Hotspot
engineering has the potential to transform the field of
surface-enhanced Raman spectroscopy (SERS) by enabling ultrasensitive
and reproducible detection of analytes. However, the ability to controllably
generate SERS hotspots, with desired location and geometry, over large-area
substrates, has remained elusive. In this study, we sculpt artificial
edges in monolayer molybdenum disulfide (MoS2) by low-power
focused laser-cutting. We find that when gold nanoparticles (AuNPs)
are deposited on MoS2 by drop-casting, the AuNPs tend to
accumulate predominantly along the artificial edges. First-principles
density functional theory (DFT) calculations indicate strong binding
of AuNPs with the artificial edges due to dangling bonds that are
ubiquitous on the unpassivated (laser-cut) edges. The dense accumulation
of AuNPs along the artificial edges intensifies plasmonic effects
in these regions, creating hotspots exclusively along the artificial
edges. DFT further indicates that adsorption of AuNPs along the artificial
edges prompts a transition from semiconducting to metallic behavior,
which can further intensify the plasmonic effect along the artificial
edges. These effects are observed exclusively for the sculpted (i.e., cut) edges and not observed for the MoS2 surface (away from the cut edges) or along the natural (passivated)
edges of the MoS2 sheet. To demonstrate the practical utility
of this concept, we use our substrate to detect Rhodamine B (RhB)
with a large SERS enhancement (∼104) at the hotspots
for RhB concentrations as low as ∼10–10 M.
The single-step laser-etching process reported here can be used to
controllably generate arrays of SERS hotspots. As such, this concept
offers several advantages over previously reported SERS substrates
that rely on electrochemical deposition, e-beam lithography, nanoimprinting,
or photolithography. Whereas we have focused our study on MoS2, this concept could, in principle, be extended to a variety
of 2D material platforms.
Cr2C is a half-metallic 2D ferromagnet possessing high Curie temperature. We disclose very high magnetoresistance and spin injection efficiency in Cr2C based magnetic tunnel junctions, making it suitable for room temperature spintronic applications.
The central rare earth cerium atom and underlying apolar B–N bonds in two-dimensional hexagonal boron nitride facilitate a unique arrangement of hydrogen molecules which leads to fairly strong adsorption of eight hydrogen molecules per metal atom.
Although the energetics of grain boundaries are more or less understood, their mechanical description remains challenging primarily because of very fast dynamics in the atomic length scale. By contrast, granular dynamics are extraordinarily sluggish. In this study, two dimensional centripetal packings of macroscopic granular particles are employed to investigate the role of geometric aspects of grain boundary formation. Using a novel sampling scheme, the extensive configuration space is well represented by a few prominent structures. Our results suggest that cohesive effects “iron out” any disorder present and enforce a transition towards a “fixed point” basin associated with a universal high density jammed hexagonal structure. Two main conjectures are advanced: (i) the appearance of grain boundary like structures is the manifestation of the kinetic instabilities of the densification process and has its origin in the structural rearrangement and (ii) the departure from six-fold coordination in the final packing is bounded from above by a sixth of the angular dispersion present in the initial configuration. If similar predictive consequences are further developed for three dimensional cases, this may have far reaching consequences in many areas of science and technology.
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