The transition of the internal structure in microwave chemical-vapor- deposited carbon nanotubes is investigated using scanning electron microscopy and high-resolution transmission electron microscopy. By controlling the thickness of the iron catalyst layer, a sequence of carbon nanotube films was obtained with diameters ranging from a few nanometers to over 100 nm. Experiments have established that by continuous reduction of the Fe layer thickness to <1 nm, single- and double-wall carbon nanotube films can be produced, whereas for an Fe film thickness <1 nm, multiwall carbon nanotube films can be synthesized. It was also found that for an Fe thickness ≥5 nm, interlayers (i.e., bamboolike or periodically compartmentalized nanotubes) were formed, while for an iron thickness <2 nm the tubes were primarily hollow. For an intermediate Fe thickness the internal structure of the carbon nanotubes was a mixture of hollow and bamboolike. A growth model which considers bulk and surface diffusions of carbon into andor onto the Fe catalyst surface is proposed to describe this transition and the internal periodic structure
