#177 from R&D Innovator Volume 4, Number 9          September 1995

Don't Ignore a “Weird” Idea
by Magnus Granström                   

Mr. Granström is completing his Ph.D. thesis in the department of physics and measurement technology, Linköping University, Sweden.     

The first years of my Ph.D. research involved studying electrically conducting polymer fibers that were grown in the pores of commercially available micro-filtration membranes.  Though the research was going well, it wasn't very exciting. And my supervisor told me to think about other possible applications. One of my colleagues suggested that I integrate my work with that of another project in the laboratory, studying polymer light-emitting diodes (LEDs).

We discussed whether the small cylindrical pores in my membranes, when filled with a suitable polymer, might work as one of the electrical contacts in a diode structure. If the diameter of the pores were small enough, say 100 nm, the size of the diodes might become smaller than the wavelength of the emitted light. Could be an interesting phenomenon!   However, this project seemed a bit too weird and I dismissed it.       

Thinking About It

Two weeks later, as I was hanging around the lab waiting for a sample to dry, I began to think that maybe the diode project would be worth at least a little effort. Also, we were expecting a visit in another month by a researcher working on sub-micron optics and scanning near-field microscopy, so I thought I would give it a try.  Since resolution is dependent on the size of the light emitting area, here was a possible application of such small diodes.                                                          

The idea was fairly simple.  Fill the pores of a filtration membrane with a conducting polymer to make the electrical contacts. Place a thin layer of an electroluminescent polymer on top of the membrane.  And then by means of vacuum evaporation, deposit calcium, which is the electron injecting material, on top of the electroluminescent polymer. This electrode injects electrons into the electroluminescent layer and holes (positive charge carriers) are injected from the conducting polymer. When the electrons and holes combine, light is emitted.                                  

There were many reasons why the experiment might fail. And, if it failed, it could take years to find out what was wrong--if, indeed, it would ever work at all. For instance, the timing for electropolymerization of the material used as a hole-injecting contact had to be such that the       appropriate contact structure was made. Would the polymer work at all as a hole-injector? The calcium contact layer had to be sufficiently thin to allow light to be emitted through it--would it then work as a contact? And so on.      

Doing It                                                            

I decided to do a Friday "quick-shot" experiment. I got to the lab early in the morning, when it's easiest to get work done, and started with my preparations. The first steps were fairly simple: attaching the membrane to a gold-covered glass substrate (the anode), and then starting the        polymerization. I chose a conjugated polymer that had suitable stability and electrical properties. Using scanning electron microscopy, I determined the time to stop the polymerization before it grew out of the pore and onto the upper membrane surface.                                              

I made samples with pore sizes 100 nm and 10 µm, the latter so I would be able to see something (perhaps light?) under an ordinary microscope. When the contact structures were prepared, I began to worry whether some pores would be only partially filled and not reach the surface, or whether perhaps the polymer had spilled over the surface. Since I didn't have the time to polish the samples in a controlled way, I just did a crude polishing.   

The next problem arose when I realized the solvent (chloroform) for the electroluminescent polymer would also dissolve the membrane. Fortunately, the person who synthesized the polymer indicated that xylene could be used as the solvent instead.                                                  

I then encountered another problem: it was impossible to make usable spin-coated films from room-temperature xylene solutions. The polymer simply vanished from the membrane surface.  (When spin-coating, your rotate the substrate at a speed of about 500 to 2000 rpm, so when the solvent evaporates too slowly, everything simply flies away.)  Could I perhaps heat the solution without destroying anything? Spending some time on this and ruining a bunch of samples, I finally determined that a warm xylene solution of the electroluminescent polymer might work. I spincoated five samples and evaporated the solvent.     

Being Watched                            

The lab had a computerized setup which made it possible to simultaneously measure current-voltage characteristics and light output. Somehow, the word had gotten out that some weird experiments were going on, and our small corner began to attract quite a crowd.                                                                  

I decided to begin with one of the 10 µm samples, since they would give more light than the 100 nm ones--if they worked at all. The test was disappointing: no measurable current went through the device, apart from very short periods where we also could detect light flashes. Since this is what might be expected with all the possibilities of bad contacts, pinholes, etc., we weren't too surprised. The next sample exhibited the same properties. The crowd began to disperse, thinking this was just another unsuccessful experiment.                                        

Since I wasn't producing light with "normal" voltages for the kinds of polymer diodes I was using, I absentmindedly decided to increase the voltage and burn them out. Perhaps I was "getting back" at the system for wasting my time.                 

Wasting My Time                                                            

That was one of the luckiest decisions of the day--because at three times higher voltage than usual, the curve showing emitted light began to shift from the zero level! Repeating the experiment gave the same result. I then switched to a 100 nm sample, and it also gave light!                     

I called back my colleagues and repeated the experiment a few more times.  I convinced them that it really was light from the diodes, and not flashes from short circuits. Later, I proved the existence of diodes by showing that the emitted light had the correct wavelength and that the light sources were sufficiently small.                                        

The diodes I got that Friday were far from perfect; but the important thing was that their existence was proven.  I recently made a second generation of nano-LEDs, using a much simpler technique. Our results have been published in Science (267: 1479, 1995) and have attracted quite a bit of interest.  The main goal now is to make the diodes individually addressable so that it will be possible to shine light on a well-defined position--say a portion of a single animal cell.                                                 

What this teaches us is that it may be easy to miss an important opportunity by disregarding weird ideas. Perhaps, more importantly, we should also not be afraid to perform "not-so-well-prepared" experiments--because sometimes we may find something completely new.

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