Most two-dimensional materials derive from layered bulk crystals, where individual atomic layers exhibit strong in-plane bonding but poor interlayer bonding with adjacent layers. In these materials, atoms in each layer are interconnected by robust covalent bonds, creating very stable and rigid two-dimensional lattices. The interactions between adjacent layers are dictated by weak van der Waals (vdW) forces. The significant disparity in bonding strength facilitates the detachment of individual layers from bulk crystals using methods such as mechanical exfoliation, liquid-phase exfoliation, or chemical vapor deposition [5].

The inadequate interlayer adhesion produces many distinctive attributes. Firstly, it enables atomically tiny layers to preserve structural integrity even when detached from the bulk crystal. The lack of dangling bonds at the surface results in chemically inert and atomically clean surfaces, thereby substantially diminishing surface scattering and flaws in electrical devices. Third, the feeble van der Waals interactions permit the stacking of various 2D materials without stringent lattice matching prerequisites, resulting in van der Waals heterostructures with customized electrical and optical characteristics [6]. Various categories of 2D materials exhibit distinct crystal formations. Graphene features a hexagonal honeycomb lattice formed by carbon atoms organized in sp² hybridization. Transition metal dichalcogenides (TMDs), including MoS₂ and WS₂, generally display a layered architecture including a transition metal layer interposed between two chalcogen layers (S-Mo-S). Hexagonal boron nitride (h-BN) possesses a structure akin to graphene, characterized by alternating boron and nitrogen atoms. These structural differences result in a spectrum of electronic characteristics, encompassing metallic, semiconducting, and insulating behaviors.

The electrical characteristics of two-dimensional materials are among their most critical attributes for nanoelectronic applications. Due to the confinement of electrons within a narrow atomic layer, two-dimensional materials frequently display distinctive band structures and carrier transport phenomena as shown in Figure 3 [7]. Graphene exhibits linear energy dispersion close to the Dirac points, leading to massless charge carriers that possess exceptionally high mobility. This characteristic enables graphene-based devices to attain exceptionally high carrier velocities and superior electrical conductivity.

Graphene, however, possesses no intrinsic bandgap, hence constraining its utility in digital circuits that necessitate a substantial on/off current ratio. To address this constraint, extensive research has concentrated on semiconducting two-dimensional materials, particularly transition metal dichalcogenides. These materials have finite bandgaps generally between 1 and 2 eV, rendering them appropriate for transistor applications. Their atomically thin channels offer robust gate control over carrier concentration, facilitating efficient switching behavior in field-effect transistors [8]. The atom-thin nature of the body of 2D semiconductors is also another benefit for transistor scaling in terms of electrostatics. The thinner channel leads to the reduction in electrostatic scaling length, which helps in improving the gate controllability while avoiding the short-channel effect even with nanometer scale of channel lengths. With the use of high-k gate dielectric layers and sub-nanometer equivalent oxide thickness (EOT), 2D channel can have good electrostatic control up to dimensions where traditional planar silicon transistors start to suffer from short-channel effects.

Charge transport in two-dimensional materials is affected by various factors, including phonon scattering, impurity scattering, and contact resistance at metal-semiconductor interfaces.

Figure 3. Electronic band structures and atomic crystal structures of representative two-dimensional materials, including NbSe₂, graphene, TiS₃, antimonene, phosphorene, SnS, MoS₂, and hexagonal boron nitride (h-BN). The figure highlights the diversity of electronic properties in 2D materials, ranging from metallic and semimetallic to semiconducting and insulating behavior [7].

The electrical characteristics of these materials can be adjusted through methods including strain engineering, chemical doping, electric field modulation, and thickness management. The assembly of heterostructures by the layering of several 2D materials enhances the potential for creating devices with tailored electrical characteristics.

The lower dimensionality in 2D materials results in pronounced quantum confinement effects. When a material’s thickness is diminished to the atomic scale, electron transport becomes restricted in the direction perpendicular to the layer, resulting in the quantization of energy levels. This confinement alters the electronic band structure and frequently results in thickness-dependent electronic characteristics.

An exemplary instance of quantum confinement is evident in transition metal dichalcogenides. Bulk MoS₂ possesses an indirect bandgap, while its monolayer variant features a direct bandgap. This transition markedly augments light-matter interactions and elevates the optical and electrical efficacy of the material. These tunable electronic architectures facilitate the creation of devices with variable optical and electrical characteristics.

