Characterization of designed cobaltacarborane porphyrins using conductive probe atomic force microscopy

© 2016 Jayne C. Garno, et al. Porphyrins and metalloporphyrins have unique chemical and electronic properties and thus provide useful model structures for studies of nanoscale electronic properties. The rigid planar structures and π-conjugated backbones of porphyrins convey robust electrical characteristics. For our investigations, cobaltacarborane porphyrins were synthesized using a ring-opening zwitterionic reaction to produce isomers with selected arrangements of carborane clusters on each macrocycle. Experiments were designed to investigate how the molecular structure influences the selforganization, surface assembly, and conductive properties of three molecular structures with 2, 4, or 8 cobaltacarborane substituents. Current versus voltage (I-V) spectra for designed cobaltacarborane porphyrins deposited on conductive gold substrates were acquired using conductive probe atomic force microscopy (CP-AFM). Characterizations with CP-AFM provide capabilities for obtaining physical measurements and structural information with unprecedented sensitivity. We found that the morphology of cobaltacarborane porphyrin structures formed on surfaces depends on a complex interplay of factors such as the solvent used for dissolution, the nature of the substrate, and the design of the parent molecule. The conductive properties of cobaltacarborane porphyrins were observed to change according to the arrangement of cobaltacarborane substituents. Specifically, the number and placement of the cobaltacarborane ligands on the porphyrin macrocycle affect the interactions that drive porphyrin self-assembly and crystallization. Interestingly, coulombic staircase I-V profiles were detected for a porphyrin with two cobaltacarborane substituents

Synthesis and Characterization of Cyclic Brush-Like Polymers by N

Cyclic brush-like polymers were synthesized by tandem organo-mediated zwitterionic polymerization and a grafting-to approach. The cyclic polymer backbone, consisting of poly(N-propargylglycine) (c-PNPG), was synthesized by an N-heterocyclic carbene (NHC)-mediated zwitterionic ring-opening polymerization of N-propargyl N-carboxyanhydride. The polymerization proceeds in a quasi-living manner, allowing access to c-PNPG of well-defined chain length. The cyclic architecture of the polymers was verified by size exclusion chromatography (SEC) and mass spectroscopy (MS), as well as scanning probe characterization. Poly(ethylene glycol) functionalized with azido end-groups was subsequently grafted onto the c-PNPG by the copper-mediated azide/alkyne cycloaddition reaction (CuAAC). The side chain grafting density was determined by 1H NMR spectroscopy and SEC analysis. The grafting efficiency is low (\u3c19%) when the cyclic backbone is comprised of a c-PNPG homopolymer. The efficiency can be significantly improved (up to 93%) by utilizing cyclic poly(N-propargylglycine)-ran-poly(N-butylglycine) random copolymers (c-PNPG-r-PNBG). This has been attributed to the ease of access to the propargyl groups in c-PNPG and c-PNPG-ran-PNBG: the strong tendency of the former to aggregate in common organic solvents (including the CuAAC reaction medium) restricts access to the propargyl groups. © 2011 American Chemical Society

Vibrational Response of FeNi 3

A hybrid imaging mode was developed for characterizing samples of magnetic FeNi3 nanoparticles, which combines contact-mode atomic force microscopy (AFM) with magnetic modulation of samples. For conventional magnetic imaging modes of AFM, magnetically coated tips are used directly as a sensor to measure the relatively long-range forces of magnetic samples in a noncontact configuration. For the magnetic sample modulation (MSM) configuration, however, the changes in sample dynamics form the basis for measurements of material properties using contact-mode AFM. Nanoparticles are driven to vibrate in response to an externally applied electromagnetic field, and a nonmagnetic tip is used as a motion sensor for directly mapping the vibration with contact-mode. Intermetallic nanoparticles of FeNi3 were used as a model nanomaterial, synthesized by either conventional oven heating or microwave preparation. By slowly scanning an AFM probe across vibrating nanoparticles, changes in the frequency and amplitude of the sample motion can be sensitively tracked by the deflection of an AFM probe. Thus, the nonmagnetic AFM tip provides a force and motion sensor for mapping the vibrational response of magnetic nanomaterials at the level of individual nanoparticles. Dynamic protocols were developed for systematic studies with changes in the magnetic field strength and field frequency. © 2013 American Chemical Society

Solvent-Responsive Properties of Octadecyltrichlorosiloxane Nanostructures Investigated Using Atomic Force Microscopy in Liquid

An emerging challenge for nanoscale measurements is to capture and quantify the magnitude of structural changes in response to environmental changes. Certain environmental parameters can affect the nanoscale morphology of samples, such as changing the pH, solvent polarity, ionic strength, and temperature. We prepared test platforms of n-octadecyltrichlorosilane ring nanostructures to study surface morphology changes at the nanoscale in selected liquid media compared to dry conditions in air. Particle lithography combined with organosilane vapor deposition was used to fabricate nanostructures of regular dimensions. Multilayer nanostructures of OTS were used as a test platform for scanning probe studies of solvent-responsive properties where the sides of designed ring structures expose a 3D interface for studying the interaction of solvents with molecular side groups. In dry, ambient conditions, nanostructures of OTS were first imaged using contact mode atomic force microscopy (AFM). Next, ethanol or buffer was introduced to the sample cell, and images were acquired using the same probe. We observed substantial changes in the lateral and vertical dimensions of the ring nanostructures in AFM topography frames; the sizes of the rings were observed to swell by tens of nanometers. Even after heat treatment of samples to promote cross-linking, the samples still evidenced swelling in liquid media. This research will have consequences for studies of the properties of nanomaterials, such as solvent-responsive organic films and polymers. © 2014 American Chemical Society