8 research outputs found
Phenyl Functionalization of Atomically Precise Graphene Nanoribbons for Engineering Inter-ribbon Interactions and Graphene Nanopores
Graphene
nanoribbons (GNRs) attract much attention from researchers
due to their tunable physical properties and potential for becoming
nanoscale building blocks of electronic devices. GNRs can be synthesized
with atomic precision by on-surface approaches from specially designed
molecular precursors. While a considerable number of ribbons with
very diverse structures and properties have been demonstrated in recent
years, there have been only limited examples of on-surface synthesized
GNRs modified with functional groups. In this study, we designed a
nanoribbon, in which the chevron GNR backbone is decorated with phenyl
functionalities, and demonstrate the on-surface synthesis of these
GNRs on Au(111). We show that the phenyl modification affects the
assembly of the GNR polymer precursors through π–π
interactions. Scanning tunneling spectroscopy of the modified GNRs
on Au(111) revealed that they have a band gap of 2.50 ± 0.02
eV, which is comparable to that of the parent chevron GNR. The phenyl
functionalization leads to a shift of the band edges to lower energies,
suggesting that it could be a useful tool for the GNR band structure
engineering. We also investigated lateral fusion of the phenyl-modified
GNRs and demonstrate that it could be used to engineer different kinds
of atomically precise graphene nanopores. A similar functionalization
approach could be potentially applied to other GNRs to affect their
on-surface assembly, modify their electronic properties, and realize
graphene nanopores with a variety of structures
Optoelectrical Molybdenum Disulfide (MoS<sub>2</sub>)î—¸Ferroelectric Memories
In this study, we fabricated and tested electronic and memory properties of field-effect transistors (FETs) based on monolayer or few-layer molybdenum disulfide (MoS<sub>2</sub>) on a lead zirconium titanate (Pb(Zr,Ti)O<sub>3</sub>, PZT) substrate that was used as a gate dielectric. MoS<sub>2</sub>–PZT FETs exhibit a large hysteresis of electronic transport with high ON/OFF ratios. We demonstrate that the interplay of polarization and interfacial phenomena strongly affects the electronic behavior and memory characteristics of MoS<sub>2</sub>–PZT FETs. We further demonstrate that MoS<sub>2</sub>–PZT memories have a number of advantages and unique features compared to their graphene-based counterparts as well as commercial ferroelectric random-access memories (FeRAMs), such as nondestructive data readout, low operation voltage, wide memory window and the possibility to write and erase them both electrically and optically. This dual optoelectrical operation of these memories can simplify the device architecture and offer additional practical functionalities, such as an instant optical erase of large data arrays that is unavailable for many conventional memories
Multilayer Graphitic Coatings for Thermal Stabilization of Metallic Nanostructures
We demonstrate that graphitic coatings,
which consist of multilayer disordered graphene sheets, can be used
for the thermal protection of delicate metal nanostructures. We studied
cobalt slanted nanopillars grown by glancing angle deposition that
were shown to melt at temperatures much lower than the melting point
of bulk cobalt. After graphitic coatings were conformally grown over
the surfaces of Co nanopillars by chemical vapor deposition, the resulting
carbon-coated Co nanostructures retained their morphology at elevated
temperatures, which would damage the uncoated structures. Thermal
stabilization is also demonstrated for carbon-coated Ti nanopillars.
The results of this study may be extended to other metallic and possibly
even nonmetallic nanostructures that need to preserve their morphology
at elevated temperatures in a broad range of applications
Solution-Synthesized Chevron Graphene Nanoribbons Exfoliated onto H:Si(100)
There
has been tremendous progress in designing and synthesizing graphene
nanoribbons (GNRs). The ability to control the width, edge structure,
and dopant level with atomic precision has created a large class of
accessible electronic landscapes for use in logic applications. One
of the major limitations preventing the realization of GNR devices
is the difficulty of transferring GNRs onto nonmetallic substrates.