Quantum confinement facilitates the creation of low-dimensional nanostructures, including nanoribbons, quantum dots, and quantum wells, originating from two-dimensional materials. These nanostructures demonstrate distinct energy states and exceptional transport phenomena, such as Coulomb blocking and quantized conductance. Experimental investigations have shown that charge carriers can be localized in quantum dots created within two-dimensional semiconductor heterostructures, facilitating the advancement of nanoscale quantum electronic devices [9].

Two-dimensional materials demonstrate exceptional mechanical capabilities owing to their robust in-plane bonding and ultrathin structure. Numerous 2D materials exhibit exceptional tensile strength, flexibility, and elasticity. Graphene exhibits a very high Young’s modulus and tensile strength, rendering it one of the strongest materials recognized. Notwithstanding its considerable strength, graphene exhibits significant flexibility and can endure substantial mechanical loads without structural compromise. As per Figure 4 the characteristics of 2D materials render them exceptionally appealing for use in flexible and wearable electrical devices [10].

Figure 4.Fabrication methodology and structural framework of the SB-BS-rGO sheet. (a) Schematic representation of the preparation process: a graphene oxide (GO) sheet obtained through filtration was subjected to biaxial stretching and subsequently infiltrated with PCO, followed by UV-induced polymerization. The resultant BS-GO-PCO was chemically reduced with hydrogen iodide (HI). Subsequently, PSE and AP molecules were infiltrated into the sheet and interacted to create PSE-AP complexes while preserving biaxial stretching. Scale bar: 1 centimeter. (b) Structural model illustrating the inter-platelet connectivity within the multilayer architecture via polydiacetylene chains and π-π bridging entities [10].

The mechanical flexibility of two-dimensional materials facilitates strain engineering, wherein mechanical deformation is employed to adjust electrical and optical properties. Strain application can alter a material’s band structure, enabling researchers to manipulate carrier mobility, bandgap energy, and optical absorption properties. Besides their mechanical durability, some two-dimensional materials demonstrate outstanding thermal characteristics.

Effective heat dissipation is essential in nanoelectronic systems, as elevated power density may result in overheating. Graphene is recognized for its exceptional thermal conductivity, facilitating efficient heat transfer and enhanced thermal management in electronic devices. Additional 2D materials, such as transition metal dichalcogenides (TMDs) and boron nitride, demonstrate advantageous thermal transport characteristics that enhance device stability and performance [11].

Mechanical exfoliation is one of the initial methods employed to isolate two-dimensional materials and was essential in the discovery of graphene. In this method, adhesive tape is used to repeatedly peel thin layers from bulk graphite and transfer them onto a suitable substrate to obtain monolayer or few-layer 2D materials. This approach involves the removal of thin layers from bulk stacked crystals by the use of adhesive tape or mechanical cleavage, as shown in Figure 5. The technique utilizes the weak van der Waals interactions between neighboring layers in materials including graphite, MoS₂, and hexagonal boron nitride [15]. The mobility values achieved in mechanically exfoliated graphene are extremely high; however, the measured mobilities are highly dependent upon the substrate, carrier concentration, temperature, sample purity, and device geometry. Suspended single-layer graphene devices made from ultraclean graphene have yielded low-temperature mobilities higher than 200,000 cm²∙V⁻¹∙s⁻¹ at carrier concentrations of about 2 × 10¹¹ cm⁻², and the graphene is suspended over the Si/SiO₂ gate stack. The mobility of supported exfoliated graphene devices is lower due to various factors, including substrate disorder, charged impurities, and surface scattering; representative room-temperature mobilities have values of the order of 10,000 - 15,000 cm²∙V⁻¹∙s⁻¹. Therefore, the mobility values obtained from suspended and supported graphene cannot be compared [12][16]. The exceptional mobility of mechanically exfoliated graphene renders it optimal for fundamental investigations of electrical transport and quantum processes.

Figure 5. Schematic illustration of the mechanical exfoliation method for fabricating two-dimensional nanomaterials. Bulk graphite is exfoliated using adhesive tape, transferred onto a Si/SiO₂ substrate, and the tape is removed to leave thin 2D material flakes on the surface, which can then be characterized using techniques such as Raman spectroscopy, UV-Vis spectroscopy, AFM, and TEM.

