A Colloidal Approach to Liquid Crystal Transflective …...A Colloidal Approach to Liquid Crystal...
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A Colloidal Approach to Liquid Crystal Transflective Displays: Making Your E-Reader Color Chris Barney, John Epling, Cheng-Yu Hung, Matthew Michie, and Jerome Nash Faculty Advisors: Professor Jeffrey Youngblood Industrial Sponsors: Dr. Johnathan Vernon Project Background Cholesteric liquid crystals (CLCs) structurally reflect light and have the potential to allow construction of transflective displays that can switch between transmitting light and selectively reflecting color. The Air Force Research Laboratory (AFRL) is interested in transflective displays as they work in reflected light and therefore have low power requirements. The goal of this project is to work towards creating a powder of CLCs where the flakes are monodisperse, have two aligned boundary conditions, have a thickness of 5 μm, and a length and width on the 10-50 μm size scale. This goal was addressed by attempting to create flakes either through a photopolymerization process or a replica molding method. Replica Molding Results Creating flakes with the photopolymerization method turned out to be much more difficult than originally anticipated. The recurring issues with this method are shown below. • Overheating the monomeric CLC causing thermopolymerization when infiltrating the cell • Judging the purity of CLC when boiling off acetone • Batching CLC and solvent at the proper levels to redshift the color without turning infrared • Finding the correct dosing time with the UV cure system The first three issues were resolved, but finding the final dosing time was abandoned for focusing the group’s efforts on the more interesting (novel) developments on the replica molding portion of this project. MSE 430 - 440: Materials Processing and Design The above image shows a macroscopic and microscopic view of the 3D printed master mold on the left and the PDMS daughter mold on the right. The optical images show how well the pattern transfers. Below are some preliminary flakes produced by using the PDMS mold and some monomeric CLC material. • Batch out monomeric CLC with solvent to dilute the chiral dopant and control pitch (color) • Heat CLC to isotropic point and infiltrate cells shearing the fluid as it cools down to increase alignment • Irradiate Sample with UV light to photopolymerize and lock in structure of flakes This work is sponsored by the Air Force Research Laboratory (AFRL) located in Dayton, OH Pitch Length Cholesteric Liquid Crystals (CLCs) self-assemble into a chiral nematic liquid crystal mesophase as shown on the right. Pitch length is defined as the length it takes to for the molecules to do a full rotation and represents the longest wavelength that the material can reflect [1]. Bragg mirrors reflect different wavelengths depending on the angle of incident light as shown in the equation below where n is the average refractive index, θ is the incident light angle, P is the pitch, and λ is the wavelength reflected [3]. An illustration of this property is shown above. = ( ) Photopolymerization Procedure As illustrated above, CLC flakes can be used to create a device that either reflects color or transmits light based on the angle of rotation [4]. Cutting the backlight in these devices reduces the power consumption for a screen with this technology. Replica Molding Procedure Photopolymerization Discussion Photopolymerization Results The images of the sample shown above were taken at different angles. Taking a photograph from different angles changes the incidence angle of light and shortens the wavelength of light reflected. These images confirm that our samples are in fact Bragg mirrors. Shown on the right above is an example of two CLC films created from the same batch of monomer where the film on the left has an alignment layer and the film on the right does not. The uniformity and strength reflection in these samples depends on the degree of molecular alignment. [2] [3] [4] • Create master mold with positive imprint on top • Cast PDMS onto top of mold to create a negative with wells to form the flakes • Remove PDMS from master mold • Fill PDMS mold with a solution of glassy CLC dissolved in toluene. Doctor blade of the excess so that the wells are filled • Evaporate off the solvent and heat to the isotropic point • Cover with glass slide and shear as it cools to help alignment Alignment can be encouraged by either directly rubbing the PDMS mold or adding a felt rubbed polyimide alignment layer. Photomask Cell Top CLC Monomer and Chiral Dopant Cell Base Ultraviolet Radiation The above images show optical micrographs of a silicon master mold fabricated at Birck Nanotechnology Center. The image on the left shows the array of features and the image on the right displays a protruding flake positive. • Use a higher resolution 3D printer for master mold • Get a more elegant UV source • Look into ink jet printing for directly printing flakes References Recommendations Replica Molding Discussion The