E. Anguiano

Electrochemical preparation of tungsten tips for a scanning tunneling microscope

We present experimental results obtained during the electrochemical preparation of tungsten tips for a scanning tunneling microscope. Experiments were done with direct current and two kinds of electrolytes widely reported in the literature: KOH and NaOH. We report the effects of the applied voltage, time of etching, tip length, electrolyte concentration, wire diameter, and immersed portion as relevant parameters in the process. From the images obtained by a metallurgical microscope and a scanning electron microscope as well as from Auger and electron diffraction x‐ray analysis the best conditions for W tip preparation were obtained. We found that KOH is better than NaOH as an electrolyte to prepare tips for scanning tunneling microscopy and that tip quality increases as the wire diameter and the immersed portion increases.

Fractal characterization by frequency analysis. I: Surfaces

A study of the quality and accuracy of the methods based on frequency analysis for the fractal characterization of surfaces as measured by scanning tunnelling microscopy (or profilometry) is made. The study is based on computer simulation of images of fractal surfaces. A discussion of the mathematical algorithms used for computer generation of fractal surfaces then follows. The main conclusion is that studies of fractal characterization by frequency analysis reported in previous papers in the STM field, as well as conclusions about the performance of the various methods, are doubtful. New methods for frequency analysis that in some cases produce more reliable results are proposed.

Fractal characterization by frequency analysis. II: A new method

A new frequency analysis method, fractal analysis by circular average (FACA), and an image replication procedure are proposed that together produce accurate measurements of the fractal dimension of surfaces and profiles, eliminating Fourier transform artefacts which arise from the lack of periodic continuity in real surfaces and profiles.

PROFILES FRACTAL CHARACTERIZATION BY FREQUENCY ANALYSIS

A discussion of the different methods for fractal profiles generation and of the methods for fractal characterization of profiles by frequency analysis is made in the whole range (1 < D < 2). We obtain the conclusion that all methods for measurement of fractal dimension that has been proposed are doubtful.

Analysis of scanning tunneling microscopy feedback system

A theoretical analysis of the feedback system in the scanning tunneling microscope (STM) is presented. The proposed model includes all the elements involved in the STM loop. The knowledge of its behavior allow one to accurately determine the region where the unstable STM operation could affect the measurements, and also to set the optimal working parameters. Each element of the feedback circuitry is analyzed and discussed as well as their mutual interactions. Thus the stability region of a STM has been obtained analytically, using all the possible elements in the feedback loop and without using simplifications or rare models for the system. The mathematical models for each element of the loop has been used and the problem has been solved using control theory. Different working conditions are simulated and analyzed. Some relations for stability conditions, considering the value of each component involved in the feedback loop, are proposed and analyzed. The good agreement between theory and experimentation ...

Optimal conditions for imaging in scanning tunneling microscopy: Theory

The scanning tunneling microscopy ~STM! feedback system is analyzed to look for the optimal conditions for measurement. The typical feedback circuit normally used in STM and the parameters involved on it are studied, and their relative importance into the loop are discussed. The analysis of the role of each parameter demonstrated the importance of a detailed knowledge of the instrument to assure that the images obtained are reliable. We obtained equations that involve the main parameters of the loop, and yield the optimal conditions for imaging taking into account stability, signal amplitude, and phase shift. Combining the stability conditions with the imaging conditions obtained in this work, we found values for the feedback parameters to perform optimal STM measurements. Moreover, with this work, we highlight the importance to include in further publications the value of the main parameters used to obtain STM images. The conclusion of our work is that in some cases ~in particular atomic resolution and fractal analysis ! STM users should pay more attention to the setting of the instrument.

Surface texture parameters as a tool to measure image quality in scanning probe microscope

The behavior of the texture parameters of surfaces imaged by scanning tunneling microscopy (STM) under different control conditions in the feedback loop is shown. The analysis of the main surface texture parameters such as the rms-roughness, the skewness, the kurtosis and the average wavelength obtained from STM images shows that they have a strong dependence on the values of the parameters used in the feedback loop for imaging regardless of the visual quality of the images that can be the same. Thus, surface texture parameters can be used to measure image quality in STM in relation to two nondimensional parameters (G and K) that described the measurement conditions.

Analysis of scanning tunneling microscopy feedback system: Experimental determination of parameters

We describe the experimental work necessary on a scanning tunneling microscopy (STM) system to obtain some important feedback parameters—to have stability and good imaging conditions—such as the mechanical resonance frequency ω0, the delay time τ, and the damping factor ζ. We study and analyze each one of the main components involved on the STM such as the feedback system, the piezoelectric elements, the rigidity of the mechanical structure (the mechanical resonance), and its relation with the scanning rate for imaging. We conclude that it is necessary to obtain these parameters with the STM in tunnel conditions, in order to consider the effects of the tip, surface contamination, and tip sample interaction. We also conclude that tripod based STM can be better than STM based on piezo tubes scanners.

A Wiener filter with circular-aperture-like point spread function to restore scanning tunneling microscopy (STM) images

Abstract With a suitable noise analysis and a relative Wiener filter, utilizing a Point Spread Function in analogy with optical cases, a good restoration of noisy fast-STM images can be achieved. Correlation between consecutive scans and integration along the scan direction could be the greatest disturbing facts.

The importance of imaging conditions in scanning tunneling microscopy for the determination of surface texture and roughness

Abstract We show that the control conditions in the feedback loop of a scanning tunneling microscope (STM) affect the values of the surface texture parameters, including fractal characterization, when they are calculated from STM images of the surface. The main surface texture parameters (such as the r.m.s. roughness, kurtosis, skewness, and average wavelength) show a strong dependence on the conditions used in the feedback loop for imaging. Fractal character changes also with feedback parameters. The quality of the STM images can be measured quantitatively by using some of the surface texture parameters.