analysis of carbon nanotubes and quantum dots in a photovoltaic device
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DESCRIPTIONAnalysis of Carbon Nanotubes and Quantum Dots in a Photovoltaic Device A poster prepared by Francis and me; presented by Francis. I modified on of the photographs used, in this copy.
- The choice of carbon nanotubes is based upon their infinitesimal diameter and resulting large surface area to volume ratio, resilient structural integrity and high-speed carrier transport capabilities facilitated by ballistic conductivity.Quantum dots will also be integrated on account of their high quantum efficiency, high surface to volume ratio, as well as their optical non-linearity , .
- Carbon Nanotubes :
- are single layer carbon atom (graphene) sheets of hexagonal structure rolled up into seamless cylinders
- are of two types Single-Walled Nanotubes (SWNTs) or Mulit-Walled Nanotubes (MWNTs)
- SWNTs consist of only one cylinder with diameter between 0.33 and 5nm and length between 2 and 10nmthis is the type of nanotube that has been used for this project
- have great strength (50 times stronger than steel while only a quarter as dense) providing for a more resilient device
- are one-dimensional structures thereby enabling ballistic transport
- Quantum Dots :
- are nanostructured semiconductors whose exitons are quantum confined in all planes of 3 dimensional space hence they canbe considered a 0-dimensional structure
- range from 2-10nm (10 to 50 atoms) in diameter
- have a very high surface to volume ratio due to their small size, thus more photons are absorbed per square centimeter by a quantum dot as opposed to bulk material (photon absorption is a surface process) resulting in a high quantum efficiency
- Methods and Materials
- Quantum Dot Preparation :
- 5ml of CdNTA and 0.6ml of decylamine were placed in one beaker and 7.5ml of Na 2 SeSO 3was placed in another beaker.
- The temperatures of the two beakers were equalized in a water bath at 55 o C for 2-3 minutes.
- The contents of the two beakers were then combined and stirred manually.
- 5ml of toluene were then added to the mixture, stirred manually and allowed to sit for 5 minutes.
- 5ml of toluene was again added, stirred and set for 5 minutes.Toluene addition, in this manner, was continued until a total of 20 ml was added to the quantum dot (QD) solution.
- After the final addition of 5ml toluene, the QD solution was left in the bath for 5 minutes and then removed to cool at room temperature for an hour.
- The CdSe QD solution was then separated from the water suspension by decantation, combined with a polymer solution (0.05g of polystyrene (PS) per 1ml of toluene), and sonicated for 48 hours before being stored in multiple, small, sealed cuvettes in a refrigerator.
- Single Walled Carbon Nanotube (SWCNT) Purification :
- 100mg of unpurified SWCNT were heated at 470 o C for 1 hour.
- The sample was cooled down in air, washed with HCL, and sonicated for 30 minutes.
- The sample was then centrifuged at 6000 RPM for 5 minutes and then at 13000 RPM for 5 minutes.
- The HCL was then pipetted out and the SWCNT were washed three times.
- The washing process included sonication with distilled water for 30 minutes, centrifuging at 6000 rpmfor 5 minutes then at 1300rpm for 5 minutes, followed by the removal of distilled water via pipette.
- Finally, the thoroughly washed SWCNT were dried at 100 o C.
- Material Combinations :
- The SWCNT and QDs were characterized in the following combinations: SWCNT, SWCNT + PS, QD, QD + PS, SWCNT + QD, and SWCNT + QD + PS.
- Film Making :
- The same film making procedure was used on all organic solutions:
- The spin-coater was spun (with a clean quartz disk) at 100 rpm.
- The sample was added drop wise to the center of the disk with a pipette until the entire surface was covered.
- The speed was then slowly increased to 1000 rpm, maintained at that level for 20 seconds and then slowly reduced to 200 rpm for 20 minutes in order to dry the sample.
- The spin speed was then quickly increased to 2000 rpm.(This instantaneous speed change was necessary to flush out any solution that may have leaked under the surface and collected at the bottom of the disk.)
- New layers were added using the same procedure.
- When a film of desired thickness was obtained, the bottom of disk was cleaned with a wipe soaked in acetone.
- Characterization :
- Each material was characterized for its absorption spectrum (using an absorption spectrometer) and two-photon absorption and non-linear refractive index characteristics (using open and closed aperture z-scans, respectively) both in solution and as film.
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