Viability of 3-D printing semiconductors: Cadmium Selenide nanoparticles for water purification purposes
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- 1 Understanding the market
- 2 Advantages of Quantum Dots Over Current Solar cells
- 3 How CdSe Quantum Dots Work
- 4 Project goals
- 5 CdSe Nanocrystal "Ink" Synthesis
- 6 Characterization of CdSe
- 7 Design
- 8 Costs
- 9 Discussion
- 10 Conclusions
- 11 References
- 12 Contact details
Understanding the market
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Advantages of Quantum Dots Over Current Solar cells
- Don’t rely on a single p-n junction configuration
- Tandem cells-stacks of p-n junctions structures
- Theoretical energy conversion efficiency of up to 66% (E.446)
- Optical and electronic properties can be tweaked by changing the size of Quantum Dots (E.446)
- Able “to inject electrons to a wider band gap material, such as TiO2”(Etgar 452)
- Higher light to energy efficiency
- Easy to manufacture
- Low cost
- Capable of layer-by-layer deposition (Webber p.7835)
- Theoretical Energy Conversion Efficiency of 66% (Etgar)
How CdSe Quantum Dots Work
Semi-conductive nano-particles also known as Quantum Dots (QDs) absorbs light in solar cells. The characteristics of QDs that are needed for transferring the absorbed light’s energy is its conduction and valence bands of the QDs permit electron injection and hole transportation through to the metal oxide and metal layers, which the QDs is between (Etgar 448). The amount of light absorbed by the Quantum dots depends on its thickness if it is too thick, the collection of photogenerated charge carriers is incomplete, while too-thin QD layers show poor light harvesting (E. 448). The QDs size also plays a factor in its performance when open circuit voltage, fill factor and photocurrent decrease with increasing the QD size; however, inner-particle electron transfer is facilitated in films made of the larger QDs (E.448). Electrons and holes move faster “by one or two orders of magnitude with an increase in QD diameter (E.448).
These solar cells don’t rely on single p-n junction design, but uses tandem cells or multi-junction solar cells with a stack of p-n junctions of low-dimensional semiconductor structures (E. 446). The p-n junction stacks can have different Eg, thus covering a very wide range of the solar spectrum, thus increasing the theoretical energy conversion efficiency from 31% to 66%.
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CdSe Nanocrystal "Ink" Synthesis
There are a variety of techniques to synthesize colloidal solutions of Cadmium Selenide. However, long, bulky ligands are also formed (and attached to the nanocrystals) during synthesization. In the reaction these Native Ligands are used advantageously, to control crystal growth, nucleation, and to prevent nanocrystals from agglomerating in solution. Once it is time for the CdSe nanoparticles to perform, for instance in a semiconducting thin film, the ligands act as insulation to the nanocrystals and destroy the carrier mobility of the semiconductor. For this reason, in conjunction with the possibilities for new nanoparticle-fueled semiconductors, much research is currently underway with the focus of Native Ligand Exchange. Click Here for more info on techniques for Native Ligand Exchange.
The Ligand Exchange technique that will be highlighted here, involves a colloidal exchange of Native Ligands for the Thiocyanate precursor, the 1,2,3,4-thiatriazole-5-thiolate anion (TTT-). CdSe nanocrystals with TTT- ligands (called CdSe(TTT)) have long term stability in solution, which would allow for synthesis of large volumes of nanoparticles without the need for immediate printing onto the water purification system. Upon mild heating of >100C, TTT- readily thermolyzes into to the small, minimally insulating ligand Thiocynate, which is commonly used in the formation of high quality nanocrystal semiconductor films.
Synthesis of Native Ligand CdSe nanocrystals, in solution:
- Mix 3.5g CdCO3, 30g Stearic Acid, and 30g TOPO, stirring at 100C for 90 minutes under flowing N2
- Stop flow of N2, and hold at 360C for 60 minutes
- In a different container, dissolve 2.3g Selenium powder in 30ml TOP under flowing N2
- Quickly add the Selenium/TOP solution to the initial solution (containing the dissolved CdCO3), while stirring rapidly
- Immediately after, quickly mix in 30mL Octadecene
- Hold this solution at constant temperature for 2 minutes, then quench the system using the air flow from a fume hood
- When the solution has cooled to nearly room temperature, add 30mL of toluene (previously nitrogen sparged)
- Transfer equal volumes of the solution to centrifuge tubes for centrifugation
- Add equal volumes of MeOH and EtOH to the centrifuge tubes, for total volumes of 60mL and 66mL MeOH and EtOH, respectively
- Centrifuge down the nanocrystals in solution at 6000 rpm for 6 minutes
- Remove and discard residual supernatant
- Wash the nanocrystals by re-dispersing them in 120mL Toluene, followed by 150mL EtOH to assist in flocculation
- Centrifuge the solution at 6000 rpm for 1 minute, then discard supernatant
- Repeat this washing procedure up to 6 additional times, while decreasing the volume of dispersant and flocculant to a minimum of 60mL each
- Perform the final redistribution in 60ml Toluene
- Filter the final dispersion through a 450nm syringe filter to separate any agglomerates that may have formed
- Store the product in the dark at 10C
Note: This process typically yields around 3.3g of CdSe nanocrystals. "Scaling-up" in production is possible given the nature of the procedure.
CdSe nanocrystal Native Ligand Exchange using the TTT- anion:
Synthesis of ligand-donor molecule through the cation exchange of (NH4)2SiF6 and NaTTT
- Preparation of NaTTT solution
- Dissolve 500mg NaN3 in 2ml distilled water and 2ml nPrOH (nPrOH added to aid in homogeneity)
- While limiting light exposure, add 0.50ml CS2, and agitate vigorously for 40 minutes to ensure a single phase
- Preparation of NH4TTT solution
- Dissolve 695mg (NH4)2SiF6 in 5ml distilled water
- Over 8 minutes and with vigorous agitation, slowly and continuously add the (NH4)2SiF6 solution to the NaTT solution
- Add 20ml nPrOH
- Centrifuge the resulting NH4TTT solution with 6000rpm for 1 minute, and filter though a 450nm syringe filter
- Store the solution in a cold (7-8C), dark place to secure colloidal stability for >2 weeks
Note: In the step above, it is important to limit exposure of heat and photons, to ensure that TTT- does not prematurely decompose into SCN-
TTT- - NL Ligand Exchange
- Dilute 2.5ml of the NH4TTT solution with 10ml nPrOH
- Using around a 0.0388 wt/vol% of CdSe nanocrystals dispersed in Toluene, inject 2.5ml into the diluted NH4TTT solution
- Mix solution, centrifuge at 6000rpm for 0.5 minutes, and remove supernatant without drying crystals
- Redisperse the CdSe(TTT) nanocrystals in 5ml Propylene Carbonate, and exchange remaining Native Ligands by adding another 2.5ml NH4TTT solution
- Allow solution to stand in the dark for 30 minutes
- Add 30ml Dimethoxymethane, and repeat the centrifugation and supernatant removal process
- Again redisperse the solid in 5ml Proplyene Carbonate, and filter through a 450nm syringe to remove any precipitates
- Again wash the CdSe nanocrystals in 30ml Dimethoxymethane, and repeat the centrifugation and supernatant removal process
- Prepare the nanacrystals for a final distribution by dispersing in 5ml Propylene Carbonate, and adding 80ml Dimethoxymethane and 20ml Pentane to facilitate flocculation
- Centrifuge solution at 6000rpm for 5 minutes, and remove supernatant
- Perform the final redistribution in a chosen dispersant (or mixture of dispersants) that is/are appropriate for the printing application. Chief considerations should be liquid viscosity, dispersibility, evaporation rate, and safety hazards. The dispersant selection process should be analogous to the selection process for spin coating of electronic wafers
- Solvents that are know to have high dispersibility with CdSe(TTT) nanoparticles include Propylene Carbonate, Sulfolane, N,N-Dimethylformamide, Dimethyl Sulfoxide, N-methylformamide, and Tetramethylurea
- For this illustration, the final dispersant is 1ml DMF
- Prepare colloidal suspension with bubbling N2 to evaporate residual Dimethoxymethane, and centrifuging (6000rpm, 0.5 minutes) and decanting to remove any small agglomerates
- Store the CdSe(TTT) colloid in a cool (7-8C) dark place
Note: This process typically yields a CdSe colloidal suspension with a concentration of around 81 mg/ml. "Scaling-up" in production is possible given the nature of the procedure.
