In-Situ Optical Absorbance Spectroscopy of Molecular Layers in Carbon Based Molecular Electronic Devices

In-situ optical absorbance spectroscopy was used to monitor transparent carbon based molecular electronic junctions with various molecular and metal oxide layers. Junctions with molecular layers consisting of N-decylamine (C10N) and fluorene (FL) did not show absorbance changes upon the application of voltage pulses. Junctions with molecular layers consisting of 4-nitroazobenzene (NAB) and 9,10anthraquinone (AQ) showed absorbance changes upon the application of voltage pulses which were reversible for at least tens of cycles. For NAB junctions, a negative voltage pulse caused an increase in absorbance at 410 nm and a decrease in absorbance at 360 nm. For AQ junctions, a negative voltage pulse caused an absorbance increase at 395 nm and a decrease in absorbance at 320-350 nm. These absorbance changes are consistent with the reduction of the NAB and AQ layers when the carbon substrate is biased negative. Positive voltage pulses reversed the absorbance changes observed during a negative pulse which is consistent with the reoxidation of the molecular layer. The persistence of the absorbance changes depended strongly on the molecule, with absorbance changes persisting for tens of minutes for NAB junctions but only several seconds for AQ junctions. The in-situ optical absorption results are supported with solution based electrochemistry of both free molecules and chemisorbed molecular layers and time-dependent density functional theory. We have shown that in-situ optical absorbance spectroscopy can be used to probe changes in energy levels through absorption changes in biased molecular junctions, which should be useful for deducing structural and electronic changes that strongly effect electron transfer in molecular electronic devices.