Electrical Rectification by a Monolayer of Hexadecylquinolinium Tricyanoquinodimethanide Sandwiched between Gold Electro
- PDF / 537,201 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 18 Downloads / 151 Views
Electrical Rectification by a Monolayer of Hexadecylquinolinium Tricyanoquinodimethanide Sandwiched between Gold Electrodes Robert M. Metzger Laboratory for Molecular Electronics, Chemistry Department, The University of Alabama Tuscaloosa, AL 35487-0336, USA, [email protected] ABSTRACT Unimolecular rectification, previously reported between oxide-bearing electrodes, was detected between oxide-free Au electrodes for a Langmuir-Blodgett monolayer of the zwitterionic D+-π-A- molecule hexadecylquinolinium tricyanoquinodimethanide, C16H33Q-3CNQ. The top gold electrode was deposited from a Au vapor pre-cooled by multiple collisions with Ar atoms. The maximum rectification ratio is 27.5 at 2.2 Volts (the average rectification ratio is 7.55). The currents are as large as 9.04 × 104 electrons molecule-1 s-1. The result reinforces previous work with oxide-bearing Al electrodes, but the currents with Au electrodes are larger by three to five orders of magnitude. INTRODUCTION Langmuir-Blodgett (LB) monolayers and multilayers of the ground-state zwitterionic donorpi-acceptor (D+-π-A-) molecule hexadecylquinolinium tricyanoquinodimethanide (C16H33Q3CNQ, 1) were first reported in 1990 to be electrically rectifying between a Pt electrode on one side and an oxide-covered Mg pad (covered by Ag) on the other side [1], and also with intervening monolayers of insulating aliphatic acids [2]. This rectification was confirmed in 1997 with oxide-bearing Al electrodes on both sides of monolayers and multilayers [3]. It was suggested that the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of 1 were involved in the enhanced forward current [3]. Most junctions rectify as described, but some cells are symmetrical, while some rectify (with much lower currents) in the opposite direction [4]. The rectification in the temperature range 370 K to 105 K has no definite temperature dependence, but the currents increase with temperature [5]. Spectroscopy and electrochemistry confirmed the zwitterionic ground state and the much less polar first excited state of 1 [6]. The large enhancement in the forward current was ascribed to resonance between the Fermi level of the Al electrodes, appropriately shifted by the applied electrical potential, and the LUMO of the molecule [3]. The relaxation of the molecule after electron transfer is most likely associated with the intervalence transfer band, (peak at 570 nm (2.17 eV) in films of 1) between the ground state (dipole moment 43 ± 8 Debyes) [3] and the first electronic excited state (dipole moment between 3 and 9 Debyes) [6]. These results have been reviewed [7-15]. A persistent question was whether the defective oxide covering of Al or Mg, unavoidably present on both sides of the organic monolayer, were responsible for the rectification [13-15]. We now show that rectification occurs also when monolayers or multilayers of 1 are placed between oxide-free gold electrodes. A preliminary report has been presented elsewhere [16], and full details have been given [17].
Data Loading...
