title: Characterization of nanoparticles
context: The characterization of nanoparticles is a branch of nanometrology that deals with the characterization of physical and chemical properties of nanoparticles.  Nanoparticles have at least one primary external dimension of less than 100 nanometers, and are often engineered for their unique properties.  Nanoparticles are unlike conventional chemicals in that their chemical composition and concentration are not sufficient metrics for a complete description, because they vary in other physical properties such as size, shape, surface properties, crystallinity, and dispersion state.
question: What unit is used to measure nanoparticles?
answers.text: nanometers