Genomic design of strong direct-gap optical transition in Si/Ge core/multishell nanowires

Finding a Si-based material with strong optical activity at the band-edge remains a challenge despite decades of research. The interest lies in combining optical and electronic functions on the same wafer, while retaining the extraordinary know-how developed for Si. However, Si is an indirect-gap material. The conservation of crystal momentum mandates that optical activity at the band-edge includes a phonon, on top of an electron-hole pair, and hence photon absorption and emission remain fairly unlikely events requiring optically rather thick samples. A promising avenue to convert Si-based materials to a strong light-absorber/emitter is to combine the effects on the band-structure of both nanostructuring and alloying. The number of possible configurations, however, shows a combinatorial explosion. Furthermore, whereas it is possible to readily identify the configurations that are formally direct in the momentum space (due to band-folding) yet do not have a dipole-allowed transition at threshold, the problem becomes not just calculation of band structure but also calculation of absorption strength. Using a combination of a genetic algorithm and a semiempirical pseudopotential Hamiltonian for describing the electronic structures, we have explored hundreds of thousands of possible coaxial core/multishell Si/Ge nanowires with the orientation of [001], [110], and [111], discovering some "magic sequences" of core followed by specific Si/Ge multishells, which can offer both a direct bandgap and a strong oscillator strength. The search has revealed a few simple design principles: (i) the Ge core is superior to the Si core in producing strong bandgap transition; (ii) [001] and [110] orientations have direct bandgap, whereas the [111] orientation does not; (iii) multishell nanowires can allow for greater optical activity by as much as an order of magnitude over plain nanowires; (iv) the main motif of the winning configurations giving direct allowed transitions involves rather thin Si shell embedded within wide Ge shells. We discuss the physical origin of the enhanced optical activity, as well as the effect of possible experimental structural imperfections on optical activity in our candidate core/multishell nanowires.     

Doping Rules and Doping Prototypes in A2BO4 Spinel Oxides

A₂BO₄ spinels constitute one of the largest groups of oxides, with potential applications in many areas of technology, including (transparent) conducting layers in solar cells. However, the electrical properties of most spinel oxides remain unknown and poorly controlled. Indeed, a significant bottleneck hindering widespread use of spinels as advanced electronic materials is the lack of understanding of the key defects rendering them as p‐type or n‐type conductors. By applying first‐principles defect calculations to a large number of spinel oxides the major trends controlling their dopability are uncovered. Anti‐site defects are the main source of electrical conductivity in these compounds. The trends in anti‐sites transition levels are systemized, revealing fundamental “doping rules”, so as to guide practical doping of these oxides. Four distinct doping types (DTs) emerge from a high‐throughput screening of a large number of spinel oxides: i) donor above acceptor, both are in the gap, i.e., both are electrically active and compensated (DT‐1), ii) acceptor above donor, and only acceptor is in the gap, i.e., only acceptor is electrically active (DT‐2), iii) acceptor above donor, and only donor is in the gap, i.e., only donor is electrically active (DT3), and iv) acceptor above donor in the gap, i.e., both donor and acceptor are electrically active, but not compensated (DT‐4). Donors and acceptors in DT‐1 materials compensate each other to a varying degree, and external doping is limited due to Fermi level pinning. Acceptors in DT‐2 and donors in DT‐3 are uncompensated and may ionize and create holes or electrons, and external doping can further enhance their concentration. Donor and acceptor in DT‐4 materials do not compensate each other, and when the net concentration of carriers is small due to deep levels, it can be enhanced by external doping.     

Particle‐Based Optical Sensing of Intracellular Ions at the Example of Calcium – What Are the Experimental Pitfalls?

Colloidal particles with fluorescence read‐out are commonly used as sensors for the quantitative determination of ions. Calcium, for example, is a biologically highly relevant ion in signaling, and thus knowledge of its spatio‐temporal distribution inside cells would offer important experimental data. However, the use of particle‐based intracellular sensors for ion detection is not straightforward. Important associated problems involve delivery and intracellular location of particle‐based fluorophores, crosstalk of the fluorescence read‐out with pH, and spectral overlap of the emission spectra of different fluorophores. These potential problems are outlined and discussed here with selected experimental examples. Potential solutions are discussed and form a guideline for particle‐based intracellular imaging of ions.     

Li‐Doped Cr2MnO4: A New p‐Type Transparent Conducting Oxide by Computational Materials Design

To accelerate the design and discovery of novel functional materials, here, p‐type transparent conducting oxides, an inverse design approach is formulated, integrating three steps: i) articulating the target properties and selecting an initial pool of candidates based on “design principles”, ii) screening this initial pool by calculating the “selection metrics” for each member, and iii) laboratory realization and more‐detailed theoretical validation of the remaining “best‐of‐class” materials. Following a design principle that suggests using d5⁵ cations for good p‐type conductivity in oxides, the Inverse Design approach is applied to the class of ternary Mn(II) oxides, which are usually considered to be insulating materials. As a result, Cr₂MnO₄ is identified as an oxide closely following “selection metrics” of thermodynamic stability, wide‐gap, p‐type dopability, and band‐conduction mechanism for holes (no hole self‐trapping). Lacking an intrinsic hole‐producing acceptor defect, Li is further identified as a suitable dopant. Bulk synthesis of Li‐doped Cr₂MnO₄ exhibits at least five orders of magnitude enhancement of the hole conductivity compared to undoped samples. This novel approach of stating functionality first, then theoretically searching for candidates that merits synthesis and characterization, promises to replace the more traditional non‐systematic approach for the discovery of advanced functional materials.     

Nanodiamonds and Their Applications in Cells

Diamonds owe their fame to a unique set of outstanding properties. They combine a high refractive index, hardness, great stability and inertness, and low electrical but high thermal conductivity. Diamond defects have recently attracted a lot of attention. Given this unique list of properties, it is not surprising that diamond nanoparticles are utilized for numerous applications. Due to their hardness, they are routinely used as abrasives. Their small and uniform size qualifies them as attractive carriers for drug delivery. The stable fluorescence of diamond defects allows their use as stable single photon sources or biolabels. The magnetic properties of the defects make them stable spin qubits in quantum information. This property also allows their use as a sensor for temperature, magnetic fields, electric fields, or strain. This Review focuses on applications in cells. Different diamond materials and the special requirements for the respective applications are discussed. Methods to chemically modify the surface of diamonds and the different hurdles one has to overcome when working with cells, such as entering the cells and biocompatibility, are described. Finally, the recent developments and applications in labeling, sensing, drug delivery, theranostics, antibiotics, and tissue engineering are critically discussed.