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Brain-like electronic devices produce vivid technicolor

09 September 2020

Researchers have successfully obtained bright colours by using semiconductor chips – not dyes – made by mimicking the human brain structure.

Structural colouration is promised to be the display technology of the future as there is no fading –  it does not use dyes – and enables low-power displays without a strong external light source. 

However, the disadvantage of this technique is that once a device is made, it is impossible to change its properties, so the reproducible colours remain fixed. Recently, a POSTECH research team has successfully obtained vivid colours by using semiconductor chips -- not dyes -- made by mimicking the human brain structure.

POSTECH's joint research team developed a technology that can freely change the structural colours using IGZO (Indium-Galium-Zinc-Oxide), a type of oxide semiconductor. IGZO is a material that is widely used, not only in flexible displays but also in neuromorphic electronic devices. This is the first study that incorporates IGZO to nanoptics.

IGZO can freely control the charge concentration within a layer through the hydrogen plasma treatment process, thereby controlling the refractive index in all ranges of visible light. In addition, nanoptical simulations and experiments have confirmed that the extinction coefficient of visible light is close to zero, thus enabling the actualisation of a transmittable colour filter in the penetrable form that can transmit exceptionally clear colours with extremely low light loss.

The IGZO-based colour filter technology developed by the research team consists of a four-layer (Ag-IGZO-SiO2-Ag) multilayer and can transmit vivid colours using the Fabry-Perot resonance3 properties. Experiments have confirmed that as the charge concentration of IGZO layer increases, the refractive index decreases which can change the resonance properties of light that are selectively transmitted.

This design method can be applied not only to colour filters for large-scale displays, but also to colour printing technique of micro (11-6, millionth) or nano (10-9, billionth) sizes.

To verify this, the research team demonstrated a colour printing technology that has a pixel size of one micrometre (µm, one-millionth of a metre).

The results proved that the colours from the centimetre or micrometre-sized colour pixels can be adjusted freely, depending on the charge concentration of the IGZO layer. It was also confirmed that the structural colour can be changed more reliably and quickly through changing the refractive index via charge concentration compared to other conventional all-solid-state variable materials like WO3 or GdOx.

"This research is the very first application of IGZO to nanoptical structural colour display technology. IGZO is the next-generation oxide semiconductor used in flexible displays and neuromorphic electronic devices," stated Professor Rho who led the research. 

"It is anticipated that this technology, which enables filtering the transmitted light by adjusting the charge concentration, can be applied to the next-generation low-power reflective display and anti-tamper display technologies."

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