5 research outputs found
Probing the Structure and Chemistry of Perylenetetracarboxylic Dianhydride on Graphene Before and After Atomic Layer Deposition of Alumina
The superlative electronic properties of graphene suggest
its use
as the foundation of next-generation integrated circuits. However,
this application requires precise control of the interface between
graphene and other materials, especially the metal oxides that are
commonly used as gate dielectrics. Toward that end, organic seeding
layers have been empirically shown to seed ultrathin dielectric growth
on graphene via atomic layer deposition (ALD), although the underlying
chemical mechanisms and structural details of the molecule/dielectric
interface remain unknown. Here, confocal resonance Raman spectroscopy
is employed to quantify the structure and chemistry of monolayers
of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) on graphene
before and after deposition of alumina with the ALD precursors trimethyl
aluminum (TMA) and water. Photoluminescence measurements provide further
insight into the details of the growth mechanism, including the transition
between layer-by-layer growth and island formation. Overall, these
results reveal that PTCDA is not consumed during ALD, thereby preserving
a well-defined and passivating organic interface between graphene
and deposited dielectric thin films
Self-Assembly and Photopolymerization of Sub‑2 nm One-Dimensional Organic Nanostructures on Graphene
While graphene has attracted significant attention from
the research
community due to its high charge carrier mobility, important issues
remain unresolved that prevent its widespread use in technologically
significant applications such as digital electronics. For example,
the chemical inertness of graphene hinders integration with other
materials, and the lack of a bandgap implies poor switching characteristics
in transistors. The formation of ordered organic monolayers on graphene
has the potential to address each of these challenges. In particular,
functional groups incorporated into the constituent molecules enable
tailored chemical reactivity, while molecular-scale ordering within
the monolayer provides sub-2 nm templates with the potential to tune
the electronic band structure of graphene via quantum confinement
effects. Toward these ends, we report here the formation of well-defined
one-dimensional organic nanostructures on epitaxial graphene via the
self-assembly of 10,12-pentacosadiynoic acid (PCDA) in ultrahigh vacuum
(UHV). Molecular resolution UHV scanning tunneling microscopy (STM)
images confirm the one-dimensional ordering of the as-deposited PCDA
monolayer and show domain boundaries with symmetry consistent with
the underlying graphene lattice. In an effort to further stabilize
the monolayer, in situ ultraviolet photopolymerization induces covalent
bonding between neighboring PCDA molecules in a manner that maintains
one-dimensional ordering as verified by UHV STM and ambient atomic
force microscopy (AFM). Further quantitative insights into these experimental
observations are provided by semiempirical quantum chemistry calculations
that compare the molecular structure before and after photopolymerization
Metal Oxide Nanoparticle Growth on Graphene via Chemical Activation with Atomic Oxygen
Chemically interfacing the inert
basal plane of graphene with other
materials has limited the development of graphene-based catalysts,
composite materials, and devices. Here, we overcome this limitation
by chemically activating epitaxial graphene on SiC(0001) using atomic
oxygen. Atomic oxygen produces epoxide groups on graphene, which act
as reactive nucleation sites for zinc oxide nanoparticle growth using
the atomic layer deposition precursor diethyl zinc. In particular,
exposure of epoxidized graphene to diethyl zinc abstracts oxygen,
creating mobile species that diffuse on the surface to form metal
oxide clusters. This mechanism is corroborated with a combination
of scanning probe microscopy, Raman spectroscopy, and density functional
theory and can likely be generalized to a wide variety of related
surface reactions on graphene
Fundamental Performance Limits of Carbon Nanotube Thin-Film Transistors Achieved Using Hybrid Molecular Dielectrics
In the past decade, semiconducting carbon nanotube thin films have been recognized as contending materials for wide-ranging applications in electronics, energy, and sensing. In particular, improvements in large-area flexible electronics have been achieved through independent advances in postgrowth processing to resolve metallic <i>versus</i> semiconducting carbon nanotube heterogeneity, in improved gate dielectrics, and in self-assembly processes. Moreover, controlled tuning of specific device components has afforded fundamental probes of the trade-offs between materials properties and device performance metrics. Nevertheless, carbon nanotube transistor performance suitable for real-world applications awaits understanding-based progress in the integration of independently pioneered device components. We achieve this here by integrating high-purity semiconducting carbon nanotube films with a custom-designed hybrid inorganic–organic gate dielectric. This synergistic combination of materials circumvents conventional design trade-offs, resulting in concurrent advances in several transistor performance metrics such as transconductance (6.5 μS/μm), intrinsic field-effect mobility (147 cm<sup>2</sup>/(V s)), subthreshold swing (150 mV/decade), and on/off ratio (5 × 10<sup>5</sup>), while also achieving hysteresis-free operation in ambient conditions
Quantitatively Enhanced Reliability and Uniformity of High‑κ Dielectrics on Graphene Enabled by Self-Assembled Seeding Layers
The full potential of graphene in integrated circuits
can only
be realized with a reliable ultrathin high-κ top-gate dielectric.
Here, we report the first statistical analysis of the breakdown characteristics
of dielectrics on graphene, which allows the simultaneous optimization
of gate capacitance and the key parameters that describe large-area
uniformity and dielectric strength. In particular, vertically heterogeneous
and laterally homogeneous Al<sub>2</sub>O<sub>3</sub> and HfO<sub>2</sub> stacks grown via atomic-layer deposition and seeded by a
molecularly thin perylene-3,4,9,10-tetracarboxylic dianhydride organic
monolayer exhibit high uniformities (Weibull shape parameter β
> 25) and large breakdown strengths (Weibull scale parameter, <i>E</i><sub>BD</sub> > 7 MV/cm) that are comparable to control
dielectrics grown on Si substrates