23 research outputs found
Identifying substitutional oxygen as a prolific point defect in monolayer transition metal dichalcogenides with experiment and theory
Chalcogen vacancies are considered to be the most abundant point defects in
two-dimensional (2D) transition-metal dichalcogenide (TMD) semiconductors, and
predicted to result in deep in-gap states (IGS). As a result, important
features in the optical response of 2D-TMDs have typically been attributed to
chalcogen vacancies, with indirect support from Transmission Electron
Microscopy (TEM) and Scanning Tunneling Microscopy (STM) images. However, TEM
imaging measurements do not provide direct access to the electronic structure
of individual defects; and while Scanning Tunneling Spectroscopy (STS) is a
direct probe of local electronic structure, the interpretation of the chemical
nature of atomically-resolved STM images of point defects in 2D-TMDs can be
ambiguous. As a result, the assignment of point defects as vacancies or
substitutional atoms of different kinds in 2D-TMDs, and their influence on
their electronic properties, has been inconsistent and lacks consensus. Here,
we combine low-temperature non-contact atomic force microscopy (nc-AFM), STS,
and state-of-the-art ab initio density functional theory (DFT) and GW
calculations to determine both the structure and electronic properties of the
most abundant individual chalcogen-site defects common to 2D-TMDs.
Surprisingly, we observe no IGS for any of the chalcogen defects probed. Our
results and analysis strongly suggest that the common chalcogen defects in our
2D-TMDs, prepared and measured in standard environments, are substitutional
oxygen rather than vacancies
Electrons imitating light: Frustrated supercritical collapse in charged arrays on graphene
The photon-like electronic dispersion of graphene bestows its charge carriers
with unusual confinement properties that depend strongly on the geometry and
strength of the surrounding potential. Here we report bottom-up synthesis of
atomically-precise one-dimensional (1D) arrays of point charges aimed at
exploring supercritical confinement of carriers in graphene for new geometries.
The arrays were synthesized by arranging F4TCNQ molecules into a 1D lattice on
back-gated graphene devices, allowing precise tuning of both the molecular
charge state and the array periodicity. Dilute arrays of ionized F4TCNQ
molecules are seen to behave like isolated subcritical charges but dense arrays
show emergent supercriticality. In contrast to compact supercritical clusters,
extended 1D charge arrays exhibit both supercritical and subcritical
characteristics and belong to a new physical regime termed frustrated
supercritical collapse. Here carriers in the far-field are attracted by a
supercritical charge distribution, but have their fall to the center frustrated
by subcritical potentials in the near-field, similar to the trapping of light
by a dense cluster of stars in general relativity
Probing the Role of Interlayer Coupling and Coulomb Interactions on Electronic Structure in Few-Layer MoSe2 Nanostructures
Despite the weak nature of interlayer forces in transition metal
dichalcogenide (TMD) materials, their properties are highly dependent on the
number of layers in the few-layer two-dimensional (2D) limit. Here, we present
a combined scanning tunneling microscopy/spectroscopy and GW theoretical study
of the electronic structure of high quality single- and few-layer MoSe2 grown
on bilayer graphene. We find that the electronic (quasiparticle) bandgap, a
fundamental parameter for transport and optical phenomena, decreases by nearly
one electronvolt when going from one layer to three due to interlayer coupling
and screening effects. Our results paint a clear picture of the evolution of
the electronic wave function hybridization in the valleys of both the valence
and conduction bands as the number of layers is changed. This demonstrates the
importance of layer number and electron-electron interactions on van der Waals
heterostructures, and helps to clarify how their electronic properties might be
tuned in future 2D nanodevices
Imaging Individual Chemical Bonds and Tuning Single-Molecule Charge States at Surfaces
In an effort to make advances in electronics through ever smaller devices, the field of molecular electronics has emerged as a natural step in achieving ultimate miniaturization of devices down to the size of single molecules. Progress in molecular electronics is intimately linked to understanding these devices at the atomic and molecular length scales at which they operate. As a move in this direction we have performed local probe studies in which we have nondestructively imaged the products of chemical reactions within molecular electronics elements. We have also imaged and tuned the orbitals and charge states of individual molecular electronics elements on surfaces. This dissertation, after introducing the field of nanoelectronics and the local probe techniques used in the study, reports on imaging of chemical structures of on-surface synthesized molecules and conductive polymers with individual-chemical-bond resolution and their relationship to the electronic structure. Depending on the specific molecules and surfaces used, the on-surface synthesized molecular structures formed single molecule products (monomers), chemically reacted intermediates, or conductive polymers exhibiting extended electronic structure along their backbone. This dissertation additionally demonstrates orbital gating of molecules on a back-gated graphene device. The energy alignment of molecular orbitals on graphene was tuned using an electrostatic back-gate, which resulted in molecules switching between neutral and negatively charged states. This control of charge states of single molecules on surfaces and identification of on-surface synthesized reaction products with sub-molecular resolution contributes to our understanding of molecular electronics elements at their natural length scales
Nanoscale Control of Rewriteable Doping Patterns in Pristine Graphene/Boron Nitride Heterostructures
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Imaging single-molecule reaction intermediates stabilized by surface dissipation and entropy.