photolithography procedure detailed below was performed by Birck Nanotechnology Center. Birck however incorrectly spun-coat 1.7 μm of positive photoresist with 5 μm of titanium vapor deposition. This means that the photoresist was completely encapsulated by the titanium preventing proper liftoff. Due to time constraints efforts were focused on 3D printing. Birck Silicon Master Mold Step 1 Clean Wafer Step 2 Positive Photoresist Deposition Step 3 UV Exposure Step 4 Development Step 5 Titanium Vapor Deposition Step 6 Lift-off with Acetone [6] 3D Printed Molds A Formlabs Form 1+ 3D printer was used in conjunction with SolidWorks CAD files of the 3D molds at 400μm x 800μm , 200μm x 400μm, 100μm x 200 μm oval sized microflakes. Oval shaped flakes were used to optimize lateral printing resolution. Flakes were produced using monomeric and glassy CLCs. Difficulties included proper wettability of the CLC on the PDMS negative mold resulting in inadequate filling of the wells. Corona surface treatments and the addition of toluene were used to improve wettability. [1] N. Tamaoki, "Cholesteric Liquid Crystals for Color Information Technology," Advanced Materials, vol. 13, no. 15, pp. 1135-1147, 2001. [2] R. H. Chen, Liquid Crystal Displays, John Wiley & Sons, Inc., 2011. [3] T. Kosc, K. Marshall, A. Trajkovska-Petkoska, C. Coon, K. Hasman, G. Babcock, R. Howe, M. Leitch and S. Jacobs, "Development of Polymer Cholesteric Liquid Crystal Flake Technology For Electro-Optic Devices and Particle Displays," in Emerging Liquid Crystal Technologies II, Rochester, 2007. [4] A. Tajkovska-Petkoska, R. Varshneya, T. Z. Kosc, K. L. Marshall and S. Jacobs, "Enhanced Electro-Optic Behavior For Shaped Polymer Cholesteric Liquid-Crystal Flakes Made Using Soft Lithography," Advanced Functional Materials, vol. 15, no. 2, pp. 217-222, 2005. [5] C. Heinen, A. Natekar, T. Schroeder and H. Zhou, "A Colloidal Approach to Liquid Crystal Transflective Displays: Making your E-Reader Color," Purdue University Materials Engineering Senior Design Group, 2014. [6] “Microelectronics: Fabrication of micro/nano structures on silicon surface” DNANOTECH. (2012). Shown on the left is an optical micrograph of the photomask used to create 20x40 μm flakes. The windows in the mask are spaced 100 μm apart to avoid light interference during polymerization.
Transcript of A Colloidal Approach to Liquid Crystal Transflective …...A Colloidal Approach to Liquid Crystal...
A Colloidal Approach to Liquid Crystal TransflectiveDisplays: Making Your E-Reader ColorChris Barney, John Epling, Cheng-Yu Hung, Matthew Michie, and Jerome NashFaculty Advisors: Professor Jeffrey YoungbloodIndustrial Sponsors: Dr. Johnathan Vernon
Project Background
Cholesteric liquid crystals (CLCs) structurally reflect light and have the potential to allow construction of transflective displays that can switch betweentransmitting light and selectively reflecting color. The Air Force Research Laboratory (AFRL) is interested in transflective displays as they work inreflected light and therefore have low power requirements. The goal of this project is to work towards creating a powder of CLCs where the flakes aremonodisperse, have two aligned boundary conditions, have a thickness of 5 µm, and a length and width on the 10-50 µm size scale. This goal wasaddressed by attempting to create flakes either through a photopolymerization process or a replica molding method.
Replica Molding Results
Creating flakes with the photopolymerizationmethod turned out to be much more difficult thanoriginally anticipated. The recurring issues withthis method are shown below.• Overheating the monomeric CLC causing
thermopolymerization when infiltrating the cell• Judging the purity of CLC when boiling off
acetone• Batching CLC and solvent at the proper levels to
redshift the color without turning infrared• Finding the correct dosing time with the UV cure
systemThe first three issues were resolved, but finding thefinal dosing time was abandoned for focusing thegroup’s efforts on the more interesting (novel)developments on the replica molding portion of thisproject.
MSE 430-440: Materials Processing and Design
The above image shows a macroscopic andmicroscopic view of the 3D printed master mold onthe left and the PDMS daughter mold on the right.The optical images show how well the patterntransfers. Below are some preliminary flakesproduced by using the PDMS mold and somemonomeric CLC material.
• Batch out monomeric CLC with solvent to dilutethe chiral dopant and control pitch (color)
• Heat CLC to isotropic point and infiltrate cellsshearing the fluid as it cools down to increasealignment
• Irradiate Sample with UV light tophotopolymerize and lock in structure of flakes
This work is sponsored by the Air Force Research Laboratory (AFRL) located in Dayton, OH
Pitc
hLe
ngth
Cholesteric Liquid Crystals (CLCs)self-assemble into a chiral nematicliquid crystal mesophase as shownon the right. Pitch length is definedas the length it takes to for themolecules to do a full rotation andrepresents the longest wavelengththat the material can reflect [1].
