Transparent electronic and electro-optical (EO) devices have become an area of increasing interest in modern day technology research. Graphene’s excellent optical and electronic properties have made it an ideal material for research in such technologies and has become a material of considerable and continuous interest for transparent conductive electrodes in liquid crystal electro-optical devices.
Researchers from Kent State University, USA, have now demonstrated an electro-optical switching method using photolithographically-patterned graphene and incorporated it into a high-density electrode pattern, for use within a high-transmission in-plane-switching (IPS) liquid crystal device.
Graphene, in its many forms, showcases a wide range of beneficial properties that make it well suited for electro-optical and transparent devices. From its myriad of benefits, graphene offers itself as an enhanced material for such applications through its high electrical conductivity, excellent optical transmittance and high mechanical flexibility.
Graphene has already been used in a wide range of optoelectronic and photonic devices including solar cells, light emitting devices (LEDs), photodetectors, smart windows, liquid crystal devices (LCDs), transparent conductive electrodes, voltage controlled liquid crystal (LC) terahertz phase shifters and polymer dispersed liquid crystal (PDLC) devices, but continues to expand into many more commercial applications.
The Ohio based Researchers have now developed a process which can produce high-resolution patterns of interdigitated electrodes suitable for modern in-plane switching (IPS), fishbone and fringe field switching (FFS) LCD modes. To achieve this, the Researchers used a liquid crystal photoalignment method to maximize the field-driven optical contrast of a prototype device.
To develop the device, the Researchers grew graphene through chemical vapor deposition (CVD) methods and spin coated a layer of poly methyl methacrylate (PMMA) onto the layer of graphene.
This was followed by etching and ‘scooping’ of the graphene-PMMA layers onto a quartz substrate. The Researchers dissolved the PMMA layer to leave graphene, which then had an indium tin oxide (ITO) layer sputtered (magnetron sputtering) on top of it, creating the ITO electrode of the device.
The ITO-graphene substrates were then subsequently dried and a combination of wet and dry photolithography steps were utilized with a positive photoresist. The graphene based substrate was also spin coated with a photoresist solution and exposed to UV light, using a photolithographic mask.
The photoresist opened areas of the substrate were exposed to acid and oxygen plasma etching techniques (Oxford 80Plus). The substrates were then spin coated with a commercial photoalignment material LIA-01 (DIC Corporation) and dried.
The then-assembled IPS devices were subjected to linearly polarized UV light to create a unidirectional planar alignment of liquid crystal molecules on the both surfaces of both the ITO and graphene layers. The layer thicknesses were also confirmed through atomic force microscopy (AFM).
The photolithographic patterning on the electrode, and the subsequent high-quality photoalignment, enabled the device to be efficiently switched using the electro-optical properties of the liquid crystals.
Upon direct comparison with reference ITO-only IPS prototypes (no graphene) with an identical design, the prototype device created by the Researchers showcases a greater optical transmittance, an equivalent electro-optical performance, an equivalent voltage response and an equivalent response time.
The non-contact and low-temperature photoalignment method allowed the Researchers to produce a treatment delicate enough to process the graphene layer into an IPS electrode structure. The device, a ‘graphene-based single pixel laboratory IPS LCD prototype’ demonstrated an electro-optic performance which has the potential to be used for commercial applications, specifically in liquid crystal electro-optic devices (with complex and high definition electrode patterns).
The ability for the new device to exhibit equivalent and greater properties than current ITO IPS devices may also aid in creating liquid crystal electro-optical devices which can operate on both rigid and flexible substrates.
“Command Electro-Optical Switching of Photoaligned Liquid Crystal on Photopatterned Graphene”- Varanytsia A. and Chien L-C, Scientific Reports, 2017, DOI:10.1038/s41598-017-11903-9.