L. Stockman

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.

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.

Submicrometer lithographic patterning of thin gold films with a scanning tunneling microscope

A scanning tunneling microscope (STM) has been used to locally expose Langmuir–Blodgett layers of a negative electron beam resist (ω‐tricosenoic acid) on top of a thin gold film. The STM operates in a dry nitrogen atmosphere at a voltage difference of about −10 V between the electrochemically etched Pt‐Ir tip and the gold surface. After development in ethanol, the unexposed areas of the gold film are removed by argon ion milling. Gold wires with a linewidth down to 15 nm have been prepared. Electrical transport measurements confirm the homogeneity and the one‐dimensional metallic character at low temperatures of the gold structures.

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.

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

Nanolithographic patterning of Au films with a scanning tunneling microscope

We present a new lithographic technique with a scannning tunneling microscope (STM), allowing us to pattern thin evaporated Au films at the nanometer scale. The electron beam produced by the STM tip is used to expose a very thin layer of ω‐tricosenoic acid. Only four monolayers of the acid, which acts as an electron sensitive, negative resist, are deposited on top of the Au films using the Langmuir–Blodgett technique. We have verified the influence of the exposure conditions on the quality of the fabricated fine‐line structures with a linewidth down to 15 nm. We can also measure the electrical properties of narrow Au lines which interconnect large predefined contact pads. As expected, the low‐temperature magnetoresistance is strongly influenced by quantum interference effects.

Quantum interference and confinement phenomena in mesoscopic superconducting systems

The superconducting field (H)-temperature (T) phase boundary has been measured in mesoscopic AI samples of different topology: lines, open and filled squares, which were made under the same conditions from the same material. These samples clearly show different superconducting H-T phase boundaries which are nicely reproducing the predictions of the theoretical calculations made for their particular confinement geometries. The confinement of the flux lines by the lattice of the submicrometer holes has been studied in the Pb/Ge multilayers. A substantial enhancement of the critical current j, has been achieved. Sharp integer and rational matching peaks in the j,(H) curve are observed. The possibility of the “quantum design” of the superconducting critical parameters (H,(T) and jc(T, H)) of the mesoscopic and nanostructured superconductors by optimizing the confinement geometry for the superconducting condensate and for the flux lines has been demonstrated.

Topographic study of thin gold films grown on SiO2

Abstract The surface topography of thin Au films (thickness ∼ 10 nm) has been studied by STM in air. The films are obtained by flash evaporation onto liquid-nitrogen cooled, oxidized Si substrates. The polycrystalline layers have a grain size comparable to the thickness. The presence of cracks and holes near the grain boundaries, where the films tend to become discontinuous, is also confirmed by transmission electron microscopy. The film smoothness and continuity can be improved dramatically by evaporating the Au layers in a reduced O 2 or He atmosphere. The topographic information is also reflected by the electrical stability of submicron lines (width comparable to the grain size).

Electron transport in mesoscopic structures

Abstract Recent advances in electron beam lithography have allowed to prepare high-quality metal structures with submicrometer dimensions. At low temperatures the size of these structures can become smaller than the coherence length over which the electrons maintain their phase memory. Therefore, a mesoscopic regime can be reached, where the quantum interference between conduction electron waves becomes important. The interference effects can be conveniently tuned by applying a perpendicular magnetic field. Using a lift-off technique we have prepared various submicrometer Al and Au structures. In the superconducting Al loops the Ginzburg-Landau coherence length, which diverges near Tc, corresponds to the relevant characteristic length scale. Due to their mesoscopic sample size the Al loops behave as superconducting quantum interference devices (SQUIDs), even when no artificial Josephson junctions are present in the branches of the loop. The mesoscopic confinement of the superconducting wave function causes in addition anomalous Little-Parks oscillations of Tc when a magnetic field is applied. We have been able to further reduce the linewidth of the mesoscopic sample by using the tip of a scanning tunneling microscope (STM) for local exposure of a very thin Langmuir-Blodgett layer used as an electron sensitive resist.

SUB-20 nm Lithographic Patterning with the STM

Thin evaporated gold films have been patterned by locally exposing a Lang-muir-Blodgett layer of an electron beam resist with the tip of an STM. The 12 nm thick resist layer consists of 4 monolayers of ω-tricosenoic acid. The STM operates in a dry nitrogen atmosphere at a voltage difference of about —10V between the electrochemically etched Pt-Ir tip and the Au film. The ω-tricosenoic acid layer acts as a negative resist and the unexposed areas can be removed by argon milling. We have written submicrometer line structures which can be connected to larger predefined contact pads. Measurements of the temperature dependence of the electrical resistance confirm the metallic character of the Au lines. The low-field magnetoresistanee is in agreement with the theory for the one-dimensional weak electron localization. While linewidths down to 15 nm can be achieved, our STM patterning also allows to prepare larger arrays of nanoscale loops and dots.