A team of scientists from the Indian Institute of Science, Bangalore have recently demonstrated the self assembly of a 2-D array of high aspect ratio, electrically conductive sharp tips fabricated on a soft, stretchable and bendable material by using capillary forces. The patented technology was developed by Ganapathy Saravanavel, Karthik Raghunandan and Prof. Sanjiv Sambandan at the Department of Instrumentation and Applied Physics at IISc.
A motivation for the use of soft materials such as Poly dimethylsiloxane (PDMS) is bio-compatibility and their elastomeric (stretchable and bendable) properties. “The development of sensors, actuators and electronics on such materials can lead to an excellent platform for bio-medical device and hand held flexible sheets that sense ambient information and display this information qualitatively. Such devices could form the backbone of low cost diagnostic tools”, says Dr. Sanjiv.
Conductive sharp tips find important applications in sensors and electronic displays. Of the various sensing and actuation platforms, field emitters are extremely versatile. Simply put, field emitters are sharp conductive tips. As Dr. Sanjiv explains, the advantage of a conductive sharp tip as compared to a more obtuse surface is that for a given potential, the tip has a high charge density. This, among several other parameters, permits easy ejection of electrons from the surface of the tip.
As the name suggests, field emitters are a critical component of field emission displays (FED). FEDs work much like the conventional cathode ray tubes (CRT) used in old television displays and computer monitors – an electron gun fires electrons accelerated to very high speeds using a high voltage which cause the excitation of a phosphor coated screen on striking it. In FEDs, arrays of numerous small conductive tips form the back end of the display and fire electrons to illuminate the screen.
Also important is the application of conducting sharp tips in sensing, specifically in profilometers which are instruments used to measure a surface’s profile. In Scanning Tunnelling Microscopy (STM) or Atomic Force Microscopy (AFM), two of the most widely used techniques for imaging surfaces, a sharp conducting tip is moved laterally over the sample, keeping it very close to the surface. To figure out the height of individual points on the surface, AFM monitors the force between the surface and the tip while STM measures the electric current that flows into the tip from the surface due to quantum mechanical tunnelling effect.
Development of field emitters with high aspect ratio (the ratio of the structure’s length to its diameter) is crucial for all the discussed applications, while developing them on soft and bendable materials would pave way to flexible or even shape shifting (morphable) electronic displays and profilometers for curved surfaces.
Typically sharp tips are fabricated using chemical etching of metals or semiconductors on a rigid flat surface. The novelty of the work is the development of close to ambient condition processes to fabricate these tips on soft materials by cleverly taking advantage of capillary action on a conducting gel.
In the process flow used for the fabrication, a structure housing a 2-D array of cylindrical pillars of dimensions in the millimetre scale was made out of PDMS. This PDMS stamp mounted on a translation stage - a structure that facilitates precisely controlled up and down movement - is dipped into a gel of metallic carbon nanotubes (CNT) in poly methyl methacrylate (maintained at 70) placed in a glass plate. The stamp is then slowly moved up for a few millimetres at a constant speed before it is brought to an abrupt stop. The subsequent wait period (about 40 s) allows for the cooling and solidification of the narrow bridge of CNT formed between the glass plate and the PDMS stamp due to capillary pressure, the same effect that lets a liquid rise up through a narrow tube despite gravity. On moving the stamp further up, this bridge like structure breaks off to form a sharp tip with a diameter of about 100 nm and length of 1mm (aspect ratio 10000). At the end of the process, the PDMS stamp is further coated with a thin film of gold to enhance conductivity from external electronics to the end of the CNT tip.
The team was successful in demonstrating their use as elastomeric field emission displays and profilometers for surfaces including the inside of a metallic pipe.