On‐surface Ullmann coupling is an established method for the synthesis of 1D and 2D organic structures. A key limitation to obtaining ordered polymers is the uncertainty in the final structure for ...coupling via random diffusion of reactants over the substrate, which leads to polymorphism and defects. Here, a topotactic polymerization on Cu(110) in a series of differently‐halogenated para‐phenylenes is identified, where the self‐assembled organometallic (OM) reactants of diiodobenzene couple directly into a single, deterministic product, whereas the other precursors follow a diffusion driven reaction. The topotactic mechanism is the result of the structure of the iodine on Cu(110), which controls the orientation of the OM reactants and intermediates to be the same as the final polymer chains. Temperature‐programmed X‐ray photoelectron spectroscopy and kinetic modeling reflect the differences in the polymerization regimes, and the effects of the OM chain alignments and halogens are disentangled by Nudged Elastic Band calculations. It is found that the repulsion or attraction between chains and halogens drive the polymerization to be either diffusive or topotactic. These results provide detailed insights into on‐surface reaction mechanisms and prove the possibility of harnessing topotactic reactions in surface‐confined Ullmann polymerization.
Real‐time spectroscopy and computed reaction pathway investigations of on‐surface Ullmann reaction of a series of aryl halides reveals topotactic kinetics in the formation of poly‐para phenylene chains, which is driven by alignment of reaction sites instead of random diffusion of the organic species.
While surface‐confined Ullmann‐type coupling has been widely investigated for its potential to produce π‐conjugated polymers with unique properties, the pathway of this reaction in the presence of ...adsorbed oxygen has yet to be explored. Here, the effect of oxygen adsorption between different steps of the polymerization reaction is studied, revealing an unexpected transformation of the 1D organometallic (OM) chains to 2D OM networks by annealing, rather than the 1D polymer obtained on pristine surfaces. Characterization by scanning tunneling microscopy and X‐ray photoelectron spectroscopy indicates that the networks consist of OM segments stabilized by chemisorbed oxygen at the vertices of the segments, as supported by density functional theory calculations. Hexagonal 2D OM networks with different sizes on Cu(111) can be created using precursors with different length, either 4,4″‐dibromo‐p‐terphenyl or 1,4‐dibromobenzene (dBB), and square networks are obtained from dBB on Cu(100). The control over size and symmetry illustrates a versatile surface patterning technique, with potential applications in confined reactions and host–guest chemistry.
On‐surface Ullmann‐type coupling with linear precursors typically forms a 1D organometallic intermediate en route to a 1D polymer. Introducing oxygen leads to the unexpected transformation of this intermediate to highly ordered 2D organometallic networks. The network size and shape can be rationally controlled by changing the precursor length and substrate symmetry, making them promising templates for host–guest systems.
Two-dimensional polymers grow up MacLean, Oliver; Rosei, Federico
Science (American Association for the Advancement of Science),
12/2019, Letnik:
366, Številka:
6471
Journal Article
Recenzirano
Silicon wafer–sized single-layer films are synthesized with a new method
Creating tunable and highly conductive two-dimensional polymers (2DPs) using organic chemistry has been a goal since the ...isolation of graphene (
1
,
2
). The high surface area and well-defined pore sizes make 2DPs attractive for a range of applications, including sensors, electronics, catalysis, energy storage, and energy conversion. Conjugated 2DPs, which have alternating single and double bonds that enable electron delocalization, are promising materials for optoelectronics. They have high predicted carrier mobilities and semiconducting behavior that contrasts with the metal-like properties of graphene (
3
). Challenges in scaling up from the laboratory have prevented different types of 2DPs from making the leap to practical applications. On page 1379 of this issue, Zhong
et al.
(
4
) report a synthetic method that solves many difficulties related to isolating and processing 2DPs and fabricate a simple large-area electronic device.
Heteroepitaxial growth is a process of profound fundamental importance as well as an avenue to realize nanostructures such as Ge/Si quantum dots (QDs), with appealing properties for applications in ...opto‐ and nanoelectronics. However, controlling the Ge/Si QD size, shape, and composition remains a major obstacle to their practical implementation. Here, Ge nanostructures on Si(111) were investigated in situ and in real‐time by low energy electron microscopy (LEEM), enabling the observation of the transition from wetting layer formation to 3D island growth and decay. The island size, shape, and distribution depend strongly on the growth temperature. As the deposition temperature increases, the islands become larger and sparser, consistent with Brownian nucleation and capture dynamics. At 550°C, two distinct Ge/Si nanostructures are formed with bright and dark appearances that correspond to flat, atoll‐like and tall, faceted islands, respectively. During annealing, the faceted islands increase in size at the expense of the flat ones, indicating that the faceted islands are thermodynamically more stable. In contrast, triangular islands with uniform morphology are obtained from deposition at 600°C, suggesting that the growth more closely follows the ideal shape. During annealing, the islands formed at 600°C initially show no change in morphology and size and then rupture simultaneously, signaling a homogeneous chemical potential of the islands. These observations reveal the role of dynamics and energetics in the evolution of Ge/Si QDs, which can serve as a step towards the precise control over the Ge nanostructure size, shape, composition, and distribution on Si(111).
