A new method for measuring excess carrier lifetime in bulk silicon: Photoexcited muon spin spectroscopy

We have measured the optically injected excess carrier lifetime in silicon using photoexcited muon spin spectroscopy. Positive muons implanted deep in a wafer can interact with the excess carriers and directly probe the bulk carrier lifetime whilst minimizing the effect from surface recombination. The method is based on the relaxation rate of muon spin asymmetry, which depends on the excess carrier concentration. The underlying microscopic mechanism has been understood by simulating the four-state muonium model in Si under illumination. We apply the technique to different injection levels and temperatures, and demonstrate its ability for injection- and temperature-dependent lifetime spectroscopy.Comment: 6 pages, 5 figures, and Supplemental Materia

Decoupling bulk and surface recombination properties in silicon by depth-dependent carrier lifetime measurements

Muons, as a bulk probe of materials, have been used to study the depth profile of charge carrier kinetics in Si wafers by scanning the muon implantation depth. The photoexcited muon spin spectroscopy technique can optically generate excess carriers in semiconductor wafers, while muons can measure the excess carrier density. As a result, carrier recombination lifetime spectra can be obtained. The depth-dependent lifetime spectra enable us to accurately measure the bulk carrier lifetime and surface recombination velocity by fitting the spectra to a simple one-dimensional diffusion model. Unlike other traditional lifetime spectroscopy techniques, the bulk and surface recombination properties can be readily de-convoluted in this method. Here, we have applied the technique to study silicon wafers both with and without passivation treatment and have demonstrated that the model can correctly describe the carrier kinetics in these two cases

In silico approach towards the identification of potential inhibitors from Curcuma amada Roxb against H. pylori: ADMET screening and molecular docking studies

Introduction: The present study attempts to identify potential targets of H. pylori for novel inhibitors from therapeutic herb, mango ginger (Curcuma amada Roxb.). Methods: Crystal structure of all the selected drug targets obtained from Protein Data Bank (PDB) were subjected to molecular docking against a total of 130 compounds (found to have biological activity against H. pylori) were retrieved from public databases. Compounds with good binding affinity were selected for Prime MM-GBSA rescoring and molecular dynamics (MD) simulation. Final list of compounds were taken for ADMET predictions. Results: Based on binding affinity denoted by glide score and ligand efficiency, mango ginger compounds were found selective to shikimate kinase and type II dehydroquinase through hydrogen bonding and salt bridge interactions. Stability of the interactions and free energy calculations by Prime MM-GBSA results confirmed the affinity of mango ginger compounds towards both shikimate kinase and type II dehydroquinase. From the above results, 15 compounds were calculated for ADMET parameters, Lipinski’s rule of five, and the results were found promising without any limitations. MD simulations identified gentisic acid as hit compound for shikimate kinase of H. pylori. Conclusion: Current study could identify the in silico potential of mango ginger compounds against shikimate kinase and type II dehydroquinase targets for H. pylori infections and are suitable for in vitro and in vivo evaluation

The new high field photoexcitation muon spectrometer at the ISIS pulsed neutron and muon source

A high power pulsed laser system has been installed on the high magnetic field muon spectrometer (HiFi) at the ISIS pulsed neutron and muon source, situated at the STFC Rutherford Appleton Laboratory in the UK. The upgrade enables one to perform light-pump muon-probe experiments under a high magnetic field, which opens new applications of muon spin spectroscopy. In this report we give an overview of the principle of the HiFi Laser system, and describe the newly developed techniques and devices that enable precisely controlled photoexcitation of samples in the muon instrument. A demonstration experiment illustrates the potential of this unique combination of the photoexcited system and avoided level crossing technique.Comment: 12 pages, 7 figures, and 2 table

Measurement of hyperfine coupling constants of muoniated radicals in small molecule semiconductors

We report the hyperfine coupling constants of muoniated radicals formed in a number of organic semiconductors, via transverse field measurements taken in the Paschen Back limit, and compare the results to avoided level crossing resonances. Five muoniated radicals are found in tetracene, despite there only being three potential non-equivalent bonding sites, and we suggest that this might be down to crystal packing effects. For 6,13-bis(triisopropylsilylethynyl) pentacene and 6,13-bis(trimethlsilylethynyl)-pentacene, we demonstrate that the transverse field data supports the previously published avoided level crossing resonances

Temporal mapping of photochemical reactions and molecular excited states with carbon specificity

Photochemical reactions are essential to a large number of important industrial and biological processes. A method for monitoring photochemical reaction kinetics and the dynamics of molecular excitations with spatial resolution within the active molecule would allow a rigorous exploration of the pathway and mechanism of photophysical and photochemical processes. Here we demonstrate that laser-excited muon pump-probe spin spectroscopy (photo-μSR) can temporally and spatially map these processes with a spatial resolution at the single-carbon level in a molecule with a pentacene backbone. The observed time-dependent light-induced changes of an avoided level crossing resonance demonstrate that the photochemical reactivity of a specific carbon atom is modified as a result of the presence of the excited state wavefunction. This demonstrates the sensitivity and potential of this technique in probing molecular excitations and photochemistry

Developing photo-μSR spectroscopy to investigate optical excitations in materials

The μSR technique is used to probe material properties, with major research on superconductors, magnetic materials, semiconductors and organic materials. This thesis is concerned with developing the photo-μSR technique (designing, building and commissioning) to investigate photophysical and photochemical processes in organic and inorganic materials. To enable photoexcitation studies, the high field muon spectrometer (HiFi) at the pulsed muon source at ISIS, Rutherford Appleton Laboratory was upgraded with a high-power laser system in 2013-14. The upgraded spectrometer now can be used to investigate the photoexcited states in a variety of materials including organic semiconductors, biologically active molecules and inorganic semiconductors. As a part of designing and building of the photo-μSR spectrometer, sample cells for photo-μSR experiments were prepared to allow muons and photons to enter the sample from opposite directions. A Monte Carlo toolkit - GEANT (GEometry ANd Tracking), which can provide accurate predictions of a muon spectrometer, was used to simulate sample cells. Capabilities of the upgraded spectrometer were demonstrated on two semiconductors- Silicon and 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-Pentacene). In the first photo-μSR experimental results on silicon, it has been interpreted that the photo-generated carriers diffuse into the bulk of silicon and interact with muonium either at bond-centre (Mu0 BC) or at tetrahedral (Mu0T ) site. This experiment offers the proof of principle of the spectrometer. Before moving on to investigations on the second experiment, a general method to optimise the experimental conditions for photo-μSR experiment on organic semiconductors is discussed. Based on this method, a second photo-μSR experiment was performed on TIPSPentacene dissolved in dichloromethane. In this part, photo-μSR technique as a probe for spatial and temporal resolution of photochemical and the dynamics of molecular excited states are discussed

Future directions of μSR—laser excitation

We discuss the general principles of laser-excited muon pump–probe spin spectroscopy (photo-μSR), including the historical origins of the technique, and discuss the overall experimental method. We review examples of past work using this technique, then discuss the future upgrade of the HiFi spectrometer with a high-power laser system. In particular, we note that performing photo-μSR experiments at high field around avoided level crossing resonances in unsaturated organic materials, offers advantages over the work previously performed at lower magnetic fields. We then present some results from some preliminary modelling of a rather simple two-electron spin system, where we see quite a complicated behaviour of the avoided level crossings. Finally, we discuss some potential applications in the biosciences, such as electron transfer in peptides and photochemistry of carotenoids, as well as magnetism which is a more traditional area for study with muons