Nanostructured carbon materials are potentially of great technological interest for the development of electronic1,2, catalytic3,4 and hydrogen-storage systems5,6. Here we describe a general strategy for the synthesis of highly ordered, rigid arrays of nanoporous carbon having uniform but tunable diameters (typically 6 nanometres inside and 9 nanometres outside). These structures are formed by using ordered mesoporous silicas as templates, the removal of which leaves a partially ordered graphitic framework. The resulting material supports a high dispersion of platinum nanoparticles, exceeding that of other common microporous carbon materials (such as carbon black, charcoal and activated carbon fibres). The platinum cluster diameter can be controlled to below 3 nanometres, and the high dispersion of these metal clusters gives rise to promising electrocatalytic activity for oxygen reduction, which could prove to be practically relevant for fuel-cell technologies. These nanomaterials can also be prepared in the form of free-standing films by using ordered silica films as the templates.

Ordered nanoporous arrays of carbon supporting high dispersions of platinum nanoparticles

http://nathan.instras.com/documentDB/paper-69.pdf

Porous silicon: a quantum sponge structure for silicon based optoelectronics

MICROELECTRONIC device integration has progressed to the point where complete 'systems-on-a-chip' have been realized1–3. Now that optoelectronics is becoming increasingly important for information and communication technologies, there is a need to develop optoelectronic devices that can be integrated with standard microelectronics. Conventional semiconductor technology is largely based on crystalline silicon, which (being an indirect bandgap semiconductor) is an inefficient light-emitting material. This has stimulated significant effort towards developing silicon-based optoelectronic components and, of the several strategies explored so far4,5, the use of porous silicon appears the most promising; porous silicon produces high-efficiency, room-temperature, visible photoluminescence6, and its material and optical properties have been studied in detail7,8. But the extreme reactivity and fragility of porous silicon have hitherto prevented its integration with conventional silicon processing technology. We have recently shown9,10 that the thermal and chemical stability of porous silicon can be greatly enhanced — while retaining desirable light-emitting and charge-transport properties — by partial oxidation. Here we take advantage of these improvements in material properties to demonstrate the successful integration of silicon-based visible light-emitting devices into a standard bipolar microelectronic circuit.

Silicon-based visible light-emitting devices integrated into microelectronic circuits

Climate models with increased levels of carbon dioxide predict that global warming causes heating in the tropics, but investigations of ancient climates based on palaeodata have generally indicated cool tropical temperatures during supposed greenhouse episodes. For example, in the Late Cretaceous and Eocene epochs there is abundant geological evidence for warm, mostly ice-free poles, but tropical sea surface temperatures are generally estimated to be only 15-23 degrees C, based on oxygen isotope palaeothermometry of surface-dwelling planktonic foraminifer shells. Here we question the validity of most such data on the grounds of poor preservation and diagenetic alteration. We present new data from exceptionally well preserved foraminifer shells extracted from impermeable clay-rich sediments, which indicate that for the intervals studied, tropical sea surface temperatures were at least 28-32 degrees C. These warm temperatures are more in line with our understanding of the geographical distributions of temperature-sensitive fossil organisms and the results of climate models with increased CO2 levels.

/pdf/warm-tropical-sea-surface-temperatures-in-the-late-u2gdc2lwqi.pdf

Warm tropical sea surface temperatures in the Late Cretaceous and Eocene epochs

There is an urgent requirement for an optical emitter that is compatible with standard, silicon-based ultra-large-scale integration (ULSI) technology. Bulk silicon has an indirect energy bandgap and is therefore highly inefficient as a light source, necessitating the use of other materials for the optical emitters. However, the introduction of these materials is usually incompatible with the strict processing requirements of existing ULSI technologies. Moreover, as the length scale of the devices decreases, electrons will spend increasingly more of their time in the connections between components; this interconnectivity problem could restrict further increases in computer chip processing power and speed in as little as five years. Many efforts have therefore been directed, with varying degrees of success, to engineering silicon-based materials that are efficient light emitters. Here, we describe the fabrication, using standard silicon processing techniques, of a silicon light-emitting diode (LED) that operates efficiently at room temperature. Boron is implanted into silicon both as a dopant to form a p-n junction, as well as a means of introducing dislocation loops. The dislocation loops introduce a local strain field, which modifies the band structure and provides spatial confinement of the charge carriers. It is this spatial confinement which allows room-temperature electroluminescence at the band-edge. This device strategy is highly compatible with ULSI technology, as boron ion implantation is already used as a standard method for the fabrication of silicon devices.

An efficient room-temperature silicon-based light-emitting diode.

Two different unmodified industrial CMOS processes have been used for the integration of highly interdigitated pn structures. Under forward bias these pn junctions emit narrow-band infrared light at 1160 nm with an electrical-to-optical power conversion efficiency of typically 10 −4 . The same junctions show broad-band visible-light emission between 450 and 800 nm in the avalanche breakdown region under reverse bias with efficiencies of the order of 10 −8 . This is already enough for a first few practical applications as light-emitting devices (LEDs). No satisfactory explanation for this emission efrect, fitting all the experimentally observed electro-optical and physical properties of our silicon LEDS, has been found yet.

Light-emitting devices in industrial CMOS technology

Visible light emission from Si materials

We compare the photoluminescence (PL) properties of silicon (Si) single quantum well structures, consisting of an either amorphous or crystalline Si layer of 3 nm thickness, embedded between silicon–nitride layers. These structures are grown by plasma‐enhanced chemical vapor deposition on Si substrates. After crystallization of the originally amorphous Si layers and passivation by hydrogen, strongly polarized PL is observed in the entire visible spectrum. In contrast, a reference structure without the 3 nm silicon layer, but otherwise identical, shows PL in the red and infrared only, but also strongly polarized. Thus, the PL in the blue and green part of the spectrum comes from recombination via Si quantum well states, while the red and infrared PL is due to states at the interface between crystalline Si and silicon–nitride.

Light emission from a silicon quantum well

Samples of porous silicon, produced electrochemically with a spatially varying current density, have have studied by Raman scattering, by spectral photoluminescence (PL) and by photoluminescence topography. The PL maximum observed at about 1.7 eV shifts to higher energies (2.0 eV) for the strongest structuring, where the highest PL is also found. These results give evidence for an effective direct bandgap in this material, which is a condition for the high PL. For a different kind of sample, produced by reactive ion etching, a very low wavelength PL peak is observed at 550 nm (2.0 eV). A model of spherical crystalline silicon particles (of up to 706 silicon atoms), saturated at their surface with hydrogen, has been studied by density functional theory. The calculation shows that this nanocrystalline silicon has a direct gap which is substantially larger than the bulk (indirect) gap and is consistent with experimental observation.

http://iopscience.iop.org/article/10.1088/0031-8949/1992/T45/066/pdf

E.F. Steigmeier

Papers

Light-emitting devices in industrial CMOS technology

Visible light emission from Si materials

Light emission from a silicon quantum well

Optical studies on silicon "quantum wires"

Light emission from recrystallized amorphous Si MQW structures