Likewise, mechanically exfoliated MoS₂ monolayers have been extensively investigated for transistor applications. Monolayer MoS₂ field-effect transistors exhibit on/off current ratios surpassing 10⁸ and carrier mobility ranging from 10 to 200 cm²∙V⁻¹∙s⁻¹, contingent upon substrate and device configuration [17]. Notwithstanding the generation of superior materials, mechanical exfoliation possesses considerable constraints. The resultant flakes often possess lateral diameters varying from a few micrometers to many tens of micrometers, and the yield of monolayer materials is quite low. Thus, this technology is predominantly employed for laboratory-scale investigations rather than industrial manufacturing [13].

Liquid-phase exfoliation is a prevalent top-down method for generating substantial numbers of 2D nanosheets in liquid suspensions. This process involves dispersing bulk layered materials in liquids and applying ultrasonic energy or shear forces to isolate individual layers. The procedure diminishes the van der Waals forces between neighboring layers, facilitating exfoliation into nanosheets that remain dispersed in the solvent [18]. This method can yield graphene nanosheets with lateral dimensions generally between 100 nm to several micrometers, contingent upon the sonication parameters and solvent employed [19]. Liquid-phase exfoliation may generate gram-scale amounts of graphene and other two-dimensional materials, rendering it appealing for industrial applications including printed electronics, energy storage systems, and composite materials [20]. In this process, graphite is dispersed in suitable solvents and subjected to ultrasonication, which weakens the van der Waals forces between graphene layers. Centrifugation and filtration then separate graphene nanosheets with controlled thickness and size as shown in Figure 6. Graphene films produced via liquid-phase exfoliation exhibit electrical conductivities ranging from roughly 10³ to 10⁴ S∙m⁻¹, rendering them appropriate for flexible electrodes and transparent conductive films [19].

Figure 6. Schematic illustration of the liquid-phase exfoliation process for producing graphene nanosheets. Bulk graphite is first ground into powder and dispersed in a solvent, followed by ultrasonication to exfoliate graphene layers. The resulting dispersion is then subjected to centrifugation and filtration to isolate graphene nanosheets and few-layer graphene suitable for various applications.

Nonetheless, the nanosheets generated by this method generally exhibit reduced lateral dimensions and increased defect density relative to mechanically exfoliated materials. Notwithstanding these constraints, liquid-phase exfoliation continues to be one of the most promising scalable methods for generating substantial quantities of two-dimensional materials for commercial use.

Chemical vapor deposition (CVD) is a prevalent bottom-up synthesis method for generating extensive two-dimensional materials. This process involves the introduction of gaseous precursor molecules into a high-temperature reactor, where they undergo chemical reactions or thermal degradation on a substrate surface to produce thin crystalline coatings [21]. Extensive graphene films have been effectively synthesized on copper surfaces utilizing methane as a carbon precursor. This approach has produced graphene films with diameters surpassing 30 inches for industrial purposes [22]. CVD-synthesized graphene generally demonstrates carrier mobility values between 1000 and 5000 cm²∙V⁻¹∙s⁻¹, contingent upon growth circumstances and transfer methodologies [21].

Comprehending and enhancing the growth mechanisms of two-dimensional (2D) materials by chemical vapor deposition (CVD) necessitates an integration of theoretical modeling, computational simulations, and experimental analysis. Advanced computational methodologies, including density functional theory (DFT), kinetic Monte Carlo simulations, molecular dynamics, and machine learning models, have emerged as formidable instruments for forecasting growth mechanisms, reaction pathways, and material characteristics. These methodologies assist researchers in examining the impact of reactor parameters, temperature, pressure, and precursor transport on the nucleation and development of high-quality two-dimensional materials as shown in Figure 7.

Figure 7. Computational and theoretical approaches used to study the chemical vapor deposition growth of two-dimensional materials, including density functional theory, kinetic Monte Carlo and molecular dynamics simulations, transport and phase-field modeling, and machine learning for predicting growth outcomes and optimizing reactor conditions [23].

CVD techniques have been extensively employed to synthesize transition metal dichalcogenides, including MoS₂ and WS₂. Monolayer MoS₂ films produced via CVD have exhibited field-effect mobilities ranging from 10 to 60 cm²∙V⁻¹∙s⁻¹ and on/off current ratios surpassing 10⁷ in transistor devices [1][24]. A primary advantage of CVD is its capacity to generate continuous monolayer films with regulated thickness and exceptional homogeneity over extensive areas. Consequently, scalable growth methodologies like CVD are deemed crucial for the incorporation of 2D materials into nanoelectronic and optoelectronic devices.

