N.R.J. Poolton

Determination of the optical band-gap energy of cubic and hexagonal boron nitride using luminescence excitation spectroscopy

Using synchrotron-based luminescence excitation spectroscopy in the energy range 4–20 eV at 8 K, the indirect Γ–X optical band-gap transition in cubic boron nitride is determined as 6.36 ± 0.03 eV, and the quasi-direct band-gap energy of hexagonal boron nitride is determined as 5.96 ± 0.04 eV. The composition and structure of the materials are self-consistently established by optically detected x-ray absorption spectroscopy, and both x-ray diffraction and Raman measurements on the same samples give independent confirmation of their chemical and structural purity: together, the results are therefore considered as providing definitive measurements of the optical band-gap energies of the two materials.

Thermo-optical properties of optically stimulated luminescence in feldspars

Abstract Optically stimulated luminescence processes in feldspars are subject to competing thermal enhancement and quenching processes: this article describes the thermal enhancement effects for orthoclase, albite and plagioclase feldspars. It is demonstrated that certain lattice vibrational modes can be selectively probed at specific optical excitation energies. The results are described in terms of the Bohr hydrogen model of the OSL donor defects.

Influence of crystal structure on the optically stimulated luminescence properties of feldspars

Abstract Work is presented showing that the crystal structure of feldspar plays an important role in determining the energy positions at which optically stimulated luminescence transitions can occur, their thermo-optical behaviour, the electrical transport properties of charge in the conduction and valence bands, and the time decay characteristics of the luminescence. First-order calculations of these effects are presented, and comparison is made with experimental results.

On the relationship between luminescence excitation spectra and feldspar mineralogy

Abstract Feldspar minerals can be used as naturally occurring radiation dosemeters, with dose assessment commonly using luminescence techniques. Since many feldspars contain radioactive 40 K, knowledge of the mineralogy of the luminescent samples being measured is of high importance. Most feldspars contain more than trace amounts of highly luminescent Fe 3+ impurities, and this article examines the relationship between features of the luminescence excitation spectrum of this ion with sample mineralogy. It is demonstrated that there is a near linear correspondence between the plagioclase feldspar composition and the separation of the 4 T 2 (D) and 4 A 1 4 E(G) Fe 3+ ion levels, and this could be used to identify plagioclase feldspar composition. In the samples tested, certain features of the spectra also allowed a distinction to be made between the alkali (KNa) and plagioclase (NaCa) feldspar groups. The results are compared with properties of the excitation spectra dose-dependent optically stimulated luminescence (OSL) in order to compare the chemical environment of the OSL donor defect, and the isolated Fe 3+ centres.

Optical luminescence from alkyl-passivated Si nanocrystals under vacuum ultraviolet excitation: Origin and temperature dependence of the blue and orange emissions

The origin and stability of luminescence are critical issues for Si nanocrystals which are intended for use as biological probes. The optical luminescence of alkyl-monolayer-passivated silicon nanocrystals was studied under excitation with vacuum ultraviolet photons (5.1–23eV). Blue and orange emission bands were observed simultaneously, but the blue band only appeared at low temperatures ( 8.7eV). At 8K, the peak wavelengths of the emission bands were 430±2nm (blue) and 600±2nm (orange). The orange and blue emissions originate from unoxidized and oxidized Si atoms, respectively.

The Mobile Luminescence End-Station, MoLES: a new public facility at Daresbury Synchrotron

A new mobile end-station is described for use on multiple beamlines at the Daresbury synchrotron radiation source (overall excitation range 5 eV to 70 keV) that allows for the detection and dispersion of photoluminescence from solid-state samples in the emission range 190-1000 nm (1.2-6.5 eV). The system is fully self-contained and includes sample-cooling facilities for the temperature range 8-330 K using a closed-cycle refrigerator, thus eliminating the need for liquid cryogens. The system also includes solid-state laser sources for use with a variety of pump-probe-type experiments, and an Ar(+) surface-cleaning facility. In order to demonstrate the various capabilities of the system, the results of a variety of experiments are summarized, carried out over the excitation range 5-5000 eV on beamlines 3.2, MPW6.1 and 4.2. These include the optical detection of XAS of L-edge structure in natural minerals and archaeological ceramics, band-gap determinations of wide-band-gap silicates, and pump-probe studies of quartz.

Luminescence excitation characteristics of Ca, Na and K-aluminosilicates (feldspars) in the stimulation range 5–40 eV: determination of the band-gap energies

The first comprehensive survey of band-edge features in the ternary group of naturally occurring aluminosilicates (the feldspars) is presented. Synchrotron-based luminescence excitation of KAlSi3O8, NaAlSi3O8 and CaAl2Si2O8 allows the measurement of the evolution of the band-gap across the system, which, at 8 K, is found to vary from 7.86 eV in NaAlSi3O8 to 7.7 eV and 7.62 eV in KAlSi3O8 and CaAl2Si2O8, respectively: the band-gap energies are typically 0.1 eV smaller at 300 K. In comparison with measurements made on natural and synthetic hydrothermal α-quartz, where both the direct and indirect band-gap structures are distinctly observable, no significant post-edge band structure is discernable in the feldspars. In Ca-rich material, the luminescence is attenuated by more than two orders of magnitude for excitation energies up to 3 eV above the band-gap, partly supporting the proposition (derived from previous cyclotron resonance experiments) that the bands are simply parabolic and that the materials may be direct band-gap insulators. The luminescence excitation experiments also allow an initial survey to be made of sub-band-gap features in the materials, including low mobility, temperature sensitive band-tail states and mid-gap defects.

Influence on donor electron energies of the chemical composition of K, Na and Ca aluminosilicates

The chemical composition of KxNa1-xAlSi3O8 and NayCa1-yAl2-ySi2+yO8 crystals (alkali and plagioclase feldspars) is shown to determine the optical transition energies of electrons trapped at donor centres within them. Optical resonances in the infrared region 1.2-1.5 eV can be interpreted as arising from the 1s-2p transition of an ideal Bohr hydrogen donor. Shifts in the energy positions with sample composition are well accounted for by variations in the dielectric constant. Within the context of the Bohr model, the effective electron mass and donor radii can be determined. The mass is found to be 0.76me in the K-Na series but slightly higher, at 0.79me in CaAl2Si2O8. The ground-state donor radii are determined as 1.62 AA, 1.63 AA and 1.68 AA in the K, Na and Ca end members, respectively; these values closely match the mean (Si,AlSi)-O bond lengths in the material for the tetrahedra with mean Al content.

Nanometer scale x-ray absorption spectroscopy and chemical states mapping of ultra thin oxides on silicon using electrostatic force microscopy

Electrostatic force microscopy (EFM) was used to obtain highly spatially resolved spectroscopic and image information of semiconductor surface region. EFM with x-ray source (X-EFM) can probe x-ray induced photoionization of near surface electron trapping. The X-EFM signal dependent on x-ray photon energy results in nanometer scale x-ray absorption spectra. Furthermore, probing tip scanning at fixed x-ray photon energy provides chemical states imaging of the trapping. The authors demonstrate characterization of substoichiometric chemical oxidation of a Si surface with less than 1nm spatial resolution.

Measuring modulated luminescence using non-modulated stimulation: ramping the sample period

Conventional methods of recording linearly modulated (LM) optically stimulated luminescence (OSL) require control over either the exciting light intensity, or the ability to pulse the source. For many light sources (e.g. constant-power CW lasers, arc lamps and synchrotrons) this can be problematic. Directly analogous results to LM-OSL can, however, be achieved with non-modulated excitation sources, by ramping the sample period (RSP) of luminescence detection. RSP-OSL has the distinct advantage over LM-OSL in that, since the excitation remains at full power, data accumulation times (that can be considerable) can be reduced by typically 50%. RSP methods are universally applicable and can be employed, for example, where the excitation source is constant heat, rather than light: here, iso-thermal decay of phosphorescence becomes recorded as a sequence of peaks, corresponding to de-trapping of charge from different defect levels, and is particularly useful for analysing shallow-trap effects. RSP methods are also useful in providing significant compaction of data sets, where signal analysis is required of overlapping systems having a wide range of decay kinetics.