Enhancing and quantifying spatial homogeneity in monolayer WS2.

Controlling the radiative properties of monolayer transition metal dichalcogenides is key to the development of atomically thin optoelectronic devices applicable to a wide range of industries. A common problem for exfoliated materials is the inherent disorder causing spatially varying nonradiative losses and therefore inhomogeneity. Here we demonstrate a five-fold reduction in the spatial inhomogeneity in monolayer WS2, resulting in enhanced overall photoluminescence emission and quality of WS2 flakes, by using an ambient-compatible laser illumination process. We propose a method to quantify spatial uniformity using statistics of spectral photoluminescence mapping. Analysis of the dynamic spectral changes shows that the enhancement is due to a spatially sensitive reduction of the charged exciton spectral weighting. The methods presented here are based on widely adopted instrumentation. They can be easily automated, making them ideal candidates for quality assessment of transition metal dichalcogenide materials, both in the laboratory and industrial environments.

Signal and noise transfer properties of CMOS based active pixel flat panel imager coupled to structured CsI:Tl.

Complementary metal-oxide-semiconductors (CMOS) active pixel sensors can be optically coupled to CsI:Tl phosphors forming a indirect active pixel flat panel imager (APFPI) for high performance medical imaging. The aim of this work is to determine the x-ray imaging capabilities of CMOS-based APFPI and study the signal and noise transfer properties of CsI:Tl phosphors. Three different CsI:Tl phosphors from two different vendors have been used to produce three system configurations. The performance of each system configuration has been studied in terms of the modulation transfer function (MTF), noise power spectra, and detective quantum efficiency (DQE) in the mammographic energy range. A simple method to determine quantum limited systems in this energy range is also presented. In addition, with aid of monochromatic synchrotron radiation, the effect of iodine characteristic x-rays of the CsI:Tl on the MTF has been determined. A Monte Carlo simulation of the signal transfer properties of the imager is also presented in order to study the stages that degrade the spatial resolution of our current system. The effect of using substrate patterning during the growth of CsI:Tl columnar structure was also studied, along with the effect of CsI:Tl fixed pattern noise due to local variations in the scintillation light. CsI:Tl fixed pattern noise appears to limit the performance of our current system configurations. All the system configurations are quantum limited at 0.23 microC/kg with two of them having DQE (0) equal to 0.57. Active pixel flat panel imagers are shown to be digital x-ray imagers with almost constant DQE throughout a significant part of their dynamic range and in particular at very low exposures.

Modeling the imaging performance of prototype organic x-ray imagers.

A unified Monte Carlo and cascaded systems model for the simulation of active-matrix flat-panel imagers is presented. With few input parameters, the model simulated the imaging performance of previously measured flat-panel imagers with reasonable accuracy. The model is used to predict the properties of conceptual flat-panel imagers based on organic semiconductors on plastic substrates. The model suggests that significant improvements in resolution and detective quantum efficiency could be achieved by operating such a detector in a back-side illuminated configuration, or by employing two imaging arrays arranged face-to-face. The effect of semiconductor properties on the conceptual imagers is investigated. According to the model, a photodiode quantum efficiency of 25% and dark current of less than 100 pA mm(-2) would be satisfactory for a prototype imager, while a competitive imager would require a photodiode quantum efficiency of 40-50% with a dark current of less than 10 pA mm(-2) to be quantum limited over the radiographic exposure range.

Efficient single photon detection by quantum dot resonant tunneling diodes.

We demonstrate that the resonant tunnel current through a double-barrier structure is sensitive to the capture of single photoexcited holes by an adjacent layer of quantum dots. This phenomenon could allow the detection of single photons with low dark count rates and high quantum efficiencies. The magnitude of the sensing current may be controlled via the thickness of the tunnel barriers. Larger currents give improved signal to noise and allow sub-mus photon time resolution.

A simplified thin-layer chromatography system for the detection of commonly abused basic drugs.

A simple method for the detection of drugs of abuse and their metabolites in urine is described. By extraction of urine (20 mL) with a buffered ToxElut solid phase column all the commonly abused opioid drugs can be detected on a single thin-layer chromatography plate. Location of the spots is achieved by spraying the plate with iodoplatinate following acidification with dilute sulphuric acid. Sensitivity of the order of 1 mg/L is attained for most drugs.

An improved method for measurement of common tricyclic antidepressant drugs and their metabolites using normal phase HPLC.

A sensitive HPLC method that allows the simultaneous determination of imipramine, amitriptyline and their desmethyl metabolites, using a 10 cm straight silica cartridge column with a mobile phase consisting of 0.004% perchloric acid in methanol is presented. Detection of the eluent is at 210 nm. The drugs are back-extracted from serum, giving both a good recovery and sensitivity of at least 5 micrograms/L. This method has proved to be extremely robust and reliable, with coefficients of variation around 3% being reported.