7.1 Carbon nanotubes and graphene-based devices

Carbon nanotubes and graphene are revolutionary nanomaterials with unique properties. These carbon-based structures exhibit incredible strength, conductivity, and flexibility, making them ideal for next-generation electronic devices and sensors.

From ballistic transport to high electron mobility, CNTs and graphene outperform traditional semiconductors. Their applications range from ultra-fast transistors to flexible, wearable sensors, promising to reshape the landscape of nanoelectronics and MEMS/NEMS technology.

Structure and Properties of Carbon Nanotubes and Graphene

Carbon Nanotubes (CNTs)

• Consist of cylindrical carbon molecules with novel properties that make them potentially useful in many applications in nanotechnology, electronics, optics and other fields of materials science
• Can be considered as rolled-up graphene sheets (graphene is an allotrope of carbon)
• Have a very high length-to-diameter ratio (up to 132,000,000:1), significantly larger than any other material
• Possess extraordinary strength and unique electrical properties
• Are efficient conductors of heat
• Come in two main varieties: single-walled (SWCNT) and multi-walled (MWCNT)

Graphene Structure and Properties

• Consists of a single layer of carbon atoms arranged in a hexagonal lattice
• Is the basic structural element of other allotropes, including graphite, charcoal, CNTs and fullerenes
• Has many unusual properties:
  • Is nearly transparent
  • Absorbs ~2.3% of white light
  • Is one of the strongest materials known (tensile strength of 130 GPa and Young's modulus of 1 TPa)
  • Conducts heat and electricity very efficiently along its plane
• Has a high electron mobility at room temperature (15,000 cm2⋅V−1⋅s−1)
• Shows an anomalous quantum Hall effect

CNT and Graphene Characteristics

• Chirality: Determines the electrical properties of CNTs (can be metallic or semiconducting based on chirality)
• Single-walled CNTs (SWCNTs):
  • Consist of a single graphene sheet rolled into a seamless cylinder
  • Are an important variety of carbon nanotube because they exhibit electric properties that are not shared by multi-walled CNTs
• Multi-walled CNTs (MWCNTs):
  • Consist of multiple rolled layers (concentric tubes) of graphene
  • Have diameters ranging from 5 to 20 nm and lengths up to several micrometers
• Mechanical strength: Individual CNT shells have strength of ~100 gigapascals, which is approximately 10-fold higher than any industrial fiber
• Thermal conductivity: All nanotubes are expected to be very good thermal conductors along the tube, exhibiting a property known as "ballistic conduction"

Electronic Properties and Transport

Ballistic Transport

• Is the unimpeded flow of charge carriers in a material
• Occurs when the mean free path of the carriers is longer than the length of the material
• Results in conductance that is independent of length, and resistance that is dependent only on the width (number of modes)
• Is observed in CNTs and graphene due to their unique band structures and high electron mobilities
  • Example: In SWCNTs, electrons can travel micron distances without scattering under ideal conditions

High Electron Mobility

• Electron mobility measures how quickly an electron can move through a metal or semiconductor when an electric field is applied
• Both CNTs and graphene exhibit very high electron mobilities compared to traditional semiconductors like silicon
  • Example: Electron mobilities > 100,000 cm2V−1s−1 have been observed in suspended graphene at low temperatures
• High mobilities are a result of the unique electronic band structures in these materials
  • Graphene has a linear dispersion relation where electrons behave as massless Dirac fermions
  • Certain SWCNTs have band gaps that allow for semiconducting behavior with mobilities far exceeding silicon

Applications in Electronic Devices

CNT Field-Effect Transistors

• CNTs can be used as the channel material in field-effect transistors (FETs)
• Advantages over silicon FETs include:
  • Higher electron mobility allowing for faster switching speeds
  • Higher current densities allowing for smaller devices
  • Ability to be deposited on a wide range of substrates including plastics
• Challenges include:
  • Difficulty in separating metallic and semiconducting CNTs
  • Imprecise placement of CNTs on substrates
• Example applications: High-frequency transistors, chemical sensors, biosensors

Graphene Sensors

• Graphene's 2D structure and high conductivity make it ideal for sensing applications
• Graphene-based sensors have been demonstrated for a wide range of analytes including:
  • Gases (NO2, NH3, H2, CO)
  • Biomolecules (DNA, proteins, glucose)
  • pH
  • Strain and pressure
• Advantages include:
  • High sensitivity due to large surface-to-volume ratio
  • Fast response times
  • Potential for miniaturization and integration with flexible substrates
• Example applications: Wearable health monitors, environmental monitoring, food safety

Flexible Electronics

• Both CNTs and graphene can be deposited on flexible plastic substrates
• Allows for electronics that can bend, fold, stretch, and conform to curved surfaces
• Potential applications include:
  • Rollable displays
  • Wearable health monitors and medical implants
  • Smart clothing
  • Flexible solar cells
  • Skin-like pressure and strain sensors
• Challenges include:
  • Developing stable and reliable flexible contacts and interconnects
  • Encapsulation to prevent degradation
  • Integration of flexible power sources