D. P. Langstaff

Aerodynamic levitator furnace for measuring thermophysical properties of refractory liquids

The development of novel contactless aerodynamic laser heated levitation techniques is reported that enable thermophysical properties of refractory liquids to be measured in situ in the solid, liquid, and supercooled liquid state and demonstrated here for alumina. Starting with polished crystalline ruby spheres, we show how, by accurately measuring the changing radius, the known density in the solid state can be reproduced from room temperature to the melting point at 2323 K. Once molten, by coupling the floating liquid drop to acoustic oscillations via the levitating gas, the mechanical resonance and damping of the liquid can be measured precisely with high-speed high-resolution shadow cast imaging. The resonance frequency relates to the surface tension, the decay constant to the viscosity, and the ellipsoidal size and shape of the levitating drop to the density. This unique instrumentation enables these related thermophysical properties to be recorded in situ over the entire liquid and supercooled range of alumina, from the boiling point at 3240 K, until spontaneous crystallization occurs around 1860 K, almost 500 below the melting point. We believe that the utility that this unique instrumentation provides will be applicable to studying these important properties in many other high temperature liquids.

A new ion detector array and digital-signal-processor-based interface

A new one-dimensional ion detector array on a silicon chip has been developed for use in mass spectrometry. It is much smaller and simpler than electro-optical arrays currently in use and in addition has a higher resolution and a zero noise level. The array consists of a one-dimensional array of metal strips (electrodes) with a pitch of 25 mu m on the top surface of a silicon chip, each electrode having its own charge pulse sensor, 8-bit counter and control/interface circuitry. The chip is mounted on a ceramic substrate and is preceded by a micro-channel plate electron multiplier. Chips are butted to give a longer array. Test results show a stable operating region. A digital-signal-processor-based interface is described, which controls the mode of operation and reads the accumulated array data at the maximum rate to avoid counter overflow.

Silicon technology in ion detection—a high resolution detector array

Abstract A new ion detecto array integrated on a silicon chip with a high spatial resolution has been designed at Aberystwyth. Integrated circuits are mounted on a ceramic substrate beneath a microchannel plate electron multiplier giving a small, light, low power, very low noise module. An array containing 384 detectors with a spatial resolution of 25 microns has been fully tested and test results are presented.

Direct observation of Schottky to Ohmic transition in Al-diamond contacts using real-time photoelectron spectroscopy

Evans D A, Roberts O R, Vearey-Roberts A R, Langstaff D P, Twitchen D J and Schwitters M 2007 Direct observation of Schottky to ohmic transition in Al-diamond contacts using realtime photoelectron spectroscopy Appl. Phys. Lett. 91 132114 doi:10.1063/1.2790779

Resolving power enhancement of a discrete detector (array) by single event detection

Abstract Experiments have been performed to demonstrate the high resolving power of a discrete detector array that can be achieved by measuring and processing single ion events (speckle mode). Measurement of the spectrum of Kr + using a miniature mass spectrometer developed at the jet propulsion laboratory equipped with a focal plane detector developed at Aberystwyth show a large resolving power enhancement in the speckle mode and good agreement with tabulated peak centroids and isotope abundances. The mode of operation of the instrument is under software control and is instantly variable. This clearly demonstrates the versatility and high performance of the miniaturised system.

Molecular organization in organic semiconductor thin films observed in real time

Post-deposition molecular rearrangement in thin organic films is revealed by in situ real-time photoelectron spectroscopy during organic molecular beam deposition. Agreement between real time spectroscopy and Monte Carlo modeling confirms the role of nearest-neighbor molecular attraction in driving a time-dependent morphology for oriented films of tin phthalocyanine (SnPc) on a range of substrates. The time-dependent molecular self-organization occurs over timescales comparable to the growth rates and is therefore an important factor in the degradation of thin films of organic semiconductors typically considered for the fabrication of multilayer semiconductor devices.

Resolving power of a discrete electrode detector array

Abstract The resolving power of a new high resolution one-dimensional ion detector array has been studied as a function of the microchannel plate electron multiplier (MCP) voltage and the detector pulse height discrimination level. Experimental data have been analysed to give a simple representation of both the resolving power and the region of stable operation of the array. The conditions necessary for high resolution and stable operation are explained and the implications of the results for future design of high resolution arrays are outlined.

Real-time monitoring of the evolving morphology and molecular structure at an organic-inorganic semiconductor interface : SnPc on GaAs(001)

An organic-III-V hybrid semiconductor interface has been studied using real-time photoelectron spectroscopy and x-ray absorption spectroscopy to reveal the evolving morphology and molecular structure within the organic layer during thin film growth. This new approach to in situ characterization has been enabled by electron detection using a direct electron-counting array detector coupled to a hemispherical electron analyzer. The nonplanar tin phthalocyanine (SnPc) molecules initially form a uniform layer within which they have a distinct molecular orientation relative to the S-passivated gallium arsenide substrate surface [GaAs:S(001)]. The critical thickness of 0.9 nm that marks the transition between layered and clustered growth, determined from the photoemission measurements, corresponds to a single molecular layer with the molecules oriented at an angle of (39 +/- 2)degrees to the substrate plane. This value is confirmed by angle-resolved near-edge x-ray absorption fine structure measurements in the same experimental environment. However, the angle is less for the thicker films as the molecule-molecule interaction dominates over the molecule-substrate interaction and the structure is close to that of the bulk triclinic SnPc crystal

High temperature photoelectron emission and surface photovoltage in semiconducting diamond

A non-equilibrium photovoltage is generated in semiconducting diamond at above-ambient temperatures during x-ray and UV illumination that is sensitive to surface conductivity. The H-termination of a moderately doped p-type diamond (111) surface sustains a surface photovoltage up to 700 K, while the clean (2 × 1) reconstructed surface is not as severely affected. The flat-band C 1s binding energy is determined from 300 K measurement to be 283.87 eV. The true value for the H-terminated surface, determined from high temperature measurement, is (285.2 ± 0.1) eV, corresponding to a valence band maximum lying 1.6 eV below the Fermi level. This is similar to that of the reconstructed (2 × 1) surface, although this surface shows a wider spread of binding energy between 285.2 and 285.4 eV. Photovoltage quantification and correction are enabled by real-time photoelectron spectroscopy applied during annealing cycles between 300 K and 1200 K. A model is presented that accounts for the measured surface photovoltage in terms of a temperature-dependent resistance. A large, high-temperature photovoltage that is sensitive to surface conductivity and photon flux suggests a new way to use moderately B-doped diamond in voltage-based sensing devices.

Simulation of a discrete electrode detector array performance

Abstract An experimental and theoretical study has been carried out in which ion peaks measured using a discrete electrode detector array placed in the focal plane of a mass spectrometer are compared with the results produced by an array simulator programmed in Mathcad. All known factors influencing a peak profile have been included in the simulator. The activation of several electrodes in single events is observed experimentally and predicted by the simulator. The results highlight the parameters which are most important in determining the performance of the array, particularly the sensitivity and resolving power.