Electron tunnelling rates as a function of intermolecular distance, measured in a Langmuir-Blodgett assembly

• Published in: Thin solid films Vol. 273; no. 1; pp. 229 - 231
• Main Authors: Donovan, K.J., Elliott, J.E., Scott, K., Wilson, E.G., Batzel, D.A., Clark, T.R., Kenney, M.E.
• Format: Journal Article Conference Proceeding
• Language: English
• Published: Lausanne Elsevier B.V 01.02.1996; Elsevier Science
• Subjects: Condensed matter: electronic structure, electrical, magnetic, and optical properties; Electronic structure and electrical properties of surfaces, interfaces, thin films and low-dimensional structures; Electronic transport in interface structures; Exact sciences and technology; Langmuir-Blodgett films; Multilayers; Organic substances; Physics; Structural properties; Tunneling; Multilayers; Organic substances; Langmuir-Blodgett films; Structural properties; Thin films; Potential barrier; Multilayer; Tunnel effect; Electron transfer; Experimental study; Structure; Phthalocyanines; Organic compounds
• ISSN: 0040-6090; 1879-2731
• DOI: 10.1016/0040-6090(95)06777-9

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, v 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 ⊥, is related to the tunnelling barrier width by: k ⊥=v 0(E)exp( −2√2mA bh 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 ln( k ⊥) vs. b reveals a straight line whose gradient is −2 √2mA bh which intercepts the k ⊥ axis at k ⊥ = v 0( E). From the data a value of A = 2.4 eV and v 0 = 7 × 10 16 Hz at an electric field of 2 × 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. v 0 is expected to depend on electric field and this is the subject of a further study.