8 research outputs found
Tuning the Band Gap of Graphene Nanoribbons Synthesized from Molecular Precursors
A prerequisite for future graphene nanoribbon (GNR) applications is the ability to fine-tune the electronic band gap of GNRs. Such control requires the development of fabrication tools capable of precisely controlling width and edge geometry of GNRs at the atomic scale. Here we report a technique for modifying GNR band gaps <i>via</i> covalent self-assembly of a new species of molecular precursors that yields <i>n</i> = 13 armchair GNRs, a wider GNR than those previously synthesized using bottom-up molecular techniques. Scanning tunneling microscopy and spectroscopy reveal that these <i>n</i> = 13 armchair GNRs have a band gap of 1.4 eV, 1.2 eV smaller than the gap determined previously for <i>n</i> = 7 armchair GNRs. Furthermore, we observe a localized electronic state near the end of <i>n</i> = 13 armchair GNRs that is associated with hydrogen-terminated sp<sup>2</sup>-hybridized carbon atoms at the zigzag termini
Step-doubling at Vicinal Ni(111) Surfaces Investigated with a Curved Crystal
Vicinal surfaces
may undergo structural transformations as a function
of temperature or in the presence of adsorbates. Step-doubling in
which monatomic steps pair up forming double-atom high staircases
is the simplest example. Here we investigate the case of Ni(111) using
a curved crystal surface, which allows us to explore the occurrence
of step-doubling as a function of temperature and vicinal plane (miscut
α and step type). We find a striking A-type ({100}-like microfacets)
versus B-type ({111}-like) asymmetry toward step-doubling. The terrace-width
distribution analysis performed from scanning tunneling microscopy
data points to elastic step interactions overcoming entropic effects
at very small miscut α in A-type vicinals, as compared to B-type
steps. For A-type vicinals, we elaborate the temperature/miscut phase
diagram, on which we establish a critical miscut α<sub>c</sub> = 9.3° for step-doubling to take place
Symmetry, Shape, and Energy Variations in Frontier Molecular Orbitals at Organic/Metal Interfaces: The Case of F<sub>4</sub>TCNQ
Near
edge X-ray absorption, valence and core-level photoemission,
and density functional theory calculations are used to study molecular
levels of tetracyano-2,3,5,6-tetrafluoroquinodimethane (F<sub>4</sub>TCNQ) deposited on Ag(111) and BiAg<sub>2</sub>/AgÂ(111). The high
electron affinity of F<sub>4</sub>TCNQ triggers a large static charge
transfer from the substrate, and, more interestingly, hybridization
with the substrate leads to a radical change of symmetry, shape, and
energy of frontier molecular orbitals. The lowest unoccupied molecular
orbital (LUMO) shifts below the Fermi energy, becoming the new highest
occupied molecular orbital (<i>n</i>-HOMO), whereas the <i>n</i>-LUMO is defined by a hybrid band with mixed Ï* and
Ï* symmetries, localized at quinone rings and cyano groups,
respectively. The presence of Bi influences the way the molecule contacts
the substrate with the cyano group. The molecule/surface distance
is closer and the bond more extended over substrate atoms in F<sub>4</sub>TCNQ/AgÂ(111), whereas in F<sub>4</sub>TCNQ/BiAg<sub>2</sub>/AgÂ(111) the distance is larger and the contact more localized on
top of Bi. This does not significantly alter molecular levels, but
it causes the respective absence or presence of optical excitations
in F<sub>4</sub>TCNQ core-level spectra
Noncovalent Dimerization after Enediyne Cyclization on Au(111)
We
investigate the thermally induced cyclization of 1,2-bisÂ(2-phenylethynyl)Âbenzene
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>âC<sup>6</sup> (Bergman) or a C<sup>1</sup>âC<sup>5</sup> cyclization
pathway. On Au(111), we find that the C<sup>1</sup>âC<sup>5</sup> cyclization is suppressed and that the C<sup>1</sup>âC<sup>6</sup> cyclization yields a highly strained bicyclic olefin whose
surface chemistry was hitherto unknown. The C<sup>1</sup>âC<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
Width-Dependent Band Gap in Armchair Graphene Nanoribbons Reveals Fermi Level Pinning on Au(111)
We
report the energy level alignment evolution of valence and conduction
