Atomic-sized lead ͑Pb͒ contacts are deposited and dissolved in an electrochemical environment, and their transport properties are measured. Due to the electrochemical fabrication process, deformation-induced mechanical strain is largely avoided, and we obtain conductance histograms with sharply resolved, individual peaks. Charge transport calculations based on density-functional theory for various ideal Pb contact geometries are in good agreement with the experimental results. Depending on the atomic configuration, single-atom-wide contacts of one and the same metal yield very different conductance values.

Atomic-scale transistors [1][2][3] based on metallic quantum point contacts were demonstrated recently. They allow controlled binary switching of an electrical current between a conducting ''on-state'' and a non-conducting ''off-state'' by means of an independent gate electrode. The devices that operate reproducibly at room temperature open fascinating perspectives towards quantum electronics and logics on the atomic scale. Even an integrated circuit consisting of atomic-scale transistors [3] as well as a nanoelectromechanical atomic switch [4] were shown. Here, we demonstrate a multilevel atomic quantum transistor that allows gate-controlled switching between different quantized conducting states. Multilevel logic and storage devices on the atomic scale are of great interest as they will allow more efficient data storage and processing with a smaller number of logical gates. Our experiments are combined with detailed computer simulations that provide a detailed understanding of the multilevel switching process. The results provide a basis for the future development of ultra-small devices for multilevel logics on the atomic scale.

We demonstrate the controlled and reproducible structuring of surfaces of muscovite mica with the tip of an atomic force microscope operated in contact mode under ambient conditions. By repeated scanning of the tip along a predefined pattern on a cleaved mica surface at forces between 100 nN and 4 µN, mechanically induced etching was observed on the atomic scale. Using silicon nitride tips, no tip wear was observed during structuring. No debris was found on the surface as a result of the structuring process, indicating the atomic-scale nature of the wear process. Line widths down to 3 nm were achieved, while at the same time patterns on a wide range of length scales between 5 nm and 100 µm were generated reproducibly. The results can be explained by abrasive wear on the atomic scale due to sliding friction. The experiments allow the study of tribochemistry and abrasive wear on the atomic and molecular scale. At the same time, they represent an approach for high-precision structuring of surfaces within a wide range of length scales.

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SummaryWe demonstrate a method that allows the controlled writing of metallic patterns on the nanometer scale using the tip of an atomic force microscope (AFM) as a “mechano–electrochemical pen”. In contrast to previous experiments, no voltage is applied between the AFM tip and the sample surface. Instead, a passivated sample surface is activated locally due to lateral forces between the AFM tip and the sample surface. In this way, the area of tip–sample interaction is narrowly limited by the mechanical contact between tip and sample, and well-defined metallic patterns can be written reproducibly. Nanoscale structures and lines of copper were deposited, and the line widths ranged between 5 nm and 80 nm, depending on the deposition parameters. A procedure for the sequential writing of metallic nanostructures is introduced, based on the understanding of the passivation process. The mechanism of this mechano–electrochemical writing technique is investigated, and the processes of site-selective surface depassivation, deposition, dissolution and repassivation of electrochemically deposited nanoscale metallic islands are studied in detail.