Optical properties of mesoporous 4H-SiC prepared by anodic electrochemical etching

Porous silicon carbide was fabricated from n-type 4H-SiC substrates via anodic electrochemical etching in HF/ethanol solution and suspended in ethanol after ultrasonication. We observed three photoluminescence bands: two at wavelengths of 303 nm and 345 nm were above the bulk bandgap and one at 455 nm was below the bulk bandgap. These blue-shifted and red-shifted emission processes reveal the interplay between quantum confinement, surface states, and band edge related optical transitions. We propose a model to explain the frequently observed deviation from the quantum confinement in the photoluminesence trends for SiC-derived nanoparticles suspended in solvents. The quantum confined properties of the SiC structures provide a route for optical tunability in the UV-blue spectrum for use in novel photonic and biomedical applications

Extremal Bundles on Calabi-Yau Threefolds

We study constructions of stable holomorphic vector bundles on Calabi–Yau threefolds, especially those with exact anomaly cancellation which we call extremal. By going through the known databases we find that such examples are rare in general and can be ruled out for the spectral cover construction for all elliptic threefolds. We then introduce a general Hartshorne–Serre construction and use it to find extremal bundles of general ranks and study their stability, as well as computing their Chern numbers. Based on both existing and our new constructions, we revisit the DRY conjecture for the existence of stable sheaves on Calabi–threefolds, and provide theoretical and numerical evidence for its correctness. Our construction can be easily generalized to bundles with no extremal conditions imposed

Surface Functionalisation of Silicon Carbide Quantum Dots

Silicon carbide (SiC) nanostructures are appealing as non-toxic, water-stable and oxidation-resistant nanomaterials. Owing to these unique properties, three-dimensionally confined SiC nanostructures, namely SiC quantum dots (QDs), have found applications in the bioimaging of living cells. Photoluminescence (PL) investigations, however, have revealed that across the polytypes: 3C-, 4H- and 6H-SiC, excitation wavelength–dependent PL is observed for larger sizes but deviates for sizes smaller than approximately 3 nm, thus exhibiting a dual-feature in the PL spectra. Additionally, nanostructures of varying polytypes and bandgaps exhibit strikingly similar PL emission centered at approximately 450 nm. At this wavelength, 3C-SiC emission is above the bulk bandgap as expected of quantum size effects, but for 4H-SiC and 6H-SiC the emissions are below bandgap. 4H-SiC is a suitable polytype to study these effects

Mechanism of the Hydrosilylation Reaction of Alkenes at Porous Silicon: Experimental and Computational Deuterium Labeling Studies

The mechanism of the formation of Si-C bonded monolayers on silicon by reaction of 1-alkenes with hydrogen-terminated porous silicon surfaces has been studied by both experimental and computational means. We propose that monolayer formation occurs via the same radical chain process as at single-crystal surfaces:1 a silyl radical attacks the 1-alkene to form both the Si-C bond and a radical center on the -carbon atom. This carbon radical may then abstract a hydrogen atom from a neighboring Si-H bond to propagate the chain. Highly deuterated porous silicon and FTIR spectroscopy were used to provide evidence for this mechanism by identifying the IR bands associated with the C-D bond formed in the proposed propagation step. Deuterated porous silicon surfaces formed by galvanostatic etching in 48% DF/D2O:EtOD (1:1) electrolytes showed a 30% greater density of Si-D sites on the surface than Si-H sites on hydrogen-terminated porous silicon surfaces prepared in the equivalent H-electrolyte. The thermal reaction of undec-1-ene and the Lewis acid catalyzed reaction of styrene on a deuterated surface both resulted in alkylated surfaces with the same C-C and C-H vibrational features as formed in the corresponding reactions at a hydrogen-terminated surface. However, a broad band around 2100 cm-1 was observed upon alkylating the deuterated surfaces. Ab initio and density functional theory calculations on small molecule models showed that the integrated absorbance of this band was comparable to the intensity expected for the C-D stretches predicted by the chain mechanism. The calculations also indicate that there is substantial interaction between the hydrogen atoms on the -carbons and the hydrogen atoms on the Si(111)-H surface. These broad 2100 cm-1 features are therefore assigned to C-D bands arising from the involvement of surface D atoms in the hydrosilylation reactions, while the line broadening can be explained partly by interaction with neighboring surface atoms/group

