Cascade Crystallization

Crysoptix improves and modifies Cascade Crystallization technology which was developed by Optiva Inc. for manufacturing Thin Crystal Films (TCF™) Polarizers. At present the patent portfolio of former Optiva Inc. on TCF™ Polarizers is a property of Nitto Denko, a leading manufacturer of LCD polarizing films.

Crysoptix has been developing a novel approach to produce coatable Thin Birefringence Film (TBF™) retarders based on new liquid self-assembling transparent materials [patents pending], for optical compensation of LCD.

The basic Cascade Crystallization technology is reviewed, analyzed and summarized by Dr. Pavel Lazarev, the founder of Optiva Inc., and the director and founder of Crysoptix, KK., in the manuscript “Cascade Crystallization” (in preparation for publication). We present below the extracts from the manuscript devoted to the main principles, features and advantages of Cascade Crystallization technology.

There is crystal growth from vapour, from solution, from melt, and we started development of crystal growth from lyotropic liquid crystal.

We started with crystalline films of conjugated aromatic compounds and here we present growth technique and applications of these films in optics and more specifically in Liquid Crystal Displays (LCD).

Crystal growth is an old technique. People have always appreciated usefulness and/or beauty of crystals. With industrial revolution, crystals became a useful industrial tool – for example, diamond cutting tool or optical elements in instrumentation and communications. Until recently, there were not many crystal film growth techniques that would allow to control the direction of crystal growth and direction of crystallographic axes in order to produce anisotropic crystalline component of useful size. We know of just one, and it is epitaxial growth. Technology presented here is the second.

People are using crystals for their optical and electrical properties. The more we use them the more we need them and the more cost reduction we desire. A new technique that we named Cascade Crystallization promises both: useful properties and cost reduction. That is why we are motivated in our work and that is why we want you to know about this new scientific and industrial topic.

Basic phenomena of Cascade Crystallization might be described as a crystalline film growth that does not depend upon substrate because crystalline order is introduced in liquid state and transferred without loss or with small losses of order onto a substrate. Substrate surface defects can not dictate the local order and so the global order introduced by deposition remains. We managed to avoid a seed formation stage in film growth that does not allow growing globally oriented anisotropic films from gas vapour or liquid solution. We circumvented seed formation and made the orientation controllable and global.

However, this technology is limited to the class of conjugated aromatic organic molecules mainly with flat plate-like molecular structure.

Crystallization of thin films has always been a challenge. Crystalline films have attractive anisotropic properties that might be used in optical and semiconductor industries. We present Cascade Crystallization method for manufacturing thin crystal films (TCF) and their applications in optical components.

Pre-ordering material in liquid phase - Lyotropic Liquid Crystals

In order to avoid effect of the substrate in crystal film growth we developed technique that pre-orders material in liquid state by self-assembly of molecules into supramolecules and forming Lyotropic Liquid Crystal with local crystalline order.

Over the last twenty years it has become increasingly clear that there is a well-defined family of Lyotropic Mesogens embracing a range of drugs, dyes, nucleic acids, antibiotics, carcinogens, and anti-cancer agents. In contrast to conventional amphiphilic mesogens, such as soaps, detergents, and biological lipids, these materials do not generally have significant surfactant properties. Lyotropic Mesogens have limited amphiphilicity and their molecules are disc-like or plank-like as opposed to rod-like. These molecules are aromatic rather than aliphatic, and the hydrophilic ionic or hydrogen-bonding solubilizing groups are arranged around the peripheries of the molecules, not at the ends. The molecules can be regarded as being insoluble in one dimension, and the basic structural unit of liquid crystal phases is a molecular stack [1]. The difference is in strong -interaction in the stack, which does not exist in conventional amphiphilic compounds. After J.-M. Lehn [2], we name these stacks Supramolecules. Lyotropic Liquid Crystals of supramolecules have characteristic phase structures, phase diagrams, optical textures, X-ray diffraction patterns, and miscibility properties.

The tendency of conjugated aromatic molecules to aggregate into columns is present even in dilute solution (just as for amphiphile systems, where micelle formation occurs before the mesophase is formed). However, although there may be a threshold concentration before aggregation begins to occur, there is no optimum column length and hence no critical concentration directly analogous to a Critical Micelle Concentration. A further distinction is the absence of a Krafft temperature. Since the process of mesophase formation does not depend on the presence of flexible alkyl chains, there is no threshold temperature below which mesophases cannot be produced because the vital flexibility of the molecules has been frozen out [1].

