Sherri Johnstone

Work-function measurement by high-resolution scanning Kelvin nanoprobe

Nanoscience promises to transform todays world in the same way that integrated semiconductor devices transformed the world of electronics and computation. In the post-genomic era, the greatest challenge is to make connections between the structures and functions of biomolecules at the nanometre-scale level in order to underpin the understanding of larger scale systems in the fields of human biology and physiology. To achieve this, instruments with new capabilities need to be researched and developed, with particular emphasis on new levels of sensitivity, precision and resolution for biomolecular analysis. This paper describes an instrument able to analyse structures that range from tenths of a nanometre (proteins, DNA) to micron-scale structures (living cells), which can be investigated non-destructively in their normal state and subsequently in chemical- or biochemical-modified conditions. The high-resolution scanning Kelvin nanoprobe (SKN) measures the work-function changes at molecular level, instigated by local charge reconfiguration due to translational motion of mobile charges, dipolar relaxation of bound charges, interfacial polarization and structural and conformational modifications. In addition to detecting surface electrical properties, the instrument offers, in parallel, the surface topographic image, with nanometre resolution. The instrument can also be used to investigate subtle work function/topography variations which occur in, for example, corrosion, contamination, adsorption and desorption of molecules, crystallographic studies, mechanical stress studies, surface photovoltaic studies, material science, biocompatibility studies, microelectronic characterization in semiconductor technology, oxide and thin films, surface processing and treatments, surfaces and interfaces characterization. This paper presents the design and development of the instrument, the basic principles of the method and the challenges involved to achieve nanometric resolution and sub-millivolt sensitivity, for both the topographic imaging of surface micromorphology and surface potential and work-function determination.

Excitation mechanisms and dispersion characteristics of guided waves in multilayered cylindrical solid media

This paper first reviews a method of simulating the propagation characteristics of guided waves in multilayered coaxial cylindrical elastic solid media. Secondly, this method is used to investigate the properties of the guided waves for the ultrasonic long-range non-destructive evaluation techniques for rockbolts. To do so, the special case of non-leaky guided modes in open waveguides is considered. The method explains how the complex dispersion function is converted into a real function: hence the bisection technique can be employed to search for all the real roots. The model is used to (i) characterize the low dispersion range and anomalous dispersion of normal and Stoneley modes and (ii) analyze the excitation mechanisms of guided waves from axisymmetric and non-axisymmetric acoustic sources. The results are used to select suitable excitation frequency ranges associated with dominant modes with large amplitudes, low dispersion, and distinguishable propagation velocities to reduce signal distortion. The results suggest the lowest order flexural mode, excited by a radial force source, has potential to be used in practice. Also, the highly dispersive Stoneley mode propagating along a cylindrical interface is defined and distinguished from the normal mode using two properties, velocity high-frequency asymptotes and amplitude distributions along the radial direction.

Anomalous dispersion of flexural guided waves in clad rods

In clad rods, the dispersion properties of firstorder flexural [F(1, 1)] modes can differ from other modes. Normal and Stoneley modes and the existence criteria for four F(1, 1) modes are investigated. We focus on low-frequency anomalous dispersion, which occurs only in F(1, 1) modes and is sensitive to shear velocity.

Anomalous dispersion of guided wave in cylindrical multi-layered solid media

This paper presents an investigation into the dispersion of guided waves in cylindrical multi-layered solid media. This will aid in the design of ultrasonic transmission rods, consisting of a metal rod surrounded by a sheath, used in liquid metal cleanliness measurements. The aim is to understand the material parameters which define the regions of normal and anomalous dispersion, such that the sheath material and transmitted frequency range can be designed to improve measurement of the surrounding medium. Three models were created for infinite length rods: (1) in a vacuum, (2) embedded in an infinite medium, and (3) surrounded by an infinite medium connected via an adhesive intermediary layer (i.e. single-, double-and three-layered models). The dispersion equations were transformed into real functions and solved numerically using the bisection technique, to give robust solutions of the phase and group velocities for all propagating modes. Simulations were performed for a variety of adhesive layers and infinite medium material properties. In the single-layered model (1) it was found that only first order modes show the attributes of anomalous dispersion. In the double-layered model (2) the materials were chosen to satisfy the Stoneley wave existence criterion and the rod density was chosen to be greater than the outer medium. In this case only the Stoneley modes were predicted to exhibit signs of anomalous dispersion. These were found to exist in two distinct frequency bands. In the three-layered model (3) the Stoneley mode was found to exhibit anomalous dispersion when the density of the inner rod was larger than that of the adhesive layer, whereas the flexural normal modes required both the shear velocity and density of the rod to be greater than those of the adhesive layer. These results suggest that the high-frequency anomalous dispersion regions could be related to the interface wave properties to aid in transmission rod design.

