NRL Research Paves Way Toward Development of New Monolayer Materials

WASHINGTON — Scientists at the U.S. Naval Research Laboratory (NRL) have discovered the reason for the large variations observed in the optical properties of new single monolayer semiconductors such as tungsten disulphide (WS2). Single monolayer transition metal dichalcogenides (TMDs), such as WS2, are an emerging class of materials that exhibit many promising optoelectronic properties, with the potential for future device applications.

By measuring spatial variations in nanoscale defect density with atomic force microscopy, the researchers were able to correlate their results with corresponding variations in the material’s optical emission. Optical emission was strongly suppressed in areas with high defect densities, and a pronounced inverse correlation was demonstrated. The ability to understand mesoscopic phenomena in terms of nanoscale defect behavior is a major step forward in the optimization and engineering of two-dimensional materials for future applications.
(a) Experimental schematic of conductive atomic force microscopy (CAFM). (b) Sample CAFM image showing eight clearly identifiable defects. (c) Photoluminescence (PL) intensity map of a WS2 triangle. (d) PL intensity and defect density along the dashed black line in (c) showing an inverse relationship between PL intensity and defect density. (U.S. Naval Research Laboratory)

(a) Experimental schematic of conductive atomic force microscopy (CAFM). (b) Sample CAFM image showing eight clearly identifiable defects. (c) Photoluminescence (PL) intensity map of a WS2 triangle. (d) PL intensity and defect density along the dashed black line in (c) showing an inverse relationship between PL intensity and defect density. (U.S. Naval Research Laboratory)

“TMD properties are expected to be strongly influenced by a variety of defects resulting from growth procedures and/or device processing,” said Dr. Berend Jonker, senior scientist and principal investigator. “Despite the importance of understanding defect-related phenomena, observation of defects over large areas has been challenging, preventing detailed understanding of TMD optical and electrical phenomena from a nano-scale perspective.”

NRL’s team of scientists addressed these issues by using conductive atomic force microscopy (CAFM) to quantify defect density in monolayer samples of WS2 over length scales up to tens of microns. They correlated these spatial variations in defect density with the intensity of the light emitted from the corresponding areas when the sample is optically excited, known as the photoluminescence (PL). A ten-fold decrease in defect density resulted in a ten-fold increase in PL intensity.

“These results show that the presence of defects limits the semiconductor’s ability to emit light, and explain the perplexing spatial variations observed in the PL,” said Jonker. “We were then able to develop a model to describe this phenomenon in which the electrically active defects serve as sites for non-radiative recombination, therefore reducing light emission.”

Dr. Matthew Rosenberger, lead author of the study, points out the importance of this discovery stating, “The ability to precisely identify and quantify defects is a critical capability for the further development and engineering of two-dimensional materials for real world devices and applications.”

Jonker further notes that the quantitative correlation of defect density with photoluminescence intensity provides key information for a fundamental understanding of the optical behavior and exciton physics at the nanoscale in this emerging class of two dimensional materials.

The results of this study pave the way for the use of TMD materials in applications relevant to the Department of Defense (DoD) mission, such as ultra-low power electronics, photodetectors and emitters, non-volatile optical memory, and quantum computation for future DoD applications in information processing and sensing. The research results are reported in ACS Nano (DOI: 10.1021/acsnano.7b08566).

The research team included Dr. Matthew Rosenberger, Dr. Hsun-Jen Chuang, Dr. Kathleen McCreary, Dr. Connie Li, and Dr. Berend Jonker from the NRL Materials Science and Technology Division. Dr. Rosenberger holds a National Research Council (NRC) fellowship at NRL. Dr. Chuang holds an American Society for Engineering Education (ASEE) fellowship at NRL.