12.5 2D materials beyond graphene

Two-dimensional materials beyond graphene offer a diverse range of properties and functionalities. These atomically thin materials, including transition metal dichalcogenides, hexagonal boron nitride, and phosphorene, exhibit unique electronic, optical, and mechanical characteristics due to quantum confinement.

The study of 2D materials explores their synthesis methods, characterization techniques, and potential applications. From electronics and sensors to energy storage and catalysis, these materials promise groundbreaking advancements across various fields, while also presenting challenges in scalable production and device integration.

Overview of 2D materials

• 2D materials are atomically thin materials with thicknesses ranging from a single atomic layer to a few nanometers
• Exhibit unique properties compared to their bulk counterparts due to quantum confinement and large surface-to-volume ratios
• Have potential applications in various fields such as electronics, optoelectronics, sensors, energy storage, and catalysis

Graphene vs other 2D materials

• Graphene, a single layer of carbon atoms arranged in a hexagonal lattice, was the first 2D material discovered and has exceptional properties (high carrier mobility, mechanical strength, and thermal conductivity)
• Other 2D materials beyond graphene offer a wider range of properties and functionalities
• Examples include transition metal dichalcogenides (TMDs), hexagonal boron nitride (hBN), silicene, germanene, phosphorene, and MXenes

Types of 2D materials

Transition metal dichalcogenides (TMDs)

• Consist of a transition metal (Mo, W, Re, Nb, etc.) sandwiched between two layers of chalcogen atoms (S, Se, or Te)
• Have a general formula of MX2, where M is a transition metal and X is a chalcogen
• Exhibit thickness-dependent electronic and optical properties (indirect-to-direct bandgap transition with decreasing thickness)
• Examples include MoS2, WS2, MoSe2, and WSe2

Hexagonal boron nitride (hBN)

• Consists of alternating boron and nitrogen atoms arranged in a hexagonal lattice
• Known as "white graphene" due to its similar structure to graphene but with a wide bandgap (~6 eV)
• Serves as an excellent substrate and encapsulation material for other 2D materials due to its atomically smooth surface and excellent insulating properties

Silicene and germanene

• Silicon and germanium analogs of graphene, respectively
• Have buckled honeycomb structures due to the larger atomic radii of Si and Ge compared to carbon
• Exhibit a mix of Dirac fermion and massive fermion behavior depending on the applied electric field
• Potential for integration with existing silicon-based electronics

Phosphorene

• Single- or few-layer form of black phosphorus
• Has a puckered honeycomb structure with anisotropic electronic and optical properties
• Exhibits a thickness-dependent direct bandgap ranging from ~0.3 eV (bulk) to ~2 eV (monolayer)
• Potential applications in high-performance electronics and optoelectronics

MXenes

• Family of 2D transition metal carbides, nitrides, and carbonitrides with a general formula of Mn+1XnTx, where M is a transition metal, X is carbon and/or nitrogen, and T represents surface terminations (O, OH, or F)
• Synthesized by selective etching of MAX phases, which are layered ternary carbides or nitrides
• Exhibit a combination of metallic conductivity and hydrophilicity, making them promising for energy storage and electromagnetic interference shielding applications

Structure and properties

Lattice structure

• 2D materials have various lattice structures depending on their composition and bonding (hexagonal, honeycomb, puckered, or orthorhombic)
• Lattice symmetry and stacking order influence the electronic, optical, and mechanical properties of 2D materials
• Presence of defects, grain boundaries, and heterostructures can modify the local structure and properties

Electronic band structure

• 2D materials exhibit diverse electronic band structures ranging from metallic to semiconducting to insulating
• Bandgap and carrier mobility can be tuned by controlling the thickness, strain, electric field, or chemical composition
• Presence of van Hove singularities in the density of states leads to enhanced light-matter interactions and nonlinear optical properties

Optical properties

• 2D materials have unique optical properties arising from quantum confinement and reduced screening
• Exhibit strong light absorption, photoluminescence, and Raman scattering
• Thickness-dependent optical properties enable applications in photodetectors, light-emitting diodes, and lasers

Mechanical properties

• 2D materials possess exceptional mechanical strength and flexibility due to strong in-plane covalent bonding
• Have high Young's moduli and breaking strengths, making them suitable for flexible electronics and reinforcement in composites
• Can withstand large strains without fracture, enabling strain engineering of electronic and optical properties

Thermal properties

• 2D materials exhibit anisotropic thermal conductivity with high in-plane and low out-of-plane values
• Thermal conductivity can be modulated by controlling the phonon scattering mechanisms (defects, grain boundaries, or interfaces)
• Potential for applications in thermal management and thermoelectric energy conversion

Synthesis methods

Mechanical exfoliation

• Also known as the "Scotch tape" method, involves peeling off single or few layers from bulk crystals using adhesive tape
• Yields high-quality 2D materials with clean surfaces and low defect densities
• Limited scalability and low throughput, primarily used for fundamental research and proof-of-concept devices

Chemical vapor deposition (CVD)

• Involves the reaction of gaseous precursors on a substrate surface to form 2D materials
• Enables the synthesis of large-area, high-quality 2D materials with controllable thickness and composition
• Can be used to grow heterostructures and alloys by sequential or co-deposition of different precursors