Speech/music discrimination by detection: Assessment of time series events using ROC graphs

This paper suggests the application of the Receiver Operating Characteristics (ROC) graph to assess the performance of any speech/music discrimination method. ROC graphs are applied in the field of speech/music discrimination to assess the Time Series Events (TSE) method. The discrimination problem is viewed as two detection problems: detection of speech and detection of music. It was found that the optimal feature for detecting speech was silence with a true positive rate of 0.9 and false positive rate of 0.14, whilst the optimal feature for music was non-zero crossing rate NZCR with a true positive rate of 0.71 and false positive rate of 0.08.

A 2D static ultrasonic array of passive probes for improved probability of detection

An experiment was conducted to investigate the potential of using the passive (awaiting active pulsing) transducers in a 2D ultrasonic array as signal receivers. This paper will demonstrate that this new technique normal probe diffraction (NPD) which can increase the probability of detection (POD), without increasing the amount of false positives. Three 10 mm diameter, 5 MHz straight beam probes were used at linear distances of 22.5 and 45 mm, from the transmitter. A V1 calibration block was used as an aid to experimental repeatability, and the 2 mm notch was utilized as a pseudo defect. The probes were used in a single transmission (Tx) with dual receiver (Rx) formation in alternate positions, to collect information from areas of the sound field that would be lost using a standard pulse echo technique. The readings were taken from the area prior to the first backwall echo because this eliminates confusion due to the sidewall reflections. The experiments will show that the diffracted echo signal can be detected using the passive probes using NPD techniques.

Stoneley waves in three-layered cylindrical solid media

This paper considers the propagation of Stoneley modes along the interfaces of three-layered concentric cylindrical solid media in order to assist in the design of ultrasonic transmission rods. The phase velocity dispersion curves and amplitude distributions are numerically analyzed. The modes are analogous to non-dispersive Stoneley waves and are confined to the vicinities of the two interfaces at high frequency. A key finding is that the peak amplitude location for each mode transfers between the two interfaces as a function of frequency. A simplified model is introduced, giving the peak amplitude locations of each mode in different frequency ranges efficiently.

Discrimination between speech and music using time series events

In this paper, a novel method for the discrimination of speech and music, motivated by the zero crossing rate (ZCR) approach, is proposed. This method has the potential to detect the presence of music by a simple count of a certain event related to non zero crossing.

Is there potential for use of the Hall effect in analytical science

This article describes the basic principles of the Hall effect and how it has been applied in solid state applications to measure both the magnetic field and material properties. This is followed by an outline of research undertaken in aqueous ionic solutions as a possible precursor to application in chemical studies. However, previous research has shown that factors such as the Nernst effect have reduced the success of reliably measuring the Hall voltage under these conditions. The possibility of a multi-sensing approach using both electrical and acoustic Hall voltages to overcome these challenges is discussed. The article concludes with a brief look at some early efforts to detect biomolecular interactions.

Coupling between flexural and Stoneley modes in cylindrical three-layered media

Theoretical and numerical analyses of propagating properties of non-asymmetric flexural modes in a three-layered cylindrical transmission rod model are presented in this paper. The first order flexural mode and segments of the higher order flexural modes are behaving assemble to the Stoneley waves propagating along the inner and outer interfaces, respectively. Investigations of mode coupling between flexural and Stoneley modes assist to understand the exhibition and characteristics of the infinite number of separated Stoneley wave segments.