Suspended Carbon Nanowire Sensors
May 24, 2012, MEMS Business Forum, Santa Clara, CA—Marc Madou from UC, Irvine described his research in nano-materials. The range of applications for his processes spans batteries, battery sensors, and nano-wire sensors.
Making a sensor from carbon nano-materials eliminates the machining needed for most MEMS. This enables the development of complex structures that are not possible in other methods. The material characteristics are easily modified through changes in processing and the addition of other materials.
By charcoaling an organic material, you can get an object that preserves the basic cell structure of the original material. Putting a polymer into an oven at 900C for an hour changes the organic material into a glassy carbon part. Changing the processing temperature to 600-750C results in a high sheet resistance material. The most interesting feature is that the original molded material shrinks by over 80 percent.
These organic precursors become mostly glassy with a graphitic surface, which makes a good electrode. By adding lithium to the precursor, you can make the device act as a battery. Changing the processing results in high resistivity, but not all precursors work for high resistance.
So far, many applications have been identified for these materials, including amplifiers, dielectrophoresis, fuel cells, bulk metallic glasses molding, and continuous writing of polymer nano-wires. This last item leads to wash-line nano-sensors.
Some of the important characteristics of these sensors are: nano-wires are almost all surface, so any surface change translates to very large resistance changes. By suspending the nano-wires, you eliminate bulk surface effects and allow access from all directions.
The processing starts with electro-spinning of mats of polymer fibers. These mats are carbonized, resulting in a carbon mat with high surface area. The use of photoresist in the electro-spinning operation retains the photo patternable characteristics before charcoaling. The starting viscosity affects the final structures, ranging from random threads to strings of beads.
The near field electrical field is used only for droplet initiation, and mechanical stage movement is applied to stretch the materials. Using very short electro-spinning periods in far field on patterned surfaces leads to suspended polymer wires that make ohmic contact with the supports.
The ability to make nanometer-dimension threads had resulted in an interesting tangential area of research. Color is a function of chemical, physical, or combination of both properties. Chemical color is the result of pigments, while structural color is due to the interaction of light with a featured structure. Butterfly wings are an example of structural color. A pure white is the result of structures that are random collections of size. One possible application for these materials is to make long-lasting paints. Another is to create pressure sensitive colors.
The field of nano-wires is just starting to open up. The possibilities for new sensors and new types of materials from polymer precursors is now proven. The next steps are to apply this new knowledge to applications and functions that are useful.