The described anisotropic systems are usually examined by polarizing optical microscopy, Small-Angle X-ray Scattering (SAXS), Wide-Angle X-ray Scattering (WAXS) and NMR spectroscopy [1-7] Extensive use of polarizing optical microscopy in these studies is primarily due to the ease, with which the mesophases can be identified by their optical textures. SAXS provides structural information describing heterogeneities with the size on the order of 1-100 nm. It can produce useful information about aggregate ordering, size, shape and separation (i.e. lyomesophase structure). WAXS, which probes the size range of 1-50 A, can also be used to determine aggregation numbers and lyomesophase structure. Thus, the WAXS-peak common to all Supramolecular mesophases is located at 3.4-3.6 A, and corresponds to the intermolecular spacing within the aggregate stack [7].

Basic principle of Cascade Crystallization

We developed the method for thin crystal film manufacturing which we refer to as Cascade Crystallization [8-16]. Cascade Crystallization process involves a chemical modification step and four steps of ordering during the crystal film formation. The chemical modification step introduces hydrophilic groups on the periphery of the molecule in order to impart amphiphilic properties to the molecule. Amphiphilic molecules stack together into supramolecules, which is the first step of ordering. By choosing specific concentration, supramolecules are converted into a liquid-crystalline state to form a lyotropic liquid crystal, which is the second step of ordering. The lyotropic liquid crystal is deposited under the action of a shear force (or meniscus force) onto a substrate, so that the shear force (or the meniscus) direction determines the crystal axis direction in the resulting solid crystal film. This shear-force- assisted directional deposition is the third step of ordering, representing the global ordering of the crystalline or polycrystalline structure on the substrate surface. The last fourth step of the Cascade Crystallization process is drying/crystallization, which converts the lyotropic liquid crystal into a solid crystal film. We will use the term Cascade Crystallization process to refer to the chemical modification and four ordering steps as a combined process demonstrated in Fig 1.

Fig. 1: Supramolecular formation and globalization of order

The film produced by the Cascade Crystallization process has a global order. The global order means that the direction of the crystallographic axis of the film over the entire substrate surface is controlled by the deposition process and, with a limited influence of the substrate surface. Molecules of the deposited material are packed into lateral supramolecules with a limited freedom of diffusion or motion. The lyotropic liquid crystal is characterized by an intrerplanar spacing of 3.4 ± 0.3 A in the direction of one of the optical axes.

Optics of Cascade Crystallization products

The resulting films of Cascade Crystallization process are crystalline films with more or less similarity to monocrystal and with global order over large areas of the film. Organic aromatic compounds rarely have symmetry higher then monoclinic syngony and they possess all features of biaxial crystal optics that put Cascade Crystallization films in potentially all applications where crystalline asymmetry is welcome.

In applications, anisotropic absorption leads to dichroic polarizers, anisotropic retardation leads to retarders, thin (in the order of 1/4 of wavelength of light) films leads to emergence of multilayer polarization sensitive optics, crystalline nature of films leads to electro-optical properties and applications in wide variety of electro-optical devices [17-22].

Crystal Optics of biaxial crystals is not developed very much and, with introduction of Cascade Crystallization, needs to be developed for use in industrial applications. Cascade Crystallization does not produce monocrystalline film – it is an eventual goal of technology development but we are not there yet. One of the first requirements is a quality control technique that would allow to compare samples and to control development and optimization process. Our group has developed methods for optical testing including measurement of 3-dimensional refraction indices. Multilayer component production requires techniques for multilayer film stack computer modelling and optimization – we developed algorithms and software for calculations and optimization of multilayer components. These results will be useful for engineers as tools for their design work.

New multilayer optical components – polarization sensitive multilayer reflectors – allow us to introduce new design of optical devices. We present example of new optical device that shows the benefits of thin crystal multilayer optics. The example is design of Liquid Crystal Display that does not contain absorptive components and does not loose the light [23]. From physics stand point we introduced new film based components with polarization sensitivity that made possible in LCD the same methods of light control as are used in fiber optics with one important difference: in fiber optics multilayer films are isotropic and are not sensitive to polarization and in LCD we introduce polarization sensitive multilayer optics. Same physics of multilayer optics with one new feature - polarization sensitivity, makes possible loss less LCD in the same way as fiber optics multiplexing and de-multiplexing does not create losses of light.

New technique for crystal growth

Technological space of crystal growth technologies has received one more technique – thin crystalline film growth which employs visco-elastic properties of thixotropic supramolecular lyotropic liquid crystals in order to eliminate the influence of substrate surface defects on the orientation of seed crystals in the early stages of film crystallization.

This technique is limited to a specific class of compounds, which can form supramolecules, but are almost unaffected by surface properties of the substrate.

This technique is named Cascade Crystallization in order to reflect the presence of separated stages of order acquisition in subsequent steps of the process.

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