Current research into semiconductor clusters is focused on the properties of quantum dots-fragments of semiconductor consisting of hundreds to many thousands of atoms-with the bulk bonding geometry and with surface states eliminated by enclosure in a material that has a larger band gap. Quantum dots exhibit strongly size-dependent optical and electrical properties. The ability to join the dots into complex assemblies creates many opportunities for scientific discovery.

Semiconductor Clusters, Nanocrystals, and Quantum Dots

Dye-sensitized solar cells (DSCs) offer the possibilities to design solar cells with a large flexibility in shape, color, and transparency. DSC research groups have been established around the worl ...

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Dye-Sensitized Solar Cells

Novel physical phenomena can emerge in low-dimensional nanomaterials. Bulk MoS2, a prototypical metal dichalcogenide, is an indirect bandgap semiconductor with negligible photoluminescence. When the MoS2 crystal is thinned to monolayer, however, a strong photoluminescence emerges, indicating an indirect to direct bandgap transition in this d-electron system. This observation shows that quantum confinement in layered d-electron materials like MoS2 provides new opportunities for engineering the electronic structure of matter at the nanoscale.

Emerging Photoluminescence in Monolayer MoS2

Bulk nanostructured materials from severe plastic deformation

Nanotechnology is a multidisciplinary field, which covers a vast and diverse array of devices derived from engineering, biology, physics and chemistry. These devices include nanovectors for the targeted delivery of anticancer drugs and imaging contrast agents. Nanowires and nanocantilever arrays are among the leading approaches under development for the early detection of precancerous and malignant lesions from biological fluids. These and other nanodevices can provide essential breakthroughs in the fight against cancer.

Cancer nanotechnology: opportunities and challenges.

In this work, nanostructured porous silicon (pSi) prepared by a metal-assisted stain etched route is investigated for its ability to act as a carrier for sustained delivery of the antibacterial drug triclosan. The morphology, analyzed by transmission electron microscopy, reveals a rather different microstructure than traditional anodized porous silicon; as a consequence, such morphology manifests a different loaded drug crystallinity, triclosan release behavior, and associated antibacterial activity versus Staphococcus aureus relative to high porosity anodized porous silicon. In addition to electron microscopies and antibacterial disk diffusion assays, a combination of x-ray diffraction, thermogravimetric analyses, and UV/Vis spectrophotometric analysis of triclosan release are employed to carry out the above investigations.

Stain Etched Nanostructured Porous Silicon: The Role of Morphology on Antibacterial Drug Loading and Release

The response of a range of porous Si and poly Si films to storage in acellular simulated body fluids is summarised and its implications discussed. It is suggested that the combination of VLSI technology, micromachining and surface microstructuring achievable with silicon, could establish this prominent semiconductor as a very useful biomaterial by the next century. The ‘biocompatibility’ of a variety of silicon microstructures, and even bulk silicon has received surprisingly little study, but now warrants detailed in-vitro and in-vivo assessment.

Silicon as an Active Biomaterial

Excitation and recombination dynamics of the photoexcited charge carriers in porous silicon membranes were studied using a femtosecond pump-probe technique. Near-infrared pulses (800 nm, 60 fs) were used for the pump while, for the probe, we employed different wavelengths in the range between 3.4 and $5\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\text{m}$ covering the medium wavelength infrared range. The data acquired in these experiments consist of simultaneous measurements of the transmittance and reflectance as a function of the delay time between the pump and probe for different pump fluences and probe wavelengths. To evaluate the results, we developed an optical model based on the two-dimensional Maxwell-Garnett formula, incorporating the free-carrier Drude contribution and nonuniformity of the excitation by the Wentzel-Kramers-Brillouin model. This model allowed the retrieval of information about the carrier density as a function of the pump fluence, time, and wavelength. The carrier density data were analyzed to reveal that the recombination dynamics is governed by Shockley-Read-Hall and Auger processes, whereas the diffusion has an insignificant contribution. We show that, in porous silicon samples, the Auger recombination process is greatly enhanced at the wavelength corresponding to the infrared-active vibrational modes of the molecular impurities on the surface of the pores. This observation of surface-vibration-assisted Auger recombination is not only for porous silicon in particular, but for low-dimension and bulk semiconductors in general. We estimate the time constants of Shockley-Read-Hall and Auger processes, and demonstrate their wavelength dependence for the excited carrier density in the range of ${10}^{18}--{10}^{19}\phantom{\rule{0.28em}{0ex}}1/{\text{cm}}^{3}$. We demonstrate that both processes are enhanced by up to three orders of magnitude with respect to the bulk counterpart. In addition, we provide a plethora of the physical parameters evaluated from the experimental data, such as the dielectric function and its dependence on the injection level of the free carriers, charge-carrier scattering time related high-frequency conductivity, and the free-carrier absorption at the midwave infrared range.

Carrier dynamics and surface vibration-assisted Auger recombination in porous silicon

Routes of Formation for Porous Silicon

This paper describes why the authors view porous silicon (PSi) as an excellent vehicle for introducing university students to both basic and advanced materials science and engineering. Its ease of fabrication offers unique possibilities for university teachers to demonstrate nanoscale phenomena. Mechanical, physical, chemical and biological properties of PSi are amenable to laboratory tests employing a variety of modern but fairly standard characterization techniques. Its radically different behaviour from bulk silicon, but compatibility with VLSI technology, can be used to give students a taste of not only materials issues in electronics, but also optoelectronics, micro-optics, sensors, solar cells, micromachining, acoustics, medicine, biotechnology and even astrophysics! By way of an example, we analyze practical experience accumulated at the Technical University of Valencia during four years of teaching courses related with materials science. The introduction of PSi research into the curriculum has led to good synergy between theoretical aspects and practical studies, and has also improved personalization of studies where each student realizes their own nanostructure, and then has a choice of which material property and application area to delve into. Some prospects for future educational developments are then suggested.

Leigh T. Canham

Papers

Stain Etched Nanostructured Porous Silicon: The Role of Morphology on Antibacterial Drug Loading and Release

Silicon as an Active Biomaterial

Carrier dynamics and surface vibration-assisted Auger recombination in porous silicon

Routes of Formation for Porous Silicon

Porous Silicon as an Educational Vehicle for Introducing Nanotechnology and Interdisciplinary Materials Science