Light-Induced Enhancement of Rectification by Monolayer of Hexadecylquinolinium Tricyanoquinodimethanide Between Al Elec
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Light-Induced Enhancement of Rectification by Monolayer of Hexadecylquinolinium Tricyanoquinodimethanide between Al Electrodes Tao Xu and Robert M. Metzger Laboratory for Molecular Electronics, Chemistry Department, The University of Alabama Tuscaloosa, AL 35487-0336 ABSTRACT Photoinduced changes in electrical rectification were measured for a 5-layer Langmuir-Blodgett multilayer of hexadecylquinolinium tricyanoquinodimethanide, sandwiched between opaque and semitransparent Al electrodes, as a function of bias and wavelength. The current increases with the light intensity, but this may be simply due to heating of the sample, rather than to a photon-induced enhancement of the rectification. INTRODUCTION Rectification by Langmuir-Blodgett (LB) monolayers and multilayers of hexadecylquinolinium tricyanoquinodimethanide (C16H33Q-3CNQ, 1), sandwiched between Pt or Ag and Mg electrodes was first reported by Sambles, Ashwell, and co-workers [1,2]. The rectification was confirmed conclusively with Al electrodes on both sides of monolayers and multilayers [3]. The mechanism of rectification was clarified [3], and it was suggested that the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of 1 were involved in the enhanced current observed in the forward direction [3]. Not all junctions rectify [4]. The rectification persisted in the temperature range 370 K to 105 K [5], and ancillary spectroscopic and electrochemical data were secured [6]. The large enhanced current can be ascribed to resonance between the Fermi level of the Al electrodes, appropriately shifted by the applied electrical potential, and the HOMO and LUMO, and probably is intimately associated with the intervalence transfer band, which is measured 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]. The results have received multiple reviews [7-9]. We present here measurements of the change in DC electrical current I as a function of applied bias V and visible light of various frequencies ν on 5 monolayers of C16H33Q-3CNQ, sandwiched between a semi-transparent Al electrode on a glass substrate and an opaque Al pad. An LB monolayer photodiode was first reported by the group of Fujihira [10]. EXPERIMENTAL DETAILS A semitransparent Al electrode (12 nm thick) was deposited on a glass substrate in an Edwards E306 evaporator. A 5-layer multilayer of freshly prepared C16H33Q-3CNQ was transferred (Ztype, with transfer ratios of 100 percent) from a monolayer of C16H33Q-3CNQ at the air-water interface held at a surface pressure of 20 mN/m in a NIMA Model 622 film balance at 10˚C in a HEPA-filtered room under a green safelight. As before [3], the LB layers were dried in a vacuum desiccator over P2O5 for three days, to remove any entrapped water between the layers. Then the film-bearing substrate was placed on a Cu cryotip cooled to 77 K, as 100 nm Al pads were evaporated atop the LB multilayer. The resulting
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