Big sparks create glowing porous silicon.
A century-old device used to demonstrate sparking has helped shed new light on the mechanism of photoluminescence in porous silicon Porous Silicon (pSi) is a form of the chemical element silicon which has an introduced nanoporous holes in its microstructure, rendering a large surface to volume ratio in the order of 500m2/cm3. .
Since first reported almost two years ago, the discovery that light causes acid-etched silicon to glow red has created much debate about the reason for this luminescence luminescence, general term applied to all forms of cool light, i.e., light emitted by sources other than a hot, incandescent body, such as a black body radiator. . Some researchers attribute the glow to quantum confinement effects in microscopic silicon "wires" left behind after etching: Electrons confined by these wires recombine re·com·bine
To undergo or cause genetic recombination; form new combinations. with positive charges to produce light. Other evidence suggests that etching modifies silicon chemically and that either a silicon compound called siloxene or bonds between hydrogen -- or perhaps oxygen -- and silicon play a key role in luminescence (SN: 5/16/92, p.324).
Now physicist Rolf E. Hummel hummel
entire, naturally polled deer. and graduate student Sung-Sik Chang of the University of Florida University of Florida is the third-largest university in the United States, with 50,912 students (as of Fall 2006) and has the eighth-largest budget (nearly $1.9 billion per year). UF is home to 16 colleges and more than 150 research centers and institutes. in Gainesville have discovered an acid-free way to make silicon porous. Their approach limits the chances that silicon will undergo the chemical changes of etching, Hummel says.
He and Chang place two silicon wafers in a Tesla transformer and generate sparks between the wafers for several hours. "You can make very, very intense, large sparks, like lightning in a thunderstorm thunderstorm, violent, local atmospheric disturbance accompanied by lightning, thunder, and heavy rain, often by strong gusts of wind, and sometimes by hail. ," says Hummel.
The porous wafers then emit red light when subjected to a laser, they report in the Oct. 19 APPLIED PHYSICS LETTERS Applied Physics Letters is a weekly peer-reviewed scientific journal published by the American Institute of Physics devoted to the publication of new experimental and theoretical papers about applications of physics to science, engineering, and modern technology. . "The longer you spark, the more porous the silicon and the better the photoluminescence," Hummel notes.
To guard against possible reactions between the wafers and the humid Florida air, they eroded a few silicon wafers in pure, dry nitrogen. "There was no difference" in luminescence between the resulting wafers, Hummel says. "In our case, the siloxene, the impurity im·pu·ri·ty
n. pl. im·pu·ri·ties
1. The quality or condition of being impure, especially:
a. Contamination or pollution.
b. Lack of consistency or homogeneity; adulteration.
c. , and the hydrogen theories can be ruled out [as the cause of photoluminescence]."
These results, coupled with others reported during the past few months, "cast a lot of doubt on models that attribute luminescence to specific [chemical] species," says Leigh T. Canham of the Defense Research Agency in Malvern, England. He, Hummel, and Chang caution that some contamination could occur as the scientists transfer the very reactive silicon from the nitrogen atmosphere to the instrument that measures light emitted.
But Canham also cites work in which German and Japanese researchers heated porous silicon to high temperatures and then exposed it to oxygen. Their results support his idea that quantum confinement may be responsible for this luminescence, Canham says.