SYSTEMS FOR ORTHOGONAL SELF-ASSEMBLY OF ELECTROACTIVE MONOLAYERS ON AU AND ITO - AN APPROACH TO MOLECULAR ELECTRONICS

Simultaneous exposure of Au and In2O3/SnO2 (ITO) electrodes to an equi molar solution of a thiol and a carboxylic acid or a thiol and phospho nic acid results in the selective attachment of the thiol to the Au el ectrode and the carboxylic or phosphonic acid to the ITO electrode. Th is selective surface-attachment chemistry is termed ''orthogonal self- assembly'' (OSA) and can be used to direct the spontaneous assembly of molecular reagents onto Au and ITO microstructures. The selectivity o f the thiols for Au and the carboxylic or phosphonic acids for ITO:is determined by a combination of cyclic voltammetry experiments using fe rrocene-tagged molecules, scanning Auger microscopy, and imaging secon dary ion mass spectrometry (SIMS) to map the distribution of thiols, c arboxylic acids, and phosphonic acids on derivatized Au and ITO micros tructures. Simultaneous exposure of Au and ITO electrodes for 30 min t o an equimolar solution of 11-mercaptoundecanoylferrocene (I) and 12-f errocenyldodecanoic acid (III) results in a coverage ratio I:LII of ap proximately 100:1 on Au and 1:45 on ITO, as determined by cyclic volta mmetry. A 30-min exposure of Au and ITO electrodes to an equimolar sol ution of I and 6-ferrocenylhexyl- phosphonic acid (V) yields a coverag e ratio of I:V of 30:1 on Au and better than 1:100 on ITO. The coverag es of I, III, and V on the Au and ITO electrodes can be determined usi ng cyclic voltammetry by virtue of the difference in redox potential b etween the acylferrocene center in I and the alkylferrocene centers in III and V. Typical converages of I on Au (4 x 10(-10) mol/cm(2)) and PPI and V on ITO (6 x 10(-10) and 3 x 10(-10) mol/cm2, respectively) a fter 30 min of derivatization correspond to approximately a monolayer of redox-active molecules in each case. Long derivatization times' (12 -15 h) result in small or insignificant changes in the coverage ratios of these reagents on both Au and ITO electrodes, demonstrating that t he OSA is essentially complete within 30 min. Surface analysis by X-ra y photoelectron spectroscopy, scanning Auger microscopy, and imaging S IMS of Au and ITO microstructures and Si3N4 surfaces exposed to equimo lar solutions of I and perfluorodecanoic acid (IV), or 12,12, 12-trifl uorodode-canethiol (II) and V, reveal the selective assembly of these reagents on the Au and ITO microstructures and their absence on the in sulating Si3N4 substrate. The orthogonal self-assembly process describ ed here provides a promising method by which individual molecules coul d be spontaneously oriented and connected between closely-spaced, exte rnally-addressable electrodes.