Bragg mirrors reflect different wavelengthsdepending on the angle of incident light as shown inthe equation below where n is the averagerefractive index, θ is the incident light angle, P is thepitch, and λ is the wavelength reflected [3]. Anillustration of this property is shown above.
𝜆𝜆 = 𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛(𝜃𝜃)
Photopolymerization Procedure
As illustrated above, CLC flakes can be used tocreate a device that either reflects color or transmitslight based on the angle of rotation [4]. Cutting thebacklight in these devices reduces the powerconsumption for a screen with this technology.
Replica Molding ProcedurePhotopolymerization Discussion
Photopolymerization Results
The images of the sample shown above weretaken at different angles. Taking a photograph fromdifferent angles changes the incidence angle oflight and shortens the wavelength of light reflected.These images confirm that our samples are in factBragg mirrors.
Shown on the right above isan example of two CLCfilms created from the samebatch of monomer wherethe film on the left has analignment layer and the filmon the right does not. Theuniformity and strengthreflection in these samplesdepends on the degree ofmolecular alignment.
[2]
[3]
[4]
• Create master mold with positive imprint on top• Cast PDMS onto top of mold to create a negative
with wells to form the flakes• Remove PDMS from master mold• Fill PDMS mold with a solution of glassy CLC
dissolved in toluene. Doctor blade of the excessso that the wells are filled
• Evaporate off the solvent and heat to theisotropic point
• Cover with glass slide and shear as it cools tohelp alignment
Alignment can be encouraged by either directlyrubbing the PDMS mold or adding a felt rubbedpolyimide alignment layer.
PhotomaskCell Top
CLC Monomer and Chiral DopantCell Base
Ultraviolet Radiation
The above images show optical micrographs of asilicon master mold fabricated at BirckNanotechnology Center. The image on the leftshows the array of features and the image on theright displays a protruding flake positive.
• Use a higher resolution 3D printer for master mold• Get a more elegant UV source• Look into ink jet printing for directly printing flakes
References
Recommendations
Replica Molding Discussion
The photolithography procedure detailed below was performed by Birck Nanotechnology Center. Birckhowever incorrectly spun-coat 1.7 µm of positive photoresist with 5 µm of titanium vapor deposition. This means that the photoresist was completely encapsulated by the titanium preventing proper liftoff. Due to time constraints efforts were focused on 3D printing.
Birck Silicon Master Mold
Step 1 Clean Wafer
Step 2 Positive Photoresist Deposition Step 3
UV Exposure
Step 4 Development
Step 5 Titanium Vapor Deposition
Step 6 Lift-off with Acetone
[6]
3D Printed Molds A Formlabs Form 1+ 3D printer was used inconjunction with SolidWorks CAD files of the 3Dmolds at 400µm x 800µm , 200µm x 400µm,100µm x 200 µm oval sized microflakes. Ovalshaped flakes were used to optimize lateral printingresolution. Flakes were produced using monomericand glassy CLCs. Difficulties included properwettability of the CLC on the PDMS negative moldresulting in inadequate filling of the wells. Coronasurface treatments and the addition of toluene wereused to improve wettability.
[1] N. Tamaoki, "Cholesteric Liquid Crystals for Color Information Technology," AdvancedMaterials, vol. 13, no. 15, pp. 1135-1147, 2001.[2] R. H. Chen, Liquid Crystal Displays, John Wiley & Sons, Inc., 2011.[3] T. Kosc, K. Marshall, A. Trajkovska-Petkoska, C. Coon, K. Hasman, G. Babcock, R.Howe, M. Leitch and S. Jacobs, "Development of Polymer Cholesteric Liquid CrystalFlake Technology For Electro-Optic Devices and Particle Displays," in Emerging LiquidCrystal Technologies II, Rochester, 2007.[4] A. Tajkovska-Petkoska, R. Varshneya, T. Z. Kosc, K. L. Marshall and S. Jacobs,"Enhanced Electro-Optic Behavior For Shaped Polymer Cholesteric Liquid-CrystalFlakes Made Using Soft Lithography," Advanced Functional Materials, vol. 15, no. 2, pp.217-222, 2005.[5] C. Heinen, A. Natekar, T. Schroeder and H. Zhou, "A Colloidal Approach to LiquidCrystal Transflective Displays: Making your E-Reader Color," Purdue UniversityMaterials Engineering Senior Design Group, 2014.[6] “Microelectronics: Fabrication of micro/nano structures on silicon surface”DNANOTECH. (2012).
Shown on the left is anoptical micrograph of thephotomask used tocreate 20x40 µm flakes.The windows in the maskare spaced 100 µm apartto avoid light interferenceduring polymerization.
