Electron Transport through Thin Organic Films in Metal−Insulator−Metal Junctions Based on Self-Assembled Monolayers

Abstract

This 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(1) 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(1) 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 d(Ag,Hg)), where d(Ag,Hg) is the distance between the electrodes, and beta 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 A(-1) for alkanethiols, 0.61 +/- 0.1 A(-1) for oligophenylene thiols, and 0.67 +/- 0.1 A(-1) for benzylic derivatives of oligophenylene thiols. The values of beta 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.

Tunneling across SAMs Containing Oligophenyl Groups

“…21 Moreover, the energy level of the HOMO of conjugated molecules aligns more favorably with the Fermi level of the metal, than does that for aliphatic molecules, and results in lower tunneling barriers. 16,21,37,38 RESULTS AND DISCUSSION these values agree with previous experimental reports using single-molecule and large-area junctions. 27,[38][39][40][41][42][43][44][45] These results presented here also agree with theoretical calculations by Ratner and coworkers; these authors predict (using density functional theory, DFT) a value of  = ~0.3 Å -1 for SAMs of polyphenyldithiolates by assuming a continuous conjugation of the molecules with the metal electrodes.…”

Photo-switchable molecular monolayer anchored between highly transparent and flexible graphene electrodes

“…S10). The current density exponentially decreased as the molecular length of the aryl alkanes increased, thus revealing the tunnelling mechanism 9,20 . Current density (J) plots obtained at several biases as a function of the molecular length (d) are described by the expression Jpexp( À bd), where b is the tunnelling decay coefficient.…”

“…Recently, stabilised fabrication methods using soft conducting layers [5][6][7] were developed for device miniaturization using molecules. Miniaturization using molecules has also been explored via microscopic and spectroscopic studies of single molecules and molecular monolayers [8][9][10][11] . Realistic organic electrodes that are conductive, flexible and even transparent should form ideal contacts to a molecular monolayer of organic molecules in two-terminal crossbar devices and should ultimately be compatible with current standard electronic circuitry.…”

Molecular Series-Tunneling Junctions

“…2,4,8 Aromatic molecules are characterized by smaller energy gaps (∼3−5 eV) between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) than those of aliphatic molecules (∼7 eV) of similar length. 2,48 Furthermore, the HOMO of aromatic molecules aligns more favorably with the Fermi level of electrodes than does that of aliphatic molecules; this alignment, in a SAM-based tunneling junction, facilitates charge transport by lowering the effective height of the tunneling barrier. 5 These differences in the electronic properties of aliphatic and aromatic carboxylates are the basis of the differences in β and rates of tunneling.…”