R. Coratger

Scanning force microscopy - With Applications to Electric, Magnetic and Atomic Forces

PART ONE: LEVERS AND NOISE 1. Mechanical properties of levers 2. Resonance enhancement 3. Sources of noise PART TWO: SCANNING FORCE MICROSCOPES 4. Tunneling detection systems 5. Capacitance detection systems 6. Homodyne detection systems 7. Heterodyne detection systems 8. Laser-Diode feedback detection systems 9. Polarization detection systems 10. Deflection detection systems PART THREE: SCANNING FORCE MICROSCOPY 11. Electric force microscopy 12. Magnetic force microscopy 13. Atomic force microscopy References Index

Investigations of surface properties of polymeric membranes by near field microscopy

Abstract Atomic force and scanning tunneling microscopes have been used to investigate the surface of polymeric ultrafiltration and microfiltration membranes. The atomic force microscope has been shown to resolve the features of the selective skin layer of the membranes providing information on both size and shape of pores as well as surface roughness of the skin. When applied to the study of ultrafiltration and microfiltration membrane for the determination of pore size characteristics, the scanning tunneling microscope has been shown to give similar results with those obtained using polyethylene glycols transport measurements. The microscopic techniques have discerned pore diameters in the range of 11 to 114 nm for 40 to 200 kDa MWCO SPS ultrafiltration membranes and confirmed a mean pore diameter of 96 nm for the 0.1 μm PVDF microfiltration membrane.

Scanning tunneling microscopy of defects induced by carbon bombardment on graphite surfaces

HOPG graphite samples are bombarded at room temperature with a 20 keV 12C+ ion beam and the surface topography is investigated by scanning tunneling microscopy (STM) for increasing doses. Lattice deformations and point defects are observed on the nanometer scale and several types of defects can be identified. The surface topography modifications appearing on the atomic scale are analyzed and theoretical damage calculations are done in order to associate the surface features after bombardment with the generation of the defects due to the ionic impact and subsequent cascade development. The experiments reported here show that the number of visible defects is not linearly dependent on the dose. Most of the defects observed by STM seem to result from interstitial migrations and lattice rearrangements following disorder generation by the cascade. It is also reported that the “atomic” resolution of the STM is lost when the graphite is supposed amorphous.

Effects of ion mass and energy on the damage induced by an ion beam on graphite surfaces: a scanning tunneling microscopy study

Abstract Scanning tunneling microscopy has been used to study the effects of ionic bombardment of a graphite surface. The low doses used allow for the characterization of single ion impacts for ions of increasing masses, as 14N+, 32S+, 40Ar+ and 131Xe+, and for various energies (from 15 to 40 keV). We show that each impact creates a small hillock on the surface whose size increases as a function of surface damage energy. These hillocks mainly result from the stresses induced in the near-surface region by the damage created during the first collisions of the cascade. Images on the atomic scale reveal local modifications of the structure such as crystallites or five-membered rings. Their presence indicates that nonlinear phenomena may occur when only a few atoms surrounding the impact area are considered. In addition, √3 × √R30° superstructures, localized next to some hillocks, are attributed to the effects of point defects in the electronic structure of graphite.

Comparative study of micro- and ultrafiltration membranes using STM, AFM and SEM techniques

Abstract In this work, Nuclepore ultrafiltration membranes are studied by various techniques (STM, AFM and SEM), in order to determine the method best suited for the study considered. AFM allows direct measurement of pore density and turns out to be highly reliable. However, in terms of surface corrugation, this technique remains limited at high magnifications. Despite the conductor film deposition needed for achieving a tunnel effect, STM yields the best resolution for defining surface corrugation. With respect to the study of ultrafiltration membranes, the resolution obtained from STM or AFM is much better than that of SEM whose electron beam also damages the polymer surface. These results are confirmed by the STM study of organic sulphonated polysulphone membranes.

