The discovery of graphene in 2004 opened a new frontier in materials science, demonstrating that stable, atomically thin crystals could exhibit extraordinary electrical, mechanical, and thermal properties. Yet as researchers quickly realized, graphene is only one member of a rapidly expanding family of two-dimensional (2D) materials. While graphene’s zero-bandgap semimetallic nature limits its use in digital logic, a rich landscape of other 2D materials has emerged, each offering unique electronic characteristics. This article explores the electrical properties of 2D materials beyond graphene, focusing on their band structures, carrier mobilities, and the mechanisms that make them promising for next-generation electronics, optoelectronics, and energy technologies.

Overview of Two-Dimensional Materials Beyond Graphene

Two-dimensional materials are crystalline solids with thicknesses of one or a few atomic layers. Their reduced dimensionality gives rise to quantum confinement effects, strong in-plane covalent bonding, and weak van der Waals interlayer coupling. Beyond graphene, the 2D family includes transition metal dichalcogenides (TMDs), black phosphorus, hexagonal boron nitride (hBN), and elemental analogues like silicene and germanene. Each material possesses distinct electrical properties that depend on its atomic structure, stoichiometry, and external perturbations such as strain, electric fields, or dielectric environment.

Transition Metal Dichalcogenides

Transition metal dichalcogenides (TMDs) are a class of layered compounds with the general formula MX2, where M is a transition metal (e.g., Mo, W, Re) and X is a chalcogen (S, Se, Te). In their bulk form, many TMDs are indirect-bandgap semiconductors. However, when exfoliated to a monolayer, the band structure transitions to a direct bandgap, dramatically enhancing their photoluminescence and making them highly attractive for optoelectronic applications.

Electrical Properties of MoS₂, WS₂, and WSe₂

Molybdenum disulfide (MoS₂) is the most studied TMD. Single-layer MoS₂ has a direct bandgap of approximately 1.8 eV, while the bulk indirect gap is near 1.2 eV. The material exhibits n-type conductivity in its pristine state, with electron mobilities in the range of 10–200 cm²/V·s for exfoliated flakes on SiO₂ substrates. Through encapsulation in hexagonal boron nitride or the use of high-κ dielectrics, mobilities exceeding 1000 cm²/V·s have been reported. Tungsten diselenide (WSe₂) similarly shows a direct bandgap in monolayer form (≈1.7 eV) and can be ambipolarly doped, enabling complementary logic functions in a single material. Tungsten disulfide (WS₂) exhibits similar behavior with a slightly larger bandgap (≈2.0 eV).

The electrical conductivity of TMDs can be tuned via electrostatic gating, chemical doping, or atomic substitution. The presence of a finite bandgap—in contrast to graphene—allows TMDs to achieve high on/off ratios in field-effect transistors, often exceeding 10⁶. This makes them ideal candidates for low-power digital electronics. Furthermore, the strong spin–orbit coupling in heavy-metal TMDs (e.g., WSe₂) gives rise to unique spintronic properties, including long spin lifetimes and the possibility of electrically controlled spin polarization.

Carrier Transport and Dielectric Screening

Carrier transport in TMDs is influenced by the interaction with underlying substrates and surface adsorbates. The atom-thin channel is extremely sensitive to charged impurities and phonons. Using van der Waals heterostructures—stacking TMDs with hBN or graphene—researchers have suppressed scattering, achieving ballistic transport at low temperatures. The dielectric environment also plays a critical role: a high-κ substrate screens Coulomb scattering and can enhance mobility by an order of magnitude.

Black Phosphorus (Phosphorene)

Black phosphorus, or phosphorene when isolated as a monolayer, has re-emerged as a remarkable 2D semiconductor. Unlike the isotropic in-plane structure of graphene and many TMDs, black phosphorus has an orthorhombic, ‘puckered’ crystal lattice. This structure imparts strong in-plane anisotropy to its electrical properties—a feature that distinguishes it from most other 2D materials.

Bandgap and Carrier Mobility

Phosphorene possesses a direct bandgap that is layer-dependent, ranging from approximately 0.3 eV (bulk) to 2.0 eV (monolayer). This tunability is unique among 2D semiconductors and covers the infrared to visible spectrum. The material exhibits high hole mobility, with reported values of up to 1000 cm²/V·s in thin flakes at room temperature. The anisotropic band structure yields different effective masses along the armchair and zigzag directions, leading to direction-dependent transport. This anisotropy can be exploited for polarized photodetectors and novel switching devices.

Stability Challenges and Encapsulation

A major drawback of black phosphorus is its ambient instability: mechanical exfoliation under ambient conditions leads to rapid degradation due to oxygen and water exposure. Encapsulation in hBN or other protective layers is essential for practical applications. With proper encapsulation, phosphorene-based transistors maintain high performance and stability over extended periods. Recent advances in large-area growth by chemical vapor transport have also improved material quality and processability.

Hexagonal Boron Nitride

Hexagonal boron nitride (hBN) is the 2D analogue of graphene but with alternating B and N atoms. Its electronic structure is that of a wide-bandgap insulator (Eg ≈ 6 eV). While not electrically conductive itself, hBN is invaluable as a dielectric substrate and encapsulation layer. It provides an atomically flat surface free of dangling bonds, high dielectric strength, and low leakage currents. When used as a gate dielectric, hBN enables high carrier mobilities in adjacent 2D semiconductors by screening charge impurities. Moreover, hBN can host single-photon emitters and exhibits excellent thermal conductivity, making it a critical component in van der Waals heterostructures.

