Chemical Vapor Deposition

Chemical Vapor Deposition (CVD) is a versatile technique often used in the semiconductor industry for deposition of materials on various substrates. In the CVD process, a gas or vapor precursor is transformed into solids, such as thin films, powders, or various structured materials, inside a reactor. Chemical vapor deposition has also been used to produce carbon fibers, filaments, and tubular carbon materials for many years. Recently, CVD has been used to synthesize a variety of nanostructured materials, including carbon nanotubes and nanowires composed of various materials, as well as graphene and thin films. It is an inherently scalable method, providing an important path from research to production.

• CVD techniques include
• Thermal Chemical Vapor Deposition (thermal CVD)
• Low Pressure Chemical Vapor Deposition (LPCVD)
• Laser-Assisted Chemical Vapor Deposition (LACVD or LCVD)
• Hot Wire/ Hot Filament/ Catalytic/ Initiated Chemical Vapor Deposition (HWCVD/ HFCVD/ Cat-CVD/ iCVD)
• Plasma Enhanced Chemical Vapor Deposition (PECVD)
• Aerosol-Assisted Chemical Vapor Deposition (AACVD)
• Microwave Plasma Chemical Vapor Deposition (MPCVD)
• Metalorganic Chemical Vapor Deposition (MOCVD)

Comparison of CVD to Other Methods Used for Making Nanotubes

CVD

Chemical vapor deposition (CVD) has been found to provide a number of benefits over other nanostructure synthesis methods. Nanotubes are obtained at relatively low temperatures with high quality and purity. The comparatively simple process can provide nanotubes of long length and controllable diameter. Catalyst deposition on substrates allows for formation of novel structures. In addition, the CVD method is scalable to industrial production levels.

Arc Discharge

In the arc discharge method, nanotubes are found in soot produced in arc discharge with catalytic metals (e.g., Fe, Ni, Co). Two graphite rods, separated by a few millimeters, are connected to power supply. At 100A, carbon vaporizes and forms a hot plasma.

Unlike CVD, in arc discharge, it is generally difficult to control the location and alignment of nanotubes. The resultant nanotubes are produced in small quantities, often short with random sizes and orientations, and typically tangled, making some applications difficult. The method requires evaporation of carbon atoms from solid targets at very high temperatures and the products generally require purification.

HiPco (High Pressure Carbon Monoxide Conversion)

In the HiPco method, high pressure carbon monoxide (CO) is heated with a volatile catalyst precursor (e.g., Fe(CO)₅) at high temperature. The SWNT form in the gas phase and are removed by filter from flowing, recirculating CO.

Unlike CVD, the HiPco method requires pressurized carbon monoxide, very high process temperatures, and a metal catalyst that is difficult to remove at the end of process.

Laser Ablation or Vaporization

In the laser ablation method, nanotubes are produced by pulsed YAG laser ablation of a graphite target in furnace at temperatures near 1200°C. The target is hit with intense laser pulses, generating carbon gas from which the nanotubes are formed.

Unlike CVD, in the laser ablation method, it is difficult to control the location and alignment of the nanotubes produced. Small quantities of nanotubes result from this process and they are generally tangled, making some applications difficult. The method also requires evaporation of carbon atoms from solid targets at very high temperatures.