22 research outputs found
Tracking and Removing Br during the On-Surface Synthesis of a Graphene Nanoribbon
The fabrication of graphene nanoribbons
(GNRs) requires a high degree of precision due to the sensitivity
of the electronic structure on the edge shape. Using Br-substituted
molecular precursors, this atomic precision can be achieved in a thermally
induced two-step reaction following Br dissociation on a Au(111) surface.
Using DFT, we find evidence that the Br atoms are bound to the intermediate
polyanthrylene chains. We employ temperature-programmed desorption
to demonstrate the associative desorption of HBr and molecular hydrogen
during the final cyclodehydrogenation step of the reaction. Both processes
are found to have similar activation barriers. Furthermore, we are
able to remove Br atoms from the polyanthrylene chains by providing
molecular hydrogen. The subsequent formation of GNR via a cyclodehydrogenation
demonstrates that Br does not influence this part of the overall reaction
Mechanisms of Halogen-Based Covalent Self-Assembly on Metal Surfaces
We
computationally study the reaction mechanisms of halogen-based
covalent self-assembly, a major route for synthesizing molecular nanostructures
and nanographenes on surfaces. Focusing on biphenyl as a small model
system, we describe the dehalogenation, recombination, and diffusion
processes. The kinetics of the different processes are also investigated,
in particular how diffusion and coupling barriers affect recombination
rates. Trends across the periodic table are derived from three commonly
used close-packed (111) surfaces (Cu, Ag, and Au) and two halogens
(Br and I). We show that the halogen atoms can poison the surface,
thus hindering long-range ordering of the self-assembled structures.
Finally, we present core-level shifts of the relevant carbon and halogen
atoms, to provide reference data for reliably detecting self-assembly
without the need for atomic-resolution scanning tunneling microscopy
CO<sub>2</sub> Hydrogenation with High Selectivity by Single Bi Atoms on MXenes Enabled by a Concerted Mechanism
Developing efficient catalysts for the capture and direct
conversion
of CO2 into various chemicals is essential to alleviate
CO2 emissions and minimize the negative environmental effects
of fossil fuels. Combining density functional theory calculations
and microkinetic analysis, we propose that single Bi atoms supported
on V2CO2 MXenes (Bi@V2CO2) are promising single-atom catalysts (SAC) for CO2 hydrogenation.
The catalytic performance of Bi SACs is ensured by the stable single-atom
dispersion of Bi atoms on V2CO2 and enhanced
adsorption of CO2. Of importance, Bi@V2CO2 exhibits remarkable selectivity toward the synthesis of formic
acid (HCOOH), in which the main competing reaction, namely, the reverse
water gas shift (RWGS) and the formation of CO, is strictly prohibited.
In contrast to conventional Cu or In2O3 catalysts,
CO2 hydrogenation exhibits a unique mechanism on Bi@V2CO2, in which the formic acid is directly generated
via a concerted pathway. As a result, the formation of both intermediate
HCOO and COOH is prevented, leading to high selectivity (nearly 100%)
toward HCOOH on Bi@V2CO2. Moreover, analysis
of the kinetic behavior suggests that the stabilization of HCOOH adsorption
would be an effective approach to promote catalyst performance toward
methanol synthesis
Intermolecular Hybridization Creating Nanopore Orbital in a Supramolecular Hydrocarbon Sheet
Molecular
orbital engineering is a key ingredient for the design of organic
devices. Intermolecular hybridization promises efficient charge carrier
transport but usually requires dense packing for significant wave
function overlap. Here we use scanning tunneling spectroscopy to spatially
resolve the electronic structure of a surface-confined nanoporous
supramolecular sheet of a prototypical hydrocarbon compound featuring
terminal alkyne (−CCH) groups. Surprisingly, localized nanopore
orbitals are observed, with their electron density centered in the
cavities surrounded by the functional moieties. Density functional
theory calculations reveal that these new electronic states originate
from the intermolecular hybridization of six in-plane π-orbitals
of the carbon–carbon triple bonds, exhibiting significant electronic
splitting and an energy downshift of approximately 1 eV. Importantly,
these nanopore states are distinct from previously reported interfacial
states. We unravel the underlying connection between the formation
of nanopore orbital and geometric arrangements of functional groups,
thus demonstrating the generality of applying related orbital engineering
concepts in various types of porous organic structures
Unraveling the Mechanism of the Covalent Coupling Between Terminal Alkynes on a Noble Metal
The
mechanism of the newly reported route for surface-assisted
covalent coupling of terminal alkynes on Ag(111) is unraveled by density
functional theory based transition state calculations. We illustrate
that the reaction path is fundamentally different from the classical
coupling schemes in wet chemistry. It is initiated by the covalent
coupling between two molecules instead of single-molecule dehydrogenation.
The silver substrate is found to play an important role stabilizing
the intermediate species by chemical bonds, although it is hardly
active electronically in the actual coupling step. The dimer intermediate
is concluded to undergo two subsequent dehydrogenation processes expected
to be rate-limiting according to the comparatively large barriers,
which origin is discussed
Topological Dynamics in Supramolecular Rotors
Artificial molecular switches, rotors,
and machines are set to
establish design rules and applications beyond their biological counterparts.