For practical nanoelectronic applications, it is imperative to transfer extensive 2D materials onto insulating or flexible substrates appropriate for device production. Typically, 2D materials produced using CVD are initially cultivated on metallic substrates like copper or nickel and subsequently require transfer to alternative substrates through polymer-assisted techniques [25]. During a standard transfer procedure, the synthesized graphene film is enveloped with a polymeric support layer, typically polymethyl methacrylate (PMMA). The metal substrate is subsequently chemically etched, facilitating the transfer of the polymer-supported graphene layer onto substrates like SiO₂/Si, glass, or flexible polymer films. Subsequent to the transfer, the polymer layer is eliminated, resulting in the graphene sheet adhering to the new substrate [25]. The transfer of CVD-grown graphene from metallic growth substrates to insulating or flexible substrates is an essential process for the construction of nanoelectronic and optoelectronic devices as shown in Figure 8. A widely employed polymer-assisted transfer method utilizing polymethyl methacrylate (PMMA) facilitates the incorporation of graphene sheets onto diverse surfaces.

Figure 8. Polymer-assisted transfer of CVD-grown graphene films. (a) Schematic of the PMMA-assisted transfer process from a Ni/Si-SiO₂ growth substrate to a target substrate after metal etching and PMMA removal. (b) Photographs of graphene films transferred onto glass, Si/SiO₂, and flexible PET substrates, demonstrating their high transparency and versatility for electronic applications [26].

Extensive graphene sheets transferred via roll-to-roll techniques have sheet resistances as low as 25 Ω/sq with 97.4% optical transparency, positioning them as viable options for transparent electrodes and flexible electronics [22]. Ongoing enhancements in transfer techniques seek to reduce structural imperfections, contaminants, and creases occurring during the transfer process. These advancements are crucial for enhancing the reliability and performance of nanoelectronic devices based on 2D materials.

Table 1compares the principal synthesis routes for 2D materials, highlighting their processing approach, advantages, limitations, and representative material properties. Mechanical exfoliation provides high-quality flakes but limited scalability, while liquid-phase exfoliation and CVD offer greater production potential, with trade-offs in defect density, transfer-related contamination, and material quality [12][13][15][18]-[25].

Table 1. Comparative analysis of synthesis techniques for two-dimensional materials.

Graphene is the most recognized two-dimensional substance, comprising a single sheet of carbon atoms organized in a hexagonal honeycomb lattice. It was initially isolated in 2004 using mechanical exfoliation of graphite, signifying a significant advancement in materials research [15]. Graphene exhibits remarkable physical and electrical characteristics. It demonstrates exceptionally high carrier mobility, attaining values of up to 200,000 cm²∙V⁻¹∙s⁻¹ at low temperatures, and around 10,000 - 15,000 cm²∙V⁻¹∙s⁻¹ at ambient temperature [12][16]. Moreover, graphene has exceptional heat conductivity surpassing 3000 W∙m⁻¹∙K⁻¹, rendering it one of the most thermally conductive materials recognized [27].

Notwithstanding these benefits, graphene possesses no intrinsic bandgap, as its conduction and valence bands converge at the Dirac points. The zero-bandgap characteristic restricts its direct employment in digital transistors, which necessitate a high on/off switching ratio. To overcome this constraint, other graphene derivatives have been created, such as graphene oxide (GO), reduced graphene oxide (rGO), and graphene nanoribbons, which display adjustable bandgaps based on their structural modifications [28]. Graphene and its derivatives have been extensively investigated for use in high-frequency transistors, transparent electrodes, flexible electronics, and sensors, owing to their exceptional electrical conductivity and mechanical flexibility [29]. While graphene is characterized by high carrier mobility, the lack of an intrinsic band gap makes the on/off ratio low, thus limiting its use for digital logic. While some methods, such as nano structuring can increase the band gap, they also reduce carrier mobility.

Transition metal dichalcogenides (TMDs) are a significant category of two-dimensional materials characterized by the generic chemical formula MX₂, whereby M denotes a transition metal (such as Mo or W) and X signifies a chalcogen atom (S, Se, or Te). These materials exhibit a stratified architecture with a layer of transition metal atoms interposed between two layers of chalcogens. In contrast to graphene, numerous transition metal dichalcogenides (TMDs) are semiconductors possessing intrinsic bandgaps between 1 and 2 eV, rendering them exceptionally appropriate for nanoelectronic applications. Monolayer molybdenum disulfide (MoS₂) possesses a direct bandgap of roughly 1.8 eV, but its bulk form has an indirect bandgap of around 1.2 eV [30].

