Ph. Niedermann

A vertical piezoelectric inertial slider

We have developed a linear translation device using piezoelectric‐induced slip‐stick motion. Reproducible single steps of about 30 A, as well as continuous stepping with an overall translation speed of 0.25 mm/s, are routinely realized. The notable feature of this device is that this performance is achieved in the vertical orientation with the translator moving against gravity. This remarkable result is made possible using cycloidal functions instead of sawtooth signals to activate the motion. We have realized a very simple translator which can be used in any orientation with a displacement onset voltage of 15 V. The instrument was successfully tested in the temperature range from 1.6 to 300 K. Since no mechanical connections are required, this design is well suited for many applications, including scanning tunneling microscopy.

Chemical vapor deposition diamond for tips in nanoprobe experiments

Diamond is the most suitable material for many experimental methods in nanoprobe microscopy and materials testing. The extreme hardness, the high Young’s modulus, the inert nature of the surface, and the electrical conductivity obtained through doping make this material particularly attractive. We have coated silicon atomic force microscope (AFM) levers with thin (100 nm) doped diamond layers by chemical vapor deposition (CVD). A continuous diamond coating was obtained, resulting in tips with 100–200 nm radii. Owing to their electrical conductivity, these tips were found to be adequate for conducting AFM and scanning tunneling microscope applications, some of which are briefly discussed and reviewed in this article. We have also demonstrated CVD diamond tips, microfabricated in a controlled fashion, that have a 20 nm apex radius. These tips are particularly promising for nanomechanics and general AFM use.

Simple piezoelectric translation device

We describe a piezoelectric device which allows continuous movement and high‐resolution micropositioning, without distance limitation. Both mechanical construction and the electronics for the device are very simple. The movement is obtained via a stick‐slip mechanism, and steps as small as 10 nm are obtained. A displacement speed of 0.4 mm/s has been attained, and the device was capable of carrying several times its own weight, exerting a horizontal force, or climbing a plane inclined by 7°. Due to its compact construction, the device shows prospects for miniaturization.

Imaging of granular high‐Tc thin films using a scanning tunnelling microscope with large scan range

STM images are presented in the micron scale, taken in UHV with a single‐tube scanner STM with SEM control of the tip position. For calibration, a carbon grid was used from which the coupling of x and y scan axes has been determined as well as the piezo‐sensitivity factors. Images taken of a YBa2Cu3O7‐δ, film display the granular structure of the crystallites formed during the post‐evaporation annealing. A direct comparison of STM scans with SEM micrographs was made which demonstrates the complementary information obtained by the two methods. The obtained resolution is compared to the tip shape which becomes crucial for the imaging of corrugated surfaces on the micron scale. With a simple geometric model, an attempt has been made to reconstruct the surface topography from STM scans based on the knowledge of the tip shape.

A versatile low‐temperature scanning tunneling microscope

We present a new scanning tunneling microscope (STM) developed specifically for use at low temperatures within a standard helium cryostat. This compact STM (diameter 25 mm and length 61 mm) has been designed to be compatible with a superconducting magnet. It incorporates a novel approach system which operates in any orientation including the vertical one, without any mechanical connections to the STM. It is able to translate a sample by 30 A steps in any orientation. The coarse and the fine approach are driven by a single piezoelectric tube via inertial slip‐stick motion of the sample holder. During the coarse approach, the translator can be moved over 15 mm at 0.25 mm/s against gravity. We have operated this STM down to 1.6 K and we have obtained images with atomic resolution on highly oriented pyrolitic graphite (HOPG) and NbSe2 from 300 K down to 90 K. This partially thermal compensated instrument is able to monitor temperature dependent surface changes on atomic scale in real time.