Herein we exemplify the role of noncovalent interactions and transient
rearrangements in the complex behavior of supramolecular rotors caged
in a 2D metal–organic coordination network. Combined scanning
tunneling microscopy experiments and molecular dynamics modeling of
a supramolecular rotor with respective rotation rates matching with
0.2 kcal mol<sup>–1</sup> (9 meV) precision, identify key steps
in collective rotation events and reconfigurations. We notably reveal
that stereoisomerization of the chiral trimeric units entails topological
isomerization whereas rotation occurs in a topology conserving, two-step
asynchronous process. In supramolecular constructs, distinct displacements
of subunits occur inducing a markedly lower rotation barrier as compared
to synchronous mechanisms of rigid rotors. Moreover, the chemical
environment can be instructed to control the system dynamics. Our
observations allow for a definition of mechanical cooperativity based
on a significant reduction of free energy barriers in supramolecules
compared to rigid molecules
Adsorption of Aromatic and Anti-Aromatic Systems on Graphene through π−π Stacking
The adsorption of neutral (poly)-aromatic, antiaromatic, and more generally π-conjugated systems on graphene is studied as a prototypical case of π−π stacking. To account for dispersive interactions, we compare the recent van der Waals density functional (vdw-DF) with three semiempirical corrections to density functional theory and two empirical force fields. The adsorption energies of the molecules binding to graphene predicted by the vdw-DF were found to be in excellent agreement with temperature desorption experiments reported in literature, whereas the results of the remaining functionals and force fields only preserve the correct trends. The comparison of the dispersive versus electrostatic contributions to the total binding energies in the aromatic and antiaromatic systems suggests that π−π interactions can be regarded as being prevalently dispersive in nature at large separations, whereas close to the equilibrium bonding distance, it is a complex interplay between dispersive and electrostatic Coulombic interactions. Moreover our results surprisingly indicate that the magnitude of π−π interactions normalized both per number of total atoms and carbon atoms increases significantly with the relative number of hydrogen atoms in the studied systems
Synthesis of Extended Graphdiyne Wires by Vicinal Surface Templating
Surface-assisted
covalent synthesis currently evolves into an important
approach for the fabrication of functional nanostructures at interfaces.
Here, we employ scanning tunneling microscopy to investigate the homocoupling
reaction of linear, terminal alkyne-functionalized polyphenylene building-blocks
on noble metal surfaces under ultrahigh vacuum. On the flat Ag(111)
surface, thermal activation triggers a variety of side-reactions resulting
in irregularly branched polymeric networks. Upon alignment along the
step-edges of the Ag(877) vicinal surface drastically improves the
chemoselectivity of the linking process permitting the controlled
synthesis of extended-graphdiyne wires with lengths reaching 30 nm.
The ideal hydrocarbon scaffold is characterized by density functional
theory as a 1D, direct band gap semiconductor material with both HOMO
and LUMO-derived bands promisingly isolated within the electronic
structure. The templating approach should be applicable to related
organic precursors and different reaction schemes thus bears general
promise for the engineering of novel low-dimensional carbon-based
materials
Steering On-Surface Self-Assembly of High-Quality Hydrocarbon Networks with Terminal Alkynes
The two-dimensional (2D) self-assembly of 1,3,5-triethynyl-benzene
(TEB) and <i>de novo</i> synthesized 1,3,5-tris-(4-ethynylphenyl)Âbenzene
(Ext-TEB) on Ag(111) was investigated by means of scanning tunneling
microscopy (STM) under ultrahigh vacuum (UHV) conditions. Both 3-fold
symmetric molecules form long-range ordered nanoporous networks featuring
organizational chirality, mediated by novel, planar 6-fold cyclic
binding motifs. The key interaction for the expression of the motifs
is identified as C–H···π bonding. For
Ext-TEB, an additional open-porous phase exists with the 3-fold motif.
The nature of the underlying noncovalent bonding schemes is thoroughly
analyzed by density functional theory (DFT) calculations including
van der Waals corrections. The comparison of calculations focusing
on isolated 2D molecular sheets and those including the substrate
reveals the delicate balance between the attractive molecule–molecule
interaction, mediated by both the terminal alkyne and the phenyl groups,
and the molecule–substrate interaction responsible for the
commensurability and the regularity of the networks. Comparison with
bulk structures of similar molecules suggests that these strictly
planar cyclic binding motifs appear only in 2D environments
Synthesis of Surface Covalent Organic Frameworks via Dimerization and Cyclotrimerization of Acetyls
The
formation of additional phenyl rings on surfaces is of particular
interest because it allows for the building-up of surface covalent
organic frameworks. In this work, we show for the first time that
the cyclotrimerization of acetyls to aromatics provides a promising
approach to 2D conjugated covalent networks on surfaces under ultrahigh
vacuum. With the aid of scanning tunneling microscopy, we have systematically
studied the reaction pathways and the products. With the combination
of density functional theory calculations and X-ray photoemission
spectroscopy, the surface-assisted reaction mechanism, which is different
from that in solution, was explored