Electron transport through thin organic films in metal-insulator-metal junctions based on self-assembled monolayers

This paper describes an experimentally simple system for measuring rates of electron transport across organic thin films having a range of molecular s tructures. The system uses a metal-insulator-metal junction based on self-a ssembled monolayers (SAMs); it is particularly easy to assemble. The juncti on consists of a SAM supported on a silver film (Ag-SAM(1)) in contact with a second SAM supported on the surface of a drop of mercury (Hg-SAM(2))-tha t is, a Ag-SAM(1)SAM(2)-Hg junction. SAM(I) and SAM(2) can be derived from the same or different thiols, The current that flowed across junctions with SAMs of aliphatic thiols or aromatic thiols on Ag and a SAM of hexadecane thiol on Hg depended both on the molecular structure and on the thickness o f the SAM on Ag: the current density at a bias of 0.5 V ranged from 2 x 10( -10) A/cm(2) for HS(CH2)(15)CH3 on Ag to 1 x 10(-6) A/cm(2) for HS(CH2)(7)C H3 on Ag, and from 3 x 10(-6) A/cm(2) for HS(Ph)(3)H (Ph = 1,4-C6H4) on Ag to 7 x 10(-4) A/cm(2) for HSPhH on Ag. The current density increased roughl y linearly with the area of contact between SAM(I) and SAM(2), and it was n ot different between Ag films that were 100 or 200 nm thick. The current-vo ltage curves were symmetrical around V = 0. The current density decreased w ith increasing distance between the electrodes according to the relation I = I(0)e(-beta dAg.Hg). where d(Ag.Hg) is the distance between the electrode s, and P is the structure-dependent attenuation factor for the molecules ma king up SAM(1). At an applied potential of 0.5 V, beta was 0.87 +/- 0.1 Ang strom (-1) for alkanethiols, 0.61 +/- 0.1 Angstrom (-1) for oligophenylene thiols, and 0.67 +/- 0.1 Angstrom (-1) for benzylic derivatives of oligophe nylene thiols. The values of P did not depend significantly on applied pote ntial over the range of 0.1 to 1 V. These junctions provide a test bed with which to screen the intrinsic electrical properties of SAMs made up of mol ecules with different structures; information obtained using these junction s will be useful in correlating molecular structure and rates of electron t ransport.