Mechanism of the Hydrosilylation Reaction of Alkenes at Porous Silicon: Experimental and Computational Deuterium Labeling Studies

The mechanism of the formation of Si-C bonded monolayers on silicon by reaction of 1-alkenes with hydrogen-terminated porous silicon surfaces has been studied by both experimental and computational means. We propose that monolayer formation occurs via the same radical chain process as at single-crystal surfaces:1 a silyl radical attacks the 1-alkene to form both the Si-C bond and a radical center on the -carbon atom. This carbon radical may then abstract a hydrogen atom from a neighboring Si-H bond to propagate the chain. Highly deuterated porous silicon and FTIR spectroscopy were used to provide evidence for this mechanism by identifying the IR bands associated with the C-D bond formed in the proposed propagation step. Deuterated porous silicon surfaces formed by galvanostatic etching in 48% DF/D2O:EtOD (1:1) electrolytes showed a 30% greater density of Si-D sites on the surface than Si-H sites on hydrogen-terminated porous silicon surfaces prepared in the equivalent H-electrolyte. The thermal reaction of undec-1-ene and the Lewis acid catalyzed reaction of styrene on a deuterated surface both resulted in alkylated surfaces with the same C-C and C-H vibrational features as formed in the corresponding reactions at a hydrogen-terminated surface. However, a broad band around 2100 cm-1 was observed upon alkylating the deuterated surfaces. Ab initio and density functional theory calculations on small molecule models showed that the integrated absorbance of this band was comparable to the intensity expected for the C-D stretches predicted by the chain mechanism. The calculations also indicate that there is substantial interaction between the hydrogen atoms on the -carbons and the hydrogen atoms on the Si(111)-H surface. These broad 2100 cm-1 features are therefore assigned to C-D bands arising from the involvement of surface D atoms in the hydrosilylation reactions, while the line broadening can be explained partly by interaction with neighboring surface atoms/group

DNA-modified silicon nanocrystals studied by X-ray luminescence and X-ray absorption spectroscopies: Observation of a strong infra-red luminescence band

Silicon nanocrystals (SiNCs) modified with 18-mer DNA oligonucleotides have been studied by X-ray excited optical luminescence (XEOL) and X-ray absorption spectroscopy (XAS) in photoluminescence yield (PLY) and total electron yield (TEY) modes. Luminescence spectra from the DNA-modified SiNCs under X-ray excitation display distinct differences from simple alkyl terminated SiNCs. The DNA-modified SiNCs show strong luminescence at 540 ± 10 nm under vacuum ultraviolet excitation which is assigned to nitrogen 1s – σ* transitions within the DNA bases. Under excitation at 130 eV the PLY spectra from the same samples show the native nanocrystal ultraviolet emission band is suppressed, and the strongest emission peak is red shifted from 430 ± 10 nm to 489 ± 10 nm which we attribute to base nitrogen 1s transitions. In addition, a strong emission band in the infrared region at 815 ± 10 nm is observed. This clearly resolved strong IR band from the DNA-modified SiNCs may provide a useful luminescence signature in cell-labeling techniques and open up a range of applications for invivo assays

Optical luminescence from alkyl-passivated Si nanocrystals under vacuum ultraviolet excitation: Origin and temperature dependence of the blue and orange emissions.

The origin and stability of luminescence are critical issues for Si nanocrystals which are intended for use as biological probes. The optical luminescence of alkyl-monolayer-passivated silicon nanocrystals was studied under excitation with vacuum ultraviolet photons (5.1-23 eV). Blue and orange emission bands were observed simultaneously, but the blue band only appeared at low temperatures ( 8.7 eV). At 8 K, the peak wavelengths of the emission bands were 430 +/- 2 nm (blue) and 600 +/- 2 nm (orange). The orange and blue emissions originate from unoxidized and oxidized Si atoms, respectivel