Aliovalent substitutions of the 2D layered semiconductor GeAs

Layered tetrel pnictides have shown promise as thermoelectrics (TEs) due to their anisotropic crystal structure and weak van der Waals interactions between layers. The binary GeAs is a p-type semiconductor with a narrow indirect bandgap of 0.57 eV and a high Seebeck coefficient (∼250 μV/K at 300 K). This work probes the limits of the aliovalent substitutions of GeAs to modify charge carrier concentration. GaxGe1-xAs (x = 0.005, 0.01, and 0.02) and GeAs1-ySey (y = 0.01, 0.02, 0.03, and 0.05) samples were synthesized to study the structure-property relationships in this system. Hole doping of GeAs via Ga-substitution increases carrier concentration resulting in the decrease in both resistivity and Seebeck coefficient. Se-substituted samples show more complex behavior related to defect chemistry. Overall, the thermoelectric power factor (S2/ρ) was significantly enhanced (up to 89%) for Ga0.005Ge0.995As as compared to pristine GeAs

Structure-property relationships of binary and ternary metal pnictides for energy applications

Binary and ternary metal pnictides have proven over the years to be structurally diverse and highly application oriented. For instance, the III-V semiconductors have been well studied since the 1950s and are widely used in electronic applications, such as GaAs or InP. The wide range of structural motifs has led to a wide range of observed properties, making some of them suited for nearly any imaginable application. In this dissertation, we examine known and novel materials that are present in binary and ternary metal phosphide and antimonide systems, focusing heavily on heavy alkali metal, late transition metal, group 13 triel, and pnictide phases. We show that many of the chosen explored systems are phase rich with undiscovered materials. The potential applications for many of these materials fall into a wide scope and range from: thermoelectrics, battery anode materials, and water splitting catalysts

Synthesis, Crystal and Electronic Structure of Layered AMSb Compounds (A = Rb, Cs; M = Zn, Cd)

Synthesis, crystal structure, thermal stability, and electronic band structure of four new metal antimonides AMSb (A = Rb, Cs; M = Zn, Cd) are reported. CsZnSb and RbZnSb crystallize in the hexagonal ZrBeSi structure type, in a P63/mmc space group (no. 194, Z = 2) and unit cell dimensions of a = 4.5588(2)/4.5466(4) Å and c = 11.9246(6)/11.0999(10) Å. CsCdSb and RbCdSb crystallize in the tetragonal PbFCl structure type in a P4/nmm space group (no. 129; Z = 2) and unit cell parameters of a = 4.8884(5)/4.8227(3) Å and c = 8.8897(9)/8.5492(7) Å. All four compounds are air‐ and water‐sensitive and are shown through DSC measurements to decompose between 975 K and 1060 K. Analysis of the calculated electronic band structure shows that the Zn‐containing antimonides are topologically trivial narrow bandgap semiconductors, whereas Cd‐containing compounds exhibit a band inversion along Γ‐Z direction

Crystallographic Facet Selective HER Catalysis: Exemplified in FeP and NiP2 Single Crystals

How the crystal structures of ordered transition-metal phosphide catalysts affect the hydrogen-evolution reaction (HER) is investigated by measuring the anisotropic catalytic activities of selected crystallographic facets on large (mm-sized) single crystals of iron-phosphide (FeP) and monoclinic nickel-diphosphide (m-NiP2). We find that different crystallographic facets exhibit distinct HER activities, in contrast to a commonly held assumption of severe surface restructuring during catalytic activity. Moreover, density-functional-theory-based computational studies show that the observed facet activity correlates well with the H-binding energy to P atoms on specific surface terminations. Direction dependent catalytic properties of two different phosphides with different transition metals, crystal structures, and electronic properties (FeP is a metal, while m-NiP2 is a semiconductor) suggests that the anisotropy of catalytic properties is a common trend for HER phosphide catalysts. This realization opens an additional rational design for highly efficient HER phosphide catalysts, through the growth of nanocrystals with specific exposed facets. Furthermore, the agreement between theory and experimental trends indicates that screening using DFT methods can accelerate the identification of desirable facets, especially for ternary or multinary compounds. The large single-crystal nature of the phosphide electrodes with well-defined surfaces allows for determination of the catalytically important double-layer capacitance of a flat surface, Cdl = 39(2) μF cm−2 for FeP, useful for an accurate calculation of the turnover frequency (TOF). X-ray photoelectron spectroscopy (XPS) studies of the catalytic crystals that were used show the formation of a thin oxide/phosphate overlayer, presumably ex situ due to air-exposure. This layer is easily removed for FeP, revealing a surface of pristine metal phosphide

Large-scale synthesis of semiconducting Cu(In,Ga)Se2 nanoparticles for screen printing application

During the last few decades, the interest over chalcopyrite and related photovoltaics has been growing due the outstanding structural and electrical properties of the thin-film Cu(In,Ga)Se2 photoabsorber. More recently, thin film deposition through solution processing has gained increasing attention from the industry, due to the potential low-cost and high-throughput production. To this end, the elimination of the selenization procedure in the synthesis of Cu(In,Ga)Se2 nanoparticles with following dispersion into ink formulations for printing/coating deposition processes are of high relevance. However, most of the reported syntheses procedures give access to tetragonal chalcopyrite Cu(In,Ga)Se2 nanoparticles, whereas methods to obtain other structures are scarce. Herein, we report a large-scale synthesis of high-quality Cu(In,Ga)Se2 nanoparticles with wurtzite hexagonal structure, with sizes of 10–70 nm, wide absorption in visible to near-infrared regions, and [Cu]/[In + Ga] ≈ 0.8 and [Ga]/[Ga + In] ≈ 0.3 metal ratios. The inclusion of the synthesized NPs into a water-based ink formulation for screen printing deposition results in thin films with homogenous thickness of ≈4.5 µm, paving the way towards environmentally friendly roll-to-roll production of photovoltaic systems.This research was funded by the Portuguese Foundation for Science and Technology (PTDC/CTM-ENE/5387/2014, PTDC/NAN-MAT/28745/2017, UID/FIS/04650/2020, UID/QUI/ 0686/2020, PTDC/FIS-MAC/28157/2017, POCI-01-0145-FEDER-028108, SFRH/BD/121780/2016); the Basque Government Industry Department (ELKARTEK, HAZITEK); the National Science Foundation (DMR-2003783 grant); the Search-ON2: revitalization of HPC infrastructure of UMinho, (NORTE07-0162-FEDER-000086), co-funded by the North Portugal Regional Operational Programme (ON.2-O Novo Norte), under the National Strategic Reference Framework (NSRF), through the European Regional Development Fund (ERDF). The use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357

