The fitting were performed first, by extracting data from experimental I-V curves of corresponding literatures using Engauge Digitizer software and the I-V plot was digitized into 501 points. Then, after mathematical description of all code segments or/ parameters, the acquired data was fitted via nonlinear fitting regression in MATLAB software using three tunneling equations one at a time, and the initial guess values of energy offsets and coupling were adjusted during fitting until the best fit was obtained.