Srest Somay is pursuing a PhD at the Indian Institute of Technology Delhi and is currently working at NIMS as part of the LOTUS program. His research interest lies in defect control and characterization in low-dimensional materials.

Defects in transition-metal dichalcogenides (TMDs) hinder the realization of high-performance devices, making controlled growth strategies that reduce defect densities essential.¹ For this purpose, a facile method for characterizing defect states is indispensable.² Here, we introduce a gate-modulated optical method for identifying defect states through photocurrent signatures. The DC gate voltage progressively fills defect states, while 820-nm sub-gap illumination excites electrons from the occupied defect states into the conduction band. A small AC modulation superimposed on the gate voltage periodically modulates the defect occupancy and the resulting photoinduced drain current. The measured AC response therefore represents the device transconductance, and its illumination-induced variation provides a direct signature of defect-state occupation and optical excitation.

1. W. Ren, et al., The 2D Materials Roadmap, 2D Mater. 12, (2025), DOI: 10.1088/2053-1583/ae2b82.
2. J. Hong, Z. Hu et al., Exploring Atomic Defects in Molybdenum Disulphide Monolayers, Nat. Commun. 6, 6293 (2015), DOI: 10.1038/ncomms7293.