Josh Lipton-Duffin

Synthesis of Polyphenylene Molecular Wires by Surface‐Confined Polymerization

The surface-mediated synthesis of epitaxially aligned and separated polyphenylene lines on Cu(110) by exploiting the Ullmann dehalogenation reaction is reported. Scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) show that the C-I bonds of 1,4-diiodobenzene and 1,3-diiodobenzene (C(6)H(4)I(2)) are catalytically cleaved when dosed onto the surface. Subsequent annealing transforms the copper-bound phenylene intermediates into covalent conjugated structures: linear chains of poly(p-phenylene) for 1,4-diiodobenzene and zigzag chains of poly(m-phenylene) as well as macrocyclic oligomers in the case of 1,3-diiodobenzene. The chains are strongly bound to the surface (likely through C--Cu bonds at the chain-ends) while the macrocycles are very mobile and can only be imaged by STM at low temperature. The detached halogens adsorb on the surface and separate the polymer chains from each other.

Insight into Organometallic Intermediate and Its Evolution to Covalent Bonding in Surface-Confined Ullmann Polymerization

We provide insight into surface-catalyzed dehalogenative polymerization, analyzing the organometallic intermediate and its evolution into planar polymeric structures. A combined study using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and first-principles calculations unveils the structural conformation of substrate-bound phenylene intermediates generated from 1,4-dibromobenzene precursors on Cu(110), showing the stabilizing role of the halogen. The appearance of covalently bonded conjugated structures is followed in real time by fast-XPS measurements (with an acquisition time of 2 s per spectrum and heating rate of 2 K/s), showing that the detaching of phenylene units from the copper substrate and subsequent polymerization occur upon annealing above 460 ± 10 K.

Step-by-step growth of epitaxially aligned polythiophene by surface-confined reaction

One of the great challenges in surface chemistry is to assemble aromatic building blocks into ordered structures that are mechanically robust and electronically interlinked—i.e., are held together by covalent bonds. We demonstrate the surface-confined growth of ordered arrays of poly(3,4-ethylenedioxythiophene) (PEDOT) chains, by using the substrate (the 110 facet of copper) simultaneously as template and catalyst for polymerization. Copper acts as promoter for the Ullmann coupling reaction, whereas the inherent anisotropy of the fcc 110 facet confines growth to a single dimension. High resolution scanning tunneling microscopy performed under ultrahigh vacuum conditions allows us to simultaneously image PEDOT oligomers and the copper lattice with atomic resolution. Density functional theory calculations confirm an unexpected adsorption geometry of the PEDOT oligomers, which stand on the sulfur atom of the thiophene ring rather than lying flat. This polymerization approach can be extended to many other halogen-terminated molecules to produce epitaxially aligned conjugated polymers. Such systems might be of central importance to develop future electronic and optoelectronic devices with high quality active materials, besides representing model systems for basic science investigations.

Synthesis and electronic structure of a two dimensional π-conjugated polythiophene

We report the synthesis and first electronic characterization of an atomically thin two dimensional π-conjugated polymer. Polymerization via Ullmann coupling of a tetrabrominated tetrathienoanthracene on Ag(111) in ultra-high vacuum (UHV) produces a porous 2D polymer network that has been characterized by scanning tunnelling microscopy (STM). High-resolution X-ray photoelectron spectroscopy (HRXPS) shows that the reaction proceeds via two distinct steps: dehalogenation of the brominated precursor, which begins at room temperature (RT), and C–C coupling of the resulting Ag-bound intermediates, which requires annealing at 300 °C. The formation of the 2D conjugated network is accompanied by a shift of the occupied molecular states by 0.6 eV towards the Fermi level, as observed by UV photoelectron spectroscopy (UPS). A theoretical analysis of the electronic gap reduction in the transition from monomeric building blocks to various 1D and 2D oligomers and polymers yields important insight into the effect of topology on the electronic structure of 2D conjugated polymers.

Substrate Effects in the Supramolecular Assembly of 1,3,5-Benzene Tricarboxylic Acid on Graphite and Graphene

The behavior of small molecules on a surface depends critically on both molecule-substrate and intermolecular interactions. We present here a detailed comparative investigation of 1,3,5-benzene tricarboxylic acid (trimesic acid, TMA) on two different surfaces: highly oriented pyrolytic graphite (HOPG) and single-layer graphene (SLG) grown on a polycrystalline Cu foil. On the basis of high-resolution scanning tunnelling microscopy (STM) images, we show that the epitaxy matrix for the hexagonal TMA chicken wire phase is identical on these two surfaces, and, using density functional theory (DFT) with a non-local van der Waals correlation contribution, we identify the most energetically favorable adsorption geometries. Simulated STM images based on these calculations suggest that the TMA lattice can stably adsorb on sites other than those identified to maximize binding interactions with the substrate. This is consistent with our net energy calculations that suggest that intermolecular interactions (TMA-TMA dimer bonding) are dominant over TMA-substrate interactions in stabilizing the system. STM images demonstrate the robustness of the TMA films on SLG, where the molecular network extends across the variable topography of the SLG substrates and remains intact after rinsing and drying the films. These results help to elucidate molecular behavior on SLG and suggest significant similarities between adsorption on HOPG and SLG.

