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

摘要

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 (Φ_BΣ) and optic measurements (Δ_o), Φ_BΣ ≈ Δ_o. The ‘electrical gap’, Φ_BΣ, consists of sum of two barriers, Φ_BLUMO and Φ_BHOMO, 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:Φ_BΣ = Φ_BLUMO + Φ_BHOMO+ ΔWF . We fabricated and tested two devices with a pair of gold (Au) and aluminum (Al) electrodes to find Φ_BHOMO (Φ_BAu) and Φ_BLUMO (Φ_BAl) using Simmons tunneling model. The symbols Φ_BLUMO and Φ_BHOMO 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 Φ_BΣ and Δ_o can identify presence of electrically active molecules between electrodes in nanoscale devices.