Chemistry of Photo Isomerization and Thermal Reset of Nitro-Spiropyran and Merocyanine Molecules on Channel of MoS2 Field Effect Transistor

We explore the chemical reaction of the photo isomerization and thermal reaction of photochromic spiropyrans (SP) (1′,3′-Dihydro-1′,3′,3′-trimethyl-6-nitrospiro[2H-1-benzopyran-2,2′-(2H)-indole] molecule deposited on atomically thin channel of MoS2 for field effect transistor (FET) through the analysis of the FET property. With four monolayer of SP molecules on the channel, we observed a clear shift of the threshold voltage in drain-current vs gate-voltage plot with the UV light injection on the molecule, which is due to the change of the SP molecule into the merocyanine (MC) molecule. A complete rest from MC to SP molecule was achieved by the thermal annealing but the green light injection failed this reset. In the process from MC to SP change, two types of decay rates were confirmed. The rapid- and slow-decay components are corresponding to the molecule attached directly to the substrate and the one in the upper layer, respectively. The activation energies for the conversion from the MC to SP molecules are estimated as 71 kJ/mol and 90 kJ/mol for the former and latter, respectively. Combined with DFT calculation, we concluded that the Id-Vg shift with the photo isomerization from SP to MC is derived from the upper layer molecules and due to the dipole moment in the surface normal direction. Based on the activation barrier of 90 kJ/mol for the reset process, we estimated the conversion rate for the controllable temperature range. From these values, we consider the chemical state of MC can be maintained and switched in a designated time period, which demonstrates the possibility of this system for the application for a logic operation.