Unprecedented transformation of tetrathienoanthracene into pentacene on Ni(111).

The imaging and characterization of single-molecule reaction events is essential to both extending our basic understanding of chemistry and applying this understanding to challenges at the frontiers of technology, for example, in nanoelectronics. Specifically, understanding the behavior of individual molecules can elucidate processes critical to the controlled synthesis of materials for applications in multiple nanoscale technologies. Here, we report the synthesis of an important semiconducting organic molecule through an unprecedented reaction observed with submolecular resolution by scanning tunneling microscopy (STM) under ultrahigh vacuum (UHV) conditions. Our images reveal a sulfur abstraction and cyclization reaction that converts tetrathienoanthracene precursors into pentacene on the Ni(111) surface. The identity of the final reaction product was confirmed by time-of-flight secondary ion mass spectrometry (TOF-SIMS). This reaction has no known literature analogue, and highlights the power of local-probe techniques for exploring new chemical pathways.

Self-assembly of rubrene on Cu(111)

We performed an ultra-high vacuum scanning tunneling microscopy (STM) investigation of the self-assembly of rubrene at room temperature on Cu(111), a metal surface with threefold symmetry. Rubrene self-assembles into two different structures called row and trimer. Both are different than the structures already observed on Cu(110) and Cu(100). Row and trimer structures have comparable molecular packing densities and are equally distributed across the surface. In the row structure the molecules are oriented with their backbone along the same high symmetry directions of the surface: [[Formula: see text]], [[Formula: see text]] or [[Formula: see text]]. The trimer structure is composed of units of three rubrene molecules, oriented along the high symmetry surface directions. These units are chiral, as revealed by height profile measurements by STM, and self-assemble in domains containing only one type of enantiomer.

Inducing Nonlocal Reactions with a Local Probe

The scanning tunneling microscope (STM) has evolved continually since its invention, as scientists have expanded its use to encompass atomic-scale manipulation, momentum-resolved electronic characterization, localized chemical reactions (bond breaking and bond making) in adsorbed molecules, and even chain reactions at surfaces. This burgeoning field has recently expanded to include the use of the STM to inject hot electrons into substrate surface states; the injected electrons can travel laterally and induce changes in chemical structure in molecules located up to 100 nm from the STM tip. We describe several key demonstrations of this phenomenon, including one appearing in this issue of ACS Nano by Chen et al. Possible applications for this technique are also discussed, including characterizing the dispersion of molecule-substrate interface states and the controlled patterning of molecular overlayers.

Photon detection with n-propanol and C2H6O isomers

We demonstrate that a Geiger–Muller-type bandpass photon detector, suitable for inverse photoemission experiments, can be constructed from a MgF2 entrance window that has a high-energy transmission threshold of 10.97 eV, and ethanol, a detection gas with an ionization potential of 10.48 eV. The photon detector has a mean detection energy of ℏωd=(10.89±0.07) eV and a bandpass of Δℏωd=(0.37±0.05) eV. A photon detector can also be constructed from n-propanol/MgF2 with a mean detection energy of ℏωd=(10.76±0.07) eV and a bandpass Δℏωd=(0.41±0.05) eV. These two new detection gas/window combinations have a higher detection energy and a narrower bandpass than the dimethylether/MgF2 detector [Δℏωd=(0.71±0.04) eV and ℏωd=10.60 eV]. Since all three detectors utilize a MgF2 entrance window, the photon bandpass can be changed straightforwardly by changing the detection gas. For systems that can be easily damaged by electron beams, having the freedom to open up the detector bandpass is an advantage because it can redu...

Mechanistic Picture and Kinetic Analysis of Surface-Confined Ullmann Polymerization

Surface-confined polymerization via Ullmann coupling is a promising route to create one- and two-dimensional covalent π-conjugated structures, including the bottom-up growth of graphene nanoribbons. Understanding the mechanism of the Ullmann reaction is necessary to provide a platform for rationally controlling the formation of these materials. We use fast X-ray photoelectron spectroscopy (XPS) in kinetic measurements of epitaxial surface polymerization of 1,4-dibromobenzene on Cu(110) and devise a kinetic model based on mean field rate equations, involving a transient state. This state is observed in the energy landscapes calculated by nudged elastic band (NEB) within density functional theory (DFT), which assumes as initial and final geometries of the organometallic and polymeric structures those observed by scanning tunneling microscopy (STM). The kinetic model accounts for all the salient features observed in the experimental curves extracted from the fast-XPS measurements and enables an enhanced understanding of the polymerization process, which is found to follow a nucleation-and-growth behavior preceded by the formation of a transient state.