J. Colchero

WSXM: A software for scanning probe microscopy and a tool for nanotechnology

In this work we briefly describe the most relevant features of WSXM, a freeware scanning probe microscopy software based on MS-Windows. The article is structured in three different sections: The introduction is a perspective on the importance of software on scanning probe microscopy. The second section is devoted to describe the general structure of the application; in this section the capabilities of WSXM to read third party files are stressed. Finally, a detailed discussion of some relevant procedures of the software is carried out.

Combined scanning force and friction microscopy of mica

A scanning force microscope using the optical lever detection method was modified to measure simultaneously the force normal to the sample surface and the friction force arising from scanning. The bending of sheet-like cantilevers is used to detect the normal force whereas the twisting of the same cantilever measures the friction force. The two effects cause, to first order, orthogonal deflections of the light beam and can therefore be measured simultaneously and independently. The relationship between normal and frictional forces and the resulting deflection angles is discussed. The authors present constant-force topographs and friction images of the surface unit-cell structure of mica and of single-layer steps on mica.

Jumping mode scanning force microscopy

In this letter, we present a new scanning probe microscopy mode, jumping mode, which allows the simultaneous measurement of the topography and of some other physical property of the sample. Essentially, at each image point first the topography of the sample is measured during a feedback phase of a cycle, and then the tip–sample interaction is evaluated in real time as the tip is moved away and towards the sample. Since the lateral motion is done out of contact the method is free, or nearly free, of shear forces. The general advantages of jumping mode are discussed. Finally, two different applications of this mode are presented. In addition to the topography, the first application measures the adhesion between the tip and the sample, while the second determines the corresponding electrostatic interaction.

DNA height in scanning force microscopy

The measured height of DNA molecules adsorbed on a mica substrate by scanning probe microscopy is always less than the theoretical diameter. In this paper we show that, when imaged in ambient conditions, the molecules are usually immersed in the salt layer used to adsorb them to the substrate. This layer distorts the measurement of DNA height and is the main source of error but not the only one. We have performed different experiments to study this problem using two scanning force techniques: non-contact tapping mode in air and jumping mode in aqueous solution, where the dehydration phenomena is minimized. Height measurements of DNA in air using tapping mode reveal a height of 0.7+/-0.2nm. This value increases up to 1.5+/-0.2nm when the salt layer, in which the molecules are embedded, is removed. Jumping experiments in water give a value of 1.4+/-0.3nm when the maximum applied force is 300pN and 1.8+/-0.2nm at very low forces, which confirms the removal of the salt layer. Still, in all our experiments, the measured height of the DNA is less than the theoretical value. Our results show that although the salt layer present is important, some sample deformation due to either the loading force of the tip or the interaction with the substrate is also present.

Contactless experiments on individual DNA molecules show no evidence for molecular wire behavior

A fundamental requirement for a molecule to be considered a molecular wire (MW) is the ability to transport electrical charge with a reasonably low resistance. We have carried out two experiments that measure first, the charge transfer from an electrode to the molecule, and second, the dielectric response of the MW. The latter experiment requires no contacts to either end of the molecule. From our experiments we conclude that adsorbed individual DNA molecules have a resistivity similar to mica, glass, and silicon oxide substrates. Therefore adsorbed DNA is not a conductor, and it should not be considered as a viable candidate for MW applications. Parallel studies on other nanowires, including single-walled carbon nanotubes, showed conductivity as expected.

Mechanical and thermal effects of laser irradiation on force microscope cantilevers

In an optical lever set-up one or two modulated laser beams of 0,1 to 6 mW modulation amplitude at a wavelength of 670 nm were focused at uncoated and gold-coated microfabricated cantilevers. The motion of the levers was analyzed by an optical lever set-up. The mechanical resonance (30 to 60 kHz) of the cantilevers was excited by the modulated light both in air and under vacuum conditions (10 6 mbar), The measured resonance frequencies and the width of the resonances were identical to the values found by exciting the cantilevers by piezo ceramics. At low frequencies under vacuum conditions, we found an increase of the oscillation amplitude with decreasing frequency. The time constant of this increase is of the order of 5 ms. At the resonance frequency of uncoated cantilevers light pressure effects dominate thermal effects; the resonance is thus excited by light pressure. Gold-coated cantilevers, however, are driven by the bimetal effect, even above 10 kHz. A possible application of the light pressure effects is the use of a modulated light beam in the attractive mode operation of a scanning force microscope to excite the cantilever oscillation.

Electrostatic force gradient signal: resolution enhancement in electrostatic force microscopy and improved Kelvin probe microscopy

In the present work the electrostatic interaction of a real scanning force microscopy (SFM) probe with a sample is studied theoretically as well as experimentally. To model the probe, a complex system composed of a macroscopic cantilever, a mesoscopic tip cone and a nanometric tip apex is proposed. The corresponding interaction is calculated analytically by means of an appropriate approximation. In most experimental situations we find that the total interaction is dominated by the cantilever and/or the tip cone and not by the tip apex. Experimental determination of tip–sample interaction supports this model. In addition, we find that a real SFM probe may lead to misinterpretation of experimental data in the so-called Kelvin probe microscopy (KPM). Again, experimental data confirm that the effects described by the model we propose may induce severe errors in KPM. As shown in this work, the resolution in KPM and electrostatic force microscopy is dramatically enhanced and data interpretation simplified if the force gradient rather than the force is used as signal source for the electrostatic interaction.

Lock‐in technique for measuring friction on a nanometer scale

A method for measuring friction forces on a nanometer scale is described. This method combines a lock‐in technique with scanning force and friction microscopy. Essentially, a lock‐in amplifier is used to determine the amplitude of the friction loop, which is measured at high frequency. To demonstrate the capability of this method, the dependence of the friction force with normal load is measured and a two dimensional image is presented.

Facets evolution and surface electrical properties of nonpolar m-plane ZnO thin films

ZnO thin films have been grown along the nonpolar [101¯0] direction by metal organic vapor phase epitaxy. The ZnO (101¯0) surface develops well defined facets. The orientation of the topographic normals reveals that the inclination angle of the facets increases as thicker films are considered, attaining a maximum value of about 28.4°. This angle corresponds to {101¯1}- and {101¯1¯}-type facets. The origin of this faceting is discussed in terms of thermodynamic stability and kinetics arguments. The surface electrical properties of the facets have been studied by Kelvin probe microscopy, showing that the surface has different contact potential domains that alternate along the polar [0001] direction.

Scanning force microscopy jumping and tapping modes in liquids

In this work theoretical considerations of the performance of scanning force microscopy jumping mode and tapping mode in liquids are discussed. A priori, jumping mode should improve in a liquid environment compared to in air while the situation for tapping mode should become worse. In order to confirm this we present jumping and tapping mode images of DNA molecules absorbed on a mica substrate immersed in water. The experiments demonstrate that jumping mode is a suitable scanning force microscopy method by which to image soft samples in liquid and that it has similar or even better performance than those exhibited by tapping, but without the complex experimental requirements of this mode.