Elemental 2D Materials: Silicene, Germanene, and Stanene

Beyond the prototypical compounds, researchers have synthesized elemental 2D analogues of graphene using silicon (silicene), germanium (germanene), and tin (stanene). These materials are predicted to have Dirac cones similar to graphene but with stronger spin–orbit coupling, potentially leading to quantum spin Hall states. However, their synthesis typically requires metallic substrates (e.g., Ag(111), Ir(111)), and the electronic states can be heavily hybridized with the substrate, obscuring the intrinsic 2D properties. Progress in decoupling these layers via intercalation or the use of epitaxial buffer layers is ongoing. For instance, freestanding silicene nanoribbons have demonstrated high carrier mobilities, and germanene on MoS₂ shows a sizeable bandgap opening.

Tuning Electrical Properties via External Stimuli

The electrical properties of beyond-graphene 2D materials can be profoundly modified by external fields, strain, and chemical functionalization. This tunability is central to their potential in reconfigurable electronics and sensing.

Electrostatic Gating and Doping

Field-effect gating allows control over carrier density and type (electrons or holes) in 2D semiconductors. In TMDs, gate voltages can shift the Fermi level across the bandgap, enabling n-type, p-type, or ambipolar conduction. Electrochemical gating with ionic liquids achieves exceptionally high carrier densities (up to 10¹⁴ cm⁻²), inducing metallic states and even superconductivity in some TMDs.

Mechanical Strain

Uniaxial and biaxial strain alter the lattice constants and symmetry of 2D materials, resulting in significant changes to the electronic band structure. For example, tensile strain in MoS₂ reduces the bandgap and can induce a direct-to-indirect transition. In phosphorene, strain modifies the anisotropic transport, enhancing mobility along specific directions. Strain engineering is a powerful tool for designing 2D materials with desired electronic properties without altering the chemical composition.

Chemical Functionalization

Covalent attachment of functional groups (e.g., hydrogen, fluorine, oxygen) can modify the electronic band structure. Hydrogenation of silicene creates silicane, a wide-bandgap semiconductor. Controlled oxidation of TMDs can create localized electronic states or heterojunctions. However, care must be taken to preserve the crystalline order, as excessive functionalization can degrade carrier mobility.

Device Applications and Emerging Technologies

The unique electrical properties of beyond-graphene 2D materials are being leveraged in a range of prototype devices, from high-performance transistors to flexible optoelectronics.

Field-Effect Transistors

Monolayer TMDs have been used to fabricate n-type and p-type transistors with subthreshold swings near the theoretical limit (60 mV/decade). The absence of dangling bonds and the ability to form clean van der Waals interfaces enable low contact resistance (<10² Ω·µm). Devices based on black phosphorus display high hole mobilities suitable for high-frequency amplifiers. Research is progressing toward fully 2D logic circuits, such as inverters and ring oscillators, combining TMDs and graphene.

Photodetectors and Light-Emitting Diodes

The direct bandgap of monolayer TMDs and the anisotropy of black phosphorus make them excellent candidates for photodetectors with fast response times and high responsivity. Heterojunctions formed between different 2D materials (e.g., MoS₂/WSe₂) exhibit efficient charge separation and photovoltaic effects. Light-emitting diodes based on TMDs have been demonstrated, with quantum efficiency improving through engineering of the dielectric environment.

Energy Storage and Conversion

The large surface area and tunable electronic properties of 2D materials are advantageous for electrodes in batteries and supercapacitors. MoS₂, for instance, is actively investigated as an anode material for lithium-ion batteries because of its high theoretical capacity. In electrocatalysis, 2D materials such as MoS₂ edges are active for hydrogen evolution, and strain can further enhance catalytic activity.

Challenges in Synthesis and Scalability

Despite the remarkable progress in fundamental research, the transition of beyond-graphene 2D materials from laboratory curiosities to commercial technologies faces significant hurdles. Large-area synthesis of high-quality monolayers remains challenging for most materials. Chemical vapor deposition (CVD) works well for MoS₂ and WS₂ but often yields polycrystalline films with grain boundaries that degrade electrical performance. Black phosphorus has been grown by pulsed laser deposition and molecular beam epitaxy, but scalability and uniformity remain issues. The exfoliation approach—while suitable for fundamental studies—is not viable for industrial production. Integration of 2D materials into existing semiconductor fabrication processes also poses challenges, particularly regarding dielectrics, contacts, and contamination control. The development of wafer-scale, clean transfer methods is critical.

Future Directions and Outlook

The field of beyond-graphene 2D materials is moving rapidly toward the rational design of heterostructures and the discovery of new compounds. Machine learning and high-throughput computation are accelerating the screening of candidate 2D materials. Experimental advances in ultrahigh-vacuum in situ techniques are revealing the intrinsic properties of air-sensitive materials. Moreover, the combination of 2D materials with topological insulators, magnetic materials, and ferroelectrics opens pathways to novel quantum phenomena, including room-temperature valleytronics and topological quantum computing.

The electrical properties of 2D materials beyond graphene—from the direct-gap TMDs and anisotropic black phosphorus to the insulating hBN and emergent elemental sheets—offer a versatile platform for electronics that are faster, thinner, and more energy-efficient than current silicon-based devices. While commercialization is still on the horizon, the fundamental understanding gained over the past decade has already reshaped how we think about materials at the atom-thin limit. Continued research into scalable synthesis, device integration, and novel physical effects will determine how quickly these materials can move beyond the laboratory and into the marketplace.

For further reading on the electrical properties of 2D materials, see reviews in Nature Reviews Materials and the Chemical Reviews. Detailed studies on black phosphorus transistors can be found in 2D Materials.