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

TitleElectron transport through thin organic films in metal-insulator-metal junctions based on self-assembled monolayers
Publication TypeJournal Article
Year of Publication2001
AuthorsR.E. Holmlin, R Haag, M.L. Chabinyc, RF Ismagilov, AE Cohen, A Terfort, M.A. Rampi, and G.M. Whitesides
Date PublishedMAY 30
AbstractThis paper describes an experimentally simple system for measuring rates of electron transport across organic thin films having a range of molecular structures. The system uses a metal-insulator-metal junction based on self-assembled monolayers (SAMs); it is particularly easy to assemble. The junction 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))-that 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 of 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(CH(2))(15)CH(3) on Ag to 1 x 10(-6) A/cm(2) for HS(CH(2))(7)CH(3) on Ag, and from 3 x 10(-6) A/cm(2) for HS(Ph)(3)H (Ph = 1,4-C(6)H(4)) on Ag to 7 x 10(-4) A/cm(2) for HSPhH on Ag. The current density increased roughly linearly with the area of contact between SAM(I) and SAM(2), and it was not different between Ag films that were 100 or 200 nm thick. The current-voltage curves were symmetrical around V = 0. The current density decreased with 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 electrodes, and P is the structure-dependent attenuation factor for the molecules making up SAM(1). At an applied potential of 0.5 V, beta was 0.87 +/- 0.1 Angstrom (-1) for alkanethiols, 0.61 +/- 0.1 Angstrom (-1) for oligophenylene thiols, and 0.67 +/- 0.1 Angstrom (-1) for benzylic derivatives of oligophenylene thiols. The values of P did not depend significantly on applied potential 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 molecules with different structures; information obtained using these junctions will be useful in correlating molecular structure and rates of electron transport.