) showed the reduce the thickness of p and n - type material which modified the interface between two materials. Fig. 25(b) showed that the depletion region in bulk p-n junction which modified the application of voltage. The depletion region's size increases under reverse bias whereas it decreases under forward voltage. The depletion zone progressively thins out with rising forward voltage to the point where the internal electric field unable to stop charge carrier motion across the p-n junction and increase the current flow. Tunnelling-mediated interlayer recombination between majority carriers at the bottom (top) of the conduction (valence) band of ntype (p-type) material control the current in atomically thin junction under forward bias. The direct recombination of electron and hole described by the Langevin recombination, which mediated by the Coulomb interaction, or the Shockley-Read-Hall (SRH) recombination, which mediated by inelastic tunnelling of majority carriers into trap states in the gap (Fig. 25(c)). Both of these physical mechanisms, or a combination of both, explain that interlayer recombination [276-278]. Each of these two processes predicts a different dependence of electron-hole recombination ratio on density of majority carriers, and it present simultaneously in 2D p-n junction. The properties of rectifying IVs could be explained by rise in interlayer recombination rate under forward bias, the photocurrent produced in 2D junction and its susceptibility to gate field could be characterised by two recombination processes [216].

8. Result and discussion

A new phase in the research of materials has begun with the discovery of graphene [279-281]. The research on 2D materials had quickly developed into a distinct field of study in the physical sciences with numerous novel applications [282-285]. Apart from graphene, other 2D materials, like TMDs: WS2, M0S2, Ga, In etc, gained substantial interest due to their tunable bandgap, surface and edge reactivity, unique electronic and optoelectronic properties, and the realisation of multilayer architectures incorporating atomically abrupt interfaces [201,286-288]. Because of their distinctive structure, 2D materials could be customised via layer number, new synthesis techniques, defects and morphological control [8,289]. There were various new, atomically thin and electrically stable materials which could be used to create ultrathin, flexible devices with a wide range of electronic properties [290]. Here, we discussed the electronic properties of 2D materials including p-n junctions and their characterization methods. Due to their high charge carrier mobilities, high conductivity, and low consumption with remarkable bandgaps that allow them to switch field-effect transistors, 2D semiconductor materials like TMDCs, MX2 etc attracting attention for use in electronic devices (FETs) [291-294]. Electrical conductivity of a semiconductor inversely proportional to charge carrier density at room temperature. Doping was not achievable on 2D materials because of the subsequent atomic thin layer. In 2D materials, charge transfer doping utilised to produce electron and hole carriers [50,295-297]. The main characteristics of 2D materials were their ability to dope common semiconductors, whether n- or p-type, without appreciably altering their fundamental properties and their capacity to alloy, allowing bandgap to be adjusted. However, unique p-n junctions with remarkable properties could be made 2D materials in a variety of ways and offering up exciting scientific directions for both fundamental research issues, as well as real-world applications. Later, research concentrated on p-n heterojunctions based on 2D materials The p-n heterojunctions based 2D materials bring about a fresh era of study, and band alignment through heterojunction production offers the chance to create new devices with desired properties.

9. Future prospects and technical challenges

Following in the footsteps of graphene research, the initial investigation of post-graphene 2D materials led to a quick demonstration of a variety of electronic devices and emerging charge transport phenomena. These days, while the underlying knowledge of electronic transport in individual 2D materials currently improving, there remain an increasing number of obstacles to wafer-scale practical implementations. The development of reliable wafer-scale growth techniques that produce uniform and predictable thickness presents substantial problems from the perspective of materials science. For instance, early studies on powder vaporisation (or CVD) led to thin films which were only applied in small areas [232,298,299]. Those difficulties were not entirely solved by atomic-layer deposition (ALD) or molecular beam epitaxy (MBE). Recently, metal-organic chemical vapour deposition (MOCVD) created monolayers of M0S2 and WS2 across 4-inch wafers with tolerable mobilities of 30 cm2/Vs (Fig. 26(a)) [122]. Another essential objective was controlled substitutional doping, which allow for contact engineering and tunable bandgaps (Fig. 26(b)). Ternary TMDCs compounds initially succeeded to achieving this goal, whereas quaternary compounds like MxW(i_x)SySe(2-y) and InxGa(i.x)SySe(i.y) were relatively rare materials. Additionally, more widely available synthetic materials would make this line of inquiry easier given the lack of knowledge regarding the effects of doping on electronic transport in two dimensions. Last but not least, some of the most promising 2D materials, like silicene and BP elemental monolayers, are unstable in ambient settings. The effects of doping on electronic transport in 2D materials, which were more accessible for synthetic materials. Last but not the least, some of the most intriguing 2D materials, like monolayer of BP and silicene were unstable in natural environment [55,56,300-302]. Thus, effective processing techniques needed for seamless integration into circuits, and reliable passivation mechanisms were essential to maintain their high efficiency (Fig. 26(c)) [303]. Thus, electronic transport was more sensitive for degradation induced traps and defects than simple spectroscopic [302].