Real‐time low energy electron microscopy observations reveal the growth dynamics and stability of Ge quantum dots formed on Si(111) at temperatures between 450°C and 600°C. A mix of metastable, flat islands and tall, faceted islands are produced at 550°C and below, whereas growth at 600°C yields uniform large, triangular islands.
We use an on‐surface synthesis approach to drive the homocoupling reaction of a simple dithiophenyl‐functionalized precursor on Cu(111). The C−S activation reaction is initiated at low annealing ...temperature and yields unsaturated hydrocarbon chains interconnected in a fully conjugated reticulated network. High‐resolution atomic force microscopy imaging reveals the opening of the thiophenyl rings and the presence of trans‐ and cis‐oligoacetylene chains as well as pentalene units. The chemical transformations were studied by C 1s and S 2p core level photoemission spectroscopy and supported by theoretical calculations. At higher annealing temperature, additional cyclization reactions take place, leading to the formation of small graphene flakes.
C−S activation is used in a surface‐supported reaction to create a reticulated network of conjugated hydrocarbon chains on Cu(111). The reaction is initiated at low annealing temperature and the product is characterized by high‐resolution scanning probe microscopy and photoelectron spectroscopy supported by DFT calculations.
The electron-induced reaction of physisorbed vinyl bromide (ViBr) and allyl bromide (AllBr) on Cu(110) at 4.6 K was studied experimentally by scanning tunneling microscopy and theoretically by ...molecular dynamics. ViBr and AllBr were found to react by two pathways: “Direct”, in which the molecule reacted under the tip, and “Delayed”, in which reaction occurred spontaneously after the molecule had diffused across the surface away from the tip. The novel pathway of Delayed reaction constituted a major route for both vinyl bromide (68%) and allyl bromide (53%). The observed reaction dynamics for ViBr and AllBr gave evidence of a long-lived vibrationally excited intermediate for both Direct and Delayed reactions. Molecular dynamics simulations with reagent excitation by way of selected vibrational normal modes resulted in either Direct or Delayed reaction, depending on the vibrational mode.
Surface-confined reactions represent a powerful approach for the precise synthesis of low-dimensional organic materials. A complete understanding of the pathways of surface reactions would enable the ...rational synthesis of a wide range of molecules and polymers. Here, we report different reaction pathways of tetrathienylbenzene (T1TB) and its extended congener tetrakis(dithienyl)benzene (T2TB) on Cu(111), investigated using scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory calculations. Both T1TB and T2TB undergo desulfurization when deposited on Cu(111) at room temperature. Deposition of T1TB at 453 K yields pentacene through desulfurization, hydrogen transfer, and a cascade of intramolecular cyclization. In contrast, for T2TB the intramolecular cyclization stops at anthracene and the following intermolecular C–C coupling produces a conjugated ladder polymer. We show that tandem desulfurization/C–C coupling provides a versatile approach for growing carbon-based nanostructures on metal surfaces.
The precise control of molecular self-assembly on surfaces presents many opportunities for the creation of complex nanostructures. Within this endeavor, selective patterning by exploiting molecular ...interactions at the solid–liquid interface would be a beneficial capability. Using scanning tunneling microscopy at the 1,2,4-trichlorobenzene/Au(111) interface, we observed selective self-assembly of 1,3,5-tris(4-methoxyphenyl)benzene (TMPB) molecules in the face-centered cubic (FCC) regions of Au(111). Density functional theory calculations suggest higher adsorption energy of TMPB molecules at FCC regions, explaining the preference for self-assembly. The molecular coverage is found to increase with the concentration of the applied solution, eventually yielding a full monolayer. Moreover, the adsorption of TMPB molecules induces a concentration-dependent lifting of the herringbone reconstruction, observed as an increase in the area of the FCC regions at higher concentrations. Our results represent a simple and cost-effective selective nanoscale patterning method on Au(111), providing a possible avenue to guide the co-adsorption of other functional molecules.
We report the systematic investigation of the effects of oxygen on the synthesis of 3
p
sub-family armchair graphene nanoribbons (3
p
-AGNRs), which revealed a strong catalytic effect with a ...reduction in the reaction temperature by approximately 180 K without degradation of the AGNRs. Poly(para-phenylene) (3-AGNR) was generated through Ullmann-type coupling of 4,4″-dibromo-
p
-terphenyl on Cu(111), which was then converted into wider 3
p
-AGNRs
via
lateral fusion. Scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy demonstrated the formation of different ribbons up to 12-AGNR, which contained regions exhibiting increased STM contrast that we attribute to the intercalation of Br atoms during lateral fusion.
PDF
