Low-temperature scanning tunneling microscope for investigations in magnetic fields

A scanning tunneling microscope (STM), equipped with an attachment for in situ sample cleavage and capable of low-temperature measurements in the presence of a strong magnetic field, is described, Details of the constructions of the cryostat insert, mechanical head, and cleavage mechanism are described. Performance of the STM is demonstrated by the results of investigation of the single-crystal surface of InAs, Ga, and high-temperature superconductor BiSiCaCuO ceramics. The tunneling gap in the magnetic field is about 15 nm/T, and the transverse STM probe point shift, about 3 nm/T

Electrical Measurements On Submicronic Synthetic Conductors - Carbon Nanotubes

The synthesis of very small samples has raised the need for a drastic miniaturization of the classical four-probe technique in order to realize electrical resistance measurements. Two methods to realize electrical contacts on very small fibers are described here. Using classical photolithography the electrical resistivity of a submicronic catalytic chemical vapour deposited filament is estimated. Scanning tunneling microscopy (STM) lithography allowed to attach small gold contacts to a small bundle (diameter 50 nm) of carbon nanotubes. This bundle is found to exhibit a semimetallic behavior at higher temperature and an unexpected drop of the electrical resistivity at lower temperature

Electrical-resistance of a Carbon Nanotube Bundle

The first direct electrical resistance measurements performed on a single carbon nanotube bundle from room temperature down to 0.3 K and in magnetic fields up to 14 T are reported. From the temperature dependence of the resistance above 2 K, it is shown that some nanotubes exhibit a semimetallic behavior akin to rolled graphene sheets with a similar band structure, except that the band overlap, DELTA almost-equal-to 3.7 meV, is about 10 times smaller than for crystalline graphite. In contrast to graphite which shows a constant low-temperature resistivity, the nanotubes exhibit a striking increase of the resistance followed by a broad maximum at very low temperatures. A magnetic field applied perpendicular to the sample axis decreases the resistance. Above 1 K, this behavior is consistent with the formation of Landau levels. At lower temperatures, the resistance shows an unexpected drop at a critical temperature which increases linearly with magnetic field. These striking features could be related to the unique quasi-one-dimensional structure of the carbon nanotubes

Quantum transport in a multiwalled carbon nanotube

We report on electrical resistance measurements of an individual carbon nanotube down to a temperature T = 20 mK. The conductance exhibits a lnT dependence and saturates at low temperature. A magnetic field applied perpendicular to the tube axis increases the conductance and produces aperiodic fluctuations. The data find a global and coherent interpretation in terms of two-dimensional weak localization and universal conductance fluctuations in mesoscopic conductors. The dimensionality of the electronic system is discussed in terms of the peculiar structure of carbon nanotubes

Nanowire bonding with the scanning tunneling microscope

We have developed a reliable lithographic method to pattern thin gold films by locally exposing a thin layer of an electron beam resist with the tip of a scanning tunneling microscope (STM). The exposure of the resist layer is induced by applying a voltage difference of ca. -10V between the STM tip and the gold film on top of which the resist layer has been deposited with the Langmuir-Blodgett technique. Our resist material is omega-tricosenoic acid which acts as a negative resist. After development, the unexposed areas of the gold film can be removed via argon ion milling. We have been able to obtain continuous gold lines having a width down to 15 nm, the linewidth being determined by the exposure dose. When reducing the tunneling voltage <5 V, exposure of the Langmuir-Blodgett resist layer no longer occurs and one switches to the classical topographic imaging mode. This switching provides us with the unique possibility to attach electrical contacts to existing nanostructures. As a nice example of this nanowire bonding, gold contacts have been attached to individual multiwalled carbon nanotubes. We have made detailed measurements of the nanotube resistance as a function of temperature down to 10 mK and in magnetic fields up to 14 T. Al low temperatures a pronounced negative magnetoresistance is observed which is consistent with the two-dimensional weak electron localization occurring in the cylindrical graphite layers forming the nanotubes. The nanotubes also show reproducible fluctuations of the magnetoresistance which can be related to the Aharonov-Bohm effect in the nanotubes. (C) 1997 Elsevier Science B.V

Nanolithographic Patterning of Metal-films With a Scanning Tunneling Microscope

We have developed a new technique, which allows to pattern thin evaporated Au films at the nanometer scale with a scanning tunnelling microscope (STM). The STM tip is used to expose a very thin layer of omega-tricosenoic acid, which has been deposited on top of the Au films using the Langmuir-Blodgett technique and acts as an electron sensitive, negative resist. We have fabricated narrow Au lines with a width down to 15 nm and we have checked that our STM lithography does not degrade the metallic properties of the Au films. The power of our lithographic patterning technique is nicely illustrated by the fact that we can attach electrical gold contacts to a small bundle (total diameter of 50 nm) of carbon nanotubes (''Buckey tubes'') and measure the electrical resistance of the bundle as a function of temperature and magnetic field