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1. The impact of site selectivity and disorder on the thermoelectric properties of Yb ₂₁ Mn ₄ Sb ₁₈ solid solutions: Yb ₂₁ Mn _4−x Cd _x Sb ₁₈ and Yb _21−y Ca _y Mn ₄ Sb ₁₈

   https://doi.org/10.1039/d1ma00497b  

   He, Allan; Cerretti, Giacomo; Kauzlarich, Susan M. (August 2021, Materials Advances)

   Thermoelectric materials can convert heat into electricity. They are used to generate electricity when other power sources are not available or to increase energy efficiency by recycling waste heat. The Yb 21 Mn 4 Sb 18 phase was previously shown to have good thermoelectric performance due to its large Seebeck coefficient (∼290 μV K −1 ) and low thermal conductivity (0.4 W m −1 K −1 ). These characteristics stem respectively from the unique [Mn 4 Sb 10 ] 22− subunit and the large unit cell/site disorder inherent in this phase. The solid solutions, Yb 21 Mn 4− x Cd x Sb 18 ( x = 0, 0.5, 1.0, 1.5) and Yb 21− y Ca y Mn 4 Sb 18 ( y = 3, 6, 9, 10.5) have been prepared, their structures characterized and thermoelectric properties from room temperature to 800 K measured. A detailed look into the structural disorder for the Cd and Ca solid solutions was performed using synchrotron powder X-ray diffraction and pair distribution function methods and shows that these are highly disordered structures. The substitution of Cd gives rise to more metallic behavior whereas Ca substitution results in high resistivity. As both Cd and Ca are isoelectronic substitutions, the changes in properties are attributed to changes in the electronic structure. Both solid solutions show that the thermal conductivities remain extremely low (∼0.4 W m −1 K −1 ) and that the Seebeck coefficients remain high (>200 μV K −1 ). The temperature dependence of the carrier mobility with increased Ca substitution, changing from approximately T −1 to T −0.5 , suggests that another scattering mechanism is being introduced. As the bonding changes from polar covalent with Yb to ionic for Ca, polar optical phonon scattering becomes the dominant mechanism. Experimental studies of the Cd solid solutions result in a max zT of ∼1 at 800 K and, more importantly for application purposes, a ZT avg ∼ 0.6 from 300 K to 800 K. 

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2. Machine learning guided microwave-assisted quantum dot synthesis and an indication of residual H ₂ O ₂ in human teeth

   https://doi.org/10.1039/D2NR03718A  

   Xu, Quan; Tang, Yaoyao; Zhu, Peide; Zhang, Weiye; Zhang, Yuqi; Solis, Oliver Sanchez; Hu, Travis Shihao; Wang, Juncheng (September 2022, Nanoscale)

   The current preparation methods of carbon quantum dots (CDs) involve many reaction parameters, which leads to many possibilities in the synthesis processes and high uncertainty of the resultant production performance. Recently, machine learning (ML) methods have shown great potential in correlating the selected features in many applications, which can help understand the relevant structure–function relationships of CDs and discover better synthesis recipes as well. In this work, we employ the ML approach to guide the blue CD synthesis in microwave systems. After optimizing the synthesis parameters and conditions, the quantum yield (QY) increases to about 200% higher than the average value of the prepared samples without ML guidance. The obtained CDs are applied as fluorescent probes to monitor hydrogen peroxide (H 2 O 2 ) in human teeth. The CD probe exhibits a linear relationship with the concentration of H 2 O 2 ranging from 0 to 1.1 M with a lower detection limit of 0.12 M, which can effectively detect the residual H 2 O 2 after bleaching teeth. This work shows that the adopted ML methods have considerable advantages in guiding the synthesis of high-quality CDs, which could accelerate the development of other novel functional materials in energy, biomedical, and environmental remediation applications. 

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3. Experimental and computational phase boundary mapping of Co ₄ Sn ₆ Te ₆

   https://doi.org/10.1039/c8ta07539e  

   Crawford, Caitlin M.; Ortiz, Brenden R.; Gorai, Prashun; Stevanovic, Vladan; Toberer, Eric S. (December 2018, Journal of Materials Chemistry A)