bands of armchair-oriented graphene nanoribbons (aGNR) as their band
gap shrinks with increasing width. We use 4,4âł-dibromo-<i>para</i>-terphenyl as the molecular precursor on Au(111) to
form extended poly-<i>para</i>-phenylene nanowires, which
can subsequently be fused sideways to form atomically precise aGNRs
of varying widths. We measure the frontier bands by means of scanning
tunneling spectroscopy, corroborating that the nanoribbonâs
band gap is inversely proportional to their width. Interestingly,
valence bands are found to show Fermi level pinning as the band gap
decreases below a threshold value around 1.7 eV. Such behavior is
of critical importance to understand the properties of potential contacts
in GNR-based devices. Our measurements further reveal a particularly
interesting system for studying Fermi level pinning by modifying an
adsorbateâs band gap while maintaining an almost unchanged
interface chemistry defined by substrate and adsorbate
Understanding Energy-Level Alignment in DonorâAcceptor/Metal Interfaces from Core-Level Shifts
The molecule/metal interface is the key element in charge injection devices. It can be generally defined by a monolayer-thick blend of donor and/or acceptor molecules in contact with a metal surface. Energy barriers for electron and hole injection are determined by the offset from HOMO (highest occupied) and LUMO (lowest unoccupied) molecular levels of this contact layer with respect to the Fermi level of the metal electrode. However, the HOMO and LUMO alignment is not easy to elucidate in complex multicomponent, molecule/metal systems. We demonstrate that core-level photoemission from donorâacceptor/metal interfaces can be used to straightforwardly and transparently assess molecular-level alignment. Systematic experiments in a variety of systems show characteristic binding energy shifts in core levels as a function of molecular donor/acceptor ratio, irrespective of the molecule or the metal. Such shifts reveal how the level alignment at the molecule/metal interface varies as a function of the donorâacceptor stoichiometry in the contact blend
Substrate-Independent Growth of Atomically Precise Chiral Graphene Nanoribbons
Contributing to the need for new
graphene nanoribbon (GNR) structures
that can be synthesized with atomic precision, we have designed a
reactant that renders chiral (3,1)-GNRs after a multistep reaction
including Ullmann coupling and cyclodehydrogenation. The nanoribbon
synthesis has been successfully proven on different coinage metals,
and the formation process, together with the fingerprints associated
with each reaction step, has been studied by combining scanning tunneling
microscopy, core-level spectroscopy, and density functional calculations.
In addition to the GNRâs chiral edge structure, the substantial
GNR lengths achieved and the low processing temperature required to
complete the reaction grant this reactant extremely interesting properties
for potential applications
Switching from Reactant to Substrate Engineering in the Selective Synthesis of Graphene Nanoribbons
The
challenge of synthesizing graphene nanoribbons (GNRs) with
atomic precision is currently being pursued along a one-way road,
based on the synthesis of adequate molecular precursors that react
in predefined ways through self-assembly processes. The synthetic
options for GNR generation would multiply by adding a new direction
to this readily successful approach, especially if both of them can
be combined. We show here how GNR synthesis can be guided by an adequately
nanotemplated substrate instead of by the traditionally designed reactants.
The structural atomic precision, unachievable to date through top-down
methods, is preserved by the self-assembly process. This new strategyâs
proof-of-concept compares experiments using 4,4âČâČ-dibromo-para-terphenyl
as a molecular precursor on flat Au(111) and stepped Au(322) substrates.
As opposed to the former, the periodic steps of the latter drive the
selective synthesis of 6 atom-wide armchair GNRs, whose electronic
properties have been further characterized in detail by scanning tunneling
spectroscopy, angle resolved photoemission, and density functional
theory calculations