화학공학소재연구정보센터
Thin Solid Films, Vol.273, No.1-2, 229-231, 1996
Electron-Tunneling Rates as a Function of Intermolecular Distance, Measured in a Langmuir-Blodgett Assembly
Results are presented that demonstrate an exponential dependence of the electron tunnelling rate on interlayer separation. Direct measurements 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 homostructures of the two molecules or a heterodimer structure incorporating both molecules, three tunnelling gaps were attainable. The attempt rate, nu(0), as determined by Franck-Condon factors, and the well depth in the three structures was identical for each, being determined by the phthalocyanine ring structure and electron affinity. It is thus possible to relate any change in measured tunnelling rate with the change in barrier width, b. The tunnelling rate, k(perpendicular to), is related 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 affinity of the conjugated rings) and m is an effective mass for the tunnelling carrier. A plot of In(k(perpendicular to)) vs. b reveals a straight line whose gradient is -2 root 2mA/(h) over bar which intercepts 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 electric field of 2 x 10(8) V m(-1) are found. The well depth is in good agreement with expectations. The attempt rate is high but there is a large uncertainty in this value and it is in overall agreement with expectations. nu(0) is expected to depend on electric field and this is the subject of a further study.