   Binary Co 4 Sb 12 skutterudite (also known as CoSb 3 ) has been extensively studied; however, its mixed-anion counterparts remain largely unexplored in terms of their phase stability and thermoelectric properties. In the search for complex anionic analogs of the binary skutterudite, we begin by investigating the Co 4 Sb 12 –Co 4 Sn 6 Te 6 pseudo-binary phase diagram. We observe no quaternary skutterudite phases and as such, focus our investigations on the ternary Co 4 Sn 6 Te 6 via experimental phase boundary mapping, transport measurements, and first-principles calculations. Phase boundary mapping using traditional bulk syntheses reveals that the Co 4 Sn 6 Te 6 exhibits electronic properties ranging from a degenerate p-type behavior to an intrinsic behavior. Under Sn-rich conditions, Hall measurements indicate degenerate p-type carrier concentrations and high hole mobility. The acceptor defect Sn Te , and donor defects Te Sn and Co i are the predominant defects and rationally correspond to regions of high Sn, Te, and Co, respectively. Consideration of the defect energetics indicates that p-type extrinsic doping is plausible; however, Sn Te is likely a killer defect that limits n-type dopability. We find that the hole carrier concentration in Co 4 Sn 6 Te 6 can be further optimized by extrinsic p-type doping under Sn-rich growth conditions. 

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4. Bandgap and strain engineering in epitaxial rocksalt structure (Ti _0.5 Mg _0.5 ) _1−x Al _x N(001) semiconductors

   https://doi.org/10.1039/d0tc03598j  

   Wang, Baiwei; Zhang, Minghua; Adhikari, Vijaya; Fang, Peijiao; Khare, Sanjay V.; Gall, Daniel (September 2020, Journal of Materials Chemistry C)

   null (Ed.)

   Rocksalt structure nitrides emerge as a promising class of semiconductors for high-temperature thermoelectric and plasmonic applications. Controlling the bandgap and strain is essential for the development of a wide variety of electronic devices. Here we use (Ti 0.5 Mg 0.5 ) 1−x Al x N as a model system to explore and demonstrate the tunability of both the bandgap and the strain state in rocksalt structure nitrides, employing a combined experimental and computational approach. (Ti 0.5 Mg 0.5 ) 1−x Al x N layers with x ≤ 0.44 deposited on MgO(001) substrates by reactive co-sputtering at 700 °C are epitaxial single crystals with a solid-solution B1 rocksalt structure. The lattice mismatch with the substrate decreases with increasing x , leading to a transition in the strain-state from partially relaxed (74% and 38% for x = 0 and 0.09) to fully strained for x ≥ 0.22. First-principles calculations employing 64-atom Special Quasirandom Structures (SQS) indicate that the lattice constant decreases linearly with x according to a = (4.308 − 0.234 x ) Å for 0 ≤ x ≤ 1. In contrast, the measured relaxed lattice parameter a o = (4.269 − 0.131 x ) Å is linear only for x ≤ 0.33, its composition dependence is less pronounced, and x > 0.44 leads to the nucleation of secondary phases. The fundamental (indirect) bandgap predicted using the same SQS supercells and the HSE06 functional increases from 1.0 to 2.6 eV for x = 0–0.75. In contrast, the onset of the measured optical absorption due to interband transitions increases only from 2.3 to 2.6 eV for x = 0–0.44, suggesting that the addition of Al in the solid solution relaxes the electron momentum conservation and causes a shift from direct to indirect gap transitions. The resistivity increases from 9.0 to 708 μΩ m at 77 K and from 6.8 to 89 μΩ m at 295 K with increasing x = 0–0.44, indicating an increasing carrier localization associated with a randomization of cation site occupation and the increasing bandgap which also causes a 33% reduction in the optical carrier concentration. The overall results demonstrate bandgap and strain engineering in rocksalt nitride semiconductors and show that, in contrast to conventional covalent semiconductors, the random cation site occupation strongly affects optical transitions. 

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5. Determining plasmonic hot-carrier energy distributions via single-molecule transport measurements

   https://doi.org/10.1126/science.abb3457  

   Reddy, Harsha; Wang, Kun; Kudyshev, Zhaxylyk; Zhu, Linxiao; Yan, Shen; Vezzoli, Andrea; Higgins, Simon J.; Gavini, Vikram; Boltasseva, Alexandra; Reddy, Pramod; et al (July 2020, Science)

   null (Ed.)

   Hot-carriers in plasmonic nanostructures, generated via plasmon decay, play key roles in applications like photocatalysis and in photodetectors that circumvent band-gap limitations. However, direct experimental quantification of steady-state energy distributions of hot-carriers in nanostructures has so far been lacking. We present transport measurements from single-molecule junctions, created by trapping suitably chosen single molecules between an ultra-thin gold film supporting surface plasmon polaritons and a scanning probe tip, that can provide quantification of plasmonic hot-carrier distributions. Our results show that Landau damping is the dominant physical mechanism of hot-carrier generation in nanoscale systems with strong confinement. The technique developed in this work will enable quantification of plasmonic hot-carrier distributions in nanophotonic and plasmonic devices. 

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