Effects of post-transfer annealing and substrate interactions on the photoluminescence of 2D/3D monolayer WS2/Ge heterostructures.

The ultraflat and dangling bond-free features of two-dimensional (2D) transition metal dichalcogenides (TMDs) endow them with great potential to be integrated with arbitrary three-dimensional (3D) substrates, forming mixed-dimensional 2D/3D heterostructures. As examples, 2D/3D heterostructures based on monolayer TMDs (e.g., WS2) and bulk germanium (Ge) have become emerging candidates for optoelectronic applications, such as ultrasensitive photodetectors that are capable of detecting broadband light from the mid-infrared (IR) to visible range. Currently, the study of WS2/Ge(100) heterostructures is in its infancy and it remains largely unexplored how sample preparation conditions and different substrates affect their photoluminescence (PL) and other optoelectronic properties. In this report, we investigated the PL quenching effect in monolayer WS2/Ge heterostructures prepared via a wet transfer process, and employed PL spectroscopy and atomic force microscopy (AFM) to demonstrate that post-transfer low-pressure annealing improves the interface quality and homogenizes the PL signal. We further studied and compared the temperature-dependent PL emissions of WS2/Ge with those of as-grown WS2 and WS2/graphene/Ge heterostructures. The results demonstrate that the integration of WS2 on Ge significantly quenches the PL intensity (from room temperature down to 80 K), and the PL quenching effect becomes even more prominent in WS2/graphene/Ge heterostructures, which is likely due to synergistic PL quenching effects induced by graphene and Ge. Density functional theory (DFT) and Heyd-Scuseria-Ernzerhof (HSE) hybrid functional calculations show that the interaction of WS2 and Ge is stronger than in adjacent layers of bulk WS2, thus changing the electronic band structure and making the direct band gap of monolayer WS2 less accessible. By understanding the impact of post-transfer annealing and substrate interactions on the optical properties of monolayer TMD/Ge heterostructures, this study contributes to the exploration of the processing-properties relationship and may guide the future design and fabrication of optoelectronic devices based on 2D/3D heterostructures of TMDs/Ge.

Machine Learning-Aided Band Gap Engineering of BaZrS3 Chalcogenide Perovskite.

The non-toxic and stable chalcogenide perovskite BaZrS3 fulfills many key optoelectronic properties for a high-efficiency photovoltaic material. It has been shown to possess a direct band gap with a large absorption coefficient and good carrier mobility values. With a reported band gap of 1.7-1.8 eV, BaZrS3 is a good candidate for tandem solar cell materials; however, its band gap is significantly larger than the optimal value for a high-efficiency single-junction solar cell (∼1.3 eV, Shockley-Queisser limit)─thus doping is required to lower the band gap. By combining first-principles calculations and machine learning algorithms, we are able to identify and predict the best dopants for the BaZrS3 perovskites for potential future photovoltaic devices with a band gap within the Shockley-Queisser limit. It is found that the Ca dopant at the Ba site or Ti dopant at the Zr site is the best candidate dopant. Based on this information, we report for the first time partial doping at the Ba site in BaZrS3 with Ca (i.e., Ba1-xCaxZrS3) and compare its photoluminescence with Ti-doped perovskites [i.e., Ba(Zr1-xTix)S3]. Synthesized (Ba,Ca)ZrS3 perovskites show a reduction in the band gap from ∼1.75 to ∼1.26 eV with <2 atom % Ca doping. Our results indicate that for the purpose of band gap tuning for photovoltaic applications, Ca-doping at the Ba-site is superior to Ti-doping at the Zr-site reported previously.

Family Physician Burnout Does Not Differ With Rurality.

PURPOSE: Burnout affects about half of family physicians (FPs). Minimal research exists which examines the impact of urban and rural practice settings on FP burnout. In this study, we examined whether rural practice is associated with FP burnout. METHODS: Data from the 2017 and 2018 American Board of Family Medicine Family Medicine Certification examination registration questionnaire were used. We limited our sample to FPs in continuity care in the United States. The questionnaire is a mandatory component of registration, resulting in a 100% response rate. Burnout was measured via 2 questions validated against the Maslach Burnout Inventory. We used logistic regression to determine associations between burnout and rural location, controlling for practice and personal characteristics. FINDINGS: Of the FPs surveyed, 2,740 met our inclusion criteria. Rural FPs were older, more likely to be male, and had a broader scope of practice than urban FPs. Rural FPs had a nonsignificantly higher burnout rate than urban FPs (45.1% vs 43.0%). Burnout was more common in younger and female FPs. We found no rural/urban differences between job satisfaction, practice environment, workload, and job stress; however, all of these characteristics were associated with burnout. In adjusted analyses, rural location was not associated with burnout (odds ratio = 1.15, 95% CI: 0.87-1.52). CONCLUSION: In a large national sample, we found no difference in burnout between rural and urban FPs. This suggests there is nothing unique about rural practice that predisposes to burnout and that a common pathway to reduce burnout may exist.