Materials – Page 2

Molybdenum selenide mirrors

Two preprints were posted to the arXiv over the weekend, both studying the exciton-mediated reflectance of monolayer MoSe₂. The research groups, one at ETH Zürich and one at Harvard, find maximum reflectance up to 0.85 for hexagonal boron nitride encapsulated monolayers. The reflectance is electrically switchable at high speed. On the other hand, the incident light must be ~755 nm (equivalently, ~1.64 eV), and the mirror must be held at cryogenic temperature. Nevertheless, this appears to be an enticing optical system.

References:
“Atomically thin mirrors made of monolayer semiconductors”, arXiv:1705.07245,
“Realization of an atomically thin mirror using monolayer MoSe2”, arXiv:1705.07317.

Titania nanotubes

Did you know that hexagonal pore ordering in anodic aluminum oxide was discovered by Rummel already in 1936? Or that under similar anodization conditions, titanium gives rise to nanotubular titanium oxide?

A recent mini-review on the subject delineates mechanisms, chemical and physical, by which these nanotubes are created. The authors claim that over 3000 papers have been published on the subject in the last decade. Their own citation list (79 references) includes both overviews and reviews.

Reference: Zhou, Nguyen, Özkan, Schmuki, “Anodic TiO2 nanotube layers: why does self-organized growth occur – A mini review”, Electrochem. Commun., 46, 2014, 157-162; also, arXiv:1610.03643.

Large-scale quantum chemistry

Nanotechnology has long depended on modeling for prediction and understanding. With this in mind, we looked some time ago at Large-scale computation of materials properties.

I would like to draw attention to a new review which appeared last week in preprint form (see reference below). It is largely devoted to delineating the realm of applicability of density functional theory (DFT). In this review, the authors take pains to highlight not only DFT’s place between near-exact calculations (applicable to very small systems) and classical methods (molecular mechanics, molecular dynamics) but also those techniques combining DFT with classical methods for very large, difficult systems (involving, for example, charge transfer over long time scales).

At 18 pages, with citations spanning the genesis of DFT around 1960 right up through the latest published developments in 2016, it is worth having a look.

Reference:
L E Ratcliff, S Mohr, G Huhs, T Deutsch, M Masella, L Genovese, “Challenges in Large Scale Quantum Mechanical Calculations”, arXiv:1609.00252.

Advance in bit patterned media

HGST reports (13 June 2016) using template-assisted epitaxial growth to create 1 Tdot / in² bit-patterned media including servo patterning. The template is established by nano-imprinting on a carefully chosen materials stack such that subsequent processing induces oxide growth between platinum pillars, enhancing magnetic separation of the islands (bits) and ensuring single domains within each one. The pattern growth process takes advantage of both chemical and topological contrast.

Embedded servo patterning permits the final media to be manufactured without a separate servo writing step. The growth process results in topography favorable to flying head aerodynamics, while maintaining adequate magnetic contrast in the servo regions.

Reference: Yang et al., “Template-Assisted Direct Growth of 1 Td/in2 Bit Patterned Media”, Nano Lett. (2016), DOI: 10.1021/acs.nanolett.6b02345. [Also, arXiv:1606.05370.]

Surface roughness and SE yield

In a recent publication in Phys Rev B, Dzhanoev et al. remind us that surface structure can strongly influence secondary electron (SE) emission yield. In the case of nanoparticles of some material, not only may the total SE yield increase relative to the flat, bulk case, but the very shape of the yield curve vs. incident electron energy may also change. The authors develop a simple sphere-on-slab model which generalizes the analytic (fitted) equation of SE yield of the slab alone.

Reference: A R Dzhanoev, et al., “Secondary Electron Emission from Surfaces with Small Structure”, Phys. Rev. B 92, 125430 (2015) (Also available as arXiv:1605.00637.)