RESUMEN
A new method for assembling organic monolayers on gold is reported that employs hafnium ions as linkers between a phosphonate headgroup and the gold surface. Monolayers of octadecylphosphonic acid (ODPA) formed on gold substrates that had been pretreated with hafnium oxychloride are representative of this new class of organic thin films. The monolayers are dense enough to completely block assembly of alkanethiols and resist displacement by alkanethiols. The composition and structure of the monolayers were investigated by contact angle goniometry, XPS, PM-IRRAS, and TOF-SIMS. From these studies, it was determined that this assembly strategy leads to the formation of ODPA monolayers similar in quality to those typically formed on metal oxide substrates. The assembly method allows for the ready generation of patterned surfaces that can be easily prepared by first patterning hafnium on the gold surface followed by alkanephosphonate assembly. Using the bifunctional (thiol-phosphonate) 2-mercaptoethylphosphonic acid (2-MEPA), we show that this new assembly chemistry is compatible with gold-thiol chemistry and use TOF-SIMS to show that the molecule attaches through the phosphonate functionality in the patterned region and through the thiol in the bare gold regions. These results demonstrate the possibility of functionalizing metal substrates with monolayers typically formed on metal oxide surfaces and show that hafnium-gold chemistry is complementary and orthogonal to well-established gold-thiol assembly strategies.
RESUMEN
We have investigated and learned to control switching of oligo(phenylene ethynylene)s embedded in amide-containing alkanethiol self-assembled monolayers on Au{111}. We demonstrate bias-dependent switching of the oligo(phenylene ethynylene)s as a function of the interaction between the dipole moment of the oligo(phenylene ethynylene)s and the electric field applied between the scanning tunneling microscope tip and the substrate. We are able to invert the polarity of the switches by altering their design-inverting their dipole moments. For appropriately designed switches and matrix molecules, the conductance states are stabilized by intermolecular hydrogen bonding. These results further support the hypothesis that conductance switching in these molecules is due to hybridization changes at the molecule-substrate bonds due to tilting of the switch molecules.
RESUMEN
Developing methods to probe the nature and structure of nanoscale environments continues to be a challenge in nanoscience. We report a cyclic voltammetry investigation of the internal, hydrogen-bond-driven phase separation of amide-containing thiols and alkanethiols. Amide-containing thiols with a terminal ferrocene carboxylate functional group were investigated in two binary monolayers, one homogeneously mixed and the other phase separated. The electrochemical response of the ferrocene probe was used to monitor adsorbate coverage, environment, and phase separation within each of these monolayers. The results demonstrate that the behavior of ferrocene-containing monolayers can be used to probe nanoscale organization.
RESUMEN
In this paper we describe a systematic study comparing the properties of self-assembled monolayers (SAMs) formed by in situ deprotection and assembly of S-triphenylmethyl (trityl)- and thiolacetate-protected alkanethiols to those of SAMs formed from the parent alkanethiols. The two in situ deprotections were carried out in trifluoroacetic acid and THF/ammonium hydroxide, respectively. Monolayers of octadecanethiol (ODT) and the peptide-containing alkanethiol 3-mercapto-N-n-pentadecylpropionamide (1ATC15) were assembled on gold using the two in situ methods and characterized by contact angle goniometry, X-ray photoelectron spectroscopy, polarization modulation infrared reflection absorption spectroscopy, and electrochemical characterization methods to assess how the monolayer properties compare to those of monolayers prepared by traditional methods. The results for the in situ deprotection of the trityl-protected molecules demonstrate that this method can afford high-quality monolayers that are nearly indistinguishable from those prepared directly from alkanethiols. The quality of the monolayers prepared using this method is shown to depend on the solubility of the trityl-protected compound in trifluoroacetic acid. The results for the in situ deprotection of acetyl-ODT indicate this method yields low-quality monolayers that contain mixtures of adsorbates bound as thiolates and thiolacetates. In situ trityl deprotection is a useful approach for monolayer formation that greatly simplifies the purification, handling, and assembly of thiol-containing monolayer precursors.
Asunto(s)
Oro/química , Membranas Artificiales , Compuestos de Sulfhidrilo/química , Estructura Molecular , Tamaño de la Partícula , Compuestos de Sulfhidrilo/síntesis química , Propiedades de SuperficieRESUMEN
We have studied oligo(phenylene-ethynylene)s inserted into amide-containing alkanethiol self-assembled monolayers using scanning tunneling microscopy to demonstrate switching based on chemical functionality of the environment of the inserted molecules. The molecules show stability in two conductance states: an ON and an OFF state. We demonstrate bias-dependent switching due to hydrogen bonding between the inserted oligo(phenylene-ethynylene) and the matrix molecules. In addition, the inserted molecules exhibit fewer switching events than previously reported for alkanethiol matrixes, which we attribute to the rigidity of the hydrogen-bonded matrix.