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Title: Electrical conductivity of single organic molecules in ultra high vacuum
Author: Pires, Ellis John
Awarding Body: Cardiff University
Current Institution: Cardiff University
Date of Award: 2013
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Measurement of the I(V ) characteristics of single molecules is the first step towards the realisation of molecular electronic devices. In this thesis, the electronic transport properties of alkanedithiol (ADT) and alkylthiol-terminated oligothiophene molecules are investigated under ultra high vacuum (UHV) using a scanning tunnelling microscope (STM). Two techniques are employed that rely upon stochastic molecular bridge formation between gold STM tip and substrate; a novel I(V; s) method is proven to be a powerful alternative to the well-known I(s) method. For ADTs, three temperature-independent (180 - 390 K) conduction groups are identified, which arise from different contact-substrate coordination geometries. The anomalous reduction of conductance at small chain lengths reported by other groups for non-UHV conditions is far less pronounced here; all groups closely follow the anticipated exponential decay with chain length, β = (0.80 ± 0.01) Å ¹, until a small deviation occurs for the shortest molecule. Thus, the likely explanation for the anomalous effect is hydration of thiol groups. The I(V ) curves were fitted using a rectangular tunnel barrier model, with parameters in agreement with literature values; m = (0.32 ± 0.02) m, φ = 2 eV. For the oligothiophene molecules, one common temperature-independent (295-390 K) conduction group was identified; the conductance decays exponentially with molecular length, with different factors of β = (0.78 ± 0.15) Å ¹ and β = (0.16 ± 0:04) Å ¹ for length changes to the alkylthiol chains and thiophene backbone, respectively. An indented tunnel barrier model, anticipated from the physical and electronic structure of the molecules, was applied to fit the measured I(V ) curves; φ1 = φ3 = 2 eV, φ2 = 1.3 to 1.6 eV, m = 0.17 to 0.24 m. These UHV measurements provide an important baseline from which to better understand recent reports indicating hydration-dependent, and hydration-induced temperature-dependent, transport properties.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics