Project data

For the manufacturing of quantum dot LED (QLED) displays, the quantum dots will have been created prior to 3D printing. The quantum dots will be made of the semiconductor, cadmium selenide/zinc sulfide (CdSe/ZnS), which consists of CdSe core and a ZnS shell. The ZnS shell on the quantum dot acts as a protective barrier between the core, which is responsible for optical emission, and the surrounding material.[1] Four sizes of quantum dots corresponding to four colors (red, yellow, orange, green) of emitted light will be purchased from mkNano. The smallest quantum dots will be purchased from Sigma-Aldrich since mknano did not offer CdSe/ZnS quantum dots in the size range that will emit blue light.

The CdSe/ZnS quantum dots will be 3D printed on a crystalline ZnO wafer which will act as the electron injection layer. ZnO was chosen for its favorable band gap energy and work function.[2] The ZnO wafer substrate will be purchased from Precision Micro-Optics.

The quantum dot suspensions used in this project will use water as the solvent. Some success has been realized by Haverinen et al.[3] in inkjet printing "CdSe core and CdS/ZnS double shell" quantum dots on a "cross-linkable poly-TPD [poly(N,N′-bis(4-butylphenyl-N,N′-bis(phenyl)benzidine)]" hole injection layer using chlorobenzene as a solvent. Table 1 compares various properties of chlorobenzene and water.

Table 1[4][5][6]
Solvent Vapor Pressure Surface Tension Density Viscosity
Chlorobenzene 8.8 Torr (20°C) 33.3 dyn/cm (20°C) 1.1 g/cm3 0.0008 Pa*s (20°C)
Water 17.5 Torr (20°C) 72.8 dyn/cm (20°C) 1.0 g/cm3 0.001 Pa*s (20°C)

For an initial trial, water is a good solvent to start with because the important properties of water are close to those for chlorobenzene, which was used as a solvent by Haverinen et al. to print CdSe core and CdS/ZnS quantum dots. The biggest difference between water and chlorobenzene are the vapor pressure and surface tension. Both the surface tension and vapor pressure for water are approximately twice as much as the vapor pressure and surface tension for chlorobenzene. Despite the seemingly significant differences in these properties, they are still close enough to justify attempting a trial using water as a solvent.

## 3D Printing Process Steps

The focus of this project is on the viability of 3D printing the quantum dot layer (CdSe/ZnS) on the electron injection layer (ZnO wafer). The other layers (cathode, hole injection layer, anode) are beyond the scope of this project. Steps involving the other layers are for clarity only.

Note: All the quantum dot colors could not be purchased from the same manufacturer. mkNano did not offer a CdSe/ZnS quantum dot capable of emitting blue light.

Steps:

• Acquire materials
• CdSe/ZnS quantum dots- red, yellow, orange, and green mkNano.
• Add 4 mg of each to 1 ml of water (each)
• CdSe/ZnS quantum dots in toluene/aliphatic amine - blue Sigma Aldrich.
• ZnO wafer (electron injection layer) Precision Micro-Optics
• Filter blue light emitting quantum dots that were purchased in toluene/aliphatic amine
• Measure 4 mg of these quantum dots and add to 1 ml of water
• 3D print the cathode layer to the desired size
• 3D print the electron injection layer (ZnO crystalline wafer Precision Micro-Optics) on top of the cathode
• The electron injection layer will be 20 mm x 20 mm
• Load the quantum dot suspensions into cartridges compatable with a printer head similar to Dimatix DMP 2800 materials printer, which was used in the study performed by Haverinen et al.[9]
• Print the quantum dots into a display array
• 3D print a hole injection layer onto the quantum dot layer
• Print the final transparent anode layer onto the hole injection layer

## Material Safety Data Sheets

Chemicals & Compounds Used

• Quantum Dots
• Solvent
• Substrate (Electron Injection Layer)

## Purification Methods

• Purification Methods
• The CdSe/ZnS quantum dots are purchased prior to 3D printing, thus, are tested to meet the specifications set by mkNano.
• The ZnO wafer acting as the electron injection layer is held to the standards put in place by Precision Micro-Optics.

## Ink Properties

There are three types of quantum dots: core-type, core-shell, and alloyed quantum dots.

• Core-type quantum dots
• Uniform internal composition
• Photoluminescence and electroluminescence properties can be altered by changing the size of the quantum dot
• Example: CdS
• Core-shell quantum dots
• Have a shell around the core which has a higher band gap than the surrounding material
• The shell increases efficiency, as well as, brightness
• A coating can also be used to increase quantum yield
• This type of quantum dot has been widely used, more research has been conducted on this type
• Example: CdSe/ZnS

This type of quantum dot was chose because it has been researched in greater depth and the shell provides greater brightness and efficiency.

• Alloyed quantum dots
• Alloying quantum dots using homogeneous and gradient internal structures
• In this case you don't have to change the size of the quantum dot for different properties, but changing the composition and internal structure
• Ex: CdSxSe1-x/ZnS.[10]

## In-Situ Analysis/Target Compound Verification

• Target compound verification
• The composition of each of the four inks can quickly and easily be verified by irradiating each suspension of quantum dots (red, orange, yellow, green, blue) with the appropriate light source and noting whether or not the emitted light matches the color claimed by the respective manufacturers. One flaw with this method of verification is that the suspension will emit an overall color, even though there may be quantum dots in each suspension that emit a color of light that is different from the overall color. A characterization method will be discussed later that will allow a more accurate measurement of the average quantum dot size in each suspension.
• Characterization of printed quantum dots
• Atomic force microscopy (AFM)
• To determine the uniformity of the printed quantum dots, AFM will be performed on the ZnO wafer before and after 3D printing the CdSe/ZnS quantum dots. The purpose of imaging the ZnO wafer with AFM before printing the quantum dots is to get a baseline for the surface topography profile of the wafer.[11] After the quantom dots are printed, AFM can be used to determine the level of uniformity with which the dots were printed by comparing the topography profile from before and after 3D printing.
• Transmission electron microscopy (TEM)
• A high resolution TEM study will be performed to verify that the size of the quantum dots match the manufacturer's claim once they are printed on the ZnO wafer.[12] TEM will also be used to determine the resistance of the printing method (and materials used) to the aggregation of the quantum dots.
• Summary of compound verification and characterization steps
• Irradiate each of the 5 quantum dot suspensions to verify the composition claimed by manufacturer
• Perform AFM on ZnO wafer to establish surface topography baseline
• Perform AFM on ZnO wafer with the printed quantum dots to measure uniformity of the printed dots
• Establish the printer's precision capabilities
• Perform TEM study to more accurately verify the size of quantum dots claimed by manufacturers
• Use TEM to determine how well the printing method resisted aggregation of quantum dots

## Applications

The application of quantum dots in LED displays is still relatively new, therefore, the initial applications may begin with larger displays. Larger displays do not require as high of a resolution, making the precision less critical. Quantum dots could be included in billboards, sports arenas, traffic management, festivals, theaters, and scoreboards.[13] Once this technology is further developed, it could be utilized in smaller scale instances, which require higher resolution. These instances could include mobile screens and watches.

## Costs

Table 2

Material Unit Price Total Price
CdSe/ZnS $396.00/1 mg$1584.00
CdSe/ZnS (in toluene/aliphatic) $160.20/1 mg$640.80
ZnO wafer $1200$1200
Total Cost \$3424.80

• Pure color: Quantum dot LED (QLEDs) displays are advantageous in that you get a more pure color than organic LEDs (OLEDs). QLEDs have been found to have 30-40% greater luminance efficiency than OLEDs.
• Require less energy: There is an inverse relationship between luminous power efficiency and energy required to operate, therefore, there QLEDs require less energy than OLEDs.
• Decreased manufacturing cost: The manufacturing process of LEDs using quantum dots allows the manufacturer to use less materials. Color filters, backlight, or glass.[14]
• Increased flexibility: Quantum dots can be absorbed in aqueous and non-aqueous solvents which allow for greater flexibility.[15]
• The color emitted by quantum dots is determined by size. Blue light emitting quantum dots are the smallest. Not only is it difficult to produce this small size, but blue quantum dots require greater emission to be able to be seen.[16]

The ZnO layer (black) is the electron injection layer, while the quantum dots are printed in three different colors. The quantum dots are printed into the shape of the pixel on the screen, and can be seen scaled up, here
//electron injection layer
color ("black",1) cube ( [40,40,1],center = true);

//print of blue quantum dot pixels
translate ([-5,-8.33,.5]) color ("blue",1) cube ( [9.3,3,.1],center = true);
translate ([5,-8.33,.5]) color ("blue",1) cube ( [9.3,3,.1],center = true);
translate ([-5,1.66,.5]) color ("blue",1) cube ( [9.3,3,.1],center = true);
translate ([5,1.66,.5]) color ("blue",1) cube ( [9.3,3,.1],center = true);
translate ([-5,-18.33,.5]) color ("blue",1) cube ( [9.3,3,.1],center = true);
translate ([5,-18.33,.5]) color ("blue",1) cube ( [9.3,3,.1],center = true);
translate ([-5,11.66,.5]) color ("blue",1) cube ( [9.3,3,.1],center = true);
translate ([5,11.66,.5]) color ("blue",1) cube ( [9.3,3,.1],center = true);
translate ([-15,-8.33,.5]) color ("blue",1) cube ( [9.3,3,.1],center = true);
translate ([15,-8.33,.5]) color ("blue",1) cube ( [9.3,3,.1],center = true);
translate ([-15,1.66,.5]) color ("blue",1) cube ( [9.3,3,.1],center = true);
translate ([15,1.66,.5]) color ("blue",1) cube ( [9.3,3,.1],center = true);
translate ([-15,-18.33,.5]) color ("blue",1) cube ( [9.3,3,.1],center = true);
translate ([15,-18.33,.5]) color ("blue",1) cube ( [9.3,3,.1],center = true);
translate ([-15,11.66,.5]) color ("blue",1) cube ( [9.3,3,.1],center = true);
translate ([15,11.66,.5]) color ("blue",1) cube ( [9.3,3,.1],center = true);

//print of green quantum dot pixels
translate ([-5,-5,.5]) color ("green",1) cube ( [9.3,3,.1],center = true);
translate ([5,-5,.5]) color ("green",1) cube ( [9.3,3,.1],center = true);
translate ([-5,5,.5]) color ("green",1) cube ( [9.3,3,.1],center = true);
translate ([5,5,.5]) color ("green",1) cube ( [9.3,3,.1],center = true);
translate ([-15,-5,.5]) color ("green",1) cube ( [9.3,3,.1],center = true);
translate ([15,-5,.5]) color ("green",1) cube ( [9.3,3,.1],center = true);
translate ([-15,5,.5]) color ("green",1) cube ( [9.3,3,.1],center = true);
translate ([15,5,.5]) color ("green",1) cube ( [9.3,3,.1],center = true);
translate ([-5,-15,.5]) color ("green",1) cube ( [9.3,3,.1],center = true);
translate ([5,-15,.5]) color ("green",1) cube ( [9.3,3,.1],center = true);
translate ([-5,15,.5]) color ("green",1) cube ( [9.3,3,.1],center = true);
translate ([5,15,.5]) color ("green",1) cube ( [9.3,3,.1],center = true);
translate ([-15,-15,.5]) color ("green",1) cube ( [9.3,3,.1],center = true);
translate ([15,-15,.5]) color ("green",1) cube ( [9.3,3,.1],center = true);
translate ([-15,15,.5]) color ("green",1) cube ( [9.3,3,.1],center = true);
translate ([15,15,.5]) color ("green",1) cube ( [9.3,3,.1],center = true);

//print of red quantum dot pixels
translate ([-5,-1.66,.5]) color ("red",1) cube ( [9.3,3,.1],center = true);
translate ([5,-1.66,.5]) color ("red",1) cube ( [9.3,3,.1],center = true);
translate ([-5,8.33,.5]) color ("red",1) cube ( [9.3,3,.1],center = true);
translate ([5,8.33,.5]) color ("red",1) cube ( [9.3,3,.1],center = true);
translate ([-15,-1.66,.5]) color ("red",1) cube ( [9.3,3,.1],center = true);
translate ([15,-1.66,.5]) color ("red",1) cube ( [9.3,3,.1],center = true);
translate ([-15,8.33,.5]) color ("red",1) cube ( [9.3,3,.1],center = true);
translate ([15,8.33,.5]) color ("red",1) cube ( [9.3,3,.1],center = true);
translate ([-5,-11.66,.5]) color ("red",1) cube ( [9.3,3,.1],center = true);
translate ([5,-11.66,.5]) color ("red",1) cube ( [9.3,3,.1],center = true);
translate ([-5,18.33,.5]) color ("red",1) cube ( [9.3,3,.1],center = true);
translate ([5,18.33,.5]) color ("red",1) cube ( [9.3,3,.1],center = true);
translate ([-15,-11.66,.5]) color ("red",1) cube ( [9.3,3,.1],center = true);
translate ([15,-11.66,.5]) color ("red",1) cube ( [9.3,3,.1],center = true);
translate ([-15,18.33,.5]) color ("red",1) cube ( [9.3,3,.1],center = true);
translate ([15,18.33,.5]) color ("red",1) cube ( [9.3,3,.1],center = true);


## References

2. http://scitation.aip.org/content/aip/journal/apl/96/15/10.1063/1.3400224 Improvement of electron injection in inverted bottom-emission blue phosphorescent organic light emitting diodes using zinc oxide nanoparticles
3. http://scitation.aip.org/content/aip/journal/apl/94/7/10.1063/1.3085771 Inkjet printing of light emitting quantum dots
4. http://en.wikipedia.org/wiki/Properties_of_water#Surface_tension Properties of Water
5. http://macro.lsu.edu/HowTo/solvents/chlorobenzene.htm Chlorobenzene Solvent Properties
6. http://macro.lsu.edu/HowTo/solvents/water.htm Water Solvent Properties
7. http://www.nanocotechnologies.com/content/CommercialApplications/QDDisplays.aspx
8. http://web.archive.org/web/20161019042954/http://www.sigmaaldrich.com:80/materials-science/nanomaterials/quantum-dots.html
9. http://scitation.aip.org/content/aip/journal/apl/94/7/10.1063/1.3085771 Inkjet printing of light emitting quantum dots
10. Quantum Dots
11. http://scitation.aip.org/content/aip/journal/apl/94/7/10.1063/1.3085771 Inkjet printing of light emitting quantum dots
12. http://scitation.aip.org/content/aip/journal/apl/94/7/10.1063/1.3085771 Inkjet printing of light emitting quantum dots
13. LED Displays-Applications
14. QLED Technology
15. The future of cadmium free QD display technology (QD TV ™)