G. C. McGonigal

Bio: G. C. McGonigal is an academic researcher from University of Manitoba. The author has contributed to research in topics: Scanning tunneling microscope & Grain boundary. The author has an hindex of 9, co-authored 11 publications receiving 587 citations.

Imaging alkane layers at the liquid/graphite interface with the scanning tunneling microscope

Abstract: We have directly imaged n‐alkane layers adsorbed at the liquid/graphite interface using a scanning tunneling microscope. The layers possessed a high degree of two‐dimensional ordering. The adsorbate was observed to enhance the tunneling current, and the atomic structure of the images was dominated by features associated with the substrate. These systems are excellent vehicles for studies concerning the imaging mechanism of adsorbed organic layers because of their stability and simplicity.

Observation of highly ordered, two‐dimensional n‐alkane and n‐alkanol structures on graphite

Abstract: We have directly imaged n‐alkane and n‐alkanol layers at the liquid–graphite interface using a scanning tunneling microscope (STM). The layers possessed a high degree of two‐dimensional ordering, and the adsorbate was observed to enhance the tunneling current. The results of this study agree with calorimetric and surface mass measurements, and show that these macroscopic measurements can aid in the selection of systems suitable for imaging with the STM.

Thermionic‐field emission from interface states at grain boundaries in silicon

Abstract: An extension of Shockley–Read–Hall kinetics is presented for interface states at grain boundaries in silicon. The emission of majority carriers by these states is generalized to include thermionic field emission (TFE), which is shown to be important in many practical cases. Comparison is made with experimental results obtained on studies of isolated grain boundaries in silicon. One of the principal results is that energy distributions of interface states deduced from electrical characteristics of grain boundaries must be interpreted using a model which includes TFE. The importance of TFE increases with the doping concentration of the silicon N and the voltage applied across the grain boundary V and decreases with temperature. It is legitimate to neglect TFE from the interface states and consider pure thermal emission only for NV≲1016 cm−3 V at a temperature of 300 K, or NV≲1015 cm−3 V for 130 K.

Two-dimensional supramolecular self-assembly probed by scanning tunneling microscopy

Abstract: Supramolecular chemistry has a very large impact on chemistry of current interest and the use of non-covalent but directional forces is appealing for the construction of 'supramolecular architectures'. The invention of scanning probe microscopy techniques has opened new doorways to study these concepts on surfaces. This review deals with recent progress in the study of two-dimensional supramolecular self-assembly on surfaces probed by scanning tunneling microscopy, with a special emphasis on structure, dynamics and reactivity of hydrogen bonded systems.

Molecular Organization of Surfactants at Solid-Liquid Interfaces

Abstract: Interactions between surfactant solutions and solid surfaces play a key role in technologically important processes such as colloidal stabilization, ore flotation, and soil removal; however, the interfacial aggregation of surfactant molecules is not yet well understood. Direct images of surfactant aggregates at solid surfaces in aqueous solutions were obtained with atomic force microscopy. The resulting structures for quaternary ammonium surfactants (above the critical micelle concentration) are consistent with half-cylinders on crystalline hydrophobic substrates, full cylinders on mica, and spheres on amorphous silica. These structures-surprisingly different from earlier models-appear to result from a compromise between the natural free curvature as defined by intermolecular interactions and the constraints imposed by specific surfactant-surface interactions. Such interfacial aggregates can potentially be used to pattern surfaces at nanometer-length scales.

Self-assembly at the liquid/solid interface: STM reveals.

Abstract: The liquid/solid interface provides an ideal environment to investigate self-assembly phenomena, and scanning tunneling microscopy (STM) is the preferred methodology to probe the structure and the properties of physisorbed monolayers on the nanoscale. Physisorbed monolayers are of relevance in areas such as lubrication, patterning of surfaces on the nanoscale, and thin film based organic electronic devices, to name a few. It's important to gain insight in the factors which control the ordering of molecules at the liquid/solid interface in view of the targeted properties. STM provides detailed insight into the importance of molecule-substrate (epitaxy) and molecule-molecule interactions (hydrogen bonding, metal complexation, and fluorophobic/fluorophilic interactions) to direct the ordering of both achiral and chiral molecules on the atomically flat surface. By controlling the location and orientation of functional groups, chemical reactions can be induced at the liquid/solid interface, via external stimul...

Electrical properties of grain boundaries in polycrystalline compound semiconductors

Abstract: In polycrystalline semiconductors the trapping of charge at the grain boundaries has a decisive influence on the electrical transport properties through the formation of electrostatic potential barriers. By proper materials processing many interesting device applications can be realised, which exploit the electrical activity of these interfaces. In this review, the authors describe both the origin and the consequences of the charge capturing at grain boundaries. Special emphasis is given to polycrystalline compound semiconductors, where they summarise the present knowledge on the interface microstructure and its electrical properties. The model of a double Schottky barrier is shown to provide a quantitative basis for understanding the wide range of electrical phenomena in this class of materials. The steady-state current-voltage characteristic becomes highly non-linear through the interplay between the applied bias and the occupation of the defect states at the interface and in the depletion regions. For large potential barriers, high doping levels and elevated bias, large electric fields build up in the depletion regions. This triggers minority carrier generation through impact ionisation by hot majority carriers and strongly enhances the non-linearities in the charge transport. The dynamic electrical properties are probed by AC admittance or pulse measurements and can be traced back to the finite relaxation times of the trapped electron and hole charges. Comparing the experimental results with the theoretical predictions allows one to obtain valuable information on the electronic grain boundary parameters. The relationship between the observed electrical properties and the electronic structure of the junctions is discussed in detail, with ZnO varistors providing the majority of the experimental data. First indications for a general picture of the grain boundary electronic structure appropriate for all compound semiconductors are presented.

In situ atomic force microscopy study of Alzheimer’s β-amyloid peptide on different substrates: New insights into mechanism of β-sheet formation

Abstract: We have applied in situ atomic force microscopy to directly observe the aggregation of Alzheimer’s β-amyloid peptide (Aβ) in contact with two model solid surfaces: hydrophilic mica and hydrophobic graphite. The time course of aggregation was followed by continuous imaging of surfaces remaining in contact with 10–500 μM solutions of Aβ in PBS (pH 7.4). Visualization of fragile nanoscale aggregates of Aβ was made possible by the application of a tapping mode of imaging, which minimizes the lateral forces between the probe tip and the sample. The size and the shape of Aβ aggregates, as well as the kinetics of their formation, exhibited pronounced dependence on the physicochemical nature of the surface. On hydrophilic mica, Aβ formed particulate, pseudomicellar aggregates, which at higher Aβ concentration had the tendency to form linear assemblies, reminiscent of protofibrillar species described recently in the literature. In contrast, on hydrophobic graphite Aβ formed uniform, elongated sheets. The dimensions of those sheets were consistent with the dimensions of β-sheets with extended peptide chains perpendicular to the long axis of the aggregate. The sheets of Aβ were oriented along three directions at 120° to each other, resembling the crystallographic symmetry of a graphite surface. Such substrate-templated self-assembly may be the distinguishing feature of β-sheets in comparison with α-helices. These studies show that in situ atomic force microscopy enables direct assessment of amyloid aggregation in physiological fluids and suggest that Aβ fibril formation may be driven by interactions at the interface of aqueous solutions and hydrophobic substrates, as occurs in membranes and lipoprotein particles in vivo.