C. Guthmann

Electrostatic forces between metallic tip and semiconductor surfaces

Atomic force microscopy used in the resonant mode is a powerful tool for measuring local surface properties: for example, the quantitative analysis of the electrical forces induced by the application of an electric field between a conductive microscope tip and a surface allows the determination of the tip/surface capacitance and the local surface work function. However, these quantitative analyses require knowledge of tip geometry. In this article, we show that the simple procedure of evaluating the tip curvature radius by fitting the variations of the electrostatic force with the tip‐surface distance is not always adapted to the case where one of the tip‐surface system elements is a semiconductor. However, particular experimental conditions are determined to overcome these difficulties.

Permanent polarization and charge injection in thin anodic alumina layers studied by electrostatic force microscopy

An electrostatic force microscope (EFM) and a Kelvin probe are used to characterize the charges embedded in thin anodic alumina layers of thickness ranging from 100 to 400 nm. Introducing a method for obtaining self-supported alumina layers, we exhibit the presence of positive charges at the metal/oxide interface of anodic alumina layers. These positive charges, together with the negative charges present at the surface of the anodic layer, induce a true polarization of the layer. The magnitude of this polarization depends on the conditions of preparation of the layers and can be well controlled. As a second step, we show the influence of this polarization on charge injection in these layers with EFM: charges of both signs may be injected in unpolarized layers whereas one cannot inject negative charges in polarized layers, which thus exhibit a diode-like behavior.

Van der Waals and capacitive forces in atomic force microscopies

In this article we show that in the atomic force microscopy experiments performed on a metallic surface, there is always a long range electrostatic force in addition to the van der Waals forces. This capacitive force is due to the contact potential between the tip and the surface and exists even without external applied potential. We have calculated this capacitive force for a real geometry of the tip–sample system and compared it to the van der Waals force calculated for the same geometry. We conclude that the electrostatic force is always dominant for a tip–surface distance larger than half of the tip radius of curvature.

In situ measurement of large piezoelectric displacements in resonant atomic force microscopy

In resonant atomic force microscopy (AFMR) the calibration of the tip–sample relative displacement remains a major problem. Commonly used PZT piezoceramics exhibit a nonlinear behavior response for large applied voltages. For low voltages applied to the piezoceramics (i.e., small corrugations), the calibration can be performed by measuring the height of known structures. For large displacements, the interferometric heterodyne detection used in the AFMR provides a relative tip–sample displacement up to 10 μm, without removing the piezo‐tube from the microscope. From these measurements, it was established that the piezosensitivity is not a constant parameter. Its averaged value during an excursion depends linearly on the applied voltage. With this system, routine controls are very easy and an example is given of the displacement corrections related to the nonlinearity of the piezo‐tube for the electrostatic interaction between the tip and a gold surface.

Contact electrification of high-K oxides studied by electrostatic force microscopy

In order to clarify the mechanisms of charge transfer on insulating surfaces by contact electrification, we performed charge-transfer experiments on high-K oxides using the tip of an electrostatic force microscope. In particular, we investigated the influence of the applied voltage between the tip and the surface and the contact duration on the amount of transferred charges on Al2O3. The electronic motion in the insulating material is analyzed in terms of hopping processes assisted by the electric field created by the tip inside the oxide. We show that this electric field must be described by a three-dimensional model. In this frame, the transfer mechanism is analyzed as an instantaneous wetting of the surface by the charges—the surface being a region of large trap concentration—followed by a progression of the charges inside the oxide.

Macroscopic two-dimensional Wigner asymmetric islands

Using two-dimensional macroscopic Wigner islands consisting of electrostatically interacting charged balls with millimetric size, we have investigated the behaviour of a finite number of particles confined in a mesoscopic elliptic frame. In particular, we have experimentally checked the influence of the asymmetry of the confinement potential on the ground state configuration. The results obtained are compared with published numerical results.

The lower end of the sublimation curve

The lower end of the solid-vapour transition curve is seldom indicated in thermodynamics textbooks, and sometimes erroneously. We show, on general model independent grounds, that the sublimation curve reaches the origin in the (T,p) plane, with a quasi-universal behaviour.

Microcanonical entropy and density of states for a macroscopic system

The microcanonical entropy, proportional to the logarithm of the number of accessible microstates of an isolated system, is given in practice by the density of states and is independent of the uncertainty delta E on the energy E. However, delta E/E is shown to determine the relative uncertainty delta S/S on the entropy.