Bright future for silicon

The Wiley-VCH journal ChemPhysChem issued an embargoed press release embargoed early on the morning of November 21, 2012, heralding "a bright future for silicon." Just eight hours later, they lifted the embargo, citing "early reporting" of the research by Brian Korgel of the University of Texas (Austin, TX) and colleagues.

Embargo breaks often indicate hot stories, and the headline hinted at an important step toward the elusive goal of efficient light emission from silicon. Yet the next line was more muted: "Ordered nanocrystal arrays may provide a new platform to study and tailor the light-emitting properties of silicon." What is the real story?

Silicon is a wonderful material for electronics, but its photonic uses have been hobbled by an indirect bandgap that makes it very hard for electrons dropping into the valence band to release their energy as photons. That leaves silicon far behind III-V compounds like gallium arsenide for LEDs and diode lasers. Yet silicon is far ahead of other semiconductors in electronics, and companies like Intel (Santa Clara, CA) want to integrate photonics into their integrated circuits.

So far they have demonstrated "silicon lasers" by optically pumping Raman lines in silicon and III-V diode laser chips bonded to silicon. Both were important advances. But neither met the real goal--electrically powered emitters based on silicon that could be integrated into standard semiconductor chip production processes.

In their ChemPhysChem paper, Korgel and colleagues take a different approach, tapping the bright luminescence produced by silicon quantum dots. They write that their major achievement is devising a chemical technique that causes self-assembly of "the first colloidal Si nanocrystal superlattices." Self-assembly is essential because individual dots are too small to fabricate by conventional photolithography, and transmission electron microscope images show the dots are closely spaced in regular face-centered-cubic arrangements (see photo).

TEM image silicon nanocrystals in the 111-oriented (c) and 112-oriented (d) plans, with depictions of the crystalline structures shown in insets. (Courtesy Yixuan Yu et al., ChemPhysChem, Wiley-VCH Verlag GmbH & Co. KGaA, http://dx.doi.org/10.1002/cphc.201200738 [2012]. Reproduced with permission)

The authors say that covalent bonds with the hydrocarbon solvent make the silicon-nanocrystal superlattices stable to 350 degrees Celsius, higher than other similar superlattices. That's encouraging news, because self-organized nanocrystals are a promising fresh approach to structuring silicon to emit light more efficiently. But so far electrical excitation--sought for integrated optoelectronics--has far to go to match the efficiency of optical excitation of isolated silicon quantum dots. So Korgel is understandably optimistic about having "a new playground for understanding and manipulating the properties of silicon in new and unique ways," and is appropriately cautious in not claiming silicon lasers are just around the corner.

Making solid-state lighting fun

Solid-state lighting is a clean, green new market for optical technology, but it's hard to get very excited about white LEDs that merely replace older incandescent and fluorescent bulbs. Now, Philips is trying to make solid-state lighting fun with wirelessly controlled color-tunable bulbs called "Hue".

A Hue bulb screws into a standard light socket and contains red, green, and blue LEDs. A smartphone or iPad app controls the bulb's output through a wireless controller and a wireless receiver in the bulb. The app matches the LED outputs colors selected from a rainbow palette in the app, or from the user's favorite photos. Users can pick bright disco colors, shades of white from candlelight to sunlight, or anything in between.

A $200 starter set including the controller and three bulbs sounds like an impulse buy at the Apple Store -- and that's exactly where Philips is selling it, as a fun gadget. A single 600-lumen Hue bulb will set you back $60, more than triple the price of a Philips Ambient bulb that emits a pleasant white light. But playing with colored lights is much more fun, as Philips shows in a video.

The Hue isn't just a party light. You can set it to emit shades of white from a bright "energize" tone to start the morning to a warm "relax" shade to unwind in the evening. You can set each bulb to turn on and off when you want it. So it's an all-purpose adjustable light ready to put into any socket in the house, without costly rewiring.

Philips is first to market, but company is coming. LiFx (San Francisco, CA) in September sought support on Kickstarter to develop their own smart bulb, and was surprised to receive $1.3 million in pledges when they had sought only $100,000. They have demonstrated a bench version and now are designing a production prototype, which will include a white LED as well as the RGB emitters.

So far press attention has focused on controls and tunable colors, but I wonder what the green sources are. Philips is using a "lime green" LED from its LumiLEDs division because it gives better color rendering than standard green LEDs, but won't disclose the wavelength or composition. Is it a hard-to-make green LED, a phosphor-LED hybrid, or something else?  If anybody out there has a spectrophotometer and a Hue at hand, it would be interesting to see a spectrum.

iPhone sets a Philips Hue bulb to "relax" for a calming evening. (Courtesy of Philips Lighting)