Society for Information Display News May/June 2014 Issue 2

Winning JSID Outstanding Student Paper Describes Novel Multi-Color Reflective Display

Compiled by Jenny Donelan

Electrochromic (EC) displays – those with reversible color changes caused by electrochemical redox reactions – have various advantages as reflective-type displays over comparable conventional displays, including high visibility under sunlight, a memory effect (they retain an image without a charge), and color-changing picture elements.  (EC displays are expected to achieve better color variation without color filters than conventional reflective displays.)  To improve on the current generation of electrochromic devices, it is necessary to develop multifunctional EC materials that enable control of multiple colors, various color densities, and specular reflection.

Ayako Tsuboi, a graduate student at Chiba University in Japan, and Dr. Kazuki Nakamura and Professor Norihisa Kobayashi of the Graduate School of Advanced Integrated Science at Chiba, recently undertook the task of developing next-generation electrochromic materials based on electrochemically sized controlled silver nanoparticles.  They eventually developed a localized surface plasmon resonance (LSPR) based display device that they believe is suitable for use in information displays and light-modulating devices such as electronic paper, digital signage, and smart windows.  In the device, the size of the electro-chemically deposited silver nanoparticles is finely controlled by applying two consecutive different voltages.  The device shows a wide variety of colors depending on the size of the silver nanoparticles.  The researchers’ report on this work, “Chromatic characterization of novel multicolor reflective display with electro-chemically size-controlled silver nanoparticles,” as described in the August 2013 issue of the Journal of the SID, recently received SID’s prize for Outstanding Student Paper of 2013.

According to JSID Editor-in-Chief John Kymissis, the team’s paper was chosen for both content and execution.  “The committee was enthusiastic about both the novelty and of the work and quality of the paper,” he says.  “Several of the reviewers noted the particularly comprehensive treatment of the problem and detail in explaining the technique used and work performed.”

The Possibilities of Silver Nanoparticles

There have been many attempts to create color reflective displays, including those based on E ink and color filters, microfluidics, tunable photonic crystals, and more.  While some of these efforts have been successful – E Ink has a working three-color display, Spectra – the goal of realizing a full-color reflective display that is commercially viable has remained elusive.  Two years ago, the team at Chiba University reported (in Advanced Materials Journal) on an Ag deposition-based device that showed three subtractive primary colors – transparent, silver mirror, and black.

“In the course of this silver deposition research,” says Kobayashi, “we found that the optical state of silver deposition was affected by the size and coalescence of silver particles deposited on an electrode.  We therefore tried to control the structure of the silver particles to represent various optical states with only silver deposition.”  Applying DC voltage to the particles induced non-uniform growth, but the team discovered they could control the nucleation of the structures, and, hence, the color through a voltage-step method.

The challenge here, notes Kobayashi, was in determining suitable first and second voltage values to control the size of the silver nanoparticles uniformly on an electrode.  Eventually they were able to determine the values necessary to represent red, magenta, purple, cyan, and blue using only silver deposition, with no dyes, pigments, or filters.  Reversible color changes between the transparent and vivid colored states were achieved (Fig. 1).


Fig. 1:  Above are photographs of the device with heart-shaped spacers: (a) before an application of the voltage and after an application of various step voltages; (b) –4.2 V for 10 msec and –1.5 V for 4 sec; (c) –3.8 V for 50 msec and –1.7 V for 17 sec; (d) –3.8 V for 50 msec and –1.7 V for 25 sec; (e) –4.2 V for 10 msec and –1.5 V for 10 sec; (f) –4.2 V for 10 msec and –1.5 V for 25 sec; and (g) –3.8 V for 50 msec and –1.7 V for 40 sec.


Fabricating the Device

The team combined two colors (red and blue) with its previously realized three colored states (transparent, silver mirror, and black), using a device composed of a flat ITO electrode and an ITO particle-modified electrode.  When a constant voltage was applied, the device entered a silver-mirror state as Ag was deposited on the flat ITO electrode.  When Ag was deposited on the rough ITO particle-modified electrode, the device turned black.  And when Ag nanoparticles were electrodeposited uniformly on the flat ITO electrode using the voltage-step method described earlier, the device turned red or blue depending on the V2 application time.  These changes in the optical states were all reversible.  By applying an oxidation voltage to the deposited silver (+0.6 V), all colored states returned to the transparent state.  The length of time required for reversing differed for each colored state (silver mirror, black, red, and blue states required 20, 25, 0.1, and 1 sec, respectively).  In previously reported Ag deposition-based devices, such reversibility was hardly observed because the Ag deposit could not be fully dissolved by electrochemical oxidation.  The bleaching properties of the team’s latest device were superior to previous ones because the Cu2+ ions in the electrolyte effectively promoted the dissolving of the Ag nanoparticles by electrochemical mediation.  After the coloration voltages were shut off, the device held its colored states for certain periods (silver mirror, black, red, and blue states for 325, 300, 5, and 25 sec, respectively).

As a result of these efforts, the team successfully fabricated an LSPR-based EC device with transparent, silver mirror, red, blue, and black states in a single device.  Various color densities and specular reflection were also demonstrated.  Future research will involve investigating the color variations to realize full-color electronic displays.

Graduate student Tsuboi said, “I am really honored to receive such a great award.  It could not have been achieved without the support of my seniors and my colleagues.  I hope that this EC device can be used to achieve colorful electronic paper or smart windows and help us lead a more vibrantly colored life.”