A- and B-Exciton Photoluminescence Intensity Ratio as a Measure of Sample Quality for Transition Metal Dichalcogenide Monolayers

The photoluminescence (PL) in monolayer transition metal dichalcogenides (TMDs) is dominated by recombination of electrons in the conduction band with holes in the spin-orbit split valence bands, and there are two distinct emission features referred to as the A-peak (ground state exciton) and B-peak (higher spin-orbit split state). The intensity ratio of these two features varies widely and several contradictory interpretations have been reported. We analyze the room temperature PL from MoS2, MoSe2, WS2, and WSe2 monolayers and show that these variations arise from differences in the non-radiative recombination associated with defect densities. Hence, the relative intensities of the A- and B-emission features can be used to qualitatively asses the non-radiative recombination, and thus the quality of the sample. A low B/A ratio is indicative of low defect density and high sample quality. Emission from TMD monolayers is governed by unique optical selection rules which make them promising materials for valleytronic operations. We observe a notably higher valley polarization in the B-exciton relative to the A-exciton. The high polarization is a consequence of the shorter B-exciton lifetime resulting from rapid relaxation of excitons from the B-exciton to the A-exciton of the valence band.Comment: Final version is published online at APL Material

Understanding Variations in Circularly Polarized Photoluminescence in Monolayer Transition Metal Dichalcogenides

Monolayer transition metal dichalcogenides are promising materials for valleytronic operations. They exhibit two inequivalent valleys in the Brillouin zone, and the valley populations can be directly controlled and determined using circularly polarized optical excitation and emission. The photoluminescence polarization reflects the ratio of the two valley populations. A wide range of values for the degree of circularly polarized emission, Pcirc, has been reported for monolayer WS2, although the reasons for the disparity are unclear. Here we optically populate one valley, and measure Pcirc to explore the valley population dynamics at room temperature in a large number of monolayer WS2 samples synthesized via chemical vapor deposition. Under resonant excitation, Pcirc ranges from 2% to 32%, and we observe a pronounced inverse relationship between photoluminescence (PL) intensity and Pcirc. High quality samples exhibiting strong PL and long exciton relaxation time exhibit a low degree of valley polarization, and vice versa. This behavior is also demonstrated in monolayer WSe2 samples and transferred WS2, indicating that this correlation may be more generally observed and account for the wide variations reported for Pcirc. Time resolved PL provides insight into the role of radiative and non-radiative contributions to the observed polarization. Short non-radiative lifetimes result in a higher measured polarization by limiting opportunity for depolarizing scattering events

Transforming multiple stakeholder insights into education action: developing a pragmatic EIT professional advisory framework

BACKGROUND This paper discusses the formation of a stakeholder engagement framework for a Professional Advisory board (PAb). These collaborative undertakings were initially conducted to review post-graduate programs in the Faculty of Engineering and Information Technology (FEIT) at the University of Technology Sydney (UTS). This led to the realisation of a pragmatic and collaborative engagement process that benefits industry and the education sector whilst developing students that are able to deal with current and emergent challenges. PURPOSE OR GOAL The PAb is a network of academics, students, alumni and industry members that undertakes to engage, advice and review discipline-specific faculty programs from multiple perspectives to ensure that programs remain relevant and valuable to industry. As the faculty moves towards reengineering their approach to teaching and learning as part of a university-wide initiative known as ‘Learning 2014’ (L2014), this provides the opportunity to shape a more engaged and collaborative teaching and learning culture within its programs. DESIGN/METHODS The collaborative stakeholder process was built upon an intensive series of mixed methods and action research initiatives as engagement mechanisms. These pragmatic and emergent mechanisms involved quantitative surveys, focus groups, in-depth interviews, industry workshops and multiple rounds of academic consultations. RESULTS The PAb framework was piloted in June 2014. Initial results from a feedback survey are also reported. The collaborative framework, whilst promising, engaging and conceptually robust, has not yet reached a stable state. The framework will be extended to other EIT disciplines in order to evaluate its outcomes across various disciplinary contexts and to optimise the framework’s efficacy for future iterations. This paper suggests a pragmatic and robust framework to integrate industry and stakeholder expectations with faculty program deliverables in a way that is valuable, relevant and rewarding. CONCLUSIONS In this initial framework, industry stakeholders and academics are given a voice to share their priorities and interests. More than that, the PAb provides a forum where areas that are less common, untapped or unknown to the rest of the group can be shared, further explored and tested. While the group is new and still evolving, different EIT aspects that have not yet been undertaken in education that may be potentially powerful could be explored in the future to ensure that we are able to support long term value in the various facets of EIT education. The PAb is proving to be a robust and balanced group of professionals and academics that provide technical and practical perspectives to ensure EIT education remains relevant and current

