New Metrology Approach for Single Molecular Device: Probing of Molecular Energy Gap by Electrical and Optical Measurements

We report on electrical and optical measurements to probe the energy gap in single molecular devices. The closeness of energy of the high occupied molecular orbital - low unoccupied molecular orbital (HOMO-LUMO) energy gap was detected by electric (ΦBΣ) and optic measurements (Δo), ΦBΣ ≈ Δo. The ‘electrical gap’, ΦBΣ, consists of sum of two barriers, ΦBLUMO and ΦBHOMO, for tunneling of electrons and holes through molecular barrier correspondingly, in devices with a different pair of electrodes at low biases and difference in work function, ΔWF, between these metal electrodes:ΦBΣ = ΦBLUMO + ΦBHOMO+ ΔWF. We fabricated and tested two devices with a pair of gold (Au) and aluminum (Al) electrodes to find ΦBHOMO (ΦBAu) and ΦBLUMO (ΦBAl) using Simmons tunneling model. The symbols ΦBLUMO and ΦBHOMO signify energy difference between HOMO and LUMO levels in a molecular system with respect to the Au and Al electrode’s Fermi level. Comparison of ΦBΣ and Δo can identify presence of electrically active molecules between electrodes in nanoscale devices.