#177 from R&D
Innovator Volume 4, Number 9
Don't Ignore a
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).
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
However, this project seemed a bit too weird and I
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
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
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.
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.
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
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.
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
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