Two-dimensional monolayer transition metal dichalcogenide semiconductors are ideal building blocks for atomically thin, flexible optoelectronic and catalytic devices. Although challenging for two-dimensional systems, sub-diffraction optical microscopy provides a nanoscale material understanding that is vital for optimizing their optoelectronic properties. Here we use the ‘Campanile' nano-optical probe to spectroscopically image exciton recombination within monolayer MoS2 with sub-wavelength resolution (60 nm), at the length scale relevant to many critical optoelectronic processes. Synthetic monolayer MoS2 is found to be composed of two distinct optoelectronic regions: an interior, locally ordered but mesoscopically heterogeneous two-dimensional quantum well and an unexpected ∼300-nm wide, energetically disordered edge region. Further, grain boundaries are imaged with sufficient resolution to quantify local exciton-quenching phenomena, and complimentary nano-Auger microscopy reveals that the optically defective grain boundary and edge regions are sulfur deficient. The nanoscale structure–property relationships established here are critical for the interpretation of edge- and boundary-related phenomena and the development of next-generation two-dimensional optoelectronic devices.

Fully dense transparent zinc aluminate ceramics with nanoscaled grain sizes were fabricated by Deformable Punch Spark Plasma Sintering (DP‐SPS). Optical transmission spectra showed high transparency, with up to 70% transmitted light in the visible spectrum. Vickers hardness was measured and grain boundary strengthening observed, showing hardness increase from 18.2 GPa up to 22.5 GPa as the grain sizes decreased from 60.3 to 10.1 nm. The trend followed the Hall‐Petch relationship, with hardness linearly proportional to the inverse of square root of grain size. A low grain size limit reported in previous literature below which hardness decreases, known as inverse Hall‐Petch relationship, was not observed within the studied grain size range. Cross‐sections of the hardness tests' indentations were prepared by focused ion beam and observed by electron microscopy and showed radically different crack patterns underneath the indentation imprint when contrasting samples with dissimilar grain sizes, shedding light on the mechanisms behind the observed grain boundary hardening mechanisms.

Measuring the surface energy of highly hygroscopic materials has remained a thorny problem for many years, mainly because obtaining an anhydrous surface state and maintaining this condition during the surface energy assessment has been considered an impractical task. In this work, we developed synthetic and calorimetric approaches that overcome these difficulties and applied them to measure the surface energy of anhydrous nanocrystalline magnesium oxide. Anhydrous MgO with specific surface area of ∼300 m2 g–1 was synthesized by laser ablation in a controlled oxygen partial pressure environment. High resolution transmission electron microscopy and X-ray diffraction showed cubic nanoparticles with sizes ranging from 5 to 10 nm (as controlled by the partial pressure) and with the periclase crystal structure. The surface energy of the anhydrous state was assessed using high temperature oxide melt drop solution calorimetry and differential scanning calorimetry; the surface energies were 1.2 ± 0.1 and 1.3 ± 0.1 J m–2, respectively. These values are slightly higher than from previously reported experiments and are consistent with a less hydrated surface.

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