13.3 Plasma-Assisted Growth of 2D Materials

Plasma-assisted growth of 2D materials revolutionizes nanomaterial synthesis. This technique enables precise control over the creation of ultra-thin layers like graphene, transition metal dichalcogenides, and hexagonal boron nitride, unlocking their unique properties for cutting-edge applications.

From electronics to energy storage, plasma-grown 2D materials are transforming technology. By manipulating plasma parameters, researchers can fine-tune material properties, paving the way for next-generation devices with enhanced performance and novel functionalities.

Plasma-Assisted Growth of 2D Materials

Key 2D materials for plasma synthesis

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• Graphene
  • Consists of a single layer of carbon atoms arranged in a hexagonal lattice structure
  • Exhibits excellent electrical conductivity (high electron mobility), thermal conductivity, mechanical strength (200 times stronger than steel), and optical transparency
• Transition metal dichalcogenides (TMDs) (MoS2, WS2, MoSe2, WSe2)
  • Feature a layered structure with a transition metal atom (Mo, W) sandwiched between two chalcogen atoms (S, Se)
  • Possess semiconducting properties with a tunable bandgap (can be adjusted by changing the number of layers) and strong light-matter interactions for optoelectronic applications
• Hexagonal boron nitride (h-BN)
  • Composed of alternating boron and nitrogen atoms arranged in a hexagonal lattice
  • Acts as a wide bandgap insulator (5.9 eV), exhibits high thermal conductivity, and offers excellent chemical stability (resistant to oxidation and acids)
• Phosphorene
  • Formed by a single layer of black phosphorus with a puckered honeycomb structure
  • Functions as a direct bandgap semiconductor (0.3-2.0 eV depending on the number of layers), displays high carrier mobility, and showcases anisotropic properties (direction-dependent electronic and optical behavior)

Growth mechanisms of 2D materials

• Plasma-enhanced chemical vapor deposition (PECVD)
  1. Decomposition of precursor gases (CH4, H2 for graphene; MoO3, S for MoS2) by plasma to generate reactive species
  2. Adsorption and surface diffusion of reactive species on the substrate surface
  3. Nucleation and growth of 2D material islands through chemical reactions and self-assembly
  4. Coalescence of islands to form a continuous film as growth progresses
• Plasma-assisted atomic layer deposition (PAALD)
  1. Sequential exposure of the substrate to precursor gases and plasma for layer-by-layer growth
  2. Self-limiting surface reactions ensure precise control over the deposited layer thickness
  3. Conformal coverage of complex substrate geometries (trenches, nanopores) due to the self-limiting nature of the process
• Plasma-surface interactions
  • Ion bombardment by energetic plasma species (Ar+, H+) leads to surface cleaning and defect creation, promoting nucleation sites for 2D growth
  • Plasma-induced surface functionalization (introduction of functional groups) enhances the adsorption and reaction of precursor molecules
  • Plasma-assisted etching selectively removes unwanted material (amorphous carbon, oxide layers) to improve the quality of the grown 2D material

Effects of plasma on 2D growth

• Plasma power
  • Higher power increases the density of reactive species and the growth rate of the 2D material
  • Excessive power can cause plasma damage (ion bombardment) and introduce defects in the 2D lattice
• Substrate temperature
  • Influences the surface diffusion and nucleation behavior of adatoms (adsorbed atoms) during growth
  • Higher temperatures generally promote larger grain sizes and better crystallinity of the 2D material
• Precursor gas composition and flow rate
  • Stoichiometry (elemental ratio) of the 2D material is determined by the ratio of precursor gases (CH4:H2 for graphene, MoO3:S for MoS2)
  • Flow rate affects the residence time of reactive species and the overall growth rate of the 2D material
• Pressure
  • Lower pressure results in a longer mean free path of plasma species, enabling more directional and anisotropic growth
  • Higher pressure increases the collision frequency and can lead to more uniform coverage of the substrate
• Scalability
  • Plasma-assisted techniques enable large-area synthesis of 2D materials beyond the limitations of mechanical exfoliation
  • Roll-to-roll processing and continuous growth methods (spatial ALD, PECVD) are promising for industrial-scale production of 2D materials

Properties of plasma-grown 2D materials

• Electronic devices
  • High-performance transistors, logic circuits, and memory devices based on plasma-grown 2D materials (graphene, MoS2)
  • Flexible and transparent electronics for wearable devices and displays using plasma-deposited graphene electrodes
• Optoelectronics
  • Photodetectors, light-emitting diodes (LEDs), and solar cells utilizing plasma-synthesized 2D materials (TMDs, phosphorene)
  • Tunable light absorption and emission properties through control of the 2D material thickness and composition during plasma growth
• Sensors
  • Gas sensors (NO2, NH3), biosensors (glucose, DNA), and chemical sensors (pH, ions) based on plasma-grown 2D materials
  • High sensitivity and selectivity due to the large surface-to-volume ratio and specific surface interactions of 2D materials
• Energy storage and conversion
  • Supercapacitors, batteries, and fuel cells incorporating plasma-synthesized 2D materials (graphene, MoS2) as electrodes or catalysts
  • Enhanced charge storage capacity and faster charge transfer kinetics compared to bulk materials
• Catalysis
  • Hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), and CO2 reduction using plasma-grown 2D materials (MoS2, WSe2) as catalysts
  • Abundant active sites and tunable electronic structure through plasma-induced defects and doping for improved catalytic performance