Study of Potential Change, Charge Distribution, Voltage Drop, Band Lineup, and Transmission Spectrum of Molecular Break Junction Under Low Bias

Using density functional theory (DFT), combined with non-equilibrium Green’s function (NEGF) method, the effect of potential change, charge distribution, voltage drop, band lineup, and evolution of the transmission spectrum under small applied bias for a memantine-functionalized gold nanogap device for DNA detection have been studied in this report. We have investigated the potential perturbation and charge distribution introduced by the electrodes and the nucleobases separately, which helps to understand the development of the potential profile throughout the molecular break junction and the effect on the transmission spectrum. The presence of electrodes is found to modify the energy levels and band lineup of the device. We have also investigated the local density of states to understand the contact- and nucleobase-modified charge distributions in the molecular states. The electronic wave functions for the HOMO transmission peak at various applied voltages are also examined to understand the physics behind the evolution of the transmission peaks. The potential drop and the charge distribution at 1 V were analyzed for the extended molecular region, and the potential drop was found to be almost uniform. The current voltage (I–V) and the differential conductance characteristics for both the nucleobases of cytosine and adenine indicate that the device will operate normally up to a maximum bias of 1 V. Beyond this voltage, the resonant peaks will become very broad, and they will start to overlap with each other (from other nucleobases), in addition to the transmission peaks becoming weak.