Scientists at the University of California, Berkeley, have built an experimental device that, they say, could speed up medical imaging using amorphous silicon and a surprising simple inexpensive ingredient—an engine lubricant called molybdenum disulfide, or MoS2, which has been sold in auto parts shops for decades. The two semiconductors form a high-speed photodetector.

Shown is an experimental photodetector made out of amorphous silicon and molybdenum disulfide (MoS2). The two semiconductors form a high-speed photodetector. (Credit: Mohammad Esmaeili-Rad)

Many photodetectors in large-area imaging devices use amorphous silicon since it absorbs light well and is relatively inexpensive to process. But, this type of silicon has defects that prevent the fast, ordered movement of electrons, leading to slower operating speeds and more exposure to radiation. Getting better performance requires more expensive, high-temperature processing, adding to the cost of the device.

Sayeef Salahuddin, UC Berkeley Assistant Professor of Electrical Engineering and Computer Sciences, and postdoctoral scholar Mohammad Esmaeili-Rad discovered that pairing a thin film of MoS2 with a sheet of amorphous silicon speeds up the collection of the photo-generated electrons. The combination forms a diode that results in a photoresponse rate that was 10 times faster than conventional amorphous silicon alone.

Since these materials are easy and inexpensive to handle, the cost of speeding up photodetectors would be minimal, they say. Unlike conventional semiconductors like silicon, MoS2 consists of individual nanosheets that can be torn off like pages in a book. These sheets can be used to make thin, novel electronic devices or to improve existing ones.