M. B. H. Breese

Silicon-based photonic crystals fabricated using proton beam writing combined with electrochemical etching method

A method for fabrication of three-dimensional (3D) silicon nanostructures based on selective formation of porous silicon using ion beam irradiation of bulk p-type silicon followed by electrochemical etching is shown. It opens a route towards the fabrication of two-dimensional (2D) and 3D silicon-based photonic crystals with high flexibility and industrial compatibility. In this work, we present the fabrication of 2D photonic lattice and photonic slab structures and propose a process for the fabrication of 3D woodpile photonic crystals based on this approach. Simulated results of photonic band structures for the fabricated 2D photonic crystals show the presence of TE or TM gap in mid-infrared range.

Spectroscopic detection of exogenous materials in latent fingerprints treated with powders and lifted off with adhesive tapes

Fingerprint evidence offers great value to criminal investigations since it is an internationally recognized and established means of human identification. With recent advances in modern technology, scientists have started analyzing not only the ridge patterns of fingerprints but also substances which can be found within them. The aim of this work was to determine whether Fourier transform infrared (FTIR) spectromicroscopy could be used to detect contamination in a fingerprint which was dusted with powder (a technique already recognized as an effective and reliable method for developing latent fingerprints) and subsequently lifted off with adhesive tape. Explosive materials (pentaerythritol tetranitrate, C-4, TNT) and noncontrolled substances (sugar, aspirin) were used to prepare contaminated fingerprints on various substrates. Freshly deposited fingermarks with powders which were lifted off with adhesive tapes (provided by Singapore Police Force) were analyzed using a Bruker Hyperion 2000 microscope at the ISMI beamline (Singapore Synchrotron Light Source) with an attenuated total reflection objective. FTIR spectroscopy is a nondestructive technique which requires almost no sample preparation. Further, the fingerprint under analysis remains in pristine condition, allowing subsequent analysis if necessary. All analyzed substances were successfully distinguished using their FTIR spectra in powdered and lifted fingerprints. This method has the potential to significantly impact forensic science by greatly enhancing the information that can be obtained from the study of fingerprints.

Nanoscale lithography of LaAlO₃/SrTiO₃ wires using silicon stencil masks.

We have developed a process to fabricate low-stress, fully crystalline silicon nanostencils, based on ion irradiation and the electrochemical anodization of p-type silicon. These nanostencils can be patterned with arbitrary feature shapes with openings hundreds of micrometers wide connected to long channels of less than 100 nm in width. These nanostencils have been used to deposit (2.5 μm- to 150 nm-wide) lines of LaAlO3 (LAO) on a SrTiO3 (STO) substrate, forming a confined electron layer at the interface arising from oxygen vacancies on the STO surface. Electrical characterization of the transport properties of the resulting LAO/STO nanowires exhibited a large electric field effect through back-gating using the STO as the dielectric, demonstrating electron confinement. Stencil lithography incorporating multiple feature sizes in a single mask shows great potential for future development of oxide electronics.

Conditioned bio-interfaces of silicon/porous silicon micro-patterns lead to the chondrogenesis of hMSCs

The interaction between cells and materials is of scientific and technological interest for the development of new biomaterials with improved functional properties. In this work the chondrogenesis (CG) of human mesenchymal stem cells (hMSCs) derived from the bone marrow of healthy donors has been achieved on Si/PSi surfaces after pre-treatment with a conditioned medium derived from hMSCs (CM). The chondrogenic process was analyzed in fixed-cell preparations by immune fluorescence, determining the cellular localization of β-catenin and the transcription factors STAT-5, Runx2 and vitamin D receptor (VDR). In a week of CG on Si/PSi or CM-Si/PSi, β-catenin showed cell–cell contacts priming a preferential chondrogenic process on a PSi surface, in contrast with the preferential proliferation and migration processes observed on hexagonal/triangular micro-designed Si areas. Interestingly, the presence of a CM bio-interface improves this differentiation process with respect to a control PSi surface. The chondroinductive effect was also observed in STAT-5, which was highly expressed and is involved in intracellular signaling of many differentiation receptors related to development and tissue repair. In good agreement with the above results, the co-localization of transcription factors Runx2/VDR at nuclei supports active transcription of chondrogenic genes. Considering these observations, we propose that the combination of a rough PSi surface and the components of the CM-hMSCs bio-interface boosts the chondrogenesis of hMSCs. These findings suggest that the tailoring of Si/PSi scaffolds with appropriate bio-interface combinations improves their therapeutic potential.

Reprogramming hMSCs morphology with silicon/porous silicon geometric micro-patterns.

Geometric micro-patterned surfaces of silicon combined with porous silicon (Si/PSi) have been manufactured to study the behaviour of human Mesenchymal Stem Cells (hMSCs). These micro-patterns consist of regular silicon hexagons surrounded by spaced columns of silicon equilateral triangles separated by PSi. The results show that, at an early culture stage, the hMSCs resemble quiescent cells on the central hexagons with centered nuclei and actin/β-catenin and a microtubules network denoting cell adhesion. After 2xa0days, hMSCs adapted their morphology and cytoskeleton proteins from cell-cell dominant interactions at the center of the hexagonal surface. This was followed by an intermediate zone with some external actin fibres/β-catenin interactions and an outer zone where the dominant interactions are cell-silicon. Cells move into silicon columns to divide, migrate and communicate. Furthermore, results show that Runx2 and vitamin D receptors, both specific transcription factors for skeleton-derived cells, are expressed in cells grown on micropatterned silicon under all observed circumstances. On the other hand, non-phenotypic alterations are under cell growth and migration on Si/PSi substrates. The former consideration strongly supports the use of micro-patterned silicon surfaces to address pending questions about the mechanisms of human bone biogenesis/pathogenesis and the study of bone scaffolds.

Characterization of thick graded Si1 xGex/Si layers grown by low energy plasma enhanced chemical vapour deposition

Thick, linearly graded-composition strained Si1� xGex/Si layers were recently developed for proton beam bending and extraction experiments. Such unrelaxed layers which are many microns thick necessitate a low maximum germanium content. Here, graded Si1� xGex epilayers, 5–20 lm thick with maximum Ge compositions of x ¼ 0:5–1.7%, grown by low energy plasma enhanced chemical vapour deposition were characterized using a recently developed mode of ion channeling analysis which is capable of quantifying the small lattice rotations along off-normal planar directions. Highquality 10 l mS i 1� xGex epilayers with bend angles along off-normal directions which agree very well with those of fully strained layers are successfully grown.

Ion beam irradiation induced fabrication of vertical coupling photonic structures

Two layer vertical coupling photonic structures can be directly fabricated on a standard SOI wafer using a combination of reactive ion etching (RIE) and proton beam irradiation followed by electrochemical etching. The top layer structures are defined by RIE on the device layer, while the bottom layer structures are defined by proton beam irradiation on the substrate. Light coupling between the structures in the two layers has been demonstrated via vertical coupling waveguides. According to simulations, the coupling efficiency mainly depends on the thickness of the two layer structure and the gap between them. In this process, the thickness of the top layer structures is fixed by the device layer thickness of the SOI wafer, which is typically 200-300 nm. The gap depends on the thickness of the oxide layer of the SOI wafer, and it can be shifted due to the natural bending of the top layer structures. The bottom layer structure thickness can vary due to different energies of proton beam. Furthermore we show the fabrication of tapered bottom waveguides, which are thin at the coupling region for higher coupling efficiency, and thick at the end for easily coupling light from an optical fiber or a focused lens.