Required Chemicals (click for MSDS)
- Cadmium Carbonate (CaCO3)
- Stearic Acid
- Tri-n-octylphosphine oxide (TOPO)
- Selenium Powder
- Tri-n-octylphosphine (TOP)
- Sodium Azide (NaN3)
- Distilled water
- Carbon disulfide
- Ammonium Hexafluorosilicate
- Propylene Carbonate
- N,N-Dimethylformamide (DMF) or chosen dispersant
Note: All pricing quotes assume bulk orders from Sigma Aldrich
Characterization of CdSe
While there are many techniques to perform a quantitative analysis, here we will focus on two of the more common tools for material analysis: X-Ray Diffraction (XRD) and Ultraviolet-visible spectroscopy (UV-Vis) analyses.
X-Ray Diffraction Analysis-XRD is a common tool used to analyze and characterize different materials and their properties. The procedure sees a small powder or thin film sample loaded into an X-Ray diffractometer and then exposed to X-Rays. The angle at which the incident rays meet the sample and the angle at which the diffracted rays are caught by the detector are changed periodically during the test. [Image of Diffractometer should be here]. This is because different materials will have different positions where the energy diffracted toward the detector is at its highest, these will form the peaks we see on diffractograms. [Need to insert short discussion on what can be learned from diffractograms and Bragg’s Law]
[Image of CdSe diffractogram should be here] The above figure is a diffractogram of a sample of CdSe Quantum Dots given by an XRD test along with an overlay of the diffraction pattern of CdSe provided by the International Center for Diffraction Data. [Need to build on discussion for results i.e. the spread of peaks vs. diffraction card].
Ultraviolet-visible spectroscopy tests examine the absorption or reflective spectroscopy of a material over the ultraviolet-visible spectral region. The test itself bombards a sample of a material with light of varying wavelengths in a spectrophotometer, as it’s exposed to different light the spectrophotometer monitors the intensity at which the sample absorbs (or reflects) the light at these different wavelengths. And with the help of logging software it can produce spectrographs like the one below: [Image of UV-Vis spectrograph and WL chart] [Need discussion points for spectrograph]
The Quantum Dot Heterojunction Junction Solar Cell
There are several different cell architectures that are currently being used in Colloidal Quantum Dot (QCD) solar cell research; they include quantum dot sensitized cells, Schottky Junction cells, and a variety of heterojunction based cell designs.
For this application, the solar cell will use a semiconductor-semiconductor heterojunction as it's effective p-n junction. Titanium Dioxide is a metal oxide that is commonly used as a transparent film with n-type semiconductor properties due to oxygen vacancies in its lattice and the presence of negatively charged charge carriers. This film material is intentionally chosen with a high band gap (~3.5eV, depending on nanoparticle size and preparation technique), and subsequently allows the majority of photons to pass through into the layer of colloidal CdSe nanocrystals. TiO2 nanoparticles are available for purchase
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Webber, David H., and Richard L. Brutchey. "Ligand Exchange on Colloidal CdSe Nanocrystals Using Thermally Labile Tert-Butythiol for Improved Photocurrent in Nanocrystal Films." Journal of the American Chemical Society 134 (2011): 1085-092. Print.
Webber, David H., and Richard L. Brutchey. "Nanocrystal Ligand Exchange with 1,2,3,4-thiatriazole-5-thiolate and Its Facile in Situ Conversion to Thiocyanate." Dalton Transaction 41.26 (2012): 7835-838. Print.
Webber, David H., and Richard L. Brutchey. "Nanocrystal Ligand Exchange with 1,2,3,4-thiatriazole-5-thiolate and Its Facile in Situ Conversion to Thiocyanate: Supplementary Information." The Royal Society of Chemistry, Electronic Supplementary Material (ESI) for Dalton Transactions (2012): n. pag. Print.
Etgar, L. “Semiconductor Nanocrystals as Light Harvesters in Solar Cells”, Materials, p. 445-455, 2013.
Luber, Erik., Mobarok, M., and Buriak, J. “Solution-Processed Zinc Phosphide (α- Zn3P2) Colliodal Semiconducting Nanocrystals for Thin Film Photovoltaic Applications ”, National Institute for Nanotechnology, p.A-K. www.ascnano.org. 2013.
Yang, Y. N.d. 0. n.p. “Best Research-Cell Efficiencies”, National Center for Photovoltaics,
http://www.nrel.gov/ncpv/images/efficiency_chart.jpg. 2013. Retrieved October 11, 2013.
Nikolenko, L. M., and V. F. Razumov. Colloidal Quantum Dots in Solar Cells. Russian Chemical Reviews 82.5 (2013): 429-48. Print.
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