However, it was observed that the performance of monolayer MoS₂ FETs is highly sensitive to dielectric environment, contact resistance, channel geometry, and mobility extraction technique. The early monolayer MoS₂ FETs showed lower mobilities; however, high-k HfO₂ gate dielectric in top gated monolayer MoS₂ FET yielded room temperature field effect mobility close to 200 cm²∙V⁻¹∙s⁻¹ along with a high on-off current ratio of around 10⁸ [17]. This can be viewed in light of the device design and not as a property of monolayer MoS₂. In general, the reported mobilities for MoS₂ are quite diverse. TMDs have robust light-matter interactions, facilitating their application in photodetectors, optoelectronic devices, and valleytronic technologies [1]. One disadvantage of TMD-based transistors is that of metal-semiconductor contact variability. The factors of Schottky barrier formation, Fermi level pinning, interface contamination, and variation in contact geometries can cause significant variation in contact resistances and carrier mobilities measured directly from the device itself.

Hexagonal boron nitride (h-BN) is a significant two-dimensional material that possesses a comparable honeycomb lattice structure to graphene. Nevertheless, the lattice is composed of alternating boron and nitrogen atoms rather than carbon atoms. This configuration produces an insulating material with a broad bandgap of roughly 5.9 eV [31]. Owing to its superior dielectric characteristics, chemical stability, and atomically clean surface, h-BN is frequently employed as an insulating substrate or dielectric layer in devices based on graphene and other two-dimensional materials. The lack of dangling bonds and surface charge traps enables h-BN to markedly enhance the carrier mobility of graphene devices by diminishing scattering effects [31].

Moreover, h-BN is essential in the synthesis of van der Waals heterostructures, wherein several 2D elements are layered to create novel artificial materials with customized electrical characteristics. These heterostructures facilitate the advancement of sophisticated nanoelectronic and optoelectronic devices [32].

Alongside graphene, transition metal dichalcogenides (TMDs), and hexagonal boron nitride (h-BN), other novel two-dimensional materials have garnered considerable interest in recent years. MXenes and black phosphorus have notably advantageous characteristics for nanoelectronic applications. MXenes constitute a class of two-dimensional transition metal carbides or nitrides, generally denoted by the formula M_n+1X_nT_x, where M signifies a transition metal, X denotes carbon or nitrogen, and T_x indicates surface functional groups such as hydroxyl or oxygen. An MXene is a kind of 2D material that is a transition-metal carbide or nitride, typically represented by the chemical formula M_n+1X_nT_x, in which M stands for a transition metal, X represents either carbon or nitrogen, and T_x stands for surface functional groups. High electrical conductivity, ability to adjust the surface chemistry, hydrophilicity, and suitability for solution-based processing make them a candidate for many applications in the field of electronics, including sensors, conductors, flexible electronics, and novel device structures. MXenes have been widely explored in the areas of energy storage and electromagnetic interference shielding. Yet these applications are not the main topics of the current review and are mentioned merely for illustrating the overall multifunctionality of MXenes [33].

Table 2summarizes the major classes of 2D materials used in nanoelectronics, comparing their crystal structures, bandgaps, carrier mobilities, key advantages, and typical applications. Graphene, TMDs, h-BN, MXenes, and black phosphorus exhibit distinctly different electronic characteristics, enabling their use in transistors, optoelectronics, flexible electronics, sensing, and heterostructure-based devices [1][12][16][17][27][30]-[34].

Table 2.Comparison of major classes of two-dimensional materials for nanoelectronics.

Another new two-dimensional substance is black phosphorus, which can be exfoliated into thin layers referred to as phosphorene. Phosphorene demonstrates a bandgap that varies with thickness, spanning from 0.3 eV in bulk form to around 2.0 eV in monolayer form. It exhibits a notable carrier mobility of up to 1000 cm²∙V⁻¹∙s⁻¹, rendering it appealing for high-performance electrical and optoelectronic applications [34]. As research advances, these novel materials are anticipated to have a progressively significant role in the evolution of next-generation nanoelectronic technology. The main drawback of black phosphorous is its instability in the environment. The presence of oxygen and water will quickly oxidize black phosphorous. Thus, it needs to be encapsulated to stabilize its properties in any application. The swift identification of novel two-dimensional materials perpetuates the diversification of accessible electronic characteristics and device potentialities.