vaportransport

Vapor transport is a materials-processing technique in which material moves in the gas phase from a source region to a deposition region within a sealed container. It is commonly realized as chemical vapor transport (CVT), in which a transport agent reacts with a solid to form volatile species that transport the material to a cooler part of the system, where the volatile compounds decompose and deposit the element as a crystal or polycrystal. The process relies on a temperature gradient; the hot end promotes volatility, while the cool end favors deposition. The overall chemistry is governed by reversible transport reactions and by the chosen agent, which can be halogen species such as iodine, chlorine, or bromine, or other gases that form volatile intermediates. Common configurations use quartz tubes with dual heating zones to achieve a well-defined gradient and a low-pressure or inert atmosphere to limit side reactions.

Vapor transport enables growth of high-purity crystals that are difficult to obtain by direct sublimation, including transition metal dichalcogenides, oxides, and halides. It offers good control over stoichiometry through the choice of transport agents and temperature profile, but progress can be slow and sensitive to gas composition, impurities, and tube sealing quality. Safety concerns include handling toxic or corrosive halogen gases and maintaining sealed vessels under pressure.