Synthesis and Characterization of Single-Phase Metal Dodecaboride Solid Solutions: Zr1–xYxB12 and Zr1–xUxB12

Single-phase metal dodecaboride solid solutions, Zr0.5Y0.5B12 and Zr0.5U0.5B12, were prepared by arc melting from pure elements. The phase purity and composition were established by powder X-ray diffraction (PXRD), energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and 10B and 11B solid-state nuclear magnetic resonance (NMR) spectroscopy. The effects of carbon addition to Zr1–xYxB12 were studied and it was found that carbon causes fast cooling and as a result rapid nucleation of grains, as well as “templating” and patterning effects of the surface morphology. The hardness of the Zr0.5Y0.5B12 phase is 47.6 ± 1.7 GPa at 0.49 N load, which is ∼17% higher than that of its parent compounds, ZrB12 and YB12, with hardness values of 41.6 ± 2.6 and 37.5 ± 4.3 GPa, respectively. The hardness of Zr0.5U0.5B12 is ∼54% higher than that of its UB12 parent. The dodecaborides were confirmed to be metallic by band structure calculations, diffuse reflectance UV–vis, and solid-state NMR spectroscopies. The nature of the dodecaboride colors—violet for ZrB12 and blue for YB12—can be attributed to charge-transfer. XPS indicates that the metals are in the following oxidation states: Y3+, Zr4+, and U5+/6+. The superconducting transition temperatures (Tc) of the dodecaborides were determined to be 4.5 and 6.0 K for YB12 and ZrB12, respectively, as shown by resistivity and superconducting quantum interference device (SQUID) measurements. The Tc of the Zr0.5Y0.5B12 solid solution was suppressed to 2.5 K

From NaZn4Sb3 to HT-Na1–xZn4–ySb3: Panoramic Hydride Synthesis, Structural Diversity, and Thermoelectric Properties

Two new sodium zinc antimonides NaZn4Sb3 and HT-Na1–xZn4–ySb3 were synthesized by using reactive sodium hydride, NaH, as a precursor. The hydride route provides uniform mixing and comprehensive control over the composition, facilitating fast reactions and high-purity samples, whereas traditional synthesis using sodium metal results in inhomogeneous samples with a significant fraction of the more stable NaZnSb compound. NaZn4Sb3 crystallizes in the hexagonal P63/mmc space group (No. 194, Z = 2, a = 4.43579(4) Å, c = 23.41553(9) Å) and is stable upon heating in vacuum up to 736 K. The layered crystal structure of NaZn4Sb3 is related to the structure of the well-studied thermoelectric antimonides AeZn2Sb2 (Ae = Ca, Sr, Eu). Upon heating in vacuum, NaZn4Sb3 transforms to HT-Na1–xZn4–ySb3 (x = 0.047(3), y = 0.135(1)) due to partial Na/Zn evaporation/elimination, as was determined from high-temperature in situ synchrotron powder X-ray diffraction. HT-Na1–xZn4–ySb3 has a complex monoclinic structure with considerable degrees of structural disorder (P21/c (No. 14), Z = 32, a = 19.5366(7) Å, b = 14.7410(5) Å, c = 20.7808(7) Å, β = 90.317(2)°) and is stable exclusively in a narrow temperature range of 736–885 K. Further heating of HT-Na1–xZn4–ySb3 leads to a reversible transformation to NaZnSb above 883 K. Both compounds exhibit similarly low thermal conductivity at room temperature (0.9 W m–1 K–1) and positive Seebeck coefficients (38–52 μV/K) indicative of holes as the main charge carriers. However, resistivities of the two phases differ by 2 orders of magnitude

Adults with learning disabilities experiences of using community dental services: Service user and carer perspectives

Accessible summary: The government and other organisations say that improving health care is important for people with learning disabilities. We asked people with learning disabilities and the people who look after them what it was like for them when they went to the dentist. Those we asked said that when they went to the dentist, they knew that those they saw knew about looking after their teeth. Some of those we talked to though said that certain things needed to be better. Background: The government alongside other health and social care organisation have identified the need to improve the care provided for people with learning disabilities. Materials and Methods: This service evaluation aimed to explore the experiences of people with learning disabilities and their carers who accessed community dental services using a qualitative research design. Adults with learning disabilities (n = 4) and their carers (n = 6) took part in one to one, face to face semi structured interviews. Results and Discussion: Generally, participants were satisfied with community dental services and in particular valued the skills and the competence of practitioners. However, when dissatisfaction was expressed this was generally as a result of poor communication and the transition from child to adult dental services. Conclusions: A number of recommendations are identified and discussed in relation to engagement with adults with learning disabilities and their carers in the development and delivery of community dental services. © 2017 John Wiley & Sons Ltd