Wide-band 'black silicon' with atomic layer deposited NbN.

Antireflection surfaces are often utilized in optical components to reduce undesired reflection and increase absorption. We report on black silicon (b-Si) with dramatically enhanced absorption over a broad wavelength range (250-2500 nm) achieved by applying a 10-15 nm conformal coating of NbN with atomic layer deposition (ALD). The improvement is especially pronounced in the near infrared (NIR) range of 1100-2500 nm where absorption is increased by >90%. A significant increase of absorption is also observed over the ultraviolet range of 200-400 nm. Preceding NbN deposition with a nanostructured ALD Al2O3 (n-Al2O3) coating to enhance the NbN texture was also examined. Such texturing further improves absorption in the NIR, especially at longer wavelengths, strong absorption up to 4-5 µm wavelengths has been attested. For comparison, double side polished silicon and sapphire coated with 10 nm thick NbN exhibited absorption of only ∼55% in the NIR range of 1100-2500 nm. The results suggest a positive correlation between the surface area of NbN coating and optical absorption. Based on the wide-band absorption, the presented NbN-coated b-Si may be an attractive candidate for use in e.g. spectroscopic systems, infrared microbolometers.

Lithography-free shell-substrate isolation for core-shell GaAs nanowires.

A facile and scalable lithography-free technique(5) for the rapid construction of GaAs core-shell nanowires incorporating shell isolation from the substrate is reported. The process is based on interrupting NW growth and applying a thin spin-on-glass (SOG) layer to the base of the NWs and resuming core-shell NW growth. NW growth occurred in an atmospheric pressure metalorganic vapour phase epitaxy (MOVPE) system with gold nanoparticles used as catalysts for the vapour-liquid-solid growth. It is shown that NW axial core growth and radial shell growth can be resumed after interruption and even exposure to air. The SOG residues and native oxide layer that forms on the NW surface are shown to prevent or perturb resumption of epitaxial NW growth if not removed. Both HF etching and in situ annealing of the air-exposed NWs in the MOVPE were shown to remove the SOG residues and native oxide layer. While both procedures are shown capable of removing the native oxide and enabling resumption of epitaxial NW growth, in situ annealing produced the best results and allowed construction of pristine core-shell NWs. No growth occurred on SOG and it was observed that axial NW growth was more rapid when a SOG layer covered the substrate. The fabricated p-core/n-shell NWs exhibited diode behaviour upon electrical testing. The isolation of the NW shells from the substrate was confirmed by scanning electron microscopy and electrical measurements. The crystal quality of the regrown core-shell NWs was verified with a high resolution transmission electron microscope. The reported technique potentially provides a pathway using MOVPE for scalable and high-throughput production of shell-substrate isolated core-shell NWs on an industrial scale.

Exceptionally strong and robust millimeter-scale graphene-alumina composite membranes.

Graphene has attracted attention as a potential strengthening material and functional component in suspended membranes as utilized in micro and nanosystems. Development of a practical and scalable fabrication process is a necessary step to allow the exceptional material properties of graphene to be fully exploited in composite structures. Using standard and scalable microfabrication processes, we fabricated free-standing chemical vapor deposition monolayer graphene-reinforced Al2O3 composite membranes, 0.5 mm in diameter, that are strong and robust. Bulge tests revealed that the graphene reinforcement increased the membrane fracture strength by a factor of at least three and maximum sustainable strain from 0.28% to at least 0.69%. We show that the graphene-reinforced membranes are even tolerant to significant cracking without loss of membrane integrity. The graphene composite membranes' freestanding area of ∼ 200 000 µm(2) is almost a thousand times larger than suspended graphene membranes reported elsewhere. The presented graphene composite membranes may be seen as representing an interesting new class of durable composite materials warranting further study and having potential for broad applicability in a variety of fields.