Making electrical contacts to molecular monolayers

Abstract

Electrical contacts between a metal probe and molecular monolayers have been characterized using conducting atomic force microscopy in an inert environment and in a voltage range that yields reversible current-voltage data. The current through alkanethiol monolayers depends on the contact force in a way that is accounted for by the change of chain-to-chain tunnelling with film thickness. The electronic decay constant, βN, was obtained from measurements as a function of chain length at constant force and bias, yielding βN = 0.8±0.2 per methylene over a ±3 V range. Current-voltage curves are difficult to reconcile with this almost constant value. Very different results are obtained when a gold tip contacts a 1,8-octanedithiol film. Notably, the current-voltage curves are often independent of contact force. Thus the contact may play a critical role both in the nature of charge transport and the shape of the current-voltage curve.

A Tunnel Current in Self‐Assembled Monolayers of 3‐Mercaptopropyltrimethoxysilane

“…We can also note that a 2-2.5 eV value for Si/SiO x /alkyl SAM/metal when the alkyl molecule form a chemical bond at the metal side is also consistent with the value found for metal/alkylthiol(or Dithiol)/metal where the molecule is similarly chemisorbed at the metal surface. [7,21,[23][24][25] The present study also yields an effective mass of electrons, m * = 0.16 m e , which is somewhat closer to m * = 0.28 m e theoretically calculated for n-alkanes [22] .…”

Molecules on Si: Electronics with Chemistry

“…An alternative approach is to study the exponential decay of the current density (J) at a fixed bias (V) as a function of molecular length (L): [29,35,156] ln J ¼ ln J C À bL (6a)…”

Charge transport across metal/molecular (alkyl) monolayer-Si junctions is dominated by the LUMO level

“…[13][14][15][16][17] In short, even for one of the simplest systems, i.e. that of alkyl chains between Au electrodes, there is a large discrepancy between the average tunnel barrier height, extracted from current -voltage (I − V ) measurements (∼ 1.2 eV), 14,[18][19][20][21] and the barrier that is expected from the experimentally determined electrode work function and alkyl monolayer ionization potential, i.e. the energy difference between the Fermi level and highest occupied molecular orbital (HOMO) as found by Ultraviolet Photoelectron Spectroscopy, UPS (∼ 5 eV).…”