Understanding defect related luminescence processes in wide bandgap materials using low temperature multi-spectroscopic techniques

Abstract

Feldspar is a dominant, naturally occurring mineral which comprises about ∼ 60% of the Earth’s crust. It is widely used in optically stimulated luminescence (OSL) dating of sediments to obtain chronologies of past events as old as ∼ 0.5 Ma, and thus, plays a crucial role in understanding Quaternary climate changes, landscape development and human evolution and dispersal. Optical properties of feldspar originate from a) a wide band gap (∼ 7.7 eV, b) crystal defects (impurity atoms and distortions) that create localized energy states within the bandgap, and c) the low-mobility band tail states, which play a role in charge transport. The main defect used in optical dating is called the infra-red dosimetric trap (IR trap), which has a thermal lifetime of millions of years, appropriate for dating Middle to Late Quaternary time scales. However, this trap is known to suffer from instability arising from tunneling loss of the trapped electrons over geological time (so called anomalous fading); this gives rise to apparent ages that underestimate the true age. Despite a rapid progress in the infra-red stimulated luminescence (IRSL) dating technique using feldspar, a clear understanding of luminescence processes is still lacking. Some unknown aspects of the feldspar dosimeter system are: the exact electron and hole trapping centers and their physical characteristics, the interaction of defects (charge trapping states) with the crystal lattice, the kinetics of tunneling loss and the time scales of intra-defect transitions. A better understanding of feldspar as a physical system is expected to lead to its improved performance as a luminescence chronometer. The purpose of my Ph.D. research is to improve the understanding of the nature of luminescence generating defects and processes in feldspar, and to test if the intra-defect relaxation transitions can be successfully exploited to improve the OSL dating technique. My Ph.D. work is entirely experimental. It includes mapping the energy levels of the defects individually and characterizing their emission process, understanding the dynamics of the excited state relaxation and tunneling, and defect interactions with the crystal lattice and the band tail states. All these experiments were carried out using the Risø station for CryOgenic LUminescence Research (COLUR) and a highly sensitive spectrometer attached to the Risø TL/OSL reader. Chapter 1 is an introduction to the thesis, while Chapter 2 describes the instrumentation and samples used to carry out this research. The key findings of my Ph.D. research are summarized in five different chapters (Chapter 3 to Chapter 7). I discovered a ‘red edge effect’ in the greenorange emission in feldspar, and demonstrated that this effect arises from interaction of a deep lying defect with the band tail states. The deep red emission is shown to vary with site dependence of Fe3+ even within a single sample. Furthermore, it is observed that there exists an excitation-energy dependence of the main radiative transition (4T1→ 6A1) in Fe3+; this is possibly related to spin-lattice interaction during resonant excitation. I also examined the YPO4:Sm,Ce system, a model analogue material for feldspar to understand the tunneling mechanism in randomly distributed defects. For the first time, a precise mapping of the energy levels of the metastable Sm2+ was carried out, and the temperature-dependent relaxation lifetime of Sm2+ excited state was determined using the defects internal radiative-transition mechanism. It was then demonstrated that OSL decay curves resulting from optically induced, sub-conduction band electron transfer (Sm2+ → Ce4+) can be adequately described using the prevalent mathematical model of excited-state tunneling. Finally, inspired by the results of YPO4:Ce, Sm, I discovered a Stokes shifted infra-red photoluminescence (IRPL) signal in feldspar. Current methods of OSL rely on transfer of electrons from the dosimetric trap to holes located elsewhere in the lattice, which is affected by sensitivity changes leading to several uncertainties in the dose measurement. In contrast, it is shown here that the IRPL signal arises from intra-defect excitation and subsequent radiative emission within the IR dosimetric trap; it is, therefore, not likely to suffer from same problems as the IRSL signal. The IRPL signal, increases with dose and can be probed non-destructively (especially at low temperatures). Preliminary dating investigations suggest that this signal does not suffer from anomalous fading. There are two important technique developments in my thesis. Firstly, based on the model of the red edge effect, a simple method is proposed for estimation of the width of the band tail states in feldspar. Secondly, it is shown that the infra-red photoluminescence (IRPL) technique can be used for non-destructive probing of dosimetric information in the IR trap; this signal opens a new window in optical dating methods. In summary, my research has significantly advanced our understanding of the feldspar luminescence dosimeter system, and has led to development of new measurement techniques, which will significantly impact the future of luminescence dating.