NOVELTY - Separating carbon nanotubes by wall number optionally by electronic type, outer wall diameter, or outer wall chirality involves centrifuging carbon nanotube composition in contact with first fluid medium contain first density gradient; and separating the carbon nanotube composition into at least two separation fractions, where the carbon nanotube composition contains at least one surface active component and carbon nanotube population contain double-walled carbon nanotubes.    USE - For separating carbon nanotubes by wall number optionally by electronic type, outer wall diameter, or outer wall chirality (claimed); for manufacture various electronic, optical, or optoelectronic devices such as field-effect transistors, transparent conductors, interconnect devices, sensors, light-emitting diodes, and solar cells.    ADVANTAGE - The method provides highly nanotube populations that are highly enriched with double-walled carbon nanotubes (DWCNT), and are also enriched chirality, diameter and/or electronic type to ensure that their electrical and optical properties are uniform. The high-purity DWCNT populations can offer various improved properties over prior art DWCNT populations such as improved properties can realized when incorporated into devices and applications such as light emitting diodes, photovoltaics, electrostatic discharge coatings, atomic force microscope tips, flat panel displays, touch screens, electromagnetic screening applications, field-emission displays, biosensors, field-effect transistors, transparent conductors or functional glass, and other functional composite materials including drug delivery materials.    DETAILED DESCRIPTION - Separating carbon nanotubes by wall number optionally by electronic type, outer wall diameter, or outer wall chirality involves centrifuging carbon nanotube composition in contact with first fluid medium contain first density gradient; and separating the carbon nanotube composition into at least two separation fractions, where the carbon nanotube composition contains at least one surface active component and carbon nanotube population contain double-walled carbon nanotubes and carbon nanotubes has wall number other than two, and at least one separation fractions comprises carbon nanotube subpopulation comprise higher percentage of double-walled carbon nanotubes than the carbon nanotube population. INDEPENDENT CLAIMS are included for the following:    (1) a carbon nanotube population comprising greater than 95% double-walled carbon nanotubes;    (2) a population (P1) of double-walled carbon nanotubes, where greater than 50% of the double-walled carbon nanotubes comprise a metallic outer wall;    (3) a transparent conductive film comprising the population (P1);    (4) a thin film transistor comprising semiconductor component contain the population (P1); and    (5) a population (P2) of double-walled carbon nanotubes, where greater than 30% of the double-walled carbon nanotubes comprise an outer wall having the same chirality (n,m) type.