January 2003 Meeting

The Optical Society of San Diego,
and host Jim Menders, are pleased to present this evening's speaker,
Professor Michael Sailor, of the Department of Chemistry at UCSD.

 

"Smart Dust: Photonic Crystals Derived
from Nanocrystalline Porous Silicon"

Professor Michael Sailor
Department of Chemistry UCSD

http://chem-faculty.ucsd.edu/sailor/research/highlights.htm

 

ABSTRACT: A method to construct small particles of silicon, each smaller than the diameter of a human hair, that contain photonic codes will be described. We refer to this material as "smart dust" because each particle can be chemically programmed to perform simple tasks such as filtering liquids, concentrating gases, testing for toxic chemicals, or detecting biological chemicals in a Petri dish. Use of such materials in high-throughput screening applications and in remote sensing of toxins or pollutants will be discussed. A silicon smart dust particle is encoded by generating sinusoidal layers of nanometer-scale porous structures in a silicon wafer using a programmed electrochemical etch. The film is released from the silicon wafer and small particles are produced by ultrasonic fracture. The layered nanostructure acts as a one-dimensional photonic crystal, or rugate filter, reflecting light of very sharp frequencies. Multiple spectral lines can be etched into a single particle, and the codes are read using a small low powered laser or with a hand-held spectrometer.

 

Speaker: Professor Michael J. Sailor received his Ph.D. in chemistry from Northwestern University in 1988 for work on the synthesis of organometallic metal clusters followed by postdoctoral research studying semiconductor photoelectrochemistry at Stanford and Caltech. He began his faculty appointment to the department of Chemistry and Biochemistry at the University of California at San Diego in 1990. Professor Sailor is a member of the Executive Steering Committee of the UCSD Materials Science division, and he is on the editorial boards of Advanced Materials and Inorganic Chemistry Communications. He is the author of 4 patents and over 90 research publications, in subjects related to nanotechnology, materials chemistry, sensors, and electrochemistry. He has supervised over 70 graduate, undergraduate, and post-doctoral students.

 

January OSSD Meeting Review

"Even Our Dust is Getting Smarter"
by Jim Menders

First we got smart bombs. Now we get smart dust. January's speaker, Mike "Professor Porous Silicon" Sailor (UCSD Chemistry) recalled that the term originally referred to particle sized systems combining sensing, processing and telemetry. Sailor's smart dust is not quite that smart. But it has a pretty high IQ in the sensing aspect. We heard about chemical sensing particles whose spectrum was affected in a tell tale way in the presence of some target chemical. Dust sensitivity was portrayed very dramatically in a video showing a shift in the color of the material in a cell from green to red upon the addition of acetone vapor. The dust itself consists of flakes of porous silicon-a result of breaking up thin sheets. The porous silicon sheets have a sinusoidally modulated refractive index, not unlike a multilayer dielectric film. The index is modulated by carefully controlling the parameters of an electro-chemical etching process that makes silicon porous. A silicon substrate is immersed in an acid etchant in the presence of a current flow, producing holes of controllable width ranging from a few nanometers to a hundred microns. The index varies approximately in proportion to the cross section of the holes. Hundreds of refractive index layers can be created by controlling the current flow during the etching.

This technique provides the smart dust designer with a greater range of index to draw from than conventional optical material systems. The simplest index structures are sinusoidally varying or rugate structures, which exhibit a single band of reflectivity at the Bragg wavelength. The acetone-sensitivity of the dust that I mentioned above is due to the increase of the refractive index of the porous silicon layers as the acetone is absorbed into the pores. More complicated structures, in which layer thicknesses are varied, can be used to produce a series of reflection bands at specific wavelengths. In this way, flakes with unique optical signatures can be produced to use as "barcodes" for biological tagging applications.

Prof. Mike Sailor (left) talking with Brian Catanzaro and
another guest at the January Meeting of the OSSD.
(Photo by K.Cummings)

 

 

I noted in my introduction of Professor Sailor, that his students has a stranglehold on our Science Fair awards. For the past three years, Mike has has welcomed high school student interns into his lab, who go on to receive OSSD sponsored awards. He related how his first student, launched into science by the experience, is now a graduate student at Cal. Thanks for a fine talk, Mike, and for your spirited educational outreach.