Tc variation in PbMo6S8: A critical analysis and a comparison with pure phases

Abstract Different methods were employed to synthesize a series of high quality PbMo 6 S 8 Chevrel-phase compounds. All the samples were characterized by means of X-ray diffraction, Auger electron spectroscopy, specific heat, acsusceptibility, resistivity, scanning electron spectroscopy and microprobe analysis. A marked decrease in the superconducting transition temperature (T c ) and the hexagonal lattice constant ratio (c/a) were noticed on the samples prepared in quartz tubes. Based on a simple correlation between T c and c/a ratio, we present some of the significant structure-property relations and phase purity of the samples. This interdependence is discussed mainly in terms of oxygen content in the phase. For instance, oxygen-free samples have T c invariably above 14.5 K. Interestingly, the optimal T c is found in samples having rather different, from each other, crystallographic parameters, but with a same c/a ratio. On the other hand, the wide variation of T c and structural parameters of PbMo 6 S 8 observed in the present investigation and those of many earlier works are explained by the presence of varying amounts of oxygen in the ternary phase. These results are checked in parallel with the oxygen-doped (PbMo 6 S 8−x O x ; x ≥ 0.2) samples.

Study of field‐emitting microstructures using a scanning tunneling microscope

Field emission from broad metal cathodes is known to be strongly enhanced at a small number of emitting sites per cm2 compared to the expected Fowler–Nordheim emission from ideal, flat surfaces. We have operated a scanning tunneling microscope (STM) in the field emission regime (typical tip voltage: +80 V) and measured the local field emission strengths and variations on niobium samples. With a modulation technique, which is an adaptation of the standard work function measurement, maps of the field enhancement factor β have been obtained. An example of an emission site is presented where STM topograph and β map are compared with a secondary electron microscope image and with field emission data obtained in a standard way using high‐voltage anodes. This demonstrates the capability of a scanning tunneling microscope to localize enhanced field emission sites (with typical β values of 50 in the present work) with high spatial resolution and to study surfaces down to the limit β=1.

A small scanning tunnelling microscope with large scan range for biological studies

We have developed a scanning tunnelling microscope specially designed for biological applications presenting some new features: the scanner tube is mounted parallel to the surface of the sample which enables a high resolution optical microscope to be brought close to the sample when working in air or liquids. The maximum scan range is 5×20 μm with a vertical range of 20 μm and the total size of the system does not exceed 10×40 mm. The piezo‐sensitivity of the scanner tube versus applied voltage was analysed by interferometry measurements and by using scanning tunnelling microscopes. We found a value for the piezoelectric constant d13 of −1·71 Å/V at low voltages (under a few volts) going up to −2 Å/V for higher voltages. Large‐scale images of a carbon grid showed a surprisingly good linearity of the scanner tube.

A low-temperature scanning tunneling microscope with in-situ sample cleaving

Abstract We present the design of a scanning tunneling microscope (STM) that is operated in an ultra-high vacuum (UHV) chamber at room temperature and can be lowered into a standard helium crystat and cooled with helium exchange gas to low temperatures. Central to the design of our system is a compact concentric tube scanning tunneling microscope in which the central tube serves as the scanner and the outer tube allows linear inertial translation of the sample by slip-stick motion. The fact that no mechanical connections are needed to approach the sample and tip allows great flexibility in the design of the system and the vibration isolation of the STM. Samples are cleaved in-situ in the UHV chamber. The stability of the instrument is demonstrated with images of both graphite and the high-temperature superconductor Bi2Sr2Ca1Cu2O8 obtained in UHV (P = 2×10-10 Torr) at room temperature.

Studies of the surface structure of YBa2Cu3O7 thin films using STM

Abstract The surface structure of magnetron-sputtered YBa 2 Cu 3 O 7 thin films has been investigated using an STM operated in air. Both c -axis and a -axis oriented films have been investigated. In the case of c -axis thin films, different types of step and plateau structures are observed depending on the deposition conditions and substrates used (SrTiO 3 and MgO), suggesting different growth behavior. In some cases, very regularly spaced parallel plateaus separated by steps are found, suggesting directional step flow; whereas in other cases, more complex surface structures with circular plateaus and screw dislocations are seen. In the case of a -axis growth, the substrate determines the grain orientation and results in a cobblestone appearance of the film surface. The tunneling conditions necessary for obtaining stable and reproducible images of the surface are discussed as well as attempts to correlate the deposition parameters with the resulting film surface structure.