Nanoscale mapping of stacking-dependent work function and local photoresponse in CVD-grown MoS2 bilayers by KPFM

Abstract

Stacking order in bilayers of transition metal dichalcogenides (TMDs) controls structural symmetry and layer-to-layer interactions, offering a direct route to tune their electronic properties and enable optoelectronic applications. The work function is a key parameter that determines the electronic and optoelectronic device performance. However, a comprehensive understanding of the influence of stacking order on work function of TMDs remains limited. Herein, we employ Kelvin Probe Force Microscopy (KPFM) to probe spatial variations in surface potential and thereby determine the work function of AA'- and AB-stacked MoS2 bilayers grown using NaCl-assisted chemical vapor deposition (CVD) technique. The work function increases with layer number in both AA'- and AB-stacked MoS2, with a larger work function difference in AB-stacked layers, reflecting their stronger interlayer coupling. KPFM measurements clearly resolve local electronic heterogeneities arising from carrier trapping at residual surface particulates from CVD growth. Photoinduced surface potential variations imply n-type doping in MoS2 due to enhanced photogating from trapped holes and Na+ ions at the MoS2/SiO2 interface. Our study demonstrates the competing effects of interlayer coupling, substrate-induced photogating, and carrier trapping by surface particulates in determining the localized optoelectronic response of MoS2 bilayers. Correlative atomic force microscopy measurements in lateral force microscopy and force modulation microscopy modes probe the nanomechanical response to electronic variations. These findings provide new insights into the localized optoelectronic response of CVD-grown AA'- and AB-stacked MoS2, with significant implications for the design and reliability of optoelectronic devices.