Glassy cholesteric structure: thickness variation induced by electron radiation in transmission electron microscopy investigated by atomic force microscopy.

During the observation of glassy cholesteric liquid crystals in transmission electron microscopy (TEM), a new contrast is created or enhanced by electron radiation which has a direct relationship with the periodic microstructure of the specimen. In this paper, we investigate the variations of the sample thickness and mass density as possible causes of this irradiation contrast. By means of observations in atomic force microscopy (AFM) coupled to TEM, we compared the surface corrugations of non-irradiated and irradiated specimens. It is shown that the final contrast is the result of several processes. including fracture during ultramicrotomy and mass loss during irradiation. Mass loss acts as an etching, and hence results in a decrease of the sample thickness. The etching depends on the initial molecular orientation, thus evidencing the latent structure. An electron channelling mechanism is suggested to explain this behaviour.

Metal/n-CdTe interfaces: A study of electrical contacts by deep level transient spectroscopy and ballistic electron emission microscopy

This paper summarises the characteristics of chemically etched CdTe surfaces obtained by photoluminescence studies and the results of extensive transport measurements involving metal contacts fabricated on them. Fermi level pinning at five discrete levels; 0.40 +/- 0.02, 0.65 +/- 0.02, 0.73 +/- 0.02, 0.96 +/- 0.04 and 1.18 +/- 0.02 eV has been observed. Deep level transient spectroscopy (DLTS) and ballistic electron emission microscopy (BEEM) experiments have been performed on contacts made under the same conditions to compare and confirm these results. DLTS reveal similar values for electron traps in the band gap confirming a relationship between Fermi level pinning and bulk defect levels. BEEM experiments performed on contacts showing I-V barrier heights of 0.96 eV reveal significant planar non-uniformity but confirm 0.96 eV as the controlling barrier height or charge transport across the junction

Schottky barrier formation at metal/n-ZnSe interfaces and characterization of Au/n-ZnSe by ballistic electron emission microscopy

Current transport and ballistic electron emission microscopy (BEEM) studies have been carried out on metal contacts fabricated on chemically etched n-ZnSe epitaxial layers grown by molecular beam epitaxy. The contact materials Ag, Sb, Au, Ge/Au, Sn, Ni, and Pd form one or more barrier heights out of the following seven discrete values: 0.90, 1.20, 1.32, 1.50, 1.67, 1.80, and 2.10±0.04 eV observed to date. BEEM work carried out on Au/n-ZnSe systems has identified four levels 1.32 [Morgan et al., J. Appl. Phys. 79, 1532 (1996)], 1.50, 1.67 [Coratger et al., Phys. Rev. B 15, 2357 (1995)] and 1.80 eV to date, confirming Fermi-level pinning at different positions. Schottky barrier formation at metal/n-ZnSe systems cannot be explained by the simple Schottky model. The strong Fermi-level pinning observed could be due to bulk and/or surface defects of the ZnSe material.

A STAGE FOR SUBMICRON DISPLACEMENTS USING ELECTROMAGNETIC COILS AND ITS APPLICATION TO SCANNING TUNNELING MICROSCOPY

Coarse motion mechanisms have proven essential for STM investigations. The new system that we have built and that is presented here uses electromagnetic forces to clamp the feet of the ‘‘louse.’’ These forces arise from a current that flows through three electromagnetic coils. This system, fully computer automated, is found to give reliable approaches and allows lateral displacements of the sample with variable step sizes and hence, can also be used in other specific applications that require nanodisplacements.

Electrical characteristics of metal/semiconductor nanocontacts using light emission in a scanning tunneling microscope

Light emission from the tunneling junction of a scanning tunneling microscope (STM) has been used to obtain the electrical characteristics of small three-dimensional gold islands deposited on flat MoS2 surfaces. It is shown that these nanocontacts behave as poor rectifying junctions whose properties are altered by leakage currents. These properties are similar to those generally observed on point contacts where a sharp STM tip is brought in contact with a semiconductor.