Professor Seong-Ju Park"s research team develops

high efficiency photodetector to improve internet performance

Figure 1. (Left) Sanning electron microscope image (right) of zinc oxide / zinc sulphide core / shell nanowire grown sideways on a flexible substrate.

□ Professor Seong-Ju Park of the Department of Materials Science and Engineering at the Gwangju Institute of Science and Technology (GIST) led a research team that has developed a photodetector * that can dramatically improve the performance of next generation multifunction sensors used in the internet of things (IoT).

  ∘ The photodetector developed by the research team is expected to contribute to the spread of "high-efficiency multi-function sensor" which can integrate various sensors by expanding the application range of pressure sensors, gas sensors, and biosensors.

  * Photodetector: Light incident to a semiconductor material generates electrons, and holes in a semiconductor material cause electrons to flow in accordance with an applied electric field to generate a current. Photodetectors are used in various fields such as smart watches, wearable electronic devices, health and medical devices, ultraviolet sterilization purifiers, and fire detection systems.

□ High efficiency optical sensors are widely used in optical communications, visual imaging, cameras, precision medical devices, etc. Especially, in order to allow the "Internet of things", which is considered to be a core technology in the 4th industrial revolution era, it is necessary to achieve energy efficiency.

However, single nanowire-based devices used in optical sensors are difficult to commercialize because they have issues with various technical problems regarding their considerably low output.

Figure 2.  Graph showing the electrical characteristics (left) and light response characteristics (right) of zinc oxide / zinc sulphide core / shell nanowire device with strain variation. When externally illuminated, the highest photocurrent (as in the pink curve in the left graph) and excellent optical response characteristics (such as the purple curve in the right graph) is seen when having a tensile strain of 0.3% at the same drive voltage.

By aligning a 1-dimensional nanostructure based on zinc oxide (ZnO) on a flexible substrate and then changing the piezoelectric property * of the nanostructure, the team succeeded in improving optical response and electrical properties at the same drive voltage.

* Piezoelectric properties: the properties of a particular material that can be converted between mechanical and electrical energy: Apply mechanical pressure to generate voltage, and apply voltage to cause mechanical deformation.

* Photoresponsiveness: The most representative factor for evaluating photodetector characteristics. It is expressed by the magnitude of the electric current generated by electrons and holes generated per unit intensity of incident light. If light response is high, light of very weak intensity can be easily detected.

In order to increase the output of zinc oxide nanowires, the researchers synthesized zinc oxide nanowires laterally on a flexible substrate and then synthesized the surface with zinc sulfide (ZnS) and adjusted the strain so that it has high light responsiveness even at a driving voltage.

  ◯ When the zinc oxide-zinc sulfide core / shell structure is formed, the probability of recombination of electrons and holes generated when external light is irradiated is lowered, and the photocurrent was greatly increased.

In this zinc oxide-zinc sulphide core / shell nanowire structure, the device was bent through external pressure to increase the output current by 8.93 times and the light responsiveness by 23.9 times compared to the control zinc oxide nanowire at a strain of 0.3% .

□ Professor Seong-Ju Park said, "This achievement will contribute to improving energy efficiency by lowering the driving voltage of the internet, human machine Interface, etc., which depend heavily on the performance of the photodetector. In addition, this will also benefit sensors as well as bio-adaptive multifunction smart sensors."

□ The research conducted by GIST Professor Seong-Ju Park (corresponding author) and Dr. Jeong Se-hui Professor Seong-Ju Park  (first author) was carried out with the support from the Industrial Technology Innovation Development Project and the GIST Research Institute Project of the Ministry of Industry and Trade. The paper appeared on September 28, 2016, in the online edition of Nano Energy, which is the authoritative site for nanomaterials.