Nanopipettes for Metal Transport K. Svensson, 1 H. Olin, 2 and E. Olsson 1 1 Department of Experimental Physics, Chalmers University of Technology and Go ¨teborg University, SE-412 96 Go ¨teborg, Sweden 2 Department of Engineering, Physics and Mathematics, Mid Sweden University, SE-851 70 Sundsvall, Sweden (Received 15 April 2004; published 30 September 2004) Here we demonstrate, for the first time experimentally, a nanopipette action for metals using multiwalled carbon nanotubes. The process relies on electromigration forces, created at high electron current densities, enabling the transport of material inside the hollow core of carbon nanotubes. In this way nanoparticles of iron were transported to and from electrically conducting substrates. DOI: 10.1103/PhysRevLett.93.145901 PACS numbers: 66.30.Qa, 61.48.+c, 68.37.Lp, 73.63.Fg The ability to manipulate materials on the nanometer scale is very important for the fabrication of future nano- scale devices. On the atomic scale the scanning tunnel- ing microscope (STM) has evolved into a powerful tool for manipulations of single atoms and molecules [1–3]. Although such manipulations may seem to be the ultimate goal, it is less useful for the manipulation and fabrication of nanometer scale features containing thousands of atoms. On this scale carbon nanotubes have been pro- posed to function as ‘‘fountain pens’’ or atomic pumps [4] for atoms, providing a continuous source of material. In their model, hot electrons created by a laser would drive material contained inside a carbon nanotube. The possi- bility of an electromigration process was discarded due to the need for, supposedly, too high electrical fields. Here we demonstrate experimentally how carbon nanotubes can be used as ‘‘nanopipettes’’ in order to deposit and retrieve solid material on a nanometer scale. The process relies on electromigration forces [5] forcing material to move inside the hollow core of a multiwalled carbon nanotube (MWNT). Our setup is based on a recently developed instrument [6] providing a movable STM probe inside a transmis- sion electron microscope (TEM) with a regular side- entry stage, here a CM200 field emission gun supertwin TEM with a Compustage. The STM is controlled by commercial software and electronics from Nanofactory Instruments AB [7]. A sharp gold tip is attached to the movable end of a piezoelectric tube (see Fig. 1), facing the sample and oriented perpendicular to the electron beam of the TEM. The sample consists of MWNTs filled with iron [8] that are attached to a metal wire by using electri- cally conducting glue. The movable tip is used to approach individual MWNTs and to make an electrical contact. By driving a high current through the nanotube the en- trapped iron will start to migrate in a direction opposing the electric field, i.e., in the direction of the electron flow. Figure 2 shows a gold tip in contact with iron filled carbon nanotube inside the TEM. As a current is passed through the nanotube, the iron core first breaks up into smaller particles [Figs. 2(a) and 2(b)] and these then begin to move in the direction of the electron flow [Fig. 2(c)]. A movie of this process is available in the EPAPS [9]. It was also observed that the moving iron particles could change the microstructure of the nanotubes. We have measured the threshold currents for iron dif- fusion in various sized nanotubes [Fig. 3(a)]. By studying the current, instead of the voltage, we can avoid the limitations of two-point measurements. The values follow a parabolic behavior, indicating that there is a threshold in the current density [of about 7  10 6 A=cm 2 , Fig. 3(b)] rather than in the current. At these high current densities electromigration processes become important [10]. The electrical resistance of the nanotubes in this study ranged from about 4–40 k (two-point measurement at the elec- tromigration threshold with biases of around 1 V). The resistance depends on the dimensions of the tube and is plotted as a function of the nanotube diameter in Fig. 3(c). The data reveal a resistance that is inversely gold tip nanotube iron TEM-holder piezoelectric tube sample FIG. 1 (color). Schematic drawing of the setup used here. A sharp metal tip is attached to a piezoelectric actuator and is used to make electrical contacts to individual, iron filled, carbon nanotubes protruding from the sample. VOLUME 93, NUMBER 14 PHYSICAL REVIEW LETTERS week ending 1 OCTOBER 2004 145901-1 0031-9007= 04=93(14)=145901(4)$22.50  2004 The American Physical Society 145901-1