Liquid phase exfoliation

• Involves the dispersion and exfoliation of bulk crystals in suitable solvents or surfactant solutions using ultrasonication or shear forces
• Yields stable colloidal suspensions of 2D material flakes, which can be processed into thin films or composites
• Scalable and cost-effective method for producing 2D materials in large quantities

Molecular beam epitaxy (MBE)

• Involves the deposition of atoms or molecules onto a heated substrate under ultra-high vacuum conditions
• Enables precise control over the growth conditions and allows for the synthesis of high-quality, epitaxial 2D materials
• Can be used to grow heterostructures and superlattices with atomically sharp interfaces

Characterization techniques

Raman spectroscopy

• Non-destructive technique that probes the vibrational modes of 2D materials
• Provides information about the thickness, stacking order, strain, doping, and defects in 2D materials
• Can be used to map the spatial distribution of properties and to study the effects of external stimuli (temperature, pressure, or electric field)

Atomic force microscopy (AFM)

• Scanning probe technique that measures the topography and local properties of 2D materials with nanometer resolution
• Can be used to determine the thickness, roughness, and mechanical properties of 2D materials
• Enables the manipulation and patterning of 2D materials at the nanoscale

Transmission electron microscopy (TEM)

• Imaging technique that uses a beam of electrons to visualize the atomic structure of 2D materials
• Provides information about the lattice structure, defects, grain boundaries, and interfaces in 2D materials
• Can be combined with spectroscopic techniques (EELS or EDS) to study the chemical composition and bonding

X-ray diffraction (XRD)

• Technique that measures the diffraction of X-rays by the periodic atomic structure of 2D materials
• Provides information about the crystal structure, lattice parameters, and stacking order of 2D materials
• Can be used to study the effects of strain, doping, or intercalation on the structure of 2D materials

Photoluminescence spectroscopy

• Optical technique that measures the emission of light from 2D materials upon photoexcitation
• Provides information about the bandgap, exciton binding energy, and defect states in 2D materials
• Can be used to study the effects of thickness, strain, doping, or electric field on the optical properties of 2D materials

Applications of 2D materials

Electronics and optoelectronics

• 2D materials can be used as channel materials in field-effect transistors (FETs) due to their high carrier mobility and low subthreshold swing
• Can be used as active layers in photodetectors, solar cells, and light-emitting diodes (LEDs) due to their strong light-matter interactions
• Enable the development of flexible, transparent, and low-power electronic and optoelectronic devices

Sensors and biosensors

• 2D materials have large surface-to-volume ratios and are sensitive to changes in their environment, making them suitable for sensing applications
• Can be used to detect various analytes (gases, chemicals, biomolecules) through changes in their electrical, optical, or mechanical properties
• Enable the development of wearable, implantable, and point-of-care sensing devices

Energy storage and conversion

• 2D materials can be used as electrode materials in batteries and supercapacitors due to their high surface area and fast ion transport
• Can be used as catalysts for hydrogen evolution, oxygen reduction, and CO2 reduction reactions due to their exposed active sites and tunable electronic properties
• Enable the development of high-performance, sustainable, and miniaturized energy storage and conversion devices

Catalysis

• 2D materials have a high density of exposed active sites and can be engineered to have specific electronic and chemical properties, making them attractive for catalytic applications
• Can be used as catalysts for various chemical reactions (hydrogenation, oxidation, coupling) and environmental remediation processes (water splitting, CO2 reduction)
• Enable the development of highly efficient, selective, and recyclable catalytic systems

Composite materials

• 2D materials can be incorporated into polymer, ceramic, or metal matrices to enhance their mechanical, thermal, electrical, or optical properties
• Can be used as reinforcing agents, conductive fillers, or barrier layers in composite materials
• Enable the development of multifunctional, lightweight, and high-performance composite materials for various applications (aerospace, automotive, construction)

Challenges and future prospects

Scalable production

• Developing cost-effective and high-throughput methods for synthesizing high-quality 2D materials with controlled thickness, composition, and defect density
• Optimizing the growth conditions and precursor materials for different 2D materials and applications
• Establishing standards and quality control measures for the production of 2D materials

Device integration

• Overcoming the challenges associated with the transfer, alignment, and patterning of 2D materials on various substrates
• Developing reliable and low-resistance electrical contacts to 2D materials
• Ensuring the compatibility and stability of 2D materials with other device components (dielectrics, electrodes, encapsulation layers)

Stability and durability

• Improving the environmental stability and chemical resistance of 2D materials, especially under ambient conditions or in harsh environments
• Developing effective passivation and encapsulation strategies to protect 2D materials from degradation and contamination
• Studying the long-term reliability and performance of 2D material-based devices under realistic operating conditions

Exploring new 2D materials

• Discovering and synthesizing new 2D materials with novel properties and functionalities beyond the currently known families
• Investigating the properties and potential applications of 2D materials with different compositions (ternary, quaternary), phases (metallic, superconducting), or structures (Janus, Janus)
• Combining 2D materials with other low-dimensional materials (1D nanowires, 0D quantum dots) to create hybrid structures with enhanced or synergistic properties