Carbon nanotubes (CNT) are a new fascinating material that have been discovered in 1991 by S. Ijiama [1]. Fabrication of nanotubes is based on the sublimation of carbon under an inert atmosphere, by using various methods such as electric arc discharge, laser ablation [2] or gashes catalytic growth [3]. Depending on the process the results are either multi-wall nanotubes (MWNT) or single-wall nanotubes (SWNT). Typically one receives structures with a length of several µm and a diameter between 1 - 5 nm or up to 50 nm for SWNT or MWNT, respectively. Geometrically carbon nanotubes can be regarded as a honeycomb lattice rolled into a seamless cylinder with buckeyballs at the ends. Depending on diameter and on chirality the electronic conductivity of carbon nanotubes exhibits both metallic and semiconducting behavior [4]. That is besides their geometry why SWNT built together in network are a promising material for future nanoelectronic-circuits.

Apart from the difficulties of creating a network, there are other problems to solve. Two of them are treated in our institute:

• Characterization of CNT in diverse gas atmospheres with a Scanning Tunneling Microscope
• Connection of CNT to existing electrodes by using a new CVD nanolithography technique

[1] Nature, 354, 56 (1991)
[2] Appl. Phys. A 67, 1 (1997)
[3] Chem. Phys. Lett. 313, 91 (1999)
[4] R. Saito, G. Dresselhaus, M.S. Dresselhaus: Physical Properties of Carbon Nanotubes, Imperial College Press (1998)

It is well known that the electronic conductivity of Carbon Nanotubes (CNT), particularly of single-wall nanotubes (SWNT), depends on the diameter and on the chirality of the tube. With a scanning tunneling microscope (STM) the group of M. Tokumoto and Y. Achiba [1] could confirm the theoretical predictions [2] by imaging a SWNT and by using the feature of Scanning Tunneling Spectroscopy (STS). STS is done by positioning the tip over the tube and then while the feedback loop is unlocked ramping the voltage and measuring the current. The measured I-V-spectra give information about the local density of states (LDOS) and let one decide whether the tube has metallic or semiconducting character. Apart from the influence of the geometrical factors on the electronic conductivity, several experiments [3-10] showed that electronic conductivity varies strongly when CNT are exposed to different gas atmospheres. In most of those experiments the change of conductivity and of thermoelectric power (TEP) were measured on mats of different nanotubes, so called 'Bucky-Papers'. The mixture of semiconducting and metallic nanotubes disables one to attribute exactly the observed behavior to one type of CNT. A better approach is either to use a SWNT transistor as a gas sensor [7] or to determine the LDOS with STS [3]. The latter method is used at our institute: A self build ultrahigh vacuum scanning tunneling microscope (UHV-STM) enables us to image CNT (-> gallery) and to determine the LDOS by STS. The STM chamber can be filled controllable with different types of gases. As it is known that oxygen has a great influence on conductivity [3-8], TEP [3-6] and LDOS [3], first experiments are done in oxygen atmosphere at our institute. Research on the sensibility of CNT to gases is motivated by physical and technical aspects. Adsorption experiments can improve the knowledge of that new material. In the great field of chemical noses CNT-sensors could play an important role as shown in [7].

[1] A. Hassanien, M.Tokumoto, Y.Kumazawa, H.Kataura, Y.Maniwa, S.Suzuki, Y.Achiba, appl. phys. lett. 73, 3839, 28.12.1998
[2] R. Saito, G. Dresselhaus, M.S. Dresselhaus: Physical Properties of Carbon Nanotubes, Imperial College Press (1998)
[3] Extreme Oxygen Sensitivity of Electronic Properties of Carbon Nanotubes, P.Collins, A. Zettl, science 287, 1801 -1804, 10.03.2000
[4] Is the Intrinsic Thermoelectric Power of Carbon Nanotubes Positive?, K. Bradley, A. Zettl, phys. rev. lett. 85, 4361-4364, 13.11.2000
[5] Carbon Nanotubes: A thermoelectric nano-nose, C.K.W. Adu, P.C. Eklund, chem. phys. lett. 337, 31-35, 30.03.2001
[6] Effects of Gas Adsorption and Collisions on Electrical Transport in SWCNT, G.U. Sumanasekera, P.C. Eklund, phys, rev. lett. 85, 1096-1099, 31.07.2000
[7] Nanotube Molecular Wires as Chemical Sensors, J. Kong, H. Dai, science, 287, 622-625, 28.01.2000
[8] Fluorination of SWCNT, E.T. Mickelson, R.E. Smalley, J.L. Margrave, chem. phys.lett. 296, 188-194, 30.10.1998
[9] Reversible Oxidation Effect in Raman Scattering from Metallic SWCNT, Z. Yu, L.E. Brus, J. of Phys. Chem. A, 104 #47, 10995-10999, 30.11.2000
[10] Conductivity enhancement in SWCNT bundles doped with K and Br

Diplomarbeit von Arnim Höchst:
Rastertunnelmikroskopische Untersuchungen an Kohlenstoff-Nanoröhren

Posterpräsentation auf der DPG-Tagung 2002:
STM-Aufnahmen und Tunnelspektroskopie an Kohlenstoff-Nanoröhren

This picture shows a bundle of CNT on a gold surface and was taken with the UHV-STM of our institute. In the rope one can distinguish several single-wall nanotubes.

In this STM-picture one can see several NT-bundles running into each other. The blue rectangled marked area is presented in the following image showing SWNT