12 research outputs found
Kinetic Control of Catalytic CVD for High-Quality Graphene at Low Temperatures
Low-temperature (∼600 °C), scalable chemical vapor deposition of high-quality, uniform monolayer graphene is demonstrated with a mapped Raman 2D/G ratio of >3.2, D/G ratio ≤0.08, and carrier mobilities of ≥3000 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> on SiO<sub>2</sub> support. A kinetic growth model for graphene CVD based on flux balances is established, which is well supported by a systematic study of Ni-based polycrystalline catalysts. A finite carbon solubility of the catalyst is thereby a key advantage, as it allows the catalyst bulk to act as a mediating carbon sink while optimized graphene growth occurs by only locally saturating the catalyst surface with carbon. This also enables a route to the controlled formation of Bernal stacked bi- and few-layered graphene. The model is relevant to all catalyst materials and can readily serve as a general process rationale for optimized graphene CVD
Observing Graphene Grow: Catalyst–Graphene Interactions during Scalable Graphene Growth on Polycrystalline Copper
Complementary in situ X-ray photoelectron
spectroscopy (XPS), X-ray
diffractometry, and environmental scanning electron microscopy are
used to fingerprint the entire graphene chemical vapor deposition
process on technologically important polycrystalline Cu catalysts
to address the current lack of understanding of the underlying fundamental
growth mechanisms and catalyst interactions. Graphene forms directly
on metallic Cu during the high-temperature hydrocarbon exposure, whereby
an upshift in the binding energies of the corresponding C1s XPS core
level signatures is indicative of coupling between the Cu catalyst
and the growing graphene. Minor carbon uptake into Cu can under certain
conditions manifest itself as carbon precipitation upon cooling. Postgrowth,
ambient air exposure even at room temperature decouples the graphene
from Cu by (reversible) oxygen intercalation. The importance of these
dynamic interactions is discussed for graphene growth, processing,
and device integration
The Phase of Iron Catalyst Nanoparticles during Carbon Nanotube Growth
We study the Fe-catalyzed chemical vapor deposition of
carbon nanotubes
by complementary in situ grazing-incidence X-ray diffraction, in situ
X-ray reflectivity, and environmental transmission electron microscopy.
We find that typical oxide supported Fe catalyst films form widely
varying mixtures of bcc and fcc phased Fe nanoparticles upon reduction,
which we ascribe to variations in minor commonly present carbon contamination
levels. Depending on the as-formed phase composition, different growth
modes occur upon hydrocarbon exposure: For γ-rich Fe nanoparticle
distributions, metallic Fe is the active catalyst phase, implying
that carbide formation is not a prerequisite for nanotube growth.
For α-rich catalyst mixtures, Fe<sub>3</sub>C formation more
readily occurs and constitutes part of the nanotube growth process.
We propose that this behavior can be rationalized in terms of kinetically
accessible pathways, which we discuss in the context of the bulk iron–carbon
phase diagram with the inclusion of phase equilibrium lines for metastable
Fe<sub>3</sub>C. Our results indicate that kinetic effects dominate
the complex catalyst phase evolution during realistic CNT growth recipes
The Phase of Iron Catalyst Nanoparticles during Carbon Nanotube Growth
We study the Fe-catalyzed chemical vapor deposition of
carbon nanotubes
by complementary in situ grazing-incidence X-ray diffraction, in situ
X-ray reflectivity, and environmental transmission electron microscopy.
We find that typical oxide supported Fe catalyst films form widely
varying mixtures of bcc and fcc phased Fe nanoparticles upon reduction,
which we ascribe to variations in minor commonly present carbon contamination
levels. Depending on the as-formed phase composition, different growth
modes occur upon hydrocarbon exposure: For γ-rich Fe nanoparticle
distributions, metallic Fe is the active catalyst phase, implying
that carbide formation is not a prerequisite for nanotube growth.
For α-rich catalyst mixtures, Fe<sub>3</sub>C formation more
readily occurs and constitutes part of the nanotube growth process.
We propose that this behavior can be rationalized in terms of kinetically
accessible pathways, which we discuss in the context of the bulk iron–carbon
phase diagram with the inclusion of phase equilibrium lines for metastable
Fe<sub>3</sub>C. Our results indicate that kinetic effects dominate
the complex catalyst phase evolution during realistic CNT growth recipes
The Phase of Iron Catalyst Nanoparticles during Carbon Nanotube Growth
We study the Fe-catalyzed chemical vapor deposition of
carbon nanotubes
by complementary in situ grazing-incidence X-ray diffraction, in situ
X-ray reflectivity, and environmental transmission electron microscopy.
We find that typical oxide supported Fe catalyst films form widely
varying mixtures of bcc and fcc phased Fe nanoparticles upon reduction,
which we ascribe to variations in minor commonly present carbon contamination
levels. Depending on the as-formed phase composition, different growth
modes occur upon hydrocarbon exposure: For γ-rich Fe nanoparticle
distributions, metallic Fe is the active catalyst phase, implying
that carbide formation is not a prerequisite for nanotube growth.
For α-rich catalyst mixtures, Fe<sub>3</sub>C formation more
readily occurs and constitutes part of the nanotube growth process.
