摘要:

In an atmospheric environment and in water, the tunneling barrier signals are usually very low compared with those measured in the ultrahigh vacuum. The tunneling barrier data presented in this paper show that the barriers in air and in distilled water have a statistical character. The barrier mean value depends not only on the tip and specimen, but also on the medium in the tunneling gap. By repeated scannings of specimen surfaces, it is found that the barrier heights connected to individual specimen locations are reproducible. This is illustrated by repeated scans on a (TiN on W)/water interface obtained with a tungsten tip. The areas of TiN and W are clearly distinguished. Measurements of tunneling barrier heights are therefore suited for nucleation studies of surface coatings and for the detection of different material components in a specimen.

ISSN:0734-211X    

DOI:10.1116/1.587749

出版商:American Vacuum Society    

年代:1994

数据来源: AIP

摘要:

With a specially developed double‐tip scanning tunneling microscope the influence of a lateral electric field on the ordering of the liquid‐crystal molecules (10CB) was observed. In this setup two independent scanning units could work on a common substrate with different tunneling voltages and currents. No interference between the two tunneling currents was observed down to tip distances of 2 μm. This result means that it is unlikely that surface currents are a major contribution to the imaging mechanism on nonconducting molecules. The additional electric field was observed to alter the alignment of the molecules on the substrate. Modified scanning tunneling spectroscopy by tip voltage modulation was used to characterize the conductivity of the phenyl and alkyl parts of the molecules at tip potentials between 0 and 1.2 V.

ISSN:0734-211X    

DOI:10.1116/1.587750

出版商:American Vacuum Society    

年代:1994

数据来源: AIP

摘要:

We have systematically studied the growth mechanism and surface topography of YBa2Cu3O7−δ(YBCO) and Bi2Sr2Ca0Cu1Oy‐2201 (BSCCO) epitaxial grown superconducting films by scanning tunneling microscopy. Here we report two growth mechanisms of screw dislocation growth and layered growth; the surface characterization; and surface modification of thin films under a controlled manner. Excimer laser ablation technique was applied to synthesize high‐quality epitaxial thin films. YBCO thin films were epitaxially grown withcaxis perpendicular to the SrTiO3(100) substrate surface. If the YBCO films were grown on the substrate which normal is slightly off the ideal [001] orientation, the films show layered growth. Whereas on the flat substrate, the films were nucleated and grown with the screw dislocation manner. By using the effect of field‐induced evaporation, we were able to analyze the initial stage of growth. Epitaxial grown BSCCO‐2201 films on ZrO2substrates were synthesized and studied by scanning tunneling microscopy (STM). Layered growth and larger atomic flat areas were observed, which are more stable under STM than YBCO films. No screw dislocations were identified in BSCCO‐2201 films. The reasons of the different behaviors of the two kinds of thin films are discussed.

ISSN:0734-211X    

DOI:10.1116/1.587751

出版商:American Vacuum Society    

年代:1994

数据来源: AIP

摘要:

An understanding of the interaction between tip and sample in atomic‐force microscopy is needed to interpret atomic‐force‐microscope (AFM) images. In contact mode, the strength of the van der Waals (vdW) force sets image resolution; in noncontact mode, local gradients in the vdW force are imaged. By immersing tip and sample in an appropriate fluid, we can decrease the vdW forces and even change their sign. Selecting a fluid that leads to a small repulsive vdW force can greatly improve image resolution and eliminates problems caused by the well‐known tip‐snapping instability. To measure the vdW interactions produced by different fluids, we have developed ways to calibrate the spring constant and sharpness of AFM tips and to measure accurately the Hamaker constant of vdW interactions. These techniques show that the AFM can be used for local force measurements with an accuracy approaching that of surface‐force apparatuses. As an example, we have observed the crossover from nonretarded to retarded vdW forces between a silicon nitride tip and a mica sample in air.

ISSN:0734-211X    

DOI:10.1116/1.587752

出版商:American Vacuum Society    

年代:1994

数据来源: AIP