Research Experience

Introduction

This summer I worked with graduate student Dhruv Sakalley on the development of nanotechnology-related lab activities for undergraduates in Drexel's Applied Engineering Technology program in the laboratory of Dr. Michael Mauk.  The labs were adapted from modules developed by the University of Wisconsin-Madison Materials Research Science and Engineering Center (UW MRSEC) http://mrsec.wisc.edu/Edetc/, and were originally designed to introduce high school students and first-year undergraduates to nanotechnology and materials science.  Dr. Mauk's goal is to further develop these labs to add a statistical analysis and optimization component (six sigma) that will allow students to approach the lab activities in a process-oriented manner.  Thus, the ultimate goal of these activities will be not only to make devices that work, but also to make the devices work as well as possible.

Part 1:  Developing an organic LED

Our first task was to modify a lab designed to develop and test organic thin film light-emitting diodes, based on a paper by Gao and Bard at the University of Texas at Austin (see attachment #1 - organic led.pdf).  Organic LEDs, or OLEDs, are LEDs that obtain their luminescence from a polymer-based layer rather than an inorganic semiconductor.  One of the advantages of OLEDs over traditional LEDs is that they are lighter and more flexible, however they are less efficient, more expensive to produce, and have shorter lifetimes.  Thus developing a method that improves the lifetime of such devices could make them more amenable to commercial applications.

Procedure and Photos

Using a homemade spin-coating device, we coated glass plates covered with a layer of indium-tin oxide (which served as the device's electrode) with a ruthenium (II) ion complex embedded in a polymer matrix.  We then dotted a gallium-indium eutectic alloy, which served as the cathode, on top of the thin film.  When a current is applied to the device, ruthenium (II) ions are oxidized to ruthenium (III) at the anode and reduced to ruthenium (I) at the cathode.  Electrons hop from one ion to another, and eventually the two ruthenium ion species combine to form an excited state ruthenium (II), which emits a reddish-orange light via phosphorescence when it returns to its ground state.  The following pictures were taken from the University of Wisconsin-Madison MRSEC web site:

 
Here are some photos I took in the lab:
 

Results

One of the goals of our research was to develop ways to optimize the lifetime of the organic LEDs.  We used a resistor to limit the amount of current running through the device.  Also, the ruthenium (II) complex is sensitive to humidity, and exposure to air limits its lifetime and light output.  Additionally, we found that the uniformity of the two films applied to the slide, as well as the contact area between the lead of the power supply and the eutectic, affected the lifetime of the device.  Another difficulty we encountered was developing a procedure to generate a uniform coating of film and eutectic -- applying the eutectic had the unfortunate effect of removing some of the ruthenium film from the surface of the slide.
 
For a later experiment, we applied the eutectic gently to the slides in a 2 x 3 array and attached wire leads to each dot in the array.  We also sealed the device with superglue to limit the amount of moisture at the surface.  We found that the lifetime of the LED is significantly reduced by the superglue, but that the LEDs that were not covered with superglue remained lit for more than 5 minutes.  Placing scotch tape over the LEDs prevented them from lighting, but covering the slide with a chemically inert, transparent tape dramatically increased the lifetime to more than 10 minutes.

Part 2:  Nanocrystalline Solar Cells

Our second lab activity involved the fabrication of small solar cells from titanium dioxide nanoparticles and a dye derived from raspberries.  Unlike conventional  photovoltaic cells, in which light absorption and charge separation are performed simultaneously by silicon crystals, the dye-based solar cells perform these processes in two different steps.  A dye that is chemically attached to a semiconducting layer of TiO2 particles absorbs light.  Electrons are then transferred from the dye to the semiconducting layer at one electrode and a mediator (iodide solution) is oxidized at the counter electrode.  When the electrodes are connected, the electrons at the semiconducting layer reduce the iodide mediator at the counter electrode, completing the circuit.
 

Procedure and Photos

For our first attempt, we modified a procedure from the "Nanocrystalline Solar Cell Kit" instructors' manual.  The first solar cell we prepared provided a maximum voltage of 0.410 V when held close to an overhead projector lamp and connected to a 500 ohm resistor.  In bright afternoon sunlight, the device provided approximately 0.315 V.  The pictures below were taken from the University of Wisconsin - Madison MRSEC web site:

 
 

Results

For our first attempt, we connected the device to a 500 ohm resistor and measured the voltage produced when the device was exposed to an overhead projector lamp.  We found that the voltage ranged from 0.205 V to 0.415 V depending on how close the device was held to the lamp.  The maximum current produced was 8.0 mA.
 
We then took the device outside in bright sunlight at about 2:30 pm.  The device generated 0.315 V when attached to the 500 ohm resistor.  The maximum current generated was 0.70 mA.  We found that the device worked best with the TiO2 layer facing the light source.


Part 3:  Cd-Se Quantum Dots

For our third lab activity, we prepared Cd-Se quantum dot nanoparticles from elemental Se powder, CdO, oleic acid, and trioctylphosphine by heating these materials in octadecene at 225 degrees C.  Quantum dots are extremely small semiconductors (roughly 2-10 nm in diameter).  Because of their size, they differ from bulk semiconductor materials in that their electron energy levels are discrete rather than continuous.  This feature gives them varying properties such as color; the nanoparticles we prepared in this lab, for example, vary in color from greenish yellow to reddish orange depending on their size.
 

Procedure and Photos

We followed the procedure for preparing quantum dots outlined on the University of Wisconsin - MRSEC web site.  After adding the Se solution, we removed 1 mL aliquots at 10 second intervals.  The fractions we isolated ranged in color from light yellow to dark orange.
 
 
Research funded by NSF grant #0836237: "Development of the Laboratory-Based Course in Lean Six Sigma Nanomanufacturing".

 
Ċ
ajconstans@yahoo.com,
Jul 17, 2009, 6:34 AM
Ċ
ajconstans@yahoo.com,
Jul 17, 2009, 6:40 AM
Ċ
ajconstans@yahoo.com,
Jul 17, 2009, 6:01 AM
Ċ
ajconstans@yahoo.com,
Jul 17, 2009, 6:35 AM
Comments