  Density-functional calculations are used to clarify the role of an ultrathin
LiF layer on Al electrodes used in molecular electronics. The LiF layer creates
a sharp density of states (DOS), as in a scanning-tunneling microscope (STM)
tip. The sharp DOS, coupled with the DOS of the molecule leads to negative
differential resistance (NDR). Electron transfer between oriented molecules
occurs via resonant tunneling. The I-V characteristic for a thin-film of tris
(8-hydroxyquinoline)- aluminum (AlQ) molecules, oriented using electric-field
poling, and sandwiched between two Al/LiF electrodes is in excellent agreement
with theory. This molecular device presents a new paradigm for a convenient,
robust, inexpensive alternative to STM or mechanical break-junction structures.