Chemical transformations at the interface between solid/liquid or solid/gaseous phases of matter lie at the heart of key industrial-scale manufacturing processes. A comprehensive study of the molecular energetics and conformational dynamics that underlie these transformations is often limited to ensemble-averaging analytical techniques. Here we report the detailed investigation of a surface-catalysed cross-coupling and sequential cyclization cascade of 1,2-bis(2-ethynyl phenyl)ethyne on Ag(100). Using non-contact atomic force microscopy, we imaged the single-bond-resolved chemical structure of transient metastable intermediates. Theoretical simulations indicate that the kinetic stabilization of experimentally observable intermediates is determined not only by the potential-energy landscape, but also by selective energy dissipation to the substrate and entropic changes associated with key transformations along the reaction pathway. The microscopic insights gained here pave the way for the rational design and control of complex organic reactions at the surface of heterogeneous catalysts
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Local electronic and chemical structure of oligo-acetylene derivatives formed through radical cyclizations at a surface.
Semiconducting π-conjugated polymers have attracted significant interest for applications in light-emitting diodes, field-effect transistors, photovoltaics, and nonlinear optoelectronic devices. Central to the success of these functional organic materials is the facile tunability of their electrical, optical, and magnetic properties along with easy processability and the outstanding mechanical properties associated with polymeric structures. In this work we characterize the chemical and electronic structure of individual chains of oligo-(E)-1,1'-bi(indenylidene), a polyacetylene derivative that we have obtained through cooperative C1-C5 thermal enediyne cyclizations on Au(111) surfaces followed by a step-growth polymerization of the (E)-1,1'-bi(indenylidene) diradical intermediates. We have determined the combined structural and electronic properties of this class of oligomers by characterizing the atomically precise chemical structure of individual monomer building blocks and oligomer chains (via noncontact atomic force microscopy (nc-AFM)), as well as by imaging their localized and extended molecular orbitals (via scanning tunneling microscopy and spectroscopy (STM/STS)). Our combined structural and electronic measurements reveal that the energy associated with extended π-conjugated states in these oligomers is significantly lower than the energy of the corresponding localized monomer orbitals, consistent with theoretical predictions
Noncovalent Dimerization after Enediyne Cyclization on Au(111)
We
investigate the thermally induced cyclization of 1,2-bis(2-phenylethynyl)benzene
on Au(111) using scanning tunneling microscopy and computer simulations.
Cyclization of sterically hindered enediynes is known to proceed via
two competing mechanisms in solution: a classic C<sup>1</sup>–C<sup>6</sup> (Bergman) or a C<sup>1</sup>–C<sup>5</sup> cyclization
pathway. On Au(111), we find that the C<sup>1</sup>–C<sup>5</sup> cyclization is suppressed and that the C<sup>1</sup>–C<sup>6</sup> cyclization yields a highly strained bicyclic olefin whose
surface chemistry was hitherto unknown. The C<sup>1</sup>–C<sup>6</sup> product self-assembles into discrete noncovalently bound
dimers on the surface. The reaction mechanism and driving forces behind
noncovalent association are discussed in light of density functional
theory calculations