BIOMEDICAL IMAGING: Inexpensive approach speeds photodetector used in medical imaging

An amorphous silicon (a-Si) metal-semiconductor-metal heterojunction photodetector with added molybdenum disulfide (MoS₂) could boost the speed of medical imaging for minimal cost.¹ MoS₂ is sold as a lubricant in auto-parts shops.

Many photodetectors in large-area imaging devices use a-Si because it absorbs light well and is relatively inexpensive to process. But because the material inherently prevents fast, ordered movement of electrons, it operates more slowly, and thus allows greater exposure to radiation. The standard approach to improving performance has involved additional high-temperature processing, which adds to the cost of the imaging device.

University of California, Berkeley engineers Sayeef Salahuddin and Mohammad Esmaeili-Rad devised their solution by pairing a thin film of MoS₂ with a sheet of a-Si. By forming a diode with the a-Si, the MoS₂ allows the photogenerated electrons that it collects to travel through the a-Si 10 times faster. The detector has a photoresponsivity of 210 mA/W for green light, which is two to four times higher than usual for a-Si devices.

The researchers say that because these materials are easy to handle and inexpensive, the cost of speeding up photodetectors would be minimal. Unlike conventional semiconductors like Si, 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.

1. M. R. Esmaeili-Rad and S. Salahuddin, Sci. Rep., 3, 2345 (2013); doi:10.1038/srep02345.