We propose that this behavior can be rationalized in terms of kinetically
accessible pathways, which we discuss in the context of the bulk iron–carbon
phase diagram with the inclusion of phase equilibrium lines for metastable
Fe<sub>3</sub>C. Our results indicate that kinetic effects dominate
the complex catalyst phase evolution during realistic CNT growth recipes
<i>In Situ</i> Observations of the Atomistic Mechanisms of Ni Catalyzed Low Temperature Graphene Growth
The key atomistic mechanisms of graphene formation on Ni for technologically relevant hydrocarbon exposures below 600 °C are directly revealed <i>via</i> complementary <i>in situ</i> scanning tunneling microscopy and X-ray photoelectron spectroscopy. For clean Ni(111) below 500 °C, two different surface carbide (Ni<sub>2</sub>C) conversion mechanisms are dominant which both yield epitaxial graphene, whereas above 500 °C, graphene predominantly grows directly on Ni(111) <i>via</i> replacement mechanisms leading to embedded epitaxial and/or rotated graphene domains. Upon cooling, additional carbon structures form exclusively underneath rotated graphene domains. The dominant graphene growth mechanism also critically depends on the near-surface carbon concentration and hence is intimately linked to the full history of the catalyst and all possible sources of contamination. The detailed XPS fingerprinting of these processes allows a direct link to high pressure XPS measurements of a wide range of growth conditions, including polycrystalline Ni catalysts and recipes commonly used in industrial reactors for graphene and carbon nanotube CVD. This enables an unambiguous and consistent interpretation of prior literature and an assessment of how the quality/structure of as-grown carbon nanostructures relates to the growth modes
<i>In Situ</i> Observations of the Atomistic Mechanisms of Ni Catalyzed Low Temperature Graphene Growth
The key atomistic mechanisms of graphene formation on Ni for technologically relevant hydrocarbon exposures below 600 °C are directly revealed <i>via</i> complementary <i>in situ</i> scanning tunneling microscopy and X-ray photoelectron spectroscopy. For clean Ni(111) below 500 °C, two different surface carbide (Ni<sub>2</sub>C) conversion mechanisms are dominant which both yield epitaxial graphene, whereas above 500 °C, graphene predominantly grows directly on Ni(111) <i>via</i> replacement mechanisms leading to embedded epitaxial and/or rotated graphene domains. Upon cooling, additional carbon structures form exclusively underneath rotated graphene domains. The dominant graphene growth mechanism also critically depends on the near-surface carbon concentration and hence is intimately linked to the full history of the catalyst and all possible sources of contamination. The detailed XPS fingerprinting of these processes allows a direct link to high pressure XPS measurements of a wide range of growth conditions, including polycrystalline Ni catalysts and recipes commonly used in industrial reactors for graphene and carbon nanotube CVD. This enables an unambiguous and consistent interpretation of prior literature and an assessment of how the quality/structure of as-grown carbon nanostructures relates to the growth modes
<i>In Situ</i> Observations of the Atomistic Mechanisms of Ni Catalyzed Low Temperature Graphene Growth
The key atomistic mechanisms of graphene formation on Ni for technologically relevant hydrocarbon exposures below 600 °C are directly revealed <i>via</i> complementary <i>in situ</i> scanning tunneling microscopy and X-ray photoelectron spectroscopy. For clean Ni(111) below 500 °C, two different surface carbide (Ni<sub>2</sub>C) conversion mechanisms are dominant which both yield epitaxial graphene, whereas above 500 °C, graphene predominantly grows directly on Ni(111) <i>via</i> replacement mechanisms leading to embedded epitaxial and/or rotated graphene domains. Upon cooling, additional carbon structures form exclusively underneath rotated graphene domains. The dominant graphene growth mechanism also critically depends on the near-surface carbon concentration and hence is intimately linked to the full history of the catalyst and all possible sources of contamination. The detailed XPS fingerprinting of these processes allows a direct link to high pressure XPS measurements of a wide range of growth conditions, including polycrystalline Ni catalysts and recipes commonly used in industrial reactors for graphene and carbon nanotube CVD. This enables an unambiguous and consistent interpretation of prior literature and an assessment of how the quality/structure of as-grown carbon nanostructures relates to the growth modes
<i>In Situ</i> Observations of the Atomistic Mechanisms of Ni Catalyzed Low Temperature Graphene Growth
The key atomistic mechanisms of graphene formation on Ni for technologically relevant hydrocarbon exposures below 600 °C are directly revealed <i>via</i> complementary <i>in situ</i> scanning tunneling microscopy and X-ray photoelectron spectroscopy. For clean Ni(111) below 500 °C, two different surface carbide (Ni<sub>2</sub>C) conversion mechanisms are dominant which both yield epitaxial graphene, whereas above 500 °C, graphene predominantly grows directly on Ni(111) <i>via</i> replacement mechanisms leading to embedded epitaxial and/or rotated graphene domains. Upon cooling, additional carbon structures form exclusively underneath rotated graphene domains. The dominant graphene growth mechanism also critically depends on the near-surface carbon concentration and hence is intimately linked to the full history of the catalyst and all possible sources of contamination. The detailed XPS fingerprinting of these processes allows a direct link to high pressure XPS measurements of a wide range of growth conditions, including polycrystalline Ni catalysts and recipes commonly used in industrial reactors for graphene and carbon nanotube CVD. This enables an unambiguous and consistent interpretation of prior literature and an assessment of how the quality/structure of as-grown carbon nanostructures relates to the growth modes
Co-catalytic Absorption Layers for Controlled Laser-Induced Chemical Vapor Deposition of Carbon Nanotubes
The concept of co-catalytic layer
structures for controlled laser-induced chemical vapor deposition
of carbon nanotubes is established, in which a thin Ta support layer
chemically aids the initial Fe catalyst reduction. This enables a
significant reduction in laser power, preventing detrimental positive
optical feedback and allowing improved growth control. Systematic
study of experimental parameters combined with simple thermostatic
modeling establishes general guidelines for the effective design of
such catalyst/absorption layer combinations. Local growth of vertically
aligned carbon nanotube forests directly on flexible polyimide substrates
is demonstrated, opening up new routes for nanodevice design and fabrication