Evaluating Consumer Sensory and Composition Attributes of Arkansas-Grown Fresh-Market Blackberries

Blackberries are grown worldwide for commercial fresh markets. Three Arkansas-grown fresh-market blackberry genotypes (‘Natchez’, ‘Ouachita’, and A-2418) were evaluated for consumer sensory and compositional attributes at the University of Arkansas Food Science Department, Fayetteville. The compositional attributes of the blackberries were within an acceptable range for commercial markets (soluble solids=8.20-11.90%, pH=2.79-3.18, titratable acidity=1.09-1.32%). In terms of soluble solids to titratable acidity ratio, ‘Ouachita’ (10.92) had the highest ratio, followed by ‘Natchez’ (8.93) and A-2418 (6.25). A consumer sensory panel (n=80) evaluated fresh-market blackberry attributes using a 9-point hedonic scale for overall impression, overall flavor, sweetness, and sourness and a 5-point Just-about-Right (JAR) scale for sweetness and sourness. The participants also ranked the blackberries in order of overall liking from most to least liked. For overall impression, overall flavor, and sweetness, ‘Natchez’ scored higher than ‘Ouachita’ and A-2418, but the panelists did not detect differences in sourness. In terms of JAR for sweetness, 64% of consumers scored ‘Natchez’ JAR, followed by ‘Ouachita’ (39%) and A-2418 (34%). Whereas, 42% percent found A-2418 “Too Sour”, followed by ‘Ouachita (33%) and ‘Natchez’ (25%). In terms of ranking the blackberries, ‘Natchez’ was the most liked blackberry followed by ‘Ouachita’ and A-2418. When looking only at blackberries ranked first, 53% of consumers ranked ‘Natchez’ as their most liked berry, compared to A-2418 (26%) and ‘Ouachita’ (21%). The results from this research suggested that fresh-market blackberries with medium-level sweetness to sourness ratios were preferred though more consumers than expected preferred the blackberries with the more extreme ratios

Who leaves and who stays? Retention and attrition in engineering education

At a time of high demand for engineering graduates, the mean graduation completion rate of engineering undergraduates in Australia has been identified as approximately 54% (with considerable variation across institutions and sectors). This proportion of non-completions has been viewed as an excessive loss to the qualified workforce of Australia. Broad brush, government-collected statistics do not, however, provide the level of detail required to understand who leaves, when and why they leave, and where they go. This paper reports on a pilot study undertaken to precede and inform final decisions on research design and methodology for a multi institutional project seeking to understand and reduce student attrition from engineering degrees across Australia. The aim of the project is to produce guidelines on curriculum formulation and delivery strategies to reduce attrition in engineering programs while meeting outcomes. The pilot study was conducted at an institution which has a relatively diverse range of students, a high proportion of whom study part time, and engineering degree structures incorporating traditional and internship-based degrees. Results from a cohort analysis, tracking pathways to completion or non-completion of the degree for the cohorts from two specific entry years will be presented. From this analysis, groups of students who "persisted over long periods", "switched to another degree" or "withdrew from the university" were identified and interviewed. Their experiences and stories, were an essential ingredient for a better understanding of the dynamics of retention/attrition and factors which required further questioning before proceeding with the multi institutional study. © 2009 Authors

A CME based channel estimation approach for MIMO-OFDM systems

A pilot-assisted, conditional model-order estimation (CME) based channel estimation algorithm is presented. The algorithm is proposed for MIMO-OFDM systems and can detect both channel frequency responses and number of multi-path taps. In addition, the modified CME estimator is also verified its capacity in determining the nonzero taps. The performance of the proposed approach is compared to the popular minimum description length (MDL) algorithm for estimation of the number of channel paths, by means of simulation in the context of a 2x2 MIMO-OFDM transceiver system. Result indicates that the new algorithm is superior in channel order estimation to the MDL algorithm in MMO-OFDM system over a noisy frequency selective fading channel. ©2009 IEEE

Introduction

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