Sj O'Shea

Microcantilever-based biosensors

The use of surface stress-based sensors as bio-chemical sensors was investigated. In principle, adsorption of biochemical species on a functionalised surface of a microfabricated cantilever will cause surface stress and consequently the cantilever bends. Two applications are presented: first lipoproteins and their oxidised form which are responsible for cholesterol accumulation in arteries were differentiated by measuring the surface stress involved in their adsorption on a sugar (heparin); secondly, the surface stress resulting from surface induced conformational changes in protein was monitored. That provided experimental evidence of long time-scale surface processes.

Atomic force microscopy of local compliance at solid-liquid interfaces

A modified atomic force microscope (AFM) is used to directly measure the local compliance of ordered liquid layers at solid—liquid interfaces. Measurements of the compliance of the solvation structure for octamethylcyclotetrasiloxane and n-dodecanol near graphite and mica surfaces are presented. We show that reasonable correlation decay lengths, molecular sizes, and material rigidities can be determined. The new method is based on a force modulation technique and can be more sensitive to weak longer range interactions compared with the more direct measurement of forces using the static deflection of the AFM cantilever.

Potentiometry and repair of electrically stressed nanowires using atomic force microscopy

Using an atomic force microscope equipped with a conducting diamond tip, the surface potential on a current carrying gold nanowire was measured with microvolt potential sensitivity and nanometer spatial resolution. Potentiometry images illustrate the stages of failure of nanowires subjected to current stressing. During this failure process, a discontinuity in the potential gradient and an enhanced resistance region were observed at the failure site until a complete fracture was formed. By increasing the repulsive force and accurately positioning the tip, gold could be manipulated into the nanoscale fracture so that the electrical conductivity of the nanowire was regained.

Measuring surface stress induced by electrode processes using a micromechanical sensor

Sensitive and fast sensors can be constructed from atomic force microscope (AFM) cantilevers for studies of interfacial processes such as adsorption and reconstruction. We have constructed a surface stress sensor with submonolayer sensitivity for use in electrochemistry, whereby simultaneous cyclic voltammograms and stress changes can be recorded. This is demonstrated with measurements of the electrocapillary curve of gold, and the stress changes associated with the underpotential deposition of silver on gold(111).

Influence of frictional forces on atomic force microscope images

Abstract In this work we simultaneously measure both the friction and applied force acting during imaging on graphite with an atomic force microscope (AFM). We show that: (1) The scan direction with respect to the cantilever is important for imaging at atomic resolution. (ii) In order to minimise the forces acting on the tip it may not be sufficient to simply reduce the applied force because if strong adhesive forces are present then both the friction forces and the contact area may be significant even at zero applied load. (iii) Adhesive forces may also influence the imaging of adsorbates which are only weakly bound to the substrate. At low scanning speeds in air adsorbed material tends to be dragged along by the tip whereas at higher speeds the adsorbates remain fixed. (iv) The friction data suggest that very high shear forces (~1 GPa) can be sustained at the tip-sample contact.

Scanning tunnelling microscopy and atomic force microscopy of carbondiamond films

Abstract We have used both scanning tunnelling microscopy (STM) and atomic force microscopy (AFM) to image and measure electrical properties of diamond films. The carbon-diamond films were deposited on {100}, 1–5 W cm, n-type Si substrates from a 13.56 MHz, capacitively coupled CH 4 Ar rf plasma. Film thicknesses ranged from 10 nm to 150 nm and were transferred immediately into UHV for measurement. In the case of the 10 nm films, images were obtained in the STM revealing three types of morphology: (i) large atomically-flat regions with an unpinned Fermi level exhibiting semiconductor-like behaviour with a band gap of ∼4 eV as determined by taking current-voltage (1 V) characteristics at selected sites. Such characteristics also showed a reverse bias stability with no evidence of breakdown up to 8 eV. (ii) Three-dimensional crystalline regions with crystal sizes up to 200 nm. (iii) Amorphous regions occupying spaces between grains and often decorating the surfaces of crystalline regions. The electrical characteristic of these regions showed an essentially metallic-like behaviour with no obvious band structure, indicating that they were probably amorphous carbon. Thicker carbon-diamond films became increasingly difficult to image in the STM indicating that most of the voltage was being dropped within the film, preventing electrons being injected into the conduction band of the silicon substrate for detection. In these cases samples were imaged in the AFM which indicated that the films were essentially flat with a roughness of ∼5 nm. Friction measurements carried out with the AFM gave a value of 0.03 which agrees favourably with previous measurements on pure diamond in air.

Conducting AFM: Applications to semiconductor surfaces

The use of Conducting Probe Atomic Force Microscopy to give nm scale electronic characterisation of surfaces is reviewed. Local conductance, Kelvin Probe work function measurements, Fowler-Nordheim tunnelling and local C-V characterisation techniques are outlined. The principle results of these and their applications to the semiconductor surface and thin film characterisation are discussed. We present tunnelling data from silicon through varying oxide thickness using conducting AFM and scanning Kelvin Probe measurements from sub micron MOS capacitors. The F-N tunnelling technique has also been used on epitaxial silicon surfaces with atomically flat topography. The inherent problems associated with quantitative, reproducible measurements are outlined, and the potential applications of the measurements to surface and thin film technology are discussed.

Micromechanical Stress Sensors for Electrochemical Studies

Cantilevers developed for atomic force microscopy can be used to construct sensitive thermal and stress sensors. We have shown how the stress changes which accompany deposition and desorption may be measured on single crystal electrodes. In this work we describe the surface stress changes associated with three processes: the electrodeposition of Pb and I and desorption of self assembled thiol monolayers on the Au(111) surface.