Submicron liquid crystal pixels on a nanopatterned indium tin oxide surface

Abstract

We have prepared a grooved indium tin oxide ͑ITO͒ surface with groove widths of ϳ40-90 nm and a variable groove separation up to 36 m using atomic force microscopy nanolithography. Twisted nematic pixels with 4-n-pentyl-4-cyanobiphenly ͑5CB͒ liquid crystal were prepared using a rubbed polyimide counter plate. The 5CB molecules align along the direction of the grooves, but no pretilt angle was observed on the ITO surface. The surface anchoring energy appeared not to depend on the groove separation nor on the scan force. The lateral correlation length of 5CB on ITO was measured to be 2.2 m. If a grooved surface is scanned in another direction, liquid crystal molecules follow the direction of the last scan.In liquid crystal displays ͑LCDs͒ liquid crystal molecules should be aligned in order to have a collective response to an electric field over a large area. Presently, the most common method of alignment is accomplished by the rubbing of a thin polymer film that is coated on a thin conductive electrode on a glass substrate. The detailed structure of the polymer alignment layer determines the anchoring strength and the pretilt angle of the liquid crystals. 1,2 The question with large practical implications is: would it be possible to merge two separate layers that are used for alignment and electrode into one conductive alignment layer? To achieve this, one might consider alignment on conductive polymer films or using conductive oxides. In this letter we will discuss the second approach.Indium tin oxide ͑ITO͒ is the most popular electrode for liquid crystal displays due to its high optical transparency and relatively good electrical conductivity. Depending on the method of preparation, ITO films can have different surface structures, conductivity and transparency. If ITO is prepared by rf sputtering, the resulting film usually will have a granular structure. The ITO surface is much harder than the polymer surface. Therefore, conventional methods that are used for rubbing polymer films can hardly deform the surface of ITO. Attempts to align liquid crystal molecules on rubbed ITO films lead to a very weak alignment. 3 Following our former approach to study the alignment of LC molecules on nanopatterned polymer surfaces, 4 we have performed a series of experiments to study the alignment of LC molecules on nano patterned ITO surfaces using the tip of an atomic-force microscope ͑AFM͒. We have found that a good alignment of LC molecules on submicron patterned ITO surfaces can be obtained, with an azimuthal anchoring energy that is comparable to that of rubbed polymer films like polyimide ͑PI͒. However, in contrast to rubbing no pretilt angle is found by the AFM technique.Indium tin oxide coated glass was purchased from Applied Film Corporation 5 and consisted out of 19 nm ITO coated by rf sputtering over a polycrystalline SiO 2 ͑20 nm͒ layer on a 1.1 mm sodalime glass substrate. The ITO layer had a granular structure with grain size of the order of 5-10 nm and was polished by the manufacturer to obtain optical quality for...

Supramolecular command surfaces for liquid crystal alignment

“…Other techniques that have shown the promise of replacing rubbing include the adsorption of thiols on gold,7–11 the bombardment of a carbonaceous surface with a low‐energy ion beam,12, 13 nano‐rubbing, in which an AFM or STM tip is used to induce surface anisotropy14–19 and the use of Langmuir–Blodgett monolayers 20–24. More comprehensive reviews of these methods were recently published 6, 25…”

Alignment of liquid crystals with periodic submicron structures ablated in polymeric and indium tin oxide surfaces

“…The anchoring energies found for PI and PMMA are of the same order as for the conventionally rubbed PI, whereas the value for ITO is a factor of 3 smaller than that obtained by AFM patterning. 6 Figure 2 shows optical micrographs of a series of TN pixels. LCs are well aligned in the grating area and form TN pixels.…”

Sub-millisecond nematic liquid crystal switches using patterned command layer

“…9,10,18 Increasing attention has been devoted to micropatterned alignment surfaces, thanks to their flexibility in controlling the main LC alignment properties, such as pretilt angle and anchoring energy, compared to unidirectional alignment methods. [19][20][21][22][23][24][25][26][27][28][29][30] Yokoyama and coworkers 19 demonstrated already more than 10 years ago that by nanorubbing a polyimide layer with an atomic force microscope, it was possible to locally manipulate the LC alignment from planar to orthogonal between neighboring domains on the same surface. More recently, nanoimprinting and photoalignment techniques have been employed to build more complex micropatterned surfaces to form bistable LC devices.…”