An apparatus comprises a scanning electron microscope (SEM) (1) positioned over a manipulation chamber (2) which houses a sample holder (3). The walls of the manipulation chamber (2) support two probes (4, 4a) and the sample holder (3) is able to hold a sample (5), such as carbon nanotubes (10a) carried on a substrate (10). The apparatus can selectively move and apply voltages and currents to the probe or probes (4, 4a) and sample holder (3) under the SEM (1). By controlling the current that is passed across a contact between the probe (4) and a carbon nanotube (10a), a conditioned weld is formed. Likewise, by controlling the current that is passed along a carbon nanotube (10a), the nanotube (10a) can be annealed. Using both the probes (4, 4a) a carbon nanotube can be held and cut at any position along its length. This allows the formation of novel carbon nanotube structures.