External Doping of Tunnel Junctions
Inelastic tunneling spectroscopy (IETS) with M-I-M diodes has been used to detect and study the vibrational modes of organic molecules contained in monolayer amounts inside the barrier layer. In previous experiments it has been necessary to introduce these molecules as an intermediate step in the fabrication process with an additional vacuum evaporation of the top metal electrode coming last. We have developed a method whereby the completed and undoped tunnel junctions can be fabricated in batches and stored in air for later doping with various organic molecules. The doping method is a vapor exposure technique in which the presence of water vapor is a condition for the infusion of molecules into the barrier region. A microscopic layer of adsorbed water apparently acts as a carrier for the introduction of molecules. Experiments with D2O show that the OH can be exchanged with OD inside the barrier. A large number of polar organic molecules (e.g. formic acid, methyl alcohol, hydrogen cyanide) have been infused in monolayer coverages in a few minutes by this method. Deuteration experiments have verified this interpretation. The sensitivity and selectivity of this technique is illustrated by the ability to detect low levels (<10 ppm) of formic acid in singly distilled or deionized tap water. Some of the species studied (methyl alcohol, pyridine) could not be studied easily by previous IETS doping methods. Further study is in progress to understand the details of the doping mechanism and determine the size and types of detectable molecular species. This approach to IETS has been found easier and more flexible than before and is capable of high sensitivity and selectivity to certain types of molecules.
KeywordsAcetone Phenol Catalysis Hydrocarbon Pyridine
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- 2.For reviews of IETS spectroscopy and analysis of these types of spectra see P.K. Hansma: Physics Reports 30C, 146 (1977) and R.G. Keil, T.P. Graham, K.P. Renker: Appl. Spectroscopy 30, 1 (1976)Google Scholar