ELECTRON-TUNNELING RATES AS A FUNCTION OF INTERMOLECULAR DISTANCE, MEASURED IN A LANGMUIR-BLODGETT ASSEMBLY

Results are presented that demonstrate an exponential dependence of th e electron tunnelling rate on interlayer separation. Direct measuremen ts of transient photocurrents with sub-nanosecond resolution have been made on a series of Langmuir-Blodgett multilayer structures assembled from two amphiphilic bis-phthalocyanine molecules. Using either homos tructures of the two molecules or a heterodimer structure incorporatin g both molecules, three tunnelling gaps were attainable. The attempt r ate, nu(0), as determined by Franck-Condon factors, and the well depth in the three structures was identical for each, being determined by t he phthalocyanine ring structure and electron affinity. It is thus pos sible to relate any change in measured tunnelling rate with the change in barrier width, b. The tunnelling rate, k(perpendicular to), is rel ated to the tunnelling barrier width by: k(perpendicular to) = nu(0)(E )exp(-2 root 2mA/(h) over bar) b where A is the well depth (electron a ffinity of the conjugated rings) and m is an effective mass for the tu nnelling carrier. A plot of In(k(perpendicular to)) vs. b reveals a st raight line whose gradient is -2 root 2mA/(h) over bar which intercept s the k(perpendicular to) axis at k(perpendicular to) = v(0)(E). From the data a value of A = 2.4 eV and nu(0) = 7 x 10(16) Hz at an electri c field of 2 x 10(8) V m(-1) are found. The well depth is in good agre ement with expectations. The attempt rate is high but there is a large uncertainty in this value and it is in overall agreement with expecta tions. nu(0) is expected to depend on electric field and this is the s ubject of a further study.