In this work, we developed a new approach for clean deposition of
solution-synthesized atomically precise chevron GNRs onto H:Si(100)
under ultrahigh vacuum. A clean transfer allowed ultrahigh-vacuum
scanning tunneling microscopy (STM) to provide high-resolution imaging
and spectroscopy and reveal details of the electronic structure of
chevron nanoribbons that have not been previously reported. We also
demonstrate STM nanomanipulation of GNRs, characterization of multilayer
GNR cross-junctions, and STM nanolithography for local depassivation
of H:Si(100), which allowed us to probe GNR–Si interactions
and revealed a semiconducting-to-metallic transition. The results
of STM measurements were shown to be in good agreement with first-principles
computational modeling
Interfacial Self-Assembly of Atomically Precise Graphene Nanoribbons into Uniform Thin Films for Electronics Applications
Because of their
intriguing electronic and optical properties, atomically precise graphene
nanoribbons (GNRs) are considered to be promising materials for electronics
and photovoltaics. However, significant aggregation and low solubility
of GNRs in conventional solvents result in their poor processability
for materials characterization and device studies. In this paper,
we demonstrate a new fabrication approach for large-scale uniform
thin films of nonfunctionalized atomically precise chevron-type GNRs.
The method is based on (1) the exceptional solubility of graphitic
materials in chlorosulfonic acid and (2) the original interfacial
self-assembly approach by which uniform films that are single-GNR
(∼2 nm) thick can be routinely prepared. These films can be
transferred to various substrates including Si/SiO<sub>2</sub> and
used for the streamlined fabrication of arrays of GNR-based devices.
The described self-assembly approach should be applicable to other
types of solution-synthesized atomically precise GNRs as well as large
polyaromatic hydrocarbon (PAH) molecules and therefore should facilitate
and streamline their device characterization
Spin Dynamics and Relaxation in Graphene Nanoribbons: Electron Spin Resonance Probing
Here we report the results of a multifrequency (∼9, 20, 34, 239.2, and 336 GHz) variable-temperature continuous wave (cw) and X-band (∼9 GHz) pulse electron spin resonance (ESR) measurement performed at cryogenic temperatures on potassium split graphene nanoribbons (GNRs). Important experimental findings include the following: (a) The multifrequency cw ESR data infer the presence of only carbon-related paramagnetic nonbonding states, at any measured temperature, with the <i>g</i> value independent of microwave frequency and temperature. (b) A linear broadening of the ESR signal as a function of microwave frequency is noticed. The observed linear frequency dependence of ESR signal width points to a distribution of <i>g</i> factors causing the non-Lorentzian line shape, and the <i>g</i> broadening contribution is found to be very small. (c) The ESR process is found to be characterized by slow and fast components, whose temperature dependences could be well described by a tunneling level state model. This work not only could help in advancing the present fundamental understanding on the edge spin (or magnetic)-based properties of GNRs but also pave the way to GNR-based spin devices
Time-Resolved Measurements of Photocarrier Dynamics in TiS<sub>3</sub> Nanoribbons
We report synthesis and time-resolved
transient absorption measurements of TiS<sub>3</sub> nanoribbons.
TiS<sub>3</sub> nanoribbons were fabricated by direct reaction of
titanium and sulfur. Dynamics of the photocarriers in these samples
were studied by transient absorption measurements. It was found that
following ultrafast injection of nonequilibrium and hot photocarriers,
the thermalization, energy relaxation, and exciton formation all occur
on a subpicosecond time scale. Several key parameters describing the
dynamical properties of photocarriers, including their recombination
lifetime, diffusion coefficient, mobility, and diffusion length, were
deduced
Nitrogen-Doping Induced Self-Assembly of Graphene Nanoribbon-Based Two-Dimensional and Three-Dimensional Metamaterials
Narrow
graphene
nanoribbons (GNRs) constructed by atomically precise bottom-up synthesis
from molecular precursors have attracted significant interest as promising
materials for nanoelectronics. But there has been little awareness
of the potential of GNRs to serve as nanoscale building blocks of
novel materials. Here we show that the substitutional doping with
nitrogen atoms can trigger the hierarchical self-assembly of GNRs
into ordered metamaterials. We use GNRs doped with eight N atoms per
unit cell and their undoped analogues, synthesized using both surface-assisted
and solution approaches, to study this self-assembly on a support
and in an unrestricted three-dimensional (3D) solution environment.
On a surface, N-doping mediates the formation of hydrogen-bonded GNR
sheets. In solution, sheets of side-by-side coordinated GNRs can in
turn assemble via van der Waals and π-stacking interactions
into 3D stacks, a process that ultimately produces macroscopic crystalline
structures. The optoelectronic properties of these semiconducting
GNR crystals are determined entirely by those of the individual nanoscale
constituents, which are tunable by varying their width, edge orientation,
termination, and so forth. The atomically precise bottom-up synthesis
of bulk quantities of basic nanoribbon units and their subsequent
self-assembly into crystalline structures suggests that the rapidly
developing toolset of organic and polymer chemistry can be harnessed
to realize families of novel carbon-based materials with engineered
properties