This illustration shows a fully transparent thin-film transistor consisting of a molybdenum sulphide monolayer, hafnium dioxide as a coating and aluminum-doped zinc oxide contacts. Image: © 2017 WILEY-VCH.See-through electronic devices such as transparent displays, smart windows and concealed circuits require completely translucent components if users are to digitally interact with their perceived surroundings and manipulate this information in real time. Now, researchers at King Abdullah University of Science & Technology (KAUST) in Saudi Arabia have devised a strategy that helps to integrate transparent conducting metal oxide contacts with two-dimensional (2D) semiconductors to produce see-through devices. They describe this strategy in a paper in Advanced Functional Materials.
Ultrathin semiconductor sheets composed of transition metals associated with chalcogen atoms such as sulfur, selenium and tellurium offer exceptional electronic properties and optical transparency. However, to date, incorporating molybdenum sulphide (MoS2) monolayers into electronic circuits has relied on silicon substrates and electrodes made of metals such as gold and aluminum. The opacity of these materials has stalled attempts to develop fully transparent 2D electronic devices.
The KAUST team, led by material scientists Xi-Xiang Zhang and Husam Alshareef, has now combined MoS2 monolayers with transparent contacts to generate a series of see-through devices and circuits, including transistors, inverters, rectifiers and sensors. The contacts consist of aluminum-doped zinc oxide (AZO), a low-cost transparent and electrically conductive material that may soon replace the widely used indium-tin oxide. "We wanted to capitalize on the excellent electronic properties of 2D materials, while retaining full transparency in the circuits," explains Alshareef.
According to Alshareef, the researchers grew the contacts over a large area by atomic-layer deposition, in which individual atomic layers precisely accumulate on a substrate. More difficult was forming high-quality MoS2 monolayers on silicon-based substrates over an equally large area. "We overcame this by using an interfacial layer that promotes MoS2 growth," says Alshareef.
The team also developed a water-based transfer process for moving the large-area monolayers onto a different substrate, such as glass or plastic. The researchers then deposited the AZO contacts on the transferred 2D sheets before fabricating the devices and circuits.
The resulting devices outperformed their equivalents equipped with opaque metal contacts for gate, source and drain electrodes, which demonstrates the high compatibility between the transparent, conducting metal oxide contacts and the MoS2 monolayers. "The transistors fabricated by the large-area process showed the lowest turn-on voltage of any reported MoS2 monolayer-based thin-film transistor grown by chemical vapor deposition," says PhD student Zhenwei Wang, first author of the study.
"Additional circuits are planned that will help demonstrate that our approach is robust and scalable," says Alshareef.
This story is adapted from material from KAUST, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.