In contrast of typical bulk semiconductors, the chemistry of 2D materials offers distinct difficulties and potential. With the lone electron pair on chalcogen atoms in TMDCs and the phosphorus atoms in BP provided opportunities for Lewis acid/base chemistry and radical-based chemistry, the high surface area and volume ratio of 2D materials suggested that chemical functionalization could dramatically affect properties [59,182]. The first challenge was regarding many of the popular degradation of 2D materials due to environmental factors. For example, black phosphorous in its ultrathin form had a tendency to absorb moisture when exposed to air, which degrades the electrical characteristics of substances [302,304, 305]. The most widely accepted method for the degradation of BP involves the reaction of materials with oxygen, which modifies the characteristics of material [306,307]. The active search for new 2D materials which did not have degrading issues and could come from either synthesis (like T1S3) or from natural sources was one alternative strategy (e.g. franckeite) [308-311].

The second main challenge was related to production of Van der waals heterostructures with well controlled interfaces. The deterministic placement methods are excellent for laboratory-scale tests, but they are not appropriate for commercial use. New and growing method like CVD growth able to grow good and high quality of 2D materials with lateral and vertical heterostructures in small scale. Whereas, Vander walls epitaxial method were more prominent for synthesis of high quality of 2D heterostructures. It is possible to scale up these growth strategies, and in the upcoming years higher-quality devices will likely become real [122]. Combining the growth of single 2D materials, such M0S2, with various doping processes (mainly chemical or electrostatic) provides a second, potential method for increasing the production of 2D p-n junctions.

There, several outstanding challenges and unexplored applications which belong to 2D p-n junctions. For example, 2D p-n junction thermoelectric applications have not yet been fully studied. A p-type and an n-type semiconductor were thermally interconnected in parallel and electrically connected in series to form the conventional Peltier device, a part that is frequently used in electronics for cooling (and less frequently for heating). With the other desirable qualities of 2D materials, like great transparency or flexibility, van der Waals heterostructures could be exploited to create atomically thin cooling (or heating) elements. Another fascinating area of novel p-n junction geometries, like circular pn n in graphene [312,313] and new devices which based on 2D p-n junctions, like memory or logic gates [195,314]. 2D p-n junctions contain numerous prospects for practical applications, and there are likely many more genuine possibilities remaining concealed in 2D materials than those that have been explored. For flexible and transparent electronic components like solar cells or light emitting diodes, these 2D junctions are particularly intriguing building blocks [50,315]. The ultrathin nature of 2D p-n junctions had advantageous for nano photonics and other applications involving light harvesting and sensing [316,317]. 2D p-n junction also used as photodetector, as well as combine with other materials that used to design devices sensitive to wavelength extending from infrared to ultraviolet [223,310,318].

10. Conclusion

In this review we described the electronic properties of 2D materials. Generally, a strong impact of defects occurs on electronic properties. Weak screening might because of considerable impact that flaws electricals properties. Here we also discussed the production of novel p-n junctions that take advantage of ultrathin nature of 2D materials. And revised the recent progress of 2D p-n junctions. 2D p-n junction belongs to eight different junction architectures and use those architectures in different applications like as rectifiers, as photodetectors, as solar cells and as light emitting diodes. As a result, 2D materials provided several opportunities to create unique p-n junctions with exceptional features, opening up fascinating scientific avenues for both basic research problems and practical applications.

Declaration of competing interest

Seeram Ramakrishna is an editorial board member for [Nano Materials Science] and was not involved in the editorial review or the decision to publish this article. All authors declare that there are no competing interests.

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