Direct Optical Determination of Interfacial Transport Barriers in Molecular Tunnel Junctions

Abstract

Molecular electronics seeks to build circuitry using organic components with at least one dimension in the nanoscale domain. Progress in the field has been inhibited by the difficulty in determining the energy levels of molecules after being perturbed by interactions with the conducting contacts. We measured the photocurrent spectra for large-area aliphatic and aromatic molecular tunnel junctions with partially transparent copper top contacts. Where no molecular absorption takes place, the photocurrent is dominated by internal photoemission, which exhibits energy thresholds corresponding to interfacial transport barriers, enabling their direct measurement in a functioning junction.

Internal Photoemission in Molecular Junctions: Parameters for Interfacial Barrier Determinations

“…The opposite sign for the case of the alkane junctions (black curve) at low energy results from electron transport through the HOSO into photogenerated holes in the Cu contact, as reported previously 24,44 and shown schematically in Figure 1. Figure 2B shows a Fowler plot for an Al/AlOx/Cu(20 nm) junction, where the extrapolated value of the electron tunneling barrier is determined to be 2.4(±0.1) eV, consistent with other reported values.…”

“…This enabled the energy level alignment of electron and hole barriers for the C12 junction to be determined. 44 Note that in Figure 3A the energy where the photocurrent changes sign (i.e., the "crossing point" of the plot in Figure 3B) depends on the structure of the molecule: the crossing points are 3.7 eV for BrP, 2.8 eV for AQ, 1.65 eV for AB, while NAB remains negative in the spectral range studied and does not cross the abscissa. In order to determine if the absorbance of the molecule correlates with these crossing points, the photocurrent yield spectra are overlaid with the absorbance spectra for each of the aromatic molecules in Figure 4 (complete details for obtaining absorbance spectra for the thin molecular layers on carbon are provided in Supporting Information, section 4 61 ).…”

“…Lock-in detection (Supporting Information Figure S3-A) was used to measure the resulting photocurrent, and an independent measure of the optical power incident on the junction with a Newport 1936-R power meter was used to determine the external quantum yield (EQE). 44 EQE is defined as the number of photoelectrons in the external circuit divided by the number of photons incident on the junction. The sign of the photocurrent was determined by calibrating the phase of the lock-in detection using a photodiode as a reference, as described previously, 44 where a positive photocurrent indicates electrons flowing from the Cu to PPF in the external circuit.…”

“…44 The generation of photocurrent by an IPE mechanism is initiated by light absorption in the conducting contacts 40,43,53−55 and results in hot electrons through the excitation and decay of surface plasmons. 56−58 These nonequilibrium carriers can then traverse the molecular layer and be collected at the second electrode when the photon energy exceeds the interfacial barrier (i.e., such that the carrier energy is resonant with the orbital energy) and if energy losses due to scattering are minimal.…”

“…44 The optical absorbance of each molecule bonded to the PPF substrate was determined and compared to the observed photocurrent spectra to determine the effects of molecular absorption. These criteria enable a full assessment of what information IPE can provide regarding energy levels in functioning molecular electronic devices.…”